U.S. patent application number 10/160016 was filed with the patent office on 2002-12-05 for interleaving optical filter.
Invention is credited to Leyva, Victor, Rakuljic, George, Yeh, Xian-Li.
Application Number | 20020181102 10/160016 |
Document ID | / |
Family ID | 24063060 |
Filed Date | 2002-12-05 |
United States Patent
Application |
20020181102 |
Kind Code |
A1 |
Leyva, Victor ; et
al. |
December 5, 2002 |
Interleaving optical filter
Abstract
An optical signal filter for providing a periodic transfer
function in transmitting signals within a selected bandwidth, by
which passbands are interleavered into groups of separate outputs.
The filter employs the transmissivity characteristic of
birefringent crystals in conjunction with splitting the input beam
into orthogonal and separate components, while compensating for
temperature variations by pairing crystals of different types. The
transmissivity functions are independent of the polarization of the
input beam, and are shaped to flatten transmissivity peaks by the
use of cascaded stages of birefringent crystal pairs.
Inventors: |
Leyva, Victor; (Pasadena,
CA) ; Yeh, Xian-Li; (Walnut, CA) ; Rakuljic,
George; (Santa Monica, CA) |
Correspondence
Address: |
George M. Cooper
JONES, TULLAR & COOPER, P.C.
Eads Station
P.O. Box 2266
Arlington
VA
22202
US
|
Family ID: |
24063060 |
Appl. No.: |
10/160016 |
Filed: |
June 4, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10160016 |
Jun 4, 2002 |
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09518218 |
Mar 3, 2000 |
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6421177 |
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Current U.S.
Class: |
359/487.02 ;
359/489.04; 359/489.05; 359/489.07; 359/489.09; 359/489.19;
359/490.02 |
Current CPC
Class: |
H04J 14/02 20130101;
G02B 27/283 20130101; G02F 1/03 20130101; G02B 27/288 20130101;
G02F 1/0147 20130101; G02B 6/29302 20130101 |
Class at
Publication: |
359/495 ;
359/497; 359/498; 359/499; 359/500 |
International
Class: |
G02B 005/30; G02B
027/28 |
Claims
1. An interleaving optical filter for wave energy, for providing a
periodic low loss transmissivity characteristic in the rate of 25
GHz to 200 GHz spacing and operating with substantial polarization
independence and with compensation for temperature variations, said
filter comprising: a support providing a generally planar surface
extending substantially parallel to a principal optical axis for
the filter; an input collimator mounted on the support at an input
region thereon to provide a collimated beam along the principal
optical axis; a first beam displacing polarizer mounted on the
support to receive the collimated beam, the polarizer transmitting
two beams of orthogonal polarization that are parallel to the
principal optical axis; a first pair of birefringent crystals
receiving the two beams and being of different thermooptic
coefficients and with lengths along the principal optical axis that
are selected to compensate for temperature-induced phase
retardation variations, the first pair being rotated 45.degree.
with respect to the planar surface about the principal optical
axis; a second pair of birefringent crystals of materials like the
first pair but of different length, and being rotated with respect
to the planar surface to provide transmissivity peaks that have
passband flatness of -0.5 dB of about 0.47 nm and a center
wavelength drift of less than .+-.0.0015 nm/.degree. C.; a second
beam displacing polarizer receiving the two beams transmitted
through the pairs of birefringent crystals for splitting each of
the two beams into two beams with different polarizations; a beam
recombining unit receiving the beams from the beam displacing
polarizer for combining the beams therefrom into two polarization
independent beams in the component beams with less than one 1 mm
path length difference.
2. An interleaving optical filter as set forth in claim 1 above,
including output collimators coupled to transmit the different
polarization independent beams and an interleaving optical filter
as set forth in claim 1 above, wherein the second beam displacing
polarizer transmits a pair of orthogonally polarized individual
beams and a combined beam having orthogonally polarized components,
and the beam recombining means directs the combined beam as one of
the outputs.
3. An interleaving optical filter as set forth in claim 1 above,
wherein the second beam displacing polarizer transmits a pair of
orthogonally polarized individual beams and a combined beam having
orthogonally polarized components, and the beam recombining unit
directs the combined beam as one of the outputs.
4. An interleaving optical filter as set forth in claim 3 above,
wherein the beam recombining means includes a third beam displacing
polarizer receiving the orthogonally polarized individual beams,
and further includes a path length compensator in one of the beam
paths to the third beam displacing polarizer.
5. An interleaving optical filter as set forth in claim 3 above,
wherein the beam recombining means includes a third beam displacing
polarizer receiving the orthogonally polarized individual beams,
and further includes a half wave plate in both beam paths to the
third beam displacing polarizer.
6. An interleaving optical filter as set forth in claim 3 above,
wherein the beam recombining means comprises a polarizing beam
splitter cube and prism means for directing the orthogonally
polarized individual beams to different faces of the beam splitter
cube.
