High-power broadband laser source tunable from 3.0 μm to 4.4 μm based on a femtosecond Yb:fiber oscillator

We describe a tunable broadband mid-IR laser source based on difference-frequency mixing of a 100?MHz femtosecond Yb:fiber laser oscillator and a Raman-shifted soliton generated with the same laser. The resulting light is tunable over 3.0?μm to 4.4?μm, with a FWHM bandwidth of 170?nm and maximum average output power up to 125?mW. The noise and coherence properties of this source are also investigated and described.     

Random distributed feedback Raman fiber laser operating in a 1.2 μm wavelength range

The random distributed feedback fiber laser operating via the stimulated Raman scattering and random distributed feedback based on the Rayleigh scattering is demonstrated in the 1.2 μm frequency band. The RDFB fiber laser generates at 1174 nm up to 2.4 W of output power with corresponding slope efficiency more than 30%. The output radiation has the spectral shape similar to the conventional Raman fiber lasers and spectral width less than 1.7 nm.     

Trace-gas sensing in the 3.3-μm region using a diode-based difference-frequency laser photoacoustic system

A compact, diode-based difference-frequency laser system combined with a photoacoustic detection scheme is presented for trace-gas sensing. It features a broad, continuous tuning range (3.2–3.7 μm), a narrow line width (154 MHz), and room-temperature operation, and thus allows numerous gas species to be measured both isolated and in mixtures of different gases. Several trace-gas species of environmental interest were detected, and gas mixtures were analysed. The detection limits are in the low-ppmV range, e.g. 1.3 ppmV for methane, 1.8 ppmV for ethane, and 1.2 ppmV for hydrogen chloride. Received: 10 April 2002 / Revised version: 5 June 2002 / Published online: 12 September 2002 RID="*" ID="*"Corresponding author. Fax: +41-1/633-1077, E-mail: Sigrist@iqe.phys.ethz.ch     

Laser diode photoacoustic and FTIR laser spectroscopy of formaldehyde in the 2.3 μm and 3.5 μm spectral range

The room temperature operating GaInAsSb/AlGaAsSb based diode laser and 66 K InAsSb/InAsSbP laser diode both operating in spectral range of formaldehyde absorption 4350-4361 cm⁻¹ and 2821-2823 cm⁻¹ have been characterized and compared. Very precise arrangement of laser absorption together with high resolution Fourier transform technique was tested. The photoacoustic technique was employed to determine the detection limit of formaldehyde (less than 1 ppmV) diluted by nitrogen for the strongest absorption line of the ν₃ + ν₅ band in the emission region of the GaInAsSb/AlGaAsSb diode laser. The detection limit (less than 10 ppbV) of formaldehyde was achieved in the 2820 cm⁻¹ spectral range in case of InAsSb/InAsSbP laser (fundamental bands of ν₁, ν₅)_.     

Line-shape study of water vapour by tunable diode laser spectrometer in the 822–832 nm wavelength region

A near-infrared tunable diode laser absorption spectrometer is set up to measure the air-induced broadening coefficients and the line-strength parameters of water-vapour overtone transitions within the (2,1,1)(0,0,0) band in the 822–832 nm wavelength region. A Hitachi HL8311 E double hetero-junction structure diode laser is used as a probe. The diode laser controller is home-built and stable within ±10 A and ±10 mK, respectively. The laser-head mount has a simple design and provides easy access whenever changing of the laser head is required. The diode laser emission wavelength is thermally tuned between 50 °C and 12 °C. Thermal tuning of the diode laser emission wavelength is used to reveal the mode structure of the diode laser and to probe the overtone-band transitions of water vapour within its operating wavelength range. Current tuning of the diode laser is used at a fixed laser temperature to study the transitions one at a time. A balanced detector is used to improve the S/N ratio of the spectrum. A phase sensitive detection technique is followed to obtain the first-derivative spectra of the overtone transitions. The first-derivative spectra have been recorded at different air pressures inside the sample cell while the water-vapour pressure is kept fixed. The first-derivative spectrum is numerically integrated to obtain the original line shape. The original line shape is fitted with a Voigt profile by using a nonlinear least-squares fit program to extract the air-broadening coefficient and the line-strength parameter. The data obtained in our work is compared with the results of the HITRAN database. PACS 33.70.-w; 33.70.Fd; 33.70.Jg     

A novel discrete mode narrow linewidth laser diode for spectroscopic based gas sensing in the 1.5 μm region

The characteristics of a novel Discrete Mode laser diode are described with particular reference to its capability in spectroscopic based gas sensing. In this regard, its emission linewidth, Side Mode Suppression Ratio and tuneability are of particular note. The measured narrow linewidth of 550?kHz has applications with respect to spectrally narrow absorption features found, for example, in saturated absorption lines, in hyperfine structure of some rotational manifolds and in whispering gallery mode features associated with optical microspheres. Other notable measured characteristics show mode-hop-free temperature tuning of 11 nm at a linear tuning rate of 0.1 nm/°C and a Side Mode Suppression Ratio in the range 56 to 46 dB and a long term (72?hours) wavelength stability of 1?pm per day. In essence, this novel Discrete Mode laser displays excellent potential for diode laser based sensing applications in the near infrared.     

