Open Access
Add to BibSonomyAbstract
This work reports on a liquid-nitrogen-cooled, SESAM mode-locked Yb:YGAG (Yb:Y3Ga2Al3O12) ceramic laser. The Yb:YGAG has a similar structure to Yb:YAG, but its emission spectrum at low temperature remains much broader, which is suitable for ultrashort pulse generation and amplification. A stable pulse train with 119-MHz repetition rate was obtained at a wavelength of 1026 nm. The measured pulse duration is 2.4 ps, which is more than four times shorter than that achieved with a cryogenically-cooled Yb:YAG. Furthermore, laser performance of the Yb:YGAG ceramics in continuous-wave operation and wavelength tunability at 80 K was investigated.
© 2016 Optical Society of America
1. Introduction
Development of highly efficient diode lasers in recent decades enabled a rapid progress in high-power solid-state laser technology, which is motivated by an ever-broadening range of applications. With high-power pump sources available, sufficient heat removal from the active medium is the main difficulty in construction of high-average-power diode-pumped lasers. To reduce the heat load, materials doped with trivalent ytterbium ion emerged as the most attractive choice for high-power laser sources in near infrared, due to their low quantum defect, simple energy level structure limiting heat generated by up-conversion processes, and the accessibility of powerful pump diode lasers [1,2]. New concepts improving heat removal such as the thin-disk or slab geometry were also demonstrated [2,3]. Other way how to mitigate the influence of the quasi-three-level nature of Yb-doped materials is cryogenic cooling of gain media, which allows reaching higher efficiency and thermal conductivity. Both high efficiency and thermal conductivity can realize good beam quality at high power level using solid-state laser media [4].
The most widely used ytterbium-doped host material for high-power operation is arguably the YAG (yttrium aluminum garnet, Y3Al5O12), mainly due to its excellent material properties and well-established fabrication providing good optical quality in both crystalline and ceramic form. However, in case of generation and amplification of picosecond and femtosecond pulses, laser media with broad emission bandwidth are required. This is not the case of Yb:YAG at cryogenic temperatures [5], whose emission bandwidth decreases more than six-fold when cooled from room temperature down to 80 K (see Table 1). Although several materials possess much broader emission bandwidth at cryogenic temperature, such as YLF, CaF2 or FP15-glass [6,7], those materials have lower thermal conductivity, narrower absorption bandwidth or they are more fragile compared to YAG.
Table 1. Comparison of stimulated emission cross sections (σe) and bandwidths of Yb:YAG (peak wavelength 1030 nm) and Yb:YGAG (peak at 1026 nm) at room and nearly liquid-nitrogen temperaturesa
Promising alternatives to these materials are mixed garnets with material properties similar to those of YAG. They derive from this material by substituting part of the aluminum ions with other metal ions, which results in inhomogeneous broadening of the lattice structure, variation of the crystal field strength, and consequently broadening the absorption and emission spectra of the material. One of these garnets is the Y3Ga2Al3O12 (YGAG), originally used with neodymium doping [8,9]; Yb:YGAG has been already proven to maintain its emission cross section bandwidth almost constant with temperature decreasing to 100 K [10]. As it is shown in Table 1, the emission bandwidth of Yb:YGAG of 7.12 nm at liquid-nitrogen temperature (≈80 K) is nearly six times higher than that of Yb:YAG. Therefore, cryogenically-cooled Yb:YGAG should support generation of ultrashort pulses with duration of several hundred femtoseconds. To our knowledge, the shortest pulse duration generated from a cryogenically-cooled mode-locked Yb:YAG oscillator was 10 ps [11]. In this work, we show that the Yb:YGAG material is capable of generating substantially shorter pulses and report on both continuous-wave and SESAM mode-locked operation of the Yb:YGAG ceramics at cryogenic temperature.
2. Free-running regime at cryogenic temperature
The 10 at.% Yb:YGAG ceramics was fabricated by Konoshima Chemical Co. Ltd., Japan, with a diameter of 18 mm. The ceramics was sliced to 9 pieces with a size of 3.6 × 3.6 × 1.5 mm3, which were anti-reflection (AR) coated for spectral range from 900 nm to 1100 nm. The ceramic sample was mounted in a copper holder inside of a small cryostat with 5 mm thick BK7 windows, AR coated for both 940 and 1030 nm, and cooled with liquid nitrogen to temperature of approximately 80 K. The sample was pumped by a fiber-coupled 40-W diode laser (Dilas M1F2S22-940.3-30C-SS2.1) at the wavelength of 940 nm with bandwidth of 2.6 nm. The pump light from the fiber (core diameter of 200 µm and NA 0.22) was imaged by two 75-mm lenses onto the ceramics with a spot diameter of approximately 200 µm.
First, we investigated laser performance of the ceramics in free-running regime at cryogenic temperature with continuous as well as pulsed pumping (results obtained at room temperature were presented in [12]). The optical resonator (Fig. 1) encompassed a concave pumping mirror with 100-mm radius of curvature (ROC), a concave folding mirror with 300-mm ROC, and a flat output coupler with reflectivity of 83% at 1030 nm; the resulting mode diameter in the laser ceramics was approximately 190 µm.
Fig. 1 Experimental set-up for CW operation of the Yb:YGAG laser (without the filter) and for CW wavelength-tunable laser. PCC – plano-concave, HR – high-reflection.
At CW pump power of 28.7 W incident on the active medium, the maximum output power of 9.9 W at a wavelength of 1026 nm was obtained with optical-to-optical efficiency of 34.6% [Fig. 2(a)]. Pump absorption near the laser threshold was approximately 81%, which gives optical efficiency with respect to the absorbed pump power of 42.7%. The M2 parameter of the output beam at pump power of 5 W was 1.04. Due to the mode-mismatching caused by the thermal lens in the ceramic, the M2 parameter increased to 1.5 in horizontal and 2.5 in vertical plane at incident pump power of 28 W. Worse beam quality in the vertical plane is also caused by the temperature gradation of copper holder. Higher beam quality is feasible by improving the copper holder and the cavity design.
Fig. 2 Yb:YGAG laser output power with respect to incident and absorbed pump power in free-running regime (a) in CW mode, (b) with pulsed pumping (4% duty cycle).
To evaluate the laser performance under high pump intensities while keeping the thermal load in the ceramics reduced, we applied pulsed pumping with 4 ms pulse duration at 10 Hz repetition rate (duty cycle of 4%). Average output power obtained in this regime is shown in Fig. 2(b). The maximum average output power was 706 mW at average incident pump power of 1.30 W, which corresponds to the peak output power of 17.7 W at the incident pump power amplitude of 32.5 W. The resulting optical-to-optical efficiency of 54.5% (67.3% with respect to the absorbed pump power) is significantly higher than that in the CW regime, which is mainly caused by mode mismatching in Yb:YGAG due to the thermal lensing in case of CW pumping. The M2 parameters in horizontal and vertical plane were 1.2 and 1.4, respectively.
We had reported the wavelength tuning range of the Yb:YGAG at room temperature from 1022 nm to 1080 nm (44-nm FWHM) [13]. The wavelength tunability at 80 K was examined using a 2.8-mm thick quartz plate as a birefringent filter. The output coupler was replaced by a flat high-reflection mirror. The output spectra were measured by a fiber-coupled spectrometer with resolution of 0.17 nm (Ocean Optics, HR4000). We found that the output spectrum of Yb:YGAG pumped at 5.2 W can be tuned from 1023.3 nm to 1031.1 nm as shown in Fig. 3.
Fig. 3 Yb:YGAG laser spectra obtained with birefringent-filter tuning and corresponding normalized output power (magenta crosses).
3. Mode-locked cryogenic Yb:YGAG laser
Ytterbium-doped media have rather high saturation energy, which makes them prone to Q-switching instabilities when employed in SESAM (semiconductor saturable absorber mirror) mode-locking [14]. The saturation energy of Yb:YAG, inversely proportional to the emission cross section, is five-fold reduced when cooled down to 80 K, while the saturation energy of Yb:YGAG decreases just by 36% (trade-off for the broader emission bandwidth). Achieving stable mode-locked regime with Yb:YGAG was therefore more difficult, making the cavity design and SESAM choice more constrained. We employed a similar set-up like for the CW operation with a modified, 1.2 m long resonator shown in Fig. 4. The flat output coupler was replaced by a flat HR mirror and the cavity was extended using a SESAM and a concave output coupler with 98.5% reflectivity and 200-mm ROC. Therefore, there are two output beams out of the cavity. The modulation depth, relaxation time and saturation fluence of the SESAM (Batop, SAM-1064-2-10ps) are 1.2%, 10 ps, and 90 µJ/cm2, respectively. The calculated mode diameters in the gain medium and the SESAM were 190 µm and 104 µm.
