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measured with a very high resolution by measuring the intensity at
the output of the sensing structure. It is expected that the sensor
may be customised for application in environments of different
refractive indices by introducing a dielectric buffer layer between
the fibre and the metal film [6]. Then, in conjunction with a
selected transducing film for specificity, this sensor may be of
great interest in numerous applications including medical science,
and environmental analysis (water-quality monitoring).
NF
Acknowledgment: This work has been supported by the Grant
Agency of the Czech Republic under contract 110219310640.
0 IEE 1996
IO Jalcllniury 1996
Electronics Letters Online No: 19960349
J. Homola and R. Slavik (Institute of Radio Engineering and
Electronics, Academy of Sciences of the Czech Republic, Cliuherslca 57,
CZ-I82 S I Prague, Czech RcJpublic)
References
A
Transient refraction index changes in UVexposed optical fibres
M.B. Danailov, T. Gasmi and P. Apai
&a
CL
TET2
I . ,
. I .
-1-
a . .
I
. .
. .
I
ti
ii.5
E .. , .
I
DETl
SCHEGGI.
A.M , and BALDINI, F.: ‘Chemical sensing with optical
fibres’, Int. J. Optoelelron., 1993, 8, pp. 133-156
LIEDBERG, B., NYLANDER, c., and SUNDSTROM, I : ‘Surface plasmon
resonance for gas detection and biosensing’. Sens. Actuators, 1983.
4, pp. 299-304
MATSUBARA, K., KAWATA, s., and MINAMI. s.: ‘Optical chemical
sensor based on surface plasmon measurement’, A ~ [ J /Opt.,
.
1988,
27, pp. 1160-1 163
ZHANG, L M., and UTTAMCHANDANI, D : ‘Optical chemical sensing
employing surface plasmon resonance’, Electron. Lett., 1988, 24.
pp. 1469-1470
JORGENSON. R.C , and YEE, s s : ‘A fiber-optic chemical sensor based
on surface plasmon resonance’, Sens. Actuutors B, 1993, 12, pp.
213-220
LAVERS, c R , and WILKINSON, J.s.: ‘A waveguide-coupled surfaceplasmon sensor for an aqueous environment’, Sens. Actuators B.
1994, 22, pp. 75-81
DIGONNET, M J F., and SHAW. H.J.: ‘Analysis of a tunable single mode
optical fiber coupler’, IEEE J. Quantum Electron., 1982. 18. pp.
746-754
DIGONNET. M J.F,
FETH. J R ,
STOKES, L F.,
and SHAW. H.J :
‘Measurement of the core proximity in polished fiber substrates
and couplers’, Opt. Lett., 1985, 10, pp. 463465
...
t
P-
.
I . I
I
UV
He-Ne;
Fig. 1 E.Yperirnentu/ setup
LI-L5: spherical lenses
CL: cylindrical lenses
BS: beam splitters
M: flat mirrors
OF: optical fibre
NF: neutral density filter
P: polariser
A: aperture
DET I . DET2: detectors
Experinzent: A Mach-Zehnder interferometric scheme (Fig. 1) is
used to detect the UV-induced phase changes. The probe beam is
delivered by a 5mW He-Ne laser. The intensity of the reference
beam is adequately attenuated by a neutral density filter (NF).
The divergences of the two beams are matched at the output of
the interferometer by the lenses L2 and L3. The axis of the polariser P is set parallel to the laser polarisation plane to cancel the
depolarised component of the probe beam. A lens (U) and an
aperture (A) are used to project the central fringe of the interference pattern onto the avalanche photodiode DETl. The transniission changes are monitored via the avalanche photodiode DET2.
The time response of both detection channels (including the fast
digitising oscilloscope) is 2ns. The W beam is delivered by the
fourth harmonic (266nm) of a Q-switched Nd:YAG laser and is
focused to a line of 15mm x 2 0 p . Typical parameters of the W
pulses are 8ns pulsewidth, energy 5mJ and lOHz repetition rate.
Both detectors are screened from the W and the UV-induced fluorescence by interference filters. In most of the experiments we
used a high Ge concentration coupler fibre from Pirelli Cavi
(NA= 0.19, core diameter = 4 ~ ) .
Iridrxing temzs: OpticolJihiw, Refractive index
Interferometric measurements of the transient refractive index
evolution in UV-exposed Ge-doped fibres are reported. It is
demonstrated that, in the nanosecond time scale both the optical
phase and the intensity of a probe beam traversing the fibre
exhibit large variations. The experimental data suggest a common
origin for the transmission and refraction index behaviour.
Introduction: W-exposure of Ge-doped fibres has proved to be a
powerful technique for inducing permanent refractive index
changes in the fibre core and in particular for inscription of Bragg
gratings [l]. The permanent refraction index and absorption
changes during the UV-exposure are well characterised in the literature [2 - 41. However, little is known about the dynamics of the
transient processes which accompany the fibre exposure to a nanosecond UV-pulse. Recently it was noted that a strong transient
gain in the near-infra-red [5] and a loss in the visible [5, 61 spectrum appeared, immediately following the exposing pulse. Studying the behaviour of the refraction index while these processes are
taking place can he very useful in better understanding the physical mechanisms involved.
In this Letter we present, for the first time to our knowledge,
interferometric visualisation of the transient (nanosecond time
scale) optical phase changes in a probe beam propagating through
LJV-exposed Ge-doped optical fibres.
