Open Access
Add to BibSonomy
Abstract
We have proposed and experimentally demonstrated an efficient method for generating high power and brightness based on an ultra-broad area laser diode (UBALD). We have developed a single-emitter UBALD capable of self-organization multi-wavelength emissions for two stripe widths of 2 and 5 mm, respectively. The 2 mm UBALD delivers an output power of 55 W with a beam quality M2 of 1.3 × 25.3 and a brightness of 179 MW/(cm2·sr). The 5 mm UBALD produces an output power of 121 W with a beam quality M2 of 2.1 × 32.7 and a brightness of 192 MW/(cm2·sr). To the best of our knowledge, these results represent the highest output power and highest brightness ever achieved from a single edge-emitting LD emitter to date.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Laser diodes (LDs) have crucial applications in material processing, biomedicine, and national defense. However, a single LD generally exhibits low output power and poor beam quality. Multiple technologies, including slab-coupled optical waveguide laser [1–4], and tapered LD [5–9], have been developed to improve the beam quality of LDs. However, the output power of single-mode LD emitters has been limited to a few watts or less. The output power of the edge-emitting LD can be scaled simply by increasing the stripe width of the emitting area. Unfortunately, in broad-area LDs (BALs), the mode profile is broken into multiple filaments due to slow-axis mode instabilities, resulting in significant degradation of beam quality that impacts laser brightness. The beam quality of BALs can be improved by using high-reflection mirror feedback self-injection locking within an external cavity. For example, with a 200 µm width emitter, an output power of 1.1 W with a beam propagation factor of ∼2.6 in the slow axis was achieved [10]. In another work, based on a 1000 µm width emitter, 2.05 W output power with a beam quality factor M2 of 2.7 was demonstrated [11]. Furthermore, an external resonant cavity with the edge-emitting LD and spectral beam combination (SBC) can achieve high output power and multi-wavelength emission while maintaining the beam quality of single LDs. By employing SBC techniques, such as off-axis feedback [12–16], V-shaped resonator structure [12,15,17], thin-film filter [18], beam transformation system [19], wavelength-chirped volume Bragg grating [20], and slow-axis gain guiding [16,21], higher output power can be obtained, but they require multiple emitters. Another approach, a single BAL based on an external cavity with wavelength self-organized architecture was realized to generate multi-wavelength output. The self-organized emitters of a single BAL were enforced by the spatially distributed wavelength selectivity of the external cavity. Using a 1000 µm BAL, a 31-line emission with a total spectrum width of 3.6 nm was obtained with near-diffraction-limited beam quality. So far, the output power was only 430 mW [22].
In this paper, we demonstrate an efficient method to attain higher brightness output based on a single ultra-broad area laser diode (UBALD) with wavelength self-organized within a grating external cavity. The UBALD has a larger emitting region, allowing the front facet of UBALD to support higher output power. Meanwhile, the larger overall area of the LD emitter allows a larger heat dissipation, effectively mitigating the thermal rollover effect. Consequently, we have designed the edge-emitting UBALD with two stripe widths of 2 and 5 mm, respectively. With 2 mm UBALD, an output power of 55 W with a beam quality M2 of 1.3 (fast axis) × 25.3 (slow axis) and a brightness of 179 MW/(cm2·sr) has been achieved. Similarly, the 5 mm UBALD has delivered an output power of 121 W with a beam quality M2 of 2.1 × 32.7 and a brightness of 192 MW/(cm2·sr).
2. Experimental setup
2.1 External cavity configuration
The experimental configuration diagram of a self-organized multi-wavelength UBALD within a grating external cavity is shown in Fig. 1. The setup consists of a UBALD, a fast axis collimator (FAC) with a 900 µm focal length, a half-wave plate (HWP), a cylindrical lens F1 with a 200 mm focal length in the slow axis, a self-imaging cylindrical lens F2 with a 300 mm focal length in the fast axis, a multi-layer dielectric grating (MLDG) and a flat output coupler (OC) with 7% reflectivity.
Fig. 1. Diagram of self-organized multi-wavelength UBALD within a grating external cavity. UBALD: ultra-broad area laser diode; FAC: fast axis collimator; HWP: half-wave plate; F1 and F2: cylindrical lens; MLDG: multi-layer dielectric grating; OC: output coupler.
