Stability of pulse emission and enhancement of intracavity second-harmonic generation in self-mode-locked quantum cascade lasers

We report the observation of stable pulse emission and enhancement of intracavity second-harmonic generation (SHG) in self-mode-locked quantum cascade (QC) lasers. Down-conversion of the detector signal by heterodyning with an RF signal allows the direct observation of the pulsed laser emission in the time domain and reveals a stable train of pulses characteristic of mode-locked lasers. The onset of self-mode locking in QC lasers with built-in optical nonlinearity results in a significant increase of the SHG signal. A pulse duration of /spl sim/12 ps is estimated from the measured increase of the SHG signal in pulsed emission compared to the power expected for the SHG signal in CW emission. This value is in good agreement with the pulse duration deduced from the optical spectral width.     

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).     

Quantum cascade lasers with double-quantum-well superlattices

A quantum-cascade laser using a double-quantum-well graded superlattice as the active region is presented. Each SL period consists of two strongly coupled quantum wells resulting in the splitting of the lowest miniband into two minibands. These two minibands can be designed to be flat and to contain delocalized, spatially symmetric wavefunctions under an applied electric field which in turn leads to a high optical dipole for the interminiband transition. In addition, the new design allows independent control of the energy levels of the lowest two minibands, their width and the splitting separating them, enhancing design flexibility. Using a cascade design of 55 pairs of alternated active regions and injectors, pulsed laser action is achieved at λ=11.6 μm. Peak output powers reach 120 mW at 7 K and approximately 12 mW at the maximum operating temperature of 195 K     

Continuous wave operation of λ~19 μm surface-plasmonquantum cascade lasers

Continuous wave laser action has been achieved in a superlattice quantum cascade device operating on surface plasmon waveguide modes. The emission wavelength λ~19 μm is by far the longest ever reported for continuous wave III-V semiconductor lasers. The output power at cryogenic temperature is of the order of the mW     

Theoretical and experimental study of optical gain and linewidth enhancement factor of type-I quantum-cascade lasers

A theoretical and experimental study of the optical gain and the linewidth enhancement factor (LEF) of a type-I quantum-cascade (QC) laser is reported. QC lasers have a symmetrical gain spectrum because the optical transition occurs between conduction subbands. According to the Kramers-Kronig relation, a zero LEF is predicted at the gain peak, but there has been no experimental observation of a zero LEF. There are other mechanisms that affect the LEF such as device self-heating, and the refractive index change due to other transition states not involved in lasing action. In this paper, the effects of these mechanisms on the LEF of a type-I QC laser are investigated theoretically and experimentally. The optical gain spectrum and the LEF are measured using the Hakki-Paoli method. Device self-heating on the wavelength shift in the Fabry-Perot modes is isolated by measuring the shift of the lasing wavelength above the threshold current. The band structure of a QC laser is calculated by solving the Schro/spl uml/dinger-Poisson equation self-consistently. We use the Gaussian lineshape function for gain change and the confluent hypergeometric function of the first kind for refractive index change, which satisfies the Kramers-Kronig relation. The refractive index change caused by various transition states is calculated by the theoretical model of a type-I QC laser. The calculated LEF shows good agreement with the experimental measurement.     

Temporal wavelength multiplexing of quantum cascade laser

A quantum cascade (QC) laser is presented that is both temporally and wavelength multiplexed, i.e. it emits two widely different wavelengths at two alternate time slots. A bidirectional and multi-wavelength QC laser source that emits at 10.8 m wavelength for a positive polarity current and 8.6 m for a negative polarity current was used. A diode based adder circuit was designed to drive the QC laser such that it added two opposite polarity current pulses with a variable time delay. Application of two polarity 100 ns pulse width current pulses at 80 kHz with time delays of 0.5 to 2.5 s resulted in a versatile time and wavelength multiplexed QC laser.     

Optimized second-harmonic generation in quantum cascade lasers

Optimized second-harmonic generation (SHG) in quantum cascade (QC) lasers with specially designed active regions is reported. Nonlinear optical cascades of resonantly coupled intersubband transitions with giant second-order nonlinearities were integrated with each QC-laser active region. QC lasers with three-coupled quantum-well (QW) active regions showed up to 2 /spl mu/W of SHG light at 3.75 /spl mu/m wavelength at a fundamental peak power and wavelength of 1 W and 7.5 /spl mu/m, respectively. These lasers resulted in an external linear-to-nonlinear conversion efficiency of up to 1 /spl mu/W/W/sup 2/. An improved 2-QW active region design at fundamental and SHG wavelengths of 9.1 and 4.55 /spl mu/m, respectively, resulted in a 100-fold improved external linear-to-nonlinear power conversion efficiency, i.e. up to 100 /spl mu/W/W/sup 2/. Full theoretical treatment of nonlinear light generation in QC lasers is given, and excellent agreement with the experimental results is obtained. For the best structure, a second-order nonlinear susceptibility of 4.7/spl times/10/sup -5/ esu (2/spl times/10/sup 4/pm/V) is calculated, about two orders of magnitude above conventional nonlinear optical materials and bulk III-V semiconductors.     

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.     

Direct liquid cooling of room-temperature operated quantum cascade lasers

A compact packaging module for direct liquid cooling of room-temperature quantum cascade lasers is implemented. The light was coupled out of the hermetically sealed package through a mid-infrared fibre epoxy-bonded to the laser. The threshold current reduces by /spl sim/10%, compared to convectional substrate-side cooled lasers when using low boiling temperature (boiling point (b.p.)=34/spl deg/C) 3M HFE-7000 coolant. No reduction was observed with high boiling point 3M FC-77 (b.p.=97/spl deg/C) coolant.