Mode-locked 05 μJ fiber laser at 976 nm
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Mode-locked 0:5 μJ fiber laser at 976 nm J. Lhermite, 1, * C. Lecaplain, 2 G. Machinet, 1 R. Royon, 1 A. Hideur, 2 and E. Cormier 1 1 Centre Lasers Intenses et Applications, Université de Bordeaux–CNRS–CEA, UMR 5107, 351 Cours de la Libération F-33405 Talence, France 2 Université de Rouen-CNRS UMR 6614 CORIA, Avenue de l’Université, BP 12,76801 Saint Etienne du Rouvray, France *Corresponding author: [email protected]‐bordeaux1.fr Received June 20, 2011; revised September 2, 2011; accepted September 5, 2011; posted September 8, 2011 (Doc. ID 149387); published September 23, 2011 We report on high-energy femtosecond pulse generation from an ytterbium-doped rod-type fiber oscillator emitting around 976 nm. Self-starting and stable single-pulse operation are demonstrated with 4:2 W of average output power at a repetition rate of 8:4 MHz. The resulting energy level reaches 0:5 μJ. Because of the all-normal dispersion of the laser cavity, output pulses are naturally chirped with a duration of 14 ps. External compression using diffraction gratings shortens the pulse duration down to 460 fs. © 2011 Optical Society of America OCIS codes: 140.3510, 140.4050, 140.3615, 320.7090, 320.7140. In a short period of time, the performance of ultrafast fi- ber oscillators based on ytterbium and emitting around 1 μm has increased significantly in terms of pulse dura- tion, energy, and average power [1–7]. This breakthrough results from two factors: the better understanding of the pulse dynamics in mode-locked fiber lasers [8] and pro- gress in development of single-mode large-mode-area fibers. Indeed, the most appropriate approach for high- energy pulse generation in a mode-locked fiber laser con- sists in operating the laser in the all-normal dispersion regime to enable chirped pulse evolution along the cav- ity. As a consequence, nonlinear effects are significantly reduced inside the fiber core, thus allowing a potential increase of the pulse energy [9]. Alternatively, nonlinea- rities may also be reduced by using large active core fibers [10]. In fact, pulse energies approaching the micro- joule level have been achieved in mode-locked oscillators featuring large-mode-area ytterbium-doped photonic crystal fibers [6] and rods [4,5]. When optically pumped with laser diodes at 976 nm, amplification in Yb-doped fibers will naturally occur in the 1020–1080 nm spectral range. Yb-doped fiber lasers also have the ability to operate in the 976 nm vicinity under specific conditions. However, achieving efficient lasing on the “zero-phonon line” at 976 nm, i.e., in the three-level configuration, is a hard task if compared to the quasi-three-level system producing radiation around 1030 nm. The major difficulty in operating Yb-doped fiber lasers at 976 nm is the competing emission at 1030 nm, which is greatly favored due to a large difference be- tween absorption and emission cross sections at this wavelength. Conventionally, Yb-doped fiber lasers are optically pumped at 976 nm because the absorption cross section is at the maximum. In order to observe amplifica- tion at 976 nm, it is necessary to pump the fiber with lower available wavelengths (commercial laser diodes at 915 or 940 nm). However, in this case, the lasing pro- cess will still occur preferentially at 1030 nm, because the absorption cross section is negligible if compared to that at 976 nm. Therefore, effective lasing at 976 nm requires (i) a sufficient population inversion above 50% to reach transparency at 976 nm (high pumping level), a limited gain at 1030 nm compared to 976 nm (specific double- clad fiber geometry), and significant cavity losses at 1030 nm to prevent lasing at this wavelength (high dy- namics intracavity filters). Precursor