We report on a simple and robust all-fiber femtosecond MOPA system based on an optimized active tapered double-clad fiber (T-DCF). The main feature of this active medium is that the thin part, acting as a pre-amplifier, allows to propagate mostly the fundamental mode LP01, while the signal experiences an extremely high gain in the thick part of the tapered fiber, where most of the pump power is absorbed. These T-DCF properties make them particularly well suited for designing high energy CPA based femtosecond amplifiers, with a high non-linear threshold (SPM, SRS) and a diffraction limited output beam. After the compressor, we obtained pulses amplified up to 40 μJ energy, 412 fs duration and 97 MW peak power at 1036 nm. To the best of our knowledge, it is the highest peak power reported in such T-DCF amplifier in the femtosecond regime.
High power diffraction-limited 1064 nm fiber lasers operating in the nanosecond regime can be used for long-range LIDAR and micromachining applications. Peak power is limited by non-linearities, there is therefore an interest to develop fibers exhibiting a very large mode field effective area. New fibers are being developed in the frame of the 4F consortium ("French laser Fibers for Factories of the Future") to fulfill this need. We report on results obtained with a new 39 μm core diameter polarization maintaining ytterbium doped fiber that has been manufactured using the powder sintering technology. It features a large cladding absorption close to 20 dB/m at 976 nm (small signal) and a mode field diameter close to 32 μm. We built a pulsed MOPA. The preamplifier generates 2.5 ns pulses at 1064 nm with 8.5 W average power at 1 MHz pulse repetition frequency. The power amplifier is based on the 39 μm core fiber with 215/230μm hexagonal cladding counterpumped at 976 nm. It features 72 % slope efficiency delivering 72.2 W average power at a pulse repetition rate of 1 MHz. An end-cap was spliced to the fiber output to increase the damage threshold. At 100 kHz a peak power of 351 kW was measured for an average power of 59.9 W. The efficiency is then 70 %. We also studied the influence of the bending radius on the slope efficiency. We do not observe any slope efficiency reduction down to 25 cm bending diameter. It decreased to 68 % for the 20 cm bending diameter. The laser shows a quasisinglemode output beam with a good quality factor M2 of 1.2.
Recent research efforts on detector arrays in the near-infrared (NIR) and the short-wave infrared (SWIR) wavelength range have led to compact and robust 3D flash lidar architectures [1]. This technique can provide real-time 3D mapping of an area in various weather conditions for surveillance, but also for detection of moving objects in guidance of unmanned vehicles. Today main size limitation for compact, portable systems is the pulsed laser source. An alternative to solid-state or fiber lasers could be laser diodes since they have a better efficiency and their beam profile is compatible with active imaging by flash illumination. This paper will present recent advances in mJ class pulsed laser diodes made by Quantel Laser, France. Experimental results will be described both in NIR and in SWIR bands.
Recent research efforts on detector arrays in the near-infrared (NIR) and the short-wave infrared (SWIR) wavelength range have led to compact and robust 3D flash lidar architectures [1]. This technique can provide real-time 3D mapping of an area in various weather conditions for surveillance, but also for detection of moving objects in guidance of unmanned vehicles. Today main size limitation for compact, portable systems is the pulsed laser source. An alternative to solid-state or fiber lasers could be laser diodes since they have a better efficiency and their beam profile is compatible with active imaging by flash illumination. This paper will present recent advances in mJ class pulsed laser diodes made by Quantel Laser, France. Experimental results will be described both in NIR and in SWIR bands.
This paper will present ultra-compact low-cost laser diode sources, including its diode driver electronics, that have been developed for integration in a photoacoustic imaging system. These laser sources deliver mJ pulses in the near-infrared range.
