1. Introduction

Ordinary lasers require a gain medium to provide amplifications via stimulated emissions. The spatial confinement of light that oscillates in the laser structure is realized with the inclusion of high reflectivity mirrors at both cavity ends [1]. Once the total gain exceeds intracavity losses, lasing is initiated. The emission has a specific direction due to the existence of a visible optical cavity. In addition, the main criteria that determine laser properties are the appropriate selection of the gain medium as well as the cavity design that characterizes the modes structure [2].

In contrast, random lasers operate based on multiple elastic scattering of photons which substitutes the role of the standard mirrors that form traditional laser cavities. This process can be observed in disordered photonic materials that include powdered laser crystals [3], translucent ceramics [4], organic composites [5], and etc. Due to the coherent nature of light scattering, the localization of propagating beam is attained through constructive interference. The pioneering work by V. S. Letokhov as early as 1968 [6] has predicted theoretically the possibility of multiple scattering to initiate light amplification responsible for a laser action. However, it was only in 1995 that the first few observations [7,8] confirmed the existence of random lasers. The concept of “random lasing” refers to the emitted radiation that is stochastically distributed over the whole solid angle. In the last decade, many theoretical and experimental studies have been developed to achieve a better insight on this physical process [9, 10].

The idea of this principle is applicable to Raman laser as well. The optical confinement in fiber waveguides allows a high-intensity intracavity light propagation that results in a strong stimulated Raman scattering (SRS) transition. This is further supported by very small core diameter of less than 10 µm and low loss coefficient, α~0.2 dB/km at 1550 nm. Moreover, the fiber composition property that implies refractive index fluctuations lead to multiple Rayleigh backscattering (RBS) effects that serve as a randomly distributed reflector. Although the influence of RBS in the fiber core is extremely negligible, it is accumulated over a very long distance. A study on the operational characteristics of an ultra-long Raman fiber laser in an open cavity has already been reported [11]. Further contribution to the development of this particular area can be achieved with the fundamental clarification of scattering processes within this waveguide. Therefore in this paper, we demonstrate a 51 km Raman laser that incorporated various pump coupling distributions along the fiber structure. In this laser scheme that consisted of an FBG at one cavity end, its detailed performances especially spectral widths and output powers are discussed with respect to the changes in pumping percentage.

2. Experimental setup

The ultra-long Raman fiber laser (ULRFL) that consisted of a Raman pump unit (RPU) as a pump source was constructed in the backward and forward pumping setups as illustrated in Fig. 1 . The terms “backward” and “forward” indicate pumping directions with respect to the wave that was reflected back from the fiber Bragg grating (FBG). If pump light co-propagated with this reflected wave, then it is called as the forward pumping scheme and vice versa. The RPU operated at 1455 nm wavelength with a maximum output power of 1585 mW. The main objective of this assessment was to divide the pump powers delivered along the two fiber-entry points by including a set of couplers with various properties. These included 10/90%, 20/80%, 30/70%, 40/60% and 50/50% that were employed individually between the pump unit and two 1480/1550 nm wavelength division multiplexers (WDM1 and WDM2). With this employment, the pumping percentage was increased towards forward direction from 10% up to 90% with 10% increments. However, no coupler was required for 100% backward and forward pumping schemes as the RPU was coupled directly to the corresponding WDM as manifested in the inset of Fig. 1.

 

Fig. 1 Experimental layout of an ULRFL arranged in backward and forward pumping configurations. The couplers of 10/90%, 20/80%, 30/70%, 40/60% and 50/50% were connected separately to the WDM’s. The dashed boxes indicate pumping geometrical designs, where those for 100% backward and forward directions are shown in the inset.

Download Full Size | PDF

For simplification, the term coupling ratio (CR) was introduced which can be defined as:

In contrast to conventional fiber lasers that incorporated two highly reflective FBGs at both cavity ends [1], only one FBG was employed in this laser design. The FBG has a high reflectivity of 98% at 1553.3 nm with 3 dB and 20 dB reflection bandwidths of 0.32 nm and 0.454 nm respectively. The laser cavity was formed inside a 51 km single-mode fiber by utilizing the FBG as the first mirror while another mirror was created virtually in the SMF. This was possible because the extremely long gain medium induced inherent RBS effects that behaved as a distributed random mirror at one end to support laser operation. Instead of only including angle-cleaved fibre ends to eliminate Fresnel reflection or back-reflected light to the setup, two isolators were also used. The isolators implied an isolation value around 40 dB in the range of 1530-1560 nm and allow the propagation of Raman photons in a single direction only.

During experiment, lasing was achieved once the Raman gain has surpassed the threshold required to overcome intra-cavity losses between the optical elements. To measure the laser attributes, namely output powers and spectral features, port (A) in Fig. 1 was connected to an optical power meter (OPM) or an optical spectrum analyzer (OSA). For a better spectral evaluation, the OSA was set at a resolution around 0.02 nm or 0.05 nm. With regards to the 13 THz SRS shift in the silica fiber, the resulting first Stokes wavelength was estimated to be around 1555 nm. By taking into account that the RPU was weakly polarized and from the experimental results obtained, the measured degree-of-polarization of the pump unit has an average value of 23%. Thus, most of the interaction between pump (1455 nm) and signal light (1553.3 nm) occur at random orientations of polarization state.

