Optics Communications 285 (2012) 2715–2718

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Optics Communications

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Wavelength tunable, high energy femtosecond laser pulses directly generated fromlarge-mode-area photonic crystal fiber

Chi Zhang, Yu-ying Zhang, Ming-lie Hu ⁎, Si-jia Wang, You-jian Song, Lu Chai, Ching-yue WangUltrafast Laser Lab, School of Precision Instruments and Optoelectronics Engineering, Key Laboratory of Optoelectronic Information Technical Science, Tianjin University, 300072 Tianjin,P.R. China

⁎ Corresponding author. Fax: +86 227404204.E-mail address: huminglie@tju.edu.cn (M. Hu).

0030-4018/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.optcom.2012.02.011

a b s t r a c t

a r t i c l e i n f o

Article history:Received 14 April 2011Received in revised form 3 January 2012Accepted 6 February 2012Available online 19 February 2012

Keywords:Mode-lockingLarge mode area photonic crystal fiberFiber laser

Wavelength tunable high energy ultrashort laser pulses are generated from a large-mode-area photonic crys-tal fiber in anomalous dispersion (AD) regime. A simplified laser cavity design with one fine polished facet ofthe fiber as a cavity mirror is used. The intra-cavity dispersion compensation is achieved by a grating pair, thespatial dispersed light from which also have optical spectrum filtering effects combined with the limited ap-erture of the fiber core. The laser system is able to generate ultrashort pulses ranging from 494 fs (with 56 nJpulse energy) to 1.24 ps (with 49 nJ pulse energy) at 55 MHz repetition rate. The filtering mechanism bene-fits the generation of high energy pulses with narrowing pulse duration in AD regime. An undulation infrequency and time domain is also observed with the increase of the pump power. Furthermore, this lasersystem is directly used as seed for supercontinuum generation.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

The ultrafast fiber lasers develop rapidly towards high power andhigh pulse energy. Traditional fibers restricted the further improve-ment of laser pulse energy because of their limited core diameter,which accumulates significant nonlinear chirp during pulse propaga-tion, and finally causes pulse breaking [1]. By increasing the mode-area of fiber, higher energy pulses were supported due to the reduc-tion of nonlinearity. However, propagation in a large-mode-area(LMA) fiber is always in multi-mode. Robust and environmentallystable fundamental mode operation can only be achieved in a trulysingle-mode fiber [2]. Microstructuring the LMA fiber by introducingair-holes and special structures can support both high pulse energyand excellent single mode operation [3,4]. This kind of fiber is so-called LMA photonic crystal fiber (PCF).

The LMA PCF has been widely used for high energy ultrafast pulsegeneration directly from laser oscillators [5–10]. In all-normal disper-sion (ANDi) regime, mode-locking pulses are highly chirped withbroad pulse widths. Based on LMA PCF, 25 nJ pulses were generatedin a fiber oscillator directly in (ANDi) regime [5]. By using rod-typeLMA PCF, pulses up to 265 nJ were reported [6]. In a recentlyupgraded system, pulse energy even reached 927 nJ [7]. However,the above LMAPCF lasers are based on complicated ring or sigma cav-ity configuration. We have simplified the bulky setup and reduced thefootprint of the LMA PCF based fiber laser system by using a direct

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fiber facet output configuration in an earlier work [8]. With an LDpump, an Yb-doped LMA PCF as gain medium, and a SESAM for satu-rable absorption and self-starting, mode-locking is achieved with46 nJ pulse energy and 1.9 ps duration in the ANDi regime.

In this work, we have applied a pair of gratings for the intracavitydispersion compensation. In this configuration, the round trip groupvelocity dispersion (GVD) of pulses is negative, leading the oscillatorwork in AD regime. Combination of grating pair and limited fiber coresize also results in spectral filtering. This filtering mechanism contrib-utes to the direct generation of high energy pulses up to 50 nJ withsub-500 fs duration in AD regime. Wide tuning range of pulse duration,spectral width and center wavelength is achieved. Filter-induced undu-lation in frequency and time domain is observed with the increase ofoutput power. Furthermore, the laser oscillator is directly applied togenerate supercontinuum, achieving a high power compact superconti-nuum resource.

