汽车倒泊防撞报警器的设计

utilize mirror face at each end of the waveguide. To achieve this process, waveguide mold equipped with 45° faces at each end of the mold is needed to form the vertical coupling structure in a single fabrication step. We made a 12 channel silicon waveguides mold, which has 45° mirror face at the ends of each waveguide. The dimension of the waveguide is 50 μm width and 50 μm height and the waveguide layout pitch is 250 μm and the length is 7 cm. With this mold, we performed UV embossing to make embedded type waveguides.

To fabricate a 12 channel silicon waveguides mold, we etched silicon substrate with KOH-saturated isopropanol solutions in two steps: First is to make a vertical coupling path for the waveguides and the other is to make 45° slope for the fabrication of mirror faces. First, a metallic mask is patterned on the silicon substrate and the silicon is vertically etched with KOH to form a waveguide pattern. In the next step to form 45° slope, a thin film of SiO2 is grown on patterned waveguide. And photoresist is patterned at the end of the each waveguide structure and the ends of the waveguides are etched with KOH-saturated isopropanol solution to form 45° slope. After the SiO2 is stripped, the process of fabricating silicon mold equipped with 45° mirror is completed.

We fabricated 12 channel embedded waveguide array by UV embossing using the

prefabricated silicon mold. Waveguide fabrication process is shown in Fig. 1. UV curable polymer, which is used as cladding layer with index as 1.45 at 850 nm wavelength, is dropped in the hollow cavity of a transparent substrate such as PDMS template. After silicon mold is pressed on template the UV light is irradiated. Silicon mold is detached and metallic film is coated on the 45° slope at the end of the waveguide to enhance coupling efficiency. And then the core polymer is dropped and a flat substrate is covered and pressed onto the core material which is also UV curable polymer with refractive index of 1.47 at 850 nm wavelength. The UV light is irradiated once again. After the upper and lower templates are detached, we can get a complete array of polymer waveguides with built-in 45° mirror face at each end of the waveguide. 3. Microlensed VCSEL

One of the approaches to collimate the light from VCSEL arrays to the waveguide is the use of microlenses [9] and [10]. This method offers an increase in coupling efficiency and alignment tolerance. The volume of a polymer drop to fabricate these lenses is approximately a few tens of picoliters. We are able to control the size of the microlenses by controlling the amount of the polymer drops and by controlling the viscosity of the materials. UV curable polymer is used for inkjetting, of which the viscosity and the refractive index are 300 cps and 1.51 at 850 nm wavelength.

Shows one of the microlensed VCSEL array and microlensed VCSEL has a microlens formed by the inkjetting method on the aperture of VCSEL. Inkjetting of UV curable resin on the VCSEL, lens material is aligned automatically on the aperture of VCSEL. Shows a view of the system where the output power from the microlensed VCSEL arrays is measured for their

divergence. The divergence angle of the laser light from the VCSEL is shown to become narrower by using microlenses by the collimating effect pf the light from VCSEL. Because of the microlens, the higher order modes from the VCSEL are suppressed by the cavity effect [10]. The emitted output from the VCSEL cavity is reflected back by microlens layer and is focused on the VCSEL cavity. During this process, the divergence angle of the VCSEL is reduced. In this case, the divergence angle of the VCSEL decreased from 18° to 15° after forming microlens. We conducted simulation study about the coupling efficiency between VCSEL and the waveguide by using the ray tracing method. As the divergence angle of the VCSEL was put into the calculation, the coupling efficiency of the VCSEL with microlens was found to be 0.44 dB is 0.96 dB which were better than that of VCSEL without microlens as ?1.40 dB. Here dimension of waveguide is 50 μm width, 50 μm height and 7 cm length.

Refractive indices of the core and the cladding are 1.47 and 1.45, respectively, at 850 nm wavelength. The distance between the VCSEL and the waveguide is 100 μm.

4. Passive alignment

Solder ball array and pin array are placed on the electrical sub-boards to bond the O-PCB and the electrical sub-boards with high precision. For precision alignment, solder ball array in diameter of 450 μm are used to thermally attach to the chip module. The solder ball array can be used for vertically alignment between the main O-PCB and the sub-boards within a mismatch below 10 μm. The size of the solder ball is 500 μm on average with standard error of ±5 μm.

