We describe an ultrasmall polarization splitter based on a simple directional coupler consisting of silicon wire waveguides. The size is only 7×16 µm 2, and the polarization extinction ratio is about 15 dB for a single coupler. A double-coupler structure improves the extinction ratio to over 20 dB. The excess loss is smaller than 0.5 dB for both types of device. In the device, the shape of the high-speed waveform is retained at any angle of polarization. Our polarization splitter represents a first step towards accomplishing an ultrasmall optical circuit with polarization diversity based on silicon wire waveguides.
©2006 Optical Society of America
Silicon wire waveguides have great potential for ultra-small optical circuits [1–3]. Their bending radius with negligible loss is as small as several microns because of their large refractive index contrast. Several kinds of passive and active devices based on Si wire have been demonstrated [4–6]. However, polarization mode dispersion (PMD) caused by structural birefringence is not negligible. This drawback narrowly limits their application range. Core-shape deviation from square and/or anisotropy of the materials increases the PMD, which degrades the wave-forms of high-speed signals and deteriorates the characteristics of functional optical devices inclusive of wavelength filter.
A solution to this problem is the pursuance of an accurate fabrication process. For instance, for waveguide length of 5 cm, the differential group delay (DGD) between cores with 300±5-nm width reaches 6.6 ps, which degrades high-speed signals, such as that with the data rates of 40 Gbps. Then again, the fluctuation of core width varies the group index and changes the operation wavelength of filters. The difference of the resonant wavelength between transverse electric (TE) mode and transverse magnetic (TM) mode is larger than 100 GHz for 10-µm-radius ring resonator with 300±1-nm-wide core. Therefore, accuracy of under a nanometer is required for devices used in polarization-independent dense wavelength division multiplexing systems.
Another solution is the use of polarization diversity system consisting of polarization splitters and rotators. If TE and TM components are separated by the splitter and the TM component is rotated 90 degrees by the rotator, we need to fabricate functional devices, such as a filter, for the TE mode only, not for both modes. Several key devices to accomplish the polarization diversity have been presented. D. Taillaert et al. developed a two-dimensional grating coupler, which couples orthogonal modes from a fiber into identical modes of two waveguides . This device is efficient for polarization splitting although the coupling efficiency is as small as approximately -7 dB. An MIT group demonstrated a polarization splitter and rotator with an asymmetric core cross section for SiN waveguides [8, 9]. However, the fabrication of such three-dimensional structures is difficult and would still be more demanding for Si wire because its core is smaller than the SiN waveguide. Polarization splitting has also been achieved by making use of structural birefringence. Devices based on this technique have been built using InGaAsP-InP waveguides  and Si rib waveguides .
In this paper, we present an ultrasmall polarization splitter based on directional coupler that consists of just simple Si wire waveguides with ordinary rectangular cores.
2. Principle and design
A schematic diagram of our polarization splitter is shown in Fig. 1. A Si wire with an oblong core produces large PMD due to structural birefringence. There is large difference between propagation constants for the TE and the TM modes. In a directional coupler fabricated using such Si wire waveguides, the coupling length for the TM mode is much shorter than for TE as shown in Fig. 1. Thus, just a simple directional coupler works as a polarization splitter.
Figure 2 shows electrical field intensities in a Si-wire-based directional coupler calculated by finite difference time domain (FDTD) method for several wavelengths. In the FDTD calculation, Si wire core was 200 nm (H)×400 nm (W) and the gap between two cores was 480 nm. The electric fields of the TM mode are transferred to the adjoining waveguide after about 10 µm propagation. The coupling length for short wavelength is longer than for long wavelength. This is reflected in the fact that the confinement is higher than for short wavelength. As a result, light with short wavelength transfers to the neighboring waveguide later than that with long wavelength. On the other hand, for any wavelength, TE light still propagates along the initial waveguide while the propagation length is shorter than 15 µm. Therefore, the directional coupler with coupling length of 10~15 µm functions to split TE and TM lights in the C-band wavelength range.
