Single-mode Quantum Cascade Lasers Research Articles

Singlemode quantum cascade lasers with power dissipation below 1 W

Presented are narrow-ridge, singlemode (λ=4.49 µm) quantum cascade lasers with low power consumption in continuous-wave operation. Episide up mounted 3 µm-wide and 1 mm-long devices with high-reflectivity backside coating show an electrical dissipation at laser threshold as low as 840 mW at 20°C. The devices can be tuned singlemode and mode-hop free between −30°C and +50°C and in a range of injected electrical power of 0.7 and 1.3 W with a sidemode suppression ratio better than 27 dB.

Single mode quantum cascade lasers with shallow-etched distributed Bragg reflector

We report the fabrication of single mode quantum cascade lasers using a shallow-etched distributed Bragg reflector as frequency selective element. Quasi-continuous single mode tuning over 15 cm-1 at room temperature and 25 cm-1 via temperature tuning at Peltier temperatures is demonstrated. The behavior of both electro-optic and spectral characteristics under variation of the segment currents is analyzed, showing a maximum peak output power at room temperature of 600 mW. Thermal crosstalk between the laser segments is investigated. The spectral resolution of a gas absorption experiment is determined to be better than 0.0078 cm-1.

Single-mode quantum cascade lasers employing a candy-cane shaped monolithic coupled cavity

We demonstrate single-mode quantum cascade lasers emitting at ∼4.5 μm by employing a monolithic “candy-cane” shaped coupled-cavity consisting of a straight section connecting at one end to a spiral section. The fabrication process is identical to those for simple Fabry-Perot-type ridge lasers. Continuously tunable single-mode emission across ∼8 cm−1 with side mode suppression ratio up to ∼25 dB and a single-mode operating current range of more than 70% above the threshold current is achieved when the lasers are operated in pulsed-mode from 80 K to 155 K.

Single-mode quantum cascade lasers based on a folded Fabry-Perot cavity

We demonstrate single-mode quantum cascade lasers employing a folded Fabry-Perot cavity consisting of two straight sections connected by a semicircular section in a “hairpin” shape. These folded cavity lasers emitting at ∼4.5 μm are fabricated with identical processes as those for plain Fabry-Perot ridge lasers, and show a strong suppression of the comb of Fabry-Perot cavity modes, leading to tunable single-mode emission with up to 27 dB side mode suppression ratio and a single-mode operating current range of up to 60% above the threshold current when operated in pulsed mode; single-mode emission is achieved from 80 to ∼240 K.

Directional single mode quantum cascade laser emission using second-order metal grating coupler

We report on the design and experimental demonstration of a substrate emitting quantum cascade laser (QCL) with low beam divergence in the two directions. A low-loss, index-coupled, distributed feedback laser is coupled to a monolithic extraction area. Both functions are performed with a top metal grating spatially differentiated for improving the divergence of the QCL in the two directions. Spectrally single-mode InGaAs/AlInAs QCL emitting at a wavelength of 5.65 μm with a low beam divergence, represented by a full width at half maximum of 2.3° and 4°, is demonstrated at room temperature with a threshold current of 2.1 kA/cm2.

High-performance midinfrared quantum cascade lasers

The design and operating principles of quantum cascade lasers (QCLs) are reviewed along with recent developments in high-power cw and broadband devices. Cw power levels of several watts at room temperature have been achieved at 4.6-µm wavelength; broadband single-mode tuning (400 cm−1) has been achieved using an external-cavity QCL with a grating as a tuning element. An alternative approach, consisting of a monolithically integrated array of single-mode QCLs individually current-driven by a microcontroller, has led to broadband single-mode tuning over a range of 200 cm−1 without requiring the use of moving parts. This spectrometer on a chip holds promise for high-brightness compact trace-gas sensors capable of operating in harsh environments.

