January 1, 2009 / Vol. 34, No. 1 / OPTICS LETTERS
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Subpicosecond pulse generation from a Nd:CLNGG disordered crystal laser G. Q. Xie,1,* D. Y. Tang,1 W. D. Tan,1 H. Luo,1 H. J. Zhang,2 H. H. Yu,2 and J. Y. Wang2 1
School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798 2 State Key Laboratory of Crystal Materials and Institute of Crystal Materials, Shandong University, Jinan 250100, China *Corresponding author:
[email protected]
Received October 10, 2008; accepted November 12, 2008; posted December 1, 2008 (Doc. ID 102647); published December 31, 2008 We report on a diode-pumped passively mode-locked subpicosecond Nd:CLNGG disordered crystal laser for the first time to our knowledge. Owing to the large inhomogeneous broadening and spectrum splitting of the disordered crystal, the Nd:CLNGG laser generated 900 fs mode-locked pulses with a repetition rate of ⬃88 MHz at 1061 nm wavelength. With a single-emitter laser diode pumping, a maximum average output power of 486 mW was achieved with a slope efficiency of 26%. Our experimental results show that the fourlevel Nd:CLNGG disordered crystal could be an excellent alternative for subpicosecond pulse generation. © 2008 Optical Society of America OCIS codes: 140.4050, 140.3380, 140.3480.
Four-level neodymium (Nd)-doped crystalline materials have been proved to be important laser gain media at 1 m wavelength. These crystalline laser materials generally have good thermal, mechanical, and optical properties. They are also suitable for direct laser diode pumping. However, their narrow fluorescence linewidths limit the available mode-locked pulse width. At present, the generated pulses from Nd-doped crystalline lasers have usually pulse widths of several to tens of picoseconds [1–4]. Although the Nd:glass lasers could generate femtosecond pulses, the low thermal conductivity and small emission cross section of Nd:glass limit the average output power level of the lasers [5,6]. Watt-level average-power output from a Nd:glass laser requires adopting complicated pump and cavity designing [7]. Therefore, it is desired to seek a four-level Nd-doped crystalline material with significant spectrum broadening for the generation of subpicosecond pulses. In the past few years, two types of Nd-doped disordered crystalline materials, the Nd3+ : Ca3Nb共1.5+1.5x兲Ga共3.5−2.5x兲 䊐xO12 (Nd:CNGG) and (Nd:CLNGG), Nd3+ : Ca3Nb共1.5+1.5x兲Ga共3.5−2.5x兲LixO12 have been developed [8–10]. Their lattice structures and spectral characteristics have also been studied [11,12]. The highly transparent Nd:CNGG crystal is a nonstoichiometric compound, and cationic vacancies exist in host lattices 共x ⫽ 0兲. The Nd:CNGG crystal has a disordered structure owing to the random distribution of Nb5+, Ga3+, and cationic vacancies in the regular sites of host lattices. The disordered structure causes a considerable inhomogeneous broadening and splitting of the spectral lines of active ions. The nonstoichiometry of Nd:CNGG crystal could be a major drawback as a laser crystal. By introducing Li+ to fill in the cationic vacancies in Nd:CNGG, a stoichiometric compound, Nd:CLNGG, is formed. The stoichiometry largely improves the crystal stability under strong pump conditions. Otherwise, owing to the different cationic distribution and crystal lattices field around Nd3+, the spectral 0146-9592/09/010103-3/$15.00
characteristic of Nd:CLNGG is different from that of Nd:CNGG. Compared with the ordered crystals, such as Nd:YAG, the disordered crystals Nd:CNGG and Nd:CLNGG have the following advantages: low melting temperature 共⬃1450 ° C兲, allowing for the Czochralski growth from platinum crucibles; broad absorption and emission bands suitable for efficient diode pumping and ultrashort pulses generation; and higher doping concentration available for active ions. Otherwise, the Nd:CNGG and Nd:CLNGG have also higher thermal conductivity 共3.5 W / m K兲 than Nd:glass [10]. The cw lasing experiments of Nd:CNGG and Nd:CLNGG at 1.06 m have shown good lasing performances under diode pumping [12–14]. Picosecond mode-locked Nd:CNGG lasers at 1.06 m wavelength have also been reported [15–17]. An actively modelocked Nd:CLNGG laser, in combination with passive Q switching, generated 10– 15 ps pulses with a peak power as high as 15 MW [18]. Very recently, passively mode-locked Nd:CLNGG laser has generated 2 ps pulses with an average output power of ⬃100 mW [19]. In this Letter we report on a subpicosecond modelocked Nd:CLNGG disordered crystal laser. The mode locking of the laser was initiated and sustained by use of a semiconductor saturable absorber mirror (SESAM). With dispersion compensation by a pair of intracavity prisms, we have achieved stable cw mode locking of the disordered crystal laser with 900 fs mode-locked pulse duration and 88 MHz repetition rate. The mode-locked laser emitted an average output power of 486 mW, with a slope efficiency of 26%. The laser setup is schematically shown in Fig. 1. A single-emitter laser diode was used as the pump source. The emission wavelength of the laser diode varied in the range of 802– 807 nm with the output power. The pump light was focused into the Nd:CLNGG crystal by two coupling lenses with the same focal length of 8 cm. The focused pump spot size in the crystal was 100 m ⫻ 250 m. The © 2009 Optical Society of America
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OPTICS LETTERS / Vol. 34, No. 1 / January 1, 2009
Fig. 1. Schematic of laser setup: the focal lengths of the lens L1 and L2 are 8 cm; radii of curvature of M1, M2, and M3 are 10, 30, and 10 cm, respectively. Transmission of the coupler is T = 8%.
