Open Access
Add to BibSonomyAbstract
We propose a novel high power random fiber laser (RFL) based on tapered fiber. It can overcome the power scaling limitation of RFL while maintaining good beam quality to a certain extent. An output power of 26.5 W has been achieved in a half-open cavity with one kilometer long tapered fiber whose core diameter gradually changes from 8 μm to 20 μm. The steady-state light propagation equations have been modified by taking into account the effective core area to demonstrate the tapered RFL through numerical calculations. The numerical model effectively describes the power characteristics of the tapered fiber based RFL, and both the calculating and experimental results show higher power exporting potential compared with the conventional single mode RFL.
© 2016 Optical Society of America
1. Introduction
Random fiber lasers (RFLs) have attracted considerable attention in recent years due to their potential applications in the fields of high power laser generation, telecommunications, and distributed sensing [1,2 ]. The propagating light in the RFL is reflected by distributed Rayleigh scattering (RS) and amplified by Raman gain [3]. Despite the small RS coefficient in the silica fiber (usually in the magnitude of 10−4 km−1), efficient lasing can still be established with sufficient gain and feedback from the long distance passive fiber. Among the various studies of RFL, the high power random fiber laser is an important branch demonstrating the possibility of achieving bright light source with simple and efficient configuration [4–6 ]. Moreover, high power random fiber laser (higher than several-ten watts) can also serve as a seed source in fiber amplification systems due to its highly stable time domain and spectral-broadening-free property [7]. Theoretically, the output power of RFL is mainly limited by the appearance of the high-order Stokes wave. One existing method to increase the achievable power of RFL is to shorten the fiber length and thus increase the threshold of high-order Stokes wave [4,6,8 ]. Numerical studies show that the maximum power increases with the decrease of the fiber length, especially in the range of hundreds of meters. At present, nearly two hundreds watts power has been reported based on G.652 fiber with length of 120 meters [8]. However, a short fiber length would cause a problem of weak feedback, which makes the cavity sensitive to the parasitic reflection in fiber ends [9]. Using multimode fiber may be another choice to increase the threshold of high-order Stokes wave instead of simply cutting down the fiber length. But the degradation of beam quality is one major defect of applying multimode fiber and needs to be solved. In this paper, we demonstrate a new kind of high power RFL based on a tapered fiber (TF), in which the core diameter gradually increases along the fiber length. Resulting from the increased core area, TF can decrease the nonlinear interaction without shortening the fiber length. It also guarantees the distributed feedback required for random lasing especially for short cavity-length RFL. Furthermore, the laser beam propagation from small mode area end to large mode area end can still preserve fundamental mode operation in tapered fiber [10,11 ].
2. Experimental setup and results
A Half-open cavity is one kind of RFL configuration, in which feedback is provided by point reflection (such as a Fiber Brag Grating, FBG) together with the distributed RS. Such structure has the property of lower lasing threshold compared with the conventional open cavity [12]. It also ensures high power laser emission from one end which is a prerequisite for further power application. The experimental setup is schematically shown in Fig. 1 , which is a half-open cavity. The pump laser, with a central wavelength of 1120 nm, is directly injected to the cavity through the FBG from the small end of the TF. The central wavelength of the FBG is 1173 nm and the reflectivity is 99%. The core diameter of the TF linearly increases from 8 μm to 20 μm in the one km fiber with constant numerical aperture (NA) of 0.12, which is fabricated by controlling the drawing speed in a parabola relationship. The output end is 8° angle cleaved to get rid of the strong point reflection.
The output power of the different waves is shown in Fig. 2 as a function of pump power. The threshold is around 14 W, where the 1st order Stokes wave increases remarkably, corresponding to the strong pump consumption. The threshold is twice the value reported in previous literature with the same fiber length [4]. The power increasing tendency is similar to conventional fiber based RFL [4,8 ]. The slope efficiency above the threshold is higher than 200% and gradually reduces to ~60% at the maximum power. When the pump power is higher than 40 W, the 2nd order Stokes wave arises in the forward direction, and limits the power scaling of first order Stokes wave.
