Abstract:
The present invention relates to an integrated light source having first and second optical waveguides defining a first optical coupling region for coupling light therebetween. At least one of the optical waveguides includes a gain medium configured to emit light upon irradiation. The light source also includes a first acoustic wave source to subject the first optical coupling region to acoustic waves having a longitudinal frequency ω AC1 , whereby a frequency of light propagating along one of the first and second waveguides differs from a frequency of light propagating along the other waveguide by an amount ω AC1 .

Description:
RELATED APPLICATIONS  
       [0001]    This application is related to U.S. application Ser. No. 10/***,***, titled Light Source For Generating Output Signal Having Evenly Spaced Apart Frequencies and filed on even date herewith, invented by Israel Smilanski, Isaac Shpantzer, Jacob B. Khurgin, Nadejda Reingand, Pak Shing Cho, and Yaakov Achiam, which application is incorporated herein. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to an acoustically tuneable light source and method for acoustically tuning a light source.  
         BACKGROUND OF THE INVENTION  
         [0003]    Tuneable light sources output light comprising at least one of a plurality of frequencies. One type of tuneable light source, the tuneable distributed feedback (DFB) laser, has found applications in optical communications. The tuning time for DFB lasers, however, is on the order of milliseconds, which is slower than the microsecond tuning times required for modern optical communication systems.  
           [0004]    Another example of a tuneable light source is a diode-pumped, packaged acousto-optically tunable Ti:Er:LiNbO3 waveguide laser described by K. Schafer et al., IEEE J. Quant. Electr., v.33, #10, pp.1636-1641. This laser provides sub-millisecond tuning capability through TE-TM mode conversion within birefringent material. It would be desirable, however, to form a tuneable laser from non-birefringent materials, such as non-crystalline materials, because birefringent materials are more complex in manufacturing and operation.  
         SUMMARY OF THE INVENTION  
         [0005]    A first embodiment of the invention relates to a light source. The light source comprises first and second optical waveguides, at least one of which waveguides comprises a gain medium. Upon excitation, such as by irradiation with light from a light source, the gain medium generates, such as by emitting, light having a plurality of frequencies, at least some of which may be output by the light source. The particular frequencies of light output by the light source may be acoustically switched at more than about 100 kHz.  
           [0006]    The first and second waveguide define a first optical coupling region, wherein light, such as the generated light, propagating along one of the waveguides may couple to the other waveguide. Preferably, only light that couples between waveguides may be output by the tuneable light source. The optical frequency that couples between waveguides may be acoustically switched by subjecting the first optical coupling region to acoustic waves having a longitudinal frequency ω AC1 . Essentially the only light that may couple is light that satisfies a matching condition of the first coupling region whereby, upon coupling, a frequency of the light is shifted by about ±ω AC1 .  
           [0007]    A second embodiment of the present invention is related to an integrated laser cavity that may be used to generate laser light. The laser cavity comprises first and second optical waveguides, which define an offset coupling region therebetween. By offset it is meant that longitudinal axes of the first and second optical waveguides are spaced apart from one another. At least one of the optical waveguides comprises a gain medium configured to, upon excitation, generate light.  
           [0008]    Light propagating along one of the first and second waveguides may couple to the other waveguide at the coupling region. The frequency of light that may couple is acoustically tuneable by varying a first longitudinal acoustic wave vector K AC  of acoustic waves impinging upon the first coupling region. Upon coupling from one waveguide to the other, a wave vector of the coupled light is shifted by an amount K AC . Preferably, only light that couples may be output by the integrated laser cavity.  
           [0009]    Another embodiment of the invention relates to an integrated interferometer having at least first and second different optical paths. The interferometer includes first and second coupling regions, whereby light propagating along the first and second optical paths couples interferingly to a first waveguide and propagates therealong. A first acoustic wave source subjects the first and second coupling regions to acoustic waves having a first longitudinal acoustic wave vector K AC1 , whereby a wave vector of light propagating along one of the first and second optical paths differs from a wave vector of light propagating along the first waveguide by an amount K AC1 .  
           [0010]    Another aspect of the invention relates to a method for producing light. In one embodiment, a gain medium within a first waveguide is irradiated with pump light to obtain generated light having an generated light frequency. The generated light is coupled to a second waveguide by subjecting at least some of the generated light to acoustic waves having a first frequency ω AC1  to thereby provide second light having a second light frequency, wherein the second light frequency differs from the emitted light frequency by an amount ω AC1 . At least some of the second light is output.  
