Chemistry Reference
In-Depth Information
main problems encountered with wet etching are the nonrectangular profile and the exten-
sive roughness of the edges [34]. Micromachining has been applied to generate ridge-like
waveguides between two parallel grooves structured by laser ablation [29], also leading to
poor edge quality. One alternative may be to use reactive ion etching (RIE), but a test
performed on ZrF
4
-based glass was not conclusive [35]. On the whole, ionic exchange is
considered to be the most suitable low-cost technique to fabricate channel waveguides.
To complete the overview, laser writing of channel waveguides has to be mentioned. This
is a very fast and powerful technique that can be applied both to films and bulk materials.
Irradiation of an area in the glass is used to achieve photo-induced refractive index changes,
up to 0.01. Practically, waveguides are written in glass with focused fentosecond laser pulses
(i.e. l
¼
800 nm [36]), or continuous waves in the case of films to avoid damage (i.e.
l
¼
244 nm [37]). Another possibility is to create a confined waveguide between two
photothermal expansion ridges (i.e. l
¼
244 nm [38]); this way, the guiding zone is sheltered
from possible radiation damage that can strongly affect the guiding properties.
11.4.1 Optical Amplifier
The most important characteristic to indicate the performance of an optical amplifier is the
net or external gain that takes into account the coupling loss (C), propagation loss (
) and
RE-absorption
a
. It is given by the following equation:
G
net
ð
dB
Þ¼
G
on
=
off
2C
ð
L
þ
a
NL
Þ
where L is the waveguide length and N the RE concentration. The relative gain G
on/off
, i.e.
the ratio of the output signal when the laser pump is on and off, is a quicker measurement
and thus gives a preliminary estimation of amplifier performance (Figure 11.4).
4
3
2
1
ZBLA
PZG
0
0
50
100
150
200
250
P @ 980 nm (mW)
Figure 11.4 'On/off' gain at 1.53
m
m as function of the incident pump power in Er
3þ
-doped
ZBLA and PZG fluoride glass waveguides pumped at 980 nm. Inserted in the graph, near field
images showing single mode (ZBLA) and multimode (PZG) waveguides at this wavelength
