JPH0522209B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The invention relates to a selective directional coupler consisting of two waveguides, preferably extending in parallel, to be coupled to each other.

Directional couplers whose degree of coupling depends on the frequency or wavelength of electromagnetic oscillations to be coupled out are required, for example, in carrier frequency communication transmission in which the same transmission medium is used in a plurality of channels by frequency multiplexing. A typical example of use is simultaneous transmission and reception using optical signals over a single glass fiber in wavelength division multiplexing. In this case, in one direction it is also transmitted at a different wavelength than in the opposite direction. At the end or repeater of a fiber section of this type, a signal is transmitted at a wavelength λ ₀ via a selective directional coupler, as shown in FIG. 1, and received in the opposite direction at another wavelength λ _e . .

The selectivity of the directional coupler helps to provide all transmit power at λ ₀ into the fiber and transmit all incoming input at λ _e to the receiver. The selectivity of the directional coupler is further facilitated by its directional action to reduce near-end crosstalk, so that even at high transmit powers, only a small amount of crosstalk can be reached with little disturbance to the receiver. do not.

Another advantage of this dual transmit/receive communication device is that the fundamental wave received from the single mode fiber for this use can have any polarization. This is because the fundamental wave passes through the selective directional coupler without actually combining.

The object of the invention is to provide a selective direction coupler of the type mentioned at the outset, which can be implemented in a simple manner.

This is solved by the features described in the claims.

A selective directional coupler for this and similar uses is constructed as shown in FIG. 2
Two successive waveguides 1 and 3 are coupled to each other in the section Z=O to Z=L via a waveguide section 2 located between them.

Successive waveguides 1 and 3 have the same cross section. For the purposes of the calculations, it is only assumed here that the waves in waveguides 1 and 3, which are to be selectively coupled together, have the same phase constants β ₁ =β ₃ =β. In practice, this condition need only be satisfied for frequencies ₀ to wavelength λ ₀ where the total power is to be overcoupled from one waveguide to the other by selective coupling.

The coupling of the waves in the waveguides 1 and 3 takes place via one of the waves guided by the intermediate waveguide 2. For calculation purposes, we assume the following: That is, the wave in waveguide 1 couples with this wave in waveguide 2 with the same strength as the wave in waveguide 3 couples with this wave in waveguide 2. However, in reality, this condition only needs to be satisfied from frequency ₀ to wavelength λ ₀ .

Under the above conditions and when losses in the coupler can be ignored, for the wave amplitudes A ₁ , A ₂ and A ₃ above, the following system of coupled differential equations arises: That is, dA ₁ /dZ=−jβA ₁ −jcA ₂ dA ₂ /dz=−jcA ₁ −jβ ₂ A _{2 −jcA 3} dA ₃ /dz= −jcA ₂ −jβA 3Then β ₂ is is the phase constant of coupled wave 2, and c is the coupling coefficient for the coupling of waves 1 and 3 with this coupled wave.

This system of three coupled waves has three natural waves moving independently of each other along the coupling section. The phase constants of these eigenwaves are β and β+
δ+√ ² +2 ² and β+δ− ^{√ 2} +2 ² , where δ=(β=β ₂ )/2 denotes half the difference in the phase constants of waves 1 and 3 and of the combined wave 2.

In the general solution for the amplitudes of waves 1, 2 and 3, the eigenwaves are superimposed as follows.

A ₁ =w ₁ e ^-j 〓 ^z +w ₂ e−j (β+δ+√ ² +2 ² )z +w ₃ e−j (β+δ−√ ² +2 ² )z A ₂ =δ+√δ ² +2c ² /cw ₂ e −j(β+δ+√ ² +2
² )z +√δ ² +2c ² −δ/cw ₃ e−j(β+δ−√ δ ²
+2c ² )z A ₃ =w ₁ e ^-j 〓 ^z +w ₂ e−j (β+δ+√ δ ² +2c ² )z +w ₃ e−j (β+δ−√ δ ² +2c ² )z Waveguide at z=0 When only one entrance is damped, the initial conditions at Z=0 are A ₁ =1 and A ₂ =A ₃ =0. In this vibration isolation wave 1
and 3 have the following amplitude values along the coupler:
That is, The following two limiting cases are of particular importance for practical use. That is, if 1 β ₂ =β The combined wave 2 has the same phase constant as both waves 1 and 3. In this case, δ=0 and the amplitude values are as follows. That is, |A ₁ |=1/2|1+cos(√2cz)| |A ₃ |=1/2|1−cos(√2cz)| Therefore, in phase-locked coupled wave 2, the output is wave 1
and 3 along the coupling interval.

