Note: Descriptions are shown in the official language in which they were submitted.
CA 02565709 2006-11-03
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TWO-STAGE OPTICAL BI-DIRECTIONAL TRANSCEIVER
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims priority from United States Patent Applications
Nos. 60/576,594 filed June 4, 2004, 60/576,595 filed June 4, 2004, and
60/577,604 filed
June 8, 2004, which are all incorporated herein by reference.
TECHNICAL FIELD
The present invention relates to a two stage optical filter, and in particular
to a
planar lightwave c ircuit (PLC) o ptical b i-directional t ransceiver f or u
se i n f iber-to-the-
home (FTTH) optical networks.
BACKGROUND OF THE INVENTION
A bi-directional transceiver, e.g. a triplexer or Voice-Data-Video (VDV)
processor, serves as an optical gateway from an FTTH optical network into a
subscriber's
home. A triplexer is an extremely compact and low-cost access device capable
of
receiving two high-speed channels (e.g. 1490 nm for telephone & internet, and
1550 nm
for video), while simultaneously transmitting on a third channel (e.g. 1310
for
information out). All these signals are multiplexed onto a single optical
fiber for simple
installation. For business purposes the video channel can be omitted forming a
two
channel bi-directional transceiver or biplexer. Alternatively, additional
outgoing
information channels can be added, as well as additional incoming data
channels.
Typical biplexer and triplexer requirements present considerable challenges to
conventional PLC design techniques. The optical architecture requires that a
laser,
nominally 1310 nm in wavelength, is coupled to a single-mode fiber for
transmitting
optical signals from the home. In the other direction on that same fiber,
light at
wavelengths of nominally 1490 nm and 1550 nm from outside the home are
captured,
demultiplexed and directed to optical detectors. The difficulty arises due to
the
operational passbands at these wavelengths. At the 1310 nm channel, a band of
50 nm to
100 nm is expected, which provides a large margin within which the laser can
operate
essentially athermally, whereas bands of only 10 nm to 20 nm width are
required for the
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detector channels. Furthermore, the laser diode operates in a single
transverse mode, and
the common input/output fiber is a single mode fiber; hence, the path followed
by the
laser channel must be at all points compatible with single-mode optics. In
other words the
laser channel's path must be reversible. In the prior art, especially those
designs using a
single d iffractive structure in a P LC, t here i s n o p ractical means o f
addressing a wide
wavelength range (- 1250 nm to 1600 nm) with channels having substantially
different
passbands.
Prior art devices, such as the one disclosed in United States Patent No.
6,493,121
issued December 10, 2002 to Althaus, and illustrated in Figure 1, achieve the
functionality of the VDV processor (triplexer 1) using a number of
individually crafted
thin film filters (TFF) 2a and 2b, placed in specific locations along a
collimated beam
path. The TFFs 2a and 2b are coupled with discrete lasers 3 and photo-
detectors 4a and
4b, and packaged in separate transistor-outline (TO) cans 6 and then
individually
assembled into one component. An incoming signal with the two incoming
channels
(1490nm and 1550nm) enter the triplexer 1 via an optical fiber 7. The first
channel is
demultiplexed by the first TFF 2a and directed to the first photo-detector 4a,
and the
second channel is demultiplexed by the second TFF 2b and directed to the
second photo-
detector 4b. The outgoing channel (1310nm) is generated in the laser 3 and
output the
optical fiber 7 via the first and second TFFs 2a and 2b. Unfortunately, the
assembly of
such a device is extremely labor intensive requiring all of the elements to be
aligned with
very low tolerances.
Attempts to simplify the housing structure and thereby the assembly process
are
disclosed in United States Patents Nos. 6,731,882 issued May 4, 2004 to
Althaus et al,
and 6,575,460 issued January 29, 2004 to Melchoir et al. Further advancements,
illustrated in Figure 2, involve mounting all of the elements on a
semiconductor
microbench ensuring repeatable and precise alignment. Unfortunately, all of
these
solutions still involve the alignment of TFFs with TO cans. An example of a
prior art
solution without TFFs is disclosed in United States Patent No 6,694,102 issued
February
17, 2004 to Baumann et al., which discloses a bi-directional multiplexer
utilizing a
plurality of Mach-Zehnder interferometers.
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In optics, a diffraction grating is an array of fine, parallel, equally spaced
grooves
("rulings") on a reflecting or transparent substrate, which grooves result in
diffractive and
mutual interference effects that concentrate reflected or transmitted
electromagnetic
energy in discrete directions, called "orders, " or "spectral orders. "
The groove dimensions and spacings are on the order of the wavelength in
question. In the optical regime, in which the use of diffraction gratings is
most common,
there are many hundreds, or thousands, of grooves per millimeter.
Order zero corresponds to direct transmission or specular reflection. Higher
orders
result in deviation of the incident beam from the direction predicted by
geometric (ray)
optics. With a normal angle of incidence, the angle 0, the deviation of the
diffracted ray
from the direction predicted by geometric optics, is given by the following
equation,
where m is the spectral order, X is the wavelength, and d is the spacing
between
corresponding parts of adjacent grooves:
(rn a
9 = ~ 5inl1.
d
Because the angle of deviation of the diffracted beam is wavelength-dependent,
a
diffraction grating is dispersive , i.e. it separates the incident beam
spatially into its
constituent wavelength components, producing a spectrum.
The spectral orders produced by diffraction gratings may overlap, depending on
the spectral content of the incident beam and the number of grooves per unit
distance on
the grating. The higher the spectral order, the greater the overlap into the
next-lower
order. Diffraction gratings are often used in monochromators and other optical
instruments.
By controlling the cross-sectional shape of the grooves, it is possible to
concentrate most of the diffracted energy in the order of interest. This
technique is called
"blazing.
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Originally high resolution diffraction gratings were ruled. The construction
of
high quality ruling engines was a large undertaking. A later photolithographic
technique
allows gratings to be created from a holographic interference pattern.
Holographic
gratings have sinusoidal grooves and so are not as bright, but are preferred
in
monochromators because they lead to a much lower stray light level than blazed
gratings.
A copying technique allows high quality replicas to be made from master
gratings, this
helps to lower costs of gratings.
A planar waveguide reflective diffraction grating includes an array of facets
arranged in a regular sequence. The performance of a simple diffraction
grating is
illustrated with reference to Figure 3. An optical beam 11, with a plurality
of wavelength
channels X1, )12, ?13 ..., enters a diffraction grating 12, with grading.pitch
A and diffraction
order m, at a particular angle of incidence 6;,,. The optical beam is then
angularly
dispersed at an angle 00õ, depending upon wavelength and the order, in
accordance with
the grating equation:
mA = A(sinein +sine ) (1)
out
From the grating equation (1), the condition for the formation of a diffracted
order
depends on the wavelength );N of the incident light. When considering the
formation of a
spectrum, it is necessary to know how the angle of diffraction ANoõt varies
with the
incident wavelength O. Accordingly, by differentiating the equation (1) with
respect to
6Noõt, assuming that the angle of incidence 9in is fixed, the following
equation is derived:
aeNo1aa. = 7/A COSeNo (2)
ut
The quantity d6Noõ,/d), is the change of the diffraction angle 6Noõt
corresponding to
a small change of wavelength k, which is known as the angular dispersion of
the
diffraction grating. The angular dispersion increases as the order m
increases, as the
grading pitch A decreases, and as the diffraction angle 6Noõt increases. The
linear
dispersion of a diffraction grating is the product of this term and the
effective focal length
of the system.
