Patent Publication Number: US-9431786-B2

Title: System and method for a delivery fiber for isolation against back reflections

Description:
This application is a divisional of and claims benefit of U.S. patent application Ser. No. 13/085,354 filed on Apr. 12, 2011, titled “HIGH-POWER LASER SYSTEM HAVING DELIVERY FIBER WITH NON-CIRCULAR CROSS SECTION FOR ISOLATION AGAINST BACK REFLECTIONS” by Matthias P. Savage-Leuchs (which issued as U.S. Pat. No. 8,736,953 on May 27, 2014), which claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 61/343,947 filed on Apr. 12, 2010, titled “HIGH-POWER LASER SYSTEM HAVING DELIVERY FIBER WITH NON-CIRCULAR CROSS SECTION FOR ISOLATION AGAINST BACK REFLECTIONS” by Matthias P. Savage-Leuchs, U.S. Provisional Patent Application No. 61/343,945 filed on Apr. 12, 2010, titled “Apparatus for optical fiber management and cooling” by Yongdan Hu et al., U.S. Provisional Patent Application No. 61/343,946 filed on Apr. 12, 2010, titled “Beam diagnostics and feedback system and method for spectrally beam-combined lasers” by Tolga Yilmaz et al., U.S. Provisional Patent Application No. 61/343,948 filed on Apr. 12, 2010, titled “HIGH BEAM QUALITY AND HIGH AVERAGE POWER FROM LARGE-CORE-SIZE OPTICAL-FIBER AMPLIFIERS; SIGNAL AND PUMP MODE-FIELD ADAPTOR FOR DOUBLE-CLAD FIBERS AND ASSOCIATED METHOD” by Matthias Savage-Leuchs et al., and U.S. Provisional Patent Application No. 61/343,949 filed on Apr. 12, 2010, titled “METHOD AND APPARATUS FOR IN-LINE FIBER-CLADDING-LIGHT DISSIPATION” by Yongdan Hu, which are each incorporated herein by reference in their entirety. 
    
    
     This invention is related to: 
     U.S. Pat. No. 6,456,756 issued Sep. 24, 2002 to Roy Mead, et al., titled “FIBER RAMAN AMPLIFIER PUMPED BY AN INCOHERENTLY BEAM COMBINED DIODE LASER,” 
     U.S. Pat. No. 7,792,166 issued Sep. 7, 2010 to Lawrence A. Borschowa, titled “APPARATUS AND METHOD FOR DRIVING LASER DIODES”, 
     U.S. Pat. No. 7,620,077 issued Nov. 17, 2009 to Angus J. Henderson, titled “APPARATUS AND METHOD FOR PUMPING AND OPERATING OPTICAL PARAMETRIC OSCILLATORS USING DFB FIBER LASERS”, 
     U.S. Pat. No. 7,701,987 issued Apr. 20, 2010 to Matthias P. Savage-Leuchs et al., titled “APPARATUS AND METHOD FOR GENERATING CHIRP-SLICE CONTROLLED-LINEWIDTH LASER-SEED SIGNALS”, 
     U.S. Pat. No. 7,471,705 issued Dec. 30, 2008 to David C. Gerstenberger et al., titled “ULTRAVIOLET LASER SYSTEM AND METHOD HAVING WAVELENGTH IN THE 200-NM RANGE”, 
     U.S. Pat. No. 7,391,561 issued Jun. 24, 2008 to Fabio Di Teodoro et al., titled “FIBER—OR ROD-BASED OPTICAL SOURCE FEATURING A LARGE-CORE, RARE-EARTH-DOPED PHOTONIC-CRYSTAL DEVICE FOR GENERATION OF HIGH-POWER PULSED RADIATION AND METHOD”, 
     U.S. Pat. No. 7,430,352 issued Sep. 30, 2008 to Fabio Di Teodoro et al., titled “MULTI-SEGMENT PHOTONIC-CRYSTAL-ROD WAVEGUIDES FOR AMPLIFICATION OF HIGH-POWER PULSED OPTICAL RADIATION AND ASSOCIATED METHOD”, 
     U.S. Pat. No. 7,379,648 issued May 27, 2008 to Christopher D. Brooks et al., titled “OPTICAL HOLLOW-CORE DELIVERY FIBER AND HOLLOW-ENDCAP TERMINATION AND ASSOCIATED METHOD”, 
     U.S. Pat. No. 7,386,211 issued Jun. 10, 2008 to Fabio Di Teodoro et al., titled “METHOD AND APPARATUS FOR SPECTRAL-BEAM COMBINING OF MEGAWATT-PEAK-POWER BEAMS FROM PHOTONIC-CRYSTAL RODS”, 
     U.S. Pat. No. 7,400,804 issued Jul. 15, 2008 to Fabio Di Teodoro et al., titled “MONOLITHIC OR RIBBON-LIKE MULTI-CORE PHOTONIC-CRYSTAL FIBERS AND ASSOCIATED METHOD”, 
     U.S. Pat. No. 7,429,734 issued Sep. 30, 2008 to Steven C. Tidwell, titled “SYSTEM AND METHOD FOR AIRCRAFT INFRARED COUNTERMEASURES TO MISSILES”, 
     U.S. Pat. No. 7,199,924 issued on Apr. 3, 2007 to Andrew J. W. Brown et al., titled “APPARATUS AND METHOD FOR SPECTRAL-BEAM COMBINING OF HIGH-POWER FIBER LASERS”, 
     U.S. Pat. No. 7,768,700 issued Aug. 3, 2010 to Matthias P. Savage-Leuchs, titled “METHOD AND APPARATUS FOR OPTICAL GAIN FIBER HAVING SEGMENTS OF DIFFERING CORE SIZES”, 
     U.S. Pat. No. 7,872,794 issued Jan. 18, 2011 to John D. Minelly et al., titled “HIGH-ENERGY EYE-SAFE PULSED FIBER AMPLIFIERS AND SOURCES OPERATING IN ERBIUM&#39;S L-BAND”, 
     U.S. patent application Ser. No. 12/624,327 titled “SPECTRALLY BEAM COMBINED LASER SYSTEM AND METHOD AT EYE-SAFER WAVELENGTHS” filed on Nov. 23, 2009 by Roy D. Mead (which issued as U.S. Pat. No. 8,441,718 May 14, 2013), 
     U.S. Provisional Patent Application 61/263,736 filed Nov. 23, 2009 by Matthias P. Savage-Leuchs et al., titled “Q-switched oscillator seed-source for MOPA laser illuminator method and apparatus”, and 
     U.S. patent application Ser. No. 12/854,868 titled “IN-LINE FORWARD/BACKWARD FIBER-OPTIC SIGNAL ANALYZER” filed on Aug. 11, 2010 by Tolga Yilmaz et al. (which issued as U.S. Pat. No. 8,755,649 on Jun. 17, 2014), 
     which are all incorporated herein in their entirety by reference. 
     FIELD OF THE INVENTION 
     The invention relates generally to optical waveguides and more particularly to delivery waveguides for reducing the amount of back-reflected light that is coupled back into an amplifying or lasing optical fiber and associated damage to high-power master-oscillator power-amplifier (MOPA) laser systems, wherein, in some embodiments, one or more delivery waveguides are formed in a delivery fiber, and the delivery fiber has a non-circular waveguide and a mode-field adaptor/beam collimator that work together to prevent back reflections from a distal end of the delivery fiber from propagating back into the power-amplifier stage, and wherein, in some embodiments, the delivery fiber is connectorized and/or sterilized and is considered disposable and replaceable. 
     BACKGROUND OF THE INVENTION 
     High-power laser systems (for example, laser systems employing a master-oscillator power-amplifier (MPOA) configuration) are subject to damage if a back-reflected signal from a distal end of a delivery fiber re-enters the power-amplifier stage. Typically, circular fibers are used as delivery fibers for pulsed and continuous-wave (CW) laser signals. However, it has been shown (M. Fermann, “Single-mode excitation of multimode fibers with ultrashort pulses”, OPTICS LETTERS/Vol. 23, No. 1/Jan. 1, 1998, which is incorporated herein by reference) that the fundamental mode in a fiber can propagate in multimode fibers over long distances. Therefore, when a reflection occurs after the laser light exits the delivery fiber, laser light can propagate backwards in the delivery fiber and maintain or nearly maintain its beam quality. Consequently, reflected signals can be coupled into the amplifier and lead to damage of the amplifier or laser. To prevent this, conventional systems typically employ an optical isolator that allows only one-directional signal propagation in a direction away from the power amplifier. Such isolators are expensive and complex. 
     U.S. Pat. No. 7,576,909 issued to Harter, et al. on Aug. 18, 2009 titled “Multimode amplifier for amplifying single mode light,” and is incorporated herein by reference. Harter et al. describe techniques for the control of the spatial as well as spectral beam quality of multi-mode fiber amplification of high-peak-power pulses, as well as using such a configuration to replace diode-pumped, Neodynium based sources. Harter et al. assert that perfect spatial beam-quality can be ensured by exciting the fundamental mode in the multi-mode fibers with appropriate mode-matching optics and techniques. The loss of spatial beam-quality in the multi-mode fibers along the fiber length can be minimized by using multi-mode fibers with large cladding diameters. Near diffraction-limited coherent multi-mode amplifiers can be conveniently cladding pumped, allowing for the generation of high average power. Moreover, the polarization state in the multi-mode fiber amplifiers can be preserved by implementing multi-mode fibers with stress producing regions or elliptical fiber cores. 
     U.S. Pat. No. 7,590,323 to Broeng et al. issued Sep. 15, 2009 titled “Optical fibre with high numerical aperture, method of its production, and use thereof” and is incorporated herein by reference. Broeng et al. describe an optical fiber, having at least one core surrounded by a first outer cladding region, the first outer cladding region being surrounded by a second outer cladding region, the first outer cladding region in the cross-section comprising a number of first outer cladding features having a lower refractive index than any material surrounding the first outer cladding features, wherein for a plurality of said first outer cladding features, the minimum distance between two nearest neighboring first outer cladding features is smaller than 1.0 μm or smaller than an optical wavelength of light guided through the fiber when in use; a method of its production, and use thereof. They also describe fibers built from performs having non-circular tubes or rods that form non-circular cores for the collection of light from laser diodes having non-symmetric non-circular beams. 
     A paper in the 30 Oct. 2006 Vol. 14, No. 22 of OPTICS EXPRESS pages 10345-10350 by J. R. Hayes et al. titled “Square core jacketed air-clad fiber” is incorporated herein by reference. Hayes et al. describe fabrication of a highly multi-mode square core jacketed air-clad fiber with a top-hat near-field intensity profile, and using this fiber to deliver Q-switched pulses to ablate square marks on indium tin oxide films. 
     There is a need for improved laser systems, particularly optical-fiber lasers and/or optical-fiber amplifiers having delivery fibers, wherein systems have improved optical isolation to prevent optical feedback due to reflections in the delivery fiber or at its ends from traveling back into, and damaging, the optical-fiber lasers and/or optical-fiber amplifiers. There is also a need to eliminate a conventional isolator to reduce the cost, size and complexity of the system. 
     SUMMARY OF THE INVENTION 
     In some embodiments, the present invention provides a method, an apparatus, and disposable parts thereof for a laser-delivery fiber that has a non-circular core cross-sectional area. In some embodiments, the signal-light output from a relatively small-core gain fiber is directed into a relatively large-core gain fiber that has a non-circular core. In some embodiments, signal light is coupled from a higher brightness fiber to a lower brightness fiber. In some embodiments, the gain fiber can be slightly multimoded. As used herein, the proximal end of the delivery fiber is the light-input end next to the gain fiber of the laser or optical amplifier that provides the signal light, while the distal end is the opposite light-output end at which the signal light is used or directed from. The forward-propagating signal light that enters the proximal delivery fiber is well mixed by the geometry of the core of the delivery fiber, and any reflections from the distal end of the delivery fiber (due, for example, to its design, its use (e.g., from, bones and/or metal pieces in case of a wounded soldier, blood or tissue that may be next to or deposit on the end), or from damage to the distal end of the delivery fiber) that become unwanted backward-propagating light will be further mixed such that multiple modes of the reflected signal light will return from the entry end of the delivery fiber, and only a very small portion (generally at a power level that is 20-30 dB less than the output beam power) will re-enter the gain fiber. In some embodiments, the end of the delivery fiber is angled relative to the output beam of the gain fiber, but has a sufficiently high numerical aperture (e.g., NA=0.2 to 0.3) that substantially all the output beam from the gain fiber enters the core (the signal waveguide) of the delivery fiber; on the other hand, in some embodiments, the gain fiber has an NA of only 0.1 or so and back-reflected light from the delivery is at too steep an angle to re-enter the gain fiber as undesired backward-traveling signal light that can damage the gain fiber or various components (such as pump laser diodes) connected to the gain fiber. In some embodiments, the delivery fiber has an endcap attached to its entry end, wherein the endcap has a non-circular or a non-centered entry aperture that is sized and/or located such that it accepts substantially all the output light from the gain fiber traveling in the forward direction, but wherein the aperture of the entry endcap has a size, shape and/or offset from the core axis of the delivery-fiber that blocks a substantial amount of any backward signal reflections from the exit end of the delivery fiber. In some embodiments, the center axis of the core of the gain fiber is offset from the center axis of the core of the delivery fiber, also with the purpose that the larger NA of the delivery fiber will receive substantially all of the output beam from the gain fiber, but the smaller NA of the gain fiber will receive only a very small portion of and backward-traveling light reflected from the far end of the delivery fiber. In some embodiments, a combination of the mode mixing of the non-circular-core delivery fiber, the angled entry end of the delivery fiber, the size/shape/offset of the entry aperture or endcap, and/or the offset of the center axes of the cores of the two fibers work together to synergistically reduce the backward-traveling reflected signal light. In some embodiments, the delivery fiber is connectorized to enable it to be disposable and easily replaced. This is an important consideration for medical and other applications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a block diagram of a prior-art optical subsystem  101  that includes an optical gain fiber having a core interfaced to a multi-mode delivery fiber. 
