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The longterm stability of the laser system is very important in many applications. In this letter, an ultra-stable, broadband, mid-infrared (MIR) optical parametric oscillator (OPO) pumped by a super-fluorescent fiber source is demonstrated. An idler MIR output power of 11.3 W with excellent beam quality was obtained and the corresponding pump-to-idler conversion slope efficiency was 15.9%. Furthermore, during 1h measurement at full power operation, the peak-to-peak fluctuation of idler output power was less than 1.9% and the corresponding standard deviation was less than 0.4% RMS, which was much better than that of a traditional single mode fiber laser pumped OPO system (10.9% for peak-to-peak fluctuation and 1.8% RMS for the standard deviation) in another experiment for comparison. To our knowledge, this is the first demonstration on a high-power, ultra-stable OPO system by using the modefree pump source, which offered an effective approach to achieve an ultra-stable MIR source and broadened the range of the super-fluorescent fiber source applications.
© 2016 Optical Society of America
1. Introduction
Fiber laser pumped MgO: PPLN optical parametric oscillator (OPO) is an effective approach to generate mid-infrared (MIR) laser [1,2], which attracts particular interest of many researchers in widely applications, such as high-resolution molecular spectroscopy [3], environmental monitoring [4], and infrared countermeasures [5]. However, in spite of that the typical power stability of the commercial fiber laser is very well (better than 2% according to reference [6] for example), the power stability of the OPO system has decreased to ~10%, inevitably [7,8]. One potential reason is that the traditional single mode fiber amplifier suffers self-pulsation effect with temporal instability and intensity fluctuations [9,10], which causes the power chaos of the OPO system. An alternative solution is to use a temporally stable pumping source – the super-fluorescent fiber source (also referred to as amplified spontaneous emission, ASE source), which is free of longitudinal mode [11]. However, the frequency down-conversion process is generally hard to occur by employing such a broadband continuous-wave (CW) pump radiation. Due to the low pumping intensities, long interaction lengths is particularly essential for nonnegligible gain, thereby the available phase-matching bandwidth (BW) for parametric interaction will be restricted and thus the use of broadband pump source is limited. Only by suitably exploiting the extended phase-matching BW in MgO:PPLN, long interaction lengths can be realized in the process of a broadband pump to provide broadband MIR output with high extraction efficiency [12]. In 2011 [13], Jelle Storteboom et al. have confirmed the feasibility of the super-fluorescent fiber source pumped signal singly resonant optical parametric oscillator (SRO), and an 1 W mid-infrared laser with 230nm spectral bandwidth has been obtained. The output power of the OPO is not high, relatively, and other characteristics of the OPO, such as the temporal stability caused by the modefree property of the pumping source, have not been discussed in details. To our knowledge, there are no corresponding studies to be reported after that. In this paper, we are committed to the temporal characteristics of an super-fluorescent fiber source pumped MIR OPO with high power operation.
The concept of super-fluorescent fiber source has attracted a great deal of attention for its features to generate ultra-stable emission, high spatial coherence, and low temporal coherence [14–20]. As the pump source of the non-linear frequency conversion, the ultra-stable emission of the super-fluorescent fiber source is beneficial for avoiding laser-induced damage of the non-linear crystals, while the high spatial coherence property makes it possible that the pump laser can be focused into a tight spot, thus allowing for the nonlinear optical conversion to occur. More important, comparing to the traditional single mode fiber laser resonator, the super-fluorescent fiber source is free of mode competition, which effectively eliminates mode hopping of the signal beam in the optical parametric process and insures the stationary broadband continuous output of the idler laser. All of these features make the super-fluorescent fiber source to be a superior pump source in non-linear domain. In this letter, we demonstrated a high power, linearly polarized, all-fiberized, MOPA structured super-fluorescent fiber source pumped MIR OPO system. Meanwhile, a traditional single mode fiber amplifier pumped the same OPO system was carried out as a comparison. It is worth noting that the OPO system has been successfully integrated, so the same OPO cavity would be used for both of the experiments, which insures the reliablity of the experiment result. Both of the experiments realized high power mid-infrared laser more than 10 W and the longterm stability of the output idler laser was measured. The peak-to-peak power stability of the ASE OPO was about 1.9% near 1h and the standard deviation was about 0.4% RMS, which is far better than the power stability of a traditional single mode fiber laser pumped OPO system with the same configuration (near 10.9% for the peak-to-peak power fluctuation, and 1.8% for the standard deviation). To our knowledge, this is the first demonstration about the stability of the OPO laser by using the modefree pump source, which may provide an effective approach to realize ultra-stable, high-power, broad-bandwidth MIR laser sources.
