Capella 2028
`JDS Uniphase v. Capella
`IPR2015-00731
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`U.S. Patent
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`Jan. 23, 2001
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`Sheet 1 of 9
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`US 6,178,284 B1
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`FIG.
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`3
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`U.S. Patent
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`Jan. 23, 2001
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`Sheet 2 of 9
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`US 6,178,284 B1
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`FIG. 2
`iPRIOH ART}
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`VOLTAGE
`SUPPLY
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`FIG. 3
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`4
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`U.S. Patent
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`Jan. 23, 2001
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`Sheet 3 of 9
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`US 6,178,284 B1
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`4
`FIG.
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`U.S. Patent
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`Jan. 23, 2001
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`Sheet 4 of 9
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`US 6,178,284 B1
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`FIG. 6
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`U.S. Patent
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`Jan. 23, 2001
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`Sheets of 9
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`US 6,178,284 B1
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`FIG. 8A
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`FIG. 33
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`FIG. 8D
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`U.S. Patent
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`Jan. 23, 2001
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`Sheet 6 of 9
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`US 6,178,284 B1
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`8
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`U.S. Patent
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`Jan. 23, 2001
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`Sheet 7 of 9
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`US 6,178,284 B1
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`FIG. 11
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`U.S. Patent
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`Jan. 23, 2001
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`Sheet 8 of 9
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`US 6,178,284 B1
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`FIG. 13
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`U.S. Patent
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`Jan. 23, 2001
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`Sheet 9 of 9
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`US 6,178,284 B1
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`FIG. 15
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`SILICON CHIP
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`US 6,178,284 B1
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`1
`VARIABLE SINGLE-MODE ATTENUAFORS
`BY SPATIAL INTERFERENCE
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`The present invention relates to a variable attenuator for
`attenuating an optical signal transmitted between an optical
`signal source and an optical signal
`receiver. More
`specifically, the present invention relates to the reflection of
`a transmitted optical signal off of divided surfaces for
`variably attenuating the optical signal.
`2. Description of the Related Art
`In optical data communications, signals are typically
`transmitted from a signal source to a signal receiver over an
`optical fiber network. FIG. 1 illustrates the general concept
`of optical signal transmission between an optical signal
`source 5 and an optical signal receiver 10, using a high
`reflectivity (HR) coated surface 15. For the sake of simplic-
`ity the various light beams illustrated in the figures are all
`shown as arcs to help in distinguishing their direction of
`travel; this illustration should not be considered as indicating
`any particular characteristic of the light beams themselves.
`Suppose light is introduced into the system through the
`optical signal source 5 (e.g., a single mode optical fiber). As
`the light exits the end of the optical signal source 5 it starts
`to spread out to form the “sending beam” 7. Sending beam
`7 is illustrated as a series of solid arcs moving from the top
`of FIG. 1 to the bottom of FIG. 1. Sending beam 7 is
`collimated by a lens 20 (or other focusing means) and then
`it falls upon the HR coated surface 15.
`The reflection of the sending beam 7 by the HR coated
`surface 15 is a “returning beam” 12 that travels to optical
`signal receiver 10 (e.g., a single mode fiber). The returning
`beam 12 is illustrated as a series of dotted-line arcs moving
`from the bottom of FIG. 1 to the top of FIG. 1. Returning
`beam 12 is refocused (by the same lens 20 as used for
`sending beam 7 or by a different focusing means, such as a
`separate lens) to be collected by optical signal receiver 10.
`It is well known that if the HR coated surface 15 is a
`nearly flat, highly reflecting surface, the optical coupling
`from the optical signal source 5 to the optical signal receiver
`10 will be very good, less than 0.5 dB loss in typical
`implementations using active alignment
`in manufacture.
`Further, it is well understood that if the reflecting surface of
`HR coated surface 15 is translated left or right by a few
`microns, the optical coupling will be changed negligibly.
`Optical signal systems have a signal intensity range in
`which they function best. If a signal falls below the opera-
`tional range, the system will either incorrectly detect the
`signal or will not detect the signal at all. If the signal is above
`the operational range, the system will saturate and may
`result in a false reading of the data in the optical signal.
