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Demonstration of a diode pumped Nd,Y co-doped SrF2 crystal based, high energy chirped pulse amplification laser system
Optics Communications 382 (2017) 201–204
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Demonstration of a diode pumped Nd,Y co-doped SrF2 crystal based, high energy chirped pulse amplification laser system Junchi Chen a,b,c, Yujie Peng b,n, Zongxin Zhang b, Hongpeng Su b,c, Yuxin Leng b,n, Dapeng Jiang d,e, Fengkai Ma d,e, Xiaobo Qian d,e, Fei Tang d,e, Liangbi Su d,e,nn a
School of Physics Science and Engineering, Tongji University, Shanghai 200092, China State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China c University of Chinese Academy of Science, Beijing 100190, China d Key Laboratory of Transparent and Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China e Synthetic Single Crystal Research Center, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 3 May 2016 Received in revised form 1 July 2016 Accepted 3 July 2016
We report, to the best of our knowledge, a chirped pulse amplification laser system based on the Nd,Y: SrF2 crystal for the first time. The incorporation of Y3 þ nonactive ions can significantly improve laser properties of Nd:SrF2 crystal, including broader emission linewidth, larger cross-section as well as longer fluorescence lifetime. Pulse laser with 5.1 mJ (uncompressed), 3.7 mJ (compressed) energy, 1.6 ps duration at 5 Hz repetition rate is demonstrated. The results indicate that the Nd,Y:SrF2 crystal is a potential candidate with excellent laser and thermal performance for developing ultra-intense laser with high repetition rate. & 2016 Elsevier B.V. All rights reserved.
Keywords: Chirped pulse amplification Regenerative amplifier Nd,Y:SrF2 crystal
1. Introduction Ultra-intense lasers are highly attractive because of their various applications, such as high field light-matter interactions, electrons acceleration, fast ignition and so on. Currently, petawatt level laser systems have been demonstrated employing the techniques, like the chirped pulse amplification (CPA) which was put forward in the 1980s [1]. Materials with emission crosssection broad enough are typically Yb:KYW [2], Yb:KGW [3], Yb:CaF2 [4], Yb:CALGO [5], however cryogenic environment is usually required and the laser threshold is high because Yb maintains three energy levels structure, and the limited active media size also restricts their applications in high energy systems as well. The other large gain media adopted in high energy CPA laser system are mainly referred to Ti:sapphire crystal and Nd:glass [6,7]. However, for the Ti:sapphire crystal based CPA system, the crystal size and parasitic oscillation still remain a challenging topic for achieving high energy; and the poor thermal conduction of the Nd:glass, generally less than 1 W/(m K), limits the repetition rate of the laser system n
Corresponding authors. Corresponding author at: Key Laboratory of Transparent and Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China. E-mail addresses: [email protected] (Y. Peng), [email protected] (Y. Leng), [email protected] (L. Su). nn
http://dx.doi.org/10.1016/j.optcom.2016.07.008 0030-4018/& 2016 Elsevier B.V. All rights reserved.
