JPS6074737A - Agc system for optical transmission - Google Patents

Abstract

PURPOSE:To attain stable AGC operation without any pilot signal by superimposing a signal of a frequency higher than a transmission signal frequency on the transmission signal to modulate a laser diode and controlling the gain of a photodetector or a reception amplifier in response to the amplitude of a modulation signal. CONSTITUTION:The high frequency signal 12 having higher frequency than the transmission signal 11 is superimposed on the signal 11 at a superimposing circuit 13, a semiconductor laser diode 14 is modulated by the signal superimposed on the high frequency signal and the signal is converted into an optical signal, which is transmitted via an optical fiber 1. The photodetector 2 converts in a photoelectric manner the optical signal at a reception side, and after the electric signal is amplified at a preamplifier 5, the superimposed high frequency signal is extracted by a BPF7. A bias voltage 4 of the detector 2 is controlled by a signal obtained by rectifying (6) the extracted high frequency signal to control the gain of the amplifier 5. Since the amplitude of the superimposed high frequency signal is increased in comparison with that of the pilot signal, stable AGC operation immune to the effect of noise is performed.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic gain control method that modulates a semiconductor laser with an analog signal and automatically suppresses fluctuations in the input signal level when receiving the optical signal.

[Background of the invention]

Conventionally, the AGC method for optical receiving circuits is Byron AG.
Method C has been frequently used. As shown in Figure 1 (b), this uses a pilot signal that is multiplexed in the frequency domain with respect to the transmission signal, and uses a light receiving element as shown in Figure 1 (axis) to keep the reception level of this pilot signal constant. 2 or the gain of the variable gain preamplifier 5.

Such a pylon AGC system requires a circuit for multiplexing a pilot signal with a transmission signal, and has the disadvantage that the transmission circuit is complicated and the cost is high. In addition, in multi-optical communications where the degree of modulation of the carrier wave is high by the amount corresponding to the pilot signal, the distortion characteristics of the semiconductor laser are degraded, and when modulating the semiconductor laser with frequency multiplexed signals, each of the pilot signals If included, the degree of modulation of the semiconductor laser light would be significantly higher than in the case without a pilot signal, which was a big problem.

[Purpose of the invention]

An object of the present invention is to provide a simple and inexpensive AG for optical fiber communication that has the same reception level fluctuation suppressing function as pilot AGC without adding a pilot signal to a transmission signal.
Our goal is to provide method C.

[Summary of the invention]

In optical communication using semiconductor lasers and multimode fibers, there is a problem in that the signal-to-noise ratio after transmission deteriorates due to the effects of modal noise and reflection noise.

In order to solve this problem, as shown in FIG.
The method for reducing the coherence of the semiconductor laser output light is different.

In the present invention, a semiconductor laser diode t-Kv! is transmitted using a transmission signal superimposed with a self-high frequency. 4. The above-mentioned variable signal, which has passed through a transmission channel such as an optical fiber, is converted into light by a light-receiving element. A gain control signal corresponding to the amplitude of the superimposed high-frequency signal is generated from this received signal, and a variable gain element such as a light receiving element or a variable gain amplifier is controlled by this control signal to perform AGC.

By performing the above-described advanced AGC, there is no need to add a pilot signal to a transmission signal.

In addition, the superimposed S1 frequency generally uses a frequency that is sufficiently high (approximately 300 MHJ or higher) than the transmission signal amplitude.
Second-order high-frequency distortion, third-order intervariable distortion, etc. due to superimposed high frequencies do not have any adverse effect on the transmission quality.

Furthermore, in the conventional system, the pilot signal is at a low level as well as the transmission signal in order to prevent deterioration of transmission signal distortion characteristics.

On the other hand, as shown in FIG. 2(b), the superimposed high frequency is at a high level as well as the transmission signal. (In the figure, a straight line 15 is the current-light conversion characteristic of the semiconductor laser 14). Therefore, it is less affected by noise than the conventional method, and it is easy to perform gentle signal processing.

[Embodiments of the invention]

The present invention will be explained below using examples.

FIG. 3 shows an embodiment of the configuration of the optical receiver of the present invention. The high frequency signal superimposed on the transmission signal shown in FIG. 2(b) is subjected to optical-to-electrical conversion by the light receiving element 2. Next [
1: After being amplified by the amplifier 5, it is separated from the transmission signal by the band pass filter 7.

The separated high frequency signal is rectified, and the gain of the preamplifier 5 or the bias voltage 4 of the light receiving element 2 is controlled by the extracted DC component. According to this embodiment, there is no membership fee for adding a new pilot signal, and the amplitude of the 1 tatami frequency signal is thicker than that of the conventional pilot signal, so it is stable and less susceptible to noise. AGC can be performed (FIG. 3(b)).

FIG. 4 shows another embodiment of the invention. The optical-to-electrical converted signal by the light-receiving element is branched and then output. A part of the signal is input to the frequency V, which is input to the converter 10, mixed with the No. 1g signal of the local oscillator 9, and transmitted at the frequency, converting the superimposed high frequency signal to a low frequency signal. Assuming that the frequency converter 10 consists of a superimposition circuit and a square characteristic circuit, the frequency converter output is as follows.

