Patent Description:
The following description relates to optical pulse compression in chirped pulse laser systems.

Chirped pulse laser systems, such as chirped pulse amplifiers (CPA), amplify short duration laser pulses by stretching a short duration optical pulse, amplifying the stretched optical pulse, and compressing the amplified optical pulse. Optical pulse compression in CPAs is typically performed by diffraction gratings, and the surfaces of these diffraction gratings may deteriorate due to the high field intensity of the compressed amplified optical pulse incident upon the diffraction gratings.

<NPL> describes the routine generation of sub-<NUM>-fs laser pulses with <NUM>-mJ energy and stable carrier-envelope phase at <NUM>-kHz repetition rate, obtained by compressing the multi-mJ output from a phase-locked Ti:sapphire amplifier in a rare-gas-filled hollow fiber.

<CIT> describes an extreme ultra violet (EUV) generation device including a light source for outputting a laser beam, a pulse width compression system for compressing a pulse width of the laser beam, a gas cell for receiving the laser beam having the compressed pulse width incident from the pulse width compression system and generating EUV light, and a vacuum chamber housing the pulse width compression system and the gas cell.

In aspects of what is described here, a chirped pulse laser system (such as a chirped pulse amplifier (CPA)) includes a programmable optical dispersive filter (PODF) and a pulse compressor, among other components. The pulse compressor includes optical elements in a vacuum chamber (e.g., a chamber that operates at less than <NUM>-<NUM> Torr, less than <NUM> × <NUM>-<NUM> Torr, or another high vacuum pressure), and the optical elements define an optical path through the pulse compressor. In some implementations, the pulse compressor may be operated such that an optical signal in the optical path of the pulse compressor is below a critical power at which self-channeling effects may appear. The optical elements of the pulse compressor are arranged to compress the optical pulse, and include diffraction gratings and a dispersive mirror. In some examples, each diffraction grating has a ridged reflective surface and the dispersive mirror has a smooth reflective surface. Because the ridged nanostructure of the diffraction gratings makes them susceptible to field enhancement, the dispersive mirror's flat reflective surface may have a higher damage threshold relative to the diffraction gratings. The smooth reflective surface of the dispersive mirror can be formed by alternating layers of dielectric materials on a substrate (e.g., alternating layers of high- and low-index materials, such as Titanium dioxide (TiO<NUM>) and Silicon dioxide (SiO<NUM>), respectively). In some instances, the dispersive mirror is the last optical element in the optical path. In some instances, the dispersive mirror provides for second order dispersion (or group delay dispersion) of approximately (+/-)<NUM> femtoseconds squared (fs<NUM>) for optical pulses with a duration of approximately <NUM>-<NUM> femtoseconds (fs).

Aspects of the present disclosure may provide one or more advantages, in some implementations. For example, a dispersive mirror may better withstand high-intensity optical pulses found in a chirped pulse laser system compared to diffraction gratings (e.g., a dispersive mirror may have a damage threshold more than four (<NUM>) times greater than the damage threshold of a diffraction grating). Thus, in some aspects, the dispersive mirror may be used in a pulse compressor of a CPA to reduce the intensity and the damage threshold fluence incident upon one or more diffraction gratings (e.g., the last diffraction grating in an optical path in the pulse compressor) in the pulse compressor by an order of magnitude or more (e.g., by approximately forty percent (<NUM>%)). In addition, in some implementations, a wave plate may be used to modify the polarization of the optical pulse in a pulse compressor, which may reduce the damage threshold of the dispersive mirror in the pulse compressor. Compression of a high-power chirped pulse (e.g., from a Petawatt class laser system) may therefore be achieved with less degradation and more lifetime of the optical elements used in the pulse compressor, which may allow for an increased duty cycle or repetition rate of the system. In some aspects, a pulse compressor comprising a dispersive mirror may produce optical pulses having higher peak power, and may do so over the same or similar lifetime as previous systems with only diffraction gratings. In some aspects, a pulse compressor comprising a dispersive mirror may produce optical pulses having the same or similar peak power as previous systems with only diffraction gratings, but with smaller optical elements and therefore reduced costs (e.g., through reduced costs related to one or more of the diffraction gratings or other optical elements in the pulse compressor). In some aspects, the dispersive mirror and diffraction gratings of a pulse compressor may operate under high vacuum pressures (e.g., less than <NUM>-<NUM> Torr), reducing cumulative non-linear effects caused by certain types of media or combinations of media (e.g., air, Helium, glass, or combinations thereof). In some aspects, the pulse compressor may be configured to produce a transform-limited optical pulse with a negligible B-Integral. In some aspects, the pulse compressor may be configured to produce a single-cycle optical pulse.

