WO2004114473A2 - Controlling pulse energy of an optically-pumped amplifier by repetition rate - Google Patents

Controlling pulse energy of an optically-pumped amplifier by repetition rate Download PDF

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Publication number
WO2004114473A2
WO2004114473A2 PCT/US2004/015835 US2004015835W WO2004114473A2 WO 2004114473 A2 WO2004114473 A2 WO 2004114473A2 US 2004015835 W US2004015835 W US 2004015835W WO 2004114473 A2 WO2004114473 A2 WO 2004114473A2
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Prior art keywords
pulse
pulses
amplifier
time
repetition rate
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PCT/US2004/015835
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French (fr)
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WO2004114473A3 (en
Inventor
Peter Delfyett
Richard Stoltz
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Raydiance, Inc.
The University Of Central Florida Research Foundation
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Priority to US47192203P priority Critical
Priority to US47197103P priority
Priority to US60/471,922 priority
Priority to US60/471,971 priority
Priority to US60/494,102 priority
Priority to US49427203P priority
Priority to US49410203P priority
Priority to US49432103P priority
Priority to US60/494,272 priority
Priority to US60/494,321 priority
Priority to US60/503,578 priority
Priority to US50357803P priority
Priority to US50365903P priority
Priority to US60/503,659 priority
Priority to US60/529,425 priority
Priority to US52942503P priority
Priority to US60/546,065 priority
Priority to US54606504P priority
Application filed by Raydiance, Inc., The University Of Central Florida Research Foundation filed Critical Raydiance, Inc.
Publication of WO2004114473A2 publication Critical patent/WO2004114473A2/en
Publication of WO2004114473A3 publication Critical patent/WO2004114473A3/en

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    • AHUMAN NECESSITIES
    • A61MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61BDIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B18/00Surgical instruments, devices or methods for transferring non-mechanical forms of energy to or from the body
    • A61B18/18Surgical instruments, devices or methods for transferring non-mechanical forms of energy to or from the body by applying electromagnetic radiation, e.g. microwaves
    • A61B18/20Surgical instruments, devices or methods for transferring non-mechanical forms of energy to or from the body by applying electromagnetic radiation, e.g. microwaves using laser
    • AHUMAN NECESSITIES
    • A61MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61BDIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B17/00Surgical instruments, devices or methods, e.g. tourniquets
    • A61B2017/00017Electrical control of surgical instruments
    • A61B2017/00022Sensing or detecting at the treatment site
    • A61B2017/00057Light
    • AHUMAN NECESSITIES
    • A61MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61BDIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B18/00Surgical instruments, devices or methods for transferring non-mechanical forms of energy to or from the body
    • A61B2018/00571Surgical instruments, devices or methods for transferring non-mechanical forms of energy to or from the body for achieving a particular surgical effect
    • A61B2018/00577Ablation
    • AHUMAN NECESSITIES
    • A61MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61BDIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B18/00Surgical instruments, devices or methods for transferring non-mechanical forms of energy to or from the body
    • A61B2018/00636Sensing and controlling the application of energy
    • AHUMAN NECESSITIES
    • A61MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61BDIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B18/00Surgical instruments, devices or methods for transferring non-mechanical forms of energy to or from the body
    • A61B2018/00636Sensing and controlling the application of energy
    • A61B2018/00904Automatic detection of target tissue
    • AHUMAN NECESSITIES
    • A61MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61BDIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B18/00Surgical instruments, devices or methods for transferring non-mechanical forms of energy to or from the body
    • A61B18/18Surgical instruments, devices or methods for transferring non-mechanical forms of energy to or from the body by applying electromagnetic radiation, e.g. microwaves
    • A61B18/20Surgical instruments, devices or methods for transferring non-mechanical forms of energy to or from the body by applying electromagnetic radiation, e.g. microwaves using laser
    • A61B2018/2035Beam shaping or redirecting; Optical components therefor
    • AHUMAN NECESSITIES
    • A61MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61BDIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B90/00Instruments, implements or accessories specially adapted for surgery or diagnosis and not covered by any of the groups A61B1/00 - A61B50/00, e.g. for luxation treatment or for protecting wound edges
    • A61B90/06Measuring instruments not otherwise provided for
    • A61B2090/061Measuring instruments not otherwise provided for for measuring dimensions, e.g. length

Abstract

The present invention provides a method for controlling pulse energy of an optically-pumped amplifier by repetition rate. For example, the present invention provides a method of material removal by optical-ablation with controlled pulse energy, and with an essentially fixed ablation spot size by utilizing an optical oscillator in the generation of a series of wavelength-swept-with-time pulses, amplifying the selected wavelength-swept-with-time pulses with a optically-pumped-amplifier, controllably varying pulse repetition rate to control pulse energy, and time-compressing the amplified pulse and illuminating a portion of an object (e.g., a body) with the time-compressed optical pulse. Preferably, the pulse energy is measured and used to control pulse energy density to a set point. Alternatively, the present invention can generate a series of wavelength-swept-with-time pulses at a fixed repetition rate and periodically select pulses from the oscillator generated series of wavelength-swept-with-time pulses.

Description

CONTROLLING PULSE ENERGY OF AN OPTICALLY-PUMPED AMPLIFIER

BY REPETITION RATE

This patent application claims priority to the following previously filed United States provisional patent applications:

Docket US Serial

Number Titlq Number Filing Date

ABI-1 Laser Machining 60/471,922 05/20/2003

ABI-7 Stretched Optical Pulse Amplification and 60/471,971 05/20/2003

Compression

AB1-8 Controlling Repetition Rate Of Fiber Amplifier 60/494,102 08/11/2003

ABI-12 Fiber Amplifier With A Time Between Pulses Of 60/494,272 08/11/2003

A Fraction Of The Storage Lifetime

ABI-13 Man-Portable Optical Ablation System 60/494,321 08/11/2003

ABI-22 Active Optical Compressor 60/503,659 09/17/2003

ABI-23 Controllώg Optically-Pumped Optical Pulse 60/503,578 09/17/2003

Amplifiers ABI-28 Quasi-Contin-uoua Current in Optical Pulse 60/529,425 12/12/2003

Amplifier Systems

ABI-34 Pulse Streaming of Optically-Pumped Amplifier 60/546,065 02/18/2004

ABI-35 Pumping of Optically-Pumped Amplifiers 60/548,216 02 26/2004

Technical Field

The present invention relates in general to the field of light amplification and, more particularly, to the control of pulse energy of an optically-pumped amplifier.

Background Art Ablative material removal is especially useful for medical purposes, either in-vi vo or on the outside surface (e.g., skin or tooth), as it is essentially non-thermal and generally painless. Moreover, ablative material removal essentially exerts no pressure on the surface of the material, so it is quite useful for many other types of cutting and machining. Ablative material removal is generally done with a short optical pulse that is stretched amplified and then compressed. A number of types of laser amplifiers have been used for the amplification, including fiber amplifiers. Fiber amplifiers have a storage lifetime of about 100 to 300 microseconds. hile some measurements have been made at higher repetition rates, these measurements have shown an approximately linear decrease in pulse energy. For ablations purposes, fiber amplifiers have been operated with a time between pulses of equal to or greater than the storage lifetime, and thus are generally run a repetition rate of less than 3-10 kHz. Laser ablation is very efficiently done with a beam of very short pulses (generally a pulse-duration of three picoseconds or less). While some laser machining melts portions of the work-piece, this type of material removal is ablative, disassociating the surface atoms.

Techniques for generating these ultra-short pulses are described, e.g., in a book entitled "Femtosecond Laser Pulses" (C. Rulliere - editor), published 1998, Springer- Verlag Berlin

Heidelberg New York. Generally large systems, such as Ti: Sapphire, are used for generating ultra-short pulses (USP). When high-power pulses are desired, they are often intentionally lengthened before amplification to avoid thermally-induced internal component optical damage. USP phenomenon was first observed in the 1970's, when it was discovered that mode-locking a broad-spectrum laser could produce ultra-short pulses. The minimum pulse duration attainable is limited by the bandwidth of the gain medium, which is inversely proportional to this minimal or Fourier-transform-limited pulse duration. Mode- locked pulses are typically very short and will spread (i.e., undergo temporal dispersion) as they traverse any medium. Subsequent pulse-compression techniques are often used to obtain USP's. Pulse dispersion can occur within the laser cavity so that compression techniques are sometimes added intra-cavity. Previous approaches have generally operated maximum-sized amplifiers at maximum power and amplified longer and longer pulses. When high-power pulses are desired, they are intentionally lengthened before amplification to avoid internal component optical damage. This is referred to as "Chirped Pulse Amplification" (CPA). The pulse is subsequently compressed to obtain a high peak power (pulse-energy amplification and pulse-duration compression).

