WO1997031278A1

WO1997031278A1 - A narrow band optical filter

Info

Publication number: WO1997031278A1
Application number: PCT/AU1997/000090
Authority: WO
Inventors: Roger Pryce Netterfield
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 1996-02-21
Filing date: 1997-02-19
Publication date: 1997-08-28
Also published as: AUPN819696A0

Abstract

A narrow band optical filter (40) receives an incident light ray (42) at a first light receiving surface (44) of an Etalon filter (30). The Etalon filter (30) transmits only those spectral components of the incident light source determined by the thickness of the component birefringent slab (12), passing from the opposed surface (46) to a plate (48) that changes the polarisation state of the light on a single pass. The light ray emanating from the Etalon filter (30) passes through the plate (48) to be reflected by a mirror (50), returning to again pass through the plate (48). The return ray (52), by the optical property of the plate (48), is in its original polarisation state rotated through 90° in relation to its principal planes of polarisation. The return ray (52) passes through the Etalon filter (30) resulting in the filtered ray (54). The Etalon filter (30) has differently spaced pass bands for each of the polarisations. The wavelength separation between the pass bands of the incident ray (42) and the return ray (52) are different, and the desired order of interference for each of the rays can be aligned at the desired wavelength to produce an optical filter having a large free spectral range.

Description

A NARROW BAND OPTICAL FILTER Field of the Invention

This invention relates to narrow band optical filters, and more particularly of the type that utilise Fabry-Perot Etalon filters.

Description of the Prior Art

In accordance with a well established convention in the field of optics, reference will hereafter be made to "wavelengths of light", rather than "frequency". It is well known that wavelength and frequency are directly related to the speed of light in the medium in question, and that the expressions can be used interchangeably. Narrow band (bandpass) optical filters have uses in the fields of optical astronomy and remote sensing, where a small frequency band characteristic of the particular phenomenon under investigation is required to be isolated from a broad spectrum light source, possibly also in the presence of unwanted background (or noise) light sources. One form of filter suitable for isolating the required frequency pass band are Fabry-Perot Etalon filters. An alternative form of instrumentation suitable to isolate a desired frequency band is spectrometers. Commercially available spectrometers are at least an order of magnitude more expensive than known Etalon filters. A detailed description of conventional Etalon filters can be found in the paper entitled "Electrooptic Fabry-Perot filter : development for the study of solar oscillations", by CH. Burton, A.J. Leistner, and D.M. Rust published in Applied Optics 26 (13), 2637-2642, (1987).

Fig. 1 shows an exaggerated cross-sectional view of an Etalon filter element 10. The filter element includes a single spacer slab 12, usually formed as a disc, most usually of a birefringent material. One suitable birefringent material is lithium niobate (LiNbO₃). A birefringent material is one that has a different refractive index for the two principal directions (planes) of polarisation (i.e. the ordinary or "o-" plane and extraordinary or "e-" plane), and thus a different effective optical path length for any given thickness of material. The two opposed major faces 14,16 have multi-layered internally reflective coatings 18,18', typically formed by alternating layers (e.g. 10 to 20 in number) of high and low refractive index materials that can be of metal and/or dielectric substances. An outer electrically conductive layer also may be applied over the coatings 18,18' .

Fig. 2 shows a general mechanical assembly of a conventional Etalon filter 30. The filter element 10 is shaped as a disc and supported in a perspex mount 20. Two silver-epoxy electrodes 22,24 have connection to the outermost conductive layer of the filter element 10, and from which two electrical leads 26,28 extend to be connected with electrical instrumentation (not shown). A DC electrical potential applied across the electrodes 22,24 can be used as a tuning mechanism in a manner that presently will be discussed.

An Etalon filter functions in the manner of an optical bandpass filter, having a plurality of narrow pass bands located at integer quotients of the optical thickness of the spacer slab 12 (neglecting phase effects due to the coatings). The optical mechanism by which the multiplicity of pass bands arise is that of partial reflections occurring at the multiple layers on the opposed faces 14,16 and passing through the birefringent slab 12, with resultant constructive and destructive interference occurring when the optical path difference between successive passes is an integer multiple (i.e. the "order of interference") of the characteristic wavelength of an incident light source finally being transmitted. It is also very important that the opposed faces 14,16 of the birefringent slab 12 be as near as possible to being parallel so that, in turn, the multilayered coatings 18,18' also are parallel. Typically this will be to within a tolerance of a few millionths of a millimetre.

