High Speed Velocity-Matched Distributed Photodetectors

In the last few years, remarkable progress has been made in ultrafast, and high-speed, high power photodetectors, as reviewed in [ 1,2]. In terms of applications, the high-speed photodetectors can be divided into four general different categories: (1) high-speed digital communication systems, (2) RF photonic systems, (3) ultrafast optoelectronic sampling, and (4) optoelectronic generation of millimeter-waves and sub-millimeter-waves. The requirements are different for different applications. For high-speed digital communications, bandwidth and efficiency are most important [2]. For analog and RF photonic systems, in addition to bandwidth and efficiency, maximum linear photocurrent is important to achieve desired RF performance [ 1,2]. Several approaches have been proposed to increase the maximum linear photocurrent, including waveguide photodetectors with reduced confinement factor [3], traveling-wave photodetectors [4,5], velocity-matched distributed photodetectors [ 1,6], and distributed absorption waveguide photodetector with non-exponential photon distribution inside the photodetector [7]. Uni-traveling-carrier photodetector (UTC-PD) has also been demonstrated to increase the saturation photocurrent density limited by the space charge effect in the intrinsic region [2,8]. The frequency of interest for RF photonic systems ranges from 1 to 100 GHz. For optoelectronic sampling or ultrashort electric pulse generation, short impulse response and flat frequency response with high 3-dB bandwidth are needed. Short-lifetime materials such as low-temperature (LT) GaAs are often used to eliminate the long tails of the impulse response, at the expense of reduced efficiency. Photodetectors with bandwidth over 500 GHz have been demonstrated using surface-illuminated metal-semiconductor-metal (MSM) [9,10] and p-i-n traveling wave photodetectors [ 1 11 with LT-GaAs absorption layer. For millimeter-wave and THz generation by optical heterodyning, the photodiodes are often referred to as photomixers. The frequencies of interest for photomixers ranges from 100 GHz to several THz. The most important parameter for photomixers is the amount of power that can be generated without damaging the photomixer. LT-GaAs photomixers with 1 p W of power at 600 GHz and somewhat lower power at 1 THz have been reported [ 12,131.