         FIG. 1B  is a block diagram of a prior-art optical subsystem  102  that includes an optical gain fiber having a core interfaced through an optical isolator  122  to a multi-mode delivery fiber. 
         FIG. 1C  is a cross-section block diagram of a prior-art single-mode optical fiber  103  that includes circular single-mode core  141  surrounded by cladding layer  140 . 
         FIG. 1D  is a cross-section block diagram of a prior-art multi-mode optical fiber  104  that includes a large-diameter circular multi-mode core  142  surrounded by cladding layer  140 . 
         FIG. 2  is a block diagram of an improved optical subsystem  201  that includes an optical gain fiber  210  having a core interfaced through a feedback-isolation-and-adaptor unit  220  to a multi-mode delivery fiber  231  having a non-circular core, according to some embodiments of the present invention. 
         FIG. 3  is a block diagram of an improved subsystem  301  that includes an amplifier system  310  for amplifying a seed pulse  91  and outputting the amplified signal  98  into a feedback-isolation-and-adaptor unit  320  that is connected to a multi-mode delivery fiber  331  having a non-circular core, according to some embodiments of the present invention. 
         FIG. 4A  is a block diagram of subsystem  401  that includes a multi-mode delivery fiber  431  having a non-circular core, according to some embodiments of the present invention. 
         FIG. 4B  is a block diagram of subsystem  402  that includes an offset multi-mode delivery fiber  432 , according to some embodiments of the present invention. 
         FIG. 4C  is a block diagram of subsystem  403  that includes a tilted multi-mode delivery fiber  433 , according to some embodiments of the present invention. 
         FIG. 4D  is a block diagram of subsystem  404  that includes a tilted and offset multi-mode delivery fiber  434 , according to some embodiments of the present invention. 
         FIG. 4E  is a block diagram of subsystem  405  that includes a multi-mode delivery fiber  431  having a reverse endcap  439 , according to some embodiments of the present invention. 
         FIG. 4F  is a block diagram of subsystem  406  that includes an offset multi-mode delivery fiber  432  having a reverse endcap  439 , according to some embodiments of the present invention. 
         FIG. 4G  is a block diagram of subsystem  407  that includes a tilted multi-mode delivery fiber  433  having a reverse endcap  439 , according to some embodiments of the present invention. 
         FIG. 4H  is a block diagram of subsystem  408  that includes a tilted and offset multi-mode delivery fiber  434  having a reverse endcap  439 , according to some embodiments of the present invention. 
         FIG. 5A  is a block diagram of optical subsystem  501  that includes an optical gain fiber  510  having a core optically coupled through a pair of collimating lens ( 521  and  523 ) to a multi-mode delivery fiber  531  having a non-circular core, according to some embodiments of the present invention. 
         FIG. 5B  is a block diagram of optical subsystem  502  that includes an optical gain fiber  510  having a core optically coupled through a pair of collimating lens ( 521  and  523 ), one or more apertures  524 , and one or more highly reflective mirrors  525  to a multi-mode delivery fiber  531  having a non-circular core, according to some embodiments of the present invention. 
         FIG. 6A  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  601  that includes a non-circular multi-mode core  641  surrounded by cladding layer  640 , according to some embodiments of the present invention. 
         FIG. 6B  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  602  that includes a non-circular multi-mode core  642  surrounded by cladding layer  640 , according to some embodiments of the present invention. 
         FIG. 6C  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  603  that includes a non-circular multi-mode core  643  surrounded by cladding layer  640 , according to some embodiments of the present invention. 
         FIG. 6D  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  604  that includes a non-circular multi-mode core  644  surrounded by cladding layer  640 , according to some embodiments of the present invention. 
         FIG. 6E  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  605  that includes a non-circular multi-mode core  645  surrounded by cladding layer  640 , according to some embodiments of the present invention. 
         FIG. 6F  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  606  that includes a non-circular multi-mode core  646  surrounded by cladding layer  640 , according to some embodiments of the present invention. 
         FIG. 6G  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  607  that includes a non-circular multi-mode core  647  surrounded by cladding layer  640 , according to some embodiments of the present invention. 
         FIG. 6H  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  608  that includes a non-circular multi-mode core  648  surrounded by cladding layer  640 , according to some embodiments of the present invention. 
         FIG. 6I  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  609  that includes a non-circular multi-mode core  645  surrounded by an inner cladding layer  640 , which is in turn surrounded by an outer protective cladding  649 , according to some embodiments of the present invention. 
         FIG. 6J  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  610  that includes a non-circular multi-mode core  641  surrounded by cladding layer  640  and a mask  638  having an offset light-entry aperture  639 , according to some embodiments of the present invention. 
         FIG. 6K  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  611  that includes a non-circular multi-mode core  651  surrounded by air-cladding or photonic-crystal layer  661  and outer cladding layer  660 , according to some embodiments of the present invention. 
         FIG. 6L  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  612  that includes a non-circular pentagonal-shaped multi-mode core  652  surrounded by air-cladding or photonic-crystal layer  661  and outer cladding layer  660 , according to some embodiments of the present invention. 
         FIG. 6M  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  613  that includes a non-circular hexagonal-shaped multi-mode core  653  surrounded by air-cladding or photonic-crystal layer  661  and outer cladding layer  660 , according to some embodiments of the present invention. 
         FIG. 6N  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  614  that includes a non-circular octagonal-shaped multi-mode core  654  surrounded by air-cladding or photonic-crystal layer  661  and outer cladding layer  660 , according to some embodiments of the present invention. 
         FIG. 6 o    is a block diagram cross-sectional view of a multi-mode optical delivery fiber  615  that includes a non-circular substantially square-shaped multi-mode core  655  surrounded by air-cladding or photonic-crystal layer  662  and outer cladding layer  660 , according to some embodiments of the present invention. 
         FIG. 6P  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  616  that includes a non-circular substantially rectangular-shaped multi-mode core  656  surrounded by air-cladding or photonic-crystal layer  662  and outer cladding layer  660 , according to some embodiments of the present invention. 
         FIG. 6Q  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  617  that includes a non-circular star-shaped multi-mode core  657  surrounded by air-cladding or photonic-crystal layer  661  and outer cladding layer  660 , according to some embodiments of the present invention. 
         FIG. 6R  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  618  that includes a non-circular star-shaped multi-mode core  658  surrounded by air-cladding or photonic-crystal layer  661  and outer cladding layer  660 , according to some embodiments of the present invention. 
         FIG. 6S  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  619  that includes a non-circular substantially square-shaped multi-mode core  664  surrounded by air-cladding or photonic-crystal layer  661 , and outer cladding layer  660 , which is in turn surrounded by an outer protective cladding  669 , according to some embodiments of the present invention. 
         FIG. 6T  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  620  that includes a non-circular multi-mode core  671  surrounded by air-cladding layer  661  and cladding layer  660  that includes a non-circular pentagonal-shaped multi-mode core  671  surrounded by air-cladding or photonic-crystal layer  661  and outer cladding layer  660 , according to some embodiments of the present invention. and a mask  638  having an offset light-entry aperture  639 , according to some embodiments of the present invention. 
         FIG. 6U  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  621  that includes a non-circular substantially square-shaped multi-mode core  665  surrounded by air-cladding or photonic-crystal layer  661  and outer cladding layer  660 , according to some embodiments of the present invention. 
         FIG. 6V  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  622  that includes a non-circular multi-mode core  641  surrounded by a cladding layer  640 , according to some embodiments of the present invention, and a mask  638  having an offset light-entry aperture  639 , according to some embodiments of the present invention. 
         FIG. 7  is a block diagram of apparatus  701  that includes non-circular core optical delivery fiber  731  that is sterilized and enclosed in sterilized package  770 , according to some embodiments of the present invention. 
         FIG. 8  is a block diagram of an instrument system  801  having a high-power mode-field-adaptor fiber-laser control system using one or more of the mode-field-adaptor fiber-laser systems as described herein. 
         FIG. 9A  is a block diagram of apparatus  901  that includes non-circular core optical delivery fiber  931  and connector  911 B, according to some embodiments of the present invention. 
         FIG. 9B  is a block diagram of apparatus  902  that includes non-circular core optical delivery fiber  931  inserted through connector  911 B, according to some embodiments of the present invention. 
         FIG. 9C  is a block diagram of apparatus  903  that includes non-circular core optical delivery fiber  931  and connector  911 B, according to some embodiments of the present invention. 
         FIG. 9D  is a block diagram of apparatus  904  that includes non-circular core optical delivery fiber  931 , connector  911 B and adaptor  911 A, according to some embodiments of the present invention. 
         FIG. 10A  is a block diagram of apparatus  1001  that includes non-circular core optical delivery fiber  1031  inserted through angled connector  1011 B, according to some embodiments of the present invention. 
         FIG. 10B  is a block diagram of apparatus  1002  that includes non-circular core optical delivery fiber  1031  and angled connector  1011 B, according to some embodiments of the present invention. 
         FIG. 10C  is a block diagram of apparatus  1003  that includes non-circular core optical delivery fiber  1031 , connector  1011 C and straight adaptor  1011 A, according to some embodiments of the present invention. 
         FIG. 10D  is a block diagram of apparatus  1004  that includes non-circular core optical delivery fiber  1031 , connector  1011 D and straight adaptor  1011 A, according to some embodiments of the present invention. 
         FIG. 11A  is a block diagram of an improved optical subsystem  1101  that includes an optical gain fiber  210  having a core interfaced through a feedback-isolation-and-adaptor unit  220  to a multi-mode delivery fiber  231  having a non-circular core, wherein the feedback-isolation-and-adaptor unit  1120  includes mirror  1175 A, according to some embodiments of the present invention. 
         FIG. 11B  is a block diagram of an improved optical subsystem  1102  that includes an optical gain fiber  210  having a core interfaced through a feedback-isolation-and-adaptor unit  220  to a multi-mode delivery fiber  231  having a non-circular core, wherein the feedback-isolation-and-adaptor unit  1120 ′ includes mirror  1175 B, according to some embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Although the following detailed description contains many specifics for the purpose of illustration, a person of ordinary skill in the art will appreciate that many variations and alterations to the following details are within the scope of the invention. Accordingly, the following preferred embodiments of the invention are set forth without any loss of generality to, and without imposing limitations upon the claimed invention. Further, in the following detailed description of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration specific embodiments in which the invention may be practiced. It is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. 
     The leading digit(s) of reference numbers appearing in the Figures generally corresponds to the Figure number in which that component is first introduced, such that the same reference number is used throughout to refer to an identical component that appears in multiple figures. Signals and connections may be referred to by the same reference number or label, and the actual meaning will be clear from its use in the context of the description. 
       FIG. 1A  is a block diagram of a prior-art optical subsystem  101  that includes an optical gain fiber having a core interfaced to a multi-mode delivery fiber. According to this configuration the optical signal  91  from a seed source is amplified in gain fiber  110  and becomes amplified forward-traveling signal  98  that is interfaced into a conventional multi-mode delivery fiber  130  by lens  121 . According to Ferman&#39;s 1998 paper (M. Fermann, “Single-mode excitation of multimode fibers with ultrashort pulses”, OPTICS LETTERS/Vol. 23, No. 1/Jan. 1, 1998) the high quality output of gain fiber  110  will be conveyed down the multi-mode fiber  130  substantially intact and without loss. This is less than desirable in certain applications such as medical laser energy delivery because of the concentration of energy in the center of the Gaussian output beam. In addition, this configuration has the undesirable quality of also conveying any reflected signal  97  (such as the reflection from human tissue, other objects in the environment, imperfections in or on the output end of the delivery fiber, and the like) substantially intact and without loss and will reenter the gain fiber in the backward traveling direction where signal  97  will be further amplified and cause undesirable heating and other damage to the gain fiber and its pump sources. One conventional way to alleviate some of these problems is to use an optical isolator, such as shown in  FIG. 1B , however this adds substantial cost to the system and also does not address the problem of having the energy of the output beam  98  concentrated at the center of the Gaussian beam when it lands on the tissue  99 . 
       FIG. 1B  is a block diagram of a prior-art optical subsystem  102  that includes an optical gain fiber having a core interfaced through an optical isolator unit  120  to a multi-mode delivery fiber. This configuration addresses the problem of backward traveling beams (described in  FIG. 1A  above) by introducing a 20 dB to 30 dB loss in the backward traveling beam (such as may re-enter the delivery fiber  130  from a reflection from human tissue or other object in the environment as described above. This configuration does not address the issue of energy concentrated at the center of the Gaussian beam and also has the undesirable result of the isolator  122  reducing power in the forward traveling beam due to the inherent properties of the isolator  122 . Subsystem  102  also includes and input ferrule  112  that releasable connects gain fiber  110  to isolator  120 , and output ferrule  111  that releasable connects delivery fiber  130  to isolator unit  120 . Isolator unit  120  further includes collimating lens  121  that collimates the forward traveling beam from gain fiber  110  and directs the collimated beam to isolator  122  and collimating lens  123  that focuses the collimated beam from isolator  122  into the core of delivery fiber  130 . 
       FIG. 1C  is a block diagram cross-sectional view of a prior-art single-mode optical fiber  103  that includes circular single-mode core  141  surrounded by cladding layer  140  and  FIG. 1D  is a block diagram cross-sectional view of a prior-art multi-mode optical fiber  104  that includes circular multi-mode core  142  surrounded by cladding layer  140 . Conventional optical systems use a delivery fiber having either a single-mode circular core (e.g., optical fiber  103 ) when a thin cut or incision is desired or a multi-mode circular core (e.g., optical fiber) when a more uniform power output is desired. 
     As used herein, “feedback isolation” is optical isolation that permits signal light propagation in a forward direction, but inhibits signal light propagation in the opposite (backward) direction. As used herein, an “adaptor unit” includes an interface from one optical subsystem (such as an optical power amplifier) and another optical subsystem (such as a delivery fiber). 
