Note: Descriptions are shown in the official language in which they were submitted.
Hagerty-Jones 4-1
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M13THOD OF FII~ELY POLISHING PIANAR OPTICAL }3Ll~TS
FI~LD OF T~ ~VENTION
The present invention relates to a method of
finely polishing planar optical elements used in
conjunction with optical waveguides.
B~CKGROUN~ OF ~E_I~VENTION
~emote information is commonly transmitted by
passing light waves through an optical waveguide, for
example, an optical fiber. one type of optical wavegulde
device of current interest is a planar optical waveguide
component. Such optical wave~uide devices include a guide
or core layer sandwiched between two cladding layers of
media with lower indices of refraction than that of the
guide layer. Increasingly, such optical waveguide devices
¦ include an integral planar optical element in the optical
path of the waveguide. Such ele~ents include lenses,
gratings, and microprisms.
I Where manufacturing integral optical elements, a
! techniquQ of reactive ionic etching is sometimes used, see
~ 25 e.g. U.S. Patent No. 4,865,453 and U.S. Patent No.
,' 4,740,551.
one problem encountered with reactive ionic
etching is the formation of rough cavity surfaces. AS a
~J i 1 1 ~ 1 0
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result, the integral optical elements formed in such
cavities tend to have rough surfaces. Due to the small and
isolated nature of such cavities, it is difficult to
eliminate such roughness. These defects cause light
scattering or loss which reduces the performance of the
planar optical element. Such performance is generally
expressed in terms of excess loss -- i.e. the amount of -
light loss above the loss in each optical channel due to
optical circuitry splitting.
In M.M. Minot, et al., "A New Guided-Wave Lens
Structure," Journal of Lightwave Technology, vol. 8, no. 12
(l990) a cavity is formed in the host waveguide by reactive
ionic etching. The walls of the resulting cavities are
then ~ade smoother by a wet chemical polishing etch. A
lens-shaped waveguide is then formed in the cavity by
evaporative deposition of SiO2 in the bottom of the cavity
as a cladding layer. Glass, which acts as a guiding layer,
is then diode sputter deposited in the cavity.
This technique is not commercially useful,
because long treatment periods must be utilized. The
present invention is directed to overcoming the surface
roughness problem encountered in integral optical elements -
in a more efficient and cost effective fashion.
SUMMARY OF THE I~V~NTION
. '
The present invention is directed to a method of
finely polishing an optically transparent surface with a
polishing liquid containing abrasive particles. The
polishing liquid, while subjected to agitation, is
contacted with an optically transparent surface under
conditions effective to polish finely the optically
~ 35 transparent surface. Such aqitation is desirably
!', ultrasonic with the abrasive particles preferably having a
size of up to 1 micron. The process is particularly useful
.,
-~ 2:~ 1` iala
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where the optically transparent surface defines a cavity
configured to define a planar optical element.
The procedure of the present invention
substantially reduces the roughness of optically
transparent cavity surfaces. When a planar optical element
is subsequently formed in the cavity, the smoothness of th~
cavity surfaces causes the conforming surfaces of the
planar optical element to be smooth. This substantially
reduces the excess loss in the waveguide.
BRIEF ~ESCRIPTION OF TH~ DRAWINGS
FIG. 1 is a schematic view of an ultrasonic
polishing apparatus in accordance with the present
invention.
FIG. 2 is a photograph taken with a scanning
electron microscope of a cavity in an optical waveguide
which has not been subjected to ultrasonic polishing.
FIG. 3 is a photograph taken with a scanning
electron microscope of a cavity in an optical waveguide
which has been subjected to one hour of ultrasonic
polishing in accordance with the present invention.
FIG. 4 is a photograph taken with a scanning
electron microscope of a cavity in an optical waveguide
which has been subjected to 2 hours of ultrasonic polishing
in accordance with the present invention.
FIG. 5 is a photograph taken with a scanning
electron microscope of a cavity in an optical waveguide
which has been subjected to ultrasonic polishing for 4
hours in accordance with the present invention.
~AILED DESCRIPTION OF THE INVENTION AN~ DRAWINÇ~
The present invention relates to a method of
finely polishing an optically transparent surface with a
polishing mixture containing abrasive particles. The
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polishing mixture, while subjected to ultrasonic agitation,
is contacted with an optically transparent surface under
conditions effective to polish finely the optically
transparent surface. This contact preferably involves
immersing the surface in the polishing mixture.
FIG. 1 is a schematic view of an apparatus for
ultrasonic polishing in accordance with the present
invention. In this device, generator 10, which produces
electrical output pulses, comprises a device for rapidly
switching high-voltage DC on and off to produce pulses.
Several known devices can accomplish such switching, and
they include blocking oscillators, multivibrators, flip-
flops, tunnel diodes, and others.
