Optics Communications 285 (2012) 2422–2425

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Optics Communications

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High-energy dissipative soliton with MHz repetition rate from an all-fiber passivelymode-locked laser

Kai Jiang a, Chunmei Ouyang a,⁎, Perry Ping Shum a,b, Kan Wu a, Jia Haur Wong a

a OPTIMUS-Photonics Centre of Excellence, School of Electrical & Electronic Engineering, Nanyang Technological University, 50 Nanyang Drive, Singapore 637553, Singaporeb CINTRA CNRS/NTU/THALES, UMI 3288, Research Techno Plaza, Singapore 637553, Singapore

⁎ Corresponding author. Tel.: +65 67904527.E-mail address: cmouyang@ntu.edu.sg (C. Ouyang).

0030-4018/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.optcom.2012.01.033

a b s t r a c t

a r t i c l e i n f o

Article history:Received 31 October 2011Received in revised form 15 January 2012Accepted 16 January 2012Available online 30 January 2012

Keywords:Passively mode-locked fiber laserDissipative solitonsLarge normal dispersionHigh pulse energy

We experimentally demonstrate pulse energy enhancement in an all-fiber passively mode-locked laser operatingin the large normal dispersion regime. By increasing the laser cavity length as well as its net cavity dispersion, theproposed laser, which is mode-locked by nonlinear polarization rotation, generates highly chirped dissipativesolitons with pulse energies up to 9.4 nJ. The fundamental repetition rate is 2.3 MHz, and the pulse duration is35 ps. Such low repetition rate as well as wide pulse width makes this mode-locked all-fiber laser a suitableoscillator to directly seed a fiber amplifier, which can be used as compact sources for high-power applications.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Fiber pulsed lasers with the advantages of simple design, low costand high stability have been proved to be strong competitor withsolid state lasers and attracted extensive attention in the last fewyears [1–5]. To scale up pulse energies extracted directly from fiberoscillators, lasers operating in large normal dispersion regime arepreferred. This type of lasers generates typical pulses with largenormal chirp and steep spectral edges that are also called dissipativesolitons (DSs). It has been shown that spectral filtering plays a curialrole in DS formation in fiber lasers with large normal dispersion. Re-cently, Chichkov et al. demonstrated an all-normal-dispersion (ANDi)fiber laser with pulse energies of 20 nJ emitting at 1.5 μm region witherbium-doped fiber (EDF) as a gain medium. In this fiber laser, a bulkbirefringent filter was used to dominate the DS pulse shaping and stabi-lize the laser operation [6]. In the same year, Liu et al. reported a com-pact all-fiber EDF laser with large normal dispersion. By nonlinearpolarization rotation the laser produced pulses with 8 nJ of the pulseenergy [7].

It is well known that pulses can be effectively enhanced in energyby lengthening cavity length together with increasing net cavitygroup velocity dispersion (GVD) [8,9]. In this letter, we propose anall-fiber-integrated EDF DSs laser with large normal dispersion,which is mode-locked by nonlinear polarization rotation. Initially,the laser produced 5.9 nJ of pulse energy. When lengthening thecavity length from 25 m to 89 m, the obtained DS pulse energies

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were enhanced to 9.4 nJ. Although the laser with similar cavity configu-ration in [10] delivers up to 12 nJ pulse energies, the lower repetitionrate as well as wider pulse duration makes the proposed laser moresuitable for seeding a fiber amplifier. This paves the way to mode-locked all-fiber master oscillator amplifiers with compact system con-figurations. Experimentally, longer cavity has also been investigated.However, due to the overdriving effect and insufficient amplitudemodulation of the effective saturable absorber, no better laser perfor-mances are realized.

2. Experimental setup

The experimental setup is shown in Fig. 1. The laser oscillator ismade of a polarization-independent isolator, a fiber based polarizer,two sets of polarization controllers (PCs), a wavelength divisionmultiplexer (WDM) combining the pump and signal at 976 nm and1550 nm, a 15-m-long EDF with GVD of 28 ps2/km and a fusedcoupler with 70% output coupling ratio. The polarization state of lightin the laser cavity can be controlled by adjusting the PCs, which workstogether with the polarizer suppressing one polarization to achievemode-locking by NPR. The pigtailed fiber of the WDM is 1.5-m-longHI1060 Flex fiber with GVD of 20 ps2/km. The polarizer and the isolatorare made with standard single mode fiber (SMF) with GVD of−22 ps2/km, and the total pigtailed fiber is 1.5 m long. The two setsof PCs and the output coupler are made of 7-m-long dispersion com-pensation fiber (DCF) with GVD of 5.2 ps2/km. The ring cavity also con-tains a 64-m-long DCF to increase the cavity length and net cavitydispersion. The total length of the laser oscillator is 89 m resulting in a~2.3 MHz fundamental repetition rate and the net cavity GVD is about0.78 ps2.

Fig. 1. Experimental setup of the large normal dispersion EDF laser mode-locked using nonlinear polarization rotation. (DCF: dispersion compensation fiber, PC: polarization con-troller, WDM: wavelength division multiplexer.).

Fig. 2. (a) Optical spectrum of the output pulse. (b)Autocorrelation trace of the chirpedpulse.

2423K. Jiang et al. / Optics Communications 285 (2012) 2422–2425

3. Experimental results

Initially, we did not add the 64-m-long DCF into the laser cavityand, therefore, the total cavity length in this case is about 25 m andthe net cavity GVD is ~0.45 ps2. By appropriately adjusting the PCswithin the cavity, self-startingmode-locking and single-pulse operationwas achieved when the pump power was beyond a threshold value of88 mW. After mode-locking, the laser generated stable pulse trainswith the fundamental cavity repetition rate of about 8.3 MHz. Fromthe measured radio frequency (rf) spectrum, a signal-to-noise ratio upto 80 dB was achieved, indicating that stable mode-locking was real-ized. When the pump power was increased above 280 mW, multi-pulse operation was observed. Fig. 2 (a) shows the optical spectrum ofthe output pulse at pump power of 280 mW. Obviously, the opticalspectrum of the pulse has the characteristic steep spectral edges ofDSs [11,12]. It has an edge-to-edge spectral width of 23 nm, centeredat 1561 nm.

The measured autocorrelation trace is shown in Fig. 2(b). It showsthat the autocorrelation trace has a full width at half maximum(FWHM) of about 33 ps, corresponding to 23 ps of pulse durationdue to its Gaussian profile. The measured average output power is48.4 mW, thus the calculated pulse energy is up to 5.9 nJ.

When the 64-m-long DCF was added into the laser cavity, the totalcavity length was increased to 89 m. Once the PCs were under propersettings, the laser was self-started and single pulse operation wasachieved when the pump power was above 62 mW. The fundamentalrepetition rate was observed to be 2.3 MHz. When the pump powerwas in the range of 62 mW to 100 mW, the laser operated on thesingle pulse state. With the pump power at 100 mW, the outputwas characterized with a 20-GHz real time oscilloscope and an auto-correlator. The measured oscilloscope trace and the rf spectrum wereshown in Fig. 3. The period of the pulse trains is 435 ns (see Fig. 3(a))which agrees well with the repetition rate of 2.3 MHz. Fig. 3(b) showsthe measured rf spectrum of the output pulse trains. It was measuredby a 2-GHz photodetector and a signal-source analyzer (SSA, Rohdeand Schwarz FSUP26). The signal-to-noise ratio is up to 80 dB at a300 Hz resolution bandwidth, which indicates stable pulse operationis attained.

Fig. 4(a) shows the output optical spectra at different pumppower. It can be seen that the output optical spectra have the charac-teristic steep edges, indicating that DSs are formed in this long lasercavity with strong net normal dispersion. With the pump power in-creasing from 62 mW to 100 mW, the edge-to-edge spectral width ofthe output spectrum was broadened from 15 nm to 19 nm due to self-phase modulation effect. The corresponding center wavelength shiftedby ~1 nm towards the short-wavelength direction. With the pumppower at 100 mW, the autocorrelation trace was measured andshown in Fig. 4(b). The pulse duration (FWHM) is 35 ps, assuming a

Gaussian profile. As we kept increasing the pump power, CW peaksappeared on the output optical spectrum and multiple-pulse operationwas further observed.

