This page contains a complete list of my academic publications, which can be directly downloaded using the PDF links. For citations etc., please visit my Google Scholar page.
|G. Hu; L. Yang; Z. Yang; Y. Wang; X. Jin; J. Dai; Q. Wu; S. Liu; X. Zhu; X. Wang; T.-C. Wu; R. C. T. Howe; T. Albrow-Owen; L. W. T. Ng; Q. Yang; L. G. Occhipinti; R. I. Woodward; E. J. R Kelleher; Z. Sun; X. Huang; M. Zhang; C. D. Bain; T. Hasan: A general ink formulation of 2D crystals for wafer-scale inkjet printing. In: Science Advances, vol. 6, no. 33, pp. eaba5029, 2020. (Type: Journal Article | | | )|
Recent advances in inkjet printing of two-dimensional (2D) crystals show great promise for next-generation printed electronics development. Printing nonuniformity, however, results in poor reproducibility in device performance, and remains a major impediment to their large-scale manufacturing. At the heart of this challenge lies the coffee-ring effect (CRE), ring-shaped nonuniform deposits formed during post-deposition drying. We present an experimental study of the drying mechanism of a binary solvent ink formulation. We show that Marangoni-enhanced spreading in this formulation inhibits contact line pinning and deforms the droplet shape to naturally suppress the capillary flows that give rise to the CRE. This general formulation supports uniform deposition of 2D crystals and their derivatives, enabling scalable and even wafer-scale device fabrication, moving them closer to industrial-level additive manufacturing.
|M. Z. Amin; M. R. Majewski; R. I. Woodward; A. Fuerbach; S. D. Jackson: Novel near-infrared pump wavelengths for dysprosium fiber lasers. In: Journal of Lightwave Technology, vol. 38, no. 20, pp. 5801, 2020. (Type: Journal Article | | | )|
We report two new near-infrared pump wavelengths at 0.8 µm and 0.9 µm for dysprosium (Dy3+)-doped mid-infrared fiber lasers, which are free from detrimental pump excited state absorption that has limited all previous Dy3+ fiber lasers using longer near-infrared pump wavelengths (i.e., 1.1 µm, 1.3 µm, and 1.7 µm). The maximum measured laser slope efficiencies were 18.5% and 23.7% with respect to launched pump power for 0.8 µm and 0.9 µm pump wavelengths, respectively. These new pump wavelengths provide higher fractional Stokes limits (which are 70% and 79% for 0.8 µm and 0.9 µm pumping wavelengths, respectively) than previous demonstrations. While our measured efficiencies are still below the Stokes limit, we have identified the causes to be background loss and multi-mode behaviour at the pump wavelengths; both easily solvable for future systems. A numerical model was used to confirm performance, paving the way to efficient future near-infrared-pumped, potentially diode-pumped Dy3+ fiber lasers emitting in the mid-infrared.
|M. R. Majewski; R. I. Woodward; S. D. Jackson: Dysprosium mid‐infrared lasers: Current status and future prospects. In: Laser & Photonics Reviews, vol. 14, no. 3, pp. 1900195, 2020. (Type: Journal Article | | | )|
With growing interest in the mid‐infrared spectral region, dysprosium has recently been revisited for efficient high‐performance infrared source development. Despite historically receiving less attention than other rare earth ions, in recent years lasers utilizing the dysprosium ion as the laser material have set record mid‐infrared performance, including tunability from 2.8 to 3.4 µm (in addition to 4.3 µm lasing), continuous wave powers exceeding 10 W, greater than 73% slope efficiencies, and even ultrafast pulsed operation. Herein, the unique energy level structure and spectroscopy of the dysprosium ion are examined and the major developments that have led to this resurgence of interest and subsequent record mid‐infrared laser performance are surveyed. Also mid‐infrared applications of emerging dysprosium lasers are highlighted, in addition to surveying the many opportunities that lie ahead.1
|O. Henderson-Sapir; N. Bawden; M. R. Majewski; R. I. Woodward; D. J. Ottaway; S. D. Jackson: Mode-locked and tunable fiber laser at the 3.5 µm band using frequency-shifted feedback. In: Optics Letters, vol. 45, no. 1, pp. 224, 2020. (Type: Journal Article | | | )|
We report on a mid-infrared mode-locked fiber laser that uses an acousto-optic tunable filter to achieve frequency- shifted feedback pulse generation with frequency tuning over a 215 nm range. The laser operates on the 3.5 µm transition in erbium-doped zirconium fluoride-based fiber and utilizes the dual-wavelength pumping scheme. Stable, self-starting mode locking with a minimum pulse duration of 53 ps was measured using a two-photon absorption autocorrelator. The longest wavelength demonstrated was 3612 nm, and the maximum average powers achieved were 50 and 167 mW in fundamental and multi-pulse mode-locking regimes, respectively. To the best of our knowledge, this is the longest wavelength rare-earth-doped mode-locked fiber laser demonstrated. The broad tunability promises potential uses for environmental sensing applications.