to 1 shot per 2–3 h. Therefore, it is desirable to develop new active media with high thermal conductivity, broad emission spectrum and large size for developing ultra-intense CPA laser system operating at high repetition rate. Alkaline earth metal (Ca, Sr, Ba) fluoride crystals as laser hosts have attracted great attention owing to their favorable properties, such as well controlled crystal growth process, high thermal conduction efficiency (over 10 W/(m K) for undoped CaF2, (9.3 W/ (m K)) for undoped SrF2, (6.7 W/(m K) for undoped BaF2 at 300 K)) and broad laser bandwidth [8–11]. Trivalent rare-earth ions doped alkaline earth metal fluoride-based crystals show potential use in developing high power laser system, especially for Nd3 þ doped crystals possessing low laser threshold and needless of cryogenic environment because of its four energy level structure. However, since concentration quenching caused by the aggregation of the Nd3 þ active ions in the Nd3 þ doped fluoride-based crystals in the form of complicated clusters, Nd3 þ doped fluoride-based crystals have been abandoned for a long time until it is found that the incorporation of nonactive buffer ions, such as Y3 þ , Sc3 þ and La3 þ , can easily dissociate the Nd3 þ –Nd3 þ quenching pairs in clusters. Further, the local coordination structures of neodymium ions can be modulated in a large scope by codoping buffer ions with different Y3 þ /Nd3 þ ratios [12,13]. Compared with Nd:CaF2 crystal, Nd:SrF2 crystal has a small number of clusters that favor energy transfer between the ions, while Nd:BaF2 crystal exhibits unacceptable low transition strength. The emission bandwidth of the
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Nd,Y:SrF2 crystal, can be tuned between 15–31 nm, and fluorescence life time can be changed between 200–1000 ms, and the emission crosssection can be tuned between 1.5–3.5 10 20 cm2. Presently Nd,Y:SrF2 crystal rod with size above Φ10 mm 100 mm and slab with size of 80 mm 80 mm 10 mm have been obtained. Nd,Y:SrF2 crystal possesses adequately high absorption, emission crosssection as well as long fluorescence lifetime. Continuous wave (CW) and pulsed operations of Nd,Y:SrF2 crystals with average output power of 380 mW and 75 mW respectively have been demonstrated [14]. A femtosecond mode-locked Nd,Y codoped SrF2 laser achieving pulse width as short as 181 fs and the average output power of 280 mW has been reported [13]. Normally, these output lasers emitting from the oscillator has limited applications due to their low output power and energy. The high pulse energy and power laser systems are usually realized by master oscillator power amplification (MOPA) structure. However, as far as we know, there is still no report on achieving high energy laser output based on the Nd,Y:SrF2 crystal MOPA system. In this paper, we report a high energy chirped pulse amplification laser system based on the diode-pumped Nd,Y co-doped SrF2 crystal for the first time. The stretched laser pulses from the fiber laser are injected into the Nd,Y co-doped SrF2 crystal based regenerative amplifier, and then the amplified pulses are compressed with dielectric grating pair. Consequently, the output pulse laser with maximum energy of 5.1 mJ at repetition rates of 5 Hz and compressed pulse width of 1.6 ps is obtained.
2. Experiment The laser system displayed in Fig. 1 is a chirped pulse amplification (CPA) architecture consisting of fiber seeder, chirped volume Bragg grating (CVBG) stretcher, laser diode (LD) pumped regenerative amplifier and dielectric grating pair compressor. The all fiber seeder is a multi-stages system starting with an amplified NPR (nonlinear polarization rotation) Yb: fiber mode-locked oscillator (FML) delivering picosecond duration pulses at 100-MHz repetition rate and with 1.053-μm central wavelength. These pulses are temporally stretched by a long fiber to avoid nonlinear effect and then amplified by two stages fiber amplifiers (FA) to 1.5nJ. Then the laser pulses are temporally stretched by CVBG (OptiGrate Corp.) which is with 5 mm 5 mm clear aperture, the chirped rate of 26.2 ps/nm, and the designed wavelength of 1050 nm. The pulse duration of the laser is stretched to about 870 ps with spectral bandwidth of 7.8 nm as shown in Fig. 4 (black line) after double-pass in CVBG. The center wavelength of the
Fig. 1. Layout of the Nd,Y:SrF2 based CPA laser system.
