June 1, 2015 / Vol. 40, No. 11 / OPTICS LETTERS
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High-power synchronously pumped femtosecond Raman fiber laser D. Churin,* J. Olson, R. A. Norwood, N. Peyghambarian, and K. Kieu College of Optical Sciences, University of Arizona, 1630 E. University Blvd., Tucson, Arizona 85721, USA *Corresponding author: [email protected] Received March 17, 2015; revised May 2, 2015; accepted May 4, 2015; posted May 5, 2015 (Doc. ID 236234); published May 21, 2015 We report a high-power synchronously pumped femtosecond Raman fiber laser operating in the normal dispersion regime. The Raman laser is pumped by a picosecond Yb3 -doped fiber laser. It produces highly chirped pulses with energy up to 18 nJ, average power of 0.76 W and 88% efficiency. The pulse duration is measured to be 147 fs after external compression. We observed two different regimes of operation of the laser: coherent and noise-like regime. Both regimes were experimentally characterized. Numerical simulations are in a good agreement with experimental results. © 2015 Optical Society of America OCIS codes: (060.2320) Fiber optics amplifiers and oscillators; (140.3550) Lasers, Raman; (140.7090) Ultrafast lasers. http://dx.doi.org/10.1364/OL.40.002529
Over the past two decades, fiber laser technology has advanced remarkably, and lasers operating in the nearIR spectral region based on ytterbium-, erbium-, and thulium-doped active fibers have replaced solid state lasers in many applications. However, existing rareearth-doped active fibers leave a significant portion of the near-IR wavelength band uncovered, and the development of compact and robust fiber light sources at these wavelengths is still ongoing. For example, nonlinear optical parametric interaction [1] and stimulated Raman scattering [2–5] in a passive fiber can be used to create signal gain at spectral regions not accessible with established rare-earth-doped active fibers. Widely tunable and high-power fiber optical parametric oscillators (FOPO) still require a significant research effort to reach the level of performance currently offered by solidstate laser systems [6]. At the same time, Raman fiber sources are already found in various applications such as multiphoton microscopy [2], laser guide stars [3], and pump sources for erbium lasers [4]. Raman scattering is an optical effect that arises from the interaction between optical photons and resonant vibrations (optical phonons) of atoms in molecules (in a gas or liquid) or in a lattice of a dense medium. An incoming “pump” photon scattering off a vibrating lattice/molecule can be frequency downshifted by giving a portion of its energy to the molecule/lattice (Stokes wave) or frequency upshifted by taking up the energy from the molecule/lattice (anti-Stokes wave). Due to the small number of optical phonons in the excited vibrational state at room temperature, the generation of the anti-Stokes wave is very weak compared to the generation of the Stokes wave. Spontaneous Raman scattering is negligible at low pump powers; however, the process can become stimulated at higher pump power levels resulting in significant conversion of pump photons into Stokes photons. Photon conversion efficiency of more than 95% has been demonstrated in a continuous wave (CW) Raman laser [5]. Stimulated Raman scattering is prevalent in optical fibers owing to the strong confinement of the optical field and the long interaction length. For that reason, optical fiber is a great platform for building Raman-based light sources. 0146-9592/15/112529-04$15.00/0
Generation of ultrashort optical pulses in a Raman laser can be achieved using a synchronous pumping scheme with a mode-locked (ML) pump laser [7–12] or with a CW pump laser and a saturable absorber (SA) [13–16]. The latter is typically called a ML Raman laser. The main difference between ultrafast Raman fiber lasers and standard ML lasers based on rare-earth-doped active fiber is the gain mechanism, but pulse evolution inside a laser cavity should be similar. For example, standard ML fiber lasers and Raman ML fiber lasers can operate in the soliton [9,10,17] or the dissipative soliton regimes [11–15,18]. The key advantage of the dissipative soliton regime is the ability to produce pulses with energies a few orders of magnitude higher than can be produced in the standard soliton regime [19]. Dissipative soliton pulses are typically generated in an oscillator with net normal cavity dispersion. The pulses propagating in such oscillator are highly chirped with lower peak power, but the pulse energy can be quite large. Due to the lower peak power, dissipative solitons can propagate in the fiber without breaking up. The chirped dissipative soliton pulses can be compressed to close to the transformlimited pulse duration using an external compressor like a grating pair. ML fiber lasers operating in the normal dispersion regime with output pulse energies of tens of nanojoules have been demonstrated [20,21]. Raman-dissipative solitons (RDS) were first reported by Aguergaray et al. [13] in a ML Raman laser with power conversion efficiency of only 0.1%. Such a low efficiency is common for ML Raman lasers with CW pumping due to the short excited state lifetime of the optical phonons (
