A low-loss, high-isolation, fiber-optic isolator for use in single-mode fiber systems utilizes a Faraday rotator and two polarizers, one at each end angularly oriented from each other at the angle of rotation for isolation, and two aspheric lens connectors to couple optical fibers to the Faraday isolator to reduce forward loss to about 2.5 dB and improve isolation to greater than 70 dB

Lutes, George F.

English

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NASA CASE NO. A/ fi 0 "I 7,
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NOTICE
The invention disclosed, in this document resulted from /n
research in aeronautical and space activities performed under
programs of the National Aeronautics and Space Administration.
The invention is owned by NASA and is, therefore, available for
licensing in accordance with the NASA Patent Licensing
Regulation (14 Code of Federal Regulations 1245.2).
To encourage commercial utilization of NASA-owned inventions,
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Address inquiries and all applications for license for this
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(KflSA-Case-NPC-17207-1-CU) ICK-LGSS, N88-25304
E1GH-ISG1ATICK, FIBEB-CFTIC ISCIMOB Patent
Application <NA£A) 11 p CSCL 20F -JPL
Unclas
G3/74 0147340
https://ntrs.nasa.gov/search.jsp?R=19880015920 2020-03-20T06:44:38+00:00Z
LOW-LOSS, HIGH-ISOLATION, FIBER-OPTIC ISOLATOR
Inventor: George F. Lutes
Contractor: Jet Propulsion
Laboaratory
JPL Case No.17207
NASA Case No. NPO-17207-1-CU
March 31, 1988
AWARDS ABSTRACT
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 ^d high-isolation3in
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mode
 fiber-optic systems, and more
o i * fiber-°Ptic isolator having a measured
forward loss of 2.6 dB and improved isolation to greater than
m IPTP V°W~10SS' high-isolation, fiber-optic isolator shown
n«f ? in"363 *" isolafcor assembly 16 comprised of a Faraday
an™?0"', f v tW° P0larizers " and 18, one at each end
angularly oriented from each other at the angle of rotation of
and £ M 7tat°r f°r lsolation' and two aspheric lenses 20
and « ,"0 th 7T? 21 ^  23 t0 C°nneCt °ptical fibers "and 15 to the isolator assembly. The system improves isola-
fea,vfr ^^ 7° dB* The aPertures into the system areby the optical fibers interfacing with the flat sur-
?o ,nH ??< !nSe3' Incident "8*»t is expanded by the lens
20 and collimated into the isolator assembly At the exit
S^ H^ Vl6"3 « focuses the collimated beam into the aperture
of the fiber 15. Light reflected in the isolator asembly is
rejected by the small acceptance angles and apertures of the
losf?^3' FIG- 2 1S 3 graph of excess transmissionloss (dB) as a function' of distance between aspheric lens
.'
 FIG
-
 3 1S a graph
 °
f
 transmission loss betweenic lens connectors as a function of offset angle.
The novelty of the invention resides in the use of
nv ?S6S St ea°h 6nd °f 3n °ptical is°lator assembly toprovide small apertures and acceptance angles for reiection of
reflected light within the isolator assembly? rejection of
Serial No. '
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DISTANCE BETWEEN CONNECTORS (cm)
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FIG 2
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0.001 0.002 0.003 0.004 0.005
OFFSET ANGLE (radians)
FIG. 3
NASA Case No. NPO-17207-1-CU
87/221
Serial
Fi;ir.g
Cc- .rftcj No,
Contractor
Pasadena
NAS7-91C
PAtEWEch/JPL
CA
- 91109
LOW-LOSS, HIGH-ISOLATION,
FIBER-OPTIC ISOLATOR
Origin of the Invention
The invention described herein was made in the perform-
ance of work under a NASA contract, and is subject to the
provisions of Public Law 96-517 (35 USC 202) in which the
Contractor has elected not to retain title.