Fig. 4 Optical scheme of the mode-locked Yb:YGAG oscillator. PCC – plano-concave, HR – high-reflection, OC – output coupler, SESAM – semiconductor saturable absorber mirror.
Using the described experimental layout and continuous-wave pumping, stable mode-locked operation with a total output power of 194 mW (sum of power in both beams) was obtained at the incident pump power of 3.9 W which was lower than the pump power of CW operation when we obtained the M2 of 1.04. The spectral bandwidth (Fig. 5) was 0.64 nm (FWHM) and the spectral shape corresponded to a transform-limited pulse with duration of 1.8 ps. The spectrum shows a partial modulation with the period of 0.2 nm, which suggests etalon effect of the 1.5-mm thick ceramics plate.
Fig. 5 Output spectrum of the mode-locked Yb:YGAG laser. Inset, transverse near-field profile of the output beam.
The laser output pulses were first characterized using a photodiode with 35-ps rise time (EOT ET-3500), a 9-GHz digital oscilloscope (LeCroy SDA9000) and an RF spectrum analyzer (Rohde&Schwarz FSL3). The pulse train with repetition rate of 119 MHz is shown in Fig. 6(a), its microwave spectrum analyzer trace confirming stable CW mode-locking is in Fig. 6(b). For accurate pulse characterization, a laboratory-made intensity autocorrelator was used. The acquired autocorrelation trace is shown in Fig. 7.
Fig. 6 (a) waveform of the mode-locked pulse train acquired using a fast photodiode; (b) RF spectrum of the CW mode-locked cryogenic Yb:YGAG laser (RBW – resolution bandwidth).
In the autocorrelation trace, peaks separated equidistantly by 18.5 ps are evident. Widths of the main autocorrelation peak as well as the side peaks are approximately 3.70 ps and its shape is very close to sech2 yielding an estimated pulse duration of 2.40 ps which is four times shorter than the value reported with a cryogenic Yb:YAG. Since the pulse fluence on the SESAM is much higher (640 µJ/cm2) than the saturation fluence of SESAM (90 µJ/cm2), multiple-pulsing effects caused by the strong saturation of SESAM should not be observed. The period of the post-pulses is 18.5 ps and it corresponds to an optical path length of a double internal reflection in a 1.5 mm thick plate with refractive index of 1.85; as the refractive index of YAG and YGG (Y3Ga5O12) at 1.06 µm is 1.822 and 1.912, respectively [15], this value is within expectations for the Yb:YGAG. The ceramic plate was slightly tilted in order to mitigate the etalon effect, however, as can be seen from both the output spectrum and autocorrelation, the internal reflection on the ceramic surface is still considerable, which might be caused by the antireflection coating ineffectiveness at low temperature. It might be possible to generate even shorter pulses with the same set-up after suppressing these reflections, since a smoother output spectrum should be obtainable. Also, employing the Kerr-lens mode-locking technique could yield further reduction of the pulse duration.
4. Conclusion
We have demonstrated a passively mode-locked cryogenic Yb:YGAG ceramic laser. Stable pulse train with 119 MHz repetition rate at the wavelength of 1026 nm was obtained. The measured pulse duration is 2.4 ps with a 1.8-ps bandwidth limit, which is a substantially shorter than the reported duration of 10 ps from a similar cryogenic Yb:YAG oscillator. Continuous wavelength tuning in a range of 8.1 nm using a birefringent filter was also demonstrated. These facts show that the Yb:YGAG is a promising alternative to Yb:YAG for short pulse generation and amplification in the low-temperature regime.
Acknowledgment
This research was supported by the Czech Science Foundation project P102/13/8888 and was co-financed by the European Regional Development Fund, the European Social Fund and the state budget of the Czech Republic (project HiLASE: CZ.1.05/2.1.00/01.0027, project DPSSLasers: CZ.1.07/2.3.00/20.0143, project Postdok: CZ.1.07/2.3.00/30.0057, project HiLASE: Superlasers for real world: LO1602).