The variation of the output power as well as the correspondingpulse energy with the pump power was also measured, as shown inFig. 5. It is clear that self-started mode-locking is achieved when thepump power is above a threshold value of 62 mW. Below 100 mW,our laser operated in a stable single pulse state with output averagepower up to 22 mW corresponding to calculated pulse energy up to9.4 nJ. The laser will operate on the multi-pulse state as the pump

Fig. 3. (a) Oscilloscope trace of the output pulse trains. (b) rf spectrum of the outputpulse trains.

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Fig. 4. (a) Output optical spectra at different pump power. (b)Autocorrelation trace ofthe chirped pulse.

Fig. 5. Pump power versus the output power and the corresponding output pulseenergy.

2424 K. Jiang et al. / Optics Communications 285 (2012) 2422–2425

power was increased above 100 mW. The spectral and the temporalwidths of pulses became narrower once an additional pulse wasgenerated, and then they became wider as the pump power increased[7]. It is necessary to mention that, a 1550 nm isolator was employedexternal to the cavity in both the short and long cavity cases and,therefore, the unabsorbed 976 nm pump light was removed fromthe measured output signals. Compared to the laser in [10], the cavitylength of the laser presented here is increased by 41% corresponding tothe repetition rate scaled down to 2.3 MHz. Besides, greatly reducedpump power required for the fundamental operation of this laser,together with the broadened pulse width, enables it be a preferredoscillator source to directly seed a fiber amplifier, although its pulseenergy is not higher than that obtained in [10]. We believe that theseeconomic and compact high power sources can find many potentialapplications, such as micromachining and laser surgery.

It is shown that both the short and the long cavities output thepulses with strong chirp. To compress these highly chirped pulses,standard SMF was employed external to the cavities. For the shortlaser cavity, a 89-m-long SMF was proved to be a proper length bycut-back method to compensate the chirp accumulated inside thecavity and the output pulses can be compressed to ~300 fs pulsewidth, 11% above the transform-limited pulse due to uncompressednonlinear chirp, as shown in Fig. 6. In contrast to the short laser cavity,a 195-m-long SMF was used to compress the output pulses from thelong laser cavity. The output pulses can be compressed to ~395 fspulse width, see Fig. 6. The calculated time-bandwidth product is 0.53,20% above the transform-limited profile indicating more nonlinear

Fig. 6. Dechirped pulses for two cavities.

2425K. Jiang et al. / Optics Communications 285 (2012) 2422–2425

chirp accumulated in the long cavity. It is clear from Fig. 6 that, both thecompressed pulses have small satellites resulting from the nonlinearchirp of the pulse edges. The satellites for the short laser cavity casecontain ~10% of the pulse energy while they contain ~20% of the pulseenergy for the long laser cavity case. Both the chirp and the spectralwidth of pulses determine the length of SMF used to compress the out-put pulses. The larger the net cavity GVD and the narrower spectralwidth, the longer SMF is needed for pulse compression.

In our experiments, without inserting any additional spectral filterthat was usually used in other large normal dispersion fiber lasers[3,4], DSs with high pulse energies were realized in the proposedfiber lasers. That is because the pulse shaping in the large normaldispersion cavities is dominated by the limited gain bandwidth to-gether with the gain saturation of the EDF. The EDF functioned as aneffective spectral filter. Experimental results showed that our laserscan operate stably more than two weeks. They can always self-startand get the same output characteristics when the pump power isabove the threshold value. In order to get insight into the laser stability,we also monitored the long term drift of the repetition rate of our longcavity laser. We define Δf as the drift of the repetition rate of the laser,namely, Δf=frep− f irep, where frep is the instant repetition rate of thelong cavity laser, and firep the initial repetition ratewhen the laser imple-mented self-started mode-locking. The repetition rate drift Δf within1000min was measured using a signal-source analyzer (SSA, Rohdeand Schwarz FSUP26) at a 10 Hz resolution bandwidth and 1 kHzspan, as shown in Fig. 7(a). Clearly, in the early hours the repetitionrate reduced slightly due to the thermal effect of the fibers within thecavity. The maximum drift is around 40 Hz, corresponding to the rela-tive repetition rate variation of 1.7×10−5. Note that the duration of1000min does not constitute any limit to the laser stability, but is mere-ly the duration of the experiment. Additionally, the phase noise of thepulse trains from the long cavity laser was also characterized andpresented in Fig. 7(b). In such an all-fiber laser design, technical noiseinfluences are minimized and, therefore, quantum noise resulting fromspontaneous emission in the amplification process dominates the

