Bidirectional router and a method of bidirectional amplification

The invention relates to a router which may be used for amplification of a bidirectional optical signal using a single optical amplifier.An advantageous embodiment of the invention comprises two 3 dB couplers which are serially connected via a delay element. According to the embodiment, the delay element comprises a difference in distance ΔL between the two optical branches which connect the two 3 dB couplers.

TECHNICAL FIELD OF THE INVENTION

The invention relates to a router and to a method of monodirectional amplification of bidirectional optical signals.

BACKGROUND OF THE INVENTION

In optical transmission systems it is frequently desired to use an optical fibre for bidirectional communication. This is achieved in most practical systems by using wavelength multiplexing so that transmission in one direction takes place at one or more wavelengths, and so that transmission in the other direction takes place at one or more other wavelengths different from the first-mentioned wavelengths.

Since the signals are transmitted through an optical fibre, they will be subjected to attenuation, which necessitates amplification of the optical signals if they are to be transmitted over great distances.

According to the prior art this bidirectional amplification may be achieved by suitable coupling of wavelength multiplex couplers and a unidirectional amplifier. This method, however, is complicated and consequently involves relatively huge costs.

SUMMARY OF THE INVENTION

When the router comprises two optical couplers interconnected serially via a delay device and wherein the optical router further comprises an optical amplifier optically connected to one of the optical couplers, a simple and economical router obtained, which may be designed according to simple dimensioning principles and be adapted to concrete applications. The property that for each optical input an optical coupler ideally divides an arriving optical signal between the outputs of the coupler means that an output signal from the first coupler contains mixed signals, which may subsequently be “mixed back” in the following optical coupler. In a suitable embodiment of the delay device, this back-mixing may have the effect that signals with different wavelength components may be fed jointly and selectively to a selected output port on the following coupler, ideally, with conservation of energy, as the interferometer properties of the delay device are utilized.

This complete signal may additionally be fed back into a port on the following coupler, whereby the input ports of the first coupler also serve as output ports.

This property is particularly advantageous in applications where a bidirectional optical signal is to be amplified with a monodirectional amplifier, as a monodirectional amplifier may be coupled between the terminals of the last coupler and amplify both optical signals, following which these, in an amplified state, may be fed back to the bidirectional port of the router. It is noted in particular that the amplified signal is routed to another bidirectional port, for which reason the complete router may be coupled between two fibre ends of a directional light guide cable having a fibre end for bidirectional router ports, amplify arriving optical signals with given wavelengths, and transmit these out on the other bidirectional port to the other fibre end and further on the light guide in the same direction as when arrived at the router.

When the delay device comprises a difference in distance ΔL between the two optical guides connecting the two couplers, a simple embodiment of the invention is obtained, as the difference in distance ΔL provides a mutual phase shift between the two optical signals on the input of the following coupler, which means that the coupler serves as an interferometer in the mixing in the coupler itself.

It will be appreciated that ΔL is not to be taken to mean a separate physical element, but is an indication of the real MZI difference in distance between the two serially connected couplers.

When 3 dB couplers are used, a particularly simple embodiment of the invention is obtained. The use of 3 dB couplers will usually be preferred, as the characteristic of the complete router is particularly simple when the optical branches of the constituent couplers are symmetrical.

When the delay device is formed by one or more pairs of electrodes arranged along the optical path, a further embodiment of the invention is obtained, wherein a desired phase shift between the optical signals may be achieved by changing the refractive index in the optical path in the delay element in response to an electrical field applied by the electrodes.

When the delay element is provided with one or more pairs of electrodes arranged along the optical path in the delay element to achieve a supplementary time delay, an advantageous embodiment of the invention is obtained, as a desired phase shift between the optical signals may be obtained at an optical difference in distance ΔL, and be finely adjusted by changing the refractive index in the optical path in the delay element in response to an electrical field applied by the electrodes.

When ΔL is equal to λ2/(2Δλπ) where λ indicates the optical wavelength used, n is the refractive index, and Δλ indicates the half-period of the power transfer function, i. e.

12⁢FSR
FSR (FSR=free spectral range), a practical embodiment of the invention is obtained.

For clarity, it should be mentioned that a selected wavelength of 1550 nm, a refractive index n=1.5, and Δλ=10 nm, result in a difference in distance of ΔL=80 μm.

When the router is made in an integrated design, an optimum design for commercial use obtained. This should be taken to mean that the actual design of the delay element is to be made with a relatively great precision, as the necessary distances ΔL are relatively small, and even small deviations therefrom give rise to a relatively great unreliability with respect to the overall system.

When the optical signals in each direction toward the router are fed to the first bidirectional port A and the second bidirectional port D. respectively, of the router and from there to the first unidirectional port B of the router, further through an optical amplifier connected to the unidirectional ports and from there through the second unidirectional port C of the router and back through the router to the second bidirectional D and the first bidirectional port A, respectively, an effective bidirectional amplification is obtained, using relatively inexpensive elements. The bidirectional amplification obtained is moreover obtained using just one naonodirectional amplifier.

