1. Introduction

The spectral range between 3-4 µm in the mid-infrared region is important for chemical sensing because it corresponds to the fundamental ro-vibrational band of C-H bonds. The availability of tunable, narrow-linewidth semiconductor lasers in this wavelength range opens up the way for the realization of compact, sensitive and selective trace sensors of the main hydrocarbon gases: methane (CH4), ethane (C2H6), propane (C3H8), ethylene (C2H4), propene (C3H6) and acetylene (C2H2). In particular, quantitative detection of the C1 to C4 alkanes (i.e., methane, ethane, propane, and butanes) is of great interest to the petrochemical industry for several applications, including natural/biogas gas composition analysis, energy content measurement (BTU measurement) and leak detection from pipelines and refineries.

As the spectra of heavier hydrocarbons are broad and strongly overlap, a wide tuning range is required to differentiate them. Optically pumped lead-chalcogenide vertical external cavity surface emitting lasers (VECSELs) with a tuning range of 150 cm⁻¹ at 3.3 µm have been developed [1] and applied to hydrocarbon sensing [2]. However, these devices only operate in pulsed mode and emit a relatively low peak power (typically ∼20 mW at a duty factor ≤ 0.1%). Broadband tuning of an external cavity quantum cascade laser (EC-QCL) in pulsed mode at 249 K over 559 cm⁻¹ from 3.3 to 4.0 µm was reported by Riedi et al. [3] using a heterogeneous cascade active region design [4]. While EC-QCLs emit significantly higher average power than the aforementioned VECSELs and continuous-wave (cw) operation was demonstrated at longer wavelengths [5], their performance decreases quickly at wavelengths below ∼3.5 µm because of carrier leakage. A tuning range of 85 nm (81 cm⁻¹) at 3.2 µm was demonstrated in pulsed mode at room temperature in high conduction band offset InAs/AlSb QCLs [6]. On the short wavelength side of the 3-4 µm window, a tuning range of 200 cm⁻¹ in cw operation at room temperature at a central at wavelength of 3.15 µm with a few milliwatts output power was demonstrated in external cavity type-I quantum well cascade diode lasers [7]. However, these lasers are of limited usefulness for hydrocarbon sensing because they are limited to wavelengths ≤ 3.25 µm.

In this paper, we report the development and characterization of an external cavity interband cascade laser (EC-ICL). Interband Cascade Lasers [8] are semiconductor lasers in which light is produced by electronic transitions between the valence and conduction bands in quantum wells (QWs), like in diode lasers, and the optical gain and output power are increased by stacking multiple active QWs in series, as in Quantum Cascade Lasers. Type-II ICLs based on InAs/GaInSb QWs on GaSb substrate operate in continuous-wave at and above room temperature in the 3-4 µm range with high wall-plug efficiency and low heat dissipation [9]. A first demonstration of EC-ICL was reported by Caffey et al. in 2010 (Ref. [10]). Continuous-wave operation near room temperature was achieved with a tuning range of 110 cm⁻¹ at 3.2 µm and a maximum output power of 4 mW. Compared to the other laser technologies mentioned above, ECICLs offer the advantage of providing a wide tuning range, average power in the mW range, low heat dissipation, cw operation at room temperature, and a full coverage of the 3-4 µm wavelength range.

2. Laser fabrication

The 5-stage active region of the interband cascade gain chip is based on a type-II “W” InAs/GaInSb/InAs quantum well and was designed for emission at 3.3 µm. The n-doping level was selected as described in Ref. [9] to minimize heat dissipation at room temperature. The claddings are made of InAs/AlSb (2.43/2.30 nm) superlattices (SLs). The epitaxial waveguide structure, starting from the substrate, is the following: lower cladding (InAs/AlSb SL, 2600 nm), lower separate confinement layer (SCL: GaSb, 500 nm), active region (240 nm), upper SCL (GaSb, 500 nm), upper cladding (InAs/AlSb SL, 1890nm), contact layer (InAs, 20 nm). The InAs/AlSb SL and GaSb regions are connected by 20 nm-thick InAs/AlSb chirped superlattice transition regions and the InAs/AlSb SL and InAs regions are connected by a 90 nm-thick InAs/AlSb chirped SL transition region.

The structure was grown in a multi-wafer molecular beam epitaxy (MBE) reactor on 3 inch-diameter Te-doped (n = 5⋅10¹⁷ cm⁻³) GaSb substrates. The emission wavelength was verified by measuring electroluminescence spectra of mesa samples. A peak wavelength of 3.3 µm was found, in good agreement with the design. The full width at 80% of maximum (FW0.8M) of the EL spectra is 345 nm (324 cm⁻¹).

A grown epi-wafer was processed in ridge waveguide lasers of 4 to 25 µm widths. The ridges were defined by dry etching. We etched all the way through the active region and stopped in the lower SCL to prevent current spreading (see Fig. 1). According to our optical mode simulations, a ridge width w ≤ 3 µm is required for the waveguide to support only one transverse mode. Nevertheless, the sidewall slope of 55° of our dry etching process limited us to a minimum width w = 4 µm. The experimental results presented in this article show that this value is sufficient to achieve robust single-transverse mode in external cavity.

 

Fig. 1. Scanning electron microscope picture of a 7 µm-wide ridge waveguide ICL.

