This paper presents the latest efforts in the development of commercial optically-pumped semiconductor disk lasers (SDLs) at M Squared Lasers. Two types of SDLs are currently being developed: an ultrafast system and a continuous wave single frequency system under the names of Dragonfly and Infinite, respectively. Both offer a compact, low-cost, easy-to-use and maintenance-free tool for a range of growing markets including nonlinear microscopy and quantum technology. To facilitate consumer uptake of the SDL technology, the performance specifications aim to closely match the currently employed systems.
An extended Dragonfly system is being developed targeting the nonlinear microscopy market, which typically requires 1-W average power pulse trains with pulse durations below 200 fs. The pulse repetition frequency (PRF) of the commonly used laser systems, typically Titanium-sapphire lasers, is 80 MHz. This property is particularly challenging for mode-locked SDLs which tend to operate at GHz repetition rates, due to their short upper state carrier lifetime. Dragonfly has found a compromise at 200 MHz to balance mode-locking instabilities with a low PRF. In the ongoing development of Dragonfly, additional pulse compression and nonlinear spectral broadening stages are used to obtain pulse durations as short as 130 fs with an average power of 0.85 W, approaching the required performance.
A variant of the Infinite system was adapted to provide a laser source suitable for the first stage of Sr atom cooling at 461 nm. Such a source requires average powers of approximately 1 W with a sub-MHz linewidth. As direct emission in the blue is not a viable approach at this stage, an SDL emitting at 922 nm followed by an M Squared Lasers SolTiS ECD-X doubler is currently under development. The SDL oscillator delivered >1 W of single frequency (RMS frequency noise <150kHz) light at 922 nm.
In recent years, M Squared Lasers have successfully commercialized a range of mode-locked vertical external cavity surface emitting lasers (VECSELs) operating between 920-1050nm and producing picosecond-range pulses with average powers above 1W at pulse repetition frequencies (PRF) of ~200MHz. These laser products offer a low-cost, easy-to-use and maintenance-free tool for the growing market of nonlinear microscopy. However, in order to present a credible alternative to ultrafast Ti-sapphire lasers, pulse durations below 200fs are required.
In the last year, efforts have been directed to reduce the pulse duration of the Dragonfly laser system to below 200fs with a target average power above 1W at a PRF of 200MHz. This paper will describe and discuss the latest efforts undertaken to approach these targets in a laser system operating at 990nm. The relatively low PRF operation of Dragonfly lasers represents a challenging requirement for mode-locked VECSELs due to the very short upper state carrier lifetime, on the order of a few nanoseconds, which can lead to double pulsing behavior in longer cavities as the time between consecutive pulses is increased.
Most notably, the design of the Dragonfly VECSEL cavity was considerably modified and the laser system extended with a nonlinear pulse stretcher and an additional compression stage. The improved Dragonfly laser system achieved pulse duration as short as 130fs with an average power of 0.85W.
Active control of the spectral and temporal output characteristics of solid-state lasers through use of MEMS scanning micromirrors is presented. A side-pumped Nd:YAG laser with two intracavity scanning micromirrors, enabling Q-switching operation with controllable pulse duration and pulse-on-demand capabilities, is investigated. Changing the actuation signal of one micromirror allows a variation of the pulse duration between 370 ns and 1.06 μs at a pulse repetition frequency of 21.37 kHz and average output power of 50 mW. Pulse-on-demand lasing is enabled through actuation of the second micromirror. To our knowledge this is the first demonstration of the use of multiple intracavity MEMS devices as active tuning elements in a single solid-state laser cavity. Furthermore, we present the first demonstration of control over the output wavelength of a solid-state laser using a micromirror and a prism in an intracavity Littman configuration. A static tilt actuation of the micromirror resulted in tuning the output wavelength of an Yb:KGW laser from 1024 nm to 1031.5 nm, with FWHM bandwidths between 0.2 nm and 0.4 nm. These proof-of-principle demonstrations provide the first steps towards a miniaturized, flexible solid-state laser system with potential defense and industrial applications.
In launching the Dragonfly, M Squared Lasers has successfully commercialized recent advances in mode-locked vertical external cavity surface emitting laser technologies operating between 920 nm – 1050 nm. This paper will describe the latest advances in the development of a new generation of Dragonfly lasers. The improved system has been engineered to utilise low-cost semiconductor gain media and integrated diode pumping, whilst exhibiting minimal footprint, diffraction limited beam quality and low intrinsic noise. Early experiments have resulted in pulses with 540mW of average output power and 150fs of duration at 200MHz pulse repetition frequency.
