We report the first demonstration, to our knowledge, of soft-aperture Kerr-lens mode locking in a diodepumped femtosecond Yb 3+ : YVO 4 laser. Near-transform-limited pulses as short as 61 fs are generated around a center wavelength of 1050 nm with an output power of 54 mW and a pulse repetition frequency of 104.5 MHz. This is, to our knowledge, the shortest pulse generated directly from an Yb laser having a crystalline host material. The femtosecond operation has a mode-locking threshold at an absorbed pump power of 190 mW. 3 From the point of view of the development of new Yb-doped crystals for reliable and efficient femtosecond lasers, materials are preferred that possess high cross sections (in particular, the emission cross section) 4 together with broad gain spectra 5-7 and good physical and mechanical properties. 8 In this context, given the strong and broad absorption peak (FWHM of ϳ8 nm) at around 985 nm, which is compatible with optical pumping by well-developed InGaAs laser diodes, the extremely low quantum defect ͑ϳ3.5% ͒, and a relatively broad and smooth 5 (glasslike) emission spectrum, the Yb: YVO 4 crystal is a good candidate gain medium for incorporation into diode-pumped femtosecond lasers for the 1-m spectral region. Indeed, 120-fs pulses with an average power of 300 mW were generated from a diode-pumped Yb: YVO 4 laser that was passively mode locked by using a semiconductor saturable absorber mirror (SESAM). 9In fact, most of the work in the field of ultrashort Yb lasers has concentrated on the employment of SESAMs 10 for passive mode locking. Kerr-lens mode locking 11 (KLM) is another well-developed technique for the generation of ultrashort pulses from lasers that are efficient, compact, and have reduced cavity component counts. 12 Reducing the intracavity losses by excluding a SESAM (nonsaturable losses) can lead to a substantial enhancement in the optical efficiency of such a femtosecond laser. In addition, the shortest pulse generated directly from a laser was produced using the KLM technique. 13 One of the critical parameters for reliable Kerr-lens mode locking is a high value of the nonlinear refractive index n 2 of a laser medium. Although, Major et al. have shown that a number of Yb-doped crystals are characterized by relatively high n 2 and are promising for KLM operation, 14,15 only the Yb:KYW laser has been mode locked successfully by using the optical Kerr effect under the conditions of direct diode pumping. 16,17 In this Letter we describe the parameters of a new Yb-doped YVO 4 crystal, which was selected for possible exploitation in KLM solid-state lasers. Impressively, our KLM Yb: YVO 4 laser produced pulses as short as 61 fs at a center wavelength of 1050 nm with an average output power of 54 mW for an absorbed pump power of just 400 mW. The nonlinear refractive indices were measured for the Yb: YVO 4 crystal by using the z-scan technique and were found to be 19 ϫ 10 −16 cm 2 / W and 15ϫ 10 −16 cm 2 / W for and polarizations, respectively, at 1080 nm.A schematic of the laser c...

We demonstrate a tunable multi-wavelength Brillouin-Raman fiber laser with 20 GHz wavelength spacing. The setup is arranged in a linear cavity by employing 7.2 and 11 km dispersion compensating fibers (DCF) in addition to a 30 cm Bismuth-oxide erbium doped fiber. In this experiment, for the purpose of increasing the Stokes lines, it is necessary to optimize Raman pump power and Brillouin pump power together with its corresponding wavelengths. At the specific Brillouin pump wavelength, it is found that the longer length of 11 km DCF with optimized parameters results in larger number of Stokes combs and optical signal to noise ratios (OSNRs). In this case, a total of 195 Brillouin Stokes combs are produced across 28 nm bandwidth at Brillouin pump power of -2 dBm and Raman pump power of 1000 mW. In addition, all Brillouin Stokes signals exhibit an average OSNR of 26 dB.

We report an ultra-long Raman laser that implemented a variable pumping scheme in backward and forward configurations. Rayleigh backscattering effects were realized in the 51 km fiber length that functioned as a virtual mirror at one fiber end. With the employment of a fiber Bragg grating that has a peak reflection wavelength at 1553.3 nm, spectral broadening effects were observed. These occurred as the pump power level was diverted more to the forward direction. Owing to this fact, a maximum width of 0.9 nm was measured at 100% forward pumping. The obtained results show that the efficient exploitation of four-wave mixing interactions as well as strong Rayleigh backscattering are beneficial to influence the lasing performances. Both of these nonlinear responses can be adjusted by varying pumping distributions along the fiber longitudinal dimension.

We discover the technique of controlling the flatness in signal amplitude of a multiwavelength Brillouin-Raman fiber laser by employing an air-gap outside of the cavity. The structure that is adjustable within sub-millimeter length behaves as flexible optical feedback that provides modifiable portions of multiple Fresnel reflectivities. This is the main benchmark that allows the efficient management of gain competition between self-lasing modes and Brillouin Stokes waves that is vital for self-flattening initiation. When setting the Brillouin pump wavelength at 1529 nm and the air-gap distance to 0.4 mm, 296 Stokes lines are produced with a channel spacing of 0.158 nm. The lasing bandwidth is 46.60 nm that covers from 1529.16 to 1575.76 nm wavelength. In this case at Raman power of 950 mW, the intense Brillouin pump power of 2 dBm saturates the cascaded higher-orders lasing lines. As a result, the overall peak power discrepancy is maintained at just 1.8 dB where an average optical-signal-to-noise ratio of 20 dB is realized. To date, this is the widest bandwidth with the flattest spectrum attained in multiwavelength fiber lasers that incorporate a single Raman pump unit.

We report an ultra-long Raman laser with a 46 km fiber length that behaved as a Rayleigh back-scattering-based optical feedback. The laser was tunable from 1550 nm to 1571 nm (3 dB bandwidth) with the insertion of an optical grating filter. Evaluations on the spectral evolution and power development were also performed from the results obtained. In fact, it was discovered that the spectral broadening effect between the modeless spectra resembled the same process that happens in a typical fiber cavity that has high reflectors at each cavity end. In addition, the output power showed a square-root development with respect to the input power.

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We demonstrate a single-spacing, multi-wavelength Brillouin-Raman fiber laser utilizing an enhanced cavity of nonlinear amplifying loop mirror. In this structure, the optimization of multi-wavelength lasing is done with proper adjustments of coupling ratio and Brillouin pump power. When setting the Raman pump power to 300 mW, up to 28 channels with an average 17 dB optical signal-to-noise ratio are achieved. In this case, the Brillouin pump power is maintained at -2.6 dBm when the splitting ratio and Brillouin pump wavelength are fixed at 99/1 and 1555 nm, correspondingly. Our achievements present high numbers of Stokes channels with an acceptable optical signal-to-noise ratio at low pump power operation.

Efficient generation of femtosecond pulses at 524 nm is demonstrated by the extracavity frequency doubling of the output of a diode-pumped femtosecond Yb3+:KY(WO4)2 laser using a periodically poled LiTaO3 crystal. An average second-harmonic power of 120 mW is produced at an internal conversion efficiency of 40%. The temporal characteristics of the frequency-doubled pulses as a function of focusing conditions in a thick nonlinear crystal are investigated experimentally, and pulses as short as 225 fs are generated at a pulse repetition frequency of 86 MHz.