JPH05150123A - Optical equalizing circuit - Google Patents

Abstract

PURPOSE:To eliminate polarization dependency and to reduce the influence of delay dispersion by insertion to an optional place of an optical fiber transmission part by distributing a light signal to a 1st and a 2nd system and coupling the signal of the 1st system side with the light signal of the 2nd system side while delaying the light signal by components through delay optical waveguides on the 1st system side. CONSTITUTION:An intermediate part 3 which is connected between input/output parts 1 and 2 is constituted by repeatedly connecting >=1 unit optical circuits 311-31n in series. The individual unit optical circuits 311-311n are of the same constitution and consist of 1st and 2nd delay optical waveguides (a) and (b) interposed in the system A side respectively and a 3dB directional coupler (c) where the 1st and 2nd delay optical waveguides (a) and (b) are connected in series on the system A side and both ends on the system B side are connected to the system B as a precedent stage. The 1st and 2nd delay optical waveguides (a) and (b) are increased or decreased by 1/2-1/4 time as long as constant length L according to target transmission band width.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical equalizer circuit that can be used for, for example, an ultrahigh-speed long-distance transmission line for lightwave communication.

[0002]

2. Description of the Related Art As is well known, in order to play an important role in constructing a communication infrastructure in an intelligent information society, an optical fiber is required to be sophisticated. Therefore, the lightwave communication technology has been rapidly advanced, and its transmission speed and multiplicity have been improved year by year. Therefore, the communication capacity of the backbone communication path, which was previously constructed by making full use of the latest technology, tends to be insufficient immediately, and it is necessary to repair it with the latest technology.

However, although changing only the optical terminal equipment is light in cost, it is costly and impossible to re-install the optical fiber line due to obsolete characteristics of the optical fiber line. near.

Specifically, an optical fiber having a minimum delay dispersion wavelength of 1.3 μm (hereinafter referred to as a normal dispersion fiber)
Let's consider how to refurbish the backbone line that was operating the Gbps optical line by using the above to a super high-speed long-distance transmission line. In this case, regarding the optical transmission loss, the required wavelength is 1.
By changing the thickness to 55 μm, the required optical reception power can be secured. Further, it is possible to take a measure to add an optical amplifier.

However, with a normal dispersion fiber, 1.55
The delay time dispersion in μm is as large as about 17 ps / nm, and it is difficult to achieve the required ultrahigh-speed long-distance transmission of several gigabits / second with the direct modulation method of the semiconductor laser which is considered to be effective at present. Here, it is conceivable to reduce the required modulation bandwidth by using an external optical modulator and reduce the effect of delay dispersion, but this will increase the cost and the bandwidth limitation due to the above-mentioned requirement for ultra-high-speed long-distance transmission. Face problems and cannot respond.

Further, even in a fiber having a minimum dispersion point in the 1.55 μm band, when the optical amplifier is used to convert the signal into an electric signal and reproduce and transmit it in the Pacific Ocean at a very high speed without relaying, a band shortage occurs. This causes a problem, and complicated control is required to solve the problem, resulting in an increase in transmission cost.

[0007]

As described above, conventionally, there is no effective means for minimizing the increase of the transmission cost in order to increase the transmission line speed and the communication capacity in the lightwave communication. It was

The present invention has been made in order to solve the above-mentioned problems and has no polarization dependence and can be inserted at an arbitrary position in an optical fiber transmission system to reduce the influence of delay dispersion. Therefore, it is an object of the present invention to provide an optical equalization circuit that can realize an ultra-high speed by using an existing lightwave communication line as it is and can suppress an increase in transmission cost to a minimum.

[0009]

In order to achieve the above object, an optical equalizing circuit according to the present invention comprises first and second systems each including an optical waveguide for transmitting an optical signal without polarization dependency. A plurality of optical coupling means for coupling the optical waveguides of the first and second systems and a plurality of delay optical waveguides interposed in the first system are connected in series, and the delay optical waveguides have a half wavelength to a quarter wavelength.
The feature is that the delay length is increased or decreased by the wavelength.

[0010]

In the optical equalization circuit having the above structure, when the optical center frequency of the optical signal is separated into, for example, a high-frequency component and a low-frequency component, the frequency-delay dispersion characteristic of the fiber used in the transmission line is determined. Focusing on the fact that the delay time of each component increases and decreases and the usable bandwidth is limited, the circuit is inserted at an arbitrary position of the fiber to transmit the optical signal to the first and second optical signals.
Of the optical signal on the second system side while delaying each component by the delay optical waveguide on the first system side to make the delay time of each component uniform, thereby Increase transmission bandwidth. At this time, since the optical waveguide does not have polarization dependency, it is not necessary to consider the polarization characteristics of the optical signal, and the versatility is extremely high.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. However, here, an example will be described in which an optical signal in the 1.55 μm (frequency f2) band with less loss is transmitted to the line using the normal dispersion fiber having the minimum delay dispersion wavelength of 1.3 μm (frequency f1).

FIG. 1 shows the configuration of an optical equalizing circuit according to the present invention, which is inserted at an arbitrary position on the above line.

In FIG. 1, A and B respectively show a pair of systems having no polarization dependence, and the whole is realized by an optical waveguide. The input section 1 and the output section 2 of each system A and B are provided with a 3 dB directional coupler that optically couples the systems A and B, respectively. The coupler of the input unit 1 takes in the optical signal from the optical signal input terminal IN from the input end on the A system side, and distributes and supplies the optical signal to the A and B system input ends of the intermediate unit 3. The coupler of the output unit 2 takes in the respective optical signals from the A and B system output ends of the intermediate unit 3 from the respective A and B system input ends and optically couples them, and outputs the combined optical signal to the B system side output. It is led to the optical signal output terminal OUT from the end.

The intermediate section 3 connected between the input / output sections 1 and 2 is constructed by repeatedly connecting one or more unit optical circuits 311 to 31n in series. Individual unit optical circuits 311 to 311
1n has the same configuration, and the first and second delay optical waveguides a and b interposed on the A system side and the first and second delay optical waveguides a and b on the A system side are connected in series, respectively. , And both ends on the B system side are connected to the B system at the front and rear stages and a 3 dB directional coupler c. The length of each of the first and second delay optical waveguides a and b is a half wavelength to a quarter wavelength (here, a quarter wavelength here) with respect to a fixed length L, depending on the target transmission bandwidth. ) Is increased or decreased.

The operation of the above configuration will be described below.

First, the amplitudes (E1, E2) of the optical signal output from the output section 2 of the optical equalization circuit are given by the following matrix display.

[0017]

[Equation 1] When the formula (1) is calculated, the following formula is obtained.

[0018]

[Equation 2] Here, U _n (α) is the Chebyshev polynomial of the second kind, and the variable α is given by the following equation.

[0019]

[Equation 3] The group delay time Tg due to the output E2 is given by the following equation.

[0020]

[Equation 4] Here, f is the optical frequency, and φ and Φ are given by the following equations, where T is the delay time corresponding to the length L.

[0021]

[Equation 5] More specifically, the delay dispersion characteristic of the applied normal dispersion fiber is minimum at a wavelength of 1.3 μm (frequency f1) and changes parabolic as shown in FIG. Therefore, when the wavelength used is 1.55 μm (frequency f2), the amount of delay dispersion changes significantly with a slight wavelength change, that is, a frequency change, resulting in a band shortage.

However, as is clear from FIG. 2, when the frequency f2 used is taken as the center, the delay dispersion amount becomes smaller at higher frequencies and becomes larger at lower frequencies. Therefore, if a large delay is given to a high frequency and a small delay is given to a low frequency to equalize the delay amounts, the transmission band can be expanded.

Therefore, in the above embodiment, first, the optical signal is distributed to the A and B systems by the input section 1, and each unit optical circuit 3 of the intermediate section 3 is divided.
In 11 to 31n, the delay optical waveguides a and b on the A system side give opposite delay times for each frequency component, and are coupled with the optical signal on the B system side by the directional coupler c. Repetition of such an operation with a multi-stage configuration is represented by a display formula using the above Chebyshev polynomial. That is, the low frequency component is given a small delay time, and the high frequency component is given a large delay time, whereby the delay dispersion of the fiber can be compensated.

FIG. 3 shows the situation. This example is a case where an optical equalization circuit with T1 = T2 = 1/125 [ns] and n = 12 is connected to a normal dispersion fiber having a minimum delay dispersion wavelength of 1.3 μm and a length of 100 km. ) Is a frequency-to-transmission coefficient characteristic of a normal dispersion fiber, FIG. 7B is a frequency-to-group delay time characteristic of an optical equalization circuit, and FIG. The frequency vs. group delay time characteristic b when an equalizer circuit is connected is shown. However, φ = 2πfT−π / 4, Φ = 2πfT + π /
Set to 4.

From FIG. 3, the center frequency f (1.55 μm)
A transmission band of about ± 10 [GHz] is obtained for
The amplitude characteristics of the pass band are Chebyshev-like, and a sufficient bandwidth can be secured even in multi-stage connection, and linear dispersion can be compensated.

Therefore, when the optical equalization circuit is used for ultrahigh-speed lightwave communication lines, the delay time of each frequency component can be made uniform, and the transmission band at the used wavelength can be expanded. The existing normal dispersion fiber line can be used as it is without using a device, and the transmission cost can be minimized. At this time, since the whole is composed of the optical waveguide having no polarization dependence, it is not necessary to consider the polarization characteristic of the optical signal, and thereby the versatility can be enhanced.

By the way, a dispersion-shifted fiber having a minimum dispersion point at 1.55 μm is used in the transmission line, and when transmitting a signal light in the 1.55 μm band to this, the f1 of FIG. Substituting the frequency corresponding to 55 μm, it is necessary to compensate for the parabolic characteristic near the zero dispersion point of the fiber.

FIG. 4 shows the configuration of an optical equalization circuit which realizes the above compensation. However, in FIG. 4, the same parts as those in FIG. 1 are designated by the same reference numerals, and the description thereof will be omitted.

In the input section 1 of FIG. 4, the input end of the directional coupler on the B system side is connected to the optical signal input terminal IN. In the output section 2, the output end of the directional coupler on the A system side is connected to the optical signal output terminal OUT.

In the intermediate section 3, the unit optical circuits 321 to 321 are provided.
2n is composed of first and second delay optical waveguides d and f and first and second directional couplers e and g, respectively.

The first delay optical waveguide d receives the optical signal from the A system in the preceding stage and has a length of a constant length L corresponding to its wavelength. The first directional coupler e optically couples the output of the first delay optical waveguide d and the optical signal from the B system of the previous stage to A,
It is distributed and output to the B system.

The second delay optical waveguide f receives the optical signal output from the A system of the first directional coupler e, and adds (or subtracts) 1/2 wavelength to the fixed length L corresponding to the wavelength. )
Has a length. The second directional coupler g has a coupling characteristic of three stages as compared with the first directional coupler e, and outputs the output of the second delay optical waveguide f and the optical signal from the B system of the previous stage. Are optically coupled and distributed and output to each system.

Further, the intermediate section 3 has a final stage optical circuit 33. This final-stage optical circuit 33 inputs the optical signal from the A-system of the preceding stage to the A-system side, and provides a third delay optical waveguide h having a length of a constant length L corresponding to the wavelength, and the B-system side. Is configured as a through state.

The operation of the above configuration will be described below.

First, the amplitudes (E1, E2) of the optical signal output from the output unit 2 of the optical equalization circuit are given by the following matrix display.

[0036]

[Equation 6] When the formula (6) is calculated, the following formula is obtained.

[0037]

[Equation 7] Here, U _n (α) is the Chebyshev polynomial of the second kind, and the variable α is given by the following equation.

[0038]

[Equation 8] The group delay time Tg due to the output E2 is given by the following equation.

[0039]

[Equation 9] Here, f is the optical frequency, and φ and Φ are given by the following equations, where T is the delay time corresponding to the length L.

[0040]

[Equation 10] More specifically, the delay dispersion characteristic of the applied normal dispersion fiber has the minimum at a wavelength of 1.55 μm (frequency f1) when the frequency of FIG. 2 is replaced, and changes in a parabolic shape. Therefore, in the vicinity of the used wavelength of 1.55 μm, the amount of delay dispersion increases due to a slight wavelength change, that is, a frequency change, resulting in insufficient bandwidth. However, if delays for compensating the delay dispersion are given to the frequency components higher than the center frequency f1 and the frequency components lower than the center frequency f1, the delay amounts become equal and the transmission band can be expanded.

Therefore, in the above embodiment, first, the input section 1 distributes the optical signal to the A and B systems, and the optical circuits 321 to 321 of the intermediate section 3 are first distributed.
In 32n and 33, the delay optical waveguides d, f,
While the reverse delay time is given to each frequency component by h, the optical signals on the B system side are coupled by the directional couplers e and g. Repetition of such an operation with a multi-stage configuration is represented by a display formula using the above Chebyshev polynomial. That is, a delay time corresponding to the frequency component is given, and thereby the delay dispersion of the fiber can be compensated.

FIG. 5 shows the situation. This example is a case where an optical equalization circuit with T1 = T2 = 10 [ps] and n = 8 is connected to a dispersion shift fiber having a minimum delay dispersion wavelength of 1.55 μm and a length of 100 km, and FIG. The frequency vs. transmission coefficient characteristic of the dispersion shifted fiber, the figure (b) shows the frequency vs. group delay time characteristic of the optical equalization circuit, and the figure (c) shows the frequency vs. group delay time characteristic c of the dispersion shifted fiber and the above optical equalization. The frequency vs. group delay time characteristic d when a circuit is connected is shown. However, φ = 2πfT−π / 4, Φ = 2πfT + π /
Set to 4.

From FIG. 5, the center frequency f (1.55 μm)
It is clear that a transmission band of about ± 125 [GHz] can be obtained with respect to and the parabolic characteristic near the zero dispersion point can be compensated. Further, as in the case of the above-mentioned embodiment, the whole is composed of the optical waveguide having no polarization dependence,
Since it is not necessary to consider the polarization characteristics of the optical signal, the versatility can be improved.

Therefore, as a result of the above-mentioned compensation, it is possible to eliminate the band shortage without the need for complicated control even when the optical amplifier is used to perform ultra-high-speed transmission in the Pacific Ocean without converting and reproducing the electrical signal for relaying. Therefore, it is possible to minimize an increase in transmission cost.

Needless to say, various modifications can be made without departing from the scope of the present invention.

[0046]

As described above, according to the present invention, there is no polarization dependence, and it is possible to reduce the influence of delay dispersion by inserting the optical fiber transmission system at an arbitrary position. An ultra-high speed can be realized by using the communication line as it is, and it is possible to provide an optical equalization circuit capable of minimizing an increase in transmission cost.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of an optical equalization circuit according to the present invention.

FIG. 2 is a characteristic diagram showing delay characteristics of a normal dispersion fiber applied in the embodiment of FIG.

FIG. 3 is a characteristic diagram showing how the transmission band is expanded when the optical equalization circuit of the embodiment of FIG. 1 is used.

FIG. 4 is a configuration diagram showing another embodiment according to the present invention.

5 is a characteristic diagram showing how the transmission band is expanded when the optical equalization circuit of the embodiment of FIG. 4 is used.

[Explanation of symbols]

1 ... Input part, 2 ... Output part, 3 ... Intermediate part, 311-31
n, 321 to 32n ... Unit optical circuit, 33 ... Final-stage optical circuit,
a, b, d, f, h ... Delay optical waveguide, c, e, g ... Directional coupler.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location H04B 10/02

Claims (3)

[Claims] 1. An optical coupling means for coupling the optical waveguides of the first and second systems, comprising first and second systems each comprising an optical waveguide for transmitting an optical signal independently of polarization, and the first system. 2. An optical equalizer circuit, wherein a plurality of delay optical waveguides interposed in the system are connected in series, and the delay optical waveguides are configured so that the delay length is increased or decreased by a half wavelength to a quarter wavelength. 2. The optical coupling means is a directional coupler, and one end of the first system side is used as an optical signal input end, and the input optical signal is distributed to the first and second systems. Is used as an input section, and the other end of the second system side is used as an optical signal output terminal, and a second directional coupler that outputs combined optical signals of the first and second systems is output as an output section. And a first delay optical waveguide having a length obtained by adding a quarter wavelength to a constant length corresponding to the wavelength of the optical signal, and a first delay optical waveguide having a length obtained by subtracting a quarter wavelength from the constant length. An optical circuit including two delay optical waveguides and a third directional coupler connecting the first and second delay optical waveguides is configured, and the optical circuit is repeatedly connected in series between the input section and the output section. The optical equalization circuit according to claim 1, wherein the optical equalization circuit is configured to be connected. 3. The optical coupling means is a directional coupler, and the one end of the second system side is used as an optical signal input end, and the input optical signal is distributed to the first and second systems. Is used as an input section, and the other end of the first system side is used as an optical signal output end, and a second directional coupler that outputs the optical signals of the first and second systems is output as an output section. And a first delay optical waveguide having a fixed length corresponding to the wavelength of the optical signal from the first stage optical system, the output of the first delay optical waveguide and the second optical system of the previous stage. Of the third directional coupler for optically coupling and outputting the optical signal from the system of No. 3 to each system, and the optical signal from the first system of the third directional coupler is input and the fixed length is set. A second delay optical waveguide having a length obtained by increasing / decreasing 1/2 wavelength, and having the characteristics of three stages as compared with the third directional coupler. An optical circuit including a fourth directional coupler that optically couples the output of the delay optical waveguide and the optical signal from the second system in the preceding stage and distributes and outputs to each system is formed. The input section and the output section are repeatedly connected in series, and the third delay optical waveguide having the fixed length is connected between the final system first output side of the optical circuit and the output section. The optical equalizer circuit according to claim 1, wherein the optical equalizer circuit is configured to: