The ability to control the temporal output from a femtosecond laser can enable the same laser to be used for multiple
functions, for example, the laser used in an optical tweezers system could be used as a constant-intensity source to trap a
biological cell and then be temporarily switched to mode-locked operation to effect photoporation. Here, we report the
rapid switching of a Cr4+:forsterite laser between mode-locked and unmode-locked continuous wave (CW) regimes via
the optical pumping of an intracavity SESAM element. Mode-locking of the laser was initiated by an intracavity
quantum well (GaInAsN) SESAM having an anti-resonant design (ΔR~0.3%, λPL~1310nm) that yielded transform-limited
89fs pulses centered around 1296nm with a repetition rate of 162MHz at an average power of 64mW. Upon
excitation of the SESAM with 600mW of extra-cavity power from an 808nm semiconductor diode laser, switching could
be induced between the unmode-locked and mode-locked regimes. Transitions free of Q-switching or relaxation
oscillations were observed with <200μs switching times for both for the initiation and cessation of mode-locking.
Periods of mode-locked operation of custom duration could be produced by appropriate control of the SESAM pump
diode enabling the generation of bursts of pulses as short as 400μs. Switching was confirmed to originate from local
pump-induced heating of the SESAM by observing the laser going through identical regime switching when the chip
temperature of the 'unpumped' SESAM was raised by ~20°C.
The direct side-pumping of Nd:YAG rods with laser diodes has been shown to be a cost effective scheme for scaling to
simultaneous high average and high peak power operation. Careful control of the parameters that govern the performance
of a multi-spatial mode MOPA system has led to the demonstration of reliable Q-switched average powers of up to
1600 W and peak powers in excess of 18 MW at the 1064 nm fundamental wavelength. The techniques used to refine the
properties of an infrared stable resonator in order to optimise intra-cavity second harmonic generation are discussed.
Average powers of up to 290 W and peak powers of up to 0.45 MW at 532 nm have been demonstrated from a single
cavity. By polarisation multiplexing the outputs of two cavities emitting at 532 nm, we have achieved single-beam
average and peak powers of up to 580 W and 0.9 MW respectively and gained flexible control of pulse duration and
temporal shape.
Many applications exist for high performance lasers in the short-wave, mid-infrared spectral regime between 1.9 and
2.5μm - from long-range communications systems through to remote atmospheric gas sensing and pollution monitoring.
However, a simple, efficient laser source offering the desired performance characteristics and flexibility has not been
available. In the last few years considerable progress has been made in the development of optically-pumped
(AlGaIn)(AsSb) quantum well semiconductor disk lasers emitting in the 2.Xμm mid-infrared spectral region -
continuous-wave and pulsed-pumped output power levels now exceed 6W and 16W respectively. Furthermore, singlefrequency
operation with linewidths <4MHz and broad tunability of up to 170nm have also been demonstrated, all at
near-diffraction-limited beam quality. Such performance metrics are only possible through the very best materials
growth, a sound understanding of the design principles of these highly multi-layered devices and, importantly, the
application of effective thermal management.
We report on recent advances in the performance of GaSb-based optically pumped semiconductor disk lasers
(OPSDLs), emitting in the 2.0 - 2.3 μm wavelength range. Both barrier pumped OPSDL (using 980 nm laser
diodes as pump source) and in-well pumped OPSDL (using 1.96 μm pump radiation) have been fabricated
and characterized. Using alternative SiC or diamond intracavity heatspreader, multiple-watt CW-output
powers have been achieved (e.g. >3W at 2.3 μm and >5W at 2.0 μm), with power efficiencies in the range of
18 % - 25 %. For an optimised resonator setup, the beam profile is close to the diffraction limit with M2
values around 1.2; and even for the highest power levels, M2 is in the range of 2-5.
Semiconductor Disk Lasers (SDLs) are compact lasers suitable for watt to multi-watt direct generation in the 670-
2350nm waveband and frequency-doubled operation in the ultraviolet and visible regions. This is, however, critically
dependent on the thermal management strategy used as, in this type of laser, the pump is absorbed over micrometer
lengths and the gain and loss are temperature sensitive. In this paper, we compare the two heat dissipation techniques that
have been successfully deployed to-date: the "thin device" approach where the semiconductor active mirror is bonded
onto a heatsink and its substrate subsequently removed, and the "heatspreader" technique where a high thermal
conductivity platelet is directly bonded onto the active part of the unprocessed epilayer. We show that for SDLs emitting
at 1060nm with pump spots of ~80µm diameter, the heatspreader approach outperforms the thin-device alternative, with
the best results being obtained with a diamond heatspreader. Indeed, the thermal resistances are measured to be 4.9, 10.4
and 13.0 K/W for diamond-bonded, SiC-bonded and flip-chip devices respectively. It is also observed, as expected, that
the thermal management strategy indirectly affects the optimum output coupling and thus the overall performance of
these lasers.
A detailed study of the high-power pulsed operation of C-band optically-pumped GaInNAsSb vertical cavity surface emitting lasers is reported.
The devices employ a resonant periodic gain structure grown by molecular beam epitaxy on a GaAs substrate with a 31-pair GaAs/AlAs bottom
distributed Bragg reflector and a 4-λ,
GaAs-based resonant cavity containing 10 GaInNAsSb quantum wells distributed among the 7 antinodes of the electric field.