The beam emitted by the UBALD in the fast axis is collimated by the FAC. Two versions of the UBALDs with different stripe widths of 2 mm and 5 mm have been implemented in this study. The HWP is used to adjust the beam polarization direction. The slow axis emission of the UBALDs is collimated by F1, which is placed one focal length away from the front facet of the LD emitter and the grating, converting the spatial distribution of the emission into an angular distribution at the MLDG. The self-imaging cylindrical lens F2 is placed 300 mm away from the front facet of the LD emitter, avoiding long-distance free-space propagation in the fast axis and improving the output power by compensating the smile of the emitter [23]. The MLDG with a line density of 1800 mm-1 is placed with the grooves perpendicular to the slow axis. The angle of incidence on the grating is approximately 56°. The dispersion effects and gain competition lead to the selection of the appropriate wavelength at different positions of the emitter through external optical path feedback. The OC is 200 mm away from the MLDG for partial feedback and output. A slit aperture with a width of 30 mm is inserted at the front of the MLDG for selecting the transversal mode. As a result, the self-organization multi-wavelength emission of the slow-axis external cavity is realized.
According to the grating equation, the relationship between the spectrum width $\Delta \lambda $ of the grating external cavity, the focal length f of F1, and the stripe width W of the UBALD can be described as follows: $\Delta \lambda = ({{W / f}} )d \cdot \cos \theta $. Here, d represents the inverse of the line density, and the $\theta $ is the incident angle. Generally, the beam quality can be effectively improved by using a grating in the external cavity. Meanwhile, the external cavity structure effectively suppresses the transverse mode instability and maintains the effective oscillation of the laser.
2.2 UBALD design
The UBALD used in our experiment was fabricated by Li Zhou, Shao-yang Tan, and Jun Wang using the metal-organic chemical vapor deposition technique. They are members of our team working at Suzhou Everbright Photonics Co., Ltd., Suzhou. Based on the characteristics of the external cavity, we made some special designs on the slow axis structure and waveguide thickness of the LD emitter to enhance output performance. The UBALD emitter used in the experiment adopts a non-etching structure, and the size of the gain region is completely controlled by electrodes through patterned electrode plating technology. The electrode widths are 2 mm for 2 mm UBALD and 5 mm for 5 mm UBALD, rendering the LDs insensitive to lateral carrier diffusion over 20 µm. To enhance the gain of the LD emitter, we increase the cavity length of the emitter chip to 4 mm. The rear facet of the emitter is coated with a high-reflecting film at 940-990 nm, while the front facet is coated with an anti-reflection film at 940-990 nm. The UBALDs possess only one electrical contact and without additional gain-guiding or wave-guiding substructure along the slow axis.
The LD emitter is designed with an asymmetric large optical cavity. The n-side and p-side waveguide layers are AlGaAs material with thicknesses of 1.34 µm and 0.66 µm, respectively. The waveguide thickness of 2 µm enlarges the size of the front facet while maintaining the output beam near the fundamental mode in the fast axis. This improves the coupling efficiency of the external cavity feedback beam and increases the higher power that the front facet of UBALD can withstand. The asymmetric waveguide design effectively reduces the series resistance and the absorption loss of the device. Moreover, it also contributes to a small divergence angle of 26.7°. The active layer consists of a single compressive strain In0.2Ga0.8As quantum well with Al0.1Ga0.9As as quantum barrier layers. The quantum well is designed with less than 6 nm thickness to get higher optical gain and expand gain spectrum width with certain carriers. The optical confinement factor for the fundamental mode is 0.63%. The material gain spectrum is calculated by a commercial software “Crosslight” using the Interband transition model. The calculated results confirm that both 2 mm and 5 mm UBALDs have the same wide gain spectrum, covering the range from 0.86 to 1.0 µm, with FWHM of ∼90 nm, as shown in Fig. 2. Moreover, we also calculate the spectrum width $\Delta \lambda $ of self-organization grating external cavity to be: 3.2 nm and 7.8 nm for 2 mm and 5 mm UBALDs, respectively.
