Canadian Patents Database / Patent 2422460 Summary

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(12) Patent Application: (11) CA 2422460
(54) English Title: MANUFACTURING A FIBER BRAGG GRATING AND A MASK USED IN THE FABRICATION OF A FIBER BRAGG GRATING
(54) French Title: FABRICATION DE RESEAUX DE BRAGG EN FIBRES ET MASQUE UTILISE DANS LA FABRICATION DESDITS RESEAUX
(51) International Patent Classification (IPC):
  • G02B 6/00 (2006.01)
  • G02B 6/02 (2006.01)
(72) Inventors :
  • ZWEIBACK, JASON (United States of America)
  • ROTHENBERG, JOSHUA E. (United States of America)
  • POPELEK, JAN (United States of America)
  • CALDWELL, ROGER F. (United States of America)
(73) Owners :
  • TERAXION INC. (Canada)
(71) Applicants :
  • PHAETHON COMMUNICATIONS (United States of America)
(74) Agent: ROBIC
(74) Associate agent:
(45) Issued:
(86) PCT Filing Date: 2001-09-19
(87) Open to Public Inspection: 2002-03-28
(30) Availability of licence: N/A
(30) Language of filing: English

(30) Application Priority Data:
Application No. Country/Territory Date
60/234,318 United States of America 2000-09-20
60/235,873 United States of America 2000-09-27
60/241,594 United States of America 2000-10-18
60/243,423 United States of America 2000-10-25
09/757,386 United States of America 2001-01-08
09/957,443 United States of America 2001-09-18

English Abstract




The invention reduces the effects of stitching errors from re-scaling or re-
positioning in the fabrication of fiber Bragg gratings or the mask used in
such fabrication. A first embodiment of the invention preferably uses
characteristics of stitching errors to compensate for the stitching errors
themselves. By increasing the number of stitching errors, errors caused by the
stitching errors can be reduced. A second embodiment uses continuous writing
of the desired pattern, wherein the desired pattern is snapped to a grid that
can be written by the fabrication equipment. Using continuous writing
eliminates stitching errors in the resulting gratings.


French Abstract

Cette invention permet d'atténuer les conséquences d'erreurs d'assemblage dues au re-dimensionnement ou au re-positionnement dans la fabrication de réseaux Bragg en fibres ou du masque utilisé pour ce processus de fabrication. Selon un premier mode de réalisation, on exploite de préférence les caractéristiques d'erreurs d'assemblage pour compenser lesdites erreurs. En augmentant le nombre d'erreurs d'assemblage, on peut réduire les erreurs dues à ces erreurs d'assemblage. Selon un second mode de réalisation, on fait intervenir une écriture en continu de motif recherché, ce motif étant déplacé vers une grille qui peut être écrite dans le matériel de fabrication. L'écriture en continu permet d'éliminer les erreurs d'assemblage dans les réseaux ainsi obtenus.


Note: Claims are shown in the official language in which they were submitted.



CLAIMS

What is claimed is:

1. A method for producing an optical grating comprising:
designing an optical pattern;
inducing a sufficient number of errors into the pattern to reduce the
average of the errors to a predetermined number; and
recording the pattern with the sufficient number of errors into an
optical element.

2. The method of claim 1 wherein the pattern comprises a
plurality of segments, and the step of inducing errors comprises:
writing an additional number of segments than are required by a
desired design.

3. The method of claim 1 wherein:
the predetermined number is about zero.

4. The method of claim 1 wherein:
the optical element is a mask, and the mask is used to form the
grating.

5. The method of claim 4 wherein the step of recording
comprises the step of:
exposing the mask with at least one beam.

24




6. The method of claim 4 wherein:
the errors are stitching errors; and
a group delay ripple error of the grating is decreased as the number of
stitching errors is increased.

7. The method of claim 1 wherein:
the pattern includes information associated with one of a linear chirp
and a non-linear chirp.

8. The method of claim 1 wherein the pattern comprises a
plurality of segments, and the step of inducing comprises:
inducing a plurality of stitching errors into the pattern.

9. The method of claim 8 wherein the step of inducing the
sufficient number of errors further comprises:
forming at least one segment to have a different period by adjusting a
scaling factor of manufacturing equipment that is used in the step of
recording.

10. The method of claim 8 wherein:
each segment has an arbitrary period with respect to at least one of a
previous segment and a subsequent segment in the pattern.




11. The method of claim 8 wherein the pattern comprises a
plurality of bars and spaces, and the step of inducing the plurality of
stitching
errors comprises:
adjusting desired locations of edges of bars and spaces to pixel
locations that are useable by manufacturing equipment used in the step of
recording.

12. The method of claim 11 wherein:
the pixel locations coincide with a periodic grid.

13. The method of claim 12 wherein:
a size of the period of the grid is 25 nm or less.

14. The method of claim 12 wherein:
a size of the period of the grid is 10 nm or less.

15. The method of claim 11 wherein the step of adjusting
comprises:
adjusting each of the desired locations to the nearest pixel location.

16. The method of claim 11 wherein:
the step of adjusting moves each desired location by up to one half of
pixel spacing.

26



17. The method of claim 8 wherein the step of inducing a plurality
of stitching errors comprises:
forming a plurality of sub-segments for each segment of the plurality
of segments.

18. The method of claim 17 wherein:
at least one segment has a different period; and
each sub-segment has the same period as the segment from which it
was formed.

19. The method of claim 17 wherein:
at least one segment has a different period; and
each sub-segment has a scaled period, such that sequential sub-
segments from a particular segment have periods that range from a period
that is greater than the period of a previous segment to a period that is less
than the period of a subsequent segment.

20. The method of claim 17 wherein:
each sub-segment has an arbitrary period with respect to at least one
of a previous sub-segment and a subsequent sub-segment.

21. The method of claim 1 wherein the pattern is continuously
recorded into the optical element and comprises a plurality of bars and
spaces, and the step of inducing comprises:
adjusting desired locations of edges of bars and spaces to pixel
locations that are useable by manufacturing equipment used in the step of
recording.

27


22. The method of claim 21 wherein:
the pixel locations coincide with a periodic grid.

23. The method of claim 22 wherein:
a size of the period of the grid is 25 nm or less.

24. The method of claim 22 wherein:
a size of the period of the grid is 10 nm or less.

25. The method of claim 21 wherein the step of adjusting
comprises:
adjusting each of the desired locations to the nearest pixel location.

26. The method of claim 21 wherein:
the step of adjusting moves each desired location by up to one half of
pixel spacing.

27. The method of claim 1 wherein the step of recording
comprises the step of:
writing the pattern with at least one raster scanned e-beam.

28. The method of claim 1 wherein the step of recording
comprises the step of:
writing the pattern with at least one raster scanned laser beam.

28


29. The method of claim 28 wherein:
the step of writing uses at least 24 beams.

30. The method of claim 28 wherein the step of writing uses a
plurality of beams in parallel, and the method further comprises:
repeating the step of writing for multiple exposures and thereby
reduce placement error.

31. The method of claim 1 wherein the step of recording
comprises the step of:
writing the pattern with at least one shaped e-beam.

32. The method of claim 31 wherein the step of writing the
pattern with at least one shaped e-beam comprises the step of:
writing a plurality of at least one type of geometrical shape.


33. The method of claim 32 wherein the step of writing the
pattern further comprises the step of:
performing the step of writing the plurality of at least one type of
geometrical shape for a sub-field of the optical element;
repositioning writing equipment after the step of performing for a
subsequent sub-field.

29



34. The method of claim 1 wherein the step of recording operates
with manufacturing equipment with a writing grid size of less than or equal
to 10 manometers.

35. The method of claim 1 wherein the step of recording operates
with manufacturing equipment with a writing grid size of less than or equal
to 25 manometers.

36. The method of claim 1 wherein:
optical element is a fiber, and the step of recording forms the grating
in the fiber.

37. The method of claim 36 wherein:
a group delay ripple error of the grating is decreased as the number of
errors is increased.

