Abstract:
An external cavity laser may be swept rapidly in frequency and cavity length to prevent formation of modes providing improved spectral response and light characteristics.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with United States government support awarded by the following agencies: NSF 0307455. The United States has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to lasers and more particularly to a laser providing a rapidly sweeping light frequency without laser modes. 
     Spectroscopic studies evaluate the “spectrographic” response of a material to different frequencies of light. The spectrographic response may be light absorption, reflectivity, scattering, fluorescence or other features. 
     Spectrographic studies may be used to investigate gases, liquids, aerosols, solids, particulates, and the like, as their physical properties change in response to temperature, pressure, velocity, composition, size, stress and strain. Similar techniques may be used to monitor sensors incorporating materials whose spectrographic responses change as a function of a physical parameter to be measured. 
     Spectroscopic studies may use a “wavelength-agile” light source providing a spectrally narrow light beam that may be quickly and controllably swept in frequency. One implementation of a wavelength-agile light source employs a laser incorporating a spectral filter into its optical cavity and changing the frequency of that filter during operation of the laser. 
     One common spectral filter is a diffraction grating. The diffraction grating may be tipped with respect to an incident laser beam to adjust the effective spacing of the grating&#39;s rule lines along the beam and hence the frequency of light preferentially reflected by the grating. As the grating is moved to change the frequency of the laser beam, the length of the optical cavity is ordinarily adjusted to match the beam&#39;s wavelength to maintain optical resonance. This optical resonance, resulting from standing light waves created by laser cavity elements such as mirrors, is termed a “mode”. At any given laser mode, stimulated emissions by the laser material produce a phase coherence in the emitted light. This phase coherence can produce a phenomenon termed “speckle” in which light from the laser constructively adds or destructively cancels at given points. 
     A laser system providing simultaneous adjustment of a diffraction grating for frequency selection and optical cavity length to preserve optical resonance is described in U.S. Pat. No. 5,319,668 hereby incorporated by reference. 
     Spectrographic analysis of short optical phenomena with a wavelength agile laser requires the ability to rapidly change the laser frequency. This speed of frequency change can be limited by mechanical constraints incident to coordinated movement of the optical grating and change in cavity length. At high speeds of frequency change, the laser may “mode hop” jumping from one mode to another mode separated by a substantial wavelength difference. Such a problem is described in U.S. Pat. No. 6,683,895, at col. 2, lines 27 through 30. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a wavelength agile laser changing its cavity length at a speed found to substantially prevent the formation of laser modes. The result is a spectrally narrow, swept-wavelength light source that is resistant to mode hopping because of the absence of laser modes. Furthermore, the output light has random phase, resulting in reduced speckle. In a preferred embodiment, a pivoting mirror design provides the high rate of cavity length change. 
     Specifically then, the present invention provides a modeless laser having a laser element (a gain medium and an associated pump). An optical system defines a cavity receiving the photons from the laser material and reflecting the photons back to the laser material. The cavity has an instantaneous effective length, which is rapidly varied so as to substantially prevent a formation of resonant modes. 
     Thus, it is one object of at least one embodiment of the invention to avoid the formation of laser modes such as may cause “mode hopping” or produce speckle. 
     The speed at which the cavity length must be changed is such that, in the time it takes light to cycle through the cavity, the length is changed by an appreciable fraction of the wavelength. Typically speeds of approximately one kilometer per second or more, or changes of one percent of the wavelength per round trip transit time of the photons are required. 
     Thus, it is another object of at least one embodiment of the invention to provide a laser cavity mechanism producing high rates of cavity length change. 
     The cavity length may change by no less than ⅛ of a wavelength of emitted photons during a period defined by the round trip transit time of the photons along the cavity length and preferably no less than 1% of that wavelength. 
     Thus, it is another object of at least one embodiment of the invention to produce rapid change in the cavity length to produce modeless operation. 
     The means for varying the length of the cavity may include a mirror pivoting about an axis to direct photons to a retro reflector having a surface with different portions of varying distance from the mirror. 
     Thus, it is an object of at least one embodiment of the invention to provide a mechanical system that can produce a virtually unbounded rate of change of cavity length with pivoting movements of a single mirror element. 
