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An electrically addressable nonlinear optical bistable device
Volume 6 1. number 6
OPTICS COMMUNICATIONS
AN ELECTRICALLY ADDRESSABLE
NONLINEAR
15 March 1987
OPTICAL BISTABLE DEVICE
J. STAROMLYNSRA, A. MILLER and R.A. CLAY Royal Signals and Radar Establishment, Malvern, Worcs., WR14 3PS, UK Received 1 October 1986
Optical studies were carried out at 647 nm in a Fabry-Perot etalon containing the liquid crystal mixture 31517PCH. The electrooptic and nonlinear effects were brought together in two sets of experiments. Firstly, optical bistable loops were obtained at different initial cavity tunings imposed by the applied voltage through the electro-optic effect. Secondly, bistable switching was achieved by fixing the input optical power and varying the voltage to obtain “voltage bistable” loops. The behaviour of these loops was found to follow the trend predicted by theory.
1. Introduction
The ability to modulate laser light from an electrical signal and in particular to impose digital information on to an optical beam is of importance in applications areas such as optical communications and digital optics. Amplitude modulation can be induced through electro-optic interactions in suitable materials engineered into appropriate geometries such as waveguides and Fabry-Perot etalons [ 11. In general, these devices produce monotonic output dependencies with applied voltage and only give switching characteristics upon the electrical feedback of the optical output [ 21. Nonlinear materials within Fabry-Perot etalons give rise to switching characteristics without any external feedback. This is the well known all-optical bistability effect [ 31 and does not involve the use of a dc electric field but relies on a change in the optical input to induce switching. This paper describes some simple theoretical and experimental studies which centered on bringing together the electro-optic and nonlinear effects to produce sharp optical switch on/off characteristics and bistability with applied voltage at constant optical input power. Optical bistability has previously been demonstrated using liquid crystals within Fabry-Perot etalons with the optical nonlinearity arising from refractive index changes induced either by laser heating or molecular reorientation in the optical field [ 41.
The material chosen for this study was the homogeneously aligned nematic liquid crystal 3/5/7 PCH incorporated into a Fabry-Perot etalon with partially reflecting silver mirrors. Using this system, optical bistability loops were obtained at different initial cavity detunings imposed by a bias voltage applied across the cavity. In addition, bistable behaviour of the output with applied voltage was achieved at constant optical input. “Voltage bistable loops” were recorded at different values of optical input power. The trend followed by these was found to be in agreement with that predicted by theory. The experimental results presented here demonstrate the possibility of transposing a digital electrical signal to a digital optical signal via a Fabry-Perot etalon containing a material which is both electrooptic and nonlinear.
2. Theory The principle of operation of this device brings together the electro-optic and nonlinear optical bistability effects. The switching mechanism is still due to an optical nonlinearity coupled with feedback from the etalon mirrors producing bistability. The major difference in this case is that the optical input power remains constant. A voltage applied across the length of the cavity changes the initial detuning via the electro-optic effect and for a constant value of input 415
OPTICS COMMUNICATIONS
Volume 6 1, number 6
power can tune the cavity such that switch-up (or down) will occur. It is possible to predict the optical bistability characteristics of a Fabry-Perot etalon containing a nonlinear material by solving the equations describing the material and cavity responses. In this work the standard graphical construction [ 51 was extended to cover the case where both the electro-optic and nonlinear mechanisms are present. The transmission, T, of an etalon is related to the single pass phase shift, 6, via the oscillatory Airy function, T=AI(l SFsin’Q,
(1)
where A is a constant which depends on mirror reflectivities, absorption and thickness of the etalon while F is the coefficient of finesse. The phase 6 can be expressed as,
(2)
6=&+yzc+?jv,
15 March 1987
where do is the initial phase of the cavity; 6,, is the phase change induced by the light intensity in the cavity, Z,; and 6,, is the phase change induced by the effective nonlinear and electro-optic constants, y and q. Fig. 1a shows an example of the transmission of an absorbing etalon as a function of cavity intensity in normalised units. The relation between cavity intensity and transmitted intensity leads to the additional criterion, TlIc = ClIi)
represented in fig. la by straight lines whose slope depends on incident intensity, Zi. Introducing a change in phase due to applied voltage effectively alters the initial cavity tuning. The condition for an incident intensity yZi=2n is shown in fig. la for a series of values of 6,, = 0 to rt (dn/d I/ assumed positive). Multiple crossing occurs at around 6,,=0.25 II. Thus for a fixed incident intensity, increasing the voltage will cause the cavity to switch up from a to b
a
b
‘IV/”
Fig. 1. Computed characteristics for ~1,= 2x.
