High-power single-mode-emission quantum cascade lasers

Several chemical species have characteristic fundamental absorptions in the mid-IR spectral region, i.e., with wavelengths in the 4–12μm range. These species include the greenhouse gases carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (NO), which are of particular interest in climate research studies.^{1, 2} Powerful mid-IR detection systems that measure such gases with high levels of precision are therefore required.

Quantum cascade (QC) laser systems can emit light in a specific spectral mode (single-mode emission) with high output power. Pulsed-wave and continuous-wave driven trace gas sensors that are based on mid-IR-emitting QC lasers are suitable systems for detecting greenhouse gases, with detection levels in the parts-per-billion range or better.^{3, 4} Current designs for high-power single-mode-emitting QC lasers achieve high optical output power. These approaches, however, result in poor discrimination and yield of the single-mode emission, an astigmatic light beam, or are limited to low-duty-cycle operations only (a duty cycle is the proportion of a period in which a signal is active).^{5, 6}

We have designed a single-mode QC laser system that combines the advantages of the previously proposed techniques. Our laser devices have a semiconductor-based monolithically integrated asymmetric master-oscillator power amplifer (MOPA) configuration. Our MOPA produces a highly coherent amplified beam and is configured with a straight waveguide in the amplifier section. The waveguide directs the emitted light into a symmetric beam. The laser we use has a relatively short (about 1.25mm in length) single-mode-emitting ‘seeding’ section, which is based on an index-coupled distributed-feedback (DFB) grating and a Fabry-Pérot (FP)-type amplifier that can be 4mm or more in length (see Figure 1).⁷

Figure 1. Top view of a typical master-oscillator power amplifier (MOPA) device (about 4.5mm in length), mounted on a copper heat sink. The individual distributed feedback (DFB) and Fabry-Pérot (FP) sections are labeled.

DFB gratings are constructed so that only a narrow band of wavelengths are reflected and a single longitudinal lasing mode is produced. Since the interaction length of our DFB grating with the mode is short, the gratings are physically strong and provide good mode discrimination. The long non-tapered amplifier section of our design enables high output powers (watt level) to still be achieved during high-duty-cycle operations. In our design the far-field region of the electromagnetic spectrum is symmetric, which makes it relatively simple to collimate the laser beam and to meet the requirements of any spectroscopic application.

We have conducted simulations, which are based on a transfer-matrix algorithm, to compare the characteristics of the one-section weakly coupled DFB design⁵—see Figure 2(a)—and our asymmetric MOPA configuration: see Figure 2(b). These simulations show that the mode discrimination (and therefore also the single-mode yield) increases by more than an order of magnitude with our MOPA approach. These simulations also compare the performance of each type of device when realistic imperfections in their fabrication are considered. The MOPA configuration is barely affected by the imperfections (indicated by the prominent stopband signature), whereas for the one-section configuration there is no single-mode emission remaining. We have also built MOPA devices, according to the results of our simulations, in a buried heterostructure configuration (where the active laser is ‘buried’ in an electrically insulating material to increase heat dissipation) to allow for narrow waveguides, which maintain a high level of performance.⁷ A typical MOPA device that is 4μm wide and 5.25mm long has watt-level peak output power at Peltier-cooled temperatures for a duty cycle of 1%.⁷

Figure 2. Comparison of simulations for the transmission (continuous lines) and modal losses (dotted lines) of (a) a weakly coupled DFB grating (i.e., the one-section approach) and (b) the MOPA configuration with strong grating coupling and anti-reflection (AR) coating. Black curves show the simulation for an unperturbed, perfect grating. Green lines are for a perturbed case, where small variations in the width of the waveguide are included in the simulations. Mode discrimination is shown at the Fabry-Pérot mode level (Δ₁) and for the next non-stopband edge mode (Δ₂).

We have analyzed the spectral properties of our devices with measurements made at various temperatures and driving currents (see Figure 3). The devices tune completely single-mode and mode-hop (sudden jumps in optical frequency associated with different modes) free over the entire temperature (263–313K) and current (700–15,000mA) ranges. We have also measured the far-field of our devices at maximum optical output power (see Figure 4). The far-field is almost perfectly symmetric with a full width at half-maximum of 25 and 27° in the vertical and horizontal directions, respectively.

Figure 3. Lasing spectra for (a) fixed temperature and (b) fixed current. arb.: Arbitrary.

Figure 4. Laser far-field at maximum optical output power.

We designed high-power single-mode QC lasers with an asymmetric two-section configuration that have significant performance advantages over conventional waveguide designs. We are able to combine a good single-mode yield, high optical output power, and a symmetric far-field pattern in one device. The next step in our work is to achieve continuous-wave operation with our MOPA devices. This will enable the full potential of our approach to be used in spectroscopic applications where high-duty-cycle operation with relatively high output power is required.

The authors are grateful for discussions with P. Jouy, A. Bismuto, and Y. Bonetti, and for expert technical assistance by M. Ebnöther. The Swiss National Science Foundation within the framework of the NCCR Quantum Photonics project provided financial support for this work.