Abstract
Keywords
1. Introduction
Photonic integrated circuits (PICs) fabricated in the silicon-on-insulator (SOI) platform are essential for massive applications in communications, military, and sensing. Due to its CMOS compatibility and ultra-high index contrast in the SOI platform, the PICs can be fabricated with low cost and compact footprint[
In this Letter, we design two novel PSRs, and one sample of them is fabricated by using electron-beam lithography (EBL). Both PSRs are dual etched in the through-port waveguide. The first level taper etching is used to meet the phase matching condition, for achieving the high-efficiency cross coupling between the mode in the etching region and the transverse electric () mode in the other waveguide. Then, after an S bend section, the etched width is gradually increased, and the full etching is formed. In this region, the residual mode will leak into the substrate. A reverse taper-etched structure is used at the end of the through port to restore the waveguide thickness to 220 nm. The cross-port waveguides of two PSRs are SWG and nanowire waveguide. One PSR is fabricated and tested, showing an ER of more than 20 or 28 dB over 1510–1580 nm for or modes. For a launched mode, the insertion loss (IL) is less than 0.6 dB within the 70 nm bandwidth. For a launched mode, the polarization conversion loss (PCL) is less than 1 or 3 dB within the bandwidth of 45 or 70 nm, respectively.
2. Design and Principle
The ultra-compact PSRs are designed based on the ADC structure. The top cladding of the PSR is specified as air to achieve a more compact footprint[
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Figure 1.Schematic diagrams of the two proposed PSRs. (a) PSR-1. (b) PSR-2. (c) Cross view.
For the two PSRs, is set as 0.628 µm. According to the fabrication process requirements, the etching depth is specified to be 70 nm. Since the cross-port waveguide should be phase matched with the through-port waveguide, the variation of the and effective refractive indices is investigated, with the etching width in the through-port waveguide. The effective refractive index of the mode is between 1.45 and 1.7. Meanwhile, the equivalent material refractive index of the SWG waveguide can be calculated by a simplified model[
Figure 2.Coupling efficiency of PSR-1 varies with the Lc.
For the nanowire waveguide in PSR-2, and are specified. With this condition, we sweep the length of Lc with different gaps, as shown in Fig. 3. The conversion loss reaches the minimum value when and .
Figure 3.Output powers from the through and cross ports vary with the length of Lc. (a) g = 100 nm, (b) g = 150 nm.
Figure 4 shows the conversion efficiency at the cross port and the through port of PSR-1 and PSR-2. Clearly, the ER is higher than 15 dB and 25 dB, and the loss is lower than 1 dB and 0.3 dB within the 130 nm and 20 nm bandwidth for PSR-1, respectively. For PSR-2, the obtained ER is higher than 15 dB, 20 dB, or 30 dB, while the loss is lower than 0.4 dB, 0.9 dB, or 1 dB within the bandwidth of 33 nm, 100 nm, or 150 nm, as shown in Figs. 4(c) and 4(d).
Figure 4.Conversion efficiency of PSR-1 at the (a) cross port and (b) through port; conversion efficiency of PSR-2 at the (c) cross port and (d) through port.
To reveal the fabrication tolerance of the two PSRs, we keep the center position constant and synchronously change the waveguide width. The results are shown in Fig. 5. PSR-1 and PSR-2 have or fabrication tolerance, respectively. Compared with PSR-2, PSR-1 based on the SWG waveguide has a larger fabrication tolerance in terms of PCL, but its errors on duty cycle will distort the performance. On the other hand, PSR-2 has better ER performance in a wider bandwidth.
Figure 5.Fabrication tolerance of (a) PSR-1 and (b) PSR-2.
3. Fabrication and Measurement
Based on the above analysis, we fabricated the PSR-2 within a SOI wafer, which has a 220 nm silicon layer and a 3 µm substrate. The EBL was used to define the patterns of grating couplers on the ZEP520A resist[
Figure 6.(a) SEM images of the reference waveguide and PSR. (b) Main structure of the PSR.
We designed two vertical coupling gratings for supporting the and modes, respectively. The period and filling factor of the grating coupler are designed as 620 nm and 45%, while 1050 nm and 47% are for the corresponding grating coupler. During the performance tests, several and reference waveguides are fabricated, and their transmission responses are shown in Fig. 7. It can be found that coupling losses of and grating couplers are 7.7 dB/port and 11.4 dB/port at the center wavelength, respectively.
Figure 7.Transmission response of the (a) TE0 and (b) TM0 vertical coupling gratings.
The experiment and simulation results for PSRs are shown in Fig. 8, after being calibrated with the reference waveguide. The IL of the mode in through port is less than 0.6 dB within the bandwidth of 1510–1580 nm, while the ER is higher than 20 dB. The conversion efficiency reaches the maximum of 96% at , with a PCL of 0.18 dB. In the 1535–1580 nm range, the PCL is less than 1 dB, and the ER is larger than 28 dB.
Figure 8.Transmission spectral responses: (a) TM0 mode launched and (b) TE0 mode launched.
In addition, a comparison among similar PSRs recently reported is listed in Table 1. It is clear that the fabricated PSR-2 has excellent comprehensive characteristics in compact footprint, PCL, ER, and bandwidth, particularly the highest ER of 28 dB within the 45 nm bandwidth.
Structure | Footprint (µm) | PCL (dB) | ER (dB) | Bandwidth (nm) | Tolerance (nm) |
---|---|---|---|---|---|
Sub-wavelength grating coupler and secondary filtering[ | 13 | 73 | |||
Sub-wavelength grating coupler[ | 35 | 0.3 | 10 | 50 | |
Discretized subwavelength nanostructure[ | 7.92 | 1 | 25 | 40 | |
Counter-tapered coupler[ | 170 | 0.8 | 18 | 60 | |
Mode-evolution and an asymmetric directional coupler[ | 170 | 1.5 | 10 | 40 | |
This work | 28 | 1 | 28 | 45 |
Table 1. Performance Comparison among PSRs (Fabricated Samples)
4. Conclusion
We have designed two novel PSRs based on the dual-etched and tapered ADC. The conversion is fulfilled by taper etching in the coupling region, and the residual mode is filtered out by the second stage etching. Using the EBL, PSR-2 is fabricated and tested. Experiment results show a maximum IL of 0.6 dB and a minimum ER of 20 dB within the 70 nm bandwidth for the mode. For conversion, the ER is higher than 28 dB over the 1510–1580 nm range, and the PCL is less than 1 dB within the 45 nm bandwidth.
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