US20030231817A1 - Method and system for acoustically tuning a light source - Google Patents
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- US20030231817A1 US20030231817A1 US10/173,581 US17358102A US2003231817A1 US 20030231817 A1 US20030231817 A1 US 20030231817A1 US 17358102 A US17358102 A US 17358102A US 2003231817 A1 US2003231817 A1 US 2003231817A1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04J—MULTIPLEX COMMUNICATION
- H04J14/00—Optical multiplex systems
- H04J14/005—Optical Code Multiplex
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- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
- G02F1/11—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on acousto-optical elements, e.g. using variable diffraction by sound or like mechanical waves
- G02F1/125—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on acousto-optical elements, e.g. using variable diffraction by sound or like mechanical waves in an optical waveguide structure
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- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
- G02F1/21—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour by interference
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
- H01S3/06—Construction or shape of active medium
- H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
- H01S3/0632—Thin film lasers in which light propagates in the plane of the thin film
- H01S3/0635—Thin film lasers in which light propagates in the plane of the thin film provided with a periodic structure, e.g. using distributed feed-back, grating couplers
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
- H01S3/1068—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using an acousto-optical device
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- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
- G02F1/21—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour by interference
- G02F1/212—Mach-Zehnder type
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
- H01S3/06—Construction or shape of active medium
- H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
- H01S3/0632—Thin film lasers in which light propagates in the plane of the thin film
- H01S3/0637—Integrated lateral waveguide, e.g. the active waveguide is integrated on a substrate made by Si on insulator technology (Si/SiO2)
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
- H01S3/16—Solid materials
- H01S3/1601—Solid materials characterised by an active (lasing) ion
- H01S3/1603—Solid materials characterised by an active (lasing) ion rare earth
- H01S3/1608—Solid materials characterised by an active (lasing) ion rare earth erbium
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/23—Arrangements of two or more lasers not provided for in groups H01S3/02 - H01S3/22, e.g. tandem arrangements of separate active media
- H01S3/2383—Parallel arrangements
Definitions
- the present invention relates to an acoustically tuneable light source and method for acoustically tuning a light source.
- 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
- DFB distributed feedback
- the tuning time for DFB lasers is on the order of milliseconds, which is slower than the microsecond tuning times required for modern optical communication systems.
- 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.
- 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.
- the 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.
- 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.
- light such as the generated light
- 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 ⁇ AC1 .
- 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 ⁇ AC1 .
- 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.
- 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.
- 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.
- K AC first longitudinal acoustic wave vector of acoustic waves impinging upon the first coupling region.
- a wave vector of the coupled light is shifted by an amount K AC .
- only light that couples may be output by the integrated laser cavity.
- 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 .
- 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 ⁇ 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 ⁇ AC1 . At least some of the second light is output.
- 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 ⁇ 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 ⁇ 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 ⁇ i , wherein a frequency of light output by the optical cavity changes upon changing the acoustic frequency ⁇ i .
- 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.
- FIG. 1 shows a tuneable light source according to the invention
- FIG. 2 shows a partial view of a coupling region of the light source of FIG. 1;
- FIG. 3 shows an example of an output spectrum of the light source of FIG. 1;
- FIG. 4 shows an example of a second embodiment of a tuneable light source according to the invention
- FIG. 5 shows a plot of available output frequencies for the tuneable light source of FIG. 4
- FIG. 6 shows spectra that contribute to the output frequencies of FIG. 5;
- FIG. 7 shows a third embodiment of a light source according to the invention.
- FIG. 8 shows a fourth embodiment of a light source according to the invention.
- FIG. 9 shows a time-frequency plot of light output by a secure communication source of the invention.
- FIG. 10 shows a secure communication source of the invention suitable for preparing the time-frequency plot of FIG. 9.
- FIG. 11 shows a receiver of the invention for receiving information transmitted by the secure communication of FIG. 10.
- 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.
- substrate materials are non-crystalline. Silica, such as amorphous silica, is an example of a suitable substrate material.
- 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 .
- Tuneable laser 20 includes an optical cavity preferably having at least two reflective elements and including the first, second, and third waveguides.
- 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.
- 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 .
- 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.
- 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 .
