Reduction of relative intensity noise in a diamond Raman laser.

The relative intensity noise (RIN) characteristics of a continuous-wave diamond Raman laser are investigated for the first time. The results reveal the parasitic stimulated Brillouin scattering (SBS) that usually occurred with higher-order spatial modes in the diamond Raman resonator is a pivotal factor impacting the Raman longitudinal modes and deteriorating the RIN level. The diamond Raman laser automatically switches to single-longitudinal-mode operation and the RIN level is significantly decreased in the frequency range of 200 Hz to 1 MHz after the parasitic SBS is effectively suppressed through inserting a spatial aperture or a χ(2) nonlinear crystal into the cavity. Due to the introduction of additional nonlinear loss to the high intensity Raman fluctuations and the non-lasing spontaneous Raman modes, the χ(2) nonlinear crystal enables better performance in the RIN-level reduction compared to the spatial aperture which can only achieve SBS inhibition. The RIN reduction routes are well suited for various crystalline Raman media to achieve high power and low intensity noise laser at different wavelengths.

Modulation depth and bandwidth analysis of planar thermo-optic diamond actuators.

Thermo-optic actuators based on bulk materials are considered too slow in applications such as laser frequency control. The availability of high-quality optical materials that have extremely fast thermal response times, such as diamond, present an opportunity for increasing performance. Here, diamond thermal actuators are investigated for configurations that use a planar thermal resistive layer applied to a heat-sinked rectangular prism. A general analytical formulation is obtained which simplifies substantially for high thermal conductivity such as diamond. Expressions for modulation depth, bandwidth and power requirements are obtained as functions of modulator dimensions and heat-transfer coefficients. For a 1 mm × 1 mm cross-section diamond at wavelength of 1 µm, around 450 W of applied heat power is needed to achieve a π phase shift at a modulation frequency of 2 kHz.

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.

Modeling and characterization of high-power single frequency free-space Brillouin lasers.

Free-space Brillouin lasers (BLs) are capable of generating high-power, narrow-linewidth laser outputs at specific wavelengths. Although there have been impressive experimental demonstrations of these lasers, there is an absence of a corresponding theory that describes the dynamic processes that occur within them. This paper presents a time-independent analytical model that describes the generation of the first-order Stokes field within free-space BLs. This model is based on the cavity resonance enhancement theory and coupled wave equations that govern the processes of stimulated Brillouin scattering (SBS). This model is validated using an experimental diamond BL to numerically simulate the influence of the cavity design parameters on the SBS threshold, pump enhancement characteristics, and power of the generated Stokes field. Specifically, the model is used to determine the SBS cavity coupler reflectance to yield the maximum Stokes field output power and efficiency, which is also a function of the pump power and other cavity design parameters. This analysis shows that the appropriate choice of Brillouin cavity coupler reflectance maximizes the Stokes field output power for a given pump power. Furthermore, the onset of higher-order Stokes fields that are undesirable in the context of single-frequency laser operation were inhibited. This study aids in understanding the relationship between the cavity parameters and resultant laser characteristics for the design and optimization of laser systems.

Narrow laser-linewidth measurement using short delay self-heterodyne interferometry.

Delayed self-heterodyne interferometry is a commonly used technique for the measurement of laser linewidth. It typically requires the use of a very long delay fiber when measuring narrow linewidth (especially linewidths in the kHz-range) lasers. The use of long fibers can result in system losses and the introduction of 1/f noise that causes spectral line broadening. In this paper, we present a calculation method for processing the output of a delayed self-heterodyne setup using a short length of delay fiber, to determine laser linewidth. The method makes use of pairs of data points (corresponding to adjacent maxima and/or minima) in the signal generated from the self-heterodyne setup to determine the laser linewidth. Here, the power ratio or amplitude difference of the signal at these data points is of importance. One of the key benefits of this method is that it avoids 1/f noise which would otherwise be introduced into the measurement through the application of long fibers. The experimental results highlight that the method has a high calculation accuracy. Furthermore, the capacity for the method to utilize different pairs of data points in the self-heterodyne output to determine the laser linewidth, imparts a high degree of flexibility and usability to the technique when applied to real-world measurements.

22.5-W narrow-linewidth diamond Brillouin laser at 1064†nm.

Stimulated Brillouin scattering (SBS), with its advantages of low quantum defect and narrow gain bandwidth, has recently enabled an exciting path toward narrow-linewidth and low-noise lasers. Whereas almost all work to date has been in guided-wave configurations, adaptation to unguided Brillouin lasers (BLs) offers a greater capacity for power scaling, cascaded Stokes control, and greater flexibility for expanding wavelength range. Here, we report a diamond Brillouin laser (DBL) employing doubly resonant technology at 1064 nm. Brillouin output power of 22.5 W with a linewidth of 46.9 kHz is achieved. The background noise from the pump amplified spontaneous emission (ASE) is suppressed by 35 dB. The work represents a significant step toward realizing Brillouin oscillators that simultaneously have high power (tens-of-watts+) and kHz-linewidths.

Integrated room temperature single-photon source for quantum key distribution.

High-purity single-photon sources (SPS) that can operate at room temperature are highly desirable for a myriad of applications, including quantum photonics and quantum key distribution. In this work, we realize an ultra-bright solid-state SPS based on an atomic defect in hexagonal boron nitride (hBN) integrated with a solid immersion lens (SIL). The SIL increases the source efficiency by a factor of six, and the integrated system is capable of producing over ten million single photons per second at room temperature. Our results are promising for practical applications of SPS in quantum communication protocols.

Integrated room temperature single-photon source for quantum key distribution: publisher's note.

This publisher's note contains a correction to Opt. Lett.47, 1673 (2022)10.1364/OL.454450.

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.

Cascaded Stokes polarization conversion in cubic Raman crystals.

We describe a theoretical approach based on Müller and tensor calculus for predicting the polarization state and gain of cascaded Stokes orders produced under coherent Raman scattering regime conditions. The formulation follows a Markovian-style implementation for F2g-type modes in Raman cubic crystals. The theoretical model is supported by experimental results that corroborate that the polarization and power of the cascaded Stokes orders can be effectively predicted using sequential calculus. We extend these results to a variety of crystal propagation directions, with the aim of facilitating the design of advanced solid-state Raman lasers.

Quantum-randomized polarization of laser pulses derived from zero-point diamond motion.

Intrinsic randomness in quantum systems is a vital resource for cryptography and other quantum information protocols. To date, randomizing macroscopic polarization states requires randomness from an external source, which is then used to modulate the polarization e.g. for quantum key-distribution protocols. Here, we present a Raman-based device for directly generating laser pulses with quantum-randomized polarizations. We show that crystals of diamond lattice symmetry provide a unique operating point for which the Raman gain is isotropic, so that the spontaneous symmetry breaking initiated by the quantum-random zero-point motion determines the output polarization. Experimentally measured polarizations are demonstrated to be consistent with an independent and identical uniform distribution with an estimated quantum entropy rate of 3.8 bits/pulse.

Widely-tunable single-frequency diamond Raman laser.

We report a diamond Raman laser that is continuously-tunable across the range from 590 nm to 625 nm producing continuous wave output with up to 8 W. The system is based on an all-fiber and tunable (1020-1072 nm) Yb-doped pump laser with a spectral linewidth of 25 GHz that is Raman-shifted and frequency doubled in a cavity containing diamond and a lithium triborate second harmonic crystal. Despite the broad pump spectrum, single frequency output is obtained across the tuning range 590-615 nm. The results reveal a practical approach to obtain tunable high-power single-frequency laser in a wavelength region not well served by other laser technologies.

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.

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.

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.

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.

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.