JPH08125414A - Delay equalizer - Google Patents

Abstract

PURPOSE: To miniaturize a equalizer by decreasing a length of a microstrip line in the delay equalizer comprising the microstrip line to compensate wavelength diffusion of an optical fiber. CONSTITUTION: In the delay equalizer made of a microstrip line in which a strip conductor 1 is formed on a substrate 2 made of a dielectric material and a ground conductor 3 is formed to a lower side of the substrate 2, a specific dielectric constant εr and a thickness (h) of the substrate 2 are decided so that a cut-off frequency fTE in the lowest degree TE mode of the microstrip line is set higher than a wavelength diffusion compensation enable frequency and a prescribed characteristic impedance Z0 is obtained by deciding an upper limit of the impedance depending on the cut-off frequency fTE and a minimum width W0 of the strip conductor 1 able to be manufactured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a delay equalizer composed of a microstrip line, and more particularly to a delay equalizer composed of a microstrip line for compensating for chromatic dispersion of an optical fiber.

The coherent optical communication system has a receiving sensitivity higher than that of the intensity modulation system by 10 dB or more, and is therefore suitable for long-distance transmission. Also, for long-distance optical transmission, an ordinary regenerative repeater is used by performing multi-stage repeater amplification by using an optical amplifier using a semiconductor element or an optical amplifier using an optical fiber doped with a rare earth element. You can do it without doing it.

When performing long-distance transmission using an optical fiber, the influence of chromatic dispersion of the optical fiber becomes large. To compensate for such chromatic dispersion of the optical fiber, an optical heterodyne detection system is used. A method of compensating using a delay equalizer after conversion into an electric signal is known.

Such a delay equalizer for wavelength dispersion compensation can be realized by using a microstrip line. In this case, the required dispersion compensation can be performed with a strip conductor length as short as possible. It is necessary to enable downsizing of equipment.

[0005]

2. Description of the Related Art In a coherent optical communication system, a heterodyne detection system is used to convert a received signal into an intermediate frequency signal and then a delay equalizer is used to compensate for chromatic dispersion in an optical fiber. You can

FIG. 3 illustrates chromatic dispersion compensation in the intermediate frequency stage. FIG. 3A shows a circuit configuration for chromatic dispersion compensation, and FIG. 3B shows optical fiber delay characteristics and compensation. And the delay characteristics of the microstrip line used for.

In (a), the signal light of frequency f _S is
It has the group delay characteristics given by the optical transmission line. This signal light and the local oscillation light of the frequency f _L from the local oscillator 11 are mixed in the optical fiber coupler 12 and output to the output ports 12 _{1 and} 12 ₂ in the opposite phase with the frequency | f _S −.
The light receiving unit 1 generates a beat frequency signal light composed of f _L |
The light receiving elements 13 _{1 and} 13 ₂ in FIG. 3 convert the signals into electric signals and generate an added intermediate frequency signal at the midpoint of both the light receiving elements. The dispersion compensator 14 compensates the group delay characteristic of the intermediate frequency signal by the opposite characteristic. The intermediate frequency signal whose group delay characteristic is compensated is demodulated in the demodulation circuit 15.

In (b), A indicates the delay characteristic of the optical fiber, and the group delay characteristic of the signal light f _S based on this is maintained as it is in the intermediate frequency signal f _S -f _L. B represents the delay characteristic of the delay equalizer composed of the microstrip line in the dispersion compensator 14, and by making the characteristic B the characteristic opposite to the characteristic A, the group delay characteristic of the signal light given in the optical transmission line is shown. Can be compensated.

The delay equalizer has, for example, a thickness h = 0.63.
The microstrip line can be realized by forming a strip conductor on the upper surface of an alumina substrate having a relative permittivity ε _r = 9.6 of 5 mm and forming a ground conductor on the lower surface of the alumina substrate. It is possible to perform chromatic dispersion compensation of 73 ps / nm per 1 cm in length.

Now, the chromatic dispersion of an optical fiber is 2 ps / n.
m / km, transmission distance by optical fiber is 10,000
If 0 km, the chromatic dispersion amount is 20,000 ps / nm
However, when the delay equalizer for compensating for this chromatic dispersion is configured by the above microstrip line, the line length is about 274 cm.

