Low coherence interferometer apparatus

Details Low coherence interferometer apparatus Low coherence interferometer apparatus

2000-07-11

2001-12-11

Turner, Samuel A. (Department: 2877)

Optics: measuring and testing

By light interference

Using fiber or waveguide interferometer

Reexamination Certificate

active

06330063

ABSTRACT:

The invention concerns a low coherence interferometer apparatus for the investigation of a sample, in particular for multi-dimensional imaging in medical applications.
Low coherence interferometer methods are used for a variety of applications. They are normally referred td in the art as LCI (Low Coherence Interferometry) methods or as OCDR (Optical Coherence Domain Reflectometry). The abbreviation LCI is used below for reasons of simplicity.
LCI methods are utilized or are at least discussed for a variety of applications. For example, reference can be made to the following citations:
1) Danielson et al: “Guide-wave Reflectometry with Micrometer Resolution”, Applied Optics, 26 (1987), 2836-2842.
2) Schmitt et al: “Measurement of Optical Properties of Biological Tissues by Low-Coherence Reflectometry”, Applied Optics, 32 (1993), 6032-6042
3) WO 95/30368
4) DE 2528209 A
5) DE 3201801 A1.
6) WO 92/19930
7
) DE 4204521C1
8) U.S. Pat. No. 5,073,024
All LCI methods have the common property that light from a low coherence (wide spectral band width emitting) light source is split into two partial beams—a measuring light beam and a reference light beam. The two beams are joined upstream of a detector to produce an interference signal containing the desired information. A principal component in the low coherence interferometry apparatus (designated below as “LCI apparatus”) is an interferometer configuration comprising, in addition to the low coherence light source, an optical coupler, a reference reflector, a probe head having a light exit opening for irradiating light into the sample, and the detector.
The optical paths between these interferometer elements form so-called interferometer arms. Light from the light source passes through the light source arm and is incident on the optical coupler where it is split. One part, constituting measuring light, passes through a sample arm and the probe head and is irradiated into the sample. The second part of the light, constituting reference light, passes through a reflector arm and is incident on the reference reflector. Both fractions of the light are reflected (the measuring light in the sample and the reference light at the reference reflector) and are guided back to the optical coupler along the same optical path (sample arm and reference arm respectively) where they are joined together and introduced through the detector arm to the detector. The light-sensitive surface of the detector can measure an interference signal caused by the interference between the two parts of the beam.
In order for an interference to occur, the optical path length in the reference arm (between the optical coupler and the reference reflector) differs by at most the coherence length of the light source from the optical path length of the measuring light between the optical coupler and the point of reflection in the sample. An interference signal is measured only if this condition is fulfilled. This fact is utilized to limit the investigation to one particular measuring depth, designated below as the LCI measuring depth, through appropriate adjustment of the length relationships between the reference arm and the sample arm.
This fundamental principle of the LCI measuring technique is used to allow various applications through variation of certain measurement details and through analysis of the interference signal.
For example, reference
1
) concerns the investigation of the structure of optical fibers, in particular for localizing optical defects. References
2
) and
3
) concern various aspects of investigations in biological tissue (in particular skin tissue). These authors are only concerned with obtaining information in dependence on the LCI measuring depth defined by the interference criterion. These publications therefore perform a pure depth scan (also termed “longitudinal scan”), i.e. the length of the reference arm is varied to adjust the LCI measuring depth.
In contrast thereto, references
4
) through
6
) describe methods and apparatuses with which an additional lateral scan is carried out in order to obtain in various ways a picture of the distribution of the information of interest in the lateral direction (parallel to the surface of the sample). These methods therefore pertain to multi-dimensional imaging. In addition to a depth scan, a scan in at least one transverse direction (“lateral scan”) is carried out. The invention is particularly concerned with methods and apparatuses for multi-dimensional imaging using the LCI principle (Optical Coherence Tomography (OTC)).
References
4
) through
6
) relate to OTC methods. Reference
4
) pertains to a surface scan of a manufactured product and reference
5
) concerns investigation of the eye, in particular the retina. Reference
6
) provides particularly detailed discussion of multi-dimensional imaging of a sample using the LCI technique, in particular—as in the present invention—for applications involving the investigation of biological samples. The most important practical example discussed in reference
6
) is investigation of the eye (as in reference
5
)). The present invention primarily concerns investigations of samples having very finely distributed structures, in particular human skin.
The OTC method has particular advantages over other imaging methods (for example, ultrasound imaging, X-ray CT and lateral scanning confocal microscopes), since it does not utilize ionizing radiation and is therefore not damaging and since it facilitates high image resolution. It is particularly well suited for the investigation of relatively fine structures near to the surface. For the case of skin, the current state of development permits a maximum LCI measuring depth of approximately 1.5 mm. A spatial resolution better than 10 &mgr;m is possible in both the axial and lateral directions.
The methods known in the art through references
1
) through
6
) are difficult to perform and require a large amount of space in the range of the probe head. For these reasons, LCI apparatus have been proposed with which portions of the interferometer configuration (at least the optical coupler and the reference arm) are both integrated into an optical chip (references
7
) and
8
).
An optical chip is an optical element made from a transparent material (normally glass) having integrated light guides. The light guides are made from a material having an index of refraction which is greater than that of the remaining optical chip. As is the case with optical fibers, the optical waveguide properties of the light guides integrated into the optical chip result from total internal reflection at the refractive index interface. Optical chips (also designated as integrated optical components) are primarily used for optical data transfer in optical fiber communication systems. Further details are given in comprehensive references such as “Optical Waveguides Advance to Meet Fiberoptic Demands”, by E. D. Jungbluth, Laser Focus World, April 1994, 99-104 or the book “Integrated Optics”, Proceedings of the Third European Conference, ECIO'85, H.-P. Noltes, R. Ulrich (Editors); Springer Verlag 1985 in which an article by P. O. Andersson et. al., “Fiber Optic Mach-Zehnder Interferometer Based on Lithium Niobate Components” is published on pages 26 through 28.
The measuring arm of an interferometer configuration always includes a part which is outside of the chip, namely the optical path between the light exit opening of the chip and the point of reflection in the sample. For this reason, the reference arm, which is completely integrated in the chip, is substantially longer than that portion of the measuring arm travelling through the chip. Since both arms depart from the same optical coupler, the reference arm cannot travel in a straight line through the optical chip. At least one beam deflection is necessary in order to accommodate the additional length by means of a zigzag or meandering travel of the reference arm in the optical chip.
Reference
7
) describes two fundamental possibilities for effecting such a deflection. Firstly, a m

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