`Thus, optical signal levels which are too high or too low
`result in unreliable transmission of data or can interfere with
`other data-carrying signals.
`The path attenuation of a fiber is a function of fiber length
`and the fiber attenuation coefficient. Further, the sensitivity
`of the receiver and the emitter output may exhibit changes
`due to aging. Thus, many optical transmission lines are
`designed with built-in attenuators which attenuate the opti-
`cal signals within the waveguide to be within the optimal
`functional range of the optical system.
`There are several known ways of providing attenuation of
`an optical signal. One method involves the use Faraday
`rotation in suitable doped Garnet films. By varying the
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`applied magnetic field from an electromagnet, the polariza-
`tion of transmitted light is changed and by using polarization
`selective optical elements, the attenuation can be varied. A
`problem with this attenuation method is that the electromag-
`net dissipates large amounts of electrical power and is quite
`large.
`Another known method of attenuation involves the use of
`motorized variable attenuators where,
`for example, an
`opaque attenuating wedge is driven into the beam path to
`block a portion of the optical signal beam. In addition to
`being bulky, however, this method also is costly and slow-
`acting.
`An additional attenuation method involves the use of
`liquid crystal designs which can work at very low electrical
`power levels and which function in a manner similar to
`Faraday rotation, but with liquid crystal rotation of polar-
`ization. Such systems are temperature and polarization sen-
`sitive and organic material in the beam path can be chemi-
`cally unstable, causing shortened device life.
`Attenuation using Micro Electro Mechanical Systems
`(MEMS) technology has been accomplished using a
`Mechanical AntiReflection Switch (MARS) modulator, an
`example of which is illustrated in FIGS. 2 and 3. These
`devices operate on the principle that varying the phase
`between two portions of a light beam allows the attenuation
`of the optical signal to be controlled, as described in more
`detail below. FIG. 2 shows a cross-wction of a typical
`MARS modulator, and FIG. 3 is a top view of the MARS
`modulator depicted in FIG. 2. Atypical MARS modulator 50
`has a conductive or semi-conductive based substrate 52 that
`
`is transparent to the operating optical band width of the
`modulator.
`
`A membrane 54 is suspended above the substrate 52,
`thereby defining an air gap 56 in between the substrate 52
`and the membrane 54. A membrane 54 is typically fabricated
`from a silicon nitride film which is a dielectric. A metal film
`58 is deposited around the top periphery of the membrane
`54. Since the metal film 58 is optically opaque, only the
`center 60 of the membrane 54 remains optically active.
`When an electrostatic potential is applied in between the
`metal film 58 and the below lying substrate 52, the metal
`film 58 becomes charged and is deflected by electrostatic
`forces toward the substrate 52. The result is that the mem-
`
`brane 58 deflects dowardwardly in the direction of arrows 59
`and the size of the air gap 56 is reduced. By applying a
`potential diflerence of about 40 volts to electrical connec-
`tions coupled to the membrane 54 and the substrate 52, large
`electric fields are developed between the substrate 52 and
`metal film 58 causing an electrostatic force between the
`membrane 54 and the underlying silicon large enough to
`how the membrane 54 closer to the underlying silicon. By
`increasing the applied voltage, the cavity width is decreased.
`By varying the cavity width, the relative phase between light
`reflected by the membrane 54 and light reflected by the
`underlying substrate 52 is also varied,
`thereby allowing
`control of the attenuation.
`
`In order to assemble the device and in order to equalize
`the gas pressure on each side of the membrane 54, and allow
`quick response time, it is necessary to perforate the mem-
`brane 54 with very small holes. In FIG. 3 the perforation of
`the membrane 54 with very small holes 62 is depicted. The
`membrane 54 has a natural mechanical
`resonance;
`the
`resonance is clamped by the gas viscosity passing through
`the holes 62. The inclusion of the holes 62 in the membrane
`54 results in an optical loss, but the size and number of the
`holes 62 is selected to minimize this optical
`loss to a
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`US 6,178,284 B1
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`negligible level. Typically such holes 62 are approximately
`3-5 ;tM in diameter and are provided merely to minimize
`vibration, i.e., they do not provide any optical functions.