Summary of the Invention

It has been found that ablative material removal with a very short optical pulse is especially useful for medical purposes and can be done either in-vivo or on the body surface, as it is essentially non-thermal and generally painless. Typically the surgical ablation event has a threshold of less than 1 Joule per square centimeter, but occasionally removal of foreign material may require dealing with an ablation threshold of up to about 2 Joules per square centimeter. Likewise, the present invention can be used for ablation laser-machining wherein at least one 0.01 to 10 microsecond-long train of pulses are generated. Each pulse has a pulse-duration of 50 femtoseconds to three picoseconds with the pulses being at intervals of 1 to 20 nanoseconds.

Directing a beam of the pulses to a work-piece with a pulse-energy-density of 0.1 to 20 Joules/square centimeter can produce one or more holes in the work-piece. In some embodiments, perforations (adjacent holes) are produced to allow removal of a portion of the work-piece. In one embodiment, the train of pulses are 0.05 to 1 microsecond-long, the pulse-duration is 50 femtoseconds to 1 picoseconds, and the pulses at intervals are 1 to 10 nanoseconds. Still further, the pulse-energy-density is preferably between 0.1 and 8

Joules/square centimeter on the work-piece and there are multiple holes and the holes are 20 to 100 micron holes on centers 15 to 200 microns.

As a result, the present invention provides a method of ablation laser-machining that generates at least one 0.01 to 10 microsecond-long train of pulses having a pulse- duration of less than three picoseconds pulses at intervals of 1 to 20 nanoseconds, and directs a beam of the pulses to a work-piece with a pulse-energy-density of 0.1 to 20 Joules/square centimeter to ablate at least one a portion of the work-piece. Preferably, the train of pulses is generated by one or more semiconductor-chip diodes, but fiber amplifiers may also be used.

Since ablation is most efficient at about three times the material's ablation threshold, it is very desirable to control the pulse energy density. If the spot size is fixed or otherwise known, this can be achieved by controlling pulse energy; or if the pulse energy is known, by controlling spot size. The present invention provides a novel control for pulse energy that is much more convenient than changing the ablation spot size. It has been found that optically-pumped amplifiers are more effective operated at a fraction (e.g., less than about half) of their maximum stored energy. When operated in this manner, the pulse energy can be varied by controlling the repetition rate.

As a result, the pulse energy of optically-pumped amplifiers can be done effectively by controlling repetition rate. Preferably, this is done by periodically selecting pulses from an oscillator operating a higher repetition rate. For example, selecting every 5th, 6th, 7th, 8l , 9* , or 10l pulse gives step-wise adjustment of the optically-pumped amplifier repetition rate (l/5th, l/6th, 1/7 th, 1/8 , l/9th, l/10th, of the oscillator repetition rate). It is preferable that the oscillator repetition rate be much higher than the optically-pumped amplifier repetition rate to allow fine adjustment. An oscillator to optically-pumped- amplifier repetition rate ratio variable between 100 and 1,000 can give energy control in steps of less than 1%.

In one example, the ablation rate is controllable independent of pulse energy. The use of more than one amplifier in a train mode (where pulses from one amplifier are delayed to arrive one or more nanoseconds, or a few picoseconds, after those from any other amplifier) allows step- wise control of ablation rate independent of pulse energy. At lower desired ablation rates, one or more amplifiers can be shut off (e.g., by stopping the optical pumping to the optically-pumped amplifier), and there will be fewer pulses per train. Thus with 20 amplifiers there would be a maximum of 20 pulses in a train, but most uses might use only tliree or four amplifiers and three or four pulses per train. Alternately, while quasi-continuous wave operation might generally be used in operating amplifiers, amplifiers might be run in a staggered fashion, e.g., on for a first period and then turned off for one second period, and a first period dormant amplifier turned on during the second period, and so forth, to spread the heat load. The use of train-mode amplifiers provides faster ablation, while providing greater cooling surface area to minimize thermal problems. In addition, one or more of the amplifiers can be shut down, allowing more efficient ablation of a variety of materials with different ablation thresholds, as, again, surfaces are most efficiently ablated at an energy density about three times threshold. The present invention provides a method of surgical material removal from a body by optical-ablation with controlled pulse energy by utilizing an optical oscillator in the generation of a series of wavelength-swept-with-time pulses at a fixed repetition rate, periodically selecting pulses from the oscillator generated series of wavelength-swept- with-time pulses, wherein the fraction of pulses selected can be controllably varied to give a selected pulse repetition rate that is a fraction of the oscillator repetition rate, amplifying the selected wavelength-swept-with-time pulse with a optically-pumped-amplifier amplifying the selected wavelength-swept-with-time pulse with a optically-pumped- amplifier, wherein controlling the pulse selection controls amplified pulse energy, and time-compressing the amplified pulse and illuminating a portion of the body with the time- compressed optical pulse, whereby controlling the pulse selection controls the pulse energy.

In addition, the present invention provides a method of material removal by optical- ablation with controlled pulse energy, and with an essentially fixed ablation spot size by utilizing an optical oscillator in the generation of a series of wavelength-swept-with-time pulses, amplifying the selected wavelength-swept-with-time pulses with a optically- pumped-amplifier, controllably varying pulse repetition rate to control pulse energy, and time-compressing the amplified pulse and illuminating a portion of an object (e.g., a body) with the time-compressed optical pulse. Preferably, the pulse energy is measured and used to control pulse energy density to a set point.

The present invention also provides a method of surgical material removal by optical-ablation with controlled pulse energy, and with an essentially fixed ablation spot size by utilizing an optical oscillator in the generation of a series of wavelength-swept- with-time pulses, controlling pulse repetition rate within a range that the amplifier operates at a fraction of its maximum stored energy, amplifying wavelength-swept-with-time pulse with a optically-pumped-amplifier, and time-compressing the amplified pulse in a pulse compressor and illuminating a portion of the body with the time-compressed optical pulse, whereby controlling the pulse repetition rate within a range that the amplifier operates at a fraction of its maximum stored energy allows the controlling of pulse repetition rate to control amplified pulse energy.

Moreover, the present invention provides a method of material removal by optical- ablation with controlled pulse energy by utilizing an optical oscillator in the generation of a series of wavelength-swept-with-time pulses, controlling pulse repetition rate within a range that the amplifier operates at a fraction of its maximum stored energy, amplifying wavelength-swept-with-time pulse with a optically-pumped-amplifier, and time- compressing the amplified pulse in a pulse compressor and illuminating a portion of an object with the time-compressed optical pulse, whereby controlling the pulse repetition rate within a range that the amplifier operates at a fraction of its maximum stored energy enables controlling the pulse repetition rate to control amplified pulse energy.

Preferably, the selected pulse repetition rate is equal to, or less than 1/10th the oscillator pulse repetition rate, and more preferably the selected pulse repetition rate is between l/100th and l/l,000th of the oscillator pulse repetition rate. In some embodiments, the oscillator, amplifier and compressor are within a man-portable system, and/or the compression is done in an air-path between gratings compressor. Preferably the compressed optical pulse has a sub-picosecond duration, and the oscillator pulse has a duration between 10 picoseconds and one nanosecond. The ablation can be from an outside surface of the body or done inside of the body. In some embodiments, more than one amplifier are used in a mode where amplified pulses from one amplifier are delayed to arrive one or more nanoseconds after those from any other amplifier, to allow control of ablation rate independent of pulse energy.

Preferably the pulse energy density applied to the body is between 2.5 and 3.6 times ablation threshold of the body portion being ablated. The system can be run either with dynamic feedback from measurement of pulse energy with a control point being varied for materials of different ablation thresholds, or open-loop.

The amplifying and compressing can be done with an optically-pumped-amplifier and air-path between gratings compressor combination, e.g., with the amplifying pulses of between 10 picoseconds and one nanosecond, and then compressing to sub-picosecond duration. With the novel system of the present invention, this can be a man-portable system (e.g., in a cart and/or a backpack).

