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We demonstrate 1.4 milliwatt average power, broadband terahertz generation extending up to 6 THz at 10 MHz repetition rate through optical rectification in the organic crystal HMQ-TMS. The pump source is based on a simple external pulse compression scheme for the generation of ultra-short pulses using compact Yb:fiber laser technology.

We demonstrate the first megahertz (MHz) repetition-rate, broadband terahertz (THz) source based on optical rectification in the organic crystal HMQ-TMS driven by a femtosecond Yb:fibre laser. Pumping at 1035 nm with 30 fs pulses, we achieve few-cycle THz emission with a smooth multi-octave spectrum that extends up to 6 THz at -30 dB, with conversion efficiencies reaching 10−4 and an average output power of up to 0.38 mW. We assess the thermal damage limit of the crystal and conclude a maximum fluence of ∼1.8 mJ·cm−2 at 10 MHz with a 1/e2 pump beam diameter of 0.10 mm. We compare the performance of HMQ-TMS with the prototypical inorganic crystal gallium phosphide (GaP), yielding a tenfold electric field increase with a peak on-axis field strength of 7 kV·cm−1 and almost double the THz bandwidth. Our results further demonstrate the suitability of organic crystals in combination with fibre lasers for repetition-rate scaling of broadband, high-power THz sources for time-domain spectroscopic applications.

We demonstrate an optical-to-THz energy conversion efficiency of 5.5 ⋅ 10−4, by pumping the organic crystal HMQ-TMS with the externally compressed output of a Yb:fibre laser, enabling several kV/cm field strengths at MHz repetition rates.

A mid-IR source based on difference frequency generation is shown capable of performing a 100 nm wide scan centred at 2.9 Um in under 3 minutes. Its use for measuring the quality factors of photonic crystal cavities is demonstrated.

Ultrafast terahertz (THz) spectroscopy is a potent tool for studying the fundamental properties of matter. Limitations of current THz sources, however, preclude the technique being applied in certain advanced configurations or in the measurement of e.g. strongly absorbing samples. In response to this problem, here we demonstrate the generation of 1.38 mW broadband THz radiation at 10 megahertz (MHz) repetition rate by combining the highly efficient nonlinear organic crystal HMQ-TMS with ultrafast pump pulses generated using a simple and stable external pulse compression of a high power, near-infrared (NIR) femtosecond ytterbium-doped fibre (Yb:fibre) laser. Utilising spectral broadening in a large core, polarisation maintaining photonic crystal fibre and a pair of SF11 prisms, we achieve a tenfold pulse compression of the Yb:fibre laser, yielding compressed 0.35 µJ pulses with a full-width at half maximum pulse duration of 22 fs, exerting a peak power of 13.8 MW at a repetition rate of 10 MHz. THz generation through optical rectification of the NIR pulses is explored in two distinct thicknesses of the organic crystal, leading to a maximum conversion efficiency of ∼5.5·10−4 , an order of magnitude higher than that achieved with inorganic nonlinear crystals, e.g. gallium phosphide, for similar pump parameters. The focused THz beam has a peak on-axis field strength greater than 6.4 kV·cm−1 in unpurged atmosphere. We believe that our moderately strong-field THz source is well suited to a variety of applications in ultrafast THz spectroscopy, in particular THz-enabled scattering-type near-field, and scanning tunnelling spectroscopy, where multi-MHz repetition rate sources are required.

Unlocking the true potential of optical spectroscopy on the nanoscale requires development of stable and low-noise laser sources. Here, we have developed a low-noise supercontinuum (SC) source based on an all-normal dispersion fiber pumped by a femtosecond fiber laser and demonstrate high resolution, spectrally resolved near-field measurements in the near-infrared (NIR) region. Specifically, we explore the reduced-noise requirements for aperture-less scattering-type scanning near-field optical microscopy (s-SNOM), including inherent pulse-to-pulse fluctuation of the SC. We use our SC light source to demonstrate the first NIR, spectrally resolved s-SNOM measurement, a situation where state-of-the-art commercial SC sources are too noisy to be useful. We map the propagation of surface plasmon polariton (SPP) waves on monocrystalline gold platelets in the wavelength region of 1.34-1.75 μm in a single measurement, thereby characterizing experimentally the dispersion curve of the SPP in the NIR. Our results represent a technological breakthrough that has the potential to enable a wide range of new applications of low-noise SC sources in near-field studies.

