Abstract:
Disclosed is an external cavity diode laser system comprising a dispersive unit having a proximal and distal end; a gain element, located proximally from the dispersive unit, and that produces coherent light incident upon the dispersive unit, and the dispersive unit dispersing the incident coherent light into dispersed light, the dispersed light comprising a reflected diffraction beam and at least one of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission; a guiding unit, located proximally from the dispersive unit, that guides the dispersed light diffracted upon it from the dispersive unit while maintaining an angular separation between the reflected diffraction beam and at least one of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission; and a physical filtering device that physically filters the reflected diffraction beam from the spatially separated at least one angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission guided to the physical filtering device by the guiding unit to produce a low-noise laser beam. Also disclosed are methods relating to producing a low-noise laser beam.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to grating-tuned external cavity lasers and more particularly to a method and apparatus for generating a continuously-tunable, low-noise laser beam in a grating-tuned external cavity laser.  
           [0003]    2. Description of Related Art  
           [0004]    Grating-tuned external cavity lasers produce continuously-tunable laser beams consisting of light with high coherence and very narrow linewidth. To obtain high coherence and narrow linewidth, a grating is generally employed to disperse the emission from a light source or gain element, and feed it back to the gain medium at a wavelength selected by a tuning device. Tunable laser beams can be produced either by rotating a grating in a Littrow-type arrangement, or a reflector in a Littman-type configuration. Littman-type tunable laser systems are described in the publications, “Spectrally Narrow Pulse Dye Laser Without Beam Expander,” by Michael G. Littman and Harold J. Metcalf,  Applied Optics,  Vol. 17, No. 14, pages 2224-2227, Jul. 15, 1978, and “Narrowband Operation Of A Pulsed Dye Laser Without Intracavity Beam Expansion” by I. Shoshan, N. N. Dannon, and U. P. Oppenheim,  Journal of Applied Physics,  Vol. 48, pages 4495-4497, 1977. A single-longitudinal-mode (very narrow linewidth) frequency tunable pulsed dye laser was described in the publication, “Single-Mode Pulsed Tunable Dye Laser,” by M. G. Littman, Optics Letters, Vol. 23, pages 138-140, 1978. This single-longitudinal mode laser provides a foundation for producing tunable narrow-bandwidth lasers.  
           [0005]    [0005]FIG. 1 shows a prior art grating-tuned external cavity laser capable of producing a laser beam which is tunable over a broad range of wavelengths by rotation of a tuning reflector. Laser system  100  comprises pivot  102 , base  104 , plane reflector  106 , gain medium  108 , diffraction grating  110 , tuning reflector  112 , rotatable unit  114 , output laser beam  116  and first-order diffracted radiation  118 .  
           [0006]    In the prior art system of FIG. 1, a proximal end of rotatable unit  114  is pivotably connected to base  104  by pivot  102 . Tuning reflector  112  is mounted on rotatable unit  114  forming an acute angle with respect to diffraction grating  110 , which is mounted on an upper surface of base  104 . Plane reflector  106  and gain medium  108  are mounted on base  104  and are disposed to produce a laser beam which is incident on diffraction grating  110  at a grazing angle, thereby generating output laser beam  116  and first-order diffracted radiation  118 .  
           [0007]    In operation, rotating arm  114  pivots around pivot  102  such that tuning reflector  112  moves relative to diffraction grating  110 . Plane reflector  106  and gain element  108  generate a laser beam which is incident on diffraction grating  110  at a grazing angle. Part of this laser beam is reflected as output laser beam  116  and exits laser system  100 . The rest of the laser beam incident on diffraction grating  110  is diffracted and reflected to generate a light radiation pattern which includes first-order diffracted radiation  118 . First-order diffracted radiation  118  retro-reflects off tuning reflector  112  and is again incident on diffraction grating  110 .  
           [0008]    Upon further diffraction and reflection by diffraction grating  110 , a portion of first-order diffracted radiation  118  enters gain element  108  and plane reflector  106 , thereby forming an external feedback laser cavity for laser system  100 . The wavelength of output laser beam  116  depends on the angle formed by grating surface  110  and the reflective surface of tuning reflector  112 , which may be adjusted by pivoting rotatable unit  114  around pivot  102 . Consequently, the wavelength of output laser beam  116  may be tuned by pivoting rotatable unit  114  around pivot  102 . Accurate positioning of pivot  102  enables mode-hop-free, continuous tuning of output laser beam  116  over the entire emission band of gain element  108 .  
           [0009]    A laser system similar to the prior art system shown in FIG. 1 is described in the publication, “Novel Geometry for Single-Mode Scanning of Tunable Lasers,” by Michael G. Littman and Karen Liu, Optics Letters, Vol. 6, No.3, pages 117, 118, March, 1981. A mode-hop-free, Littman cavity laser system with broad-range tuning capabilities is set forth in the publication, “Synchronous Cavity Mode and Feedback Wavelength Scanning in Dye Laser Oscillators with Gratings,” by Harold J. Metcalf and Patrick McNicholl,  Applied Optics,  Vol. 24, No. 17, pages 2757-2761, Sep. 1, 1985. The publication “Scanning Geometry for Broadly Tunable Single-Mode Pulsed Dye Lasers,” by Guangzhi Z. Zhang and Kohzo Hakuta,  Optics Letters,  Vol. 17, No. 14, pages 997-999, Jul. 15, 1992, describes a dye laser system capable of continuously tuning a single-longitudinal-mode laser beam over a range of more than 190 cm −1  by employing a predefined rotation pivot for the tuning reflector and grating.  
           [0010]    Various configurations of grating-tuned, Littman-type, external laser cavity systems capable of providing continuous, broadband, mode-hop-free laser beams have been disclosed in U.S. Pat. No. 5,319,668 to Luecke, U.S. Pat. No. 5,867,512 to Sacher, U.S. Pat. No. 5,771,252 to Lang, U.S. Pat. No. 5,802,085 to Lefevre, et al and the publication “Continuously Tunable Diode Lasers,” by Timothy Day, Frank Luecke, and Michel Brownell,  Lasers  &amp;  Optronics,  No. 6, June, 1993, pp.15-17. According to these publications, accurate positioning of the pivot is paramount to obtain continuous, broadband tuning capability over the entire emission bandwidth of the gain medium.  
           [0011]    Lowering the lasing threshold for grating-tuned external cavity lasers increases the laser power output in the presence of optical power loss occurring inside the laser cavity due to grating diffraction. A method for reducing power loss was described in the publication, “Lasing Threshold Reduction for Grating-Tuned Laser Cavities,” by Guangzhi Z. Zhang and Dennis Tokaryk,  Applied Optics,  vol. 36, No. 24, pages 5855-5858, Aug. 20, 1997. This publication introduced a laser system that utilized potentially wasted optical power in an effective feedback configuration.  
           [0012]    Mode-hop-free, broadband tunable lasers have been extensively used in a wide range of applications, including laser spectroscopy, optical metrology, in-situ process monitoring and test and measurement of optical passive components in Dense Wavelength Division Multiplexing, Wavelength Division Multiplexing and optical fiber systems.  
           [0013]    The output of grating-tuned, external cavity laser systems in the prior art generally consists of two spectral components: (1) a laser beam; and (2) background light radiation comprising Source Spontaneous Emission (“SSE”) and Amplified Spontaneous Emission (“ASE”) light radiation. The laser beam is the desired output component and consists of substantially coherent, narrow-linewidth laser light. The SSE and ASE radiation, however, constitutes an undesirable incoherent noise background which is emitted directly by the gain element.  
           [0014]    The laser beam component of the laser output couples with the SSE and ASE background radiation component in space and time. Although the SSE and ASE background radiation is usually weak in power as compared to the laser output, it has a significant effect in many sensitive applications including test and evaluation of optical passive components and fibers and Dense Wavelength Division Multiplexing, Wavelength Division Multiplexing and optical fiber data-transmission systems. Consequently, there is a need to filter out SSE and ASE background radiation from the output of grating-tuned, external cavity laser systems to obtain a coherent, narrow-linewidth, noise-free output laser beam.  
           [0015]    A few types of grating-tuned external cavity laser systems that could suppress SSE and ASE background noise have been described by the publications, “Using Diode Lasers for Atomic Physics” by Carl E. Wieman and Leo Hollberg, Review of Scientific Instruments, vol. 62, Pages 1-19, January, 1991 and “Impact of Source Spontaneous Emission (SSE) on the Measurement of DWDM Components” by Edgar Leckel et al. These systems insert a beam coupler, usually consisting of an optical flat, into the grating-tuned external feedback cavity, along the laser beam path, between the gain element and the diffraction grating. The beam coupler partially reflects the laser beam out the cavity.  
           [0016]    [0016]FIG. 2 shows a schematic representation of a tunable laser source constructed by Hewlett-Packard Co. based on the concept described in the above-cited publications. Laser system  200  consists of diffraction grating  210 , waveguiding device  232 , laser diode  250 , tuning reflector  260 , beam splitter  292 , reflection mirror  294  and optical lens  296 .  
           [0017]    Laser diode  250  is disposed to generate a laser beam which is incident at a grazing angle upon diffraction grating  210 . Beam splitter  292  is located along an optical path between laser diode  250  and diffraction grating  210  such that it intercepts a feedback light radiation component diffracted by diffraction grating  210 . Reflection mirror  294  is disposed to intercept a light radiation component diverted by beam splitter  292 . Optical lens  296  is disposed along an optical path between reflection mirror  294  and waveguiding device  232 .  
           [0018]    In operation, laser diode  250  generates a laser beam which is incident on diffraction grating  210  at a grazing angle. Part of this beam is reflected by diffraction grating  210  to provide a conventional laser output (not shown in FIG. 2). Diffraction grating  210  also diffracts a feedback light radiation component, which propagates back into laser diode  250  from the retroreflection of tuning reflector  260 . Beam splitter  292  intercepts and partially reflects the feedback light radiation component, thereby giving rise to a diverted light radiation component. The diverted light radiation component consists of a laser beam, an angularly-separated SSE light component and an angularly-separated ASE light component. The diverted light radiation component reflects off reflection mirror  294  and is incident on optical lens  296 . Optical lens  296  refracts the incident diverted light radiation while maintaining the angular separation between its three constituent components. Upon refraction by optical lens  296 , the laser beam component of the diverted light radiation is coupled into waveguiding device  232  while the angularly-separated SSE and ASE components are filtered out, thereby giving rise to a low-noise laser beam (not shown in FIG. 2).  
           [0019]    The laser system described above and shown in the embodiment of FIG. 2 has a number of disadvantages. A disadvantage of the laser system of FIG. 2 is that both the conventional output laser beam and the low-noise laser beam coupled into waiveguiding device  232  have reduced optical power due to optical power losses and additional optical dispersion which occur in the laser cavity due to the introduction of beam splitter  292 . A further disadvantage of this laser system is that the introduction of beam splitter  292  in the laser cavity modifies the cavity length, and consequently, component positions have to be carefully adjusted to achieve mode-free tuning for the output laser beams. Another disadvantage of the laser system shown in FIG. 2 is that introduction of beam splitter  294  into the laser cavity increases the lasing threshold of the laser cavity, therefore increasing the instability of the laser operation of laser diode  250 .  
           [0020]    Considering the limitations associated with grating-tuned, external cavity laser systems in the prior art, including the disadvantages described above, there is a need for a grating-tuned, external cavity laser system which can produce a continuously-tunable laser output with suppressed SSE and ASE background noise over the entire laser tuning range and with automatic wavelength and power tracking capability.  
         SUMMARY OF THE INVENTION  
         [0021]    In an aspect, the invention relates to an external cavity diode laser system comprising a dispersive unit having a proximal and distal end; a gain element, located proximally from the dispersive unit, and that produces coherent light incident upon the dispersive unit, and the dispersive unit dispersing the incident coherent light into dispersed light, the dispersed light comprising a reflected diffraction beam and at least one of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission; a guiding unit, located proximally from the dispersive unit, that guides the dispersed light diffracted upon it from the dispersive unit while maintaining an angular separation between the reflected diffraction beam and at least one of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission; and a physical filtering device that physically filters the reflected diffraction beam from the spatially separated at least one angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission guided to the physical filtering device by the guiding unit to produce a low-noise laser beam.  
           [0022]    In another aspect, the invention relates to a laser system comprising an external cavity diode laser that comprises a dispersive unit and a gain medium, and the dispersive unit having proximal and distal sides, and the gain medium located proximally from the dispersive unit, the external cavity diode laser emitting dispersed light, and the dispersed light comprising a reflected diffraction beam and at least one of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission; a guiding unit, positioned along the beam path of the reflected diffraction beam proximally from the dispersive unit; and a physical filtering device positioned along a beam path of the reflected diffraction beam that physically filters the reflected diffraction beam from the at least one of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission to produce a low-noise laser beam.  
           [0023]    In still another aspect, the invention relates to a method comprising the steps of providing an external cavity diode laser that comprises a dispersive unit; and a gain element located proximally from the dispersive unit, and the external cavity diode laser emitting a reflected diffraction beam and at least one of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission; guiding the reflected diffraction beam along a propagation direction using a guiding unit located proximally from the dispersive unit; and physically filtering the reflected diffraction beam from angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission.  
           [0024]    In an aspect, the invention relates to an external cavity diode laser system comprising dispersive means; coherent light producing means, located proximally from the dispersive unit, for producing coherent light incident upon the dispersive means, the dispersive means dispersing the incident coherent light into dispersed light, the dispersed light comprising a reflected diffraction beam and at least one of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission; guiding means, located proximally from the dispersive means, for guiding the dispersed light diffracted by the dispersive means while maintaining an angular separation between the reflected diffraction beam and the at least one of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission; and physically-filtering means for physically filtering the reflected diffraction beam from the spatially separated the at least one angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission guided to the physically-filtering means by the guiding means to produce a low-noise laser beam. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    [0025]FIG. 1 shows a prior art grating-tuned, external cavity laser system.  
         [0026]    [0026]FIG. 2 shows another prior art grating-tuned, external cavity laser system including a beam splitter.  
         [0027]    [0027]FIG. 3 shows a continuously-tunable, low-noise, grating-tuned, external cavity laser system according to the present invention with a guiding element comprising a flat reflection mirror and a beam collector comprising an optical lens.  
         [0028]    [0028]FIG. 4 shows a representation of the light radiation pattern in the X-Y focal plane of the beam collector shown in FIG. 3  
         [0029]    [0029]FIG. 5 shows a simulation of the effectiveness of SSE and ASE filtering achieved by an embodiment of the present invention.  
         [0030]    [0030]FIG. 6 shows another embodiment of the present invention with a laser diode acting as a light source and a collimation lens acting as a light collimating device.  
         [0031]    [0031]FIG. 7 shows another embodiment of the present invention with a concave mirror acting as a beam collector.  
         [0032]    [0032]FIG. 8 shows another embodiment of the present invention with a laser diode acting as a light source, a collimation lens acting as a light collimating device and a concave mirror acting as beam collector.  
         [0033]    [0033]FIG. 9 shows another embodiment of the present invention with a concave mirror acting as both a guiding element and a beam collector.  
         [0034]    [0034]FIG. 10 shows another embodiment of the present invention with a laser diode acting as a light source, a collimation lens acting as a light collimating device and a concave mirror acting as both a guiding element and beam collector.  
         [0035]    [0035]FIG. 11 shows another embodiment of the present invention with a dispersion unit acting as both a guiding element and beam collector.  
         [0036]    [0036]FIG. 12 shows another embodiment of the present invention with a laser diode acting as a light source, a collimation lens acting as a light collimating device and a dispersion unit acting as both a guiding element and beam collector.  
         [0037]    [0037]FIG. 13 shows another embodiment of the present invention with an optical transmission pinhole acting as a narrow, band-pass filter.  
         [0038]    [0038]FIG. 14 shows another embodiment of the present invention with an alternative disposition of certain elements.  
         [0039]    [0039]FIG. 15 shows a schematic representation of the embodiment of FIG. 14.  
         [0040]    [0040]FIG. 16 shows a representation of the light radiation pattern in the X-Y focal plane of the beam collector shown in FIG. 14  
         [0041]    [0041]FIG. 17 shows a simulation of the effectiveness of SSE and ASE filtering achieved by an embodiment of the present invention.  
         [0042]    [0042]FIG. 18 shows another embodiment of the present invention with a laser diode acting as a light source and a collimation lens acting as a light collimating device.  
         [0043]    [0043]FIG. 19 shows another embodiment of the present invention with a concave mirror acting as a beam collector.  
         [0044]    [0044]FIG. 20 shows another embodiment of the present invention with a laser diode acting as a light source, a collimation lens acting as a light collimating device and a concave mirror acting as beam collector.  
         [0045]    [0045]FIG. 21 shows another embodiment of the present invention with a dispersion unit acting as both a guiding element and beam collector.  
         [0046]    [0046]FIG. 22 shows another embodiment of the present invention with a laser diode acting as a light source, a collimation lens acting as a light collimating device and a dispersion unit acting as both a guiding element and beam collector.  
         [0047]    [0047]FIG. 23 shows another embodiment of the present invention with an optical transmission pinhole acting as a narrow, band-pass filter. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0048]    According to the present invention, a grating-tuned, external cavity laser system and a method to suppress SSE and ASE background light noise is described. The system and method disclosed herein recycle and effectively employ optical power dissipated and wasted by prior art external cavity laser systems to produce a continuously-tunable, narrow-bandwidth laser beam with low SSE and ASE light noise in addition to the conventional laser beam associated with prior art external cavity laser systems.  
         [0049]    The present invention has numerous advantages over the laser systems in the prior art. For example, an advantage of the current invention over the laser system shown in FIG. 2 is that since the present invention does not insert any optical devices into the laser cavity, the present invention avoids perturbing the operation of the master laser cavity operation in general. As a result, the grating-tuned external cavity can directly and fully utilize its mode-hop-free tuning capacity to tune the laser wavelength over large bandwidths without any further adjustments and repositioning of optical components. In contrast, the presence of an optical beam splitter into the laser cavity of the laser system shown in FIG. 2 modifies the length of the laser cavity and requires repositioning of different components, as discussed above. Additional advantages of the present invention over the prior art, including over the system of FIG. 2, will be discussed below in conjunction with different embodiments of the present invention, or will be apparent to one skilled in the art.  
         [0050]    [0050]FIG. 3 shows a tuning arrangement for SSE and ASE suppression in a grating-tuned external cavity laser with dual laser beam output. Dual-beam laser system  300  comprises pivot  302 , base  304 , plane reflector  306 , gain medium  308 , dispersion unit  310 , tuning reflector  312 , rotatable unit  314 , output laser beam  316 , first-order diffracted radiation  318 , reflected diffraction beam  320 , guiding mirror  322 , collector incident light  324 , beam collector  326 , focused light spot  328 , optical coupling device  330 , waveguiding device  332 , low noise laser beam  334 , angularly-separated SSE  336 , angulary-separated ASE  338 , and coupling unit  390 .  
         [0051]    A proximal end of rotatable unit  314  is pivotably connected to base  304  by pivot  302 . Tuning reflector  312  is mechanically coupled to rotatable unit  314  forming an acute angle with respect to dispersion unit  310 , which is mechanically coupled to an upper surface of base  304 . In a preferred embodiment, dispersion unit  310  comprises a diffraction grating and tuning reflector  312  comprises a Porro prism. Use of Porro prisms as reflectors is well-known in the art and is described in Eugene Hecht,  Optics,  Addison-Wesley Publishing Company, Inc. (1987), p. 168. In an alternative embodiment, tuning reflector  312  comprises a reflection mirror.  
         [0052]    Beam collector  326  is mechanically coupled to rotatable unit  314  distally from tuning reflector  312  with respect to pivot  302 . Guiding mirror  322  is mechanically coupled to base  304  and is located in line-of-site of beam collector  326 . Coupling unit  390  comprises guiding mirror  322 , beam collector  326  and optical coupling device  330 . Plane reflector  306  and gain medium  308  are mechanically coupled to base  304  and are disposed to produce a laser beam which is incident on dispersion unit  310  at a grazing angle, thereby generating output laser beam  316 , first-order diffracted radiation  318 , reflected diffraction beam  320 , angularly-separated SSE  336  and angulary-separated ASE  338 . In a preferred embodiment, plane reflector  306  comprises a rear facet of a laser diode. Beam collector  326  is disposed along an optical path between guiding mirror  322  and optical coupling device  330  of waveguiding device  332 . In a preferred embodiment, optical coupling device  330  comprises an optical fiber aperture or the tip of a fiberoptic cable, and waiveguiding device  332  comprises a single-mode or a multi-mode fiberoptic cable.  
         [0053]    In operation, rotating arm  314  pivots around pivot  302  such that tuning reflector  312  and beam collector  326  move relative to dispersion unit  310  and guiding mirror  322 . Plane reflector  306  and gain element  308  generate coherent light radiation comprising a laser beam which is incident on dispersion unit  310  at a grazing angle. Part of this laser beam is reflected as output laser beam  316 . Output laser beam  316  exits dual-beam laser system  300  and represents a conventional laser beam generally associated in the art with grating-tuned external cavity lasers. The rest of the laser beam incident on dispersion unit  310  is diffracted and reflected to generate a light radiation pattern which includes first-order diffracted radiation  318 , reflected diffraction beam  320 , angularly-separated SSE  336  and angulary-separated ASE  338 . First-order diffracted radiation  318  retro-reflects off tuning reflector  312  and is again incident on dispersion unit  310 . Upon further diffraction and reflection by dispersion unit  310 , a portion of first-order diffracted radiation  318  enters gain element  308  and reflects off plane reflector  306  thereby forming an external feedback laser cavity for the dual-beam laser system  300 .  
         [0054]    Reflected diffraction beam  320  comprises a laser beam with a wavelength equal to the wavelength of output laser beam  316 . Angularly-separated SSE  336  and angulary-separated ASE  338  comprise incoherent light radiation which spans a broad range of wavelengths and which propagates away from dispersion unit  310  on optical paths which form acute angles with the direction of propagation of reflected diffraction beam  320 . Alternatively stated, angularly-separated SSE  336  and angulary-separated ASE  338  diverge from reflected diffraction beam  320  as they propagate away from dispersion unit  310 .  
         [0055]    Reflected diffraction beam  320 , angularly-separated SSE  336  and angulary-separated ASE  338  propagate away from dispersion unit  310  on diverging optical paths and reflect off guiding mirror  322  to generate collector incident light  324 . Beam collector  326  refracts collector incident light  324  and concentrates it into a number of discrete light spots including focused light spot  328 . A spatial propagation separation of reflected diffraction beam  320  with respect to and angularly-separated SSE  336  and angulary-separated ASE  338  is maintained by coupling unit  390  upon reflection by guiding mirror  322  and refraction by beam collector  326 , and is transposed into a spatial or angular separation of the discrete light spots formed by beam collector  326 .  
         [0056]    Each light spot comprises light with a narrow range of wavelengths. Focused light spot  328  comprises light from reflected diffraction beam  320 , which has a narrow wavelength band centered around the wavelength of output laser beam  316 . The total light energy contained in focused light spot  328  is significantly higher than the total light energy of any of the other light spots. Focused light spot  328  is coupled into optical coupling device  330  and propagates through waveguiding device  332 , thereby generating low noise laser beam  334 . The other focused light spots comprise light from angularly-separated SSE  336  and angulary-separated ASE  338 . These focused light spots are physically filtered out by coupling unit  390  by not being coupled into waiveguiding device  332 , and therefore the SSE and ASE light noise is suppressed from low noise laser beam  334 .  
         [0057]    The propagation angles with respect to base  304  and the wavelengths of both output laser beam  316  and reflected diffraction beam  320  depend on the angle formed by dispersion unit  310  with the reflecting surface of tuning reflector  312 , which may be adjusted by pivoting rotatable unit  314  around pivot  302 . The propagation angles of reflected diffraction beam  320  with respect to base  304  and guiding mirror  322  determine the optical propagation pattern of collector incident light  324 . As previously discussed, collector incident light  324  is refracted by beam collector  326  to generate focused light spot  328 . Focused light spot  328  is located in the focal plane of beam collector  326  and its position depends on the optical characteristics of beam collector  326  and on the propagation pattern and wavelength structure of collector incident light  324 . Since reflected diffraction beam  320  which is comprised in collector incident light  324  has substantially the same wavelength as output laser beam  316 , and since the wavelength of output laser beam  316  may be tuned by pivoting rotatable unit  314  around pivot  302 , the spatial distribution of focused light spot  328  can be adjusted by pivoting rotatable unit  314 . The topography and elements of dual-beam laser system  300  are selected such that pivoting of rotatable unit  314  results both in controlled wavelength tuning of output laser beam  316  and in stable coupling of focused light spot  328  into optical coupling device  330 .  
         [0058]    The following discussion provides a mathematical description for the structure and operation of the embodiment shown in FIG. 3. Despite the specific nature of the following discussion, it may be applied generally in principle to other embodiments of the present invention. As discussed above, reflected diffraction beam  320  comprises a desired coherent light radiation component (i.e., a laser beam with a wavelength substantially identical with the wavelength of output laser beam  316 ). In contrast, angularly-separated SSE  336  and angularly-separated ASE  338  comprise undesired incoherent noise background light radiation which generally covers the full emission band of gain medium  308  and couple with reflected diffraction beam  320  in space and time. To suppress angularly-separated SSE  336  and angularly-separated ASE  338 , spatial (i.e. angular) separation of these components and spatial narrow band-pass filtering are required. In the present invention, spatial (i.e. angular) separation is provided by the angular and spectral dispersion introduced by dispersion unit  310  and spatial narrow band-pass filtering is provided by coupling unit  390  through adequate placement of optical coupling device  330  relative to the location of focused beam spots  328 .  
         [0059]    Upon incidence on dispersion unit  310 , the laser beam generated by gain medium  308  and plane reflector  306  is dispersed into a radiation pattern which includes output laser beam  316 , first-order diffracted radiation  318  and reflected diffraction beam  320 . Light comprising these three components propagates along different wavelength-dependent paths, forming angles θ(λ) with respect to dispersion unit  310 . If the laser beam generated by gain medium  308  and plane reflector  306  forms an angle of incidence θ 0  with respect to the dispersion unit  310  and if the spatial period of dispersion unit  310  is denoted by d, the angle θ(λ) can be expressed as,  
         θ        (   λ   )       =       arcsin        [       λ   d     -     sin                   θ   0         ]       .                           
 
