Weak feedback assisted random fiber laser from 45 °-tilted fiber Bragg grating

We have demonstrated the realization of a high-polarization random fiber laser (RFL) output based on the hybrid Raman and Erbium gain with the tailored effect provided by a 45°-tilted fiber Bragg grating (45°-TFBG), revealing an improvement in the polarization extinction ratio (PER) and achieving a PER of ~15.3 dB. The hybrid RFL system incorporating the 45°-TFBG has been systematically characterized. The random lasing wavelength can be fixed under the extremely weak feedback effect of the 45°-TFBG with reflectivity of 0.09%. In addition, numerical simulation has verified that the weak feedback can boost the random lasing emission with fixed wavelength using a power balance model, which is in good accordance with the experiment results. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement


Introduction
To solve two problems-high threshold and non-directivity-of random lasers (RLs), optical fibers have been used to confine the random laser system, bringing about the development of random fiber lasers (RFLs) [1].Furthermore, a coherent RFL based on nanoparticles (NPs) scattering in the extremely weakly scattering regime has been obtained [2][3][4], which extends RFL from incoherent to coherent random lasing.To extend the application of RFL, D. V. Churkin and S. K. Turitsyn et al. have demonstrated RFLs in the telecommunication fiber based on a random distributed feedback due to the Raman amplified Rayleigh backscattering [5,6].And X. Bao et al. have reported Rayleigh scattering-assisted Brillouin lasing with single frequency and narrow linewidth in cascaded low-loss communication fiber [7,8].However, the threshold of random laser is still high in the telecommunication single mode fiber (SMF) due to the low back-scattering coefficient.Therefore, there are two main ways to decrease the random lasing threshold: 1) bring active fibers in the RFL system, e.g., Erbium doped fibers [9]; 2) add a fiber Bragg grating (FBG) to form a half-open cavity [6].Therefore, H. Wu et al. have used the hybrid Erbium-Raman fiber system (ERFS) with assistance of a FBG to obtain low threshold, high efficient RFL [10].In the hybrid Erbium-Raman fiber with FBG system, the role of FBG is to reflect the fixed wavelength light based on the Bragg law and boost random lasing with the fixed wavelength.Nevertheless, in the hybrid system without FBG, there are multi-modes stochastic lasing due to nearly equal probability of the dense multi-longitudinal modes in ultra-long fiber cavity.The reflectivity of FBG used in the random fiber laser system is >90% to obtain narrow random lasing with fixed wavelength.They also have investigated the role of point reflector with reflectivity of 1%-90% on the lasing threshold and power distribution in random fiber lasing process.The power distribution and threshold tend to be stable when the reflectivity reaches a relatively high level [11].In our previous work [2], the waveguide effect can boost the random lasing in the extremely weakly scattering regime.To date, super-low reflectivity (<0.1%) feedback how to influence the random lasing process and stabilize random lasing wavelength have not been reported in the RFL system.
The polarized lasers are important in the applications of sensor and optical communication.Nevertheless, it is hard to obtain the polarized RFL based on Rayleigh scattering since the local birefringence along SMFs is strongly influenced by external perturbations, leading to the deterioration of the RLs characteristics.L. Zhang et al. reported the linearly polarized multi-wavelength Brillouin laser comb by cascading multiple Brillouin random lasing oscillations in a semi-open polarization maintaining fiber-based composite cavity [12].B. Yao et al. have obtained singly-polarized pulse RFL using the broadband saturable absorption of monolayer graphene [13]. A. E. Budarnykh et al. have demonstrated an operation of a linearly polarized Raman fiber laser with random distributed feedback based on a polarization maintaining twin-core fiber [14].H. Wu et.al. have reported a polarizationmodulated RFL that generates pulsed output is proposed due to the different lasing threshold for two polarization states [15].S. Babin et.al. have proposed approach enables highefficiency generation of high-quality linearly-polarized laser radiation in a polarizationmaintaining fiber [16].The hitherto reported polarized RFLs are mainly based on adding saturable materials or polarization-maintaining devices in the fiber system.To decrease the complexity and extend the application of polarized RFL, we focus on the research of low-cost and all fiber polarized RFLs.
To simplify the fabrication procedures and decrease the cost of polarizers, the ultraviolet (UV) inscribed 45°-tilted fiber Bragg grating (45°-TFBG) has been fabricated as a polarizer in a standard commercial SMF that possesses significant advantages, e.g.low-cost, effective and all fiber system, over traditional polarizers [17][18][19].In principle, the TE light through such grating shows large transmission loss whereas the TM-light loss remains small due to Brewster's Law.Therefore, the 45°-TFBGs have been applied in optical communications and fiber laser systems as a broadband polarizer [20].Moreover, the reflectivity of 45°-TFBGs is weak.Therefore, the 45°-TFBGs comes into our sight to research how super-low reflectivity influences the random laser.Furthermore, this could also provide a simple way to realize polarized RFL in the all fiber hybrid ERFS system.
In this paper, firstly, characteristics of the RLs in the hybrid ERFS without 45°-TFBG have been investigated.The wavelengths of random lasing fluctuate randomly with time with the absence of the 45°-TFBG.Interestingly, when the 45°-TFBG with super-low reflectivity of 0.09% has been integrated in the hybrid ERFS, the RLs wavelength can be fixed and the threshold decreases comparing with the hybrid system without 45°-TFBGs.In our experiment, the reflectivity of Rayleigh backscattering of SMF is 0.05% based on the Rayleigh backscattering coefficient ε = 4.5 × 10 −5 km −1 [21] and fiber length of 11 km, which is similar with that of the 45°-TFBG.Therefore, we prove that the weak feedback can boost random lasing with fixed wavelength in ERFS.Moreover, a low-cost, effective and all fiber polarized RFL with PER of 15.3 dB has been obtained since only TM light can transmit through the 45°-TFBG.Finally, the influence of weak feedback on boasting the random lasing process and fixing the lasing wavelength is simulated.

