Improvement is made in the traditional spectral beam combining structure by adding beam shaping element, namely Beam Transformation System (BTS). Spectral beam combination is performed in a horizontal direction by the external cavity after beam shaping. The effect of smile and the divergence of the slow axis are reduced. A standard semiconductor laser array is used in this experiment. A CW output power of 58.8 W and an electro-optic conversion efficiency of 51% are achieved. The spectral line-width is 12.7 nm. M2 of 1.3 × 11.6 in horizontal and vertical directions are obtained. The beam quality of the output is close to that of a single emitter of the array in two directions.
© 2014 Optical Society of America
High-power and high-brightness semiconductor diode laser sources are required in many applications. With the development of the semiconductor industry, high-power semiconductor diode lasers have been obtained. For instance, >700 W continuous-wave output power has been demonstrated from a single 1 cm-wide laser bar (940 nm) with 77% fill factor . But due to the structure of a semiconductor laser bar, the entire beam quality and the spatial brightness are very low. Spectral beam combining (SBC) has been proven to be an effective way to improve the beam quality and the spatial brightness for laser arrays . Several demonstrations of SBC at different wavelengths with semiconductor diode laser arrays have been made. Daneu et al. combined an 11-element diode laser array at 2.05μm, yielding 1.8 W CW with 50% beam-combining efficiency (ratio of SBC output to free running output). The M2 was 20 (slow axis) in combining direction . In another demonstration, Chann et al. combined a 3-bar commercial stack at 915nm, yielding 89.5 W CW with 75% beam-combining efficiency. The M2 was 26 on the slow axis (wavelength-beam-combined dimension) and 21 on the fast axis (no wavelength-beam-combined dimension) . An array of single-mode emitters has also been used in wavelength beam combining. A 100-element, 100-micron pitch array of slab-coupled optical waveguide lasers at 915 nm was combined to yield 35 W of output power with an M2 of 1.35 in both directions . A −1st-order transmission grating is introduced to spectral beam combining of a 970 nm diode laser bar. A CW output power of 50.8 W, an electro-optical conversion efficiency of 45%, and M2 value of 10.9 on the slow axis (wavelength-beam-combined dimension) are achieved . From all of the above SBC experimental results, it can be seen that the beam combining is performed on the slow axis. The beam quality is improved on the slow axis (wavelength-beam-combined dimension), however, the beam quality in the non-combining dimension degrades [4,5,7] since diode bars manufactured to the strictest tolerance suffer from a phenomenon called “smile” (packaging-induced distortion of the elements in a bar). A telescope can eliminate the loss in feedback caused by smile and improve the system efficiency . Unfortunately, it does not improve the system beam quality. On the other hand, slow axis collimation in these SBC experiments is less successful, leaving large divergence, which will reduce the feedback. To avoid the overlap in the emission plane, further collimation is impossible .As a result, the intensity of the edge emitters obviously decreases [3,6,8], which can be observed in the figure of the spectrum. This phenomenon limits the number of emitters involved in the spectral beam combination. In addition, in order to provide adequate feedback to lock each emitter in different wavelengths in these SBC experiments, the output coupler must have a minimum reflectivity of about 10% [3,5]. The lower the reflectivity, the better the overall conversion efficiency of the spectral beam combination.
To solve these problems, we made an improvement in the spectral beam combination structure. A beam shaping element (BTS) is inserted, which is usually used for making the beam parameter product of diode laser bars symmetrical for high-brightness fiber coupling [10,11]. As a result, the beams are rotated by 90 degrees. First and foremost, the output beam has the same spatial beam quality as that of a single emitter’s fast axis in the horizontal direction and slow axis in the vertical direction respectively. Secondly, the intensity of each emitter is substantially the same, owing to the excellent collimation in both the fast and slow axes. Finally, output coupler with lower reflectivity can also give sufficient feedback, so that the overall conversion efficiency is increased.
A 940-nm standard laser bar containing 19 laser elements has been combined to yield an output power of 58.8 W with an M2 of 1.3 (horizontal direction) x 11.6 (vertical direction). The electro-optic conversion efficiency is 51% thanks to the low reflectivity of the output coupler which is about 4%.
2. Experimental setup
Fig.1 shows our improved SBC cavity, which is similar to the former setups, including an AR-coated diode laser bar, a transform lens, a −1st order transmission grating and an output coupler. In addition, an optical path rotating device (BTS) for beam shaping is also inserted to rotate the positions of the laser beams from the emitters substantially for a right angle, converting the laser beams from the emitters into laser beams lined up in the form of ladder rungs.
