Fan, Jintao

Gu, Chenglin

Hu, Minglie

Reid, Derryck Telford

Song, Youjian

Wang, Chingyue

Zhang, Limeng

English

We report on the highly efficient generation of widely tunable femtosecond pulses based on intracavity second harmonic generation (SHG) and sum frequency generation (SFG) in a MgO-doped periodically poled LiNbO3 optical parametric oscillator (OPO), which is pumped by a Yb-doped large-mode-area photonics crystal fiber femtosecond laser. Red and near infrared from intracavity SHG and SFG and infrared signals were directly obtained from the OPO. A 2 mm beta - BaB2O4 is applied for Type I (oo -&gt; e) intracavity SHG and SFG, and then femtosecond laser pulses over 610 nm similar to 668 nm from SFG and 716 nm similar to 970 nm from SHG are obtained with high efficiency. In addition, the oscillator simultaneously generates signal and idler femtosecond pulses over 1450 nm similar to 2200 nm and 2250 nm similar to 4000 nm, respectively. (C) 2013 Optical Society of America</p

Heriot Watt Pure

    Heriot-Watt University Research Gateway                                                          Heriot-Watt UniversityHigh average power, widely tunable femtosecond laser source from red to mid-infraredbased on an Yb-fiber-laser-pumped optical parametric oscillatorGu, Chenglin; Hu, Minglie; Zhang, Limeng; Fan, Jintao; Song, Youjian; Wang, Chingyue;Reid, Derryck TelfordPublished in:Optics LettersDOI:10.1364/OL.38.001820Publication date:2013Link to publication in Heriot-Watt Research GatewayCitation for published version (APA):Gu, C., Hu, M., Zhang, L., Fan, J., Song, Y., Wang, C., & Reid, D. T. (2013). High average power, widely tunablefemtosecond laser source from red to mid-infrared based on an Yb-fiber-laser-pumped optical parametricoscillator. Optics Letters, 38(11), 1820-1822. 10.1364/OL.38.001820High average power, widely tunable femtosecond lasersource from red to mid-infrared based on anYb-fiber-laser-pumped optical parametric oscillatorChenglin Gu,1 Minglie Hu,*,1 Limeng Zhang,1 Jintao Fan,1 Youjian Song,1 Chingyue Wang,1 and Derryck T. Reid21Ultrafast Laser Laboratory, Key Laboratory of Opto-electronic Information Science and Technology of Ministry of Education,College of Precision Instruments and Opto-electronics Engineering, Tianjin University, Tianjin 300072, China2Scottish Universities Physics Alliance (SUPA), Department of Physics, School of Engineering and Physical Sciences,Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, UK*Corresponding author: huminglie@tju.edu.cnReceived March 12, 2013; revised April 21, 2013; accepted April 22, 2013;posted April 24, 2013 (Doc. ID 186925); published May 21, 2013We report on the highly efficient generation of widely tunable femtosecond pulses based on intracavity secondharmonic generation (SHG) and sum frequency generation (SFG) in a MgO-doped periodically poled LiNbO3 opticalparametric oscillator (OPO), which is pumped by a Yb-doped large-mode-area photonics crystal fiber femtosecondlaser. Red and near infrared from intracavity SHG and SFG and infrared signals were directly obtained fromthe OPO. A 2 mm β − BaB2O4 is applied for Type I (oo → e) intracavity SHG and SFG, and then femtosecond laserpulses over 610 nm ∼ 668 nm from SFG and 716 nm ∼ 970 nm from SHG are obtained with high efficiency. In addi-tion, the oscillator simultaneously generates signal and idler femtosecond pulses over 1450 nm ∼ 2200 nm and2250 nm ∼ 4000 nm, respectively. © 2013 Optical Society of AmericaOCIS codes: (190.4970) Parametric oscillators and amplifiers; (320.7090) Ultrafast lasers.http://dx.doi.org/10.1364/OL.38.001820Femtosecond laser sources in visible, infrared, andmid-infrared ranges are of great importance in many ap-plications including biophotonics, optical microscopy,and time-resolved spectroscopy [1,2]. Optical parametricoscillators (OPOs) or amplifiers (OPAs) pumped byfemtosecond Ti:sapphire lasers have been the preferredsource in the past for this range [3,4]. The first demon-stration of a high-repetition-rate, femtosecond OPOsbased on periodically poled LiNbO3 (PPLN) pumped bya femtosecond Ti:sapphire laser is reported by Burr et al.[5]. Theoretical and experimental work on OPO withintracavity second harmonic generation (SHG) and sumfrequency generation (SFG) was reported by Koch et al.