IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 27, NO. 20, OCTOBER 15, 2015 2205

Up to the 1552nd Order Passively HarmonicMode-Locked Raman Fiber Laser

Qingqiang Kuang, Li Zhan, Zhiqiang Wang, and Meizhen Huang

Abstract— An ultrahigh order passively harmonic mode-lockedRaman fiber laser based on nonlinear polarization rotationhas been proposed and demonstrated experimentally. Differentorders harmonic mode-locking (HML) was observed from thefirst to the 1552nd order by changing the pump power withadjusting polarization states. Up to the 1552nd order HML,corresponding to the repetition rate of 587.2 MHz, was obtainedwith the output power of 168.3 mW. The stability of HML isattested by the supermode suppression level as large as 40 dB.Different from the pulse energy quantization in low-order HML,the mechanism of high-order HML contributes to the acousticstabilization effect.

Index Terms— Fiber lasers, Raman fiber laser, mode locking,ultrafast, simulated Raman scattering.

I. INTRODUCTION

MODE-LOCKED fiber lasers (MLFLs) had attractedmuch interest due to their simplicity and compactness

in the past two decades [1]–[4]. One of important featuresof MLFLs is harmonic mode-locking (HML), which is oftenexploited to generate high-repetition-rate pulses [5]. Thehigh-repetition- rate pulses are widely useful in manyfields, including frequency combs, optical sensing, opticalcommunications, biomedical researches and optical frequencymetrology [6].

The generation of high-performance ultrashortpulses [7]–[12] were mostly achieved through mode-lockingin rare-earth doped fiber lasers. Usually, the doped fibers limitthe emission wavelength at the particular gain bands of thedopants. However, this limitation can be broken by using thesimulated Raman scattering (SRS) in optical fibers, which canbe operated at any wavelength by providing a suitable pumpsource. This is great potential to access new wavelengths forfiber lasers [13], [14]. The continuous-wave (CW) Raman

Manuscript received June 15, 2015; accepted July 14, 2015. Date ofpublication July 30, 2015; date of current version September 17, 2015. Thiswork was supported in part by the National Natural Science Foundation ofChina under Grant 61178014, Grant 11274231, Grant 61178083, and GrantE050802, and in part by the Young Scientist Project of Jiangxi Province underGrant 20133BCB23010. (Corresponding author: Li Zhan.)

Q. Kuang is with the Key Laboratory for Laser Plasmas, State KeyLaboratory of Advanced Optical Communication Systems and Networks,Department of Physics and Astronomy, Ministry of Education, Shanghai JiaoTong University, Shanghai 200240, China, and also with the Key Laboratoryof Photoelectronics and Telecommunication of Jiangxi Province, Collegeof Physics and Communication Electronics, Jiangxi Normal University,Nanchang 330022, China (e-mail: qqkuang@sjtu.edu.cn).

L. Zhan, Z. Wang, and M. Huang are with the Key Laboratory forLaser Plasmas, State Key Laboratory of Advanced Optical CommunicationSystems and Networks, Department of Physics and Astronomy, Ministryof Education, Shanghai Jiao Tong University, Shanghai 200240,China (e-mail: lizhan@sjtu.edu.cn; ewangzhiqiang1987@hotmail.com;mzhuang@sjtu.edu.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.2015.2457238

fiber lasers have been investigated, in particular occurring inlong fiber cavities [15]. For pulsed operation, the researchesof Raman fiber lasers are more recent. The first reportedpassively mode-locked Raman fiber laser was realized using afigure-of- eight cavity [13]. Mode-locked Raman fiber lasersemploying NPR were also reported [16], [17]. However, allthese Raman lasers were mode-locked at the fundamentalcavity frequency.

Also, various approaches for HML have been exploitedto generate high repetition rate pulses, in which, the laseroperates at a multiple of the fundamental frequency. Althoughactive mode-locking has allowed to generate HML up to the165,000th order [18], passively mode-locking is difficult toachieve such a high order HML. Since the passively HML wasdemonstrated in fiber lasers in 1993 [19], numerous researcheshave followed [20]–[24]. Of them, self-induced modulationinstability generates ultrahigh order HML in doped fiber lasers[25], [26] and also in Raman fiber lasers [27], [28]. Er-dopedfiber lasers operating at the 322nd- and 337th- order harmonichave been reported [29], [30]. To date, the highest order HMLin a Raman fiber laser ever reported is the 693rd order [24].The limit of the HML order in fiber lasers still needs furtherinvestigation.

