The optical detector used in pulsed LIDAR, range finding and optical time domain reflectometry systems is typically the limiting factor in the system's sensitivity. It is well-known that an avalanche photodiode (APD) can be used to improve the signal to noise ratio over a PIN detector, however, APDs operating at the eye-safe wavelengths around 1550 nm are limited in sensitivity by high excess noise. The underlying issue is that the impact ionization coefficient of InAlAs and InP used as the avalanche region in current commercial APDs are very similar at high gain, leading to poor excess noise performance. Recently, we have demonstrated extremely low noise from an Al(Ga)AsSb PIN diode with highly dissimilar impact ionization coefficients due to electron dominated impact ionization.In this paper, we report on the first low noise InGaAs/AlGaAsSb separate absorption, grading and multiplication APDs operating at 1550 nm with extremely low excess noise factor of 1.93 at a gain of 10 and 2.94 at a gain of 20. Furthermore, the APD's dark current density was measured to be 74.6 µA/cm 2 at a gain of 10 which is competitive with commercial devices. We discuss the impact of the excess noise, dark current and responsivity on the APDs sensitivity and, project a noise-equivalent power (NEP) below 80 fW/Hz 0.5 from a 230 µm diameter APD and commercial transimpedance amplifier (TIA). The prospects for the next generation of extremely low noise APDs for 1550 nm light detection are discussed.

Avalanche photodiodes (APDs) made with the material AlGaAsSb (lattice-matched to InP) exhibit very low excess noise characteristics. We demonstrate a Separate Absorption and Multiplication APD (SAM-APD) incorporating a GaAs0.52Sb0.48 (GaAsSb) absorption region and an Al0.85Ga0.15As0.56Sb0.44 (AlGaAsSb) avalanche region. Our GaAsSb/AlGaAsSb SAM-APD exhibits a cut-off wavelength of 1.70 μm at room temperature and a responsivity of 0.39 A/W at 1.55 μm wavelength (with no antireflection coating). Temperature dependence of the breakdown voltage was obtained from avalanche gain data from multiple devices operated at 77 to 295 K. This produced a temperature coefficient of breakdown voltage of 4.31±0.33 mV/K, a factor of 10 and 5 smaller than values for comparable InP and InAlAs SAM-APDs. The very small temperature coefficient of this work is consistent with the extremely weak temperature dependence of avalanche breakdown previously observed in AlGaAsSb diodes.

An extremely low noise Separate Absorption and Multiplication Avalanche Photodiode (SAM-APD), consisting of a GaAs0.52Sb0.48 absorption region and an Al0.85Ga0.15As0.56Sb0.44 avalanche region, is reported. The device incorporated an appropriate doping profile to suppress tunneling current from the absorption region, achieving a large avalanche gain, ∼130 at room temperature. It exhibits extremely low excess noise factors of 1.52 and 2.48 at the gain of 10 and 20, respectively. At the gain of 20, our measured excess noise factor of 2.48 is more than three times lower than that in the commercial InGaAs/InP SAM-APD. These results are corroborated by a Simple Monte Carlo simulation. Our results demonstrate the potential of low excess noise performance from GaAs0.52Sb0.48/Al0.85Ga0.15As0.56Sb0.44 avalanche photodiodes.

The optical detector used in pulsed LIDAR, range finding and optical time domain reflectometry systems is typically the limiting factor in the system's sensitivity. It is well-known that an avalanche photodiode (APD) can be used to improve the signal to noise ratio over a PIN detector, however, APDs operating at the eye-safe wavelengths around 1550 nm are limited in sensitivity by high excess noise. The underlying issue is that the impact ionization coefficient of InAlAs and InP used as the avalanche region in current commercial APDs are very similar at high gain, leading to poor excess noise performance. Recently, we have demonstrated extremely low noise from an Al(Ga)AsSb PIN diode with highly dissimilar impact ionization coefficients due to electron dominated impact ionization.In this paper, we report on the first low noise InGaAs/AlGaAsSb separate absorption, grading and multiplication APDs operating at 1550 nm with extremely low excess noise factor of 1.93 at a gain of 10 and 2.94 at a gain of 20. Furthermore, the APD's dark current density was measured to be 74.6 µA/cm 2 at a gain of 10 which is competitive with commercial devices. We discuss the impact of the excess noise, dark current and responsivity on the APDs sensitivity and, project a noise-equivalent power (NEP) below 80 fW/Hz 0.5 from a 230 µm diameter APD and commercial transimpedance amplifier (TIA). The prospects for the next generation of extremely low noise APDs for 1550 nm light detection are discussed.

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