Fig. 2 The output power of 1st Stokes, 2nd Stokes, and residual pump waves as a function of total input pump power for the TF RFL (dots: experimental data; solid line: numerical simulation result); Inset: the dependence of Aeff on the fiber position.
The temporal behaviors of the 1st order Stokes wave have also been detected in the output end by a detector with bandwidth of 1.2 GHz and an oscilloscope with bandwidth 1 GHz. A stable state can be observed in the time scale of microsecond, which is shown in Fig. 3 . The radio frequency (RF) spectrum (calculated by Fast Fresnel Transformation, enclosed in Fig. 3) also reveals that no characteristic frequency dominates the signal, which validates the random lasing behavior [4]. This kind of light source is fit for applying as the seed of the high power amplification system, as the temporal stability and lasing incoherence may effectively increase the threshold of nonlinear effects [7].
The spectra of the 1st order Stokes wave at different pump powers are also recorded in Fig. 4(a) . All spectra show only one peak, and correspond to the reflection spectrum of the FBG. It is the same as conventional RFL assisted by FBG, which usually has one peak corresponding to the central wavelength of the FBG [13]. The full width at half maximum (FWHM) was measured in the condition of RFL and Raman oscillator assisted by Fresnel reflection, as shown in Fig. 4 (b). The FWHM of the FBG is about 1.2 nm for the Raman oscillator the FWHM of the generated spectrum is almost constant and follows the spectral width of the FBG. But for the RFL, the spectrum gradually broadens as the power increase and exceed the bandwidth of FBG when pump power is higher than 28 W. The spectral broadening of RFL comes from the interaction of Kerr nonlinearity and dispersion which is similar to the processes in conventional Raman fiber laser [14]. But the difference in the experiments needs to be further studied.
Fig. 4 (a) The output spectra of the tapered fiber based RFL under different pump power; (b) the FWHM of tapered fiber based RFL and flat-cutting-end assisted Raman oscillator.
Another advantage of the TF based RFL is that the laser can preserve fundamental mode operation. Most of the applications in relation to tapered double-clad Yb-doped fiber adopt the scheme of pumping from the large mode area end in order to reduce the requirement of pump brightness [10]. In our case, the RFL is core pumped and the pump laser is injected from the small end. It can help to avoid the possibility of excitation of high order modes in the condition of mismatched splice, because the core diameter of the FBG is also about 8 μm. Meanwhile, injection from the large end would probably introduce loss for the backward propagated light (related to the pump direction) in the mismatched splice point if no other technique is taken. We collimated the output laser beam and measured the beam distribution in the far field. The result is shown in Fig. 5 . It can be found that the output laser operates in the fundamental mode with an almost Gaussian profile, even though the normalized frequency is about 6.4 in the output end, revealing that this pump scheme can maintain the generation of fundamental mode.
3. Theoretical study and discussion
To make a comprehensive theoretical analysis on the TF RFL, we have modified the steady-state light propagation equations by taking into account the gradually changing core area [4,9,12 ].
The subscripts 0, 1, 2 represent the pump, 1st order Stokes, 2nd order Stokes waves, respectively. Superscripts ‘ + ’ and ‘−’ denote the forward and backward waves. P 0,1,2 is the power in different position. gR 1,2 is the Raman gain coefficient; Aeff is the effective core area; v 0,1,2 corresponds to the wave frequency; Δv 1,2 is the laser bandwidth and set to be 0.22 THz; α 0 = 0.4 km−1, α 1,2≈0.36 km−1 are the fiber background losses that are determined through experiment; ε 0,1,2≈0.002α 0,1,2 is the Rayleigh backscattering coefficient [9]. T = 298K is the fiber temperature and KB is the Boltzmann’s constant, h is the Plank’s constant. The boundary conditions are P0(0) = Pin, P + 1,2(0) = RL 1,2 P - 1,2(0), P - 1,2(L) = RR 1,2 P + 1,2(L), where Pin is the input pump power, RL 1,2 and RR 1,2 donate the reflectivity at the left and right end, respectively.
