Multipass Cavity Research Articles

Trace gas sensing using diode-pumped collinearly detected spontaneous Raman scattering enhanced by a multipass cell

A simple but highly effective method for enhancement of spontaneous Raman scattering from gases is described. It employs a multimode blue laser diode that receives feedback from a near-concentric bidirectional multipass cavity in such a way as to generate a circulating power on the order of 100 W for a sample volume of 10 m m 3 . The feedback, provided via a volume Bragg grating, reduces the laser bandwidth to 4 c m − 1 . Spectra of spontaneous Raman scattering from ambient atmospheric air, detected collinearly with the pump, were recorded with a limit of detection below 1 ppm. The performance of the setup is further illustrated through detection of acetone, methane, and isotopic carbon dioxide in breath.

Impacts of laser cooling for low earth orbit observation satellites: An analysis in terms of size, weight and power

Laser cooling in solids is a breakthrough technology allowing vibration-free cooling down to a temperature of 100 K in a miniaturized way. It appears as a promising technology to improve future observation satellites performances e.g. in SWIR and NIR domains.In this paper, integration of a laser cooler onboard an observation satellite is studied for the first time. Our study focuses on size, weight and power (SWaP) criteria, at both satellite payload and platform levels. Its goal is to assess the interest of using an optical cryocooler over a mechanical cryocooler for low earth orbit (LEO) infrared observation missions.A preliminary space-borne laser cooler (LC) architecture is proposed. It is composed of two parts. The first part is the cooling head, based on state-of-the-art cooling crystals 10%Yb:YLF and an astigmatic multipass cavity. The second part is the cryocooler opto-electronics, based on redundant laser diodes and fiber coupled to the cooling head. The cooling power is estimated for a small focal plane, taking into account the thermal load of an infrared detector and the parasitic heat fluxes inside the cryostat. The required optical and electrical powers of the laser cooler are then estimated considering the crystal efficiency, the thermal link losses and the opto-electronics efficiency.Assuming a 5-year long LEO microsatellite mission, the sizing of the electrical power systems (PCDU, solar array, batteries) and thermal control systems (heatpipes, radiators) is performed. An additional mass margin is added to take mechanical support structures into account. At the end, payload and platform masses and volumes are summed respectively to obtain a SWaP balance at satellite level, representative of the overall impact of a laser cooler. The study is repeated for the case of a Miniature Pulse Tube Cooler (MPTC) architecture under the same mission and platform assumptions. Finally, the two architectures are compared. It is shown that even if the power requirement of a laser cooler is high, the reduction of mass and internal volume makes it possible for small satellite payloads.

Pulse duration adjusted by changing the cavity length with a multi-pass cavity in a Q-switched Nd:YAG laser.

We report a compact, long nanosecond (ns) pulse duration stretched laser source by a multi-pass cavity (MPC). Based on the combination of the MPC and pump power, a high-power high beam quality 1064nm Q-switched Nd:YAG laser with a pulse duration adjustable over the range of 160-1000ns was obtained at a pulse repetition frequency of 10kHz for the first time, to the best of our knowledge. At a typical pulse width of 560ns, an average output power of 10.6W was successfully achieved. The beam quality factor M2 was measured to be 1.45 with a good Gaussian mode.

Limits on magnetically induced Faraday rotation from polarized He3 atoms

Faraday rotation has become a powerful tool in a large variety of physics applications. Most prominently, Faraday rotation can be used in precision magnetometry. Here we report the first measurements of gyromagnetic Faraday rotation on a dense, hyperpolarized $^3$He gas target. Theoretical calculations predict the rotations of linearly polarized light due to the magnetization of spin-1/2 particles are on the scale of 10$^{-7}$ radians. To maximize the signal, a $^3$He target designed to use with a multipass cavity is combined with a sensitive apparatus for polarimetry that can detect optical rotations on the order of 10$^{-8}$ radians. Although the expected results are well above the sensitivity for the given experimental conditions, no nuclear-spin induced rotation was observed.

The efficiency of one-line versus multi-line excitation of boron isotopes within the method of selective laser assisted retardation of condensation

The influence of multi-line pulsed excitation of target isotopes within the method of selective laser assisted retarded condensation (SILARC) on the efficiency of boron isotopes separation is investigated. In pulsed mode simultaneous excitation of all four isotopologues 11B35Cln37Cl3−n is possible. In order to study it, a new formula for multi-line excitation rate is derived. The total number of photons absorbed by target isotopologues for proposed physical parametrization of the laser pulse is compared with continuous wave excitation case, corresponding to the minimal level of laser intensity, required to excite all four isotopologues. It is found that irradiation efficiency of the pulsed mode is 20.59 times smaller than for CW-mode one. Influence of multi-pass cavity on the efficiency of excitation has been also considered.