works have been published in the late 1980s reporting on continuous emission at 976 nm by exploiting the three-level operation [11]. Since then, many varia- tions and configurations have been demonstrated in bulk or fiber media for the continuous and pulsed regime [12–15]. Recently, we succeeded in producing 94 W of continuous wave at 976 nm by implementing a laser op- erating on the three-level system in a rod-type fiber [12,13], thanks to the optimal fiber geometry. Later, thanks to the achievement of a new flexible fiber design, a nanosecond Q-switch millijoule-class laser at 976 nm has been demonstrated in a MOPA configuration [13]. In the quest of ever-increasing peak powers at the exo- tic wavelength of 976 nm, operating on the mode-locking regime is by far the best strategy. Ti:sapphire lasers have the potential to generate femtosecond pulses at 976 nm, thanks to the broad amplification bandwidth centered around 800 nm (see, for instance, products from MIRA of Coherent). However, such lasers only deliver pulses with a few nanojoules of energy. An alternative consists in associating a short Yb-doped fiber with a semiconduc- tor saturable absorber mirror (SESAM) to build a mode- locked laser delivering a few milliwatts and ∼100 pJ pulses with a duration of 1:6 ps [16]. More recently, based on a specific fiber design [14], we have demonstrated the first energetic mode-locked fiber laser emitting at 976 nm and delivering 480 mW of 12 nJ pulses [17]. Mode locking was achieved by nonlinear polarization evolution techni- que leading to pulses as short as 286 fs after external compression. Here, we report on the demonstration and character- ization of an all-normal dispersion mode-locked fiber laser featuring an Yb-doped rod-type fiber and emitting high-energy short pulses at 976 nm. The experimental setup is presented in Fig. 1. The gain medium consists of an Yb-doped rod-type fiber with a core diameter of 80 μm and a cladding of 200 μm. The active fiber is angle cleaved on both ends in order to avoid any parasitic lasing. It is pumped through a di- chroic mirror (M 1 ) by a laser diode emitting at 915 nm. The pump beam size is adapted to the fiber-clad dia- meter thanks to a couple of lenses. Because of the limited absorption at 915 nm, a significant amount of unabsorbed pump power emerges at the opposite side of the fiber. Thus, in order to increase the laser efficiency, the resi- dual pump is recycled and injected back into the fiber October 1, 2011 / Vol. 36, No. 19 / OPTICS LETTERS 3819 0146-9592/11/193819-03$15.00/0 © 2011 Optical Society of America
Transcript of Mode-locked 05 μJ fiber laser at 976 nm
Mode-locked 0:5 μJ fiber laser at 976 nmJ. Lhermite,1,* C. Lecaplain,2 G. Machinet,1 R. Royon,1 A. Hideur,2 and E. Cormier1
1Centre Lasers Intenses et Applications, Université de Bordeaux–CNRS–CEA, UMR 5107, 351 Cours de la Libération F-33405 Talence, France2Université de Rouen-CNRS UMR 6614 CORIA, Avenue de l’Université, BP 12,76801 Saint Etienne du Rouvray, France
*Corresponding author: [email protected]‐bordeaux1.fr
Received June 20, 2011; revised September 2, 2011; accepted September 5, 2011;posted September 8, 2011 (Doc. ID 149387); published September 23, 2011
We report on high-energy femtosecond pulse generation from an ytterbium-doped rod-type fiber oscillator emittingaround 976nm. Self-starting and stable single-pulse operation are demonstrated with 4:2W of average output powerat a repetition rate of 8:4MHz. The resulting energy level reaches 0:5 μJ. Because of the all-normal dispersion of thelaser cavity, output pulses are naturally chirped with a duration of 14ps. External compression using diffractiongratings shortens the pulse duration down to 460 fs. © 2011 Optical Society of AmericaOCIS codes: 140.3510, 140.4050, 140.3615, 320.7090, 320.7140.
