Taking advantage of relatively strong Rayleigh scattering and Raman gain of dispersion compensated fiber (DCF), three configurations to form efficient random fiber lasers (RFL) are proposed in this paper. Compared with the reported RFL formed by single-mode fiber (SMF) solely, lasing threshold and length of the proposed RFL are effectively reduced through combination of DCF and SMF. In addition, FBGs with central wavelengths at the 1st and 2nd -order Raman Stokes wavelengths are also added to the hybrid SMF/DCF cavity to further reduce the lasing threshold, leading to realization of a new kind of 2nd-order RFL.
© 2013 OSA
Since the first fiber laser was developed in 1960s, various types of fiber lasers have been studied for their potential applications in material processing, medicine, sensing and telecommunications [1–3]. Recently, a new kind of fiber laser, distributed feedback random fiber laser (RFL) solely based on standard single-mode fiber (SMF), was reported by Turistsyn et al [4, 5]. The SMF performs as gain medium to provide both optical amplification and distributed feedback for trapping light. Compared with conventional fiber lasers, the RFL has unique features of simple structure without any ‘mirrors’, good relative intensity noise (RIN) transfer characteristics due to its incoherent radiation, stable output with little thermal sensitivity and wide wavelength tenability [5–9]. Due to these advantages, attention has been paid to their potential applications in fiber-optic communication and sensing [8–11].
Recently, studies have been focused on the issues of threshold reduction and high order emission of RFLs. Firstly, 2nd-order random lasing (RL) was realized through cascaded Raman amplification in SMF . Secondly, 2nd-order RL with a half-opened cavity (i.e., positive feedback origins from point reflection of a FBG and random Rayleigh scattering (RS) of SMF) was demonstrated to reduce lasing threshold effectively . In addition, Raman fiber laser using dispersion compensated fiber (DCF) as dual-random mirrors was also proposed [15, 16]. Due to relatively large Raman gain and strong RS of the DCF, RL was enhanced efficiently.
In this paper, a novel idea to enhance RL efficiency of RFL through mixing of SMF and DCF is proposed and verified experimentally. This hybrid DCF/SMF scheme provides a more flexible way to design and optimize RFL ‘cavity’ by using specialty fibers, such as length, internal power distribution and output characteristics, etc. In addition, by adding or removing FBGs with central wavelengths at the 1st and 2nd -order Raman Stokes wavelengths, three configurations of improved RFLs are demonstrated in this work. It is found that the significant Raman amplified RS provided by the DCF reduces lasing threshold and length of the RFL considerably. Furthermore, half-opened cavity formed by adding FBGs at one side of the RFL can further reduce the lasing threshold, enabling 2nd-order RL in such a short cavity.
2. Operating principle and experimental results
The operating principle of enhancing RL performance through different combinations of SMF and DCF is that the DCF can be inserted at some position in the lasing ‘cavity’ to form a new mixed ‘cavity’, reducing both the length and the lasing threshold of the RFL, when compared with the RFL based on SMF solely. This is because the DCF provides more efficient distributed feedback with improved Raman gain, and the output of RFL can be modified by controlling the length or/and the position of DCF (i.e., design the longitudinal distribution of gain and feedback).
In this paper, a hybrid cavity with 1 km DCF and 9 km SMF is demonstrated to verify the idea proposed. The experimental setup of the hybrid RFL is shown in Fig. 1 . A Raman fiber laser with central wavelength of 1365nm is used as the pump laser. The pump is launched into the fiber spool though a 1365/1461 nm wavelength division multiplexer (WDM). A roll of 9 km SMF and a roll of 1 km DCF are connected in sequence to the common port of the WDM and perform as the laser medium. Two FBGs (removable in the experiment) with central wavelengths at the 1st and 2nd -order Raman Stokes wavelengths respectively are spliced between the common port of the WDM and the SMF. To monitor output of the RFL, a 1:99 coupler is located after the FBGs and a 1454/1550 nm WDM is used to separate the 1st and 2nd -order RL. The output spectrum is monitored at the right end of the DCF after a 16 dB attenuation.
In this work, three configurations of RFL are studied. As indicated by Table 1 , Regimes I, II, and III correspond to the configurations with no FBG, with one 1455 nm FBG and with both 1455 nm and 1550 nm FBGs, respectively. In addition, two cases for each of the three regimes are considered, i.e. with DCF (case 1) and without DCF (case 2).
Figures 2(a) and 2(b) correspond to cases 1 and 2 of regime I, respectively. In Fig. 2(a), pump power of 32.5 dBm is slightly beyond the threshold of 1st-order RL. However, no evident chaotic spikes, which appear in spectrum of SMF-based RFL , are observed in the spectrum. It is suspected that both the shorter length of the fiber and the different Brillouin frequencies of DCF and SMF induce relatively large stimulated Brillouin scattering threshold, suppressing the chaotic dynamics of the spectrum. With further increase of pump power, the lasing peak becomes dominant, however, no 2nd-order Stokes light is observed for pump power of up to 34.3 dBm. In Fig. 2(b), the DCF is removed and only the SMF is used as the lasing medium. Compared with Fig. 2(a), the peak power of the output spectrum in Fig. 2(b) is about 15 dB lower under the same pump value. This indicates that the 9 km SMF cannot support efficient RL generation.
Figure 3 shows the output power of regime I as a function of pump power. The lasing threshold (efficiency) of case 1 is much lower (higher) than that of case 2. This is because the residual pump power after the 9 km SMF provides Raman gain to the DCF, further amplifies the Stokes light, giving birth to additional positive feedback to enhance RL. In this case, the DCF performs as an efficient distributed feedback mirror with improved Raman gain.
As we know, enhanced RS in the DCF induces more attenuation for the pump light, while it provides positive feedback for the emission light. In our case, a 9 km SMF is mounted between the DCF and the pump, which is useful to obtain trade-off between attenuation of the pump light and positive feedback of the emission light in the DCF. We also studied the case when the DCF and SMF exchange their positions, and no obvious RL is observed. Besides, Fig. 3 is redone when the 9 km long SMF is replaced by a 50 km long SMF. It is found that that influence of the DCF is insignificant. All these indicate that the arrangement of the DCF greatly influences the performance of the hybrid DCF/SMF based RFL.
In the successive studies, we put forward new kinds of RFL (regimes II and III) based on both FBGs and combination of SMF and DCF. By adding FBGs to the pump side of the laser, the backward output (1st or/and 2nd-order random emission) can be selectively feedback into the fibers to further enhance the laser efficiency. Point reflection from the FBGs and RS reflection from the SMF and DCF form positive feedback for lasing. Besides, as the cavity is ‘half-opened’, the output is still RL .
Figure 4 shows the output power of regime II as a function of pump power. Compared with the threshold in Fig. 3, the laser threshold is ~1 W and ~0.7 W lower for cases 1 and 2 (see Tab. 1) respectively thanks to the additional 1455nm FBG.
Figure 5 shows the output power of regime III as a function of pump power. In Fig. 5(a), the backward output power (measured as shown in Fig. 1) is given. The results are similar to that of Fig. 4, except that 2nd-order RL is observed. To further explain the 2nd-order emission, the output power monitored at the right end of the DCF is also given in Fig. 5(b). As the pump power increases beyond 2.5 W (i.e., 2nd-order RL threshold), the 2nd-order RL starts to emit and the 1st-order RL starts to decrease. When the DCF is removed (see the triangle line in Fig. 5(a)), no 2nd-order RL is observed within the same range of pump power. This also reflects the important role of the DCF.
Figure 6 gives the corresponding spectra for Fig. 5. Figure 6(a) corresponds to the 1st-order RL. When pump power is slightly beyond the threshold, i.e., 30.1 dBm, numerous stochastically changed spikes exist in the output spectrum. This is caused by cascaded Brillouin scattering effect, and is in accordance with the observations of Ref . With pump power increasing, the spectrum becomes uniform due to broadening and superposing of the spikes caused by complex nonlinear interactions . Figure 6(b) corresponds to the 2nd-order RL. When the pump power is increased beyond a critical value, a quasi-ASE spectrum, a chaotic lasing spectrum, and a stable lasing spectrum occur in sequence.
3. Discussion and conclusions
Owing to the relatively large Raman gain factor as well as the strong RS of DCF, an optimized combination of DCF and SMF is very useful to enhance RL. Based on this principle, we realized efficient RL using relatively short segments of SMF (9 km) and DCF (1 km). In fact, length ratio and pump method are also important factors to influence performance of the RFL. Besides, the DCF can also be replaced by other specialty fibers with higher nonlinearity or higher RS feedback.
In summary, we have studied three configurations of RFL through mixing SMF and DCF. In Regime I, the mixed cavity proposed has a much lower (higher) lasing threshold (efficiency) than the RFL based on SMF solely. In regime II, one 1454 nm FBG is added to the pump side of the laser, and the lasing threshold is further reduced to about half of the threshold of regime I. In regime III, one 1454 nm and one 1550 nm FBGs are added to the pump side of the laser, and 2nd-order RL is obtained in such a short cavity. This provides a new way to realize high order random lasing. These results are useful to reveal the role of DCF and FBGs in RL, and also provide theoretical support for flexible design of RFLs.
The authors would like to thank Dr. X. F. Chen and Prof. L. Zhang in Aston University for providing the FBGs. This work is supported by National Natural Science Foundation of China under Grants (61106045, 61290312, & 61205048) and the Fundamental Research Funds for the Central Universities under Grants (ZYGX2011J001).
References and links
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