Alan J. Kemp

We report a continuous-wave Ti:sapphire laser with an output power of 1.03 W, achieved with two low-cost single-emitter diode pumps, both of blue wavelength (448 and 468 nm). Using a novel strategy of combining blue-wavelength pumping with a long, low-doping Ti:sapphire crystal, we maximise the available pump power while minimising deleterious effects associated with blue pump wavelengths, demonstrating Watt-level output powers.

A time-resolved Raman spectrometer is demonstrated based on a 256 × 8 single-photon avalanche diodes fabricated in CMOS technology (CMOS SPAD) line sensor and a 573-nm fiber-coupled diamond Raman laser delivering pulses with duration below 100-ps full-width at half-maximum (FWHM). The collected backscattered light from the sample is dispersed on the line sensor using a custom volume holographic grating having 1800 lines/mm. Efficient fluorescence rejection in the Raman measurements is achieved due to a combination of time gating on sub-100-ps time scale and a 573-nm excitation wavelength. To demonstrate the performance of the spectrometer, fluorescent oil samples were measured. For organic sesame seed oil having a continuous wave (CW) mode fluorescence-to-Raman ratio of 10.5 and a fluorescence lifetime of 2.7 ns, a signal-to-distortion value of 76.2 was achieved. For roasted sesame seed oil having a CW mode fluorescence-to-Raman ratio of 82 and a fluorescence lifetime of 2.2 ns, a signal-to-distortion value of 28.2 was achieved. In both cases, the fluorescence-to-Raman ratio was reduced by a factor of 24-25 owing to time gating. For organic oil, spectral distortion was dominated by dark counts, while for the more fluorescent roasted oil, the main source of spectral distortion was timing skew of the sensor. With the presented postprocessing techniques, the level of distortion could be reduced by 88%-89% for both samples. Compared with common 532-nm excitation, approximately 73% lower fluorescence-to-Raman ratio was observed for 573-nm excitation when analyzing the organic sesame seed oil.

Energy scaling of the 1st Stokes oscillation is compared in micro-lensed and plane-plane monolithic diamond Raman lasers under 532 nm pumping. A maximum Raman pulse energy of 92 μJ at 573 nm was achieved with the micro-lensed device, while in a plane-plane configuration the maximum Raman pulse energy was 3.1 mJ. 2nd Stokes generation at 620 nm in 2 and 1 mm long micro-lensed monolithic diamond Raman lasers is also reported. The best conversion efficiency from the pump at 532 nm, namely 63 %, was observed in a 2 mm long crystal at the pump pulse intensity of 4.5 GW/cm2. By measuring the output Raman laser beam caustic it was found that the 2nd Stokes intracavity beam radius at the output coupler of the micro-lensed device is at least two times smaller than that expected from the ABCD matrix calculations of the resonator mode. A Raman gain-guiding mechanism is suggested to explain this difference.

We report a broadly wavelength-tunable femtosecond diode-pumped Ti:sapphire laser, passively mode-locked using both semiconductor saturable absorber mirror (SESAM) and Kerr-lens mode-locking (KLM) techniques. Using two pump laser diodes (operating at 450 nm), an average output power as high as 433 mW is generated during mode-locking with the SESAM. A tunability range of 37 nm (788-825 nm) was achieved with the shortest pulse duration of 62 fs at 812 nm. In the KLM regime, an average output power as high as 382 mW, pulses as short as 54 fs, and a tunability of 120 nm (755-875 nm) are demonstrated.

The longest wavelength (~1.4 µm) emitted by a diamond Raman laser pumped by a semiconductor disk laser (SDL) is reported. The output power of the intracavity-pumped Raman laser reached a maximum of 2.3 W with an optical conversion efficiency of 3.4% with respect to the absorbed diode pump power. Narrow Stokes emission (FWHM 40 nm was achieved via rotation of an intracavity birefringent filter that tuned the SDL oscillation wavelength.