`FIG. 4 is a partial cross-sectional view of the prior art
`MARS modulator of FIG. 2. Light traveling from top to
`bottom, identified as 64 in FIG. 4, will be partially reflected
`by the membrane 54 and partially transmitted beyond the
`membrane 54. The partially reflected light is identified as 66
`in FIG. 4. The light transmitted beyond the membrane 54 is
`reflected by the floor of the cavity; this reflected light is
`identified as 68 in FIG. 4. Depending upon the cavity width
`and the wavelength of light used, the reflections will inter-
`fere constructively or destructively when they are received
`by an optical receiver (not shown). Constntctive interference
`occurs when the wavelengths of the two reflected signals are
`in sync with each other, thereby enhancing the strength or
`power of the returned signal, i.e., the signal is not attenuated.
`Destructive interference refers to the effect caused by the
`receipt at the light collector of the two reflected signals in an
`“out of sync” state, which results in a signal of lesser
`strength or power, i.e., an attenuated signal. Thus, by vary-
`ing the cavity width, the attenuation of the optical signal can
`be increased or decreased selectively.
`The cavity widths for maximum total reflectivity and for
`minimum total reflectivity diifer by Era wavelength. Thus, by
`applying a suitable change of voltage between the metal film
`58 and the substrate 52, the membrane 58 can be moved
`from one extreme in reflectivity to the other, thus passing
`through the total range of possible attenuations by moving
`only about 0.4 microns (for radiation at 1545 nm). Further
`degradation in performance (e.g., high attenuation occurring
`at the minimum attenuation point) is likely to occur where
`there are membrane holes in the optical path, although the
`degradation is negligible.
`SUMMARY OF THE INVENTION
`
`The present invention utilizes the concept of destructive
`and constructive interference to enhance the ability to
`attenuate the light signal sent between an optical transmitter
`and an optical receiver. Rather than completely blocking out
`a portion of the light signal or utilizing a membrane coated
`with a partially reflecting material as is known in the prior
`art, the present invention utilizes a very high reflectivity
`coating on divided surfaces, and controls the relative dis-
`tances between each of the divided surfaces and the optical
`transmitterfreceiver.
`In a first embodiment, a moveable
`membrane is coated with a highly reflective coating and an
`opening or openings are provided in the membrane of a size
`large enough to allow a portion of an optically transmitted
`signal to pass beyond the membrane and be reflected oil‘ of
`a second highly reflective surface underneath the membrane.
`In a second embodiment, a fixed surface is coated with
`highly reflective material and a second surface that
`is
`moveable with respect to the first surface, and which is also
`coated with highly reflective material, is situated such that
`each of the first and second surfaces receive a portion of a
`light beam and reflects the same back to the optical receiver.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 illustrates the general concept of optical signal
`transmission between an optical signal source and an optical
`signal receiver;
`FIG. 2 is a crossrsectional view of a prior art MARS
`modulator;
`FIG. 3 is a top view of the prior art MARS modulator
`shown in FIG. 2;
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`FIG. 4 is a partial cross-section view of the prior art
`MARS modulator of FIG. 2;
`FIG. 5 is a top view of a variable attentuator in accordance
`with a preferred embodiment of the present invention;
`FIG. 6 is a cross-sectional view of a first embodiment of
`
`an attentuator in accordance with the present invention;
`FIG. 7 is a partial cross-sectional view of the attentuator
`of FIG. 6;
`FIGS. 8(a) through 8(a') illustrate alternative configura-
`tions for the large hole of the device depicted in FIG. 7;
`FIG. 9 is a top view of a second embodiment of an
`attenuator in accordance with the present invention;
`FIG. 10 is a cross-sectional side view of the embodiment
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`depicted in FIG. 9, taken along line 10-10;
`FIG. 11 is a top view of a third embodiment of an
`attenuator in accordance with the present invention;
`FIG. 12 is a cross—sectional side view of the attentuator
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`depicted in FIG. 11 taken along line 12—12;
`FIG. 13 is a top view of a fourth embodiment of an
`attenuator in accordance with the present invention;
`FIG. 14 is a cross-sectional view of the embodiment
`depicted in FIG. 13, taken along line 14-14;
`FIG. 15 is a top view of a fifth embodiment of an
`attenuator in accordance with the present invention; and
`FIG. 16 is a cross-sectional view of a sixth embodiment
`
`of an attenuator in accordance with the present invention.
