## 2 WDM Photodetector Array for Performance Monitoring  
The opportunities presented by a spectrometer on a chip device were exploited to design integrated components which performed multiple functions or placed a complete WDM subsystem onto a single substrate. Optical multiplexers and demultiplexers are the basic components of WDM systems. Demultiplexers using a Rowland circle grating construction seemed to be very promising because of their compactness, simplicity [18] and significant reductions in interconnections and assembly complexity. Therefore, the integration of these demultiplexers with active elements such as detectors and amplifiers is a major step toward a fully integrated receiver module. Figure 4: Photodetector monolithically integrated with optical waveguide by the filling material in the TIR region. We presented an advanced 1.55-\(\upmu\)m wavelength based WDM photodetector array (PDA) including the photodiode array monolithically integrated with a channel/planar waveguide, grating and TIR, that took advantage of GaAs optoelectronic devices, as shown in Figure 5. Light coupled from the input fiber was guided in the five-layer slab AlGaAs/GaAs waveguide onto the blazed and curved grating. The light was diffracted such that each wavelength was focused onto a different location on the Rowland circle, shown as the solid beam path line in the top view of Figure 5. The focused light was then reflected by the TIR mirror through the 1.7-\(\upmu\)m InP buffer layer between the GaAs and In\({}_{0.53}\)Ga\({}_{0.47}\)As in order to be detected by the In\({}_{0.53}\)Ga\({}_{0.47}\)As pin photodiode situated above the Rowland circle. The dashed line of the beam path in the side view of Figure 5 demonstrated that the two transverse-electric (TE) and transverse-magnetic (TM) polarization modes in the AlGaAs/GaAs ridge waveguides were completely reflected by the TIR mirrors. This means that there was no polarization dependent loss (PDL) from the TIR. A TIR mirror was utilized to couple the optical signal from the waveguide into the detector. The typical waveguide photodetector utilizes the optical waveguide to fan out for signal detection. To avoid optical channel cross talk, the distance between the waveguides is usually 30-50 \(\upmu\)m, which limits the resolution. Our receiver module detector array was designed to be placed onto the Rowland focus circle to increase the detector resolution for dense WDM applications. The 50 \(\times\) 50 \(\upmu\)m\({}^{2}\) In\({}_{0.53}\)Ga\({}_{0.47}\)As p-i-n photodiode mesas were defined by etching the 1.4 \(\upmu\)m thick i-region. The mesa etch was performed in an inductively coupled plasma (ICP) system utilizing a chlorine based chemistry. A platinum p-metal layer was used as the etch mask. A plasma containing Cl\({}_{2}\)/BCl\({}_{3}\) at a substrate bias voltage of -100 V, inductive power of 500 W, pressure of 5 mTorr and substrate temperature of 170 \({}^{\circ}\)C was used to achieve high anisotropic etching. A low substrate bias value was used to reduce the ion damage and hence improve the detector leakage current. Following n-metal deposition, Si\({}_{3}\)N\({}_{4}\) was deposited using PECVD for photodiode passivation and dielectric  
Figure 5: A schematic is the diagram that represents the WDM photodetector module. The insets are the scanning electron microscope (SEM) pictures for grating teeth and the photo-detector array with TIR mirrors [20]. isolation for the metal coplanar waveguide. The InP buffer layer was then removed using wet etching, 1 HCl: 1 H\({}_{2}\)O, from the areas surrounding the detector. The chemically assisted ion beam etching (CAIBE) technique [19] allowed the mirror angle to be set by adjusting the wafer orientation with respect to the incident ion beam. The angle reflector opening angle was \(\sim\)29\({}^{\circ}\). The teeth grating and ridge waveguides were then etched in separate CAIBE steps. The echelle grating and ridge optical waveguide depths for AlGaAs/GaAs were 4 \(\upmu\)m and 1.5 \(\upmu\)m, respectively.