The optically-pumped amplifier can be an erbium-doped fiber amplifier, and the air-path between gratings compressor preferably is a Tracey grating compressor. Preferably, more than one optically-pumped amplifiers are used in parallel, or more than one semiconductor optical amplifiers are used in parallel. More than one optically-pumped amplifier may be used with one compressor.

High ablative pulse repetition rates are preferred and the total pulses per second

(the total system repetition rate) from the one or more parallel optical amplifiers is preferably greater than 0.6 million.

Brief Description of the Drawings

No Figures.

Description of the Invention

While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention.

To facilitate the understanding of this invention, a number of terms are defined below. Terms defined herein have meanings as commonly understood by a person of ordinary skill in the areas relevant to the present invention. Terms such as "a", "an" and "the" are not intended to refer to only a singular entity, but include the general class of which a specific example may be used for illustration. The terminology herein is used to describe specific embodiments of the invention, but their usage does not delimit the invention, except as outlined in the claims.

Laser machining is most efficiently done with a beam of very short pulses (generally a pulse-duration of three picoseconds or less) in a controlled range of energy density (generally about 0.1 to 20 Joules/square centimeter, and preferably 0.1 to 8

Joules/square centimeter). While lasers can remove a slit of material, e.g., 500 microns wide, it has been found that most cutting tasks on most materials (including metals), can be much more efficiently done as a line of small diameter perforations (e.g., 25 micron holes on 40 micron centers), which allows the material to break cleanly along the line with little or no application of additional force. Thus, the amount of material that needs to be removed is greatly reduced and the small spot size reduces the required power and allows machining with smaller and less expensive lasers (including portable semiconductor-chip- diode systems). Perforation machining with tapered holes is also much more practical as channeling of the energy within a hole generally causes the hole diameter to taper down with depth (while the hole diameter can be made relatively constant, this is generally unnecessary and consumes more energy).

It has also been found that with the controlled energy density, however, that in many instances, holes formed by a single pulse (and often even several pulses) do not sufficiently penetrate the work-piece to give a clean break. Further, due to the small diameter of the laser beam, relative motion (e.g., vibration) between the laser beam and the work-piece can prevent successive pulses from hitting the same hole, thus preventing sufficient penetration. Even in other laser machining, e.g., when the surface is being ablated, rather than a hole produced, movement such as vibration can cause uneven ablation. Note that other uses such as surgical procedures can use surface ablation or cutting, and can use overlapping ablation to produce a cut surface, rather than a series of holes. In all such uses, a train of pulses is preferably generated by one or more semiconductor-chip diodes. Note also, the train of pulses allows a quasi-continuous wave operation that improves system efficiency, e.g., lessening the number of power up-ramps and down-ramps.

Typically a line of laser-produced holes (including a circle of small holes to create a large hole) is desired. There are, however, applications where a single laser-produced hole completely penetrating a work-piece is desired. Again, vibration or other motion can interfere with efficient production of such a hole.

It has also been found that the smaller and less expensive lasers (e.g., semiconductor-chip diodes) can generate a train of femtosecond pulses at intervals of a few nanoseconds for up to a few microseconds without overheating. As there are generally only a few nanoseconds between the pulses, and as channeling guides energy down the hole even if the beam and hole centerlines are offset by a few microns, relative motion would have to be many times supersonic to prevent multiple pulses from entering each laser-produced hole. One embodiment of the present invention provides a method of perforation laser- machining that includes generating at least one 0.01 to 10 microsecond-long train of pulses, each pulse having a pulse-duration of 50 femtoseconds to three picoseconds, with said pulses being at intervals of 1 to 20 nanoseconds, and directing a beam of said pulses to a work-piece with a pulse-energy-density of 0.1 to 20 Joules/square centimeter to produce one or more holes in the work-piece. The holes may be, e.g., 10 to 150 micron holes on centers 15 to 300 microns. Preferably, the train of pulses are 0.05 to 1 microsecond-long; the pulse-duration is 50 femtoseconds to 1 picoseconds; pulses at intervals are 1 to 10 nanoseconds; and the pulse-energy-density is between 1 and 8 Joules/square centimeter on the work-piece. The holes are preferably 20 to 100 micron holes on centers 15 to 200 microns.

For example, a 100 femtosecond pulse can be time-stretched to make an optical pulse signal ramp (of, e.g., increasing, wavelength) which is amplified (at comparatively low instantaneous power), and time-compressed into an amplified 100 femtosecond pulse. Generally a series of pulses are generated, and thus a series of wavelength-ramps are used (e.g., a "saw-tooth" waveform with 50 "teeth" may be amplified by the SOAs without turning the current off between the teeth). Thus although the amplifiers are amplifying continuously during the 50-tooth waveform, the time-compression will separate the optical output into 50 separate pulses.

Semiconductor laser diodes are highly preferred for generating the ultra-short pulses. Semiconductor laser diodes typically are of III-V compounds (composed of one or more elements from the third column of the periodic table and one or more elements from the fifth column of the periodic table, e.g., GsAs, AlGaAs, InP, InGaAs, or InGaAsP). Other materials, such as II- VI compounds, e.g., ZnSe, can also be used. Typically lasers are made up of layers of different III-V compounds (generally, the core layer has higher index of refraction than the cladding layers to generally confine the light to a core).

Semiconductor lasers have been described (see Rulliere, Chapter 5). It should be noted that this method works especially with semiconductor-chip diodes. Semiconductor-chip diodes can have high efficiency (e.g., about 50%) and have short energy-storage-lifetimes

(e.g., a few nanoseconds). With a small, e.g., 20 micron spot, the ablating energy can be furnished by a single semiconductor optical amplifier (SOA) putting out less than 10 micro- Joules per pulse (which low energy density also limits collateral damage). The other types of lasers (e.g., Ti:sapphire) generally have energy-storage-lifetimes (e.g., in the hundreds of microsecond range), and this is convenient for accumulating energy and releasing it in a short period of time as a high-energy pulse. These other type of lasers have generally been used for generating short, high energy pulses, but their efficiencies are low (generally less than 1%) and the pulse energies drop off rapidly when operated at high repetition rates (when they begin to heat up, and when time between pulses becomes short and starts to reduce the time for accumulating energy for the next pulse). Conversely, semiconductor-chip diodes can provide a microsecond long train of pulses of nearly constant energy with nanosecond spacings. Thus while other types of lasers could be used, semiconductor-chip diodes are preferred.

The examples used herein are to be viewed as illustrations rather than restrictions, and the invention is intended to be limited only by the claims. For example, the invention applies not only to GaAs and InP (which generates light within it III-V semiconductor structure at a wavelength of about 1550 nm) laser diodes, but also to other semiconductor materials such as II- VI compounds.

Ablation is most efficient at about three times the material's ablation threshold, and thus control of pulse energy density for optimum removal efficiency is very desirable. If the spot size is fixed or otherwise known, this can be achieved by controlling pulse energy; or if the pulse energy is known, by controlling spot size. The present invention uses a novel method of controlling the pulse energy by controlling the amplified pulse energy, which is much more convenient than changing the ablation spot size. It has been found that optically-pumped amplifiers are more effective operated at a fraction (e.g., less than about half) of their maximum stored energy. When operated in this manner, the pulse energy can be varied by controlling the repetition rate, as the amount of stored energy in the amplifier increases with the time between pulses. It has been found that in fiber amplifiers, pulse energy control can be done step- wise by controlling repetition rate and can be fine-tuned by controlling optical pumping power. The pulse energy of a semiconductor optical amplifier (SOA) can be adjusted by changing the current thru the amplifier. While the compressors in either type of system can be run with inputs from more than one amplifier, reflections from other of the parallel amplifiers can cause a loss of efficiency, and thus should be minimized (as used herein, "parallel" includes train mode). The loss is especially important if the amplifiers are amplifying signals at the same time, as is the case with the SOAs. Thus each off the parallel SOAs preferably has its own compressor and while the amplified pulses may be put into a single fiber after the compressors, reflections from the joining (e.g., in a star connector) are greatly reduced before getting back to the amplifier. With the fiber amplifiers, however, a nanosecond spacing of sub-nanosecond stretched pulses eliminates any amplifying of multiple signals at the same time, and a single compressor is preferably used. The present invention uses one or more fiber amplifiers of moderate-power, with a short optical pulse that is amplified and then compressed into a very short pulse, and the light pulse focused onto a very small area spot. The system rapidly scans the spot over an area to be ablated and controls the pulse power to maximize ablation efficiency.