2. Experimental setup

The experimental setup is shown in Fig. 1, which is a typical Fabry-Perot cavity. A LMA PCF is used as the active fiber and laser deliveringpigtail. The cross section of the fiber is shown in Fig. 1a. It has a typicaldouble cladding structure. The outer air cladding contributes to anumerical aperture (NA) of 0.6 for the inner cladding, which acts asthe core for pump laser at 976 nm. The inner cladding has a smallNA=0.03 at 1040 nm. The inner core has a mode field area of660 μm2, which can effectively reduce the accumulation of the nonli-nearity during pulse propagation, and thus support high energy ultra-short pulses. The LMA PCF has a pump light absorption of 9 dB/m.By adding anisotropic material in the inner cladding, the LMA PCF

Fig. 1. Experimental setup. LD, laser diode; DM, dichroic mirrors; λ/2, half wave plate;HR, high reflector. AL, asphere lens. Insets, a is the cross section of LMA PCF; b is 8°polished end; c is 0° polished end; d is the cross section of polished LMA PCF; e isthe output mode near field distribution; and f is the fluoresce distribution after grating.

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supports single polarization operation with a polarization extinctionratio of 15 dB and a bandwidth of 100 nm around 1040 nm. Thegroup velocity dispersion (GVD) of the fiber is about 0.019 ps2/mat 1040 nm. One facet of the fiber is polished to 8° (inset b inFig. 1) to avoid parasitic reflection and self-excitation. The otherfacet is fine polished to 0° (insets c and d in Fig. 1) after collapsingand serves as one cavity mirror. It can provide enough feedback togenerate high power laser and deliver it with nice fundamentalmode (the inset e in Fig. 1, the M2 value of the laser beam isabout 1.2). Laser beam from 8° end is spatially dispersed by thegrating pair to elliptical shape (inset f in Fig. 1, measured at fluo-rescence operation condition) before being focused on the semi-conductor saturable absorber mirror (SESAM), which works asthe other cavity mirror. In this cavity, together with the limitedfiber core size, the gratings not only act as the dispersion compen-sator, but also as an ambulatory and variable bandwidth spectrumfilter. The laser beam is spatially expanded via the grating pair.After passing the lens and reflected by the SESAM, the short wave-length component and long wavelength component exchangetheir spatial positions. Then the beam passes gratings again result-ing in an even wider spread (as shown in the dashed square inFig. 1). Due to the small NA of the fiber core, only part of the spec-trum components can feed back into the core to achieve lasing andmode-locking, thus the spectral filtering occurs. Such configura-tion is also used in another paper for high repetition rate solitonmode locking [11]. However, it is the first time that we study andutilize its filtering effect to achieve high power operation in soli-ton mode-locked lasers.

The LMA PCF is pumped by a 976 nm high power laser diodethrough a fiber coupler (NA=0.22). The AL1 and AL2 work as anoptical collimator for pump and laser light, respectively. At 20°incidence angle, the dichroic mirror (DM) has a reflection ratioof 99% at 1040 nm, and a transmission ratio of 96% at 976 nm. Itis used to separate laser beam from pump light. A pair of 600lines/mm diffraction gratings is used for dispersion compensa-tion and spectrum filtering. A half-wave plate is inserted betweenthe grating pair and DM to achieve the optimized diffraction effi-ciency. The SESAM (BATOP GmbH) has a small signal absorptionof 65%, a modulation depth of 35%, a saturation fluence of 20 μJ/cm2, and a relaxation time of 500 fs. AL3 (f=8 mm) is used tofocus laser on SESAM to achieve the desired saturation intensity.A 200 ps rising time photodiode and a 4-GHz sampling oscillo-scope are used to monitor pulse train and to check multi-pulseoperation. A high resolution spectrometer and a long scan-rangeautocorrelator are used to measure optical spectrum and pulseduration.

