On wave-breaking free fiber lasers mode-locked with two saturable absorber mechanisms

On wave-breaking free fiber lasersmode-locked with two saturable

absorber mechanisms

Axel Ruehl,1 Dieter Wandt,1 Uwe Morgner1,2 and Dietmar Kracht1

1Laser Zentrum Hannover e.V., Hollerithallee 8, 30419 Hannover, Germany

[email protected]

2Institut fur Quantenoptik, Leibniz Universitat Hannover, Welfengarten 1, 30167 Hannover,Germany

Abstract: We propose a hybrid mode-locking scheme for wave-breakingfree fiber lasers based on a saturable Bragg reflector and the nonlinearpolarization evolution in the fiber section. With this scheme, the self-startingoperation is ensured by the saturable Bragg reflector while the nonlinearpolarization evolution acts as an additional pulse shaper in the steady state.Owing to the sensitivity of the pulse dynamics to filtering effects, a tuningrange of more than 10 nm as well as the suppression of undesired modesof operation became possible. The impact of the modulation depth andthe non-saturated losses is discussed via comparative measurements withdifferent saturable Bragg reflectors.

© 2008 Optical Society of America

OCIS codes: (140.3510) Lasers, fiber; (140.4050) Mode-locked lasers.

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#94559 - $15.00 USD Received 1 Apr 2008; revised 14 May 2008; accepted 15 May 2008; published 20 May 2008

(C) 2008 OSA 26 May 2008 / Vol. 16, No. 11 / OPTICS EXPRESS 8181

12. A. Ruehl, O. Prochnow, D. Wandt, and D. Kracht, “Hybrid mode-locking scheme for similariton fiber lasers,” inConference on Laser and Electro-Optics, CLEO Europe 2007 (Optical Society of America, 2007), paper CJ1-5-WED.

13. A. Ruehl, O. Prochnow, M. Engelbrecht, D. Wandt, and D. Kracht, “Similariton fiber laser with a hollow-corephotonic bandgap fiber for dispersion control,” Opt. Lett. 32, 1084-1086 (2007).

14. http://www.batop.de/products/saturable absorber/SAM/SAMs 1040.html.15. R. Paschotta and U. Keller, “Passive mode locking with slow saturable absorbers,” Appl. Phys. B 73, 653-662

(2001).16. A. Komarov, H. Leblond, and F. Sanchez, “Multistability and hysteresis phenomena in passively mode-locked

fiber lasers,” Phys. Rev. A 71, 053809 (2005).17. F. Krausz, T. Brabec, and Ch. Spielmann, “Self-starting passive mode-locking,” Opt. Lett. 16, 235-237 (1991).18. O. Prochnow, A. Ruehl, M. Schultz, D. Wandt, and D. Kracht, “All-fiber similari-

ton laser at 1μm without dispersion control,” Opt. Express 15, 6889-6893 (2007),http://www.opticsinfobase.org/abstract.cfm?URI=oe-15-11-6889.

19. H. Leblond, M. Salhi, A. Hideur, T. Chartier, M. Brunel, and F. Sanchez, “Experimental and theoretical study ofthe passively mode-locked ytterbium-doped double-clad fiber laser,” Phys. Rev. A 65, 063811 (2002).

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21. M. Horowitz, Y. Barad, and Y. Silberberg, “Noiselike pulses with a broadband spectrum generated from anerbium-doped fiber laser,” Opt. Lett. 22, 799-801 (1997).

1. Introduction

Passively mode-locked fiber lasers were studied intensively during the last decades and becameuseful light sources for various applications. Passively mode-loked fiber lasers generating par-abolic or more general wave-breaking free pulses drawed particular attention as reduced peakintensities inside the resonator offer the possibility to scale up the pulse energy [1,2]. The pulseformation is based on an interplay between positive group-velocity dispersion (GVD) and self-phase modulation (SPM). During propagation in a fiber section or amplifier this interplay leadsto optical wave-breaking unless the pulses are reshaped in the nonlinear regime resulting ina self-similar propagation [3]. In a laser the evolution towards wave-breaking is interruptedby filtering effects of the limited gain bandwidth and the mode-locking mechanism, respec-tively [4]. This is in contrast to the stretched-pulse regime where the self-amplitude modulation(SAM) of the mode-locking mechanism is only needed for the initialization and stabilizationof the inherently stable pulses. The steady state itself relies only on the reciprocal phase be-haviour of SPM and GVD inside the fibers. In the positive dispersion regime where parabolicand wave-breaking free pulses can be generated, the phase behaviour of SPM and GVD is cu-mulative which explains the necessity for and sensitivity to (self-) amplitude modulation in thesteady-state. Filtering effects can therefore be used to control the output characteristic.

