Ultrafast all-optical asymmetric Fabry-Pérot switch based on bulk beryllium-doped InGaAsP grown by He-plasma-assisted epitaxy

We demonstrate ultrafast polarization-independent switching, using an asymmetric Fabry-Pérot device, based on bulk InGaAsP, which can operate at high repetition rates. Our preliminary device exhibited 5.5ps switching time, 12dB contrast, and 0.5pJ/μm switching energy density. © 1999 Optical Society of America OCIS codes: (230.1150) All-optical devices; (190.7110) Ultrafast nonlinear optics; (230.4320) Nonlinear optical devices Ultrafast all-optical switching devices are essential for signal processing in high-bit-rate communications systems. Semiconductor-based devices have the advantages of being compact, easy to integrate, and having negligible latency. Asymmetric Fabry-Pérot (AFP) switching devices utilizing resonant nonlinearity in semiconductors require low switching energy and offer high contrast ratio. Two such devices have recently been demonstrated, one based on low-temperature-grown bulk GaAs [1], operating around 0.8μm, and another based on low-temperature-grown InGaAs/InAlAs strained multiple quantum wells (MQW) [2-3], operating around 1.5μm, compatible with fiber communications systems. However, MQW-based devices must be compensated for their polarization dependency and have narrower bandwidth than bulk-based devices. We report herein ultrafast switching by an AFP device based on bulk semiconductor operating around 1.5μm. Fig. 1. Switching window of the AFP device after excitation by a 1.1ps pump pulse. The inset shows the AFP device structure. We use InGaAsP grown by He-plasma-assisted epitaxy (HELP InGaAsP) with 1×10cm Be doping as the active nonlinear material for our AFP device. As previously shown [4-6], this material is well suited for fast switching devices, due to its large resonant nonlinearity and subpicosecond to picosecond response times. The device structure (inset of Figure 1) consists of a high-reflection stack made of 13 l/4 pairs of InP/InGaAsP which gives a reflection of ~60% over 60nm centered at 1550nm, a 2.5μm-thick active layer, and a l/4-thick SiON dielectric layer as the low-reflection surface of the AFP. The low finesse of the device ensures a wide bandwidth for operation. The reflections from the front and back surfaces are designed to cancel each other by interference when the pump pulse is absent, whereas a strong pump pulse can saturate the absorption of the active layer, resulting in high reflection. Figure 1 shows a 5.5-ps switching window with a contrast ratio of ~12dB obtained by optical excitation using a 1.1-ps pump pulse. The contrast ratio of this preliminary device can be further improved by more accurate controls on the active layer thickness and on the composition and thickness of the low-reflection layer. The switching time can also be shortened using a heavier doping concentration [5]. We have also experimentally verified that, as expected for a bulk device, the contrast ratio does not depend on the polarization of either pump or probe. 0.0 0.5 1.0 1.5 0 5 10 15 20