7. An interleaving optical filter as set forth in claim 1 above,
wherein the birefringent crystals are of opposite sign, and wherein
the second pair of crystals have a negative angular rotation
relative to the first pair.
8. An interleaving optical filter as set forth in claim 1 above,
wherein the pairs of birefringent crystals each comprise a YV04
crystal and an LiNbO.sub.3 crystal having length ratios of 6.60:1
and wherein the crystals of the second pair are twice the length of
those in the final pair.
9. An interleaver filter as set forth in claim 1 above, wherein the
first and second beam displacing polarizers are of YV04 crystal and
the beam recombining unit comprises a prism and polarizing beam
splitter cube.
10. An interleaving optical filter as set forth in claim 2 above,
wherein the input collimator and output collimators comprise
gradient index lenses, wherein the filter further includes housings
attached to the collimators and the housings are attached to the
support, wherein the collimators are disposed along the principal
axis, and the filter further comprises input and output optical
fibers in communication with the input and output collimators
respectively.
11. An optical assembly for retaining a number of birefringent
elements, polarizing elements and collimating elements in precise
axial and rotational positions along an optical axis, comprising:
an optical bench having a principal planar surface, the surface
including inset recesses disposed at spaced locations along the
length of the optical axis; the recesses having angles relative to
the planar surface to define the rotational orientation of the
birefringent elements; adjustable attachment mechanisms attached to
the optical bench at collimator positions along the optical axis;
collimator housings each supporting a different collimator and each
attached to a different one of the attachment mechanisms; and a
containment housing encompassing the optical assembly and including
optical fiber feedthroughs along the optical axis and in alignment
with collimating elements therein.
12. An optical assembly in accordance with claim 11 above, wherein
the optical bench is of stainless steel, and wherein in addition to
the attachment mechanisms are laser welded to the optical bench and
at least one of the birefringent elements are conductively coupled
to the optical bench.
13. An optical assembly in accordance with claim 12 above, wherein
the housing includes a tray and a lid in a sealed configuration,
and a layer of resilient material supporting the optical bench in
the tray.
14. An interleaving optical filter for introducing a periodic
transfer function with flattened apices within transfer in a wider
band spectrum of an input optical beam, comprising: a first
polarizer in the path of the input optical beam; at least two
stages of pairs of birefringent crystals of opposite thermooptic
coefficients, the crystals of each pair having a like length
proportionality but the lengths of the crystals in the first pair
having a selected ratio to the lengths of the crystals of the
second pair, the longer pair being disposed closer to the first
polarizer; and a second polarizer in the path of the optical beam
subsequent to the two stages.
15. An optical filter as set forth in claim 14 above, wherein the
like crystals within each stage have like optical axes and
orientations relative to the polarizer direction and wherein the
filter further includes beam displacing means between the first
polarizer and the at least two stages for directing beams of
orthogonal polarization through the at least two stages, and
wherein the filter further includes an optical circuit for
recombining the beams of orthogonal polarization into two beam
sets, each including both polarizations.
16. An optical filter as set forth in claim 15 above, wherein the
lengths of the crystals are selected to provide a selected
periodicity in the transfer function, and wherein the optical
circuit recombines the beams in the beam sets with equal path
lengths.
17. An optical filter as set forth in claim 16 above, wherein each
pair of crystals comprise a YVO4 crystal and an LiNbO.sub.3
crystal, and wherein the C axes of the first pair are at 45.degree.
to the direction of the first polarizer and the C axes of the
second pair are at .quadrature.14.8.degree., and wherein the length
proportionality of the crystals in each pair is 6.60:1 and the
ratio between the two pairs is 1:2.
18. An Optical filter for introducing an interleaving function into
a signal beam occupying an optical band, comprising: a polarizing
beam splitting cube for dividing the signal beam into two
orthogonally polarized beams; first and second birefringent crystal
sets, each including an initial polarizer sheet and two pairs of
serially disposed birefringent crystals of opposite thermooptic
sign; and beam recombining means including a second polarizing beam
splitting cube receiving the two orthogonally polarized beams and
providing two output beams, each including both polarization
components.
19. A filter for separating an input band of optical signal
frequencies into a number of periodicity varying transmissive
frequency bands divided into at least two groups, comprising: at
least one pair of birefringent crystals disposed serially along an
optical axis for receiving the optical signals, at least one of the
crystals having a pyroelectric characteristic; polarizing means
disposed along the optical axis and bordering the at least one pair
of optical crystals; and means coupled to crystals having a
pyroelectric characteristic for dissipating electric charges
induced thereon.
20. A filter as set forth in claim 19 above, wherein the crystals
of a pair have optical axes that are similarly aligned relative to
the optical axis of the device, and wherein the polarizing means
comprises a beam displacing polarizer for dividing the optical
signals into two beams directed along the optical axis.