Attenuation of ultrashort THz pulses poses a significant technological challenge due to the broadband nature of such light pulses. Several methods exist for this purpose, including crossed wire grid polarizers, high refractive index, high resistivity silicon wafers, and ultrathin metal films. In this review, we discuss the operational principles of these methods, and highlight some of the advantages and potential pitfalls of the methods. We describe the limits of high-frequency operation of wire grid polarizers, relevant for contemporary ultra-broadband THz sources in air photonics. We discuss the effects of multiple reflections and interference in sequences of silicon wafers for attenuation, and finally discuss the potential of using ultrathin metallic films for broadband attenuation.

We demonstrate a gain-switched thulium fiber laser that can be continuously tuned over 140 nm, while maintaining stable nanosecond single-pulse operation. To the best of our knowledge, this system represents the broadest tuning range for a gain-switched fiber laser. The system simplicity and wideband wavelength tunability combined with the ability to control the temporal characteristics of the gain-switched pulses mean this is a versatile source highly suited to a wide range of applications in the eye-safe region of the infrared, including spectroscopy, sensing and material processing, as well as being a practical seed source for pumping nonlinear processes., 4 pages, 5 figures

We use liquid phase exfoliation to produce dispersions of molybdenum disulfide (MoS2) nanoflakes in aqueous surfactant solutions. The chemical structures of the bile salt surfactants play a crucial role in the exfoliation and stabilization of MoS2. The resultant MoS2 dispersions are heavily enriched in single and few (6) layer flakes with large edge to surface area ratio. We use the dispersions to fabricate free-standing polymer composite wide-band saturable absorbers to develop mode-locked and Q-switched fiber lasers, tunable from 1535 to 1565 and 1030 to 1070 nm, respectively. We attribute this sub-bandgap optical absorption and its nonlinear saturation behavior to edge-mediated states introduced within the material band-gap of the exfoliated MoS2 nanoflakes.

We report an environmentally stable nanotube mode-locked fibre laser producing linearly-polarized, nanosecond pulses. A simple all-polarization-maintaining fibre ring cavity is used, including 300 m of highly nonlinear fibre to elongate the cavity and increase intracavity dispersion and nonlinearity. The laser generates scalar pulses with a duration of 1.23 ns at a centre wavelength of 1042 nm, with 1.3-nm bandwidth and at 641-kHz repetition rate. Despite the long cavity, the output characteristics show no significant variation when the cavity is perturbed, and the degree of polarization remains at 97%.

Due to their strong light-matter interaction, monolayer transition metal dichalcogenides (TMDs) have proven to be promising candidates for nonlinear optics and optoelectronics. Here, we characterize the nonlinear absorption of chemical vapour deposition (CVD)-grown monolayer MoSe2 in the 720–810 nm wavelength range. Surprisingly, despite the presence of strong exciton resonances, monolayer MoSe2 exhibits a uniform modulation depth of ∼80 ± 3% and a saturation intensity of ∼2.5 ± 0.4 MW/cm2. In addition, pump-probe spectroscopy is performed to confirm the saturable absorption and reveal the photocarrier relaxation dynamics over hundreds of picoseconds. Our results unravel the unique broadband nonlinear absorptive behavior of monolayer MoSe2 under ultrafast excitation and highlight the potential of using monolayer TMDs as broadband ultrafast optical switches with customizable saturable absorption characteristics.

© 2015.Two-dimensional (2D) nanomaterials are an emergent and promising platform for future photonic and optoelectronic applications. Here, we review recent progress demonstrating the application of 2D nanomaterials as versatile, wideband saturable absorbers for Q-switching and mode-locking fibre lasers. We focus specifically on the family of few-layer transition metal dichalcogenides, including MoS2, MoSe2 and WS2.

