Hitless Wavelength Switching of Semiconductor Optical Amplifier-Integrated Reflection-Type Transversal Filter Laser With Suppressed Frequency Error

A spurious wavelength (λ
_spurious
) from a tunable laser during tuning is an issue in optical systems based on dynamically controlled laser wavelengths. Output shuttering synchronized with tunable-laser tuning is promising way to suppress λ
_spurious
. A semiconductor optical amplifier (SOA) is an ideal shutter for laser outputs with λ
_spurious
in terms of low-wavelength-independent extinction ratio. We developed an electro-optically tunable reflection-type transversal filter (RTF) laser monolithically integrated with an SOA. Thanks to the nanosecond scale tuning speed of the RTF laser, the required shuttering time of the integrated SOA is also a corresponding nanosecond scale, which leads to little temperature change in the laser cavity. As a result, the laser frequency drift after turning off/on the SOA is suppressed to within +/−2.5 GHz without any additional laser control algorithm to compensate for the thermal crosstalk from the SOA. Using the laser, we demonstrated a “hitless” wavelength-switching subsystem based on a coherent format. Namely, outputs from an SOA-RTF laser coded with 32-GBd dual-polarization quadrature phase-shift keying (128 Gbps) are dynamically switched with λ
_spurious
eliminated by SOA shuttering. In addition to practical bit error rates (BERs) of wavelength channels from the SOA-RTF laser, we observe no interference with the BER of a wavelength channel from another monitor laser in the wavelength-switching subsystem.