The U.S. Army Research Laboratory (ARL) has developed a number of near-infrared, prototype laser detection and ranging (LADAR) Systems based on the chirp, amplitude-modulated LADAR (CAML) architecture. The use of self-mixing detectors in the receiver, that have the ability to internally detect and down-convert modulated optical signals, have significantly simplified the LADAR design. Recently, ARL has designed and fabricated single-pixel, self-mixing, InGaAs-based, metal-semiconductor-metal detectors to extend the LADAR operating wavelength to 1.55 mm and is currently in the process of designing linear arrays of such detectors. This paper presents fundamental detector characterization measurements of the new 1.55 mm detectors in the CAML architecture and some insights on the design of 1.55 μm linear arrays.
We analyze the optoelectronic mixing characteristics of InAlAs, Schottky-enhanced, InGaAs-based, metal-semiconductor-metal photodetectors. For devices with Schottky-enhancement layers (SELs) of about 500 Å, the measured frequency bandwidth is less than that of a corresponding photodetector. The mixing efficiency decreases with decrease in optical power, decreases with increase in local oscillator frequency and decreases with decrease in mixed signal frequency. We attribute this behavior to the band-gap discontinuity associated with the SEL. For devices with thinner SELs (≈100 Å), the mixing characteristics are greatly improved: the bandwidth of the optoelectronic mixer (OEM) is similar to that of a corresponding photodetector and the mixing efficiency decreases only slightly with decrease in optical power. We attribute these results to the enhancement of the thermionic/tunneling current through the thinner SEL. We also present a circuit model of the Schottky-enhanced, InGaAs-based OEM to explain the experimental results.
Interdigitated-finger metal-semiconductor-metal photodetectors (MSM-PDs) are widely used for high-speed optoelectronic applications. Recently, GaAs MSM-PDs have been utilized as optoelectronic mixers (OEMs) in an incoherent laser radar (LADAR) system. InGaAs MSM-PDs would allow LADAR operation at eye-safe wavelengths, mainly 1.55 μm. Unfortunately, the Schottky barrier height on InGaAs is quite low (~0.1-0.2eV) leading to high dark current and, hence, low signal-to-noise ratio. To reduce dark current, the Schottky barrier is typically “enhanced” by employing a high-band-gap lattice-matched Schottky enhancement layer (SEL). Detectors using SELs yield low dark current, high responsivity, and high bandwidths. In this paper we analyze the mixing effect in InAlAs Schottky-enhanced InGaAs-based MSM-PDs. We find that the measured frequency bandwidth of such a mixer is smaller than when used as a photodetector. Moreover, the mixing efficiency depends on the light modulation and mixed signal frequencies and decreases non-linearly with decrease in optical power. This is not observed in GaAs-based and non-Schottky-enhanced InGaAs MSM-PDs. We present a circuit model of the MSM-PD OEM to explain the experimental results.
Finite difference analysis was used to determine the thermal characteristics of continuous wave (CW) 850 nm AlGaAs/GaAs implant-apertured vertical-cavity surface-emitting lasers. A novel flip-chip design was used to enhance the heat dissipation. The temperature rise in the active region can be maintained below 40 °C at 4 mW output power with 10 mA current bias. In contrast, the temperature rise reaches above 60 °C without flip-chip bonding. The transient-temperature during turn-on of a VCSEL was also investigated. The time needed for the device to reach the steady-state temperature was in the range of a few tenths of a milli-second, which is orders of magnitude larger than the electrical or optical switch time. Flip-chip bonding will reduce the shift of the wavelength, peak power, threshold current and slope efficiency during VCSEL operations.