JPH05226901A - Equalizer for high frequency communication system - Google Patents

Abstract

PURPOSE:To provide the equalizer for a high frequency communication system by applying the equalizer to a broad band at a low loss in order to allow the equalizer to be compatible with an optical communication system capable of long distance transmission at a high transfer speed. CONSTITUTION:A center conductor 15 is made of a superconducting oxide thin film, the entire delay line is formed in a meandering form and coupling lines 20 being basic components of the delay line and are formed in a meandering form, then a low loss is realized and miniaturization is realized, the degree of coupling of a high frequency signal to be propagated is set to a proper degree and frequency dispersion is provided to the signal delay line, a relative delay time required for the equalizer is obtained and reproduction of the optical fiber dispersion over a broad band is compensated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an equalizer which constitutes a demodulation system of an optical communication system using a single mode fiber in a backbone communication line such as on the ground or in the sea, and more specifically, for example, 10 GBits / s. The present invention relates to a high-frequency communication system equalizer applied to optical communication for transmitting information at an extremely high transfer rate.

[0002]

2. Description of the Related Art FIG. 10 shows an optical communication system of this type.
FIG. 1 shows an example of a system configuration of coherence communication which is one of the methods. As shown in the figure, an optical signal 1 input from a modulation system and containing communication information is supplied to a demodulation system 8 via an optical transmission line 2 made of a single mode fiber and received as an output signal 3 which is an electric signal. Supplied to the system.

Here, the demodulation system 8 includes the optical transmission line 2
Receiver 5 for photoelectrically converting an optical signal from the receiver and the local light signal 4 into an electric signal of several hundred GHz, an equalizer 6 for compensating and reproducing waveform distortion of the output of the receiver 5, and a communication signal The demodulator 7 is generally included.

In this case, the equalizer 6 is provided because the electrical signal from the light receiver 5 retains the dispersion generated in the optical signal propagating through the optical transmission line 2 as it is. This is because it is necessary to reproduce and compensate the waveform distortion in the signal state.

FIG. 11 shows an example of dispersion characteristics of a single mode fiber that constitutes the optical transmission line 2. As shown in FIG. 11, in the case of this fiber, the relative delay time ns at a wavelength of 1.3 μm. / Km is 0, in other words, there is no dispersion in the wavelength band. However, the wavelength band with the lowest transmission loss is 1.5
Since it is 5 μm, the optical signal is normally used in the 1.55 micron wavelength band.

On the other hand, when an optical signal propagates in a fiber with a wavelength band of 1.55 μm as a carrier, as shown in FIG. 11, since it has a relative delay time depending on the wavelength, the propagated optical signal has waveform distortion. Occurs. The dispersion characteristic of the fiber is 15 ps / km / nm, which is 125 GHz after mixing at a wavelength of 1.55 μm.
The optical signal after propagating 00 km is subjected to photoelectric conversion, and the electrical signal contains waveform distortion corresponding to a relative delay time of about 24 ps / GHz.

In order to reproduce and compensate such waveform distortion, 2
An equalizer consisting of a wideband delay line with a linear frequency dispersion of 4 ps / GHz is needed. In addition, since the frequency band required for such an equalizer is twice the transfer rate, for example, 10 GBits / s at present, a delay line that can be used in a wide band of at least 20 GHz is required.

Conventionally, as this type of equalizer, there is known an equalizer composed of a microstrip type delay line as shown in FIG. As shown in the figure, this equalizer comprises a center conductor 9 made of a normal conductive metal thin film, a 5 cm × 5 cm dielectric substrate 10 to which the center conductor 9 is attached, and a normal conductive metal thin film supporting the dielectric substrate 10. The ground plane 11 is formed.

In this case, in order to avoid frequency dispersion due to λ / 4 side coupling, the distance between the lines is widened to about 10 mm and the line length is about 21 cm.

FIG. 13 shows the frequency characteristic of the relative delay time for the equalizer constructed as described above. As shown in the figure, the relative delay time linearly increases as the frequency increases.

This is mainly because in the microstrip structure, the high-frequency mode changes from the quasi-TEM mode to the TM mode as the frequency increases, so that the effective dielectric constant near the center conductor changes.

The dispersion characteristic of the fiber is 0.12 ps / GH.
Since it is z / km, assuming a transfer rate of 10 GBits / s, 2.4 ps / k in the band of 5 to 20 GHz.
A relative delay time of m that varies linearly with frequency is required. Therefore, as shown in FIG. 13, when the line length L of the delay line is 6 cm, the dispersion of the transmission line of about 30 km is
When the line length L is 21 cm, the dispersion of about 100 km fiber can be regeneratively compensated.

[0013]

However, in the equalizer of the above-mentioned conventional configuration, the maximum applicable frequency is the TE mode cutoff frequency of the microstrip structure, which is 3 dB lower than that of DC. Assuming a transfer rate of 10 GBits / s and a fiber dispersion of 15 ps / km / nm, the length of the fiber is 256 km even when the delay line is made of an ideal normal-conducting metal, since it depends on the frequency. I can only deal with things.

Therefore, the equalizer using a normal conductive metal as a conductor as described above has a large loss and a relatively low cutoff frequency. Therefore, the equalizer is applied to a repeaterless optical transmission communication system having a high transfer rate and a long distance. Had technical limitations.

Further, in the conventional equalizer, although the dielectric substrate has a relatively large shape of 5 cm × 5 cm, only a line length of about 20 cm is obtained in order to prevent inter-line coupling. As a result, one equalizer can only perform regenerative compensation of 100 km fiber dispersion.

Furthermore, in order to equalize the dispersion of the optical fiber line, the delay time is frequency-dispersed by using the mode change with the increase of the frequency. It is necessary to change the line length of the delay line according to the above, and the degree of freedom in design is low.

The present invention has been made to solve the above-mentioned problems of the prior art, and can be applied to a wide band with low loss in order to support an optical communication system capable of long-distance transmission at a high transfer rate. It is an object of the present invention to provide an equalizer for a demodulation system.

[0018]

The present invention relates to a high-frequency signal delay line having a stripline structure or a microstrip structure, in which a superconducting thin film is used as a central conductor and a superconducting thin film or a normal conducting metal thin film is used as a ground plane. Further, the whole pattern of the central conductor is formed in a meandering state, and the coupling line of the basic unit forming the central conductor is formed in a meandering shape.

[0019]

Since at least the central conductor is made of an oxide superconducting thin film, low loss can be realized, the entire delay line is formed in a meandering state, and further, the coupling line of the basic unit constituting the delay line is formed in a meandering shape. Therefore, miniaturization can be realized, the degree of coupling of propagating high frequency signals can be set to an appropriate value, and the signal delay line can have frequency dispersion. The delay time can be obtained, and thus the reproduction compensation of the optical fiber dispersion in the wide band can be performed.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows the structure of an equalizer according to the present invention. As shown in FIGS. 1A and 1B, the high frequency delay line is arranged in a stripline structure. Further, the entire center conductor pattern 15 is formed in a meandering state in which a square well type is repeated, and as shown in an enlarged portion 14 of FIG. It is composed of the coupling lines 20, 21, 22 and the like. Here, the central conductor 15 is formed of an oxide superconducting thin film.

One end of the center conductor 15 is the input part 12, and the other end is the output part 13. The center conductor 15 is mounted on the dielectric substrate 19 on the lower ground plane 17, and is located above the center conductor 15. Is provided with an upper ground plane 16 having a dielectric substrate 18.

Here, the central conductor is 0.16
An EuBaCuO oxide superconducting thin film having a width of mm and a copper thin film having a thickness of 3 μm are used for the ground plane, and the dielectric substrates 18 and 19 are, for example, 2.5 with a thickness of 0.5 mm.
A 4 cm x 2.54 cm MgO substrate is used.

As described above, the signal path is composed of the coupling lines 20, 21, 22, etc. of the basic unit, and when the total length of the signal path is 28 cm and the length of the coupling line as the basic unit is 1 mm, The frequency at which the first dispersion peak where this coupled line is a λ / 4 coupler appears is about 24 GHz.

The line spacing of the coupled line is set to 0.64.
In mm, the coupling degree of the coupling line is about 24 dB from the relationship shown in FIG. Further, the delay time per unit length in this coupled line is about 8.9 from the relationship shown in FIG.
ns / m. From the relationship shown in FIG. 3, a relative delay time of about 25% is obtained, and the effective total length of the coupled line is about 1%.
Considering that it is 5 cm, 8.9 ns / m × 0.
20m relative delay time of 15m × 0.25 = 330ps
It can be expected in the band below Hz.

The degree of coupling can be easily adjusted by adjusting the mutual spacing of the coupled lines 20, 21, 22.

A signal delay line for an equalizer for compensating for dispersion of an optical fiber is required to have a low loss and a wide band, but as a basic method for obtaining a relative delay time with frequency,
(1) Dispersion by coupled line, (2) Dispersion of dielectric substrate,
(3) The dispersion of the effective dielectric constant due to the mode change accompanying the high frequency is used.

In the case of the dispersion of the coupling line of (1), the relative delay time can be widely selected in the band of 50 GHz or less, which is suitable for use as a demodulation system equalizer. Also used this.

Next, an equalizer having a stripline structure will be supplementarily described.

FIG. 2 shows an example of frequency characteristics of delay time in the distributed delay line. The coupled line length is 6 mm, the coupling degree is 23 dB, and the peak of dispersion with λ / 4 is repeated at about 4.1 GHz. The ratio (Δt / t ₀ ) of the relative delay time by the coupled line to the delay time of all the lines caused by the coupled line is about 24%. The smaller the degree of coupling, the narrower the line spacing and the stronger the coupling.

As shown in FIG. 3, the relationship between the ratio of the relative delay times obtained from delay lines having various degrees of coupling and the degree of coupling can be understood, and the degree of coupling can be obtained by adjusting the meander line spacing of the stripline structure. It is changing. Increasing the coupling to about 15 dB gives a relative delay time of about 80%.

From FIG. 4, the dependency of the delay time on the coupling line on the coupling degree can be understood, and the capacitance and the inductance of the mutual lines change due to the coupling between the lines, so that the delay time dependency as shown in the same figure is shown. As shown in FIG. 5, by reducing the line spacing to 0.2 mm, 10 dB
The above coupled line can be easily realized.

If 0.5 mm thick MgO (relative permittivity 9.4) is used as the dielectric substrate and the center conductor has a line width of 0.16 mm, the characteristic impedance is 50.
It becomes Ω.

When the signal delay line is formed of a superconducting thin film, its loss is about 0.7 dB at 10 GHz and 77 K, which is remarkably small as about 1/10 of the loss of the copper delay line. In the case of a superconducting delay line, the loss is 2 dB even at 20 GHz.
It is below. Further, if the ground plane is also formed of a superconducting thin film, the loss can be further reduced, and the loss and the band can be further reduced as compared with the conventional equalizer.

FIG. 7 shows the relationship between the relative delay time and the frequency. In the case of the coupling degree of 24 dB set in the configuration of the present embodiment, a relative delay time of about 340 ps is obtained in the band of 20 GHz or less, and in the case of the coupling degree of 29 dB, a relative delay time of about 120 ps is obtained. The total line length of both equalizers is approximately 28 cm, and the line intervals of the coupled lines are different.
Since the dispersion characteristic of the fiber is 0.12 ps / GHz / km, an equalizer with a coupling degree of 24 dB can compensate dispersion of about 140 km fiber, and an equalizer with a coupling degree of 29 dB can compensate dispersion of about 50 km fiber. It is possible. Coupling degree 2
The 4 dB equalizer is about 1/4 the size of the conventional microstrip type equalizer (capable of compensating 100 km fiber with a size of 5 cm x 5 cm), but the fiber distance that can be compensated is about 1.4 times. Therefore, it was confirmed that it was compact and had an excellent equalization function.

Next, an equalizer having a microstrip structure will be described. FIG. 8 shows the relationship between the coupling degree and the line spacing in a 50Ω microstrip line composed of a 0.5 mm thick MgO substrate and a 0.5 mm line width. As in the case of the stripline structure, the coupling degree of the coupled line can be easily changed by adjusting the line spacing.

FIG. 9 shows the relationship between delay time and coupling degree. From this, it can be understood that the delay time per unit length becomes shorter as the coupling between the lines becomes stronger, similar to the stripline structure. Since the relationship between the relative delay time and the coupling degree is almost the same as in the case of the stripline structure shown in FIG. 3, basically, the microstrip structure can also obtain the same performance as the equalizer of the stripline structure. Can understand.

[0038]

As described above, according to the present invention,
By using an oxide superconducting thin film for the center conductor of the equalizer and the ground plane, the loss is significantly reduced, which enables a wider bandwidth. In addition, a wide band distributed delay line can be realized by meandering the entire signal path and further forming a coupling line of a basic unit arranged in the center conductor in a meandering shape.

In addition, regardless of the stripline or microstrip structure, the frequency dispersion of the delay characteristic due to the coupled line can obtain a wide range of relative delay time by changing the interval between the coupled lines or the total length of the coupled line. Therefore, it can be used for dispersion compensation of optical fibers at various distances. As a result, the communication system using optical fiber,
The degree of freedom in designing is increased, and an excellent effect is achieved in that the repeaterless optical transmission can be performed at a higher transfer rate and longer. Furthermore, a compact equalizer can be realized with the above-described configuration.

[Brief description of drawings]

FIG. 1 is a plan view (a) showing a preferred embodiment of the present invention,
It is the sectional view (b) along the AA 'line, and its partial enlarged view (c).

FIG. 2 is a graph showing a characteristic example of a distributed delay line.

FIG. 3 is a graph showing the relationship between the relative delay time ratio and the coupling degree.

FIG. 4 is a graph showing a relationship between delay time and coupling degree.

FIG. 5 is a graph showing line spacing dependence of coupling degree.

FIG. 6 is a graph showing a relationship between insertion loss and frequency in a distributed delay line for an equalizer.

FIG. 7 is a graph showing a characteristic example of a superconducting equalizer.

FIG. 8 is a graph showing the dependency between the degree of coupling and the line spacing in the microstrip structure.

FIG. 9 is a graph showing the dependence of delay time on coupling degree.

FIG. 10 is a block diagram showing an example of typical coherent communication.

FIG. 11 is a graph showing a characteristic example of an optical fiber.

FIG. 12 is a perspective view showing a configuration example of a conventional equalizer using a microstrip type delay line.

FIG. 13 is a graph showing the relationship between the relative delay time and the frequency of the microstrip type equalizer.

[Explanation of symbols]

 12 signal input part, 13 signal output part, 14 part of coupling line, 15 center conductor, 16 upper ground plane, 17 lower ground plane, 18 upper dielectric substrate, 19 lower dielectric substrate, 20, 21, 22 coupling line.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Naofumi Suzuki 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Inside Nippon Telegraph and Telephone Corporation

Claims (2)

[Claims] 1. A stripline structure in which a high-frequency signal delay line is interposed between a pair of ground planes and provided as a conductor on a dielectric substrate on one of the ground planes, or on one ground plane. In an equalizer for a high frequency communication system configured by a microstrip structure provided as a conductor on a dielectric substrate, at least the conductor is formed of a superconducting thin film, and the entire signal path of the high frequency signal delay line is meandered. An equalizer for a high frequency communication system, characterized in that the signal path is composed of a plurality of coupled lines of basic units formed in a meandering shape. 2. The basic coupling line has a line length of at least 4 of a high frequency signal propagating in the signal path.
The equalizer for a high frequency communication system according to claim 1, wherein the equalizer has a length that can avoid a frequency corresponding to a dispersion peak generated in the lowest frequency region due to the coupling on the one-half wavelength side.