Mid-infrared trace gas detection using continuous-wave difference frequency generation in periodically poled RbTiOAsO4

A tunable mid-infrared continuous-wave (cw) spectroscopic source in the 3.4–4.5 μm region is reported, based on difference frequency generation (DFG) in a quasi-phase-matched periodically poled RbTiOAsO₄ (PPRTA) crystal, DFG power levels of 10 μW were generated at approximately 4 μm in a 20-mm long PPRTA crystal by mixing two cw single-frequency Ti:Al₂O₃ lasers operating near 713 nm and 871 nm, respectively, using a laser pump power of 300 mW. A quasi-phase-matched infrared wavelength-tuning bandwidth (FWHM) of ∼12 cm^-1 and a temperature tuning rate of 1.02 cm^-1/°C were achieved. Experimental details regarding the feasibility of trace gas detection based on absorption spectroscopy of CO₂ in ambient air using this DFG radiation source are also described. Received: 23 October 2000 / Revised version: 22 January 2001 / Published online: 27 April 2001     

Continuous-wave tunable Cr2+:ZnS laser

We report the first continuous-wave tunable over ∼280 nm around 2.3 μm room-temperature operation of a chemical vapor transport-grown and diffusion-doped Cr²⁺:ZnS laser, pumped by a Co:MgF₂ laser at 1.67 μm and generating over 100 mW of output power at 16% slope efficiency. The self-consistent results of the laser and spectroscopic analysis demonstrate a large potential of this crystal as an active medium for diode-pumped tunable mid-infrared sources. Received: 7 January 2002 / Revised: 14 March 2002 / Published online: 2 May 2002     

Investigation of intracavity third-harmonic generation at 1.06 μm in YCa4O(BO3)3 crystals

Intracavity sum-frequency mixing of 1.06 μm and 532 nm in YCa₄O(BO₃)₃ (YCOB) crystals cut for different type-I phase-matching directions of (θ,ϕ)=(106°,77.2°), (111°, 79.6°) and (65°, 82.8°) was investigated in a compact diode-end-pumped acousto-optical Q-switched Nd:YVO₄/KTP laser formed with a three-mirror folded resonator. The maximum 355-nm average output power of 124 mW was obtained in the phase-matching direction of (106°, 77.2°) with a pump-to-ultraviolet conversion efficiency of 3.3% at the repetition frequency of 20 kHz. Received: 17 September 2001 / Revised version: 27 November 2001 / Published online: 17 January 2002     

A 9.5-W 82-MHz-repetition-rate picosecond optical parametric generator with cw diode laser injection seeding

We report on an injection-seeded 9.5-W 82-MHz-repetition-rate picosecond optical parametric generator (OPG) based on a 55 mm long crystal of periodically poled lithium niobate (PPLN) with a quasi-phase-matching (QPM) grating period of 29.75 μm. The OPG is excited by a continuously diode pumped mode-locked picosecond Nd:YVO₄ oscillator-amplifier system. The laser system generates 7 ps pulses with a repetition rate of 82.3 MHz and an average power of 24 W. Without injection-seeding the total average output power of the OPG is 8.9 W, which corresponds to an internal conversion efficiency of 50%. The wavelengths of the signal and idler waves were tuned in the range 1.57–1.64 μm and 3.03–3.3 μm, respectively, by changing the crystal temperature from 150 °C to 250 °C. Injection seeding of the OPG at 1.58 μm with 4 mW of single frequency continuous-wave radiation of a distributed-feedback (DFB) diode laser increases the OPG output to 9.5 W (53% conversion efficiency). The injection seeding increases the pulse duration and reduces the spectral bandwidth. When pumped by 10 W of 1.06 μm laser radiation, the duration of the signal pulses increased from 3.6 ps to 5.5 ps while the spectral bandwidth is reduced from 4.5 nm to 0.85 nm. Seeding thus improved the time-bandwidth product from 1.98 to a value of 0.56, much closer to the Fourier limit. Received: 29 April 2002 / Published online: 8 August 2002     