In this paper, we analyse the prospects for using nitrogen-vacancy centre (NV) containing diamond as a laser gain material by measuring its key laser related parameters. Synthetic chemical vapour deposition grown diamond samples with an NV concentration of ~1 ppm have been selected because of their relatively high NV concentration and low background absorption in comparison to other samples available to us. For the samples measured, the luminescence lifetimes of the NV- and NV0 centres were measured to be 8 ± 1 ns and 20 ± 1 ns, respectively. The respective peak stimulated emission cross-sections were (3.6 ± 0.1) × 10⁻¹⁷ cm² and (1.7 ± 0.1) × 10⁻¹⁷ cm². These measurements were combined with absorption measurements to calculate the gain spectra for NV- and NV0 for differing inversion levels. Such calculations indicate that gains approaching those required for laser operation may be possible with one of the samples tested and for the NV- centre.

One of the most common impurities in synthetic diamond is single substitutional nitrogen, which is incorporated in the diamond lattice substituting a carbon atom. If the nitrogen is adjacent to a vacancy in the diamond lattice, it forms the nitrogen-vacancy (NV) colour centre (CC). The negatively charged state of this CC, NV-, is particularly well studied since its quantum properties are suitable for applications such as quantum information processing, single-photon sources and optical magnetometry. NV CCs in the neutral state (NV0) are less widely studied. This CC exhibits broadband luminescence at slightly shorter wavelengths than NV-, and hence is also potentially of interest for tuneable and ultrafast visible laser applications. In this report, we present a detailed study of the laser-related spectroscopic properties of a diamond containing NV0 and NV-CCs.

Nitrogen vacancy (NV) centres are a type of defect in diamond that exhibit a number of interesting properties. NV centres can be optically excited around 450-650 nm and emit fluorescence at around 550-800 nm [1]. The optical excitation also spin polarises the negatively charged NV centres, permitting optical detection of magnetic resonance [2]. This allows nanotesla scale sensitive magnetometry to be performed with nanometre scale spatial resolution [3].To date, a direct measurement of stimulated emission from ensembles of NV centres in diamond has yet to be demonstrated experimentally. Despite this, stimulated emission depletion microscopy has been utilised in superresolution imaging that relies on the stimulated emission of single NV centres to quench fluorescence [4]. Recently, there has also been similar indirect experimental evidence of stimulated emission in NV ensembles [5]. We thus aimed to show both optical gain of a probe beam in a pumped NV-rich diamond sample and a modulation of this gain via microwave excitation of its ground level spin transitions. The experimental set-up consisted of a pump (532 nm) and probe (723-950 nm) laser beam combined with a dichroic mirror and focused into a bulk diamond specimen to a spot-size of around 90 microns. Pump excitation powers of up to 3 W and probe powers up to 0.5 W were used. The beams were then collimated and separated with a prism and the probe power measured with a photodiode. The diamond specimen was placed on an antenna to allow simultaneous excitation with microwaves. A double modulation scheme was employed to measure either the change in probe power due to the pump laser or the microwave excitation during any one experiment.

Summary form only given. Here we demonstrate, to the best of our knowledge, the first monolithic diamond Raman laser, converting pump pulses with duration of 86 ps at 532 nm to 39 ps pulses at 573 nm. With ~400 mW of average pump power, 150 mW of 1st Stokes 573 nm signal was achieved yielding a 37% conversion efficiency. As a Raman gain material, we used a synthetic single-crystal diamond (Element 6). The diamond was 0.5 mm thick and its surfaces were coated to form a monolithic plane-plane Raman laser cavity. The input coupler was highly transparent (HT) for the pump wavelength (532 nm) and highly reflective (HR) for the 1st Stokes (573nm). The output coupling was 82% reflective for 573 nm and HR at 532 nm to double-pass the pump light. Our pump source was a frequency-doubled SESAM Q-switched microchip MOPA producing 86 ps pulses and ~500 mW of average power at 100 kHz repetition rate.

Summary form only given. Recent advances in the growth of low loss single crystal diamond [1] coupled with its high Raman gain and high thermal conductivity have led to the material becoming an established Raman laser material. With applications often having to adapt to available laser wavelengths, Raman lasers can be used as a simple means to shift ubiquitous commercial laser sources to the hard to reach but application rich spectral regions. The authors recently demonstrated a compact, robust monolithic diamond Raman laser shifting 20μJ nanosecond pulses from a Q-switched 532nm laser to Raman wavelengths of 573nm, 620nm and 676nm with a conversion efficiency of 84% [2]. This work investigates the energy scalability of such a system.