21. A filter as set forth in claim 20 above, wherein the at least
one pair of birefringent crystals comprises two pairs, the second
pair having alignments of their optical axes that are alike within
the pair but different from the other pair, and have a selected
length relationship to the crystals of the other pair.
22. A filter as set forth in claim 19 above, wherein the means for
dissipating electric charges comprises conductive coating material
disposed on selected surfaces of the crystals.
23. An optical filter for transferring optical signals in either
direction between a terminal at one side and a pair of terminals at
the other side, in either direction, the optical signals
periodically spaced in wavelength, the filter transferring the
signals with a selected periodic passband function, comprising: at
least two serial stages of birefringent light propagating elements
arranged to provide differential retardations between orthogonal
polarization components of the optical beams, each stage being
configured with at least two elements of different thermooptic
characteristics to be substantially athermal over a selected
temperature band, the differential retardation relationships being
selectively varied between the stages to provide selected passband
widths at the selected passband periodicity; at least one
polarization responsive beam path juncture device between the
terminal at one side and the at least two stages and transferring
optical signals therebetween, splitting the signals in the beam
path in accordance with polarization in one direction, and
combining the signals transferred in the other direction; and at
least two beam path juncture devices each between the terminal of a
different one of the pair of terminals and the at least two stages
for processing optical signals therebetween, said beam path
juncture devices being polarization responsive and disposed in
series, and including elements arranged to cross-combine signals of
orthogonal polarization, such that the filter is polarization
insensitive.
24. A filter as set forth in claim 23 above, wherein the light
propagating elements are arranged in two sets of at least two
stages disposed in separate paths, and like elements in the
parallel paths are of matched lengths and thermooptic
characteristics.
25. A filter as set forth in claim 23 above, wherein the filter
further includes separate lens collimators at the outputs of the
separate ones of the at least two beam path juncture device, and
means associated with the at least one beam path juncture device
for establishing the input polarizations of the beams to minimize
the polarization dependent loss to below 0.1 dB.
26. A filter as set forth in claim 23 above, wherein the relative
angles of the fast axes of the stages are selected to vary the
retardation relationships, and wherein the filter includes elements
for equalizing the path lengths of the separate optical beams to
minimize PMD.
27. A filter as set forth in claim 23 above wherein the stages have
an athermal characteristic such that the center wavelength drift is
less than 0.0015 nm/.degree. C.
28. A filter as set forth in claim 23 above, wherein the athermal
compensation conditions is approximately in accordance with: 6 L 1
L 2 := - ( ( n 1 ) T + 1 n 1 ) n ( ( n 2 ) T + 2 n 2 ) where L is
the length of each crystal, .DELTA.n is the birefringence for a
crystal, and .alpha. is the thermal expansion coefficient.
29. A multiplexer/demultiplexer functioning in accordance with an
interleaver transfer function for processing interleaved,
wavelength multiplexed signals of a first inter-channel wavelength
periodicity present at one terminal and signals at half the first
inter-channel wavelength periodicity present at a pair of
terminals, comprising: a pair of optical beam paths, each
comprising at least two stages of optical delay elements disposed
in series, each stage of each path including at least two optical
elements in series, whose lengths and thermooptic characteristics
are selected to provide a differential phase retardation between
orthogonally polarized beam components that is athermal over a
selected temperature range to define a periodic transmission
characteristic of chosen periodicity, the lengths and fast axis
orientations of each stage being selected to broaden the passbands
of the transmission characteristic; a first beam splitter/combiner
in the optical communication path between the one terminal and the
pair of beam paths for (1) directing and separating received
signals of arbitrary state of polarization from the one terminal
into the two beam paths, the signals on the two beam paths being
orthogonally polarized in a fixed relation to one another, and for
(2) combining orthogonally polarized optical beams from the beam
paths into signals of arbitrary state of polarization at the said
one terminal; and a second and third beam splitter/combiner means
in the optical communication path between the pair of beam paths
and the pair of terminals, for (1) cross-combining orthogonally
polarized beam components received from the beam paths and
exhibiting periodic polarization characteristics at half the first
wavelength periodicity into separate beams exhibiting periodic
transmission characteristics at half the first wavelength
periodicity and transferring them to the pair of terminals, and for
(2) separating signals received at the pair of terminals, signals
exhibiting periodic transmission characteristics at half the first
periodicity, into orthogonally polarized signals, and transferring
orthogonally polarized signals to the beam paths through stages as
individual beams.
30. A multiplexer/demultiplexer as set forth in claim 29 above,
wherein multiplexed signals applied to the terminals have arbitrary
states of polarization and where the beam splitter/combiners are
polarization sensitive and divide or combine beams in accordance
with polarization and direction.
Description
FIELD OF THE INVENTION
[0001] This application is a division-of U.S. application Ser. No.
