Abstract:
A compact signal source including: a semiconductor-based, pulsed optical energy source for providing a series of pulses at a given frequency; a selector being optical fiber coupled to the pulsed optical energy source and for down-selecting the pulses to a lower frequency; a stretcher being optical fiber coupled to the selector and for temporally stretching the selected pulses; at least one semiconductor-based optical amplifier being optical fiber coupled to the stretcher and for amplifying the selected pulses; a compressor being optical fiber coupled to the at least one semiconductor-based amplifier and for temporally compressing the amplified, stretched, selected pulses; and, a portable housing containing the pulsed optical energy source, stretcher, at least one semiconductor-based optical amplifier and compressor.

Description:
RELATED APPLICATIONS  
       [0001]     This Application claims priority of U.S. patent application Ser. No. 60/571,355, filed May 15, 2004, entitled COMPACT SEMICONDUCTOR-BASED CHIRPED-PULSE AMPLIFICATION SYSTEM, and is a continuation-in-part application of U.S. patent application Ser. No. 10/859,553, filed Jun. 1, 2004 entitled COMPACT, HIGH-POWER, LOW-JITTER, SEMICONDUCTOR MODELOCKED LASER MODULE, the entire disclosures of each of which are hereby incorporated by reference as if being set forth in their respective entireties herein. 
     
    
     GOVERNMENT RIGHTS  
       [0002]     This invention was made with Government support under Contract No. MDA-972-03-C-0043 awarded by DARPA. The Government has certain rights in this invention. 
     
    
     FIELD OF INVENTION  
       [0003]     The present invention relates generally to optical systems, and more particularly to photonic systems.  
       BACKGROUND OF THE INVENTION  
       [0004]     Semiconductor-based optical sources are desired in many applications, due in part to their compact and transportable nature, high operating speeds, and relative low cost, for example. Optical pulse signals having energies in the nano-joule (nJ) and micro-joule (pJ) range may be particularly useful in microscopy, high frequency (e.g., THz) signal generation and/or micro-machining applications, for example. However, when generating and amplifying short optical pulses using semiconductor-based sources, optical peak intensities may conventionally be sufficiently high to cause significant nonlinear pulse distortion and/or damage or destroy the semiconductor gain medium.  
         [0005]     There are applications that require, or would otherwise benefit from, a compact source of nJ or μJ-level, high repetition-rate, short duration (e.g., picosecond (ps)) optical pulses, such as material modification, non-thermal ablation, electromagnetic pulse directed energy, and others.  
       SUMMARY OF INVENTION  
       [0006]     A compact signal source including: a semiconductor-based, pulsed optical energy source for providing a series of pulses at a given frequency; a selector being optical fiber coupled to the pulsed optical energy source and for down-selecting the pulses to a lower frequency; a stretcher being optical fiber coupled to the selector and for temporally stretching the selected pulses; at least one semiconductor-based optical amplifier being optical fiber coupled to the stretcher and for amplifying the selected pulses; a compressor being optical fiber coupled to the at least one semiconductor-based amplifier and for temporally compressing the amplified, stretched, selected pulses; and, a portable housing containing the pulsed optical energy source, stretcher, at least one semiconductor-based optical amplifier and compressor. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0007]     Understanding of the present invention will be facilitated by consideration of the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings, wherein like numerals refer to like parts and:  
         [0008]      FIG. 1  illustrates a block-diagrammatic representation of a system according to an aspect of the present invention;  
         [0009]      FIG. 2  illustrates signal processing according to an aspect of the present invention;  
         [0010]      FIG. 3  illustrates a block-diagrammatic representation of a system according to an aspect of the present invention;  
         [0011]      FIG. 4  illustrates a graphical representation of output signal intensity versus wavelength for a system according to an aspect of the present invention;  
         [0012]      FIG. 5  illustrates a graphical representation of output signal intensity versus time for a system according to an aspect of the present invention;  
         [0013]      FIG. 6  illustrates a graphical representation of a system configuration according to an aspect of the present invention; and,  
         [0014]      FIG. 7  illustrates a graphical representation of a device according to an aspect of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for purposes of clarity, many other elements found in typical optical systems and methods of making and using the same. Those of ordinary skill in the art will recognize that other elements are desirable and/or required in order to implement the present invention. However, because such elements are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements is not provided herein.  
