KEYWORDS: Single photon avalanche diodes, Quantum sensors, Sensors, Active imaging, Photon counting, Photodetectors, Non line of sight propagation, Cameras, Detector arrays, Time correlated photon counting
Single photon avalanche diode (SPAD) sensors can detect the occurrence of a single photon and record binary intensities, count rates or event timing with virtually no noise. This revolutionary new optical sensing concept has the potential to trigger a paradigm shift that will change the way we talk and think about optical sensing. In recent years, this technology has made significant advances, and the latest sensors offer high-resolution arrays with high temporal and spatial resolution.
In this paper, we present results achieved at ISL that demonstrate how single-photon imaging combined with computational methods differs from classical imaging methods. We show how we can extract and reconstruct new, previously unattainable information from scenes.
ISL has investigated passive single photon counting to reconstruct the photon flux imaging the sensor array. We could reconstruct image information and obtained up-scaling by application of convolutional neural networks, reduced noise and motion blur by computer vision algorithms. Finally, we extracted modulation frequencies by Fourier analysis and obtained event-based neuromorphic imaging.
Further, we have studied laser-based active imaging of single photons to measure the round-trip path length of light pulses for ranging and 3D imaging. We have analyzed multi-bounce photon path to estimate the size of cavities and to improve vision through scattering media such as dense fog. Finally, we investigated SPAD sensing for the reconstruction of objects outside the direct line of sight in non-line of sight (NLOS) sensing approaches.
In 2019, the French-German Research Institute of Saint-Louis (ISL) is celebrating its 60th anniversary, but the roots of the institute are going back to the early end of the WWII when a team of German scientists led by Prof. Schardin from the Air Force Technical Academy (Technische Akademie der Luftwaffe) in Berlin-Gatow came in the Upper Rhine to work for the French Army. There, the Prof. Schardin was working in the field of ballistics and was world-wide renowned for his works in high-speed physics. In his early years, as a permanent assistant to the eminent German ballistics Professor Carl Cranz, he developed the famous Cranz-Schardin camera, a revolutionary electro-optical high-speed cinematography method using electric sparks or flash x-rays for illumination and able to work at frame rates over 106 images/second. This technique brought huge improvements in the comprehension of ballistic phenomenon and moreover in high-speed physics.
Since 1945, the LRSL, renamed ISL in 1959, maintained a leading position in the domain of high-speed phenomenon. In the beginning of the 60's, the invention of the laser was a true revolution that permits the emergence of new techniques like holography and interferometric holography. With the introduction of semiconductor lasers, ISL has a leading position on range-gated active imaging and deploys a significant research effort in a new emerging domain: computational imaging which includes scientific thematic such as see around the corner, compressed sensing or imaging with multiple scattered photons.
Active imaging is an emerging technology in the field of surveillance for security and military applications. Numerous works have demonstrated the value of active imaging in target recognition and identification, vision through fog, underwater vision or three-dimensional (3D) imaging. However, surveillance applications in civilian and military fields need the use of an eye-safe illumination. Unfortunately in this spectral region, there is still a lack of ITAR-free, commercially available and efficient intensified cameras or laser sources which are the two main components of an active imaging system. Though, a few years ago, ISL showed the feasibility of a portable SWIR night-vision goggle with the PELIS system. This goggle was based on a continuous illumination associated with an InGaAs camera, but with this technique, the fundamental properties of range-gating were not exploited.
In this paper, we report on a new portable and range-gated night-vision goggle in the SWIR spectral region. This goggle will be a useful eye-safe device for surveillance and imaging under all weather conditions. At 1.5 μm, it is well known that human skin appears black making face recognition ineffective. For applications which need facial identification as a legal proof, we implemented a bi-wavelength laser where it is also possible to extract one pulse of light at a second wavelength, where the skin appears with the same reflectance as in the visible spectrum (1.06 μm). After a theoretical analysis, we will describe the goggle technology and show some lab and outdoor recordings.
Laser range-gated viewing experiments in the eye-safe spectral region are conducted where a semiconductor based laser illuminator is associated with a corresponding image detector able to operate in high frequency and high sensitivity shutter mode. After experimental validations of the camera developed for accumulation operation, a high power semiconductor based illuminator has been designed and realized. This technology can be used to develop efficient, compact and high average power SWIR illuminators. In a first step, images of different scenes were recorded in a test tunnel and the results are compared to those recorded simultaneously with a solid state based laser illuminator working in flash mode. Both results are similar in terms of image intensity whereas semiconductor based recordings exhibits lower speckle noise and better homogeneity. In a second step, outdoor experiments were conducted during daylight conditions like full sunshine and cloudy weather and also in night conditions. No significant image degradation is measured even with longer integration time. These results underline the potential of SWIR accumulation mode for outdoor and long range active imaging applications.
