High-Power External Spatial Beam Combining of 7-Channel Quantum Cascade Lasers Emitting at ~8.5 μm

: Based on the demand for high-power output, a spatial beam combining 7-channel quantum cascade lasers (QCLs) is demonstrated in this paper. A “2 + 3 + 2” stepped structure is designed to convert the seven beam spots into a circular arrangement. An aspherical lens with a large numerical aperture (NA) of 0.85 and a focal length of 1.873 mm is used in each single QCL for collimation, and seven reflectors are utilized in the 7-channel QCLs combined in the spatial beam. After combining the spatial beam, the maximum continuous output power of the system is 3.6 W, and the beam quality M 2 is 5.59 in the fast axis and 8.3 in the slow axis, respectively


Introduction
Since QCLs were demonstrated by Faist et al. at the Bell labs in 1994 [1], they have become an important semiconductor laser source for the mid-wave infrared (MWIR) to terahertz wavelength [2,3].The 8-14 μm band in particular is an important atmospheric window, and the laser in this band is not easily absorbed by particles in the atmosphere, resulting in the low spatial transmission loss of laser.In addition, many molecules such as benzene, toluene, ethylbenzene, and other benzene compounds have an absorption intensity in this band that is one order of magnitude higher than that in the MWIR band.Therefore, long-wave infrared (LWIR) QCL (8~12 μm) has natural advantages and can be used in gas sensing [4], free-space optical communication [5], directional infrared countermeasure (DIRCM) [6] and many other fields.These applications have significant demands for high-power LWIR QCLs.For example, higher power means higher detection sensitivity for gas detection and a higher transmission efficiency for optical space communication, etc.
At present, the continuous-wave (CW) working performance of MWIR QCLs has reached a high level, with a wall plug efficiency up to 22% and a 5.6 W CW output power at room temperature [7].However, for LWIR QCLs, due to the decrease in the internal quantum efficiency and voltage efficiency and the increase in the waveguide loss caused by free carrier absorption [8][9][10], the CW output power and efficiency are significantly lower than those of MWIR QCLs [11], which greatly limits the development of LWIR QCLs.To increase the limited optical power of a single laser, it is necessary to use laserbeam-combining technology in practical applications.Generally, external laser beam combining is divided into coherent beam combining and incoherent beam combining.For coherent beam combining, as the number of combining units increases, the conditions for achieving interference become more stringent, and the complexity of the system increases sharply [12,13].The incoherent beam combining methods available for QCLs include polarization beam combining [14], waveguide beam combining [15,16], spectral beam combining [17,18], and spatial beam combining [19][20][21][22].Most of these incoherent beam-combining schemes are costly and require the use of gratings, and are difficult to assemble.Spatial beam combining is the simplest structure among the incoherent combining techniques, and the combining efficiency is also relatively high, meaning that the performance of each single laser can be fully utilized.Compared with the previous work using spatial beam combining, this paper adopts a new spatial beam-combining structure with a wavelength of ~8.5 μm in the LWIR region; this has more combining units and a higher total optical power after beam combining.
In this paper, we present an external spatial beam-combining scheme with a "2 + 3 + 2" three-layer stepped layout that is used to concentrate the laser beams of 7-channel QCLs together, achieving a CW output power of 3.6 W at room temperature, with a wavelength of ~8.5 μm in the LWIR region.The beam quality M 2 of the single laser and combined beam of the QCLs is also studied.The scheme further provides high power in the LWIR region easily.

Optical Design and Experimental Setup
The optical path for external incoherent beam combining was designed and simulated by ZEMAX, as shown in Figure 1a.Different colors of light represent lasers located on different step surfaces.The scheme structure of the experimental setup is shown in Figure 1b.Seven QCLs with F-mount heat sinks were fixed on the oxygen-free copper base, according to the "2 + 3 + 2" stepped layout.Each single QCL was collimated by a circular aspherical lens.The angle and position of the lenses were adjusted by a precise six-dimensional adjustment frame, ensuring the directionality and quality of the beam.Also, the gold-plated reflectors were adjusted to change the optical path.Moreover, after a mirror was adjusted each time, it needed to be fixed to the ceramics (adhered to an oxygen-free copper base before) with UV adhesive and solidified under UV light irradiation simultaneously, ultimately forming a combined structure of 7-channel QCL external spatial beams.The volume of the whole device was 99 mm × 109 mm × 8 mm after combination.For the narrow ridge of a single QCL, the size of the transverse waveguide is nearly identical to that of the lateral waveguide, which causes a small astigmatism.Consequently, only a single circular aspheric lens can be used for the collimation of the fast and slow axis [22].The adopted lasing wavelength of the QCL was about 8.5 μm, and the average output power of each single QCL was 500 mW.The measured horizontal divergence angle of the full width half maximum (FWHM) was 46°, as shown in Figure 2a.Due to the small emission size and large divergence angle of laser chips, the laser chip was collimated using an aspherical lens with a large numerical aperture.The diameter of the aspherical lens was 4 mm.The numerical aperture (NA) was 0.85, and the focal length was 1.873 mm.After collimation, the remaining divergence angle of the single QCL was reduced from 100° to 0.2°, as shown in Figure 2b and Figure 2c.Therefore, the collimated beam of the single QCL can be approximated as parallel laser beams.Furthermore, the collimated beam passes through the reflector, so as to adjust the direction of the beam spot and the spacing between different beam spots, ensuring that the spacing between the seven beam spots is minimized as much as possible and that the overall size of the beam spot is also minimized.In this simulation, because the spot size of a single chip after collimation was 4.2 mm, as shown in Figure 3a, the height difference between the three different steps was designed to be 4.3 mm, which can avoid beam occlusion and ensure the minimum spot spacing.In the horizontal direction, the distance between the beam spots is controlled by adjusting the angle of the reflector.And in the vertical direction, a three-layer external spatial combining distribution is presented, forming a "2 + 3 + 2" external spatial combining structure.The overall spot diameter after external beam combining is about 12 mm, as shown in Figure 3b.The effective area of the beam spot accounts for 85.75%.

