(12) United States Patent
`Sparks et al.
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 6,625,340 B1
`Sep. 23, 2003
`
`US006625340B1
`
`(54) OPTICAL SWITCH ATTENUATOR
`
`(56)
`
`References Cited
`
`(75)
`
`Inventors: Adrian P Sparks, Ongar (GB); Robert
`Slmgllflletti, Herffmdshife (GB);
`Gordon D Henshall, Harlow (GB)
`
`( *) Notice:
`
`(73) Assignee; None] Networks Limited, st, Laurent
`(CA)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`Appl. No.: 09/474,544
`.
`.
`F11ed'
`
`(21
`
`(22)
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`Dec’ 29’ 1999
`
`(51)
`
`Int Cl-7 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -- G023 6/265 G02B 6/42
`
`.......................................... .. 385/18; 398/45
`(52) US. Cl.
`(58) Field of Search ............................... .. 359/117, 128;
`385/16-18
`
`US‘ PATENT DOCUMENTS
`6,031,946 A *
`2/2000 Bergmann etal.
`.......... .. 385/18
`6,256,430 B1 *
`7/2001 Ji11 et al.
`.......... ..
`385/18
`
`6,424,757 B1 *
`7/2002 Sparks et al.
`385/16
`............... .. 385/16
`2002/0168131 A1 * 11/2002 Walter et al.
`* cited by examiner
`Primary ExamirLer—Kinfe-Michael Negash
`(74) Attorney, Agent‘, or Firm—Barnes & Thornburg
`(57)
`ABSTRACT
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`An 0 tical switch com risin switchin means arran ed to
`switch an optical signal by redirection of the optical beam
`path of said signal, wherein said optical switch is arranged
`to misalign the optical beam path so as to provide a
`predetemiined optical output power. Thus a switch can be
`directly used to attenuate a signal,
`instead of utilising
`separate attenuators
`
`12 Claims, 4 Drawing Sheets
`
`TELECOMMUNICATIONS NETWORK 110
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`OPTICAL SWITCH ‘I00
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`PATH
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`CONTROL MEANS
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`POWER MEASURING MEANS
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`130
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`OPTICAL
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`SW|TCH|NG MEANS
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`OPTICAL
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`120
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`FNC 1006
`
`

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`U.S. Patent
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`sep. 23, 2003
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`sep. 23, 2003
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`sep. 23, 2003
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`US 6,625,340 B1
`
`1
`OPTICAL SWITCH ATTENUATOR
`
`FIELD OF THE INVENTION
`
`The invention relates to a optical switch, and in particular
`to an optical switch and a method of controlling an optical
`switch for attenuation of an optical signal.
`
`BACKGROUND OF THE INVENTION
`
`The control of optical power levels in optical communi-
`cations systems is critical
`in obtaining optimum perfor-
`mance. The power level needs to be sufficient to establish a
`signal to noise ratio which will provide an acceptable bit
`error rate but without the power level exceeding a level at
`which limiting factors (e.g. the onset of non-linear effects)
`result in degradation of the signal or other co-propagating
`signals.
`In wavelength division multiplexed (WDM) transmission,
`it is desirable to control the power of the individual optical
`channels or wavelengths. Channels could be controlled to
`provide constant system signal to noise ratio. One of the
`simplest methods of control is to maintain each of the power
`levels of the individual wavelength components (channels)
`at substantially the same level.
`Alternate schemes provide different power levels for each
`channel, e.g. lower bit rate signals and/or signals propagat-
`ing over a relatively short distance may be transmitted at
`lower powers.
`The network components that the optical multiplex signal
`passes through typically have attenuation or amplification
`characteristics which vary with wavelength. In particular,
`optical amplifiers may not have a flat gain profile, providing
`different amounts of gain on different wavelengths. As an
`optical multiplex signal may pass through many amplifiers
`before the signal is regenerated, even small variations in
`gain flatness can produce large differences in the relative
`amplitudes of the individual optical channels.