|R. I. Woodward; M. R. Majewski; N. Macadam; G. Hu; T. Albrow-Owen; T. Hasan; S. D. Jackson: Q-switched Dy:ZBLAN fiber lasers beyond 3 μm: comparison of pulse generation using acousto-optic modulation and inkjet-printed black phosphorus. In: Optics Express, vol. 27, no. 11, pp. 15032, 2019. (Type: Journal Article | | | )|
We report high-energy mid-infrared pulse generation by Q-switching of dysprosium-doped fiber lasers for the first time. Two different modulation techniques are demonstrated. Firstly, using active acousto-optic modulation, pulses are produced with up to 12 μJ energy and durations as short as 270 ns, with variable repetition rates from 100 Hz to 20 kHz and central wavelengths tunable from 2.97 to 3.23 μm. Experiments are supported by numerical modeling, identifying routes for improved pulse energies and to avoid multi-pulsing by careful choice of modulator parameters. Secondly, we demonstrate passive Q-switching by fabricating an inkjet-printed black phosphorus saturable absorber, simplifying the cavity and generating 1.0 μJ pulses with 740 ns duration. The performance and relative merits of each modulation approach are then critically discussed. These demonstrations highlight the potential of dysprosium as a versatile gain medium for high-performance pulsed sources beyond 3 μm.
|M. R. Majewski; R. I. Woodward; S. D. Jackson: Ultrafast mid-infrared fiber laser mode-locked using frequency-shifted feedback. In: Optics Letters, vol. 44, no. 7, pp. 1698, 2019. (Type: Journal Article | | | )|
We demonstrate ultrashort pulse generation from a fluoride fiber laser co-doped with holmium and praseodymium. To date the majority of work focused on short pulse generation from this class of fiber laser has employed loss modulators in the cavity, both real and artificial. In this work we alternatively employ a frequency shifting element: an acousto-optic modulator (AOM) in the cavity. This results in mode-locked output of sub-5 ps pulses with 10 nJ of energy at a center wavelength of 2.86 μm, and a pulse repetition frequency of 30.1 MHz, equating to a peak power of 1.9 kW. Additional experimental investigation of the relationship between frequency shift and cavity round trip offer insight into the complex underlying dynamics. As a complementary mode-locking technique to conventional loss modulation, this method of pulse formation may greatly expand the design flexibility of pulsed mid-infrared fiber lasers.
|R. I. Woodward; M. R. Majewski; D. D. Hudson; S. D. Jackson: Swept-wavelength mid-infrared fiber laser for real-time ammonia gas sensing [Invited]. In: APL Photonics, vol. 4, pp. 020801, 2019, (Editor's Pick). (Type: Journal Article | | | )|
The mid-infrared (mid-IR) spectral region holds great promise for new laser-based sensing technologies, based on measuring strong mid-IR molecular absorption features. Practical applications have been limited to date, however, by current low-brightness broadband mid-IR light sources and slow acquisition-time detection systems. Here, we report a new approach by developing a swept-wavelength mid-infrared fiber laser, exploiting the broad emission of dysprosium and using an acousto-optic tunable filter to achieve electronically controlled swept-wavelength operation from 2.89 to 3.25 μm (3070-3460 cm^-1). Ammonia (NH3) absorption spectroscopy is demonstrated using this swept source with a simple room-temperature single-pixel detector, with 0.3 nm resolution and 40 ms acquisition time. This creates new opportunities for real-time high-sensitivity remote sensing using simple, compact mid-IR fiber-based technologies.