Cavity enhanced absorption spectroscopy for N_2O detection at 2.86 μm using a continuous tunable color center laser

The cavity enhanced absorption technique is applied to N2O detection around 2.86 μm using a continuous-wave color center laser. A high-finesse triangular ring cavity is used in this technology. Transmission through the cavity is obtained by jittering the cavity-length with a piezo on one of the cavity mirrors. A minimum detectable absorption coefficient of 2 × 10-6 cm-1 is achieved with a mirror reflectivity of 99.24%, corresponding to a N2O detection limit of 600 parts per billion.     

Dual-beam laser heterodyne spectrometer: Ethylene absorption spectrum in the 10 μm range

A laser heterodyne spectrometer for the 10 m region has been built. A 5 MHz apparatus function is obtained. The improvement of this spectrometer using a dual-beam technique is described. The folding effects in heterodyne spectroscopy are discussed. The recorded lineshapes are explained. An ethylene spectrum is shown as an example.     

Collinear and non-collinear sum-frequency mixing inβ-BBO for a tunable 195–198 nm all-solid-state laser system

We attained tunable UV laser radiation between 195 and 198 nm by sum-frequency mixing two synchronized flashlamp-pumped solid-state Q-switch lasers, a Nd:YAG laser frequency quadrupled to 266 nm and a tunable (730–770 nm) alexandrite laser. UV pulse energies of 0.12 mJ with repetition rates of 10 Hz were attained in collinear, as well as non-collinear sum-frequency interaction in a-Barium Borate (BBO) crystal with a conversion efficiency of 2.5%. Theoretical models for the non-collinear phase-matching interaction were investigated at UV wavelengths below 200 nm.     

Wavelength-modulation-spectroscopy for real-time, in situ NO detection in combustion gases with a 5.2 μm quantum-cascade laser

A mid-infrared absorption strategy with calibration-free wavelength-modulation-spectroscopy (WMS) has been developed and demonstrated for real-time, in situ detection of nitric oxide in particulate-laden combustion-exhaust gases up to temperatures of 700 K. An external-cavity quantum-cascade laser (ECQCL) near 5.2 μm accessed the fundamental absorption band of NO, and a wavelength-scanned, 1f-normalized WMS with second-harmonic detection (WMS-2f/1f) strategy was developed. Due to the external-cavity laser architecture, large nonlinear intensity modulation (IM) was observed when the wavelength was modulated by injection-current modulation, and the IM indices were also found to be strongly wavelength-dependent as the center wavelength was scanned with piezoelectric tuning of the cavity. A quantitative model of the 1f-normalized WMS-2f signal was developed and validated under laboratory conditions. A sensor was subsequently designed, built and demonstrated for real-time, in situ measurements of NO across a 3 m path in the particulate-laden exhaust of a pulverized-coal-fired power plant boiler. The 1f-normalized WMS-2f method proved to have better noise immunity for non-absorption transmission, than wavelength-scanned direct absorption. A 0.3 ppm-m detection limit was estimated using the R15.5 transition near 1927 cm⁻¹ with 1 s averaging. Mid-infrared QCL-based NO absorption with 1f-normalized WMS-2f detection shows excellent promise for practical sensing in the combustion exhaust.     

Saturable absorbers for passive Q-switching of erbium lasers emitting in the region of 3 μm

The phototropic properties of Fe:ZnSe, Co:ZnSe, and Co:ZnS single crystals have been investigated. It is shown that these crystals can be used to advantage as the saturable absorbers in solid-state erbium lasers emitting in the region of the 3-μm range. The absorption cross sections of the ground states of the Co²⁺ ion in the ZnSe (σ_GSA = 11·10⁻²⁰ cm²) and ZnS (σ_GSA = 5.6·10⁻²⁰ cm²) crystals and of the Fe²⁺ ion in the ZnSe (σ_GSA = 50·10⁻²⁰ cm²) crystal at λ = 2.79 μm were determined experimentally. It has been established that the above-indicated crystals in the excited state absorb light weakly. The use of these crystals as passive Q switches made it possible to realize, for the first time, the regime of Q-switching of a Cr,Er:YSGG laser emitting at a wavelength of 2.79 μm. Single pulses with an energy of 60 mJ and a duration of 170 nsec were obtained in the regime of passive Q-switching. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 6, pp. 747–751, November–December, 2005.     

