Fiber optics signal attenuator

Details Fiber optics signal attenuator Fiber optics signal attenuator

1998-03-18

2001-04-24

Pascal, Leslie (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S227000, C385S019000, C385S033000, C385S140000

Reexamination Certificate

active

06222656

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to fiber optics communications and, in particular, to an attenuator for optical signals in an optical fiber.
BACKGROUND
The relatively wide bandwidth of light that may be transmitted through a conventional optical fiber enables multiple light signals, each at a different wavelength, to be multiplexed and transmitted simultaneously over the same optical fiber. Such a technique is called wavelength division multiplexing (WDM). It is common for a single optical fiber to simultaneously transmit
16
or more multiplexed channels for any form of communication, including telephone communications and cable television.
In WDM, the signals (either electrical or optical) to be conveyed on each channel are converted into light signals within a narrow band of wavelengths (e.g., 2 nanometers) associated with a particular channel. A 16 channel WDM would use a total bandwidth of about 32 nanometers. A common center wavelength is on the order of 1500-1600 nanometers.
Converting an electrical or optical signal into a particular narrow band of wavelengths is well known. For example, an electrical signal may be applied to a particular type of laser diode which generates wavelengths within a particular bandwidth. Other techniques may include converting the electrical signal into a light signal and eliminating unwanted wavelengths. Some devices for extracting a specific narrow band of wavelengths from an optical signal include: 1) a tuned waveguide; 2) a diffraction grading; 3) a taper filter; and 4) other types of filters, such as a coated silica substrate where certain wavelengths are refracted and other wavelengths are reflected.
The process of causing the optical signals to be within a particular narrow bandwidth also typically causes the optical intensities to differ for each channel. As a result, after the optical signals for the channels have been limited to their respective optical bandwidths, such as shown in
FIG. 1
, each of these optical signals must be attenuated so that the light intensity transmitted is equal for each channel and is of a predetermined level. This is so that the transmission performance for each channel is predictable. Such attenuators for each of the three channels (
1
,
2
, and n) shown in
FIG. 1
include attenuators
12
,
13
, and
14
for attenuating the optical signals in optical fibers
16
,
17
, and
18
, respectively. Similar attenuators reside in a demultiplexer
19
.
FIG. 2
illustrates the intensity levels of the optical signals in each of the three channels, each optical signal being within a different narrow bandwidth of light. As seen, the intensity of the optical signal in channel
1
prior to attenuation is greater than that of the optical signals in channels
2
and
3
, and the optical signal in channel
3
is greater than the intensity of the optical signal in channel
2
. Attenuators
12
,
13
, and
14
serve to equalize the intensity levels of the three channels by selectively lowering the overall intensity of the higher intensity signals to equal that of the lowest intensity signal. One such attenuator will be discussed later with respect to
FIGS. 3
,
4
A, and
4
B.
The light outputs from the attenuators
12
-
14
are then applied to optical fibers
20
,
21
, and
22
and combined into a single optical fiber
24
so as to multiplex the n channels onto a single optical fiber. Hence, the device of
FIG. 1
acts as a multiplexer to simultaneously transmit multiple channels, each at a different light bandwidth, along the same optical fiber. Additional multiplexers may be employed to multiplex additional channels on other optical fibers. The optical fibers may then be bundled in a cable for transmitting many optical signals.
Ultimately, the signals on the optical fiber
24
are demultiplexed by a demultiplexer
19
to separate out the various wavelengths of light into separate channels using well known means. These separate channels are then attenuated to have equal, predetermined intensities and converted into electrical signals, if required, for various applications such as by using photodetectors. Such demultiplexers include detraction gratings and filters which may be tuned to transmit a narrow range of predetermined wavelengths.
The attenuation levels in the multiplexer and demultiplexer may be determined empirically.
One popular prior art technique for attenuating the intensity of a light output within a narrow band of wavelengths uses a neutral density filter for each of the wavelength bands of interest. Such a filter removes a selected amount of light depending on where the light impinges upon the filter.
FIG. 3
illustrates a neutral density filter
30
composed of a silica substrate
32
with a coating
34
composed of material for progressively absorbing the light output of a fiber optic cable
36
as filter
30
is moved in the direction of arrow
38
. The percentage of absorption of light output from cable
36
with respect to each area of filter
30
is identified in FIG.
3
. The light exiting filter
30
is received by a fiber optic cable
40
. It would be understood that additional optics, such as collimators, may be used at the ends of the fiber optic cables
36
and
40
to cause the light between the two cables to be collimated.
The filter
30
is adjusted in the direction of arrow
38
using a micrometer to select the desired amount of attenuation.
FIG. 4A
illustrates the ideal light energy versus time for a number of pulses of the attenuated light received by fiber optic cable
40
. In reality, however, this light signal contains ripples and other distortions, as shown in
FIG. 4B
, due to reflections at the interface of filter
30
causing constructive and destructive interference. Further, an inherent property of the silica
32
and the coating
34
is that there is always some attenuation even at the minimum attenuation level of filter
30
.
What is needed is a light attenuator for a WDM system which is inexpensive, reliable, and does not suffer from the performance drawbacks of the prior art attenuators.
SUMMARY
In one embodiment, an attenuator for use in a wavelength division multiplexer (WDM) uses an opaque (e.g., metal) wedge-shaped device, referred to as a knife-edge, having a substantially triangular face which controllably blocks the light output of an optical fiber whose light output is to be attenuated. By selectively moving the knife-edge of the triangular face in front of the optical fiber, the attenuator can block any amount of the light output. The position of the attenuator in one embodiment is adjusted by means of a fine screw (e.g., a micrometer) which acts as a potentiometer control.
The use of such an attenuator instead of a neutral density filter includes the advantages of: 1) no noise (ripple) due to reflections and interference; 2) no residual attenuation so that the attenuation can be zero; 3) a wide dynamic range (0%-100%); 4) high stability; and 5) compact size.
A preferred embodiment attenuator includes a wedge-shaped knife-edge attenuator where the substantially triangular face has a beveled light blocking portion so as not to be directly orthogonal to the light output. Any reflections of light from the beveled portion do not reflect back into the impinging light so as to avoid any interference between the impinging and reflected light.
To minimize reflections, the knife-edge attenuator is essentially a black color, such as anodized aluminum.


REFERENCES:
patent: 4591231 (1986-05-01), Kaiser et al.
patent: 4697869 (1987-10-01), So et al.
patent: 4989938 (1991-02-01), Tamulevich
patent: 5513286 (1996-04-01), Easley
patent: 5642456 (1997-06-01), Baker et al.
patent: 5790289 (1998-08-01), Taga et al.
patent: 5877879 (1999-03-01), Naito
patent: 5930441 (1999-07-01), Betts et al.

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