Tapered Quantum Cascade Laser Arrays Integrated with Talbot Cavities
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Power scaling in broad area quantum cascade laser (QCL) usually leads to the deterioration of the beam quality with an emission of multiple lobes far-field pattern. In this letter, we demonstrate a tapered QCL array integrated with Talbot cavity at one side of the array. Fundamental supermode operation is achieved in the arrays with taper straight-end connected to the Talbot cavity. Lateral far-field of the fundamental supermode shows a near diffraction limited beam divergence of 2.7°. The output power of a five-element array is about three times as high as a single-ridge laser with an emission wavelength of around 4.8 μm. However, arrays with the taper-end connected to the Talbot cavity always show a high-order supermode operation whatever Talbot cavity length is.
KeywordsPhase locked arrays Quantum cascade lasers Semiconductor lasers
Full width of half maximum
Molecular beam epitaxy
Metal organic vapor phase epitaxy
Plasma-enhanced chemical vapor deposition
Quantum cascade laser
Quantum cascade laser (QCL), invented in 1994, has been one of the most important light sources in mid- and far-infrared for its wavelength flexibility and portability [1, 2, 3]. Popular applications of QCLs have covered many areas such as free space optical communication and directed infrared countermeasure (DRICM), trace chemical sensing of explosives, toxins, pollutants, and medical testing [4, 5, 6, 7]. Some applications always demand high output light power for better jamming effect and detection accuracy. High power QCLs can be obtained by broadening the width of active region area. However, a simple broadening of the ridge without waveguide engineering design or external optics will deteriorate the beam quality of QCLs with an emission of multiple lobes far-field pattern . Single-lobe emission is obtained in the past with the methods such as photonic crystal distributed-feedback (PCDFB) QCLs, angled cavity QCLs, master-oscillator power-amplifier QCLs, and broad area QCLs via external feedback mechanisms [9, 10, 11, 12]. Recently, phase-locked arrays have been popular approaches to keep wide ridge QCL emitting with coherent narrow beam patterns.
Phase-locked arrays have been skillfully applied in the wide ridge and low divergence semiconductor lasers since the 1980s . In previous works, phase-locked QCL arrays have been studied in the Y-junction arrays, resonant leaky-wave-coupled arrays, and evanescent wave-coupled arrays, as the near-infrared laser did in the past [14, 15, 16, 17, 18]. These structures either bring about large losses in the waveguide  or result in heat accumulation by pursuing a short adjacent distance to obtain the coupling [16, 17, 18]. Recently, diffraction-coupled QCL arrays that integrated a side cavity based on diffraction-coupled Talbot effects were reported . In the diffraction-coupled structure, the coupling occurs in the Talbot cavity by the diffraction of the ridge end and reflection of the cavity facet. The diffraction-coupled phase-locked QCL array elements can be placed for a wide space, which will decrease the heat accumulation.
The output power of Talbot cavity phase-locked QCL arrays is limited because of a low coupled efficiency between the Talbot cavity and the array channels. To further increase the output power of the Talbot cavity QCL arrays, the filling factor (ratio of ridge width to period) should be increased. Whereas, widening the channel width will arise a high-order mode emission of the array elements. Reducing the center-to-center distance will increase the heat accumulation. Taper structure is one of the best methods to increase the filling factor at the same time ensuring a fundamental mode operation of the single ridge itself. In this letter, taper structures are exploited and the Talbot cavities are integrated at one side of taper structures respectively. The devices with straight-end connected to Talbot cavity show a fundamental supermode operation with a near diffraction limited (D.L.) far-field divergence of 2.7°. In the contrast, the devices with taper-end connected to Talbot cavity show high-order supermode operation whatever Talbot cavity length is. A maximum peak power of 1.3 W is obtained for the devices with straight-end connected to Talbot cavity with a threshold current density of 3.7 kA/cm2 and a slope efficiency of 0.6 W/A at 298 K.
The QCL wafer was grown on an n-doped (Si, 2 × 1017 cm−3) InP substrate wafer by solid-source molecular beam epitaxy (MBE). The active region (AR) structure consists of 35 periods of strain-compensated In0.67Ga0.33As/In0.37Al0.63As quantum wells and barriers. The whole wafer structure before the fabrication is 4 μm lower InP cladding layer (Si, 3 × 1016 cm−3), 0.3-μm-thick n-In0.53Ga0.47As layer (Si, 4 × 1016 cm−3), 35 active/injector stages, 0.3-μm-thick n-In0.53Ga0.47As layer (Si, 4 × 1016 cm−3), 2.6-μm InP upper cladding layer (Si, 3 × 1016 cm−3), 0.15-μm InP gradually doped layer (changing from 1 × 1017 to 3 × 1017 cm−3), and 0.4-μm highly doped InP cladding layer (Si, 5 × 1018 cm−3).
