The field of nonlinear optics (NLO) has been continuously growing over the past decades, and several NLO data tables were published before the turn of the century. After the year 2000, there have been major advances in materials science and technology beneficial for NLO research, but a data table providing an overview of the post-2000 developments in NLO has so far been lacking. Here, we introduce a new set of NLO data tables listing a representative collection of experimental works published since 2000 for bulk materials, solvents, 0D-1D-2D materials, metamaterials, fiber waveguiding materials, on-chip waveguiding materials, hybrid waveguiding systems, and THz NLO materials. In addition, we provide a list of best practices for characterizing NLO materials. The presented data tables and best practices form the foundation for a more adequate comparison, interpretation, and practical use of already published NLO parameters and those that will be published in the future.
Diamond Raman lasers (DRLs) are novel high power, narrow linewidth sources with output wavelengths challenging to generate by other means, such as the 589 nm guide star line. While DRLs have been demonstrated to operate single-frequency at high powers for short durations, locking to a frequency reference has not yet been achieved. Here, we report an intracavity frequency doubled, 589 nm DRL stabilised to a wavemeter. Frequency fluctuations of +/- 75 MHz of the frequency setpoint were obtain over a 15 minute period. The effective and intrinsic linewidths were also measured, yielding 8 MHz and <900 Hz (resolution limited) respectively.
The field of Nonlinear Optics (NLO), launched about 60 years ago, has gained considerable momentum over the past two decades, resulting in an enormous growth in NLO publications for a wide range of material categories, including bulk materials, 0D-1D-2D materials, metamaterials, fiber waveguiding materials, on-chip waveguiding materials, and hybrid waveguiding systems. However, a convenient summary of NLO data collected since 2000 for these different material types has been lacking and would be a valuable resource for researchers in the field. Here, we present a new set of data tables showcasing a representative list of NLO properties taken from the literature since 2000 on the above-mentioned material categories. Furthermore, we provide best practices for performing and reporting NLO experiments. These best practices underpin the selection process that we used for including papers in the tables, and also form the foundation for a more adequate comparison, interpretation, and use of the NLO parameters published today and those that will be published in the future.
We report on a paratellurite (TeO2) Brillouin laser utilising an etalon as a dichroic input coupler in a three-mirror standing wave cavity. In this free-space crystal Brillouin laser arrangement, the oscillator transitions between two modes to produce either a Brillouin frequency comb (mode one) or cascade-suppressed single laser line output (mode two) by adjustment of the cavity length. In mode one, the cavity is multiply-resonant for pump enhancement and cascaded Stokes orders. In this mode, the generation of a Brillouin frequency comb spanning 104 GHz was demonstrated. In mode two, a singly resonant configuration for only the first Stokes order is selected. We obtain a factor of four reduction in linewidth was measured from the pump seed laser to the Brillouin laser output.
Diamond Raman lasers are an emerging high-power laser technology offering kW-level beam powers with high brightness. Here we report experimental observation of a diamond thermal lens at output powers up to 1.1 kW in a quasi-steady-state regime. It is found that a thermal lens of up 16 diopters is induced, which causes the beam quality to evolve over the investigated power range. The results aid design at higher power and clarify conditions under which beam quality degradation is expected when scaling output power.
High-power single-longitudinal-mode (SLM) lasers are crucial for applications such as LIGO, sodium guide stars, isotope separation and atom cooling, but are challenging due to gain profile inhomogeneities and spatial hole burning. This work shows that a SLM 620 nm laser is firstly achieved using a standing-wave diamond Raman resonator with intracavity second harmonic generation (SHG). In addition to the benefits of the spatial hole burning-free gain medium [1], SHG provides a valuable additional mechanism for increasing gain competition and therefore enhancing SLM stability. Nascent secondary modes experience double the nonlinear loss in the SHG crystal due to the role of sum frequency generation with the primary mode [2]. A further feature of the current external standing-wave cavity design is that the cavity is non-resonant at the pump frequency, thereby allowing the laser to be pumped without mutual control of the wavelength and cavity length and by using multi-longitudinal-mode (MLM) lasers with a spacing different to the free-spectral-range of the diamond Raman laser.
