The PDs, as one of the key components in optical communication systems, have been investigated passionately all the time with respect to the quantum efficiency, bandwidth, saturation current and dark current etc. The major limitations on the speed of PIN PDs are: (1) the diffuse time it takes carriers to get out of undepleted regions, (2) the drift time it takes carriers to get across the depletion region, (3) charge trapping at heterojunctions, and (4) the time it takes to charge and discharge the inherent capacitance of the diode plus any parasitic capacitance. However, quantum and coupling efficiency and bandwidth are mutually constrained in terms of the width of the depletion layer, so much effort has been concentrated on increasing data rates of PD’s for systems operating at high efficiency.
From 1987 to 1990, a series performance enhancement of the optical receiver was reported including increased bandwidth and optimized structure. For example, a high-impedance pin FET receiver with an 8 GHz bandwidth and a similar design with a 16 GHz bandwidth but using HEMTs was demonstrated by the Gimlett group[14, 15], a 10 GHz PIN detector design comprised by GalnAs which followed by a three-stage, high-impedance amplifier and interconnected by impedance matching networks was demonstrated and in Ref. , a 14.5 GHz bandwidth with a transimpedance design was also obtained.
Generally, there exists tradeoff between the quantum efficiency and bandwidth, which are two important figures of merit for high-speed PDs. Faced with such a situation, an edge-coupled waveguide photodiode (WGPD) has been widely studied for overcoming the efficiency-bandwidth tradeoff in the vertically illuminated photodetectors (VPD). As the name implies, the WGPD has an optical waveguide structure which nearly permits the efficiency and bandwidth to be specified independently. In 1986, the first high speed edge-coupled WGPD with a coupling efficiency of 25% and a bandwidth of 28 GHz was demonstrated. However, the coupling efficiency of the edge-coupled WGPD is relatively low. To improve the coupling efficiency, Kato et al. proposed a double-core multimode waveguide structure in 1991, which was the first breakthrough for the WGPD. Then, WGPDs using the double-core multimode waveguide structure were reported successively with the bandwidth of 40 and 50 GHz, respectively[20, 21]. The CR time constant is another limiting factor for PD’s bandwidth. To overcome the degradation introduced by the parasitic effect, a mushroom-mesa waveguide structure was reported in 1994 by the Kato group, which realized a bandwidth above 100 GHz and the bandwidth-efficiency product was 70–90 GHz, this structure was considered as the second breakthrough for large-bandwidth WGPD’s.
However, WGPD has not made a breakthrough in terms of high power output. With the increasing of signal speed, ultra-bandwidth PDs with a high saturation power become more significant for applications such as ultrafast measurement and high-speed optical fiber communications. In 1997, a novel structure called uni-traveling carrier photodiode (UTC-PD) was first proposed by the NTT Photonics Laboratories in Japan. In the UTC-PD structure, only an electron is used as the active carrier, which is an essential difference from the traditional pin PD. The UTC-PD has two main advantages, the one is a higher operation speed due to the higher electron velocity compared with hole velocity in the depletion layer, and the other one is the higher output saturation current because of much less space charge effect in the depletion layer. For UTC-PD research, the NTT Corporation in Japan has been in a leading position. In Ref. , a UTC-PD at a wavelength of 1.55 μm which had a 3 dB bandwidth of 220 GHz was fabricated by the Ito group in 1999, only in the next year, they realized an InP/InGaAs UTC-PD with a bandwidth of 310 GHz. In 2003, a packaged receiver module with a 1 mm connector and a chip with 310 GHz bandwidth has been fabricated; the module has a 3 dB bandwidth of 80 GHz at an average photocurrent of 10 mA. In the same year, they presented a UTC-PD with a monolithically integrated matching circuit, which showed a high saturation output power of 20.8 mW at 100 GHz. In 2007, Wu et al. discussed a PD that was composed of a UTC-PD and an improved evanescently coupled optical waveguide. The integrated UTC-PD realized a high responsivity of 1.04 A/W and a wide electrical 3 dB bandwidth up to 60 GHz.
Compared with PIN PD, APDs can achieve higher 5–10 dB sensitivity. Therefore, many works have been reported on APDs. In 1994, a first waveguide superlattice APD was reported by the Shishikura group. Then, an APD with a thinner photoabsorption layer was fabricated, which reached a 3.2 multiplication factor and 0.9 A/W responsivity at 20 GHz in 1997. Using a similar approach, a broader bandwidth of 35 GHz was realized for APD by Nakata et al. in 2001, its gain-bandwidth product was 140 GHz. In Ref. , the gain-bandwidth product of the APD was greatly improved up to 320 GHz, while the quantum efficiency only was 16% in the same year. In 2003, a broad bandwidth of 48 GHz, very high responsivity (> 1 A/W), and 11 mA saturation current were demonstrated.
In the high capacity communication system, a single discrete device is not conducive to the system cost reduction, therefore, an integrated optoelectronic device is a nice choice to reduce system cost, make the system small and easy to achieve. In 1992, M. Tabasky et al. reported a four-channel hybrid receiver with a modulation bandwidth up to 400 MHz and the corresponding data rate in the NZR arrived at 800 Mb/s using a silicon substrate for packaging. The 10-channel transmitter and receiver modules with the rate of 800 Mb/s/ch were fabricated in 1996, which were composed of a low capacitance pin PD array, a low threshold current LD array and high-speed Si-bipolar array ICs. In 2003, the NTT Corporation demonstrated a 1.25 Gb/s × 8 channel photoreceiver using silica-based AWG.