           [0011]    Another aspect of the invention relates to an optical transmitter that includes an optical cavity comprising an optical coupling region between first and second waveguides. An acoustic wave source is disposed to subject the optical coupling region to acoustic waves having an acoustic frequency ω 1 , whereby, upon coupling from one waveguide to the other, a frequency of light oscillating within the optical cavity is shifted by an amount of about ±ω i . The optical cavity is configured to output at least some of the oscillating light. The transmitter also includes an acoustic wave source driver for changing the acoustic frequency ω i , wherein a frequency of light output by the optical cavity changes upon changing the acoustic frequency ω i .  
           [0012]    Light transmitted by the optical transmitter may be received by a receiver that simultaneously detects the transmitted light with light output by an acousto-optically tuneable optical cavity local to the receiver. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The present invention is discussed below in reference to the drawings in which:  
         [0014]    [0014]FIG. 1 shows a tuneable light source according to the invention;  
         [0015]    [0015]FIG. 2 shows a partial view of a coupling region of the light source of FIG. 1;  
         [0016]    [0016]FIG. 3 shows an example of an output spectrum of the light source of FIG. 1;  
         [0017]    [0017]FIG. 4 shows an example of a second embodiment of a tuneable light source according to the invention;  
         [0018]    [0018]FIG. 5 shows a plot of available output frequencies for the tuneable light source of FIG. 4;  
         [0019]    [0019]FIG. 6 shows spectra that contribute to the output frequencies of FIG. 5;  
         [0020]    [0020]FIG. 7 shows a third embodiment of a light source according to the invention; and  
         [0021]    [0021]FIG. 8 shows a fourth embodiment of a light source according to the invention.  
         [0022]    [0022]FIG. 9 shows a time-frequency plot of light output by a secure communication source of the invention;  
         [0023]    [0023]FIG. 10 shows a secure communication source of the invention suitable for preparing the time-frequency plot of FIG. 9; and  
         [0024]    [0024]FIG. 11 shows a receiver of the invention for receiving information transmitted by the secure communication of FIG. 10. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    Referring to FIGS. 1 and 2, a light source, which in this embodiment is a laser  20 , which preferably includes first, second, and third waveguides  24 ,  26 , and  28 . Laser  20  is preferably tuneable, by which it is meant that a frequency of light output by laser  20  may be varied. The laser  20  may be integral with a substrate  22 , such as by having the waveguides formed therein by, for example, diffusive doping. In general, preferred substrate materials are non-crystalline. Silica, such as amorphous silica, is an example of a suitable substrate material.  
         [0026]    First and second waveguides  24 ,  26  define a first coupling region  38 , wherein light propagating along one of the first and second waveguides may couple to the other waveguide to provide coupled light, which propagates therealong. First and third waveguides  24 ,  28  define a second coupling region  40 , wherein light propagating along one of the first and third waveguides may couple to the other waveguide to provide coupled light, which propagates therealong. Preferred coupling regions of devices in accordance with the invention are essentially free of crystalline material. First and second coupling regions  38 ,  40  are preferably spaced apart from one another along a general propagation dimension of the first waveguide  24 .  
         [0027]    Tuneable laser  20  includes an optical cavity preferably having at least two reflective elements and including the first, second, and third waveguides. By “including the first, second, and third waveguides” it is meant that light oscillating within the optical cavity propagates along at least portions of each of the first, second, and third waveguides. Preferably, only light satisfying a matching condition may oscillate within the cavity.  
         [0028]    Oscillation within the optical cavity preferably comprises propagation of the light between respective ends  60 ,  61  of second and third waveguides  26 ,  28 . A first reflective element  42  may be optically associated with end  60  and preferably operates as an output coupler that is only partially reflective at output wavelengths of tuneable laser  20 . A second reflective element  44  may be associated with end  61  and preferably reflects substantially all light at output wavelengths of tuneable laser  20 .  
         [0029]    First waveguide  24  has a propagation constant different from a propagation constant of second and third waveguides  26 ,  28 , which may have the same propagation constant. As understood in the art, a propagation constant of a waveguide depends upon the dimensions, such as the height and width of the waveguide. Dimensions of first waveguide  24  may be different from dimensions of second and third waveguides  26 ,  28 . The propagation constant also depends upon the refractive index of the material forming the waveguide. First waveguide  24  may have a refractive index that is different from respective refractive indexes of second and third waveguides  26 ,  28 .  
         [0030]    The polarization of light is preferably substantially maintained upon coupling at coupling regions of devices in accordance with light sources of the invention. For example, an angular difference between (1) coupled light that has coupled from one of the waveguides (here termed the origin waveguide) to another waveguide and (2) the light propagating along the origin waveguide is less than about π/2, such as less than about π/8.  
