1914 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES,VOL. 49, NO. 10, OCTOBER2001 Velocity-Matched Distributed Photodetectors and Balanced Photodetectors with p-i-n Photodiodes M. Saif Islam, Student Member, IEEE, Sanjeev Murthy, Student Member, IEEE, Tatsuo Itoh, Fellow, IEEE, Ming C. Wu, Senior Member, IEEE, Dalma Novak, Member, IEEE, Rodney B. Waterhouse, Member, IEEE, Deborah L. Sivco, and Alfred Y. Cho, Fellow, IEEE Abstract—We report on the first demonstration of ve- locity-matched distributed photodetectors and balanced pho- todetectors with p-i-n photodetectors. Record-high linear dc photocurrent of 45 mA has been achieved without suffering from thermal damage, thanks to the superior power handling capability of p-i-n photodiodes. A novel fiber alignment technique has been developed to achieve high linear photocurrent. More than 37 dB of common-mode-rejection ratio and 45-dB suppression of laser relative intensity noise over a broad frequency range have been achieved using the distributed balanced photodetectors in an RF fiber-optic link. The frequency response is flat from 1 to 35 GHz. Index Terms—Analog fiber-optic links, balanced photodetectors, high power photodetector, microwave photonics, noise suppres- sion, optical receivers, p-i-n photodetectors, RF photonics. I. INTRODUCTION I N AN externally modulated fiber-optic link, high-speed photodetectors (PDs) with high saturation photocurrent can improve the overall link performance, including the link gain, noise figure, and spurious free dynamic range (SFDR) [1]. When PDs have sufficiently high saturation power, this improvement is limited by the relative intensity noise (RIN) of the laser source and the amplified spontaneous emission noise (ASE) from erbium-doped fiber amplifiers (EDFA). It is known that the laser RIN and EDFA-added noise can be suppressed by balanced receivers [1], [2]. High power balanced receivers are also important for optical heterodyned receivers and optoelectronic generation of high power microwaves and millimeter-waves. Therefore, PDs and, in particular balanced PDs with high linear photocurrent are critical components of high-performance RF photonic links [1]–[3]. Several approaches have been proposed to increase the max- imum linear photocurrent of high-speed PDs, including wave- Manuscript received January 22, 2001; revised May 28, 2001. This work was supported in part by Office of Naval Research under a Multiuniversity Research Initiative on RF photonics, and the University of California Microelectronics Innovation and Computer Research Opportunities. M. S. Islam was with the Electrical Engineering Department, University of California at Los Angeles, Los Angeles CA 90095 USA. He is now with Optical Network Research, JDS Uniphase Corporation, Santa Clara, CA 95054 USA (e-mail: saif.islam@jdsuniphase.com). S. Murthy, T. Itoh, and M. C. Wu are with the Electrical Engineering Depart- ment, University of California at Los Angeles, Los Angeles, CA 90095 USA. D. Novak and R. B. Waterhouse are with the Australian Photonics Coopera- tive Research Center, Department of Electrical and Electronic Engineering, The University of Melbourne, Melbourne, Vic. 3010, Australia. D. L. Sivco and A. Y. Cho are with Lucent Technologies, Bell Laboratories, Murray Hill, NJ 07974 USA. Publisher Item Identifier S 0018-9480(01)08713-0. guide PDs with low confinement factors [4]–[6], traveling-wave photodetectors [7], [8] phototransistors [9], uni-traveling-carrier photodiode [10], and velocity-matched distributed photodetec- tors (VMDP) [11]–[13], and parallel fed VMDP [14]. Using the VMDP with metal–semiconductor–metal (MSM) PDs, we have previously achieved a saturation photocurrent of 33 mA at 1.55- m wavelength [15]. Bimberg et al. also reported on an MSM-based VMDP with a bandwidth above 78 GHz [13]. We have also demonstrated a novel monolithic distributed balanced photodetector with MSM PDs [16] that successfully suppressed broad band (1–12 GHz) laser RIN [17]. Maximum noise sup- pression of 36 dB was observed at the relaxation oscillation fre- quency of the DFB laser. The maximum linear photocurrent in our MSM-VMDP was limited by the catastrophic damage caused by thermal runaway. p-i-n PDs have higher threshold for thermal runaway than MSM PDs. It was previously shown that MSM PDs fail at junction temperatures of 700 K [18], whereas p-i-n can stand junc- tion temperatures above 900 K [19]. We developed a theoretical model to carry out detailed analysis of the thermal runaway is- sues and found that the dark current ( ) and the effective bar- rier height ( ) are the fundamental parameters in the failure mechanism of high power PDs. Our investigation also showed that junction PDs (such as p-i-n) with high were expected to perform better than metal–semiconductor contact PDs (such as MSM or Schottky) for high power operation. Further details of our analysis are reported elsewhere [20]. The uniform electric field distribution in p-i-n is also advantageous for linear oper- ation under high power illumination. Moreover, the fabrication of high bandwidth MSM PDs demands sophisticated submicron e-beam writing and thin MSM fingers are vulnerable to failure caused by high photocurrents. Therefore, VMDP with p-i-n PDs are of great interest for high power photodetection. In this paper, we report on the first demonstration of the VMDP with p-i-n PDs. Record-high linear photocurrent of 45 mA has been achieved. No thermal runaway was observed for photocurrent above 55 mA. We have also found that the fiber position for maximum responsivity is different from that for maximum linear photocurrent. A novel technique of offset launching of the input power helped attain more uniform distribution of photocurrents. Our VMDP has a flat frequency response up to 35 GHz, though there is an initial drop at low frequency due to slow carrier diffusion. We also fabricated a distributed balanced photodetectors with p-i-n PDs and successfully suppressed broad-band RIN and EDFA added noises by more than 43 dB. 0018–9480/01$10.00 © 2001 IEEE