Strong Enhancement in Saturation Power of Sub-THz Photodiode by Using Photonic Millimeter-Wave Femtosecond Pulse Generator

Abstract

A photonic MMW femto-second pulse-generator has been demonstrated. Using it, we achieve strong (6.4 dB) saturation-power enhancements, which result in +3.9 dBm maximum output of UTC-PD at 160 GHz, as compared to that under sinusoidal excitation.OCIS codes: (320.5540) Pulse shaping, (230.5170) Photodiodes. I. IntroductionHigh-power photodiode (PD) based photonic millimeter-wave (MMW) generation technique attracts lots of attention in the recent years due to its application in MMW over fiber communication system [1] and photonic sub-THz measurement system [2]. Compared with the other millimeter-wave (MMW) signal generation techniques [3], although this approach may not offer the highest output power in the sub-THz regime (0.1 to 1 THz), it can provide almost infinite bandwidth for MMW signal generation and delivery due to the characteristics of low-loss and wide optical window of glass fiber. To further improve the O-E bandwidth (~300 GHz) and saturation power (~mW level) of PD thus becomes an important issue to extend the MMW bandwidth of such signal generation scheme for practical application, such as, MMW network analyzer [2], which needs to cover a wide bandwidth (hundreds of GHz) and provide a wide dynamic range [2].The key point to pursuit the ultimate high speed with high saturation current (power) of PD is to downscale the photo-absorption active area also its depletion layer thickness. However, the large amount of injected optical power results in serious device heating and thermal failure in these miniaturized PDs. An elegant approach in reducing the PD DC output current and device heating is to increase the effective optical modulation depth during fiber delivery through optical signal processing techniques [4-6]. Recently, a higher effective modulation depth as compared to using conventional optical sinusoidal excitation has been achieved using short (~1 ps) optical pulse train with ~100 GHz repetition rate [5]. A 4 dB improvement in output power under the same output photocurrent has been successfully demonstrated in the linear operation mode of PD [5]. Furthermore, due to the reduction in duty cycle under such pulse-mode operation scheme, a significant minimization in device heating and improvement in maximum saturation power may also be expected. However, it becomes more difficult to further narrow down the pulse-width to femto-second regime when the repetition rate of pulse train reaches hundreds of GHz. This is because that the peak optical power of each pulse significantly decreases with the increase of repetition rate of pulse train, which would impede the happening of nonlinear pulse compression process. In this paper, we demonstrate novel photonic MMW sources at 160 GHz using spectral line-by-line pulse shaping technique [6] for power enhancement of MMW signal generations [5]. The line-by-line pulse shaper is capable of generating ~400 fs optical pulse train with tunable repetition-rates multiplied with integer multiples of 20 GHz [6]. By use of this novel optical MMW source, we demonstrate a 6...