A compact,widely tunable intracavity PPLN optical parameter oscillator driven by an Nd:YAG/Cr:YAG composite crystal laser

Based on periodically poled lithium niobate (PPLN), a mini intracavity optical parameter oscillator (IOPO) driven by an diode-end-pumped composite Nd³⁺:YAG/Cr⁴⁺:YAG laser was demonstrated. The PPLN wafer has 20 domain reversal periods from 27.8 to 31.6 μm with a step of 0.2 μm between the neighbor periods. The output signal laser of OPO can be widely tunable in the range of 1402–1676 nm by changing the period at a certain temperature of 50°C. Under the diode pump power of 14 W, the maximum average output power of 600 mW at 1534 nm with pulse width of 2.0 ns and repetition rate of 16 kHz was obtained, corresponding to a peak power of 18.7 kW and a single pulse energy of 37.5 μJ, respectively.     

A tunable diode-laser spectrometer for the MIR region near 7.2 μm applying difference-frequency generation in AgGaSe2

2 and two diode lasers as pump sources are presented. A single-mode Fabry–Pérot-type tunable diode laser (TDL) and an external-cavity diode laser (ECL) were combined to generate radiation in the mid-infrared region near 7.2 μm. With a TDL at a wavelength of approximately 1290 nm and an ECL emitting between 1504 and 1589 nm it was possible to carry out spectroscopic experiments concerning SO₂ at five different phasematching points between 1350 and 1400 cm^-1 by fixing the wavelength of one pump laser and tuning the wavelength of the other. With an input power of 8 mW for the single-mode Fabry–Pérot-type diode laser and 6 mW for the external-cavity laser an output power of about 10 nW was generated. Using the tuning capabilities of the external-cavity laser a spectral region up to 5 cm^-1 could be covered within one scan. Measurements of SO₂ absorption lines at low pressure demonstrate the high-resolution features of the spectrometer. Moreover, these data provide new direct experimental phasematching data for the rarely investigated spectral region at 7.2 μm. Received: 27 October 1997/Revised version: 8 May 1998     

Efficient generation of orange light in a quasi-periodically poled LiTaO3 crystal

We present an approach to generating a tunable orange laser from 0.601 to 0.604 μm based on a quasi-periodically poled superlattice in LiTaO₃ and a Q-switched 1.064 μm Nd:YVO₄ laser as pump. The orange laser was generated in a cavity by a parametric process cascaded by a frequency mixing with a maximum output of 310 mW using 15 W pump power.     

Fabrication and modulation characteristics of InGaAsP/InGaAsP MQW DCPBH lasers at λ ∼ 1.57 μm

InGaAsP/InGaAsP multiple-quantum-well (MQW) double-channel planar-buried heterostructure (DCPBH) lasers emitting at λ∼ 1.57 μm were fabricated by optimizing the epitaxial growth with material characterization. At 25 °C (85 °C), a 1.8-μm-wide and 300-μm-long antireflectivity/high reflectivity coated laser exhibits a threshold current of 8 mA (23 mA) and a slope efficiency of 0.34 mW/mA (0.24 mW/mA) in continuous-wave mode (cw) as a result of the optimized thickness of the p-InP filling layer in the PBH structure with p-n-p-n current blocking layers. The maximum cw output power was approximately 20 mW at 25 °C, which was reduced to 17 mW at 85 °C. The peak wavelength varied from 1572 nm at 25 °C to 1602 nm at 100 °C for a fixed output power of 5 mW, indicating a temperature coefficient of ∼ 0.4 nm/K. The resonance frequency in the small-signal modulation response of devices depends on the etching depth of the U-shaped double channel; it increases from 0.4 GHz without channel etching to 4.3 GHz with 7-μm-thick etching. The full-width at half maximum values in the horizontal and vertical far-field patterns were approximately 24.5° (25.2°) and 29.4° (30.1°), respectively, at 25 °C (85 °C). PACS 42.55.Px; 73.61.Ey; 81.05.Ea     

