## 3 10 Gb/s High Speed Performance Monitor for Optical Interconnection  
The p-i-n photodiodes, AlGaAs/GaAs ridge waveguide and TIR were processed as described in the WDM photodetector module section. DuPont PI 2771 polyimide replacing previous Si\({}_{3}\)N\({}_{4}\) layer became another excellent detector passivation and resulted in electrical, mechanical and stress relief properties for improved performance [21, 22]. A SEM picture of the final monolithic integration device is shown in Figure 10. The detector area was a 30 \(\upmu\)m by 30 \(\upmu\)m square. The trench after the detector was not completely filled in by the polyimide PI 2771. The reason was that the polyimide thickness  
Figure 9: 10-\(\upmu\)m diameter photodetector with coplanar metal waveguide. was about 3.15 \(\upmu\)m smaller than the detector mesa depth plus the TIR etching, 8 \(\upmu\)m. Since there was a slowly varying slope at the filled trench, the metal coplanar waveguide could be deposited continuously and its profile was compatible with a high-speed electric interconnection. Generally, the overall photodetector speed is determined by both its intrinsic bandwidth caused by the photo-induced carrier transit time and its RC circuit-limit bandwidth. The higher i-region thickness would facilitate the more absorption in the photodiode, but degrade the intrinsic bandwidth. To have reasonable sensitivity, a 1.4 \(\upmu\)m thick i-region in the InGaAs photodiode was selected to meet a quantum efficiency over 0.6 at 1,550 nm wavelength. The photo-responsivity and quantum efficiency performance for the only p-i-n photodetectors in free-space illumination were experimentally demonstrated to be 0.7 A/W and 56%, respectively, at 1,550 nm wavelength without any antireflection coating [6]. In the reflectively waveguide coupled photodetector, the measured internal responsivity at 1,550 nm wavelength, excluding any coupling losses, was shown to be 0.42 A/W, which could be derived as 0.34 internal quantum efficiency [21]. The total loss from the optical waveguide and TIR reflector was \(\sim\)2.5 dB after considering the quantum efficiency 0.6 from the 1.4 \(\upmu\)m thick i-region photodiode. The propagation loss from AlGaAs/GaAs optical waveguides was verified using the Fabry-Perot resonance method at 0.98 dB/cm. The waveguide coupled detector length was around 6 mm. The coupling loss from the TIR reflector was estimated to be 1.9 dB. The tilt surface smoothness and angle with respect to the incident ion beam could be further optimized for better responsivity. The lines in the Smith chart in Figure 11 demonstrated smaller capacitances from -kHz to 10 GHz at different bias voltages on the monolithically integrated photodetector. The capacitance _vs._ frequency measurement showed a detector capacitance with polyimide processing at 0.2 pF at 2 GHz. A picosecond fiber laser (Pri Tel, Inc.) was used to measure the temporal response of 30 \(\times\) 30 \(\upmu\)m\({}^{2}\) photodetectors. The full width half maximum (FWHM) of the laser pulse is 1 ps at 1,543.5 nm. A peak power of 1.5 kW was generated with a 66 MHz repetition rate. The FWHMs of the measured photocurrents are 29.5 ps for 30 \(\times\) 30 \(\upmu\)m\({}^{2}\) photodetector using \(-\)10 V reverse bias shown in the inset of Figure 12. After Fourier transform was carried on the characterized temporal response, a 3-dB bandwidth of 11 GHz was obtained for the frequency response curve as shown in Figure 12[21]. Figure 10: Reflectively coupled waveguide photodetector with metal coplanar waveguide [22]. From the overall photodetector bandwidth equation [21], the 3-dB bandwidth from the 1.4 \(\upmu\)m i-region and 30 \(\times\) 30 \(\upmu\)m\({}^{2}\) photodetector area was transit time limited and demonstrated as 11 GHz.