' 92/04653 PCT/US91/05609
Tunable Liquid-Crystal Etalon Filter
SPECIFICATION
Field of the Invention
The invention relates generally to optical filters; in particular, it
w 5 relates to liquid-crystal optical filters.
Background Art
A need has arisen for low-cost, tunable optical filters. For
example, one proposed architecture for the future telephone network
based on optical fiber uses wavelength-division multiplexing (WDM). In
WDM, the data of different communication channels (e.g., multiple voice
channels, video channels, high-speed data channels) modulate optical
carriers of different wavelength, and all the optical carriers are impressed
upon a single optical fiber. The multiplexed optical signals are all
distributed to many customer sites, each with its own receiver. Each
receiver must be able to pick out one of the multiplexed signals. In a
direct detection ~;cheme, an optical filter passes only the selected optical
carrier, and an optical detector detects the time-varying (data modulated)
intensity of the filtered optical carrier. Preferably, this filter should be
tunable so as to easily select different data channels. Channel spacings of
as little as 1 nm are being proposed in the infra-red band of 1.3 to
1.5 wm.
Diffraction gratings provide the required resolution but are too
expensive and fragile for customer-premise use. It is desired that the
tunability be purely electrical and include no moving mechanical parts.
Acousto-optic filters have been proposed. They offer superior resolution,
tuning range, and ruggedness. However) their cost remain moderately
high, and they require significant amounts of expensive RF electrical
power.
A liquid-crystal light modulator has been disclosed by Sounders
in U.S. Patent 4,779,959 and by Sounders et al. in "Novel optical cell
design for liquid crystal devices providing sub-millisecond switching,"
Optical and Quantum Electronics, volume 18, 1986, pages 426-430. A
c ) ~ Y~ v
WO 92/04653 2 PCT/US91/0560°
modulator blocks or passes the input light. A filter performs a more
complicated function by frequency selecting from a broad input spectrum,
passing some components while blocking others. Saunders defines a
wm optical cavity between two partially reflecting metallic mirrors and
5 fills the cavity with a nematic liquid crystal. The mirrors also act as
electrodes for the standard liquid-crystal display configuration in which an
applied bias rearranges the liquid-crystal orientation. However, in the
etalon configuration of Saunders, the applied bias in changing the
effective refractive index of the liquid crystal also changes the resonance
10 condition for the cavity. Both references include a graph showing a bias-
dependent optical filtering. Saunders relies upon this effect to intensity
modulate a beam of well defined frequency between two intensity values.
The liquid-crystal modulator of Saunders could be modified to
be used as a tunable filter for a wide bandwidth signal. However, it
would operate poorly. For a simple Fabry-Perot etalon, the transmission
at a given wavelength ~ for radiation incident to the normal of the surface
is given by
)= Z ~ 2 (
(1-p) +4psin 8
where 8=~+kodn. Here T and p are the transmittance and the
reflectance and, c~ is the phase shift experienced upon reflection, d is the
thickness of the uniaxial material, n is the refractive index along the
director axis, and ko is the magnitude of the wave-vector outside the
layer. Equation (1) shows that the width of the transmission peak 0~
depends essentially on the reflectivity of the surfaces ) while the overall
transmission is dictated by the absorption losses.
Saunders uses silver mirrors having reflectivities in the range of
85- 90% . H is illustrated transmissions peak at about 50% , an acceptable
value for some applications, but the peaks are relatively wide. The
widths present little problem when the structure is used as a modulator
for a well defined wavelength. However, Saunder's peaks are separated
by only a few times the peak widths. Therefore, his structure would be
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effective at filtering only a very few channels. For a practically useful
device such as those useful in multichannel systems, the reflectivity of the
mirrors must be kept above 95% . The peak width of Saunders could be
reduced by increasing the thickness of the silver mirrors, thus increasing
the mirror reflectivity to above 90°l0 . However, the increased
thickness
would increase tile mirror absorption losses to the point where the peak
transmission is unacceptably lowered.
In liquid crystal optical filters as heretofore proposed, the input
light directed at the filter must be linearly polarized in a particular
direction relative: to the orientation of the liquid crystal molecules.
Otherwise) the filter cannot be electrically tuned. In some applications,
the polarization requirement imposed on the light directed at the filter is
easily met. In other applications, however, such as in an optical fiber
communication system wherein propagating light is generally elliptically
polarized, such a requirement constitutes an undesirable and
disadvantageous :limitation.
Many applications, particularly in a fiber-optic communication
network, require that an optical filter be polarization-independent, that is,
that the spectral characteristics of the filter be independent of the
polarization of lil;ht being filtered.
Summary of the Invention
A first aspect of the invention can be summarized as a liquid-
crystal etalon optical filter in which the liquid crystal is sandwiched
between two dielectric stack mirrors defining the optical cavity.
Electrodes associated with each mirror apply a bias to the liquid crystal,
thereby changing; its dielectric characteristics and thus the resonance
conditions of thf: optical cavity. Preferably, the electrodes are placed
behind the mirrors, thus avoiding absorption loss. V ariation of the bias
applied to the electrodes causes the pass frequency of the filter to be
electrically changed.
According to a second aspect of the present invention, the
molecular orientation of a nematic liquid crystal material included in a
Fabry-Perot etalon is twisted in a prescribed fashion to achieve an
electrically tunable polarization-insensitive optical filter. In particular,
the
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WO 92/04653 ~ ~ PCT/US91/0560°
twist imparted to the molecules of the liquid crystal is established at n2 ,
where n is a positive odd integer.
A third aspect of the invention can be summarized as a liquid
crystal etalon filter or modulator comprising a liquid-crystal filling a
Fabry-Perot cavity defined between two end mirrors. One of the
alignment layers at the ends of the cavity is a homogeneous aligning agent
patterned into two areas aligning the liquid crystal in perpendicular
directions within the plane of the alignment layer. The other alignment
layer may be similarly patterned and homogeneous layer or preferably
may be a uniform and homeotropic alignment layer aligning the liquid
crystal perpendicularly to the alignment layer. An input light beam is
divided between two areas of the liquid crystal so that both its
polarization states are equivalently filtered. Preferably, a birefringent
crystal, such as calcite, is affixed to the input side of the filter so as to
polarization divide the input beam to the lateral side of the filter with the
corresponding polarization.
