OPTICAL WAVELENGTH CONVERTER

Abstract: A wavelength converter comprises a loop of optical fibre having an entry end (14) and an exit end (16). The entry end of the loop forms an input for a pump wavelength and the exit end forms an outlet for a converted signal. The loop is arranged to produce Stokes shifted wavelength components from a pump wavelength by Raman effect. A wavelength selective coupler (18) is provided between the entry and exit end (14, 16) of the loop. The coupler permits exit of a predetermined wanted Stokes shifted wavelength component but causes coupling of lower order Stokes wavelengths to the entry for recirculating in the loop to enhance generation of the predetermined wavelength component.

Description: OPTICAL WAVELENGTH CONVERTER This invention relates to an optical wavelength converter suitable for providing from a laser a high power converted wavelength suitable for use as a pump for a remote optically pumped amplifier (ROPA) . The converter of the invention may also be adapted to provide signal amplification by stimulated Raman Scattering and may be employed in an optical signal transmission system. This invention has resulted from a consideration of the difficulty of obtaining a high power pump for a ROPA e.g. 1 Watt at 1480 nm. Such a power and wavelength is not currently obtainable by means of semiconductor devices. Accordingly, the invention seeks to enable such a drive power to be achieved and has led to the provision of a wavelength converter. According to the invention there is provided a wavelength converter comprising a loop of optical fibre an entry end of which loop forms an input for a pump wavelength and an exit end of which loop forms an outlet for a converted signal, which loop is arranged to produce Stokes shifted wavelength components from a pump wavelength by Raman effect, and a wavelength selective coupler between the entry and exit ends of the loop, which coupler permits exit of a predetermined wanted Stokes shifted wavelength component but causes coupling of lower order Stokes wavelengths to the entry for recirculation in the loop to enhance generation of the predetermined wavelength component. The converter may permit exit to line of the predetermined wavelength component . In a refinement of the invention it is arranged that from the entry and exit ends of the loop the fibre extends to form a second loop, one end of the second loop forms an entry end which forms a loop input for pump wavelength and the other end of the second loop forms a loop output for a converted signal and a second wavelength selective coupler is provided between the entry and exit ends of the second loop, which second coupler permits exit of a predetermined second Stokes shifted component of wavelength longer than said predetermined wavelength but causes coupling of the other wavelengths to the entry of the second loop for recirculation through the loops to enhance generation of the second predetermined wavelength. The second coupler may permit exit to line of the second predetermined wavelength. In a further refinement it is arranged that from the entry and exit ends of the second loop the fibre extends to form one or more additional loops each having an entry end which forms a loop input for the pump wavelength and an exit end of the loop for a converted signal, each loop having a wavelength selective coupler between its entry and exit ends which permits exit of a predetermined Stokes Shifted wavelength component of wavelength longer than that at the exit of the preceding loop but causes coupling back of other wavelengths into the entry of the loop for recirculation through the loops to enhance generation of the shorter wavelength Stokes shift components and the coupler of the last loop permits exit to line of the longest predetermined wavelength. The or each wavelength selective coupler may be a wavelength division multiplexer for example a fused fibre device. The invention also extends to a pump for an optical fibre transmission system comprising the combination of a laser and a wavelength converter, as hereinbefore defined, wherein the laser is coupled to the input of the loop or loops of the wavelength converter to provide at the outlet a wavelength longer than that of the laser wavelength. Yet another aspect of the invention is that it extends to an optical transmission system including a pump as hereinbefore defined provided with an additional input coupled to the input of the loop or loops for introduction of a signal, thereby to permit signal amplification in the loop or loops by stimulated Raman scattering. An extension of the invention is in the provision of an optical fibre signal transmission system comprising a remote optically pumped amplifier (ROPA) and a pump as hereinbefore defined having its outlet coupled to the pump input of the ROPA. In order that the invention and its various other preferred features may be understood more easily, some embodiments thereof will now be described, by way of example only, with reference to the drawings, in which: - Figure 