- the polarization of light is preferably substantially maintained upon coupling at coupling regions of devices in accordance with light sources of the invention.
- 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 ⁇ /2, such as less than about ⁇ /8.
- 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.
- the tuneable laser 20 is not limited to gain media generating light in the infrared.
- 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.
- At least one of the waveguides is preferably configured to receive pump light from a pump source 63 .
- 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.
- each waveguide comprising gain medium receives pump light from a pump source.
- 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.
- 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.
- 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.
- 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 .
- propagating light that fails to satisfy the matching condition is substantially prevented from continued propagation by attenuation region 71 .
- 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 ⁇ 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 ⁇ 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 ⁇ AC1 and ⁇ AC2 may be the same or different.
- 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.
- 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 .
- 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.
- ⁇ 23 is a frequency of optical wave 23
- c is the speed of light in a vacuum
- n 24 is the index of refraction of waveguide 24 for light having a frequency ⁇ 23 .
- Acoustic wave source 30 subjects first coupling region 38 to acoustic waves 46 having a frequency ⁇ AC , which waves travel with a longitudinal velocity V AC with respect to first coupling region 38 .
- longitudinal frequency it is meant the component of the acoustic waves taken along a longitudinal axis 54 of first waveguide 24 .
- ⁇ 27 is a frequency of optical wave 27 and n 26 is a refractive index of waveguide 26 for light having a frequency ⁇ 27 .
- ⁇ n is a refractive index difference given by n 26 ⁇ n 24 and ⁇ is a frequency difference given by ⁇ 27 ⁇ 23 .
- acoustic sources 30 , 32 determine the frequency of light that is output by tuneable laser 20 for any given acoustic wave frequency.
- ⁇ out c ⁇ ⁇ ⁇ AC ⁇ ⁇ ⁇ n ⁇ ⁇ V AC Eq . ⁇ 5
- 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 ⁇ 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.
- tuneable light source 20 may be acoustically tuned by varying ⁇ AC to output light having any one of the frequencies generated by the gain medium.
- the acoustic frequency ⁇ 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 may be varied, by changing the frequency of acoustic waves ⁇ AC , between first and second frequencies in less than about 50 ⁇ s and preferably in less than about 10 ⁇ s.
- the spectrum of light output by tuneable laser 20 includes a output spectrum 51 light having a range of frequencies centered at ⁇ out .
- 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.
- 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.
- 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 .
- 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.
- 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.
- 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.
- 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 .
- 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.
- light satisfying a matching condition will couple between fourth waveguide 110 and second and third waveguides 106 , 108 .
- At least one of the waveguides of tuneable laser 100 is configured to output light that has propagated along the waveguide.
- 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.
- 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.
- 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.
- FIG. 6 shows two acoustic grating spectra 134 a and 134 b.
- the optical cavity defined by just the output coupler 117 and the reflector 119 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 .
- 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.
- 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 .
- 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.
- interferometer 115 includes at least two optical paths, each having a respective different length.
- 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 .
- 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 .
- ⁇ F the frequency spacing between adjacent maxima of interferometer spectrum 132
- n the refractive index of the longer waveguide
- ⁇ p the absolute path length difference.
- a 25 GHz spacing ⁇ F 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.
- 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.
- Maxima such as maxima 133 , 135 , and 137 , of interferometer spectrum 132 are centered at frequencies for which constructive interference occurs.
- 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 1 ⁇ 2 of the wavelength of the light propagating therealong.
- 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.
- 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.
- 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.
- 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.
- substantially all light coupling between the waveguides has a frequency corresponding to a matching condition of a respective coupling region.
- 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 .
- 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.
- Light is output from at least one of the waveguides, such as through output coupler 117 of fourth waveguide 110 .
- 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.
- tuneable laser 100 outputs light having one of a discrete set of frequencies corresponding to maxima of interferometer spectrum 132 .
- 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 .
- 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 .
- 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 .
- 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 .
- the second optical path includes first interferometer 215 .
- 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 .
- 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.
- 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 .
- 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.
- 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 .
- 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 .
- 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.
- 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 .
- 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.
- the output frequency of tuneable laser 300 can be varied between discrete frequencies corresponding to frequencies of light 331 .
- 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.
- 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.
