All-optical modulator for gated range finding and active imaging in LWIR

Since the 1960s range finding through the use of Laser Imaging, Detection, and Ranging (LIDAR)^1-4 and Range Gated Active Imaging (RGAI)^{5, 6} has been explored. Presently, these processes of range finding are considered as fast and reliable methods for determining distance in the domestic, commercial and defence environments. Both technologies share benefits such as fast measurement time, high accuracy and resolution, and the ability to carry out measurements remotely. While both methods utilise a laser and some form of detector to measure distance, this is where the similarities end. LIDAR typically creates a 3D point cloud of the scene by scanning the laser across the scene and measuring the travel time for the laser pulse, usually recorded by a single pixel detector. RGAI on the other hand, illuminates a scene at a fixed repetition rate and moves the gate of the sensor, operating at the same frequency as the laser, in time with respect to the output of the laser. The sensor, usually a focal plane array (FPA), records the incident light while the gate is open. A depth map can then be built where the observed depth is related to the delay between the laser firing and the gate opening. The depth of the image correlates to the time the gate is open; the longer the exposure time, the deeper the image. These methods for range finding predominantly use lasers and detectors that operate in the Near Infrared (NIR)⁷ and Short Wave Infrared (SWIR),^{8, 9} usually around the telecom wavelengths. For these wavelengths, there exist multiple options for single pixel detectors that are inherently fast, leading to a high temporal resolution for LIDAR, or in the case of RGAI, detectors that can be gated with short on-time windows.^{10, 11} The choice to move from the visible range to longer wavelengths is due to the reduced effect of atmospheric scattering, relative saftey for the retina of the eye and mature market of fast lasers and detectors necessary for high temporal resolution. The reduced effect of atmospheric scattering and transmission through obscurants to the visible range is a huge benefit.¹² The use of Medium Wave Infrared (MWIR) and Long Wave Infrared (LWIR) in range finding is a relatively unexplored area, apart from a handful of experiments and government projects in the 80s^13-16 and later on in the late 2000s.¹⁷ There exists numerous high power, short rise time IR lasers which will enable a high resolution measurement. The main factor delaying the development in the MWIR and LWIR is attributed to the fact that detectors currently on the market are generally either very sensitive, but have a long rise time, or they are relatively fast but tend to sacrifice sensitivity.^{18, 19} This leads to either obtaining inaccurate results or needing a very high power laser and longer integration time to achieve reliable results. However, the use of the longer wavelengths could herald many advantages such as an increased range because of greatly reduced scattering - the MWIR and LWIR bands both include large transmission windows spanning 3 µm and 8 − 12 µm spectral windows.²⁰ The use of longer wavelength range will reduce the likelihood of damage caused to the eye retinas due to inability to penetrate the outer tissue.^{21, 22} Our research aims to overcome this problem by externally gating a slow but sensitive detector to achieve high resolution at low laser power illuminating the scane. To accomplish this we designed and built an all-optical modulator comprising of an 808 nm diode laser to illuminate a semiconductor window. The transparency of the window can be modulated between the states of nearly full transparency to opaqueness when the laser pulse is absorbed. The rate of change of transparency follows that of the laser pulse's leading edge. This group of components is further referred to as the modulator. Since there are no moving parts involved in the modulator this would be a perfect drop-in replacement for more traditional mechanical style shutters. A solid state shutter, with all-optical modulation would be more rugged and less susceptible to shocks, g-forces and offer fewer points of wear and tear, increasing logenvity of the device with benefits offering faster activation times. This could be used for shorter exposures and faster reaction times to overexposure, potentially reducing damage caused to sensors under high illumination. Another added advantage of the modulator is the possibility to modulate the signal between totally on and totally off. IR modulators available today are either mechanical, such as optical choppers and shutters, or electro-optical such as LCD style technology,²³ Pockels cells and acousto-optic modulator. While LCD technology offers a solid state solution to IR modulation, it does not offer the speed that our technology could achieve. This paper looks into the feasibility of an active imaging LWIR system for use in RGAI applications and LWIR time-of- ight range finder using the all-optical solid state modulator technology. The signal is provided by a LWIR, 10:07 µm Hamamatsu Quantum Cascade Laser (QCL). To assess the abilities of the range-finding a Mercury Cadmium Telluride (MCT) detector was used to detect the LWIR laser pulse. The setup included two separate optical paths, with one being approximately 18m longer than the other. It was found that we were able to reliably discern a difference between the two optical paths. For active imaging an uncooled Thermoteknix camera was used to image an object placed in the beam path before the modulator. It was found that it was possible to gate the camera with such timing that the image of the object, with an illumination time of 20 µs, would almost disappear entirely from the scene.