This invention provides polarization-independent optical filters that are Fabry-Perot filters. The inventive filters incorporate a phase modulator within the FP cavity and two quarter-wave plates, one on either side of the phase modulator, in the cavity. The quarter-wave plates are perpendicular to each other and oriented at .+-.45.degree. with respect to the optic axis of the phase modulator. A variety of phase modulators including those that incorporate nematic and smectic liquid crystal optical modulators can be employed in these FP filters. The filters of this invention are particularly useful in fiber optic systems and in telecommunications applications at 1550 nm.

FIELD OF THE INVENTION 
This invention relates to polarization-insensitive tunable Fabry-Perot FB 
optical filters, particularly those employing liquid crystal materials in 
tuning elements. 
BACKGROUND OF THE INVENTION 
The use of optical fibers in telecommunication networks is gaining favor as 
a method for increasing the capacity, and providing longer communication 
links and system economy for the network. Since the bandwidth, or the 
information carrying capacity of optical fiber, is about 200 nm (25 THz) 
at 1550 nm, wavelength division multiplexing can fit 256, 0.8 nm channels 
within this bandwidth. Inexpensive, high speed, wide range tunable optical 
filters are needed to take advantage of this fiber capacity. 
Polarization-sensitivity is a characteristic of many optical filters and 
represents a severe limitation in certain filtering applications. Light 
coming out of an optical fiber has its polarization drifted in time due to 
the temperature and stress variations. Thus, filtering applications in 
optical fibers, for example in wavelength division multiplexing, where 
input polarization is unknown or cannot be selected, are significantly 
limited by polarization sensitivity. 
For arbitrary polarized input, an optical filter typically decomposes the 
polarization into two eigen states: one which interacts with the ordinary 
index of refraction (n.sub.o) and the other which interacts with the 
extra-ordinary index of refraction (n.sub.e). In such cases, the filter 
transmits two peaks, one associated with each polarization eigen state. 
Use of an entrance polarizer eliminates the untuned resonant peak (the 
ordinary wave), but also eliminates a significant portion of the input 
intensity. With arbitrary input polarization from a fiber, a polarizer may 
cause more than 3 dB loss of polarization perpendicular to the input 
polarizer. 
Conventionally, polarization sensitivity has been solved by use of 
polarization-diversity techniques. (See, for example a description of such 
techniques applied to FP filters in U.S. Pat. No. 5,493,426) Such 
techniques split the input light into orthogonal polarizations, modulate 
each component separately and then recombine the two polarizations. For FP 
filters, polarization diversity effectively constitutes the use of two FP 
cavities (or at least spatial separation in a single cavity) . Careful 
fabrication process and balanced electronic drivers are required to 
achieve substantially identical resonance conditions for both 
polarizations. These requirements increase production difficulties, 
increase manufacturing cost and can make high-volume commercial device 
manufacturing impractical. 
U.S. Pat. No. 5,068,749 of Patel, issued Nov. 26, 1991, relates to an 
electronically tunable polarization-independent FP filter employing 
nematic liquid crystal materials. A nematic liquid crystal layer is 
introduced between the opposed mirrors of a FP cavity and electrodes are 
provided to allow a variable electric field to be applied across the 
layer. A n.pi./2 twist (where n is a positive odd integer) of the 
principal axis of the LC material is introduced across the LC material by 
selecting the relative orientation of LC alignment layers on either side 
of the LC layer. In the specifically exemplified filter, alignment layers 
on either side of the LC layer were oriented perpendicular to each other. 
In this device, wavelength tuning is said to be independent of input light 
polarization. 
U.S. Pat. No. 5,111,321 of Patel issued May 5, 1992, reports a tunable 
dual-polarization FP filter having a twisted nematic liquid crystal layer 
between opposing interference mirrors. The twisted nematic LC layer is 
divided into two portions by use of a two-portion alignment layer on at 
least one side of the nematic LC layer. The two-portion alignment layer is 
homogeneous, but the alignment of the portions are orthogonal to each 
other. The orthogonal portions of the LC layer are said to operate in 
equal amounts on orthogonal polarizations of light with resultant 
polarization insensitive tuning when input light is divided between the 
two portions of the LC layer. 
