GB2417789A

GB2417789A - Scanning delay line for interferometer

Info

Publication number: GB2417789A
Application number: GB0419383A
Authority: GB
Inventors: Adrian G H Podoleanu
Original assignee: OTI Ophthalmic Technologies Inc Canada
Current assignee: OTI Ophthalmic Technologies Inc Canada
Priority date: 2004-09-01
Filing date: 2004-09-01
Publication date: 2006-03-08
Also published as: GB0513773D0; GB0419383D0

Abstract

A scanning delay line, an apparatus for optical coherence tomography (OCT) and a method of scanning an optical path difference. A scanning delay line for an interferometer comprising an input aperture and an output aperture different from said input aperture, a dispersive element 2, an optical element 3 and a controllable tilt mirror 4. The scanning delay line is configured to let the incident light traverse the dispersive element twice, traverse the optical element four times and be reflected twice from the tilt mirror. The tilting of the tilt mirror results in scanning of the optical path. The scanning delay line can have additional mirrors for redirection of the light. The apparatus incorporates the scanning delay line.

Description

TRANSMISSIVE SCANNING DELAY LINE, FOR OCT

FIELD OF THE, INVENTION

10001 J This invention relates to the field of path scanning, optical measurements, and more specifically, to the field of high resolution optical imaging by means of'optical s coherence tomography (OCT). In particular, the invention relates to a scanning delay line for use he an OCT interferometer.

BACKG ROU N D OF TH E INVF,NTION j00021 Optical coherence tomography (OCT) is an hnaghig technique that produces high resolution cross sectional images of'optical reflectivity. It is based on the principle ol'low coherence hiterl'erometry where distance hf'ormation concerning various ocular structures Is extracted fiom thee delays of'reflected signals.

100031 Dit'f'erent depth scanning procedures have been devised to be incorporated into optical coherence tomography set-ups. One of the most successful procedures uses the introduction of a phase term linearly dependent on the optical frequency. 'I'he Fourier transformation leads to an equivalent optical path. The method is inspired t'rom research on processing of'femtosecond pulses. 'I'he method has the added advantage of allowing for dispersion compensation. Known under the name of'spectral delay line, the method uses a diffiaction grating, a lens and a galvanometer scanner, as disclosed in the US patent US611645, Grating based phase control optical delay line by, G. learney, E. Bourna, J. Fujhnoto and by US6421164 by, G. Tearney,'Interferometeric imaging with a grathig based phase control optical delay line", US patent US62820 1 I B I, Grathig based phase control optical delay line by, G. Tearney, E. 13Ouma, J. Fujhnoto.

10004J The Hear variation of phase versus optical fiequency is introduced by tilthig a galvanometer mirror behind a lens, where the galvanometer mirror is situated in the Fourier plane of'the lens. 'I'ilting the mirror causes the returned beam to be deviated prom the grating along a direction parallel to the incident beam. Usually, OCT configurations use reflective set-ups, where the same fiber aperture is employed in launching light towards and from the depth scanning device. This means that during scanning he depth, the amount of light reinfected back Into the fiber aperture varies, lo the beam walks off'the 3() main direction (where the loss is at a minimum) and the phenomenon itself'is called walk off. Therefore, as described in the US patent20030137669Al, by A. M. Rollins,'Aspects of basic OCT engine technologies for high speed optical coherence tomography and light source and other Improvements ha optical coherence tomography'', in order to reduce the walk-ol'f, a mirror is added to return the beam bacl; to the scanning delay line, the beam is s re-circulated four thnes via the diffraction grathg which finally leads to a dc-scanning of the lateral movement of tle output beam.

1000S] Such configurations have been devised to operate in transmission as well, where a dif't'erent f her aperture is used to capture the output beam, as disclosed in the US patent 6,564,089 B2, by J. A. Izatt, "Optical Imaging Device". Ilowever, this transmissive scanning delay line descans the beam by flour thuc diffraction of'fthe diffraction grating which leads to losses of'tilc optical beam. It would thus be desirable if the high loss element such as the diff'racthg grating were used less in the configuration

SUMMARY OF THE INVENTION

100061 'I'he present invention permits the walk-off'can be reduced and the beam to be de scamped by using the dispersive element, typically a diffraction grating, twice only. This means a reduction of losses to half of those of'previously known configurations which employ the diffraction grating four thnes.

