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Single-walled carbon nanotube saturable absorber for a diode-pumped passively mode-locked Nd,Y:SrF2 laser
Optics Communications 372 (2016) 76–79
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Single-walled carbon nanotube saturable absorber for a diode-pumped passively mode-locked Nd,Y:SrF2 laser Chun Li a, Wei Cai a, Jie Liu a,n, Liangbi Su b, Dapeng Jiang b, Fengkai Ma b, Qian Zhang b, Jun Xu c, Yonggang Wang d a
Shandong Provincial Key Laboratory of Optics and Photonic Device, School of Physics and Electronics, Shandong Normal University, Jinan 250014, China Key Laboratory of Transparent and Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China c School of Physics Science and Engineering, Institute for Advanced Study, Tongji University, Shanghai 200092, China d State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi’an 710119, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 16 August 2015 Received in revised form 3 April 2016 Accepted 6 April 2016 Available online 12 April 2016
A reflective single-walled carbon nanotube as saturable absorber has been firstly adopted to a passively mode-locked Nd,Y:SrF2 crystal. Without any dispersion compensation, the stably mode-locked laser delivers pulses with pulse width as short as 1.7 ps, repetition rate of 107.8 MHz and center wavelength of 1056 nm. The oscillator produces maximum average output power of 319 mW corresponding with a high slope efficiency of 20.2%. The single pulse energy and the peak power are 2.96 nJ and 1.74 kW, respectively. The experimental results show that single-walled carbon nanotube is an excellent saturable absorber for mode-locked lasers. & 2016 Elsevier B.V. All rights reserved.
Keywords: Single-walled carbon nanotube Nd,Y:SrF2 disordered crystal Mode-locked laser
1. Introduction As everyone knows, saturable absorber is an important component of passively mode-locked laser. In the past several decades, semiconductor saturable absorber mirror (SESAM) has been extensively employed because of high stability. However, some fatal flaws limit its applications, such as low damage threshold, narrow operation wavelength, complex epitaxy growth techniques and expensive fabrication costing. Compared to SESAM, the rapid-developed 2D materials and carbon nanotubes saturable absorber provide a new opportunity for mode-locked laser. 2D materials saturable absorber mainly have graphene, the insulating hexagonal boron nitride (hBN), the topological insulator (TI, as Bi2Se3), the transition metal dichalcogenides (TMDCs, like MoS2) and black phosphorus (BP). Their outstanding physical and chemical properties make them possess some unique advantages for specific applications [1–6]. To carbon nanotubes saturable absorber, they exhibit fast recovery times, superbly chemical stability, and broad spectrum range. Moreover, they can be fabricated by simple and economy-costed methods [7–14]. In particular, single-walled carbon nanotubes have been successfully employed in a variety of lasers for the generation of ultra-short pulses [15–20]. n
Corresponding author. E-mail address: [email protected] (J. Liu).
http://dx.doi.org/10.1016/j.optcom.2016.04.012 0030-4018/& 2016 Elsevier B.V. All rights reserved.
Gain medium is an another major component of passively mode-locked laser. In recent years, because of significant broad spectrum and excellent thermal effect, Nd-doped disordered crystals have generated much attention for the generation of the diode pumped solid-state high-power femtosecond laser. In this kind of crystals, the cationic ions are randomly distributed in the lattice sites to form multiple types of substitutional sites, which provides strong inhomogeneous lattice field for rare earth dopants and correspondingly leads to large ground-state stark splitting and broad emission spectra. The Nd3 þ , Y3 þ codoped SrF2 crystals are a typical disordered crystal, which belongs to MeF2–MF3–NdF3 family (Me ¼Ca, Sr, Ba, etc, M¼ Y, Sc, La, Gd, Lu, etc). The Nd3 þ and the Y3 þ ions displace the Sr2 þ ions of fluoride, resulting in multiple Nd3 þ optical centers. Since various Nd3 þ centers have different lattice field, the crystals have effectively inhomogeneous spectral broadening. Meanwhile, the Nd3 þ , Y3 þ doped SrF2 disordered crystals also have long fluorescence lifetime, high damage threshold, large size growth, and better thermal conductivity [21,22]. To date, the Nd,Y:SrF2 disordered crystals have been reported about the laser performance [23–27]. In [25–27], femtosecond pulses have been obtained with SESAM as saturable absorber and Ti:sapphire laser as pumping source. But the SESAM and Ti:sapphire laser are both high cost, which is harmful for the application. In this paper, employing a low-cost reflective single-walled
C. Li et al. / Optics Communications 372 (2016) 76–79
Fig. 1. W-folded cavity setup of passively mode-locked Nd,Y:SrF2 laser.
