High resolution spectroscopy of environmental and medical gases requires reliable, fast tunable laser light sources in the mid-infrared (MIR) wavelength regime between 3 and 5 mu m. Since this wavelength cannot be reached via direct emitting room temperature semiconductor lasers, additional techniques like difference frequency generation (DFG) are essential. Tunable difference frequency generation relies on high power, small linewidth, fast tunable, robust laser diode sources. We report a new, very compact, alignment insensitive, robust, external cavity diode laser system in Littman/Metcalf configuration with an output power of 1000 mW and an almost Gaussian shaped beam quality (M-2 < 1.2). The coupling efficiency for optical waveguides as well as single mode fibers exceeds 70%. The center wavelength is widely tunable within the tuning range of 20 nm via remote control. This laser system operates longitudinally single mode with a mode-hop free tuning range of up to 150 GHz without current compensation and a side-mode-suppression better than 50 dB. This concept can be realized within the wavelength regime between 750 and 1060 nm. We investigated this light source for high resolution spectroscopy in the field of Cavity Ring-Down Spectroscopy (CRDS). Our high powered Littman/Metcalf laser system was part of a MIR-light source which utilizes difference-frequency generation in Periodically Poled Lithium Niobate (PPLN) crystals. At the wavelength of 3.3 mu m we were able to achieve a high-resolution absorption spectrum of water with four resolved isotopic H2O components. This application clearly demonstrates the suitability of this laser for high-precision measurements.

We report on recent progress on external cavity diode lasers (ECDL) using a new concept of a Littman/Metcalf configuration. Within this concept one facet of the diode laser chip is used for coupling to a high quality Littman/Metcalf resonator whereas the other side of the diode laser chip emits the output beam. The alignment of the external resonator is independent from the alignment of the output beam and there is no need for any compromise in the alignment. This results in an improved behavior of the external resonator with the benefit of a drastic increase in power and single mode tuning.

In this paper, we report a new compact design of an external cavity diode laser system (ECDL) to combine high output power with high brilliance emission with broad area diodes. To achieve an excellent phase coupling to the external resonator one facet of the broad area diode is unreflection coated and the other facet is high reflection coated. The very compact laser design offers an output power of up to 500 mW with a sidemode suppression ratio better than 50 dB in both longitudinal and spatial singlemode operation.

An unstable external cavity diode laser is presented with an AR-coated stripe-array diode as gain medium which yields up to 1 W with a bandwidth below 3 MHz and M 2 < 2 over the tuning range 940 nm through 980 nm.

Abstract Für zahlreiche Anwendungen in der industriellen Messtechnik werden durchstimmbare Laser im mittleren infraroten Spektralbereich benötigt. Aufgrund charakteristischer Spektren vieler umwelt- und medizinisch relevanter Moleküle ist dieser Bereich von besonderem Interesse. Um einen Laser mit einem großen monomodig spektral durchstimmbaren Wellenlängenbereich zu realisieren, sind Quantenkaskadenlaser entspiegelt und in einem externen Resonator betrieben worden.

The combination of high power, small linewidth and rapid tuneability is essential for many fields in high resolution spectroscopy. Furthermore these optical features are essential for laser-cooling techniques. Enhancement of high power lasers with excellent spectral and spatial quality is currently an important research subject. The requirements for a laser system applied in both fields of application are demanding: a mode-hop free tuning range of a few GHz, with a linewidth in the order of 1MHz and an output power of a few 100mW. We report a very compact external cavity diode laser system (ECDL) with an output power of up to 800mW with an almost Gaussian shaped beam quality (M2<1.2). The coupling efficiency for a single mode fibre exceeds 60%. The centre wavelength can be preadjusted within the tuning range of 20 nm. This laser operates single mode with a mode-hop free tuning range of up to 15GHz without current compensation and a side-mode-suppression better than 50dB at different wavelength between 730 and 1060nm. To demonstrate the suitability for neutral atom cooling we used this laser as light source in the production of a BEC of over a million 87Rb atoms. In addition we approved this light source for high resolution spectroscopy, more precisely for the Cavity-Ring-Down-Spectroscopy (CRDS). Our ECDL was part of a MIR-light source which utilizes difference-frequency-generation in PPLN. At the wavelength of 3.3μm we were able to perform a high resolution absorption measurement of 50ppb Ethane. Both applications clearly demonstrate the suitability of this laser for high-precision measurements.

Quantum cascade laser (QCL) are an excellent tool for MIR-spectroscopy. We report on the design and realization of antireflection coated pulsed and cw-QCL in external cavity (EC) configurations and investigate their performance.

