Rainer Strzoda

Wavelength modulation spectroscopy with a widely tunable InP-based 2.3 μm vertical-cavity surface-emitting laser

We report on the successful application of recently developed 2.3 microm InP-based vertical-cavity surface-emitting lasers with a buried tunnel junction in a wavelength modulation spectroscopy measurement for carbon monoxide (CO) detection. The electrically pumped devices operate at room temperature under cw operation with stable single-mode emission and should allow for parts in 10(6) (ppm) level resolution measurement of CO with a standard optical setup.

An open-path, hand-held laser system for the detection of methane gas

We have developed an open-path hand-held gas detector incorporating a distributed feedback InGaAs laser diode at 1.65 µm. Incorporated into a hand-held transceiver unit, the emitted laser beam is backscattered from nearby surfaces, collected and focused onto an amplified InGaAs detector using a 150 mm diameter plastic Fresnel lens. At ranges of 4–5 m, a typical backscattered signal is tens of nanowatts of laser light. Applying second derivative wavelength modulation spectroscopy gives a sensitivity to methane of better than 10 parts per million over a one metre path length. A number of demonstration units have been fabricated and successfully evaluated by end users.

Experimental characterization of the frequency modulation behavior of vertical cavity surface emitting lasers

The frequency response of the current-to-wavelength tuning rate (FM response) was measured for two different vertical cavity surface emitting lasers (VCSELs) up to frequencies of 1MHz: GaAs-based VCSEL (763nm) and InP-based VCSEL (1853nm). Both of them show the same qualitative FM response behavior, which can be described by a square root law and therefore cannot be modeled by a first order low pass. The square root law behavior is a significant advantage for laser spectroscopy applications with VCSELs because the decrease of the current-to-wavelength tuning coefficient is less severe as in the case of the first order low pass.

The Frequency Modulation Response of Vertical-Cavity Surface-Emitting Lasers: Experiment and Theory

The FM response of vertical-cavity surface-emitting lasers (VCSELs), i.e., the dynamic wavelength tuning behavior, is scrutinized. The FM amplitude and phase shift are measured up to 80 MHz for GaAs-, InP-, and GaSb-based VCSELs from 763 to 2300 nm. From measurements, it is found that the FM response consists of three components: intrinsic thermal tuning (dominating to several megahertz) with characteristic 1/√(if) behavior, the plasma effect (dominating from several megahertz), and a small effect (10-100 Hz) caused by the interaction of laser chip and submount. All effects are modeled and the measurement data are fitted to obtain effective thermal diffusivities, strength of the plasma effect, and time constant of the laser chip submount interaction. Comparing thermal models with different asymptotic behaviors, an approximation of the heat source in the laser with a nonzero thickness turned out to be necessary. Due to the plasma effect, with influence starting at 100 kHz, VCSELs cannot be considered a minimum phase system, which makes separate amplitude and phase measurements essential for device characterization. The “ N time constants model” is the proper choice for empirical description of the intrinsic thermal tuning component. The best fit coefficients to a rational frequency response are given for use in time-domain simulation programs.

Imaging of methane gas using a scanning, open-path laser system

We have developed an imaging system for the detection and visualization of methane gas leaks. The system is based on a distributed feedback InGaAs laser diode emitting at 1.65 μm, the beam from which is directed at neighbouring objects. The backscattered light is collected by a Fresnel lens and the gas concentration is deduced from the reduction in collected intensity as measured using a second derivative wavelength modulation technique. The incident laser and the collected beam are both scanned over an area to form an image of the gas emission. To ease the task of locating the source of the emission, we combine the resulting low-resolution image of the gas emission with a high-resolution colour image of the scene. Our results show that the system can image a gas cloud of 1 mm effective thickness at a range of several metres, sufficient to detect a gas leak of 1 litre min−1 in light to moderate winds.

Low-level and ultralow-volume hollow waveguide based carbon monoxide sensor

We demonstrate an ultralow sample volume optical carbon monoxide sensor with detection sensitivity of 180 parts in 10(9) (1σ at 1 Hz). The utilization of a 2.3 μm surface-emitting laser directly coupled to a 3 m hollow capillary fiber as the gas cell is proven to be a compact, sensitive, and cost-efficient gas sensing concept. By mechanical vibration of the fiber, an absorbance resolution of 10(-5) is achieved, which is comparable to single-reflective (double-pass) cells. An improvement of sensitivity over the conventional single-reflective cell is thus approximately linearly scaled with the enhancement of the optical path length, which is usually more than 1 order of magnitude.

