Room-temperature continuous-wave external cavity interband cascade laser tunable from 3.2 to 3.6†µm.

We report on wide tuning of external cavity interband cascade lasers (EC-ICLs) in continuous-wave operation at room temperature. The antireflection coated ICL gain chips were tuned with a diffraction grating in the Littrow configuration. A tuning range of 313 cm-1 (360 nm) from 2789 cm-1 to 3102 cm-1 (3.22 to 3.58 µm) in continuous wave at 293 K was demonstrated with a 5 µm-wide, 1.5 mm-long gain chip. A maximum output power of 13 mW and a minimum threshold current of 62 mA were measured at the peak gain. The heat dissipation of the chip was 0.2 W at threshold and 0.8 W at the maximum current of 200 mA.

High performance quantum cascade laser frequency combs at λ ∼ 6 µm based on plasmon-enhanced dispersion compensation.

We demonstrate quantum cascade laser (QCL) optical frequency combs emitting at λ ∼ 6 µm. A 5.5 µm-wide, 4.5 mm-long laser exhibits comb operation from -20 °C up to 50 °C. A maximum output power of 300 mW is achieved at 50 °C showing a robustness of the system. The laser output spectrum is ∼80 cm-1 wide at the maximum current, with a mode spacing of 0.334 cm-1, resulting in a total of 240 modes with an average power of 0.8 mW per mode. To achieve frequency comb operation, a plasmonic-waveguide approach is utilized. A thin, highly-doped indium phosphide (InP) layer is inserted in the top cladding design to compensate the positive dispersion of the system (material and waveguide). This approach can be further exploited to design QCL combs at even shorter wavelengths, down to 4 µm. Different ridge widths between 2.8 and 5.5 µm have been fabricated and characterized. All of the devices exhibit frequency comb operation. These observations demonstrate that the plasmonic-waveguide is a robust and reliable method for dispersion compensation of a semiconductor laser systems to achieve frequency comb operation.

Quantum cascade lasers with discrete and non equidistant extended tuning tailored by simulated annealing.

In this work, we report a new superstructure grating design method for broad, non-equidistant discrete tuning in quantum cascade lasers using the Vernier effect. Our approach is applied to a wafer with gain centred at ∼7.8 µm. Measurements of a 3.75 mm long device are presented yielding 3.66% tuning around the central frequency and a peak optical power over 200 mW at 0 ∘C heat sink temperature. In addition, we show that taking into account the optical dispersion of the material is crucial to fulfill narrow specifications. Our device is particularly well suited for multi absorption line spectroscopic measurements requiring high resolution and small form factor for high volume production.

Electrically-driven pure amplitude and frequency modulation in a quantum cascade laser.

We present pure amplitude modulation (AM) and frequency modulation (FM) achieved electrically in a quantum cascade laser (QCL) equipped with an integrated resistive heater (IH). The QCL output power scales linearly with the current applied to the active region (AR), but decreases with the IH current, while the emission frequency decreases with both currents. Hence, a simultaneous modulation applied to the current of the AR and IH sections with a proper relative amplitude and phase can suppress the AM, resulting in a pure FM, or vice-versa. The adequate modulation parameters depend on the applied modulation frequency. Therefore, they were first determined from the individual measurements of the AM and FM transfer functions obtained for a modulation applied to the current of the AR or IH section, respectively. By optimizing the parameters of the two modulations, we demonstrate a reduction of the spurious AM or FM by almost two orders of magnitude at characteristic frequencies of 1 and 10 kHz compared to the use of the AR current only.

Frequency stability of a dual wavelength quantum cascade laser.

We characterized the dual wavelength operation of a distributed Bragg reflector (DBR) quantum cascade laser (QCL) operating at 4.5 µm using two independent optical frequency discriminators. The QCL emits up to 150 mW fairly evenly distributed between two adjacent Fabry-Perot modes separated by ≈11.6 GHz. We show a strong correlation between the instantaneous optical frequencies of the two lasing modes, characterized by a Pearson correlation coefficient of 0.96. As a result, we stabilized one laser mode of the QCL to a N2O transition using a side-of-fringe locking technique, reducing its linewidth by a factor 6.2, from 406 kHz in free-running operation down to 65 kHz (at 1-ms observation time), and observed a simultaneous reduction of the frequency fluctuations of the second mode by a similar amount, resulting in a linewidth narrowing by a factor 5.4, from 380 kHz to 70 kHz. This proof-of-principle demonstration was performed with a standard DBR QCL that was not deliberately designed for dual-mode operation. These promising results open the door to the fabrication of more flexible dual-mode QCLs with the use of specifically designed gratings in the future.

Plasmon-enhanced waveguide for dispersion compensation in mid-infrared quantum cascade laser frequency combs.