- information is encoded, such as by amplitude or, preferably, phase modulation of light having a frequency ⁇ 1 .
- information is encoded by modulation of light having a frequency ⁇ 2 , which may be the same as or different from ⁇ 1 .
- information is encoded, during the ith time period, by modulation of light of a frequency ⁇ 1 .
- the encoding step is repeated for a number N t times until all the information has been transmitted.
- 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.
- the information is encoded upon light having a number of frequencies N ⁇ , 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.
- 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 .
- 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.
- phase modulator 416 modulates a phase of light having a frequency ⁇ 1 of output beam 412 to encode information from data source 419 .
- phase modulator 416 modulates a phase of light having a frequency that may be the same as or different from ⁇ 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.
- 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.
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Abstract
Description
- 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.
- The present invention relates to an acoustically tuneable light source and method for acoustically tuning a light source.
- 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.
- 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.
- 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.
- 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 ωAC1. 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 ±ωAC1.
- 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.
- 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 KAC 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 KAC. Preferably, only light that couples may be output by the integrated laser cavity.
- 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 KAC1, 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 KAC1.
- 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 ωAC1 to thereby provide second light having a second light frequency, wherein the second light frequency differs from the emitted light frequency by an amount ωAC1. At least some of the second light is output.
- 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 ω1, 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 ±ωi. 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 ωi, wherein a frequency of light output by the optical cavity changes upon changing the acoustic frequency ωi.
- 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.
- The present invention is discussed below in reference to the drawings in which:
- FIG. 1 shows a tuneable light source according to the invention;
- FIG. 2 shows a partial view of a coupling region of the light source of FIG. 1;
- FIG. 3 shows an example of an output spectrum of the light source of FIG. 1;
- FIG. 4 shows an example of a second embodiment of a tuneable light source according to the invention;
- FIG. 5 shows a plot of available output frequencies for the tuneable light source of FIG. 4;
- FIG. 6 shows spectra that contribute to the output frequencies of FIG. 5;
- FIG. 7 shows a third embodiment of a light source according to the invention; and
- FIG. 8 shows a fourth embodiment of a light source according to the invention.
- FIG. 9 shows a time-frequency plot of light output by a secure communication source of the invention;
- FIG. 10 shows a secure communication source of the invention suitable for preparing the time-frequency plot of FIG. 9; and
- FIG. 11 shows a receiver of the invention for receiving information transmitted by the secure communication of FIG. 10.
- Referring to FIGS. 1 and 2, a light source, which in this embodiment is a
laser 20, which preferably includes first, second, andthird waveguides laser 20 may be varied. Thelaser 20 may be integral with asubstrate 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. - First and
second waveguides 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 andthird waveguides second coupling regions 38, 40 are preferably spaced apart from one another along a general propagation dimension of thefirst waveguide 24. -
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. - Oscillation within the optical cavity preferably comprises propagation of the light between respective ends60, 61 of second and
third waveguides reflective element 42 may be optically associated withend 60 and preferably operates as an output coupler that is only partially reflective at output wavelengths oftuneable laser 20. A secondreflective element 44 may be associated withend 61 and preferably reflects substantially all light at output wavelengths oftuneable laser 20. -
First waveguide 24 has a propagation constant different from a propagation constant of second andthird waveguides first waveguide 24 may be different from dimensions of second andthird waveguides First waveguide 24 may have a refractive index that is different from respective refractive indexes of second andthird waveguides - 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.
- Portions of at least one and preferably all of the first, second, and
third waveguides 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. - At least one of the waveguides is preferably configured to receive pump light from a pump source63. 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. - As best seen in FIG. 2,
waveguide portions 50, 52 of respective first andsecond waveguides first coupling region 38 define respectivelongitudinal axes waveguide portions 50, 52 propagates generally along its respectivelongitudinal axis - 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 includeattenuation regions waveguides Attenuation regions first waveguide portion 50 towardattenuation region 71 couples to second waveguide portion 52 ofsecond waveguide 26. Preferably, however, propagating light that fails to satisfy the matching condition is substantially prevented from continued propagation byattenuation region 71. -
Tuneable laser 20 includes a firstacoustic wave source 30 to facilitate variable wavelength coupling between first andsecond waveguides acoustic wave source 30 subjectsfirst coupling region 38 toacoustic waves 46 having a variable frequency ωAC1 and propagating generally along apropagation axis 48. A secondacoustic wave source 32 subjects second coupling region 40 to acoustic waves (not shown) also having a variable frequency ωAC2 and propagating with a velocity VAC generally along apropagation axis 49 to thereby facilitate variable wavelength coupling between first andthird waveguides acoustic wave sources - 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.