       FIG. 2  is a block diagram of an improved optical subsystem  201  that includes an optical gain fiber  210  having a core interfaced through a feedback-isolation-and-adaptor unit  220  to a multi-mode delivery fiber  231  having a non-circular core, according to some embodiments of the present invention. In some embodiments of the present invention, optical subsystem  201  addresses the both the problem of backward traveling beams (described in  FIG. 1A  above) and the issue of energy concentrated at the center of the Gaussian output beam (also described above in  FIG. 1A ). Optical subsystem  201  addresses the issue of optical energy being concentrated at the center of a Gaussian beam (i.e., one wherein the spatial cross-section exhibits a Gaussian profile) by using a delivery fiber  231  that has a non-circular core which has the effect of causing the single-mode forward-traveling amplified beam  98  to be mode mixed in the non-circular core delivery fiber  231 , even over short distances. In contrast to the Gaussian beam output by prior-art optical subsystems  101  and  102  using optical fiber  103  and  104 , optical subsystem  201  of the present invention will output a beam with a significantly degraded beam quality, wherein the optical energy of the output beam will be substantially uniform over the area of the beam spot. Having a beam spot with uniform output energy is very desirable in many applications, including medical laser treatment procedures. The issue of backward traveling beams caused by reflections of the output beam from human tissue, other objects in the operational environment, imperfections in or on the output end of the delivery fiber, or the like, is also addressed in the present invention with the use of the non-circular core delivery fiber  231 . In the situation of the backward traveling reflected beam, the non-circular core delivery fiber  231  is used to protect a optical subsystem  201  because, when the light output from the non-circular delivery fiber  231  is reflected back into the delivery fiber  231  propagates in a backward direction towards the amplifying gain fiber  210 , only a small amount of light is coupled back into the gain fiber  210 . The worst case of coupling of the backward traveling beam into the gain fiber  210  corresponds to the brightness difference between the delivery fiber  231  and the gain fiber  210 . As described in the present invention, laser systems with non-circular delivery fibers can achieve isolation of about 20 dB to 30 dB without requiring the use of an isolator unit and such isolation is capable and sufficiently high to isolate and protect against feedback reflections. 
     In some embodiments, the present invention provides isolation against optical feedback by using non-circular delivery fibers, isolation levels of 20 dB-40 dB, a reduction in insertion loss even without the use of an isolator, a reduction in the laser power required because an isolator is not used, and further, non circular deliver fibers have the added benefit of producing a top-hat intensity profile and provides an optimum interaction of the optical radiation with treated tissue. 
     Optical subsystem  201  of the present invention also includes a seed source  209  optically coupled to fiber amplifier  210  that provides an optical signal into the core of the gain fiber  210 , input ferrule  212  that releasable connects gain fiber  210  to isolator and adaptor unit  220 , and output ferrule  211  that releasable connects delivery fiber  231  to isolator and adaptor unit  220 . Isolator and adaptor unit  220  further includes lens  221  that focuses the forward traveling beam  98  from gain fiber  210  and into the non-circular core of delivery fiber  231 . Additionally, in some embodiments, optical subsystem  201  further includes a mode stripper  214  coupled to the gain fiber  210  to remove unwanted optical light from the cladding of the gain fiber  210  and a matching single-clad fiber  215  connected between the output end of the gain fiber  210  and the input ferrule  212  which acts like a pinhole and eliminates light which would otherwise propagate in the pump cladding of the gain fiber  210  and might lead to damage of the pump diodes. In some embodiments, the cladding of the delivery fiber  231  is single-clad and is anti-guiding and therefore strips any light coupled into its cladding. 
       FIG. 3  is a block diagram of an improved optical subsystem  301  that includes an amplifier system  310  for amplifying a seed pulse  91  and outputting the amplified signal  98  into a feedback-isolation-and-adaptor unit  320  that is connected to a multi-mode delivery fiber  331  having a non-circular core, according to some embodiments of the present invention. In some embodiments, amplifier system  310  is a two-stage amplifying system that includes first optical pump  318  optically connected to amplifier  317  through optical fiber  316  and a second optical pump  318  optically connected to amplifier  319  through optical fiber  316 . In some embodiments, seed source  309  is configured to output optical seed pulse  91  and provide optical seed source  91  to amplifier system  310 . Amplifier system  310  is configured to receive optical seed pulse  91  and amplify the seed pulse in the first amplifier  317  and the second amplifier  318  and output amplified optical pulse to the isolation and adaptor unit  320 . In some embodiments, amplified optical pulse  98  propagates through isolation and adaptor unit  320  and is output into non-circular multi-mode delivery fiber  331 , wherein the amplified optical pulse  98  is mode mixed and output through endcap  329  as output pulse  92  from the exit end of the non-circular multi-mode delivery fiber  331 , resulting in output pulse  92  having a generally uniform power density. 
       FIG. 4A  is a block diagram of subsystem  401  that includes a multi-mode delivery fiber  431  having a non-circular core, a connector  411 B, according to some embodiments of the present invention. In some embodiments, subsystem  401  is similar to optical subsystem  201  of  FIG. 2 , described above, in that subsystem  401  also addresses both the problem of backward traveling beams (described in  FIG. 1A  above) and the issue of energy concentrated at the center of the Gaussian output beam (also described above in  FIG. 1A ), through the use of non-circular delivery fiber  431 . In some embodiments, subsystem  401  also includes a connector  411 B that is attached to the entrance end of the delivery fiber  431  and connector  411 B is configured to connect and disconnect to adaptor  411 A. In some embodiments, adaptor  411 A and connector  411 B combine together as ferrule  411  and provide a means for connecting and disconnecting the delivery fiber to the isolator and adaptor unit  420 . In some embodiments, subsystem  401  includes amplifying fiber  410  configured to output amplified light  98  into isolator and adaptor unit  420 . Isolator and adaptor unit  420  further includes lens  421 , which is configured to focus amplified light signal  98  that is output by a fiber amplifier  410  (as described above in  FIG. 2 ) into the entry end of non-circular delivery fiber  431 . 
       FIGS. 4B-4H , show additional embodiments according to the present invention reduces the amount of backward traveling reflected light that enters the amplifier fiber, thereby improving the achievable isolation to even more sufficiently isolate and protect the optical subsystem against feedback reflections. In some embodiments, the subsystems in  FIGS. 4B-4H  also address the issue of energy concentrated at the center of the Gaussian output beam (also described above in  FIG. 1A ) by using a non-circular core delivery fiber.  FIGS. 4B-4H  each include fiber amplifier  410  configured to output amplified light into isolator and adaptor unit  420  as described above for  FIG. 4A  and further includes lens  421  configured to focus amplified light  98  into the delivery fiber and adaptor  411 A configured to connect and disconnect from connector  411 B and together with connector  411 B forms a ferrule. In some embodiments, each subsystem described in  FIGS. 4B-4H  include an aperture (not shown) that allows substantially all of amplified light  98  to pass through, but prevents a portion of the backward traveling reflected light from passing through and thus further reduces the amount of reflected light that enters the fiber amplifier  410 . 
       FIG. 4B  is a block diagram of subsystem  402  that includes an offset multi-mode delivery fiber  432 , according to some embodiments of the present invention. In some embodiments, subsystem  402  further reduces the amount of backward traveling reflected light from entering the amplifying stage of the optical subsystem and damaging the optical subsystem by offsetting the placement of the entry end of delivery fiber  432  in connector  411 B a linear distance away from an optical axis  96  created by amplified light  98  being focused by lens  421 . That is, in some embodiments, the delivery fiber connector is configured such that the axis of the core of the delivery fiber is substantially parallel to the axis of the core of the amplifying fiber and/or to a central axis  96  of the focused beam from lens  421  and/or to a central axis of the lens  421 , but wherein the axis of the core of the delivery fiber is radially offset from the axis of the core of the amplifying fiber and/or from the central axis of the focused beam from lens  421  and/or from a central axis of the lens  421 . In some embodiments, this radial offset is accomplished by mounting the fiber into its connector  411 B such that the central axis of the fiber is radially offset from the central axis of the connector, and the adaptor  411 A is concentric with the central axis of lens  421  and the end of the core of amplifying fiber  410 , while in other embodiments, the fiber is mounted into its connector  411 B such that the central axis of the fiber is concentric with the central axis of the connector, and the adaptor  411 A is radially offset to the central axis of lens  421  and the end of the core of amplifying fiber  410 , while in still other embodiments, the fiber is mounted into its connector  411 B such that the central axis of the fiber is concentric with the central axis of the connector and the adaptor  411 A is concentric to the end of the core of amplifying fiber  410 , but the central axis of lens  421  is radially offset from these axes. In some embodiments, delivery fiber  432  has a numerical aperture (NA) sufficiently large (in some such embodiments, a diffraction-limited beam at the output of a 20-micron-core-diameter gain fiber has an approximate NA of about 0.04 (when the signal beam has a 1-micron wavelength)-0.08 (when the signal beam has a 2-micron wavelength), therefore a delivery fiber having a numerical aperture that is at least two times the NA of the gain fiber makes sense, and thus some embodiments use a delivery fiber having an NA of at least 0.2) to accept substantially all of the focused amplified light  98  into the core of delivery fiber  432  and amplifying fiber  410  has a sufficiently small NA such that substantially none of the backward-traveling reflected light enters fiber amplifier  410 . In some embodiments, delivery fiber  432  has a non-circular core to cause mixing of the single-mode amplified light  98  such that the light output by delivery fiber  432  has a more uniform energy profile. In some embodiments, the lateral (in the radial direction) distance by which the optical axis of the entry end of deliver fiber  432  is offset from optical axis  96  by an amount that is one to two times the diameter of the mode profile of the signal beam at the output of the gain fiber, or one to two times the diameter of the core of the gain fiber. In other embodiments, the linear distance by which the center longitudinal axis of the entry end of deliver fiber  432  is offset from the center longitudinal optical axis  96  is more than 0.25 times, but less than one (1) times the mode field diameter of the signal beam at the output of the gain fiber. In still other embodiments, the linear distance by which the center longitudinal axis of the entry end of deliver fiber  432  is offset from the center longitudinal optical axis  96  is at least 1.2 times, or at least 1.4 times, or at least 1.6 times, or at least 1.8 times, or at least 2.0 times the mode field diameter of the signal beam at the output of the gain fiber. 
     For example, in some embodiments having a gain fiber with an NA of about 0.04, the delivery fiber has an NA of at least 0.07, while in other embodiments having a gain fiber with an NA of about 0.04, the delivery fiber has an NA of at least 0.08, an NA of at least 0.09, an NA of at least 0.10, an NA of at least 0.11, an NA of at least 0.12, an NA of at least 0.14, an NA of at least 0.16, an NA of at least 0.18, an NA of at least 0.20, an NA of at least 0.22, or even an NA of at least 0.24. 
     For another example, in some embodiments having a gain fiber with an NA of 0.08, the delivery fiber has an NA of at least 0.15, while in other embodiments having a gain fiber with an NA of about 0.08, the delivery fiber has an NA of at least 0.16, an NA of at least 0.18, an NA of at least 0.20, an NA of at least 0.22, an NA of at least 0.24, an NA of at least 0.26, an NA of at least 0.28, or even an NA of at least 0.3. 
       FIG. 4C  is a block diagram of subsystem  403  that includes a tilted-end multi-mode delivery fiber  433 , according to some embodiments of the present invention. In some embodiments, subsystem  403  further reduces the amount of backward traveling reflected light from entering the amplifying stage of the optical subsystem and damaging the optical subsystem by tilting the entry end of delivery fiber  433  relative to the central axis of connector  411 B such that an angle is formed between the tilted end of delivery fiber  433  and an optical axis  96  created by amplified light  98  being focused by lens  421 , however the entry end of delivery fiber  433  (i.e., the end surface of delivery fiber  433  wherein amplified light signal  98  enters delivery fiber  433 ) continues to be lined up with optical axis  96 . In other embodiments, the delivery fiber connector is configured such that the axis of the core of the delivery fiber is substantially parallel to the central axis of connector  411 B, but the central axis of adaptor  411 A is tilted relative to optical axis  96 , the core of the amplifying fiber and/or to a central axis of the focused beam from lens  421  and/or to a central axis of the lens  421 . In some other embodiments, this tilt is accomplished by mounting the fiber into its connector  411 B such that the central axis of the fiber is parallel to and centered on the central axis of the connector, and the adaptor  411 A is concentric with the end of the core of amplifying fiber  410 , but the central axis of lens  421  is tilted relative to the axis of delivery fiber  433 . In some embodiments, delivery fiber  433  has a numerical aperture (NA) sufficiently large (e.g., an NA of at least 0.15) to accept substantially all of the focused amplified light  98  into the tilted core of delivery fiber  432  and amplifying fiber  410  has a sufficiently small NA such that substantially none of the backward-traveling reflected light exiting delivery fiber  433  at an angle with respect to optical axis  96  enters fiber amplifier  410 . In some embodiments, delivery fiber  433  has a non-circular core to cause mixing of the single-mode amplified light  98  such that the light output by delivery fiber  433  has a more uniform energy profile. In some embodiments, the angle between the axis of the delivery fiber at the entry end of deliver fiber  433  and the optical axis  96  of the output end of the gain fiber is about 0.05 radians (wherein the delivery fiber has an NA of at least about 0.05 (which is the sine of 0.05 radians)) to about 0.5 radians (wherein the delivery fiber has an NA of at least about 0.48 (which is the sine of 0.5 radians)). This range is equal to about 3 degrees, wherein the delivery fiber has an NA of at least about 0.052; to about 30 degrees, wherein the delivery fiber has an NA of at least 0.5. In other embodiments, this angle is about 0.1 radians, about 0.15 radians, about 0.2 radians, about 0.25 radians, about 0.3 radians, about 0.35 radians, about 0.4 radians, or about 0.45 radians. 