3. Experimental results and discussion
The output power is measured by a power meter (OPHIR FL600A-LP2-65). Figure 3 shows the measured output power for the 2 mm UBALD emission as the function of the pump current. The threshold current is ∼10 A and the output power increases monotonically with the pump current. The slope efficiency is ∼1.0 W/A. At a maximum pump current of 60 A, a maximum output power of 55 W is achieved, which corresponds to an electro-optical efficiency of ∼64.5%. In addition, it is found from Fig. 3 that the output power does not show any saturation effect up to the maximum pump current, which seems to indicate a considerable scaling potential of the output power.
The spectrum is tested by a spectrometer (Ocean Optics, HR4000). The measured output spectrum of the 2 mm UBALD emission at a pump current of 60 A with the external cavity is shown in Fig. 4. The total spectrum width is approximately 3.2 nm. There is a slight variation in the spectrum intensity at different wavelengths. To determine whether the inside of the emitter locks at different wavelengths depending on the position of the emitting region, we insert another narrow slit with a 0.1 mm width at the front facet of the emitter and translate it along the slow axis in 0.1 mm steps to examine the spectrum distribution of the near-field for the UBALD emission at pump current of 15 A.
Figure 5 shows the measured wavelength distribution at the near-field position for the 2 mm UBALD emitter. As seen in Fig. 5 that the spacing between adjacent spectrum peaks varies from 0.15 nm to 0.19 nm, which is consistent with our design. Within the 2 mm range, the measured longest wavelength is 968.0 nm and the shortest wavelength is 964.8 nm, in accordance with the previously calculated 3.2 nm spectrum width. Also, the oscillating inside the emitter conforms to a continuous spectrum. Due to the angular wavelength dependency of the grating, the different wavelength components are separated at the emitter front facet. Therefore, each lateral position of the emitter is linked to a specific wavelength.
The beam quality factor is performed by a laser beam analyzer (GENTEC-EO, BEAMAGE-4 M). The beam spot sizes of the 2 mm UBALD emission with external cavity are measured at various distances from a focusing lens. Typical beam quality factor M2 measurement for 2 mm UBALD with external cavity under the maximum pump current is shown in Fig. 6(a). The measured beam quality values at the slow and the fast axes are $M_s^2$ = 25.3 (BPP = 7.8 mm·mrad) and $M_f^2$ = 1.3, respectively. The inset in Fig. 6(a) displays the two-dimensional (2D) far-field spatial beam intensity distribution under the maximum pump current. The ellipticity of the laser mode is due to the asymmetry in the LD emitting dimensions. The beam quality factor M2 and brightness as the function of pump current is shown in Fig. 6(b). The beam quality factor $M_f^2$ consistently remains close to the diffraction limit, while the beam quality factor $M_s^2$ degrades as the pump current increases. The brightness of 2 mm UBALD is estimated to be 179 MW/(cm2·sr) at the maximum pump current. This brightness is more over two times higher than that of a typical 220 µm BAL emitter, which has a brightness of only ∼80 MW/(cm2·sr) [24].
Fig. 6. Beam quality and brightness of the 2 mm UBALD with external cavity. (a). Typical beam quality factor M2 measurement under the maximum pump current; Inset: 2D far-field spatial beam intensity distribution. (b). Beam quality factor M2 and brightness of the 2 mm UBALD versus the pump current.
Similarly, the performance of the 5 mm UBALD emission with a grating external cavity is also studied as follows. The output power as the function of the pump current is measured as shown in Fig. 7. It suggests that the threshold current is ∼30 A and the output power increases monotonically with the pump current. At a pump current of 150 A, a maximum output power of 121 W is obtained, which corresponds to an electro-optical efficiency of ∼57.6% and a slope efficiency of ∼0.95 W/A. Compared to the 2 mm emitter, the 5 mm emitter has a higher threshold current and a slightly lower slope efficiency.
The spectrum of the external cavity self-organization multi-wavelength emission of the 5 mm single UBALD at a pump current of 150 A is shown in Fig. 8. It can be observed that the output wavelength spans from 953 to 961 nm with a total spectrum width of approximately 8 nm.
