Investigation of intracavity MgO:LiNbO3 terahertz polariton laser gain over an extended 1-5 THz range.

Most intracavity MgO:LiNbO3 terahertz polariton lasers generate output in the 1-3 THz range. In this study, we investigate the potential to extend this tuning range toward 6 THz. We predict broader tuning using a modified polariton gain theory that takes account of additional polariton damping at 3.15 THz and the angle-dependent field overlap in the crystal. We achieved continuous tuning between 1.5 and 5.4 THz and characterised the gain in this range-so far the widest THz frequency range achieved from a nonlinear crystal via intracavity SPS.

Monte Carlo modelling for elastic and Raman signals in oceanic LiDAR.

We investigate Raman returns in oceanic light detection and ranging (LiDAR), and explore the similarities and differences to conventional elastic returns. We show that Raman returns have much more complex behavior than elastic returns, which likely cannot be well matched by simple models, making Monte Carlo simulations essential. We investigate the correlation between the time of signal arrival and the depth of the Raman event, and find that a linear correlation only exists for judicious choice of system parameters.

Secondary Raman and Brillouin mode suppression in two- and three-mirror-cavity diamond Raman lasers.

We report an investigation into secondary mode suppression in single longitudinal mode (SLM) 1240 nm diamond Raman lasers. For a three-mirror V-shape standing-wave cavity incorporating an intra-cavity LBO crystal to suppress secondary modes, we achieved stable SLM output with a maximum output power of 11.7 W and a slope efficiency 34.9%. We quantify the level of χ(2) coupling necessary to suppress secondary modes including those generated by stimulated Brillouin scattering (SBS). It is found that SBS-generated modes often coincide with higher-order spatial modes in the beam profile and can be suppressed using an intracavity aperture. Using numerical calculations, it is shown that the probability for such higher-order spatial modes is higher for an apertureless V-cavity than in two-mirror cavities due its contrasting longitudinal mode-structure.

Mid-term results of the treatment of complex acetabular fractures with combined acute fixation and total hip replacement in the older patient.

INTRODUCTION: The incidence of acetabular fractures in older patients is increasing. The management of these patients is evolving due to the recognition of risks associated with prolonged immobility with conservative treatment. MATERIALS AND METHODS: Consecutive patients undergoing fixation and total hip replacement (THR) for displaced acetabular fractures undergoing single operation with acetabular fixation and THR were identified. Outcomes were assessed using radiographs, clinical notes, Oxford Hip Score and EuroQol-5L. RESULTS: 77 patients were identified with 51 completing outcome scores. Mean age 68 years at time of injury. Mean follow-up 5 (2-12) years, OHS 40, EQ-5D 0.78. Revision surgery performed in 7 patients (9%). DISCUSSION: Acute fixation combined with THR for acetabular fractures in the elderly patient, offers good functional outcomes and a low complication rate in the mid-term.

Cavity design with single-mirror THz frequency tuning for polariton lasers.

We demonstrate a new, to the best of our knlowledge, cavity design for terahertz (THz) lasers based on stimulated polariton scattering (SPS). The design simplifies the angle tuning of these lasers, which require non-collinear cavity fields at fundamental and Stokes wavelengths to cross in an SPS crystal with adjustable crossing angle. A mirror shared by both the fundamental and Stokes cavities ensures stationary overlap of the fields within the crystal, with the angle between the fields tunable by adjustment of one axis of a single mirror. We demonstrate the design for an intracavity SPS laser using a rubidium titanyle phosphate (RTP) crystal, and achieve single-mirror tuning of the THz output in bands between 3 and 5.8 THz, with a maximum output of 78 µW at 4.08 THz.

Diamond Raman laser and Yb fiber amplifier for in vivo multiphoton fluorescence microscopy.

Here we introduce a fiber amplifier and a diamond Raman laser that output high powers (6.5 W, 1.3 W) at valuable wavelengths (1060 nm, 1250 nm) for two-photon excitation of red-shifted fluorophores. These custom excitation sources are both simple to construct and cost-efficient in comparison to similar custom and commercial alternatives. Furthermore, they operate at a repetition rate (80 MHz) that allows fast image acquisition using resonant scanners. With our system we demonstrate compatibility with fast resonant scanning, the ability to acquire neuronal images, and the capability to image vasculature at deep locations (>1 mm) within the mouse cerebral cortex.

Generation of sub-100 fs ultraviolet pulses from a Kerr-lens mode-locked Ce:LiCAF laser.

We report the direct generation of mode-locked pulses as short as 91 fs from the broad-bandwidth gain medium of LiCaAlF6 (Ce:LiCAF) by combining Kerr-lens mode locking with synchronous pumping. The latter of these schemes, and the broad bandwidth of Ce:LiCAF, resulted in dispersion tuning of wavelength via cavity length in the spectral region of 290 nm; this mechanism facilitated a practical means of estimating intra-cavity dispersion, which was compensated for using a Brewster's-cut prism pair. The pulse duration was measured via split-beam asynchronous cross-correlation using a Ti:sapphire reference laser and a known time reference. From the Ce:LiCAF laser cavity, output powers of 110 mW and a 9% slope efficiency were achieved.