Nd:CLNGG disordered crystal was grown by the Czochralski method. The Nd:CLNGG rod used in our experiments had a Nd3+-doping concentration of 0.5 at. %, and a dimension of 3 mm⫻ 3 mm in cross section and 8 mm in length. About 80% of the incident pump power was absorbed by the Nd:CLNGG rod. To remove the generated heat in the crystal for effective laser operation, the Nd:CLNGG rod was wrapped in indium foil and mounted tightly in a water-cooled copper holder that was maintained at a temperature of 15° C. Both sides of the crystal were antireflective coated at the pump and laser wavelengths. In the experiment the crystal was placed with a small incidence angle with respect to the cavity axis to suppress the Fabry–Perot etalon effect. A standard X-folded cavity was used in the experiment to achieve a suitable focusing spot on the SESAM and simultaneously an optimum mode matching with the pump in the laser crystal. The laser mode sizes in the crystal and on the SESAM were calculated to be ⬃45 m and ⬃35 m in radius, respectively. A pair of SF10 prisms, with a tip–tip distance of 49 cm, was used to compensate for the cavity dispersion. A SESAM with a saturable absorption of 1.2% was employed to start and sustain the mode locking of the laser. The transmission of the laser output coupler was 8%. With appropriate alignment of the cavity, stable cw mode locking was achieved in the laser. The cw mode locking could be established only when the absorbed pump power was beyond ⬃3 W. By use of a highspeed detector and a 1 GHz bandwidth oscilloscope, we measured the cw mode-locked pulse trains. Figure
Fig. 2. Typical cw mode-locked pulse train.
Fig. 3. Average output power of the mode-locked laser as a function of absorbed pump power.
2 shows a typical cw mode-locked pulse train in a total time scale of 2 s. The pulse repetition rate is 88 MHz, corresponding to the laser cavity length of 1.7 m. The pulse-to-pulse intensity fluctuation is estimated to be less than 3%. The stable cw mode locking could be sustained for long term in the experiment. The average output power of the mode-locked laser as a function of the absorbed pump power is shown in Fig. 3. In the experiment, we tried output couplers with different couplings. We found that 8% was the optimal output coupling. With the optimal coupler, the lasing threshold was 2.27 W of the absorbed pump power. Near the threshold, the laser operated in cw regime. When the absorbed pump power increased to ⬃2.6 W, Q-switched mode locking was observed. As the absorbed pump power increased beyond 3 W, cw mode locking of the laser occurred. The average output power increased linearly with the absorbed pump power, with a slope efficiency of 26%. The maximum average output power of 486 mW was obtained under an absorbed pump power of 4.11 W, and higher output power level was limited only by the available pump power. The laser output beam had a good TEM00 transverse mode 共M2 ⬇ 1.1兲. With a commercial noncollinear autocorrelator, we also measured the pulse duration of the mode-locked pulses. The autocorrelation trace of the shortest pulses obtained is shown in Fig. 4. The autocorrelation trace has a FWHM of 1.39 ps. Assuming a sech2
Fig. 4. Autocorrelation trace of the mode-locked pulses. Inset, corresponding pulse spectrum.
January 1, 2009 / Vol. 34, No. 1 / OPTICS LETTERS
pulse shape, the pulse duration of the mode-locked pulses is ⬃900 fs. The mode-locked pulse spectrum was measured by an optical spectrum analyzer with a resolution of 0.05 nm. The measured pulse spectrum is shown in the inset of Fig. 4. The pulse spectrum has a FWHM width of 1.8 nm, centered at 1061 nm. The time-bandwidth product of the modelocked pulses is 0.43, which is 1.4 times of the transform-limit value for the sech2-shape pulses. However, the mode-locked pulses could not be further shortened by changing intracavity dispersion. The generated mode-locked pulses from the Nd:CLNGG laser are significantly shorter than those obtained from the Nd:CNGG lasers [15–17]. The stimulated emission spectrum of Nd:CNGG shows that there exist three separate gain bands, centered at 1059.3, 1061.5, and 1066 nm, respectively [10]. Consequently, the mode-locked Nd:CNGG lasers generated longer pulses at multiple wavelengths [16,17]. Owing to different crystal lattices field around Nd3+ in Nd:CLNGG, the two strong gain bands of 1059.3 nm and 1061.5 nm become connected for Nd:CLNGG crystal. Therefore, the mode-locked Nd:CLNGG laser can emit a wider pulse spectrum and shorter mode-locked pulses. Subpicosecond pulses have also been generated from the additivepulse mode-locked Nd:GSAG-YSGG mixed crystal lasers; however, the mode-locked pulse spectra typically showed a multiple-peak structure [20,21]. In conclusion, we have experimentally demonstrated a subpicosecond mode-locked Nd:CLNGG disordered crystal laser for what we believe to be the first time. With a SESAM as mode locker, the disordered crystal laser emitted stable 900 fs mode-locked pulses at a repetition rate of 88 MHz. By pumping of a single-emitter diode, the maximum average output power of the mode locked laser was as high as 486 mW, with a slope efficiency of 26%. These results indicate that the four-level Nd:CLNGG disordered crystal could be an excellent candidate for the generation of subpicosecond pulses. The work is partially supported by the National Research Foundation Singapore under the contract NRF-G-CRP 2007-01, the National Natural Science Foundation of China (NSFC) (50672050, 50721002), and the Grant for State Key Program of China (2004CB619002). References 1. G. J. Spuhler, R. Paschotta, U. Keller, M. Moser, M. J.
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