1. Introduction

External-cavity (EC) quantum cascade (QC) lasers [1, 2] have been developed as promising light sources for mid-infrared (MIR) spectroscopic applications [3]. Their broadband single-mode tuning permits high accuracy gas detection for various species. Taking notice of the second atmospheric window of MIR, the wavelength region around 10 μm is characterized by fundamental stretching modes of several molecules (C-C, C-O, C-N) [4] and some absorption lines of gases as typified by ammonia [5]. However, in such an attractive wavelength region, broadband wavelength tuning with EC QC lasers in continuous wave (cw) operation at room temperature has been difficult because of absence of a high performance broad-gain QC laser. In the long wavelength range over 10 μm, it is hard to build a population inversion, unlike the short wavelength case, because of its shorter upper laser state life time; besides this, a huge internal loss exists. These disadvantages lead to poor laser performance with a high threshold current density and, as a result, a cw operation is difficult at room temperature. In fact, to the author’s knowledge, although several kinds of broad-gain QC lasers have been developed by some research groups [6–9], no wavelength tuning has been reported so far in the long wavelength range of greater than 11 μm under the condition of cw operation at room temperature. In these situations, the simultaneous realization of a stable cw operation at room temperature and a broadband wavelength tuning of such long wavelength range is a valuable target to be attained. In order to overcome such obstacles, we optimized the anti-crossed dual-upper-state (DAU) design [10–13] emitting at a center wavelength of 10 μm for broadband tuning in cw operation. As we reported in [14], an EC QC laser operated in cw mode was demonstrated with a wide tuning range of up to 17% of the center wavelength by using a DAU QC gain medium emitting around 6.8 μm which shows broadband and spectrally homogeneous gain profile [11]. Simultaneously, the DAU QC lasers with an identical (homogeneous cascade) active region exhibit low threshold current density compared to heterogeneous QC lasers [2, 7]. These features of anti-crossed DAU design are highly suitable for broadband and stable single-mode tuning of EC QC lasers in cw operation. In this paper we present a high performance broadband QC laser emitting around 10 μm and wide wavelength tuning of 18% (176 cm⁻¹) of the center wavelength in cw operation at room temperature without any anti-reflection (AR) coating.

2. Design of active region

The conduction band diagram of the active region is shown in Fig. 1.Anti-crossed DAU was designed to produce a broad electroluminescence (EL) spectrum. In order to obtain high laser performance for long wavelength range, the wave function of the lower upper state 1 was engineered to be diagonal, while the transition from the higher upper state 2 was engineered to be vertical, at the operating electric field. The designed wavelengths from two upper laser states, levels 2 and 1, were 9.5 μm and 11 μm, respectively. The energy spacing between them was configured at ~20 meV which is smaller than the longitudinal-optical (LO) phonon energy of ~34 meV. In this situation, the electron populations of both upper laser states are basically equal due to very fast elastic scattering. The electron distribution, depending on the ultra-fast elastic scattering, has been calculated with so-called energy-diffusion model which is described in ref [15, 16]. The relaxation times due to LO phonon emissions and elastic scattering by interface roughness and alloy disorder were estimated as: τ₂~0.9 ps, τ₁~1.7 ps [17, 18]. The relaxation time τ₁ is approximately twice the value of τ₂. Thus, stronger population inversion would be expected for the long wavelength range, i.e. greater than 10 μm.

 

Fig. 1 Schematic conduction band diagram and moduli squared of the relevant wave functions of injector/active/injector parts in the active region. An electric field of 34 kV/cm was applied to align the structure. The In_0.53Ga_0.47As/In_0.52Al_0.48As layer sequence of one period of the active layers, in angstroms, starting from the injection barrier (toward the right side) is as follows: 38/38/23/85/10/69/11/56/12/48/13/45/14/42/16/41/18/40/23/40/26/40 where InAlAs barrier layers are in bold, InGaAs quantum well layers are in roman, and doped layers (Si, 8 × 10¹⁰ cm⁻²) are underlined.