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Since light of different wavelengths XN are diffracted at different angles
6N01ti each
order m is drawn out into a spectrum. The number of orders that can be
produced by a
given diffraction grating is limited by the grating pitch A, because AN õt
cannot exceed
90 . The highest order is given by AAN. Consequently, a coarse grating (with
large A)
produces many orders while a fine grating may produce only one or two.
The free spectral range (FSR) of a diffraction grating is defined as the
largest
bandwidth in a given order which does not overlap the same bandwidth in an
adjacent
order. The order m is important in determining the free spectral range over
which
continuous dispersion is obtained. F or a given input-grating-output
configuration, with
the grating operation at a preferred diffraction order m for a preferred
wavelength X, other
wavelengths will follow the s ame path at other diffraction orders. The first
overlap of
orders occurs when
m Am _ (m + 1)zlm+l (3)
mA
m (4)
,n+l - (m + 1)
0.1. _ A. (5)
m+l
A blazed grating is one in which the grooves of the diffraction grating are
controlled to form right triangles with a blaze angle w, as shown in Figure 3.
The
selection of the blaze angle w offers an opportunity to optimize the overall
efficiency
profile of the diffraction grating, particularly for a given wavelength.
Planar waveguide diffraction based devices provide excellent performance in
the
near-IR (1550 nm) region for Dense Wavelength Division Multiplexing (DWDM). In
particular, advancements in Echelle gratings, which usually operate at high
diffraction
orders (40 to 80), high angles of incidence (approx 60 ) and large grading
pitches, have
lead to large phase differences between interfering paths. Because the size of
grating
facets scales with the diffraction order, it has long been considered that
such large phase
differences are a necessity for the reliable manufacturing of diffraction-
based planar
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waveguide devices. Thus, existing devices are limited to operation over small
wavelength ranges due to the high diffraction orders required (see equation
5).
Furthermore, for diffraction grating-based devices fabricated in a planar
waveguide platform, a common problem encountered in the prior art is
polarization
dependent loss arising from field exclusion of one polarization caused by the
presence of
conducting metal S (a reflective coating) adjacent to the reflective facets F.
An optical signal propagating through an optical fiber has an indeterminate
polarization state requiring that the (de)multiplexer be substantially
polarization
insensitive so as to minimize polarization dependent losses. In a reflection
grating used
near Littrow condition, and blazed near Littrow condition, light of both
polarizations
reflects equally well from the reflecting facets (F in Fig. 3). However, the
metalized
sidewall facet S introduces a boundary condition preventing light with
polarization
parallel to the surface (TM) from existing near the surface. Moreover, light
of one
polarization will be preferentially absorbed by the metal on the sidewall S,
as compared
to light of the other polarization. Ultimately, the presence of sidewall metal
manifests
itself in the device performance as polarization-dependent loss (PDL).
There are numerous methods and apparatus for reducing the polarization
sensitivity of diffraction gratings. Chowdhury, in United States Patents Nos.
5,966,483
and 6,097,863 describes a reduction of polarization sensitivity by choosing to
reduce the
difference b etween first and second diffraction efficiencies o f a wavelength
within the
transmission bandwidth. This solution can be of limited utility because it
requires
limitations on election of blaze angles and blaze wavelength.
Sappey et al, in United States Patent No. 6,400,509, teaches that polarization
sensitivity can be reduced by including reflective step surfaces and
transverse riser
surfaces, separated by a flat. This solution is also of limited utility
because it requires
reflective coating on some of the surfaces but not the others, leading to
additional
manufacturing steps requiring selective treatment of the reflecting
interfaces.
The free spectral range of gratings is proportional to the size of the grating
facets.
It has long been thought that gratings with a small diffraction order could
not be formed
reliably by means of photolithographic etching, because low order often
implies steps
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smaller or comparable to the photolithographic resolution. The
photolithographic
resolution and subsequent processing steps blur and substantially degrade the
grating
performance. Therefore, the field of etched gratings has for practical reasons
limited itself
to reasonably large diffraction orders typically in excess of order 10.
Devices with orders
ranging close to order 1 have long been thought to be impractical to realize.
Other important considerations in the design of.a triplexer is the optical
isolation
of the 1310 nm channel from the 1490 nm and 1550 nm channels, and the
insertion loss
of each channel, which must be kept at a minimum. This is particularly true
for the 1310
nm laser channel, since the coupling of the laser diode to the waveguide chip
is a difficult
process and requires a relaxed tolerance afforded by the filter loss.
Furthermore, a very
flat and wide passband is required for all channels.
In the VDV processor, isolation of close to 50 dB is sometimes required
between
the laser source at 1310 nm and the receiver channels at 1490 and 1550 nm. In
a grating-
based device the main source of background light arises from scattering from
defects on
the facet profile. The facets themselves are arranged to create phase coherent
interference
to disperse and focus light in a wavelength specific manner. Corner rounding
between the
reflective facet and the non-reflective sidewall will also be periodic, and
therefore
spatially coherent, but with an inappropriate phase, leading to periodic ghost
images with
low intensity. Facet roughness will be spatially incoherent, leading to random
low-level
background light. Thus, if a strong laser signal is incident on a grating and
receiver
channels are also obtained from that grating, the receiver channels will have
a strong
background contributed from the laser, at a level typically 30 dB below the
strength of the
laser. Isolation of - 50 dB is closer to the requirement for a practical VDV
processor.
An object of the present invention is to overcome the shortcomings of the
prior art
by providing a two-stage optical filter planar lightwave circuit bi-
directional transceiver
with high isolation and low insertion loss.
In a conventional reflective-grating device, the spectrometer output angles
are
selected to maximize the throughput of the intended wavelengths to the
intended
locations. Little consideration is given to Littrow radiation that may be
quite intense,
almost as intense as the intended output emission. In the realm of optical
telecommunications, light that returns along an input path can be disastrous
to the overall
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performance of an optical system. Accordingly, reflective grating-based
devices may
introduce problems to telecommunications systems. As a result, nearly all
components
for telecommunications have a specification for maximum "Return Loss", or
"Back-
reflection", which has been particularly difficult to achieve using reflective
grating
technology, in which the device has a fundamental layout that is, by design,
optimized for
reflecting high intensities of light directly back towards the input fiber.
