- Email: [email protected]
93 fs pulses at watt-level powers from a diode pumped Yb:CaGdAlO4 laser
Optics and Laser Technology 125 (2020) 106020
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
93 fs pulses at watt-level powers from a diode pumped Yb:CaGdAlO4 laser ⁎
T
Xianghao Meng , Chong Lv, Baozhen Zhao, Xiaofeng Xi, Qiushi Liu, Xiaohua Zhang, Yuchen Li China Institute of Atomic Energy, Beijing 102413, China
H I GH L IG H T S
study on the performances of laser in the net normal or net anomalous dispersion region. • Experimental mode-locked pulses with a duration of 94 fs centered at 1050 nm. • Stable • The pulse energy is 59 nJ and the peak power is 0.56 MW.
A R T I C LE I N FO
A B S T R A C T
Keywords: Mode-locked lasers Ultrafast laser Diode pumped
We investigated the performance of diode pumped mode-locked Yb:CGA laser operating in the net normal or net anomalous dispersion region. With 26 W pump power, stable femtosecond laser pulses with average power of 4.5 W are obtained, yielding a pump power slope efficiency of 17%. Operating at 80.5 MHz repetition rate, the laser delivers as short as 93 fs pulse duration with average output power of 4.5 W, corresponding to a pulse energy of 56 nJ and a peak power of 0.59 MW. The oscillator exhibits a passive rms power stability of ~0.38% rms over 5 h in high beam quality.
1. Introduction High-power all-solid-state femtosecond laser sources have created profound impact in the fields of time-resolved spectroscopy, nonlinear optics, industrial processing and pump-probe measurements [1–8]. For the past few years, ytterbium-doped (Yb3+-doped) materials have become the candidates for diode-pumped femtosecond laser oscillators and regenerative amplifiers. It can be attributed to high quantum efficiency, good thermal properties, broad emission bandwidths and consequent small (~6%) quantum defect. As the most important advantage, they can be directly pumped by the commercially high brightness InGaAs laser diodes because the absorption band of Yb3+ ions matches well with the emission band of the InGaAs laser diodes. Recently, a various of results have been reported from diode pumped mode-locked femtosecond oscillators [9–18]. In 2008, Berger et al. report a diode-pumped Yb:KGW oscillator which delivered 250 fs pulse duration with average output power of 3.5 W [19]. In 2012, Agnesi et al. demonstrated 15 mW average output power with 40 fs from a mode-locked Yb:CGA oscillator [20]. Next year, Haitao et al. reported 67 fs pulses generation with 3 W output power from a diode pumped Kerr-lens mode-locked Yb:KGW laser [14]. In 2018, Tian et al. reported 30 nJ pulse energy with 68 fs pulse duration from a mode-locked Yb:CYA laser [21]. Among those Yb3+-doped materials, Yb:CaGdAlO4 ⁎
(Yb:CGA) is one of the most promising gain media for short pulse duration and high-power oscillators owing to its large emission bandwidths and relatively high thermal conductivity (Ka = 6.9 W m−1 K−1 and Kc = 6.3 W m−1 K−1). The Yb:CGA has a full width at half-maximum (FWHM) spectral bandwidth of about 60 nm at σ polarization, supporting sub-40 fs laser generation. In addition, the crystal presents favorable high mechanical strength (Mohs hardness 6), which is beneficial to large size growth. Utilizing the semiconductor saturable absorber mirrors (SESAM) for passive mode-locking, the Yb:CGA lasers have generated 38 mW of output power with 47 fs pulses and 520 mW with 68 fs pulses [22,23]. As an alternative to SESAM, the Kerr-lens mode-locking (KLM) is another approach to produce ultrashort pulses. For example, the 115 MHz oscillator is mode-locked with Kerr-lens effect, providing 60 fs pulses at an output power of 66 mW [12]. Recently researches reported pulse generation as short as 30 fs in this gain medium, which is the shortest pulse duration ever achieved from an Ybdoped bulk material-based oscillator [11]. In 2012, Greborioa et al. report on the generation of 94 fs pulses with 12.5 W average output power from an 80 MHz Yb:CGA oscillator, which is the highest average power for diode-pumped bulk oscillators [24]. However, these modelocked short pulse oscillators based on SESAM and KLM generate average output power between several hundreds of milliwatts and a dozen watts, limiting the applications in the fields of ultrafast lasers,
Corresponding author. E-mail address: [email protected] (X. Meng).
https://doi.org/10.1016/j.optlastec.2019.106020 Received 21 August 2019; Received in revised form 29 October 2019; Accepted 19 December 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
Optics and Laser Technology 125 (2020) 106020
X. Meng, et al.
mirror (DM), a SESAM and a 10% plane output coupler (OC). To obtain high output power, an OC with 10% transmittance is used in the cavity. The pump laser is focused into the Yb:CGA crystal through a DM, which is deliberately coated to support high transmission at 980 nm for the pump and high reflection for the oscillation pulses in the range of 1000–1100 nm. The concave mirrors and GTI mirrors have high reflection coating (R > 99.9%) over 1000–1100 nm range. The modelocked pulses are generated using a SESAM (Batop GmbH) operated at 1064 nm with a modulation depth of ΔR = 2.4%, a saturation fluence of 70 μJ/cm2 and a relaxation time of 1 ps, which is fastened at a onedimensional translation stage. The distance between the M3 and SESAM could be precisely optimized through a micrometer driven translation stage. This allowed for continuous control of the beam size on the surface of the SESAM. The total cavity length is set to about 1.86 m corresponding to the repetition rate of 80.5 MHz.
