Investigation of Narrowband Acousto-Optic Tunable Filters for Semiconductor Laser Tuning

Abstract

Wavelength tunable sources find applications in areas of laser surgery, optical coherence tomography, material processing, and differential absorption light detection and ranging. Acousto-optic tunable filters (AOTFs) are wavelength selective devices which allow for better stability than mechanical tuning due to their lack of mechanical movement. Agile tunability is achieved by avoiding inertial effects. The aim of this thesis was to investigate the performance and limitations of AOTF wavelength controlled external semiconductor laser cavities. The three key components of a typical tunable source are outlined. Conventional tuning mechanisms are compared with AOTFs, and different gain media discussed, with semiconductors being identified as suitable for building AOTF tuned sources. Initially, two diffraction grating tuned sources were constructed. The first was a Thulium doped fibre laser which produced >1.3 W over 1920 – 2060 nm wavelength range which has since been delivered to OzGrav for characterising the absorption of low hydroxide OH bulk fused silica. The second source was constructed from a superluminescent diode with an Indium Phosphide (InP) active layer and produced >5 mW over 1650 – 1750 nm. This source was subsequently used for characterising a frequency shifting, quasi-collinear AOTF for its tuning relation, response to different driving conditions, and temporal response. Two tunable laser cavities were built using the quasi-collinear AOTF. The first was based on the InP superluminescent diode and demonstrated >10 mW over 1615 – 1775 nm. However, the laser displayed significant etalon effects in its spectral output. This motivated the construction of a ring-cavity based on an InP semiconductor optical amplifier. This laser produced >5 mW over 1600 – 1750 nm, with ~0.3 nm linewidths and no etalon effects observed in the output. Sweep rates of 10⁵ nm/s were demonstrated for both lasers without significant spectral broadening or power modulation. This is of similar magnitude to the fastest previously reported quasi-collinear AOTF tuned semiconductor lasers. Wavelength agility was demonstrated by switching between any two wavelengths in the gain band within the 29 μs interaction time of the AOTF. Finally, a frequency compensating AOTF device was utilised in the ring-cavity to investigate the effect of frequency shift on laser performance. Operating with no frequency shift allowed for a reduction in linewidth, but made the laser more sensitive to residual etalon effects which had not previously been observed with the frequency shifting AOTF ring-cavity. A +30 kHz frequency shift was significant enough to disturb these intracavity interference effects while being small enough to reduce the linewidth from the 0.3 nm observed with the frequency shifting AOTF to <0.1 nm. Future directions of research are discussed. These include operation at other wavelengths, and investigating dual wavelength operation.