Known Etalon filters typically will have a thickness of the birefringent slab in the range 0.1 to 0.5 mm, and are in the range 10 to 100 mm in diameter. An Etalon filter of this construction can cost of the order of A$ 30,000. Fig. 3 shows a plot of transmittance versus wavelength for a conventional Etalon filter for light polarised in a plane parallel to one of the principal optical axes of the slab 12 material, where the birefringent slab is a 150 μm thickness of lithium niobate material. Another set of bandpasses will be produced for the orthogonal polarisation direction. Light incident on the Etalon filter is usually polarised (i.e. to reject one plane of polarisation) prior to entering the filter to avoid ambiguity of the passbands.

A measure of the quality of an Etalon filter is its Finesse (FN), defined as the "Free Spectral Range" (FSR), which is the separation in wavelength between the centre wavelength of adjacent pass bands, divided by the full width half maximum (FWHM) value of the bandpass. Thus FN = FSR/FWHM . The higher the Finese, the better quality the filter.

It is also known that by applying a voltage potential across the birefringent material the refractive index (and thus the effective optical path length), and in turn the transmitted wavelength, will change, meaning (in terms of Fig. 3) that the centre wavelength of the two pass bands shown can be shifted (adjusted) to higher or lower wavelengths, serving as a form of tailing.

A shortcoming of a single Etalon filter is the limited FSR. For example, a filter with FN = 20 and FWHM = 0.01 nm will have adjacent bandpasses 0.2 nm from the desired transmitted wavelength. Rejection of undesired bandpasses requires the use of very narrow band interference filters or additional Etalon filters. An advantage of multiple tunable Etalon filters is that the effective free spectral range can be significantly increased beyond the one order attainable with a single Etalon filter. Fig. 4 shows the series arrangement of two Etalon filters 30,30' foπning a known so-called tandem Etalon filter. The two Etalon filters are configured so that each has a birefringent slab of different physical thickness, and thus exhibit different FSRs. The respective thicknesses typically will be arranged in a vernier ratio (e.g. 5:4), and the Etalon filters arranged so that the respective pass bands coincide at (at least) one desired wavelength, and otherwise are slightly offset for the next adjacent multiples, coinciding again only many multiples later. In this way the transmittance of the next adjacent pass band is greatly reduced to the point of having little practical effect, thereby increasing the overall filter FSR. The tuning of the desired pass band again can be achieved by the application of a DC voltage across the birefringent material for one or both of the filters 30,30' .

It is also known to rotate one or both of the Etalon filters 30,30' to tune the pass band by means of adjusting the physical, and thus the effective optical, path length through the birefringent material. This practice also avoids direct reflections between the two filters, known as decoupling.

When the lithium niobate material is fabricated with the Z crystallographic axis perpendicular to the polished faces, the passbands of the Etalon filter are polarisation insensitive since the light traverses the filter parallel to its optic axis. However, the taning range of Z-cut Etalon filters is somewhat restricted (less than an order) for typical applications where the thickness of the lithium niobate is in the range 0.2-0.5 mm, so usually the filter must also be rotated to cover all wavelengths. It is possible to tune over more than an order if the filter is fabricated from Y-cut material since the electro-optic coefficient for the o-ray is several times greater than for Z-cut lithium niobate. However, the filter now has separate sets of passbands for the principal orthogonal polarisation directions so usually it is operated in polarised light (corresponding to the o-ray).

Since the light passing through the two Etalon filters must be polarised, at least half the intensity of incident randomly polarised light is lost. In addition, since there are deviations from the mean optical thickness of the two filters, the transmitted light will not usually be correlated. The final component of the light source transmitted also has a reduced photon count, hence data analysis and downstream post-processing time will be increased. The present invention seeks to overcome or at least ameliorate one or more of the problems or disadvantages associated with the discussed prior art narrow band optical filters.

One preferred object of the invention is to provide a narrow band optical filter having an improved Free Spectral Range.