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Fig. 7. (a) Drift of the repetition rate and (b) Phase-noise power spectral density of thepulse trains of the presented laser.

phase noise [13]. Because of the wide pulse duration and large disper-sion, the laser exhibits higher phase noise induced by quantum noisethan soliton lasers. Calculation of the root mean square (rms) timingjitter is typically performed over a specified frequency band using thefollowing equation

Δtrms ¼1

2πnf rep

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2∫f H

f LSn fð Þdf

rð1Þ

where n denotes the harmonic order of the carrier measured, frep is thefundamental repetition rate of the pulse trains, and Sn(f) is the powerspectral density of phase fluctuations per Hz. Here, fH and fL are bound-aries of the frequency range. From Eq. (1), the integrated timing jitterbetween 10 Hz and 1 MHz is calculated to be 9.2 ps.

4. Conclusion

We have proposed an all-fiber-integrated DS EDF laser generatingultrashort high energy pulses. When the laser cavity is 25 m long, itspulse energies can be up to 5.9 nJ. The output chirped pulses arecompressed to 300 fs in duration. In order to scale up the pulse energies,we have lengthened the cavity to 89 m by inserting a 64-m-long DCFwithin the cavity. Experimental results show that the pulse energiescan be enhanced by 60% (up to 9.4 nJ) in this case. The output highlychirped pulses have duration of 35 ps and have been compressed to395 fs. For the long cavity laser, its repetition rate drift as well asphase noise has also been investigated in the experiment.

References

[1] E.J.R. Kelleher, J.C. Travers, E.P. Ippen, Z. Sun, A.C. Ferrari, S.V. Popov, J.R. Taylor,Optics Letters 34 (22) (2009) 3526.

[2] K.K. Chen, S.U. Alam, J.R. Hayes, H.J. Baker, D. Hall, R. Mc Bride, J.H.V. Price, D. Lin,A. Malinowski, D.J. Richardson, IEEE Photonics Technology Letters 22 (12) (2010)893.

[3] C.M. Ouyang, L. Chai, H. Zhao, M.L. Hu, Y.J. Song, C.Y.Wang, IEEE Journal of QuantumElectronics 45 (10) (2009) 1284.

[4] Y.Y. Zhang, C. Zhang, M.L. Hu, Y.J. Song, S.J. Wang, L. Chai, C.Y. Wang, IEEE PhotonicsTechnology Letters 22 (5) (2010) 350.

[5] E.J.R. Kelleher, J. Travers, Z. Sun, A. Rozhin, A. Ferrari, S. Popov, J. Taylor, AppliedPhysics Letters 95 (11) (2009) 111108.

[6] N.B. Chichkov, K. Hausmann, D. Wandt, U. Morgner, J. Neumann, D. Kracht, OpticsLetters 35 (16) (2010) 2807.

[7] X.M. Liu, D. Mao, Optics Express 18 (9) (2010) 8847.[8] L.L. Chen, M. Zhang, C. Zhou, Y. Cai, L. Ren, Z.G. Zhang, CLEO/Pacific Rim 2009, 30–3

Aug, 2009.[9] X.L. Tian, M. Tang, P.P. Shum, Y.D. Gong, C.L. Lin, S.N. Fu, T.S. Zhang, Optics Letters

34 (9) (2009) 1432.[10] C.M. Ouyang, P. Shum, K. Wu, J.H. Wong, X. Wu, H.Q. Lam, S. Aditya, IEEE Photonics

Journal 3 (5) (2011) 881.[11] C.K. Nielsen, S.R. Keiding, Optics Letters 32 (11) (2007) 1474.[12] Y.J. Song, M.L. Hu, C. Zhang, L. Chai, C.Y. Wang, Chinese Science Bulletin 53 (23)

(2008) 3741.[13] H.A. Haus, A. Mecozzi, IEEE Journal of Quantum Electronics 29 (3) (1993) 983.