Furthermore, if multiple diffraction orders are intended for use, such that
the same
wavelength emerges from a spectrometer at several different angles, there is
the
likelihood that the intensity of the secondary diffraction orders may be
extremely weak
(down to infinitesimal amounts). Therefore products such a s integrated
demulitiplexer-
channel monitors will achieve poor and possibly insufficient responsivity in
the
secondary diffraction order channels.
Presently, wavelength separating devices used in optical telecommunications
systems are ultimately transmissive in nature, e.g. employing arrayed
waveguide gratings
or thin-film filters, in which there are no strong interferences caused by
light rebounding
directly backwards from the component.
An object of the present invention is to overcome the shortcomings of the
prior art
by providing a multiplexer/demultiplexer with input and output ports optimally
positioned
in accordance with the grating facet diffraction envelope to minimized back
reflection to
the input ports and maximize output light collected from different diffraction
orders.
Ideally MUX/DEMUX systems perform consistently in spite of small fluctuations
in laser wavelength, which requires that the MUX/DEMUX be designed with flat
passbands in the frequency domain.
Numerous designs exist for both arrayed-waveguide grating (AWG) and echelle-
grating etched w aveguide s pectrometers, w hich are u sed for optical
MUX/DEMUX or
optical channel monitors/performance monitors (OCM/OPM) in the field of
optical
telecommunications. Conventionally, flat-passband performance of the
spectrometer unit
is achieved at the expense of higher insertion loss, by degrading the shape of
the passband
from the ideal narrow-peaked Gaussian bandshapes, which are common to
spectrometers
in waveguide based devices. The bandshape is degraded by widening the optical
aperture
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at the entrance to or exit of the spectrometer unit, and/or by introducing
aberrations, e.g.
de-focus, c oma, s pherical, t o t he i nterference e lement. E ven f or a n i
deal d esign, a f lat
passband top with sharp band cutoffs will come only at the expense of
spectrometer
transmission. Furthermore, the passband flattening will not result in temporal
narrowing
in the existing designs.
With conventional grating-based devices, such as the ones disclosed in United
States Patents Nos. 6,298,186 issued October 2, 2001 to Jian-Jun He and
6,188,818 issued
February 1 3, 2 001 t o Han et a 1, a f lat p assband p erformance can o nly
be achieved b y
sacrificing transmission at the peak of each channel. Moreover, there is no
shortening of
impulse in the time domain accompanying the flattening of the passband in the
frequency
domain.
Accordingly, an object of the present invention is to overcome the
shortcomings
of the prior art by providing a MUX/DEMUX including a pair of gratings to be
used in
sequence such that the emission from the first grating achieves a cyclic
offset of incidence
angle into the second grating of the system.
SUMMARY OF THE INVENTION
Accordingly, the present invention relates to a two stage optical filter
planar
lightwave circuit device for receiving first and second input channels from a
system
waveguide and for transmitting an output channel onto the system waveguide
comprising:
a laser transmitter for transmitting the output channel;
a non-diffractive filter, having a first passband for multiplexing the output
channel onto
the system waveguide, and for separating the first and second input channels
from the
output channel; and
a diffraction grating filter ~for demultiplexing the first and second input
channels, each of
the first ands second input channels having a second passband narrower than
the first
passband.
The diffraction grating filter comprising an input port for receiving the
first and
second input channels; a diffraction grating receiving the first and second
input channels
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at an incident angle; and first and second output ports for outputting the
first and second
input channels from the diffraction grating filter, respectively.
The two stage optical filter planar lightwave circuit device further
comprising first
and second output waveguides optically coupled to the first and second ports,
respectively
for transmitting the first and second input channels, respectively; and
first and s econd p hoto-detectors optically coupled t o t he f irst a nd s
econd o utput ports,
respectively, for converting the input channels into electrical signals.
Accordingly, the present invention relates to a planar waveguide optical
device
comprising:
an input port for launching an input optical signal, which is comprised of a
plurality of optical channels;
a reflective waveguide diffraction grating for dispersing the optical signal
into a
diffraction envelope having a principle diffraction maximum, a plurality of
higher order
diffraction maxima, and a plurality of diffraction minima therebetween; and
a first plurality of output ports outputting said optical channels;
wherein said input port is positioned at one of said diffraction minima to
limit the
amount of light reflected from the reflective waveguide diffraction grating
from re-
entering the input port.
Another aspect of the present invention relates to a planar waveguide optical
2 0 device comprising:
an input port for launching an input optical signal, which is comprised of a
plurality of optical channels;
a reflective waveguide diffraction grating for dispersing the optical signal
into a
diffraction envelope having a principle diffraction maximum, a plurality of
higher order
diffraction maxima, and a plurality of diffraction minima therebetween; and
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a first plurality of output ports positioned along the principle diffraction
maximum
for outputting said optical channels.
Another aspect of the present invention relates to a planar waveguide optical
device comprising:
an input port for launching an input -optical signal, which is comprised of a
plurality of optical channels;
a reflective waveguide diffraction grating for dispersing the optical signal
into a
diffraction envelope having a principle diffraction maximum, a plurality of
higher order
diffraction maxima, and a plurality of diffraction minima therebetween;
a first plurality of output ports for outputting said optical channels; and
second plurality of output ports positioned along one of the higher order
diffraction maxima for outputting light therefrom.
Accordingly, the present invention relates to an optical channel demultiplexer
device for separating an input optical signal into a plurality of output
channel bands with
a given channel spacing comprising:
an input port for launching an input optical signal including a plurality of
optical
channel bands at the given channel spacing;
a f irst o ptical g rating h aving a f irst o rder a first F SR s ubstantially
equal to the
given channel spacing, for dispersing each optical channel band over
substantially a same
range of output angles;
a second optical grating having a second order and a second FSR for receiving
the
optical channel bands from the first reflective grating, for directing each
wavelength in
one of the optical channel bands at a same output angle, and for directing
each optical
channel band at a different output angle; and
a plurality of output ports for outputting a respective one of the plurality
of optical
channel bands.
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Another aspect of the present invention relates to an optical channel
multiplexer
device for combining a plurality of input channel bands with a given channel
spacing into
a single output signal comprising:
a plurality of input ports for inputting a respective one of the plurality of
optical
channel bands;
a first reflective grating having a first FSR and a first order for receiving
each of
the optical channel bands at different input angles from their respective
input ports, and
for directing each optical channel band over substantially a same range of
output angles;
a second reflective grating having a second order and a second FSR
substantially
equal to the given channel spacing for combining each optical channel band
into the
output signal; and
an output port for outputting the output signal.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described in greater detail with reference to the
accompanying drawings which represent preferred embodiments thereof, wherein:
Figure 1 illustrates a conventional thin film filter based triplexer;
Figure 2 illustrates a conventional thin film filter based triplexer utilizing
a semiconductor
substrate;
Figure 3 illustrates a conventional reflective diffraction grating;
Figure 4 illustrates a diffraction grating according to the present invention;
Figure 5 illustrates a reflective concave diffraction grating PLC filter a
ccording to the
present invention;
Figure 6 illustrates a two-stage optical filter according to the present
invention;
Figure 7 illustrates an output spectrum from a second stage of the optical
filter of Fig. 6;
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Figure 8 illustrates an output spectrum from a first stage of the optical
filter of Fig. 6.