3. Results and discussion

In the geometrical design that incorporated 100% backward pumping ($CR$ = 0), the asymmetric spectral profiles around the central Bragg wavelength were observed after reaching threshold. These are believed to occur due to thermal effects clarified before (see Fig. 2(b) in [12]). The full-width at half maximum (FWHM) acquired from this spectral feature was also plotted as a function of Raman pump power that is depicted in Fig. 2(b).

 

Fig. 2 Lasing performances in the 100% backward pumping architecture, (a) spectral profiles and (b) the corresponding spectral width and output power developments.

Download Full Size | PDF

In general, two regimes of operation were observed which indicate a narrowing effect at small power levels and a broadening effect at higher power levels. For the pump power of less than 1300 mW, the spectral width decreased down to 0.12 nm from its maximum value of 0.2 nm approximately. This implies that the spectral wing envelopes that were still influenced by spontaneous emissions had been gradually suppressed. As a result, the output power improved from 7.6 mW up to 45 mW. After this series, the next optical bandwidths contained stronger transitions between stimulated Raman photons at the peak FBG wavelength. However, the spectral width just increased slightly to a maximum of 0.13 nm which demonstrates the initiation of weak four-wave mixing process. When assessing continuous wave (CW) laser operation, a linear growth of output power was produced. This linearity shows the vastly available backreflected Raman photons that could be exploited if more backward pump powers were given to this laser. A maximum output power of 194 mW was generated which relates to an optical-to-optical efficiency of nearly 12%. Moreover, from the linear fit of the graph, a reasonable threshold at 1197 mW was also fulfilled which is almost comparable to that reported in [13].

The underlying physics behind this physical phenomenon is outlined in Fig. 3 . In the backward pumping mechanism, the Raman gain is maximum at the initial fiber length, $L_{0,b}$ before reducing exponentially at the maximum fiber length, $L_{\max ,B}$. This complies to the longitudinal field distribution of $g_R{P}_p(z)$ where the pump power attenuation along the fiber length z is$P_p(z)=P_o\exp (-{\alpha }_{p}z)$. From this formula, $g_R$ is the distributed Raman gain coefficient, $P_0$ is the total input power, and ${\alpha }_p$ is the loss coefficient at the pump frequency. Due to the molecular vibration of the fiber, the pump wavelength at 1455 nm is converted to a broadband Raman photon around 1555 nm according to the power evolution $\raisebox{1ex}{$d{P}^{\pm }$}\!\left/ \!\raisebox{-1ex}{$dz$}\right.$ (see Eq. (1) in [11]). In this case, more generated waves propagate in the backward direction compared to those in the forward direction with respect to the pump photons. As a result, a stronger Rayleigh scattering influence is induced around $L_{0,b}$ compared to that around $L_{\max ,b}$ as manifested in Fig. 3. With the inclusion of the FBG, an optical cavity that allows selective beam amplifications at 1553.3 nm is completed. Together with a stronger Rayleigh scattering effect that forms an efficient distributed virtual mirror around$L_{0,b}$, a significant CW laser generation at port (A) is initiated. However, its narrow laser spectra property implies weak four wave mixing interactions. These interactions occur between the backreflected photons from the FBG with the backscattered photons that propagate in the reverse direction to that of the pump photon.

 

Fig. 3 (Above) Simplified diagram of the backward-pumped fibre laser. (Below) Propagation of Raman photons in the laser structure where $L_{0,b}$ and $L_{\max ,B}$ represent the initial and maximum fiber lengths, respectively. The subscript $"b"$ in these parameters signifies “backward” which refers to the pumping setup.

Download Full Size | PDF

Interestingly, a greater broadening effect was induced in the laser signal when the pumping arrangement was changed to 100% forward pumping ($CR$=1). This is manifested by the spectral evolutions that were plotted in Fig. 4(a) which comprised of nearly symmetrical shapes at different pump power levels. This effect, which is associated to multiple four wave mixing processes between the interacting modes involves the same characteristics that occurred in the conventional fiber lasers [12,14]. When examining its bandwidth changes as a function of incoming pump power, a linear increase was produced. The FWHM was wider compared to that attained in the previous result (see Fig. 2(b)), where the maximum value was estimated to be 0.9 nm approximately. In addition, the lasing threshold was reached at a pump power of 788 mW and the maximum output power was measured to be 67 mW. The threshold is almost 1.6 times higher in comparison to that achieved in the conventional RFL that consisted of equivalent fiber lengths [12]. This discrepancy is acceptable by taking into account the absence of a high reflector at another end of the cavity. In the prior research work carried out by S. K. Turitsyn and associates [11], a comparison between $P_{th}^{FBG}$ to that only caused by $P_{th}^{RBS}$ was simulated. In this case, $P_{th}^{FBG}$ represents the threshold determined by two FBG-inclusions laser cavity and $P_{th}^{RBS}$ is the threshold initiated by RBS impacts. At a short resonator length, $P_{th}^{RBS}$ is much higher than that induced by$P_{th}^{FBG}$. Nevertheless, $P_{th}^{RBS}$ reduces slowly before approaching the value of $P_{th}^{FBG}$ at extended cavity lengths of more than 250 km. This ascertains the considerable impact of RBS in fiber lasers that are designed in an extreme cavity length dimension. From the trend of power development depicted in Fig. 4(b), it can be inferred that a saturation level can be achieved at higher pump powers than those provided in this assessment.