2529
High-power synchronously pumped femtosecond Raman fiber laser D. Churin,* J. Olson, R. A. Norwood, N. Peyghambarian, and K. Kieu College of Optical Sciences, University of Arizona, 1630 E. University Blvd., Tucson, Arizona 85721, USA *Corresponding author: [email protected] Received March 17, 2015; revised May 2, 2015; accepted May 4, 2015; posted May 5, 2015 (Doc. ID 236234); published May 21, 2015 We report a high-power synchronously pumped femtosecond Raman fiber laser operating in the normal dispersion regime. The Raman laser is pumped by a picosecond Yb3 -doped fiber laser. It produces highly chirped pulses with energy up to 18 nJ, average power of 0.76 W and 88% efficiency. The pulse duration is measured to be 147 fs after external compression. We observed two different regimes of operation of the laser: coherent and noise-like regime. Both regimes were experimentally characterized. Numerical simulations are in a good agreement with experimental results. © 2015 Optical Society of America OCIS codes: (060.2320) Fiber optics amplifiers and oscillators; (140.3550) Lasers, Raman; (140.7090) Ultrafast lasers. http://dx.doi.org/10.1364/OL.40.002529
Over the past two decades, fiber laser technology has advanced remarkably, and lasers operating in the nearIR spectral region based on ytterbium-, erbium-, and thulium-doped active fibers have replaced solid state lasers in many applications. However, existing rareearth-doped active fibers leave a significant portion of the near-IR wavelength band uncovered, and the development of compact and robust fiber light sources at these wavelengths is still ongoing. For example, nonlinear optical parametric interaction [1] and stimulated Raman scattering [2–5] in a passive fiber can be used to create signal gain at spectral regions not accessible with established rare-earth-doped active fibers. Widely tunable and high-power fiber optical parametric oscillators (FOPO) still require a significant research effort to reach the level of performance currently offered by solidstate laser systems [6]. At the same time, Raman fiber sources are already found in various applications such as multiphoton microscopy [2], laser guide stars [3], and pump sources for erbium lasers [4]. Raman scattering is an optical effect that arises from the interaction between optical photons and resonant vibrations (optical phonons) of atoms in molecules (in a gas or liquid) or in a lattice of a dense medium. An incoming “pump” photon scattering off a vibrating lattice/molecule can be frequency downshifted by giving a portion of its energy to the molecule/lattice (Stokes wave) or frequency upshifted by taking up the energy from the molecule/lattice (anti-Stokes wave). Due to the small number of optical phonons in the excited vibrational state at room temperature, the generation of the anti-Stokes wave is very weak compared to the generation of the Stokes wave. Spontaneous Raman scattering is negligible at low pump powers; however, the process can become stimulated at higher pump power levels resulting in significant conversion of pump photons into Stokes photons. Photon conversion efficiency of more than 95% has been demonstrated in a continuous wave (CW) Raman laser [5]. Stimulated Raman scattering is prevalent in optical fibers owing to the strong confinement of the optical field and the long interaction length. For that reason, optical fiber is a great platform for building Raman-based light sources. 0146-9592/15/112529-04$15.00/0
Generation of ultrashort optical pulses in a Raman laser can be achieved using a synchronous pumping scheme with a mode-locked (ML) pump laser [7–12] or with a CW pump laser and a saturable absorber (SA) [13–16]. The latter is typically called a ML Raman laser. The main difference between ultrafast Raman fiber lasers and standard ML lasers based on rare-earth-doped active fiber is the gain mechanism, but pulse evolution inside a laser cavity should be similar. For example, standard ML fiber lasers and Raman ML fiber lasers can operate in the soliton [9,10,17] or the dissipative soliton regimes [11–15,18]. The key advantage of the dissipative soliton regime is the ability to produce pulses with energies a few orders of magnitude higher than can be produced in the standard soliton regime [19]. Dissipative soliton pulses are typically generated in an oscillator with net normal cavity dispersion. The pulses propagating in such oscillator are highly chirped with lower peak power, but the pulse energy can be quite large. Due to the lower peak power, dissipative solitons can propagate in the fiber without breaking up. The chirped dissipative soliton pulses can be compressed to close to the transformlimited pulse duration using an external compressor like a grating pair. ML fiber lasers operating in the normal dispersion regime with output pulse energies of tens of nanojoules have been demonstrated [20,21]. Raman-dissipative solitons (RDS) were first reported by Aguergaray et al. [13] in a ML Raman laser with power conversion efficiency of only 0.1%. Such a low efficiency is common for ML Raman lasers with CW pumping due to the short excited state lifetime of the optical phonons (
