GB2264792A - Optical filter - Google Patents

Abstract

An optical system is provided, comprising first and second waveguides 1, 2 constituting a transmission path for optical signals. Interposed in a gap 3 between the ends of the two fibres is an insert 4 which extends halfway down the gap. A wavefront of wavelength lambda propagating between the waveguides encounters the insert and shears, and the portion passing through the insert is retarded with respect to the remainder. Depending on wavelength lambda , the two portions may constructively or destructively interfere. For two propagating wavefronts at lambda 1 and lambda 2, one may be coupled into waveguide 2 whilst the other may be coupled out of the transmission path. An effective filter is thus provided. An application in a passive optical network is described. The insert may alternatively consist of a slot cut halfway into a single fibre. <IMAGE>

Description

OPTICAL COMMUNICATIONS The present invention relates to optical communications. It has particular application to optical communications systems with wavelength selectivity, and to optical filters.

In optical systems it is frequently desired to filter out a particular wavelength. In order to make the most effective use of investment in communications media and networks, it is sought to use as much of potential bandwidth as possible. In optical systems, one way of improving use of available bandwidth is to multiplex a number of waves of different wavelengths, each carrying different signals, on a single waveguide.

There is an increasing need for optical wavelength filters to separate out different signals in optical wavelength multiplexed systems. In such systems, telephony signals may be carried at a wavelength around 1300nm, and video and other broadband signals at around 1550nm, and optical filters are needed to separate out the two bands of signals. For example, it may be desired to reject 1550nm so that signals at that wavelength do not adversely affect reception by a telephony customer's receiver, of telephony signals at 1300nm.

It is expected that such systems will have widespread commercial use. Optical filters will need to be cheap to manufacture in bulk and to have good performance characteristics. For certain applications, a good rejection ratio will be important; that is, the ratio of desired signal transmitted to undesired signal rejected will be very large. In different wavelength multiplexed systems, different wavelengths will be used for transmission depending on the optical sources and receivers used, and the nature of the systems. It will be convenient to be able to manufacture filters adapted to transmit well in a selected wavelength region, and to reject effectively in another desired region, so that the system will not be constrained by the availability of suitable filters.

One system which requires optical filters at customers' premises is the present applicants' Passive Optical Network (PON) described in British Telecom Technology Journal, April 1989, Volume 7 No 2, pages 89-99. One approach which is described is to transmit a set of time division multiplexed (TDM) telephony signals. These signals are at a wavelength of around 1300nm and are carried on a single optical fibre from an exchange. A sequence of passive splitters is used to feed the full set of TDM signals to individual customers. The bits for a given customer aretime accessed for him at his receiver. In addition to TDM telephony signals, the system is used for transmitting broadband signals to those customers who have subscribed to a broadband service. These signals are carried at around 1550nm, together with some maintenance signals.An optical filter adjacent to the customer' s telephony receiver enables the network operator to provide the customer with telephony signals, but filters-out maintenance and broadband signals.

The present applicants have conducted a trial of this system in which a single fibre is split 128 ways. A laser at the exchange end transmits TDM telephony signals in the 1260 to 1340nm window. In addition broadband signals are transmitted at 1500, 1525 and 1550nm and maintenance signals at 1575nm. Each telephony-only customer is provided with a filter which passes only the telephony wavelengths. Currently, multilayer dielectric interference filters are used in the PON system, although there can be problems with spurious reflections. A lmm square filter is precision-mounted in a ferrule, with optical fibres inserted on either side. A 1300nm signal has a typical loss of only 1. 5dB in transmission via the filter from one fibre to the other, whereas signals between 1500 and 1600nm are attenuated by around 20dB.There can be reflection problems which can be serious, especially in networks incorporating optical amplifiers.

Attempts have been made, in Japan and elsewhere, to develop improved filters with reduced reflection problems.

An alternative possibility is to provide a multilayer filter integrally on an optical detector. However, there are certain disadvantages with such filters; for example, there may be relatively high rejection of the desired signal.

Depending on the intended application for the filter, cost of manufacture may be an important factor. Certain prior art filters can be costly to produce.

The present invention provides, in one form, a filter which has a region of non-uniform refractive index.

According to a first aspect, the present invention provides an optical communications system with wavelength selectivity.

According to a second aspect, the invention provides an optical system in which first and second portions of a wavefront take different lengths of time to pass from a first position to a second position, such that at the second position the portions reinforce or cancel each other to an extent dependent on the wavelength of the wave.