         [0031]    Portions of at least one and preferably all of the first, second, and third waveguides  24 ,  26 , and  28  are doped with a gain medium. The gain medium preferably generates light, such as fluorescence with a plurality of wavelengths in at least the C-band, when irradiated with pump light. Of course, the tuneable laser  20  is not limited to gain media generating light in the infrared. For example, gain media generating light in the visible may also be used. A preferred gain medium, such as a gain medium comprising Er(Yb, Nd), exhibits population inversion and lasing under suitable pumping conditions.  
         [0032]    At least one of the waveguides is preferably configured to receive pump light from a pump source  63 . For example, first waveguide  24  receives pump light  21  from a light source  63 , which generates light that has a wavelength suitable to generate light from the gain medium. Preferably, each waveguide comprising gain medium receives pump light from a pump source. Of course, all of the waveguides receiving pump light may receive the pump light from a single pump source. An example of a pump source suitable for irradiating Er(Yb, Nd) is a diode laser emitting light in the infrared, such as at about 1480 nm. A waveguide may receive pump light via, for example, a facet at an end of the waveguide, side coupling, or grating coupling. Pump light received by a waveguide propagates therealong to thereby irradiate gain medium associated with the waveguide. Light sources in accordance with the present invention are not limited to optical pumping so that, for example, electrically pumped gain media may also be used.  
         [0033]    As best seen in FIG. 2, waveguide portions  50 ,  52  of respective first and second waveguides  24 ,  26  that are adjacent to first coupling region  38  define respective longitudinal axes  54 ,  56 , which are preferably essentially parallel. Light propagating along one of waveguide portions  50 ,  52  propagates generally along its respective longitudinal axis  54 ,  56 , which axes are preferably offset from one another. A coupling region in accordance with the invention may be described as an “offset” coupling region where light that is propagating along one waveguide of the coupling region translates laterally upon coupling to the other waveguide of the coupling region.  
         [0034]    Coupling regions of the invention are preferably configured to substantially prevent light that does not couple from one waveguide to another from continuing to propagate along the waveguide. First and second waveguide portions  50 ,  52  preferably include attenuation regions  71 ,  73  to attenuate light that has not coupled from one of the waveguides  24 ,  26  to the other. Attenuation regions  71 ,  73  may be shaped, such as by tapering, to attenuate light. Thus, for example, light satisfying a matching condition discussed below and propagating along first waveguide portion  50  toward attenuation region  71  couples to second waveguide portion  52  of second waveguide  26 . Preferably, however, propagating light that fails to satisfy the matching condition is substantially prevented from continued propagation by attenuation region  71 .  
         [0035]    Tuneable laser  20  includes a first acoustic wave source  30  to facilitate variable wavelength coupling between first and second waveguides  24 ,  26 . First acoustic wave source  30  subjects first coupling region  38  to acoustic waves  46  having a variable frequency ω AC1  and propagating generally along a propagation axis  48 . A second acoustic wave source  32  subjects second coupling region  40  to acoustic waves (not shown) also having a variable frequency ω AC2  and propagating with a velocity V AC  generally along a propagation axis  49  to thereby facilitate variable wavelength coupling between first and third waveguides  24 ,  28 . First and second acoustic wave sources  30 ,  32  may be piezo-electric transducers. Frequencies ω AC1  and ω AC2  may be the same or different.  
         [0036]    First and second acoustic wave sources are operably associated with at least one acoustic wave source driver, which provides an acoustic frequency signal to the acoustic wave sources to vary the respective acoustic frequency output by each source.  
         [0037]    An acoustic absorber  37  may be disposed to absorb or otherwise prevent acoustic waves  46  that have passed through first coupling region from returning therethrough, such as by reflection. A second acoustic absorber  36  may be disposed to absorb or otherwise prevent acoustic waves emitted by second acoustic wave source  32  that have passed through second coupling region  40  from returning therethrough, such as by reflection. Additional acoustic absorbers may be positioned to substantially prevent propagation of the acoustic waves lateral to propagation axes  48 ,  49 .  
         [0038]    Suitable coupling conditions for the coupling of light from one waveguide to another are discussed next using light propagating along first waveguide  24  and coupling to second waveguide  26  at first coupling region  38  as an example. It should be understood, however, the following coupling conditions also pertain coupling at coupling region  40  as well as coupling region of other light sources of the invention. A suitable condition for coupling is defined herein as a matching condition.  
         [0039]    A wave vector K 2  of an optical wave  23  propagating along waveguide  24  is given by:  
               K   2     =         ω   23     c          n   24               Eq   .              1                               
 
         [0040]    where ω 23  is a frequency of optical wave  23 , c is the speed of light in a vacuum, and n 24  is the index of refraction of waveguide  24  for light having a frequency ω 23 . Acoustic wave source  30  subjects first coupling region  38  to acoustic waves  46  having a frequency ω AC , which waves travel with a longitudinal velocity V AC  with respect to first coupling region  38 . By longitudinal frequency, it is meant the component of the acoustic waves taken along a longitudinal axis  54  of first waveguide  24 .  