Widely tunable PPLN/IOPO driven by a diode-end-pumped Q-switched Nd:YVO<Subscript>4</Subscript> laser

This paper reported a broadband tuning intracavity optical parametric oscillator (IOPO), based on the multiple grating periodical poled lithium niobate (PPLN) pumped by a acoustic-optical (AO) Q-switched Nd:YVO₄ laser. The widely tunable OPO output signal wavelength range from 1390 to 1605 nm, which was obtained by changing PPLN poling period from 27.8 to 31.6 μm at a certain temperature of 46°C, while the continuous tuning range was measured from 1475 to 1592 nm with the PPLN poling period of 30 μm by varying the temperature of nonlinear crystal PPLN from 50 to 120°C. The maximum output power of 0.92 W at 1534 nm with the minimum pulse width of 5.17 ns was generated under the incident pump power of 9.6 W at 808 nm. The corresponding peak power and single pulse energy were calculated to be 5.94 kW and 30.7 μJ, respectively. The M ₂-factor was measured to be 2.01 at the signal power of 0.4 W.     

$90^{\circ}$ phase-matched parametric frequency conversion in AgGa1-xInxS2

AgGa_1-xIn_xS₂ with x=0.14±0.01 was found to be 90° phase-matchable for type-I difference-frequency generation (DFG) by mixing the dual-wavelength pulses emitted from an electronically tuned Ti:sapphire laser. Infrared radiation continuously tunable over the range of 4.80–6.98 μm was generated by independently varying the two wavelengths in the 705–932 nm spectral range, and 4.04 μm radiation by mixing a Nd:YAG laser with the Ti:sapphire laser. In addition, this material was found to be noncritically phase-matchable for the second harmonic generation (SHG) of CO₂ laser radiation at 10.591 μm at 203 °C. Sellmeier equations that reproduce well these experimental data are presented. PACS  42.65.-k; 42.65.ky     

Efficient, kHz repetition rate, gain-switched Cr:forsterite laser

We have developed a gain-switched room-temperature Cr:forsterite laser operating at repetition rates of between 1 and 34 kHz, and pumped by a continuous wave, Q-switched Nd:YAG laser. With optimised output coupling, an output pulse energy of 52 μJ was measured at 1.5 kHz repetition rate, corresponding to 11% efficiency and 13% slope efficiency. Threshold pulse energy was 53 μJ. Output power of 370 mW was obtained at 10 kHz repetition rate and 4.4 W pump power. Water cooling was not required for repetition rates up to 10 kHz. In a tunable, folded resonator, the Cr:forsterite wavelength tuned between 1173 and 1338 nm. This laser operated with maximum pulse energy of 34 μJ, efficiency of 13%, and power of 307 mW. The laser output was close to diffraction-limited with M² of 1.2. Received: 6 January 1999 / Published online: 29 July 1999     

A cw room-temperature Ho,Tm:YLF laser pumped at 1.682 μm

We present the results obtained with a Ho,Tm:YLF crystal grown at a new crystal growth facility in Pisa. The optical quality of the sample has been tested by studying its performance as the active medium of a laser operating at 2.06 μm. We employed three different pump laser sources: a Ti:sapphire, a diode (both tuned at 793 nm) and, for the first time, a continuous-wave Co:MgF₂ laser, tuned at 1.682 μm. At room temperature the best slope efficiency was 30 % in the case of “red” pumping, and 59 % in the case of “infrared” excitation. The typical lasing threshold is about 100 mW. Received: 14 March 2001 / Revised version: 15 June 2001 / Published online: 19 September 2001     

High-power picosecond LiB3O5 optical parametric oscillators tunable in the blue spectral range