Figure 9 is a plot of output angle vs. frequency for a reflective diffraction
grating;
Figure 10 is a top view of a double-grating subtractive-dispersion MUX/DEMUX
according to the present invention;
Figure 11 illustrates a plot of input angle vs. frequency and a plot of output
angle vs.
frequency for the second diffraction grating of the device of Fig. 10;
Figure 12 is a plot of the angle error vs. frequency for the second
diffraction grating of the
device of Fig. 10;
Figure 13 illustrates an alternative embodiment of an optical device
incorporating the
planar waveguide reflective diffraction grating according to the present
invention with the
input waveguide positioned at a minimum of the diffraction envelope;
Figure 14 illustrates a diffraction envelope from the central facet for the
device of Figure
13;
Figure 15 illustrates an alternative embodiment of an optical device
incorporating the
planar waveguide reflective diffraction grating according to the present
invention with the
input waveguide positioned at a minimum of the diffraction envelope and the
first and
second sets of output waveguides positioned at maximums of the diffraction
envelope;
Figure 16 illustrates a spectrum for a situation in which an input waveguide
is located
physically near an output waveguides; and
Figure 17 i llustrates a s pectrum f or a s ituation in which a n i nput
waveguide h as b een
located at a third diffraction envelope minimum.
DETAILED DESCRIPTION
.One of the m ajor concerns in the design of planar lightwave circuit (PLC)
diffraction
gratings is the manufacturability of the reflecting and sidewall facets F and
S,
respectively. Furthermore, a major limit to the manufacturability of the
facets heretofore,
has been the photolithographic resolution limitations. Typical
photolithographic
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procedures are limited to resolutions in the range of 0.5 to 1.0 pm, so the
minimal
requirement to achieve reasonable performance from a grating is that the
reflecting facet
size F must be larger than this resolution, say 2.5 to 5 m or more in size.
In Figure 4, the light path is simplified by the assumption that the input and
output angles
Oiõ and ANoõt, respectively are identical. This assumption is only to simplify
the
mathematical treatment of the facet geometry. Accordingly:
F,;z A cos 9;n ; and (6)
Equation (1) simplifies to
YiZA ~ 2A sin Brn (7)
Combining equations 6 and 7 yields
(8)
0
From Figure 1:
S ;:t: tan 9;,, (9)
F
Historically, incidence and output angles of 45 to 65 have been used
inevitably leading
to grating facet aspect ratio of FIS to be about 1 (see Figure 3 and Equation
9). At a
wavelength of 1550 nm, one finds from equation (6) that facet sizes, for both
reflecting F
and non-reflecting surfaces S, of 10-17 m are easily achievable in the prior
art, for
DWDM applications. This makes grating facets F manufacturable, but at the
expense of
large non-reflecting facets (or sidewalls) S contributing to the polarization
dependent loss.
In the prior art, facet size variation is also done by varying the diffraction
order m, i.e.
adjusting the numerator of equation (8).
Telecommunications networks have evolved from DWDM to CWDM and FTTH
networks. The latter two network architectures have channels spanning large
wavelength
ranges, from - 1250 nm to - 1630 nm. These wide ranges cannot be served by a
high-
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diffraction order device, and often require orders as low as 1. Practitioners
of the prior art
have not been aware of, or taken advantage of equation (8). At low diffraction
orders m
and operating angles 6;,, and 0 õt of 45 to 65 the resulting facet size F
for a planar
waveguide diffraction grating would be too small to be practically
manufacturable.
Existing planar waveguide diffraction based devices include AWGs and echelle
gratings.
Both rely on high diffraction orders; the AWGs need high order operation for
guide
routing reasons, the echelle technique employs high orders to maintain large
facet sizes
that are more easily manufactured. Hence, prior art has intrinsic limitations
in addressing
the CWDM or FTTH network architectures in a planar waveguide platform.
The present invention recognizes the importance of equation (8), in particular
the fact
that it is possible to increase the grating facet aspect ratio FIS through
angular
dependence of the denominator. As the diffraction angle is reduced, the facet
size
increases linearly with tan0;,,. Additionally, inventors recognize that the
increase of the
facet aspect ratio FIS yields devices with improved polarization dependent
loss and
larger free spectral range.
For example, in silica-on-silicon, a diffraction order of 5 or less (yielding
the smallest
practical free spectral range for CWDM or FTTH networks), at a wavelength of
1550 nm,
and size of reflecting facet F to exceed 5.0 m, would require F/S to be
increased to
more than 3, which can be accomplished by lowering the diffraction angle to
about 25 .
Thus, the present invention encompasses all planar waveguide diffraction
grating designs
with the ratio of reflecting to non-reflecting f acets (or sidewalls) of at
least 3. Other
planar w aveguide m aterials include silica, silicon oxynitride, s ilicon n
itride, silicon on
insulator, or indium phosphide
The amount of PDL is strongly dependent on the aspect ratio F/S and the length
of the
non-reflecting facet S. Conventional echelle designs have an aspect ratio of -
1, and are
strongly subjected to sidewall dependent PDL; however, for FIS in excess of 3,
the non-
reflecting facets make substantially smaller contribution to the PDL. By
further increasing
FIS, it is possible to design manufacturable facets with the non-reflecting
grating facet
sizes S at or smaller than the wavelength of the reflected light, e.g.
S<3000nm, preferably
CA 02565709 2006-11-03
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<2500nm, even more preferably <2000nm, and ultimately preferably <1550nm. For
such
gratings, the interaction length of light with the metallized sidewall is so
small that PDL-
free operation of the device becomes possible.
Therefore, when we enter a regime in which tan(O) is small, i.e. to achieve a
1/3 ratio or 0
< 25 , we can reduce sidewall dependent PDL.
From a manufacturability standpoint, if reflecting facets F are large, the
facets themselves
are reproduced faithfully despite photolithographic resolution limits. Small
non-reflecting
facets S will likely not be reproduced faithfully, and will be slightly
rounded, but grating
performance i s not affected. Practitioners o f prior art no doubt have
realized that the
pitch governs dispersion as per equation (1). However, it is quite common to
equate the
pitch of a grating to the normal distance between reflecting facets (the
sidewall S in Fig.
3). With that thinking, a distortion to the sidewall S could be equated with a
distortion to
the pitch. This is a mistaken conception, and in fact the pitch is given by
equation (6).
Counter-intuitively, the pitch increases with F, not S. The present inventors
recognize this
fact and can increase the aspect ratio, i.e. decrease S/F, shown in equation
(9) without risk
of affecting the pitch. In fact, the fidelity of the grating reproduction is
limited not by
photolithography but by the accuracy of the features on the mask itself. This
limit is
several orders of magnitude (100-fold) smaller than the photolithographic
resolution.