<FIG> is a block diagram showing aspects of an example pulse amplification system <NUM>. In the example shown, the pulse amplification system <NUM> includes a pulse generator <NUM>, a pulse stretcher <NUM>, a pulse amplifier <NUM>, and a pulse compressor <NUM>. A pulse amplification system may include additional or different components. In some implementations, the pulse amplification system <NUM> amplifies broad-spectrum, short duration chirped optical pulses, such as, for example, optical pulses with a duration of less than <NUM> fs. The duration of the pulse may be measured as the full width of the pulse at half maximum intensity (FWHM). For instance, the pulse amplification system <NUM> may be configured to produce a pulse with a duration of approximately <NUM>-<NUM> fs with a peak power of approximately <NUM> TeraWatts (TW).

The example pulse generator <NUM> generates an optical pulse <NUM> for amplification by the pulse amplification system <NUM>. In some implementations, the optical pulse <NUM> is a Fourier transform limited (or transform-limited) optical pulse. In some implementations, the pulse generator <NUM> includes a femtosecond laser oscillator, such as, for example, a femtosecond Ti:sapphire laser. The pulse generator <NUM> may include additional or different components. For example, in some implementations the pulse generator <NUM> includes a chirped pulse amplifier (CPA) in addition to a femtosecond laser oscillator.

The example pulse stretcher <NUM> receives the optical pulse <NUM> generated by the pulse generator <NUM>, and stretches the optical pulse <NUM> to produce the stretched optical pulse <NUM>. The stretched optical pulse <NUM> may be a chirped optical pulse, with a frequency that is time dependent. The frequency of the stretched optical pulse <NUM> may increase with time (an up-chirped pulse) or decrease with time (a down-chirped pulse). The pulse stretcher <NUM> stretches the optical pulse <NUM> using one or more dispersive optical elements, such as diffraction gratings, optical glass with chromatic dispersion characteristics (e.g., SCHOTT SF57 glass). The dispersive optical elements of the pulse stretcher <NUM> cause the different wavelength components of the optical pulse <NUM> to disperse spatially and temporally from one another while travelling through the pulse stretcher <NUM>, causing the stretched optical pulse <NUM> to have a lower intensity (e.g., an intensity below the damage threshold of the pulse amplifier <NUM>). The dispersive optical elements of the pulse stretcher <NUM> may positively or negatively disperse the chirped optical pulse received from the pulse generator <NUM>.

In some implementations, the spectral phase of the optical signal produced by the pulse stretcher <NUM> can be modified. For example, the pulse amplification system <NUM> may include a programmable optical dispersive filter (PODF) that modifies the spectral phase of the optical signal provided to the pulse amplifier <NUM>. In some cases, an optical signal can be described in the frequency domain, for example, as <MAT> where |E(ω)| represents the spectral intensity and φ(ω) represents the spectral phase of the optical signal. An optical signal may have another form, for example, with the spectral intensity and spectral phase having another representation. In some cases, a PODF is used to modify the spectral phase of an optical signal such that the optical signal output from the pulse compressor <NUM> has a flat spectral phase (with all frequency components having the same phase), or to modify the spectral phase of an optical signal such that the optical signal output from the pulse compressor <NUM> has another spectral phase profile.

