Abstract:
A method and apparatus for modulating light, wherein a light source provides light of a certain wavelength to be modulated by a layer of superconducting material which forms part of a specifically configured plate assembly. The superconducting layer is placed in the optical path of the light source. Further the superconducting layer is switched between a partially transparent non-superconducting state and a substantially non-transparent superconducting state by a modulation circuit. The resulting optical pulses transmitted through the superconducting layer are converted from the original wavelength to a lower wavelength by a frequency converting device.

Description:
This is a Divisional application of prior application Ser. No. 09/208,326, filed on Dec. 9, 1998, now U.S. Pat. No. 6,115,170, which is a continuation of prior application Ser. No. 08/965,880, filed on Nov. 7, 1997 now issued as U.S. Pat. No. 5,886,809, which is a divisional of application Ser. No. 08/643,642 filed May 6, 1996 now issued as U.S. Pat. No. 5,768,002, from which priority under 35 U.S.C. §120 is claimed the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to fiber optic communication, and more particularly to modulation of light for use with fiber optics in fiber optic communication schemes. 
     With the advent of dispersion compensating fibers, erbium doped fiber amplifiers, high speed amorphous silicon detectors, and all optical demultiplexing, fiber optic transmission speed is limited principally by the modulation speed of the optical transmitter. 
     High speed modulators have been invented that take advantage of the properties of superconducting materials. Superconducting materials are in there superconducting state if the current density in the material, and the temperature of the material, and the magnetic field around the material are all below certain critical values. The critical current density (J c ), critical temperature (T c ), and the critical magnetic field (H c ) are all dependent on the chemical composition of the material and on the presence or absence of defects and impurities. If any of these quantities rise above the critical values the material leaves its superconducting state and enters its “normal” state. The material has properties similar to a semiconductor in it&#39;s normal state and is characterized by a normal-state resistivity. The superconducting state has many of the properties of a theoretically perfect conductor. The electrical resistance is zero and electromagnetic fields are reflected by it. Thus in the superconducting state a superconducting thin film acts like a mirror with 100% reflectivity [1, 2]. In the normal state light is partially transmitted [3]. The bracketed end note reference numbers [1, 2] and all other such end notes and reference numbers cited herein appear at the end of this specification along with the reference notes themselves. 
     U.S. Pat. No. 5,210,637 which is incorporated herein by reference issued May 11, 1993 to Puzey for OHigh Speed Ught Modulationn discloses a device for the high speed modulation of light wherein a layer of superconducting film is used to modulate the light. U.S. Pat. No. 5,036,042 which is incorporated herein by reference issued Jul. 30, 1991 to Hed for “Switchable Superconducting Mirrors” and discloses a device that can be used for the high speed modulation of light. 
     FIG. 1 herein illustrates one embodiment of U.S. Pat. No. 5,210,637 as indicated by reference numeral  10 . A DC power supply  15  is connected to a light source  13  to provide constant light output. A superconducting film  14  is placed in the path of the optical output and its reflectivity is altered by a modulating circuit  16  which switches the film between its superconductivity and non-superconductive states, as described In the Puzey patent. The altered reflectivity results in optical pulses  20  which are carried away by an optical fiber  25 . The superconducting film is kept cool by placing it in a dewar  22  which is cooled by means of a refrigerating device  26 . 
     A key drawback of device  10  and the corresponding device in the Hed Patent is that they are both limited to creating optical pulses in the far infrared range (approximate wavelength of 14 microns). This is because at higher frequencies the photon energy of the light is high enough to break the binding energy of the Cooper electron pairs responsible for the phenomena of superconductivity. In order for the device to work properly the photon energy of the light must be less than the binding energy (or energy gap) of the cooper pairs. This relation is given by the formula below: 
     
       
         hv&lt;2Δ  {1} 
       
     
     Where h is Planck&#39;s constant, v is the frequency of light, and 2Δ is the energy gap of the superconductor. The energy gap of the superconductor can be found from Mattis-Bardeen [4]. 
