Extremely efficient two-section polarization converter for InGaAsP-InP photonic integrated circuits

Summary form only given. Polarization converters are important components to manipulate polarization in photonic integrated circuits [1]. Several polarization converters were reported [2-6]. Their problem is tight fabrication tolerances to have efficiency of polarization conversion more than 95 %. The authors of [7] suggested a way to improve the fabrication tolerances using a two-section polarization converter. This paper shows the first fabrication and measurements of the two-section polarization converter which confirms the result predicted in [7]. Figure 1a shows the cross section of a one-section polarization converter. A deviation from the 1.24-μm top width reduces polarization conversion. If we introduce two trapezoidal sections with opposite slopes (Fig. 1b), we can Ind the lengths of the sections at which the deviations from the 1.24-μm width in the Irst section is compensated by the deviation from the 1.24-μm width in the second section. This brings the original 1.24 ± 0.05 μm top width to 1.24 ± 0.08 μm for conversion above 95 %. The Irst section should have a half length of the one-section converter and the second section should have a three-half length of the one-section converter [7]. Figure 1c shows the fabrication process of the converter. We described the fabrication in [6] and show only key steps here. Step 1 is masking the top width of the converter with SiNx. We use electron beam lithography and reactive ion etching. Step 2 is Br2-CH3OH wet etching of the slopes on both sides of the converters and waveguides. Step 3 is protection of the area where we need the slopes of the converter with the AZ4533 photoresist. Step 4 is etching straight walls of the waveguides and converters by inductively-coupled plasma. We made 1.24-μmwide and 275-μm-long one-section polarization converters, 275-μm-long one-section converters with different top widths, 1.24-μm-wide and 554-μm-long two section converters, and 554-μm-long two section converters with different top widths. We measure efIciency of polarization conversion and loss of the converters using a Fabry-Perot setup described in [6]. The 1.24-μm-wide and 554-μm-long two-section converters show the record polarization conversion 99.8 % at a wavelength of 1.55 μm. As we can see from Fig. 1d, the curve of polarization conversion for the two-section devices is not only higher than the curve of the one-section converters, but also the shape is much wider. This proves that the two-section converters are much more tolerant. The loss of the two-section converters is compared with the loss of a straight waveguide as described in [6]. The loss is 0.6dB (one-section converters have <; 0.5 dB).