U.S. patent number RE35,337 [Application Number 08/355,928] was granted by the patent office on 1996-09-24 for temperature compensation of liquid-crystal etalon filters.
This patent grant is currently assigned to Bell Communications Research, Inc.. Invention is credited to Jayantilal S. Patel, John R. Wullert, II.
United States Patent |
RE35,337 |
Patel , et al. |
September 24, 1996 |
Temperature compensation of liquid-crystal etalon filters
Abstract
A compensator for thermal or other uncontrollable effects in a
liquid-crystal etalon filter. The narrow pass band of the filter is
controlled by adjusting the amplitude of an AC drive signal applied
to the electrodes on either side of the liquid crystal in the
filter. An optical detector detects the intensity of light from a
narrow-bandwidth input beam passed by the detector. Electrical
circuitry determines the bipolar amplitude of the component of the
light intensity that is at twice the frequency of the AC drive
signal (the doubled-frequency amplitude) and adjusts the amplitude
of the AC drive signal in response to the doubled-frequency
amplitude so as to reduce the doubled-frequency amplitude toward
zero.
Inventors: |
Patel; Jayantilal S. (Red Bank,
NJ), Wullert, II; John R. (Colonia, NJ) |
Assignee: |
Bell Communications Research,
Inc. (Morristown, NJ)
|
Family
ID: |
24914143 |
Appl.
No.: |
08/355,928 |
Filed: |
December 14, 1994 |
Current U.S.
Class: |
349/72;
349/104 |
Current CPC
Class: |
G02F
1/216 (20130101); G02F 1/133382 (20130101); G02F
2203/055 (20130101); G02F 1/0123 (20130101); G02F
1/213 (20210101) |
Current International
Class: |
G02F
1/133 (20060101); G02F 1/21 (20060101); G02F
1/13 (20060101); G02F 1/01 (20060101); G02F
001/133 (); G02F 001/137 () |
Field of
Search: |
;359/85,86 ;340/713
;332/123 ;345/48,50 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
J S. Patel et al., "Electrically tunable optical filter for
infrared wavelength using liquid crystals in a Fabry-Perot etalon,"
Appl. Phys. Lett., Oct. 1990, vol. 57, No. 17, pp.
1718-1720..
|
Primary Examiner: Mai; Huy
Attorney, Agent or Firm: Suchyta; Leonard Charles Falk;
James W.
Claims
What is claimed is: .[.1. A compensator for a liquid-crystal filter
for filtering a beam of light having an optical frequency that is
thereafter detected in an optical detector, comprising:
an oscillator circuit providing a first oscillatory electrical
signal at a driving frequency f and a second oscillatory electrical
signal at a frequency proportionally related to f;
a phase-sensitive detector receiving a detection output from said
optical detector and said second oscillatory electrical signal and
providing a compensation signal representing an amplitude of a
component of said detection output having said related frequency;
and
a driving circuit receiving said first electrical signal and said
compensation signal and providing a driving signal for said
liquid-crystal filter having said driving frequency and an
amplitude related to said compensation signal..]..[.2. A
compensator as recited in claim 1, wherein said related frequency
is 2f..]..[.3. A compensator as recited in claim 2, wherein said
oscillator circuit comprises an oscillator providing an oscillator
output and a frequency multiplier receiving said oscillator
output..]..[.4. A compensator as recited in claim 3, wherein said
oscillator circuit includes a power supply input controlled in
response to said compensation signal..]..[.5. A compensator as
recited in claim 1, wherein said driving circuit further receives a
tuning signal and wherein said amplitude of said driving signal is
related to an additive combination of said tuning signal and said
compensation signal..]..[.6. A compensated tunable optical
receiver, comprising:
a liquid-crystal etalon filter receiving on a first side an optical
signal and having electrodes for impressing a voltage across a
liquid crystal in said filter;
an optical detector disposed on a second side of said filter,
receiving a portion of said optical signal filtered by said filter,
and providing a detection signal;
means for applying a first oscillatory signal at a frequency f
across said electrodes of said filter; and
means for changing an amplitude of said first oscillatory signal
according to an amplitude of a component of said detection signal
at a frequency related to said frequency f..]..[.7. A receiver as
recited in claim 6, wherein said related frequency is 2f..]..[.8. A
receiver as recited in claim 6, further comprising tuning means for
tuning said filter to a peak
of said optical signal independently of said changing means..].9. A
method of .[.compensating a.]. .Iadd.using the internal modulating
refractive index variations within a liquid-crystal filter from an
applied oscillatory signal to provide temperature compensation to
