A BIDIRECTIONAL ROUTER AND A METHOD OF MONODIRECTIONAL AMPLIFICATION

Abstract: 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 DELTA L between the two optical branches which connect the two 3 dB couplers.

Description: A BIDIRECTIONAL ROUTER AND A METHOD OF MONODIRECTIONAL AMPLIFICATION Background art The invention relates to a router as stated m the introductory portion of claim 1 and to a method of monodirec- tional amplification of bidirectional optical signals as stated m the introductory portion of claim 8. In optical transmission systems it is frequently desired to use an optical fibre for bidirectional communication. This is achieved m most practical systems by using wave- length multiplexing so tnat transmission m one direction takes place at one or more wavelengths, and so that transmission m 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, nowever, is complicated and consequently involves relatively huge costs. Summary of the invention When, as stated m claim 1, the router comprises two op- tical 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 is 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 monodirec- tional 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 an- other 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 fur- ther on the light guide m the same direction as when it arrived at the router. When, as stated m claim 2, the delay device comprises a difference m distance ΔL between the two optical guides connecting the two couplers, a simple embodiment of the invention is obtained, as the difference m 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 m the mixing m 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 m distance between the two serially connected couplers. When, as stated m claim 3, 3 dB couplers are used, a particularly simple embodiment of the invention is ob- tamed. The use of 3 dB couplers will usually be pre ¬ ferred, as the characteristic of the complete router is particularly simple when the optical branches of the constituent couplers are symmetrical. When, as stated m claim 4, 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 m the optical path m the delay element m response to an electrical field applied by the electrodes. When, as stated m claim 5, the delay element is provided with one or more pairs of electrodes arranged along the optical path m the delay element to achieve a supplementary time delay, an advantageous embodiment of the mven- tion 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, as stated in claim 6, ΔL is equal to λ 2 / (2Δλn) 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. ^ FSR (FSR = free spectral range) , a practical embodiment of the invention is ob ¬ tained. 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 μ . When, as stated in claim 7, the router is made in an in- tegrated design, an optimum design for commercial use is 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, as stated in claim 8, the optical signals in each direction toward the router are fed to the first bidirec- tional 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, re- spectively, an effective bidirectional amplification is obtained, using relatively inexpensive elements. The bidirectional amplification obtained is moreover obtained using just one monodirectional amplifier. When, as stated in claim 9, λ rl and λ r2 are allocated on the power transfer function of the router in one trans ¬ mission direction on each side of a maximum of λ R , and λu and λi 2 are allocated on the power transfer function of the router in the other transmission direction on each side of a maximum of λ L , said bidirectional optical signals having the wavelengths λn and λ 12 in one direction and having the wavelengths λ ri and λ r ; in the other direction, said λ L and λ R indicating 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 tech- nique, as a two-channel signal may thus be transmitted and amplified each way through the router. The invention will be described below with reference to the drawing, in which fig. 1 shows a communications system consisting of two network elements, fig. 2 shows an MZI router according to the invention, fig. 3 shows a known coupler, fig. 4 shows a preferred embodiment of the invention, fig. 5 shows a first channel coupling characteristic for an MZI router of the invention, and fig. 6 shows an additional channel coupling characteristic for an MZI router of the invention. Example Fig. 1 shows a communications system consisting of two network elements 1 and 2 connected by a wavelength multiplexed bidirectional optical connection 3. The network element 2 transmits at the wavelength λ L , and the network element 1 transmits at the wavelength λ . Since the connection is wavelength-divided, it is possible to transmit communications signals from 1 to 2 while transmitting from 2 to 1. In practical systems, the connection 3 is an optical fibre which subjects the transmitted signals from both 1 and 2 to 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 connection 3. If there is one or more locations on the connection 3 where the signals, which