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We demonstrate the spectral beam combining of a diode laser stack, which contains three 970nm Mini-Bars along the fast-axis direction, in an external cavity. At the pump current of 60 A, the output power of 127 W, the spectral bandwidth of 12 nm and the Electro-optical conversion efficiency of 48.35% are achieved. The measured beam qualities after the spectral beam combining are M2 ≈10.2 along the slow axis and M2 ≈11.5 along the fast axis. Under a maximum injection current of 75A, the laser output power of more than 159W is achieved. The beam quality deteriorated slightly with the rising of the current from 60A to 75A, but it is enough to be coupled into a 50µm core / 0.22NA fiber.
© 2015 Optical Society of America
1. Introduction
The spectral brightness as well as spatial brightness of diode lasers (DL) are the important parameters for material processing, fiber laser pumping, diode-pumped solid state lasers (DPSSLs), industrial manufacturing and national defense. How to get high brightness and high power direct diode laser source have attracted intense research interests [1,2]. Because of the poor beam quality and “smile” effect of diode laser array, its power density is so low that it is difficult to achieve higher power that is able to launch into a smaller core-size fiber. Lincoln Laboratory has demonstrated a spectral-beam-combining (SBC) technique that significantly improves the brightness and intensity of diode laser systems [3]. After that, several demonstrations of diode laser SBC with different wavelengths and different structures have been implemented. By the SBC employing a 1450-nm broad area diode laser (BAL) bar with 25 elements, Gopinath et al. achieved output power of 20W in continuous-wave (CW) mode with the M2 (fast axis) of 1.9 and M2 (slow axis) of 10 [4]. J Zhang et. al. introduced a transmission grating and a 980-nm broad area diode laser array into spectral beam combining system, an output power of 50.8W(CW), electro-optical conversion efficiency of 45%, and M2 value of 10 (at 60A) on the slow axis were achieved [5]. Vijayakumar et. al. achieved an off-axis SBC of a 980-nm high power broad area diode laser array containing 12 emitters. The demonstration yields the optical power of 9.0 W and the measured M2 of 1.9 and 6.4 along the fast axis and the slow axis respectively [6]. A 940-nm standard laser bar containing 19 laser elements has been used to yield the output power of 58.8 W with the M2 of 1.3 (slow axis) × 11.6 (fast axis) and the electro-optic conversion efficiency of 51% [7]. Self-organized emitters were realized in a single broad area diode laser (1000μm wide stripe) by utilizing a spectral beam combining external cavity. In this setup, the laser emission of 31 individual spectral lines and the total spectral width of 3.6 nm were realized, The beam quality after the SBC was measured to be M2 < 3.0 ± 0.5 along the slow axis and M2 < 1.5 ± 0.3 along the fast axis in all cases up to the pump current of 10A [8]. Huang et. al. demonstrated the DL SBC with the recording output power greater than 2 kW. This system achieved the beam quality as low as M2≈12 that is the best beam quality for the multi-kW-class direct diode lasers so far [9].
SBC has also been extended to the DLs with the special structures such as slab-coupled optical waveguide or tapered amplifier. Huang et al. reported the SBC by an array of high-power high-brightness 970-nm slab-coupled optical waveguide diode lasers. The 50W peak out power under quasi-continuous wave (QCW) operation was achieved with the beam quality of M2x·y = 1.2, and the 30 W average output power under CW operation was obtained with the beam quality of M2x·y = 2.0 [10]. In another attempt, a 980-nm tapered diode laser bar containing 12 tapered emitters has been used to demonstrate SBC that yields the output power of 9.3 W and the M2 value of 5.3 [11].
From all the above mentioned experimental results, we can conclude that the beam qualities along the fast axis and slow axis are unequal [4–8], since the SBC is utilized only on the slow axis. With respect to the principle of fiber coupling, it is advantageous that the diode laser has the same beam quality in both directions to realize high coupling efficiency from the diode laser system to the output fiber. Another problem is that the improved beam quality by SBC accompanies with the effect of spectral width broadening, which leads to a spectrum span of up to several tens of nanometers, and is at the expense of sacrificing the spectral brightness. Therefore, the SBC source is not suitable for application of the narrow bandwidth requirements, such as the pumping sources of fiber lasers.