The 940-nm COTS (commercial off-the-shelf) semiconductor laser array is used in this paper. The multi-stripe array has arranged 19 pieces (but three pieces are shown in the figure for simplicity's sake) of active layer stripes for emitting laser beams in a row being 10mm long. The cross section of each active layer stripe is 100μm wide and 1μm thick. In a laser beam radiated from one stripe, the divergence angle expanding in the thickness direction(the fast-axis direction) of the stripe is 70 degrees and divergence angle expanding in the width direction(the slow-axis direction) is about 7 degrees. The smile effect bending of the laser array is about 0.5μm, which can been seen from Fig. 2.
An aspheric cylindrical lens (fF = 500μm) is used to collimate the laser beams from the multi-stripe array semiconductor laser in the fast axis direction substantially in the vertical direction, and therefore the lens changes the radiant into nearly parallel beams in the vertical direction. The divergence angle of the laser beams remains about 7 degrees in its horizontal direction since the first cylindrical lens has a uniform length in this direction, allowing the light to travel virtually straight.
An optical path rotating device (BTS) rotates the cross section of the laser beams incident from the first cylindrical lens by 90 degrees with respect to the cross section at the position of the incident light. The BTS has joined together a plurality of optical elements made of S-TIH53 (Ohara) each having cylindrical faces of incidence and emergence, parallel side faces, and a dense interior. Each optical element is inclined at 45 degrees to the horizontal and the elements are arranged linearly so as to correspond to the respective active layer stripes. In the flat beam incident horizontally on the face of incidence, the axis of the flat beam is rotated by being subjected to different refracting powers at different positions of incidence, which are produced on the cylindrical face of the 45-degree-inclined face of incidence, and the axis of the flat beam is further rotated at the cylindrical face of the 45-degree-inclined face of emergence for a total of 90 degrees, and the beam emerges from the face of emergence. After the optical path rotating devices, the laser beams are changed into nearly parallel beams in the horizontal direction but have a divergence angle of about 7 degrees expanding in the vertical direction. And the deviation of vertical space due to smile effects becomes the horizontal displacement. The FAC and BTS we used are mounted on a bottom tab, which is shown in Fig. 3(a). A larger view of three elements and the process of rotating beams by BTS is illustrated in Fig. 3(b).
The second cylindrical lens (fS = 100mm), whose function is the same as the first cylindrical lens, is placed after the BTS to collimate all the beams in the slow axis simultaneously, for the slow axis light distribution of all the emitters is in the vertical direction. And the selection of the collimating lens is no longer constrained by space. The beams can be collimated very well in the vertical direction (the slow axis). After the second cylindrical lens, the laser beams are now nearly parallel beams as viewed both in the vertical and horizontal directions, which can’t be achieved in the previous SBC setups.
The transform lens (fT = 200mm) is placed one focal length away from both the front face of the laser array and the transmission grating. It transforms the position of an array element into the angle of incidence on the grating. As a result, the beams from the laser array are spatially overlapped at the grating.
The output coupler, combined with the transmission grating (1850 lines/mm), forces each element of the array to lasers at a unique wavelength that varies linearly across the array when the output beam has normal incidence on the output coupler. The horizontal displacement leads to only an insignificant deviation from the wavelength that each emitter should be locked at, having no effect on the beam quality of the combining output beam in the non-combining dimension. Thus, the cavity spatially overlaps the beams from all the laser elements into a single output beam both in the near and far field. This single output beam has the same spatial beam quality as that of a single laser of the array. The length of the common arm is 48 cm. The wavelength spread Δλis shown in Eq. (1),
Where Δλis the cavity wavelength spread, x is the width of the array, f is the focal length of the transform lens, a is the line spacing of the grating, and θ is the incidence angle. Based on the experimental parameter, the calculated spectrum span of the laser bar is 13.5nm.
3. Experimental results and analysis
Figure 4 shows the output power, electro-optical conversion efficiency and voltage as a function of current for the laser array at the coolant temperature of 18 °C, flow of 16L/min and CW operating mode. A maximum output of 58.8 W from the beam-combined cavity is obtained. A maximum electro-optical conversion efficiency of 51% is measured at 65 A, the corresponding slop efficiency is 0.893 W/A. From the measuring result, we found a significant fraction of the loss is caused by the grating, which is designed to have a maximum diffractive efficiency at 980nm. A further improvement in efficiency should be possible if the grating was optimized at the right wavelength of 940nm. The loss of SBC excluding the effect of the grating is approximately 2.5%. To our knowledge, this is the lowest-loss of SBC with a broad-area diode-laser system.