 

Fig. 4 Lasing properties in the 100% forward pumping design, (a) spectral evolutions and (b) the corresponding spectral width and CW performance.

Download Full Size | PDF

The basic principle that elucidates these circumstances at 100% forward pumping is shown in Fig. 5 . Analogous to the physical development in Fig. 3, the Raman gain is highest at $L_{0,f}$ before decreasing exponentially as it reaches$L_{\max ,f}$. With reference to the pumping direction, this results in the generation of more backward signal photons compared to the forward signal photons. As a result, a strong Rayleigh scattering is induced around $L_{0,f}$ which behaves as a virtual high reflector. The backscattered Raman photons at this region propagate together with the amplified 1553.3 nm photons in the same transmission line. This leads to the initiation of considerable four wave mixing interactions which instigate broadening to the laser spectra. The same observation was also reported by S. A. Babin and associates in conventional fiber laser architecture known as turbulence-induced broadening effect [15]. However, due to the formation of a weak distributed mirror around $L_{\max ,f}$, the average output power emitted at port (A) is lower. The utilization of laser power to support the energy conversion during broadening effects is also another factor that clarifies this condition.

 

Fig. 5 (Above). Simplified diagram of the fiber laser that incorporated 100% forward pumping. (Below) Propagation of Raman photons in the laser structure where $L_{0,f}$ and $L_{\max ,f}$ represent the initial and maximum fiber lengths, respectively. The subscript $"f"$ in these parameters signifies “forward” that refers to the pumping setup.

Download Full Size | PDF

In the next analysis, the complete results that cover output power operation and spectral widening effect obtained at all pump coupling ratios are presented in Fig. 6 . These parameters could be controlled depending on the specific applications. From the graph, it was observed that the increase in $CR$ from 0 to 1 resulted in the continuous increase of the spectral width. To maintain a quadratic increment of these linewidths with maximum values above 0.6 nm, the coupling ratios from 0.8 to 1 were chosen. In order to introduce ultrashort pulse generation, wider spectra are required. D. J. Spence and co-authors have reported previously, detailed modeling and mode-locking attempts by exploiting stimulated Raman scattering in simple and compact device footprints [16,17]. The fiber lasers comprised of a dichroic mirror that can support the whole stimulated Raman emission spectrum, and an output coupler that completed the cavity. Pulses within a twentieth of the cavity round trip time were generated in this ordinary laser architecture. However with advanced modifications to these laser designs [16], sub-picoseconds pulse train is expected to be realized. The potential success of this future attempt will also provide the basis of understanding towards the initialization of ultrashort pulses between modeless beating spectra. Therefore in this research assessment, by replacing the FBG with a broadband mirror that has a high reflectivity at all Raman wavelengths, broader spectra of more than 1 nm can be generated. In addition, by introducing a higher power pump source of more than 1.6 W, the nonlinear response that initiates the broadening effect can further be intensified. This assists the generation of ultrashort pulses in the cavity that utilizes a saturable absorber. Their high peak power and wide optical spectra features are favourable for many new emerging eye-safe applications within this wavelength regime. These include precision spectrometry, optical-soliton communication, supercontinuum generation, biomedical sensing and so on.

 

Fig. 6 Comparisons between spectral and output power progressions at various coupling ratios.

Download Full Size | PDF

From Fig. 6, in order to attain the highest degree of monochromaticity with linewidths below 0.22 nm, the employment of coupling ratios from 0 to 0.3 is suggested. The linewidth evolution was also in a full analogy to that exhibited in Fig. 2(b). Moreover, for applications that necessitate a power scaling attempt, a coupling ratio of 0.4 is preferable. In this case, the output power reached up to 220 mW with a narrow bandwidth around 0.27 nm. In the future, by retaining the CR at 0.4, further advancement to high output power operation can be realized. This is done by incorporating several times longer SMF with more powerful pump unit to optimize the strengthening of RBS impact.

4. Conclusion

In summary, the results have demonstrated the changes in laser parameters such as spectral widths and output powers as pumping directions were varied. These parameters can also be adjusted to meet special requirements by varying the pump coupling distribution along the fiber end-facets. In this experiment, the strengthening of turbulence-induced broadening between these modeless spectra was fulfilled for CR = 0 to 1. This is because at CR = 1 the nonlinear interaction between the amplified 1553.3 nm photons with the backscattered photons that has a higher backscattering coefficient is maximum. In addition, at CR = 0.4 high output power operation is favourable. With better understanding of these issues, more advanced design and performance to this novel class of lasers can be developed which will open up various beneficial applications in the future.