Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
wo93/2n47s ~ i5~ PCI/A1193/00146
IMPP~OVE~MENTS TO OPTICAL PHASE SHIFTING
Technical Field
The present inv~ntion relates to optical systems, including phas~
shifters, isolators, circulators and bi-directional ~ommunication systems,
5 particularly but not exclusively for optical fibre communication systems.
Background Art
An opticai isolator is an important componen~ in many optical
systems, including communications applications and bulk lasers. The role
of the isolator is to allow transmission of light in only one dir~ction.
10 Isolators are used in communications system~ to prevent feedback
resultin~ from refi~ctions returning to a laser dio~e, and are used in
conjunction with optical amplifiers to ensure thers is no lasing or noise
degradations due to feedback. A typical amplifier may have a
conventional isolator at its input, its output or both input and output. It is
15 not practically possible t~ operatB a very high gain amplifier without
isolation because of residual refiections and scattering.
The most common type of prior art isolator relies on the
Faraday ~ffect to achieve non reciprocal rotation o~ the polarisation state
of light. Ih the presence of an applied magnetic field the pslarisation of
20 light is rotated in a direction independ~nt of the propagation direction,
and proportional to the Verde constant of the material in whi~h the light
propagates. ~ By using a crystal~ with a high V~rde constant, a rotation of
45 ~degrees can be effected in a short distance. A single polarisation
isolator can be constructed~ as shown in Figurs 1. Light travelling from
25 1 eR to right is polarised in the vertical direction, th~n rotated by 45
degrees by the Faraday rotator~ A second polariser is placed at this
an~le, allowing the light to pass undisturbed, Light travelling from right
~to Ieft is polarised in the 45 degres direction, then rotated by the
~ ~ Faraday elemsnt a further 45 degrees, so that it is now in the horizontai
30 plane. This light will be completely block~d by the vertical polarisation
so there will be no transmission in the right to left direction.
One technique for achi~ving polarisalion independent isolation is
WO 93/21)475 ~ 1 3 3 5 5 1~ PCl ~AU93/00
to split incident light in~o two poiarisa~ion components, isolating ~ach of
ths components, and then recombining the two polarisation ccmponents.
The polarisatîon splitt?rs th~mselves may act as the polarisers for the
isolators, so a polarisation independent isolator can bs constructed as
5 shown in Figure 2.
Whilst such iselators are essentTal in many amplification
applications, they negate the possibility of two w~y communicatlon down
a single fibre, and of bi-directionai ampli~ication. It wolJld be
advantageous to be able to provide singl~ fibre two way comrnunication
10 ~ in systems which use optical amplifiers.
It is one obj~ct of the pres~nt inv~ntion to provi~e an improved
isolator which at least ameliorates the disadvantag~s of the prior art.
According to one aspect ~he present inv~ntion proYides a non-
reciprocal phass shifter, comprising at least one first polarisa~ion rotating
15 means, the direction of polarisation rotation being dependant on the
direction of propagation~ of transmTtted light, and at least one s~ccnd
~ means for altering polaris~ation, the direction of polarisation alteratlon
: ~ being independent of the direction of propagation of transmitted light,c hara~terised in that substantially half of any light propagating trom either
20 end of the phase shifter havin~ an arbitrary polarisation travels through
: each of: the first polarisation rotating means and s~cond means for
a5tering poiarisation.
According to:~ another aspect the invBntion comprises a non-
reciprocal opticai phase shitter, comprising m~ans tor transmitting incident
25 light through first and second optical paths and recombining said paths at
an output, said paths including respPctively first ~nd second means for
altering the polarisatlon of: incident light having arbitrary polarisation, at
ieast one of said means for altering polarisation having a first rotation in
one direction of propagation and another rotation in a second direction of
30 propagation, and the o~her: or one of said rneans for aitering having a
polarisation change independent of the direction of propagation,
the arrangement being such that a first reiative phase shift
wo s3~2047s 2 1 3 3 S ~ li PCI /AU93/001~6
between the paths occurs for light propagating in on~ direction, and a
second relative phase shift betw~an the paths occurs tor light propagating
in a second direction.
According to a Surthsr aspect the invention ~omprises a non-
5 reciprocal optical phase shifter, cemprising means for transmitting incidentlight through first and s~cond optical paths and r~combining said paths at
an output, said first path includin~ In a tirst p~opagation direction
su~cessively first polarisatlon rotating m~ans having a rotation dependent
on propagation dire~tion and second polarisation altering means having a
: : 10 change independent of the dir~ction of propasatl~n~ said second path
including in said first propagation direction successively thTrd p~larisation
: alteriny means haYing a change independQnt of the direction of
propagation and ~ourth polarisation rotating means having a rotation
dependent on propagation directionj
tS ~ ~ ~ the arrangement being ~ such that substantially half of the
incident: light trav~ls through each of said first and second paths, and the
; relative: phase shi~t of ~ ~output light is dependent on the dir~ction of
pr~pagation of the: incident light.