Further, since the size of the dielectric substrate forming the microstrip line is limited to about 50 mm × 50 mm, in order to realize a long microstrip line, the microstrip line is arranged in a meandering shape and the microstrip is as long as possible. Layout design that can accommodate the tracks becomes important.

FIG. 4 shows an example of the layout of the microstrip line, where (a) shows the case where the width of the strip conductor is wide, and (b) shows the case where the width of the strip conductor is narrow. There is.

The number of turns in the meandering type microstrip line is determined by the radius of curvature allowed at the meandering portion. Since the allowable radius of curvature is approximately proportional to the width of the strip conductor, a microstrip line with a narrow strip conductor can be designed in a meandering type with many turns, increasing the microstrip line length. can do.

In a delay equalizer using a microstrip line, in order to improve group delay characteristics (dispersion characteristics) per unit length of the microstrip line, a substrate having a high relative dielectric constant is used, or It is known that the thickness of the substrate may be increased.

When a delay equalizer is designed using a substrate having a high relative dielectric constant, not only can the group delay characteristic per unit length of the microstrip line be improved, but also the width of the strip conductor can be narrowed, so that the number of turns can be reduced. A large number of meandering designs are possible, and the microstrip line length can be increased.

On the other hand, when the delay equalizer is designed by increasing the thickness of the substrate, the group delay characteristic per unit length of the microstrip line can be improved, but the width of the strip conductor becomes wide, and therefore the meandering type of A layout design with a small number of turns is forced, and the microstrip line length cannot be increased.

[0017]

A conventional delay equalizer for compensating chromatic dispersion has a predetermined thickness and a relative permittivity ε _r of 9.
A microstrip line in which a strip conductor is formed on an alumina substrate of 5 to 9.9 (10 GHz), which has a low chromatic dispersion compensation capability and compensates chromatic dispersion of an optical fiber for long distance transmission. Therefore, a microstrip line having a length of several meters is required. Therefore, there is a problem in that the microstrip line is inevitably increased in size even if it is formed on the alumina substrate in a folded manner.

Further, as a result of the length of the microstrip line being increased, the insertion loss is increased and the frequency characteristic of the insertion loss is likely to be non-uniform. Therefore, there has been a problem that a compensation circuit is newly required for such frequency characteristic compensation of insertion loss.

The present invention is intended to solve the problems of the prior art as described above, and delay equalization is performed by shortening the microstrip line necessary for realizing the required chromatic dispersion compensation characteristic. The purpose is to enable downsizing of vessels.

[0020]

[Means for Solving the Problems]

(1) In a delay equalizer using a microstrip line, in which a strip conductor is formed on a dielectric substrate and a ground conductor is formed on the bottom surface of the substrate, the lowest TE mode of this microstrip line is cut off. The frequency f _TE becomes higher than the frequency that can be compensated for chromatic dispersion by the delay equalizer, and the upper limit value is defined by the cutoff frequency f _TE and the minimum width W ₀ of the strip conductor that can be manufactured.
The relative permittivity ε _r and the thickness h of the substrate are determined so that a predetermined characteristic impedance Z ₀ can be obtained.

(2) In (1), the relative permittivity ε of the substrate
_r

(Equation 3) The value given by is defined as the upper limit.

(3) In (1), the thickness h of the substrate is

[Equation 4] The value given by is defined as the upper limit.

(4) In any one of (1) to (3),
A substrate is manufactured using barium titanate (BaTiO ₃ ) as a dielectric.

[0024]

FIG. 1 shows a basic structure of the present invention, and is a perspective view of a main part of a delay equalizer composed of a microstrip line. Reference numeral 1 denotes a strip conductor having a width W, which is provided on the upper surface of a substrate 2 made of a dielectric material having a thickness h and a relative permittivity ε _r . Reference numeral 3 is a ground conductor, which is provided on the lower surface of the substrate 2.

It is known that the delay equalizer composed of a microstrip line can be miniaturized by increasing the relative permittivity ε _r of the substrate 2 and increasing the thickness h thereof. The first restriction is that the cutoff frequency f _TE of the lowest TE mode of the microstrip line is
It is not possible to compensate chromatic dispersion for frequencies above.

When the maximum frequency that can be compensated for chromatic dispersion is determined, the shape of the microstrip line can be determined by making the higher frequency the cutoff frequency f _TE . Further, there is a second limitation in realizing a desired characteristic impedance due to the limit that the width of the strip conductor 1 can be made uniform.

