In this paper, we reported the realization of an ultra-long ring fiber laser (RFL) with hybrid emission related to both random lasing and cavity resonance. Compared with a linear random fiber laser (LRFL), the Rayleigh scattering (RS) inducting distributed feedback effect and the cavity inducting resonance effect exist simultaneously in the laser, which reduces the lasing threshold considerably and provides a hybrid way to form random lasing (RL). The laser output can be purely modeless RL when pump power is high enough. It is also discovered that the laser is insensitive to temperature variation and mechanical disturbance, this is unique and quite different from conventional RFLs which are environmentally unstable due to existence of the cavity modes.
©2012 Optical Society of America
As a typical kind of fiber lasers, ring fiber lasers (RFLs) have drawn significant attention in optical communication and sensing. In the ring cavity, various gain media, such as erbium-doped fiber amplifiers (EDFAs) and Raman amplifiers [1–6], are used to generate lasing. The Raman amplification based RFL is one of the attractive research topics due to its low noise and lasing wavelength flexibility. However, conventional Raman RFLs usually use a relative short (compared with the 125km cavity length used in this work) ring cavity with a high nonlinear fiber to enhance the Raman scattering and a wavelength filtering component (i.e., FBG) to select light. Hence, the emitting light has mode resonance characteristics that induce mode competition instability and environmental perturbation instability, as described in reference  and references therein.
In this paper, we propose a modeless RFL with hybrid emission characteristics, where random lasing (RL) arise from Raman amplified Rayleigh scattering (RS) and the ring resonance play different roles in laser generation, depending on the length of the cavity and the value of the pump power. For conventional random lasers, the feedback is provided by light scattering in a disordered gain medium [7–9], wherein the output is irregular and unidirectional. Low-dimensional structures were proposed to guide the output direction [10, 11]. Recently, Turitsyn, et al. realized a stable CW random fiber laser in a standard single-mode fiber (SMF) based on Raman amplification and distributed RS feedback [12–16]. In addition, another quasi-random lasing regime is reported in an ultra-long fiber Fabry-Perot cavity formed by two FBGs , wherein the so called weak wave turbulence effect between numerous cavity modes provides a way to realize modeless quasi-random lasing.
To the best of our knowledge, RL with the company of cavity resonance in a RFL has not been investigated yet. Hence, this paper reported a hybrid-emitting RFL (HERFL) with an ultra-long cavity. Such a novel HERFL has a much lower threshold compared with linear random fiber lasers (LRFL). When the pump power of the laser is high enough, the dominant RL makes the laser to become purely modeless and environmentally stable in spectrum and output power, providing an efficient and simple way of generating low threshold, spectrum stable, broadband RL. These characteristics make the HERFL an ideal light source for potential applications to low-coherence interferometry [18, 19], such as optical coherence tomography (OCT) and optic fiber gyroscopes etc [20–22].
2. Experimental results
The schematic diagram of the HERFL is given in Fig. 1 . A Raman fiber laser with central wavelength of 1366 nm is used as the pump. The pump is launched into the fiber spool through a 1365/1461 nm wavelength division multiplexer (WDM). A 125 km SMF spool is spliced between the common port and the 1461 nm port of the WDM, forming a resonant ring cavity for the 1st-order Raman Stokes light. To monitor the laser generation, a 1: 99 coupler is spliced between the 1461 nm port of the WDM and the SMF. Thus, clockwise-propagating light coupled to the 1% port of the coupler is used as the output of the laser.
With the increase in pump power, the Raman Stokes light occurs. In the fiber ring, there exist two regimes of resonance modes of the Stokes light. The first regime is due to the ring cavity effect. Namely, the generated light propagates periodically in the ring cavity, giving birth to resonance modes with frequencies, mc /nL (where n ≈1.45 is the refractive index of the fiber, L is the fiber length, and m is an integer). The second regime corresponds to the RS effect. Namely, the generated light is scattered forwardly and backwardly due to the distributed RS effect, which forms a multitude of resonance modes with random frequencies [12, 13]. Obviously, with the increase in fiber length, the first (second) regime would play a more subordinate (dominant) role in lasing generation due to the increased cavity attenuation and the RS feedback, and meanwhile this also depends on the pump power applied.
We first studied the output characteristics of the HERFL. In a 125 km long cavity, the ring resonance effect of the cavity is much weakened due to large attenuation of light per circle, while a considerable amount of RS feedback plays a more important role in lasing generation. Figure 2 shows the output power of the laser as a function of the pump power, while the output power of a corresponding LRFL is also presented for comparison. The LRFL has a similar setup as the HERFL, except that the 99% port of the coupler and the SMF are disconnected. In addition, the ends of LRFL were also angle cleaved to eliminate Fresnel reflection, so the feedback of LRFL is only from RS. It is observed that the threshold power of the HERFL is ~0.85 W, which is about three fifth of the threshold of the LRFL. But, the HERFL has smaller slope efficiency. This is due to the power distribution differences in the two cavities that induce different spontaneous emission rates.