U.S. Pat. Nos. 5,469,279; 5,381,253; 5,552,912 and 5,493,426 all of which 
are incorporated in their entirety by reference herein, relate to folded 
path configurations of high resolution optical filters and modulators, 
i.e., within a FP cavity, including those that use LC materials as tuning 
elements. These filters, all of which are employed in combination with 
polarized input light, are not described, used, or applied as 
polarization-independent filters. The multi-pass filter configuration of 
U.S. Pat. No. 5,469,279 has an etalon (FP cavity) containing two 
quarter-wave plates with a central retarder (at .GAMMA. (retardance) of 
.pi. or .pi./2, full or half wave, respectively) between the quarter-wave 
plates and an optional intracavity isotropic space. The QHQ 
(quarter-half-quarter) configuration of this patent modulates phase by 
changing the polarization of light. In the "folded Solc filter" the 
central retarder is a half-wave retarder oriented at a rocking angle, 
.rho., and the quarter-wave plates are parallel to the input polarization. 
In an alternate "fan Solc filter" configuration, the central retarder is a 
full wave plate (.GAMMA.=.pi.) orientated at angle, .rho.. 
The optical modulator of U.S. Pat. Nos. 5,381,253, 5,552,912 and 5,493,426 
provide modulation of phase, intensity, and wavelength of polarized light. 
The exemplified configurations have chiral smectic liquid crystal cells 
which are planar aligned, homeotropically aligned or tilted-layer aligned. 
QHQ configurations in which the half-wave plate is a planar aligned chiral 
smectic liquid crystal are also provided. 
SUMMARY OF THE INVENTION 
This invention provides polarization-insensitive Fabry-Perot filters for 
use with any arbitrary polarized input light. These filters comprise a 
phase modulator positioned between crossed quarter-wave plates (QWPs) 
within an FP cavity. The QWPs are oriented at an angle + or - 45.degree. 
from the optic axis of the phase modulator. 
Most polarization-dependent phase modulators can be employed as an element 
in the inventive filters. Phase modulators which achieve modulation by 
varying the polarization of input light will not, however, function in 
this invention. Apart from this limitation, phase modulators containing 
active, passive or both active and passive birefringent materials as 
elements can be employed in filters of this invention. Phase modulators 
containing active liquid crystal elements alone or in combination with 
passive birefringent elements can be employed in the filters of this 
invention. Phase modulators comprising liquid crystals, liquid crystal 
polymers or nonlinear electro-optic polymers, among others, can function 
in this invention. 
Any QWPs can be employed as elements in the polarization-insensitive 
filters of this invention. Zero-order QWPs are preferred. Passive QWPs are 
also preferred. For WDM systems in telecommunications applications at 1550 
nm, preferred QWPs are those with thicknesses narrow enough to construct 
FP cavities less than or equal to about 20 .mu.m. Zero-order QWPs with 
thickness less than about 5 .mu.m are preferred for applications in 
optical fiber telecommunication. 
Polarization-insensitive FP of this invention do not require the use of 
entrance or exit polarizers. Because of this feature, they are well suited 
for use in fiber optic systems. 
The invention is also directed to methods of filtering light of arbitrary 
unselected polarization employing an FP filter consisting essentially of 
an FP cavity containing a phase modulator positioned between two 
quarter-wave plates wherein the QWP's are oriented perpendicular to each 
other and at an angle of + or -45.degree. to the optic axis of the phase 
modulator. 
The polarization insensitive FP filters of this invention can be employed 
in any optical system device including filters, sensors, spectrometers, 
fiber optical systems or communication systems in which FP filters can be 
employed. Preferred applications are those where the insensitivity of the 
filter to polarization benefits the performance of the device or systems, 
for example, by enhanced signal intensity or enhanced signal resolution.