         [0016]     According to an aspect of the present invention, chirped pulse amplification (CPA) may be used in combination with a semiconductor-based (e.g., diode) laser source to provide a high peak power, short duration optical pulse generating laser system. CPA may be used to provide high peak power laser pulses by temporally stretching (chirping) ultrashort pulses prior to amplification. This effectively reduces the peak power to an acceptable level so as to efficiently extract energy from an optical amplifier without damaging the gain material. After amplification, the chirp is removed and the signal temporally re-compressed to provide short duration, high-power pulses. Typically low-gain solid state gain media are utilized with upper-state lifetimes much greater than the stretched pulse duration. Multi-pass amplifier systems are utilized to extract optical energy, resulting in large-scale laser systems.  
         [0017]     According to an aspect of the present invention, semiconductor based, high efficiency, high gain, compact amplifiers may be used in combination with extreme CPA (x-CPA) techniques to provide a stretched pulse that is longer than the upper-state lifetime, such that energy extraction beyond the saturation energy can be achieved. X-CPA is a variant on CPA technique, in which high-gain short upperstate lifetime diode amplifier is utilized as the gain media. X-CPA is discussed in “X-CPA (extreme chirped pulse amplification)—Beyond The Energy Storage Limit Of Semiconductor Gain Media”, by Kyungbum Kim, Shinwook Lee, Delfyett, P. J., Jr., Lasers and Electro-Optics, 2004, (CLEO), ISBN: 1-55752-777-6, the entire disclosure of which is hereby incorporated by reference herein. Briefly, pulse stretching is used mainly for high energy extraction as the stretched pulse duration is longer than the upper-state lifetime, allowing pulse amplification over many lifetimes. Further, if pulse repetition rate is such that stretched pulses nearly overlap, utilized semiconductor amplifier experience CW as opposed to pulsed optical injection.  
         [0018]     Referring now to  FIG. 1 , there is shown a system  100  according to an aspect of the present invention. System  100  generally includes an in-line source and X-CPA system, and may be packaged in a compact enclosure, such as an enclosure having an interior volume of less than about 1500 cubic inches (in 3 ), for example.  
         [0019]     More particularly, the illustrated system  100  includes an actively locked, high-frequency mode-locked laser (MLL) source  110  (that may provide a pulse-train on the order of about 1 GHz or higher), a pulse selector  150  (that may down-select the pulse-train to be on the order of about 1.5 MHz), a fiber Bragg grating (FBG) stretcher  160  (that may provide temporal pulse stretching on the order of about 500 ps/nm), cascaded pulse-bias semiconductor optical amplifiers (SOAs)  180 ,  220 , and a FBG compressor  250  (that may provide temporal pulse compression pulse stretching on the order of about −500 ps/nm). Such as system may produce 10 pJ, 50 ps pulse trains with a 1 MHz repetition rate, for example. Pulse energies of 10 nJ or higher energies may be achievable by incorporating an Erbium Doped Fiber Amplifier (EDFA)  240  prior to compressor FBG  250 , for example. Elements  110 ,  150 ,  160 ,  180 ,  220  (optionally  240 ) and  250  may be communicatively coupled together using polarization maintaining (PM), single-mode optical fiber patch cables, for example.  
