A High Repetition Rate, Short Pulse, Self-Starting Laser
This novel design of laser substantially improves on conventional mode-locked lasers. It permits very high repetition rate and reliable self-starting. This is achieved by incorporation of an Semiconductor Optical Amplifier (SOA) in the "Figure of Eight" configuration.
Fibre lasers provide an alternative to conventional bulk lasers and offer high efficiency, high beam quality, excellent heat dissipation, robusteness and are typically of a relatively small size. For some applications pulsed laser sources are required and recently a mode-locked figure-eight laser has been developed. The figure-eight laser includes a non-linear optical fibre and an amplifying medium in a first loop, which provides an amplifying loop mirror for light that is guided in an optically coupled second loop.
The phase shift that light will experience when guided through the non-linear optical fibre of the first loop depends on the light intensity. Consequently light that passes through the amplifying medium before passing through the non-linear optical fibre will experience a phase shift that is different to that of light guided in an opposite direction. Interference of light guided in both directions and a suitable optical coupler is used to generate pulsed light having desired properties.
In a known passively mode-locked laser, the pulse that initiates the lasing originates from optical noise fluctuation. In an actively mode-locked laser, mode-locking is typically achieved using an optical modulator which is electronically controlled and is used to generate a more intense initiating laser pulse from the optical noise. In particular passively mode-locked figure-eight fibre lasers have been of interest for many applications as they are of relatively low cost and simple construction. However, as the initiating pulse originates from noise, the lasing performance, which depends on the properties of the initiating pulse, is of poor repeatability and the laser often has unpredictable performance. Furthermore the repetition rate is governed by the cavity round-trip time. For fibre lasers this is typically dozens of metres and thus the repetition rate is fairly slow.
In the new invention, the inclusion of an SOA in the laser cavity not only results in self-starting and cleaner pulses but also provides a passive phase modulation that stabilized the laser operating in high order harmonic mode. As high as 1.2 GHz repetition rate stable pulse train has been achieved in our preliminary experiment. The SOA provides the nonlinear phase shift and additive gain that effect nonlinear switching and enable self-starting of the optical pulse by self amplitude modulation. The slight asymmetric gain experienced across a pulse imparts a small frequency drift to that pulse. The magnitude of this drift depends on the separation between pulses and the recovery time scale of the SOA. The result is an effective repulsion force between adjacent pulses and a steady-state condition consisting of equally spaced pulses. The time dependence of the free-carrier density reflects the phase modulation effect in the SOA. This novel design can in principal be extended to a free-space version which has the potential to achieve a repetition rate of over ~10 GHz. It is more valuable for commercialisation due to its higher repetition rate, shorter pulse duration performance and compact size.
- Optical sources for telecommunications;
- OCDMA, OTDMA, WDM, and other communication schemes that need return-to-zero (RZ) optical pulses;
- Optical sources for non-destructive testing in engineering and manufacturing;
- Optical sources for medical applications such as photo-dynamic therapy, diagnostics and surgery.