DETAILED DESCRIPTION OF THE INVENTION 
This invention relates to optical filters that are polarization-insensitive 
(i.e., polarization-independent) in which spectral filtering is not 
affected by differences in polarization of input light. These filters do 
not require selective polarization of input light and thus in general 
result in higher levels throughput of a given amount of input light of 
arbitrary polarization. These filters provide spectral filtering of 
unpolarized input light or of light of any arbitrary polarization. 
The filters of this invention are Fabry-Perot filters in which selected 
optical elements are positioned within a FP cavity, i.e., an etalon with 
reflective surfaces separated by a fixed distance, d, wherein the 
intracavity light can be treated as an infinite number of partial waves 
produced by the reflections at the two surfaces. At optical resonance, the 
phase delay attributed to one round trip is an integral number of 
wavelengths, i.e. 2d=m.lambda./n, where n is the average index of 
refraction of the cavity and m is an integer. This resonance condition is 
satisfied at a series of wavelengths and these transmissions are called FP 
fringes of order m. The free spectral range (FSR) is the spacing between 
these peaks. The general operation of FP filters, particularly those 
containing LC optical elements is described, for example, in U.S. Pat. 
Nos. 5,469,279 and 5,381,253. FP filters of this invention can be operated 
in transmission-reflection mode or reflection-only mode. 
In transmission-reflection mode, both reflective surfaces of the FP etalon 
are partially reflective, i.e. the reflectivity, R, of each surface is 
less than one, such that light can enter through one reflective surface, 
and undergo multiple internal reflections before the portion of light 
satisfying the resonance condition exits through the other reflective 
surface. In reflection mode, one of the reflective surfaces of the etalon 
is partially reflective and the other is a completely reflective surface, 
R.about.1, where it is understood that this is an approximation since 
totally reflective surfaces can not be manufactured. Light enters the 
reflection mode etalon through the partially reflective surface, undergoes 
multiple internal reflection and then exits through the same partially 
reflective surface. In the reflection mode configuration, input and output 
light can be spatially separated at the partially reflective surface to 
discriminate against light that does not satisfy the resonance condition. 
In etalon configurations exemplified herein, the reflective surfaces are 
planar and parallel and light enters at normal incidence. As is known in 
the art, the same function can be obtained with other cavity geometries 
which comprise, for example, angled or curved mirrors and non-zero angle 
of incidence of input light. 
A liquid crystal wave plate is a liquid crystal cell containing an aligned 
liquid crystal layer confined between transparent or semi-transparent 
substrates provided with electrodes for application of an electric field 
across the LC layer. A LC layer can be parallel (i.e., homogenous or 
"bookshelf") aligned or homeotropically aligned. The LC wave plate 
functions to retard light passing therethrough. 
A half-wave plate satisfies the equation: .increment.nd=.lambda./2 at the 
design wavelength .lambda..sub.d. LC wave plates are generally formed from 
uniformly-spaced transparent or semi-transparent inert substrate walls, 
each wall optionally having an alignment layer on its inside surface in 
contact with the LC layer between the walls to induce desired alignment. 
Parallel aligned cells typically can have transparent or semi-transparent 
electrodes or reflective electrodes at the walls to apply an electric 
field perpendicular to the LC layer. Homeotropically aligned LC cells have 
lateral electrodes. LC wave plates of this invention that function as 
phase modulators or function as elements of phase modulators can employ 
smectic liquid crystals or various types of nematic liquid crystals. 
Smectic liquid crystal materials of this invention include chiral smectic 
C (SmC*), smectic A (SmA), distorted helix ferroelectric liquid crystals 
(DHF), antiferroelectric and other related liquid crystal materials. 
The terms "quarter wave plate" (Q or QWP) and "half wave plate" (H or HWP) 
are used broadly, herein, unless otherwise stated and take their standard 
meaning in the art. The qualifying terms "quarter" or "half" refer to a 
condition at a given design wavelength at which the filter is intended to 
be operated. Those of ordinary skill in the art can design or select 
quarter or half-wave plates for a given design wavelength. 