[6] in detail. Nowadays, the rapid progress in ultrafastfiber lasers leads to the development of a new generationof compact and robust femtosecond laser sources andinspires the development of fiber-based OPOs [7]. AnOPOs pumped by a fiber laser, which has a high averagepower, efficiency, and environmental stability, is suitableto generate tunable and powerful ultrashort pulses. Thefirst Yb-fiber-laser pumped OPO offering substantialpower was reported by Lamour et al. [8], producing thehighest pulse energy (72 nJ) obtained from a free-spaceresonator design. Recently, a Yb-fiber-laser-pumped,picosecond parametric oscillator, together with intracav-ity SHG tunable across 752 nm ∼ 860 nm, was reported[9]. A femtosecond OPO based on LiB3O5 pumped by afrequency-doubled Yb-fiber amplifier at 525 nm providedsignal tuning over 780 nm ∼ 940 nm with more than250 mW and idler tuning over 1190–1630 nm with morethan 300 mW [10]. These ultrashort OPOs pumped byhigh-powerultrafast fiberlasersnear1 μm with intracavityfrequency doubling and mixing, are able to provide thespectral range as that of Ti:sapphire lasers but are morerobustandeconomical,togetherwithhigheroutputpower[8–10]. As a result, extending the tunable wavelengthrange of a fiber laser-based OPO is an important andinteresting topic for this technology.In this Letter, we demonstrate the highly efficientgeneration of a widely tunable femtosecond pulses, redand near-infrared, from intracavity SHG and SFG in β −BaB2O4 (BBO) of an OPO, based on a 5% MgO-dopedPPLN (MgO:PPLN) and infrared signal, and idler pulsesdirectly obtained from an OPO. Owing to the high non-linear gain in both PPLN and BBO, we achieve an averagepower of 470 mW at 23.5% conversion efficiency tuningacross 716 nm ∼ 970 nm for SHG, and 372 mW at 18.6%conversion efficiency tuning across 610 nm ∼ 668 nmfor SFG. Moreover, we obtain an average power of694 mW average power for a 4.1 W pump, representinga 16.9% conversion efficiency for maximum SFG outputat 615 nm. The SFG and SHG act as a signal output losscaused by the nonlinear process with hardly any affecton the stability of the original OPO.The configuration of the femtosecond OPO is shownin Fig. 1. A homemade high-power Yb large-mode-areaphotonics crystal fiber (PCF) femtoseocond laser operat-ing at a repetition rate of 51 MHz is used as the pumpsource. Its central wavelength is 1040 nm with an FWHMFig. 1. Experimental setup of intracavity SHG and SFG inMgO:PPLN OPO. M1–M6, mirrors; L1–L3, lenses; HWP, half-wave plate; PBS, polarizing beam-splitter; BS, beam-splitter.1820 OPTICS LETTERS / Vol. 38, No. 11 / June 1, 20130146-9592/13/111820-03$15.00/0 © 2013 Optical Society of Americaof 40 nm, and pulse duration of 70 fs. The nonlinear crys-tal for the OPO is a 1 mm long, 8.5 mm wide, 5% MgO:PPLN. In the experiment, the crystal is housed in an ovenmaintained at 80°C in order to avoid the photorefractiveeffect. The crystal contains seven gratings with periodsranging from 28.5 to 31.5 μm, in steps of 0.5 μm.The OPO cavity is a bifocal ring, comprised of fourconcave and two plane mirrors. In order to enhance theintracavity nonlinear efficiency, the OPO has no outputwhen operating for intracavity SHG and SFG. Gold-coated concave mirrors M1 and M2, with r  150 mm,provide the focus for the OPO crystal, whereas mirrorsM5 and M6, with r  100 mm, allow focusing into theSHG or SFG crystal. M5 is an SHG or SFG output mirror,which is 99% reflective over 1400 nm ∼ 2100 nm and95% transmittive over 600 nm ∼ 1000 nm, SHG or SFG isoutput through M5 and collimated by lens L3, M3, andM4 are dichroic pump mirrors, which are 99% reflectiveover 1400 nm ∼ 2100 nm, 95% transmittive for the pumpwavelength at 1.04 μm and highly transmittive for atidler wavelength. The crystal faces are AR-coated over1000 nm ∼ 1100 nm (R<1%) and 1420 nm ∼ 2000 nm(R<1.5%). The crystal length is chosen to be 1 mmconsidering group-velocity mismatch (GVM). A BBOcrystal is used for the SHG or SFG because of the weakdependence of the phase matching angle on the wave-length and small GVM in the near infrared region [11].In order to characterize the OPO’s spectral coverage,we tune the cavity length of the OPO with seven gratingperiods of the