GB2529195A

GB2529195A - Variable precompensator for multi-photon microscopy

Info

Publication number: GB2529195A
Application number: GB1414305.1A
Authority: GB
Inventors: Iain Gavin Cormack; Ian Macgillivray
Original assignee: Coherent Scotland Ltd
Current assignee: Coherent Scotland Ltd
Priority date: 2014-08-12
Filing date: 2014-08-12
Publication date: 2016-02-17
Anticipated expiration: 2034-08-12
Also published as: GB201414305D0; GB2529195B

Abstract

A pre-compensator for a laser beam may be delivered to a microscope such as a multi-photon microscope. It includes two prisms P1,P2 with opposing apexes for introducing negative group delay dispersion (GDD) into the beam. Reflective surfaces, such as plane mirrors M1-5 and roof-reflector R4 are arranged to fold the laser-radiation beam in a horizontal plane. Reflective surfaces, such as roof-reflectors R1-3 produce a polarization-orientation perpendicular to the vertical plane. The reflective surfaces and prisms cause a beam introduced into the pre-compensator to make an equal number of passes (i.e. 8 passes) through each of the prisms P1,P2, with the beam being folded horizontally and vertically by the reflective surfaces R1,R2,R3,R4 in a passage through the pre-compensator. Surfaces folding the beam have either all-dielectric reflective coatings or dielectric-enhanced metal reflective coatings.

Description

VARIABLE PRECOMPENSATOR FOR MULTI-PHOTON MICROSCOPY

TECHMCAL FIELD OF THE INVENTION

The present invention relates in general to ultra-short laser-pulse illumination S sources for multi-photon microscopes. The invention relates in particular to pre-compensators for changing group delay dispersion (GDD) of the laser pulses to compensate for positive GDD introduced in illuminating pulses by optical elements of a microscope.

DTSCUSSION OF BACKGROUND ART

In microscopes employing multi-photon excitation (MPE) of tissue (multi-photon microscopes), tissue samples being imaged are illuminated by ultra-short pulses of laser-radiation, These pulses typically have a pulse-duration less than about one picosecond and a wavelength in a range between about 680 and 1300 nanometers.

1 5 Optical elements of the microscopes introduce positive ODD into the illuminating-pulses. Positive GDD increases the pulse-duration and distorts the temporal profile of the pulses unless the pulses are pre-compensated for the positive GDD by adding negative GDD using a suitable optical arrangement. Such an arrangement, not surprisingly, is referred to as a pre-compensator by practitioners of the art.

The amount of positive GDD introduced by a microscope depends, inter alia, on the configuration of a particular microscope and the wavelength of the pulses. Because of this, an illumination source that can be tunable over a broad range, for example about an octave, and used with a variety of different microscopes, must be provided with a variable pre-compensator that can be adjusted to match pre-compensating negative GDD to a particular microscope illuminated at a particular wavelength.

A typical such variable pre-compensator uses a prism-pair (one prism canceling spectral dispersion of the other) in a multi-pass configuration. Negative GDD (NODD) is varied by selectively moving one or both of the prisms to vary the thickness of the prisms through which radiation passes, In order to provide the multiple passes several niirrors are required, with multiple reflections on sonic of the mirrors. There may be twenty-five or more reflections in total. Each mirror must include some degree of GDD control to avoid comproniising the function of the prisms.

ODD contrcil mirrors are very complex multi-layer structures that can have as many as fifty layers or more, niany with a non-integer thickness relationship. A comprehensive description of such mirrors is provided in US. Patent No. 6,154,318 granted to Austin et al. Further, when niany reflections are involved, it is generally considered necessary to deposit such multi-layer mirrors by ion-beam sputtering (IBS) which is a slow process, generally applicable only to small batches.