Experiments and Results
Seven QCLs with a wavelength of ~8.5 μm were used.The power-current-voltage (PIV) curve of the seven QCLs (labeled as QCL1~QCL7) after external beam combining is shown in Figure 4a.The output power was measured with a thermal power sensor head in CW mode at 20 °C.The max output power of QCL1~7 was 0.63, 0.6, 0.43, 0.45, 0.5, 0.5, and 0.59 W, respectively, at a current of 1.5 A to 1.6 A. Figure 4b shows the total optical power after external beam combining.The total power of the seven QCLs is 3.6 W after external beam combining.As the current increases, the output power of the combined beam no longer increases after 1.6A, due to the thermal saturation effect.If the current continues to increase, the power may even decrease.The insert of Figure 4b displays the lasing wavelength of the external beam-combining system, which was measured with a Fourier-transform infrared (FTIR) spectrometer with a scanning speed of 1 cm/s and a resolution of 1 cm −1 .
(a) (b) Based on the size of the beam spots at different positions, the relationship between the beam width and the propagation distance of the beam can be established, and this can be used to fit the beam quality.By observing the shape of the beam spot on the CCD beam analyzer, the collimation effect can be preliminarily judged.The beam width of the single QCL and the external beam-combining system are shown in Figure 5.The beam quality of the single QCL is 1.302 in the fast axis and 1.336 in the slow axis, and the external beamcombining system is 5.59 in the fast axis and 8.3 in the slow axis, respectively.The insert of Figure 5a

Discussion
Due to the use of F-mount packaging for each quantum cascade laser, when this packaging is used for spatial beam combining, compared with other layout structures, the vertical stepped layout can minimize the spacing between the light spots, which is less than 0.1mm.This can maximize space utilization and integrate more quantum cascade lasers per unit area, thereby increasing the total output power.Regarding beam quality, the system has a beam quality of around 1.3 for both fast and slow axes for single laser.After external beam combining the beam quality deteriorates to 5.59 for the fast axis and 8.3 for the slow axis .This is due to the fact that the spatial combining corresponds to the arrangement of all the QCLs.The beam quality factor will increase exponentially.This needs to be improved by using spectral beam combining in the future.

Conclusions
In this work, a high-power external spatial beam-combining system was demonstrated; this consisted of seven QCLs emitting around 8.5 μm.Seven QCLs are fixed in a "2 + 3 + 2" stepped layout, using circular aspherical lenses with a large numerical aperture (NA) of 0.85 and a focal length of 1.873 mm for collimation.High-reflectivity gold-plated reflectors were used to change the direction of the optical path.After collimation, an M 2 of 1.302 in the fast axis and 1.336 in the slow axis for the single QCL was achieved.After external beam combining, the total CW output power of the system was 3.6 W, and the M 2 was 5.59 in the fast axis and 8.3 in the slow axis.This is promising for the delivery of high powers in the LWIR region over short to medium distances (<100 m).

Figure 1 .
Figure 1.Beam-combining configuration.(a) Zemax simulation schematic of the experimental scheme.(b) Scheme of the experimental setup.

Figure 2 .
Figure 2. Far-field divergence angle of a single QCL for experimentation and simulation.(a) Experimental divergence angle.(b) Simulated divergence angle of QCL before collimation.(c) Simulated divergence angle of QCL after collimation.Note: The simulated value of this divergence angle is defined by 1/e 2 .

Figure 4 .
Figure 4. (a) The PIV curve of the seven single QCLs after external beam combining.(b) Optical power curve of the system after external beam combining, as shown in the dotted line graph of the red square in this figure.The inset shows the lasing spectrum of the system, the wavelength at the peak, as well as the full width at half maximum of the spectrum.Note: As shown in Figure (a), the meaning represented by the circled part is consistent with the color of the corresponding coordinate axis, and is indicated by arrows.The curve adopts the form of a dotted line graph, where the square represents the voltage curve and the triangle represents the power curve.In addition, the values of the current transverse coordinates in both Fig.(a) and Fig.(b) are the result of powering up each laser independently at the same time.