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A ridge waveguide technology exhibiting high polarization dependency is developed for new efficient multi-section passive polarization rotator applications. In the presented configuration, the calculated mode coupling between the waveguide sections is very efficient and allows a polarization rotation with a high extinction ratio at λ=1,55 µm. Experimental results show efficient polarization rotation with low cross-talk levels (-16dB) and no significant excess losses between sections. However, the overall transmission efficiency is limited by propagation losses and coupling losses to standard optical fibers.
©2007 Optical Society of America
1. Introduction
Birefringent materials/structures such as LiNbO3 or InGaAs are generally used in integrated optical devices of integrated phase and intensity modulators or in integrated laser sources. High-Index-Contrast (HIC) dielectric waveguides are intensively used to realize highly integrated devices. However, as the refractive index contrast is increased, the difference between the propagation properties of transverse electric (TE) and transverse magnetic (TM) modes becomes more pronounced, causing detrimental polarization effects such as polarization dependent loss. As a consequence, many integrated optical components are sensitive to the polarization state of light. A lot of research has been performed to reduce these effects but it remains very difficult to solve all polarization problems. As a result, integrated components able to manipulate or convert the optical polarization are of a great interest, and recent progress in e-beam lithography gives an opportunity to realize such components [1]. In this paper, we will present a passive polarization rotator for mode conversions between TE and TM modes. In the last few years, many integrated polarization rotators have been proposed. These are based on mode coupling due to longitudinally periodically modified structures [2–3], bend structures [4] or single section waveguides with one vertical wall and one slanted wall [5]. In principle, these structures allows a complete power transfer between TE and TM propagative modes, but modes must be phase-matched and precisely tuned to the structure length, resulting in wavelength sensitive devices. Recently a new polarization rotator based on mode evolution has been simulated and realized using SOI technology [6]. In contrast with the structures based on mode coupling, the length and wavelength dependencies are insignificant. But several steps of high precision e-beam lithography are necessary to realize them, as they involve features as small as 55 nm.
In this paper, we propose a polarization converter based on mode coupling and composed of longitudinal periodic perturbations. The waveguide is composed by a very thin Ta2O5 stripe deposited by Dual Ion Beam Sputtering (DIBS) on a thermally oxidized Silicon wafer and covered by a silica sol-gel thin film [7–8]. A similar structure has already been proposed using HIC technology but the poor mode matching between two successive waveguide sections induced high losses which decreased the conversion efficiency (extinction ratio is only about -7dB) [9]. On the contrary, by using the technology described in this paper, mode profiles could be easily adjusted between the alternative sections. In that case, the very low excess losses between asymmetric waveguide sections (-0,005 to -0,007dB losses between sections) allow high polarization conversion efficiency with low extinction ratio. The polarization converter has been fabricated and the experimental performances show an efficient conversion of the polarisation state with a high extinction ratio (-16dB), but total insertion losses (coupling with fiber, propagation losses) are strong and reduce significantly the power transmission efficiency.
2. Principle of the polarization rotator
To obtain an efficient polarization rotator, we propose a structure based on mode coupling and a periodic alternating of left shifted and right shifted waveguides sections of period 2Lb as described in Fig. 1 and Fig. 3. A silicon wafer covered by 8 µm of thermally-grown silica is used as the optical bench. The waveguide core is composed of pairs of oppositely shifted sections made of two Ta2O5 layers deposited by DIBS. The polarization rotation properties depend on the value of the shift ec. The parameter is the width of Ta2O5 waveguides and is the total thickness of the asymmetric core. The component input and output are composed of a rectangular Ta2O5 ridge waveguide. Two 2 µm thick SiO2 layers made of organic-inorganic low loss thin films of refractive index respectively equal to 1.495 and 1.442 are deposited on top of the core. The physical principle has been extensively described in the case of InGa/InGaAS structures in Ref [9]. The asymmetry of the cross sections of the waveguide sections produces a rotation of the optical axis φ around the propagation direction. To achieve phase matching between TE and TM modes, the length Lb of the waveguide sections is chosen equal to beat-length between the modes defined as the ration between the wavelength and the difference between the effectives indices of TE00 and TM00 modes (Eq. 1).
with neff TE00 the effective index of TE00 mode and neff TM00 the effective index of TM00 mode.
With this condition, if an x-polarized signal is launched in the left shifted waveguide section, the output polarization state at the end of this section is rotated by an angle of +2φ. At the end of the right shifted section the output polarization state is rotated by an angle of -4φ (Fig. 2). The total polarization rotation θ after n alternated left shifted and right shifted section is given by Eq. (2).
3 Theoretical performances and fabrication tolerances
The goal is to obtain a high polarization extinction ratio and low propagation losses. These losses have two origins: coupling losses between the successive sections, absorption and scattering losses due to the material and structure imperfections. In this section we will concentrate on coupling losses between asymmetric waveguide sections. An optimal design has been defined by using the Field Mode Matching (FMM) method [10] (for the modal analysis) and the Scattering Matrix method [11] (to calculate the component transmittance as a function of the polarization state of light). The optimum values of the asymmetric waveguide section parameters are: for the Ta2O5 ridge a thickness and a width , a lateral shift ec=0,5µm and a section length L b=31.5 µm. Figure 4 shows that the electromagnetic fields and effective indices of TE00 and TM00 modes in the right and left shifted sections are very similar, which enables efficient mode coupling between left and right shifted sections: the coupling losses between “left” and “right” sections are of only - 0,007dB and -0,005dB respectively in TE and TM polarizations. This is the main advantage of the proposed architecture, and we will discuss later the influence of “technical” losses due to structure imperfections.
Fig. 4. Near field intensity profiles and effective indices of TE00 and TM00 modes in “right” and “left” shifted sections (computation performed using the Field Mode Matching method).
