2. Device Concept

Contrary to traditional fixed v-groove designs obtained by wet chemical etching [78-83, 149], the fiber alignment mechanism developed in this research creates a dynamic v-groove using opposing sloped, silicon wedge structures to hold the optical fiber in a particular alignment location. The basic alignment mechanism is illustrated in Figure 5.1. In Figure 5.1(a), the system is “at rest” with the fiber lying at the bottom of the dynamic v-groove. However, in Figure 5.1(b), after an in-plane displacement of one silicon alignment wedge, the bottom of the dynamic v-groove has been translated in both the in-plane and out-of-plane directions, altering the alignment of the optical axis. Thus, through coupled in-plane motion of opposing wedge structures, alignment of an optical fiber in the X-Y plane can be achieved.

3-D and top-view schematics of the 2-axis optical fiber alignment system are shown in Figure 5.2 and Figure 5.3. A flexible fiber cantilever is created by anchoring one end of the fiber in a static v-groove or trench located a few millimeters away. The static v-groove provides approximate passive alignment such that the free end of the flexible fiber cantilever rests between two sets of 3-D shaped wedges. Each set of wedges is attached to an in-plane MEMS actuator, such as comb-drives, which provide the requisite forces. The movement of each in-plane actuator allows the position of the fiber tip to be changed; improving alignment to a target device - in the case of Figure 5.2, the target is a chip with a waveguide. After achieving the desired alignment, the fiber could be secured using various types of epoxy or possibly a clamping mechanism (more discussion on this topic in Chapter 7). It is anticipated that fiber tip actuation of >10p,m will be required to compensate for fabrication and assembly errors within an optoelectronic module [152].

Figure 5.3: Top view schematic of the 2-axis optical fiber actuator [157]. The opposing actuators are aligned with a static v-groove trench to provide approximate passive alignment.

3. Fiber Coupling Loss Analysis

The goal of the gray-scale 2-axis fiber aligner is to eliminate axial misalignment by bending the fiber to an appropriate position. However, bending the fiber inherently introduces some loss as well. Thus, three primary sources of optical loss, shown in Figure 5.4, should be considered and analyzed: longitudinal (along the axis of light propagation), axial (perpendicular to light propagation), and angular. The coupling analysis in this section is based on the Gaussian coupling model presented by Joyce and

As mentioned earlier, the sloped alignment wedges are fabricated using gray-scale technology. Since the integration of gray-scale technology with an SOI MEMS actuator process flow has already been developed in Chapter 3 of this dissertation, only the results of the process will be given later when the fabrication is discussed. Additionally, since the gray-scale alignment wedges are purely mechanical elements, they are not limited too conductive or magnetic materials, as may be the case in other types of actuators.

DeLoach in 1984 [158]; however adaptations have been introduced to specifically model the behavior of the gray-scale fiber aligner. This approach requires beams to be represented by their nearest equivalent Gaussian mode, which while an approximation, provides useful insight to the coupling for a variety of optical and mechanical configurations of the gray-scale fiber aligner. The following analysis will assume fiber- fiber coupling, but can be applied to other source/sink combinations with approximately Gaussian modes. Similar treatment of Gaussian coupling can be found in [159, 160].

b) Axial

Figure 5.4: Three primary sources of loss in fiber-fiber coupling.

The simplest case to consider initially is that of purely longitudinal separation between two co-axial fibers, as shown in Figure 5.4(a). Since the optical mode is no longer confined upon entering the gap between the two fibers, the beam waist (W) will expand as it propagates in the z-direction according to:

where k=2n/X and w₀ is the original beam waist inside the fiber core. The term w is known as the half-width or beam waist, where the amplitude of the electric field drops to 1/e of the peak, or where the intensity drops to 1/e . For the simulations below, and most subsequent experiments in the following chapter, 8.2p,m core single mode optical fibers were used (Corning SMF-28e), with 2w=10.4jum^f, and an operating wavelength of X = 1550 nm to match the preferred low loss window of optical fibers [161].

For elliptical mode profiles, the coupling efficiency (t) between co-axial fibers for either the x or у primary axes, can be calculated separately to be [158]:

where w₀₁ and w₀₂ are the original beam waists for the input and output fibers respectively, and 7_Total is the separation distance between them. Assuming circular symmetry and identical input/output fibers, the coupled power transmission coefficient (TLongitudinal) can be simplified to:

We can plot this transmission as a function of separation to evaluate the anticipated loss resulting from only longitudinal separation, see Figure 5.5 below. From the graph it is clear that the magnitude of separation (Izl) has a large influence over the coupled power between fibers, and should therefore be kept as small as possible. However, small changes (Az) about a certain separation have little effect on the total transmission (T(z + Az) ~ T(z)). For example, assuming only 5^m longitudinal placement accuracy for lzl=20^m, the difference in coupled power between 20^m and

Material data sheet (www.corning.com/photonicmaterials/pdf/pi1446.pdf, accessed 3/16/05).25pm separation is <0.08dB. As will be shown shortly, this difference in coupling is virtually negligible compared to the change in coupling that would be caused by similar levels of axial misalignment. Other studies have also shown the longitudinal axis to be the least critical of the misalignment components considered here [159, 160].