1 illustrates graphically the trace of a Raman Spectrum after a single pass through an optical fibre, Figure 2 illustrates schematically a single loop wavelength converted constructed in accordance with the invention, Figure 3 illustrates schematically a double loop wavelength converter constructed in accordance with the invention, Figure 4 illustrates schematically a triple loop wavelength converter constructed in accordance with the invention, Figure 5 illustrates graphically the pass band response of the three types of couplers in the triple loop wavelength converter of Figure 4, Figure 6 is a block diagram of part of an optical fibre signal transmission system employing a ROPA and incorporating a pump employing the principles of this invention and, Figure 7 is a block diagram of part of an optical fibre signal transmission system including an optical signal amplifier employing the principles of this invention. In the drawings, the same reference numbers are given to similar components throughout to aide description by avoiding repetition. This invention is based on the non linear Raman effect. Photons within a fibre may be scattered by collisions with the lattice. In doing so, they will impart energy in the form of a characteristic lattice vibration and by conservation of energy will themselves be downshifted in wavelength. The Raman absorption spectrum in Optical fibre is quite broad and typically centred around a lOOn shif . The effect is probabilistic and so observable as a function of optical power density. Given a single length of fibre and a sufficiently powerful pump (of arbitrary wavelength) , it is possible to observe Raman scattering as the formation of downshifted peaks. Power is transferred from the pump to the lattice by scattering. These photons having lost energy are down¬ shifted in wavelength resulting in the first peak. The photons may then further inter-act with the lattice to produce yet another downshifted peak, and so on. The probability of interaction once is obviously greater than that for two interactions, thus the intensity will be respectively less for higher order peaks, such peaks are shown in the graphical trace illustrated in Figure 1. Each peak is commonly termed as a Stoke's shift. Figure 2 illustrates a basic optical wavelength converter using the Raman effect in a loop 10 of fibre, which may be standard silica fibre or any other form of optical fibre. An input from a laser is provided at input 12 at a fundamental wavelength. This wavelength causes generation or transfer of power from one wavelength to another by Stokes shifting. The loop has an entry end 14 and a loop exit end 16 and a wavelength selective coupler 18, e.g. a four port fused fibre wavelength division multiplexer (WDM) , is coupled between the two ends. The WDM is arranged to permit transfer of the fundamental wavelength SO and also the first Stokes Shift wavelength SI. The fundamental drive wavelength may be provided by a laser e.g. an ND:YAG Laser and when introduced to the loop from the end 14 the Raman effect causes some of the pump energy to be transferred to the first Stokes wavelength and is subsequently circulated within the loop. Energy from the first Stokes wavelength is then transferred to the second Stokes wavelength. Energy at the first Stokes shift wavelength is coupled between the exit and entry ends of the loop so that it continues to circulate and is converted to the second Stokes shift wavelength to enhance generation of the second Stokes shift wavelength to provide a source of high power at the second Stokes wavelength at the loop exit end which may be onwardly coupled to line 20. The raised closine characteristic (typical of fused fibre devices) provides coupling of the second Stokes wavelength to the exit end of the loop. The arrangement of Figure 2 would be achieved by using the shortest coupler period indicated in Figure 5 and as such using 1064nm ND:YAG Laser, 1184nm would be available at the loop exit. Figure 3 shows an alternative embodiment of the invention in which two loops are employed and an output wavelength four Stokes shifts from the fundamental wavelength is achieved. The entry and exit ends 14 and 16 respectively of the loop 10 of Figure 2 are extended to form a second loop 22. One end 24 of the second loop forms an entry end for receiving an input at a fundamental wavelength from a laser. The other end of the second loop forms a loop output 26 for a converted signal. A second wavelength selective coupler 28 is provided between the entry and exit ends of the loop 22. The second coupler which is similar to the first coupler is however arranged to permit transfer of the fundamental wavelength SO and the second Stoke shifted wavelength S2 but not the fourth Stokes shifted wavelength 54 which is available at the exit end 26. It will be appreciated that the wavelengths SO and S2 are thereby circulated through the loops and are converted to the fourth Stokes shifted wavelength to enhance generation of that wavelength to provide a source of high power at the fourth