Diode-pumped Tm : KY ( WO 4 ) 2 laser passively modelocked with a GaSb-SESAM

We present the first diode-pumped modelocked thulium (Tm) laser based on a double-tungstate crystalline gain material. The solid-state laser consists of a Tm:KY(WO4)2 crystal as gain medium and a GaInSb/GaSb quantum well saturable absorber for self-starting passive mode locking. The laser is pumped by a multi-mode fiber-coupled laser diode at a wavelength of 793 nm. An average output power of 202 mW is achieved at a center wavelength of 2032 nm. Pulses with duration of 3 ps are generated at a repetition rate of 139.6 MHz. We also report on the first noise evaluation of a modelocked solid-state laser operating in the 2-μm wavelength range. We measured a timing jitter of sub-100 fs and a relative intensity noise of only 0.04% (frequency range from 500 Hz to 1 MHz). © 2017 Optical Society of America OCIS codes: (140.4050) Mode-locked lasers; (140.3580) Lasers, solid-state; (140.3480) Lasers, diode-pumped; (140.5680) Rare earth and transition metal solid-state lasers References and links 1. M. Gaponenko, N. Kuleshov, and T. Südmeyer, “Passively Q-switched thulium microchip laser,” IEEE Photonics Technol. Lett. 28, 147–150 (2016). 2. J. M. Serres, X. Mateos, P. Loiko, K. Yumashev, N. Kuleshov, V. Petrov, U. Griebner, M. Aguiló, and F. Díaz, “Diode-pumped microchip Tm:KLu(WO4)2 laser with more than 3 W of output power,” Opt. Lett. 39(14), 4247– 4250 (2014). 3. M. Gaponenko, N. Kuleshov, and T. Südmeyer, “Efficient diode-pumped Tm:KYW 1.9-μm microchip laser with 1 W cw output power,” Opt. Express 22(10), 11578–11582 (2014). 4. 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. 37(8), 1376–1378 (2012). 5. J. Ma, G. Q. Xie, P. Lv, W. L. Gao, P. Yuan, L. J. Qian, H. H. Yu, H. J. Zhang, J. Y. Wang, and D. Y. Tang, “Graphene mode-locked femtosecond laser at 2 μm wavelength,” Opt. Lett. 37(11), 2085–2087 (2012). 6. A. A. Lagatsky, P. Koopmann, P. Fuhrberg, G. Huber, C. T. A. Brown, and W. Sibbett, “Passively mode locked femtosecond Tm:Sc2O3 laser at 2.1 μm,” Opt. Lett. 37(3), 437–439 (2012). 7. W. B. Cho, A. Schmidt, J. H. Yim, S. Y. Choi, S. Lee, F. Rotermund, U. Griebner, G. Steinmeyer, V. Petrov, X. Mateos, M. C. Pujol, J. J. Carvajal, M. Aguiló, and F. Díaz, “Passive mode-locking of a Tm-doped bulk laser near 2 microm using a carbon nanotube saturable absorber,” Opt. Express 17(13), 11007–11012 (2009). 8. F. Fusari, A. A. Lagatsky, G. Jose, S. Calvez, A. Jha, M. D. Dawson, J. A. Gupta, W. Sibbett, and C. T. A. Brown, “Femtosecond mode-locked Tm and Tm-Ho doped 2 μm glass lasers,” Opt. Express 18(21), 22090– 22098 (2010). 9. A. A. Lagatsky, O. L. Antipov, and W. Sibbett, “Broadly tunable femtosecond Tm:Lu2O3 ceramic laser operating around 2070 nm,” Opt. Express 20(17), 19349–19354 (2012). 10. A. A. Lagatsky, S. Calvez, J. A. Gupta, V. E. Kisel, N. V. Kuleshov, C. T. A. Brown, M. D. Dawson, and W. Sibbett, “Broadly tunable femtosecond mode-locking in a Tm:KYW laser near 2 μm,” Opt. Express 19(10), 9995–10000 (2011). 11. A. Schmidt, P. Koopmann, G. Huber, P. Fuhrberg, S. Y. Choi, D.