JPH06334631A - Analog optical transmission method and device thereof - Google Patents

Abstract

PURPOSE:To reduce noise and distortion generated by the multiple reflection within a transmission line by converting a multiplexed electric signal into an optical signal by a semiconductor laser via a filter after the signal is converted higher by a prescribed frequency by a mixer and transmitting the signal by an optical filer. CONSTITUTION:The signal where the input signals 101 to 103 of an optical transmitter 3 are modulated by modulation circuits 11 to 13 by carrier frequencies f1 to f3 is outputted as a multiplexed signal A by a multiplexor 5. An input mixer 130 takes product of this signal A and the signal B of the frequency f0 from a local oscillator 150, and a filter 300 extracts a signal C where each f0 is added to f1 to f3 from the mixed signals. A semiconductor laser 10 transmits an optical signal where an optical intensity modulation is performed for the signal C to an optical fiber transmission line 200 and the light receiving circuit 20 of a light receiver 4 converts this optical signal into an electric signal. A mixer 140 takes the product of this electric signal and the frequency f0 from a local oscillator 160, a filter 310 extracts the components of frequencies f1 to f2 and a demodulation circuit 31 performs a demodulation. Thus, noise and distortion due to the multiple reflection within a transmission line is reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser (L) by frequency-division multiplexing a plurality of signals modulated by AM, FM and the like.
The present invention relates to an analog optical transmission method and device for performing optical intensity modulation in D), transmitting the optical signal through an optical fiber, receiving the optical signal, and demodulating.

[0002]

2. Description of the Related Art A block diagram of a conventional analog optical transmission device is shown in FIG. In FIG. 2, 1 is an optical transmitter that transmits an optical signal, 2 is an optical receiver that receives the transmitted optical signal, and 70 is an optical fiber transmission line that transmits the optical signal.
Further, in the optical transmitter 1, 10 is a semiconductor laser for converting an electric signal into a laser beam, 5 is a multiplexer for multiplexing a plurality of electric signals, and 11, 12, 13 have carrier frequencies of f ₁ respectively. , F ₂ , f ₃ modulation circuits, 101 to 10
Reference numeral 3 is an input signal to each of the modulation circuits 11 to 13. On the other hand, in the optical receiver 2, 20 is a light receiving circuit for receiving an optical signal transmitted through the optical fiber transmission line 70 and converting it into an electric signal, 30 is a demodulation circuit for demodulating the converted electric signal, 110 Is an output signal from the demodulation circuit 30. In the optical fiber transmission line 70, 701,
Reference numeral 702 denotes a reflection point existing in the transmission path, and 240 denotes two reflection points 7 of the optical signal output from the semiconductor laser 10.
Reference numeral 250 denotes an optical signal transmitted through 01 and 702, and reference numeral 250 denotes an optical signal multiply reflected between two reflection points 701 and 702.

Next, the operation of the above conventional analog optical transmission device will be described.

Here, the number of input signals to be transmitted is three. First, the input signals 101 to 103 are input to the modulation circuits 11 to 13, respectively, and are modulated by the carrier wave frequencies f ₁ , f ₂ and f ₃ . The modulation method is A
M, FM, etc. Next, the signals modulated by the modulation circuits 11 to 13 are frequency-division multiplexed by the multiplexer 5.
Thereafter, the frequency-division multiplexed signal is transmitted to the semiconductor laser 1
The light intensity is modulated by 0 and converted into an optical signal. The optical signal output from the semiconductor laser 10 is transmitted through the optical fiber transmission line 7
The optical signal emitted to 0 and passed through the optical fiber transmission line 70 enters the optical receiver 2. In the optical receiver 2, the incident optical signal is converted into an electric signal by the light receiving circuit 20,
The converted electric signal is demodulated by the demodulation circuit 30 into one of the three-channel signals and output as the output signal 110.

[0005]

In the above-described optical fiber transmission line 70, a plurality of optical fiber cords may actually be connected in multiple stages. As a method of connecting these optical fiber cords, There are methods such as fusing the wires and using an optical connector.

However, the normal connection point becomes the reflection point as shown in FIG. 2 described above, and if the reflectance of the connection point is large to some extent, the signal light undergoes multiple reflection between the connection points,
There are problems that the SN ratio of the modulated signal after reception deteriorates and distortion increases.

This is described in, for example, [1] Kobayashi et al., Influence of interference noise in analog optical transmission, 1990 IEICE Spring National Convention, B-958, [2] JHAngenent et.a.
l., "Distortion of a multicarrier signal due to opti
cal reflections. ", ECOC91 & IOOC, WeC8-4, pp.569-571,19
91, [3] Domon et al., "Study on distortion due to light reflection and fiber dispersion in optical feeders", IEICE Technical Report, OCS91-79,1
991, [4] Yamamoto et al., Study on distortion caused by multiple reflection in optical fiber, 1992 IEICE Spring Conference, B-104
It is shown at 0.

