Waveguide Interconnection in Silica-Based Planar Lightwave Circuit Using Femtosecond Laser

The interconnection of waveguides inside a silica-based planar lightwave circuit (PLC) is demonstrated both two- and three-dimensionally by using a femtosecond laser. The waveguides written with a femtosecond laser can be successfully connected to waveguides inside a PLC with low loss. Unlike previous work on the direct writing of 2-D and 3-D waveguides in bulk glass, the waveguide must be written a few tens of micrometers beneath the surface of a PLC composed of multilayers of different glasses. To realize a low-loss waveguide, we studied the sensitivity difference for femtosecond pulses in each layer in detail and developed a multiple-scanning technique with a femtosecond laser for writing waveguides. In addition, we investigated a mode-field (MF) diameter control technique to allow us to achieve the low-loss interconnection of PLC and written waveguides. We also studied particular problems caused by nonlinear optical effects, such as the positional displacement of written waveguides from the focal point. As a result, we achieved a low-loss waveguide with almost the same MF diameter as a PLC waveguide and performed the first demonstration of interconnection between PLC waveguides. The excess losses at 1550 nm were 1.4 and 2.3 dB for 2-D and 3-D interconnection, respectively.

I. Introduction

Femtosecond micromachining [1] has been used to fabricate a variety of photonic devices using transparent materials. Waveguide fabrication is an attractive application of this technique in relation to fabricating photonic devices [2]. Femtosecond pulses can directly increase the refractive index inside a transparent dielectric material as a result of multiphoton absorption [1], [3], [4]. This phenomenon allows us to fabricate waveguides directly in the material and realize passive and active optical devices. For example, Y-junction splitters [5], [6], X-couplers [7], periodic gratings [8]–[10], waveguide amplifiers [11], [12], couplers [22], and photonic interconnections [23] have been fabricated by using 2-D or 3-D waveguides. Writing a waveguide with a femtosecond laser is a very flexible and attractive way to make simple optical circuits, but it takes time to fabricate large-scale optical circuits.