the .Iaddend.liquid-crystal filter .Iadd.when .Iaddend.irradiated
with a beam of light, comprising the steps of:
applying .[.a first.]. .Iadd.said .Iaddend.oscillatory signal at a
frequency f across electrodes of said liquid-crystal filter;
detecting a component of said beam of light filtered by said filter
and having a frequency proportionally related to said frequency f;
and
a first step of adjusting said oscillatory signal in response to
said detected component .Iadd.as modulated by the internal
variations within
said liquid-crystal filter..Iaddend.10. A method as recited in
claim 9, wherein said detecting step detects an amplitude of said
component in fixed phase relationship with a signal oscillating at
said related
frequency. 11. A method as recited in claim 10, wherein said
related
frequency is 2f. 12. A method as recited in claim 11, wherein said
detecting step comprises the steps of:
detecting an intensity of said beam of said light filtered by said
filter;
generating a second oscillatory signal at said related frequency
2f; and
detecting a component of said intensity having a fixed phase
relationship with said second oscillatory signal and thereby
providing said detected
component. 13. A method as recited in claim 11, further comprising
the steps of:
detecting said beam while said first adjusting step is disabled and
thereby providing a measure of an intensity of said beam;
a second step of adjusting said oscillatory signal in response to
said measure of said intensity of said beam; and
enabling said first adjusting step after said second adjusting
step.
Description
FIELD OF THE INVENTION
The invention relates generally to liquid-crystal devices. In
particular, the invention relates to temperature compensation of
liquid-crystal etalon filters.
BACKGROUND ART
Electrically tunable, liquid-crystal, optical filters have been
proposed, for example, by Patel et al. in "An electrically tunable
optical filter for infra-red wavelength using liquids crystals in a
Fabry-Perot etalon." Applied Physics Letters, volume 57, 1990, pp.
1718-1720 and by Patel in U.S. patent application, Ser. No.
07/677,769, filed Mar. 29, 1991. Although different types have been
proposed, the high-performance types share the structure
illustrated in FIG. 1 for a liquid-crystal etalon filter 10. Two
dielectric interference mirrors 12 and 14 are formed on transparent
substrates 16 and 18 as two separate assemblies. Semi-transparent
electrodes 22 and 24 are deposited on the mirrors 12 and 14. The
two assemblies are then fixed together with a small predetermined
gap between them, and a liquid crystal 26 is filled into the gap.
The size of the gap is chosen such that the corresponding optical
length between the mirrors 12 and 14 (taking into account the
relevant refractive index of the liquid crystal 26) nearly equals
the wavelength of the light being filtered or a multiple thereof.
That is, the mirrors 12 and 14 and intervening liquid crystal 26
form a Fabry-Perot cavity and thus an etalon filter for transmitted
light. A voltage generator 28 electrically tunes the liquid-crystal
by imposing a variable voltage, determined by a tuning signal TUNE,
across the electrodes 22 and 24 and thus imposing an electric field
across the liquid crystal 26. At least one of the refractive
indices of the liquid crystal 26 is changed by the electric field.
Thereby, the optical length of the resonant cavity is changed, and
the filter 10 will pass an optical band of the input light 20 into
an output light 30 in correspondence to the voltage imposed across
it. This description has neglected alignment layers adjacent to the
liquid crystal and polarizing components which vary among the
various liquid-crystal filters, but preferred examples may be found
in the Patel references.
A liquid-crystal filter of this type is not only easy to fabricate
and to operate, it also offers a very narrow bandwidth of the order
of 1 nm because of the high reflectivity (greater than 98%) and the
low loss provided by the dielectric interference mirrors. However,
this narrow bandwidth raises difficulties. The refractive indices
of the liquid crystal depend not only on electric field but also
upon the temperature of the liquid crystal. Some experiments, to be
described later, have determined that a temperature variation of
.+-.0.5.degree. C. can shift the pass band by as much as half the
width of the pass band. Although temperature can be controlled to
these small variations, such controlling equipment is expensive and
limits the usefulness of liquid-crystal etalon filters.
SUMMARY OF THE INVENTION
Accordingly, an object of the invention is remove the temperature
dependence of a liquid-crystal optical filter.
Another object is to do so at minimal cost and without having to
finely control the temperature.
