Mode-locked Tm,Ho:YAP laser around. μm K. J. Yang,, * D. C. Heinecke, C. Kölbl, T. Dekorsy, S. Z. Zhao, L. H. Zheng, J. Xu, and G. J. Zhao Department of Physics and Center of Applied Photonics, University of Konstanz, 757 Konstanz, Germany School of Information Science and Engineering, Shandong University, Jinan, 5, China Key Laboratory of Transparent and Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 5 Chengbei Road, Shanghai,, China Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, No.9 Qinghe Road, Shanghai, China * k.j.yang@sdu.edu.cn Abstract: A passively mode-locked Tm,Ho:YAP laser around. μm waveleng employing a semiconductor saturable absorber mirror is demonstrated. Stable continuous wave mode-locking operation was achieved at variable center wavelengs of 6.5 nm, 6.5 nm, 95.5 nm,.5 nm, and nm, respectively. Pulses as short as. ps were obtained at 6.5 nm wi a spectral FWHM of.5 nm at output powers of mw and a repetition rate around 7 MHz. A maximum output power of mw was obtained at nm wi a pulse duration of 66 ps. Optical Society of America OCIS codes: (.79) Ultrafast lasers; (.5) Mode-locked lasers; (.7) Infrared and far-infrared lasers. References and links. T. M. Taczak and D. K. Killinger, Development of a tunable, narrow-linewid, cw.66-μm Ho:YLF laser for remote sensing of atmospheric CO and H O, Appl. Opt. 7(6), 6 76 (99).. P. A. Budni, L. A. Pomeranz, M. L. Lemons, C. A. Miller, J. R. Mosto, and E. P. Chicklis, Efficient midinfrared laser using.9-μm pumped Ho:YAG and ZnGeP optical parametric oscillators, J. Opt. Soc. Am. B 7(5), 7 7 ().. C. H. Zhang, P. B. Meng, B. Q. Yao, G. Li, Y. L. Ju, and Y. Z. Wang, Efficient Cr:ZnSe Laser wi a Volume Bragg Grating, Laser Phys. (), 7 ().. K. J. Yang, H. Bromberger, H. Ruf, H. Schäfer, J. Neuhaus, T. Dekorsy, C. V. B. Grimm, M. Helm, K. Biermann, and H. Künzel, Passively mode-locked Tm,Ho:YAG laser at µm based on saturable absorption of intersubband transitions in quantum wells, Opt. Express (7), 657 65 (). 5. J. Ma, G. Q. Xie, W. L. Gao, P. Yuan, L. J. Qian, H. H. Yu, H. J. Zhang, and J. Y. Wang, Diode-pumped modelocked femtosecond Tm:CLNGG disordered crystal laser, Opt. Lett. 7(), 76 7 (). 6. I. A. Denisov, N. A. Skoptsov, M. S. Gaponenko, A. M. Malyarevich, K. V. Yumashev, and A. A. Lipovskii, Passive mode locking of.9 μm Cr,Tm,Ho:Y Sc Al O laser using PbS quantum-dot-doped glass, Opt. Lett. (), 5 (9). 7. A. A. Lagatsky, X. Han, M. D. Serrano, C. Cascales, C. Zaldo, S. Calvez, M. D. Dawson, J. A. Gupta, C. T. A. Brown, and W. Sibbett, Femtosecond (9 fs) NaY(WO ) Tm,Ho-codoped laser at 6 nm, Opt. Lett. 5(), 7 9 ().. A. Schmidt, P. Koopmann, G. Huber, P. Fuhrberg, S. Y. Choi, D. I. Yeom, F. Rotermund, V. Petrov, and U. Griebner, 75 fs Tm:Lu O laser at.7 µm mode-locked using single-walled carbon nanotubes, Opt. Express (5), 5 5 (). 9. K. J. Yang, H. Bromberger, D. Heinecke, C. Kölbl, H. Schäfer, T. Dekorsy, S. Z. Zhao, L. H. Zheng, J. Xu, and G. J. Zhao, Efficient continuous wave and passively mode-locked Tm-doped crystalline silicate laser, Opt. Express (7), 6 65 ().. H. Bromberger, K. J. Yang, D. Heinecke, T. Dekorsy, L. H. Zheng, J. Xu, and G. J. Zhao, Comparative investigations on continuous wave operation of a-cut and b-cut Tm,Ho:YAlO lasers at room temperature, Opt. Express 9(7), 655 65 ().. R. C. Stoneman and L. Esterowitz, Efficient.9-μm Tm:YAlO laser, IEEE J. Sel. Top. Quantum Electron. (), 7 (995).. I. F. Elder and M. J. P. Payne, Lasing in diode-pumped Tm:YAP, Tm,Ho:YAP and Tm,Ho:YLF, Opt. Commun. 5(-6), 9 9 (99).. B. Q. Yao, Y. Tian, G. Li, and Y. Z. Wang, InGaAs/GaAs saturable absorber for diode-pumped passively Q- switched dual-waveleng Tm:YAP lasers, Opt. Express (), 57 579 ().. J. Li, S. H. Yang, C. M. Zhao, H. Y. Zhang, and W. Xie, High efficient single-frequency output at 99 nm from a diode-pumped Tm:YAP coupled cavity, Opt. Express (), 6 67 (). (C) OSA January / Vol., No. / OPTICS EXPRESS 57
