U.S. patent application number 09/242720 was filed with the patent office on 2002-04-18 for fabricating optical waveguide gratings.
Invention is credited to COLE, MARTIN, LAMING, RICHARD IAN.
Application Number | 20020044358 09/242720 |
Document ID | / |
Family ID | 10798853 |
Filed Date | 2002-04-18 |
United States Patent
Application |
20020044358 |
Kind Code |
A1 |
LAMING, RICHARD IAN ; et
al. |
April 18, 2002 |
FABRICATING OPTICAL WAVEGUIDE GRATINGS
Abstract
A method of fabricating an optical waveguide grating having a
plurality of grating lines of refractive index variation comprises
the steps of: (i) repeatedly exposing a spatially periodic writing,
light pattern onto a photosensitive optical waveguide, and (ii)
moving the writing light pattern and/or the waveguide between
successive exposures of the writing light pattern, so that each of
at least a majority of the grating lines is generated by at least
two exposures to different respective regions of the writing light
pattern.
Inventors: |
LAMING, RICHARD IAN;
(EDINBURGH, GB) ; COLE, MARTIN; (DELRAY,
FL) |
Correspondence
Address: |
FINNEGAN HENDERSON FARABOW
GARRETT & DUNNER
1300 I STREET NW
WASHINGTON
DC
20005
|
Family ID: |
10798853 |
Appl. No.: |
09/242720 |
Filed: |
January 13, 2000 |
PCT Filed: |
August 4, 1997 |
PCT NO: |
PCT/GB97/02099 |
Current U.S.
Class: |
359/570 |
Current CPC
Class: |
G02B 6/02152 20130101;
Y10S 359/90 20130101; G02B 6/02138 20130101 |
Class at
Publication: |
359/570 |
International
Class: |
G02B 005/18 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 23, 1996 |
GB |
9617688.8 |
Claims
1. A method of fabricating an optical waveguide grating having a
plurality of grating lines of refractive index variation, the
method comprising the steps of: (i) repeatedly exposing a spatially
periodic writing light pattern onto a photosensitive optical
waveguide (10); and (ii) moving (20) the writing light pattern
and/or the waveguide (10) between successive exposures or groups of
exposures of the writing light pattern, characterised in that the
successive exposures or groups of exposures overlap so that each of
at least a majority of the grating lines is generated by at least
two exposures to different respective regions of the writing light
pattern.
2. A method according to claim 1, in which step (i) comprises
moving (20) the writing light pattern and/or the waveguide (10)
between exposures a by a distance, in a substantially longitudinal
waveguide direction, substantially equal to an integral number of
spatial periods of the writing light pattern.
3. A method according to claim 2, in which step (i) comprises
moving (20) the writing light pattern and/or the waveguide (10)
between exposures a by a distance, in a substantially longitudinal
waveguide direction, substantially equal to one spatial period of
the writing light pattern.
4. A method according to any one of claims 1 to 3, in which step
(ii) comprises: detecting (55) the relative position of the writing
light pattern and the waveguide (10); comparing the detected
relative position to predetermined switching positions related to
the spatial period of the writing light pattern; and controlling
exposure of the writing light pattern in response to that
comparison.
5. A method according to any one of the preceding claims, in which:
the writing light pattern is generated from one or more source
light beams (40); and exposure of the writing light pattern is
controlled by directing the one or more source light beams through
one or more optical modulators (50).
6. A method according to claim 5, in which the writing light
pattern is generated by directing the source light beam through a
phase mask (30).
7. A method according to claim 5 or claim 6, in which the one or
more source light beams are substantially continuously generated
(CW) light beams (40).
8. A method according to any one of the preceding claims, in which
step (i) comprises moving the writing light pattern and/or the
waveguide (10) at a substantially uniform relative velocity.
9. A method according to claim 8, in which step (i) comprises
substantially periodically exposing the writing light beam onto the
waveguide (10), the exposures having a substantially constant
temporal duty cycle.