In this Letter, we use a high nonlinear fiber (HNLF) Ramanring laser to investigate HML dynamics. The laser is found tooperate stably at various HML orders, from the 1st to 1552nd,with the super-mode suppression level better than 40 dB. Themaximum 1552nd order HML was obtained. Different frompulse energy quantization in low order HML, the mechanismof high order HML contributes to the acoustic stabilizationeffect. This work contributes to a further understanding ofthe complex temporal and pulsed dynamics of passively HMLfiber laser.

II. EXPERIMENTAL SETUP AND OPERATION PRINCIPLE

The experimental scheme for the proposed mode-lockedRaman fiber laser is shown in Fig. 1, which is a typicalstructure of fiber laser mode-locked by NPR. In the cavity, a500-m HNLF is used as Raman medium, which is pumpedby a 1.5 W CW multi-mode fiber laser at the wavelengthof 1539.0 nm through a 1550/1650 nm wavelength divisionmultiplexer (WDM). The linewidth of the pump laseris ∼0.07 nm. Two polarization controllers (PCs) are used toselect suitable polarization states, and a 10:90 coupler is tooutput the signal for measurement. A polarization dependentisolator (PDI) with single mode fiber (SMF) pigtails sand-wiched by two PCs is to ensure the unidirectional operationand also to act as a polarizer. The whole cavity length is 540 m.The Raman medium is a zero-slope HNLF that has a disper-sion value of D = −1.0 to +1.5 ps/(nm·km) and dispersion

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2206 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 27, NO. 20, OCTOBER 15, 2015

Fig. 1. Schematic setup of ultrahigh-order Raman harmonic mode-lockedfiber ring laser.

slope value of s = 0.006±0.004 ps/(nm2·km). The otherfibers are standard SMFs with group velocity delay (GVD)of 17ps/(nm·km) at 1550 nm. The total cavity is net negativedispersion at 1650 nm regime. The output signal from the10/90 coupler was measured by a powermeter, and also weredetected by a 20-GHz photodetector (New Focus 1014) thatwas connected to a radio frequency (RF) spectrum analyzer(Anritsu MS2601B) and a oscilloscope with 2-GHz bandwidth(LeCroy waveRunner 6200). The laser spectrum was measuredby an optical spectrum analyzer (Yokogawa AQ6370C). Withan appropriate setting of the PCs, the mode- locking operationat either the fundamental cavity frequency or its harmonics canbe achieved.

The backward pumping is essential to inhibit the polariza-tion instability and amplitude noise which may be induced bythe pump laser. Thus, we deployed a counter-pumping Ramanlaser. Stimulated Brillouin scattering (SBS) and backwardSRS are generally in competition in the laser. The SBSthreshold usually is much lower than the SRS one withoutconsidering the pump linewidth. In order to better stimulateSRS by suppressing SBS, we use a CW multiple longitudinalmode laser source as the Raman pump source. Its linewidthof ∼0.07nm is much above the gain bandwidth of SBS, result-ing in a higher SBS threshold and leading to the inhibitionof SBS. Therefore, we can better stimulate backward SRS bysuppressing SBS. It is important that this multi-mode pumplaser is almost polarization independent. This is to avoid theNPR laser cavity susceptible to the polarization state of thepump source, especially for a long cavity laser. The impactof the polarization instability of pump source on the cavitycan be neglected. This benefits to produce stable high orderHML pulses. The Raman media is a zero-slope HNLF. The lowdispersion fiber is helpful to ensure ultrashort pulse generationin a long cavity fiber laser.

III. EXPERIMENTAL RESULTS AND DISCUSSION

Overdriving of NPR is necessary to generate HML [31].This can be realized by increasing the nonlinear phase shiftthat is proportional to the cavity length and intra-cavity power.In the laser, first, a CW pump source of up to 1.5 W poweris employed. Second, the 500-m HNLF accounts for gainmedium. The intracavity polarization plays an important roleto obtain stable HML. NPR relies on the intensity dependent

Fig. 2. Mode-locked operation at the fundamental cavity frequency:(a) optical spectrum; (b) mode-locked pulse train.