5-nJ Femtosecond Ti3+:sapphire laser pumped with a single 1 W green diode

We report a Kerr-lens mode-locked, extended-cavity femtosecond Ti3+:sapphire laser directly pumped at 520 nm with a 1 W AlInGaN green diode. To obtain energy scaling, the short x-cavity was extended with a q-preserving multi-pass cavity to reduce the pulse repetition rate to 5.78 MHz. With 880 mW of incident pump power, we obtained as high as 90 mW of continuous-wave output power from the short cavity by using a 3% output coupler. In the Kerr-lens mode-locked regime, the extended cavity produced nearly transform-limited 95 fs pulses at 776 nm. The resulting energy and peak power of the pulses were 5.1 nJ and 53 kW, respectively. To our knowledge, this represents the highest pulse energy directly obtained to date from a mode-locked, single-diode-pumped Ti3+:sapphire laser.

Soot measurement in diluted methane diffusion flames by multi-pass extinction and laser-induced incandescence

Multi-pass cavity line-of-sight extinction (MPC-LOSE) and laser-induced incandescence (LII) techniques are deployed to measure the soot volume fraction in a series of nitrogen-diluted flames, which produce only ppm volume mass fractions of soot. The separate suppression effects on soot formation of direct fuel dilution and indirect effects of temperature and residence time are interpreted by using a numerically calculated flow velocity and temperature field using a one-step fast chemistry model. The experimentally determined rate of soot formation is shown to obey approximately the same function of the local temperature for all dilution cases. The results show that a simple one-step reaction model using previously measured activation energies can account for the dilution effect with good accuracy. The results show that the direct effect of dilution on concentration is comparable to the effects of changing the temperature estimated local temperature and residence time.

Toward the measurement of the hyperfine structure of muonic hydrogen in the FAMU experiment: Multi-pass cavity optimization for experiments with pulsed sources.

We consider a simplified model of the optical multi-pass cavity that is being currently developed by the FAMU collaboration for the measurement of the hyperfine splitting in the ground state of muonic hydrogen and of the Zemach radius of the proton. The model is focused on the time distribution of the events of laser-stimulated hyperfine transitions in the muonuc atom and may be helpful in the preliminary design of the FAMU experimental set-up and, more generally, in the optimization of multi-pass optical cavities for experiments with pulsed lasers.

Optical ray paths in a quasi-periodic waveguide or a multipass cavity with smoothly varying properties

Ray paths in a quasi-periodic optical system (waveguide of cavity) with smoothly varying properties have been constructed along the ray path. The ray-transfer matrix is constructed for a large number of ray paths; it has been shown that the conventional stability condition (the modulus of the ray-transfer matrix trace for a ray path is smaller than two) does not ensure the boundedness of the beam path after a large number of paths.

Potassium tune-out-wavelength measurement using atom interferometry and a multipass optical cavity

The longest tune-out wavelength for potassium atoms, ${\ensuremath{\lambda}}_{\mathrm{zero}}=768.9701(4)$ nm, was measured using an atom interferometer with a large irradiance gradient supported in a multipass optical cavity. Systematic errors in ${\ensuremath{\lambda}}_{\mathrm{zero}}$ measurements that arise from laser light, Doppler shifts, and the Earth's rotation are described. The ratio of oscillator strengths for the potassium $D2$ and $D1$ lines inferred from this ${\ensuremath{\lambda}}_{\mathrm{zero}}$ measurement is $\ensuremath{\rho}={f}_{D2}/{f}_{D1}=2.0066(11)$, and the ratio of line strengths is $R={S}_{D2}/{S}_{D1}=1.9977(11)$.

Mirrors with designed spherical aberration for multi-pass cavities.

We present a novel multi-pass cavity design based on the use of a rotationally symmetric end mirror having a specifically designed spherical aberration so that its focal length varies inversely as the ray height from the optical axis. We provide a detailed discussion of how ray tracing can be done for this system and show with numerical simulations that a very rich set of exotic spot patterns can be formed on the end mirrors. We further show a specific q-preserving configuration where the q-parameters of the input and output beams remain the same. Finally, we derive the polar form of the mirror surface profile that gives this offset-dependent focal length.