Disclosure of the Invention

The invention discloses a narrow band optical filter comprising: a birefringent element for receiving and passing, in a series of spaced harmonic pass bands, an incident light source; and optical means for effecting a change in principal polarisation of said passed light and retarning the changed polarised light to said birefringent element, the effective optical path lengths for the incident light and returned light in the birefringent element being different, resulting in the returned light being passed by the birefringent element in greater spaced pass bands than said passed light.

Preferably, the optical filter further comprises means for effecting a change to a property of said first birefringent element so that at least one of said respective pass bands for the incident light and the returned light coincide.

The property changing means can include any one or more of controlling the surface temperature of the birefringent element, applying a DC electrical potential across the birefringent element, and orienting the birefringent element at some angle to the incident light source.

In one preferred form, the incident light is filtered to have only one plane of polarisation. The optical filter can further comprise a second birefringent element adjacent to, and in optical alignment with, said first birefringent element. The respective birefringent elements can have different physical thicknesses. Where the property changing means comprises an electrical potential applied across the faces of the birefringent element, this is effective to move the transmitted pass band in wavelength. The rates of change of the pass band wavelengths for the incident light and the returned light are different. Advantageously, the birefringent elements and/or the mirror are arranged to be normal to, or tilted equally with respect to the incident light by said property changing means.

Further preferably, the birefringent elements are rotatable about a central axis lying at the intersection of a plane in a direction parallel to the incident light source and a plane parallel to the mirror. In this way, the pass bands of the birefringent material are independent of the polarisation state of the incident light.

In the form where the optical filter further comprises a second birefringent element having a different effective optical path length so that the spacing between each sequential harmonic pass band is different to said first element, both the first and second birefringent elements are tuned by said property changing means so that at least one desired pass band of each substantially coincides, and thus both said elements pass only incident light falling in said desired pass band.

In another form, the invention discloses a method for producing narrow band filtered light from an incident light source, the method comprising the steps of: passing said incident light through a birefringent element resulting in light passed in a series of harmonic pass bands; changing the principal polarisation of said passed light; and returning and passing said changed light through said birefringent element, the birefringent element having different effective optical path lengths for different planes of polarisation, resulting in the transmitted light being filtered in greater spaced pass bands. The method can comprise the further step of changing a property of the birefringent element so that at least a respective one of said pass bands for said incident light and said returned light coincide.

The property changing can include any one or more of controlling the surface temperature of, applying a DC electric potential across, and orienting at some angle to the incident light source, the birefringent element.

Embodiments of the invention result in a marked increase in the FSR over conventional Etalon filters, to the point of being comparable with high quality spectrometers, which, as previously noted, are at least an order of magnitude more expensive.

Embodiments also can result in narrower pass bands then for conventional Etalon filters.

Further, the optical filters embodying the invention have ease of set-up and are readily adjusted or tuned to the narrow band of wavelengths to be studied. The filters having a single birefringent element structure (with the mirror and polarisation changing member) also have a greater throughput (e.g. a higher percentage of photons for a given light source) than conventional tandem Etalon filters, resulting in faster post-processing of the finally filtered light source.

Brief Description of the Drawings

Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

Fig. 1 is a cross-sectional view of a conventional Etalon filter element;

Fig. 2 shows a cut-away perspective view of a conventional Etalon filter; Fig. 3 is a plot of transmittance versus wavelength for the filter of Fig. 2;

Fig. 4 shows a known tandem Etalon filter arrangement;

Fig. 5a is a cross-sectional view of the elements of a narrow band optical filter embodying the invention for incident polarised light; Fig. 5b is a cross-sectional view of the elements of a narrow band optical filter embodying the invention for incident unpolarised light;

Fig. 5c shows the arrangement of Fig. 5a with the addition of a polarising beamsplitter; Fig. 5d shows an arrangement where the filtered beam emanates in the same direction as the incident beam;

Fig. 6 is a perspective view of a mechanical arrangement for the optical filter of Fig. 5;

Figs. 7, 8a and 8b are plots of transmittance versus wavelength for the filter of Fig. 6;

Fig. 9 is a cross-sectional view of the elements of another narrow band optical filter embodying the invention;

Fig. 10 is a perspective view of a mechanical arrangement for the optical filter of Fig. 9; Fig. 11 shows a plot of transmittance versus wavelength for each element of the filter of Fig. 10;

Fig. 12 shows a plot of resultant transmittance versus wavelength for the filter of Fig. 10;

Fig. 13 is a plot comparing the performance of the filters of Figs. 1 and 6; and Fig. 14 is a schematic view of instrumentation associated with a filter embodying the invention.