`DETAILED DESCRIPTION OF THE
`PRESENTLY PREFERRED EMBODIMENTS
`
`FIGS. 5-? illustrate different views of an attenuator
`according to a first embodiment of the present invention.
`Referring to FIGS. 5-6, the reflective portions are divided
`by including a “large hole” 170 in the membrane 154 after
`coating the membrane with a high reflectivity (HR) coating.
`The floor 152 of the cavity is also coated with an HR
`coating. Note that
`it is not necessary to coat the entire
`regions indicated but only the regions that are illuminated by
`the incident beam, namely the middle region of the mem-
`brane 154- and the cavity floor 152 that is visible through the
`large hole 170. The illuminated area 172 (shown as a dotted
`circle) of the attenuator includes two portions or regions: (1)
`the crescent shaped area 174 on the membrane 154, and (2)
`the portion of the cavity floor 152 illuminated by the beam
`through the large hole 170. In a preferred embodiment,
`emphasizing high altenuations, the reflections from these
`two portions are approximately equal.
`FIG. 7 illustrates the light reflection paths generated by
`the attenuator shown in FIGS. 5 and 6. A sending beam 164
`from an optical signal source (not shown)
`is directed
`towards the membrane 154 such that a portion of the sending
`beam 164 falls upon the highly reflective coating on the
`membrane 154 and a second portion of the sending beam
`164 continues through the hole 170 and falls upon the
`reflecting surface of substrate 15.2. This forms a divided
`reflected signal comprising a first returning beam 166 and a
`second returning beam 168.
`Depending upon the cavity width (and slightly upon the
`angle of incidence), the phase difference between the two
`reflections will constructively or destructively interfere. By
`varying the electrostatic forces on the membrane 154, one
`can cause the cavity width to be varied. The amount of
`interference between the two reflected portions can be
`adjusted to achieve the degree of attenuation from input
`single-mode fiber to output single-mode fiber by voltage
`control.
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`It is not necessary to use a single round hole as in the hole
`170 in FIG. 5; the shape, size and number of holes in the
`membranes can be varied and still achieve a high level of
`attenuation. For example, in FIG. 8(a), a non-round hole is
`shown. This shape may be used to split the illuminated
`portions (shown as dotted lines) of each reflecting surface
`more evenly. In FIG. 8(b) two holes are used so that slight
`errors in locating the illuminated area or in sizing the
`illuminated area will have less efiect upon the fraction of the
`illumination returned from the cavity floor 152. FIG. 8(c)
`shows the use of numerous, smaller holes, and FIG. 8(d)
`shows the use of a moderate sized hole that is overfilled by
`the illumination beam.
`In a second embodiment, of the present invention reflec-
`tive surfaces that are side-by-side are used in the attenuator
`as shown in, e.g., FIGS. 9 and 10, with one of the surfaces
`being moveable with respect to the other. Referring to FIGS.
`9 and 10, the silicon chip 200 is seen from above and in
`cross-section, respectively. A moving surface 210, called a
`“see-saw” herein, pivots on torsion hinge mountings 212. As
`can be seen in FIG. 10, the right side of moving surface 210
`is further away from the floor of the silicon chip 200 than the
`left side thereof. Maximum reflective coupling occurs for a
`light beam directed at the silicon chip 200 when the see-
`saw’s top right edge is even with the top of the neighboring
`surface 214 of the chip 200; then the reflecting surface will
`appear to be an ordinary HR plane with a slight gap 218
`(vertical slit) in the middle of the illuminated area 216. To
`maximize this coupling, the width of gap 218 relative to the
`light beam’s diameter should be minimized.
