Laser ranging and burst illumination imaging (BIL) in the atmospheric transmission window at 1.5µm are made difficult by speckle effects which are observed when a rough surface is illuminated by a laser beam with a coherence length greater than the characteristic surface feature size. For the narrowband pulsed lasers currently used, this length is of the order of a few millimetres which leads to observable speckle effects for many common surfaces. In this context we describe progress towards the development a short-coherence length laser source operating at 1.5µm and based on optical parametric amplification of broadband seed pulses from a modelocked femtosecond erbium-doped fibre laser. Our optical parametric amplifier (OPA) system comprises a compact actively Q-switched 1047nm Nd:YLF laser, operating at ~1kHz repetition frequency, to which a 54MHz femtosecond 1.55pm Er:fibre laser is synchronised. The fibre laser produces bandwidth-limited 100fs pulses which are stretched by chromatic dispersion in a spool of SMF28 fibre to match the 3.5ns duration of the Q-switched pulses. Pulses from the Nd:YLF and Er:fibre lasers act as the pump and seed respectively for an OPA based on an aperiodically-poled crystal of MgO:PPLN containing a single linearly chirped grating. The chirp grating enables broadband parametric amplification across a wavelength range comparable with the spectral bandwidth of the seed pulses, amounting to ~150nm in the wings of the spectrum. Early results from this system have demonstrated output energies of 2.55µJ and a single-pass gain ~51dB and are expected to be increased with continued development of the project.
|Journal||Proceedings of SPIE - the International Society for Optical Engineering|
|Publication status||Published - 2005|
|Event||Unmanned/Unattended Sensors and Sensor Networks II - Bruges, Belgium|
Duration: 26 Sep 2005 → 28 Sep 2005
- Burst illumination imaging
- Optical parametric amplification
- Remote sensing