Detailed Description of Embodiments and Best Mode

Figs. 5a and b show a narrow band optical filter 40 embodying the invention, that receives an incident light ray 42 at a first light receiving surface 44 of an Etalon filter 30. For light polarised parallel to either the ordinary or extraordinary directions with respect to the optical axis of the slab 12, the Etalon filter 30 transmits only those spectral components of the incident light source determined by the thickness of the component birefringent slab 12 (as previously described), passing from the opposed surface 46 to a plate 48 having particular polarisation changing optical qualities.

These properties are such that the plate changes the polarisation state of the light on a single pass, in that linearly polarised light becomes circularly polarised, and vice versa. For the configuration shown, the light ray emanating from the Etalon filter 30 passes through the plate 48 to be reflected by a mirror 50, of any suitable construction, retarning to again pass through the plate 48. The return ray 52, by the optical property of the plate 48, is again in its original polarisation state although now rotated through 90° in relation to its principal planes of polarisation. Thus for a vertically polarised light source 42 the return ray 52 will be horizontally polarised. As follows, a horizontally polarised incident ray 42 will result in a vertically polarised return ray 52.

The plate 48 is commercially available and commonly known as a "quarter wave plate" . In the case shown in Fig. 5a, the rnirror 50 is at a slight angle away from the plane of the Etalon filter 30 and the plate 48. This configuration is advantageous in separating the optical components associated with the incoming and outgoing light, and ensures decoupling of the two Etalon filter passes when the filter is normal to the mcoming beam. Conversely, the mirror 50 may be normal to the incoming light provided the filter 30 is rotationally offset. In such a configuration the mcoming and outgoing light paths can be separated with a polarising beamsplitter since the incoming and outgoing light are polarised in orthogonal directions.

In the case shown in Fig. 5b, the mirror 50 and Etalon filter 30 are angled so that the incident ray 42 and return ray 52 make equal angles of incidence with respect to the filter.

The return ray 52 (in the case of both Figs. 5a and 5b), now having the opposite polarisation, passes through the Etalon filter 30 resulting in the filtered ray 54. The birefringent slab 12 of the Etalon filter 30 has a different effective optical path Iength for the opposed polarisations. The result is that the Etalon filter 30 has differently spaced pass bands for each of the polarisations. The wavelength separation between the pass bands of the incident ray 42 and the return ray 52 are different, and the desired order of interference for each of the rays can be aligned at the desired wavelength to produce an optical filter having a large FSR. This is achieved by rotational and/or voltage tuning of the Etalon filter 30.

For an incident ray 42 that is orthogonal to the plane of the light-receiving surface 44, it is necessary that the incident light source firstly pass through a polariser (not shown) so that the resulting incident ray 42 is polarised only in one direction. This is the arrangement shown in Fig. 5c, that includes a polarising beamsplitter 58 in the incident path. The Etalon filter 30 is an angle to the mirror 50.

In the embodiment of Fig. 5d, a polarising beamsplitter again interrupts the incident beam 42. Once passing the Etalon filter 30 the filtered beam then passes a plate 49 that has the property of rotating polarisation by 180°. Such plates are generally known as "half wave plates". The rotated beam is deflected by another beamsplitter 58', passes two mirrors 56,58 and the original beamsplitter 58 to pass the Etalon filter 30 a second time. Following rotation in polarisation by a further 180°, the now oppositely polarised beam passes the second beamsplitter 58' to become the exiting beam 54. This arrangement provides for the incident and exiting beams to be in the same direction, such that such an optical filter is suited to being retrofitted to an optical telescope or other such instrument.

