An example for fiber array coupling to Laser Diode
1. Coupling theory of semiconductor laser fiber array beams
The light coming out of the semiconductor laser directly from the chip has a large beam divergence angle, the divergence in two directions is very different, and the beam quality is very poor. As shown in Figure 1, the length of this experiment is 10mm, the power is 60W, and the fill factor is 30%. The semiconductor laser array has 19 light-emitting points, the width of the light-emitting area is 150μ, and the spacing between adjacent light-emitting areas is 500μ. The full angle of divergence in the direction of the fast axis of the laser beam is 70°, and the full angle of divergence in the direction of the slow axis is 8°. In order to obtain good optical quality, the fast and slow axes must be collimated and optically shaped before they can be efficiently coupled into the fiber.
1.1 Principle of fast axis alignment and fiber coupling of semiconductor lasers
According to the luminous characteristics of the laser array, we designed a D-type cylindrical lens for fast-axis collimation, as shown in Figure 2. In order to improve the coupling efficiency, the coupling surface of the cylindrical lens needs to be coated with an 808nm antireflection coating.
The coupling method of the fiber D-type lens refers to coupling each light-emitting unit of the semiconductor laser array with the end face of the fiber array one by one. From the basic theory of laser beams and the common sense of semiconductor lasers, we know that the numerical aperture of the beam in the fast axis direction is much smaller than the divergence angle, but there is no such situation in the slow axis direction, so we only need to focus on the divergence in the fast axis direction. A full-angle collimation compression can be achieved for collimating the entire laser system.
The far-field speckle pattern after collimating the fast axis of the 19 laser beams with a cylindrical lens is the far-field speckle pattern after imaging with a CCD. As can be seen from the figure, the linearity of the array beam is good. The Smile<1μ of the semiconductor laser array was calculated by the Beamview analysis software. Using NanoScan beam quality analyzer to measure the light field distribution of the far-field spot along the fast and slow axes at a distance of 300mm from the luminous point, the measured value shows that the width of the collimated fast-axis beam is less than 1mm at the peak energy 1/e2. Through simple calculation, it can be concluded that the divergence angle of the fast axis after collimation is less than 0.2°, which fully meets the requirements of fiber-coupled semiconductor lasers.
In the application of semiconductor lasers, the end-face coupling of large-core multi-mode fibers can be divided into adapter connection between fiber end faces and lens coupling alignment connectionWe can set the distribution of laser power on the fiber end face section to be uniform, then the polarization distribution and intensity distribution are also uniform.
In the previous basic knowledge of optical fibers, we learned that the critical refractive index distribution of multimode fibers is also uniform, so the coupling loss between the end faces can be calculated by the method of applied optics.
When the lateral deviation between the two fiber axes is x, the coupling efficiency of optical power is:
If there is only a lateral distance d between the laser beam and the fiber, that is, there is only a lateral error, then the coupling efficiency is:
If there is only an angle α between the axes of the laser beam and the fiber, the coupling efficiency is approximated as:
In the above three formulas, the two end faces are in the air, the value of n is 1.458, △ is the refractive index difference between the core and cladding, and α is the core radius.
1.2 Array fiber bundle coupling process
The basic principle of fiber array coupling using fiber cylinder lens is to use a single fiber cylinder as a lens to compress the fast axis of the laser. Each laser unit of the entire bar is shaped by the fast axis, and the corresponding fiber end face is coupled. In theory, a higher coupling efficiency, a smaller diameter, and a better uniform light spot can be obtained, as shown in Figure 3.
In semiconductor lasers, due to the poor beam quality of high-power diodes, the divergence in two directions is very different, so it is necessary to obtain high beam quality and high coupling efficiency. Therefore, the fiber coupling technology of high-power fiber-output semiconductor lasers has always been one of the important topics that researchers pay attention to and study.
The coupling between the laser beam and the transmission fiber must satisfy the following basic conditions: ω<d/2; θ<2NA In the formula,
ω is the radius angle of the laser spot at the end face of the fiber;
d is the fiber core diameter;
θ is the full angle of divergence after the laser beam is focused;
NA is the numerical aperture of the fiber.
In general, when a laser beam passes through an ideal optical system, the product of beam parameters is an invariant. When the divergence angle is increased, the diameter of the spot will decrease. This is the basic cognition and design concept that laser engineering designers always have. In general, the following factors should be considered for energy transmission fibers. At present, the coupling conditions that are relatively recognized at home and abroad are as follows:
The first optimal coupling condition is obtained by mode matching considering the incident light source:
In the formula, ω is the beam waist diameter of the incident beam; α is the inner diameter of the fiber.
Coupled in this way, from the previous theoretical analysis, the theoretical coupling efficiency is very high.
The second ideal coupling condition is obtained by applying an optical method when the incident light source is parallel light. The specific structure is shown in Figure 4.
Under the conditions, the angle φ between the incident ray and the optical fiber geometric axis should satisfy: φ=COS-1/(λ /2a) Obtained from the figure: θ=90°-φ, tanθ=ω/2f to obtain the coupling lens of the focal length f should be:
According to the above theory, when the coupling conditions are met, the beam intensity does not exceed the rated damage threshold of the fiber. Because the optical field mode of the semiconductor laser is poor, only the second coupling method can be used.
