December 1, 2006 / Vol. 31, No. 23 / OPTICS LETTERS 3489

Passively mode-locked Raman fiber laser with100 GHz repetition rate

Jochen Schröder, Stéphane Coen, and Frédérique VanholsbeeckDepartment of Physics, The University of Auckland, Private Bag 92019, Auckland, New Zealand

Thibaut SylvestreDépartement d’Optique P. M. Duffieux, Institut FEMTO-ST, Université de Franche-Comté, CNRS UMR 6174,

F-25030 Besançon, France

Received August 29, 2006; revised September 10, 2006; accepted September 11, 2006;posted September 12, 2006 (Doc. ID 74497); published November 9, 2006

We experimentally demonstrate the operation of a passively mode-locked Raman fiber ring laser with anultrahigh repetition rate of 100 GHz and up to 430 mW of average output power. This laser constitutes asimple wavelength versatile pulsed optical source. Stable mode locking is based on dissipative four-wavemixing with a single fiber Bragg grating acting as the mode-locking element. © 2006 Optical Society ofAmerica

OCIS codes: 140.3550, 140.4050, 140.3510, 190.4380.

In recent years, significant research has gone into in-creasing the repetition rate of laser sources for appli-cations ranging from telecommunications to ultrafastspectroscopy. An ideal source would provide high av-erage powers at a very high repetition rate, while atthe same time exhibiting very low amplitude andtiming jitter. Passive mode locking naturally leads tothe desired high repetition rates since, unlike activemode locking, it is not limited by electronic band-widths. Different passive mode-locking mechanismshave been implemented both in Er- and Yb-doped fi-ber lasers.1 Although repetition rates well above100 GHz have been obtained,2,3 these sources exhibitrelatively low average output powers. Additionally,their spectral operation is limited to the gain band ofthe rare-earth dopant. Raman lasers on the otherhand do not suffer from such a limitation, as Ramangain exists at all wavelengths, while providingenough bandwidth to support subpicosecond pulses.4

It is also well known to facilitate high average outputpowers, and continuous-wave (cw) commercial Ra-man fiber lasers (RFLs) have been widely availablefor some time now. For these reasons, the combina-tion of the high-power levels of Raman technologywith the mode-locking capabilities of fiber lasers hasa tremendous potential. Despite this, it is only re-cently that a passively mode-locked RFL has beendemonstrated.5 Although ultrashort femtosecondpulses have been successfully generated, the use of afigure-of-eight configuration restricted the repetitionrate to the megahertz region.

In this Letter, we report what we believe to be thefirst demonstration of a mode-locked RFL with an ul-trahigh repetition rate of 100 GHz. The passivemode-locking mechanism is dissipative four-wavemixing (DFWM).6,7 Similarly to the self-inducedmodulation instability laser,8 DFWM relies on four-wave mixing (FWM) to achieve the phase locking ofcavity modes. The pulse train is stabilized by the use

of a dissipative element that ensures a unidirectional

0146-9592/06/233489-3/$15.00 ©

power transfer from the fundamental modes to thehigher-order harmonics. In our case, this consists of aspecific fiber Bragg grating (FBG). However, despitethe similarities to the modulation instability laser,DFWM mode locking works in both the normal andanomalous dispersion regime.6,7

The experimental setup (Fig. 1) is similar to theEr-doped fiber laser reported in Ref. 3. Our fiber laseris made up of a ring cavity pumped by a cw RFL op-erating between 1448 and 1454 nm, with a maximumoutput power of approximately 4.5 W. Tuning thepump wavelength allows us to maximize the signalgain. The pump laser is coupled into the cavity bya 1450/1550 nm wavelength division multiplexer(WDM) and a second WDM is used to reject the de-pleted pump from the cavity. Note that the cavity ispumped in the backward direction, which we found toyield significantly better results, both in terms of am-plified spontaneous emission noise and output power.One kilometer of highly nonlinear fiber (HNLF) pro-vided by Sumitomo fibers acts as the main nonlinear

Fig. 1. Experimental setup. WDM, wavelength divisionmultiplexer; OC, output coupler; HNLF, highly nonlinearfiber; RFL, Raman fiber laser; FBG, fiber Bragg grating.

The inset shows the reflectivity of the FBG.

2006 Optical Society of America

3490 OPTICS LETTERS / Vol. 31, No. 23 / December 1, 2006

element. This fiber has a zero dispersion wavelengthof 1555 nm and a small dispersion slope of0.0323 ps nm−2 km−1, leading to slightly normal dis-persion at 1550 nm with D=−0.16 ps nm−1 km−1. Thefiber nonlinearity and the Raman gain coefficient arevery high, �=14 W−1 km−1 and CR=7 W−1 km−1, re-spectively, permitting the high output power and therelatively low threshold of the laser. The laser outputis monitored behind a 90/10 output coupler using anoptical spectrum analyzer or an intensity–autocorrelator. Finally, a polarization insensitive cir-culator reflects the intracavity light off a specially de-signed FBG, while simultaneously acting as anoptical isolator to ensure unidirectional operation ofthe laser.

The utilized FBG sets the central wavelength ofthe laser and functions as the main mode-locking el-ement by selecting a subset of the cavity modesaround 1550 nm (see inset of Fig. 1) in such a waythat only the central frequencies experience net posi-tive gain. These frequencies subsequently transfertheir energy to their higher-order harmonics byFWM. As a result of the phase-sensitive nature ofthis process, the phases of the generated modes arelocked and pulses are formed.7 The repetition rate ofthe laser is determined by the frequency-spacing be-tween the reflecting bands of the FBG, here 100 GHz.The 3 dB bandwidth of the individual bands is ap-proximately 3.75 GHz while the full width at half-maximum (FWHM) of the entire grating is 250 GHz.The maximum reflectivity of the two central modes isclose to unity, minimizing the loss.

