Investigation on extreme frequency shift in silica fiber-based high-power Raman fiber laser Download: 686次
1 Introduction
Raman fiber laser (RFL) can theoretically achieve emission at almost arbitrary wavelength with the help of proper pump wavelength[1]; thus, the emission range of RFL is much broader than lasers based on rare earth-doped fibers. Therefore, RFL has been explored to achieve high-power output at specialized wavelengths[2–10], which are rather challenging for lasing or amplifying efficiently by rare earth-doped fibers[11–13]. To date, high-power RFLs have been widely investigated and applied in optical pumping, frequency conversion, optical communications, biology and scientific research[14–22]. It is well known that the Raman gain in silica fibers extends over a large frequency range up to 40 THz[23]. In general, the frequency shift between pump wavelength and Raman wavelength is approximately 13.2 or 14.7 THz corresponding to the double-peak structure of Raman gain spectrum for silica fiber[23, 24]. The frequency shift between the pump light and target Raman light is usually designed to be well matched to fulfill the peaks of Raman gain spectrum. For example, hundred-watt-level high-power RFLs operating at 1120 nm and 1150 nm were demonstrated by using 1070 nm and 1090 nm fiber lasers as pump sources, respectively[3, 17]. It is to be noted that, for application that requires RFLs with some specific wavelengths, it is usually not easy to obtain high-power pump source. The central wavelength of the pump source should match well with a 13.2 THz frequency shift compared with the output wavelength of RFL. For example, single mode 1178 nm RFL, which can be used for frequency doubling to the yellow, is often pumped by 1120 nm fiber laser. However, it is relatively difficult to achieve high-power lasing at 1120 nm based on Yb-doped fiber because of smaller net gain[25–27] compared with shorter wavelength[28, 29]. Therefore, it is interesting to explore the feasibility of generating high-power 1178 nm by pumping with a powerful Yb-doped fiber laser (YDFL) operating at (or even shorter wavelength) corresponding to the frequency shift of , which has a significant difference compared with 13.2 THz. In fact, efficient lasing from a fiber Raman oscillator by fully exploring the broadband gain spectrum was studied as early as in 1977[30, 31]. In the visible band, tunable Raman oscillator pumped by a 4 W argon ion laser at 514.5 nm was tuned over 8 nm using a prism, which corresponds to the frequency shift from 6.2 to 14.9 THz. In the infrared band, Stokes oscillation tuning from 1085 to 1130 nm corresponding to the frequency shift of 5.5 to 16.5 THz was obtained pumped by a 5 W Nd:YAG laser at 1064 nm. In 2008, Belanger
Fig. 1. The experimental schematic of the linearly polarized Raman fiber laser. LP: linearly polarized; AMP: amplifier; GDF: germanium-doped fiber; CMS: cladding mode stripper.
In this paper, efficient high-power Raman lasing with extreme frequency shift between the pump and Stokes light was explored experimentally, based on an RFL cavity with fixed central wavelength. A homemade high-power wavelength-tunable master oscillator power amplifier (MOPA) with tunable working range was employed as pump source. All the fiber and fiber components involved are polarization maintained to eliminate the influence of polarization state. It is found that frequency shift located within 10.6 THz and 15.2 THz can ensure efficient Raman lasing, where the conversion efficiency is more than 95% of the maximal value of 71.3%. In addition, a maximum output power of 147 W was obtained with an optical efficiency of 71.3% at the pump wavelength of 1062.5 nm. The polarization extinction ratio (PER) of RFL is around 20 dB. This is the highest power ever reported in LP Raman fiber oscillators to the best of our knowledge. Before this, Surin
Fig. 3. (a) The output power of the first-order Stokes wave versus pump wavelength; (b) the output spectrum as a function of pump wavelength.
Fig. 4. The output power of first-order Stokes wave and corresponding conversion efficiency.