38. The method of claim 1 further comprising:
including at least one phase shift in the pattern;
wherein the step of recording is operative to record the pattern with
the at least one phase shift into the optical element.

30



39. An optical mask that is useable to produce a grating
comprising:
a pattern of bars and spaces, wherein the pattern includes a sufficient
number of errors in the pattern to reduce the average of the errors to a
predetermined number.

40. The mask of claim 39 wherein:
edges of the bars and spaces are locations coinciding with a periodic
grid.

41. The mask of claim 40 wherein:
a size of the period of the grid is 25 rim or less.

42. The mask of claim 40 wherein:
a size of the period of the grid is 10 nm or less.

43. The mask of claim 39 wherein the pattern comprises a
plurality of segments, and a number of the plurality of segments is greater
than a number of segments required by a desired design.

44. The mask of claim 39 wherein:
the predetermined number is about zero.

31



45. The mask of claim 39 wherein:
the pattern includes information associated with one of a linear chirp
and a non-linear chirp.

46. The mask of claim 39 wherein:
the errors are stitching errors; and
a group delay ripple error of the grating is decreased as the number of
stitching errors is increased.

47. The mask of claim 39 wherein:
the pattern comprises a plurality of segments.

48. The mask of claim 47 wherein
at least one segment has a period that is different by a scaling factor.

49. The mask of claim 47 wherein:
each segment has an arbitrary period.

50. The mask of claim 47 wherein:
the errors are stitching errors induced by adjusting edges of the bars
and spaces from desired locations of the edges of bars and spaces.

32



51. The mask of claim 50 wherein:
the edges of the bars and spaces are locations coinciding with a
periodic grid.

52. The mask of claim 51 wherein:
a size of the period of the grid is 25 nm or less.

53. The mask of claim 51 wherein:
a size of the period of the grid is 10 nm or less.

54. The mask of claim 47 wherein:
each segment comprises a plurality of sub-segments.

55. The mask of claim 54 wherein:
at least one segment has a different period; and
each sub-segment has the same period as its associated segment.

56. The mask of claim 39 wherein:
the errors are induced by adjusting edges of the bars and spaces from
desired locations of the edges of bars and spaces.

57. The mask of claim 39 wherein:
the pattern includes at least one phase shift.

33


58. A system that produces an optical grating, the system
comprising:
means for designing an optical pattern;
means for inducing a sufficient number of errors into the pattern to
reduce the average of the errors to a predetermined number; and
means for recording the pattern with the sufficient number of errors
into an optical element.

59. The system of claim 58 wherein the pattern comprises a
plurality of segments, and the means for inducing errors comprises:
means for writing additional segments than are required by a desired
design.

60. The system of claim 58 wherein:
the predetermined number is about zero.

61. The system of claim 58 wherein:
the optical element is a mask, and the mask is used to form the
grating.

62. The system of claim 61 wherein the means for recording
comprises:
means for exposing the mask with at least one beam.

63. The system of claim 61 wherein:
the errors are stitching errors, and

34


a group delay ripple error of the grating is decreased as the number of
stitching errors is increased.

64. The system of claim 58 wherein:
the pattern includes information associated with one of a linear chirp
and a non-linear chirp.

65. The system of claim 58 wherein the pattern comprises a
plurality of segments, and the means for inducing comprises:
means for inducing a plurality of stitching errors into the pattern.

66. The system of claim 65 wherein the means for inducing the
sufficient number of errors further comprises:
means for forming at least one segment to have different a period by
adjusting a scaling factor of the means for recording.

67. The system of claim 65 wherein:
each segment has an arbitrary period with respect to at least one of a
previous segment and a subsequent segment in the pattern.

68. The system of claim 65 wherein the pattern comprises a
plurality of bars and spaces, and the means for inducing the plurality of
stitching errors comprises:
means for adjusting desired locations of edges of bars and a spaces to
pixel locations that are useable by the means for recording.

35


69. The system of claim 68 wherein:
the pixel locations coincide with a periodic grid.

70. The system of claim 69 wherein:
a size of the period of the grid is 25 nm or less.

71. The system of claim 69 wherein:
a size of the period of the grid is 10 nm or less.

72. The system of claim 68 wherein the means for adjusting
comprises:
means for adjusting each of the desired locations to the nearest pixel
location.

73. The system of claim 68 wherein:
the means for adjusting moves each desired location by up to one half
of pixel spacing.

74. The system of claim 65 wherein the means for inducing a
plurality of stitching errors comprises:
means for forming a plurality of sub-segments for each segment of
the plurality of segments.

75. The system of claim 74 wherein:
at least one segment has a different period; and

36



each sub-segment has the same period as the segment from which it
was formed.

76. The system of claim 74 wherein:
at least one segment has a different period; and
each sub-segment has a scaled period, such that sequential sub-
segments from a particular segment have periods that range from a period
that is greater than the period of a previous segment to a period that is less
than the period of a subsequent segment.

77. The system of claim 74 wherein:
each sub-segment has an arbitrary period with respect to at least one
of a previous sub-segment and a subsequent sub-segment.

78. The system of claim 58 wherein the pattern is continuously
recorded into the optical element and comprises a plurality of bars and
spaces, and the means of inducing comprises:
means for adjusting desired locations of edges of bars and spaces to
pixel locations that are useable by the means for recording.

79. The system of claim 78 wherein:
the pixel locations coincide with a periodic grid.

80. The system of claim 79 wherein:
a size of the period of the grid is 25 rim or less.

37


81. The system of claim 79 wherein:
a size of the period of the grid is 10 nm or less.

82. The system of claim 78 wherein the means for adjusting
comprises:
means for adjusting each of the desired locations to the nearest pixel
location.

83. The system of claim 78 wherein:
the means for adjusting moves each desired location by up to one half
of pixel spacing.

84. The system of claim 58 wherein the means for recording
comprises:
means for generating at least one raster scanned e-beam.

85. The system of claim 58 wherein the means for recording
comprises:
means for generating at least one raster scarred laser beam.

86. The system of claim 85 wherein:
the means for generating at least one raster scanned laser beam
generates at least 24 beams.

38



87. The system of claim 85 wherein:
the means for generating at least one raster scanned laser beam
generates a plurality of beams in parallel and are used for multiple exposures
and thereby reduce placement error.

88. The system of claim 58 wherein the means for recording
comprises:

means for generating at least one shaped e-beam.

89. The system of claim 88 wherein the at least one shaped e-
beam writes a plurality of at least one type of geometrical shape.

90. The system of claim 89 wherein the means for generating at
least one shaped e-beam writes the plurality of at least one type of
geometrical shape for a sub-field of the optical element, and repositions
after
writing for a subsequent sub-field.

91. The system of claim 58 wherein the means for recording has a
writing grid size of less than or equal to 10 nanometers.

92. The system of claim 58 wherein the means for recording has a
writing grid size of less than or equal to 25 nanometers.

93. The system of claim 58 wherein:
the optical element is a fiber, and means for recording forms the
grating in the fiber.


39


94. The system of claim 93 wherein:
a group delay ripple error of the grating is decreased as the number of
errors is increased.

95. The system of claim 58 wherein:
the pattern includes at least one phase shift; and
the means for recording is operative to record the pattern with the at
least one phase shift into the optical element.


40

Note: Descriptions are shown in the official language in which they were submitted.


