Chirped pulse amplification
Encyclopedia
Chirped pulse amplification (CPA) is a technique for amplifying an ultrashort
laser
pulse up to the petawatt level with the laser pulse being stretched out temporally and spectrally prior to amplification. CPA is the current state of the art technique which all of the highest power lasers (greater than about 100 terawatts, with the exception of the ~500 TW National Ignition Facility
) in the world currently utilize. Some examples of these lasers are the Vulcan Petawatt Upgrade at the Rutherford Appleton Laboratory
's central laser facility, the Diocles Laser at the University of Nebraska-Lincoln, the Gekko Petawatt laser at the Gekko XII facility in the Institute of Laser Engineering at Osaka University
, the OMEGA EP laser at the University of Rochester's Lab for Laser Energetics and the now dismantled petawatt line on the former Nova laser
at the Lawrence Livermore National Laboratory
. Apart from these state-of-the-art research systems, a number of commercial manufacturers sell Ti:sapphire
-based CPAs with peak powers of 10 to 100 gigawatts.
Chirped-pulse amplification was originally introduced as a technique to increase the available power in radar in 1960. CPA for lasers was invented by Gérard Mourou
and Donna Strickland at the University of Rochester
in the mid 1980s . Before then, the peak power
of laser pulses was limited because a laser pulse at intensities
of gigawatts per square centimeter causes serious damage to the gain medium through nonlinear processes
such as self-focusing
. For example, some of the most powerful compressed CPA laser beams, even in an unfocused large aperture (after exiting the compression grating) can exceed intensities of 700 gigawatts/cm2, which if allowed to propagate in air or the laser gain medium would instantly self focus and form a plasma
or cause filament propagation
, both of which would ruin the original beam's desirable qualities and could even cause back-reflection potentially damaging the laser's components. In order to keep the intensity of laser pulses below the threshold of the nonlinear effects, the laser systems had to be large and expensive, and the peak power of laser pulses was limited to the high gigawatt level or terawatt level for very large multi beam facilities.
In CPA, on the other hand, an ultrashort laser pulse is stretched out in time prior to introducing it to the gain medium using a pair of gratings
that are arranged so that the low-frequency component of the laser pulse travels a shorter path than the high-frequency component does. After going through the grating pair, the laser pulse becomes positively chirp
ed, that is, the high-frequency component lags behind the low-frequency component, and has longer pulse duration
than the original by a factor of 103 to 105. Then the stretched pulse, whose intensity is sufficiently low compared with the intensity limit of gigawatts per square centimeter, is safely introduced to the gain medium and amplified by a factor 106 or more. Finally, the amplified laser pulse is recompressed back to the original pulse width through the reversal process of stretching, achieving orders of magnitude higher peak power than laser systems could generate before the invention of CPA.
In addition to the higher peak power, CPA makes it possible to miniaturize laser systems (the compressor being the biggest part). A compact high-power laser, known as a tabletop terawatt laser (T3 laser), can be created based on the CPA technique.
In principle, the dispersion of an optical device is a function
, where 
is the time delay experienced by a frequency component 
. (Sometimes the phase 
is used, where c is the speed of light and 
is the wavelength.) Each component in the whole chain from the seed laser to the output of the compressor contributes to the dispersion. It turns out to be hard to tune the dispersions of the stretcher and compressor such that the resulting pulses are shorter than about 100 femtoseconds. For this, additional dispersive elements may be needed.
This setup is normally used as a compressor, since it does not involve components that could lead to unwanted side-effects when dealing with high-intensity pulses. The dispersion can be tuned easily by changing the distance between the two gratings.
Figure 2 shows a more complicated grating configuration that involves focusing elements, here depicted as lenses. The lenses are placed at a distance
from each other (they act as a 1:1 telescope), and at a distance 
from the gratings. If 
, the setup acts as a positive-dispersion stretcher and if 
, it is a negative-dispersion stretcher. And the case 
is used in the pulse shaper
. Usually, the focusing element is a spherical or cylindrical mirror rather than a lens. As with the configuration in Figure 1, it is possible to use an additional mirror and use a single grating rather than two separate ones. This setup requires that the beam diameter is very small compared to the length of the telescope; otherwise undesirable aberrations will be introduced. For this reason, it is normally used as a stretcher before the amplification stage, since the low-intensity seed pulses can be collimated to a beam with a small diameter.