A :furth~r aspect of the present invention provides a non-
: 20~ reciprocal phase shifter,; comprising means for transmitting substantiallyh~alf of incident ~ light into two optical paths each having arbitrary
: polarisation, and recombining: the paths to produce an outputl each of
said: ~paths: comprising ~ means for altering the polarisation of the
t ransmitted light,: characterise~ in that in a first direction of propagation
2S the~paths have outputs~:which ars have a first relatiY6 phase shift, and in
th reverse :direction the :paths have outputs which have a different
: relative phase shift.
:~ : : Another aspect of the present invention provides an optical
,
: isolator, comprising ~a non-reciprocal phase shifter characterised in that
30 ~ for a selected waveiength, in a first propa9ation dirsction the output ~ight
is substantially transmitted,~ and in the reverse direction the output light is
: substantially attenuated.
WO 93/2047~ 1 3 3 5 5 6 PCI /A U93/00!
A further aspect of the present invention proYides bidirectional
optical isolator, cornprising m~ans for ~ransmi~ting substantially half of
incident light through each of ~irst and second optical paths, at least
said first path including p~larisation rotating means having a rotation
5 dependent on propagation dir~c~ion and at least said second path
including second polarisation altering means having a rotation
independent of tha diraction of propagation, sàid first and second paths
having a path length difference, and means for re~ornbining said first and
second paths,
10the arrangement being such that for llght haYing a first
wavelength propagating in a tirst direction total attenuation occurs, while
for light having said first wavelength propagating in a second reverse
direction substantia3 transmission occurs; and for Jight having a second
:: wavelength propagating in said first direction substantial transmission
~15 occurs, while ~r iight having said second wavelength propagating in
~: : said second reverse direction substantial attenuatiQn occurs.
: ~
A further aspect of the present invention provides an optical
circulator9 comprising at~ieast one input to an non-reciprocal isolator, and
means for coupling the: ~isolator to two output~, th~ arrangement being
20;: such that~light having one: wavelength is output through onB output, and
ght~ having a second wavelength is output through the second output.
A further aspect of the present invention provides bidirectional
optlcal fibre communications~ system, in which signals travelling in a tirst
direction ~have a first wavelength, and signals travelling in the other
25~: direction have a second w~velength, both signais travelling in the same
optical fibre, characierised~ in that the system includes one or mors
wavelength selective bidirectional isolators~ .
A further aspect of ~the present in~ention provides a bidirectional ~-
optical amplifier for allowing amplification of signals at a first wavelength
:30 in a first direction, and at a second waveien9th in the second, reverse
direction, comprising means for inducing gain at said ~irst and second
wavelengths, and bldirectional wavelength dependent isolation means
:.' '
. .
WO 93/20475 PCI/AU93/00146
~)13~S5~
arranged such that
signals at said first wavelength travelling in said first direction,
and signals at said sscond wavelength travelling in said second direction,
are transmitted, and
signals at saict firs~ wavelength travelling in said second
direction, and signals at said second wave,ength trav~lling in said first
direction, are attenuated,
and such that undasired teedback at said first and second
wav~lengths is substantially suppr0ssed.
A further aspect of the present invention provides an optica
isolator, comprising means for transmitting substantially half ot incident
light into two op~ical paths each halving arbitrary polarisation, and
recombining the paths to produce an output, each of said paths
comprising means for alterin~ the polarisation of the transmitted light,
t 5 characterised in that in a first direction of propagation the paths have
outputs which are in phase, ~nd so transmit the incident light, and in the
r~verse direction the paths have outputs whic~ are 180~ out of phase9
and ;so do not transmit tha incident light.