`The voltage sensitivity of motion of the see-saw depends
`upon the electrostatic forces that can be produced for a given
`voltage (as with the membrane designs) and, more
`specifically, upon the torque (unbalanced force) that can be
`supplied. The logical way to supply torque is to provide an
`electrical field only on the right or only on the left side of the
`see-saw 210. For a given torque, the deflection will be
`proportional to the radius (distance from the point of rotation
`of the see-saw) and the weakness of the torsional restoring
`force of the torsional hinges. Narrowing the hinges and
`making them longer are two ways to reduce the restoring
`forces of the see-saw. Thus one can expect that very high
`voltage sensitivities can be achieved. If more precision is
`desired, a damping mechanism can be included in the
`attenuator so that any mechanical oscillation that would be
`set up each time the voltage was changed would dissipate
`quickly. The damping mechanism could be the viscous drag
`of the surrounding gas, and it will be necessary to provide
`the right mix and size of holes in the see-saw 210 to provide
`suficient damping. The damping will also be influenced by
`the amount of clearance between the see-saw 210 and the
`
`surrounding walls.
`The sensitivity and symmetry of the attenuator can be
`improved by using two neighboring see-saws 2240 and 222 as
`shown in, e.g., FIGS. 11 and 12 illustrates an attenuator
`according to a third embodiment of the present invention. By
`directing the light beam 216 to straddle both see-saws 220
`and 222 as shown, and applying the same forces to each one,
`twice the phase shift can be achieved.
`Another group of variations comes from exploiting the
`transparency of silicon in the region of the infrared used for
`the variable attenuator. If the silicon is transparent and one
`(optionally) applies an AR (Anti-Reflecting) coating to the
`underside of the silicon chip and elsewhere as needed, one
`can make a variable reflective coupling through the under-
`side.
`
`It is not necessary for any of the designs to have the edges
`of the HR coatings be coincident with the edges of the holes
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`or of the see-saws; the edges can be lithographically defined
`instead so that there is less danger of edge curling. The edges
`formed in this manner are less likely to produce as much
`scattering The HR coating can be on the underside of the
`membranelsee-saw, or it can be on the top side or on both
`sides.
`
`Further variations are to use coatings which are a little less
`highly refiective, i.e., coatings which are designed to trans-
`mit a fixed fraction, (e.g., 1%) of the incident light. In this
`manner the amount of power in the incident beam can be
`measured, in exchange for a slightly higher coupling loss. If
`the HR coatings have been replaced by coatings having a
`light transmitting property, a “tapped” portion of the beam
`emerges that can be measured with a photodetector. Regard-
`less of the cavity width variation (in the order of half a
`wavelength), the photodetector will report the same power
`level; in efiect, an “input” monitor or fiber lap is created. The
`power that will be coupled to the output single-mode fiber
`will see the interference efiects of the reflected beams, but
`the photodetector observing the transmitted beams will not.
`The total reflected light is not affected by the interference
`condition; the coupling of the reflected power to the par-
`ticular collection fiber is what is varying with interference.
`The technique of providing an input monitor can be used for
`variable attenuators that are illuminated from the top as well,
`provided that the underlying material is transparent. One
`application of this fiber tap would be closed-loop control.
`The above method of attenuation, using two divided
`surfaces, results in a substantial
`increase in attenuation
`control over the prior art.
`An attenuator based upon two surfaces of equal illumi-
`nation and with separation 8 has a linear coupling C of
`approximately:
`C-COS7(2:s6fn) [on reflection from the divided surface]
`
`where the wavelength in the surrounding medium is n. The
`attenuation using this method is wavelength independent at
`low attenuation values (C almost 1.0). However, for higher
`attenuation values a “ ' t" occurs, i.e., the attenuation varies
`with wavelength. Nearly flat wavelength response is achiev-
`able by using unequal
`illumination as set forth in the
`equation:
`C-|(1-f)+1"=¥PU(4 =XPU(4ar6fn)]|’
`
`where f is the fractional amplitude of light falling on one
`surface and (1-0 is the fractional amplitude of light falling
`on the other surface. Nearly constant wavelength response
`can be achieved around where 45;’-n, namely where the
`separation of the surfaces is a multiple of a quarter wave-
`length. When a multiple of a quarter wavelength exists, then
`exp[j(4rtE'J;’n)] is +1 or -1, for u even or odd, respectively. For
`the even 11 case, C-1 (very little attenuation) and for the odd
`n case, C-1(1-2,,-)2. By adjusting the value of f, any value
`of attenuation (with a more or less flat response) can be
`achieved.