However, since the light is collimated by an aspheric cylindrical lens, its divergence angle is much smaller than the numerical aperture of the optical fiber, so direct coupling can be used without the focusing lens as shown in the figure. At this time, the optical fiber coupling using a semiconductor laser array also has Two methods: fiber bundle method and shaped coupling method.
2. Coupling module experiment of fiber array
2.1 The experimental process and test of the coupling module
The size of a single light-emitting unit in the active layer of the semiconductor laser array is 150μ×1μ, and the arrangement period of each light-emitting unit is 500μ. According to the structural characteristics of the semiconductor laser array, we added a glass rod with a diameter of 80 μ to the end faces of 19 fibers with a core diameter of 150 μ to shape the semiconductor laser array.
The V-shaped grooves are precisely arranged into a circular fiber array, and the arrangement period is equal to the unit interval period of the semiconductor laser line array of 500μ. It matches the 19 light-emitting units of the semiconductor laser array, and calculates the distance from the semiconductor laser array to the glass rod. Fixed Glass rod, adjust the distance between the end face of the fiber array and the light-emitting surface of the semiconductor laser array to achieve the highest coupling efficiency, and fix the relative positions of the semiconductor laser array and the fiber array.
The whole experimental process adopts water cooling method to dissipate heat. Figure 5 is a schematic diagram of the coupling between the semiconductor laser array and the fiber array.
We measured the relationship between the output power P0 (W) before coupling of the semiconductor laser array and the output power P1 (W) after coupling with the current I (A), as shown in Table 1.
Table 1 Comparison of output power before and after LDA coupling
It can be seen that at I(A)=64 A, the output power after coupling is P1(W)=50.44W. Then gradually increase the current value, the coupled output power will not increase, and tends to be saturated. The coupling efficiency when I(A)=64A can be obtained is 89%, that is, the coupling efficiency at the maximum power point is 89%.
Figure 6 is the far-field output light spot after the semiconductor laser array is coupled with the fiber array. It can be seen from the figure that the uniformity of the output light spot is very good and the brightness is moderate.
Figure 7 is a photo of the assembled fiber coupler suitable for use in a module.
After the fiber array is coupled to the semiconductor laser array, the output test in the fast and slow axis directions is shown in Figure 8.
From the test results, it can be seen that the angle of the fast axis is close to that of the slow axis after the angle is compressed, indicating that the purpose of the experiment is achieved, and it is also in line with the theoretical analysis of the characteristics of the fast and slow axes of the semiconductor laser array.
The coupling of the array is completed. The power and reliability of the coupled semiconductor laser array depend on its matching with the fiber array. Therefore, the ABI data of the test module after the coupling is completed has become an important information basis for mastering the later work of the module. In the experiment, the 50W module After the ABI test, the data is shown in Figure 9 below.
It can be seen from the spectrum test in Figure 9 that the laser can output 50W stably, and its photoelectric conversion efficiency is 43%. At this time, the center wavelength is 811.60 nm, and the wavelength shift (FWHM at half waist) is less than 2.4 nm. This solves the technical difficulty of “narrow spectrum”, meets the technical requirements of the product, and also makes the product reach the international advanced level.
The key to the packaging of high-power semiconductor lasers lies in how to better achieve and achieve low thermal resistance technology, good welding packaging technology and efficient cooling technology, which can effectively improve the heat dissipation of the laser chip during operation, and increase the output power of the laser. Improve working life.
With the use of fiber array devices, the transmission power increases, and the thermal effect during operation also increases the influence of the device. In order to achieve high power output, its heat dissipation technology has become the key to laser packaging technology. We know that the more light-emitting points of the bar, the greater the direct output power of the laser, but at the same time, the heat dissipation is poor and the filling factor is high, which greatly reduces the coupling efficiency of the fiber, or even cannot be coupled at all. After many experimental comparisons and verifications, only a special semiconductor material with a fill factor of 20% is selected to form a semiconductor laser array with 19 light-emitting points.
This reduces the fill factor, effectively solves the problem of heat dissipation, and at the same time improves the fiber coupling efficiency, which can reach more than 89%. The high-power laser output with a total loss of only 0.7dB can be achieved. For the heat dissipation, the infrared temperature measurement CCD is used to test the maximum temperature of only 46.82 °C, as shown in Figure 10.
The heat and heat dissipation problems of semiconductor lasers are the key to their reliability and life. Effective control solutions are the driving force for the development of high-power semiconductor lasers. The research on conductive fiber arrays in this topic is in line with its future development requirements and progress. It can adapt to the long-term use of high-power semiconductor lasers.
In this paper, from the optical fiber device coupling theory to the device laser coupling experimental design and operation, the coupling performance and accumulated heat of the fabricated device were measured separately. In terms of coupling performance, the efficiency and spectrum fully meet the current international coupling efficiency and spectral characteristics of high-power semiconductor lasers. The accumulation of heat, because of the good coupling efficiency, the accumulated heat makes the temperature of the device not increase significantly, and the solution of the heat problem also makes the laser wavelength drift problem controlled.