       FIG. 4D  is a block diagram of subsystem  404  that includes a tilted-and-offset-end multi-mode delivery fiber  434 , according to some embodiments of the present invention. In some embodiments, subsystem  404  further reduces the amount of backward traveling reflected light from entering the amplifying stage of the optical subsystem and damaging the optical subsystem by both offsetting the placement of the entry end of delivery fiber  434  with respect to an optical axis  96  created by amplified light  98  being focused by lens  421  and by tilting the entry end of delivery fiber  434  in connector  411 B such that an angle is formed between the tilted end of delivery fiber  434  and an optical axis  96 . In other embodiments, the delivery fiber connector is configured such that the axis of the core of the delivery fiber is substantially parallel to the central axis of connector  411 B, but the central axis of adaptor  411 A is tilted and offset relative to optical axis  96 , the core of the amplifying fiber and/or to a central axis of the focused beam from lens  421  and/or to a central axis of the lens  421 . In some other embodiments, this tilt and offset is accomplished by mounting the fiber into its connector  411 B such that the central axis of the fiber is parallel to and centered on the central axis of the connector, and the adaptor  411 A is concentric with the end of the core of amplifying fiber  410 , but the central axis of lens  421  is radially offset and tilted relative to the axis of delivery fiber  433 . In some embodiments, delivery fiber  434  has a numerical aperture (NA) sufficiently large (e.g., in some embodiments, an NA of at least about 0.16) to accept substantially all of the focused amplified light  98  into the offset and tilted core of delivery fiber  434  and amplifying fiber  410  has a sufficiently small NA such that substantially none of the backward-traveling reflected light exiting delivery fiber  434  offset and at an angle with respect to optical axis  96  enters fiber amplifier  410 . In some embodiments, delivery fiber  434  has a non-circular core to cause mixing of the single-mode amplified light  98  such that the light output by delivery fiber  434  has a more uniform energy profile. In some embodiments, the linear distance by which the center longitudinal axis of the entry end of deliver fiber  434  is offset from the center longitudinal optical axis  96  is at least one times the mode field diameter of the signal beam as it exits the gain fiber. In other embodiments, the linear distance by which the center longitudinal axis of the entry end of deliver fiber  434  is offset from the center longitudinal optical axis  96  is at least 1.2 times, or at least 1.4 times, or at least 1.6 times, or at least 1.8 times, or at least 2.0 times the mode field diameter. In some embodiments, the angle between the entry end of deliver fiber  434  and the optical axis  96  is about 0.05 radians to about 0.5 radians. In other embodiments, the angle between the entry end of deliver fiber  434  and the optical axis  96  is at least about 0.1 radians, at least about 0.15 radians, at least about 0.2 radians, at least about 0.25 radians, at least about 0.3 radians, at least about 0.35 radians, at least about 0.4 radians, or at least about 0.45 radians, but in each of these cases less than about 1 radian. 
       FIG. 4E  is a block diagram of subsystem  405  that includes a multi-mode delivery fiber  431  having a reverse endcap  439  connected between connector  411 B and delivery fiber  431 , according to some embodiments of the present invention. In some embodiments, subsystem  405  is substantially similar to subsystem  401  described above, however subsystem  405  also includes reverse endcap  439  that further degrades the backward-traveling reflected light and thus further reduces the amount of backward-traveling reflected light that enters fiber amplifier  410 . In some embodiments, reverse endcap  439  includes a short section of fiber having a core (either substantially circular or non-circular) that is larger than the core of non-circular delivery fiber  431 . In some embodiments, the reverse endcap  439  having its larger core fiber section degrades the reflected light propagating backwards in delivery fiber  431 . In some embodiments, the maximum length of the reverse endcap  439  is chosen so that when the amplified light  98  from fiber amplifier  410  propagates (in the forward direction) through reverse endcap  439 , the brightness of the amplified light  98  does not degrade in the endcap more than the brightness of amplified light that propagates through the smaller core-size delivery fiber  431  would degrade. However, reverse endcap  439  further degrades the brightness of reflected light propagating backwards and therefore reverse endcap  439  will reduce the coupling of the reflected light into the fiber amplifier  410 . 
       FIG. 4F  is a block diagram of subsystem  406  that includes an offset multi-mode delivery fiber  432  having a reverse endcap  439 , according to some embodiments of the present invention. In some embodiments, subsystem  406  is substantially similar to subsystem  402  described above, however subsystem  406  also includes reverse endcap  439  that further degrades the backward-traveling reflected light and thus further reduces the amount of backward-traveling reflected light that enters fiber amplifier  410 . 
       FIG. 4G  is a block diagram of subsystem  407  that includes a tilted multi-mode delivery fiber  433  having a reverse endcap  439 , according to some embodiments of the present invention. In some embodiments, subsystem  407  is substantially similar to subsystem  403  described above, however subsystem  407  also includes reverse endcap  439  that further degrades the backward-traveling reflected light and thus reverse endcap  439  further reduces the amount of backward-traveling reflected light that enters fiber amplifier  410 . 
       FIG. 4H  is a block diagram of subsystem  408  that includes a tilted and offset multi-mode delivery fiber  434  having a reverse endcap  439 , according to some embodiments of the present invention. In some embodiments, subsystem  408  is substantially similar to subsystem  404  described above, however subsystem  408  also includes reverse endcap  439  that further degrades the backward-traveling reflected light and thus reverse endcap  439  further reduces the amount of backward-traveling reflected light that enters fiber amplifier  410 . 
       FIG. 5A  is a block diagram of optical subsystem  501  that includes an optical gain fiber  510  having a core optically coupled through a pair of collimating lens ( 521  and  523 ) to a multi-mode delivery fiber  531  having a non-circular core, according to some embodiments of the present invention. In some embodiments, optical subsystem  501  includes a optical fiber amplifier  510  configured to amplify optical light signal  91  and output amplified light signal  98  from the output end of gain fiber  510 . In some embodiments, collimating lens  521  is optically coupled to the output end of gain fiber  510  and receives and collimates amplified light signal  98 . In some embodiments, lens  523  is optically coupled to the input end of delivery fiber  531  and receives the collimated amplified light signal  98  from lens  521  and focuses the amplified light signal  98  into the input end of the non-circular core delivery fiber  531 . Amplified light signal  98  propagates through multi-mode delivery fiber  531  where amplified light signal  98  is mixed and is output through the output end of non-circular core delivery fiber  531  where the amplified light signal  98  interacts with sample  99 . In some embodiments, the mode mixing of the amplified light signal  98  that occurs during propagation through the non-circular core multi-mode delivery fiber  531  causes the output amplified light signal to have a more uniform energy density. In some embodiments, backward-traveling light  97  is generated by reflections of the light signal  98  with the sample  99  as well as other interfaces and imperfections within optical system  501  and propagates through the delivery fiber in a direction opposite to the amplified light signal  98  where it will exit from the input end of delivery fiber  531 . In some embodiments, the beam quality of backwards propagating light signal  97  is highly degraded due to the non-circular core of delivery fiber  531  and therefore backward-traveling light  97  will have a Rayleigh range that is shorter than the Rayleigh range of the forward-traveling amplifier light signal  98 . In some embodiments, the distance between lens  521  and lens  523  is selected such that backward-traveling light  97  will significantly diverge and overfill lens  521 , whereby the fraction of the coupled power of backward-traveling light  97  into gain fiber  510  is significantly reduced. In some embodiments, the distance d between lens  521  and lens  523  is d≧NA DF *f 523 /(M 2 *λ) wherein NA DF  the NA of the non-circular delivery fiber  531 , f 523  the focal length of lens  523 , M 2  is the beam quality of the light coming back from the delivery fiber  531 , λ, is the laser wavelength of the signal beam. In some further embodiments, an aperture  524  is inserted between lens  521  and lens  523  to provide further reduction of backward-traveling light  97 , for example, when the fiber used on the output of the amplifier  510  is a double clad fiber. In still further embodiments, an enclosure  528  includes an adaptor  411 A (such as shown in  FIG. 4A ) configured to receive a connector  411 B, which is used on the end of the delivery fiber  531  (such as shown in  FIG. 4A ) and the offset and/or tilting mechanism described above in  FIGS. 4B-4D  is used for tilting and/or offsetting lens  521  and/or  523  and/or for tilting and/or offsetting connector  411 B and/or adaptor  411 A to further reduce the amount of backward-traveling reflected light  97  that can enter fiber amplifier  510 . 
       FIG. 5B  is a block diagram of optical subsystem  502  that includes an optical gain fiber  510  having a core optically coupled through a pair of collimating lens ( 521  and  523 ), one or more apertures  524 , and one or more highly reflective mirrors  525  to a multi-mode delivery fiber  531  having a non-circular core, according to some embodiments of the present invention. In some embodiments, optical subsystem  502  utilizes the increased Rayleigh range of the backward-traveling light  97 , as discussed above for  FIG. 5A , in a compact arrangement. In some embodiments, optical subsystem  502  includes isolator and adaptor unit  520  which includes collimating lens  521  configured to collimate amplified light signal  98 , one or more highly reflective mirrors  525  configured to reflect forward-traveling amplified light signal  98 , and collimating lens  523  configured to focus forward-traveling light signal  98  into the non-circular core of delivery fiber  531 . In some embodiments, the one or more highly-reflecting mirrors  525  increase the path length that amplified light signal  98  and backward-traveling light signal  97  travel between lens  521  and lens  523  in a compact manner. Using the configuration of subsystem  502 , the optical path length between lens  521  and lens  523  is, in some embodiments, increased in order to further decrease the amount of backward-traveling light  97  that is coupled into the gain fiber  510  in a compact manner without sacrificing performance. In some embodiments, optical subsystem further includes input ferrule  512 , which includes connector  512 B and adaptor  512 A and allows gain fiber  510  to be connected and disconnected to isolator and adaptor unit  520 . In some embodiments, optical subsystem further includes output ferrule  511 , which includes connector  511 B and adaptor  511 A and allows delivery fiber  531  to be connected and disconnected to isolator and adaptor unit  520 . In some embodiments, isolator and adaptor unit  520  further includes one or more apertures  524  for further reducing the amount of backward-traveling light  97  that is coupled into gain fiber  510 . In some embodiments, the optical path length between lens  521  and lens  523  is greater than the Rayleigh length of the backward-traveling light signal  97 , but shorter than the forward-traveling amplified light signal  98 . In still further embodiments, the offset and/or tilting mechanism described above in  FIGS. 4B-4D  is used for tilting and/or offsetting lens  521  and/or  523  and/or for tilting and/or offsetting connector  411 B and/or adaptor  411 A to further reduce the amount of backward-traveling reflected light  97  that can enter fiber amplifier  510 . 
       FIG. 6A  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  601  that includes a non-circular multi-mode core  641  surrounded by cladding layer  640 , according to some embodiments of the present invention. In some embodiments, the non-circular core  641  is substantially oval in shape. 
       FIG. 6B  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  602  that includes a non-circular multi-mode core  642  surrounded by cladding layer  640 , according to some embodiments of the present invention. In some embodiments, the non-circular core  642  is substantially a polygon in shape (e.g., in some embodiments, a regular polygon such as an equilateral triangle, a square, a pentagon, a hexagon, a heptagon, an octagon or other regular polygon, while in other embodiments, the core is a non-regular polygon such as a non-square rectangle or other polygonal shape). In yet other embodiments, the core has any non-circular shape such as an oval or wavy curved circumference. In still other embodiments, the core is round but has stress rods, areas of higher or lower index of refraction, or similar elements in it. Such elements lead to enhanced mode coupling. In yet still other embodiments, an airclad or photonic crystal structure is used around the core (e.g., forming a core that is quasi round), for example, such as the embodiments described in  FIG. 6K through 6V . 
       FIG. 6C  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  603  that includes a non-circular multi-mode core  643  surrounded by cladding layer  640 , according to some embodiments of the present invention. In some embodiments, the non-circular core  641  is substantially hexagonal in shape. 
       FIG. 6D  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  604  that includes a non-circular multi-mode core  644  surrounded by cladding layer  640 , according to some embodiments of the present invention. In some embodiments, the non-circular core  641  is substantially octagonal in shape. 
       FIG. 6E  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  605  that includes a non-circular multi-mode core  645  surrounded by cladding layer  640 , according to some embodiments of the present invention. In some embodiments, the non-circular core  641  is substantially square in shape. 
       FIG. 6F  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  606  that includes a non-circular multi-mode core  646  surrounded by cladding layer  640 , according to some embodiments of the present invention. In some embodiments, the non-circular core  641  is substantially rectangular in shape. 
       FIG. 6G  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  607  that includes a non-circular multi-mode core  647  surrounded by cladding layer  640 , according to some embodiments of the present invention. In some embodiments, the non-circular core  647  is shaped as substantially a star-shaped polygon having both concave and convex vertices. 
       FIG. 6H  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  608  that includes a non-circular multi-mode core  648  surrounded by cladding layer  640 , according to some embodiments of the present invention. In some embodiments, the non-circular core  648  is shaped as substantially a star-shaped polygon having both concave and convex vertices, but with fewer vertices than the embodiment shown in  FIG. 6G . 
       FIG. 6I  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  609  that includes a non-circular multi-mode core  649  surrounded by cladding layer  640  and cladding layer  640  is surrounded by additional cladding, “cabling,” and/or armor layers (e.g., in some embodiments, a triple-clad fiber) layer  649 , according to some embodiments of the present invention. In some embodiments, the non-circular core  645  is shown as being substantially square in shape, however, other embodiments use a non-circular core of any suitably non-circular shape, including those described in the previous figures. 
       FIG. 6J  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  610  that includes a non-circular multi-mode core  641  surrounded by cladding layer  640  and a mask  638  having an offset light-entry aperture  639 , according to some embodiments of the present invention. In some embodiments, the aperture is located such that a majority of the signal light from the gain fiber can enter the delivery fiber through the aperture  639 , but a majority of the reflected signal light from the delivery fiber is blocked by the mask so it cannot re-enter the gain fiber, according to some embodiments of the present invention. In some embodiments, the aperture  639  is located and offset from the optical center of the optical delivery fiber  610  such that only a portion  637  of the aperture  639  overlaps with the non-circular core  641 , such that a majority of the signal light from the gain fiber can enter the delivery fiber through the aperture  639 , but a majority of the reflected signal light from the delivery fiber is blocked by the mask so it cannot re-enter the gain fiber. 