PPLN crystal. First, we obtain the signalby inserting a BS (beam splitter) as the output coupler.The signal and idler output powers of the OPO for differ-ent wavelengths are shown in Fig. 2(a). The maximumsignal output power is 374 mW at 1502 nm correspondingto an 18.7% conversion efficiency, and the autocorrela-tion duration is 202 fs [Fig. 2(b)], corresponding to a144 fs duration. The calculated transform-limit durationis about 90 fs. The duration is larger than the pump laser,mainly because of the GVM in the PPLN crystal. The idlerpower is measured at the back of M3. The spectra of thesignal and idler pulses are shown in Figs. 2(c) and 2(d),respectively. The signal and idler spectra are continu-ously tuned from 1450 to 2200 nm and 2250 to 4000 nm,respectively, by changing the PPLN grating and thecavity length. The spectrum is noisy at 2200 nm, which isdue to the instability of the OPO at degeneracy [12].For the SHG and SFG processes, the BS output coupleris removed to enhance the intracavity power. The secondharmonic (SH) signal is obtained simply by insertingthe BBO crystal in the cavity. The BBO crystal is 2 mmthick, cut at θ  23.2° for type I interaction. The crystalfaces are AR-coated centered at 1.06 μm( R<1%). TheSH of the signal is obtained simply by adjusting thecavity length and crystal angle. The tuning range ofthe SHG is 716–970 nm as shown in Fig. 3(a), and thewavelength limitation is mainly due to the reflectionbandwidth of the mirrors. Figure 3(b) shows the depend-ence of SHG power on the wavelength at a fixed pumppower of 2 W. The maximum output power is 470 mWwith a 165 fs pulse duration [inset of Fig. 3(b)] and 12 nmspectrum width at 757 nm, corresponding to a time-bandwidth product of ΔυΔτ ∼ 1. The net conversion effi-ciency is 23.5% for SH and the total efficiency includingthe idler is 35.2%. The SH power is above 200 mW over716–850 nm, and above 100 mW at 970 nm.In the case of SFG, an additional pump beam is usedto overlap the intracavity signal beam instead of theresidual pump light as before [13]. In order to get maxi-mum SFG output, a half-wave plate (HWP) and a polar-izing beam-splitter (PBS) are used to distribute the pumppower for the OPO and SFG, and a second HWP is usedto control the pump polarization for SFG phase-matching(Fig. 1). The additional pump beam is mode-matchedwith the oscillating signal beam by L1 (f  200 mm)and L2 (f  100 mm). In the time domain. the pumppulses are synchronized with the oscillating signal pulsesin the crystal by adjusting the delay line.The alignment of the SFG is performed from theSHG configuration simply by rotating the BBO a few de-grees. The pump beam for SFG is adjusted to coincidewith the signal beam line, and the SFG output is easilyobserved by scanning the delay line. At the optimumangle of SFG, only a few milliwatt of SH output remains.In a previous report [2], residual pump light from theOPO is used for intracavity SFG, and so the pump lightis depleted in both parametric and SFG crystals. Theoutput behavior of the OPO becomes complicatedFig. 2. (a) Output powers of the signal and idler, (b) auto-correlation of signal at 1514 nm, (c) spectrum tuning of thesignal from 1450 to 2200 nm, and (d) spectrum tuning of theidler from 2250 to 4000 nm.Fig. 3. (a) Spectrum tuning of the intracavity SHG from716 to 970 nm and (b) dependence of output power of intra-cavity SHG on wavelength. Inset: autocorrelation of SHG signalat 757 nm.June 1, 2013 / Vol. 38, No. 11 / OPTICS LETTERS 1821because of its dependence on the degree of pump lightdepletion in both crystals. Using an additional pump lightmakes the behavior of the OPO simple and stable, and itis easy to obtain an optimum SFG. The distribution ratioof the OPO is a key parameter of the SFG power. If theratio of the OPO pump light is too small, the intracavitysignal power will be too low or unable to maintain theoscillation. If the ratio of the SFG pump light power istoo small, the nonlinear interaction of the fundamentaland signal will also be inefficient. So, it has an optimumvalue for the maximum output power of SFG. Figure 4(a)shows the dependence of the SFG output power on thedistribution at a fixed total pump power of 2 W. The OPOstarts oscillating at 868 mW, which is much higher