Four-wave mixing (FWM) in optical fibre is a leading technique for generating high-quality photon pairs. We report the generation of photon pairs by spontaneous FWM in photonic crystal fibre pumped by a 1.5 GHz repetition-rate vertical-external-cavity surface-emitting laser (VECSEL).The photon pairs exhibit high count rates and a coincidence-to-accidental ratio of over 80. The VECSEL's high repetition-rate, high average power, tunability, and small footprint make this an attractive source for quantum key distribution and photonic quantum-state engineering. * Electronic address: p.mosley@bath.ac.ukThe drive towards photonic quantum-enhanced technologies is placing ever more stringent demands on the performance of nonclassical light sources [1][2][3]. However due to their size, cost, complexity, and limited generation rates it is challenging to incorporate state-of-the-art photon sources into turn-key systems. Nevertheless in recent years, huge advances have been made in producing higher-quality single photons from straightforward equipment operating at room temperature. Optical nonlinearity has been at the forefront of these efforts [4][5][6][7][8][9][10].By propagating a high-intensity laser pulse through a nonlinear medium, pairs of photons can be spontaneously generated, either through parametric downconversion (PDC), a threewave mixing process that requires the presence of χ (2) nonlinearity, or by χ (3) -mediated fourwave mixing (FWM). The highest-performance sources are usually pumped by Ti:Sapphire oscillators [11][12][13][14][15][16], placing them orders of magnitude higher in both complexity and cost than the attenuated laser sources typically used in commercially-available quantum technologies. Furthermore, the repetition rate of these lasers, usually 80 MHz or so, places a limit on the rate at which high-quality photon pairs can be delivered by a single source [17]. These factors present significant obstacles to implementing real-world photonic quantum-enhanced technologies.In this paper we demonstrate photon-pair generation through four-wave mixing in a photonic crystal fibre (PCF) driven by a 1.5 GHz modelocked tunable vertical-external-cavity surface-emitting laser (VECSEL) [18]. VECSELs have not previously been used for photonpair generation but are attractive for a number of reasons [19]. Modelocked VECSELs produce transform-limited ultrafast pulses with peak power in the correct range for photonpair generation by FWM in fibre [20]. Modelocking a VECSEL requires a compact cavity containing only three or four components providing the possibility of very small-footprint photon-pair sources that require little maintenance. The short cavity results in a high pulse repetition frequency which could allow an order of magnitude increase in the rate of pair generation relative to 80 MHz laser systems. VECSELs are intrinsically flexible: the wavelength can be continuously tuned by an intracavity etalon [18]; the repetition rate can be adjusted without interrupting modelocking by translating the output couple...
Abstract:We describe time-resolved measurements of the evolution of the spectrum of radiation emitted by an optically-pumped continuous-wave InGaAs-GaAs quantum well laser, recorded as lasing builds up from noise to steady state. We extract a fitting parameter corresponding to the gain dispersion of the parabolic spectrum equal to −79 ± 30 fs 2 and −36 ± 6 fs 2 for a resonant and anti-resonant structure, respectively. Furthermore the recorded evolution of the spectrum allows for the calculation of an effective FWHM gain bandwidth for each structure, of 11 nm and 18 nm, respectively.
We report a mode-locked Vertical-External-Cavity Surface-Emitting Laser (VECSEL) that exhibits 13.7 nm of tuning around a centre wavelength of 1042 nm. The wavelength tuning is achieved by incorporating an uncoated, 25 µm thick, fused silica etalon into the cavity of the laser at Brewster's angle. The etalon is then tilted with respect to the cavity axis. The etalon has a calculated free spectral range of 14 nm at normal incidence. The repetition rate of the laser is measured to be 1.88 GHz. The pulse duration, averaged over the tuning range, is 1.9 ps corresponding to a mean time bandwidth product of 0.46. For a sech 2 pulse this is 1.46 times larger than the transform limit. The average power of the laser does not fall below 2.6 mW and, over the tuning range, averages 3.5 mW. With appropriate amplification, such a laser would be highly suited to the generation of heralded single photons in photonic crystal fibre.
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