When λr1and λr2are allocated on the power transfer function of the router in one transmission direction on each side of a maximum of λR, and λ11and λ12are allocated on the power transfer function of the router in the other transmission direction on each side of a maximum of λL, said bidirectional optical signals having the wavelengths λ11and λ12in one direction and having the wavelengths λr1and λr2in the other direction, said λLand λRindicating a maximum in a specific frequency band for the power transfer function of the router in one direction and the power transfer function of the router in the other direction, respectively, an effective amplification of a bidirectional signal is obtained, using a relatively simple and inexpensive technique, as a two-channel signal may thus be transmitted and amplified each way through the router.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1shows a communications system consisting of two network elements1and2connected by a wavelength multiplexed bidirectional optical connection3. The network element2transmits at the wavelength λL, and the network element1transmits at the wavelength λR. Since the connection is wavelength-divided, it is possible to transmit communications signals from1to2while transmitting from2to1. In practical systems, the connection3is an optical fibre which subjects the transmitted signals from both1and2to attenuation through the fibre. If the system is to be used over great distances, it is necessary to insert one or more amplifiers in the connection3.

If there is one or more locations on the connection3where the signals, which are transmitted from both1and2, have travelled such a great distance through the optical fibre as makes it necessary to amplify them, then a router is inserted so that a single traditional unidirectional amplifier may be used transmitted from both1and2for amplifying signals transmitted from both1and2.

The router10has two bidirectional ports5and6and two unidirectional ports7and8. An amplifier9is inserted between the two unidirectional ports7and8. The input of the amplifier is connected to the unidirectional port7, and the output of the amplifier is connected to the unidirectional port8.

On the ports5and6, the router10is connected to two optical fibres3′ and3″ which are connected to the ports5and6, respectively.

The router10is arranged such that a signal transmitted at the wavelength λLinto the router through the port5has maximum power on the port7and minimum power on the port8. Correspondingly, a signal transmitted at the wavelength λRinto the router through the port6has maximum power on the port7and minimum power on the port8. The amplifier may therefore amplify the signals at both λRand λL. The amplified signals are transmitted via the same router10through the port8. The amplified signal at λRis transmitted out through the port5, and, correspondingly, the amplified signal at λLis transmitted out through the port6. Such a router10thus ensures that a traditional unidirectional amplifier may be used for amplifying bidirectional signals.

In the figure, a unidirectional amplifier9is connected to the unidirectional output port9of the router10and the unidirectional input port8of the router10.

The router comprises four wavelength multiplex couplers15,16,17and18. The wavelength multiplex couplers are also called WDM couplers.

The wavelength multiplex coupler15is connected to the wavelength multiplex coupler17via an optical connection11. The wavelength multiplex coupler16is connected to the same wavelength multiplex coupler17via an optical connection12. The wavelength multiplex coupler17subsequently optically connected to the port7.

The wavelength multiplex coupler15filters such that the optical signal λL, received on the port5via the connection11, is fed to the wavelength multiplex coupler17, while the wavelength multiplex coupler16filters such that the optical signal λR, received on the port6via the connection12, is fed to the wavelength multiplex coupler17. The complete signal consisting of λRand λLis thus fed to the port7, which may subsequently be connected to an optical amplifier capable of amplifying the complete received signal from the fibre3′ and3″, respectively.

Subsequently, an input port8feeds the complete amplified signal to the wavelength multiplex coupler18, which separates the received amplified optical signal again into two amplified signals consisting of λRand λL, respectively, which are fed via the connections14and13to the wavelength multiplex coupler15and the wavelength multiplex coupler16, respectively, which subsequently feed the amplified signals at λRand λL, respectively, out to the ports5and6connected to them.

FIG. 4shows a preferred embodiment of the invention.

The shown router10of the invention comprises two 3 dB couplers21and22.

The coupler21comprises ports A, A′, D and D′, and the coupler22comprises ports B′, B, C′ and C.

The ports A′ and B′ are interconnected optically by a delay device23that may include a pair of electrodes24according to an example embodiment of the invention, and also the ports D′ and C′ are interconnected optically.

The central aspect of the invention is the transmission matrix T of the optical 3 dB coupler. With reference toFIG. 4an optical field Ē1(λ1)at the wavelength λ1applied to the port A and a second field Ē2(λ2) at the wavelength λ2applied to the port D of an ideal 3 dB coupler will give rise to an optical field on the port A′, D′.

