Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2001-08-02
2003-02-04
Lester, Evelyn A (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S245000, C359S320000, C359S321000, C359S326000, C385S070000, C385S016000, C385S088000, C505S181000, C505S182000, C505S702000
Reexamination Certificate
active
06515788
ABSTRACT:
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'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 “High Speed Light Modulation” 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
<2&Dgr; {1}
Where h is Planck's constant, v is the frequency of light, and 2&Dgr; is the energy gap of the superconductor. The energy gap of the superconductor can be found from Mattis-Bardeen [
4
].
2&Dgr;=8
kT
{2}
Where k is Boltzman'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.
&agr;=
Ae
−a/&lgr;
+B/&lgr;
4
{3}
Where &agr; is the attenuation; A, a, and B are constants that are material dependent. &lgr; 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 '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 inventi
Lester Evelyn A
Morita Yoriko
Pritzkau Michael
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