Rüdeger Köhler

Dependence of the device performance on the number of stages in quantum-cascade lasers

The cascading scheme is a characteristic feature of quantum cascade (QC) lasers. It implies that electrons above threshold generate one photon per active region they successively traverse. This paper presents a study of the cascading behavior as a function of the number N of stacked active regions. Experimental results are presented for devices with N=1, 3, 6, 12, 20, 30, 45, 60, and 75 active stages. The highest optical power and lowest threshold current density are obtained for laser devices with N as high as possible. However, the lowest threshold voltage and the lowest dissipated power at laser threshold are achieved for N=3 and N=22, respectively. We further present the highest power QC lasers so far, which, using N=75 stages, show in pulsed mode peak powers of 1.4, 1.1, and 0.54 W at 50 K, 200 K, and room temperature, respectively. Finally, we also demonstrate the first few-stage (N<10) QC lasers. These QC lasers show strongly reduced operating voltages. A threshold voltage around 1.5 V is achieved for N=3. This makes the lasers very well compliant with conventional laser diode drivers, which in turn will simplify their immediate use in systems and applications.

Single-mode tunable, pulsed, and continuous wave quantum-cascade distributed feedback lasers at λ≅4.6–4.7 μm

Single-mode tunable quantum-cascade distributed feedback lasers emitting at 4.6–4.7 μm wavelength are reported. The lasers employ strained heterostructure material with global strain compensation to provide the large band offset needed for high-performance short wavelength operation. Pulsed, continuously tunable single-mode emission is achieved from 90 to 300 K with a tuning range of 65 nm. Peak output power levels of 100 mW at room temperature are obtained. In continuous-wave operation, current tunable single-mode emission is demonstrated around liquid-nitrogen temperature with a tuning range of 20 nm (over a current range of 450 mA). The maximum output power in continuous wave at 80 K is 150 mW.

Thermoelectrically cooled quantum-cascade-laser-based sensor for the continuous monitoring of ambient atmospheric carbon monoxide

We report the first application of a thermoelectrically cooled, distributed-feedback quantum-cascade laser for continuous spectroscopic monitoring of CO in ambient air at a wavelength of 4.6 microm. A noise-equivalent detection limit of 12 parts per billion was demonstrated experimentally with a 102-cm optical pathlength and a 2.5-min data acquisition time at a 10-kHz pulsed-laser repetition rate. This sensitivity corresponds to a standard error in fractional absorbance of 3 x 10(-5).

Transportable automated ammonia sensor based on a pulsed thermoelectrically cooled quantum-cascade distributed feedback laser

A compact ammonia sensor based on a 10-microm single-frequency, thermoelectrically cooled, pulsed quantum-cascade laser with an embedded distributed feedback structure has been developed. To measure NH3 concentrations, we scanned the laser over two absorption lines of its fundamental v2 band. A sensitivity of better than 0.3 parts per million was achieved with just a 1-m optical path length. The sensor is computer controlled and automated to monitor NH3 concentrations continuously for extended periods of time and to store data in the computer memory.

Single-mode tunable quantum cascade lasers in the spectral range of the CO 2 laser at /spl lambda/=9.5-10.5 μm

Widely tunable, single-mode quantum cascade distributed feedback (QC-DFB) lasers based on a complex coupling scheme and operating in the wavelength range of the CO/sub 2/ laser (/spl lambda//spl ap/9.5-10.5 /spl mu/m) are reported. Dynamic single-mode emission up to high current levels is obtained. The continuous single-mode tuning range is 150 nm, while the tuning range of the equivalent Fabry-Perot laser is /spl sim/400 nm. By homogeneously reducing all layer thicknesses by 10%, the wavelength coverage of a single QC-laser design can be extended to cover one entire regular band of the fundamental CO/sub 2/ laser spectrum.

Characterization of a quantum cascade laser as local oscillator in a heterodyne receiver at 2.5 THz

A quantum cascade laser operating at 2.5 THz has been investigated with respect to performance parameters which are relevant for a local oscillator in a heterodyne receiver. The beam profile has been measured and transformed in to a close to Gaussian profile resulting in a good matching between the field patterns of the quantum cascade laser and the antenna of a superconducting hot electron bolometric mixer. Noise temperature measurements with the hot electron bolometer and a 2.5 THz quantum cascade laser yielded the same result as with a gas laser as local oscillator.