# Patent

## Title
Laser amplification system

## Abstract

A high peak intensity laser amplification system and the method therein implemented are provided. In a first aspect of the invention, the laser system includes at least one optical member (27) operably introducing a phase function into a high peak intensity laser pulse (25). A further aspect includes introducing destructive interference in an unchirped laser pulse prior to amplification and reconstructive interference in the output laser pulse after amplification. Dynamic pulse shaping is employed in another aspect of the present invention.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This is a U.S. National Phase entry application based on International Application No. PCT/US2010/025564, filed on Feb. 26, 2010, which claims the priority of U.S. Provisional Application Ser. No. 61/157,718, filled on Mar. 5, 2009, all of which are incorporated by reference herein.


BACKGROUND AND SUMMARY
The present invention generally relates to laser systems and more particularly to a laser amplification system.
It is known to employ chirp pulse amplification (“CPA”) for ultrashort lasers. Chirping overcomes concerns with intense ultrashort laser pulses inducing nonlinear optical processes in transparent media which would otherwise damage the amplification equipment. When chirp pulse amplification is used, the pulses are stretched by a factor greater than 1000 and therefore their intensity is significantly reduced. Examples of chirp pulse amplification can be observed in the following U.S. Pat. Nos. 5,862,287 entitled “Apparatus And Method For Delivery Of Dispersion Compensated Ultrashort Optical Pulses With High Peak Power” which issued to Stock et al. on Jan. 19, 1999; 5,633,885 entitled “Frequency Chirp Control And Compensation For Obtaining Broad Bandwidth Ultrashort Optical Pulses From Wavelength-Tunable Lasers” which issued to Galvanauskas et al. on May 27, 1997; and 5,572,355 entitled “Optical System For Stretching, Compressing And Amplifying Ultrashort Optical Pulses” which issued to Cotton et al. on Nov. 5, 1996; all of which are incorporated by reference herein. Chirp pulse amplification, however, requires an expensive set of optics.
An experiment has also been conducted which provides one SLM pulse shaper before a regenerative amplifier and another SLM pulse shaper after the amplifier. This experiment is disclosed in I. Pastirk, B. Resan, A. Fry, J. Mackay and M. Dantus, “No Loss Spectral Phase Correction and Arbitrary Phase Shaping of Regeneratively Amplified Femtosecond Pulses using MIIPS,” Optics Express, Vol. 14, No. 20, 9537 (2006). CPA amplifiers, having a large stretcher and compressor, were used in this experiment. Furthermore, symmetric and single binary phase step functions were used and only for multiphoton intrapulse interference phase scan purposes in the chirped pulse.
In accordance with the present invention, a laser amplification system is provided. In another aspect, a laser system and method include at least one optic member operably introducing a phase function into a high peak intensity laser pulse. A further aspect includes introducing destructive nonlinear optical interference in an unchirped laser pulse prior to amplification and reconstructive interference in the output laser pulse after amplification. Dynamic pulse shaping is employed in an aspect of the present system. In yet another aspect, a minimal correlation binary phase function is introduced into a laser pulse prior to amplification.
The present laser system is advantageously less expensive and more efficient than traditional chirped devices. The present laser system is also more compact and is less sensitive to air turbulence as compared to conventional chirped devices. Moreover, the present system maintains a more intense pulse but without harmful nonlinear optical processes therein. Additional advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic view showing a first embodiment of the laser amplification system;
FIG. 2 is a diagrammatic view showing a second embodiment of the laser amplification system;
FIG. 3 is a diagrammatic view showing a preferred pulse shaper employed in either of the laser amplification system embodiments;
FIG. 4 is a diagrammatic view of deconstruction and reconstruction of a minimal correlation binary phase function used in either of the laser amplification system embodiments;
FIG. 5 is a graphical representation of the minimal correlation binary phase function as it is imposed on a spectrum of the laser pulse;
FIG. 6 is a graphical representation of how an intense laser pulse labeled as Original Pulse is able to generate additional frequencies through nonlinear optical processes;
FIG. 7 is a graphical representation of how different frequencies in a fundamental laser pulse labeled as ω0, can interfere nonlinearly at 2 ω0; and
FIG. 8 is a graphical representation of how minimum correlation phase functions break a single TL pulse into a larger number of pulses that do not have constructive nonlinear optical interference; these effects shown are expected to occur on a laser pulse employed with either of the laser amplification system embodiments (CPA and the method disclosed here and labeled as BP).