-I. Yeom, F. Rotermund, V. Petrov, and U. Griebner, “175 fs Tm:Lu2O3 laser at 2.07 μm mode-locked using single-walled carbon nanotubes,” Opt. Express 20(5), 5313–5318 (2012). Vol. 25, No. 21 | 16 Oct 2017 | OPTICS EXPRESS 25760 #300642 https://doi.org/10.1364/OE.25.025760 Journal © 2017 Received 18 Aug 2017; revised 26 Sep 2017; accepted 26 Sep 2017; published 10 Oct 2017 12. A. Gluth, Y. Wang, V. Petrov, J. Paajaste, S. Suomalainen, A. Härkönen, M. Guina, G. Steinmeyer, X. Mateos, S. Veronesi, M. Tonelli, J. Li, Y. Pan, J. Guo, and U. Griebner, “GaSb-based SESAM mode-locked Tm:YAG ceramic laser at 2 μm,” Opt. Express 23(2), 1361–1369 (2015). 13. Y. Wang, G. Xie, X. Xu, J. Di, Z. Qin, S. Suomalainen, M. Guina, A. Härkönen, A. Agnesi, U. Griebner, X. Mateos, P. Loiko, and V. Petrov, “SESAM mode-locked Tm:CALGO laser at 2 μm,” Opt. Mater. Express 6, 131 (2016). 14. A. Schmidt, S. Y. Choi, D.-I. Yeom, F. Rotermund, X. Mateos, M. Segura, F. Diaz, V. Petrov, and U. Griebner, “Femtosecond pulses near 2 μm from a Tm:KLuW laser mode-locked by a single-walled carbon nanotube saturable absorber,” Appl. Phys. Express 5, 092704 (2012). 15. A. A. Lagatsky, Z. Sun, T. S. Kulmala, R. S. Sundaram, S. Milana, F. Torrisi, O. L. Antipov, Y. Lee, J. H. Ahn, and C. T. A. Brown, “2 μm solid-state laser mode-locked by single-layer graphene,” Appl. Phys. Lett. 102, 013113 (2013). 16. N. Coluccelli, G. Galzerano, D. Gatti, A. D. Lieto, M. Tonelli, and P. Laporta, “Passive mode-locking of a diode-pumped Tm:GdLiF4 laser,” Appl. Phys. B 101, 75–78 (2010). 17. T. Feng, K. Yang, J. Zhao, S. Zhao, W. Qiao, T. Li, T. Dekorsy, J. He, L. Zheng, Q. Wang, X. Xu, L. Su, and J. Xu, “1.21 W passively mode-locked Tm:LuAG laser,” Opt. Express 23(9), 11819–11825 (2015). 18. Z. Qin, G. Xie, L. Kong, P. Yuan, L. Qian, X. Xu, and J. Xu, “Diode-pumped passively mode-locked Tm:CaGdAlO4 laser at 2-μm wavelength,” IEEE Photonics J. 7, 1–5 (2015). 19. K. Yang, H. Bromberger, H. Ruf, H. Schäfer, J. Neuhaus, T. Dekorsy, C. V. Grimm, M. Helm, K. Biermann, and H. Künzel, “Passively mode-locked Tm,Ho:YAG laser at 2 microm based on saturable absorption of intersubband transitions in quantum wells,” Opt. Express 18(7), 6537–6544 (2010). 20. K. J. Yang, D. C. Heinecke, C. Kölbl, T. Dekorsy, S. Z. Zhao, L. H. Zheng, J. Xu, and G. J. Zhao, “Mode-locked Tm,Ho:YAP laser around 2.1 μm,” Opt. Express 21(2), 1574–1580 (2013). 21. K. Yang, D. Heinecke, J. Paajaste, C. Kölbl, T. Dekorsy, S. Suomalainen, and M. Guina, “Mode-locking of 2 μm Tm,Ho:YAG laser with GaInAs and GaSb-based SESAMs,” Opt. Express 21(4), 4311–4318 (2013). 22. V. Aleksandrov, A. Gluth, V. Petrov, I. Buchvarov, G. Steinmeyer, J. Paajaste, S. Suomalainen, A. Härkönen, M. Guina, X. Mateos, F. Díaz, and U. Griebner, “Mode-locked Tm,Ho:KLu(WO4)2 laser at 2060 nm using InGaSb-based SESAMs,” Opt. Express 23(4), 4614–4619 (2015). 23. V. Aleksandrov, A. Gluth, V. Petrov, I. Buchvarov, S. Y. Choi, M. H. Kim, F. Rotermund, X. Mateos, F. Díaz, and U. Griebner, “Tm,Ho:KLu(WO4)2 laser mode-locked near 2 μm by single-walled carbon nanotubes,” Opt. Express 22(22), 26872–26877 (2014). 24. A. A. Lagatsky, F. Fusari, S. Calvez, J. A. Gupta, V. E. Kisel, N. V. Kuleshov, C. T. A. Brown, M. D. Dawson, and W. Sibbett, “Passive mode locking of a Tm,Ho:KY(WO4)2 laser around 2 μm,” Opt. Lett. 34(17), 2587– 2589 (2009). 25. A. A. Lagatsky, F. Fusari, S. Calvez, S. V. Kurilchik, V. E. Kisel, N. V. Kuleshov, M. D. Dawson, C. T. A. Brown, and W. Sibbett, “Femtosecond pulse operation of a Tm,Ho-codoped crystalline laser near 2 μm,” Opt. Lett. 35(2), 172–174 (2010). 26. 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 (191 fs) NaY(WO4)2 Tm,Ho-codoped laser at 2060 nm,” Opt. Lett. 35(18), 3027–3029 (2010). 