such as seed for high-energy amplifiers, frequency comb and nonlinear frequency conversion [25,26]. For oscillators using Yb:CGA as gain medium, the sub-100 fs with pulse energies of hundreds of nano-joules remains a very challenging frontier although they present excellent laser performance in the femtosecond regime. Compared to nonlinear Schrödinger equation (NLSE) type solitons, dissipative solitons (DS) have larger pulse energy owing to large capacity and difficulty in soliton splitting. DS operation of the mode-locked femtosecond lasers have been reported in all normal dispersion Yb3+-doped fiber lasers [27]. However, the DS operation of solid-state lasers is less reported in the Yb3+-doped solid state lasers. In this letter, we investigated the preference of ultrashort pulse generation based on diode pumped mode-locked Yb:CGA laser, which operated in the net normal or net anomalous dispersion region. Optimizing the laser operation conditions based on soliton theory, the Yb:CGA laser can generate as high as 4.5 W average output power, delivering as short as 93 fs pulse duration centred at 1050 nm. Operating at 80.5 MHz repetition rates, the pulse energy is 56 nJ and the peak power is 0.59 MW, respectively. The output pulses exhibit a passive rms power stability of 0.38% over 5 h. In addition, the M2 factors in the horizontal and vertical directions are 1.13 and 1.15, respectively.
3. Experimental results and disscussion Based on this high-brightness diode laser, we first aligned continuous-wave operation in the cavity. The maximum output power is 3 W with 15 W pump power incident on the crystal. Then a Q-switched mode-locking (QML) operation is established by using the SESAM instead of the HR mirror. Large spot size on the SESAM is the main reason that caused the instability of the QML operation. With finely optimizing the position and angle of the SESAM, the QML gradually gives way to the stable self-starting CW mode-locking regime. It is noticed that once the mode-locking is established, the laser pulses are always stable without any Q-switched envelope in the condition of maximum pump power level although the Yb:CGA oscillator is currently built directly on the optical table without any housing. Initially, without any dispersion compensation elements (GTI) in the cavity, we measured autocorrelation trace of the short pulses by using a commercial intensity autocorrelator (A. P. E. GmbH, pulse Check USB). As shown in Fig. 2, assuming a sech2-shaped temporal intensity profile, the laser delivers 8.4 ps pulses at an output power of 4.8 W. As shown in the inset of Fig. 2, the mode-locked spectra have a rectangular-shape with the characteristic sharp steep edges, which indicates the laser operates in the net normal dispersion region. The central wavelength is 1046 nm with a full width at half maximum (FWHM) bandwidth of 7 nm. The corresponding time-bandwidth products (Δτ, Δv) is 16.1, which is 51 times of the Fourier transform-limited value (0.315). The strongly chirped pulses with distinctive characteristics of DS shows the DS operation in the Yb:CGA oscillator.
2. Experimental setup The experimental setup of mode-locked Yb:CGA oscillator is shown in Fig. 1. A 4-mm-long, 5-at. % Yb3+-doped, c-cut Yb:CGA is used as the gain medium, which end faces are coated with high transmission at 980–1100 nm. The crystal is wrapped with indium film and then placed tightly on a water-cooling copper heat-sink block. The water temperature is maintained at 14 °C. The pump source is a commercial highbrightness fiber-coupled multi transverse mode diode laser (100 μm core diameter, 0.22 NA, M2 ≈ 40) with a maximum power of 40 W at the central wavelength of 980 nm. The pump laser output from the fiber is coupled into the Yb:CGA crystal by a coupling system with a magnification of 1:2, forming a beam waist of 200 µm. For optimizing the spatial matching between the pump and oscillation pulses, the Z-fold cavity is designed. Based on the ABCD matrix calculation, the laser beam waist is about 210 μm in the Yb:CGA crystal, which is slightly larger than the pump laser for effectively enhancing the KLM effect. The Yb:CGA oscillator is a linear standing-wave cavity comprising three concave mirrors, M1, M2 and M3 (300 mm, 500 mm and 500 mm ROC), two Gires-Tournois interferometer (GTI) mirrors, a dichroic
Fig. 1. Schematic of the experimental setup for Yb:CGA laser. LD: fiber-coupled diode laser. DM: dichroic mirror. GTI: Gires-Tournois interferometer mirror. SESAM: semiconductor saturable absorber mirror. OC: 10% plane output coupler. 2
Optics and Laser Technology 125 (2020) 106020
