1. Introduction

Semiconductor lasers possess many outstanding advantages, like small volume, light weight, high electro-optical conversion efficiency, narrow spectral half width and so on. In recent years, with the development of international science and technological advances, such as the semiconductor material growth technology, high efficiency cooling technology and the big breakthrough process of the related technology, the development of semiconductor laser has been into a more mature stage, so the semiconductor laser diode is widely used in optical fiber communication^[1-4], optical fiber sensing, optical storage, laser display, laser manufacturing, military, medical, and many other important applications. The semiconductor laser diode has become one of the core components of optoelectronic technology devices, of which 980 and 915 nm high-power semiconductor laser have been widely used in pumping solid state laser and fiber laser^[5-8]. However, due to the special structure of the semiconductor laser diode, its beam quality is very poor, the beam divergence angle in the horizontal and vertical directions are obviously different, so how to obtain a diode laser source with both high power and high beam quality has become an international major bottleneck. In the actual applications, we often adopt the method of optical fiber coupling to improve beam quality, so that the light spot which outputs from the fiber becomes uniform^{[9, 10]}.

Semiconductor laser has the small beam waist, and angles of the beam in the fast axis are greatly different from those in the slow axis, the beam in the spatial distribution is very asymmetrical. The laser beam is highly divergent, and the characteristics of semiconductor laser on the inherent structure have a bad influence on the coupling efficiency^{[11, 12]}. In order to get high coupling efficiency, the technology of beam reshaping and focusing have been proven to be a key solution and an effective way. In the recent reporting literature, the studying about 915 nm semiconductor laser single chip fiber coupling module is very rare, the output power of the modules are less than 12 W, and the modules have strict requirements in the parameters of the device chip; an output power of more than 13 W from a single chip optical fiber coupling module has not been reported^{[13, 14]}.

This paper is based on the development of 915 nm GaAs high-brightness and high-power single chip semiconductor laser diode, we design a single chip fiber coupling output module which can realize high brightness and high power. As the components are domestic, the module can easily realize industrialization.

2. Experiment process

This article is based on the high power GaAs 915 nm semiconductor laser diode which is independently designed and researched by our group. The 10 nm active layer is AlGaInAs, the $\rm Al_{0.24}\rm Ga _{0.76}$ As is for the 550 nm waveguide layer structure, the optical cladding layer is 1.2 μm $\rm Al_{0.37}\rm Ga _{0.63}$ As structure, 230 nm GaAs is for the buffer layer, and the contact layer is 250 nm GaAs. The substrate material is GaAs. The whole structure is shown in Fig. 1.

After the crafting processes which are cleavage, coating and packaging, we get the 915 nm semiconductor laser chip of which the cavity length is 4500 μm and the stripe width is 120 μm. When the current is 14.5 A, the output power of the diode is 13.91 W and divergence angle in the fast axis and slow axis are 34.5° and 10.4°, respectively. Its output power and voltage diagram are shown in Fig. 2, the output light spot and the divergence angle as Figs. 3 and 4.

After using ZEMAX software to simulate the whole structure of the 915 nm single chip fiber coupling module, we could use the equipment in the laboratory to realize the result of the simulation experiment.

2.1 The simulation of ZEMAX

Due to the poor beam quality of the semiconductor, before processing the optical fiber coupling technology, we need to reshape the beam in order to decrease the divergence angle in two directions. In the process of selecting the lens requiring the collimation lens can achieve the best effect of the light spot, and the whole lens should be placed coaxially. The focal length for the fast axis collimating lens is 200 μm, refractive index is 1.84, the thickness of the lens is 0.27 mm, and the material of lens type is S-TIH53. The focal length of the lens for the slow axis is 4.8 mm, the refractive index is 1.51, lens thickness is 2 mm, and the material type is BK7. The length of the self-grin lens is 2.1 mm, the diameter is 1.8 mm and center active index is 1.6. The calculating process using the following formula^[15]:

$\begin{equation} \label{eq1} {\theta }'=\arctan \left[\frac{\omega }{{\omega }'}\tan\theta\right], \end{equation}$

(1)

