JPH02170732A - Optical communication system - Google Patents

Abstract

PURPOSE:To completely suppress modulating distortion generated at the time of intensity modulation by utilizing an optical feed forward method, and using the combination between a delay mechanism, which has not so high delay accuracy but has a broad regulating range, and a delay mechanism, which has the narrow regulating range and the high delay accuracy, for the regulation of the delay time difference between a main signal and a correcting signal. CONSTITUTION:The delay mechanism consists of two delay mechanisms 16a and 16b, one of them is the mechanism 16a which has not so high delay accuracy but has the broad regulating range, and the other is the mechanism 16b which his the narrow regulating range and the high delay accuracy. The rough regulation is executed by one delay mechanism 16a, fine regulation is executed by the other delay mechanism 16b, and a necessary delay time can be correctly obtained over the broad range. Thus, the optical communication system, which attains the complete suppression of the modulating distortion, can be obtained.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a communication system and a communication method, and in particular, it is a system that directly intensity-modulates and transmits an analog signal by optical communication, which uses feedforward control to The present invention relates to an optical communication system that suppresses modulation distortion in communication signals.

[Prior art]

With the development of optical fiber communication networks, optical cable television (hereinafter referred to as optical CATV), which transmits video and audio information via optical fibers, has been considered instead of coaxial cables.

By using optical fibers in this way, it is possible to dramatically increase the amount of transmitted information, increase the number of transmission channels, and further extend the transmission distance.

As future CATV transmission methods, a wideband frequency modulation method (hereinafter referred to as FM method) and a vestigial sideband amplitude modulation method (hereinafter referred to as VSB/AM method) are known.

In the FM system, the signal-to-noise ratio (SN ratio) is large and the intermodulation distortion (INTEI?MO[)tJLATIONdlsi
oslon) has the advantage of being small, but
When injecting an optical signal into an optical fiber, A changes an electrical amplitude modulation signal (hereinafter referred to as an AM signal) into an FM signal.
It requires an M/FM converter, and further requires an FM/AM converter that converts the optical FM signal transmitted through the optical fiber into an electrical AM signal after receiving the light. Therefore, the entire communication system becomes expensive.

On the other hand, the VSB/AM method does not require the above-mentioned converter, resulting in a communication system that is inexpensive and has a simple configuration. In this VSB/AM system, an optical signal is generated by directly modulating the current injected into a light emitting element used for optical communication, such as a semiconductor laser. However, since the input current-optical output characteristics of light-emitting devices such as semiconductor lasers have nonlinearity, the optical output waveform contains high-order distortion and the CN ratio (equivalent to the SN ratio) deteriorates. There is a problem with this.

Specifically, the output optical power does not increase in proportion to the current injected into the semiconductor laser, and a modulation distortion component proportional to the square of the injected current is included, so information is transmitted using a carrier wave with a frequency of [1]. In this case, a frequency component other than this carrier frequency, for example 2
Frequency components such as f, 3f2, f10f, 2f - f2 are generated, and if other carrier waves exist in the vicinity of these frequencies, this modulation distortion component acts on the other carrier waves as a noise component and the CN ratio increases. will deteriorate. Therefore, the following restrictions are set when transmitting image information, etc.

108N oglG (amplitude of carrier wave/amplitude of modulation distortion)
≧58 (dB)...■ In order to comply with such a restriction, it is necessary to limit the number of channels to be transmitted.

As a method of preventing this deterioration of the CN ratio, there is a method of applying a constant current bias to a semiconductor laser serving as a light source and subjecting the emitted light to external modulation using an amplitude modulator that exhibits linear modulation characteristics.
Volume 12 of 1984 Electronic Telecommunications Day,
A paper entitled "Analog transmission of TV channels over optical fibers with nonlinear correction by adjustable feedforward" published by Frankart, Jie, Bi, et al. in issue 9 (-ANALOG TRANSMISSION)
OF TV-CHANNELsON 0PTICAL
['1BER, revenge ITI (N0N-LINIEARI
TIESCORRECTION BY REGULAT
ED PEEDPOνARDPRANKART, J, P
etal, REV, l(, F, ELECTRON
ECOMMIJNICATION VOL, 12 N
The so-called optical feed forward (hereinafter referred to as optical FF) method shown in 2007, 91H4) is known. In this optical feedforward method, the electrical signal to be transmitted is applied to a semiconductor laser, a part of the main signal light that has passed through the semiconductor laser is compared with the original electrical signal, and the obtained correction signal is added to the main signal. In this way, so-called feedforward control is performed to suppress modulation distortion generated in the semiconductor laser.

[Problem to be solved by the invention]

If the above optical feedforward method is configured as designed,
Theoretically, it is possible to minimize (suppress) modulation distortion. However, when transmitting optical and electrical signals, the signal propagation time varies depending on the transmission path of each signal and the signal processing mechanism inserted therein. This difference in signal propagation must be taken into consideration.Especially, in the optical feedforward method shown in the above literature, in which a correction signal is added to the main signal to remove modulation distortion, this difference in signal propagation must be taken into account. When a signal delay occurs due to a difference in signal propagation time, a problem arises in that modulation distortion components cannot be suppressed sufficiently, or conversely, modulation distortion components increase.

Therefore, in the optical feedforward method described above, in order to adjust the signal delay, it has been considered to provide an electrical delay circuit immediately before the signal merging of the transmission signals to perform relative adjustment of the propagation time. However, this signal delay has not been sufficiently considered, and in particular, the signal delay range and delay accuracy have not been considered at all. Therefore,
Modulation distortion could not be suppressed sufficiently.

Furthermore, since signal delays were adjusted on the receiving side, the structure of the receiving station became large, making it difficult to downsize the receiving station.

Therefore, in order to solve the above-mentioned problems, the present invention provides an optical communication system that employs an intensity modulation method with a delay mechanism with low delay accuracy and a delay mechanism with high delay accuracy to completely eliminate modulation distortion. The purpose is to provide an optical communication system that enables suppression.

[Means to solve the problem]

In order to achieve the above object, the optical communication system of the present invention is an optical communication system that transmits a signal by directly intensity modulating an analog signal, and branches the analog signal to be transmitted into a first signal and a second signal. a first branching means, a main light emitting means for intensity modulating the first signal and emitting an optical signal, and a second branching means for branching the optical signal emitted from the main light emitting means into first and second optical signals. an optical signal delay mechanism connected to the second branching means and capable of delaying the first optical signal with a first delay accuracy; and a first optical signal delaying mechanism that generates an electrical signal in response to the second optical signal. a light receiving means, a comparing means for comparing the electric signal from the first light receiving means and the second electric signal and outputting the comparison result, and a comparison means for comparing the electric signal from the first light receiving means and outputting the comparison result. auxiliary light emitting means for emitting light;
a second light receiving means for generating an electrical signal in response to an optical signal from the optical signal delay mechanism; a third light receiving means for generating an electrical signal in response to an optical signal emitted by the auxiliary light emitting means; an electrical signal delay mechanism for delaying the electrical signal from the second light receiving means with a delay accuracy higher than the first delay precision; and a merging and flowing means for superimposing the electrical signals from the third light receiving means and the electrical signal delaying mechanism. It is characterized by suppressing modulation distortion contained in the transmission signal during intensity modulation.

[Effect]

The optical communication system of the present invention is configured as described above, employs an optical feedforward method, and uses a delay mechanism with a wide adjustment range but low delay accuracy to reduce the signal delay between the main signal and the correction signal. By providing a delay mechanism with a narrow width but high delay accuracy, highly accurate signal delay is achieved and modulation distortion that occurs during intensity modulation can be suppressed.

〔Example〕

Embodiments according to the present invention will be described below with reference to the drawings.

Elements with the same reference numerals have the same functions, so duplicate explanations will be omitted.

FIG. 1 shows a schematic configuration of an example of an optical communication system according to the present invention.

The optical communication system shown in Fig. 1 basically consists of a main semiconductor laser 2 that directly intensity-modulates an analog signal to be transmitted inputted from an input section 1 into an optical signal, and a main semiconductor laser 2 that directly intensity-modulates the intensity-modulated optical signal for a predetermined period of time. An optical fiber delay line 16a having a predetermined length for delay, a main optical fiber line 4 for transmitting the delayed optical signal, a main light receiving element 5 for demodulating the intensity modulated optical signal into an electrical signal, and a main light receiving element. 5
Electrical signal delay mechanism 1 that delays the electrical signal demodulated by
6b. The optical fiber delay line 16a is configured to have a maximum adjustment range of 5 to 200 nsec and a delay accuracy of ±0, 5 n5ec. An optical fiber having a certain length is selected and inserted, or an optical fiber of a predetermined length is cut and inserted. The electrical signal delay mechanism 16b is constructed of an electrical delay circuit so as to have a delay adjustment function of 0.5 psec in terms of adjustment accuracy with a maximum adjustment range of 1 to 2 n5ec. In this way, 2
Signal delay mechanisms 16a and 116b with different delay accuracies are provided at each stage, and the precision of the delay time is increased while adjusting over a wide range. Generally, it is very difficult to form a delay mechanism with high delay accuracy over a wide range. Therefore, in the present invention, by adjusting the signal delay with a delay mechanism with low delay accuracy in a rough range and with a delay mechanism with high delay accuracy in a minute range, high delay can be achieved over a wide range. This realizes a highly accurate delay mechanism.

Furthermore, this optical communication system includes a branch line 30 that branches part of the analog signal to be transmitted, and a branch line 31 that branches part of the optical signal intensity-modulated by the semiconductor laser 2.
is provided. A semiconductor laser 2 is connected to the branch line 31.
e.g., one of the intensity modulated optical signals
A splitter 3 that branches 1/2 of the optical signal to the branch line 31 side and the other 1/2 to the main optical fiber line 4, a first sub-light receiving element 9 for demodulating the branched optical signal into an electrical signal, and An amplifier 10 is provided which amplifies the demodulated signal to a predetermined amplification factor and transmits it to the branch line 30 described above. At the input end of this branch line 30, there is provided a branch arithmetic circuit 1a that performs a branch operation on the human input signal, and the input signal is branched in the branch arithmetic circuit 1a and flows to the branch line 30.

Furthermore, a signal delay section 6 is provided in the middle of this branch line 30 to match the phase of the signal transmitted from the branch line 31. Further, a subtracter 32 is provided at the junction of the branch line 30 and the branch line 31 to calculate the difference between the signals transmitted from the respective branch lines 30 and 31.

A feedforward line 33 is connected to the output terminal of this subtracter 32, and this feedforward line 33 is connected to an amplifier 11 that amplifies the transmitted electric signal, and the amplified electric signal is intensity-modulated into an optical signal. A sub-semiconductor laser 12 that transmits a sub-semiconductor laser 12, a sub-optical fiber line 13 that transmits an optical signal emitted by the sub-semiconductor laser 12, and a second sub-light receiving element 14 that demodulates the optical signal transmitted by the sub-optical fiber line 13 into an electrical signal. It is. Further, an attenuator 15 is provided to adjust the amplitude of the electrical signal demodulated by the second sub-light receiving element 14.

One input terminal of an adder 17 is connected to the other end of this feedforward line 33.
The other input terminal of the electrical delay circuit 16 described earlier is connected to the other input terminal of the
An output line 34 is connected thereto. The output terminal of this adder 17 is connected to the receiving section 2 of this optical communication system.
1 is connected.

With this configuration, when the main light emitting element 2 performs intensity modulation, the modulation distortion contained in the intensity modulation signal is removed by the correction signal transmitted through the sub optical fiber line 13.

Here, if the phase of this correction signal differs from the phase of the main signal due to signal delay or the like, modulation distortion cannot be eliminated. Therefore, it is necessary to construct and insert a signal delay mechanism with high delay precision and a wide delay range to completely eliminate such signal delays. In the above embodiment, two delay mechanisms 16a and 16b are inserted to enable highly accurate delay over a wide range.

The principle of the optical feedforward method of the above embodiment is described in detail in the above-mentioned literature, so a detailed explanation will be omitted.

Hereinafter, a model system for studying the delay adjustment range and delay accuracy of the delay mechanisms 6.16a and 16b used in the optical communication system of the above embodiment is shown in Fig. 2, and the discussion will be made with reference to this figure. .

Among the constituent elements shown in FIG. 2, those having the same reference numerals as the constituent elements in FIG. 1 are equivalent to those shown in FIG. And the delay mechanism 16a of the above embodiment
, 16b are collectively represented as the delay mechanism 16.

First, let S (t) be the time function of the human input signal, and let T be the delay time between input and output signals, then the output signal can be expressed as Z(t+T).

Here, the optical outputs 01(x), 02 for the input terminals X of the main semiconductor laser 2 and the sub semiconductor laser 2 are
The relationships of (X) are assumed as follows.

0 (x) = P (x I tht )−
Q (x I tht) ■ 0 (x) =P (x-1th2)10
(x I th2) (Here, P , P SQ , Q are characteristic parameters, 1th and 1th2 are threshold current values) ■ Also, the branching ratio of the splitter 3 is set to 1 = 1° Main light receiving element 5, first sub The output Y for each optical input X of the light receiving element 9 and the second sub light receiving element 10 is expressed as Y
-rx, Y-rlx.

Y-r x and also the main optical fiber line 4,
The attenuation factors of the sub-optical fiber line 13 are fl and T2, respectively.
Assume that

Furthermore, assuming that the delay time of the delay mechanism 5 is τ and the delay time of the delay mechanism 16 is τ2, the output Y of the amplifier 10 is Y4-blx with respect to the input X, and the output Y of the attenuator 15 is Y with respect to the input X. −b 2 X, the human signal S (
t), the main semiconductor laser 2 at the branch point 1
Assuming that an electric signal of 1 (t)■ flows to I + on the side and 2 (t) to I + on the branch line 30 side, the function Z (t + T) of the output signal Z is
Find each constant b 1b2, k Sk SP SP2, l
1 2 1f % f
2S rls rls T2, "old 11 with 3,
■ b −1/(p Xr ), b −”(flx
P.

l 1 1 2 Xr )/(f xp Xr Xr ), k
2- has a relationship with 1/2 and output function Z(t+T)
When calculating, Z (10T) = (f xp ”2+r1)X(
1-1th +k 5(t-Δτ1))(f x
P x Q x r +P 2 ÷ “1) ×i
f −1th −(1−1th1)/212−4−
(f Xk xP Xr2÷21 ×(S(t-
Δτ) −3(t) 1+ (f
X Q X r 2 + 21 Xll [T5(t)l −1s(t−Δτ ))2]・・
・It becomes ■.

In the above formula ■, Δτ −T −(T +T2)Δτ −T
-(T2+T5 TMmTl+T2+T5 T1; Delay time from branch arithmetic circuit 1a to branch 3, T2: Delay time from branch 3 to subtracter 32, T3;
Delay time from the output part of branch calculation circuit 1 to subtracter 32, T4: Delay time from branch 3 to adder 17, T:
The delay time from the subtracter 32 to the adder 17 is as follows.

Here, when we investigated changes in the output signal with respect to changes in the delay times Δτ and ΔT2 of the delay mechanism 6.16, we were able to obtain the results shown in the table shown in FIG.

As can be seen from this table, the allowable delay accuracy of the delay mechanism 16 is 1 gpsec, and therefore it has been found that adjustment must be made within this delay accuracy range.

On the other hand, when considering the adjustment delay range of the delay mechanism 6.16, it is as follows.

First, in the delay mechanism 6, it is necessary to delay the signal coming through the branch line 30 by a delay time corresponding to the drive delay time of the main light emitting element 2, the response time of the first sub light receiving element 9 and the amplifier 10, etc. However, the delay time is at most 2n
It is about sec.

On the other hand, in the delay mechanism 16, the response time of the adder 8, the delay time due to the response time of the amplifier 11, the delay time due to the response time of the sub-semiconductor laser 12,
, the delay time due to the response time of the second sub light receiving element 14 and the delay time due to the response time of the attenuator 15, and the relative signal between the main optical fiber line 4 and the sub optical fiber line 13. The signal must be delayed by the total delay time. Of this total delay time, the largest one is the relative signal delay time between the main optical fiber line 4 and the sub-optical fiber line 13. Although it depends on the signal transmission method, for example, when transmitting signals over a distance of about 20 km, Even if two optical fibers in the same optical fiber cable are used, there will be a difference in transmission time of about 200 nsec at the maximum due to the difference in the twisting ratio, and a difference in transmission time of about several n5ee at the maximum when optical signals of different wavelengths are used.

From the above study, the maximum adjustment range for the delay mechanism 6 is 2 ns.
A delay mechanism with performance of ec and delay accuracy of ±130 psec is required. It is easy for those skilled in the art to construct a delay mechanism with such performance using an electrical delay circuit.

On the other hand, the maximum adjustment range of the delay mechanism 16 is 200n.
sec, delay accuracy of ±8 psec is required. It is difficult to realize a delay mechanism with such a wide range and high precision by simply constructing a conventional electrical delay circuit with multiple stages of tl.

Therefore, based on the results of the above study, the present invention has a delay mechanism 16.
is composed of two delay mechanisms 16a and 16b, one of which is mechanism 16a which does not have very high delay accuracy but has a wide adjustment range, and the other mechanism 16b which has a small adjustment range but high delay accuracy. With this configuration, rough adjustment is performed by one delay mechanism 16a, and fine, 1! The adjustment is performed by the other delay mechanism 16b, making it possible to accurately set the required delay time over a wide range.

An electrical delay circuit may be used as the delay mechanism for making the general adjustment, but if the optical signal is to be delayed, there is also a method using an optical fiber as shown in the embodiment described above. In the case of optical fiber, a signal delay of 200 nsec corresponds to the time it takes for an optical signal to travel through an optical fiber that is approximately 41 m long. Therefore, a delay time of 0.5 nsec corresponds to the time required for the optical fiber to pass through approximately 0.1 m (10 cm) of optical fiber. Then, several tens of meters of optical fiber can be connected to ±0
．． Since it is extremely easy to measure and cut out with an accuracy of 1 m, it is possible and simple to construct the delay mechanism (corresponding to the optical fiber delay line 16a in the above embodiment) with an optical fiber for making approximate adjustments. be. Furthermore, in the case of using an optical fiber, there is an advantage that the delay time can be adjusted within the required accuracy extremely easily by adjusting the delay time using an optical pulse tester as described later. A delay mechanism (corresponding to the electrical delay circuit 16b in the embodiment) that performs fine adjustment can be easily constructed using an electrical circuit.

Here, if the general delay mechanism is constructed by winding a long optical fiber or electric wire into a coil, or if it is constructed by a multistage electric delay circuit, it is difficult to miniaturize it. Therefore, by providing it on the transmitting station side as in the above embodiment, the receiving station can be made smaller. In this case, the delay mechanism for fine adjustment is configured with an electric circuit as in the above embodiment, and the fine adjustment is made just before the input to the adder 17 in the receiving station, thereby reducing the size and height of the receiving station. Precise adjustment becomes possible.

An optical communication system according to the above embodiment was prototyped with the following specifications.

The main semiconductor laser 2 has an oscillation wavelength of 1308 nm.
The sub-semiconductor laser 12 is an InGaAsP Fabry-Perot semiconductor laser with an oscillation wavelength of 1314 nm, the main light-receiving element 5, and the first and second sub-light-receiving elements 9.14 are InGaAs-PIN photodiodes. , the optical fiber line 4.13 has a core diameter of 9 μm and a cladding diameter of 125
Length of single mode optical fiber for 1.3 μm band: 15 μm
km was used. And the transmission signal is 40
Channel (Channel: 4~12.M5~M10, S
24 to 833) were synthesized and used.

Then, the delay adjustment between the main signal transmitted through the main optical fiber line 4 and the correction signal transmitted through the sub optical fiber line 13 was performed as follows.

First, when a pulse signal is input from the input section 1, the time difference between the optical signal input to the main optical fiber line 4 and the optical signal input to the sub optical fiber line 13 is measured.D
I (2.8 nsec) was measured. Next, the input end of the main optical fiber line 4 and the sub optical fiber line 13
An optical pulse signal with a wavelength of 1310 nm is transmitted from the input end of the
DR (pulsed light tester) is used to measure the optical path length to the Fresnel reflection section at the output end of the main optical fiber line 4 and the respective output and inner ends of the sub optical fiber line, and The delay time D2 (30.8 nsec) corresponding to the relative difference between The delay time D3 (0.2 nsec) was calculated. Then, the sum of these delay times D + D 2 +
D 3-33-8 ■ n5ec is the delay time per 1 m of optical fiber F-4,8
Divide by 6nsec and calculate the required optical fiber delay line 16a.
Find the length of. The length of the optical fiber delay line 16a was 6.95 m.

An optical fiber of this length was cut out and connected and inserted into the input end of the main optical fiber line 4 as the optical fiber delay line 16a shown in FIG. After that connection insertion, adder 17
When the optical pulse signal was applied to the input section 1 again before the , and the delay time difference between the main signal and the correction signal was measured, it was 0.8n.
sec, and this delay time was readjusted using the electrical delay circuit 16b. When modulation distortion was measured in this state, it was confirmed that modulation distortion could be suppressed by 18.5 dB compared to the case where the feedforward method was not used.

The present invention is not limited to the above embodiments, and various modifications may be made.

For example, in the above embodiment, the main optical fiber line and the sub optical fiber line are separate optical fiber lines for transmitting the main signal and the correction signal, but the optical fiber line may be shared. . In this case, the optical signals of the main signal and the correction signal are made different, and each signal is combined by a multiplexer and introduced into the optical fiber line, while the transmitted combined signal is demultiplexed by a demultiplexer. After that, the demultiplexed optical signals may be converted into electrical signals and then combined.

〔Effect of the invention〕

In the optical communication system of the present invention, as described above, when suppressing modulation distortion using the optical feedforward method in a system that transmits analog signals using an intensity modulation method, the delay time difference between the main signal and the correction signal is By using a combination of a delay mechanism that does not have very high delay accuracy but a wide adjustment range and a delay mechanism that has a small adjustment range but has high delay accuracy, it is possible to achieve a wide range of delay time control with high accuracy. , modulation distortion that occurs during intensity modulation can be kept extremely small at all times.

Therefore, it is effective for optical transmission of image signals such as CATV.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram of an optical communication system according to the present invention;
Figure 2 shows a model system used when examining the signal delay time in the optical communication system shown in Figure 1, and Tx3 shows a table of the results of the study of the model system shown in Figure 2. It is a diagram. DESCRIPTION OF SYMBOLS 1... Input part, 1a... Branch calculation circuit, 2... Main semiconductor laser, 3... Brancher, 4... Main optical fiber line, 5... Main light receiving element, 6.10 .11... Amplifier, 9... First sub light receiving element, 12... Sub semiconductor laser, 13... Sub optical fiber line, 14... Second sub light receiving element, 15... Attenuator , 16... Delay mechanism, 16a... Optical fiber delay line, 16b... Electric delay circuit. Patent applicant: Sumitomo Electric Industries, Ltd.

Claims (1)

[Claims] 1. In an optical communication system that transmits a signal by directly intensity modulating an analog signal, the first branching means branches the analog signal to be transmitted into first and second signals; a main light emitting means for intensity modulating the signal of the main light emitting means and emitting an optical signal; a second branching means for branching the optical signal emitted from the main light emitting means into first and second optical signals; and a second branching means connected to the second branching means. an optical signal delay mechanism that is capable of delaying the first optical signal with a first delay accuracy; a first light receiving means that generates an electrical signal in response to the second optical signal; Comparing means for comparing the electrical signal from the light receiving means and the second electrical signal and outputting the comparison result; auxiliary light emitting means for emitting light in response to the comparative electrical signal obtained from the comparing means; a second light receiving means that generates an electrical signal in response to an optical signal from the signal delay mechanism; a third light receiving means that generates an electrical signal in response to an optical signal emitted by the auxiliary light emitting means; an electric signal delay mechanism that delays the electric signal from the light receiving means with a delay accuracy higher than the first delay accuracy; and a merging means that superimposes the electric signals from the third light receiving means and the electric signal delay mechanism, An optical communication system that can suppress modulation distortion included in transmission signals during intensity modulation. 2. The optical communication system according to claim 1, wherein the optical signal delay mechanism is an optical fiber line having a predetermined length, and the electrical signal delay mechanism is constituted by an electrical delay circuit. 3. The optical communication system according to claim 1 or 2, wherein the optical signal delay mechanism is provided on the transmission side of the optical communication system.