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with −1300 fs2 group delay dispersion (GDD) in the range of 1020–1080 nm for each piece are used to compensate positive intracavity dispersion. The GDD introduced by the GTI mirrors is about −10400 fs2 per round trip in the cavity, which ensures that the total intracavity GDD is net anomalous dispersion and closes to zero. As shown in the Fig. 3, with pump power of 26 W, assuming that the pulse has a sech2-shaped temporal intensity profile, the laser delivers 93 fs pulses with 4.5 W of average output power. As displayed in the inset of Fig. 3, the FWHM spectral bandwidth of the laser is measured to be 14.3 nm at the central wavelength of 1050 nm, and the corresponding Fourier-transform-limited pulse duration is 81 fs, calculated by Fourier transforming the spectrum without dispersion. The two measured experimental results indicate a time-bandwidth product (Δτ, Δv) of about 0.361, not far from the transform limit for soliton pulses. As shown in Fig. 4, operating at 80.5 MHz repetition rate, the maximum output power is 4.5 W with 26 W incident pump power. This corresponds to a pulse energy of 56 nJ and a peak power of 0.59 MW, respectively. The linear fit to the data results in an estimated slope efficiency of 17% by using a 10% OC. Obvious damage to the SESAM is observed when the pump power is more than 26 W. With a fixed dispersion compensation in the cavity, as the pump power is increased above pump threshold, the mode-locked pulses spectra broaden and the pulses duration decrease. To estimate the stability of the mode-locking operation, the output pulse trains of Yb:CGA laser are detected by focusing about 10 mW optical power into a 26 GHz spectrum analyzer (R&S FSW26) with a photo-detection. As depicted in the Fig. 5, the radio-frequency (RF) spectrum of the fundamental beat note shows the high signal-to-noise of the laser is more than 65 dB, indicating a clean mode-locked pulse trains at repetition rate of 80.5 MHz. The high harmonics of the fundamental beat note at a wide span from 0 to 1 GHz is shown as an inset in Fig. 5 with a resolution bandwidth of 100 kHz. The absence of other harmonic frequencies and the high contrast in the wide span illustrate a very clean and stable mode-locking operation of the Yb:CGA oscillator. Beam-quality measurements of the oscillator are important for evaluating the usefulness of the laser for various applications and understanding problems in the system. As shown in the Fig. 6(a), we measured the long-time stability of the average output power of the Yb:CGA oscillator. The laser exhibits a stability better than 0.38% rms over 5 h at 4.5 W maximum output power. The fluctuations of output power are due to disturbance of the environment, such as air turbulence and temperature drift. The power stability can be improved by isolation and feedback control system to optimize the pump power and direction. To obtain an accurate M2 measurement, images of the output beam at various distances from a focusing element are used. The spatial profile
Fig. 2. Autocorrelation and optical spectrum (inset) of the measured pulses with fit curves assuming ideal sech2 pulses.
Fig. 3. The intensity autocorrelation trace of laser pulses at 1050 nm with a duration of 93 fs (×1.54, assuming a sech2 pulse shape). Inset: The corresponding mode-locked spectrum.
Fig. 4. Output power of the Yb:CGA laser at 1050 nm versus pump power with a 10% OC.
To obtain the ultrashort pulses from the Yb:CGA oscillator, it is necessary to balance the intracavity dispersion, saturated gain and loss, nonlinear phase shift and self-phase modulation. A pair of GTI mirrors
Fig. 5. Typical radio frequency spectrum of the Yb:CGA laser. Inset: wide spectrum of 1 GHz with the RBW of 100 kHz. 3
Optics and Laser Technology 125 (2020) 106020
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Declaration of Competing Interest The author declare that there is no conflict of interest. Acknowledgement We are grateful for the funding provided by the National Natural Science Foundation of China (Grant Nos. 61905287), Continue Basic Scientific Research Project (Grant Nos.WDJC-2019-02). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optlastec.2019.106020. References [1] M. Drescher, M. Hentschel, R. Kienberger, M. Uiberacker, V. Yakovlev, A. Scrinzi, Th. Westerwalbesloh, U. Kleineberg, U. Heinzmann, F. Krausz, Time-resolved atomic inner-shell spectroscopy, Nature 419 (2002) 803–807, https://doi.org/10. 1038/nature01143. [2] J.J. Macklin, J.K. Trautman, T.D. Harris, L.E. Brus, Imaging and time-resolved spectroscopy of single molecules at an interface, Science 272 (1996) 255–258 https://10.1126/science.272.5259.255. [3] R. Gattass, E. Mazur, Femtosecond laser micromachining in transparent materials, Nat. Photon. 2 (2008) 219–225, https://doi.org/10.1038/nphoton.2008.47. [4] D. Jones, S. Diddams, J. Ranka, A. Stentz, R. Windeler, J. Hall, S. Cundiff, Carrierenvelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis, Science 288 (2000) 635–639 https://10.1126/science.288.5466.635. [5] R. Jones, K. Moll, M. Thorpe, J. Ye, Phase-coherent frequency combs in the vacuum ultraviolet via high-harmonic generation inside a femtosecond enhancement cavity, Phys. Rev. Lett. 94 (2005) 193201, https://doi.org/10.1103/PhysRevLett. 94. 193201. [6] R.L. Fork, C.V. Shank, C. Hirlimann, R. Yen, W.J. Tomlinson, Femtosecond whitelight continuum pulses, Opt. Lett. 8 (1983) 1–3, https://doi.org/10.1364/OL.8. 000001. [7] S. Woutersen, U. Emmerichs, H.J. Bakker, Femtosecond mid-IR pump-probe spectroscopy of liquid water: evidence for a two-component structure, Science 278 (1997) 658–660 https://10.1126/science.278.5338.658. [8] A. Baltuška, T. Fuji, T. Kobayashi, Controlling the carrier-envelope phase of ultrashort light pulses with optical parametric amplifiers, Phys. Rev. Lett. 88 (2002) 133901, https://doi.org/10.1103/PhysRevLett. 88.133901. [9] J. Aus der Au, G.J. Spühler, T. Südmeyer, R. Paschotta, R. Hövel, M. Moser, S. Erhard, M. Karszewski, A. Giesen, U. Keller, 16.2-W average power from a diodepumped femtosecond Yb:YAG thin disk laser, Opt. Lett. 25 (2000) 859–861, https://doi.org/10.1364/OL.25.000859. [10] S. Ricaud, A. Jaffres, K. Wentsch, A. Suganuma, B. Viana, P. Loiseau, B. Weichelt, M. Abdou-Ahmed, A. Voss, T. Graf, D. Rytz, C. Hönninger, E. Mottay, P. Georges, F. Druon, femtosecond Yb:CaGdAlO4 thin-disk oscillator, Opt. Lett. 37 (2012) 3984–3986, https://doi.org/10.1364/OL.37.003984. [11] J. Ma, H. Huang, K. Ning, X. Xu, G. Xie, L. Qian, K.P. Loh, D. Tang, Generation of 30 fs pulses from a diode-pumped graphene mode-locked Yb:CaYAlO4 laser, Opt. Lett. 41 (2016) 890–893, https://doi.org/10.1364/OL.41.000890. [12] Z. Gao, J. Zhu, J. Wang, Z. Wang, Z. Wei, X. Xu, L. Zheng, L. Su, J. Xu, Diodepumped Kerr-lens mode-locked Yb:CaGdAlO4 laser with tunable wavelength, Laser Phys. Lett. 13 (2016) 015302, https://doi.org/10.1088/1612-2011/13/1/015302. [13] F. Hoos, T.P. Meyrath, S. Li, B. Braun, H. Giessen, Femtosecond 5-W Yb:KGW slab laser oscillator pumped by a single broad-area diode and its application as supercontinuum source, Appl. Phys. B 96 (2009) 5–10, https://doi.org/10.1007/s00340009-3421-3. [14] H. Zhao, A. Major, Powerful 67 fs Kerr-lens mode-locked prismless Yb:KGW oscillator, Opt. Express 21 (2013) 31846–31851, https://doi.org/10.1364/OE.21. 031846. [15] P. Wasylczyk, P. Wnuk, C. Radzewicz, Passively modelocked, diode-pumped Yb:KYW femtosecond oscillator with 1 GHz repetition rate, Opt. Express 17 (2009) 5630–5636, https://doi.org/10.1364/OE.17.005630. [16] A. Schmidt, V. Petrov, U. Griebner, R. Peters, K. Petermann, G. Huber, C. Fiebig, K. Paschke, G. Erbert, Diode-pumped mode-locked Yb:LuScO3 single crystal laser with 74 fs pulse duration, Opt. Lett. 35 (2010) 511–513, https://doi.org/10.1364/ OL.35.000511. [17] W. Li, Q. Hao, H. Zhai, H. Zeng, W. Lu, G. Zhao, L. Zheng, L. Su, J. Xu, Diodepumped Yb:GSO femtosecond laser, Opt. Lett. 15 (2007) 2354–2359, https://doi. org/10.1364/OE.15.002354. [18] W. Tian, Z. Wang, J. Liu, J. Zhu, L. Zheng, X. Xu, J. Xu, Z. Wei, Dissipative soliton and synchronously dual-wavelength mode-locking Yb:YSO lasers, Opt. Express 23 (2015) 8731–8739, https://doi.org/10.1364/OE.23.008731. [19] J.A. Berger, M.J. Greco, W.A. Schroeder, High-power, femtosecond, thermal-lensshaped Yb:KGW oscillator, Opt. Express 16 (2008) 8629–8640, https://doi.org/10. 1364/OE.16.008629.
Fig. 6. (a) Average output power and temporal stability of the Yb:CGA oscillator. (b) Spatial profile of the laser beam with maximum power.
and beam quality of the laser beam in high-power output (4.5 W) situation is shown in Fig. 6(b). The M2 factors are measured to be M2x = 1.13, M2y = 1.15 in the horizontal and vertical directions, respectively. The beam spatial profile is also measured with a chargecoupled device (CCD) camera, which exhibits a Gaussian shape to a good approximation.
4. Conclusion In summary, we have demonstrated a diode-pumped passively mode-locked Yb:CGA laser. By optimizing the intracavity dispersion, the stable operation is achieved with a maximum output power of 4.5 W at repetition rate of 80.5 MHz, the corresponding pules energy is 56 nJ and peak power is 0.59 MW, respectively. The laser pulses duration is 93 fs at the central wavelength of 1050 nm with 14.3 nm bandwidth. Shorter pulse duration may be achieved by further controlling the intracavity dispersion as well as compensating outside the cavity. Limited by the threshold of the SESAM, the passively modelocked Yb:CGA oscillator generates average power of several watt level. Future works will be focused on improving the output power by optimizing the laser power density on the SESAM. Furthermore, it should be possible to generate sub-100 fs pulses in the green by means of selffrequency doubling because of the birefringent property of Yb:CGA cyrstal. Such a high-power, sub-100 fs all-solid-state femtosecond laser source will be applied in many applications.
4
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Yb:CALGO based oscillators, 823511, Proc. SPIE 8235 (2012), https://doi.org/10. 1117/12.906575. [25] X. Meng, Z. Wang, W. Tian, H. He, S. Fang, Z. Wei, Watt-level widely tunable femtosecond mid-infrared KTiOAsO4 optical parametric oscillator pumped by a 1.03 μm Yb:KGW laser, Opt. Lett. 43 (2018) 943–946, https://doi.org/10.1364/OL. 43.000943. [26] T. Lang, T. Binhammer, S. Rausch, G. Palmer, M. Emons, M. Schultze, A. Harth, U. Morgner, High power ultra-widely tuneable femtosecond pulses from a noncollinear optical parametric oscillator (NOPO), Opt. Express 20 (2012) 912–917, https://doi.org/10.1364/OE.20.000912. [27] K. Kieu, W.H. Renninger, A. Chong, F.W. Wise, Sub-100 fs pulses at watt-level powers from a dissipative-soliton fiber laser, Opt. Express 34 (2009) 593–595, https://doi.org/10.1364/OL.34.000593.