Combining equation (8) and (9), we find that:
S MA (10)
2
Thus, by choosing a small diffraction order (m= 3, 2 or 1, if necessary) one
can nearly
eliminate PDL, because the sidewall size S becomes less than the wavelength.
In a preferred embodiment, illustrated in Figures 4 and 5, a dispersive PLC
optical filter
19 includes a concave reflective diffraction grating 20 is formed at an edge
of a slab
waveguide 21 provided in chip 22. An input port is defined by an end of a
waveguide 23,
which extends from an edge of the chip 22 to the slab waveguide 21 for
transmitting an
input wavelength division multiplexed (WDM) signal, comprising a plurality of
wavelength channels Q1, %2d13 ...), thereto. The diffraction grating 20, as
defined above
with reference to Figure 4, has an aspect ratio (F/S) greater than 5, and a
sidewall length S
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less than or equal to the average wavelength of the wavelength channels (%1,
X2, X3 ...).
The input waveguide 23 is positioned to ensure that the incident angle 9;,, is
less than 45 ,
preferably less than 30 ,and more preferably less than 15 , and the grating
pitch A is
selected to ensure that the grating 20 provides diffraction in an order of 5
or less. The
diffraction grating 20 disperses the input signal into constituent wavelengths
and focuses
each wavelength channel on a separate output port in the form of an output
waveguide 25,
the ends of which are disposed along a focal line 26 of the grating 20 defined
by a
Rowland circle, for transmission back to the edge of the chip 22. The
illustrated device
could also be used to multiplex several wavelength channels, input the
waveguides 25,
into a single output signal transmitted out to the edge of the chip 22 via the
input
waveguide 23. The input and output ports represent positions on the slab
waveguide 21 at
which light can be launched or captured; however, the ports can be optically
coupled with
other transmitting devices or simply blocked off.
In a preferred embodiment, illustrated in Figure 3, a concave reflective
diffraction grating
10 is formed at an edge of a slab waveguide 11 provided in chip 12. An input
port is
defined by an end of a waveguide 13, which extends from an edge of the chip 12
to the
slab waveguide 11 for transmitting an input wavelength division multiplexed
(WDM)
signal, comprising a plurality of wavelength channels (a,l, )12, X3 ...),
thereto. The
diffraction grating 10, as defined above with reference to Figure 2, has an
aspect ratio
(F/S) greater than 5, and a sidewall length S less than or equal to the
average wavelength
of the wavelength channels (kl, XZ, )13 ...). The input waveguide 13 is
positioned to ensure
that the incident angle 9;,, is less than 30 , and the grating pitch A is
selected to ensure
that the grating 10 provides diffraction in an order of 5 or less. The
diffraction grating 10
disperses the input signal into constituent wavelengths and focuses each
wavelength
channel o n a s eparate o utput p ort i n the f orm o f a n output w aveguide
15, the ends of
which are disposed along a focal line 16 of the grating 10 defined by a
Rowland circle,
for transmission back to the edge of the chip 12. The illustrated device could
also be used
to multiplex several wavelength channels, input the waveguides 15, into a
single output
signal transmitted out to the edge of the chip 12 via the input waveguide 13.
The input
and output ports represent positions on the slab waveguide 11 at which light
can be
launched or captured; however, the ports can be optically coupled with other
transmitting
devices or simply blocked off.
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Specific examples for operating the aforementioned optical device are:
9;,,-5 50 50 6
m=1 2 3 2
4,,g= 1550nm 1550nm 1550nm 1550nm
A=8892nm 17784run 26676nm 14828nm
F=8858nm 17716nm 26574nm 14747nm
S=775nm 1550nm 2325nm 1550nm
F/S=11.4 11.4 11.4 9.5
For a biplexer or a triplexer the relevant passbands are 100 nm for the laser,
and - 20 nm
for the detector channels. Such a device would be impractical to implement
with a single
diffractive structure because the various channels would share a common
physical
dispersion. Assume that a spectrometer slab region has been chosen such that
the smallest
reasonable guiding waveguide widths handle the 20 nm passbands at the grating
output.
The waveguide width necessary for the 100 nm passband channel would be so wide
as to
support innumerable modes, creating a device with high sensitivity to
fabrication
tolerances if a reversible path is necessary for this channel.
With reference to Figure 6, the two-stage optical filter according to the
present invention
includes a non-dispersive filter 31, a dispersive filter 32, a laser source
33, and first and
second photo-detectors 34 and 35 formed in a planar lightwave circuit (PLC)
chip 36. A
single photo-detector 34 can be provided, when one of the detector channels.is
omitted.
Preferably, the non-dispersive filter 31 is a wavelength selective directional
coupler, i.e.
two parallel waveguides of specific width, spacing and coupling length, which
separates
the receiver channels from the laser channel. Alternatively, the non-
dispersive filter 31
2 5 can be a wavelength dependent modal interference (MMI) filter or a phase
dependent
wavelength splitter, e.g. a Mach Zehnder interferometer designed for splitting
wavelength bands. Instead of a single-stage coupler, a multi-stage coupler or
MMI can be
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used, which provides flatter passbands than those commonly produced by single-
stage
filters, which slightly improves the insertion loss at the outer edges of the
channels, where
the passbands from the single-stage filters begin to roll off.
The laser source 33 transmits the data channel along waveguide 41 to the non-
dispersive
filter 31, which multiplexes the data channel onto output waveguide 42. A
system
waveguide 43, e.g. an optical fiber, is optically coupled to the output
waveguide 43 at the
edge of the PLC chip 36. A monitor photodiode 46 can be positioned proximate
the back
facet of the laser source 33; however, the structure of the present invention
enables the
monitor photodiode 46 to be positioned upstream of the laser source 33
optically coupled
thereto via a tap coupler 47, which separates a small portion (2%) of the
laser light. Back
facet monitors measure the light produced by the laser, but not what is
actually coupled to
the waveguide 41, i.e. into the PLC chip 36; however, the downstream
photodiode 46 is
able to directly measure what light has been coupled in the waveguide 41.
The detector channels must pass through both stages of the filter, i.e. the
non-dispersive
filter 31 and the dispersive filter 32, and are processed by the grating-based
dispersive
filter 32. Preferably, the dispersive filter 32 is similar the dispersive
filter 19, as disclosed
with reference to Figure 5, including a concave reflective diffraction grating
50 with a
focal line 56, preferably defined by a Rowland circle. As above, a launch
waveguide 53
extending between the non-dispersive filter 31 and the dispersive filter 32 is
positioned to
ensure that the incident angle 6iõ is less than 45 , preferably less than 30
,and more
preferably less than 15 . Furthermore, the diffraction grating 50 has a pitch
A selected to
ensure that the diffraction grating 50 provides diffraction in an order of 5
or less.