In a short period of time, the performance of ultrafast fi-ber oscillators based on ytterbium and emitting around1 μm has increased significantly in terms of pulse dura-tion, energy, and average power [1–7]. This breakthroughresults from two factors: the better understanding of thepulse dynamics in mode-locked fiber lasers [8] and pro-gress in development of single-mode large-mode-areafibers. Indeed, the most appropriate approach for high-energy pulse generation in a mode-locked fiber laser con-sists in operating the laser in the all-normal dispersionregime to enable chirped pulse evolution along the cav-ity. As a consequence, nonlinear effects are significantlyreduced inside the fiber core, thus allowing a potentialincrease of the pulse energy [9]. Alternatively, nonlinea-rities may also be reduced by using large active corefibers [10]. In fact, pulse energies approaching the micro-joule level have been achieved in mode-locked oscillatorsfeaturing large-mode-area ytterbium-doped photoniccrystal fibers [6] and rods [4,5].When optically pumped with laser diodes at 976 nm,
amplification in Yb-doped fibers will naturally occur inthe 1020–1080 nm spectral range. Yb-doped fiber lasersalso have the ability to operate in the 976 nm vicinityunder specific conditions. However, achieving efficientlasing on the “zero-phonon line” at 976 nm, i.e., in thethree-level configuration, is a hard task if compared tothe quasi-three-level system producing radiation around1030 nm. The major difficulty in operating Yb-doped fiberlasers at 976 nm is the competing emission at 1030 nm,which is greatly favored due to a large difference be-tween absorption and emission cross sections at thiswavelength. Conventionally, Yb-doped fiber lasers areoptically pumped at 976 nm because the absorption crosssection is at the maximum. In order to observe amplifica-tion at 976 nm, it is necessary to pump the fiber withlower available wavelengths (commercial laser diodesat 915 or 940 nm). However, in this case, the lasing pro-cess will still occur preferentially at 1030 nm, because theabsorption cross section is negligible if compared to thatat 976 nm. Therefore, effective lasing at 976 nm requires(i) a sufficient population inversion above 50% to reachtransparency at 976 nm (high pumping level), a limitedgain at 1030 nm compared to 976 nm (specific double-clad fiber geometry), and significant cavity losses at1030 nm to prevent lasing at this wavelength (high dy-namics intracavity filters).
Precursor works have been published in the late 1980sreporting on continuous emission at 976 nm by exploitingthe three-level operation [11]. Since then, many varia-tions and configurations have been demonstrated in bulkor fiber media for the continuous and pulsed regime[12–15]. Recently, we succeeded in producing 94W ofcontinuous wave at 976 nm by implementing a laser op-erating on the three-level system in a rod-type fiber[12,13], thanks to the optimal fiber geometry. Later,thanks to the achievement of a new flexible fiber design,a nanosecond Q-switch millijoule-class laser at 976 nmhas been demonstrated in a MOPA configuration [13].
In the quest of ever-increasing peak powers at the exo-tic wavelength of 976 nm, operating on the mode-lockingregime is by far the best strategy. Ti:sapphire lasers havethe potential to generate femtosecond pulses at 976 nm,thanks to the broad amplification bandwidth centeredaround 800 nm (see, for instance, products from MIRAof Coherent). However, such lasers only deliver pulseswith a few nanojoules of energy. An alternative consistsin associating a short Yb-doped fiber with a semiconduc-tor saturable absorber mirror (SESAM) to build a mode-locked laser delivering a few milliwatts and ∼100 pJpulses with a duration of 1:6 ps [16]. More recently, basedon a specific fiber design [14], we have demonstrated thefirst energetic mode-locked fiber laser emitting at 976 nmand delivering 480mW of 12 nJ pulses [17]. Mode lockingwas achieved by nonlinear polarization evolution techni-que leading to pulses as short as 286 fs after externalcompression.
Here, we report on the demonstration and character-ization of an all-normal dispersion mode-locked fiberlaser featuring an Yb-doped rod-type fiber and emittinghigh-energy short pulses at 976 nm.
The experimental setup is presented in Fig. 1. The gainmedium consists of an Yb-doped rod-type fiber with acore diameter of 80 μm and a cladding of 200 μm. Theactive fiber is angle cleaved on both ends in order toavoid any parasitic lasing. It is pumped through a di-chroic mirror (M1) by a laser diode emitting at 915 nm.