09/518,218, filed Mar. 3, 2000, now U.S. Pat. No. ______ issued
______ 2002.
[0002] This invention relates to wavelength division mutiplexed
(DWDM) systems used in optical fiber communications, and more
particularly to optical signal filters which separate a WDM channel
stream into groups of channels on separate fibers.
BACKGROUND OF THE INVENTION
[0003] Narrow band optical filters are essential in wavelength
division (WDM) communication systems in order to process signals at
different precisely spaced wavelengths. Low insertion loss, flat
top filter response, sharp cutoff, and the ability to scale to high
channel counts and dense channel spacing are all critical
parameters. An interleaving filter is a device or subsystem which
can separate multiple channels in a WDM transmission into groups. A
1.times.2 interleaving filter divides a WDM channel stream,
periodically spaced in optical frequency, in a manner such that
every other channel is launched into one of two separate fibers.
More generally a 1.times.N interleaving filter, separates every Nth
channel into one of N fibers.
[0004] The interleaving function, more broadly speaking, includes
establishing a periodic transmissivity characteristic within a
given wider frequency band, so that there is virtually lossless
transmission within incrementally spaced frequency channels, and in
effect full signal rejection between the channels. Preferably, the
transmissive pass bands are shaped with flat top response, so that
laser wavelength shifts and other variations within the pass bands
can be tolerated, thus reducing the stringency of performance
specifications imposed on such active elements. Therefore, in
multiplexing, channel spacings can be reduced with improved
performance, while in demultiplexing closely spaced channels can be
separated without requiring prohibitively precise individual
components, such as add/drop filters. In demultiplexing,
interleaving filters can also serve to reduce the component counts
and serial insertion losses, because they separate signals in
parallel fashion and can be cascaded to divide channels into a
number of smaller groups before wavelength selective devices are
used to add or drop individual wavelength signals.
[0005] The most common approach to interleaving filter design is
based upon using unbalanced Mach-Zender interferometers. These are
adequately responsive but are large, costly units that are
difficult to adapt to many system requirements. In addition, they
are subject to inherent instability problems that require extra
measures to overcome. Thin film 200 GHz filters are now being
offered, but thin films require costly and precise processes. Other
periodic optical transmission functions are known, such as those
exhibited by birefringent crystals, as delineated in detail by Yeh
and Yariv in "Optical Waves in Crystals", John Wiley and Sons
(1983). As the authors explain, a birefringent element sandwiched
between parallel polarizers has a transmission characteristic that
is periodic in optical frequency, and effectively without loss at
transmissive peaks. Much analytical work, of both theoretical and
practical natures has been directed to using the properties of
birefringent crystals. In 1964, for example, Harris et al proposed
a procedure for the synthesis of optical networks in an article in
the Journal of the Optical Society of America, Vol. 54, No. 10
(October 1964), pp. 1267-1279, entitled "Optical Network Synthesis
Using Birefringent Crystals". This article treats some of the
considerations fundamental to synthesizing specific transfer
functions using a series of birefringent crystals between entry and
exit polarizers. Subsequently, Kimura et al discussed a technique
for reducing thermally induced variations in an article entitled
"Temperature Compensation of Birefringent Optical Filters", in the
Proceedings of the IEEE, August 1971, pp. 1271-2. They disclosed
that if the signs of the birefringence of two different crystal are
opposite, the retardation of the series is less dependent on
temperture. Although the intended purpose of the device described
is as a filter for frequency stabilization, one of the articles
cited, "Wide-band Optical Communication Systems, Part I--Time
Division Multiplexing", by T. S. Kinsel, Proc. IEEE, Vol. 58,
October 1970, pp. 1666-1683 is referenced in regard to the use of
birefringent optical filters to multiplex or demultiplex carriers
of different frequencies in the field of wide-band optical
communications.
[0006] A usage of crystals that is somewhat more related to the
interleaving filter context is disclosed in a letter published in
Electronics Letters, Vol. 23, No. 3 dated Jan. 29, 1987, at pp. 106
and 107, by W. J. Carlsen et al, discussing the use of a series of
birefringent crystals configured to improve the characteristics of
systems disclosed by articles on prior tunable
multiplexers/demultiplexer- s (referenced therein). All of these
multiplexers are intended to be used with either of two lasers
about 15 to 25 nm apart in optical wavelength, but they do not
suggest features suitable for an interleaving function or operation
at the now common 100 to 200 GHz spacings. A 100 GHz interleaving
filter, for example, requires a passband of the order of 0.2 nm
(vs. about 10 for the Carlsen et al system) and like intermediate
stop bands. Carlsen et al do discuss a modification which achieves
a flattened passband using five retardation plates of selected
orientations relative to the end polarizers, and achieving
polarization independence by splitting the beam so as to direct
polarization components separately through the filter.
[0007] A need thus exists for a wideband interleaving filter having
multiple narrow channel spacings and functioning with wide and
flattened passband characteristics, insensitivity to polarization,
temperature stabilization and very low insertion loss. The need
includes a configuration made of readily available materials that
can be readily assembled with the necessary precision, and that is
of compact size and also mechanically stable.
