- Email: [email protected]
Femtosecond laser technologies for linear collider designs
Nuclear Instruments and Methods in Physics Research A 472 (2001) 86–93
Femtosecond laser technologies for linear collider designs Katsuyuki Kobayashia,*, Akira Endob b
a R&D Center, Sumitomo Heavy Industries, 2-1-1 Yato Tanashi, Tokyo 188-8585, Japan The Femtosecond Technology Research Association, 5-5 Tokodai, Tsukuba, Ibaraki 300-2635, Japan
Abstract A highly stabilized high-energy femtosecond laser system was developed for Compton X-ray experiments. The laser system is based on the chirped pulse amplification, and each component is actively or passively stabilized. A master oscillator with less than 100 fs timing jitter, two independent oscillators with 300 fs relative timing lag, a new measurement technique of timing fluctuation of low-repetition amplified pulse, and a special designed regenerative amplifier with high quality beam were developed. New technical options for linear collider are proposed based on these expertises. The options are temporally square pulse for low emittance electron generation, a timing stabilized seeder for CO2 amplifier, and multi-pulse high-energy lasers for g–g collision and for multi-bunch electron generation. r 2001 Elsevier Science B.V. All rights reserved. PACS: 41.50.+h; 42.60.Da; 42.60.Lh; 42.60.Mi Keywords: Compton X-ray; Mode-locked laser; Timing stability; Linear collider
1. Introduction Rapid and remarkable progress in ultrashort lasers has made new scientific fields. IR, visible and UV femtosecond pulses are widely applied as diagnostic tools for material science, chemical reactions, communication devices and so on. Also, femtosecond deep-UV sources have been demonstrated by means of higher harmonics generation [1]. However, generation of shorter wavelength femtosecond X-rays [2] is still in its infancy. Since 1996, the Femtosecond Technology Project [3] in Japan has researched femtosecond X-ray generation. In this project, we are developing a 901 Compton *Corresponding author. E-mail address: kty [email protected] (K. Kobayashi).
scattering scheme [4]. Compton X-rays have the advantage of short pulsewidth, high peak power, tunability, small divergence and monochromaticity. The pulsewidth of generated X-rays is approximately the same as the interaction time between electron and laser beams. There are two choices for short pulse Compton X-ray generation. One is a head-on collision with a very short electron pulse and a very short laser pulse. The other is a 901 collision with a tightly focused electron pulse and a very short laser pulse. Although very short electron pulses have been reported [5], they are very difficult to generate. In a 901 collision, the pulsewidth of generated X-rays is almost the same as the transit time that the laser pulse crosses the electron pulse. Tightly focused electron pulse and a femtosecond laser pulse lead to a very short X-ray pulse.
0168-9002/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 1 6 6 - 4
K. Kobayashi, A. Endo / Nuclear Instruments and Methods in Physics Research A 472 (2001) 86–93
The parameters of the electron pulse and the laser pulse in our project are listed in Table 1. A low emittance electron pulse is generated from a photocathode. The pulse is accelerated in a 1.6-cell RF cavity to 6 MeV, and again accelerated in a Microtoron to 150 MeV. The charge and the pulsewidth are 1 nC and 1 ps, respectively. The pulse is focused to a 50 mm diameter at the collision point. A femtosecond laser pulse is generated by a chirped pulse amplification (CPA) system. The energy and pulsewidth are 100 mJ and 100 fs, respectively. The laser pulse is also focused to 50 mm in diameter at the collision point. Table 2 lists the parameters of the generated X-ray pulse predicted from a numerical calculation [6]. The wavelength and the photon energy are 4:6 10 3 nm and 268 keV, respectively. The pulsewidth is predicted to be 268 fs, and the number of X-ray photons is 2:6 106 per each pulse. In Section 2, we describe our achievements on the laser system for Compton X-ray generation: stabilization of a master oscillator, synchronization of two mode-locked oscillators, timing stability measurement of a regenerative amplifier, and high quality beam regenerative amplifier using a special
Table 1 Parameters of the electron pulse and the laser pulse Electron pulse Energy Charge Pulsewidth Focused beam size Laser pulse Energy Wavelength Pulsewidth Focused beam size
150 MeV 1 nC=pulse 1 ps 50 mm 100 mJ=pulse 800 nm 100 fs 50 mm
Table 2 Parameters of generated X-ray pulse X-ray pulse Photon energy Wavelength Pulsewidth Photon number
268 keV 4:6 10 3 nm 248 fs 2:6 106 pulse
87
design. In Section 3, we propose new laser options for linear colliders, namely temporally square profile of the laser pulse for low emittance photo-cathode, stabilized picosecond–femtosecond seeder for CO2 amplifiers, and multipulse lasers for g–g collisions. Section 4 summarizes the major results.