10 Technical Field
The invention relates to a low-loss, high-isolation,
fiber-optic isolator for use in single-mode fiber systems, and
more particularly to an isolator having a measured forward
loss of 2.6 dB and improved isolation to greater than 70 dB.
15
20
25
30
Background Art
Reflections cause intolerable instabilities in fiber-
optic systems used in some precision applications, such as
precise reference frequency distribution and microwave trans-
mission. The reflections have the same effect on a semicon-
ductor laser source as an unstable.external cavity, and causes
changes in the laser's output wavelength and amplitude. In
some systems, external reflections must be reduced by 60 dB or
more before the resulting instabilities are reduced to toler-
able levels.
Optical isolators are currently used to alleviate this
problem. However, the best single-mode fiber-optic isolators
which have been reported have losses of 3 or 1 dB and isola-
tion of 30 to 40 dB. (See Okamoto, K., Miyazawa, H., Noda, J.,
and Saruwatari, M., "Novel optical isolator consisting of a
YIG spherical lens and PANDA-fiber polarizers," Elect. Lett.,
1985, 21, pp. 36-38; Green, A.E., Georgiou, G., "Compact bulk
optical isolator with monomode fibre pigtails for use at 1.3,"
Elect. Lett., 1986, 22, pp. 10H5-1046; and Gauthier, D.J.,
87/221
Narum P., Boyd, R.W., "Simple, compact, high-performance per-
manent-magnet Faraday isolator," Optics Lett., 1986, 11, pp.
623-625. It is necessary to use two such isolators in series
to obtain the isolation of greater than 60 db required in some
5 fiber-optic systems, such as those used for precise reference
frequency or microwave frequency distribution.
There are disadvantages in using two isolators in
series: good optical isolators are very expensive, and the
forward loss is increased.
10
Statement of the Invention
Accordingly, it is an object of this invention to pro-
vide an optical isolator with greater than about 70 dB isola-
tion of reflected light from a source and less than about 3 dB
15 loss in the forward direction.
The low-loss and high-isolation apparatus of this
invention is comprised of an isolator assembly consisting of a
Faraday rotator and polarizers on each side oriented with
respect to each other at an optimum angle between their direc-
20 tions of polarization equal to the rotation angle of the
rotator, and two aspheric lenses facing each other, one on
each side of the isolator assembly.
Each of the aspheric lenses has an aspheric surface
facing the isolator assembly and a planar surface on the
25 outside. Together with the isolator assembly positioned in a
gap of several centimeters between the aspheric lens, the
aspheric lenses permit light to be transmitted with low loss
and high isolation from one side to the other by accepting
incident light through small apertures and expanding the light
30 to collimated beams larger than the apertures, and providing a
small acceptance angle and small aperture to reflected light
within the isolator assembly.
The novel features that are considered characteristic
of this invention are set forth with particularity in the
87/221 3
appended claims. The invention will best be understood from
the following description when read in connection with the
accompanying drawings.
5 Brief Description of the Drawings
FIG. 1 is a schematic block diagram of the present
invention.
FIG. 2 is a graph of excess transmission loss (dB) as a
function of distance between aspheric lens connectors (cm).
10 FIG. 3 is a graph of transmission loss between connect-
ors as a function of offset angle.
Detailed Description of the Invention
FIG. 1 is a schematic drawing of the. invention com-
15 prised of two aspheric lens connectors 10 and 12 for optically
coupling to the ends of single-mode optical fibers 1*J and 15.
Positioned between the connectors is an optical isolator 16
comprised of two polarizers 17 and 18 oriented at an optimum
angle a from each other, depending on the angle of rotation of
20 a transparent isotropic medium 19 having a Verdet constant o>
in a magnetic field of strength H. The angle a of rotation is
equal to a •* &>4H, where I is the length of the path traversed
through the medium. The connectors 10 and 12 consist of
aspheric lenses 20 and 22, and ferrules 21 and 23 on flat
25 sides opposite the aspheric surfaces. The flat surfaces are
oriented parallel to planar surfaces of the isolator 16 in
order to align the axes of the lenses parallel to each other
and with minimum lateral offset from each other.