         [0041]    Acoustic waves  46  have an acoustic wave vector K AC  and form an acoustic grating having a period Δ K  given by:  
               K   AC     =       Δ                 K     =       ω   AC       V   AC                 Eq   .              2                               
 
         [0042]    The acoustic grating interacts with optical wave  23 , such as by scattering at least some of optical wave  23 , to provide an optical wave having a wave vector K 1 =K 2 +ΔK. At least some of the optical wave with wave vector K 1  couples into waveguide  26  and propagates therealong with wave vector K 1 , where the wave vector K 1  is given by:  
               K   1     =           ω   27     c          n   26       =       K   2     +     Δ                 K                 Eq   .              3                               
 
         [0043]    where ω 27  is a frequency of optical wave  27  and n 26  is a refractive index of waveguide  26  for light having a frequency ω 27 .  
         [0044]    The matching condition, ΔK, is the difference between the wave vectors of optical waves  23  and  27 , is given by the period of the acoustic grating:  
               Δ                 K     =         ω   AC       V   AC       =             ω   27     c          n   26       -         ω   23     c          n   24         =           ω   27     c        Δ                 n     +         Δ                 ω     c          n   24                     Eq   .              4                               
 
         [0045]    where Δn is a refractive index difference given by n 26 −n 24  and Δω is a frequency difference given by ω 27 −ω 23 .  
         [0046]    When the matching condition is satisfied, light having a wave vector K 2  will couple from first waveguide  24  to second waveguide  26 . Likewise, light having a wave vector K 1  will couple from second waveguide  26  to first waveguide  24 . A similar matching condition must be met before light will couple in either direction between first waveguide  24  and third waveguide  28  at second coupling region  40 . Oscillation, and therefore lasing, will only occur at frequencies for which the matching condition is satisfied. Thus, acoustic sources  30 ,  32  determine the frequency of light that is output by tuneable laser  20  for any given acoustic wave frequency.  
         [0047]    Assuming that (Δω/c) n 24  is negligible compared to other terms in Eq. 4, a frequency ω out  of light that may be output by tuneable laser  20  is given by  
               ω   out     =       c                   ω   AC         Δ                 n                   V   AC                 Eq   .              5                               
 
         [0048]    During use, tuneable light source  20  may operate as follows. Gain medium within at least one of first, second, and third waveguides  24 ,  26 ,  28  is irradiated with pump light  21 . Upon pumping, the gain medium generates light having a plurality of frequencies. At least one of first and second coupling regions are subjected to acoustic waves having a frequency ω AC  so that light that is generated and propagates along, for example, first waveguide  24  may couple to, for example, the second waveguide  26 . Because only one frequency of the generated light may satisfy a given matching condition, the coupled light comprises essentially only light having the satisfying frequency. Thus, tuneable light source  20  may be acoustically tuned by varying ω AC  to output light having any one of the frequencies generated by the gain medium. The acoustic frequency ω AC  is preferably varied using the acoustic wave source driver associated with the acoustic wave generators. The frequency of light output by tuneable laser  20 , like all tuneable light sources of the invention, may be varied, by changing the frequency of acoustic waves ω AC , between first and second frequencies in less than about 50 μs and preferably in less than about 10 μs.  
         [0049]    As an example of using Eq. 5 to determine ω out , substitute c=3×10 8  m/s, Δn=1×10 −2 , V AC =3×10 4  m/s, and ω AC =2×10 8  Hz to predict an output frequency of 2×10 14  Hz, which corresponds to a wavelength of about 1.5 μm. Varying the acoustic wave frequency ω AC  over a range of about 191 to 196 MHz allows the output wavelength to be tuned over the range of about 1.53 to 1.57 μm. Of course, the acoustic wave frequency may be varied over wider ranges, such as about 170 to about 220 MHz or even wider ranges, to provide output wavelengths of less than 1.53 μm or greater than 1.57 μm.  
         [0050]    Referring to FIG. 3, the spectrum of light output by tuneable laser  20  includes a output spectrum  51  light having a range of frequencies centered at ω out . Preferably, however, the output spectrum  51  includes substantially fewer frequencies than light emitted by the gain medium of the waveguides. A width w 1  of output spectrum is preferably sufficiently narrow that the output spectrum  51  can be considered, as referred to above, to consist essentially of a single frequency of light. In terms of wavelength, for example, a line width of single frequency light output by tuneable light sources of the invention is less than about 0.5 nanometers, such as less than about 0.1 nanometers.  
