A 3rd-order cascaded Raman fiber laser has been demonstrated without using fiber Bragg gratings at all in the entire laser system. More than 2 W output power at the 3rd Stokes emission wavelength of 1307 nm, a bandwidth of 4.2 nm and a slope efficiency of ~46% was measured when a 500 m long moderately Ge-doped silica fiber was pumped by a free running broadband Yb3+-doped fiber laser. This method is simple, versatile and cheap when compared with conventional methods employing narrow band pumping and fiber Bragg gratings to resonate the Stokes wavelengths. A slope efficiency of ~72% and an output power of over 4 W was also achieved at the 1st order Stokes wavelength of 1168 nm when a 130 m long Ge-doped silica fiber was used.
© 2005 Optical Society of America
Recently remarkable progress has been made in the development of highly efficient CW fiber lasers using stimulated Raman scattering (SRS) as the gain mechanism . The SRS process in optical fibers is an effective method for amplifying optical signals and for generating laser radiation at practically any wavelength in the 1 µm–1.7 µm spectral region . Successes include the recent demonstration of 13.2 W output power from a phosphosilicate-fiber-based 2nd order cascaded Raman fiber laser . Higher order Raman Stokes radiation was generated in a phosphosilicate fiber resonator formed by pairs of Bragg gratings  and the highest reported slope efficiency is currently ~84.2% . Vitreous germania-doped silica fiber has a higher nonlinearity and lower loss than phosphosilicate fiber but it has a smaller Raman Stokes shift , however, Raman fiber lasers (RFL) as short as 3 m have been demonstrated using very highly germania-doped fiber  and a “grating-free” cascaded Raman fiber laser has also been demonstrated using narrow band pumping .
In an extension to these demonstrations of Raman fiber lasers that utilize narrow band pump sources and fiber Bragg gratings (FBG) for the Raman fiber laser resonator, we present a three-wavelength high power and highly efficient Raman fiber laser that employs high Raman gain Ge-doped optical fibers and broadband pumping. A slope efficiency of ~72% and an output power of over 4 W have been achieved at a wavelength of 1168 nm using 130 m Ge-doped Raman fiber. This result demonstrates that the broadband pumped free-running cascaded Raman laser configuration is a very effective approach. The simplicity, overall efficiency and versatility of this Raman fiber laser makes it highly practical for a number of applications including Raman amplification, laser pumping applications and distributed sensing systems.
A schematic diagram of the experimental arrangement is shown in Fig. 1. The Yb3+-doped fiber laser consisted of a 975 nm pump laser, a high reflectivity (~100% at 1060 nm) broadband dielectric mirror M butted against the input end to the double clad Yb3+-doped fiber and the ~4% Fresnel reflection fiber end reflector FER1. The Yb3+-doped fiber laser is used to pump the cascaded Raman fiber laser after launching into the Ge-doped silica fiber using a pair of objective lenses. With an appropriate alignment, the coupling induced power variation is less than ~2%. The cascaded Raman fiber laser uses the ~4% Fresnel reflection at the two ends of the Ge-doped silica fiber as the resonator. As we show later, the center wavelength and bandwidth of the Raman fiber laser output depends strongly upon the pump power level.
Germania glass has the highest Raman cross-section amongst the widely used vitreous Raman materials SiO2, GeO2, B2O3 and P2O5 . We fabricated an ~18 mol% concentration germano-silicate fiber as the Raman fiber to increase the Raman gain per unit length of fiber. The Raman fiber had a small mode-field-diameter (MFD) of 3.87 µm and the effective area was Aeff=12 µm2 at the 1300 nm wavelength. The estimated loss and Raman gain coefficient were measured to be ~3.3 dB/km and ~10.1 dB/km W respectively at 1230 nm. The cascaded Raman fiber laser was pumped by an Yb3+-doped fiber laser operating at ~1112 nm which delivered a maximum output power of up to ~20 W with a slope efficiency of ~46%. The free-running Yb3+-doped fiber laser was not optimized for overall efficiency and consequently operated at a longer wavelength. The maximum launched pump power into the single mode Ge-doped fiber after the imaging optics was ~10 W.
The first experiment was conducted with the experimental setup as shown in Fig. 1. Initially, a 500 m long Ge-doped fiber was used as the Raman gain medium. The dependence of the total output power and the output power of each individual Stokes emission versus the pump power from the Yb3+-doped silica fiber laser is shown in Fig. 2. The individual Stokes and total output power characteristics shown in Fig. 2 are expected and are well known . The measured total slope efficiency with respect to the launched pump power is ~46% for the 500 m long Raman fiber laser. The maximum total output power and the maximum 3rd order Stokes power were 3.3 W and 2.2 W, respectively. In a separate experiment, a 130 m length of Ge-doped fiber was used as the gain medium. In this experiment, only the 1st Stokes was generated and the output power versus the launched pump power is depicted with filled square symbols in Fig. 2. Approximately 4.2 W of the 1st Stokes output at 1168 nm was generated at a slope efficiency of ~72%. As can be seen, the overall slope efficiency and threshold features of the broadband pumped free-running Raman laser are similar with the narrow band pumping scheme .
