1. Introduction

Polarization control of the light signal transmitted through an optical fiber is an essential process for adjusting the time-varying polarization states of the light. Whenever a single-mode optical fiber is used to transmit a light signal, polarization controllers must be inserted for various applications such as coherent optical communications [1], optical coherence tomography [2], fiber-optic sensors [3], and optical polarization scramblers [4].

Most widely adopted polarization controllers are based on rotating optical fibers with a certain amount of strain-induced optical birefringence [5]. Electro-optic wave plates in lithium niobate were investigated to increase the speed of polarization control [6]. Moreover, an integrated-optical polarization controller having liquid crystal thin layers inserted in the middle of the waveguide was demonstrated [7].

In this work, we propose an integrated-optic polarization controller based on polymeric waveguide technology, which has drawn considerable attention through the demonstration of various functional devices [8–10] and has provided essential components for wavelength-division multiplexing in optical communications [11]. By virtue of the design flexibility of organic materials, a novel polymer material with high birefringence has been synthesized and utilized for demonstrating birefringence modulators (BMs) in which the difference in phase retardation for TE and TM guided modes could be controlled through the thermo-optic effect [12]. The polymer waveguide polarization controllers consist of three BM sections and 45°-inclined quarter-wave plates (QWPs) inserted between them. By applying combinations of control signals to the BMs, general polarization conversion covering the entire surface of the Poincaré sphere is demonstrated.

2. Device configuration and operating principle

The thermo-optic effect in polymer waveguide devices has the great advantage of large refractive index tuning at a small heating power owing to the significant thermo-optic (TO) coefficient and low thermal conductivity. The heat-induced refractive index change is an isotropic effect with no dependence on the optical polarization. However, when the polymer material is coated on a rigid substrate, a strain-induced birefringence is imposed on the polymer film as it shrinks in the curing process. The birefringence decreases at higher temperatures because of strain relaxation. In our previous experiment, a micro-heater was placed on top of a polymeric straight waveguide in order to demonstrate BMs that control the phase retardation between the TE and TM polarizations [12].

The polymer waveguide BM is used in this work to demonstrate general polarization controllers. As shown in Fig. 1 , the polarization controller consists of three BM sections and two QWPs whose optic axis is inclined 45° to the substrate. Depending on the applied heating power, the BM produces a certain amount of phase retardation between the TE and TMpolarizations along the fast and slow axes. Depending on the phase retardation imposed on each BM, the resultant polarization conversion could be calculated by using the Jones matrix, and the results confirmed that the cascading structure of three BMs and two QWPs is sufficient for producing arbitrary output polarization states.

 

Fig. 1 Schematic diagram of polarization controllers consisting of polymer waveguide birefringence modulators and thin-film quarter-wave plates.

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The highly birefringent polymer prepared for the core layer of the polymer waveguide had a birefringence of 0.0074 at 1550 nm for the TE and TM polarizations. The large birefringence makes it hard to satisfy the single-mode condition for both polarizations because the index contrast with the cladding material becomes significantly different for the two polarizations. An oversized rib waveguide structure was designed in order to obtain the single-mode condition for both polarizations simultaneously [13].

3. Fabrication procedures

The proposed polarization controllers were fabricated by following well-established polymer waveguide device fabrication procedures, as illustrated in Fig. 2 . A straight waveguide was formed on a silicon wafer with two kinds of UV-curable polymer materials (ZPUs) available from ChemOptics Co. in Korea. The cladding polymer had a refractive index of 1.480, whereas the core polymer had indices of 1.4989 and 1.4915 for TE and TM polarization, respectively. The core layer was spin-coated to a thickness of 5 μm; it was then etched by 2.5 μm in oxygen plasma, resulting in a lateral cladding thickness of 2.5 μm. Three BM sections were defined by heating electrodes made of Cr/Au metal with a length of 5 mm. Each heater exhibited a resistance of 202 Ω.

 

Fig. 2 Schematic of fabrication procedures for general polarization controllers consisting of polymer waveguide and thin-film QWPs.

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In the middle of the BMs, two QWPs were inserted at an angle of 45° so as to introduce a λ/4 phase retardation between the two polarization components parallel and perpendicular to the optic axis of the QWP. Grooves for inserting the wave plates were formed by a dicing saw, which produced grooves 30 μm in width. SEM image of the diced groove and a photograph of fabricated chip with a length of 20 mm are shown in Fig. 3 .

 

Fig. 3 (a) SEM photograph of the groove formed by a dicing saw, and (b) photograph of the completed polarization controller chip.