     
       
         2Δ=8kT  {2} 
       
     
     Where k is Boltzman&#39;s constant, and T is the critical temperature of the superconducting material. High critical temperature Thallium compounds have critical temperatures around 128 Kelvin. Plugging this into equations {1} and {2}, the operation of the device is limited to light with a wavelength around 14 microns. 
     The attenuation of light in silica glass fiber (the most common material for long haul fibers) can be calculated from the formula below. 
     
       
         α=Ae −a/λ +B/λ 4   {3} 
       
     
     Where α is the attenuation; A, a, and B are constants that are material dependent. λ is the wavelength. The attenuation of 14 micron light in silica glass fiber is approximately 7.32×10 10  dB per km using formula {3} and data from reference [5]. Therefor, applicant has found that the attenuation of 14 micron light in glass fiber is to high to be useful for telecommunication. Modern telecommunication systems are optimized for wavelengths around 1.3 or 1.55 microns and have attenuation around 0.15 dB per km. Unfortunately, light at these wavelengths (i.e. at these higher photon energies) are not compatible with the devices described in the Puzey and Hed Patents. The present invention to be described hereinafter provides a solution to this problem which has remained unsolved since as long ago as December 1988, the filing date of the &#39;042 Hed patent. 
     SUMMARY OF THE INVENTION 
     The present invention provides an apparatus and method for the modulation of light. A layer of superconducting material is placed in the optical path of a light source. The light source emits light with a wavelength which in a preferred embodiment is long enough that the energy of the individual photons is less than the superconducting gap of the superconductor. At least a portion of the superconducting layer is then switched between a substantially non-transparent superconducting state and a partially transparent non-superconducting state by predetermined means The optical pulses transmitted through the portion of the superconducting layer are then converted from the original wavelength to a different wavelength, a shorter wavelength in the preferred embodiment, by a frequency converting device. The shorter wavelength in the preferred embodiment has been chosen to allow an optical fiber to efficiently carry the optical pulses without significant attenuation or dispersion. 
     In a specific embodiment of the present invention, the predetermined switching means is designed for switching a particular portion of the superconducting layer between its superconducting and non-superconducting states using a modulating circuit that intermittently raises the current density in the particular portion above the critical current density of the superconducting material, or raises the magnetic field of the portion above the critical field of the superconducting material, or raises the temperature of the portion above the critical temperature of the superconducting material. The frequency converting device can be a parametric amplifier, parametric oscillator, Nth harmonic generator, four wave mixer, frequency upconverter, or any other frequency converting device. Means for keeping the superconducting layer below its critical temperature can be provided by placing the device in a dewar where at least a portion of the dewar is transparent to the longer wavelength and the dewar is actively cooled by a cryogenic refrigerator. 
     The present invention may also include certain other features described below. The present invention can include an optical fiber, optically coupled to the frequency converting device to conduct the optical pulses away from the device, for example to a receiver, optical demultiplexer, or other useful device. The present invention can include a number of fiber optic links used to provide input data for the modulating circuit. This is useful because glass optical fibers conduct less heat than metal electrical wires. Alternatively, free space optical links may be used to provide data to the modulating circuit, eliminating a heat conducting path into the dewar all together. 
     It is an object of the present invention to provide an improved light modulation device which increases the range of wavelengths which can be modulated at an increased rate. 
     It is another object of the present invention to increase the rate at which data bits can be transmitted on a fiber optic link. 
     It is another object of the present invention to create high speed optical pulses that are not limited to wavelengths with photon energies lower than the superconducting gap of the superconducting material. 
     It is an object of the present invention to reduce the amount of heat introduced by incoming data sent to the modulating circuit. 
     The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of the preferred embodiment of the invention as illustrated in the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic illustration of a prior art light modulation device employing superconducting material. 
     FIG. 2 is a diagrammatic illustration of a light modulating system designed in accordance with the present invention and employing a superconducting layer and a frequency converting device to modulate light. 