27. O. Silvestre, J. Grau, M. C. Pujol, J. Massons, M. Aguiló, F. Díaz, M. T. Borowiec, A. Szewczyk, M. U. Gutowska, M. Massot, A. Salazar, and V. Petrov, “Thermal properties of monoclinic KLu(WO4)2 as a promising solid state laser host,” Opt. Express 16(7), 5022–5034 (2008). 28. M. S. Gaponenko, P. A. Loiko, N. V. Gusakova, K. V. Yumashev, N. V. Kuleshov, and A. A. Pavlyuk, “Thermal lensing and microchip laser performance of Ng-cut Tm:KY(WO4)2 crystal,” Appl. Phys. B 108, 603– 607 (2012). 29. A. E. Troshin, V. E. Kisel, A. S. Yasukevich, N. V. Kuleshov, A. A. Pavlyuk, E. B. Dunina, and A. A. Kornienko, “Spectroscopy and laser properties of Tm:KY(WO4)2 crystal,” Appl. Phys. B 86, 287–292 (2006). 30. M. Petrov, J. Liu, M. C. Pujol, U. Griebner, M. Aguilo, D. Galan, and G. Viera, “Efficient 2-μm continuouswave laser oscillation of Tm:KLu(WO4)2,” IEEE J. Quantum Electron. 42, 1008–1015 (2006). 31. J. Paajaste, S. Suomalainen, A. Härkönen, U. Griebner, G. Steinmeyer, and M. Guina, “Absorption recovery dynamics in 2 μm GaSb-based SESAMs,” J. Phys. Appl. Phys. 47, 065102 (2014). 32. D. von der Linde, “Characterization of the noise in continuously operating mode-locked lasers,” Appl. Phys. B 39, 201–217 (1986). 33. P. Li, A. Ruehl, U. Grosse-Wortmann, and I. Hartl, “Sub-100 fs passively mode-locked holmium-doped fiber oscillator operating at 2.06 μm,” Opt. Lett. 39(24), 6859–6862 (2014). 34. A. E. Akosman and M. Y. Sander, “Low noise, mode-locked 253 MHz Tm/Ho fiber laser with core pumping at 790 nm,” IEEE Photonics Technol. Lett. 28, 1878–1881 (2016). 35. M. Mangold, S. M. Link, A. Klenner, C. A. Zaugg, M. Golling, B. W. Tilma, and U. Keller, “Amplitude noise and timing jitter characterization of a high-power mode-locked integrated external-cavity surface emitting laser,” IEEE Photonics J. 6, 1–9 (2014). 36. V. J. Wittwer, R. van der Linden, B. W. Tilma, B. Resan, K. J. Weingarten, T. Sudmeyer, and U. Keller, “Sub60-fs timing jitter of a SESAM modelocked VECSEL,” IEEE Photonics J. 5, 1400107 (2013). Vol. 25, No. 21 | 16 Oct 2017 | OPTICS EXPRESS 25761


Introduction
Thulium (Tm 3+ ) lasers allow accessing the spectral region of 1900-2100 nm and are therefore highly attractive for atmosphere monitoring, spectroscopy, gas analysis, remote sensing, semiconductor and plastic materials processing, and medical applications.Efficient crossrelaxation processes in adjacent Tm 3+ ions enable the use of commercially-available laser diodes emitting at wavelengths of ~800-nm as pump sources.This allows the realization of laser systems with a luminescence quantum yield of ~2 (with the corresponding theoretical quantum efficiency of ~80%) and a reduced thermal load in the gain medium.Recently, diode-pumped solid-state Tm-lasers emitting several watts of output power in continuouswave operation with diffraction-limited beam quality and slope efficiencies exceeding 70% (to the absorbed pump power) were reported [1][2][3].However, the state-of-the-art modelocked solid-state lasers [Fig.1] emitting in this spectral region (based either on Tm-doped or Tm,Ho-codoped gain materials) still mainly rely on Ti:sapphire lasers as pump sources .Ti:sapphire lasers are expensive and complex solutions for such an application, and, moreover, can become a limiting factor for future power scaling.Switching to compact, costefficient, and powerful multi-mode fiber-coupled laser diodes does not seem to be a trivial task and only two reports with sub-ps pulse duration were presented so far [4,5].