Brief Description of the Drawings
A complete understanding of the present invention and of the
above and other features and advantages thereof will be apparent from a
consideration of the detailed description set forth below taken in
conjunction with the accompanying drawing, not drawn to scale, in which:
FIG. 1 is a cross-section of a first embodiment of the liquid
crystal etalon filter of the invention.
FIG. 2 is a graph illustrating the spectral dependence of
transmission of the filter of FIG. 1 at two bias voltages.
FIG. 3 is a graph illustrating the bias dependence of the
transmission peaks of the filter of FIG. 1.
FIG. 4 is a cross-section of a second embodiment.
FIG. S is a plan view of the alignment layer of a third
embodiment.
FIG. 6 is a simplified diagrammatic side-view representation of
an optical filter made in accordance with the fourth embodiment of the
present invention .
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FIG. T schematically depicts the contours of the longitudinal
axes of liquid cry;~tal molecules included in the FIG. 6 arrangement.
FIG. 8 is a cross-sectional view of a fifth embodiment of a
liquid-crystal etalon filter of the present invention.
FIG. 9' is a plan view of a patterned alignment layer taken
along sectional lines 9-9 of FIGS. 8 and 11.
FIG. 10 is a illustration of the transmission wavelength as a
function of voltage for an example of the invention.
FIG. 11 is a cross-sectional of a sixth embodiment of the
invention.
FIG . 12 is a schematic illustration of an optical drop circuit
using the polarization diversity filter of the invention.
FIG. 13 is a plan view of an arrayed alignment layer taken
along sectional lines 9-9 of FIGS. 8 and 11.
Detailed Description of the Preferred Embodiments
The optical filter of a first embodiment of the invention is a
liquid-crystal etalon filter, in which the end mirrors of the optical cavity
are highly reflective, preferably being dielectric interference mirrors.
Example 1
A first example of the first embodiment of the invention is
illustrated in cross-section in FIG. 1. Two glass plates 10 and 12 of
1.62 mm thick soda-lime glass are used as substrates. Transparent
electrodes 14 and 16 of indium-tin-oxide are deposited on the substrates
10 and 12. Dielectric stack mirrors 18 and 20 are formed on the electrodes
14 and 16. The mirrors 18 and 20 may be either commercially procured
from Virgo Optics, Inc, of Port Richey, Florida, in which case they are
juxtaposed to the existing structure. Alternatively, they may be deposited
on the electrodes 14 and 16 by sputtering. The deposited mirrors 18 and
20 are designed for an infrared filter around 1.5 p,m although we have
fabricated others that have been centered around 0.3 wm and 1.9 wm.
The mirrors we f~ibricated consisted of four pairs of quarter-wavelength
thick layers of different refractive index, specifically A1203 (an insulator)
and Si. For the 1.5 p,m mirrors, the A1203 layers were ~-240 nm thick and
the Si layers were -120 nm thick. The transmission curves of FIG. 2, to
r
-6-
be described later, were fit to theoretical expressions including the
reflectance of the mirrors.
The so calculated mirror reflectance was ~98%, compared to the maximum
reflectance of
90% for the modulator of Saunders. Thus, the system requirement of 95% is
easily satisfied.
Dielectric mirrors of themselves are well known. For example, Yoo et al.
disclose two interference mirrors defining the ends of an optical cavity for a
surface-emitting
semiconducting laser :in U.S. Patent No. 5,034,958 issued July 23, 1991. One
of the minors
consisted of alternating layers of Si and AlzO,, and the other consisted of
alternating layers
of semiconductingAl.4s and GaAs.
Alignment layers 22 and 24 are formed on the dielectric stacks 18 and 20 by
the
procedure described by Patel et al. in "A reliable method of alignment for
smectic liquid
crystals", Ferroelectrics, volume 59, 1984, pages 137-144. The two assembled
structures are
then assembled with a precise gap between them according to the following
method. Four
UV curable epoxy dola were placed over the alignment layer 24 at the corners
of one of the
structures. The epo~:y is previously mixed with 10 p.m rod spacers available
from EM
Chemicals of Hawthorne, New York. The second structure is then placed on the
first
structure having the e~?oxy with the alignment directions of the two alignment
layers 22 and
24 being parallel. Manual pressure is gently applied to the structures while
observing optical
interference patterns under monochromatic light. The interference fringes are
minimized.
This structure is capW red by hardening the UV curable epoxy by exposing the
structure to
UV radiation. The assembled structure is heated to about 100°C and a
liquid crystal material
26 is flowed into the gap by capillary action. A nematic liquid crystal, E7,
available from
EM Chemicals, is used in its isotropic state. The gap is estimated to produce
a Fabry-Perot
cavity length of about 11 pm between the dielectric stacks 18 and 20. The
alignment layers
22 and 24 cause liquid-crystal molecules 28 in the liquid-crystal material 26
to orient with
their long axes paralL~l to the reflectors 18 and 20 and also one set of their
short axes to
orient parallel to the reflectors 18 and 20 but perpendicular to the long
axes. These
orientations apply only for no applied bias. In this embodiment, there is no
significant twist
of the liquid-crystal molecules from one reflector to the other.
208-8372
_7_
Electrical leads are connected between the electrodes and a voltage generator
29.
For our tests, the gener~~tor 29 was a computer-controlled programmable
voltage source, such
as, Wavetek~' Model 7~~, which produced a square wave at 1 kHz.
A sheet polarizes 30 can be formed on the outside of either glass substrate 10
or
12 with its polarization direction aligned with the long axes of the molecules
28. However, the
filter used in our tests used either unpolarized light or external means for
controlling the
polarization. Furthermore, the filter can be designed to operate in a band of
radiation that
avoids the need of a polarizes.