         [0051]    Referring to FIG. 4, a discretely tuneable light source, which in this embodiment is a tuneable laser  100 , is configured to output light having one of a set of discrete frequencies. Tuneable laser  100  includes a substrate  102 , which preferably includes first, second, third, and fourth wave guides  104 ,  106 ,  108 , and  110 . An interferometer  115  defines, at least in part, the set of discrete frequencies available to be output by tuneable laser  100 . Substrate  102  may be formed of material identical to that of substrate  22 .  
         [0052]    At least one and preferably both of second and third waveguides  106 ,  108  include a gain medium, which can be identical to the gain medium discussed above for tuneable laser  20 . At least one of the waveguides  104 ,  106 ,  108 ,  110  is configured to receive light from a pump source  121  to irradiate gain medium of the first and second waveguides. For example, second and third waveguides  106 ,  108  of tuneable laser  106  are configured to receive pump light  120 , which propagates along waveguides  106 ,  108  irradiating gain medium therein. The pump source  121  may be identical to the pump source described above for tuneable laser  20 . The output from a single pump source may be split, such as by a beam splitter  171  and steered, such as by a mirror  173  to respective waveguides.  
         [0053]    Tuneable laser  100  includes a first coupling region pair  112 . A coupling region pair preferably comprises a pair of coupling regions where light may couple between each of two waveguides and a third waveguide. For example, coupling region pair  112  includes first and second coupling regions  112   a,    112   b.  A first acoustic wave source  116  subjects the first coupling region pair  112  to acoustic waves. Upon activation of acoustic wave source  116 , light satisfying a matching condition will couple between first waveguide  104  and second and third waveguides  106 ,  108 .  
         [0054]    Tuneable laser  100  includes a second coupling region pair  114  comprising third and fourth coupling regions  114   a,    114   b.  A second acoustic wave source  118  subjects the second coupling region pair  114  to acoustic waves. Upon activation of the acoustic wave source  118 , light satisfying a matching condition will couple between fourth waveguide  110  and second and third waveguides  106 ,  108 .  
         [0055]    At least one of the waveguides of tuneable laser  100  is configured to output light that has propagated along the waveguide. For example, fourth waveguide  110  includes an output coupler  117 , which allows a first portion of light propagating along fourth waveguide  110  to be emitted as output light  122 . A second portion of light propagating along fourth waveguide  110  is reflected by output coupler  117  to thereby return along fourth waveguide  110 . The returning light may couple into second and third wave guides  106 ,  108 , where the light propagates along until coupling into first waveguide  104 , where the light propagates therealong until reaching a reflector  119 , which preferably reflects substantially all light incident upon it. Thus, output coupler  117 , reflector  119  and the waveguides define an optical cavity, which may be a laser cavity supporting oscillation within a gain medium therein. As discussed below, oscillation only occurs when the light propagating within laser  100  satisfies both a matching condition for coupling and experiences constructive interference upon coupling.  
         [0056]    Referring to FIGS. 5 and 6, the available output frequencies  129  of laser  100  define a “comb” of essentially equally spaced frequencies. The particular frequency that is output by tuneable laser  100  is determined by a gain spectrum  130  of the gain medium, an interferometer spectrum  132  of interferometer  115 , and an acoustic grating spectrum. For convenience of the following discussion, FIG. 6 shows two acoustic grating spectra  134   a  and  134   b.    
         [0057]    It should be understood that the optical cavity defined by just the output coupler  117  and the reflector  119  (without the waveguides) creates a comb of frequencies, which are narrowly spaced compared to the available output frequencies  129  of laser  100 . Therefore, the frequency comb defined by the optical cavity does not substantially affect the output frequencies available from laser  100 .  
         [0058]    The gain spectrum  130  is determined by frequencies at which the gain medium of tuneable laser  100  generates light upon pumping, such as by irradiation with pump light. For a laser, such as tuneable laser  100 , a gain spectrum represents a broad envelope of frequencies at which lasing may occur, as understood in the art. A preferred gain spectrum covers at least a portion, and preferably all, of the C-band of frequencies. Interferometer  115  determines which frequencies of gain spectrum  130  are available for output by tuneable laser  100 .  
         [0059]    Interferometer spectrum  132  is defined by interferometer  115 . Tuneable laser  100  may output light at frequencies corresponding to each maximum of interferometer spectrum  132 . Essentially no light may be output at frequencies corresponding to minima of interferometer spectrum  132 . The maxima and minima are determined by frequencies for which interferometer  115  causes constructive or destructive interference of light propagating therein.  