Among the available Tm-doped gain media suitable for ultrafast laser applications, double-tungstate crystals such as Tm:KY(WO 4 ) 2 and Tm:KLu(WO 4 ) 2 , cut along their N g principal axis, are particularly attractive.Although, these materials possess moderate thermal properties (thermal conductivity of ~3 W m −1 K −1 [27], thermo-optic coefficient of 6.6 × 10 −6 K −1 [28]), they benefit from a broad tunability range over 170 nm [10], high absorption and emission cross sections (favorable for efficient laser operation) [29,30], and smooth gain cross section spectra (favorable for short pulse generation) [10,14].Up to date, modelocked operation with such gain media was only achieved by Ti:sapphire pumping.An average output power of 360 mW with 750-fs pulses at a wavelength of 2030 nm was reported with Tm:KY(WO 4 ) 2 [10].The shortest pulses with a duration of 141 fs were achieved at a wavelength of 2037 nm and at average output power of 26 mW, using a Tm:KLu(WO 4 ) 2 crystal [14].In this paper, we present the first diode-pumped modelocked Tm-laser based on doubletungstate crystalline gain materials.The Tm:KY(WO 4 ) 2 laser incorporates a GaInSb/GaSbbased semiconductor saturable absorber mirror (SESAM) for self-starting passive modelocking operation.We also report on the first noise evaluation of a modelocked solidstate laser operating in the 2-µm wavelength range.The achieved integrated timing jitter of ≤ 94 fs and relative intensity noise of ≤ 0.04% (in the frequency range from 500 Hz to 1 MHz) demonstrate the high stability of the laser and can serve as a benchmark for future developments.

Experimental set-up and results
The schematics of the laser cavity for continuous-wave (cw) operation is shown in Fig. 2. The cavity consists of a flat output coupler (T = 1% at the laser wavelength), three flat highly reflective (HR) mirrors M1, M4, M6, and three concave HR mirrors M2, M3, M5.During modelocking experiments, the HR mirror M1 is replaced by a SESAM.The cavity length is 1.05 m.As a gain medium, we use a wedged Tm(5%):KY(WO 4 ) 2 (Tm:KYW) crystal with a thickness of 2 mm.The crystal faces are AR-coated for the pump and laser radiation.The gain medium is mounted on a water-cooled copper heat sink and is pumped through the flat dichroic mirror M4, which is highly-reflective for the laser radiation and highly transmissive at the pump wavelength.The pump source is a 793-nm multimode fiber-coupled (100-µm diameter, N.A. = 0.22) laser diode with an output power up to ~10 W. The pump radiation is focused inside the gain medium to a spot with a radius of ~50 µm (measured in air).The calculated intracavity mode radius in the gain medium is ~55 µm.About 60% of pump power is absorbed in the Tm:KYW crystal during laser operation.Fig. 2. Schematics of the diode-pumped Tm:KYW laser.OC: output coupler with transmission of T = 1%; M1, M4, M6: flat HR mirrors; M2, M3, M5: concave HR mirrors.Mirror M6 is replaced by a short-wave pass filter (SWPF) to provide a laser operation at the wavelength of 2032 nm.HR mirror M1 is replaced by a SESAM for modelocking operation.FS is a 6-mm fused silica plate for introducing a negative GDD in the cavity during modelocking laser operation.
