WO2009059406A1

WO2009059406A1 - Optical coherence tomography with tandem interferometer

Info

Publication number: WO2009059406A1
Application number: PCT/CA2008/001944
Authority: WO
Inventors: Mark Hathaway; John A. Rogers
Original assignee: Oti Ophthalmic Technologies Inc.
Priority date: 2007-11-06
Filing date: 2008-11-06
Publication date: 2009-05-14

Abstract

Optical Coherence (OCT) tomography apparatus includes a first interferometer receiving light from a source and having a first and second arms. Light from the first and second arms is combined to produce a first output beam, there being an optical path difference between the first and second arms. A second interferometer in tandem with the first receives as an input the output beam from the first interferometer. The second interferometer has a reference arm and an object arm. Light from the reference and object arms is combined to produce a second output beam, there being an optical path difference between said reference and object arms. The optical path difference in said first interferometer is matched to the optical path difference in the second interferometer to produce an output signal. Alternatively, the object arm can be in the first interferometer.

Description

OPTICAL COHERENCE TOMOGRAPHY WITH TANDEM INTERFEROMETER

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 USC 119(e) of US provisional application no. 60/985,697, filed November 6, 2007, the contents of which are herein incorporated by reference.

FIELD OF THE INVENTION

This invention relates to the field of optical coherence tomography (OCT), and in particular to OCT imaging apparatus employing a tandem interferometer.

BACKGROUND OF THE INVENTION

Optical coherence tomography is useful in the three dimensional imaging of transparent or semi-transparent objects, such as the eye. OCT uses interferometery to extract depth information from the sample, and thus permit the construction of a three dimensional image. OCT can operate in either the time domain or the frequency domain (spectral OCT). A time domain system is described in US Patent No. 6,769,769, the contents of which are herein incorporated by reference. A frequency domain system is described in DE 4309056, the contents of which are also incorporated by reference. Whereas in a time domain system, it is necessary to scan the object in the depth direction by moving the coherence gate along the z axis, in a spectral system, an entire line of data in the z direction, an A-scan, can be derived at once by analyzing the spectrum of the returned signal. The light source can either be a broadband source containing multiple frequencies, or alternatively it can be a tunable single frequency source that whose frequency is swept of the wavelength range of interest. In this case, the detector can be a simple photodetector. The frequency of the source varies with time as the source frequency is swept through the range of interest.

Current interferometer designs for OCT use a single interferometer. In any interferometer, fiber is a major part of the interferometer, and this leads to problems with polarization and broadband sources. In addition, the need to have a small optical path length between the reference and object arms of the interferometer leads to manufacturing difficulties in the construction of OCT equipment.

It is known to use tandem interferometers in sensing applications as illustrated, for example, in for example, US published application no. 20060061768. US U.S. Patent No. 7259862 also describes a low-coherence interferometery optical sensor using a single wedge polarization readout interferometer.

The article entitled "Low-coherence tandem interferometer for measurement of group refractive index without knowledge of the thickness of the test sample" by T.J. Eom et al., Lasers and Electro-Optics - Pacific Rim, 2007. CLEO/Pacific Rim 2007, ISBN: 978-1- 4244-1173-3, INSPEC Accession Number: 10020907, describes an OCT system using a kind of tandem interferometer wherein a conventional interferometer is used as a readout interferometer and a second interferometer employs a Wollaston prism to create two beams of different polarization which are then combine in a polarizer. The readout interferometer compensates the optical path length differences in the sensing interferometer within a given depth interval, but in this case the object is scanned with the first interferometer. Another paper by Akiko Hirai and Hirokazu Matsumoto, Optics Letters, Vol. 28, Issue 21, pp. 2112-2114, describes a tandem interferometer for measurement of group refractive index.

SUMMARY OF THE INVENTION

In accordance with the present invention, an optical coherence tomography apparatus is implemented using a tandem interferometer arrangement instead of a single interferometer.