High precision magnetometry is important for a range of applications from the monitoring of biologically generated magnetic fields (e.g. magnetoencephalography and magnetocardiography), to navigation in GPS denied environments, to the detection of gravitational waves. Diamond containing the negatively-charged nitrogen vacancy colour centre (NV-) has emerged as a powerful room-temperature sensing solution. Here we explore NV- centres as a laser medium for a new form of magnetometry: laser threshold magnetometry (LTM). LTM works by placing NV- inside an optical cavity and uses the coherent laser output as a potentially more sensitive readout channel than is possible using conventional (incoherent) optically detected magnetic resonance. Here we show progress towards LTM with diamond. We show twolaser excitation and stimulated emission in free space, and report progress towards diamond-cavity experiments. Our studies highlight the need for different NV- optimisation for laser applications, rather than those conventionally used for quantum information applications
A quasi-continuous-wave external cavity diamond Raman laser with 1.2 kW output power was demonstrated using gated pump pulses of 100 µs duration, which was 7 times longer than the time constant for the establishment of steady-state temperature gradients. An 83% slope efficiency and a 53% optical-to-optical efficiency were obtained in conversion from a 1.064 µm Nd:YAG pump to a 1.24 µm first Stokes. The transient Stokes behavior from the sharp turn-on was consistent with calculations for the first Stokes build-up time. A brightness enhancement factor of 56 was obtained from the M2 = 15 pump beam. An observed increase in the first Stokes beam quality from M2 = 2.95 to M2 = 1.25 with increase of the Raman laser output power indicated the presence of a steady-state thermal lens in diamond.
We demonstrate a continuous-wave diamond Brillouin laser (DBL) in a ring cavity, operating near 532 nm with a 167 GHz Stokes shift. The DBL is pumped by a narrow-line (<1 MHz), frequency-doubled ytterbium fiber laser with the intracavity power resonantly enhanced via Hänsch-Couillaud locking. The measured threshold enabled the Brillouin gain coefficient in diamond to be determined for the first time, yielding a value of 60 cm/GW for pump and Brillouin polarizations aligned parallel to the 〈111〉 crystallographic direction in diamond (determined by the cut of our diamond in this case). Analysis of diamond’s photoelastic tensor shows that for polarization aligned to 〈110〉 for maximum gain, a coefficient of 115 cm/GW can be deduced, the highest bulk Brillouin gain coefficient reported for any material. The high Brillouin gain coefficient in combination with outstanding optical and thermal properties, indicates great potential for realizing diamond lasers and stimulated Brillouin scattering-enabled devices of performance far exceeding other materials.
We report a second-Stokes diamond Raman laser in eye-safe wavelength 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 output power is the highest single-mode power reported for Er-doped and Raman fiber lasers (~300 W). The measurements are in good agreement with model calculations, which we use to optimize and predict performance over wider range of power and input beam quality. The results highlight a novel pathway to high brightness eye-safe lasers based on relatively incoherent 1.0–1.1 μm pumps. The concept may be adapted and extended to other wavelength regions by using other pumps or via higher-order (3+) cascading. For example, to generate high brightness red output near 0.62 – 0.67 m by using second harmonic pumps near 0.53 μm. A large number of wavelength options are conceivable as a result of the wide transparency of diamond (0.23–3.8 µm, and > 6 µm).
We present our recent advances on power scaling of a high-power single-pass pumped CVD-diamond Raman oscillator at 1.2 μm. The single pass scheme reduced feedback to the high gain fiber amplifier, which pumps the oscillator. The Yb-doped multi-stage fiber amplifier itself enables up to 1 kW output power at a narrow linewidth of 0.16 nm. We operate this laser in quasi-cw mode at 10% duty cycle and on-time (pulse) duration of 10 ms. With a maximum conversion efficiency of 39%, a maximum steady-state output power of 380 W and diffraction limited beam quality was achieved.
We present our recent advances on power scaling of a high-power single-pass pumped CVD-diamond Raman oscillator at 1.2 μm. The single pass scheme reduced feedback to the high gain fiber amplifier, which pumps the oscillator. The Yb-doped multi-stage fiber amplifier itself enables up to 1 kW output power at a narrow linewidth of 0.16 nm. We operate this laser in quasi-cw mode at 10% duty cycle and on-time (pulse) duration of 10 ms. With a maximum conversion efficiency of 39%, a maximum steady-state output power of 380 W and diffraction limited beam quality was achieved.
We present our recent advances in the field of Raman frequency conversion using high-optical quality CVD-diamond. Different diamond Raman lasers were developed for efficiently generating multi-Watt output at specific wavelengths from the visible to the eye-safe spectral range, while single-frequency operation was accomplished by exploiting an intrinsic mode stability mechanism.