       FIG. 6K  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  611  that includes a non-circular multi-mode core  651  surrounded by air-cladding or photonic-crystal layer  661  and outer cladding layer  660 , according to some embodiments of the present invention. In some embodiments, fiber  611  of  FIG. 6K  is substantially similar to fiber  601  of  FIG. 6A  described above, except that fiber  611  includes an air-cladding layer  661  to provide additional mode mixing of the optical signal in the optical delivery fiber  611 . 
       FIG. 6L  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  612  that includes a non-circular pentagonal-shaped multi-mode core  652  surrounded by air-cladding layer  661  and cladding layer  660 , according to some embodiments of the present invention. In some embodiments, fiber  612  of  FIG. 6L  is substantially similar to fiber  602  of  FIG. 6B  described above, except that fiber  612  includes an air-cladding layer  661  to provide additional mode mixing of the optical signal in the optical delivery fiber  612 . 
       FIG. 6M  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  613  that includes a non-circular hexagonal-shaped multi-mode core  653  surrounded by air-cladding layer  661  and cladding layer  660 , according to some embodiments of the present invention. In some embodiments, fiber  613  of  FIG. 6M  is substantially similar to fiber  603  of  FIG. 6C  described above, except that fiber  613  includes an air-cladding layer  661  to provide additional mode mixing of the optical signal in the optical delivery fiber  613 . 
       FIG. 6N  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  614  that includes a non-circular octagonal-shaped multi-mode core  654  surrounded by air-cladding layer  661  and cladding layer  660 , according to some embodiments of the present invention. In some embodiments, fiber  614  of  FIG. 6N  is substantially similar to fiber  604  of  FIG. 6D  described above, except that fiber  614  includes an air-cladding layer  661  to provide additional mode mixing of the optical signal in the optical delivery fiber  614 . 
       FIG. 6 o    is a block diagram cross-sectional view of a multi-mode optical delivery fiber  615  that includes a non-circular substantially square-shaped multi-mode core  655  surrounded by air-cladding layer  662  and cladding layer  660 , according to some embodiments of the present invention. In some embodiments, fiber  615  of  FIG. 6 o    is substantially similar to fiber  605  of  FIG. 6E  described above, except that fiber  615  includes an air-cladding layer  662  to provide additional mode mixing of the optical signal in the optical delivery fiber  615 . 
       FIG. 6P  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  616  that includes a non-circular substantially rectangular-shaped multi-mode core  656  surrounded by air-cladding layer  662  and cladding layer  660 , according to some embodiments of the present invention. In some embodiments, fiber  616  of  FIG. 6P  is substantially similar to fiber  606  of  FIG. 6F  described above, except that fiber  616  includes an air-cladding layer  662  to provide additional mode mixing of the optical signal in the optical delivery fiber  616 . 
       FIG. 6Q  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  617  that includes a non-circular star-shaped multi-mode core  657  surrounded by air-cladding layer  661  and cladding layer  660 , according to some embodiments of the present invention. In some embodiments, Figure fiber  617  of  6 Q is substantially similar to fiber  607  of  FIG. 6G  described above, except that fiber  617  includes an air-cladding layer  661  to provide additional mode mixing of the optical signal in the optical delivery fiber  617 . 
       FIG. 6R  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  618  that includes a non-circular star-shaped multi-mode core  658  surrounded by air-cladding layer  661  and cladding layer  660 , according to some embodiments of the present invention. In some embodiments, Figure fiber  618  of  6 R is substantially similar to fiber  608  of  FIG. 6H  described above, except that fiber  618  includes an air-cladding layer  661  to provide additional mode mixing of the optical signal in the optical delivery fiber  618 . 
       FIG. 6S  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  619  that includes a non-circular substantially square-shaped multi-mode core  664  surrounded by air-cladding layer  661 , and cladding layer  660 , which is in turn surrounded by an outer protective cladding  669 , according to some embodiments of the present invention. In some embodiments, fiber  619  of  FIG. 6S  is substantially similar to fiber  609  of  FIG. 6I  described above, except that fiber  619  includes an air-cladding layer  661  to provide additional mode mixing of the optical signal in the optical delivery fiber  619 . 
       FIG. 6T  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  620  that includes a non-circular multi-mode core  671  surrounded by air-cladding layer  661  and cladding layer  660  that includes a non-circular pentagonal-shaped multi-mode core  671  surrounded by air-cladding layer  661  and cladding layer  660 , according to some embodiments of the present invention, and a mask  638  having an offset light-entry aperture  639 , according to some embodiments of the present invention. In some embodiments, fiber  620  of  FIG. 6T  is substantially similar to fiber  610  of  FIG. 6J  described above, except that fiber  620  includes an air-cladding layer  661  to provide additional mode mixing of the optical signal in the optical delivery fiber  620 . In some embodiments, the aperture  639  is located and offset from the optical center of the optical delivery fiber  620  such that only a portion  637  of the aperture  639  overlaps with the non-circular core  671 , such that a majority of the signal light from the gain fiber can enter the delivery fiber through the aperture  639 , but a majority of the reflected signal light from the delivery fiber is blocked by the mask so it cannot re-enter the gain fiber. In some embodiments, the aperture  639  is oriented at an “apogee” end of core  637  as is shown in  FIG. 6J  rather than as shown in this  FIG. 6T . 
       FIG. 6U  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  621  that includes a non-circular substantially square-shaped multi-mode core  665  surrounded by air-cladding layer  661  and cladding layer  660 , according to some embodiments of the present invention. In some embodiments, fiber  621  of  FIG. 6U  is substantially similar to fiber  605  of  FIG. 6E  described above, except that the non-circular substantially square-shaped core  665  of fiber  621  is smaller than the substantially-square core  645  of  FIG. 6E , and in addition fiber  621  further includes an air-cladding layer  661  to provide additional mode mixing of the optical signal in the optical delivery fiber  611 . 
       FIG. 6V  is a block diagram cross-sectional view of a multi-mode optical delivery fiber  622  that includes a non-circular multi-mode core  641  surrounded by a cladding layer  640 , according to some embodiments of the present invention. and a mask  638  having an offset light-entry aperture  639 , according to some embodiments of the present invention. In some embodiments, delivery fiber  622  is substantially identical to delivery fiber  610  of  FIG. 6J , however mask  638  is oriented such that aperture  639  is aligned to receive core light from the flat side of oval core  641 , rather than from the “apogee” end of the oval core  641 , as was the case in  FIG. 6J . 
       FIG. 7  is a block diagram of apparatus  701  that includes a delivery fiber assembly  741 , having a non-circular core optical delivery fiber  731 , that is sterilized and enclosed in package  770 , according to some embodiments of the present invention. In some embodiments, apparatus  701  is used in a sterilized environment (e.g., a surgical operating room or the like) and the non-circular core optical delivery fiber  731  is removed from the sterilized package  770  and is connected to a laser system (e.g., those systems shown in  FIG. 2, 3, 4A-4H, 5A, 5B, 8, 11A , or  11 B) using adaptor  711 B for the purpose of delivering an output optical pulse or signal through distal endcap  729  to a target (e.g., animal or human tissue, or the like). In some embodiments, after use, delivery fiber  731  can be disposed of (or, in some embodiments, sterilized and re-packaged). In various embodiments, each one of the embodiments described herein uses a delivery fiber assembly having a distal endcap as described for  FIG. 7  here. 
       FIG. 8  is a block diagram of an instrument system  801  having a high-power fiber-laser control system using one or more of the optical subsystems as described herein. In some embodiments, instrument system  801  (e.g., a medical instrument such as a laser scalpel, optical stimulator for evoking nerve-action potentials in nerves of a human, skin or corneal ablator, or other medical instrument, or a material processor instrument (such as for heat treatment of a surface, or welding or cutting) or the like) using one or more of the fiber amplifiers  810  as described herein. In some embodiments, system  801  includes the instrument and/or facility enclosure  880  and its other contents (e.g., engines and control systems), one or more battery and/or electrical power supplies  881 , a laser controller  882  that provides the control of wavelength, pulse timing and duration for embodiments using pulse signals (other embodiments use CW signal beams), output-power control, direction control of the output beam and the like, optionally an imaging-calculation microprocessor and/or circuitry  884  that obtains an image signal from imager  886  and calculates such data as target location and size that is then delivered to laser controller  882 , one or more signal processor  883  that, in some embodiments, receives wavelength-determination signals and/or directional-drift signals from the beam pointer module  885  (with its associated wavelength-, beam-off-axis and beam-off-angle detection sensors and circuitry, as described in U.S. Pat. No. 7,199,924 issued Apr. 3, 2007 to Andrew J. W. Brown et al., titled “Apparatus and method for spectral-beam combining of high-power fiber lasers,” which is incorporated herein by reference), and that delivers wavelength-correction control data to laser controller  882 . In some embodiments, laser controller  882  generates the control and power signals that are sent to fiber-laser module  200 , which then delivers the high-power optical beam to beam pointer module  885 , that points and outputs a single output laser beam  92  that is directed toward target  99  (e.g., a tissue of a person to be treated or analyzed, or a material to be conditioned, welded or cut), according to the control information that was generated based on image information obtained from imager  886 , or as directed by manual control of the device  880  or its beam pointer  885 . In some embodiments, system  801  is part of, and used as an element of, a nerve-stimulation and surgical cutting/ablation medical instrument whose output is automatically controlled to prevent undesired damage to collateral tissue (such as described in commonly assigned U.S. Patent Application Publication 2008/0077200 titled “APPARATUS AND METHOD FOR STIMULATION OF NERVES AND AUTOMATED CONTROL OF SURGICAL INSTRUMENTS” by Bendett et al., which is incorporated herein by reference). In some embodiments, system  801  is an entire system that uses the delivery fiber  831  according to the present invention, and benefits by not allowing reflections from the patient (i.e., the target of the laser radiation from the distal end of the delivery fiber) from traveling back into the delivery fiber and thence into the gain fiber. System  801  also benefits from having an easily replaceable and sterile delivery fiber (such as described above in  FIG. 7 ) that does not need an expensive isolator to provide this isolation function. 
     In some embodiments, the present invention provides a method for manufacturing an optical delivery fiber having a non-circular core and a connector, such that the delivery fiber is configured to be connected and disconnected to a system (e.g., a medical-treatment laser system, or the like).  FIG. 9A  is a block diagram of apparatus  901  that includes non-circular core optical delivery fiber  931  and connector  911 B, according to some embodiments of the present invention. In some embodiments, connector  911 B further includes one or more short radially oriented pegs  960  (used for an insert-twist-lock connection and removal capability) and has a cylindrical hole  962  that extends from connector surface  963  through connector  911 B to connector surface  964  configured to accept optical delivery fiber  931 . In some embodiments, the diameter of cylindrical hole  962  is slightly larger than the diameter of delivery fiber  931  (e.g., in some embodiments, about 10 microns to 30 microns larger diameter). In some embodiments, the insertion end of hole  962  is flared or chamfered to facilitate insertion of the delivery fiber  931 . 
       FIG. 9B  is a block diagram of apparatus  902  that shows non-circular core optical delivery fiber  931  inserted through connector  911 B and extending out through connector surface  964  a distance, d,  969 . In some embodiments, optical delivery fiber  931  is secured in cylinder  962 . In some embodiments, the delivery fiber  931  is secured in hole  962  by laser welding, or using adhesive, solder, or the like. In some embodiments, the exposed end  969  of the delivery fiber  931  is cleaved or polished as described below. 
       FIG. 9C  is a block diagram of apparatus  903  that shows non-circular core optical delivery fiber  931  extending through cylinder  962  of connector  911 B such that the end of delivery fiber  931  is coplanar with connector surface  964 . In some embodiments, delivery fiber  931  and connector surface  964  are made coplanar by removing the section of delivery fiber  931  that extended beyond connector surface  964  in  FIG. 9B . In various embodiments, exposed fiber end section  969  is removed by cleaving, polishing, cutting, or the like, such that the end of the fiber  931  is flush with the inside end of connector  911 B. 
       FIG. 9D  is a block diagram of apparatus  904  that shows non-circular core optical delivery fiber  931  attached to connector  911 B. Connector  911 B is configured to releasably attach to adaptor  911 A (in some embodiments, pegs  960  insert and twist into slots  966  in adaptor  911 A), and together, connector  911 B and adaptor  911 A combine to form ferrule  911  which allows delivery fiber  931  to be connected and disconnected to a system capable of outputting laser light. In some embodiments (e.g., laser systems having very high power), the delivery fiber is typically not butt-coupled, as the power levels are too high, and thus in such systems having high average power, a lens is added adjacent the delivery fiber and used to relay an image of the signal beam with a magnification onto the entry end of the delivery fiber, and this image can be greater than one times the spotsize of the signal beam as it exits the gain fiber. In some embodiments, adaptor  911 A further includes optical fiber  910  attached to adaptor  911 A, diverging lens  975  configured to expand forward-propagating light signal  98  to fill first focusing lens  976  having a first focal length, focusing lens  976  is configured to receive forward-propagating light signal  98  output from plano-concave lens  975  (or other optical element that causes the signal beam to diverge) such that the focused light from focusing lens  976  passes through aperture  977 , and a second focusing lens  978  having a second focal length (e.g., a shorter focal length, in some embodiments) is configured to pass the focused signal light  98  received from the first focussing lens  976  and to then focus the signal light  98  onto the core of the delivery fiber  931 . In some embodiments, the first focal length of the first focusing lens  976  is substantially longer than the second focal length of the second focusing lens  978 . In some embodiments, backward-propagating reflected-light  97  will be substantially blocked or otherwise prevented from traveling from the delivery fiber  931  into optical fiber  910  because the backward-propagating reflected-light  97  will pass through the second focusing lens  978  and, because the second focusing lens  978  has a short focal length, will be substantially blocked by aperture  976 . In  FIG. 9D , dotted line  95  represents the location of connector side  964  when the connector  911 B and the adaptor  911 A have been connected together. In some embodiments, optical fiber  910  further includes an integrated beam expanding end cap (e.g., as described in U.S. Pat. No. 7,835,068 titled “PHOTONIC-CRYSTAL-ROD OPTICAL AMPLIFIER WITH SEALED-HOLE ENDCAP AND ASSOCIATED METHOD” that issued Nov. 16, 2010 to Christopher D. Brooks, et al.) to prevent damage to the end of optical fiber  910  due to high-power laser pulses output by the optical fiber  910 . 