The present invention preferably uses parallel amplifiers (optically pumped or semiconductor optical amplifiers (SOAs)) to increase the ablation rate by further increasing the effective repetition rate (while avoiding thermal problems and allowing control of ablation rate by adjusting the number of operating amplifiers), and allowing control of ablation rate by the use of a lesser number of operating amplifiers). The use more than one amplifier in parallel train mode (pulses from one amplifier being delayed to arrive one or more nanoseconds after, or a few picoseconds after, another amplifier. This allows the ablation rate to be controlled largely independent of repetition rate.

The pump diodes and their power supply can be operated efficiently when operated with the electrical current is being supplied continuously (CW). In the past, ablation systems have been operated with laser materials with a long storage lifetime, e.g., one millisecond, and at low repetition rates, e.g., 1 KHz.

It has been found that an optically-pumped amplifier is more effectively operated at higher repetition rates such that the amplifiers reach only a fraction (e.g., less than about half) of their maximum stored energy, and that ablation is most effective an energy density of about three times the ablation threshold for the material being ablated. The system of the present invention preferably uses a moderately short storage lifetime laser material (e.g., Cr.'YAG with a few microsecond storage lifetime), with the amplifiers optically- pumped quasi-CW (e.g., pumping and amplifying in 1 millisecond long streams of pulses with 100 streams per second). During such streams, the pulse repetition rate may be, e.g., 1 MHz, and thus 1,000 pulses per stream.

With quasi-CW, there is a pause between streams, which allows adjustment of the ratio of pause duration to stream duration, e.g., for amplifier temperature control. Operated in this manner, the pulse energy can be optimized by controlling the repetition rate within a range of increased amplifier efficiency, without over-heating system components.

During each stream (but not during the pause), continuous current is supplied to the diodes that supply the optically-pumped amplifier. As the pump diodes are turned off during the pauses, the temperature of the pump diodes can also be limited or otherwise controlled. In the optically-pumped amplifier, the input optical signal is a series pulses (each pulse, e.g., a 1 nanosecond long, wavelength-swept-with-time ramp). The pulses to the amplifier may be continued during the pause as amplification will stop due to lack of pumping, or SOAs used as preamplifiers may be turned off during the pauses to reduce temperature of the SOAs.

Thus, for example, with amplifiers directly pumped by pump diodes, optical pumped amplifiers can be effectively operated to amplify intermittent sub-millisecond streams (or bursts) of 1 MHz pulses (streams separated by pauses). The ratio of streaming time to pause time can be controlled to control amplifier temperature, andor the ablation rate, and/or the temperature of the pump diodes). In the case where the system is to be operated at lower ablation rates, the ratio of streaming-time to pause-time can be controlled to vary the ablation rate.

Quasi-CW operation improves system efficiency. As the number of pulses per stream in CW can be large (there are comparatively few current up-ramps and down- ramps) and there is little lost in efficiency compared to CW, and there are significant increases in energy storage efficiency and the optimizing of pulse energy density. Preferably, 1550 nm light is used both for safety purposes, and for greater pulse compression efficiency. At 1550 nm compression is much more efficient than at shorted wavelengths. With longer distances between elements and/or more diffractions or reflections, higher stretching/compression factors can be obtained. The improved diffracting grating stretching/compressing method of the present invention uses inputting a tilted collimated beam, spatial-spreading the beam, spatial-narrowing the beam, spatial- spreading the beam, spatial-narrowing the beam, and collimating the beam output, wherein the beam hits at least one of the gratings more than once (because of the tilt, the beam hits such a grating at a different line or point each time around). The gratings may be transmissive and/or reflective (a device having transmissive gratings disposed at right angles, but with an un-tilted beam is shown in Figure 4 of U.S. Patent 5,822,097 by Tournois). In systems with larger tilts, e.g., more than % degree, collimating of an intermediate spatially-narrowed beam, then manipulating the beam to correct spatial-chirp can be done with, e.g., a retro-reflector mirror-pair or prism.

In one embodiment, the present invention uses an optically-pumped-amplifier (e.g., a erbium-doped fiber amplifier or a Cr:YAG amplifier) and compressed by an air-path between gratings compressor (e.g., a Tracey grating compressor), with the compression creating a sub-picosecond ablation pulse. Generally a semiconductor oscillator is used to generate pulses and in some embodiments a SOA preamplifier is used to amplify the selected pulses before introduction into the optically-pumped amplifier.

Fiber amplifiers have a storage lifetime of about 100 to 300 microseconds. While measurements have been made at higher repetition rates, these measurements have shown an approximately linear decrease in pulse energy. For ablations purposes, power fiber amplifiers have generally been operated with a time between pulses about equal to than the storage lifetime (or at greater than the storage lifetime, to avoid thermal problems in the fiber), and thus are generally run a repetition rate of less than 3-10 kHz. Optically-pumped amplifiers are available with average power of 30 W or more. A moderate-power 5 W average power optically-pumped amplifiers have been operated to give pulses of 500 microJoules or more, as energy densities above the ablation threshold are needed for non- thermal ablation, and increasing the energy in such a system, increases the ablation rate in either depth or allows larger areas of ablation or both. The present invention, however, generally runs the optically-pumped amplifier with a time between pulses of a fraction (e.g., one-half or less) of the storage lifetime and uses a smaller ablation spot. Preferably, the spot is less than about 50 microns in diameter, but the diameter can be 60 or 75 microns and with sufficient power per amplifier, possibly even more (spot sizes herein are given as circle diameter equivalents, a "50 micron" spot has the area of a 50 micron diameter circle, but the spot need not be round). The smaller spot is preferably scanned to get a larger effective ablation area.

The present invention also preferably uses parallel optically-pumped amplifiers to generate a train of pulses to increase the ablation rate by further increasing the effective repetition rate (while avoiding thermal problems and allowing control of ablation rate by the use of a lesser number of operating optically-pumped amplifiers). The present invention may use a SOA preamplifier to amplify the initial pulse before splitting to drive multiple parallel optically-pumped amplifiers and another SOA before the introduction of the signal into each optically-pumped amplifier (which allows rapid shutting down of individual optically-pumped amplifiers). Further, the present invention generally operates with pulse energy densities at about three times the ablation threshold for greater ablation efficiency.

Although very-high power SOA's can be built, they are quite expensive and generally require large cooling systems. As a result, to be practical, an SOA generally needs somewhat lower power and a longer period of amplification, generally from 1 to 20 nanoseconds, and preferably between 5 and 20 nanoseconds. Air-grating compressors are unpractically large at these time periods, and thus the man-portable SOA amplifier systems use chirped fiber gratings (such gratings are commercially available from 3M). Fiber amplifiers could also use chirped fiber gratings, and generally these fiber gratings can be shorter, with less compression than those used with the SOAs.

The use of a 1 nanosecond selected-pulse with an optically-pumped amplifier and air optical-compressor (e.g., a Tracey grating compressor) typically gives compression with -40% losses. At less than 1 nanosecond, the losses in a Tracey grating compressor are generally lower. If the other-than-compression losses are 10%, 2 nano Joules are needed from the amplifier to get 1 nanoJoule on the target. Preferably, for safety purposes, 1550 nm light is preferably used. The use of much greater than 1 nanosecond selected- pulses in an air optical-compressor, in the past, presented two problems; the difference in path length for the extremes of long and short wavelengths needs to be more 3 cm and thus the compressor is large and expensive and generally not man-portable, and the losses increase with a greater degree of compression.

Ultra-short-pulse ablation can provide efficient material removal with very high ablation rates. Ablative removal of material is generally done with a sub-picosecond optical oscillator pulse that is stretched in duration, amplified, and then compressed. The optical amplifiers are generally power limited thus can give greater pulse energy at longer pulse durations, the compressor gives significant losses of pulse energy of per nanosecond of compression. Air-grating compressors can handle high power and while they can give multiple-nanosecond amplification in a reasonably small size by using multiple-pass configurations, but losses are greatly increased and little is gained. Thus usable pulse power is significantly limited.