The organization of the low-loss, high-resolution,
30 single-mode fiber-optic isolator shown in FIG. 1 will now be
described in terms of the functions of the components. The
isolator 16 is essentially generic and is the primary means
for isolation between the fibers 11 and 15. The isolator used
in this assembly is a model IO-1-IR manufactured by Optics For
35 Research, Box 82, Caldwell, New Jersey 07006, who manufactures
87/221
a variety of optical isolators using the Faraday rotator
principle. The model used is designed for 1300 nm operation
and has an aperture of M mm. Isolation is specified as great-
er than 30 dB. One better than about MO dB has not heretofore
5 been known. Forward loss is specified to be less than 0.5 dB.
As noted hereinbefore, such an isolator is not suffi-
cient. Two such isolators in series would normally be re-
quired for isolation greater than 60 dB, but forward loss is
then apt to be greater than 3 dB. Accordingly, only one such
10 isolator is used with the connectors 10 and 12 one on each end
of the isolators facing each other. These connectors permit
light to be transmitted with low loss from the single-mode
optical fiber 11 to the single-mode optical fiber 15 with a
gap of several centimeters between them. A plot of the opti-
15 cal loss as a function of the distance between connectors is
shown in FIG. 2. The connectors are placed an appropriate
distance for the optical isolator to be installed between
them.
The'aspheric lenses 20 and 22 are shaped to convert a
20 beam entering through a small aperture (diameter of the opti-
cal fiber) into a larger columnar beam, and are provided with
the ferrules 11 and 15. These aspheric lenses with ferrules
attached are commercially available from Lamdek Fiber Optics,
a division of Eastman Kodak Company. The lenses are made of
25 glass to facilitate mass production, to render them capable of
excellent performance over a broad range of environmental
conditions (temperature, humidity, etc.), and to provide long
life operation (20 years). Flat surfaces are provided around
the aspheric surfaces of the lenses 20 and 22 which are paral-
30 lei to their planar surfaces in order to facilitate aligning
the axes of the two connectors to within less than 15 seconds
of arc. Means (not shown) are provided to maintain the con-
nector and isolator assemblies together, and in alignment with
the isolator. Such means may include precision steel balls
35 between the surrounding flat surfaces of the lenses and the
87/221
planar surfaces of the Isolator, as suggested by Laradek Fiber
Optics. The isolator surfaces should also be aligned perpen-
dicular to the connector axes with the same degree of accu-
racy. Once aligned angularly, the axis of one connector
5 through the isolator assembly should be adjusted to a minimum,
within 0.05 mm lateral displacement. This angular and lateral
alignment is important because the function of the connectors
10 and 12 is to virtually eliminate internal reflections by
providing the small apertures and acceptance angles needed for
10 high isolation by the Lamdek connectors.
Each aspheric lens is ideally provided with an aspheric
surface that is an ellipse of revolution with a conic constant
P
K = 1/N , where N is the index of refraction of the lens
material (glass) to eliminate spherical aberration in the beam
15 entering the connector 10 as it is transmitted through the
connector 12. Aspheric lenses are reported by the manufac-
turer to eliminate over O.1! dB insertion loss, as compared to
spherical lenses. They are used in the present invention in
order to transmit a substantially collimated 1.5 mm beam
20 through the isolator 16, and then to refocus the beam to a
small aperture (9 ym diameter) of the fiber 15. As the beam
enters the isolator 16, it is polarized in a particular direc-
tion. That direction is then rotated through an angle a by the
rotator 19, and the rotated and collimated beam is then passed
25 through the polarizer 18 having its direction of polarization
oriented at the angle a in respect to the polarization direc-
tion of the polarizer 17. The connectors 10 and 12 virtually
eliminate all reflections at the ends of the fibers 14 and 15
by providing a small aperture at their interfaces with the
30 planar back of the aspheric lenses.
