第十二讲 啁啾脉冲放大器

Laser intensity evolution
CPA Lasers
引自G. Mourou ICUIL 2008
Chirped pulse amplification Scheme
CPA: Why?
1. Efficient amplification 2. Avoid optical damage in amplification medium due to high optical intensity 3. Reduce the nonlinear effect in medium
• CPA based on Nd:glass: 1053 nm, 400 fs, >200 J, one
shot per hours;
• OPCPA based on KD*P: 40-80 fs; 910 nm or 1053 nm,
35 J; single shot
Petawatt example: petawatt hybrid OPCPA-Glass laser-Texas
Multi-pass amplifier (1)
Power amplifier
Gain narrowing
Suppression of gain narrowing
OPCPA
Nonlinear crystal
OPCPA characteristics: (1) high gain ( >1000 for single pass) (2) large amplifying bandwidth, wavelength designable (3) Low energy conversion efficiency (4) requires synchronization between pumping and seeding laser
FWHM
0.44 02 / c0 0.44 02 / c0 t L GVM
GVM
0 1 n ( ) n ( / 2) 0 0
c0 2
FWHM
0.44 0 / L n(0 ) 1 2 n (0 / 2)
Calculating the bandwidth by considering the GVM yields the same result as the phase-matching bandwidth!
t = t [d/d]–1 = 0.44 [d/d]–1 = 0.44 2/c0
where we’ve neglected the minus sign since we’re computing the bandwidth, which is inherently positive. So the bandwidth is:
Grating #2

Lse p d 2 3 0 2 d 0 2c 2 d 2 cos2 (' )
where d = grating spacing (same for both gratings)
Lsep
Note that, as in the prism pulse compressor, the larger Lsep, the larger the negative GDD.
Coupling wave equation
Take the second-harmonic pulse to have a Gaussian intensity, for which t = 0.44. Rewriting in terms of the wavelength,
Group-velocity mismatch limits bandwidth.
Double CPA design to boost contrast
CPA Stage 1 Nonlinear Pulse Cleaning CPA Stage 2
Nonlinear polarization rotation, Jullien, OL 2004
SHG boost contrast
12. Chirped Pulse Amplication(CPA) and Petawatt Lasers
Chirped pulse amplication (CPA) Pulse stretching method Pulse compression Regenerate amplification Multiple amplification Gain narrowing Optical parametric CPA Petawatt laser example Pulse contrast
Petawatt scheme
OPCPA scheme
Nd:glass hybrid amplifier
Suppression gain narrowing by hybrid amplification
Compression grating
Results
Petawatt laser facilities
Pulse stretching by grating pair
Pulse to be strecthed due to GDD
30 fs + 109 fs2
3.08 ns
Diffraction-grating pulse compressor
The grating pulse compressor also has negative second-order phase.
The pulse contrast can be reduced significantly during its generation, propagation and amplifications. Among all the processes, contrast degradation due to ASE is fundamental (always accompanied with laser amplification)
引自 Amplitude Data Sheet
Pump noise OPG
引自 V. Bagnoud OE 2007
The noise pedestal will be more complicated in OPCPA. Temporal ripples in pump pulse will also degrade the contrast significantly.
560 TW OPCPA by Russia
Main Routes for Petawatt laser
• CPA based on Ti:Sapphire: 800 nm wavelength; broad
bandwidth (30 fs), repetitive (0.1 Hz); 30 J
Grating #1
Mode locked oscillator as seeding light
Regenerate amplifier
四分之一波电压放大器
Regenerate amplifier
半波电压再生放大器
Optical Isolator
Function: (1) avoid the amplifed light feeding back to oscillator (2) Split seeding and amplified beam
Petawatt laser facilities in the world
Pulse Contrast: intensity of main pulse/ intensity of noise pulse
Definition of pre-pulse contrast
引自 Annual Report 2007 LLE, Rochester
I 2
0 2 ( (2) )2 ( I )2 L2
2c0 n3
2
sinc2 (kL 2)
Hale Waihona Puke Baidu
Questions
1. Please briefly indicate the characteristics of OPCPA in comparison with CPA based on Ti:Sapphire and Nd:glass. 2. Why CPA is necessary for obtaining ultrahigh power pulses?