DETAILED DESCRIPTION
The first embodiment of a laser amplification system 21 is shown in FIG. 1. A Ti:Sapphire laser oscillator 23 emits a series of high peak intensity laser beam pulses 25 through a pulse shaper 27, to an amplifier 29 and through a second pulse shaper 30. A satisfactory oscillator can be obtained from KM Labs, and has an 86 mhz repetition rate at  ̃100 nm FWHM bandwidth. A satisfactory amplifier can be obtained from Coherent Inc., namely the Legend USP Model which generates pulses at or less than 35 femtoseconds at 800 nm. The high peak intensity laser pulse has a typical peak intensity greater than about 1 kW and each laser pulse preferably has a duration of less than 100 picoseconds, more preferably less than 10 picoseconds, and even more preferably equal to or less than 25 femtoseconds. Alternately, a fiber oscillator followed by a fiber amplifier can be employed.
A computer controller 31 is electrically connected to control oscillator 23 and at least one of the pulse shapers 27 and 29. The computer controller has an input device such as a keyboard, an output device such as a display screen and wireless transmitter, a microprocessor and memory. Appropriate software is stored in the memory of the computer and includes programmed instructions which control the properties of the output pulse through pulse shaping. A spatial light modulator (“SLM”) in the pulse shaper can introduce different phase functions based on the computer software control of such, however, a fixed and preset pulse shaper or manually adjustable pulse shaper can be used without computer control in a different construction as will be further discussed hereinafter.
Referring to FIG. 2, a second embodiment laser amplification system 41 is set up in a multi-pass manner. An oscillator 43 emits a series of high peak intensity and ultrafast laser pulses 45 in a first direction transmitting through a pulse shaper optical member 47 and through an amplifier 49. The pulse is then reflected by a mirrored optic member 51. Thereafter, pulse 45 reverses direction and travels back along essentially the same path through amplifier 49 and through the same pulse shaper 47. A mirror 53 picks off the reversed and amplified pulse 45, and redirects it toward its target. This multi-task configuration provides a much less expensive and smaller sized system than with the first embodiment, and use of the same pulse shaper before and after amplification further ensures identical deconstructing and reconstructing phase functions, thereby improving accuracy. It is noteworthy that for both embodiment systems 21 and 41, the laser pulses 25 and 45 do not use chirped pulse amplification, and no stretcher or compressor is necessary in these system embodiments, yet the high peak intensity pulses will not damage the optics within the amplifier due to the present pulse shaping of the pulse.
It should be appreciated that “without chirping” includes substantially or essentially without intentional chirping since optical fibers and crystals may introduce some chirp. Furthermore, “without chirping” as used herein can also be expressed by stating that chirping requirements are preferably reduced by at least two orders of magnitude and more preferably less than a factor of 100. By way of comparison, in conventional CPA systems, the factor is typically more than 1000.
In traditional CPA, 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 chirped, 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. Stretching the pulses by factors of 103 to 105 requires extremely high quality gratings that are large, high grove density and can take very high powers. In addition, the grating spacing needs to be very large and this can cause instabilities in the presence of air drafts or temperature gradients. In contrast, the present system reduces and/or eliminates the need for introducing large quantities of linear chirp. The present system replaces the linear chirp by a different approach to pulse shaping that is more efficient at reducing the detrimental nonlinear optical processes the CPA was designed to mitigate. While chirp reduces the peak intensity of the pulses and therefore very large chirp values are required, destructive nonlinear optical intrapulse interference prevents the processes from occurring.
As an example of this difference, a laser system using an oscillator, such as the Coherent Micra titanium sapphire oscillator, is stretched so that its pulse duration is increased to 2000 times its TL pulse duration using a grating stretcher. Stretching is used to mitigate nonlinear optical processes such as self phase modulation and self focusing. The pulses are amplified by a factor of a million and then compressed using a grating compressor. The stretcher, amplification and compression portions are included in a single instrument—the amplifier, such as the Legend USP amplifier from Coherent. The burden on the stretcher and compressor are greatly reduced or eliminated by the use of a minimum correlation binary phase that breaks the pulse into hundreds of much less intense pulses that are not able to cause detrimental nonlinear optical processes. In principle, a 100 bit binary phase that is designed to have a minimum correlation series would reduce the need for linear chirp stretching from 2000 times pulse duration to only 20 times pulse duration. This requirement is much smaller and can be easily accomplished with the same pulse shaper that introduces the binary phase. If the binary phase has 1000 bits, then no linear chirp would be required at all. Binary phases can be designed to prevent nonlinear optical processes, therefore, are more efficient than linear chirp.
