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A tunable polarization diversity silicon waveguide based optical filter was demonstrated. With the polarization diversity scheme, less than 0.5dB polarization dependent loss of the silicon optical filter was achieved in the wavelength range from 1525nm to 1600nm. The insertion loss of the whole polarization diversity circuits is 6.3dB. The extinction ratio of the optical filter is more than 27dB.
©2011 Optical Society of America
1. Introduction
Silicon photonics received much attention in the recent years. Silicon waveguides have great potential as a platform for ultra-small photonic circuits [1–3]. However, silicon waveguide has large structural birefringence which causes polarization mode dispersion, polarization dependent loss, and polarization dependent wavelength characteristics. The polarization mode dispersion will affect devices application in high data rates. The difference in the effective polarization modes indices makes the filters’ resonance wavelengths different. The polarization dependent characteristics limit the application of silicon photonics devices.
To make a photonic circuit polarization independent, the simplest way is to use a square core waveguide. However, for high-index-contrast waveguides, such as silicon waveguide, fabrication errors of just a couple of nanometers are critical and result in birefringence. Another way is to implement polarization diversity scheme. The light with arbitrary polarization from the source will be split into orthogonal component by polarization splitter. By further rotating one of the components, a single polarization is achieved. The two paths may be operated in parallel with identical structures. Fukuda et al. designed and fabricated mode-coupling-based polarization splitters [4] and rotators [5] in silicon waveguides. Watts et al. in MIT published mode-evolution-based polarization splitters [6] and rotators [7]. Their designs were silicon nitride based. The waveguides had core indices of 2.2 and dimensions of 800nm(w) × 400nm(h) for TE mode waveguide and 400nm(w) × 800nm(h) for TM mode waveguide. A silicon nitride based integrated mode-evolution-based polarization splitter and rotator (PSR) was first reported in [8]. In 2006, a polarization-transparent add-drop filter was demonstrated with the mode-evolution-based polarization splitter-rotator (PSR) in silicon nitride waveguide [9]. In 2008, Romagnoli et al. in Pirelli demonstrated an integrated PSR in silicon channel waveguide with cross-section of 450nm × 220nm [10].
The mode-coupling-based splitter and rotator have the advantage of single layer of silicon waveguide. NTT applied the mode-coupling based polarization splitter and rotator in a wavelength filter and achieved improvement for 10Gbps data transmission [11]. The demonstration was a polarization transparent ring resonator based optical filter. The polarization dependent loss was 1.2dB at the peak wavelength. However, the polarization rotator has very tight fabrication tolerances. The mode-evolution-based splitter and rotator have larger wavelength operation window and larger fabrication tolerances.
In this paper, we present a polarization diversity ring resonator circuit for the application of a tunable optical fillter. The polarization diversity components are in 400nm × 200nm silicon waveguide. Rather than the mode-coupling based components in NTT’s demonstration, the key polarization diversity components are a mode-coupling-based splitter to split TE and TM modes, a polarization mode converter to convert the TM/TE mode between the horizontal waveguide and vertical waveguide, a hybrid polarization rotator to rotate the light between TM mode and TE mode. Experimental results are presented.
2. Designs the polarization diversity ring resonator circuit
We designed two configurations of the polarization diversity circuit as shown in Figs. 1(a) and 1(b), respectively. The functional component is a ring resonator. There are two types of silicon waveguides in the polarization diversity circuit. One is horizontal waveguide, with the dimension of 400nm (w) by 200nm (h). The other is vertical waveguide, with the dimension of 200nm (w) by 400nm (h). The horizontal waveguide is the main waveguide in the circuit, which makes the input waveguide, ring resonator, and output waveguide. The vertical waveguide is used between the polarization rotator and polarization mode converter.
The ring resonator in silicon waveguide has different wavelength response for TE and TM polarization modes. The light with arbitrary polarization from the source is split into orthogonal components by a polarization splitter. Generally, there are two kinds of polarization splitter in silicon waveguide. The mode coupling based polarization splitter is easier to fabricate than the mode-evolution based polarization splitter in the waveguide of 200nm by 400nm due to the wider gap between the two waveguides. Therefore, a mode-coupling based polarization splitter and a combiner were used in the circuit. We used the design by NTT in [4]. Basically, it is a directional coupler with horizontal waveguides. It takes longer distance for TE mode to fully couple into the neighbouring waveguide than TM mode. The input light consists of TE and TM modes. Therefore, in this splitter, the TM mode will couple into the neighbouring waveguide, while the TE mode will remain in the same waveguide.
