Hybrid Integrated Module on PLC Platform

It is widely recognized that the bandwidth in customer premises will only be possible if the fiber-based network is extended from metro areas to customer premises. Bringing optics closer to the end-users, especially in fiber-to-the-home (FTTH), can be realized based on the availability of the low-cost,

Fig. 13.22 Picture of the 10 G receiver (APD and pre-amplifier) module,

architecture of such kind complicates the assembly process and increases the number of components required, which results in increasing the cost.

Besides FTTH application, it is also essential to increase the module functionality in many applications where the size and shape are the main concerns. In last few years, much work has been going on to avoid complication in the assembly, and also to increase the module's functionality by integrating several functional devices in a single module. Photonics and electronics devices are to be integrated inside the single module, either monolithically or hybrid, to increase functionality. Monolithic integration is unfortunately not readily achieved with today's fabrication technology. Today's hybrid integration using the Si-platform can only allow to have integrated optical module with higher electronics and optical functionalities [65]. The technology for surface-mounting optical devices onto a platform such as silica-based PLC is very attractive [66,67] due to its potential for cost reduction in the optical module assembly process and increases in the number of optical components required with easy coupling technique.

Bi-Directional Transmitter/Receiver Module

In FTTH application, a bi-directional module is necessary to transmit and receive separate wavelength optical signals over the single fiber. Figure 13.23 shows an example of the bi-directional module consisting of 3 dB Y-branch silica waveguide, an LD transmitter, a PD receiver,

Fig. 13.23 Schematic diagram showing the integrated bi-directional module on the Si-platform for FTTH application. Comer-illuminated PD is used in the module.

a monitor PD to control the LD power automatically, and a 1.3/1.55-|Am optical WDM filter [45]. The module can multiplex and demultiplex the 1.3/1.55 |im wavelength optical signals, and transmit and receive the 1.3 (Am bi-directional signal. Optical waveguide is used to mux/demux the optical wavelengths and also to guide the optical signal to/from the PD/LD.

Silica-on-silicon technology, as mentioned earlier, is used in fabrication of the bi-directional transmitter/receiver modules. Alignment markers are generally used on the PLC platform for accurate positioning of the waveguide in the lateral direction, necessary in passive alignment technique for assembling the LD/PD. Flip-chip self-alignment bonding can be used in attachment of LD/PD on the PLC platform. The coupling loss and alignment tolerance can be adjusted by using the LD with spot-size converter, as described in Section 13.3.1, and also by using the refractive index gel around the LD and fiber [38]. Si-resin or per-fluoride liquid having lower refractive index (than optical fiber) can be used as the index matching gels. Of them, Si-resin, having a refractive index of 1.39, is suitable for the module assembly because of its gelatinous and stable chemical and electrical characteristics. Fiber with micro-spherical lens is also used to increase the coupling efficiency and the alignment tolerance. To achieve coupling loss as low as 5 dB and high alignment tolerance (>12 (im horizontally, and 2 )im vertically), fiber with spherical lens, and the refractive index matching gels (refractive index of 1.39) must be used [38]. Typical surface-emitting type PD or corner-illuminated PD can be used for this module. In the case of corner-illuminated type PD, as shown in Fig. 13.23, the light beam is illuminated at the back of the substrate, a portion of which is formed into the angle surface, and it is guided to the absorption region. This provides high coupling efficiency (making >0.85 A/W) and good uniformity [45]. For the purpose of fiber alignment to couple with waveguide, V-groove approach, as mentioned in the earlier section, can be made on the Si-bench. Sufficient grooves length is necessary to ensure sufficient strength in the fiber mount, and generally 3 mm [38] or more groove length is necessary. In this method, the excess coupling loss between the fiber and the waveguide caused by displacement is as low as 0.1 dB.

Using a similar approach, bi-directional transmitter/receiver module incorporated with receiver IC onto the PLC platform can also be thought about for the subscriber-end device (e.g., FTTH) application. Figure 13.24

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