246 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 3, FEBRUARY 1, 2014

Output Characterization of Random Fiber LaserFormed by Dispersion Compensated Fiber

Ye Yu Zhu, Wei Li Zhang, Senior Member, IEEE, Yun Jiang Rao, Zi Nan Wang, and Xin Hong Jia

Abstract— Random lasing (RL) characteristics of second-orderrandom fiber laser formed solo by dispersion compensated fiberare studied. It is found that the threshold of first-order RL is only0.45 W. In addition, a special route to stable second-order lasingis revealed, i.e., a special arc-shape output spectrum of second-order RL and three chaotic regimes during evolution from thefirst RL to the second-order one with increased pump power.

Index Terms— Fiber laser, Raman laser, Rayleigh scattering,dispersion compensated fiber.

I. INTRODUCTION

M IRRORLESS random lasers formed by multiple scatter-ing in a disordered gain medium have attracted signifi-

cant interest among researchers over the past decades [1], [2].In 1990s, random lasers formed by disordered semiconductor[3] or organic materials [4] were reported experimentally[3]–[5].

Recently, a concept of a new type of random laser basedon extremely weak random Rayleigh scattering (RS) in astandard single-mode fiber (SMF) has been demonstrated byTuritsyn et al. [6], [7]. The work has triggered manyexperimental and theoretical studies on random fiber lasers(RFLs) [8]–[13]. Besides, lots of attentions have been paid totheir potential applications in fiber-optic communication andsensing [14]–[16].

Furthermore, multiple scattering can be provided byother disordered gain medium, dispersion compensated fiber(DCF), in which random lasing (RL) is enhanced efficientlydue to relatively large Raman gain and strong RS. Severalstudies have been focused on this kind of RFLs. Firstly,a multi-wavelength fiber laser using DCF as dual-randommirrors was presented [16], [17]. Secondly, a modulatedrandom mirror laser was experimentally realized by utilizinga ∼2.5 km DCF, in which no distortion of the modulatingfrequency or self-mode-locking effects were measured [18].

Manuscript received September 25, 2013; revised October 25, 2013;accepted November 6, 2013. Date of publication November 19, 2013; dateof current version January 8, 2014. This work was supported in part bythe National Natural Science Foundation of China under Grants 61106045,61290312, and 61107073, in part by the Seeding Project of Scientific andTechnical Innovation of Sichuan Province (20132006), in part by the Programfor Changjiang Scholars and Innovative Research Team in University, and inpart by the Fundamental Research Funds for the Central Universities underGrant ZYGX2011J001.

The authors are with the Key Laboratory of Optical Fiber Sensing andCommunications, Education Ministry of China, University of ElectronicScience and Technology of China, Chengdu 611731, China (e-mail:rainyleafyy@gmail.com; wl_zhang@aliyun.com; yjrao@uestc.edu.cn;znwang@uestc.edu.cn; jxh_0@yahoo.com.cn).

Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LPT.2013.2291575

Fig. 1. The schematic setup of the RFL. OSA: optical spectrum analyzer;OPM: optical power meter.

In addition, by adding FBGs to the hybrid SMF/DCF cavity2nd-order RFL was also obtained [19]. Through differentcombination of SMF and DCF, the laser performance canbe enhanced by an optimized combination of SMF and DCF(i.e., length and position). These studies imply that DCF canbe used to reduce threshold of RFL.

Besides the above studies, it is suspected that RFLs takingadvantage of DCF will show emission characteristics differentfrom RFLs based on SMF, especially for relatively long DCF,while this hasn’t been addressed before. In this letter, a noveltype of 1st and 2nd -order RFL formed solo by 11 km DCF isdemonstrated. RL with sharply reduced threshold is obtained.In addition, a different formation route from the 1st RL to the2nd -order one is revealed.

II. EXPERIMENTAL RESULTS

The schematic diagram of the random fiber laser with amirrorless open cavity is depicted in Fig. 1. A Raman fiberlaser with central wavelength of 1365 nm is used as the pumplaser. The pump is launched into a fiber spool through a1365/1461nm wavelength division multiplexer (WDM). A rollof 10 km DCF is connected to the common port of the WDMwhile the other 1 km DCF is mounted at the 1461 nm port ofthe WDM. The 1 km DCF is used as an additional randomfeedback mirror. It reflects the back-propagating light back intothe 10 km DCF, which helps reducing the laser threshold. Tomonitor output of the RFL, a 1:99 coupler is located after the10 km DCF and a 1454/1550 nm WDM is used to separatethe 1st and 2nd -order RL. The output spectrum is monitoredat the 1% port of the coupler. The fiber ends are angle cleavedto eliminate facet reflection. The DCF has the characteristicdispersion of −130.135 ps/nm/km at 1545 nm.

It is worth noting that there exists an optimal length ofDCF, which corresponds to the point of fiber where Ramangain equals the fiber loss [6]. This optimal length will changewith pump power increasing. In our discussion, a 10 km DCFis chosen as an example to study RL subjected to differentpump power. Besides, DCF has anomalous dispersion regime,which can partly compensate the effect of the nonlinearity,

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ZHU et al.: OUTPUT CHARACTERIZATION OF RFL FORMED BY DCF 247

Fig. 2. Output power as a function of pump power.

causing modulation instability. This effect will result in richdynamical phenomenon, like the formation of soliton etc.In this letter, we mainly focused on the output power andspectrum characteristics of RFL. Hence, Rayleigh scatteringand Raman gain are key factors discussed.

With the increase of pump power, the 1st-order Stokeslight generates. Due to distributed RS the generated light isfeedback forwardly and backwardly in the DCF. When theRaman amplification overcomes the fiber loss, the 1st-orderRL begins to radiate. Multiple modes with random frequenciesare formed.