2. Experimental setup
A schematic of the experimental setup was illustrated in Fig. 1. The pump source we employed was a homemade high power linearly polarized super-fluorescent fiber source, which was composed of a backward pumped ASE seed source and two-stage high power polarization maintaining (PM) fiber amplifier systems, similar to the master-oscillator power amplifier system that described in reference [19] except that all fiber devices used in the amplifiers were characterized by PM physical properties. The pump laser was collimated by an optical fiber collimator, and a linearly polarized beam of 4 mm diameter in TEM00 spatial mode was finally obtained.
Figure 2(a) showed the spectrum of the super-fluorescent fiber source at different output powers. The output spectrum was centered on 1076nm with the maximum linearly polarized output power of 84.84W. The 10dB spectrum line-width as a function of output power was depicted in the insertion graph of Fig. 2(a), can be seen that the spectrum line-width almost had no obvious broadening varying with the output power. Figure 2(b) showed the power stability of the super-fluorescent fiber source at high power operation. The peak-to-peak power fluctuation was computed to 1.43%.
Fig. 2 Spectral evolution (a) Spectral details of the super-fluorescent fiber source at different power levels; (b) Power stability at full power.
An optical isolator was placed behind the super-fluorescent fiber source to protect the pump source from back reflections, and then the linearly polarized output laser was focused into the OPO cavity with a beam radius of 48 µm at the center of the crystal. The OPO cavity has been designed to be a signal bow-tie ring singly resonant (SRO) based on a 50 × 4 × 1mm MgO: PPLN crystal with 31μm poling period. The cavity was constructed of two concave mirrors M1 and M2 (r = 100 mm) and two plane mirrors M3 and M4 [7]. All the mirrors except the output coupling mirror M2 had a high reflectivity (R>99.9%) over 1.4~1.7μm, and were anti-reflection coated (T>95%) for the pump 1.0~1.1μm and idler 2.5~4.1μm. The coupling output mirror M2 was coated with partly transparency (T = 1.5%) around the signal laser 1.4~1.7μm wavelength region. The total optical length of the cavity including the crystal is 450 mm. A convex-plan lens was used to collimate the output laser in order to the following accurate measurement. Two dichroic mirrors were utilized after the collimating lens for separating the three kinds of lasers from each other. Firstly, M5 had a high reflectivity (R>99%) over the pump 1.0~1.1μm, and was anti-reflection coated (T>95%) for signal 1.4~1.7μm and idler 2.5~4.1μm, thus the pump laser was separated from the signal and idler lasers. Then the mirror M6 had high reflectivity coating for the signal and high transmission for the idler, so that the signal laser was separated from the idler laser. Three power meters were used to synchronously measure the power of the un-depleted pump laser, the signal laser and the idler laser. Furthmore, the OPO system has been integrated and moduled with the last fiber laser amplifier successfully, which insures the reliablity of the experiment result under different seed laser conditions. The size of the modular OPO system is displayed in right of Fig. 3. Pictured left was the machine containing both fiber laser amplifier and OPO system.
Furthermore, as a comparison, we also investigated the output characteristics of the same OPO system pumped by a traditional single mode fiber laser. The only difference between two pump sources was that the seed source here we used was a typical single mode fiber resonator. The corresponding spectrum of the traditional single mode fiber laser at different output power was charted in Fig. 4(a). As can be seen, the central wavelength of output spectrum is 1070nm with the maximum linearly polarized output power of 83.1W. The insertion graph of Fig. 4(a) also illustrated the 10dB spectrum line-width as a function of output power. Compared with the inset of Fig. 3(a), it was clearly to see that the spectrum line-width was narrow but increased from 0.64nm to 0.94nm, respectively, with the increasement of output power. The major reason leading to the spectral broadening is induced by the non-linear effects. Traditional single mode fiber lasers usually suffers self-pulsation effect, which cause high peak powers and result in strong non-linear effect under high output powers. However, the super-fluorescent fiber source can suppress the non-linear spectral broadening due to its good temporal stability. Figure 4(b) showed the power stability of the conventional fiber amplifier at high power operation. The peak-to-peak power fluctuation was computed to 1.37%, which was as well as the super-fluorescent fiber source.
Fig. 4 Spectral evolution (a) Spectral details of a conventional fiber amplifier at different power levels; (b) Power stability at full power.
3. Experimental results and discussion
The measured idler output power of the OPO system as a function of pump power and the corresponding slope efficiencies were shown in Fig. 5. Red line showed the characteristics of the super-fluorescent fiber source pumped SRO, while blue line fitted the characteristics of the traditional single mode fiber laser pumped the same SRO. As can be seen, that the OPO realized a maximum idler output power of 11.3W for given 84.84W pump power with a corresponding pump-to-idler conversion efficiency of 13.3% and had a slope efficiency of 15.9%, a threshold of 20W under the super-fluorescent fiber pump radiation. While that of the traditional single mode fiber laser pumping condition, a maximum idler output power of 12.3W for a pump power of 83.1W with a corresponding pump-to-idler conversion efficiency of 14.8% was obtained, and the slope efficiency was up to 17.1% with a threshold only of 15W. The results of comparison revealed that OPO’s properties dropped to some degree pumped by the super-fluorescent fiber source, which mainly because that broad bandwidth spectrum of the pumping source led to a lower power spectral density for the frequency conversion. Meanwhile, both of lines indicated that there was no obvious appearance of thermal effect phenomenon at the maximum power levels that even higher power could be achieved with more powerful pump source. However, limited by the power durability of the collimator, experiments of higher pumping power had not been carried out for the time being. According to the ref [21], the power of the OPO system was also bellow to the instability threshold (4.61 times oscillation threshold).