|G. Bharathan; T. T. Fernandez; M. Ams; R. I. Woodward; D. D. Hudson; A. Fuerbach: Optimized laser written ZBLAN fiber Bragg gratings with high reflectivity and low loss. In: Optics Letters, vol. 44, no. 2, pp. 423, 2019. (Type: Journal Article | | | )|
We report the direct femtosecond (fs) laser inscription of Type-I fiber Bragg gratings (FBGs) into the core of soft-glass ZBLAN fibers. We investigate and compare various fabrication methods such as single pass (line-by-line), double pass and stacking (plane-by-plane) to create the highest reflectivity FBGs (99.98%) for mid-infrared (mid-IR) applications. In addition, we experimentally demonstrate how the parameters that influence the coupling coefficient, i.e. refractive index modulation and overlap factor, can be controlled in these gratings to specifically tailor the FBG properties. The performance of the direct-written Type-I gratings after 6 hours of annealing is further analyzed and the reflectivity increased by approximately 10 dB. To the best of our knowledge, this is the first demonstration of temperature-stable mid-IR FBGs with the highest coupling coefficient (464 m^−1) and lowest loss (< 0.5 dB/cm) without the use of an expensive phase mask.
|R. I. Woodward; M. R. Majewski; S. D. Jackson: Mode-locked dysprosium fiber laser: picosecond pulse generation from 2.97 to 3.30 µm. In: APL Photonics, vol. 3, pp. 116106, 2018, (Editor Featured Article.
Highlighted by Laser Focus World: www.bit.ly/dy_lfw). (Type: Journal Article | | | )|
Mode-locked fiber laser technology to date has been limited to sub-3 µm wavelengths, despite significant application-driven demand for compact pulse sources at longer wavelengths. Erbium- and holmium-doped fluoride fiber lasers incorporating a saturable absorber are emerging as promising pulse sources for 2.7–2.9 µm, yet it remains a major challenge to extend this coverage. Here, we propose a new approach using dysprosium-doped fiber with frequency shifted feedback (FSF). Using a simple linear cavity with an acousto-optic tunable filter, we generate ∼33 ps pulses with up to 2.7 nJ energy and 330 nm tunability from 2.97 to 3.30 µm (∼3000–3400 cm −1 )—the longest wavelength mode-locked fiber laser and the most broadly tunable pulsed fiber laser to date. Numerical simulations show excellent agreement with experiments and also offer new insights into the underlying dynamics of FSF pulse generation. This highlights the remarkable potential of both dysprosium as a gain material and FSF for versatile pulse generation, opening new opportunities for mid-IR laser development and practical applications outside the laboratory.
|G. Bharathan; D. D. Hudson; R. I. Woodward; S. D. Jackson; A. Fuerbach: In-fiber polarizer based on a 45-degree tilted fluoride fiber Bragg grating for mid-infrared fiber laser technology. In: OSA Continuum, vol. 1, pp. 56, 2018. (Type: Journal Article | | | )|
We report the direct femtosecond laser inscription of a 45-deg tilted fiber Bragg grating (TFBG) into fluoride fiber, creating an in-fiber mid-infrared polarizer. Utilizing a 16 mm long intracavity TFBG, we demonstrate a 2.862 µm Ho3+Pr3+:ZBLAN fiber laser with 21.6 dB output polarization extinction ratio (PER), up to 0.37 W output power and 31.3% slope efficiency. In addition, we experimentally demonstrate that the laser PER is a linear function of grating length. Our results show that fluoride TFBGs are a promising route to replace bulk polarizers in mid-IR laser cavities, paving the way to all-fiber mid-infrared laser systems.
|M. R. Majewski; R. I. Woodward; J.-Y. Carreé; S. Poulain; M. Poulain; S. D. Jackson: Emission beyond 4 μm and mid-infrared lasing in a dysprosium-doped indium fluoride (InF3) fiber. In: Optics Letters, vol. 43, no. 8, pp. 1926, 2018. (Type: Journal Article | | | )|
Optical emission from rare-earth-doped fluoride fibers has thus far been limited to less than 4 μm. We extend emission beyond this limit by employing an indium fluoride (InF3) glass fiber as the host, which exhibits an increased infrared transparency over commonly used zirconium fluoride (ZBLAN). Near-infrared pumping of a dysprosium-doped InF3 fiber results in broad emission centered around 4.3 μm, representing the longest emission yet achieved from a fluoride fiber. The first laser emission in an InF3 fiber is also demonstrated from the 3 μm dysprosium transition. Finally, a frequency domain excited state lifetime measurement comparison between fluoride hosts suggests that multiphonon effects are significantly reduced in indium fluoride fiber, paving the way to more efficient, longer wavelength lasers compared to ZBLAN fibers.