3. Results and discussion

Mode-locking is achieved by optimizing saturation properties ofthe SESAM in terms of the spot size and the pulse energy focusedon it. The spectral width of the laser output can be tuned by adjustingthe distance of gratings while fixing the incident angle. The larger thegrating pair distance is, the wider the laser beam is expanded, and themore spectrum components are filtered. The mode-locking can betuned fromwide-spectrum filtering (WSF) to narrow-spectrum filter-ing (NSF). Correspondingly, the pulse duration can be tuned from~500 fs to several ps. At 0° incident angle of gratings and 21 mm grat-ing distance, the laser works in theWSF condition. The net-dispersionof the cavity is −0.006 ps2 (0.057 ps2 from LMAPCF and −0.063 ps2

from the grating pair) per round trip. The spectrum at the maximumoutput is shown in curve 1 of Fig. 2a. The spectrum width is about6.2 nm. Curve 1 of Fig. 2b shows the auto-correlation trace indicating494 fs pulse duration (assuming a sech2

fitting). The highest outputpower is 3.1 W with 55 MHz repetition rate, corresponds to 56 nJpulse energy. At the same incident angle, with 30 mm grating dis-tance, the net dispersion of the cavity is −0.033 ps2 (0.057 ps2 fromLMAPCF and −0.090 ps2 from the grating pair) per round trip. Thespectrum and pulse autocorrelation trace of the maximum outputare shown in curve 4 of Fig. 2a and b. The spectrum is much narrower(about 1.5 nm) and the output pulse is wider (about 1.24 ps) due tothe stronger soliton formation at larger negative cavity dispersiontogether with stronger filtering of larger grating pair geometric sepa-ration. The laser works in the NSF condition with the maximumoutput power of 2.7 W at 55 MHz repetition rate (49 nJ pulse energy).

The spectra and pulse autocorrelation traces of the selected inter-mediate states in their maximum output conditions are shown incurves 2 and 3 of Fig. 2a and b. The 3.5 nm spectrum corresponds to596 fs output pulse, with 2.8 W average power and 51 nJ pulse ener-gy. The 2.8 nm spectrum corresponds to 951 fs, with 3.0 W averagepower and 54 nJ pulse energy. The maximum output power is limitedby the damage of SESAM. The output power properties of the lasersystem are shown in Fig. 2c. All operation regions (ranging fromWSF to NSF) almost follow the same slope efficiency. Fig. 2d showsthe radiofrequency spectrum of the pulse train. The first harmonicof the optical–electrical converted pulse train centers at 55 MHz.The signal to noise ratio reaches 75 dB at 1 kHz resolution bandwidth,which shows the good stability of the mode-locking. By slightlyadjusting the angle of the grating pair to change the spectral compo-nent focused on the SESAM, the central wavelength of the mode-locking can be continuously tuned to some extent in both WSFand NSF conditions. As shown in Fig. 3a, inWSF condition, the tunablecentral wavelength covers 15 nm bandwidth. In NSF condition, thecentral wavelength can also be tuned within a 10 nm range, as shownin Fig. 3b.

In mode-locking operation, the filter-induced undulations in spec-trum width and time duration are observed following the increase ofpump power. As shown in Fig. 4a, the spectrum width decreases from4.2 nm to 3.7 nm as the pump power increases from 5.4 W to 5.8 W.The corresponding pulse width also decreases from 457 fs to 444 fs.While continuing to increase the pump power, spectrum width in-creases steadily to 6.1 nm, as the pulse width increases to 462 fs.There is another significant decrease in both spectrum width andpulse width at the pump power of 6.9 W. The same phenomenon oc-curs at the pump power of 9 W. This effect is brought by the uniqueambulatory spectrum filtering. The increased pump power tends tobroaden the optical spectrum width due to the stronger self phasemodulation (SPM). However, the optical spectrum broadening is re-stricted by the limited filter bandwidth. The two contrary effects com-pete severely as the increase of pump power, which results in adisturbance for the mode-locking. The mode-locking is re-built withspectrum and pulse duration preferred for the balance betweenSPM effect and spectrum filtering. It is similar to the formation of

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properties of output power from WSF to NSF. (d) Radiofrequency spectrum of the mode-locking laser.