Typical grating-based demultiplexers exhibit relatively sharp passbands that
are difficult
to make wide and flat, as required for the bi-directional transceiver
application.
Accordingly, the present invention incorporates multi-mode output waveguides
51 and 52
at output ports along the focal line 56. The multi-mode waveguides 51 and 52
support an
innumerable collection of modes, which serves to flatten the spectral response
of the
grating output, as shown in Figure 7. Alternatively, the first and second
output
waveguides 51 and 52 include a multimode section adjacent to the first and
second ports,
respectively, and a single mode section remote therefrom for providing the
diffraction
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grating filter 31 with a flattened spectral response. The waveguides 51 and 52
direct the
light from the output ports to the first and second photo-detectors 34 and 35,
respectively.
The present invention achieves the varying passbands for the detector and
signal channels
by incorporating a dual-stage filter, in which the laser channel is separated
from the
detector channels; which are further demultiplexed with a dispersive element
of higher
resolution. The passband of the laser channel is therefore determined by the
first stage of
the filter, e.g. the wavelength-selective directional coupler 31, while the
passband of the
detector channels is determined predominantly by the second stage of the
filter, e.g.
grating-based dispersive element 32. The directional coupler 31 can be
designed to easily
cover a passband of 100nm, as shown in Figure 8. The detector channels undergo
further
processing by the grating.
As demonstrated in Figures 7 and 8, narrow transmission passbands are achieved
for
detector channels, whereas the laser channel is quite broad. The detector
channels at 1490
and 1552 nm encounter both stages of the filter, and they are dispersed into
narrow bands
by the dispersive filter 32. The output waveguides 51 and 52 used in the
dispersive filter
32 enable the passbands to be extremely flat and wide across the whole range
of interest.
The 1310 n m r adiation is e xtracted f ollowing o nly the f irst s tage o f t
he filter, e.g. the
wavelength-selective directional coupler, with extremely low loss. The loss
for the laser
channel is therefore far superior to other Triplexer filters in which the
laser channel must
pass through one or several grating-based elements. The present two-stage
configuration
ensures that there is no direct path from the laser source 33 to the first and
second photo-
detectors 34 and 35, and the two channels are always counter-propagating,
resulting in
extremely high isolation of the laser source 33 from the first and second
photo-detectors
34 and 35. The level of isolation is significantly improved from the typical
level of 30 dB
from a standard grating, and can exceed the 50 dB specification required by
some
customers.
Rearranging equation (1) to yield the output angle versus the optical
frequency, produces:
mc
sineou~ _ -sine,n (11)
fnA
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With reference to Figure 9, the output angle varies in a smooth monotonic
manner with
respect to optical frequency. If the diffraction grating is designed for sharp
imaging, and
the input and output apertures are sharply defined, then the optical passband
shape for this
grating device w ill be a n arrow p assband s hape, w ith virtually no i
nsertion loss a t the
peak. In traditional designs, the passband is widened by deforming the grating
or
widening the optical apertures, such that as the frequency is swept, the
response over the
output aperture is dulled. The result can be a flat, and potentially sharp-
sided passband, at
the expense of insertion loss at the peak.
It can be seen from Equation 11 that for a given optical frequency, the output
angle can be
made to vary by changing the input angle. In fact this is an element of
coarse/fine
refractive index error correction for standard echelle-grating based optical
DEMUX's and
OCMJOPMs. Also, from Equation 1, for a given (fixed) output angle, the optical
frequency (or wavelength) can be made to vary with the input angle.
Typically, as the optical frequency varies over the passband of a ITU-grid
channel,
normally the output angle of the light would vary (as in Figure 9), and the
light would
sweep past the output waveguide. However, if the input angle could be made to
vary in a
complementary direction, i.e. introduce some frequency insensitivity, then the
output
angle could be held fixed in place. To be useful as a MUX/DEMUX, by the time
the next
frequency on the ITU grid is tuned, the light must image onto the next output
waveguide,
with the same insensitivity to frequency variation over the new passband.
In accordance with the present invention an angular dependence versus
frequency is
introduced, as in Figure 9, but with a pattern that repeats with a controlled
period, e.g.
every 100 G Hz as with the ITU grid spacing. To accomplish this a second
diffraction
grating is inserted prior to the first diffraction grating of Figure 9, having
a Free-Spectral-
Range (FSR) of the required period, e.g. 100 GHz, with a geometry chosen to
achieve the
required angular variations.
Recasting equation (1) in terms of frequency, and subtracting the frequencies
of
consecutive diffraction orders for the same input/output angle combination,
the difference
being a constant frequency (disregarding index variations with optical
frequency), which
is the FSR of the grating.
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mc (12)
f nA sin e,n + sin eou, (f FSR=, c f (13)
m+~-f m nA sinetn+sineoõr m
The required diffraction order for a given FSR is then given by
rn = s (14)
For a FSR of 100 G Hz and a c entral f requency f of 194.0 T Hz, t he required
order i s
m=1940. Index dispersion of the waveguide material will result in a slight
error in the
FSR as the frequency deviates substantially from the point at which the FSR
calculation
was performed. This can easily be compensated by a slight adjustment to the
diffraction
order.
For a similar geometry, the grating facet size will scale as the order.
Whereas standard
DEMUX's in low diffraction orders (m-20) have facets of - 10 m in size, the
high order
grating will have a facet of - 1 mm in size.
To understand how a frequency insensitive design might work, imagine a high
order
(FSR=100 GHz) grating spectrometer with a Rowland Circle geometry. For
convenience
of calculation, the output angle of the high-order spectrometer is chosen to
be the same as
the input angle used in the standard order (m-20) design. We place the Rowland
circle of
the high-order spectrometer such that the output of this spectrometer is
located at the
input of a standard spectrometer. The gratings and the input to the high-order
spectrometer are arranged such that the coupling of light from the high-order
spectrometer t o t he standard spectrometer i s optimum. T he c hoice o f
input a nd output
angles, and the grating geometries are for convenience of calculation only.