10. A method according to claim 9, in which step (i) comprises
varying the time at which each exposure of the writing light beam
is made to vary the spatial alignment along the waveguide (10) of
successive exposures, thereby varying the contrast of grating lines
generated by those exposures.
11. A method according to any one of the preceding claims,
comprising varying the spatial period of the writing light beam
during fabrication of the grating.
12. A method according to claim 6 and claim 11, comprising
directing the source light beam onto different regions of a chirped
phase mask (30) in order to vary the spatial period of the writing
light beam during fabrication of the grating.
13. A method according to any one of the preceding claims, in which
the waveguide (10) is an optical fibre.
14. Apparatus for fabricating an optical fibre grating having a
plurality of grating lines of refractive index variation, the
apparatus comprising: a writing light beam source (40) for
repeatedly exposing a spatially periodic writing light pattern onto
a photosensitive optical waveguide (10); and means for moving the
writing light pattern and/or the waveguide (10) between successive
exposures or groups of exposures of the writing light pattern,
characterised in that the successive exposures or groups of
exposures overlap so that each of at least a majority of the
grating lines is generated by at least two exposures to different
respective regions of the writing light pattern.
Description
[0001] Dispersion compensation is an attractive technique allowing
the upgrade of the existing installed standard fibre network to
operation at 1.5 .mu.m where it exhibits a dispersion of
.about.(about) 17 ps/nm.km which would otherwise prohibit high
capacity (eg. 10 Gbit/s) data transmission.
[0002] Chirped fibre gratings are currently the most attractive
technique for fibre dispersion compensation [1]. This is because
they are generally low loss, compact, polarisation insensitive
devices which do not tend to suffer from optical non-linearity
which is the case with the main competing technology, dispersion
compensating fibre.
[0003] For present practical applications chirped gratings must
exhibit both high dispersion, .about.1700 ps/nm, sufficient to
compensate the dispersion of around 100 km of standard fibre at a
wavelength of 1.55 .mu.m, and a bandwidth of around 5 nm. This
implies a need for a chirped grating of length 1 m.
[0004] Fibre gratings are generally created by exposing the core of
an optical fibre to a periodic UV intensity pattern [2]. This is
typically established using either an interferometer or a phase
mask [3]. To date, phase masks are the preferred approach owing to
the stability of the interference pattern that they produce. The
length of the grating can be increased by placing the fibre behind
the phase mask and scanning the UV beam along it. Techniques for
post chirping a linear grating after fabrication include applying
either a strain [1] or temperature gradient [4] to it. However
these techniques are limited due to the length of the initial
grating (.about.10 cm with available phase masks) and the length
over which a linear temperature or strain gradient can be applied.
Alternatively more complex step chirped phase masks can be employed
[5]. However, all of these techniques are currently limited to a
grating length of about 10 cm.
[0005] In addition to chirping the grating, it is also sometimes
desirable to be able to apodise (window) the gratings to reduce
multiple reflections within them and to improve the linearity of
the time delay characteristics. A powerful technique has been
developed which allows chirped and apodised gratings to be written
directly in a fibre, referred to as "the moving fibre/phase mask
scanning beam technique" [6]. This technique is based on inducing
phase shifts between the phase mask and the fibre as the phase mask
and fibre are scanned with the UV beam. Apodisation is achieved by
dithering the relative phase between the two at the edges of the
grating. Like all the previous techniques the one draw back with
this technique is that it is again limited to gratings the length
of available phase masks, .about.10 cm at present.
[0006] This problem has been overcome in one approach by Kashyap et
al using several 10 cm step-chirped phase masks [5]. These are
scanned in series to obtain a longer grating. The phase "glitch" or
discontinuity between the sections is subsequently UV "trimmed" to
minimise its impact. However this is a time consuming and costly
process. In addition the effect of the UV trimming will vary with
grating ageing.