The invention can be summarized as a method and apparatus of
compensating for temperature and other variations in an
electrically tunable liquid-crystal etalon filter by applying an
electrical potential oscillating at the frequency f across the
electrodes of the liquid-crystal filter and adjusting the amplitude
of the oscillatory potential so as to minimize the amplitude of one
of the frequency components of a light beam passed by the filter.
Preferably, this frequency component is the 2f component.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-section of a liquid-crystal etalon filter.
FIG. 2 is a schematic illustration of the circuitry of an
embodiment of a temperature compensator of the invention for
compensating variations associated with the illustrated
liquid-crystal etalon filter.
FIG. 3 is a schematic diagram of a preferred circuit of the
feedback and drive circuit of FIG. 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
It is standard practice to electrically bias a liquid crystal used
in an optical modulator (display) or in an optical filter, not with
a DC voltage, but with an AC voltage. A DC voltage causes charge
migration in the liquid crystal resulting in a depolarization field
that decreases the electric field across the liquid crystal as a
function of time. As a result, the voltage generator 28 in FIG. 1
is an AC voltage generator producing an oscillatory signal of a
generally fixed frequency f and of an amplitude determined by the
tuning signal TUNE. Typically in the prior art, the applied signal
was a symmetrical bipolar square wave. For nematic liquid crystals,
the dielectric torque on the liquid-crystal molecules is
independent of the direction of the field since the torque is
proportional to the square of the electric field. Consequently, the
response should primarily depend on the RMS value of the applied
voltage. However, at least two effects create an AC modulation by
the applied AC voltage. First, ion migration causes time-dependent
depolarization fields. Second, the flexo-electric effect causes
structural relaxation and distortion of the director close to the
surfaces. Both of these effects modulate the refractive index and
result in a resonance peak having finitely sloped sides. When the
filter is tuned on one of the sides, the transmitted intensity is
modulated at twice the applied frequency, that is, at 2f. However,
the phase of the modulation changes by 180.degree. when the
resonance of a narrow-band liquid-crystal etalon filter is tuned
from one side of a very narrow-band light source to the other side
so that the 2f component disappears at the resonance peak. The
signed amplitude of the 2f component represents the derivative of
the resonance with respect to the applied voltage. In narrow-band
filters, such an effect is generally undesirable. However, the
invention uses this effect to tune to the peak of the resonance,
which may be changing with temperature.
In an embodiment of the invention illustrated schematically in FIG.
2, the temperature variation of the liquid-crystal etalon filter 10
is compensated by an active feedback circuit. It is initially
assumed that the liquid-crystal filter 10, irradiated with an
optical signal 34 preferably having a bandwidth less than the pass
band of the filter 10, has its resonance at least partially tuned
to the optical frequency of that signal 34.
The light 30 transmitted through the filter 10 is directly detected
in an optical detector 36. The resulting electrical signal measures
the intensity of the transmitted light 30 and may be directly
received by a receiver 38 for which the data signal carried by the
optical input signal 34 is intended. However, the electrical signal
is also connected to the signal input SIG of a phase-sensitive
detector 40 which has a frequency response at a considerably lower
frequency than that of the receiver 38. The phase-sensitive
detector 40 determines the component of the input signal SIG that
is in phase with an oscillatory reference signal REF. Its output
OUT is the signed amplitude of that oscillatory portion of the
input signal SIG, although the output may be intentionally offset
from zero. This signed amplitude represents an error signal.
A feedback and drive circuit 42 electrically drives the
liquid-crystal filter 10 at a frequency f, generally about 1 kHz.
Within it, an oscillator 44 produces an oscillatory output at the
frequency 2f. This oscillatory signal is connected not only to the
reference input REF of the phase-sensitive detector 40 but also to
a frequency divider 46 which outputs a signal at only half the
frequency of its input. That is, the frequency divider 40
multiplies the input frequency 2f by 0.5 and outputs at the
frequency f. The f signal, having constant amplitude, is connected
to one input of a multiplier 48. The other input of the multiplier
48 receives the error signal from the output OUT of the
phase-sensitive detector 40, to which an analog adder 50 has added
a DC tuning voltage TUNE. The output of the multiplier 50 drives
the liquid-crystal filter 10 with an oscillatory signal having a
frequency f and an amplitude determined by the bipolar error signal
from the phase-sensitive detector 34 and by the tuning voltage
TUNE.
