Method and apparatus for performing scanning optical coherence confocal microscopy through a scattering medium

• US 6,307,633 B1
• Filed: 03/31/2000
• Issued: 10/23/2001
• Est. Priority Date: 03/13/1998
• Status: Expired due to Fees

• Current Assignee: Optical Biopsy Technologies LLC
• Original Assignee: Optical Biopsy Technologies LLC
• Inventors: Kino, Gordon S., Mandella, Michael J., Garrett, Mark H.
• Primary Examiner(s): Font, Frank G.
• Assistant Examiner(s): Lee, Andrew H.
• Application Number:
  US09/541,433
• Time in Patent Office: 571 Days
• Field of Search: 356/479, 356/497
• US Class Current: 356/479
• CPC Class Codes: A61B 3/102 for optical coherence tomography [OCT]

Abstract

An apparatus and method for performing optical coherence domain reflectometry. The apparatus preferably includes a single output light source to illuminate a sample with a probe beam and to provide a reference beam. The reference beam is routed into a long arm of an interferometer by a polarizing beamsplitter. A reflected beam is collected from the sample. A 90° double pass polarization rotation element located between the light source and the sample renders the polarizations of the probe beam and reflected beam orthogonal. The polarizing beamsplitter routes the reflected beam into a short arm of the interferometer. The interferometer combines the reference beam and the reflected beam such that coherent interference occurs between the beams. The apparatus ensures that all of the reflected beam contributes to the interference, resulting in a high signal to noise ratio.

Claims

1. An apparatus for performing scanning optical coherence confocal microscopy on a sample, the apparatus comprising:

a) a light source having a predetermined coherence length for producing a probe beam and a reference beam, wherein the light source is located such that a light path exists between the light source and the sample;

b) a scanning head disposed along the light path between the light source and the sample, comprising;

i) at least one silicon micromachined scanning mirror for deflecting the probe beam to the sample;

ii) a lens assembly comprising at least one lens for focusing the probe beam to a focal point at a predetermined depth within the sample, wherein the lens is disposed along an optical path between the silicon micromachined scanning mirror and the sample, and wherein a reflected beam emanates from the focal point;

iii) and

iv) a translation means for causing the silicon micromachined scanning mirror and the lens assembly to move relative to the sample along a direction substantially parallel to the probe beam, to provide vertical scanning;

wherein the silicon micromachined scanning mirror can be pivoted such that the probe beam is able to scan the sample along a direction substantially perpendicular to the probe beam, thereby providing transverse scanning;

c) an interferometer having at least two arms, wherein the interferometer is disposed in optical communication with the reference beam and the reflected beam, and wherein the arms have an optical path length difference selected to restore optical coherence between at least a portion of the reference beam and at least a portion of the reflected beam;

wherein a coherence gate interval at the focal point of the probe beam is established by means of the interferometer.

2. The apparatus of claim 1 wherein the light source further comprises means for providing optical amplification of the reflected beam.

3. The apparatus of claim 2 wherein the light source is a semiconductor optical amplifier.

4. The apparatus of claim 1 wherein the interferometer comprises two optical fibers of unequal length coupled on each end by a fiber optic coupler.

5. The apparatus of claim 1 further comprising a phase modulator for modulating the phase of either at least said portion of the reference beam or at least said portion of the reflected beam.

6. The apparatus of claim 5 wherein the phase modulator is located within the light path.

7. The apparatus of claim 5 wherein the phase modulator is located within one arm of the interferometer.

8. The apparatus of claim 5 wherein the phase modulator is a piezoelectric fiber stretcher, an electrooptic crystal, or an acousto-optic modulator.

9. The apparatus of claim 1 further comprising a frequency shifting means for shifting the frequency of either at least said portion of the reference beam or at least said portion of the reflected beam.

10. The apparatus of claim 9 wherein the frequency shifting means is located within the light path.

11. The apparatus of claim 9 wherein the frequency shifting means is located within one arm of the interferometer.

12. The apparatus of claim 1 wherein the light source has a coherence length less than 3000 microns.

13. The apparatus of claim 1 wherein the light source is of the type which produces light in the wavelength range of 0.8 to 1.6 microns.

14. The apparatus of claim 1 further comprising a flexible optical fiber disposed along the light path and delivering the probe beam to the scanning mirror.

15. The apparatus of claim 14 wherein an output end of the flexible optical fiber and the lens assembly constitute a confocal microscope.

16. The apparatus of claim 1 further comprising a mirror disposed along the light path for diverting the probe beam to the silicon micromachined scanning mirror.

17. The apparatus of claim 1 wherein said scanning head further comprises a fluid filling a space between the lens assembly and the sample, wherein the fluid is substantially transparent to the probe beam and has an index of refraction closely matching an index of refraction of the sample.

18. The apparatus of claim 1 wherein the silicon micromachined scanning mirror can be pivoted about one axis, to provide a line-scan at the predetermined depth within the sample.

19. The apparatus of claim 1 wherein the silicon micromachined scanning mirror can be pivoted about two axes, to provide a two-dimensional scan at the predetermined depth within the sample.

20. The apparatus of claim 1 wherein the lens has a numerical aperture in the range of 0.4 to 1.4.

21. The apparatus of claim 1 wherein the lens assembly includes a gradient-index (GRIN) lens.

22. The apparatus of claim 21 wherein the GRIN lens has a substantially flat bottom such that there is a close spacing between the GRIN Lens and the sample.

