Ajmal Mohamed

High-speed cavity ringdown spectroscopy with increased spectral resolution by simultaneous laser and cavity tuning

Cavity ringdown spectroscopy is an efficient gas-sensing method, but improvement in measurement speed is required before this method can be applied to the analysis of fast phenomena. We present a new continuous-wave cavity ringdown design, involving fast tuning of the laser frequency and a rapidly swept optical cavity, to allow high-speed sensing with spectral resolution refinement. This approach, which provides a simple and versatile instrument, is investigated numerically and experimentally. By performing detection of a forbidden transition of molecular oxygen near 766 nm during a 2-ms single sweep of the laser frequency, we show that our system fulfils the requirements for probing rapid chemical processes.

High-energy single-longitudinal mode nearly diffraction-limited optical parametric source with 3 MHz frequency stability for CO2 DIAL

We report on a 2.05 microm nanosecond master oscillator power amplifier optical parametric source for CO2 differential-absorption lidar. The master oscillator consists of an entangled-cavity nanosecond optical parametric oscillator based on a type II periodically poled lithium niobate crystal that provides highly stable single-longitudinal-mode radiation. The signal emission is amplified by a multistage parametric amplifier to generate up to 11 mJ in a nearly diffraction-limited beam with an M2 quality factor of approximately 1.5 while maintaining single-longitudinal-mode emission with a frequency stability better than 3 MHz rms. This approach can be readily applied to the detection of various greenhouse gases.

A Compact Tunable Diode Laser Absorption Spectrometer to Monitor CO2 at 2.7 µm Wavelength in Hypersonic Flows

Since the beginning of the Mars planet exploration, the characterization of carbon dioxide hypersonic flows to simulate a spaceship’s Mars atmosphere entry conditions has been an important issue. We have developed a Tunable Diode Laser Absorption Spectrometer with a new room-temperature operating antimony-based distributed feedback laser (DFB) diode laser to characterize the velocity, the temperature and the density of such flows. This instrument has been tested during two measurement campaigns in a free piston tunnel cold hypersonic facility and in a high enthalpy arc jet wind tunnel. These tests also demonstrate the feasibility of mid-infrared fiber optics coupling of the spectrometer to a wind tunnel for integrated or local flow characterization with an optical probe placed in the flow.

High-enthalpy hypersonic project at ONERA

Abstract This article deals with the research performed in the framework of the internal ONERA Hyperenthalpic Hypersonic Project. This project involved fifteen researchers specialised in different domains, which are the following: fundamental studies on shock/wave and shock/boundary layers interactions; study of the laminar/turbulent transition; real gas effects and associated physical modelling; the development of non-intrusive optical diagnostic methods usable in cold and hot wind tunnels; flow rebuilding in a hot shot wind tunnel such as F4; conception, realisation and validation of computational fluid dynamics (CFD) solvers.

Tunable ultraviolet intracavity tripled Ti:sapphire laser

We present what we believe to be the first intracavity tripled Ti:sapphire laser. This source is tunable in the 275-285-nm range and will be useful for applications in the detection of important atmospheric species such as OH and NO radicals. Single-frequency operation and high optical yield (>50%) are obtained in this system after it has been injected by a laser diode.

High Brightness 2 µm Source Based on A Type II Doubly Resonant ECOPO

For CO2 DIAL, the single mode output of a type II PPLN, entangled-cavity nanosecond OPO is amplified to 11 mJ at 2.05 µm, with 3MHz frequency stability and a M2quality factor better than 1.9.

Development of airborne/satellite lidars using non linear optics for characterization of greenhouse gases

Active optical sensing of the atmosphere through long distances from airborne or satellite platforms requires specific and demanding features for the necessary laser sources in terms of wavelength addressing and power level. Many molecules of atmospheric interest for nowadays urgent issues like climate change and contrails (CO2, CH4, H2O) have their best relevant detection features in the near and mid infrared spectral bands whereas most present days robust lasers have limited emission in the visible or near infrared at a few fixed wavelengths imposed by the laser media. Photonic solutions like DFG, OPO or Raman shift have to be employed to provide the necessary frequency conversion to cover the spectral fingerprints of molecules to be probed. This presentation will review some of the developments undertaken in our laboratory to provide attractive solutions using these schemes. One of promising solutions is 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 (MOPA architecture) as depicted in Figure1. This scheme enables the generation of tunable single-frequency (<; 60 MHz) high energy nanosecond pulses (tens of mJ) suitable for differential absorption lidar (DIAL) probing of the atmosphere (Figure 2). Development is also being carried out with picosecond/femtosecond frequency comb lasers to provide extended spectral coverage for more species and to enable detection schemes like Fourier transform spectroscopy with a dual comb. Ultra-sensitive trace level local probing on a balloon or airplane carrier is also being investigated through enhanced geometry for quartz-enhanced photoacoustic spectroscopy.

Remote gas leak detection using a portable mini-Lidar based on a doubly-resonant OPO

We report on remote gas leak detection using a portable mini-Lidar. The Lidar transmitter is based on a micro-laser pumped, entangled cavities, doubly-resonant optical parametric oscillator, allowing specific tuning procedures in the 3.8–4.3μm range.

Compact Electron Gun Based on Secondary Emission Through Ionic Bombardment

We present a new compact electron gun based on the secondary emission through ionic bombardment principle. The driving parameters to develop such a gun are to obtain a quite small electron gun for an in-flight instrument performing Electron Beam Fluorescence measurements (EBF) on board of a reentry vehicle in the upper atmosphere. These measurements are useful to characterize the gas flow around the vehicle in terms of gas chemical composition, temperatures and velocity of the flow which usually presents thermo-chemical non-equilibrium. Such an instrument can also be employed to characterize the upper atmosphere if placed on another carrier like a balloon. In ground facilities, it appears as a more practical tool to characterize flows in wind tunnel studies or as an alternative to complex electron guns in industrial processes requiring an electron beam. We describe in this paper the gun which has been developed as well as its different features which have been characterized in the laboratory.

Vernier frequency sampling - a new approach for broadband high-resolution spectroscopy

We present a novel method for broadband spectroscopy. Due to the entanglement of two cavities within an optical parametric oscillator, frequency resolution and span are widely adjustable. The technique is applied to CO2 remote sensing.