A light-emitting diode producing light at 1.5 pm was used as a light source 32
to
test the filter of the embodiment of FIG. 1. In an initial test, no polarizes
was used and the
transmission spectrum was measured for applied AC bias of 0 V and 4 Y, as
illustrated in the
graphs of FIG. 2. The units of transmission are arbitrary and the baselines
have been
suppressed.
It is seen that some peaks shift with bias while others remain fairly
stationary. By
separate experiments, i~t has been demonstrated that the stationary peaks
correspond to light
polarized parallel to the short axes of the liquid-crystal molecules 28 and
the tunable peaks
correspond to light polarized parallel to the long axes. As a result, the
filter can also be thought
of as a narrow-band polarizes.
Because of the high reflectivity of the dielectric stacks 18 and 20, the width
~7~ of
the transmission peaks is relatively small, ~1 - 2 nm, measured as a full
width at half maximum.
However, the free spectral range (FSR), which is the wavelength separation
between successive
transmission peaks, is rc;latively large, ~75 nm. It is determined by the
optical thickness n ~d of
the etalon and by the spectral regions in which it is being operated. This
range
depends on the order in which the interferometer is being operated, and it is
given by
WO 92/04653 n ~,~ c~'~ ~3 ~ ~( ~ 8 PCT/US91/0560~
FSR = 2w~m~m+1)
where m is the order. The refractive index of liquid crystals is typically in
the range of 1.5 to 1.7. Thus, 1.5 wm light will pass through the etalon
having the 11 p,m gap when the etalon is used in the 22-nd order. In this
wavelength region ) the free spectral range for a 11 ~,m etalon would be
about 75 nm. The choice of using an 11 wm thickness was simply due to
convenience of fabrication of the actual device. This thickness should
ideally be chosen such that the wavelength range of interest is the same as
the free spectral range. The tuning range is estimated to be order of
200 nm which corresponds to a change of about 0.2 in the refractive
index. The maximum change in the index is equal to the birefringence of
the uniaxial liquid crystal.
The tunability of the filter of FIG. 1 is demonstrated by the
bias dependence of the peaks' wavelengths, as illustrated in the graph of
FIG. 3. The dashed lines represent the transmission of light having a
polarization along the short axes of the liquid-crystal molecules 28. There
is virtually no tunability of these peaks, and thus this polarization cannot
be tunably filtered. The solid lines represent the transmission of light
having a polarization along the long axes of the liquid-crystal molecules
28. There is a threshold below which no wavelength shift is observed.
This plateau is due to the Freedricks effect and has been observed by
Saunders et al. as well. It exists when the directors at both surfaces are
parallel to the surfaces. It is possible to eliminate this threshold and
control its characteristics by changing the surface tilt of the molecules.
For example) no threshold would be exhibited in a hybrid aligned sample
in which the liquid-crystal molecules at one of the surfaces lie parallel to
the surface and those at the other lie perpendicular. Perpendicular
orientation can be achieved by the use of a homeotropic alignment agent,
such as, octadecyltriethoxysilane. For such a structure in the low voltage
regime, the index will change almost linearly with the applied field.
The tunable operation of the filter can be understood from
FIG. 3. If the bias voltage is varied between approximately 1.6 V and
2.6 V) a single transmission peak varies between about 1.528 wm and
V 92/04653 9 PCT/US91/05609
1.472 Wm without interference from any other peaks) a tunability of
56 nm. To obtain tunability over the range of 1.592 wm to 1.532 ~,m
would require removing the peak at 1.537 Wm with the polarizer 40.
These tuning ranges can be shifted and widened with optimization of the
design.
A calculated transmission spectrum has been fit to the
transmission data of FIG. 2. For the zero electric field, the ordinary
refractive index is no = 1.5 and the extraordinary refractive index is
ne = 1.7. On th.e other hand, for the 4 V applied across 11.32 wm,
no = 1.5 and ne~'(E) = 1.536, where net' is the effective refractive index.
The power required to operate the filter has been estimated to
be in the microwatt range. The switching speed is of the order of
milliseconds.
A filtE:r of the present invention has been used in the
demonstration of the polarization scrambler disclosed by Maeda et al. in
"New polarization-insensitive direction [sic] scheme based on fibre
polarisation scram.bling," Electronics Letters, volume 27, pp. 10-12, 1991.
The liquid-crystal etalon filter of FIG. 1 is not laterally
patterned, but the invention is not so limited.
Illustrated in FIG. 4 is a second embodiment of a liquid-crystal
etalon 1-dimensia~nal or 2-dimensional filter array. It differs from the
filter of FIG. 1 in that at least one of the electrodes is patterned into
pixels 40 which extend across the substrate 12. The pixels 40 can be
individually contacted at the side of the filter structure. If the associated
dielectric stack 2(l is deposited on the electrode pixels 40, a planarizing
layer 42 must first be deposited to insure the optical flatness of that
mirror stack 20.
In a third embodiment, at least one of the alignment layers 22
and 24 is patterned. The aligning procedure) described by Patel er al.)
involves depositing a nylon aligning material on the substrate and then
rubbing the align ing material in the direction in which the liquid-crystal
molecules are to be oriented. Illustrated in FIG. S is a plan view of an
alignment layer 4~~ having first portions 46 rubbed in a first direction and
second portions 48 rubbed in a different direction, preferably orthogonal
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WO 92/04653 PCT/US91/0560°
to the first direction. The differential rubbing is easily accomplished for
the aligning materials, such as nylon and 1,4 polybutyleneterephathalate.
The entire alignment layer 44 is rubbed in a given direction, say the first
direction, so that the first portions 46 are given the correct alignment.
Then, the entire structure is covered with photoresist and processed using
standard lithographic techniques so as to keep the first portions 46 coated
with a photoresist material. The exposed second portions 48 are then
rubbed in the second direction with the photoresist protecting the already
rubbed first portions 46. R emoval of the photoresist does not affect the
alignment of rubbed first and second portions 46 and 48.