         [0020]     Referring now to  FIG. 2 , there is shown a series of graphical representations of signals that may be processed according to an aspect of the present invention. More particularly, source  110  may provide a signal  115  having a plurality of pulses of about 1 ps duration and at a 1 pulse/ns repetition rate. Pulse selector  150  may down-convert signal  115  to provide a signal  155  having a plurality of pulses at an about 1 pulse/ps repetition rate. Stretcher  160  may temporally stretch each of the pulses of signal  155  to have a duration of about 1 nsec or greater in signal  165 . Amplifiers  180 ,  220  (and optionally  240 ) may amplify signal  165  to provide amplified pulse containing signal  205 . Finally, FBG compressor  250  may temporally recompress the amplified pulses of signal  205  to have a duration on the order of about 1 psec or less in signal  255 , thus converting the amplification energy into higher-peak, shorter duration pulse envelopes.  
         [0021]     As will be understood by those possessing an ordinary skill in the pertinent arts, the example of  FIG. 2  is for non-limiting purposes of explanation only. Pulses of other durations may be effectively used. For example, signal  115  may include pulses having fs to ps durations. Signal  155  may include pulses having ns to ps durations. Signal  165  may include pulses having durations greater than a ns. And, signal  255  may include pulses having fs to ps durations.  
         [0022]     Referring now to  FIG. 3 , there is shown a block diagrammatic view of a system  100 ′ according to an aspect of the present invention. Like elements in systems  100  ( FIG. 1 ) and  100 ′ ( FIG. 3 ) have been identically labeled for clarity of discussion.  
         [0023]     The illustrated system  100 ′ includes a pulse source  110 . By way of non-limiting example only, pulse source  110  may take the form of a low-capacitance, curved-waveguide containing semiconductor source. Source  110  may incorporate two-section gain elements and angle-striped semiconductor optical amplifiers. Such a source is disclosed in co-pending U.S. patent application Ser. No. 10/859,553, entitled “COMPACT, HIGH-POWER, LOW-JITTER, SEMICONDUCTOR MODELOCKED LASER MODULE”, the entire disclosure of which is hereby incorporated by reference herein. Such a source may be packaged within standard sized butterfly packages utilizing lensed-tipped single-mode fiber, for example.  
         [0024]     Source  110  may provide a high-power, low-jitter pulse train to an isolator  120 . For example, the mode-locked laser (MLL) source  110  may provide 15 ps, 2 nm bandwidth pulses with a 1.5 GHz repetition rate. Isolator  120  may serve to prevent reflections from the remainder of system  100 ′ from adversely affecting source  110 . Source  110  may be coupled to isolator  120  using polarization maintaining (PM), single-mode optical fiber, for example. Isolator  120  may take the form of a dual stage component providing greater than about 45 dB optical isolation. For example, isolator  120  may take the form of a commercially available Faraday isolator, such as model no. PDSI-2-56-P-1-4-L-1, which is available from Novawave Technologies.  
         [0025]     Isolator  120  may feed a polarization control component  130 . Polarizer  130  may be coupled to isolator  120  using polarization maintaining, single-mode optical fiber, for example. Polarizer  130  may serve to better ensure that the pulse-train provided by source  110  includes electromagnetic energy of a single polarization well-suited for amplification. Polarizer  130  may take the form of a commercially available polarizer, such as model no. PC100-15-F/A, which is commercially available from Fiberpro, for example. The polarized pulse-train may be provided to amplifier  140 . Amplifier  140  may be coupled to polarizer  130  using polarization maintaining, single-mode optical fiber, for example.  
         [0026]     Amplifier  140  may take the form of a semiconductor optical amplifier (SOA). SOA  140  may include a single-mode, ridge-guided structure operating at a center wavelength of about 1560 nm and having a bandwidth greater than about 20 nm, and introducing a small signal gain on the order of about 25 dB or more. Such a device may present a seeded, saturated output power greater than about 10 mW.  
         [0027]     Amplifier  140  may be packaged in a form that allows insertion into transportable, fiberized, optical systems. The amplifier package may be of a 14-pin “butterfly” variety, containing thermoelectric (TE) based cooling and a Kovar mounting plate. The SOA and a thermistor for facilitating temperature control may be bonded to a patterned aluminum nitride submount, which is attached to the Kovar mounting plate. Lensed optical fiber may be attached to a Kovar clip and sub-micron aligned to SOA emission before being attached in place (via laser welding, for example). Thermal cycling/repositioning of fiber weld allows for rigid positioning of fiber lens tip with respect to the SOA. Wirebonds to package pin configurations allow for outside electrical connections to the hermetically sealed package.  