         [0060]    The intensity of collector incident light  324  in the X-Y focal plane of beam collector  326  is described by a two-dimensional (x, y) equation which includes an angular-cone distribution-function representing the beam focusing effect of beam collector  326 ,  
           I (λ,  x, y,  Ω)= I (λ)ζ( x−x   λ   , y−y   λ   , f,  Ω), 
         [0061]    where a normalized arbitrary distribution function ζ(x−x λ , y−y λ , f, Ω) provides a light intensity distribution for focused light spot  328  with beam center (x λ , y λ , f) in the X-Y focal plane of beam collector  326  as illustrated in FIG. 4.  
         [0062]    If the grating of diffraction grid  310  exhibits a one-dimensional variation, coordinates x λ  and y λ  can be expressed as x λ =f tan θ(λ) and y λ =0, where f represents the focal length of the beam collector  326 . In a preferred embodiment, beam collector  326  comprises a convex lens and f represents the focal length of the convex lens. In the X-Y focal plane of beam collector  326 , optical coupling device  330  is aligned with the center of focused light spot  328  to receive the light with laser wavelength λ L . In a preferred embodiment, optical coupling device  330  comprises a single-mode fiber, a multi-mode transparent fiber or a waveguide designed for coupling a light beam. Through proper alignment with focused light spot  328 , optical coupling device  330  receives only light with wavelength λ L  by filtering out light with other wavelengths than λ L .  
         [0063]    The light energy E(λ) coupled into waveguiding device  332  by optical coupling device  330  is determined by convolution of the X-Y focal light intensity distribution function I(λ, x, y, Ω) with both an optical aperture function ρ(x−x λ     L   , y−y λ     L   , z−f, Ω) characteristic to optical coupling device  330  and with a numerical aperture function κ(λ) representing the average insertion or surface-reflection light coupling loss over the entire area of optical coupling device  330 ,  
                 E     (   fiber   )            (   λ   )       =                  I        (   λ   )            (     1   -     κ        (   λ   )         )          ∫     ∫     ∫       ρ        (       x   -     x     λ   L         ,     y   -     y     λ   L         ,   f   ,   Ω     )       ·                                        ζ        (       x   -     x   λ       ,     y   -     y   λ       ,   f   ,   Ω     )               x             y             Ω                   =                  E        (   λ   )            (     1   -     κ        (   λ   )         )          σ        (   λ   )                                     
 