00071 According to a first aspect of'the invention there is provided a scanning delay Idle l'or an interf'eromete'; comprising an input aperture and an output aperture different from said input aperture; a dispersive element; an optical element for altering the convergence of light passing theretilrougil; and a controllable tilt mirror; wherein said dispersive element, said optical element and said tilt mirror are configured such incident light undergoes dispersion by traversing said dispersive element twice, undergoes alteration of its convergence by traversing said optical element four times, and is reflected twice from said tilt mirror; and wherein tilting said tilt mirror results in scanning of the optical path difference for said interferometer.

100081 According to a second aspect of the invention there is provided transmissive scanning delay lilac foi optical coherence tomography, comprising: an input aperture; an output aperture; a dispersive element; an optical element for changing the convergence of 3() light passing therethrough; redirector means; and a controllable tilt mirror; and wherein said dispersive element, said optical element, said redirector means and said controllable tilt mirror are configured such that an Incident light beam passing through said input aperture strikes said dispersive element a f rst thee and is directed by said dispersive element through said optical element toward said tilt mirror, said beam reflected from said tilt mirror passes back through said optical element to strike said dispersive element a second time, said beam striking said dispersive element a second time is directed by said dispersive element toward redirect.or means, said redirector means redirects said beam after striking said dispersive element a second time through said optical element so as to strike said tilt mirror a second time, and said beam reflected from said tilt mirror a second thee passes through said optical element toward said output aperture unobstructed by said dispersive element.

100091 'I'he optical element Is typically a lens although it could be made up of lens components. In an alternative embodiment it could be a concave mirror, or concave mirror sections. The dispersive element is typically a diffraction grating although it could be a prism.

1001 l According to a third aspect the invention provides a method of scanning an optical path difference nor optical coherence tomography, comprising directing an Incident light beam toward a dispersive element a first thee; directhg the beam returned by said dispersive element after striking said dispersive element a first time through an optical convergence element toward said tilt mirror; directing said beam reflected from said tilt mirror back through optical element to strike said dispersive element a second time; redirecting said beam aflter striking said dispersive element a second time through said optical element so as to strike said tilt mirror a second time; and allowing said beam reflected fi om said tilt mirror a second time to pass through said optical element toward said output aperture unobstructed by said dispersive element.

BRIEF DESCRIPTION OF THE DRAWINGS

100111 The above and further advantages of the present invention may be better understood by referring to the fo!!owi,.6 dra-wh',s.

[00121 Fig. I is a detailed diagram of'an embodhnent ofthe invention.

100131 Fig.2 is a block diagram of a balanced OCT configuration using the scanning delay line in the reference arm.

100141 Fig. 3 shows the orientation of A, T. B and C-scans.

100151 Fig. 4 shows the utilization of the transverse scanners and the scanning delay line to generate B and C scans.

DETAILED DESCRIPTION

100161 As shown in Fig. I a collimated input beam I is dit'fracted by a dit'fraction grating 2, the dil'fracted beam, diffracted once, ID, passes through a lens, 3, and the beam refracted once, 11,, passes towards a galvanometer scanne'; 4. The light reflected once l o Tom the galvanometer scanner 4, beam I G. is refracted a second time by the lens 3 as beam 2L towards the diffraction grating 2. The beam is de-scanned by the grating so diffracted second time 2D into a beam parallel with the input beam 1. Rotating the galvanometer scanner 4 leads to deviation of the beam parallel with itself. To de-scan the beam and insure that the returned beam does not oscillate transversally, the beam 2L) is redirected via mirrors 5 and 6 through the lens 3, 3r'' time refracted as beam 3L back towards the galvanometer mirror, wherefrom it is reflected a 2t'i time as beam 2G. The beam is then retracted a 4" time into the output beam 4L.

