We present our last numerical and experimental results on a mid-infrared source based on a tunable Yb-based hybrid MOPA pump and a Backward Wave Optical Parametric Oscillators (BWOPO). The BWOPO has a record-low oscillation threshold of 19.2 MW/cm2 and generates mJ-level output with an overall conversion efficiency exceeding 70%. The BWOPO acts a frequency shifter of the pump radiation toward the forward wave, maintaining the pump spectral properties. The demonstrated tuning range of 10 GHz is already compliant for DIAL applications. We have also developed advanced numerical modelling of the BWOPO taking into account spectral and, for the first time, spatial beam profiles.
A highly efficient mirrorless OPO tunable in the mid-infrared around 2 μm has been developed and characterized in an original pumping configuration comprising a tunable high power hybrid Ytterbium laser MOPA (Master Oscillator Power Amplifier) in the nanosecond regime. The hybrid pump laser is based on a fiber laser seeder continuously tunable over several GHz at 1030 nm, which is shaped in the time domain with acousto-optic modulators (AOM), and power amplified in a dual stage Ytterbium doped fiber amplifiers, followed by two Yb:YAG bulk amplifiers. The pump delivers up to 3.5 mJ of energy within narrowband 15 ns pulses with a 5 kHz repetition rate. The output was focused into Periodically Poled KTP (PPKTP) crystals with a quasi-Phase Matching (QPM) period of 580 nm, producing Backward Optical Parametric Oscillation (BWOPO), with a forward signal wave at 1981 nm and a backward traveling idler at 2145 nm. We report significant optical to optical efficiencies exceeding 70 % depending on crystal length and input power. As theoretically expected, the forward wave could be continuously tuned over 10 GHz following the pump frequency sweep, while the backward wave remains almost stable, both being free from mode hops. These properties obtained from an optical arrangement without free-space cavities are attractive for future space Integrated Path Differential Absorption (IPDA) Lidar applications, which require robust and efficient tunable frequency converters in the mid-infrared. Additional presentation content can be accessed on the supplemental content page.
We present design and first performance results of an airborne differential absorption lidar laser transmitter that can measure CO2 and water isotopes at different wavelengths around 2 µm with the same setup. This laser will be integrated into an airborne lidar, intended to demonstrate future spaceborne instrument characteristics with high-energy (several tens of millijoules nanosecond-pulses) and high optical frequency-stability (less than a few hundreds of kilohertz long-term drift).
The transmitter consists of a widely tunable OPO with successive OPA that are pumped by a Nd:YAG MOPA and generates the on- and offline wavelength of the addressed species with narrow bandwidth.
We present our activities on the development of narrow linewidth tunable optical parametric sources and their integration in lidar systems. In particular, we present different implementations of the nested cavity optical parametric oscillator (NesCOPO) that enables tunable single-frequency emission from the SWIR to the LWIR, when pumped by a fixed or a tunable wavelength laser beam. We show how to amplify the output energy and while preserving the spectral linewidth to perform standoff detection of greenhouse gases and toxic chemicals with direct detection lidars.
We report on the current design and preliminary developments of the airborne Lidar Emitter and Multi-species greenhouse gases Observation iNstrument (LEMON), which is aiming at probing H2O and its isotope HDO at 1982 nm, CO2 at 2051 nm, and potentially CH4 at 2290 nm, with the Differential Absorption Lidar method (DIAL). The infrared emitter is based on the combination of two Nested Cavity OPOs (NesCOPOs) with a single optical parametric amplifier (OPA) line for high-energy pulse generation. This configuration is enabled by the use of high-aperture periodically poled KTP crystals (PPKTP), which provide efficient amplification in the spectral range of interest around 2 μm with slight temperature adjustments. The parametric stages are pumped with a Nd:YAG laser providing 200 mJ nanosecond double pulses at 75 Hz. According to parametric conversion simulations supported by current laboratory experiments, output energies in the 40 - 50 mJ range are expected in the extracted signal beam whilst maintaining a good beam quality (M² < 2). The ruler for all the optical frequencies involved in the system is planned to be provided by a GPS referenced frequency comb with large mode spacing (1 GHz) against which the emitter output pulses can be heterodyned. The frequency precision measurement is expected to be better than 200 kHz for the optical frequencies of interest. The presentation will give an overview of the key elements of design and of preliminary experimental characterizations of sub-systems building blocks.
In the scope of the preparation of spaceborne lidar missions to measure the concentration of greenhouse gases with differential absorption LIDAR techniques, we report on the development of a high energy 2.05 μm optical parametric source based on a versatile architecture enabling multiple wavelengths generation in the vicinity of the R30 absorption line of CO2. The multi-wavelength configuration is under study for a few greenhouse gas active detection missions, such as Ascend.
