Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-06-30
2001-10-16
Chan, Jason (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06304355
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a system and method for modulating a combined light information signal and light carrier wave signal for transmission through an optical transmission link and demodulating the combined signal after reception to extract the transmitted information. More particularly, the invention relates to such a system and method of extremely simple and robust construction and which allow use of commercially available lasers and low frequency switching electronics for conversion of electronic domain information input signals to light signals and for retrieval of information from the light signals with reconversion to the electronic domain.
2. State of the Art
Optical communication systems allow information to be transmitted in the form of light. Fibre optic cables may be used to transmit the information from a transmitter to a receiver. Fibre optic cables can transmit light at extremely high speed and with relatively small power loss.
Referring to
FIG. 1
, a typical fibre optical cable consists of an optical core
14
surrounded by an optical cladding
16
. The light is transmitted through core
14
. As used herein, “light” refers to electromagnetic radiation that may be effectively transmitted through fibre optic cable and associated components, or other optic transmission systems known or contemplated in the art.
All materials that allow the transmission of electromagnetic radiation including light have an associated refractive index n, which is the ratio of the speed of light in a vacuum to the speed of light in the material. The speed of light in a vacuum is normalized to 1. The speed of light in a vacuum is constant regardless of the wavelength of the light. By contrast, the speed of light in a material is a function of wavelength and the structure of the material. Accordingly, the refractive index is a function of the wavelength of the light and the structure of the material.
Refraction refers to bending of light due to variations in the refractive index. As a ray of light passes from one material (or a vacuum) to another material, it is possible for the ray to refract, reflect, or partially refract and partially reflect. (The ray may also be partially absorbed.) Refracted rays are sometimes called transmitted rays, which term will be used herein to avoid confusion of subscripts.
The following three laws govern the relationship between incident, reflected, and transmitted (refracted) rays. First, the incident, reflected, and transmitted rays all reside in a plane, known as the plane of incidence, which is normal to the interface of the materials. Second, the angle of incidence &thgr;
I
equals the angle of reflection &thgr;
R
, where each angle is measured with respect to a line normal to the interface. Third, the angle of incidence &thgr;
I
and the angle of transmittance &thgr;
T
are related by Snell's law shown in equation (1), below:
n
I
sin &thgr;
I
=n
T
sin &thgr;
T
(1),
where n
I
is the refractive index of the material through which the incident ray travels, n
T
is the refractive index of the material through which the transmitted ray travels, &thgr;
I
is the angle of the incident ray with respect to the normal, and &thgr;
T
is the angle of the transmitted ray with respect to the normal.
An example of refraction is shown in
FIGS. 2A and 2B
. Referring to
FIGS. 2A and 2B
, a ray travels from Material A, having refractive index n
I
, to Material B, having a refractive index n
T
. The ratio of the angle of incidence &thgr;
I
to the angle of transmittance &thgr;
T
is governed by Snell's law, shown in equation (1). Generally, where n
T
>n
I
(as in FIG.
2
A), &thgr;
T
<&thgr;
I
. Where n
T
<n
I
(as in FIG.
2
B), &thgr;
T
>&thgr;
I
. (Of course, a larger &thgr;
I
also results in a larger &thgr;
T
.) At &thgr;
T
=90°, &thgr;
I
is defined to be at critical angle, denoted &thgr;
C
. The critical angle &thgr;
C
is defined in equation (2) below:
&thgr;
C
=sin
−1
(
n
T
I
) (2),
For &thgr;
I
>&thgr;
C
, all of the incident ray is totally internally reflected, remaining in the incident medium. An ideal fibre optic cable has total internal reflection, which leads to a relatively small amount of loss in the transmission of light through the cable.
Referring to
FIG. 3
, one end of fibre optic cable
10
interfaces with air, which has a refractive index n
1
(which happens to be about 1.00027). Core
14
has a refractive index n
2
, where n
2
>n
1
. Cladding
16
has a refractive index n
3
. Dashed lines show the normal with respect to the air-core interface and the core-cladding interface. An incident ray hits the air-core interface at angle &thgr;
I1
. The transmitted (refracted) ray is referred to as ray TI to designate the ray as both a transmitted ray with respect to the air-core interface and an incident ray with respect to the core-cladding interface. The angle of transmittance &thgr;
T
may be derived according to Snell's law, shown in equation (1).
An angle of incidence &thgr;
I2
inside core
14
equals 90° minus &thgr;
T
. If &thgr;
I
>&thgr;
C
, there will be total internal reflection and ray TI will continue to transmit through core
14
at angle &thgr;
I2
until another interface is reached. Further, there is no loss of radiated power at the reflection (although there is loss as the light passes through core
14
).
If &thgr;
I
is too large, &thgr;
I2
cannot be greater than &thgr;
C
, and there will not be total internal reflection. The maximum incident angle &thgr;
MAX
is derived in equation (3), below:
&thgr;
MAX
=sin
−1
((1
1
)(
n
2
2
−n
3
2
)
½
) (3),
where n
1
, n
2
, and n
3
are the refractive indices defined above in connection with FIG.
3
. Accordingly, if &thgr;
I
>&thgr;
MAX
, there will not be total internal reflection.
Interference refers to the consequence which arises when two light waves starting from the same point source or from two identical point sources arrive at some point P after having travelled two trajectories with different lengths. Generally, the two light waves have the same frequency, but different phases at the time they reach point P. However, the inventor has discovered that it is possible to employ the interference phenomenon with laser light waves of different frequencies and from different sources, as the description of the present invention will hereinafter show.
Modulation is used to impress information from one signal into another signal to create a modulated signal. There are various types of modulation, including amplitude modulation and frequency modulation.
Amplitude modulation is a method of transmitting an information signal by superimposing it on a carrier signal which has a much higher frequency. Consider the following simple example. A carrier signal cos &ohgr;
C
t is varied in amplitude by a modulating information signal cos &ohgr;
M
t, where &ohgr;
M
is much less than &ohgr;
C
. The resulting modulated signal I
Mod
is shown in equation (4), below:
I
Mod
=(1
+M cos &ohgr;
M
t)cos •
C
t (4),
where M is the modulating index, which is less than or equal to 1, &ohgr;
M
=2&pgr;f
M
=2&pgr;/&lgr;
M
, and &ohgr;
C
=2&pgr;f
C
=2&pgr;/&lgr;
C
. I
Mod
may be rewritten as in equation (5), below:
I
Mod
=cos &ohgr;
C
t+
m
/2(cos(&ohgr;
C
+&ohgr;
M
)
t
+cos(&ohgr;
C
−&ohgr;
M
)
t
) (5).
Equation (5) illustrates that the modulated carrier has power at frequencies &ohgr;
C
, &ohgr;
C
+&ohgr;
M
, and &ohgr;
C
−&ohgr;
M
. In amplitude modulation, the frequency of the information signal remains constant while the amplitude varies to convey information. In frequency modulation, the frequency of the modulated signal varies depending on the frequency of the information signal.
Where the information (modulating) signal is a complex waveform f(t), the amplitude modulated waveform may be (K+f(t))*cos &ohgr;
C
t, w
Chan Jason
Sedighian M. R.
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