         [0028]     Select ones of the amplified pulses output from amplifier  140  may be provided to a pulse temporal stretching device  160 . For example, amplifier  140  may be coupled via a polarization maintaining, single-mode optical fiber to a pulse selector  150 , in turn coupled via a polarization maintaining, single-mode optical fiber to temporal stretching device  160 . Selector  150  may serve to down-convert the pulse repetition frequency of pulses provided by source  110  and amplified by amplifier  140 , such as by selectively passing one out of every 1000 optical pulses received to stretching device  160 .  
         [0029]     By way of further non-limiting example, 15 ps, 2 nm bandwidth pulses with a 1.5 GHz repetition rate may be down-selected by selector  150  to a 1.5 MHz repetition rate using a LiNbO 3  modulator. A 1.5 MHz triggering signal for selectively picking amplified pulses to pass for stretching may be derived from the source  110  master 1.5 GHz signal, divided by a factor of 1000 using two trigger countdown circuits, for example. The lower repetition rate pulse-train allows for extraction of higher pulse energy from a 100 milli-watt (mW) class Erbium Doped Fiber Amplifier (EDFA), for example. Pulse selector  150  may take the form of a commercially available device, such as a device utilizing a high-speed Mach-Zehnder (MZ) modulator with pulsed-bias. For example, modulator model no. AZ-0k1-12-PFA-PFA-UL, which is commercially available from EOSpace and pulse bias source AVM-1-P which is commercially available from Avtech, may be used. As will be understood by those possessing an ordinary skill in the pertinent arts, due to the high repetition rate of pulse provide by source  110 , pulse down-selection is performed prior to pulse stretching to mitigate the deleterious effects that would otherwise result from temporally adjacent pulses overlapping after stretching.  
         [0030]     Stretching device  160  may take the form of a chirped, fiber Bragg grating (FBG). As is understood by those possessing an ordinary skill in the pertinent arts, Chirped Fiber Bragg Gratings (CFBG) are an extension of FBG commonly used to stabilize, and select a single optical tone from a laser. The grating “chirp” (controlled, linear increase or decrease in grating period) allows for reflection of a continuous band of wavelength. Due to the grating chirp, different wavelength components satisfy the Bragg condition at different points of propagation into the fiber grating. This results in a time delay of reflection of the various spectral-band components, such that an initially Fourier transform limited ultrashort pulse propagating into the CFBG results in an output pulse having a temporal spread in bandwidth, and a broadened, i.e., stretched output pulse. Characteristics of CFBG include degree of chirp linearity and uniformity of spectral reflection. Such a FBG has a dispersion of around 500 ps/nm, centered at 1563 nm with a 4 nm reflection band or greater. Where source  110  includes a harmonically mode locked laser (MLL), an intra-cavity tunable filter may be used to facilitate matching the MLL center wavelength and full bandwidth to the stretcher  160  FBG band. By way of further, non-limiting example only, the aforementioned down-selected 1.5 MHz pulses may be stretched to have durations of about 1.2 ns using FBG  160  in combination with optical circulators to separate input/output pulse streams. Following stretching, pulse energy may be on the order of about 0.1 pJ/pulse, for example.  
         [0031]     Stretcher  160  may be coupled using a polarization maintaining, single-mode optical fiber to a polarizer  170 . Like polarizer  130 , polarizer  170  may serve to better ensure that the propagating pulse-train includes electromagnetic energy of a single polarization well-suited for further processing. Polarizer  130  may take the form of a commercially available polarizer, such as model no. PC1100-15-F/A, which is commercially available from Fiberpro, for example. The polarized pulse-train may be provided to an amplifier  180 . Amplifier  180  may be coupled to polarizer  170  using polarization maintaining, single-mode optical fiber, for example.  