`However, the choice of f is usually determined at the time
`of manufacture and is therefore not easily controlled by prior
`art methods. To overcome this problem, interferometry by
`reflection from more than 2 surfaces with independent
`motion can be used. Thus, instead of only one separation
`parameter, such as the 8, above, at least two separation
`parameters can be controlled electrically. Following are two
`exemplary ways that four independently controlled surfaces
`can be used; it is understood that a difierent number, such as
`three, six, or more surfaces could be used.
`FIGS. 13 and 14 show the use of four reflecting surfaces
`in an attenuator according to a fourth embodiment of the
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`US 6,178,284 B1
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`present invention with the illumination equally divided four
`ways between the four surfaces. The close proximity of the
`torsional hinges for the upper and lower pairs of see-saw
`structures is somewhat limiting; thus, as shown in, e.g., FIG.
`15, the shape of the ends of the four see-saw structures is
`modified, enabling the four ends to be situated together
`without bringing the torsional hinges as close as in FIG. 13.
`Although a particular shape is shown in FIG. 15, other
`shapes for the ends of the four see-saw structures are
`contemplated as part of the present invention. Thus, using
`the configuration of FIG. 15 showing an attenuator accord-
`ing to a fifth embodiment of the present invention, the
`torsional hinges can be lengthened if desired, to reduce their
`stifiness for a given width.
`The electrical connections needed for providing electro-
`static control of the tilt of each see-saw structure are not
`shown. One way to provide these connections is by con-
`ductive filrn traces that
`lead from the see-saw structure
`across one or both torsional hinges of a given structure onto
`the non-moving remaining surface of the silicon chip, where
`electrical bonding pad areas can be provided. As shown in
`FIG. 15,
`the illumination does not have to be equally
`supplied to all four structures.
`Although in the above description the light source is
`directed from above the substrate, with the membrane or
`see-saw positioned between the light source and the
`substrate, as an alternative, asshown in FIG. 16, a hole could
`be formed in the substrate and the see-saw positioned such
`that the substrate is situated between the light source and the
`see-saw. In this embodiment, the underside of the see-saw
`and the side of the substrate nearest to the light source would
`optimally be the coated surfaces. As is clear to one of
`ordinary skill in the art,
`the theory of operation of this
`embodiment is essentially the same as that of the previously
`described embodiments, the primary diiference being that
`the optical signal is transmitted past the substrate through
`the hole formed therein and is refiected off of the see-saw.
`
`While there has been described herein the principles of
`the invention, it is to be understood by those skilled in the
`art that this description is made only by way of example and
`not as a limitation to the scope of the invention. Accordingly,
`it is intended by the appending claims, to cover all modifi-
`cations of the invention which fall within the true spirit and
`scope of the invention.
`We claim:
`
`1. An attenuator for variably attenuating an optical signal,
`said attenuator comprising:
`a plurality of reflective portions for entirely refiecting any
`portion of an optical signal impinging thereon and
`thereby generating a plurality of reflected optical sig-
`nals which are combined as an attenuated optical
`signal, wherein at least one of said reflective portions is
`moveable with respect to the rest of said plurality of
`reflective portions.
`2. An attenuator as set forth in claim 1, wherein said
`plurality of reflective portions comprises a micro-
`mechanical device for selectively moving said at least one
`moveable reflective portion using voltage signals.