We report on a DIAL emitter for remote sensing of greenhouse gases, capable of addressing the three species of interest (CO2, CH4 and H2O) for space applications with a single optical source. It is based on an amplified Nested Cavities Optical Parametric Oscillator (NesCOPO) around 2 μm. The source is single frequency over a wide range of tuneability between 2.05 – 2.3 μm, and shows a typical energy conversion efficiency of 20% toward the signal wave. Spectral analysis shows a linewidth better than 100 MHz. These performances are measured in the vicinity of absorption lines of interest for space remote sensing of the three gases.
High-energy mid-infrared nanosecond sources are required in a number of applications including biomedicine, remote sensing, and standoff countermeasures, to name just a few. Sources which serve these applications include mid-infrared fiber and solid-state lasers, quantum cascade lasers, as well as optical parametric oscillators (OPO).
Several possible future spatial lidar missions (MERLIN EXCALIBUR ASCENDS) are intended to measure major greenhouse gases (CO2, H2O, CH4) in order to better understand their cycle and their impact on climate change.
Prevision of climate change is presently one of the main research goals. In order to improve the accuracy of current climate models, it is necessary to better characterize the main greenhouse gases concentration and fluxes (CO2, CH4, and water vapor) at a global scale. For this purpose one promising solution is the space-borne integrated path differential absorption lidar (IP-DIAL) technique, which is currently investigated by space agencies in the preparation of future missions such as MERLIN (CNES-DLR) for methane, or ASCENDS (NASA) for carbon dioxide. One of the challenges for these missions is to have high energy laser sources which can emit specific wavelength to address the species of interest. At ONERA, a high energy transmitter based on a broadly tunable parametric source has been developed in the 2 μm spectral region to address the main greenhouse gases absorption lines that are well-suited for space application1. This source has been recently implemented on the R30 CO2 absorption line at 2051 nm for ground-based range resolved measurements in the atmosphere. In our set-up the source emits 10 mJ pulses at a 30 Hz repetition rate. The backscattered light from aerosols is collected with a Newton telescope and a direct detection scheme based on an InGaAs photodiode. CO2 concentration has been estimated with a precision better than 25 ppm for a 200-meter spatial resolution in the 100-500 m range and a 10 minutes acquisition time.
Coherent beam combining (CBC) by active phase control could be useful for power scaling fiber-laser-pumped optical frequency converters like optical parametric oscillators (OPOs). However, a phase modulator operating at the frequency-converted wavelength would be needed, which is a non-standard component.
Fortunately, nonlinear conversion processes rely on a phase-matching condition, correlating not only the wave-vectors of the coupled waves, but also their phases. It is therefore possible to control the phase indirectly, using more standard phase modulators.
Feasibility of this technique was previously demonstrated for second harmonic generators (SHG). Controlling the phase of the fundamental wave, excellent harmonic wave combining efficiency was achieved in both cases of phase matching and quasi phase matching, with lower than λ/30 residual phase error.
In this paper, coherent combining of difference frequency generators (DFG) is experimentally tested. Even if DFG is more challenging than SHG as it implies handling three waves instead of two, phase control of the sole 1-μm pump waves is sufficient to combine the 3.4-μm waves generated.
The mid-infrared DFG crystals are pumped and signal-seeded with standard all-fiber sources at 1 μm and 1.5 μm respectively. Phase control is performed with an electro-optic phase modulator which is a standard all-fiber component operating at 1 μm.
CBC of mid-infrared DFG modules is a first step towards combining continuous wave OPOs.
We report on a widely tunable synchronously-pumped picosecond OPO combining an aperiodically poled MgOdoped LiNbO3 crystal as a broadband gain medium and an intracavity axially chirped volume Bragg (VBG). Owing to the high dispersion induced by the chirped VBG, only a narrow spectral band, corresponding to a thin slice of the VBG, satisfies the synchronous-pumping condition. At a fixed position, the VBG is thus a narrow-band filtering element; variation of its position along the cavity axis enables to tune the idler wavelength over 215nm around 3.82 μm. Rapid continuous tuning over 150nm in 100 ms is also demonstrated.
We report on the first single-frequency nanosecond optical parametric oscillator (OPO) emitting in the longwave infrared, and use it to perform standoff detection of ammonia vapor by differential spectrometry. The OPO is based on orientation-patterned GaAs (OP-GaAs) pumped by a pulsed single-frequency Tm:YAP microlaser. Single-longitudinal mode emission is obtained owing to a nested cavity OPO (NesCOPO) scheme. The OPO is tuned over 700 nm around 10.4 μm, allowing to measure the absorption spectrum of ammonia across several lines at atmospheric pressure. The potential of this OPO for standoff detection of hazardous gases is also discussed.
We present our activities on the development of narrow linewidth tunable optical parametric sources and their integration in gas sensing instruments. In particular, we have introduced the nested cavity optical parametric oscillator (NesCOPO) scheme that enables to implement very compact devices. The NesCOPO was successfully demonstrated in the microsecond to nanosecond regime and in spectral ranges from short- to long-wave infrared. Its high potential was demonstrated both for local photoacoustic spectroscopy and standoff detection using lidar instruments. We also present our recent advances on rapidly tunable picosecond OPOs based on aperiodic quasi-phase matching and their application to gas detection.