carbon nanotube (RSWCNT) saturable absorber and a inexpensive diode pumping source, the stably mode-locked performances of Nd,Y:SrF2 laser have been demonstrated. The RSWCNT saturable absorber was firstly applied to Nd,Y:SrF2 laser. Without any dispersion compensation, the minimum pulse width of 1.7 ps was achieved at the central wavelength of 1056 nm with an average output power of 319 mW. The repetition rate was 107.8 MHz, corresponding to the single pulse energy and the peak power are 2.96 nJ and 1.74 kW, respectively.
2. Experiment setup for nd,Y:SrF2 laser The RSWCNT was fabricated by vertical evaporation method. The carbon nanotube powders were rinsed using the H2SO4/HNO3 and dissolved in sodium dodecyl sulfate (SDS) aqueous solution whose concentration was 0.1%. In order to obtain SWCNT aqueous dispersion with high absorption, the SDS aqueous solution was vibrated for 10 h by the ultrasonic wave. After using the centrifuge to quickly precipitate the chunks of carbon nanotube, the upper section of the solution was poured into a hydrophilic polystyrene glass unit. Then they were slow evaporating for 2 weeks in the oven of 40 °C. Finally, to this experiment, SWCNT was coated for protective film and high reflection film in 1 μm waveband. As saturable absorber, the modulation depth was measured to be 2%–3%. The Nd,Y:SrF2 disordered crystals with concentrations of 0.65 at% Nd3 þ doping and 10 at% Y3 þ codoping were grown by the mature temperature gradient technique (TGT). The room-temperature absorption and emission spectra is presented in [23]. The strongest absorption peak locates at 796 nm, which suits the commercial laser diode pumping. The broad emission spectra has
77
a full width of half maximum (FWHM) of 15 nm, which make them particularly suitable for the amplification of ultra-short laser. The strong emission peaks are at 1050 and 1056 nm, respectively. So, the crystals are also appropriate to study the dual-wavelength lasers. And, the emission and absorption cross section are about 4.16 10 20 cm2 and 5.73 10 20 cm2, respectively. In the laser experiment, a W-folded cavity was designed for the mode-locked laser, as presented in Fig. 1. The pump source was a fiber-coupled diode laser (LD) whose emission wavelength was around 796 nm by adjusting the LD temperature, in order to match the absorption peak of Nd,Y:SrF2 crystal. The core diameter and the numerical aperture of LD were 400 μm and 0.22, respectively. The incident laser was focused by a 1:0.8 optical system into a uncoated Nd,Y:SrF2 crystal of 3 3 5 mm3. The absorption efficiency of the crystal was 88.1%. To remove the heat and reduce the thermal effect, the crystal was wrapped in indium foil and cooled with a constant temperature of 12 °C. The cavity consisted of a RSWCNT saturable absorber, an output coupler (OC) and three mirrors M1, M2 and M3. The curvature radii of M1, M2 and M3 were 1, 500 and 100 mm, respectively. The input mirror M1 was the dichroic mirror. The concave mirrors M2 and M3 had high reflection in a waveband around 1064 nm. An OC mirror, with transmission of 2% at 1064 nm, was a plane mirror. The RSWCNT was selected as saturable absorber to obtain the stably modelocked laser. As described previously, it was coated for high reflection at 1 mm to meanwhile act as a reflected mirror. According to the ABCD matrix, the mode radii at the center of crystal and on the RSWCNT were calculated to be 190 μm and 30 μm, respectively. In this experiment, no dispersion compensation element was used.
3. Results and discussions The average output power as a function of the absorbed pump power for Nd,Y:SrF2 laser is shown in Fig. 2(a). After careful adjusting the cavity mirrors and the position of the RSWCNT, the Q-switched mode-locking regime was achieved at the absorbed pump power of 412 mW. When the absorbed pump power was beyond 1.05 W, the laser turned to continuous wave (CW) modelocked operation as Fig. 2(a) identified. And by increasing the pump power slightly further, the stable mode-locking was more and more perfect. In the process of the whole experiment, the mode-locked state had almost 100% depth of modulation. At the
Fig. 2. (a) Average output power versus absorbed pump power for the laser. (b) The output power changed over time at the absorbed power of 1.97 W. (inset) 2D and 3D images of far-field intensity distribution for the CW mode-locking.
