Optical waveguide grating filter

Abstract: The isolation between stop and pass bands afforded by an optical fibre normally reflective Bragg grating (5) reflective over a waveband λ₃ in an optical fibre (2) is enhanced by preceding it with two blazed Bragg gratings (3, 4) that are spectrally selectively mode coupling respectively over wavebands λ₁ and λ₂, where the λ₃ waveband compasses the spectral waveband separating waveband λ₁ from waveband λ₂. The fibre is connected to an optical circulator (40) or one port of an optical 4-port 3dB coupler (50).

Description: This invention relates to optical waveguide grating filters and in particular to such filters that exhibit a relatively high degree of isolation between stop and pass bands. A transmission type filter with a defined pass-band can be constructed in a length of single mode optical waveguide by creating in the waveguide two Bragg gratings providing stop-bands respectively on the long-wavelength side and the short wavelength side of the intended pass-band. These two Bragg gratings may be normally reflective Bragg gratings whose elements extend in parallel planes normal to the waveguide axis, in which case their stop-bands are created by reflection of light within those wavebands reversing the direction of propagation of the guided light. Alternatively, they may be blazed Bragg gratings whose grating elements extend in parallel planes inclined at an oblique angle to the waveguide axis, in which case their stop-bands are created by mode conversion of light propagating in the waveguide from the guided mode into an unguided, and hence radiating, mode or group of modes. By way of example, normally reflective Bragg gratings may be made in optical fibre waveguides by the method described in US 4,725,110, and blazed Bragg gratings by the method described in US 5,042,897. A transmission type filter of this type may be constructed to provide an isolation of about 30 dB between its stop and pass bands. A similar degree of isolation can be obtained in a reflective type filter in single mode waveguide by the use of a single normally reflective Bragg grating. An example of an optical waveguide Bragg grating filter comprising an optical fibre provided with one or more normally reflective Bragg gratings is to be found for example in US 5 283 686. The present invention is directed to the provision of a filter affording the possibility of improved isolation. According to the present invention there is provided a single mode optical waveguide filter which includes the series combination comprising two optical waveguide blazed Bragg gratings optically in tandem with an optical waveguide normally reflective Bragg grating, wherein the periodicities of the three Bragg gratings are such that the blazed gratings are spectrally selectively mode coupling respectively over spectrally separated wavebands λ 1 and λ 2 , while the normally reflective grating is spectrally selectively reflecting over a waveband λ 3 which at least compasses the spectral waveband separating waveband λ 1 from waveband λ 2 , which series combination is optically coupled with one port of an optical multiport device consisting of an optical circulator or an optical 4-port 3dB coupler. It may be noted that the improved isolation afforded by such a structure is generated at least in part by the fact that the light transmitted by the filter makes a full double passage through each of the two blazed Bragg gratings. Such improved isolation may for instance be required in filters for use in association with optical waveguide amplifiers for separating wanted signal from spontaneous emission noise. Another illustrative application is in wavelength division multiplexed (WDM) systems extending between one or more optical transmitters and receivers, and where the wavelength spacing of the multiplexed channels is found to be such that the side-lobes of a normally reflective Bragg filter designed for one channel register with, or at least extend into, the spectral bands of other channels. It will be appreciated that in some circumstances the presence of one of the side-lobes presents a problem, while that of the other does not. In such circumstances one of the blazed Bragg gratings can clearly be dispensed with. Accordingly the invention also provides a single mode optical waveguide filter which includes the series combination comprising an optical waveguide blazed Bragg grating optically in tandem with an optical waveguide normally reflective Bragg grating wherein the periodicities of the two Bragg gratings are such that the blazed grating is spectrally selectively mode coupling over spectral waveband λ 1 , while the normally reflective grating is spectrally selectively reflecting over a waveband λ 3 a side lobe of which extends into spectral waveband λ 1 , which series combination is optically coupled with one port of an optical multiport device consisting of an optical circulator or an optical 4-port 3dB coupler. There follows a description of optical fibre filters embodying the invention in preferred forms. The description refers to the accompanying drawings in which:- Figure 1 schematically depicts a length of single mode fibre in which there is a series combination of two blazed Bragg gratings in tandem with a normally reflective Bragg grating; Figure 2 schematically depicts the spectral transmission characteristics of the series combination of the two blazed Bragg gratings of Figure 1; Figure 3 schematically depicts the spectral reflection characteristics of the normally reflective grating of Figure 1; Figures 4 and 5 schematically depict the fibre of Figure 1 optically coupled respectively with a three-port optical circulator and with a four-port 3dB tapered fused coupler, and Figure 6 depicts a filter comprising a 3dB tapered fused fibre coupler with the arrangement of three Bragg gratings as in Figure 1 in both fibres on one side of its coupling region. Referring to Figure 1, in the core 1 of a single mode fibre 2 are photo-induced two blazed Bragg gratings 3 and 4, and a normally reflective grating 5. The periodicities of blazed gratings 3 and 4 are such as to provide them with wavebands λ 1 and λ 2 (Figure 2) over which they respectively couple light from the propagating mode guided by optical fibre 2 into radiating modes whose power is dissipated. Accordingly, for a single passage of light through the two blazed gratings 3 and 4, they provide a spectral transmission characteristic as depicted by curve 20. Curve 30 shows the corresponding spectral reflection characteristic for the normally reflective Bragg grating 5. This Bragg grating, on its own, functions in reflection as a spectrally selective band-pass reflector reflective over a waveband λ 3 , but typically the construction of such filters is liable to produce a spectral characteristic with side-bands 30a and 30b. The construction of the Bragg grating 5 is such that the spectral waveband λ 3 over which it is selectively reflective at least compasses the waveband separating waveband λ 1 from waveband λ 2 , and may be, as depicted in Figure 3, slightly larger than this. The wavebands λ 1 and λ 2 of Bragg gratings 3 and 4 are preferably chosen to compass respectively the side-bands 30a and 30b. In this way, light launched into optical fibre 2 from the end nearest grating 3, the combination exhibits a spectral reflection characteristic in which the spectral discrimination of curve 30 provided by the spectral reflective characteristic of grating 5 is twice enhanced by the spectral discrimination of curve 20 provided by the spectral transmission characteristic of gratings 3 and 4. For use as a transmission type filter, the end of the combination nearer grating 3 is connected to a circulator 40 as depicted in Figure 4, or to one port of a 4-port 3dB optical coupler 50 as depicted in Figure 5. This coupler 50 may conveniently be constituted by a single mode tapered fused optical fibre type 3dB coupler. The arrangement of Figure 5 provides an additional 6dB loss which can be avoided by a Mach Zehnder type arrangement as depicted in Figure 6 in which identical grating elements 3, 4 and 5 are located in both fibres of a 3dB tapered fused coupler 60. In the case of the normally reflective gratings 5, it will be appreciated the light reflected by these gratings will interfere, and hence the phase relationship between the two reflected signals is critical in ensuring that when they interfere, they do so in a manner that directs substantially no output power back into the input port. Such a phase relationship can be ensured by the method described in the specification of United Kingdom Patent Application No. 2 283 831 that involves creating the gratings in both fibres simultaneously at a point in the coupler where the two fibres remain tacked together in side-by-side contact so that the resulting gratings are automatically formed optically equidistant from the coupling region of the coupler.