2. Laser developments for Compton X-ray generation In order to collide an electron pulse and a laser pulse, both pulses must arrive at the collision point simultaneously. Precise timing control is strictly required both for the electron pulse and for the laser pulse. We use a stable RF electrical signal as a reference, and both pulses are synchronized to the same RF reference. Therefore, both pulses are synchronized to each other. Our CPA laser system [7,8] is constructed with a mode-locked Ti : sapphire oscillator, a pulse stretcher, a regenerative amplifier, a four-pass amplifier, and a pulse compressor. Also, timing stabilizing units and a wave-front correcting apparatus are built in. In this section, we describe our developments on this laser system. 2.1. Timing stabilization of a mode-locked master oscillator A home-made mode-locked femtosecond laser [9,10] is used as a master oscillator. Nearly transform-limited pulses with 50 fs duration were generated by this oscillator. The output power and the center wavelength were about 100 mW and 800 nm, respectively. The repetition frequency of this oscillator was set to 119 MHz, 1=24 of the RF frequency for an s-band accelerator. For better stabilization, the room temperature of the laboratory was kept within 0.11: The oscillator was covered with a plastic box to avoid airflow. To reduce vibrations, the floor under the optical bench was isolated from the building and the pile of the floor was driven deep into the ground. The cavity optical length determines the pulse repetition time. One of the cavity mirrors was controlled by a piezoelectric transducer
88
K. Kobayashi, A. Endo / Nuclear Instruments and Methods in Physics Research A 472 (2001) 86–93
(PZT) to adjust the cavity optical length. Also, the phase of the pulse train was locked to the reference RF signal with a phase locked loop (PLL). We measured the timing fluctuation of this oscillator by a power spectral analysis technique [11] and a digital demodulation technique [12]. The short-term timing jitter, mostly caused by mechanical vibrations of optical components, was 77 fs (rms). The long-term timing drift, caused by thermal expansion of a base plate, was about 100 mHz over one hour measuring time [10]. 2.2. Synchronization of two independent oscillators Two lasers are employed in the femtosecond Xray system. One is a high power femtosecond laser for collision with an electron pulse, and the other is a UV picosecond laser for photocathode irradiation. Both lasers should be synchronized each other. We synchronized two femtosecond mode-locked Ti : sapphire lasers as a basic experiment. The center wavelength of these oscillators were 800 and 850 nm, respectively. Each laser was synchronized to the same reference signal. The timing jitter of each laser were 127 and 378 fs (rms), respectively. Since each oscillator was synchronized to the same reference signal, these two oscillators were synchronized each other. The cross-correlation trace was measured, and the relative timing lag was about 300 fs (rms) [13]. Generally, synchronization of two lasers is achieved using one oscillator as a seeder. One oscillator pulse is divided into two and the divided pulses are amplified. But in this case, the wavelength of two lasers must be the same. In our case, the wavelengths of the two lasers can be different. For example, a Ti : sapphire laser and a Cr : YAG can be synchronized. This system can be applied to two-wavelength pump and probe experiments [14], wavelength conversion with two laser pulses, and so on. 2.3. New technique to measure the timing stability of low repetition amplified pulses The timing jitter of mode-locked oscillators can be measured with statistical methods, such as
power spectrum analysis or digital demodulation, since the typical repetition frequency is around 80– 100 MHz. But these statistical methods cannot be applied to the amplified pulses, because the repetition is much lower. We proposed [15] and achieved [16] the jitter measurement for low repetition rate amplified laser pulses, based on a modified cross-correlation technique. We used a home-made Ti : sapphire regenerative amplifier as a test laser. A low noise all-solid-state Nd : YLF green laser (Positive Light, Evolution) was used as a pump source. The stabilized mode-locked Ti : sapphire master oscillator, transform limited 50 fs pulse, was amplified at 1 kHz repetition frequency. For better stabilization, the temperature of the intracavity Pockels cell was stabilized within 0.11: The laboratory temperature was also stabilized within 0.11: The pulse energy and the pulse to pulse energy stability were 0.3 mJ and 0.4% (rms), respectively. The amplified pulse was linearly stretched to 500 fs. This stretched amplified pulse and the transform limited oscillator pulse were put into a cross-correlator (Fig. 1). Relative timing lag of these two pulses corresponds to the wavelength shift of the upconverted light. Using a spectrometer with 0.025 nm resolution, the resolution of the timing fluctuation was approximately 1 fs. The timing fluctuation was measured in every 6 ms. Even without stabilization, the timing fluctuation of the amplifier was very small. The short-term timing jitter was less
Fig. 1. Modified cross-correlator to measure the timing fluctuation of low-repetition ratio amplified pulse. A stretched, amplified pulse and a transform limited oscillator pulse are incident on a BBO crystal. The relative timing lag of two pulses corresponds to the wavelength of the upconverted pulse.
K. Kobayashi, A. Endo / Nuclear Instruments and Methods in Physics Research A 472 (2001) 86–93
89
than 10 fs, and the long-term timing drift over a one hour time span was about 200 fs. 2.4. High power regenerative amplifier with high beam quality The pointing stability and beam focusability of an amplified laser pulse are the key factors for spatial overlapping of a laser pulse and an electron pulse. Although high power, all-solid-state laser has been reported [17], flash-lamp pumped lasers are generally used as the pump source for terawatt laser systems. Flashlamp-pumped lasers generally exhibit highly structural spatial profile and poor energy stability. When used to pump an amplifier, this causes thermal inhomogeneities in Ti : sapphire amplifier crystal. Thermal inhomogeneity results in wave-front distortion [18,19] and pointing instability. The focusability becomes worse because of these wave-front distortion. We designed and constructed a high power regenerative amplifier with high beam quality [20]. The seed pulse generated by a Ti : sapphire mode-locked oscillator was stretched to about 400 ps and injected into the regenerative amplifier. The regenerative amplifier consists of a z-folded cavity and a 10 mm long, Brewster-cut Ti : sapphire crystal. A small part of the pumping laser for four-pass power amplifier (Spectra-Physics Quanta-Ray PRP-290-50, second harmonics of Nd : YAG, flush-lamp pumped, Q-switched) was divided and injected to the regenerative amplifier. This regenerative amplifier is designed to be very stable against thermal effects and to be able to sustain a large diameter TEM00 mode. The output energy was about 6 mJ with 2.5% (rms) stability at 50 Hz repetition rate. The pointing stability was measured by a laser beam profiler (Spiricon LBA100A), to be 5:4 mrad in the horizontal and 6:6 mrad in the vertical direction (Fig. 2). At this moment, only passive stabilization (special cavity design, stable environmental conditions, carefully selected components) have been performed. For better pointing stability, adaptive optics will be used as a vector scanning system. The wave-front was measured with a Shack–Hartmann type [21] wave-front sensor (AOA Wavescope). Fig. 3(a) shows the wave-front of the amplified pulse. We
Fig. 2. Pointing stability of 50 Hz regenerative amplifier. Standard deviation of horizontal (top) and vertical (bottom) are 5:4 and 6:6 mrad; respectively.
used a telescope in order to extract the spherical components of the wave-front. Fig. 3(b) shows the wave-front with focused term removed. Higherorder aberration of the wave-front were below l=10 at the center wavelength of 800 nm. In this case also, only passive stabilization have been performed. We plan to use a deformable mirror to remove the remaining aberration.