The wavelength shift and bandwidth broadening of the pump output versus the launched diode power were measured after FER1 and the results are shown in Fig. 3(a). As can be seen, the wavelength of the output shifts to longer wavelength and the 3 dB bandwidth becomes wider as the pump power increases which we believe relates to the broadband Yb3+-doped fiber gain spectrum combined with the broadband mirror M and thermal effects. The relationship of the spectral position and the associated bandwidth of the Stokes emissions with the increase in the launched pump power is shown in Figs. 3(b), 3(c) and 3(d) for the 1st, 2nd and 3rd Stokes emissions, respectively. The broadband nature of the pump spectrum may cause a broadening of the spectrum of the Raman fiber laser output. The Raman gain spectral bandwidth is ~13 nm at the 1113 nm pump wavelength. The nonlinear variation in the bandwidth with the increase in the pump power is due to the fact that the bandwidth limited Ge-doped Raman gain spectrum is narrow and sets an upper limit to the maximum bandwidth of the output. The change in the centre wavelength of the Stokes emissions follows the change in the center wavelength of the Yb3+-doped fiber laser output and is therefore thermally related. Since no narrowband filters are included for the 1st, 2nd and 3rd Stokes emissions, their 3 dB bandwidths are between 1 nm and 4 nm.
The spectral characteristics of each stokes emission is shown in Fig. 4(a) for a 500 m long Raman fiber laser at launched power values of 6.5 W and 9.2 W. As can be noted from these two curves, higher pump powers leads to wider 3 dB bandwidth of each individual Stokes emission. Due to the narrow gain spectrum of Ge-doped fiber, the overall bandwidth of each Stokes is narrower than the Yb3+-doped fiber laser. However, the higher order Stokes bandwidths are wider than lower order Stokes bandwidths. This broadening is due to several competing nonlinear processes, such as self-phase modulation and four-photon mixing .
There are actually five possible cavities CP, C1, C2, C3 and CS in this configuration. The cavities are only set up because the lenses and fiber facets have reflectivities at the laser wavelengths. To investigate the influences of these five cavities on the spectrum of the output from the fiber laser system, a number of experiments were carried out. The fiber end reflectors of FER1 and FER2 were attached to translation stages and the cavity lengths C1 and C3 were varied. The influence of cavity C1 on Yb3+-doped fiber laser spectrum is shown in Fig. 5 which was measured with an optical spectrum analyzer at 0.01 nm resolution. We observed a wavelength modulation period of 0.09 nm corresponding to the axial mode spacing 22.06 GHz which relates to a 6.8 mm long cavity C1 as shown in the inset of Fig. 5. We estimate the feedback ratio of the cavities C1, C2 and C3 is 1:0.7:0.3, therefore the influences of cavities C2 and C3 are weaker than the influence of C1. The corresponding axial modes relating to cavities C2 and C3 were not observed. Experimental results show therefore, that the modulation in the pump spectrum is due to the cavity C1 only . It is observed that the small modulation in pump spectrum does not show up in the lasing Stokes line as shown in Fig. 4(b). Therefore, cavity CS plays the major role as the Raman fiber laser resonator.
In this broadband pumping scheme, the overall slope efficiency (~46%) for the ~500 m long RFL is higher than the slope efficiency (~28%) for the same length of narrow band pumped “grating free” RFL reported in Ref. . The high efficiency in this configuration is due to the efficient Raman resonator CS which has lower cavity loss than the previous experimental setup . In this configuration, the Yb3+-doped fiber was not included in the Raman resonator and hence the overall cavity loss is low. Therefore, this broadband pumping RFL is effective and simple for multi-wavelength lasing. If narrowband output is required from the laser, then a single fiber Bragg grating at the output wavelength can be incorporated. This greatly reduces the overall complexity of cascade Raman fiber lasers. If a tunable grating is inserted to replace FER1, then the pumping will be tunable, and a tunable Yb3+-doped fiber laser will result in broadband tunable Raman fiber laser output.
In conclusion, we have experimentally demonstrated a 3rd order cascaded Raman fiber laser that uses broadband pumping from a free-running Yb3+-doped silica fiber laser. Since we did not use pairs of gratings to force the output wavelength, for a fixed pump power, the Stokes lines can be adjusted by setting the length of fiber. The fiber length and pump power play an important role in determining the Raman laser output when broadband reflection is used. A maximum output power of 4.2 W has been obtained at a slope efficiency of ~72%. The simplicity of this configuration combined with its overall high efficiency makes this laser very attractive as a pump source for a number of applications including Raman amplification and distributed sensing systems.
This work was partly funded by the Australian Research Council and the Australian Photonics Cooperative Research Centre.
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