Optical transmission device and system

Details Optical transmission device and system Optical transmission device and system

1997-11-26

2001-08-07

Pascal, Leslie (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200

Reexamination Certificate

active

06271942

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to analog optical transmission devices by an optical SCM method, and more specifcally to a device capable of suppressing noise or distortion which increases due to reflected light, and noise or distortion which increases due to reflected return light to a light source and unstableness of the light source. Further, it relates to an analog optical transmission system capable of reducing optical beat interference noise which causes a problem when optical signals from a plurality of light sources are received in totality.
2. Description of the Background Art
An optical SCM (Sub-Carrier Multiplexing) transmission method is a method of converting a frequency-modulated electrical signal to be transmitted into laser light which is intensity-modulated by the signal and then transmitting the light, not requiring A/D and D/A converters unlike digital transmission by on/off of light and further having a characteristic of an extremely broad band and low loss compared to the conventional transmission method using a coaxial cable. Therefore, in recent years, its practical use has been highly expected.
In this optical SCM transmission method, the following problems are known.
Firstly, when multiple reflected light occurs in an optical fiber, noise and distortion increase to deteriorate a transmission characteristic.
Secondly, when reflected light is coupled to a light source, the state of the light source becomes unstable to increase noise and distortion.
Thirdly, when optical signals outputted from a plurality of light sources are received in totality, if wavelengths of optical signals are close to each other, optical beat interference (OBI) noise occurs to deteriorate the transmission characteristic.
The optical signal is reflected in an end surface of an optical component, a connector end surface of the optical fiber, etc. Or, Rayleigh scattering in the optical fiber, etc., also causes reflected light.
Among the above three problems, as an example of a device of suppressing the first transmission characteristic deterioration, an analog optical transmission device and an optical fiber amplifier are disclosed in Japanese Patent Laying-Open No. 5-291671.
FIG. 17
is a block diagram showing the structure of the conventional analog optical transmission device disclosed in Japanese Patent Laying-Open No. 5-291671. The device in
FIG. 17
includes a multiplex portion
501
, an adding portion
502
, a picture signal input terminal
503
, amplifiers
504
and
508
, a semiconductor laser device
505
, an optical fiber
506
, a photo-receptive device
507
, a picture signal output terminal
509
, and optical connectors
510
and
511
.
Analog electrical signals of N channels (ch
1
to N) are RF-multiplexed in the multiplex portion
501
, further in the additional portion
502
a pilot signal is added thereto, and then inputted to the picture signal input terminal
503
. The electrical signal inputted in the picture signal input terminal
503
is amplified in the amplifier
504
and then converted into an optical signal by the semiconductor laser device
505
. The optical signal obtained by the semiconductor laser device
505
is transmitted through the optical fiber
506
and the optical connectors
510
and
511
to the receiving side. The transmitted optical signal is again converted into an electrical signal in the photo-receptive device
507
, further amplified in the amplifier
508
, and then outputted from the picture signal output terminal
509
.
In the above operation, part of the optical signal outputted from the semiconductor laser device
505
is reflected in the optical connectors
510
and
511
or subjected to Rayleigh scattering in the optical fiber
506
. Further, part of reflected light is re-reflected, causing multiple reflected light which proceeds in the same direction as that of the optical signal. Generally, in the semiconductor laser, since wavelength chirping exists associated with electrical-optical conversion, the reflected light which proceeds in the same direction interferes with the optical signal at the time of optical-electrical conversion to cause unfavorable electrical intensity modulation, resulting in interference noise. Therefore, as it is, it is expected that noise or distortion will occur in the electrical signal outputted from the picture signal output terminal
509
. For this reason, the device in
FIG. 17
adds a pilot signal to the electrical signal to be transmitted, converts the electrical signal into an optical signal and then transmits the optical signal, thereby dispersing the power of the interference noise over a wide frequency band. This reduces the power of the interference noise in the transmission frequency band, resulting in reduction in noise or distortion by reflected light.
Also disclosed in Japanese Patent Laying-Open No. 5-291671 there are conditions in which the frequency of the pilot signal is not more than the frequency corresponding to the spectrum line width of the semiconductor laser. It is expressed in
FIG. 17
that adding a pilot signal which satisfies the conditions sufficiently reduces noise or distortion by reflected light which occurs in the optical fiber
506
and the optical connectors
510
and
511
. However, in the device of
FIG. 17
, adding a pilot signal newly causes a second order intermodulation distortion (hereinafter referred to as IM
2
) between the RF-modulated analog electrical signal and the pilot signal.
Furthermore, a structure like that of Japanese Patent Laying-Open No. 5-291671 is used in U.S. Pat. No. 5,373,385. In this patent, the frequency of the additional signal is defined to be outside the band of the signal to be transmitted. Therefore, although the additional signal does not directly have an adverse effect on the signal to be transmitted, as is the same in Japanese Patent Laying-Open No. 5-291671, IM
2
newly occurs to have an adverse effect on the signal to be transmitted.
On the other hand, U.S. Pat. No. 5,430,569 discloses a structure capable of reducing IM
2
which newly occurs by an additional signal. In this patent, when IM
2
occurs in the band allotted for transmission of the signal to be transmitted, a predistorter is used for reduction in its effect. When IM
2
occurs only outside the band, the predistorter is omitted from the structure.
As a method of suppressing the second transmission characteristic deterioration, a method of inserting an optical isolator between a light source and an optical fiber so as not to couple reflected light to the light source is generally adapted.
As a method of suppressing the third transmission characteristic deterioration, Japanese Patent Laying-Open No. 6-177840 discloses an optical communications method of suppressing OBI noise.
FIG. 18
is a block diagram showing the structure of the conventional optical transmission system using the optical transmission method described in Japanese Patent Laying-Open No. 6-177840. The system in
FIG. 18
includes transmitting terminals
600
to
602
, receiving terminals
603
and
604
, optical fibers
605
and
606
, and an optical star coupler
607
. Each of the transmitting terminals
600
to
602
has oscillators
608
1
to
608
3
, electrical modulators
609
1
to
609
3
, and optical modulators
610
1
to
610
3
. The receiving terminal
603
includes an optical demodulator
611
, frequency selective filters
612
1
to
612
3
, electrical demodulators
613
1
to
613
3
, and oscillators
614
1
to
614
3
. The receiving terminal
604
includes an optical demodulator
615
, frequency selective filters
616
1
to
616
3
, electrical demodulators
617
1
to
617
3
, and oscillators
618
1
to
618
3
.
Each of the oscillators
608
1
to
608
3
(f1 to 3 shown in the drawing),
614
1
to
614
3
, and
618
1
to
618
3
outputs a sub-carrier with the frequency of f1 to f3 (electrical signal), respectively. Each of the electrical modulators
609
1
to
609
3
modulates the sub-carrier by the input signal. Each of the optical modulators

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