100171 Let us consider the galvanometer scanner mirror 4 tilted as shown by the dashed line. The beam I G becomes I G', 2L becomes 2L' and as mentioned before, irrespective of the scanner angle, the beam diffracted from the diffraction grating second time, 2D', is parallel with the incident beam, 1. Now 21)'generates a beam 31,' which makes a smaller angle with the axis, 7, but the same angle with the scanner mirror 4, so the resulting reflected beam 2G' superposes on the beam 2G and the output beam 41,' superposes the initial beam 41, and no walk-off is registered.

2s 100181 It will be appreciated by one skilled ha the art that the single lens 3 could be formed of separate lens components corresponding to the different portions through which the various beams pass.

1OOl9l Fig. 2 gives an example of using the transmissive delay line in the reference arm of an OCT interferometer. Light from a low coherence source, 50, is sent via the I s' 3() splitter, 11 toward the object path, 19 and reference path, 1, which via the transmissive delay line, 10, reaches its output, 4L, and is injected in the I s' port of the second splitter 13, whose 2nt port receives light returned from the object, 14, via the interface optics, 15 and transverse scampers 16' and the I s' splitter, l l.The conf guration in Fig. 2 with re- circulathig rel'erencc power is essential in order to prevent the signal t'rom the reference path, usually strong, from returning to the low coherence source, 50, prone to oscillation and even damage il'light if sufficient light is returned back into the source. This configuration allows balance detection, implemented at the output of the 2r/ splitter, using two photodetectors, 17 and 19 and a differential amplifier 20. Thc signal is processed in the demodulator 21 and displayed and measured by the displayed measuring means 22.

The Corporation ofthc transmissive delay line in the OCT configuration is compatible with application of phase modulation, using the a phase modulator, 25 driven by a generator, 26. Thc fight input to the transmissive delay line is collimated using a focusing element 27' a convergent lens or a concave mirror. Shnilarly, the output light, 41, is l'ocused mto the fiber using another focusing element 28, in the form ol'a convergent lens or a concave mirror.

100201 Those skilled in the art will realise that the lens 3 could equally be implemented using a mirror and the diffraction grating 2 using a prism.

Modes of operation 100211 Thc transmissivc scanning delay line can be used to generate A, B and C-scan hnages.

A-scan ha.sel B-.scan 100221 The scanning delay line is used to generate A-scans. B-scan images, analogous to ultrasound B-scan are generated by collecting many such A- scans for different and adjacent transverse positions, selected by the transverse scanners. 'I'he lines in the raster 2s generated correspond to A-scans, i.e. the lines are oriented along the depth coordinate.

Tile transverse seamier (operating along X or Y. or along the angle 0 in polar coordinates in Fig. 3, with X shown h1 1 ig. 4 top) advances at a slower pace to build a B-scan image 100231 In this regime, by displacing the beam away fiom the axis oi'rotation of the tilting mirror, a high f'requcncy carrier can be generated, which can be used to carry the square root of reflectivity of the pixel volume interrogated. s

100241 The scanning line can also be used in tandem with a phase modulator, if the beam is arranged to fall on the axis of'rotation of the tilting mirror. In this case, the processing electronics after photodetection consists oi'band pass filtering on the phase modulation freq uency.

T-scan based B-scan 100251 In this case, the transverse scampers (or scamper) determine(s) the fast lines in the image; each image line is a T-scan (pig. 3). This can be produced by controlling either the transverse scamper along the X - coordinate, or along the Y - coordinate with the other two scanners fixed, or controlling both transverse scanners, along the polar angle 6, with the delay line scanner fixed. The example in the middle of Fig. 4 illustrates the generation of a B-scan using several 'I'-scans, where the X-scanner produces the'L-scans and the scamping delay line advances slower in depth, along the Z-coordinate. This procedure has a net advantage in comparison with the B-scan generated Tom several A-scans procedure as it allows production of'C-scans, lo of OCT transverse (or en- face) images for a fixed reference path, as presented below.