thanthe threshold (287 mW) without the SFG pump in. This isbecause the SFG increases the loss of the cavity. So, theSFG power rises with the increase of OPO pump power,and reaches a maximum output of 372 mW correspond-ing to an 18.6% conversion efficiency at an OPO pumppower of 1.1 W. With further increases in the OPO pumppower, the SFG power decreases. Figure 4(b) shows thedependence of the SFG power on the SFG pump powerata fixed OPO pump power of 2 W. Initially, the SFG powergrows with the SFG pump power linearly, and then tendsto be saturated. The maximum SFG power is 694 mW ata SFG pump power of 2.1 W, corresponding to a 16.9%conversion efficiency. It is because the increase of theSFG power indicates more intracavity signal depletion,and the SFG power stops increasing when it reaches abalance. The tuning property of the SFG is also recordedby tuning the cavity length of the OPO with seven gratingperiods of the PPLN crystal. For optimum SFG outputpower, a slight phase-matching angle rotation of theBBO is also performed. Figure 4(c) shows the SFG powerat a fixed OPO pump power of 2 W and an SFG pumppower of 1 W, the maximum output power being545 mW corresponding to an 18.1% conversion efficiency.Figure 4(d) shows the spectrum tuning coverage from610 to 668 nm continuously. An abrupt drop by a factorof 2 for the SFG intensity at wavelengths around 640 nmis mainly caused by the decreasing signal intensity dueto the increasing GVM between the signal and pump,which reduce the intracavity signal power and increasethe duration of signal, which both reduces the intensityof the signal. The temporal walk-off between the signaland SFG in the 2 mm BBO is about 230 fs, and the tem-poral walk-off between the pump and signal is 30 fs,which is relatively small, and so the duration of the SFGis estimated to be about 300 fs.In summary, we have demonstrated a femtosecondsource tunable across 610–668 nm and 716–970 nmbased on intracavity SHG and SFG in a MgO:PPLN OPO,together with signal and idler tuning in the near- andmid-infrared regions. We have obtained 470 mW of powerfor a 2 W Yb:fiber laser pump with a 23.5% conversionefficiency for SHG and 372 mW with a 18.6% conversionefficiency for SFG. Future work will be focused on in-creasing the output power and extending the operatingwavelength of fiber femtosecond laser based OPOs. Adouble pumped OPO setup and an additional SHG proc-ess can be applied to obtain the high average power tun-able ultraviolent femtosecond pulse. It can be expectedthat these techniques will lead to a feasible scenario inmany applications.This research is supported by the National BasicResearch Program of China (Grant 2011CB808101 and2010CB327604) and the National Natural ScienceFoundation of China (Grant 61078028, 61205131, and11274239).References1. M. Jurna, J. P. Korterik, H. L. Offerhaus, and C. Otto, Appl.Phys. Lett. 89, 251116 (2006).2. A. Zumbusch, G. R. Holtom, and X. S. Xie, Phys. Rev. Lett.82, 4142 (1999).3. A. Shirakawa, H. W. Mao, and T. Kobayashi, Opt. Commun.123, 121 (1996).4. A. Esteban-Martin, O. Kokabee, and M. Ebrahim-Zadeh,Opt. Lett. 33, 2650 (2008).5. K. C. Burr, C. L. Tang, M. A. Arbore, and M. M. Fejer, Appl.Phys. Lett. 70, 3341 (1997).6. K. Koch, G. T. Moore, and E. C. Cheung, J. Opt. Soc. Am. B12, 2268 (1995).7. J. Limpert, F. Roser, T. Schreiber, and A. Tunnermann,IEEE J. Sel. Top. Quantum Electron. 12, 233 (2006).8. T. P. Lamour, L. Kornaszewski, J. H. Sun, and D. T. Reid,Opt. Express 17, 14229 (2009).9. S. Chaitanya Kumar, O. Kimmelma, and M. Ebrahim-Zadeh,Opt. Lett. 37, 1577 (2012).10. C. Cleff, J. Epping, P. Gross, and C. Fallnich, Appl. Phys. B103, 795 (2011).11. R. J. Ellingson and C. L. Tang, Opt. Lett. 18, 438(1993).12. J. Falk, IEEE J. Quantum Electron. 7, 230 (1971).13. A. Shirakawa, H. W. Mao, and T. Kobayashi, Opt. Commun.123, 121 (1996).Fig. 4. (a) Dependence of SFG (at 615 nm) output poweron the OPO and SFG pump powers, at a fixed total input powerof 2 W, (b) SFG (at 615 nm) power with an increase of SFGpump power at a fixed OPO pump power of 2 W, (c) SFG powerdependence on wavelength at a fixed OPO pump power of2 W and SFG pump power of 1 W, and (d) spectrum tuningof SFG from 610 to 668 nm.1822 OPTICS LETTERS / Vol. 38, No. 11 / June 1, 2013