In some implementations, the example pulse stretcher <NUM> includes or is coupled to an acousto-optic programmable dispersive filter (AOPDF), which modifies the spectral phase of the stretched optical pulse <NUM> while travelling through the AOPDF. The spectral phase modification may be based on an acoustic signal received by the AOPDF. For instance, the AOPDF may include a piezoelectric medium having mechanical properties (e.g., mechanical stress) that are controlled by an applied acoustic signal and influence a spectral phase modification applied to the optical signal. The AOPDF may modify the spectral phase of the stretched optical pulse <NUM> such that the pulse compressor <NUM> produces an optical signal that has a flat spectral phase over the different wavelength components of the optical signal. In some instances, the AOPDF may be programmed based on properties of the pulse compressor <NUM>. For example, the AOPDF may be programmed to modify the spectral phase of the optical signal based on the optical properties of dispersive optical elements in the pulse compressor <NUM>.

The example pulse amplifier <NUM> receives the stretched optical pulse <NUM> from the pulse stretcher <NUM>, and increases the peak power of the stretched optical pulse. The example pulse amplifier <NUM> increases the power of the stretched optical pulse using one or more optical elements having a gain medium (e.g., Ti:sapphire) which transfers energy input to the gain medium (e.g., electrical energy) to the stretched optical pulse <NUM> to produce the amplified optical pulse <NUM>. In some implementations, the pulse amplifier <NUM> includes a multipass amplifier.

The example pulse compressor <NUM> is a system of one or more components that receives the amplified optical pulse <NUM> from the pulse amplifier <NUM>, and compresses the amplified optical pulse <NUM> using optical dispersion. The example pulse compressor <NUM> compresses the pulse using one or more dispersive optical elements, such as, for example, diffraction gratings or dispersive mirrors. In the example shown, the dispersive optical elements of the pulse compressor <NUM> cause an opposite magnitude dispersion of the optical pulse than the dispersive optical elements of the pulse stretcher <NUM>. For example, where the pulse stretcher <NUM> positively disperses the optical pulse, the pulse compressor <NUM> negatively disperses the optical pulse. In some implementations, the pulse compressor <NUM> is configured to at least approximately cancel out the amount of dispersion imparted to the chirped optical pulse <NUM> by the pulse stretcher <NUM>. In some implementations, the pulse compressor <NUM> includes a highly dispersive mirror (HDM) that has a smooth reflective surface and is composed of layers of dielectric materials. For example, in some implementations, the HDM includes alternating layers of high- and low-index dielectric materials, such as Titanium dioxide (TiO<NUM>) and Silicon dioxide (SiO<NUM>), respectively. In some implementations, the total physical thickness of the high-index material is approximately <NUM>, while the total physical thickness of the low-index material is approximately <NUM>. Other materials and layer thickness can be used.

<FIG> is a diagram showing aspects of an example chirped pulse amplifier (CPA) system <NUM>. In the example shown, the CPA system <NUM> includes a pulse generator <NUM>, a pulse stretcher <NUM>, a programmable optical dispersive filter (PODF) <NUM>, a pulse amplifier <NUM>, and a pulse compressor <NUM>. Like the pulse amplification system <NUM> of <FIG>, the example CPA system <NUM> amplifies broad-spectrum, short duration chirped optical pulses, such as, for example, those with durations of less than <NUM> fs. For instance, the CPA system <NUM> may be configured to produce an amplified optical pulse having a duration of approximately <NUM>-<NUM> fs, a spectrum of <NUM>-<NUM>, and peak power of approximately <NUM> TW. In some implementations, the CPA system <NUM> may be configured to generate single-cycle optical pulses. For instance, the CPA system <NUM> may be configured to produce an amplified optical pulse having a duration of approximately <NUM> fs at a wavelength of approximately <NUM>.

In the example shown, the pulse generator <NUM> generates an optical pulse that is transmitted to the pulse stretcher <NUM>. The example pulse generator <NUM> includes one or more components that generate a broad-spectrum ultrashort optical pulse. In some cases, the pulse generator <NUM> can produce Fourier transform limited ultrashort optical pulses. For example, the pulse generator <NUM> may generate an optical pulse having a duration of <NUM>-<NUM> fs and spectrum of <NUM>-<NUM>. In some implementations, the example pulse generator <NUM> is implemented similar to the pulse generator <NUM> of <FIG>. For instance, the pulse generator <NUM> may include a femtosecond laser oscillator, such as a femtosecond Ti:sapphire laser.