We demonstrate dispersion compensation in mid-infrared quantum cascade laser frequency combs (FCs) emitting at 7.8 µm using the coupling of a dielectric waveguide to a plasmonic resonance in the top cladding layer of the latter. Devices with group velocity dispersion lower than 110 fs2/mm were fabricated, and narrow beatnotes with FWHM linewidths below 1 kHz were measured on the entire operation range. At -20°C, the optical output power reaches 275 mW, and the optical spectrum spans 60 cm-1. The multi-heterodyne beating spectrum of two devices was measured and spans 46 cm-1, demonstrating the potential of dispersion-engineered waveguides for the fabrication of highly stable and reliable quantum cascade laser FCs with high output power across the mid-infrared.

Mid infrared quantum cascade laser operating in pure amplitude modulation for background-free trace gas spectroscopy.

We present a single mode multi-section quantum cascade laser source composed of three different sections: master oscillator, gain and phase section. Non-uniform pumping of the QCL's gain reveals that the various laser sections are strongly coupled. Simulations of the electronic and optical properties of the laser (based on the density matrix and scattering matrix formalisms, respectively) were performed and a good agreement with measurements is obtained. In particular, a pure modulation of the laser output power can be achieved. This capability of the device is applied in tunable-laser spectroscopy of N2O where background-free quartz enhanced photo acoustic spectral scans with nearly perfect Voigt line shapes for the selected absorption line are obtained.

Gain-guided broad area quantum cascade lasers emitting 23.5 W peak power at room temperature.

We report gain-guided broad area quantum cascade lasers at 4.55 µm. The devices were processed in a buried heterostructure configuration with a current injector section much narrower than the active region. They demonstrate 23.5 W peak power at a temperature of 20°C and duty cycle of 1%, while their far field consists of a single symmetric lobe centered on the optical axis. These experimental results are supported well by 2D numerical simulations of electric currents and optical fields in a device cross-section.

High power and single mode quantum cascade lasers.

We present a single mode quantum cascade laser with nearly 1 W optical power. A buried distributed feedback reflector is used on the back section for wavelength selection. The laser is 6 mm long, 3.5 µm wide, mounted episide-up and the laser facets are left uncoated. Laser emission is centered at 4.68 µm. Single-mode operation with a side mode suppression ratio of more than 30 dB is obtained in whole range of operation. Farfield measurements prove a symmetric, single transverse-mode emission in TM00-mode with typical divergences of 41° and 33° in the vertical and horizontal direction respectively. This work shows the potential for simple fabrication of high power lasers compatible with standard DFB processing.

Extended tuning of mid-ir quantum cascade lasers using integrated resistive heaters.

We present single mode quantum cascade lasers including a microscopic heater for spectral emission tuning. Through the use of a buried heater element, the active region temperature can be modified without changing the submount one. Emission frequency tuning in continuous-wave as large as 9 cm(-1) at 1270 cm(-1) and 14 cm(-1) at 2040 cm(-1) are observed, corresponding to an increase of the active region temperatures of ∼ 90 K. Due to the proximity of the heaters to the active region, emission can be modulated at several kHz range and the absence of moving parts guarantees the mechanical stability of the system. This method can be successfully applied to all buried heterostructure lasers, becoming an attractive solution for molecular spectroscopy in the IR. Using the presented devices, molecular absorptions of N(2)O have been measured between 1270 cm(-1) and 1280 cm(-1) and are in agreement with data from the HITRAN database.

High performance, low dissipation quantum cascade lasers across the mid-IR range.

In this work, we present the development of low consumption quantum cascade lasers across the mid-IR range. In particular, short cavity single-mode lasers with optimised facet reflectivities have been fabricated from 4.5 to 9.2 µm. Threshold dissipated powers as low as 0.5 W were obtained in continuous wave operation at room temperature. In addition, the beneficial impact of reducing chip length on laser mounting yield is discussed. High power single-mode lasers from the same processed wafers are also presented.

All-electrical frequency noise reduction and linewidth narrowing in quantum cascade lasers.

A novel all-electrical method of frequency noise reduction in quantum cascade lasers (QCLs) is proposed. Electrical current through the laser was continuously adjusted to compensate for fluctuations of the laser internal resistance, which led to an active stabilization of the optical emission frequency. A reduction of the linewidth from 1.7 MHz in the standard constant current mode of operation down to 480 kHz is demonstrated at 10-ms observation time when applying this method to a QCL emitting at 7.9 µm.

Surface Plasmon Resonance sensor showing enhanced sensitivity for CO2 detection in the mid-infrared range.

We present the first optical sensor based on Surface Plasmon Resonance (SPR) operating in the mid-infrared range. The experimental setup is based on a Kretschmann geometry with Ti/Au layers deposited on a CaF(2) prism where light excitation is provided by a Quantum Cascade Laser (QCL) source. Evidence of SPR is presented and the sensing capability of the system is demonstrated by using CO(2) and N(2) mixtures as test samples. Due to the absorption of CO(2) at this wavelength, it is shown that the sensitivity of this configuration is five times higher than a similar SPR sensor operating in the visible range of the spectrum.