Intracavity Laser Intensity Research Articles

Trace Gas Detection System Based on Photoacoustic and Photothermal Spectroscopy Using Ring Fiber Laser and Quartz Tuning Fork

In this paper, a dual-spectroscopy gas sensing system based on ring cavity laser and quartz tuning fork (QTF) is designed. 15 m erbium-doped fiber with 1500 ppm doping concentration is used as gain medium, piezoelectric transducer (PZT) and acousto-optic modulator (AOM) are used as detection method. The first fiber collimator (FC) emits laser beam, laser is received by the second FC after passing through an off-beam acoustic micro-resonator (AmR) for quartz-enhanced photoacoustic spectroscopy (QEPAS) signal excitation and enhancement. The received laser is transmitted through the optical fiber and then incident on the QTF prongs by the third FC, which excites and quartz-enhan-ced photothermal spectroscopy (QEPTS) signal. Combined with the ring intra-cavity fiber laser and wavelength scanning and intensity modulation technology, dual-spectroscopy gas detection is realized. Compared with the single detection of QEPAS or QEPTS signals, the detection limit is enhanced. Under the conditions of 400mW pump power and 2500 ppmv acetylene (C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) gas concentration, the signal to noise ratio (SNR) of QEPAS, QEPTS and dual-spectroscopy signals are 838.07, 1117.47 and 2096.26, respectively, and the minimum detection limits (MDL) of 2.98 ppmv, 2.24 ppmv, 1.19 ppmv are achieved, respectively.

Optical-feedback cavity ring-down spectroscopy for NO2 extinction coefficient measurement using a continuous wave laser diode

Optical-feedback (OF) cavity ring-down spectroscopy consisting of a linear cavity is developed by employing a continuous wave laser diode (LD) with multi-longitudinal modes. Due to the OF effect caused by the cavity output laser back into the LD, the laser frequency is locked, and the intracavity laser intensity is enhanced. We use different concentrations of N O 2 gases to test the apparatus, and the results show good agreement with theoretical values. Owing to the compactness of the laser source and high detection accuracy, the device can be used for detection of low-concentration absorbent gases in the environmental monitoring field.

Tunable laser operation of Tm3+:KY3F10 near 1.9 µm via upconversion pumping at 1064 nm

We experimentally demonstrate, for the first time to our knowledge, that upconversion pumping can be employed with a 1064-nm fiber laser to achieve tunable laser operation with 8 at. % T m 3 + : K Y 3 F 10 crystal near 1.9 µm, in addition to the previously explored upconversion pumping of the 2.3-µm laser transition. Two different sets of cavity optics were used to investigate lasing at 1.9 and 2.3 µm, corresponding to the laser transitions 3 F 4 − 3 H 6 and 3 H 4 − 3 H 5 , respectively. In the case of the 3 F 4 − 3 H 6 laser transition, an x-cavity laser oscillator was constructed with a 2.3% output coupler and as high as 142 mW of output power was obtained at an incident pump power of 1.9 W at 1064 nm. Tunable laser operation could be obtained between 1849 and 1994 nm by using C a F 2 and suprasil prisms. The highest slope efficiency of 29% with respect to average absorbed pump power was obtained with a 5.5% output coupler. For the case of 1.9 µm lasing, it was further demonstrated that the nonlinear absorption of the crystal at 1064 nm depends on the intracavity laser intensity as well as the pump intensity. The 3 H 4 − 3 H 5 laser transition was also investigated with the same crystal in a z-cavity configuration under upconversion pumping, giving as high as 130 mW of output power at the central wavelength of 2344 nm with 1.6 W of incident pump power.

Short-pulse gain-switched Raman fiber laser based on conventional silica fibers

We report a high-energy 1666 nm short-pulse Raman fiber laser (RFL) based on gain-switching scheme. The RFL cavity is formed by conventional single mode fibers (SMFs), which is with low connection loss, low cost and ultrahigh compactness. To reduce the intra-cavity laser intensity and group velocity mismatch, the laser cavity and pump source parameters (i.e. laser output coupling, single-pass cavity length and pump pulse duration) are carefully selected to be 90%, 1 km and 10 ns, respectively. At last, an average output power of 670 mW, a single pulse energy of 223 nJ and a laser conversion efficiency of 66.9% have been successfully achieved. This work can clear a path towards high-performance and low cost 1666 nm fiber laser sources.

Active modulation of intracavity laser intensity with the Pound-Drever-Hall locking for photoacoustic spectroscopy.