That is, the lower limit of the width W of the strip conductor 1 that can be manufactured is, for example, about 100 μm, and depending on the future progress of manufacturing technology, about 50 μm,
Alternatively, it may be possible to set it to be less than that. In the present invention, with respect to the cutoff frequency f _TE and the minimum width W ₀ within the manufacturable range, the maximum values of the relative permittivity ε _r and the thickness h of the substrate 2 that can realize the desired characteristic impedance Z ₀ are set. Ask.

By adopting a structure that meets such conditions of relative permittivity ε _r and thickness h, it is possible to increase the amount of chromatic dispersion compensation per unit length of the microstrip line. The delay equalizer can be downsized.

Further, the cutoff frequency f _TE of the lowest TE mode of the microstrip line can be defined as a frequency which is 3 dB lower than the flat frequency characteristic. Therefore, in order to use the flat frequency characteristic region, for example, the frequency corresponding to about 80% of the cutoff frequency f _TE is set as the maximum value of the frequencies capable of chromatic dispersion compensation.

Further, the desired characteristic impedance Z ₀ is
It is generally set to 50Ω, but the effective dielectric constant ε _eff, of the substrate 2 for realizing the characteristic impedance Z ₀ is
It is known that the width W of the strip conductor 1, the thickness h of the substrate 2, and the relative permittivity ε _r of the substrate 2 have the following relationships.

When W / h ≦ 1, Z ₀ = (60 / ε _eff ^1/2 ) ln (8h / W + 0.25W / h) (1) ε _eff = (ε _r +1) / 2 + [( ε _r −1) / 2] · [(1 + 12h / W) ^−1/2 +0.04 (1-W / h) ² ] ... (2)

When W / h ≧ 1, Z ₀ = (120π / ε _eff ^1/2 ) · [(W / h) +1.393 + 0.667ln (W / h + 1.444)] ^-1 (3) ε _eff = (ε _r +1) / 2 + [(ε _r −1) / 2] · (1 + 12 h / W) ^−1/2 (4)

Further, the group delay characteristic ε _{eff at} the signal frequency f [GHz] of such a microstrip line.
(F) can be expressed as ε _eff (f) = ε _r − (ε _r −ε _eff ) / [1 + G (f / f _p ) ² ] ... (5). Here, the unit of the thickness h of the substrate is c
As m, f _p = Z ₀ / 8πh (6) G = 0.6 + 0.009Z ₀ (7)

From these relational expressions (1) to (7), it is possible to obtain a desired characteristic impedance Z ₀ when the strip conductor 1 has a width W that can be uniformly manufactured.
It is possible to determine the relative permittivity ε _r and the thickness h thereof.

[0035]

EXAMPLES In the present invention, the above formulas (1) to (7) are used.
The configuration of the microstrip line that realizes the delay equalizer is determined by the following. At this time, in order to reduce the size of the delay equalizer, the relative dielectric constant ε _r
It is necessary to use the substrate 2 having a large thickness and to increase the thickness h of the substrate 2.

It is known that the cutoff frequency f _TE of the lowest TE mode of the microstrip line can be expressed by f _TE = c / 4hε _r ^1/2 (8) where the speed of light is c. In this case, the cutoff frequency f _TE corresponds to a frequency that is 3 dB lower than the flat characteristic, and therefore, the maximum value of the frequency that can be chromatic dispersion compensated is
It shall be selected to be about 80% of the cutoff frequency f _TE .

The microstrip line can be manufactured as follows. That is, for example, on both surfaces of the substrate 2 made of a dielectric material such as alumina, chromium oxide (C
rO) is vapor-deposited on 50 Å, chromium (Cr) is vapor-deposited thereon by, for example, 0.05 μm, and copper (Cu) or gold (Au) is further deposited thereon by, for example, about 5 μm to form a conductor layer.

Next, a photoresist is applied to the entire surface of the conductor layer, the pattern of the strip conductor 1 is exposed and developed, and then etching is performed with hydrofluoric acid or the like using the photoresist as a mask, and finally the unexposed photoresist is removed. By strip conductor 1 according to the mask pattern
Can be formed.