In the case of z=(2m+1)π/(√2c) (if m=0, 1, 2...), the output is completely wave 3.
and if z=2mπ/(√2c) (where m=0, 1, 2...), the output is completely guided by wave 1. For complete power conversion from wave 1 to wave 3, the coupler is optimally made to have a length of L=π/(√2c) (2).

2 If |δ|≫c A complete power conversion from wave 1 to wave 3 is only possible for δ=0, ie in a phase-locked coupled wave. For δ≠0 only part of the input of wave 1 is converted into wave 3 in each case. Moreover, the second limit example δ
≫It is very small in c. That is, under this condition, approximately from equation (1) for the amplitude value, |A ₁ |1−jc ² /2〓 ² sin δz e ^-j 〓 ^z |A ₃ |c ² /2〓 ² sin δz Established.

According to the above approximation formula, only the part c ⁴ /(4δ ⁴ ) of the input is converted into wave 3 in the best case. The remainder remains mainly in wave 1, but a small portion is also guided into coupled wave 2. However, wave 1 loses only a portion of its input c ² /δ ² in the best case under these conditions.

By the way, in order to obtain the desired selectivity, that is, to perform lossless output conversion from frequency ₀ to wavelength λ ₀ , and to convert as little output as possible at a predetermined frequency different from that, the following An intermediate waveguide such as That is, the combined wave 2 is phase-locked with waves 1 and 3 at = ₀ , but has a sufficient phase difference at the cut-off frequency, so that the condition |δ|≫c
This is an intermediate waveguide that satisfies the following conditions.

For optical frequencies, these requirements are met by using dielectric films or strips as waveguides. These optical films or strip waveguides, for example according to FIG. 3, are embedded in a transparent material with a refractive index n ₀ . Waveguides 1 and 3 have the same cross section and the same refractive index n ₁ >n ₀ in the embodiment of FIG. The intermediate waveguide must have a relatively large refractive index n ₁ >n ₀ and its cross section must also be larger than the cross section of waveguides 1 and 3, depending on the selectivity requirements.

FIG. 4 shows the phase constant β of the waves in the intermediate waveguide, which can be used as the fundamental wave in waveguides 1 and 3 as well as the combined wave, as a function of frequency in a dispersion diagram. All phase curves have their starting point at their respective cutoff frequency on a straight line n ₀ 2π/c ₀ for the wavenumber in the surrounding medium with index of refraction n ₀ . In this case, c ₀ is the speed of light in vacuum. As the frequency increases, the phase curve asymptotes to a straight line for the wave number of each waveguide material. Except for the phase curve of the fundamental wave, the phase curves of all higher-order waves in the intermediate waveguides intersect the phase curves of the fundamental wave in waveguides 1 and 3. Therefore all these higher order waves are transmitted through waveguides 1 and 3.
It can be used as a coupling wave between fundamental waves in . At the crossing frequency with the fundamental-phase curves of waveguides 1 and 3, a total power conversion between the fundamentals in 1 and 3 becomes possible.

Which waves are selected as coupling waves and how the waveguides 1 and 3 and the intermediate waveguide 2 are formed accordingly depends on the location of the frequencies to be coupled or decoupled. If the frequency spacing is relatively large, a lower-order coupled wave is selected; if the frequency spacing is relatively small and the selectivity is correspondingly relatively high, a higher-order coupled wave is selected. The selectivity can be increased both by increasing the refractive index in the intermediate waveguide and by enlarging its cross section. That is, the phase curve of the coupled wave in the intermediate waveguide intersects the phase curve of the fundamental wave in waveguides 1 and 3 at gradually increasing angles. The phase difference between these waves is then δ at the intersection of the curves
Starting from =0, it increases rapidly as the frequency deviation from the synchronous frequency increases.