         [0064]    where the filtering effect of the beam coupling device is represented by a filtering function  
               σ        (   λ   )       =                ∫     ∫     ∫       ρ        (       x   -     x     λ   L         ,     y   -     y     λ   L         ,   f   ,   Ω     )       ·                                      ζ        (       x   -     x   λ       ,     y   -     y   λ       ,   f   ,   Ω     )               x             y               Ω     .                                   
 
         [0065]    If the numerical aperture of optical coupling device  330  is larger than the divergence of focused light spot  328  and if the area of optical coupling device  330  is larger than the size of focused light spot  328  at the laser wavelength λ=λ L , all the light of the respective beam spot is coupled into waveguiding device  332 . The energy of the light coupled into the fiber can therefore be expressed as,  
           E   (fiber) (λ)≈ E (λ L )(1−κ(λ L )). 
         [0066]    [0066]FIG. 4 shows a representation of the X-Y focal plane of beam collector  326  from FIG. 3 and illustrates how the present invention achieves spatial narrow band-pass filtering for light coupled into waveguiding device  332 . FIG. 4 shows the focal plane  400  of beam collector  326 . Focal plane  400  includes an aligned light spot  402 , a misaligned light spot  404  and an optical aperture  406 . Consistent with the previous discussion, beam collector  326  refracts collector incident light  324  and concentrates it into a number of discrete light spots in the focal plane  400  of beam collector  326 . The spatial position of each light spot depends upon the wavelength of the light associated with that particular light spot.  
         [0067]    Aligned light spot  402  represents focused light spot  328  from FIG. 3 which comprises light of substantially wavelength λ L . Aligned light spot  402  is centered at coordinates (x λ     L   , y λ     L   ) and has a radius of r λ     L   . The radius r λ     L    is selected such that the resulting circular area includes only light with an intensity of at least 1/e of the peak value existing within aligned light spot  402 . Aligned light spot  402  is concentrically collocated with optical aperture  406 . If the radius r 0  of optical aperture  406  is larger than the radius r λ     L    of aligned light spot  402 , aligned light spot  402  is fully contained within optical aperture  406  and light from aligned light spot  402  may be fully coupled into waveguiding device  332 . In contrast, misaligned light beam  404  (which includes SSE and ASE radiation) is centered at coordinates (x λ , y λ ) and is not fully contained within optical aperture  406 . Consequently, light associated with misaligned light spot  404  cannot be fully coupled into waveguiding device  332  and is therefore at least partially filtered out.  
         [0068]    As a result of spatial narrow band-pass filtering, therefore, for both light and SSE and ASE background noise radiation with wavelengths other than λ L  (λ≠λ L ), the energy coupled into waveguiding device  332  upon proper alignment of optical aperture  406  with aligned light spot  402  is minimized such that E (fiber) (λ)≈0.  
         [0069]    Referring to FIG. 4, for r λ ≦r 0 , the filtering function associated with optical aperture  406  can be expressed as  
               σ        (   λ   )       =                  π     -   1              r   λ     -   2       ·     ∫     ∫     ∫       ρ        (       x   -     x     λ   L         ,     y   -     y     λ   L         ,   f   ,   Ω     )       ·                                          ζ        (       x   -     x   λ       ,     y   -     y   λ       ,   f   ,   Ω     )               x             y               Ω     .                                   
 
         [0070]    For r λ ≧r 0 , however, the filtering function can be represented by  
               σ        (   λ   )       =                  π     -   1              r     λ   L       -   2       ·     ∫     ∫     ∫       ρ        (       x   -     x     λ   L         ,     y   -     y     λ   L         ,   f   ,   Ω     )       ·                                          ζ        (       x   -     x   λ       ,     y   -     y   λ       ,   f   ,   Ω     )               x             y               Ω     .                                   
 
         [0071]    The optical aperture function of optical aperture  406  and the normalized distribution function describing the light intensity distribution for aligned light spot  402  can then be approximated by,  
         ρ( x−x   λ     L     , y−y   λ     L     , f,  Ω)≅μ( x−x   λ     L     , y−y   λ     L     , f )Θ(Ω)Γ( r   λ     L     −r ), 
         [0072]    and respectively,  
         ζ( x−x   λ   , y−y   λ   , f,  Ω)≅τ( x−x   λ   , y−y   λ   , f )Θ FIBER (Ω)Γ( r   λ   −r′ ), 
         [0073]    where Ω L  represents the spherical angle of the light intensity distribution of aligned light spot  402 , Ω FIBER  represents the numerical aperture of optical aperture  406 , and the following formulas apply:  
         r   =           (     x   -     x     λ   L         )     2     +       (     y   -     y     λ   L         )     2           ;               r   ′     =           (     x   -     x   λ       )     2     +       (     y   -     y   λ       )     2           ;                         
 
           Γ        (       r     λ   L       -   r     )       =     {         1             r     λ   L       -   r     ≥   0             0             r     λ   L       -   r     &lt;   0           }       ;               Γ        (       r   λ     -   r     )       =     {         1             r   λ     -   r     ≥   0             0             r   λ     -   r     &lt;   0           }       ;             Θ        (   Ω   )       ≈     {         1         Ω   ≤     Ω   L               0         Ω   &gt;     Ω   L             }             (     This                 formula                 represents                 the                 angular                 distribution                           
 
         [0074]    function of aligned light spot  402 );  
           Θ   FIBER          (   Ω   )       ≈     {         1         Ω   ≤     Ω   FIBER               0         Ω   &gt;     Ω   FIBER             }             (     This                 formula                 represents                 the                 numerical                                      
 
