GB2216678A - Optical filters - Google Patents

Optical filters Download PDF

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Publication number
GB2216678A
GB2216678A GB8610879A GB8610879A GB2216678A GB 2216678 A GB2216678 A GB 2216678A GB 8610879 A GB8610879 A GB 8610879A GB 8610879 A GB8610879 A GB 8610879A GB 2216678 A GB2216678 A GB 2216678A
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GB
United Kingdom
Prior art keywords
filter
waveband
reflection
filters
reflection filters
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
GB8610879A
Other versions
GB8610879D0 (en
GB2216678B (en
Inventor
James Allister Mcquoid
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Qioptiq Ltd
Original Assignee
Pilkington PE Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Pilkington PE Ltd filed Critical Pilkington PE Ltd
Publication of GB8610879D0 publication Critical patent/GB8610879D0/en
Publication of GB2216678A publication Critical patent/GB2216678A/en
Application granted granted Critical
Publication of GB2216678B publication Critical patent/GB2216678B/en
Anticipated expiration legal-status Critical
Expired - Lifetime legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/42—Coupling light guides with opto-electronic elements
    • G02B6/4201—Packages, e.g. shape, construction, internal or external details
    • G02B6/4246—Bidirectionally operating package structures
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/20—Filters
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29304—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by diffraction, e.g. grating
    • G02B6/29316—Light guides comprising a diffractive element, e.g. grating in or on the light guide such that diffracted light is confined in the light guide
    • G02B6/29317—Light guides of the optical fibre type
    • G02B6/29322—Diffractive elements of the tunable type
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29379—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device
    • G02B6/29395—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device configurable, e.g. tunable or reconfigurable
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29346—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by wave or beam interference
    • G02B6/29361—Interference filters, e.g. multilayer coatings, thin film filters, dichroic splitters or mirrors based on multilayers, WDM filters

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Diffracting Gratings Or Hologram Optical Elements (AREA)