In this paper, we will show the spectral beam combining of the DL stack consisting of 3-mini-bars around 980nm. The mini-bar brings a couple of advantages compared to the conventional 10mm-width diode bar. The first one is due to the total spectral width after SBC is the function of bar width. Then the smaller the width of the bar is the smaller the spectral width will be. As the resulted benefit due smaller width of the mini-bars, we can get a small wavelength extension by using a transformation lens with shorter focal length to obtain a small size SBC system. Another advantage is the lower “smile” effect of the mini-bars, which can lead to the more efficient feed-back and wavelength locking for each emitter without any “smile” compensation elements. Because of the imperfect beam collimation along the slow axis, the feed-back beam from output coupler can be captured not only by the corresponding emitter but also by adjacent emitters. Consequently, this “cross-talk” beam forms a multi-lobed output and deteriorates the beam quality significantly [12]. To overcome the “cross-locked” problems, we improve the design of output coupler. It can eliminate influence of any adjacent emitters and realize satisfied inject-locked for individual emitters.
With the help of that improved output coupler, we achieve the beam quality M2 after SBC to be 10.2 (slow axis) × 11.5 (fast axis) at the output power of 127 W. This demonstrate almost the same beam qualities along the fast axis and slow axis respectively.
2. Experimental setup
Compared with other incoherent beam combining methods, such as spatial or polarization beam combining, the SBC of DLs combines external cavity diode laser (ECDL) technology with wavelength-division multiplexing (WDM) technology in optical fiber communications, and can increase the spatial brightness greatly. The SBC utilizes the external optical element and internal laser oscillation to realize multiple emitters beam combining and wavelength stabilization of single emitter. Figure 1 shows our experiment setup. It is known as a standard closed-loop beam combining structure, which includes a 3-Mini-Bar diode laser stack with 1.8mm stack pitch. Each mini-bar is collimated by fast-axis collimator (FAC) and slow-axis collimator (SAC) with the effective focal length (EFL) of 1mm and 3mm respectively. The transformation lens has the effective focal length of 150mm, the dielectric transmission grating has the groove density of 1600 lines/mm and the diffraction efficiency around 94%, and the cylindrical output coupler with 10% reflectivity and 500mm EFL is used to collimate the diffracted beam by the grating again and eliminates influence of adjacent emitters. The grating is placed at the common focus of the transformation lens and the output coupler. The coolant temperature of the system is 20°C in all operations.
Fig. 1 Schematic of SBC with external cavity feedback. FAC: fast axis collimating lens. SAC: slow axis collimating lens.
In this paper, three Mini-Bar stack is chosen in order to realize an equal beam quality in both the fast axis and slow axis and to get a small spectrum region by the transformation lens with shorter focal length. Each bar in the stack is 4.5mm long along the slow axis and contains 9 elements spaced at a 500-micron pitch. The gain region of each element is 100-microns wide and 3 mm length. In order to eliminate the impact of inner-cavity feedback, similar to the previous references, the Mini-Bars (manufactured by QPC lasers) are anti-reflection (AR) coated at the front facet with the reflectivity less than 0.1%. Then the back facet of each emitter and the external cylindrical output coupler form an external optical resonator, as shown by Fig. 2(a), where L and Ls are the lengths of the inner-cavity of diode laser and the external cavity, respectively.
The feedback light is injected into corresponding emitter and in turn coupled with the local light field in the active region. That coupling may generate a gain difference among various longitudinal modes of the external resonance cavity. Consequently the longitudinal modes which satisfy the threshold condition are excited, while the other longitudinal modes are suppressed. As the result of gain competition and dispersion effects of grating, the light wavelength emitted by each individual emitter varies with its position along the array. The position determines the angle that the corresponding beam incidents on the grating in turn, as shown by Fig. 2(b). The individual beams spatially overlap on the grating by the transformation lens and fully superpose as they propagate to the near field and far field. The diffractive angles of all of the emitter must be the same and is equal to the Littrow angle of the grating to achieve the highest diffractive efficiency. For the wavelength around λ = 976nm, θi = θLittrow = θd = 51.33° according to the well-known grating equation,
where d is the grating period, θi and θd are the incidence angle and diffractive angle, respectively, and λi denotes the wavelength emitting from the i-th emitter and m = −1. For the geometry given by Fig. 2(b), the i-th emitter’s incidence angle θi must satisfy:where p = 500μm is the interval of emitter positions along the laser bar, f = 150mm is the effective focal length of the transformation lens. Recalling , Eqs. (1) and (2) give the i-th emitter’s central operating wavelength as follow:Then applying the parameters mentioned above, we obtain that the whole spectrum region after the SBC of the laser stack is about 11.72 nm.3. Experiment results and discussions
Figure 3 shows the measured spectrum of emission light of the diode laser stack under free running at a pump current of 60 A. The free running operation means that the feedback of the external cavity is blocked by removing the grating. The spectrum in Fig. 3(a) is measured without FAC and SAC, where the resulted spectral bandwidth is about 4.9 nm. The FAC and SAC induce a coupling cavity into the diode stack and cause the extension of spectrum region. The measured spectrum with the bandwidth of 16.97 nm is shown by Fig. 3(b).
