An optical sub-assembly of MUX/DEMUX where optical devices are hybrid-integrated on a silicon optical bench (SiOB)
using a low cost passive alignment method was reported. A tight tolerance of positional and tilting angular accuracy is
required for optical devices attachment in order to maximize the coupling efficiency. The critical positioning transverse
to the optical axis merely depends on the symmetry, and accuracy of the position and shape of trenches. Any inaccuracy
primarily affects the non-critical positioning, i.e., z-axis & θz, in the direction along the optical axis; misalignment
accumulated and causes undesired insertion loss. All the piece parts, i.e., mirror, thin-film filters (TFFs), ball lens, SiOB
etc., have a defined tolerance in their dimensions and surfaces which increases the challenge in achieving high placement
accuracy along the optical axis. The effects from these inherent inaccuracies of the position and shape of trenches and
piece parts could be minimized by improve the bottom flatness, and proper procedure selection. Misalignment at each
axis, e.g. x-, y-, z-, θx, θy & θz was characterized and its effect to the coupling efficiency was discussed.
Silicon Optical Bench (SiOB) is a popular solution for passive assembly of optical module. In order to realize an optical
transmitter or receiver module, it is necessary to integrate high frequency optoelectronic components such as signal
photodiodes (PD) or laser diodes (LD) onto the SiOB. In this way, the module's electrical and optical performances can
be further improved, and a higher degree of miniaturization can be achieved. The challenge for this integration is not
only on the assembly accuracy for the LD and PD, it required the design of low loss electrical interconnect at high
frequency. However, the standard silicon substrate used in the SiOB has a high electrical loss especially at high
frequency. This imposed a limitation on the electrical interconnection length between the optoelectronic components and
their I/O interfaces. It is proposed here to design the electrical interconnection using a layer of SiO<sub>2</sub> sandwiched between
two layers of metal layer. Simulations have demonstrated that by varying the thickness of the SiO<sub>2</sub> layer, an optimum
electrical performance can be achieved.
In this study, a low-cost (with bare chips) and high (optical, electrical, and thermal) performance optoelectronic system with
a data rate of 10Gbps is designed and analyzed. This system consists of a rigid printed circuit board (PCB) made of FR4
material with an optical polymer waveguide, a vertical cavity surface emitted laser (VCSEL), a driver chip, a 16:1 serializer,
a photo-diode detector, a Trans-Impedance Amplifier (TIA), a 1:16 deserializer, and heat spreaders. The bare VCSEL, driver
chip, and serializer chip are stacked with wire bonds and then solder jointed on one end of the optical polymer waveguide on
the PCB via Cu posts. Similarly, the bare photo-diode detector, TIA chip, and deserializer chip are stacked with wire bonds
and then solder jointed on the other end of the waveguide on the PCB via Cu posts. Because the devices in the 3D stacking
system are made with different materials, the stresses due to the thermal expansion mismatch among various parts of the
system are determined.
In this paper, the optical design of 4-channel WDM Transmission Optical Subassemblies (TOSA)/Receiver Optical
Subassemblies (ROSA) is reported. The TOSA and ROSA are being developed for uncooled modules for CWDM
applications and are compatible with the SFP/SFF form factor TOSA and ROSA. The physical dimension of OSA
together with the electronic circuitries is limited to 10×6×5 mm<sup>3</sup>. The designs of TOSA and ROSA are employed using
four thin film filters (TFFs) to select the specific channel wavelength, four 500 μm ball lenses, one 2.5 mm ball lens and
a high reflection mirror using folded optical configuration. The optical elements are to be assembled on a SiOB, except
the 2.5 mm ball lens. The simulation results are used to estimate the required optical components assembly accuracy.
Based on the simulation results, the tolerance requirement for tilting the mirror and first thin film filter is approximately
± 0.2° for the longest optical path namely Channel 4.
A new method of coupling the light from a laser diode to a Single Mode Fiber (SMF) with large alignment tolerances
and without using coupling lenses is presented. A pseudo vertical tapered coupler is designed for light coupling between
laser diode and single mode fiber. It has a large input aperture which is about 100 times the size of the laser waveguide
cross-section. The tapered coupler provides single mode output and matches the mode size with the single mode fiber.
The tapered coupler is fabricated on a silicon optical bench and is located between the laser and the fiber through the
silicon micrfabrication process. The misalignment between the fiber and taper coupler can be very small since this is
controlled by high precision silicon optical bench patterning processes. The coupler relaxes the laser diode placement
accuracies and eliminates the need for a coupling lens. Design Studies showed that the tolerance between the laser diode
and taper coupler can be more than +/-5μm misalignment at x-y, and +/-0.5degree tilting angle tolerance and the
fabricated assembly results are encouraging with good placement tolerances and coupling efficiency. The laser to single
mode fiber coupling tolerances is greatly improved and passive alignment for laser and single mode fiber is realized. The
technology can be useful for multi channel optical assembly where significant device and process cost saving can be
achieved and is suitable for functional integration for silicon photonics.