Fig. 5 Idler output power as a function of pump power (blue:traditional fiber laser; red:ASE fiber source).
Figure 6 showed the optical idler spectrum at the maximum output power of the OPO by using a MIR wavelength meter (Bristol, 621A). Figure 6(a) showed the results pumped by the super-fluorescent fiber source. The idler spectrum was centered on 3325nm with a full width half maximum (FWHM) line-width of 72nm. For comparison, the spectrum pumped by a traditional fiber laser was showed in Fig. 6(b). A central wavelength at 3251nm with a FWHM line-width of 4nm was obtained. The difference between the idler central wavelength stemed from different central wavelength of two pumping sources. The measurements indicated that it was an effective approach to generate broad bandwidth MIR lasers by using the super-fluorescent fiber pumping sources.
Besides, the corresponding signal spectra of the OPO were shown in Fig. 7. As can be seen, the FWHM line-widths of both cases were at a similar level. The reason was that in the singly resonant OPO, only the signal beam is oscillated in the cavity. Although the pump sources had different features of spectral and temporal characterizations, the signal beam would be with longitudinal modes defined the cavity. Since the signal frequency in a SRO is constrained to an axial mode of the optical cavity, in order to maintain energy conservation, any frequency spread in the pump will be directly transferred to the nonresonant idler. So that the use of the super-fluorescent fiber source can lead to broadband idler output.
Further, we recorded the longterm power stability of the idler laser with a maximal output power at room temperature operation. The corresponding idler power exhibited a peak-to-peak power fluctuation better than 1.9% about 1h continuous operating at an idler power >11W pumped by the super-fluorescent fiber source, as shown in Fig. 8(a), while that of the traditional single mode fiber laser pumped OPO was up to 10.9%, as shown in Fig. 8(b). With a standard deviation calculating methods, the idler power stability was 0.4% RMS (the ASE pumped OPO) versus 1.8%(the traditional fiber laser pumped OPO). By contrast, the results clearly showed that it was an effective approach to generate ultra-stabe MIR lasers by using super-fluorescent fiber pump sources. One possible reason was that the modefree property of super-fluorescent fiber sources eliminated the effects of mode jump and phase variation among the optical parametric process. In both cases, the short-term stability of the idler output laser was measured on the 8 microsecond class domain, as shown in the inset of Fig. 8(a) and Fig. 8(b). We can probably ignore the impact of electromagnetic noise, the OPO pumped by the super-fluorescent fiber presented much better short-term stability than that of OPO pumped by the traditional single-mode fiber laser.
Fig. 8 Idler power stability (a) pumped by ASE fiber source; and (b) pumped by a traditional fiber laser.
Beam quality of the idler laser at the maximum output power pumped by the ASE fiber source was measured with knife-edge method by Ophir-Spiricon beam propagation analyzer software with a pyrocam-III, as depicted in Fig. 9(a). The beam quality M2 factors were ~1.37 and ~1.60 in the horizontal and vertical directions, respectively. Figure 9(b) showed the corresponding beam near-field intensity distribution of the mid-infrared laser at 11.3W.
4. Conclusion
In conclusion, we demonstrate a 11.3W all-fiberized linear polarized MOPA structured super-fluorescent source pumped singly resonant MIR optical parametric oscillator. The pump source is a homemade high power super-fluorescent source which consists of a backward pumped ASE source and three-stage high power fiber amplifier systems. High power MIR emission is generated with maximal output power of 11.3W with a beam quality of Mx2 = 1.37 and My2 = 1.60. The corresponding pump-to-idler conversion slope efficiency was computed to be 15.9%. The peak-to-peak idler power stability of the ASE pumped OPO (near 1.9%) is far better than the power stability of a traditional single mode fiber laser pumped the same OPO (near 10.9%) with a little loss on the efficiency. Experimental results provide an effective approach to realize ultra-stable, high-power, wide-bandwidth MIR laser sources.
Funding
National Natural Science Foundation of China (NSFC, 61322505);Program of China for the New Century Excellent Talents in University.
Acknowledgments
We are particularly grateful to Zhejiang University, Fujian Institute of Research on the Structure, and Nanjing University for their crystal supports on this work.
We appreciate for the help from laboratory assistant of Lichun Liu and PhD candidate of Wei Liu.
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