Patent Application
20090273828
1. Technical Field
The present invention relates generally to the field of light amplification and, particularly to systems useful in athermal ablation.
2. Description of Related Art
An ultra-short pulse (USP) laser emits pulses with a temporal pulse length in the range of picoseconds (psec, 10−12 seconds) to femtoseconds (fsec, 10−15 seconds) resulting in a very high electric field for a short duration of time. Typical techniques for generating these ultra-short pulses are well known. Generally, large systems, such as Ti:Sapphire, are used for generating ultra-short pulses.
USP phenomena were first observed in the 1970's. It was discovered that mode-locking a broad-spectrum laser could produce ultra-short pulses. As produced, an ultra-short pulse has significantly lower power compared to optical pulses having greater temporal lengths. When high-power, ultra-short pulses are desired, the pulses are intentionally lengthened temporally, or chirped, prior to amplification to avoid damaging system components. This process is referred to as chirped pulse amplification (CPA). Subsequent to chirping and amplification, the pulse is compressed temporally to obtain both high peak power and ultra-short pulse duration.
Generally, ablation refers to removal of material, for example, by an erosive process. Lasers can be implemented to ablate material in a selective manner. Two broad classes of laser ablation are thermal and athermal. Thermal ablation is dependent of thermal effects, such as melting. Athermal ablation can occur when an ultra-short pulse is focused on a material as a result of the high electric fields associated with the ultra-short pulse. There are several advantages of athermal ablation over other means of material removal. Compared to conventional mechanical machining, athermal ablation permits more accurate removal without mechanical damage of surrounding material. Conventional laser machining (e.g., thermal ablation), which uses continuous wave (cw) or long-pulsed lasers (e.g., pulse durations greater than roughly 1 nsec, or nanoseconds, 10−9 seconds) can be more precise and flexible as compared to mechanical machining, but can damage surrounding materials. Material removal by athermal ablation is especially useful for medical purposes, either in-vivo or on the outside surface (e.g., skin or tooth), as it is generally painless.
Despite the advantages of athermal ablation, there is a trade-off between average pulse power and pulse quality. Higher pulse powers enable higher material removal rates, but are subject to pulse aberrations and distortions. Conversely, lower pulse powers result in low material removal rates that render the technique impractical for most applications.
In one embodiment, a system may comprise an optical pulse stretcher, an optical splitter, an optical amplifier, and an optical pulse compressor. The optical pulse stretcher may be configured to chirp an optical pulse to produce a chirped optical pulse. The optical splitter may be configured to optically split the chirped optical pulse to produce a plurality of split optical pulses. The optical amplifier may be configured to optically amplify one of the plurality of split optical pulses to produce an optically amplified split optical pulse. The optical pulse compressor may be configured to compress the optically amplified split optical pulse to produce a compressed optically amplified split optical pulse.
In another embodiment, a method may comprise optically splitting a chirped optical pulse to produce a plurality of split optical pulses, optically amplifying one of the plurality of split optical pulses to produce an optically amplified split optical pulse, and optically compressing the optically amplified split optical pulse to produce a compressed optically amplified split optical pulse.
An ultra-short pulse (USP) laser system emits optical pulses resulting in a very high electric field for an ultra-short short period of time. In this context, “ultra-short” refers to durations in the range of picoseconds (psec, 10−12 seconds) to femtoseconds (fsec, 10−15 seconds). Although the peak power of a USP may be high, the average power contained by the USP may be relatively low, as a result of the pulse duration being ultra-short.
As discussed herein, the optical pulse 110 generated by the seed source 105 may have a small average power and require subsequent amplification for certain applications. Prior to amplification, the pulses may be temporally stretched, or “chirped,” by an optical pulse stretcher 115. Chirping the pulse reduces the peak power and permits subsequent amplification without damage to the optical amplifiers and other system components. Temporal pulse stretching may be achieved with various grating and/or prism arrangements, although other methods exist and are known in the art. In one embodiment, the optical pulse 110 propagates through a thick slab of glass to be stretched temporally. In another embodiment, the optical pulse stretcher 115 may include an optical fiber.