         [0032]     Like amplifier  140 , amplifier  180  may take the form of a packaged semiconductor optical amplifier (SOA). SOA  180  may include a single-mode, ridge-guided structure operating at a center wavelength of about 1560 nm and having a bandwidth greater than about 20 nm, and introducing a small signal gain on the order of about 25 dB or more. Such a device may present a seeded, saturated output power greater than about 10 mW. Amplifier  180  may be coupled via polarization maintaining, single-mode optical fiber to an isolator  190 .  
         [0033]     Like isolator  120 , isolator  190  may serve to prevent reflections from the remainder of system  100 ′ from adversely affecting those elements discussed heretofore. Isolator  190  may take the form of a dual stage component providing greater than about 45 dB optical isolation. For example, isolator  190  may take the form of a commercially available Faraday isolator, such as model no. PDSI-2-56-P-1-4-L-1, which is available from Novawave Technologies Isolator  190  may be communicatively coupled to a filter  200  using polarization maintaining, single-mode optical fiber.  
         [0034]     Filter  200  may take the form of a pass-band filter, for example. In the illustrated system  100 ′, filter  200  may provide for pass-band filtering on the order of 7-10 nm also centered at the source center wavelength. This may serve to remove ASE components and other optical noise components outside the band of interest that may adversely affect downstream amplifiers. Filter  200  may be communicatively coupled to a polarizer  210  using polarization maintaining, single-mode optical fiber. For example, model no. TF-11-11-1555/1565-9/125-S-40-3A3A-1-7-SP-CSP, which is commercially available from Oz Optics may be used.  
         [0035]     Like polarizer  130 , polarizer  210  may serve to better ensure that the propagating pulse-train includes electromagnetic energy of a single polarization well-suited for further processing Polarizer  130  may take the form of a commercially available polarizer, such as model no. PC1100-15-F/A, which is commercially available from Fiberpro, for example The polarized pulse-train may be provided to amplifier  220  using polarization maintaining, single-mode optical fiber, for example.  
         [0036]     Like amplifiers  140 ,  180 , amplifier  220  may take the form of a packaged semiconductor optical amplifier (SOA). SOA  220  may include a single-mode, ridge-guided structure operating at a center wavelength of about 1560 nm and having a bandwidth greater than about 20 nm, and introducing a small signal gain on the order of about 25 dB or more. Such a device may present a seeded, saturated output power greater than about 10 mW.  
         [0037]     Commercial current pulsers delivering 800 mA, 12 ns drive pulses may be used to drive amplifiers  140 ,  180  and/or  220 . Of course, other current pulser schemes may be used though. The current pulsers provide drive pulses being temporally synchronized with the stretched optical pulses such that the SOA amplifiers are powered only during the times that pulse amplification is intended to occur, i.e., to coincide with the arrival of the low duty cycle stretched optical pulse stream.  
         [0038]     Amplifier  220  may be communicatively coupled to a pass-band filter  230  using polarization maintaining, single-mode optical fiber. Like filter  200 , filter  230  may provide for pass-band filtering on the order of 7-10 nm also centered at the source center wavelength. This may serve to remove ASE components and other optical noise components outside the band of interest that may adversely affect downstream amplifiers. For example, model no. TF-11-11-1555/1565-9/125-S-40-3A3A-1-7-SP-CSP, which is commercially available from Oz Optics may be used.  
         [0039]     Thus, according to an aspect of the present invention, the stretched pulse may be amplified in two packaged, cascaded pulse-bias SOA amplifiers  180 ,  220 . This amplification may be to around a level of about 20 pJ/pulse (as opposed to the 0.1 pJ/pulse energy provided by stretcher  160 ). Pulse-biasing and pass band optical filtering may mitigate background amplified spontaneous emissions (ASE), that may otherwise deteriorate system performance.  
