U.S. patent application number 14/024984 was filed with the patent office on 2015-03-12 for low insertion loss, low back reflection fiber optic rotary joint with increased data throughput.
This patent application is currently assigned to PRINCETEL INC.. The applicant listed for this patent is PRINCETEL INC.. Invention is credited to Louis D. VIOLANTE, Boying B. ZHANG, Hong ZHANG.
Application Number | 20150071588 14/024984 |
Document ID | / |
Family ID | 52473050 |
Filed Date | 2015-03-12 |
United States Patent
Application |
20150071588 |
Kind Code |
A1 |
ZHANG; Hong ; et
al. |
March 12, 2015 |
LOW INSERTION LOSS, LOW BACK REFLECTION FIBER OPTIC ROTARY JOINT
WITH INCREASED DATA THROUGHPUT
Abstract
A low insertion loss, low back reflection fiber optic rotary
joint with increased data throughput capabilities has been
invented. This is accomplished by using a dispersion free prism
with optical coatings to minimize the amount of back reflection
caused by the optical beam passing through the prism-air
interface.
Inventors: |
ZHANG; Hong; (North
Brunswick, NJ) ; ZHANG; Boying B.; (Lawrenceville,
NJ) ; VIOLANTE; Louis D.; (Monroe, NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
PRINCETEL INC. |
Hamilton |
NJ |
US |
|
|
Assignee: |
PRINCETEL INC.
Hamilton
NJ
|
Family ID: |
52473050 |
Appl. No.: |
14/024984 |
Filed: |
September 12, 2013 |
Current U.S.
Class: |
385/26 |
Current CPC
Class: |
G02B 6/3604
20130101 |
Class at
Publication: |
385/26 |
International
Class: |
G02B 6/36 20060101
G02B006/36 |
Claims
1. A multi-channel fiber optic rotary joint comprising: a rotor; a
stator; a plurality of bears; a first collimator array with a
rotary axis; a second collimator array with a rotary axis; a Pechan
prism with an plurality optical coatings to minimize back
reflection; wherein said first collimator array and said second
collimator array are aligned with said rotary axes and are
relatively rotatable along said rotary axes and having a coated
Pechan prism positioned in the optical path between said first
collimator array and said second collimator array wherein is
arranged for rotation around said rotary axes relative to each of
said first and second collimator arrays at a rotary speed equal to
one-half the relative rotational rate between said first and second
collimator arrays; and a speed reduction mechanism for providing
the rotation between said Pechan prism and said first and second
collimator array to rotate the Pechan prism at a rotational rate
half the rotational rate of said first and second collimator
array.
2. A multi-channel fiber optic rotary joint of claim 1, further
said optic rotary joint having a plurality of environmental
seals
3. A multi-channel fiber optic rotary joint of claim 1, wherein
said speed reduction mechanism is a gearing mechanism with a gear
ratio of 2:1; or any other passive mechanical system.
Description
BACKGROUND OF THE INVENTION
[0001] A typical rotary joint consists of a fixed collimator holder
and a rotatable collimator holder which are relatively rotatable to
each other to allow uninterrupted transmission of optical signals
through the rotational interface from collimators in any one of the
holders to the collimators in the other holder.
[0002] A multi-channel fiber optic rotary joint typically utilizes
a dove prism and a gear box as the de-rotating mechanism between
the fixed collimator holder and the rotatable collimator holder. It
typically rotates at half the rotational speed of the rotatable
collimator holder; effectively de-rotation the image.
[0003] The dove prism relies on refraction to bend the optical beam
as part of the de-rotation, which theoretically should not cause a
problem. However, in practice this refraction is a source of
chromatic dispersion because optical signals have a finite spectral
range. Since the angle of refraction is a partial function of the
wavelength itself, each discrete wavelength in the optical signal
travels a different distance in the dove prism which causes part of
the signal to effectively fall behind other parts of the signal.