`3. An attenuator as set forth in claim 1, wherein said at
`least one moveable reflective portion is positioned at a
`predetermined distance above another one of said plurality
`of reflective portions, said at least one moveable reflective
`portion including an aperture formed therein for enabling a
`portion of said optical signal to be transmitted through said
`aperture and refiected olf of said another reflective portions
`to generate one of said refiected optical signals.
`4. Art attenuator as set forth in claim 3, wherein said
`predetermined distance is a function of an electrostatic
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`potential that exists between said at least one moveable
`reflective portion and said another reflective portion.
`5. An attenuator as set forth in claim 4, further comprising
`a controller, coupled to an optical signal source that gener-
`ates said optical signal, for controlling said attenuator as a
`function of said optical signal.
`6. An attenuator as set forth in claim 5, wherein said
`controller selectively varies said electrostatic potential as a
`function of said optical signal.
`7. An attenuator as set forth in claim 1, wherein said at
`least one moveable reflective portion comprises at least one
`pivotable surface pivotably moveable relative to said other
`reflective portions.
`8. An attenuator as set forth in claim 7, wherein said at
`least one moveable reflective portion comprises a see-saw
`shaped member having said at least one pivotable surface
`and wherein said at least one moveable reflective portion is
`positioned adjacent to one of said other reflective portions so
`that said optical signal straddles and is reflected off of said
`at least one moveable reflective portion and said adjacent
`one of said other reflective portions.
`9. An attenuator as set forth in claim 8, wherein said
`adjacent one of said other reflective portions is stationary.
`10. An attenuator as set forth in claim 8, wherein said
`adjacent one of said other reflective portions is pivotably
`moveable with respect to said at least one moveable reflec-
`tive portion.
`11. An attenuator as set forth in claim 1, further compris-
`ing a substrate disposed a predetermined distance fi'om said
`plurality of reflective portions, said substrate including a
`through-aperture formed therein enabling said optical signal
`to be transmitted past said substrate and impinge on said
`plurality of reflective portions.
`12. An attenuator as set forth in claim 1, wherein each of
`said plurality of refiective portions has a fixed refiectivity.
`13. An attenuator as set forth in claim 1, wherein said at
`least one moveable reflective portion includes four move-
`able reflective portions that are pivotably moveable, wherein
`ends of said four moveable reflective portions are positioned
`adjacent to each other so that said optical signal can be
`focused on said ends.
`14. An attenuator as set forth in claim 13, wherein said
`four moveable reflective portions are positioned in an
`X-shaped configuration.
`15. An attenuator as set forth in claim 1 further compris-
`ing a power monitor for measuring the power of the optical
`signal.
`16. An attenuator as set forth in claim 15, wherein said
`power monitor comprises a fiber tap.
`17. A method of variably attenuating an optical signal
`incident on a plurality of reflective portions, said reflective
`portions entirely nefiecting any portion of said optical signal
`impinging thereon, said method comprising the steps of:
`focusing said optical signal onto said reflective portions
`and thereby generating a plurality of reflected optical
`signals, wherein at least one of said reflective portions
`is moveable with respect to the rest of said reflective
`portions; and
`combining said refiected optical signals to generate an
`attenuated optical signal.
`18. A method as set forth in claim 1'7, further comprising:
`pivotably moving said at least one moveable reflective
`portion so as to control
`interference among said
`refiected optical signals.
`19. A method as set forth in claim 17, wherein each of said
`plurality of reflective portions has a fixed refiectivity.
`20. An attenuator for variably attenuating an optical beam,
`said attenuator comprising:
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`US 6,178,284 B1
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`a first reflective portion for receiving a first portion of said
`optical beam and entirely reflecting said first portion of
`said optical beam; and
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`a second reflective portion, positioned adjacent to and 5
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`pwotably movable wflh respect to Sam fim reflecnve
`portion, for receiving a second portion of said optical
`beam and entirely reflecting said second portion of said
`optical beam, whereby said refleeted first and second
`
`portions of said optical beam are combined as an
`attenuated °P“°a1 beam-
`21. An attenuator as set forth in claim 20, wherein said
`first refiective portion is pivotably moveable with respect to
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