input spectrum is located at 1053 nm, which is the same with the fiber oscillator since the spectral width of the CVBG keeping high reflectivity is 20 nm, which is large enough to stretch the entire spectrum of the input laser. The LD pumped regenerative amplifier (RA) is followed by polarization beam splitter (PBS) for delivering pulses out of stretcher to amplify the chirped pulses. The pulses is switched into the regenerative cavity with the assistance of the Pockels cell (PC), quarter wave plate (QWP) and the thin film polarizer (TFP). A Faraday rotator (FR) and half-wave plate (HWP) are inserted between the two thin film polarizes to isolate the amplified pulses from the injected pulses. The cavity is designed to work at a stable location, which is composed of two plane high reflectors (HR) and an intra-cavity convex lens in the middle part of the cavity. According to calculation based on the ABCD matrix, the beam radius of the fundamental laser at PC is about 1 mm. To obtain fundamental mode laser output, a pinhole with aperture diameter of 3 mm is inserted in the cavity to suppress high order transverse mode. The pulses are switched out of the regenerative cavity by the Pockels cell (PC) at the peak of the energy buildup when the up inversion of the gain is exhausted (saturated). The gain module adopted in the RA is a LD side-pumped Nd,Y:SrF2 crystal doped with 0.5% Nd3 þ , 5% Y3 þ and in size of Φ4 mm 53 mm. There are five LD arrays around the crystal, and each array consists of four LD bars. The pumping laser is directly coupled into the crystal without any focusing component. Both end faces of the rod are coated with antireflection coating @802 nm and 1053 nm, and the lateral side of the crystal is unpolished. The threshold and maximum driving currents of the LD are about 25 A and 250 A respectively, and the LD emitting 802 nm laser can provide a peak power as high as 4 kW at 5 Hz repetition rate. The LD and crystal both are cooled by the fast flow water sustaining at 20 °C.
3. Results and discussion The gain performance of a laser medium is critically important for designing the laser system. So before the building of the CPA system, the small signal gain (SSG) of Nd,Y:SrF2 based laser head is firstly investigated. The SSG is dependent on the volume and stimulated emission crosssection of the active medium as well as the energy storage density directly determined by the duration and power of the pump LD. Under the constant driving current of 200 A, the SSG of Nd,Y:SrF2 crystal versus pumping duration is investigated and plotted in Fig. 2(a). With the increase of the pump duration, the SSG firstly increases but then decreases. The maximum small signal gain can reach about 1.9 when the pump duration is 500 μs. It is known that long pump duration can provide high pump energy (Ep ¼Pp τp, Pp: pumping power; τp: pumping duration), but low storage efficiency ( ηst=[1 − exp( − τp/τf )] /(τp/τf ), τf : fluorescence lifetime) for a LD pumped laser head. Given the Nd,Y:SrF2 crystal properties employed in our laser system, the optimal pump duration is selected to be 500 μs. Moreover, the SSG values of Nd,Y:SrF2 crystal versus different driving currents are also measured with the pumping duration of 500 μs, which is displayed in Fig. 2(b). It can be obviously seen that the SSG nearly linearly increases with respect to the driving currents. Based on the aforementioned gain performance of Nd,Y:SrF2 the crystal, we studied the RA system. The RA boosts the seed pulse energy (about 0.5 nJ) to the maximum after a number of traveling round trips determined by the gain saturation effect. For the laser system reported in this paper, the maximum output energy and the required saturation round-trips at different driving currents is shown in Fig. 3(a). It is easily confirmed that the
J. Chen et al. / Optics Communications 382 (2017) 201–204
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Fig. 2. (a) Small signal gain of Nd,Y:SrF2 crystal versus pump duration under the driving current of 200 A. (b) Small signal gain of Nd,Y:SrF2 crystal versus the driving current with the pumping duration of 500 μs.
