Optical waveguides – Accessories – Attenuator
Reexamination Certificate
1999-04-20
2001-07-03
Spyrou, Cassandra (Department: 2872)
Optical waveguides
Accessories
Attenuator
C385S073000
Reexamination Certificate
active
06256446
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a variable attenuator for fiber optic transmission cables, and in particular to a variable attenuator of the type in which attenuation is achieved by varying an air gap between ends of two optical fibers. A cam arrangement is included to achieve a linear response between turning of an actuator and attenuation of an optical signal transmitted through the gap between the fiber optic transmission cables.
2. Description of Related Art
Attenuators are used to regulate the strength of an optical signal in a fiber optic transmission system. It is desirable to maintain the strength of an optical signal within a certain range to avoid receiver saturation, to compensate for variable distances of various receivers from the source, or to compensate for aging or other changes in the system.
The present invention involves a variable attenuator that is in the form of a fiber optic coupler arranged such that a controllable air gap exists between the ends of the fibers in the transmission cables coupled by the coupler. By mechanically varying the air gap length, the amount of attenuation can be controlled. Examples of variable attenuators of this type are disclosed in U.S. Pat. Nos. 4,145,110 and 5,066,094. A similar arrangement, involving interception of the air gap by a screw rather than direct adjustment of the air gap length is disclosed in U.S. Pat. No. 5,734,778.
FIG. 1
illustrates one such conventional variable attenuator of the type in which the air gap length in varied by turning a screw, and which is available from Amphenol Corporation, Fiber Optics Products Division, Lisle, Ill. The variable attenuator couples two transmission cables
1
,
2
, each of which is terminated in conventional fashion by: (1) stripping the respective cable jackets
3
,
4
, and strength members
5
,
6
to expose fiber buffers
7
,
8
; (2) further stripping portions of the fiber buffers
7
,
8
to expose the fibers and inserting the exposed fibers in alignment ferrules
9
,
10
; (3) polishing ends of alignment ferrules
9
,
10
so that the ends of the exposed fibers are flush with the facing surfaces of the alignment ferrules; (4) capturing the alignment ferrule holding members
11
,
12
and bias springs
13
,
14
within externally threaded front portions
15
,
16
of rear bodies
17
,
18
by securing the front portions to internally threaded rear portions
19
,
20
of front bodies
21
,
22
; (5) crimping exposed portions of strength members
7
,
8
between rear portions
23
,
24
of rear bodies
17
,
18
and crimp ferrules
25
,
26
; and securing boots
27
,
28
to the rear bodies
17
,
18
.
Front body
21
of the first transmission cable is arranged in conventional fashion to support a coupling nut
29
, but slide body
22
of the second transmission cable termination has been modified to be cylindrical in shape and to include external threading for cooperation with an internally threaded thumb wheel
30
. Thumb wheel
30
includes a collar
31
that is captured between a front housing
32
and a rear housing
33
which together form the attenuator housing upon threading of rear housing
33
onto an externally threaded extension
34
of front housing
32
, and as a result thumb wheel
30
can rotate relative to front housing
32
but cannot move axially. On the other hand, slide body
22
is free to slide axially within front housing
32
, causing alignment ferrule
10
to also move axially in response to rotation of thumb wheel
30
. Rotation of slide body
22
is prevented by a dowel
35
extending inwardly from front housing
32
and slidably fitted in a slot in slide body
22
, while an attenuation spring
36
captured between the front housing
32
and slide body
22
eliminates tolerances between the threading of the thumb wheel
30
and slide body
22
.
Alignment of ferrules
9
,
10
is provided by an alignment sleeve
37
fitted in a holder
38
, which in turn is captured between front body
32
and an adapter
39
upon threading of adapter
39
into the front housing. Extending from adapter
39
is a cylindrical coupling section
40
to which the coupling nut
29
is secured to thereby fix cable
1
relative to the housing. In order to vary the length of the air gap between the ends of ferrules
9
,
10
, it is simply necessary to turn thumb wheel
30
, causing corresponding axial movement of slide body
22
and ferrule
10
relative to the fixed position of ferrule
9
and front housing
32
.
A problem with attenuators of the type illustrated in
FIG. 1
is that the relationship between air gap length and attenuation is non-linear, making it difficult to predict the attenuation that will result for a given number of turns of the thumb wheel
30
. For example, adjustment of the attenuation from 10 dB to 15 dB might require 1.3 turns of the wheel, while adjustment of the attenuations from 20 dB to 25 dB might require 0.8 turns of the thumb wheel. Adjustment of the attenuation would be much easier to achieve if each turn of the thumb wheel resulted in the same variation in attenuation, i.e., if the relationship between turns of the wheel and attenuation were linear, but such a linear relationship is not possible in conventional screw arrangements of the type illustrated in
FIG. 1
due to the non-linear relationship between the air gap length and attenuation of the light signal across the air gap.
A second problem with attenuators of the type illustrated in
FIG. 1
is that the thumb wheel or adjustment screw can easily be turned beyond the minimum and maximum attenuation points, causing damage to the attenuator.
The present invention solves these problems by replacing just five parts of the conventional variable attenuator mechanism illustrated in FIG.
1
. The five parts are the slide body
22
, thumb wheel
30
, rear housing
33
, a wave washer (not shown) which fits between the slide body
22
and front housing
32
, and a Teflon™ washer (not shown) which fits between the thumb wheel
30
and rear housing
33
. These five parts are replaced by a modified slide body, a cam body, a worm and worm gear mechanism, and a modified rear housing which cooperate to cause the air gap length to be adjusted in such a way that each turn of an actuator will result in a proportional change in the attenuation, as will be described below, thereby simplifying the adjustment procedure without an undue increase in complexity of the attenuator. The cam surface is arranged such that the position of the slide body will reset when the actuator is turned beyond the maximum or minimum positions, making it impossible to damage the attenuator by over-turning of the actuator.
SUMMARY OF THE INVENTION
It is accordingly an objective of the invention to provide a variable attenuator for fiber optic transmission cables that achieves a linear response between turning of an actuator and attenuation of a light signal across a air gap.
It is a further objective of the invention to provide a variable attenuator for fiber optic transmission cables in which attenuation is achieved by mechanically varying a air gap between ends of respective transmission cables in response to turning of an actuator, the relationship between air gap length and attenuation being non-linear, and yet in which the relationship between turning of the actuator and the resulting attenuation is linear.
It is a still further objective of the invention to provide a variable attenuator for fiber optic transmission cables in which a linear relationship is achieved between a mechanical input and the resulting attenuation, and yet which is easily assembled and which uses standard fiber optic transmission cable terminations.
It is yet another objective of the invention to provide a variable optical attenuator with a non-destructive feature which allows the input actuator to be turned beyond the maximum range of the attenuator without damaging the attenuator.
These objectives are achieved, in accordance with the principles of the invention, by providing a linear attenuator of the type in whi
Brunsting Albert
Olson Thomas Clifford
Walter Eric
Amphenol Corporation
Blank Rome Comisky & McCauley LLP
Parvin Nasreen
Spyrou Cassandra
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