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We would like to point out that, unless there is an experimental proof that this coincidence exists, the experimental
results shown in that letter are not sufficient to support the
conclusion that the fibre preserves the s.o.p.: the experimental
results are also compatible with the behaviour of the output
s.o.p. when polarisation mode coupling exists.BC In particular,
there always exist two orthogonal input polarisation directions
leading to linear output polarisations. But these s.o.p. are not
eigenstates and these directions depend on fibre length,
mechanical and thermal environment etc. Thus to ensure that
the fibre really maintains the s.o.p., one has to check that these
particular input directions are independent of the fibre length,
temperature and more general environmental conditions.
M. MONERIE
L. JEUNHOMME
C. VASSALLO
CNET Lannion B
Department MER/FOG, BP 40
22301 Lannion, France
17 th October 1980
of temperature fluctuations within ± 1°C during the
measurements.
Our assumption has also been ascertained by measuring the
state of polarisation while applying uniaxial stress on a short
piece of fibre. If the direction of the stress applied on the fibre
coincides with the major or minor axis of the fibre, the state of
polarisation changes little.
Thus it can be concluded that the 21 km v.a.d. single-mode
fibre used in the articleA has major and minor axis polarisation. The experimental results concerning the stress effect on
state of polarisation will be submitted to Electronics Letters at
a later date.
K.
Y.
T.
M.
T.
OKAMOTO
SASAKI
MIYA
KAWACHI
EDAHIRO
12th November 1980
Ibaraki Electrical Communication Laboratory
Nippon Telegraph & Telephone Public Corporation
Tokai, Ibaraki 319-11, Japan
REPLY
References
We would like to thank Monerie, Jeunhomme and Vassallo for
their thoughtful comment on our recent articleA concerning
state of polarisation in a 21 km v.a.d. single-mode fibre.
They point out that we did not give experimental
verification of the assumption that the input direction for
which linear output polarisation was obtained coincides with
the major or minor axis of the fibre.
In fact, we found the output polarisation angle at which the
maximum degree of polarisation is detected in the single-mode
fibre used in the articleA to be stable even under the existence
EFFICIENT LASER-DIODESINGLE-MODE-FIBRE COUPLING USING
TWO CONFOCAL LENSES
A
OKAMOTO, K., SASAKI, Y., MIYA, T., KAWACHI, M., a n d EDAHIRO, T.:
'Polarisation characteristics in long length v.a.d. single-mode
fibres', Electron. Lett., 1980, 16, pp. 768-769
B
MONERIE, M., MOUTONNET, D., and JEUNHOMME, L. : 'Polarisation stu-
dies in long length single mode fibres'. Sixth ECOC, York, England, 16-19 Sept. 1980, pp. 107-109
C MONERIE, M., and JEUNHOMME, L.: 'Polarisation mode coupling in
long single mode fibres', Opt. & Quantum Electron., to be published
0013-5194/80/250954-02$!.50/0
on the ratio, an appropriate combination of lenses can be
selected. The optimal magnification is defined by wo/\/(vvx wn),
where w± and wn are the spot sizes perpendicular and parallel
to the junction plane, respectively.
Indexing terms: Optical connectors, Optical fibres
Lens I
A new method for coupling a laser diode to a single-mode
fibre is described. It uses a spherical ruby lens and a g.r.i.n.
rod lens, which are positioned nearly under confocal conditions. A high coupling efficiency of more than — 3 dB has been
realised, and stringent alignment tolerances required for conventional coupling circuits have been overcome.
Introduction: Optical fibre transmission using single-mode
(s.m.) fibres is now becoming feasible, especially for high capacity and long haul communication systems. In order to fully
utilise s.m. fibre systems, coupling of laser diodes (l.d.) to s.m.
fibres must be improved. Recently, various coupling methods,
such as cylindrical lens,12 combination of cylindrical lens and
g.r.i.n. rod lens,3 hemispherical lens fabricated on the fibre
endface4 and tapered hemispherical end s.m. fibre5 coupling
methods have been reported. Although coupling efficiencies for
these methods can be improved, these impose stringent alignment tolerances because of the extremely small lenses used. In
this letter, a new efficient coupling circuit which consists of a
relatively large spherical ruby lens and a graded-index (g.r.i.n.)
rod lens positioned nearly confocally, and which overcomes
the usual stringent alignment tolerances, is proposed.
Coupling principle: To increase the coupling efficiency, the
elliptical l.d. mode must be transformed to match the circular
s.m.fibremode. For a single-lens circuit, optimal focal length is
derived by the following equation: fopt = TTW, WO/X, where Wj
and w0 are spot sizes of l.d. and s.m.fibre,respectively, and X is
a wavelength. By inserting the typical values of w, = 1 (xm,
w0 = 5 //m, and X = 1-3 /mi, the optimal focal length is calculated to be about 10 pm. This is the reason why the conventional microlens approach imposes stringent alignment
tolerances.
By adopting the confocal lens system, where lenses of focal
lengths/i and/ 2 are positioned as shown in Fig. 1, the l.d. spot
size can be magnified by/ 2 //i. Because the magnification does
not depend on the absolute values of the focal lengths but only
ELECTRONICS LETTERS 4th December 1980
Lens 2
2wix(fa/fi)
2wi
LD CHIP
RUBY LENS
SINGLE-MODE FIBRE
GRIN ROD LENS
Fig. 1 Schematic diagram and photograph of proposed laser-diodesingle-mode fibre coupling circuit
Table 1 COUPLING LOSS FOR PROPOSED
LENS CIRCUIT
Item
Coupling loss
Lens insertion
loss
1
2
3
Proposed circuit
Butt joint
experiment theory experiment theory
3-5 dB2
1-4 dB1
96 dB
90 dB1
10 dB
11 dB3
—
—
Gaussian beam approximation used
Including lens insertion loss
Calculated by Fresnel reflection loss
Vol. 16 No. 25
955
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