Terahertz heterodyne spectrometer using a quantum cascade laser

A terahertz (THz) heterodyne spectrometer is demonstrated based on a quantum cascade laser (QCL) as a local oscillator (LO) and an NbN hot electron bolometer as a mixer, and it is used to measure high-resolution molecular spectral lines of methanol (CH3OH) between 2.913–2.918 THz. The spectral lines are taken from a gas cell containing methanol gas and using a single-mode QCL at 2.9156 THz as an LO, which is operated in the free running mode. By increasing the pressure of the gas, line broadening and saturation are observed. The measured spectra showed good agreement with a theoretical model.

Surface-emitting mid-infrared quantum cascade lasers with high-contrast photonic crystal resonators

We have developed surface-emitting single-mode quantum cascade lasers which employ high-contrast photonic-crystal resonators. The devices operate on band-edge states of the photonic band-structure. The mode profile and polarization characteristics of the band-edge modes are calculated by three-dimensional finite-difference time-domain simulation. Experimentally, the spectral properties, the far-field patterns, and the polarization characteristics of the lasers are determined and compared with simulations. The good agreement between the simulations and the experiments confirms that the hexapolar mode at the Gamma-point band-edge gives rise to lasing. By using a novel and advanced fabrication method, deep and vertical PhC holes are fabricated with no metal redeposition on the sidewalls, which improves the laser performance with respect to the current status. The angular of the output beam is approximately 15 masculine, and the side mode suppression ratio of the single mode emission is about 25 dB. The threshold current density at 78 K and the maximum operation temperature are 7.6 kA/cm2 and 220 K, respectively. The performance is mainly limited by the loss induced by surface plasmon waveguide, which can be overcome by using an optimized dielectric waveguide structure.

A wrench of wavelength

Mechanically manipulating the lateral size of the lasing mode of a single-mode terahertz quantum cascade laser provides a new and robust method for widely and continuously tuning its emission wavelength.

Bound-to-continuum terahertz quantum cascade laser with a single-quantum-well phonon extraction/injection stage

A terahertz quantum cascade laser design that combines a wide gain bandwidth, large photon-driven transport and good high-temperature characteristics is presented. It relies on a diagonal transition between a bound state and doublet of states tunnel coupled to the upper state of a phonon extraction stage. The high optical efficiency of this design enables the observation of photon-driven transport over a wide current density range. The relative tolerance of the design to small variations in the barrier thicknesses made it suitable for testing different growth techniques and materials. In particular, we compared the performances of devices grown using molecular-beam epitaxy with those achieved using organometallic chemical vapor deposition. The low-threshold current density and the high slope efficiency makes this device an attractive active region for the development of single-mode quantum cascade lasers based on third-order-distributed feedback structures. Single-mode, high power was achieved with good continuous and pulsed wave operation.

Focus on lasers and optics

Focus on lasers and optics

Theory of the Intrinsic Linewidth of Quantum-Cascade Lasers: Hidden Reason for the Narrow Linewidth and Line-Broadening by Thermal Photons

We have developed a theory of the intrinsic linewidths of laser output of single-mode quantum-cascade (QC) lasers in mid-infrared and terahertz (THz) ranges. In the theoretical treatment, the concept of an effective coupling efficiency of spontaneous emission, given by a fractional rate of spontaneous emission coupled into a lasing mode to total nonlasing relaxation, is introduced to clarify a hidden reason for the narrowness of the linewidths. A narrow linewidth (12-kHz) reported with a frequency-stabilized 8.5- distributed-feedback QC laser is successfully interpreted in terms of an extremely small effective coupling efficiency of spontaneous emission, caused by ultrafast nonradiative scatterings. The present theory predicts the presence of a minimum ldquolinewidth floorrdquo in a high-injection-current region and the independence of linewidth on detuning between gain-peak and emission wavelengths. The theoretical treatment is expanded to derive the further modified Schawlow-Townes formula including the line-broadening by black body radiation in a THz QC laser. The linewidth of a THz QC laser is predicted to be considerably broadened by black body radiation.