This invention can provide a novel "no-pulse-energy-loss" air-grating-compressor configuration that gives a practical pulse compressor that compresses a pulse to sub- picosecond duration without reducing the energy of the pulse. It utilizes a single "active- grating" device that compresses and supplies energy to the pulse to at least make up for the inherent losses during the pulse compression. The active grating can be fabricated by placing a grating on the face of a novel low-gain active mirror.

Air-grating compressors typically bounce the beam between two gratings and use several reflections (e.g., 8) per pass and pulses lose some energy (e.g., 5%) in each reflection. Multiple-pass air-grating compressors also use at least one additional reflection between each pass. Thus such an 8-reflection single-pass grating would get -66% of the input energy of the light out (.958) and the 34% loss/66% output gives losses of about 50% of the output. An 8-reflection active-grating device with similar reflection loss would get losses of about 40% of the output. A conventional two-pass 17-reflection grating would get ~42% of the input energy of the light out and gives losses of about 138% of the output. A 17-reflection two-pass active-grating device with similar reflection loss would get losses of about 85% of the output. In these two-pass configurations the active-grating losses are less than the output energy, while the conventional compressor heat-generating losses are significantly more than the output energy. A conventional 35-reflection four-pass grating would get ~16.6% of the input energy of the light out and gives losses of about 502% of the output. A 35-reflection four- pass active-grating device with similar reflection loss would get losses of about 175% of the output. Thus, in these 35-reflection four-pass configurations, the active grating compressor reduces the losses by almost a factor of three. Even if one uses a low-gain active mirror without a mirror and one conventional grating the losses are significantly reduced compared to the normal two grating compressor.

The active grating can be fabricated by placing a grating on the face of a novel low- gain active mirror. In one embodiment, this low-gain active mirror has a gain of only two or three, compared to the very high pulse energy conventional active mirrors that gains of thousands (see U.S. Patent No. 6,339,605 entitled "Active mirror amplifier system and method for a high-average power laser system" and U.S. Patent No. 6,610,050 entitled

"Laser beam delivery system with multiple focal points"). The '605 Patent describes an active-mirror system (figure numbers deleted) as follows: "Typically, the laser gain medium disk may have a thickness ranging approximately from 1 mm to 10 mm and transverse dimensions ranging from about 10 mm to 300 mm. The material of the laser gain medium disk comprises a suitable solid-state laser gain medium such as, but not limited to neodymum doped yttrium aluminum garnet (Nd:YAG), yitterbium doped yttrium aluminum garnet (Yb:YAG), neodymum chromium codoped gadolinium gallium garnet (Nd:Cr:GGG or "GGG" for short), or neodymum doped glass (Nd:Glass).

"Referring further to FIGS., the back planar surface has a dielectric optical coating with high reflectivity at a laser wavelength and at optical pump wavelengths. The front surface has a dielectric optical coating that is antireflective at the laser wavelength and at the optical pump wavelength. The back surface is in contact with a surface of a cooled, rigid substrate. The surface contains an array of interconnected vacuum microchannels extending generally over, but not beyond, the contact area between the disk and the substrate." The '050 Patent describes a active mirror system in which a master beam produced by the laser source is directed into a lenslet array to partition the master beam into a plurality of beams, with each of the beams having a separate focal point. In one embodiment of the '050 Patent, the lenslets are on an active mirror.

In one embodiment, the active grating of the present invention uses two novel low- power, low-gain active mirrors with Cr:YAG gain media, and with the mirror backsides directly air-cooled. The grating can be etched in the front surface using known semiconductor fabrication lithographic techniques. As in conventional grating compressors, the beam will be fanned out into a broad beam and then focused back into a narrow beam, with different wavelengths having different path lengths and thus different time delays (such that the spreading in time introduced in the stretching is generally eliminated).

In an alternate embodiment of the present invention, one low-gain active mirror with a surface grating and one conventional grating is used. In still another alternate embodiment, one low-gain active mirror without a surface grating and one conventional grating is used.

Note that the active mirror can be controlled by repetition rate and/or diode- pumping current in the manner described (generally using fiber amplifiers as examples) in co-pending provisional applications cited below. Note further that lamp-pumped optical amplifiers can be controlled by controlling lamps in a manner similar to that of controlling pump diode current. Preferably, diode pump-current is used to control the amplification of the active mirror. Generally optical pump device (diode or lamp) current is controlled either directly or indirectly by controlling voltage, power, and/or energy. As used herein, controlling current can include shutting off one or more optical pump devices, when multiple pump devices are used.

The alternate configuration with a semiconductor optical amplifier (SOA) and a with a chirped fiber compressor, and with pulses stretched to 1 to 20 nanosecond during amplification is run at repetition rates with a time between pulses of more that the very short semiconductor storage lifetime. Preferably the present invention uses a semiconductor generated initial pulse. The present invention may use a SOA preamplifier to amplify the initial pulse before splitting to drive multiple amplifiers. The present invention preferably scans the ablation a smaller spot to get a larger effective ablation area, and in many cases the scanned spot is smaller than the above optically-pumped-amplifier case. In addition, the present invention preferably uses parallel amplifiers to generate a train of pulses to increase the ablation rate by further increasing the effective repetition rate (while avoiding thermal problems and allowing control of ablation rate by the use of a lesser number of operating amplifiers).

Generally the present invention operates with pulse energy densities at about three times the ablation threshold for greater ablation efficiency. The system can be run either with dynamic feedback from measurement of pulse energy with a control point being varied for materials of different ablation thresholds, or open-loop. The open-loop control could be a selector switch where the selector switch is used to directly or indirectly select a repetition rate. The selector switch could be a multi-position switch, but could also be a high low switch.

The use more than one amplifier in parallel train mode (pulses from one amplifier being delayed to arrive one or more nanoseconds after those from another amplifier) allows step-wise control of ablation rate independent of pulse energy. At lower desired powers, one or more amplifiers can be shut off (e.g., the optical pumping to a optically- pumped amplifier), and there will be fewer pulses per train. Thus with 20 amplifiers there would be a maximum of 20 pulses in a train, but most uses might use only three or four amplifiers and three or four pulses per train. While CW operation might normally be used for operating amplifiers, amplifiers might be run for e.g., one second and then turned off and a dormant amplifier turned on to spread the heat load.

Generally the fiber amplifiers are optically-pumped CW (and are amplifying perhaps 100,000 times per second in 1 nanosecond pulses). Alternately, non-CW-pumping might be used in operating amplifiers, with amplifiers run in a staggered fashion, e.g., one on for a first half-second period and then turned off for a second half-second period, and another amplifier, dormant during the first-period, turned on during the second period, and so forth, to spread the heat load.

In such systems, the present invention can control input optical signal power, optical pumping power of optically-pumped amplifiers, timing of input pulses, length of input pulses, and timing between start of optical pumping and start of optical signals to control pulse power, and average degree of energy storage in optically-pumped amplifier.

Many fiber amplifiers have a maximum power of 4 MW, and thus a 10-micro Joule- ablation pulse could be as short as 2 picoseconds. Thus e.g., a 10 picosecond, 10 microJoule pulse, at 500 kHz (or 50 microJoule with 100 kHz), and, if heating becomes a problem, operating in a train mode and switching fiber amplifiers. Thus one might rotate the running often fiber amplifiers such that only five were operating at any one time (e.g., each on for 1/10th of a second and off for 1/10th of a second). Again one can have ten fiber amplifiers with time spaced inputs, e.g., by 1 nanosecond, to give a train of one to 10 pulses. With 5 W amplifiers operating at 100 kHz (and e.g., 50 microJoules) this could step between 100 kHz and 1 MHz. With 50% post-amplifier optical efficiency and 50 microJoules, to get 6 J/sq. cm on the target, the spot size would be about 20 microns.

Another alternative is to have 20 optically-pumped amplifiers with time spaced inputs, e.g., by 1 nanosecond, to give a train of one to 20 pulses. With 5 W amplifiers operating at 50 kHz (and e.g., 100 microJoules) this could step between 50 kHz and 1 MHz. With 50% post-amplifier optical efficiency and 100 microJoules, to get 6 J/sq. cm on the target, the spot size would be about 33 microns. The selected pulse might be 50 to 100 picoseconds long. A similar system with 15 optically-pumped amplifiers could step between 50 kHz and 750 kHz. Another alternative is to have 10 optically-pumped amplifiers with time spaced inputs, e.g., by 1 nanosecond, to give a train of one to 20 pulses. With 5 W amplifiers operating at 20 kHz (and e.g., 250 microJoules) this could step between 20 kHz and 200 kHz. With 50%) post-amplifier optical efficiency and 250 microJoules, to get 6 J/sq. cm on the target, the spot size would be about 50 microns. The selected pulse might be 100 to

250 picoseconds long. A similar system with 30 optically-pumped amplifiers could step between 20 kHz and 600 kHz.