- An
acoustic absorber 37 may be disposed to absorb or otherwise preventacoustic waves 46 that have passed through first coupling region from returning therethrough, such as by reflection. A secondacoustic absorber 36 may be disposed to absorb or otherwise prevent acoustic waves emitted by secondacoustic 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 topropagation axes - 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 tosecond waveguide 26 atfirst 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. -
- where ω23 is a frequency of
optical wave 23, c is the speed of light in a vacuum, and n24 is the index of refraction ofwaveguide 24 for light having a frequency ω23.Acoustic wave source 30 subjectsfirst coupling region 38 toacoustic waves 46 having a frequency ωAC, which waves travel with a longitudinal velocity VAC with respect tofirst coupling region 38. By longitudinal frequency, it is meant the component of the acoustic waves taken along alongitudinal axis 54 offirst waveguide 24. -
- The acoustic grating interacts with
optical wave 23, such as by scattering at least some ofoptical wave 23, to provide an optical wave having a wave vector K1=K2+ΔK. At least some of the optical wave with wave vector K1 couples intowaveguide 26 and propagates therealong with wave vector K1, where the wave vector K1 is given by: - where ω27 is a frequency of
optical wave 27 and n26 is a refractive index ofwaveguide 26 for light having a frequency ω27. -
- where Δn is a refractive index difference given by n26−n24 and Δω is a frequency difference given by ω27−ω23.
- When the matching condition is satisfied, light having a wave vector K2 will couple from
first waveguide 24 tosecond waveguide 26. Likewise, light having a wave vector K1 will couple fromsecond waveguide 26 tofirst waveguide 24. A similar matching condition must be met before light will couple in either direction betweenfirst waveguide 24 andthird 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 tuneable laser 20 for any given acoustic wave frequency. -
- During use, tuneable
light source 20 may operate as follows. Gain medium within at least one of first, second, andthird waveguides 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 ωAC so that light that is generated and propagates along, for example,first waveguide 24 may couple to, for example, thesecond 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, tuneablelight source 20 may be acoustically tuned by varying ωAC to output light having any one of the frequencies generated by the gain medium. The acoustic frequency ωAC is preferably varied using the acoustic wave source driver associated with the acoustic wave generators. The frequency of light output bytuneable laser 20, like all tuneable light sources of the invention, may be varied, by changing the frequency of acoustic waves ωAC, between first and second frequencies in less than about 50 μs and preferably in less than about 10 μs. - As an example of using Eq. 5 to determine ωout, substitute c=3×108 m/s, Δn=1×10−2, VAC=3×104 m/s, and ωAC=2×108 Hz to predict an output frequency of 2×1014 Hz, which corresponds to a wavelength of about 1.5 μm. Varying the acoustic wave frequency ωAC 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.