         [0060]    Interference occurs because interferometer  115  includes at least two optical paths, each having a respective different length. Preferably, interferometer  115  is a Mach Zehnder interferometer having respective optical paths along second and third waveguides  106 ,  108 . An optical path length along third waveguide  108  is greater than an optical path length along second waveguide  106 . As defined herein, the optical path lengths of each of the second and third waveguides  106 ,  108  is the length of the respective waveguide between first and second coupling region pairs  112 ,  114 . The frequency spacing, ΔF, between adjacent maxima of interferometer spectrum  132  is given by ΔF=c/(n×Δp), where n is the refractive index of the longer waveguide and Δp is the absolute path length difference. For LiNbO 3 , a 25 GHz spacing ΔF corresponds to about 5.4 mm path length distance difference. A 12.5 GHz spacing corresponds to a 10.8 mm path length difference. A 200 GHz spacing corresponds to a 1.35 mm path length difference.  
         [0061]    As an example of interference caused by interferometer  115 , consider light, propagating along first waveguide  104 , which light couples, at first coupling region pair  112 , into both second and third waveguides  106 ,  108  to propagate therealong. The light then propagating along each of the second and third waveguides  106 ,  108  couples, at second coupling region pair  114 , into fourth waveguide  110 , whereupon interference occurs. The light may be said to have interferingly coupled by way of the coupling region pair. The interference is selective, either constructive or destructive, because of the path length difference along second and third waveguides  106 ,  108 . Of course, interference also occurs at first coupling region pair  114  for light propagating in the opposite direction.  
         [0062]    Maxima, such as maxima  133 ,  135 , and  137 , of interferometer spectrum  132  are centered at frequencies for which constructive interference occurs. Thus, the maxima correspond to light for which the optical path length difference along second and third waveguides  106 ,  108  is an integral multiple of the wavelength of the light propagating therealong. Minima, such as minima  139 ,  141 , and  143 , of interferometer spectrum  132  are centered at frequencies for which destructive interference occurs. The frequency minima correspond to light for which the optical path length difference along second and third waveguides  106 ,  108  is a integral multiple of ½ of the wavelength of the light propagating therealong.  
         [0063]    The acoustic grating spectrum of tuneable laser  100 , is defined by the narrowest range of frequencies that will couple efficiently between (1) first waveguide  104  and second, third waveguides  106 ,  108 , or (2) fourth waveguide  110  and second, third waveguides  106 ,  108 . Light exactly satisfying a matching condition will couple efficiently from one waveguide to another. However, light that has a frequency only slightly different will also couple at least in part because of the finite size of the acoustic grating and coupling region. In practical terms, therefore, a distribution of frequencies will couple from one waveguide to the other. Of course, only a small range of frequencies (the acoustic grating spectrum) will couple efficiently enough to lase. A central frequency of the acoustic grating spectrum may be varied by varying the frequencies of acoustic waves impinging upon the first and second coupling regions.  
         [0064]    During operation, gain medium within tuneable laser  100  may be irradiated with pump light from a pump source to thereby obtain emitted light that propagates along waveguides of tuneable laser  100 . First and second coupling region pairs are subjected to acoustic waves from first and second acoustic wave sources, respectively. The acoustic waves from the first and second acoustic wave sources may have the same frequency. Preferably, substantially all light coupling between the waveguides has a frequency corresponding to a matching condition of a respective coupling region.  
         [0065]    To select an output frequency corresponding to a particular one of the maxima of interferometer spectrum  115 , the frequency of acoustic waves impinging upon the coupling pairs are varied so that the acoustic grating spectrum overlaps the particular maximum. For example, first acoustic grating spectrum  134   a  has a central frequency  145  that coincides with a maximum  137  of interferometer spectrum  132 . Tuneable laser  100  would output light corresponding to central frequency  145 . On the other hand, second grating spectrum  134   b  has a central frequency  147  that coincides with a minimum  139  of interferometer spectrum  132 . Essentially no light would be output for this condition.  
         [0066]    Upon coupling, light having a wavelength that provides destructive interference is attenuated compared to light having a wavelength that provides constructive interference. Thus, even if the acoustic wave frequency is tuned so that light propagating within the waveguides of tuneable laser  100  satisfies a matching condition allowing the light to couple, oscillation will not occur unless the frequency of the light experiences constructive interference at first and second coupling pairs  112 ,  114 .  
         [0067]    Light is output from at least one of the waveguides, such as through output coupler  117  of fourth waveguide  110 . Preferably, substantially all of the light output from tuneable laser  100  satisfies both a matching condition of the first and second coupling regions and has a wavelength corresponding to constructive interference. Tuning the wave vector K AC  of acoustic waves output by first and second acoustic wave sources allows light corresponding to a particular one of the discrete set of frequencies to be obtained. Thus, tuneable laser  100  outputs light having one of a discrete set of frequencies corresponding to maxima of interferometer spectrum  132 .  
