Optical waveguides – Optical waveguide sensor
Reexamination Certificate
2000-01-07
2002-05-07
Ullah, Akm E. (Department: 2874)
Optical waveguides
Optical waveguide sensor
Reexamination Certificate
active
06385358
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a birefringence insensitive fiber optic optical coherence domain reflectometry (OCDR) system. In particular, the system is designed to provide a disposable section of non-polarization maintaining optical fiber in the sample arm while achieving high resolution by matching the dispersion between the sample arm and the reference arm.
2. Description of Related Art
Optical coherence domain reflectometry (OCDR) is a technique developed by Youngquist et al. in 1987 (Youngquist et al., “Optical Coherence-Domain Reflectometry: A New Optical Evaluation Technique”, 1987
, Optics Letters
12(3):158-160). A similar technique, optical coherence tomography (OCT), was developed and used for imaging with catheters by Swanson et al. in 1994 (See U.S. Pat. Nos. 5,321,501 and 5,459,570). OCDR and OCT have been applied to imaging and diagnoses of biological tissues, such as dental tissue (See U.S. Pat. No. 5,570,182 to Nathel et al.). OCT systems have been miniaturized to enable their use with guidewires. OCDR and guidewire systems are disclosed in WO 99/02113 (PCT/US98/14499) to Winston et al. and U.S. patent application Ser. No. 09/050,571 to Everett et al.
A diagram of a prior art OCDR scanning system is shown in FIG.
1
. Light from a low coherence source
10
is input into a 2×2 fiber optic coupler
12
, where the light is split and directed into a sample arm
14
and a reference arm
16
. An optical fiber
18
in the sample arm
14
extends into a device
20
that scans an object
22
. The reference arm
16
provides a variable optical delay. Light input into the reference arm
16
is reflected bade by a reference mirror
24
. A piezoelectric modulator
26
may be included in the reference arm
16
with a fixed reference mirror
24
, or the modulator
26
may be eliminated by scanning the mirror
24
in the Z-direction. The reflected reference beam from reference arm
16
and the scattered sample beam from sample arm
14
pass back through the coupler
12
to detector
28
(including processing electronics), which processes the signals by techniques that are known in the art to produce a backscatter profile or image on a display unit
30
.
Standard fiber optic OCDR systems currently use non-polarization maintaining (non-PM.) fiber throughout, leading to loss of signal and to artifacts associated with mismatches between the polarization states of the light from the reference and sample arms (polarization fading). These mismatches are caused by birefringence in the sample and reference arms and the sample itself.
Several attempts have been made to eliminate this polarization fading through the use of polarization diversity receivers, where the light returning from the sample and reference arms is split into two orthogonal polarization modes each mode is detected by a separate detector. To minimize costs, such system would ideally have non-PM fiber in the sample arm. However all polarization insensitive systems developed to date with non-PM fiber in the sample arm have either suffered from dispersion caused by PM fiber in the reference arm, or variations in the polarization state of light returning from the reference arm, caused by changes in the birefringence of non-PM reference arm fiber.
Co-pending U.S. patent application Ser. No. 09/050,571 to Everett et al. describes a sensing system, shown in
FIG. 2
, in which the polarization of the light through the system is controlled by polarization maintaining (PM) fibers and optics. Linearly polarized light is introduced into the system either through use of a linearly polarized broadband light source
40
or by placing linear polarizer
42
directly after an unpolarized source
40
. The linear polarization of the light is then maintained through the use of PM fibers and a PM fiber optic coupler
44
, where the linear polarization is one of the two modes of the PM fiber and PM coupler
44
. The polarization state of the light in the reference arm
46
is modified by either a waveplate or a faraday rotator
48
so as to be equally split between the two modes (orthogonal polarizations) of the PM fiber upon reflection. A polarization beam splitter
50
in the detector arm
52
splits the two polarization modes and directs them to two separate detectors
54
,
56
connected to the image processing and display unit
58
.
In one embodiment shown in
FIG. 2
, the multiplexed optical fibers
60
in the sample arm
62
are polarization maintaining (PM). The sample arm
62
contains a multiplexer
66
for switching between the plurality of fibers
60
, allowing sequential spatially distinct regions to be observed consecutively using the OCDR system. The fibers
60
can be oriented such that the light leaving the fibers is linearly polarized at an angle approximately 45° relative to the fast axis of birefringence of the sample
64
. Alternatively, a quarter waveplate can be placed at the distal end of each fiber
60
to cause the light entering the sample
64
to be circularly polarized. In either case, the total light in all polarization states returning from the sample
64
is determined by summing the signal from the two detectors
54
,
56
. In addition, processing and display unit
58
includes means for ratioing the output signals from detectors
54
,
56
; the birefringence of the sample
64
is determined based on the arc tangent of the ratio of the signals from the two detectors
54
,
56
.
In another embodiment described in U.S. patent application Ser. No. 09/050,571 to Everett et al., the optical fibers
60
in the sample arm
62
are not polarization maintaining (non-PM). In this case, the polarization beam splitter
50
ensures that the polarization state of the light from the reference arm
46
and the sample arm
62
is matched on each detector
54
,
56
, thus eliminating the losses due to depolarization of the light. The light returning from the sample arm
62
is then measured by summing the signals from the two detectors
54
,
56
.
It was found that the hybrid system described above containing non-PM fiber in the sample arm and PM fiber in the reference arm suffered from path length offsets between the two polarization modes, and reduced resolution caused by a difference in chromatic dispersion between the sample arm
62
non-PM fiber and the reference arm
46
PM fiber. Chromatic dispersion causes pulse broadening due to unequal speeds of different wavelength components of light in the reference arm fiber that are not matched by the sample arm non-PM fiber. The difference in the group velocity between the two polarization modes in the reference arm also lead to a path mismatch between the two polarization modes, which causes additional problems.
An alternate design for a fiber optic polarization insensitive OCDR system with non-PM fiber in the sample arm has previously been described (Kobayashi et al, “Polarization-Independent Interferometric Optical-Time-Domain Reflectometer”, 1991
, J. Lightwave Tech.
9(5):623-628). The reference arm in this system consists of all PM optical fiber, leading to loss of resolution due to mismatched dispersion between the sample and reference arms. The system also requires a specialized 50/50 coupler.
Another design of a polarization insensitive OCDR system is described by Sorin et al. in U.S. Pat. No. 5,202,745. In this design, a linear polarizer in the reference arm is adjusted to compensate for birefringence in the reference arm so as to equal signal powers on each detector in the detector arm in the absence of a signal from the test, or sample, arm. The problem with this approach is that the polarizer needs to be adjusted as the birefringence in the reference arm changes. As the birefringence in the non-PM reference arm fiber is strongly affected by temperature and stress, the system must be recalibrated with each use, and suffers from polarization drift during use.
Despite the problems with the systems described above, there is strong motivation to incorporate non-PM fiber into the sample arm, particularly to accommodate
Davis Joseph G.
Everett Matthew J.
Connelly-Cushwa Michelle R.
Staggs Michael C.
The Regents of the University of California
Thompson Alan H.
Ullah Akm E.
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