- Referring to FIG. 3, the spectrum of light output by
tuneable laser 20 includes aoutput spectrum 51 light having a range of frequencies centered at ωout. Preferably, however, theoutput spectrum 51 includes substantially fewer frequencies than light emitted by the gain medium of the waveguides. A width w1 of output spectrum is preferably sufficiently narrow that theoutput 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. - 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 asubstrate 102, which preferably includes first, second, third, and fourth wave guides 104, 106, 108, and 110. Aninterferometer 115 defines, at least in part, the set of discrete frequencies available to be output bytuneable laser 100.Substrate 102 may be formed of material identical to that ofsubstrate 22. - At least one and preferably both of second and
third waveguides tuneable laser 20. At least one of thewaveguides pump source 121 to irradiate gain medium of the first and second waveguides. For example, second andthird waveguides tuneable laser 106 are configured to receivepump light 120, which propagates alongwaveguides pump source 121 may be identical to the pump source described above fortuneable laser 20. The output from a single pump source may be split, such as by abeam splitter 171 and steered, such as by amirror 173 to respective waveguides. -
Tuneable laser 100 includes a firstcoupling 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 andsecond coupling regions acoustic wave source 116 subjects the firstcoupling region pair 112 to acoustic waves. Upon activation ofacoustic wave source 116, light satisfying a matching condition will couple betweenfirst waveguide 104 and second andthird waveguides -
Tuneable laser 100 includes a secondcoupling region pair 114 comprising third andfourth coupling regions acoustic wave source 118 subjects the secondcoupling region pair 114 to acoustic waves. Upon activation of theacoustic wave source 118, light satisfying a matching condition will couple betweenfourth waveguide 110 and second andthird waveguides - 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 anoutput coupler 117, which allows a first portion of light propagating alongfourth waveguide 110 to be emitted as output light 122. A second portion of light propagating alongfourth waveguide 110 is reflected byoutput coupler 117 to thereby return alongfourth waveguide 110. The returning light may couple into second and third wave guides 106, 108, where the light propagates along until coupling intofirst waveguide 104, where the light propagates therealong until reaching areflector 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 withinlaser 100 satisfies both a matching condition for coupling and experiences constructive interference upon coupling. - Referring to FIGS. 5 and 6, the
available output frequencies 129 oflaser 100 define a “comb” of essentially equally spaced frequencies. The particular frequency that is output bytuneable laser 100 is determined by again spectrum 130 of the gain medium, aninterferometer spectrum 132 ofinterferometer 115, and an acoustic grating spectrum. For convenience of the following discussion, FIG. 6 shows twoacoustic grating spectra - 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 theavailable output frequencies 129 oflaser 100. Therefore, the frequency comb defined by the optical cavity does not substantially affect the output frequencies available fromlaser 100. - The
gain spectrum 130 is determined by frequencies at which the gain medium oftuneable laser 100 generates light upon pumping, such as by irradiation with pump light. For a laser, such astuneable 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 ofgain spectrum 130 are available for output bytuneable laser 100. -
Interferometer spectrum 132 is defined byinterferometer 115.Tuneable laser 100 may output light at frequencies corresponding to each maximum ofinterferometer spectrum 132. Essentially no light may be output at frequencies corresponding to minima ofinterferometer spectrum 132. The maxima and minima are determined by frequencies for which interferometer 115 causes constructive or destructive interference of light propagating therein. - 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 andthird waveguides third waveguide 108 is greater than an optical path length alongsecond waveguide 106. As defined herein, the optical path lengths of each of the second andthird waveguides 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 LiNbO3, 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. - As an example of interference caused by
interferometer 115, consider light, propagating alongfirst waveguide 104, which light couples, at firstcoupling region pair 112, into both second andthird waveguides third waveguides coupling region pair 114, intofourth 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 andthird waveguides coupling region pair 114 for light propagating in the opposite direction. - Maxima, such as
maxima 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 andthird waveguides minima 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 andthird waveguides - 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 fourth waveguide 110 and second,third waveguides - 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 oftuneable 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. - 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 acousticgrating spectrum 134 a has acentral frequency 145 that coincides with a maximum 137 ofinterferometer spectrum 132.Tuneable laser 100 would output light corresponding tocentral frequency 145. On the other hand, secondgrating spectrum 134 b has acentral frequency 147 that coincides with aminimum 139 ofinterferometer spectrum 132. Essentially no light would be output for this condition. - 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. - Light is output from at least one of the waveguides, such as through
