U.S. patent application number 09/783529 was filed with the patent office on 2002-08-15 for fast attenuator.
Invention is credited to Colbourne, Paul, Lonjoux, Phillipe, MacDonald, Robert I., Mala, Mohiuddin, Yang, Lianxiang.
Application Number | 20020109904 09/783529 |
Document ID | / |
Family ID | 25129565 |
Filed Date | 2002-08-15 |
United States Patent
Application |
20020109904 |
Kind Code |
A1 |
MacDonald, Robert I. ; et
al. |
August 15, 2002 |
Fast attenuator
Abstract
The invention provides a fast variable optical attenuator
including a reflective membrane that serves as a varifocal mirror.
Electrostatic deflection of the mirror defocuses the optical system
and attenuates an optical signal launched to the mirror from an
input port to an output port. Since the mirror is generally limited
to deflections less than 1 micron, fast attenuation within a small
(<5 dB) dynamic range is achieved. The dynamic range is improved
by combining the fast attenuator with a slower attenuator. In the
preferred embodiment, the slow and fast attenuator are not coupled
via optical waveguides, but rather are optically coupled via free
space or are integrated in the same device. Advantageously, this
compact arrangement reduces the excess loss associated with optical
fiber coupling and lowers packaging costs.
Inventors: |
MacDonald, Robert I.;
(Manotick, CA) ; Yang, Lianxiang; (Ottawa, CA)
; Lonjoux, Phillipe; (Kanata, CA) ; Colbourne,
Paul; (Nepean, CA) ; Mala, Mohiuddin; (Ottawa,
CA) |
Correspondence
Address: |
JDS UNIPHASE CORPORATION
INTELLECTUAL PROPERTY DEPARTMENT
3000 MERIVALE ROAD
OTTAWA
ON
K2G 6N7
CA
|
Family ID: |
25129565 |
Appl. No.: |
09/783529 |
Filed: |
February 15, 2001 |
Current U.S.
Class: |
359/291 ;
359/290; 359/292 |
Current CPC
Class: |
G02B 26/0825
20130101 |
Class at
Publication: |
359/291 ;
359/290; 359/292 |
International
Class: |
G02B 026/00 |
Claims
What is claimed is:
1. An optical attenuator comprising: an input port and an output
port; a deformable membrane having a reflective surface disposed to
reflect light launched from the input port substantially towards
the output port, the membrane supported by a frame having an
opening; and deforming means for controllably deflecting the
membrane into the opening such that the reflective surface forms a
concave mirror having a concavity that determines the optical
coupling efficiency between the input and output ports, the
membrane deformable between a first position in which light
launched from the input port is received at the output port with
substantially no attenuation, and a plurality of other positions in
which light launched from the input port is inefficiently coupled
into the output port.
2. An optical attenuator according to claim 1, wherein the
deforming means comprises a capacitor for electrostatically
deforming the membrane, a first electrode of the capacitor
corresponding to the reflective surface and a second electrode
corresponding to a substrate disposed below the membrane.
3. An optical attenuator according to claim 2, wherein the opening
has a diameter from about 100 microns to about 300 microns.
4. An optical attenuator according to claim 3, wherein a deflection
of the membrane is restricted to less than about 1 micron.
5. An optical attenuator according to claim 4, wherein the input
and output ports are optically coupled to input and output optical
fibres, respectively.
6. An optical attenuator according to claim 5, further comprising
at least one lens optically coupled to the input and output optical
fibres.
7. An optical attenuator according to claim 6, comprising a first
chip including the frame and a second chip including a slot for
support the at least one lens.
8. An optical attenuator according to claim 2, wherein the opening
has one of a substantially circular and square shape.
9. An optical attenuator according to claim 2, wherein in the first
position the reflective surface corresponds to a focussing mirror,
and wherein the input and output ports are disposed about first and
second foci of the focussing mirror, respectively.
10. An optical attenuator according to claim 2, wherein in the
first position the reflective surface corresponds to a
substantially flat mirror.
11. An optical attenuator according to claim 2, wherein the
membrane and deforming means are designed such that the optical
attenuator has a dynamic range less than about 10 dB.
12. An optical attenuator according to claim 11, further comprising
another attenuator designed to have a dynamic range greater than
about 10 dB.