After a chirped optical pulse 120 is produced by the optical pulse stretcher 115, the chirped optical pulse 120 may be amplified by an optical amplifier 125. The optical amplifier 125 may be a component that amplifies the optical power of the pulse directly without converting it to an electrical signal. According to various embodiments, the optical amplifier 125 may be a single component or include a serial array of amplifiers, where the output of one amplifier is received directly by the input of another amplifier and so on. In other embodiments, the optical amplifier 125 may include any combination of laser amplifiers, optical fiber based optical amplifiers (e.g., doped fiber amplifier), semiconductor optical amplifiers, Raman amplifiers, and/or parametric optical amplifiers.
After an optically amplified chirped optical pulse 130 is produced by the optical amplifier 125, the optically amplified chirped optical pulse 130 may be compressed temporally by an optical pulse compressor 155. Temporal compression of an optical pulse may be achieved using similar approaches as may be used with the optical pulse stretcher 115 (e.g., grating, prism, and/or fiber configuration). According to an exemplary embodiment, a compressed optically amplified optical pulse 160 produced by the optical pulse compressor 155 may have duration similar to the duration of the optical pulse 110 (i.e., ultra-short duration) and with a peak power increased by several orders of magnitude. Finally, a delivery system 185 may receive the compressed optically amplified optical pulse 160 and deliver it to a location. In some embodiments, the delivery system 185 may include, for example, optical fibers, focusing optics, beam modulators, and beam steerers.
In one alternative embodiment, the optical splitter 235 may be a temporal splitter. The temporal splitter may direct different pulses from a high-repetition pulse train into different fibers. The temporal splitter may result in reduced loss of optical power at the optical splitter 235. One skilled in the art will recognize that in some embodiments, the temporal splitter may comprise an acousto-optic switch or a series of binary switches.
Subsequent to the optically amplified chirped optical pulse 130 being split by the optical splitter 235, each of the plurality of split optical pulses 240 may be received by a separate optical amplifier 245. The optical amplifiers 245 may have any number of physical configurations. The configuration illustrated in
Each of the plurality of split optical pulses 240 may have a reduced peak power relative to that of the optically amplified chirped optical pulse 130. To regain the power lost as a result of splitting, the plurality of split optical pulses 240 may be further amplified. In the embodiment illustrated in
Following amplification by the optical amplifiers 245, the optically amplified split optical pulse 250 may be temporally compressed by an optical pulse compressor 255. In an exemplary embodiment, the optical pulse compressor 255 may include a plurality of individual optical pulse compressors (e.g., similar to the optical pulse compressor 155), each of which may separately receive a pulse. A compressed optically amplified split optical pulse 260 may be produced by the optical pulse compressor 255. The compressed optically amplified split optical pulse 260 may have duration similar to the optical pulse 110, but with much higher peak power. The compressed optically amplified split optical pulse 260 may then be received by a delivery system 285.
The delivery system 285 may include a plurality of independent delivery systems which may each be similar to the delivery system 185. The delivery system 285 may deliver one or more of the compressed optically amplified split optical pulses 260 to at least one location. The delivery system 285 is discussed further herein.
Further, in the embodiment illustrated in
Additionally, in the embodiment illustrated in
Various other embodiments at least include substituting or combining the components illustrated in
Since each of the pair of split optical pulses 440 has a reduced power relative to the optically amplified chirped optical pulse 130, the pair of split optical pulses 440 may be further amplified. Subsequent to the optically amplified chirped optical pulse 130 being optically split by the optical splitter 235, each of the pair of split optical pulses 440 may be received by an optical amplifier 445. According to the embodiment illustrated in
Following optical amplification by the optical amplifiers 445, each of the pair of optically amplified split optical pulses 450 may be temporally compressed by an optical pulse compressor 455. Each of the optical pulse compressors 455 may include at least one optical pulse compressor similar to the optical pulse compressor 155. A pair of compressed optically amplified split optical pulses 460 may be produced by the optical pulse compressors 455. Each of the pair of compressed optically amplified split optical pulses 460 may have duration similar to the optical pulse 110, but with much higher peak power.
