Reliability Study of Fiber Coupling Efficiency of 980 nm Semiconductor Laser

Abstract

In order to improve the stability of semiconductor laser fiber coupling efficiency, based on the coupling principle, the optimal parameters for semiconductor laser fiber coupling were simulated to be θ = 45°, r = 3.25 μm, and z = 5.65 μm. By optimizing the structure and position of the lens fiber, it has been experimentally proven that the maximum fiber coupling efficiency of the 980 nm semiconductor laser can reach 87.1%, and the average coupling efficiency can also reach 84%. After temperature cycling and aging experiments, the average coupling efficiency of the device was 81.7%, indicating a decrease in coupling efficiency. At the same time, the effect of fiber stress on the reliability of coupling efficiency was analyzed, and the stability and consistency of the device before and after temperature cycling were explored. In future work, it will be necessary to further optimize the thermal stress caused by UV glue curing and tail pipe soldering, find suitable process parameters, and obtain stable and reliable coupling modules.

1. Introduction

With the rapid development of information technology, high-speed communication has become the key to communication technology [1,2,3,4,5]. High-power semiconductor laser diodes (LDs) operating at a 980 nm wavelength are used to pump erbium-doped fiber amplifiers (EDFAs). The high coupling efficiency between the LD and fiber, as well as the high power of the LD itself, are crucial for maximizing the performance of EDFAs and dense wavelength division multiplexing systems. As the output power of the LD increases, thermal problems that affect beam quality and can cause catastrophic optical damage will appear on the end face of the LD [2,6,7,8,9]. Most high-power LDs are designed to have a large emission width to prevent these issues, and therefore, they have an elliptical mode field at the end face of the LD. Usually, the ratio of the longitudinal and transverse directions of an elliptical mode field is between 3 and 5. At present, some methods are being adopted to couple elliptical mode fields with single-mode fibers (SMFs), such as the combination of cylindrical lenses or special lens fibers with different shaped end faces. However, the coupling loss between the light source and the optical fiber still needs to be further improved [10,11,12,13,14,15,16]. Therefore, studying laser coupling transmission technology to provide a theoretical basis for optical signal transmission and reception module packaging is of great significance.

In recent years, research on fiber optic coupling systems at home and abroad has mainly focused on developing new optical lenses and new lens fibers to improve the optical coupling performance of the system. By establishing a more effective optical coupling model, the influence of various parameters on coupling efficiency was studied. By optimizing structural and coupling parameters, high coupling efficiency and good process parameters can be ensured [17,18,19]. In 2022, Lu et al. [20] prepared conical microlenses with fiber tips by grinding and polishing. After optimizing parameters such as incident wavelength, laser waist radius, and fiber numerical aperture, the optimal coupling distance of a single-mode fiber was found to be 15 μm, proving that the conical tip coupling was a tightly sealed structure. In 2023, Liu et al. [17] successfully prepared a high-power 792 nm fiber-coupled semiconductor laser module with an output power of 232 W and an electro-optical conversion efficiency of 48.6% at a 10 A continuous current. The laser module was aged for more than 4000 h at 12 A and 25 °C without significant power loss. In 2024, Ma et al. [18] designed a novel lens combination consisting of a right-angled prism and a trapezoidal prism and analyzed the beam propagation and coupling efficiency of the system in detail using numerical analysis and ZEMAX, respectively. The results show that at a current of 35 A, the output power of the fiber laser reaches 620.9 W, and the coupling efficiency is 86.24%, which is consistent with the simulation result of 89.65%.

In this work, the influence of various parameters of a wedge-shaped lens fiber on coupling efficiency was first theoretically analyzed, and the optimal half wedge angle, arc radius, and axial distance were obtained. In the experiment, the influence of the fast axis direction, slow axis direction, and axial direction of the laser chip on the fiber coupling efficiency was systematically studied, and it was determined that the fiber coupling efficiency was maximized when the arc radius was approximately equal to the axial distance. At the same time, the effect of fiber stress on the reliability of coupling efficiency was analyzed, and the stability and consistency of the device before and after temperature cycling were explored.