[20] A. Agnesi, A. Greborio, F. Pirzio, G. Reali, J.A. Au, A. Guandalini, 40-fs Yb3+:CaGdAlO4 laser pumped by a single-mode 350-mW laser diode, Opt. Express 20 (2012) 10077–10082, https://doi.org/10.1364/OE.20.010077. [21] W. Tian, Z. Yi Peng, Z. Zhang, J. Yu, X. Zhu, Z. Xu, We, Diode-pumped power scalable Kerr-lens mode-locked Yb:CYA laser, Photon. Res. 6 (2018) 127–131, https://doi.org/10.1364/PRJ.6.000127. [22] Y. Zaouter, J. Didierjean, F. Balembois, G.L. Leclin, F. Druon, P. Georges, J. Petit, P. Goldner, B. Viana, 47-fs diode-pumped Yb3+:CaGdAlO4 laser, Opt. Lett. 31 (2006) 119–121, https://doi.org/10.1364/OL.31.000119. [23] J. Boudeile, F. Druon, M. Hanna, P. Georges, Y. Zaouter, E. Cormier, J. Petit, P. Goldner, B. Viana, Continuous-wave and femtosecond laser operation of Yb:CaGdAlO4 under high-power diode pumping, Opt. Lett. 32 (2007) 1962–1964, https://doi.org/10.1364/OL.32.001962. [24] A. Greborioa, A. Guandalinib, J.A. der Au, Sub-100 fs pulses with 12.5-W from
5
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
93 fs pulses at watt-level powers from a diode pumped Yb:CaGdAlO4 laser ⁎
T
Xianghao Meng , Chong Lv, Baozhen Zhao, Xiaofeng Xi, Qiushi Liu, Xiaohua Zhang, Yuchen Li China Institute of Atomic Energy, Beijing 102413, China
H I GH L IG H T S
study on the performances of laser in the net normal or net anomalous dispersion region. • Experimental mode-locked pulses with a duration of 94 fs centered at 1050 nm. • Stable • The pulse energy is 59 nJ and the peak power is 0.56 MW.
A R T I C LE I N FO
A B S T R A C T
Keywords: Mode-locked lasers Ultrafast laser Diode pumped
We investigated the performance of diode pumped mode-locked Yb:CGA laser operating in the net normal or net anomalous dispersion region. With 26 W pump power, stable femtosecond laser pulses with average power of 4.5 W are obtained, yielding a pump power slope efficiency of 17%. Operating at 80.5 MHz repetition rate, the laser delivers as short as 93 fs pulse duration with average output power of 4.5 W, corresponding to a pulse energy of 56 nJ and a peak power of 0.59 MW. The oscillator exhibits a passive rms power stability of ~0.38% rms over 5 h in high beam quality.
1. Introduction High-power all-solid-state femtosecond laser sources have created profound impact in the fields of time-resolved spectroscopy, nonlinear optics, industrial processing and pump-probe measurements [1–8]. For the past few years, ytterbium-doped (Yb3+-doped) materials have become the candidates for diode-pumped femtosecond laser oscillators and regenerative amplifiers. It can be attributed to high quantum efficiency, good thermal properties, broad emission bandwidths and consequent small (~6%) quantum defect. As the most important advantage, they can be directly pumped by the commercially high brightness InGaAs laser diodes because the absorption band of Yb3+ ions matches well with the emission band of the InGaAs laser diodes. Recently, a various of results have been reported from diode pumped mode-locked femtosecond oscillators [9–18]. In 2008, Berger et al. report a diode-pumped Yb:KGW oscillator which delivered 250 fs pulse duration with average output power of 3.5 W [19]. In 2012, Agnesi et al. demonstrated 15 mW average output power with 40 fs from a mode-locked Yb:CGA oscillator [20]. Next year, Haitao et al. reported 67 fs pulses generation with 3 W output power from a diode pumped Kerr-lens mode-locked Yb:KGW laser [14]. In 2018, Tian et al. reported 30 nJ pulse energy with 68 fs pulse duration from a mode-locked Yb:CYA laser [21]. Among those Yb3+-doped materials, Yb:CaGdAlO4 ⁎
(Yb:CGA) is one of the most promising gain media for short pulse duration and high-power oscillators owing to its large emission bandwidths and relatively high thermal conductivity (Ka = 6.9 W m−1 K−1 and Kc = 6.3 W m−1 K−1). The Yb:CGA has a full width at half-maximum (FWHM) spectral bandwidth of about 60 nm at σ polarization, supporting sub-40 fs laser generation. In addition, the crystal presents favorable high mechanical strength (Mohs hardness 6), which is beneficial to large size growth. Utilizing the semiconductor saturable absorber mirrors (SESAM) for passive mode-locking, the Yb:CGA lasers have generated 38 mW of output power with 47 fs pulses and 520 mW with 68 fs pulses [22,23]. As an alternative to SESAM, the Kerr-lens mode-locking (KLM) is another approach to produce ultrashort pulses. For example, the 115 MHz oscillator is mode-locked with Kerr-lens effect, providing 60 fs pulses at an output power of 66 mW [12]. Recently researches reported pulse generation as short as 30 fs in this gain medium, which is the shortest pulse duration ever achieved from an Ybdoped bulk material-based oscillator [11]. In 2012, Greborioa et al. report on the generation of 94 fs pulses with 12.5 W average output power from an 80 MHz Yb:CGA oscillator, which is the highest average power for diode-pumped bulk oscillators [24]. However, these modelocked short pulse oscillators based on SESAM and KLM generate average output power between several hundreds of milliwatts and a dozen watts, limiting the applications in the fields of ultrafast lasers,
Corresponding author. E-mail address: [email protected] (X. Meng).