         [0068]    Referring to FIG. 7, a tuneable laser  200  includes first and second interferometers to thereby provide a comb of output frequencies having teeth with a narrower width than teeth of a comb of a laser having only a single interferometer. Tuneable laser  200 , which is preferably integral with a substrate  202 , includes first, second, third, and fourth, waveguides  204 ,  206 ,  208 , and  210  having a configuration similar to waveguides  104 ,  106 ,  108 , and  110  of tuneable laser  100 . Tuneable laser  200  defines an optical cavity between a first reflector  219  and a second reflector  221 .  
         [0069]    A first interferometer of tuneable laser  200  includes a first coupling region pair  212 , including first and second coupling regions  212   a,    212   b  and a second coupling region pair  214 , including third and fourth coupling regions  214   a,    214   b.  An optical path along third waveguide  208  is greater than an optical path along second waveguide  206 . Thus, light propagating along second and third waveguides  206 ,  208  and coupling at first or second coupling region pair  212 ,  214  will experience interference, as discussed above for interferometer  115 .  
         [0070]    A second interferometer of tuneable laser  200  includes third and fourth coupling regions  223 ,  225 . Third coupling region  223  couples light between first waveguide  204  and a fifth waveguide  211 . Fourth coupling region  225  couples light between fifth waveguide  211  and fourth waveguide  210 . At each of coupling regions  223 ,  225 , coupling may occur in either direction. A first optical path, between third and fourth coupling regions  223 ,  225 , along fifth waveguide  211  is different than a second optical path, between third and fourth coupling regions  223 ,  225 . Thus, upon coupling at either of third and fourth coupling regions  223 ,  225 , interference occurs between light having traveled along the first and second optical paths. The second optical path includes first interferometer  215 .  
         [0071]    A first acoustic wave source  216  subjects third coupling region  223  and first coupling region pair  212  to acoustic waves. A second acoustic wave source  218  subjects second coupling region pair  214  and fourth coupling region  225  to acoustic waves. The frequencies of acoustic waves impinging upon third coupling region  223  and second coupling region pair  214  can be varied to select the output frequency of tuneable laser  200 . Although FIG. 7 shows that acoustic wave sources  216  and  218  each subject more than 1 coupling region to acoustic waves, it should be understood that each coupling region may be provided with a dedicated acoustic wave source. Also, devices in accordance with the present invention may be configured with an acoustic wave source that subjects more than 2 coupling regions to acoustic waves. Light source  200  may also include acoustic wave absorbers, which may be similar to acoustic wave absorbers  36  and  37  of light source  20 .  
         [0072]    Referring to FIG. 8, a tuneable light source, which in this embodiment is a tuneable laser  300 , outputs light having one of a set of discrete optical frequencies. The available output frequencies of tuneable laser  300  are determined by the output frequencies of a secondary light source, such as a comb generator  330 , which preferably emits light  331  having a plurality of equally spaced frequencies. The output of tuneable laser  300  can be acoustically tuned to output light at any one of the frequencies received from the external source. A comb generator suitable for use as a secondary light source is described in U.S. application No. 10/***,***, titled Light Source For Generating Output Signal Having Evenly Spaced Apart Frequencies, filed on even date herewith, invented by Israel Smilanski, Isaac Shpantzer, Jacob B. Khurgin, Nadejda Reingand, Pak Shing Cho, and Yaakov Achiam, which application is incorporated herein.  
         [0073]    Tuneable laser  300 , includes a substrate  302 , which preferably includes first, second, and third waveguides  324 ,  326 , and  328 . First and second waveguides  324 ,  326  define a first coupling region  338 , wherein light propagating along one of the first and second waveguides may couple to the other waveguide to propagate therealong. First and third waveguides define a second coupling region  340 , wherein light propagating along one of the first and third waveguides may couple to the other waveguide to propagate therealong. The coupling regions  338 ,  340  are identical to coupling regions  38 ,  40 .  
         [0074]    Tuneable laser  300  includes a first and second acoustic wave sources  330 ,  332 , which operate identically to acoustic wave sources  30 ,  32  to thereby facilitate variable wavelength coupling of light at first and second coupling regions  338 ,  340 . Acoustic absorbers  334 ,  336  operate identically to acoustic absorbers  36 ,  37  of tuneable laser  20 .  
         [0075]    First and second reflective elements  317 ,  319  define an optical cavity including first, second, and third waveguides  324 ,  326 , and  328 . First element  317  is preferably an output coupler. Second reflective element  319  is preferably sufficiently transmissive to allow at least some of the light  331  output by comb generator  330  to be received by waveguide  328 . Light having a frequency that satisfies a matching condition determined by a frequency of acoustic waves output by acoustic wave sources  330 ,  332 , may couple between waveguides at first and second coupling regions  338 ,  340  and, therefore, oscillate within the optical cavity.  
