High power fiber isolator

Details High power fiber isolator High power fiber isolator

2001-04-02

2004-03-23

Ngo, Hung N. (Department: 2874)

Optical waveguides

Polarization without modulation

C385S034000

Reexamination Certificate

active

06711310

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to the field of optical isolators and more specifically to an optical isolator for use in high power applications.
BACKGROUND OF THE INVENTION
Optical isolators are one of the most ubiquitous of all passive optical components found in optical communication systems. Generally, optical isolators are used to allow signals to propagate in a forward direction but not in a backward direction. They are frequently used to prevent unwanted back reflections from being transmitted back to a transmitting source such as a laser.
Referring to
FIG. 1
a
, there is shown a polarization insensitive optical isolator. The isolator
2
includes an isolator core
4
comprising a first birefringent crystal
8
, a non-reciprocal rotator in the form of a Faraday rotator
10
, a reciprocal rotator in the form of a half-waveplate
12
, and a second birefringent crystal
14
. The Faraday rotator
10
is typically formed from doped garnet or YIG, and is placed in a permanent magnet. On one side of the isolator core is placed an input optical fiber
18
and on the other side is placed an output optical fiber
20
. The jacket
18
c
of input optical fibre
18
is stripped away and an exposed end of the core
18
a
and cladding
18
b
of the input
18
optical fibre is secured in a ferrule
26
with epoxy
30
. Similarly, an exposed end of the core
20
a
and cladding
20
b
of the output
20
fibre is secured in a ferrule
28
with epoxy
31
. Preferably, the ferrules
26
and
28
have a predetermined angle selected to reduce backreflection. Lens
40
and optional spacer
42
are optically coupled to the ferrule
26
and optical fibre
18
, and held in place with a sleeve
44
. Similarly, lens
50
and optional spacer
52
are optically coupled to ferrule
28
and optical fibre
20
, and are held in place with sleeve
54
.
In operation, an optical signal launched from the core
18
a
of the input fibre is collimated and transmitted through the first birefringent crystal
8
, where it is separated into two orthogonally polarized sub-beams of light. More specifically, the two orthogonal sub-beams are depicted as the O-ray and the E-ray components, wherein the E-ray experiences a spatial displacement as it traverses the birefringent crystal
8
. The two rays pass through the Faraday rotator
10
wherein the polarization of each sub-beam is rotated by 45° and the half waveplate
12
wherein the polarization of each sub-beam is rotated another 45°, for a total rotation of about 90°. Since the polarization of each ray is rotated by 90°, the E-ray component is unaffected as it passes through the second birefringent crystal
14
, whereas the O-ray component experiences spatial displacement. The two rays are recombined and focussed onto the core of the output fibre
20
a.
Referring to
FIG. 1
b
, a ray diagram showing an optical signal launched from the core of the output fibre
20
a
to the second birefringent crystal
14
is shown. The second birefringent crystal separates the optical signal into two orthogonal rays corresponding to the O-ray and the E-ray components, wherein the E-ray experiences a spatial displacement as it traverses the birefringent crystal
8
. The two rays of light are then passed through the half waveplate
12
and the Faraday rotator
10
. Since the half waveplate
12
is a reciprocal device, whereas the Faraday rotator
10
is a non-reciprocal device, a total rotation of about 0° is observed and the first birefringent crystal
8
does not recombine the two rays. More specifically, the E-ray component experiences a further spatial displacement, whereas the O-ray component passes through the second birefringent crystal
14
unaffected, such that the two rays are focussed on points away from the core
18
a
of the first optical fibre, thus providing isolation in the reverse direction.
Improvements, or modifications, in optical isolators include adding a reflector, replacing the half waveplate with a third birefringent crystal, designing the optical components with wedge angles, adding lenses, adding polarization diversity, and/or adding additional components for improving isolation (e.g., multi-stage optical isolators). For example, see U.S. Pat. Nos. 5,033,830, 5,208,876, 5,345,329, 5,734,762, and 6,088,153 incorporated herein by reference.
As described above, a disadvantage of the isolator shown in
FIGS. 1
a
and
1
b
is that it directs the backward propagating light to points surrounding the input core
18
a
. For example, the backward propagating light is directed to locations ranging from the input optical fibre cladding
18
b
to the ferrule
26
, depending on the length of the first birefringent crystal (e.g.,
8
) and the optical arrangement. For low power applications this may be acceptable, however, for high power applications this results in significant damage of the optics and/or system degradation/failure. For example, one significant problem arises when backward propagating high power light is transmitted to the ferrule and degrades the epoxy (e.g.,
30
) used to secure and align the optical components. The degradation of epoxy in optical isolators is discussed in U.S. Pat. No. 5,661,829, incorporated herein by reference. Other examples include the burning or damaging of other optics due to excessive heating and/or high power radiation.
In U.S. Pat. No. 5,546,486 to Shih et al. there is disclosed an optical isolator including an input fiber having a reflective optical barrier layer, such as gold, that covers the end surface of the fiber with an aperture exposing the core and covering the cladding of the fiber. Although, this improvement substantially reduces light transmission into the end of the optical fiber via the cladding, thus improving the isolation, it does not protect the other optical components from the high power radiation. In fact, the device taught by Shih et al. is not compatible with high power applications, since the reflective layer produces additional reflections that may introduce noise and/or damage other optical components. For example, if the reflective layer is soiled with dust or another impurity, it might ignite in high power applications.
It is an object of this invention to provide an optical isolator that obviates the above mentioned disadvantages.
It is a further object of this invention to provide an optical isolator for use in high power applications.
SUMMARY OF THE INVENTION
The instant invention provides an optical isolator for use in high power applications that includes a light collector for redirecting and/or absorbing backward propagating radiation. In the preferred embodiment, the light collector does not interfere with forward propagating radiation, but collects or gathers the backward propagating light.
In accordance with the invention there is provided a method for protecting isolator components from high intensity backreflections in an optical isolator comprising a first port for launching light in a forward propagating direction, a second port for receiving the light launched from the first port and for transmitting light in a backward propagating direction towards the first port, and an isolator core optically disposed between the first and second ports including a first birefringent crystal, a non-reciprocal rotator, and a second birefringent crystal, the method comprising the step of: providing light collecting means for substantially unaffecting forward propagating light launched from the first port and for collecting and isolating backward propagating light transmitted from the isolator core.
In accordance with the invention there is provided an optical isolator comprising: a first port; a second port optically coupled to the first port; an isolator core optically disposed between the first port and the second port comprising a first birefringent crystal, a non-reciprocal rotator, and a second birefringent crystal for transmitting forward propagating light from the first port to the second port and for preventing backward propagating light transmitted from the second po

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