|R. I. Woodward; M. R. Majewski; G. Bharathan; D. D. Hudson; A. Fuerbach; S. D. Jackson: Watt-level dysprosium fiber laser at 3.15 µm with 73% slope efficiency. In: Optics Letters, vol. 43, no. 7, pp. 1471, 2018. (Type: Journal Article | | | )|
Rare-earth-doped fiber lasers are emerging as promising high-power mid-infrared sources for the 2.6–3.0 μm and 3.3–3.8 μm regions based on erbium and holmium ions. The intermediate wavelength range, however, remains vastly underserved, despite prospects for important manufacturing and defense applications. Here, we demonstrate the potential of dysprosium-doped fiber to solve this problem, with a simple in-band pumped grating-stabilized linear cavity generating up to 1.06 W at 3.15 μm. A slope efficiency of 73% with respect to launched power (77% relative to absorbed power) is achieved—the highest value for any mid-infrared fiber laser to date, to the best of our knowledge. Opportunities for further power and efficiency scaling are also discussed.
|M. R. Majewski; R. I. Woodward; S. D. Jackson: Dysprosium-doped ZBLAN fiber laser tunable from 2.8 μm to 3.4 μm, pumped at 1.7 μm. In: Optics Letters, vol. 43, no. 5, pp. 971, 2018, (Editors' Pick). (Type: Journal Article | | | )|
We demonstrate a mid-infrared dysprosium-doped fluoride fiber laser with a continuously tunable output range of 573 nm, pumped by a 1.7 μm Raman fiber laser. To the best of our knowledge, this represents the largest tuning range achieved to date from any rare-earth-doped fiber laser and, critically, spans the 2.8–3.4 μm spectral region, which contains absorption resonances of many important functional groups and is uncovered by other rare-earth ions. Output powers up to 170 mW are achieved, with 21% slope efficiency. We also discuss the relative merits of the 1.7 μm pump scheme, including possible pump excited-state absorption.
|R. I. Woodward: Dispersion engineering of mode-locked fibre lasers [Invited]. In: Journal of Optics, vol. 20, pp. 033002, 2018. (Type: Journal Article | | | )|
Mode-locked fibre lasers are important sources of ultrashort pulses, where stable pulse generation is achieved through a balance of periodic amplitude and phase evolutions. A range of distinct cavity pulse dynamics have been revealed, arising from the interplay between dispersion and nonlinearity in addition to dissipative processes such as filtering. This has led to the discovery of numerous novel operating regimes, offering significantly improved laser performance. In this Topical Review, we summarise the main steady-state pulse dynamics reported to date through cavity dispersion engineering, including average solitons, dispersion-managed solitons, dissipative solitons, giant-chirped pulses and similaritons. Characteristic features and the stabilisation mechanism of each regime are described, supported by numerical modelling, in addition to the typical performance and limitations. Opportunities for further pulse energy scaling are discussed, in addition to considering other recent advances including automated self-tuning cavities and fluoride-fibre-based mid-infrared mode-locked lasers.
|R. I. Woodward, D. D. Hudson, A. Fuerbach, S. D. Jackson: Generation of 70-fs pulses at 2.86 μm from a mid-infrared fiber laser. In: Optics Letters, vol. 42, pp. 4893, 2017, (Editors' Pick). (Type: Journal Article | | | )|
We propose and demonstrate a simple route to few-optical-cycle pulse generation from a mid-infrared fiber laser through nonlinear compression of pulses from a holmium-doped fiber oscillator using a short length of chalcogenide fiber and a grating pair. Pulses from the oscillator with 265-fs duration at 2.86 μm are spectrally broadened through self-phase modulation in step-index As2S3 fiber to 141-nm bandwidth and then re-compressed to 70 fs (7.3 optical cycles). These are the shortest pulses from a mid-infrared fiber system to date, and we note that our system is compact, robust, and uses only commercially available components. The scalability of this approach is also discussed, supported by numerical modeling.