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3. Laser fabrication and performances

All the layer structures were grown on an n type InP substrate by metal organic vapor phase epitaxy technique. The cascade stages with 40 repetitions inserted between two n-In_0.53Ga_0.47As layers (Si, 5 × 10¹⁶ cm⁻³) of 0.6 μm thick were grown on the lower cladding layer of a 5 μm thick n-InP (Si, 1 × 10¹⁷ cm⁻³). The wafer was etched (wet etching) to form 10 μm wide stripes and buried with a semi-insulating InP (Fe doped) layer for planarization of the wafer surface. In order to construct a low loss wave guide, the upper cladding layer of 5 μm thick InP (Si, 1 × 10¹⁷ cm⁻³) was grown on the processed wafer followed by a 0.1 μm thick n⁺-InP (Si, ~10¹⁸ cm⁻³) cap layer. Then the evaporation of the top Ti/Au contact was followed by electroplating of a thick 10 μm Au layer on top of the QC laser structure. The bottom Ti/Au contact was evaporated after the wafer was polished up to the thickness of ~100 μm. Finally, the lasers were cleaved into 3 mm lengths and one of the cleaved (CL) facets was coated with dielectric multilayer for high reflection (HR). To test the performance, the laser chip was mounted episide down on a copper heat sink.

EL spectra of a simple mesa device measured in pulsed operation (a width of 500 ns and a repetition rate of 100 kHz) for various voltages at room temperature are indicated in Fig. 2.Though the actual width of the EL spectra was difficult to obtain because of the sensitivity of the detector turns down at the long wavelength region (> 10 μm), the full-widths at half-maximum of the EL spectra were over 300 cm⁻¹ for all voltage range. Figure 3(a) shows the pulsed current–light output and current–voltage characteristics measured with a calibrated thermopile detector at different temperatures for a HR-CL laser. A threshold current density at 300 K was observed to be 1.6 kA/cm² and an peak optical output power of 220 mW with a slope efficiency of ~0.9 W/A was obtained in the pulsed operation (a width of 100 ns and a repetition rate of 10 kHz). The cw current–light output and current–voltage characteristics of the identical device are indicated in Fig. 3(b). In order to prevent serious damage to the device, the maximum current density was limited at ~2.2 kA/cm². At the heat sink temperature of 300 K, we observed a threshold current density of 2.1 kA/cm² and a slope efficiency of ~0.7 W/A, and an optical output power of 16 mW at the current density of 2.2 kA/cm². The threshold current density of the device obtained in pulsed and cw operation as a function of temperature is exhibited in the inset of Fig. 3(b). The characteristic temperature T₀ can be estimated as 276 and 210 K in pulsed and cw mode by fitting the experimental results to the empirical formula J_th = J₀exp(T/T₀).

 

Fig. 2 Intersubband EL spectra of the mesa device for various voltages at 300 K measured in pulsed operation (a width of 500ns and a repetition t rate of 100 kHz).

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Fig. 3 (a) Current–light output characteristics of the 10 μm wide and 3 mm long buried heterostructure laser (HR-CL) at different heat sink temperatures measured in pulsed mode (a width of 100 ns and a repetition rate of 10 kHz). The voltage–current characteristics at 300 and 400 K are also shown. (b) cw current–light output characteristics of the present laser at different heat sink temperatures. The voltage–current characteristics at 260 and 300 K are also shown. The inset shows temperature dependent threshold current densities for pulsed and cw operations for the present device. The experimental data is fitted by the empirical formula J_th = J₀exp(T/T₀) with a T₀ of 276 and 210 K for pulsed and cw operation.