Two types of pin arrays are used. One array with diameter of 1 mm is for alignment and the other with diameter of 200 μm is for electrical interconnection. The 1 mm pin array is used for lateral alignment between the main O-PCB and the sub-boards. Because of the impedance match, the pin array of the electrical interconnection is limited. Similar to solder ball array alignment tolerance of the pin array, about 10 μm, depends on variation of diameter of pin. The size of the pin is 1 mm on average with standard error of 10 μm.

We conducted simulation study about the coupling efficiency between the VCSEL-waveguide pair and the waveguide-PD pair by ray tracing. With the variation of misalignment of x, y, and z axis we calculated the coupling efficiencies. From the calculation we obtained the total coupling loss within 2.30 dB for the worst case of having position errors as large as 10 μm in the x–z axis and in the y axis, respectively. For example, when the position misalignment is 10 μm in the x–z axis and in the y axis, the coupling loss between

VCSEL-waveguide is 1.59 dB and the coupling loss between VCSEL-waveguide is 0.71 dB. From the previous results, one can achieve the alignment between solder ball array and pin array can be achieved for alignment between main O-PCB and sub-boards with precision as about 10 μm in x–z axis and in y axis, respectively. Here the dimension of the waveguide is 50 μm width and

50 μm height. The refractive indices of the core and the cladding are 1.47 and 1.45, respectively, at 850 nm wavelength. The distance between the VCSEL and the waveguide is 100 μm in the y axis.

5. Optical interconnect modules

We demonstrated the use of optical interconnection module for the assembly of O-PCB having four 2.5 Gbps channels. The optical interconnection module, which includes E/O (electrical/optical) conversion unit, is attached to the O-PCB with solder ball. The solder ball bonding is designed to accomplish the alignment between the waveguide structure and the electric circuit with high precision. The O-PCB prototype consists of main body of O-PCB and two electrical sub-boards. The main O-PCB has embedded waveguide which is the medium of optical interconnection. The two sub-boards are used for electrical-to-optical (E/O) or optical-to-electrical (O/E) conversion. The VCSEL array and the PD array are bonded to interconnect the waveguide to the bottom of the sub-board. The driving circuits are placed on the opposite side to VCSEL array and PD array. The power, ground and other electrical control signal are supplied through the pin grid. The main O-PCB is placed on the E-PCB within a rectangular area of 70 mm × 10 mm at the center of the E-PCB.The overall planar size of the O-PCB is 200 mm × 80 mm and thickness is 1 mm. The UV embossed waveguide including the 45° mirror for vertical coupling is inserted into the E-PCB and is glues with UV-epoxy. The sub-boards including VCSEL array/PD array are designed and fabricated using conventional analysis of microstrip line.

We finally evaluated the quality of the optical interconnection module. First, we tested the waveguide array with 45° mirror face. The total losses of the waveguide include the

propagation loss, the coupling loss, the 45° mirror loss and the insertion loss. And an average total loss is 7.9 dB for a waveguide of 7 cm length and their variation is within 1 dB. For the worst case, in 12 channel, the total loss was 8.9 dB.

To demonstrate the data transmission performance, we utilized aligned optical

interconnection module .A 2.5 Gbps psudo-random binary system (PRBS) pattern were put in to the VCSEL driver via the pin grid and the electrical output signal of the module were connected to a wide-band oscilloscope. An eye pattern of 2.5 Gbps transmission was clearly observed without any significant distortion.

6. Conclusion

We performed micro-fabrication for optical interconnection module. The optical waveguide array is fabricated by UV imprint process. The 45° mirrors faces are fabricated as an integrated part of the silicon waveguide mold for low-cost one-step processing. We fabricated microlensed VCSELs by micro-inkjetting method and found a significant increase in the

improvement of the coupling efficiency reaching 0.96 dB. Use of solder ball array and pin array for the alignment between the O-PCB and the sub-boards could be achieved with a precision below 10 μm in the x–y axis and in the z axis. This passive alignment is designed for coupling loss induced by of misalignment within 2.3 dB in total. We designed and fabricated a 2.5 Gbps × 4 channels optical interconnecting micro-module for optical printed circuit board (O-PCB) application. This optical interconnection module transmits data at the rate of 2.5 Gbps per channel.

This work has been supported by the Engineering Research Center Grant No. R11-2003-022 for OPERA (Optics and Photonics Elite Research Academy).