         [0075]    function of optical aperture  406 );  
           x   λ   =f  tan θ(λ);  
           y   λ =0;  
           x   λ     L     =f  tan θ(λ L ); and  
           y   λ     L   =0. 
         [0076]    (r λ , r λ     L    and r 0  have been previously defined).  
         [0077]    [0077]FIG. 5 shows a simulation of the effectiveness of SSE and ASE filtering achieved by an embodiment of the present invention for r λ     L   =r 0 , f=1000·r 0 , d=1 μm, Ω L ≦Ω FIBER  and assuming a Gaussian light intensity distribution for aligned light spot  402 . Over the emission band of gain medium  308  of FIG. 3, the present invention filters out SSE and ASE background noise radiation at all wavelengths other than the desired laser wavelength λ L , which is shown in FIG. 5 to be approximately 1.54 μm. Consequently, the only light coupled into waveguiding device  332  is light with the desired wavelength, λ L .  
         [0078]    The present invention provides numerous advantages over the prior art. For convenience, and to take advantage of the detailed description provided in connection with the embodiment shown in FIG. 3, a number of advantages of the present invention will be discussed here with particular reference to the embodiment of FIG. 3. These advantages, however, may also apply to other embodiments of the present invention disclosed herein. Additionally, embodiments of the present invention may have additional advantages, some of which may be further described below.  
         [0079]    An advantage of the embodiment of FIG. 3 is that it provides a means for the low noise laser beam  334  to track the laser wavelength of output laser beam  316  with automatic power coupling control as the wavelength of output laser beam  316  is continuously tuned through a broad range of wavelengths. Alternatively stated, the embodiment of FIG. 3 can maintain a maximum and constant level of light coupled into waveguiding device  332  while the wavelength of output laser beam  316 , and implicitly of low noise laser beam  334 , is tuned across a wide range of wavelengths. In the embodiment shown in FIG. 3, this advantage is achieved by appropriate selection of the physical dimensions of dual-beam laser system  300 .  
         [0080]    As shown in FIG. 3, dispersion unit  310  and guiding mirror  322  are mechanically coupled to base  304  such that their normals form an angle φ 0 . Beam collector  326  is mechanically coupled to rotatable unit  314  such that its focal axis forms an angle φ 0  with the normal of tuning reflector  312 . As a result, upon reflection off guiding mirror  322 , reflected diffraction beam  324  forms an angle α(λ L ) with the focal axis of beam collector  326 , where  
         α(λ L )=180°−2φ 0 ±φ 0 . 
         [0081]    The angle α(λ L ) is maintained constant as rotatable unit  314  pivots around pivot  302  to tune wavelength λ L  through a broad range of wavelengths. Consequently, focused light spot  328  can be continuously coupled into waveguiding device  332  while wavelength λ L  is tuned.  
         [0082]    The automatic wavelength and power tracking features of the present invention could also be achieved through an active tracking system which would move optical coupling device  330  in response to positional variations of focused light spot  328  due to wavelength tuning in the laser system. Such a system might employ a computer system coupled with a light sensor located in the proximity of optical aperture  330 . The light sensor would provide feedback data to the computer system to permit dynamic relocation of optical coupling device  330  in response to movement of focused light spot  328  to maintain stable light coupling into waiveguiding device  332 .  
         [0083]    Such a system would be difficult and expensive to implement, however, considering that the optical sensor would have to be inserted into the laser system and located in the proximity of optical aperture  330 . Further, optical aperture  330  would have to be independently mobile with respect to rotatable unit  314 , thereby requiring a complex mechanical coupling device with full two-dimensional movement capability. Such a coupling device would be difficult to implement considering the high degree of precision required for proper optical alignment of optical aperture  330  with focused light spot  328 . The inclusion of a complex mobile mechanical coupling device for optical aperture  330  would also significantly complicate the design and functionality of rotatable unit  314 , whose pivoting around pivot  302  must be accurately controlled but is highly sensitive to the mass and moment of the components coupled to rotatable unit  314 . In contrast, the present invention provides a system which automatically tracks and fully couples focused light spot  328  into optical aperture  330  without any active tracking components, therefore circumventing the limitations associated with an active tracking system.  
         [0084]    Another advantage of the embodiment of FIG. 3 is that it may suppress SSE and ASE noise in a laser output of a grating-tuned, external cavity laser system. Referring to FIG. 3, the SSE and ASE noise present in the laser beam generated by gain medium  308  and plane reflector  306  is dispersed upon its incidence on dispersion unit  310 . Since most of the SSE and ASE noise consists of light with wavelengths that are different from the desired laser wavelength and cover the entire emission band of gain medium  308 , the grating dispersion redirects angularly-separated SSE  336  and angularly-separated ASE  338  in propagation directions divergent from the propagation path of reflected diffraction beam  320 . Coupling unit  390 , which comprises guiding mirror  322 , beam collector  326  and optical coupling device  330 , translates the angular separation of these beam propagation directions into a spatial distribution of light beam energy, which is coupled into waveguiding device  332  to generate low noise laser beam  334 . Consequently, the embodiment of FIG. 3 produces low noise laser beam  334  which is essentially free of SSE and ASE background noise.  
         [0085]    Yet another advantage of the embodiment of FIG. 3 is that it provides an additional laser beam output for grating-tuned, external cavity laser systems by recovering optical energy traditionally wasted by grating-tuned, external cavity laser systems in the prior art and efficiently employing it in a novel approach to generate a non-conventional laser beam with an extremely low level of SSE and ASE noise. This additional laser beam is tuned at the same laser wavelength as the conventional laser beam, but exits the laser system through a separate output port.  
         [0086]    A further advantage of the embodiment of FIG. 3 is that it provides a significant number of benefits without interfering with the functionality and classic design of conventional grating-tuned, external cavity laser systems. More specifically, since the embodiment of FIG. 3 generates low noise laser beam  334  by recycling previously-wasted optical energy, the power and general characteristics of output laser beam  316  are generally not affected. Additionally, the embodiment of FIG. 3 deviates from the classic design of conventional grating-tuned, external cavity laser systems only minimally, therefore decreasing the cost and uncertainties associated with radical design alterations.  
         [0087]    [0087]FIG. 6 shows an alternative embodiment of the present invention. Dual-beam laser system  600  comprises pivot  602 , base  604 , dispersion unit  610 , tuning reflector  612 , rotatable unit  614 , output laser beam  616 , first-order diffracted radiation  618 , reflected diffraction beam  620 , guiding mirror  622 , collector incident light  624 , beam collector  626 , focused light spot  628 , optical coupling device  630 , waveguiding device  632 , low noise laser beam  634 , angularly-separated SSE  636 , angulary-separated ASE  638 , laser diode  650  and collimation lens  652 .  
         [0088]    The structure of the embodiment shown in FIG. 6 is substantially identical with the structure of the embodiment shown in FIG. 3 except that the embodiment of FIG. 6 employs a laser diode  650  to replace the combination of the plane reflector  306  and the gain medium  308  from FIG. 3 and introduces a collimation lens  652  disposed along an optical path between laser diode  650  and dispersion unit  610 . Since the output of laser diode  650  generally exhibits an undesirable elliptically-divergent shape, collimation lens  652  is employed to collimate the light incident on dispersion unit  610  at a grazing angle. In a preferred embodiment, both facets of collimation lens  652  are treated with an anti-reflection coating to reduce light feedback from internal reflection within collimation lens  652 . Further, a facet of laser diode  650  oriented towards dispersion unit  610  is also treated with an anti-reflection coating to maximize power output of laser diode  650 .  
         [0089]    In operation, the embodiment of FIG. 6 functions substantially the same as the embodiment of FIG. 3 because the light beam generated by laser diode  650  in conjunction with collimation lens  652  is substantially identical with the light beam produced by gain element  308  and plane reflector  306 . Consequently, the light incident at a grazing angle on diffraction grid  610  is substantially identical with the light incident at a grazing angle on diffraction grid  310 , and therefore the description provided for the embodiment of FIG. 3 generally applies to the embodiment of FIG. 6.  
         [0090]    [0090]FIG. 7 shows another alternative embodiment of the present invention. Dual-beam laser system  700  comprises pivot  702 , base  704 , plane reflector  706 , gain medium  708 , dispersion unit  710 , tuning reflector  712 , rotatable unit  714 , output laser beam  716 , first-order diffracted radiation  718 , reflected diffraction beam  720 , guiding mirror  722 , mirror incident light  724 , focused light spot  728 , optical coupling device  730 , waveguiding device  732 , low noise laser beam  734  angularly-separated SSE  736 , angulary-separated ASE  738 , and concave mirror  754 .  
         [0091]    The structure of the embodiment of FIG. 7 is substantially identical with the structure of the embodiment of FIG. 3, except that the embodiment of FIG. 7 substitutes a concave mirror  754  for beam collector  326 . Concave mirror  754  is mechanically coupled to rotatable unit  714  such that a concave reflective surface of concave mirror  754  is oriented in the general direction of guiding mirror  722  to intercept mirror incident light  724 . Optical coupling device  730  and waveguiding device  732  are mechanically coupled to rotatable unit  714  on the same side of concave mirror  754  as guiding mirror  722 .  
         [0092]    In operation, the embodiment shown in FIG. 7 functions substantially the same as the embodiment of FIG. 3. Mirror incident light  724  is substantially identical with collector incident light  324  from FIG. 3. Unlike in the embodiment of FIG. 3, however, mirror incident light  724  is not refracted by beam collector  326 , which comprises a lens, but is instead reflected by concave mirror  754 .  
         [0093]    Concave mirror  754  is designed to reflect and focus mirror incident light  724  in a pattern substantially identical with the pattern experienced by the light refracted by beam collector  326  in the embodiment of FIG. 3. Consequently, concave mirror  754  reflects and concentrates mirror incident light  724  into focused light spot  728  which is substantially identical to focused light spot  328  of FIG. 3. The optical axis of concave reflection mirror  754  forms an angle ψ 0  with the normal of tuning reflector  712 . To take advantage of the spatial filtering technique previously discussed in connection with the embodiment of FIG. 3, optical coupling device  730  is mechanically coupled to rotatable unit  714  such that focused light spot  728  is coupled into optical coupling device  330 .  
         [0094]    Upon reflection by concave mirror  754 , reflected diffraction beam  720  propagates at an angle β(λ L ) with respect to the focal axis of concave mirror  754 . Referring to FIG. 7, angle β(λ L ) can be expressed as,  
         β(λ L )=180°−2φ 0 ±ψ 0 . 
         [0095]    As indicated by this formula, angle β(λ L ) does not exhibit any dependence on wavelength or on the position of rotatable unit  714 , but is instead fully determined by initial selection and alignment of the components of dual-beam laser system  700 . To ensure full coupling of focused light spot  728  into optical coupling device  730 , the area of optical coupling device  730  must be larger than the effective size of focused light spot  728  and the numerical aperture of optical coupling device  730  must be larger than the convergence of focused light spot  728 . If these conditions are satisfied, proper initial design of dual-beam laser system  700  results in continuous and stable coupling of selected focused light spot  728  into waveguiding device  732  with simultaneous and effective filtering of SSE and ASE background light in the presence of laser tuning.  
         [0096]    [0096]FIG. 8 shows yet another embodiment of the present invention. Dual-beam laser system  800  comprises pivot  802 , base  804 , dispersion unit  810 , tuning reflector  812 , rotatable unit  814 , output laser beam  816 , first-order diffracted radiation  818 , reflected diffraction beam  820 , guiding mirror  822 , collector incident light  824 , focused light spot  828 , optical coupling device  830 , waveguiding device  832 , low noise laser beam  834 , angularly-separated SSE  836 , angulary-separated ASE  838 , laser diode  850 , collimation lens  852  and concave mirror  854 .  
         [0097]    The structure of the embodiment shown in FIG. 8 is substantially identical with the embodiment shown in FIG. 3, except that the embodiment of FIG. 8 includes the modifications introduced by the embodiments shown in FIG. 6 and FIG. 7. More specifically, the embodiment of FIG. 8 employs a laser diode  850  to replace the combination of the plane reflector  306  and the gain medium  308  from FIG. 3 and introduces a collimation lens  852  disposed along an optical path between laser diode  850  and dispersion unit  810 , as discussed in connection with FIG. 6. Additionally, the embodiment of FIG. 8 substitutes a concave mirror  854  for beam collector  326 , as described in conjunction with FIG. 7. In a preferred embodiment, both facets of collimation lens  852  are treated with an anti-reflection coating to reduce light feedback from internal reflection within collimation lens  852 . Further, a facet of laser diode  850  oriented towards dispersion unit  810  is also treated with an anti-reflection coating to maximize power output of laser diode  850 .  
         [0098]    In operation, both modifications operated to the embodiment shown in FIG. 8 perform substantially identical functions as the original elements they replace, as discussed in connection with the embodiments of FIG. 6 and FIG. 7. Consequently, the descriptions provided for the embodiments shown in FIGS. 3, 6 and  7  also apply to the embodiment of FIG. 8.  
         [0099]    [0099]FIG. 9 shows yet another embodiment of the present invention. Dual-beam laser system  900  comprises pivot  902 , base  904 , plane reflector  906 , gain medium  908 , dispersion unit  910 , tuning reflector  912 , rotatable unit  914 , output laser beam  916 , first-order diffracted radiation  918 , reflected diffraction beam  920 , mirror incident light  924 , focused light spot  928 , optical coupling device  930 , waveguiding device  932 , low noise laser beam  934 , angularly-separated SSE  936 , angulary-separated ASE  938 , and concave guiding mirror  956 .  
         [0100]    The structure of the embodiment shown in FIG. 9 is substantially identical with the structure of the embodiment shown in FIG. 3, except that the embodiment of FIG. 9 employs a concave guiding mirror  956  to replace both guiding mirror  322  and beam collector  326  of FIG. 3. Concave guiding mirror  956  is mechanically coupled to base  904  such that its concave reflecting surface is directed in the general direction of rotatable unit  914  and its optical axis forms an angle φ 0  with respect to the normal of dispersion unit  910 .  
         [0101]    In operation, the embodiment shown in FIG. 9 functions substantially the same as the embodiment of FIG. 3. Concave mirror  956  is designed to reflect and focus reflected diffraction beam  920  in a pattern substantially identical with the pattern exhibited by collector incident light  324  upon its refraction by beam collector  326  in the embodiment of FIG. 3. Essentially, concave guiding mirror  956  is designed to operationally substitute both guiding mirror  322  and beam collector  326  of FIG. 3. Consequently, concave guiding mirror  956  reflects and concentrates reflected diffraction beam  920  into focused light spot  928  which is substantially identical with focused light spot  328  from FIG. 3. Analogously with the arrangement of FIG. 3, optical coupling device  930  is mechanically coupled to rotatable unit  914  such that it is oriented in the direction of concave guiding mirror  956  and is aligned to permit coupling of focused light spot  928  into optical coupling device  930 .  
         [0102]    Upon reflection by concave guiding mirror  956 , reflected diffraction beam  924  forms an angle γ(λ L ) with the normal of tuning reflector  912 , where  
         γ(λ L )=180°−2φ 0 . 
         [0103]    As indicated by this formula, angle γ(λ L ) does not exhibit any dependence on wavelength or on the position of rotatable unit  914 , but is instead fully determined by initial selection and alignment of the components of dual-beam laser system  900 . Proper initial design of dual-beam laser system  900  results in continuous coupling of focused light spot  928  into optical coupling device  930  with simultaneous and effective filtering of SSE and ASE background light regardless of wavelength variations in the system as a result of laser tuning.  
         [0104]    The particular arrangement of the embodiment of FIG. 9 results in an advantage. Specifically, as rotatable unit  914  pivots around pivot  902 , the distance between concave guiding mirror  956  and optical coupling device  930  varies because optical coupling device  930  is attached to, and moves together with, rotatable unit  914 . Consequently, since the focal length of concave guiding mirror  956  is fixed, and since optical coupling device  930  is initially located in the focal plane of concave mirror  956 , pivoting of rotatable unit  914  removes optical coupling device  930  from the focal plane of concave guiding mirror  956 . As a result, due to the inherent divergence of the light reflected by concave guiding mirror  956 , the size of focused light spot  328  will increase as it projects upon the optical coupling device  930  out of focus. This general concept may be applied to other embodiments of the present invention.  