Imaging based on laser illumination is present in various fields of applications such as medicine, security, defense, civil engineering and in the automotive sector. In this last domain, research and development to bring autonomous vehicles on the roads has been intensified the recent years. Among the various technologies currently studied, automotive lidars are a fast-growing one due to their accuracy to detect a wide range of objects at distances up to a few hundreds of meters in various weather conditions. First commercialized devices for ADAS were laser scanners. Since then, new architectures have recently appeared such as solid-state lidar and flash lidar that offer a higher compactness, robustness and a cost reduction. Flash lidars are based on time-of-flight measurements, with the particularity that they do not require beam scanners because only one short laser pulse with a large divergence is used to enlighten the whole scene. Depth of encountered objects can then be recovered from measurement of echoed light at once, hence enabling real-time 3D mapping of the environment. This paper will bring into the picture a cutting edge laser diode source that can deliver millijoule pulses as short as 12 ns, which makes them highly suitable for integration in flash lidars. They provide a 100-kW peak power highly divergent beam in a footprint of 4x5 cm2 (including both the laser diode and driver) and with a 30-% electrical-to-optical efficiency, making them suitable for integration in environments in which compactness and power consumption are a priority. Their emission in the range of 800-1000 nm is considered to be eye safe when taking into account the high divergence of the output beam. An overview of architecture of these state-of-the-art pulsed laser diode sources will be given together with some solutions for their integration in 3D mapping systems. Future work leads will be discussed for miniaturization of the laser diode and drastic cost reduction.
Active imaging based on laser illumination is used in various fields such as medicine, security, defense, civil engineering and in the automotive sector. In this last domain, research and development to bring autonomous vehicles on the roads has been intensified these last years with an emphasis on lidar technology that is probably the key to achieve full automation level. Based on time-of-flight measurements, the profile of objects can be measured together with their location in various conditions, creating a 3D mapping of the environment. To be embedded on a vehicle as advanced driver assistance systems (ADAS), these sensors require compactness, low-cost and reliability, as it is provided by a flash lidar. An attractive candidate, especially with respect to cost reduction, for the laser source integrated in these devices is certainly laser diodes as long as they can provide sufficiently short pulses with a high energy.
A recent breakthrough in laser diode and diode driver technology made by Quantel (Les Ulis, France) now allows laser emission higher than 1 mJ with pulses as short as 12 ns in a footprint of 4x5 cm2 (including both the laser diode and driver) and an electrical-to-optical conversion efficiency of the whole laser diode source higher than 25% at this level of energy. The components used for the laser source presented here can all be manufactured at low cost. In particular, instead of having several individual laser diodes positioned side by side, the laser diodes are monolithically integrated on a single semiconductor chip. The chips are then integrated directly on the driver board in a single assembly step. These laser sources emit in the range of 800-1000 nm and their emission is considered to be eye safe when taking into account the high divergence of the output beam and the aperture of possible macro lenses so that they can be used for end consumer applications. Experimental characterization of these state-of-the-art pulsed laser diode sources will be given. Future work leads will be discussed for miniaturization of the laser diode and drastic cost reduction.
The ageing population faces today an increase of chronic diseases such as rheumatism/arthritis, cancer and cardio vascular diseases for which appropriate treatments based on a diagnosis at an early-stage of the disease are required. Some imaging techniques are already available in order to get structural information. Within the non-invasive group, ultrasound images are common in these fields of medicine. However, there is a need for a point-of-care device for imaging smaller structures such as blood vessels that cannot be observed with purely ultrasound based devices. Photoacoustics proved to be an attractive candidate. This novel imaging technique combines pulsed laser light for excitation of tissues and an ultrasound transducer as a receptor. Introduction of this technique into the clinic requires to drastically shrink the size and cost of the expensive and bulky nanosecond lasers generally used for light emission. In that context, demonstration of ultra-short pulse emission with highly efficient laser diodes in the near-infrared range has been performed by Quantel, France. A multi-wavelength laser source as small as a hand emitted more than 1 mJ per wavelength with four different wavelengths available in pulses of about 90 ns. Such a laser source can be integrated into high sensitivity photoacoustic handheld systems due to their outstanding electrical-to-optical efficiency of about 25 %. Further work continues to decrease the pulse length as low as 40 ns while increasing the pulse energy to 2 mJ.