The example pulse stretcher <NUM> includes one or more dispersive optical elements that stretch an optical signal as it traverses an optical path through the pulse stretcher <NUM>. The dispersive optical elements of the example pulse stretcher <NUM> temporally broaden the pulse by spatially dispersing the different wavelength components of the optical pulse received from the pulse generator <NUM>. The dispersive optical elements may positively or negatively disperse the optical pulse. In some implementations, the pulse stretcher <NUM> is implemented similar to the pulse stretcher <NUM> of <FIG>. For instance, the pulse stretcher <NUM> may include one or more diffraction gratings that act to disperse the optical pulse from the pulse generator <NUM>. In the example shown, the pulse stretcher <NUM> receives the optical pulse from the pulse generator <NUM>, stretches the optical pulse, and transmits the stretched optical pulse to the PODF <NUM>.

The example PODF <NUM> includes one or more components that modify the spectral phase of the optical pulse as it travels through the PODF <NUM>. In some implementations, the PODF <NUM> includes an acousto-optic programmable dispersive filter (AOPDF), which receives an acoustic signal and modifies the spectral phase of an optical pulse travelling through the PODF based on the acoustic signal. In some implementations, the PODF <NUM> is implemented using a DAZZLER system available from FASTLITE. Another type of PODF may be used, and the PODF <NUM> may include additional or different components. In the example shown, the PODF <NUM> receives the stretched optical pulse from the pulse stretcher <NUM>, modifies the spectral phase of the stretched optical pulse, and transmits the phase-modified optical pulse to the pulse amplifier <NUM>.

The example pulse amplifier <NUM> includes one or more components that increase the power of an optical pulse using one or more optical elements having a gain medium (e.g., Ti:sapphire), which transfers energy input to the gain medium (e.g., electrical energy) to the optical pulse. In some implementations, the pulse amplifier <NUM> includes a multipass amplifier. In the example shown, the pulse amplifier <NUM> receives the phase-modified optical pulse from the PODF <NUM>, amplifies the optical pulse, and transmits the amplified optical pulse to the pulse compressor <NUM>.

The example pulse compressor <NUM> includes one or more dispersive optical elements that compress an optical signal as it traverses an optical path through the pulse compressor <NUM>. The dispersive optical elements of the example pulse compressor <NUM> temporally compress the pulse by spatially dispersing the different wavelength components of the optical pulse received from the pulse amplifier <NUM>. In some implementations, the dispersive optical elements of the pulse compressor <NUM> are in a vacuum chamber. The example pulse compressor <NUM> includes an optical inlet <NUM>, a first diffraction grating <NUM>, a second diffraction grating <NUM>, a third diffraction grating <NUM> a fourth diffraction grating <NUM>, a wave plate <NUM>, a dispersive mirror <NUM>, and an optical outlet <NUM>. In some implementations, the pulse compressor <NUM> includes additional optical elements, such as, for example, one or more mirrors or thin optics between the fourth diffraction grating <NUM> and the dispersive mirror <NUM>. In the example shown, the pulse compressor <NUM> receives the amplified optical pulse from the pulse amplifier <NUM> at the optical inlet <NUM>, compresses the amplified optical pulse, and transmits the compressed optical pulse to the chamber <NUM> from the optical outlet <NUM>. In some implementations, the optical pulse produced from the optical outlet <NUM> is a transform-limited pulse. In some implementations, the pulse compressor <NUM> produces a transform-limited pulse with a negligible B-Integral. In some examples, the beam diameter of the optical pulse at the outlet <NUM> is approximately <NUM> millimeters (mm); or the pulse compressor <NUM> may produce an optical signal having another beam diameter. In some implementations, the optical inlet <NUM> comprises glass, while the optical outlet <NUM> comprises an optical tunnel to the chamber <NUM>. The optical tunnel of the optical outlet <NUM> may be operated at a vacuum pressure, such as, for example, the vacuum pressure of the pulse compressor (e.g., less than <NUM>-<NUM> Torr).

