1. Introduction

Over the last decade, quantum cascade lasers (QCLs) [1] have become the most attractive light source in the Mid-Infrared (Mid-IR) spectral region due to outstanding improvement of their performance. Multi-watt level output powers at room temperature [2–4] as well as high efficiency [5, 6] and low consumptions [7] are nowadays available across the whole region. For most of the spectroscopic usages, laser sources with mW-level emission are generally sufficient due to the large intensity of the roto-vibrational molecular resonances in the Mid-IR. Nevertheless, in some cases, e.g. photo-acoustic sensing, remote sensing, chemical imaging, both high optical power and high spectral stability are advantageous [8]. While extensive research has been done on the development of high power Fabry-Perot(FP) QCLs, optimizing the size of the device, ridge width and length, as well as the facet reflectivity [9], less work has been done on single mode laser sources. The latter ones require a more careful optimization of the optical field profile in the cavity in order to maintain the spectral purity. Record output powers have been demonstrated in the surface grating configuration [5], where the grating is etched on the top of the device far from the optical mode in order to reduce the coupling strength. In the buried-grating configuration, the output power are mainly limited due to the difficulty to reduce the coupling strengh sufficiently [10]. This configuration, which is the most commonly used for QCL DFBs, benefits both from lower waveguide losses and better heat extraction. Unfortunately, reduction of coupling strength is for these devices more difficult due to the large overlap between the optical mode and the grating. To obtain single-mode high power devices, one possibility is to insert an amplifying section outside the DFB cavity [11]. In this case, the QCL cavity is separated in two sections. In the first section a standard grating is etched and the lasing occurs as in a standard DFB, this section is referred as master oscillator (MO). In the second section, the active medium is used as a single pass power amplifier (PA). Anti-reflection coating of the PA section front facet is required in order to avoid self lasing. This strategy is imported from laser diodes where tapered PA sections allowed to reach record output power. In QCLs, however, electrical dissipation is much higher and the large currents necessary to drive the tapered PA sections prevent them to reach continuous wave operation [12, 13]. In this work we propose to use a configuration similar to the MOPA but where the lasing action occurs in the Fabry-Perot section. A distributed bragg reflector (DBR) is etched in the back of the device that behaves as a spectrally selective mirror. Compared to the MOPA case, the optical mode inside the FP section is nearly constant reducing the onset of optical gain saturation [14]; also in this configuration the presence of front Anti-Reflection coating (AR) is not mandatory. The details of the optical mode inside the cavity will be presented below. The fabrication process presented is fully compatible with the standard buried-heterostructure DFB QCL fabrication and low-dissipation single mode devices can be fabricated at the same time, as it will be shown later in the text.

2. Results

We focused the present work on the demonstration of single mode high power QCLs mounted in episide-up configuration. This has both the advantage of a higher mounting yield and also that additional features could be added to the laser device, e.g. integrated heater for improved tuning [15]. As mentioned above, the devices are processed in a buried heterostructure configuration using Metalorganic Vapour Phase Epitaxy (MOVPE) for the selective regrowth of Iron-doped InP. Ridge widths as narrow as 2.5 μm were obtained through wet etching. A DBR host layer of InGaAs was deposited on the top of the active region and etched using Deep-UV lithography with an etch depth of 100 nm. This etching depth has been carefully designed in order to guarantee the fabrication of low consumption devices in the same fabrication batch. DBR sections and FP sections of with a respective length of 0.75 mm and 5.25 mm were defined for the high power devices. In order to avoid spectral instabilities, a single top contact was defined on both sections. Devices were mounted episide-up on copper submounts. In Fig. 1 the light-voltage-current characterists in continuous wave operation (cw) for different submount temperatures are shown for a 6 mm long and 4.5 μm wide cavity. Optical powers of nearly 0.35 W are observed at 50°C. Temperature tuning of the laser emission is shown in Fig. 2(a). The DFB tuning as a function of the laser current is shown in Fig. 2(b) for a submount temperature of 0°C. A spectral coverage of over 11 cm⁻¹ is observed and side-mode suppression ratios of over 30 dB are shown. In Fig. 3 measured far field pattern is also shown for a cw injection current of 1.2 A. For far field emission, assuming a Gaussian profile, full widths at half maximum (FWHM) of 33° and 41° were fitted in the horizontal and vertical direction respectively. Measurements as a function of the increasing current revealed no beam-steering effect.