`It is also possible that wavelengths from different sources
`need to be placed in the same multiplex. An optical network
`may use optical time division multiplexing to compliment
`the wave division multiplexing functionality, routing differ-
`ent wavelength packets on different channels to different
`destinations. Consequently, optical signal packets or wave-
`lengths from different sources may share common routes,
`and in order to ensure optimal transmission along those
`routes relatively frequent channel power equalisation may
`be necessary. If the paths traversed by the wavelengths
`through the network are different then even if the wave-
`lengths started with the same amplitude at their respective
`origins, the amplitudes may be very different at the point
`they are placed on the same multiplex. If a channel is of
`much lower power than the adjacent channels interference
`from these channels due to fibre non-linearity will have a
`greater effect than if the channels had approximately equal
`powers.
`It is therefore desirable to provide variable attenuators in
`communications system to allow the control of optical signal
`powers.
`
`Variable optical attenuator systems are well-known in the
`telecommunications field. For instance, U.S. Pat. No. 5,956,
`437 describes an electrically controlled optical attenuator.
`The attenuator is formed by a mach-zehnder waveguide
`configuration with a variable refractive index element in one
`arm to modify the spectral characteristics of network to give
`a specific attenuation at a specific wavelength. Co-pending
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`U.S. application Ser. No. 09/361,950 describes a method and
`apparatus for determining control signals of a filter for
`optimisation (i.e. attenuation) of the gain profile of an
`optical amplifier, and indicates how equalisation of an
`optical signal profile may be made over a relatively broad
`spectral range.
`All of these instances require at least one attenuator/filter
`element to be incorporated into the system. If a range of
`channels need to be equalised (i.e. ensure that an approxi-
`mately equal power is within each optical channel) then due
`to the roll off of the profiles of normal filters,
`it can be
`necessary to demultiplex a WDM system, equalise each
`channel individually using a separate attenuator, and then
`remultiplex the system. This significantly increases the
`component count and cost of any equalisation system.
`The present invention aims to address such problems.
`
`SUMMARY OF THE INVENTION
`
`In one aspect, the present invention provides a method of
`controlling an optical switch comprising switching means
`arranged to switch an optical signal by redirection of the
`optical beam path of said signal, the method comprising
`controlled misalignment of the optical beam path so as to
`achieve a predetermined optical output power.
`By controlling the misalignment of the optical beam path
`through the switch, the optical signal can be attenuated in a
`controlled manner. Utilising an optical switch in this format
`alleviates the requirement for separate optical attenuators to
`be incorporated into the system. If the optical system is
`being used as part of a WDM system, it is typical for the
`signal to be dernultiplexed into the separate optical channels
`prior to input to the switch. If desired, each of the channels
`passing through the switch may be attenuated to whatever
`degree necessary to achieve the desired effect, e.g. equali-
`sation of optical power across all channels. If the signal is
`demultiplexed into groups of channels, equalisation of
`power could also be applied to these groups of wavelengths.
`If desired, normal operation of the switch could be
`performed with each of the signals attenuated by
`misalignment, thus permitting the optical power of any one
`or more signals to be increased by improving the beam
`alignment to the degree required to obtain the desired power.
`Preferably,
`the method further comprises the steps of
`measuring the power of an optical signal; and controlling
`said switching means to misalign said optical beam path and
`achieve said predetermined output power.
`Preferably, said step of measuring the optical signal power
`comprises at least one of measuring the input optical signal
`power and measuring the output optical signal power.
`If the optical switch is calibrated such that a predeter-
`mined misalignment produces a predetermined attenuation,
`then only a single indication of the optical signal power is
`necessary. If desired, such a power measurement could be
`performed substantially upstream or downstream of the
`optical switch, at a different point within the network if the
`attenuation characteristics of any intervening components
`are known. Alternatively, both the input and the output
`optical signal to the switch could be measured in order to
`directly indicate the degree of the attenuation of the optical
`signal as it passes through the switch. This information
`could be used to provide a closed loop feedback control
`system to ensure that the desired degree of attenuation is
`achieved for each optical signal (or channel).
