Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making named article
Reexamination Certificate
1999-10-22
2003-04-15
Angebranndt, Martin (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making named article
C430S001000, C430S002000, C356S340000, C359S034000, C359S035000, C385S037000, C385S122000
Reexamination Certificate
active
06548225
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the creation of patterns on photosensitive optical materials by placing the materials in the interference pattern generated by the intersection of at least two beams of light, preferably ultra violet light.
BACKGROUND OF THE INVENTION
International Patent Application No. PCT/AU96/00782 filed Dec. 2, 1996 entitled “Ring Interferometer Configuration for Writing Gratings” (“the PCT Application”) discloses a system for writing gratings in photosensitive optical materials. The disclosed system has significant advantages in the reduction of noise characteristics in gratings.
In
FIG. 1
of the drawings attached to this specification, there is illustrated a perspective view of an arrangement
30
constructed in accordance with the principles disclosed in the PCT Application. In the arrangement
30
a narrow UV beam is projected onto a phase mask
32
. The phase mask
32
produces at
41
two coherent mode beams
33
and
38
. One of the beams
33
is reflected at
34
by mirror
35
before again being reflected at
36
by mirror
37
before projecting upon an optical fibre
40
placed at position
45
. The second diffracted beam
38
traverses a counter propagating route (not shown) by reflection from mirror
37
and mirror
35
. Both of the diffracted beams are constructed so as to impinge upon area
45
, resulting in a “Sagnac” type of arrangement. The two beams
33
and
38
, being coherent, form an interference pattern at the point
45
. The optical fibre
40
, which exhibits photosensitivity, is thereby provided with a modulated refractive index in accordance with the interference pattern. The modulated refractive index is then utilised to form a Bragg grating at the point
45
.
The PCT Application further discloses the step of sweeping the UV beam
31
along the phase mask
32
so as to create an extended Bragg grating structure.
FIG. 2
of the attached drawings illustrates the embodiment of
FIG. 1
in schematic form and more clearly illustrates the path followed by the reflected beam
33
,
34
,
36
and the reflected beam
38
,
42
and
43
.
The PCT Application also discloses rotation of mirrors
35
,
37
so as to “chirp” the interference pattern
45
and further discloses moving the optical fibre
40
to a staging area where the maximum interference contrast is obtained. This arrangement provides the advantage that the wavelength of the Bragg grating is thereby tunable and, utilising a single phase mask, the wavelength can be lowered by means of increasing the crossing angle of the writing beams. Of course, altering the crossing angle may cause the overlap region of interference
45
to move away from the optical material
40
but, as disclosed in the PCT Application, the movement can be corrected by moving the fibre to a new location of maximum overlap. This can be achieved by utilising, for example, a small translation stage to mount and move the fibre
40
. However, the process of translation of the fibre is extremely complex, requiring the turning off of the laser and the accurate repositioning of the fibre with respect to the interference pattern.
Unfortunately, as noted in the arrangement of the PCT application any movement of the reflecting mirrors results in a corresponding movement of the intersection point of the interfering beams and a change in the angle of intersection of the beams. The change in the angle of intersection will result in a consequential change in the Bragg Wavelength written as the Bragg Wavelength is proportional to the angle of intersection.
Hence, as the mirror angles are changed to, for example, chirp the grating, the point of intersection will move away from or towards the mirrors
35
,
37
. The fact that the point of intersection of the writing beams is directly related to the angles of each mirror
35
,
37
means that it is impossible to vary the Bragg wavelength of the grating without the point of intersection moving. If the fibre is positioned parallel to the phase mask, this orthogonal movement in the beam intersection reduces the effective fringe contrast and apodises the grating in a known but unwanted manner. While this effect can be reduced by aligning the axis of the fibre
80
along the path followed by the intersection of the writing beams, this is only effective if you wish to linearly chirp the grating. Therefore, in the arrangement of FIG.
1
and
FIG. 2
, the orthogonal movement in the writing beam intersection and the Bragg wavelength are not independently controllable, thereby limiting the amount and type of chirp that can be written into a grating without some degree of unwanted self apodisation.
It has also been found in practice that translation of the UV beam
31
along the phase mask
32
results in a corresponding translation of diffracted beams
33
,
38
across the surfaces of mirrors
35
,
37
. Unless the mirrors
35
,
37
are perfectly flat, the path of beams
33
,
38
will undergo slight variations in angle and intensity as each beam traverses its mirror surface. This results in the introduction of a “noise” factor which can show up in a grating within fibre
40
as unwanted fluctuations in the frequency response of the grating. In particular, where the grating is a chirped Bragg grating and the fibre
40
is utilised as a dispersion compensator in a telecommunication circuit, the variation from a purely linear response can become significant. This is often evidenced as a significant group delay ripple.
If the fibre
40
is positioned parallel to the phase mask
32
, the orthogonal movement of the beam intersection point, as a result of mirror movement, can reduce the effective fringe contrast and apodise the grating in a known but unwanted manner. While this effect may be reduced by aligning the axis of the fibre along the path followed by the intersection of the writing beams, this reduction is generally only effective when a linearly chirped blazed grating is required as the non-perpendicular fibre results in a Blazed grating being produced.
Further, due to the geometry of the writing system
30
, it is clear that, as the UV beam is scanned from one end of the phase mask to the other, the distances travelled by the two beams from the phase mask to the point of intersection will not be identical. The difference in path length will vary from approximately minus half the scan length to plus half the scan length. For long grating periods, this path length difference can place high demands on the temporal coherence requirements of the UV source and also can effect the spatial stability of the intersecting beams, and is a further potential source of noise in the written interference pattern.
Additionally, because the fibre
40
must be in a plane either below or above the plane of the UV beam
31
the path length difference between the beams, in combination with the small vertical tilt applied to both mirrors, can cause the intersecting beams to move apart vertically as the UV beam is scanned along the phase mask. This vertical separation can also lead to unwanted self apodisation, loss of grating strength and contrast at the ends of the grating.
Further problems exist with the system
30
when the phase mask is “dithered” so as to apodise the grating in a desired manner. As the mask
32
is dithered, the fringe contrast will be reduced. This effect can be used to apodise the grating within fibre
40
. Dither control is important if the noise on the apodised grating profile is to be reduced. If the dither amplitude is not exactly right then unwanted fringes may be written in the fully apodised regions of the grating. Unfortunately, chirped gratings used for dispersion compensation can be extremely sensitive to imperfections in the apodisation profile. It is, therefore, important that the fringe pattern be smoothly extinguished at each end of the grating. The present utilisation of the process of dithering the phase mask is thought to perhaps introduce both unwanted phase and aperture noise in the apodisation profile, leading to unwanted noise on the transmis
Bulman Jonathan Mark
Hammon Timothy Edward
Stephens Thomas David
Angebranndt Martin
JDS Uniphase Pty Limited
Ladas & Parry
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