The use of light pipes is an efficient and low-cost technique to get a homogeneous illumination for laser-gated viewing systems. However, this technique suffers from drawbacks when used with coherent sources like solid-state lasers. Compacting light pipe-based laser illuminators involves working with small light pipe sections, and experiments show that interference fringes appear on the laser illumination profiles. The principle of light pipe homogenization has been reviewed using geometrical optics to understand the phenomenon better, and a pragmatic light pipe design method, based on laser-gated viewing system parameters, is proposed. Another original solution based on optical stepper is studied to reduce both interference fringes and speckle noise to increase the homogeneity of laser illumination profiles.
One of the most efficient ways to decrease the speckle contrast in the field of laser illumination is to increase the
spatial diversity of coherent laser sources. For very short laser pulses such as those required for flash laser
imaging, the spatial diversity should take place instantaneously and no time averaging effect can be used. The
spatial diversity is realized by sampling the laser beam into m beamlets with increased optical path length. This
path length has to be greater than or equal to the coherence length of the laser beam. In this case, the beamlets
are no longer able to create interferences which each other. According to the Goodman’s theory of speckle
reduction, the speckle contrast is then reduced by a factor of 1/√m. Unfortunately, in the case of multimode
lasers, the number of uncorrelated beamlets is not infinite but is limited by a periodicity function resulting from
the laser resonator length itself. The speckle reduction possibility is therefore limited and is directly linked to
each laser source where the coherence length and cavity length are defined.
In this work we present a methodology to determine experimentally the optical path length difference as well as
the number of beamlets for de-speckling a laser source. An experimental realization is presented where both,
coherence length and periodicity function are measured with a Michelson interferometer where only the speckle
contrast of the two beams from each arm is analyzed. For the validation of the method, the chosen laser source is
a single emitter 660 nm laser diode. Two cylindrical steppers made with diamond turned PMMA have been
realized. Both elements yield interesting results with close values and in accordance with the theory of spatial
diversity. The speckle contrast could be reduced from about 10% to a value close to 4%. These values confirm
and validate the methodology presented in this work.
Steppers can also be a promising solution for the reduction of interference fringes which appear when using a
lightpipe in a laser illuminator design.
Laser diode stacks are interesting laser sources for active imaging illuminators. They allow the accumulation of large amounts of energy in multi-pulse mode, which is well suited for long-range image recording. Even when laser diode stacks are equipped with fast-axis collimation (FAC) and slow-axis collimation (SAC) microlenses, their beam parameter product (BPP) are not compatible with a direct use in highly efficient and compact illuminators. This is particularly true when narrow divergences are required such as for long range applications. To overcome these difficulties, we conducted investigations in three different ways. A first near infrared illuminator based on the use of conductively cooled mini-bars was designed, realized and successfully tested during outdoor experimentations. This custom specified stack was then replaced in a second step by an off-the-shelf FAC + SAC micro lensed stack where the brightness was increased by polarization overlapping. The third method still based on a commercial laser diode stack uses a non imaging optical shaping principle resulting in a virtually restacked laser source with enhanced beam parameters. This low cost, efficient and low alignment sensitivity beam shaping method allows obtaining a compact and high performance laser diode illuminator for long range active imaging applications. The three methods are presented and compared in this paper.
Homogeneous and speckle-free laser illumination devices are the key technology for high resolution active imaging and range-gated viewing systems. At ISL, a waveguide technology was developed to effectively reduce laser speckle of solid-state laser sources by a spatial or angular diversity approach, respectively. Further, a distant scene is illuminated with a homogeneous rectangular top-hat profile. In the present publication we give a theoretical description of the waveguide laser illumination devices and present results from ray tracing simulations and experimental investigation.
We present simple illumination devices with built-in speckle reduction using the spatial diversity approach. These devices are based on a waveguide homogenization technique with a transmission efficiency of up to 95%. Even for single-laser pulse illumination from a solid-state laser source a reduction of the speckle contrast by a factor of 4.5 was demonstrated. In detail, we present two different illumination devices based on either a solid-state laser source or an array of semiconductor laser diodes. These illumination devices are used for range-gated imaging and active polarimetry with speckle-free and homogeneous illumination.
For very long-range flash active imaging applications, a powerful and narrow beam divergence pulsed laser illuminator is
needed. Conventional high fill factor 1-centimeter diode bars are not well suited to produce a homogeneous and narrow
beam divergence in an easy way. In the present paper, a concept based on a QCW mini-bars laser diode stack, focusing
lenses and beam homogenisation is presented. With fifteen 1.5 mm conductively cooled fast axis collimated mini-bars, a
peak power of 803 Watt at a wavelength of 810 nm is achieved with a maximum duty cycle of 2 percent. After
collimation, the laser beam on the scene shows very homogeneous rectangular illumination geometry with a ratio of 4:3
well adapted for active imaging purposes. The beam divergence is reduced to 3.35 × 2.53 mrd with a two lenses objective
of focal length f = 573 mm and f-number f# = 4.8.
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