3. New laser options for linear collider designs In this section, we propose new laser options for LC designs, based on our achievements for Compton X-ray generation. 3.1. Temporally square pulse for low emittance photocathode Generally, the temporal profile of laser pulses are Gaussian, sech2 ; and so on. But in some applications, other profiles are required. Generation of a low emittance electron pulse is a key technology for high efficiency X-ray generation. In this case, square pulse shape is better for the photocathode. We built a pulse shaping apparatus [22] which consisted of a grating pair, a lens pair and a liquid crystal spatial light module (LC-SLM) [23,24]. The optimum pulse energy for the photocathode is on the order of several hundreds of mJ in
90
K. Kobayashi, A. Endo / Nuclear Instruments and Methods in Physics Research A 472 (2001) 86–93
shaping apparatus. The phase of each wavelength element was modulated so that the amplified pulse would be a square pulse with 1 ps duration. Then the modulated pulse was amplified by an 1 kHz regenerative amplifier. The output energy was 300 mJ at the center wavelength of 800 nm. We are now developing a third harmonic generation apparatus for a photocathode in combination with this scheme. 3.2. Timing stabilized seeder for CO2 amplifier
Fig. 3. (a) Wave-front of the high-energy pulse from the regenerative amplifier, (b) wavefront with focused term removed.
the UV region, therefore an optical amplifier is required. The pulse shaping apparatus was inserted before the amplifier to avoid damage to the LC-SLM. There are two choices for the design, modulate the amplitude and the phase simultaneously or modulate only the phase. Amplitude modulation of laser pulses before the chirped pulse amplification may cause damages of the optical elements in the regenerative amplifier. Therefore, we selected phase only modulation. Fig. 4 shows our design to make a square shaped pulse. A transform limited 50 fs pulse was put into the pulse
In some applications, such as polarized positron generation [25], a CO2 laser will be used. The wavelength of CO2 lasers are 10 times longer than that of Nd : YAG. Because of its longer wavelength, the number of photons is much larger compared to a solid-state laser with the same output power. Furthermore, the electrical to optical efficiency of CO2 laser is very high. Therefore, CO2 laser is the best choice when the cost of generating photons is a issue. The timing stability of a CO2 laser is very poor because the laser medium is pumped by a gas discharge. We are developing a stable all-solid-state laser as a seeder for a CO2 laser. In this case, the CO2 medium will be used only as an amplifier. The pulse timing will be determined by the seeder. Fig. 5 shows the setup for the seeder. Two Ti : sapphire mode-locked oscillators are synchronized with each other. The center wavelength of these oscillators are 745 and 801 nm, respectively. The pulses generated by these oscillators are amplified by regenerative amplifiers. Finally, the amplified pulses are incident on a AgGaS2 crystal. Pulses of 10:6 mm will be generated by difference frequency mixing [26], and delivered to a CO2 amplifier. The pulses from two oscillators must arrive at the crystal simultaneously to generate the seed pulse. Synchronization of two oscillators is the essential point for this technique. We have described our technique for synchronizing two oscillators in Section 2.2. 3.3. Multipulse laser for g–g collider For a g–g collider, two types of laser systems are required. One is for the collision with a GeV
K. Kobayashi, A. Endo / Nuclear Instruments and Methods in Physics Research A 472 (2001) 86–93
91
Fig. 4. Pulse shaping apparatus with a liquid crystal spatial light module (LC-SLM).
Fig. 5. Timing stabilized seeder for CO2 amplifier. Seven hundred and forty five nm and 801 nm pulses from synchronized two mode-locked oscillator are amplified, and introduced to AgGaS2 crystal. A seed pulse for CO2 amplifier is generated by difference frequency mixing.
electron pulse to generate Compton g photons. The other is used for the photocathodes of multipulse electron bunch. The laser energies are much different, but the time structures are the same. For example, let us consider the g–g collider at the JLC [27]. The time structure of the electron bunch is shown in Fig. 6. One macro-pulse contains 85 micro-pulses with 2.8 ns time interval, corresponding to 357 MHz repetition rate. The pulse length and the repetition rate of the macro-
Fig. 6. Time structure of JLC g–g collider. 238 ns macro-pulse, contains 85 micro-pulse with 2.8 ns interval, repeats in 150 Hz.
pulse are 238 ns and 150 Hz, respectively. The time structures of both laser systems are the same as that of the electron bunch. To generate g photons with electron-laser Compton scattering, high-energy laser pulses are required. The required energy and the pulsewidth of each micro-pulse are 1 J and 1 ps, respectively. It is not realistic to generate such high energy pulses with 357 MHz repetition rate. Therefore, we propose to build 85 lasers for one arm of the linac. The repetition rate of each laser should be 150 Hz. For better stabilization, a low-noise
92
K. Kobayashi, A. Endo / Nuclear Instruments and Methods in Physics Research A 472 (2001) 86–93
all-solid-state laser is desirable. Recent progress in all-solid-state lasers made it possible to build 1 J, 150 Hz lasers. However, this kind of laser is very expensive. Since 170 lasers are required for two arms, the cost for the lasers is the key issue for the g–g collider. One possible solution is to use a crystal which has a long upper state lifetime. Fewer diodes are necessary to produce the same energy output compared to short lifetime crystals, because the pumping energy can be stored in the crystal over a longer period of time. Ytterbium doped Sr5 (PO4 )3 F (Yb : S-FAP) [28] is one promising material. The upper state lifetime of Yb : SFAP is 1.26 ms, five times longer than Nd : YAG. A reduction of the number of diodes significantly reduces the laser cost. Another attractive feature of Yb : S-FAP is the 1047 nm lasing wavelength. It exactly overlaps the wavelength of Nd : YLF. The well established Nd : YLF technology can be applied for the seed laser, and the Yb : S-FAP crystal can be used as an amplifier. Relatively lower energy is required for photocathode irradiation. We have developed a 357 MHz mode-locked Nd : YAG oscillator, and a high-speed electro-optical modulator (EOM). The 10–90% rise time and fall time of the EOM are 0.8 and 0.9 ns, respectively. Eighty-five micropulses are picked up in 150 Hz with the high-speed EOM. The macro-pulse is amplified with a Nd : YAG amplifier, and is converted to the fourth harmonics.
were about 10 and 200 fs, respectively. A high energy and high stability regenerative amplifier with a specially designed cavity was built. The pointing stability was about 6 mrad; and the higher order wave-front aberration was only l=10; only with passive stabilization. Also, new technical options for linear collider were introduced. Square temporal pulses were generated with a liquid crystal spatial light module. Timing stabilized seed pulses for a CO2 amplifier was proposed. Two systems for a g–g collider were studied. One is a high-energy multipulse laser. Using a long upper-level lifetime crystal, the total cost will be significantly reduced. The other one is a multi-pulse laser for the photocathode. Multi-pulse seeder, generated by using a high repetition mode-locked oscillator and a high-speed EOM, will be amplified and converted to fourth harmonics.