SUMMARY OF THE INVENTION
[0008] Interleaving circuits for optical networks in accordance
with the invention utilize a series of birefringent crystals in
varying electrooptic property combinations and orientations to
provide densely packed periodic transmission peaks which
nonetheless have very low insertion loss, polarization
independence, flattened passband peaks and temperature
compensation. Pairs of dissimilar birefringent elements in cascaded
(series) relationship broaden the transmissive peaks while
compensating out the effects of temperature variations. By mounting
the elements on a planar reference structure having preset recesses
in which adjustments can be made, the unit can be aligned and
adjusted with respect to retardation, spacings and orientation for
best performance.
[0009] In a more specific example of an interleaving filter in
accordance with the invention, birefringent crystals are arranged
in series between an input and output beam displacing polarizers,
together with beam combining elements at the output. The input beam
is divided into two beams of orthogonal polarization, which are
successively incident on two stages of paired birefringent
crystals, the crystals of each pair being of opposite sign of
thermooptic coefficient and of specific length ratios, and the
crystals of the second pair being twice the length of the first.
With crystals of yttrium orthovandate (YVO.sub.4) and lithium
niobate (LiNbO.sub.3), respectfully, the ratio used is 6.60 to 1,
and the crystals are precisely spaced apart and provided with
anti-reflection coatings on the beam-incident surfaces. The lengths
used are inversely related to the desired channel spacing. The
optical (c) axes of the crystals are angled relative to the
polarized input signals and to each other to utilize the
retardation difference of the birefringent crystals, providing two
temperature compensated output beams having flatband maxima, which
are then split into another set in an output beam splitting
polarizer. One combined beam of both polarization components is
collimated for direction to one output fiber, while two separate
beams of orthogonal polarization are combined in a group of prisms
and a polarizing beam splitter cube, for direction through a
collimator to a second output fiber.
[0010] This interleaving filter, enclosed is a sealed housing is
less than 20 cm long and 5 cm wide. Placement and angular
orientation of the optical elements is facilitated by the shaping
of receiving recesses in the optical bench, along the optical beam
path. Where a pyroelectric crystal such as LiNbO.sub.3 is used,
buildup of surface changes due to temperature cycling is avoided by
current conduction from the crystal faces that do not receive the
optical beams. To achieve precise tuning, crystal faces may be
angled so that relative translation of beams with respect to the
crystals changes the path length within crystals.
[0011] In accordance with other features of the invention, beam
displacing polarizers are used in recombination of beams which have
passed through the birefringent crystal system. Path length
differences can be equalized by employing a half wave plate in both
beams or a compensating plate in one of the beams incident on the
beam displacing polarizer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] A better understanding of the invention can be had by
reference to the following description, taken in conjunction with
the accompanying drawings, in which:
[0013] FIG. 1 is a somewhat simplified perspective view of an
example of an interleaving optical filter in accordance with the
invention;
[0014] FIG. 2 is a schematic side view representation of the filter
of FIG. 1 within a housing;
[0015] FIG. 3 is a schematic perspective representation of the
orientation of crystals and polarizers in the filter of FIGS. 1 and
2, and an optical bench on which they are mounted;
[0016] FIG. 4 is a simplified plan view of the filter of FIGS. 1-3,
illustrating beam paths along the structure;
[0017] FIG. 5 is a waveform diagram showing the periodic
characteristic of a 100 GHz interleaving filter with passband
flattening;
[0018] FIG. 6 is a waveform diagram of characteristics of a series
of birefringent crystals, showing passband flattening;
[0019] FIG. 7 is a simplified block diagram view showing further
features as to interleaving filter design and construction;
[0020] FIG. 8 is a simplified block diagram view showing further
features as to interleaving filter design and construction, and
[0021] FIG. 9. is a simplified block diagram view showing further
features as to interleaving filter design and construction.
DETAILED DESCRIPTION
[0022] Birefringent or polarization filters are described in
"Optical Waves in Crystals", Yeh and Yariv, referenced above. The
transmission through a birefringent element sandwiched between
parallel polarizers is periodic in frequency and is given by: 1 I (
) := [ cos [ ( n e - n 0 ) L 10 6 ] ] 2 Eqn . 1
[0023] where I is the intensity, .lambda. is the wavelength in
nanometers, n.sub.e is the extraordinary index of refraction,
n.sub.o is the ordinary index of refraction and L is the thickness
of the crystal in mm. The crystal is oriented with its optic axis
at 45 degrees to the input polarization. Note that in this
governing relationship the transmission is periodic in optical
frequency (i.e. inverse wavelength) and the transmission is
lossless at the peak. The present systems use this fundamental
controlling Eqn. 1 together with a number of others in providing
shaped transmissivity characteristics with low insertion loss,
polarization insensitivity and compensation for temperature
variation.