For a practical design, we have to pay attention to the bending radius around the directional coupler. In the case of a rectangular core 200-nm high and 400-nm wide, the bending radius without bending loss for light with TE and that with TM mode is quite different: the former can propagate through a sharp bend but the latter can not. Although light with either polarization modes can propagate through slight bends without loss, sharp bends are better for high-density integration. Therefore, we designed one waveguide (for TE) to bend sharply and the other (for TM) to be straight along the directional coupler.
Figure 3 shows the transmittance of the cross port simulated by eigenmode expansion (EME) calculation for TE and TM modes as a function of Si core width and coupler gap. Even when the core widened from 400 to 420 nm and gap narrowed from 480 to 440 nm, the changes of the transmittance are smaller than 0.1 dB and 0.15 dB for TE and TM modes, respectively. Consequently, our polarization splitter has great tolerance against fabrication error.
3.1. Sample preparation
An SEM image of a fabricated device is shown in Fig. 4. Devices were fabricated on SOI wafer with a 200-nm-thick Si layer and a 3-µm-thick SiO2 buried layer. The Si wire waveguides are 400-nm wide and the gap between waveguides is 480 nm. The coupling length and the bending radius of the directional coupler are 10 µm and 3 µm, respectively.
For efficient coupling between the Si wire and external fiber, we made spot-size converters (SSCs) at the ends of the Si wires . Each SSC has a Si reverse taper with an 80-nm-wide tip, a 3-µm-square silicon-rich silicon oxide core, and a 7-µm-thick SiO2 overcladding. The propagation loss was measured by the cutback method and found to be 2.2 dB/cm for the TE mode and 1.7 dB/cm for the TM mode. Coupling loss was 0.5 dB/point for a small-core fiber with a diameter of 4.3 µm for both polarizations. Si wires and SSCs were fabricated by electron beam lithography and electron cyclotron resonance plasma etching to form smooth surfaces. Details are described in Ref. .
3.2. Polarization extinction ratio
Light from broadband light source was filtered through polarizer, and linear polarized light was coupled into a polarization maintaining fiber (PMF). At the coupling between the fiber and SSC of Si wire, the angle of polarization of incident light was measured by a near-infrared CCD camera through a rotatable Glan-Thompson prism and set to TE or TM mode by rotating the fiber. Output light was coupled into a fiber through an SSC and measured by an optical spectrum analyzer.
Transmission spectra of bar and cross ports for input light with TE and TM modes are shown in Fig. 5. The transmittance is defined as the output power ratio between the polarization splitter and a simple Si wire waveguide with same length as splitter. The light with TM mode can pass across the directional coupler and be output at the cross port. On the other hand, the light with TE mode propagates along the initial waveguide and appears at the bar port. Measured extinction ratios of the cross and bar ports in the C-band wavelength range are 10 and 13 dB, respectively. The excess loss for the pass light is less than 0.5 dB for each port. The unlevel structure in the spectra of the stop light is caused by slight misalightment of the rotation angle of the input fiber. In this experiment, the length between the end of SSC and the start of the coupler is 250 µm. This section is the normal Si wire with a 400-nm×200-nm core. Thus, if the polarization plane of incident light is tilted toward the substrate, ripples appear in the transmission spectrum. The wavelength interval of ripples Δλ can be calculated as
where λ is operating wavelength, nTE the group index of the TE mode, nTM the group index of the TM mode, and L waveguide length. For a Si wire with a 400-nm×200-nm core, nTE and nTM are 4.2 and 2.8, respectively. Calculated Δλ is 6.9 nm, which agrees with the value in Fig. 5.
The spectra simulated by EME are also plotted in Fig. 5. For the cross port, the measured TM spectrum exhibits nearly ideal performance. On the other hand, the measured extinction ratio of TE mode is about 5 dB smaller than the simulated one. This difference is due entirely to the shift of the operating wavelength because of fabrication error. For the bar port, the measured transmission level of TM is smaller than the simulated level. This would be caused by bending loss in the TM mode, which is much larger than for TE. The residual TM component in the bar port is eliminated around the sharp bend at the end of coupler. Therefore, the experimental extinction ratio becomes larger than the simulated one. The insertion loss in the TE mode is slightly large compared with the simulation result. There are two possible origins of this excess loss. One is a bending loss of TE light around the sharp bend, and the other is a bending loss of TM light converted from TE in the sharp bend. It is difficult to separate these in both experiment and simulation.
