The recent surge in demand for adaptive cruise control (ACC) and commercial autonomous vehicles has drawn a lot of attention to these innovative applications. Based on pulsed laser diodes and hybrid receivers with Avalanche Photodiodes (APDs) for laser range finding (LRF), these systems are often classified under the general acronym LiDAR (Light Detection and Ranging).

LiDAR systems focus light beams on targets to allow measurement of distance by detecting the faint signals echoed back to the launch point and timing the difference between the launch and detection timestamps. This method is similar to focused sound waves used in sonar applications or radio waves used in radar applications.

It is used in a broad range of end applications, from adding topography through three-dimensional mapping of still images, to general surveying of urban settings, telemetry for final docking stages of capsules after reaching the International Space Station (ISS), and mapping density of the forest canopy from the same outer-space vantage point.

High-Volume, Smaller Size Packaging

Figure 1. SMD packaging enables simple solder-reflow processing of components alongside all other electronics required, rather than a secondary step of manual placement and hand soldering.

Military UAV systems have long depended on highly hermetic Transistor Outline (TO) cans packaging to survive rugged environments. Yet, the increasing diversity of roles played by unmanned aerial vehicles (UAVs) has brought forward new design challenges centered mainly around the size, weight, power requirements, and the desire to reduce development and unit cost (a.k.a. SWaP-c) for the optical payloads added on to UAVs, and to enable remote sensing and ranging.

Very large unmanned aerial combat vehicles (UACV) can handle larger and heavier payloads, and aim for very highlevels of performance throughout their lifetime. Development of smaller, lighter LiDAR components is not as high a priority compared to getting the very best performance possible and largest possible collection area of optics for the systems deployed.

The increased use of miniature, battery-power and over-the-shoulder soldier-launched units comes with very different requirements due to their almost disposable nature. These systems are much smaller, with less available power, but they still are expected to be rugged and designed to meet several military standards and unique qualification requirements, including a wider range of operating temperature, vibration and shock requirements, etc.

Robust TO-can designs remain the preferred go-to solution to meet these requirements, but through-hole assembly and hand soldering do not scale easily to very high volumes. Further, small UAVs are leaning towards using COTS sensors and lasers to help reduce weight and cost, with minimal performance degradation.

High-volume production also typically favors components that are Surface-mount compatible (SMD). SMD packaging enables simple solder-reflow processing of components alongside all other electronics required, rather than a secondary step of manual placement and hand soldering. In both cases, multi-channel options for lasers and APDs are desirable, in low-cost packaging that can meet the needs of the higher-volume, and disposable miniature UAV market (Figure 1).

LiDAR systems design, therefore, highly depends on the end application to select the proper laser wavelength and detection material, power budgets and weight restrictions.

Wavelength Options for Lasers

While most commercial LiDAR systems are currently using 905 nm pulsed laser diodes, which match the peak sensitivity of silicon APDs, they are migrating towards longer wavelengths to provide eye-safe precision systems, suitable in environments with background visible light.

Silicon vs. InGaAs APDs

Figure 2. Spectral Response Comparison

Typical Silicon APDs can easily detect 905 nm light, while some material improvements are required to increase the quantum efficiency at the YAG wavelength, 1064 nm, used mostly in military applications as laser designators for PGMs. Most LRF systems in the defense market are focused on use of eye-safe 1550 nm lasers, which are not detectable by Silicon APDs, but compatible with InGaAs APDs (Figure 2). InGaAs APDs are unable to amplify signals as much as reach-through silicon APDs, which can easily reach gains of over 100. Even with lower gain levels of 10 to 30, the lower transmission losses through the atmosphere and higher potential resolution of range are the main benefits of longer wavelength systems, while cost and much smaller detector active areas that can be offered in an economical fashion remain the main challenges for OEM suppliers.

The ultimate goal of any LiDAR system is to maintain a sufficient Signal-to-Noise Ratio (SNR) under the expected operating conditions. Careful tradeoffs are required to optimize the often-conflicting parameters of the ideal detector for each application. Special care is needed to optimize the overall active area, its impact on overall speed of the detector, the desired spectral range of operation and the impact on the noise associated with the avalanche phenomenon within APDs, which converts a single photon detected into a much larger photocurrent.

Novel designs and photodiode architectures are being developed to reduce the amplification noise and obtain ever-lower Excess Noise Factor, F. Continuous improvement of production will control dopant levels and minimize material defects and, in turn, minimize the effective ionization coefficient, keff, in the McIntyre equation. The Excess Noise Factor is a representation of the deviation from a noiseless, ideal amplifier circuitry as a given gain, M:

Some of the lowest noise APDs developed by Excelitas are based on a reach-through structure that offers the best available combination of high speed, low noise and capacitance, and extended IR response, but typically requires a much larger bias voltage to “reach through” to the junction where the avalanche phenomenon occurs. A slightly noisier approach, where doped Silicon is deposited epitaxially onto a Silicon substrate often offers a “good enough” F or SNR at a lower operating voltage, thus not taxing the limited power budget of the battery-operated miniature UAVs.

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