A dual-pump-band SiO2/TiO2 dielectric top mirror allows efficient optical pumping via low reflectivities at 808nm and 1064nm
while providing very high reflectivity at the 1.55μm target emission wavelength. The laser characteristics were evaluated using both a Q-switched Nd:YAG
1064nm pump and a 20W-peak 180ns-pulsed 850nm diode laser. The importance of the gain-cavity detuning was evident from time-dependent spectral
measurements of laser material
subjected to post-growth annealing at different temperatures between 725 and 775°C. The highest annealing temperature produces the largest blue shift of the
gain peak relative to the cavity resonance, resulting in the best power transfer characteristics as well as reduced temperature sensitivity.
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
also observed.
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.
We present an overview of diode-pumped solid-state lasers with negative feedback stabilization. Active stabilization by means of direct modulation of cavity losses was utilized in several Nd-based quasi-continuous-wave lasers mode-locked by saturable Bragg reflectors (SBR). The stabilization acts to eliminate the relaxation-oscillation-driven spiking after laser oscillation turn-on. Stable continuous-wave modelocking was observed in as little as 10 μs after laser turned on where the leading spike was reduced or completely eliminated dependent on the gain material and the operating wavelength. Stabilization was also used to prevent Q-switching instabilities in a continuous-wave SBR-modelocked Nd:KGW laser resulting in an extended operational parameter range and pulse shortening by up to 35%. Stabilization of passively mode-locked lasers could alternatively be achieved using an intra-cavity element exhibiting nonlinear absorption, which was demonstrated using an Indium Phosphide plate. Pulse shortening by up to 30% was permitted as the modulation depth of the SBR could be increased whilst maintaining stable cw modelocking. Active stabilization was also employed for Tm-based lasers emitting in the 1.9 - 2 μm wavelength region as these lasers suffer from inherent amplitude instability and sensitivity to cavity perturbation. Passive stabilization of a Tm:YAlO laser utilizing a germanium plate was also investigated.
The technique of operating picosecond modelocked laser systems quasi-CW is presented. Higher modelocked output power results at the expense of reduced effective pulse repetition rate. Passively modelocked quasi-CW lasers at 1.0 and 1.3 μm with high 'on-time' powers (~65W) are described. Active stabilization of these systems is demonstrated via negative feedback using an intracavity loss modulator. Stabilization suppresses relaxation-oscillation driven spiking on reaching threshold and prevents Q-switching of the laser caused by the presence of a saturable absorber for modelocking. In this way, the usable parameter range of a saturable absorber for stable CW modelocking is extended such that stable Q-switch free modelocking of a quasi-CW laser with pulse repetition rates over 900MHz, is possible. Numerical modeling confirms the flexible nature of stabilization via negative feedback applied to unstable laser systems incorporating saturable absorbers.
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.
High thermal conductivity intra-cavity crystalline heatspreaders are used to control the pump-induced temperature increase limiting the power scaling of vertical external-cavity surface-emitting lasers (VECSELs). Pump-power-limited output of greater than 0.4 W was achieved from a GaAs-based VECSEL at room temperature with the use of a silicon carbide heatspreader bonded to the surface of the gain element and 0.5 W by water-cooling the system to 7.5°C.
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.
A high damage threshold, strain compensated, double quantum well InGaAs saturable Bragg reflector has been developed and successfully used to modelock a high average power, all- solid-state Nd:YVO4 laser. A methodology for obtaining single transverse mode oscillation at high output powers was coupled with a `lens relay' approach to access practical cavity configurations. Ultrashort pulses of 21 ps duration were recorded at a repetition rate of 90 MHz and a diffraction limited average output power of greater than 20 W. By extending the laser resonator to give a pulse repetition rate of 36 MHz, a pulse duration of 25 ps was recorded and the Q-switching instability could be eliminated for all output power levels. In this configuration the peak power and pulse energy were in excess of 24 kW and 0.6 (mu) J respectively.
We report on the development of practical and user friendly lasers for multiphoton imaging of biological material. The laser developed for the work is a laser diode pumped Cr:LiSAF source modelocked using a saturable Bragg reflector as the passive modelocking element. For this system we routinely obtain 100 fs pulses at a repetition rate 200 MHz with an average output power of 20 mW. The laser has a single operator control and is particularly suitable for use by non-laser specialists. We have used the source developed to image a range of biologically significant samples. The initial work has centered on the imaging of intact human dental tissue. The first two-photon images of dental tissue are reported showing the development of early dental disease from depths up to 500 micrometers into the tooth. These results demonstrate the detection of carious lesions before the more conventional techniques currently used by dental practitioners. Work on other living intact biological tissue is also reported, in particular plants containing a genetically bred fluorescent marker to enable the examination of complete and intact living plant tissue.
We report on a side-pumped and passively mode-locked all- solid-state laser. The laser consists of an astigmatically compensated resonator with a saturable Bragg reflector to achieve mode locking and a Brewster-cut Nd:YVO4 rod, which is side-pumped by a diode-laser bar. At 17 W of pump power a fundamental-mode average output power of 4.4 W was attained. Pulses as short as 33 ps have been measured at pulse repetition rates of 235 MHz and 440 MHz. When synchronously pumping an optical parametric oscillator (OPO), these pulse durations lead to wide cavity length detuning tolerances and a comparatively narrow spectral bandwidth of < 15 GHz which is very suitable for applications such as molecular spectroscopy and pollutant detection. A pump depletion of 78% and 1 W of signal output power between 1461 nm and 1601 nm were obtained from an OPO based on periodically poled lithium niobate.
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