After the splitter, the TE part of the light (TE1) propagates to the ring resonator. The TM part of the light (TM2) in the horizontal waveguide is rotated to TE mode (TE2) in the horizontal waveguide by using a hybrid ultra-small polarization rotator and a polarization mode converter. In configuration A (Fig. 1(a) and Fig. 2(a) ), the TM mode in horizontal waveguide is converted to TM mode in vertical waveguide, and then rotated to TE mode in horizontal waveguide. In configuration B (Fig. 1(b) and Fig. 2(b)), the TM mode in horizontal waveguide is rotated to TE mode in vertical waveguide, and then converted to TE mode in horizontal waveguide. Hence, a single polarization is achieved at the ring resonator. The dropped wavelengths pass the ring resonator. The TE1 is rotated to TM1 and combined with the TE2 at the combiner. The paths of the light components propagating in the circuit to the drop output are shown in Figs. 1(a) and 1(b). Figures 2(a) and 2(b) show the paths of the light components propagating in the circuit to the through output.
The polarization mode convertor [12] between horizontal waveguide and vertical waveguide is shown in Fig. 3 . It connects horizontal waveguide with vertical waveguide while it maintains light at its polarization status. The mode converter works for both TE and TM mode in bi-direction.
The polarization rotator rotates the polarization mode between vertical waveguide and horizontal waveguide. The hybrid polarization rotator (Fig. 4 ) has a main transition region with an asymmetrical cross-section of 400nm (w) by 400nm (h) with an etched area of 200nm (w) by 200nm (h). The length of the main rotation region is only 3.6 long. The main rotation region is similar to the design in [13]. The fabrication errors will affect the polarization rotation in the main rotation region and the dimension control would be a challenge. To improve the fabrication tolerance while maintaining the good polarization rotation efficiency, two tapering structures (taper 1 and taper 2) were used to connect the rotation region with the horizontal and vertical waveguides, which would adjust the rotation in the way of mode-evolution. The polarization rotator works for both TM to TE rotation and TE to TM rotation in bi-direction. Figure 5 shows the combined mode converter and rotator. In configuration A and configuration B, the combined mode converter and rotator is used in opposite directions.
3. Fabricate and characterization of the polarization diversity ring resonator circuit
The polarization diversity circuits were fabricated on SOI wafer. The bottom oxide layer (BOX SiO2) was 2μm thick. The top silicon was 400nm thick. The two-layer waveguide structure was processed by two-step dry etching. 2um top SiO2 cladding layer was deposited by High Density Plasma PECVD (HDP SiO2) in Applied Material PECVD chamber. The total waveguide was 3mm long. The waveguide was tapered to 180nm width at the both ends. The vertical waveguide is 400nm (w) by 200nm (h). The horizontal waveguide for the polarization diversity components is 200nm (w) by 400nm (h). While the horizontal waveguide for the ring resonator has a dimension of 500nm (w) by 200nm (h), which has smaller propagation loss than the horizontal waveguide of 200nm (w) by 400nm (h) due to better mode confinement.
The ring resonator in this circuit has a diameter of 20 µm. The gap between the ring and the straight waveguides is 0.2 µm. The coupling length at the ring and the straight waveguide coupling region is 1µm. Figure 6(a) shows the SEM picture of the ring resonator fabricated. A micro-heater (Fig. 6(b)) was fabricated on the top of the ring resonator to make the optical filter tunable. The micro-heater was made with TiN wire of 120nm thick and 1µm wide.
Figure 7 shows the polarization splitter. The waveguides are horizontal waveguides. The gap between the two waveguide in the coupling region is 480nm. The length of the coupling region is 10 µm. The polarization mode converter is shown in Fig. 8 . Figure 9 shows the transition region of the polarization rotator. The polarization extinction ratio (PER) of the polarization splitter is about 15dB. The PER of the polarization rotator is more than 15dB and the PER of the mode converter is more than 17dB.
A broadband ASE source was used to characterize the circuit. It covered the wavelength range from 1520nm to 1610nm. Polarization maintenance fibers were used in connection. The polarization status of the light source was adjusted using a polarization controller. The output power after the polarization controller was −1dBm. The power fluctuation caused by changing the polarization status was less than 0.1dB. Lensed fibers were used to couple the light between the waveguide and optical fibers.
We compared the insertion loss of a 3mm-long straight waveguide with partial-etched surface and a waveguide with the same design without partial-etched surface and found that there was no obvious difference in insertion loss between the two waveguides for TE mode. The coupling loss plus propagation loss in the 3mm-long waveguide for TE mode was 4dB. However, the 3mm-long partial etched straight waveguide showed about 0.2dB higher loss for TM mode than TE mode, which may be due to the partial etched top surface.