According to various embodiments, a pair of optical pulses may have approximately orthogonal polarization relative to one another to facilitate polarization combination. In the optical amplification system 400, the polarization orientation of one of the pair of compressed optically amplified split optical pulses 460 may be rotated by approximately 90 degrees by a polarization rotator 465. The polarization rotator may include any number of polarization rotating elements (e.g., a ½-wave plate). According to another embodiment, the polarization rotation of one of the pair of compressed optically amplified split optical pulses 460 may be achieved by physically rotating an optical fiber which contains the pulse. A pair of compressed optically amplified split optical pulses 470 results, having approximately orthogonal polarization relative to one another. In
Subsequent to polarization rotation, the pair of compressed optically amplified split optical pulses 470 may be polarization combined by, for example, a polarization combiner 475. According to various embodiments, the polarization combiner 475 may be fiber-based or a bulk element. The polarization combined pulse 480 may be received by a delivery system 485.
According to various embodiments, a delivery system, such as the delivery system 285, delivery system 385, and delivery system 485, may include any combination of optical fibers, focusing optics, beam modulators, and beam steerers.
According to other embodiments, a delivery system, such as the delivery systems 285, 385, and 485, may be configured to focus the plurality of compressed optically amplified split optical pulses to different areas, for example, as illustrated in
In yet another embodiment, illustrated in
In alternative embodiments, the delivery systems 285, 385, and 485 may include a temporal splitter. The temporal splitter may combine different pulses from, for example, different fibers. As mentioned herein, one skilled in the art will recognize that in some embodiments, the temporal splitter may comprise an acousto-optic switch or a series of binary switches. Additionally, one skilled in the art will further recognize that a spatial or temporal optical splitter may be located at other positions in the optical amplification systems described herein (e.g., between the optical amplifier 345 and the optical pulse compressor 355), in accordance with some embodiments.
At step 620, the optically amplified chirped optical pulse is optically split to produce a plurality of split optical pulses (e.g., the plurality of split optical pulses 240). As discussed in detail herein, step 620 may be performed by an optical splitter, such as the optical splitter 235.
At step 630, at least one of the plurality of split optical pulses is optically amplified to produce an optically amplified split optical pulse (e.g., the optically amplified split optical pulse 250). As discussed in detail herein, step 630 may be performed by an optical amplifier, such as one of the optical amplifiers 245 and the optical amplifier 345.
At step 640, the optically amplified split optical pulse is optically compressed to produce a compressed optically amplified split optical pulse (e.g., the compressed optically amplified split optical pulse 260). As discussed in detail herein, step 640 may be performed by an optical pulse compressor, such as the optical pulse compressor 255 and the optical pulse compressor 355.
At step 650, the polarization of one of two compressed optically amplified split optical pulses is rotated by approximately 90 degrees to produce a pair of approximately orthogonally polarized compressed optically amplified split optical pulses (e.g., the pair of compressed optically amplified split optical pulses 470). As discussed in detail herein, step 650 may be performed by a polarization rotator, such as polarization rotator 465.
At step 660, the pair of approximately orthogonally polarized compressed optically amplified split optical pulses is polarization combined. As discussed in detail herein, step 660 may be performed by a polarization combiner, such as polarization combiner 475.
As mentioned herein, the process shown in the flowchart 600 is exemplary. For example, steps 650 and 660 may be omitted according to some embodiments. In other embodiments, steps may be added which describe certain delivery techniques as may be implemented by delivery systems, such as the delivery systems 285, 385, and 485.
Those skilled in the art would appreciate that waveguides other than optical fibers may be used for some or all components of the optical amplification systems discussed herein. Examples of other waveguides may include planar, or “chip-based,” waveguides. These waveguides may have a substantially rectangular cross-section and allow the same or similar guiding techniques to be utilized as with traditional optical fiber.
The above description is illustrative and not restrictive. Many variations of the invention will become apparent to those of skill in the art upon review of this disclosure. The scope of the invention should, therefore, be determined not with reference to the above description, but instead should be determined with reference to the appended claims along with their full scope of equivalents.