     The mirror may pivot using a reciprocating actuator or may be a polygonal prism having a reflective periphery and a motor for pivoting the prism about a central axis. 
     Thus, it is another object of at least one embodiment of the invention to provide a system that may be flexibly implemented to produce varying functions of frequency agility. 
     The retro reflector may be a diffraction grating. 
     Thus, it is one object of at least one embodiment of the invention to combine the functions of frequency selectivity and retro-reflector for cavity length change in a single optical element. 
     The optical system may separately provide a means for varying the wavelength of the photons and for varying the cavity length independently of the wavelength of the photons to substantially prevent formation of resonant modes. 
     Thus, it is another object of at least one embodiment of the invention to create the possibility of suppressing resonant modes by independently varying photon frequency and cavity length so as to upset the formation of standing resonant modes. 
     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a laser of the present invention providing modeless frequency-agile operation; 
         FIG. 2  is a diagrammatic representation of the cavity length of the laser of  FIG. 1  showing a rate of change of cavity length in proportion to the wavelength of photons along the cavity during a round trip passage of the photos through the cavity; 
         FIG. 3  is an alternative embodiment of the mirror assembly of  FIG. 1  such as provides for a linear sweeping of frequency versus time; and 
         FIG. 4  is a plot of frequency versus time for the embodiments of  FIG. 1  and  FIG. 2  showing regions of modeless operation for each. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , a frequency agile modeless laser  10  includes a laser source  12  providing a laser medium  14  such as supports the stimulated emission of photons, and an energy pump  16  and power supply  18  exciting the of the laser medium  14  into a stimulated state. 
     The laser medium  14  is preferably a solid-state material forming part of a solid-state laser diode, for example, in which case the pump  16  is an electrode of the diode. More generally, the laser medium  14  may be any suitable laser material and the pump  16  may be an optical or electrical pump for stimulating the electrons of the laser medium  14  as will be understood in the art. 
     The laser medium  14  may emit photons along an optical axis  20  extending through a front and rear surface of the laser medium  14 . At the rear surface of the laser medium, the optical axis  20  is intercepted by a mirror  22  which reflects emitted photons back into the laser medium  14 . The photons emitted from the front surface of the laser medium  14  may be received by a lens assembly  24  of a type well known in the art to direct a focused beam  26  of photons further along the optical axis  20 . 
     The beam  26  is received by a beam splitter  28  directing a portion  30  of the beam  26  at right angles to the optical axis  20  for use in spectrographic purposes. The remainder of the beam  26  passes to a front surface, pivoting mirror  34  which may direct a diverted beam  26 ′ at an acute angle θ with respect to the optical axis  20 . Pivoting mirror  34  turns about a pivot point  36  so that the angle θ may be changed by an amount Δθ during reciprocation of the mirror about the pivot point  36  by a piezoelectric transducer  38  or the like. 
     The diverted beam  26 ′ from pivoting mirror  34  may be received by a diffraction grating  40  having a ruled face toward the diverted beam  26 ′ and being arrayed generally (but not necessarily) parallel to the optical axis  20 . The pivoting of the pivoting mirror  34  changes a point at which the center of the diverted beam  26 ′ strikes the diffraction grating  40  from point A furthest from the pivoting mirror  34  to point B closest to the pivoting mirror  34 , both points A and B being on the ruled face of the diffraction grating  40 . 
     The incident angle at which diverted beam  26 ′ intersects the surface of diffraction grating  40  will vary as a function of where the diverted beam  26 ′ intersects the surface of diffraction grating  40 . This angle (θ in the case of an axis parallel diffraction grating  40 ) is smaller at point A than at point B. Generally this angle determines the dominant frequency of the reflection of the diverted beam  26 ′ off of the diffraction grating  40 . 
     The frequency selecting qualities of the diffraction grating  40  result from the constructive adding of light frequencies reflected off of each ruling of the diffraction grating  40  for a particular frequency as determined by the grating periodicity projected onto the axis of diverted beam  26 ′. Thus, generally at point A, the diffraction grating  40  will selectively reflect lower frequency light and at point B will selectively reflect higher frequency of light. 