High average power, widely tunable femtosecond laser source from red to mid-infrared based on an Yb-fiber-laser-pumped optical parametric oscillator

Chenglin Gu

Minglie Hu

Limeng Zhang

Jintao Fan

Youjian Song

Chingyue Wang

Derryck T. Reid

Crossref

Optics Letters

High average power, widely tunable femtosecond lasersource from red to mid-infrared based on anYb-fiber-laser-pumped optical parametric oscillatorChenglin Gu,1 Minglie Hu,*,1 Limeng Zhang,1 Jintao Fan,1 Youjian Song,1 Chingyue Wang,1 and Derryck T. Reid21Ultrafast Laser Laboratory, Key Laboratory of Opto-electronic Information Science and Technology of Ministry of Education,College of Precision Instruments and Opto-electronics Engineering, Tianjin University, Tianjin 300072, China2Scottish Universities Physics Alliance (SUPA), Department of Physics, School of Engineering and Physical Sciences,Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, UK*Corresponding author: huminglie@tju.edu.cnReceived March 12, 2013; revised April 21, 2013; accepted April 22, 2013;posted April 24, 2013 (Doc. ID 186925); published May 21, 2013We report on the highly efficient generation of widely tunable femtosecond pulses based on intracavity secondharmonic generation (SHG) and sum frequency generation (SFG) in a MgO-doped periodically poled LiNbO3 opticalparametric oscillator (OPO), which is pumped by a Yb-doped large-mode-area photonics crystal fiber femtosecondlaser. Red and near infrared from intracavity SHG and SFG and infrared signals were directly obtained fromthe OPO. A 2 mm β − BaB2O4 is applied for Type I (oo → e) intracavity SHG and SFG, and then femtosecond laserpulses over 610 nm ∼ 668 nm from SFG and 716 nm ∼ 970 nm from SHG are obtained with high efficiency. In addi-tion, the oscillator simultaneously generates signal and idler femtosecond pulses over 1450 nm ∼ 2200 nm and2250 nm ∼ 4000 nm, respectively. © 2013 Optical Society of AmericaOCIS codes: (190.4970) Parametric oscillators and amplifiers; (320.7090) Ultrafast lasers.http://dx.doi.org/10.1364/OL.38.001820Femtosecond laser sources in visible, infrared, andmid-infrared ranges are of great importance in many ap-plications including biophotonics, optical microscopy,and time-resolved spectroscopy [1,2]. Optical parametricoscillators (OPOs) or amplifiers (OPAs) pumped byfemtosecond Ti:sapphire lasers have been the preferredsource in the past for this range [3,4]. The first demon-stration of a high-repetition-rate, femtosecond OPOsbased on periodically poled LiNbO3 (PPLN) pumped bya femtosecond Ti:sapphire laser is reported by Burr et al.[5]. Theoretical and experimental work on OPO withintracavity second harmonic generation (SHG) and sumfrequency generation (SFG) was reported by Koch et al.[6] in detail. Nowadays, the rapid progress in ultrafastfiber lasers leads to the development of a new generationof compact and robust femtosecond laser sources andinspires the development of fiber-based OPOs [7]. AnOPOs pumped by a fiber laser, which has a high averagepower, efficiency, and environmental stability, is suitableto generate tunable and powerful ultrashort pulses. Thefirst Yb-fiber-laser pumped OPO offering substantialpower was reported by Lamour et al. [8], producing thehighest pulse energy (72 nJ) obtained from a free-spaceresonator design. Recently, a Yb-fiber-laser-pumped,picosecond parametric oscillator, together with intracav-ity SHG tunable across 752 nm ∼ 860 nm, was reported[9]. A femtosecond OPO based on LiB3O5 pumped by afrequency-doubled Yb-fiber amplifier at 525 nm providedsignal tuning over 780 nm ∼ 940 nm with more than250 mW and idler tuning over 1190–1630 nm with morethan 300 mW [10]. These ultrashort OPOs pumped byhigh-power ultrafast fiber lasers near 1 μmwith intracavityfrequency doubling and mixing, are able to provide thespectral range as that of Ti:sapphire lasers but are morerobust and economical, together with higher output