 

Fig. 1 Top: Sketch of the fabricated DBR-QCL device. Bottom: Light-Voltage-Current characteristics as a function of the temperature for a 6 mm long, 4.5 μm wide DBR-QC laser emitting at 4.68 μm

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Fig. 2 (a): Single mode emission for submount temperatures from −30° C to 50° C for a cw injection current of I_l=0.8 A. A spectral coverage of over 11 cm⁻¹ is observed. (b): Single mode spectral emission as function of the injection current for a submount temperature T_hs= 0 ° C. Single mode suppression ratios of over 30 dB are observed. The spectra were measured using an FTIR Vertex 80(Bruker) with a resolution of 0.2 cm⁻¹. and side mode suppression ratios larger than > 30 dB are observed for all the spectra. Our experience with DFB show that the linewidth is comparable with the one observed on standard DFB devices.

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Fig. 3 Measured far field pattern in cw for a current of 1.2A. As shown in the inset, the laser submount is on an overhanging Cu plate, which causes the vertical asymmetry.

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The longitudinal mode profile for the fabricated structure is shown in the Fig. 4(a). The mode has been computed solving the Helmholtz equation assuming outgoing boundary conditions with a real wavevector outside the cavity and using the finite difference approximation. The lasing modes are the so called constant-flux states [16], these differ from quasi-bound states in that they conserve energy flux and don’t diverge at infinity. As pointed out in [17] by Tureci et al, quasi-bound states are often wrongly thought of as being lasing modes. However, only constant-flux states exist in all space and are therefore suited to predict the laser’s output power. The mode profile shows that the intensity of the light in the device is nearly constant in the FP section while it decreases sharply in the DBR section. This has the positive effect to limit the spatial inhomogeneity of the gain saturation, which in turn limits undesired spectral instabilities often observed in long DFB devices and attributed to spatial hole burning. Also observed is a strong asymmetry of the predicted extracted power at the back and front facets which is due to the highly asymmetric mirror losses. In order to estimate the accuracy of the optical longitudinal mode simulations presented, we measured the optical emission from both facets, see Fig. 4(b). We can see that optical power emitted from the front facet is nearly 10 times higher than the one emitted from the back facet. Using the common expression dP/dI|_FP/DBR = (hν) · α_mFP/DBR/(α_tot) [18], where dP/dI|₁ and α_mFP/DBR are the slope efficiency and mirror losses of the facets, the ratio of the slopes can be used to estimate α_mDBR/α_mFP; the obtained value of 9.15. is in good agreement with the ratio of the optical losses simulated using the constant flux model, 9.4.

 

Fig. 4 (a): Longitudinal mode profile of the optical field in the DBR-QCL. The intensity is nearly constant in the FP section(in green) while decreases exponentially in the DBR section(in red). (b): Front (FP) and back (DBR) facet emission power measured at −30°C for the device presented in Fig. 1. Back facet emission power is nearly an order of magnitude lower than front facet one, in good agreement with mode profile simulations shown in the top figure.

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In order to show that the fabrication of the presented devices is compatible with the low power consumption fabrication, we fabricated low dissipation devices on the same wafer. Best performance, were obtained for a 750 μm long and 3.5 μm wide DFB. Lasing threshold powers in cw at room temperature below 0.8 W were observed and optical powers of more than 10 mW from a single facet were measured for 1 W of dissipation.

3. Conclusions

To summarize, high power single mode DBR QCLs were presented emitting at 4.68 μm with cw optical powers of nearly 1 W. The lasers are mounted epi-side up and facets are left un-coated. The introduced devices are fully compatible with low consumption laser fabrication.