This signal lag ultimately determines the maximum frequency at
which a signal can be sent through the fiber optic rotary joint.
One way to envision this problem is to picture a square wave, Each
peak of the wave represents a discrete piece of information and the
width of each peak represents the amount of dispersion in the
signal. Now the only way to send the same information faster is to
reduce the distance between signal peaks; however, if the distance
between peaks is reduced to the point that the waves start to
overlap then the information in the two waves starts to become
scrambled. Therefore, the frequency at which a signal can be sent
is limited by the width of each wave, or the amount of dispersion
that occurs in the signal.
[0004] Another effect of dispersion is it effectively limits the
wavelengths at which a particular device can be used. One of the
results of different wavelengths traveling slightly different
distances through the prism is each wavelength will emerge at
different angles on the other side of the prism. As such, a device
tuned at 1625 nm will not perform the same if used at 1310 mu. The
degree to which the performance will degrade is directly related to
the difference between the wavelengths in question. As such the
unit will have better performance the closer the operating
wavelength is to the wavelength at which the unit was tuned.
Conversely, the performance of the unit will degrade the farther
the operating wavelength is from the wavelength at which the unit
was tuned.
[0005] This phenomena also sets a floor for the lowest achievable
insertion loss. The cause of this lower limit is that in practice
every light source has a finite spectral range around the nominal
wavelength. The slight variation in the distance traveled by each
wavelengths in the spectral range cause slight variations in the
focal point for each wavelength in the spectral range. This
theoretically reduces the efficiency with which the opposing
collimator could recapture the signal, thereby putting a
theoretical limit on the lowest achievable insertion loss.
[0006] All of the aforementioned problems can be resolved by using
a dc-rotating prism that does not rely on refraction, such as the
Pechan prism or the K-prism. However, as Ames correctly indicated
in U.S. Pat. No. 5,157,745; the Pechan prism suffers from high back
reflection. This problem leads Ames to conclude that the dove prism
is preferable. The configuration embodied herein, solves the back
reflection problem by applying an optical coating to the surfaces
of the Pechan prism through which the optical signal must pass.
This optical coating will eliminate/reduce the reflection at zero
degree and function as a mirror at 45 degrees as well. Similarly,
high reflection optical coatings can be applied to the surfaces of
the K-prism through which the optical signal must pass for the same
net result.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 Is the schematic drawing of de-rotating Dove
prism;
[0008] FIG. 2 Is an outline diagram a coated Pechan prism in the
present invention;
[0009] FIG. 3 Illustrates the principles of a coated Pechan prism
for a multi-channel fiber optic rotary joint in the present
invention;
[0010] FIG. 4 Depicts the position of coated Pechan prism relative
to a stationary collimator array and a fiber collimator array in
the present invention;
[0011] FIG. 5 Is a cross-sectional view of a multi-channel fiber
optic rotary joint in the present invention,
DETAIL DESCRIPTION OF INVENTION
[0012] Dove prisms are used to invert an image and when they are
rotated about their longitudinal axis, the transmitted image
rotates at twice the rate of the prism (see FIG. 1). Therefore, if
the prism rotates at half the rate of a rotating object, the image,
after passing through the prism, will appear to be stationary. FIG.
1 is the schematic drawing of de-rotating Dove prism in the prior
art. The image (2) of an object (1) is inverted by the Dove prism
(10). Furthermore, if the prism (10) is rotated about the optic
axis (3), the image (2) rotates at twice the rate of rotation of
Dove prism (10).
[0013] FIG. 2 illustrates the imaging principle of the coated
Pechan prism in the present invention. The image (152) of an object
(151) on the entrance side of a coated Pechan prism (157) is
inverted in a similar way as the Dove prism (10) in FIG. 1.
However, there is one critical difference; there are no refractive
elements in the optical path of the coated Pechan Prism. As a
result, there will not be any dispersion of the optical beam. It is
also important to note that the prism surfaces through which the
beam passes (153, 154, 155 and 156) must be coated with an optical
coating to reduce the back reflection to an acceptable number.
[0014] FIG. 3 depicts how the coated Pechan prism (101) can be used
as a de-rotating mechanism for the multi-channel fiber optic rotary
joint in the present invention. Suppose the coated Pechan prism
(101) rotates an angle "b" around its axis "Z" from position "1" to
position "2", e.g., from 101"1" to 101"2". The co-ordinates of the
object (4) in position "1", e.g., 4 "1", is (X1, Y1). According to
FIG. 2, because the image (5) is inverted symmetrically relative to
the axis "Z", the co-ordinates of the image (5) in position "1" are
(-X1, Y1). If the object rotates an angle "2b" around axis "Z" in
the same direction as the coated Pechan prism (101), the
co-ordinates of the object (4) in position "2", 4 "2", are (X2,
Y2). It's easy to get that the co-ordinates of the image (5) in
position "2" are (-X2, Y2). So the absolute position of the image
(5) remains s the same before and after the rotation. That means
that if the coated Pechan prism rotates at half the speed of a
rotating object (4), its image (5) passing through the coated
Pechan prism (101), will remain to be stationary.
[0015] In FIG. 4, a coated Pechan prism de-rotating mechanism (12)
its the present invention is positioned between a stationary
collimator array (13) and a rotary collimator array (11). The
rotary collimator array (11), the stationary collimator array (13)
and the coated Pechan prism de-rotating mechanism (12) are
rotatable around a common axis (15). All the collimators (111, 112,
113, 114, 115, 116 . . . ) in said stationary collimator array (13)
and said rotary collimator array (11) are arranged parallel to the
common axis (15). If the coated Pechan prism de-rotating mechanism
(12) rotates at half the speed of rotation of said rotary
collimator array (11) around the common axis (15), the optical
signals from the rotary collimator array (11) would be passed
through the coated Pechan prism de-rotating mechanism (12) and be
transmitted to the related channel of the stationary collimator
array (13) respectively, e.g., the first channel optical signal can
be transmitted between collimator (111) and (112); the second
channel optical signal can be transmitted between collimator (115)
and (116); the third channel optical signal can be transmitted
between collimator (113) and (114), so as to provide a continuous,
bi-directional, multi-channel electro-magnetic signal transmission
between two collimator arrays.
[0016] FIG. 5 depicts one of embodiments of a multi-channel fiber
optic rotary joint of the present invention. A speed reduction
mechanism includes gears (24, 25, 26, and 27) in which two gears
(26 and 27) are rotatable around the common axis (15), while the
other two gears (24 and 25) are rotatable around a parallel axis
(16). The gear ratio i from gears 26 to gear 27 can be determined
as follows:
i = Z 24 Z 27 Z 26 Z 28 ##EQU00001##
where, Z.sub.24, Z.sub.25, Z.sub.26 and Z.sub.27 are the number
gear teeth number for gears 24, 25, 26 and 27 respectively. If the
gear ratio i=2:1, that means gear 27 will rotate at half the speed
of the rotation of gear 26.
[0017] As illustrated in FIG. 5, the coated Pechan prism
de-rotating mechanism (12) is in the center of the cylinder (28).
The relative position between the coated Pechan prism de-rotating
mechanism (12), the stationary collimator array (13) and the rotary
collimator array (11) are the same as depicted in FIG. 4. The rotor
(21) is part of a gear (26), which is rotatable relative to the
stator (22) through the bearings (31 and 32), The cylinder (28) is
part of a gear (27), which is rotatable relative to the stator (22)
through the bearings (33 and 34). Two gears (24 and 25) are
physically connected to the common shaft (23), which is rotatable
around the parallel axis (16) relative to the stator (22) through
two bearings (35 and 36). As stated above, a gear ratio i=2:1 would
assure that the coated Pechan prism de-rotating mechanism (12) will
rotate at half the speed of the rotation of the rotary collimator
array (11).
* * * * *