Third-order random lasing operating in 1670nm spectral band is experimentally demonstrated for the first time to the best of our knowledge, with only 2.45W pump threshold. The lasing cavity is formed by G.652 fiber and fiber loop mirrors (FLMs), while the former acts as the distributed reflector and the latter acts as the point reflector. The G.652 fiber and the FLMs are connected via a multi-band wavelength-division-multiplexer, which ensures each of the three Raman Stokes components generated in the long fiber is routed to one FLM and then reflected back with minimum loss. Unlike existing half-open random lasing cavities using fiber Bragg gratings, the reflection bandwidth of FLMs is wide enough to preserve the intrinsic spectral features of each lasing bands, providing a valuable platform to study the mechanism of high-order random lasing in fibers. Also, the reflection efficiency can be treated as an invariant as the pump power grows, significantly reducing the threshold of high-order random lasing. The stationary model is used to calculate the output power, and the results fit the experimental data well.
© 2013 Optical Society of America
Since the demonstration of random lasing from distributed feedback within telecom fibers , the concept of random fiber lasers has attracted a lot of attentions. Various aspects of their lasing features, as well as their applications in fiber-optic communication and sensing were studied [2–14].
Up till now, there are still a lot of open questions regarding the physics of random lasing in fibers. There are some proposals about reducing the lasing threshold by introducing strong point-reflection at one end of the fiber cavity, mostly with fiber Bragg grating (FBG). Such a regime is quite effective and it can reduce the 1st-order random lasing threshold in long G.652 fiber by ~50%, and the 2nd-order random lasing can be observed with ~2W pump power . However, it should be noted that, as the laser works above threshold, its intrinsic lasing bandwidth grows significantly and becomes broader than the bandwidth of the FBG, making the point-reflection less effective. As a result, the pump efficiency is constrained, and the generation of higher-order lasing may be hindered. Also, as the FBG significantly alters the spectral profile of lasing to a Gaussian shape, it is unable to know how the intrinsic spectral profile evolves itself and how it would pass on to the next order. Note that the 2nd-order random lasing in a completely open cavity was demonstrated, however the pump power is as high as 7.5W . In our very recent paper , fiber loop mirror was used as the point-reflector for a half-open Brillouin-Raman random fiber laser, and it helps to efficiently generate the multiple Brillouin Stokes components via hybrid Raman-Brillouin gain, suggesting its potential value in high-order random fiber lasers operating via Raman gain.
In this paper, we experimentally demonstrated the third-order random lasing via Raman gain for the first time, with a half-open all-fiber cavity. The cascaded generation of random lasing within the same lasing cavity could provide a useful basis to reveal unknown physical features of high-order random fiber lasers, which also has the potential to be utilized as a modified high-order Raman amplification scheme for long-haul transmission .
2. Experimental set-up and results
Figure 1 shows the experimental setup of the half-open RFL. A 1365nm Raman fiber laser is used as the pump laser. The pump is launched into a coil of 94km G.652 fiber via the Port 1 of a wavelength division multiplexer (WDM). Port 2 of the WDM is the pass channel for 1455nm and 1670nm light-waves, while Port 3 is the pass channel for 1555nm light-wave. Port 2 and 3 each are connected with a fiber loop mirror (FLM) formed by a wide-band 3dB coupler. Point A, B and C indicate the monitoring points for power and spectra of the system.
As the 1365nm pump power launched into the fiber spool is increased, the 1455nm Stokes light-wave builds up; and the portion moving towards the pump side will be selected out by Port 2 of the WDM, reflected by the FLM1, and enters back into the fiber spool. The distributed Rayleigh backscattering and FLM1 feedback constitute a half-open cavity, and lasing at 1455nm will be generated once the pump power exceed the threshold, which is ~0.75W in our case. Then the presence of 1455nm light-wave provides the Raman pump source for 1555nm light-wave. Again, the 1555nm light-wave propagate towards the pump side will be routed out to FLM2 by the WDM, bouncing back into the fiber spool, acting as a positive feedback and so forth the random lasing at 1670nm will be generated in succession.
The spectra evolution with the increase of pumps power is shown in Fig. 2. For the 1st-order lasing, the typical broadband shape of the Raman gain curve is firstly appeared, and then the two peaks which localized at the two Raman gain maxima get narrowing and the left peak (1453.3nm) with the narrowest 3dB bandwidth (BW) of 1.01nm is well toned at 0.81W pump power. By further increasing the pump power, the spectrum broadens continuously, and at the same time, the energy will transfer from the left peak to the right peak (1462.52nm). As a consequence, the right peak starts to dominate when pump power is increased to 1.11W. The central wavelength of the left peak exhibits a red shift with the increase of pump power; similar phenomenon for the 1st-order lasing has also been observed in the conventional Raman fiber laser with broadband Fresnel reflection  and the random fiber laser with mirror-less open cavity . But note that the wavelength shift of the right peak is not significant. With regard to the 2nd-order random lasing, as the pump power (2.05W) is well above the threshold level (1.45W), the spectrum becomes stabilized and the 3dB BW of the 1563.9nm peak is 1.94nm. The right peak grows higher and the two peaks separated by 10.3nm reach almost the same level with 2.18W pump power, then the right peak starts drops when the pump power reach the threshold of the 3rd-order random lasing (2.45W). The spectrum evolution of the 3rd-order random lasing is different from the 2nd-order one, but similar to the 1st-order one. The narrowest 3dB BW (2.24nm) at 1666.3nm is achieved with 2.65W pump power. Then the spectrum broadens and the energy transfers to the right peak with the central wavelength of 1677.5nm. Comparing Fig. 2(a)-2(c), we can also infer that for the higher order of random lasing, the spectrum becomes broader.