: 20 Br~e~ Description of Drawings
Several ernbodiments of the invention will be described with
~- ~ r eference to the drawings, in which:
:~ Figure 1 is a schematic view of a prior art isolator;
~: ~ Figure 2 is a schematic view of another prior art isolator;
Figure 3 is a sch~matic view of an optical circulator;
Figure 4 iilustrates a first technique for bi-directional isolation in
a wavelength diversity transmission system;
Figure 5 i!lustrates a second techniqus for bi-directional isolation
in a wavelength diversity transmission system;
Figure 6 illustrates conceptually a techniqu~ for bi-directional
isolation in a wavelength diversity system;
Figure 7A and 7B illustrate interference within the system ~f
wo s3/2047s PCr/AUg3JOO-
~33556 6
figure 1 0,
Figure 8 illustrates ona implem~nta~ion of the systsm ~ figure
10;
Figure 9 illustrat~s anoth~r impl~mentation of the system of
5 figure 10;
Figur~s 10A and 10B illus~rate a preferred implem~ntation of a
non-reciprocai phase shifter;
Figure 11 illustrates a polarisation~ dispersion fr~s isolator;
Figure 12 illustrates a balanced polarisation dispersion free
~0 :design;
Figure 13 illustrat~s an integrated impiementathn o~ a
ci~ulator/isolator according :to tho pres~nt inventi~n;
Figure 14 is a graph showing wavelangth dependence in an
exp~rimental device;::
15 : ~ Figure 15 illustrates a prefsrred implementati~n of the system of
`
figure 10;
~: Figure 16 illustrates schematically a bidire~ional amplifier. : -
Figure 17 illustrates schematically a fibr~ ~mbedd~d circulat~r
20~ according to th~ present invention; ~:
Figure 18 ~ illustrates an implementation ~f the device of Figure
Figure 19 illustrates~ a mod~&onv~rting circulator;
:: : Figure 20 iilustrates a fused Mach Zender irnplementatlon of an
:25 ~ isolato~/circulator: according to the preseni invention; : ::
- ~.
~; Figure 21 illustrates a filtered isolator accordin~ to the pr~sent
inYention;
Figure 22 illustrates a high isolation devica;: ~;j
: : Figure 23~ illustrates ~ a network utilising a bi-dlr~ctional i::
30 ~ amplifier,and ` ;;
Figure 24 illustra~es schematically a low palarisation dispersion
amplifler.
W093/20475 ~13~55~ PCI/AU93/00146
Detailed description
This aspect of the pres~nt invention is particularly adapted to be
implemented in bi-dir~ctional networks. In tha transmission system
envisaged ~sing this form of isoîation, transmission in one direction takes
5 place at one wavelength, and in tha opposite direction a~ a second
wavelength, as is illustrat~d in principle in Figuro 23. A partlcular difficul~yin this arrangemen~ is in cons~tuc~in~ a simple isolatQr wh}ch achi3ves
wavelength s~lectiv0 isolation. H such an isolator could be constructed, ~t
is clear that the amplifie-: would be adequately isolated from refl~ctions,
10 as such refiections would in general maintain the wavalength of the light
: and so be passed out of the system upon meetin~ an isolator.
There are several approaches that can be taken to achieve the
desired end. A first embodimen~ of this type o~ isolator depends upon
~ ~ splRting the light into the:two wavelengths using a grating or wavelength
5 ~ division~: multiplexer 34, isolating each wavelength separately in each
direction using isolators~ 32, 33, th~n recombining the light afterwards 31,
as shown in Figure ~4. A: slightly more refin~d approach can be
constructe:d with an optical clrculator 42 and a single waveiength
multiplexing elemsnt 41 as :shown in Figure ~.
20~ This aspect :of the: pr~sent invention is based on a new
approach to isolation~ which is inter~erometric instead Ql being polarisation
dependent. One immediate ~advantage that arises from this is that all of
the embodiments~ described~: are in general polarisation independent, :
with~ut the need t~ split ~ and: recombine polarisatJons. Sorne o~ the
25~ mplementations illustrafed ~aré for single moded waYeguiding applications
only, but the genera3 concepi is~ equalîy applicabls to bulk Isoia~ors. ~ -
A general: form of this aspect of the invention is shown in
~: ; Figure~ 6.~ A ~ingle ~beam o f light entering at: point 1 is split by -
beamsp!itter 51 into two paths ~formed using mirrors 54, 55. In one path
30~ ~a~ 90 degree Faraday rotator ~3 is placed, and in~ the second path a pair
of half-waYe retardation~ plates 52 at a relative angle of 4~ degrees ~s ::
placed. The pair of reta ders 52 rotates ~he polarisation of any arbitrary ::
:
,,,:
WO 93/21)47~ PCl/AU93/00
2133S5~
polarisation of incident light by 9Q d~grees.
This can be d~monstrat~d by the appropriate Jones' Matrix
multiplications~ The two polarisations are now in ~he same direction, and
the optical path length can now be adjusted to b~ Qt:lU~II in both arms
5 ensuring constructive interfererlce on ~h~ throvgh pa~h ~1) to (4). 3n the
reverse path with light incident at por~ 4, ths polarlsatlon of the two
b~ams will be opposits because the F~raday rotation Is independent of
the prvpagation direction, whereas ~he birefringent ro~a~ion is reciprocal.
This corresponds to a 1 8û degrees phase chan~e in the electric field
10 vector between the two be~rns and dsstructivs int~rfsrence wTII oc~ur at
port 1. The light will instead exit thfough port 2 formiQg an isslator.
.
The d~vice created is r~lated to a Mach-Zender interferometer.
It will be apparent that wavelength dependence in the isolation can be
;achieved. If an optical ;path length difference is introduced between the
15; two arms of the Mach-Zender, then there will be only certain wavelengths
;~ ` for which th~ int~rference will be constructiv~ in light travelling ~rom port
to port 4. This will give a sin2 de,oQndencs to the intensity of light
emerging In port 4 as~ a function ~of ~, with a peri~)dicity proportional to
1, the difference in optical path lengths, with the remaining light being
~20 ~ redirected to port 3. Because of the 180 degree phas~ shift between
beams travelling right~to left, the tfansmission curve at port 1, Figure 7A,
wili ~ be complimentary ~to that obtained at port 4, Figure 7B. For
wavelengths having ~destructive intPr~erence at port 4 for forward
propagation, constructive interference wili be obtained at port 1 for reverse
25 propagation.
This is precisely the characteristics required ~or a wavelength
dependent isolator. ;For instance, by using an optical path difference of
0.12 mm a device can be constructed to be forward propagating at 1535
nn (corresponding to the first gain peak of the erbium-doped fibre
3û amplifier) and revers~ propagating at 1555 nrn (the second gain peak of
the erbium doped fibre arnplifier). Isolation is provided for both
wavelengths.
wo 93/2047s ~ ~ 3 ~ 5 ~ 6 Pcr/Aug3/l)0146
The main obsta~le to be overcome is the stabillty of the device
to external perturbations. As an int~rfQrometric device i~ is potentially
much more sensitive to any ~herrnally Qt mechanically induced variations
in the relative path lengths. Althou~h th~rmai and rnechanical
5 stabilisation is possible, it is unlik~ly tha~ any bulk d~vice built accoFdirlg
to Figurs 6 would have the required stability tor a ~ield devlce.
One solution is to build .the entire device from s~lid
componants, with a balance in ~he construction of ~he arms such that any
therrnal expansion affects each arm equally (excspt for the very small
:: 10 path length difference 1/ 1. This is shown in Figure 8. Incident light 62
is incident on silvered face ~3, and passes either through rotat~r 67 and
half wav~ plate 68, or rotator 64 and half wav~ plate 65, to eventually
exit 66. It will be apparent that an isolator similar to figure 6 is created.
It is :noted that instead of a~ 90 degree Faraday ro~ation in one arm we
: :t5 now:hav~ a 4~ degree~rotation in one arrn ~nd minus 45 degr~es in the
second arm,and one of the hal~ wave plates ts Tn one arm and the
second i5 in the second arm. It can a~ain be shown by Jones matrices
~: : ; :that this is equivalent :to ~the~ previous arrangement. It is, however, very
difficult t o produce a ~50% beam-splitting cube which is polarisation
20~ ~insensitive~ at~ many of the wavelengths of particular interest to optical
: communi~ations. ~:
A:second :solution is to physicaOly split the beam into the top
half ~and bottom half and to~ use:~the same rotators and birefringent plates
as ~ before ,as~ shown in ~Figure 9. Lens 73, 71 ~collimate/focus the
: 25~: incident~: 1ight to~ and from~ beam 75 and project the light onto mirror set
70. It is now much less obvious that this should work as an isolator, or
: even that there shQuld ~ be~ any interfersnce at all. In fact this will no
: longer :act as a bulk isolatot, and it is only when the~input 74 and output
~ ~ 72 are both single mode: fibres that the interference or isolation will take
:: ; 30 place. This is because ~of the quantum nature o~ the modes of a fibre.
A qualitùtive theoretical understanding can be obtainad by
: considering the device as a black box, which provides ~two paths ~f equal
WO 93/20475 P(:~/AU93/00'
213 3~ ~fi lo
probability and the sante phase for photons travelling in the forward
direction. The expectation value of a photon arriving at a point in the
second fibre is the sum of ~the diftsr~nt possible wave functions times the
probabiiity of that wave ~unction~. In the forward case the wave functions
5 will be in phase for a ~iven wav~leng~h photon and the normalised
expectation value will be ~ (100% probabili~.of the photon arriving). In
the reverse direction for that same wavel~nQih the waYe functions will be
opposite sign and so the probability of the photon arriving will be 0.
