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We propose and demonstrate a method for improving optical-frequency sweepers that uses a disk-shaped optical bandpass filter operated under the synchro-scan mode. We optimize the filter layer structure to achieve high linearity in optical-frequency tuning with the rotation angle of the disk, while maintaining a constant band-inhibition performance. We introduce an optical system that narrows the bandwidth by using an optimized beam coupler and improves the filtering performance by using a multiple-path method. The optimized filter enables a stable linear optical-frequency sweep when it is used with a motor stabilized by phase locking. The improved optical frequency sweepers can extend the sweep range to above 2 THz, while suppressing the accumulation of spontaneous emission noise.

We realized a lightwave synthesized frequency sweeper which has a >1-THz sweep span and operates stably. We employed an actively controlled tunable bandpass filter together with a Fabry-Perot cavity, which consisted of a Faraday rotator mirror and a polarization rotation mirror. We confirmed that the frequency information of an input pulse was preserved after 1000 round-trips in the cavity and that signal pulses could circulate stably for 12 h.

A broadband lightwave synthesised frequency sweeper has been realised. An acousto-optic frequency shifter with a large frequency shift (1 GHz) and a synchronously tuned bandpass filter in an amplified fibre optic ring were employed, and a frequency sweep span of 1.1 THz was acheived.