For the Raman interaction in a step index fiber, the effective mode area is usually different from the fiber core area [15]. Assuming that the Stokes and the pump waves have the same mode distribution, such as a Gaussian distribution, the effective mode area can be calculated by the following equations [16].
where, V is the normalized frequency. For a conventional single mode fiber (SMF), Aeff is a constant value. However, it changes along the fiber length in the TF. The core radius of our TF linearly increases along the fiber, so the Aeff can be calculated by Eqs. (5),(6) and the result is shown in the inset of Fig. 2. It should be noted that this assumption is reasonable in our case for the observation of the fundamental mode at the maximum power in the experiment, showing in Fig. 5.To obtain the Raman gain coefficient, we flatly cleave the output end to form a 99%-4% Raman oscillator by the assistant of Fresnel reflection. The output power of the oscillator is measured and shown in Fig. 6 . It is found that the calculated results by the present model fit well with the experiment when the Raman gain coefficient gR 1 is set to be 5.8 × 10−14 m/W, which is a reasonable value compared with other reports [4]. Then we apply this parameter to calculate the random cavity with the boundary conditions of RL 1 = 0.99, RL 2 = RR 1,2 = 10−6. In the calculation, we assume gR2 = 0.9 gR 1 [4]. As shown in Fig. 2, the numerical results can describe well the threshold and power evolution of the TF RFL, indicating the functionality of the proposed theoretical model.
Fig. 6 The output power of 1st order Stokes, 2nd order Stokes, and residual pump waves as a function of total input pump power in the case of 99%-4% cavity (dots: experimental data; solid line: numerical simulation result).
To further evaluate the power scaling potential of the TF, the threshold and the maximum power of the 1st order Stokes wave was calculated based on the proposed model. The threshold refers to the pump power in the condition that the output 1st order Stokes wave power is 1% of the input power. And the maximum power is defined as the highest power of the 1st order Stokes wave when further power increase is limited by the 2nd order threshold. For comparison, the threshold and maximum power of conventional SMF based RFL are also calculated. The effective mode diameter of SMF is set to be 9 μm and other parameters are the same as the TF used here. The calculated results are shown in Fig. 7 . With decreasing fiber length, the maximum power of both the TF and SMF increase exponentially, while the maximum achievable power of the TF is obviously higher than that of the SMF. The ratio of maximum power between TF and SMF is plotted in Fig. 8 . It can be found that the advantage of TF become notable when the fiber length is gradually shortened. Accordingly, TF is more suitable to apply in the high power short cavity-length cases.
Fig. 7 The calculated threshold and maximum power of 1st Stokes wave for tapered fiber (blue) and SMF (red).
4. Conclusion
In conclusion, we proposed a tapered fiber based high power random laser in a half-open cavity configuration. An output power of 26.5 W in a one-kilometer-length fiber was achieved. The numerical model was established with the modified steady-state light propagation equations by taking into account the gradually changing core area. We firstly conducted the experiment of a Raman oscillator assisted by the 4% Fresnel reflection of the fiber end to obtain the Raman gain coefficient of the proposed model. The collected Raman gain coefficient was then applied in the numerical calculation of the RFL case. The calculated results are supported well with the experiment. The TF based RFL is capable of increasing the maximum output power comparing with SMF based RFL with the same fiber length, while in the meantime maintaining fundamental mode laser operation. The result has shown convincing high power potential of the TF in short cavity-length RFL.
Acknowledgments
The authors would like to acknowledge Professor Zhiyong Pan for the supply of the tapered fiber. The work is supported by the National Natural Science Foundation of China (grant NO. 61322505) and Graduate Student Innovation Foundation of the National University of Defense Technology (grant No. B130702).