We present an OPO pumped mid IR diamond Raman laser with tuneable output from 3.49 μm to 3.78 μm, which to our knowledge is the longest wavelength produced in a solid state Raman laser. Up to 59 μJ is generated with a conversion efficiency of 10%. We also determine the Raman gain coefficient of diamond at 1.864 μm through measurement of the amplification of a seed signal. With pump and probe polarisations aligned with the <110< crystal axes a value of 4.8 cm/GW is measured, which corresponds to 6.4 cm/GW for polarisation aligned with the <111< crystal axes. Achievable conversion efficiencies were limited by multi-phonon absorption at the Stokes wavelength. Numerical modelling shows that increasing the output coupling factor of the cavity reduces the impact of multi-phonon absorption and leads to higher conversion efficiencies. By reducing the output coupler reflectivity from 55% to 5% and eliminating Fresnel reflections from cavity components, 30% conversion efficiency (44% quantum conversion efficiency) is predicted.
We report external cavity Raman lasers using a 9.5-mm-long low-loss CVD diamond pumped by a 35 W q-switched
Nd:YVO4 laser with approximately 22 ns pulses at 36 kHz pulse repetition frequency. Two systems were investigated.
The first demonstrated first and second Stokes simultaneously with more than 14.5 W of combined optical power. For an
output coupler optimized for second Stokes only output (1485 nm), 11.1 W at 38% conversion efficiency was obtained.
This output power is comparable to the maximum output powers reported for competing pulsed eye-safe technologies
such as optical parametric oscillators and Er:YAG lasers.
The performance of 100W small-scale "second-generation" kinetically-enhanced copper vapour lasers of active length im long and volume O.25-O.8L are reviewed. These systems have substantially reduced plasma tube insulation so that the lasers operate in a regime of high heat flux. The specific output powers are a factor of 2 higher than "first-generation" kinetically-enhanced copper vapour lasers and five-fold higher than copper vapour lasers operating without
HCl additive. It is shown that the spatio-temporal gain characteristics remain well suited to efficient extraction of high
beam quality output, as an amplifier or oscillator using unstable resonators. Radially-resolved measurements of the Cu density indicate that the plasma properties are much closer to equilibrium than expected from thermal models, indicating promise for further increases in specific output power. The implications for specific output power of H2:CuBr and Cu HyBrID lasers are also considered.
We review our recent studies into external cavity and intracavity potassium gadolinium tungstate Raman lasers generating output wavelengths in the range 555 nm to 669 nm. We have characterised the performance external cavity Raman lasers pumped by Q-switched 532 nm pump lasers at 5-10 kHz pulse repetition rates generating either single output wavelengths or multiple wavelengths simultaneously. Single output wavelengths are obtained with slope efficiencies up to 68% and maximum output powers ~0.5 W. Simultaneous output at 5 wavelengths (eg., 532 nm, 559 nm, 589 nm, 622 nm and 658 nm) is demonstrated with ~100 mW output power for at least 3 lines. Using the intracavity Raman laser scheme, we demonstrate a 0.3-1.8 W laser that is "user switchable" amongst wavelengths spanning the green to red eg., 532 nm - 555 nm - 579 nm - 606 nm, the wavelengths corresponding to frequency sums and mixing of Stokes and fundamental intra-cavity fields.
Development of pulsed excitation techniques for high-pressure dielectric barrier discharges (DBD) has led to a short-pulsed, high-peak-power, spatially uniform source of UV7VUV radiation -- a preferred type of output for materials processing and many other applications. Results of such a Xe2* DBD source at 172 nm for removing mountants from optical surfaces and for removing hydrocarbon contamination from optical and polymer surfaces are presented.
We have investigated the power scaling behavior of a kinetically enhanced copper vapor laser as a function of the plasma tube thermal insulation. By reducing the insulation to levels much lower than typically used for conventional copper vapor lasers we obtained increases in the specific output power of a small-to-medium scale device (0.8L) up to 130W/litre (i.e., total output power 104W). The laser wall-plug efficiency remains approximately constant (~1.4%) when the supplied power is increased from 5.9kW to 7.4kW. Radially-resolved Cu density measurements show that the depletion of Cu atoms is similar to other devices having much lower specific input power. The results show that kinetic enhancement reduces the depletion of atoms from the axial region by ion-pumping due to reduced fractional ionization and rapid charge recombination during the interpulse period.