Acknowledgements The authors would like to thank to Mr. Taisuke Miura, Mr. Shinji Ito, Dr. Kazuya Takasago, Mr. Hideyuki Nagaoka for their fine experiments, Prof. Zhigang Zhang and Dr. Kenji Torizuka for their fruitful discussions, Dr. Mark Bowers for carefully read this manuscript. This work was performed under the management of the Femtosecond Technology Research Association, supported by New Energy and Industrial Technology Development Organization.
4. Summary We have developed a highly stabile laser system under the research project for Compton X-ray generation. A Ti : sapphire master oscillator was stabilized, resulting a short-term timing jitter less than 100 fs and long-term timing drift of about 100 mHz: Two independent mode-locked oscillators were synchronized. The relative timing lag was about 300 fs. A new measurement technique for the timing fluctuation of low-repetition, amplified pulse was performed. The resolution of this technique is less than 1 fs. The short-term timing jitter and long-term timing drift of the regenerative amplifier, with no active stabilization,
References [1] Y. Kobayashi, T. Sekikawa, Y. Nabekawa, S. Watanabe, Opt. Lett. 23 (1998) 64. [2] R.W. Schoenlein, W.P. Leemans, A.H. Chin, P. Volfbeyn, T.E. Glover, P. Balling, M. Zorotorev, K.J. Kim, S. Chattopadhyay, C.V. Shank, Science 274 (1996) 5285. [3] H. Yajima, T. Kamiya, IECIE Trans. Electron. E81-c (1998) 95. [4] A. Endo, in: T. Kamiya, F. Saito, O. Wada, H. Yajima (Eds.), Femtosecond Technology From Basic Research to Application Prospects, Springer Series in Photonics, Vol. 2, Springer, Berlin, 1999. [5] M. Uesaka, K. Kinoshita, T. Watanabe, T. Ueda, K. Yoshii, H. Harano, J. Sugahara, K. Nakajima,
K. Kobayashi, A. Endo / Nuclear Instruments and Methods in Physics Research A 472 (2001) 86–93
[6] [7] [8] [9] [10] [11] [12] [13]
[14] [15] [16] [17]
A. Ogata, F. Sakai, H. Dewa, M. Kando, H. Kotaki, S. Kondo, AIP Conf. Proc. 427 (1999) 908. J. Yang, M. Washio, A. Endo, T. Hori, Nucl. Instr. and Meth. A 428 (1999) 556. A. Endo, K. Kobayashi, S. Ito, WQ6, CLEO=Pacific Rim’99, Seoul Korea, 30 August–3 September 1999. A. Endo, Nucl. Instr. and Meth. A 455 (2000) 228. K. Kobayashi, T. Miura, Z. Zhang, K. Torizuka, Proc. SPIE 3616 (1999) 156. K. Kobayashi, T. Miura, S. Ito, Z. Zhang, K. Torizuka, A. Endo, Nucl. Instr. and Meth. A 455 (2000) 239. D. von der Linde, Appl. Phys. B 39 (1986) 201. H. Tsuchida, Opt. Lett. 24 (1999) 1641. H. Nagaoka, T. Miura, K. Kobayashi, K. Takasago, Z. Zhang, F. Kannari, K. Torizuka, M. Washio, Proceedings of the Seventh International Workshop on Femtosecond Technology, Tsukuba, Japan, 29–30 June 2000, p. 125. T. Asano, M. Tamura, S. Yoshizawa, K. Tomoda, S. Noda, Physica E 7 (2000) 704. T. Miura, K. Kobayashi, Z. Zhang, K. Torizuka, F. Kannari, Proc. SPIE 3616 (1999) 165. T. Miura, K. Kobayashi, K. Takasago, Z. Zhang, K. Torizuka, F. Kannari, Opt. Lett. 25 (2000) 1795. Y. Akiyama, A. Takada, T. Takase, M. Sasaki, H. Yuasa, N. Nishida, Advanced Solid-State Lasers, OSA Technical Digest, Optical Society of America, Washington, DC, 2000, pp. 19–21.
93
[18] J.-C. Chanteloup, F. Druon, M. Nantel, A. Masksimchuck, G. Mourou, Opt. Lett. 23 (1998) 621. [19] C.L. Blanc, E. Baubeau, F. Salin, J.A. Squier, C.P.J. Barty, C. Spielmann, IEEE J. Quantum. Electron. 4 (1998) 407. [20] S. Ito, T. Miura, Z. Zhang, A. Endo, K. Torizuka, CWB 0005, CLEO=Europe-IQEC2000, Nice, France, 10–15 September 2000. [21] E. Hecht, Optics, Addison-Wesley, Reading, MA, pp. 233–234. [22] K. Takasago, A. Yada, T. Miura, M. Washio, F. Kannari, Z. Zhang, K. Torizuka, A. Endo, CTuK 104, CLEO/ Europe-IQEC2000, Nice, France, 10–15 September 2000. [23] A.M. Weiner, D.E. Leaird, J.S. Patel, J.R. Wullert II, IEEE J. Quantum Electron. 28 (1992) 908. [24] M.M. Wefers, K.A. Nelson, Opt. Lett. 20 (1995) 1047. [25] T. Omori, Proceedings of the International Conference of Lasers’99, Quebec, Canada, 13–16 December 1999, pp. 273. [26] M.R.X. Barros, R.S. Miranda, T.M. Jedju, P.C. Becker, Opt. Lett. 20 (1995) 480. [27] T. Matsui, The Fifth Japan–China Joint Symposium on Accelerators for Nuclear Science and Their Applications, Osaka, Japan, 18–20 October 1993. [28] S.A. Payne, L.D. Deloach, L.K. Smith, W.L. Kway, L.B. Tassano, W.F. Krupke, B.H.T. Chai, G. Loutts, J. Appl. Phys. 76 (1994) 497.
Femtosecond laser technologies for linear collider designs Katsuyuki Kobayashia,*, Akira Endob b
a R&D Center, Sumitomo Heavy Industries, 2-1-1 Yato Tanashi, Tokyo 188-8585, Japan The Femtosecond Technology Research Association, 5-5 Tokodai, Tsukuba, Ibaraki 300-2635, Japan
Abstract A highly stabilized high-energy femtosecond laser system was developed for Compton X-ray experiments. The laser system is based on the chirped pulse amplification, and each component is actively or passively stabilized. A master oscillator with less than 100 fs timing jitter, two independent oscillators with 300 fs relative timing lag, a new measurement technique of timing fluctuation of low-repetition amplified pulse, and a special designed regenerative amplifier with high quality beam were developed. New technical options for linear collider are proposed based on these expertises. The options are temporally square pulse for low emittance electron generation, a timing stabilized seeder for CO2 amplifier, and multi-pulse high-energy lasers for g–g collision and for multi-bunch electron generation. r 2001 Elsevier Science B.V. All rights reserved. PACS: 41.50.+h; 42.60.Da; 42.60.Lh; 42.60.Mi Keywords: Compton X-ray; Mode-locked laser; Timing stability; Linear collider
1. Introduction Rapid and remarkable progress in ultrashort lasers has made new scientific fields. IR, visible and UV femtosecond pulses are widely applied as diagnostic tools for material science, chemical reactions, communication devices and so on. Also, femtosecond deep-UV sources have been demonstrated by means of higher harmonics generation [1]. However, generation of shorter wavelength femtosecond X-rays [2] is still in its infancy. Since 1996, the Femtosecond Technology Project [3] in Japan has researched femtosecond X-ray generation. In this project, we are developing a 901 Compton *Corresponding author. E-mail address: kty [email protected] (K. Kobayashi).
scattering scheme [4]. Compton X-rays have the advantage of short pulsewidth, high peak power, tunability, small divergence and monochromaticity. The pulsewidth of generated X-rays is approximately the same as the interaction time between electron and laser beams. There are two choices for short pulse Compton X-ray generation. One is a head-on collision with a very short electron pulse and a very short laser pulse. The other is a 901 collision with a tightly focused electron pulse and a very short laser pulse. Although very short electron pulses have been reported [5], they are very difficult to generate. In a 901 collision, the pulsewidth of generated X-rays is almost the same as the transit time that the laser pulse crosses the electron pulse. Tightly focused electron pulse and a femtosecond laser pulse lead to a very short X-ray pulse.
0168-9002/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 1 6 6 - 4
K. Kobayashi, A. Endo / Nuclear Instruments and Methods in Physics Research A 472 (2001) 86–93
The parameters of the electron pulse and the laser pulse in our project are listed in Table 1. A low emittance electron pulse is generated from a photocathode. The pulse is accelerated in a 1.6-cell RF cavity to 6 MeV, and again accelerated in a Microtoron to 150 MeV. The charge and the pulsewidth are 1 nC and 1 ps, respectively. The pulse is focused to a 50 mm diameter at the collision point. A femtosecond laser pulse is generated by a chirped pulse amplification (CPA) system. The energy and pulsewidth are 100 mJ and 100 fs, respectively. The laser pulse is also focused to 50 mm in diameter at the collision point. Table 2 lists the parameters of the generated X-ray pulse predicted from a numerical calculation [6]. The wavelength and the photon energy are 4:6 10 3 nm and 268 keV, respectively. The pulsewidth is predicted to be 268 fs, and the number of X-ray photons is 2:6 106 per each pulse. In Section 2, we describe our achievements on the laser system for Compton X-ray generation: stabilization of a master oscillator, synchronization of two mode-locked oscillators, timing stability measurement of a regenerative amplifier, and high quality beam regenerative amplifier using a special
Table 1 Parameters of the electron pulse and the laser pulse Electron pulse Energy Charge Pulsewidth Focused beam size Laser pulse Energy Wavelength Pulsewidth Focused beam size
150 MeV 1 nC=pulse 1 ps 50 mm 100 mJ=pulse 800 nm 100 fs 50 mm
Table 2 Parameters of generated X-ray pulse X-ray pulse Photon energy Wavelength Pulsewidth Photon number
268 keV 4:6 10 3 nm 248 fs 2:6 106 pulse
87
design. In Section 3, we propose new laser options for linear colliders, namely temporally square profile of the laser pulse for low emittance photo-cathode, stabilized picosecond–femtosecond seeder for CO2 amplifiers, and multipulse lasers for g–g collisions. Section 4 summarizes the major results.
2. Laser developments for Compton X-ray generation In order to collide an electron pulse and a laser pulse, both pulses must arrive at the collision point simultaneously. Precise timing control is strictly required both for the electron pulse and for the laser pulse. We use a stable RF electrical signal as a reference, and both pulses are synchronized to the same RF reference. Therefore, both pulses are synchronized to each other. Our CPA laser system [7,8] is constructed with a mode-locked Ti : sapphire oscillator, a pulse stretcher, a regenerative amplifier, a four-pass amplifier, and a pulse compressor. Also, timing stabilizing units and a wave-front correcting apparatus are built in. In this section, we describe our developments on this laser system. 2.1. Timing stabilization of a mode-locked master oscillator A home-made mode-locked femtosecond laser [9,10] is used as a master oscillator. Nearly transform-limited pulses with 50 fs duration were generated by this oscillator. The output power and the center wavelength were about 100 mW and 800 nm, respectively. The repetition frequency of this oscillator was set to 119 MHz, 1=24 of the RF frequency for an s-band accelerator. For better stabilization, the room temperature of the laboratory was kept within 0.11: The oscillator was covered with a plastic box to avoid airflow. To reduce vibrations, the floor under the optical bench was isolated from the building and the pile of the floor was driven deep into the ground. The cavity optical length determines the pulse repetition time. One of the cavity mirrors was controlled by a piezoelectric transducer
88
K. Kobayashi, A. Endo / Nuclear Instruments and Methods in Physics Research A 472 (2001) 86–93
(PZT) to adjust the cavity optical length. Also, the phase of the pulse train was locked to the reference RF signal with a phase locked loop (PLL). We measured the timing fluctuation of this oscillator by a power spectral analysis technique [11] and a digital demodulation technique [12]. The short-term timing jitter, mostly caused by mechanical vibrations of optical components, was 77 fs (rms). The long-term timing drift, caused by thermal expansion of a base plate, was about 100 mHz over one hour measuring time [10]. 2.2. Synchronization of two independent oscillators Two lasers are employed in the femtosecond Xray system. One is a high power femtosecond laser for collision with an electron pulse, and the other is a UV picosecond laser for photocathode irradiation. Both lasers should be synchronized each other. We synchronized two femtosecond mode-locked Ti : sapphire lasers as a basic experiment. The center wavelength of these oscillators were 800 and 850 nm, respectively. Each laser was synchronized to the same reference signal. The timing jitter of each laser were 127 and 378 fs (rms), respectively. Since each oscillator was synchronized to the same reference signal, these two oscillators were synchronized each other. The cross-correlation trace was measured, and the relative timing lag was about 300 fs (rms) [13]. Generally, synchronization of two lasers is achieved using one oscillator as a seeder. One oscillator pulse is divided into two and the divided pulses are amplified. But in this case, the wavelength of two lasers must be the same. In our case, the wavelengths of the two lasers can be different. For example, a Ti : sapphire laser and a Cr : YAG can be synchronized. This system can be applied to two-wavelength pump and probe experiments [14], wavelength conversion with two laser pulses, and so on. 2.3. New technique to measure the timing stability of low repetition amplified pulses The timing jitter of mode-locked oscillators can be measured with statistical methods, such as
power spectrum analysis or digital demodulation, since the typical repetition frequency is around 80– 100 MHz. But these statistical methods cannot be applied to the amplified pulses, because the repetition is much lower. We proposed [15] and achieved [16] the jitter measurement for low repetition rate amplified laser pulses, based on a modified cross-correlation technique. We used a home-made Ti : sapphire regenerative amplifier as a test laser. A low noise all-solid-state Nd : YLF green laser (Positive Light, Evolution) was used as a pump source. The stabilized mode-locked Ti : sapphire master oscillator, transform limited 50 fs pulse, was amplified at 1 kHz repetition frequency. For better stabilization, the temperature of the intracavity Pockels cell was stabilized within 0.11: The laboratory temperature was also stabilized within 0.11: The pulse energy and the pulse to pulse energy stability were 0.3 mJ and 0.4% (rms), respectively. The amplified pulse was linearly stretched to 500 fs. This stretched amplified pulse and the transform limited oscillator pulse were put into a cross-correlator (Fig. 1). Relative timing lag of these two pulses corresponds to the wavelength shift of the upconverted light. Using a spectrometer with 0.025 nm resolution, the resolution of the timing fluctuation was approximately 1 fs. The timing fluctuation was measured in every 6 ms. Even without stabilization, the timing fluctuation of the amplifier was very small. The short-term timing jitter was less
Fig. 1. Modified cross-correlator to measure the timing fluctuation of low-repetition ratio amplified pulse. A stretched, amplified pulse and a transform limited oscillator pulse are incident on a BBO crystal. The relative timing lag of two pulses corresponds to the wavelength of the upconverted pulse.
K. Kobayashi, A. Endo / Nuclear Instruments and Methods in Physics Research A 472 (2001) 86–93
89
than 10 fs, and the long-term timing drift over a one hour time span was about 200 fs. 2.4. High power regenerative amplifier with high beam quality The pointing stability and beam focusability of an amplified laser pulse are the key factors for spatial overlapping of a laser pulse and an electron pulse. Although high power, all-solid-state laser has been reported [17], flash-lamp pumped lasers are generally used as the pump source for terawatt laser systems. Flashlamp-pumped lasers generally exhibit highly structural spatial profile and poor energy stability. When used to pump an amplifier, this causes thermal inhomogeneities in Ti : sapphire amplifier crystal. Thermal inhomogeneity results in wave-front distortion [18,19] and pointing instability. The focusability becomes worse because of these wave-front distortion. We designed and constructed a high power regenerative amplifier with high beam quality [20]. The seed pulse generated by a Ti : sapphire mode-locked oscillator was stretched to about 400 ps and injected into the regenerative amplifier. The regenerative amplifier consists of a z-folded cavity and a 10 mm long, Brewster-cut Ti : sapphire crystal. A small part of the pumping laser for four-pass power amplifier (Spectra-Physics Quanta-Ray PRP-290-50, second harmonics of Nd : YAG, flush-lamp pumped, Q-switched) was divided and injected to the regenerative amplifier. This regenerative amplifier is designed to be very stable against thermal effects and to be able to sustain a large diameter TEM00 mode. The output energy was about 6 mJ with 2.5% (rms) stability at 50 Hz repetition rate. The pointing stability was measured by a laser beam profiler (Spiricon LBA100A), to be 5:4 mrad in the horizontal and 6:6 mrad in the vertical direction (Fig. 2). At this moment, only passive stabilization (special cavity design, stable environmental conditions, carefully selected components) have been performed. For better pointing stability, adaptive optics will be used as a vector scanning system. The wave-front was measured with a Shack–Hartmann type [21] wave-front sensor (AOA Wavescope). Fig. 3(a) shows the wave-front of the amplified pulse. We
Fig. 2. Pointing stability of 50 Hz regenerative amplifier. Standard deviation of horizontal (top) and vertical (bottom) are 5:4 and 6:6 mrad; respectively.
used a telescope in order to extract the spherical components of the wave-front. Fig. 3(b) shows the wave-front with focused term removed. Higherorder aberration of the wave-front were below l=10 at the center wavelength of 800 nm. In this case also, only passive stabilization have been performed. We plan to use a deformable mirror to remove the remaining aberration.
3. New laser options for linear collider designs In this section, we propose new laser options for LC designs, based on our achievements for Compton X-ray generation. 3.1. Temporally square pulse for low emittance photocathode Generally, the temporal profile of laser pulses are Gaussian, sech2 ; and so on. But in some applications, other profiles are required. Generation of a low emittance electron pulse is a key technology for high efficiency X-ray generation. In this case, square pulse shape is better for the photocathode. We built a pulse shaping apparatus [22] which consisted of a grating pair, a lens pair and a liquid crystal spatial light module (LC-SLM) [23,24]. The optimum pulse energy for the photocathode is on the order of several hundreds of mJ in
90
K. Kobayashi, A. Endo / Nuclear Instruments and Methods in Physics Research A 472 (2001) 86–93
shaping apparatus. The phase of each wavelength element was modulated so that the amplified pulse would be a square pulse with 1 ps duration. Then the modulated pulse was amplified by an 1 kHz regenerative amplifier. The output energy was 300 mJ at the center wavelength of 800 nm. We are now developing a third harmonic generation apparatus for a photocathode in combination with this scheme. 3.2. Timing stabilized seeder for CO2 amplifier
Fig. 3. (a) Wave-front of the high-energy pulse from the regenerative amplifier, (b) wavefront with focused term removed.
the UV region, therefore an optical amplifier is required. The pulse shaping apparatus was inserted before the amplifier to avoid damage to the LC-SLM. There are two choices for the design, modulate the amplitude and the phase simultaneously or modulate only the phase. Amplitude modulation of laser pulses before the chirped pulse amplification may cause damages of the optical elements in the regenerative amplifier. Therefore, we selected phase only modulation. Fig. 4 shows our design to make a square shaped pulse. A transform limited 50 fs pulse was put into the pulse
In some applications, such as polarized positron generation [25], a CO2 laser will be used. The wavelength of CO2 lasers are 10 times longer than that of Nd : YAG. Because of its longer wavelength, the number of photons is much larger compared to a solid-state laser with the same output power. Furthermore, the electrical to optical efficiency of CO2 laser is very high. Therefore, CO2 laser is the best choice when the cost of generating photons is a issue. The timing stability of a CO2 laser is very poor because the laser medium is pumped by a gas discharge. We are developing a stable all-solid-state laser as a seeder for a CO2 laser. In this case, the CO2 medium will be used only as an amplifier. The pulse timing will be determined by the seeder. Fig. 5 shows the setup for the seeder. Two Ti : sapphire mode-locked oscillators are synchronized with each other. The center wavelength of these oscillators are 745 and 801 nm, respectively. The pulses generated by these oscillators are amplified by regenerative amplifiers. Finally, the amplified pulses are incident on a AgGaS2 crystal. Pulses of 10:6 mm will be generated by difference frequency mixing [26], and delivered to a CO2 amplifier. The pulses from two oscillators must arrive at the crystal simultaneously to generate the seed pulse. Synchronization of two oscillators is the essential point for this technique. We have described our technique for synchronizing two oscillators in Section 2.2. 3.3. Multipulse laser for g–g collider For a g–g collider, two types of laser systems are required. One is for the collision with a GeV
K. Kobayashi, A. Endo / Nuclear Instruments and Methods in Physics Research A 472 (2001) 86–93
91
Fig. 4. Pulse shaping apparatus with a liquid crystal spatial light module (LC-SLM).
Fig. 5. Timing stabilized seeder for CO2 amplifier. Seven hundred and forty five nm and 801 nm pulses from synchronized two mode-locked oscillator are amplified, and introduced to AgGaS2 crystal. A seed pulse for CO2 amplifier is generated by difference frequency mixing.
electron pulse to generate Compton g photons. The other is used for the photocathodes of multipulse electron bunch. The laser energies are much different, but the time structures are the same. For example, let us consider the g–g collider at the JLC [27]. The time structure of the electron bunch is shown in Fig. 6. One macro-pulse contains 85 micro-pulses with 2.8 ns time interval, corresponding to 357 MHz repetition rate. The pulse length and the repetition rate of the macro-
Fig. 6. Time structure of JLC g–g collider. 238 ns macro-pulse, contains 85 micro-pulse with 2.8 ns interval, repeats in 150 Hz.
pulse are 238 ns and 150 Hz, respectively. The time structures of both laser systems are the same as that of the electron bunch. To generate g photons with electron-laser Compton scattering, high-energy laser pulses are required. The required energy and the pulsewidth of each micro-pulse are 1 J and 1 ps, respectively. It is not realistic to generate such high energy pulses with 357 MHz repetition rate. Therefore, we propose to build 85 lasers for one arm of the linac. The repetition rate of each laser should be 150 Hz. For better stabilization, a low-noise
92
K. Kobayashi, A. Endo / Nuclear Instruments and Methods in Physics Research A 472 (2001) 86–93
all-solid-state laser is desirable. Recent progress in all-solid-state lasers made it possible to build 1 J, 150 Hz lasers. However, this kind of laser is very expensive. Since 170 lasers are required for two arms, the cost for the lasers is the key issue for the g–g collider. One possible solution is to use a crystal which has a long upper state lifetime. Fewer diodes are necessary to produce the same energy output compared to short lifetime crystals, because the pumping energy can be stored in the crystal over a longer period of time. Ytterbium doped Sr5 (PO4 )3 F (Yb : S-FAP) [28] is one promising material. The upper state lifetime of Yb : SFAP is 1.26 ms, five times longer than Nd : YAG. A reduction of the number of diodes significantly reduces the laser cost. Another attractive feature of Yb : S-FAP is the 1047 nm lasing wavelength. It exactly overlaps the wavelength of Nd : YLF. The well established Nd : YLF technology can be applied for the seed laser, and the Yb : S-FAP crystal can be used as an amplifier. Relatively lower energy is required for photocathode irradiation. We have developed a 357 MHz mode-locked Nd : YAG oscillator, and a high-speed electro-optical modulator (EOM). The 10–90% rise time and fall time of the EOM are 0.8 and 0.9 ns, respectively. Eighty-five micropulses are picked up in 150 Hz with the high-speed EOM. The macro-pulse is amplified with a Nd : YAG amplifier, and is converted to the fourth harmonics.
were about 10 and 200 fs, respectively. A high energy and high stability regenerative amplifier with a specially designed cavity was built. The pointing stability was about 6 mrad; and the higher order wave-front aberration was only l=10; only with passive stabilization. Also, new technical options for linear collider were introduced. Square temporal pulses were generated with a liquid crystal spatial light module. Timing stabilized seed pulses for a CO2 amplifier was proposed. Two systems for a g–g collider were studied. One is a high-energy multipulse laser. Using a long upper-level lifetime crystal, the total cost will be significantly reduced. The other one is a multi-pulse laser for the photocathode. Multi-pulse seeder, generated by using a high repetition mode-locked oscillator and a high-speed EOM, will be amplified and converted to fourth harmonics.