To explain the roles of cavity effect and RS effect in the RL. A numerical simulation based on the stable light propagation Eqs [14, 15, 23, 24]. is also give in Fig. 2. The curves simulated with (the solid curve) and without (the broken curve) the consideration of the RS effect have the same threshold, indicating that reduction in the lasing threshold of the HERFL is mainly due to the cavity effect. However, once the laser begins to emit, RS related random distributed feedback takes effect (i.e., light propagating in the cavity enhances stimulated Raman scattering as well as RS, increasing the output power considerably), which is verified by the comparison that the broken line has a much smaller slope efficiency than the solid line. In fact, the RS effect becomes increasingly important in lasing with increase in the pump power. As also indicated by the simulation results in Fig. 3 that the power distribution of the Stokes light becomes more dependent on the RS effect when the pump power increases.
Figure 4(a) and 4(b) give the experimental RF spectra of the HERFL under different values of the pump power. In Fig. 4(a), when the pump is near the lasing threshold, a peak at 1.65 kHz can be seen, which exactly corresponds to the cavity effect. As pump power increases (larger than ~1.25 W), the cavity resonance disappears. This is because the cavity effect supports large amount of resonance modes with spacing c/nL, while the RS distributed feedback builds up numerous resonance modes with random frequencies. The output of HERFL is the sum of these two effects. At low pump power, the cavity resonance manifests itself as a peak in the RF spectrum of the output, seeing Fig. 4(a). As the pump power increases, the fiber nonlinearity (e.g. four-wave mixing, phase modulation, etc) causes interacting, de-phasing, broadening and superimposing of these modes [6, 13, 17]. This provides a route towards modeless RL. Hence, no resonance peak is observed in the RF spectrum, seeing Fig. 4(b).
Since the RS (cavity) effect is enhanced (weakened) with increase in the cavity length, there exists one critical length regime beyond which the RS feedback (cavity resonance) plays the dominant (negligible) role and the output power does not change with further increase in the cavity length. Figure 4(c) indicates that the critical lengths for pump at 1, 1.5 and 3 W are ~200, 175 and 130 km, respectively. In our case, the cavity length is 125 km, thus the output of the RRFL is hybrid RL for the most pump power applied.
Figure 5 shows the spectra of the laser output. In Fig. 5(a), the pump power varies from 0.933 to 1.318 W. For pump at 0.933 W, the spectrum shows a single peak with a central wavelength nearby 1454 nm. When the pump power increases, a second peak with a central wavelength nearby 1462 nm appears in the spectrum. The amplitude of the second (first) peak changes increasingly (decreasingly) as the pump power increases. This result is similar to that in the reference  wherein a LRFL is studied. However, such a phenomenon appears at lower pump power. Besides, the output is stable in both spectrum and output power for the most pump power applied, which is quite different from the reported LRFL in which the output exhibits stochastic narrow-band spectra and dynamical fluctuations for pump power close to the threshold. In LRFL, the stochastic appearance of narrow-band spectra is related to the cascade stimulated Brillouin scattering (SBS). In our case, the long-period ring feedback is supposed to be one important factor that suppresses these dynamics. When the pump power increases from 1.413 to 3.631 W, the spectrum broadens continuously, seeing Fig. 5(b) where a relatively large bandwidth of >10 nm is achieved. According to the theory of H. Cao et. al. [25, 26], light emission arose from multiple scattering in a disordered medium can be thought of as RL, and there is no strict definition of random laser’s bandwidth as that of conventional lasers. Hence, we still treat the broadband emission as lasing rather than amplified spontaneous emission (ASE) that means light amplification in a gain medium without cavities and light scattering.
We also investigated the thermal and mechanical stability of the 125 km HERFL. Figure 6 gives the output spectra of the laser operating under different environmental temperatures. It is seen that the output spectrum as well as the power keeps almost unchanged for the temperature variation from −40 to 50 °C. To test the mechanical stability, we vibrated the 125 km fiber spool, and stretched a section of the fiber, and little change was observed in the output spectrum. Since the acceleration, strain, and refractive index would vary accordingly while shaking the fiber, the HERFL is believed to be insensitive to these parameters, i.e., it is environmentally stable in both spectrum and output power.
4. Discussions and conclusions
Compared with the reported LRFLs [12, 15], the HERFL has similar output spectrum, however, its threshold pump power is much lower thanks to its hybrid emission characteristic. In the HERFL, the contribution of the cavity resonance and the RS feedback varies with the cavity length and the pump power, hence, the coherence degree of the output light as well as the route towards the modeless RL can be controlled by these two parameters. Besides, the spectrum of HERFL is determined by the Raman gain profile rather than a point reflector as in an ultra-long fiber laser with a liner cavity formed by FBGs [14, 17], so it has larger bandwidth if the pump power is high enough.
In summary, we have studied the output characteristics of a HERFL with a very long cavity length. Thanks to the cavity effect, the threshold of the laser is reduced considerably. Moreover, once the laser starts emitting, the RS effect plays an important role. With the increase in the pump power, the RS-related distributed feedback and the cavity-related resonance modes form a hybrid way to generate modeless RL. A relatively large bandwidth of >10 nm is achieved when the laser is pumped with a high power. Furthermore, it is found that such a laser is insensitive to temperature variation and mechanical disturbance and it can overcome the bottleneck problem of conventional RFLs and would find important applications in low-coherence interferometry.
This work is supported by National Natural Science Foundation of China under Grants (61106045, 61205048)
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