《超快光学 第19章 放大》由会员分享，可在线阅读，更多相关《超快光学 第19章 放大（50页珍藏版）》请在金锄头文库上搜索。
1、The Amplification of Ultra- short Laser Pulses,Francois Salin Center for Intense Lasers and Applications (CELIA) Universit Bordeaux I, France www.celia.u-bordeaux.fr salincelia.u-bordeauxfr Gilles Darpentigny (CELIA), Vincent Bagnoud (LLE) Antoine Courjaud, Clemens Honninger, Eric Mottay (Amplitude
2、Systemes), Luc Vigroux (Amplitude Technologies) and some additional stuff from Dan Mittleman, Rice,Pulse compressor,t,t,Solid state amplifier,t,Dispersive delay line,t,Short pulse oscillator,Most of this lecture courtesy of,Pulse energy vs. Repetition rate,Rep rate (pps),Pulse energy (J),Oscillator,
3、Cavity-dumped oscillator,RegA,Regen,Regen + multipass,Regen + multi-multi-pass,1 W average power,A t,What are the goals in ultrashort pulse amplification?,Ipeak =,E,Increase the energy (E), Decrease the duration (t), Decrease the area of the focus (A).,Maximum intensity on target,Needed to start the
4、 experiment,Needed to get useful results,Pave = E r,Signal is proportional to the number of photons on the detector per integration time.,Maximum average power at the detector,Pulse energy,Rep rate,Pulse energy,Beam area,Pulse length,Issues in Ultrafast Amplification and Their Solutions,Pulse length
5、 discrepancies: Multi-pass amplifiers and regenerative amplifiers (“Regens”). Damage: Chirped-Pulse Amplification (CPA) Gain saturation: Frantz-Nodvick Equation Gain narrowing: Birefringent filters Thermal effects: cold and wavefront correction Satellite pulses, Contrast, and Amplified Spontaneous E
6、mission: Pockels cells Systems cost lots of money: Earn more money,Cavity Dumping,Before we consider amplification, recall that the intracavity pulse energy is 50 times the output pulse energy. So we have more pulse energy. How can we get at it?,What if we instead used two high reflectors, let the p
7、ulse energy build up, and then switch out the pulse? This is the opposite of Q-switching: it involves switching from minimum to maximum loss, and its called “Cavity Dumping.”,Cavity dumping: the Pockels cell,A Pockels cell is a device that can switch a pulse (in and) out of a resonator. Its used in
8、Q-switches and cavity dumpers. A voltage (a few kV) can turn a crystal into a half- or quarter-wave plate.,V,If V = 0, the pulse polarization doesnt change.,If V = Vp, the pulse polarization switches to its orthogonal state.,Abruptly switching a Pockels cell allows us to extract a pulse from a cavit
9、y. This allows us to achieve 100 times the pulse energy at 1/100 the repetition rate (i.e., 100 nJ at 1 MHz).,Pockels cell (voltage may be transverse or longitudinal),Polarizer,Amplification of Laser Pulses, in General,Very simply, a powerful laser pulse at one color pumps an amplifier medium, creat
10、ing an inversion, which amplifies another pulse.,Nanosecond-pulse laser amplifiers pumped by other ns lasers are commonplace.,Laser oscillator,Amplifier medium,Pump,Energy levels,Jpump (lpump/lL),lL,lpump,Single-pass Amplification Math,Assume a saturable gain medium and J is the fluence (energy/area
11、). Assume all the pump energy is stored in the amplifier, but saturation effects will occur.,At low intensity, the gain is linear:,At high intensity, the gain “saturates” and hence is constant:,Intermediate case interpolates between the two:,Jsto= stored pump fluence = Jpump (lpump/lL) Jsat= saturat
12、ion fluence (material dependent),Single-pass Amplification Math,where the small signal gain per pass is given by:,This differential equation can be integrated to yield the Frantz-Nodvick equation for the output of a saturated amplifier:,Frantz-Nodvick equation,G,0,exp(,g,0,L),exp(,J,sto,J,sat,),High
13、er pumping (Jsto) means higher efficiency and higher saturation and so lower gain. So you can have high gain or high extraction efficiency. But not both.,Gain,Extraction efficiency (Jout/Jsto),J,sto,/J,sat,Jout/Jin,Another problem with amplifying ultrashort laser pulses,Another issue is that the ult
14、rashort pulse is so much shorter than the (ns or ms) pump pulse that supplies the energy for amplification.,So should the ultrashort pulse arrive early or late?,Early:,Late:,Pump energy arrives too late and is wasted.,time,pump,pump,time,Energy decays and is wasted.,In both cases, pump pulse energy
15、is wasted, and amplification is poor.,So we need many passes.,All ultrashort-pulse amplifiers are multi-pass.,This approach achieves much greater efficiency.,The ultrashort pulse returns many times to eventually extract most of the energy.,Two main amplification methods,Multi-pass amplifier,Regenera
16、tive amplifier,Another multi-pass amplifier,A Pockels cell (PC) and a pair of polarizers are used to inject a single pulse into the amplifier.,Regenerative amplifier geometries,This is used for 10-20-Hz repetition rates. It has a larger spot size in the Ti:sapphire rod.,The Ti:Sapphire rod is 20-mm long and doped for 90% absorption.,This design is often used for kHz-repetition-rate amplifiers.,Pulse intensities inside an amplif