+N person i genus (ω0+→)t+oos(ω0-ω2)t)
+AIA! (focus (ω1 + ω1) to ten width (ωl - ω2) t
)...(1) However, Ao008ωo1 is the transmission signal 1. A10[ISω! t superimposed high frequency signal, A, 003
ω2t was used as a local oscillation signal. When this output is passed through a filter 7 that passes ω1-ω-value, the output AtI'dXR
(ω!-ω2)t is obtained. This is rectified and smoothed and used as a gain control signal for the preamplifier 5. In this example,
The control signal strength is proportional to A1 and A3. Unlike general pylon AGC, in the method according to the present invention,
The pilot signal amplitude A1 is also larger than the transmission signal amplitude AO.

Also, since A2 can be set arbitrarily large enough, A
1. Two A-wires are sufficiently large and have the characteristic that AGO can be applied without being affected by noise. Furthermore, according to the present %M example, by converting the low frequency to a low frequency, it is possible to avoid the difficulties associated with high frequency circuits.

FIG. 5 shows another embodiment of the invention. In this embodiment, an amplifier 2 sensor 7 photodiode (APD) is used as the light receiving element 2. The multiplication factor M of the APD has a dependence on the bias turtle pressure V as follows: M = M o M t V + M s V ''.The received photocurrent is expressed as ■ = IoOO
The bias voltage of Sωot+J and coSω1tAPD is set to V ””V o +V tcOS”z 1.

However, I(100Sω(I L is transmission number 18, 1loo
sct+mt is a superimposed high frequency signal. The photodetection output current i in this case is: i=MI = (M(+ +M t (Vo + V*Cm ω2
t)+M*(Vo +V*0[ISωi tF)
X (IoQFi% t+I*00sa+10==Cx
I*(KIGIa t + CCl11oζt+CgI
o(00B (ω0+ωz)t-1・C()3(ω0-
ω2n t)+CzIt(008(ωt+ω2)t+(
XIS(Qll-ω2)i)+Csl+(cO8(ωo
+2ωz) to 10Os(ωo 2G'z)tl+Cl1
1(IXIs(ωt+2 GJI)i+oO8(ω1−
2ω, )t ) ・(SQ. Branch this and ω0
The transmission signal CIJ6cosω is transmitted through the bandpass filter 8 of
Take out the OT. On the other hand, the bandpass filter 7 of ω1-ω2
Then, CtIt(X18(ωt ``ritotori'') is output. This 1&:im is smoothed and used as the gain control signal of the preamplifier 5. According to this embodiment, the strength of the gain control amplitude is Hidden is C*It=1 (M Sumire Vz + 2M tVo Vs)
I is proportional to l. Il is larger in this embodiment than in the conventional pylon AGC.

Therefore, even if the amplitude Vz of the APD AC bias 9 is small, the control signal is large f! , it is possible to apply a stable AGO.

In the embodiments shown in FIGS. 4 and 5, the variable gain intensifier 5 is controlled, but AGCt- can also be applied by controlling the bias voltage 41t- of the light receiving element.

Although the above embodiments are concerned with an AGC system for analog optical transmission, it goes without saying that the present invention can also be applied to an AGC system for digital optical transmission.

〔Effect of the invention〕

As explained in the embodiments above, in order to suppress modal noise in analog optical transmission, the high-frequency signal superimposed on the transmission signal is used as a pilot signal for AGC, so the transmission side There is no need to add a pilot signal, the transmission circuit is inexpensive, and it can be used in the dark.

Furthermore, in the conventional pilot AGC, pylon H
Normally, the strength of the 1D and 1D signals was about 10 dB lower. On the other hand, the superimposed high-frequency signal used as a pilot signal in the present invention is at a lower level than the transmission signal, and therefore has the effect of being able to perform stable AGC that is less susceptible to the influence of noise. .

[Brief explanation of drawings]

FIG. 1<a) shows the frequency spectrum of the conventional pilot AGC1(b). FIGS. 2(a) and 2(b) are explanatory diagrams of Koshoha Tei Tatami. FIG. 3(b) shows jI according to the present invention.
Fig. 3 (ω, Fig. 4) shows an embodiment of the present invention. 1... Optical fiber, 3... Load resistance, 4 and 4'
... Light-receiving element bias power supply, 6... Carrier flow smoothing circuit,
7...Band pass filter, 8...Low pass filter, 9...Local oscillator, 10...Frequency converter, 11
...Transmission signal oscillator, 12...Superimposed 1g oscillator,
13... Breath folding 1 Figure 2 ■ (L) (b) ■ 3 Figure (0-) ■ 4 Figure

Claims (1)

[Claims] 1. A high-frequency wave No. 18 having a frequency higher than the frequency of the transmission signal is superimposed on an analog or digital transmission signal, a semiconductor laser diode is modulated with the superimposed signal, and the modulated optical signal is In an optical transmission method in which the optical signal is transmitted through an optical transmission path such as an optical fiber, the optical signal is received by a light receiving element, photoelectric conversion is performed, and the transmitted signal is regenerated and amplified, the above-mentioned A for optical transmission characterized by controlling the gain of a light receiving element or a receiving amplifier circuit
GC method. 2. In the AGC method for optical transmission described in item 1, after converting the frequency of the received high-frequency signal into a low-frequency signal, the gain of the light-receiving element or receiving amplifier circuit is adjusted according to the low-frequency signal scanning width. An AGC method for optical transmission whose main feature is to control