5. Z. S. Qu, Y. G. Wang, J. Liu, L. H. Zheng, L. B. Su, and J. Xu, Passively mode-locked -µm Tm:YAP laser wi a double-wall carbon nanotube absorber, Chin. Phys. B (6), 6 (). 6. J. Liu, Y. G. Wang, Z. S. Qu, L. H. Zheng, L. B. Su, and J. Xu, Graphene oxide absorber for μm passive mode-locking Tm:YAlO laser, Laser Phys. Lett. 9(), 5 9 (). 7. B. Q. Yao, L. J. Li, L. L. Zheng, Y. Z. Wang, G. J. Zhao, and J. Xu, Diode-pumped continuous wave and Q- switched operation of a c-cut Tm,Ho:YAlO laser, Opt. Express 6(7), 575 5 ().. Introduction Apart from e applications in e fields of light detection and ranging (LIDAR), frequency metrology, time-resolved spectroscopy, laser microsurgery, plastics material processing, and free space optical communication [, ], ultrafast laser sources around µm wi high peak power are gaining more and more attention as potential pumping of solid state lasers such as Cr + :ZnSe [] and optical parametric oscillators (OPOs) for e mid- and far-infrared spectral regions. Based on e above listed wide potential applications, waveleng tunable laser sources near µm are potentially of high interest. Due to a strong absorption band around nm covered by commercial AlGaAs diode lasers and a quantum efficiency up to two via cross relaxation, passively mode-locked Tm + or Tm + -Ho + doped lasers have become prevalent options to obtain µm ultrashort pulses efficiently. To date passively mode-locked Tm + - doped or Tm + -Ho + co-doped garnet [ 6], tungstate [7], sesquioxide [], and silicate [9] crystalline lasers at µm have been successfully realized based on semiconductor saturable absorber mirrors (SESAMs) [7] or saturable absorption in carbon nanotubes (CNTs) [], intersubband transitions (ISBTs) in quantum wells [], graphene [5], as well as PbS quantum dots [6]. Meanwhile, on-going efforts explore novel ultrashort µm laser systems combining different gain media and mode-locking meods. The biaxial crystal yttrium aluminium oxide (YAlO ), short YALO or YAP, has a natural birefringence at dominates ermally induced birefringence due to e anisotropic lattice structure. Thus ermally induced degradation is suppressed to a certain extent and e generation of linearly polarized emission becomes easy []. In comparison wi YAG crystals, Tm + ions doped in a YAP host have a broader strong absorption peak wi a FWHM of about nm along e b-axis at around 795 nm [], which makes YAP crystals a promising laser host for ulium or ulium-holmium doping. Very recently, we have demonstrated a maximum slope efficiency of nearly 6% and a quantum efficiency of above. from a Tm,Ho:YAP laser []. Alough e continuous wave (CW) and Q-switched operations wi Tm + -doped or Tm + -Ho + co-doped YAP crystals have been reported [, ], e modelocking regimes were less explored. Only recently mode-locked Tm:YAP lasers wi CNTs and graphene oxides were demonstrated in [5, 6]. However, ere are no reports on e mode-locking operation of a Tm,Ho:YAP laser. Here we report - for e first time to our best knowledge - a passively mode-locked Tm,Ho:YAP laser around μm wi a semiconductor saturable absorber mirror. Stable CW mode-locking was achieved at variable wavelengs of 6.5 nm, 6.5 nm, 95.5 nm,.5 nm, and nm wi pulse durations of ps, 66 ps,. ps,. ps and.6 ps, respectively. The shortest pulse wi duration of. ps was obtained at 6.5 nm wi an output power of mw and a repetition rate around 7 MHz.. al setup and results The employed schematic laser setup is shown in Fig.. A b-cut 5 at.% ulium and.at.