It has been shown that additional amplitude modulation provided by spectral filters leadsto a stabilization of the pulse train and dispersion control could be set aside [5]. Even linearfilters reduce the influence of the nonlinear polarization evolution (NPE) in the steady statenow acting as an additional pulse shaper [6, 7]. The corresponding approach with nonlinearfilters (namely saturable absorbers) can additionally stabilize the pulse train during built-up.The utilization of two saturable absorber mechanisms in ultrafast fiber lasers has indeed beendiscussed in previous publications [8–10] but their interplay in the parabolic pulse and wave-breaking free pulse regime, respectively has, to the best of out knowledge, not been addressedso far.

In this work we investigate the role of saturable Bragg reflectors (SBR) as nonlinear tem-poral filters implemented in a wave-breaking free fiber laser acting together with the NPE in thefiber section. To point out the role of the SBRs, a comparison with the same oscillator solelymode-locked by NPE has been made. We will demonstrate that the nonlinear characteristic ofthe SBR strongly influences the pulse build-up leading to an enhanced self-starting capability.



Owing to the strong preference for the mode-locked state, a cw-operation of the laser becamerather impossible. The hybrid mode-locked laser was tunable over more than 10 nm by chang-ing the feedback of the NPE. In addition, the spectral width could be adjusted from 18.5 nmto 34 nm. The adjustable nonlinear characteristic allows the suppresion of competitive chaoticpulsation. A short description of some parts of this work was previously reported in [12].

2. Experimental setup

The fiber laser used for our experiments is sketched in Fig. 1 and was also described in previ-ous publications [4, 12, 13]. For the experiments we used different commercial SBRs (BatopGmbH) based on GaAs/AlAs Bragg mirrors and InGaAs quantum wells in front of the mirrorimplemeted in a sigma-branch. Owing to their non-resonant design, the dispersion slopes arealmost flat and negligible. The relaxation time constants of the SBRs were measured by pump-probe technique in a Mach-Zehnder interferometer and the other parameters were taken fromthe datasheets [14]. The basic data of the AR-coated devices are summarized in Table 1. Toovercome their saturation threshold, tight focussing with AR-coated lenses was used. The focallengths were choosen in a way that all SBRs were operated at a factor of about 6 above theirsaturation fluence, so the SAM can be regarded as beeing saturated [15]. The comparison withthe laser mode-locked by NPE only was realized by replacing the SBR with a highly reflectingmirror.

Fig. 1. Experimental setup. QWP: quarter wave-plate; HWP: half wave-plate; PBS: polariz-ing beam splitter; ISO: polarization-dependent Faraday isolator; SBR: saturable Bragg re-flector; HR: highly reflecting mirror; SMF: single-mode fiber; WDM: wavelength-divisionmultiplexer.

In order to adjust the polarization state for the NPE in the fiber section (4.8 m longsingle-mode fiber (SMF), 0.14 m long highly doped ytterbium gain fiber and 0.44 m SMF)wave-plates were applied. The energy exchange between the polarization modes owing tocross-phase modulation leads to an intensity dependent rotation of the polarization state whichis transfered into SAM by the polarization beamsplitter behind the fiber section. To change thefeedback of the NPE we varied the half wave-plate behind the fiber section (HWP) and thequarter-wave-plate in front of the fiber section (QWP 2) while the quarter wave-plate behindthe fiber section (QWP 1) remained fixed [16]. The position of QWP 1 was aligned in a waythat the polarization state was linear in the central part of the pulse. To control the cavitydispersion, a compressor based on diffraction gratings (900 grooves/mm) was implemented.Throughout the experiments presented here, the cavity dispersion was fixed at 0.015 ps 2 whichwas in center of the dispersion region for stable parabolic pulse operation found without the



SBRs [4].

Table 1. Overview about the basic data of the saturable Bragg reflectors.

