King Fahd University of Petroleum & Minerals Electrical Engineering Department

EE 633: Optical Fiber Communications Assignment Report 2009/2010 (092)

Subwavelength-diameter optical fibres (SDFs) Ahmad Suhail Salim

EE 633: Optical Fiber Communications

Abstract— Interest in subwavelength-diameter optical fibre

(SDF or SDOF) has been remarkably growing. These fibres have

diameters smaller than the wavelength of the transmitted light

which essential for the future generation of photonic devices.

Many techniques have been proposed to fabricate these fibres by

using a wide range of bottom-up techniques such as chemical or

physical vapor deposition and top-down processes such as fibre

drawing. In this paper we briefly introduce SDFs with spotting

the light on their features, applications, and fabrication.

I. INTRODUCTION

Many names have been used in the literature to refer to the

SDFs, some of these are photonic nanowires, optical fibre

nanowires (OFNs), submicron-diameter silica fibres, ultra-thin

optical fibres, etc. We will use these names interchangeably.

Prior to 2003 only two attempts to manufacture

submicrometer wires by using a top-down process were

reported in the literature [1]. Interest in OFNs has been limited

mainly because of the perceived difficulties in manufacturing

suitably low-loss structures. Although several SDFs were

fabricated by using a variety of bottom-up methods, all of

them exhibited an irregular profile and a surface roughness

that appear to have limited the loss levels that could be

reliably achieved [1]. In 2003 a two-step process to fabricate

low-loss submicrometric silica wires was presented [2]; it

involved wrapping and drawing a pretapered section of

standard fibre around a heated sapphire tip. Although the

measured loss was orders of magnitude higher than that

achieved later with flame-brushing techniques [3], it was low

enough to allow the use of SDFs for optical devices and gains

researchers’ interest in the technology.

The remainder of this report is organized as follows. In

section II, the fabrication methodologies of SDFs are

discussed. Section III presents the features of SDFs, followed

by the applications of SDFs in section IV. Section V

illustrates the constraints on the taper length. Section VI draws

the report’s conclusions.

II. FABRICATION

In the past few years, three different methodologies have

been used to fabricate SDFs from optical fibres:

A. Tapering the fibre by pulling it around a sapphire rod

heated by a flame

B. The flame-brushing technique

C. The modified flame-brushing technique

We will explain the first technique in this report. Special

pulling machines are used for it. An optical fibre usually

consists of a core, a cladding and a protective coating. Before

pulling a fibre, its coating is removed (the fibre is stripped).

Then the fibre is fixed at two ends on the movable translation

stages of the pulling machine. The middle of the fibre between

the stages is then heated with a flame or a laser beam and at

the same time the translation stages move in the opposite

directions [2,4]. The glass melts and the fibre is elongated so

that its diameter decreases as shown in Fig 1.

Fig. 1 Schematic diagram of drawing a wire from a micrometre-

diameter silica wire wound around the tip of a sapphire taper.

The flame or laser beam usually moves in order to obtain

waist of significant length and constant thickness. The

resulting structure comprises a narrow stretched filament (the

taper waist of 1-10 mm in length and diameter down to

100 nm), each end is linked to an unstretched fibre by a

conical section (the taper transition region), as shown in Fig. 2.

Fig. 2 Optical fibre taper.

III. FEATURES

SDFs are of interest for a range of emerging fibre optic

applications, since they offer a number of enabling optical and

mechanical properties, including the following:

1. Strong confinement. Light can be confined to a very

small area over long device lengths, allowing the ready

observation of nonlinear interactions.

2. High power in the evanescent field. A considerable

fraction of the power can propagate in the evanescent field

outside the SDF physical boundary.

3. Great configurability. As shown in Fig 3, these fibres do

not break when bent and/or twisted, indicating that they have

excellent flexibility and mechanical properties [2].

4. Low-loss connection. Low-loss connection to other

optical fibres and fibreized components is possible; since

SDFs are manufactured by adiabatically stretching optical

fibres, they maintain the original fibre size at their input and

output, allowing ready splicing to standard fibres and

fibreized components. Insertion losses smaller than 0.1dB are

commonly observed.