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The QWP film was fabricated by using a reactive mesogen (RM) material with a birefringence of 0.137 at visible wavelengths [13]. The thinner film is the better for reducing the diffraction loss of the guided mode at the disconnected waveguide. A free-standing flexible RM film was prepared by using additional layers of polymer materials. Selective adhesion property of SU-8 was utilized to lift off the film. Polyimide layer was used to produce an anchoring force for aligning the RM liquid crystal along the rubbing direction. ZPU polymer from ChemOptics Co. with a low optical loss was spin-coated to increase the total thickness of the film to 16.2 μm. During this process, strain was built up on the film by the shrinkage of the ZPU; this made it convenient to separate the film from the polyimide layer by applying a slight shear force. The birefringence of the RM film was measured by transmitting linearly polarized 1550 nm light through it. The ellipticity of the output light was measured and plotted as a function of the film thickness, as shown in Fig. 4 . The birefringence of RM film was proportional to the thickness. When the thickness of film was adjusted to 3.2 μm, the film produced the peak ellipticity with the phase retardation of λ/4 to be the QWP. The birefringence of RM at 1550 nm was calculated to be 0.121.

 

Fig. 4 Characteristics of the thin-film QWPs made of RM material; the ellipticity of the transmitted light was measured as a function of the film thickness.

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4. Characterization of the polymer waveguide polarization controllers

To measure the device performance, a 1550 nm DFB laser was connected to a PM fiber pigtailed polarizer in order to precisely define the input polarization. Before inserting the QWP film, to confirm the operation of the BM, 45°-inclined linear polarization was launched on the polymer waveguide by adjusting the angle of the PM fiber, and the output light was passed through an analyzer. The polarization modulation with respect to the applied heating power was clearly observed with an extinction ratio of more than 20 dB, as shown in Fig. 5 . A 1 Hz, 8 Vp-p electrical signal was used, and the measured heating power for π phase retardation was 73 mW (3.84 V for a 202 Ω electrode).

 

Fig. 5 Polarization modulation signal with respect to the applied voltage of the birefringence modulator, which was measured by a crossed polarizer-analyzer configuration.

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The output polarization state of the integrated optic polarization controller was monitored by a polarization analyzer. With the input polarization adjusted to TE, a fiber-optic polarization controller inserted in front of the polarization analyzer was adjusted to define the initial polarization state at a certain point on the Poincaré sphere. Because the first BM has no effect on the TE input polarization, the second and third BMs were controlled by electrical signals. By monitoring the periodic output polarization variation in response to the triangular AC signal applied to BM2, the DC voltage applied to BM3 was gradually increased. To demonstrate that the device could produce arbitrary polarization conversion, we tried to find the condition of input signals that would cover the entire surface of the Poincaré sphere. When a 5 Hz, 6.85 V AC signal was applied, the DC signal increased from 0.97 V to 3.3 V, and then the output polarization changed gradually, as shown in Fig. 6(a) . The polarization change produced circles on the sphere connecting the TE and TM states as a response to the AC signal applied to BM2. Then, as the DC signal to BM3 increased, the angle of the circle changed gradually, until eventually it covered the entire surface of the sphere. An additional experiment was performed by exchanging the AC and DC signal application (applying an AC signal to BM3 and a DC signal to BM2). Under these conditions, the polarization change shown in Fig. 6(b) occurred. The initial polarization was adjusted to the left-handed circular polarization (LHCP) point of the sphere by using the fiber-optic polarization controller. Then, the variation in the polarization state due to the AC signal produced a circle with the center fixed on the LHCP point. As the DC signal increased, the circle was shifted to higher latitudes, finally approaching the right-handed circular polarization (RHCP) point.

 

Fig. 6 Output polarization states of the polarization controller covering the entire surface of the Poincaré sphere depending on the applied AC and DC signals: (a) AC signal applied to BM2 and DC signal applied to BM3, and (b) DC signal applied to BM3 and AC signal applied to BM2. Multimedia file for Fig. 6(b) is uploaded as Media 1.

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When the AC modulation frequency was reduced to 1 Hz, the required voltage was reduced to about 3.84 V. However, the polarization change in response to the triangular signal did not show a symmetric response. The trajectory of the polarization change responding to the the rising and falling time of the triangular signal was different. . It may be caused by the visco-elastic effect of the polymer material, which is known to have a very slow response time. Hence, we increased the signal frequency to 5 Hz in order to avoid thermal expansion of the polymer film.

The insertion loss of the device was about 6 dB from fiber to fiber before the groove was formed. Additional loss of 2 dB was occurred after the grooving and insertion of the QWPs. The loss could be reduced to be less than 2 dB by incorporating state-of-the-art polymer waveguide fabrication technique.

5. Conclusion

An integrated-optic polarization controller consisting of thermo-optic birefringence modulators and thin-film wave plates was demonstrated. A highly birefringent polymer material was prepared for efficient birefringence modulation proportional to the applied heat. The wave plates were fabricated of a reactive mesogen liquid crystal material coated on a thin polymer substrate and inserted between the sections of the birefringence modulating polymer waveguide. By applying a 6.85 V (230 mW) triangular AC signal and a DC signal of up to 3.3 V on another birefringence modulator, it was possible to generate arbitrary polarization states covering the entire surface of the Poincaré sphere. A general polarization controller would be regarded as an important building block for integration with various polymeric optical waveguide devices.