As a second example, a passively mode-locked Yb:KGW oscillator producing 250 fs at 1040 nm is stretched by using a step-index single-mode fiber that causes the pulses to stretch to 1.9 ps. The stretched pulses are then launched into a high gain amplifier fiber. A length of 2 m of air-guiding photonic bandgap fiber is used to recompress the amplified positively chirped pulses. The dispersion compensation is finely optimized by changing the length of the single-mode fiber in front of a fiber amplifier. The best compression is found at a stretcher fiber length of 1.9 m. With the present system, the step-fiber stretcher can be replaced by a binary phase pulse shaper, and the compressor fiber can be replaced by an identical binary phase pulse shaper. The advantages are that much higher mitigation of nonlinear optical processes can be achieved by binary phase shaping, and that the pulse shaper can be used to correct for high order dispersion and other variations in the performance of the laser caused by changes in temperature and humidity. A similar system, but having a 20 m long single mode fiber, can be used to achieve an order of magnitude greater chirp stretching, however, this introduces additional nonlinear optical distortions. In contrast, with binary phases, the only requirement is to introduce more binary phase bits and this does not add to the nonlinear optical distortions.
FIG. 3 illustrates the preferred pulse shaper 27/29/47 used with either of the first or second embodiments of the laser amplification system of FIGS. 1 and 2. The pulse shaper includes the following optics: a grating 61, a first reflective mirror 63, a curved reflective mirror 65, a binary phase mask 67, an optimally bent mirror 69, and a second reflective mirror 65. In use, the laser beam pulse enters the shaper, passes over mirror 65 and 63 and then impinges on grating 61. The grating disperses the pulse and then mirror 63 reflects the dispersed pulse onto curved mirror 65, collimates the expanded beam, and then reflects it to binary phase mask 67 and optimally bent mirror 69. The beam thereafter passes over mirror 63. The binary phase mask, at the Fourier Plane, breaks or separates the pulse into multiple sub-pulses of lower intensity as is illustrated in FIGS. 6-8, and the optimally bent mirror corrects for high order dispersion. The laser beam pulse is subsequently reflected at the Fourier Plane to retrace its path back to grating 61 and then travels at a lower height before reflecting off mirror 65 to exit the pulse shaper. The optimally bent mirror includes a regular, thick substrate mirror of at least 1 mm thickness, which is slightly bent, curved or deformed by about 0.2 mm.
Multiphoton intrapulse interference phase scan, known as MIIPS®, software and phase functions can be optionally introduced into optimally bent mirror 69 so as to characterize and compensate for undesired distortions and to suppress and control nonlinear optical processes, also known as multi-wave mixing, and multiphoton intrapulse interference within the laser beam pulse. Such multiphoton intrapulse interference phase scan and the preferably associated binary phase shaping (“BPS”), are conducted in a calculated and/or predetermined manner, optionally based on sensed pulse characteristics from one to five prior pulses, and not based on time consuming genetic learning algorithms. MIIPS and BPS are disclosed in U.S. Patent Publication Nos. 2004/0233944 entitled “Laser System Using Ultrashort Laser Pulses” to Dantus et al.; 2006/0056468 entitled “Control System And Apparatus For Use With Ultra-Fast Laser” to Dantus et al.; and PCT/US2008/087707 entitled “Direct Ultrashort Laser System” to Dantus et al.; all of which are incorporated by reference herein.
With regard to the preferred version of binary phase mask 67, the smallest feature must be greater than the optical resolution of the system setup, otherwise some light will be undesirably lost by diffraction. Notwithstanding, it may alternately be desirable to intentionally cause diffraction losses by introducing small features in the phase mask, especially in the center of the spectrum. This reduces gain-narrowing and takes advantage of space-time coupling to cause amplitude modulation.
Referring now to FIGS. 2-4, binary phase mask 67 and optimally bent mirror 69 of shaper 47 introduce a minimal correlation binary phase function into laser beam pulse 45. This causes nonlinear optical constructive or destructive interference within the high peak intensity and unchirped pulse, thereby breaking the pulse into lower intensity sub-pulses (see FIG. 8). This still unchirped pulse can thereafter be amplified but without damage to the amplifier. The amplified pulse is then reconstructed and is subject to constructive interference by way of essentially the same minimal correlation binary phase function being introduced into the pulse using the same pulse shaper (or a second pulse shaper for the first embodiment). This undoes or neutralizes the effects of the initially introduced phase function to the pulse in a π+π=2 π=0 phase reconstruction manner, before the pulse exits the laser amplifier system. As shown in FIG. 5, it is desired to intentionally use pseudo-random sequences in order to achieve the minimal correlation