         [0105]    To maximize the amount of optical power coupled into waveguiding device  932 , the area of optical coupling device  930  must be larger than the effective size of focused light spot  328 , and the numerical aperture of optical coupling device  930  must be larger than the convergence of focused light spot  328 . Depending on the amplitude of movement of rotatable unit  914 , however, the size of focused light spot  928  could potentially exceed the effective size of optical coupling device  930 , therefore resulting in reduced coupling efficiency. This apparent inconvenience can be remedied by simultaneously moving optical coupling device  930  along the optical axis of concave guiding mirror  956  to compensate for any focal plane translation induced by pivoting of rotatable unit  956 .  
         [0106]    [0106]FIG. 10 shows an alternative embodiment of the present invention. Dual-beam laser system  1000  comprises pivot  1002 , base  1004 , dispersion unit  1010 , tuning reflector  1012 , rotatable unit  1014 , output laser beam  1016 , first-order diffracted radiation  1018 , reflected diffraction beam  1020 , mirror incident light  1024 , focused light spot  1028 , optical coupling device  1030 , waveguiding device  1032 , low noise laser beam  1034 , angularly-separated SSE  1036 , angulary-separated ASE  1038 , laser diode  1050 , collimation lens  1052  and concave guiding mirror  1056 .  
         [0107]    The structure of the embodiment shown in FIG. 10 is substantially identical with the structure of the embodiment shown in FIG. 9 except that the embodiment of FIG. 10 employs a laser diode  1050  to replace the combination of plane reflector  906  and gain medium  908  from FIG. 9 and introduces a collimation lens  1052  disposed along an optical path between laser diode  1050  and dispersion unit  1010 . Since the output of laser diode  1050  generally exhibits an undesirable elliptically-divergent shape, collimation lens  1052  is employed to collimate the light incident on dispersion unit  1010  at a grazing angle. In a preferred embodiment, both facets of collimation lens  1052  are treated with an anti-reflection coating to reduce light feedback from internal reflection within collimation lens  1052 . Further, a facet of laser diode  1050  oriented towards dispersion unit  1010  is also treated with an anti-reflection coating to maximize power output of laser diode  1050 .  
         [0108]    In operation, the embodiment of FIG. 10 functions substantially the same as the embodiment of FIG. 9 because the light beam generated by laser diode  1050  in conjunction with collimation lens  1052  is substantially identical with the light beam produced by gain element  908  and plane reflector  906 . Consequently, the light incident at a grazing angle on diffraction grid  1010  is substantially identical with the light incident at a grazing angle on diffraction grid  910 , and therefore the description provided for the embodiment of FIG. 9 also applies to the embodiment of FIG. 10.  
         [0109]    [0109]FIG. 11 shows yet another alternative embodiment of the present invention. Dual-beam laser system  1100  comprises pivot  1102 , base  1104 , plane reflector  1106 , gain medium  1108 , dispersion unit  1110 , tuning reflector  1112 , rotatable unit  1114 , output laser beam  1116 , first-order diffracted radiation  1118 , reflected diffraction beam  1120 , beam collector incident light  1124 , beam collector  1126 , focused light spot  1128 , optical coupling device  1130 , waveguiding device  1132 , low noise laser beam  1134 , angularly-separated SSE  1136 , angulary-separated ASE  1138 , and guiding dispersion unit  1160 .  
         [0110]    The structure of the embodiment of FIG. 11 is substantially identical with the structure of the embodiment of FIG. 3, except that in the embodiment of FIG. 11 a guiding dispersion unit  1160  substitutes guiding mirror  322  of FIG. 3 and beam collector  1126 , optical coupling device  1130  and waveguiding device  1132  are removed from rotatable unit  1114 . Guiding dispersion unit  1160  is mounted to the base  1104 , and substantially parallel with, dispersion unit  1110 , and is oriented towards dispersion unit  1110  to intercept reflected diffraction beam  1120 . Beam collector  1126 , optical coupling device  1130  and waveguiding device  1132  are mounted above dispersion unit  1110  and are fixed with respect to guiding dispersion unit  1160 .  
         [0111]    In operation, the embodiment shown in FIG. 11 functions substantially the same as the embodiment of FIG. 3. Reflected diffraction beam  1120  is substantially identical with reflected diffraction beam  320  from FIG. 3. Unlike in the embodiment of FIG. 3, however, reflected diffraction beam  1120  is not reflected by guiding mirror  322 , but is instead diffracted by guiding dispersion unit  1160 .  
         [0112]    Guiding dispersion unit  1160  is designed to diffract reflected diffraction beam  1120  in a pattern substantially identical with the pattern experienced by the light reflected by guiding mirror  322  in the embodiment of FIG. 3. Consequently, beam collector incident light  1124  is substantially identical with beam collector incident light  324  from FIG. 3. As a result, beam collector  1126  refracts and focuses beam collector incident light  1124  into focused light spot  1128  which is substantially identical to focused light spot  328  of FIG. 3.  
         [0113]    Reflected diffraction beam  1124 , which is comprised in collector incident radiation  1124  forms an angle χ(λ L ) with the normal of guiding dispersion unit  1160 , where  
         χ(λ L )=θ 0 . 
         [0114]    As indicated by this formula, angle χ(λ L ) does not exhibit any dependence on wavelength or on the position of rotatable unit  1114 , but is instead fully determined by initial selection and alignment of the components of dual-beam laser system  1100 . To ensure full coupling of focused light spot  1128  into optical coupling device  1130 , the area of optical coupling device  1130  must be larger than the effective size of focused light spot  1128  and the numerical aperture of optical coupling device  1130  must be larger than the convergence of focused light spot  1128 . If these conditions are satisfied, proper initial design of dual-beam laser system  1100  results in continuous and stable coupling of selected focused light spot  1128  into waveguiding device  1132  with simultaneous and effective filtering of SSE and ASE background light regardless of wavelength variations in the system as a result of laser tuning.  
         [0115]    [0115]FIG. 12 shows an alternative embodiment of the present invention. Dual-beam laser system  1200  comprises pivot  1202 , base  1204 , plane reflector  1206 , gain medium  1208 , dispersion unit  1210 , tuning reflector  1212 , rotatable unit  1214 , output laser beam  1216 , first-order diffracted radiation  1218 , reflected diffraction beam  1220 , beam collector incident light  1224 , beam collector  1226 , focused light spot  1228 , optical coupling device  1230 , waveguiding device  1232 , low noise laser beam  1234 , angularly-separated SSE  1236 , angulary-separated ASE  1238 , laser diode  1250 , collimation lens  1252  and guiding dispersion unit  1260 .  
         [0116]    The structure of the embodiment shown in FIG. 12 is substantially identical with the structure of the embodiment shown in FIG. 11 except that the embodiment of FIG. 12 employs a laser diode  1250  to replace the combination of plane reflector  1106  and gain medium  1108  from FIG. 11 and introduces a collimation lens  1252  disposed along an optical path between laser diode  1250  and dispersion unit  1210 . Since the output of laser diode  1250  generally exhibits an undesirable elliptically-divergent shape, collimation lens  1252  is employed to collimate the light incident on dispersion unit  1210  at a grazing angle. In a preferred embodiment, both facets of collimation lens  1252  are treated with an anti-reflection coating to reduce light feedback from internal reflection within collimation lens  1252 . Further, a facet of laser diode  1250  oriented towards dispersion unit  1210  is also treated with an anti-reflection coating to maximize power output of laser diode  1250 .  
         [0117]    In operation, the embodiment of FIG. 12 functions substantially the same as the embodiment of FIG. 11 because the light beam generated by laser diode  1250  in conjunction with collimation lens  1252  is substantially identical with the light beam produced by gain element  1108  and plane reflector  1106 . Consequently, the light incident at a grazing angle on diffraction grid  1210  is substantially identical with the light incident at a grazing angle on diffraction grid  1110 , and therefore the description provided for the embodiment of FIG. 11 also applies to the embodiment of FIG. 12.  
         [0118]    [0118]FIG. 13 shows yet another embodiment of the present invention. The preceding description of various embodiments of this invention taught how an optical aperture coupled to a waveguiding device can be employed as a spatial narrow band-pass filter to suppress SSE and ASE background light with wavelengths other than a desired wavelength. The embodiment of FIG. 13 illustrates how an optical transmission pinhole can be employed as a narrow band-pass filter to either replace or supplement and enhance the filtering effect of an optical coupling device.  
         [0119]    [0119]FIG. 13 shows a simplified representation of the complete laser system described in prior embodiments. Laser system  1300  includes light generator  1370 , pinhole incident light  1372 , pinhole  1374 , beam collector  1326 , focused light spot  1328 , optical coupling device  1330  and waveguiding device  1332 .  
         [0120]    Light generator  1370  and optical coupling device  1330  are mounted at opposite ends of laser system  1300 . Referring to the embodiment of FIG. 3 for example, light generator  1370  could include a subsystem comprising plane reflector  306 , gain medium  308 , dispersion unit  310 , tuning reflector  312  and guiding mirror  322 . Pinhole  1374  and beam collector  1326  are disposed along an optical path between light generator  1370  and optical coupling device  1330  such that optical aperture  1330  is located distally from light generator  1370  with respect to pinhole  1374 . In an alternative embodiment, beam collector  1326  could be replaced by a concave mirror, as disclosed in the embodiment shown in FIG. 7 for example. Optical aperture  1330  is operationally connected to waveguiding device  1332  to permit coupling of light.  
         [0121]    In operation, light generator  1370  projects pinhole incident light  1372  towards pinhole  1374 . Pinhole  1374  includes a transparent area which permits part of pinhole incident light  1372  to propagate beyond pinhole  1374  and illuminate beam collector  1326 . Beam collector  1326  focuses the incident light radiation into focused light spot  1328  which is pre-aligned with optical coupling device  1330  to permit efficient light coupling into waveguiding device  1332 . Pinhole  1374  acts as a physical spatial narrow band-pass filter effectively suppressing SSE and ASE background light, and therefore provides a first-order filtering stage for light propagating towards beam collector  1326 . Consequently, the light incident on beam collector  1326  is already filtered prior to being concentrated into focused light spot  1328 . This advantage could be employed, among others, to relax the design constraints imposed on the optical characteristics of beam collector  1326  and the alignment requirements associated with narrow band-pass filtering as taught by the present invention. Further, the SSE and ASE radiation may be cut off by the spatial filter so that a broader optical coupling device  1330  is still adequate to couple the laser beam into waveguiding device  1332 . A larger receiving aperture decreases the probability that optical coupling device  1330  is damaged by the heat produced by the high optical energy being coupled into waiveguiding device  1332 .  
         [0122]    [0122]FIG. 14 shows yet another embodiment of the present invention. Dual-beam laser system  1400  comprises pivot  1402 , base  1404 , plane reflector  1406 , gain medium  1408 , dispersion unit  1410 , tuning reflector  1412 , rotatable unit  1414 , output laser beam  1416 , first-order diffracted radiation  1418 , reflected diffraction beam  1420 , guiding mirror  1422 , collector incident light  1424 , beam collector  1426 , focused light spot  1428 , optical coupling device  1430 , waveguiding device  1432 , angularly-separated SSE  1436 , angularly-separated ASE  1438 , and low noise laser beam  1434 .  
         [0123]    The structure of the embodiment shown in FIG. 14 is substantially identical with the structure of the embodiment shown in FIG. 3, except that the relative position of a number of elements is changed in FIG. 14. Specifically, beam collector  1426  is mechanically coupled to rotatable unit  1414  proximally to pivot  1402  with respect to tuning reflector  1412 . In contrast, in the embodiment of FIG. 3, beam collector  326  is mechanically coupled to rotatable unit  314  distally from tuning reflector  312  with respect to pivot  302 . Additionally, in the embodiment of FIG. 14, dispersion unit  1410  is disposed between gain medium  1408  and pivot  1402 , whereas in the embodiment of FIG. 3 gain medium  308  and pivot  302  are collocated on the same side of dispersion unit  310 . Further, in the embodiment of FIG. 14, guiding mirror  1422  is disposed between rotatable unit  1414  and gain element  1408 , whereas in the embodiment of FIG. 3, both rotatable unit  314  and gain medium  308  are collocated on the same side of guiding mirror  322 .  
         [0124]    In operation, the embodiment of FIG. 14 functions substantially the same as the embodiment of FIG. 3 except for certain differences associated with the topographical modifications described above. For example, in the embodiment of FIG. 14, the laser cavity of dual-beam laser system  1400  is formed by a feedback path defined by plane reflector  1406 , gain medium  1408 , dispersion unit  1410 , guiding mirror  1422  and tuning reflector  1410  and is denoted as M 1 -G-M 2 -M 3 . In contrast, the corresponding feedback path in FIG. 3 does not include guiding mirror  1422 .  
         [0125]    [0125]FIG. 15 provides a simplified schematic diagram for the embodiment shown in FIG. 14 together with a number of geometrical relationships existing between various elements of that embodiment. Dual-beam laser system  1500  comprises pivot  1502 , base  1504 , plane reflector  1506 , gain medium  1508 , dispersion unit  1510 , tuning reflector  1512 , rotatable unit  1514 , output laser beam  1516 , first-order diffracted radiation  1518 , reflected diffraction beam  1520  and guiding mirror  1522 .  
         [0126]    The X-Y coordinate system in FIG. 15 is defined such that the Y-axis coincides with the normal of dispersion unit  1510  and points away from dispersion unit  1510 , while the X-axis lies in the plane of the diffracting surface of dispersion unit  1510 . The origin of the X-Y coordinate space is denoted at point G. The center of tuning reflector  1512  is denoted as point T.  
         [0127]    Pivot  1502  is denoted as point O and has X-Y coordinates (x 0 , y 0 ). The distance between points O and G is denoted L o . Similarly, the distance between points S and G is denoted L d . Further, the distance between points O and T is denoted as L R .  
         [0128]    For mode-hop-free laser tuning while rotatable unit  1514  pivots around pivot  1502 , the total length L(λ) of laser cavity M 1 -G-M 2 -M 3  must stay constant over the whole range of tunable wavelengths and must be an integer multiple of the mode number, i.e.  
           L (λ)= Nλ/ 2. 
         [0129]    The laser wavelength X must satisfy the m th -order diffraction equation for dispersion unit  1510 ,  
           mλ=d [sin θ(λ)+sin θ 0 ], 
         [0130]    where θ 0  represents the angle of incidence of the laser beam generated by gain medium  1508  and plane reflector  1506  on dispersion unit  1510  and d represents the spatial grating period of dispersion unit  1510 .  
         [0131]    For a laser wavelength λ, the total cavity length L(λ) can be expressed as  
           L (λ)=|{overscore ( M   1   G )}|+|{overscore (GM 2 )}|+| {overscore (M 2 M 3 )}|+[   n   1 (λ)−1] d   1 , 
         [0132]    A portion of the M 1 -G optical path included in this equation is located inside gain medium  1508 , whose optical index or dispersion figure is n 1 (λ).  
         [0133]    In an alternative embodiment of the present invention, tuning reflector  1512  may be replaced by a Porro prism. In still another embodiment of this invention, a laser diode and collimation lens may replace plane reflector  1506  and gain medium  1508  as described, for example, in connection with the embodiment shown in FIG. 6. Both of these two alternative embodiments are substantially identical with the embodiment of FIG. 14 from a functional point of view, except that the total cavity length L(λ) may vary due to additional dispersion introduced into the optical system.  
         [0134]    In general, medium dispersion is a function of light wavelength and may be expressed as,  
           n   1 (λ)= n   10   +a   1   λ+a   2 λ 2   +a   3 λ 3 + . . . , 
         [0135]    where n 10  represents a constant and a1, a2, . . . represent coefficients associated with higher orders of dispersion. Generally,  
           n   10   &gt;&gt;a   1   λ&gt;&gt;a   2 λ 2   &gt;&gt;a   3 λ 3  . . . . 
         [0136]    Taking into consideration the dispersion introduced by gain medium  1508  and by any other dispersion-inducing elements present in the embodiment of FIG. 15, the mode number N may be expressed as,  
             N   =                  2      m        {           (       L   o     +       L   d        cos                 α       )          cos        (     α   +     2      Δ       )         +       L   d          (     1   +     sin                 α                        sin                 Δ              )       +       a   1          d   1         d     }       +                              2      m        {             (       L   o     +       L   d        cos                 α       )          sin        (     α   +     2      Δ       )         -       L   d        sin                 α                 cos                 Δ       λ     ·                                        1   -       (       λ   d     -     sin                   θ   0         )     2         +     (         a   2        λ     +       a   3          λ   2       +   …                )       }     +                            2      m        {                            M   1        G     _          +       (       n   10     -   1     )          d   1       -       L   R        sin                 β     -     [         L   d     (     1   +     sin                 α               sin                 Δ            )     +                     (       L   o     +       L   d        cos                 α       )        cos                   (     α   +     2      Δ       )       ]        sin                   θ   0             λ     }                                   
 