output coupler 117 offourth waveguide 110. Preferably, substantially all of the light output fromtuneable 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 KAC 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 ofinterferometer spectrum 132. - 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 asubstrate 202, includes first, second, third, and fourth,waveguides waveguides tuneable laser 100.Tuneable laser 200 defines an optical cavity between afirst reflector 219 and asecond reflector 221. - A first interferometer of
tuneable laser 200 includes a firstcoupling region pair 212, including first andsecond coupling regions coupling region pair 214, including third andfourth coupling regions third waveguide 208 is greater than an optical path alongsecond waveguide 206. Thus, light propagating along second andthird waveguides coupling region pair interferometer 115. - A second interferometer of
tuneable laser 200 includes third andfourth coupling regions Third coupling region 223 couples light betweenfirst waveguide 204 and afifth waveguide 211.Fourth coupling region 225 couples light betweenfifth waveguide 211 andfourth waveguide 210. At each ofcoupling regions fourth coupling regions fifth waveguide 211 is different than a second optical path, between third andfourth coupling regions fourth coupling regions - A first
acoustic wave source 216 subjectsthird coupling region 223 and firstcoupling region pair 212 to acoustic waves. A secondacoustic wave source 218 subjects secondcoupling region pair 214 andfourth coupling region 225 to acoustic waves. The frequencies of acoustic waves impinging uponthird coupling region 223 and secondcoupling region pair 214 can be varied to select the output frequency oftuneable laser 200. Although FIG. 7 shows thatacoustic wave sources Light source 200 may also include acoustic wave absorbers, which may be similar toacoustic wave absorbers light source 20. - 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 oftuneable laser 300 are determined by the output frequencies of a secondary light source, such as acomb generator 330, which preferably emits light 331 having a plurality of equally spaced frequencies. The output oftuneable 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. -
Tuneable laser 300, includes asubstrate 302, which preferably includes first, second, andthird waveguides second waveguides 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 asecond coupling region 340, wherein light propagating along one of the first and third waveguides may couple to the other waveguide to propagate therealong. Thecoupling regions coupling regions 38, 40. -
Tuneable laser 300 includes a first and secondacoustic wave sources acoustic wave sources second coupling regions Acoustic absorbers acoustic absorbers tuneable laser 20. - First and second
reflective elements third waveguides First element 317 is preferably an output coupler. Secondreflective element 319 is preferably sufficiently transmissive to allow at least some of the light 331 output bycomb generator 330 to be received bywaveguide 328. Light having a frequency that satisfies a matching condition determined by a frequency of acoustic waves output byacoustic wave sources second coupling regions - Portions of at least one and preferably all of the first, second, and
third waveguides pump source 321. The gain medium and pump source may be identical to those described fortuneable laser 20. - At least one of the
waveguides comb generator 330.Light 331 fromcomb generator 330 seeds the gain medium oftuneable laser 300 such that lasing occurs preferentially at frequencies oflight 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 andsecond coupling regions tuneable laser 300 can be varied between discrete frequencies corresponding to frequencies oflight 331. - In order to prevent the frequencies of
light 331, and, therefore the frequency of light output bytuneable laser 300, from varying, thecomb generator 330 may be locked, such as to a frequency stabilized reference laser 360. Lockingcomb 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. - 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 t1, information is encoded, such as by amplitude or, preferably, phase modulation of light having a frequency ω1. During a second time period t2, information is encoded by modulation of light having a frequency ω2, which may be the same as or different from ω1. In general, information is encoded, during the ith time period, by modulation of light of a frequency ω1. The encoding step is repeated for a number Nt 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 Nω, which number may be less than Nt. 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.
- 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.
- Referring to FIG. 10, a
transmitter 408 having alight 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 acousticwave source driver 414 varies an acoustic wave frequency oflight source 410 to prepare anoutput beam 412 that switches between a plurality of frequencies as a function of time.Output beam 412 is received by amodulator 416, which modulatesoutput beam 412 with information from adata source 419 to prepare a modulatedoutput beam 418. -
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 ω1 ofoutput beam 412 to encode information fromdata 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 ωi.Modulated output beam 418 is transmitted bytransmitter 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. - Referring to FIG. 11, a
heterodyne receiver 450 includes alocal oscillator 451 providing anoscillator beam 452 having a variable frequency corresponding to the variable frequency of receivedoutput beam 418.Local oscillator 451 preferably comprises any of the light sources of the invention.Oscillator beam 452 andoutput beam 418 are combined 453 and detected by anoptical detector 454. It is preferable that a frequency mismatch betweenbeams - 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.
Claims (54)
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US10/462,834 US7418209B2 (en) | 2002-06-18 | 2003-06-17 | System and method for multi-dimensional encoding |
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US5610760A (en) * | 1994-03-11 | 1997-03-11 | U.S. Philips Corporation | Device for raising the frequency of electromagnetic radiation |
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