     FIGS. 2A and 2B are diagrammatic illustrations of modified light modulating systems designed in accordance with the present invention; 
     FIGS. 3A-3H are detailed enlarged, not to scale diagrammatic illustrations of one embodiment of the superconducting layer shown in FIG.  2 . 
     FIGS. 4A-4H are detailed enlarged, not to scale diagrammatic illustrations of an alternative embodiment of the superconducting layer shown in FIG.  2 . 
     FIG. 5 is a diagrammatic illustration of an alternative embodiment for the superconducting layer. 
     FIGS. 6A and 6B are diagrammatic illustrations of still other alternative embodiments of the superconducting layer. 
    
    
     DETAILED DESCRIPTION 
     Turning to the drawings, attention is immediately directed to FIG. 2, inasmuch as FIG. 1 was discussed above. FIG. 2 shows a light modulation system  10 ′ designed in accordance with one embodiment of the present invention. This system includes most of the components described above in conjunction with FIG. 1 (designated by the same reference numbers primed) plus additional components to be described hereafter. With particular regard to the superconducting arrangement  14 ′, as stated in the Puzey patent electrical current of a certain minimum critical level can be used to switch superconducting arrangement  14 ′ from its superconducting state to the normal state. Removing the electrical current allows arrangement  14 ′ to return to its superconducting state. When the current is below the critical current, the arrangement  14 ′ is in the superconducting state and the optical output  20 ′ is zero because of the 100% reflectance. When the current is above the critical current the material is in the normal state and the optical output  20 ′ is non-zero, that is some measurable level. Thus electrical current pulses can be used to amplitude modulate the light from an optical source  13 ′. Note that this modulator has the ideal extinction ratio of zero. 
     Still referring to FIG. 2, in system  10 ′, a plurality of fiber optic transmitters  31  and  32  are arranged to transmit fight pulses in parallel to a plurality of receivers  35  and  36  through individual optical fibers  33  and  34 , see Van Zehgbroeck [6]. The optical transmitters  31  and  32  may be current modulated laser diodes or LEDs. The receivers  35  and  36  may be MSM (metal semiconductor metal) detectors. The electrical data from the signals are then read in parallel and serialized via a high speed shift register  16 ′. The shift register is made from Josephson Junction circuitry, see Martens et. al. [7]. A light source  13 ′is used to generate fight which is then amplitude modulated by arrangement or device  14 ′ under the control of the serialized signals from the shift register  16 ′. Electrical energy is supplied to the light source  13 ′by a power supply  15 ′. The light source  13 ′may be an LED, laser, etc., as is commonly known in the art. The modulating device  14 ′ will be described in more detail later. 
     The optical pulses  20 ′ from the modulating device  14 ′ enter a frequency converting device  41  which replicates the incoming pulses  20 ′ in a different frequency of light  42  in accordance with one feature of the present invention. The frequency converting device  41  may be a parametric amplifier, parametric oscillator, Nth harmonic generator, four wave mixer, frequency upconverter, etc., some of the principles of which are described in Yariv [8] and Saleh and Teich [9]. The frequency converting device  41  should be made from material that is transparent at the wavelength of the modulated light  20 ′ and the desired wavelength of the outgoing pulses  42 , and has a high nonlinear conversion efficiency. The incoming pulses  20 ′ are preferably on the order of 14 microns so as to be compatible with superconducting device  14 ′. The outgoing pulses  42  are on the order of about 0.5 to 2 microns, preferably on the order of 1.3 or 1.5 microns so as to be compatible with silica glass fibers. Suitable materials for the frequency converting device  41  are GaAs, ZnGeP 2 . AgGaSe 2 , TI 3 AsSe 3 , CdSe, AgGaS 2 , Ag 3 AsS 3 . These new pulses  42  then enter an optical fiber  25 ′ which carries the pulses and is typically made of silica glass. The pulses  44  exiting the fiber are then received by an optical receiver  45 . By way of illustration and not limitation, the optical receiver  45  may be a high speed amorphous silicon detector or an all optical demultiplexer and a plurality of low speed detectors. Such techniques have resulted in 100 Gbts receiver capability, see Ronson et.al. [10]. 