Fig. 3 The original emission spectrum of the diode stack at pump current 60.00A. (a) without FAC and SAC. (b) with FAC and SAC.
A typical output spectrum of SBC is depicted in Fig. 4. It possesses a comb-like structure including nine resonance peaks. The total spectrum bandwidth is about 12 nm and agrees with the theoretical prediction obtained in previous section. Each resonance peak originates from corresponding three emitters (each bar contributing one) at the same position of the slow-axis. The beam collimation by the cylindrical output coupler guarantees that the feed-back beam from output coupler is captured only by the corresponding emitter and hence the “cross-talk” among neighboring emitters is avoided. Figure 4(a) implies that the peaks have the approximate same intensities. It means that all of the nine emitters in each bar achieve almost the same feed-back strength due to the lower “smile” effect of Mini-Bar diode laser (see insert figure of Fig. 4(a)). In contrary, since the conventional bars including nineteen emitters usual exhibit larger “smile” effect, the peaks in the locked spectrum are rather irregular (Fig. 4(b)).
Fig. 4 after spectral beam combining model at pump current 60.00A. (a) Spectrum characteristics of mini-bar stack. (b) Spectrum characteristics of conventional10mm bars.
A typical input-output (or P-I) curve as well as electro-optical conversion efficiency are shown in Fig. 5, in which the results after the SBC of the diode stack and under the free running mode are compared. In the SBC mode, the output power more than 127W and the maximum electro-optical conversion efficiency of 48.35% are achieved at the pump current of 60 A.The beam quality of the combining laser at 60A is measured by Beam Propagation Analyzer M2-200S-FM (Ophir-Spiricon). M2 values of the combining beam are 10.2 and 11.5 for the slow and fast axis, respectively. The slow axis beam quality is approximately equal to the that obtained from a single emitter in free running mode at the same current level. As the pumping current is increased from 60 A to 75 A, more than 159W laser output was achieved. But the beam quality degenerates slightly at the same time. It can be explained as the results that grating surface is deformed by heat and the divergence angle of laser emission from the diode laser stack increases with the pumping current.
4. Conclusion
In summary, we demonstrate a robust method to scale output power and improve beam quality of diode lasers by the spectral beam combining of Mini-Bar diode laser stack. Our experiment has confirmed that the smaller “smile” effect of Mini-Bar may yielded the efficient and the same amplitude external cavity feedback for different emitters, and the second collimation to the diffractive beam by the grating can efficiently suppress the “cross-talk” among adjacent emitters. Both of them are significantly important in maintaining the beam quality as well as the output power after the spectral beam combining of DLs. Finally, the output power more than 127 W is achieved at the pump current of 60 A, and Electro-optical conversion efficiency increases up to 48.35%. As our expectation, the beam quality M2 of combining beam are M2y = 11.5 (fast axis) and M2x = 10.2 (slow axis). They are almost equal to each other in both directions. In principle, it is possible to realize kW level output by incorporating the increase of the number of Mini-Bar stacks along slow axis and the method of the polarization beam combining, while the beam qualities along both directions are maintain to be the same. They can be used as the direct diode laser sources in material processing.
Acknowledgment
This work was supported by funding from the Key Laboratory of Science and Technology on High Energy Laser, CAEP, under the Grant No. HEL-2014-08. The authors would also like to thank all peoples in DL group of Institute of Applied Electronics, CAEP, and ChangXi Xue from ChangChun University of Science and Technology for their helpful technical discussions.
References and links
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