       FIG. 10A  is a block diagram of apparatus  1001  that includes non-circular core optical delivery fiber  1031  inserted through angled connector  1011 B, according to some embodiments of the present invention.  FIG. 10A  is substantially the same as  FIG. 9B , described above, except that connector surface  1064  is not perpendicular to the longitudinal axis of fiber  1031 . In some embodiments, the outer connector surface  1063  is substantially perpendicular to the axis of delivery fiber  1031 , and the angle between a plane that is perpendicular to the longitudinal axis of fiber  1031  and the plane of inner connector surface  1064  is a suitable angle between about 5 to about 30 degrees. 
       FIG. 10B  is a block diagram of apparatus  1002  that includes non-circular core optical delivery fiber  1031  and angled connector  1011 B, according to some embodiments of the present invention.  FIG. 10B  is substantially the same as  FIG. 9C , described above, except that, as described in  FIG. 10A  above, connector surface  1064  is not perpendicular to the longitudinal axis of fiber  1031  and therefore the end of delivery fiber  1031  is coplanar with connector surface  1064  and is not perpendicular to the longitudinal axis of fiber  1031 . In some embodiments, angled connector  1011 B is configured to connect to an adaptor  1011 A such as shown in  FIG. 10C . 
     As shown in  FIG. 10C  and  FIG. 10D , a plurality of types and geometries of adaptors are used in various embodiments for connecting to angled connectors  1011 B,  1011 C or  1011 D.  FIG. 10C  is a block diagram of apparatus  1003  that shows non-circular core optical delivery fiber  1031 , angled connector  1011 C and straight adaptor  1011 A, according to some embodiments of the present invention.  FIG. 10D  is a block diagram of apparatus  1004  that includes non-circular core optical delivery fiber  1031 , connector  1011 D and straight adaptor  1011 A, according to some embodiments of the present invention. In  FIG. 10C , connector  1011 C (and in  FIG. 10D , connector  1011 D), the respective connectors are configured to releasably attach to adaptor  1011 A, and together, connector  1011 C and adaptor  1011 A combine to form ferrule  1011  which allows delivery fiber  1031  to be connected and disconnected to a system capable of outputting laser light. (As used herein, the “ferrule” refers to a combination of a “connector” at the end of the fiber, and an “adaptor” on a fixture or enclosure, wherein the adaptor releasably receives and holds the connector.) In some embodiments, adaptor  1011 A further includes optical fiber  1010  attached to adaptor  1011 A, diverging optical element such as a concave lens  1075  configured to expand forward-propagating light signal  98  to fill a first focusing lens  1076  having a first focal length, wherein focusing lens  1076  is configured to receive forward-propagating light signal  98  output from diverging lens  1075  such that the focused light passes through aperture  1077 , and a second focusing lens  1078  having a second focal length that is configured to pass the focused light received from the first focussing lens  1076  and to then focus the signal light  98  onto the core of the delivery fiber  1031 . In some embodiments, the first focal length of the first focusing lens  1076  is substantially longer than the second focal length of the second focusing lens  1078 . In some embodiments, backward-propagating reflected-light  97  will be substantially prevented from traveling from the delivery fiber  1031  to optical fiber  1010  because the backward-propagating reflected-light  97  will pass through the second focusing lens  1078  and, because the second focusing lens  1078  has a short focal length, will be expanded and the periphery will be substantially blocked by aperture  1076 . In  FIG. 10  C (and  FIG. 10D ), dotted line  95  represents the location of connector side  1064  when the connector  1011 C (or connector  1011 D) and the adaptor  1011 A have been connected together. In some embodiments, optical fiber  1010  further includes an integrated beam expanding end cap (e.g., as described in U.S. Pat. No. 7,835,068 titled “PHOTONIC-CRYSTAL-ROD OPTICAL AMPLIFIER WITH SEALED-HOLE ENDCAP AND ASSOCIATED METHOD” that issued Nov. 16, 2010 to Christopher D. Brooks et al.) to prevent damage to the end of optical fiber  1010  due to high-power laser pulses output by the optical fiber  1010 . 
       FIG. 11A  is a block diagram of an improved optical subsystem  1101  that includes an optical gain fiber  210  having a core interfaced through a feedback-isolation-and-adaptor unit  1120  to a multi-mode delivery fiber  231  having a non-circular core, wherein the feedback-isolation-and-adaptor unit  1120  includes mirror  1175 A configured to transmit a majority of forward-propagating amplified signal  98  and to reflect a minority of amplified signal  98  for analysis purposes, according to some embodiments of the present invention. In some embodiments,  FIG. 11A  is substantially similar to feedback-isolation-and-adaptor unit  220  of  FIG. 2 , described above, except that additional lenses  223 ,  1173 , and  1174 , and mirror  1175 A have been added to the feedback-isolation-and-adaptor unit  1120  such that a small portion of forward-propagating amplified signal  98  is reflected to one or more sensors  1176  and a portion of the backward-propagating reflected amplified light  96  is provided to one or more sensors  1177 . Delivery fiber  231  is, in some embodiments, is configured according to any of the methods and apparatus described above for  FIGS. 2, 3, 4A, 4B, 4C, 4D, 4E, 4F, 4G, 4H, 5A, 5B, 6A, 6B, 6C, 6D, 6E, 6F, 6G, 6H, 6I ,  6 J,  6 K,  6 L,  6 M,  6 N,  6   o ,  6 P,  6 Q,  6 R,  6 S,  6 T,  6 U,  6 V,  7 ,  8 ,  9 A,  9 B,  9 C,  9 D,  10 A,  10 B,  10 C, or  10 D. 
     In some embodiments, beamsplitting mirror  1175 A transmits about 99.5% of the incident amplified signal  98  and reflects about 0.5% of the incident amplified signal. In some other embodiments, mirror  1175 A transmits about 99.0% of the incident amplified signal  98  and reflects about 1.0% of the incident amplified signal. In some other embodiments, mirror  1175 A transmits about 98.5% of the incident amplified signal  98  and reflects about 1.5% of the incident amplified signal. In some other embodiments, mirror  1175 A transmits about 98.0% of the incident amplified signal  98  and reflects about 2.0% of the incident amplified signal. In some other embodiments, mirror  1175 A transmits about 95.0% of the incident amplified signal  98  and reflects about 5.0% of the incident amplified signal. In some other embodiments, mirror  1175 A transmits about 90.0% of the incident amplified signal  98  and reflects about 10.0% of the incident amplified signal. 
     In some embodiments, feedback-isolation-and-adaptor unit  1120  includes focusing lens  1173  configured to focus the reflected portion  98 ′ of the amplified light  98  that is reflected from mirror  1175 A into ferrule  1171  in order to provide the reflected portion  98 ′ to the one or more sensors  1176 . In some embodiments, feedback-isolation-and-adaptor unit  1120  further includes focusing lens  1174  configured to focus the reflected portion  97 ′ of light reflected by mirror  1175 A from a backward-propagating signal  97  into ferrule  1172  in order to provide the reflected light to one or more sensors  1177 . In some embodiments, mirror  1175 A provides the ability to simultaneously tap forward-propagating and backward-propagating optical signals in optic laser system  1101  for sensing, analysis and/or control (e.g., in some embodiments, the tapped signals are used to control a feedback loop that controls operation of laser system  1101 ). 
     In some embodiments, optical laser system  1101  further includes a seed source  209  (e.g., in some embodiments, seed source  209  includes a laser such as a diode laser or an optical-fiber laser that is optically pumped using light from a diode-laser system) that generates an optical seed signal (in other embodiments, a controlled-bandwidth ASE source is used for generating the seed signal, such as described in U.S. Pat. No. 7,539,231 issued May 26, 2009 to Eric C. Honea et al. titled “Apparatus and method for generating controlled-linewidth laser-seed-signals for high-powered fiber-laser amplifier systems,” which is incorporated herein by reference). In some embodiments, an optical fiber carrying seed signal is connected between seed source  209  and the input port of a pump dump  1191 , which passes light of the wavelength of the seed signal but which “dumps” (blocks, diverts or otherwise absorbs) light of the wavelength of the pump light from seed laser  209  or backward-propagating pump light from optical power amplifier  1196  (also called an optical-amplifier subsystem  1196 ). In some embodiments, system  1101  omits a two-port optical isolator  1122  that would be used for isolating high-power optical signals (e.g., in various embodiments, such high-power pulses are pulses having a peak power of at least 1000 watts, or even at least 10,000 watts, at least 100,000 watts, or even at least 1,000,000 watts); and instead uses the isolation techniques and mechanisms as described in  FIGS. 2, 3, 4A, 4B, 4C, 4D, 4E, 4F, 4G, 4H, 5A, 5B, 6A, 6B, 6C, 6D, 6E, 6F, 6G, 6H, 6I ,  6 J,  6 K,  6 L,  6 M,  6 N,  6   o ,  6 P,  6 Q,  6 R,  6 S,  6 T,  6 U,  6 V,  7 ,  8 ,  9 A,  9 B,  9 C,  9 D,  10 A,  10 B,  10 C, or  10 D. In other embodiments, isolator  1122  is included and supplements those additional isolation techniques and mechanisms. In some embodiments, system  1101  also includes a power optical amplifier  1196  that is capable of amplifying (and which, when supplied with suitable seed signals and optical pump light, does amplify) high-power optical signals (e.g., such that, in various embodiments, the output pulses have a peak power of at least 10,000 watts, or at least 100,000 watts or even at least 1,000,000 watts). 
     In some embodiments, power amplifier  1196  includes a plurality of pump lasers  1192  that are connected by optical fibers  1193  to a pump combiner  1194  that inserts the seed signal into a core of gain fiber  210 . In some embodiments, a “backward tap” to obtain a portion of any backward-propagating light coming from delivery fiber  231  is provided, and this backward-propagating light is coupled to one or more sensors  1177 . In some embodiments, the sensors  1177  generate one or more electrical signals to controller  1195  that are each indicative of the different respective optical properties of the backward-tap signal that were measured by sensors  1177 . 
     In some embodiments, sensors  1176  receive a portion of the forward-propagating light coming from gain fiber  210  and generate one or more electrical signals to controller  1195  that are each indicative of the different respective optical properties of the forward-tap signal that were measured by sensors  1176 . In some embodiments, controller  1195  is used to control one or more operations of system  1101  (e.g., in some embodiments, controller  1195  can turn off the electrical power to the pump lasers  1192  via electrical control or power line(s)  1197 , and/or can alter the operation of the seed source  209  via electrical control or power line(s)  1198  (e.g., in some embodiments, if the backward tap and sensors  1177  indicate excess reflected power (or ASE (amplified spontaneous emission) or SBS (stimulated Brillouin scattering) or other noise from downstream optical amplifiers) from delivery fiber  231  into the gain fiber  210 , controller  1195  can force the seed source  209  to emit a continuous-wave (CW) signal that would bleed the excess stored energy from the gain fiber). 
       FIG. 11B  is a block diagram of an improved optical laser system  1102  that includes an optical gain fiber  210  having a core interfaced through a feedback-isolation-and-adaptor unit  1120 ′ to a multi-mode delivery fiber  231  having a non-circular core, wherein the feedback-isolation-and-adaptor unit  1120 ′ includes mirror  1175 B, according to some embodiments of the present invention. In some embodiments, optical laser system  1102  is substantially similar to optical laser system  1101  shown in  FIG. 11A  and described above expect that mirror  1175 B of  FIG. 11B  is configured to reflect a majority of forward-propagating amplified signal  98  and to transmit a minority portion  98 ′ of the amplified signal  98  for analysis purposes, according to some embodiments of the present invention. In some embodiments, reflecting a majority of the forward-propagating amplified signal  98  with mirror  1175 B, as opposed to passing a majority of the amplified signal  98  through mirror  1175 B, prevents the possible overheating of the mirror  1175 B and distortion and degradation of the amplified optical signal  98  due to mirror  1175 B absorbing power from amplified signal  98 . 
     In some embodiments, mirror  1175 B reflects about 99.5% of the incident amplified signal  98  and transmits about 0.5% of the incident amplified signal. In some other embodiments, mirror  1175 B reflects about 99.0% of the incident amplified signal  98  and transmits about 1.0% of the incident amplified signal. In some other embodiments, mirror  1175 B reflects about 98.5% of the incident amplified signal  98  and transmits about 1.5% of the incident amplified signal. In some other embodiments, mirror  1175 B reflects about 98.0% of the incident amplified signal  98  and transmits about 2.0% of the incident amplified signal. In some other embodiments, mirror  1175 B reflects about 95.0% of the incident amplified signal  98  and transmits about 5.0% of the incident amplified signal. In some other embodiments, mirror  1175 B reflects about 90.0% of the incident amplified signal  98  and transmits about 10.0% of the incident amplified signal. 