An output power of 1.6 W in TEM 00 mode is achieved in cw laser operation at a wavelength of 1960 nm [Fig.3(a)].The laser output is linearly polarized parallel to the N m optical axes of the gain medium.However, in this spectral region, stable cw modelocking operation cannot be realized due to the presence of strong water absorption lines in the ambient atmosphere.In order to shift the laser emission wavelength to the spectral region beyond 2000 nm, we replaced the flat HR mirror (M6) by a short-wavelength pass filter (SWPF) with a cut-off wavelength of ~1990 nm.This allows generation of 0.92 W of output power in a TEM 00 mode at a central wavelength of 2022 nm [Fig.3(b)].The incident pump power is measured to be 5.6 W, which corresponds to 3.6 W of absorbed pump power during laser operation.We limited the pump power to this level, corresponding to a pump intensity at the input laser crystal surface of 35 kW/cm 2 , to avoid crystal damage which was previously observed at higher pump intensities [1].To operate the laser in modelocking regime we exchanged mirror M1 with a GaSb-based SESAM.The SESAM comprises GaInSb/GaSb quantum wells and a distributed Bragg reflector consisting of 18.5 pairs of lattice-matched AlAsSb/GaSb quarter-wavelength layers, grown on a GaSb substrate.As revealed in Fig. 4, it exhibits an unsaturated reflectivity of 98% in the wavelength region of 2020-2040 nm.The typical absorption recovery dynamics for such GaSb-based SESAMs follows a double-exponential decay with the fast component shorter than 1 ps and the slow component in the range of 5-10 ps [31].Data on the exact saturation fluence of the SESAM used are currently unavailable due to difficulties in performing accurate measurements.However, we estimated the saturation fluence to be in the range of 20-40 µJ/cm 2 .The SESAM is mounted on a copper heatsink.In this configuration, we did not observe stable modelocking operation.Thus, we introduced additional negative group delay dispersion (GDD) by placing an AR-coated 6-mm fused silica plate into the cavity (added GDD = −1286 fs 2 at 2030 nm per cavity roundtrip).The exact amount of intracavity GDD cannot be estimated at the moment due to unknown values of dispersion for several optical components, including the SESAM and AR-coatings of the gain element.After this, the laser switches to stable self-starting cw modelocked operation at a threshold of 2.7 W of absorbed pump power [Figs.4(c) and 5].The laser operates in a fundamental TEM 00 mode with a maximum output power of 202 mW at an absorbed pump power of 3.6 W [Fig. 4(c)].The laser slope efficiency in the modelocked operation decreased to 12% due to  the additional non-saturable loss introduced by the SESAM.Modelocked laser pulses are separated by the cavity roundtrip time of 7.16 ns [Fig.5(a)] corresponding to the pulse repetition rate of ~139.58MHz [Fig.5(c)].The radio-frequency (RF) spectrum of the laser output reveals a signal to noise ratio of 95 dB (at a resolution bandwidth of 100 Hz) and indicates stable single-pulse mode locking without Q-switching instabilities.The pulse duration of 3 ps is deduced from the intensity autocorrelation traces, assuming a sech 2 temporal laser pulse profile [Fig.5(d)].The optical spectrum is centered at 2032 nm and has a full width at half-maximum (FWHM) of 2.3 nm [Fig.5(e)], thus giving a corresponding timebandwidth product of 0.5.Assuming sufficient bandwidth of the gain medium and operation range of the SESAM, most likely that the pulse duration is limited by the intracavity GDD.We expect that precise engineering of intracavity dispersion and non-linear phase shift should enable generation of sub-ps pulses.
In order to evaluate the stability of the modelocked diode-pumped Tm:KYW laser we performed characterization of its phase and amplitude noise at the maximum output power.The measurements were done using a 2-µm InGaAs 12.5-GHz photodiode (model ET-5000, EOT Inc.) connected via a low-noise 1-GHz amplifier (model ZFL-1000LN + , Mini-Circuits) to the phase noise analyzer (model FSWP26, Rohde&Schwarz).Characterization of the laser noise using a higher harmonic of the repetition rate allows for more accurate phase noise measurements (without affecting the accuracy of amplitude noise measurements) [32].The phase and amplitude noise power spectral densities (PSDs) were measured in the range of offset frequencies from 1 Hz to 50 MHz on the 7th harmonic of the laser repetition rate (977.06MHz).The results are presented in the Fig. 6(a).The root mean square values of timing jitter [Fig.6(b)] and relative intensity noise [Fig.6(c)] were calculated by integrating, respectively, the measured phase and amplitude noise PSDs from the lower limit frequency to the higher limit frequency of 1 MHz.In the range of frequencies from 500 Hz to 1 MHz, the modelocked Tm-laser exhibits an integrated timing jitter of ≤94 fs (and ≤60 fs in the range of frequencies from 1 kHz to 1 MHz) and relative intensity noise of ≤0.04%.These values are significantly affected by the shot noise of the photodiode (seen as a constant level for frequencies ≥0.1 MHz) and represent the upper limit, thus the genuine laser noise is even lower.The low-frequency part of the noise from 1 Hz to 500 Hz has mainly mechanical nature.The water-cooled Tm-laser is built on an optical breadboard and operates in the ambient environment under standard room conditions.Despite this, the integrated relative intensity noise at output power of 202 mW remains below 0.04% in the frequency range from 10 Hz to 1 MHz and does not exceed 0.1% in the frequency range from 1 Hz to 1MHz [Fig.6(c)].These values are at least 3 times lower than parameters of the state-of-the-art 2-μm modelocked fiber lasers operating in the range of output powers below 40 mW [33,34].Packaging the Tm:KYW laser in a mechanically stable housing should further reduce amplitude noise due to decreasing the influence of acoustic noise and noise introduced by air currents.Furthermore, similar to previous ultrafast lasers [35,36], the remaining phase noise in the frequency range below 1 kHz should be drastically reduced by stabilizing the laser cavity length with an actuator.

Conclusion
In conclusion, we demonstrated the first modelocked operation of a diode-pumped laser based on Tm-doped double-tungstate crystalline gain media.The laser comprises a Tm:KYW gain crystal and a GaInSb/GaSb quantum well saturable absorber and is pumped at the wavelength of 793 nm by a multi-mode fiber-coupled laser diode.During modelocked operation, the Tm:KYW laser emits pulses with a duration of 3 ps at a repetition rate of 139.58 MHz.The average output power is 202 mW and the central emission wavelength is 2032 nm.The noise characterization proofs excellent performance for the free-running (not-stabilized) laser system.The values of integrated timing jitter and relative intensity noise in the range of frequencies from 500 Hz to 1 MHz are 94 fs and 0.04%, respectively.

Fig. 3 .
Fig. 3. Output power of the Tm:KYW laser vs. absorbed pump power in cw operation: a) operation at a wavelength of 1960 nm without any spectrally-selective element inside the laser cavity; b) operation at a wavelength of 2022 nm (with a short-wavelength pass filter inside the laser cavity).

Fig. 5 .
Fig. 5. Characteristics of the modelocked Tm:KYW laser at the maximum output power: a pulse train (a), a RF-spectrum measured with a resolution bandwidth of 3MHz (b), a RFspectrum measured with a resolution bandwidth of 100 Hz (c), an autocorrelation trace with a sech 2 fit (d), an optical spectrum (e).

Fig. 6 .
Fig. 6.Phase and amplitude noise of the modelocked Tm-laser measured on the 7th harmonic of the repetition rate (a).Integrated timing jitter (b) and relative intensity noise (c) values calculated for the shown range of lower limit frequencies.