Because of this, the cost of the mirrors, in particular the cost of the multi-layer 1 5 mirror coatings, dominates the cost of a selectively variable pre-compensator. This results in the cost of the selectively-variable pre-compensator being as much as 25% of the cost of a microscope-illuminating system. Accordingly, there is a need for an approach to pre-compensator design that can help reduce the cost of GDD control mirrors therein,

SUMMARY OF THE INVENTION

The present invention is directed to optical apparatus for introducing negative group delay dispersion (GDD) into a plane-polarized beam of laser-radiation to be delivered to a microscope. One aspect of an apparatus in accordance with the present invention comprises first and second prisms arranged with apexes thereof opposed for introducing the negative ODD. A first plurality of reflecting surfaces is arranged to fold the laser-radiation beam in a horizontal plane. The plane-polarized laser-radiation beam has a polarization-orientation parallel to the horizontal plane. A second plurality of reflecting surfaces is arranged to fold the laser-radiation beam in a direction perpendicular to the horizontal plane. The laser-radiation beam has a polarization-orientation perpendicular to the vertical plane. The first and second pluralities of reflecting surfaces and the prisms are cooperatively arranged such that the beam makes an equal plurality of passes through each of the prisms before exiting the apparatus.

Each of the first plurality of reflective surfaces has an all-dielectric coating, and each of the second plurality of reflective surffices has a dielectric-enhanced metal layer reflective coating.

BRTEF DESCRIPTTON OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain principles of the present invention.

FIG. 1 is a plan view from above schematically illustrating a selectively variable pre-compensator in accordance with the present invention including a plurality of plane mirrors for folding the beam being compensated in a horizontal plane and a plurality of roof-reflectors folding the beam vertically, and two prisms one of which is translatable in the horizontal plane for introducing selectively variable negative GDD in to the beam, FIG. 1A is an "unfolded" view schematically illustrating the sequence of interaction of the beam with the plane mirrors, roof-reflectors, and prisms of FIG. 1.

FIG. 2 is a three-dimensional view schematically illustrating further detail of the interaction of the beam with the plane mirrors, roof-reflectors, and prisms of FIG. 1.

FIG. 3A is graph schematically illustrating reflection as a function of S wavelength for all-dielectric reflective coatings for plane mirrors of the variable pre-compensator of FTGS I and 2.

FIG. 3B is graph schematically illustrating ODD as a function of wavelength for the all-dielectric reflective coatings of FTG. 3A.

FIG. 4 is graph schematically illustrating reflection as a function of wavelength for dielectric enhanced metal reflective coatings for roof reflectors of the variable pre-compensator of FTGS I and 2.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to the drawings, wherein like features are designated by like reference numerals, FIG. 1, FIG. 1A, and FIG. 2 schematically illustrate a preferred embodiment 10 of selectively variable pre-compensator apparatus in accordance with the present invention, Apparatus 10 includes plane mirrors Ml, M2, M3, M4, and M5, and a roof-reflector R4 "folding" a beam being pre-conipensated in a horizontal plane, i.e., in the plane of FIG. I Roof-reflectors RI, R2, and R3 fold the beam vertically, i.e., in a direction perpendicular to the plane of FIG. 1. The beam being folded is plane-polarized in the plane of FIG. las indicated by arrows PH.

Prisms P1 and P2 are a complementary prism pair with apexes thereof opposed.

In such an arrangement, the prisms provide pre-compensating negative ODD which is selectively variable by selectively translating prism P2 in the plane of FIG. 1, as indicated by arrows A. Lenses Li and L2 are provided between Ml and M2 for causing the beam to have a beam-waist on exiting the apparatus.

FIG. iA depicts apparatus 10 in an "unfolded" condition to show the sequence of interactions of the beam being pre-compensated with all of the optical components, Here, it should be noted that single lines RI, R2, R3, and R4, designating roof reflectors, each represent two reflections, one from each of two mirrors inclined at 90° to each other reflector, FIG. 2 provides detail of the vertical beam-folding provided by roof-reflectors Ri, R2, andR3, A detailed description of the beani-interaction is set 1 0 forth below.