The mode power confined in the Ta2O5 part of the waveguides is low (11% and 2% respectively for TE00 and TM00 modes). On the one hand, it is necessary to keep an efficient coupling between the “left” and “right” waveguide sections to minimize losses, on the other hand, in this configuration, guided modes are not very sensitive to the asymmetry introduced by the shifted Ta2O5 core waveguide. As a consequence the polarization state turn of only 2,24° with respect to the Cartesian coordinate system in a single “left” or “right” waveguide section. An alternation of “left” and “right” sections along the optical axis is thus absolutely necessary to accumulate polarization rotation and obtain a 90° rotation of polarization state. Figures 5(a) and 5(b) shows TE to TM and TM to TE power transfer as a function of the number of periods. By using 25 periods, 97 % of the mode power is transferred from TE00 to TM00 guided mode and, the extinction ratio of TE00 mode is as high as -30dB. The same performance is obtained for the TM00 to TE00 power transfer. The coupling losses between the input and the output rectangular waveguides of width and the polarization rotator core are of only 0,1dB. Tolerances for parameters have been computed to yield -10 dB extinction ratio and 90 % power transfer efficiency at λ=1,55 µm. The deviation that can be tolerated are ±0.1 µm for ec and , and ±2 nm for parameter .
Fig. 5. The TE to TM (a) and the TM to TE (b) power exchanges versus the number of periods in the polarization rotator.
4. Fabrication
A photolithography mask comprising 30 different polarization rotators has been designed. This mask is composed of 25 periods rotators with asymmetric sections lengths Lb ranging between 22 and 37 µm by step of 1 µm. After a first optical photolithography step, the oxidized silicon wafer is etched in a CF4/O2 RIE process specially tuned to give low surface roughness. The depth of optical substrate removed is 60±2nm along the core of the rotator (1.1 mm). Then a 60 nm thick Ta2O5 DIBS layer is deposited and a lift-off process allows forming a ridge waveguide buried in the SiO2 oxide [as shown in Fig. 6(a)]. A second lift-off step is then performed to obtain a pattern symmetric to the one realized in the first step with respect to the propagation axis [Fig. 6(b)]. This second pattern is aligned with the first pattern with a 0.1 µm tolerance. Two 2 µm thick SiO2 layers made of organic-inorganic low losses materials [12] of refractive index 1.495 and 1.442 are then deposited by spin-coating, at 25°C, on top of the Ta2O5 waveguide core. Cutting and specific side-polishing are then performed to obtain low roughness waveguide end-faces.
Fig. 6. Optical Nomarsky microscopy images of the waveguide patterns fabricated after a first step of photolithographic process (a) and after a second one (b).
5. Experimental results and discussions.
A wavelength tunable laser (1550–1565 nm) is used to inject TE or TM polarized light via a polarization maintaining (PM) fiber in the input waveguide of the polarization rotator. The output power is collected via another PM fiber and the TE and TM components are measured through an analyser by using a standard photodetector. Figure 7 shows the theoretical and experimental output powers in TE and TM polarizations versus wavelengths ranging between 1550 and 1565 nm in the case of TE polarized light launched in a converter having Lb=24,5µm. The maximal conversion efficiency of this polarization rotator is obtained at a working wavelength equal to 1558nm (about 90±10%; output power is -15±0.5dBm for TM polarization at λ=1558nm and -14.5±0.5dBm for TE polarization at λ=1570 nm). The crosstalk level between TE and TM polarization is better than -16±1,5 dB [see Fig. 7(a)]. The knowledge of the working wavelength and the period allows calculating the experimental effective index difference Δneff between the TE00 and TM00 modes thanks to Eq. (1). The experimental Δneff value is equal to 0,031±10-3. Fabrication errors result in an 8 nm wavelength shift between the experimental and the theoretically predicted working wavelengths (1550 nm), and in a +0.006 shift between the experimental and theoretical Δneff values.
The total length of the whole component equals to 1,5 cm. Total insertion losses are about -12±1.5 dB in TM polarization (with -6±1.5 dB coupling losses and -6dB propagation losses) and -15±1,9dB in TE polarization (with -8±1.5dB coupling losses and -7dB propagation losses). The coupling losses are high because no taper is used to have efficient coupling with PM fibers. The absorption in Ta2O5 and scattering losses along the waveguide result in the high propagation losses. Finally, Fig. 8 shows the insertion losses of the rotator compared to those of a reference rectangular waveguide. Excess losses introduced by the pairs of oppositely right and left shifted sections are found to be negligible. These results show that the proposed multi-section structure allows obtaining a good conversion efficiency and a high extinction ratio.
Fig. 7. Experimental TE to TM power conversion (a) and theoretical TE to TM power conversion (b) versus wavelength.
Fig. 8. Insertion losses of the rotator compared to those of a reference waveguide in TE and TM polarizations.
6. Conclusion
Integrated periodic passive polarization rotator using mode coupling has been fabricated. The TE to TM power conversion is obtained with -16±1, 5 dB extinction ratio at λ=1.558 µm for a converter composed of fifty 24.5 µm long asymmetric waveguide sections. The periodic structure introduces no coupling losses as opposed with other periodic structures proposed in the literature [9]. Experimentally, we verified that the excess propagation losses due to the periodic structure are negligible. However, the propagation losses in the highly birefringent waveguides and their coupling losses with PM fibers are still very high. A taper structure allowing a better coupling with fibers is under study and further experiments are under process to reduce propagation losses. These preliminary results indicate that this kind of polarization rotator, associated with a polarization beam splitter could be hybridized with LiNbO3 active waveguide to obtain polarization insensitive electro-optic and nonlinear components.
References and links
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