CA 02422460 2003-03-13
WO 02/25327 PCT/USO1/29178
MANUFACTURING A FIBER BRAGG GRATING AND A MASK USED IN THE
FABRICATION OF A FIBER BRAGG GRATING
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims benefit of priority to Provisional
Application
Serial Nos. 60/234,318 entitled "EFFICIENT PERIODIC SUPER-STRUCTURES IN
WAVE-GUIDES TO PRODUCE SPECTRAL RESPONSE OVER MANY CHANNELS
AND FABRICATION METHODS FOR THESE STRUCTURES" filed September 20,
2000, 60/243,423 entitled "SAMPLED FIBER BRAGG GRATING BASED ON
MULTILEVEL PHASE CHANGE TECHNOLOGY" filed October 25, 2000, 60/235,873
entitled "EFFICIENT PERIODIC SUPER-STRUCTURES IN WAVEGUIDES" filed
September 27, 2000, and 60/241,594 entitled "SAMPLED FIBER BRAGG GRATING
BASED ON MULTILEVEL PHASE CHANGE TECHNOLOGY" filed October 18, 2000,
the disclosures of which are hereby incorporated herein by reference; and is
related to
commonly assigned, co-pending U.S. Application Serial Nos. 09/757,386 entitled
"EFFICIENT SAMPLED GRATINGS FOR WDM APPLICATIONS" filed January 8,
2001, and 09/883,081 entitled "LITHOGRAPHIC FABRICATION OF PHASE MASK
FOR FIBER BRAGG GRATINGS" filed June 15, 2001, the disclosures of which are
hereby
incorporated herein by reference.


CA 02422460 2003-03-13
WO 02/25327 PCT/USO1/29178
BACKGROUND OF THE INVENTION
[0002] The present invention relates in general to fiber Bragg gratings, and
in
specific to methods and apparatuses for producing masks that are used to
create fiber Bragg
gratings.
[0003] Normal optical fibers are uniform along their lengths. A slice from any
one
point of the fiber looks lilce a slice taken from anywhere else on the fiber,
disregarding tiny
imperfections. However, it is possible to modify fibers in such a way that the
refractive
index varies regularly along their length. These fibers are called fiber Bragg
gratings
(FBG). The periodic refractive index variation causes different wavelengths of
light to
interact differently with the fiber, with certain wavelengths being reflected
and certain
wavelengths being transmitted.
[0004] Whenever there is a change in the index of refraction within the fiber,
there
is a slight reflection from the transition. In an FBG there are many of these
slight
reflections. The locations of these reflections are arranged such that the
reflections all
interfere with each other to create a strong reflection at a certain
wavelength. This is the so
called Bragg condition, and is satisfied when the wavelength of light is equal
to twice the
period of the index modulation times the overall index of refraction of the
fiber. Light that
does not meet this Bragg condition will be transmitted.
[0005] FBGs can be used to compensate for chromatic dispersion in an optical
fiber. Dispersion is the spreading out of light pulses as they travel on the
fiber. Dispersion
occurs because the speed of light through the fiber depends on its wavelength,
polarization,
and propagation mode. The differences are slight, but accumulate with
distance. Thus, the
longer the fiber, the more dispersion. Dispersion can limit the distance a
signal can travel
through the optical fiber because dispersion cumulatively blurs the signal.
After a certain
point, the signal has become so blurred that it is unintelligible. The FBGs
are used to
compensate for chromatic (wavelength) dispersion by serving as a selective
delay line. The
FBG delays the wavelengths that travel fastest through the fiber until the
slower
wavelengths catch up. The spacing of the grating is chirped, varying along its
length, so
that different wavelengths are reflected at different points along the fiber.
These points
2


CA 02422460 2003-03-13
WO 02/25327 PCT/USO1/29178
correspond to the amount of delay that the particular wavelengths need to have
so that
dispersion is compensated. Suppose that the fiber induces dispersion such that
a longer
wavelength travels slower than a shorter wavelength. Thus, a shorter
wavelength would
have to travel farther into the FBG before being reflected back. A longer
wavelength would
travel less far into the FBG. Consequently, the longer and shorter wavelengths
can be made
coincidental, and thus without dispersion. FBGs are discussed further in Feng
et al. United
States Patent Number 5,982,963, which is hereby incorporated herein by
reference in its
entirety. A circulator is used to separate the light reflected from the FBG
onto a different
fiber from the input. With a properly designed FBG, the group delay is a
function of the
wavelength of the reflected beam and has the desired shape to compensate for
dispersion
(group delay) accumulated in propagation through an optical communication
transmission
system. One practical problem encountered with such FBG devices is that the
group delay
fluctuates around the desired functional shape. This deviation shall be
referred to as group
delay ripple (GDR) and is generally deleterious to the quality of transmitted
optical signals.
[0006] FBGs are typically fabricated in two manners. The first manner uses a
phase mask. The phase mask is a quartz slab that is patterned with a grating.
The mask is
placed in close proximity with the fiber, and ultraviolet light, usually from
an ultraviolet
laser, is shined through the maslc and into the fiber. As the light passes
through the mask,
the light is primarily diffracted into two directions, which then forms an
interference pattern
in the fiber. The interference pattern comprises regions of high and low
intensity light. The
high intensity light causes a change in the index of refraction of that region
of the fiber:
Since the regions of high and low intensity light are alternating, a FBG is
formed in the
fiber. See also Kashyap, "Fiber Bragg Gratings", Academic Press (1999), ISBN 0-
12-
400560-8, which is hereby incorporated herein by reference in its entirety.
[0007] The second manner is known as the direct write FBG formation. In this
manner two ultraviolet beams are impinged into the fiber, in such a manner
that they
interfere with each other and form an interference pattern in the fiber. At
this point, the
FBG is formed in the same way as the phase mask manner. One of the fiber or
the writing
system is moved with respect to the other such that the interference pattern
is scanned and
the fiber exposed. Note that the two beams are typically formed from a single
source beam


CA 02422460 2003-03-13
WO 02/25327 PCT/USO1/29178
by passing the beam through a beam separator, e.g. a beamsplitter or a
grating. Also, the
two beams are typically controlled in some mamier so as to allow control over
the locations
of the high and low intensity regions. For example, Laming et al., WO
99/22256, which is
hereby incorporated herein by reference in its entirety, teaches that the beam
separator and
part of the focusing system are moveable to alter the angle of convergence of
the beams,
which in turn alters the fringe pitch on the fiber. Another example is
provided by Glenn,
United States Patent Number 5,388,173, and Stepanov et al., WO 99/63371, both
of which
are hereby incorporated herein by reference in their entirety. Both teach the
use of an
electro-optic module, which operates on the beams to impart a phase delay
between the
beams, which in turn controls the positions of the high and low intensity
regions.
[0008] Note that whichever manner is used, it is still difficult to
manufacture
FBGs. The period of the spacing of the index modulation of the fiber Bragg
grating is
typically about one-half micron. When a phase mask is used to fabricate an
FBG, the period
of the mask grating is chosen to be twice that of the FBG, or about 1 micron.
Thus, the
etched bars and spaces which comprise the phase mask are about five hundred
nanometers
in width. For example, one application of the FBG is dispersion compensation.
In this
application FBGs must have a chirp (a slow variation) in the period, which is
typically a
very small change (~ lnm) over the length of the FBG. Thus, the spacing would
ideally
need to be adjusted on a picometer scale to have the period change
appropriately over the
length of the grating. This presents a serious challenge in design of any
grating writing
system. Inaccuracies in forming the chirp can cause group delay ripple in the
output of the
FB G.
[0009] Each FBG writing manner has advantages and disadvantages when
compared with each other. For example, the first maalner, the phase mask
manner, is
relatively inflexible, as changes cannot be made to the mask. However, since
the phase
mask is permanent, the phase mask manner is stable, repeatable, and aside from
the cost of
the mask, relatively inexpensive to operate. On the other hand, the direct
write manner is
very flexible, and can write different gratings. However, this manner is less
repeatable and
is costly to operate. Also, the direct writing process must be very strictly
controlled. Any
variation will lead to differences between gratings. This is difficult because
the coherence
4