Typical beam quality factor M2 measurement for the 5 mm UBALD with external cavity under the maximum pump current is depicted in Fig. 9(a). The measured beam quality values at the slow and fast axes are $M_s^2$ = 32.7 (BPP = 10.0 mm·mrad) and $M_f^2$ = 2.1, respectively. The inset in Fig. 9(a) shows the 2D far-field spatial beam intensity distribution under the maximum pump current. The beam quality M2 and brightness as the function of pump current is shown in Fig. 9(b). The variation trend of fast and slow axes beam quality is the same as that of 2 mm UBALD. However, the degradation in beam quality for the 5 mm UBALD is more than that of the 2 mm UBALD due to the increased stripe width, in which it suffers from more serious lateral lasing [11]. At the maximum output power, the corresponding brightness is estimated to be 192 MW/(cm2·sr), which is slightly more than that of the 2 mm emitter. Additionally, the output power of the 5 mm emitter is significantly higher as mentioned above. Therefore, the 5 mm UBALD is more advantageous in the field for requiring higher power.
Fig. 9. Beam quality and brightness of the 5 mm UBALD with external cavity. (a). Typical beam quality factor M2 measurement under the maximum pump current; Inset: 2D far-field spatial beam intensity distribution. (b). Beam quality factor M2 and brightness of the 5 mm UBALD versus the pump current.
The output power of the 2 mm emitter under free-running at 60 A is only 3.72 W and the free-running power of the 5 mm emitter is only 3.81 W at 120 A. We refrain from further increasing the pump current during free-running to protect the LD. It is much lower than the power of the self-organized external cavity. However, the slow axis divergence angle of the UBALD is about 8°∼10°. It leads to the beam quality of greater than 69 mm·mrad for the 2 mm emitter and 174 mm·mrad for the 5 mm emitter. It means that M2-factors for the free-running UBALD are far beyond the measurement range of the laser beam analyzer. Therefore, we use BPP instead of the M2-factors to compare the beam quality of the equivalent free-running with of the self-organized external cavity. The self-organized external cavity significantly improves the output power of the emitter and optimizes the beam quality.
4. Conclusion
In conclusion, we have proposed and experimentally demonstrated an attractive way for achieving high power and brightness LD based on UBALD with a grating external cavity. We have designed the UBALD with two stripe widths of 2 and 5 mm, respectively, and each single-emitter UBALD is utilized for self-organization multi-wavelength emission. With the 2 mm single emitter LD, we have achieved a laser output with a power of 55 W, a beam quality M2 of 1.3 (fast axis) × 25.3 (slow axis), a spectrum width of 3.2 nm, and a brightness of 179 MW/(cm2·sr). With the 5 mm single emitter UBALD, it delivers a laser output with a power of 121 W and a brightness of 192 MW/(cm2·sr). These results, to the best of our knowledge, are the highest output power and the highest brightness for a single edge-emitting LD emitter so far. Meanwhile, further power scaling can be expected by forming a stack with multiple UBA emitters placed in the fast axis.
Funding
Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences (552023000179).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. R. K. Huang, J. P. Donnelly, L. J. Missaggia, et al., “High-power nearly diffraction-limited AlGaAs–InGaAs semiconductor slab-coupled optical waveguide laser,” IEEE Photonics Technol. Lett. 15(7), 900–902 (2003). [CrossRef]
2. J. P. Donnelly, R. K. Huang, J. N. Walpole, et al., “AlGaAs–InGaAs slab-coupled optical waveguide lasers,” IEEE J. Quantum Electron. 39(2), 289–298 (2003). [CrossRef]
3. J. J. Plant, P. W. Juodawlkis, R. K. Huang, et al., “1.5-µm InGaAsP–InP slab-coupled optical waveguide lasers,” IEEE Photonics Technol. Lett. 17(4), 735–737 (2005). [CrossRef]
4. G. M. Smith, R. K. Huang, J. P. Donnelly, et al., “High-power slab-coupled optical waveguide lasers,” 2010 23rd Annual Meeting of the IEEE Photonics-Society, 479–480 (2010).
5. D. Feise, G. Blume, H. Dittrich, et al., “High-brightness 635 nm tapered diode lasers with optimized index guiding,” Proc of SPIE 7583, 75830 V (2010).