Generally it is the pulse generator that controls the input repetition rate of the optically-pumped amplifiers to tune energy per pulse to about three times threshold per pulse.

Another alternative is generating a sub-picosecond pulse and time-stretching that pulse within semiconductor pulse generator to give the wavelength-swept-with-time initial pulse for the fiber amplifier. Another alternative is to measure light leakage from the delivery fiber to get a feedback proportional to pulse power and/or energy for control purposes. Measurement of spot size, e.g., with a video camera, is useful, and can be done with a stationary spot, but is preferably done with a linear scan. Preferably, the spot is less than about 50 microns in diameter.

The camera is preferably of the "in- ivo" type using an optical fiber in a probe to convey an image back to, e.g., a vidicon-containing remote camera body. This is especially convenient with a handheld beam-emitting probe and can supply its own illumination. Other cameras using an optical fiber in a probe to convey an image back to a remote camera body, e.g., a vidicon-containing camera with a GRIN fiber lens, can also be used. Endoscope type cameras can also be used.

Smaller ablation areas may be scanned by moving the beam without moving the probe. Large areas may be scanned by moving the beam over a first area, and then stepping the probe to second portion of the large area and then scanning the beam over the second area, and so on. The scanning may be by beam deflecting mirrors mounted on piezoelectric actuators. Preferably the system actuators scan over a larger region but with the ablation beam only enabled to ablate portions with defined color and/or area. A combination of time and, area and/or color, can be preset, e.g., to allow evaluation after a prescribed time. Ablative material removal is especially useful for medical purposes either in-vivo or on the body surface and typically has an ablation threshold of less than 1 Joule per square centimeter, but may occasionally require surgical removal of foreign material with an ablation threshold of up to about 2 Joules per square centimeter. The use of more than one amplifier in parallel train mode (pulses from one amplifier being delayed to arrive one or more nanoseconds after those from another amplifier. At lower desired powers, one or more amplifiers can be shut off (e.g., the optical pumping to a fiber amplifier), and there will be fewer pulses per train. Thus with 20 amplifiers there would be a maximum of 20 pulses in a train, but most uses might use only three or four amplifiers and three or four pulses per train. While CW operation might normally be used for operating amplifiers, amplifiers might be run for e.g., one second and then turned off and a dormant amplifier turned on to spread the heat load. The present invention also provides a method of system operation for material removal from a body portion being ablated by optical-ablation with controlled pulse energy from an amplifier by utilizing an optical oscillator in the generation of a series of wavelength-swept-with-time pulses, wherein the system has a repetition rate that gives a time between pulses of less than Vi the storage time of the fiber amplifier, amplifying the wavelength-swept-with-time pulse with the fiber-amplifier, controlling pulse energy of the fiber-amplifier pump diodes to give a pulse energy density applied to the body of between 2.5 and 3.6 times ablation threshold of the body portion being ablated, and time- compressing the amplified pulse and illuminating a spot on a portion of an object with the time-compressed optical pulse, wherein the system has a spot size between 10 and 60 microns diameter.

In addition, the present invention provides a method of surgical material removal from a body by optical-ablation with controlled pulse energy from a fiber amplifier by inputting a nominal spot size signal and a pulse-energy-for-material-being-ablated signal, utilizing an optical oscillator in the generation of a series of wavelength-swept-with-time pulses, primarily controlling pulse energy based on the pulse-energy-for-material-being- ablated signal by either selecting pulses from the oscillator generated series of wavelength- swept-with-time pulses, wherein the fraction of pulses selected can be controllably varied to give a selected pulse repetition rate that is a fraction of the oscillator repetition rate, or passing electrical current through at least one pump diode to generate pumping light, optically pumping the fiber amplifier with the pumping light, and controlling pump diode current, using an ablation spot-size sensor to measure the ablation spot size and dynamically adjusting either the fraction of pulses selected or the pump diode current for changes in ablation spot size from the nominal spot size; amplifying the wavelength-swept- with-time pulse with the fiber-amplifier, and time-compressing the amplified pulse and illuminating a portion of the body with the time-compressed optical pulse, whereby controlling the pulse selection controls the pulse energy.

Furthermore, the present invention provides a method of system operation for surgical material removal from a body by optical-ablation with controlled pulse energy from a fiber amplifier by determining a size of a spot to be used by the system, wherein the spot is between 10 and 60 microns diameter; setting a repetition rate to give time between pulses of between l and 1/10th the storage time of the fiber amplifier into the system, inputting a pulse-energy-for-material-being-ablated signal into the system; controlling current through one or more fiber-amplifier pump diodes to give a pulse energy density applied to the body is between 2.5 and 3.6 times ablation threshold of the body portion being ablated, utilizing an optical oscillator in the generation of a series of wavelength- swept-with-time pulses, amplifying the wavelength-swept-with-time pulse with the fiber- amplifier, time-compressing the amplified pulse and illuminating the spot on a portion of the body with the time-compressed optical pulse; and scanning the spot over an area, whereby removal over the area is even due to the high repetition rate, while the pulse energy is at a near optimum efficiency level. Preferably the fiber-amplifier repetition rate is at least 0.6 million pulses per second. Preferably the ablation spot size is between 20 and 50 microns in diameter. In some preferred embodiments, the ablation spot size is between 20 and 40 microns in diameter.

Moreover, the present invention provides a method of surgical material removal from a body by optical-ablation with controlled pulse energy from a optically-pumped pulse amplifier by inputting a nominal spot size signal and a pulse-energy-for-material- being-ablated signal, utilizing an optical oscillator in the generation of a series of wavelength-swept-with-time pulses, primarily controlling pulse energy based on the pulse- energy-for-material-being-ablated signal by either selecting pulses from the oscillator generated series of wavelength-swept-with-time pulses, wherein the fraction of pulses selected can be controllably varied to give a selected pulse repetition rate that is a fraction of the oscillator repetition rate, or passing electrical current through at least one pump diode to generate pumping light, optically pumping the optically-pumped pulse amplifier with the pumping light, and controlling pump diode current; using an ablation spot-size sensor to measure the ablation spot size and dynamically adjusting either the fraction of pulses selected or the pump diode current for changes in ablation spot size from the nominal spot size, amplifying the wavelength-swept-with-time pulse with the fiber- amplifier, and time-compressing the amplified pulse and illuminating a portion of the body with the time-compressed optical pulse, whereby controlling the pulse selection controls the pulse energy. The present invention also provides a method of ablative material removal, from a surface or with a short optical pulse that is amplified and then compressed by generating an initial pulse in a pulse generator within a man-portable system, amplifying the initial pulse and then compressing the amplified pulse within the man-portable system, wherein the amplifying and compression are done with either a fiber-amplifier and a 10 picosecond- 1 nanosecond pulse-compressor combination, or a SOA and chirped fiber compressor combination, and applying the compressed optical pulse to the surface.

The amplifying and compressing can be done with a fiber-amplifier and air-path between gratings compressor combination, e.g., with the oscillator pulses of between 10 picoseconds and one nanosecond, or the amplifying and compressing can be done with a chirped fiber compressor combination, e.g., with the amplified pulses between 1 and 20 nanoseconds in duration.

In one embodiment, repetition rate is used to control pulse energy, the pre- compression optical amplifier's temperature is controlled by, and an active mirror is used in the compressor with the amplification of the active mirror being controlled by current of the active mirror's pump-diodes.

Generally a semiconductor oscillator is used to generate pulses and in some embodiments a SOA preamplifier is used to amplify the selected pulses before introduction into the optically-pumped pulse amplifier. In one embodiment, sub-picosecond pulses of between 10 picoseconds and one nanosecond are used, followed by pulse selection, with the selected pulses amplified by a fiber-amplifier (e.g., a erbium-doped optically-pumped pulse amplifier or EDFA) and compressed by an air-path between gratings compressor (e.g., a Treacy grating compressor), with the compression creating a sub-picosecond ablation pulse.