|G. Bharathan, R. I. Woodward, M. Ams, D. D. Hudson, S. D. Jackson A. Fuerbach
: Direct inscription of Bragg gratings into coated fluoride fibers for widely tunable and robust mid-infrared lasers. In: Optics Express, vol. 25, pp. 30013, 2017. (Type: Journal Article | | | )|
We report the development of a widely tunable all-fiber mid-infrared laser system based on a mechanically robust fiber Bragg grating (FBG) which was inscribed through the polymer coating of a Ho3+-Pr3+ co-doped double clad ZBLAN fluoride fiber by focusing femtosecond laser pulses into the core of the fiber without the use of a phase mask. By applying mechanical tension and compression to the FBG while pumping the fiber with an 1150 nm laser diode, a continuous wave (CW) all-fiber laser with a tuning range of 37 nm, centered at 2870 nm, was demonstrated with up to 0.29 W output power. These results pave the way for the realization of compact and robust mid-infrared fiber laser systems for real-world applications in spectroscopy and medicine.
|G. Hu, T. Albrow-Owen, X. Jin, A. Ali, Y. Hu, R. C. T. Howe, K. Shehzad, Z. Yang, X. Zhu, R. I. Woodward, T. Wu, H. Jussila, J. Wu, P. Peng, P. Tan, Z. Sun, E. J. R. Kelleher, M. Zhang, Y. Xu, T. Hasan: Black phosphorus ink formulation for inkjet printing of optoelectronics and photonics. In: Nature Communications, vol. 8, pp. 278, 2017. (Type: Journal Article | | | )|
Black phosphorus is a two-dimensional material of great interest, in part because of its high carrier mobility and thickness dependent direct bandgap. However, its instability under ambient conditions limits material deposition options for device fabrication. Here we show a black phosphorus ink that can be reliably inkjet printed, enabling scalable development of optoelectronic and photonic devices. Our binder-free ink suppresses coffee ring formation through induced recirculating Marangoni flow, and supports excellent consistency (< 2% variation) and spatial uniformity (< 3.4% variation), without substrate pre-treatment. Due to rapid ink drying (< 10 s at < 60 °C), printing causes minimal oxidation. Following encapsulation, the printed black phosphorus is stable against long-term (> 30 days) oxidation. We demonstrate printed black phosphorus as a passive switch for ultrafast lasers, stable against intense irradiation, and as a visible to near-infrared photodetector with high responsivities. Our work highlights the promise of this material as a functional ink platform for printed devices.
|R. I. Woodward; E. J. R. Kelleher: Genetic algorithm-based control of birefringent filtering for self-tuning, self-pulsing fiber lasers. In: Optics Letters, vol. 42, no. 15, pp. 2952, 2017, (Editors' Pick). (Type: Journal Article | | | )|
Polarization-based filtering in fiber lasers is well-known to enable spectral tunability and a wide range of dynamical operating states. This effect is rarely exploited in practical systems, however, because optimization of cavity parameters is non-trivial and evolves due to environmental sensitivity. Here, we report a genetic algorithm-based approach, utilizing electronic control of the cavity transfer function, to autonomously achieve broad wavelength tuning and the generation of Q-switched pulses with variable repetition rate and duration. The practicalities and limitations of simultaneous spectral and temporal self-tuning from a simple fiber laser are discussed, paving the way to on-demand laser properties through algorithmic control and machine learning schemes.
|R. I. Woodward; R. T. Murray; C. F. Phelan; R. E. P. de Oliveira; T. H. Runcorn; E. J. R. Kelleher; S. Li; E. C. de Oliveira; G. J. M. Fechine; G. Eda; C. J. S. de Matos: Characterization of the second- and third-order nonlinear optical susceptibilities of monolayer MoS2 using multiphoton microscopy. In: 2D Materials, vol. 4, no. 1, pp. 011006, 2017. (Type: Journal Article | | | )|
We report second- and third-harmonic generation in monolayer MoS2 as a tool for imaging and accurately characterizing the material’s nonlinear optical properties under 1560 nm excitation. Using a surface nonlinear optics treatment, we derive expressions relating experimental measurements to second- and third-order nonlinear sheet susceptibility magnitudes, obtaining values of |χ(2)s|=2.0×10^−20 m^2 V^−1 and, for the first time for monolayer MoS2, χ(3)s|=1.7×10^−28 m^3 V^−2. These sheet susceptibilities correspond to effective bulk nonlinear susceptibility values of 2.9×10^-11 m V^−1 and 2.4×10^-19 m^2 V^−2, accounting for the sheet thickness. Experimental comparisons between MoS2 and graphene are also performed, demonstrating∼3.4 times stronger third-order sheet nonlinearity in monolayer MoS2, highlighting the material’s potential for nonlinear photonics in the telecommunications C band.