2. Design of Semiconductor Laser Fiber Coupling System

2.1. Structural Design of Fiber Optic Coupling Control System

As shown in Figure 1, the optical module part of the device mainly includes the tube shell, the substrate, a thermoelectric cooler (TEC), a laser chip, a wedge-shaped lens fiber, UV glue, an airtight sealing device, and other parts. The wedge-shaped lens fiber mainly has two parameter controls, namely the wedge angle 2θ and the top arc radius r. The center wavelength λ_c of the wedge-shaped lens fiber is 974.5 nm, the peak reflectivity at the center wavelength is 3%, the full width at half maximum is 0.25 nm, the sidelobe suppression ratio (SLSR) is 10 dB, and the length of the rear grating fiber is 70 ± 8 mm. There is a grating region at the tail of the optical fiber to stabilize the laser spectrum. Detuning with the optimal wavelength affects the coupling efficiency of the lens fiber by changing the spot radius and the curvature radius of the beam. The reflectivity of the semiconductor laser output mirror is 6%. The wedge-shaped lens fiber produced by YOFC company was used in the experiment.

2.2. Coupling Principle and Simulation Results

A wedge-shaped lens fiber is made by processing the fiber end face perpendicular to the PN junction direction into a wedge shape through grinding, polishing, and other methods to create a wedge angle. Then, a micro lens is produced at the top of the wedge angle through discharge or grinding, which plays a role in phase transformation and can reduce the phase mismatch between the LD and single-mode fiber (SMF) wavefront caused by excessive laser divergence. The formula for expressing the coupling efficiency between the semiconductor laser and single-mode wedge-shaped lens fiber is as follows:

$$\eta ={\displaystyle \frac{{\left|{\displaystyle {\int }_{-\infty }^{+\infty }{\displaystyle {\int }_{-\infty }^{+\infty }{E}_1(x,y,z){[E_f(x,y)t(x)]}^{\ast }dxdy}}\right|}^2}{{\displaystyle {\int }_{-\infty }^{+\infty }{\displaystyle {\int }_{-\infty }^{+\infty }{\left|E_1(x,y,z)\right|}^{2}dxdy{\displaystyle {\int }_{-\infty }^{+\infty }{\displaystyle {\int }_{-\infty }^{+\infty }{\left|E_f(x,y)t(x)\right|}^{2}dxdy}}}}}}$$

(1)

In the above equation, $t(x)=\exp [-ik(n-1)Z_c(x)]$ represents the phase change caused by the wedge-shaped lens; $Z_c(x)$ is the surface contour function of the wedge-shaped lens. When $\left|x\right|\le r\cos \theta$, $Z_c(x)=r-\sqrt{r^2-x^2}$; when $\left|x\right|\ge r\cos \theta$, $Z_c(x)=r(1-\sin \theta )+(\left|x\right|-r\cos \theta )\cot \theta$; n = 1.464 is the refractive index of the fiber core; θ is the half wedge angle; r is the radius of the top arc of the optical fiber; and E₁ is the fundamental mode field distribution of a semiconductor laser, expressed as follows:

$$E_1(x,y,z)=\exp (-ikz){\left[{\displaystyle \frac{{\omega }_{ox}{\omega }_{oy}}{{\omega }_x(z){\omega }_y(z)}}\right]}^{1/2}\exp \left(-{\displaystyle \frac{x^2}{{\omega }_{ox}^2}}-{\displaystyle \frac{y^2}{{\omega }_{oy}^2}}\right)\exp \left\{-ik\left[{\displaystyle \frac{x^2}{2R_x(z)}}+{\displaystyle \frac{y^2}{2R_y(z)}}\right]\right\}\exp \left[i{\displaystyle \frac{{\varphi }_x(z)+{\varphi }_y(z)}{2}}\right]$$

(2)

In the above formula, ${\omega }_x(z)$ and ${\omega }_y(z)$ are the spot radii of the laser beam transmitted to z in the fast and slow axis directions, respectively. Their expressions are as follows:

$${\omega }_x(z)={\omega }_{ox}\sqrt{1+{\left({\displaystyle \frac{\lambda z}{\pi {\omega }_{ox}^2}}\right)}^2}$$

(3)

$${\omega }_y(z)={\omega }_{oy}\sqrt{1+{\left({\displaystyle \frac{\lambda z}{\pi {\omega }_{oy}^2}}\right)}^2}$$