It is possible to use prisms
rather than gratings as a dispersive elements, as in Figure 3. Despite such a simple change the set-up behaves quite differently, as to first order no group delay dispersion is introduced. Such a stretcher/compressor can have both a positive or negative dispersion, depending on the geometry and the material properties of the prisms. With lenses, the sign of the dispersion can be reversed, similar to Figure 2. For a given distance between the dispersive elements, prisms generate much less dispersion than gratings. Prisms and gratings are sometimes combined to correct higher order dispersion ("grisms"), in which case the distance between the prisms is on the order of 10 meters rather than 50 cm as with a grating compressor. Gratings lose power into the other orders while prisms lose power due to Rayleigh scattering
.
Ultrashort pulse
In optics, an ultrashort pulse of light is an electromagnetic pulse whose time duration is of the order of a femtosecond . Such pulses have a broadband optical spectrum, and can be created by mode-locked oscillators...
laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
pulse up to the petawatt level with the laser pulse being stretched out temporally and spectrally prior to amplification. CPA is the current state of the art technique which all of the highest power lasers (greater than about 100 terawatts, with the exception of the ~500 TW National Ignition Facility
National Ignition Facility
The National Ignition Facility, or NIF is a large, laser-based inertial confinement fusion research device located at the Lawrence Livermore National Laboratory in Livermore, California. NIF uses powerful lasers to heat and compress a small amount of hydrogen fuel to the point where nuclear fusion...
) in the world currently utilize. Some examples of these lasers are the Vulcan Petawatt Upgrade at the Rutherford Appleton Laboratory
Rutherford Appleton Laboratory
The Rutherford Appleton Laboratory is one of the national scientific research laboratories in the UK operated by the Science and Technology Facilities Council . It is located on the Harwell Science and Innovation Campus at Chilton near Didcot in Oxfordshire, United Kingdom...
's central laser facility, the Diocles Laser at the University of Nebraska-Lincoln, the Gekko Petawatt laser at the Gekko XII facility in the Institute of Laser Engineering at Osaka University
Osaka University
, or , is a major national university located in Osaka, Japan. It is the sixth oldest university in Japan as the Osaka Prefectural Medical College, and formerly one of the Imperial Universities of Japan...
, the OMEGA EP laser at the University of Rochester's Lab for Laser Energetics and the now dismantled petawatt line on the former Nova laser
Nova laser
Nova was a high-power laser built at the Lawrence Livermore National Laboratory in 1984 which conducted advanced inertial confinement fusion experiments until its dismantling in 1999. Nova was the first ICF experiment built with the intention of reaching "ignition", a chain reaction of nuclear...
at the Lawrence Livermore National Laboratory
Lawrence Livermore National Laboratory
The Lawrence Livermore National Laboratory , just outside Livermore, California, is a Federally Funded Research and Development Center founded by the University of California in 1952...
. Apart from these state-of-the-art research systems, a number of commercial manufacturers sell Ti:sapphire
Ti-sapphire laser
Ti:sapphire lasers are tunable lasers which emit red and near-infrared light in the range from 650 to 1100 nanometers. These lasers are mainly used in scientific research because of their tunability and their ability to generate ultrashort pulses...
-based CPAs with peak powers of 10 to 100 gigawatts.
Chirped-pulse amplification was originally introduced as a technique to increase the available power in radar in 1960. CPA for lasers was invented by Gérard Mourou
Gérard Mourou
Gérard Mourou is a French pioneer in the field of electrical engineering and lasers. Along with Donna Strickland, he co-invented a technique called chirped pulse amplification, or CPA, which was later used to create ultrashort-pulse, very high-intensity laser pulses...
and Donna Strickland at the University of Rochester
University of Rochester
The University of Rochester is a private, nonsectarian, research university in Rochester, New York, United States. The university grants undergraduate and graduate degrees, including doctoral and professional degrees. The university has six schools and various interdisciplinary programs.The...