SBR Modulation Non-saturable Saturation Relaxation timedepth loss fluence constants

SAM-1040-30 20 % 10 % 120 μJ cm−2 320 fs, 2.0 ps

SAM-1040-40 24 % 16 % 130 μJ cm−2 680 fs, 2.4 ps

SAM-1040-65 35 % 30 % 20 μJ cm−2 610 fs, 2.5 ps

3. Enhancement of the self-starting capability

In general, the formation of ultrashort pulses results from a continuous shortening of the mostintensive mode-beating fluctuations of the free-running laser. The saturable absorber mecha-nism equalizes the distance between the longitudinal modes which is inter alia inhibited bydispersion and Fabry-Perot effects leading to uneven mode-spacing. The tendency of a laser toself-start can therfore be determined by the correlation time of the modes which characterizestheir tendency to loose coherence by random phase pertubations [17].

Krausz et al. proposed a scheme to measure this correlation time τ c by observing the3dB-width of the first RF-beat note and the number of modes present in the free-runninglaser [17]. The intracavity power P required for self-starting can be related to τ c accordingto κ · P > 1

lnm · TRτc

where m is the initial number of modes and κ a constant of the nonlinearity.

In a typical fiber laser mode-locked by NPE using 1-m length of fiber, κ ≈ 10 −4 W−1. Detailshow to determine κ for other mode-locking schemes were not given. The correlation time isτc = 1

π · 1Δν3dB

where Δν3dB is the 3-dB full width of the first beat note in the RF-spectrum.It should be noted that this approach acts on the assumption of zero cavity dispersion and ab-sent Fabry-Perot effects. Nevertheless, the comparison of different mode-locking schemes inthe same setup allows for reasonable conclusions about their influence on the mode-lockingthreshold.

The RF-beat note measured with a resolution bandwidth of 10 Hz and the optical spectrumare shown in Fig. 2(a) and (b) for the SAM-1040-40. The sharp beat note in (a) gives a correla-tion time of 600 μs. The number of modes was estimated at -20 dB to 76,000, whereas we en-sured that the intensity of amplified spontaneous emission (measured by blocking the resonator)was at least 5 dB below that level. The number of modes was estimated close to the ASE back-ground as these spectral components can contribute significantly to the duration of the modebeat fluctuation. The number of modes is indeed not exactly determinable but the logarithm ofthis value changes the result only slightly. A similar behaviour was also oberserved with theother SBRs and the results are summarized in Table 2. For comparison, the measurements ofthe laser mode-locked only by NPE are displayed in Fig. 2 (c) and (d). The beat note is muchbroader and the corresponding correlation time is reduced to 150 μs. The optical spectrum hasindeed a much broader FWHM but the intensity drops down rapidly so that only 11,000 modeshave been measured at -20 dB. Assuming κ ≈ 5 ·10−4 W−1m−1 this corresponds to a intracav-ity power of about 60 mW required for self-starting. The laser starts its mode-locked operationwith an output power of 23 mW. Taking the required intracavity power of 60 mW into account,this corresponds to an output coupling ratio of about 30 % which is a small but realistic valuefor NPE mode-locking.



Fig. 2. (a) First RF-beat note and (b) optical spectrum of the free-running hybrid mode-locked laser with SAM-1040-40. (c) and (d) corresponding measurement of the laser mode-locked by NPE only.

The required power for self-starting with the hybrid mode-locking scheme could not bededuced as κ could not be quantified. However, the minimum κ ·P required for self-startingis decreased by nearly one order of magnitude by all SBRs. Consequently the laser producedultrashort pulses just above laser threshold. Cw-operation could only be found in a very limitedrange of wave-plate settings and was unstable. Usually, the laser switches either to pulsedoperation or runs out within a few seconds. The hysteresis region between cw and pulsedoperation which is a characteristic of passively mode-locked fiber lasers vanished completely.

Table 2. Experimental results on the self-starting capability.

Mode-locking scheme number of modes correlation time minimum κ ·PSAM-1040-30 & NPE 82,000 440 μs 6.5·10−6

SAM-1040-40 & NPE 76,000 600 μs 4.8·10−6

SAM-1040-65 & NPE 110,000 530 μs 5.3·10−6

NPE only 11,000 150 μs 3.4·10−5

The different behaviour during pulse build-up becomes obvious by recalling that the pulseshortening ratio of a saturable absorber whose net gain window is determined by the pulseenergy like an SBR does not depend on the pulse duration. In contrast, the pulse shortening ratioof an artificial saturable absorber based on a Kerr-nonlinearity like the NPE indeed depends



on the pulse duration. Therefore, the pulse build-up occurs much slower resulting in a highermode-locking threshold.