King Fahd University of Petroleum & Minerals Electrical Engineering Department

EE 633: Optical Fiber Communications Assignment Report 2009/2010 (092)

5. SDFs can be fabricated with very good uniformity of

diameter and surface smoothness compared to submicrometre- or

nanometre-width wires, strips or other structures obtained by

earlier developed fabrication methods [2].

Fig. 3 (scanning electron microscope (SEM) images)

a, A 15-mm diameter micro-ring made by 520-nm-diameter SDF

b, Two twisted 330-nm-diameter wires.

IV. APPLICATIONS

Applications of SDFs can be classified into three main

groups according to what property they exploit:

1. Evanescent field

2. Confinement

3. Transition regions

Evanescent field applications harnesses the power propagating

outside the physical boundary of the wire and include high-Q

knot, loop, and coil resonators, particle manipulation [1], and

chemical and bio-sensing in liquid media [2]. Applications exploiting the confinement properties of SDFs

include supercontinuum generation, particle trapping, and

nonlinear switching [1].

On the other hand, transition regions have been exploited to

convert and filter modes [1]. Fig 4 represents an idealized

SDF for higher-order mode filtering, If the conical transition

tapers are adiabatic, guided modes in the core of the

multimode fibre (LP01, LP11 in Fig. 1(a)) are continuously

mode converted to guided cladding modes in the SDF on a

one-to-one basis by the down-taper and are then coupled back

into guided modes in the multimode fibre by the up-taper.

However, higher-order modes can be effectively suppressed

by controlling the SDF diameter in the uniform waist region.

In addition to that, SDFs might be used for the next

generation of computer processing. Also, SDFs are suitable

for applications where tight waveguide bends are desired due

to their mechanical flexibility [2].

Fig. 4 (a) Non-filtered taper (b) Filtered taper

V. CONSTRAINTS ON THE TAPER LENGTH

Creating an optical fibre nanowire involves heating and

drawing a section of a larger-diameter fibre to create a section

of nanofibre having a taper at one or both ends (depending on

application). Knowledge essential for proper design includes

not just the mode intensity profiles in and around the

nanofibre but also the efficiency of the light transfer from the

section of larger-diameter fibre through the taper and into the

nanofibre. The researchers calculated this for various

situations, based on a previously developed criterion [5].

The researchers determined the taper length required for an

adiabatic (lossless) transmission of light as shown in Fig 5.

They phrased their results in terms of the ratio of the

nanofibre diameter to the light wavelength (λ); as a

consequence, the results are useful at other wavelengths as

well.

For a diameter equal to 0.6λ or more, the taper length need

only be 10 µm for adiabatic coupling, while for a diameter of

0.29λ, the taper should be about 1 mm long. For a diameter of

0.16λ, however, the required minimum taper length for true

adiabatic coupling is an enormous 1 km.

Fig. 4 Adiabatic (lossless) coupling from a standard optical fibre into a

nanofibre is achieved only if the taper length is above a certain minimum.

VI. CONCLUSIONS

In this report, the characteristics of subwavelength-diameter

optical fibre were investigated. the features that these fibres

can give shows that these fibres will be promising components

in the future photonic devices to provide various applications.

REFERENCES

[1] Brambilla et al., ―Optical fibre nanowires and microwires: fabrication

and applications‖, Advances in Optics and Photonics, Vol. 1, Issue 1,

pp. 107-161 (2009) [Online]. Available:

http://www.opticsinfobase.org/DirectPDFAccess/C0FB626F-BDB9-

137E-CE797CEFD6EFAA4C_176227/107-fulltext.xhtml.

[2] Tong et al., ―Subwavelength-diamter silica wires for low-loss optical

wave guiding‖, Nature 426, 816 (2003)

[3] G. Brambilla, V. Finazzi, and D. J. Richardson, ―Ultra-low-loss optical

fibre nanotapers,‖ Opt. Express 12, 2258–2263 (2004).

[4] Subwavelength-diameter-optical-fibre,

http://en.wikipedia.org/wiki/Subwavelength-

diameter_optical_fibre#cite_note-2

[5] Alexander Hartung et al., Opt. Express 18, 3754 (2010).