U.S. patent application number 12/356854 was filed with the patent office on 2009-05-14 for method and apparatus for writing grating structures using controlled phase delay between beams.
This patent application is currently assigned to THE UNIVERSITY OF SYDNEY. Invention is credited to Mark Sceats, Dmitrii Stepanov.
Application Number | 20090121392 12/356854 |
Document ID | / |
Family ID | 3808063 |
Filed Date | 2009-05-14 |
United States Patent
Application |
20090121392 |
Kind Code |
A1 |
Stepanov; Dmitrii ; et
al. |
May 14, 2009 |
METHOD AND APPARATUS FOR WRITING GRATING STRUCTURES USING
CONTROLLED PHASE DELAY BETWEEN BEAMS
Abstract
A method of writing a grating structure with at least one of
predetermined amplitude, period and phase properties in a
photosensitive waveguide, the method comprising providing at least
two light beams which overlap in an overlap region to form an
interference pattern; moving the photosensitive waveguide through
the overlap region; and modulating the phase of at least one of the
light beams relative to the phase of the other light beams using a
non-mechanical beam modulator so that the interference pattern
appears to move through the overlap region, the apparent movement
being variably controlled in response to the movement of the
photosensitive waveguide such that a grating structure is written
with the at least one of predetermined amplitude, period and phase
properties. The apparent movement of the interference pattern may
be variably controlled to match the movement of the waveguide, or
to be deliberately detuned. The grating structure may be chirped or
apodized.
Inventors: |
Stepanov; Dmitrii; (Croydon
Park, AU) ; Sceats; Mark; (Pyrmont, AU) |
Correspondence
Address: |
FOLEY AND LARDNER LLP;SUITE 500
3000 K STREET NW
WASHINGTON
DC
20007
US
|
Assignee: |
THE UNIVERSITY OF SYDNEY
|
Family ID: |
3808063 |
Appl. No.: |
12/356854 |
Filed: |
January 21, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11361987 |
Feb 27, 2006 |
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12356854 |
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10338884 |
Jan 9, 2003 |
7018745 |
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11361987 |
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09674302 |
Jan 16, 2001 |
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PCT/AU99/00417 |
May 29, 1999 |
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10338884 |
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Current U.S.
Class: |
264/400 ;
425/174.4 |
Current CPC
Class: |
G03H 2001/0482 20130101;
G02B 2006/02157 20130101; G03H 1/0476 20130101; G02B 5/1857
20130101; G02B 6/02133 20130101; G02B 6/02152 20130101; G02B
6/02138 20130101; Y10S 430/146 20130101 |
Class at
Publication: |
264/400 ;
425/174.4 |
International
Class: |
B29C 35/08 20060101
B29C035/08 |
Foreign Application Data
Date |
Code |
Application Number |
May 19, 1998 |
AU |
PP 3816 |
Claims
1. A method of writing a grating structure with at least one of
predetermined amplitude, period and phase properties in a
photosensitive waveguide, the method comprising: providing at least
two light beams which overlap in an overlap region to form an
interference pattern; moving the photosensitive waveguide through
the overlap region; and modulating the phase of at least one of the
light beams relative to the phase of the other light beams using a
non-mechanical beam modulator so that the interference pattern
appears to move through the overlap region, the apparent movement
being variably controlled in response to the movement of the
photosensitive waveguide such that a grating structure is written
with the at least one of predetermined amplitude, period and phase
properties.
2. A method as claimed in claim 1, further comprising sensing the
movement of the photosensitive waveguide relative to the apparent
movement of the interference pattern and variably controlling the
apparent movement of the interference pattern in response to the
sensed movement of the photosensitive waveguide.
3. A method as claimed in claim 1, in which the apparent movement
of the interference pattern is variably controlled to be
substantially the same as the movement of the photosensitive
waveguide over at least a portion of the grating structure.
4. A method as claimed in claim 1, in which the apparent movement
of the interference pattern is variably controlled to be different
to the movement of the photosensitive waveguide over at least a
portion of the grating structure.
5. A method as claimed in claim 1, in which the apparent movement
of the interference pattern is variably controlled to be different
to the movement of the photosensitive waveguide over at least a
portion of the grating structure and to be substantially the same
as the movement of the photosensitive waveguide over at least
another portion of the grating structure.
6. A method as claimed in claim 2, in which the sensing and the
variably controlling comprises a feedback loop for improving noise
properties of the grating structure.
7. A method as claimed in claim 1, in which the at least two light
beams are provided by splitting a single coherent light beam and
directing the at least two light beams to overlap in the overlap
region.
8. A method as claimed in claim 7, in which the modulation of the
phase of the at least one of the light beams is performed before
the splitting of the single coherent light beam.
9. A method as claimed in claim 7, in which the modulation of the
phase of the at least one of the light beams is performed after the
splitting of the single coherent light beam.
10. A method as claimed in claim 7, in which the modulation of the
phase of the at least one of the light beams is performed during
the splitting of the single coherent light beam.
11. A method as claimed in claim 1, in which the non-mechanical
beam modulator comprises at least one of a group selected from: an
electro-optic phase modulator; a magneto-optic phase modulator; a
frequency shifter; an acousto-optic frequency shifter; a
controllable optical retarder; and an optical delay line.
12. A method as claimed in claim 1, in which: two of the light
beams have substantially orthogonal polarization states; the
non-mechanical beam modulator modulates the phase of at least one
of the polarization states relative to the other polarization
states; and the method further comprises aligning the polarization
states of the light beams after the modulation to form the
interference pattern.
13. A method as claimed in claim 1, in which the grating structure
is written during a single continuous writing process.
14. A method as claimed in claim 1, in which at least a portion of
the grating structure includes at least one of an apodized
structure and a chirped structure.
15. An apparatus for variably controlling the apparent movement of
an interference pattern with reference to a moving photosensitive
waveguide to write a grating structure in the photosensitive
waveguide, the apparatus comprising: at least one light beam source
configured to provide at least two light beams; a beam director
configured to direct at least one of the light beams so that the
light beams overlap in an overlap region to form the interference
pattern; at least one non-mechanical beam modulator configured to
modulate the phase of at least one of the light beams relative to
the phase of the other light beams so that the interference pattern
appears to move through the overlap region; and a beam modulator
controller configured to control the modulation of the beam
modulator so that the apparent movement of the interference pattern
is variably controlled in response to the movement of the
photosensitive waveguide.
16. An apparatus as claimed in claim 15, the apparatus further
comprising a waveguide mover configured to move the photosensitive
waveguide through the overlap region.
17. An apparatus as claimed in claim 15, the apparatus further
comprising a position sensor configured to sense the position of
the interference pattern relative to the photosensitive
waveguide.
18. An apparatus as claimed in claim 15, in which the beam
modulator controller is configured to variably control the apparent
movement of the interference pattern to be substantially the same
as the movement of the photosensitive waveguide over at least a
portion of the grating structure.
19. An apparatus as claimed in claim 15, in which the beam
modulator controller is configured to variably control the apparent
movement of the interference pattern to be different to the
movement of the photosensitive waveguide over at least a portion of
the grating structure.
20. An apparatus as claimed in claim 17, in which the beam
modulator controller includes a feedback arrangement configured to
improve the noise properties of the grating structure by
controlling the modulation of the at least one non-mechanical beam
modulator in response to the movement of the photosensitive
waveguide, the feedback arrangement being in communication with the
position sensor.
21. An apparatus as claimed in claim 15, in which the at least one
beam source includes a beam splitter configured to split a single
coherent light beam to form the at least two light beams.
22. An apparatus as claimed in claim 17, in which the at least one
beam splitter and the at least one non-mechanical beam modulator
are arranged so that, in use, the modulation of the phase of the at
least one of the light beams is performed after the splitting of
the single coherent light beam.
23. An apparatus as claimed in claim 17, in which the at least one
beam splitter and the at least one non-mechanical beam modulator
are arranged so that, in use, the modulation of the phase of the at
least one of the light beams is performed before the splitting of
the single coherent light beam.
24. An apparatus as claimed in claim 17, in which the at least one
beam splitter and the at least one non-mechanical beam modulator
are arranged so that, in use, the modulation of the phase of the at
least one of the light beams is performed during the splitting of
the single coherent light beam.
25. An apparatus as claimed in claim 15, in which: the at least one
beam source includes a polarizer configured to polarize a single
coherent light beam into a linear superposition of at least two
polarisation states, wherein the at least one non-mechanical beam
modulator is arranged to modulate the phase of at least one of the
at least two polarization states relative to the other polarization
states; and a polarization beam splitter configured to split the
linear superposition of the at least two polarization states into
the at least two light beams after the modulation; and the
apparatus further comprises polarization manipulation element
configured to align the polarization states of the at least two
light beams after the splitting.
26. An apparatus as claimed in claim 15, in which the
non-mechanical beam modulator comprises at least one of: an
electro-optic phase modulator; a magneto-optic phase modulator; a
frequency shifter; an acousto-optic frequency shifter; a
controllable optical retarder; and an optical delay line.
Description
RELATED APPLICATIONS
[0001] This application is a Continuation application of U.S.
patent application Ser. No. 11/361,987 filed Feb. 27, 2006, which
is a Continuation application of U.S. application Ser. No.
10/338,884, filed Jan. 9, 2003 now U.S. Pat. No. 7,018,745, which
is a Continuation of U.S. application Ser. No. 09/674,302, filed
Jan. 16, 2001, which is the U.S. National Stage of PCT/AU99/00417
filed May 29, 1999, which claims priority from Australian
Application No. PP3816 filed May 19, 1998. The subject matter of
which are incorporated fully by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to the field of constructing
Bragg gratings or the like in UV or like photosensitive waveguides
utilizing a UV or like interference pattern.
BACKGROUND OF THE INVENTION
[0003] The present invention is directed to writing gratings or
other structures in a photosensitive optical waveguide. The
creation of a grating utilizing the interference pattern from two
interfering coherent UV beams is well known. This technique for
construction of Bragg gratings is fully described in U.S. Pat. No.
4,725,110 issued to W H Glenn et al and U.S. Pat. No. 4,807,950
issued to W H Glenn et al.
[0004] Bragg grating structures have become increasingly useful and
the demand for longer and longer grating structures having higher
and higher quality properties has lead to the general need to
create improved grating structures.
SUMMARY OF THE INVENTION
[0005] In accordance with a first aspect of the present invention
there is provided a method of writing an extended grating structure
in a photosensitive waveguide comprising the steps of utilising at
least two overlapping beams of light to form an interference
pattern, moving the waveguide through said overlapping beams,
simultaneously controlling a relative phase delay between the beams
utilising a phase modulator, thereby controlling the positions of
maxima within said interference pattern to move at approximately
the same velocity as the photosensitive waveguide, wherein the
phase modulator does not comprise a mechanical means for effecting
the phase modulation, and modifying the relative phase delay
between the beams during the writing of the grating structure,
whereby a deliberate detuning of the velocity of the positions of
maxima within said interference pattern and the velocity of the
photosensitive waveguide is utilised to vary a period of the
written grating structure in the photosensitive waveguide.
[0006] Preferably, the at least two overlapping beams are formed by
the splitting of a single coherent beam of light.
[0007] In one embodiment, the steps of controlling and modifying of
the relative phase delay is performed before the splitting of the
single coherent beam.
[0008] In one embodiment, the steps of controlling and modifying of
the relative phase delay may be performed after the splitting of
the single coherent beam.
[0009] In one embodiment, the steps of controlling and modifying of
the relative phase delay is performed prior to the splitting of the
single coherent beam.
[0010] Said modulator may comprise one or more of a group
comprising an electro-optic phase modulator, a magneto-optic phase
modulator, a frequency shifter, an acousto-optic frequency shifter,
a controllable optical retarder, and an optical delay line.
[0011] In one embodiment, the method further comprises, after the
splitting of the single coherent beam, the step of reflecting said
beams at a series of reflection elements for effecting the
overlapping of the beams to form the interference pattern.
[0012] The method may further comprise utilising a feedback loop in
controlling and modifying of the phase delay to improve the noise
properties of the grating structure. The feedback loop comprises an
opto-electronic feedback loop.
[0013] The grating structure may comprise a chirped grating and/or
an apodized grating.
[0014] The grating structure may have one or more of a group
comprising a predetermined strength profile, a predetermined period
profile, and a predetermined phase profile.
[0015] In one embodiment, the two beams have substantially
orthogonal polarization states and wherein the modulator modulates
the relative phase delay between the polarization states and
wherein the method further comprises the step of aligning the
polarization states of the beams subsequent to modulating the
relative phase delay for forming the interference pattern.
[0016] The two beams having the substantially orthogonal
polarization states may initially from a single beam of light, and
a polarization splitter element is utilised to separate the two
beams from each other. The modulator may modulate the relative
phase delay between the polarisation states in the single beam.
[0017] According to one embodiment, there is provided a method of
writing a grating structure with at least one of predetermined
amplitude, period and phase properties in a photosensitive
waveguide. The method comprises: providing at least two light beams
which overlap in an overlap region to form an interference pattern;
moving the photosensitive waveguide through the overlap region; and
modulating the phase of at least one of the light beams relative to
the phase of the other light beams using a non-mechanical beam
modulator so that the interference pattern appears to move through
the overlap region, the apparent movement being variably controlled
in response to the movement of the photosensitive waveguide such
that a grating structure is written with the at least one of
predetermined amplitude, period and phase properties.
[0018] In accordance with a second aspect of the present invention,
there is provided a device for writing an extended grating
structure in a photosensitive waveguide comprising an
interferometer arranged to form an interference pattern utilising
at least two overlapping beams of light; a phase modulator for
controlling a relative phase delay between the beams whereby, in
use, the positions of maxima within said interference pattern are
controlled to move at approximately the same velocity as the
photosensitive waveguide moving through said overlapping beams,
wherein the phase modulator does not comprise a mechanical means
for effecting the phase modulation, and wherein the phase modulator
is arranged, in use, to modify the relative phase delay between the
beams during the writing of the grating structure, whereby a
deliberate detuning of the velocity of the positions of maxima
within said interference pattern and the velocity of the
photosensitive waveguide is utilised to vary a period of the
written grating structure in the photosensitive waveguide.
[0019] Preferably, the device comprises a beam splitter element for
splitting of a single coherent beam of light into said at least two
overlapping beams.
[0020] In one embodiment, the device is arranged, in use, such that
the controlling and modifying of the relative phase delay is
performed before the splitting of the single coherent beam.
[0021] In one embodiment, the device is arranged, in use, such that
the controlling and modifying of the relative phase delay is
performed after the splitting of the single coherent beam.
[0022] In one embodiment, the device is arranged, in use, such that
the controlling and modifying of the relative phase delay is
performed prior to the splitting of the single coherent beam.
[0023] Said modulator may comprise one or more of a group
comprising an electro-optic phase modulator, a magneto-optic phase
modulator, a frequency shifter, an acousto-optic frequency shifter,
a controllable optical retarder, and an optical delay line.
[0024] In one embodiment, the device further comprises a series of
optical reflection elements for effecting the overlapping of the
beams to form the interference pattern.
[0025] The device may further comprise a feedback unit for
facilitating the controlling and modifying of the phase delay to
improve the noise properties of the grating structure. The feedback
unit may comprise an opto-electronic feedback loop.
[0026] In one embodiment, the two beams have substantially
orthogonal polarization states and the modulator is arranged, in
use, to modulate the relative phase delay between the polarization
states and wherein the device further comprises a polarisation
manipulation element for aligning the polarization states of the
beams subsequent to modulating the relative phase delay for forming
the interference pattern.
[0027] The two beams having the substantially orthogonal
polarization states may initially from a single beam of light, and
the device compresses a polarization splitter element for
separating the two beams from each other.
[0028] The modulator may be arranged, in use, to modulate the
relative phase delay between the polarisation states in the single
beam.
[0029] According to one embodiment, there is provided an apparatus
for variably controlling the apparent movement of an interference
pattern with reference to a moving photosensitive waveguide to
write a grating structure in the photosensitive waveguide. The
apparatus comprises: at least one light beam source for providing
at least two light beams; a beam director for directing at least
one of the light beams so that the light beams overlap in an
overlap region to form the interference pattern; at least one
non-mechanical beam modulator for modulating the phase of at least
one of the light beams relative to the phase of the other light
beams so that the interference pattern appears to move through the
overlap region; and a beam modulator controller for controlling the
modulation of the beam modulator so that the apparent movement of
the interference pattern is variably controlled in response to the
movement of the photosensitive waveguide.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] Notwithstanding any other forms which may fall within the
scope of the present invention, preferred forms of the invention
will now be described, by way of example only, with reference to
the accompanying drawings in which:
[0031] FIG. 1 illustrates schematically a first embodiment of the
present invention;
[0032] FIG. 2 illustrates one form of driving of the electro-optic
modulator in accordance with the principles of the present
invention;
[0033] FIG. 3 illustrates an alternative embodiment of the present
invention; and
[0034] FIG. 4 illustrates a further alternative embodiment of the
present invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0035] Turning initially to FIG. 1, there is illustrated the
arrangement 1 of a preferred embodiment which is similar to the
aforementioned arrangement of Glenn et al with the additional
inclusion of an optical phase modulating element 2. The basic
operation of the arrangement of FIG. 1 is that a UV source 3
undergoes beam splitting by beamsplitter 4 so as to form two
coherent beams 5, 6. A phase mask placed appropriately into a setup
can be used to split the beam. Each beam is reflected by a suitably
positioned mirror e.g. 7, 8 so that the beams interfere in the
region 9. In this region, there is placed a photosensitive optical
waveguide 10 on which an extended grating structure is to be
written. The essence of the preferred embodiment is to utilize the
phase modulator 2 so as to modulate the relative phase difference
between the two beams 5, 6 at the point of interference 9 such that
the interference pattern remains static in the reference frame of
the optical waveguide 10 as the waveguide is moved generally in the
direction 12. The phase modulator 2 can be an electro-optic
modulator of a known type including an ADP, KD*P, BBO crystal type
transparent at the UV source wavelength. Suitable electro-optic
crystals are available from many optical components manufacturers
including Leysop Limited under the model numbers EM200A and EM200K.
The modulator operates so as to provide for a controlled phase
delay of the beam 5 relative to the beam 6. In a first example, the
control is achieved by setting the level of an input signal given
the fibre 10 is moving at a constant velocity. The input signal in
this case can comprise a saw tooth wave form as illustrated in FIG.
2, the maximum saw tooth magnitude being set to be equivalent to a
2.pi. phase delay. The slope of the saw tooth wave form is set so
as to closely match the velocity of the changing maxima of the
interference pattern to that of the fibre 10.
[0036] Hence, prior known mechanical methods of movement of the
portion of the apparatus is dispensed with and long or stitched
interference patterns can be obtained through the utilization of
the phase modulating device 2 to introduce the required optical
phase difference between the interfering UV beams 5 and 6. AS the
phase is invariant with respect to a 2.pi. change, there is no need
to introduce large phase differences thus limiting the required
amplitude of the phase change to 2.pi. and allowing it to operate
near the balance point of the interferometer. The electro-optically
induced phase change will make the interference pattern move along
the fibre as the fibre itself moves and the direction and velocity
of the move can be set in accordance with requirements. The saw
tooth wave form achieving the effect of "running lights".
[0037] Electro-optic modulators such as those aforementioned can
operate with very low response time and extremely high cut off
frequencies. Hence, the saw tooth edge fall can be practically
invisible and a near perfect stitch can be achieved. At 6 mm per
minute scanning speed, the modulation frequency can be about 200
Hz.
[0038] Further, by applying a differential velocity between the
fibre and the pattern or through appropriate control of the phase
delay, a wavelength shift with respect to the static case can be
obtained. An acceleration or appropriate control of the phase delay
can be used to produce a chirp and so on. Apodisation can also be
provided by proper additional modulation of the electro-optic
modulator.
[0039] The embodiment described has an advantage of having all
optical elements static except for the moving fibre. Therefore, it
allow for focussing of the interfering beams tightly onto the fibre
and achieving spatial resolution reaching fundamental limits (of
the order of the UV writing wavelength, the practical limit being
the fibre core diameter). The static interferometer arrangement
itself leads to reduced phase and amplitude noise of the
interference pattern. Additionally, the ability to control the
phase and amplitude of the pattern using a feedback loop provides a
means to improve the noise properties of the interferometer
substantially.
[0040] A number of further refinements are possible. For example,
in order to accurately match the velocity of the fibre 10 and the
electro-optic modulator frequency, a simple scanning Fabry-Perot
interferometeric sensor can be used to measure the relative
positions of the fibre and the interference pattern 9. A high
finesse (F) resonator can be used to achieve the accuracy of
distance measurements much better than the wavelength of the narrow
line width source which would be employed in the sensor.
[0041] By scanning the Fabry-Perot at a constant rate or sweeping
the laser frequency the position can be precisely (1/2F)
determined. To increase the resolution further a conversion of the
interferometer into a laser at threshold may be needed. In this
case the finesse F of the cavity is close to infinity and the
resolution is enhanced. Other types of interferometric sensors such
as a Michelson interferometer can be used to accurately determine
the fibre position with respect to the interference pattern.
[0042] Of course, other arrangements utilizing this principle are
possible. For example, the teachings of PCT patent application no.
PCT/AU96/00782 by Ouellette et al discloses an improved low noise
sensitivity interferometric arrangement which operates on a "Sagnac
loop" type arrangement. Turning now to FIG. 3 there is illustrated
a modified form of the Ouellette arrangement to incorporate the
principles of the present invention. In this modified form, an
initial input UV beam 20 is diffracted by phase mask 21 so as to
produce two output beams 22, 23. The beam 23 is reflected by
mirrors 24, 25 so as to fall upon the fibre 26 in the area 27.
Similarly, beam 22 is reflected by mirror 25 and mirror 24 before
passing through an electro-optic modulator 28 which modifies the
phase of the beam relative to the beam 23. The two beams interfere
in the area 27. The phase of the interference patterns can be
controlled by the modulator 28 in the same manner as the
aforementioned. In this manner, the advantages of the previous
Ouellette arrangement can be utilized in a stable mechanical
arrangement in that it is not necessary to sweep the beam across
the phase mask 21 or perform any other movements other than the
electrical modulation of the modulator element 28 whilst forming an
extended grating structure. Moreover, the interferometer can be
adjusted to operate near its balance point and a low coherence
length UV source can be used in the arrangement.
[0043] Further, a phase modulator based on a magneto-optic effect
could be used instead of an electro-optic modulator. In the Sagnac
interferometer arrangement, it can be placed such that both of the
interfering beams pass the Faraday cell in opposite directions such
that a non-reciprocal controlled relative phase delay is introduced
between the counter propagating beams.
[0044] Turning now to FIG. 4 there is illustrated an alternative
arrangement to incorporate the principles of the present invention.
In this arrangement, the output from a UV laser 30 is initially
linearly polarized 31 before passing through an electro-optic
modulator 32 which modifies the polarization state of the beam. The
polarization plane of the UV beam with respect to the birefringent
axes of the electro-optic modulator 32 is such that two orthogonal
polarization eigenstates with equal intensities propagate in the
modulator, with one of the eigenstates being phase modulated while
the other one being not. The arrangement uses polarization beam
splitter 33 to separate the polarization states and half-wave plate
34 is used to 90 degree rotate the polarization of one of the
resulting beams to allow for the interference taking place between
the beams. The beams are further reflected by mirrors 36 and 37 so
as to fall upon the fibre 38 in the area 39 to produce an
interference pattern in conjunction with movement of the fibre 38.
The phase of the interference pattern can be controlled by the
modulator 32 in the same manner as the aforementioned to produce an
extended grating structure.
[0045] In a further alternative embodiment, a travelling wave
acousto-optic (AO) modulator transparent at the wavelength of the
UV source 3 can be used as a modulating element 2 to frequency
shift the diffracted light. The interference between the two beams
at different frequencies in region 9 will result in a interference
pattern travelling at a velocity v=.DELTA..nu..LAMBDA./2. For
example, for .DELTA..nu.=200 Hz frequency shift and .LAMBDA./2=1
.mu.m interference pattern period the velocity of the pattern is
v=6 mm/min and the optical waveguide 10 should be translated at
this speed in the same direction. No special modulation waveforms
need to be applied in this case, with the control parameter being
the frequency shift. As most commercial acousto-optic modulators
operate in a MHz range, a frequency shift of the second interfering
beam may be required to achieve the differential frequency shift in
the Hz-kHz range. There may be also need for a minor adjustment
compared to the electro-optic modulator arrangement of FIG. 2 as
the Bragg angle will vary with the frequency of the applied to the
AO modulator signal resulting in a displacement of the diffracted
beam. However the effect of this displacement can be reduced by
making the setup compact. There could also be a further adjustment
since AO modulators may exhibit resonances.
[0046] In a modified embodiment, an optical phase mask, optical
wedge or an optical waveplate can be utilized. The optical phase
mask can also have a function of the beamsplitter. The embodiment
utilizing the phase mask works for all known phase-mask based
interferometer arrangements, such as phase mask direct writing
technique, or for a Sagnac interferometer writing technique (such
as that due to Ouellette disclosed on PCT application number
PCT/AU96/00782) or when utilizing the aforementioned system due to
Glenn et al.
[0047] It would be appreciated by a person skilled in the art that
numerous variations and/or modifications may be made to the present
invention as shown in the specific embodiments without departing
from the spirit or scope of the invention as broadly described. The
present embodiments are, therefore, to be considered in all
respects to be illustrative and not restrictive.
* * * * *