2 Experimental setup
The extreme frequency shift between the pump wavelength and Raman wavelength with high efficiency in high-power RFL was investigated in the experiment. As is shown in Figure
The tuning range of the pump source was from 1055 nm to 1080 nm. 1067.4 nm laser wavelength would be preferred because of the well-matched frequency shift (13.2 THz) compared with 1120 nm. Nevertheless, deviating from 1067.4 nm provides a flexible platform to investigate the gain property of RFL. The output properties of the tunable pump source were measured and recorded. As is shown in Figure
3 Results and discussion
By comparing the output power and the corresponding conversion efficiency at different pump wavelengths, the extreme frequency shift between the pump wavelength and Raman wavelength in RFL was studied. The total output power (residual pump plus RFL output power) is around 155 W, corresponding to the slope efficiency of . Figure
Fig. 5. (a) The output power as a function of pump power; (b) the spectrum at maximum power in the linear coordinate.
The difference among the first-order Stokes wave at different frequency shifts is further explored in Figure
Fig. 6. (a) The experimental setup of the comparative experiment; (b) the spectrum of comparative Raman fiber laser at 1062.5 and 1070 nm.
The whole system is specially designed to provide almost equal conditions for all the pump wavelengths. Sufficient conversion to the first-order Stokes wave and valid suppression of the second-order Stokes wave should be fulfilled at the same time. Taking the pump wavelength of 1070 nm as an example, the output properties of the Raman laser are recorded and analyzed. The power evolutions of the total output, residual pump, first-order Stokes wave and second-order Stokes wave are shown in Figure
Since the stimulated Raman scattering stems from the amplification of spontaneous Raman scattering, we also conducted a comparative experiment in order to further eliminate the effect of the FBGs. As shown in Figure
The PER of the output laser is also measured based on the setup shown in Figure
The PER of the output laser can be calculated according to , where and are the values of minor and major axes of the polarization ellipse, which correspond to the powers measured by the two power meters. The PER of the pump source can be obtained by measuring the PER of the residual pump when the pump power is less than the threshold power of , as is shown in Figure
Fig. 8. The PER of the residual pump and first-order Stokes wave at the pump wavelength of 1070 nm.
The power stability of the laser system was also measured and recorded, as is shown in Figure
As indicated in the introduction section, it is interesting to explore the feasibility of generating 1178 nm Raman laser with pump source. The experimental results obtained before have indicated the probability, and now we will validate it through a proof-of-concept experiment. As shown in Figure
Fig. 11. (a) The output power and (b) spectra versus pump power of 1178 nm random laser.
The output power and spectra as a function of pump power are shown in Figure
4 Conclusion
In summary, we report an experimental investigation on the extreme frequency shift between the pump wavelength and Raman wavelength in a high-power LP RFL. A pair of FBGs whose central wavelength is 1120 nm are adopted to achieve fixed target Raman wavelength. A LP wavelength-tunable pump source with 25 nm tuning range is used to change the frequency shift. A piece of 31-m-long PM silica passive fiber is selected to provide Raman gain. By comparing the properties of power, efficiency and spectra as the pump wavelength tunes from 1055 to 1080 nm, the range of frequency shift that can demonstrate high-power and high-efficiency Raman lasing is achieved. The experimental results verify that Raman laser with the frequency downshifted from the pump frequency by 10.6 to 15.2 THz operates with high efficiency, which is higher than 95% of the maximum one of 71.3%. At the same time, the maximum output power of 142.7 W is obtained with an optical efficiency of 71.3%, which is the highest power ever reported in LP RFLs to the best of our knowledge. As a typical application example of the results obtained in this paper, we experimentally verify that the 1120 nm pump laser could be replaced with a pump laser for a 1178 nm RFL, which is more beneficial to provide high-power pump lasers. The results could help expand the frequency shift in cascaded RFL or select appropriate frequency shift to generate Raman laser at special waveband, while the efficiency would not be reduced compared to the classic frequency shift. The FWM phenomenon observed in the experiment will be further investigated in the near future. The extreme frequency shift in other configurations, such as integrated ytterbium-Raman fiber amplifier and pure Raman amplifiers, could also be investigated in the future.
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[34]
[35]
[36]
Jiaxin Song, Hanshuo Wu, Jun Ye, Hanwei Zhang, Jiangming Xu, Pu Zhou, Zejin Liu. Investigation on extreme frequency shift in silica fiber-based high-power Raman fiber laser[J]. High Power Laser Science and Engineering, 2018, 6(2): 02000e28.