PHASE MODULATOR SYSTEM FOR GENERATING MILLIJOULE LEVEL FEW- CYCLE LASER PULSES

RELATED APPLICATION

[0001] The present non-provisional application claims the benefit of U.S.

Provisional Application No. 61/043,824, entitled "PHASE MODULATOR FOR GENERATING MILLEJOULE LEVEL FEW-CYCLE LASER PULSES," filed April 10, 2008. The identified provisional application is incorporated herein in its entirety by specific reference.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT PROGRAM [0002] The present invention was developed with support from the U.S.

Government under Grant No. DE-FG02-86ER13491 awarded by the Department of Energy and Grant No. 0457269 awarded by the National Science Foundation. Accordingly, the U.S. Government has certain rights in the present invention.

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

[0003] Embodiments of the present invention relate to chirped pulse amplifier laser systems. More particularly, embodiments of the present invention relate to a phase modulator system utilized with a chirped pulse amplifier laser system for generating millijoule energy level, few-cycle laser pulses with a stabilized carrier envelope phase and a compensated spectral phase.

DESCRIPTION OF THE RELATED ART

[0004] Chirped pulse amplifier (CPA) laser systems are often utilized to amplify laser pulses to millijoule level or higher energies For example, CPA laser systems may be employed to generate high-power laser pulses to study various aspects of atomic physics, such as attosecond pulse generation, above threshold ionization, and molecular dissociation, among other high field applications, in some instances, it may be desirable to broaden the spectral bandwidth of the output of the CPA laser system. The broadening may be accomplished by focusing the optical output of the CPA laser system into one or more gas-filled, hollow-core optical fibers. While the output of the optical fibers may have the desired property of greater

spectral bandwidth, the output pulse may be undesirably stretched or elongated in time. The stretched pulse, which may be considered to be positively chirped, may be compressed or shortened in time by pulse compressors, such as chirped mirrors or prism pairs. However, conventional pulse compressors may suffer from limited spectral bandwidth, low energy output, less stable carrier envelope phase, or uncompensated spectral phase.

SUMMARY OF THE INVENTION

[0005] Embodiments of the present invention solve the above-mentioned problems and provide a distinct advance in the art of chirped pulse amplifier laser systems. More particularly, embodiments of the invention provide a phase modulator system utilized with a chirped pulse amplifier laser system for generating millijouie energy level, few-cycle laser pulses with a stabilized carrier envelope phase and a compensated spectral phase.

[0006] Various embodiments of the invention may include an adaptive phase modulator which comprises a first and second grating, a first and second cylindrical mirror, and a spatial light modulator. The first grating may receive laser pulses from an external source and diffract the pulses. The first mirror may receive laser pulses from the first grating and may collimate the spectrum of pulses to the second mirror, which in turn may reflect the pulses to the second grating which may recombine the spectrum of the pulses for an external destination. The spatial light modulator may be positioned between the first and second mirrors and may adjust an optical property of the laser pulses.

[0007] Other embodiments of the invention may include a phase modulator system which comprises a chirped-puise amplifier (CPA) laser, an optical fiber, an adaptive phase modulator, a phase measurement element, and a control element. The CPA laser provides laser pulses that are coupled into the optical fiber to increase the spectral bandwidth of the pulses. The pulses from the optical fiber are compressed by the adaptive phase modulator. The phase measurement element receives a portion of the energy of the laser pulses from the adaptive phase modulator and sends phase information about the laser pulses to the control element. The control element sends a control signal to the CPA laser to stabilize the carrier envelope phase and a control signal to the adaptive phase modulator to compensate the spectral phase.

[0008] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

[0009] Other aspects and advantages of the present invention will be apparent from the following detailed description of the embodiments and the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0010] Embodiments of the present invention are described in detail below with reference to the attached drawing figures, wherein:

[001 i] FIG. i is a schematic block diagram of a phase modulator system constructed in accordance with at least a first embodiment of the present invention;

[0012] FIG. 2 is a plot of the electric field vs. time for a laser pulse illustrating the carrier envelope phase;

[0013] FIG. 3 is a plot of the electric field vs. time for a chirped iaser pulse illustrating the spectral phase;

[0014] FIG. 4 is a schematic block diagram of a chirped pulse amplifier laser;

[0015] FIG. 5 is a schematic block diagram of a laser pulse stretcher;

[0016] FIG. 6 is a schematic block diagram of an adaptive phase modulator;

[0017] FIG. 7 is a schematic block diagram of a phase measurement element;

[0018] FIG. 8 is a schematic block diagram of a second embodiment of the phase modulator system; and

[0019] FIG. 9 is a schematic block diagram of a third embodiment of the phase modulator system.

