Method and apparatus for optical filtering with two passbands

An apparatus includes an optical filter having first and second passbands that are different, the optical filter including selectively operable first passband adjusting structure that varies a characteristic of the first passband without influencing the second passband. According to a different aspect, a method includes filtering radiation with an optical filter having first and second passbands that are different, and selectively varying a characteristic of the first passband without influencing the second passband.

FIELD OF THE INVENTION

This invention relates in general to filtering techniques and, more particularly, to optical filtering techniques.

BACKGROUND

There are applications for which it is desirable to have an optical bandpass filter with two distinct passbands. One approach would be to fabricate a multi-layer coating that has two separate passbands. But as a practical matter, it can be difficult to actually manufacture such a coating with two separate passbands that each have the desired frequency range. Further, the spacing between the passbands becomes fixed at the time of manufacture. In addition, although the filter can be tilted from normal incidence in order to achieve a degree of tuning, this causes both passbands to move simultaneously toward shorter wavelengths. It is not possible to significantly vary the center wavelength of one passband without affecting the other passband, nor is it possible to vary the bandwidth of either passband. Thus, although existing dual-passband optical filters have been generally adequate for their intended purposes, they have not been entirely satisfactory in all respects.

DETAILED DESCRIPTION

FIG. 1is a diagrammatic view of an apparatus10that is part of a fluorescence microscope, and that includes a dual-passband optical filter12embodying aspects of the invention. A source14of a known type emits a broadband beam within a selected waveband, and this radiation is filtered by the filter12in a manner discussed in more detail later. The filtered radiation from the source14is ultimately directed by the filter12to a specimen or target16that has been treated with a conventional fluorescent dye. This radiation causes the dye in the target16to fluoresce, and to emit radiation at a wavelength different from the wavelength of the radiation received through the filter12from the source14. Radiation emitted due to the fluorescence is filtered by the filter12in a manner discussed in more detail later, and then directed through imaging optics18of a known type to a detector array19of a known type.

The optical filter12includes two cascaded optical filter sections26and28that operate independently. The optical filter section26includes a gear33that is supported for limited rotational movement about a pivot axis34. A drive mechanism36is provided to selectively pivot the gear33. In the disclosed embodiment, the drive mechanism36includes a not-illustrated stepper motor having a rotatable shaft with a not-illustrated pinion gear thereon, the pinion gear engaging the teeth of the gear33. The drive mechanism36includes two not-illustrated switches that can be manually actuated to cause the stepper motor to rotate its shaft in either of two opposite directions, and thus pivot the gear33in either one direction or the other. The drive mechanism36could, however, have some other configuration. InFIG. 1, the gear33is shown in a center position. The drive mechanism36can selectively pivot the gear33a few degrees away from the illustrated center position about the axis34, in either of two opposite rotational directions. The drive mechanism36can also releasably maintain the gear33in any angular position.

A V-shaped support member41is fixedly secured on the gear33for pivotal movement therewith. Two substrates43and44each have one end fixedly but detachably secured to a respective leg of the V-shaped support member41. The facing surfaces of the substrates43and44extend at an angle of approximately 45° with respect to each other, and intersect at a line that is coincident with the axis34.

A bandpass filter element46is provided on the surface of substrate43that faces the substrate44. The bandpass filter element46is a multi-layer filter coating of a known type, and in particular a multi-cavity Fabry-Perot structure, but it could alternatively have some other suitable structure. The filter element46is transmissive to radiation inside a passband having a center wavelength, and is reflective to radiation above and below this passband.FIG. 2is a graph of a curve51representing the transmissivity of the filter element46ofFIG. 1with respect to radiation arriving from the source14, when the gear33is in the center position shown inFIG. 1. For clarity, the curve51inFIG. 2has a somewhat idealized shape. When the gear33and the filter element46are in the center position, the filter element46has a passband that is between two wavelengths A and C, and has a bandwidth52and a center wavelength B. The filter element46is transmissive to radiation within the passband (between wavelengths A and C), and reflective to wavelengths above or below the passband.