A beam (pulse from a laser) being pre-compensated is input into apparatus I 0 and is reflected (folded) from plane-mirror Mi at about 930 to the input direction, The beam then traverses lenses LI and L2, and is incident on mirror M2, which folds the beam again at 90°. The beam is then folded by mirror M3 and passes under roof- reflector M4 (see FIG. 2) into prism P1, which is arranged at about a minimum-deviation condition with respect to the incident beam.

The beam from prism P1 is twice reflected by roof-reflector Ri, which causes the beam to be reflected back to prism P1 but displaced vertically downward (see FIG. 2), The beam is again refracted (dispersed) by prism P1 and transmitted under mirror M3 and over roof reflector R3 to prism P2, which is also in about a minimum-deviation condition with respect to the incident beam. The beam is transmitted through prism P2 to roof reflector R2, which folds the beam vertically downward, and directs the beam back through prism P2 to roof reflector R3. Roof reflector R3 directs the beam vertically upward and back through prism P2 to roof reflector R2. Roof reflector R2 directs the beam upward, then back through prism P2. Prism P2 directs the beam back to roof prism P1 but at a height below that of the beam originally leaving prism P1 for prism P2.

The beam is transmitted through prism P1 to roof-reflector Ri, which directs the beam upward and back through prism P1 Because of the action of roof reflector RI, the beam is now at a height sufficient to cause the beam to interact with roof reflector R4, Roof-reflector R4 translates the beam in the horizontal plane and directs the beam back to prism P1 at a wider point on the prism.

The beam then makes a reverse series of interactions with prism P1, roof-reflector RI, prism P1, prism P2, roof-reflector R2, prism P2 roof reflector RI, prism P2, roof-reflector R2, prism P2, prism P1, roof-reflector Ri and prism P1. The beam emerges from prism P1 at the same height as the beam originally directed into the prism 1 0 from mirror MI but laterally displaced. The laterally displaced beam is reflected from mirror MI and is intercepted by mirror M4. Mirror M4 directs the beam to mirror M5, which directs the beam out of apparatus I 0 as output to be delivered to a microscope (not shown).

All reflections from plane-mirrors and roof-reflectors are at non-normal 1 5 incidence, here about 45°, and are accordingly polarization-dependent. Reflections from mirrors Ml, M2, M3, M4, and Mi, and from roof-reflector R4 (seven mirrors total) are in p-polarization, i.e., with the polarization-orientation of the beam parallel to the plane of incidence. All reflections from roof-reflectors Ri, R2 and R3 (six mirrors total) are in s-polarization, i.e., with the polarization-orientation of the beam perpendicular to the plane of incidence. Tn the complete sequence of reflections described above, there are eight (8) p-polarization reflections and twenty (20) s-polarization reflections.

FIG. IA is a graph schematically illustrating reflection as a function of wavelength for exemplary all-dielectric p-polarization, reflective NGDD coatings for plane mirrors Mi-Mi and roof-reflector R4 of apparatus 10. The reflectivity is greater than 99.8 between about 650 nm and about 1350 nm. The many ripples in the reflection curve are indicative of multiple layers with non-integer thickness relationship that are necessary to cover the one-octave bandwidth. Such coatings are available from Layertec GmbH of Mellingen, Germany FIG. 3B is graph schematically illustrating GDD as a function of wavelength for the all-dielectric reflective coating of FIG, 3A. It can be seen that the GDD has a mean value of about minus-eighty (-80) femtoseconds squared (fs2) with fluctuations of about +80 fs2 about the mean, It was found that for s-state polarization all-dielectric reflecting coatings, a much higher reflectivity (about 99.9%) was obtainable over the 650 nm to 1350 nm spectral range. This is to be expected, because of the relatively high s-polarization effective-index contrast-ratio of high and low refractive index material used in the coatings. This same high effective-index contrast-ratio, however, prevented the realization of an acceptable NGDD performance over the same spectral range.

Typically the GDD value was about -100 fs2 but with fluctuation of about +500 fs2 about the mean.

1 5 NGDD mirrors comprise a first plurality of hyers that provides the required reflectivity, surmounted by a second plurality of layers that provides the negative ODD.