The second alignment layer 22 or 24 can be made to have the
same patterned alignments as the patterned alignment layer 44. When the
two structures are then assembled, the alignment directions of the
opposed portions are made parallel. This requires precise physical
alignment of the two alignment layers 22 and 24. Alternatively, the
second alignment layer can be made to uniformly induce a perpendicular
orientation of the liquid crystals, that is, along the normal of the
alignment layer. A homeotropic aligning agent, described previously, will
provide this effect. No patterning of the homeotropic aligning agent is
required.
The third embodiment of FIG. 5 advantageously provides a
polarization independent tunable filter for a single-mode optical fiber.
The filter is assumed .to be designed to place the untunable passbands
(dashed lines of FIG. 3) outside the desired wavelength tuning range. A
graded-index lens on the output of the fiber is aligned to the boundary
between a paired first and second portions 46 and 48 so that each portion
46 and 48 receives half the light intensity. One portion 46 or 48 blocks a
first light polarization but tunably passes a second polarization. Likewise,
the other portion 48 or 46 blocks the second light polarization but tunably
passes the first polarization. A single direct photo-detector detects the
intensity of light passed by both portions. Alternatively, the two
frequency-filtered polarizations can be recombined by a graded-index lens
onto a second single-mode optical fiber. The intensity passed through
both portions 46 and 48 is independent of the polarization of the light. A
~~ =3 ri ~.
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s
~' 92/04653 ~ ~ PCT/US91/05609
3dB loss is incurred by this polarization diversity technique.
The filter of the invention can be used as a short optical pulse
generator. A narrow-band light source 32, such as a laser, irradiates the
tunable liquid-crystal etalon filter with monochromatic CW light of
wavelength J~. The filter of, for example, FIG. 1 is designed to have at
least a limited tuning range extending from one side of a to the other.
The bandwidth of the light source 32 should preferably be less than the
width of the transmission peak of the filter. To generate the light pulse, a
step function DC bias is applied by the voltage generator 29 to the
electrodes so as to cover the limited tuning range about ~. Only in the
short but finite time required for the finite-width transmission peak to
transit a will the filter transmit the narrow-band light.
In a fourth embodiment of the invention, a liquid-crystal filter
can be made insensitive to polarization by imparting a 90° twist (or
odd
multiples thereof) to the liquid crystal between the two alignment layers.
A specific illustrative optical filter 60 made in accordance with
the principles o:F the fourth embodiment of the present invention is
represented in FIG. 6. Optical signals from a source 62 are directed at
the left-hand side: of the filter 60, as indicated by dash-line arrow 64. By
way of example, the source 62 comprises a standard light-emitting diode
and an associated optical fiber for applying signals to the filter 60.
The source 62 of FIG. 1 simultaneously supplies multiple input
wavelengths in) :for example, the range 1.4-to-1.6 wm. Only a selected
one of these wavelengths is passed by the filter 60 and delivered in the
direction of arrow 66 to a utilization device 68. The device 68 includes,
for example, an optical fiber of the type included in a conventional
wavelength-division-multiplexed (WDM) communication system.
In accordance with the invention, a signal source 70 shown in
FIG. 6 is utilized to apply electrical control signals to the filter 60. In
the
absence of an applied control signal, the filter 60 will pass to the device
68 an optical sil;nal of a particular wavelength, as determined by the
geometry and properties of the constituent parts of the filter 60. Above a
specified operating threshold value, control voltages applied to the filter
60 are effective to change its electro-optic properties such that
c I
V it
WO 92/04653 ~ 2 PCT/US91/0560~
respectively associated wavelengths supplied by the source 62 are passed
by the filter 60 to the device 68. In that way, an electrically tunable
optical filter is realized.
The filter 60 of the fourth embodiment shown in FIG. 6
comprises on its left-hand side a glass plate 72 through which input optical
signals are transmitted and on its right-hand side another glass plate 74
through which output optical signals are transmitted. Disposed on the
inner or facing surfaces of the plates 72 and 74 are optically transparent
electrodes 76 and 78, respectively, which comprise, for example, standard
layers of indium tin oxide. A s indicated in FIG . 6, electrical leads 80 and
81 respectively connect the layers 76 and 78 to the control signal source
70.
The optical filter 60 of FIG. 6 further includes mirrors 82 and
83. Illustratively, the mirrors each include multiple layers of dielectric
material) which is a conventional known design. By way of example,
each of the mirrors 82 and 83 is designed to have a reflectivity of about
94 to 99.99 percent for the range of wavelengths to be transmitted by the
filter 60.
The filter 60 of FIG. 6, including the spaced-apart mirrors 82
and 83, comprises in effect a Fabry-Perot etalon. As is well known, the
geometry of such a device can be designed to be resonant at a particular
wavelength (and multiples thereof). In that way, the device can be
utilized to provide an output only at specified frequencies.
In accordance with the present invention, the filter of FIG. 6 is
electrically tunable by changing the electro-optic properties of a layer 84
of a liquid-crystal material that is contained between the mirrors 82 and
84. Moreover, due to the imposition initially of a particular orientation on
the molecules of the liquid-crystal material (described later below), the
operation of the filter 60 over its tuning range is independent of the
polarization of input optical signals. In other words, the wavelength
selected by the control source 70 for transmission by the filter 60 will be
delivered to the utilization device 68 with the same output intensity
regardless of the polarization of the input optical signals.
A
' "' 92/04653 ~ 3 PCT/US91/05609
The layer 84 of liquid-crystal material represented in FIG. 6 is,
of course) retained in the indicated space by conventional spacer and
sealing members (not shown). Illustratively, the layer 84 comprises a
standard nematic liquid-crystal material having elongated rod-like
molecules characterized by positive dielectric anisotropy. In one
particular illustrative example, the thickness of the layer 84 in the
indicated Z direction is only about 10 Wm.
Interposed between the liquid crystal layer 80 of FIG. 6 and
the mirrors 82 and 83 are so-called alignment layers. A variety of
alignment materials suitable for use with liquid crystals are well known in
the art. In particular) the mirror 82 includes on its right-hand face a layer
86 of a conventional alignment material, whereas the mirror 83 includes
on its left-hand face a layer 88 of a conventional alignment material.