To initially tune the liquid-crystal filter 10 to the resonance
corresponding to the input signal 34, a double-throw switch 52
substitutes a grounded potential for the output OUT of the
phase-sensitive detector 40, and the tuning voltage TUNE is changed
until the receiver 38 or other monitoring device detects that the
filter 10 is passing the optical signal. Thereby, the cavity of the
liquid-crystal filter 10 is at least partially tuned to the optical
frequency of the optical input signal 34 under the conditions
occurring during the tuning operations. Thereafter, the switch 52
is set back to the output OUT and feedback control starts.
Any non-zero output from the phase-sensitive detector 40 (that is,
the presence of any detection signal at 2f) indicates that the
liquid-crystal filter 10 is not tuned to the peak of the resonance.
The sign of the output OUT indicates on which side of the frequency
of the resonance peak is the optical frequency of the optical input
signal 34. The polarity of the output voltage signal OUT must be
chosen so that the feedback and driver circuit 42 drives the
resonance peak back to coincidence with the optical frequency of
the optical input signal 34. The magnitude of the output signal OUT
measures the amount of deviation between the resonance peak and the
optical frequency.
The feedback control illustrated in FIG. 2 is proportional feedback
control since the amount of the correcting signal OUT is
proportional to the amplitude of the 2f signal. As a result, if the
resonance has shifted, the compensation will be unable to return
the liquid-crystal filter to the peak of the resonance, where there
is no 2f signal, but will only return it toward the peak. More
elaborate types of feedback control would eliminate this problem.
For example, proportional-integral control would include partial
control by a time integral of the correcting signal OUT. Yet more
complex control would include a derivative term. Stability of the
feedback loop must always be insured by inserting appropriate time
constants.
The type of feedback control described above resembles well-known
feedback control of a laser that is DC biased and is additionally
biased by a small AC signal oscillating at a dither frequency. Then
a detected signal is phase-sensitively detected at twice the dither
frequency. The detected dither component then corrects the DC bias
applied to the laser.
FEEDBACK AND DRIVE CIRCUIT
A circuit 42 has been built to provide the feedback and driving
functions illustrated in FIG. 2 but with different components, as
illustrated in the schematic diagram of FIG. 3. A 555-type timer 60
was connected with capacitors and resistors so as to oscillate at 2
kHz with a 50% duty cycle. The 2 kHz output both is connected to
the REF input of the phase-sensitive detector 40 and controls a
D-type flip/flop 62, which acts as a frequency divider producing a
signal at 2 kHz. The power supply inputs V.sub.cc of both the timer
60 and the flip-flop 62 are connected to the combined tuning and
error signal from the adder 50. The adder 50 is an operational
amplifier and feedback resistor 66 receiving the tuning signal TUNE
from a voltage source through a variable resistor 68 and the error
signal from the OUT output of the phase-sensitive detector 40
through a fixed resistor 70. Thus, the amplitudes of both the 1 kHz
and the 2 kHz outputs depend on the tuning and error signal. The
output of the flip/flop 62 is a symmetric 1 kHz square wave, but
oscillating between the variable controlled amplitude and zero. A
level shifter 72 shifts the square wave to be bipolar, oscillating
between equal positive and negative voltages. The final stage of
the level shifter 72 is an operational amplifier 74. When both a
capacitor 76 and a resistor 78 are connected in parallel in its
feedback loop, the operational amplifier 74 integrates the square
wave input so as to output a bipolar triangular waveform. When the
capacitor 76 is removed from the feedback loop, the operational
amplifier 74 only amplifies its input signal so as to output a
bipolar square wave.
EXPERIMENTS
A series of experiments were performed upon a liquid-crystal etalon
filter fabricated as described in the Patel et al. article cited
above. The mirrors had reflectivities of 98.5% in a broad band from
1.4 .mu.m to 1.6 .mu.m. The cell gap was about 10 .mu.m, and the
liquid crystal was nematic, Type E7, available from EM Chemicals.
This liquid crystal has a transition from the nematic phase to the
higher-temperature isotropic phase at 60.5.degree. C. The pass band
of the liquid-crystal 10 was about 0.5 nm. The filter was
irradiated with laser light from a solid-state DFB laser operating
at 1.5464 .mu.m and having a line width considerably less than 0.5
nm. Single-mode fibers were coupled to each side of the filter. The
filter was mounted on a temperature-controlled holder. An
electronic amplifier was inserted between the optical detector and
a PAR Model 121 lock-in amplifier, which acted as the
phase-sensitive detector. The decay time on the lock-in amplifier
was set to 3 seconds, which determined the feedback time
constant.