The term phase modulator is used generally and broadly herein for any 
optical device, particularly an electro-optic device that allows discrete 
or analog phase modulation of input light. A number of phase modulators 
that can be rendered polarization-independent are described in the 
discussion below and by reference to art-known phase modulator 
configurations. 
Referring now to the drawings, where like numbers indicate like features 
and the same number appearing in more than one drawing refers to the same 
feature, the invention is further illustrated. 
FIG. 1 provides a schematic configuration of a polarization-independent FP 
filter 100 of this invention in which phase modulator 10 is positioned 
within an FP cavity formed by reflective surfaces (e.g., mirrors) 15 and 
17. QWPs 20 and 30 are also within the FP cavity on either side of the 
phase modulator. The optic axes of the pair of QWPs are oriented 
perpendicular to each other and at .+-.45.degree. (.+-..pi./4) to the 
optic axis of the phase modulator. In phase modulators composed of 
multiple optical elements, the optic axis relative to which the QWPs are 
oriented is that of the composite phase modulator. No input or exit 
polarizers are needed to achieve filtering. Input light of arbitrary 
polarization is modulated by the filter, independent of input 
polarization. 
The operation of the polarization-independent filters of this invention is 
illustrated by reference to FIG. 2 where polarization propagation of R- 
and L-polarized light on several passes through the FP structure is shown. 
Elliptically polarized light incident upon the FP filter can be viewed as 
a superposition of two orthogonal eigen polarizations, for example, left- 
and right-handed circularly polarized light as illustrated in FIG. 2 
where: 
EQU E.sub.r =A.sub.r (a.sub.x +ja.sub.y ); E.sub.l +=A.sub.l e.sup.i.phi. 
(a.sub.x -ja.sub.y ), (1) 
and where .phi. is the phase difference between the two circular eigen 
states and a.sub.x and a.sub.y are unit vectors in the x and y directions 
as shown in FIG. 2 and 
##EQU1## 
As the right-handed circularly polarized light (R) progresses through 
filter 100, input R light is converted to linearly polarized (S) light by 
passage through QWP 20. S polarized light is parallel to the optic axis of 
phase modulator 10, and is modulated without rotation of the polarization 
to give S polarized modulated light, S'. The S' light is then converted to 
modulated left-handed circularly polarized light (L') by passage through 
QWP 30. Modulated L' light can exit the filter after one pass. Reflection 
of L' light at the etalon converts the light to right-handed polarization 
(R' light) and passage back through QWP 30 transforms polarization to give 
P' linearly polarized light. P polarized light is perpendicular to the 
optic axis of phase modulator 10 and is not modulated. P' light passes 
through QWP 20 resulting in L' light. A third pass one-way through the 
filter gives polarized light that is twice modulated (L"). Five one-way 
passes give thrice modulated light (L'") light, etc. Right-handed 
circularly polarized light entering the filter exits only as left-handed 
circularly polarized light. In transmission-reflection mode and 
reflection-only mode, the exiting field due to entering R-polarized light 
is described by the summation E.sub.L' +E.sub.L" +E.sub.L'" + . . . . 
Again referring to the Scheme of FIG. 2, left-handed circularly polarized 
light (L) progresses through filter 100 in an analogous fashion to 
right-handed light. Input L polarized light is converted to linearly 
polarized R light by passage through QWP 20. P polarized light is not 
modulated by the phase modulator 10. P polarized light is converted to R 
polarized light on passage through QWP 30. Unmodulated R light (E.sub.R) 
can exit the filter on the first one-way pass. Reflection of R-handed 
circularly polarized light at the etalon 17 gives L polarized light and 
passage back through QWP 30 gives S linearly polarized light. S light is 
then modulated on passage through the phase modulator to give S' light 
(once modulated) and passage through QWP 20 gives R' light. A third 
one-way pass through the filter results in singly modulated R' light. 