C'-scan 100261 C-scans are made from many 'f - scans along either of X, Y. p or 0 coordinates, repeated for different values of the other transverse coordinate, Y. X, 0 or p respectively in the transverse plane. 'I'he repetition of'L-scans along the other transverse coordinate is performed at a slower rate (pig. 4 bottom), which determines the frame rate. In this way, a complete raster is generated. Dit't'erent transversal slices are collected for different depths 7., either by advancing the optical path difference in the OCT via the scanning delay line in steps after each complete transverse (XY) or (p,0) scan, or continuously at a mucl slower speed than the frame rate.

2s 100271 Nor the 'f-scan based B-scan and C-scan regimes, the scanning line operates at a low speed, the carrier t'or the square root of reflectivity is created by either the phase modulation introduced by the external phase modulator. or by the,nath modulation generated by the transverse scanner or scanners determining the line in the final image. In this case, the modulation frequency could be relatively low, kl Iz or tens of kHz. In order to reduce the 1 /f noise and the stray modulation due to the walk-off, a high pass f Itcr is used after photodetection.

Claims

Claims 1. A scanning delay line for an interferometer, comprising: an

input aperture and an output aperture different from said input aperture; a dispersive element; an optical element for altering the convergence ol'light passing therethrough; and a controllable tilt mirror; wherein said dispersive element, said optical element and said tilt mirror are configured such that incident light undergoes dispersion by traversing said dispersive element twice, undergoes alteration of its convergence by traversing said optical element 1() four times, and is reflected twice Mom said mirror; and wherein tilting said tilt mirror results in scanning ofthe optical path difference t'or said interferometer.

2. A scanning delay line according to claim 1, where the said dispersive element is a prism.

3. A scanning delay line according to claim 1, where the said dispersive element is a diffraction grating.

4. A scanning delay line according to any one of claims I to 3, where the said optical element is a convergent Ions.

5. A scanning delay Ihie according to claim 1, where the said optical element is a concave mirror.

6. A scanning delay line according to claim 1, where the said mirror is a galvanometer scanner.

7. A scanning delay line according to claim 1, which is configured such that said input beam Is incident on said dispersive element, redirected via said optical element to said mirror, then bounced back through said optical element towards said dispersive element, wherefrom, via at least one additional mirror, the beam is redirected again via tile same said optics! eleme:.t to the said mirror, wherefrom said beam is processed tilrougil said optical element towards said output aperture.

8. A scanning delay line according to claim I wherein respective beams passing through said input and output apertures are collimated.

9. A scamping delay line according to claim 1, further comprising an input fibre providing said the input beam.

s 10. A scanning delay line according to claim I, further comprising an output fibre for receiving an output beam.

11. An apparatus for optical coherence tomography, equipped with an interferometer producing an interference signal that Incorporates the scanning delay line according to claim l, and which is terminated by a photodetection unit followed by an electronic processh1g unit, said apparatus comprising: an interferometer driven by a low coherence source, transverse scanning means for scanning the object angularly or transversally, h1terf'ace optics to convey the scanned beam to the object, and display means for displaying an OC]'hnage and measurements, and wherein the scanning delay line and the transverse scanning means are syncl1ronised with the display means; 12. An apparatus as clawed in claim 1 1, further comprising a phase modulator driven at a sufficient high frequency to modulate the interference signal.

13. An apparatus for optical coherence tomography according to claim 11, wherein 2() said transmissive scanmng delay line and the transverse scanning means are configured to generate B-scans by scanning the scanning delay line fast and the transverse scanning means slow, wherein the line h1 the raster displayed by the displaying means corresponds to the scanning delay Ih1e movement, and wherein the said electronic processing unit comprises a electronic bandpass filter.

14. An apparatus for optical coherence tomography according to clahn 13, wherein said band pass filter is tuned on the frequency generated by said scanning delay line.