https://doi.org/10.1016/j.optlastec.2019.106020 Received 21 August 2019; Received in revised form 29 October 2019; Accepted 19 December 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
Optics and Laser Technology 125 (2020) 106020
X. Meng, et al.
mirror (DM), a SESAM and a 10% plane output coupler (OC). To obtain high output power, an OC with 10% transmittance is used in the cavity. The pump laser is focused into the Yb:CGA crystal through a DM, which is deliberately coated to support high transmission at 980 nm for the pump and high reflection for the oscillation pulses in the range of 1000–1100 nm. The concave mirrors and GTI mirrors have high reflection coating (R > 99.9%) over 1000–1100 nm range. The modelocked pulses are generated using a SESAM (Batop GmbH) operated at 1064 nm with a modulation depth of ΔR = 2.4%, a saturation fluence of 70 μJ/cm2 and a relaxation time of 1 ps, which is fastened at a onedimensional translation stage. The distance between the M3 and SESAM could be precisely optimized through a micrometer driven translation stage. This allowed for continuous control of the beam size on the surface of the SESAM. The total cavity length is set to about 1.86 m corresponding to the repetition rate of 80.5 MHz.
such as seed for high-energy amplifiers, frequency comb and nonlinear frequency conversion [25,26]. For oscillators using Yb:CGA as gain medium, the sub-100 fs with pulse energies of hundreds of nano-joules remains a very challenging frontier although they present excellent laser performance in the femtosecond regime. Compared to nonlinear Schrödinger equation (NLSE) type solitons, dissipative solitons (DS) have larger pulse energy owing to large capacity and difficulty in soliton splitting. DS operation of the mode-locked femtosecond lasers have been reported in all normal dispersion Yb3+-doped fiber lasers [27]. However, the DS operation of solid-state lasers is less reported in the Yb3+-doped solid state lasers. In this letter, we investigated the preference of ultrashort pulse generation based on diode pumped mode-locked Yb:CGA laser, which operated in the net normal or net anomalous dispersion region. Optimizing the laser operation conditions based on soliton theory, the Yb:CGA laser can generate as high as 4.5 W average output power, delivering as short as 93 fs pulse duration centred at 1050 nm. Operating at 80.5 MHz repetition rates, the pulse energy is 56 nJ and the peak power is 0.59 MW, respectively. The output pulses exhibit a passive rms power stability of 0.38% over 5 h. In addition, the M2 factors in the horizontal and vertical directions are 1.13 and 1.15, respectively.
3. Experimental results and disscussion Based on this high-brightness diode laser, we first aligned continuous-wave operation in the cavity. The maximum output power is 3 W with 15 W pump power incident on the crystal. Then a Q-switched mode-locking (QML) operation is established by using the SESAM instead of the HR mirror. Large spot size on the SESAM is the main reason that caused the instability of the QML operation. With finely optimizing the position and angle of the SESAM, the QML gradually gives way to the stable self-starting CW mode-locking regime. It is noticed that once the mode-locking is established, the laser pulses are always stable without any Q-switched envelope in the condition of maximum pump power level although the Yb:CGA oscillator is currently built directly on the optical table without any housing. Initially, without any dispersion compensation elements (GTI) in the cavity, we measured autocorrelation trace of the short pulses by using a commercial intensity autocorrelator (A. P. E. GmbH, pulse Check USB). As shown in Fig. 2, assuming a sech2-shaped temporal intensity profile, the laser delivers 8.4 ps pulses at an output power of 4.8 W. As shown in the inset of Fig. 2, the mode-locked spectra have a rectangular-shape with the characteristic sharp steep edges, which indicates the laser operates in the net normal dispersion region. The central wavelength is 1046 nm with a full width at half maximum (FWHM) bandwidth of 7 nm. The corresponding time-bandwidth products (Δτ, Δv) is 16.1, which is 51 times of the Fourier transform-limited value (0.315). The strongly chirped pulses with distinctive characteristics of DS shows the DS operation in the Yb:CGA oscillator.