[0007] A technique for potentially writing longer gratings has been
reported by Stubbe et al [7]. In this case a fibre is mounted on an
air-bearing stage and continuously moved behind a stationary
grating writing interferometer. The position of the fibre is
continuously monitored with a linear interferometer. The UV laser
is pulsed to write groups of grating lines with period defined by
the writing interferometer. A long grating can be written by
writing several groups of grating lines in a linearly adjacent
series, with controlled phase between the sections. The phase shift
between each group of grating lines is controlled via the linear
interferometer and a computer which sets the time the laser pulses.
A short pulse, .about.10 ns, is required such that the position of
the writing lines is effectively stationary and accurately
controlled with respect to fibre motion. Having said this, however,
jitter in the pulse timing and in the linear interferometer
position will give detrimental random phase errors in the grating.
Chirped gratings can potentially be fabricated by continuously
introducing phase shifts between adjacent groups along the grating.
Obviously the maximum translation speed is limited by the number of
grating lines written with one laser pulse and the maximum
repetition rate of the pulsed laser. It is also proposed in this
paper that apodisation is achieved by multiple writing scans of the
grating.
[0008] This invention provides a method of fabricating an optical
waveguide grating having a plurality of grating lines of refractive
index variation, the method comprising the steps of:
[0009] (i) repeatedly exposing a spatially periodic writing light
pattern onto a photosensitive optical waveguide; and
[0010] (ii) moving the writing light pattern and/or the waveguide
between successive exposures or groups of exposures of the writing
light pattern, characterised in that the successive exposures or
groups of exposures overlap so that each of at least a majority of
the grating lines is generated by at least two exposures to
different respective regions of the writing light pattern.
[0011] Embodiments of the invention provide a number of advantages
over previous techniques:
[0012] 1. The realisation that the laser does not have to be pulsed
but just has to be on for a particular duty cycle--preferably less
than 50% of the period. This allows an externally modulated CW
(continuous wave) laser to be used.
[0013] 2. With this technique the grating lines are re-written by
several successive exposures of the writing light beam at every
grating period (or integral number of grating periods). Thus the
footprint defined by the writing light beam is significantly
overlapped with the previous lines. Significant averaging of the
writing process is achieved thus improving the effective accuracy
and resolution of the system, compared to that of [7] where a group
of lines is written in a single exposure, and the fibre is then
advanced to a fresh portion where a further group of lines is
written in a single exposure.
[0014] 3. Effectively controlling the grating writing process on a
line-by-line basis allows accurate apodisation to be achieved. This
may be performed in embodiments of the invention by dithering the
grating writing interferometer position in the fibre to wash out or
attenuate the grating strength whilst keeping the average index
change constant.
[0015] 4. The technique offers the further advantage that the CW
laser may be extremely stable, whereas pulsed lasers (e.g. those
used in [7]) may suffer from pulse-to-pulse instability which is
not averaged. In addition the high peak powers of the pulsed laser
may cause non-linear grating writing effects.
[0016] 5 Arbitrary phase profiles and in particular a linear chirp
can be built up by inducing phase shifts electronically along the
grating as it grows. In a similar manner to the "Moving fibre/phase
mask" technique [6] the maximum wavelength is inversely
proportional to the beam diameter. This can be further improved in
particular embodiments of the invention by incorporating a short,
linearly chirped phase mask. Thus as the fibre is scanned the UV
beam may be also slowly scanned across the phase mask, an
additional small phase shift is induced, whilst most significantly
we have access to writing lines of a different period allowing
larger chirps to be built up.
[0017] This invention also provides apparatus for fabricating an
optical fibre grating having a plurality of grating lines of
refractive index variation, the apparatus comprising:
[0018] a writing light beam source for repeatedly exposing a
spatially periodic writing light pattern onto a photosensitive
optical waveguide; and
[0019] means for moving the writing light pattern and/or the
waveguide between successive exposures or groups of exposures of
the writing light pattern, characterised in that
[0020] the successive exposures or groups of exposures overlap so
that each of at least a majority of the grating lines is generated
by at least two exposures to different respective regions of the
writing light pattern.