Previous high-order lasing cavities formed by incorporating FBGs have been studied [7,9]; however, the reflection spectrum of the FBG will significantly alter the lasing spectrum from its intrinsic form. In this work, the FLMs and the Rayleigh scattering provide wide-band reflection for the three bands of interest; therefore the features of lasing spectrum are well retained. On the other hand, the wavelength drift of FBGs induced by temperature will shift the lasing wavelength significantly, but the FLMs are more stable reflectors.
We have also compared the spectrum of the random lasing at different ports of the fiber cavity, i.e., port A, B and C in Fig. 1. As shown in Fig. 3, each of the three orders of random lasing has the same shape both at the near end and the far end of the fiber spool (i.e., port A and C). Moreover, the forward and the backward waves measured at the same point have the same shape (comparing port B and C). It is because both the FLMs and distributed Rayleigh mirror are broadband, the feedback couples essentially all the light waves and generates identical spectra for all points inside the lasing cavity .
3. Numerical results and discussions
The experimentally measured output lasing powers have been compared to theoretical predictions. To calculate numerically the lasing characteristics, we extend the well-known steady-state light propagation equations to three orders [5,9,10,18].Table 1.
The boundary conditions are and , where Pin denotes the pump power, and R1,2,3 is the reflectivity of the FLMs. Taking these conditions into consideration, the model can be solved numerically through the shooting method. Thanks to the broad reflection BW provided by the FLMs, we can simply fix the reflectivity of the fiber loop mirror to 0.6 (considering the insert loss of 3-dB coupler and WDM) in our simulation. While in the simulation of FBG-based half-open configuration, the effective reflectivity should be carefully tuned with pump variation due to the spectral broadening effect of the Stokes light. That is the reason why there is significant discrepancy between the experimental results and the numerical model with fixed FBG reflectivity .
Figure 4 shows the output Stokes power measured at Port A as a function of 1365nm pump power. The threshold for the 1st, the 2nd and the 3rd-order random lasing is measured as 0.75W, 1.45W and 2.45W, respectively. As the 2nd random lasing start to appear, the output power of the 1st-order lasing decreases rapidly and a nearly full depletion of the transmitted 1st-order lasing power is observed with 1.85W pump power. Similarly, when the pump power reaches 2.45W, the 3rd-order random lasing occurs and the power of the 2nd-order lasing decreases. The output power at the far-end computed from the numerical simulation nicely coincides with the experimental results, validating the effectiveness of our theoretical model. It should be noted that if the length of G.652 fiber is reduced to 50km, the lasing thresholds are only subtly different from the case with 94km G.652 fiber, which is verified both experimentally and numerically. In this paper we focus on the 94km case, because a longer lasing cavity would be more relevant to the application of distributed amplification for optical telecommunication and fiber sensing.
Figure 5 shows the calculated power distribution of each order of lasing with 2.7 W pump power. It is evident that the majority of the power flow is towards the far-end of the fiber spool, and as the order of lasing increases the position of power maximum is further shift to longer distance. Therefore, such a configuration of random fiber laser can be utilized as a valuable platform of high-order distributed Raman amplification , featuring high pump-Stokes conversion efficiency and structural simplicity. Given a more powerful pump laser with a shorter wavelength, it is possible to observe even higher order of random lasing by extending the work in this paper.
In this paper, we experimentally demonstrated and numerically analyzed the third-order random Raman fiber laser with a half-open cavity. With the high reflectivity of fiber loop mirrors, the threshold for random lasing is considerably reduced and the third-order random lasing occurs with only 2.45W pump power. Also, the intrinsic spectral features are well preserved; because both the fiber loops mirrors and the Rayleigh-scattering mirrors are broad-band. This work provides a useful platform to further investigate the physics of high-order random fiber lasers. Moreover, with the simplicity and the efficiency of the setup, it could be utilized in optical fiber communication and sensing as a high-order Raman amplification scheme.
This work is supported by Natural Science Foundation of China (61205048, 61106045, 61290312), Research Fund for the Doctoral Program of Higher Education of China (20120185120003), Fundamental Research Funds for the Central Universities (ZYGX2012J002, ZYGX2011J001), and PCSIRT.
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