Xiaogang Bai

Low-cost compact MEMS scanning ladar system for robotic applications

Future robots and autonomous vehicles require compact low-cost Laser Detection and Ranging (LADAR) systems for autonomous navigation. Army Research Laboratory (ARL) had recently demonstrated a brass-board short-range eye-safe MEMS scanning LADAR system for robotic applications. Boeing Spectrolab is doing a tech-transfer (CRADA) of this system and has built a compact MEMS scanning LADAR system with additional improvements in receiver sensitivity, laser system, and data processing system. Improved system sensitivity, low-cost, miniaturization, and low power consumption are the main goals for the commercialization of this LADAR system. The receiver sensitivity has been improved by 2x using large-area InGaAs PIN detectors with low-noise amplifiers. The FPGA code has been updated to extend the range to 50 meters and detect up to 3 targets per pixel. Range accuracy has been improved through the implementation of an optical T-Zero input line. A compact commercially available erbium fiber laser operating at 1550 nm wavelength is used as a transmitter, thus reducing the size of the LADAR system considerably from the ARL brassboard system. The computer interface has been consolidated to allow image data and configuration data (configuration settings and system status) to pass through a single Ethernet port. In this presentation we will discuss the system architecture and future improvements to receiver sensitivity using avalanche photodiodes.

32 x 32 Geiger-mode ladar camera

For the wide applications of LAser Detection and Ranging (LADAR) imaging with large format Geiger-mode (GM) avalanche photodiode (APD) arrays, it is critical and challenging to develop a LADAR camera suitable to volume production with enough component tolerance and stable performance. Recently Spectrolab and Black Forest Engineering developed a new 32x32 Read-Out Integrated Circuit (ROIC) for LADAR applications. With a specially designed high voltage input protection circuit, the ROIC can work properly even with more than 1 % of pixels shorted in the APD array; this feature will greatly improve the camera long-term stability and manufacturing throughput. The Non-uniform Bias circuit provides bias voltage tunability over a 2.5 V range individually for each pixel and greatly reduces the impact of the non-uniformity of an APD array. A SMIA high speed serial digital interface streamlines data download and supports frame rates up to 30 kHz. The ROIC can operate with a 0.5 ns time resolution without vernier bits; 14 bits of dynamic range provides 8 μs of range gate width. At the meeting we will demonstrate more performance of this newly developed 32x32 Geiger-mode LADAR camera.

Geiger-mode ladar cameras

The performance of Geiger-mode LAser Detection and Ranging (LADAR) cameras is primarily defined by individual pixel attributes, such as dark count rate (DCR), photon detection efficiency (PDE), jitter, and crosstalk. However, for the expanding LADAR imaging applications, other factors, such as image uniformity, component tolerance, manufacturability, reliability, and operational features, have to be considered. Recently we have developed new 32×32 and 32×128 Read-Out Integrated Circuits (ROIC) for LADAR applications. With multiple filter and absorber structures, the 50-μm-pitch arrays demonstrate pixel crosstalk less than 100 ppm level, while maintaining a PDE greater than 40% at 4 V overbias. Besides the improved epitaxial and process uniformity of the APD arrays, the new ROICs implement a Non-uniform Bias (NUB) circuit providing 4-bit bias voltage tunability over a 2.5 V range to individually bias each pixel. All these features greatly increase the performance uniformity of the LADAR camera. Cameras based on these ROICs were integrated with a data acquisition system developed by Boeing DES. The 32×32 version has a range gate of up to 7 μs and can cover a range window of about 1 km with 14-bit and 0.5 ns timing resolution. The 32×128 camera can be operated at a frame rate of up to 20 kHz with 0.3 ns and 14-bit time resolution through a full CameraLink. The performance of the 32×32 LADAR camera has been demonstrated in a series of field tests on various vehicles.

High-performance InP Geiger-mode SWIR avalanche photodiodes

LAser Detection And Ranging (LADAR) is a promising tool for precise 3D-imaging, which enables field surveillance and target identification under low-light-level conditions in many military applications. For the time resolution and sensitivity requirements of LADAR applications, InGaAsP/InP Geiger-mode (GM) avalanche photodiodes (APDs) excel in the spectrum band between 1.0~1.6 μm. Previously MIT Lincoln Laboratory has demonstrated 3D LADAR imaging in the visible and near infrared (1.06 μm) wavelengths with InP/InGaAsP GM-APD arrays. In order to relieve the design tradeoffs among dark count rate (DCR), photo detection efficiency (PDE), afterpulsing, and operating temperature, it is essential to reduce the DCR while maintaining a high PDE. In this paper we will report the progress of GM-APD detectors and arrays with low DCR and high PDE at 1.06 μm. In order to improve both DCR and PDE, we optimized the multiplication layer thickness, substrate, and epitaxial growth quality. With an optimized InP multiplier thickness, a DCR as low as 100 kHz has been demonstrated at 4V overbias at 300 °C. and at 240 K, less than 1 kHz DCR is measured. A nearly 40% PDE can be achieved at a DCR of 10 kHz at the reduced temperature.