Each of the alignment layers 86 and 88 is effective to impose a
particular orientation on molecules in adjacent portions of the liquid
crystal layer 84. Illustratively, each of the alignment layers 86 and 88 is
initially rubbed in a particular direction to impose a corresponding
orientation on adjacent liquid-crystal molecules. Such rubbing of
alignment layers to control the molecular orientation of liquid-crystal
materials is well :known in the art.
In accordance with the present invention) the alignment layers
86 and 88 of F(G. 6 are designed to impose quiescently a particular
twisted structure on the liquid-crystal molecules included in the layer 84.
The nature of this twist is schematically depicted in FIG . 7 which shows
some of the rod-like liquid-crystal molecules in the layer 84 disposed
between the alignment layers 86 and 88.
Illustratively, the liquid-crystal molecules in the layer 84 of
FIG. 7 are oriented to have a 90° or 2 twist between the alignment
layers 86 and 88. In this twisted orientation, molecules at the surface of
the left-hand layer 86 are established to have their longitudinal axes
parallel to the indicated X axis, whereas molecules at the surface of the
right-hand layer 88 are established to have their longitudinal axes parallel
to the indicated I~' axis. As shown, the longitudinal axes of the molecules
~c~Jc~r~~
WO 92/04653 ~ 4 PCT/US91/0560~'
at these two surfaces are displaced 90° with respect to each other.
Between these surfaces) the longitudinal axes of the liquid-crystal
molecules gradually change in the Z direction from parallelism with the X
axis to parallelism with the Y axis.
In accordance with the principles of the present invention, the
amount of twist imparted to the molecules of a liquid-crystal material
included in an optical filter is defined by the term n2 . For the particular
illustrative case represented in FIG . 7, n equals 1. M ore generally ) n can
be any positive odd integer. Thus, for example, for n= 3, the longitudinal
axes of the liquid-crystal molecules would undergo a rotation of 270°
in
the liquid-crystal layer 84 of FIG . 2 in the direction of the Z axis between
the surfaces of the alignment layers 86 and 88.
In the absence of an electrical control voltage applied to the
electrodes 76 and 78 of the optical filter 60 shown in FIGS. 6 and 7, both
constituent orthogonal polarizations of a particular wavelength (and
multiples thereof) will not be resonantly supported by the Fabry-Perot
etalon) as determined; for example, by mirror spacing and the electro-
optic properties of the liquid-crystal layer 84. Only those particular
wavelengths which are resonant will appear at the output of the filter 60.
For each polarization condition, however, a different wavelength will be
resonantly selected.
Assume now that an electrical control voltage is applied to the
depicted filter to establish a Z-direction electric field. Until the
magnitude of the electric field in the liquid-crystal layer 84 reaches the
well-known Freedricks threshold, the molecular orientation represented in
FIG. 7 remains substantially unchanged. Above that threshold (for
example, above about 2.0 V for a 10-wm-thick liquid-crystal layer 84), the
longitudinal axis of molecules in the central portion of the layer 84 begin
to align parallel to the Z axis. Molecules at the surfaces of the alignment
layers 86 and 88, however, remain unaffected by the applied field because
of strong anchoring forces at these surfaces. The thickness of the
unaffected or substantially unaffected surface regions in the liquid-crystal
layer 84 is a function of the magnitude of the applied electric field. Thus,
~~J~~r~
PCT/US91 /05609
' ~ 92/04653
in response to the application thereto of an applied control voltage above
the Freedricks ~:hreshold, the liquid-crystal layer 84 includes two field-
dependent variable-thickness birefringent regions at and near the
respective surfaces of the alignment layers 86 and 88. Significantly) the
principal optic axis in one such region is disposed at 90° with respect
to
the principal oF~tic axis in the other region. Accordingly, each of the
constituent orthogonal polarization states of an input optical signal will be
affected by the same amount irrespective of the input polarization state.
At are operating field strength above that of the Freedricks
threshold (for example) above about 2.0 V RMS at lkHz for E7 nematic
liquid-crystal material from EM Chemicals, Hawthorne, N.Y., for a 10
Wm-thick liquid.-crystal layer 84), the thickness and electro-optic
characteristics of the two aforespecified birefringent regions become
substantially the same and then track each other as the applied control or
tuning voltage is increased further. As a result, for a range of voltages
above an operating or high-field value, the herein-described filter
structure imparla the same phase change to each of the constituent
orthogonal polarizations of an input optical signal. Thus, the wavelength
passed by the structure is determined by a particular value of the applied
voltage and is independent of the state of input polarization. In one of
the birefringent regions, one constituent input polarization disposed
approximately parallel to the principal axes of liquid-crystal molecules in
the one region is affected while it is unaffected in the other birefringent
region. The other or orthogonal polarization component of the input
optical signal is substantially unaffected in the first region while it is
affected in the other birefringent region. A tunable polarization
insensitive optical filter is thereby realized. In such a filter, the
wavelength selecaed by and corresponding to a particular applied voltage
will be transmitted at the same intensity level regardless of its polarization
condition.
A spf:cific illustrative embodiment of the present invention
includes a 10-Wm-thick twisted nematic liquid-crystal layer 84 of the type
represented in FIG. 7. The index of refraction of such a layer can be
varied between approximately 1.5 and 1.7 by applying thereto high-field
WO 92/04653 ~ 1 ~ ~1 ~ ~~ ~ ~~ ~ 6 PCT/US91/0560~
operating voltages in the range of 2 to 10 V. (For such an embodiment,
the Freedricks threshold voltage is about 2 V ). For each of selected input
wavelengths, there is a corresponding operating control voltage that when
applied to the filter will permit a particular input wavelength to be
transmitted therethrough. Changing the control voltage by a specified
amount will cause another input wavelength to be selected for
transmission. In the particular specified example) wavelengths spaced
apart by 2 nm in a tuning range of 15 nm are respectively selected by
changing the control voltage in steps of about 0.5 V. W avelengths passed
by such a filter are characterized by a spectral passband width of about 1
nm.