[0024] FIGS. 1 to 4 depict the arrangement and relationships of the
components of a 1.times.2 passband flattened and temperature
compensated interleaving filter 10, the components of which are
seated on a generally planar surface 13 of a stainless steel
optical bench 12 (see FIGS. 2 and 3). The optical bench 12 (which
alternatively can be of other materials such as silicon) contains
shaped recesses 14 formed by electron discharge machining (EDM), in
which recesses 14 in the planar surface 13 the polarizing
components and crystals are mounted and precisely aligned and
angled. A gradient index (GRIN) lens responsive to the wideband
input stream from an optical fiber is a collimator 16 for the WDM
channel stream. The collimator lens 16 is secured within a
cylindrical metal housing 20 which is welded within a stainless
steel clip 22 of general U-shape, where base legs 24, 25 are welded
to the optical bench 12. The clip 22 design allows for precise and
stable tilt and translation adjustment of the collimator lens
during assembly.
[0025] After collimation, the input beam is transmitted through a
beam displacing polarizer 28, here of YVO4 crystal, which splits
the input beam into two parallel beams (FIG. 4 particularly) with
crossed polarizations that are shifted 1 mm with respect to the
other. Splitting the input beam into separate polarizations and
then recombining after filtering results in a polarization
independent device.
[0026] The two beams are then incident on a first birefringent
crystal stage comprised of a Yttrium orthovanadate (YVO4) crystal
30 and a Lithium niobate (LiNbO.sub.3 crystal, 32 configured to
form a first a temperature compensated pair. There are a number of
different birefringent materials which can be used for temperature
compensation, including YVO4 and LiNbO3. YVO4 has high
birefringence, .DELTA.n=0.2039 at 1550 nm, and is readably
available commercially. LiNbO3 has a large thermooptic coefficient
opposite in sign to YVO4 and is also readably available
commercially. The required length ratio for temperature
compensation of YVO4 to LiNbO3 is 6.60:1. The lengths scale
inversely with channel spacing, and 50 Ghz and 25 GHz spacing are
achievable. The optical (C) axis of the YVO4 crystal and the LiNbO3
crystal 32 are oriented at 45.degree. relative to the input
polarizer 28. For a 100 Ghz to 200 Ghz channel spacing the YVO4
crystal 30 is 7.370 mm long and the lithium niobate crystal 32 is
1.116 mm long.
[0027] The second stage, which is also temperature compensated but
employed to flatten the peak of maximum transmissivity of the
passband is comprised of another set of YVO4 36 and lithium niobate
crystals 38. The lengths of each of these are twice that of the
like crystals used in the first stage. The optical axes of each
separate crystal 36,38 of the second pair are oriented along a
crystal edge and the crystals are tilted--14.8 degrees with respect
to the top surface 13 of the optical bench 13. The edges of the
lithium niobate crystals 32 and 38 are electrically shorted with a
conductive coating 40 such as silver epoxy which conducts away
charges built up due to the pyroelectric property of LiNbO.sub.3.
Such electric charges would otherwise tend to build up on the
surface of crystals 32, 38 as the temperature is cycled, and the
result could be uncontrolled hysteresis of the index of refraction.
Such charges must be eliminated for LiNbO.sub.3 to be used as a
temperature compensating element. Conductive silver epoxy 40 (or
metallization or anti static liquid) of the +/- c faces of a
crystal electrically shorts the crystal and dissipates charges.
Dopants such as MgO which reduce the electrical conductivity would
also reduce the pyroelectric effects.
[0028] The two beams after being transmitted through the
birefringent crystals 30, 32, 36, 38 are incident on another YVO4
beam displacing polarizer 41. Each input beam is split up into two
beams with different polarizations, as best seen in FIG. 4. Two of
the beams with crossed polarization overlap and are coupled
directly into one of two output GRIN lens collimators 47, 48. The
two other beams are combined in part by using a single prism 42 to
direct the s polarized beam to one side of a polarizing beam
splitter cube, 43 with the p polarized beam being redirected off a
pair of prisms 44, 45 to an orthogonal side of the beam splitter
cube 43. The path lengths of the two combined beams are matched to
better than 1mm in order to minimize polarization mode dispersion
(PMD). The resulting overlapping beams are then coupled into the
second output collimator 48. The output collimators 47, 48 are
laser welded to clips which are in turn laser welded to the optical
bench 12.
[0029] Referring specifically to FIG. 2, the optical bench 12 is
mounted inside a tray 50 with a fiber feedthrough 52 in the end
wall 54 receiving an input optical fiber 18 in line with the input
collimator 16. Although the output side is not shown in this view
it differs in having only pairs of elements for delivering the two
output beams. The bench 12 is attached to the base of the tray 50
with RTV adhesive or, as shown, a silicone sheet 58 can be used to
provide cushioning from shock and vibration. The input fiber 18 and
two output fibers are fed through the fiber feedthroughs and sealed
with epoxy. A lid 58 is attached to the body of the tray 50 and the
waist is sealed with epoxy in a dry nitrogen atmosphere.
