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fibre array. The extinction ratio between the signals going out at
the output and the add ports is 17dB before and after the exposure. This value may be improved by optimising the coupling ratio
of the directional couplers and by adjusting the phase between the
signals propagating in the two arms of the Mach-Zehnder interferometer. The latter may be achieved by UV-trimming. The power
back-reflected to the input port at the Bragg wavelength is also
17dB lower than the one exiting the drop port. This indicates that
the Bragg gratings do not create significant phase variation
between the signals reflected in the two arms and that W-trimming is therefore not necessary.
,
1559.6,
,
,
.
,
.
,
.
,
I60
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KOHNKE, G.E.,
MILBRODT, M.A.,
ERDOGAN, T.,
HENRY, c.H.,
STRASSER, T.A,
WHITE, A.E.,
and LASKOWSKI, E.J.: ‘Planar
waveguide Mach-Zehnder bandpass filter fabricated with single
exposure
W-induced
gratings’.
Proc.
Optical
Fiber
Communication, San Jose, CA, 1996, pp. 277
5
KOHNKE, G.E.,
STRASSER, T.A.,
HENRY, C.H.,
LASKOWSKI, E.J.,
CAPPUZZO, M.A.,
and WHITE, A.E.: ‘Silica based Mach-Zehnder adddrop filter fabricated with UV-induced gratings’, Electron. Lett.,
1996, 32, (17), pp. 1579-1580
6 RUNE, R.J., and JOERGENSEN, B.F.: ‘Impact Of coherent cross-talk on
usable bandwidth of a grating-MZI based OADM’, submitted to
IEEE Photonics Technol. Lett.,
7 STRASSER, T A , CHANDONNET, P J., DEMARCO, J., SOCCOLICH, C.E.,
PEDRAZZANI, J.R., DIGIOVANNI, D.J., ANDREJCO, M.J., and SHENK, D.S :
‘UV-induced fiber grating OADM devices for efficient bandwidth
utilization’. Optical Fiber Communication, San Jose, CA, 1996,
Post-deadline paper 8
8 JOUANNO, J.-M , HUBNER, J., and KRISTENSEN, M.: ‘60-dB Bragg
gratings in planar waveguides’. Proc. Optical Fiber
Communication, Dallas, Texas, 1997, pp. 228-229
L
4-
& 1558.8
1558.61
.-Q
U
4
I
100
.
I
.
I
.
I
200
300
400
annealing temperature,deg
.
I
500
4I 20
Fig. 3 Bragg wavelength and tvansmission dip of UV written Bvagg
gratings against annealing tcmperature
Spectral method applied to design of
spotsize converters
A. Vukovic, P. Sewell, T.M. Benson and P.C. Kendall
The thermal behaviour of the Bragg gratings written in the planar waveguides was also investigated. As indicated previously, the
wafer was first heated to 80°C for 10h. Then the sample was
annealed for 2h at temperatures up to 500°C and the changes in
Bragg wavelength and in transmission dip were recorded at room
temperature after each annealing. Both the Bragg wavelength and
the transmission dip are almost linearly decreasing with the
annealing temperature as shown in Fig. 3. One of the critical
issues for WDM components used in telecommunications is to
achieve wavelengths in accordance with I.T.U. requirements. The
dependence of the Bragg wavelength on the annealing temperature
may be used in order to permanently tune the operating wavelength of the OADM.
Conclusion: We present an optical add-drop multiplexer based on
a symmetrical Mach-Zehnder interferometer with UV-written
Bragg gratings in the arms. The device does not need a subsequent
optimisation by UV-trimming. The -30dB bandwidth in transmission is 0.46nm, resulting in a positive BWU parameter. We thus
achieved both a low inter-channel crosstalk and a low intra-channe1 crosstalk which is a significant progress in Mach-Zehnder
based planar add-drop filters enabling use of such devices in
multi-wavelength networks.
0 IEE 1997
Electronics Letters Online No: 19971437
20 August 1997
J.-M. Jouanno, D. Zauner and M. Kristensen (Mikroelektronik
Centret, Technical University of Denmark, Bldg. 345 East, DK-2800
Lyngby, Denmark)
References
1
VENGHAUS, H.,
GLADISCH, A.,
JnRGENSEN, B.F.,
JOUANNO, I.-M.,
KRISTENSEN,M.,
PEDERSEN, R.J.,
TESTA, F.,
TROMMER, D.,
and
WEBER, J.-P.:
‘Optical addidrop multiplexers for WDM
communication systems’. Proc. Optical Fiber Communication,
Dallas, Texas, 1997, pp. 280-281
2
KASHYAP, R., MAXWELL, G.D., and AINSLIE, B.J.: ‘Laser-trimmed fourport bandpass filter fabricated in single-mode photosensitive Ge-
doped planar waveguide’, IEEE Photonics Technol. Lett., 1993, 5,
(2), pp. 191-194
3
HIBINO, Y.,
ALBERT, J.,
KITAGAWA, T.,
HILL, K.O.,
BILODEAU, F.,
MALO, B.,
and JOHNSON, D.c.: ‘Wavelength division multiplexer
with photoinduced Bragg gratings fabricated in a planar-lightwavecircuit-type asymmetric Mach-Zehnder interferometer on Si’, IEEE
Photonics Technol. Lett., 1996, 8, (l), pp. 8 4 8 6
ELECTRONICS LETTERS
4th December 1997
Vol. 33
Indexing terms: Integrated optics, Optical waveguides, Optical
couplers
A new spectral method is proposed and applied to the analysis
and design of integrated optical spotsize transformers. Results
presented are of the same accuracy as those from a finite
difference method. The new method is simple, efficient and gives
propagation constants and fields over a wide range of structures
in seconds, making it well suited to the class of optical waveguide
problems involving interacting rib and buried waveguides.
Both rib and buried rectangular waveguides are widely used in
integrated optical technology. One important application vertically
couples these structures to produce spotsize transformers which
allow more efficient coupling between semiconductor lasers and
optical fibres. These function by tapering down the width of the
small spotsize rib waveguide, which adiabatically forces the optical
field into a large spotsize mode of the passive buried waveguide [1,
21. Accurate design of these spotsize transformers is of great
importance as they offer improved coupling efficiency and reduced
alignment sensitivity. Although numerical solvers such as finite
differences, (FD) [3], and finite elements, (FE) [4], can be used for
analysing these devices, they are often too computationally cumbersome for use within an interactive design environment.
A new, computationally efficient and accurate spectral method
for the analysis of coupled rib and buried waveguides is reported
in this Letter. The method combines two previously developed
methods, namely the spectral index (SI) method [5],used for the
analysis of single and coupled rib waveguides, and the free space
radiation mode (FSRM) method [6] used for buried waveguide
analysis. Although both of these methods have proved highly successful in their own right, neither is individually suitable for the
analysis of the composite structure described above. The new
method takes advantage of the simplicity and speed of both SI
and FSRM, to provide the propagation constants and the field
profiles of the supermodes of the structure in a matter of seconds.
These can then be used to optimise the design of devices such as
spotsize transformers, making the new method of calculation
timely. The accuracy of the new approach is confirmed by comparison with results from large scale FD calculations.
Consider the coupled rib and buried waveguide structure illustrated in Fig. 1, for convenience divided into the separate regions
shown. The air-semiconductor part of the rib structure may be
simplified by use of an effective width and depth, W and K ,
respectively, which form the basis of the SI approximation and
allow the polarised field in the rib to be expressed as [5]
No.25
2121
(1)
The conventional spectral index variational principle is applied
at the base of the effective rib (j
= 0), linking the solutions in the
two regions above and below eqns. 1 and 2, giving a transcendental equation for p. The variational condition of the form:
where
where h is the operating wavelength, p is the waveguide propagation constant, D,are the amplitudes of the rib terms (to be determined) and N, is the number of terms necessary for convergence.
IV
Results: The new method is applied to a structure with dimensions
H = 0.5pn, a = 1pn, d = 0.1pn, 2c = 4 ~with, refractive indices
n, = 3.4382, n, = 3.1665, n3 = 3.2819 and h = 1 . 3 ~The
. modal
indices (pik,,) for the two quasi-TE supermodes supported by the
structure are shown in Table 1, obtained from the present method
and from a semi-vectorial FD method [3]. The structure supports
two modes, the rib dominant mode (pr) and the slab dominant
mode (p,). The results from the present method are obtained in a
matter of seconds on a Pentium PC, whereas the FD method
required significantly longer calculation time on a work station,
particularly for the slab dominant mode which, by design, is close
to cutoff and weakly confiied, thus requiring that many FD mesh
parts are used. Example field profdes for the case with W = 0 . 8 ~
are shown in Fig. 2a and b, respectively.
y
2c
m
n2
becomes a system of N, x N, equations, the non-trivial solutions of
which yield the propagation constants of the composite structure.
These procedures are straightforward to implement as a consequence of the spectral nature of both SI and FSRM methods.
Fig. I Coupled rib and buried waveguide geometry
In region 111, referred to for convenience as the 'buried slab
region', the field is expressed as a superposition of the N, guided
slab modes (supported by the symmetric waveguide of core width
2c) and a radiation field (unguided), which is assumed to exist in a
uniform medium of a suitable refractive index, typically n2.This
FSRM approach has proved valid for situations where the indices
differ by no more than 10% [7],which is the case for many practical structures. The fields in regions I1 and N are simply expressed
as superpositions of plane waves.
Let E,(s),As), g(s) and C,(s) denote the Fourier transform,
with respect to x,of the ith buried slab guided mode, the radiation
spectrum on the planes y = -a' and y = <U' + d) and each rib
term, respectively. Let A, and B, be the amplitudes of the ith slab
mode on the planes y = -d and y = <a' + d).The propagation
constant in the y direction for the plane waves in regions I1 and
IV and also for the radiation field in region I11 is:
y = (p2 - sg + s 2 ) 1 / 2
The propagation constants in the y direction for the slab
waveguide modes are
7 2
= (P2 - P
y2
where pzis the z th slab mode propagation constant.
Therefore, the transformed field in region I1 can be written as
32612/
Fig. 2 Field profile (10% contours) of rib dominant mode and slab dominant mode for W = 0 . 8 (quasi-TE
~
mode)
a Rib dominant mode
b Slab dominant mode
Table 1: Modal indices of two quasi-TE supermodes obtained with
present method and with finite difference (FD) method
where 6'= (a' + 4. The transformed field in region I11 is
and finally, the field in region IV is
[email protected](S,Y)
[
g(s)
N,
1
+ CBzEt(')
exp(y(y + b'))
a=l
(4)
Enforcing continuity of tangential electric and magnetic fields at
the planes y = -a', and y = -(U' + d),and requiring that the radiation field in region 111 is orthogonal to each guided slab mode,
gives a set of 2N, equations from which A, and B, can be expressed
in terms of the unknown constants D,.
2122
In conclusion, a new spectral method has been formulated for
the analysis and design of mode spot tapers. The method is
simple, fast and gives excellent agreement with benchmark FD
solutions.
Acknowledgment: The authors are grateful to BT Laboratories and
EPSRC (Grant ref. GR/L28753) for financial support and to J.
Heaton, DERA for helpful suggestions.
ELECTRONICS LETTERS
4th December 1997
Vol. 33
No. 25
0 IEE 1997
16 September I997
Electronics Letters Online No: 19971405
A. Vukovic, P. Sewell, T.M. Benson and P.C. Kendall (Department of
Electrical and Electronic Engineering, University of Nottingham,
Nottingham NG7 2RD, United Kingdom)
References
SELTZER, c.P.,
RIVERS, L.J.,
HARLOW, M.J.,
and
‘Low threshold current 1.6 mm InGaAsPiInP tapered
active layer multiquantum well laser with improved coupling to
cleaved singlemode fibre’, Electron. Lett., 1994, 30, pp. 973-974
2 LEALMAN, I.F., SELTZER, c.P., RIVERS, L.J., HARLOW, M.J., and
PERRIN, s.D.: ‘InGdAsP/InP tapered active layer multiquantum well
laser with 1.8 dB coupling loss to cleaved singlemode fibre’,
Electron. Lett., 1994, 30, pp. 1685-1687
1
LEALMAN, I.F.,
PERRIN, s.D.:
3
STERN, M.s.:
‘Semivectorial polarised finite difference method for
optical waveguides with arbitrary index profile’, IEE Proc. J.,
1988, 135, pp. 56&63
4 RAHMAN, B.M.A., and DAVIES, J.B.: ‘Finite-element solution in
integrated optical waveguides’, IEEE J. Lightwave Technol., 1984,
2, pp. 682-688
5 MCILLROY P.W.A : ‘Spectral index method: Single rib waveguide’, in
ROBSON, P.N.,
and
KENDALL, P.C.
(Eds.): ‘Rib waveguide theory by
the spectral index method’ (John Wiley & Sons Inc., 1990),Chap. 5
6
REED, M., BENSON, T.M., SEWELL, P., KENDALL, P.c., BERRY, G.M., and
DEWAR, s.v.: ‘Free space radiation mode analysis of rectangular
dielectric waveguides’, Opt. Qia“mm Electron., 1996, 28, pp. 11751179
M.J.:‘Semiconductor laser facet reflectivity using free
space radiation modes’, ZEE Proc. J., 1993, 140, pp. 49-55
ROBERTSON,
cations. To demonstrate these improvements we report wavelength
conversion of a 40Gbitis signal over a maximum of 24.6nm.
MQ W SOA: The operating conditions of an SOA when used as a
linear amplifier, and when used for nonlinear signal processing
(such as FWM), are very different. In the linear regime, the carrier
density is very high and, due to band filling effects, the wavelength
of highest gain can be as much as 5Onm from the bandedge. However, for good FWM performance, the amplifier is very strongly
saturated i.e. low carrier density. Thus, an MQW SOA with an
active region optimised for linear applications will not be optimised for FWM, and vice versa. We have designed and fabricated
an MQW SOA specifically for FWM.
We use strained ternary MQW material for the active layer as it
has a lower density of states than bulk or quaternary material.
Thus, the inversion factor close to the bandedge will be higher,
which results in a lower noise figure.
For a given carrier density fluctuation, larger index fluctuations
(which are beneficial to FWM efficiency) are produced when the
linewidth enhancement factor (aH)is large. We have positioned
the bandgap wavelength close to the operating wavelength (-1 550
nm) to obtain a large a,. This also ensures that the saturated gain
slope at the operating point with respect to wavelength is either
flat or negative, thus avoiding a strong reduction of the pump-signal power ratio as the beams propagate through the SOA. This could
lead to data pattern dependent effects through gain inodulation.
25
c
20 -15
=/-’
--
mym
i o --
40Gbit/s wavelength conversion over
24.6nm using FWM in a semiconductor
optical amplifier with an optimised MOW
active region
D -15
m
0
5-
<
a
A.E. Kelly, D.D. Marcenac and D. Nesset
Indexing terms: Semiconductor optical amplifiers, Opticalfrequency
Introduction: Semiconductor optical amplifiers (SOAs) are likely
to be key components in future optical fibre networks. This is
because, as well as performing linear amplification, SOAs exhibit
nonlinearities which have proven useful in numerous applications.
An important nonlinear process is four wave mixing (FWM) in
which a signal is mixed in the SOA with a ‘pump’ beam. This
results in the generation of optical sidebands which can be isolated
by filtering and used for demultiplexing [l], wavelength conversion
[2] and dispersion compensation [3].
To avoid waveform distortion caused by gain modulation in the
SOA, the pump power must be high in comparison to the signal.
Furthermore, to reduce ASE noise, the amplifier should be operated in saturation. With these constraints, the FWM efficiency
(and signal-noise ratio) can be too low for high bit rate systems.
Hence, there has been a great deal of interest recently in improving the eficiency of the FWM process in SOAs. Optimisation of
the SOA length has recently led to CW performance improvements [4] and 40 Gbitis transmission over 400km of standard fibre
using mid-span spectral inversion [5]. Since the efficiency of the
FWM process decreases rapidly with increasing pump probe
detunings, the maximum possible wavelength translation is a good
indication of device performance. Recently a record wavelength
conversion range of 18nm has been demonstrated at lOGbit/s [6].
In this Letter, we report the improvements obtained when the
SOA active region has been designed specifically for FWM appli-
ELECTRONICS LETTERS
4th December 1997
Vol. 33
-I?:/
-20 .=
i
i
~
The MQW active region co;sists of seven periods of 40A
unstrained ternary wells and 60A (-1 YOstrain) ternary barriers,
resulting in a photoluminescence wavelength of 1.553pn. SOA
devices were fabricated using buried heterostructure technology. A
2mm long device with multilayer antireflection coated facets was
packaged with lensed fibres. Fig. 1 shows the fibre-to-fibre gain
and the noise figure against current at 1.55pn. The device exhibited 20dB maximum fibre-to-fibre gain and a noise figure (2n,/c,)
of 7.6dB which includes at least a 3dB coupling loss. The increase
in the noise figure at currents 90mA is caused by residual reflections.
4OGbit/s OTDM system performance: To demonstrate the
enhanced performance of the FWM optimised SOA, it was used
to wavelength convert a 4OGbit/s, 27-1 PRBS, optical time division multiplexed (OTDM) data stream. Full details of the OTDM
system can be found in [SI. The power penalty against wavelength
shift at 10-9 bit error ratio (BER) is shown in Fig. 2. The circles
denote wavelength downshifts with an input data signal wavelength of 1557nm and the squares denote wavelength upshifts with
the data signal wavelength at 1543.5nm. Downshifts of up to
24.6nm and upshifts of up to 8.6nm are possible with BERs of <
lVY, indeed a negative penalty is observed for wavelength downshifts of -9nm. The negative penalty is thought to be due to the
No. 25
2123
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