type-II Interband Cascade Lasers Research Articles

Toward robust and practical interband cascade laser frequency combs: A perspective

Multimode emission from a semiconductor laser can sometimes take the form of a low-noise frequency comb with equidistant separation between the modes. Two general types of “passive” comb operation have been explored experimentally: (1) the periodic short-pulse mode achieved by intracavity mode locking, usually obtained by incorporating a saturable absorber section into the cavity, and (2) the so-called frequency-modulated (FM) mode in which the output intensity can vary within wide bounds but does not completely turn off between pulses, and the instantaneous frequency is linearly chirped over a round trip. The FM mode sometimes manifests as a “sparse” harmonic state, with individual modes spaced by multiples of the cavity free spectral range. This Perspective reviews the current understanding of these modes, along with the conditions under which they may arise in practical devices. We then consider in detail the case of type-II mid-infrared interband cascade laser (ICL) frequency combs. Our simulations clarify the roles of carrier dynamics and group velocity dispersion and identify design modifications that may substantially improve the device performance. We find no fundamental roadblocks to the development of practical mode-locked ICL frequency combs that emit short pulses with broad spectral bandwidth for dual comb spectroscopy and other applications, alongside the FM combs that have already been demonstrated experimentally.

Room-temperature continuous-wave interband cascade laser emitting at 3.45 μm**Project supported by the Major Program of the National Natural Science Foundation of China (Grant No. 61790580), the National Natural Science Foundation of China (Grant No. 61435012), and the National Basic Research Program of China (Grant No. 2014CB643903).

We report a type-II GaSb-based interband cascade laser operating a continuous wave at room temperature. The cascade region of interband cascade laser was designed using the ‘W’ configuration of the active quantum wells and the ‘Carrier Rebalancing’ method in the electron injector. The devices were processed into narrow ridges and mounted epitaxial side down on a copper heat sink. The 25-μm-wide, 3-mm-long ridge without coated facets generated 41.4 mW of continuous wave output power at T = 15 °C. And a low threshold current density of 267 A/cm2 is achieved. The emission wavelength of the ICL is 3452.3 nm at 0.5 A.

IHWG-ICL: Methane Sensing with Substrate-Integrated Hollow Waveguides Directly Coupled to Interband Cascade Lasers

The development of a compact iHWG-ICL gas sensor combining innovative substrate-integrated hollow waveguides (iHWG) with mid-infrared emitting type-II interband cascade lasers (ICL) is presented. Hence, tunable laser absorption spectroscopy (TLAS) with iHWGs in direct absorption mode is enabled. Using a room-temperature distributed feedback (DFB) ICL emitting at approximately 3.366 μm, quantitative sensing of methane was demonstrated. Wavelength scanning was obtained via current tuning for monitoring an isolated line in the v3 fundamental band of CH4. The obtained spectra were compared to calculated spectra derived from the HITRAN2012 database. Furthermore, the performance of iHWGs simultaneously serving as miniaturized gas cell and as efficient optical waveguide at various absorption path lengths was tested and optimized. Calibration functions in the concentration range of 50 to 400 ppmv were established enabling limits of detection ranging from 6 to 28 ppmv. Hence, the combination of iHWGs with ICLs fac...

Design of type-II interband cascade lasers for terahertz emission

We report the design of type-II interband cascade (IC) lasers emitting at terahertz (THz) range. The band structures are obtained using an eight-band k · p finite difference method with strain effects, which accounts for the coupling between the conduction and valence bands. Also, a detailed analysis of the interband transition energy in the type-II IC laser structures as a function of layer thickness is carried out using the self-consistent Schrodinger–Poisson calculations. The calculations show that the promising type-II IC lasers operating at 2.3 THz can be reached.

Widely tunable type-II interband cascade laser

We discuss an ultrawide, voltage-tunable type-II mid-IR interband cascade laser. Its design has a charge accumulation layers outside of the optically active quantum wells that unclamps the electron-hole concentrations and facilitates above-threshold Stark shifts. Our results demonstrate laser tuning of 120nm (120cm−1).

Analysis of the temperature performance of type-II interband cascade lasers

The temperature performance of type-II semiconductor lasers has been analyzed by comparing the temperature-concentration dependence for a charge-carrier subsystem at the threshold with steady-state temperature-concentration relationship implied by the carrier heating process. The low material gain characteristic of type-II heterostructures and the high resistance of the thermal link to the heat sink are primarily responsible for limiting the continuous-wave laser operation to low temperatures. We show also that the number of cascades for type-II interband cascade lasers can be optimized with respect to the highest achievable operating temperature.