|R. I. Woodward; E. J. R. Kelleher: Towards 'smart lasers': self-optimisation of an ultrafast pulse source using a genetic algorithm. In: Scientific Reports, vol. 6, pp. 37616, 2016. (Type: Journal Article | | | )|
Short-pulse fibre lasers are a complex dynamical system possessing a broad space of operating states that can be accessed through control of cavity parameters. Determination of target regimes is a multi-parameter global optimisation problem. Here, we report the implementation of a genetic algorithm to intelligently locate optimum parameters for stable single-pulse mode-locking in a Figure-8 fibre laser, and fully automate the system turn-on procedure. Stable ultrashort pulses are repeatably achieved by employing a compound fitness function that monitors both temporal and spectral output properties of the laser. Our method of encoding photonics expertise into an algorithm and applying machine-learning principles paves the way to self-optimising `smart' optical technologies.
|R. I. Woodward; E. J. R. Kelleher: Dark solitons in laser radiation build-up dynamics. In: Physical Review E, vol. 93, no. 3, pp. 032221, 2016. (Type: Journal Article | | | )|
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 Schrodinger 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.
|R. C. T. Howe; R. I. Woodward; G. Hu; Z. Yang; E. J. R. Kelleher; T. Hasan: Surfactant-aided exfoliation of molybdenum disulfide for ultrafast pulse generation through edge-state saturable absorption. In: Physica Status Solidi (B) , vol. 253, no. 5, pp. 911, 2016. (Type: Journal Article | | | )|
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-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.
|R. I. Woodward; E. J. R. Kelleher: 2D Saturable Absorbers for Fibre Lasers [Invited]. In: Applied Sciences, vol. 5, no. 4, pp. 1440-1456, 2015. (Type: Journal Article | | | )|
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.
|R. I. Woodward; R. C. T. Howe; T. H. Runcorn; G. Hu; F. Torrisi; E. J. R. Kelleher; T. Hasan: Wideband saturable absorption in few-layer molybdenum diselenide (MoSe2) for Q-switching Yb-, Er- and Tm-doped fiber lasers. In: Optics Express, vol. 23, no. 15, pp. 20051, 2015. (Type: Journal Article | | | )|
We fabricate a free-standing molybdenum diselenide (MoSe2) saturable absorber by embedding liquid-phase exfoliated few-layer MoSe2 flakes into a polymer film. The MoSe2-polymer composite is used to Q-switch fiber lasers based on ytterbium (Yb), erbium (Er) and thulium (Tm) gain fiber, producing trains of microsecond-duration pulses with kilohertz repetition rates at 1060 nm, 1566 nm and 1924 nm, respectively. Such operating wavelengths correspond to sub-bandgap saturable absorption in MoSe2, which is explained in the context of edge-states, building upon studies of other semiconducting transition metal dichalcogenide (TMD)-based saturable absorbers. Our work adds few-layer MoSe2 to the growing catalog of TMDs with remarkable optical properties, which offer new opportunities for photonic devices.
| R. I. Woodward; R. C. T. Howe; G. Hu; F. Torrisi; M. Zhang; T. Hasan; E. J. R. Kelleher: Few-layer MoS2 saturable absorbers for short-pulse laser technology: current status and future perspectives [Invited]. In: Photonics Research, vol. 3, no. 2, pp. A30–A42, 2015. (Type: Journal Article | | | )|
Few-layer molybdenum disulfide (MoS2) is emerging as a promising quasi-two-dimensional material for photonics and optoelectronics, further extending the library of suitable layered nanomaterials with exceptional optical properties for use in saturable absorber devices that enable short- pulse generation in laser systems. In this work, we catalog and review the nonlinear optical properties of few-layer MoS2, summarize recent progress in processing and integration into saturable absorber devices, and comment on the current status and future perspectives of MoS2-based pulsed lasers.
| R. I. Woodward; E. J. R. Kelleher; T. H. Runcorn; S. Loranger; D. Popa; V. J. Wittwer; A. C. Ferrari; S. V. Popov; R. Kashyap; J. R. Taylor: Fiber grating compression of giant-chirped nanosecond pulses from an ultra-long nanotube mode-locked fiber laser. In: Optics Letters, vol. 40, no. 3, pp. 387–390, 2015. (Type: Journal Article | | | )|
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.