Compressors could be run with overlapping inputs from more than one amplifier, but reflections from other of the parallel amplifiers can cause a loss of efficiency. With the optically-pumped pulse amplifiers, a nanosecond spacing of sub-nanosecond pulses minimizes amplifying of multiple signals at the same time, and a single compressor is preferably used. High ablative pulse repetition rates are preferred and the total pulses per second (the total system repetition rate) from the one or more parallel optical amplifiers is preferably greater than 0.6 million.

Another alternative is generating a sub-picosecond pulse and time-stretching that pulse within semiconductor pulse generator to give the wavelength-swept-with-time initial pulse for the optically-pumped pulse amplifier. Another alternate is to measure light leakage from the delivery fiber to get a feedback proportional to pulse power and/or energy for control purposes.

Note also that optically-pumped optical pulse amplifiers (including, and those used to pump other optical devices) in general (including, and in such shapes as slabs, discs, and rods) can be controlled. N ote further that lamp-pumped can be controlled by controlling the pumping lamps in a manner similar to that of controlling pump diode current. Preferably, active-diode diode pump-current is used to control the amplification of an active mirror. Generally optical pump device (diode or lamp) current is controlled either directly or indirectly by controlling voltage, power, and/or energy. As used herein, controlling current can include shutting off one or more optical pump devices, when multiple pump devices are used.

The present invention provides a method of system operation, wherein the spot is between 10 and 60 microns diameter, and the system is controlled current through an amplifier or pump diodes, and gives a pulse energy density applied to the body is between 2.5 and 3.6 times ablation threshold of the body portion being ablated. Generally it can utilize an optical oscillator in the generation of a series of wavelength-swept-with-time pulses, amplifying the wavelength-swept-with-time pulse with an amplifier, time- compressing the amplified pulse and illuminating the spot on a portion of the body with the time-compressed optical pulse, and scanning the spot over an area. Preferably the pulse repetition rate is at least 0.6 million pulses per second. Preferably the ablation spot size is between 20 and 50 microns in diameter. In some preferred embodiments, the ablation spot size is between 20 and 40 microns in diameter.

Another alternative is generating a sub-picosecond pulse and time-stretching that pulse within semiconductor pulse generator to give the wavelength-swept-with-time initial pulse for the optically-pumped pulse amplifier. For example, the pulse generator can generate pulses at a 50 MHz rate, and the pulses stretched to 20 nanoseconds. The pulse generator need not be shut down during the pause, as the amplifier will only operate while the current is supplied. At 1550 mn compression is much more efficient than at shorted wavelengths, and long stretches can be used, for example operation with a 100 nanosecond stretch or more may be possible.

A beam of high energy, ultra-short (generally sub-picosecond) laser pulses can literally vaporize any material (including steel or even diamond). Such a pulse has an energy-per-unit-area that ionizes the atoms of spot on a surface and the ionized atoms are repelled from the surface. A series of pulses can rapidly create a deep hole. Some machining applications can be done with small (e.g., 10 to 20 micron diameter) spots, but other applications need larger (e.g., 40 to 100 micron) spots. While solid-state laser systems can supply enough energy (in a form compressible to short-enough pulses) for the larger spot sizes, the efficiency of such systems has been very low (generally less than

1%), creating major heat dissipation problems, and thus requiring very expensive systems that provide only slow machining (due to low pulse repetition rates). This system uses a beam pattern within the amplifier and can essentially eliminate heating due to amplified spontaneous emission. The system can operate at a wavelength such that the optical amplifier can be directly pumped by laser diodes emitting wavelengths of greater than 900 nm, further increasing the efficiency. This system can obtain efficiencies of over 30%, lowering the size and cost of the system and greatly increasing machining speed.

In the past, solid-state amplifiers have either been pumped essentially continuously (e.g., for at least many seconds) or pumped on a pulse-by-pulse basis. To operate the system efficiently at high power, the starting and stopping of current from the pump diode power supply should be a small fraction of the number of pulses, and stream of pulses should be limited to sub-millisecond duration (to limit thermal spikes) and pauses are preferably longer than the solid-state material's storage lifetime, but preferably reasonably short (e.g., sub-second, and more preferably sub-millisecond). The present invention also provides a method of controlling an optical pumped amplifier capable of optical ablation at an ablation rate by directly pumping the amplifier with pump diodes, introducing sub-millisecond streams (bursts) of pulses separated by pauses into amplifier, controlling the ratio of streaming-time to pause-time to control and least one of amplifier-operating-temperature and ablation-rate. The amplifier is operated at a lower than maximum ablation rate, and the ratio of streaming-time to pause-time can be used to vary the ablation rate. Preferably, the pause is sub-millisecond in duration, the pulses 1 to 20 nanoseconds in duration during amplification (and are later compressed to sub-picosecond in duration), and the amplifier is a solid-state optical amplifier, especially a Cr:YAG amplifier.

In some embodiments, current through the pump diodes is used to control pump diode operating temperature. The repetition rate can be controlled to vary energy of the pulses and preferably the repetition rate of pulses within a stream is at least 220 kHz, and more preferably the repetition rate of pulses within a stream is between 230 kHz and 6

MHz.

Generally the amplifier's optical input signal is a series of light wavelength swept with time pulses (wavelength either increasing or decreasing during the pulse). In the case of the optically pumped amplifier, the input optical signal pulses the ramps are end-to- end), and the repetition rate and/or pump-diode current are controlled to prevent the amplifier from exceeding its maximum stored energy. Thus the pumping power and timing between pulses are controlled such that pumping does not saturate the amplifier material and thus ASE is reduced.

In the past, optically pumped optical amplifiers (e.g., solid-state amplifiers) have generally been pumped by lamps, or occasionally, have been pumped by narrow-band (e.g., 30 nm or less) emitting, thermoelectric-cooled, pump diodes. It has now been found that amplifier systems for such use can be more effective using un-cooled pump diodes where the diodes have a much higher efficiency than lamps, and system efficiency is improved by the elimination of power-cooling devices such as thermoelectric coolers. Preferably, the pump diodes are broadband emission (e.g., bandwidth of 50 nm or more) diodes, such as super-luminescent diodes, and preferably the amplifier is a solid-state amplifier, especially a Cr:YAG amplifier (which also has the advantage of a relatively broad absorption spectrum). The solid-state amplifier can contain a co-dopant, and when the amplifier is a Cr:YAG amplifier, Nd can be used as the co-dopant. In some embodiments, the amplifier contains a co-dopant and the diodes are broadband emission diodes whose emission spectrum overlaps the emission of the co-dopant, and the emission of the diode directs the emission of the co-dopant to more uniformly activate the primary (e.g., Cr) dopant.

Note that the optical amplifier is cooled in many embodiments, including by a heat pipe or by forced air. In some embodiments, a fan is intermittently used to cool the diodes. Note that as used herein, the term "un-contiolled-temperature pump diode pumping" means the temperature is not controlled within a narrow temperature range (e.g., within less 5 oC) by a powered "cooler" (such as a thermoelectric cooler which may in some cases be heating, rather than cooling), and not directly or indirectly water-cooled (e.g., not cooled by submersion of the unit in water). In some high power embodiments, broadband pump diodes are controlled only within a wide range (preferably with only intermittent cooling), such as between -25 and

+125 °C. The broadband diodes give more uniform penetration of pump light, do not require precise temperature control (they are effective even if their emission spectrum shifts), and they can direct emission from co-dopants. Ablative material removal previously has been done using systems with optical benches weighing perhaps 1,000 to 2,000 pounds and occupying about 300 cubic feet. The present invention provides a novel system that can weigh less than 100 pounds and occupy less than 2.5 cubic feet. In some embodiments, the man-portable system comprises a cart and or a backpack, in addition to the probe (and connecting cables). The combination of an efficient amplifier system with a small pulse-compressor enables practical, and significant size reduction, which in turn enables a system in accordance with the present invention to be man-portable, e.g., capable of being moved reasonably easily by one person, such as wheeling a wheeled cart from room to room or even being carried in a backpack. It has been found that two laser-amplifier/compressor combinations enable practical, and significant size reduction, which in turn enables the system to be man- portable. A used herein, the term "man-portable" means capable of being moved reasonably easily by one person, e.g., as wheeling a wheeled cart from room to room or possibly even being carried in a backpack. In one embodiment, the present invention uses sub-picosecond pulses stietched to between 10 picoseconds and one nanosecond, with the stietched pulse either amplified by a fiber-amplifier (e.g., a erbium-doped fiber amplifier or EDFA) and compressed by an air-path between gratings compressor (e.g., a Treacy grating compressor), with the compression creating a sub-picosecond ablation pulse. Alternately, the present invention uses a semiconductor optical amplifier (SOA) and a with a chirped fiber compressor, generally with pulses stretched to 1 to 20 nanosecond during amplification. Generally, the present invention uses a semiconductor generated initial sub- picosecond pulse in either case and preferably a chirped fiber stretcher in either case (to reduce system size for man-portability), and preferably uses a SOA preamplifier to amplify the initial pulse before introduction into the fiber amplifier.