6. L. J. Wang, Z. Li, C. Z. Tong, et al., “Near-diffraction-limited Bragg reflection waveguide lasers,” Appl. Opt. 57(34), F15 (2018). [CrossRef]
7. B. Sumpf, P. Adamiec, M. Zorn, et al., “Nearly diffraction-limited tapered lasers at 675 nm with 1-W output power and conversion efficiencies above 30%,” IEEE Photonics Technol. Lett. 23(4), 266–268 (2011). [CrossRef]
8. Y. X. Lei, Y. Y. Chen, F. Gao, et al., “High-power single-longitudinal-mode double-tapered gain-coupled distributed feedback semiconductor lasers based on periodic anodes defined by i-line lithography,” Opt. Commun. 443, 150–155 (2019). [CrossRef]
9. C. Fiebig, G. Blume, C. Kaspari, et al., “12 W high-brightness single-frequency DBR tapered diode laser,” Electron. Lett. 44(21), 1253–1254 (2008). [CrossRef]
10. V. Raab and R. Menzel, “External resonator design for high-power laser diodes that yields 400 mW of TEM00 power,” Opt. Lett. 27(3), 167–169 (2002). [CrossRef]
11. M. Chi, B. Thestrup, and P. M. Petersen, “Self-injection locking of an extraordinarily wide broad-area diode laser with a 1000-µm-wide emitter,” Opt. Lett. 30(10), 1147–1149 (2005). [CrossRef]
12. Y. F. Zhao, F. Y. Sun, C. Z. Tong, et al., “Going beyond the beam quality limit of spectral beam combining of diode lasers in a V-shaped external cavity,” Opt. Express 26(11), 14058–14065 (2018). [CrossRef]
13. F. Y. Sun, S. L. Shu, Y. F. Zhao, et al., “High-brightness diode lasers obtained via off-axis spectral beam combining with selective feedback,” Opt. Express 26(17), 21813–21818 (2018). [CrossRef]
14. D. Vijayakumar, O. B. Jensen, and B. Thestrup, “980 nm high brightness external cavity broad area diode laser bar,” Opt. Express 17(7), 5684–5690 (2009). [CrossRef]
15. A. Jechow, V. Raab, and R. Menzel, “High cw power using an external cavity for spectral beam combining of diode laser-bar emission,” Appl. Opt. 45(15), 3545–3547 (2006). [CrossRef]
16. O. B. Jensen, B. Thestrup, P. E. Andersen, et al., “Near-diffraction-limited segmented broad area diode laser based on off-axis spectral beam combining,” Appl. Phys. B 83(2), 225–228 (2006). [CrossRef]
17. A. Jechow, V. Rabb, R. Menzel, et al., “1 W tunable near diffraction limited light from a broad area laser diode in an external cavity with a line width of 1.7 MHz,” Opt. Commun. 277(1), 161–165 (2007). [CrossRef]
18. M. Haas, A. Killi, C. Tillkorn, et al., “Thin-film filter, wavelength-locked, multi-laser cavity for dense wavelength beam combining of broad-area laser diode bars,” Opt. Lett. 40(17), 3949–3952 (2015). [CrossRef]
19. D. Zhu, L. Gou, M. H. Jiang, et al., “High beam quality in two directions and high efficiency output of a diode laser array by spectral-beam-combining,” Opt. Express 22(15), 17804–17809 (2014). [CrossRef]
20. B. Chann, A. K. Goyal, T. Y. Fan, et al., “Efficient, high-brightness wavelength-beam-combined commercial off-the-shelf diode stacks achieved by use of a wavelength-chirped volume Bragg grating,” Opt. Lett. 31(9), 1253–1255 (2006). [CrossRef]
21. A. Heuer, A. Sagahti, A. Jechow, et al., “Multi-wavelength, high spatial brightness operation of a phase-locked stripe-array diode laser,” Laser Phys. 22(1), 160–164 (2012). [CrossRef]
22. C. Zink, N. Werner, A. Jechow, et al., “Multi-wavelength operation of a single broad area diode laser by spectral beam combining,” IEEE Photonics Technol. Lett. 26(3), 253–256 (2014). [CrossRef]
23. J. T. Gopinath, B. Chann, T. Y. Fan, et al., “1450-nm high-brightness wavelength-beam combined diode laser array,” Opt. Express 16(13), 9405–9410 (2008). [CrossRef]
24. Y. Kaifuchi, Y. Yamagata, R. Nogawa, et al., “Ultimate high power operation of 9xx-nm single emitter broad stripe laser diodes,” Proc of SPIE 10086, 100860D (2017).