Development of DFB Quantum Cascade Lasers and their Applications

We have developed single-mode quantum cascade lasers (QCLs) emitting at 5.2μm and 7.7 Em. Single-mode emission with narrow line width is achieved using a distributed feedback (DFB) technology. A narrow line width is interpreted in terms of an extremely small effective coupling efficiency of spontaneous emission, caused by ultrafast nonradiative scatterings. We have demonstrated absorption signal detection of NO, N2O, and CH4. The highest spectral resolution can be achieved with a CW mode DFB-QCL. We have observed lines of isotopomer of nitrous oxide, and signal detection limit is about 50 ppb.

Widely tunable single-mode quantum cascade laser source for mid-infrared spectroscopy

We demonstrate a compact, single-mode quantum cascade laser source continuously tunable between 8.7 and 9.4μm. The source consists of an array of single-mode distributed feedback quantum cascade lasers with closely spaced emission wavelengths fabricated monolithically on a single chip and driven by a microelectronic controller. Our source is suitable for a variety of chemical sensing applications. Here, we use it to perform absorption spectroscopy of fluids.

Low-consumption (below 2 W) continuous-wave singlemode quantum-cascade lasers grown by metal-organic vapour-phase epitaxy

Continuous-wave operating singlemode quantum-cascade lasers with a threshold current of 75 mA and total power consumption of less than 2 W are presented. The lasers are grown by metal-organic vapour-phase epitaxy; they function up to temperatures higher than 50°C and the emission wavelength is 5.25 µm.

Widely tunable single-mode quantum cascade lasers with two monolithically coupled Fabry-Pérot cavities

The authors report on tunable, miniaturized, two-segment coupled cavity quantum cascade lasers fabricated in a process with a single dry etch step. They observed controllable vernier-effect-based mode switching, induced by the spectral shift of the short cavity’s Fabry-Pérot mode comb due to Joule heating. With the help of this discontinuous tuning mechanism, single-mode operation with routinely ∼20dB side mode suppression ratio was achieved over a broad spectral range. The authors measured a quasicontinuous tuning range of 12cm−1 (138nm) and an entire single-mode tuning range of 16.5cm−1 (190nm), centered around 933cm−1 (10.7μm).

23 GHz operation of a room temperature photovoltaic quantum cascade detector at 5.35μm

We present a room temperature operated 5.35μm quantum cascade detector which was tested at high frequencies using an optical heterodyne experiment. Two slightly detuned continuous wave distributed feedback single mode quantum cascade lasers were used to generate a beating signal. The maximum frequency at which the resulting microwave signal could be detected was 23GHz. The cutoff behavior of our device was modeled with a simple RLC circuit and showed excellent agreement with the experimental data.

High-performance operation of single-mode terahertz quantum cascade lasers with metallic gratings

A periodic array of thin slits opened on a metallic surface can act as a one-dimensional photonic crystal for the propagation of surface-plasmon waves. We have used such structure for the implementation of distributed feedback resonators in quantum cascade lasers emitting near 2.5THz. Single-mode emission, stable at all injection currents and operating temperatures, was achieved both in pulsed and continuous wave. The devices exhibited output powers of several milliwatts with low threshold current densities of ∼100A∕cm2.

High-power spatial singlemode quantum cascade lasers at 8.9 µm

High performance of InP-based quantum cascade lasers emitting at λ≃9 µm is reported. A thick electroplated gold layer was deposited on top of the laser to improve heat dissipation. Devices with one facet high reflection coated produce a maximum output power of 175 mW at 40% duty cycle at room temperature and continuous-wave operation up to 278 K.

Long-wavelength (λ≈16 μm), room-temperature, single-frequency quantum-cascade lasers based on a bound-to-continuum transition

Room-temperature operation of long-wavelength, Fabry–Perot and single-mode quantum-cascade lasers at λ≈16 μm is reported. Multimode emission with pulsed peak power up to 400 mW at −40 °C and 220 mW at 30 °C is demonstrated. Single-mode emission up to 60 mW peak power has been achieved at 30 °C.