1.5-W 520 nm continuous-wave output from a 50μm/0.22NA fiber-coupled laser diode module

: In this paper, a high brightness fiber-coupled module with a central wavelength of 520nm is simulated and designed by ray-tracing software ZEMAX, and then is experimentally implemented. Three 1-w continuous-wave green LD single emitters based on TO-9-package are successively collimated, spatially combined, and focused into an optical fiber with a core diameter of 50 μm and a numerical aperture of 0.22. The final output power of 1.53w is obtained, corresponding to an optical-optical conversion efficiency of 51% and an electro-optical conversion efficiency of 10%, and the tolerance between the simulation and the experimental result is analyzed and explained.


Introduction
Laser diodes (LDs) in the range of visible wavelength play an increasingly important role in biomedical applications, pumping sources, and laser-based displays, because of the advantages of compactness, long lifetimes, and good reliability. The output power of the red LD stack can be as high as 56W and the optical-coupled module constructed by incoherent combination has a maximum output power of 100W (Andreas et al. 2013). The blue LD bar has reached 98w, and the fiber-coupled module based on the bar has reached 1KW presented by Laserline and OSRAM in February 2019 (Baumann et al. 2019). And in September 2020, Nuburu launch the latest blue fiber-coupled module (Feve J et al. 2020), AI-1500, which increase the power to 1.5KW under 11 mm·mrad from a 100μm /0.22 numerical aperture (NA) fiber, taking a different approach that mounts Osram's GaN single-emitter diodes on a ship in series.
For green direct diode laser, it has become an attractively alternative pump source of Ti: sapphire oscillator because of its low cost and without using liquid nitrogen cooling at high power compared to blue LD module. However, the research progress of highpower green LD is very slow due to the "green gap" difficulty. Until 2009, Sumitomo, OSRAM, and Nichia successively achieve green output based on GaN substrate (Miyoshi et al. 2009;Queren et al. 2009;Enya et al. 2009;Yoshizumi et al. 2009) but all LDs are single-mode with low power which is not enough for scale applications. In 2013, Nichia successfully manufactures a multimode green LD single emitter with a power up to 1W and a central wavelength of 520nm based on AlInGaN grown on c-face GaN substrate (Masui et al. 2013). Up to date, the maximum output power of a green LD is merely 1.5-W produced by Nichia (NICHIA 2020 (Zhao et al. 2018). However, the beam quality of such lasers is not good enough for some applications. For example, when the above laser diode is used for pumping a laser medium, the beam is focused on a spot with a diameter of around 150 μm, its corresponding Rayleigh length is about 1 mm, which is not suitable for pumping a gain medium with a length of more than 2 mm. In the previous work, we obtained a wattlevel continuous-wave (CW) Ti: sapphire laser oscillator where a 10-mm-long Ti: sapphire crystal was end-pumped by a 200 μm/ NA 0.22 fiber-coupled green LD module that can deliver a power of more than 20 watts (Miao et al. 2020). The experimental results show that the final optical-to-optical efficiency is heavily dependent on the beam quality of the pumping source. Therefore, based on the requirement of end-pumping of Ti: sapphire with a length of 5mm or more, it is necessary to further reduce the core diameter of coupling fiber, for example to an optical fiber with a core diameter of 50 μm, and improve its beam quality.
In this paper, we show a high beam quality fiber-coupled module by combining three