Macro-Raman spectroscopy for bulk composition and homogeneity analysis of multi-component pharmaceutical powders

A new macro-Raman system equipped with a motorized translational sample stage and low-frequency shift capabilities was developed for bulk composition and homogeneity analysis of multi-component pharmaceutical powders. Different sampling methods including single spot and scanning measurement were compared. It was found that increasing sample volumes significantly improved the precision of quantitative composition analysis, especially for poorly mixed powders. The multi-pass cavity of the macro-Raman system increased effective sample volumes by 20 times from the sample volume defined by the collection optics, i.e., from 0.02μL to about 0.4μL. A stochastic model simulating the random sampling process of polydisperse microparticles was used to predict the sampling errors for a specific sample volume. Comparison of fluticasone propionate mass fractions of the commercial products Flixotide® 250 and Seretide® 500 simulated for different sampling volumes with experimentally measured compositions verified that the effective sample volume of a single point macro-Raman measurement in the multi-pass cavity of this instrument was between 0.3μL and 0.5μL. The macro-Raman system was also successfully used for blend uniformity analysis. It was concluded that demixing occurred in the binary mixture of l-leucine and d-mannitol from the observation that the sampling errors indicated by the standard deviations of measured leucine mass fractions increased during mixing, and the standard deviation values were all larger than the theoretical lower limit determined by the simulation. Since sample volume was shown to have a significant impact on measured homogeneity characteristics, it was concluded that powder homogeneity analysis results, i.e., the mean of individual test results and absolute and relative standard deviations, must be presented together with the effective sample volumes of the applied testing techniques for any measurement of powder homogeneity to be fully meaningful.

Modeling Herriott cells using the linear canonical transform.

We demonstrate a new way to analyze stable, multipass optical cavities (Herriott cells), using the linear canonical transform formalism, showing that re-entrant designs reproduce an arbitrary input field at the output, resulting in useful symmetries. We use this analysis to predict the stability of cavities used in interferometric delay lines for temporal pulse addition.

Some Features of Boron Isotopes Separation by the Laser-Assisted Retardation of Condensation Method in Multipass Irradiation Cell Implemented as a Resonator

More cheap production of boron isotopes is needed to provide further progress in nuclear engineering, microelectronics, and nuclear medicine. Separation of boron isotopes by laser-assisted retarded condensation scheme is considered. Continuous irradiation of steady gas flow is considered. In this case, interaction time of gas flow with laser beam (gas flow length) is desirable to make as long as possible in order to increase excitation probability. It can be provided by suitable choice of gas flow expansion conditions, such as small molar fraction of target gas in carrier gas, gas flow, and ambient small gas pressure and temperature. We propose irradiation cell design as a resonant multipass cavity. In order to estimate its length that can be used for efficient isotopes separation, we calculated laser field distribution inside the cavity for different assumed values for mirror wall reflectivities and average photo-absorption cross sections. It has been shown, that significant saving of electricity (4.2-4.6 times less) can be expected.

Green pulsed lidar-radar emitter based on a multipass frequency-shifting external cavity.

This paper investigates the radio frequency (RF) up-conversion properties of a frequency-shifting external cavity on a laser beam. We consider an infrared passively Q-switched pulsed laser whose intensity modulation results from the multiple round-trips in the external cavity, which contains a frequency shifter. The output beam undergoes optical second-harmonic generation necessary to reach the green wavelength. We model the pulse train using a rate-equation model to simulate the laser pulses, together with a time-delayed interference calculation taking both the diffraction efficiency and the Gaussian beam propagation into account. The predictions are verified experimentally using a diode-pumped Nd:YAG laser passively Q-switched by Cr4+:YAG whose pulse train makes multiple round-trips in a mode-matched external cavity containing an acousto-optic frequency shifter driven at 85 MHz. Second-harmonic generation is realized in a KTP crystal, yielding RF-modulated pulses at 532 nm with a modulation contrast of almost 100%. RF harmonics up to the 6th order (1.020 GHz) are observed in the green output pulses. Such a RF-modulated green laser may find applications in underwater detection and ranging.