| M. Zhang; R. C. T. Howe; R. I. Woodward; E. J. R. Kelleher; F. Torrisi; G. Hu; S. V. Popov; J. R. Taylor; T. Hasan: Solution processed MoS2-PVA composite for sub-bandgap mode-locking of a wideband tunable ultrafast Er:fiber laser. In: Nano Research, vol. 8, no. 5, pp. 1522–1534, 2015. (Type: Journal Article | | | )|
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.
| D. J. J. Hu; R. T. Murray; T. Legg; T. H. Runcorn; M. Zhang; R. I. Woodward; J. L. Lim; Y. Wang; F. Luan; B. Gu; P. P. Shum; E. J. R. Kelleher; S. V. Popov; J. R. Taylor: Fiber-integrated 780 nm source for visible parametric generation. In: Optics Express, vol. 22, no. 24, pp. 29726, 2014. (Type: Journal Article | | | )|
We report the development of a fully fiber-integrated pulsed master oscillator power fibre amplifier (MOPFA) source at 780 nm, producing 3.5 W of average power with 410 ps pulses at a repetition rate of 50 MHz. The source consists of an intensity modulated 1560 nm laser diode amplified in an erbium fiber amplifier chain, followed by a fiber coupled periodically poled lithium niobate crystal module for frequency doubling. The source is then used for generating visible light through four-wave mixing in a length of highly nonlinear photonic crystal fiber: 105 mW at 668 nm and 95 mW at 662 nm are obtained, with pump to anti-Stokes conversion slope efficiencies exceeding 6% in both cases.
| V. Mamidala; R. I. Woodward; Y. Yang; H. H. Liu; K. K. Chow: Graphene-based passively mode-locked bidirectional fiber ring laser. In: Optics Express, vol. 22, no. 4, pp. 4539, 2014. (Type: Journal Article | | | )|
We present an all-fiber bidirectional passively mode-locked soliton laser with a graphene-based saturable absorber for the first time to the best of our knowledge. Our design includes a four-port circulator to introduce different sections of cavity for the two counter-propagating pulses, so they have distinct output characteristics. Simultaneous bidirectional operation is achieved by appropriately adjusting the net cavity birefringence and loss. In the clockwise direction, the laser emits ~750 fs pulses at 1561.6 nm, with a repetition rate of 7.68 MHz. In the counter clockwise direction, the central wavelength, pulse width, and repetition rate are 1561.0 nm, ~850 fs, and 6.90 MHz, respectively.
| R. I. Woodward; E. J. R. Kelleher; R. C. T. Howe; G. Hu; F. Torrisi; T. Hasan; S. V. Popov; J. R. Taylor: Tunable Q-switched fiber laser based on saturable edge-state absorption in few-layer molybdenum disulfide (MoS2). In: Optics Express, vol. 22, no. 25, pp. 31113, 2014. (Type: Journal Article | | | )|
We fabricate a few-layer molybdenum disulfide (MoS2) polymer composite saturable absorber by liquid-phase exfoliation, and use this to passively Q-switch an ytterbium-doped fiber laser, tunable from 1030 to 1070 nm. Self-starting Q-switching generates 2.88 $mu$s pulses at 74 kHz repetition rate, with over 100 nJ pulse energy. We propose a mechanism, based on edge states within the bandgap, responsible for the wideband nonlinear optical absorption exhibited by our few-layer MoS2 sample, despite operating at photon energies lower than the material bandgap.
| R. I. Woodward; E. J. R. Kelleher; S. V. Popov; J. R. Taylor: Stimulated Brillouin scattering of visible light in small-core photonic crystal fibers. In: Optics Letters, vol. 39, no. 8, pp. 2330–2333, 2014. (Type: Journal Article | | | )|
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.
| R. I. Woodward; E. J. R. Kelleher; D. Popa; T. Hasan; F. Bonaccorso; A. C. Ferrari; S. V. Popov; J. R. Taylor: Scalar nanosecond pulse generation in a nanotube mode-locked environmentally stable fiber laser. In: IEEE Photonics Technology Letters, vol. 26, no. 16, pp. 1672–1675, 2014. (Type: Journal Article | | | )|
We report an environmentally stable nanotube mode-locked fiber laser producing linearly-polarized, nanosecond pulses. A simple all-polarization-maintaining fiber ring cavity is used, including 300 m of highly nonlinear fiber to elongate the cavity and increase intracavity dispersion and nonlinearity. The laser generates scalar pulses with a duration of 1.23 ns at a center 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%.