Stokes Orders Research Articles

Dual-wavelength eye-safe Nd:YAP Raman laser.

We report on a diode-pumped, Q-switched intracavity-Raman-shifted Nd-doped yttrium-aluminum-perovskite (Nd:YAP) laser using potassium titanyl phosphate (KTP) or potassium titanyl arsenate (KTA) nonlinear crystals to produce eye-safe radiation. In both cases, we obtained simultaneous laser emission at two wavelengths corresponding to distinct Raman gain peaks and different Stokes orders for the nonlinear crystal used. From the Nd:YAP/KTP laser source, we obtained 3 ns pulses of combined 21 kW peak power at 1478 and 1503 nm. From the Nd:YAP/KTA laser source, we obtained up to 220 mW total average output power, equally split between 1474 and 1480 nm emission lines.

377 μm fiber laser based on cascaded Raman gain in a chalcogenide glass fiber

Laser emission is demonstrated at a wavelength of 3.766 μm in a cascaded Raman gain device. The laser cavity is made of two nested pairs of fiber Bragg gratings inscribed in a 2.8 m length of low-loss As2S3 fiber. An erbium-doped fluoride glass quasi-CW fiber laser emitting at 3.005 μm is used to pump the cascaded Raman cavity, which converts the pump wavelength successively to the first and second Stokes orders, respectively at 3.340 and 3.766 μm. A laser output peak power in excess of 100 mW is obtained with a lasing efficiency of about 8.3% with respect to the launched pump power. This represents the highest emission wavelength delivered by a fiber laser operating at room temperature.

An efficient 14.5 W diamond Raman laser at high pulse repetition rate with first (1240 nm) and second (1485 nm) Stokes output

Efficient operation of a pulsed diamond Raman laser at output powers up to 14.5 W is reported. An external-cavity Raman resonator 2.5 cm long employing a low-loss CVD diamond 9.5 mm long generated first (1240 nm) and second (1485 nm) Stokes output powers of up to 3 W and 13 W respectively when pumped with a 30 W q-switched Nd:YVO4 laser with approximately 22 ns pulses at a 36.5 kHz pulse repetition frequency. We show that this approach to efficient wavelength conversion is promising for a range of practical pulsed laser systems with high average power at first and cascaded Stokes orders in the near-infrared and including eye-safe laser output.

Control of cascading in multiple-order Raman lasers

In this work we explore a method for controlling Raman cascade within an intracavity Raman laser, with a view to maximizing output power at desired visible wavelengths. Sum-frequency generation is used to suppress unwanted Stokes orders, and prevent their build up. Using this method to control the Raman cascade, we demonstrate increases in output power of 40% at 532 nm, 42% at 559 nm, and 67% at 586 nm.

Spectral gain control using shaped pump pulses in a counter-pumped cascaded fiber Raman amplifier

We investigate the Raman gain spectra produced from pulse-pumping a highly nonlinear fiber with shaped optical pulses delivered from a Yb-doped fiber MOPA pump source. Cascaded Raman wavelength shifting up to seven Stokes orders is demonstrated and the counter-propagating gain is measured across all seven Stokes orders. Step-shaped optical pulses with varying instantaneous powers are then used to pump the highly nonlinear fiber, generating a controllable gain spectrum across multiple Stokes orders. Furthermore, we extend this work by using multiple pump wavelengths along with step-shaped pulses to increase the bandwidth of the Raman gain spectrum.

Power Scaling of Single-Frequency Hybrid Brillouin/Ytterbium Fiber Lasers

A coupled-wave rate-equation model, including multiple-order stimulated Brillouin scattering (SBS), is used to study power scaling of hybrid Brillouin/ytterbium fiber lasers. To validate the model, a single-frequency, Brillouin/ytterbium fiber laser was built with a laser output of 40 mW and an optical signal-to-noise ratio greater than 50 dB. The numerical model simulation agrees with the measurements in both fully and partially injection locked regimes. To scale up the laser's output power, a dual-clad architecture is proposed. In this new configuration, the active Yb-doped fiber provides the nonlinear SBS gain as well as the gain resulting from the excited Yb ions. Numerical modeling including three Stokes orders shows that over 5 W of single-frequency laser output can be achieved with a side-mode suppression ratio (SMSR) of greater than 80 dB. Beyond this power, multi-order SBS affects the laser efficiency and SMSR.

Extreme-value statistics in supercontinuum generation by cascaded stimulated Raman scattering

We study experimentally the statistical fluctuations observed in a supercontinuum generated in the normal dispersion regime through cascaded stimulated Raman scattering. Specifically, we show that the statistical distribution of shot-to-shot spectral variations evolves from a quasi-Gaussian in the saturated regime for Stokes orders near the pump to a long-tailed extreme-value distribution for Stokes orders at a large separation from the pump in the unsaturated regime.

Increased wavelength options in the visible and ultraviolet for Raman lasers operating on dual Raman modes

We report increased wavelength options from Raman lasers for Raman media having two Raman modes of similar gain coefficient. For an external-cavity potassium gadolinium tungstate Raman laser pumped at 532 nm, we show that two sets of Stokes orders are generated simultaneously by appropriate orientation of the Raman crystal, and also wavelengths that correspond to sums of the two Raman modes. Up to 14 visible Stokes lines were observed in the wavelength range 555-675 nm. The increase in Stokes wavelengths also enables a much greater selection of wavelengths to be accessed via intracavity nonlinear sum frequency and difference frequency mixing. For example, we demonstrate 30 output wavelength options for a wavelength-selectable 271-321 nm Raman laser with intracavity sum frequency mixing in BBO. We also present a theoretical analysis that enables prediction of wavelength options for dual Raman mode systems.

Multi-port beam combination and cleanup in large multimode fiber using stimulated Raman scattering

We demonstrated the successful combination and cleanup of four laser beams via stimulated Raman scattering (SRS) using a multi-port fiber combiner and a large multimode fiber. Multiple Stokes orders were observed in the output, but loss at longer wavelengths reduced the transmission of the higher Stokes orders and limited the SRS conversion efficiency. SRS beam cleanup was also investigated using a single laser beam. The output beam had a measured M(2) better than 2 for fiber lengths from 400-1400 meters.

Third order cascaded Raman wavelength shifting in chalcogenide fibers and determination of Raman gain coefficient

Cascaded Raman wavelength shifting up to three orders from 1553 nm to 1867 nm is demonstrated in As(2)S(3)-chalcogenide fibers. Due to a long zero dispersion wavelength for the sulfide fiber (>4.5 mum), pumping the fiber at 1553 nm results in generation of cascaded Stokes orders based on stimulated Raman scattering. Using the threshold power for the Raman orders, we estimate the Raman gain coefficient for the As(2)S(3) fibers to be ~5.7x10(-12) m/W at 1550 nm. Observation of higher Raman orders is limited by damage to the fiber at input intensities >1 GW/cm(2).

Continuous-wave pumping in the anomalous- and normal-dispersion regimes of nonlinear fibers for supercontinuum generation

Supercontinuum (SC) growth in highly nonlinear fibers is compared for cw pumping in the anomalous- and normal-dispersion regimes. For anomalous-dispersion pumping, the combined effects of modulation instability (MI) and stimulated Raman scattering (SRS) contribute to spectral broadening. Furthermore, breakup of the cw light into ultrashort pulses by MI leads to the formation of a Raman pulse that evolves into a soliton, as evidenced by the observation of soliton self-frequency shift. Blueshifted, nonsolitonic radiation associated with the fission of higher-order solitons is also present in the SC spectra. For normal-dispersion pumping, SRS seeds the SC growth by generating several cascaded Stokes orders. When the Stokes orders are shifted into the anomalous-dispersion regime at higher launch powers, MI again causes soliton formation. Broadband continua are generated when the laser is positioned as far away as 191 nm from the zero-dispersion wavelength in normal dispersion.

Experimental optimization of high power Raman fiber lasers at 1495 nm using phosphosilicate fibers

We report in this paper on the experimental optimization of high power Raman fiber lasers at the second Stokes order of a phosphosilicate (P-doped) fiber doped with 13 mol% P 2O 5. The lasers were pumped with a 20 W Yb double clad fiber laser at 1070.75 nm and the cavity used two pairs of fiber Bragg gratings (FBGs). One pair is highly reflective at 1248 nm to fully cascade the pump wavelength to the first Stokes order, and the other includes a partial reflective (PR) FBG at 1495 nm to couple the laser out of the cavity. A total of 20 laser configurations were constructed and tested, consisting of four PR FBGs couplers with reflection from 4% to 45% and five fiber lengths from 200 to 1000 m. It is found that the optimal configuration is that with a fiber length of 300 m and a 33% PR FBG. The maximum power obtained from this laser is 6.66 W at 1495 nm with a threshold of ∼1 W and a slope efficiency of 39.18%. The highest slope efficiency achieved with pump power up to 18 W is 55.6% in the shortest fiber laser of 200 m without any PR FBG but a cleaved end. It is observed that the input–output characteristics are almost linear for all the configurations with feedback of PR FBGs ⩾15%, indicating the stimulated scattering to higher Stokes orders is negligible. Laser line width is also observed to become broader with increase in pump power, fiber length and the reflection of the PR FBGs.