After the fifth one-way pass, doubly modulated R" light can exit the 
filter. In transmission mode, right-handed circularly polarized light 
exits the filter according to the equation E.sub.R +E.sub.R' +E.sub.R" 
+E.sub.R'" + . . . . In reflection-only mode, right-handed circularly 
polarized light exits the filter according to the equation E.sub.R' 
+E.sub.R" +E.sub.R"' + . . . . 
As the two eigen fields propagate through FP filter 100 the passage of 
right- and left-handed circularly polarized light through QWP 20 results 
in linear polarized light of S or P polarization, respectively. The 
linearly polarized light is converted to left-handed linearly polarized 
light and is then rotated by 90.degree. upon reflection from the back 
mirror due to QWP 30. By changing the state of polarization during a round 
trip within the cavity, the P or S polarization sees the extraordinary or 
ordinary, index of refraction, respectively, when propagating forward in 
the etalon configuration, and the ordinary or extra-ordinary index of 
refraction, respectively, when propagating in the backward direction in 
the etalon configuration. The total round trip phase .phi. for both eigen 
polarizations is equal to: 
##EQU2## 
where .lambda. is the optical wavelength n.sub.x and n.sub.y are the 
indices of refraction of the fast and slow axes, respectively, of the QWP; 
n.sub.e (V) and n.sub.o are the extraordinary and ordinary indices of 
refraction of the liquid crystal, respectively; d.sub.Q is the thickness 
of the QWPs and d.sub.l is the thickness of the liquid crystal phase 
modulator. Because the two eigen polarizations exchange states, P&lt;=&gt;S, 
during a round trip, one is modulated in the forward propagation path and 
the other in the backward propagation path. The two polarizations, 
therefore, experience the same accumulated absolute phase, resulting in 
polarization-insensitive tuning. 
The basic principles of this filter design can be illustrated using the 
simplified 2.times.2 Jones calculus, as a closed-form solution for the 
transmission function and the resulting polarization states. The matrix 
equation describing the field transmitted by an etalon containing 
isotropic and/or anisotropic media can be written as: 
##EQU3## 
where E'(.lambda.) and E(.lambda.) are the incident and transmitted 
complex field amplitudes, and Q.sup.f (.lambda.) and Q.sup.b (.lambda.) 
represents the matrix for forward and backward propagations, respectively. 
The field transmission and reflection coefficients of the mirrors at 
normal incidence are given by t and r, respectively. By summing the 
series, equation (3) can be further reduced to: 
##EQU4## 
where I is the 2.times.2 identity matrix. In this analysis, mirror 
absorption and other cavity losses are neglected and it is assumed that 
the mirror reflectivity is neither polarization dependent, nor wavelength 
dependent over the spectral range of operation. Under these assumptions, 
the complex transmission and reflection coefficients at normal incidence 
can be written as: 
##EQU5## 
where R is mirror reflectivity. 
For the structure considered, the Jones matrix representing a single pass 
through the cavity is given by: 
##EQU6## 
are the Jones matrices for the two QWPs oriented at .+-.45.degree. to the 
phase modulator and liquid crystal phase modulator oriented at 0.degree. 
with optical phase retardation .theta.. 
Polarization of arbitrary polarized input can be followed as light makes a 
round trip within the cavity. The Jones matrix is equal to Q.sup.f 
rQ.sup.b r. Substituting equations (5)-(7) into the above expression, 
gives: 
##EQU7## 
It is clear from the identity matrix of equation (8) that after one round 
trip the polarization of the input light is maintained. Here, x-polarized 
light returns to x and y-polarized light returns to y, with an increase of 
absolute phase .gamma.. We now substitute equations (5)-(8) into equation 
(4), yielding the transmission function of the Fabry-Perot resonator: 
##EQU8## 
The FP transmission function can be removed from the matrix, yielding only 
a rotation matrix. This rotation matrix results because the initial 
conditions (the first path) for the two eigen states are different. Thus, 
provided that a polarization insensitive receiver follows the FP 
structure, all of the filtered light transmitted by the filter is 
available at the detector plane, regardless of incident polarization. The 
transmitted electromagnetic field is represented by: 
##EQU9## 
The total transmitted intensity for the QPQ (QWP-Phase Modulator-QWP) 
Fabry-Perot tunable filter is then equal to: 
##EQU10## 
where .gamma. is described in equation (9), and A.sub.x, and A.sub.y are 
the amplitudes of the input electromagnetic field along the x and the y 
axes, respectively. The QPQ Fabry-Perot filter is clearly insensitive to 
the input polarization. 
The QPQ-FP structure was tested in a computer analysis program, based on 
Berreman's 4.times.4 matrix formalism, for evaluating the anisotropic 
multi-layer structures. This comprehensive program allows the inclusion of 
reflections at the interfaces between layers, the polarization dependence 
of all elements, and the effect of oblique incidence on all layers. 
Results of this simulation are shown in FIG. 3 for FP transmission versus 
the wavelength. FIG. 3a has an input of linearly polarized light with an 
azimuth angle of 30.degree. , and FIG. 3b has a left-handed elliptically 
polarized input with 40.degree. azimuth angle and 30.degree. of 
ellipticity. The two plots show exactly the same traces indicating that 
the QPQ-FP filter is polarization insensitive. 
The polarization-independent filter of this invention can employ a planar 
aligned nematic liquid crystal cell as a phase modulator. The structure 
and operation of nematic liquid crystal cells, i.e. electro-optic 
modulators, are described, for example, in U.S. Pat. No. 4,779,959, issued 
Oct. 25, 1988, which is incorporated in its entirety by reference herein, 
and in references cited therein. 
FIG. 4 illustrates a cross-sectional view of a polarization-independent 
filter employing a planar aligned nematic cell 10 as a phase modulator. 
The phase modulator comprises a nematic liquid crystal layer 33 aligned 
between appropriate alignment layers 34, which induce desired planar 
alignment of the LC layer and parallel electrodes 35 across which an 
electric field is applied to modulate phase. The FP filter of FIG. 4 also 
includes quarter-wave plates 20 and 30 oriented perpendicular (i.e., 
orthogonal) to each other and at .+-.45.degree. with respect to the optic 
axis of the phase modulator. The FP cavity is formed by reflective 
surfaces 15 and 17, deposited, for example, on transparent substrates 40 
and 41. 
The polarization-independent filter of this invention can employ 
homeotropically aligned LC cells as phase modulators. In particular, a 
polarization-independent FP filter employing a lateral electrode smectic 
liquid crystal cell phase modulator is illustrated in FIG. 5. The 
structure and operation of homeotropically aligned lateral electrode 
smectic liquid crystal devices is described, for example, in U.S. patent 
application Ser. No. 08/056,415, filed May 3, 1993 no. 5,493,426 FIG. 5 
(in end view or cross-section B) shows phase modulator 10 comprising a 
smectic liquid crystal layer 36 homeotropically aligned between 
appropriate alignment layers 37 which induce desired homeotropic 
alignment. An electric field is applied across the LC layer as indicated 
via lateral electrodes 38. The polarization-independent FP 100 includes 
QWPs 20 and 30 oriented perpendicular to each other and at + or 
-45.degree. with respect to the optic axis of the LC cell. The FP cavity 
is formed by reflective surfaces 15 and 17 which are, for example, 
deposited on substrate walls 40 and 41. 