Stokes wavelength at the loop exit 26 which may be onwardly coupled to the line 20. Figure 4 shows another alternative embodiment of the invention in which three loops are employed and an output 8 wavelength 8 Stokes shifts from the fundamental wavelength is achieved. The entry and exit ends 24 and 26 respectively of the second loop 22, shown in Figure 3, are extended to form a third loop 30 having an entry end 32 for the fundamental wavelength from a laser. The other end 34 of the third loop forms a loop output 34 for a converted signal. A third wavelength selective coupler 36 is provided between the entry and exit ends of the loop 30. The third coupler which is similar to the first and second couplers is however arranged to permit transfer of the fundamental wavelength SO and the fourth Stokes shifted wavelength S4 but not the eighth Stokes shifted wavelength S8 which is available at the exit end 34. It will be appreciated that the wavelengths SO and S4 are thereby circulated through the loops and are converted to the eighth Stokes shifted wavelength to enhance generations of that wavelength to provide a source of high power at the eighth Stokes wavelength at the loop exit 34 which may be onwardly coupled to the line 20. Figure 5 illustrates graphically the response of the three optical couplers used by the Figure 4 arrangement. The exact wavelength response of the WDM may be adjusted to either lengthen or shorten the peak converted wavelength (this will reduce efficiency) . It will be appreciated that the arrangement can be expanded to include additional loops to introduce higher Stokes shift to the output wavelength and such arrangements are considered to fall within the scope of this invention. Figure 6 is a block diagram of part of an optical fibre signal transmission system employing a ROPA provided by an erbium doped amplifier 40 provided in a signal transmission line 42. The amplifier 40 is pumped by a pump 44 constructed in accordance with the invention which is coupled to the line and to the amplifier by a coupler 46. The pump comprises a laser 48 and a wavelength converter 50 as previously described. A typical arrangement by way of example is to employ an ND:YAG Laser operating at 1319nm and a single loop converter such as illustrated in Figure 3 to derive 1480nm for pumping the amplifier 40. (Given that WDM response is tailored to be 1319 and 1400nm i.e. 80nm spacing) . At this point nothing has been stated on the Amplification capabilities of the design. If a Pump photon, a Lattice vibration and a Signal photon are all within the same vicinity in a fibre, then an effect called SRS may occur. SRS (Stimulated Raman Scattering) is where the presence of a signal photon will resonantly convert a pump photon to a signal one by the influence of a lattice vibration. Thus the signal will undergo Gain. If in Figure 3 for example a 1300nm signal is coupled in with the pump, then it will undergo gain via SRS between the 3rd Stokes (the SRS pump) and lattice vibrations - assuming the system is pumped by an alternative ND:YAG wavelength of 1064nm as previously described. The resulting Optical amplifier' will have a fairly broad gain spectrum and is suitable for the 1300nm optical window. Figure 4 could similarly be used at 1550nm with a suitable pump wavelength and in-fact the scheme is completely arbitrary and extendible to any wavelength or Stoke's shift. Figure 7 illustrates the employment of the invention as an optical amplifier in an optical fibre signal transmission system. In this system a line input 52 for an optical signal is coupled to a converter 54 such as previously described together with a fundamental pump drive wavelength from a pump 56. The converter 54 acts as an amplifier of the optical signal by SRS action and the output of the converter 54 is routed via an optional filter 58 e.g. a Bragg grating which passes the amplified signal but not the pump wavelength to line 60. Accordingly, the principles of this invention are considered to provide: 1) A wavelength converter taking an arbitrary pump wavelength and shifting it by an integral number of Stokes shifts. 2) Simple construction based on one or more couplers and a length of fibre. 3) The possibility of being used as a high gain and high powered broadband amplifier. 4) Application as a ROPA pump in an unrepeatered optical signal transmission system. The invention enables the provision of a simple all fibre based Raman Laser than can be easily manufactured from present technology components which may have dual functionality either as an amplifier or wavelength shifting laser source. The design is arbitrary and only dependant on the fibre properties and pump wavelength used. The WDM as a frequency selective element permits a relatively wide gain spectrum when used as an amplifier and also a broad pump spectrum as a Laser. The invention is usable in unrepeatered optical signal transmission systems as a high powered pump source for a ROPA or in Repeatered systems which require high power and a broad gain spectrum e.g. Solitons.

