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Abstract
In this report, we present a stepped laser-ablation method for the fabrication of concave micromirrors in rectangular optical waveguides. The numerically simulated vertical coupling loss of the reflection of the concave micromirror can be reduced to 1.53 dB. The processing parameters of the utilized excimer laser, such as the step number, width, and depth, were optimized to fabricate the concave micromirrors. After the thermal reflow process, the measured curve of the circular concave micromirrors obtained using a 3D optical profiler agreed well with a standard circle with a surface roughness of 39.56 nm. Furthermore, vertical coupling for 62.5 µm MMF revealed that the loss of the circular concave micromirror coated with a 50 nm thick Au film is as low as 1.83 dB, corresponding to a high coupling efficiency of 65.61%. This new, convenient, and efficient fabrication technology for the fabrication of concave micromirrors can be applied to vertical coupling for optical printed circuit board (OPCB) interconnection technology.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With the development of broadband communication [1], big data processing [2], and high-performance computing (HPC) [3], the traditional printed circuit board (PCB) interconnection technology has become a bottleneck in high-speed bandwidth capacity beyond 56 Gbps [4]. In recent years, optical printed circuit board (OPCB) interconnection technologies [5–11] have evolved to play a critical role in advanced communication equipment. This is attributed to their advantages such as high bandwidth, low energy consumption, and the absence of electromagnetic interference. The coupling technology that is used between optical waveguides and fibers (single-mode fiber, SMF or multi-mode fiber, MMF), or between optical waveguides and optical devices, is a key technology [12–28], especially for vertical coupling [14–28]. Vertical coupling is important for the integration of optical devices, full utilization of inter-board space, flexible design of photoelectric systems, and numerous other applications. At present, there are many available methods for vertical coupling such as using 45° tilted micromirrors [13–21], embedded coupling elements [19,22,27–28], soft waveguide [23–24], bending fibers [25], and waveguide gratings [26]. The use of 45° tilted micromirrors is widely implemented because it is simple, highly efficient, and the mirrors are easy to fabricate. The application of photolithography [14–17] in the fabrication of these mirrors results in several advantages that include a high precise structure profile and smooth optical surfaces of the micromirrors. However, some practical processes including the precise control of the tilt angle and accurate positioning are quite challenging using this fabrication approach. Recently, laser-ablation technology [18–20] has proven to be very promising because of its simple process, accurate positioning, and flexible operation. Currently, the vertical coupling loss using 45° tilted micromirrors can be less than 0.36 dB at 850 nm [18]. In this case, the numerical aperture (NA) and the diameter between the optical devices should be strictly matched.
However, in some cases, it is difficult to match the NA and the diameter for practical devices. Therefore, specially designed optical coupling devices embedded in concave micromirrors [27] or convex lenses have been developed [19] for efficient vertical coupling. However, these optical devices can also cause great difficulties in the process of fabrication, positioning, and packaging. Direct fabrication of a concave micromirror in a rectangular optical waveguide (ROW) is desirable. However, the fabrication of a concave micromirror in a ROW based only on deep proton writing (DPW) [28] has been reported. The fabrication process is very complex, and its efficiency is low. Recently, the fabrication of concave or convex microlenses based on CO2 [29,30] or femtosecond lasers [31,32] has been demonstrated, which is very important for fabricating concave micromirrors. However, further investigation using these two methods is needed for concave micromirrors fabricated in a ROW.
In this report, a new method for the fabrication of concave micromirrors in a ROW is presented, based on stepped laser-ablation technology. First, an analysis model for circular concave micromirrors is established using the ray-tracing method and the vertical coupling characteristics are discussed in detail. A fabrication method for the concave micromirrors is then developed, where each arc section is composed of several steps. Subsequently, circular concave micromirrors are fabricated using an excimer laser based on optimized parameters. The roughness and the curves of the circular concave surface are then measured using a 3D optical profiler. Finally, the experimental results for circular concave micromirrors coated with Au film agree well with theoretical predictions. The method presented in this work can be applied to vertical coupling between optical devices with high efficiency.