Development of high-sensitivity SWIR APD receivers

Emerging short wavelength infrared (SWIR) LIght Detection And Ranging (LIDAR) and long range laser rangefinder systems, require large optical aperture avalanche photodiodes (APDs) receivers with high sensitivity and high bandwidth. A large optical aperture is critical to increase the optical coupling efficiency and extend the LIDAR sensing range of the above systems. Both APD excess noise and transimpedance amplifier (TIA) noise need to be reduced in order to achieve high receiver sensitivity. The dark current and capacitance of large area APDs increase with APD aperture and thus limit the sensitivity and bandwidth of receivers. Spectrolab has been developing low excess noise InAlAs/InGaAs APDs with impact ionization engineering (I2E) designs for many years and has demonstrated APDs with optical gain over 100 utilizing multiple period I2E structures in the APD multiplier. These high gain I2E APDs have an excess noise factor less than 0.15. With an optical aperture of 200 μm, low excess noise multiple periods I2E APDs have capacitances about 1.7 pF. In addition, optical gains of InAlAs based APDs show very little temperature dependence and will enable APD photoreceivers without thermal electric cooling.

Three-dimensional imaging with 1.06μm Geiger-mode ladar camera

Three-dimensional (3D) topographic imaging using Short wavelength infrared (SWIR) Laser Detection and Range (LADAR) systems have been successfully demonstrated on various platforms. LADAR imaging provides coverage down to inch-level fidelity and allows for effective wide-area terrain mapping. Recently Spectrolab has demonstrated a compact 32×32 LADAR camera with single photon-level sensitivity with small size, weight, and power (SWAP) budget. This camera has many special features such as non-uniform bias correction, variable range gate width from 2 microseconds to 6 microseconds, windowing for smaller arrays, and shorted pixel protection. Boeing integrated this camera with a 1.06 μm pulse laser on various platforms and had demonstrated 3D imaging. In this presentation, the operation details of this camera and 3D imaging demonstration using this camera on various platforms will be presented.

GHz low noise short wavelength infrared (SWIR) photoreceivers

Next generation LIDAR mapping systems require multiple channels of sensitive photoreceivers that operate in the wavelength region of 1.06 to 1.55 microns, with GHz bandwidth and sensitivity less than 300 fW/√Hz. Spectrolab has been developing high sensitivity photoreceivers using InAlAs impact ionization engineering (I2E) avalanche photodiodes (APDs) structures for this application. APD structures were grown using metal organic vapor epitaxy (MOVPE) and mesa devices were fabricated using these structures. We have achieved low excess noise at high gain in these APD devices; an impact ionization parameter, k, of about 0.15 has been achieved at gains >20 using InAlAs/InGaAlAs as a multiplier layer. Electrical characterization data of these devices show dark current less than 2 nA at a gain of 20 at room temperature; and capacitance of 0.4 pF for a typical 75 micron diameter APD. Photoreceivers were built by integrating I2E APDs with a low noise GHz transimpedance amplifier (TIA). The photoreceivers showed a bandwidth of 1 GHz and a noise equivalent power (NEP) of 150 fW/rt(Hz) at room temperature.

Large format geiger-mode avalanche photodiode LADAR camera

Recently Spectrolab has successfully demonstrated a compact 32x32 Laser Detection and Range (LADAR) camera with single photo-level sensitivity with small size, weight, and power (SWAP) budget for threedimensional (3D) topographic imaging at 1064 nm on various platforms. With 20-kHz frame rate and 500- ps timing uncertainty, this LADAR system provides coverage down to inch-level fidelity and allows for effective wide-area terrain mapping. At a 10 mph forward speed and 1000 feet above ground level (AGL), it covers 0.5 square-mile per hour with a resolution of 25 in2/pixel after data averaging. In order to increase the forward speed to fit for more platforms and survey a large area more effectively, Spectrolab is developing 32x128 Geiger-mode LADAR camera with 43 frame rate. With the increase in both frame rate and array size, the data collection rate is improved by 10 times. With a programmable bin size from 0.3 ps to 0.5 ns and 14-bit timing dynamic range, LADAR developers will have more freedom in system integration for various applications. Most of the special features of Spectrolab 32x32 LADAR camera, such as non-uniform bias correction, variable range gate width, windowing for smaller arrays, and short pixel protection, are implemented in this camera.

Single photon sensitive Geiger-mode LADAR cameras

Three-dimensional (3D) imaging with Short wavelength infrared (SWIR) Laser Detection and Range (LADAR) systems have been successfully demonstrated on various platforms. It has been quickly adopted in many military and civilian applications. In order to minimize the LADAR system size, weight, and power (SWAP), it is highly desirable to maximize the camera sensitivity. Recently Spectrolab has demonstrated a compact 32x32 LADAR camera with single photo-level sensitivity at 1064. This camera has many special features such as non-uniform bias correction, variable range gate width from 2 microseconds to 6 microseconds, windowing for smaller arrays, and short pixel protection. Boeing integrated this camera with a 1.06 μm pulse laser on various platforms and demonstrated 3D imaging. The features and recent test results of the 32x128 camera under development will be introduced.

Development of low excess noise SWIR APDs

There is a strong interest in developing sensitive Short Wavelength Infrared (SWIR) avalanche photodiodes (APDs) for applications like eye safe laser ranging and robotic vision. The excess noise associated with the avalanche process is critical in dictating the sensitivity of APDs. InGaAs APDs that are commonly used in the SWIR region have either InP or InAlAs as an avalanche layer and these materials have excess noise factor of 0.5 and 0.22, respectively. Earlier, Spectrolab had developed APDs with impact ionization engineering (I2E) structures based on InAlAs and InGaAlAs heterostructures as avalanche layers. These I2E APDs showed an excess noise factor of 0.15. A photoreceiver based on the I2E APD exhibited an noise equivalent power (NEP) of 150 fW/rt(Hz) over 1 GHz bandwidth at 1.06 μm. In this paper, a new multiplier structure based on multiple stages of I2E is studied. The APDs show optical gains over 100 before device breakdown. The increased gain and low excess noise will improve the sensitivity of InGaAs APDs based photoreceivers.