(4)

$R_x(z)$ and $R_y(z)$ are the curvature radii of the laser beam transmitted to z in the fast and slow axis directions, respectively. Their expressions are as follows:

$$R_x(z)=z\left[1+{\left({\displaystyle \frac{\pi {\omega }_{ox}^2}{\lambda z}}\right)}^2\right]$$

(5)

$$R_y(z)=z\left[1+{\left({\displaystyle \frac{\pi {\omega }_{oy}^2}{\lambda z}}\right)}^2\right]$$

(6)

${\omega }_{ox}$ and ${\omega }_{oy}$ are the waist beams in the x and y directions at the cavity surface, λ is the free-space propagation wavelength, k is the wave vector, $k={\displaystyle \frac{2\pi }{\lambda }}$ (λ = 974 nm), $E_f(x,y)=\exp \left(-{\displaystyle \frac{x^2+y^2}{{\omega }_f^2}}\right)$ is the mode field distribution of the flat end fiber, and ${\omega }_f$ is the mode field radius.

Matlab is used to optimize three variables—the half wedge angle θ, top arc radius r, and axial distance z—through the variable control method. This method of controlling variables is used to study the relationship between the coupling efficiency of the lens fiber and the wedge angle θ, top arc radius r, and lateral distance z from the laser chip to the wedge-shaped fiber of the wedge-shaped lens fiber. When studying the relationship between the wedge angle and coupling efficiency of wedge-shaped lens fibers, r = 3 μm and z = 6 μm are set as parameters. When studying the relationship between the top arc radius and coupling efficiency, θ = 45° and z = 6 μm are set as parameters; When studying the relationship between the lateral distance of the laser chip from the wedge-shaped fiber and the coupling efficiency, θ = 45° and r = 3 μm are set as parameters. Figure 2 shows the variation curves of coupling efficiency with respect to θ, r, and z. During the simulation, Gaussian beams are approximated as laser beams, resulting in a coupling efficiency of over 90%. For each variable, the point at which the coupling efficiency reaches its maximum can be found. Thus, through simulation, it can be determined that the optimal parameters for the fiber coupling of semiconductor lasers are θ = 45°, r = 3.25 μm, and z = 5.65 μm.

3. Coupling Experiment Results and Discussion

The semiconductor laser used in our experiment was a 980 nm ridge waveguide semiconductor laser. Its special ridge structure can suppress high-order modes of the optical field and achieve reliable transmission of the fundamental mode. The LD output laser has a slope efficiency of about 85%, a fast axis divergence angle of 37°, a slow axis divergence angle of 12°, and a chip ridge width of 4 μm. Figure 3 is a schematic diagram of the variation in LD output optical power with current. During the experiment, the optical fiber was fixed using two sets of fixtures. The rear fixture mainly fixed the metallized part of the optical fiber, while the front fixture mainly clamped the bare core part of the optical fiber to keep it horizontal. Two sets of fixtures can adjust the position of the fiber from eight directions—up, down, left, right, front, back, clockwise rotation, and counterclockwise rotation—maintaining the horizontal relationship between the fiber wedge surface and the workbench. The fiber optic end is connected to the power meter, and by adjusting the relative position of the lens fiber and LD, the power meter can display the current coupling power in real time. As shown in Figure 4, Figure 4a is a schematic diagram of fixing optical fibers with two sets of fixtures at the front and rear, and Figure 4b shows results derived from testing the laser power after coupling. The ratio of this power to the output power of the laser chip is the coupling efficiency of the coupling system. The key components of the experimental plan include the optical module, clamp 1, clamp 2, the wedge-shaped lens fiber, the LD laser chip, and UV glue curing, which have been labeled in Figure 4. Figure 5a,b show the laser far-field profile distributions of the slow and fast axes at 80 mA, respectively. Figure 5c,d show the laser far-field profile distributions of the slow and fast axes at 780 mA, respectively. As the current increases, the far-field divergence angles of both the fast and slow axes increase.