`two
`least
`Preferably,
`the optical switch comprises at
`inputs and two outputs, and said measurement step com-
`
`

`
`US 6,625,340 B1
`
`3
`prises determining tl1e relative ratios between the optical
`powers of at least any two optical signals.
`Prcfcrably, said optical switch comprises at least two
`inputs and two outputs arranged to switch the optical beam
`path of different wavelength optical signals,
`the method
`comprising misaligning respective optical beam paths so as
`to achieve a predetermined ratio of output optical power
`between at least any two of said different wavelength optical
`signals.
`Preferably, said predetermined ratio is substantially unity.
`Hence, channel equalisation is achieved.
`Preferably, said redirection of the optical beam path is
`achieved by reflection, refraction and/or diffraction.
`In another aspect, the present invention provides a com-
`puter program on a machine-readable medium, said com-
`puter program being capable of performing a method of
`controlling an optical switch comprising switching means
`arranged to switch an optical signal by redirection of the
`optical beam path of said signal,
`the method comprising
`controlled misalignment of the optical beam path so as to
`achieve a predetermined optical output power.
`In a further aspect,
`the present invention provides an
`optical switch comprising switching means arranged to
`switch an optical signal by redirection of the optical beam
`path of said signal, wherein said optical switch is arranged
`to misalign an optical beam path so as to provide a prede-
`termined optical o11tp11t power.
`Preferably, the switch further comprises control means
`capable of receiving an input signal indicative of the power
`of an optical signal, the control means being arranged to
`control the functioning of said switching means for achiev-
`ing misalignment of said optical beam path.
`Preferably, the switch further comprises power measuring
`means arranged to provide a signal indicative of the power
`of an optical signal.
`In another aspect, the present invention provides a tele-
`communications system comprising an optical switch com-
`prising switching means arranged to switch an optical signal
`by redirection of the optical beam path of said signal,
`wherein said optical switch is arranged to misalign the
`optical beam path so as to provide a predetermined optical
`output power.
`The preferred features may be combined as appropriate,
`as would be apparent
`to a skilled person, and may be
`combined with any aspects of the invention.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`In order to show how the invention may be carried into
`effect, embodiments of the invention are now described
`below by way of example only and with reference to the
`accompanying figurcs in which:
`FIG. 1 illustrates a schematic diagram of an optical path
`through two modules in an optical cross—connect (OXC)
`switch (PRIOR ART);
`FIGS. 2a and 2b illustrate respectively the manner in
`which the optical beam is coupled into the output fibre in
`normal operation and according to an embodiment of the
`present invention;
`FIGS. 3a and 3b illustrate respectively how the beam may
`be reflected from a mirror in normal operation and in
`accordance with another embodiment of the present inven-
`tion.
`FIG. 4 shows an embodiment of a switch in a telecom-
`munications network.
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`4
`DETAILED DESCRIPTION OF PREFERRED
`EMBODIMENTS
`
`Many tclccommunications nctworks arc moving in thc
`direction of all optical networks,
`incorporating optical
`switching. “Performance of a 576x576 optical cross con-
`nect” H Laor, A Richards, E Fontenot, Proceedings of the
`National Fibre Optic Enginccrs Confcrcncc Scp. 28”’, 1999,
`Chicago, USA, incorporated herein by reference, describes
`a recently developed optical switch. As is common with
`many such optical switches,
`it
`is fabricated using
`microclcctro-mcchanical systcms (MEMS) tcchnology. It
`may be used to switch WDM signals as a group, or the
`WDM signals may be demultiplexed outside the switch and
`switched individually or as groups of channels if desired.
`This particular optical switch is constructed of a number
`of modules or units. FIG. 1 shows the optical path which
`represents a connection between the two modules 10, 20.
`Light from a fibre 2 passes through a focussing lens 12. The
`light which forms the beam is rellected off a fixed mirror 14
`to keep device packaging small. It is then reflected off a
`movable mirror 16 which precisely directs the beam in two
`axes. If tl1e movable mirror is in position 16', the beam will
`move along an alternative path shown as 32.
`To make an optical connection between modules 10,20,
`the movable mirror 16 directs the beam at the movable
`
`mirror 26 of a targeted second module 20. At the same time
`the movable mirror 26 of the second module 20 is controlled
`to dellect the beam 30 towards the fixed mirror 24, into the
`lens 22 and hence into the fibre 4 completing the connection.
`It is the co-ordinated control of deflection angles by the two
`movable mirrors 16,26 that creates the optical connection
`between two transmission fibres 2,4.
`By having two arrays of such modules, optical signals
`coming in from a first array may be directed into any of the
`output fibres of the second array. It should also be noted that
`the terms input and output are used for convenience, the
`optical path through any two modules in a connection being
`bi-directional.
`
`In normal operation a closed—loop servo control system is
`employed. This control system is normally used to provide
`high optical coupling efliciency between the fibres and to
`protect the optical signal against vibration and drift. The
`system operates by controlling the movable micromirrors
`(16,26), which are fabricated using MEMS technology and
`are capable of two axis movement, to carefully align the
`beams so as to ensure that the maximum possible input
`optical signal is received at the output of the switch.
`The present invention utilises a control system to control
`the mirrors so as to deliberately misalign the optical beam
`path 30 through the switch. By non-optimally aligning the
`optical beam path, the optical beam will be attenuated as it
`passes through the switch due to a reduction in the power of
`the beam coupled into the output fibre. This permits the
`switch to be utilised to achieve any desired optical beam
`power output
`less than the maximum. Consequently,
`if
`desired, WDM system channels may be equalised. Such
`attenuation is achieved without
`incorporating separate
`attenuator(s) within the system.
`FIG. 4 shows an optical switch 100 as part of a telecom-
`munications network 110, the switch having an optical path,
`a switching means 120 a control means 130 capable of
`receiving an input signal
`indicative of the power of an
`optical signal, and being arranged to control the functioning
`of said switching means for achieving misalignment of said
`optical beam path. A power measuring means 140 is
`arranged to provide a signal indicative of the power of the
`optical signal to the switching means.
`
`

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`US 6,625,340 B1
`
`5
`FIG. 2a illustrates how the optical beam 30 would nor-
`mally be coupled into the optical fiber core 4a, which is
`surrounded by optical fibre cladding 4b, by the focussing
`lens 22. If, in accordance with an embodiment of the present
`invention, the optical beam path is misaligned, e.g. either to
`misalignment of one of the mirrors 16, 26 or movement of
`the lens 22, then FIG. 2b illustrates how only a portion of the
`beam 30 will be coupled into the optical fibre core 4a.
`Consequently, only the fraction of the beam profile 30
`coupled into the output forms the output signal, and hence
`the optical signal is attenuated.
`FIG. 3a shows how the beam 30 is reflected from a mirror
`
`(in this example mirror 26) during normal operation. The
`lines 30a and 30b represent the two extremes of the beam
`width, with the arrows indicating the beam direction. FIG.
`3b shows how such a beam could be attenuated according to
`another embodiment of the present invention, whereby the
`incoming beam is misaligned so that only part of the beam
`profile is reflected from the mirror (and hence subsequently
`coupled into the output fiber 4). 'lhe dotted arrows represent
`the extremes of the profile of width X of the beam that is lost
`from the signal due to the misalignment of the beam on the
`refiective surface of mirror 26. Absorbent material may
`surround 26, or be placed elsewhere in the switch for
`prevention of interference by the signal X with other signals.
`Whilst the preferred embodiment has been described as
`utilising a reflective surface (the mirrors) to misalign the
`optical beam path, it will of course be appreciated that by
`controlling the position and/or orientation of a refractive unit
`(e.g. a focussing lens within a switch), the beam path may
`be similarly misaligned by misaligning the lens in a con-
`trollable manner, a desired optical output power less than
`maximum (i.e. the optical output power if the optical path is
`perfectly aligned) can be achieved.