482
W
0
04
-
a
-
20 0
I
i 0 8 4
0-0
’
0
\
n
I
I
5
10
i
15
time, n s
I
20
I
I
25
30
@
Fig. 2 Transmission and intevferomelric signals in 30ns timescale
Up: transmission
Down: interferometric
Results and discussion: In general, the observed signal exhibits two
distinct time phases. The first phase is shown in Fig. 2, where the
lower trace is the interferometric signal. The exposure energy is
ELECTRONICS LETTERS
29th February 1996
Vol. 32
No. 5
3mJ. There is a fast drop in transmission coinciding with the
arrival of the UV pulse. At the same time the interference signal
exhibits fast oscillations, indicating changes of the optical phase of
the probe beam. We have observed up to eight fringes of this type.
The time interval between successive interference maxima is typically 3 4 n s . As the exposure energy grows, the contrast of the
fringes decreases and they are not well resolvable; most probably
their period goes below the resolution of the detection system. Fig.
3 presents a longer time scale signal and allows us to see the second phase of the transmission and interferometric fringe evolution
with an exposure energy of 5mJ. The transmission starts recovering, and the length of this process varies from a few microseconds
to tens of microseconds, depending on the irradiating W energy.
ever, its contribution would be at least an order of magnitude
smaller than that of the refraction index change due to the same
thermal effect (see, for example, [7],Chap. 10). So, one can conclude that the observed signal indicates refractive index changes;
the maximuni value computed was 5 >: 10 4. The reported transient
behaviour is well reproducible as long as the same exposure conditions were kept.
Conclzaions: We have monitored simultaneously the transient optical phase and intensity changes of a probe beam propagating
through a fibre exposed to nanosecond UV pulses. Two distinct
phases of the refractive index change were observed. The presented data suggest a common origin of the tratnsmission and
refractive index changes. A faster (< 1ns) detection system is
required in order to further investigati: the index behaviour during
the interaction of the laser pulse with the fibre. The reported
effects may find applications in generating nanosecond long Brdgg
gratings.
Acknowledgments: The authors are grateful to Prof. G. Denardo
for his constant support and guidance.
0 IEE 1996
I 9 Deceinbev 1995
Electronics Letters Online No: 1W6O305
M.B. Ilanailov, T. Gasmi and P. Apai (Luser Laboratory,
Internutionul Centre f o r Tlzeoretical I’lzysic.s, P O Box 586, 34100
Trieste, Italy)
0
200
T. Garmi: Pcrmanent address: Laboratoire des Lasers, C.D.T.A.,
Algiers, Algeria
P. Apai: Permanent address: Research Institute for Solid State Physics,
H-152.5, Budapest, Hungary
6 00
400
time, ns
Fig. 3 Transmission (up) and interferometric (down) signal,^ in 600ns
timescale
Up: transmission
Down: interferometric
References
‘Photosensitive optical fibers: devices and
applications’, Opt, Fiher Technol., 1994, 1, (I), pp. 17-34
IIAND. D . P , and RUSSEL, 1.’ ~ 7 . 1 :Opi. Lett., 1989, 15, (2), pp. 102KA~SHYAP,R.:
104
The number of the observed interferometric fringes depends nearly
linearly on the exposing energy, starting from a single one (1mJ)
and reaching 15 (5mJ). The time delay between the successive
fringes increases monotonically with time. Quantitatively this
behaviour is illustrated in Fig. 4. The Y-axis of the plot gives the
optical phase change, which was computed from the intervals
between the peaks in Fig. 3. In addition, we have shown the normalised change of attenuation (derived from the transmission
curve) for this time interval. It is seen that the time decay constants of the two processes are very close (35011s at lle of the maximum). The optical pathlength variation producing the observed
, and OIJELETTE, F : ‘Correlation
between ultraviolet-induced index change and photoluminescence
in Oe-doped fiber’, Appl. Phys. Lett. 1992, 61, (25), pp. 2955-2957
KARLITSCI-IEK, P.,
HILLRICHS, G ,
and
KLEIN, K -F.:
‘Photodegeneration and nonlinear effects in optical fibers induced
by pulsed uv-laser radiation’, Opt. Cominun., 1995, 116, (3), pp.
DLVAL, Y.. KASHYAP, R., FLEMING. s
21 5J-230
and RUSSEL, P. ST.J : ‘Excimer laser
production of fibre Bragg gratings’, Proc. SPZE, 1993, 2044, pp.
69--75
DANAILOV. M B , and AI”,
r.: ‘UV-induced transmission frustration
in optical fibres’, Appl. Plzys. Lett., 1995 (in press)
UDD, E. (Ed): ‘Fiber optic sensors’ (John Wiley & Sons, New York,
1991)
AR~CHAMBAULT,ILL., REEKIE, L.,
Long wavelength pin/HBT optical receiver
moriolithically integrated with HBT
comparator
H. Y a n o , S. S a w a d a , T. K a t o , (3.
Sasaki, K. D o g u c h i
and Ic1. M u r a t a
--
0
2 00
600
400
time, ns
z7
Fig. 4 Time decay of norinalised phase change and normali.red attenuation
Time origin is at 40ns after arrival of exposing pulse
normalised phase change
.~
normalised attenuation
interferometric signal can be caused both by refraction index and
length changes. One could suppose that there is a lengthening of
the fibre due to heating during the exposure. For silica fibre, how-
ELECTRONICS LETTERS
Indexing terms: Olitical rereiwrs, Integrated optoelectronics.
Integrated circuibs
F-
29th February 1996
Vol. 32
A OaTnAs pin PD, a GaInAtJInP HBT transimpedance-type
preamplifier, and a GaInAs/lnP HBT comparator have been
monolithically integrated on an InP substrate with pin/HBT
integration technoloby. The receiver OElC with a comparator
showed a sensitivity of -36.0dBm at a bit error rate of 1 X
for 125Mbit/s signals.
Introduction: Optical interconnections are expected to play a
major role in the data processing industry in the future, owing to
optical interconnections providing longer transmission distance,
No. 5
483
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