2. Vertical coupling characteristics based on concave micromirrors
Generally, a circular concave micromirror has excellent convergence properties compared to a 45° tilted plane micromirror. Based on geometrical optics, the analysis model for the ray-tracing method is as shown in Fig. 1. The O-point (0, 0, 0) is the origin of the coordinate system. A random ray from the B-point (xB, yB, zB) on the end-face of the ROW output is reflected by the 45° tilted plane micromirror or a circular concave micromirror at the C-point (xC, yC, zC), then enters the MMF at the D-point (xD, yD, zD). The QC-line is the normal line of the reflective surface. In addition, the Q-point (xQ, yQ, zQ) is the center of the circle and R is the radius of the concave micromirror. φ is the angle between the DC-line and the y-axis. The thickness of the ROW H is 150 µm. The concave micromirror has only one concave surface along the z-direction, and the plane surface is along the x-direction.
Fig. 1. Analysis model for the ray-tracing method. (a) 45° tilted plane micromirror, (b) circular concave micromirror.
To allow CD-rays to enter the MMF, the core diameter and NA of the MMF must satisfy a particular relationship, which is written as follows:
It is necessary to determine the distribution of random rays at the MMF input end-face. To detect the light pattern consisting of random rays, the detector is set at the ROW output end-face, the MMF input end-face, and the MMF output end-face, respectively, as shown in Figs. 2(a) and 2(b). In simulation, the light source emits tens of thousands of random rays with the divergence angle range of 17.44°. And it is beyond the accepted light angle range limited by the waveguide NA. The cross section size of the rectangular light beam from the ROW output end-face in Fig. 2(c)–① is equal to that of the ROW. After vertical reflection by the 45° tilted plane micromirror, an enlarged Gaussian light beam with a diameter of approximately 80 µm is formed at the MMF input end-face, as shown in Fig. 2(c)–②. It mismatches the diameter (less than 80 µm) of the MMF core. Moreover, it depicts an elliptical light beam with a short axis length of approximately 30 µm, that is reflected by the concave micromirror in Fig. 2(c) – ③. It marginally matches the diameter of the MMF core along the z-direction. The formation of an elliptical light beam is due to the special structure of the concave micromirror with only one concave surface along the z-direction. The light beams at the MMF output end-face due to the two vertical optical reflectors are shown in Fig. 2(c)–④ and Fig. 2(c)–⑤. The vertical coupling loss Lc can be expressed as follows:
where Iout is the intensity of rays from the MMF input end-face, and Iin is the intensity of the rays from the ROW output end-face.Fig. 2. Simulation of transmission characteristics for the vertical coupling system. (a) with a 45° tilted plane micromirror, (b) with a concave micromirror, (c) light beams at different end-faces.
When H = 128 µm, the vertical coupling loss Lc of the circular concave micromirrors with radius R is calculated as shown in Fig. 3. Lc always decreases initially, then slowly increases with the increase in R. This is attributed to the matching characteristics of the NA and the diameter between the light beam induced by micromirrors and the MMF core. When R is 350 µm and 300 µm, Lc has minimum values of 3.36 dB for a receiver of 50 µm MMF and 1.53 dB for a receiver of 62.5 µm MMF. Compared to the values of 4.56 dB and 2.73 dB for the 45° tilted plane micromirrors, the vertical coupling characteristics of the circular concave micromirrors are significantly improved.
3. Fabrication principle of concave micromirrors based on stepped laser-ablation
We hereby introduce an effective method for the fabrication of concave micromirrors with only one concave surface along the z-direction. The total processing area with a length L is divided into m identical sub-processing areas (M1, M2, …, Mm), as shown in Fig. 4. The corresponding arc is also divided into m arc sections, and each arc section is composed of multiple steps. For No. i (i=1, 2, …, m) arc sections, Ni, wi, h, Hi correspond to the step number, width, depth, and maximum depth, respectively. Thus, wi and h can be expressed as follows:
Fig. 4. Schematic of the concave micromirror. (a) Concave shape, (b) step-shaped concave micromirror.
There are the relationships for N1>N2>…>Nm, w1<w2<…<wm, and H1<H2<…<Hm. The larger the sub-processing area number and the step number, the higher the fitness between the step-shaped curve and the standard arc.