% holmium doped YAP crystal wi size of mm was grown by e Czochralski technique (Shanghai Institute of Ceramics, China). Bo faces of e crystal were antireflection coated from 75 to 5 nm (reflectivity < %) and 9- nm (reflectivity <.%). The laser crystal was wrapped in indium foil and water-cooled to C. A CW linearly polarized Ti:sapphire laser tunable from 76 nm to 59 nm was used as e pump source. Mirrors M and M had e same radii of curvature of mm and reflectivity of 99.9% from to 5 nm. The front surface of mirror M was also anti-reflection coated at e wavelengs of 75-5 nm wi a reflectivity less an.5%. Concave mirrors M, M, and M 5 wi respective curvature radii of mm, 5 mm and mm were all high reflectivity (C) OSA January / Vol., No. / OPTICS EXPRESS 575
coated from to 5 nm (reflectivity >99.9%). All e cavity mirrors except e output couplers (OCs) were chirped wi group delay dispersion (GDD) of ~- fs in e nm to 5 nm spectral region. OCs wi transmissions of % (nm - 5 nm, ±.%) and % (95 nm-5 nm, ±.%) were employed for comparison. A commercial InGaAs- SESAM (Batop Inc.) wi a saturation fluence of 7 μj/cm and a modulation dep of about.6% at nm was employed to start and stabilize e mode-locking. The total cavity leng was. m. This leng was maintained wheer a silicon prism pair was inserted into e cavity or not. The corresponding repetition rate was about 7 MHz. To compensate e astigmatism, e folding angles of e mirrors were about 5. Wi ABCD matrix propagation eory, e laser mode waist radii in e laser crystal were calculated to be μm and μm in sagittal and tangential planes, respectively, and e beam waist radii on e SESAM was about μm in e sagittal plane and μm in e tangential planes. Fig.. Schematic setup of mode-locked Tm,Ho:YAP laser. OC: output coupler Wi e Ti:Sapphire laser tuned to 79.7 nm, e Tm,Ho:YAP laser was first investigated wiout e silicon prisms in e cavity. A GHz bandwid digital oscilloscope (DPO 5, Tektronix Inc.) and a 6 MHz bandwid extended-ingaas PIN photodiode (G, Hamamatsu Inc.) were used to monitor e pulse train leaking from mirror M to optimize e stability of CW mode-locking. Wi careful alignment of e cavity, passive mode-locking operation was achieved. The output spectra were recorded by a laser spectrometer wi a resolution of. nm (APE WaveScan, APE Inc.), and a collinear autocorrelation setup using second harmonic generation wi a PPLN crystal was employed to measure e pulse durations. Figures (a) (f) summarize e output characteristics including average output powers, optical spectra, and autocorrelations of e mode-locked Tm,Ho:YAP lasers wi % and % OCs. Wi % OC used, CW mode-locking was achieved at.5 nm and nm, respectively by aligning e OC carefully, wi e reshold absorbed pump powers of 6 mw and mw. At e reshold pump powers for CW mode-locking, e output powers at.5 nm and nm were 7. mw and. mw, respectively corresponding to e fluences of 75 μj/cm and μj/cm on e SESAM. However, when e laser ran in e CW mode-locking regime, e slope efficiencies decreased to 6.9% and 6.5%, respectively, as shown in Figs. (a) and (d). The maximum output powers of 6 mw at.5 nm and mw at nm were obtained wi a repetition rate of about 7 MHz. During e experiment, we observed double pulse mode-locking at bo.5 nm and nm under e maximum pump power when a % OC was used, which were attributed to e much higher intracavity fluences of 7 μj/cm and 795 μj/cm on e SESAM an e saturation fluence of 7 μj/cm. However, when a % OC was employed, CW mode-locking could only be achieved at nm. From Fig. (d), we can see at e reshold absorbed pump power for CW modelocking was increased to be.55 W, corresponding to an output power of about 55 mw, which generated a fluence of 5 μj/cm on e SESAM and was much higher an e case (C) OSA January / Vol., No. / OPTICS EXPRESS 576
