On the mechanism of non-linear optical attenuation at 1.3–1.5 μm in arsenic sulfide and tellurium oxide glasses
Introduction
Non-resonant ultrafast (10–100 fs timescale) optical non-linearity is of interest for all-optical switching in telecommunication networks operating at 1.31 and 1.55 μm (see e.g., [1], [2], [3]). A large interaction length available in fibres allows using non-resonant non-linearity despite the fact that it may be several orders of magnitude less than resonant non-linearity [1], [2], [3], [4]. Silica-based fibres are not suitable for all-optical switching since their non-linear non-resonant refraction at these wavelengths is too small (about ). Doping of silica with metals can result in enhancement of non-linear refraction, but in this case it is resonant and therefore its response time is limited in the nano-second scale [4], which is too slow. Hence, it is necessary to examine other types of glasses with larger non-resonant non-linearity while getting more insight into mechanism of such non-linearity.
It was shown that As2S3- (see for example, [1], [2]) and TeO2-based (see for example, [5]) glasses are suitable for fibre drawing. Non-linear non-resonant refraction about 100 times larger than in silica fibres was measured in As2S3 glass fibres at 1.55 μm and a fibre device for ultrafast demultiplexing was demonstrated [1], [2]. Non-linear refraction about 2–5 times less than in As2S3 glass was also measured in TeO2-based glasses at 1.5 [6] and 1.3 [7], [8] μm with a response time about 100 fs. Therefore these fibres can be an alternative to silica-based fibres for an application in all-optical fibre switches.
In this paper, we discuss the mechanism of non-resonant non-linear attenuation (loss) in As2S3- and TeO2-based glasses which have a linear refractive index n=2.44 and 2.1±0.1, respectively, at 1.3 and 1.5 μm. Non-linear absorption is a main impediment in the application of non-linear glass fibres in ultrafast optical switching [3]. We argue that non-linear scattering also contributes to the non-linear attenuation (loss) in these glasses at 1.3–1.5 μm. Non-linear (small-angle) scattering appears at the irradiance at which non-linear absorption is not present yet. We show that a single-beam Z-scan technique can be a tool for an evaluation of contributions of non-linear absorption and non-linear scattering to the total non-linear attenuation (loss).
Section snippets
Z-scan technique for measurement of optical non-linear absorption and refraction
Procedures for the preparation of glasses and fibres were described in [1], [2], [5]. Z-scan technique for measurements of optical non-linear absorption and refraction is described in [9]. In this technique, a 200–400 μm thick [7], [8] sample is moved along the Z-axis passing the focal point of the lens, which focuses the laser beam. The focal point is situated at Z=0 in Fig. 1. The intensity of the transmitted beam is detected with two photodetectors placed behind a beam splitter and open (an
Discussion
The field-dependent refractive index n is defined [3] by the relationship (1)where n0 is the linear and n2 is the non-linear refractive index. I is the light intensity. Two-photon non-linear absorption coefficient, β, is determined [3] from Eq. (2):
Non-resonant refraction, n2, is due to a non-linear motion of bound electrons in the electromagnetic field of an intensity, I, and it has a femtosecond response time (see e.g., [12]). An intensity-independent figure-of-merit
Conclusion
We identified two contributions to the optical non-resonant non-linear attenuation (loss) at 1.3 to 1.5 μm in large linear refractive index glasses, such as As2S3- and TeO2-based glasses. We showed that a single beam Z-scan technique resolves distinguish the contributions which are due to non-linear absorption and non-linear scattering.
Acknowledgements
This work was supported by a travel grant of The Royal Society, UK. We acknowledge Professor Banfi, University of Pavia, Italy for useful comments about this work and bringing to our attention [19].
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