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The wafer was cleaved into bars. An anti-reflective (AR) coating was deposited on one facet to enhance the external cavity operation and a protective coating was deposited on the other facet to prevent degradation. The reflectivity of the AR coating was estimated to be 0.03% at 3.4 µm by extrapolating from reflectance measurements on a Si sample. The bars were then cleaved into chips and the chips were bonded in epitaxial-side-up configuration onto copper mounts with In solder.

3. Free running operation

To evaluate the performance of the processed devices, we mounted and characterized free-running Fabry-Pérot (FP) lasers with as-cleaved facets. Measured output power and voltage as function of current (LIV characteristics) of a 10 µm-wide, 1.5 mm-long chip in continuous-wave operation at various temperatures from 253 K (−20°C) to 323 K (+50°C) are shown in Fig. 2. The maximum cw output power is 63 mW per facet at 253 K and 28 mW per facet at 293 K. The threshold current density J_th is equal to 380 A/cm² at 253 K and 620 A/cm² at 293 K. The wallplug efficiency (WPE) for the emission from the 2 facets is 8.4% at 253 K and 5.1% at 293 K.

 

Fig. 2. Measured continuous wave LIV characteristics of a 10 µm-wide, 1.5 mm-long FP ICL at various temperatures between 253 K and 323 K.

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By fitting the temperature dependences between 253 K and 323 K with exponential functions, we find the characteristic temperatures T₀ = 63 K and T₁ = 86 K for the threshold current and the slope efficiency, respectively. We expect to be able to improve the temperature stability and increase the maximum operation temperature in future production batches by adding a thick layer of electroplated gold to enhance heat dissipation.

4. External cavity laser operation

To achieve tunable single-mode operation, the beam emitted from the AR coated back facet of a 5 µm-wide, 1.5 mm-long ICL gain chip was collimated with a high-numerical-aperture aspheric lens (NA = 0.85, effective focal length EFL = 1.873 mm) and the first order diffracted beam from a 450 groove/mm diffraction grating in Littrow configuration provided wavelength-selective optical feedback. The distance between the chip facet and the diffraction grating was ∼40 mm. The output beam emitted from the front facet was collimated by an identical aspheric lens.

The output power was measured with a calibrated thermopile detector and the emission spectrum was measured using a Fourier transform infrared (FTIR) spectrometer with a resolution of 0.125 cm⁻¹.

Measured emission spectra in continuous wave operation at room temperature (293 K) are shown in Fig. 3. We observed single mode operation over a tuning of range of 313 cm⁻¹ (360 nm) from 2789 cm⁻¹ to 3102 cm⁻¹ (3.22 to 3.58 µm) by adjusting the grating angle between 46.50° and 53.75°. When the angle was set beyond these limits, the spectrum became multimode as in free-running operation. The EC tuning range is comparable to the FW0.8M of the electroluminescence spectrum. Figure 4 shows one of the single-mode emission spectra in logarithmic scale. The small peaks ∼52 cm⁻¹ away from the main peak are artefacts of the spectrometer. Neglecting those, the side mode suppression ration (SMSR) is > 35 dB.

 

Fig. 3. Measured emission spectra of an EC-ICL in cw operation at 293 K for various angles of the diffraction grating.

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Fig. 4. Logarithmic scale spectrum in cw operation at a current of 150 mA and a temperature of 293 K showing the high side-mode suppression ratio.

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The measured cw output power of the EC-ICL as a function of the emission wavenumber at a constant current I = 150 mA at 293 K is shown in Fig. 5. The output power is ≥ 1 mW over most of the tuning range (> 310 cm-1) with a maximum of 9 mW near the central wavelength of 3.4 µm.

 

Fig. 5. Measured cw output power as a function of emission wavenumber of the EC-ICL at a current of 150 mA and a temperature of 293 K.

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Measured LIV characteristics of the EC-ICL at various emission wavelengths are shown in Fig. 6. The threshold current reaches a minimum value of 62 mA (threshold current density = 825 A/cm²) at an emission wavenumber of 2934 cm⁻¹, i.e. near the central wavelength of 3.4 µm and increases monotonically towards the edges of the tuning range because the optical gain decreases. The output power reaches a maximum of 13 mW for a current of 192 mA at 2891 cm⁻¹. The heat dissipated by the laser is 0.19 W at the minimum threshold and 0.79 W at the maximum current of 200 mA. At 150 mA, the heat dissipation is 0.56 W.

 

Fig. 6. LIV characteristics of the EC-ICL at various emission wavelengths.

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The modulation which is superimposed to the usual linear behavior of the output power versus current curves is due to mode hops. When the current is increased, the refractive index of the gain chip changes, while the distance between the chip and the grating remains constant. As a result, the relative phase of the partial beams reflected by the AR coated facet and by the grating changes which leads to mode hops. The same behavior was also observed in cw external cavity QCLs [5].

5. Conclusions

We report an external cavity interband cascade laser (EC-ICL) with a tuning range of 313 cm⁻¹ (360 nm), i.e. 10.6% of the 3.4 µm central wavelength, in continuous wave at room temperature. The output power is ≥ 1 mW over the entire tuning range with a maximum of 13 mW at the central wavelength and the heat dissipation is 0.8 W. FTIR spectra show excellent spectral purity with a SMSR > 35 dB. These results represent an improvement of a factor of 3 on tuning range and output power compared to the previous state of the art.

The high performance and wide spectral coverage from 3.2 to 3.6 µm of our EC-ICLs make them a very interesting option for the detection and analysis of hydrocarbons. The continuous-wave operation allows the realization of mode-hop-free tuning by adjusting the cavity length while changing the grating angle.