Optical communication equipment and system

Details Optical communication equipment and system Optical communication equipment and system

1999-09-17

2003-03-04

Tweel, John (Department: 2632)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C359S341100

Reexamination Certificate

active

06529304

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an information communication technique using light signals and to an optical communication apparatus and an optical communication system suitable for speeding-up of signal transmission and an increase in the capacity for the transmission of a signal or an increase in reliability of signal transmission.
BACKGROUND ART
The development of research toward the speeding-up of signal transmission and an increase in the capacity for the signal transmission has recently been active in an information communication technique (hereinafter called optical transmission technique) which performs the transmission of a light signal through an optical fiber. An optical communication system wherein the transmission rate reaches about 2.4 Gb/s (giga-bit/s) at present, has been put into practical use upon the transmission of a light signal between base stations (communication equipment centers), for example. Further, the development of research intended for a high-speed capacity increase of 40 Gb/s or more is in progress.
On the other hand, the introduction of a so-called optical communication system for replacing a transmission signal with light is proceeding even at information communications made between base stations and users (home, office, etc.), and an increase in the reliability of an optical network unit (ONU) on the user side remains a problem. This problem is of importance even to an optical communication apparatus adopted in the transmission of a light signal between the base stations.
The speeding up of the light signal and the great increase in the capacity for the transmission thereof in the optical communication system are problems which are integrally inseparably related to each other. The above-described transmission rate is known as one reference for evaluating these performance. Parameters for improving the transmission rate exist in specifications of, for example, a transmitting device or transmitter for transmitting a light signal, a receiving device or receiver for receiving the light signal therein, and a signal processing device for converting the received signal to an electric signal or decoding it. As to the transmitting device, the shortening of each pulse of a light signal generated according to an electric signal (including an electric signal temporarily converted from a signal inputted by light) without impairing the S/N ratio (signal-to-noise ratio) is taken as one parameter (the present parameter will hereinafter be called “first viewpoint”). Further, one of other parameters is that a plurality of light signals different in wavelength from each other are used and signals to be transmitted are distributed and transmitted every wavelength (hereinafter the present parameter will hereinafter be called “second viewpoint”). Thus, the background art related to the improvement in the transmission rate in the optical communication system will be viewed from the first and second viewpoints respectively and summarized below.
1. First Viewpoint (Shortening of Pulse of Light Signal)
In view of trends of the background art from the first viewpoint, an improvement in performance of a so-called optical modulator for modulating light oscillated from a laser light source according to a transmission signal is now mainstream. The present optical modulator is constructed as a so-called semiconductor optical device formed by stacking semiconductor layers on each other. Most of the optical modulators are (monolithically) formed over the same substrate as a semiconductor laser device used as the laser light source. Most of these optical modulators intermittently apply an electric field to a semiconductor layer (hereinafter called “wave-guiding layer”) for wave-guiding laser light or a semiconductor layer (hereinafter called “cladding layer”) jointed thereto, modulate an absorption coefficient of at least one of the semiconductor layers with respect to the laser light, and repeat transmission and cutting off of a light signal incident to an optical fiber corresponding to a signal transmission line.
The principle of operation of this type of optical modulator is that a forbidden band width Eg of each semiconductor layer is reduced by the application of the electric field, and the semiconductor layer is allowed to absorb light having a much longer wavelength as compared with light &lgr;g of the longest wavelength (corresponding to the forbidden band width of the corresponding semiconductor layer) originally absorbable by the semiconductor layer. This principle will be explained typically with reference to FIG.
23
. The following relation is established between a forbidden band width Eg (unit: eV) of each semiconductor layer and the longest wavelength &lgr;g (unit: &mgr;m) of light absorbed thereby:
Eg
=1.24
/&agr;g
  (equation 1)
When the non-applied state of the electric field or the strength of the applied electric field is sufficiently small as shown in FIG.
23
(
a
), Eg of the semiconductor layer is determined according to the difference between an upper end Va of an energy level in a valence band and a lower end Ca of an energy level in a conduction band. If the absorption of the light by the semiconductor layer is discussed by band-to-band transition alone, then the light incident to the semiconductor layer transitions electrons in the valence band to the conduction band by its own energy, so that the light is absorbed into the semiconductor layer. Therefore, light of energy (wavelength longer than &lgr;g) lower than Eg is not substantially absorbed. However, when the electric field to be applied to the semiconductor layer is strengthened, respective degeneration of the energy levels are released so that they are split into a plurality of energy levels. This phenomenon is called “Stark effect”. As a result of its splitting, the transition of electrons between the valence band and the conduction band due to the energy levels Va
2
and Ca
1
is newly added. Since an energy difference Eg′ between the levels is smaller than Eg, the range of the wavelength of the light absorbed by the corresponding semiconductor layer is shifted to the long-wave side up to &agr;g′ equivalent to Eg′.
On the other hand, when the degree of the absorption of light by the semiconductor layer depends on an absorption coefficient a and light having an intensity I
0
enters into the semiconductor layer by a distance x, a light intensity I(x) attenuated by the absorption of the semiconductor layer is represented like the following equation:
I
(
x
)=
I
0
exp{−&agr;
x}=I
0
exp{−2
&ohgr;&kgr;x/c}
  (equation 2)
where &ohgr;(s
−1
) indicates an angular frequency (&ohgr;=2&pgr;&ngr;, &ngr;: a light's wave number), &kgr; (dimensionless) indicates an attenuation coefficient (also called extinction coefficient), and c indicates the velocity of light in vacuum (about 3.0×10
8
m/s), respectively. The extinction coefficient is a parameter indicative of an imaginary part of a refractive index (n+i&kgr;=C/v, v: the velocity of light traveling or propagating in a medium) of the medium accompanied by light absorption.
The absorption coefficient &agr; depends on the energy (hv) of light as shown in FIG.
23
(
b
). The absorption coefficient a satisfies the relations in the following equation in a state free of the splitting of the above-described energy levels (it is illustrated as a curve a). Here, h indicates a Planck constant (about 6.63×10
−34
J·s).
&agr;∝ (
h&ngr;−Eg
)
½
/&ohgr;  (equation 3)
On the other hand, when the energy levels are split, the relationship between &agr; and h&ngr; is illustrated as a curve b in which the rise of the curve is shifted to Eg′ lower in energy than Eg. Eg′ is further shifted to the low energy side as the strength of an electric field to be applied increases.
Of the conventional optical modulators, the principle of the above-described light absorption has been general

Affiliated with

Also associated with

Rating

Optical communication equipment and system does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Optical communication equipment and system, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical communication equipment and system will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3062935