13. An optical attenuator according to claim 12, wherein the other
attenuator comprises one of a neutral density filter, an
attenuating prism, a beam blocker, and a tilting mirror, and is
optically coupled to the reflective surface via free space
optics.
14. An optical attenuator according to claim 11, further comprising
a tiltable substrate for supporting the optical attenuator, the
tiltable substrate designed to further affect optical coupling
efficiency between the input and output ports such that variable
attenuation greater than about 10 dB is provided.
15. An optical attenuator comprising: a first port and a second
port; a deformable membrane having a reflective surface disposed to
reflect light launched from the first port substantially towards
the second port, the membrane supported by a frame having a
radially symmetric opening; and deforming means for controllably
deflecting the membrane into the opening such that the reflective
surface forms a concave mirror having a concavity that determines
the optical coupling efficiency between the first and second
ports.
16. An optical attenuator comprising: an input port and an output
port; a deformable membrane having a reflective surface disposed to
reflect light launched from the input port substantially towards
the output port, the membrane supported by a frame having an
opening; deforming means for controllably deflecting the membrane
into the opening such that the reflective surface forms a concave
mirror having a concavity that determines a first order optical
coupling efficiency between the first and second ports; and a
movable member for controllably moving the frame and membrane
between a plurality of positions that determine a second order
optical coupling efficiency between the first and second ports.
17. An optical attenuator according to claim 16, wherein the
deforming means comprises a capacitor for electrostatically
deforming the membrane, a first electrode of the capacitor
corresponding to the reflective surface and a second electrode
corresponding to a substrate disposed below the membrane.
18. An optical attenuator according to claim 17, wherein the
movable member comprises an electrostatically controlled tiltable
substrate for supporting the frame.
19. A variable optical attenuator comprising: a first attenuator
having a dynamic range less than about 10 dB and a response time of
less than about one microsecond; and a second attenuator having a
dynamic range greater than about 10 dB at a response time greater
than about one millisecond, wherein the first and second
attenuators are optically coupled in series.
20. A variable optical attenuator according to claim 19, wherein
the first attenuator is a MEMS varifocal mirror and the second
attenuator comprises one of a neutral density filter, an
attenuating prism, a beam blocker, and a tilting mirror.
Description
FIELD OF THE INVENTION
[0001] This invention relates generally to the field of optical
attenuators and more specifically to a fast optical attenuator.
BACKGROUND OF THE INVENTION
[0002] Optical attenuators are used to adjust the optical power
level in optical signals, for example to equalize levels among WDM
channels or to prevent saturation of receivers. Such applications
generally require the attenuator to be adjustable over more than 20
dB. Low cost examples of this type of optical attenuator include
neutral density filters, attenuating prisms, beam blockers, tilting
mirrors and/or systems to bend or off-set optical fibres.
Typically, the relatively large mass of the movable attenuating
optics in these devices limit the response time to
milliseconds.
[0003] Optical attenuators may also be used to impose information
upon the optical signal passing through them, for example telemetry
information. However, such applications typically require a
response time of microseconds with a dynamic range of about 5 dB.
Conventionally, optical attenuators that are capable of achieving
fast speed in the order of MHz are based on electro-optics,
acousto-optics, or magneto-optics. Traditionally, these fast
attenuators are wavelength sensitive, have a relatively high
insertion loss, and a relatively high cost.
[0004] Ideally, an optical attenuator should have a fast response
time and be able to adjust the optical power level over a wide
range of intensity levels. Unfortunately, the design of a low cost
attenuator that has simultaneously high speed and wide dynamic
range is very difficult.
[0005] It is an object of this invention to provide to provide a
fast optical attenuator that is relatively low cost.
[0006] It is another object of this invention to provide an optical
attenuator having a wide dynamic range and that can also provide
attenuation that varies at high speeds.
SUMMARY OF THE INVENTION
[0007] The instant invention provides a variable optical attenuator
that attenuates over a wide dynamic range and that operates at high
speeds. The optical attenuator includes a slow attenuator having a
large dynamic range and a fast attenuator having a smaller dynamic
range. In the preferred embodiment, the slow and fast attenuator
are not coupled via optical waveguides, but rather are optically
coupled via free space or are integrated in the same device.
Advantageously, this compact arrangement reduces the excess loss
associated with optical fibre coupling and lowers packaging costs.