Fig. 3. (a) Maximum output energy and required round trips versus the driving current; the inset shows the beam profile of output laser at maximum pulse energy of 5.1 mJ. (b) Buildup process of pulse amplifying in the laser regenerative cavity, the inset shows the injected seeder pulse shape (blue line) and amplified pulse shape (red line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
maximum output energy increases in terms of the driving current. Since higher driving current means higher small signal gain as mentioned above in Fig. 2(b), therefore, the required number of round trips determined by the injected pulse energy and small signal gain becomes less with the increasing driving current and pumping energy when the pulses being completely amplified to saturation. Because the electric power supply can only work at the maximum repetition rate of 5 Hz, so the laser system is operated at 5 Hz. Under the pumping energy of 1000 mJ, the maximum output pulse energy of 5.1 mJ is obtained at repetition rate of 5 Hz, and the corresponding conversion efficiency is about 0.5%. Since a side pumped module is employed, and the energy transferring efficiency and energy store efficiency are lower compared with that of the end pumped module, additionally, the laser mode is not matched well with the mode of the pumping laser in the side pumped module, consequently the conversion efficiency is low. The beam profile of the output laser is nearly a Gaussian distribution with 1.5 mm diameter. The cavity and the pinhole inserted in the cavity are for obtaining fundamental mode TEM00 laser. The beam quality denoted by M2 factor is about 1.3 measured with the knife edge method. A typical example of the pulse dynamic buildup process in the RA cavity is shown in Fig. 3(b). However, due to the gain narrowing effect caused during the amplifying process, the pulse duration of the output laser (red line) is about 170 ps measured with a high speed photodetector (ET-3500, EOT Corp.) and oscilloscope (DPO71254C, Tektronix Corp.) becomes narrower
compared with stretched seeder laser presented in inset of Fig. 3 (b). Under the situation of 170 ps pulse duration and 5.1 mJ energy, the laser fluence and intensity reach about 0.29 J/cm2 and 1.7 GW/ cm2, which possibly leads optical damage to the DKDP crystal in the PC, so the pumping energy is limited to below 1000 mJ. The spectrum of amplified laser is presented in Fig. 4(a) (blue line). It can be easily observed that the output spectrum of RA is with two almost fragmented peaks centered at 1050 nm and 1055 nm, which are very well matched with the two emission peaks of the fluorescence spectrum as Fig. 4(a) shows (red line). That is caused by spectral gain narrowing effect that longitudinal modes close to the emission peak are firstly amplified and extract more energy, so the intensity of the laser located in the middle part of the spectrum is weaker than the intensity of the two peaks. And because of the red shift caused by gain saturation effect, the spectral intensity of the 1055 nm peak is much higher than that at 1050 nm. In consideration of the spectrum of the laser from RA and the emission spectrum of the Nd,Y:SrF2 crystal, an Nd doped phosphate glass with a single emission peak centered at 1053 nm can be introduced in the RA with the Nd,Y:SrF2 crystal to compensate for the gain narrowing effect to yield a flat and more broad output spectrum. Out from the RA, the laser pulses are delivered to the dielectric coating grating pair compressor after a 2 beam expander. After being compressed, laser pulses with 3.7 mJ energy are obtained, corresponding to the compressing efficiency of about 72%. The compressed spectrum is almost the same with the spectrum of the
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Fig. 4. (a) Fluorescence spectrum of Nd,Y:SrF2 (red line), output laser spectrum (blue line) and seeder laser spectrum (black line). (b) Measured autocorrelation trace of the compressed pulses (black line) and example trace of sech2 function (red line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
amplified output laser. The compressed pulses duration is measured by a single shot autocorrelator. As is presented in Fig. 4(b), the measured autocorrelation trace curve (black line) assuming sech2 function pulse intensity profile (red line) indicates that the pulse duration of 1.6 ps (FWHM) can be derived. The poor compressing result is due to the residual high-order dispersion brought by the fiber amplifier and CVBG, which can't be compensated very well.
4. Conclusions We have developed a chirped pulse amplification laser system of laser diode directly pumped Nd,Y:SrF2 crystal for the first time to our knowledge. The gain performance of the new crystal based LDH is investigated and measured. The RA delivers maximum energy of 5.1 mJ operating at the repetition rates of 5 Hz. The pulse duration pulses can be compressed to 1.6 ps with 3.7 mJ energy. Profiting from the high heat conductivity and excellent laser performance of the Nd,Y:SrF2 crystal, it is suggested that this new crystal may be a promising candidate as gain medium for high repetition and ultra-intense laser system. Further work focusing on improving the repetition rates, optical conversion efficiency and further pulse compression will be carried out.
Acknowledgments This
work
is
supported
by
Shanghai
Sailing
Program
(15YF1413500), National Natural Science Foundation of China (NSFC) (11127901, 11134010, 61422511and 51432007).
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Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Demonstration of a diode pumped Nd,Y co-doped SrF2 crystal based, high energy chirped pulse amplification laser system Junchi Chen a,b,c, Yujie Peng b,n, Zongxin Zhang b, Hongpeng Su b,c, Yuxin Leng b,n, Dapeng Jiang d,e, Fengkai Ma d,e, Xiaobo Qian d,e, Fei Tang d,e, Liangbi Su d,e,nn a
School of Physics Science and Engineering, Tongji University, Shanghai 200092, China State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China c University of Chinese Academy of Science, Beijing 100190, China d Key Laboratory of Transparent and Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China e Synthetic Single Crystal Research Center, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 3 May 2016 Received in revised form 1 July 2016 Accepted 3 July 2016
We report, to the best of our knowledge, a chirped pulse amplification laser system based on the Nd,Y: SrF2 crystal for the first time. The incorporation of Y3 þ nonactive ions can significantly improve laser properties of Nd:SrF2 crystal, including broader emission linewidth, larger cross-section as well as longer fluorescence lifetime. Pulse laser with 5.1 mJ (uncompressed), 3.7 mJ (compressed) energy, 1.6 ps duration at 5 Hz repetition rate is demonstrated. The results indicate that the Nd,Y:SrF2 crystal is a potential candidate with excellent laser and thermal performance for developing ultra-intense laser with high repetition rate. & 2016 Elsevier B.V. All rights reserved.
Keywords: Chirped pulse amplification Regenerative amplifier Nd,Y:SrF2 crystal
1. Introduction Ultra-intense lasers are highly attractive because of their various applications, such as high field light-matter interactions, electrons acceleration, fast ignition and so on. Currently, petawatt level laser systems have been demonstrated employing the techniques, like the chirped pulse amplification (CPA) which was put forward in the 1980s [1]. Materials with emission crosssection broad enough are typically Yb:KYW [2], Yb:KGW [3], Yb:CaF2 [4], Yb:CALGO [5], however cryogenic environment is usually required and the laser threshold is high because Yb maintains three energy levels structure, and the limited active media size also restricts their applications in high energy systems as well. The other large gain media adopted in high energy CPA laser system are mainly referred to Ti:sapphire crystal and Nd:glass [6,7]. However, for the Ti:sapphire crystal based CPA system, the crystal size and parasitic oscillation still remain a challenging topic for achieving high energy; and the poor thermal conduction of the Nd:glass, generally less than 1 W/(m K), limits the repetition rate of the laser system n
Corresponding authors. Corresponding author at: Key Laboratory of Transparent and Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China. E-mail addresses: [email protected] (Y. Peng), [email protected] (Y. Leng), [email protected] (L. Su). nn
http://dx.doi.org/10.1016/j.optcom.2016.07.008 0030-4018/& 2016 Elsevier B.V. All rights reserved.