Designs and simulations
In this paper, the beam parameter product (BPP) is used to describe the beam quality of LDs, which is defined as the product of beam waist half-width and divergence half-angle (Qi et al. 2017). For a given optical-fiber, the beam quality is half of the product of core-diameter ( core d ) and numerical aperture (NA) of the fiber, that is . According to the coupling conditions between a rectangular spot and a circular fiber, to effectively couple the beam into an optical fiber, the BPPs of the fast axis and the slow axis of the rectangular beam should be less than 22 times of BPP of the optical fiber, respectively  (2) It is clear from Eq. (1) and (2) that for an optical fiber if the numerical number NA remains the same, a smaller diameter of the fiber core core d would improve the BPP directly. For example, if the core diameter can be reduced from 150μm to 50μm while keeping a constant NA , the BPP can be improved by a factor of 3. And this can be achieved based on our following calculations: The specifications of a 1-W green LD manufactured by Nichia are given in Table 1 Table 1). we need to design a set of mutually orthogonal fast and slow axis collimators (FACs and SACs) to collimate the beams emitted from the LDs. A nearly diffraction-limited aspheric cylindrical lens with a focal length of 3.5mm is designed as FAC, and a set of positive and negative lens groups with a focal length of 32 mm is designed as SAC. These collimators have been used previously to construct the 12-W green LD module in our group and it worked well (Zhao et al. 2018). The beam sizes of the fast and slow axes after collimation can be obtained from the detector viewer of ZEMAX, which are 2.6mm and 6.8mm respectively. The relationship between radiance and the residual divergence angle, as well as the output beam spot after collimation, is given as shown in   The overall optical configuration of the fiber-coupled module is shown in Fig.2(a).
Three LDs are arranged parallelly along the fast axis direction (with an interval of 10 mm) and they are collimated by the FACs and SACs respectively, and then deflected 90-degrees by three 45-degrees high-reflection (HR) mirrors with the same size (Length: 5mm; Width: 2mm; Height:9 mm). By adjusting three HR mirrors, the beams can be tightly stacked and spatially combined along the fast axis direction. Then, to reduce the volume of the module, the beams are again reflected 90 degrees by a larger HR mirror (Length: 10 mm, width: 2mm, height: 8 mm). The beam spot size is 8mm×6.8mm as the beam reaches the coupling lens as shown in Fig.2(b). Finally, the beams pass through the coupling lens are focused into the optical fiber. As a thumb of rule, the beam footprint on the facet of the fiber and the beam divergence must meet the coupling condition, that is: where dia W and dia  represent the diagonal length and divergence half-angle of the rectangular beam respectively. In the simulation, we use a single aspherical focusing lens to focus the rectangular beam. The focal length of the lens can be derived from Eqs. (4) and (5) Due to the small size of the green LD single emitter, the focal length range satisfying the condition of Eq. (4) is larger than that of Eq. (5). It can be concluded that the divergence angle has more rigorous restrictions on the focal length of the lens, compared with the beam size without considering spherical aberration of the lens. And when Eq. (5) takes the mark of equality, the module has the maximum brightness. Thus, the effective focal length (EFL) of the coupling lens is determined, and a nearly diffraction-limited aspheric lens with the EFL of 25mm from Thorlabs (AL1225G-A) is selected. In Fig. 2(b), the diagonal length of the combined beam is about 10.5mm.
According to Eq. (6), the divergence half-angle of the focused beam could be equal to 0.21.
The beam divergences in the fast and slow axis are simulated using ZEMAX, in the simulation, a rectangular detector is placed at the image plane of the coupling lens.
The numerical irradiance of the ZEMAX simulation is illustrated in Fig. 3(a) and the image of the focused spot of the rectangular beam obtained by the detector is shown in Fig. 3(b), whose size is 28μm×40μm in the fast axis and slow axis, respectively. As we expected, astigmatism in the focus is observed because the diode laser beam is the inherent asymmetry between the fast and slow axes. If it is necessary to be eliminated, the beam should be focused separately in the two axes. Here we can find an optimal position that is closed to the image plane of the coupling lens by using the global optimization function of ZEMAX, where the beam sizes in both fast and slow axes are less than 50 μm as shown in Fig.3(b). It can be seen that the beam divergence half-angle in the fast and slow axes after coupling lens are about 8.5-degrees and 8-degrees, corresponding to 0.15 rad and the 0.14 rad. Consequently, based on the simulation, the focusing spot can be efficiently coupled into a fiber with a core diameter of 50 μm and NA of 0.22.  62mm×28mm. The rays in Fig. 5(a) illustrates the process of three beams being coupled into the optical fiber. Fig. 5(b) illustrates a photograph of the module that can produce1.53W-green laser from a 50μm /0.22NA fiber.  The power and loss ratio in each step of the module and simulation are illustrated in Fig.9. There is a significant difference between the experimental result and the simulation result, especially for the fiber-coupling efficiency of only 70% in the experiment which is far lower than the simulation data of 96%. This can be explained from the following:

Experimental results and discussion
First, the initial output power of each LD is set to 1W in ZEMAX, but in the experiment, the actual output power of three LD is measured as 0. 95W、0.96W、 0.96W, respectively.
Second, the reflectivity of all mirrors can reach 100% in ZEMAX, but the reflectivity of HR-mirrors in the experiment can only reach 99.5%.
Third, the surface of the focusing lens is coated with an AR coating of 99.9999% in ZEMAX, but in practice, it is only 99.5%. And the end facet of optical fiber is coated with an AR coating of 99.9999% in ZEMAX, but the optical fiber used in the experiment is not coated. The theoretical value of fiber-coupling efficiency can only reach 92%.
In addition, the global optimization function of ZEMAX can be used to find the optimal positions of FACs, SACs, HR mirrors, focusing lens, and fiber. However, due to the limitation of assembly accuracy, it is difficult to find the optimal position in the experiment. Therefore, the spot diameter near the focus point may have exceeded 50 microns because of the existence of assembly error. The CCD detector head cannot be placed near the focusing spot for a compact volume of the module, so the spot diameter cannot be accurately obtained. Later, we replace a fiber with a core diameter of 100μm and NA of 0.22, and the output power has been increased to 1.83W, corresponding to a coupling efficiency of 87%, which indirectly verify our conjecture.

Conclusion
In this paper, by collimating in the fast and slow axes, spatial combination, and focusing, three LDs coupled into an optical-fiber with 50μm core diameter and 0.22 NA, a fiber-coupled diode module is presented both stimulatingly and experimentally.
The output power of about 1.53W at 520 nm and brightness of ( ) 2 0.608MW cm sr  is obtained in the experiment. The optical-optical conversion efficiency is 51% and the electrooptical conversion efficiency is 10%. The tolerance between the simulated data and the experimental results are also analyzed. The design presented here can be further combined with polarization multiplexing for coupling more LDs into an optical fiber to double the output power with the same beam quality.