The example chamber <NUM> includes components that interact with the compressed optical pulse from the optical outlet <NUM>. In some implementations, the chamber <NUM> comprises a vacuum chamber operated at a vacuum pressure (e.g., less than <NUM>-<NUM> Torr). In some implementations, the vacuum chamber of the chamber <NUM> is operated at the same vacuum pressure as the vacuum chamber of the pulse compressor <NUM>. For example, in some implementations, the chamber <NUM> includes crystalline materials or other elements that can be used to produce high-energy particles based on the optical pulse from the pulse compressor <NUM>. For instance, the chamber <NUM> may produce a particle beam, a gamma ray beam, an x-ray beam or another type of output. As an example, the chamber <NUM> may be configured to produce a x-ray beam by Betatron, Compton, or K-alpha scattering. As another example, the chamber <NUM> may be configured to produce high duty cycle particle beams or radiation beams.

The diffraction gratings <NUM>-<NUM>, wave plate <NUM>, and the dispersive mirror <NUM> define an optical path within the pulse compressor <NUM> that includes path portions <NUM>-<NUM>. The optical path generally extends from the optical inlet <NUM> to the optical outlet <NUM> and includes the series of optical elements and path portions between respective pairs of the optical elements. The path portions <NUM>-<NUM> are regions of the vacuum chamber that reside between the optical elements in the pulse compressor <NUM>, and during operation of the pulse compressor <NUM>, the path portions <NUM>-<NUM> contain gas (e.g., air, Helium, or another gas) at vacuum pressure (e.g., in the range of <NUM> × <NUM>-<NUM> to <NUM> × <NUM>-<NUM> Torr).

The example diffraction gratings <NUM>-<NUM> and the dispersive mirror <NUM> are arranged in the pulse compressor <NUM> such that they disperse the optical pulse in the optical path. The diffraction gratings <NUM>-<NUM> and the dispersive mirror <NUM> may impart positive or negative dispersion on the optical pulse, depending on the dispersion imparted by the pulse stretcher <NUM> in the example CPA system <NUM>. For example, where the pulse stretcher <NUM> positively disperses the optical pulse, the pulse compressor <NUM> negatively disperses the optical pulse. In some implementations, the pulse compressor <NUM> is configured to at least approximately cancel out the amount of dispersion imparted to the optical pulse by the pulse stretcher <NUM>. In the example shown, the diffraction gratings <NUM>-<NUM> are arranged before the dispersive mirror <NUM> in the optical path. In some implementations, the diffraction gratings <NUM>-<NUM> may each have <NUM> lines per millimeter (l/mm), and may be displaced from one another by <NUM>. Where the diffraction gratings <NUM>-<NUM> are displaced from one another by <NUM>, the diffraction gratings <NUM>-<NUM> may each induce a dispersion of <NUM> fs<NUM> and <NUM>,<NUM> fs<NUM> onto an optical pulse in the optical path.

The example wave plate <NUM> modifies a polarization of the optical pulse as it travels therethrough. For example, the wave plate <NUM> may modify the polarization of the optical pulse such that it is entirely s-polarized (e.g., by converting p-polarized components to s-polarized components of the optical pulse), which may impart less damage when incident upon the dispersive mirror <NUM>. The wave plate <NUM> may modify the polarization of the optical pulse in another manner, such as, for example, modifying the optical pulse to be circularly polarized. In some implementations, the wave plate <NUM> is a half-wave plate. In some implementations, the wave plate <NUM> is a quarter-wave plate. In some implementations, the wave plate <NUM> is a segmented wave plate (e.g., with a mosaic pattern) that creates radial-typed polarizations. In the example shown, the wave plate <NUM> is between the last diffraction grating <NUM> (the final diffraction grating in the optical path) and the dispersive mirror <NUM>. Although the example CPA system <NUM> includes one wave plate <NUM> as shown in <FIG>, the pulse compressor <NUM> may include no wave plates or may include additional wave plates.

