1 Introduction

Since the random lasers (RLs) were demonstrated [1], RLs have attracted considerable interest due to their unique properties. Compared with conventional lasers requiring a cavity formed by the point-based highly reflective mirrors, RLs only rely on an active medium and a scattering structure, in which an optical feedback is achieved by multiple scattering. In recent years, the effects of two-dimensional confinement on the lasing properties of a classical RL system have received increased attention because of the birth of random fiber lasers (RFLs) [2]. What is the definition of a RFL? Similar to the definition of the random laser [3], the RFL is an optical structure in an optical fiber and its definition satisfies the following two criteria: (1) light is multiply scattered of photons owing to randomness and is amplified by stimulated emission and (2) there exists a threshold, above which the total gain is larger than total loss. This definition includes all of multiple scattering systems with gain in an optical fiber. So far there are three main types of RFLs. Firstly, multiple scattering is Rayleigh scattering induced by inhomogeneities of refractive index present in a standard communication optical fiber. Multiple scattering is amplified by stimulated Raman scattering [4, 5].

Secondly, multiple scattering is induced by the distributed laser cavity formed by a multitude of randomly spaced Bragg gratings. Multiple scattering is amplified by doped gain fiber [6, 7]. Thirdly, multiple scattering is induced by nanoparticles which were disorder dispersed in solution with gain filled a hollow optical fiber. Multiple scattering is amplified by gain media which were dissolved in solution [2, 8, 9]. In RFLs, laser emission results from random distributed feedback mechanism. Because the reasons of formation of random distributed feedback are different, the lasing mode formation process and controlling methods are also different.

In principle, even a weak Rayleigh backscattering might be important for laser devices and may influence its lasing characteristics. Especially, in optical fibers there is an additional possibility for controlling the Rayleigh backscattering level. Namely, the backscattering light can be easily amplified, thus providing much more visible impact. In April 2010, Turitsyn et al. [4] proposed a new one-dimensional random distributed feedback fiber laser. A distributed feedback potentially can be provided by a naturally present Rayleigh backscattered radiation which is captured by the fiber waveguide. At first glance, because the Rayleigh backscattering coefficient is extremely small having a typical value of only ε ~10-5 km-1, this is rather difficult to implement. However, in a long optical fiber such small random distributed feedback can be amplified by the Raman gain. As a result, the continuous wave (CW) generation of stable narrow spectrum near 1,550 nm was demonstrated in an 83-km fiber span without any point reflectors. They used two equal-power 1,455 nm Raman fiber laser as pump source, namely bidirectional pumping scheme, and achieved a stable continuous lasing as high as 150 mW from each fiber end with a mirrorless open cavity in the standard communication optical fiber, and the total slope efficiency is up to 30 %. The properties of such random distributed feedback laser appear different from those of both traditional random lasers and conventional fiber lasers. Fotiadi [5] appraised this work via publishing an article in Nature: the work of Turitsyn et al. had made an outstanding contribution to the basic laser science, provided a new platform to explore the laser physics, nonlinear optics and the applications of lasers.

Extended wavelength range of RFL is always a research content. It is very difficult for shorter wavelength of the RFL because of the higher loss in the communication optical fiber. Recently, a RFL operating near 1.2 μm has been proved [10]. In their scheme, a bidirectional pumped ytterbium doped fiber laser (YDFL) operating at 1,115.6 nm with the bandwidth of 0.05 nm is used as the pump wave, when an 10.7 km span of OFS True Wave XL fiber is used as laser medium generating random distributed feedback, the laser generates up to 3.8 W of the quasi-CW radiation at 1,175 nm with the narrow spectrum of 1 nm corresponding to slope efficiency that reaches 60 %. In this paper, one-arm scheme is investigated, in which the pump wave is coupled into the 50 km standard communication optical fiber only from one side. The pump wave wavelength is 1,064 nm generated by a 975 nm LD laser pumping ytterbium doped fiber. The conversion efficiency from LD laser to YDFL can be as high as 85.7 %. We achieve the RFL operating at 1,115 nm using a LD-pumped Yb-doped fiber laser as the pump source. The laser can generate maximum power of 274 mW CW radiation with slope efficiency more than 28 %.