When constructing a delay equalizer, several tens of cm
It is necessary to form the strip conductor 1 having a uniform width W over a certain length.
Depending on the etching error etc., the width W may be several μm to several tens μm
Fluctuations occur. Therefore, the minimum manufacturable width W to the extent that such an error in the width W can be ignored is 10 at present.
It is about 0 μm. Therefore, when the width W of the strip conductor 1 is 100 μm and the characteristic impedance Z ₀ is 50Ω, the cutoff frequency f _TE when the relative permittivity ε _r of the substrate 2 and the thickness h of the substrate are maximum. The relationship between and can be obtained by using the equations (1) to (8).

FIG. 2 shows an embodiment of the present invention, in which the relative permittivity ε _{r of the} substrate thus obtained is obtained.
And the thickness h of the substrate. The curves a, b, and c show that the cutoff frequency f _TE is 9 GHz,
The case of 18 GHz and 36 GHz is shown.

In FIG. 2, the transmission speed is B _r, and the cutoff frequency of the lowest TE mode of the microstrip line is f _TE.
Then, the relative permittivity ε _r of the substrate and the thickness h of the substrate in the delay equalizer applied when the transmission speed is determined by the relationship of B _r = f _TE /3.6 (9) It shows the relationship with. That is, FIG. 2 shows transmission rates of 2.5 GHz and 5 GH, assuming that the center frequency of the intermediate frequency signal after the heterodyne detection is twice the transmission rate.
An example in which delay equalizers corresponding to z and 10 GHz are configured is shown.

The upper right side of each of these curves a, b, c shows the above-mentioned first constraint based on the cutoff frequency f _TE , and each curve a, b, c shows the maximum value for each transmission rate. Will be shown. For example, in the case of the PSK transmission method with a transmission speed of 2.5 GHz, the intermediate frequency signal band is 2.5 GHz to 7.5 G.
When the frequency is Hz, the upper limit frequency of 7.5 GHz corresponds to about 80% of the cutoff frequency f _TE = 9 GHz.

In addition, the inclination direction with respect to the curves a, b and c is
The opposite curves d and e are the widths of the strip conductors 1, respectively.
Characteristic impedance when W is 100 μm and 50 μm
Space Z ₀ (1) or (2), where is set to 50Ω
It is a curve that satisfies the requirement that strip conductor 1 can be manufactured.
Within the range, the desired characteristic impedance Z₀ Get
Is necessary for the relative permittivity of the substrate ε_rAnd the thickness h of the substrate
2 illustrates a second constraint for Of curves d and e
The lower right region shows the manufacturable range, and therefore the curve a,
The lower area surrounded by b and c and the curves d and e is the microphone.
It shows the range in which a loss lip line can be manufactured.

In FIG. 2, the curves a, b, and c descending to the right.
And the point of intersection with the curves d and e that rise to the right are the points that give the conditions for maximizing the amount of chromatic dispersion compensation per unit length of the microstrip line. From FIG. 2, when the cutoff frequency of the lowest TE mode of the microstrip line is f _TE and the minimum width of the strip conductor 1 that can be manufactured is W ₀ , the chromatic dispersion per unit length of the microstrip line is shown. When an approximate expression showing the relative permittivity ε _r of the substrate 2 which maximizes the compensation amount is obtained,

(Equation 5) Becomes

Further, from FIG. 2, when the cutoff frequency of the lowest TE mode of the microstrip line is f _TE and the minimum width of the strip conductor 1 that can be manufactured is W ₀ , per unit length of the microstrip line. When an approximate expression showing the thickness h of the substrate 2 that maximizes the chromatic dispersion compensation amount of

(Equation 6) Becomes

Therefore, a microstrip line is formed in which the relative permittivity ε _r of the substrate 2 and the thickness h of the substrate 2 given by the equations (10) and (11) are set as upper limits, and a signal is applied to the microstrip line. By propagating, a delay equalizer having a desired chromatic dispersion compensation amount can be realized.

For example, if the transmission rate is 2.5 GHz (curve a), the strip conductor 1 can be manufactured in a range of 10
If 0 μm (curve d), the relative permittivity ε _r of the substrate 2 is 5
3, The thickness h of the substrate 2 is 1.14 mm.

The chromatic dispersion compensable amount per 1 cm of the length of the microstrip line under the condition is 505 p.
s / nm / cm. Therefore, the length of the conventional example is 1
The chromatic dispersion compensation amount per cm is 73 ps / nm, which is about 7 times, so the length of the microstrip line is set to about 40 cm by a delay equalizer and an optical fiber with a transmission distance of 10,000 km is used. The chromatic dispersion that occurs can be compensated. As described above, the delay equalizer of the present invention is remarkably miniaturized as compared with the conventional example.