Based on the example of a selective directional coupler consisting of a dielectric strip placed on a dielectric substrate S as shown in FIG. 5, what dimensions can be selected in relation to the wavelength λ of light waves or microwaves? I will explain about it. Two parallel beams of width b=3.5λ and height h=1.75λ with refractive index n ₁ =1.5, provided at mutual spacing a=b on substrate s with refractive index n ₀ =n ₁ /1.1 A simple directional coupler consisting of strips st1 and st2 couples the fundamental waves of the strips by a coupling coefficient c=0.002λ. When the same strip on the same substrate is selected for the selective directional coupler and a strip two to several times wider with a refractive index somewhat greater than n ₁ for the intermediate waveguide ZWL, the strips are By selecting the spacing between a=b to be somewhat smaller than a=b, the same coupling coefficient is set for the coupling of the fundamental wave in the outer strip and the phase-locked combined wave in the intermediate waveguide. Then, condition (2) is satisfied if L=1110λ. For the wavelength of light, L is on the order of mm. In order to require shorter couplers in integrated optical devices, the strips must be packed more closely together. Since the coupling coefficient depends exponentially on the spacing of the strips, even a slight reduction in the spacing will significantly shorten the coupler.

The substrate and film or strip of the selective directional coupler for optical frequencies can be made from quartz glass or another silicate glass.
In order to increase the refractive index of the film or strip compared to the refractive index of the substrate, and in particular to obtain a higher refractive index in the intermediate waveguide than in the two outer waveguides, quartz is used. The glass can be doped with germanium oxide or phosphorous oxide.

Larger refractive index differences can be obtained, for example, by using a substrate glass with a low refractive index, making the outer waveguide from a transparent polymer such as polyurethane, and making the intermediate waveguide from zinc sulphide. . Many different types of materials are conceivable for selective directional couplers of this type to operate at optical frequencies, but they must be sufficiently transparent to the optical wavelengths to be transmitted to keep coupler losses small. must be taken into account.

If neither the form of the waveguides nor the form of the intermediate waveguides to which electromagnetic energy is to be selectively converted between each other is limited to simple films or strips in or on the substrate, ridges and ridges may be used instead. Film waveguides with shaped conductors as well as strips are conceivable. FIG. 6 shows, as a typical example, a sectional view of a selective directional coupler, in particular for optical frequencies, in which the two outer waveguides are ridge-shaped waveguides RL1 and RL2 and an intermediate waveguide. Film waveguide with strip as wave body
A FWL is used, and the substrate of this film waveguide is formed by the same dielectric film to which the outer ridge waveguide also belongs. The refractive index n ₁ of the film and the ridge should be somewhat greater than the refractive index n ₀ of the substrate S, and the strip should have a refractive index n ₂ even greater than n ₁ .

Selective directional couplers for microwaves can also be constructed using dielectric strip waveguides, especially when using millimeter waves. This is because the dielectric strip has a smaller cross section. However, dielectric image lines and hollow waveguides are also conceivable.
FIG. 7 is a sectional view of a selective directional coupler comprising a dielectric image line. The three dielectric image lines BL1, BL2 and BL3 then extend parallel to each other on a common metal plate P. The two outer dielectric image lines have the same dimension cross-section and the same refractive index, while the inner dielectric image line, which is the intermediate waveguide, has a larger cross-section and a refractive index n greater than n ₁ Also has ₂ .

FIG. 8 shows in cross section a selective directional coupler consisting of rectangular hollow conductors H1, H2, H3. The intermediate hollow conductor is connected to the outer hollow conductor by, for example, a common spacing of hole rows L1 and L2. The intermediate hollow conductor H2 has a wider cross section than the outer hollow conductor or is partially or completely filled with dielectric material. In the embodiment of FIG. 8, both the configuration of having a wider cross-section than the outer hollow conductor and at the same time filling the intermediate hollow conductor H2 with dielectric material serve to increase the selectivity.