The mode-locking process is self-starting. Once weincrease the pump power above the lasing thresholdof approximately 150 mW we observe a clear train ofpulses, which remains stable over time. Figure 2shows (a) the spectral density and (b) the autocorre-lation trace of the laser output for a pump power of2.2 W, yielding 77 mW of output power. The spectrumis made up of a collection of discrete sidebands sepa-rated by approximately 0.8 nm corresponding to the100 GHz mode separation of the FBG. However, thenumber of sidebands is significantly larger than thenumber of reflection bands of the filter, demonstrat-ing the efficiency of the FWM process. The autocorre-lation clearly shows a train of pulses separated by10 ps in agreement with the expected 100 GHz rep-etition rate. The central peak corresponds to the au-tocorrelation of a single pulse. It has a measuredFWHM of 930 fs, i.e., the generated pulses have atemporal duration of �600 fs (FWHM), assuming asech2 pulse shape. The side peaks in the autocorrela-tion trace are the cross-correlations between subse-quent pulses. Interestingly, the temporal width ofthese side peaks do not significantly differ from thatof the central autocorrelation peak. As these signalsresult from an average over a large number of pulses,this is a good indication that the timing jitter is low.Note that these side peaks have a smaller amplitudethan the central one. This is partly due to a delay-dependent loss of our autocorrelator but also to someresidual amplitude jitter caused by supermode noise.

Additionally, the autocorrelation trace is superim-

posed on a slight background, which is caused mainlyby a slight asymmetry of the central modes, resultingin a small cw contribution. This can, however, beavoided by careful adjustment of the pump wave-length.

As the cavity does not contain any polarization-selective elements, the laser output is almost com-pletely unpolarized. We measured a very low maxi-mum polarization extinction ratio of 0.35 dB that isstable over time. The random polarization of the la-ser can be attributed to the pump. The cw RFL emitsvirtually randomly polarized light. Therefore all pos-sible polarizations experience the same gain, andmode-lock simultaneously. Theoretically the stabilityof the laser should be improved by introducing a po-larizing element and polarization controllers into thecavity, but we could not observe any appreciable dif-ference. Finally, we would like to stress that eventhough the generated spectra look very similar, thereis a fundamental difference between our laser andsome multiwavelength lasers that are also designedwith an intracavity FBG.9 The FBGs used in thosemultiwavelength lasers have the same reflectivity ateach wavelength, i.e., they do not provide any differ-ential loss between the modes. This dissipation of thehigher-order sidebands that is present in our laser isprecisely the key for stable DFWM mode-locking.6

When we increase the cw pump power, we observean increase in the number of excited modes and awidening of the spectrum, whose width grows almost

Fig. 2. (a) Spectrum (linear scale) and (b) autocorrelationtrace of the mode-locked laser output at 2.2 W pump powerwith 77 mW output power.

linearly with pump power [see Figs. 3(a) and 3(c)].

December 1, 2006 / Vol. 31, No. 23 / OPTICS LETTERS 3491

This can simply be understood by considering thatFWM gets more efficient at higher power levels andstimulates a cascade of mixing processes that trans-fer power to higher and higher harmonics. In thetime domain, the widening of the spectrum is associ-ated with a shortening of the pulses. The autocorre-lation traces plotted in Fig. 3(b) for two differentpump powers clearly show this trend. We have alsoplotted in Fig. 3(d) the FWHM of the autocorrelationpeak versus pump power. Clearly, the pulse width de-creases with increasing pump power. From the spec-tral and the autocorrelation widths, Figs. 3(c) and3(d), we can also calculate the time–bandwidth prod-uct of the pulses. Assuming a sech2 pulse shape, weget values between 0.53 and 0.36 over the range ofpump powers shown, which reveals that the mode-locked pulses generated by our laser are almosttransform limited.

Fig. 3. (a) Spectra (linear scale) and (b) autocorrelationsfor 1.1 W (gray bottom curve) and 2.6 W (black top curve)pump power. (c) Spectral and (d) autocorrelation width as afunction of pump power.

Fig. 4. Laser output power as a function of pump powerwith a 20% output coupler.

Finally we were able to increase the output powerof our mode-locked laser further by replacing the90/10 output coupler with an 80/20 coupler. Figure 4shows the output power as a function of pump power.Due to the higher cavity loss introduced by the newoutput coupler, the laser threshold has now increasedto 320 mW. From there, we observe the expected lin-ear increase of the output power with pump powerwith a slope efficiency of 11%. The laser reaches amaximum output power of 430 mW at 4.3 W pumppower without showing any signs of saturation. Wehave checked that the laser was still mode locked atthat power level. To the best of our knowledge thisoutput power is the highest reported for a mode-locked fiber laser with a similar repetition rate.10

In conclusion we have demonstrated mode lockingof a Raman fiber laser at a repetition rate of 100 GHzwith a stable average output power of 430 mW. Thepower can still be increased further by decreasing thecavity losses and more careful dispersion manage-ment. The mode-locking process is very simple andself-starting, relying only on a specifically designedfiber Bragg grating. As the mode-locking process re-lies on four-wave mixing and Raman gain, it is inde-pendent of the gain bands of rare-earth dopants andcan easily be adapted to other wavelengths. In futurestudies, we also plan to incorporate supermode noisereduction techniques into the cavity design to reducethe amplitude jitter and to improve the pulse charac-teristics. We believe that this laser has tremendouspotential as a source for future optical applications,as it delivers stable high average output powers atultrahigh repetition rates.

The authors thank Sumitomo Electric for providingthe HNLF. S. Coen and J. Schröder(j.schroeder@auckland.ac.nz) acknowledges supportfrom the Marsden Fund of the Royal Society of NewZealand.

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