416
I
rb
05 INPUT
(3)
Fig. 2. Computed characteristics for yli = 3~.
Volume 6 1, number 6
OPTICS COMMUNICATIONS
and down from c to d. This may be replotted as a function of voltage to give the bistability loop of fig. 1b. If dnld V is of opposite sign to dnldl then a clockwise loop is obtained (fig. 1c) . This construction also reveals that the higher the optical input power, the wider the voltage bistability loop, fig. 2a. In addition under certain initial tuning conditions, the optical output does not return to its original value on removal of the voltage. This is illustrated in figs. 2b and 2c. All of these predicted features have been verified experimentally and are reported below. Note too that a useful aspect of having electrical control of an otherwise all-optical nonlinear bistable device allows resetting of the device with an electrical input rather than by interrupting the beam.
3. Experimental details Fig. 3 shows the sample geometry. The Fabry-Perot etalon was formed by evaporating silver films of approximately 200 A thickness onto IT0 coated glass substrates. The silver films acted as partially reflecting mirrors and the IT0 as rugged transparent electrodes. Liquid crystal alignment layers of 100 8, thickness SO were subsequently evaporated at an angle of 30 8, to induce homogeneous alignment of the liquid crystal. Final steps of the construction consisted of clamping together the two substrates separated by spacers to give a cavity length of approximately 7 pm and capillary filling the etalon with the liquid crystal 3/5/7 PCH. This is a nematic
15 March 1987
liquid crystal mixture of positive dielectric anisotropy having n, and n, values of 1.49 and 1.61 respectively at 20 “C. The molecular structure and properties of this mixture have been documented by Bradshaw et al. [ 61. The sample parameters and molecular orientation were chosen to enable tuning of the etalon through several (3-4) fringes by the application of a bias voltage across the cavity. All measurements were made cw at a wavelength of 647 nm produced by an Innova 90-K krypton-ion laser. Fig. 4 is a diagram of the experimental arrangement. An acousto-optic modulator ramped the optical input power to the sample, two large area silicon photodiode detectors D 1 and D2 monitored the input and transmitted powers respectively and the halfwave plate and polariser allowed control of the polarisation of the input beam. Lens Ll was a x 5 microscope objective giving a spot size of 12 c ( l/e2 radius); lens L2 collected all of the transmitted light and weakly focussed it onto D2: The sample was mounted on an x-y-z translation stage which allowed accurate control over the focus and position of the beam on the sample. The orientation of the molecules was such that their long axis lay in the y-direction as defined in fig. 4. Two types of experiment were carried out. Firstly, optical bistability loops were recorded by taking the outputs from Dl and D2 directly onto a digital oscilloscope. Bistability characteristics were obtained with different ac bias voltages applied across the cavity. ACbias voltages of frequency 1 to 2 kHz appear dc to a liquid crystal and were used to avoid any sample degradation. Results were taken with the input polarisation both parallel and perpendicular to the molecular orientation. In the second set of experi-
i’
I.
-
Ll /\
I
I
I/
v
L2 --D2
+J+
sawa Fig. 3. Sample geometry.
Fig. 4. Experimental arrangement.
417
Volume 6 1. number 6
OPTICS COMMUNICATIONS
ments output characteristics of the etalon at a constant optical input power were recorded as a function of bias voltage. For these experiments the desired input power to the cavity was fixed by the variable attenuator. For experimental convenience the applied voltages in these experiments were dc and controlled either manually or by means of a ramp generator. The output power versus voltage characteristics were obtained for different values of optical input power.
15 March 1987
.t_11_7 25v
P-P
4. Results Figs. 5 and 6 show the nonlinear characteristics obtained with different bias voltages applied as described in section 3. The polarisation of the input beam was parallel to the long axis of the molecules and hence the effective low power refractive index at zero volts was n, = 1.6. The bias voltage changed the initial cavity detuning by changing the effective refractive index in the y-direction. This occurred through the reorientation of the molecules which tend to line up with their long axis parallel to the electric field. In theory the maximum change in refractive index achievable by this method is the birefringence n,-n,, which in this case was 0.12. In practice the molecules do not completely reorient and the maximum change is smaller than 0.12. Maximum reorientation occurred at < 10 V with intermediate changes occurring at lower voltages. Fig. 5 is a series
Fig. 5. Optical bistability loops obtained at different bias voltages. 1: 0 Volt, 2: 1.2vp-p, 3: 2.2v p-p.
418
4VP-P
7v P-P
*t--L!-10
INPUT
POWER
mW
Fig. 6. Bistability characteristics obtained by electrically tuning the cavity through two fringes.
of bistability loops obtained at different initial detunings by electrically tine tuning the cavity relative to the fringe. In this case increasing the bias voltage tuned the cavity closer to a transmission maximum, this being reflected in the loops moving to lower powers and becoming narrower. Fig. 6 is a complimentary set of characteristics achieved by coarsely tuning the cavity through two complete fringes. These results are consistent with dnld V and dnldZ being of the same sign. The unconventional characteristic of a decrease in output power with increasing power above the b&able region (fig. 6) was a feature consistently present in this work and may be due to the mirror material being metallic rather than dielectric.
Volume 6 1, number 6
OPTICS COMMUNICATIONS
Identical experiments carried out with the polarisation in the z-direction, (i.e. perpendicular to the long axis of the molecules), yielded the expected result of no change in the nonlinear characteristic with applied voltage. Fig. 7a shows the temporal profiles of the applied bias voltage and transmitted intensity obtained at constant optical input power. Fig. 7b is the companion characteristic of the transmitted intensity plotted against applied voltage. These demonstrate the behaviour of the optical output with applied voltage. Two anti-clockwise voltage bistable loops occur, the first at _ 1.4 V and the sec-
(a)
e 6
0.5-
z
15 March 1987
ond at N 2.2 V. These correspond to the cavity being electrically tuned to a point where, for a given optical input power, switch up (or down) can occur through the positive feedback mechanism brought about by the nonlinearity of the material. The two loops correspond to tuning through two fringes. A series of voltage characteristics obtained as a function of optical input power ar presented in fig. 8. The last characteristic was obtained by fixing the input power just below the switch up point on an optical bistability loop and manually ramping the voltage from a dc supply. In agreement with theory, these results show that an increase in input power results in the loops becoming wider and that under certain conditions, switch up will occur with no switch down on removal of the voltage (fig. 8~). Switch down in this case was achieved by interrupting the input beam.
E
5. Conclusions
Fig. 7. (a) Temporal profiles of the optical output and applied dc voltage at constant optical input. (b) Optical output versus applied dc voltage at constant optical input power.
This work has demonstrated the possibility of imposing digital electrical information on to an optical beam via a device consisting of a Fabry-Perot etalon containing a material which is both electro-optic and optically nonlinear. Regenerative switching with optically bistable memory was demonstrated with electrical input. This device requires no external feedback or change in the input power to produce the switching but has a minimum value of input power below which it will not operate. This minimum power level will be governed by the magnitude of the nonlinearity of the material and the cavity parameters. For experimental convenience and demonstration purposes the material used here was a nematic liquid crystal. The electro-optical and nonlinear effects in this material are a consequence of the reorientation of birefringent molecules, produced electrically in the first case and thermally in the second. The switching times are therefore long, being of the order of milliseconds (although the regenerative switching should lead to faster sweeping than for the purely electrically tunable filter). Faster response times may be accomplished by replacing the liquid crystal with thin layers of other materials such as ZnSe interference filters [ 71 or GaAs multiple quantum wells [ 81 in which low power all-optical bistability has been demonstrated. The liquid crystal device demon419
0.15-
15 March 1987
OPTICS COMMUNICATIONS
Volume 6 1, number 6
@)
0
1.0
0.5
DC VOLTAGE
1.5
vdtr
Fig. 8. Optical output versus applied dc voltage at an optical input power of (a) 7.9, (b) 8.3 and (c) 8.5 mW. strated here may be suitable for application binary electrically addressable spatial modulator.