Moreover, each of the slow and fast attenuators work together to
modulate an optical signal from the other such that the total
attenuation of the device is the product of the two separate
attenuations.
[0008] In a preferred embodiment, the fast attenuation is provided
by a varifocal mirror. Electrostatic deflection of the mirror
defocuses the optical system and attenuates an optical signal
launched to the mirror from an input port to an output port. Since
the mirror is generally limited to deflections less than 1 micron,
fast attenuation within a small (<5 dB) dynamic range is
achieved.
[0009] In accordance with the invention there is provided an
optical attenuator comprising an input port and an output port, a
deformable membrane having a reflective surface disposed to reflect
light launched from the input port substantially towards the output
port, the membrane supported by a frame having an opening, and
deforming means for controllably deflecting the membrane into the
opening such that the reflective surface forms a concave mirror
having a concavity that determines the optical coupling efficiency
between the input and output ports, the membrane deformable between
a first position in which light launched from the input port is
received at the output port with substantially no attenuation, and
a plurality of other positions in which light launched from the
input port is inefficiently coupled into the output port.
[0010] In accordance with the invention there is provided an
optical attenuator comprising a first port and a second port, a
deformable membrane having a reflective surface disposed to reflect
light launched from the first port substantially towards the second
port, the membrane supported by a frame having a radially symmetric
opening, and deforming means for controllably deflecting the
membrane into the opening such that the reflective surface forms a
concave mirror having a concavity that determines the optical
coupling efficiency between the first and second ports.
[0011] In accordance with the invention there is provided an
optical attenuator comprising: an input port and an output port; a
deformable membrane having a reflective surface disposed to reflect
light launched from the input port substantially towards the output
port, the membrane supported by a frame having an opening;
deforming means for controllably deflecting the membrane into the
opening such that the reflective surface forms a concave mirror
having a concavity that determines a first order optical coupling
efficiency between the first and second ports; and a movable member
for controllably moving the frame and membrane between a plurality
of positions that determine a second order optical coupling
efficiency between the first and second ports.
[0012] In accordance with the invention there is provided a
variable optical attenuator comprising: a first attenuator having a
dynamic range less than about 10 dB and a response time of less
than about one microsecond; and a second attenuator having a
dynamic range greater than about 10 dB at a response time greater
than about one millisecond, wherein the first and second
attenuators are optically coupled in series.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] Exemplary embodiments of the invention will now be described
in conjunction with the drawings in which:
[0014] FIG. 1 is a schematic diagram of a prior art optical
attenuator based on the electro-optic effect;
[0015] FIG. 2 is a schematic diagram of a prior art optical
attenuator based on the acoustic-optic effect;
[0016] FIG. 3 is a schematic diagram of a prior art optical
attenuator based on the magneto-optic effect;
[0017] FIG. 4 is a schematic diagram of a prior art acoustic
transducer;
[0018] FIG. 5 is a schematic diagram of an embodiment of a fast
attenuator in accordance with the instant invention including a
MEMS membrane having a reflective surface;
[0019] FIG. 6a is a schematic diagram of another embodiment of a
fast attenuator in accordance with the instant invention;
[0020] FIG. 6b is a schematic diagram the embodiment shown in FIG.
6a where the membrane is deformed into a concave reflector;
[0021] FIG. 7 is a schematic diagram of an embodiment of a fast
attenuator having a wide dynamic range;
[0022] FIG. 8 is a schematic diagram of another embodiment of a
fast attenuator having a wide dynamic range;
[0023] FIG. 9 is a schematic diagram showing the shape of a beam
spot at the position of the receiving end after defocusing for both
circular and square membranes; and
[0024] FIG. 10 is a schematic diagram showing another embodiment of
a fast attenuator having an axial design.
DETAILED DESCRIPTION OF THE INVENTION
[0025] As described above, optical attenuators that have a
relatively fast (e.g., less than a microsecond) response time are
frequently based on electro-optics, acousto-optics, or
magneto-optics.