Photoacoustics is a novel medical imaging technique with high potential for early detection of different diseases such as skin cancer or rheumatology. It is a hybrid modality with pulsed laser light for excitation of the tissue, and ultrasound as response. One of the hurdles for its introduction into the clinic, or even in clinical pilot studies and larger trials, is the bulkiness and price of existing photoacoustic systems. This presentation describes how recent developments in diode laser technology lead to a compact ultrasound scanner with built-in photoacoustic functionality. This is a key for the introduction of photoacoustic technology in the clinic and future point of care systems. We have developed a diode laser system and driver that deliver pulse energies which up to now were only achievable with Nd:YAG lasers. The efficiency and compactness allows integration in a handheld probe. The paper will highlights the laser technology and its radical integration with a medical ultrasound scanner, leading to a first prototype for clinical pilot studies.
Experiments have been performed to demonstrate that near infrared (NIR) transmission through a wide range of clothing
materials is possible. Studies have shown that the characteristics of NIR transmission are affected by both the type of
fibre used, and the weave pattern. A series of experiments has indicated that NIR transmission is also dependent on other
variables such as fabric porosity and dye colour. It is shown that, in many cases, transmission coefficients are sufficiently
high that imaging and spectroscopy of objects hidden behind clothing samples should be possible. However, while
transmission through clothing at NIR wavelengths in the 750-1,700 nm range is often more effective than in the visible or
IR regions, the fabrics themselves will modify the transmitted signal in terms of spatial effects, intensity and spectral
content. The paper also describes the possible use of near infrared signals to identify objects that are hidden behind
clothing layers. This can be done using spectroscopy. It is important, however, to distinguish the various contributions
that exist within the backscattered signal. A set of careful laboratory experiments have demonstrated that transmission
through a set of different clothing fabrics does modify the spectral content of signals, but that the spectrum of a particular
chemical can still be identified, provided certain steps are taken. These involve a set of careful calibration measurements,
and the use of processing techniques for the retrieval of data. It will be shown that this is possible for both granular solids
and selected liquids.
KEYWORDS: Calibration, Near infrared, Data modeling, Hydrogen, Signal to noise ratio, Chemical analysis, Spectroscopy, Neural networks, Statistical modeling, Liquids
The detection of specific chemicals when concealed behind a layer of clothing is reported using near infrared (NIR)
spectroscopy. Concealment modifies the spectrum of a particular chemical when recorded at stand-off ranges of three
meters in a diffuse reflection experiment. The subsequent analysis to identify a particular chemical has involved
employing calibration models such as principal component regression (PCR) and partial least squares regression (PLSR).
Additionally, detection has been attempted with good results using neural networks. The latter technique serves to
overcome nonlinearities in the calibration/training dataset, affording more robust modelling. Finally, lock-in
amplification of spectral data collected in through-transmission arrangement has been shown to allow detection at SNR
as low as -60dB. The work has been shown to both allow detection of specific chemicals concealed behind a single
intervening layer of fabric material, and to estimate the concentration of certain liquids.
This paper will demonstrate that near infrared (NIR) signals at wavelengths in the range 0.9 to 2.5 microns can be used
for personal screening applications. At these wavelengths, there is sufficient spectral information to provide chemical
identification, while still providing transmission through many types of common clothing materials. Thus, chemical
identification in diffuse reflection is possible.
Initial measurements on selected clothing materials have indicated that there is sufficient transmission to allow NIR
spectra from concealed chemicals to be collected. The effect of the clothing material on the observed spectra has also
been quantified. The clothing materials ranged from cotton to man-made fibres. Spectra have been collected at stand-off
distances of several metres or more, using a suitable lens system and an NIR spectrometer. The optics required to
achieve this will be described, and some spectra from chemicals hidden behind clothing will be presented. The further
steps necessary to provide correct identification of chemicals such as ammonium nitrate in granular form will also be
given, involving signal analysis methods. A set of spectra will be shown that have been collected and analysed, for a
wide range of clothing fabric materials, indicating that the technique could have wide application to personal screening
situations.
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