Since the introduction of laser-cooling techniques for neutral atoms, the enhancement of high-power lasers with excellent spectral and spatial quality has been an important research subject. We report a new principle of using high-power laserdiodes directly in an external cavity. The very compact design offers an output power of up to 1 W and an excellent beam quality (M2 < 1.2). The coupling efficiency for a single mode fiber exceeds 60%. The center wavelength can be tuned between 775 nm and 785 nm. This laser operates single mode with a mode-hop free tuning range of up to 15 GHz without current modulation and a side-mode suppression better than 55 dB. Demonstrating the suitability for neutral atom cooling we used this laser as light source in the production of a BEC of over a million 87Rb atoms.

Commercially available GaN-based laser diodes were antireflection coated in our laboratory and operated in an external cavity in a Littrow configuration. A total tuning range of typically 4 nm and an optical output power of up to 30 mW were observed after optimization of the external cavity. The linewidth was measured with a beterodyne technique, and 0.8 MHz at a sweep time of 50 ms was obtained. The mode-hop-free tuning range was more than 50 GHz. We demonstrated the performance of the laser by detecting the saturated absorption spectrum of atomic indium at 410 nm, allowing observation of well-resolved Lamb dips.

We report a portable mid-infrared spectrometer for trace-gas analysis which is based on an all-solid-state difference-frequency-generation laser. The spectrometer provides in situ absorption path lengths of more than 3 km by means of the cavity leak-out method, a cw variant of the cavity ring-down technique. The design, performance, and application of this spectrometer are presented. The light source utilizes difference-frequency generation in a periodically poled lithium niobate (PPLN) crystal pumped by two single-frequency solid-state lasers. A maximum power of 27 μW in the wavelength region near 3.3 μm is achieved using a pump power of 20 mW at 808 nm, a signal power of 660 mW at 1064 nm, and a 50-mm-long PPLN crystal. This corresponds to a conversion efficiency of 0.42 mW/(W2 cm). We demonstrate that this portable laser system is suitable as a light source in a cavity leak-out spectrometer. We achieved a minimum detectable absorption coefficient of 1×10-8/cm (integration time: 2 s), corresponding, for example, to a detection limit of 1 part per billion ethane. This compact trace-gas analyzer with high sensitivity and specificity is promising for various environmental and medical applications.

We demonstrate real-time analysis of ethane fractions in exhaled human breath using laser absorption spectroscopy. Our measurements are carried out by means of mid-infrared cavity leak-out spectroscopy, a ring-down technique utilizing a cw laser. This method proves to be an unique tool with very high sensitivity and specificity for rapid and precise breath testing. The detection limit achieved is 300 volume parts per trillion ethane in human breath (integration time: 5s). In contrast to conventional gas chromatographic analysis our method enables ethane monitoring without preconcentration of the breath sample.

Summary form only given. In the last years, laser-based spectroscopic techniques for trace gas detection on the ppb level have received increasing attention. Cavity leak-out spectroscopy (CALOS) is a fast, accurate and universal method for online measurement of absolute trace gas concentrations,e.g., the concentration of small hydrocarbons in ambient air samples. The sample gas is filled into an optical cavity formed by a pair of high-reflectivity mirrors. This results in a virtual absorption length of several kilometers depending on the mirror reflectivity, leading to detection limits on the ppb level. In this work we developed a difference frequency laser system, which is tunable from 2797 cm/sup -1/ to 3196 cm/sup -1/ with a fast, modehop free tuning range.

We present measurements of the absolute absorption intensity of the weak fifth overtone of the CH-stretch vibration of benzene at 16 550 cm(-1). By using cavity ring-down spectroscopy, it is possible to obtain directly the absolute value of the oscillator strength of the detected transition (f = (5.9 +/- 0.8) x 10(-11)). For this purpose, we had to develop a means of heating the cavity, mirrors. The influence of the mirror heating on the measured spectra is discussed. (C) 1999 Elsevier Science B.V. All rights reserved.

We report on our recent advances with cavity ring-down spectroscopy using mid-infrared cw lasers. An external high- finesse cavity is excited on a single fundamental mode with a tunable laser operating in the 3 micrometers region. After excitation the laser power is turned off for a short time and the subsequent decay of the field stored inside the cavity is observed. The effective pathlength covered by the laser light inside the cavity during the decay amounts to several km, depending on the mirror reflectivity. Measurement of the decay time gives the photon losses and thus enables the detection of weakly absorbing species inside the cavity. This approach is closely related to cavity ring-down spectroscopy with pulsed lasers. However the cw approach exhibits several advantages concerning spectral resolution and detection sensitivity. Application of this method to online monitoring of trace gases seems to be very promising. We demonstrate detection of hydrocarbons, like membrane and ethylene on the ppb level.