         [0076]    Portions of at least one and preferably all of the first, second, and third waveguides  324 ,  326 , and  328  are doped with a gain medium. At least one of the waveguides is configured to receive pump light  320  from a pump source  321 . The gain medium and pump source may be identical to those described for tuneable laser  20 .  
         [0077]    At least one of the waveguides  324 ,  326 ,  328  is configured to receive light  331  from the comb generator  330 . Light  331  from comb generator  330  seeds the gain medium of tuneable laser  300  such that lasing occurs preferentially at frequencies of light  331 . Oscillation and, therefore, lasing, will only occur, however, at frequencies which also satisfy the matching condition as discussed above. Thus, by varying the frequency of acoustic waves impinging upon first and second coupling regions  338 ,  340 , the output frequency of tuneable laser  300  can be varied between discrete frequencies corresponding to frequencies of light  331 .  
         [0078]    In order to prevent the frequencies of light  331 , and, therefore the frequency of light output by tuneable laser  300 , from varying, the comb generator  330  may be locked, such as to a frequency stabilized reference laser  360 . Locking comb generator  330  substantially prevents the frequencies of light  331  from shifting from one optical frequency to another. Locking may be performed by, for example, either injection-locking or phase-locking. An example of a comb generator and method for locking a laser to a comb generator is discussed by C. F. Silva et al. in “Exact Optical Frequency Synthesis Over 1 THz Using SG-DBR Lasers,” Proceedings CLEO-Europe-IQEC 2000 conference, Nice, France, September, 2000, which proceeding is incorporated by reference herein to the extent necessary to understand the present invention.  
         [0079]    Referring to FIG. 9, a method for secure optical communication includes varying, as a function of time, a frequency of light encoding transmitted information. This secure optical communication system uses optical spread spectrum techniques. During a first time period t 1 , information is encoded, such as by amplitude or, preferably, phase modulation of light having a frequency ω 1 . During a second time period t 2 , information is encoded by modulation of light having a frequency ω 2 , which may be the same as or different from ω 1 . In general, information is encoded, during the ith time period, by modulation of light of a frequency ω 1 . The encoding step is repeated for a number N t  times until all the information has been transmitted. At each successive time period, information may be encoded upon light having a frequency different or the same as a frequency of light encoded upon during the previous time segment. Thus, the information is encoded upon light having a number of frequencies N ω , which number may be less than N t . The length of the time periods may be the same or may vary from period to period. The encoded information is transmitted to the receiver.  
         [0080]    Light encoding the information upon the plurality of frequencies is transmitted, such as through free space or a fiber optic network to a receiver, where the information is decoded. Because the frequency of the transmitted light switches from frequency to frequency, one without knowledge of the transmission frequency sequence is prevented from decoding the transmitted information.  
         [0081]    Referring to FIG. 10, a transmitter  408  having a light source  410 , which may be a light source in accordance with the present invention, is preferably used to provide the light which is modulated to encode the information. An acoustic wave source driver  414  varies an acoustic wave frequency of light source  410  to prepare an output beam  412  that switches between a plurality of frequencies as a function of time. Output beam  412  is received by a modulator  416 , which modulates output beam  412  with information from a data source  419  to prepare a modulated output beam  418 .  
         [0082]    Modulator  416  is preferably a phase modulator, which prepares an optical signal that encodes information by, for example, phase shift keying, binary phase shift keying or quaternary phase shift keying. During the ith time period, phase modulator  416  modulates a phase of light having a frequency ω 1  of output beam  412  to encode information from data source  419 . During the jth time period, where j=i+1, phase modulator  416  modulates a phase of light having a frequency that may be the same as or different from ω i . Modulated output beam  418  is transmitted by transmitter  408  to be received and decoded by one having knowledge of the successive frequencies used to encode the information. The information may be decoded using, for example, homodyne or heterodyne detection.  
         [0083]    Referring to FIG. 11, a heterodyne receiver  450  includes a local oscillator  451  providing an oscillator beam  452  having a variable frequency corresponding to the variable frequency of received output beam  418 . Local oscillator  451  preferably comprises any of the light sources of the invention. Oscillator beam  452  and output beam  418  are combined  453  and detected by an optical detector  454 . It is preferable that a frequency mismatch between beams  452  and  418  is less than about 1 GHz, such as less than about 250 MHz.  
         [0084]    While the above invention has been described with reference to certain preferred embodiments, it should be kept in mind that the scope of the present invention is not limited to these. Thus, one skilled in the art may find variations of these preferred embodiments which, nevertheless, fall within the spirit of the present invention, whose scope is defined by the claims set forth below.