Fig. 3. Pulse trains in the oscilloscope: (a)∼(f) corresponds to the 2nd orderHML at 758.46 KHz, 4th HML at 1.5139 MHz, 10th HML at 3.7752 MHz,209th HML at 79.08 MHz, 1075th HML at 406.7 MHz, and 1552nd HMLat 587.2 MHz, respectively.

rotation of elliptical polarization state in the cavity [2]. Withproper settings of the polarization ellipse and phase bias, thetransmission rate of NPR increases with the pulse intensity,which can initiate pulse shortening and mode-locking.

In the experiment, when the pump power was increasedto 540 mW, an obvious CW Raman spectrum was visible.The threshold for mode-locking is 680 mW. Figure 2 showsa typical fundamental mode-locked operation under the pumppower of 960 mW. As shown in Fig. 2(a), this Raman laseroperates at 1651.30 nm and the spectral bandwidth is 5.37 nm.Fig. 2(b) shows the mode-locked pulse train. The fundamentalrepetition rate is 378.35 KHz.

A typical feature of this laser is that the pulse repetitionrate increases with increasing the pump power. By adjustingthe PCs appropriately at different pump power, we can obtaina series of HML pulse trains with different repetition rates.In Fig. 3, the optimized pulse trains varying from fundamentalto the 1552nd order HML are achieved by enlarging thepumping power from 680 mW to 1.5 W. Stable HML pulsesat other orders, such as 3rd, 5th, 6th, 7th, 8th, 9th, 10th, 11th,12th, 13th, 209th, 1075th, 1480th, 1552nd, and so on, are

KUANG et al.: UP TO THE 1552nd ORDER PASSIVELY HARMONIC MODE-LOCKED RAMAN FIBER LASER 2207

Fig. 4. The features of the 1552nd order HML pulses: (a) the optical spectraon linear scale and logarithmic scale (inset); (b) the autocorrelation trace.

also obtained under different pump power with appropriatepolarization states. Figure 3(f) shows the 1552nd-order HMLpulse trains at 587.2 MHz repetition rate, which was obtainedwith an output power of 168.3 mW under the 1.5 W pumppower. To the best of our knowledge, it is the highest orderHML pulse in a passively mode-locked Raman fiber laser. Thesingle pulse energy of the 1552nd order HML is 286.6 pJ.The energies of lower order HML pulses are larger than thatof the 1552nd order. We also measured the optical spectrumand pulse width. Fig. 4(a) shows the spectra of 1552nd orderpulses. The center wavelength and the spectral width are1644.47 nm and 15.04 nm, respectively. This wavelength liesin the anomalous dispersion regime of the fiber, in whichthe Raman gain is shifted by 105.5 nm from the pumpwavelength. The pulse duration was measured as 712.8 fs asshown in Fig. 4(b), corresponding to a time-bandwidth-productof 1.18, which is far from the transform limited value (0.315)of sech2 pulse in soliton fiber lasers, since the large anomalousdispersion and the nonlinear effects in the laser cavity lead tochirped pulse output.

Usually, when the pump power is enlarged, the single pulseenergy is constant and cannot be increased due to energyquantization of soliton pulses [5], [19]. Increasing pumppower results in pulse splitting or harmonic order increasing.However, the generation of high order HML pulses in our laserdoesn’t result from the energy quantization of soliton pulses.To better show the mechanism of the laser, we measuredthe radio frequency (RF) spectra at different order HMLpulses as shown in Fig. 5. All these HML pulses, includingthe fundamental mode-locking, were suppressed with asuper-mode suppression ratio (SMSR) higher than 40 dB.

Well known to all, there are different operation modes fora MLFL based on NPR: single pulse train, tightly bunchedpulses [32], harmonic mode-locking [33], and so forth. Theseoperation regimes can be switched by adjusting polarizationstates. It is evident that the low order HML in our laser isa result of pulse energy quantization [19], [31]. When theaccumulated pulse power is larger than the maximal saturableenergy of NPR, the pulse split up. High harmonic oscillationoccurs, which results in the continuous resonant frequencyfrom the 1st order to the 13th order HML. In this regime, theHML order could be continuously increased if the pump powerwas further increased. It should be noted that the orientationof PCs, which determines the linear birefringence bias, mustbe adjusted to achieve different HML. This adjustment of theintra-cavity polarization in turn changes the extracted gain andnonlinear phase shift of the pulses, and allows for differentstabilization mechanisms for passive HML.