as a
References
light
[ 1] W.J. Stewart, I. Bennion and M.J. Goodwin, Phil. Trans. R. Sot. Land. A 313 (1984) 401. [2] P.W. Smith, Opt. Eng. 19 (1980) 456.
[ 3 ] H.M. Gibbs, Optical bistability: controlling light with light Acknowledgements We thank K.J. Harrison and the RSRE liquid crystal fabrication unit for supplying the sample. 0
Controller,
London,
1986.
Her
Majesty’s
Stationary
Offlice,
(Acad. Press, N.Y., 1985). [4] Y.R. Shen, Phil. Trans. R. Sot. Lond. A 313 (1984) 327. [ 51J.H. Marburger and F.S. Felber, Phys. Rev. A 17 (1978) 335. [ 61 M.J. Bradshaw, J. Constant, D.G. McDonnel and E.P. Raynes, Molec. Cry& Liq. Cryst. 97 (1983) 177. [ 71 S.D. Smith, J.G.H. Mathew, M.R. Taghizadeh, A.C. Walker and B.S. Wherrett, Optics Comm. 51 (1984) 357. [ 8 J H.M. Gibbs, S.S. Tamg, J.L. Jewell, D.A. Weinberger, K. Tai, A.C. Gossard, S.L. McCall, A. Passner and W. Wiegmann, Appl. Phys. Lett. 41 (1982) 221.
OPTICS COMMUNICATIONS
AN ELECTRICALLY ADDRESSABLE
NONLINEAR
15 March 1987
OPTICAL BISTABLE DEVICE
J. STAROMLYNSRA, A. MILLER and R.A. CLAY Royal Signals and Radar Establishment, Malvern, Worcs., WR14 3PS, UK Received 1 October 1986
Optical studies were carried out at 647 nm in a Fabry-Perot etalon containing the liquid crystal mixture 31517PCH. The electrooptic and nonlinear effects were brought together in two sets of experiments. Firstly, optical bistable loops were obtained at different initial cavity tunings imposed by the applied voltage through the electro-optic effect. Secondly, bistable switching was achieved by fixing the input optical power and varying the voltage to obtain “voltage bistable” loops. The behaviour of these loops was found to follow the trend predicted by theory.
1. Introduction
The ability to modulate laser light from an electrical signal and in particular to impose digital information on to an optical beam is of importance in applications areas such as optical communications and digital optics. Amplitude modulation can be induced through electro-optic interactions in suitable materials engineered into appropriate geometries such as waveguides and Fabry-Perot etalons [ 11. In general, these devices produce monotonic output dependencies with applied voltage and only give switching characteristics upon the electrical feedback of the optical output [ 21. Nonlinear materials within Fabry-Perot etalons give rise to switching characteristics without any external feedback. This is the well known all-optical bistability effect [ 31 and does not involve the use of a dc electric field but relies on a change in the optical input to induce switching. This paper describes some simple theoretical and experimental studies which centered on bringing together the electro-optic and nonlinear effects to produce sharp optical switch on/off characteristics and bistability with applied voltage at constant optical input power. Optical bistability has previously been demonstrated using liquid crystals within Fabry-Perot etalons with the optical nonlinearity arising from refractive index changes induced either by laser heating or molecular reorientation in the optical field [ 41.