We measured an individual ring resonator as a reference. Figure 10 shows the spectrum at the drop output of the individual ring resonator without polarization diversity circuit launched with both TE and TM modes in the waveguide. Since TE and TM resonate at different wavelengths and have different free space range (FSR) in the ring resonator, this ring resonator based filter doesn’t work properly if the polarization status of the input light is not well controlled. However, in the real optical system, it is difficult to predict the polarization status of the incoming light. The polarization transparent circuit becomes necessary.
Fig. 10 Spectra of the ring resonator at TE mode and TM mode without polarization diversity circuit.
The polarization diversity circuit with the ring resonator worked at TE mode. To characterize the polarization dependency of the filter circuit, the polarization status of the input light was adjusted by the polarization controller to be TE and TM linear polarization, respectively, and the optical spectra at the drop output were recorded with TE and TM linear polarization inputs, respectively. Both the polarization diversity circuits of Configuration A and Configuration B were measured. The circuits had the same wavelength response in the range from 1525nm to 1600nm regardless of the input polarization status. The FSR is about 12nm. Figure 11 shows the spectra of the ring resonator with polarization diversity circuit of Configuration A measured with TE input and TM input, respectively. The spectra have uniform peak transmittances and extinction ratios from 1525nm to 1600nm. The polarization dependent loss is less than 0.5dB at the peak wavelengths. The extinction ratio of the filter is more than 24dB in the range of 1525nm to 1600nm (Fig. 11 (a)). Figure 11(b) shows the resonance peak at 1544.7nm wavelength. The total insertion loss of the circuit (including coupling loss and propagation loss) is 13.3dB at 1544.7nm. After removing the 4dB loss from coupling and propagation in the 3mm-long straight waveguide, the propagation loss of the polarization diversity circuit at the peak wavelength is 9.3dB. The 3dB bandwidth of the resonance peak at 1544.7nm is 0.3nm. The 10dB bandwidth is 0.7nm and the 20dB bandwidth is 2.1nm.
Figure 12 shows the spectra of the ring resonator with polarization diversity circuit of Configuration B measured with TE input and TM input, respectively. The polarization dependent losses at the peak wavelengths are less than 0.5dB in the wavelength range from 1525nm to 1610nm. With polarization diversity circuit, the extinction ratio of the filter is more than 27dB in the range of 1525nm to 1600nm (Fig. 12 (a)). Figure 12(b) shows the resonance peak at 1545.2nm wavelength. The total insertion loss of the circuit (including coupling loss and propagation loss) is about 10.3 dB at 1545.2nm. After removing the 4dB loss from coupling and propagation in the 3mm-long straight waveguide, the propagation loss of the polarization diversity circuit at the peak wavelength is about 6.3dB. The 3dB bandwidth is 0.32nm. The 10dB bandwidth is 0.68nm and the 20dB bandwidth is 2.0nm.
The results show that the both polarization diversity circuits of configuration A and B solve the polarization dependency in the wavelength range from 1525nm to 1600nm. However, the polarization diversity circuit of Configuration B has about 3dB less insertion loss and 3dB higher extinction ratio than the circuit of Configuration A, which is due to the 3dB higher insertion loss of the combined converter and rotator used as in configuration A than that used as in configuration B. The difference in insertion loss may be caused by the imperfect surface of the two-layer waveguide structure.
We observed that the ER of the polarization diversity ring resonator was higher than the PER of the individual polarization diversity components. The PER of the components should affect the extinction ratio (ER) of the filter since TE and TM modes resonate at different wavelength peaks in the ring resonator. However, the PER of the circuit can be enhanced by the polarization dependent loss (PDL) of the combined polarization rotator and mode converter. The insertion loss of the combined polarization rotator and mode converter is less than 2dB in Configuration B. The combined polarization rotator and mode converter has 3dB higher insertion loss in Configuration A than in Configuration B. Besides, the TM mode experience higher loss than TE mode in the ring resonator which may be due to the partial etched top surface. This helps to further suppress the minor TM mode in the ring resonator and improve the ER of the filter. The overall extinction ratio of the polarization diversity filter is achieved to be more than 27dB in Configuration B. We also observed that optical filters with broader pass band in the polarization diversity circuit will have less polarization dependent loss.
By injecting current to the micro-heater, the resonance peak of the ring resonator will shift toward longer wavelength. With 15mA current, the peak resonance wavelength shifted 11nm. The tuning range covers the FSR.
4. Conclusions
A tunable polarization transparent optical filter in silicon waveguide was demonstrated. Less than 0.5dB polarization dependent loss was achieved in the wavelength range from 1525nm to 1600nm. The insertion loss of the polarization diversity circuit is 6.3dB at the peak wavelength. The extinction ratio of the circuit is more than 27dB. The tuning range covers the FSR. The silicon photonics tunable filter can be used in wavelength tunable WDM network. With the polarization diversity scheme, silicon photonics based optical circuit can be connected with fiber optics and will have more applications.
References and links
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