2 Experimental setup and results

The experimental setup for studied RFL is schematically illustrated in Fig. 1. To demonstrate a RFL working in the low wavelength range, we use a LD laser operating at 975 nm with the maximum output power of 7.4 W as the pump wave. An YDFL, which is composed of a span of about 4.5 m ytterbium doped fiber and two fiber Bragg gratings (FBG), a high reflectivity (R > 90 %) FBG and a high transmittance (R > 30 %) FBG, seen in Fig. 1, operates at 1,064 nm with the maximum output power of 4.5 W. The conversion efficiency from LD laser to YDFL can be as high as 85.7 %. Figure 2 gives the YDFL output power versus the LD input pump power.

Fig. 1
figure 1

Experimental setup

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Fig. 2
figure 2

The YDFL output power as a function of the LD input pump power

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The YDFL will provide the Raman amplification for the RFL in the 1.1 μm wavelength range. The right end of fiber span is cut at polished angle to prevent 4 % Fresnel reflection which is badly serious to the RFL operation. A 1,064/1,115 nm WDM filter is used as the port of RFL output. Figure 3 shows the output power of RFL versus the LD pump power. It clearly demonstrates lasing with a threshold pump power of ~6.5 W and a typical linear growth of the generated output power above threshold. An output power as high as 274 mW was observed from the laser output port of the WDM filter, limited mainly by the available pump power. The slope efficiency is more than 28.7 %.

Fig. 3
figure 3

Random fiber laser power as a function of the LD input pump power

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In our experiment, the power stability of RFL was observed over the time-domain. From Fig. 4, we can see that when the pump power is below or near the 6.5 W, output power of the RFL shows an unstable oscillation with stochastic spikes and burr appeared in the power curves (Fig. 4a, b). But when the pump power is high enough, at a pump power of >6.5 W, the laser begins to operate in the CW regime with strongly suppressed amplitude fluctuation over the time domain (Fig. 4c, d). So, we can conclude that the threshold of our RFL is about 6.5 W.

Fig. 4
figure 4

Time-domain behavior at different LD pump power: a below the threshold (5.0 W), b near threshold (6.2 W), c above the threshold (6.5 W), and d well above the threshold (6.9 W)

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In order to estimate the threshold of the RFL, we have studied spectral characteristics at different LD input power. When the pump power is lower than the generation threshold (Fig. 5a), only a broad spectrum of amplified spontaneous scattering is observed. When the pump power is near the threshold, a typical broadband shape of the Raman gain curve of the amplified spontaneous emission (ASE) superimposed with the random spikes and dips is shown in Fig. 5b. However, when the pump power is above threshold, the impact of Rayleigh feedback scattering grows rapidly. As a result, the optical spectrum is also stabilized (Fig. 5c), showing two narrow (full-width at half-maximum, FWHM, ~1 nm) laser lines localized near 1,115 nm. So, combining the time-domain and frequency-domain characteristics, we can infer that the laser threshold is about 6.5 W. The spectral characteristics described above are similar to paper in [4].

Fig. 5
figure 5

Optical spectra measured at different LD pump power: a below the threshold (5.3 W), b near threshold (6.2 W), c above the threshold (6.7 W)

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3 Conclusions

We demonstrate a RFL operating at 1,115 nm in the long standard communication optical fiber using LD-pumped Yb-doped fiber laser as the pump source. The YDFL can generate power as high as 4.5 W with slope efficiency of 85.7 %. When the LD laser power is high enough, as high as 6.5 W, the RFL will be lasing with stable power and spectral properties. We have obtained the maximum power of 274 mW CW lasing with slope efficiency more than 28.7 %. Moreover, our experimental system could reduce the cost largely. Recently, the random lasers have attracted more and more attentions because of several newly emerging applications such as photodynamic therapy and tumor detection, speckle-free, full-field laser imaging and sensing [11–13]. We believe that our work will make a significant contribution to the development of RFLs.