Demonstration of a signal enhanced fast Raman sensor for multi‐species gas analyses at a low pressure range for anesthesia monitoring

The spontaneous Raman scattering technique is an excellent tool for a quantitative analysis of multi‐species gas mixtures. It is a noninvasive optical method for species identification and gas phase concentration measurement of Raman active molecules, since the intensity of the molecule specific Raman signal is linearly dependent on the concentration. Applying a continuous wave (cw) laser it typically takes a few seconds to capture a gas phase Raman spectrum at room temperature. Nevertheless in contrast to these advantages the weak Raman signal intensity is a major drawback. Thus, it is still challenging to detect gas phase Raman spectra in a low‐pressure regime with a temporal resolution of only a few 100 ms. In the presented study a fully functional gas phase Raman system for measurements in the low‐pressure regime (p ≥ 980 hPa (absolute)) is presented; it overcomes the drawback of the weak Raman effect by using a multipass cavity to enhance the Raman signal. The signal amplification of a retro‐reflecting cavity is experimentally compared to a near‐confocal cavity. A description of this sensor setup as well as of the calibration procedure, which also allows the quantification of condensable gases, is presented. Moreover the functionality of the sensor system is demonstrated in a measurement campaign at an anesthesia simulator under clinical relevant conditions and in comparison to a conventional gas monitor. Copyright © 2015 John Wiley & Sons, Ltd.

A signal enhanced portable raman probe for anesthetic gas monitoring

The spontaneous Raman scattering technique is an excellent tool for a quantitative analysis of multi-species gas mixtures. It is a noninvasive optical method for species identification and gas phase concentration measurement of all Raman active molecules, since the intensity of the species specific Raman signal is linearly dependent on the concentration. Applying a continuous wave (CW) laser it typically takes a few seconds to capture a gas phase Raman spectrum at room temperature. Nevertheless in contrast to these advantages the weak Raman signal intensity is a major drawback. Thus, it is still challenging to detect gas phase Raman spectra in alow-pressure regime with a temporal resolution of only a few 100 ms. In this work a fully functional gas phase Raman system for measurements in the low-pressure regime (p ≥ 980 hPa (absolute)) is presented. It overcomes the drawback of a weak Raman signal by using a multipass cavity. A description of the sensor setup and of the multipass arrangement will be presented. Moreover the complete functionality of the sensor system will be demonstrated by measurements at an anesthesia simulator under clinical relevant conditions and in comparison to a conventional gas monitor.

Pulse Energy Optimization in Multipass-Cavity Mode-Locked Femtosecond Lasers

We describe a comprehensive model, which accounts for reflection and clipping losses of notched multipass-cavity (MPC) mode-locked lasers and determines the optimum q-preserving MPC configuration, which maximizes the output pulse energy for a given pump power. In order to make realistic predictions, an initial experimental MPC resonator with negligible clipping losses was built and its power performance was measured in order to determine the values of the fixed model parameters such as the round-trip loss of the short cavity and reflection coefficient of the MPC mirrors. Then, by using these parameters and by taking into account all possible reflection and clipping loss mechanisms, the output pulse energy for various q-preserving configurations was calculated. The simulation results indicated that, a mode-locked Ti 3+ :Sapphire laser based on the optimum MPC configuration should produce 30 nJ of pulse energy with a 3% output coupler at the pump power of 2 W. The experimental setup constructed at the optimum MPC configuration generated nearly transform-limited 74 fs pulses with 33 nJ of output pulse energy, in very good agreement with the model predictions. We believe that the model and the experimental procedure described in this paper should prove very useful in the practical design and optimization of energy-efficient MPC mode-locked lasers.

Exploitation of an external unstable multi-pass cavity to enhance the second harmonic conversion efficiency

Attempts to overcome the low conversion efficiency often encountered in second harmonic generation process by increasing the interaction length through usage of a number of crystals in tandem or in parallel or a single crystal in the intra-cavity or external resonant enhanced configuration have been made although with mixed success. We report here on the gainful exploitation of an unstable cavity to greatly enhance nonlinear conversion efficiency by integrating it with the stable pump cavity. This integration allowed transport of the pump radiation into the external cavity that at the same time exhibited high ‘Q’, prerequisite for multi-pass of the unabsorbed pump through the crystal. The unstable cavity not only ensured longer interaction length in the crystal without exposing it to high optical flux thereby safeguarding the crystal even in the pulsed operation but also eliminated the feedback problem. This scheme is demonstrated in the mid-IR region by frequency doubling of pulsed CO2 laser emission but its advantage should persist across UV, visible, and near-IR regions too.

Multicomponent gas analysis using broadband quantum cascade laser spectroscopy.

We present a broadband quantum cascade laser-based spectroscopic system covering the region between 850 and 1250 cm(-1). Its robust multipass cavity ensures a constant interaction length over the entire spectral region. The device enables the detection and identification of numerous molecules present in a complex gas mixture without any pre-treatment in two minutes. We demonstrate that we can detect sub-ppmv concentration of acetone in presence of 2% of water at the same wavenumber region.