The efficient nonlinear conversion of Yb-doped fiber laser systems using a combination of stimulated Raman scattering and second-harmonic generation is an effective method for developing sources for biophotonic applications in the yellow–green spectral region. In this paper, we review recent progress in the development of these sources, compare the relative benefits of differing source architectures, and demonstrate stimulated emission depletion microscopy using an exemplar source.

We develop an analytical model to describe sub-band-gap optical absorption in two-dimensional semiconducting transition metal dichalcogenide (s-TMD) nanoflakes. The material system represents an array of few-layer molybdenum disulfide crystals, randomly orientated in a polymer matrix. We propose that optical absorption involves direct transitions between electronic edge states and bulk bands, depends strongly on the carrier population, and is saturable with sufficient fluence. For excitation energies above half the band gap, the excess energy is absorbed by the edge-state electrons, elevating their effective temperature. Our analytical expressions for the linear and nonlinear absorption could prove useful tools in the design of practical photonic devices based on s-TMDs., Royal Academy of Engineering, Collaborative Research Center 767

We apply the concept of natural selection to intelligently find optimum operating regimes in a fiber laser — a multi-parameter space. Using a compound fitness function we reliably achieve self-optimizing mode-locked operation.

We reveal the existence of slowly-decaying dark solitons in the radiation build-up dynamics of bright pulses in all-normal dispersion mode-locked fiber lasers, numerically modeled in the framework of a generalized nonlinear Schr\"odinger equation. The evolution of noise perturbations to quasi-stationary dark solitons is examined, and the significance of background shape and soliton-soliton collisions on the eventual soliton decay is established. We demonstrate the role of a restoring force in extending soliton interactions in conservative systems to include the effects of dissipation, as encountered in laser cavities, and generalize our observations to other nonlinear systems.

We present results on high-average power difference frequency generation of pulsed Yb/Er fiber systems to the mid-IR (6.2 W at 3.35 µm), and use focused Gaussian beam theory to validate our experimental results.

We report the first broadband nonlinear absorption characterization of CVD-grown intrinsic monolayer molybdenum diselenide (MoSe 2 ) from 720–810 nm. A constant modulation depth of ∼80% is revealed across the lowest energy excitonic absorption feature.

We report multi-Watt-level average power 3.28-3.45 μm difference frequency generation using two sychronous picosecond master oscillator power fiber amplifiers. Greater than 3.4 W of idler power is generated across the entire spectral tuning range.

The effect of pump coherence on the evolution of continuous-wave (CW) supercontinua is explored within the framework of the generalised nonlinear Schrodinger equation. Recent results of experimental studies, using an amplified spontaneous emission (ASE) based CW pump source, with a selectable coherence time, confirm the trend observed numerically [1]: an optimum degree of partial pump coherence generates the broadest spanning continuum. The effect of third order dispersion on continuum expansion is discussed in the context of an optimum degree of pump coherence. Finally, a design guideline for CW pumped supercontinuum light sources is suggested, relating the optimum pump bandwidth to the modulation instability period.

W e present an all-fiber passively mode-locked Ra- man laser using graphene as a saturable absorber. Different lengths of a highly non-linear fiber presenting normal disper- sion at both pump and signal wavelengths are used in the cav- ity to provide Raman amplification. The cavity is pumped by a continuous wave Raman laser at 1450 nm, and generates short pulses around 1550 nm, which depending on polarization and filtering parameters can be either at the repetition rate of the cavity or at higher harmonics of it.

We demonstrate gain-guided dissipative soliton operation of a linearly polarized Yb-doped fiber laser, mode-locked by carbon nanotubes. In the regime of low nonlinear phase shift per cavity roundtrip, a dissipative soliton pulse can be supported by the finite gain bandwidth, without the need for strong spectral selection. The gain-guiding dynamics are confirmed by simulations.