Fig. 5. RF spectrum: (a) fundamental cavity frequency mode-lockingat 378.35 KHz, (b) 4th HML at 1.5139 MHz, (c) 1075th HML at 406.7 MHz,and (d) 1480th HML at 560 MHz.

In addition to low order HML pulse trains below the 13thorder, stable high order HML pulse trains, such as the 209th,1075th, 1480th, and the 1552nd order, are obtained. Clearly,Figs. 5(a) and 5(b) show a noisy background existed inRF spectra of low order HML. These noises typically resultfrom the super-mode noises [22], [30]. The RF spectra ofthe low order HML from 1st to 13th order are all like this.It is noteworthy that Figs. 5(c) and 5(d) show almost nosidebands and cleaner in the high order HML spectra, whichmeans that the high order trains are uniform with excellentpulse quality. We attribute this phenomenon to the differentstabilizing mechanism from the low order HML. The acousticwave modulation of refractive index is a well-known effectduring the pulse propagation in optical fibers [5], [20], [34].When the pulses are almost uniformly distributed along thering cavity, each pulse circulating inside the cavity excites aweak transverse acoustic wave by the electro-strictional effect.The acoustic wave causes the density changes in the fiber core,which perturbs and induces the index change for followingpulses and imposes phase modulation. When the acoustic waveis periodically excited at an appropriate eigen-frequency, theindex perturbation is enhanced in the resonator, and thus along-range interaction is induced to stabilize pulse train. Therelevant eigen-frequencies show up as a series of peaks in theacoustic response spectrum, which is excited due to differenttypes of acoustic wave. The pulse train is most stronglystabilized when the pulse frequency coincides with the oneof acoustic resonant frequencies. Thus the induced phasemodulation is enhanced. The high order HML easily appearsat these discrete resonant frequencies of transverse acousticwave in the fiber, which is in agreement with the result in [20].Consequently, the acoustic effect is the dominant mechanismfor stabilizing high order HML. Because the repetition rateof the pulses should exactly meet the one of discrete acousticresonant frequencies, the RF spectra of high order HML pulsesshow almost no super-mode sidebands and cleaner on the noisebackground.

Here, the HML pulses with some specific repetition ratesare enhanced in accord with the acoustic resonant frequencies.The enhanced factor is estimated to be 103 at the frequencies

2208 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 27, NO. 20, OCTOBER 15, 2015

near 500 MHz, which is the strongest in the whole range [20].It is relatively easy to generate stable high order HML atthis range. This is the reason why we observed the HMLgeneration at the 1075th, 1480th, and the 1552nd order. TheHML is originated from the overdriving of the nonlinearpolarization rotation in the cavity [31]. The acoustic effectsonly act as a mechanism for stabilizing high-order HML withuniform separations and free super-mode noise. Because theeigen-frequencies of transverse acoustic waves are independenton the cavity length, the stable HML pulses should stillkeep on this frequency range even if slightly shorting orlengthening the fiber in the cavity. Certainly, the conditionis that the HML repetition rate can precisely meet the discreteeigen-frequencies of transverse acoustic waves. However, it isdifficult to obtain a higher order HML by greatly increasingthe cavity length. To support our viewpoint, we redo theexperiment of a Raman fiber laser with a 4400-m long cavity.The experiment shows that the higher order HML can’t beobtained. If simply increasing the cavity length, the only resultwas failed on mode-locking. Certainly, it is far from high-orderHML. In fact, it is very difficult for a long cavity fiber laserto obtain stable high order HML.

IV. CONCLUSION

In conclusion, we have proposed and demonstrated an ultra-high order passively HML Raman fiber laser based on NPR.The laser can produce stable pulses up to the 1552nd orderHML with the output power of 168.3 mW under the pumppower of 1.5 W. Different orders HML can also be obtainedat the range from the 1st to the 1552nd order. The SMSR of thelaser is better than 40 dB. Compared with previous works, thislaser offers an unprecedented combination of high harmonicorder, high pulse energy, and excellent pulse quality. Owingto its simple structure and high repetition rate, this laser couldhave potential applications in many areas.

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