         [0040]     According to an aspect of the present invention, the filtered output from filter  230  may be provided via polarization maintaining, single-mode optical fiber to an amplifier  240  for further amplification. Amplifier  240  may take the form of a 10 mW class EDFA. Due to low duty cycle, 10 mW average power produces pulses with nanojoules of energy. EDFA  240  may take the form of a pre-amplification amplifier and power amplifier. EDFA  240  may provide amplification on the order of greater than about 30 dB and seeded, saturation powers greater than 10 mW, such as up to about 400 mW or more.  
         [0041]     Amplifier  240  may be communicatively coupled via polarization maintaining, single-mode optical fiber to an FBG compressor  250 . Compressor  250  may be a matching compressor for stretcher  160 , i.e. similarly fabricated CFBG operated such as to provide the opposite pulse dispersion—so as to remove the temporal effects introduced by stretcher  150 . As will be understood by those possessing an ordinary skill in the pertinent arts, with access to both fiber ends, a single CFBG may be utilized as both the stretcher and compressor. However, due to out of band optical power coupling, independent CFBG may be desirable.  
         [0042]     Following re-compression by compressor  250 , 50 ps, 7 nJ/pulse (11 mW average power) may be obtainable, limited by EDFA  240  ASE, for example. Increased seed energy or mid-span EDFA filtering may optionally be used to achieve higher pulse energy extraction from EDFA  240 , for example.  
         [0043]     Referring now to  FIG. 4 , there is shown a pulse spectrum after EDFA amplification and FBG compression. Referring now to  FIG. 5 , there is shown a sampling scope profile after EDFA amplification and FBG compression. As may be ascertained therefrom, according to an aspect of the present invention a coherent (i.e., compressible) broadband signal may be provided. As will be appreciated by those possessing an ordinary skill in the art, bandwidth of the MLL source is preserved.  
         [0044]     Referring now to  FIG. 6 , there is shown a configuration  600  according to an aspect of the present invention. Configuration  600  may include each of the elements illustrated in and discussed with regard to  FIG. 3 . For example, configuration  600  may include a source  110 , pulse selector  150 , FBG stretcher  160  and FBG compressor  250  in the illustrated relative positions. The other elements of  FIG. 3  may be positioned in a polarization, filtering and amplification region  610 . Configuration  600  may be well suited for being placed within an enclosure. By way of non-limiting example only, configuration  600  may be suitable for being placed in an enclosure measuring about 7 inches (dimension A)×about 13.375 inches (dimension B)×about 13 inches (dimension C).  
         [0045]     Referring now to  FIG. 7 , there is shown a compact and portable source device  700  according to an aspect of the present invention. Device  700  may incorporate configuration  600  of  FIG. 6 , and hence system  100 ′ of  FIG. 3 , for example. Device  700  may include a panel  705 , that provides fiber outputs (e.g., monitor taps for system diagnostics)  710 - 760 . Tap  710  may provide a signal associated with source  110  (e.g., signal  115 ,  FIG. 2 ). Tap  720  may provide an output associated with pulse selector  150  (e.g., a signal tapped from between isolator  120  and polarizer  130 ). Tap  730  may also provide an output associated with pulse selector  150  (e.g., a signal tapped from between selector  150  and stretcher  160 , e.g., signal  155  of  FIG. 2 ). Tap  740  may provide an output associated with FBG stretcher  160  (e.g., signal  165 ,  FIG. 2 ). Tap  750  may provide an output associated with compressor  250  (e.g., a system output or a signal tapped before or after compressor  250 ). Tap  760  may provide an output associated with amplifiers  180 ,  220  (e.g., a signal tapped from between filter  200  and polarizer  210 ).  
         [0046]     Another panel (not shown), such as a panel being oppositely disposed from panel  705 , may provide electrical connections for the elements of system  600 . Such a compact CPA mode locked laser system incorporating packaged semiconductor gain elements may be used to produce 7 nJ, 50 ps output pulses at a 1.5 MHz repetition rate, for example.  
         [0047]     It will be apparent to those skilled in the art that various modifications and variations may be made in the apparatus and process of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modification and variations of this invention provided they come within the scope of the appended claims and their equivalents.