to 1 shot per 2–3 h. Therefore, it is desirable to develop new active media with high thermal conductivity, broad emission spectrum and large size for developing ultra-intense CPA laser system operating at high repetition rate. Alkaline earth metal (Ca, Sr, Ba) fluoride crystals as laser hosts have attracted great attention owing to their favorable properties, such as well controlled crystal growth process, high thermal conduction efficiency (over 10 W/(m K) for undoped CaF2, (9.3 W/ (m K)) for undoped SrF2, (6.7 W/(m K) for undoped BaF2 at 300 K)) and broad laser bandwidth [8–11]. Trivalent rare-earth ions doped alkaline earth metal fluoride-based crystals show potential use in developing high power laser system, especially for Nd3 þ doped crystals possessing low laser threshold and needless of cryogenic environment because of its four energy level structure. However, since concentration quenching caused by the aggregation of the Nd3 þ active ions in the Nd3 þ doped fluoride-based crystals in the form of complicated clusters, Nd3 þ doped fluoride-based crystals have been abandoned for a long time until it is found that the incorporation of nonactive buffer ions, such as Y3 þ , Sc3 þ and La3 þ , can easily dissociate the Nd3 þ –Nd3 þ quenching pairs in clusters. Further, the local coordination structures of neodymium ions can be modulated in a large scope by codoping buffer ions with different Y3 þ /Nd3 þ ratios [12,13]. Compared with Nd:CaF2 crystal, Nd:SrF2 crystal has a small number of clusters that favor energy transfer between the ions, while Nd:BaF2 crystal exhibits unacceptable low transition strength. The emission bandwidth of the
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Nd,Y:SrF2 crystal, can be tuned between 15–31 nm, and fluorescence life time can be changed between 200–1000 ms, and the emission crosssection can be tuned between 1.5–3.5 10 20 cm2. Presently Nd,Y:SrF2 crystal rod with size above Φ10 mm 100 mm and slab with size of 80 mm 80 mm 10 mm have been obtained. Nd,Y:SrF2 crystal possesses adequately high absorption, emission crosssection as well as long fluorescence lifetime. Continuous wave (CW) and pulsed operations of Nd,Y:SrF2 crystals with average output power of 380 mW and 75 mW respectively have been demonstrated [14]. A femtosecond mode-locked Nd,Y codoped SrF2 laser achieving pulse width as short as 181 fs and the average output power of 280 mW has been reported [13]. Normally, these output lasers emitting from the oscillator has limited applications due to their low output power and energy. The high pulse energy and power laser systems are usually realized by master oscillator power amplification (MOPA) structure. However, as far as we know, there is still no report on achieving high energy laser output based on the Nd,Y:SrF2 crystal MOPA system. In this paper, we report a high energy chirped pulse amplification laser system based on the diode-pumped Nd,Y co-doped SrF2 crystal for the first time. The stretched laser pulses from the fiber laser are injected into the Nd,Y co-doped SrF2 crystal based regenerative amplifier, and then the amplified pulses are compressed with dielectric grating pair. Consequently, the output pulse laser with maximum energy of 5.1 mJ at repetition rates of 5 Hz and compressed pulse width of 1.6 ps is obtained.
2. Experiment The laser system displayed in Fig. 1 is a chirped pulse amplification (CPA) architecture consisting of fiber seeder, chirped volume Bragg grating (CVBG) stretcher, laser diode (LD) pumped regenerative amplifier and dielectric grating pair compressor. The all fiber seeder is a multi-stages system starting with an amplified NPR (nonlinear polarization rotation) Yb: fiber mode-locked oscillator (FML) delivering picosecond duration pulses at 100-MHz repetition rate and with 1.053-μm central wavelength. These pulses are temporally stretched by a long fiber to avoid nonlinear effect and then amplified by two stages fiber amplifiers (FA) to 1.5nJ. Then the laser pulses are temporally stretched by CVBG (OptiGrate Corp.) which is with 5 mm 5 mm clear aperture, the chirped rate of 26.2 ps/nm, and the designed wavelength of 1050 nm. The pulse duration of the laser is stretched to about 870 ps with spectral bandwidth of 7.8 nm as shown in Fig. 4 (black line) after double-pass in CVBG. The center wavelength of the
Fig. 1. Layout of the Nd,Y:SrF2 based CPA laser system.