Pulses from generator 10 are supplied to
transducer 8 which moves mechanically in response to the
pulses. Several generally known devices can be made pulse-
responsive to serve as transducer 8, and these include
crystals, piezo-electrics, electrostrictive,
magnetostrictive devices, and others.
Generator 10 is designed to operate in the
ultrasonic frequency range of 20-45 kilohertz at an
agitation power level of 150 to 200 watts. ~ransducer 8 is
caused to operate under these conditions as a result of its
being coupled to generator 10. In turn, transducer 8 is
attached to the base or a ~ide of tank 6 to vibrate liquid
L under these conditions. Transducer 8, generator 10, and
tank 6 are preferably together embodied in a conventional -~-
I ultrasonic cleaning unit. one example of such a unit is
the Branson D-150 Ultrasonic cleaner manufactured by
Branson Equip~ent Co., Shelton, CT.
Planar optical device D is placed in container 2
I for fine polishing in accordance with the present
I invention. A weighted cover 4 is then placed over
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container 2 to keep it in contact with the bottom of tank
6. Also held within container 2 is polishing mixture P
which is subjected to ultrasonic conditions as the high
intensity positive displacements produced in liquid L are
transmitted through the wall of container 2. In turn, such
displacements of polishing mixture P impinge planar optical
device D. During operation of the apparatus of FIG. 1, the
level of liquid L should be maintained at a height
sufficient to ensure transmission of such positive
displacements. Generally, a depth of 2.0 to 2.6
centimeters, preferably 2.54 centimeters, of liquid in tank
6 is sufficient.
Instead of utilizing the arrangement of FIG. 1,
~5 the ultrasonic polishing process of the present invention
can employ a workholder or clamp to immerse planar optical
device D in polishing mixture P of container 2. This
technique is disclosed in U.S. Patent Nos. 2,796,702 and
3,564,775 to Bodine, Jr. which are hereby incorporated by
reference.
In another alternative embodiment of the present
invention, the process can be carried out without utilizing
~ container 2 and cover 4. Polishing mixture P and planar
3 25 optical device D can be placed directly in tank 6 for fine
polishing.
Polishing mixture P is prepared from a mixture of
a liquid and abrasive particles. The polishing mixture has
a volumetric ratio of liquid to abrasive particles of 1:0.4
to 1:2.5, preferably 1:1.
, :
Polishing can be carried out at any temperature
which is not detrimental to tne optically transparent
surface being polished. Temperatures of not more than 20
to 50-C should be used with room temperature being
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a l ~
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preferred. If need be, ice can be added to liquid L to
ensure that it is not overheated by transducer 8.
Generally, the longer polishing is carried out,
the smoother the optically transparent surfaces become.
Polishing times of about 1 to 4 hours are usually
satisfactory.
The abrasive particles preferably have a size of
up to 1 micron, more preferably .l to .3 microns. These
particles are made from aluminum oxide, glass, diamond
dust, carborundum, tungsten carbide, silicon carbide, boron
carbide, and mixtures thereof. Preferably, aluminum oxide
powder with a .3 micron particle size is utilized.
The liquid component of polishing mixture P can
be any liquid suitable for slurrying the above abrasive
particles. Such liquids include tap water and deionized
water.
The fine polishing method of the present
invention is useful in treating optically transparent
surfaces which define a cavity or sidewall in an optical
waveguide. Such cavities or sidewalls are formed by
subjecting optical waveguides to reactive ionic etching and
have a configuration corresponding to that of a planar
optical element. After the cavity or sidewall is formed,
it is finely polished in accordance with the present
invention. A planar optical element is then formed in the
polished cavity in accordance with the procedure of U.S.
Patent Nos. 4,868,453 to Gidon, et al. and 4,740,951 to
Lizet, et al. and M.M. Minot, "A New Guided-Wave Lens
Structure, n Journal of Lightwave Technology, vol. 8, no. 12
(1990), all of which are discussed above. Suitable planar
optical element configurations are those of a geodesic
component, a Luneberg lens, a Fresnel lens, a grating lens,
a TIPE lens, and other similar microcomponent devices.
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Several techniques are known for producing such
planar optical elements in planar integrated optical
devices. The following references disclose suitable
procedures: U.S. Patent No. 4,712,856 to Nicia for
geodesic components; Suhara, et al., "Graded-Index Fresnel ~ -~
Lenses for Integrated opticS," Ap~lied O~tics, vol. 21, no. ~-
11, pp. 1966-71 (1982) for Fresnel lenses: Columbini,
"Design of Thin-film Luneberg-type Lenses for Maximum Focal
Length Control," A~Dlied optics, vol. 20, no. 20, pp. 3589-
93 (1981) for Luneberg lenses; and Hatakoshi et al.,
"Waveguide Grating Lenses for Optical Couplers," A~plied
Optics, vol. 23, no. 11, pp. 1749-53 (1984) for grating
lenses. Another technique has been developed where planar
optical waveguides and components therein are fabricated
using polymers, e.g., Fan, et al., EPO Patent Publication
No. 0,446,672.