     A dewar  22 ′ is used in accordance with another feature of the invention to thermally isolate the device  14 ′ and shift register  16 ′ from the outside room temperature. The dewar  22 ′ must be at least partially transparent to the optical energy  20 ′ or have a window that is substantially transparent to the optical energy  20 ′. A second window could be used to direct pulses to detectors  35  and  36  in lieu of optical fibers  33  and  34  which extend into the dewar or the dewar could be entirely transparent. A cryogenic cooler  26 ′ is used to keep the temperature in the dewar below the critical temperature. The cryogenic cooler may be a sterling cycle refrigerator, Gifford-McMahon refrigerator, tank of liquid nitrogen, etc. 
     FIGS. 3A-3H show an embodiment of the superconducting arrangement  14 ′. The modulator  14 ′ is made by depositing a thin superconducting layer  50  on a transparent or partially transparent substrate  49  such as silicon or diamond[11]. A film thickness of 480 angstroms transmits 6% of the incident light [3]. The critical current of the film depends on the product of the width, thickness, and critical current density. For a 480 angstrom thick layer with a 100 micron bridge width and a critical current density of 10,000 Amps per square centimeter the critical current would be 480 micro Amps. This switch current would lead to dissipative heating of about 96 nW for the same bridge 100 microns long with a normal resistivity of 200 micro Ohms per centimeter. The switching speed of the film is limited by Abrikosov vortices nucleation. The modulation speed is given by [12] 
     
       
         t s−n =3t D (W/2Ω)(I c /I) 2 ,  {4} 
       
     
     where t s−n  is the superconducting to normal switching time, t D  is the order parameter relaxation time, W is the bridge width, Ω is the depairing ratio, I c  is the critical current, I is the switching current. Kozyrev estimates t s−n  to be on the order of a picosecond for a 70 micron bridge. The FWHM spectral width of a transform limited pulse is the reciprocal of the FWHM temporal width. For a pulse width of about a ps this gives a spectral width of a nanometer or so. 
     Still referring to FIGS. 3A-3H, substrate  49  which is at least partially transparent to the optical pulses  20 ′ is used to support the thin film of superconducting material  50  which is H shaped so as to include legs  50   a  and a bridge  50   b . At the same time, segments  49   a  of substrate  49  remain exposed in FIG. 3C. A dielectric layer  51  is used to electrically isolate the superconducting layer  50  from a reflective layer  52  which covers segments  49   a  along with most of layer  51  as best seen in FIGS. 3G and 3H. A conducting layer  53  which is elongated in configuration is placed over and in direct contact with each leg  50 A of material  50  and is used to provide electrical contact to the device  14 ′. The substrate  49  may be made from MgO, silicon, diamond, etc. The superconducting layer  50  may be made from niobium, yttrium, thallium, or mercury based superconductors. Preferably the superconducting layer  50  is made from a superconducting material with a high critical temperature, low normal resistivity, and low critical current density. The dielectric layer  51  may be composed of silicon dioxide, spin on glass, polyimide, etc. The reflective layer  52  is composed of a material that reflects optical energy  20 ′ such as gold, copper, silver, metal, good conductors, etc The reflective layer  52  prevents light from “leaking” around the superconducting bridge  50   b . The conducting layers  53  are used to make a good electrical contact between the shift register  16 ′ and the superconducting layer  50  via leads  54 . The conducting layer  53  may be the same material used for the reflecting layer  52 . The conducting layer  53  should have low electrical resistance and be substantially unreactive. Gold is a suitable material for both the reflecting layer  52  and conducting layer  53 . 
     Superconducting material only superconducts when the temperature of the material is below a certain temperature (called the critical temperature) and the magnetic field passing through the material is below a certain value (called the critical magnetic field) and the electrical current density passing through the material is below a certain value (called the critical current density). Raising any of these three parameters above the critical value causes the superconductor to enter a non-superconducting state. In the superconducting state the superconducting material  50  is very conductive and thus highly reflective. Electromagnetic energy is reflected in this state. In the non-superconducting state the superconducting material  50  has properties similar to a semiconductor. 