References and links
1. H. W. Bruesselbach, D. S. Sumida, R. A. Reeder, and R. W. Byren, “Low-heat high-power scaling using InGaAs-diode-pumped Yb:YAG lasers,” IEEE J. Sel. Top. Quantum Electron. 3(1), 105–116 (1997). [CrossRef]
2. A. Giesen, H. Hügel, A. Voss, K. Wittig, U. Brauch, and H. Opower, “Scalable concept for diode-pumped high-power solid-state lasers,” Appl. Phys. B 58(5), 365–372 (1994). [CrossRef]
3. B. J. Comaskey, R. Beach, G. Albrecht, W. J. Benett, B. L. Freitas, C. Petty, D. VanLue, D. Mundinger, and R. W. Solarz, “High average powers diode pumped slab laser,” IEEE J. Quantum Electron. 28(4), 992–996 (1992). [CrossRef]
4. T. Y. Fan, D. J. Ripin, R. L. Aggarwal, J. R. Ochoa, B. Chann, M. Tilleman, and J. Spitzberg, “Cryogenic Yb3+-doped solid-state lasers,” IEEE J. Sel. Top. Quantum Electron. 13(3), 448–459 (2007). [CrossRef]
5. V. Jambunathan, T. Miura, L. Těsnohlídková, A. Lucianetti, and T. Mocek, “Efficient laser performance of a cryogenic Yb:YAG laser pumped by fiber coupled 940 and 969 nm laser diodes,” Laser Phys. Lett. 12(1), 015002 (2015). [CrossRef]
6. V. Jambunathan, J. Koerner, P. Sikocinski, M. Divoký, M. Sawicka, A. Lucianetti, J. Hein, and T. Mocek, “Spectroscopic characterization of various Yb3+ doped laser materials at cryogenic temperatures for the development of high energy class diode pumped solid state lasers,” Proc. SPIE 8780, 87800G (2013). [CrossRef]
7. J. Kawanaka, K. Yamakawa, H. Nishioka, and K. Ueda, “Improved high-field laser characteristics of a diode-pumped Yb:LiYF4 crystal at low temperature,” Opt. Express 10(10), 455–460 (2002). [CrossRef] [PubMed]
8. B. M. Walsh, N. P. Barnes, R. L. Hutcheson, R. W. Equall, and B. Di Bartolo, “Spectroscopy and lasing characteristics of Nd-doped Y3GaxAl(5-x)O12 materials: application toward a compositionally tuned 0.94-μm laser,” J. Opt. Soc. Am. B 15(11), 2794–2801 (1998). [CrossRef]
9. Y. Oishi, K. Okamura, K. Miyazaki, N. Saito, M. Iwasaki, and S. Wada, “Amplifying high energy pulses at 1062.78 nm with diode pumped Nd:YGAG ceramic,” in Advanced Solid-State Lasers Congress, G. Huber and P. Moulton, eds. (Optical Society of America, 2013), paper ATu3A.40. [CrossRef]
10. V. Jambunathan, L. Horáčková, T. Miura, J. Šulc, H. Jelínková, A. Endo, A. Lucianetti, and T. Mocek, “Spectroscopic and lasing characteristics of Yb:YGAG ceramic at cryogenic temperatures,” Opt. Mater. Express 5(6), 1289–1295 (2015). [CrossRef]
11. K. F. Wall, D. E. Miller, and T. Y. Fan, “Cryo-Yb:YAG lasers for next-generation photoinjector applications,” Proc. SPIE 8235, 823512 (2012). [CrossRef]
12. J. Mužík, V. Jambunathan, M. Jelínek, V. Kubeček, T. Miura, A. Endo, and T. Mocek, “Laser properties of Yb:YGAG ceramic in comparison with crystalline Yb:YAG,” presented at the CLEO/Europe - EQEC 2015, Munich, Germany, 21–25 June 2015.
13. J. Šulc, H. Jelínková, V. Jambunathan, T. Miura, A. Endo, A. Lucianetti, and T. Mocek, “Wavelength tunability of laser based on Yb-doped YGAG ceramics,” Proc. SPIE 9342, 93421T (2015). [CrossRef]
14. C. Hönninger, R. Paschotta, F. Morier-Genoud, M. Moser, and U. Keller, “Q-switching stability limits of continuous-wave passive mode locking,” J. Opt. Soc. Am. B 16(1), 46–56 (1999). [CrossRef]
15. R. Adair, L. L. Chase, and S. A. Payne, “Nonlinear refractive index of optical crystals,” Phys. Rev. B Condens. Matter 39(5), 3337–3350 (1989). [CrossRef] [PubMed]