Embodiments of the invention will now be described, by way of non-limiting example, with reference to the accompanying drawings in which: Figure 1 is a schematic drawing in cross-section of a first embodiment of the invention; Figure 2 is an enlarged view of part of the arrangement of Figure 1, from the direction A-A shown in Figure 1; Figure 3 is a slightly enlarged version of Figure 1 showing a propagating wavefront at various instants; Figure 4 is a transmission characteristic for the filter of Figure 1; Figure 5 is a transmission characteristic for a second filter; Figure 6 is a schematic drawing of an embodiment of a network; Figure 7 is a schematic sectional view of a second embodiment of a filter; and Figure 8 is an enlarged perspective view of the central portion of the filter of Figure 7.

Referring to Figure 1, an optical communications system according to a first embodiment of the invention comprises a first monomode optical fibre having a core 1 of diameter around 10m and a second monomode optical fibre having a core 2 of similar diameter. Together, the fibre cores 1, 2 constitute a transmission path for signals propagated along the two fibres. Interposed in a gap 3 of about 50m between the ends of the two fibres is a piece of silica material 4 which is conveniently rectangular in shape and is about 1. 25m thick. As shown in Figure 2, the material 4 extends half-way down the gap between the cores 1 and 2.

Figure 2 shows the end of the core 2 (which has radius r and centre 0), and also the mode field 5 of a propagating wave which has radius R about 10m (R - 2r), at the 1/e level. As the mode propagates from fibre 1 towards fibre 2, approximately half the energy of the mode encounters material 4. Material 4 has refractive index n around 1. 5, which is higher than that of the surrounding air, and the portion of the mode which propagates through the material 4 is retarded relative to the portion propagating through air in the gap, thus shearing the wave front of the propagating wave.

Figure 3 shows wavefront 6 at four instants, to, tl, t2 and t3. At to, the wavefront is propagating along core 1 in guided fashion. By a subsequent time tl, the wavefront has left the guided region and entered unguided region 3, where the medium is air. Next, part of the wave front encounters material 4 and shears as one part of the wavefront is retarded with respect to the remainder, which advances through air, having a lower refractive index. At t3, the sheared portions continue to propagate through air, until they reach core 2. Thus, because the portion which passes through the material 4 is slowed by the higher refractive index, it takes longer to pass from core 1 to core 2 than the portion which does not pass through material 4.

Depending on the phase difference between the two portions of wavefront, a proportion of the energy of the mode will be coupled into the core 2 and guided along it. If the two portions destructively interfere (phase difference is an odd multiple of s) then little energy is coupled into core 2, whereas if there is constructive interference (phase difference is an integer multiple of 2s) then most energy is coupled into core 2 and continues along the transmission path.

Material 4 should be carefully positioned so as to extend 50% into the mode field of propagating waves at and 2 for good transmission/rejection ratios TA2/TAl where TA is the transmission at frequency A. Ideally, a point on the lower edge of material 4 will be within around 0. 1m of the centre of the mode field of a propagating wave from core 1.

The distance between the ends of the two fibre cores 1, 2 can be relatively great. In this embodiment, material 4 is of similar refractive index to the fibre, and is surrounded by air in region 3, and the fibres ends are around 50pom apart. This distance could be increased to 100cm, and the additional transmission loss for a transmitted wavelength resulting from the increased spacing will typically be only about ldB.

The filter illustrated in Figure 1 can be manufactured by first aligning all the components to a reasonable degree of accuracy, and then injecting some u.v. curable adhesive onto each component. Light is then transmitted down fibre 1, and the positioning of the components adjusted until a satisfactory rejection ratio is achieved for two selected wavelengths A 3t2. The adhesive is then cured.

Figure 4 shows a transmission characteristic for the filter of Figure 1. This is the measured transmission across a range of wavelengths for light transmitted from the core of fibre 1, via region 3 and into the second fibre. It will be seen that wavelengths around 1250nm will be substantially coupled out, as the transmission loss is around 20dB, whilst wavelengths between 1400 and 1500nm will suffer only 1-2 dB loss.

Figure 5 shows a transmission characteristic for another similar filter with a thicker piece of material corresponding to material 4 in Figure 1. The phase difference wf introduced into a wavefront of a mode of wavelength A travelling through region 3 is much larger than in the Figure 1 embodiment. There is now a range of wavelengths in the optical transmission region which can be coupled into fibre core 2 with only a low loss - for example #A, #B, #C. Wavelengths A 0' A E and A F are partially coupled out, with losses around 12-16 dB.