The paper reports on an experimental investigation and numerical analysis of noncritically and critically phasematched LiB₃O₅ (LBO) optical parametric oscillators (OPOs) synchronously pumped by the third harmonic of a cw diode-pumped mode-locked Nd:YVO₄ oscillator–amplifier system. The laser system generates 9.0 W of 355-nm mode-locked radiation with a pulse duration of 7.5 ps and a repetition rate of 84 MHz. The LBO OPO, synchronously pumped by the 355-nm pulses, generates a signal wave tunable in the blue spectral range 457–479 nm. With a power of up to 5.0 W at 462 nm and 1.7 W at 1535 nm the conversion efficiency is 74%. The OPO is characterized experimentally by measuring the output power (and its dependence on the pump power, the transmission of the output coupler and the resonator length) and the pulse properties (such as pulse duration and spectral width). Also the beam quality of the resonant and nonresonant waves is investigated. The measured results are compared with the predictions of a numerical analysis for Gaussian laser and OPO beams. In addition to the blue-signal output visible-red 629-nm radiation is generated by sum-frequency mixing of the 1.535-μm infrared idler wave with the residual 1.064-μm laser radiation. A power of 1.25 W of 1.535-μm idler radiation and 5.7 W of 1.064-μm laser light generated a red 629-nm output power of 2.25 W. Received: 2 February 2000 / Revised version: 28 July 2000 / Published online: 22 November 2000     

Synchronously pumped femtosecond optical parametric oscillator of congruent and stoichiometric MgO-doped periodically poled lithium niobate

A synchronously pumped femtosecond optical parametric oscillator based on congruent MgO-doped periodically poled lithium niobate (c-MgO:PPLN) is reported. The system, operating at room temperature, was pumped by a mode-locked Ti:sapphire laser. The wavelengths of the signal and idler waves were tuned from 870 nm to 1.54 μm and 1.58 to 5.67 μm, respectively, by changing the pump wavelength, the grating period or the cavity length. Pumped by 1.1 W of 755 nm laser radiation, the OPO generated 310 mW of 1080 nm radiation. This signal output corresponds to a total conversion efficiency of 50%. Without dispersion compensation the OPO generated phase-modulated signal pulses of 200 fs duration. Besides the OPO of c-MgO:PPLN, an OPO of stoichiometric (s) MgO:PPLN was investigated. Because of the reduced sensitivity to photorefractive damage, both crystals allowed efficient OPO operation at room temperature. Received: 19 August 2002 / Revised version: 11 December 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-631/205-3906, E-mail: andres@physik.uni-kl.de     

High-performance,continuous-wave quantum-cascade lasers operating up to 85°C at <Emphasis Type="Italic">λ</Emphasis>∼8.8 μm

High-temperature, high-power, and continuous-wave (CW) operation of quantum-cascade lasers with 35 active/injector stages at λ∼8.85 μm above room temperature is achieved without using a buried heterostructure. At this long wavelength, the use of a wider ridge waveguide in an epilayer-down bonding scheme leads to a superior performance of the laser. For a high-reflectivity-coated 21 μm×3 mm laser, the output power of 237 mW and the threshold current density of 1.44 kA/cm² at 298 K under CW mode are obtained with a maximum wall-plug efficiency of 1.7%. Further improvements were observed by using a 4-mm-long cavity. The device exhibits 294 mW of output power at 298 K and it operates at a high temperature, even up to 358 K (85°C). The full widths at half-maximum of the laser beam in CW operation for the parallel and the perpendicular far-field patterns are 25°and 63°, respectively.     

2.2 μm InGaAsSb/AlGaAsSb laser diode under continuous wave operating at room temperature

2.2 μm InGaAsSb/AlGaAsSb Sb-based type-I laser diodes (LDs) were fabricated, with cavity lengths of 1000 μm and stripe width of 150 μm. The high output performance was achieved with the threshold current density of the device is as low as 187 A/cm², slope efficiency of 0.2 W/A, and vertical and parallel divergent angles ϑ_⊥ = 42° and ϑ_| = 10°, respectively. The continuous wave operating up to 320 mW at room temperature (RT) were achieved.     