23. The scanning apparatus of claim 1 wherein the translation means comprises:

a) a motor comprising at least one magnet and at least one coil, wherein the coil is magnetically coupled to the magnet; and

b) a movable carriage coupled to the motor such that the motion of the carriage is driven by the motor.

24. The apparatus of claim 23 wherein the coil and the magnet constitute a voice coil motor.

25. The apparatus of claim 23 wherein the silicon micromachined scanning mirror and the lens assembly are mechanically coupled to the movable carriage.

26. The apparatus of claim 1 wherein the light source is a polarized light source and wherein:

a) the light path comprises a 90°

double pass polarization rotation element so that the probe beam and the reflected beam have orthogonal polarizations; and

b) the apparatus further comprises a polarizing beamsplitter located such that the polarizing beamsplitter routes the reflected beam into at least one arm of the interferometer.

27. The apparatus of claim 26 wherein the 90°

double pass polarization rotation element comprises a Faraday rotator.

28. The apparatus of claim 26 wherein the polarizing beamsplitter is a polarizing beamsplitter evanescent wave optical fiber coupler.

29. The apparatus of claim 26 wherein the light path comprises a polarization maintaining optical fiber capable of supporting two independent orthogonal polarization modes.

30. The apparatus of claim 26 wherein the interferometer comprises a polarization maintaining optical fiber.

31. The apparatus of claim 26 further comprising a polarization rotator in the interferometer such that the reference beam and the reflected beam have substantially the same polarization when combined.

32. The apparatus of claim 1 further comprising an optical delay device in at least one arm of the interferometer.

33. The apparatus of claim 1 further comprising an optical delay device disposed in the light path.

34. The apparatus of claim 1 further comprising an optical detector.

35. A method for performing scanning optical coherence confocal microscopy on a sample, the method comprising the steps of:

a) producing a reference beam and a probe beam from a light source having a predetermined coherence length;

b) transmitting the probe beam along a flexible optical fiber disposed between the light source and the sample;

c) using a silicon micromachined scanning mirror to deflect the probe beam emerging from an output end of the flexible optical fiber to the sample;

d) focusing the probe beam to a focal point at a predetermined depth within the sample by use of a lens assembly disposed along an optical path between the silicon micromachined scanning mirror and the sample;

e) using an interferometer to establish a coherence gate interval comprising a region of the sample substantially centered about the focal point;

g) pivoting the silicon micromachined scanning mirror such that the probe beam scans the sample along a direction substantially perpendicular to the probe beam, while collecting a reflected beam comprising a portion of the probe beam reflected from within the coherence gate interval;

h) passing the reflected beam into the output end of the flexible optical fiber;

i) sending the reference beam and the reflected beam through the interferometer, wherein the interferometer has at least two arms, and wherein an optical path length difference between the arms is selected to restore optical coherence between at least a portion of the reference beam and at least a portion of the reflected beam; and

j) moving the silicon micromachined scanning mirror, the output end of the flexible fiber, and the lens assembly relative to the sample along a direction substantially parallel to the probe beam such that the focal point of the lens moves further into or away from the sample, while maintaining the coherence gate interval about the focal point, and repeating the steps of g) through j).

36. The method of claim 35 further comprising the step of performing spatial filtering of the reflected beam.

37. The method of claim 35 wherein the silicon micromachined scanning mirror is pivoted about one axis, such that a line-scan at the predetermined depth within the sample is achieved.

38. The method of claim 35 wherein the silicon micromachined scanning mirror is pivoted about two axes, such that a two-dimensional scan at the predetermined depth within the sample is achieved.

39. The method of claim 35 further comprising the step of modulating the phase of either at least said portion of the reference beam or at least said portion of the reflected beam with a predetermined temporal pattern such that coherent interference between at least said portion of the reference beam and at least said portion of the reflected beam is modulated with the predetermined temporal pattern.

40. The method of claim 35 further comprising the step of shifting the frequency of either at least said portion of the reference beam or at least said portion of the reflected beam such that coherent interference between at least said portion of the reference beam and at least said portion of the reflected beam is modulated at a beat frequency.

41. The method of claim 35 further comprising the step of detecting an interference between at least said portion of the reference beam and at least said portion of the reflected beam with an optical detector.

42. The method of claim 35 further comprising the step of adjusting an optical path length of at least one arm of the interferometer to maintain the coherence gate interval substantially centered about the focal point.

43. The method of claim 35 further comprising the steps of:

a) polarizing the reference beam and the probe beam such that the reference beam and the probe beam have the same polarization;

b) rotating the polarization of the reflected beam compared to the probe beam such that the reflected beam and the probe beam have orthogonal polarizations;

c) rotating the polarization of either the reference beam or the reflected beam such that the reference beam and the reflected beam have substantially the same polarization within the interferometer; and

d) combining at least a portion of the reference beam and at least a portion of the reflected beam such that coherent interference is produced.

44. The method of claim 35 further comprising the step of filling a space between the lens assembly and the sample with a fluid such that the fluid is disposed along an optical path between the lens assembly and the sample, wherein the fluid is substantially transparent to the probe beam and has an index of refraction closely matching an index of refraction of the sample.