Polarization Rotation Locked Vector Solitons (PRLVSs) are experimentally observed for the first time in a fiber ring laser passively mode-locked by a single-wall carbon nanotube (SWCNT) saturable absorber. Period-doubling of these solitons at certain birefringence values has also been observed. We show that fine adjustment to the intracavity birefringence can swing the PRLVSs from period-doubled to period-one state without simultaneous reduction in the pump strength. The timing jitter for both states has also been measured experimentally and discussed analytically using the theoretical framework provided by the Haus model.

We characterize the saturable absorption of the second (E22) electronic transition of a sample of single-walled carbon nanotubes and use it to mode-lock an ytterbium fiber ring laser. The modulation depth of ~ 15% was found to be similar to the corresponding E11 transition (~ 13%), but the saturation intensity (~ 220 MW cm-2) about an order of magnitude larger (~ 10 MW cm-2). We achieved a 15 MHz mode-locked pulse train with an output pulse duration of 6.5 ps. For comparison we also demonstrate stable mode-locking on the E11 transition, of the same nanotubes, with an erbium fiber ring laser, producing 1.1 ps pulses. Using the E22 transition should enable the use of carbon nanotube saturable absorbers at shorter wavelengths than currently possible with the E11 transition, which are limited by the smallest achievable nanotube dimensions.

Relatively high power supercontinuum sources were available from the mid 1980s based upon picosecond pulse pumping by Nd:YAG lasers of conventional step index fibres and utilizing soliton-Raman generation, however, it wasn't until the introduction of photonic crystal fibres (PCF) and the application of master oscillator power fibre amplifier (MOPFA) configurations that high average power supercontinuum sources became practical and subsequently, a commercial success. With the Yb-doped fibre amplifier being the most extensively developed high power fibre amplifier system the vast majority of all-fibre based supercontinuum source studies utilized pumping around 1060 nm. Consequently, PCF became indispensible as the nonlinear medium since the zero dispersion wavelength of the fibre could be accurately tailored, as well as the nonlinear coefficient, to optimize pumping in this wavelength range. Early schemes provided average powers of ∼2 W and spectral power densities in the visible of ∼ 2 mW/nm. Over the past six years the average operational powers have gradually increased to ∼50 W. However, as a result of phase and group velocity matching criteria introduced by the necessary condition of having the zero dispersion wavelength of the nonlinear fibre in the region of the pump wavelength such that modulational instability and subsequent soliton evolution played the major role in determining supercontinuum generation, the short wavelength extent of 1060nm pumped sources only extended to about 600nm. Extension to the blue was achieved by generation in cascaded systems where PCFs with gradually decreasing zero dispersion wavelength were employed, where taken to its most extreme, single fibres with tapered profiles (manufactured at the University of Bath) were utilized. This allowed spectral extension to 320 nm and spectral power densities greater than 5 mW/nm below 400 nm.

We demonstrate passive mode-locking of a Bi-doped fiber laser using a nanotube-based saturable absorber. We achieve stable mode-locking in both the all-normal and net anomalous dispersion regimes. Near transform-limited 4.7 ps soliton pulses are generated in the average-soliton regime, with a chirped fiber Bragg grating used for dispersion compensation to retain an all-fiber format.

A pulse source at 560 nm that is tunable in duration between 50 ps and 2.7 ns with >1 W of average power and near diffraction-limited beam quality is demonstrated. The source is based on efficient (up to 50%) second-harmonic generation in a periodically poled lithium tantalate crystal of a linearly polarized fiber-integrated Raman amplifier operating at 1120 nm. A duration-tunable ytterbium master-oscillator power-fiber amplifier is used to pulse-pump the Raman amplifier, which is seeded by a continuous-wave distributed-feedback laser diode at 1120 nm. The performance of the system using two different master oscillator schemes is compared. A pulse energy of up to 765 nJ is achieved with a conversion efficiency of 25% from the ytterbium fiber pump, demonstrating a compact and turn-key architecture for obtaining high peak-power radiation at 560 nm.

Few-layer transition metal dichalcogenides such as molybdenum disulfide (MoS2) present new opportunities for photonic devices. Here, we review recent progress in few-layer MoS2-based nonlinear optics and state-of-the-art pulsed lasers using this new nanomaterial.