The aboYe argum~nt is easily v~rified by using the formalism ~f
t 0 Fourier optic~. An assumed Gaussian moda is transformed by the l~ns
into a Gaussian beam in Fourier space. In the absence o~ any phase
delay, the second lens transforms the be~m back to the original
Gaussianf which eXCitBS ~ully the ~undamental mode of the second fibre~
In quantum mechanical terms, the overlap int~gral of the mode and the
15 exciting~light is 1. We~have 100% transmiss!on~ If a phase delay of ~
is: incurred in the top half: of th~ beam, then this is equivaient to
~: multiplying by -t the top half ot the Fourier transform. Upon performing
~ ths Fourier transform corresponding t~ the second lans, an odd function is
:: : obtained. The overlap integral of an odd functiQn with an e~ en function
20 ~ is~ necessarily zero. it is ~now clear what is happening to the photons
which are lost: to the system. They are simply trying to excite higher
order modes which are cut off and so cannot propagate along the ~ibre.
These photons are lost very~quickly into the cladding ~t the flbre.
This principla has been demonstrated in the laboratory by
25 constructing a very simple interferometer, which has proved surprisingly
stable. An optical flat was inserted so as to occupy ha~ of the beam in
a beam expander between two single mode fibres. Ther~ is a clear
modul:ation of the received light as a function of wavelength,
corresponding to the two paths being in phase and out of phase. This
30 is shown in Figure 14.
This leads us to another device ~or bi-directional isolation~ In
this the expanded beam is nsver physically separated. A compound
WO 93/20475 PCI/AV93/00146
~133SS6
11
element consisting of a 90 ~egree Faraday 7~ ro~a~or and a pair of
crossed half wave plates 7~ is made up, by polishing and joining on one
side. This cornpound element is now located in ~he bRam so as ~o
occupy exactly one half of the beam as shown in Figure ~ ~. The path
S difference can ba achi~ved by having a slightly different optical path
between tha Faraday rotator and the birefringent elemont. This is
conceptually identical to the davice outlined in Figure 9. The exact
wavelength of transmission can be tuned by varying slightly the angie at
which the elemen~ is inserted into the beam. Ths thickness of this
10 compound element need not be much larger than 500 llm with stat~of-
th~art Faraday rotators. Insertion loss should be negligible (c0.5 dB)
with good beam expansion optics, and stabiliîy is maximised because
there is only a small length of crystal (~1 mm) generating the path
difference.
:1 5One disadvantage of She embodiments de~cribed is that the
~: construction of the non-reciprocal phase shi~ter used different materials to
act upon: each half of the beam which is split. The device is ther~fore
.
susceptible to temperature dependence as the thermal expansion
coefficients nd thermal refractive index coefficients for each half will be
20 dfflerent, thus shifting the~ wavelength of maximum isolatlon.
Figures tûA, 10B illustrate a preferred stabl2 design for the non- :
reciprocal phase shifting element.
:: :
:: This implementation: uses a 45 degree Faraday rotator 101 in
~: both::halves and a haif-wave plate 100, 102 in both halves. In one half
: 25 the half waveplate 100 is to the left of the Faraday rotator 101, and in
the second half, the half waveplate 102 is to the right of tha Faraday
rotator 101. The ~second waveplate 102 is orientated to have its QptiC
axis at 45 degree relative to the first waveplate 100.
To understand the operation of the device ~onsider the path of
30 light travelllng left to right in the vertical polarisation 104 for a wavelength
independent device~ In the top half of the beam, tha light tra~els through
the fast axis of the half-wave plate 100 then through the Faraday rotator
WO 93/2047~ PCr/AU93/001
~ ;~ '3 S S 6 12
101 to rotate the polarisation 45 degre~ c~ockwise. The light in the
bottom half of th~ beam passss ~irst lhrough the Faraday rotator 101, to
be rotated cleckwise into line with the fast axis of th~ half wave plate
102. The two beams upon recombining wil~ be in phase. Simitarly, for
5 light in the horizontal polarisa~ion 105, light travelling through both the top
and bottom halves wil~ travel through . !hè slow axis of the half wave
plates 100, 102 and so will be in phase. ~ight travelling from right to
left, incident at 45 dagree clockwise to ~he verti~al in the top halt wlll
travel through the Faraday rotator 101 first to ~e rotated to the horizontal
10~ axis and then pass through the slow axis of the half wava plate 100.