CA 02422460 2003-03-13
WO 02/25327 PCT/USO1/29178
(i.e. the relative position of the index modulation on a nm scale) of the
entire pattern, e.g. 20
cm or greater, must be maintained. Little changes in alignment, temperature,
etc. can result
in the loss of coherence.
[0010] Another problem with the phase mask manner resides in the fabrication
of
the masks. Masks axe fabricated by lithographic ox holographic techniques.
More
specifically, the exposure of the resist that coats the maslc may be done
holographically, as
well as lithographically. In the lithograplucal method, a small beam (of width
smaller than
the minimum mask feature size - 0.5 micron) is used to directly expose the
resist with the
desired pattern. In the holographic method, two large (large meaning having a
beam section
that is approximately the same size as the masle) beams are interfered with
each other to
produce a periodic intensity pattern that exposes the resist on the mask
substrate. While this
process is used for simple masks, it is limited in its capabilities since the
phase fronts of the
interfering beams cannot be easily varied arbitrarily. For complicated masks,
containing
phase shifts and complex (nonlinear) chirp functions, current art holographic
methods are
not effective and lithographic methods are preferred.
[0011] The mask slab is coated on its surface with resist, which is a light
(photo)
or particle (electron or ion) sensitive material. Under the resist, the slab
may also be coated
with a metallic layer (e.g. chrome) to assist conduction of charged particles
away from the
exposed regions. Regions or bars of the resist are illuminated by light or
particle beams
according to a desired pattern, which is generally an array of parallel bars
along a straight
line with precisely selected positions. This illumination causes chemical
changes in the
exposed regions of resist. The exposed resist can be preferentially removed
from the slab
by a chemical or plasma, which does not strongly affect the unexposed resist
(or vice versa).
After the preferential removal of the resist according to the desired pattern,
the slab may
then be etched by a different chemical or plasma, which preferentially etches
the slab where
the resist has been removed. The etched portions of the slab have a difference
in thickness
or height from the un-etched portions. When the etched (bars) and un-etched
(spaces)
portions are patterned to form an array along a substantially straight line,
the differences in
thickness form a phase grating. Thus, by etching an array of bars and spaces
on the slab to
form a grating, a phase mask is fabricated. Other lithography tools can
directly etch the bars


CA 02422460 2003-03-13
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and spaces onto the mask rather than in resist. In another embodiment, these
regions can
have alternate transmittance properties, such as by the presence or absence of
an opaque
material (e.g. chrome), and thus form an amplitude grating. Note that in all
these cases, the
critical part of the fabrication is the exposure of the bars and spaces (or
direct etching of the
bars and spaces). The resulting mashc is limited by the quality and precision
of the exposure
process.
[0012] Current lithographic techniques use segmenting to encode the chirp into
the
mask. Due to the limitations of the lithographic writing machines, the period
of the grating
cannot be continuously varied. Fortunately, the grating can be written as a
series of butt-
coupled uniform period gratings which approximate a grating with a
continuously varying
period in a stepwise mariner. A first series of bars, e.g. 500 bars, are
written at a first period.
A second series of bars are written at a second period, which is slightly
different from the
first period, and so on, until the desired variation of period (chirp) is
written into the entire
mask. The lithographic machines typically have a scaling feature that allows a
segment to
be scaled in size to picometer accuracy. Thus, a first segment is written at a
first scale, and
then the segment is rescaled to a different scale, which is slightly different
from the first
scale, and so on, until the mask is completely written.
[0013] This solution might be adequate for creating the proper pitch, but
still
suffers from a positioning error that occurs when the position is changed to
write subsequent
segments. This type of error is known as a'stitching error'. Thus, each time
the machine is
rescaled and repositioned for a different segment, another stitching error is
added to the
mask. This, in turn, introduces an error into the grating that is written into
the fiber. These
errors cause group delay ripple in the optical signal reflected from the FBG.
Consequently,
the prior art attempts to write as few segments as possible, thus minimizing
the number of
stitching errors. For example, a typical mask would need about 100-200 scaled
segments to
encode the chirp into the mask. Thus, the prior art would only write about 100-
200 scaled
segments. -
[0014] Note that the current technology for lithography does have the
capability to
write continuous patterns (so called cursive writing) effectively without such
stitching
errors. However, this cursive writing camiot be used to make maslcs for
chirped FBGs


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and/or FBGs with arbitrary phase shifts (positional shifts of the bars or
spaces, or changes in
the bar or space widths or period), without the introduction of stitching
errors. This is
because such cursive writing methods would not allow for rescaling of the
grating period
along the length of the phase maslc. In addition, the locations of the bars
and spaces on the
mask are limited to fit on an address unit grid which is much too coarse to
allow the
picometer scale positioning required of the varying grating period.


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BRIEF SUMMARY OF THE INVENTION
[0015] These and other objects, features, and technical advantages are
achieved by
a system and method system which uses current lithography tools to fabricate
masks with
greatly reduce the effects of stitching errors from re-scaling or re-
positioning. The masks
fabricated by the invention will generate the linear or non-linear chirp, and
other phase
shifts as desired, in the fiber Bragg grating (FBG) in the core of the fiber.
[0016) A first embodiment of the invention preferably uses a characteristic of
stitching errors to compensate for the stitching errors themselves. Each
stitching error is
typically random. Some stitching errors are formed when the segments are too
far apart,
thereby having too wide a space between the end bars of the adjacent segments.
Other
stitching errors are formed when the segments are too close together, thereby
having too
narrow a space between the end bars of the adjacent segments. Consequently,
error induced
by one stitching error may be offset by another stitching error. The invention
preferably
takes advantage of the characteristic that the effective error introduced into
the grating from
the mask with the stitching errors is the root mean square (RMS) of the
stitching errors,
when averaged over a length determined by the characteristics of the FBG
design. This
averaging length is typically on the order of 1 mm, and thus since the mask
period is about 1
micron, the averaging occurs over about 1000 periods of the mask. Thus,
increasing the
number of stitching errors, by increasing the number of segments, can increase
the number
of errors being averaged by the light passing through the FBG. This increases
the
population of stitching errors and normalizes the mean, by bringing the median
value closer
to the mean value of pool of stitching errors. Thus, the overall average is
brought closer to
zero or no error. In other words, this increase in the number of stitching
errors tends to
reduce the RMS of the effective net stitching error by averaging out the
stitching errors, and
hence reduces the group delay ripple of a FBG created from the mask. Thus, a
mask with
an increased number of stitching errors, so long as these additional errors
occur over the
effective averaging length of the FBG, produces a grating with a lower group
delay ripple
error. If 1000 such errors are introduced over the effective averaging length
in the FBG,
then the net effective stitching error should be reduced by about 1000 or
about 30 times.


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[0017] The first embodiment is preferably implemented in one of two ways. In
the
first way, each segment is split into a plurality of segments that have the
same scaling. For
example, assume the example of the prior art segments, wherein 200 segments
are used to
form a 10 cm long mask, with each segment having a slightly different scaling
factor. Each
segment may be further split into 4 segments for a total of 800 segments, with
each sub-
segment within a particular group having the same scaling factor. In the
second way, the .
scaling factor is adjusted for each of the smaller segments. For example,
assume the
example of the prior art segments, wherein 200 segments are used to form a
mask, with each
segment having a different scaling factor, as compared with an adjacent
segment. This
scaling change is generally extremely small. For a typical application of a 10
cm grating
with 200 segments, the period may change about 3 pm per segment, as compared
to the
nominal 1000 nm period, or a scale change of about 3 x 10-~. Each segment may
then be
further split into 4 segments for a total of 800 segments, with each segment
having a smaller
change in scaling factor of about 7.5 x 10-x, as compared with an adjacent
segment.
[0018] A second embodiment uses continuous writing of the desired pattern.
Instead of writing a series of scaled segments, the entire grating or mask is
written in one
cursive writing cycle at a single scale, i.e. one continuous single-scale
segment. Thus, there
should not be any stitching errors as the writing equipment is not stopped for
resealing and
re-alignment. In writing a grating pattern that includes a fme scale chirp,
the desired size of
the bars and spaces (i.e. the location of the edges) may not be achievable on
the address unit
or pixel grid required by the writing equipment. The invention has the bar
and/or spacing
lines moved or snapped to the nearest address unit or grid. The error of
placement of bar
edges would accumulate as a difference between the ideal and the desired
pattern until the
error at most equals one-half of a pixel width, and then the edge would snap
to the next grid
location. While the misalignment between the designed edges and the actual
edges will
induce many more errors in the resulting fiber Bragg grating than the current
art of using
resealed segments, the effective net error is minimized by the averaging
described above
with regards to the first embodiment. For a uniform distribution of error up
to ~ one-half of
the grid spacing (or pixel size) p, the expected statistical RMS error for
each edge placement
is found to be about ~0.29p. In this case, for example, with the current
lithography tools,
the pixel size is p = 5 nm, and the expected RMS error of an edge is ~ ~1.5
nm. Since these