[0020] The drawing figures do not limit the present invention to the specific embodiments disclosed and described herein. The drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0021] The following detailed description of the invention references the accompanying drawings that illustrate specific embodiments in which the invention

can be practiced. The embodiments are intended to describe aspects of the invention in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments can be utilized and changes can be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense. The scope of the present invention is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled.

[0022] A phase modulator system 10 for generating millijoule energy level, few-cycle laser pulses constructed in accordance with at least a first embodiment of the current invention is shown in FIG. 1. The system 10 may broadly comprise a chirped pulse amplifier (CPA) laser 12, an optical fiber 14, an adaptive phase modulator 16, a feedback element 18, a phase measurement element 20, a control element 22, a carrier-envelope (CE) phase feedback signal 24, and a spectral phase feedback signal 26. The system 10 may provide stabilization of a CE phase 28 as well as compensation for a spectral phase 30, as shown in FIGs. 2 and 3. [0023] The system 10 may also include components not shown such as mirrors, lenses, secondary optical fibers, and the like to reflect, steer, couple, focus, and so forth, optical energy within the system 10.

[0024] The electric field of radiation emitted from a laser, such as the CPA laser 12, is generally characterized as shown in FIG. 2, with a higher-frequency carrier wave 32 oscillating within a lower-frequency envelope 34. The CE phase 28, φc _E , is the difference in time between the peak of the envelope wave 34 and the closest peak of the carrier wave 32. The electric field of a chirped or time-stretched laser pulse (discussed in more detail below) demonstrates the spectral phase 30 of the pulse, as shown in FIG. 3.

[0025] The CPA laser 12 may generally provide a laser pulse that has an energy on the order of millijoules and a time duration on the order of tens of femtoseconds. The level of energy of the laser pulse may be produced by chirping, a process in which a relatively low-energy pulse is produced by a laser source and elongated in time by a stretcher. The stretched pulse may be optically amplified and compressed in time. The resulting pulse may be of roughly the same time duration as the original pulse, but the energy level may be orders of magnitude greater. As shown in FIG. 4, the CPA laser 12 may include a laser source 36, a Pockels cell 38, a stretcher 40, an amplifier 42, and a compressor 44.

[0026] The CPA laser 12 may further include a plurality of planar mirrors, not shown in the figures, implemented and positioned at various points within the CPA laser 12 to reflect, steer, direct, aim, or align a laser signal 46, pulses thereof, or beams thereof. For example, one or more planar mirrors may be used to align the output of the Pockels cell 38 with the input of the stretcher 40. The planar mirror may include at least one substantially reflective surface that is generally flat or planar.

[0027] The laser source 36 generally provides the laser signal 46 comprising pulses or beams of electromagnetic radiation, as is known in the art. The laser signal 46 may have a generally stabilized CE phase 28. The laser source 36 may include any elements or combination of elements operable to generate or pump laser light. In various embodiments, the laser source 36 may include a pump laser such as a Verdi 6 laser and/or the laser source disclosed by U.S. Patent No. 7,050,474, which is incorporated herein by reference.

[0028] The Pockels cell 38 generally receives the laser signal 46 from the laser source 36 and provides laser pulses of the same CE phase 28 to the stretcher 40. Providing pulses having the same or similar CE phase 28 to the stretcher 40 may simplify measurement and enable CE phase 28 to be more readily corrected. The Pockels cell 38 may include electro-optic crystals, electro-optic modulators, voltage-controlled wave plates, and the like,