[Brief explanation of the drawing]

FIG. 1 is a diagram explaining the action of a selective directional coupler provided on the fiber end face, and FIG. 2 is a diagram illustrating the selective directional coupler of the present invention used for the purpose shown in FIG. 3 is a diagram illustrating the configuration of an embodiment of a directional coupler for optical frequencies, and FIG. 4 is a diagram for explaining the frequency and waveguides 1 and 3.
FIG. 5 is a cross-sectional view of one embodiment of the selective directional coupler of the present invention, and FIG.
The figure is a cross-sectional view of one embodiment of the selective directional coupler of the present invention suitable for optical frequencies, and FIG. 7 is a cross-sectional view of the selective directional coupler of the present invention comprising a dielectric image line.
FIG. 8 is a sectional view of a selective directional coupler of the present invention comprising a rectangular hollow waveguide. 1, 3...Waveguide to be coupled, 2...Intermediate waveguide, st1, st2...Strip conductor, ZWL...Intermediate waveguide, S...Dielectric substrate, RL1, RL2...
Rib-shaped conductor, FWL...film waveguide with strip, BL1, BL2, BL3...dielectric image line, H1, H2, H3...hollow waveguide, L1, L2...hole, n ₀ , n ₁ , n ₂ ... refractive index.

Claims (1)

[Claims] 1. A selective directional coupler having two waveguides to be coupled to each other, in which another optical waveguide 2 is arranged between the two optical waveguides 1 and 3. ,
Over its entire length, the further optical waveguide forms a unique coupler together with the other two waveguides 1, 3, and the coupled wave of the further waveguide has a desired coupling frequency _0. A selective directional coupler characterized in that it is phase synchronized with the waves in the two other waveguides 1 and 3. 2 Both waveguides 1 and 3 to be combined are intermediate waveguide 2
2. A selective directional coupler according to claim 1, wherein the selective directional coupler is coupled only with: 3. Both waveguides 1 and 3 to be coupled are dielectric films, and a dielectric film as an intermediate waveguide 2 is present between the waveguides.
Selective directional coupler as described in section. 4. The selection according to claim 1, in which both waveguides 1 and 3 to be coupled are dielectric strip conductors, and a dielectric film as intermediate waveguide 2 is present between the strip conductors. Directional coupler. 5. The selective directional coupler according to claim 1, wherein both waveguides 1, 3 and the intermediate waveguide 2 to be coupled are dielectric strips. 6. The selection direction according to claim 5, wherein both waveguides 1 and 3 to be coupled and the intermediate waveguide 2, which is a dielectric strip, are placed on a dielectric substrate with a low refractive index. combiner. 7. The selection direction according to claim 5, wherein both waveguides 1 and 3 to be coupled and the intermediate waveguide 2, which is a dielectric strip, are embedded in a dielectric substrate with a low refractive index. combiner. 8. The waveguides 1 and 3 to be coupled are formed of a ridge on a dielectric film, and the intermediate waveguide 2 is also formed of a ridge on a dielectric film. Selective directional coupler. 9 The waveguides 1 and 3 to be coupled and the intermediate waveguide 2 as a ridge-like conductor are placed on a dielectric substrate with a low refractive index together with the dielectric film of the waveguide. 9. The selective directional coupler according to item 8. 10 The two waveguides RL1, RL2 to be coupled consist of a ridge on a dielectric film and the intermediate waveguide is a film waveguide FWL with a strip.
2. A selective directional coupler as claimed in claim 1, formed from a strip on a film. 11 Waveguide RL to be coupled which is a ridge-shaped conductor
1. The intermediate waveguide, which is a film waveguide FWL with RL2 and a strip, is placed on a dielectric substrate S with a relatively low refractive index together with the common film of the waveguide. Range 10th
Selective directional coupler as described in section. 12. The selective directional coupler according to claim 1, wherein the waveguides BL1, BL3 and the intermediate waveguide BL2 to be coupled are dielectric image lines on a common metal plate P. 13 Claim 1 in which the coupling interval is variable
Selective directional coupler as described in section. 14. The selective directional coupler according to claim 6, wherein the dielectric has a variable refractive index. 15 The waveguides to be coupled are hollow waveguides H1 and H3
, and the intermediate waveguide is also a hollow waveguide H2, and the intermediate waveguide has a hole row L provided in a common partition wall.
2. A selective directional coupler according to claim 1, wherein the selective directional coupler is coupled to the waveguides H1, H3 by means of L1, L2. 16. The selective directional coupler according to claim 15, wherein the waveguide and intermediate waveguide to be coupled are rectangular intermediate waveguides. 17. Selective directional coupling according to claim 15, wherein the intermediate waveguide H2 is provided with a dielectric insert along the entire coupler, which at least partially fills the cross section of the intermediate waveguide. vessel.