When the gear33and filter element46are in the center position ofFIG. 1, the beam of radiation from the source14propagates along a path of travel71and then impinges on the filter element46at an angle of approximately 22.5° with respect to a not-illustrated reference line perpendicular to the filter element46. If the gear33and filter element46are pivoted away from the center position ofFIG. 1in either direction, there will be a change in the angle at which radiation from the source14impinges on the filter element46. As a result, the entire passband will move leftwardly or rightwardly inFIG. 2, without any significant change in the bandwidth52. This means that the center wavelength will either increase or decrease by the same amount.FIG. 3is a graph that is similar toFIG. 2, but that also shows in broken lines two curves56and58that represent shifted positions of the passband51caused by limited pivotal movement of the gear33away from the center position in respective directions that are opposite. The curves56and58have respective bandwidths57and59that are each approximately equal to the bandwidth52. When the passband shifts from51to56or58, the center wavelength also increases or decreases from wavelength B, as indicated diagrammatically by arrows61and62inFIG. 3. The curves56and58inFIG. 3each represent a small and exemplary amount of shift of the passband51in each direction, but do not represent upper and lower limits on the shifting of passband51. The passband51can in fact be shifted significantly farther in either direction.

A reflective element66is provided on the surface of the substrate44that faces the substrate43. In the disclosed embodiment, the reflective element66is a mirror coating of a known type that has a multi-layer design utilizing multiple dielectric materials. Alternatively, however, the coating66could be made from any other suitable material or combination of materials, and could for example be made from a metallic material. The reflective element66is reflective to all wavelengths within the operating range of the optical filter section26. Wavelengths that are emitted by the source14and that are outside the current passband of the filter element46are reflected by the filter element46, then travel to and are reflected by the reflective element66, and then propagate along a path of travel73to an optical beam dump74of a known type. The geometry of the optical filter section26is such that, as the filter element46and reflective element66rotate with the gear33, the path of travel73does not move.

The substrate43is made of a material that is transmissive to all wavelengths within all possible wavelength ranges of the passband of the filter element46. Wavelengths emitted by the source14that are within the current wavelength range of the passband of filter element46pass through the filter element46and through the substrate43, and then propagate along a path of travel72. The pivotable support41, substrates43-44, filter element46and reflective element66are equivalent to an arrangement disclosed in U.S. Ser. No. 12/363,527 filed Jan. 30, 2009, the entire disclosure of which is hereby incorporated herein by reference.

The optical filter section26also includes a further gear81supported for limited rotational movement about an axis82that is spaced from and parallel to the axis34. The gear81has a diameter substantially equal to that of the gear33, and has teeth that engage the teeth on gear33. Thus, when the gear33is pivoted, it pivots the gear81through an equal but opposite angular movement. A compensating element86is fixedly supported on the gear81for pivotal movement therewith, at a location so that the path of travel72extends through the compensating element. The compensating element86is made of the same material and has the same thickness as the substrate43. The compensating element86is oriented so that the facing surfaces of the substrate43and compensating element86form equal but opposite angles with respect to the path of travel72. The compensating element86realigns radiation propagating along the path72, in order to compensate for any deviation that may be caused by refraction as the beam travels through the substrate43. After passing through the compensating element86, the radiation continues along a path of travel88to the optical filter section28.

The optical filter section28includes a gear101that is supported for limited rotational movement about a pivot axis102that is spaced from and parallel to the axes34and82. A drive mechanism104is similar to but independent from the drive mechanism36, and can effect limited pivotal movement of the gear101about the axis102. The gear101is shown in a center position inFIG. 1. The drive mechanism104can selectively pivot the gear101a few degrees away from the illustrated center position about the axis102, in either of two opposite rotational directions. The drive mechanism104can also releasably maintain the gear101in any angular position.