Fig. 6 shows the mechanical configuration of the optical filter 40. As shown in relation to the prior art arrangements, a DC supply 32 has connection with the Etalon filter element 10, and can be used to tune the pass bands of the incident ray 42 and return ray 52 to coincide. For the same voltage adjustment, a different rate of change in the pass band centre wavelengths occurs for the respective rays. Thus it is possible to adjust the applied electrical potential so that the pass band for one of the rays 'catches-up' the pass band for the other ray. A tuning stub 60 also is connected with the mount 20, by which the Etalon filter can be rotated in the plane of the filter element to adjust the physical, and hence effective optical path lengths. This form of tuning can be used independently of, or in conjunction with, the applied electrical potential tuning. Another form of tuning as a supplement to, or replacement for the methods discussed above is the use of ambient temperature, and thus the temperature of the Etalon filter, as a control variable given that the refractive index of the birefringent slab is a function of temperature. The rate of change of the refractive index with temperature is usually different for the two principal polarisation directions or planes. It can be desirable to operate the Etalon filter at normal incidence since the transmitted wavelength will be relatively insensitive to the angular divergence of the incident radiation. This is usually achieved by electrically taning the filter to the desired wavelength or changing the operating temperature. A change of about 70°C tunes the o-ray one order, about 20°C tunes the e-ray one order, and about 25 °C will result in the e-ray tuning one more order than the o-ray.

Fig. 7 shows transmittance as a function of wavelength for the ordinary and extraordinary rays, where the two sets of rays coincide at approximately 765.8 nm for a 150 μm LiNbθ₃ Etalon filter having a finesse of FN = 43. The effective optical path lengths for the incident ray 42 and the returned ray 52 are approximately 3.8 percent different. As is apparent, there is only small overlap between subsequent pass bands. The passbands next very closely coincide 24 orders away.

Fig. 8a shows the resultant transmittance for the filter 40 over the same wavelength range. The immediately adjacent harmonic pass bands have approximately 1/8 lower magnitude than the desired pass band centred on 765.8 nm. Fig. 8b shows an exploded view of the wavelength window shown in Fig. 8a.

If the mirror and Etalon filter are arranged so that the angle of incidence for the incident and return rays is the same, as shown in Figs, 5b and 6, then a symmetric arrangement arises that can support incident light having both polarisation phases (e.g. for linear polarisation, the vertical and horizontal component phases), thereby doubling throughput over the pre-polarised arrangement discussed above.

Materials other than lithium niobate exhibit birefringent behaviour and are suitable for use in an optical filter embodying the invention. The materials include quartz, calcite, mica and PLZT (a compound of lead, lanthium, zirconium and titanium).

The optical filter shown in Figs. 5a, 5b and 6 illustrates one embodiment of the optical filter of the present invention, and it is to be understood that any other kind of optical apparatus that results in a 90° rotation of the polarisation between successive passes can be used as in substitation for the quarter wave plate 48 and the mirror 50 assembly.

Figs. 9 and 10 show another embodiment of a narrow band optical filter 70, in which component parts in common with the embodiment of Figs. 5 and 6 are indicated by like reference numeral. The optical filter has two spaced-apart parallel Etalon filters 30,30' , and to that extent is similar to the prior art arrangement shown in Fig. 4. The

Etalon filters 30,30' have different physical thicknesses, hence different effective optical path lengths. The thicknesses typically will be in a vernier ratio, such as 24:25.

The introduction of the second Etalon filter 30' adds a further level of filtering over the embodiment shown in Figs. 5 and 6, resulting in yet further rejectance of next adjacent pass bands. Another advantage is that the width of the coincident pass bands is narrower than conventional Etalon filters.

The incident ray 42 can be normal to, or at an angle to the plane of the receiving surface 44' of the first Etalon filter 30' , in a manner, and for the purpose previously described, concerning the need for pre-polarisation. Tuning of the optical filter 70 so that at least two respective incident and returned light pass bands coincide can be by various combinations of the electrical potential applied across the respective Etalon filter 30,30' , rotational tuning, and/or temperature tuning. In terms of the rotational tuning, it may be preferred to decouple the two Etalon filter 30,30' to avoid internal reflections therebetween, this being achieved by rotationally tuning the filter elements in opposite directions.