The material chosen for this study was the homogeneously aligned nematic liquid crystal 3/5/7 PCH incorporated into a Fabry-Perot etalon with partially reflecting silver mirrors. Using this system, optical bistability loops were obtained at different initial cavity detunings imposed by a bias voltage applied across the cavity. In addition, bistable behaviour of the output with applied voltage was achieved at constant optical input. “Voltage bistable loops” were recorded at different values of optical input power. The trend followed by these was found to be in agreement with that predicted by theory. The experimental results presented here demonstrate the possibility of transposing a digital electrical signal to a digital optical signal via a Fabry-Perot etalon containing a material which is both electrooptic and nonlinear.
2. Theory The principle of operation of this device brings together the electro-optic and nonlinear optical bistability effects. The switching mechanism is still due to an optical nonlinearity coupled with feedback from the etalon mirrors producing bistability. The major difference in this case is that the optical input power remains constant. A voltage applied across the length of the cavity changes the initial detuning via the electro-optic effect and for a constant value of input 415
OPTICS COMMUNICATIONS
Volume 6 1, number 6
power can tune the cavity such that switch-up (or down) will occur. It is possible to predict the optical bistability characteristics of a Fabry-Perot etalon containing a nonlinear material by solving the equations describing the material and cavity responses. In this work the standard graphical construction [ 51 was extended to cover the case where both the electro-optic and nonlinear mechanisms are present. The transmission, T, of an etalon is related to the single pass phase shift, 6, via the oscillatory Airy function, T=AI(l SFsin’Q,
(1)
where A is a constant which depends on mirror reflectivities, absorption and thickness of the etalon while F is the coefficient of finesse. The phase 6 can be expressed as,
(2)
6=&+yzc+?jv,
15 March 1987
where do is the initial phase of the cavity; 6,, is the phase change induced by the light intensity in the cavity, Z,; and 6,, is the phase change induced by the effective nonlinear and electro-optic constants, y and q. Fig. 1a shows an example of the transmission of an absorbing etalon as a function of cavity intensity in normalised units. The relation between cavity intensity and transmitted intensity leads to the additional criterion, TlIc = ClIi)
represented in fig. la by straight lines whose slope depends on incident intensity, Zi. Introducing a change in phase due to applied voltage effectively alters the initial cavity tuning. The condition for an incident intensity yZi=2n is shown in fig. la for a series of values of 6,, = 0 to rt (dn/d I/ assumed positive). Multiple crossing occurs at around 6,,=0.25 II. Thus for a fixed incident intensity, increasing the voltage will cause the cavity to switch up from a to b
a
b
‘IV/”
Fig. 1. Computed characteristics for ~1,= 2x.
416
I
rb
05 INPUT
(3)
Fig. 2. Computed characteristics for yli = 3~.
Volume 6 1, number 6
OPTICS COMMUNICATIONS
and down from c to d. This may be replotted as a function of voltage to give the bistability loop of fig. 1b. If dnld V is of opposite sign to dnldl then a clockwise loop is obtained (fig. 1c) . This construction also reveals that the higher the optical input power, the wider the voltage bistability loop, fig. 2a. In addition under certain initial tuning conditions, the optical output does not return to its original value on removal of the voltage. This is illustrated in figs. 2b and 2c. All of these predicted features have been verified experimentally and are reported below. Note too that a useful aspect of having electrical control of an otherwise all-optical nonlinear bistable device allows resetting of the device with an electrical input rather than by interrupting the beam.