Modelling and characterisation of continuous wave resonantly pumped diamond Raman lasers.

We present experimental results and modeling of continuous wave resonantly pumped Raman lasers. The first Stokes diamond Raman ring laser generated 0.6 W at 960 nm with an efficiency of 18%; the second Stokes laser generated 1.5 W at 1485 nm at 9% efficiency. The analytical model, extended to arbitrary Stokes orders, shows the importance of modelling imperfect mode matching and guides the optimization of input and output coupler reflectivity to predict practical watt-level Raman converters of few-watt pump lasers.

Analysis of a thermal lens in a diamond Raman laser operating at 1.1 kW output power.

We report experimental observations of thermal lens effects in a diamond Raman laser operating up to 1.1 kW output power in a quasi- steady-state regime. Measured changes in the output beam parameters as a function of output power, including beam quality factor and beam divergence after a fixed focusing lens, are compared to modelling enabling us to track the development of a thermal lens up to 16 diopters at maximum output power. Analysis shows that good agreement between model and experiment is obtained by considering the power deposition profile and the spatial overlap with the laser mode. The results clarify previous work that raised questions about thermal lens effects in the diamond gain medium and provides increased confidence in thermal models for determining the power limits for the current design.

Broadly tunable linewidth-invariant Raman Stokes comb for selective resonance photoionization.

We demonstrate a continuously tunable, multi-Stokes Raman laser operating in the visible range (420 - 600 nm). Full spectral coverage was achieved by efficiently cascading the Raman shifted output of a tunable, frequency-doubled Ti:Sapphire laser. Using an optimized hemi-spherical external Raman cavity composed only of a diamond crystal and a single reflecting mirror, producing high power output at high conversion efficiency (>60 % from pump to Stokes) for a broad range of wavelengths across the visible. Enhancement of the cascading was achieved by controlling the polarization state of the pump and Stokes orders. The Stokes outputs exhibited a linewidth of 11 ± 1 GHz for each order, resembling the pump laser linewidth, enabling its use for the intended spectroscopic applications. Furthermore, the Raman laser performance was demonstrated by applying it for the resonance excitation of atomic transitions in calcium.

Diamond sodium guide star laser.

Laser guide stars based on the mesospheric sodium layer are becoming increasingly important for applications that require correction of atmospheric scintillation effects. Despite several laser approaches being investigated to date, there remains great interest in developing lasers with the necessary power and spectral characteristics needed for brighter single or multiple guide stars. Here we propose and demonstrate a novel, to the best of our knowledge, approach based on a diamond Raman laser with intracavity Type I second-harmonic generation pumped using a 1018.4 nm fiber laser. A first demonstration with output power of 22 W at 589 nm was obtained at 18.6% efficiency from the laser diode. The laser operates in a single longitudinal mode (SLM) with a measured linewidth of less than 8.5 MHz. The SLM operation is a result of the strong mode competition arising from the combination of a spatial-hole-burning-free gain mechanism in the diamond and the role of sum frequency mixing in the harmonic crystal. Continuous tuning through the Na D line resonance is achieved by cavity length control, and broader tuning is obtained via the tuning of the pump wavelength. We show that the concept is well suited to achieve much higher power and for temporal formats of interest for advanced concepts such as time-gating and Larmor frequency enhancement.

Investigating single-longitudinal-mode operation of a continuous wave second Stokes diamond Raman ring laser.

We report a diamond Raman ring cavity laser resonantly pumped by a tunable Ti:sapphire continuous wave laser. We characterize the laser operation generating first Stokes output and, for the first time, generate second Stokes lasing at a maximum output power of 364 mW with 33.4% slope efficiency at 1101.3 nm. Single longitudinal mode operation is achieved for all first Stokes output powers, but only for lower output powers for second Stokes operation. We discuss possible reasons preventing single longitudinal mode operation.

Continuously tunable diamond Raman laser for resonance laser ionization.

We demonstrate a highly efficient, tunable, ∼5 GHz linewidth diamond Raman laser operating at 479 nm. The diamond laser was pumped by a wavelength-tunable intracavity frequency-doubled titanium sapphire (Ti:Sapphire) laser operating at around 450 nm, at a repetition rate of 10 kHz with a pulse duration of 50 ns. The Raman resonator produced a continuously tunable output with high stability, high conversion efficiency (28%), and beam quality (M2<1.2). We also demonstrate that the linewidth and tunability of the pump laser is directly transferred to the Stokes output. Our results show that diamond Raman lasers offer great potential for spectroscopic applications, such as resonance laser ionization, in an all-solid-state platform.

Generalised theory of polarisation modes for resonators containing birefringence and anisotropic gain.