We report the development of a fiber-integrated picosecond source at 560 nm by second harmonic generation of a Raman fiber laser. A picosecond ytterbium master oscillator power fiber amplifier is used to pulse-pump a Raman amplifier, which is seeded by a continuous wave distributed feedback laser diode operating at 1120 nm. The pulse train generated at 1120 nm is frequency-doubled in a fiber-coupled periodically-poled lithium niobate crystal module, producing 450 mW of average power at 560 nm with a pulse duration of 150 ps at a repetition rate of 47.5 MHz. The near diffraction-limited (M(2) = 1.02) collimated output beam is ideal for super-resolution microscopy applications.

Second harmonic generation (SHG) is a ubiquitous technique for extending the spectral coverage of laser sources into regions that would otherwise be technologically challenging to access. SHG schemes typically rely on the use of bulk optical components, resulting in systems with large footprints requiring precise optical alignment. Integration of the SHG components into a single unit facilitates the implementation of compact, robust and turn-key sources, suitable for applications in biophotonic imaging, amongst others. We report on the development of fiber-coupled frequency doubling modules and their application to novel fiberintegrated picosecond pulse sources in the visible and near-visible. The modules employ a simple, single-pass configuration using a periodically-poled lithium niobate (PPLN) crystal as the nonlinear conversion medium. They are readily adaptable for different fiber pump laser configurations and are configurable with either fiber-coupled or collimated free-space outputs. Two sources using the modules are presented, operating at 780 nm and 560 nm. The 780 nm source utilizes an erbium master oscillator power fiber amplifier (MOPFA) scheme. SHG was performed in a 35 mm long crystal, generating 3.5 W of 780 nm radiation with a pulse duration of 410 ps at 50 MHz and conversion efficiencies exceeding 20%. Results of this source being used for parametric wavelength conversion in photonic crystal fiber are discussed. The 560 nm source was based on SHG of a Raman amplified CW diode pumped by a pulsed ytterbium-fiber MOPFA. This source generated 450 mW of average power with conversion efficiencies greater than 20%.

We demonstrate that the giant chirp of coherent, nanosecond pulses generated in an 846 m long, all-normal dispersion, nanotube mode-locked fiber laser can be compensated using a chirped fiber Bragg grating compressor. Linear compression to 11 ps is reported, corresponding to an extreme compression factor of ∼100. Experimental results are supported by numerical modeling, which is also used to probe the limits of this technique. Our results unequivocally conclude that ultra-long cavity fiber lasers can support stable dissipative soliton attractors and highlight the design simplicity for pulse-energy scaling through cavity elongation.

We fabricate a free-standing few-layer molybdenum disulfide (MoS2)-polymer composite by liquid phase exfoliation of chemically pristine MoS2 crystals and use this to demonstrate a wideband tunable, ultrafast mode-locked fiber laser. Stable, picosecond pulses, tunable from 1,535 nm to 1,565 nm, are generated, corresponding to photon energies below the MoS2 material bandgap. These results contribute to the growing body of work studying the nonlinear optical properties of transition metal dichalcogenides that present new opportunities for ultrafast photonic applications.

We characterize stimulated Brillouin scattering (SBS) of visible light in small-core photonic crystal fiber (PCF). Threshold powers under 532 nm excitation agree with established theory, in contrast to measured values up to five times greater than expected for Brillouin scattering of 1550 nm light. An isolated, single-peaked signal at a Stokes shift of 33.5 GHz is observed, distinct from the multi-peaked Stokes spectra expected when small-core PCF is pumped in the infrared. This wavelength-dependence of the Brillouin threshold, and the corresponding spectrum, are explained by the acousto-optic interactions in the fiber, governed by dimensionless length scales that relate the modal area to the core size, and the pump wavelength to PCF hole pitch. Our results suggest new opportunities for exploiting SBS of visible light in small-core PCFs.