Similarly, if the transmission rate is 5 GHz (curve b), the relative permittivity ε _{r of} the substrate 2 is 39, and the thickness h of the substrate 2 is h.
= 0.67 mm, and the transmission speed is 10 GHz
Assuming (curve c), the relative permittivity ε _{r of} the substrate 2 is 28 and the thickness h of the substrate 2 is h = 0.39 mm.

When the strip conductor 1 can be manufactured in a range of 50 μm (curve e), the transmission speed is 2.5 GHz.
In the case of (curve a), the relative permittivity of the substrate 2 ε _r = 6
9, the thickness of the substrate 2 h = 1mm, the transmission speed is 5GH
In the case of z (curve b), the relative permittivity of the substrate 2 ε _r = 5
3, the thickness h of the substrate 2 is 0.57 mm, and when the transmission speed is 10 GHz (curve c), the relative permittivity ε _{r of} the substrate 2 is 39, and the thickness h of the substrate 2 is 0.33 mm. Becomes

For forming the substrate, various relative permittivity ε
_rDielectrics are known and of any thickness
Since it is possible to form a substrate, as described above,
The best way to increase the compensable amount of chromatic dispersion in an optical fiber.
Relative permittivity ε that meets appropriate conditions_rAnd has a thickness h
It is possible to realize the substrate 2. Under such conditions
Suitable substrates include, for example, titanic acid as the dielectric.
Barium (BaTiO ₃ ) Is preferable.

Needless to say, the dielectric loss of the substrate 2 in this case must be small in the frequency band used. Further, it is considered that the width W of the strip conductor 1 can be reduced to 50 μm or less by the progress of the manufacturing technique, but the delay equalizer can be further miniaturized by finding the optimum conditions corresponding to this. Can be achieved.

[0053]

As described above, according to the present invention, the shape of the strip conductor 1 is selected so that the cutoff frequency f _TE is higher than the frequency at which the wavelength dispersion compensation of the optical fiber can be compensated. The width W is within a manufacturable range and a desired characteristic impedance Z ₀
Since the relative permittivity ε _r and the thickness h of the substrate 2 are selected to be maximum, the chromatic dispersion compensation amount can be increased, and therefore the delay equalizer can be downsized. .

[Brief description of drawings]

FIG. 1 is a diagram showing a basic configuration of the present invention.

FIG. 2 is a diagram showing an embodiment of the present invention.

FIG. 3 is a diagram for explaining chromatic dispersion compensation at an intermediate frequency stage, in which (a) shows a circuit configuration for chromatic dispersion compensation;
(B) shows the delay characteristic of the optical fiber and the delay characteristic of the microstrip line used for compensation.

FIG. 4 is a diagram showing an example of a layout of a microstrip line, where (a) shows a case where the width of the strip conductor is wide, and (b) shows a case where the width of the strip conductor is narrow.

[Explanation of symbols]

 1 Strip conductor 2 Board 3 Ground conductor

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication H04B 10/26 10/14 10/04 10/06

Claims (4)

[Claims] 1. A delay equalization by a microstrip line in which a strip conductor (1) is formed on a substrate (2) made of a dielectric material and a ground conductor (3) is formed on a lower surface of the substrate (2). In this device, the cutoff frequency f _TE of the lowest TE mode of the microstrip line is higher than the wavelength dispersion compensable frequency by the delay equalizer, and the cutoff frequency f _TE and the strip conductor (1) that can be manufactured are The relative permittivity ε _r and the thickness h of the substrate (2) are determined so that a predetermined characteristic impedance Z ₀ can be obtained with an upper limit value that is determined from the minimum width W _0. And a delay equalizer. 2. The delay equalizer according to claim 1, wherein
The relative permittivity ε _r of the substrate (2) is A delay equalizer characterized in that the value given by is defined as an upper limit value. 3. The delay equalizer according to claim 1, wherein
The thickness h of the substrate (2) is A delay equalizer characterized in that the value given by is defined as an upper limit value. 4. The barium titanate (B) is used as the substrate (2).
4. The delay equalizer according to claim 1, wherein the delay equalizer is made of aTiO ₃ ) as a dielectric.