These measured extinction ratios are probably limited by the experimental setup because the ideal extinction ratio for polarization error of ±5 degrees is -21.2 dB. To polish up the experiment, we measured the extinction ratios for a fixed wavelength (1552.52 nm) by rotating the half waveplate installed in front of the input fiber. The PMF was replaced with a normal single-mode fiber. The measured results for the cross and bar ports as a function of the rotation angle of the waveplate are shown in Fig. 6. The maximum extinction ratios are found to be 25 and 13 dB for bar and cross ports, respectively. The larger value for the bar port is reflected by the effect of bending loss in the TM mode discussed above.
To improve the performance, we also fabricated a polarization splitter with the two-stage directional coupler depicted in Fig. 7(a). The measured transmission spectrum for this device is shown in Fig. 7(b) along with that for single-stage one. The extinction ratio rises from 10 to 15 dB for the cross port. We also measured the extinction ratio in the same manner as above. The measured results for the cross port as a function of the rotation angle of the waveplate are shown in Fig. 8. The maximum extinction ratios for both ports are found to be 23 dB, which is close to the value for commercial products.
3.3. Response for high-speed signals
In general, PMD degrades the waveform of high-speed signals in intensity-modulation/ direct-detection (IM/DD) systems. In the case of 200-nm high and 400-nm wide Si wire core, the group indices of TE and TM mode are 4.2 and 2.8, respectively. Therefore, the TE component is delayed compared to TM. The Si wire with a length of only 1 cm produces differential group delay of about 50 ps, which is quite serious for IM/DD systems with data rates exceeding 10 Gbps.
Our device can split two orthogonal polarization components of high-speed signals without waveform degradation. We measured the high-speed signal response of our device. The experimental setup is shown in Fig. 9. Light with the wavelength of 1535.0 nm from a tunable laser diode was modulated by a lithium-niobate intensity modulator with the data rate of 10 Gbps (NRZ, PRBS 231-1, Mark ratio 0.5). It was then coupled into a waveplate which controlled the angle of polarization. Separating the waveforms between TE and TM mode by over a half of bit requires at least 50-ps delay time, which corresponds to 1-cm-long Si wire with core of 200×400 nm2. In this experiment, in order to make up the deficit in the delay, a 30-m-long PMF was inserted between the waveplate and our device, because the fabricated polarization splitter was equipped with only a 2-mm-long Si wire for run-up. The light from the PMF was launched into normal Si wire waveguide or the two-stage polarization splitter. The slow axis of the PMF was set to be same direction as that of the Si wire. The output waveforms from Si wires with/without the polarization splitter were measured with an O/E-converter and sampling oscilloscope.
The measured eye-diagrams for several conditions of polarization, which is defined at the entrance of the PMF, are shown in Fig. 10. Without the polarization splitter, eye-diagrams are deformed around TE/TM ratio of one. On the other hand, the waveforms transmitted through the polarization splitter retain their shapes, except for the amplitude. These results indicate that our polarization splitter is already practical for high-speed data transmission.
We have demonstrated an ultra-small polarization splitter consisting of simple silicon wire waveguides. The polarization extinction ratio reaches 23 dB for a two-stage coupler configuration. The excess loss is smaller than 0.5 dB. The efficient wavelength range covers the C-band. The shape of a 10-Gbps waveform is retained at any angle of polarization. Our device requires no complex fabrication steps such as three-dimensional structure formation. This technology is a significant step towards the development of a polarization-independent ultrasmall optical circuit based on Si wire waveguides.
This work is partly supported by the SCOPE program of the Ministry of Internal Affairs and Communications, Japan.
References and links
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