$\begin{equation} \label{eq2} f=\frac{{\omega }' }{\tan\theta}, \end{equation}$

(2)

$\begin{equation} \label{eq3} r =(n-1)f, \end{equation}$

(3)

$\begin{equation} \label{eq4} t =f -\frac{d}{n}. \end{equation}$

(4)

The f is the focal length of lens, $\omega '$ is the half size of the spot after the collimating lens, $\theta$ as half angle of divergence at fast and slow axis, r is curvature radius of the lens for reshaping, t is the distance between collimating lens and the semiconductor laser, d is the thickness of the lens, n is the refractive index for the lens. Then calculating based on Eqs. (1)--(4), we can get ${\omega }_{\rm fast}^{'}=0.062$ mm, $r_{\rm fast}=0.168$ , $t_{\rm fast}=53$ $\mu m$ , $\theta'=4.804$ rad, $\omega_{\rm slow}'=0.436$ mm, $r_{\rm slow}=2$ , $t_{\rm slow} =3.47$ mm.

In order to describe and evaluate the characters of the beam from the semiconductor laser diode precisely, we need to know the concept of beam parameter product (BPP). The BPP is defined by half size of the semiconductor laser beam product half divergence angle of the beam. The full name is expressed as the Beam Parameter Product, referred to as the BPP, relation as shown in type (4)^{[16, 17]}:

$\begin{equation} {\rm BPP}=\omega \times \theta. \end{equation}$

(5)

The numerical aperture (NA) of the fiber is 0.18, and diameter of the fiber core is 105 μm fiber. We can know BPP_fast < BPP_slow < BPP_Fiber from Table 1. The parameter of the optical fiber coupling technology is satisfied. Through the ZEMAX software to simulate the process of reshaping beam and optical fiber coupling, due to the result of the simulation and optimization, we can conclude that the distance between the slow axis of the lens and self-focusing lens is 2.27 nm, the distance between the self-grin lens and fiber front surface is 2 mm. Through the simulation result we can know that when the output power of the 915 nm semiconductor laser is 13.91 W, through the beam shaping and the coupling processes, the output power from the optical fiber is 13.88 W, concluding that optical fiber coupling efficiency of the single chip coupling module is 99.8%. The simulation figure of the overall structure and the output light spot of the fiber-coupling module are shown in Figs. 5 and 6.

3. The result of the experiment and analysis

During the experiment, we use the 6-axis platform to reshape the light spot in fast axis direction and slow axis direction. The light spot after reshaping is as Fig. 7, distance between the ground screen and the module is 50 cm.

Usually, many semiconductor devices including the semiconductor laser single chip will encounter the COD if the device needs to work above 10 A. In order to solve this problem, we put forward to change the way to connect the chip and the whole structure of the module as shown in Fig. 8. We use the indirect connecting gold wire to substitute the direct connecting through one gold shell. This way we can decrease the length of the gold wire and make it difficult for the gold wire to burn out when the current is above 10 A.

After reshaping the beam from the laser diode, we use the self-grin lens to focus the beam into the optical fiber; when we achieve the largest output power from the fiber, we fix the whole components. The optical fiber coupling module as shown in Fig. 9. Under the 14.5 A, the fiber coupling module can output 13.22 W, calculating the coupling efficiency to be 95.03%, and the output power and voltage of the module as shown in Fig. 10.

For high-power semiconductor laser, the brightness is a very important parameter, the expression is as follows^[18]:

$\begin{equation} B=\frac{P}{\pi^{2}\frac{D^{2}}{4}{\rm NA}^{2}}, \end{equation}$

(6)

where B is brightness, P is the output power of fiber, D is the fiber core diameter, and NA is optical fiber numerical aperture. After computing we can know the brightness of the single chip coupling module is 1.5 MW/cm² -str.

4. Conclusion

In this paper, we design a 915 nm semiconductor laser single chip fiber-coupling module. The output power of the module is 13.22 W which is the leader in recent reports. The coupling efficiency of the module is 95.03% and the brightness is 1.5 MW/cm² -str. For the further major work, we will aim to realize the eight single chip coupling module, and realize high-power pumping applications.