Claims: CLAIMS : 1. A wavelength converter comprising a loop (10) of optical fibre an entry end (14) of which loop forms an input for a pump wavelength and an exit end (16) of which loop forms an outlet for a converted signal, which loop is arranged to produce Stokes shifted wavelength components from a pump wavelength by Raman effect, and a wavelength selective coupler (18) between the entry and exit ends (14, 16) of the loop, which coupler permits exit of a predetermined wanted Stokes shifted wavelength component but causes coupling of lower order stokes wavelength(s) to the entry for recirculation in the loop to enhance generation of the predetermined wavelength component. 2. A wavelength converter as claimed in claim 1, wherein the coupler (18) permits exit to line (20) of the predetermined wavelength component. 3. A wavelength converter as claimed in claim 1, wherein from the entry and exit ends (14, 16) of the loop the fibre extends to form a second loop (22) , one end (24) of the second loop forms an entry end which forms a loop input for pump wavelength and the other end (26) of the second loop forms a loop output for a converted signal and a second wavelength selective coupler (28) is provided between the entry and exit ends of the second loop, which second coupler (28) permits exit of a predetermined second Stokes shifted component of wavelength longer than said predetermined wavelength but causes coupling of the other wavelengths to the entry (24) of the second loop for recirculation through the loops (22, 10) to enhance generation of the second predetermined wavelength. 4. A wavelength converter as claimed in claim 3, wherein the second coupler (28) permits exit to line of the second predetermined wavelength. 5. A wavelength converter as claimed in claim 3, wherein from the entry and exit ends (24, 26) of the second loop the fibre extends to form one or more additional loops (30) each having an entry end (32) which forms a loop input for the pump wavelength and an exit end (34) of the loop for a converted signal, each loop having a wavelength selective coupler (36) between its entry and exit ends which permits exit of a predetermined Stokes Shifted wavelength component of wavelength longer than that at the exit of the preceding loop but causes coupling back of other wavelengths into the entry (32) of the loop for recirculation through the loops (30, 22, 10) to enhance generation of the shorter wavelength Stokes shift components and the coupler (36) of the last loop (30) permits exit to line (20) of the longest predetermined wavelength. 6. A wavelength converter as claimed in any one of the preceding claims, wherein the or each wavelength selective coupler (18, 28, 36) is a wavelength division multiplexer (WDM) . 7. A wavelength converter as claimed in any one of the preceding claims, wherein the or each wavelength selective coupler (18, 28, 36) is a fused fibre device. 8. A pump for an optical fibre transmission system, comprising the combination of a laser (48) and a wavelength converter (50) as claimed in any one of the preceding claims wherein the laser is coupled to the input (14, 24, 32) of the loop or loops (10, 22, 30) of the wavelength converter to provide at the outlet a wavelength longer than that of the laser wavelength. 9. A pump as claimed in claim 8, wherein the laser wavelength is 1319nm. 10. A pump as claimed in claim 8 or 9, wherein the outlet wavelength is 1480nm. 11. A pump as claimed in claim 8 or 9, wherein the outlet wavelength is 1550nm. 12. An optical signal amplifier, comprising a pump (54, 56) as claimed in any one of claims 8 to 11, provided with an additional input (52) coupled to the input of the loop or loops (10, 22, 30) for introduction of a signal, thereby to permit signal amplification in the loop or loops by stimulated Raman scattering. 13. An optical fibre signal transmission system, comprising a remote optically pumped amplifier (ROPA) (40) and a pump (44) as claimed in any one of claims 8 to 11 having its outlet coupled via a coupler (46) to the ROPA.

Inventors: WEBB STEPHEN MICHAEL, WEBB, STEPHEN, MICHAEL
Applicants: STC SUBMARINE SYSTEMS LTD, WEBB STEPHEN MICHAEL, STC SUBMARINE SYSTEMS LIMITED, WEBB, STEPHEN, MICHAEL
Application year: 1997
Main classification: H01S3
IPC codes: H01S3