Step-type optical delay line using silica-based planar light-wave circuit (PLC) technology

We successfully fabricated a step-type optical delay line using a silica-based planar lightwave circuit (PLC) with a total delay of 51.0 mm and an optical power crosstalk of -49.1 dB. No other example of an optical lightwave circuit-type optical delay line with an optical delay of several tens of mm, as well as low crosstalk has been reported previously. To ensure that the fabricated step-type optical delay line could be used in a mode-locked Er³⁺-doped fiber ring laser, we aimed for: an optical delay of several tens of mm so that any optical pulse repetition frequency could be set, as-low-as-possible optical crosstalk so that the mode-locked ring laser would oscillate stability, and low optical insertion loss. We integrated an optical circuit consisting of 8 optical delay units possessing three thermo-optic switches, which have a Mach-Zehnder interferometer configuration, and two waveguides; the difference in the optical path of the waveguides corresponds to the optical delay in a silicon wafer, and we set the optical delay of each unit to 0.5 0.4, 0.8, 1.6, 3.3 6.4, 12.8 and 25.6 mm, to give a maximum delay of 51.0 mm. Moreover, the total insertion loss was 5.2 dB. Precision measurement of the optical delay of each unit gave good results with a manufacturing error of less than 0.47%. Based on the above results, the fabricated PLC step-type optical delay line can be used in a mode-locked ring laser