Stéphane Blaser

Mid-infrared frequency comb based on a quantum cascade laser

Optical frequency combs act as rulers in the frequency domain and have opened new avenues in many fields such as fundamental time metrology, spectroscopy and frequency synthesis. In particular, spectroscopy by means of optical frequency combs has surpassed the precision and speed of Fourier spectrometers. Such a spectroscopy technique is especially relevant for the mid-infrared range, where the fundamental rotational–vibrational bands of most light molecules are found. Most mid-infrared comb sources are based on down-conversion of near-infrared, mode-locked, ultrafast lasers using nonlinear crystals. Their use in frequency comb spectroscopy applications has resulted in an unequalled combination of spectral coverage, resolution and sensitivity. Another means of comb generation is pumping an ultrahigh-quality factor microresonator with a continuous-wave laser. However, these combs depend on a chain of optical components, which limits their use. Therefore, to widen the spectroscopic applications of such mid-infrared combs, a more direct and compact generation scheme, using electrical injection, is preferable. Here we present a compact, broadband, semiconductor frequency comb generator that operates in the mid-infrared. We demonstrate that the modes of a continuous-wave, free-running, broadband quantum cascade laser are phase-locked. Combining mode proliferation based on four-wave mixing with gain provided by the quantum cascade laser leads to a phase relation similar to that of a frequency-modulated laser. The comb centre carrier wavelength is 7 micrometres. We identify a narrow drive current range with intermode beat linewidths narrower than 10 hertz. We find comb bandwidths of 4.4 per cent with an intermode stability of less than or equal to 200 hertz. The intermode beat can be varied over a frequency range of 65 kilohertz by radio-frequency injection. The large gain bandwidth and independent control over the carrier frequency offset and the mode spacing open the way to broadband, compact, all-solid-state mid-infrared spectrometers.

Bound-to-continuum and two-phonon resonance, quantum-cascade lasers for high duty cycle, high-temperature operation

Recent advances in quantum-cascade (QC) laser active-region design are reviewed. Based on a rate equation model of the active region, we show why new gain regions. based on a two-phonon resonance or a bound-to-continuum transition exhibit significantly better performance than the traditional design based on a three-quantum-well active region. Threshold current densities as low as 3 kA/cm/sup 2/ at T=300 K, operation with a peak power of 90 mW at 425 K, single-mode high-power operation up to temperatures above 330 K at /spl lambda//spl ap/16 /spl mu/m and continuous wave operation up to T=311 K are demonstrated. QC lasers able to operate at high duty cycles (50%) on a Peltier cooler were used in a demonstration of a 300-MHz free-space optical link between two buildings separated by 350 m.

Dual-comb spectroscopy based on quantum-cascade-laser frequency combs

Mid-infrared dual-comb spectroscopy by means of quantum cascade laser frequency combs is demonstrated. Broadband high resolution molecular spectroscopy is performed, showing the potential of quantum cascade laser combs as a compact, all solid-state, chemical sensor.

Room-temperature, continuous-wave, single-mode quantum-cascade lasers at λ≃5.4μm

We demonstrate room-temperature, single-mode, continuous-wave operation of a λ≃5.4μm quantum-cascade laser up to the temperature of 30°C. Processing is done using standard lithography in a ridge waveguide mounted junction-up. The active region is based on a bound-to-continuum transition. The high performances were achieved with a low active region doping and a thick electroplated gold deposition, resulting in a characteristic temperature of T0=155K in continuous-wave with a threshold current density of jth=2.05kA∕cm2 at 300K.

Continuous-wave distributed-feedback quantum-cascade lasers on a Peltier cooler

Continuous-wave operation of λ∼9 μm distributed-feedback quantum-cascade lasers is reported up to a temperature of 260 K. Single-frequency emission with a side mode suppression ratio of ⩾27 dB and with a tuning range of 5 cm−1 between 200 and 245 K (a tunability of −0.078 cm−1/K and −0.764 cm−1/W) is obtained for the junction-down mounted buried heterostructure devices. Uncoated lasers display an output power of up to 18 mW at 180 K and still 1 mW at 250 K. Lasers with high-reflection coated facets could be operated up to 260 K.

Direct measurement of the linewidth enhancement factor by optical heterodyning of an amplitude-modulated quantum cascade laser

A measurement of the linewidth enhancement factor α of a distributed feedback quantum cascade laser is presented. The measurement is based on a heterodyning experiment, in which one of the lasers is modulated at radio frequency. A value of α=0.02±0.20 is obtained for a modulation frequency of 500MHz. As the frequency is decreased, α increases and is consistent with a thermal chirp effect.

Distributed-Feedback Quantum-Cascade Lasers at 9

Single-mode lasers operating at lambda ap 9 mum in continuous wave up to 423 K (150degC) were achieved by the combination of strong distributed-feedback coupling, a narrow gain active region design, low intersubband, and free-carrier losses as well as a good thermal management. Tuning of 10 cm-1 or 0.9% of the center frequency was achieved by heating the device. The threshold current density varies from 1.1 kA/cm2 at 303 K to 2.4 kA/cm2 at 423 K. Other devices with low electrical power consumption of 1.6 and 3.8 W for an optical output power of 16 and 100 mW have been demonstrated at 263 K.

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Lasing properties of room temperature, continuous wave operated distributed feedback (DFB) quantum cascade lasers are reported. A bound-to-continuum active region was used to generate a broad gain spectrum. As a result, first-order DFB lasers employing different periods allowed us to achieve single mode continuous wave emission at several wavelengths ranging from 7.7to8.3μm at a temperature of +30°C. The frequency span corresponds to 8% of the center frequency.

m Operating in Continuous Wave Up to 423 K

We report on power, spectral linewidth, and mode purity for a cw 5.3 microm quantum cascade laser operated on a thermo-electric cooler. A totally noncryogenic nitric oxide monitor was constructed by integrating this laser with an astigmatic multipass cell and a thermo-electrically cooled infrared detector. The resulting instrument is capable of continuous unattended monitoring of ambient, atmospheric nitric oxide for several weeks with no operator intervention. The detection method was rapid sweep, direct absorption spectroscopy. A detection sensitivity of 0.03 parts in 10(9) is achieved with 30 s averaging time with a path length of 210 m, corresponding to an absorbance path length product of 1.5 x 10(-10) cm(-1).

Room temperature, continuous wave operation of distributed feedback quantum cascade lasers with widely spaced operation frequencies

Room-temperature, continuous-wave operation of an external-cavity quantum cascade laser (EC-QCL) is reported. Single-mode tuning range of 120 cm(-1) was achieved, from 7.96 to 8.84 microm. The gain chips utilized are based on the bound to continuum design and were fabricated as buried heterostructure lasers. Gap-free tuning (mode hops only on the external-cavity modes) is demonstrated for an antireflection-coated laser, just by grating rotation. The EC-QCL was implemented in a Littrow setup and an average power of 1.5 mW was obtained at 20 degrees C, while a peak power of 20 mW was obtained for a modified Littrow setup with the back extraction of light.