The pump beam size is adapted to the fiber-clad dia-meter thanks to a couple of lenses. Because of the limitedabsorption at 915 nm, a significant amount of unabsorbedpump power emerges at the opposite side of the fiber.Thus, in order to increase the laser efficiency, the resi-dual pump is recycled and injected back into the fiber
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cladding by both the dichroic mirror (M2) and the normalincidence mirror with a maximum reflectivity at 915 nm(M3). The laser architecture consists of a ring cavity builtaround the active rod-type fiber. The mode-locked re-gime is obtained by inserting a SESAM with high modu-lation depth (∼30%) and fast relaxation time (∼500 fs) inthe linear part of the cavity. The spot size on the SESAMis adjusted in order to optimize the pulsed regime stabi-lity. The ring cavity includes an isolator set within half-and quarter-wave plates. A 20m long passive fiber with acore diameter of 20 μm is inserted in the laser cavity toprovide positive group velocity dispersion.Two dichroic mirrors (M4 and M9) reject the parasitic
signal at 1030 nm out of the cavity to prevent its ampli-fication, to the detriment of the 976 nm emission. Finally,the signal is injected in the Yb-doped rod-type fiber coreby a short focal lens. The laser beam is extracted fromthe cavity by a polarizing beamsplitter (BS), and the out-put coupling is adjusted by the half-wave plate locatedjust before.The present ring cavity configuration allows self-
starting operation of the mode-locked regime at a pumppower of 41W. A stable single-pulse emission is obtainedby optimizing the saturation threshold on the SESAM to-gether with the output coupling. The highest averagepower measured on the output of the fiber laser reaches4:2W at a repetition rate of 8:4MHz. The efficiency of thelaser is around 8%. The corresponding energy level is0:5 μJ per pulse. The pulse-to-pulse peak power stabilityhas been recorded over 2 h runs on a large-bandwidthoscilloscope coupled with an ultrafast photodiode andreveals RMS values below 1%. The shape of the outputspectrum, given in Fig. 2, exhibits steep edges as usuallyobserved in typical all-normal dispersion lasers. Thespectrum FWHM is 5:4 nm with a total width of 7 nm(edge to edge). A very good contrast above 25 dB isobserved between the emission at 976 nm and the emis-sion at 1030 nm (Fig. 2).Because of the all-normal dispersion operation of the
laser, the outgoing pulses are naturally chirped. Theduration is estimated to ∼14:2 ps from the autocorrela-tion trace, assuming a sech2 pulse profile. The compres-sion is achieved in a set of two transmission gratings with
1740 lines=mm. After optimization, a second-harmonicgeneration frequency-resolved optical gating measure-ment has allowed retrieval of the pulse temporal profiledisplayed in Fig. 3. A pulse duration as short as 460 fshas been retrieved, corresponding to 1.05 times thetransform limit of the spectrum. Note here that the asym-metric retrieved pulse contains the remaining third-orderdispersion introduced by both the compressor (12:4 ps3)and the nonlinearities (0:2ps3), which unfortunately addup. The low efficiency of our compressor (about 50%due to optimized gratings for 1030 nm radiation insteadof 976 nm) allows us to deliver only 2:1W of averagepower after compression, corresponding to a measuredpeak power of 508 kW and a potential peak power of914 kW with a 90% compressor efficiency (multilayer di-electric gratings), which is close to the megawatt level.
To investigate the pulse evolution in the cavity, we per-formed numerical simulations [18] using a standard split-step Fourier method with the arrangement of the lasercavity elements shown in Fig. 4. Absorption of the satur-able absorber mirror (SAM) is described by the rate equa-tion model with a relaxation time of 500 fs. Suggestingthat the NPE mechanism plays a key role in pulse shap-ing, an ideal saturable absorber with instantaneousresponse is introduced just after the gain fiber. Note thatour scalar model describes only the amplitude modula-tion provided by the nonlinear polarization evolution(NPE) effect. Losses of optical elements and fiber cou-pling are added between the laser cavity elements.