     FIG. 2, which has been described, illustrates the use of the critical current density to control device  14 ′. FIGS. 2A and 2B illustrate the use of a critical magnetic field and a critical temperature respectively to control the superconducting device  14 ′. Referring to FIG. 2A, element  56  is a magnetic coil which is placed in proximity to device  14 ′. This magnetic coil  56  when provided with electrical current from shift register  16  via leads  54  raises the magnetic field above the critical magnetic field of material  50  causing it to enter a non-superconducting state. Removal of the current from shift register  16  allows the material  50  to re-enter a superconducting state. Similarly referring to FIG. 2B, element  57  is a resistor or other heating element placed in close proximity to device  14 ′. This resistor  57  heats the material  50  above its critical temperature when provided with electrical current from shift register  16  via leads  54 . Removal of the current from shift register  16  allows the material  50  to re-enter its superconducting state. In the non-superconducting state electromagnetic energy can be transmitted through the material  50 . In the superconducting state material  50  substantially blocks transmission. Thus by placing the superconducting layer  50  in the path of the light from the light source  13 ′the superconducting layer  50  can be used to control the transmission or reflection of light under the influence of the electrical signals from the shift register  16 ′. Recall that light is an electromagnetic wave. 
     An alternative embodiment of the modulating device  14 ′ is shown in FIGS. 4A-4H. A substrate  49 ′ that is substantially opaque to the optical energy  20 ′ is used to support H shaped superconducting layer  50 . Substrate  49 ′ may be either highly reflective or absorptive. The substrate  49 ′ may be made from sapphire, lanthanum aluminate, gallium arsenide, or the like. The superconducting material may be any superconducting compound such as the niobium, yttrum, thallium or mercury based superconductors. An area  49   c  of the substrate under the bridge  50   b  of superconducting material  50  is at least partially removed to allow optical energy  20 ′ to be transmitted through this area  49   c . The substrate material may be removed by ion milling, chemical etching, drilling, etc. A conducting layer  53  is used to provide a low resistance electrical contact between the legs  50   a  of superconducting layer  50  and the shift register  16 ′ as before. 
     Returning to FIG. 2, the addition of the frequency converting device  41  allows the present system to overcome the problem of large attenuation described above. A parametric amplifier as device  41  can be used to take the high speed pulses  20 ′ at 14 microns and convert them to high speed pulses  42  at a wavelength with lower attenuation and dispersion characteristics (such as 1.3 microns or 1.55 microns). Parametric amplifiers are capable of reproducing even femtosecond pulses. Other devices may be used to perform this frequency conversion as mentioned earlier. 
     Using the fiber optic arrangement ( 31 ,  32 ,  33 ,  34 ,  35 ,  36 ) to communicate signals to be multiplexed reduces heat loss as glass does not conduct as much heat into the dewar as copper electrical wires would. In addition, the fiber optic arrangement has better bandwidth, lower crosstalk, and avoids ground-level feed through. An alternative embodiment that eliminates the optical fibers  33  and  34  is also advantageous. A free space optical communication link is established through the transparent dewar (or a transparent window of the dewar). This eliminates heat loss because there is no physical link to carry heat into the dewar. A vertical cavity surface emitting laser (VCSEL) array and charged coupled device (CCD) array would be especially desirable in this type of arrangement. 
     Returning to FIGS. 3A-3H and  4 A- 4 H the “H” configuration of the superconducting layer  50  provides several advantages. The two legs  50   a  of the H shape allow for low resistance electrical contact with the shift register  16 ′. The narrower bridge  50   b  part of the H allows this part of the superconducting layer to switch faster. The switching speed is linearly related to the width of the bridge as shown by equation {4}. In addition, the narrower bridge reduces the amount of current required to switch the switch to its partially transparent non-superconducting state. This reduces the dissipative heating in the switch. 
     A more detailed explanation of the current flow through the modulating device  14 ′ is given below. The following dimensions concern the superconducting layer  50 . 