input spectrum is located at 1053 nm, which is the same with the fiber oscillator since the spectral width of the CVBG keeping high reflectivity is 20 nm, which is large enough to stretch the entire spectrum of the input laser. The LD pumped regenerative amplifier (RA) is followed by polarization beam splitter (PBS) for delivering pulses out of stretcher to amplify the chirped pulses. The pulses is switched into the regenerative cavity with the assistance of the Pockels cell (PC), quarter wave plate (QWP) and the thin film polarizer (TFP). A Faraday rotator (FR) and half-wave plate (HWP) are inserted between the two thin film polarizes to isolate the amplified pulses from the injected pulses. The cavity is designed to work at a stable location, which is composed of two plane high reflectors (HR) and an intra-cavity convex lens in the middle part of the cavity. According to calculation based on the ABCD matrix, the beam radius of the fundamental laser at PC is about 1 mm. To obtain fundamental mode laser output, a pinhole with aperture diameter of 3 mm is inserted in the cavity to suppress high order transverse mode. The pulses are switched out of the regenerative cavity by the Pockels cell (PC) at the peak of the energy buildup when the up inversion of the gain is exhausted (saturated). The gain module adopted in the RA is a LD side-pumped Nd,Y:SrF2 crystal doped with 0.5% Nd3 þ , 5% Y3 þ and in size of Φ4 mm 53 mm. There are five LD arrays around the crystal, and each array consists of four LD bars. The pumping laser is directly coupled into the crystal without any focusing component. Both end faces of the rod are coated with antireflection coating @802 nm and 1053 nm, and the lateral side of the crystal is unpolished. The threshold and maximum driving currents of the LD are about 25 A and 250 A respectively, and the LD emitting 802 nm laser can provide a peak power as high as 4 kW at 5 Hz repetition rate. The LD and crystal both are cooled by the fast flow water sustaining at 20 °C.
3. Results and discussion The gain performance of a laser medium is critically important for designing the laser system. So before the building of the CPA system, the small signal gain (SSG) of Nd,Y:SrF2 based laser head is firstly investigated. The SSG is dependent on the volume and stimulated emission crosssection of the active medium as well as the energy storage density directly determined by the duration and power of the pump LD. Under the constant driving current of 200 A, the SSG of Nd,Y:SrF2 crystal versus pumping duration is investigated and plotted in Fig. 2(a). With the increase of the pump duration, the SSG firstly increases but then decreases. The maximum small signal gain can reach about 1.9 when the pump duration is 500 μs. It is known that long pump duration can provide high pump energy (Ep ¼Pp τp, Pp: pumping power; τp: pumping duration), but low storage efficiency ( ηst=[1 − exp( − τp/τf )] /(τp/τf ), τf : fluorescence lifetime) for a LD pumped laser head. Given the Nd,Y:SrF2 crystal properties employed in our laser system, the optimal pump duration is selected to be 500 μs. Moreover, the SSG values of Nd,Y:SrF2 crystal versus different driving currents are also measured with the pumping duration of 500 μs, which is displayed in Fig. 2(b). It can be obviously seen that the SSG nearly linearly increases with respect to the driving currents. Based on the aforementioned gain performance of Nd,Y:SrF2 the crystal, we studied the RA system. The RA boosts the seed pulse energy (about 0.5 nJ) to the maximum after a number of traveling round trips determined by the gain saturation effect. For the laser system reported in this paper, the maximum output energy and the required saturation round-trips at different driving currents is shown in Fig. 3(a). It is easily confirmed that the
J. Chen et al. / Optics Communications 382 (2017) 201–204
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Fig. 2. (a) Small signal gain of Nd,Y:SrF2 crystal versus pump duration under the driving current of 200 A. (b) Small signal gain of Nd,Y:SrF2 crystal versus the driving current with the pumping duration of 500 μs.