Linewidth enhancement factor of a type-II interband-cascade laser

Experimental results using the amplified spontaneous emission spectroscopy of a type-II interband-cascade laser are presented. Using the Hakki-Paoli method, the optical gain spectra of the laser are extracted for the wavelength of 3.13μm at various subthreshold current levels. A group index of around 3.33 is determined from Fabry-Pérot modal spacing. The change in refractive index with increased bias current is obtained from the peak wavelength shifts of the Fabry-Pérot spectrum. A low value of 0.71 for the linewidth enhancement factor at the lasing wavelength near threshold is found.

Effects of anisotropic k·p interactions on energy bands and optical properties of type-II interband cascade lasers

The validity of an eight-band k·p method with the axial approximation for modeling the band structures and optical properties of type-II interband cascade lasers is assessed by comparing its results with those from the standard eight-band k·p method. Explicit expressions for anisotropic k·p interaction terms, which are neglected by the axial approximation, are obtained. Then the band structures and optical properties of practical type-II interband cascade lasers are calculated using both eight-band k·p methods and the numerical results are discussed in detail. It is found that the axial approximation may be problematical for the simulation of type-II interband cascade lasers.

Type-II interband cascade lasers emitting at wavelengths beyond 5.1 µm

Type-II interband cascade lasers have been demonstrated at emission wavelengths longer than 5.1 µm at temperatures up to 163 and 240K in continuous wave and pulsed modes, respectively. The emission wavelengths are the longest attained among III–V interband diode lasers. The threshold current density is as low as 36 A/cm2 at 80K.

Type-II Interband Cascade Lasers in the 4.3–4.7-<tex>$muhboxm$</tex>Wavelength Region

Operation of type-II interband cascade lasers in the 4.3-4.7-μm wavelength region has been demonstrated at temperatures up to 240 K in pulsed mode. These lasers fabricated with 150-μm-wide mesa stripes operated in continuous-wave (CW) mode up to a maximum temperature of 110 K, with an output power exceeding 30 mW/f and a threshold current density of about 41 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 90 K. The maximum CW operation temperature of 110 K is largely limited by the high specific thermal resistance of the 150-μm-wide broad area lasers. A 20-μm-wide mesa stripe laser was able to operate in CW mode at higher temperatures up to 125 K as a result of the reduced specific thermal resistance of a smaller device.

Recent progress in the development of type II interband cascade lasers

Type-II interband cascade lasers combine the advantage of an interband optical transition with interband tunneling to enable the cascading of type-II quantum well active regions as is done in type-I quantum cascade laser. The relatively high radiative efficiency resulting from interband optical transitions translates into very low-threshold current densities, and when combined with the high quantum efficiency of cascade lasers, this diode laser design has the potential to operate under cw conditions at room temperature with high output power. Experimental results have already demonstrated some of this potential including high differential external quantum efficiency (>600%), high peak output power ( ∼6 W/facet at 80 K ), high cw power conversion efficiency (>17% at 80 K ), and operation at 300 K under pulsed conditions. Recent work aimed at reducing device thermal resistance and increasing cw operating temperature is reviewed including the demonstration of significant reductions in thermal resistance (averaging 25 K/W or 40% for 1-mm-long devices), 80 K cw operation at 3.4 μm with high-power conversion efficiency (23%) and high differential external quantum efficiency (532%), and cw operation up to 214 K .

Low-threshold interband cascade lasers operating above room temperature

Mid-IR type-II interband cascade lasers were demonstrated in pulsed mode at temperatures up to 325 K and in continuous mode up to 200 K . At 80 K , the threshold current density was 8.9 A/ cm 2 and a continuous wave output power of ∼140 mW/ facet was obtained.

Room-temperature type-II interband cascade lasers near 4.1 μm

Operation of type-II interband cascade lasers has been demonstrated in pulsed mode at room temperatures at wavelengths near 4.1 μm. These lasers fabricated with 150 μm wide mesa stripes also operated in cw mode at temperatures up to 145 K. Threshold current densities as low as 9 A/cm2 and output powers exceeding 100 mW/f have been achieved in cw mode at 80 K.

