3. Ultrashort mode-locked lasers and their application to frequency standards and microwave photonics

A mode locked fiber laser, with a ring-shaped resonator composed of optical fibers and an erbium-doped fiber gain medium, can generate ultra-short pulses. Passive mode locking, which takes advantage of nonlinear optical effects, makes it possible to generate femto-second optical pulses but the repetition rate is limited to about 10 MHz, whereas active mode locking based on high-speed optical modulators can increase the repetition rate to about 10 GHz but the pulse width cannot be decreased below the pico second level. As an example of an actively mode locked laser, Fig. 1 shows the configuration of a regeneratively and harmonically mode locked fiber laser, and Fig. 2 shows the output waveform. Since fiber lasers generate nearly transform-limited pulses with their pulse width ranging from ps to fs, their expected light source applications may include optical communication as well as signal processing, measurement, and spectroscopy.

Fig. 1 Regeneratively and harmonically mode-locked fiber laser.

Fig. 2 Output waveform

Distribution of frequency standards by fibers

The linewidth of the longitudinal mode in a laser output spectrum is below 1 kHz. This exceptionally stable regeneratively mode locked laser is used for frequency measurement with extremely high precision by employing the equally-spaced longitudinal modes as a "ruler" (Fig. 3). Furthermore, a laser driven by a 9.1926 GHz signal generated from a Cs atom as a clock provides optical signals with a frequency of 9.1926 GHz, namely an "optical atomic clock". This frequency standard in optical communication band can be distributed directly to photonic networks. The application of these stable mode-locked lasers to frequency standards is a goal of our research.

Fig. 3 Providing frequency standards through optical fibers

Fig. 4 Cesium optical atomic clock