It was found that for s-polarization coatings of apparatus 10, the first plurality of all-dielectric layers could be replaced by a metal (silver) layer with reflectivity of the silver layer enhanced by relatively few die'ectric layers, albeit with reflectivity being less than that of an all-dielectric reflector.

FIG. 4 is graph schematically illustrating reflection as a function of wavelength for exemplary dielectric enhanced metal reflective coatings for roof-reflectors of the variable pre-compensator of FIGS I and 2. Such reflective coatings are available from Laseroptik GmbH of Garbsen Germany. The metal layer is a silver layer and reflectivity is greater than 99.7% over the spectral range from 650 nm to 1350 nm.

Surprisingly the GDD performance is superior to that of the all-dielectric version.

GDD over the 650 nm to 1350 nm range is nominally 50 fs2 with fluctuations of 125 fs2. This was found to be tolerable in practice when deposited on all reflective surfaces of roof-reflectors RI, R2, andR3, Superior ODD performance notwithstanding, the cost of the dielectric-enhanced metal coating was less than about 20% of the cost of the all-dielectric equivalent S coating. A total of twenty s-polarization reflections in a pass through the pre-compensator resulted in a throughput loss of less than 10%, compared with what would have been achieved with the same surfaces having the all-dielectric coating of FTGS 3A and 3B, This was found deemed to be acceptable in view of the coating cost savings.

The present invention is described above with reference to a preferred 1 0 embodiment, The invention is not limited, however, to the embodiment described and depicted herein. Rather the invention is limited only by the claims appended hereto.

Claims

CLAIMS1. Optical apparatus for introducing negative group delay dispersion (GDD) into a plane-polarized beam of laser-radiation to be delivered to a microscope, the apparatus comprising: first and second prisms arranged with apexes thereof opposed for introducing the negative GDD; a first plurality of reflecting surfaces arranged to fold the laser-radiation beam in a horizontal plane, the plane-polarized laser-radiation beam having a polarization-orientation parallel to the horizontal plane; a second plurality of reflecting surfaces arranged to fold the laser- radiation beam in a direction perpendicular to the horizontal plane, the laser-radiation beam having a polarization-orientation perpendicular to the vertical plane, the first and second pluralities of reflecting surfaces and the prisms being cooperatively arranged such that the beam makes an equal plurality of passes through each of the prisms before exiting the apparatus; and wherein each of the first plurality of reflective surfaces has an all-dielectric coating, and each of the second plurality of reflective surfaces has a dielectric-enhanced metal-layer reflective coating.
2. The apparatus of claim 1, wherein the laser-radiation beam makes eight passes through each of the prisms.
3. The apparatus of claim 1 or claim 2, wherein the laser radiation-beam is incident on each of the reflecting surthces at an angle of about 45-degrees.
4, The apparatus of my preceding claim, wherein the all-dielectric coating has a reflectivity greater than about 998% and a GDD of about minus 80 femtoseconds squared, plus or minus about 80 fenitoseconds squared, in a wavelength range between about 650 nanometers and 1350 nanometers.
5, The apparatus of any preceding claim, wherein the dielectric-enhanced metal-layer reflective coating has a reflectivity greater than about 99.7% and a GDD of about minus 50 fenitoseconds squared, plus or minus about 125 fenitoseconds squared, in a wavelength range between about 650 nanometers and 1350 nanometers.
6. The apparatus of claim 5, wherein the metal layer is a silver layer.
7. The apparatus of any preceding claim, wherein there are seven reflecting surfaces in the first plurality thereof, and six reflecting surfaces in the second plurality thereof.
8. The apparatus of claim 7, wherein the laser-radiation beam makes eight passes 1 5 through the prisms before exiting apparatus.
9. The apparatus of claim 8, wherein the laser radiation beam makes a total of eight reflections from the first plurality of reflecting surfaces, and a total of twenty reflections form the second plurality of reflecting surffices.
10. Optical apparatus as herein described with reference to, and as illustrated by, the accompanying drawings.