Patent Application: US-92105401-A

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:
turning to the drawings , attention is immediately directed to fig2 inasmuch as fig1 was discussed above . fig2 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 fig1 ( 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 fig2 in system 10 ′, a plurality of fiber optic transmitters 31 and 32 are arranged to transmit light 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 light 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 , tl 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 gb / s 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 . fig3 a - 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 = 3 t 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 fig3 a - 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 fig3 c . 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 fig3 g 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 . fig2 which has been described , illustrates the use of the critical current density to control device 14 ′. fig2 a and 2b illustrate the use of a critical magnetic field and a critical temperature respectively to control the superconducting device 14 ′. referring to fig2 a , 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 fig2 b , 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 fig4 a - 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 , yttrium , 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 fig2 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 fig3 a - 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 w 1 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 solution 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 fig5 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 light 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 fig6 a and 6b . this allows analog control of the light amplitude sent . it can be seen from fig6 a 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 . 1 . collins , r . t . et . al . “ infrared studies of the normal and superconducting states of yba 2 cu 3 o 7 - x .” ibm journal of res .& amp ; dev . vol . 33 , no . 3 , may 1989 , pgs 238 - 244 . 2 . schlesinger , z et . al . “ infrared studies of the superconducting energy gap and normal - state dynamics of the high - t c superconductor yba 2 cu 3 o 7 .” physical review b , vol . 41 , no . 16 , jun . 1 , 1990 , pgs 11237 - 11259 . 3 . tanner , d . b . “ far - infrared transmittance and reflectance studies of oriented yba 2 cu 3 o 7 - d .” physical review b , vol . 43 , no . 13 , may 1 , 1991 , pgs 10383 - 10389 . 4 . mattis , d . c ., bardeen j . phys rev 111 , 412 ( 1958 ). 5 . lines , m . e ., nassau k “ calculations of scattering loss and dispersion related parameters for ultralow - 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612 . 12 . kozyrev , a . b . “ fast current s - n switching in yba 2 cu 3 o 7 - x films and it &# 39 ; s application to an amplitude modulation of microwave signal .” sverkhprovodimost april 1993 , pgs 655 - 667 .