The polarization-independent filter of this invention can employ a smectic 
liquid crystal analog phase modulator with a chevron or titled layer 
alignment as described in U.S. Pat. No. 5,510,414, which is incorporated 
in its entirety by reference herein. The FP devices described therein are 
made polarization-independent by introducing properly oriented QWPs into 
the FP cavity on either side of the analog phase modulator. As shown in 
FIG. 6, a polarization-independent FP filter 100 containing an analog 
phase modulator 10 comprises substrates 40 and 41 with reflective surfaces 
15 and 17 to form the FP cavity. QWPs 20 and 30 are inserted between the 
reflective surfaces and the phase modulator. The phase modulator 10 
comprises a transparent electrode layer, and an alignment layer 42 on 
either side of the liquid crystal layer 43 (the alignment layers are 
positioned adjacent to the LC layer). Preferably the reflective surface, 
quarter-wave plate, transparent electrode, and an alignment layer will be 
sequentially layered or deposited on each substrate. The relative 
orientations of the QWP plates with respect to each other and the phase 
modulator which are required for polarization-independent operation can be 
readily achieved by known methods of deposition and alignment. The LC 
layer is aligned as described in the U.S. Pat. No. 5,510,914 between the 
two multi-layered substrates to form the polarization-independent filter. 
The analog phase modulators of the application Ser. No. 08/153,079 comprise 
a pair of substrates, smectic layers of liquid crystal aligned so that 
they are not parallel to the cell walls, and a means for applying an 
electric field perpendicular to the cell walls. The smectic liquid crystal 
forms an oblique angle with either substrate (cell wall). The phase 
modulator employs a means for aligning the smectic layers which can 
comprise an alignment layer. This alignment layer can, for example, be a 
rubbed polymer coating such as nylon 66, PVA, PBT or silane or obliquely 
evaporated SiO.sub.x. The substrates coated with the alignment layer are 
aligned anti-parallel to produce the quasi-bookshelf structure or parallel 
to produce a chevron structure. The substrate itself can be rubbed to 
promote a quasi-bookshelf or chevron alignment. The liquid crystal layer 
employed in the phase modulator can be a smectic C*, a smectic A or DHF 
liquid crystal layer. Alignment layers can promote quasi-bookshelf or 
chevron layer structures and the liquid crystal layer thickness can be 
chosen to be larger than that which can be surface stabilized into a 
bookshelf alignment (i.e, larger than 2-3 .mu.m). 
U.S. Pat. No. 5,361,320 issued Nov. 11, 1994, which is incorporated by 
reference in its entirety herein, describes optical fiber waveguides with 
liquid crystal cores aligned parallel to, orthogonal to or in a tilted 
alignment with respect to the fiber axis. When such fiber waveguides are 
provided with means for applying an electric field with a component in a 
direction orthogonal to the fiber axis, they can function as a LC cell to 
modulate the phase, polarization or both phase and polarization of 
elliptically polarized light guided through the fiber. These fiber 
waveguide modulators can incorporate nematic or smectic liquid crystals. 
Smectic liquid crystals useful in such modulators include, among others 
smectic A, smectic C*, or DHF materials. The fiber waveguides are 
described in combination with polarizers, optical cavities (FP cavities) 
and birefringent elements to provide amplitude modulation and spectral 
filtering. Fiber waveguide phase modulator configurations can be employed 
in the polarization-independent FP filters of this invention by 
introduction of QWPs into the fiber wave guide FP cavity configurations 
shown in U.S. Pat. No. 5,361,320. 
In general any passive or active QWPs can be employed in the FP filters of 
this invention, including among others those that are LC cells, those that 
are passive birefringent elements, or those that are fabricated by 
deposition on substrates or mirrors. 
For certain applications the thickness of the QWPs can be important. For 
telecommunications at 1550 nm, a free spectral range (FSR) of about 40 nm 
or more is preferred. To achieve the preferred (FSR), a FP cavity 
thickness of less than about 20 .mu.m is needed, assuming an average index 
of refraction of about 1.5. True zero-order QWPs made of quartz have a 
thickness of about 40 .mu.m at .lambda.=1550 nm (1.55 .mu.m). A FP filter 
of FIG. 1 with two intracavity quartz QWPs and one liquid crystal phase 
modulator (at a thickness of about 10 .mu.m) would have a free spectral 
range (FSR) of only about 10 .mu.m. This range is too narrow for 
application to EDFA WDM systems. Assuming the phase modulator has a 
thickness of 10 .mu.m (as in an LC phase modulator), the preferred QWP 
thickness for EDFA WDM systems will be equal to or less than about 5 .mu.m 
with optical birefringence (.increment.n)=0.08. In preferred narrow FP 
cavity configuration, the QWPs are deposited or otherwise attached at the 
intra cavity surfaces of the reflective surfaces that form the FP. 