are transmitted from both 1 and 2, 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 for amplifying signals transmitted from both 1 and 2. Fig. 2 shows a known router 10. The router 10 has two bidirectional ports 5 and 6 and two unidirectional ports 7 and 8. An amplifier 9 is inserted between the two unidirectional ports 7 and 8. The input of the amplifier is connected to the unidirectional port 7, and the output of the amplifier is connected to the unidirectional port 8. On the ports 5 and 6, the router 10 is connected to two optical fibres 3' and 3" which are connected to the ports 5 and 6, respectively. The router 10 is arranged such that a signal transmitted at the wavelength λ L into the router through the port 5 has maximum power on the port 7 and minimum power on the port 8. Correspondingly, a signal transmitted at the wavelength λ R into the router through the port 6 has maximum power on the port 7 and minimum power on the port 8. The amplifier may therefore amplify the signals at both λ R and λ L . The amplified signals are transmitted via the same router 10 through the port 8. The amplified sig- nal at λ R is transmitted out through the port 5, and, correspondingly, the amplified signal at λ L is transmitted out through the port 6. Such a router 10 thus ensures that a traditional unidirectional amplifier may be used for amplifying bidirectional signals. In the figure, a unidirectional amplifier 9 is connected to the unidirectional output port 9 of the router 10 and the unidirectional input port 8 of the router 10. Fig. 3 shows how a known router 10 is constructed. The router comprises four wavelength multiplex couplers 15, 16, 17 and 18. The wavelength multiplex couplers are also called WDM couplers. The wavelength multiplex coupler 15 is connected to the wavelength multiplex coupler 17 via an optical connection 11. The wavelength multiplex coupler 16 is connected to the same wavelength multiplex coupler 17 via an optical connection 12. The wavelength multiplex coupler 17 is subsequently optically connected to the port 7. The wavelength multiplex coupler 15 filters such that the optical signal λ L , received on the port 5 via the connection 11, is fed to the wavelength multiplex coupler 17, while the wavelength multiplex coupler 16 filters such that the optical signal λ R , received on the port 6 via the connection 12, is fed to the wavelength multiplex coupler 17. The complete signal consisting of λ R and λ L is thus fed to the port 7, which may subsequently be con- nected to an optical amplifier capable of amplifying the complete received signal from the fibre 3' and 3", respectively. Subsequently, an input port 8 feeds the complete ampli- fied signal to the wavelength multiplex coupler 18, which separates the received amplified optical signal again into two amplified signals consisting of λ R and λ L , respectively, which are fed via the connections 14 and 13 to the wavelength multiplex coupler 15 and the wavelength multiplex coupler 16, respectively, which subsequently feed the amplified signals at λ R and λ, respectively, out to the ports 5 and 6 connected to them. Fig. 4 shows a preferred embodiment of the invention. The shown router 10 of the invention comprises two 3 dB couplers 21 and 22. The coupler 21 comprises ports A, A', D and D', and the coupler 22 comprises ports B', B, C and C. The ports A' and B' are interconnected optically, 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 to fig. 4 an optical field E, (Λ λ ) at the wavelength λi ap ¬ plied to the port A and a second field E 2 ( l 2 ) at the wavelength λ 2 applied to the port D of an ideal 3 dB coupler will give rise to an optical field on the port A' , D' where the transmission matrix Ti of the 3 dB coupler is defined: 1 e J Without loss of generality, losses in the transmission A 1 to B' and D' to C and the absolute time delay in the transmission may be disregarded. The only important pa ¬ rameter in the transmission is therefore the difference in distance Δ L between the two optical connections A' to B' and D' to C. The transmission matrix T 2 for the four- port A', B', C, D' may be written: Since B', C in fig. 4 are connected to another ideal 3 dB coupler, the transmission matrix T 3 for the port B', C, B, C is known, since T 3 = T x . The overall transmission matrix T s for the port A, D, B, C may be written T Ξ = T 3 T 2 T ! and the fields on the ports B and C may thereby be calcu- lated Owing to the symmetry of the optical circuit, the transmission matrix T s may 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 T s For this field to be extinguished, the coefficients of £ " ,(/!,,) and E 2 {λ 2 ) must be zero. This is satisfied if ΔL is selected so that 2π — nAL = p2π A, and 2τ — nAL = p2π + π λ 2 where p e N, the set of natural numbers. Similar calculations give the resulting field on the port B: E B = -E 1 (λ l ) + E 2 (λ 2 This means that the fields E, (λ, ), E 2 (λ 2 ) 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 E B is amplified and coupled on the port C, the transmission matrix T s may 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: E c = ] (Λ i J + E 2 (A 2 and results in a field on the port A: E , = E 2 λ 2 2 Similarly, the field out of the port D is calculated: E D = E 1 (λ 1 This means that the field received e.g. on the port A at the wavelength λ L may be amplified and transmitted out of the port D, and a field received on the port D at the wavelength λ R may 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 op- tical insulators are used m connection with the two ter ¬ minals of the optical amplifier. If it is defined that E 2 (2 2 ) = 0 on the port D and E x λ, ) on the port A have the power Pi, the resulting power P s and P c on the port B and the port C, respectively, may be calculated and where frequency is substituted for wavelength. It will be seen that the two power transfer functions are offset π with respect to each other and are period with the period Δf = FSR, the free spectral range FSR = nAL Fig. 5 shows a first channel coupling characteristic for an MZI router. The figure shows a first example of how two frequency multiplexed channels m each direction may be allocated m relation to the power transfer function. The power transfer function of the MZI router has two minima/maxima m a specific frequency band at λ R and λ ^ , respectively. The four channels are positioned two by two m terms of frequency so that the two wavelengths λ Lι and λ L -> associated with λ R are positioned on each side of mm- lma/maxima λ κ and so that the two wavelengths λn and λ associated with λ L are positioned on each side of minima/maxima λ . It is noted that the shown allocation windows Δrl, Δr2 and Δll, Δ12 indicate the wavelengths which may be selected for each of the above-mentioned four channels λ 1± , λ r2 , λ x and λi?. 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 cer- tain minimal spacing between the channels on each side of λ R and λ, respectively, since there is a certain tolerance on the laser sources used for each channel. Fig. 6 shows 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 Δrl, Δr2, Δll and Δ12 may 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.

Claims: P A T E N T C L A I M S 1. An optical router, c h a r a c t e r i z e d in that the router comprises two optical couplers (21, 22) which are serially interconnected via a delay device and wherein the optical router further comprises an optical amplifier (9) optically connected to one of the optical couplers (21, 22 ) . 2. An optical router according to claim 1, c h a r a c t e r i z e d in that the delay device (23) comprises a difference in distance ΔL between the two optical guides which connect the two couplers (21, 22) . 3. An optical router according to claim 1 or 2, c h a r a c t e r i z e d in that the couplers (21, 22) are 3 dB couplers. 4. An optical router according to claims 1-3, c h a r - a c t e r i z e d in that the delay device is formed by one or more pairs of electrodes arranged along the optical path. 5. An optical router according to claim 3, c h a r a c - t e r i z e d in that the delay element is additionally provided with one or more pairs of electrodes arranged along the optical path in the delay element to achieve a supplementary time delay. 6. An optical router according to claim 2, c h a r a c t e r i z e d in that ΔL = λ 2 / (2Δλn) , where λ indicates the optical wavelength used, n is the refractive index, and Δλ indicates the half-period of the power transfer function in each direction, i.e. ^FSR (FSR = free spectral range) . 7. An optical router according to claims 1-6, c h a r - a c t e r i z e d in that the router is made in an integrated design. 8. A method of monodirectional amplification of bidirectional optical signals with given wavelengths λu and λ r in an optical guide (3) by means of an optical router according to claims 1 to 7 having a first bidirectional port (A) and a second bidirectional port (D) and a first unidirectional port (B) and a second unidirectional port (C) , c h a r a c t e r i z e d in that the optical sig- nals 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 (9) connected to the unidi- rectional ports and from there through the second unidirectional port (C) of the router and back through the router to the second bidirectional port (D) and the first bidirectional port (A) , respectively. 9. A method according to claim 8, c h a r a c t e r i z e d in that λ r ι and λ r2 are allocated on the power transfer function of the router in one transmission direction on each side of a maximum of λ R , and that λn and λ 12 are allocated on the power transfer function of the router in the other transmission direction on each side of a maximum of λ L , said bidirectional optical signals having the wavelengths λn and λ i2 in one direction and having the wavelengths λ rl and λ, 2 in the other direction, and said λ and λ R indicating 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.

Inventors: KRISTIANSEN RENE ENGEL, KRISTIANSEN, RENE, ENGEL
Applicants: DSC COMMUNICATIONS AS, KRISTIANSEN RENE ENGEL, DSC COMMUNICATIONS A/S, KRISTIANSEN, RENE, ENGEL
Application year: 1997
Main classification: H04J14
IPC codes: G02B6, H04Q11