! 5. ,^;. apparatus fiat olJiicai coherence tomography according to claw 13, wherein when said scanning delay line generates frequency zero, said band pass filter is tuned on the frequency generated by said phase modulator. lo

16. An apparatus for optical coherence tomography according to claim I I, wherein said transmissive scanning delay line and said transverse scanning means are used to generate B-scans by scanning the scanning delay line slow and the transverse scanning means fast, wherein the Idle he the raster displayed by the displaying means corresponds to the transverse scanning means.

17. An apparatus for optical coherence tomography according to claim I 1, which is configured such that said transmissive scanning delay line and said transverse scanning means generate stacks of C-scans at different depths by generating f rst a raster of a C- scan based by exclusive control ol'the transverse scanning means followed by movement ofthc scanning delay line to a new depth position and where collection a C-scan raster is repeated until the desired number ol'C- scans are collected.

18. An apparatus for optical coherence tomography according to claim I 1, which is configured such that the said transmissive scanning delay line and the said transverse scanning means generate stacks of C-scans at different depths by acquiring rasters based IS by exclusive control ofthe transverse scanning means while the scanning delay line is moved continuously at a pace much slower than that of completing a given said raster until all depth range has been scanned by the scanning delay line.

19. /Ln apparatus for optical coherence tomography according to any of the claims 16, 17 or 18 wherein the said electronic processing means includes at least a high pass filter 2() with the frequency cut-ot'f at least 10 times larger than the frequency of the ramp signal driving the transverse scanner he the said transverse scanning means which determines the line in the final image, in order to attenuate the l/f'noise and the walk-ol'f due to the scanning delay line.

20. A transmissive scanning delay line for optical coherence tomography, comprising: 2s an input aperture; an output aperture; a dispersive element; an optical ele''cni for changing the convergence of light passing therethrough; redirector means; and a controllable tilt mirror; and 1() wherein said dispersive element, said optical element, said redirector means and said controllable tilt mirror are configured such that an incident light beam passing through said input aperture strikes said dispersive element a first thee and is directed by said dispersive element through said optical element toward said tilt mirror, said beam s reflected from said tilt mirror passes back through said optical element to strike said dispersive element a second time, said beam striking said dispersive element a second thee is directed by said dispersive element toward redirector means, said redirector means redirects said beam after striking said dispersive element a second time through said optical element so as to strike said tilt mirror a second time, and said beam reflected from said tilt mirror a second time passes through said optical element toward said output aperture unobstructed by said dispersive element.

21. A transmissive scanning delay Ihie as claimed in claim 20, wherein said redirector means comprises an arrangement of'mirrors.

22. A transrnissive scanning delay line as claimed in claim 21, wherein one of said pair of mirrors and said dispersive element are laterally offset relative to said optical element so as to permit said beam directed toward said optical element after striking said dispersive element a first thee to reach said optical element unobstructed.

23. A transmissive scanning delay line as claimed in claim 22, wherein said dispersive element is laterally offset relative to said a portion of said optical element through which said beam passes alter bemg reflected prom said tilt mirror a second time so as not to obstruct said beam passing from said optical element toward said output aperture.

24. A transmissive scanning delay line as claimed in any one of claims 20 to 23, wherein said optical element is a lens.

25. A transmissive scanning delay line as claimed in any one of claims 20 to 24, Us which is conf gured such that said beam reflected from said tilt mirror a second time is returned along the same path regardless of the tilt angle of the tilt mirror.

26. A method of scamping an optical path difference tor optics! coherer ce tomography, comprising: directing an incident light beam toward a dispersive element a t'irst thee; directing the beam returned by said dispersive element after striking said dispersive element a first time through an optical convcrgencc element toward said tilt mirror; directing said beam reflected Mom said tilt mirror back through optical convergence element to strike said dispersive element a second thee; s redirecting said beam after striking said dispersive element a second thee through said optical convergence clement so as to strike said tilt mirror a second time; and allowing said beam reflected from said tilt mirror a second time to pass through said optical element toward said output aperture unobstructed by said dispersive element.

27. method as claimed in clahn 26, wherein said beam, after striking said dispersive element a second thee, is redirected to said first portion of'said optical element by an arrancment of m Errors.