According to the present invention there is provided optical coherence tomography (OCT)apparatus comprising a light source; a first interferometer receiving light from said source and having a first arm and a second arm, said first interferometer producing an output beam, and there being an optical path difference between said first and second arms; a second interferometer coupled to said first interferometer in tandem, said second interferometer producing an output beam and having a third arm and a fourth arm, there being an optical path difference between said third and fourth arms; wherein one of said interferometers forms a sensing interferometer, and one of the arms in the sensing interferometer forms an object beam; a scanner for scanning an object with the object beam and returning light scattered from the object along the arm forming the object beam; and a detector for detecting a signal in one of said output beams; and wherein the optical path difference in said first interferometer is matched to the optical path difference in said second interferometer to produce said signal.

Typically, the object arm is in the second interferometer, with an adjustable mirror in one of the arms of the first interferometer to match the optical path length differences, although it is also possible to put the object in the first interferometer, and place the adjustable mirror for matching the optical path lengths in the second interferometer.

It will be understood by one skilled in the art that the reference to "light" throughout the specification is not restricted to visible light, and can include light in both the infrared and ultraviolet portions of the spectrum.

The interferometers can operate in either the time domain or the frequency domain, in which latter case the detector may be a spectrometer. In the latter case, the source could be a broadband source, or alternatively it is possible to implement spectral OCT using a swept frequency source. In other words, instead of analyzing the spectrum with a spectrometer, it is possible to rapidly sweep the frequency and look at the response at each frequency. The two methods are equivalent, and permit a line of depth information to be obtained using spectral analysis.

The two interferometers operated in tandem. Tandem interferometry refers to two interferometers connected together so that low coherence light passes through one then the other. One interferometer is normally called the processing interferometer and the other is called the sensing interferometer. The interferometer that includes the patient's eye would be called the sensing interferometer.

In a conventional single interferometer, fringes are visible only when the optical path difference between the object arm and the reference arm is less than the coherence length of the beam at the detector, where the coherence length l_c is inversely proportional to the spectral width of the source, Δλ. Thus, l_c ~λ²/Δλ. A typical diode laser source has a coherence length of tens of mm to m, whereas an LED may have a coherence length tens to hundreds of μm. Thus, in the case of a conventional time domain interferometer with a broadband source, the OPD between the reference path and the object paths need to be in the order of microns in order to obtain a signal. This requirement imposes severe restrictions on the design of the apparatus, which has to accommodate two arms of substantially the same length.

However, by using a tandem interferometer arrangement, it is possible to obtain interference fringes when there is a large difference between the paths in the reference and object arms of each interferometer provided the OPD of the first interferometer is matched to the OPD of the second interferometer. In effect, the large OPDs of the two interferometers cancel each other out so that a signal is obtained.

In accordance with a second aspect the invention provides a method of performing an optical coherence tomography (OCT) scan of an object, comprising passing a light beam from a broadband source through a first interferometer having a first optical path difference between first and second arms thereof to produce a first output beam; launching the output beam of the first interferometer into a second interferometer in tandem with the first interferometer and having a third arm and a fourth arm; one of said arms providing an object beam; scanning a sample with said object beam; and returning light from the object beam through the interferometer to a detector; and matching the optical path length difference between the first and second arms of the first interferometer with the optical path length difference between the reference and object arms of the second interferometer in order to obtain a signal at said detector.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which:- Figure 1 is a block diagram of a first embodiment of the invention; and Figure 2 is a block diagram of a second embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT.

In Figure 1, a broadband light source 10, such as a super-luminescent diode (SLD), directs light via optical fiber 11 through a collimator 12 into a reference interferometer 20 comprising a beam splitter 13, which divides the light into a reference beam 14 terminating in mirror 15, and an object beam 16 terminating in mirror 17.

Dispersion compensation unit 18 is inserted in the object beam 16 to compensate for additional components in the object interferometer descried below.

Mirror 17 is displaceable along the axis of the object arm 16. In a single interferometer, no interference fringes will be observed if the OPD between the reference arm 14 and object arm 16 exceeds the coherence length of the light. However, in the case of interferometer 20, which is used in tandem with the interferometer 30, the OPD can greatly exceed the coherence length, and, for example, be in the order of 30 cms.