A filter can be produced so as to allow good transmission between core 1 and core 2 in one wavelength region, and to couple out very substantially another wavelength region. With reference to the filter shown in Figure 1, suppose the refractive index difference between the air in region 3 and material 4 is an, the distance travelled through the material 4 (ie its thickness) is L and the wavelength of a signal propagating along fibre 1 is A, then a phase difference + is introduced between the delayed and non-delayed portions of the sheared wavefront after transmission through region 3 where: = = 2s x An x L # If, for example, an = 0. 5, L = 1.25pm and A = 1250nm then ## = it, and there is destructive interference.The effect is that the mode of a signal at A = 1250nm from core 1 is very substantially coupled out of the transmission path, and only a residual fraction of the signal at A is coupled into core 2. The loss at A = 1250nm is about 20dB between the two fibres, as shown in figure 4.

It will normally be important to select the length and refractive index of material 4 so that An and L combine to maximise transmission of wavelengths in the region of one wavelength A1 and minimise transmission of another set of wavelengths in the region of 2. This will be achieved if the ratio A 2 is approximately equal to ##2/##1, where A+2 is the phase difference introduced between the two parts of the wave front of a wave at A2 after transmission through region 3, and ##1 is the corresponding phase difference for the wavefront of a wave at A1. A for A1 will need to be - 2sd where d is (1, 2, 3 ), and t+2 for a wave at 12 will need to be - it(2d + 1). For example, if A1 . 1300nm and A2 .= 1550nm then A2/A1 .= 1. 2 and a good choice for A4)1 is 6n and for A is 5#. In this case, the thickness of the material interposed between the fibre ends will be around 7.8pm if its refractive index is 1. 5. It will be appreciated that by selecting the refractive index and thickness of material d, a filter can be produced to transmit in one selected wavelength region and reject in another.

Figure 6 shows schematically a network which includes a number of filters embodying the present invention. A transmitter 10 and receiver 11 for transmitting and receiving telephony signals in the range 1260 - 1340nm are provided at the exchange end. Telephony signals are multiplexed by multiplexer 12 with broadband and maintenance signals in the 1550nm window, and the signals are split at a number of optical splitters in the cabinet 13 and at the distribution point ld. Each of these divides arriving optical power between a number of points. At the customers premises, a wavelength demultiplexer 15 splits 1300nm telephony signals from the 1550nm signals, transmitting mainly telephony signals at 1300nm on line 16.

Filter 17 is located adjacent receiver 18, and provides further rejection in the 1550nm region. By itself, WDM 15 provides rejection at 1550nm of only around lldB. A much higher rejection ratio is required for adequate performance by the receiver 18, because received power at 1550nm would be likely to be much higher than that at 1300nm. For adequate isolation for the telephony service, a rejection of 36dB is required in the 1550nm region. After lldB rejection at the WDM 15, another 25dB needs to be rejected at the filter 17. This level of rejection can be provided by filters embodying the present invention.

It is also important that the loss introduced by filter 17 at 1300nm is small, because losses in the network as a whole are already high. Filters embodying the invention can provide loss below 2dB and, with careful selection of transmitted wavelength and filter material, transmission can be maximised and losses reduced to below ldB. It will be appreciated that a filter should be provided at each receiver at a customer' s premises, and also at receiver 11 at the exchange. Consequently, it is desirable to minimise the cost of each device.

The phase shift a introduced by shearing the wavefront of a mode at 1300nm is selected to be 2nd where d = 1, 2, 3 ... and the phase shift in the 1550nm region to be it (2d ffi 1). This is achieved by choosing an and L for the filter appropriately as described above.

In the system of Figure 6, a second filter 19 is provided in the customers premises to reject telephony signals, but to transmit broadband signals in the 1550nm region to receiver 20. Again, lldB loss at 1300nm has already been achieved at the WDM 15. The rejection requirements for filter 19 are less stringent than those for filter 17 because of the lower power of the signals at 1300nm. Filtering out at 1300nm can be achieved following the same principles as at 1550nm. Thus at 1300nm will be s(2d i 1) and at 1500nm A will be 2nd.

The Fibres having cores 1 and 2 in Figure 1 are monomode silica fibres, and it is anticipated that the principal use of this invention will be in a monomode silica environment. However, other suitable guiding means may be provided on each side of the filtering region. For example, waveguides could be integrated. If multimode fibres are used steps should be taken to avoid interference between modes - for example a single mode at each wavelength may be propagated along the incoming waveguide.

Filtering occurs where part of a propagating wave front encounters a material of higher refractive index than the medium encountered by the remainder of the wavefront.

