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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.

Superconducting rotating machines have looked promising since multifilamentary niobium-titanium (NbTi) superconductors became available in the mid-1960s. Both dc homopolar and ac synchronous machines were successfully tested from the 1970s to the 1990s. Three different 70-MW generators were recently demonstrated by the SuperGM project in Japan. However, economic considerations with respect to competitive cost combined with the requirement for liquid helium cooling did not make these machines commercially attractive. On the other hand, high-temperature superconductors (HTSs) can operate at much higher temperatures (30-40 K), providing much larger thermal margin and simpler cooling systems. This refrigeration advantage has provided new impetus to the development of such machines for commercial applications. In the last few years, a number of superconducting rotating machines with HTS field windings have been demonstrated and several projects are currently transitioning to advanced development stages. HTS machines with ratings from a few kilowatts to several megawatts have been demonstrated in the United States and Europe. Currently, large high-torque ship propulsion motors, large generator prototypes, and synchronous condensers are under development and are expected to be commercially available in the next few years. Prospects for improved life cycle cost, smaller size, less weight, and higher efficiency benefits are providing incentives for the development of these larger rating HTS machines. This paper reviews the past and recent progress on the worldwide development of industrial-grade superconducting rotating machines utilizing low-temperature superconductor and HTS field windings and provides an outlook on the benefits and opportunities of this new technology.

In this letter, we demonstrate uncooled and no mode-hop induced signal deterioration operation of an external cavity diode laser (ECDL) employing a fiber grating. The ECDL achieved 2.5-Gb/s direct modulation and good lasing wavelength stability (< 1 nm) in a temperature range of -5/spl deg/C to +45/spl deg/C. Our ECDL is suitable for next generation coarse-wavelength-division-multiplexing access network applications.

We propose a novel node configuration for a dense wavelength-division multiplexing photonic access ring that uses a centralized multiwavelength light source. The proposed node, which we call a wavelength channel modulator, features a simple configuration that achieves both low crosstalk and optical stability.

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.

We describe an effective method for stabilizing a pulsed lightwave circulating around an amplified fiber-optic ring containing an acoustooptic frequency shifter. By installing a Lyot depolarizer with a large birefringence in the ring, we can effectively suppress the change in the maximum number of the pulse circulations caused by a fluctuation in the ring birefringence. We experimentally confirmed that the change in the number of circulations was suppressed to less than 30, which is very small compared with the maximum number of pulse circulations of 730.

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.