binary phase function without a point of symmetry. At least 100 bits are desired in the minimal correlation binary phase function to ensure no amplifier damage, and approximately 213 bits are preferred. This is somewhat different than the prior multiphoton intrapulse interference approach, by itself, since with the present disclosure, it is now only desired to employ destructive interference with the initial pulse shaping, although MIIPS can additionally be employed at the same time to compress the output pulses after they are reconstructed. The present pulse shaping approach will avoid damage to the amplifier optics with a high peak intensity and ultrafast laser beam pulse and is highly advantageous over conventional chirping approaches by significantly reducing costs, reducing equipment complexity, and providing a smaller packaging size for the system. FIG. 8 illustrates the expectation that the performance of a 304 bit binary of the present system is equivalent to a 300× chirp of conventional CPA amplifiers.
The binary phase mask 67 and mirror 69 portion of the pulse shaper is preferably manufactured as a single optical component as follows. First, a mirror substrate of BK7 glass or quartz is made. Second, binary steps, depressions or other formations are etched into a surface of the substrate. 213, 304 or even 1,000 bits or binary steps can be provided without increasing the size of the shaper. In one example, a 0 (zero) value has no depth and a π value has 1⁄4 of the optical wavelength depth. The density or spacing of the binary steps is proportional to the pulse intensity to be shaped; in other words, the more intense areas have more density of binary steps (0 and π).
Third, a reflective metallic coating is applied to the stepped substrate surface. Silver or gold is used for visible or infrared pulses and aluminum is used for ultraviolet pulses, chromium or titanium is used for improved adhesion to the substrate. The reflective coating is about 10-100 nm thick if chromium or titanium, and 10μ if silver or gold. If a silver coating is employed then a 0.5μ thick protective MgO or SiO2 overcoating is applied. Fourth, the finished shaper or mask is then bent or curved to the desired final shape according to the laser output desired.
The present laser amplification system is ideally suited for use in telecommunications where the first pulse shaper between the fiber oscillator and fiber amplifier is controlled by the computer controller to introduce a coded communication signal into the pulse during shaping at the same time as the minimal correlation binary phase function is also introduced. This encoded communication signal is based on a voice, video or data signal from a communications transmitter such as a telephone, television, computer or the like. After amplification and reconstructive shaping of the pulse, the still encoded pulse is then transmitted through a fiber optic cable over great distances of many meters or kilometers to a communications receiver including a spectrometer, or other sensor, and connected computer controller. This receiver then decodes the communications signal and transmits it to a receiving station connected to telephones, televisions, computers or the like.
The present laser amplification system is advantageous for communications uses over prior constructions since the present system obtains synergistic and multi-functional benefits of using the same pulse shaping optics to achieve many functions in a simultaneous manner. The present system advantageously transmits intense laser pulses through optical fibers, whether for communication or even portable medical uses, without harmfully causing extreme pulse broadening and power reduction, and possible destruction of the optical fiber. In other words, the present system suppresses and removes harmful self-phase modulation and other nonlinear optical processes while also eliminating self-focusing but with minimal phase manipulation. It should be appreciated that some form of chirping may slightly occur with the present amplification system, but such is not due to an unnecessary stretcher or traditional compressor.
The present system is also well suited for all situations where an amplified ultrafast laser is used. For example, the present system can be used for laser machining, surgery, material processing, and sensing.
Although various embodiments of the present invention have been disclosed, it should be appreciated that other modifications can be made. For example, other types of oscillators, pulse shapers and amplifiers may be employed. An exemplary computer controlled alternative would be to employ an SLM from Boulder Nonlinear Systems having 12,288 pixels, although some of the advantages of the preferred systems may not be realized. A reflective SLM, with liquid crystal on silicone, or a two-dimensional SLM pulse shaper can alternately be used, although certain advantages may not be obtained. Moreover, a fiber oscillator and/or a fiber amplifier may be employed. It is envisioned that certain aspects of the present disclosure (e.g., pseudo-random minimal correlation binary phase functions) can be used in combination with CPA, however, this will not obtain many of the advantages discussed hereinabove. The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. It is intended by the following claims to cover these and any other departures from the disclosed embodiments which fall within the true spirit of the present invention.

