Laser doppler optical sensor for use on a monitoring probe

A laser Doppler probe for measuring blood flow, comprising a transmit fiber, and one or more receive fibers, which are configured to direct and receive energy orthogonal to the fiber axis.

FIELD OF THE INVENTION 
The present invention relates generally to an optical sensor or probe for 
measuring fluid flow, and more particularly to a laser Doppler blood 
perfusion monitoring probe for measuring blood flow in tissue. 
BACKGROUND OF THE INVENTION 
Blood flow is an important parameter which is measured as a part of certain 
surgical and diagnostic procedures. Laser Doppler flow meters utilize 
fiber optics to distribute coherent light to an anatomic site. The 
relative motion of the blood cells modulates the coherent light, and the 
frequency shift associated with the motion of the corpuscles can be 
extracted from a photodiode signal connected to a receiving fiber. 
SUMMARY OF THE INVENTION 
The probe of the present invention utilizes a single transmitting fiber 
connected to a source of laser light, and one or more receiving fibers 
which are connected to a photodiode detector. The distal ends of the 
fibers are cut to form facets. Each facet is then rendered reflective by 
metalization or the like, so that the light emerging from the transmit 
fiber leaves at an angle with respect to the fiber axis. One or more 
receive fibers are provided and each receive fiber is also cut at an angle 
to form a facet. This faceted geometry permits light scattered from tissue 
to be returned along an axis perpendicular to the long axis of the receive 
fiber. This creates a lateral or "side looking" sensor arrangement, which 
is useful in a variety of settings. The orientation of the fibers are 
important for the operation of the sensor, and it is generally desired to 
have the transmit fiber extend distally of the receive fibers and several 
configurations are contemplated. Although the unique sensor can be used in 
isolation, a sheath or cover may be applied to house the sensor for some 
applications.

DETAILED DESCRIPTION 
FIG. 1 shows the blood perfusion probe 10 in a schematic fashion. The 
optical sensor assembly 14 is formed of a single transmit fiber 18, and 
one or more receive fibers. In the illustrative embodiment shown through 
out the figures a pair of receive fibers are shown as receive fiber 20 and 
receive fiber 22 respectively. It should be apparent that a single receive 
fiber is operable as well as embodiments which include three receive 
fibers. However for clarity in description only two receive fibers are 
shown in the various figures. 
The two receive fibers are coupled to a photodetector 50, while the 
transmit fiber 18 is coupled to a source of optical radiation such as a 
laser 52. In this figure, three orthogonal directions are defined by 
coordinate 15 (X); the coordinate 13 (Y); and the coordinate 11 (Z). In 
the particular configuration depicted in FIG. 1 the two receive fibers lie 
in the YZ plane while the single transmit fiber 18 is displaced along the 
X axis from the plane of the receive fibers. 
The distal tip of the transmit fiber 18 is cut to form a facet 16. The 
receive fibers typified by receive fiber 20 also are cut to form facets 
shown as facet 17 on fiber 20 and as facet 19 on fiber 22. The facets 
serve to direct light out of each fiber. For example the light is emitted 
from fiber 18 along the direction indicated by direction path 21. The 
light received by the receive fiber 20 enters along the centerline 
indicated by path 23. Similarly the light enters fiber 23 along the path 
shown as path 25. In each instance the centerline of the path is 
approximately orthogonal to the centerline or major axis of the fiber. 
In general the single relatively small diameter (typically 50-100 .mu.m, 
preferably 62.5 .mu.m) transmit fiber 18, is positioned distal and 
"behind" the illustrative pair of relatively large diameter (typically 
50-200 .mu.m, preferably 100 .mu.m) receive fibers shown as receive fiber 
20 and receive fiber 22. The collective fiber orientation defines an 
aperture shown by the loop 27. 
As seen in connection with FIG. 1, the fibers may be embedded in a 
transparent optical block 26, which stabilizes and retains the distal tips 
of the fibers in a fixed relationship with respect to each other. The 
optical block 26 may extend the entire length of the sheath 12 or the 
fibers may transition into a flexible fiber optic cable assembly as known 
in this industry. Although the optical sensor assembly 14 is shown in a 
schematic form for clarity, the fibers are typically embedded in the block 
26 in optical epoxy to permanently position them. The optical sensor 
assembly 14 is then permanently covered by an extension of the sheath 12 
to complete the fabrication of the probe 10. 
FIG. 2 is a composite diagram showing a central sensor axis 29. This figure 
shows an elevation view and a plan view of an alternate fiber 
configuration. In this schematic diagram the transmit fiber 18 is "in 
front" of the plane defined by the two receive fibers shown as fiber 20 
and fiber 22. In general, each fiber has a facet angle "theta" formed with 
respect to the central axis 29 which serves to direct light away from the 
central axis 29 as indicated by path arrow 31 for the transmit fiber 18 
and path arrow 33 for the receive fiber 22. 
A reflective coating 42 is provided at each oblique facet surface of each 
fiber. The preferred reflective coating is gold. Gold is preferred because 
of its high reflectivity at the wave lengths available for commercial 
laser sources. However silver is an alternative choice, and multilayer 
dielectric mirrors can also be formed on the distal facets to form a 
reflective mirror structure. 
In general, the facet angle will be near forty five degrees but need not be 
equal to forty five degrees (forty five plus or minus 15 degrees for 
example). In the configuration of FIG. 1 the transmit facet angle "theta" 
will preferably be greater than forty five degrees, while in the 
configuration of FIG. 2 the transmit fiber facet angle will preferably be 
less than forty five degrees. In each of these examples the angle is 
selected to minimize internal reflection which returns light to the laser 
52 which is undesirable. In general the configuration of FIG. 1 maximizes 
the amount of light emitted from the transmit fiber which improves overall 
system efficiency. 
The relative displacement of the emitted light from fiber 18 and the 
received light from fiber 22, as determined by projecting the fibers on to 
a plane perpendicular to the path arrows 31 and 33 and measuring the 
distance between fibers 18 and 22 in this plane, is shown in FIG. 2 as the 
distance "D". This distance "D" may be selected to suit particular 
applications. It will preferably be approximately 0.5 mm, but may range 
from about 0.25 mm to 3.0 mm. In general, the depth of tissue which can be 
observed with the probe is a function of "D", with the identified range 
representing a compromise between laser power and tissue monitoring 
volume. 
FIG. 3 shows an alternate and generally planar configuration where the 
transmit fiber 18 lies along the central axis 29 and is in the same plane 
as the receive fiber 20 and receive fiber 22. In this configuration the 
distance between the emitted light path and the received light path is 
shown as "L". In this configuration the preferred angle for the distal tip 
facet may be greater than or less than 45.degree.. In this alternative 
configuration, all the fibers have the same length and are in the same 
plane. 
In each of the foregoing embodiments it is generally preferred to have all 
the facets aligned to the same direction so that the beam centers of each 
facet are parallel to each other indicated in FIG. 4. However it should be 
understood that other alignments are possible as well. Although the 
configuration seen in FIG. 5 has the two receive fibers at an acute angle 
with respect to the transmit path 33, any angle from zero degrees to 360 
degrees may be selected. This geometry may be especially useful where it 
is important to have as small a diameter probe as possible. In these 
instances the fibers will be clustered and abut each other in a minimum 
space filling configuration as seen in FIG. 5. 
Although two specific applications of the sensor are shown it should be 
understood that the sensor can be used alone of integrated into a variety 
of devices without departing from the scope of the invention.