The output of the first interferometer 20 is input through an optical fiber 21 into a second interferometer 30, comprising beam splitter 22, reference arm 23, and mirror 24, and object arm 25, XY scanner 26, and objective 27 including lenses Ll and L2, of which lens L2 is adjustable. The objective 27 projects the object beam onto the eye of the patient, and movable lens L2 can be used to focus the object beam within the eye. Returned light scattered from the eye is directed back along the object path and combined with the light traversing the reference arm 23 in beam splitter/combiner 22. The beam splitter/combiner 22 of the second interferometer 30 produces an output beam, which is directed to spectrometer 32 preferably over an optical fiber 31, although the beam can be also directed using bulk components.

As in the case of the first interferometer 20, the path difference between the reference arm 23 and the object arm 25, is significantly greater than the coherence length of the light so that if the second interferometer were directly supplied by light from the light source 10, no interference fringes would be observed.

However, by supplying the second interferometer 30 with light output from the first interferometer 20, and by matching the optical path differences in the two interferometers, the first interferometer will compensate for the path difference in the second interferometer, and as a result interference fringes will be observed despite the large OPD in each of the interferometers. For example, if the OPD in the each of the interferometers is 30 cms, acting alone, neither would produce interference fringes, but when the output light output from the first interferometer is used as an input to the second interferometer, interference fringes are observed at the output of the second interferometer as if the OPD of the second interferometer was within the coherence length of the light source.

The output of the beam splitter 22 is fed over an optical fiber 31 to spectrometer 32, from which OCT data can be extracted in a known manner. For each x, y point on the object, a line of data in the z direction (A scan) is obtained using spectral OCT methods. By scanning transversely with the XY scanner 26, a B-scan can be obtained, and from multiple B-scans, it is possible to obtain a complete three dimensional image of the object. An important advantage of this arrangement is that the second interferometer 30 can be mounted on a common platform 50 with the scanner 26 providing the actual moveable head of the equipment that is brought up to the patient's eye, whereas the first interferometer 20 and other components can be located anywhere and connected to the second interferometer 30 via the optical fiber 21. The length of reference arm 23 can be made very small, and in practice the mirror 24 can be placed right up against the output of the beam splitter 22, or actually deposited on its surface. This makes arrangement the active head of the equipment much more compact and robust than would be the case if the two arms in the second interferometer were required to be of substantially the same length, as would be the case for a single interferometer.

In an alternative embodiment, the source 10 can be a tunable laser, for example, whose frequency is swept over a defined range of frequencies during each data capture in the z direction. In this case, the spectrometer 32 can be replaced by a simple photodetector because by sweeping the source frequency the spectral response of the sample is still obtained.

The embodiment shown in Figure 2 works in a similar manner to Figure 1 except the first interferometer is formed by beam splitter/combiner 40 and mirror 41, and the second interferometer is formed by beam splitter/combiner 43 and the eye 28.

In the case of the first interferometer 20, one arm is formed by the path between beam splitter 40 and mirror 41 and the other arm is formed by the light reflected off the backside of beam splitter/combiner 40. This is combined with the light returned from the first arm and in effect the beam splitter/combiner 43 of the second interferometer 30 sees light coming from an interferometer with two arms of greatly unequal path length. The same is true of the second interferometer 30, which forms an object path between beam splitter/combiner 43 and the eye 28, and a reference path by the light reflected off the backside of beam splitter/combiner 43. The returned light exiting the beam splitter/combiner 43 is directed through collimator 45 into single mode fiber 46, single mode fused fiber coupler 47, and single mode fiber 48 to spectrometer 32, which as in the first embodiment can be replaced by a photodetector if a frequency swept source, such as a tunable laser, is used instead of the SLD 10.

The spectrometer sees the second interferometer as having two arms of greatly unequal length, but of course by supplying the input of the second interferometer with the output of the first interferometer, and adjusting the position of the mirror 41, the large optical path differences of the two interferometers can be canceled out.

In Figure 2, the source 10 supplies light through the single mode fiber 49, single mode fused fiber coupler 50 and single mode fiber 51 to collimator52.

The fused fiber couplers 47 and 50 are interconnected by single mode fiber 53.

Angle polished fiber ends 54, 55 permit the output power of the apparatus to be monitored. This is particularly important when the equipment is used for scanning the eye.

Light returning from the second Interferometer is fed to the spectrometer 32. The returned light is analyzed in a conventional manner to permit the construction of three dimensional images of the eye, for example, by collecting A-scans to form B-scans, and collecting B- scans to form a complete three dimensional image. The data and image processing can be done in a standard computer with a display. As noted by the optical path difference of the first interferometer is roughly matched to the optical path difference of the second interferometer to produce a signal.

In the described embodiments, the object, the eye in the example, is placed in one of the arms of the second or downstream interferometer. It will be appreciated, however, that the object could also be placed in the first interferometer, for example, at the location of mirror 17 in Figure 1. In this case, the eye 28 would be replaced by an adjustable mirror, and this mirror would be displaced to match the optical path length differences of the two interferometers.

There are several other benefits arising from the use of a pair of interferometers in a tandem arrangement as described.

First, the fiber does not need to be part of the arms of either interferometer. This means that polarization issues, which arise when fiber is used within the interferometers, can be easily controlled or removed. Without fiber forming part of the interferometers, it is also easier to implement ultra broadband systems.

Second, the two interferometers can be separated by any distance and can be linked be fiber to provide an easy connection between them. The fiber connection between the interferometers does not cause polarization/dispersion problems because of the nature of a tandem interferometer setup.

Third, the nature of the fiber components is not critical as there are no tolerance issues with a custom array. Instead, off the shelf couplers and patch cords can be used.

Fourth, by using the back reflection at the end of a fiber as the reference light, there is no need to re-couple this light and therefore no costly alignment issues. Fifth, the first interferometer can be of fixed length, and only the sensing interferometer need be moved back and forth on the equipment head. This arrangement significantly reduces the complexity and cost of this part compared to the reference arm of current designs

Sixth, the only components that need to be in the optical head (for the OCT) are the scanners, interface lenses, and the fiber. All the other components (spectrometer, processing interferometer, SLD, and SLD driver etc..) can be located anywhere, even in a separate cart. This results in a substantial reduction in the size of the equipment head.

Seventh, the tandem design is very modular, and the individual components will be simple to make and test before they are assembled into a system. This will reduce manufacturing costs.

These benefits result in a performance advantage with respect to higher and higher resolutions, manufacturability advantages, and component design cost advantages.

Claims

We claim:

1. Optical Coherence tomography (OCT)apparatus comprising: a light source; a first interferometer receiving light from said source and having a first arm and a second arm, said first interferometer producing an output beam, and there being an optical path difference between said first and second arms; a second interferometer coupled to said first interferometer in tandem, said second interferometer producing an output beam and having a third arm and a fourth arm, there being an optical path difference between said third and fourth arms; one of said interferometers forming a sensing interferometer, and one of the arms in the sensing interferometer forming an object beam; a scanner for scanning an object with the object beam and returning light scattered from the object along the arm forming the object beam; and a detector for detecting a signal in one of said output beams; and wherein the optical path difference in said first interferometer is matched to the optical path difference in said second interferometer to produce said signal.

2. The optical coherence tomography apparatus as claimed in claim 1, wherein one of said arms is of adjustable length.

3. The optical coherence tomography apparatus as claimed in claim 1 or 2, wherein the first interferometer produces an output beam that serves as an input beam to said second interferometer, wherein said second interferometer provides said sensing interferometer, and wherein said detector detects a signal in the output beam of said second interferometer.

4. The optical coherence tomography apparatus as claimed in claim 1 or 2, wherein the first interferometer provides said sensing interferometer and produces an output beam that serves as an input to said second interferometer, and wherein said detector detects a signal in the output beam of said second interferometer.

5. The optical coherence tomography apparatus as claimed in claim 1, wherein the path length of the reference arm in the second interferometer is substantially shorter than the path length of the object arm in the second interferometer such that said second interferometer alone would not produce interference fringes.

6. The optical coherence tomography apparatus as claimed in claim 5, further comprising a first beam splitter/combiner in said first interferometer producing said first output beam and a second beam/splitter combiner in said second interferometer receiving said first output beam as an input to said second interferometer.

7. The optical coherence tomography apparatus as claimed in claim 6, wherein said reference arm of the second interferometer includes a mirror located on said second beam splitter/combiner.

8. The optical coherence tomography apparatus as claimed in any one of claims 1 to

7, wherein the second interferometer and said scanner are mounted on a common moveable platform for positioning in front of an object, and the remaining components are located off said platform and connected thereto by an optical fiber.

9. The optical coherence tomography apparatus as claimed in any one of claims 1 to

8, wherein said detector is a spectrometer.

10. The optical coherence tomography apparatus as claimed in claim 1, wherein the source is a swept frequency source, wherein the frequency of the source is varied over a defined frequency range for each line of data.

11. The optical coherence tomography apparatus as claimed in claim 10, wherein the detector is a photodetector.

12. The optical coherence tomography apparatus as claimed in claim 8, further comprising an objective mounted on said common platform.

13. The optical coherence tomography apparatus as claimed in claim 12, wherein said objective includes a displaceable lens to focus the object beam onto the object.

14. The optical coherence tomography apparatus as claimed in claim 1 , wherein the first interferometer comprises a first beam splitter/combiner receiving input light from said light source, and a mirror, and wherein light reflected back of said beam splitter forms said first arm, and the path between said beam splitter and said mirror forms said second arm.

15. The optical coherence tomography apparatus as claimed in claim 14, further comprising a dispersion compensator in said second arm.

16. The optical coherence tomography apparatus as claimed in claim 14 or 15, wherein the second interferometer comprises a second beam splitter/combiner receiving light from said first beam splitter/combiner, and wherein light reflected back of said beam splitter forms said reference arm, and the path between said beam splitter and an object forms said object arm.

17. The optical coherence tomography apparatus as claimed in claim 16, wherein said second interferometer and said scanner are mounted on a common moveable platform for positioning in front of an object, and the remaining components are located off said platform and connected thereto by an optical fiber.

18. The optical coherence tomography apparatus as claimed in claim 17, wherein light output from said light source is connected via optical fiber to a collimator aligned with said first beam splitter/combiner, and wherein light returned through said first beam splitter/combiner is conducted via optical fiber to a second collimator aligned with said second beam splitter/combiner.

19. The optical coherence tomography apparatus as claimed in claim 18, wherein said second collimator is coupled to said detector via optical fiber.

20. The optical coherence tomography apparatus as claimed in any one of claims 14 to 19, further comprising angle polished optical fiber ends coupled respectively to said first and second collimators to monitor optical power.

21. The optical coherence tomography apparatus as claimed in any one of claims 1 to 20, wherein the light paths within said interferometers use bulk components free of optical fiber, and wherein said interferometers are coupled together by optical fiber.

22. A method of performing an optical coherence tomography (OCT) scan of an object, comprising: passing a light beam from a broadband source through a first interferometer having a first optical path difference between first and second arms thereof to produce a first output beam; launching the output beam of the first interferometer into a second interferometer in tandem with the first interferometer and having a third arm and a fourth arm; one of said arms providing an object beam; scanning a sample with said object beam; and returning light from the object beam through the interferometer to a detector; and matching the optical path length difference between the first and second arms of the first interferometer with the optical path length difference between the reference and object arms of the second interferometer in order to obtain a signal at said detector.

23. A method as claimed in claim 22, wherein the path length of the reference arm in the second interferometer is substantially shorter than the path length of the object arm in the second interferometer such that said second interferometer alone would not produce interference fringes.

24. A method as claimed in claim 23, wherein the second arm of the first interferometer includes a displaceable mirror which is adjusted to match the optical path length differences of the two interferometers.

25. A method as claimed in any one of claims 22 to 24, wherein the second interferometer and said scanner are mounted on a common moveable platform which is positioned in front of an object, and the remaining components are located off the platform and connected thereto by an optical fiber.

26. A method as claimed in any one of claims 22 to 25, wherein said detector is a spectrometer and scans are obtained using spectral OCT.