1. Introduction

Tunable laser absorption spectroscopy (TLAS) is a versatile and robust method for gas sensing with applications ranging from industry to Earth and planetary science [1–4]. The method relies on the absorption of laser emission in a gas mixture, where the exact wavelength coincides with the absorption of molecular transitions of the constituent molecules. The mid-infrared region from 3 to 5 µm presents particular promise since it contains several strong absorption features of scientifically important species, such as methane, hydrogen chloride, and water [5]. Together with techniques such as wavelength modulation spectroscopy, TLAS with simple absorption cells is capable of detection limits on the order of parts per million, which is sufficient for many applications of interest [6]. However, for weaker-absorbing molecules or measurements that require higher sensitivity, a cavity-enhanced method, such as cavity ring-down spectroscopy, is often used and requires light sources capable of emitting tens of milliwatts of single-mode power due to the low coupling of the light into a high-finesse cavity. Standard GaSb-based diode lasers perform well only up to wavelengths around 3.3 µm, due largely to an increase in non-radiative recombination rates, whereas interband cascade lasers (ICLs) do not suffer from the same limitations and are an excellent choice for high-power single-mode emission in the 3 to 5 µm wavelength range [7,8].

A common method of realizing single-mode operation of ICLs is to incorporate a distributed feedback (DFB) Bragg grating. As typical buried-grating designs are impractical for GaSb-based devices due to issues with overgrowth in this material system, the grating must be incorporated into the design using different approaches. For instance, a metal Bragg grating can be deposited on top of a ridge waveguide (RWG), and a single-mode ICL of this design has been able to emit 27 mW at 40 °C at a wavelength of 3.80 µm [9]. However, the close vicinity of the metal grating to the optical mode introduces losses through evanescent coupling, limiting the potential of this design for even higher output power. Another design is based on etching vertical corrugations in the sidewalls of the RWG. These corrugations cannot only serve as a Bragg grating, but can also suppress higher-order lateral optical modes by introducing scattering losses at the edges of the RWG. ICLs with such corrugated sidewall gratings have been capable of emitting up to 55 mW of optical output power at 25 °C at wavelengths close to 3.6 µm [10,11]. However, the low side-mode suppression ratio (SMSR) of the latter devices suggests the emission is only “effectively” single-mode due to the wide 13.2-µm RWG, and the authors of this previous work mention a lack of reproducibility with this grating design, due in part to difficulties in maintaining high-fidelity grooves throughout the deep etching process. Recently, a new design with laterally-coupled gratings in a double-ridge system was presented and shown to emit up to 18 mW in single-mode operation at 46 °C at 3.57 µm wavelength, with the high operating temperature being a requirement due to misalignment of optical gain spectrum and Bragg grating resonance wavelength [12]. This design minimizes optical losses and employs well-proven and reproducible grating fabrication techniques, in that the grating etch depth in the double-ridge design is 300 to 400 nm as compared to typically several micrometers for the sidewall grating design. Here, we report on double-ridge ICLs capable of emitting in a single-mode up to 20 mW at 20 °C and 40 mW at 0 °C. These devices were developed for hydrogen chloride detection at a wavelength of 3.3746 µm in an integrated cavity output spectroscopy system.

2. Laser fabrication

Our ICLs were fabricated from an epitaxial wafer similar in design to ones described elsewhere [13]. The n-type cladding is composed of InAs/AlSb layers with a total thickness of 1.5 µm above the active region and 2.8 µm below, and the active region consists of seven ICL stages between two GaSb separate confinement layers (SCLs). Devices were processed by first defining a narrow-width (~3 µm) RWG by contact lithography. The waveguide was then etched into the semiconductor to the top SCL by reactive ion etching (RIE). After the RWG was fabricated, a second-order lateral Bragg grating was patterned directly adjacent to the ridge sidewalls by first defining the grating with electron-beam lithography and then transferring the pattern into the remaining top SCL by the same RIE fabrication process. The dimensions of the grating structure were 925 nm pitch with a 75% duty cycle and a depth of 300 nm, just enough so that the grating etch did not penetrate the ICL stages. For current confinement, a second 9-µm-wide ridge was defined with the RWG at the center. The second ridge was etched to the bottom SCL, as shown in the inset in Fig. 1. Simulations show that the fundamental optical mode supported by the first ridge is not perturbed by the wider ridge, since the optical mode does not extend significantly beyond the narrow RWG. After fabricating the RWG, lateral grating, and current confinement ridge, the devices were insulated with silicon nitride except for a narrow strip on the top of the RWG, followed by deposition of an ohmic contact and a thick gold electroplated layer for heat extraction. The backside of the wafer was then thinned by mechanical lapping to a thickness of approximately 100 µm to facilitate cleaving into individual device chips. Finally, backside ohmic contacts of Pd/Ge/Au were deposited and annealed at 260°C.

