Download 485-146 - Wseas.us

Integrated optical Bragg grating with electrically controllable spectral
transfer function
A.V. CHAMRAI, A.S. KOZLOV, I.V. ILICHEV, M.P. PETROV
Laboratory of Quantum Electronics
Ioffe Physico-Technical Institute
26 Polytekhnicheskaya, St. Petersburg, 194021
RUSSIA
Abstract: - A novel versatile integrated optical device based on an electrically controlled Bragg grating in the
lithium niobate waveguide has been designed and fabricated. Fast electrooptic control of the device spectral
transfer function has been demonstrated.
Key-Words: - Optical telecommunications, Optical Networks, Integrated optical circuits, Electrooptic devices,
Gratings.
Introduction
Principle of operation
It is well known that a Bragg grating works as a
reflection (stop-band or notch) optical filter with very
high wavelength selectivity. The grating of 5 mm in
length can provide FWHM about 0.1 nm at 1550 nm. If
a Bragg grating is formed in an electrooptical material,
the central reflection wavelength can be tuned by
applying an external electric field that changes the
average refractive index (electrically controlled
diffraction) [1]. However, since the electrooptical
parameters of available materials (in particular LiNbO3)
are very limited the wavelength range of tuning is rather
narrow.
Transmission, a.u.
80
60
40
20
0
1553.6
1553.8
1554
1554.2
Wavelength detuning, nm
1554.4
1553.6
1553.8
1554
1554.2
Wavelength detuning, nm
1554.4
a.
100
80
Transmission, a.u.
We have developed a novel integrated optical device
based on new original technique of the electrooptical
control of the spectral transfer function of Bragg gratings
in LiNbO3 single-mode channel waveguides. The device
could provide new functional capacity of the optical
components for wavelength control in WDM systems.
Potentially a high wavelength selectivity (0.1  0.01
nm), fast electrooptic control (up to 20 GHz), relatively
low controlling voltage together with the integrated
optical implementation, compatibility with other
components in modern optical networks, and possibility
of mass production make this device very promising as a
key building block of various optical systems for control
of narrow-band spectral channels. For instance, it could
be used for building wavelength selective electrically
controlled optical attenuators for optical power
equalizers,
electrooptic
modulators
(allowing
modulation of one specific wavelength channel without
affecting the other channels), Add/Drop multiplexers,
and other wavelength switches.
100
60
40
20
0
b.
Fig. 1. The spectral transfer functions of the
homogeneous Bragg grating (a) and the same
grating with  phase shift at the midpoint (b).
The parameters of the grating are as follows: n0
= 2.2., n1 = 10-4, l = 6 mm, 0 = 1554 nm.
We suggest to exploit a new and more flexible
technique for the control of the spectral transfer function
of Bragg gratings. The unique spectral properties of the
gratings with phase-, spacing- and average refractive
index discontinuities [2] are used. From the theory [3, 4]
it follows that if the reflection Bragg grating gets a phase
shift equal to  in the midpoint of the grating the spectral
transfer function is changed from stop-band to pass-band
mode (Fig.1).
Similar results can be obtained in the case of average
refractive index discontinuity. For instance, If the
average refractive index n0 of the homogeneous grating
transforms into n0 + n/2 for the first half of the grating
and n0 - n/2 for the second half (where n = /(2l) is
the refractive index shift, l is the grating length, and  is
the Bragg wavelength) the spectral transfer function is
also modified from reflection to transmission mode.
The goal of our work was to show experimentally
that such methods of the control of the spectral transfer
function can be implemented in integrated optical
devices. In the device presented in this report an average
refractive index discontinuity in the Bragg grating was
produced and electrically controlled by choosing a
proper spatial distribution of the applied external electric
field. In this case the electrooptical nature of
reconfiguration provides very high speed of control, and
simple fixed gratings in LiNbO3 waveguides that are
very convenient for practical application can be used.
Device fabrication
At the first stage a channel single mode waveguide in the
LiNbO3 substrate was fabricated by the standard
technique of thermal indiffusion of titanium stripes [5].
The orientation of the waveguide (light propagation
direction) was along the optical C axis of the LiNbO3
crystal. The fabricated waveguides had excellent optical
quality and low losses (< 1dB). Then thermal indiffusion
of a Cu layer was performed to dope the LiNbO3
substrate and provide photosensitivity for Bragg grating
recording. At the next stage, Cu electrodes were
deposited on the substrate surface by magnetron
sputtering. The electrodes of specially designed
configuration provided a phase discontinuity by
application of the external electric field with a specified
spatial distribution in the geometry of the transverse
electrooptic effect. Two single-mode optical fibers were
connected to the front and back edges of the waveguide
and provided input and output of the optical signal. Then
the device was packed in the protective coverage. The
layout of the device is shown in Fig. 2.