Typically this occurs as a guided mode enters a non-guided region. In the embodiment of Figure 1, a material 4 was impinged by 50% of the energy of a propagating mode, bisecting the wavefront. The remainder of the wavefront propagated through air from fibre core 1 to fibre core 2.

The material 4 was described as a silica insert, but the present invention is not limited to such a construction.

For example, Figures 7 and 8 show, schematically, an alternative construction. In Figures 7 and 8 a single circular waveguide of refractive index n has a small semicircular section 100 removed. The gap may be air-filled, or filled with a material of refractive index nl different from the adjacent waveguide.The phase difference introduced for a mode at A propagating along portion 102 of the waveguide and passing through unguided region 103 before entering portion 102 is, as before, = = 2s (n - n1) A So far, the propagation of a pair of wavelengths, A1 and 12, through a first guided region has been described, with the filtering out, or rejection, of one wavelength in an unguided region and onward transmission into a second guided region for the other. It will often be desirable to optimise the device for two particular wavelengths in order to achieve low loss for one and high rejection for another.

However, as described with reference to Figure 5, it is possible to propagate several wavelengths (Ao, AEl AF) which will all suffer high loss and several (A A8, XC) which will be substantially unaffected by the presence of the filter.

Also, in practice, good rejection and good transmission will each be achieved for a range of wavelengths. Thus, in the example illustrated in Figure 6, telephony signals in the 1300nm range are rejected at filter 19, but four different broadband channels in the regions 1500, 1520, 1550 and 1575nm are transmitted to receiver 20. If rejection of several significantly different wavelengths is desired, then a series of filters could be provided, each adapted to transmit a selected desired wavelength As but to reject a respective one of the wavelengths to be rejected.

In the embodiment of Figure 1, a propagating mode is sheared on encountering material 4, which extends half-way between fibre cores 1 and 2. Various alternatives are possible, provided that roughly 50% of the mode field encounters a material of a first refractive index and the remainder encounters a material of a different refractive index. For example, material d could be annular.

In certain applications, or for ease of manufacture, it may be desirable to expand the mode field.

Claims (10)

1. An optical filter having a region of non-uniform refractive index such that a wavefront propagating through the region is sheared into portions which, for a certain wavelength or set of wavelengths At, destructively interfere at least partially so as to reduce onward transmission at At.

2. An optical communications system comprising an optical transmission path, an element of which is provided by a waveguide over which are transmitted first signals at wavelength A1 and second signals at a second wavelength 12 and an optical filter in the transmission path to filter out said second wavelength to a significant extent without substantially affecting said first signals characterised in that the filter comprises a region in the mode field of the signals of non-uniform refractive index located so as to shear a propagating wave front.

3. An optical system in which part of the energy of an optical signal takes a first length of time to pass from one place to another and part of the energy takes a second length of time to pass between the two places, so that the signal is reinforced or cancelled hy interference in a wavelength dependant manner.

4. An optical filter comprising first and second guiding regions defining a transmission path and an intermediate region of non-uniform refractive index such that a substantial part of the energy of a mode at wavelength At propagating along the first guiding region is coupled out of the transmission path and a substantial part of the energy of a mode of wavelength Aj is transmitted from the first to the second guiding regions.

5. An optical filter as claimed in claim 4, wherein said first and second guiding regions comprise single mode optical waveguides.

6. A filter as claimed in claim 4 or claim 5, wherein the intermediate region is adapted so that the filter transmits a substantial proportion of the energy of a mode in the 1300nm window, and rejects a substantial proportion of the energy of a mode in the 1550nm window.

7. A filter as claimed in claim 4 or claim 5, wherein the intermediate region is adapted so that the filter transmits a substantial proportion of the energy of a mode in the 1550nm window and rejects a substantial proportion of the energy of a mode in the 1300nm window.

8. A filter as claimed in any one of claims 5 to 7, wherein the intermediate region comprises a first section of refractive index n which receives approximately 50% of the energy of a mode propagating from the first guiding region and a second section of refractive index nl, wherein nl < n, which receives the remainder of the energy of the mode.

9. A filter as claimed in claim 8, wherein the refractive index difference (n-nl) and the length of the region of nonuniform refractive index traversed by the mode portions arie selected so that mode portions at a wavelength A1 constructively interfere and mode portions at a wavelength A2 destructively interfere after passing through said region.

10. A filter as claimed in claim 8 or claim 9 wherein the first section comprises material of similar refractive index to that of the adjacent guiding regions and the second section comprises air.