Based on the stepped laser-ablation method, the fabrication process for the concave micromirrors is depicted in Fig. 5(a). Figure 5(b) displays the set of fabrication parameters including the processing area, rectangular laser, moving path, and moving direction. The size of the rectangular laser (L1×L2, L1=L) is the same as that of the m sub-processing areas (M1, M2, …, Mm). The length of the moving path A1Am+1 is equal to that of the rectangular laser. The starting point A1 is located at the center of the rectangular laser. The key working principle is that the rectangular laser moves a fixed distance of Di along the moving direction each time after etching. Thus, there is an overlapping area after etching is performed twice. Finally, the steps appear after the etching process. For different moving path sections (A1A2, A2A3, …, AmAm+1), Di is also different. There is a relationship given by Di = wi.
Fig. 5. Stepped laser-ablation method. (a) Diagram showing stepped laser etching, (b) fabrication parameters.
The parameter Si is defined as the ratio of the rectangular laser length L1 and the laser moving distance Di, and can be expressed as:
Ni is assumed to be 5, 4, …, 2. Therefore, Si is calculated as 20, 16, …, 8, respectively. Figure 6(a) depicts the fabrication process of the stepped laser-ablation on area M1. To obtain a concave micromirror with only one concave surface, the metal block is placed on the right of the neighbor area Mm. When the rectangular laser performs an etch, a step with a depth of h is formed in area M1. The rectangular laser then moves a distance of D1 = L/20 along the moving direction, and 4/5 of the area M1 is etched again. Thus, the depth of the overlapping area changes to 2h. There are 2 steps in area M1. As the etching process is implemented, the ratio of the etching area changes to 4/5, 3/5, 2/5 and 1/5. After etching in area M1, steps N1 = 5, H1 = 5h and w1 = L/20 appear. Furthermore, there is a flat surface with a depth of 5h in area M2, …, Mm because these areas are always etched by the rectangular laser. For other areas, the etching process is similar to that of area M1. However, the parameter Si and the steps obtained are different, as shown in Fig. 6(b) and Fig. 6(c), respectively. Finally, the etching shape assumes a concave surface consisting of many steps.
Fig. 6. Etching process of the stepped laser-ablation method. (a) appearance of area M1 after etching, (b) appearance of area M2 after etching, (c) appearance of area Mm after etching. The inset depicts the etching process for etching area M1 in detail.
4. Experiment and analysis
4.1 Fabrication of circular concave micromirror
The polymer multi-mode ROW (NA=0.3) was fabricated on a standard PCB using a photolithography method. The refractive indices of the core material (LightLink XP-6701A) and the refractive indices of the cladding material (LightLink XH-100145) are 1.51 and 1.48, respectively. Figure 7(a) shows the microscopic morphology of the ROW end-face with a core size of 50 µm × 50 µm obtained using a 3D optical profiler (Sensofar-Tech, SNEOX, Spain).
Fig. 7. (a) Microscopic morphology of the ROW end-face, (b) 3-dimension microscopic morphology of etching square groove, (c) the relationship between etching depth and etching number.
Due to the higher absorption rate of ultraviolet light, the molecular bond of the polymer is broken, and the polymer is wiped out [33]. Therefore, an ArF excimer laser (LSV 3, OPTEC, Belgium) with a working wavelength of 193 nm, a pulse duration of 5 ns and a repetition rate of 50 Hz was chosen to fabricate the circular concave micromirror in the ROW. Taking the ROW thickness into account, the size of the rectangular laser was chosen as 128 µm × 128 µm, and the intensity of the rectangular laser is more uniform. Figure 7(b) shows the 3-dimension microscopic morphology of the etching square groove. The relationship between the etching depth and the etching number is shown in Fig. 7(c). When the laser pulse energy was 4.5 mJ, 5.0 mJ and 5.5 mJ, the minimum etching depth was measured as 0.101 µm, 0.115 µm and 0.130 µm respectively. During fabrication process, the pulse power, energy density and the time of laser ablation with the average moving speed of 3.65 µm/s were 1 MW/pulse, 5 J/cm2 and 35 s, respectively, with the laser pulse energy as 5 mJ. Moreover, the debris generated in laser-ablation process could affect laser beam propagation thus reducing fabrication accuracy [32]. And debris was in-situ cleaned up by the blowing equipment. As such, concave micromirror with better surface quality was obtained.