CA 02422460 2003-03-13
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errors occur at every edge, which are typically separated by ~l ~,m, in the
above example of
an effective averaging length of ~ 1 mm, ~ 1000 edge errors are averaged and
therefore the
effective net averaged error is reduced by 1000 ~30 and is thus only ~ 50 pm.
[0019] Note that the above described inventive embodiments can also be used
for
FBGs that have a second periodic pattern superimposed on the basic pattern. In
general this
pattern is introduced as an amplitude pattern or periodic set of phase shifts
(see for example,
U.S. Application Serial No. 09/757,386 entitled "EFFICIENT SAMPLED GRATINGS
FOR WDM APPLICATIONS" filed January 8, 2001, the disclosure of which is hereby
incorporated herein by reference). This pattern serves to sample the initial
grating, creating
duplicate reflective channels at a spacing dependent on the period of the
secondary
(sampling) pattern.
[0020] The foregoing has outlined rather broadly the features and technical
advantages of the present invention in order that the detailed description of
the invention
that follows may be better understood. Additional features and advantages of
the invention
will be described hereinafter which fomn the subject of the claims of the
invention. It
should be appreciated by those skilled in the art that the conception and
specific
embodiment disclosed may be readily utilized as a basis for modifying or
designing other
structures for carrying out the same purposes of the present invention. It
should also be
realized by those skilled in the art that such equivalent constructions do not
depart from the
spirit and scope of the invention as set forth in the appended claims. The
novel features
which are believed to be characteristic of the invention, both as to its
organization and
method of operation, together with further objects and advantages will be
better understood
from the following description when considered in connection with the
accompanying
figures. It is to be expressly understood, however, that each of the figures
is provided for the
purpose of illustration and description only and is not intended as a
definition of the limits
of the present invention.


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BRIEF DESCRIPTION OF THE DRAWINGS
[0021] For a more complete understanding of the present invention, reference
is
now made to the following descriptions taken in conjunction with the
accompanying
drawing, in which:
[0022] FIGURE 1 depicts a plot of the period of the bars of a mask with the
location along the mask for a mask formed according to the first embodiment of
the
invention;
[0023] FIGURES 2A-2D depict a portion of the profiles of the bars and space of
different masks according to the first embodiment of the invention;
[0024] FIGURES 3A-3E depict graphs showing the group delay ripple for the
masks having different number of segments according to the first embodiment of
the
invention;
[0025] FIGURES 4A and 4B depict examples of systems configured to use the
inventive mask of the first embodiment of the invention to record FBGs into
fiber cores;
[0026] FIGURE 5 depicts an example of a system configured to produce the
inventive mask of the first embodiment of the invention;
[0027] FIGURE 6 depicts an example of snapping a desired pattern to a pattern
usable by manufacturing equipment according to a second embodiment of the
invention;
[0028] FIGURE 7 depicts an example of a calculation of the group delay ripple
error in a FBG manufacturing according to the second embodiment of the
invention; and
[0029] FIGURE 8 depicts a flowchart of the inventive snapping mechanism
according to the second embodiment of the invention.
11


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DETAILED DESCRIPTION OF THE INVENTION
[0030] FIGURE 1 depicts a plot 100 of the period of the bars 104 (and spaces)
of a
mask with the location along the mask 103 for a mask formed according to the
first
embodiment of the invention. Note that FIGURE 1 is not showing the length of
the
segments, but rather the period of the bars within the segments. As shown
FIGURE l, the
period of the bars is increased in a step-wise fashion 101, with incremental
changes as the
mask is traversed. Note that the number of steps is shown by way of example
only as the
inventive mask could comprise many more steps. The steps approximate the
desired
continuous curve 102. Note that the steps 101 and curve 102 depict a non-
linear chirp. This
is by way of example only, as the invention would operate with a linear chirp
or no chirp, as
well. Also, note that the segment lengths can be arbitrarily chosen and do not
have to be
equal. Still further note that the invention is described in terms of a mask,
but could work
for direct writing of index modulation into the FBG as well. However, direct
writing a very
large number of segments into the FBGs may require more time than is practical
for a
commercial production environment.
[0031] FIGURES 2A to 2D depict a portion of the profiles of the bars and
spaces
of different masks. FIGURE 2A depicts an ideal profile 200. This profile is
shown to have
three different segments, 201, 202, and 203, with each segment having two
cycles. Each
segment has a different period. Note that in FIGURES 2A to 2D, the number and
size of the
segments, and the number and size of the cycles is by way of example only, as
other
numbers and sizes could be used. FIGURE 2B depicts a profile 204 similar to
that of
FIGURE 2A, but with a stitching error 205. Error 205 is formed by having
segment 202 too
close to segment 201. Note that space 206 is not a separate error, but rather
is a result of
error 205 as the entire segment 202 has been shifted. Even though the spacing
between 202
and 203 is not correct in profile 204, segment 203 is located in the same
location as it is in
profile 200. Note that a stitching error is an error from the correct absolute
placement of the
segment as compared with the desired profile, not the relative placement from
segment to
segment. Thus, space 206 is not a stitching error with respect to the proper
placement of
segment 203.
12


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[0032] The stitching errors are assumed to be random. Thus, additional errors
may
be present or fewer errors may be present. The errors may occur at arbitrary
locations in the
profile. This invention makes use of the characteristic that the light which
reflects from the
FBG is not strongly affected by the individual stitching errors, but rather by
the average of
the errors over an interaction length. The average of random stitching errors
will tend to be
reduced to zero, however there is some standard deviation associated with the
distribution of
errors. The difference between the expected average and average of the actual
errors in a
particular mask will be inversely proportional to the square root of N, where
N is the number
of stitching errors over the effective averaging length. In the current art, N
is a relatively
small number, and hence the average of the stitching error can vary widely and
the light can
be subject to a large net stitching error.
[0033] This invention increases the value of N to a large value in order to
reduce
the effective averaged stitching error towards the expected average value,
namely zero.
Assuming each stitching error has an expected value (RMS) of ~so , then the
expected value
of the average of N such errors is given by s~,,e = ~ ~ E p ~ l N and since
the expected
value of all the stitching errors is the same (so), one finds E~"~ = sp / ~ .
Thus, for
example, if the effective interaction length for averaging of the stitching
errors is 1 mm, the
segment length is O.S mm, then the number of segments and stitching errors
averaged is N=
2, and if the individual stitching error is ~S nm, then one expects an
averaged error of
S / ~ = 3.S mn. If instead, for example, one chooses a segment length of 10
p,m, then N=
100, and the effective averaged stitching error over 1 mm is only S / 100 =
O.S mn. Thus,
the first embodiment of the invention is to increase the number of stitching
errors, by
increasing the number of segments in the maslc to reduce the expected averaged
error, and
hence, the group delay ripple of an FBG written from the mask.
[0034] FIGURE 2C depicts one way to increase the number of stitching errors.
The profile 207 of FIGURE 2C has each segment of FIGURE 2A divided in sub-
segments.
For example segment 201 has been divided into 201a and 201b, while segment 202
has been
divided into 202a and 202b. Similarly segment 203 has been divided into 203a
and 203b.
Note that each of the sub-segments created from a single segment has the same
period as the
13