Abstract

A tunable transmission filter (3) is provided which has first and second reflection fibers, optionally holographic, (1) (2) each of which selectively reflects different wavebands at different positions along the filter, the two reflection filters (1, 2) being disposed so that a position on the first filter which reflects a first waveband is optically aligned with a position on the second filter which reflects a second waveband spaced from the first waveband so that wavelengths between the first and second wavebands are transmitted through both reflection filters at the optically aligned position. The invention also provides optical apparatus incorporating such a filter, (figure 5, not shown), for example for demultiplexing, the optical apparatus including a fibre-optic input (5) and a photo-detector (6) aligned across the filter, and means (4) arranged to afford relative movement between the filter and the fibre-optic 1 photo-detector so as to thereby vary the waveband transmitted through the filter to the photo-detector. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO OPTICAL FILTERS This invention concerns improvements in or relating to optical filters and relates more particularly to wavelength selective optical filters which are tunable to vary the transmitted waveband.
Wavelength selective reflection filters can be tuned to and capable of high efficiency over a well defined waveband, especially if they are of holographic form. In some circumstances, however, a wavelength selective transmission filter can be more convenient than, or otherwise advantageous over, a reflection filter and there is therefore a requirement for tunable transmission filters.
According to the present invention there is provided an optical transmission filter comprising a first reflection filter which selectively reflects different wavebands at different positions along the filter, and a second reflection filter which selectively reflects different wavebands at different positions along the filter, the two reflection filters being disposed so that a position on said first filter which reflects a first waveband is optically aligned with a position on said second filter which reflects a second waveband spaced from said first waveband so that wavelengths between said first and second wavebands are transmitted through both reflection filters at said optically aligned positions.Each of said first and second reflection filters is preferably graded so that the reflected waveband gradually changes progressively along the filter.Preferably the reflection filters are holographic.
The reflection filters may be disposed in adjacent relationship and may be secured, for example cemented, together.
The invention further provides optical apparatus, for example for demultiplexing, comprising an optical transmission filter as set forth above, light input means for introducing light to the filter, light receiving means for receiving light transmitted through the filter, and means for effecting relative movement between the filter and the light input means so as to vary the waveband transmitted through the filter to the light receiving means.
In order that the invention may be better understood, reference will now be made to the accompanying drawings illustrating certain features by way of example in which:- Figure 1 is a graph showing the reflectivity/ wavelength characteristics of a reflection filter, Figure 2 is a graph showing the reflectivity/ wavelength characteristics of two reflection filters in combination, Figure 3 is a graph showing the transmission/ wavelength characteristics of a transmission filter resulting from the combination of reflection filters of Figure 2, Figure 4 schematically shows the transmission filter, and Figure 5 schematically shows optical apparatus employing the transmission filter.
A holographic wavelength selective reflection filter has reflectivity/wavelength characteristics as graphically shown in Figure 1 where R represents reflectivity and W represents wavelength. Notable features are a highly reflected waveband centred on a wavelength WX and of width DW with steep sides, and usually undesirable, but generally unavoidable, side bands S of lower reflectivity. The peak reflection central wavelength WX is dependent on the manner of making the filter, and particularly on the wavelength and angle of incidence of the illuminating beams when constructing the hologram, and as more fully mentioned later, the value of WX may progressively vary along the length of a tunable holographic reflection filter.
Figure 2 graphically shows two reflectivity/ wavelength curves R1 and R2 representing first and second spaced reflected wavebands having respective different peak reflection central wavelengths WX1 and WX2 displaced by a distance approximating to the waveband width DW such that there is a space or gap G between the adjacent steep sides of the two curves R1 and R2. This gap represents wavelengths between, and therefore not significantly reflected in either of, the two reflection wavebands. Such wavelengths can therefore be transmitted and the equivalent transmission/ wavelength characteristics are graphically shown in Figure 3 where T represents transmission and the width of the transmitted waveband is indicated as dW.It will be seen that this transmitted waveband can be relatively narrow, and, as can be appreciated from Figure 2, sufficiently narrow for the side bands S of the reflection curves RI and R2 not to affect the transmission curve since the right hand side bands of curve R1 lie within the reflected waveband of curve R2 and the left hand side bands of curve R2 lie within the reflected waveband of curve R1. If desired, however, the curves R1 and R2 could be further displaced to give a wider gap G and transmitted bandwidth dW, in which case the transmission curve would generally be affected by the sidebands, although such affect would be minimised if the peaks of one set of sidebands coincide with the troughs of the other set.
