Laser pulse control of a Q-switched Nd:YVO₄ bounce geometry laser using a secondary cavity

Pulsed laser operation is desirable for a wide range of applications such as laser micromachining in industrial manufacturing and laser marking for product identification. In these cases it is beneficial to have flexibility in the parameters of the laser pulse to suit the specific application. This can include the ability to achieve a wide range of pulse repetition rates, with some applications requiring variation of laser pulse rate from high rate (multi-kHz) to low rate or even an off-state in a fast timescale. To generate ultrahigh pulse rates requires Q-switched lasers with ultrahigh gain, but problems can arise if the modulation element is insufficient to prevent laser action or hold-off lasing at low repetition rates. In these cases, lasing output can occur when it is not desired. In this work we present a novel method for pulse control in a high gain bounce amplifier Q-switched system by using a secondary cavity to clamp the gain and allow for clean single pulse operation from very high (800kHz) to very low (e.g.1kHz) repetition rates

Enhanced pump absorption in double-clad fibres using localised laser-machined mode scramblers

A novel method for improving pump absorption efficiency in circular double-clad fibres by laser machining localised pump scramblers at strategic positions along the fibre is reported. Preliminary results for a thulium-doped double-clad fibre indicate that the pump absorption coefficient attainable via this approach is comparable to an equivalent octagonal fibre

Extreme short wavelength operation (1.65 - 1.7 µm) of silica-based thulium-doped fiber amplifier

We report the first demonstration of silica-based thulium-doped fiber amplifier (TDFA) working in the 1.65 - 1.7 µm waveband. Up to 29 dB small signal gain and noise figure as low as 6.5 dB are achieved

Exploiting the short wavelength gain of silica-based thulium-doped fiber amplifiers

Short wavelength operation (1650-1800 nm) of silica-based thulium-doped fiber amplifiers (TDFAs) is investigated. We report the first demonstration of in-band diode-pumped silica-based TDFAs working in the 1700-1800 nm waveband. Up to 29 dB of small-signal gain is achieved in this spectral region, with an operation wavelength accessible by diode pumping as short as 1710 nm. Further gain extension toward shorter wavelengths is realized in a fiber laser pumped configuration. A silica-based TDFA working in the 1650-1700 nm range with up to 29 dB small-signal gain and noise figure as low as 6.5 dB is presented

Cladding shaping of optical fibre preforms via CO₂ laser machining

Double-clad silica fibres used in high power lasers typically comprise a core doped with a laser active ion, a silica inner-cladding pump guide and a low refractive index outer polymer coating for protection and low loss pump guidance. For efficient pump absorption in the active-ion doped core, the inner-cladding must be shaped in order to scramble the pump radiation to achieve a high spatial overlap with the core. This shaping is traditionally undertaken via diamond milling of the fibre preform into an octagon or hexagon, leaving a rough surface that is subsequently fire polished. We report on a new approach for shaping the inner-cladding using a CO2 laser to machine the fibre preform. This process is shown to allow fabrication of novel cladding structures, which include concave and convex surfaces, as well as a significant increase in the processing speeds and avoids the need for fire polishing prior to fibre drawing

Null-frequency-shift acousto-optic tunable filter for wavelength tuning of a Tm fibre laser

Acousto-optic tunable filters (AOTFs) are used in many applications where fast agile tuning capabilities combined with conservation of spatial coherence are required; such as for selection of wavelength from a broadband source for applications in confocal microscopy, hyperspectral imaging and spectroscopy [1]. For generation of higher spectral intensity they can also be used inside a laser cavity in order to select the operating wavelength [2]. AOTFs utilise a travelling acoustic wave to diffract phase-matched wavelengths, with the diffracted wave frequency-shifted by the acoustic drive frequency. When used intracavity as a tuning element in a fibre laser, this continual wavelength shift on each pass through the AOTF has a number of undesirable consequences including, poor power stability and, in some situations, pulsing, which can lead to damage

Optimising Tm-doped silica fibres for high lasing efficiency

Two-micron lasers are of great interest for a range of applications, from spectroscopy to polymer machining and laser surgery, as well as being an important stepping stone for wavelength generation further into the mid infra-red band. Tm-doped silica fibre lasers are especially attractive as they are capable of operating at wavelengths from below 1700nm to more than 2100nm, and can be pumped by commercially available high power laser diodes operating around ~793nm. Moreover, by optimising the dopant concentration within the Tm fibre core a beneficial two-for-one cross-relaxation process can be exploited allowing efficiencies far-above the quantum limit for 793nm pumped Tm fibre lasers [1]. Indeed, efforts to optimise the core composition to enhance this process have been the subject of many studies, but as yet the best slope efficiencies reported for high power cladding-pumped Tm doped silica fibre lasers remain well below the theoretical maximum (~80%)

Coherent beam combining of self-adaptive lasers

The novel technique of phase conjugate self-organised coherent beam combination is reported. It is demonstrated that due to the spectral mode freedom of a phase conjugate self-adaptive laser that efficient "all-passive" combination of many high quality lasers can be achieved. Efficient combination (>92%) of two modules has been experimentally demonstrated to >30W in continuous wave operation. Recent results demonstrating pulsed operation are also reported

Acousto-optic devices for operation with 2 µm fiber Lasers

Fibre lasers operating in the 2µm region are of increasing interest for a range of applications, including laser machining and biomedical systems. The large mode area compared to 1µm fibre lasers combined with operation in an “eye-safe” region of the spectrum makes them particularly attractive. When developing fibre lasers at 1µm and 1.5µm manufacturers were able to call upon enabling technologies used by the telecoms industry, but at longer wavelengths, including 2µm, many such components are either unavailable or immature. We report on recent developments of Acousto-Optic Modulators and Tunable Filters that are specifically optimised for use with fibre systems operating at or around 2µm. AO devices are interesting due to their ability to conserve spatial-coherence, making them appropriate for use with single-mode optical fibres. We describe how the choice of interaction medium is an important consideration, particularly affecting the drive power and the polarisation behaviour of the device – the latter being an important parameter when used in a fibre system. We also describe two designs of AO Tunable Filter intended for laser tuning. Both designs have been demonstrated intracavity in 2µm fibre lasers. The first gives exceptionally narrow resolution (δλ/λ<0·1%). The second design is of a novel type of AOTF where a matched pair of AOTFs is configured to give a substantially net zero frequency-shift with little or no loss of pointing stability, any minor deviations in manufacture being self-compensated. Furthermore, small controlled frequency-shifts (up to about 10kHz) may be introduced with little or no detriment to the alignment of the system

Narrow-band wavelength-tunable thulium fiber ring laser

Laser sources with flexibility in operating wavelength have applications in a number of areas including: remote sensing and monitoring, spectroscopy and metrology. For many of these applications the requirements for wavelength flexibility and good beam quality are very often accompanied by the need for a relatively narrow bandwidth and, in some cases, also by the need for rapid wavelength scanning. Rare earth doped fiber lasers offer versatility in operating wavelength due to the broad linewidths associated with glass hosts, but simultaneously achieving wavelength agility and a narrow lasing bandwidth is rather challenging