Various methods are available for generation of thin zero order QWPs, 
including, for example, those made from liquid crystal polymers, high 
angle evaporated metal oxide thin films, and stretched polymer films. 
Liquid crystal polymers have relatively large optical birefringence, 
.about..increment.n=0.15, of the same order of magnitude as liquid crystal 
materials. A zero order QWP at 1550 nm prepared from LC polymers would 
only be about 2.6 .mu.m in thickness. LC polymers have been used primarily 
for nonoptical molding and extrusion applications. Cross-linkable LC 
silicone materials that can be polymerized into a glassy state using UV 
light are commercially available. Once cured, a thin LC retarder film is 
left on the supporting substrate. 
Optical anisotropy of an obliquely evaporated metal oxide thin film is a 
well-known phenomenon in thin film coating applications. Motohiro and Yaga 
(1989), Applied Optics 28:2466-2482 reported the construction of wave 
retardation plates from Ti.sub.2 O.sub.5 by this method. The QWP 
illustrated therein had a thickness of 2 .mu.m in the visible with a large 
optical birefringence .increment.n of about 0.07 to 0.08 and very small 
loss. Similar techniques have been used to achieve .increment.n of about 
0.10. (See: Hodgkinson (1991), Applied Optics 30:1303-1312). 
Stretched polymers can also be used to achieve a thin QWP at .lambda.=1550 
nm. Low cost stretched polymers with optical birefringence (.increment.n) 
of about 0.05 are commercially available. Stretch polymer QWP's may be 
less preferred for certain applications due to possible poor wavefront 
distortion, and integration problems associated with cementing the film 
onto a mirror. Stretched polymer QWP can be thicker than evaporated thin 
layers so that higher voltages may be required to address LC with ITO. 
The method disclosed herein for obtaining polarization-independence applies 
to LC-FP filters, as well as to other birefringence-based FP tunable 
filters. Multi-layer QPQ FP structures will enhance and broaden the 
usefulness of liquid crystal based tunable filters relative to other 
technologies, such as acousto-optic tunable filters. 
The FP filters of this invention can be used in fiber Fabry-Perot filter 
(FFP) design configurations (both in lensed FFP configurations and FFP 
ferrule assembly configurations). The FP filter configurations of this 
invention can be readily adapted to such fiber systems. FIG. 7 illustrates 
an FFP configuration of this invention in which a polarization-independent 
filter .apprxeq.100 is introduced into a fiber optic system. FIG. 7 
illustrates one means for introducing light from a fiber (53) to the 
filter 100, lens 51. Modulated light exits the filter, and returns to 
fiber 54 in transmission mode via lens 52. Other means for conducting 
light from fibers into Fabry-Perot filters and back into fibers after 
modulation are known in the art. 
The FP filters of this invention can be employed generally in any optical 
devices or optical systems where polarization-insensitive filtering is 
beneficial. The inventive FP filters can be combined with any desired 
optical elements or devices for use in a given application. 
The FP filters of this invention are useful in fiber-based optical sensors 
particularly in those employing long (0.5 meters or longer) fibers and in 
remote sensing applications. 
Those of ordinary skill in the art will appreciate that alternative 
methods, configurations, materials, optical elements and combination of 
elements other than those specifically described herein can be employed in 
view of the descriptions herein to generate functionally equivalent 
polarization-independent filters. All such alternatives are within the 
scope and spirit of this invention.