Pulse energy vs. Repetition rate 10 0 Regen + multipass Pulse energy (J) 10-3 10-6 Regen + multimulti-pass RegA Regen 1 W average power 10-9 Cavity-dumped oscillator Oscillator 10-3 10 0 10 3 10 6 10 9 Rep rate (pps)
Pulse intensities inside an ampliier can become so high that damage (or at least small-scale selocusing) occurs. Solution: Expand the beam and use large ampliier media. Okay, we did that. But that s still not enough. Solution: Expand the pulse in time, too.
Deormable-Mirror Pulse-Shaper Grating Δz x Spherical Mirror Deormable mirror This modulates the phase but not the amplitude. 2π Δ ϕ( x) = 2 Δzx ( ) λ A. Eimov, and D. H. Reitze, Proc. SPIE 2701, 190 (1996) K. F. Wong, D. Yankelevich, K. C. Chu, J. P. Heritage, and A. Dienes, Opt. Lett. 18, 558 (1993)
Micro-Machined Deormable Mirror (MMDM) 600 nm Silicon Nitride Membrane Gold or Silver Coated 1 ms Response Time ~280 V Drive Voltage Computer Controlled 3x13 or 1x19 Actuator Layout G.V. Vdovin and P.M. Sarro, ``Flexible mirror micromachined in silicon'', Applied Optics 34, 2968-2972 (1995) E. Zeek, et. Al., Pulse compression using deormable mirrors, Opt. Lett. 24, 493-495 (1999)
An All-Optical Fourier Transorm: The Zero-Dispersion Stretcher x λ( x) grating grating Fourier Transorm Plane John Heritage, UC Davis Andrew Weiner, Purdue How it works: The grating disperses the light, mapping color onto angle. The irst lens maps angle (hence wavelength) to position. The second lens and grating undo the spatio-temporal distortions. The trick is to place a mask in the Fourier transorm plane.
The Fourier-Synthesis Pulse-shaper Amplitude mask Transmission = T(x) = T(λ) E% in ( λ ) Phase mask Phase delay = ϕ(x) = ϕ(λ) E% out ( λ ) grating Fourier Transorm Plane grating ( λ ) = ( λ)exp[ ϕ( λ)] H T i We can control both the amplitude and phase o the pulse. The two masks or spatial light modulators together can yield any desired pulse.
Why pulseshape? ( λ ) % Masks E in grating E% out ( λ ) grating To compress pulses with complex phase To generate pulses that control chemical reactions or other phenomena To generate trains o pulses or telecommunications To precompensate or distortions that occur in dispersive media
Front view Liquid crystal arrays Liquid crystal modulators (LCMs) consist o two liquid crystal arrays at 90 to each other and at 45 to the incoming light. Dead Space Pixel The irst array rotates the polarization o the light in one direction and the second in the opposite direction. Rotating each the same amount (in opposite directions) yields a phase only modulation. Rotating one more than the other yields an amplitude and phase modulation o the light. The pixels in LCMs limit the resolution o the modulation. The inite width covers a range o wavelengths, reducing the idelity o the shaping. The dead spaces (gaps between electrodes) also add artiacts to the pulse train (eectively an unshaped pulse).
A shaped pulse or telecommunications Ones and zeros Andrew Weiner and coworkers
Chirped-Pulse Ampliication Short pulse oscillator CPA is THE big development. t Dispersive delay line G. Mourou and coworkers 1983 t Chirped-pulse ampliication involves stretching the pulse beore ampliying it, and then compressing it later. Solid state ampliier(s) Pulse compressor t We can stretch the pulse by a actor o 10,000, ampliy it, and then recompress it! t
Ampliication o o Laser Pulses, in in General Very simply, a powerul laser pulse at one color pumps an ampliier medium, creating an inversion, which ampliies another pulse. pump Energy levels Laser oscillator Ampliier medium Nanosecond-pulse laser ampliiers pumped by other ns lasers are commonplace.
What s dierent about ampliying ultrashort laser pulses? The irst issue is that the ultrashort pulse is so much shorter than the (ns) pump pulse that supplies the energy or ampliication. So should the ultrashort pulse arrive early or late? Early: Pump energy arrives too late and is wasted. Late: Energy decays and is wasted. pump pump time time In both cases, pump pulse energy is wasted, and ampliication is poor.
So we need many passes. All ultrashort-pulse ampliiers are multi-pass. pump The ultrashort pulse returns many times to eventually extract most o the energy. time This approach achieves much greater eiciency.
CPA vs. Direct Ampliication 100 10 Alexandrite Fluence (J/cm 2 ) 1 0,1 0,01 Direct Ampliication Nd:Glass Ti:sapphire Excimers 0,001 0,0001 1 10 100 1000 10 4 10 5 10 6 Pulse Duration (s) Dyes CPA achieves the luence o long pulses but at a shorter pulse length!
Cavity Dumping Beore we consider ampliication, recall that the intracavity pulse energy is ~50 times the output pulse energy. E intracavity R=100% R=98% E E = T output E intracavity Transmission o output coupler: ~2% What i we instead used two high relectors, let the pulse energy build up, and then switch out the pulse. This is essentially Q-switching, but it s called Cavity Dumping.
Cavity dumping: the Pockels cell A Pockels cell is a device that can switch a pulse (in and) out o a resonator. It s used in Q-switches and cavity dumpers. A voltage (a ew kv) can turn a crystal into a hal- or quarter-wave plate. Pockels cell (voltage may be transverse or longitudinal) V Polarizer I V = 0, the pulse polarization doesn t change. I V = V π, the pulse polarization switches to its orthogonal state. Abruptly switching a Pockels cell allows us to extract a pulse rom a cavity. This allows us to achieve ~100 times the pulse energy at 1/100 the repetition rate (i.e., 100 nj at 1 MHz).
Two Main Ampliication Methods Multi-pass ampliier Regenerative ampliier input pump output pump gain input/output gain polarizer Pockels cell
A Multi-Pass Ampliier A Pockels cell (PC) and a pair o polarizers are used to inject a single pulse into the ampliier
Regenerative Ampliier Geometries Faraday rotator Two regens. The design in (a) is oten used or khzrepetition-rate ampliiers and the lower (b) at a 10-20- Hz repetition rate. Pockels cell The Ti:sapphire rod is usually ~20-mm long and doped or 90% absorption. thin-ilm polarizer