Endless smectic A* liquid crystal polarization controller

Endless smectic A* liquid crystal polarization controller

1 August 2002 Optics Communications 209 (2002) 101–106 www.elsevier.com/locate/optcom Endless smectic A liquid crystal polarization controller Laur...

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1 August 2002

Optics Communications 209 (2002) 101–106 www.elsevier.com/locate/optcom

Endless smectic A liquid crystal polarization controller Laurent Dupont *, Thierry Sansoni, Jean-Louis de Bougrenet de la Tocnaye Ecole Nationale Sup erieure des T el ecommunications de Bretagne, Technop^ ole de Brest-Iroise, BP 832, 29285 Brest Cedex, France Received 5 February 2002; received in revised form 4 April 2002; accepted 1 May 2002

Abstract We propose and demonstrate an endless polarization controller that uses the electroclinic effect in the smectic A phase with homeotropic orientation. This kind of device has a great importance in optical telecommunications for polarization mode dispersion compensation, polarization multiplexing or to compensate polarization dependent losses. Its main characteristics are both high speed and endless control. To realize a liquid crystal waveplate with an arbitrary birefringence orientation, we use an in-plane rotating electric field. Ó 2002 Elsevier Science B.V. All rights reserved.

1. Introduction Polarization controllers are at present one of the most important devices in long haul optical telecommunication. Their potential applications are in polarization multiplexing and for both polarization dependent loss (PDL) and polarization mode dispersion (PMD) compensation. The main challenge is the compensation of strong PMD with a pure optical compensator module. First order PMD is currently one of the main limitations on the growth of time division multiplexing (TDM), in long haul optical fibre transmission. PMD results from natural or accidental mechanical stresses in the fibre, that induces optical birefringence and two different polarization group velocities, for the signal along two orthogonal states of polar-

*

Corresponding author. Fax: +33-2-98-00-10-25. E-mail address: [email protected] (L. Dupont).

ization. The difference between the arrival time of the two polarization states is called the differential group delay (DGD). Depending on the input polarization state, PMD results in pulse broadening leading to a varying degradation of the bit-error rate. To compensate PMD, a pure optical technique has been introduced, that uses an active polarization controller [1]. The resulting device should have quick response time, endless polarization control and should transform any polarization state to any each other [2]. We present a device using liquid crystals in a smectic A homeotropic alignment as the birefringent material.

2. Polarization controller The polarization controller should transform any polarization state to any other with endless control. This last property is obtained by the use of endless rotatable waveplate with fixed birefringence. We will focus on two possible configura-

0030-4018/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 ( 0 2 ) 0 1 5 6 0 - 2

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Fig. 1. Two possible configuration of polarization controller.

tions that allow endless control: two quarter wave plates or two quarter wave plates with half wave plate (see Fig. 1). With these controllers, a good birefringence accuracy of each wave plate is necessary (especially for the first configuration). Then the application of an analog electroclinic effect allows adaptative waveplates. Each cell is connected with optical fibre including lenses with a focal length well adapted to the glass thickness.

3. The device The polarization controller should have a short response time: a few ls and should have an endless control. For rapid response times, the birefringent material used is a smectic A liquid crystal (W415) of the Alkenyl family in homeotropic configuration and confined between two glass plates of 0.7 mm thickness. This material has a giant electroclinic effect at room temperature ScA phase: 23–

(a)

32 °C) and the induced tilt angle is 18° for 10 V=lm. The in-plane electric field is applied within liquid crystal and the electroclinic effect induces birefringence modulation (see Figs. 2(a) and (b)). The ITO electrode system consists of six electrodes which has the hexagonal geometry. This design of liquid crystal cell has been already studied with nematic liquid crystal [3]. On one glass plate, several groups of electrodes are etched on one glass plate: two opposite electrodes are spaced by d ¼ 15, 20, 25, 30 lm. The maximum voltage applied on the electrode is þ= 40 V. With six electrodes, that allows an electric field magnitude of 120 V=d within the liquid crystal. By applying phase shifted voltage on each electrode the amplitude and the orientation control electric field can be arbitrarily changed (see Fig. 2(b)). The homeotropic alignment is obtained by the use of polysiloxane (Techneglass). This is a polymer which is deposited by spin coating and cured at 200 °C for 1 h. The texture shows a good

(b)

Fig. 2. (a) Geometry of liquid crystal cell. (b) Applied voltage on electrodes.

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Fig. 3. Full view liquid crystal waveplate between crossed polarizers with star-like electrode design (50) and active area (400). The liquid crystal in the centre of the electrode is clearly in the homeotropic orientation.

homeotropic alignment on the glass substrate and in the active area. However some planar defects are observed near and on the ITO electrode and around the active area (see Fig. 3). A possible explanation of this effect, is the perpendicular electric field orientation near electrodes that induce a transition from homeotropic to planar orientation. The cell thickness of 18 lm is obtained by a joint of cement including 18 lm spacers. We used the best-aligned cell with 10 lm

spaced electrodes. For this cell, only one glass plate has electrodes. The liquid crystal cell is studied as a birefringent plate. A polarized He–Ne laser light (632 nm) passes through the liquid crystal cell and a polarizer. Light intensity is detected with a PIN photodiode. By rotating both the in-plane electric field direction and its amplitude, we controlled the polarization change and then the intensity change through the polarizer (see Fig. 4). With a uniform

Fig. 4. Optical set-up.