With reference to Figure 10, a WDM optical signal comprising a plurality of
optical
channel bands is input an optical waveguide 109 at the edge of a planar
lightwave circuit
chip 110 and enters a first slab waveguide 111 at input port 112. A first
concave
reflective grating 113 has a relatively high order, e.g. greater than 1000,
preferably
greater than 1500, and even m ore preferably greater than 1800, and a r
elatively small
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FSR, e.g. substantially the same as the channel spacing of the optical channel
bands to be
output. Due to the small FSR, the first grating 113 disperses each channel
band over the
same small range of output angles through an aperture 114 into a second slab
waveguide
116. A second concave reflective grating 117 is positioned at one side of the
second slab
waveguide 116 opposite the first reflective grating 113 in a face-to face
relationship. The
first and second reflective gratings 113 and 117 have optical power and focus
the light
along the same line defined by a Rowland Circle 118. The second reflective
grating 117
has a much lower order than the first reflective grating 113, e.g. less than
100, preferably
less than 50 and even more preferably less than 25, a much higher FSR, e.g. 10
times
greater than the FSR of the first grating, and is designed to convert the
small range of
input angles (corresponding to the small range of output angles from the first
grating 113)
into a single output angle for each channel band, i.e. for the small range of
wavelengths
the output angle of the second grating 117 remains the same. Accordingly, each
wavelength in the band of wavelengths in a single channel will be directed at
exactly the
same spot on an output port, e.g. output port 119a, corresponding to an output
waveguide,
e.g. output waveguide 120a. When the next channel band hits the second grating
117, the
frequency has increased, but the input angle returns to the lower end of the
range,
resulting in the output angle of the second grating 117 changing. The new
output angle
from the second grating 117 will remain fixed for all wavelengths in the new
channel
band, which is output a second output port, e.g. output port 119b. Other
waveguides,
such as optical fibers, are attached to the edge of the planar lightwave
circuit chip 110 for
transmitting the optical signals.
The device can also be used in a reciprocal fashion for multiplexing a
plurality of input
optical channel bands into a single output signal. In this case the second
reflective grating
117 r eceives e ach c hannel b and at a d ifferent i nput angle, w hich the s
econd reflective
grating 117 converts into the same small range of output angles for
transmission through
the aperture 114. The first reflective grating 113 then converts the small
range of input
angles into a s ingle output angle, thereby c ombining all of the channels
onto a s ingle
output waveguide 109.
In this double-grating configuration, as the input frequency tunes, the output
angle of the
first spectrometer will vary in a cyclic pattern according to a desired
channel spacing of
the second grating, i.e. the input signal and the output signals. If the
geometry and facet
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spacing of the first spectrometer are chosen properly, the pattern will repeat
every 100
GHz (or other desired channel spacing), with a variation in output angle that
becomes a
variation in input angle to the second spectrometer. The input angle variation
with optical
frequency provides a constant output angle for all wavelengths in the band of
wavelengths in each channel, which can nearly exactly pin the output image to
the
designated output waveguide. With reference to equation 11, the second grating
117 is
designed so that the change in input angle 6iõ compensates for the change in
frequency f
providing a constant output angle 00õt over the given range of wavelengths in
the channel
band. For the next channel band, the frequency keeps increasing, but the input
angle 0;n
reverts back to the lower end of the repeating range, which results in a new
0oõt for the
next channel.
In actual fact, due to the index dispersion of silica, i.e. the index varies
with optical
frequency, the output from the first spectrometer 113 is not exactly cyclic at
a 100 GHz
period resulting in a gradual drift in the output angle as the frequency is
tuned. over the
entire ITU grid. The walk-off can be partially compensated for by re-
positioning the
output ports 119a and 119b of the second spectrometer 117 relative to their
usual
positions for a fixed input aperture location. Furthermore, as explained
previously, a
modification can be made to the diffraction order of the first grating 7 in
order to tune its
period to the required value.
Figure 11 illustrates the near-cyclic behavior of the input angle Oin to the
second
spectrometer 117, i.e. the output angle 0o,,t of the first spectrometer 113,
versus frequency,
as well as the stepped behavior of the output angle 00õt of the second
spectrometer 117
versus optical frequency. A refractive index dispersion of
n =1.452061-1.342485x10 5(A-1545)
where k is stated in nanometers is used for these calculations.
Figure 11 graphically relates the input angles to the second grating 117, and
the output
angles from the second grating 117, as a function of optical frequency,
illustrating the
cyclic nature of the input angles, and the resulting stepped response of the
output angle.
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The slight wavelength dependence of the refractive index (of the silica
waveguides) leads
to a barely perceptible shift in mean input angle to the second spectrometer
117 over the
wide frequency range of the C-band; however, in general, the output angles of
the second
spectrometer 117 do show the expected stepped performance, i.e. over large
fractions of
each ITU grid the steps show little slope. The angular content of typical
waveguide
modes in a silica-on-silicon design will have a magnitude on the order of a
few degrees. If
the angle of coupling into these output guides can be held fixed to a small
fraction of this
mode angular content, the coupling should remain unchanged.
The graph in Figure 12 illustrates the deviation of the output angles from
their mean
position across the g rid. A s c an be seen from the f igure, t he output
angles a re indeed
pinned t o t heir r equired mean position t o within 2 m illi-degrees. The p
hysical spacing
between the output guide is approximately 15 m, so the physical error in the
o utput
position from the second grating will correspond to - 0.3 m.
The double-grating subtractive-dispersion design according to the present
invention has
benefits i n t he time domain as well a s the f requency domain. H owever, i n
a s tandard
single-grating design with well optimized sharp (Gaussian) passbands, improved
performance will be limited when transformed between the time and frequency
domains.
A temporal impulse broadening arises because the optical path from the input
to any
output v ia the n ear e dge o f t he g rating v ersus the p ath v ia t he f ar
e dge o f the g rating
differs by a non-zero length, which is indicative of the impulse broadening.
As stated
above, flat-top passbands are usually obtained by introducing aberrations to
the grating or
by increasing the input or output apertures; however, none of these solutions
reduces the
spread in time for different paths across the grating, i.e. the standard flat-
top design does
not narrow the temporal response. In the double-grating subtractive-dispersion
configuration according to the present invention, a ray which follows a short
path off the
first grating 113 to the input of the second grating 117, will then follow a
long path off
the second grating 117 to the output port 119a of the second grating. The
reverse holds
for rays initially taking a long path from the first grating 113. As a result,
temporal
compression is achieved at the same time as frequency-domain broadening.
Accordingly,
a subtractive dispersion double-grating device can be utilized at
significantly higher data
bit rates than a standard design flat passband device.
CA 02565709 2006-11-03
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The device illustrated in Figure 10 with two gratings each operating in a
Rowland circle
geometry was a first embodiment provided as an example for simplicity of
calculation;
however, there are a few other options that may be more favorable. One option
is to
design the first grating with a shape that is more appropriate for imaging
along a chord at
the second grating's Rowland circle centered for the input to the second
grating. A
second option is to create the first grating to collimate its diffracted
light, i.e. imaging to
infinity, and the second grating would be shaped in the same manner to re-
focus its
diffracted light.
The output of the first grating will need to be collected efficiently by the
second grating.
At the same time some form of aperturing will be needed between the first and
second
gratings because there will be light from multiple orders emanating from the
first grating
near the intended input to the second grating. Much of the aperturing will be
accomplished simply by the fact that the large-order grating facets are
physically quite
large, leading to a narrowing of the diffraction envelope from the first
grating. If blazed
properly, only the intended diffraction orders should arrive with any
reasonable intensity
onto the second grating. In order to prevent order overlap from confounding
the spectrum
of the second grating, aperturing would also be needed to restrict the angular
range, which
enters the second grating from the first one.
