Simultaneous enhancement of the bandwidth and responsivity in high-speed avalanche photodiodes with an optimized flip-chip bonding package.

The enhancement in responsivity of photodiodes (PDs) or avalanche photodiodes (APDs) with the traditional flip-chip bonding package usually comes at the expense of degradation in the optical-to-electrical (O-E) bandwidth due to the increase of parasitic capacitance. In this work, we demonstrate backside-illuminated In0.52Al0.48As based APDs with novel flip-chip bonding packaging designed to relax this fundamental trade-off. The inductance induced peak in the measured O-E frequency response of these well-designed and well-packaged APDs, which can be observed around its 3-dB bandwidth (∼30 GHz), effectively widens the bandwidth and becomes more pronounced when the active diameter of the APD is aggressively downscaled to as small as 3 µm. With a typical active window diameter of 14 µm, large enough for alignment tolerance and low optical coupling loss, the packaged APD exhibits a moderate damping O-E frequency response with a bandwidth (36 vs. 31 GHz) and responsivity (3.4 vs. 2.3 A/W) superior to those of top-illuminated reference sample under 0.9 Vbr operation, to attain a high millimeter wave output power (0 dBm at 40 GHz) and output current (12.5 mA at +8.8 dBm optical power). The excellent static and dynamic performance of this design open up new possibilities to further improve the sensitivity at the receiver-end of the next-generation of passive optical network (PON) and coherent communication systems.

Separated electrodes for the enhancement of high-speed data transmission in vertical-cavity surface-emitting laser arrays.

In this work, a novel design for the electrodes in a near quasi-single-mode (QSM) vertical-cavity surface-emitting laser (VCSEL) array with Zn-diffusion apertures inside is demonstrated to produce an effective improvement in the high-speed data transmission performance. By separating the electrodes in a compact 2×2 coupled VCSEL array into two parts, one for pure dc current injection and the other for large ac signal modulation, a significant enhancement in the high-speed data transmission performance can be observed. Compared with the single electrode reference, which parallels 4 VCSEL units in the array, the demonstrated array with its separated electrode design exhibits greater dampening of electrical-optical (E-O) frequency response and a larger 3-dB E-O bandwidth (19 vs. 15 GHz) under the same amount of total bias current (20 mA). Moreover, this significant improvement in dynamic performance does not come at the cost of any degradation in the static performance in terms of the maximum near QSM optical output power (17 mW @ 20 mA) and the Gaussian-like optical far-field pattern which has a narrow divergence angle (full-width half maximum (FWHM): 10° at 20 mA). The advantages of the separated electrode design lead to a much better quality of 32 Gbit/sec eye-opening as compared to that of the reference device (jitter: 1.5 vs. 2.8 ps) and error-free 32 Gbit/sec transmissions over a 500 m multi-mode fiber has been achieved under a moderate total bias current of 20 mA.

Reconfigurable scan lens based on an actively controlled optical phased array.

Integrated photonics provides a path for miniaturization of an optical system to a compact chip scale and offers reconfigurability by the integration of active components. Here we report a chip-scale reconfigurable scan lens based on an optical phased array, consisting of 30 actively controlled elements on the InP integrated photonic platform. By configuring the phase shifters, we show scanning of a nearly diffraction-limited focused spot with a full width at half maximum spot size down to 2.7 µm at the wavelength of 1550 nm. We demonstrate the key functions needed for a laser-scanning microscope, including light focusing, collection, and steering. We also perform confocal measurements to detect reflection at selective depths.

Enhancement in speed and responsivity of uni-traveling carrier photodiodes with GaAs0.5Sb0.5/In0.53Ga0.47As type-II hybrid absorbers.

We demonstrate a top-illuminated high-speed uni-traveling carrier photodiode (UTC-PD) with a novel design in the p-type absorber, which can effectively shorten the photon absorption depth at telecommunication wavelengths (1.31~1.55 µm) and further enhance the bandwidth-efficiency product of UTC-PD. In our proposed new UTC-PD structure, the p-type In0.53Ga0.47As absorption layer is replaced by the type-II GaAs0.5Sb0.5 (p)/In0.53Ga0.47As (i) hybrid absorber. Due to the narrowing of the bandgap and enhancement of the photo-absorption process at the type-II interface between the GaAs0.5Sb0.5 and In0.53Ga0.47As layers, our device shows an over 16.7% improvement in the responsivity compared with that of UTC-PD with the same thickness of pure In0.53Ga0.47As absorber (0.7 µm) and a zero optical coupling loss. Our demonstrated device with a simple top-illuminated structure offers a large active mesa (25 µm), a wide optical-to-electrical (O-E) bandwidth (33 GHz), a high responsivity (0.7 A/W), and a high saturation current (>5 mA) under 1.31 µm optical wavelength. These promising results suggest that our proposed PD structure can fundamentally overcome the trade-off among bandwidth, efficiency, and device active diameter of high-speed PDs.

Avalanche photodiodes with multiple multiplication layers for coherent detection.

We demonstrate a novel avalanche photodiode (APD) design which fundamentally relaxes the trade-off between responsivity and saturation-current performance at receiver end in coherent system. Our triple In0.52Al0.48As based multiplication (M-) layers with a stepped electric (E-) field inside has more pronounced avalanche process with significantly less effective critical-field than the dual M-layer. Reduced E-field in active M-layers ensures stronger E-field allocation to the thick absorption-layer with a smaller breakdown voltage (Vbr) resulting in less serious space-charge screening effect, less device heating at high output photocurrent. Compared to the dual M-layer reference sample, the demonstrated APD exhibits lower punch-through (- 9 vs. - 24 V)/breakdown voltages (- 43 vs. - 51 V), higher responsivity (19.6 vs. 13.5 A/W), higher maximum gain (230 vs. 130), and higher 1-dB saturation-current (> 5.6 vs. 2.5 mA) under 0.95 Vbr operation. Extremely high saturation-current (> 14.6 mA), high responsivity (7.3 A/W), and decent O-E bandwidth (1.4 GHz) can be simultaneously achieved using the demonstrated APD with a 200 µm active window diameter. In coherent FMCW LiDAR test bed, this novel APD exhibits a larger signal-to-noise ratio and high-quality 3-D images than the reference dual M-layer and high-performance commercial p-i-n PD modules, while requiring significantly less optical local-oscillator (LO) power (0.5 vs 4 mW).