MEMS scanners are of interest for their potential as low-cost, low operating power devices for use in various photonic systems. The devices reported here are actuated by the electromagnetic force between a static external magnetic field and a current flowing through an SOI MEMS scanner. These scanners have several modes of operation: their mirrors may be rotated and maintained at a static angle (up to ± 1.4 degrees), scanned rapidly (up to 500 Hz); or may be operated in a resonance mode, at the device’s mechanical resonance frequency (~1.2 kHz) for higher rate scanning. The use of these scanners as a Q-switching element within a Nd:YAG laser cavity has been demonstrated. Pulse durations of 400 ns were obtained with a pulse energy of 58 μJ and a pulse peak power of 145 W. The use of an external magnetic field, generated by compact rare-earth magnets, allows a simple and cost-effective commercial fabrication process to be employed (the multi-user SOI process provided by MEMSCAP Inc) and avoids the requirement to deposit magnetic materials on the MEMS structure.
Multiple individually-controllable Q-switched laser outputs from a single diode-pumped Nd:YAG module are presented by using an electrostatic MEMS scanning micromirror array as cavity end-mirror. The gold coated, 700 μm diameter and 25 μm thick, single-crystal silicon micromirrors possess resonant tilt frequencies of ~8 kHz with optical scan angles of up to 78°. Dual laser output resulting from the actuation of two neighboring mirrors was observed resulting in a combined average output power of 125 mW and pulse durations of 30 ns with resulting pulse energies of 7.9 μJ and 7.1 μJ. The output power was limited by thermal effects on the gold-coated mirror surface. Dielectric coatings with increased reflectivity and therefore lower thermal stresses are required to power-scale this technique. An initial SiO2/Nb2O5 test coating was applied to a multi-mirror array with individual optical scan angles of 14° at a resonant tilt frequency of 10.4 kHz. The use of this dielectric coated array inside a 3-mirror Nd:YAG laser cavity led to a single mirror output with average Q-switched output power of 750 mW and pulse durations of 295 ns resulting in pulse energies of 36 μJ.
An adaptive optics system was developed to reduce the time taken to reach full brightness of a solid-state laser. This
system was based on the translation of the end-mirror during the turn-on time of the laser. It was implemented on a
simple laser configuration featuring a side-pumped Nd:YLF rod and resulted in the reduction of the transient time by a
factor of 15. Several limitations such as the innate astigmatism of Nd:YLF and the inertia of the moving mirror were
Confocal techniques allow the user to achieve optically sectioned images with significantly enhanced axial and improved lateral resolution compared to widefield methods. Unfortunately, as one images more deeply within a sample, sample induced aberrations lead to a significant reduction in image resolution and contrast. Using adaptive optic techniques, we report on the effectiveness of a number of algorithms for removing sample induced aberrations. The viability and efficiency at a number of fitness parameters used in the optimisation routines is also considered.
An intra-cavity deformable membrane mirror (DMM) has been used to optimise the brightness of a 15W, diode-pumped, grazing incidence Nd:GdVO4 laser. In one configuration an order of magnitude improvement of laser beam quality was recorded with negligible drop in output power. Local and global optimum-locating algorithms have been developed to enable automatic optimisation of the laser quality, and have been tested in both intra- and extra-cavity configurations. A novel laser brightness sensor based on second-harmonic-generation has also been developed to assess the progress of the laser towards optimisation. A tip & tilt mirror was also incorporated in the laser resonator cavity and initial tailoring of the algorithm procedure was performed in order to enhance the optimisation capabilities.
Preliminary investigations into the potential for automatic spatial-mode optimization in tehrmally distorted soild-sate lasers using deformable mirrors are presented. A 37 element adaptive optic mirror has been used intracavity to control the oscillation mode profile of a diode-laser pumped Nd:YVO4 laser. Spatial mode and output power optimization are demonstrated by closed loop computer control of the deformable mirror using a modified hill-climbing algorithm.
The use of semiconductor saturable absorbers has emerged as an enabling technology in modern passively modelocked laser systems. Their application to high power picosecond lasers, most notably Nd-doped lasers, has produced systems with average power levels of a few tens of watts. In this paper, the development of these laser systems to the 100W level and above will be outlined.