Fig. 11 shows a plot of transmittance versus wavelength for a double pass of each element of the optical filter 70, where the first Etalon filter 30 is of 150 μm thickness, and the second Etalon filter 30' is of 120 μm thickness, being in the vernier ratio 5:4. The desired pass bands have been tuned to be centred on 550.0 nm, with the individual finesses of the Etalon filters 30,30' equal to 20.

Fig. 12 shows the plot of transmittance versus wavelength for the whole filter, with transmission indicated on a logarithmic scale. The next long wavelength pass band having a transmittance of greater than 1 percent is more than 50 nm from the desired pass band.

Fig. 13 is a plot of transmittance versus wavelength comparing the embodiment of Figs. 5 and 6 with a single Etalon filter, indicating that the rejection outside of the pass bands is almost three orders of magnitude greater. The schematic diagram of Fig. 14 shows an instrumentation arrangement whereby a remote light source 80 has its incident rays focused by a first focusing lens 82 onto an object lens 84 to be passed as a substantially parallel beam 92 to an optical filter 40,70 embodying the invention. The return filtered beam 94 passes through the object lens 84 to be reflected by a plane mirror or reflector 86 and passed to a detector 88 by a further intermediate focusing lens 90. The finally filtered light 96 can then be detected and post-processed. As will be apparent, other optical filters, such as polarisers, also can be introduced in the optical path.

Claims

CLAHVIS:

1. A narrow band optical filter comprising: a first birefringent element for receiving and passing, in a series of spaced harmonic pass bands, an incident light source; and optical means for effecting a change in principal polarisation of said passed light and retarmng the changed polarised light to said birefringent element, the effective optical path lengths for the incident light and returned light in the birefringent element being different, resulting in the returned light being passed by the birefringent element in greater spaced pass bands than said passed light.

2. An optical filter as claimed in claim 1, further comprising means for effecting a change to a property of said first birefringent element so that at least one of said respective pass bands for said incident light and said returned light coincide.

3. An optical filter as claimed in claim 2, wherein said property changing means includes one or more of: temperature control means to control surface temperature of said first birefringent element; power supply means to apply an electrical potential across opposed surfaces of said first birefringent element; and means for adjusting angular position of said first birefringent element.

4. An optical filter as claimed in any of the preceding claims, wherein said optical means comprises a quarter wave plate and mirror aligned on the path of said passed light.

5. An optical filter as claimed in claim 4, wherein said quarter wave plate and/or mirror are rotatable or tiltable with respect to the plane of said first birefringent element.

6. An optical filter as claimed in claim 4 or claim 5, further comprising a first polarising filter for receiving and filtering said incident light in advance of said first birefringent element.

7. An optical filter as claimed in any one of claims 1 to 3, further comprising a first polarising beamsplitter for receiving and filtering said incident light in advance of said first birefringent element, and wherein said optical means comprises the series arrangement of a half wave plate, second polarising beamsplitter and one or more mirrors to return said passed light to said first polarising beamsplitter than directs the returned light again to said first birefringent element.

8. An optical filter as claimed in any one of claims 1 to 6, further comprising a second birefringent element adjacent to said first birefringent element and aligned on the path of said passed light.

9. An optical filter as claimed in claim 8, wherein said first and second birefringent elements have different physical thickness.

10. An optical filter as claimed in any one of the preceding claims wherein said first and second birefringent elements are fabricated from lithium niobate material.

11. A method for producing narrow band filtered light from an incident light source, the method comprising the steps of: passing said incident light through a birefringent element resulting in light passed in a series of harmonic pass bands; changing the principal polarisation of said passed light; and retarning and passing said changed light through said birefringent element, the birefringent element having different effective optical path lengths for different planes of polarisation, resulting in the transmitted light being filtered in greater spaced pass bands.

12. A method as claimed in claim 11, comprising the further step of changing a property of the birefringent element so that at least a respective one of said pass bands for said incident light and said returned light coincide.

13. A method as claimed in claim 12, whereby said property changing includes any one or more of: controlling the surface temperature of, applying a DC electric potential across, and orienting at some angle to the incident light source, the birefringent element.

14. A method as claimed in any one of claims 11 to 13, comprising the further step of passing said passed light through a second birefringent element adjacent to said first birefringent element.