Integrated-path differential absorption lidar (IPDIAL) is an attractive technique to monitor greenhouse gases from space. For that purpose, suitable absorption lines have been identified as good candidates around 2.05 μm for CO2, 2.29 μm for CH4, and 2.06 μm for H2O. In this context, we have developed a high energy transmitter around 2 μm based on frequency conversion in a nested cavity doubly resonant optical parametric oscillator (NesCOPO) followed by high energy parametric amplification. This master oscillator power amplifier (MOPA) architecture enables the generation of tunable single-frequency high energy nanosecond pulses (tens of mJ) suitable for atmospheric DIAL applications. Moreover, taking advantage of the wide spectral coverage capability of the NesCOPO, we demonstrate the potential for this single emitter to address the aforementioned spectral lines, without the use of additional seeding devices. The emitter provides energies up to 20 mJ for the signal waves in the vicinity of CO2 and H2O lines, and 16 mJ at 2290 nm for the CH4 line. By implementing a control loop based on a wavemeter frequency measurement, the signal fluctuations can be maintained below 1 MHz rms for 10 s averaging time. Finally, from optical heterodyne analysis of the beat note between our emitter and a stabilized laser diode, the optical parametric source linewidth was estimated to be better than 60 MHz (Full width at half maximum).
We present our results on the first nanosecond single-frequency optical parametric oscillator (OPO) emitting in the
longwave infrared. It is based on orientation-patterned GaAs (OP-GaAs), and can be pumped by a pulsed singlefrequency
Tm:YAP microlaser thanks to its low oscillation threshold of 10 μJ. Stable single-longitudinal mode emission
of the OPO is obtained owing to Vernier spectral filtering provided by its nested cavity OPO (NesCOPO) scheme.
Crystal temperature tuning covers the 10.3-10.9 μm range with a single quasi-phase-matching period of 72.6 μm. Shortrange
standoff detection of ammonia vapor around 10.4 μm is performed with this source. We believe that this
achievement paves the way to differential absorption lidars in the LWIR with increased robustness and reduced footprint.
AgGaGeS4 compound (AGGS) is a promising nonlinear material for mid-IR applications. The different steps of this
materials processing are presented. The chemical synthesis of polycrystals and the single crystal growth process are
described. Compounds volatility can induce stoichiometry deviation and reduce the quality of obtained single crystals.
Nevertheless, 28 mm diameter and 70 mm length single crystals have been grown by Bridgman-Stockbarger method, cut
and polished AGGS crystal is obtained. The crystal has good homogeneity and absorption coefficient of less than 0.1
cm-1 in the 0.5-11.5 μm range which make it usable in nonlinear devices.
Spectrally tunable narrow-linewidth mid-infrared sources are used in a variety of spectrometric optical systems for
detection, identification, and/or quantification of chemical species. However, in the pulsed regime they often display a
varying spectrum in time, either from shot-to-shot or during the pulse itself, with consequences on the measurement
accuracy, resolution, and repeatability. This is, for instance, the case of pulsed quantum cascade lasers (QCL), mainly
because of strong transient thermal effects in the optical waveguide. Unfortunately, little information has been published
on this subject because mid-infrared time-resolved spectrometers are extremely scarce. In this paper, we explain how this
can be circumvented by using time-gated frequency upconversion in a nonlinear crystal. We apply this principle to
characterize a pulsed external cavity QCL (EC-QCL) at 7.8 μm, using AgGaS2 as the nonlinear crystal and a Q-switched
Nd:YAG laser as the pump source. The upconverted near infrared spectrum is conveniently analyzed with a high
resolution lambdameter and an optical spectrum analyzer. We evidence frequency chirp at an average rate of -50 MHz/ns
and mode hops spanning 15 GHz for the EC-QCL. These results are compared to published data.
We present, in a first part, the complete elaboration process of AgGaS2 to obtain 4×4×15 mm3 slabs and the first results
on ZnGeP2. A two zone oven was designed for the chemical synthesis and the vertical Bridgman method was chosen for
the growth. These slabs were tested on optical benches to produce mid-IR laser emissions from near IR sources: results
on Difference Frequency Generation are presented in a second part.
We report on the development and characteristics of infrared solid state laser as compact and robust light sources for
Directed Infrared Countermeasures (DIRCM).
DIRCM against infrared missile seekers requires wavelength tunable laser sources. When adding an optical parametric
oscillator to a pump laser source, it is possible to cover the 2-5 μm wavelength transmission windows. For more than
five years, CILAS has developed critical technologies for the development of a laser source adapted for DIRCM
application. This includes: The choice of the crystals, the optical coatings in band II, the thermal management and an
optimized oscillator configuration insensitive to repetition rate variation in a wide range. The scope of this presentation is
to recall performances of the technologies developed and to present the technical base line of our laser concept. This
work is supported by the French MOD (DGA).
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