         [0137]    Parameters {overscore (M 1 G)}, L o , L d , L R , α, β, Δ(Δ=ψ 0 −90°) are only dependent on the physical design and static setup of dual-beam laser system  1500  and do not vary as rotatable unit  1514  pivots around pivot  1502 . Consequently, for a laser wavelength λ, the cavity mode number N may be expressed as,  
           N=N   0   +ΔN   0   +ΔN (λ), 
         [0138]    where the cavity constant mode number N 0  is given by,  
         N   0     =     2      m          {           (       L   o     +       L   d        cos                 α       )          cos        (     α   +     2      Δ       )         +       L   d          (     1   +     sin                 α             sin                 Δ              )       +       a   1          d   1         d     }     .                             
 
         [0139]    The mode shift ΔN 0  induced by initial alignment and setup of dual-beam laser system  1500  may be expressed as,  
               Δ                     N   0          (   λ   )         =                2      m        {                          M   1        G     _          +       (       n   10     -   1     )          d   1       -       L   R        sin                 β     -               [         L   d          (     1   +     sin                 α             sin                 Δ              )       +                   (       L   o     +       L   d        cos                 α       )          cos        (     α   +     2      Δ       )       ]   sin                 θ           λ     +                                  2      m        {                   (       L   o     +       L   d        cos                 α       )          sin        (     α   +     2      Δ       )         -                 L   d        sin                 αcosΔ           λ     ·       1   -       (       λ   d     -     sin                   θ   0         )     2           }                                   
 
         [0140]    Analogously, the cavity mode shift ΔN(λ) induced by dispersion associated with the optical components of dual-beam laser system  1500  is provided by,  
         Δ N (λ)=2 m ( a   2   λ+a   3 λ 2 + . . . ): 
         [0141]    Mode-hop-free laser tuning over the entire tuning band of gain medium  1508  and dispersion unit  1510  can be achieved only when  
         Δ N   0 (λ)+Δ N (λ)=0. 
         [0142]    This equation suggests that if dual-beam laser system  1500  exhibits sufficiently-high nonlinear dispersion or misalignment, mode-hop-free laser tuning might not be achievable. Mode-hop-free tuning may only be maintained if the ΔN 0 (λ)+ΔN(λ)&lt;&lt;1 over the entire tuning range.  
         [0143]    An advantage of the present invention is that mode-hop free tuning can be achieved by proper selection of certain parameters during the design of dual-beam laser system  1500  such as the position of plane reflector  1506  (|{overscore (M 1 G)}|), the position of pivot  1502  (L o , α), the position of guiding mirror  1522  (L d ), or the location of tuning reflector  1512  (L R , β). Adjustment of any combination of these parameters can provide the necessary condition,  
         Δ N   0 (λ)+Δ N (λ)&lt;&lt;1. 
         [0144]    For example, the position of tuning reflector  1512  may be selected such that, |{overscore (M 1 G)}|+(n 10 −1)d 1 −(L d  sin ψ 0 +L o  cos α+L R  sin β)=0. At the same time, the position of pivot  1502  can be defined such that the mode shift ΔN 0 (λ) compensates the dispersion shift up to high orders. For a practical device, the compensation of high order dispersion yields and guarantees the continues tuning of the laser from the grating-tuned external cavity, i.e.  
         Δ N   0 (λ)+Δ N (λ)&lt;&lt;1. 
         [0145]    The following discussion provides a mathematical description for the structure and operation of the embodiment shown in FIG. 14. The intensity of collector incident light  1424  in the X′-Y′ focal plane of beam collector  1426  is described by a two-dimensional (x′, y′) equation which includes an angular-cone distribution-function representing the beam focusing effect of beam collector  1426 ,  
           I (λ,  x′, y′ )= I (λ)ζ( x′−x′   λ   , y′−y′   λ   , f,  Ω), 
         [0146]    where a normalized arbitrary distribution function ζ(x′−x′ λ , y′−y′ λ , f, Ω) provides a light intensity distribution for focused light spot  1428  with beam center (x λ , y λ , f) in the X′-Y′ focal plane of beam collector  1426  as illustrated in FIG. 16.  
         [0147]    If the grating of diffraction grid  1410  exhibits a one-dimensional variation, coordinates x′ λ  and y′ λ  can be expressed as x′ λ =f tan(θ(λ)−θ(λ L )) and y′ λ =0 where f represents the focal length of the beam collector  1426 . In a preferred embodiment, beam collector  1426  comprises a convex lens and f represents the focal length of the convex lens. In the X-Y focal plane of beam collector  1426 , optical coupling device  1430  is aligned with the center of focused light spot  1428  to receive the light with laser wavelength λ L . In a preferred embodiment, optical coupling device  1430  comprises a single-mode fiber, a multi-mode transparent fiber or a waveguide designed for coupling a light beam. Through proper alignment with focused light spot  1428 , optical coupling device  1430  receives only light with wavelength λ L  by filtering out light with other wavelengths than λ L .  
         [0148]    The light energy E(λ) coupled into waveguiding device  1432  by optical coupling device  1430  is determined by convolution of the X-Y focal light intensity distribution function I(λ, x′, y′) with both an optical aperture function ρ(x′−x′ λ     l   , y′−y′ λ     L   , z′−f, Ω) characteristic to optical coupling device  1430  and with a numerical aperture function κ(λ) representing the average insertion or surface-reflection light coupling loss over the entire area of optical coupling device  1430 ,  
                 E     (   fiber   )            (   λ   )       =                  I        (   λ   )            (     1   -     κ        (   λ   )         )          ∫     ∫     ∫       ρ        (         x   ′     -     x     λ   L     ′       ,       y   ′     -     y     λ   L     ′       ,   f   ,   Ω     )       ·                                        ζ        (         x   ′     -     x   λ   ′       ,       y   ′     -     y   λ   ′       ,   f   ,   Ω     )                 x   ′                 y   ′               Ω                   =                  E        (   λ   )            (     1   -     κ        (   λ   )         )          σ        (   λ   )                                     
 