Claims: A single mode optical waveguide filter which includes an optical waveguide normally reflective Bragg grating (5) that reflects back into the optical waveguide, the normally reflective Bragg grating being optically coupled with one port of an optical multiport device consisting of an optical circulator (40) or a 4-port 3dB coupler (50) characterised in that optically interposed between the normally reflective Bragg grating (5) and the optical multiport device is an optical waveguide blazed Bragg grating (3), wherein the periodicities of the two Bragg gratings are such that the blazed grating (3) is spectrally selectively mode coupling over spectral waveband λ 1 , while the normally reflective grating (5) is spectrally selectively reflecting over a waveband λ 3 a side lobe of which extends into spectral waveband λ 1 . An optical waveguide filter as claimed in claim 1, wherein the optical multiport device is a 3dB tapered fused optical fibre coupler. An optical waveguide filter as claimed in claim 1 or claim 2, wherein optically interposed between the optical waveguide blazed grating and the optical waveguide normally reflective Bragg grating is an additional optical waveguide blazed Bragg grating (4), wherein the periodicities of the three Bragg gratings are such that the blazed gratings are spectrally selectively mode coupling respectively over spectrally separated wavebands λ 1 and λ 2 , while the normally reflective grating is spectrally selectively reflecting over a waveband λ 3 which at least compasses the spectral waveband separating waveband λ 1 from waveband λ 2 . An optical fibre filter including a 3dB tapered fused optical fibre coupler (60) having an optical coupling region formed between a pair of single mode fibres, on one side of which coupling region, in both fibres is formed a series combination including an optical fibre blazed Bragg grating (3) optically in tandem with an optical waveguide normally reflective Bragg grating (5) that reflects back into the respective fiber, wherein the periodicities of the two Bragg gratings are such that the blazed grating is spectrally selectively mode coupling over spectral waveband λ 1 , while the normally reflective grating is spectrally selectively reflecting over a waveband λ 3 a side lobe of which extends into spectral waveband λ 1 . An optical fibre filter as claimed in claim 4, wherein the series combination comprises two optical fibre blazed Bragg gratings (3, 4) optically in tandem with an optical waveguide normally reflective Bragg grating (5), wherein the periodicities of the three Bragg gratings are such that the blazed gratings are spectrally selectively mode coupling respectively over spectrally separated wavebands λ 1 and λ 2 , while the normally reflective grating is spectrally selectively reflecting over a waveband λ 3 which at least compasses the spectral waveband separating waveband λ 1 from waveband λ 2 . An optical fibre filter as claimed in any preceding claim, wherein the Bragg gratings are photo-induced gratings. A method of operating an optical transmission system which method of operation includes transmission of optical signals from at least one transmitter to at least one receiver through one or more filters as claimed in any preceding claim.

Inventors: BYRON KEVIN C, BYRON, KEVIN C.
Applicants: NORTEL NETWORKS LTD, NORTEL NETWORKS LIMITED
Application year: 1995
Main classification: G02B6
IPC codes: G02B6, H01S3