     Referring now to  FIGS. 1 and 2 , an effective optical cavity length  50  is defined as the apparent optical distance between the front surface of mirror  22  and the point of intersection along a line between A or B on the front surface of the diffraction grating  40 . The effective optical cavity length  50  will generally be the geometric length as modified by the light speed of materials interposed into the cavity. During mode operation of a typical laser, this cavity length is an integer number of wavelength of the frequency of the light of the beam  26 ,  26 ′ such as creates a standing wave  52 . 
     The movement of the pivoting mirror  34  described above changes the point of intersection of diverted beam  26 ′ and diffraction grating  40 , and thus the cavity length  50  of the laser  10  by an distance  42 . It will be understood that by decreasing angle θ, the distance  42  may be arbitrarily increased for a given value of Δθ. Thus, small motion of pivoting mirror  34  may create extremely large change in cavity length  50 . Thus, for example, with audio frequency oscillation of the pivoting mirror  34 , for example, at 10,000 hertz or greater, a distance  42  of 1/10 of a meter will provide a change in the optical cavity length  50  of one kilometer per second or greater. 
     During operation of the device of  FIG. 1 , the cavity length  50  is rapidly varied to prevent the formation of modes. The necessary speed of change of the optical cavity length  50  is believed to be a function of the wavelength of the light of beam  26 . A suitable change in optical cavity length  50 , during a round trip propagation of light from mirror  22  to diffraction grating  40  and back, is at least ⅛ of a wavelength or at least 1% of the wavelength. 
     While the inventors do not wish to be bound by a particular theory, it is believed that this rapid change in cavity length and light frequency prevents the cascading stimulation of coherent photons such as are necessary to create a mode while still allowing sufficient stimulated emission to promote acceptable energy at the given wavelength frequency. The lack of mode formation is believed to prevent mode-hopping and to allow a smoother and more reliable sweeping of light frequency with reduced amplitude variations. 
     Movement of the pivoting mirror  34  not only changes the cavity length but also changes the preferential frequency of the photons reflected back from the diffraction grating  40 . The relative geometry of pivoting mirror  34  and diffraction grating  40  described with respect to  FIG. 1  may match wavelength of the photons to cavity length. That is, increases in cavity length caused by movement of the pivoting mirror  34  may cause the diverted beam  26 ′ to strike the diffraction grating  40  at an angle to promote a frequency whose wavelength times an integer substantially equals the cavity length. 
     Alternatively, it will be understood that the frequency selectivity of the diffraction grating  40  may be made independent of the instantaneous length of the cavity length by adjustment of the geometry of the diffraction grating  40  and pivoting mirror  34 , so that frequency selected by the diffraction grating  40  may diverge from an integer division of the cavity length. While the inventors do not wish to be bound by a particular theory, it may be a mismatch between frequency and cavity length helps promote modeless operation. 
     Referring now to  FIG. 4 , in an alternative embodiment to the mirror mechanism of  FIG. 1 , pivoting mirror  34  may be replaced with rotating mirror  60  being a polygonal prism, in this case having an octagonal cross section rotated about its axis  62  such as forms a pivot, and having a reflective outer periphery  64  for reflecting beam  26 ′. The diverted beam  26  may be received by a retro reflector  66  allowing cavity length changes with changing angle θ. Note that the retro reflector  66  need not be parallel to the optical axis  20  and need not be a diffraction grating, but may be other retro reflective material including a series of corner reflectors or transparent sphere type retro reflector surfaces. Frequency selection in this case may be provided by other means well known in the art. 
     Referring now to  FIG. 4 , the cavity length, as a function of time for the embodiment of  FIG. 1 , will be a generally sinusoidal-shaped curve  72  defined by the reciprocation of the pivoting mirror  34 . Curve  72  provides a variable rate of change of cavity length possibly limiting modeless operation to restricted range  68  permitting the laser  10  to revert to a modal operation at times near when the pivoting mirror  34  changes direction. These periods of modal operation may be reduced by rapid oscillation or may be acceptable during spectrographic scanning representing only the limits of the frequency range. 
     In contrast, the polygonal mirror of  FIG. 3  provides for a set of discontinuous ramp-shaped curves  74  having a constant rate of cavity length change believed to provide no lapse into modal operation. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. For example, other methods of changing the effective optical cavity length, including electronically or acoustically modulated elements, may be used.