Our experiment of coupling semiconductor lasers with optical fibers tested the output laser power values of LD chips at different input currents; subsequently, we recorded the output power of the laser when passing through the lens fiber at different input currents. Figure 6 is a schematic diagram of the relative position between the lens fiber and the ridge semiconductor laser. From the figure, it can be seen that the distance between the lens fiber and the semiconductor laser-emitting surface is about 3.5 μm, which is approximately equal to the radius of the circular arc at the front end of the lens fiber. As shown in Figure 7, the power output of the laser through the lens fiber was measured at currents of 80 mA, 180 mA, 280 mA, 380 mA, 480 mA, 580 mA, 680 mA, and 780 mA. From the graph, it can be seen that the maximum coupling efficiency of the coupled system can reach 87.1%, and the average coupling efficiency can also reach 84%. However, after UV glue fixation and tail pipe soldering, the coupling efficiency is reduced, which is caused by the thermal stress generated by the UV glue curing and tail pipe soldering processes exerting an effect on the optical fiber. The welded optical module needed to undergo temperature cycling experiments to verify the stability of the coupling system. In the heating stage, the temperature was increased from room temperature to 85 °C at a rate of 5 °C/min; during the high-temperature maintenance phase, the temperature was maintained at 85 °C for 2 h; during the cooling phase, cooling down to −40 °C took place at a rate of 3 °C/min; and during the low-temperature holding phase, the temperature was maintained at −40 °C for 2 h. We repeated these steps 10 times. From Figure 8, it can be seen that the coupling efficiency changed before and after the temperature cycling experiment. The average coupling efficiency of the first seven groups of devices in the temperature cycling experiment was 83.4%, and the average coupling efficiency of the devices after the temperature cycling experiment was 81.7%, with a decrease in coupling efficiency of 1.66%.

Although the highest coupling efficiency value achieved by the device actually prepared is 81.7%, there is still a significant gap compared to the previously simulated coupling efficiency of over 90%. The main reason for this is that the Gaussian beam is processed as the transmission beam of the semiconductor laser, and the actual mode field form is very complex. Once the coupling system is fixed by UV glue curing and tail pipe soldering, the thermal stress caused by it will cause the position of the optical fiber to shift. This requires continuous adjustment of the position of the optical fiber during the UV glue curing process to approach the output power before welding. After the coupling module is cured using UV glue, the curing stress will cause a displacement of the relative position between the lens fiber and the semiconductor laser, resulting in a decrease in coupling efficiency. The offset between optical fibers and semiconductor lasers includes axial, vertical, and angular offsets. Whether it is vertical or angular offset, a small offset can have a significant impact on coupling efficiency. Under vertical axis offset, the decrease in coupling efficiency caused by slight offset in the x direction (fast axis direction) is greater than that caused by slight offset in the y direction (slow axis direction). Under angular displacement, the coupling efficiency in the x and y directions decreases to a similar extent. This requires the wedge-shaped lens fiber to be fully aligned with the semiconductor laser during actual coupling to avoid a decrease in coupling efficiency caused by axial (z direction), vertical, and angular offsets.

The highest coupling efficiency achieved in the experiment was 81.7%, but there is still a certain gap compared to the coupling efficiency values from abroad. The reason for this is that the divergence angle of LD chips abroad is often less than 30°. In this experiment, the fast axis divergence angle of the LD chip was 37°, and the slow axis divergence angle was 12°. The high divergence angle of the fast axis has a significant impact on the coupling efficiency. In addition, welding processes and fiber end face fabrication processes also have a certain impact on coupling efficiency. In subsequent work, it will be necessary to change the process parameters, optimize the welding process, and achieve higher coupling efficiency, so that the prepared devices can be applied to more fields.

4. Conclusions

A model for the coupling of a semiconductor laser and wedge-shaped lens fiber was designed, and the results showed that when the coupling system simultaneously satisfied mode field matching and phase matching, the coupling efficiency was relatively high. Optimal values of three independent variables were obtained through theoretical analysis and software simulation, with a half wedge angle of 45°, a top arc radius of 3.25 μm, and an axial distance of 5.65 μm. The optimization results are relatively accurate. The experiment used a 980 nm semiconductor laser coupled with a single-mode wedge-shaped lens fiber, achieving a maximum coupling efficiency of 81.7% at a current of 100 mA. In future work, it will be necessary to study the thermal stress caused by UV glue curing and tail pipe soldering, find suitable process parameters, and obtain stable and reliable coupling modules.