[0030] All of the optical surfaces, including the crystals, are
antireflection coated to minimize optical loss.
[0031] The experimentally measured transmission (using an LED and
an optical spectrum analyzer) of a fiber coupled, passband
flattened 100 Ghz interleaver is shown in FIG. 5. The unit uses two
stages of lithium niobate and YVO4 crystals. This measured response
is charted in FIG. 5. The spacing between transmissivity peaks is
that of a 100 Ghz channel spacing to 200 Ghz channel spacing
interleaving filter.
[0032] Wider passband flatness in a filter (i.e. a broadening of
the width of the transmissivity peaks), prevents narrowing of the
transmission spectra when filters are cascaded and reduces the
required wavelength accuracy of the WDM source lasers. It also
improves system performance by reducing the attenuation of the
information content of a modulated signal. By adding additional
birefringent elements, the passband of the interleaving filter is
flattened to a selectable degree. As shown in FIG. 6, which depicts
response variations between one, two and three crystals in series,
the passband of a single element birefringent filter is 0.35 nm
wide at the -0.5 dB bandwidth By adding a second and third
birefringent element, shown by dotted and dashed lines
respectively, the maximum is progressively broadened. The first
element is of length L and oriented with its C axis at 45 degrees.
For a series of two, the second element is of length 2L and has an
orientation of--14.8 degrees, substantially widening the amplitude
at maximum without broadening the cutoff point. Addition of a third
element of length 2L and orientation =+10 degrees broadens the
maximum even further, but introduces intermediate dips of minor
magnitude. For a 100 Ghz/200 ghz interleaver the bandwidth for a
passband flatness of -0.5 dB is 0.35 nm for the single stage, 0.47
nm for the two stage, and 0.60 nm for the three stage design. Even
better flatness can be achieved by adding more elements; however,
this comes at the expense of additional cost and insertion loss.
Although the curves depict the results of measurements with
crystals of only one type, they are equally valid for temperature
compensated combinations using different crystal types.
[0033] The polarization dependent loss (PDL) of the interleaving
filter must be minimized to a value below 0.1 dB. This is achieved
during coupling of the two crossed polarization beams incident on
each output GRIN lens collimator 46 or 48. In order to minimize PDL
the beams must be coupled into the output fiber with the same
efficiency. This is not necessarily at the peak coupling efficiency
of each beam. This can be determined by varying or switching the
input polarization to the interleaving filter 10 until no variation
of power on the output fibers is measured.
[0034] In order to make a polarization independent fiber based
interleaving filter, the signal is split up into two beams using
the lossless beam displacing polarizer 28. The two beams are
transmitted through the birefringent elements 30, 32, 36, 38 and
recombined into two outputs using the additional lossless beam
displacing polarizer 42. The s polarized output of one of the beams
is recombined with the p polarization of the other beam. Every
other channel of a WDM stream is thus separated into one of the two
output collimators 47, 48 and the output fibers to which they
couple.
[0035] Devices typically operate over a 0-70.degree. C. temperature
range and should be passively temperature compensated. For a 100
Ghz filter response the center wavelength drift should typically be
less than +/-0.0015 nm/.degree. C. The use of different crystals
with opposite signs of the birefringence or thermooptic coefficient
in the manner described achieves this result. The retardance of two
crystals in series is given by: 2 1 + 2 := 2 ( L 1 n 1 + L 2 n 2
)
[0036] Where .GAMMA. is the phase retardance, L is the length of
each crystal, and the birefringence is given by
.DELTA.n.sub.1=n.sub.e,1-n.sub- .o,1 for the first crystal. For the
second crystal .DELTA.n.sub.2=n.sub.e-- n.sub.o if the crystal axis
is parallel to that of the first crystal and is
.DELTA.n.sub.2=n.sub.o-n.sub.e if it is rotated 90 degrees. The
change of retardance with temperature is given by: 3 ( 1 + 2 ) T :=
( 2 ) ( L 1 ( n 1 ) T + L 2 ( n 2 ) T + L 1 1 n 1 + L 2 2 n 2 )
[0037] Where .alpha. is the thermal expansion coefficient. The
condition for compensation is given by: 4 L 1 L 2 := - ( ( n 1 ) T
+ 1 n 1 ) ( ( n 2 ) T + 2 n 2 )
[0038] Usually the thermal expansion coefficient term can be
neglected. With the optic axis of the crystals in alignment,
compensation is achieved using two crystals with different signs of
the thermooptic coefficient. If the crystals are rotated 90 degrees
with respect to each other, materials can be used with the same
sign of the thermooptic coefficient.
