Natural and man-made obscurants like fog, cloud, smoke and dust are an impediment to the conduct of military operations, preventing effective pilotage, denying the ability to carry out surveillance and reconnaissance, and restricting situational awareness. Additionally, there is a growing interest in the ability to penetrate haze and fog for the safe navigation of autonomous vehicle applications.
There are several electro-optic technologies that offer improved ability to image through obscurants [1,2]. In this study the authors assessed 4 different active imaging technologies in the presence of an artificial smoke, and obtained 3D imagery of targets at ranges of 100m out to 1400m. The four systems tested were:
• a scanned time-correlated single photon counting (TCSPC) sensor using a InGaAs/InP single-photon avalanche diode (SPAD) detector operating at ~ 1.55 µm ;
• a 32 32 InGaAs/InP SPAD array using TCSPC at ~ 1.55 µm;
• a coherent frequency modulated continuous wave (FMCW) scanned lidar system ~ 1.55 µm , ;
• a CMOS SPAD array camera operating as a time gated imager operating at ~ 670nm.
The selection of sensors enables comparisons to be drawn between scanning and staring systems and direct detection and coherent detection, and between short-wave infrared and visible wavelengths.
Three-dimensional structured targets were placed at ranges of 100 – 150m and smoke was introduced between the targets and the sensors. The smoke transmission was measured with a separate laser device to correlate the imagery with the level of attenuation presented by the smoke and thereby relate the image quality to the degree of optical loss in the system. For the coherent lidar system, long range 3D images were obtained out to a distance of 1400m, and imaging through smoke of a target at 900m was achieved. Under the test conditions at least 2 of the systems have demonstrated the ability to obtain images through greater than 4 attenuation lengths of obscurant between transceiver and target, and work is progressing on image processing approaches to reconstruct images at greater levels of loss.
Imagery from the systems will be presented, the relative merits of the different techniques discussed, and the prospects for future practical systems will be explored.
 “Demonstration of frequency modulated continuous wave (FMCW) eye-safe, coherent LIDAR to See Through Clouds”, M.Silver, P.Feneyrou, L.Leviander, A.Martin and J Parsons, Optro, Jan 2018.
 “Depth imaging through obscurants using time-correlated single-photon counting”, R.Tobin, A.Halimi, A.McCarthy, M.Laurenzis, F.Christnacher and G.S.Buller, SPIE Vol 10659, April 2018
Compact eye-safe laser rangefinders (LRFs) are a key technology for future sensors. In addition to reduced size, weight and power (SWaP), compact LRFs are increasingly being required to deliver a higher repetition rate, burst mode capability. Burst mode allows acquisition of telemetry data from fast moving targets or while sensing-on-the-move. We will describe a new, ultra-compact, long-range, eye-safe laser rangefinder that incorporates a novel transmitter that can deliver a burst capability. The transmitter is a diode-pumped, erbium:glass, passively Q-switched, solid-state laser which uses design and packaging techniques adopted from the telecom components sector. The key advantage of this approach is that the transmitter can be engineered to match the physical dimensions of the active laser components and the submillimetre sized laser spot. This makes the transmitter significantly smaller than existing designs, leading to big improvements in thermal management, and allowing higher repetition rates. In addition, the design approach leads to devices that have higher reliability, lower cost, and smaller form-factor, than previously possible. We present results from the laser rangefinder that incorporates the new transmitter. The LRF has dimensions (L x W x H) of 100 x 55 x 34 mm and achieves ranges of up to 15km from a single shot, and over a temperature range of -32°C to +60°C. Due to the transmitter’s superior thermal performance, the unit is capable of repetition rates of 1Hz continuous operation and short bursts of up to 4Hz. Short bursts of 10Hz have also been demonstrated from the transmitter in the laboratory.
Lasers intended for application to man-portable and hand-held laser target designators are subject to significant constraints on size, weight, power consumption and cost. These constraints must be met while maintaining adequate performance across a challenging environmental specification. One of the challenges of operating a Nd3+:YAG laser over a broad ambient temperature range is that of diode-pump-tuning. This system is specified to operate over an ambient temperature range of –46°C to +71°C, and the system electrical power consumption requirements preclude active temperature control. As a result the laser must tolerate a 32.8nm pump wavelength range. The optical absorption of Nd3+:YAG varies dramatically over this wavelength range. This paper presents a laser that minimizes the effect of this change on laser output. A folded U-shaped geometry laser resonator is presented, made up of a corner cube at one end and a plane mirror substrate at the other. The action of the corner cube coupled with this configuration of end mirrors results in a resonator that is significantly less sensitive to misalignment of the end mirror and/or the corner cube. This Ushaped resonator is then further folded to fit the laser into a smaller volume. Insensitivity of this compact folded resonator to mirror misalignments was analyzed in Zemax via a Monte-Carlo analysis and the results of this analysis are presented. The resulting laser output energy, pulse duration and beam quality of this athermally pumped, misalignment insensitive folded laser resonator are presented over an ambient temperature range of –46°C to +71°C.
We present a passively Q-switched, diode end-pumped, 1μm Nd:YAG laser with a single pulse energy in excess of 40mJ. To our knowledge, this is the highest single pulse energy reported for a passively Q-switched end-pumped laser. We achieved this with a novel pump scheme, which uses an engineered diffuser to create the necessary uniform gain distribution for efficient passive Q-switching. The system consists of a 3kW, 808nm, diode-laser stack pump source, and a set of collimating optics, with the engineered diffuser, to homogenise and couple the pump beam into the end of a 20mm diameter Nd:YAG laser rod. Q-switching is achieved with a Cr:YAG saturable absorber within a plane-parallel cavity. The 40mJ value was achieved despite a pump coupling efficiency of only 55%; hence we believe higher energies are achievable. The beam parameter product and pulse width were measured to be 12mm mRad and 18ns, respectively, which are consistent with those required for designation. We have investigated the pulse-to-pulse timing jitter of our system, which has been previously cited as the main drawback when implementing passive Q-switching for designation applications. We have achieved a reduction in timing jitter from 16 μs to 3.2 μs by environmental isolation of the laser resonator.
We report development activities towards realization of fully integrated 1×2, 2×2 and 4×4 cross-point optical switches
for WDM-packet based data networking. Two enabling technologies, quantum well intermixing and etched turning
mirrors, are developed and demonstrated in InGaAs/InAlGaAs InP-based material at a wavelength of 1.55 &mgr;m. We
describe the use of both technologies to fabricate switch chips with different port counts.
Quantum well intermixing (QWI) can bring considerable benefits to the reliability and performance of high power laser diodes by intermixing the facet regions of the device to increase the band-gap and hence eliminate absorption, avoiding catastrophic optical damage (COD). The non-absorbing mirror (NAM) regions of the laser cavity can be up to ~20% of the cavity length, giving an additional benefit on cleave tolerances, to fabricate very large element arrays of high power, individually addressable, single mode lasers. As a consequence, large arrays of single mode lasers can bring additional benefits for packaging in terms of hybrization and integration into an optics system. Our QWI techniques have been applied to a range of material systems, including GaAs/AlGaAs, (Al)GaAsP/AlGaAs and InGaAs/GaAs.