We demonstrate wide-band ultrafast optical pulse generation at 1, 1.5, and 2 μm using a single-polymer composite saturable absorber based on double-wall carbon nanotubes (DWNTs). The freestanding optical quality polymer composite is prepared from nanotubes dispersed in water with poly(vinyl alcohol) as the host matrix. The composite is then integrated into ytterbium-, erbium-, and thulium-doped fiber laser cavities. Using this single DWNT–polymer composite, we achieve 4.85 ps, 532 fs, and 1.6 ps mode-locked pulses at 1066, 1559, and 1883 nm, respectively, highlighting the potential of DWNTs for wide-band ultrafast photonics.

We demonstrate a nanotube mode-locked fiber laser with low repetition rate (244 kHz), enabling supercontinuum generation in photonic crystal fiber spanning 600 to 2000 nm, at a low average pump power of 87 mW.

A MoS2-based saturable absorber is fabricated using wet chemistry techniques. We use it to passively Q-switch a fiber laser at 1068 nm.

A reduced stimulated Brillouin scattering threshold power in small-core PCFs is achieved using visible wavelength excitation. We explain this in the context of acousto-optic interactions at length-scales relative to the fiber geometry.

Graphene mode-locked and self-sychronized fiber lasers are used for sum-frequency mixing in a graphene-polymer composite.

We report a novel narrow linewidth Bismuth-doped all-fiber ring laser, operating at 1177 nm. We achieve a 10-mW output in a 4-GHz linewidth, direct from the oscillator under 7 W of pumping, delivered by an Yb-fiber laser. Mode suppression in the 50-m unidirectional ring cavity is provided by a narrowband fiber Bragg grating and two fiber integrated Fabry-Perot (FP) filters. The FP filters provide a factor of 20 mode suppression, while improving the relative intensity noise by up to 20 dB/Hz. The optical signal-to-noise ratio (OSNR) is ~60 dB.

We report a near-visible parametric wavelength converter comprising a polarization-maintaining photonic crystal fiber (PM-PCF) pumped by a highly versatile diode-seeded master-oscillator power amplifier system based around 1.06 μm. The device is broadly tunable in wavelength (0.74–0.81 μm), pulse duration (0.2–1.5 ns) and repetition rate (1–30 MHz). A maximum anti-Stokes slope conversion efficiency of 14.9% is achieved with corresponding anti-Stokes average output powers of 845 mW, at a wavelength of 0.775 μm.

We report a PM-PCF based near-visible parametric wavelength converter, pumped by a diode-seeded master-oscillator power amplifier. The system is broadly tunable in wavelength (740–810 nm), pulse duration (0.3–2 ns) and repetition rate (1–30 MHz).

We use numerical simulations to revisit the generation of fiber supercontinua pumped by partially coherent continuous-wave (CW) sources. Specifically, we show that intensity fluctuations characteristic of temporal partial coherence can be described as a stochastic train of high-order solitons, whose individual dynamics drive continuum formation. For sources with sufficiently low coherence, these solitons actually undergo fission rather than modulation instability, changing the nature of the CW supercontinuum evolution.

For low degrees of partial coherence soliton fission-based dynamics rather than modulation instability are shown to be the driver of continuous-wave supercontinuum generation. Intensity fluctuations can be described in terms of an effective soliton order, and an optimum degree of partial pump coherence for strong fission evolution is outlined.

We demonstrate mode-locking of a thulium-doped fiber laser operating at 1.94 μm, using a graphene-polymer based saturable absorber. The laser outputs 3.6 ps pulses, with ~0.4 nJ energy and an amplitude fluctuation ~0.5%, at 6.46 MHz. This is a simple, low-cost, stable and convenient laser oscillator for applications where eye-safe and low-photon-energy light sources are required, such as sensing and biomedical diagnostics.

We report the development of a bismuth-doped fiber master oscillator power fiber amplifier system. The system operates at 1177 nm, producing 28 ps pulses at 9.11 MHz repetition rate, with an output power of 150 mW and a peak pulse power of 580 W. We subsequently frequency double the output, resulting in a picosecond pulsed visible source operating at 588.5 nm, with a maximum average output power of 13.7 mW.