2. Experimental setup The experimental setup of mode-locked Yb:CGA oscillator is shown in Fig. 1. A 4-mm-long, 5-at. % Yb3+-doped, c-cut Yb:CGA is used as the gain medium, which end faces are coated with high transmission at 980–1100 nm. The crystal is wrapped with indium film and then placed tightly on a water-cooling copper heat-sink block. The water temperature is maintained at 14 °C. The pump source is a commercial highbrightness fiber-coupled multi transverse mode diode laser (100 μm core diameter, 0.22 NA, M2 ≈ 40) with a maximum power of 40 W at the central wavelength of 980 nm. The pump laser output from the fiber is coupled into the Yb:CGA crystal by a coupling system with a magnification of 1:2, forming a beam waist of 200 µm. For optimizing the spatial matching between the pump and oscillation pulses, the Z-fold cavity is designed. Based on the ABCD matrix calculation, the laser beam waist is about 210 μm in the Yb:CGA crystal, which is slightly larger than the pump laser for effectively enhancing the KLM effect. The Yb:CGA oscillator is a linear standing-wave cavity comprising three concave mirrors, M1, M2 and M3 (300 mm, 500 mm and 500 mm ROC), two Gires-Tournois interferometer (GTI) mirrors, a dichroic
Fig. 1. Schematic of the experimental setup for Yb:CGA laser. LD: fiber-coupled diode laser. DM: dichroic mirror. GTI: Gires-Tournois interferometer mirror. SESAM: semiconductor saturable absorber mirror. OC: 10% plane output coupler. 2
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with −1300 fs2 group delay dispersion (GDD) in the range of 1020–1080 nm for each piece are used to compensate positive intracavity dispersion. The GDD introduced by the GTI mirrors is about −10400 fs2 per round trip in the cavity, which ensures that the total intracavity GDD is net anomalous dispersion and closes to zero. As shown in the Fig. 3, with pump power of 26 W, assuming that the pulse has a sech2-shaped temporal intensity profile, the laser delivers 93 fs pulses with 4.5 W of average output power. As displayed in the inset of Fig. 3, the FWHM spectral bandwidth of the laser is measured to be 14.3 nm at the central wavelength of 1050 nm, and the corresponding Fourier-transform-limited pulse duration is 81 fs, calculated by Fourier transforming the spectrum without dispersion. The two measured experimental results indicate a time-bandwidth product (Δτ, Δv) of about 0.361, not far from the transform limit for soliton pulses. As shown in Fig. 4, operating at 80.5 MHz repetition rate, the maximum output power is 4.5 W with 26 W incident pump power. This corresponds to a pulse energy of 56 nJ and a peak power of 0.59 MW, respectively. The linear fit to the data results in an estimated slope efficiency of 17% by using a 10% OC. Obvious damage to the SESAM is observed when the pump power is more than 26 W. With a fixed dispersion compensation in the cavity, as the pump power is increased above pump threshold, the mode-locked pulses spectra broaden and the pulses duration decrease. To estimate the stability of the mode-locking operation, the output pulse trains of Yb:CGA laser are detected by focusing about 10 mW optical power into a 26 GHz spectrum analyzer (R&S FSW26) with a photo-detection. As depicted in the Fig. 5, the radio-frequency (RF) spectrum of the fundamental beat note shows the high signal-to-noise of the laser is more than 65 dB, indicating a clean mode-locked pulse trains at repetition rate of 80.5 MHz. The high harmonics of the fundamental beat note at a wide span from 0 to 1 GHz is shown as an inset in Fig. 5 with a resolution bandwidth of 100 kHz. The absence of other harmonic frequencies and the high contrast in the wide span illustrate a very clean and stable mode-locking operation of the Yb:CGA oscillator. Beam-quality measurements of the oscillator are important for evaluating the usefulness of the laser for various applications and understanding problems in the system. As shown in the Fig. 6(a), we measured the long-time stability of the average output power of the Yb:CGA oscillator. The laser exhibits a stability better than 0.38% rms over 5 h at 4.5 W maximum output power. The fluctuations of output power are due to disturbance of the environment, such as air turbulence and temperature drift. The power stability can be improved by isolation and feedback control system to optimize the pump power and direction. To obtain an accurate M2 measurement, images of the output beam at various distances from a focusing element are used. The spatial profile
Fig. 2. Autocorrelation and optical spectrum (inset) of the measured pulses with fit curves assuming ideal sech2 pulses.
Fig. 3. The intensity autocorrelation trace of laser pulses at 1050 nm with a duration of 93 fs (×1.54, assuming a sech2 pulse shape). Inset: The corresponding mode-locked spectrum.
Fig. 4. Output power of the Yb:CGA laser at 1050 nm versus pump power with a 10% OC.
To obtain the ultrashort pulses from the Yb:CGA oscillator, it is necessary to balance the intracavity dispersion, saturated gain and loss, nonlinear phase shift and self-phase modulation. A pair of GTI mirrors
Fig. 5. Typical radio frequency spectrum of the Yb:CGA laser. Inset: wide spectrum of 1 GHz with the RBW of 100 kHz. 3