The causes of the deterioration of the SN ratio and the increase of distortion due to the multiple reflections in the optical fiber transmission path are as follows.

When the semiconductor laser 10 is directly intensity-modulated,
The output light from the semiconductor laser 10 is also subjected to frequency modulation in the light region at the same time, and the spectrum of the optical signal has a large sideband and is widely generated. Now, as shown in FIG. 2, it is assumed that there are two reflection points on the optical fiber transmission line 70. This reflection point 70
Reference numerals 1 and 702 are, for example, connection points of optical fiber cords. At this time, the optical signal incident on the light receiving circuit 20 is composed of an optical signal (E1) 240 transmitted through the two reflection points 701 and 702 and an optical signal (E2) 250 subjected to multiple reflection. The beat of these two optical signals is detected by the light receiving circuit 20 as an electric signal. This beat is noise or distortion due to multiple reflection. Here, the magnitude of the sideband of the optical signal determines the magnitude of noise and distortion due to multiple reflection. In addition, two reflection points 701 and 702
When the interval is shorter than the coherent length, distortion appears remarkably, and when the interval is long, noise increases.

In view of the above problems in conventional optical transmission, the present invention provides an analog optical transmission method and apparatus capable of reducing noise and distortion caused by multiple reflection in an optical fiber transmission line. The purpose is.

[0011]

According to the present invention of claim 1, a plurality of electric signals having different frequencies are multiplexed, and the multiplexed electric signal is increased in frequency by a predetermined frequency. The converted electrical signal is converted into an optical signal by a semiconductor laser, the optical converted optical signal is transmitted through an optical fiber, the received optical signal is converted into an electric signal, and the converted electric signal is converted. Is an analog optical transmission method for converting so as to be lowered by a predetermined frequency.

According to the present invention of claim 2, a plurality of modulation signals having different frequencies are frequency-division-multiplexed into a multiplexed signal,
Transmission mixing means for mixing the transmission station-originated signals having a predetermined frequency f ₀ , and transmission filter means for extracting the sum signal of the frequency components of the sum of the frequency of the multiplex signal and the frequency of the transmission station-originated signal from the mixed signal. And an optical output means for outputting an optical signal whose optical intensity is modulated according to the extracted sum signal to an optical transmission line.

According to a fourth aspect of the present invention, there is provided light receiving conversion means for receiving an optical signal output from the analog optical transmitter of the second aspect and transmitted through an optical transmission line, and converting the optical signal into an electric signal. Reception mixing means for mixing the converted electric signal with a reception station-originated signal having substantially the same frequency as the predetermined frequency f _0, and a frequency of a difference between the frequency of the mixed signal and the frequency of the reception-station-originated signal. It is an analog optical receiver provided with a receiving filter means for extracting a component signal and a demodulating means for demodulating the extracted signal.

[0014]

According to the present invention, the multiplexed electric signal is converted so that its frequency is increased by a predetermined frequency, and the converted electric signal is converted into an optical signal by the semiconductor laser. The modulation index becomes small, and distortion and noise caused by multiple reflections in the optical fiber can be reduced.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings showing its embodiments.

FIG. 1 is a block diagram of an analog optical transmission device according to an embodiment of the present invention. That is, the analog optical transmission device is provided with an optical transmitter 3 that transmits an optical signal and an optical receiver 4 that receives the transmitted optical signal. The optical transmitter 3 and the optical receiver 4 transmit the optical signal. They are connected by an optical fiber transmission line 200 for transmission.

One optical transmitter 3 has a plurality (here, 3
Modulation circuits 11, 12, and 13 for modulating the input signals 101, 102, and 103 with different carrier frequencies f ₁ , f ₂ , and f ₃ , respectively. It is connected to a multiplexer 5 that frequency-division-multiplexes the modulated signal thus generated. The multiplexer 5
Is connected to a mixer 130 that multiplies (called mixing) the signal B output from a local oscillator 150 that oscillates a signal of frequency f ₀ and the multiplexed signal A, and the mixer 130 is mixed. It is connected to a filter 300 that extracts a signal C having a sum frequency of the frequency of the signal B and the frequency of the signal A from the signal. The filter 300 is connected to the semiconductor laser 10 that converts the signal C into an optical signal, and the semiconductor laser 10 outputs an optical signal whose light intensity is modulated. The above-mentioned mixer 130 constitutes the transmission mixing means,
The filter 300 constitutes transmission filter means, and the semiconductor laser 10 constitutes light output means.