[0026] Referring to FIG. 1, there is shown an example of a prior
art fast attenuator based on the electro-optic effect, e.g.,
Pockels or Kerr effects. A non-polarized beam of light is launched
through a first polarizer 10, the electro-optic material 12 (e.g.,
lithium niobate), and a second polarizer (analyzer) 14. When no
voltage is applied to the electro-optic material 12, there is
substantially no loss in intensity observed. When a voltage is
applied to the material 12 via ring electrodes 16a and 16b, the
direction of the polarized light leaving the first polarizer 10 is
rotated by an angle related to the applied voltage, thus affecting
the amount of light transmitted through the analyzer 14. The
disadvantages of this type of optical attenuator include relatively
high voltage requirements, limited beam diameter capabilities that
lead to high insertion loss, material unreliability, and
sensitivity to wavelength.
[0027] Referring to FIG. 2, there is shown an example of a prior
art fast attenuator based on acousto-optics. The attenuator
includes a piezoelectric transducer 20, an acousto-optic material
22, and an acoustic absorber 24. The transducer 20 provides a sound
wave that travels through the acousto-optic material 22, e.g. as a
series of compressions and rarefractions. In regions where the
sound pressure is high, the acousto-optic material 22 is compressed
slightly, thus locally increasing the index of refraction. This
series of local increases in index of refraction forms a grating
that defracts an incident beam of light as it travels through the
acousto-optic material 22. The angle of deflection is related to
the radio-frequency power applied to the transducer. When no
acoustic wave is provided, no light is deflected. The disadvantages
of this optical attenuator include wavelength sensitivity and an
excessively high cost.
[0028] Referring to FIG. 3, there is shown an example of a prior
art fast attenuator based on magneto-optics. A non-polarized beam
of light is launched through a first polarizer 30, a magneto-optic
material 32, and a second polarizer (analyzer) 34. When no magnetic
field is applied, there is substantially no loss observed. When a
magnetic field is applied to the material 12 via magnets 36a and
36b, the direction of the polarized light leaving the first
polarizer 10 is rotated by an angle related to the applied voltage,
thus affecting the amount of light transmitted through the analyzer
14. The disadvantages of this type of optical attenuator include
high insertion loss and wavelength sensitivity.
[0029] The prior art fast attenuators described heretofore are
based on systems with non-moving parts. Presumably, this is due to
the fact that the speed of moving an attenuating optic is limited
by the inertia of the object moved. Accordingly, fast attenuators
have traditionally been limited to relatively costly systems with
non-moving parts.
[0030] One solution to this is to provide an attenuating object
with a relatively small inertia. For example, it is well known that
ultrasonic transducers fabricated by MEMS technologies can be
driven electrostatically at speeds in the 10 MHz region. Moreover,
these transducers are simple and thus show remarkable potential for
reliablity.
[0031] Referring to FIG. 4, there is shown a MEMS ultrasonic
transducer including a thin membrane 100, typically made of silicon
nitride (SiN), stretched over the periphery of a frame 102, which
is also typically made of SiN. The structure is formed by MEMS
technology on a silicon substrate 104, thus forming a cavity 106 in
the order of 5-50 microns deep between the substrate 104 and the
membrane 100. An electrically conductive material 108, such as
aluminum, is coated on the membrane 100. The silicon substrate 104
and conductive material 108, each serve as an electrode of a
capacitor for providing a measure of membrane deflection resulting
from pressure changes near the membrane 100. For maximum efficiency
the ultrasonic transducers are usually operated at a mechanical
resonance. Typically the structures are tensioned by applying a DC
voltage of 10-100 volts, such that the mechanical resonance
frequencies are very high, e.g., in the 10 MHz range. Preferably,
the SiN layer 100 is uniform and the tensioned membrane has a
radially symmetric concave shape. Advantageously, MEMS ultrasonic
transducer technology produces the cavity 106 without perforating
the membrane. The uniform membranes are deflectable into an
approximately parabolic shape by an applied DC voltage.
[0032] When the uniform MEMS membrane disclosed above includes a
reflective surface, the deformable membrane becomes a deformable
mirror having a shape dependent on the degree of deflection, the
number of electrodes, and the placement of the electrodes. More
specifically, the deformable mirror can be constructed to form a
concave mirror having a variable focal point. This type, or
similar, deformable mirrors have been proposed for various spatial
light modulators (e.g., U.S. Pat. Nos. 4,441,791 and 6,147,789,
herein incorporated by reference). However, in general, the design
of these prior art deformable mirrors is not compatible with the
wavelength insensitive, polarization sensitive requirements for
optical attenuators used in telecommunication systems. Moreover,
these prior art devices are generally limited to on/off states and
do not recognize the advantageous of providing variable
attenuation.