GaSb and InP-based VCSELs at 2.3 μm emission wavelength for tuneable diode laser spectroscopy of carbon monoxide

We present long-wavelength buried tunnel junction (BTJ) VCSELs for emission wavelengths around 2.3 μm. Two different device concepts have been realized utilizing either InP- or GaSb-based materials. The InP-VCSELs are based on a BTJ-design which has been well-proven for wavelengths up to 2 μm in recent years. To extend this range up to emission wavelengths around 2.3 μm, the main focus is set on an optimization of the active region. In this context, we use a graded and heavily strained quantum well design in conjunction with optimized growth conditions. The photoluminescence and x-ray characterization shows a very good material quality. Room-temperature operated VCSELs exhibit around 0.5 mW of output power with singlemode-emission at 2.36 μm representing the longest wavelength that has been achieved with InP-based interband lasers so far. GaSb-based devices comprise an epitaxial back mirror and a dielectric output mirror while the basic BTJ-principle is maintained. Using GaInAsSb quantum wells, the active region reveals excellent gain characteristics at 2.3 μm. Singlemode VCSELs show room temperature threshold currents around 1 mA and output powers of 0.7 mW, respectively. Both laser types have been implemented in a tuneable diode laser spectroscopy (TDLS) setup to evaluate their capability for sensing of carbon monoxide. Using an absorption path length of only 10 cm, concentration measurements down to a few ppm have been successfully demonstrated.

Resolution limits of laser spectroscopic absorption measurements with hollow glass waveguides.

In this paper, resolution limits of laser spectroscopy absorption measurements with hollow capillary fibers are investigated. Furthermore, a concept of sensitive near-infrared sensing utilizing hollow fiber directly coupled with vertical-cavity surface-emitting lasers is developed. By performing wavelength modulation spectroscopy, the smallest absorbance that can be detected by the fiber sensor was determined to be 10(-4), limited by a random modulation of the fiber transmission function (modal noise). By mechanically vibrating the fiber, a sensor resolution of 10(-5) in absorbance is achieved. Because the random modulation on the fiber transmission function limits the detection sensitivity, its physical reasons are analyzed. One contribution is found to be the partial integration of the far field, and the amplitude of the spectral features is inversely proportional to the square root of the integrated speckle points number. Therefore, careful design of the fiber-detector outcoupling is necessary. It turned out that incoupling alignment is not of much influence with respect to the spectral background. The residual spectral background is caused by mode-dependent effects and can be lowered by vibrating the fiber mechanically.

Studies on the butt-coupling of InGaAsP-waveguides realized with selective area metalorganic vapour phase epitaxy

Abstract A method for the characterisation of the process-induced optical loss in butt-coupled passive waveguides will be described. In principle the results are valid for any active-passive waveguide butt-coupling as well (e.g. laser-waveguide). The method was applied to butt-coupled waveguides, which were grown by selective area metal organic vapour phase epitaxy (MOVPE). Different etching procedures for the preparation of the masked mesa stripes were examined, which considerably influenced the regrowth behaviour close to the mesa edge. The optical loss at the interface was found to depend primarily on the thickness difference of the waveguides. The excess layer thickness due the regrowth near the mesa is proportional to the width of the masked stripe. It reaches a factor of 1.7 at 50 μm wide stripes and decreases exponentially with distance from the stripe ( 1 e length = 26 μ m ). In general the measured loss values agree with the numerical simulation at least for stripe widths ≥30 μm. For stripe widths of ≤30 μm loss values ≤1 dB have been obtained in all cases, but deviations of the calculated loss values occurred, which can be attributed to the regrowth geometry at the interface.

Long-wavelength VCSELs for sensing applications

InP-based tunable singlemode VCSELs in the 1.3 -2.3 mum wavelength range are presented. The relevant laser parameters are discussed and several applications in trace-gas-sensing are demonstrated.