in the mid 1980s . Before then, the peak power
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...
of laser pulses was limited because a laser pulse at intensities
Irradiance
Irradiance is the power of electromagnetic radiation per unit area incident on a surface. Radiant emittance or radiant exitance is the power per unit area radiated by a surface. The SI units for all of these quantities are watts per square meter , while the cgs units are ergs per square centimeter...
of gigawatts per square centimeter causes serious damage to the gain medium through nonlinear processes
Nonlinear optics
Nonlinear optics is the branch of optics that describes the behavior of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light...
such as self-focusing
Self-focusing
Self-focusing is a non-linear optical process induced by the change in refractive index of materials exposed to intense electromagnetic radiation. A medium whose refractive index increases with the electric field intensity acts as a focusing lens for an electromagnetic wave characterised by an...
. For example, some of the most powerful compressed CPA laser beams, even in an unfocused large aperture (after exiting the compression grating) can exceed intensities of 700 gigawatts/cm2, which if allowed to propagate in air or the laser gain medium would instantly self focus and form a plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
or cause filament propagation
Filament propagation
In nonlinear optics, filament propagation is propagation of a beam of light through a medium without diffraction. This is possible because the Kerr effect causes an index of refraction change in the medium, resulting in self-focusing of the beam....
, both of which would ruin the original beam's desirable qualities and could even cause back-reflection potentially damaging the laser's components. In order to keep the intensity of laser pulses below the threshold of the nonlinear effects, the laser systems had to be large and expensive, and the peak power of laser pulses was limited to the high gigawatt level or terawatt level for very large multi beam facilities.
In CPA, on the other hand, an ultrashort laser pulse is stretched out in time prior to introducing it to the gain medium using a pair of gratings
Diffraction grating
In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as...
that are arranged so that the low-frequency component of the laser pulse travels a shorter path than the high-frequency component does. After going through the grating pair, the laser pulse becomes positively chirp
Chirp
A chirp is a signal in which the frequency increases or decreases with time. In some sources, the term chirp is used interchangeably with sweep signal. It is commonly used in sonar and radar, but has other applications, such as in spread spectrum communications...
ed, that is, the high-frequency component lags behind the low-frequency component, and has longer pulse duration
Pulse duration
In signal processing and telecommunication, the term pulse duration has the following meanings:#In a pulse waveform, the interval between the time, during the first transition, that the pulse amplitude reaches a specified fraction of its final amplitude, and the time the pulse amplitude drops,...
than the original by a factor of 103 to 105. Then the stretched pulse, whose intensity is sufficiently low compared with the intensity limit of gigawatts per square centimeter, is safely introduced to the gain medium and amplified by a factor 106 or more. Finally, the amplified laser pulse is recompressed back to the original pulse width through the reversal process of stretching, achieving orders of magnitude higher peak power than laser systems could generate before the invention of CPA.
In addition to the higher peak power, CPA makes it possible to miniaturize laser systems (the compressor being the biggest part). A compact high-power laser, known as a tabletop terawatt laser (T3 laser), can be created based on the CPA technique.
Stretcher and compressor design
There are several ways to construct compressors and stretchers. However, a typical Ti:sapphire-based chirped-pulse amplifier requires that the pulses are stretched to several hundred picoseconds, which means that the different wavelength components must experience about 10 cm difference in path length. The most practical way to achieve this is with grating-based stretchers and compressors. Stretchers and compressors are characterized by their dispersion. With negative dispersion, light with higher frequencies (shorter wavelengths) takes less time to travel through the device than light with lower frequencies (longer wavelengths). With positive dispersion, it is the other way around. In a CPA, the dispersions of the stretcher and compressor should cancel out. Because of practical considerations, the stretcher is usually designed with positive dispersion and the compressor with negative dispersion.In principle, the dispersion of an optical device is a function
, where is the time delay experienced by a frequency component . (Sometimes the phase is used, where c is the speed of light and is the wavelength.) Each component in the whole chain from the seed laser to the output of the compressor contributes to the dispersion. It turns out to be hard to tune the dispersions of the stretcher and compressor such that the resulting pulses are shorter than about 100 femtoseconds. For this, additional dispersive elements may be needed.With gratings
Figure 1 shows the simplest grating configuration, where long-wavelength components travel a larger distance than the short-wavelength components (negative dispersion). Often, only a single grating is used, with extra mirrors such that the beam hits the grating four times rather than two times as shown in the picture.This setup is normally used as a compressor, since it does not involve components that could lead to unwanted side-effects when dealing with high-intensity pulses. The dispersion can be tuned easily by changing the distance between the two gratings.