Fig. 1. Schematic representation of the mode-locked fiberlaser emitting around 976nm: L1–10, lenses; M1 and M2, dichroicmirrors 915=976nm; M3, Rmax at 915 nm; M4 and M9, dichroicmirror Tmax at 1030 and Rmax at 976 nm; M5–8, Rmax mirrors;λ=2, half-wave plates; λ=4, quarter-wave plates; BS, beam split-ting polarizer cube.
Fig. 2. Experimental spectrum and simulated spectrum withFiberdesk. Inset, extinction ratio between 976 and 1030nmradiations.
Fig. 3. (Color online) Temporal profile of the compressedpulse compared to the calculated transform-limited pulse andthe computed pulse, including the remaining TOD from thecompressor and nonlinearities. Inset, far-field image of theoutput beam.
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The simulation starts from quantum noise, and afterconvergence, the intracavity pulse evolution and the out-put pulse characteristics are calculated. The results of anumerical simulation performed with accurate fiberparameters and 0:5 μJ pulse energy are shown in Fig. 4.The solution calculated after the gain fiber has a sech2
profile with a FWHM width of ∼12:4 ps and a spectralwidth of ∼7nm, which are close to the experimentalvalues. The accumulated spectral phase terms are ϕð2Þ ¼1:4ps2 (0:48 ps2 material dispersion and 0:92 ps2 non-linear phase) and ϕð3Þ ¼ 0:98 ps3 (0:78 ps3 material dis-persion and 0:2 ps3 nonlinear phase). The calculatedpulses are dechirped down to ∼494 fs duration. This cor-responds to ∼1:18 times the Fourier limit, indicating thatthe calculated pulse suffers from an excessive nonlinearchirp compared to experiments. Let us note that the B-integral accumulated along the cavity is ∼3:7π. The slightdiscrepancy between theory and experiments could beattributed to the additional spectral discrimination pro-vided by NPE, which is not accurately described byour scalar model. This could also explain the little devia-tion observed on the shape of the optical spectrum, asshown in Fig. 2.It is clear from the evolution of the pulse in the cavity
that gain filtering plays a dominant role in pulse shapingboth in the temporal and spectral domains. The pulseduration and the spectral width increase almost monoto-nically in the passive fiber. Gain filtering shortens signif-icantly the spectral width and the pulse duration. TheSAM and NPE have a small effect on the steady-state re-gime, but they are needed to ensure self-starting and self-consistency. This behavior is governed both by the highnet cavity dispersion [5] and by the strong spectral filter-ing due to the limited gain bandwidth.In conclusion, we have demonstrated a mode-locked
Yb-doped rod-type fiber laser operating around 976 nm.
Our laser is stable and self-starting for a pump powerof 41W. It delivers chirped pulses of 14:2ps for an outputaverage power of 4:2W at 8:4MHz, corresponding topulse energies as high as 0:5 μJ. Output pulses have beenrecompressed down to a duration of 460 fs. Amplificationarchitectures will be investigated to reach a very high en-ergy level in order to generate 488, 325, and 244 nm bydoubling, tripling, or quadrupling the IR beam.
We acknowledge support from the Agence Nationalede la Recherche IMFINI ANR-09-BLAN-0065 and PICF-APUS, the European Community through the I3 LASER-LAB II JRA2 HAPPIE, the Conseil Régional d’Aquitainewith the FEMTO+ Program, Aquitaine Valorisation,and the Centre National de la Recherche Scientifique(CNRS).
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Fig. 4. (Color online) Numerical results calculated for 0:5 μJenergy: (a) pulse dynamics inside the cavity: OC, outputcoupler; NP, nonlinear polarization evolution; SA, saturable ab-sorber, (b) optical spectrum, (c) output pulse, and (d) dechirpedpulse.
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