Figure 4 schematically shows a first holographic reflection filter 1 having at the relevant position reflectivity/wavelength characteristics as shown by curve R1 and a second holographic reflection filter 2 having at the relevant position reflectivity/ wavelength characteristics as shown by curve R2. An incident light beam of wavelengths encompassing the wave bands of curves R1 and R2 striking the first filter has the R1 waveband reflected off. The light transmitted through the first filter 1 then strikes the filter 2 which reflects off the R2 waveband. The light transmitted through the second filter 2 is therefore of wavelengths in the space or gap G between reflection curves R1 and R2, i.e.
accords with the transmission curve of Figure 3. The two reflection filters in combination therefore provide a wavelength selective transmission filter 3 giving a narrow transmitted waveband.
The filters 1 and 2 are not uniform but reflect different wavebands at different positions along the filter, i.e. the peak reflection central wavelength WX in Figure 1 is dependent on the position along the filter. Conveniently the filters are graded so that the reflected waveband, with its peak reflection central wavelength WX, changes gradually and progressively along the filter.
Holographic reflection filters of this type, and a way of making them, are described in British Patent Application No. 8428855 the relevant teachings of which are incorporated herein by reference. With two such filters 1 and 2, one can be moved, e.g. slid, relatively to the other until a position on the first filter having the required reflection curve R1 is optically aligned with a position on the second filter having the required reflection curve R2. By 'optically aligned' is meant that the beam being filtered strikes the filters at the respective positions so that light transmitted through both filters is of the required waveband, and such alignment of the filters may in practice be achieved by detecting the transmitted waveband and adjusting the relative positions of the filters 1 and 2 until it has the required characteristics.
Conveniently the filters 1 and 2 are disposed in closely adjacent relationship without any intermediate optical elements so that the optical alignment is effectively also mechanical alignment, i.e. the respective positions are simply straight in line with one other. If a transmission filter permanently with the required characteristics is wanted, then the two reflection filters 1 and 2 may be secured together, for example cemented together as a laminate, the respective holographic coatings preferably being in the centre sandwiched between the substrates for protective purposes.
It will be seen that two graded reflection filters arranged together can produce a graded transmission filter. As the reflected waveband, with its peak reflection central wavelength WX, gradually changes progressively along each of the reflection filters, then the transmitted waveband will progressively change correspondingly gradually along the combined filter. In other words, as the curves R1 and R2 in Figure 2 both move along the wavelength axis W, then the transmission peak of Figure 3 will correspondingly move along the wavelength axis W.
A graded transmission filter which thus transmits different wavelengths at different positions along the filter can be used, for example, in a simple demultiplexer as schematically shown in Figure 5. The transmission filter 3, formed by cementing together two reflection filters 1 and 2, is movable by appropriate means 4 past an input fibre optic cable 5 disposed opposite a detector or other receiver 6 on the other side of the filter. Wavelength multiplexed optical signals in the beam emitted by the cable 5 can be demultiplexed by appropriate different positioning of the filter 3 to transmit the different respective multiplexed wavelengths to the detector or other receiver 6.
It will be understood that there may be practical limitations on the efficiency of reflection, and transmission, and use of these and related terms herein are to be construed accordingly so that, for example, reference to a 'reflected waveband' is not to be taken as necessarily meaning 100% reflection of that waveband. It will further be understood that the filter may operate with wavelengths in the visible part of the electromagnetic spectrum, and/or may operate with wavelengths outside the visible such as in the infra red or ultraviolet, and the terms 'light', 'optical' and the like when used herein are to be construed accordingly. Also, references to the 'length' of or 'along' a filter are not to be construed in limited fashion as necessarily referring to the major dimension.
It will be appreciated that, while holographic reflection filters are generally highly desirable because of their steep sided reflectivity curves and relatively high efficiency, other forms of reflection filter could be employed in circumstances where their generally inferior characteristics are tolerable. Notably, multilayer coating interference filters may be used in some situations, and such filters can be graded by having alternating high and low refractive index layers whose thickness progressively increases along the filter. The progressive increase in layer thickness can be achieved for example by appropriately inclining the substrate relative to the axis of a uniform density diverging stream of the coating material as each layer is applied. Although graded reflection filters which gradually change the reflected waveband progressively along the filter are generally preferable for a tunable arrangement, other forms of reflection filter which reflect different wavelengths at different positions along the filter could be used. In particular, 'stepped' reflection filters (whether holographic or multilayer coating interference) effectively having a series of successive filter strips with different reflectivity/wavelength characteristics could be used. For example, the transmission filter could be formed from two stepped reflection filters arranged so that the optically aligned filter strips have the required reflectivity charactersitics with displaced reflected wavebands (R1 and R2 in Figure 2), the different pairs of aligned strips in combination transmitting different respective wavebands.

