5. Testing Summary and Discussion

While the exact time required to align to particular device depends on its location, our tests show that an accuracy of <1.6p,m could be routinely achieved on the order of 10 seconds to InP waveguides initially misaligned by ~20p,m. Direct comparison of such alignment times to previous research is difficult because externally actuated systems typically optimize degrees of freedom beyond the 2-axis optimization performed by our gray-scale fiber aligner. Although previous simulations showed that our gray-scale fiber aligner is optimizing alignment along the two most important axes. Nonetheless, the alignment speed of the gray-scale fiber aligner (at ~10 seconds for 2-axes) compares favorably to active alignment times reported using external actuators (~30 seconds for 3- axes) [174]. Many algorithm parameters could also be adjusted to tailor the alignment speed and/or resolution for particular applications.

The alignment resolution achieved with the gray-scale fiber aligner (<1.25p,m) is competitive with the best reported passive alignment techniques [84], with the advantage that extreme control over all fabrication and assembly tolerances is not required. Pulse testing results imply that continuous small displacements by the gray-scale fiber aligner are limited by friction between the wedges and fiber. Thus, improving the sloped wedge surface morphology should lead to more continuous movement and finer alignment accuracy. Re-design of the gray-scale slope using more gray levels could minimize wedge roughness in photoresist at the expense of increasing optical mask cost. Alternatively, techniques such as short isotropic silicon etching or hydrogen annealing [129] are candidates for post-process smoothing of the surface, but would require careful process control to avoid effecting other geometries on the device. However, as shown with the pulse actuation method, fibers can already be positioned with accuracy below the minimum continuous movement threshold.

Use of a 9p,m core cleaved fiber makes evaluating the desired sub-micron resolution nearly impossible with the current optical setup. As stated previously, the FHWM is currently ~10p,m, so <1p,m misalignment corresponds to <0.1dB of loss, which is too near the noise threshold of our system (<1% ~ 0.04dB) to be reliable. It would be preferable to work with a setup similar to Kang et al [155] where they reported a 3dB loss for only 1-1.2p,m misalignment of a lensed fiber to an InP chip. However, manually assembly of lensed fibers in the gray-scale fiber aligner would be exceedingly difficult and expensive given the equipment available. (Assembly yield would be extremely low since lensed fiber tips are delicate and manual insertion/epoxying of the fiber often does not result in good contact between the fiber and both alignment wedges.)

5. Conclusion

The static and auto-alignment testing presented in this chapter was able to clearly demonstrate three key abilities of the gray-scale fiber aligner. First, controlled actuation of the optical fiber in both the horizontal and vertical directions was achieved over a range >35p,m in each axis with switching speeds of ~1ms. Second, auto-alignment results illustrated that standard search algorithms could be implemented using the gray­scale fiber aligner with predictable and intuitive behavior; optimizing alignment to fiber and waveguide targets on the order of 10 seconds. Thirdly, alignment using the pulse actuation method was able to confirm that an alignment resolution <1.25 p,m was achievable over a 20 by 20p,m area. Gray-scale fiber aligners have also proven robust, in some instances actuating >10⁵ times in numerous testing configurations without any observed change in performance.

The developed gray-scale fiber aligner system is a significant step towards in­package alignment of optoelectronic components. The most realistic packaging configuration would likely include flip-chip bonding of III-V or SOI photonic circuits onto a silicon substrate containing one or more fiber alignment devices (and possibly relevant control electronics). The gray-scale fiber aligners would then provide individually optimized alignment to minimize optical losses. While only basic device configurations and control algorithms were presented here, there remain numerous avenues for optimizing active alignment time and accuracy for particular applications. In addition, testing has shown that nearly arbitrary control methods and search algorithms could be adapted to work with this device.

CHAOTER 7: CONCLUSION