Figure 2 shows a more complicated grating configuration that involves focusing elements, here depicted as lenses. The lenses are placed at a distance
from each other (they act as a 1:1 telescope), and at a distance from the gratings. If , the setup acts as a positive-dispersion stretcher and if , it is a negative-dispersion stretcher. And the case is used in the pulse shaperFemtosecond pulse shaping
In optics, femtosecond pulse shaping is a technique that modifies the temporal profile of an ultrashort pulse from a laser. Pulse shaping can be used to shorten/elongate the duration of optical pulse, or to generate more complex pulses.-Introduction:...
. Usually, the focusing element is a spherical or cylindrical mirror rather than a lens. As with the configuration in Figure 1, it is possible to use an additional mirror and use a single grating rather than two separate ones. This setup requires that the beam diameter is very small compared to the length of the telescope; otherwise undesirable aberrations will be introduced. For this reason, it is normally used as a stretcher before the amplification stage, since the low-intensity seed pulses can be collimated to a beam with a small diameter.
With prisms
Prism (optics)
In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use...
rather than gratings as a dispersive elements, as in Figure 3. Despite such a simple change the set-up behaves quite differently, as to first order no group delay dispersion is introduced. Such a stretcher/compressor can have both a positive or negative dispersion, depending on the geometry and the material properties of the prisms. With lenses, the sign of the dispersion can be reversed, similar to Figure 2. For a given distance between the dispersive elements, prisms generate much less dispersion than gratings. Prisms and gratings are sometimes combined to correct higher order dispersion ("grisms"), in which case the distance between the prisms is on the order of 10 meters rather than 50 cm as with a grating compressor. Gratings lose power into the other orders while prisms lose power due to Rayleigh scattering
Rayleigh scattering
Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...
.
Other techniques
Some other techniques can be used for stretching and compressing pulses, but these are not suitable as the main stretcher/compressor in CPA due to their limited amount of dispersion and due to their inability to handle high-intensity pulses.- A pulse can be stretched simply by letting it propagate through a thick slab of transparent material, such as 200 mm glass. As with the prisms, only a limited amount of dispersionDispersion (optics)In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or alternatively when the group velocity depends on the frequency.Media having such a property are termed dispersive media...
can be achieved within physically practical dimensions. Outside the visible-light spectrum, materials exist both for positive and negative dispersion. For visible and near-infrared wavelengths, almost all transparent materials have positive dispersion. However, glass fibres can have their dispersion tailored to meet the needs.
- One or multiple reflections between a pair of chirped mirrorsDielectric mirrorA dielectric mirror is a type of a mirror composed of multiple thin layers of dielectric material, typically deposited on a substrate of glass or some other optical material. By careful choice of the type and thickness of the dielectric layers, one can design an optical coating with specified...
or similar device allow any form of chirp. This is often used in conjunction with the other techniques to correct for higher orders.
- The Dazzler is a commercial pulse shaper in which light is diffracted from an acoustic wave. By tuning the timing, frequency, and amplitude of the acoustic wave, it is possible to introduce arbitrary dispersion functions with a maximum delay of a few picoseconds.
- A phase-shifting mask can be placed in the focal plane of the stretcher in Fig. 2, which introduces additional dispersion. Such a mask can be an LCD array, where the phase shift can be tuned by changing the voltage on the pixels. This can generate arbitrary dispersion functions with a maximum of a few tens of picoseconds of delay. Such a setup is called a pulse shaperFemtosecond pulse shapingIn optics, femtosecond pulse shaping is a technique that modifies the temporal profile of an ultrashort pulse from a laser. Pulse shaping can be used to shorten/elongate the duration of optical pulse, or to generate more complex pulses.-Introduction:...
.