[0039] During assembly, both the frequency period and absolute
wavelength of the peaks must be adjusted. This can be controlled by
tight tolerances of the thickness of the polished crystals. The
crystals or the input beam angle can also be tilted to adjust the
wavelength. Another approach is to polish the crystal to form a
slight wedge shape, with the beam-incident faces thus being
non-orthogonal to beam direction. Then the wavelength and period
can be adjusted by translating the beam on the crystal. In order
for both the parallel beams to see the same thickness of crystal,
the wedge angle should be transverse to the plane of the two
incident beams. Two crystals with opposing wedges can be translated
relative to each other to adjust the thickness and minimize any
beam steering. Another approach to tune the wavelength is to choose
from a set of LiNbO3 crystals at slightly different thickness, and
tuning by substituting for the best response. A spacing of 10
microns can allow for tuning while only slightly changing the
temperature compensation condition. Adjustment of the absolute
wavelength peaks of the filter can also be achieved by using a zero
order half wave plate after each of the stages. By rotating the
waveplates additional birefringence is introduced which tunes the
filter. Zero order waveplates are used to minimize temperature
dependence of the waveplate.
[0040] The alignment and tolerance of the optical components are
critical. Both insertion loss and manufacturing assembly cost need
to be minimized. An alternative to the stainless steel optical
bench is to use a silicon bench as a platform to mount all of the
components. Precise V-grooves are etched onto the silicon substrate
and components are dropped into them and attached with epoxy.
[0041] 1.times.2 interleavers can be cascaded to split every Nth
channel into one of N output fibers. For the second stage a crystal
of half the thickness of the first stage is required. In general
the transmission through an N stage interleaver will have a
transmittance at one of the output fibers given by: 5 I ( ) := i =
1 N n [ cos [ ( n e - n 0 ) L i 10 6 ] ] 2 4
[0042] The other fibers will have the same wavelength dependent
transmittance with the peaks shifted by a multiple of the input
channel spacing.
[0043] Each stage of a birefringent filter is not limited to
separating every other channel. More generally a single stage can
group every Nth channel onto a single fiber and the remaining (N-1)
adjacent channels in each period onto a separate fiber. One
approach is to use a Solc type filter described in the Yeh and
Yariv treatise referred to above. There are two designs, folded and
fanned, which rely on a stack of rotated birefringent plates of
equal thickness.
[0044] Another example of an arrangement in accordance with the
invention, referring now to FIG. 7, divides the input beam from an
input collimater 60 into an s polarized beam and an orthogonal p
polarized beam at a first beam splitting polarizer cube 62. The p
polarized beam is directed back into parallelism for compactness at
a prism 63, and both beams then pass separately through sheet
polarizers 64, 65 to temperature compensating birefringent crystal
pairs 70, 71 and 73, 74 as described above, after which separate
beam pairs are recombined. S polarized components are angled off a
prism 80 to one face of a second polarizer cube 82, which receives
the p polarized beam at another face. From the second cube 82 two
orthogonal beams merge, each combining s and p components, and the
two combined beams are directed to first and second collimators 84,
85 respectively. This arrangement simplifies beam recombination
but, because of the characteristics of polarizing beam splitters,
the cross-talk between adjacent channels is higher, even though
reduced somewhat by the sheet polarizers. In addition, the costs of
using separate crystals of equal lengths must be considered.
[0045] Other examples of arrangements in accordance with the
invention, referring to FIG. 8 and FIG. 9 make use of different
optical arrangements to recombine the s and p polarized beams into
the second output fiber. In FIG. 8 the optical layout from the
input collimator through the birefringent crystals and the second
beam displacing polarizer is the same as described previously.
Here, however, a prism 90 is used to pick off the center beam
emerging from the second beam displacer 41 which contains both
required polarizations. This beam is reflected with another prism
92 and coupled directly into an output collimator 47. The other two
beams emerging from the second beam displacer 41 are recombined
within a third beam displacing polarizer 94. The length of the last
beam displacer 90 is twice that of the first two 28, 41 due to the
need for twice the displacement. Since the path lengths of the two
beams would not otherwise be matched, a compensating plate 96 is
inserted in one of the beam paths to match the optical path length.
A high index material such as lithium niobate, with the crystal
axis aligned with the input polarization, is used in the s
polarized beam.
[0046] Another approach to match the optical path lengths is shown
in FIG. 9. The two beams emerging from the second beam displacer 41
are transmitted through a half wave plate 98 and then recombined
using a third beam displacement polarizer 100. The half wave plate
98 rotates the polarization 90 degrees, which ensures that the
overall path lengths of the two beams are matched after going
through the final beam displacer 100. The half wave plate 98 is a
zero order design to reduce the temperature dependence. The
combined beams in the midregion of the second beam displacing
polarizer 41 are angled off a common prism 102 to the second output
collimator 48.
[0047] Although a number of variants and alternatives have been
described, the invention is not limited thereto but emcompasses all
forms and modifications within the scope of the appended
claims.
* * * * *