Figure 5 shows the beam quality of the combining laser focused by an objective with a focal length of 300mm at 65A. The M2 in the vertical direction is 11.6 which corresponds with the slow axis, and in the horizontal direction is 1.3 which corresponds with the fast axis. Compared with the spectral beam combining in the slow axis, the improvement has made the overall output to obtain the approximate beam quality of a single emitter in both directions. The brightness is increased consequently.
Figure 6 shows a spectrum of the output beam at 60 A, and the total wavelength spread is 12.7 nm which agrees with the theoretical calculation. This highly periodic spectrum indicates that the cavity is controlling the wavelength of each emitter separately, without the influence of any neighboring emitters, and that each emitter has approximately the same intensity. The edge effect that the intensity of the edge emitters obviously decreases, which can be seen if the spectral beam combining in the slow axis, is eliminated. Thus it allows more emitters to take part in the beam combining which can increase the overall output power.
A beam shaping element is inserted to reduce the effect of smile and the divergence of the slow axis in a spectral beam combination structure. As a result, the electro-optical conversion efficiency increases up to 51%, and the combining beam has the approximate beam quality of the single emitter in both direction, M2h = 1.3, M2v = 11.6. The intensity of all the emitters exhibits uniform distribution. This method provides a significant potential of higher power spectral beam combining diode laser arrays with high beam quality.
This work is supported by National Natural Science Foundation of China under grant 61378023, and the Beijing Municipal Natural Science Foundation under grant 4112005.
References and links
1. H. Li, I. Chyr, X. Jin, F. Reinhardt, T. Towe, D. Brown, T. Nguyen, M. Berube, T. Truchan, D. Hu, R. Miller, R. Srinivasan, T. Crum, E. Wolak, R. Bullock, J. Mott, and J. Harrison, “>700W continuous-wave output power from single laser diode bar,” Electron. Lett. 43(1), 27–28 (2007). [CrossRef]
2. C. C. Cook and T. Y. Fan, “Spectral beam combining of Yb-doped fiber lasers in an external cavity,” Advanced Solid-State Lasers 26, 163–166 (1999).
3. V. Daneu, A. Sanchez, T. Y. Fan, H. K. Choi, G. W. Turner, and C. C. Cook, “Spectral beam combining of a broad-stripe diode laser array in an external cavity,” Opt. Lett. 25(6), 405–407 (2000). [CrossRef] [PubMed]
4. B. Chann, A. K. Goyal, T. Y. Fan, A. Sanchez-Rubio, B. L. Volodin, and V. S. Ban, “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] [PubMed]
5. B. Chann, R. K. Huang, L. J. Missaggia, C. T. Harris, Z. L. Liau, A. K. Goyal, J. P. Donnelly, T. Y. Fan, A. Sanchez-Rubio, and G. W. Turner, “Near-diffraction-limited diode laser arrays by wavelength beam combining,” Opt. Lett. 30(16), 2104–2106 (2005). [CrossRef] [PubMed]
6. J. Zhang, H. Peng, X. Fu, Y. Liu, L. Qin, G. Miao, and L. Wang, “CW 50W/M2 = 10.9 diode laser source by spectral beam combining based on a transmission grating,” Opt. Express 21(3), 3627–3632 (2013). [CrossRef] [PubMed]
7. R. K. Huang, B. Chann, L. J. Missaggia, J. P. Donnelly, C. T. Harris, G. W. Turner, A. K. Goyal, T. Y. Fan, and A. Sanchez-Rubio, “High-brightness wavelength beam combined semiconductor laser diode arrays,” IEEE Photon. Technol. Lett. 19(4), 209–211 (2007). [CrossRef]
9. P. Y. Wang, A. Gheen, and Z. Wang, “Beam shaping technology for laser diode arrays,” Proc. SPIE 4770, 131–135 (2002). [CrossRef]
10. A. Timmermann, J. Meinschien, P. Bruns, C. Burke, and D. Bartoschewski, “Next generation high-brightness diode lasers offer new industrial applications,” Proc. SPIE 6876(68760U), 1–12 (2008). [CrossRef]
11. W. Hill, D. Hauschild, T. Mitra, and V. Lissotschenko, “Micro-optics gets your photons to work,” Laser + Photonik, Hanser Verlag Issue 3, 24–27 (2005).