Experiments
Figure 1 shows the setup of RFL based on hybrid ERFS.A 1455 nm Raman fiber laser (IPG) with a maximum output power of 5 W is used to pump both the Erbium doped fiber (EDF, 2 m length) and the SMF (11 km length) through a 1455/1550-1600 nm wavelength division multiplexer (WDM).The EDF provides active amplification, while the SMF provides both amplification based on simulated Raman scattering and feedbacks through distributed random Rayleigh scattering.And all the fiber ends are angle cleaved to avoid the Fresnel reflection his can be attrib an that of [22] in [22], never m lasing spikes mportant featur to be 0.05% b iber length of 1 edback, and thu       Based on (max) and mi Figure 7 give Port 1 (2).W random laser of feedback ra

Simulatio
The weak fe theoretically.considers both to mention th measured refl and Stokes li simulated.
Equations ( 1  ear-ideal polar because the po is further inv r balance mod uations [22].It point reflector p wavelength (1 utput spectrum q. (3) is the Ra esents the forw ht waves with bes the stokes l be 0 and for sto maximum g the PC.The relationship of reflection strength and lasing process is analyzed numerically by increasing the reflectivity of the 45°-TFBG (R) at different pump power, as shown in Fig. 8.For pump power under the lasing threshold, the peaks in the spectrum are amplified simultaneously with the increase of reflectivity, as shown in Fig. 8(a).However, for pump power higher than the threshold, the lasing wavelength (1555.19nm, the highest reflective peak in the 45°-TFBG reflection profile) dominates in the output spectrum, while the background light, the power of the peaks with lower reflectivity and the 3-dB bandwidth of the lasing wavelength are all strongly suppressed with the increase of reflection, as shown in Fig. 8(d).Therefore, weak feedback in this level has a remarkable impact on the random lasing process and the dominant lasing wavelength is only selected by the highest reflective peak of the 45°-TFBG.Figure 9 gives the simulated backward output spectra.The evolutionary process of the backward spectra follows the main characteristics of the forward ones, except with a much lower output power and background light which both coincide with the experimental results as shown in Fig. 4.

Summary
In summarize, we have reported a fixed wavelength random fiber laser emission with the weak feedback assistance of 45°-TFBG that located in the reflection of 45°-TFBG in the hybrid Raman and erbium gain fiber.Meanwhile, we prove the weak feedback of 45°-TFBG plays a key role to obtain fixed wavelength random lasing in experiment and simulation.Moreover, we obtain random laser with high polarization extinction ratio of ~15.3 dB.Therefore, this work provides a way to control the emission wavelength by weak scattering feedback.

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Fig. 7 .
Fig. 7.The PER sp ntal setup with power of rand d random laser t the 45°-TFBG om Port 1 is lo disturbed by th