 

Fig. 1 Light-current characteristics of several double-ridge DFB ICLs measured at 10 °C with different facet coating configurations: both facets uncoated (U/U); front facet AR-coated, back facet passivated (AR/P); and front facet AR-coated, back facet HR-coated (AR/HR). Inset: scanning electron micrograph of a double-ridge ICL. The top RWG is about 3 µm wide and serves to contain the optical mode, and the bottom 9-µm-wide ridge is used for lateral current confinement.

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Before cleaving the wafer into individual laser chips, anti-reflection (AR) and high-reflection (HR) coatings were deposited onto the front and back facets. The multi-layered coatings were designed and optimized for the wavelength of interest with reflectivity of approximately 1% and 73% for the AR and HR coatings, respectively. The effect of different facet coating configurations is clear from Fig. 1, which shows single-mode light-current characteristics of several DFB ICLs employing the double-ridge design. Although there are slight variations in fabrication, there is consistency in device-to-device performance for a given facet coating configuration. As compared to devices with uncoated facets (U/U), having the front facet AR-coated increases the mirror loss and thus the current threshold. For these AR-coated devices, the back facet was passivated with a thin film of alumina, resulting in approximately the same reflectivity as the uncoated facets. The increase in threshold current was reduced with a back-facet HR coating in addition to the AR coating on the front facet.

3. Results

The ICLs were hermetically sealed in TO-3 packages with internal thermal control for efficient heat extraction. The optical output power was measured with a Molectron thermopile detector. At 20 °C, the thermal rollover-limited output power was 20 mW and at the lowest tested temperature of 0 °C, the maximum output power reached 40 mW, as shown in Fig. 2. At maximum output power, the wall-plug efficiency based on emission from the AR facet was 1.9%. The spectral characteristics were collected at a resolution of 0.125 cm⁻¹ using a Thermo Nicolet FTIR spectrometer with a cooled InSb detector. Over the entire range of testing conditions, single-mode emission was observed with a SMSR of more than 25 dB and current and temperature tuning rates of 50 cm⁻¹/A and 0.35 cm⁻¹/°C, respectively, as shown in Fig. 3.

 

Fig. 2 Performance of an AR/HR-coated DFB ICL measured in 5 °C increments, with maximum observed optical output power of 20 mW at 20°C and 40 mW at 0°C. The device exhibited single-mode emission over the entire range of temperature and current shown.

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Fig. 3 Emission spectra collected from a double-ridge DFB ICL with SMSR of at least 25 dB. Inset: current and temperature tuning characteristics of the same device. The current and temperature tuning rates were 50 cm⁻¹/A (57 nm/A) and 0.35 cm⁻¹/°C (0.39 nm/°C), respectively. The dotted lines represent the targeted absorption feature of hydrogen chloride at 2963.3 cm⁻¹.

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The reason for the increased output power for these devices, as compared to the previously reported devices in Ref. 12, is the larger overlap of the fundamental RWG mode and the DFB grating due to optimized waveguide dimensions. More specifically, the RWG is narrower and the grating is etched about 100 nm closer to the active region. Based on scanning electron micrographs and electromagnetic simulations, the grating overlap has increased by nearly a factor of three and the grating coupling factor, κ, has increased accordingly from approximately 7 cm⁻¹ to 18 cm⁻¹, based on the expression found in Ref. 14. The result is that DFB ICLs are over-coupled with κL ≈3.5, where L = 2 mm is the cavity length, allowing for single-mode operation over a broader range of temperature and current compared with previously reported devices [12]. Sub-threshold Hakki-Paoli measurements of devices with and without gratings indicate that the grating only introduces additional optical losses of around 2-3 cm⁻¹.

4. Summary

We have reported on laterally coupled ICLs with a double-ridge design capable of emitting 20 mW of single-mode output power at 20 °C and 40 mW at 0 °C, which represents a significant improvement over previous results. Improved fabrication has led to increased coupling to the second-order Bragg grating, extending the range of operating conditions for single-mode emission. The laser emission wavelength is targeted for a hydrogen chloride absorption feature at 2963.3 cm⁻¹ (3.3746 µm), and the emission is continuously tunable over 20 cm⁻¹ without mode hops. At 5 °C, the target wavenumber is achieved with more than 34 mW of single-mode output power.