A first experiment was performed with no feedback and using
oscillators other than those in the feedback and drive circuit.
Both the DC and 2f amplitude signals from the optical detector were
monitored as a function of the amplitude of the AC drive signal at
the frequency f. The DC signal showed two peaks at voltages for
which the filter cavity was in resonance for the laser light. The
2f amplitude showed a positive peak immediately followed by a
negative peak at these two voltages, that is, the 2f amplitude
corresponded to the derivative of the DC signal. It was found that
a sine wave or a triangular AC drive signal produced larger
derivative signals than a square wave AC drive signal. Also the
magnitude of the derivative signal decreased with increasing
frequency f of the AC drive signal. The frequency f is picked so
that the feedback loop is stably operated without severely
affecting the detection of the optical signal being filtered.
A second experiment was performed with feedback, as illustrated in
FIGS. 2 and 3, and using a triangular oscillatory waveform. The
filter was initially tuned to resonance at 49.degree. C., and then
the feedback was turned on. The temperature was reduced to
25.degree. C. and then gradually raised to above 65.degree. C. The
DC optical intensity remained fairly constant from 25.degree. C. to
just above 55.degree. C., at which point it fell but remained
locked until about 60.degree. C. When the temperature was then
lowered with the filter not being locked to the input signal, no
output signal was obtained until the filter cavity came into an
uncompensated resonance, from which point the intensity remained
fairly constant down to 25.degree. C.
A third experiment was performed using a bit-error ratio (BER)
tester to impress pseudo-random data at 155 Mb/s upon the laser.
The optical output signal was optically split between the optical
detector of the BER tester and the optical detector of the
temperature compensator. In order to reduce the size of the 2f
amplitude, a square-wave drive signal was applied to the filter.
After initial tuning, the BER was measured to be about 10.sup.-8
for a received laser power of -37.6 dBm. Thereafter, the BER was
measured as a function of temperature with and without feedback
control. Without any feedback, a temperature change of
.+-.0.5.degree. C. from 25.degree. C. caused the BER to increase to
10.sup.-2. With feedback, as the temperature was raised from
25.degree. C. to 40.degree. C., the BER gradually increased to
about 10.sup.-3. Thereby, the temperature compensation of the
invention extended the thermal operating range of the 0.5 nm filter
by more than a factor of ten.
The tracking range of the temperature compensator used in the
experiments is believed to be limited by the gain-bandwidth product
of the feedback loop. However, increasing the loop gain of the
described circuitry sends the loop into oscillation. The tracking
range also depends on the parameters of the liquid crystal. The
E7liquid crystal melts at 60.5.degree. C. Tracking is difficult
even near the phase transition, where the change of refractive
indices is most steep. A liquid crystal of higher melting point is
desirable.
The last described experiment is related to a possible use of the
liquid-crystal filter in a wavelength-division multiplexing
communication systems in which multiple optical carriers are
carried on a single optical fiber. The liquid-crystal filter would
be tuned to the one desired optical carrier frequency. Thereafter,
that carrier can be tracked by the temperature compensation of this
invention as long as the carrier continues to carry enough energy
to excite the temperature compensator. The initial tuning to that
carrier frequency at an unknown driving voltage in the presence of
other carriers will require an automatic scanning and recognition
of a carrier identifier.
Although the described embodiment detected the doubled-frequency
component at 2f other harmonics of the driving frequency f can be
detected and minimized. If the fundamental harmonic frequency f is
to be used, it is necessary to provide asymmetry with a DC bias or
with asymmetrical surface alignment of the liquid crystal. Although
the experiment has been described for the temperature compensation
of a liquid-crystal filter, the invention can be used to compensate
variations of the liquid-crystal filter caused by other factors,
for example, variations in the drive circuit. Indeed, the invention
can be used to compensate frequency drifts of the incoming
light.
Although the invention involves temperature compensating the
liquid-crystal by adjusting its biasing amplitude, it may be
preferred to use biasing adjustment only for fine feedback control
and for rough feedback control to control the actual temperature by
resistive heating and thermoelectric cooling. The temperature
compensator of the invention is simple and inexpensively
implemented. It requires no modification to the liquid-crystal
filter and no application of additional signals to the filter.
Nonetheless, it greatly extends the thermal operating range of a
narrow-bandwidth liquid-crystal etalon filter.
* * * * *