A V-shaped support member108is fixedly supported on the gear101for pivotal movement therewith. Two substrates111and112each have one end fixedly but detachably coupled to a respective leg of the V-shaped support member108. The facing surfaces of the substrates111and112extend at an angle of 45° with respect to each other, and intersect at a line that is coincident with the axis102. The facing surfaces of the substrates111and112each have thereon a respective edge filter element113or114. The edge filter elements113and14are each a multi-layer coating of a known type. The edge filter elements113and114are discussed in more detail later.

The optical filter section28includes a further gear118supported for limited rotational movement about an axis119that is spaced from and parallel to the axes34,82and102. The gear118has a diameter substantially equal to the diameter of the gear101, and has teeth that engage the teeth on gear101. Thus, whenever the gear101is pivoted, it pivots the gear118through an equal but opposite angular movement. A compensating element122is fixedly supported on the gear118for pivotal movement therewith. The compensating element122is made of the same material as the substrate111, and has the same thickness as the substrate111. The substrate111, the substrate112and the compensating element122are each transmissive to all radiation within the operating range of the filter12.

The compensating element122is positioned so that radiation propagating along the path of travel88will pass through the compensating element122, and then continue along a path of travel126. The facing surfaces of the compensating element122and the substrate111are oriented so that they form equal but opposite angles with respect to the path of travel126. The compensating element122realigns beams of radiation that pass through it, in order to compensate for any deviation or offset that may possibly be induced into the radiation by refraction as the beam travels through the substrate111. The compensating elements86and122and the associated gears81and118are optional, but are included in the apparatus10ofFIG. 1for enhanced beam alignment.

FIG. 4is a graph depicting a broken line curve131that represents the transmission characteristic of the edge filter element113, and also depicting a solid line curve132that represents the transmission characteristic of the edge filter element114, when the gear101is in the center position shown inFIG. 1.FIG. 5is a graph that is similar toFIG. 4except that it shows the same two curves131and132from the perspective of reflection rather than transmission. In other words, inFIG. 5the broken line curve131represents the reflection characteristic of the edge filter element113, and the solid line curve132represents the transmission characteristic of the edge filter element114, when the gear101is in the center position shown inFIG. 1. For clarity, the curves131and132are each shown with a somewhat idealized shape inFIGS. 4 and 5. For the purpose of this discussion, the transmission characteristic inFIG. 4of each edge filter element113and114(considered alone) identifies the wavelengths that are transmitted through that edge filter element, and the reflection characteristic inFIG. 5of each edge filter element identifies the wavelengths that are reflected by that edge filter element.

As evident from the curve131inFIGS. 4 and 5, the edge filter element113is transmissive and reflective to wavelengths respectively below and above an edge wavelength D. Similarly, as evident from the curve132inFIGS. 4 and 5, the edge filter element114is reflective and transmissive to wavelengths respectively below and above an edge wavelength F. In the embodiment ofFIG. 1, the wavelengths D and F are both significantly longer than the wavelengths A, B and C inFIGS. 2 and 3. The pivotable support108, substrates111-112, and filter elements113-114are equivalent to an arrangement disclosed in U.S. Ser. No. 12/169,808, filed Jul. 9, 2008, the entire disclosure of which is hereby incorporated herein by reference.

With reference toFIG. 1, radiation from the optical filter section26that is propagating along the path88will arrive at the optical filter section28, and will pass successively through the compensating element122, substrate111and edge filter element113, and then travel along a path of travel136to the target16. This radiation will cause the fluorescent dye in target16to fluoresce and emit radiation with a longer wavelength that is approximately halfway between wavelengths D and F. This radiation propagates away from the target16along a path of travel138. In the disclosed embodiment, the path of travel138is actually coincident with the path of travel136but, for clarity, these paths are shown inFIG. 1with a slight lateral offset.