power[8–10]. As a result, extending the tunable wavelengthrange of a fiber laser-based OPO is an important andinteresting topic for this technology.In this Letter, we demonstrate the highly efficientgeneration of a widely tunable femtosecond pulses, redand near-infrared, from intracavity SHG and SFG in β −BaB2O4 (BBO) of an OPO, based on a 5% MgO-dopedPPLN (MgO:PPLN) and infrared signal, and idler pulsesdirectly obtained from an OPO. Owing to the high non-linear gain in both PPLN and BBO, we achieve an averagepower of 470 mW at 23.5% conversion efficiency tuningacross 716 nm ∼ 970 nm for SHG, and 372 mW at 18.6%conversion efficiency tuning across 610 nm ∼ 668 nmfor SFG. Moreover, we obtain an average power of694 mW average power for a 4.1 W pump, representinga 16.9% conversion efficiency for maximum SFG outputat 615 nm. The SFG and SHG act as a signal output losscaused by the nonlinear process with hardly any affecton the stability of the original OPO.The configuration of the femtosecond OPO is shownin Fig. 1. A homemade high-power Yb large-mode-areaphotonics crystal fiber (PCF) femtoseocond laser operat-ing at a repetition rate of 51 MHz is used as the pumpsource. Its central wavelength is 1040 nm with an FWHMFig. 1. Experimental setup of intracavity SHG and SFG inMgO:PPLN OPO. M1–M6, mirrors; L1–L3, lenses; HWP, half-wave plate; PBS, polarizing beam-splitter; BS, beam-splitter.1820 OPTICS LETTERS / Vol. 38, No. 11 / June 1, 20130146-9592/13/111820-03$15.00/0 © 2013 Optical Society of Americaof 40 nm, and pulse duration of 70 fs. The nonlinear crys-tal for the OPO is a 1 mm long, 8.5 mm wide, 5% MgO:PPLN. In the experiment, the crystal is housed in an ovenmaintained at 80°C in order to avoid the photorefractiveeffect. The crystal contains seven gratings with periodsranging from 28.5 to 31.5 μm, in steps of 0.5 μm.The OPO cavity is a bifocal ring, comprised of fourconcave and two plane mirrors. In order to enhance theintracavity nonlinear efficiency, the OPO has no outputwhen operating for intracavity SHG and SFG. Gold-coated concave mirrors M1 and M2, with r  150 mm,provide the focus for the OPO crystal, whereas mirrorsM5 and M6, with r  100 mm, allow focusing into theSHG or SFG crystal. M5 is an SHG or SFG output mirror,which is 99% reflective over 1400 nm ∼ 2100 nm and95% transmittive over 600 nm ∼ 1000 nm, SHG or SFG isoutput through M5 and collimated by lens L3, M3, andM4 are dichroic pump mirrors, which are 99% reflectiveover 1400 nm ∼ 2100 nm, 95% transmittive for the pumpwavelength at 1.04 μm and highly transmittive for atidler wavelength. The crystal faces are AR-coated over1000 nm ∼ 1100 nm (R < 1%) and 1420 nm ∼ 2000 nm(R < 1.5%). The crystal length is chosen to be 1 mmconsidering group-velocity mismatch (GVM). A BBOcrystal is used for the SHG or SFG because of the weakdependence of the phase matching angle on the wave-length and small GVM in the near infrared region [11].In order to characterize the OPO’s spectral coverage,we tune the cavity length of the OPO with seven gratingperiods of the PPLN crystal. First, we obtain the signalby inserting a BS (beam splitter) as the output coupler.The signal and idler output powers of the OPO for differ-ent wavelengths are shown in Fig. 2(a). The maximumsignal output power is 374 mW at 1502 nm correspondingto an 18.7% conversion efficiency, and the autocorrela-tion duration is 202 fs [Fig. 2(b)], corresponding to a144 fs duration. The calculated transform-limit durationis about 90 fs. The duration is larger than the pump laser,mainly because of the GVM in the PPLN crystal. The idlerpower is measured at the back of M3. The spectra of thesignal and idler pulses are shown in Figs. 2(c) and 2(d),respectively. The signal and idler spectra are continu-ously tuned from 1450 to 2200 nm and 2250 to 4000 nm,respectively, by changing the PPLN grating and thecavity length. The spectrum is noisy at 2200 nm, which isdue to the