[0033] Referring to FIG. 5, there is shown an embodiment of a
variable optical attenuator in accordance with the instant
invention having a structure similar to the acoustic transducer
shown in FIG. 4. The attenuator includes a thin membrane 200,
preferably made of silicon nitride (SiN), stretched over the
periphery of a frame 202, which is also preferably made of SiN. For
example, the structure can be formed by MEMS technology on a
silicon substrate 204 to form a cavity 206 about 1-10 microns deep
between the substrate 204 and the membrane 200. An electrically
conductive and reflective material 208, such as gold, is coated on
the membrane 200. Preferably, the surface has a reflectivity
greater than about 99%. The silicon substrate 204 and conductive
material 208, each serve as an electrode of a capacitor that
provides deforming means for deforming the membrane 200. When a
potential is applied between the two electrodes, electrostatic
forces cause the membrane to be attracted to the substrate such
that the reflective surface 208 becomes a focussing mirror.
Referring to FIG. 5, the deformable membrane becomes an
approximately elliptical mirror designed to reflect a beam of light
launched from one of its two foci, e.g., 212, onto the other focus,
e.g., 214. Accordingly, an input optical fibre 210 is positioned at
a first focus 212 of the elliptical mirror, whereas an output
optical fibre 216 is positioned at the other focus 214. The
arrangement shown in FIG. 5, corresponds to a maximum optical
coupling arrangement or the low loss state. When the potential
applied to the two electrodes is varied, the membrane is deflected
to various positions, thus changing the position of the foci and
providing variable attenuation for a beam of light launched from
the input port 210 and collected at the output port 216. In other
words, when the shape of the mirror 208 changes, the efficiency of
the coupling changes, thus providing a controllable variable
attenuation. Optionally, the shape of the mirror is further
controlled by using multiple electrodes.
[0034] Preferably, the thickness of the membrane including the
reflective layer, the diameter of the deformable portion, and the
applied voltage and are selected such that the maximum deflection
is less than about 1 .mu.m to achieve a maximum attenuation of
about 3-10 dB, or more preferably of about 3-5 dB. Although this
relatively small deflection generally limits the dynamic range of
the fast attenuator, many applications of fast attenuators only
require small dynamic range. For example, when the fast attenuator
is used to provide a small amplitude modulation on an optical pulse
stream, only small amplitude modulation is desired.
[0035] The diameter of the deformable membrane is also related to
the response time. Preferably, the diameter of the membrane is
designed to be about 100 to 300 .mu.m. Membrane diameters varying
from about 150 .mu.m to 300 .mu.m have been calculated to be
particularly useful. Although it is preferred that the mirror be
substantially spherical when deformed, the membrane can also take
on other shapes, such as a square.
[0036] Advantageously, the circular membrane is radially symmetric
to provide the wavelength insensitive and polarization insensitive
requirements for optical attenuators used in telecommunication
systems.
[0037] Problems arising from the diameter of the beam being larger
than the diameter of the membrane, may be circumvented by using a
lens designed to reduce the beam diameter (e.g., a SELFLOC having
less than one-quarter pitch). Alternatively the membrane diameter
is designed to be larger than 300 .mu.m, e.g., about 500 .mu.m, but
wherein only an area having a diameter of about 300 .mu.m is
deformable. Alternatively, a plurality of deformable membranes is
disposed in close proximity to one another to receive a single beam
of light.
[0038] Referring to FIG. 6, there is shown another embodiment of a
fast attenuator in accordance with the instant invention including
lenses 330 and 336 optically coupled to the input 310 and output
316 fibres. FIG. 6a illustrates the low loss state, wherein the
reflective surface 308 is not deformed, i.e., is substantially
flat, and wherein light launched from the input fibre 310 is
substantially coupled into the output fibre 316. In FIG. 6b, the
reflective surface is electrostatically deformed to form a concave
reflective surface. As a result, the beam of light launched from
the input fibre 310 is not effectively coupled into the output
fibre 316, thus providing attenuation of the optical signal.
Variable attenuation is provided by varying the applied potential
(e.g., typically by applying a voltage from about 10-100 V). In one
embodiment, the focussing mirror is designed to provide the extra
optical power required for efficient coupling, and the lenses are
somewhat shorter or longer than those required for efficient
coupling with reflection off a flat mirror. Alternatively, the
optical fibres are thermally expanded core fibres, and/or include
other means for increasing the optical coupling efficiency.