`Equally, if the switch is of a diffraction type, such as
`might be achieved by an LC (liquid crystal) providing a
`controllable diffraction grating, or of a different refiective
`type (eg. use of MEMS pop-up mirrors to act as a switch),
`by controlling the switching media to misalign the optical
`beam path(s), optical attenuation can be similarly achieved.
`An example of such a MEMS pop-up mirror is disclosed in
`“Free—Space Micromachined Optical Switches for Optical
`Networking”, F Y Lin, E L Goldstein and R W Tkach, IEEE
`Journal of Selected Topics in Quantum Electronics, Vol. 5,
`No. 1, January/February 1999.
`For the purposes of this specification, the terms “optical”
`and “light” should be understood as pertaining not only to
`the visible part of the electromagnetic spectrum, but also to
`the infra-red and ultra-violet parts that bound the visible
`part.
`Any range or device given herein may be extended or
`altered without losing the effect sought, as will be apparent
`to a skilled person from an understanding of the teaching
`herein. For instance, whilst the preferred embodiment of the
`present invention has been described in conjunction with a
`specific type of optical switch, it will of course be appreci-
`ated that the invention may equally be applied to any optical
`switch utilising any one or more of refiection, refraction
`and/or diffraction, in which the optical beam path through
`the switch can be misaligned so as to attenuate the resultant
`output signal.
`
`What is claimed is:
`
`6
`
`1. A method of controlling an optical switch comprising
`switching means arranged to switch an optical signal by
`redirection of the optical beam path of said signal,
`the
`method comprising the steps of:
`
`measuring the power of an optical signal; and
`controlling said switching means to misalign the optical
`beam path so as to achieve a predetermined optical
`output power.
`2. A method as claimed in claim 1, wherein said step of
`measuring the optical signal power comprises at least one of
`measuring the input optical signal power and measuring the
`output optical signal power.
`3. A method as claimed in claim 1, wherein the optical
`switch comprises at least two inputs and two outputs, and
`said measurement step comprises determining the relative
`ratios between the optical powers of at least any two optical
`signals.
`4. A method as claimed in claim 1, wherein said optical
`switch comprises at
`least
`two inputs and two outputs
`arranged to switch the optical beam path of different wave-
`length optical signals, the method comprising misaligning
`respective optical beam paths so as to achieve a predeter-
`mined ratio of output optical power between at least any two
`of said different wavelength optical signals.
`5. A method as claimed in claim 4, wherein said prede-
`termined ratio is substantially unity.
`6. A method as claimed in claim 1, wherein said redirec-
`tion of the optical beam path is achieved by at least one of
`refiection, refraction or diffraction.
`7. A computer program on a machine-readable medium,
`said computer program being arranged to carry out
`the
`method of claim 1.
`
`8. An optical switch comprising switching means
`arranged to switch an optical signal by redirection of the
`optical beam path of said signal, wherein said optical switch
`is arranged to misalign the optical beam path so as to provide
`a predetermined optical output power and further comprises
`control means capable of receiving an input signal indicative
`of the power of an optical signal, the control means being
`arranged to control the functioning of said switching means
`for achieving misalignment of said optical beam path.
`9. A switch as claimed in claim 8, further comprising
`power measuring means arranged to provide a signal indica-
`tive of the power of an optical signal.
`10. A telecommunications system comprising an optical
`switch according to claim 8.
`11. The switch as claimed in claim 8, having at least two
`inputs and two outputs, and the input signal having a relative
`ratio between the optical powers of at least any two optical
`signals.
`12. The switch as claimed in claim 8, having at least two
`inputs and two outputs arranged to switch the optical beam
`path of different wavelength optical signals,
`the control
`means being arranged to misalign respective optical beam
`paths so as to achieve a predetermined ratio of output optical
`power between at least any two of said different wavelength
`optical signals.
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