Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2001-02-28
2002-11-05
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S281000, C359S237000, C359S245000
Reexamination Certificate
active
06476956
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to optical modulation. More particularly, it relates to a device for high data rate modulation of an optical signal. Even more particularly, the invention relates to a device that provides a high data rate of magneto-optical modulation.
BACKGROUND OF THE INVENTION
Because of the high data rates available, optical fiber is preferred for high-speed transmission of data, audio, and video. Binary optical signals consist of low and high to intensity signals traveling through the fiber. A limiting factor in optical fiber communication networks has been the speed at which light can be electrically switched or modulated to provide change from a high intensity signal to a low intensity signal and back to a high intensity signal. This conversion from electrical to optical signal is slower than the capability of the fiber. While the optical fiber can accommodate much higher data rates, commercial techniques for creating high-speed modulation are presently achieving approximately 40 billion bits per second, or 40 GHz.
One method of modulating an optical signal involves providing a digital optical signal directly from a light source. In this method light is directly modulated by turning on and turning off power to a laser source. It is difficult, however, to make these transitions quickly without introducing non-linear effects that can degrade the signal. These effects include changes in index of refraction of material in the laser cavity which effectively changes the optical path length of the cavity during the pulse, leading to an effect called chirp, and provides greater dispersion of the signal as it travels down optical fiber.
Alternatively, a continuous wave light source can be externally modulated to provide a digital optical signal. One such method is electroabsorption modulation. Continuous wave light is directed through a semiconductor. When current flows in the semiconductor, enough electrons are moved from valence to conduction band to provide a population inversion. Light traveling through the semiconductor with the population inversion is amplified by stimulated emission. On the other hand, when no electric current flows, electrons move back to the valence band. Now the light is absorbed by the low energy electrons, so the light intensity is diminished as it travels through the semiconductor. The substantial difference in light intensity when current is flowing and when current is not flowing provides the on and off signals. However, this scheme is limited by the time for generation and relaxation of excited states in the semiconductor.
A third method, a Mach-Zehnder modulator, provides another external modulation technique in which a light beam traveling in a waveguide is split into two paths and then recombined into a single path where the two beams interfere. A material is provided along one path that has a refractive index sensitive to applied voltage. The change in phase introduced by the changing voltage applied to the material provides for constructive or destructive interference where the signals recombine. Currently, however, 10-15V is needed to provide the phase shift, and it has been difficult to provide high frequency signals at a high voltage to drive the phase modulator.
An alternative approach to increase the amount of data that can be transmitted through an optical fiber is Dense Wave Division Multiplexing (DWDM), in which many individual signals, each with a slightly different wavelength, are transmitted through a single optical fiber at one time. Each of the dozens of signals in the fiber runs at the 40 GHz data rate, providing a substantially higher overall data rate. While DWDM increases the data rate provided by a fiber, the equipment cost for transmission capacity is higher for DWDM than for faster modulators. Also, errors may be introduced into the data as a result of a process known as four wave mixing, in which photons of different wavelengths in a fiber combine, so data is lost in two channels in the fiber. Two other photons are generated at different wavelengths, and these may contribute to noise and errors in other channels in the fiber. Thus, faster modulation for each wavelength is desirable.
Two additional techniques to greatly increase modulation frequency and increase the data rate for transmission in a fiber have been proposed in commonly assigned U.S. Pat. No. 5,768,002 to K. A. Puzey, and in a paper “Magneto-Optical Modulator for Superconducting Digital Output Interface,” by Roman Sobolewski, et al, given at the Applied Superconducting Conference held Sep. 17-22, 2000 (“the Sobolewski paper”). Superconductors allow low voltage high speed current switching.
The Puzey technique rapidly switches a superconducting film between superconducting and non-superconducting states and takes advantage of the difference in optical properties of the material in the two states. In the superconducting state, more far-infrared light is reflected from the material, while in the non-superconducting state, more is transmitted. Continuous wave far-infrared light is modulated by an electrical signal provided to such a superconducting film. After modulation of this far-infrared light, the signal is then parametrically converted to a shorter wavelength in the near-infrared range for transmission in a standard optical fiber. Well known frequency up-conversion nonlinear optics are used for the conversion.
The technique described in the Sobolewski paper stimulates magneto-optic material
10
, such as europium monochalcogenides (EuS, EuTe, EuO, and EuSe) by providing magnetic field
12
from current pulse
14
in adjacent superconducting signal electrode
16
driven by a Josephson junction, as shown in
FIGS. 1
a
,
1
b
. Continuous light wave
18
is coupled into magneto-optic material
10
through fiber optic input
19
a
and exits through fiber optic output
19
b.
Portion of light wave
18
traveling in magneto-optical material
10
in magnetic field
12
has its polarization rotated, a property known as the Faraday effect. An interferometer is used to provide pulses of light based on this rotation of the polarization. Because the excitation of magneto-optical materials occurs in a time measured in pico-seconds, as shown in
FIG. 2
a
from a paper, “Femtosecond Faraday rotation in spin-engineered heterostructures,” by J. J. Baumberg, et al, J. Appl. Phys. 75 (10), May 15, 1994 (“the Baumberg paper”), early investigators recognized that such magneto-optical microstriplines might provide a way to modulate signals in the THz (trillion bits per second) range, about two orders of magnitude higher than present modulation.
The curves in the Baumberg paper, however, show a problem with the slow relaxation from the excited state that limits the overall transition time. The relaxation time of magneto-optical materials from their excited state back to ground state can be much longer than the time for excitation, as also shown in
FIG. 2
a
from the Baumberg paper. Thus, there is a very fast excitation rate, on the order of one picosecond, for Faraday rotation in an applied magnetic field for a heterostructure. There is also a slow exponential relaxation rate extending over 250 ps. The slow relaxation limits the speed at which a magneto-optical material can operate as an optical modulator. No way to avoid the slow relaxation has been demonstrated. This lengthy relaxation time substantially limits the speed of operation of such devices as compared to the promise of the much more rapid excitation.
Similarly, in a paper, “Ultrafast magneto-optic sampling of picosecond current pulses, ” by A. Y. Elazzabi and M. R. Freeman, Appl. Phys. Lett. 68 (25) Jun. 17, 1996, data is presented showing current pulses having a rise time of 15 ps and an exponential fall time of 250 ps obtained by triggering a photoconductive switch with an ultrashort laser pulse. The current pulse is used to change the refractive index of a Bi-substituted yttrium-iron-garnet ferromagnetic film, and this causes a rotation in the plane of polarization of pola
Cottrell William J.
Ference Thomas G.
Puzey Kenneth A.
Epps Georgia
Leas James M.
O'Neill Gary
TeraComm Research Inc.
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