3. Experimental details Fig. 3 shows the sample geometry. The Fabry-Perot etalon was formed by evaporating silver films of approximately 200 A thickness onto IT0 coated glass substrates. The silver films acted as partially reflecting mirrors and the IT0 as rugged transparent electrodes. Liquid crystal alignment layers of 100 8, thickness SO were subsequently evaporated at an angle of 30 8, to induce homogeneous alignment of the liquid crystal. Final steps of the construction consisted of clamping together the two substrates separated by spacers to give a cavity length of approximately 7 pm and capillary filling the etalon with the liquid crystal 3/5/7 PCH. This is a nematic
15 March 1987
liquid crystal mixture of positive dielectric anisotropy having n, and n, values of 1.49 and 1.61 respectively at 20 “C. The molecular structure and properties of this mixture have been documented by Bradshaw et al. [ 61. The sample parameters and molecular orientation were chosen to enable tuning of the etalon through several (3-4) fringes by the application of a bias voltage across the cavity. All measurements were made cw at a wavelength of 647 nm produced by an Innova 90-K krypton-ion laser. Fig. 4 is a diagram of the experimental arrangement. An acousto-optic modulator ramped the optical input power to the sample, two large area silicon photodiode detectors D 1 and D2 monitored the input and transmitted powers respectively and the halfwave plate and polariser allowed control of the polarisation of the input beam. Lens Ll was a x 5 microscope objective giving a spot size of 12 c ( l/e2 radius); lens L2 collected all of the transmitted light and weakly focussed it onto D2: The sample was mounted on an x-y-z translation stage which allowed accurate control over the focus and position of the beam on the sample. The orientation of the molecules was such that their long axis lay in the y-direction as defined in fig. 4. Two types of experiment were carried out. Firstly, optical bistability loops were recorded by taking the outputs from Dl and D2 directly onto a digital oscilloscope. Bistability characteristics were obtained with different ac bias voltages applied across the cavity. ACbias voltages of frequency 1 to 2 kHz appear dc to a liquid crystal and were used to avoid any sample degradation. Results were taken with the input polarisation both parallel and perpendicular to the molecular orientation. In the second set of experi-
i’
I.
-
Ll /\
I
I
I/
v
L2 --D2
+J+
sawa Fig. 3. Sample geometry.
Fig. 4. Experimental arrangement.
417
Volume 6 1. number 6
OPTICS COMMUNICATIONS
ments output characteristics of the etalon at a constant optical input power were recorded as a function of bias voltage. For these experiments the desired input power to the cavity was fixed by the variable attenuator. For experimental convenience the applied voltages in these experiments were dc and controlled either manually or by means of a ramp generator. The output power versus voltage characteristics were obtained for different values of optical input power.
15 March 1987
.t_11_7 25v
P-P
4. Results Figs. 5 and 6 show the nonlinear characteristics obtained with different bias voltages applied as described in section 3. The polarisation of the input beam was parallel to the long axis of the molecules and hence the effective low power refractive index at zero volts was n, = 1.6. The bias voltage changed the initial cavity detuning by changing the effective refractive index in the y-direction. This occurred through the reorientation of the molecules which tend to line up with their long axis parallel to the electric field. In theory the maximum change in refractive index achievable by this method is the birefringence n,-n,, which in this case was 0.12. In practice the molecules do not completely reorient and the maximum change is smaller than 0.12. Maximum reorientation occurred at < 10 V with intermediate changes occurring at lower voltages. Fig. 5 is a series
Fig. 5. Optical bistability loops obtained at different bias voltages. 1: 0 Volt, 2: 1.2vp-p, 3: 2.2v p-p.
418
4VP-P
7v P-P
*t--L!-10
INPUT
POWER
mW
Fig. 6. Bistability characteristics obtained by electrically tuning the cavity through two fringes.
of bistability loops obtained at different initial detunings by electrically tine tuning the cavity relative to the fringe. In this case increasing the bias voltage tuned the cavity closer to a transmission maximum, this being reflected in the loops moving to lower powers and becoming narrower. Fig. 6 is a complimentary set of characteristics achieved by coarsely tuning the cavity through two complete fringes. These results are consistent with dnld V and dnldZ being of the same sign. The unconventional characteristic of a decrease in output power with increasing power above the b&able region (fig. 6) was a feature consistently present in this work and may be due to the mirror material being metallic rather than dielectric.