wi % OC. A slope efficiency of.5% and a maximum output power of mw wi respect to an absorbed pump power of. W were obtained, which is shown in red in Fig. (d). The maximum fluence on e SESAM was about 5 μj/cm, and we did not observe double pulse formation. Figure (e) shows e optical spectrum centered at nm wi a spectral bandwid of. nm. A pulse duration of 66 ps was obtained by assuming a sech pulse shape as shown in Fig. (f). Due to e spectrometers low resolution, we omit to give a time-bandwid product (TBP) here..5. T=%, η=6.9% (a).5 nm...6 (b)..5 CWML....5..5..5 6.5 T=%,η=6.5%. (e) T=%,η=.5%.. (d) nm.5 CWML.6. nm.. 7 6 5-5--5 5 5 (f) 7 τ 6 p =66 ps 5.5 CWML.....5..5..5-5--5 5 5.5 nm (c) τ p = ps Fig.. Average output powers, optical spectra and autocorrelations (from left to right) of mode-locked Tm,Ho:YAP laser wi % and % OCs. To investigate e influences of e intracavity dispersions on e pulse duration, a pair of silicon prisms wi tip to tip separation of 55 mm was inserted into e cavity. Considering e GDD introduced by e cavity mirrors and Tm,Ho:YAP crystal as well as e silicon prism pair, e maximum compensated dispersion was ~- fs, which increased by 6 fs /mm wi e insertion leng of silicon prism into e cavity. However, e Tm,Ho:YAP laser could not oscillate eier at.5 nm or at nm any more after e insertion of e silicon prisms wi % or % OCs, while e CW mode-locking operations at 6.5 nm, 6.5 nm and 95.5 nm were achieved by aligning e OCs carefully. By using % OC e Tm,Ho:YAP laser could only oscillate in CW regime at 6.5 nm. However, e modelocking operation could be realized at 6.5 nm wi a reshold absorbed pump power of. W, corresponding to an output power of 6. mw and a fluence of μj/cm on e SESAM when % OC used. The output power characteristics are shown in Fig. (a), from which we can see at a maximum output power of mw wi respect to an absorbed pump power of.6 W was obtained, corresponding to a slope efficiency of 7.5%. Under CW mode-locking, Fig. (b) shows e spectral bandwid of.6 nm centered at 6.5 nm wi e corresponding pulse duration of. ps as shown in Fig. (c). Wi e OCs aligned carefully, CW mode-locking operations at e oer wavelengs of 6.5 nm and 95.5 nm could be realized bo for % and % OCs. When e laser ran at 6.5 nm, e reshold absorbed pump powers for CW mode-locking were. W and.5 W, corresponding to e output powers of 6. mw and 5.9 mw as well as e fluences of 57 μj/cm and 7 μj/cm on e SESAM, respectively, for e % and % OCs. The slope efficiencies of 5.% for % OC and % for % OC were obtained, wi e maximum (C) OSA January / Vol., No. / OPTICS EXPRESS 577
B B output powers of 5 mw and mw as shown in Fig. (d), respectively. The spectrum centered at 6.5 nm wi a spectral FWHM of about.5 nm is shown in Fig. (e), corresponding to a pulse duration of. ps as demonstrated in Fig. (f). This was e shortest pulse obtained from e mode-locked Tm,Ho:YAP laser in e experiment.. T=%, η=7.5%. (b) 7 (c).6 (a) 6.5 nm. 6 τ p =. ps.6.6 nm 5. CWML........6.... 6.. T=%, η=5.%. (e) 7 (f).. T=%, η=% 6 τ p =. ps. (d) 6.5 nm.6.5 nm 5.6.. CWML... CWML..6.9..5... 6 6 66 6.. T=%, η=.% (h) 7 (i). T=%, η=7.6%. 6 τ p =.6 ps. (g) 95.5 nm B.6 nm 5.6 CWML..... CWML..9..5... 9 9 96 9-5 - -5 5 5-5 - -5 5 5-5 - -5 5 5 Fig.. Average output powers, optical spectra and autocorrelations (from left to right) of mode-locked Tm,Ho:YAP laser wi dispersion compensated by silicon prisms for % and % OCs. Figure shows e first beat note of e radio frequency (RF) spectrum of e stable CW mode-locking at 6.5 nm at e maximum pump power when e OC of T = % was employed, which was recorded by a spectrum analyzer wi a bandwid of. GHz and a resolution bandwid of KHz (E5B, Agilent Inc.). The RF spectrum obtained under a span of 5 khz shows a clean peak at e repetition rate of about 7 MHz wiout side peaks, which exactly agrees wi e roundtrip time of e cavity and reveals stable CW mode-locking operation of e laser as well as e absence of Q-switching instabilities. In addition, e wide-span RF measurement indicated e single pulse operation of e modelocked Tm,Ho:YAP laser, as shown inset of Fig.. (C) OSA January / Vol., No. / OPTICS EXPRESS 57