The other optical receiver 4 is provided with a light receiving circuit 20 which receives an optical signal transmitted through the optical fiber transmission line 200 and converts the optical signal into an electric signal. , Is connected to a mixer 140 that takes the product of a converted electrical signal and a local signal output from a local oscillator 160 that oscillates a signal of frequency f ₀ . The mixer 140 is connected to a filter 310 that extracts a signal of a difference between the local signal and the converted electric signal in the mixed signal, and the filter 310 is connected to a demodulation circuit 31 that demodulates the extracted signal. It is connected. The above-mentioned light receiving circuit 20 constitutes a light receiving conversion means, the mixer 140 constitutes a reception mixing means, the filter 310 constitutes a reception filter means, and the demodulation circuit 31 constitutes a demodulation means.

Next, the operation of the analog optical transmission device of the above embodiment will be described.

First, in the optical transmitter 3, the input signal 10
1, 102, 103 are modulation circuits 11, 12, respectively.
13 and the carrier frequencies f ₁ , f ₂ , and f, respectively.
Modulated by ₃ . The modulation method is AM, FM, or the like. The carrier frequencies f ₁ , f ₂ , f ₃ are, for example, several tens of MHz to several GHz, and the respective frequencies are different from each other. Next, the modulation signals output from the modulation circuits 11 to 13 are frequency division multiplexed by the multiplexer 5. This signal is referred to as signal A. On the other hand, the local oscillator 150
Outputs a signal B having a frequency f ₀ , and the signals B and A are input to the mixer 130. The mixer 130 takes the product of the signal A and the signal B, and the mixed signal is input to the filter 300. The filter 300 extracts the signal C having the frequency component of the sum of the frequencies of the signals A and B from the output signal of the mixer 130. Next, the extracted signal C is input to the semiconductor laser 10 and
From the optical fiber transmission line 200.
Sent to.

Next, the optical signal propagating through the optical fiber transmission line 200 is input to the light receiving circuit 20 of the optical receiver 4.
The light receiving circuit 20 converts the input optical signal into an electric signal and outputs the electric signal to the mixer 140. On the other hand, the signal of frequency f ₀ is output from the local oscillator 140 to the mixer 140.
Is output to. The mixer 140 takes the product of the electric signal input from the light receiving circuit 20 and the signal of frequency f ₀ , and inputs the product to the filter 310. The filter 310 extracts the signal of the frequency component of the difference between the frequency of the electrical signal and the signal of the frequency f ₀ , and the extracted signal is input to the demodulation circuit 31. The demodulation circuit 31 demodulates the frequency division multiplexed signal and outputs it as the output signal 110.

As described above, by making the signal input to the semiconductor laser 10 a signal having a frequency higher than that of the modulation signal to reduce the modulation index of light, the sideband of the optical signal is suppressed, and noise and distortion due to multiple reflections are suppressed. Can be reduced.

At this time, the carrier frequency of the signal input to the semiconductor laser 10 has an upper limit. That is, the strain generated in the semiconductor laser 10 increases as the carrier frequency increases due to the effect of the relaxation oscillation frequency. Therefore, the carrier frequency of the signal input to the semiconductor laser 10 must be set lower than the relaxation oscillation frequency so that the distortion generated in the semiconductor laser 10 does not become a problem.

Next, the principle of the present invention will be described.

Now, in a semiconductor laser, the carrier frequency is f
_{It is} assumed that N frequency division multiplexed signals _{1 to} f _N are input.
The electric field of the signal light at this time is given by the following equation (Equation 1).

[0026]

[Equation 1]

However,

[0028]

Β _i = (ΔF / ΔI) (I _b −I _t ) m / f _i where m is the modulation degree per carrier, ψ _i is the phase of the i-th carrier, and ν is when there is no modulation. optical frequency, [Delta] F / [Delta] I of the semiconductor laser shows a chirp amount, I _b, I _t are each bias current and the threshold current of the semiconductor laser, beta _i is the frequency modulation index of the light per unit current of the semiconductor laser.

When there are two reflection points on the optical fiber transmission line, the frequency of distortion caused by multiple reflection is f ₁
It is determined by the combination of ~ f _N and is given by the following equation (Equation 3).

[0030]

F _k = | n ₁ f ₁ + n ₂ f ₂ + ... + n _N f _N | (n ₁
... n _N : integer) The distortion caused by multiple reflection at the frequency f _k is given by the following equation (Equation 4) when normalized by the carrier signal.

[0031]

## EQU4 ## D (f _k ) = (2ξ / m) ² R ₁ R ₂ exp (−2πΔντ) {h (f _k )} ² where

[0032]

[Equation 5]

[0033]

[Equation 6] B _i = 2β _i · sin (πf _i τ)

[0034]

Τ = 2L / v where ξ is the coupling coefficient of the polarization plane of the transmitted light and the reflected light, R ₁ and R ₂ are the reflectances at the reflection points in the fiber, and Δν is the spectral line width of the semiconductor laser. , Τ is the time difference between the transmitted light and the reflected light, L is the distance between the reflection points, and v is the speed of light in the fiber.

Further, the noise caused by multiple reflection is converted into the relative noise intensity of the semiconductor laser,

[0036]

[Equation 8]

However,

[0038]

Equation 9] _{S (f, f k) =} (Δν / π) / {(f-f k) 2 + (Δν) 2} Here, assuming the spectral distribution Lorentz distribution of the semiconductor laser beam at the time of non-modulation is doing.

As can be seen from (Equation 4) and (Equation 8), it is effective to reduce the value of h (f) near the carrier frequency in order to reduce distortion and noise due to multiple reflection near the carrier frequency. is there. In order to reduce h (f) near the number of carriers, the absolute value of B _i or β _i in (Equation 6) may be decreased. The FM modulation efficiency ΔF / ΔI and the threshold current I _t of the semiconductor laser are values specific to the laser. The modulation degree m and the bias current I _b are inevitably determined from the required transmission performance. The delay time τ of transmitted light and reflected light takes various values depending on the system. Therefore, with the configuration described above,
By increasing the carrier frequency f _i , the modulation index β _{i of} light is reduced, and distortion and noise due to multiple reflection are reduced. In other words, this means that the minimum frequency of the signal C, which is the sum of the maximum frequency deviation given by the modulation index × carrier frequency and the modulated signal A and the signal B of the frequency f ₀ output from the local oscillator 150 ( In FIG. 1, f ₀ +
f _1, the ratio of the smallest) of _{f 0 + f 2, f 0} + f 3 is that it may be set to be sufficiently smaller than 1.

In the above embodiment, the number of modulated signals having different frequencies is three, but the number is not limited to this.

Further, in the above embodiment, in order to convert the frequency of the multiplexed electric signal to be high, the two signals are mixed and the signal of the sum frequency of the two signals is extracted. The method is not limited to this, and another method may be used as long as the relationship between the multiplexed electric signals can be maintained as it is.

[0042]

As is apparent from the above description, according to the present invention, a multiplexed electric signal is converted so that its frequency is increased by a predetermined frequency, and the converted electric signal is converted into a semiconductor laser. Since the optical signal is converted into an optical signal by the optical fiber and the optical signal converted by the optical fiber is transmitted, there is an advantage that noise and distortion caused by multiple reflection in the optical fiber transmission line can be reduced.

[Brief description of drawings]

FIG. 1 is a block diagram of an analog optical transmission device according to an embodiment of the present invention.

FIG. 2 is a block diagram of a conventional analog optical transmission device.

[Explanation of symbols]

 1, 3 Optical transmitter 2, 4 Optical receiver 5 Multiplexer 10 Semiconductor laser 11, 12, 13 Modulation circuit 20 Light receiving circuit 30, 31 Demodulation circuit 70, 200 Optical fiber transmission line 130, 140 Mixer 150, 160 Local oscillator 300, 310 filters

Continuation of front page (51) Int.Cl. ⁵ Identification number Office reference number FI technical display location 9372-5K H04B 9/00 Y

Claims (4)

[Claims] 1. A semiconductor laser in which a plurality of electric signals having different frequencies are multiplexed, the multiplexed electric signal is converted so that the frequency is increased by a predetermined frequency, and the converted electric signal is converted into a semiconductor laser. Optical signal is converted into an optical signal by an optical fiber, the optical signal converted by the optical fiber is transmitted, the received optical signal is converted into an electric signal, and the converted electric signal is converted to be lower by the predetermined frequency. An analog optical transmission method characterized by: 2. A transmission mixing means for mixing a transmission station-originated signal of a predetermined frequency f ₀ with a multiplexed signal obtained by frequency division multiplexing a plurality of modulated signals having different frequencies, and the mixed signal from the mixed signal. Transmitting filter means for extracting the sum signal of the frequency components of the sum of the frequency and the frequency of the transmitting station signal, and light for outputting an optical signal whose optical intensity is modulated according to the extracted sum signal to the optical transmission line. An analog optical transmitter comprising an output means. 3. The ratio of the maximum frequency shift amount of the oscillation light frequency of the optical output means to the minimum frequency of the sum signal is sufficiently smaller than 1, and the predetermined frequency f ₀ of the transmission station-originated signal is equal to the sum signal. The analog optical transmitter according to claim 2, wherein the highest frequency is set to be equal to or lower than the relaxation oscillation frequency of the light output means. 4. Light receiving conversion means for receiving an optical signal output from the analog optical transmitter of claim 2 and transmitted through the optical transmission line, and converting the optical signal into an electrical signal.
Reception mixing means for mixing the converted electric signal with a reception station-originated signal having substantially the same frequency as the predetermined frequency f ₀ , and a difference between the frequency of the mixed signal and the frequency of the reception-station-originated signal. An analog optical receiver comprising: reception filter means for extracting a signal of a frequency component; and demodulation means for demodulating the extracted signal.