Polarisation eigenmode theory is well established for laser cavities in which the principal axes for gain and polarisation elements are parallel. Here we generalise the theory to include the case for gain axes at arbitrary angle to the birefringence, which is the case for Raman lasers based on cubic-class gain crystals that contain stress-induced birefringence. The theory describes regimes dominated by gain, linear or circular birefringence, and the intermediate regime in which elliptically polarised output modes are obtained. Previously reported behaviour for diamond Raman lasers are found to be in accord with the findings. Design criteria are obtained to enable prediction of polarisation behaviour as functions of birefringence and resonator design.

1.2 kW quasi-steady-state diamond Raman laser pumped by an M2 = 15 beam.

An external cavity diamond Raman laser with 1.2 kW output power is demonstrated for durations 7 times longer than the thermal lens time constant. An 83% slope efficiency and a 53% optical-to-optical efficiency were obtained for conversion from a 1.06 µm pump to the 1.24 µm first Stokes. The pump had an M2 of 15, demonstrating that efficiency is maintained at the highest levels even when using exceptionally poor quality pumps. We show that a measured decrease in the output beam quality factor from M2=2.95 to M2=1.25 as power increased is evidence for thermal lens development in the diamond. The results foreshadow development of continuous-wave kilowatt-class lasers or amplifiers based on single diamond elements and pumped efficiently by lasers having poor spatial coherence such as line-narrowed diode laser arrays.

A single-frequency intracavity Raman laser.

A continuous-wave (CW) single-longitudinal-mode (SLM) intracavity Raman laser is demonstrated for the first time, by virtue of the spatial hole-burning free nature of stimulated Raman scattering (SRS) gain. By using a single etalon in the Nd:GdVO4 fundamental laser cavity, the spectral linewidth of the multimode fundamental field is suppressed below the Raman linewidth of Raman crystal BaWO4; hence power in all the longitudinal modes of fundamental field can be extracted by one single Stokes mode. Therefore, the hole-burning free SRS gain exhibits a spectral cleanup effect whereby a stable SLM Stokes field is derived from the multimode fundamental field within a simple standing-wave cavity arrangement. The low-threshold SLM Raman laser delivered 3.42 W SLM Stokes and 1.53 W SLM one-way yellow harmonic at the guide-star wavelength of 589.16 nm. The results here provide a new approach to SLM laser operation with good simplicity and power dynamic range. Further engineering for power scaling and better stability is also discussed.

Single-frequency 620 nm diamond laser at high power, stabilized via harmonic self-suppression and spatial-hole-burning-free gain.

Single longitudinal mode (SLM) operation of a 620 nm diamond Raman laser is demonstrated in a standing-wave cavity that includes a second-harmonic generation element. Mode competition provided by the harmonic mixing is shown to greatly increase mode stability, in addition to the benefits of the spatial-hole-burning-free gain medium. Using a multi-longitudinal mode 1064 nm Nd:YAG pump laser of power 321 W and linewidth 3.3 GHz, SLM powers of 38 W at 620 nm and 11.8 W at 1240 nm were obtained. The results indicate that simple standing-wave oscillators pumped by multimode Yb or Nd pumps compose a promising practical route towards the generation of high-power SLM beams in the yellow-red part of the spectrum.

302 W quasi-continuous cascaded diamond Raman laser at 1.5 microns with large brightness enhancement.

We report a second Stokes diamond Raman laser at 1.49 µm capable of high power and large-scale-factor brightness enhancement. Using a quasi-continuous 1.06 µm pump of power 823 W (0.85% duty cycle) and M2 up to 6.4, a maximum output power of 302 W was obtained with an M2 = 1.1 providing an overall brightness enhancement factor of 6.0. The pulse length of ~210 µs was selected to ensure operation was representative of steady-state continuous lasing conditions in the diamond bulk. Accompanying theoretical calculations indicate that even more strongly aberrated pumps may be used to efficiently generate high beam quality output and with higher brightness enhancement factors. This diamond-based beam conversion technique addresses needs for high brightness and efficient eye-safe sources using low-brightness 1 µm pumps and reveals a widely-applicable route to practical high brightness lasers of increased wavelength range.

Tunable THz polariton laser based on 1342 nm wavelength for enhanced terahertz wave extraction.

We detail the operation of a THz laser source based on non-linear stimulated polariton scattering (SPS) in Mg:LiNbO3. This system utilizes a fundamental wavelength of 1342 nm to completely avoid the negative effect of free-carrier generation within high-resistivity silicon (Si) prisms used to extract THz radiation from the Mg:LiNbO3 crystal. THz power of up to 23.6 µW (62.3 µW when chopped at 50% duty cycle) was detected at 1.33 THz, and frequency tunability across the range 1.05-2.2 THz was achieved.

Single-longitudinal-mode ring diamond Raman laser.

We demonstrate a single-longitudinal-mode ring diamond Raman laser, pumped by a tunable single-mode Ti:sapphire laser. Two methods to achieving unidirectional operation have been demonstrated: increasing gain for one direction using a reinjecting mirror and increasing loss for one direction using sum frequency mixing in BBO. Both methods result in a stable single-longitudinal-mode operation.