     W 1  is the width of the first contact segment  50   a.    
     L 1  is the length of the first contact segment  50   a.    
     T 1  is the thickness of the superconducting thin film in the first contact segment  50   a.    
     W 2  is the width of the light impinging segment  50   b.    
     L 2  is the length of the light impinging segment  50   b.    
     T 2  is the thickness of the superconducting thin film in the light impinging segment  50   b.    
     W 3  is the width of the second contact segment  50   a.    
     L 3  is the length of the second contact segment  50   a.    
     T 3  is the thickness of the superconducting thin film in the second contact segment  50   a.    
     A good conductor (such as gold) is deposited on the surface of the first and second contact segments  50   a  The layer of gold should at least partially cover the surface area of the superconductor defined by (W 1 ×L 1 ) for the first contact segment and (W 3 ×L 3 ) for the second contact segment. This provides a low resistance contact to the superconducting layer. 
     The critical current density J is defined by the electrical current I flowing through a cross sectional area (W 1 ×L 1 ) A Initially the current flows substantially vertically through the area defined by the gold-superconductor contact. Then the current travels substantially in a horizontal direction through a cross section of area (W 1 ×T 1 ). The current then enters the light impinging segment  50   b . In at least one embodiment the width w 2  of the cross sectional area of the light impinging segment  50   b  is substantially smaller than the width wi of the contact segment  50   a  and the thickness of the films are the same (T 1 =T 2 ). Thus the cross sectional area A in the light impinging segment  50   b  (W 2 ×T 2 ) is smaller and since I is conserved J increases. This increase in J is due to the restriction of the cross sectional area and causes the light impinging segment to enter its non-superconducting state at a lower electrical current than the contact segments. The electrical current then moves into the second contact segment traveling substantially in a horizontal direction through a cross sectional area (W 3 ×T 3 ). The current then moves substantially in a vertical direction into the gold through the area where the gold and superconductor are in contact. This area at least partially covers (W 3 ×L 3 ). 
     In addition, the present system is compatible with wave division multiplexing (WDM) and soliton transmission. 
     Another alternative embodiment of the present invention uses different shapes in the light impinging section of layer  50  to allow for discrete regions to switch. An example of this is shown in FIG. 5 which illustrates a modified H shaped configuration  50 ′. Here the legs of the H shape  50   a ′ serve as contact areas in the same manner as legs  50   a . The bridge  50   b ′ is divided into discrete sections  1 ,  2  and  3 , as shown. This allows amplitude shift keying (ASK). ASK allows a single pulse to carry multiple bits of information. By way of illustration and not limitation, when the electrical current passing through the superconducting layer is low, the light output is zero and this can be used to represent the bit string “00”. Notice that section  1  has the most restricted width and therefore constricts the electrical current to a smaller cross sectional area increasing the critical current density in this region for a given amount of electrical current The current can then be raised to a level just high enough to cause section  1  (but not the other sections) to enter its non-superconducting state, allowing only the light impinging section  1  to pass light. This small amount of light could be used to represent the bit string “01”. An even higher current could cause section  2  and section  1  to enter its non-superconducting state. Then only light impinging section  1  and  2  would pass light. This greater amount of light could be used to represent the binary string “10”. Finally an even greater current could be used to cause section  3  to enter its non-superconducting state, preferably this current is not high enough to cause section  50   a ′ (the contact section) to switch. light would then pass through sections  1 ,  2 , and  3  which could be used to represent the binary string “11”. Thus current pulses with different magnitude can be used to create fight pulses with different magnitude. 
     Yet another alternative embodiment of the present invention uses different shapes in the light impinging segment to allow for continuous regions to switch, examples of which are shown in FIGS. 6A and 6B. This allows analog control of the light amplitude sent. It can be seen from FIGS. 6A and 6B that the amount of area in section B that is in its non-superconducting state increases with an increase in the current passing through the superconducting layer. Thus the amount of light allowed to pass through the superconducting layer is proportional to the current passing through the superconducting layer and can be varied in a continuous or analog manner. 
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