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original segment, as well as with each other, i.e. the sub-segments have the
same scaling.
Note that the increased number of segments has resulted in an increased number
of stitching
errors 208, 209, 210, 211. Changing scale and repositioning of the lithography
tool for each
segment or sub-segment can increase the individual (sub-)segment stitclung
errors. In this
manner, the scale is not adjusted for every sub-segment and thus any error due
to a scaling
change may be systematically present in the entire group of sub-segments.
Therefore the
contribution to stitching error from rescaling may not be averaged out with
this manner.
[0035] FIGURE 2D depicts another way to increase the number of stitching
errors.
The profile 212 of FIGURE 2D, separates the segments of FIGURE 2A into sub-
segments,
but has a different period for each sub-segment. Thus, the scaling factor is
adjusted for each
of the sub-segments according to the desired chirp function. For example, sub-
segments
201 a, 202a, and 203a have the same period as segments 201, 202, and 203,
respectively, but
the other sub-segments have different periods. For example, sub-segment 213
has a period
that is smaller than 201 (or 201 a), but larger than 202 (or 202a). Similarly,
sub-segment 214
has a period that is between that of 202 (or 202a) and 203 (or 203a), while
sub-segment 215
has a period that is between 203 (or 203a) and the next segment (or sub-
segment). Note that
only one stitching error 216 is shown for simplicity, as additional stitching
errors would
hinder the understanding of the different periods between the sub-segments.
Further note
that this embodiment has been described in terms of dividing the segments of
an existing
design, however, additional segments may be formed on the mask in an arbitrary
manner,
each with a period selected according to the desired chirp design. Since in
this manner the
scaling changes every sub-segment, averaging of the errors from rescaling is
obtained.
[0036] FIGURES 3A to 3E depict graphs showing the RMS and maximum (peak
to valley) group delay ripple (measured over .1 nm of bandwidth) for 8 cm long
FBGs made
from masks having different number of segments. Note that these graphs are by
way of
example only, as other FBGs would have different values. FIGURE 3A depicts the
graph
300 for I60 segments. Note that for 20 nanometers (nm) stitching errors,
meaning that the
individual errors are normally distributed about zero with an RMS value of 20
nm. The
RMS curve 301 results in group delay ripple error of about 50 picosecond (ps).
FIGURE
3B depicts the graph 302 for 320 segments. Note that for 20 nm stitching
error, the RMS
14


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curve 303 results in group delay RMS ripple error of about 35 ps. FIGURE 3C
depicts the
graph 304 for 640 segments. Note that for 20 nm stitching error, the RMS curve
305 results
in group delay ripple error of about 25 ps. FIGURE 3D depicts the graph 306
for 1280
segments. Note that for 20 ntn stitching error, the RMS curve 307 results in
group delay
ripple error of about 18 ps. FIGURE 3E depicts the graph 308 for 2560
segments. Note
that for 20 nm stitching error, the RMS curve 309 results in group delay
ripple error of
about 12 ps. This is approximately the variation of GDR given by the 1/ ~N
scaling of
effective stitching error.
[0037] FIGURE 4A depicts an example of a system 400 configured to use the
inventive mask 402 to record a FBG onto the core of a fiber 404. A light
source 401, e.g.
ultraviolet laser, would provide the input beam. A phase mask 402, constructed
in
accordance with a preferred embodiment of the invention, as described herein,
separates the
light beam into two first order diffracted beams, which form the grating with
the fiber core.
A stop 405 may be used to block a zero order beam emanating from the mask 401.
Additional steps (not shown) may be used to the ~2"a order, and higher orders,
if present,
emanating from mask 402. Note that if there is no zero order or other unwanted
higher
orders, then a stop (or stops) does not need to be used. A lens or lens system
( a multiplicity
of lenses) is used to image the mask and the ~1 St diffracted orders on to the
core of the fiber.
The FBG may be written either using a large illuminating beam, which
illuminates the entire
FBG at once, or a smaller beam may be used to scan the fiber and thereby
incrementally
write the FBG. To use the scanning method either the beam and imaging system
(lens(es),
stops, and associated positioning hardware) are scanned in unison while the
fiber and mask
are stationary, or the mask and fiber are scanned in unison, while the imaging
system and
illuminating beam are stationary.
[0038] FIGURE 4B depicts an alternative embodiment of the arrangement of
FIGURE 4A, wherein the imaging system is not used and the fiber 404 is placed
in close
proximity to the mask 402. This embodiment has an advantage of better
mechanical
stability, owing to the close coupling of the fiber and mask. However, it has
the
disadvantage of the additional diffracted orders which may be present, and the
imperfect


CA 02422460 2003-03-13
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imaging (i.e. the diffraction over the short distance between the mask and the
core of the
fiber).
[0039] FIGURE 5 depicts an example of a system S00 configured to form the
inventive mask of the different embodiments of the invention. The lithographic
system 500
includes a light source or particle source SO1, that provides a beam of light
or stream of
particles, respectively. The beam or stream is used to alter a characteristic
of resist 503 in
portions 504. The altered portions 504 will be used to form either spaces or
bars for the
mask, depending on the type of resist being used. In any event, after
processing bars and
spaces will be formed onto the substrate 502 to form the mask. Control system
505 controls
the movement of either the substrate 502 or the source 501, or both, and the
beam intensity
during the bar/space writing process. Instead of exposing the resist 503, a
fiber core can be
directly exposed at the same location.
[0040] Note that the semiconductor industry has been the primary driver of
mask
writing equipment technology for more than 10 years. In the pursuit of smaller
integrated
circuit (IC) geometry's, multiple pieces of equipment are available which are
capable of
achieving the resolution required to make masks suitable for FBG. As already
discussed,
FBG requires resolution on the mask of about 0.5 micron. The capability to
write 0.5
micron features on a mask was first available with ,e-beam mask writing
equipment (such as
that produced by Applied Materials, Etec Inc; Hitachi Corp.; Toshiba Corp.,
Leica Inc., and
JEOL Inc.). However, this resolution is now available with optical laser mask
writing
equipment (such as that produced by Applied Materials, Etec Inc., and Micronic
Laser,
Inc.). In recent years, the IC industry has driven the requirement for
placement precision of
features on the mask from more than 10 nanometers to about 1 nanometer.
However, even
with this tremendous progress, the placement precision still falls about 1000
times short of
the requirement necessary for non-linearly chirped FBG.
[0041] Common to all lithographic mask writing machines is the necessity to
have
the desired pattern reduced to a set of representative digital data, which
when presented to
the machine in the appropriate format instructs the machine where to expose
and where not
to expose. The writing tools accept design data with some minimum resolution
which
16


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varies from machine to machine, but which is currently measured on a scale of
nanometers.
The highest resolution accepted by any suitable and available writing tool is
1 nanometer.
[0042] All of the machines have the capability to have scaling factors which
can be
tuned. Typically, a scaling factor is not programmable, as its purpose in
making IC masks,
is to ensure that a nanometer written is really equal to a traceable standard
nanometer. A
scaling check is typically performed on some periodic basis to ensure that
calibration of the
machine has not drifted. However, the inventive embodiments take advantage of
the
characteristic that making small adjustments to the scaling factor of a
writing tool in the
middle of a writing process can result in very small (e.g. picometer scale)
changes in
placement precision. In normal operation, when a writing tool scaling factor
is changed, the
machine stops and measures itself to ensure the accuracy of the scaling. After
stopping and
moving to another location for scaling measurements, the machines typically
cannot find
their original positions to an accuracy better than about one manometer, which
results in the
aforementioned stitclung error. If, however, a writing tool is configured to
make scaling
changes "on the fly", for example by ignoring the re-calibration procedure
when the scaling
adjustment is made, then stitching errors can be reduced. Furthermore, if the
writing tool is
configured to have a programmable scaling factor, then arbitrarily complex FBG
masks can
be fabricated with reduced errors. Specifically, where the lithography tool
operates in a
continuous writing manner and where the scaling factor changes in a continuous
or stepwise
continuous fashion. Note that the smaller the size of the stitching errors,
the better the
resulting FBG will be. The invention involves averaging out, to the extent
possible, any
unavoidable stitching errors, whatever their size may be. Thus, the invention
involves the
number of errors, and not their size. Moreover, reducing the size of the
errors, while
maintaining the number of errors would improve the final result of the
averaging effect by
having more errors with smaller values. For example, assuming a RMS size of
the errors of
20 mm and having a total number of 100 errors would result in a net averaged
error of
20 / 100 = 2 mm. By shrinking the RMS error to 10 mm, and still averaging over
100 errors
would result in a net averaged error of 10 / 100 =1 mm.
[0043] Most of the advanced writing tools have the capability to do some sort
of
averaging or gray-scaling within the tool itself to reduced size errors and
placement
17