instability of the OPO at degeneracy [12].For the SHG and SFG processes, the BS output coupleris removed to enhance the intracavity power. The secondharmonic (SH) signal is obtained simply by insertingthe BBO crystal in the cavity. The BBO crystal is 2 mmthick, cut at θ  23.2° for type I interaction. The crystalfaces are AR-coated centered at 1.06 μm (R < 1%). TheSH of the signal is obtained simply by adjusting thecavity length and crystal angle. The tuning range ofthe SHG is 716–970 nm as shown in Fig. 3(a), and thewavelength limitation is mainly due to the reflectionbandwidth of the mirrors. Figure 3(b) shows the depend-ence of SHG power on the wavelength at a fixed pumppower of 2 W. The maximum output power is 470 mWwith a 165 fs pulse duration [inset of Fig. 3(b)] and 12 nmspectrum width at 757 nm, corresponding to a time-bandwidth product of ΔυΔτ ∼ 1. The net conversion effi-ciency is 23.5% for SH and the total efficiency includingthe idler is 35.2%. The SH power is above 200 mW over716–850 nm, and above 100 mW at 970 nm.In the case of SFG, an additional pump beam is usedto overlap the intracavity signal beam instead of theresidual pump light as before [13]. In order to get maxi-mum SFG output, a half-wave plate (HWP) and a polar-izing beam-splitter (PBS) are used to distribute the pumppower for the OPO and SFG, and a second HWP is usedto control the pump polarization for SFG phase-matching(Fig. 1). The additional pump beam is mode-matchedwith the oscillating signal beam by L1 (f  200 mm)and L2 (f  100 mm). In the time domain. the pumppulses are synchronized with the oscillating signal pulsesin the crystal by adjusting the delay line.The alignment of the SFG is performed from theSHG configuration simply by rotating the BBO a few de-grees. The pump beam for SFG is adjusted to coincidewith the signal beam line, and the SFG output is easilyobserved by scanning the delay line. At the optimumangle of SFG, only a few milliwatt of SH output remains.In a previous report [2], residual pump light from theOPO is used for intracavity SFG, and so the pump lightis depleted in both parametric and SFG crystals. Theoutput behavior of the OPO becomes complicatedFig. 2. (a) Output powers of the signal and idler, (b) auto-correlation of signal at 1514 nm, (c) spectrum tuning of thesignal from 1450 to 2200 nm, and (d) spectrum tuning of theidler from 2250 to 4000 nm.Fig. 3. (a) Spectrum tuning of the intracavity SHG from716 to 970 nm and (b) dependence of output power of intra-cavity SHG on wavelength. Inset: autocorrelation of SHG signalat 757 nm.June 1, 2013 / Vol. 38, No. 11 / OPTICS LETTERS 1821because of its dependence on the degree of pump lightdepletion in both crystals. Using an additional pump lightmakes the behavior of the OPO simple and stable, and itis easy to obtain an optimum SFG. The distribution ratioof the OPO is a key parameter of the SFG power. If theratio of the OPO pump light is too small, the intracavitysignal power will be too low or unable to maintain theoscillation. If the ratio of the SFG pump light power istoo small, the nonlinear interaction of the fundamentaland signal will also be inefficient. So, it has an optimumvalue for the maximum output power of SFG. Figure 4(a)shows the dependence of the SFG output power on thedistribution at a fixed total pump power of 2 W. The OPOstarts oscillating at 868 mW, which is much higher thanthe threshold (287 mW) without the SFG pump in. This isbecause the SFG increases the loss of the cavity. So, theSFG power rises with the increase of OPO pump power,and reaches a maximum output of 372 mW correspond-ing to an 18.6% conversion efficiency at an OPO pumppower of 1.1 W. With further increases in the OPO pumppower, the SFG power decreases. Figure 4(b) shows thedependence of the SFG power on the SFG pump power ata fixed OPO pump power of 2 W. Initially, the SFG powergrows with the SFG pump power linearly, and then tendsto