All-fibre Brillouin laser based on holey fibre yielding comb-like spectra

An all-fibre integrated Brillouin ring laser including a 100-m holey fibre is presented and yields comb-like spectra due to four-wave mixing between different Brillouin Stokes orders. Two spectral combs were simultaneously observed, corresponding to odd- and even-order Brillouin Stokes and anti-Stokes, and could be separately extracted from the cavity. More than 60 spectral lines were observed in total. In the time domain the laser output consisted of pulses with duration of a few nanoseconds. A study of the single-pass holey fibre Brillouin characteristics is also presented.

Cascaded Raman generation in optical fibers: influence of chromatic dispersion and Rayleigh backscattering.

We study experimentally the influence of chromatic dispersion and Rayleigh backscattering on cascaded Raman generation in silica optical fibers. Effects ranging from enhanced spectral broadening of the Stokes orders to generation of higher Stokes order at unexpected wavelengths are observed. Additionally, we show that four-wave-mixing processes can quench the noisy Rayleigh lasing lines generated in power Raman amplifiers. Our observations are confirmed by numerical simulations.

Efficient, all-solid-state, Raman laser in the yellow, orange and red

We report efficient operation of a KGd(WO(4))(2) Raman laser pumped by a small, 1 W, 532 nm laser module. By changing the output coupler and Raman crystal orientation, more than 8 wavelengths in the yellow-to-red spectral region were generated including 555 nm, 559 nm, 579 nm, 589 nm, 606 nm, 622 nm, 636 nm and 658 nm, ie., the first 4 Stokes orders on the two orthogonal high-gain Raman shifts of KGd(WO(4))(2). We have also demonstrated spectrally pure output (typically >90% pure) for selected Stokes order with output power up to 400 mW. High slope efficiency (up to 68%) and high beam quality (M(2)~1.5) of Stokes output are obtained even at the highest pump power.

Raman fiber lasers for optical communication application

We review the principle and characteristics of cascaded Raman fiber lasers in their telecommunication applications. The fundamentals of Raman fiber lasers are described, such as pumping scheme, fiber type and reflectors. We explain simple equations forcw laser operation as well as a possible way to calculate spectral width. We investigate the common issues in the use of Raman fiber lasers: theoretical optimization, control of the linewidth, suppression level of the other Stokes orders and relative intensity noise. Finally the more complex multiple-wavelength Raman fiber lasers are reviewed.

Design of widely tunable Raman fibre lasers supported by switchable FBG resonators

A tunable Raman fibre laser (RFL) has been designed such that a wide continuous spectral tuning range spanning several Stokes orders is achieved and the lasing of undesired parasitic resonators is suppressed. A practical design for an RFL, which is continuously tunable over 350 nm spanning the full optical communications wavelength range, is presented.

Influence of classical pump noise on long-pulse multiorder stimulated Raman scattering in optical fiber

We present a combined experimental and theoretical study of the effect of pump pulse noise on the growth and statistics of multiorder stimulated Raman scattering in optical fiber. Because of the intensity dependence of stimulated Raman scattering, fluctuations in the detailed temporal structure of the pump pulse amplitude strongly affect the growth and statistics of the Stokes orders, even when dispersive effects are not important. By comparing experimental results with a detailed model including the frequency dependence of the Raman gain and the pump pulse temporal structure, we show that the pump pulse temporal fluctuations play a pivotal role in determining the growth and pulse energy statistics of the Stokes orders.

Effects of pump pulse temporal structure on long-pulse multi-order stimulated Raman scattering in optical fiber

We present an experimental and theoretical investigation of the effects of pump pulse temporal structure on cascade Raman generation in the quasi-continuous wave regime. The growth and saturation of the Stokes pulse energies depend very strongly on the pump pulse temporal envelope and substructure. Experiments and simulations on the growth of the total Stokes pulse energy and the energy in a narrow central slice highlight the importance of knowing the detailed pump pulse temporal structure. Experimentally measured growth curves for different Stokes orders could be transformed by a simple scaling to lie on a universal curve.

Output Characteristics of High-Power Continuous Wave Raman Fiber Laser at 1484 nm Using Phosphosilicate Fiber

A continuous wave (CW) high-power Raman fiber laser (RFL) with maximum output power of 2.24 W and slope efficiency of 32.8% at 1484 nm is obtained using a CW 8.4 W/ 1064 nm Yb-doped double-clad fiber laser as a pump, 700 m phosphosilicate fiber, and cascaded cavities with two pairs of fiber Bragg grating mirrors for the first and the second Stokes orders. Theoretical simulation of the RFL using a very efficient and rapidly converging collocation method is done to understand and optimize the fiber length to obtain maximum second Stokes power. RFL performance for the 300, 700 and 1150 m phosphosilicate fiber lengths was experimentally investigated by observing maximum output power, slope efficiency, threshold power, and full-width at half maximum at the second Stokes wavelength.