The example diffraction gratings <NUM>-<NUM>, wave plate <NUM>, and dispersive mirror <NUM> of the pulse compressor <NUM> reside in a vacuum chamber. In some implementations, one or more diffraction gratings (e.g., the diffraction gratings <NUM>, <NUM>) may reside outside of the vacuum chamber. The vacuum chamber may be configured to operate at a vacuum pressure of less than <NUM>-<NUM> Torr. For example, in some implementations, the diffraction gratings <NUM>-<NUM>, wave plate <NUM>, and dispersive mirror <NUM> reside in a vacuum chamber that operates at approximately <NUM> × <NUM>-<NUM> Torr (in the range of <NUM> × <NUM>-<NUM> to <NUM> × <NUM>-<NUM> Torr). By operating the optical elements in a vacuum chamber under vacuum pressure, non-linear effects caused by media in the optical path between the optical elements (e.g., air) may be reduced or avoided. In addition, in some implementations, the vacuum chamber may be configured to operate at a vacuum pressure that is based on the peak power of the optical pulses travelling in the pulse compressor <NUM>. For example, the vacuum chamber of the pulse compressor <NUM> may be operated at a particular vacuum pressure such that the peak power of the optical pulses is below a critical power, and the optical pulses may avoid self-channeling effects while traversing the optical path through the pulse compressor <NUM>. By operating the vacuum chamber of the pulse compressor <NUM> at vacuum pressures less than <NUM>-<NUM> Torr, the optical pulses produced by the pulse compressor <NUM> may have higher peak power. For example, in some instances, the optical pulse travelling in the pulse compressor <NUM> has a peak power of greater than <NUM> GigaWatts (GW) (which is approximately the critical power at which self-channeling effects appear for optical pulses in air at atmospheric pressure). The critical power at which self-channeling effects may appear can be described by the following equation: <MAT> where n is the refractive index of the medium (e.g., n = <NUM> for vacuum), and n<NUM> is the non-linear refractive index of the medium that is density dependent.

In the example CPA system <NUM> of <FIG>, the diffraction gratings <NUM>-<NUM> have a ridged reflective surface that define part of the optical path in the pulse compressor <NUM>, while the dispersive mirror <NUM> has a smooth reflective surface that defines part of the optical path in the pulse compressor <NUM>. The example dispersive mirror <NUM> can be implemented as a highly dispersive mirror (HDM) composed of layers of dielectric materials disposed on a substrate (e.g., silica or sapphire). In some implementations, the dispersive mirror <NUM> includes alternating layers of high- and low-index dielectric materials. In the example shown, the high-index material has a refractive index greater than <NUM>; for example, Titanium dioxide (TiO<NUM>) having a refractive index of <NUM> may be used. In the example shown, the low-index material has a refractive index less than1. <NUM>; for example, Silicon dioxide (SiO<NUM>) having a refractive index of <NUM> may be used. In some implementations, the total physical thickness of the high-index dielectric material of the dispersive mirror <NUM> is approximately <NUM>, while the total physical thickness of the low-index dielectric material of the dispersive mirror <NUM> is approximately <NUM>. In some implementations, the dispersive mirror <NUM> imparts a second order dispersion (or group delay dispersion) of approximately (+/-)<NUM> fs<NUM> onto an optical pulse with a spectrum centered around <NUM> (e.g., having a spectrum of <NUM>-<NUM>). In some implementations, the second order dispersion characteristics of the dispersive mirror <NUM> are wavelength dependent.

<FIG> is a plot <NUM> showing example intensities incident upon a last diffraction grating in a pulse compressor of a pulse amplification system. The example plot <NUM> includes traces <NUM> and <NUM>, which indicate relative intensities with respect to the peak intensity of trace <NUM>. Trace <NUM> is an example measurement of temporal intensity incident upon a last diffraction grating in a pulse compressor of a pulse amplification system, such as a CPA, where the pulse amplification system does not include a dispersive mirror in the optical path of the pulse compressor. Trace <NUM> is an example measurement of temporal intensity incident upon a last diffraction grating in a pulse compressor of a pulse amplification system, such as a CPA, that includes a dispersive mirror as the last optical element in the optical path of the pulse compressor. For instance, the measurements of trace <NUM> may be indicative of the intensity incident upon diffraction grating <NUM> of <FIG> relative to a system similar to what is shown in <FIG> except that it does not include the dispersive mirror <NUM>. As indicated by the plot <NUM>, the inclusion of a dispersive mirror <NUM> as the last optical element in the optical path of the pulse compressor <NUM> may reduce the intensity of the optical pulse incident upon the diffraction grating <NUM>, which may allow for a longer lifespan and increased duty cycle of the diffraction grating.