Fig. 2 shows the output power of two configurations of RFLas a function of pump power. The cross and the dot curvescorrespond to respectively the 1st and 2nd -order RL of theconfiguration with 11 km DCF as shown in Fig. 1. As pumppower increases, four regions correspond to different emissionregimes are observed in sequence.

For comparison, we investigate the RFL with only 10 kmDCF. As shown in Fig. 2, the star and the fort correspond tothe 1st and 2nd -order RL of the configuration, respectively.The input-output curve is similar to that of the configurationof 11 km DCF. However, without the 1 km DCF, stationary2nd-order lasing isn’t observed, indicating that the randomfeedback from the 1 km DCF plays an important role toenhance the lasing efficiency.

In region I, only 1st-order RL happens. The threshold of1st-order RL is ∼0.45 W, which is even lower than thatobtained in a half-opened fiber cavity formed by an FBG and aspool of 50 km SMF [9]. When the pump power is increased tobe ∼0.72W, a discontinuity appears in the linear curves. This isbecause the spectrum dislocates to higher wavelength resultingin a slightly low-value of the maximum output power [17].

Fig. 3 gives the output spectra of the RFL for pump power at0.63 W and 0.87 W, respectively. For pump power of 0.63 W,the operation is near the RL threshold. Chaotic spectrumis observed due to cascade Brillouin scattering effect [7].When the pump power is increased to be 0.87 W, the outputspectra become stable with a single emission peak. The 3dBmbandwidth is ∼1.4 nm (1451.8 ∼1453.2 nm) and broadensslightly with increased pump power. These observations aresimilar to the operation regime of a RFL using a completely-opened SMF cavity [6].

Fig. 3. Output spectra of 1st-order random lasing.

Fig. 4. Output spectra of the RFL for different value of pump power (Pp).(a) Pp = 1.35 W; (b) Pp = 1.70 W; (d) Pp = 2.40 W.

In region II, as the pump power increases to ∼1.35 W,the 2nd-order Stokes light begins to emit. Power of2nd-order emission changes slightly (less than 0.65 mW) whenpump increases from 1.35 W to 2.35 W. This is becausespectrum broadening of the chaotic output (caused by relativelarge nonlinearity of DCF) prevents energy transfer from the1st Stokes light to the 2nd–order one. Fig. 4(a) shows thatthe 1st and 2nd -order emissions exhibit chaotic spectra witha double-peak and an arc-shape envelope, respectively. Thespectrum ranges are 1445 − 1490 nm and 1535 − 1580 nm,respectively. Such a width bandwith is helpful to achieve largewavelength range tunable lasers [20].

In region III, when the pump power is increased tobe ∼2.40W, the 2nd-order Stokes light almost disappears.As shown in Fig. 4(c), the 1st-order emission becomes stable,and the 2nd-order emission almost disappears. This abnormalphenomenon isn’t observed in SMF [6]. It might be causedby complex nonlinear effects which broad the spectrum of the1st-order RL/laser, and thus enhance the threshold of

248 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 3, FEBRUARY 1, 2014

Fig. 5. Output spectra of the RFL for different value of pump power.(a) 1st-order Stokes light; (b) 2nd-order Stokes light.

2nd-order RL/laser, causing disappearance of 2nd-order RL.The 3dBm bandwidth of 1st-order RL is 1.9 nm (1452.4∼1454.3 nm).

In region IV, when the pump power increases beyond∼2.60W, the 2nd-order emission reappears and the spectra ofboth the 1st and 2nd-order emission become chaotic again, asshown in Fig. 5. Energy of the 1st-order lasing transfers tothe 2nd-order lasing efficiently with increased pump power.When the pump power is increased to be 2.95 W, the spectrabecome stable. The threshold of 2nd-order RL is ∼2.5 W,which is higher than that achieved in a half-opened fiber cavityformed by an FBG and a spool of 50 km SMF [9]. It isshown that the threshold of both the 1st and 2nd -order RL hasbeen reduced apparently, compared with that of RFLs using acompletely-opened SMF cavity.

We find that the formation process of stationary 2nd-orderlasing is more complex than that of SMF and the process(regimes II and III) exists within a relative wide range ofpump power. During this process the power and the bandwidthof 1st-order lasing increase continually, while the 2nd-orderemisssion keeps at a low power. The characteristics aren’tobserved in SMF. Besides, RL of DCF can be fully modeledthrough the well-known Non Linear Schrodinger Equation[13], including dispersion and nonlinear effects of the fiber.The numerical method is similar to the statistic method givenby S. V. Smirnov to analysze RL in SMF.

III. CONCLUSION

According to our previous study [19], taking advantage ofrelatively strong Rayleigh scattering and Raman gain of DCF,and 1 km DCF performing as random mirror, threshold ofRFL is reduced, and efficiency of the laser is improved. Basedon this principle, we realize efficient 2nd-order RL usingcompletely-open cavity formed entirely by DCF. The thresholdof 1st-order RL is reduced to 0.45 W, which is much smallerthan that of other RFLs demonstrated up to data. Besides, adifferent formation route of stationary 2nd-order lasing withrandom distributed feedback is revealed. Before the stable2nd-order RL emitted, three chaotic regimes are obtainedduring the process when the 1st-order RL evolves to the2nd-order one with increased pump power. These emissioncharacteristics are different from RFLs based on SMF, whilethese haven’t been addressed before. The results are useful toreveal the role of DCF in RL, and also provide theoreticalsupport for flexible design of RFLs.

ACKNOWLEDGMENT

The authors would like to thank Prof. Adrian Podoleanu forhis helpful guidance and stimulating discussions.

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