[E⇀A″E⇀D′]=12⁡[1ⅇj⁢⁢π2ⅇj⁢⁢π21]⁡[E⇀1⁡(λ1)E⇀2⁡(λ2)]
where the transmission matrix T1of the 3 dB coupler is defined:

Without loss of generality, losses in the transmission A′ to B′ and D′ to C′ and the absolute time delay in the transmission may be disregarded. The only important parameter in the transmission is therefore the difference in distance ΔLbetween the two optical connections A′ to B′ and D′ to C′. The transmission matrix T2for the four-port A′, B′, C′, D′ may be written:

Since B′, C′ inFIG. 4are connected to another ideal 3 dB coupler, the transmission matrix T3for the port B′, C′, B, C is known, since T3=T1. The overall transmission matrix Tsfor the port A, D, B, C may be written
Ts=T3T2T1
and the fields on the ports B and C may thereby be calculated

Owing to the symmetry of the optical circuit, the transmission matrix Tsmay also be used for calculating the fields which will occur on the ports A and D as a function of the fields applied to the ports B and C, i.e. the opposite way back through the router. It is noted that, ideally, no field is applied to B but just to the port C according to the invention.

As another object of the invention is the extinction of the field on the port C caused by the fields on the port A and D, the conditions of this extinction are made in the light of the transmission matrix Ts

For this field to be extinguished, the coefficients of Ē1(λ1) and Ē2(λ2) must be zero. This is satisfied if ΔL is selected so that

2⁢⁢πλ1⁢n⁢⁢Δ⁢⁢L=p⁢⁢2⁢⁢π
and

2⁢⁢πλ2⁢n⁢⁢Δ⁢⁢L=p⁢⁢2⁢⁢π+π
where pεN, the set of natural numbers.

Similar calculations give the resulting field on the port B:
ĒB=−Ē1(λ1)+Ē2(λ2

This means that the fields Ē1(λ1)+Ē2(λ2) are transmitted out of the port B with full amplitude, and that the fields will be extinct on the port C, thereby allowing a unidirectional amplifier to be used between the terminals B and C.

If the field EBis amplified and coupled on the port C, the transmission matrix Tsmay be used for calculating the field which occurs on the port A and D as a consequence of the amplified field on the port C. The fields on the ports A and D caused by the field applied to the port C are calculated relatively to the field on the port C:

The field into the port C is defined:
ĒC=−Ē1(λ1)÷Ē2(λ2
and results in a field on the port

Similarly, the field out of the port D is calculated:
ĒD=−Ē1(λ1

This means that the field received e.g. on the port A at the wavelength λBmay be amplified and transmitted out of the port D, and a field received on the port D at the wavelength λRmay be amplified with the same amplifier and transmitted out of the port A.

A power consideration illustrates how an MZI router may directionally couple several channels at various wavelengths in each direction. This is possible, provided that complete extinction of the fields on the port C is not necessary. This may be achieved particularly when optical insulators are used in connection with the two terminals of the optical amplifier.

If it is defined that Ē2(λ2)=0on the port D and Ē1(λ1) on the port A have the power P1, the resulting power and PBand PCon the port B and the port C, respectively, may be calculated

PB=12⁢P1⁡(1+cos⁡(2⁢⁢π⁢⁢fc⁢n⁢⁢Δ⁢⁢L÷π))
and

PC=12⁢P1⁡(1+cos⁡(2⁢⁢π⁢⁢fc⁢n⁢⁢Δ⁢⁢L))
where frequency is substituted for wavelength. It will be seen that the two power transfer functions are offset with respect to each other and are period with the period Δf=FSR, the free spectral range

FIG. 5shows a first channel coupling characteristic for an MZI router. The figure shows a first example of how two frequency multiplexed channels in each direction may be allocated in relation to the power transfer function. The power transfer function of the MZI router has two minima/maxima in a specific frequency band at λRand λL, respectively. The four channels are positioned two by two in terms of frequency so that the two wavelengths λr1and λr2associated with λRare positioned on each side of minima/maxima λRand so that the two wavelengths λ11and λ12associated with λLare positioned on each side of minima/maxima λL. It is noted that the shown allocation windows Δr1, Δr2and Δ11, Δ12indicate the wavelengths which may be selected for each of the above-mentioned four channels λr1, λr2, λ11and λ12.

In the shown embodiment, one boundary of the allocation window is selected in consideration of the fact that the difference between the transmission of the power transfer function from A to D and vice versa must be at least 10 dB. It is noted that this boundary may vary from application to application.

The other boundary of each allocation window is selected in consideration of the fact that there should be a certain minimal spacing between the channels on each side of λRand AL, respectively, since there is a certain tolerance on the laser sources used for each channel.

FIG. 6shows another channel coupling characteristic for an MZI router. The figure shows another example of how two channels in each direction may be allocated in relation to the power transfer function. The power transfer function of the MZI router has four minima/maxima in a specific frequency band in which the four channels are positioned.

The allocation windows Δr1, Δr2and Δ11, Δ12may be selected in this case separately in consideration of the fact that the difference between the transmission of the power transfer function from A to D and vice versa must be at least 10 dB. It should be noted that this limit may vary from application to application.