<FIG> is a flow diagram showing an example process <NUM> for amplifying chirped optical pulses. The process <NUM> may be implemented using the pulse amplification system <NUM> of <FIG>, the CPA system <NUM> of <FIG> or another system. In some implementations, one or more operations of the process <NUM> are performed by optical elements or other components operating under high vacuum pressure. For example, the diffraction gratings and dispersive mirror that compress the optical pulse at <NUM> may reside in a vacuum chamber that operates at pressures below <NUM>-<NUM> Torr.

At <NUM>, a chirped optical pulse is generated using a femtosecond oscillator, such as a femtosecond Ti:sapphire laser. At <NUM>, the optical pulse is stretched using one or more diffraction gratings. The diffraction gratings may impart either positive or negative dispersion upon the optical pulse. At <NUM>, the spectral phase of the stretched optical pulse is modified using a programmable optical dispersive filter (PODF). The PODF may be implemented using an AOPDF, which modifies the spectral phase of the optical pulse based on a received acoustic signal. At <NUM>, the optical pulse is amplified using a multipass amplifier. At <NUM>, the optical pulse is compressed using one or more diffraction gratings and dispersive mirrors. The diffraction gratings and dispersive mirrors may impart either positive or negative dispersion upon the optical pulse, which is the opposite of the dispersion imparted upon the optical pulse at <NUM>. For instance, where positive dispersion is imparted upon the optical pulse at <NUM>, negative dispersion may be imparted upon the optical pulse at <NUM>.

In a general aspect of the examples described here, a chirped pulse laser system includes a pulse compressor with a dispersive mirror.

In a first example, a chirped pulse laser system includes a programmable optical dispersive filter (PODF) and a pulse compressor that receives an optical pulse based on an output of the PODF. The PODF is operable to modify a spectral phase of optical pulses. The pulse compressor includes optical elements in a vacuum chamber. The optical elements define an optical path through the pulse compressor, and are arranged to disperse the optical pulse in the optical path. The optical elements include diffraction gratings and a dispersive mirror. The dispersive mirror has a smooth reflective surface that defines a portion of the optical path.

Implementations of the first example may include one or more of the following features. The dispersive mirror may include one or more dielectric materials on a substrate. The dispersive mirror may include alternating layers of a high-index dielectric material and a low-index dielectric material. The high-index material may include Titanium dioxide (TiO<NUM>) and the low-index material may include Silicon dioxide (SiO<NUM>). The total thickness of the layers of Titanium dioxide (TiO<NUM>) may be approximately <NUM>, and the total thickness of the layers of Silicon dioxide (SiOz) may be approximately <NUM>. The dispersive mirror may be operable to impart a second order dispersion of approximately (+/-)<NUM> fs<NUM> to the optical pulse.

Implementations of the first example may include one or more of the following features. The vacuum chamber may be configured to operate at pressures less than <NUM>-<NUM> Torr. The PODF may include an acousto-optic programmable dispersive filter (AOPDF), which may be operable to modify the spectral phase of an optical signal based on an acoustic signal received by the PODF. The optical elements of the pulse compressor may include a wave plate operable to modify a polarization of the optical pulse. The wave plate may reside in the optical path between the dispersive mirror and the last diffraction grating. The diffraction gratings may be arranged before the dispersive mirror in the optical path.

Implementations of the first example may include one or more of the following features. The system may include a pulse generator, a pulse stretcher that receives an output of the pulse generator, and a pulse amplifier that receives an output of the PODF. The pulse stretcher may include one or more optical elements that disperse optical pulses, and the pulse amplifier may be operable to increase a power of optical pulses. The pulse compressor may be arranged to receive an output of the pulse amplifier. The pulse generator may include a femtosecond laser oscillator.

Implementations of the first example may include one or more of the following features. The optical elements may define an optical path between an optical inlet of the pulse compressor and an optical outlet of the pulse compressor. The pulse compressor may include a first diffraction grating arranged to receive the optical pulse from the optical inlet, a second diffraction grating arranged to receive the optical pulse from the first diffraction grating, a third diffraction grating arranged to receive the optical pulse from the second diffraction grating, and a fourth diffraction grating arranged to receive the optical pulse from the third diffraction grating. The dispersive mirror may be arranged to provide the optical pulse to the optical outlet.