## Claims

1. A method of using a laser, the method comprising:
(a) emitting a high peak intensity laser beam pulse;
(b) introducing at least one minimal correlation phase function into the high peak intensity pulse spanning a spectrum of the pulse without symmetry; and
(c) amplifying the pulse substantially without pre-chirping and without damage to an amplifier.

2. The method of claim 1, further comprising introducing a substantially identical phase function into the pulse after the amplification step to cancel the effects of the phase function originally introduced in the pulse before the amplification.

3. The method of claim 1, further comprising using a pulse shaper at a Fourier Plane to introduce the phase function prior to the amplification.

4. The method of claim 1, further comprising using binary phase shaping to introduce the phase function.

5. The method of claim 1, further comprising using at least 100 bits in a binary series of the minimal correlation phase function to deter nonlinear optical processes from being created by the pulse which would otherwise damage the amplifier.

6. The method of claim 1, further comprising transmitting the amplified high peak intensity pulse through an optical fiber.

7. The method of claim 1, further comprising encoding a communications signal into the pulse prior to the amplification.

8. The method of claim 1, further comprising sending the pulse between an oscillator and an output of the amplifier, wherein the requirement for chirping the input pulse is reduced by at least two orders of magnitude.

9. The method of claim 1, further comprising using a double pass optical path so the same pulse shaper interacts with the pulse before and after amplification.

10. The method of claim 1, further comprising using pulse shaping to break the single high intensity pulse into multiple less intense pulses prior to the amplification.

11. The method of claim 1, further comprising using pulse shaping to deter second or higher nonlinear optical processes induced by the pulse through introduction of a phase that causes destructive nonlinear optical interference in a calculated manner, without use of a genetic learning algorithm, prior to the amplification.

12. The method of claim 1, further comprising using an optical member to compensate for high order dispersion and self-phase modulation in a fiber amplifier.

13. A method of using a laser, the method comprising:
(a) shaping a high peak intensity laser beam pulse, which is substantially unchirped, with a minimal correlation binary phase function without symmetry;
(b) amplifying the pulse after step (a); and
(c) shaping the amplified pulse with a substantially identical minimal correlation binary phase function.

14. The method of claim 13, wherein the amplifying of the high peak intensity pulse is conducted with chirping requirements that are reduced by at least two orders of magnitude and without damage to an amplifier.

15. The method of claim 13, further comprising using the prior minimal correlation binary phase function to prevent nonlinear optical processes from being created in the pulse which would otherwise damage the amplifier.

