Mode-locked Raman Fiber Laser Research Articles

Cylindrical vector beam generation from a passively mode-locked Raman fiber laser

We experimentally investigate cylindrical vector beam (CVB) generation from a passively mode-locked Raman fiber laser. The mode-locked Raman fiber lasers based on nonlinear polarization rotation (NPR), utilize the Raman gain resulted from the stimulated Raman scattering (SRS) effect in passive fibers, instead of stimulated emission amplification of active fibers in conventional mode-locked fiber lasers. With a mode-selective coupler (MSC) incorporated into the mode-locked Raman fiber laser, the generation of CVBs has been realized. In experiment, using a Raman pump source of 1454 nm, mode-locked CVB output with central wavelength of 1562 nm was obtained. The output polarization state can be switched between the radial and azimuthal polarization states, and the beam purity is above 90%. Due to the broadband Raman gain in optical fiber, the CVB output with the desired wavelength is available in principle by selecting a suitable pump source. This kind of flexible wavelength CVB Raman fiber laser may have potential applications in optical communications and fiber optics.

Actively Mode Locked Raman Fiber Laser with Multimode LD Pumping

We present our recent experimental results on the pulsed regimes of Raman conversion of highly multimode laser diode (LD) pump radiation into the 1st and higher order Stokes radiation in multimode graded-index fibers. Three different linear cavities of Raman fiber laser with the modulation of losses (by acousto-optic modulator, AOM) or gain (by LD current) are explored and compared. An LD with wavelength of 976 nm is used for pumping enabling Raman lasing at wavelength of the 1st (1018 nm) and 2nd (1064 nm) Stokes orders. At ~27.2-kHz repetition rate corresponding to the laser cavity round-trip frequency (i.e., in the mode-locking regime), nanosecond pulses have been observed for both Stokes orders having the highest peak power of ~300 W in the scheme with bulk AOM and the shortest duration of 5–7 ns in the scheme with fiber-pigtailed AOM. At the same time, the beam quality of generated pulses is greatly improved as compared to that for pump diode (M2 > 20) reaching the best value (M2 = 2.05) for the 2nd order Stokes beam in the scheme with the gain modulation and demonstrating also the most stable regime.

Cylindrical Vector Beam Generation from a Passively Mode-Locked Raman Fiber Laser

Cylindrical Vector Beam Generation from a Passively Mode-Locked Raman Fiber Laser

Passively mode-locked cascaded Raman fiber laser in a linear cavity

Passively mode-locked cascaded Raman fiber laser is proposed and demonstrated in a linear cavity. The mode-locking is based on nonlinear polarization rotation (NPR) with a polarization beam splitter (PBS) in the proposed laser. The passively mode-locked Raman fiber laser with a spectral range of 220 nm operates in a low pump power level, and up to the three orders Raman frequency shift are observed under our pump power conditions. The maximum output average power and single pulse energy of the proposed laser are 119.3 mW and 63.97 nJ with a slope efficiency of 41.7%. Multi-pulse mode-locked operation can also be generated by precisely adjusting polarization controller (PC) under suitable pump power conditions.

Ultra-high order harmonic mode-locking of a Raman fiber laser

An ultra-high order harmonically mode-locked Raman fiber laser with a high signal-to-noise ratio has been proposed and demonstrated. We apply the nonlinear polarization rotation (NPR) technique as the mode-locker in the Raman fiber laser. The underlying harmonic mode-locking (HML) mechanism is discussed to achieve ultra-high order HML in the Raman fiber laser. Up to 5044th ultra-high order HML has been obtained with a signal-to-noise ratio of up to 42.5 dB. To the best of our knowledge, this is the highest order of passive HML achieved so far from a Raman fiber laser using NPR or similar techniques.

Rectangular Pulse Generation From a Mode Locked Raman Fiber Laser

Rectangular and step-like pulses are generated in a mode-locked Raman fiber laser with a nonlinear optical loop mirror. The Raman laser has a typical figure-8 configuration. Stable rectangular pulses with a fundamental repetition rate of 733 kHz are obtained. As the pump power increases, the Raman pulse evolves to a step-like rectangular pulse. The maximum pulse energy is 64.1 nJ with a pulsewidth of 25.1 ns. After spectrally resolved measurements, the step-like pulse is proved to be induced by the dual wavelength oscillation. It is the first report of rectangular pulsed output in the mode-locked Raman fiber laser to the best of our knowledge.

Up to the 1552nd Order Passively Harmonic Mode-Locked Raman Fiber Laser

An ultrahigh order passively harmonic mode-locked Raman fiber laser based on nonlinear polarization rotation has been proposed and demonstrated experimentally. Different orders harmonic mode-locking (HML) was observed from the first to the 1552nd order by changing the pump power with adjusting polarization states. Up to the 1552nd order HML, corresponding to the repetition rate of 587.2 MHz, was obtained with the output power of 168.3 mW. The stability of HML is attested by the supermode suppression level as large as 40 dB. Different from the pulse energy quantization in low-order HML, the mechanism of high-order HML contributes to the acoustic stabilization effect.

Duration Switchable High-Energy Passively Mode-Locked Raman Fiber Laser Based on Nonlinear Polarization Evolution

We experimentally demonstrate a stable passively mode-locked Raman fiber laser delivering high-energy pulses that can be switched between the regime of hundreds of nanoseconds and that of picoseconds by the nonlinear polarization rotation technique. Maximum average output power values of 304 and 53 mW are obtained, respectively, for the two typical mode-locking states with the pulse duration of 500 ns and 180 ps at the fundamental repetition rate of 275 kHz. The corresponding single pulse energy is as much as 1.1 μJ and 193 nJ, respectively. To the best of our knowledge, this is the highest pulse energy achieved from mode-locked Raman fiber lasers reported so far.

