Method and equipment for writing a Bragg grating in a waveguide

Details Method and equipment for writing a Bragg grating in a waveguide Method and equipment for writing a Bragg grating in a waveguide

2001-11-16

2003-07-08

Sanghavi, Hemang (Department: 2874)

Optical waveguides

With optical coupler

Input/output coupler

C359S563000, C359S566000, C359S569000, C430S290000

Reexamination Certificate

active

06591039

ABSTRACT:

BACKGROUND OF INVENTION
The present invention relates to a method an equipment for writing a Bragg grating, particularly an apodized Bragg grating, in a waveguide. In the course of the present description, reference will be made to optical fibres, but this reference should be understood as providing an example rather than being limiting, since the technology described is equally applicable to waveguides in integrated optical systems.
Typically, the optical fibres used for telecommunications are doped with germanium, which induces a property of photosensitivity to UV radiation. In order to write a Bragg grating in an optical fibre, this property is used to modify the refractive index locally by means of UV illumination.
As is known, an in-fibre Bragg diffraction grating is a length of fibre which has an essentially periodic longitudinal modulation of the refractive index in its core. This structure has the property of retroreflecting the light in a wavelength band centred on the Bragg wavelength. The Bragg wavelength, as is know (for example, from Report 3.3 in the publication “Fiber Bragg Gratings”, by Andreas Othonos and Kyriacos Kalli, Artech House, Boston/London, 1999) can be expressed as follows:
&lgr;
B
=2
·n
eff
·&Lgr;  (1)
where n
eff
is the effective refractive index and &Lgr; is the spatial period of the diffraction grating.
Additionally, as is know (from Report 3.4 of the aforesaid publication “Fiber Bragg Gratings”, for example), in the most general case the variation of the refractive index n along the axis z of the fibre core can be expressed by the following relation (which shows all the possible dependences of the variable z):
n
(
z
)=
n
0
(
z
)+&Dgr;
n
(
z
)sin(2&pgr;
z/
&Dgr;(
z
))  (2)
where n
0
(z) is the mean local value of the refractive index and &Dgr;n(z) represents the local envelope of the refractive index. The effective refractive index n
eff
of relation (1) is proportional to the mean refractive index n
0
(z) of relation (2) by a term defining the confinement (typically indicated by &Ggr;) of the fundamental mode of the fibre.
Uniform gratings, gratings known as “chirped”, and apodized gratings are known and can be distinguished by the variation of the refractive index.
In uniform gratings, the terms n
0
(z), &Dgr;n(z) and &Lgr;(z) are constant, as shown in
FIG. 1
a
, which shows the typical variation of the refractive index n (normalized to 1) as a function of the co-ordinate z (expressed in arbitrary units). Additionally, as shown in
FIG. 1
b
, the reflection spectrum of a uniform grating typically has a central peak at the Bragg wavelength and a plurality of secondary lobes. These secondary lobes can be disadvantageous is some applications, for example when the Bragg grating is used to filter a channel (at a corresponding wavelength) in a multi-channel optical transmission system. This is because, in this case, the secondary lobes of the reflection spectrum introduce an undesired attenuation into the transmission channels adjacent to those which are to be filtered.
In apodized gratings, the term &Dgr;n(z) is variable, and the refractive index n(z) has a variation of the type shown qualitatively in
FIG. 2
a
(in which n is normalized to 1 and z is expressed in arbitrary units). The refractive index therefore shows an envelope corresponding to a predetermined curve. A typical variation of the reflection spectrum of an apodized grating is shown in
FIG. 2
b
. It is clear that a suitable modulation of the term &Dgr;n(z) enables the secondary lobes to be reduced around the principal reflection peak. A grating of this type can therefore be used advantageously for channel filtering in a multi-channel system, thus reducing the aforesaid problem of the attenuation of the channels adjacent to the filtered channel.
In chirped gratings, one or the other of the terms n
0
(z) and &Lgr;(z) is variable. Owing to this variability, and since the Bragg wavelength is proportional, for the reasons stated above, to the product of n
0
(z) and &Lgr;, the reflection bands of these gratings are wider than those of uniform gratings.
FIGS. 3
a
,
3
b
,
3
c
show, respectively, the qualitative variation of the refractive index in the case in which the term n
0
(z) is modulated, the variation of the same parameter in the case in which the term &Lgr;(z) is modulated (with a continuous variation from approximately 500 nm to approximately 502 nm, for example), and the typical reflection spectrum of a chirped grating. As can be seen in the spectrum of
FIG. 3
c
, the reflection peak is considerably widened. A grating of this type can therefore be used as a wide-band reflection filter or, more typically, as a chromatic dispersion compensation device. If the term &Dgr;n(z) is also modulated, the grating bercomes a chirped apodized grating.
There are various known techniques for writing an apodized Bragg grating. In these techniques, the fibre is exposed to suitably shaped UV interference fringes, to produce a corresponding variation of the refractive index, and particularly of the local envelope &Dgr;n(z).
The known techniques essentially fall into two categories: interferometric techniques and phase mask techniques.
Interferometric techniques essentially consist in dividing a UV beam into two components and making them strike the fibre at a predetermined relative angle, thus generating the interference fringes which induce the desired variation of the refractive index. These techniques are highly versatile, since by varying the relative angle between the two components it is possible to vary the parameters of the grating, particularly its period.
However, interferometric techniques are poorly suited to serial production, since the “set-up” for writing is particularly sensitive to external factors (temperature, vibrations, etc.), so that the parts require frequent realignment. The application of these techniques is therefore essentially limited to the research field.
The phase mask techniques are generally considered more suitable for large-scale production, owing to the high repeatability, the lower susceptibility to external factors, and the fact that the UV beams require a lower coherence length.
A phase mask is a quartz substrate on whose surface there is a series of rectilinear projections running in a principal direction and parallel to each other, and forming, in section, a profile which is essentially of a square wave type. These projections are typically equally spaced and of equal height in a uniform mask, at variable intervals in the case of a chirped mask, and of variable height in an apodized mask.
To write the grating, the phase mask is usually positioned facing the portion of fibre concerned, and orientated in such a way that its principal direction (as defined above) is parallel to the fibre axis. When the UV radiation passes through it, the phase mask generates at its output interference fringes with an essentially sinusoidal variation and with a period &Lgr; equal to half of the period &Lgr;
m
of the projections of the mask. In greater detail, the electromagnetic radiation leaving the phase mask can be subdivided into different orders m associated with corresponding propagation angles &thgr;
m
according to the relation
sin
⁢
 
⁢
θ
m
=
m
⁢
λ
Λ
(
3
)
The aforesaid fringes are generated from the orders +1 and −1 (values+1 and −1 of m), while the other orders, particularly the zero order, are unwanted, since they tend to diminish the visibility v of the fringes. The visibility v is defined, in a first approximation, by the following relation:
v
=
I
max
-
I
min
I
max
+
I
min
(
4
)
where I
max
and I
min
are, respectively, the peak intensity and the valley intensity of the fringes.
In general, phase masks are designed in such a way as to reduce (typically by approximately 1% to 3%) the transmitted zero order and to maximize (typically with an increase of approximately 30% to 40%) the quantity of light of the orders +1 and −1.
The article by J. Hübner, M.

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