Based on the simulated results in Fig. 3, the radius R of the circular concave micromirrors is chosen as 300 µm. The expression for the standard circle can be written as y2+z2 = 3002. It is assumed that the total length along the z-direction and the maximum depth along the y-direction are both 128 µm, which matches the size of the rectangular laser. Therefore, when m = 10, the optimization laser processing parameters (Hi, Ni, Si) can be calculated, as shown in Table 1.
Table 1. Optimization of laser processing parameters
Figure 8(a) shows scanning electron microscopy (SEM) images of the circular concave micromirror (by SU-1500, HITACHI). This result agrees well with the design configuration in Fig. 6(c). Figure 8(b) further describes the 3-dimension morphology of a circular concave micromirror using a 3D optical profiler. The curves of the circular concave micromirror and the designed circle equation y2+z2 = 3002 are plotted in Fig. 8(c). The results indicate that there is improved agreement. And it is identical for the curves before and after thermal reflow, indicating the small deforming effect induced by thermal reflow process. The width and depth of the circular concave micromirror after thermal reflow are 127.65 µm and 127.72 µm, respectively, which is close to the design values.
Fig. 8. Microscopic morphology of the circular concave micromirror. (a) SEM images of the circular concave micromirror, (b) image of circular concave micromirror obtained using a 3D optical profiler, (c) comparison of the curve of the fabricated and the designed micromirrors.
The roughness varies randomly and reflects the degree of smoothness of the sample surface, which can lead to significant scattering loss [34,35], which can influence the reliability and stability of optical devices. The roughness of the circular concave micromirror surface was measured as 54.78 nm over an area of 30 µm × 30 µm using a 3D optical profiler, as shown in Fig. 9(a). Thermal reflow is a very effective method to reduce the roughness. As such, the scattering loss induced by its rough surface can be further reduced. After investigation on the roughness characteristics of waveguide cores under different temperatures for different time periods, the optimized thermal reflow temperature is 170 °C, and the optimized thermal reflow time period is 30 min. In addition, the transmission loss is further reduced after thermal reflow experiment. Compared to the surface morphology identified using the red dashed line, it became smoother after thermal reflow [36] at 170 °C for 30 min, as shown in Fig. 9(b). Correspondingly, the roughness decreased to 39.56 nm, and the scattering loss also decreased significantly.
Fig. 9. Roughness measurements of the circular concave micromirror surface. (a) before thermal reflow, (b) after thermal reflow.
4.2 Vertical coupling experiment
A diagram of the vertical coupling test system based on circular concave micromirrors is shown in Fig. 10. The optical signal with a wavelength of 850 nm propagates from a light source (S4FC852, Thorlabs), and is coupled into the SMF, followed by the ROW with a length of 20 mm. After vertical reflection by the circular concave micromirrors, the optical signal is coupled into the MMF. Finally, it is detected by a dual-channel optical power and energy meter (PM320E, Thorlabs). An enlarged detailed view of the vertical coupling component is shown in Fig. 10(a).
Fig. 10. (a) Diagram of vertical coupling structure, (b) setup for testing vertical coupling, and (c) vertical coupling test system.
Figure 10(b) shows the experimental setup for testing of vertical coupling. The image shows that the red light travels into the SMF and the MMF, which indicates that the circular concave micromirror can achieve vertical optical coupling. To significantly increase the reflectivity, the circular concave micromirror surface was coated with a 50 nm thick Au film. Because of the Fresnel’s law, when coated with Au film, incident light can be totally reflected on the concave surface, thus the coupling loss is reduced. The inset in Fig. 10(b) depicts the image of the convergent light spot reflected by the circular concave micromirror. This mirror has superior convergence and reflection performance. To further decrease the coupling loss, index-matching liquid was applied to the connection between the SMF and the ROW. Experimental results indicated a decrease of coupling loss as 0.5 dB. Thus, the coupling loss in this region can be neglected.