[0029] The stretcher 40 generally stretches or elongates the pulses of the laser source 36 in the time domain, as discussed above. The stretcher 40 may include a first grating 48, a second grating 50, a first concave mirror 52, a second concave mirror 54, a positioning element 56, and a positioning input 58, as seen in FIG. 5. The stretcher 40 may output the stretched pulses to the amplifier 42. [0030] The first concave mirror 52 and the second concave mirror 54 may have a generally concave reflecting surface and may be positioned with the reflective surfaces facing one another at a first distance 60 apart. The first grating 48 and the second grating 50 may be diffraction gratings and generally include a plurality of parallel and equally spaced grooves that are typically etched on glass. The first grating 48 and the second grating 50 may be positioned at appropriate angles between the first concave mirror 52 and the second concave mirror 54. The positioning element 56 may be coupled to the first concave mirror 52 and may be operable to adjust the position of the first concave mirror 52 so as to change the

value of the first distance 60. Thus, the positioning element 56 may be used to increase the first distance 60 or decrease the first distance 60. The positioning element 56 may also be coupled to the second concave mirror 54 in order to accomplish the task. The positioning element 56 may include any electronically- controlled mechanical translation device, such as a piezoelectric transducer. In various embodiments, the positioning element 56 may include one or more electronically powered piezoelectric transducer translation stages, such as the MAX311 manufactured by Thorlabs, Inc. The positioning input 58 is coupled to the positioning element 56 and generally provides information to adjust the positioning element 56 in order to control the first distance 60. The positioning input 58 may include an electronic signal.

[0031] The amplifier 42 generally amplifies the stretched laser pulses that are received from the stretcher 40. For example, the amplifier 42 may amplify a 3 nJ laser pulse received from the stretcher 40 to a 5 mJ laser pulse. The amplifier 42 may utilize generally conventional amplification elements to amplify the stretched laser pulse to any desired power or energy level. In some embodiments, the amplifier 42 may employ a 14-pass Ti:Sapphire crystal amplifier to amplify pulses. The amplifier 42 may also include liquid nitrogen cooling to facilitate amplification to desired levels and employ any conventional carrier phase envelope stabilization techniques as is known in the art.

[0032] The compressor 44 generally compresses or shortens in time the laser pulses that are stretched by the stretcher 40 and amplified by the amplifier 42. For example, the compressor 44 may compress an 80 ps laser pulse to a 25 fs laser pulse. The compressor 44 may include generally known pulse modification components, such as gratings, prisms, mirrors, lenses, combinations thereof, and the like that are arranged in a generally known compressor configuration. [0033] Returning to the system 10 of FIG. 1 , the optical fiber 14 generally receives laser pulses from the CPA laser 12 and broadens the spectrum of the laser pulses. For example, laser pulses that exit the optical fiber 14 may have a spectral bandwidth of more than one octave and may be in the range from 400 nm to 1000 nm. In addition, the laser pulses may be stretched or elongated in time after exiting the optical fiber 14. The optical fiber 14 may be a hollow-core fiber with a length of approximately 0.9 m and an inner diameter of approximately 400 μm. The core may be filled with a pressurized noble gas, such as approximately 2 bars of Neon gas.

[0034] The adaptive phase modulator 16 generally compresses or shortens in time the broad spectrum laser pulses produced from the optical fiber 14. The adaptive phase modulator 16, as seen in FIG. 6, may include a third grating 62, a fourth grating 64, a first cylindrical mirror 66, a second cylindrical mirror 68, a first planar mirror 70, a second planar mirror 72, a spatial light modulator 74, and a spatial light modulator input 76.

[0035] The third grating 62 and the fourth grating 64 may be similar to the first grating 48 and the second grating 50 with a groove density of 235/mm and may further include a protective silver coating to achieve a high diffraction efficiency of approximately 70% to approximately 80%. The first cylindrical mirror 66 and the second cylindrical mirror 68 may include at least one reflective surface that has a cylindrical cross-sectional shape and may be coated with silver. The first cylindrical mirror 66 and the second cylindrical mirror 68 may have a focal length of approximately 50 cm. The first planar mirror 70 and the second planar mirror 72 may include at least one substantially reflective surface that is generally flat or planar and may be coated with silver. The spatial light modulator 74 generally provides electronically-controlled modulation of the light that passes through a transmissive planar surface. The spatial light modulator 74 may include an array of pixels whose index of refraction is electronically adjustable, and may further include an anti- reflective coating. An example of the spatial light modulator 74 may include the liquid crystal 640-pixel SLM S640 by Jenoptik of Germany. The spatial light modulator input 76 may be coupled to the spatial light modulator 74 and may adjust or modulate one or more physical characteristics of the spatial light modulator 74 in order to control the amplitude, polarization, or phase of the laser pulses. The spatial light modulator input 76 may include an electronic signal.

