JPH07107070A - Method for reducing optical beat interference - Google Patents

Abstract

PURPOSE:To reduce optical beat interference generated by the plural lights of the same wavelength band by converting the modulation frequency of transmission signals by an external modulator. CONSTITUTION:The lights generated in light emitting devices 5 and 6 are modulated by the transmission signals whose oscillation frequencies are omega1 and omega2 from oscillators 3 and 4 and modulated to the lights of omegaC1 and omegaC2 and both modulated lights are synthesized in a photocoupler 7 and transmitted. In a reception part, the modulated lights are modulated in a modulator 2 by external modulation signals omegaL from an optical external modulation source 10 and then detected in a light receiving device 1. At the time, by using either omega1+ or -omegaL or omega2+ or -omegaL obtained as conversion modulation signals, interference by the beat component of omegaC1-omegaC2 can be reduced. Thus, by optionally selecting omegaL, the change of an optical modulation frequency can be coped with as well, the beat interference by the difference component of light emitting wavelength is reduced and optical transmission quality is secured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The optical beat interference reduction method of the present invention is an optical SCM (Sub Carrier Multiple).
x) It can be used for transmission and reduces optical beat interference that occurs when a plurality of lights modulated by a transmission signal are received and demodulated.

[0002]

2. Description of the Related Art When light of a plurality of frequencies modulated by a transmission signal is received and demodulated, conventionally two modulated signal sources a having different oscillation frequencies (ω ₁ and ω ₂ ) as shown in FIG. 2 are used.
The light emitted from the two light sources (light emitters) b ₁ and b ₂ such as LD is modulated by the two transmission signals oscillated from ₁ and a ₂ , and the light of the optical frequencies ω _C1 and ω _C2 is transmitted. In this case, the light having the optical frequencies ω _C1 and ω _C2 was directly detected by the light receiving element g such as a photodiode. Then the two optical frequencies ω
Occurs becomes optical beat difference omega _C1 - [omega] _{C2 C1} and omega _C2, it has been a disturbance occurs in the vicinity of the modulation frequency omega _C1, omega _C2. In FIG. 2, c is an optical coupler, and d is an optical transmission line using an optical fiber.

Therefore, conventionally, when a plurality of lights modulated by a transmission signal are received and demodulated, the emission wavelengths of a plurality of light sources are selected as a combination in which a beat is unlikely to occur.

[0004]

However, the beat disturbing wave is generated at the time of detection of the receiving section, and the main disturbing wave is the difference component of the emission wavelength. Therefore, when a plurality of lights are received and demodulated as described above. However, due to the interaction of the respective signal lights, the difference component of the emission wavelength (emission frequency) appears as a beat at the time of detection, and when this is close to the modulation frequency of each signal light, it becomes an interfering wave and the transmission quality is impaired.

It is an object of the present invention to reduce optical beat interference caused by a plurality of lights in the same wavelength band by simple means.

[0006]

As shown in FIG. 1, a method for reducing optical beat interference according to the present invention receives a plurality of lights (lights of optical frequencies ω _C1 and ω _C2 in FIG. 1) modulated by a transmission signal. Bowl 1
In the optical transmission system in which the light is received and demodulated by, the frequency of the light is converted using the optical external modulator 2 before the light is received.

[0007]

In the optical beat interference reduction method of the present invention, the modulation frequency of the transmission signal can be converted into the sum or difference with the external modulation frequency by applying the intensity modulation by the optical external modulator. By using this conversion component as the desired wave, it is possible to avoid the generation frequency of the beat interfering wave caused by the above-mentioned emission wavelength difference that occurs during detection. That is, beat interference is reduced by appropriately selecting the modulation frequency of the optical external modulator.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the optical beat interference reducing method of the present invention will be described using the optical transmission device shown in FIG. In FIG. 1, 3 and 4 are oscillators with different oscillation frequencies (ω ₁ and ω ₂ ), 5 and 6 are light emitters such as LDs, 7 is an optical coupler that combines the light emitted from both light emitters 5 and 6, 8 is an optical transmission line using an optical fiber, 2 is an optical external modulator, 10 is an optical external modulation source,
Reference numeral 1 is a light receiver such as a photodiode.

When optical transmission is performed by the optical transmission device of FIG. 1, the light emitted from the light emitters 5 and 6 of FIG.
4 is modulated by _two transmission signals of oscillation frequencies ω ₁ and ω ₂ to be modulated into lights of optical frequencies ω _C1 and ω _C2 , and the both modulated lights are combined by an optical coupler 7 to form an optical transmission line. Transmit at 8.

Then, the light having the optical frequencies ω _C1 and ω _C2 is modulated by the optical external modulator 2 by the external modulation signal ω _L from the optical external modulation source 10, and then received by the light receiver 1 and detected. At this time, ω ₁ ± ω _L and ω ₂ ± ω _L are obtained as the modulation signals converted by modulating with the external modulation signal ω _L. In this case, since the main component of the beat component generated at the optical frequencies ω _C1 and ω _C2 is still ω _C1 − ω _C2 ,
Even if beat interference occurs due to _C1 − ω _C2 ≈ ω ₁ (or ω ₂ ), using ω ₁ ± ω _L (ω ₂ ± ω _L ) can reduce the influence of beat interference. Here, if the optical external modulation source 10 is a light source capable of arbitrarily selecting ω _L , even if ω _C1 −ω _C2 changes, the optical external modulation source 10 can cope with it.

[0011]

According to the optical beat interference reduction method of the present invention, beat interference due to the difference component of the emission wavelength can be reduced, and good optical transmission quality can be secured.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of an optical beat interference reduction method of the present invention.

FIG. 2 is an explanatory diagram of a conventional optical beat interference reduction method.

[Explanation of symbols]

 1 is a light receiver 2 is an optical external modulator

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location H04B 10/18

Claims (1)

[Claims] 1. An optical transmission system in which a plurality of lights modulated by a transmission signal are received by a light receiver (1) and demodulated, and the light is frequency-converted by using an optical external modulator (2) before light reception. An optical beat interference reduction method characterized by the above.