Here we report a novel, to the best of our knowledge, method of active intracavity intensity modulation for cavity-enhanced photoacoustic spectroscopy (PAS) without the need for any external optical modulators. Based on the Pound-Drever-Hall (PDH) locking technique, a dither is added to the PDH error signal to periodically vary the locking point between the laser frequency and optical cavity within a sub-MHz frequency range. While significantly enhancing the intracavity laser intensity, the optical cavity also acts as an intensity modulator. As a proof-of-principle, we demonstrated the PAS of ${{\rm C}_2}{{\rm H}_2}$C2H2 by placing a photoacoustic cell ($Q$Q-factor $\sim{10}$∼10) inside a Fabry-Perot cavity (finesse $\sim{628}$∼628) and adopting the proposed intracavity intensity modulation scheme. By detecting the weak ${{\rm C}_2}{{\rm H}_2}$C2H2 line at ${6412.73}\;{{\rm cm}^{ - 1}}$6412.73cm-1, the sensor achieves a normalized noise equivalent absorption (NNEA) coefficient of ${1.5} \times {{10}^{ - 11}}\;{{\rm cm}^{ - 1}}{{\rm WHz}^{ - 1/2}}$1.5×10-11cm-1WHz-1/2. This method enables the continuous locking of laser frequency and optical cavity, and it achieves the intracavity intensity modulation with an adjustable modulation depth as well.

Study of a high output coupling ratio Q-switched erbium-doped fibre laser using MoS2 saturable absorber

In the work here, the performance of a Q-switched erbium-doped fibre laser using a MoS2 saturable absorber and operating with a high output coupling ratio has been studied. It is found that the pump conversion efficiency for the higher output coupling ratio is higher in the current setup. Q-switched laser oscillation is observed over the set wavelength range from 1525–1565 nm with a maximum pulse energy of ~0.50 µJ achieved at a pump power of ~83 mW, corresponding to a maximum peak power of ~186 mW. With lower intra-cavity laser intensity achieved using the higher output coupling ratio, the second order absorption of the MoS2 is mitigated. However, scaling of the peak power and pulse energy seems to be limited by the interplay between the pulse duration, pulse energy and repetition rate.

Single-frequency CW Ti:sapphire laser with intensity noise manipulation and continuous frequency-tuning.

We present a tunable single-frequency CW Ti:sapphire laser with intensity noise manipulation. The manipulation of the laser intensity noise is realized by varying the frequency of the modulation signal loaded on the electrodes of an intracavity electro-optic etalon. A lithium niobate (LiNbO3) crystal is used to act as the electro-optic etalon, and its electro-optic effect is utilized to modulate the intracavity laser intensity for locking itself to the oscillating wavelength of the laser to implement continuous frequency-tuning. When the electro-optic etalon is locked to the oscillating mode of the Ti:sapphire laser with arbitrarily selected modulation frequency, the maximal continuous frequency-tuning range can reach to 20GHz, and the laser intensity noise is successfully manipulated simultaneously.

Numerical analysis of an end-pumped Yb:YAG thin disk laser with variation of a fractional thermal load.

An analytical model is developed to describe the dynamic behavior of an end-pumped Yb:YAG thin disk laser. Within the model, the rate equations, including the nonradiative relaxation process, are calculated taking into account the dependence of the fractional thermal load on the temperature of the thin disk crystal and intracavity laser intensity. The fractional thermal load is analyzed, or can be evaluated clearly, under lasing or nonlasing conditions. The stable temperature and fractional thermal load in a thin disk crystal for different radiative quantum efficiencies are obtained using the numerical iterative method. Furthermore, the dependence of the laser output intensity on variables such as pumping intensity, coupler reflectivity, radiative quantum efficiency, and the temperature of thin disk crystal is discussed.

Bistability and spatial hysteresis in an Nd:GdVO4 laser with an intracavity twisted-nematic liquid crystal

In this study, optical bistability accompanied by spatial hysteresis was demonstrated in an Nd:GdVO4 laser with an intracavity twisted-nematic liquid crystal. The low branch of power hysteresis mainly has the spatial distribution of the fundamental mode as the pump power increases, whereas a four-mode and varying transverse pattern exist in the high branch of power hysteresis as the pump power decreases. The result revealed that the power and spatial hysteresis is controlled by the director axis reorientation and the order parameter modification in twisted-nematic liquid crystals, which is determined by the non-uniform transverse distribution of the intracavity laser intensity.

Time-Resolved Investigations of Electronic Transport Dynamics in Quantum Cascade Lasers Based on Diagonal Lasing Transition

In this study, the nature of electronic transport in quantum cascade lasers (QCLs) has been extensively investigated using an ultrafast time-resolved, degenerate, pump-probe optical technique. Our investigations enable a comprehensive understanding of the gain recovery dynamics in terms of a coupling of the electronic transport to the oscillating intracavity laser intensity. In QCLs that have a lasing transition diagonal in real space, studies of the near-threshold reveal that the transport of electrons changes bias region from phonon-limited relaxation (tens of picoseconds) below threshold to photon-driven transport via stimulated emission (a few picoseconds) above threshold. The gain recovery dynamics in the photon-driven regime is compared with conventional four-level lasers such as atomic, molecular, and semiconductor interband lasers. The depopulation dynamics out of the lower lasing state is explained using a tight-binding tunneling model and phonon-limited relaxation. For the superlattice relaxation, it is possible to explain the characteristic picosecond transport via dielectric relaxation; Monte Carlo simulations with a simple resistor model are developed, and the Esaki-Tsu model is applied. Subpicosecond dynamics due to carrier heating in the upper subband are isolated and appear to be at most about 10% of the gain compression compared with the contribution of stimulated emission. Finally, the polarization anisotropy in the active waveguide is experimentally shown to be negligible on our pump-probe data, supporting our interpretation of data in terms of gain recovery and transport.

Determination of the stimulated-emission cross section in an end-pumped solid-state laser from laser-induced pump saturation data

A novel method for the determination of the stimulated-emission cross section in an end-pumped solid-state laser is presented. The technique is based on the fact that increasing intracavity laser intensity stimulates faster decay of excited atoms and reduces the saturation of pump absorption. The use of this laser-induced pump saturation method is demonstrated with experimental data collected from a Cr(4+): YAG laser and analyzed by use of a numerical model that calculates the change in the pump transmission as a function of the intracavity laser intensity. The predicted cross-section value of 1.1x10(-19)cm(2) agrees well with previously reported results.

Bistable output from a coupled-resonator vertical-cavity laser diode

We report a monolithic coupled-resonator vertical-cavity laser with an ion-implanted top cavity and a selectively oxidized bottom cavity which exhibits bistable behavior in the light output versus injection current. Large bistability regions over current ranges as wide as 18 mA have been observed with on/off contrast ratios of greater than 20 dB. The position and width of the bistability region can be varied by changing the bias to the top cavity. Switching between on and off states can be accomplished with changes as small as 250 μW to the electrical power applied to the top cavity. The bistable behavior is the response of the nonlinear susceptibility in the top cavity to the changes in the bottom intracavity laser intensity as the bottom cavity reaches the thermal rollover point.

Ultrahighly sensitive laser-Doppler velocity meter with a diode-pumped Nd:YVO4 microchip laser

Extremely high sensitivity of a diode-pumped microchip Nd:YVO4 laser to external light injection is demonstrated in optical detection of Doppler shift from a moving object. Intensity modulation induced by reflected scattered light from a rotating object at the Doppler-shift frequency is measured for a 1-mm-long diode-pumped Nd:YVO4 laser. When adjusting the diode pump power so that the relaxation oscillation frequency is resonant to the Doppler-shift frequency, the minimum detectable intensity of scattered light injected back into the microchip laser can be &amp;lt;−170 dB weaker than the intracavity laser intensity.

Parametrization of the electron beam induced energy spread in free electron laser dynamics

We discuss a formula providing a parametrization of the energy spread induced on the electron beam in a free electron laser (FEL) oscillator. The formula we propose depends on the intracavity laser intensity and on the saturation intensity of the system. We show that such a parametrization can usefully be exploited in the analysis of FEL storage ring dynamics.

Simple model of gain saturation in free-electron lasers.

In this paper we analyze the dynamics of a free-electron laser in the strong-signal regime, drawing a parallel with that of a conventional laser. A simple model of gain saturation is developed and a scaling relation of the gain versus the intracavity laser intensity is derived.

Spectral dynamics in dye lasers: A new picosecond source

A new method of producing single picosecond dye laser pulses at different wavelengths is described. It is based on the spectro-temporal evolution of the intracavity laser intensity above threshold. Spectral filtering of the output from a sweeping oscillator pumped with a standard 8 ns Nd: YAG pulse is shown to produce, in a two-stage system, a nearly transform-limited, single, 3 μJ, 16 ps pulse.

Xe2Cl fluorescence and absorption in self-sustained discharge XeCl lasers

Fluorescence at 490 nm from the triatomic excimer Xe2Cl* has been investigated to determine the 308 nm absorption due to this species in an x-ray preionized, self-sustained gas discharge XeCl laser. The dependence of Xe2Cl* density on laser intensity (at 308 nm), buffer gas and Xe and HCl partial pressures has been determined for discharges with a peak electrical power deposition of 2.5 GWl−1. Xe2Cl* absorption is estimated to reach 0.6% cm−1 under non-lasing conditions but decreases to a non-saturable 0.2% cm−1 for intracavity laser intensity>1 MW cm−2. XeCl* and Xe2Cl* fluorescence intensities were found to be a similar for both helium and neon buffer gases but laser output was a factor of two greater with a neon buffer.

Intracavity Laser Intensity Modulation at Frequencies from 0.01 Hz to 1.2 GHz

An acoustooptic output coupler (cavity-dumper) is employed to intensity modulate the radiation produced by argon ion or dye lasers. By using an amplitude modulated rf carrier, heterodyning two acoustically upshifted beams, heterodyning one upshifted beam against the unshifted beam, or heterodyning an upshifted beam against a downshifted beam, a continuous range of frequencies from 0.01 Hz to 1.2 GHz could be generated. Phase fluorimetric determinations of subnanosecond lifetimes demonstrate the utility of the source.