[0036] One possible configuration of the adaptive phase modulator 16, as shown in FIG. 6, includes the first cylindrical mirror 66 and the second cylindrical mirror 68 positioned at a certain distance apart with their reflective surfaces facing one another. Positioned between the first cylindrical mirror 66 and the second cylindrical mirror 68 are the first planar mirror 70, the spatial light modulator 74, and the second planar mirror 72. Positioned along the side of the path between the first cylindrical mirror 66 and the second cylindrical mirror 68 are the third grating 62 and the fourth grating 64. The components may be oriented such that laser pulses are received by the third grating 62 and diffracted to the first planar mirror 70, which

reflects the pulses to the first cylindrical mirror 66, which in turn collimates the pulses to the spatial light modulator 74. The pulses may pass through and be modulated by the spatial light modulator 74. The laser pulses may then be reflected by the second cylindrical mirror 68 and the second planar mirror 72, and recombined by the fourth grating 64 in order to exit the adaptive phase modulator 16.

[0037] Returning to the system 10 of FIG. 1 , the feedback element 18 may optically divert a fraction of the energy of the laser pulses exiting the adaptive phase modulator 16 to provide a feedback control signal 78. The feedback control signal 78 may include approximately 3% or less of the energy of the laser pulses and may be forwarded to the phase measurement element 20. The feedback element 18 may include optical beam-splitting components such as a fused silica plate. [0038] The phase measurement element 20 generally measures the spectral phase 30 and the CE phase 28 of the laser pulses received from the feedback element 18. As shown in FIG. 7, the phase measurement element 20 may include one or more of the following: a beta-barium borate (BBO) crystal 80, a polarizer 82, a BG3 filter 84, and a spectrometer 86. The phase measurement element 20 may also include components not shown such as mirrors, lenses, optical fibers, and the like to reflect, steer, couple, focus, and so forth, the laser pulses within the phase measurement element 20.

[0039] The BBO crystal 80 generally provides second order harmonics (SHG) of various wavelengths of the laser pulses. The wavelengths may be determined by the thickness and the phase matching angle of the crystal. Generally, thicker BBO crystals 80 produce stronger but more narrow SHG. The wavelengths that are chosen may also correspond to the type of control that is desired. For example, spectral phase 30 compensation may require SHG of broader spectral bandwidth, thus, a thinner BBO crystal 80 may be used. Alternatively, CE phase 28 stabilization may require more narrow spectral bandwidth and accordingly a thicker BBO crystal 80 may be used. A BBO crystal 80 with a thickness of approximately 10 μm may be used for spectral phase 30 compensation, while a BBO crystal 80 with a thickness of approximately 100 μm may be used for CE phase 28 stabilization. The phase measurement element 20 may include one or more BBO crystals 80 with varying thicknesses.

[0040] The polarizer 82 generally passes or filters laser pulses received from the BBO crystal 80, according to the polarization of the pulse. The polarizer 82 may

be manufactured from an α BBO crystal. The BG3 filter 84 generally reduces the magnitude of fundamental wavelengths of the laser pulses received from the polarizer 82. This filtering may be necessary to reduce saturation of the input to the spectrometer 86.

[0041] The spectrometer 86 generally measures the spectrum of the laser pulses from the BG3 filter 84. The spectrometer 86 may include the ability to detect a wide range of wavelengths, may have a high resolution, and a variety of exposure times. An example of the spectrometer 86 may be the HR2000+ by Ocean Optics of Florida. The spectrometer 86 may require a first configuration for the spectral phase measurement, such as wavelength detection between approximately 200 nm and approximately 600 nm, and a second configuration for the CE phase measurement, such as wavelength detection between approximately 380 nm and approximately 580 nm. Thus, the phase measurement element 20 may include more than one spectrometer 86 to provide information about the phase of the received laser pulses to the control element 22.