References and links
1. D. V. Churkin, S. Sugavanam, I. D. Vatnik, Z. Wang, E. V. Podivilov, S. A. Babin, Y. Rao, and S. K. Turitsyn, “Recent advances in fundamentals and applications of random fiber lasers,” Adv. Opt. Photonics 7(3), 516–569 (2015). [CrossRef]
2. S. K. Turitsyn, S. A. Babin, D. V. Churkin, I. D. Vatnik, M. Nikulin, and E. V. Podivilov, “Random distributed feedback fibre lasers,” Phys. Rep. 542(2), 133–193 (2014). [CrossRef]
3. S. K. Turitsyn, S. A. Babin, A. E. El-Taher, P. Harper, D. V. Churkin, S. I. Kablukov, J. D. Ania-Castañón, V. Karalekas, and E. V. Podivilov, “Random distributed feedback fibre laser,” Nat. Photonics 4(4), 231–235 (2010). [CrossRef]
4. Z. Wang, H. Wu, M. Fan, L. Zhang, Y. Rao, W. Zhang, and X. Jia, “High power random fiber laser with short cavity length: theoretical and experimental investigations,” IEEE J. Sel. Top. Quantum Electron. 21(1), 900506 (2015).
5. S. A. Babin, I. D. Vatnik, A. Y. Laptev, M. M. Bubnov, and E. M. Dianov, “High-efficiency cascaded Raman fiber laser with random distributed feedback,” Opt. Express 22(21), 24929–24934 (2014). [CrossRef] [PubMed]
6. H. Zhang, P. Zhou, H. Xiao, and X. Xu, “Efficient Raman fiber laser based on random Rayleigh distributed feedback with record high power,” Laser Phys. Lett. 11(7), 075104 (2014). [CrossRef]
7. X. Du, H. Zhang, P. Ma, H. Xiao, X. Wang, P. Zhou, and Z. Liu, “Kilowatt-level fiber amplifier with spectral-broadening-free property, seeded by a random fiber laser,” Opt. Lett. 40(22), 5311–5314 (2015). [CrossRef] [PubMed]
8. X. Du, H. Zhang, X. Wang, P. Zhou, and Z. Liu, “Short cavity-length random fiber laser with record power and ultrahigh efficiency,” Opt. Lett. 41(3), 571–574 (2016). [CrossRef] [PubMed]
9. I. D. Vatnik, D. V. Churkin, and S. A. Babin, “Power optimization of random distributed feedback fiber lasers,” Opt. Express 20(27), 28033–28038 (2012). [CrossRef] [PubMed]
10. V. Filippov, J. Kerttula, Y. Chamorovskii, K. Golant, and O. G. Okhotnikov, “Highly efficient 750 W tapered double-clad ytterbium fiber laser,” Opt. Express 18(12), 12499–12512 (2010). [CrossRef] [PubMed]
11. J. Kerttula, V. Filippov, Y. Chamorovskii, V. Ustimchik, K. Golant, and O. G. Okhotnikov, “Principles and performance of tapered fiber lasers: from uniform to flared geometry,” Appl. Opt. 51(29), 7025–7038 (2012). [CrossRef] [PubMed]
12. D. V. Churkin, S. A. Babin, A. E. El-Taher, P. Harper, S. I. Kablukov, V. Karalekas, J. D. Ania-Castañón, E. V. Podivilov, and S. K. Turitsyn, “Raman fiber lasers with a random distributed feedback based on Rayleigh scattering,” Phys. Rev. A 82(3), 033828 (2010). [CrossRef]
13. S. Sugavanam, N. Tarasov, X. Shu, and D. V. Churkin, “Narrow-band generation in random distributed feedback fiber laser,” Opt. Express 21(14), 16466–16472 (2013). [CrossRef] [PubMed]
14. S. V. Smirnov and D. V. Churkin, “Modeling of spectral and statistical properties of a random distributed feedback fiber laser,” Opt. Express 21(18), 21236–21241 (2013). [CrossRef] [PubMed]
15. R. H. Stolen, “Fiber Raman lasers,” Fiber Integr. Opt. 3(1), 21–51 (1980). [CrossRef]
16. W. P. Urquhart and P. J. Laybourn, “Effective core area for stimulated Raman scattering in single-mode optical fibres,” IEE Proc., J Optoelectron. 132(4), 201–204 (1985). [CrossRef]