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Fig. 5. Typical optical response of the LC waveplate at 23 °C (10 Hz).

rotating frequency f, the signal detected has a frequency four times higher: 4f (see Fig. 5). The maximum of the optical signal is obtained when the neutral axes of the cell are oriented at 45° to the polarizer axis. The optical response gives both the induced birefringence and the response time. The distortions observed in the optical signal are strongly dependent on the position and the size of the spot. The planar liquid crystal optical response can be mixed to the homeotropic oriented liquid crystal optical response so, an high precision should be reached on the focal of fibres with lens.

4. Optical behavior We characterized the dynamics of electroclinic effect by two ways. First, by applying sweep frequency of rotating electric field, second, by applying an electric pulse within the liquid crystal. With the first method, the optical response exhibits relaxation curves. Near the phase transition temperature, an important nonlinearity behavior is observed due to the high-induced tilt angle [4]. The linear model of electroclinic effect cannot explain that the relaxation frequency depends of the electric field amplitude (see Figs. 6 and 7). Upon plotting relaxation frequency versus applied voltage in Fig. 7, we observe a quasi-linearity relation between both parameters. By applying a square pulse voltage, the electroclinic effect exhibits the same nonlinear behav-

Fig. 6. Optical signal amplitude versus rotating field frequency.

ior near to the phase transition. The divergence of the response times is also observed near the SA –SC transition. The Jones matrix of the rotatable birefringent plate is: ! expðiD/=2Þ 0 RðhÞ RðhÞ; 0 expðiD/=2Þ where D/ is the phase shift of the birefringent material and h is the angle between neutral axis of birefringent material and the input polarization. By measuring amplitude between parallel and crossed polarizer, it is straightforward to calculate the induced phase shift by the formula: rffiffiffiffiffi! I? 1 D/ ¼ 2 sin ; Ik

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5. Discussion

Fig. 7. Relaxation frequencies versus applied voltage. k where Imax is the intensity maximum with polarizer ? parallel to the input polarizations and Imax is the maximum of intensity with polarizer crossed with respect to the input polarizations. The phase shift magnitude rise continuously by increasing applied voltage. The phase shifts obtained at 24 °C are resumed in the following table:

Applied voltage (V)

Phase shift (p)

105 30 15

0.9 0.89 0.86

These values of phase shift are not enough for a polarization controller in near infrared range. Some solution are proposed in the nest discussion.

The wave plate should be quarter wave plate at k ¼ 1:55 lm to be used in the polarization controller. Experiments shows an induced birefringence phase shift lower than p at the HeNe wavelength that gives an effective birefringence about: Dn ¼ 0:03. This low value of birefringence (below typical values of smectic liquid crystals) is the proof of the strong decrease in electric field magnitude in the thickness of the plate. Straightforward calculation shows that this measured birefringence is too low to obtain the required phase shift in the near infrared range. Experimental measurements of intrinsic birefringence of this liquid crystal in a planar alignment are currently in progress. Moreover, the electric field cannot be increased because instabilities are induced within the smectic layers for high field values confirmed by observations. The simply way to enhance birefringence is to use star-like electrodes on each glass plate and then, to decrease the applied voltage on each electrode. Another possible solution is to use thick electrodes (about 20 lm) to induce an homogeneous electric field in the thickness and hence an high effective phase shift. The response time remains the most critical problem: At high temperatures, we observe a very

Fig. 8. Optical response times with square pulse voltage (140 V) and oscillogram of optical response at 27.1 °C.

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fast dynamic variation but the induced birefringence is too low to obtain a quarter waveplate. Near the transition, the dynamic slows down and optical parameters have a critical behavior versus temperature (see Fig. 8). Thermal control with a Pelletier module is then needed. However, the birefringence characteristics of the waveplate are good and well adaptative. Using the smectic C phase could be an interesting solution for a waveplate: the birefringence is high and can be controlled with temperature. Moreover, the waveplate characteristics remain in the absence of an applied electric field. The main challenge for this solution is to obtain a good and homogenous homeotropic alignment in this phase.

Acknowledgements The authors would like to thank Pr. Walba for providing us the electroclinic material and they acknowledge both Glaverbel Ltd. and Techneglass Ltd. for providing us thin ITO glass plate and polysiloxane delivery.

References [1] C. Francia, F. Bruyere, J.-P. Thiery, D. Penninckx, Electron. Lett. 35 (5) (1999) 414. [2] L. Dupont, J.L. de Bougrenet de la Tocnaye, M. Le Gall, D. Penninckx, Opt. Commun. 176 (2000) 113. [3] T. Chiba, Y. Ohtera, S. Kawakami, JLT 17 (5) (1999) 885. [4] S.-D Lee, J.S. Patel, Appl. Phys. Lett. 55 (2) (1989) 122.