[0042] The control element 22 generally receives spectral phase 30 information and CE phase 28 information about the laser pulses from the spectrometer 86 and provides feedback control signals to control various parameters of the system 10. Specifically, the control element 22 provides the CE phase feedback signal 24 to the positioning element 56 of the CPA laser 12 and the spectral phase feedback signal 26 to the spatial light modulator 74 of the adaptive phase modulator 16.

[0043] The control element 22 may include general-purpose or specific purpose processing elements such as desktop or laptop computers, workstations, microprocessors, microcontrollers, programmable logic devices, field-programmable gate arrays, application-specific integrated circuits, combinations thereof, and the like. The processing elements may be hard wired to perform the control function or may execute firmware, software, or combinations thereof.

[0044] The control element 22 may perform any generally known feedback control method to compensate for the spectral phase 30 and stabilize the CE phase 28. The control element 22 may also include the hardware and operating system or software to implement a Multiphoton lntrapulse Interference Phase Scan (MIIPS) setup. The control element 22 may utilize MIIPS to provide control data through the

spectral phase feedback signal 26 and the carrier envelope phase feedback signal 24.

[0045] The control element 22 may send the spectral phase feedback signal

26 to the spatial light modulator input 76 to adjust one or more optical properties of the spatial light modulator 74 to compensate for the spectral phase 30. The control element 22 may also send the carrier envelope phase feedback signal 24 to the positioning element 56 in order to position the first concave mirror 52 and adjust the first distance 60.

[0046] The system 10 may operate as follows. The CPA laser 12 may produce narrowband laser pulses with an energy level on the order of millijoules and a pulse duration on the order of tens of femtoseconds For example, the pulses may be approximately 2 mJ and 30 fs. The pulses may be coupled into the hollow- core, gas-filled optical fiber 14 to broaden the spectral bandwidth For example, pulses exiting the optica! fiber 14 may have a bandwidth of approximately 400 nm to approximately 1000 nm. The pulse from the optical fiber 14 may be coupled into the adaptive phase modulator 16 to narrow their pulse width. For example, pulses exiting the adaptive phase modulator 16 may have a pulse width of approximately 5 fs and an energy level of approximately 0.5 mJ. In order to compensate for the spectral phase and to stabilize the CE phase of the output pulses, a fraction of the energy of each pulse is diverted by the feedback element 18. [0047] The feedback control signal 78 from the feedback element 18 may be forwarded to the phase measurement element 20, which may provide spectral phase 30 and CE phase 28 information to the control element 22. The control element 22 may compensate the spectral phase 30 of the laser pulses by adjusting one or more optical properties of the spatial light modulator 74 through the spectral phase feedback signal 26. Generally, compensation of the spectral phase 30 occurs only periodically. Thus, the control element 22 may send a signal to the spatial light modulator 74 on the order of every few hours

[0048] The control element 22 may stabilize the CE phase 28 of the laser pulses by sending control data through the carrier envelope phase feedback signal 24 to the positioning element 56 of the stretcher 40 in the CPA laser 12. The CE phase 28 may be adjusted by positioning the first concave mirror 52, which adjusts the first distance 60 between the first concave mirror 52 and the second concave

mirror 54. The control element 22 may send control data through the carrier envelope phase feedback signal 24 in a continuous or near-continuous fashion. [0049] A second embodiment 200 of the system is shown in FIG. 8. The second embodiment 200 may perform the function of CE phase 28 stabilization only. Thus, the second embodiment 200 may not include the spectral phase feedback signal 26 and the phase measurement element 20 may be configured to measure the CE phase 28 only. Otherwise, the second embodiment 200 of the system may include substantially the same structure and function substantially the same as the first embodiment of the system 10.

[0050] A third embodiment 300 of the system is shown in FIG. 9 The third embodiment 300 may perform the function of spectral phase 30 compensation only. Thus, the third embodiment 300 may not include the carrier envelope phase feedback signal 24 and the phase measurement element 20 may be configured to measure the spectral phase 30 only Otherwise, the third embodiment 300 of the system may include substantially the same structure and function substantially the same as the first embodiment of the system 10.

[0051] Although the invention has been described with reference to the embodiments illustrated in the attached drawing figures, it is noted that equivalents may be employed and substitutions made herein without departing from the scope of the invention as recited in the claims

[0052] Having thus described various embodiments of the invention, what is claimed as new and desired to be protected by Letters Patent includes the following: