We report a fully packaged silicon passive waveguide device designed for a tunable filter based on a ring-resonator. Polarization diversity circuits prevent polarization dependant issues in the silicon ring-resonator. For the device packaging, the YAG laser welding technique has been used for pigtailing both of the input and output fibers. Post welding misalignment was compensated by mechanical fine tuning using the seesaw effect via power monitoring. Packaging loss less than 1.5dB with respect to chip measurement has been achieved using 10 µm-curvature radius lensed fibers. In addition, the packaging process and the module performance are presented.
© 2011 OSA
Silicon-based photonic waveguides have high potential to be employed as an integrated platform for devices in many areas such as telecommunications, optical interconnects, and bio-medical sensors . Due to the unique properties of their high refractive index contrast, Si-waveguide devices can be fabricated with bending radii as small as 1.5 µm on silicon-on-insulator (SOI). Thus Si-waveguides enable high-density integrated optical circuits, which can be fabricated by standard complementary metal-oxide semiconductor (CMOS) technology. Although Si-waveguide devices on the SOI platform have many advantages, problems exist. For example, high index contrast results in large mode size mismatch between the fundamental mode of Si-waveguide and that of single mode fiber (SMF). This leads to high coupling loss. For practical use of Si-waveguide, the waveguide must be connected to SMFs with high coupling efficiency. Therefore, mode size converters and grating couplers have been intensively studied. An expanded guiding mode from the output of mode size converters and grating couplers can make fiber coupling efficiently with large lateral tolerance. A low loss polymer converter has been demonstrated by Shoji et al.  where a polymer interface was used between the Si-waveguide and SMF to reduce coupling loss. The polymer covers the tapered Si-tip. The width of Si-tip can be as small as 60nm using electron-beam (E-beam) processing. However, this method requires an e-beam process that is costly and the polymer process is not compatible with CMOS technology. Vertical grating couplers are another technology that are excellent candidates for interconnecting SMFs. The grating coupler has large alignment tolerance  and large bandwidth up to several tens of nanometers. However, even though the grating coupler has large bandwidth, the designed bandwidth can be easily shifted by the variation of collecting fiber angle and fabrication error. The wavelength shift causes higher insertion loss which is undesirable. Moreover, high polarization dependency makes such couplers unsuitable for many applications. Vertical coupling may be less favorable than edge coupling given that it does not free the top surface for electrical and thermal connections and active device . Therefore, it is preferred to fabricate edge-coupled mode size converters using standard CMOS technology without e-beam lithography. For example, in  a Si-taper tip with width of approximately 175nm was covered with a low refractive index polymer layer. The Si-tip was fabricated using 248-nm-deep UV lithography. This converter has 1.9 dB per facet coupling loss with a conventional lensed fiber. Another example of an E-beam-free process was shown in , where a Si-tip was tapered down to 80nm by means of deep UV-193nm lithography and reactive ion etching. This tapered tip was covered with SiOx layer. Less than 1 dB-loss was reported when coupling between the taper and a conventional lensed fiber.
For the packaging of preferable schemes, an assembly process development is required due to the tight alignment tolerance for SMFs. There are intensive research activities on Si-photonics packaging for enhanced coupling efficiency and tolerance, such as using epoxy welding between a grating coupler and SMF [1, 7–10]. Laser-welded packaging is a well known method for active devices (e.g., laser diode and photo diode) [11–15] and it can offer better strength, cleanliness, and long-term reliability than an epoxy welding method . However, Si-photonic packaging using laser welding technique and fully packaged Si-based passive devices have been rarely reported. In this paper, we demonstrated fully packaged Si-waveguide passive device using laser welding technique on conventional lensed fibers as input and output ports. Its packaging process, fiber coupling, and the module performance are discussed.
2. Silicon photonic integrated circuit
Si-waveguides have a large structural birefringence which gives rise to polarization dependent characteristics. The polarization dependent characteristics can be problematic when connecting the Si-photonics circuit to fiber-optic systems. In this work, the Si-waveguide filter was designed with a polarization diversity circuit as proposed in [16, 17]. Figure 1(a) shows the structure of polarization diversity circuits integrated symmetrically for in- and outputs. The ring-resonator is located in between the symmetric polarization diversity circuits. The circuit was fabricated on SOI wafer. The bottom oxide layer (SiO2) was 2µm-thick. 400nm-thick silicon top layer was covered with 1µm-thick SiO2 upper cladding layer. The device was fabricated by standard CMOS technology. The polarization diversity circuit consists of a polarization splitter, a polarization rotator, and a polarization mode converter. TE and TM modes are separated by the splitter and then the TM mode rotates to be TE mode. The polarization mode is converted by the structure as shown in Fig. 1 (b).
The polarization dependent loss (PDL) of a polarization diversity circuit integrated ring resonator can be as low as 0.5dB [16, 17]. Therefore, the packaging design of a Si- photonics chip does not need to consider the polarization issue. Figure 2 shows the fabricated ring-resonator. The micro-heater was integrated using 120nm-thick and 1µm-wide TiN wire for wavelength tuning as shown in Fig. 2. The propagation loss of the silicon waveguide is approximately 2dB/cm. Total length of the circuit is 3 mm, the ring diameter is 20 µm, and the lengths of polarization rotator and mode converter are 26µm and 25µm, respectively.
3. Coupling system design and module assembly
Figure 3 shows our packaging structure for the Si-based ring resonator. This device has tapered mode size converters covered with 1µm-thick SiO2 upper cladding layer. For efficient fiber coupling, the spot size of the Si-tapered waveguide tip must be known. In order to recognize the spot size, 3D-FDTD (finite difference time domain) simulation has been implemented as shown in Fig. 4 . The expanded spot diameter is approximately 1.8µm at 1/e 2. The width and the height of the tapered tip are 180nm and 200nm, respectively. A tapered tip width of 180nm is our experimental limit of the fabrication using 248nm lithography.
To couple efficiently, the proper curvature radius of the lensed fiber needs to be chosen . Figure 5 (a) shows the coupling efficiency as a function of lateral offset with various curvature radii, R. The case of R = 5 µm, which can be focused to a spot diameter of 2.0 µm, shows coupling loss of 0.1 dB when coupling a 1.8 µm spot from the Si-waveguide. However, the lateral tolerance is too small. If a lateral misalignment of 1.0 µm occurs, optical power of 5.6dB will be lost. For the case of R = 10 µm equivalent to a 3.8 µm-focused spot, 1.0 µm lateral offset causes optical power loss of 2.4dB. Even without offsets, the coupling loss of R = 10µm is 2.3 dB, which is 2.1 dB higher than that of R = 5µm. When R is infinite (cleaved fiber), the lateral tolerance is wide but it cannot couple efficiently for a 1.8 µm guiding mode as shown in Fig. 5(a). Therefore, we chose the lensed fiber curvature radius of 10 µm. Figure 5(b) shows the lateral, longitudinal, and angular tolerances when the 10µm-curvature lensed fiber collects 1.8 µm spot light from the Si-waveguide. The 3dB-lateral tolerance is ± 1.25 µm. Angular and longitudinal offsets are relatively large in the simulation. Additionally, experimental lateral tolerance in vertical and horizontal directions is approximately 500nm larger than the simulated lateral tolerance as shown in Fig. 5 (b). This discrepancy between experimental and theoretical data may be from longitudinal and angular misalignments.
Before laser welding, wire bonding has been carried out using 1mil (25µm)-thick gold wires between metal pads on the micro heater and the printed circuit board for wavelength tuning.
The laser welding station and a sub-assembly loaded on the station are shown in Fig. 6 . The first lensed fiber was passively aligned by a precision vision system on the laser welding system and then welded by YAG laser. The 10µm-lensed fiber was surrounded by a nickel based metal ferrule. This metal ferrule was joined with a nickel based weld clip by YAG laser. Nickel is a laser-weldable material. The laser welding sequences were the same as those demonstrated in . A total of four welds are placed between the weld clip and the metal ferrule in pairs, two welds in the front near the Si-waveguide and two in the rear. The welding was implemented from front to rear side with a pre-welding offset [13, 14] of 2.0 µm. The weld joint (between weld-clip and metal ferrule) is positioned at the same height as the center-line of the metal ferrule to minimize Post-Weld-Shift (PWS). For the reliable joining between the metal ferrule and weld clip, the choice of YAG laser energy power is critical. The large joining area will be more reliable than small joining area. Figure 7 shows the characterization of YAG laser spot used in the experiments. The diameter of joining area is larger than 200 µm when YAG laser power is greater than 9 J for the 150 µm thick nickel plate. Therefore, the YAG laser power of approximately 9 J was used for joining between the metal ferrule and weld clip. Before this joining, the clip was welded on the Kovar plate with a YAG laser power of 12-15 J.
During welding, rapid solidification of the welded region and the associated material shrinkage cause the PWS between the metal weld clip and the ferrule. As a result of PWS, misalignment has occurred. To compensate the misalignment by PWS for the first fiber, mechanical tuning with monitoring on an Infra-Red (IR) camera was implemented as shown in Fig. 8 . The mechanical tuning can be performed using seesaw effects . The clip welded with the ferrule acts as a pivot. When the rear side of metal ferrule surrounding the lensed fiber is tuned to the up-, down-, left-, and right- side, the tip of lensed fiber moves to down-, up-, right- and left- side, respectively. This tuning can be performed due to the large angular tolerance as shown in Fig. 5(b). Figure 8 shows the IR camera view after mechanical tuning. The laser welding station we employed does not equip an IR camera inside. Therefore, we implemented the mechanical tuning on the other setup. If the IR camera is equipped in the laser welding station, this mechanical tuning process can be replaced by laser hammering process . The laser hammering process can also compensate misalignment using the seesaw effect. For example, the tip of lensed fiber will move to up-side when laser hammering is implemented on the rear side of between the metal ferrule and the weld clip. The pivot is composed of two front welding points in this case.
The second lensed fiber was aligned actively and then attached by laser welding. Due to the PWS, misalignment compensation was required. Thus, the mechanical tuning has been implemented via IR and power monitoring. It can be also done by laser hammering process. In our experiment, we did the mechanical tuning process for convenience. The optical power measurement is needed for the sub-assembly after pigtailing input and output fibers. The additional mechanical tuning can be performed for both fibers to check the misalignment.
Finally the housing process has been done by loading the sub-assembly on a designed metal box and then bonding the two by a thermal epoxy as shown in Fig. 9 . It takes typically more than 1 hour for curing with over 80 °C in the thermal epoxy process. In our packaging process, the curing temperature and time were 90 °C and 2 hours, respectively. Additional micro-tuning process via power monitoring will be required if misalignment is occurred by the thermal curing process. The packaging process is summarized on the flow chart as shown in Fig. 10 .
4. Results and discussion
The optical power of the ring resonator chip and module has been compared as shown in Fig. 11 . The chip measurement has been performed by active alignment on the precision motorized stages. The chip loss, including coupling loss, of both sides was approximately 12.0dB.
The optical loss of module after performing the mechanical tuning has been measured to be 1.4 dB with respect to the chip data. Namely, the insertion loss of 0.7 dB on each port has been occurred by misalignment. The misalignment can be expressed as. ΔX, ΔY, and ΔZ indicate horizontal, vertical, and longitudinal offsets, respectively. The angular offset expressed as may be less than 1.0 degree equivalent to 0.2dB-loss as shown in Fig. 5(b). Here, ΔθX and ΔθY are angular offsets in horizontal and vertical directions, respectively. Assuming that ΔZ would be nearly zero due to large tolerance as shown in Fig. 5(b). Ideally, the Δr at each port is approximately 0.6 µm. However, most of 1.4dB-loss is expected to come from the first pigtailed fiber because of passive alignment. Even though we use the precision vision system for the passive alignment, it is difficult to be observed by the system when the distance between the tip of fiber and the facet of waveguide is very short (a few micrometers). Therefore, the tip of the first fiber may not be located at the optimal working distance of 19 µm for the 10 µm-lensed fiber. The micro mechanical tuning process can align the fiber precisely on horizontal and vertical directions via power monitoring. However, it cannot micro-tune the longitudinal position of the fiber tip. In order to overcome the limitation of micro-tuning, a vision system with enhanced magnification and high resolution is required to be equipped in the laser welding station.
The total insertion loss of the module is 13.4 dB including coupling loss, propagation loss and circuit excess loss. The circuit excess loss including two polarization diversity circuits and one ring resonator was estimated to be 6.3 dB  after removing propagation loss and coupling loss. The propagation loss for 3 mm-long waveguide and theoretical coupling loss for in- and out-puts were approximately 0.7 and ~4.6 dB, respectively. Therefore, the module insertion loss is from the sum of these ( = 11.6 dB) and the misalignment ( = 1.4 dB). The loss difference between the analysis and measurement is 0.4 dB. It may be due to the reflection or/and the phase error of coupling between the tapered tip and lensed fiber.
The module has a free spectral range of 11.72nm and 3 dB-bandwidth of 0.2 nm. The wavelength can be tuned by micro heater with applied voltage as shown in Fig. 12 (a) and (b) .
We demonstrated the feasibility of Si-based passive waveguide device packaging using YAG laser welding technique. The misalignment due to the PWS can be compensated by using mechanical fine tuning. The packaging loss was less than 1.5 dB with respect to chip data. The module has low polarization dependence and large wavelength tuning characteristics due to the use of polarization diversity circuits and integrated micro heater, respectively. We also showed the packaging process development and discussed loss-analysis of the packaged device. This packaging process is simple and the module assembled by the process will be reliable due to the use of joining metal structures using a laser welding. The packaging structure will be an attractive solution for interconnecting a photonic chip to the system.
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