An optical filter of the type specified above can be switched
relatively rapidly from a condition in which it passes one wavelength to a
condition in which it passes another wavelength. The switching speed is
dictated primarily by how fast the liquid-crystal molecules in the layer 84
can be re-oriented. In turn, this depends on a variety of factors such as
the dielectric anisotropy of the liquid-crystal material, the value of the
control voltage and the thickness of the liquid-crystal layer 84.
Illustratively, switching speeds in the order of milliseconds are feasible in
practice. Moreover, the power required to switch such a compact
microminiature filter is typically less than a microwatt.
Finally, it is to be understood that the above-described
arrangements are only illustrative of the principles of the fourth
embodiment of the present invention. In accordance with these
principles, numerous modifications and alternatives may be devised by
those skilled in the art without departing from the spirit and scope of the
invention. For example, in one modification a polarization-insensitive
filter is achieved by combining two layers of liquid-crystal material that
are physically separated from each other. In this modification, the two
layers are contained within the optical cavity formed by the
aforedescribed mirrors and electrode structure and are retained in place
by a sandwich structure that includes three spaced-apart glass plates. The
optic axes of the two layers are arranged respectively orthogonally to each
other so that light of any polarization experiences the same phase shift.
2u~~~~l
'~ '" 92/04653 ~ 7 PCT/US91/05609
In this modification, it is important that the thicknesses of the two liquid-
crystal layers be the same to insure polarization-insensitive operation.
Additionally, it is apparent that the devices described herein
can be used a;~ polarization-insensitive spatial light modulators. In
operation, two voltages are applied to such a modulator. One voltage is
chosen to allow a selected wavelength to propagate through the device.
The other operating voltage is designed to cause the device to block the
selected wavelength from passing therethrough.
One of the variants of the previously described third
embodiment involved perpendicular and parallel alignments of the liquid
crystal at the opposed alignment layers so as to produce a polarization
independent tun~ible liquid-crystal filter. Patel et al. provide a similar
disclosure in "E:lectrically tunable optical filter for infrared wavelength
using liquid crystals in a Fabry-Perot etalon," Applied Physics Letters,
volume 57, 1990, pp. 1718-1720. This concept is further expanded in a
fifth embodiment of a polarization independent tunable liquid-crystal
filter 110, as illustrated in cross-section in FIG. 8. The filter 110 is
fabricated on two glass substrates 112 and 114, onto which are deposited
transparent indium-tin-oxide electrodes 116 and 118. Dielectric stack
mirrors 120 and 122 are formed on electrodes 116 and 118, and each
consists of multiple pairs of quarter-wavelength thick layers of differing
refractive indices to thereby act as interference mirrors for the wavelength
of interest.
A homogeneous alignment layer 124 and a homeotropic
alignment layer 126 are deposited and buffed on top of the respective
stack mirrors 120 and 122. The alignment layers 124 and 126 cause
liquid-crystal molecules disposed adjacent to the respective alignment
layer to be alignE:d in a particular direction. In particular) nematic liquid
crystals are characterized by orientational order along the average
direction of the long axes of the liquid-crystal molecules, called the
director n. The alignment layers 124 and 126 establish the director n at
the interface with the liquid crystal. The director n then varies smoothly
in the liquid crystal between the two alignment layers 124 and 126. If an
electrical field is applied across the liquid crystal, the director n becomes
208837 2
_, g_
increasingly aligned with the electrical field in the gap between the
alignment layers
124 and 126.
According to the fifth embodiment of the invention, the homogeneous alignment
layer 124, as illustrated in plan view in FIG. 9, is divided into two portions
130 and 132 divided
by an interface 134. Both portions 130 and 132 are formed of a homogeneous
aligning agent,
e.g., a nylon or polyester such as 1,4 polybutyleneterephthalate,that causes
the director n at that
point, and therefore the adjacent liquid-crystal molecules, to be aligned
parallel to the surface
of the alignment layer 2.4. However, the two portions 130 and 132 are buffed
in perpendicular
directions so that one homogeneousportion 130 aligns the liquid-crystal
molecules perpendicular
to the interface 134 while the other homogeneous portion 132 aligns them
parallel to the
interface 134. On the ether hand, the other alignment layer 126 is formed of a
homeotropic
aligning agent, e.g., octadecyltriethoxysilane, that causes the director n and
the adjacent
liquid-crystal molecules. to be aligned perpendicularlyto the surface of the
alignment layer 126.
Following the formation of the critical alignment layers 124 and 126, the two
substrates 112 and 114 are formed into an assembly having a gap between the
two alignment
layers of about 10 um and a nematic liquid-crystal 136 is filled into the gap,
all according to
the procedure detailed above.
When a voltage generator 138 is connected between the electrodes 116 and 118,
the applied electric field will cause a change in the effective refractive
index in the liquid crystal
136 for light polarized in the direction of the director n, thus affecting the
effective optical
length of the Fabry-Perot cavity formed between the two mirrors 120 and 122.
The direction
of the director n trans~rerse to the optical propagation is determined by the
alignment
direction of the homogeneous portion 130 or 132. The electric field, on the
other
hand, has no effect on the refractive index for light polarized in the
direction
perpendicular to the director direction. Thus, the change in effective
refractive index
depends strongly upon the polarization of the light. Because of the
differential buffing
c
~ ~~~~~'~lF
'~ ' 92/04653 ~ 9 PCT/US91/05609
directions in the alignment layer 124, light passing through the portion
130 will be affected only for its electrical polarization component that is
perpendicular to the interface 134 while that passing through the portion
132 will be affecaed only for its electrical polarization component that is
parallel to the interface 134. Because of the symmetry, both of the
polarization com ponents will be equally affected by the applied voltage as
they pass through the respective portions 130 and 132 and the
corresponding portions of the liquid crystal 136.