A subtractive dispersion spectrometer pair can also be designed using AWGs;
however,
in this case the high-FSR first spectrometer will have many drawbacks in terms
of phase
control. For etched grating based devices, facet shaping is a parameter that
has no direct
analog for AWGs, i.e. straight, circular, parabolic, elliptical, or other
facet shapes can be
implemented to control the phase of the radiation as it emerges from the high-
FSR first
grating.
The overall transmission of the double grating device, i.e. the height of any
passband, can
be quite high. Diffraction gratings that are designed to be astigmatic over
limited angular
regions and blazed for that region can be as efficient as - 0.5 dB excess
loss..A theoretical
insertion loss of ~- IdB is not unexpected for the grating-pair device.
Traditional channel
flattening techniques often require over twice that loss to achieve much less-
optimal
performance. The sharpness of the passband, i.e. the steepness of the band
walls, can be
increased by narrowing the frequency span covered by a given optical waveguide
mode
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width. One simple means to do this is to increase the diameter of the Rowland
circle of
the second grating system, or more generally to increase the physical
dispersion of the
second grating system. The first grating system would have to be altered
appropriately as
well. The width of the passband will be limited only by the aperturing that
has just been
mentioned. For a 40 channel, 100 GHz design, widths of - 40 - 50 GHz should be
achievable. Depending on the steepness of the walls, these numbers could
represent -0.5
dB, 1 dB and -3 dB widths all within.a few GHz of each other.
The performance of the double-grating configuration is expected to be near
transform-
limited, thereby providing good optical performance at higher bit-rates than
standard flat-
top designs allow.
The present invention can be used to create high-transmission, ultra-flat
ultra-sharp
passband, high bit rate compatible MUX/DEMUX's. The present invention could be
applied to DWDM, CWDM, 1310/1550 nm splitters, comb filters or optical channel
monitors, all by proper choice of diffraction order for the first and second
gratings.
The efficiency of the diffraction from a grating is a coherent superposition
of the
diffraction envelope from individual facets. The positioning of the multitude
of facets
dominates the mode shape of the emissions from the gratings at specific
wavelengths,
while the size of the individual facets dominates the relative intensity of
different modes
at different angles/wavelengths. This diffraction envelope is essentially
a(sin(x)/x)2
intensity distribution. By carefully choosing the location of the input to the
spectrometer
at a m inimum o f the d iffraction e nvelope, and the r equired outputs c
entered a bout the
maximum of the distribution, it should be possible to have optimum
transmission to the
output of the spectrometer with minimal reflection of light towards the input
of the
spectrometer.
If secondary diffraction orders are employed as well as the primary orders,
then it would
be desirable to place the secondary outputs at other maxima of the diffraction
envelope,
which would improve the signal captured at the secondary outputs, while at the
same time
reducing the sensitivity of the secondary signal strength to slight changes in
grating facet
orientation.
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The intentional utilization of minima and secondary (or higher) maxima of the
grating
facet diffraction envelope is new. A design using these minima or maxima
explicitly
positions inputs and outputs of the grating spectrometer by accounting for the
performance of the grating as a whole.
With reference to Figure 13, a simple optical demultiplexer is designed with
one input
channel 221 and four output channels 222a to 222d for Coarse Wavelength
Division
Multiplexing (CWDM). An optical signal with a plurality of optical channels,
defined by
center wavelengths ki to X4, is launched via input channel 221 into a slab
waveguide
region 223 to be incident on a grating 224. The grating 224 disperses the
optical channels
according to wavelength, whereby each optical channel X1 to X4 is captured by
one of the
output channels 222a to 222d.
A diffraction envelope from the central facet for the device of Figure 13 is
displayed in
Figure 14. Note the high principle maximum 231 and the multiple higher-order
maxima
232 with minima 233 between them. By moving the input guide 221 to a minimum
233 of
the diffraction envelope the return light intensity is greatly reduced.
Moreover, by
moving the o utput channels 2 22a t o 222d t o the principle maximum 231 or at
least a
higher order maximum 232 the transmitted light is maximized. Obviously,
positioning
both the input guide 221 and the output guides 222a to 222d in minima 233 and
maximum 231 is preferred.
With reference to Figure 15, since the position of the diffraction envelope,
the design of
grating 241, and the position of the input port 242 are all inter-related, the
design of a
demultiplexer device 240 is an iterative process starting with the design of
the grating 241
to generally provide a diffraction envelope with a sufficient amount of higher
order
minima and maxima. Preferably, the grating 241 is a concave reflective
grating, as
2 5 disclosed above w ith reference to Figures 4 and 5, with a focal l ine a
long a Rowland
circle 243. Next, an initial trial position for the input port 242 is
selected, and the
resulting diffraction envelope is examined. Assuming the input port 242 was
not
positioned correctly in the desired higher-order minima, a second trial
position is selected.
The process continues until the input port 242 matches the desired higher-
order minima.
Now the primary output ports 244, e.g. Order n, can be chosen based on the
primary order
maximum, and the secondary output ports 246, which are optically coupled to
output
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CA 02565709 2006-11-03
WO 2005/119954 PCT/CA2005/000834
waveguides 247, e.g. for optical channel monitoring detector array 248 made up
of photo
detectors, are selected based on the position of the higher order maximums,
e.g. Order n-
1. Ideally all of the output ports 244 and 246 are positioned along the focal
line 243 of
the grating 241, defined by a Rowland Circle.
Figure 16 illustrates a spectrum for a situation in which the input port 242
is located
physically near the output ports 244, such that the input port 242 falls
somewhere within
the principle diffraction maximum, which is very typical for demultiplexers
based on
Echelle Gratings. The intensity 251 of the return light signal is comparable
to the
intensity 252 of the main output signals, and can result in very high Return
Loss that is
unacceptable for telecommunications-grade optical components. A similar
spectrum is
calculated and illustrated in Figure 17, in which the input port 242 has been
located at the
third diffraction envelope minimum. Note the nearly 240 dB reduction o f the
intensity
253 of light returning along the input channel.
The sharp dip in the middle of the input channel is a result of the
diffraction envelope
dramatically minimizing within the span of the input port 242 itself.
The s econdary set o f o utput ports 246 are I ocated at h igher-order
diffraction envelope
maxima 232, see Fig. 14, to capture duplicate signals in parallel with the
capture of light
into the primary output waveguides 244a via primary output ports 244, which is
useful,
inter alia, as an integrated Demultiplexer/Optical Channel Monitor. In this
case, the
primary Demultiplexer output ports 244 would fit in the region of the
principle diffraction
envelope maximum 231, while the secondary output ports 246, e.g. channel
monitor
guides, for the same wavelengths but at a different diffraction order off the
grating 241,
would fit in the region of a secondary or higher diffraction envelope maximum
232.
Accordingly, monitoring of the optical power in each channel X1 to X4 of the
2 5 Demultiplexer can be performed by measurement of the light coupled into a
different
order, instead of through the insertion of a tap coupler and the subsequent
demultiplexing/monitoring of that light signal.
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