Novel in-line fiber-optic filters and polarisers.
Antonio Diez, Miguel V. Andres, Jose L. Cruz,
Departamento de Fisica Aplicada, Universidad de Valencia, Dr. Moliner 50,
46100 Burjassot (Valencia), Spain. Tel. (34-6) 3864760
D.O. Culverhouse,
Optoelectronics Research Centre, University of Southampton, Southampton
S017 1BJ, United Kingdom
Abstract
Novel in-line devices, based on a tapered fiber whose uniform waist has been metal-coated, are reported. The
resonant excitation of a surface plasma mode of the metal film allows for the design of wavelength filters and
polarisers.
We have developed a novel technique to fabricate in-line fiber-optic devices, such as wavelength tunable
filters and polarisers. Our method is based on a fusion-pulling technique [1] to produce tapered single-mode
fibers whose uniform waists are coated with a metal layer.
A theoretical analysis [2], using a standard boundary-value method, has been carried out to obtain the mode
spectrum of a cylindrical dielectric/ dielectric/metal/dielectric waveguide, whose structure corresponds to the
core and the cladding of the fiber, the gold coating and the external medium. This analysis shows that
tapering a standard 125 m single-mode fibre, to give an uniform waist of about 20-40 m diameter and 10
mm length, and coating, the waist with a gold layer of about 10-30 nm, one can achieve the conditions for
resonant coupling between the core-mode and a hybrid surface plasma mode of the metal layer.
The devices were fabricated from a standard single-mode fiber with a core diameter of 7.5 m, a cladding
diameter of 125 m and a cutoff wavelength of 1.2 m. A thin layer of gold was evaporated onto the waist
after narrowing the fiber using a fusion-pulling technique. Some devices were metal-coated on two opposite
sides to obtain a quasi-cylindrical symmetry, while the other devices were coated only on one side.
Figure 1 gives the transmission spectra of a quasi-cylindrical device showing that the resonant coupling can
be controlled, changing the external refractive-index. The characteristics of these devices allow for the
design of tunable wavelength filters and external refractive-index sensors.
Figure 2 gives the transmission spectra of a one-side coated device changing the polarisation of the input
light. In this case, the spectra for two orthogonal linear polarisation states were recorded to illustrate the
polarising characteristics of this asymmetric device.
All the results have been recorded using a halogen lamp and a monochromator with a 6 nm spectral
bandwidth. A linear polariser was inserted before coupling the light into the fiber in the case of figure 2. In
this case, the input fiber was placed in a straight line to insure no polarisation change.
Conclusion:
Novel fiber-optic spectral filters and polarisers have been fabricated by coating the waist of a tapered fiber
with a gold layer.
Acknowledgement:
Financial support for DGCYT of Spain (grant TIC93- 1203) is acknowledged.
References.
[1]
R.P. Kenny, T.A. Birks, K.P. Oakley, "Control of the optical fibre taper shape", Electronics Letters,
Vol. 27, No. 18, pp. 1654-1656, 1991.
[2]
A. Diez and M.V. Andres, "Cylindrical multilayer optical waveguides: Applications.", Second
Iberoamerican Meeting on Optics, SPIE Proc., Vol. 2730, pp. 514-517,1996.
Figure 1. Transmission spectra of a taper with a waist diameter of 30 m and a gold layer of 24 nm, for
three different external refractive indices (nominal values of commercially available Cargille fluids): (i)
1.436, (ii) 1.438 and (iii) 1.440.
Figure 2. Transmission spectra of a taper with a waist diameter of 34 m and a gold layer of 25 nm
evaporated only on one side, for two orthogonal linear polarisations oriented at (a) 0o and (b) 90o