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resolution. In the discussion below, when a minimum writing grid or pixel is
discussed, it is
refernng to the effective writing grid after averaging and gray-scaling has
taken place. An
example of an averaging or gray-scaling would be a writing beam that comprises
multiple
beams which axe set on separate grids and which are off set by some fraction
of their beam
widths. By controlling the dose of each beam independently, fractional beam
width
precision can be achieved in the resulting pattern.
[0044] Although there are many different commercial lithographic mask writing
machines currently in use, each of which has different configurations and
features, for the
purpose of this discussion, they can be separated into three categories,
namely raster e-
beam, raster laser, and vector (or shaped) e-beam.
[0045] The first category uses a raster scamled e-beam or electron beam. The
writing time for this type of tool scales with the inverse square of the
writing grid. For
practical mask writing, a grid of 25 nanometers or more is required. Using
this type of
writing tool, the data is described as a digital field of 25nm (minimum)
pixels. For the
inventive method described below, where each bar is rounded (snapped) to the
nearest pixel
location, placement errors occur in every period (~l ~,m). The RMS of the
individual errors
is +(0.29)25~7.Snm, 1000 errors are averaged in a lmm effective length, and
thus the error
averaging formula predicts a reduction in the effective error of 1000 ~30, or
a net
averaged error of about 7.5 nm/30= 250 pm. These tools generally operate by
scanning one
or multiple beams in a raster pattern across the mask, where the beam is
effectively turned
on and off, on the fly, according to the pattern in the design data file. For
further
information see www.etec.com/products/pdf/pb_mS500.pdf the disclosure of which
is
hereby incorporated herein by reference.
[0046] The second category uses a laser raster scan. These machines have
writing
grids as small as Smn, or possibly even smaller. These machines use multiple
beams in
parallel, for example 24 or 32 beams, wluch implement elaborate multiple
exposure and
averaging schemes to reduce placement error. Because of the multiplicity of
beams, it is
more difficult to implement an arbitrary on the fly scaling approach for
adjusting placement
resolution. However, with the smaller effective grid, the minimum net average
error is
i8


CA 02422460 2003-03-13
WO 02/25327 PCT/USO1/29178
greatly reduced. For further information see
www.etec.com/products/pdf/pb_a3700.pdf the
disclosure of which is hereby incorporated herein by reference.
[0047] The third category uses a shaped e-beam. In this type of writing
system,
instead of having a grid which is rasterized with an on/off beam, each
geometrical shaped is
written separately. The design data is broken down into a set of shapes (e.g.
rectangles or
trapezoids). The writing time for this type of machine is a function of the
number of shapes
that are required to write the complete pattern. The effective writing grid
for this type of
tool is as low as 1 nanometer. As a result, using the inventive grid snapping
embodiment
described below is very effective to reduce the GDR. These tools will
typically write a
series of shapes in one field, called a sub-field, while the writing stage is
stationary, then
move onto another sub-field for the next set of shapes. Since these machines
have the same
nanometer scale reposition accuracy as other writing tools, the sequence of
moving and
stopping at each sub-field has the potential to induce stitching errors. By
reducing the size
of the sub-field in accordance with the first embodiment of the invention, the
net average
error can be reduced. For more information, see U.S. Patent 6,114,071 issued
to Chen et al.
September 5, 2000, the disclosure of which is hereby incorporated herein by
reference.
[0048] The second embodiment continuously writes the desired pattern into a
fiber
or mask. For example, the system shown in FIGURE 5 could be used to write such
a
continuous pattern. In writing a grating pattern that includes a fine scale
chirp, the desired
period of the bars and spaces may not be achievable using the address unit or
pixel grid of
the writing equipment. The prior art used segments with re-scaling to
accomplish the fine
chirp. For example, in a typical grating, the width and spacing of the bars
are on the order
of 500 nm. A fine scale chirp may have a change in period of only 0.01
picometers per bar
(note that 1000 picometers equals 1 nanometer). The address unit or pixel size
of the
system shown in FIGURE 5 is typically 25 nanometers or larger, although some
systems
today have 1-5 nanometer pixel size. Consequently, the pixel size camlot
achieve the fine
subpicometer scale needed for a typical design variation of mask period, e.g.
chirp.
[0049] As discussed above, with respect to the stitching errors, the light
that passes
through an FBG with many fme scale period errors is not fully subjected to
these small scale
irregularities, but rather is effected by the average of the errors over some
effective
19


CA 02422460 2003-03-13
WO 02/25327 PCT/USO1/29178
averaging length. Thus, this embodiment of the invention moves (or snaps)
every edge of
the bars and spaces of the desired pattern to the nearest address unit or
pixel grid. Previous
to this invention the prior art belief was that such a large number of errors
would increase
the GDR of FBGs fabricated with such a mask. However, based on the discovery
of the
described averaging effect of the edge errors, tlus embodiment can be used to
reduce
stitching error and GDR.
[0050] For example, as shown in FIGURE 6 a desired pattern 601 has bars and
spaces that are 2.2 pixels in width, where the exact address unit or pixel
size is dependent on
the fabrication equipment. Thus some of the edges of the desired pattern will
have to be
shifted or snapped, in processing the desired pattern, to a grid of locations
which are
separated by a single pixel. One example of the shifted or snapped pattern is
pattern 602.
Snapped pattern 602 is the result of moving the edges to the nearest pixel
grid location.
Note that this pattern is by way of example only, as other snapped patterns
could be formed.
For example, always snapping to a particular side, e.g. left or right. Thus,
for always
snapping right, the line at 2.2 would be snapped to location 3. Another
example is to always
snap to increase the size of the bars (and thereby always decrease the size of
the spaces).
Alternatively, another example is to always snap to decrease the size of the
bars (and
thereby always increase the size of the spaces).
[0051] Snapping to the nearest pixel location shall be referred to as
'rounding' and
is preferable as it introduces the least amount of error into the formed
pattern. Rounding
also tends to keep the line widths and duty cycle of the bars and spaces as
close as possible
to the desired design. For example, as shown in FIGURE 6, the duty cycle for
the desired
pattern 601 is 50%, meaning half of the pattern is bars and half of the
pattern is spaces, i.e.
11 blocks of bars and 11 blocks of spaces. The duty cycle of the snapped
pattern 602 is also
50%, with 11 blocks of bars and 11 blocks of spaces.
[0052] An example of the calculated GDR of an FBG fabricated from a 10 cm
linearly chirped mask made by snapping all the edges of the mask to the next
nearest grid
line (spaced at a pixel size of 5 nm) is shown in FIGURE 7. As shown, the flat
central
region 701 of this plot 700 has a very small (~ 1 psec) GDR. Note that this
GDR is smaller
than the GDR of the embodiment shown in FIGURE 3. In the embodiment of FIGURE
3,


