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
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
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.
Beyond the amplification of noise upon detection of incident light, the conversion of this generated photocurrent into a useable signal requires a transimpedance amplifier (TIA). Individually packaged APDs can be installed onto a printed circuit board (PCB) with special care used to offset the temperature-sensitive operating voltage (Vop) and avoid exceeding over the breakdown voltage (Vbr).
A slightly larger TO-can with a built-in hybrid receiver circuit allows significant improvements by minimizing parasitics associated with long leads needed to reach an external TIA integrated circuit (IC). These hybrid receiver designs are often easier to integrate into the final system layout as they are precalibrated. Enhanced designs even offer built-in thermoelectric coolers (TECs) or heaters to help stabilize the APD temperature (Figure 3).
The military market often prefers and relies solely on battlefield-proven analog signal systems; next-generation UAV designs may benefit from a more digital approach, with smarter sensors and lasers. Excelitas has integrated additional software features and advanced signal processing through field-programmable gate arrays (FPGAs) within modules, also offering high-speed analog output or even a digitized stream of data via an Inter-Integrated Circuit (I2C) or Serial Peripheral Interface (SPI) bus connection. These modules ease the first steps taken with APD devices and facilitate rapid development of new concepts, which can further be miniaturized and integrated with laser driver circuitry as a custom laser-plus-detector module, similar to the HeliX™ module (Figure 4).
As for pulsed laser diodes, ever-higher power levels, through integration of multiple lasing cavities within a single laser die or multi-channel options in unique packaging options, - enable unique, compact designs to be considered for LiDAR applications in UAVs. Although large fiber lasers are ideal for some applications, especially very long ranging applications, pulsed laser diodes remain much smaller, lighter and often sufficient for the mission at hand. Integration of a larger number of laser drive components within the laser packaging, be it TO-can or SMD, will also continue to benefit UAV designers.
Recent years have been challenging for SWaP reduction efforts in the defense market, with expected cost of requalification often blocking such efforts before they are fully laid out for review. The commoditization of LiDAR systems now creates a different type of pressure on OEM suppliers, since cost reduction is now a prerequisite for all production runs and any design revisions. Reduced military budgets also force designers to find solutions that will not affect the mission objectives and will attempt to qualify COTS-based designs by analogy to similar component designs. Companies with a vertically integrated structure are able to design custom variants at lower price points by pulling in resources across their different divisions, and can potentially offer higher integration levels in a more compact form factor.
This article was written by Eric Desfonds, Product Line Manager, Sensors - Defense and Aerospace, Excelitas Canada Inc. (VaudreuilDorion, Québec, Canada). For more information, Click Here .