Generation of mode-locked optical pulses at 1035 nm from a fiber Bragg grating stabilized semiconductor laser diode

We report the generation of transform-limited, ~18 ps optical pulses from a fiber Bragg grating (FBG) stabilized semiconductor laser diode. Up to 7.2 pJ of pulse energy and a peak power of 400mW were achieved when operating at a repetition frequency of 832.6 MHz, a multiple of the cavity (diode + FBG) free spectral range (FSR). A small detuning in the repetition frequency resulted in broader optical pulses. We have shown experimentally the transition from a gain-switched regime of operation to mode-locked operation once the injection current modulation frequency is set to match a harmonic of the cavity FSR. The transition also results in a reduction in the timing jitter of the optical pulses

Pulse energy packing effects on material transport during laser processing of < 1

The effects of energy pulse packing on material transport during single-pulse laser processing of silicon is studied using temporarily shaped pulses with durations from 50 to 150 ns. Six regimes of material transport were identified and disambiguated through energy packing considerations over a range of pulse durations. Energy packing has been shown to shift the interaction to energetically costlier regimes without appreciable benefit in either depth, material removal or crater morphology and quality.The authors would like to thank the UK Technology Strategy Board under project TP14/HVM/6/I/BD5665. The authors acknowledge the EPSRC Centre for Doctoral Training in Photonic Systems Development for their generous support

High performance pulsed fiber laser systems for scientific & industrial applications

This thesis reports an investigation of the power scaling of pulsed fiber laser systems towards the hundreds of Watts regime whilst keeping the impact of fiber nonlinearities such as Stimulated Raman Scattering (SRS) at a manageable level. Two regimes of pulsed operation are investigated: the nanosecond pulse regime and the picosecond pulse regime. Some of the work reported in this thesis was carried out in collaboration with SPI Lasers and Institute for Manufacturing, University of Cambridge under the TSB funded SMART LASER programme.In the nanosecond regime, two kinds of MOPA configurations are investigated. In the first instance a high accuracy active pulse shaping technique is implemented. Using the combination of a fast electrical Arbitrary Waveform Generator (AWG) and an Electro-Optic Modulator (EOM), optical pulses can be shaped into various custom defined pulse shapes with high temporal resolution feature definition, allowing faster pulse rise and fall times than previously possible. This MOPA has the capability to generate a maximum average output power of ~70 W, pulse energy close to 1 mJ, all within a diffraction limited output beam. The second instance a fully-fiberized system capable of producing up to 45 W of average output power with a pulse energy of ~1 mJ was developed in collaboration with SPI Lasers Ltd. Unlike the first system, which uses an EOM for optical pulse shaping, an Acousto-Optic Modulator (AOM) is instead used to pre-shape the leading edge of the optical seed pulse in order to reduce the impact of nonlinear effects caused by the high peak powers otherwise associated with gain-saturation assisted reshaping of long nanosecond square input pulses, providing a cost-effective solution for the SMART Laser system. The successful development of the SMART Laser system allowed SPI Lasers Ltd to introduce a new product line, namely the G4 pulsed fiber laser system. Both types of fiber laser system were used in material processing experiments to investigate their performance and capabilities.Using a nanosecond fiber MOPA as a pump source, a synchronously pumped, tuneable, Raman fiber laser is demonstrated both in the near infrared (NIR) and visible regions. A continuous tuning range of 28nm in the NIR and 2.8nm in the visible region are achieved with efficiencies in the range of 12% to 18% respectively. The conversion efficiency can be increased further with the use of a feedback signal. Furthermore, with the presence of a feedback signal, the linewidth of the Raman Stokes lines in both visible and NIR regions shows a significant narrowing effect. This technique will allow the generation of wavelengths which are not easily generated with rare-earth doped fiber lasers and will be useful in the fields of spectroscopy, archaeology, biomedical and many more.Next, optical pulses in the picosecond regime are investigated. A gain-switching technique is used to generate a stable train of picosecond optical pulses from a semiconductor laser diode (SLD). Gain switching of different types of commercially available SLDs shows different temporal and spectral characteristics which are primarily influenced by the design of the specific chip used. The shortest pulse durations achieved through direct gain switching resulted in ~50 ps pulses; however these were far from transform-limited. However, an external FBG seeded gain switched SLDs was shown to be capable of producing transform-limited optical pulses. I show that a mode-locking mechanism is responsible for the short, transform limited optical pulses observed. This is the first demonstration of a mode locked SLD at the 1.06 µm waveband. With this technique, 18 ps optical pulses with pulse energy of 7.2 pJ and peak power of 400 mW were obtained.The single polarization, stable, picosecond optical pulses were fed into a chain of polarization maintaining fiber amplifiers to investigate the power scaling capability of this system. A maximum average output power of 513 W is demonstrated in a diffraction-limited output beam. The system operated at a repetition frequency of 215 MHz, corresponding to an estimated pulse energy of 2.4 µJ and a peak power of ~ 69 kW. At the maximum operating output power, the OSNR is measured to be well above 26 dB with a polarization extinction ratio (PER) of 17 dB. A pulse energy of 3.23 µJ is achieved from a similar system at a reduced operating frequency of 53 MHz and an average optical power of 200 W, corresponding to a pulse peak power of 107 kW. In both cases, further power scaling is limited by the SRS.. These results represent the highest optical power demonstrated from a fiber MOPA producing tens of picosecond optical pulses

Generation of transform-limited picosecond pulses at 1.0 µm from a gain switched semiconductor laser diode

We report the generation of short, transform-limited, ~18 ps optical pulses from an external fiber Bragg grating (FBG) stabilized semiconductor laser diode. Up to 7.2 pJ of pulse energy and a peak power of 400mW were achieved when operating at a repetition frequency of 832.6 MHz, a multiples of the cavity round trip frequency. A small detuning in the repetition frequency resulted in broader optical pulses. We have shown experimentally that an active mode-locking rather than gain switching mechanism is behind the generation of transform limited optical pulses at the optimum operating frequency

A fiber based synchronously pumped tunable Raman laser in the NIR

Raman lasers have attracted much interest since they allow a very wide range of wavelengths to be generated [1]. Operation in the pulsed regime is compromised by the power dependence of the gain - different parts of the pulse with different instantaneous power will undergo differing amounts of Raman scattering leading to major variations in spectral content across the pulse (varying amounts of light in different Raman orders). To address this issue active pulse shaping technique can be applied to obtain rectangular shaped output pulses so that constant Raman gain can be ensured across the pulse profile [2]. Here we demonstrate a new approach combining pulse shaping with a synchronously pumped scheme [3] to produce narrower linewidths as well as higher extinction ratios between neighboring Raman Stokes lines. Several tens of nanometer of tuning range for each Stokes order and a total tuning range of over 200 nm was achieved using resonant feedback from an external bulk grating

1.06 µm picosecond pulsed, normal dispersion pumping for generating efficient broadband infrared supercontinuum in meter-length single-mode tellurite holey fiber with high Raman gain coefficient

We investigate efficient broadband infrared supercontinuum generation in meter-length single-mode small-core tellurite holey fiber. The fiber is pumped by 1.06µm picosecond pulses in the normal dispersion region. The high Raman gain coefficient and the broad Raman gain bands of the tellurite glass are exploited to generate a cascade of Raman Stokes orders, which initiate in the highly normal dispersion region and quickly extend to longer wavelengths across the zero dispersion wavelength with increasing pump power. A broadband supercontinuum from 1.06µm to beyond 1.70µm is generated. The effects of the pump power and of the fiber length on the spectrum and on the power conversion efficiency from the pump to the supercontinuum are discussed. Power scaling indicates that using this viable normal dispersion pumping scheme, 9.5 W average output power of infrared supercontinuum and more than 60% conversion efficiency can be obtained from a 1 m long tellurite fiber with a large mode area of 500µm2

200W diffraction limited, single-polarization, all-fiber picosecond MOPA

A fully fiberized, single-polarization, gain-switched diode-seeded fiber master oscillator power amplifier (MOPA) system is demonstrated delivering 28ps pulses at variable repetition frequencies ranging from 53 MHz up to 858 MHz. An average signal output power of 200 W was achieved with good OSNR for all operating frequencies. A maximum pulse energy of 3.23 µJ at a repetition frequency of 53 MHz was achieved, corresponding to a pulse peak power of 107 kW. The extraction of higher pulse energy was limited primarily by the onset of nonlinear effects such as SRS which lead to compromised pulse quality at higher peak powers

Simultaneous excitation of selective multiple Raman Stokes wavelengths (green-yellow-red) using shaped multi-step pulses from an all-fibre MOPA system

We report the simultaneous excitation of multiple Raman Stokes lines in a 250 m long fiber using multi-step pump pulses. The frequency doubled output of a single polarization all-fiber Yb-doped MOPA operating at 1060 nm was used as the pump source. By adjusting the pump power and the pulse profiles we achieved the simultaneous excitation of green (1st Stokes), yellow (4th Stokes) and red light (6th Stokes) using 3-step pulses or the combination of any two using 2-step pulses. Through the use of pulse shaping we generate sequences of colored pulses with the flexibility of providing dynamic, agile frequency tuning between well-defined wavelengths

Single polarization picosecond fiber MOPA power scaled to beyond 500 W

We demonstrate a gain-switched diode-seeded, ytterbium-doped fiber based master oscillator power amplifier (MOPA) system, capable of delivering 2.4µJ, 35ps pulses at a repetition frequency of 215MHz in a single-polarization, close to diffraction limited beam. We are aware that the corresponding average power of > 500W is a record for such a MOPA system. Further pulse energy scaling was limited primarily by the onset of nonlinear effects such as self-phase modulation and stimulated Raman scattering which led to a compromised pulse quality at higher peak powers (> 70kW)

Tunable synchronously-pumped fiber Raman laser in the visible and near-infrared exploiting MOPA-generated rectangular pump pulses

We report a tunable synchronously pumped fiber Raman laser (SPFRL) in the near-infrared (NIR) and visible wavebands pumped by a pulsed, all-fiber PM 1060 nm master oscillator power amplifier (MOPA) and its frequency-doubled output, respectively. The seed was adaptively shaped to deliver rectangular output pulses, thereby enabling selective excitation of individual Raman Stokes lines. Using filtered synchronous feedback of the desired Raman Stokes line, the linewidth of the SPFRL was reduced by a factor of 4 and the extinction ratio of the desired Raman Stokes was improved by more than 3 dB relative to a simple single-pass conversion scheme. A continuous tuning range of 2.2 THz was obtained for each of the Raman Stokes orders in the visible (spanning from green to orange - first to fifth Stokes lines). A larger 5.0 THz tunable range was achieved in the NIR spectral region