     In some embodiments, the non-circular core of the deliver fiber is defined by photonic crystal structures such as U.S. Patent Publication 2003/0165313 by Broeng et al., which published Sep. 4, 2003 titled “Optical fibre with high numerical aperture, method of its production, and use thereof” which is incorporated herein by reference, (except having a non-circular core defined by the photonic crystal structures) or such as longitudinal holes similar to those as described in U.S. Pat. No. 7,391,561 titled “Fiber- or rod-based optical source featuring a large-core, rare-earth-doped photonic-crystal device for generation of high-power pulsed radiation and method” that issued Jun. 24, 2008 to Fabio Di Teodoro et al., except using larger hole sizes and/or smaller hole spacings (in order to create a large numerical aperture and thus promote mixing and/or multimode propagation) and having substantially no doping with species that absorb light of the signal wavelength. A PCF (photonic-crystal fiber) core having relatively small photonic-crystal holes and relatively large hole-to-hole spacings will have a relatively low numerical aperture and is typically single moded and the concept of isolation used in some embodiments of the present invention is based on a multimode core. In some embodiments, the photonic crystal structures define a non-circular core having a relatively high numerical aperture. 
     In some embodiments, the present invention provides a method that includes amplifying an optical signal in an optical-gain-fiber subsystem, wherein the optical-gain-fiber subsystem includes an output end, outputting the amplified optical signal in a forward-propagating direction from the output end of the optical-gain-fiber subsystem as a high-brightness optical beam having a first Rayleigh range, receiving the amplified optical signal from the output end of the optical-gain-fiber subsystem into a first end of a delivery fiber having a non-circular waveguide core, wherein the delivery fiber is interfaced to the optical-gain-fiber subsystem, outputting the amplified optical signal from a second end of the delivery fiber wherein the delivery fiber includes a non-circular waveguide core, and, without the use of a non-linear optical isolator, inhibiting light traveling in a backward-propagating direction in the delivery fiber from entering the optical-gain-fiber subsystem in the backward-propagating direction. 
     In some embodiments of the method, inhibiting of light traveling in a backward-propagating direction further includes offsetting a light-propagation axis of light exiting the output end of the optical-gain-fiber subsystem from a light-propagation axis of light exiting the first end of the delivery fiber relative to one another such that a majority of the light traveling in the backward-propagating direction and emitted from the first end of the delivery fiber does not enter a core of the optical-gain-fiber subsystem. 
     In some embodiments of the method, the light traveling in the backward-propagating direction in the delivery fiber that exits the first end of the delivery fiber toward the optical-gain-fiber subsystem has a second Rayleigh range that is shorter than the first Rayleigh range of the optical signal output from the optical-gain-fiber subsystem. 
     Some embodiments of the method further include providing an aperture located between the output end of the optical-gain-fiber subsystem and the first end of the delivery fiber, passing a majority of the forward-propagating optical signal through the aperture, blocking a majority portion of the light traveling in the backward-propagating direction that exits the first end of the delivery fiber toward the optical-fiber amplifier subsystem, and preventing a majority portion of the light traveling in the backward-propagating direction from entering the output end of the optical-gain-fiber subsystem. 
     Some embodiments of the method further include providing an endcap attached to the first end of the delivery fiber, providing an aperture in the endcap and located between the output end of the optical-gain-fiber subsystem and the first end of the delivery fiber, passing a majority of the forward-propagating optical signal through the aperture, blocking a majority portion of the light traveling in the backward-propagating direction that exits the first end of the delivery fiber toward the optical-fiber amplifier subsystem; and, preventing a majority portion of the light traveling in the backward-propagating direction from entering the output end of the optical-gain-fiber subsystem. 
     Some embodiments of the method further include providing a first lens and a second lens separated by a first distance, optically coupling the first lens to the output end of the optical-gain-fiber subsystem and to the second lens, optically coupling the second lens to the first lens and to the first end of the delivery fiber, collimating the forward-propagating optical signal from the optical-gain-fiber subsystem using the first lens, and receiving the collimated forward-propagating optical signal from the first lens and focusing the forward-propagating optical signal using the second lens such that the forward-propagating optical signal enters the first end of the delivery fiber. 
     In some embodiments of the method, the first distance is greater than the second Rayleigh range, and the first distance is not greater than the first Rayleigh range. In other embodiments of the method, the first distance is greater than the second Rayleigh range, and the first distance is less than the first Rayleigh range. 
     Some embodiments of the method further include providing a reflecting optical element located between the first lens and the second lens, reflecting the forward-propagating optical signal and the light traveling in the backward-propagating direction using the reflecting optical element in order to increase a distance the forward-propagating optical signal and the light traveling in the backward-propagating direction travel between the first lens and the second lens. 
     Some embodiments of the method further include folding an optical path of the forward-propagating light between the first lens and the second lens. 
     Some embodiments of the method further include providing a reflecting optical element located between the optical-gain-fiber subsystem and the delivery fiber, reflecting the forward-propagating optical signal and the light traveling in the backward-propagating direction using the reflecting optical element in order to increase a distance the forward-propagating optical signal and the light traveling in the backward-propagating direction travel between the optical-gain-fiber subsystem and the delivery fiber. 
     Some embodiments of the method further include providing a reflecting optical element located between the optical-gain-fiber subsystem and the delivery fiber, reflecting the forward-propagating optical signal and the light traveling in the backward-propagating direction using the reflecting optical element in order to shorten a footprint of the optical-gain-fiber subsystem while providing a suitable distance the forward-propagating optical signal and the light traveling in the backward-propagating direction travel between the optical-gain-fiber subsystem and the delivery fiber. In some embodiments, the suitable distance is a distance that is longer than a spacing between the optical-gain-fiber subsystem and the delivery fiber. 
     Some embodiments of the method further include folding an optical path between the optical-gain-fiber subsystem and the delivery fiber. 
     In some embodiments of the method, the delivery fiber has a sufficiently large numerical aperture such that substantially all of the light output from the optical-gain-fiber subsystem enters the non-circular core of the delivery fiber. 
     In some embodiments of the method, the numerical aperture of the delivery fiber is between about 0.2 and about 0.6. 
     In some embodiments, the present invention provides an apparatus that includes an optical-fiber amplifier subsystem configured to amplify an optical signal, wherein the optical-fiber amplifier subsystem includes an output end, and wherein the output end of the optical-fiber amplifier subsystem is configured to output the optical signal in a forward-propagating direction as a high-brightness optical beam having a first Rayleigh range, and a delivery fiber that has an interface to the optical-fiber amplifier subsystem and that has a first end and a second end, wherein the delivery fiber is configured to receive the optical signal from the output end of the optical-fiber amplifier subsystem into the first end of the delivery fiber, and wherein the delivery fiber is configured to output the optical signal from the second end of the delivery fiber, wherein the delivery fiber includes a non-circular waveguide core, and wherein the apparatus is configured such that light traveling in a backward-propagating direction in the delivery fiber is substantially prevented from entering the optical-fiber amplifier subsystem in the backward-propagating direction. 
     In some embodiments, the apparatus is configured such that an axis of the output end of the optical-fiber amplifier subsystem and an axis of the first end of the delivery fiber are offset relative to one another such that a majority of the light traveling in the backward-propagating direction and emitted from the first end of the delivery fiber does not enter a core of the optical-fiber amplifier subsystem. 
     In some embodiments, the light traveling in the backward-propagating direction in the delivery fiber that exits the first end of the delivery fiber toward the optical-fiber amplifier subsystem has a second Rayleigh range that is shorter than the first Rayleigh range of the forward-propagating optical signal output from the optical optical-fiber amplifier subsystem. 
     In some embodiments, the apparatus is configured such that an optical axis of the output end of the optical amplifier and an axis of the first end of the delivery fiber are aligned offset relative to one another such that a majority of light reflected from the second end of the delivery fiber and emitted from the first end of the delivery fiber does not enter a core of the optical-amplifier subsystem. 
     In some embodiments, the apparatus further includes an aperture located between the output end of the optical-fiber amplifier subsystem and the first end of the delivery fiber, wherein the aperture is configured to pass a majority of the forward-propagating optical signal through the aperture and wherein the aperture is configured to block a majority portion of the light traveling in the backward-propagating direction that exits the first end of the delivery fiber toward the optical-fiber amplifier subsystem from entering the output end of the optical-fiber amplifier subsystem. 
     In some embodiments, the apparatus further includes a first lens optically coupled to the output end of the optical-fiber amplifier subsystem, wherein the first lens is configured to substantially collimate the forward-propagating optical signal from the optical-fiber amplifier subsystem, a second lens optically coupled to the first end of the delivery fiber, wherein the second lens is configured to receive the collimated forward-propagating optical signal from the first lens and to focus the forward-propagating optical signal such that the forward-propagating optical signal enters the first end of the delivery fiber, and wherein the first lens and the second lens are separated by a first distance. 
     In some embodiments, the first distance is greater than the second Rayleigh range, and the first distance is not greater than the first Rayleigh range. 
     In some embodiments, the first distance is greater than the second Rayleigh range, and the first distance is less than the first Rayleigh range. 
     In some embodiments, the apparatus further includes a reflecting optical element located between the optical-fiber amplifier subsystem and the delivery fiber, wherein the reflecting optical element is configured to provide at least one reflection of the forward-propagating optical signal and of the light traveling in the backward-propagating direction in order to fold an optical path of the forward-propagating optical signal and of the light traveling in the backward-propagating direction between the optical-fiber amplifier subsystem and the delivery fiber. 
     In some embodiments, the apparatus further includes a reflecting optical element located between the first lens and the second lens, wherein the reflecting optical element is configured to provide at least one reflection of the forward-propagating optical signal and of the light traveling in the backward-propagating direction in order to fold an optical path of the forward-propagating optical signal and of the light traveling in the backward-propagating direction between the first lens and the second lens. 
     In some embodiments, the delivery fiber is configured to have a sufficiently large numerical aperture (e.g., in some embodiments, an NA that is no less than 0.1) such that substantially all of the light output from the signal fiber enters the non-circular core of the delivery fiber. 
     In some embodiments, the apparatus further includes a partially reflecting mirror located between the first lens and the second lens, wherein the partially reflecting mirror is configured to reflect a minority portion of the collimated forward-propagating optical signal from the first lens to a first sensor and to transmit a majority portion of the collimated forward-propagating optical signal to the second lens, and wherein the reflecting mirror is further configured to reflect a minority portion of the light traveling in the backward-propagating direction to a second sensor. 
     In some embodiments, the apparatus further includes a partially reflecting mirror located between the first lens and the second lens, wherein the partially reflecting mirror is configured to reflect a majority portion of the collimated forward-propagating optical signal from the first lens to the second lens, and wherein the reflecting mirror is further configured to transmit a minority portion of the forward-propagating light toward a first sensor and to transmit a minority portion of the backward-propagating optical signal to a second sensor. 
     In some embodiments, the present invention provides a method that includes providing a delivery fiber assembly having a reverse endcap and a fiber portion, optically coupling the reverse endcap and the fiber portion, wherein the reverse endcap has a solid first core having a first core diameter, wherein the fiber portion has a non-circular second core having a second core diameter; and wherein the first core diameter is larger than the second core diameter. 
     Some embodiments of the method further include receiving a forward-propagating optical signal into the solid first core of the reverse endcap, passing a majority of the forward-propagating optical signal into the non-circular second core with substantially no degradation in brightness, wherein light traveling in a backward-propagating direction has a brightness that is degraded by the reverse endcap. 
     Some embodiments of the method further include providing an optical-amplifier subsystem, wherein the optical-amplifier subsystem has an output adaptor attaching a connector to the first endcap of the delivery fiber, wherein the adaptor and the connector are configured to releasably connect and disconnect with each other. 
     Some embodiments of the method further include providing a removable sterile enclosure, inserting the delivery fiber into the removable sterile enclosure, sealing the removable sterile enclosure and sterilizing both such that the delivery fiber is kept in a sterile condition until use, and wherein the delivery fiber is replaceably connectable to the optical-amplifier subsystem, and wherein the delivery fiber is optionally disposable. 
     In some embodiments of the method, the first endcap has a multi-mode core and the fiber portion has a multi-mode core. 
     In some embodiments of the method, the non-circular second core has a substantially polygonal-shaped. 
     In some embodiments of the method, the first endcap and the fiber portion delivery fiber each have a sufficiently large numerical aperture such that substantially all of the forward-propagating optical signal enters the non-circular core of the fiber portion. 
     In some embodiments of the method, the numerical aperture of the delivery fiber is between about 0.2 and about 0.3. In some embodiments of the method, the numerical aperture of the delivery fiber is between about 0.3 and about 0.4. In some embodiments of the method, the numerical aperture of the delivery fiber is between about 0.4 and about 0.5. In some embodiments of the method, the numerical aperture of the delivery fiber is between about 0.5 and about 0.6. 
     In some embodiments of the method, the non-circular second core has a substantially square shape. 
     In some embodiments of the method, the non-circular second core includes a substantially oval shape. 
     In some embodiments, the present invention provides an apparatus that includes a delivery fiber having a reverse endcap and a fiber portion optically coupled to the reverse endcap, wherein the reverse endcap has a solid first core having a first core diameter, wherein the fiber portion has a non-circular second core having a second core diameter, and wherein the first core diameter is larger than the second core diameter. 
     In some embodiments of the apparatus, the delivery fiber is configured to receive a forward-propagating optical signal into the solid first core of the reverse endcap and pass a majority of the forward-propagating optical signal into the non-circular second core with substantially no degradation in brightness, while the reverse endcap does degrade brightness of light traveling in a backward-propagating direction opposite the forward-propagating optical signal. 
     In some embodiments of the apparatus, the apparatus further includes an optical-amplifier subsystem having an output adaptor mechanically connected to the optical-amplifier subsystem, wherein the delivery fiber has a connector mechanically connected to the delivery fiber, and wherein the adaptor and the connector are configured to releasably connect to and disconnect from each other. 
     In some embodiments of the apparatus, the first endcap has a multi-mode core and the fiber portion has a multi-mode core. 
     In some embodiments of the apparatus, the apparatus further includes a removable sterile enclosure surrounding the delivery fiber for holding the delivery fiber in a sterile condition until use, wherein the delivery fiber is replaceably connectable to the optical-amplifier subsystem, and wherein the delivery fiber is disposable. 