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the dissipation soliton in all-normal dispersion regime [8,12–14]. OnWSF condition, the laser tends to work in a stretched pulse regimeand the pulses accumulate significant amount of chirp. The filter-induced spectrum narrowing effect on the chirped pulse directlyshortens the output pulse duration. However, on NSF condition, onthe contrary, the soliton formation dominates. The narrow spectrumwidth corresponds to nearly transform limited pulses. Therefore, thepulse width increases when the spectrum width decreases at certainpump power level, as shown in Fig. 4b. At the intermediate state, theoutcome brings similarity to both WSF and NSF conditions, as shownin Fig. 4c.

The filter induced undulation in WSF is brought by the interplaybetween SPM broadening and filter bandwidth restriction, whichnarrows the output pulse duration automatically when the broaden-ing spectrum is wide enough to destabilize the mode-locking. InNSF mode locking condition, the spectrum is narrow and the undula-tion is determined by the near transform-limited intra-cavity lasers'pulses, resulting in a contrary phenomenon compared with WSFmode-locking condition. With unique self-starting properties ofSESAM, mode-locking can automatically reconstruct at the jumpingpoint of the undulations. The filtering effect in our cavity configura-tion in either WSF or NSF condition can stabilize the soliton andbenefits the generation of higher energy pulses in AD regime, i.e.higher peak power of pulses. However, if there is no intra-cavityfiltering effect, the optical spectrum will continue broadening with

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un-bounded pulse shortening, which results in soliton break up andrestrict the highest intra-cavity pulse energy.

To qualify this laser system, the tunable fiber laser is used as seedfor the generation of supercontinuum (SC) [15]. By using a 3 m highnonlinear PCF, SC is obtained directly with the femtosecond laserpulses in WSF condition. The typical experimental result is shown inFig. 4d. The central wavelength of the femtosecond laser pulses isaround 1040 nm, which is in the anomalous GVD regime of the non-linear PCF (λ0=1036 nm). The 2W femtosecond laser pulses with450 fs duration are coupled into the nonlinear PCF. An SC with thespan of more than one octave is generated from 650 nm to 1550 nm,as shown in Fig. 4d. The average power of the SC is about 550 mW,which results in a high power and compact SC resource.

4. Conclusion

In conclusion, we reported a high energy mode-locked fiber laserbased on LMA PCF with fiber facet output working in AD regime.The mechanism of filtering brought by a grating pair and limitedfiber core size is analyzed. By altering grating distance, the spectrumwidth of the mode-locking was tuned from WSF to NSF condition,with tunable output pulse duration. By slightly adjusting the incidentangle of gratings, the central wavelength can also be tuned to someextent. The laser directly generated 494 fs ultrashort pulses with56 nJ pulse in WSF condition and 1.24 ps pulses with 49 nJ in NSF

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condition. Filter-induced undulations of spectrum width and pulsewidth are observed and discussed. Experiment of supercontinuum ispresented to testify the performance of the fiber laser and to showits promising applications in nonlinear optics.

Acknowledgements

This work was supported by the National Basic Research Programof China (Grant Nos. 2011CB808101 and 2010CB327604), NationalNatural Science Foundation of China (Grant Nos. 60838004 and61078028), the Doctoral Program Foundation of Institution of HigherEducation of China (Grant No. 20110032110056), and FANEDD(Grant No. 2007B34).

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