The enhancement of the self-starting capability is beneficial when the absorber action of theNPE is too weak to mode-lock a laser alone. As already mentioned this is a particular problemfor oscillators with a large cavity dispersion. To demonstrate this, we set the dispersion controlaside and operated the oscillator at a cavity dispersion of 0.167ps 2. The 3 dB-width of theattained spectrum was 4.9 nm corresponding to a Fourier-limited pulse duration of 428 fs. InRef. [18] we also demonstrated mode-locking of an all-fiber version of the proposed setupwith a cavity dispersion of 0.15 ps2. In both setups, the initialization of pulsed operation wasnot possible without the additional amplitude modulation of an SBR.

4. Tuning of the spectral output characteristic

Another advantage of the proposed setup is the tunability of the spectral output resulting fromthe pulse shaping of the NPE as soon as the steady state is reached. In Fig. 3(a), the stabilitydiagram introduced in Ref. [19] in the plane of the position of HWP and QWP 2 (whereasQWP 1 was fixed at the same position) is displayed for SAM-1040-30 and SAM-1040-65. Themeasurement with SAM-1040-40 showed similar stability regions and is not displayed for thesake of clarity. As can be seen, the NPE feedback can be variied over a wide range withoutleaving the mode-locked state. It is to mention that the mode-locked region also contains smallregions with irregular pulse trains. These states depend on various experimental parameters andcould not be fully reproduced in subsequent measurements. Therefore, no distinction betweenregular and irregular pulse trains was drawn in Fig. 3(a). The graph shows also clear hysteresisregions between the mode-locked state and no laser operation. Naturally, self-starting of thelaser is not possible in the bistability regions. Cw-operation occurs only in a limited range ofwave-plate positions and was unstable as mentioned above. Therefore it could not be resolved inthis measurement. It is remarkable that again the modulation depth as well as the non-saturatedlosses of the SBRs had only minor influence once the laser is mode-locked. Beside the enhance-ment of the self-starting capability, this is another indication that the SBR is dominant duringpulse built-up whereas the steady-state is mainly influenced by the NPE. This conclusion isconsistent with observations made in other operating regimes of ultrafast oscillators [8, 11]. Incontrast, the stability diagram of the laser mode-locked by NPE only (displayed in Fig. 3(b))showed only two localized regions of wave-plate settings for mode-locked operation so thepotential of tuning the output characteristic was minimal.

The hybrid mode-locked laser could be tuned by changing the feedback of the NPE. In themeasurement shown in Fig. 4(a) we varied QWP 2 in the setup with SAM-1040-40 and shiftedthe central wavelength from 1022 nm to 1032 nm. Also the spectral FWHM can be adjusted byvariing QWP 2 as can be seen from Fig. 4(b). In this measurement, HWP and QWP 1 were atdifferent positions related to Fig. 4(a) and the FWHM could be tuned from 18.5 nm to 34 nm.Other experimental series showed similar tuning ranges. For all settings of the wave-plates, thepulses could be externally dechirped within 10 % of the Fourier-limit as known for the parabolicand wave-breaking free regime, respectively [4]. Within the tuning ranges presented, the pulseenergy variied between 2 nJ and 3.5 nJ which is due to changed output coupling ratios. In bothmode-locking schemes, the obtainable pulse energy was only limited by the available pumppower.

Tunability is of particular importance when subsequent amplifier media have only smallgain bandwidths like Yb:KGW or Yb:KYW. With the additional SBR the gain of this amplifiermedia can be utilized completely by adapting the central wavelength as well as the spectralenergy density. Another possible applications of tunable fs-pulses is the optimization of thespectral output during supercontinuum generation in microstructured fibers or frequency con-



(a) Hybrid mode-locked laser with SAM-1040-30(blue +) and SAM-1040-65 (red x), respectively.

(b) Laser mode-locked by NPE only.

Fig. 3. Stability diagram of the laser in the plane of the positions of the half wave-plate andthe quarter wave-plate 2.

(a) Tuning of the central wavelength. (b) Tuning of the spectral FWHM.

Fig. 4. Evolution of the optical spectra when variing the position of QWP 2 in the setupwith SAM-1040-40.

version in nonlinear crystals.