RF Power (dbm) - - -6 - - Span:5 khz RBW: khz RF Power (dbm) - - - - -5-6 -7 Span: GHz RBW: MHz 6 Frequency (MHz) - 7. 7. 7.6 Frequency (MHz) Fig.. RF spectrum of continuous wave mode-locked Tm,Ho:YAP laser at 95.5 nm. RBW: resolution bandwid. The CW mode-locked Tm,Ho:YAP laser could run at 95.5 nm wi e reshold absorbed pump powers of.5 W and.6 W, corresponding to e output power of mw and 5. mw as well as e fluences of on μj/cm and 7 μj/cm on SESAM, respectively for % and % OCs. As shown in Fig. (g), e obtained maximum output powers were 95 mw and mw, corresponding to e slope efficiencies of.% and 7.6% for % and % OCs, respectively. Under CW mode-locking, e spectrum centered at 95.5 nm was recorded wi a spectral FWHM of.6 nm as shown in Fig. (h). A recorded autocorrelation signal for pulse wi duration of.6 ps is shown in Fig. (i). In e experiment, we did not observe obvious variation of e pulse duration when changing e introduced dispersion from about ~- fs to about fs by aligning e silicon prisms in e cavity. According to e measured emission spectra of a c-cut Tm,Ho:YAP crystal [7], e narrow peaks of e emission spectra in our previous experimental spectral range may be e reason for e raer long pulse durations here. Table. Output characteristics of mode-locked Tm,Ho:YAP laser at variable wavelengs Waveleng (nm) 6.5 6.5 95.5.5 OCs Transmission % % % % % % % % Threshold Absorbed...5.5.6.6..55 Pump Power (W) Threshold Output Power 6. 6. 5.9 5. 7.. 55 (mw) Threshold Fluence on 57 7 7 75 5 SESAM (μj/cm ) Maximum Output 5 95 6 Power (mw) Slope Efficiency (%) 7.5 5.. 7.6 6.9 6.5.5 Pulse Duration (ps)...6 66 FWHM (nm).6.5.6.5. Table summarizes e output characteristics of e mode-locked Tm,Ho:YAP laser wi SESAM at variable wavelengs for % and % OCs. From e Table, we can see at when e fluence on e SESAM was increased to about twice e saturation fluence, CW modelocking could be obtained except at nm wi % OC. It should be also noted at when e fluence on e SESAM reaches ten times e saturation fluence, double pulse modelocking occurs, i.e., e case at.5 nm and nm wi % OC under maximum pump (C) OSA January / Vol., No. / OPTICS EXPRESS 579
power. In oer words, a fluence on e SESAM ranging from twice to ten times of e saturation fluence can allow for stable single pulse mode-locking of e Tm,Ho:YAP laser.. Conclusion In conclusion, a passively mode-locked Tm,Ho:YAP laser around μm wi a semiconductor saturable absorber mirror is reported to e best of our knowledge for e first time. Stable continuous wave mode-locking was achieved at variable wavelengs of 6.5 nm, 6.5 nm, 95.5 nm,.5 nm, and nm, respectively. Pulses as short as. ps were obtained at 6.5 nm wi a spectral FWHM of.5 nm.the coresponding output power was mw and e repetition rate was around 7 MHz. A maximum output power of mw was obtained at nm wi pulse duration of 66 ps. Acknowledgments The work was supported by e Ministry of Science, Research and e Arts of Baden- Württemberg, National Natural Science Foundation of China (6, 69, 69), Research Award Fund for Outstanding Middle-aged and Young Scientist of Shandong Province (BSDX), Independent Innovation Foundation of Shandong University, IIFSDU (JC5), and Innovation Project of Shanghai Institute of Ceramics (YZC55G). Kejian Yang acknowledges support from e Alexander-von-Humboldt Foundation. (C) OSA January / Vol., No. / OPTICS EXPRESS 5