Previous approaches have generally operated maximum-sized amplifiers at maximum-power and amplifying longer-and-longer pulses. The present invention provides a much smaller and lighter system. A man-portable unit in accordance with the present invention for use in a hospital might include a handheld probe, a vest and control-cart (e.g., a wheeled cart), and receive 120 V power from a wall plug. The handheld probe can contain beam-scanners and optical delivery fibers. The vest can contain optical compressors, and possibly the optical amplifiers (the amplifiers might also be in the cart). The cart can contain the control module, the control panel, the pulse generator, the power supplies, a video camera, a video monitor, air flush system, suction system, and a marker beam generator.

Generally, optical-fiber-containing umbilical cables are used between pieces (e.g., a probe-vest umbilical and a vest-cart umbilical). The umbilical can include a hollow ablation fiber (for pulses compressed to sub-picosecond duration, hollow optical fibers are preferred), a video-camera fiber, an illumination fiber, a marker-beam fiber, an air flush tube, a suction tube and wiring for the scanners.

Alternately, the battery-powered unit could contain a probe, vest, backpack and one or two satchels. The handheld probe could again contain beam-scanners and optical delivery fibers. The vest could contain optical compressors, the optical amplifiers and control devices (e.g., control knobs, switches, etc., that were on the control panel in the cart). The backpack could contain the contiol module, the pulse generator, the power supplies, a marker beam generator, and a battery pack. A satchel might contain a video camera, a video monitor, an illumination source, and additional batteries. The system might be operable without the satchel, but have additional capabilities including longer operation, with the satchel connected (through a wiring-containing umbilical). In a variation of this alternative, the video camera could be in the backpack and a heads-up display used to provide a video monitor (and a display of control settings) without using a satchel. The handheld probe preferably contains piezoelectrically-driven-mirror beam- scanners and optical delivery fibers. One delivery fiber has a lens on the fiber-end near the probe tip and can transmit a video image back to the video camera (e.g., in the vest, backpack, satchel or cart). Another fiber can convey illumination to the ablation region. A hollow optical fiber can bring ablation pulses to the beam-scanner mirrors. A fiber can also be used to bring a laser marker beam to the beam-scanner mirrors (where it is scanned in the same manner as the ablation beam). While the laser marker beam can show the entire scan area, it is preferably turned off and on by the specifications of area, color, and distance from target, such that it shows the area that would be ablated if the ablation beam were on (again the marker beam can also change color to indicate whether the ablation beam is on or off). The probe can also contain tubes for suction and/or gas flush.

Initially man-portable units may include several pieces, e.g., a handheld probe, handheld probe, vest/backpack and 2 satchels. Such a unit can be relatively inexpensive and might be used by emergency personnel (e.g., EMTs) in the field. The unit can do emergency cutting of a victim and any needed cauterizing of wounds (it can also be run with longer, e.g., microsecond long, thermally-inducing, pulses to cauterize a wound, either with the same, or a different laser). It can also cut through any obstacles in the way of getting to, or freeing the victim, for example cutting the top of a car loose, or cutting through an I-beam. The size and number of pieces may be reduced later, to a handheld probe, vest, and backpack, for example.

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification, but only by the claims.

Claims

1. A method of surgical material removal by optical-ablation with controlled pulse energy, and with an essentially fixed ablation spot size, comprising the steps of: utilizing an optical oscillator in the generation of a series of wavelength-swept- with-time pulses; selecting pulses from the series of pulses to control pulse repetition rate within a range that an amplifier operates at a fraction of its maximum stored energy; amplifying the selected wavelength-swept-with-time pulses with the optically- pumped-amplifier; and time-compressing the amplified pulse in a pulse compressor and illuminating a portion of a body with the time-compressed optical pulses, wherein contiolling the pulse repetition rate within the range operates the amplifier at a fraction of its maximum stored energy and controls amplified pulse energy.
2. The method of claim 1 , wherein an optical oscillator is used in the generation of a series of wavelength-swept-with-time pulses at a fixed repetition rate, and pulses are periodically selected from the oscillator generated series of wavelength-swept-with-time pulses, wherein a fraction of pulses selected can be controllably varied to give a selected pulse repetition rate that is a fraction of the oscillator repetition rate, and the selected wavelength-swept-with-time pulse are amplified with the optically-pumped-amplifier.
3. The method of claim 2, wherein the selected pulse repetition rate is equal to, or less than 1/10th the oscillator pulse repetition rate.
4. The method of claim 2, wherein the selected pulse repetition rate is between 1/100 and l/l,000th of the oscillator pulse repetition rate.
5. The method of claim 1, wherein the oscillator, amplifier and compressor are within a man-portable system.
6. The method of claim 1, wherein the compression is done in an air-path between gratings compressor.
7. The method of claim 1, wherein the compressed optical pulse has a sub-picosecond duration.
8. The method of claim 1, wherein the oscillator pulse has a duration between 10 picoseconds and one nanosecond.
9. The method of claim 1, wherein the ablation is from an outside surface of the body.
10. The method of claim 1, wherein the ablation is done inside of the body.
11. The method of claim 1 , wherein more than one amplifiers are used in a mode where amplified pulses from one amplifier are delayed to arrive one or more nanoseconds after those from any other amplifier, to allow control of ablation rate independent of pulse energy.
12. The method of claim 1, wherein the pulse energy density applied to the body is between 2.5 and 3.6 times ablation threshold of the body portion being ablated.
13. A method of material removal by optical-ablation with controlled pulse energy, and with an essentially fixed ablation spot size, comprising the steps of: utilizing an optical oscillator in the generation of a series of wavelength-swept- with-time pulses; amplifying the selected wavelength-swept-with-time pulse with a optically- pumped-amplifier; contiolling pulse repetition rate to control pulse energy density; and time-compressing the amplified pulse and illuminating a portion of an object with the time-compressed optical pulse.
14. The method of claim 13, wherein the pulse repetition rate is controllably varied to give a pulse energy density applied to the body between 2.5 and 3.6 times ablation threshold of the body portion being ablated
15. The method of claim 12, wherein a selector switch is used to directly or indirectly select a pulse repetition rate.
16. A method of material removal by optical-ablation with controlled pulse energy, comprising the steps of: utilizing an optical oscillator in the generation of a series of wavelength-swept- with-time pulses; controlling pulse repetition rate within a range that the amplifier operates at a fraction of its maximum stored energy; amplifying wavelength-swept-with-time pulse with a optically-pumped-amplifier; and time-compressing the amplified pulse in a pulse compressor and illuminating a portion of an object with the time-compressed optical pulse, whereby controlling the pulse repetition rate within a range that the amplifier operates at a fraction of its maximum stored energy enables controlling the pulse repetition rate to control amplified pulse energy.
PCT/US2004/015835 2003-05-20 2004-05-19 Controlling pulse energy of an optically-pumped amplifier by repetition rate WO2004114473A2 (en)

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US49427203P true 2003-08-11 2003-08-11
US49410203P true 2003-08-11 2003-08-11
US49432103P true 2003-08-11 2003-08-11
US60/494,272 2003-08-11
US60/494,321 2003-08-11
US60/494,102 2003-08-11
US50357803P true 2003-09-17 2003-09-17
US50365903P true 2003-09-17 2003-09-17
US60/503,659 2003-09-17
US60/503,578 2003-09-17
US52942503P true 2003-12-12 2003-12-12
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