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C. Li et al. / Optics Communications 372 (2016) 76–79
Fig. 3. (a) The pulse trains of 1 ms/div and 10 ns/div for CW mode-locked laser. (b) The autocorrelation trace of 1.7 ps. (inset) The corresponding spectrum of CW modelocked pulse.
absorbed power of 1.97 W, the maximum output power was obtained. In this state, the fluctuations of output power were measured as time goes on. In 1 h, the output power was measured every 10 min. As showed in Fig. 2(b), the output power had a small fluctuations. It was decreased from 321 mW to 317 mW and the average output power was calculated to be 319 mW. So, the experiment obtained maximum average output power of 319 mW with a good stability. And the CW mode-locked laser had a high slope efficiency of 20.2%. In whole experiment, RSWCNT was not damage and, in order to protect the laser crystal from damage, the pump power was not further increased. The inset of Fig. 2(b) was the 2D and 3D images of far-field intensity distribution for CW mode-locking at the output power of 319 mW. The beam profile was very close to fundamental transverse mode (TEM00). By a fast photodiode (New Focus 1611) and a digital storage oscilloscope with 1 GHz bandwidth (Tektronix TDS 5104), the CW mode-locked pulse trains were measured. Fig. 3(a) shows the typical CW mode-locked pulse trains in 100 ns and 10 μs time span, respectively. It displayed good stability with the pulse repetition rate of 107.8 MHz which matched well with the cavity length of 1.4 m. At the same time of measuring the fluctuations of maximum output power, the experiment also surveyed the pulse-to-pulse amplitude fluctuations. The perfectly mode-locked state was with time jitter less than 2%. So, the CW mode-locked state could be sustained for more than 1 h. And the stably mode-locked laser could be reproducible. The autocorrelation trace of the stably mode-locked pulse was measured with a commercial autocorrelator (FR-103XL). It is shown in Fig. 3(b). From the autocorrelation trace, the pulse duration was estimated to be 1.7 ps assumed a Gaussian pulse profile. The spectrum of mode-locked pulse was measured by an optical spectrum analyzer (Avaspec-3648-USB2), which is shown in the inset of Fig. 3(b). The pulse spectrum had a FWHM of 1.7 nm, centering at 1056 nm. The single pulse energy and the peak power were estimated to be 2.96 nJ and 1.74 kW, respectively. The time-bandwidth product of the mode-locked pulses was 0.78, which was 1.8 times of the transform-limit value for the Gaussian-shape pulses. It indicated that the mode-locked pulses could be further narrowed with dispersion compensation. In this experiment, the crystal was uncoated and vertically incident by the pump laser, which added more losses. If the crystal was coated for antireflection at the lasing wavelength and placed with Brewster angle, higher slope efficiency and average output power may be obtained. If we used dispersion compensation device (like Gires–Tournois interferometer mirror), optimized the
cavity parameters and improved the quality of RSWCNT saturable absorber, the femtosecond pulse may be generated. Moreover, the stability of mode-locked laser would be further and more exactly measured by using spectrum analyzer.
4. Conclusions In summary, with a low-cost and simply fabricated RSWCNT as saturable absorber, for the first time to our knowledge, a LD pumped Nd,Y:SrF2 laser at picosecond passively mode-locked operation was demonstrated. Without any dispersion compensation, the average output power of 319 mW and the slope efficiency as high as 20.2% were obtained at the central wavelength of 1056 nm. The pulse duration was 1.7 ps with the repetition rate of 107.8 MHz, corresponding to the pulse energy of 2.96 nJ and the peak power of 1.74 kW, respectively. The experimental results proved that Nd,Y:SrF2 disordered crystal was a very promising crystal for diode pumped ultrashort lasers, and pulse duration as short as femtosecond could be hopefully obtained in a further research. Moreover, the RSWCNT was a kind of superb saturable absorber to acquire the stably CW mode-locked laser of high efficiency.
Acknowledgments The authors acknowledge support from the National Natural Science Foundation of China (Nos. 61475089, 61178056, 61422511 and 51432007) and Development projects of Shandong Province Science and Technology (Grant no. 2013GGX10108).
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Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Single-walled carbon nanotube saturable absorber for a diode-pumped passively mode-locked Nd,Y:SrF2 laser Chun Li a, Wei Cai a, Jie Liu a,n, Liangbi Su b, Dapeng Jiang b, Fengkai Ma b, Qian Zhang b, Jun Xu c, Yonggang Wang d a
Shandong Provincial Key Laboratory of Optics and Photonic Device, School of Physics and Electronics, Shandong Normal University, Jinan 250014, China Key Laboratory of Transparent and Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China c School of Physics Science and Engineering, Institute for Advanced Study, Tongji University, Shanghai 200092, China d State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi’an 710119, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 16 August 2015 Received in revised form 3 April 2016 Accepted 6 April 2016 Available online 12 April 2016
A reflective single-walled carbon nanotube as saturable absorber has been firstly adopted to a passively mode-locked Nd,Y:SrF2 crystal. Without any dispersion compensation, the stably mode-locked laser delivers pulses with pulse width as short as 1.7 ps, repetition rate of 107.8 MHz and center wavelength of 1056 nm. The oscillator produces maximum average output power of 319 mW corresponding with a high slope efficiency of 20.2%. The single pulse energy and the peak power are 2.96 nJ and 1.74 kW, respectively. The experimental results show that single-walled carbon nanotube is an excellent saturable absorber for mode-locked lasers. & 2016 Elsevier B.V. All rights reserved.
Keywords: Single-walled carbon nanotube Nd,Y:SrF2 disordered crystal Mode-locked laser
1. Introduction As everyone knows, saturable absorber is an important component of passively mode-locked laser. In the past several decades, semiconductor saturable absorber mirror (SESAM) has been extensively employed because of high stability. However, some fatal flaws limit its applications, such as low damage threshold, narrow operation wavelength, complex epitaxy growth techniques and expensive fabrication costing. Compared to SESAM, the rapid-developed 2D materials and carbon nanotubes saturable absorber provide a new opportunity for mode-locked laser. 2D materials saturable absorber mainly have graphene, the insulating hexagonal boron nitride (hBN), the topological insulator (TI, as Bi2Se3), the transition metal dichalcogenides (TMDCs, like MoS2) and black phosphorus (BP). Their outstanding physical and chemical properties make them possess some unique advantages for specific applications [1–6]. To carbon nanotubes saturable absorber, they exhibit fast recovery times, superbly chemical stability, and broad spectrum range. Moreover, they can be fabricated by simple and economy-costed methods [7–14]. In particular, single-walled carbon nanotubes have been successfully employed in a variety of lasers for the generation of ultra-short pulses [15–20]. n
Corresponding author. E-mail address: [email protected] (J. Liu).
http://dx.doi.org/10.1016/j.optcom.2016.04.012 0030-4018/& 2016 Elsevier B.V. All rights reserved.
Gain medium is an another major component of passively mode-locked laser. In recent years, because of significant broad spectrum and excellent thermal effect, Nd-doped disordered crystals have generated much attention for the generation of the diode pumped solid-state high-power femtosecond laser. In this kind of crystals, the cationic ions are randomly distributed in the lattice sites to form multiple types of substitutional sites, which provides strong inhomogeneous lattice field for rare earth dopants and correspondingly leads to large ground-state stark splitting and broad emission spectra. The Nd3 þ , Y3 þ codoped SrF2 crystals are a typical disordered crystal, which belongs to MeF2–MF3–NdF3 family (Me ¼Ca, Sr, Ba, etc, M¼ Y, Sc, La, Gd, Lu, etc). The Nd3 þ and the Y3 þ ions displace the Sr2 þ ions of fluoride, resulting in multiple Nd3 þ optical centers. Since various Nd3 þ centers have different lattice field, the crystals have effectively inhomogeneous spectral broadening. Meanwhile, the Nd3 þ , Y3 þ doped SrF2 disordered crystals also have long fluorescence lifetime, high damage threshold, large size growth, and better thermal conductivity [21,22]. To date, the Nd,Y:SrF2 disordered crystals have been reported about the laser performance [23–27]. In [25–27], femtosecond pulses have been obtained with SESAM as saturable absorber and Ti:sapphire laser as pumping source. But the SESAM and Ti:sapphire laser are both high cost, which is harmful for the application. In this paper, employing a low-cost reflective single-walled
C. Li et al. / Optics Communications 372 (2016) 76–79
Fig. 1. W-folded cavity setup of passively mode-locked Nd,Y:SrF2 laser.
carbon nanotube (RSWCNT) saturable absorber and a inexpensive diode pumping source, the stably mode-locked performances of Nd,Y:SrF2 laser have been demonstrated. The RSWCNT saturable absorber was firstly applied to Nd,Y:SrF2 laser. Without any dispersion compensation, the minimum pulse width of 1.7 ps was achieved at the central wavelength of 1056 nm with an average output power of 319 mW. The repetition rate was 107.8 MHz, corresponding to the single pulse energy and the peak power are 2.96 nJ and 1.74 kW, respectively.
2. Experiment setup for nd,Y:SrF2 laser The RSWCNT was fabricated by vertical evaporation method. The carbon nanotube powders were rinsed using the H2SO4/HNO3 and dissolved in sodium dodecyl sulfate (SDS) aqueous solution whose concentration was 0.1%. In order to obtain SWCNT aqueous dispersion with high absorption, the SDS aqueous solution was vibrated for 10 h by the ultrasonic wave. After using the centrifuge to quickly precipitate the chunks of carbon nanotube, the upper section of the solution was poured into a hydrophilic polystyrene glass unit. Then they were slow evaporating for 2 weeks in the oven of 40 °C. Finally, to this experiment, SWCNT was coated for protective film and high reflection film in 1 μm waveband. As saturable absorber, the modulation depth was measured to be 2%–3%. The Nd,Y:SrF2 disordered crystals with concentrations of 0.65 at% Nd3 þ doping and 10 at% Y3 þ codoping were grown by the mature temperature gradient technique (TGT). The room-temperature absorption and emission spectra is presented in [23]. The strongest absorption peak locates at 796 nm, which suits the commercial laser diode pumping. The broad emission spectra has
77
a full width of half maximum (FWHM) of 15 nm, which make them particularly suitable for the amplification of ultra-short laser. The strong emission peaks are at 1050 and 1056 nm, respectively. So, the crystals are also appropriate to study the dual-wavelength lasers. And, the emission and absorption cross section are about 4.16 10 20 cm2 and 5.73 10 20 cm2, respectively. In the laser experiment, a W-folded cavity was designed for the mode-locked laser, as presented in Fig. 1. The pump source was a fiber-coupled diode laser (LD) whose emission wavelength was around 796 nm by adjusting the LD temperature, in order to match the absorption peak of Nd,Y:SrF2 crystal. The core diameter and the numerical aperture of LD were 400 μm and 0.22, respectively. The incident laser was focused by a 1:0.8 optical system into a uncoated Nd,Y:SrF2 crystal of 3 3 5 mm3. The absorption efficiency of the crystal was 88.1%. To remove the heat and reduce the thermal effect, the crystal was wrapped in indium foil and cooled with a constant temperature of 12 °C. The cavity consisted of a RSWCNT saturable absorber, an output coupler (OC) and three mirrors M1, M2 and M3. The curvature radii of M1, M2 and M3 were 1, 500 and 100 mm, respectively. The input mirror M1 was the dichroic mirror. The concave mirrors M2 and M3 had high reflection in a waveband around 1064 nm. An OC mirror, with transmission of 2% at 1064 nm, was a plane mirror. The RSWCNT was selected as saturable absorber to obtain the stably modelocked laser. As described previously, it was coated for high reflection at 1 mm to meanwhile act as a reflected mirror. According to the ABCD matrix, the mode radii at the center of crystal and on the RSWCNT were calculated to be 190 μm and 30 μm, respectively. In this experiment, no dispersion compensation element was used.
3. Results and discussions The average output power as a function of the absorbed pump power for Nd,Y:SrF2 laser is shown in Fig. 2(a). After careful adjusting the cavity mirrors and the position of the RSWCNT, the Q-switched mode-locking regime was achieved at the absorbed pump power of 412 mW. When the absorbed pump power was beyond 1.05 W, the laser turned to continuous wave (CW) modelocked operation as Fig. 2(a) identified. And by increasing the pump power slightly further, the stable mode-locking was more and more perfect. In the process of the whole experiment, the mode-locked state had almost 100% depth of modulation. At the
Fig. 2. (a) Average output power versus absorbed pump power for the laser. (b) The output power changed over time at the absorbed power of 1.97 W. (inset) 2D and 3D images of far-field intensity distribution for the CW mode-locking.
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C. Li et al. / Optics Communications 372 (2016) 76–79
Fig. 3. (a) The pulse trains of 1 ms/div and 10 ns/div for CW mode-locked laser. (b) The autocorrelation trace of 1.7 ps. (inset) The corresponding spectrum of CW modelocked pulse.
absorbed power of 1.97 W, the maximum output power was obtained. In this state, the fluctuations of output power were measured as time goes on. In 1 h, the output power was measured every 10 min. As showed in Fig. 2(b), the output power had a small fluctuations. It was decreased from 321 mW to 317 mW and the average output power was calculated to be 319 mW. So, the experiment obtained maximum average output power of 319 mW with a good stability. And the CW mode-locked laser had a high slope efficiency of 20.2%. In whole experiment, RSWCNT was not damage and, in order to protect the laser crystal from damage, the pump power was not further increased. The inset of Fig. 2(b) was the 2D and 3D images of far-field intensity distribution for CW mode-locking at the output power of 319 mW. The beam profile was very close to fundamental transverse mode (TEM00). By a fast photodiode (New Focus 1611) and a digital storage oscilloscope with 1 GHz bandwidth (Tektronix TDS 5104), the CW mode-locked pulse trains were measured. Fig. 3(a) shows the typical CW mode-locked pulse trains in 100 ns and 10 μs time span, respectively. It displayed good stability with the pulse repetition rate of 107.8 MHz which matched well with the cavity length of 1.4 m. At the same time of measuring the fluctuations of maximum output power, the experiment also surveyed the pulse-to-pulse amplitude fluctuations. The perfectly mode-locked state was with time jitter less than 2%. So, the CW mode-locked state could be sustained for more than 1 h. And the stably mode-locked laser could be reproducible. The autocorrelation trace of the stably mode-locked pulse was measured with a commercial autocorrelator (FR-103XL). It is shown in Fig. 3(b). From the autocorrelation trace, the pulse duration was estimated to be 1.7 ps assumed a Gaussian pulse profile. The spectrum of mode-locked pulse was measured by an optical spectrum analyzer (Avaspec-3648-USB2), which is shown in the inset of Fig. 3(b). The pulse spectrum had a FWHM of 1.7 nm, centering at 1056 nm. The single pulse energy and the peak power were estimated to be 2.96 nJ and 1.74 kW, respectively. The time-bandwidth product of the mode-locked pulses was 0.78, which was 1.8 times of the transform-limit value for the Gaussian-shape pulses. It indicated that the mode-locked pulses could be further narrowed with dispersion compensation. In this experiment, the crystal was uncoated and vertically incident by the pump laser, which added more losses. If the crystal was coated for antireflection at the lasing wavelength and placed with Brewster angle, higher slope efficiency and average output power may be obtained. If we used dispersion compensation device (like Gires–Tournois interferometer mirror), optimized the
cavity parameters and improved the quality of RSWCNT saturable absorber, the femtosecond pulse may be generated. Moreover, the stability of mode-locked laser would be further and more exactly measured by using spectrum analyzer.
4. Conclusions In summary, with a low-cost and simply fabricated RSWCNT as saturable absorber, for the first time to our knowledge, a LD pumped Nd,Y:SrF2 laser at picosecond passively mode-locked operation was demonstrated. Without any dispersion compensation, the average output power of 319 mW and the slope efficiency as high as 20.2% were obtained at the central wavelength of 1056 nm. The pulse duration was 1.7 ps with the repetition rate of 107.8 MHz, corresponding to the pulse energy of 2.96 nJ and the peak power of 1.74 kW, respectively. The experimental results proved that Nd,Y:SrF2 disordered crystal was a very promising crystal for diode pumped ultrashort lasers, and pulse duration as short as femtosecond could be hopefully obtained in a further research. Moreover, the RSWCNT was a kind of superb saturable absorber to acquire the stably CW mode-locked laser of high efficiency.
Acknowledgments The authors acknowledge support from the National Natural Science Foundation of China (Nos. 61475089, 61178056, 61422511 and 51432007) and Development projects of Shandong Province Science and Technology (Grant no. 2013GGX10108).
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