Light travelling from right to left, incident at 45 d~gree clockwise to the
vertical in the bottom half wilt pass first through the fast axis of the half
wave pla~e 102 then through the Faraday rotator 101 to be in the
: horkontal axis. As: the top beam has trav~lled through the fast axis and
15 the bottom beam has~ travelled through tha slow axls th~re is a 180
: degree relative: phase shifl~ between both halves of the beam. In the
forward direction, recombination of the light leads to constructive
interference and hence ~ transmission, and in the revers2 direction
recombination of the light~ leads to destructive interf~rencs and hence
20~ attenuatlon. This is~the:~basis:for~non-reciprocal transmission, or isolation.
:This~may~ be readily ~extended ~to a circulator application, as will be
apparent to the reader. ~
The isolator describ2d with reference to Fi~ure 10 has a very
small~ (yet :: finite~ polari~s~ation:dispersion of one wa~elength difference
25~:::between :the~ two polarisaiion states in the forward directiQn. This is
because one polarisation travels in the fast axis ~top and bottom halves)
: ~ and the other polarisation travels in the slow axis of ths half wave plates.~The -~resultant polarisation~ dispersion is for 1.5 ~m light equal to 5 fs.
: ~` : Although this value is much iower than even the best commercially
~:: ;30 a~railable isolators, it is: possible to design the present isolator to have
intrinsically zero polarisation dispersion. This is done by making the
forward transmissive palh equivalent to a fast axis and a slow axis transit.
;
Wo 93/2047s ~ 5 5 ~ PCI/AU93iO0146
Figure 1t shows hvw this can b~ achieY~d. The top half consists of for
the vertical polarisation.
fast axis: Faraday rotator: slow axis
for th~ hwizon~al pol~fisa~ion
slow axis: Faraday rotator: fast axis
The bottom half consists for both polarisations ot an optical path
length equivalent to fast axis + Faraday rotatQr + slow axis.
There is no intrinsic polarisation disp~rsion.
Figure 12 is equivalQnt to Figure 11 except that the design is
10 entir~ly balanced again.
A simpie opti~al circulatQr using the b~am splitting isolator as
the basis may be readily implernented as shown in Fi~ure 19. In the
isolator the light which is rejected in the non-transmission direction is
; ~ antisymmetric about the~ axis of the interface and so cannot excite th~
15 fundamental mode o~ a single mode fibre. It can however excite the tirst
higher~order mode~of a~ mul~imode fibre 110 which can have the same
symmetry. By using~a ~fibre ~which supports this hi~her otder mode and
then~produGin g a coupler~t~t5 which coup~es the higher order mode to a
se~ond~fibre~ we can~ per{~rm the ~unction of a thre~ or four port circulator.
20 ~ Single~ ~ mode fibres 116 ar~ coupled 114, 115 to multimode fibre t 18,
110.~ ~ ~Lens ~111, 113 and~ phase shifter 1t2 fofm~ a beam-expanded
Isolator~ as descrlbed above.~ Such a coupl~r ~an ~uple light ~rom the
;correct~ symmetry higher~ order~ mode ~o the fund~mental mode of a sin~le
mode fibre, where~ tl~e~ ~ propagation coefficients are matched in the
25~coupling ~region. Ei~her~fused or polished ~coupling techniqu~s can be
used. Note that with the crystal interface in tha parall~l ~xpanded beam,
non-op~imal excitation o f~ the higher order mode is achieved with ths
remaining light lost to the system This can be improved by having the
interfa~e where the beam of light is in transîtion from the near-field to far-
30 field~ image. This is achieved by~focussing ~he light between the lens~s
~ ~ snd placing the non reciprocal phas~ shifting crystals at an appropri~t~
; ~ ~position.
.
WO 93~2047S P~/AU93/0~ .
2 i3~S~
Referring to Figure 13, in th~s impl~m~nta~ion a Mach-Zander
wave guide is mad~ in integr~tsd optics. Thes~ ars commercially
available. A slot is c~tt thr~ugh bo~h ~tms of the MachaZender 113 and
the two halves of the non-r~ciprocal phase shifter t 12 are insert3d into
the slot so that light trQm one arm passes through one half and light
from the second arm passes through th~ second half. Beam ~xpansion
techniques can be used if nacessary:lo reduce the toss due to the non-
wave~uiding propag~tion through thls ragion. As usual a magne~ic field
mus~ b~ supplied around the F~rad~y rotator. This device could be
10 ccmbir~ed with other integrat~d op~ic device~ in a useful fashion, such as
combining it with an inlegrated splitt~r.
Figure 20 illustrates a further Ma~h~;~ender implementation
involving two fibres :using ~used coupler t~chnolo~y. The implemerltation
illustrated insert two fibres 115 Into a glass tube 114 of lower refractive
15 index than th~ fibr~ :cladding Tndex, and tap~r the tuba down in ~w~
closely spaced rsgions 113~ to form two 50:50 couplers. ~th the fibres
115 held firmly: in place ~y the surrounding collapsed glass tubing 114,
: the ~ devi~ :can be cut and ~ polished, ba~ore the n~n-reciprocal phase
shifting elements t12 ~ are placed between the fibre waveguides.
20 ~AIternatl~ely, a s!ot~ l~16 ~can be~ used as for the integrated optic
impiementation :shown:~::; in Figure 13. This implementation has ths
potentiai :to provide ~low~ loss coupling of the light through the non-
reciproca! phase :shifter~ ::: By expanding the ~undamentai mode, the
di~fraction~ eflects: are ~reduced and propagation through non-waveguiding
25~ regions can be achieved with iow loss. Beam expansion is achisved
~: through: using a tapere~ region of the fibre, or by core diffusion
techniques~ :
A single polarisation ~ibre embedded isolator has be0n
described in the :s~ientifi~ literature~ The application ot single polarisation
~30 devices :is, however, extremely limit~d. The davice shown in Figure 15
provides a polarisation~ insensitive isolator. Light frvm a singl~ mode ~ibre
74 is expanded via lens 73 into an expanded beam. Non-reciprocal
:
:: '
wo 93/2047~ 2 1 ~ b PCI/AU93/~0~46
phase shifter 112 is formed from a Faraday ro~ator 7~ in one half of
beam 75 and a pair of half wav~ pla~es 77 in the other half. Lens 71
conveys the light into fibre 7~ will be appreciated that the principle of
operation is analogous to Ihe devi~e of Figuro 6. These could be
5 potentially made into isola~ing connsctors. Such deYices can be sither
wavelength independent or wav~length selective
Another feature which can be incorporated into a isoiator ~f the
split beam typ5 (or a standard isolator for ~hat matter) is filt~ring using
the split beam technique. By splitting the beam with a non-bire~ringent
10 reciproc~l ~lement for instance a non-bire~ring~nt wave plata 117 with
different optical path iengths on both sides a sinusoidal tiltering can be
,
applied. This can be seen in Figure 21. This is particularly U~BfLII in
amplifier applications to equalise the gain over a certain band width. To
combine this with an isoiator of the typa dls~ussed above th~ int~rface of
1:5 th~ reciproçal tiltering elem~nt should be ortho~onal to the interface o~ the
non-reciprocal phase shifter 112. By splitting the beam using ratios other
than 50:50 the required degree of extinctîon can be obtained.
The ability: to casoade these d~Yk:eS to ~orm higher i505ation or
temperature independent features is remarkably simple. Two non~
20 : reciprQcal phase shifters 11 2 1 22 can be cascaded in the same beam
expander by ensuring that the~ interfaces of the devices are perpendicular
to each :other. This is:~ shown in Figure 22. Some tunirlg ot th~
characterîstics of the ;device can also be achiev~d with small variations
from ~9O degree relative: orientation. Two identical devices can be
~ ~ ,
: 25~ : cascaded to increase: ~the peak isolation and isolation bandwidth of thodevice or two devices; with siightly different c~ntral wavelengths can bs
cascaded to achievs a broàder isolation bandwidth~
~Peak isolation wavelength tuning
It is useful in: manufacture to be able to tune the wavelength of
30 peak isolation to be: different to the waYelengths of 45 degree Faraday
rotation. This can be achieved according to the present invention by
choosing the relative angle between the optic axis ~f the half wave plates
:::
'
WO 93~20475 PCI /A U93JI)0 '
2133~56
16
to be equal to the Faraday rotation ang~e at the wavelengths of desired
peak isolation. This featur~ can ba combined with th~ previous ~eature
for broadband isolation, or to achieve hi~h isolation in opposite directions
for wavelengths which ars wsll saparated.
5 Tuning of device
One important practical issue, distinct from the wavelength of
peak isolation, is the tuning ot ~he devîce ~o ensure that the maximum
extlnction occurs at the wavalength(s~ which are re~lJired. This i5
achieved by ehsuring that tha light from sach half of the non-reciprocal
:10 phase element is exac~ly in phase at the wavelen~th(s) of interest for the
transmission direction. The relative phase between each half is most
easily tuned by an angul~r ~wariation ~f the crystals. Where the optical
path length îs different or there is a difference in the angular orientation
of both halves, this: can be ~achieved by rotating the whole device. This
~:15~ should be rotated in a plane such that the interta~e of the crystals
remains perpendicular to ~he beam, for the split beam implementation.
A separate ~uning mechanism is possible~ A small phase shiR
in haîf of the Fourier plane (ie :in th~ expand~d bsam) is equivalent to
:~: ; a lateral displacement in the~direction perpendicular to the interface in the
: 20 image plane :So an alternative is to laterally displace the fibre from the
:: ~
true~ image position to ~tune the phase. This ~uning mechanism can incur
some~ small losses, but :is useful for fine tuning.
Température independence of phase change through material
alancing ~::
:~ 25 : It should be noted that although the balance design has exaGtly
the sama material on~ both: haives of the non-reciprocal phase shifter for
the broad band case, the bi-directional design requires an additional
:: :
:: : optlcal path length, and: so there will be potsntially son ~ temperatura
: dependence to the phase, due to the thermal expansiorl, and therrnal
~; ~: 30 refractive index dependence in this unbalanced portion. At least two
approaches could be taken. One is to uso a material with a very small
temperature dependent phase shifter Another is to use two dNferent
:
: :~
wo ~3~2047s ~ .i 5 b PCr/AUg3/0014~
materials in each haif of the non-reciprocal phase shifter, to produce the
optical path difference. These materials are chosen to have slightly
different thermal coefficients such that the smaller optical length has the
larger therrnal coeffici~n~s. Usîng this m~thod the relative phase can be
5 made to remain essentially cons~ant ov~r a wids tamperature range.
Fibre embeddod optical circulator
Figu~e 17 illustrat~ a circulator using the principle of the
devices described sarlier, for instanca Figure 6, except that the beam
splitting will be done in fibre using an optical coùpl~r. A 90 degree
10 Faraday rotator 88 is embedded in ona arm of the l~lach-Zender and a
90 degree birefringent rotator 88 in the o~her arm (~r other combinations
as described earlier), to form a circulator.
One way to manufacture thls and keap the arm lengths down to
an absolute rninimum is described with ref~rence to Figure 18. A fused
:: : 15 silica V-grove 90, 92 is used to align each of the ~ibres, and a 90
: : desree Faraday rotator 93 is embedded into one fibre 130 half way
along the fused silica and a 90 degree polarisation rotating birefringent
plate 94 is embedded into the other fibr~ 131. The silica V-grooves are
then used to make a polished coupler, by continually polishing unSil 100
20i~ :percent coupling is achieved in the forward direction. This should
correspond to 50% coupling before and after the rotating devices. The
~; ~: device~ is ~ :now identical in operation to that shown in Figure 17, but is
extremely compact and resistant to the environment. The device will
behave as a circulator and is capable of mass pro~uction. This is of
25 cours~ an alternative implementation of the isolator, as all ~irculators are
also isolators. It may be necessary to bury slightly the fibre in the silica
at the point wher~ the cr~stal embedding takes plac~, but this will havo
no reai effect on the operation of the d~vice.
Figure 24 illustrates an amplifier arrangement using an isolator
30 according to the present invention . This arrangernent is low polarisation
dispersive. Pump source 131 and input signal 142 enter wavelength
division muitipiexer 135, pass through low birefringencs erbium doped
.- ~..
WO 93/20475 PCl/AU93/00
3556 1~
~ibre 132, and enter polarisation dispersion isola~or 138 (as described with
referencs to figures 11 and 1 ~ for example). The signal then passes
through low birefringence erbium doped fibre 134, wavelength division
multiplexer 136, and is outpu~ 143. Pump source 137 drives the erbium
5 fibrs amplifier.
Figure 25 i!lus~rat~s a bidirectional ampiifier, of the type which
enables bidirectional communications down a single fibre, utilising the
` ~ same amplifiers for both wavelengths. The arrangement is similar tv figure
24, but incorporates a bidirectional wavel~ngth dependant isolator 133
10 (as described, for example, with refer6nce to tigure 6). The isolator 133 is
positioned between two erbium dop~d fibre amplifiers 132, 134. At t40,
signals at ~ are input, and at ~2 are output. At 141, signais at ~2 are
input, and at ~ are output. Signals counterpropagating to the allowed
directions at the different frequencies, f~r instance at ~l input at .141, are
15 suppressed. Thus, a true bidirectional system is possible, as is a true
bidirectional amplifier, without undesired feedback to the amplifiers
.~
:~ ~ becoming a probiem.:
: ~ :
lt will be appreciated that variations and additions are possible
within the spirit and scope of the invention.
: :
. :
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