The tunable liquid-crystal etalon filter 110 can be used by
optically aligning; an input optical fiber 140 with the interface 134 of the
homogeneous alignment layer 124. A rod graded-index lens 142)
represented fun<aionally in FIG. 8, disperses the beam so that equal
amounts of optical energy fall upon the two portions 130 and 132. On the
output side, a corresponding lens 144 recombines the filtered light onto an
output fiber 146.
Example 2
A dual-polarization liquid-crystal etalon filter 110 was
fabricated according to the above procedure. It had a cell gap of 10 Wm.
Its lateral dimensions were 1 cm x 1 cm. Poly 1,4 butyleneterephathalate
and octadecyltri~~thoxysilane were used as the aligning agents. The
nematic liquid crystal was the previously mentioned E7. The liquid
crystal was filled. while in its isotropic state into the cell gap of 10 p,m
using a vacuum i"filling technique. The alignment layers cannot absolutely
determine the alignment direction of the liquid-crystal molecules since
there is a degenE:racy between the parallel and anti-parallel directions in
the homogeneous aligning agent) which would likely result in the
formation of multiple domains in each of the homogeneous portions 130
and 132. MultiF~le domains can be avoided by breaking the symmetry
using finite tilt angles at the homogeneous alignment layer 124. However,
in the example, a single domain was obtained in each of the homogeneous
portions 130 and 132 by filling the gap with liquid crystal 136 while in its
isotropic phase and then cooling it while an electric field is applied across
it by the electrodes 116 and 118.
r~ < ;~
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WO 92/04653 2 0 PCT/US91/0560~
The device was characterized at room temperature using an
optical spectrum analyzer, a 1.5 wm light emitting diode as a light source)
and mufti-mode optical fibers. The mufti-mode fiber and graded-index
rod lens produces a highly collimated with having a diameter of about
400 wm. Use of a single-mode fiber produced a beam diameter of about
250 wm. A programmable and computer-controlled voltage source
provided a square wave potential at 1 kHz. The spectral location for the
maxima of the transmission peaks are illustrated in the graph of FIG. 10
as a function of the applied voltage. The transmission peaks had widths
or pass bands of about 0.9 nm. The transmission spectra show two bands
P and Q. The P band does not change with voltage and corresponds to
polarization perpendicular to the director n. The Q band, however, does
change significantly with voltage. It corresponds to polarization parallel
to the director n. The position of the P band is determined by the
resonance condition
m~ = nfd, (3)
where m is the mode number, ~ is the optical wavelength, ns is the
refractive index along the short axis of the liquid crystal, and d is the
physical thickness of the Fabry-Perot cavity. The applied voltage changes
ns by about 12°lo The figure shows a tuning range of about 100 nm and a
free spectral range of about 70 nm for the Q peak. The spectra were
remeasured with a polarizer inserted on the input side. Both when the
light was polarized perpendicular to the interface 134 and when parallel to
the interface 134, the spectra did not differ significantly from those of
FIG . 10.
Because the P band is not tunably filtered, the filter needs to
be designed with the desired free spectral range avoiding this band.
The filter of FIG. 8 suffers the disadvantage that there is a
3 dB loss associated with the unfiltered polarization or P band. That is,
the lens 142 distributes equal amounts of both optical polarizations to the
two portions 130 and 132. However, in each portion 130 or 132, only that
polarization corresponding to the buffing direction is selected accorded to
the wavelength. The other polarization is not affected by the applied
' ~ 92/04653 2 ~ PCT/US91/05609
field and is thus necessarily blocked.
A general method of avoiding this 3 dB loss is to recognize that
the lens 142 acts as a spatial beam splitter that is polarization insensitive.
There are many well known types of polarization beam splitters which
separate an input beam into two output beams according to the
polarization, for example, a Wollaston prism. Such a polarization beam
splitter would re~~eive light from the input optical fiber 140 and deliver the
respective polari:aation components to the respective portions 130 and 132
which filter that polarization.
A sia;th embodiment of the invention is a particularly
advantageous tunable dual-polarization liquid-crystal etalon filter 150 that
uses a polarization beam splitter) as illustrated in the cross-sectional view
of FIG 11. It is fabricated on substrates 152 and 154 of birefringent
material which divides a beam according to its polarization state. On the
input side ) the b irefringent substrate 152 splits the beam from the input
fiber into an orf,inary beam 156 and an extraordinary beam 158. If the
birefringent optical axis is set perpendicularly to the long axis of the
interface 134 of the homogeneous alignment layer 124 and preferably at
45° to the input optical axis, the ordinary beam 156 is undeflected
while
the extraordinary beam 158 is deflected. The light in the ordinary beam
156 is electrica113~ polarized parallel to the interface 134 while that in the
extraordinary beam 158 is polarized perpendicularly to it. At the
interface between the birefringent substrate 152 and the transparent
electrode 116, the two beams 156 and 158 return to their original
directions and pass through the two portions 130 and 132 respectively
containing the homogeneous aligning agent of the proper alignment.
Importantly, the two beams 156 and 158 are well separated at this point
so that the diameter and alignment of the input beam are not critical.
After passing through the liquid crystal 136, the two beams are
recombined by the other birefringent substrate 154 set with its
birefringent optical axis set to mirror that of the first birefringent
substrate 152. Although a non-polarization beam combiner could be used
instead) the two birefringent substrates 152 and 154 are easily matched
and provide an integrated assembly. Rather than using birefringent
< v c ,~
~~ 4 (~ ~ ~ B I J
WO 92/04653 2 2 PCT/US91/0560a
substrates, birefringent layers can be permanently fixed on glass
substrates. The liquid-crystal dual-polarization filter 150 avoids the 3 dB
loss because all of the polarization component is delivered to that portion
130 or 132 that can completely filter it. This filter 150 has the further
advantage that the P band is eliminated. If a broad-area photodetector is
used, the birefringent layer 154 on the output side is not needed.