         [0149]    where the filtering effect of the beam coupling device is represented by a filtering function  
               σ        (   λ   )       =                ∫     ∫     ∫       ρ        (         x   ′     -     x     λ   L     ′       ,       y   ′     -     y     λ   L     ′       ,   f   ,   Ω     )       ·                                      ζ        (         x   ′     -     x   λ   ′       ,       y   ′     -     y   λ   ′       ,   f   ,   Ω     )                 x   ′                 y   ′                 Ω     .                                   
 
         [0150]    If the numerical aperture of optical coupling device  1430  is larger than the divergence of focused light spot  1428  and if the area of optical coupling device  1430  is larger than the size of focused light spot  1428  at the laser wavelength λ=λ L , all the light of the respective beam spot is coupled into waveguiding device  1432 . The energy of the light coupled into the fiber can therefore be expressed as,  
           E   (fiber) (λ)≈ E (λ L )(1−κ(λ L )). 
         [0151]    [0151]FIG. 16 illustrates how the present invention achieves spatial narrow band-pass filtering for light coupled into waveguiding device  1432  of FIG. 14.  
         [0152]    [0152]FIG. 16 shows the focal plane  1600  of beam collector  1426 . Focal plane  1600  includes an aligned light spot  1602 , a misaligned light spot  1604  and an optical aperture  1606 . Consistent with the previous discussion, beam collector  1426  refracts collector incident light  1424  and concentrates it into a number of discrete light spots in the focal plane  1600  of beam collector  1426 . The spatial position of each light spot depends upon the wavelength of the light associated with that particular light spot.  
         [0153]    Aligned light spot  1602  represents focused light spot  1428  from FIG. 14 which comprises light of substantially wavelength λ L . Aligned light spot  1602  is centered at coordinates (x λ     L   , y λ     L   ) and has a radius of r λ     L   . The radius r λ     L    is selected such that the resulting circular area includes only light with an intensity of at least 1/e of the peak value existing within aligned light spot  1602 . Aligned light spot  1602  is concentrically collocated with optical aperture  1606 . If the radius r 0  of optical aperture  1606  is larger than the radius r λ     L    of aligned light spot  1602 , aligned light spot  1602  is fully contained within optical aperture  1606  and light from aligned light spot  1602  may be fully coupled into waveguiding device  1432 . In contrast, misaligned light beam  1604  is centered at coordinates (x λ , y λ ) and is not fully contained within optical aperture  1606 . Consequently, light associated with misaligned light spot  1604  cannot be fully coupled into waveguiding device  1432  and is therefore at least partially filtered out.  
         [0154]    As a result of spatial narrow band-pass filtering, therefore, for both light and SSE and ASE background noise radiation with wavelengths other than λ L  (λ≠λ L ), the energy coupled into waveguiding device  1432  upon proper alignment of optical aperture  1606  with aligned light spot  1602  is minimized such that E (fiber) (λ)≈0.  
         [0155]    Referring to FIG. 16, for r λ ≦r 0 , the filtering function associated with optical aperture  1406  can be expressed as  
               σ        (   λ   )       =                  π     -   1              r   λ     -   2       ·     ∫     ∫     ∫       ρ        (         x   ′     -     x     λ   L     ′       ,       y   ′     -     y     λ   L     ′       ,   f   ,   Ω     )       ·                                          ζ        (         x   ′     -     x   λ   ′       ,       y   ′     -     y   λ   ′       ,   f   ,   Ω     )                 x   ′                 y   ′                 Ω     .                                   
 
         [0156]    For r λ ≧r 0 , however, the filtering function can be represented by  
               σ        (   λ   )       =                  π     -   1              r     λ   L       -   2       ·     ∫     ∫     ∫       ρ        (         x   ′     -     x     λ   L     ′       ,       y   ′     -     y     λ   L     ′       ,   f   ,   Ω     )       ·                                          ζ        (         x   ′     -     x   λ   ′       ,       y   ′     -     y   λ   ′       ,   f   ,   Ω     )                 x   ′                 y   ′                 Ω     .                                   
 
         [0157]    The optical aperture function of optical aperture  1606  and the normalized distribution function describing the light intensity distribution for aligned light spot  1602  can then be approximated by,  
         ρ( x′−x′   λ     L     , y′−y′   λ     L     , f,  Ω)≅μ( x′−x′   λ     L     , y′−y′   λ     L     , f )Θ(Ω)Γ( r   λ     L     −r ), 
         [0158]    and respectively,  
         ζ( x′−x′   λ   , y′−y′   λ   , f,  Ω)≅τ( x′−x′   λ   , y′−y′   λ   , f )Θ FIBER (Ω)Γ( r   λ   −r′ ), 
         [0159]    where Ω L  represents the cone angle of the light intensity distribution of aligned light spot  1602 , Ω FIBER  represents the numerical aperture of optical aperture  1606 , and the following formulas apply:  
         r   =           (       x   ′     -     x     λ   L     ′       )     2     +       (       y   ′     -     y     λ   L     ′       )     2           ;               r   ′     =           (       x   ′     -     x   λ   ′       )     2     +       (       y   ′     -     y   λ   ′       )     2           ;               Γ        (       r     λ   L       -   r     )       =     {         1             r     λ   L       -   r     ≥   0             0             r     λ   L       -   r     &lt;   0           }       ;               Γ        (       r   λ     -   r     )       =     {         1             r   λ     -   r     ≥   0             0             r   λ     -   r     &lt;   0           }       ;             Θ        (   Ω   )       ≈     {         1         Ω   ≤     Ω   L               0         Ω   &gt;     Ω   L             }             (     This                 formula                 represents                 the                 angular                 distribution                           
 
         [0160]    function of aligned light spot  1602 );  
           Θ   FIBER          (   Ω   )       ≈     {         1         Ω   ≤     Ω   FIBER               0         Ω   &gt;     Ω   FIBER             }             (     This                 formula                 represents                 the                 numerical                           
 