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4. Wavelength tuning with EC

By use of the broad-gain medium described above, we constructed an EC setup with Littrow configuration. The collimating lens which introduces the laser light to the external grating (150 grooves/mm) was an AR coated ZnSe aspheric one with a working distance of 1.44 mm and numerical aperture of 0.74. The output beam was provided by the zeroth-order reflection of the grating. The heat sink temperature was stabilized with a thermoelectric cooler, and the lasing wavelength of the EC QC laser was measured with a Fourier transform infrared spectrometer. Figure 4(a) shows the result of the external spectral tuning with a HR-CL device in cw operation with a specified current density of 2.1 kA/cm² at 17 °C. The tuning range of the EC single-mode was determined to be 18% (176cm⁻¹; corresponding from 9.5 to 11.4 μm) of the center wavelength. The average output power corresponding to each spectrum is also plotted in Fig. 4(a). The strong side-mode suppression between the shortest and longest wavelength is indicated in Fig. 4(b) and the stable single-mode operation is observed over the whole tuning range.

 

Fig. 4 (a) Wavelength tuning with a HR-CL device in cw operation with a fixed current density of 2.1 kA/cm² at 17 °C. The tuning range is 18% (176cm⁻¹; corresponding from 9.5 to 11.4 μm) of the center wavelength. The output power is also plotted as a function of the emission wavelength. (b) Spectra of the present EC QC laser for the shortest and longest wavelength obtained with a cw operated HR-CL device.

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The highly stable broadband single-mode tuning without any AR coating was obtained by the spectrally homogeneous gain profile of the DAU QC laser. In the case of a heterogeneous QC laser, simultaneous lasing tends to occur in each gain section, which may hinder the stable single-mode operation during wavelength tuning. As a result, in order to obtain a stable single-mode wavelength tuning, an AR coating of extremely low reflectivity in broadband is required for suppression of the several Fabry-Perot (FP) modes. Additionally, a several-wavelength-stack active region may give rise to spectral hole burning because of a longer time for electron transfer between spatially separated wavelength stacks [11]. On the other hand, in identical stack QC lasers based on an anti-crossed DAU design, depression of electron population in one of the upper subbands, caused by stimulated emissions is expected to be compensated very quickly by relaxation from the other subband due to almost complete spatial overlapping of wave functions of both subbands. Thus, gain spectra of DAU QC lasers can be regarded as spectrally homogeneous, and in the EC case, would be smoothly concentrated in an EC mode, overcoming a FP mode. In fact, as shown in the experimental results, the stable single mode lasing in broadband wavelength range was obtained by external feedback without any AR coating. Thus, the anti-crossed DAU design providing wide homogeneous electroluminescence as well as high device performance is promising for highly stable broadband wavelength tuning of EC QC lasers.

In order to maximize the tuning range, one of the CL facets was coated with dielectric multilayer for AR and the other facet was coated for HR. In this trial, we employed another QC laser different from the device described so far. The calculated reflectivity of the AR coating was less than 1% at the center wavelength. Figure 5(a) exhibits the tuning result of pulsed operation (a width of 100 ns and a repetition rate of 100 kHz) with a specified current density of 2.6 kA/cm² at 15 °C.The tuning range was determined to be 27% (273 cm⁻¹) of the center wavelength, corresponding to 8.8 to 11.6 μm. The wide wavelength tuning of 19% (190 cm⁻¹) of the center wavelength in cw operation with a locked heat sink temperature of 10 °C and a fixed current density of 2.5 kA/cm² is exhibited in Fig. 5(b). This tuning result in cw mode is slightly greater than that shown in [14] and [19].

 

Fig. 5 (a) Wavelength tuning of 273 cm⁻¹ (from 863 to 1136 cm⁻¹) in pulsed operation (a width of 100 ns and a repetition rate of 100 kHz) with a fixed current density of 2.6 kA/cm² at 15 °C. (b) Wavelength tuning of 190 cm⁻¹ (from 888 to 1078 cm⁻¹) in cw operation with a fixed current density of 2.5 kA/cm² at 10 °C.

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5. Conclusion

In summary, we demonstrated the broadband tunable EC QC laser by using the high performance DAU QC gain medium emitting at a center wavelength of 10 μm. The highly stable single mode tuning range of 18% (176 cm⁻¹) around the center wavelength was obtained in cw operation at room temperature without any AR coating. The technique of broadband single mode tuning in cw operation around the 10 μm wavelength range is useful for extensive applications in the second atmospheric window.