     In some embodiments of the apparatus, the non-circular second core has a substantially polygonal-shaped. 
     In some embodiments of the apparatus, the first endcap and the fiber portion delivery fiber each have a sufficiently large numerical aperture such that substantially all of the forward-propagating optical signal enters the non-circular core of the fiber portion. In some embodiments, the numerical aperture of the delivery fiber is between about 0.2 and about 0.3. In some embodiments, the numerical aperture of the delivery fiber is between about 0.3 and about 0.4. In some embodiments, the numerical aperture of the delivery fiber is between about 0.4 and about 0.5. In some embodiments, the numerical aperture of the delivery fiber is between about 0.5 and about 0.6. In some embodiments, the numerical aperture of the delivery fiber is between about 0.6 and about 0.75. 
     In some embodiments of the apparatus, the non-circular second core has a substantially square shape. 
     In some embodiments of the apparatus, the non-circular second core has a substantially oval shape. 
     In some embodiments of the apparatus, the non-circular second core has a substantially polygonal-shaped. 
     In some embodiments of the apparatus, the non-circular second core is defined by and surrounded by an airclad region. 
     In some embodiments of the apparatus, the non-circular second core includes stress rods. 
     In some embodiments, the present invention provides an apparatus that includes for delivering a forward-propagating signal to a destination from a signal-providing fiber having an output end, the apparatus comprising, a delivery fiber having a first end and a second end, wherein forward-propagating light travels to the second end from the first end and backward-propagating light travels to the first end from the second end, and wherein the delivery fiber includes a non-circular waveguide core and a cladding layer surrounding the non-circular waveguide core, and a first endcap that is connected to the first end of the delivery fiber, wherein the endcap operates to pass a majority of forward-propagating signal light output from the signal-providing fiber into the delivery fiber and pass less than a majority of backward-propagating light into the output end of the signal-providing fiber from the second end of the delivery fiber. In some embodiments, the endcap operates to pass at least 60% of the forward-propagating signal light output and pass no more than 40% of backward-propagating light. In some embodiments, the endcap operates to pass at least 70% of the forward-propagating signal light output and pass no more than 30% of backward-propagating light. In some embodiments, the endcap operates to pass at least 80% of the forward-propagating signal light output and pass no more than 20% of backward-propagating light. In some embodiments, the endcap operates to pass at least 90% of the forward-propagating signal light output and pass no more than 10% of backward-propagating light. In some embodiments, the endcap operates to pass at least 95% of the forward-propagating signal light output and pass no more than 5% of backward-propagating light. In some embodiments, the endcap operates to pass at least 98% of the forward-propagating signal light output and pass no more than 2% of backward-propagating light. 
     In some embodiments, the endcap includes an aperture, wherein the aperture passes the majority of forward-propagating signal light that is output from the signal-providing fiber into the delivery fiber and wherein the aperture blocks a majority of the backward-propagating signal light. In some embodiments, the aperture operates to pass at least 60% of the forward-propagating signal light output and pass no more than 40% of backward-propagating light. In some embodiments, the aperture operates to pass at least 70% of the forward-propagating signal light output and pass no more than 30% of backward-propagating light. In some embodiments, the aperture operates to pass at least 80% of the forward-propagating signal light output and pass no more than 20% of backward-propagating light. In some embodiments, the aperture operates to pass at least 90% of the forward-propagating signal light output and pass no more than 10% of backward-propagating light. In some embodiments, the aperture operates to pass at least 95% of the forward-propagating signal light output and pass no more than 5% of backward-propagating light. In some embodiments, the aperture operates to pass at least 98% of the forward-propagating signal light output and pass no more than 2% of backward-propagating light. 
     In some embodiments of the apparatus, the non-circular waveguide core is defined by and surrounded by an air-clad region. 
     In some embodiments of the apparatus, the non-circular waveguide core includes stress-rods. 
     In some embodiments of the apparatus, the non-circular waveguide core has a shape that is substantially polygonal. 
     In some embodiments of the apparatus, the non-circular waveguide core has a shape that is substantially square. 
     In some embodiments of the apparatus, the non-circular waveguide core has a shape that is substantially oval. 
     In some embodiments, an optical axis of the first end of the delivery fiber is tilted relative to an optical axis of the output end of the signal-providing fiber in order to reduce back reflected light from the delivery fiber from entering the signal-providing fiber. 
     In various embodiments, the tilt angle is substantially 1°, or substantially 2°, or substantially 3°, or substantially 4°, or substantially 5°, or substantially 6°, or substantially 7°, or substantially 8°, or substantially 9°, or substantially 10°. In some embodiments, the tilt angle is at least 5°, or at least 10°, or at least 15°, or at least 20°. 
     In some embodiments, a central optical axis of the first end of the delivery fiber is substantially parallel but laterally offset relative to a central optical axis of the output end of the signal-providing fiber in order to reduce back reflected light from the delivery fiber from entering the signal-providing fiber. 
     In various embodiments, the lateral offset amount between the central optical axis of the first end of the delivery fiber and the central optical axis of the output end of the signal-providing fiber is at least 5 microns, or at least 10 microns, or at least 15 microns, or at least 20 microns, or at least 25 microns, or at least 30 microns, or at least 35 microns, or at least 40 microns, or at least 45 microns, or at least 50 microns, or at least 75 microns, or at least 100 microns, or at least 250 microns. 
     In some of the embodiments, the delivery fiber is configured to have a sufficiently large numerical aperture such that substantially all of the light output from the signal fiber enters the non-circular core of the delivery fiber. 
     In various ones of the embodiments, the numerical aperture of the delivery fiber is between about 0.1 and about 0.6, between about 0.1 and about 0.12, between about 0.12 and about 0.15, between about 0.1 and about 0.15, between about 0.15 and about 0.2, between about 0.2 and about 0.3, between about 0.3 and about 0.4, between about 0.4 and about 0.5, between about 0.5 and about 0.6, or between about 0.6 and about 0.7. In general, by using non-circular fibers the NA fills very fast, i.e. in a short distance. For circular fibers the fundamental mode can propagate a long distance and then one should specify the NA of the fundamental mode. 
     In various ones of the embodiments, the numerical aperture of the output end of the signal source is about 0.01, about 0.02, about 0.05, about 0.1, about 0.12, about 0.15, about 0.18, about 0.2, about 0.25, about 0.3, or larger than about 0.3. 
     In some of the embodiments, the delivery fiber is configured to propagate a plurality of modes of light through the delivery fiber (i.e., in some embodiments, the delivery fiber is a multimode fiber). 
     In some of the embodiments, the entry aperture of the delivery fiber is non-circular. 
     In some of the embodiments, the entry aperture of the delivery fiber is laterally offset from an optical axis of the endcap. 
     In some of the embodiments, the entry aperture is non-centered with respect to the longitudinal axis of the non-circular core of the delivery fiber. 
     In some of the embodiments, the delivery fiber is connectorized and disposable. 
     In some of the embodiments, the delivery fiber is sterilized and enclosed in a sealed removable package. 
     Some embodiments of the apparatus further include an optical-amplifier subsystem having a first output connector mechanically connected to the optical-amplifier subsystem, wherein the delivery fiber has a second connector mechanically connected to delivery fiber, and wherein the first connector and the second connector are configured to releasably connect and disconnect with each other. 
     Some embodiments of the apparatus further include a medical instrument that includes an electrical power supply, a controller operably coupled to receive power from the electrical power supply, an optical-signal source operably coupled to be controlled by the controller and having a first output connector mechanically connected to the optical-signal source, wherein the delivery fiber has a second connector mechanically connected to delivery fiber, and wherein the first connector and the second connector are configured to releasably connect and disconnect with each other. 
     In some embodiments, the present invention provides a system that includes an optical-fiber amplifier subsystem that amplifies an optical signal, wherein the optical-fiber amplifier subsystem includes an output end, and wherein the output end is outputs the optical signal; and a delivery fiber having a first end and a second end, wherein the delivery fiber receives the optical signal from the output end of the optical-fiber amplifier subsystem into the first end of the delivery fiber and outputs the optical signal from the second end of the delivery fiber, wherein the delivery fiber includes a non-circular waveguide core. 
     In some embodiments of the system, light traveling in the backward-propagating direction from the second end of the delivery fiber toward the first end of the delivery fiber exits the first end of the delivery fiber toward the output end of the optical-amplifier subsystem having a reflected-light-signal Rayleigh range that is shorter than the first Rayleigh range of the optical beam of the optical-amplifier subsystem. 
     In some embodiments, the system is configured such that a longitudinal axis of the output end of the optical-amplifier subsystem and a longitudinal axis of the first end of the delivery fiber are aligned offset relative to one another such that a majority of light traveling in the backward-propagating direction and emitted from the first end of the delivery fiber does not enter a core of the optical-amplifier subsystem. 
     In some embodiments, the system is configured such that an axis of the output end of the optical-amplifier subsystem and an axis of the first end of the delivery fiber are aligned offset relative to one another such that a majority of light reflected from the second end of the delivery fiber and emitted from the first end of the delivery fiber does not enter a core of the optical-amplifier subsystem. 
     In some embodiments, the present invention provides a method for delivering a forward-propagating signal to a destination from a signal-providing fiber having an output end through a delivery fiber, the method including providing the delivery fiber, the delivery fiber having a first end and a second end, wherein forward-propagating light in the delivery fiber travels to the second end from the first end and backward-propagating light travels to the first end from the second end, wherein the delivery fiber includes a non-circular waveguide core, a cladding layer surrounding the non-circular waveguide core, and a first endcap that is connected to the first end of the delivery fiber, and wherein the delivery fiber and the endcap are free of any non-linear optical isolator. This method further includes passing a majority of forward-propagating signal light output from the signal-providing fiber into the delivery fiber; and passing less than a majority of light of back-reflected light into the output end of the signal-providing fiber from the second end of the delivery fiber. 
     In some embodiments of the method, the forward-propagating signal light is not optically isolated with non-linear optics. In some embodiments of the method, the providing of the delivery fiber includes forming the non-circular waveguide core having a shape that is substantially polygonal. In some embodiments of the method, the providing of the delivery fiber includes forming the non-circular waveguide core having a shape that is substantially square. In some embodiments of the method, the providing of the delivery fiber includes forming the non-circular waveguide core having a shape that is substantially oval. 
     In some embodiments of the method, the providing of the delivery fiber includes tilting an optical axis of the first end of the delivery fiber relative to an optical axis of the output end of the signal-providing fiber in order to reduce back reflected light from the delivery fiber from entering the signal-providing fiber. In some other embodiments of the method, the providing of the delivery fiber includes positioning a central optical axis of the first end of the delivery fiber to be substantially parallel but laterally offset relative to a central optical axis of the output end of the signal-providing fiber in order to reduce back reflected light from the delivery fiber from entering the signal-providing fiber. 
     In some embodiments of the method, the providing of the delivery fiber includes providing the delivery fiber with a sufficiently large numerical aperture such that substantially all of the light output from the signal fiber enters the non-circular core of the delivery fiber. In some embodiments of the method, the numerical aperture of the delivery fiber is between about 0.2 and about 0.3. In some embodiments of the method, the numerical aperture of the output end of the signal source is between about 0.01 and about 0.15. 
     In some embodiments of the method, the passing of the majority of forward-propagating signal light output includes propagating multiple modes of light through the delivery fiber. In some embodiments of the method, the providing of the delivery fiber includes making the entry aperture of the delivery fiber non-circular. 
     In some embodiments, the method further includes laterally offsetting the entry aperture of the delivery fiber from a central optical axis of the endcap. In some embodiments, the method further includes non-centering the entry aperture with respect to the non-circular core of the delivery fiber. In some embodiments, the method further includes connectorizing the delivery fiber, wherein the delivery fiber is single-use and disposable. In some embodiments, the method further includes sterilizing the delivery fiber and enclosing the delivery fiber in a sealed removable package. In some such embodiments, the sterilizing of the delivery fiber is done after the enclosing of the delivery fiber in a sealed removable package. In some embodiments, the delivery fiber is then stored until needed, then removed from the package, attached to the gain fiber, and used. 
     In some embodiments, the method further includes providing an optical-amplifier subsystem having a first output connector mechanically connected to the optical-amplifier subsystem; mechanically connecting a second connector to the delivery fiber; and releasably connecting the first connector and the second connector with each other. 
     In some embodiments, the method further includes providing a medical instrument that includes: an electrical power supply, a controller operably coupled to receive power from the electrical power supply, an optical-signal source operably coupled to be controlled by the controller and having a first output connector mechanically connected to the optical-signal source, mechanically connecting a second connector to the delivery fiber; and releasably connecting the first connector and the second connector with each other. 
     In some embodiments, the present invention provides an apparatus that includes means for amplifying an optical signal, the means (as described herein and equivalents thereof) for amplifying having an output end; means (as described herein and equivalents thereof) for outputting the amplified optical signal in a forward-propagating direction from the output end as a high-brightness optical beam having a first Rayleigh range; non-circular waveguide means for delivering an optical signal; means (as described herein and equivalents thereof) for receiving the amplified optical signal from the output end of the means for amplifying into a first end of the non-circular waveguide means for delivering; means (as described herein and equivalents thereof) for interfacing the non-circular waveguide means for delivering to the means for amplifying; means (as described herein and equivalents thereof) for outputting the amplified optical signal from a second end of the non-circular waveguide means for delivering; and means (as described herein and equivalents thereof) for inhibiting light traveling in a backward-propagating direction in the non-circular waveguide means for delivering from entering the means for amplifying in the backward-propagating direction. 
     It is to be understood that the above description is intended to be illustrative, and not restrictive. Although numerous characteristics and advantages of various embodiments as described herein have been set forth in the foregoing description, together with details of the structure and function of various embodiments, many other embodiments and changes to details will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein,” respectively. Moreover, the terms “first,” “second,” and “third,” etc., are used merely as labels, and are not intended to impose numerical requirements on their objects.