Actively mode-locked Raman fiber laser.

Active mode-locking of Raman fiber laser is experimentally investigated for the first time. An all fiber connected and polarization maintaining loop cavity of ~500 m long is pumped by a linearly polarized 1120 nm Yb fiber laser and modulated by an acousto-optic modulator. Stable 2 ns width pulse train at 1178 nm is obtained with modulator opening time of > 50 ns. At higher power, pulses become longer, and second order Raman Stokes could take place, which however can be suppressed by adjusting the open time and modulation frequency. Transient pulse evolution measurement confirms the absence of relaxation oscillation in Raman fiber laser. Tuning of repetition rate from 392 kHz to 31.37 MHz is obtained with harmonic mode locking.

High-Energy Passively Mode-Locked Raman Fiber Laser Pumped by a CW Multimode Laser

A passively mode-locked Raman fiber laser based on nonlinear polarization rotation (NPR) with a high nonlinear fiber cavity is proposed and demonstrated. A polarization-independent continuous wave (CW) multimode laser is employed as the pump source in order to avoid stimulated Brillouin scattering generation. This Raman laser generates 890 ps pulses at 1651.3 nm. Under the pump power of 960 mW, the single pulse energy is up to 290.7 nJ with the peak power of 326.7 W. The average output power is up to 110 mW with a conversion efficiency of 11.5%.

Linearly Polarized 1180-nm Raman Fiber Laser Mode Locked by Graphene

In this paper, we demonstrate a linearly polarized 1180-nm passively mode-locked Raman fiber laser using graphene-based saturable absorber (SA). The pump source is a linearly polarized ytterbium-doped fiber laser at 1120 nm. Stable nanosecond mode-locked pulses with a repetition rate of 0.4 MHz are generated. The combination of Raman technology and the graphene-based ultrawide bandwidth SA offer a prospect of real wavelength-versatile mode-locked laser source. The mode-locked laser operating at 1180 nm is suitable for frequency doubling to yellow light after amplification.

Raman fiber laser harmonically mode-locked by exploiting the intermodal beating of CW multimode pump source

We report here the first demonstration of a harmonic mode-locked Raman fiber laser using the intermodal beating of a continuous-wave (CW) multiple-longitudinal-mode pump laser. By matching the Raman-cavity round-trip frequency with the intermodal-beating one of a 1064 nm CW pump source, harmonic mode-locking in phosphosilicate Raman fiber laser is stably initiated at the first-order Stokes of 1239.5 nm, and generates rectangular-shape nanosecond pulses with the pulse energy up to 4.25 nJ. Using the new type of mode-locking, the harmonic order can be discretely tuned from 78 th- to 693 rd-order, and the cavity supermode is suppressed up to 51.1 dB with the signal-to-noise ratio of more than 65 dB.

138-µm mode-locked Raman fiber laser pumped by semiconductor disk laser

A mode-locked Raman fiber laser pumped by 1.3 µm semiconductor disk laser is demonstrated. Direct Watt-level core-pumping of the single-mode fiber Raman lasers and amplifiers with low-noise disk lasers is demonstrated to represent a highly practical solution as compared with conventional scheme using pumping by Raman wavelength convertors. Raman laser employing passive mode-locking by nonlinear polarization evolution in normal dispersion regime produces stable pedestal-free 1.97 ps pulses at 1.38 µm. Using semiconductor disk lasers capable of producing high power with diffraction-limited beam allows Raman gain to be obtained at virtually any wavelength of interest owing to spectral versatility of semiconductor gain materials and wafer-fusing technology.

Raman fiber laser pumped by a semiconductor disk laser and mode locked by a semiconductor saturable absorber mirror

A 1.6µm mode-locked Raman fiber laser pumped by a 1480nm semiconductor disk laser is demonstrated. Watt-level core pumping of the single-mode fiber Raman lasers with low-noise disk lasers together with semiconductor saturable absorber mirror mode locking represents a highly practical solution for short-pulse operation.

Dynamics of an ultrahigh-repetition-rate passively mode-locked Raman fiber laser

We present an extended study of an ultrahigh-repetition-rate Raman fiber laser passively mode-locked through dissipative four-wave mixing. We demonstrate mode locking at 100 and 160 GHz repetition rates, corresponding to harmonic frequencies more than 400,000 times the cavity resonance frequency. By varying the output coupling ratio we are able to change the threshold and slope efficiency of the laser. The maximum average output power achieved was 926 mW, a further twofold increase of the output power reported previously for this type of laser. Further numerical analysis reveals that the main factor limiting the pulse quality of our setup is supermode noise. Subsequently, we experimentally investigate the use of all-fiber subcavity Fabry-Perot filters to reduce the noise and demonstrate a significant improvement in the laser operation.

Passively mode-locked Raman fiber laser with 100 GHz repetition rate

We experimentally demonstrate the operation of a passively mode-locked Raman fiber ring laser with an ultrahigh repetition rate of 100 GHz and up to 430 mW of average output power. This laser constitutes a simple wavelength versatile pulsed optical source. Stable mode locking is based on dissipative four-wave mixing with a single fiber Bragg grating acting as the mode-locking element.