Output
SMF
Input
SMF
Electrodes
Wires
Bragg grating in
optical waveguide
Fig. 2. Device layout.
Finally a Bragg grating has to be produced in the
sample. Several techniques of the Bragg grating
formation have been developed. One of the ways is a
holographic recording. We used a conventional scheme
for recording transmitted holograms in LiNbO3 crystals
with active stabilisation. A Nd-YAG laser (532 nm) was
used. The grating spacing was determined by the angle
of incidence of the recording beams. An angle of 49.5
deg gives a grating spacing of about 349.8 nm, which
corresponds to the Bragg wavelength of about 1554 nm
for the reflection geometry of readout. The excellent
Bragg grating with the diffraction efficiency up to 90 %
have been recorded in the waveguide.
Experimental results
Fig. 3a shows the spectral transfer function of a
simple homogeneous holographic Bragg grating in a
LiNbO3 photorefractive waveguide. The light intensity
transmitted through the waveguide versus wavelength
was measured using the sweeping mode of operation of
a tunable semiconductor laser (PRO 800). The
diffraction efficiency higher than 90% and FWHM of
about 0.1 nm have been achieved. The device allows
simple tuning of the central wavelength of the Bragg
grating transfer function without changing its shape by
application of a spatially uniform electric field. The
wavelength shift of about 0.1 nm has been obtained for
the 75 kV/cm of applied electric field. However, fast
electrooptical tuning even in this narrow spectral range
could be very interesting for wavelength locking and
laser stabilization.
The selected electrode configuration allows
formation of the average refractive index discontinuity,
which is equivalent to the  phase shift at the midpoint
of the grating. Theoretical analysis shows that in this
case the transmission at the central wavelength of the
grating transfer function should be equal to 100 %. Our
experiments proved the validity of the theory. Fig. 3b
shows the device spectral characteristic when an external
electric field of the same magnitude (75 kV/cm) but
opposite polarity was applied to different halves of the
grating. The maximum of transmittance is observed at
the middle of the grating stop band. So the electrooptical
switching from the stop band mode to the pass band
mode has been experimentally demonstrated. This mode
of operation is very interesting for building optical
Add/Drop multiplexers, wavelength selective electrically
controlled optical attenuators for optical power
equalizers, and electrooptic modulators (allowing
modulation of one specific wavelength channel without
affecting the other channels).
Transmission, %
100
80
60
40
E=75
E=0
20
1549.5
a.
1550
Wavelength, nm
kV
cm
1550.5
Transmission, %
100
80
E=75
kV
cm
60
40
E=0
20
0
b.
1554,0
1554,5
Wavelength, nm
Fig. 3. Experimental demonstration of the
electric control of the transfer function of a
Bragg grating in the LiNbO3 waveguide. a tuning of the central wavelength. b - switching
from stop-band mode to pass-band mode.
Summary
A novel versatile integrated optical device based on an
electrically controlled holographic Bragg grating in the
lithium niobate waveguide has been designed and
fabricated. New original technique for the control of
spectral transfer function of Bragg grating has been
used. A high wavelength selectivity (~0.1 nm) and fast
electrooptic control of the device spectral transfer
function has been experimentally demonstrated. This
device is very promising as a key building block of
wavelength selective electrically controlled optical
attenuators for optical power equalisers, electrooptic
modulators (allowing modulation of one specific
wavelength channel without affecting the others
channels), and Add/Drop multiplexers.
References:
[1]. V.M. Petrov, C. Denz, A.V. Shamray, M.P. Petrov,
T. Tschudi, “Electrically controlled volume LiNbO3
holograms for wavelength demultiplexing system,” Opt.
Materials 18, 191-194 (2001).
[2]. G.P. Agrawal, S. Radic, “Phase-shift fiber Bragg
gratings and their application for wavelength
demultiplexing,” IEEE Photon. Technol. Lett. 6, 995997 (1994).
[3]. V.M. Petrov, C. Karaboue, J. Petter, T. Tschudi,
V.V. Bryksin, M.P. Petrov, Appl. Phys. B 76, 41-44
(2003).
[4]. V.M. Petrov, S. Lichtenberg, J. Petter, T. Tschudi,
A.V. Chamrai, M.P. Petrov, “A dynamic wavelength
Bragg-filter with an on-line controllable transfer
function,” in Advances in Photorefractive Materials,
Effects and Devices, Vol. 87 of OSA TOPS, pp. 564570.
[5]. J. Hukriede, D. Kip, E. Krätzig, “Investigation of
titanium- and copper indiffused channel waveguides in
lithium niobate and their application as holographic
filters for infrared light,” J. Opt. A: Pure Appl. Opt. 2,
484-487 (2000).