[0039] The fast optical attenuators depicted in FIGS. 5 and 6 have
been calculated to provide insertion losses varying from about 0.02
dB to 0.6 dB, maximum attenuation less than about 10 dB, and
microsecond response times. More specifically, it has been
calculated that a deflection of 0.5 .mu.m can correspond to an
attenuation to about 2 dB.
[0040] Advantageously, the varifocal optical attenuators described
above provide modulation with a rate in the 1 MHz range, and small
dynamic range (up to 30%), for imposing dither, telemetry and other
in-band modulation on optical data.
[0041] In accordance with another aspect of the instant invention,
there is provided an optical attenuator including the MEMS
attenuator described above and a slow, high dynamic range (>20
dB) attenuator for providng both fast modulation with the slow,
precise and high range attenuation properties characteristic of the
conventional slow attenuators.
[0042] Referring to FIG. 7, there is shown an embodiment of an
optical attenuator including a MEMS varifocal mirror 440 for
providing fast attenuation with a dynamic range less than about 5
dB within about one microsecond, and a graded neutral density
filter 442 for providing slow attenuation with a dynamic range
greater than about 20 dB within about a millisecond. A mirror 446
directs a beam of light launched along an optical path 448 to the
neutral density filter 442, which is controllably moved in and out
of the optical path 448 with a stepping motor 444 to provide the
slow attenuation. The varifocal mirror 440 is disposed in the
optical path in series with the neutral density filter 442 to
provide the fast modulation. The two attenuators are controlled
independently.
[0043] Referring to FIG. 8, there is shown another embodiment of an
optical attenuator in accordance with the instant invention,
wherein a varifocal mirror 540 for providing fast modulation is
supported on a tilting substrate 542 for providing the slow
attenuation. Separate input control is provided for each attenuator
(i.e., the fast 540 and the slow 542). Preferably, the substrate
542 is tilted using a piezoelectric crystal, thermal deformation,
or electronic charge as shown in FIG. 8. The optical attenuator
further includes a single lens 560 disposed between the combination
attenuator 540,542 and the input 510 and output 516 optical fibres,
which are supported by a double bore tube 550.
[0044] In operation, a beam of light launched from input optical
fibre 510 is transmitted through the lens 560 and is incident on
the reflective surface of the varifocal mirror 540. The reflected
light is retransmitted through the lens 560 and is directed to the
output optical fibre 516 with a coupling efficiency related to the
degree of deflection of the varifocal mirror 540 and an angle of
the tilting substrate 542. Low speed attenuation input is provided
via electrodes 580a and 580b, which electrostatically attract one
side of the substrate 542 closer to the electrodes. High speed
attenuation input is provided via the electrodes within the
varifocal mirror as discussed above. Advantageously, this
embodiment allows the fast and slow attenuation to be provided
simultaneously and controlled independently.
[0045] Notably, a square membrane is particularly attractive for
the embodiment illustrated in FIG. 8. Referring to FIG. 9, the beam
spot produced at the output port by the square membrane is
elliptical in shape. Accordingly, only small deflections of the
substrate 542 in a direction that moves the beam spot in a
direction parallel to the minor axis of the ellipse, are necessary
to provide very sensitive slow attenuation. Notably, when the
membrane is square only two opposing sides of the membrane need to
be supported.
[0046] Referring to FIG. 10, there is shown another embodiment of a
fast attenuator having axial design. More specifically, the axial
design includes a wafer bonded membrane chip 601 and top chip 602.
The membrane chip 601 supports membrane 603. The top chip 602
includes a slot for supporting a lens 604, which is secured within
the slot with an adhesive. A double bore tube 605 supporting input
606 and output 607 optical fibres is secured to the top chip 602
with an adhesive. A cavity 608, or air gap, separates the lens 604
and the double bore tube 605. This embodiment has the advantage of
being relatively compact (e.g., less than 7 mm in length) and
relatively stable.
[0047] Of course, numerous other embodiments can be envisaged
without departing from the spirit and scope of the invention. For
example, the fast and slow attenuators are not limited to those
disclosed herein, other attenuators are also possible.
* * * * *