[0021] The various sub-features defined here are equally applicable
to each aspect of the present invention.
[0022] The invention will now be described by way of example with
reference to the accompanying drawings, throughout which like parts
are referred to by like references, and in which:
[0023] FIG. 1 is a schematic diacram of a fibre grating fabrication
apparatus;
[0024] FIGS. 2a to 2c are schematic diagrams showing a grating
fabrication process by repeated exposures;
[0025] FIGS. 3a and 3b are schematic timing diagrams showing the
modulation of a UV beam; and
[0026] FIGS. 4a and 4b are schematic graphs characterising a 20 cm
grating produced by the apparatus of FIG. 1.
[0027] FIG. 1 is a schematic diagram of a fibre grating fabrication
apparatus. An optical fibre (e.g. a single mode photorefractive
fibre) 10 is mounted on a crossed roller bearing translation stage
20 (such as a Newport PMLW160001) which allows for a continuous
scan over 40 cm. The fibre 10 is positioned behind a short
(.about.5 mm) phase mask 30 (e.g a mask available from either QPS
or Lasiris).
[0028] The fibre is continuously and steadily linearly translated
or scanned in a substantially longitudinal fibre direction during
the grating exposure process.
[0029] Ultraviolet (UV) light at a wavelength of 244 nm from a
Coherent FRED laser 40 is directed to the fibre/phase mask via an
acoustic-optic modulator 50 (e.g. a Gooch & Housego,
M110-4(BR)) operating on the first order.
[0030] The relative position of the fibre to the interference
pattern of the phase mask is continuously monitored with a Zygo,
ZMI1000 differential interferometer 55. The interferometer
continuously outputs a 32-bit number (a position value) which gives
the relative position with a .about.1.24 nm resolution. This output
position value is compared by a controller 70 with switching
position data output from a fast computer 60 (e.g. an HP Vectra
series 4 5/166 with National Instruments AT-DIO-32F) in order that
the controller can determine whether the UV beam should be on or
off at that position. Whether the UV beam is in fact on or off at
any time is dependent on the state of a modulation control signal
generated by the controller 70 and used to control the
acousto-optic modulator 50.
[0031] So, as each position value is output by the interferometer,
the controller 70 compares that position value with the switching
position data currently output by the computer 60. If, for
illustration, the interferometer is arranged so that the position
values numerically increase as the fibre scan proceeds, then the
controller 70 detects when the position value becomes greater than
or equal to the current switching position data received from the
computer 60. When that condition is satisfied, the controller 70
toggles the state of the modulation control signal, i.e. from "off"
to "on" or vice-versa. At the same time, the controller 70 sends a
signal back to the computer 60 requesting the next switching
position data corresponding to the next switching position.
[0032] If the fibre was scanned with the UV beam continuously
directed onto the fibre, no grating would be written since the
grating lines would be washed out by the movement.
[0033] However if the UV beam is strobed or modulated (under
control of the switching position data generated by the computer
60) with a time period matching or close to: 1 phase mask projected
fringe pitch fibre translation speed
[0034] then a long grating would grow.
[0035] This expression is based on a time period of a temporally
regular modulation of the UV beam, and so assumes that the fibre is
translated at a constant velocity by the translation stage.
However, more generally, the switching on and off of the UV beam is
in fact related to the longitudinal position of the fibre, so that
in order to generate a grating the UV beam should be turned on and
off as the fibre is translated to align the interference pattern
arising from successive exposures through the phase mask.
[0036] FIGS. 2a to 2c are schematic diagrams showing a grating
fabrication process by repeated exposures of the fibre to the UV
beam.
[0037] In FIG. 2a, the UV beam from the acousto-optic modulator 50
passes through the phase mask 30 to impinge on the fibre 10. During
the exposure process, the fibre 10 is being longitudinally
translated by the translation stage 20 in a direction from right to
left on the drawing. FIG. 2a illustrates (very schematically) a
refractive index change induced in the fibre by a first exposure
through the phase mask.