Acknowledgements The authors would like to thank to Mr. Taisuke Miura, Mr. Shinji Ito, Dr. Kazuya Takasago, Mr. Hideyuki Nagaoka for their fine experiments, Prof. Zhigang Zhang and Dr. Kenji Torizuka for their fruitful discussions, Dr. Mark Bowers for carefully read this manuscript. This work was performed under the management of the Femtosecond Technology Research Association, supported by New Energy and Industrial Technology Development Organization.
4. Summary We have developed a highly stabile laser system under the research project for Compton X-ray generation. A Ti : sapphire master oscillator was stabilized, resulting a short-term timing jitter less than 100 fs and long-term timing drift of about 100 mHz: Two independent mode-locked oscillators were synchronized. The relative timing lag was about 300 fs. A new measurement technique for the timing fluctuation of low-repetition, amplified pulse was performed. The resolution of this technique is less than 1 fs. The short-term timing jitter and long-term timing drift of the regenerative amplifier, with no active stabilization,
References [1] Y. Kobayashi, T. Sekikawa, Y. Nabekawa, S. Watanabe, Opt. Lett. 23 (1998) 64. [2] R.W. Schoenlein, W.P. Leemans, A.H. Chin, P. Volfbeyn, T.E. Glover, P. Balling, M. Zorotorev, K.J. Kim, S. Chattopadhyay, C.V. Shank, Science 274 (1996) 5285. [3] H. Yajima, T. Kamiya, IECIE Trans. Electron. E81-c (1998) 95. [4] A. Endo, in: T. Kamiya, F. Saito, O. Wada, H. Yajima (Eds.), Femtosecond Technology From Basic Research to Application Prospects, Springer Series in Photonics, Vol. 2, Springer, Berlin, 1999. [5] M. Uesaka, K. Kinoshita, T. Watanabe, T. Ueda, K. Yoshii, H. Harano, J. Sugahara, K. Nakajima,
K. Kobayashi, A. Endo / Nuclear Instruments and Methods in Physics Research A 472 (2001) 86–93
[6] [7] [8] [9] [10] [11] [12] [13]
[14] [15] [16] [17]
A. Ogata, F. Sakai, H. Dewa, M. Kando, H. Kotaki, S. Kondo, AIP Conf. Proc. 427 (1999) 908. J. Yang, M. Washio, A. Endo, T. Hori, Nucl. Instr. and Meth. A 428 (1999) 556. A. Endo, K. Kobayashi, S. Ito, WQ6, CLEO=Pacific Rim’99, Seoul Korea, 30 August–3 September 1999. A. Endo, Nucl. Instr. and Meth. A 455 (2000) 228. K. Kobayashi, T. Miura, Z. Zhang, K. Torizuka, Proc. SPIE 3616 (1999) 156. K. Kobayashi, T. Miura, S. Ito, Z. Zhang, K. Torizuka, A. Endo, Nucl. Instr. and Meth. A 455 (2000) 239. D. von der Linde, Appl. Phys. B 39 (1986) 201. H. Tsuchida, Opt. Lett. 24 (1999) 1641. H. Nagaoka, T. Miura, K. Kobayashi, K. Takasago, Z. Zhang, F. Kannari, K. Torizuka, M. Washio, Proceedings of the Seventh International Workshop on Femtosecond Technology, Tsukuba, Japan, 29–30 June 2000, p. 125. T. Asano, M. Tamura, S. Yoshizawa, K. Tomoda, S. Noda, Physica E 7 (2000) 704. T. Miura, K. Kobayashi, Z. Zhang, K. Torizuka, F. Kannari, Proc. SPIE 3616 (1999) 165. T. Miura, K. Kobayashi, K. Takasago, Z. Zhang, K. Torizuka, F. Kannari, Opt. Lett. 25 (2000) 1795. Y. Akiyama, A. Takada, T. Takase, M. Sasaki, H. Yuasa, N. Nishida, Advanced Solid-State Lasers, OSA Technical Digest, Optical Society of America, Washington, DC, 2000, pp. 19–21.
93
[18] J.-C. Chanteloup, F. Druon, M. Nantel, A. Masksimchuck, G. Mourou, Opt. Lett. 23 (1998) 621. [19] C.L. Blanc, E. Baubeau, F. Salin, J.A. Squier, C.P.J. Barty, C. Spielmann, IEEE J. Quantum. Electron. 4 (1998) 407. [20] S. Ito, T. Miura, Z. Zhang, A. Endo, K. Torizuka, CWB 0005, CLEO=Europe-IQEC2000, Nice, France, 10–15 September 2000. [21] E. Hecht, Optics, Addison-Wesley, Reading, MA, pp. 233–234. [22] K. Takasago, A. Yada, T. Miura, M. Washio, F. Kannari, Z. Zhang, K. Torizuka, A. Endo, CTuK 104, CLEO/ Europe-IQEC2000, Nice, France, 10–15 September 2000. [23] A.M. Weiner, D.E. Leaird, J.S. Patel, J.R. Wullert II, IEEE J. Quantum Electron. 28 (1992) 908. [24] M.M. Wefers, K.A. Nelson, Opt. Lett. 20 (1995) 1047. [25] T. Omori, Proceedings of the International Conference of Lasers’99, Quebec, Canada, 13–16 December 1999, pp. 273. [26] M.R.X. Barros, R.S. Miranda, T.M. Jedju, P.C. Becker, Opt. Lett. 20 (1995) 480. [27] T. Matsui, The Fifth Japan–China Joint Symposium on Accelerators for Nuclear Science and Their Applications, Osaka, Japan, 18–20 October 1993. [28] S.A. Payne, L.D. Deloach, L.K. Smith, W.L. Kway, L.B. Tassano, W.F. Krupke, B.H.T. Chai, G. Loutts, J. Appl. Phys. 76 (1994) 497.