Single-longitudinal-mode emission from interband cascade DFB laser with a grating fabricated by interferometric lithography

A distributed-feedback, type-II interband cascade laser is demonstrated in pulsed mode, emitting near 3.145 µm at temperatures between 50 and 80 K. Feedback is provided by a surface-etched grating formed using interferometric lithography. Between 50 and 80 K, the device&apos;s single longitudinal mode red-shifts with temperature at ∼0.1 nm/K. At 70 K and 190 mA of pulsed injection current, side-mode suppression exceeds 30 dB. The characteristics of related Fabry–Perot IC lasers are also described and contrasted with those of the distributed feedback device.

Room temperature type-II interband cascade laser

A mid-IR (3.3–3.5 μm) type-II interband cascade laser has been demonstrated at temperatures up to 300 K in pulsed mode and 150 K in cw mode. Threshold current densities as low as 13.2 A/cm2 and power efficiencies as large as 17% have been achieved under cw conditions at 80 K.

Mid-infrared type-II interband cascade lasers

Interband cascade (IC) lasers that utilize optical transitions between the conduction and valence bands in a staircase of Sb-based type-II quantum wells (QWs) represent a new class of mid-IR diode lasers. By combining the advantages of quantum cascade lasers and type-II QW interband lasers, type-II IC lasers show promise of operating in continuous-wave (CW) mode up to room temperature with high output powers. Significant advances toward such high performance have been reported in terms of record-high differential external quantum efficiency (DEQE>600%), peak output power (/spl sim/6 W/facet at 80 K), CW power conversion efficiency (>16% at 80 K), and room-temperature operation under pulsed conditions. Here, we will review the progress made in the past few years and discuss the issues encountered during the development. Also, the current status of type-II IC lasers and the remaining challenges will be discussed.

Modeling of Sb-based type-II quantum cascade lasers

The interband Sb-based quantum cascade lasers at mid-IR range are studied. A self-consistent method that solves the Schr\odinger's equation and Poisson's equation simultaneously to obtain the band structure and calculate the optical gain for type-II quantum cascade lasers is presented for the first time to the best of our knowledge. We start with the formulation of band structure based on the $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ method with the inclusion of the conduction-band--valence-band coupling effect. The 8\ifmmode\times\else\texttimes\fi{}8 Hamiltonian for the k\ensuremath{\cdot}p method is block diagonalized into two 4\ifmmode\times\else\texttimes\fi{}4 blocks under the axial approximation. Explicit expressions for momentum-matrix elements for interband transitions are given. The optical gain formulation including many-body effects is then presented. The carrier accumulation in different layers and its screening effects are shown. For the lasers of our study, the carrier screening affects the peak gain wavelength by about 0.3 \ensuremath{\mu}m and the peak gain magnitude by 10%. The self-consistent model is shown to be important to give a good agreement with experimental data. The temperature dependence of the optical gain is also examined for the regular and the W-shaped type-II quantum-well structures. The optical gain for the W-shaped type-II quantum well is found to have a 50% enhancement over the regular type-II quantum well. Our model is important for designing high-power, high-efficiency, and high-temperature type-II interband cascade lasers.

Power, efficiency, and thermal characteristics of type-II interband cascade lasers

High-performance mid-infrared type-II interband cascade lasers have been demonstrated under continuous-wave (CW) conditions with record-high wall-plug efficiencies (>14%) and output powers (>100 mW/facet) above 77 K. Device characteristics of these type-II interband cascade lasers are investigated systematically in terms of their output powers and efficiencies. Also, by comparing the temperature dependence of the threshold currents under pulsed and CW conditions, the thermal resistance and maximum heat sink temperature for CW operation are estimated for several mesa sizes. The limiting factors due to device heating for high-power/high-efficiency operation are identified and discussed in connection with device dimensions and packaging for the purpose of assessing further improvements.

Enhanced CW performance of the interband cascade laser using improved device fabrication

Continuous-wave (CW) operation of a mid-infrared type-II interband cascade (IC) laser has been demonstrated at temperatures up to 142 K by improving device processing and fabrication. Also, the IC laser exhibited record-high wall-plug efficiencies (/spl sim/18% at 60 K) with considerable CW output powers. An analysis of the thermal resistance partially explains the still low maximum CW operating temperature and suggests further potential for improvement with continued development of fabrication/packaging techniques.

Continuous wave operation of type-II interband cascade lasers

Continuous wave operation of interband cascade lasers based on type-II InAs/Ga(In)Sb heterostructures is reported. The interband cascade lasers, consisting of 23 cascaded active regions, lased at wavelengths near 4 /spl mu/m. Continuous wave operation was observed from 60-110 /spl mu/m wide by 1.02 mm long mesa-stripe lasers at temperatures up to 70 K.