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Declaration of Competing Interest The author declare that there is no conflict of interest. Acknowledgement We are grateful for the funding provided by the National Natural Science Foundation of China (Grant Nos. 61905287), Continue Basic Scientific Research Project (Grant Nos.WDJC-2019-02). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optlastec.2019.106020. References [1] M. Drescher, M. Hentschel, R. Kienberger, M. Uiberacker, V. Yakovlev, A. Scrinzi, Th. Westerwalbesloh, U. Kleineberg, U. Heinzmann, F. Krausz, Time-resolved atomic inner-shell spectroscopy, Nature 419 (2002) 803–807, https://doi.org/10. 1038/nature01143. [2] J.J. Macklin, J.K. Trautman, T.D. Harris, L.E. Brus, Imaging and time-resolved spectroscopy of single molecules at an interface, Science 272 (1996) 255–258 https://10.1126/science.272.5259.255. [3] R. Gattass, E. Mazur, Femtosecond laser micromachining in transparent materials, Nat. Photon. 2 (2008) 219–225, https://doi.org/10.1038/nphoton.2008.47. [4] D. Jones, S. Diddams, J. Ranka, A. Stentz, R. Windeler, J. Hall, S. Cundiff, Carrierenvelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis, Science 288 (2000) 635–639 https://10.1126/science.288.5466.635. [5] R. Jones, K. Moll, M. Thorpe, J. Ye, Phase-coherent frequency combs in the vacuum ultraviolet via high-harmonic generation inside a femtosecond enhancement cavity, Phys. Rev. Lett. 94 (2005) 193201, https://doi.org/10.1103/PhysRevLett. 94. 193201. [6] R.L. Fork, C.V. Shank, C. Hirlimann, R. Yen, W.J. Tomlinson, Femtosecond whitelight continuum pulses, Opt. Lett. 8 (1983) 1–3, https://doi.org/10.1364/OL.8. 000001. [7] S. Woutersen, U. Emmerichs, H.J. Bakker, Femtosecond mid-IR pump-probe spectroscopy of liquid water: evidence for a two-component structure, Science 278 (1997) 658–660 https://10.1126/science.278.5338.658. [8] A. Baltuška, T. Fuji, T. Kobayashi, Controlling the carrier-envelope phase of ultrashort light pulses with optical parametric amplifiers, Phys. Rev. Lett. 88 (2002) 133901, https://doi.org/10.1103/PhysRevLett. 88.133901. [9] J. Aus der Au, G.J. Spühler, T. Südmeyer, R. Paschotta, R. Hövel, M. Moser, S. Erhard, M. Karszewski, A. Giesen, U. Keller, 16.2-W average power from a diodepumped femtosecond Yb:YAG thin disk laser, Opt. Lett. 25 (2000) 859–861, https://doi.org/10.1364/OL.25.000859. [10] S. Ricaud, A. Jaffres, K. Wentsch, A. Suganuma, B. Viana, P. Loiseau, B. Weichelt, M. Abdou-Ahmed, A. Voss, T. Graf, D. Rytz, C. Hönninger, E. Mottay, P. Georges, F. Druon, femtosecond Yb:CaGdAlO4 thin-disk oscillator, Opt. Lett. 37 (2012) 3984–3986, https://doi.org/10.1364/OL.37.003984. [11] J. Ma, H. Huang, K. Ning, X. Xu, G. Xie, L. Qian, K.P. Loh, D. Tang, Generation of 30 fs pulses from a diode-pumped graphene mode-locked Yb:CaYAlO4 laser, Opt. Lett. 41 (2016) 890–893, https://doi.org/10.1364/OL.41.000890. [12] Z. Gao, J. Zhu, J. Wang, Z. Wang, Z. Wei, X. Xu, L. Zheng, L. Su, J. Xu, Diodepumped Kerr-lens mode-locked Yb:CaGdAlO4 laser with tunable wavelength, Laser Phys. Lett. 13 (2016) 015302, https://doi.org/10.1088/1612-2011/13/1/015302. [13] F. Hoos, T.P. Meyrath, S. Li, B. Braun, H. Giessen, Femtosecond 5-W Yb:KGW slab laser oscillator pumped by a single broad-area diode and its application as supercontinuum source, Appl. Phys. B 96 (2009) 5–10, https://doi.org/10.1007/s00340009-3421-3. [14] H. Zhao, A. Major, Powerful 67 fs Kerr-lens mode-locked prismless Yb:KGW oscillator, Opt. Express 21 (2013) 31846–31851, https://doi.org/10.1364/OE.21. 031846. [15] P. Wasylczyk, P. Wnuk, C. Radzewicz, Passively modelocked, diode-pumped Yb:KYW femtosecond oscillator with 1 GHz repetition rate, Opt. Express 17 (2009) 5630–5636, https://doi.org/10.1364/OE.17.005630. [16] A. Schmidt, V. Petrov, U. Griebner, R. Peters, K. Petermann, G. Huber, C. Fiebig, K. Paschke, G. Erbert, Diode-pumped mode-locked Yb:LuScO3 single crystal laser with 74 fs pulse duration, Opt. Lett. 35 (2010) 511–513, https://doi.org/10.1364/ OL.35.000511. [17] W. Li, Q. Hao, H. Zhai, H. Zeng, W. Lu, G. Zhao, L. Zheng, L. Su, J. Xu, Diodepumped Yb:GSO femtosecond laser, Opt. Lett. 15 (2007) 2354–2359, https://doi. org/10.1364/OE.15.002354. [18] W. Tian, Z. Wang, J. Liu, J. Zhu, L. Zheng, X. Xu, J. Xu, Z. Wei, Dissipative soliton and synchronously dual-wavelength mode-locking Yb:YSO lasers, Opt. Express 23 (2015) 8731–8739, https://doi.org/10.1364/OE.23.008731. [19] J.A. Berger, M.J. Greco, W.A. Schroeder, High-power, femtosecond, thermal-lensshaped Yb:KGW oscillator, Opt. Express 16 (2008) 8629–8640, https://doi.org/10. 1364/OE.16.008629.
Fig. 6. (a) Average output power and temporal stability of the Yb:CGA oscillator. (b) Spatial profile of the laser beam with maximum power.
and beam quality of the laser beam in high-power output (4.5 W) situation is shown in Fig. 6(b). The M2 factors are measured to be M2x = 1.13, M2y = 1.15 in the horizontal and vertical directions, respectively. The beam spatial profile is also measured with a chargecoupled device (CCD) camera, which exhibits a Gaussian shape to a good approximation.
4. Conclusion In summary, we have demonstrated a diode-pumped passively mode-locked Yb:CGA laser. By optimizing the intracavity dispersion, the stable operation is achieved with a maximum output power of 4.5 W at repetition rate of 80.5 MHz, the corresponding pules energy is 56 nJ and peak power is 0.59 MW, respectively. The laser pulses duration is 93 fs at the central wavelength of 1050 nm with 14.3 nm bandwidth. Shorter pulse duration may be achieved by further controlling the intracavity dispersion as well as compensating outside the cavity. Limited by the threshold of the SESAM, the passively modelocked Yb:CGA oscillator generates average power of several watt level. Future works will be focused on improving the output power by optimizing the laser power density on the SESAM. Furthermore, it should be possible to generate sub-100 fs pulses in the green by means of selffrequency doubling because of the birefringent property of Yb:CGA cyrstal. Such a high-power, sub-100 fs all-solid-state femtosecond laser source will be applied in many applications.
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