Claims (6)

1. An optical transmission filter comprising a first reflection filter which selectively reflects different wavebands at different positions along the filter, and a second reflection filter which selectively reflects different wavebands at different positions along the filter, the two reflection filters being disposed so that a position on said first filter which reflects a first waveband is optically aligned with a position on said second filter which reflects a second waveband spaced from said first waveband so that the wavelengths between said first and second wavebands are transmitted through both reflection filters at said optically aligned positions.
2. A filter according to Claim 1 in which each of said first and second reflection filters is graded so that the reflected waveband gradually changes progressively along the filter.
3. A filter according to Claim 1 or Claim 2 in which the reflection filters are holographic.
4. A filter according to any preceding claim in which the reflection filters are disposed in adjacent relationship.
5. A filter according to Claim 4 in which the reflection filters are secured together, for example, cemented together.
6. Optical apparatus having an optical transmission filter substantially as hereinbefore described with reference to and as shown in Figure 5 of the accompanying drawings.
6. Optical apparatus, for example for demultiplexing, comprising an optical transmission filter according to any preceding claim, light input means for introducing light to the filter, light receiving means for receiving light transmitted through the filter, and means for effecting relative movement between the filter and the light input means so as to vary the waveband transmitted through the filter to the light receiving means.
7. A transmission filter substantially as hereinbefore described with reference to and as shown in Figure 4 of the accompanying drawings.
8. Optical apparatus including a transmission filter substantially as hereinbefore described with reference to and as shown in Figure 5 of the accompanying drawings.
Amendments to the claims have been filed as follows 1. Optical apparatus, for example for demultiplexing, having an optical transmission filter comprising a first reflection filter which selectively reflects different wavebands at different positions along the filter, and a second reflection filter which selectively reflects different wavebands at different positions along the filter, the two reflection filters being disposed so that a position on said first filter which reflects a first waveband is optically aligned with a position on said second filter which reflects a second waveband spaced from said first waveband so that the wavelengths between said first and second wavebands are transmitted through both reflection filters at said optically aligned positions, light input means for introducing light to the filter, light receiving means for receiving light transmitted through the filter, and means for effecting relative movement between the filter and the light input means so as to vary the waveband transmitted through the filter to the light receiving means.
2. Apparatus according to Claim 1 in which each of said first and second reflection filters is graded so that the reflected waveband gradually changes progressively along the filter.
3. Apparatus according to Claim 1 or Claim 2 in which the reflection filters are holographic.
4. Apparatus according to any preceding claim in which the reflection filters are disposed in adjacent relationship.
5. Apparatus according to Claim 4 in which the reflection filters are secured together, for example, cemented together.
GB8610879A 1985-05-16 1986-05-02 Improvements in or relating to optical apparatus with tuneable filter. Expired - Lifetime GB2216678B (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
GB8512411 1985-05-16

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GB8610879D0 GB8610879D0 (en) 1989-07-05
GB2216678A true GB2216678A (en) 1989-10-11
GB2216678B GB2216678B (en) 1990-02-21

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2224133B (en) * 1988-10-21 1993-02-10 Pilkington Perkin Elmer Ltd Method and apparatus for the use of holographic optical elements
WO1996010762A1 (en) * 1994-09-30 1996-04-11 Cambridge University Technical Services Limited Wavelength selective filter

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB714955A (en) * 1952-03-31 1954-09-08 Technicolor Motion Picture Improvements in or relating to light dividing systems
GB716332A (en) * 1951-07-21 1954-10-06 Technicolor Motion Picture Optical beam splitting or combining systems
GB768036A (en) * 1954-09-17 1957-02-13 Technicolor Motion Picture Improvements in or relating to optical beam splitting or combining systems
GB2014752A (en) * 1978-01-31 1979-08-30 Nippon Telegraph & Telephone Element for use in optical multiplexer or de-multiplexer
EP0146957A2 (en) * 1983-12-22 1985-07-03 Alcatel N.V. Optical multiplexer/demultiplexer

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB716332A (en) * 1951-07-21 1954-10-06 Technicolor Motion Picture Optical beam splitting or combining systems
GB714955A (en) * 1952-03-31 1954-09-08 Technicolor Motion Picture Improvements in or relating to light dividing systems
GB768036A (en) * 1954-09-17 1957-02-13 Technicolor Motion Picture Improvements in or relating to optical beam splitting or combining systems
GB2014752A (en) * 1978-01-31 1979-08-30 Nippon Telegraph & Telephone Element for use in optical multiplexer or de-multiplexer
EP0146957A2 (en) * 1983-12-22 1985-07-03 Alcatel N.V. Optical multiplexer/demultiplexer

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2224133B (en) * 1988-10-21 1993-02-10 Pilkington Perkin Elmer Ltd Method and apparatus for the use of holographic optical elements
WO1996010762A1 (en) * 1994-09-30 1996-04-11 Cambridge University Technical Services Limited Wavelength selective filter
US6141361A (en) * 1994-09-30 2000-10-31 British Technology Group Limited Wavelength selective filter

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Publication number Publication date
GB8610879D0 (en) 1989-07-05
GB2216678B (en) 1990-02-21

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PCNP Patent ceased through non-payment of renewal fee

Effective date: 19970502