Results and Discussion
The far-field patterns of the Talbot cavity phase-locked arrays were measured from the array waveguide facet using the lock-in technique with a room temperature mercury-cadmium-telluride (MCT) detector. The QCL array mounted on a rotation stage was placed ~ 25 cm away from the MCT detector and controlled by a home-built software for data collection. The measured far-field patterns of Talbot cavity arrays are shown in Fig. 3c, d, corresponding to the straight-end connected to the Talbot cavity device and the taper-end connected to the Talbot cavity device. The far-field distributions in Fig. 3c show strong central lobes at 0°, indicating the existence of fundamental supermode operation according to couple-mode theory. The full width of half maximum (FWHM) is around 2.7°, which shows a diffraction-limited (D.L.) divergence angle according to the D.L. formula: sin θ = 1.22λ/d, where θ is the D.L. angle, λ is wavelength, and d is the light output width of the array. For a tapered single emitter with a light output width of 16 μm, the D.L. FWHM divergence is around 21°. The side-lobes appear around ~ 12° which are very close to the FWHM location of the single emitter far-field envelope. The intensities of the central lobe and side-lobes are corresponding to the distribution of single emitter far-field pattern. Thus, the side-lobes have the half of intensity of the central lobe. Furthermore, single-lobe far-field profile array can be obtained by increasing ridge width to decrease the divergence of array elements. The wider ridge width can be achieved by widening the taper. The far-field patterns in Fig. 3d have no lobe at center 0° position, but are primarily double-lobed, showing the operation of higher order supermodes, which are corresponding to the three-order supermode in Fig. 3b. In order to obtain the fundamental supermode operation, we fabricated the devices with the different Talbot cavity length from 90 to 110 μm stepping 1 μm. Unfortunately, the fundamental supermode operation in the device with taper-end connected to the Talbot cavity cannot be obtained whatever Talbot cavity length is.
Output Characteristic of the Three Different Devices
Total Peak Power (W)
Threshold Current Density (kA/cm2)
Slope Efficiency (W/A)
Relative power (a.u)
In conclusion, we have demonstrated the tapered QCL arrays integrated with Talbot cavities in straight-end and taper-end respectively. The devices with the Talbot cavity integrated at the straight-end shows a fundamental mode far-field patterns with a D.L. divergence of 2.7° at an emission wavelength of 4.8 μm. An output power of 1.3 W is obtained for the straight-end array with a slope efficiency of 0.6 W/A. Since the Talbot cavity phase-locked array does not require a very close coupling distance, the heat accumulation is lower than the evanescent wave-coupled arrays. Such devices have a potential for high brightness QCL arrays of high duty cycle operation with D.L. divergence. Future work should focus on the selection of an appropriate array element ridge width and interspace, the use of buried ridge waveguides, and the thermal management with micro-impingement coolers . In addition, the reduced cascade number of the AR will make great contribution to the high duty cycle operation of high brightness QCLs .
The authors would like to thank Ping Liang and Ying Hu for their help in device processing.
This work was supported by the National Basic Research Program of China (Grant No. 2017YFB0405303, 2018YFA0209103), National Natural Science Foundation of China (Grant Nos. 61790583, 61574136, 61435014, 61774146, 61674144), and Beijing Natural Science Foundation (Grant No. 4172060).
Availability of Data and Materials
All data are fully available without restriction.
YZ designed the structure of the device, calculated the theoretical model, performed the testing, and wrote the paper. JCZ and FQL provided the concept, polished the paper, and supervised the project. FMC and DBW fabricated the device. SQZ and SML improved the design. LJW and JQL completed the MOCVD growth. NZ and CWL modulated the active region structure and completed the MBE growth. ZGW supervised the project. All authors read and approved the final manuscript.
FQL is a professor in Key Laboratory of Semiconductor Materials Science at the Institute of Semiconductors, Chinese Academy of Sciences. He earned his MSc degree in Solid State Physics at University of Science and Technology China in 1990 and obtained his Ph.D. degree in the Department of Physics Nanjing University in 1996. He has studied quantum cascade laser since 1996 using a solid-source MBE in Beijing and realized a laser emitting at 5.1 μm in the end of 1999 and the room temperature operated quantum cascade laser emitting at ~ 3.54 μm in 2000. Recently, he has demonstrated the quantum dot cascade laser by two-step strain-compensation active region and material grown technique. He is a winner of the National Outstanding Youth Fund in China.
The authors declare that they have no competing interests.
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