CA 02422460 2003-03-13
WO 02/25327 PCT/USO1/29178
the number of stitching errors was increased to as many as 2500 over the
length of the mask.
In the snapping embodiment, the equivalent number of errors is determined by
the actual
total number of bars, which is about 100,000. As a result, the number of
stitching errors is
increased by about 40 times and the statistical argument for averaging of
errors would
indicate an improvement of about l l 40 -1 l 6 over the GDR found in FIGURE
3E. The
RMS GDR for a 5 nm stitching error in FIGURE 3E is about 3 ps, and thus based
on square
root scaling, an estimate of the GDR for the snapping embodiment is about 0.5
ps, which is
consistent with the results shown over the central region 701 of the plot 700
of FIGURE 7.
The calculated result for an ideal grating, i.e. perfect scaling without
snapping, is shown by
line 702.
[0053] Other schemes could also be used to make snapping decisions for any
particular edge or group of edges. For example, a decision mechanism could
determine that
a particular region of a snapped pattern will have an error in the duty cycle
(owing to
statistical variations), such that there is an excess in overly wide bars. The
mechanism
could then skew the snapping decisions so as to balance duty cycle for the
pattern over the
region. Note that the mechanism may move one or more edges to balance out the
duty cycle
or to equalize the bar widths.
[0054] FIGURE 8 depicts an example of a rounding mechanism according to the
above described embodiments. The mechanism would start for a particular
desired edge
901. The mechanism determines whether the desired edge is located less than
one half pixel
spacing from a grid line 902. If so, then the mechanism snaps the desired line
to the grid
line that is less than one half pixel spacing. Optionally, a counter or
monitor may be
incremented 906 to account for duty cycle or line width differences. The
mechanism checks
the counter 903 to determine whether to round up or down. The mechanism the
snaps the
desired edge to the grid line as indicated by the counter 904. The counter is
then
incremented 905 to indicate either the amount of duty cycle or line width
difference. The
mechanism then proceeds to the next desired edge 908. An optional block (not
shown)
could be inserted before block 906 to determine whether the snapping will be
up or down,
based upon the duty cycle. Thus, bloclc 907 would be modified to cause
snapping to the
21


CA 02422460 2003-03-13
WO 02/25327 PCT/USO1/29178
grid line indicated by the optional (not shown) block. This would allow for
the regional
errors in the duty cycle or line widths to be compensated.
[0055] Note that for the sake of simplicity, the pixel size has been described
in the
above examples in terms of 1 unit. However, this unit may be of any size, e.g.
25
manometers, etc. Also note that the patterns, duty cycles, bar widths, spacing
widths, chirps
described above are by way of example only as other patterns and values,
including both
linear and/or non-linear chirps, could be used. Also note that the patterns
have been
numbered in units from left to right by way of example only, as other
numbering
convention, e.g. right to left, etc. could be used.
[0056] Note that the feathering technique described in U.S. Patent Application
Serial No. 09/883,081 entitled "LITHOGRAPHIC FABRICATION OF PHASE MASK
FOR FIBER BRAGG GRATINGS" filed June 15, 2001, the disclosure of which is
hereby
incorporated herein by reference in its entirety, can be used to adjust the
boundaries of the
snapped grid.
[0057] Note that invention is operative for masks and/or gratings having phase
shifts. A phase shift is a space between bars in the grating that is larger or
smaller than
usual. Thus, a phase shift is similar to a stitching error, but larger in
scale. These shifts are
valuable fox making mufti-chatmel FBGs for use in WDM commuucation systems. A
number of techniques are available induce phase shifts. For example, in
holographic
systems, the phase shifts can be generated by movement of the mask/fiber or
illumination
system with respect to each other, or by changing the relative phase between
the two beams
of the interferometer. In lithographic systems, the phase shift is merely
included in the
written pattern. The phase shift can be chosen in a manner to generate a
particular envelope
of WDM channels. Such a technique is described in US Application serial No.
09/757,386,
which is hereby incorporated herein by reference. For the embodiments
described herein, a
phase shift can be introduced by adjusting the position of the bars/spaces as
needed, or by
adjusting the spacing between the different segments with respect to each
other.
[0058] Further note that the first and second embodiments of the invention are
useable together. For example, the stitching errors of the segments and/or sub-
segments
22


CA 02422460 2003-03-13
WO 02/25327 PCT/USO1/29178
may be induced by snapping or rounding the edges of the bars and spaces to the
nearest
pixel locations.
[0059] When implemented in software, the elements of the present invention may
be code segments that are able to perform the necessary tasks. The program or
code
segments can be stored in a processor readable medium or transmitted by a
computer data
signal embodied in a carrier wave, or a signal modulated by a earner, over a
transmission
medium. The "processor readable medium" may include any medium that can store
or
transfer information. Examples of the processor readable medium include an
electronic
circuit, a semiconductor memory device, a ROM, a flash memory, an erasable ROM
(EROM), a floppy diskette, a compact dislc CD-ROM, an optical disk, a hard
disk, a fiber
optic medium, a radio frequency (RF) link, etc. The computer data signal may
include any
signal that can propagate over a transmission medium such as electronic
network channels,
optical fibers, air, electromagnetic, RF links, etc. The code segments may be
downloaded
via computer networks such as the Internet, Intranet, etc. The code aspects of
the present
invention may be operated on a general purpose computer and/or personal
computer.
[0060] Although the present invention and its advantages have been described
in
detail, it should be understood that various changes, substitutions and
alterations can be
made herein without departing from the spirit and scope of the invention as
defined by the
appended claims. Moreover, the scope of the present application is not
intended to be
limited to the particular embodiments of the process, machine, manufacture,
composition of
matter, means, methods and steps described in the specification. As one of
ordinary skill in
the art will readily appreciate from the disclosure of the present invention,
processes,
machines, manufacture, compositions of matter, means, methods, or steps,
presently
existing or later to be developed that perform substantially the same function
or achieve
substantially the same result as the corresponding embodiments described
herein may be
utilized according to the present invention. Accordingly, the appended claims
are intended
to include within their scope such processes, machines, manufacture,
compositions of
matter, means, methods, or steps.
23

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Admin Status

Title Date
Forecasted Issue Date Unavailable
(86) PCT Filing Date 2001-09-19
(87) PCT Publication Date 2002-03-28
(85) National Entry 2003-03-13
Dead Application 2007-09-19

Abandonment History

Abandonment Date Reason Reinstatement Date
2006-09-19 FAILURE TO REQUEST EXAMINATION
2006-09-19 FAILURE TO PAY APPLICATION MAINTENANCE FEE

Payment History

Fee Type Anniversary Year Due Date Amount Paid Paid Date
Application Fee $300.00 2003-03-13
Registration of a document - section 124 $100.00 2003-07-17
Registration of a document - section 124 $100.00 2003-07-17
Maintenance Fee - Application - New Act 2 2003-09-19 $100.00 2003-09-11
Maintenance Fee - Application - New Act 3 2004-09-20 $100.00 2004-09-09
Maintenance Fee - Application - New Act 4 2005-09-19 $100.00 2005-09-01
Current owners on record shown in alphabetical order.
Current Owners on Record
TERAXION INC.
Past owners on record shown in alphabetical order.
Past Owners on Record
CALDWELL, ROGER F.
PHAETHON COMMUNICATIONS
POPELEK, JAN
ROTHENBERG, JOSHUA E.
ZWEIBACK, JASON
Past Owners that do not appear in the "Owners on Record" listing will appear in other documentation within the application.

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Date
(yyyy-mm-dd)
Number of pages Size of Image (KB)
Abstract 2003-03-13 1 58
Claims 2003-03-13 17 380
Drawings 2003-03-13 6 95
Description 2003-03-13 23 1,332
Cover Page 2003-06-06 1 37
PCT 2003-03-13 3 166
Assignment 2003-03-13 6 189
Correspondence 2003-06-04 1 25
Assignment 2003-07-17 6 205
Assignment 2003-07-17 7 297
Correspondence 2003-08-28 1 32
Correspondence 2003-08-28 1 2
Fees 2003-09-11 1 30
Assignment 2003-09-24 16 661
Fees 2004-09-09 1 29
Fees 2005-09-01 1 35