Volume 6 1, number 6
OPTICS COMMUNICATIONS
Identical experiments carried out with the polarisation in the z-direction, (i.e. perpendicular to the long axis of the molecules), yielded the expected result of no change in the nonlinear characteristic with applied voltage. Fig. 7a shows the temporal profiles of the applied bias voltage and transmitted intensity obtained at constant optical input power. Fig. 7b is the companion characteristic of the transmitted intensity plotted against applied voltage. These demonstrate the behaviour of the optical output with applied voltage. Two anti-clockwise voltage bistable loops occur, the first at _ 1.4 V and the sec-
(a)
e 6
0.5-
z
15 March 1987
ond at N 2.2 V. These correspond to the cavity being electrically tuned to a point where, for a given optical input power, switch up (or down) can occur through the positive feedback mechanism brought about by the nonlinearity of the material. The two loops correspond to tuning through two fringes. A series of voltage characteristics obtained as a function of optical input power ar presented in fig. 8. The last characteristic was obtained by fixing the input power just below the switch up point on an optical bistability loop and manually ramping the voltage from a dc supply. In agreement with theory, these results show that an increase in input power results in the loops becoming wider and that under certain conditions, switch up will occur with no switch down on removal of the voltage (fig. 8~). Switch down in this case was achieved by interrupting the input beam.
E
5. Conclusions
Fig. 7. (a) Temporal profiles of the optical output and applied dc voltage at constant optical input. (b) Optical output versus applied dc voltage at constant optical input power.
This work has demonstrated the possibility of imposing digital electrical information on to an optical beam via a device consisting of a Fabry-Perot etalon containing a material which is both electro-optic and optically nonlinear. Regenerative switching with optically bistable memory was demonstrated with electrical input. This device requires no external feedback or change in the input power to produce the switching but has a minimum value of input power below which it will not operate. This minimum power level will be governed by the magnitude of the nonlinearity of the material and the cavity parameters. For experimental convenience and demonstration purposes the material used here was a nematic liquid crystal. The electro-optical and nonlinear effects in this material are a consequence of the reorientation of birefringent molecules, produced electrically in the first case and thermally in the second. The switching times are therefore long, being of the order of milliseconds (although the regenerative switching should lead to faster sweeping than for the purely electrically tunable filter). Faster response times may be accomplished by replacing the liquid crystal with thin layers of other materials such as ZnSe interference filters [ 71 or GaAs multiple quantum wells [ 81 in which low power all-optical bistability has been demonstrated. The liquid crystal device demon419
0.15-
15 March 1987
OPTICS COMMUNICATIONS
Volume 6 1, number 6
@)
0
1.0
0.5
DC VOLTAGE
1.5
vdtr
Fig. 8. Optical output versus applied dc voltage at an optical input power of (a) 7.9, (b) 8.3 and (c) 8.5 mW. strated here may be suitable for application binary electrically addressable spatial modulator.
as a
References
light
[ 1] W.J. Stewart, I. Bennion and M.J. Goodwin, Phil. Trans. R. Sot. Land. A 313 (1984) 401. [2] P.W. Smith, Opt. Eng. 19 (1980) 456.
[ 3 ] H.M. Gibbs, Optical bistability: controlling light with light Acknowledgements We thank K.J. Harrison and the RSRE liquid crystal fabrication unit for supplying the sample. 0
Controller,
London,
1986.
Her
Majesty’s
Stationary
Offlice,
(Acad. Press, N.Y., 1985). [4] Y.R. Shen, Phil. Trans. R. Sot. Lond. A 313 (1984) 327. [ 51J.H. Marburger and F.S. Felber, Phys. Rev. A 17 (1978) 335. [ 61 M.J. Bradshaw, J. Constant, D.G. McDonnel and E.P. Raynes, Molec. Cry& Liq. Cryst. 97 (1983) 177. [ 71 S.D. Smith, J.G.H. Mathew, M.R. Taghizadeh, A.C. Walker and B.S. Wherrett, Optics Comm. 51 (1984) 357. [ 8 J H.M. Gibbs, S.S. Tamg, J.L. Jewell, D.A. Weinberger, K. Tai, A.C. Gossard, S.L. McCall, A. Passner and W. Wiegmann, Appl. Phys. Lett. 41 (1982) 221.