[0038] FIGS. 2a to 2c illustrate a feature of the normal operation
of a phase mask of this type, namely that the pitch of the lines or
fringes of the interference pattern projected onto the fibre (which
gives rise to the lines of the grating) is half that of (i.e. twice
as close as that of) the lines physically present (e.g. etched) in
the phase mask. In this example, the phase mask has a "physical"
pitch of 1 .mu.m, and the lines projected onto the fibre have a
pitch of 0.5 .mu.m.
[0039] The UV beam is modulated by the acousto-optic modulator in a
periodic fashion synchronised with the translation of the fibre. In
this way, successive exposures, such as the two subsequent
exposures shown in FIGS. 2b and 2c, generate periodic refractive
index changes aligned with and overlapping the first exposure of
FIG. 2a. Thus, the refractive index change providing each
individual grating "element" or fringe is actually generated or
built up by the cumulative effects of multiple exposures through
different parts of the phase mask as the fibre moves along behind
the phase mask. This means (a) that the optical power needed to
generate the grating can be distributed between potentially a large
number of exposures, so each exposure can be of a relatively low
power (which in turn means that the output power of the laser 40
can be relatively low); and (b) the grating can be apodised by
varying the relative positions of successive exposures (this will
be described below with reference to FIG. 3b).
[0040] Although each of the successive exposures of the fibre to UV
light through the phase mask 30 could be a very short pulse (to
"freeze" the motion of the fibre as the exposure is made), this has
not proved necessary and in fact the present embodiment uses an
exposure duty cycle in a range from below 10% to about 50%,
although a wider range of duty cycles is possible. An example of a
simple regular exposure duty cycle is shown schematically in FIG.
3a, which in fact illustrates the state of the modulation control
signal switching between an "on" state (in which light is passed by
the acousto-optic modulator) and an "off" state (in which light is
substantially blocked by the acousto-optic modulator). The period,
.tau., of the modulation corresponds to the time taken for the
fibre 10 to be translated by one (or an integral number) spatial
period of the interference pattern generated by the phase mask
30.
[0041] As the duty cycle for the UV exposure increases, the grating
contrast decreases (because of motion of the fibre during the
exposure) but the writing efficiency increases (because more
optical energy is delivered to the fibre per exposure). Thus,
selection of the duty cycle to be used is a balance between these
two requirements.
[0042] Assuming linear growth, the index modulation, n.sub.g(z) in
an ideal grating can be described as a raised cosine profile:
[0043] n.sub.g(z).varies.1+sin(2.pi.z/.LAMBDA.)
[0044] where z is the position down the fibre and .LAMBDA. the
grating period. With the new technique we obtain:
[0045]
n.sub.g(z).varies.(.DELTA..LAMBDA..sub.ON/.LAMBDA.)[1+{sin(.pi..DEL-
TA..LAMBDA..sub.ON/.LAMBDA.)/(.pi..DELTA..LAMBDA..sub.ON/.LAMBDA.)}sin(2.p-
i.(z+.DELTA..LAMBDA..sub.ON/2)/.LAMBDA.)]
[0046] where .DELTA..LAMBDA..sub.ON/.LAMBDA. is the fraction of the
period that the beam is on (i.e. the duty cycle).
[0047] For small values of .DELTA..LAMBDA..sub.ON/.LAMBDA. a near
100% grating contrast is obtained however the efficiency of the
grating writing is reduced to
.about..DELTA..LAMBDA..sub.ON/.LAMBDA. because most of the UV beam
is prevented from reaching the fibre.
[0048] The maximum grating strength is obtained for
.DELTA..LAMBDA..sub.ON/.LAMBDA.=0.5 however the ratio of dc to ac
index change is worse. For .DELTA..LAMBDA..sub.ON/.LAMBDA.>0.5
the grating begins to be reduced whilst the dc index change
continues to build.