Examp le 3
The design of the integrated dual-polarization liquid-crystal
filter with polarization beam splitters and combiners was verified by
clamping calcite plates of 4 mm thicknesses to the outsides of the soda
lime glass substrates 112 and 114 of the dual-polarization liquid-crystal
filter 10 of Example 2. The filter was tested by inputting a beam from a
laser and detecting its output. When a manually operated polarization
controller rotated the polarization direction of the input light, the output
intensity varied by less than 1 dB. On the other hand, when a polarizing
sheet was additionally inserted across the input beam, the polarization
controller produced 25 dB variations of the output intensity.
Because the dual polarization filter 150 of FIG. 11 is insensitive
to polarization, it can be advantageously used in an optical drop circuit.
WDM communication systems impress multiple optical signals at
wavelengths al . . .kN on a single optical fiber. The N-fold optical signal
will be represented as .~k. Various types of filters can be used to select
the channel ak from the N channels of Via. However, a simple filter will
discard the other N - 1 channels. The optical equivalent of an electronic
drop circuit would be advantageous in which the kk channel is removed
from the fiber but the other N - 1 channels remain on the fiber. An
optical drop circuit 170, illustrated in FIG. 12 and a seventh embodiment
of the invention, performs such a function.
An input fiber 172 carries the WDM signal ~k, which is
divided by a first polarization beam splitter 174 into a first beam 176 and
a second beam 178. The illustrated type of polarization beam splitter 174
is a cube of quartz split along a diagonal to form a planar interface. The
interface is covered with one or more dielectric layer, and the cube is
assembled. Light polarized parallel to the interface is reflected at
90°
-23- ~ 8 8 ~ 7 2
while the orthogonal polarization is transmitted. The first beam 176
initially carries the polarization components of the WDM signal that are
polarized within the plane of the illustration while the second beam 178
carries the orthogonally polarized components. When the first beam 176
passes through a quarter-wave plate 180, all its frequency components
become circularly polarized. The circularly polarized beam 176 is incident
on a first liquid-crystal dual-polarization filter 182 having the structure of
the filter 150 of FIG. 11 and tuned to the selected wavelength ~~. The
filter 182 spatially splits the circularly polarized first beam 176 into its
two
linearly polarized components, passes both linearly polarized components
at ax, and recombines the J~k components into a circularly polarized beam.
Another quarter-wave plate 184 converts the circularly polarized ~k
channel to the linear polarization perpendicular to the original
polarization, that is, perpendicular to the illustration.
The other N - 1 optical channels E~- T~k which do not pass the
filter 182 are refle~~ted with minimal loss of energy. Upon their reverse
passage through the first quarter-wave plate 180, they become linearly
polarized but perpendicularly to their original polarization. Therefore,
the polarization beam spIitter 174 reflects them to an unselected output
beam 186 received by an output fiber 188.
The quarter-wave plates 180 and 184 can be replaced by
magneto-optical devices that rotate the linear polarization of a light beam
by 45° and rotate by the same angle when the beam travels in the
reverse
direction, that is, a 90° rotation for a double passage. Thereby, equal
amounts of the two polarizations are presented to the filter 182, and the
polarization beam splitter 174 directs the reflected beam away from the
input beam. The second magneto-optical device reestablishes the original
polarization direction, if this is required.
The structure described so far would operate as an optical drop
circuit if the linear polarization of the WDM signal could be controlled.
However, to compensate for a lack of such control, similar quarter-wave
plates 190 and 192 and dual-polarization filter 194 are needed to perform
similar filtering an~3 reflection of the orthogonally polarized components
in the second beam 178. The reflected components have their linear
C
WO 92/04653 ~ ~~ ~.~ ~~ ~~ ~' % 2 4 PCT/US91/0560~
polarization rotated so that they pass through the first polarization beam
splitter 174 to the output fiber 188. That is, the beam sputter 174 also
acts as a combiner of the two linearly polarized components of the
unselected signal ~~- ~k. Mirrors 196 and 198 bring the two polarization
components of the selected channel ak to a second polarization beam
splitter 200, which combines them into a selected output beam 202
received by another output fiber 204. Regardless of the polarization state
of the input channels Via, the drop circuit 170 performs the same selection
into the selected output beam 202 and the same reflection into the
unselected output beam 186.
In one array embodiment illustrated in plan view in FIG . 13,
the homogeneous alignment layer 124 is divided into a plurality of
orthogonally buffed homogeneous portions 130 and 132. The dual-
polarization filter 110 of FIG. 8, when combined with the array of
FIG . 13) can filter three beams at the interfaces 134 and 160. However,
the dual-polarization filter 150 of FIG. 11 can filter only two beams at the
equivalent interfaces 134. In another array embodiment, the multiple
beams are arrayed along the interface 134 between the two alignment
layer portions 130 and 132. One electrode 116 or 118 is patterned to
provide separate electrodes for the beams.
Although the above embodiments provide a filter having a
narrow enough tunable pass band so as to separate out a portion of a
wide spectrum, the invention is equally applicable to a modulator having a
relatively wide but tunable pass band. For instance, the mirrors may be
metallic, as disclosed by Saunders above, and combined with the
electrodes. For purposes of the fifth and sixth embodiments, a modulator
will be considered as a special case of a filter.
In the fifth and sixth embodiments, the homogeneous
alignment layer was buffed parallel and perpendicular to the interface 134
between the two homogeneous portions 130 and 132. However, the
dual-polarization effect can be obtained for orthogonal buffing at
different angles with respect to the interface. The embodiments described
above used a uniform homogeneous aligning agent in the second
alignment layer 120. However, the same effect can be obtained if the
~~~''~~r~~
' 92/04653 2 5 PCT/US91/05609
second alignment layer is formed of a homeotropic aligning agent that is
patterned similarly to the first alignment layer. Then, the two alignment
layers need to be precisely assembled so that their respective interfaces
134 are aligned. The homogeneous aligning agent can be aligned in
parallel between the two alignment layers or aligned perpendicularly so as
to impart a 90° twist to the liquid crystal across the gap.
The liquid-crystal etalon optical filters of the invention
involving interference mirrors provides for a narrow pass band with wide
electrical tunability. They are low-powered) economical to fabricate, and
rugged.