be saturated. The maximum SFG power is 694 mW ata SFG pump power of 2.1 W, corresponding to a 16.9%conversion efficiency. It is because the increase of theSFG power indicates more intracavity signal depletion,and the SFG power stops increasing when it reaches abalance. The tuning property of the SFG is also recordedby tuning the cavity length of the OPO with seven gratingperiods of the PPLN crystal. For optimum SFG outputpower, a slight phase-matching angle rotation of theBBO is also performed. Figure 4(c) shows the SFG powerat a fixed OPO pump power of 2 W and an SFG pumppower of 1 W, the maximum output power being545 mW corresponding to an 18.1% conversion efficiency.Figure 4(d) shows the spectrum tuning coverage from610 to 668 nm continuously. An abrupt drop by a factorof 2 for the SFG intensity at wavelengths around 640 nmis mainly caused by the decreasing signal intensity dueto the increasing GVM between the signal and pump,which reduce the intracavity signal power and increasethe duration of signal, which both reduces the intensityof the signal. The temporal walk-off between the signaland SFG in the 2 mm BBO is about 230 fs, and the tem-poral walk-off between the pump and signal is 30 fs,which is relatively small, and so the duration of the SFGis estimated to be about 300 fs.In summary, we have demonstrated a femtosecondsource tunable across 610–668 nm and 716–970 nmbased on intracavity SHG and SFG in a MgO:PPLN OPO,together with signal and idler tuning in the near- andmid-infrared regions. We have obtained 470 mW of powerfor a 2 W Yb:fiber laser pump with a 23.5% conversionefficiency for SHG and 372 mW with a 18.6% conversionefficiency for SFG. Future work will be focused on in-creasing the output power and extending the operatingwavelength of fiber femtosecond laser based OPOs. Adouble pumped OPO setup and an additional SHG proc-ess can be applied to obtain the high average power tun-able ultraviolent femtosecond pulse. It can be expectedthat these techniques will lead to a feasible scenario inmany applications.This research is supported by the National BasicResearch Program of China (Grant 2011CB808101 and2010CB327604) and the National Natural ScienceFoundation of China (Grant 61078028, 61205131, and11274239).References1. M. Jurna, J. P. Korterik, H. L. Offerhaus, and C. Otto, Appl.Phys. Lett. 89, 251116 (2006).2. A. Zumbusch, G. R. Holtom, and X. S. Xie, Phys. Rev. Lett.82, 4142 (1999).3. A. Shirakawa, H. W. Mao, and T. Kobayashi, Opt. Commun.123, 121 (1996).4. A. Esteban-Martin, O. Kokabee, and M. Ebrahim-Zadeh,Opt. Lett. 33, 2650 (2008).5. K. C. Burr, C. L. Tang, M. A. Arbore, and M. M. Fejer, Appl.Phys. Lett. 70, 3341 (1997).6. K. Koch, G. T. Moore, and E. C. Cheung, J. Opt. Soc. Am. B12, 2268 (1995).7. J. Limpert, F. Roser, T. Schreiber, and A. Tunnermann,IEEE J. Sel. Top. Quantum Electron. 12, 233 (2006).8. T. P. Lamour, L. Kornaszewski, J. H. Sun, and D. T. Reid,Opt. Express 17, 14229 (2009).9. S. Chaitanya Kumar, O. Kimmelma, and M. Ebrahim-Zadeh,Opt. Lett. 37, 1577 (2012).10. C. Cleff, J. Epping, P. Gross, and C. Fallnich, Appl. Phys. B103, 795 (2011).11. R. J. Ellingson and C. L. Tang, Opt. Lett. 18, 438(1993).12. J. Falk, IEEE J. Quantum Electron. 7, 230 (1971).13. A. Shirakawa, H. W. Mao, and T. Kobayashi, Opt. Commun.123, 121 (1996).Fig. 4. (a) Dependence of SFG (at 615 nm) output poweron the OPO and SFG pump powers, at a fixed total input powerof 2 W, (b) SFG (at 615 nm) power with an increase of SFGpump power at a fixed OPO pump power of 2 W, (c) SFG powerdependence on wavelength at a fixed OPO pump power of2 W and SFG pump power of 1 W, and (d) spectrum tuningof SFG from 610 to 668 nm.1822 OPTICS LETTERS / Vol. 38, No. 11 / June 1, 2013

High average power, widely tunable femtosecond laser source from red to mid-infrared based on an Yb-fiber-laser-pumped optical parametric oscillator

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