[0049] Experimentally, a good value for
.DELTA..LAMBDA..sub.ON/.LAMBDA. has been found to be
.about.0.3-0.4.
[0050] Thus, with embodiments of this technique, exposure of the
grating lines or elements is repeated every grating period. Thus
the footprint defined by the UV beam, which might for example for a
500 .mu.m diameter beam, .phi..sub.beam, consists of
.phi..sub.beam/.LAMBDA.(.about.1000) lines, is significantly
overlapped with the previously exposed lines. Significant averaging
of the writing process given by (.phi..sub.beam/.LAMBDA.).sup.1/2
is therefore achieved, thus improving the effective accuracy and
resolution of the system.
[0051] The computer in this embodiment actually generates the
switching positions internally as "real" numbers (obviously subject
to the limitation of the number of bits used), but then converts
them for output to the controller into the same unit system as that
output by the Zygo interferometer, namely multiples of a "Zygo
unit" of 1.24 nm. This internal conversion by the computer makes
the comparison of the actual position and the required switching
position much easier and therefore quicker for the controller. A
random digitisation routine is employed in the computer 60 to avoid
digitisation errors during the conversion from real numbers to Zygo
units. This involves adding a random amount in the range of .+-.0.5
Zygo units to the real number position data before that number is
quantised into Zygo units. Thus an effective resolution can be
obtained of:
[0052] 1.24 nm/(.phi..sub.beam/.LAMBDA.).sup.1/2.apprxeq.0.03
nm.
[0053] The technique offers the further advantage that the CW laser
is extremely stable whereas pulsed lasers (as required in the
technique proposed by Stubbe et al [7]) may suffer from
pulse-to-pulse instability which, in the Stubbe et al technique, is
not averaged over multiple exposures. In addition the high peak
powers of a pulsed laser may cause non-linear grating writing
effects, which are avoided or alleviated by using longer and
repeated exposures in the present technique.
[0054] A refinement of the above technique, for producing apodised
gratings, will now be described with reference to FIG. 3b.
[0055] Using the techniques described above, effectively
controlling the grating writing process on a line-by-line basis
allows accurate apodisation to be achieved.
[0056] Apodisation is achieved by effectively dithering the grating
writing interferometer position in the fibre to wash out or
attenuate the grating strength. However, if the overall duty cycle
of the exposure is kept the same, and just the timing of each
exposure dithered, the average index change along the grating is
kept constant.
[0057] To completely wash out the grating subsequent on periods of
the UV laser are shifted in phase (position) by
.+-..pi./2(.+-..LAMBDA./4). To achieve a reduced attenuation the
amplitude or amount of dither is reduced. FIG. 3b illustrates an
applied dither of about .+-..pi./3 from the original (undithered)
exposure times.
[0058] This technique of apodising is better with an exposure duty
cycle of less than 50%, to allow a timing margin for 100%
apodisation.
[0059] One example of the use of this technique is to generate a
grating with a contrast increasing at one end of the grating
according to a raised cosine envelope, and decreasing at the other
end of the grating in accordance with a similar raised cosine
envelope, and remaining substantially constant along the central
section of the grating. This apodisation can be achieved
particularly easily with the present technique, as the central
section requires no phase shift between successive exposures, and
the two raised cosine envelopes require a phase shift that varies
linearly with longitudinal position of the fibre.
[0060] The required phase shifts can be calculated
straightforwardly by the computer 60, under the control of a simple
computer program relating required phase shift to linear position
of the fibre (effectively communicated back to the computer 60 by
the controller 70, whenever the controller 70 requests a next
switching position data value).
[0061] Other apodisation schemes are also possible. Compared with
previous methods of dithering [6] this technique is not limited by
the dynamics of a mechanical stage used for dithering, but instead
simply adjusts the switching time of a non-mechanical modulator
element 50. It can also achieve substantially instantaneous phase
shifts.
[0062] Furthermore, arbitrary phase profiles and in particular a
linear chirp can be built up by the computer 60 inducing phase
shifts along the grating as it is fabricated. In a similar manner
to the "Moving fibre/phase mask" technique [6] the maximum
wavelength is inversely proportional to the beam diameter. However,
with the present technique an improvement can be obtained (with
respect to the technique of [6]) by incorporating a short, linearly
chirped phase mask. Thus as the fibre is scanned the UV beam is
also slowly scanned (by another PZT translation stage, not shown)
across the phase mask. This scanning of the position of the UV beam
in itself induces a small chirp, in accordance with the techniques
described in reference [6], but more significantly the translated
beam accesses writing lines of a different period allowing larger
chirps to be built up. This has been tested using a 19 mm diameter,
.about.20 nm chirped phase mask (sourced from Lasiris) with its
central period around 1070 nm. This allows .about.30 nm chirped
gratings centred around a central wavelength of 1550 nm to be
fabricated.
[0063] FIGS. 4a and 4b are schematic graphs showing the
characterisation of a 20 cm linearly chirped grating written at a
fibre translation speed of 200 .mu.m/s with the basic technique
described earlier, i.e. with a fixed mask. At this fibre
translation speed, for a projected fringe pitch of 0.5 .mu.m the
writing light beam is switched at a switching rate of 400 Hz. In
other words, the fibre advances by one projected fringe between
exposures. (It is noted that the limitation on fibre translation
speed in these prototype experiments is the calculation speed of
the computer 60 used in the experiments, and that given a faster
computer such as a Pentium or subsequent generation PC, much higher
translation speeds of, say, 10 mm per second or more would be
possible).
[0064] In particular, therefore, FIG. 4a is a graph of reflectivity
against wavelength, and FIG. 4b is a graph of time delay against
wavelength. The wavelength (horizontal) axes of the two graphs have
the same scale, which for clarity of the diagram is recited under
FIG. 4b only.
[0065] A .about.4nm bandwidth and dispersion of .about.500 ps/nm
are observed.
[0066] Such results have not been reported by any other method.
Gratings up to 40 cm aend writing speeds up to 1 mm/s have been
demonstrated. Lengths in excess of in and writing speeds up to 10
mm/s are feasible.
[0067] In the above description, the fibre has been translated with
respect to the phase mask, and in the later description the UV beam
is translated with respect to the phase mask. However, it will be
clear that the important thing is relative motion, and so the
choice of which component (if any,) remains "fixed" and which is
translated is relatively arbitrary. Having said this, however, the
arrangement described above has been tested experimentally and has
been found to be advantageously convenient to implement. It will
also be apparent that in other embodiments each "exposure" could in
fact involve a group of two or more exposures, with the position of
the fibre with respect to the writing light beam being constant or
substantially constant for exposures within a group, but different
from group to group.
Publication References
[0068] 1. D. Garthe et al, Proc. ECOC, vol. 4, (post-deadline
papers), pp. 11-14 (1994).
[0069] 2. G. Meltz et al, Opt. Lett., 14(15), pp. 823-825,
1989.
[0070] 3. K. O. Hill et al, Appl. Phys. Lett., 62(10), pp.
1035-1037, 1993.
[0071] 4. R. I. Laming et al, Proc.ECOC'95, Brussels, Vol 2, Paper
We.B.1.7, pp 585-8, Sep. 17-21 1995.
[0072] 5. R. Kashyap et al, Electronics Letters, Vol 32 (15), pp.
1394-6, 1996.
[0073] 6. M. J. Cole et al, Electronics Letters, Vol 31 (17), pp
1488-9, 1995.
[0074] 7. R. Stubbe et al, postdeadline paper 1, Proc.
Photosensitivity and Quadratic Nonlinearity in Glass Waveguides,
Portland, Oreg., Sep. 9-11, 1995.
* * * * *