In a second example, compressing a chirped optical pulse includes modifying a spectral phase of an optical pulse by operation of a programmable optical dispersive filter (PODF) and compressing the optical pulse by operation of a pulse compressor. The pulse compressor includes optical elements in a vacuum chamber. The optical elements define an optical path in the pulse compressor, and are arranged to disperse the optical pulse in the optical path. The optical elements include diffraction gratings and a dispersive mirror. The dispersive mirror has a smooth reflective surface that defines a portion of the optical path.

Implementations of the second example may include one or more of the following features. The dispersive mirror may include one or more dielectric materials on a substrate. The dispersive mirror may include alternating layers of a high-index dielectric material and a low-index dielectric material. The high-index material may include Titanium dioxide (TiO<NUM>) and the low-index material may include Silicon dioxide (SiO<NUM>). The total thickness of the layers of Titanium dioxide (TiO<NUM>) may be approximately <NUM>, and the total thickness of the layers of Silicon dioxide (SiO<NUM>) may be approximately <NUM>. The dispersive mirror may be operable to impart a second order dispersion of approximately (+/-)<NUM> fs<NUM> to the optical pulse. The vacuum chamber may be operated at pressures less than <NUM>-<NUM> Torr.

Implementations of the second example may include one or more of the following features. The method may include amplifying the optical pulse by operation of a pulse amplifier before compressing the optical pulse. The method may include generating the optical pulse using a femtosecond laser oscillator. The method may include stretching the optical pulse by operation of a pulse stretcher before modifying the spectral phase of the optical pulse. The pulse stretcher may include dispersive optical elements. The PODF may include an acousto-optic programmable dispersive filter (AOPDF), and the method may include modifying the spectral phase of the optical pulse base on an acoustic signal received by the AOPDF.

Implementations of the second example may include one or more of the following features. Compressing the optical pulse may include compressing the optical pulse by operation of the diffraction gratings before compressing the optical pulse by operation of the dispersive mirror. Compressing the optical pulse may include modifying a polarization of the optical pulse using a wave plate. Compressing the optical pulse may include imparting a second order dispersion of approximately (+/-)<NUM> fs<NUM> to the optical pulse by operation of the dispersive mirror. The compressed optical pulse may have a peak power of greater than <NUM> GigaWatts (GW).

Implementations of the second example may include one or more of the following features. The optical elements may define an optical path between an optical inlet of the pulse compressor and an optical outlet of the pulse compressor. The pulse compressor may include a first diffraction grating arranged to receive the optical pulse from the optical inlet, a second diffraction grating arranged to receive the optical pulse from the first diffraction grating, a third diffraction grating arranged to receive the optical pulse from the second diffraction grating, and a fourth diffraction grating arranged to receive the optical pulse from the third diffraction grating. The dispersive mirror may be arranged to provide the optical pulse to the optical outlet.

While this specification contains many details, these should not be understood as limitations on the scope of what may be claimed, but rather as descriptions of features specific to particular examples. Certain features that are described in this specification or shown in the drawings in the context of separate implementations can also be combined. Conversely, various features that are described or shown in the context of a single implementation can also be implemented in multiple embodiments separately or in any suitable subcombination.

Claim 1:
A chirped pulse laser system (<NUM>) comprising:
a programmable optical dispersive filter, PODF, (<NUM>) operable to modify a spectral phase of optical pulses; and
a pulse compressor (<NUM>) that receives an optical pulse based on an output of the PODF (<NUM>), and in response, produces a transform-limited optical pulse, the pulse compressor comprising optical elements in a vacuum chamber, the optical elements defining an optical path (<NUM>-<NUM>) through the pulse compressor, the optical elements arranged to disperse the optical pulse in the optical path, the optical elements comprising diffraction gratings (<NUM>-<NUM>) and a dispersive mirror (<NUM>), the dispersive mirror having a smooth reflective surface that defines a portion of the optical path.