The theory of operation of the low-loss, high-isola-
tion, fiber-optic apparatus of FIG. 1 will now be further des-
cribed, but first it should be recalled from the discussion of
the background art that reflections cause instabilities in
35 fiber-optic systems which are intolerable in some precision
87/221
applications, such as precise reference frequency distribu-
tions and microwave transmission. Reflections have the same
effect on a semiconductor laser source as an unstable external
cavity, and causes changes in the laser's output wavelength
5 and amplitude. In some systems, external reflections must be
reduced by 60 dB or more before the resulting instabilities
are reduced to tolerable levels.
The isolator 16 has much greater isolation potential
than expected. However, the isolation reported previously by
10 the manufacturer of the isolator 16 was evidently limited by
scattering due to internal reflections. The polarization of
this scattered light at various locations within the isolator
16 is not in the direction required for high attenuation.
Consequently, such scattered light was a problem in achieving
15 the full potential of the isolator. A secondary problem is
the nonuniformity of the polarizers across their diameters.
The polarizers have a much higher extinction ratio when the
optical beam size is smaller because they are more uniform
across a small area.
20 Fortunately the scattered light, which limits the
isolation, is not parallel to the axis of the isolator assem-
bly 16 when it exits. Therefore, it can be virtually elimi-
nated by collecting the optical output of the isolator
assembly 16 using a lens with a small aperture and small
25 acceptance angle. The aspheric lenses 20 and 22 provide such
a small aperture (9 ym at the interface with the optical
fibers 14 and 15) and a small acceptance angle needed for high
isolation. Thus, when used on both sides of the isolator,
these aspheric lenses set the collimated beam diameter through
30 the isolator assembly 16 by expanding the beam from an input
. aperture of 9 ym diameter to about 1.5 ram in diameter. The
isolator assembly 16 presents the collimated beam to a 9 ym
aperture at the output of the aspheric lens 22 through a small
acceptance angle for high isolation.
87/221 7
The transmission loss in dB of optical power at the
receiving connetor is related to the offset angle of a light
beam or ray by
Loss = 20 log [e"(fenuo)2],
5 where f = the focal length of the lens = 7 mm,
G = the offset angle (radians),
n = the index of refraction = 1.4995, and
o>0 = 1/2 the mode field diameter = .00523 mm.
A plot of transmission loss as a function of the offset
10 angle is shown in FIG. 3- At an angle of .00^ 5 radians the
loss is 70 dB. This accounts for the extremely large rejec-
tion of scattered light coming out of the optical isolator.
A low-loss, high-isolation fiber-optic isolator assem-
bled as described has been measured for loss and isolation and
15 found to be 2.6 dB and greater than 70 dB, respectively.
These results were achieved using commercially available
components. The availability of such high quality single-mode
fiber-optic isolators will make it possible to achieve sub-
stantial improvements in precision fiber-optic systems such as
20 those used for stable reference frequency distribution and
microwave frequency transmission.
Although particular embodiments of the invention have
been described and illustrated herein, it is recognized that
modifications and variations may readily occur to those
25 skilled in the art. Consequently, it is intended that the
claims be interpreted to cover such modifications and varia-
tions.
87/221
LOW-LOSS, HIGH-ISOLATION,
FIBER-OPTIC ISOLATOR
ABSTRACT OF THE DISCLOSURE
A low-loss, high-isolation, fiber-optic isolator for
5 use in single-mode fiber systems utilizes a Faraday rotator
and two polarizers, one at each end angularly oriented from
each other at the angl-i of rotation for isolation, and two
aspheric lens connectors to couple optical fibers to the
Faraday isolator to reduce forward loss to about 2.6 dB and
10 improve isolation to greater than 70 db.


Low-loss, high-isolation, fiber-optic isolator

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