         [0161]    function of optical aperture  1606 );  
           x′   λ   =f  tan[θ(λ)−θ(λ L )];  
           y′   λ =0;  
           x′   λ     L   =0;  
           y′   λ     L   =0. 
         [0162]    [0162]FIG. 17 shows a simulation of the effectiveness of SSE and ASE suppression achieved by an embodiment of the present invention for r λ     L   =r 0 , f=1000·r 0 , d=1 μm, Ω L ≦Ω FIBER  and assuming a Gaussian light intensity distribution for aligned light spot  1602 . Over the emission band of gain medium  1408  of FIG. 14, the present invention filters out SSE and ASE background noise radiation at all wavelengths other than the desired laser wavelength λ L , which is shown in FIG. 17 to be approximately 1.54 μm. Consequently, the only light coupled into waveguiding device  1432  is light with the desired wavelength, λ L .  
         [0163]    An advantage of the present invention, as illustrated in the numerous embodiments discussed herein, is that it can maintain a maximum and constant level of light coupled into waveguiding device  1432  while the wavelength of output laser beam  1416  and of low noise laser beam  1434  is tuned across a wide range of wavelengths, i.e., it provides automatic wavelength and power tracking for the low-noise output laser beam. In the embodiment shown in FIG. 14, this advantage is achieved by appropriate selection of the physical dimensions of dual-beam laser system  1400 .  
         [0164]    [0164]FIG. 18 shows an alternative embodiment of the present invention. Dual-beam laser system  1800  comprises pivot  1802 , base  1804 , dispersion unit  1810 , tuning reflector  1812 , rotatable unit  1814 , output laser beam  1816 , first-order diffracted radiation  1818 , reflected diffraction beam  1820 , guiding mirror  1822 , collector incident light  1824 , beam collector  1826 , focused light spot  1828 , optical coupling device  1830 , waveguiding device  1832 , low noise laser beam  1834 , angularly-separated SSE  1836 , angulary-separated ASE  1838 , laser diode  1850  and collimation lens  1852 .  
         [0165]    The structure of the embodiment shown in FIG. 18 is substantially identical with the structure of the embodiment shown in FIG. 14 except that the embodiment of FIG. 18 employs a laser diode  1850  to replace the combination of the plane reflector  1406  and the gain medium  1408  from FIG. 14 and introduces a collimation lens  1852  disposed along an optical path between laser diode  1850  and dispersion unit  1810 . Since the output of laser diode  1850  generally exhibits an undesirable elliptically-divergent shape, collimation lens  1852  is employed to collimate the light incident on dispersion unit  1810  at a grazing angle. In a preferred embodiment, both facets of collimation lens  1852  are treated with an anti-reflection coating to reduce light feedback from internal reflection within collimation lens  1852 . Further, a facet of laser diode  1850  oriented towards dispersion unit  1810  is also treated with an anti-reflection coating to maximize power output of laser diode  1850 .  
         [0166]    In operation, the embodiment of FIG. 18 functions substantially the same as the embodiment of FIG. 14 because the light beam generated by laser diode  1850  in conjunction with collimation lens  1852  is substantially identical with the light beam produced by gain element  1408  and plane reflector  1406 . Consequently, the light incident at a grazing angle on diffraction grid  1810  is substantially identical with the light incident at a grazing angle on diffraction grid  1810 , and therefore the description provided for the embodiment of FIG. 18 generally applies to the embodiment of FIG. 18.  
         [0167]    [0167]FIG. 19 shows another alternative embodiment of the present invention. Dual-beam laser system  1900  comprises pivot  1902 , base  1904 , plane reflector  1906 , gain medium  1908 , dispersion unit  1910 , tuning reflector  1912 , rotatable unit  1914 , output laser beam  1916 , first-order diffracted radiation  1918 , reflected diffraction beam  1920 , guiding mirror  1922 , focused light spot  1928 , optical coupling device  1930 , waveguiding device  1932 , low noise laser beam  1934 , angularly-separated SSE  1936 , angulary-separated ASE  1938 , and concave mirror  1954 .  
         [0168]    The structure of the embodiment of FIG. 19 is substantially identical with the structure of the embodiment of FIG. 14, except that the embodiment of FIG. 19 substitutes a concave mirror  1954  for beam collector  1426 . Concave mirror  1954  is mechanically coupled to rotatable unit  1914  such that a concave reflective surface of concave mirror  1954  is oriented in the general direction of guiding mirror  1922  to intercept reflected diffraction beam  1920 . Optical coupling device  1930  and waveguiding device  1932  are mechanically coupled to rotatable unit  1914  on the same side of concave mirror  1954  as guiding mirror  1922 .  
         [0169]    In operation, the embodiment shown in FIG. 19 functions substantially the same as the embodiment of FIG. 14. Reflected diffraction beam  1920  is substantially identical with collector incident light  1424  from FIG. 14. Unlike in the embodiment of FIG. 14, however, reflected diffraction beam  1920  is not refracted by beam collector  1426 , which comprises a lens, but is instead reflected by concave mirror  1954 .  
         [0170]    Concave mirror  1954  is designed to reflect and focus reflected diffraction beam  1920  in a pattern substantially identical with the pattern experienced by the light refracted by beam collector  1426  in the embodiment of FIG. 14. Consequently, concave mirror  1954  reflects and concentrates reflected diffraction beam  1920  into focused light spot  1928  which is substantially identical to focused light spot  1428  of FIG. 14. To take advantage of the spatial filtering technique previously discussed in connection with the embodiment of FIG. 14, optical coupling device  1930  is mechanically coupled to rotatable unit  1914  such that focused light spot  1928  is coupled into optical coupling device  1930 .  
         [0171]    Upon reflection by concave mirror  1954 , reflected diffraction beam  1924  propagates at an angle ξ(λ L ) with respect to the focal axis of concave mirror  1954 . Angle ξ(λ L ) does not exhibit any dependence on wavelength or on the position of rotatable unit  1914 , but is instead fully determined by initial selection and alignment of the components of dual-beam laser system  1900 . To ensure full coupling of focused light spot  1928  into optical coupling device  1930 , the area of optical coupling device  1930  must be larger than the effective size of focused light spot  1928  and the numerical aperture of optical coupling device  1930  must be larger than the convergence of focused light spot  1928 . If these conditions are satisfied, proper initial design of dual-beam laser system  1900  results in continuous and stable coupling of selected focused light spot  1928  into waveguiding device  1932  with simultaneous and effective filtering of SSE and ASE background light regardless of wavelength variations in the system as a result of laser tuning.  
         [0172]    [0172]FIG. 20 shows yet another embodiment of the present invention. Dual-beam laser system  2000  comprises pivot  2002 , base  2004 , dispersion unit  2010 , tuning reflector  2012 , rotatable unit  2014 , output laser beam  2016 , first-order diffracted radiation  2018 , reflected diffraction beam  2020 , guiding mirror  2022 , focused light spot  2028 , optical coupling device  2030 , waveguiding device  2032 , low noise laser beam  2034 , angularly-separated SSE  2036 , angulary-separated ASE  2038 , laser diode  2050 , collimation lens  2052  and concave mirror  2054 .  
         [0173]    The structure of the embodiment shown in FIG. 20 is substantially identical with the embodiment shown in FIG. 14, except that the embodiment of FIG. 20 includes the modifications introduced by the embodiments shown in FIG. 18 and FIG. 19. More specifically, the embodiment of FIG. 20 employs a laser diode  2050  to replace the combination of the plane reflector  1406  and the gain medium  1408  from FIG. 14 and introduces a collimation lens  2052  disposed along an optical path between laser diode  2050  and dispersion unit  2010 , as discussed in connection with FIG. 18. Additionally, the embodiment of FIG. 20 substitutes a concave mirror  2054  for beam collector  1426 , as described in conjunction with FIG. 19. In a preferred embodiment, both facets of collimation lens  2052  are treated with an anti-reflection coating to reduce light feedback from internal reflection within collimation lens  2052 . Further, a facet of laser diode  2050  oriented towards dispersion unit  2010  is also treated with an anti-reflection coating to maximize power output of laser diode  2050 .  
         [0174]    In operation, both modifications operated to the embodiment shown in FIG. 20 perform substantially identical functions as the original elements they replace, as discussed in connection with the embodiments of FIG. 18 and FIG. 19. Consequently, the descriptions provided for the embodiments shown in FIGS. 14, 18 and  19  also apply to the embodiment of FIG. 20.  
         [0175]    [0175]FIG. 21 shows yet another alternative embodiment of the present invention. Dual-beam laser system  2100  comprises pivot  2102 , base  2104 , plane reflector  2106 , gain medium  2108 , dispersion unit  2110 , tuning reflector  2112 , rotatable unit  2114 , output laser beam  2116 , first-order diffracted radiation  2118 , reflected diffraction beam  2120 , beam collector incident light  2124 , beam collector  2126 , focused light spot  2128 , optical coupling device  2130 , waveguiding device  2132 , low noise laser beam  2134 , angularly-separated SSE  2136 , angulary-separated ASE  2138 , and guiding dispersion unit  2160 .  
         [0176]    The structure of the embodiment of FIG. 21 is substantially identical with the structure of the embodiment of FIG. 14, except that in the embodiment of FIG. 21 a guiding dispersion unit  2160  substitutes guiding mirror  1422  of FIG. 14 and beam collector  2126 , optical coupling device  2130  and waveguiding device  2132  are removed from rotatable unit  2114 . Guiding dispersion unit  2160  is mounted to the base  2104 , and substantially parallel with, dispersion unit  2110 , and is oriented towards dispersion unit  2110  to intercept reflected diffraction beam  2120 . Beam collector  2126 , optical coupling device  2130  and waveguiding device  2132  are mounted above dispersion unit  2110  and are fixed with respect to guiding dispersion unit  2160 .  
         [0177]    In operation, the embodiment shown in FIG. 21 functions substantially the same as the embodiment of FIG. 14. Reflected diffraction beam  2120  is substantially identical with reflected diffraction beam  1420  from FIG. 14. Unlike in the embodiment of FIG. 14, however, reflected diffraction beam  2120  is not reflected by guiding mirror  1422 , but is instead diffracted by guiding dispersion unit  2160 .  
         [0178]    Guiding dispersion unit  2160  is designed to diffract reflected diffraction beam  2120  in a pattern substantially identical with the pattern experienced by the light reflected by guiding mirror  1422  in the embodiment of FIG. 14. Consequently, beam collector incident light  2124  is substantially identical with beam collector incident light  1424  from FIG. 14. As a result, beam collector  2126  refracts and focuses beam collector incident light  2124  into focused light spot  2128  which is substantially identical to focused light spot  1428  of FIG. 14.  
         [0179]    Reflected diffraction beam  2124  comprised in collector incident radiation  2124  forms an angle χ(λ L ) with the normal of guiding dispersion unit  2160 , where  
         χ(λ L )=θ 0 . 
         [0180]    As indicated by this formula, angle χ(λ L ) does not exhibit any dependence on wavelength or on the position of rotatable unit  2114 , but is instead fully determined by initial selection and alignment of the components of dual-beam laser system  2100 . To ensure full coupling of focused light spot  2128  into optical coupling device  2130 , the area of optical coupling device  2130  must be larger than the effective size of focused light spot  2128  and the numerical aperture of optical coupling device  2130  must be larger than the convergence of focused light spot  2128 . If these conditions are satisfied, proper initial design of dual-beam laser system  2100  results in continuous and stable coupling of selected focused light spot  2128  into waveguiding device  2132  with simultaneous and effective filtering of SSE and ASE background light regardless of wavelength variations in the system as a result of laser tuning.  
         [0181]    [0181]FIG. 22 shows an alternative embodiment of the present invention. Dual-beam laser system  2200  comprises pivot  2202 , base  2204 , plane reflector  2206 , gain medium  2208 , dispersion unit  2210 , tuning reflector  2212 , rotatable unit  2214 , output laser beam  2216 , first-order diffracted radiation  2218 , reflected diffraction beam  2220 , beam collector incident light  2224 , beam collector  2226 , focused light spot  2228 , optical coupling device  2230 , waveguiding device  2232 , low noise laser beam  2234 , angularly-separated SSE  2236 , angulary-separated ASE  2238 , laser diode  2250 , collimation lens  2252  and guiding dispersion unit  2260 .  
         [0182]    The structure of the embodiment shown in FIG. 22 is substantially identical with the structure of the embodiment shown in FIG. 21 except that the embodiment of FIG. 22 employs a laser diode  2250  to replace the combination of plane reflector  2106  and gain medium  2108  from FIG. 21 and introduces a collimation lens  2252  disposed along an optical path between laser diode  2250  and dispersion unit  2210 . Since the output of laser diode  2250  generally exhibits an undesirable elliptically-divergent shape, collimation lens  2252  is employed to collimate the light incident on dispersion unit  2210  at a grazing angle. In a preferred embodiment, both facets of collimation lens  2252  are treated with an anti-reflection coating to reduce light feedback from internal reflection within collimation lens  2252 . Further, a facet of laser diode  2250  oriented towards dispersion unit  2210  is also treated with an anti-reflection coating to maximize power output of laser diode  2250 .  
         [0183]    In operation, the embodiment of FIG. 22 functions substantially the same as the embodiment of FIG. 21 because the light beam generated by laser diode  2250  in conjunction with collimation lens  2252  is substantially identical with the light beam produced by gain element  2108  and plane reflector  2106 . Consequently, the light incident at a grazing angle on diffraction grid  2210  is substantially identical with the light incident at a grazing angle on diffraction grid  2110 , and therefore the description provided for the embodiment of FIG. 21 also applies to the embodiment of FIG. 22.  
         [0184]    [0184]FIG. 23 shows yet another embodiment of the present invention. The preceding description of various embodiments of this invention taught how an optical aperture coupled to a waveguiding device can be employed as a spatial narrow band-pass filter to suppress SSE and ASE background light with wavelengths other than a desired wavelength. The embodiment of FIG. 23 illustrates how an optical transmission pinhole can be employed as a narrow band-pass filter to either replace or supplement and enhance the filtering effect of an optical coupling device.  
         [0185]    [0185]FIG. 23 shows a simplified representation of the complete laser system described in prior embodiments. Laser system  2300  includes light generator  2370 , pinhole incident light  2372 , pinhole  2374 , beam collector  2326 , focused light spot  2328 , optical coupling device  2330  and waveguiding device  2332 .  
         [0186]    Light generator  2370  and optical coupling device  2330  are mounted at opposite ends of laser system  2300 . Referring to the embodiment of FIG. 14 for example, light generator  2370  could include a subsystem comprising plane reflector  1406 , gain medium  1408 , dispersion unit  1410 , tuning reflector  1412  and guiding mirror  1422 . Pinhole  2374  and beam collector  2326  are disposed along an optical path between light generator  2370  and optical coupling device  2330  such that optical aperture  2330  is located distally from light generator  2370  with respect to pinhole  2374 . In an alternative embodiment, beam collector  2326  could be replaced by a concave mirror, as disclosed in the embodiment shown in FIG. 19 for example. Optical aperture  2330  is operationally connected to waveguiding device  2332  to permit coupling of light.  
         [0187]    In operation, light generator  2370  projects pinhole incident light  2372  towards pinhole  2374 . Pinhole  2374  includes a transparent area which permits part of pinhole incident light  2372  to propagate beyond pinhole  2374  and illuminate beam collector  2326 . Beam collector  2326  focuses the incident light radiation into focused light spot  2328  which is pre-aligned with optical coupling device  2330  to permit efficient light coupling into waveguiding device  2332 . Pinhole  2374  acts as a physical spatial narrow band-pass filter effectively suppressing SSE and ASE background light, and therefore provides a first-order filtering stage for light propagating towards beam collector  2326 . Consequently, the light incident on beam collector  2326  is already filtered prior to being concentrated into focused light spot  2328 . This advantage could be employed, among others, to relax the design constraints imposed on the optical characteristics of beam collector  2326  and the alignment requirements associated with narrow band-pass filtering as taught by the present invention. Further, the SSE and ASE radiation may be cut off by the spatial filter so that a broader optical coupling device  2330  is still adequate to couple the laser beam into waveguiding device  2332 . A larger receiving aperture decreases the probability that optical coupling device  2330  is damaged by the heat produced by the high optical energy being coupled into waiveguiding device  2332 .  
         [0188]    It will be manifest that various additions, modifications and rearrangements of the features of the invention may be made without deviating from the spirit and scope of the underlying inventive concept. It is intended that the scope of the invention as defined by the appended claims and their equivalents cover all such additions, modifications, and rearrangements. The appended claims are not to be interpreted as including means-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase “means-for.” Expedient embodiments of the invention are differentiated by the appended claims.