When the gear101is in the center position shown inFIG. 1, the radiation traveling along path138impinges on the edge filter element113at an angle of approximately 22.5° with respect to a not-illustrated reference line perpendicular to the edge filter element113. The portion of this radiation below the wavelength D passes through the edge filter element113and the substrate111, and travels through the compensating elements122and86and through the substrate43to the bandpass filter element46, where it is reflected because it is outside the passband of the bandpass filter element46. This reflected radiation then travels to and is absorbed by an optional beam dump141.

In contrast, radiation that arrives along the path138and that is above wavelength D is reflected by the edge filter element113, and then travels to the edge filter element114, where it impinges on the edge filter element114at an angle of incidence of 22.5° with respect to a not-illustrated reference line perpendicular to the filter element114. The portion of this radiation that is above wavelength F (FIGS. 4 and 5) passes through the filter element114and the substrate112, and then travels to and is absorbed by an optional beam dump144. In contrast, radiation that impinges on the edge filter element114and that is below wavelength F is reflected by the edge filter element114and then propagates along a path of travel147through the imaging optics18to the detector array19.

The edge filter elements113and114, operating together, serve as a bandpass filter as to radiation that arrives on path138and departs on path147, where the passband is between wavelengths D and F when the gear101and filter elements113and114are in the center position shown inFIG. 1.FIG. 6is a graph showing a curve161that represents the transmissivity of the bandpass filter section28, or in other words the combined reflectivity of the filter elements113and114with respect to radiation arriving on path138and departing on path147. For clarity, the curve161inFIG. 6has a somewhat idealized shape. When the gear101and filter elements113-114are in the center position ofFIG. 1, the filter elements113and114together define a passband that is between two wavelengths D and F, and that has a bandwidth162and a center wavelength E. Radiation between wavelengths D and F is transmitted through the bandpass filter and exits at147, whereas radiation outside this passband does not exit at147but instead is routed to either the beam dump141or the beam dump144.

If the gear101and edge filter elements113and114are pivoted in either direction away from the center position ofFIG. 1by the drive mechanism104, the path147will not move, but the angles of incidence of radiation impinging on each of the edge filter elements113and114will change.FIG. 7is a graph that is similar toFIG. 6and shows the same curve161depicted inFIG. 6, but that also shows the effect on the passband of pivotal movement in either direction away from the center position.

More specifically, if the gear101and edge filter elements113and114are pivoted in one direction away from the center position, the edge wavelength of the filter element113will increase from D while the edge wavelength of the filter element114will decrease from F, as indicated by a broken-line curve163inFIG. 7. In contrast, if the gear101and filter elements113and114are pivoted away from the center position in the opposite direction, the edge wavelength of the filter element113will decrease from D and the edge wavelength of the filter element114will increase from F, as indicated by a broken-line curve164inFIG. 7. Consequently, as shown inFIG. 7, if the gear101and filter elements113and114are pivoted away from the center position in one direction, the bandwidth of the passband will decrease from the bandwidth162to a bandwidth166, whereas if they are pivoted away from the center position in the opposite direction, the bandwidth of the passband will increase from the bandwidth162to a bandwidth167. During pivotal movement of the gear101and filter elements113and114in either direction, the center wavelength E does not change significantly.

FIG. 8is a graph showing both passbands of the dual-passband filter12ofFIG. 1, including the curve51ofFIGS. 2-3involving a passband with a fixed bandwidth and a tunable frequency, and the curve161ofFIGS. 6-7involving a passband with a tunable bandwidth and a fixed center wavelength.

FIG. 9is a diagrammatic view of an apparatus210that is a portion of a fluorescence microscope, and that is an alternative embodiment of the apparatus10ofFIG. 1. Components inFIG. 9that are similar or identical to components inFIG. 1are identified inFIG. 8with the same reference numerals used inFIG. 1. The following discussion focuses primarily on differences between the embodiments ofFIGS. 1 and 8.

