## 2 WDM Photodetector Array for Performance Monitoring
Figure 5 shows the top view of a six-layer device which includes the input ridge/slab waveguides, teeth grating and photodetectors with TIR mirrors [20]. In this case, a series of detectors was designed at different grating orders with each grating order sharing some areas on the Rowland focus circle. The insets in Figure 5 show the enlargement SEM pictures for the teeth grating and photodetector array with TIR built-in where the arrows show the optical beam propagation. The echelle grating was designed as the highest grating efficiency (blazing) and the largest grating period to avoid the corner rounding effect (low loss). For the photodetectors with TIR, the Platinum was the p-metal for the p-i-n photodetector and also the mask for ICP dry etching. Therefore, we can definitely reduce the detector size to a small area due to the novel self-alignment processing. Because the WDM PDA uses multi-layer processing, the alignment is extremely important in controlling the wavelength accuracy, the integrated component accuracy and maintaining high optical performance. Therefore, the alignment tolerance was very critical to the process development in reflectively coupled waveguide photodetectors. Verniers were utilized to insure that every pattern formed a line in different layers, basically a fraction of a micron. Figure 6 shows the novel vernier tool used for alignment purposes. The roughness of the side walls is higher than the roughness of the top and bottom interfaces, causing additional propagation loss for the TE mode as its field is much higher at the side walls. That is the reason why the propagation loss is typically higher for TE modes than for TM modes. Therefore, the PDL obtained from the waveguide coupled photodetector was measured around 0.4 dB. The two TE and TM polarization modes in AlGaAs/GaAs ridge waveguides are completely reflected by TIR mirrors. This means that there is no PDL from TIR [21]. The five-layer AlGaAs/GaAs optical waveguides were theoretically designed for the fairly round mode to achieve better birefringence control. The single mode condition of 4-\(\upmu\)m wide waveguides can be maintained with the etch depth  
Figure 6: Verniers for different layer alignments [21]. lower than 1.7 \(\upmu\)m. The birefringence of 9 \(\times\) 10\({}^{-4}\) and its variation at 4 \(\times\) 10\({}^{-5}\) were demonstrated at triplexing wavelengths between 1,310 and 1,550 nm with etch depths from 1.5 to 1.7 \(\upmu\)m, as shown in Figure 7[21]. The 50 \(\times\) 50 \(\upmu\)m\({}^{2}\) photodiodes exhibited a dark current of 28 nA. The fiber to detector photocurrent response as a function of wavelengths is shown in Figure 8. The channel spacing was \(\sim\)2 nm. The pass band of the filter was approximately 1 nm at a \(-\)1 dB level. The crosstalk into the next channel was about \(-\)7 dB. The reason for this was that the detector size was large enough to pick up the wavelength signal from the adjacent channels. In order to reduce the channel crosstalk and also approach the dense WDM, small area detectors are necessary. In Figure 8, the photo-current response could be reduced to \(-\)35 dB if the channel side lobe is minimized by increasing the Rowland circle radius and decreasing the photodetector area. A 10-\(\upmu\)m diameter photodetector was successfully fabricated using ICP with high speed probing, as shown in Figure 9 and a crosstalk of \(\sim\)20 dB could be expected. Figure 8: Fiber to detector photocurrent response as a function of wavelengths (detector channel). Figure 7: Birefringence as a function of wavelengths for AlGaAs/GaAs optical waveguides with the same width (W) of 4 \(\upmu\)m for different etch depths (h) of 1.5 \(\upmu\)m, 1.6 \(\upmu\)m, and 1.7 \(\upmu\)m [21]. The optical insertion loss of the echelle grating is significantly related to the corner rounding of the grating teeth, the vertical angle and the roughness of the grating facets.