Absorption spectrum of HDO in the 1.06 μm region at a temperature of 1000 K

In the 9387–9450 cm^–1 region at temperatures of 300–1000 K, we have used an intracavity laser spectrometer based on a neodymium laser with threshold sensitivity to absorption 10^–8 cm^–1 and spectral resolution 0.035 cm^–1 to study the absorption spectrum of D₂¹⁶O, H₂¹⁶O, and HD¹⁶O vapor. The high-temperature spectrum contains more than 450 absorption lines, 240 of which are assigned to the HDO isotopomer. The absorption lines of HDO were identified and belong to nine vibrational transitions: 3ν₂ +ν₃, 2ν₁ + 3ν₂, 2ν₁ + ν₃, 4ν₂ + ν₃, 7ν₂, ν₁ + 2ν₂ + ν₃, ν₁ + 5ν₂, ν₁ + 2ν₂, and 3ν₃ – ν₂.     

Diode laser measurement of line strengths and air-broadening coefficients of CO2 and CO in the 1.57 μm region for combustion diagnostics

Spectral measurements of two line pairs of CO₂ and CO in the temperature range 300–1000 K at 1.573 µm were performed using a fiber-coupled distributed feedback (DFB) diode laser. The two line pairs can be used in a tunable diode laser (TDL) absorption sensor for simultaneously detecting CO₂ and CO gas in a single scan of the diode laser. The spectral parameters (line strengths, air-broadening coefficients and the temperature exponent n) of the two pairs are presented. The measured data agree well with existing databases (HITRAN 2004 and HITRAN 2008), the discrepancies being less than 5% for most of the probed transitions. Although the HITRAN database is a useful tool for sensor design, we found that laboratory measurements of the spectroscopic data for the line pair selected for high-temperature sensors are necessary for establishing the uncertainty for accurate measurements.     

Antimonide-based lasers and DFB laser diodes in the 2–2.7 μm wavelength range for absorption spectroscopy

We report on Fabry–Pérot semiconductor lasers and single frequency distributed feedback lasers based on GaInAsSb/AlGaAsSb quantum wells. The laser structures were grown by molecular beam epitaxy on GaSb substrates. The devices were etched either by wet process or by inductively coupled plasma (ICP) process. Electron-beam lithography was used to deposit a metal Bragg grating on each side of the laser ridge to fabricate the DFB lasers. The devices all operate in the continuous wave regime at room temperature with a single frequency emission above 2.6 μm and good tuning properties, making them well adapted to tunable diode laser absorption spectroscopy. PACS 42.55.Px; 42.62.Fi     

Gold nanorods as saturable absorbers for the passively Q-switched Nd:LLF laser at 1.34 μm

Gold nanorods(GNRs) with two different aspect ratios were successfully utilized as saturable absorbers(SAs) in a passively Q-switched neodymium-doped lutetium lithium fluoride(Nd:LLF) laser emitting at 1.34 μm. Based on the GNRs with an aspect ratio of five, a maximum output power of 1.432 W was achieved, and the narrowest pulse width was 328 ns with a repetition rate of 200 kHz. But, in the case of the GNRs with the aspect ratio of eight, a maximum output power of 1.247 W was achieved, and the narrowest pulse width was 271 ns with a repetition rate of 218 kHz. Our experimental results reveal that the aspect ratios of GNRs have different saturable absorption effects at a specific wavelength. In other words, for passively Q-switched lasers at a given wavelength, we are able to select the most suitable GNRs as an SA by changing their aspect ratio.     

Measurement of absorption spectra for atmospheric methane by a lidar system with tunable emission wavelength in the range 1.41–4.24 μm

We have developed, built, and tested an automated differential lidar system for measuring low concentrations of atmospheric gases, based on an optical parametric oscillator tunable in the near IR region. We have calculated the spectral shift of the relative intensities of the individual lines in the ν₃ absorption band of methane. In comparing the measured and calculated spectra, we did not observe any shifts in the ν₃ absorption band of methane. At the same time, in the experimental spectra we observe broadening of the Q branch and the individual lines of the P branch. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 2, pp. 285–290, March–April, 2009.     

Tunable laser via F 2 + and F 2 − colour centers in the spectral region 0.88–1.25 μm

Frequency-tunable generation by means of F ₂ ⁺ and F ₂ ^? colour centers in a LiF crystal is reported. Colour centers were created by illuminating LiF crystals with electrons of 3 meV energy at the electron current density of 1 μA/cm². The pumping source was a ruby laser with a peak power of 20 MW, a pulse duration of 20 ns, and a repetition rate of 1 Hz. The frequency tuning is obtained in the range of 0.88–1.25 μm. Discussed are the ways of pumping of colour centers and the possibility of lasing in the spectral region of 0.85 to 2 μm in the type of colour centers under investigation.     

Tunable large angular aperture TeO2 acousto-optic filters for use in the 1.0–1.6 μm region

The design of tunable large angular aperture TeO₂ noncollinear acousto-optic filters for use in the 1.0–1.6m region is discussed. It is shown that it is possible to maximize the angular aperture of such a filter by the correct choice of the direction for the acoustic wave vector and that this required direction is independent of the operating wavelength in the range 1.0–1.6m. The theoretical model adopted is verified by comparison with experimentally measured filter characteristics and the design of filters with narrow bandwidths and low power requirements is discussed.