Parallel
optical module QSFP+ PSM and
AOC products are mainly based on multi-mode fiber optical interconnection technology, it has the advantage of high-bandwidth, low loss, no crosstalk and electromagnetic compatibility and matching problems, and has been gradually replaced copper-based electrical interconnect products and applications in between cabinets, high-speed interconnect plate frame and connect the distance up to 300 meters in
OM3 fiber. Meanwhile, in order to apply to more long-distance transmission solutions, PSM parallel optical modules have emerged, mainly used in single-mode fiber transmission FP laser 2KM, DFB 10KM transmission applications, it is more difficult than having a multimode interconnect technology.
First, the parallel multi-mode fiber coupling technology accumulation and breakthrough
Parallel
optical module interconnect technology the optical coupler is a big problem, compared to the popular Plastic lens solution Gigalight adopted a more simple and efficient and reliable fiber coupling technology. This technology uses a near 45 degree angle (Fig.1) of the total reflection face VCSEL emitting reflected light path is formed products needed, but such a simple application of the principle, Gigalight in actual production to be the exact product The theoretical calculations and rigorous experimental verification. For the reflection angle of choice, we passed to the lasing VCSEL optical properties (Figure 2), the use of optical simulation software analysis (Figure 3), and the principle of total reflection of the light incident angle VCSEL determine our optimal angle of reflection at the interface approaches a value of 45°at the same time through a special fiber surface material processing (Figure 4), the optical fiber coupling efficiency from the initial design (43% to 47%) raised to (75% -80%).
Second, parallel single-mode fiber coupling product realization
To achieve long distance transmission must use single-mode optical fiber dispersion loss, but the single-mode optical fiber and semiconductor to achieve high coupling efficiency of the light to be emitted from a semiconductor laser field shaping incident light field and the fiber intrinsic optical field Possible matches maximized. We can use the optical coupling lens (i.e., added between the lens and the optical fiber laser, FIG.5) and an optical fiber directly coupled (i.e., fiber laser source and direct coupling, FIG.6) in two ways. But the optical lens coupling, due to the coupling system is separate from the optical elements of the semiconductor laser, a lens, coaxial quasi-linear optical fiber between the three requirements are very high. To ensure coaxial collimation requirements, often need to make a special surface shape, but also an array of lenses, which makes the high cost, and space requirements of the larger product, is not conducive to miniaturization of the package. Our technical accumulation process based on the parallel multi-mode optical module products, the use of optical fiber direct coupling to achieve a parallel single-mode product development and production. In this process, we have experienced flat end of the fiber coupling and tapered fiber coupling. The flat end of the fiber coupler production process is simple, easy to implement, but because of the light emitting area of the light source and single-mode fiber core diameter area and a light source divergence angle and fiber numerical matching relation severe mismatch, resulting in low coupling efficiency, light reflection is likely to cause large optical eye diagram Scatter affect the transmission quality. Therefore, we used a fiber-optic splice connection arcing ball end face of the optical fiber obtained (Figure 7) coupling. Using ball coupling end face, not only help to improve fiber coupling efficiency, and help to change the reflected light is reflected path, help block the light path of debugging. Figures 8 and 9 for the optical eye diagram comparison level end of the fiber end face of the optical fiber coupling and coupling ball.
Third, COB (CHIP ON BOARD) technology to improve the accuracy
Existing programs for parallel optical modules are based on VCSEL array and the fiber array coupling scheme, using CHIP ON BOARD process to achieve cost-efficient production requirements. In this process, the chip placement accuracy of a direct impact on the efficiency of light coupling. We passed comparison of the data analysis, placement accuracy deviation light on the impact of the loss (Figure 10, Figure 11). From the figure we understand, multimode coupling can guarantee the accuracy of ±5um coupling efficiency. According to the study, we used a high-precision chip technology, the guarantee of quality products coupled.
In order to ensure placement accuracy and high efficiency of production, we have introduced automated precision placement equipment (Fig.12), to ensure accuracy within ±3um and automatic Wire BOND equipment.
Fourth, the necessary thermal design to ensure good use of parallel optical modules
As we all know, parallel optical module is highly integrated device products, increase the number of internal multi-channel integration, so that power and I/O pins greatly increased. For example, in QSFP- PSM module, four drive IC's power consumption accounts for more than nearly 50% of the entire module of the heat. QSFP in a small space, heat is a big problem. If the long-term cooling, can cause aging of the internal components and shorten life. And more directly reflect the laser power will increase as the temperature changes, the characteristics of the laser (Figure 13) shows: Ith becomes larger, in order to get sufficient transmission power, the module will increase the bias current so that the overall power consumption of the module directly increase, creating a vicious cycle, which is very unfavorable for the module. Therefore, in the product development cycle, we will be concerned about thermal design of the product. Layout and thermal path between the chips inside the module attention is good. In product design has been rigorous thermal design (Fig.14), and then to revise and improve the cooling effect (Fig.15) by experimental thermal testing.