Fig. 3. (a) Maximum output energy and required round trips versus the driving current; the inset shows the beam profile of output laser at maximum pulse energy of 5.1 mJ. (b) Buildup process of pulse amplifying in the laser regenerative cavity, the inset shows the injected seeder pulse shape (blue line) and amplified pulse shape (red line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
maximum output energy increases in terms of the driving current. Since higher driving current means higher small signal gain as mentioned above in Fig. 2(b), therefore, the required number of round trips determined by the injected pulse energy and small signal gain becomes less with the increasing driving current and pumping energy when the pulses being completely amplified to saturation. Because the electric power supply can only work at the maximum repetition rate of 5 Hz, so the laser system is operated at 5 Hz. Under the pumping energy of 1000 mJ, the maximum output pulse energy of 5.1 mJ is obtained at repetition rate of 5 Hz, and the corresponding conversion efficiency is about 0.5%. Since a side pumped module is employed, and the energy transferring efficiency and energy store efficiency are lower compared with that of the end pumped module, additionally, the laser mode is not matched well with the mode of the pumping laser in the side pumped module, consequently the conversion efficiency is low. The beam profile of the output laser is nearly a Gaussian distribution with 1.5 mm diameter. The cavity and the pinhole inserted in the cavity are for obtaining fundamental mode TEM00 laser. The beam quality denoted by M2 factor is about 1.3 measured with the knife edge method. A typical example of the pulse dynamic buildup process in the RA cavity is shown in Fig. 3(b). However, due to the gain narrowing effect caused during the amplifying process, the pulse duration of the output laser (red line) is about 170 ps measured with a high speed photodetector (ET-3500, EOT Corp.) and oscilloscope (DPO71254C, Tektronix Corp.) becomes narrower
compared with stretched seeder laser presented in inset of Fig. 3 (b). Under the situation of 170 ps pulse duration and 5.1 mJ energy, the laser fluence and intensity reach about 0.29 J/cm2 and 1.7 GW/ cm2, which possibly leads optical damage to the DKDP crystal in the PC, so the pumping energy is limited to below 1000 mJ. The spectrum of amplified laser is presented in Fig. 4(a) (blue line). It can be easily observed that the output spectrum of RA is with two almost fragmented peaks centered at 1050 nm and 1055 nm, which are very well matched with the two emission peaks of the fluorescence spectrum as Fig. 4(a) shows (red line). That is caused by spectral gain narrowing effect that longitudinal modes close to the emission peak are firstly amplified and extract more energy, so the intensity of the laser located in the middle part of the spectrum is weaker than the intensity of the two peaks. And because of the red shift caused by gain saturation effect, the spectral intensity of the 1055 nm peak is much higher than that at 1050 nm. In consideration of the spectrum of the laser from RA and the emission spectrum of the Nd,Y:SrF2 crystal, an Nd doped phosphate glass with a single emission peak centered at 1053 nm can be introduced in the RA with the Nd,Y:SrF2 crystal to compensate for the gain narrowing effect to yield a flat and more broad output spectrum. Out from the RA, the laser pulses are delivered to the dielectric coating grating pair compressor after a 2 beam expander. After being compressed, laser pulses with 3.7 mJ energy are obtained, corresponding to the compressing efficiency of about 72%. The compressed spectrum is almost the same with the spectrum of the
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Fig. 4. (a) Fluorescence spectrum of Nd,Y:SrF2 (red line), output laser spectrum (blue line) and seeder laser spectrum (black line). (b) Measured autocorrelation trace of the compressed pulses (black line) and example trace of sech2 function (red line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
amplified output laser. The compressed pulses duration is measured by a single shot autocorrelator. As is presented in Fig. 4(b), the measured autocorrelation trace curve (black line) assuming sech2 function pulse intensity profile (red line) indicates that the pulse duration of 1.6 ps (FWHM) can be derived. The poor compressing result is due to the residual high-order dispersion brought by the fiber amplifier and CVBG, which can't be compensated very well.
4. Conclusions We have developed a chirped pulse amplification laser system of laser diode directly pumped Nd,Y:SrF2 crystal for the first time to our knowledge. The gain performance of the new crystal based LDH is investigated and measured. The RA delivers maximum energy of 5.1 mJ operating at the repetition rates of 5 Hz. The pulse duration pulses can be compressed to 1.6 ps with 3.7 mJ energy. Profiting from the high heat conductivity and excellent laser performance of the Nd,Y:SrF2 crystal, it is suggested that this new crystal may be a promising candidate as gain medium for high repetition and ultra-intense laser system. Further work focusing on improving the repetition rates, optical conversion efficiency and further pulse compression will be carried out.
Acknowledgments This
work
is
supported
by
Shanghai
Sailing
Program
(15YF1413500), National Natural Science Foundation of China (NSFC) (11127901, 11134010, 61422511and 51432007).
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