16. The method of claim 13, further comprising transmitting the amplified high peak intensity pulse through an optical fiber.

17. The method of claim 13, further comprising sending the pulse between an oscillator and an output of the amplifier without passing through a linear chirp stretcher or compressor.

18. The method of claim 13, further comprising using a double pass optical path for the pulse wherein the pulse follows a first direction through a pulse shaper, through an amplifier and to a reflective optic, and then reverses to follow a second and substantially opposite direction back through the amplifier and pulse shaper.

19. The method of claim 13, further comprising using pulse shaping to break the single high intensity pulse into multiple less intense pulses prior to the amplification.

20. The method of claim 13, further comprising using pulse shaping to deter second or higher order nonlinear optical processes by the pulse through destructive interference in a predetermined manner, without use of a genetic learning algorithm, prior to the amplification.

21. The method of claim 13, further comprising emitting the pulse from a fiber oscillator and amplifying the pulse with a fiber amplifier.

22. A laser system comprising:
a laser oscillator emitting a high peak intensity laser pulse;
at least one optical member introducing a phase function into the high peak intensity laser pulse to suppress nonlinear optical processes in the pulse; and
an amplifier amplifying the high peak intensity laser pulse which is substantially unchirped prior to the amplification, the amplifier being located downstream of the optical member;
the at least one optical member introducing the same phase function to the high peak intensity laser pulse after amplification.

23. The laser system of claim 22, wherein the at least one optical member includes a single pulse shaper.

24. The laser system of claim 22, wherein the at least one optical member includes two pulse shapers, one located before the amplifier and the other located after the amplifier.

25. The laser system of claim 22, wherein the at least one optical member includes a optimally bent mirror and a phase mask located adjacent each other.

26. The laser system of claim 22, wherein the phase function is a minimal correlation binary phase function applied on a spectrum of the pulse without symmetry.

27. The laser system of claim 22, wherein the at least one optical member separates the high intensity pulse into multiple less intense pulses prior to the amplification.

28. The laser system of claim 22, further comprising:
a reflective member located downstream of the amplifier;
the at least one optical member, the amplifier and the reflective member being arranged in a double pass manner wherein the pulse interacts with the same at least one optical member before and after it passes through the amplifier.

29. The laser system of claim 22, further comprising an optical fiber transmitting the amplified pulse and a communications receiver operably receiving the pulse from the optical fiber to decode a communications signal carried in the pulse.

30. The laser system of claim 22, wherein the pulse is substantially unchirped and is sent from the oscillator to the amplifier without the addition of positive or negative chirp through a stretcher or compressor, but the substantially unchirped high intensity laser pulse does not damage the amplifier.

31. A laser system comprising:
a substantially unchirped laser pulse having a duration of less than 10picoseconds;
at least one pulse shaper introducing a minimal correlation binary phase function into the pulse; and
an amplifier amplifying the unchirped pulse after the introduction of the phase function;
the at least one pulse shaper neutralizing the minimal correlation binary phase function of the unchirped pulse after the amplification.

32. The laser system of claim 31, wherein the at least one pulse shaper includes a mirror and a phase mask having binary depressions therein.

33. The laser system of claim 31, wherein the at least one pulse shaper changes the pulse into multiple less intense sub-pulses prior to the amplification.

34. The laser system of claim 31, further comprising:
a reflective member located downstream of the amplifier;
the at least one pulse shaper, the amplifier and the reflective member being arranged in a double pass manner wherein the pulse interacts with the same at least one pulse shaper before and after it passes through the amplifier.

35. The laser system of claim 31, further comprising an optical fiber transmitting the amplified pulse and a communications receiver operably receiving the pulse from the optical fiber to decode a communications signal carried in the pulse.

36. The laser system of claim 31, wherein the substantially unchirped pulse is sent from an oscillator to the amplifier without passing through a stretcher or compressor, the substantially unchirped pulse has an intensity of at least 1 kW, but does not damage the amplifier due to the minimal correlation binary phase function effects in the pulse without symmetry.