Telecom-grade fiber laser-based difference-frequency generation and ppb-level detection of benzene vapor in air around 3 μm

We report on the development of a field deployable compact laser instrument tunable over ∼232 cm⁻¹ from 3.16 to 3.41 μm (2932.5–3164.5 cm⁻¹) for chemical species monitoring at the ppb-level. The laser instrument is based on widely tunable continuous-wave difference-frequency generation (DFG), pumped by two telecom-grade fiber lasers. DFG power of ∼0.3 mW near 3.3 μm with a spectral purity of ∼3.3 MHz was achieved by using moderate pumping powers: 408 mW at 1062 nm and 636 mW at 1570 nm. Spectroscopic performance of the developed DFG-based instrument was evaluated with direct absorption spectra of ethylene at 3.23 μm (∼3094.31 cm⁻¹). Absorption spectra of vapor-phase benzene near 3.28 μm (∼3043.82 cm⁻¹) were recorded with Doppler-limited resolution. Line intensities of the most intense absorption lines of the ν ₁₂ band near 3043.8 cm⁻¹ were determined to support development of sensitive mid-infrared trace gas detection of benzene vapor in the atmosphere. Detection of benzene vapor in air at different concentration levels has been performed for the first time using multi-pass cell enhanced direct absorption spectroscopy at ∼3.28 μm with a minimum detectable concentration of 50 ppb (1σ).     

Efficient blue light generation by frequency doubling of a broad-area diode laser in a compact external cavity

An antireflection-coated broad-area laser diode is frequency stabilized in a compact unstable external cavity that was designed off-axis for transversal mode selection. The external cavity laser yielded up to 1 W near-diffraction-limited and tunable light centered around a wavelength of 970 nm. This light was frequency doubled by the use of periodically poled magnesium doped lithium niobate in a single-pass setup. With a 4-cm-long bulk crystal 57.5 mW of blue light could be generated out of 850 mW of infrared light resulting in a conversion efficiency of 6.7%. By using a 1.2-cm waveguide crystal it was possible to couple up to 64% of the infrared light into a 3.5 μm×5 μm channel and 142 mW of blue light at 488 nm could be obtained. An internal conversion efficiency of 44.1% and a wall-plug efficiency of 5.9% were achieved. Furthermore, the blue light was tunable over more than 5 nm. PACS 42.55.-f; 42.55.Px; 42.60.Da     

An all-diode-laser-based, dual-cavity AgGaS2 cw difference-frequency source for the 9–11 μm range

2 ) nonlinear crystal is reported. The device uses a standing-wave dual-resonator scheme to enhance both near-infrared lasers. For 7 mW of 778.2-nm and 275 mW of 842.5-nm input powers, 40 nW of 10.2-μm radiation is generated by a preliminary setup. A μW-level range output power is expected after optimization of the device optical components. This compact tunable source will allow Fabry–Pérot high-resolution Doppler-free spectroscopy of spherical molecules, such as OsO₄ or SF₆, in the 9–11 μm range as an alternative source to frequency-stabilized CO₂ lasers. Received: 17 March 1998     

Difference-frequency generation by mixing dual-wavelength pulses emitting from an electronically tuned Ti:sapphire laser

Broadly tunable difference-frequency generation (DFG) in AgGaS₂ was achieved by mixing dual-wavelength oscillating pulses from an electronically tuned Ti:sapphire laser with a two-frequency-driven acousto-optic device. Continuous tuning from 6.5 to 8.5 μm was achieved by simultaneous dual-wavelength-tuned DFG without crystal rotation. In the dual-pulse oscillation, the shorter and longer wavelength pulses were tuned from 700 to 775 nm and from 763 to 880 nm, respectively, while keeping the phase-matching relationship for DFG. When crystal rotation was adopted, however, the tunable output range was extended from 5.3 to 12 μm by tuning the longer wavelength pump pulse, while the shorter wavelength pulse was fixed. Received: 18 November 1998 / Revised version: 5 February 1999 / Published online: 26 May 1999