5. Suppression of undesired modes of operation

NPE pulse shaping can not only be used to control the output properties of the pulses but alsofor the suppression of undesired modes of operation. Parabolic or wave-breaking free pulses aregenerally unstable against the cw-state before a certain cavity dispersion threshold is exceeded,as theoretically shown in Ref. [20]. The elimination of cw-operation was already mentionedin section 3 and is also underlaid with the fact that the minimum cavity dispersion for stablepulsed operation decreased from 0.010 ps2 to 0.005 ps2 when an SBR was implemeted. Evenabove this threshold, cw-backgrounds occured in the setup without the SBR which were highlysensitive to many experimental parameters like wavelength dependent coupling into the SMFor losses. These parameters can change by and by owing to mechanical instabilities of thehardware. With the SBR implemented in the cavity these problems vansished and the laserbecame much more long term stable. Also mechanical or temperature induced disturbances of



Fig. 5. Transition from bunched noise-like operation to wave-breaking free operation takenwith a photonic bandgap fiber for dispersion control by changing the position of QWP 2.Single pulse operation corresponds to the lowest spectrum whereas the other correspondsto bunched noise-like pules.

the fiber section did not affect the mode-locked pulses as much as in the setup without the SBR.But even within the mode-locked states there are competing regimes. The most trouble-

some in the laboratory was the build up of chaotic pulsation without any order structure referedto as bunch noise-like pulse formation. It can be attributed to the combined effect of polariza-tion mode dispersion, gain response and a nonlinear transmission element [21]. Owing to theadditional birefringence of a photonic bandgap fiber (PBF) implemented for dispersion control,this mode of operation caused particular trouble [13]. Nevertheless, it occured also in the setupwith the grating compressor for intracavity dispersion control.

When the bulky grating compressor was replaced by the 4.05 m long piece of PBF (CrystalFibre A/S HC-1060-02), the roundtrip time was approximately doubled. The initialization ofpulsed operation was not possible without the SBR (SAM-1040-40) as the NPE action alonewas too weak [7]. The output characteristic of the bunched noise-like pulses is dislayed inFig. 5 where the top spectrum corresponds to the autocorrelation trace shown in Fig. 6(a). Thebroad spectrum contains many pulses which were generated and destroyed randomly. This isexpressed by the sharp coherence spike apearing on a broad shoulder in the autocorrelationtrace indicating incomplete mode-locking. By turning QWP 2 this mode of operation can beconvicted into wave-breaking free operation shown in Fig. 5 by the lowest spectrum and inFig. 6(b), respectively. The optical spectrum showed indeed some structure (see Ref. [13] fordetails) but the chirped as well as the dechirped autocorrelation trace was a Gaussian. No ad-ditional pulse was observable even with the long-range autocorrelator (150 ps scanning range)or the fast photodiode/oscilloscope (2 GHz resolution). Owing to an optimized feedback of theNPE the bunch of pulses merged together to a single pulse circulating in the resonator.



(a) Bunched noise-like pulse operation. (b) Wave-breaking free pulse operation.

Fig. 6. Autocorrelation traces taken with a photonic bandgap fiber for dispersion control.

6. Conclusion

In conclusion, we discussed the application of two saturable absorber mechanisms in wave-breaking free fiber lasers at 1 μm. The additional saturable Bragg-reflector reduced themode-locking threshold drastically, stabilized the laser against competitive cw-operation andfacilitated the suppression of chaotic pulsations. Based on our results we conclude that themodulation depth and non-saturated loss of the saturable Bragg-reflector has only a minorinfluence on this behaviour. The nonlinear polarization evolution in the fiber section acts asan additional pulse shaper. Owing to the sensitivity of the pulse evolution in the steady-statethis filtering effect can be used to tune the spectral output characteristic. A tuning range from1022 nm to 1032 nm has been demonstrated. Also the spectral width could be adjusted from18.5 nm and 34 nm. The tunable oscillator presented here is a versatile fs-light source allowingfor spectral adaption to subsequent amplifiers or frequency conversion stages. Strikinglyspeaking, with the proposed setup one can benefit from the performance of a slow saturableabsorber during pulse built-up without loosing the advantages of a fast one in the steady-state.

Acknowledgment

The authors thank Marcel Schultze (Leibniz Universitat Hannover, Germany) for measuringthe relaxation time constants of our SBRs and the Deutsche Forschungsgemeinschaft (DFG)for their financial support under grant SFB 407.



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On wave-breaking free fiber lasers mode-locked with two saturable absorber mechanisms