As shown in Fig. 10(a), the system loss Lsum mainly arises from the transmission loss Lw of the ROW, the Fresnel reflection loss Lf1 at the interface of the ROW output end-face and air, the loss Lc induced by the circular concave micromirror, and the Fresnel reflection loss Lf2 at the interface of the MMF input end-face and air. The system loss can be written as.
As the receiver, the NA of the 50 µm MMF and the 62.5 µm MMF was calculated as 0.2 and 0.25, respectively, according to the measurement results using an optical fiber index analyzer (S14, Photon Kinetics Inc., USA). MMF was mismatched to the NA (0.3) of the ROW. Due to its convergence properties, the circular concave micromirrors can improve the matching performance of the NA and the diameter between optical devices. Correspondingly, the coupling efficiency between the circular concave micromirror and the MMF also increases significantly.The measurement results in Fig. 11 show that Lc for 62.5 µm MMF is less than that for 50 µm MMF. This is because the diameter matching performance of 62.5 µm MMF core and the ROW core is better than that of the 50 µm MMF and the ROW. Moreover, it is demonstrated that the vertical coupling loss of circular concave micromirrors is lower than the theoretical results for the 45° tilted plane micromirrors. This value is higher compared to the theoretical results obtained using circular concave micromirrors. Considering the No. 3 waveguide channel in Fig. 11(b) as an example, based on the aforementioned test method, Lsum is 2.31 dB. Lw is measured as 0.14 dB by exciting the ROW with a length of 20 mm using a 9 µm SMF, followed by the collection of the transmitted light using a photodetector (S145C, Thorlabs). Lf1 and Lf2 are calculated as 0.16 dB and 0.18 dB, respectively. According to Eq. (4), Lc is calculated as 1.83 dB, corresponding to a coupling efficiency of 65.61%. It is slightly higher than the theoretical result of 1.53 dB for the circular concave micromirror, and is lower than the theoretical result of 2.73 dB for a 45° tilted plane micromirror. The main factor that influences Lc is the roughness of the circular concave micromirror surface. The next important aspect of this research is to further reduce the roughness of the concave micromirrors by optimizing the fabrication and the thermal reflow processes.
Fig. 11. Vertical coupling loss measurement results. (a) 50 µm MMF as the receiver, (b) 62.5 µm MMF as the receiver.
Furthermore, the 45° tilted plane micromirror can be fabricated based on the stepped laser-ablation method. And the arc section m for the 45° micromirror is 1. According to Eq. (5) and Eq. (6), the parameters N1 and S1 are determined as 1110. Then thermal reflow process was conducted to realize the high reflection efficiency. After thermal reflow process, the minimum coupling loss based on the 45° tilted plane surface of 3.4 dB was larger than the one based on circular concave micromirror, which was 1.83 dB.
5. Conclusion
In this work, a stepped laser-ablation method for the fabrication of concave micromirrors in a ROW is proposed and demonstrated for low-loss vertical coupling. Simulated results show that the concave micromirror can reduce vertical coupling losses to a greater extent compared to 45° tilted plane micromirrors. By optimizing the processing parameters of an excimer laser, circular concave micromirrors with high performance were fabricated, where the measured curve agreed well with the standard circle. The surface roughness after thermal reflow decreased to 39.56 nm. Finally, the circular concave micromirror coated with a 50 nm thick Au film was used in a vertical coupling experiment. Compared to the theoretical value of 2.73 dB for a 45° tilted plane micromirror, the loss of the circular concave micromirror is only 1.83 dB, corresponding to a coupling efficiency of 65.61%. Optimization of thermal reflow parameters can further reduce roughness, which will be carried out in our near future work. In summary, this new method for the fabrication of concave micromirrors based on stepped laser-ablation is convenient and effective, and has great potential for application to OPCB interconnection.
Appendix
In above equations, β0 is the angle between OQ-line and y-axis, while α, β, γ is included angle of BC-line and x-axis, y-axis, z-axis, respectively. xB, yB and zB can be determined according to the emission position of ray at ROW endface.
Funding
Science and Technology Commission of Shanghai Municipality (16511104300); National Natural Science Foundation of China (61735009, 61875116).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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