The imaging optics18and detector array19ofFIG. 1have been replaced with a source213in the embodiment ofFIG. 9. The source213is a known device that emits a broadband beam within a selected waveband. In the disclosed embodiment, the source213is essentially identical to the source14.

InFIG. 9, the dual-passband filter12ofFIG. 1has been replaced by a dual-passband filter212. The filter212ofFIG. 9is effectively identical to the filter12ofFIG. 1, except for differences discussed below. The filter212includes an optical filter section226that differs from the optical filter section26ofFIG. 1only in that the filter section226does not include the beam dump141. The filter212also includes an optical filter section228that differs from the optical filter section28ofFIG. 1only in that the beam dump144has been slightly repositioned, and an optional beam dump215has been added for radiation that passes through the edge filter element113and the substrate111.

The apparatus210ofFIG. 9operates as follows. Radiation emitted by the source14enters the filter212, and is filtered in basically the same manner discussed above in association with the filter12ofFIG. 1. The resulting filtered radiation exits the filter212along the path136and impinges on the target16.

Radiation from the source213propagates along a path of travel238to the edge filter element114, where radiation above the wavelength F (FIGS. 4 & 5) passes through the edge filter element114and substrate112, and then travels to and is absorbed by the beam dump144. In contrast, radiation arriving along the path238that is below the wavelength F is reflected and travels to the filter element113. The portion of this radiation below the wavelength D passes through the filter element113and the substrate111, and then travels to and is absorbed by the beam dump215. In contrast, radiation that arrives at filter element113and is above wavelength D is reflected, and then travels along a path of travel247to the target16. The edge filter elements113and114together function as a bandpass filter as to radiation arriving at238and departing at247.

The target16is thus illuminated by radiation within two different passbands that are each separately and independently controlled by a respective one of the two optical filter sections226and228. This radiation arriving at the target16may illuminate and/or cause fluorescence from the target, and radiation emitted by the target16as a result of illumination and/or fluorescence can then be collected and imaged in a known manner by known structure that is not shown inFIG. 9.

The radiation beam traveling along path73inFIGS. 1 and 9is a notch that is the inverse of the radiation beam traveling along the path72. In each ofFIGS. 1 and 9, it would be possible to replace the beam dump74with the not-illustrated detector of a known spectrometer. The spectrometer could then be used as an optical wavelength monitor that indicates the range of wavelengths currently within the passband of the bandpass filter section26or126. This information could then in turn be used to operate the drive mechanism36in order to adjust the rotational position of the gears33and81, and thereby tune the bandpass filter section26or126so as to change the range of wavelengths falling within the passband.

As evident from the foregoing discussion, the embodiments ofFIGS. 1 and 9are each configured so that the bandpass filter section26or226with tunable wavelength and fixed bandwidth passes wavelengths that are shorter than the wavelengths passed by the bandpass filter section28or228with fixed wavelength and variable bandwidth. Alternatively, however, either embodiment could be configured so that the bandpass filter section28or228with fixed wavelength and variable bandwidth passes wavelengths that are shorter than the wavelengths passed by the bandpass filter section26or226with tunable wavelength and fixed bandwidth.

In the embodiments ofFIGS. 1 and 9, the substrate44supports the reflective element66that reflects radiation to the beam dump74. Alternatively, however, the reflective element66and the beam dump74could be omitted, and the substrate44could be made from a material that is absorptive to the wavelengths of radiation reflected by the bandpass filter element46, so that the substrate44serves as a beam dump. This would avoid the expense of the reflective element66and the beam dump74. Similarly, in the embodiments ofFIGS. 1 and 9, radiation passing through the edge filter element114also passes through the substrate112and then travels to the beam dump144. Alternatively, however, the beam dump144could be omitted, and the substrate112could be made from a material that is absorptive to the wavelengths of radiation that pass through the edge filter element114, so that the substrate112serves as a beam dump. This would avoid the expense of the beam dump144.

Although selected embodiments have been illustrated and described in detail, it should be understood that a variety of substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the claims that follow.