We are presently developing nonlinear spectroscopic techniques for measuring the spatio-temporal density behaviour of H and halogen atoms species to investigate the action of these species in halogen enhanced copper lasers. In this paper, we report time resolved measurements of ground-state H atom density in a Cu HyBrID laser obtained using two-photon allowed laser induced fluorescence. Ultraviolet probe radiation (205nm) was focussed through an end window to excite the n=3 level and the resultant Balmer fluorescence detected via a side window. The results indicate that the H atom density is depleted to less than half its prepulse value during the excitation pulse and then recovers almost fully during the first 20 microseconds of the interpulse period. By interrupting the discharge, we also have used the technique to investigate the H reassociation rate at the tube wall. The results suggest the technique, which may also be adapted to obtain halogen densities and gas temperature measurements, is a promising diagnostic for investigating halogen enhanced Cu laser vapor plasmas.
The performance of a small-scale KE-CVL operating at a pulse repetition rate of 50kHz has been investigated by comparing modelling results and experimental data to understand the underlying plasma kinetics issues for pulse repetition rate scaling. Comparison between initial modelling predictions and experimental results relating to the laser output power and the population densities of the metastable lower laser levels (Cu* 4s2 2D3/2,5/2 ) suggests that there is an additional de-excitation mechanism for the metastables in the KE-CVL. The most likely de-excitation mechanism involves vibrationally-excited H2(v) molecules Cu*2D+H2(v) => CuH+H. A rate-coefficient of 2x10-16m3s1 is obtained by comparing modelling results with experimental data relating to the laser output power and the time-evolution of the axial 2D3/2 metastable population density.
We have measured the efficiency and spatial characteristics of output radiation as a function of fill pressure for a Xe excimer lamp employing a short voltage pulse (approximately 100 ns) excitation circuit, and compared the results with those obtained using conventional AC (ie. sinusoidal voltage waveform). When using pulsed excitation, VUV output is obtained from a homogeneous discharge at efficiencies which increase linearly with Xe pressure in the range 50 - 750 torr up to 3.2X the maximum efficiency obtained when using AC. When using AC excitation, the efficiency saturates with increasing pressure > 350 torr for which pressures the discharge appearance is altered from a diffuse discharge to one which is comprised of stochastic or stationary filaments. We have also recorded discharge spectra which highlight the different character of the homogeneous and filamented types of discharges. It is deduced that the enhanced efficiency arises due to the capability of pulsed excitation to produce a homogeneous (glow-like) discharge at higher pressure, which brings about more optimal electron density and temperature conditions for exclusively exciting Xe metastables than possible using AC. We attribute the homogenizing effect of short-pulsed excitation to the rapid rate at which the applied E-field increases to the necessary value for homogeneous discharge breakdown to proceed at a faster rate than the formation of filaments.
Investigations of the factors that limit average power scaling of elemental copper vapor lasers (CVLs) have demonstrated that decay of the electron density in the interpulse period is critical in restricting pulse repetition rate and laser aperture scaling. We have recently developed the 'kinetic enhancement' (or KE) technique to overcome these limitations, whereby optimal plasma conditions are engineered using low concentrations of HCl/H2 additive gases in the Ne buffer. Dissociative electron attachment of HCl and subsequent mutual neutralization of Cl- and Cu+ promote rapid plasma relaxation and fast recovery of Cu densities, permitting operation at elevated Cu densities and pulse rates for given apertures. Using this approach, we have demonstrated increases in output power and efficiency of a factor of 2 or higher over conventional CVLs of the same size. For a 38 mm- bore KE-CVL, output powers up to 150 W have been achieved at 22 kHz, corresponding to record specific powers (80 mW/cm3) for such a 'small/medium-scale' device. In addition, kinetic enhancement significantly extends the gain duration and restores gain on-axis, even for high pulse rates, thereby promoting substantial increases (5 - 10x) in high- beam-quality power levels when operating with unstable resonators. This has enabled us to achieve much higher powers in second-harmonic generation from the visible copper laser output to the ultraviolet (e.g. 5 W at 255 nm from a small- scale KE-CVL). Our approach to developing KE-CVLs including computer modeling and experimental studies will be reviewed, and most recent results in pulse rate scaling and scaling of high-beam-quality power using oscillator-amplifier configurations, will be presented.
We present results of single shot ablation experiments for a variety of metal samples (In, Al, Cu, Mo, W, Ti) using visible, nanosecond lasers at fluences up to approximately 104 J cm-2. At low fluences, usually less than 102 J cm-2, small amounts of material were removed and removal was approximately uniform across the ablation crater. As the fluence increased above approximately 102 J cm-2, substantially more material was removed and a conical pit developed in the center of the ablation crater. The appearance of these conical pits is consistent with material removed by phase explosion mechanisms. In this paper, this ablation phenomenon will be investigated by presenting the crater morphology as a function of fluence. Consequences for micromachining with visible, high repetition rate, nanosecond lasers will be discussed.
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