Geodesic lenses are characterized by a surface
indentation in the top of the planar optical waveguide.
Geodesic lenses require tight control during the
manufacture of this surface indentation in order to keep
scattering losæes at transition points to a minimum.
Luneberg lenses, which are a subclass of geodesic
lenses, require the use of a lens material which has a
higher index of refraction than the planar optical
wave~uide substrate with which it is used.
Fresnel lenses, which are similar to zone plates
in bulk optics, rely on phase shifting and/or absorption to
obtain the desired focusing effect. This phase shifting is
achieved through a series of half-period zones which are
applied to a planar optical waveguide. For a more detailed
1 35 discussion of the use of Fresnel lenses in planar optical
waveguides, see Ashley et al., "Fresnel Lens in a Thin-film
~ 2 i ~ 0
-8-
Waveguide", Applied Physics Lette~s, vol. 33, pages 490-92 ---
(1978).
Other techniques for producing planar optical
elements in optical waveguides are disclosed in U.S. Patent
Application Serial No. 840,74g to Bhagavatula, which is
hereby incorporated by reference.
Optically transparent surfaces treated in
accordance with the present invention generally have a
surface roughness which is substantially smoother than that
encountered after reactive ionic etching. When surfaces of
an optical waveguide cavity are finely polished in this
fashion before formation of a planar optical element in the
cavity, the waveguide has substantially less excess loss
than waveguides which have not been polished in this
fashion. Thus, optical waveguides treated in accordance
with the present invention exhibit substantially better
performance than those not subjected to polishing.
EXAMPLES
Exampl~ 1
An optical waveguide with a cavity formed by
reactive ionic etching was subjected to a buffered oxide
etch with a modified HF buffered solution for 30 seconds at
an etch rate of 400 Angstroms per minute. After completion
of the buffered oxide etch treatment, a photograph of the
optical waveguide was taken with a scanning electron
microscope. See Figure 2. This photograph shows that the
cavity surfaces have significant roughness.
~xample 2
An optical waveguide with a cavity like that of
Example 1 was subjected to a buffered oxide etch according
to the procedures set forth in Example 1.
~7`~s ~
0 1 0
The waveguide was then placed in a 30 milliliter
beaker to which has been added a polishing mixture -~
formulated from 10 milliliters of 0.3 micron alumina powder
and 10 milliliters of water. The beaker was then placed in
the tank of a Branson D-lso ultrasonic cleaner and a
weighted plastic cover was put on top of the beaker to
ensure that it remained in good contact with the base of
the ultrasonic cleaner. The ultrasonic cleaner tank was
then filled to a height of 2.54 centimeters of water so
that ultrasonic vibrations were transmitted to the beaker
contents. The ultrasonic cleaner was then turned on and
operated for a period of 1 hour.
:
After completion of the ultrasonic treatment, the
optical waveguide was removed and a photograph of it was
taken with a scanning electron microscopeO This photograph
is FIG. 3. A comparison of FIGS. 2 and 3 show that the
fine polishing procedure of the present invention produces
a substantially smoother cavity.
Exampl* 3
The test procedure of Example 2 was repeated
except that the ultrasonic polishing stage was carried out
for 2 hours. After completion of ultrasonic polishing, the
optical waveguide was removed from the polishing mixture
and a photograph of it was taken with a scanning electron
microscope. This photograph is FIG. 4. A comparison of
FIG. 4 with FIGS. 2 and 3 shows that increasing the
ultrasonic polishing time enhances the smoothness of the
cavity walls.
~xample 4
The process of Example 2 was repeated except that
the ultrasonic polishing time was 4 hours. After
completion of ultrasonic polishing, the optical waveguide
was removed from the polishing mixture and a photograph of
it was taken with a scanninq electron microscope. This
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photograph is FIG. 5. A comparison of FIG. 5 with FIGS. 2,
3, and 4 indicates that the 4 hour polishing time achieved
increased smoothness of the cavity walls.
Example 5
lx8 coupler/splitter devices were made according
to the design and process set forth in copending U.S. . :
Patent Application Serial No. 07/840,749, which is
incorporated herein by reference. One such device was
measured after processing according to Example 1 for
optical performance. The mean ratio between the input
power and the output power for the 8 outputs was
approximately 12 dB (theoretical ratio is 9 dB for lx8
splitting).
Another such device was additionally treated
according to the polishing process of Example 4. The mean
excess loss was about 11 dB, an improvement of 33% from the
excess loss without the inventive polishing treatment.
