Chapter 5
Exploited Fibers
Fibers described in this chapter exhibit strong nonlinear properties. This reduces the required input power and fiber length, thus making possible the implementation of more stable and compact devices.
5.1 OFS Highly Nonlinear Fiber
This fiber was developed by OFS Denmark [9, 10]. The fiber design includes a germanium doped high-index core, a deeply depressed ring yielding an effective area around 11-12 µm2 and a nonlinear coefficient around 10.5 W-1km-1. In addition the fiber dispersion is around 0 ps/nm/km at 1550 nm. The fiber non-linearity is summarized in the non-linear coefficient γ = 2πn2/λAeff. It follows directly that γ increases directly with n2 and inversely with Aeff. Because
the OFS HNLF is designed for low dispersion in the C-band, the geometry of the fiber is essentially fixed, which in turn fixes the effective area. Consequently, n2 must increase to
increase γ. This can be done by higher Ge doping of the core. Unfortunately, this also increases the fiber loss and the variation of n2 for practical core indices is less than a factor of
two for silica fibers. Increasing fiber loss decreases the fiber non-linear efficiency, as expressed by the effective length:
L 1 e Leff −α − = α
so, there is a trade off between effective area and loss.
The trade off between linearity, loss and dispersion has led to a design with a typical non-linear coefficient of 10-11 W-1km-1, a Kerr coefficient n2 of 3.0-3.25×10-20m2/W and an
effective area of 11.5 µm2.
Fig.5.1 Physical Characteristic and Optical Specifications at 1550nm.
As specified by OFS itself, this fiber is suitable for: • Pulse compression • Optical regeneration
• Super continuum generation • Parametric amplification • Optical sampling
• NOLM optical time domain demultiplexing • Wavelength conversion
5.2 Bismuth-based nonlinear fiber
The Bismuth-based nonlinear fiber we tested was developed by Asahi Glass. Since the nonlinear coefficient γ of the fiber is written as γ = 2πn2/λAeff, there are two possible
approaches to enhance the fiber nonlinearity γ. One is to reduce the effective core area Aeff
and the other is to use a glass material whose n2 is high. The fiber belongs to the step-index
family with an Aeff of 3.3 μm2 using a newly developed Bi2O3-based glass (Bi-NLF), which
has high refractive index more than 2.2 at 1.55 μm and a suitable cladding glass material for the step-index structure with small Aeff. Bismuth-oxide glasses were chosen because of their
high nonlinear refractive index (~15 × higher than silica), good mechanical and thermal stability and fast response among 3rd order nonlinear optical materials [13]. The Bi-NLF we
used exhibits an extremely high nonlinearity (γ =1250 W-1km-1) because of high nonlinearity
of the glass material and the small effective core area. In order to fabricate fiber with higher nonlinearity γ, core glass with n=2.22 and clad glass with n=2.13 at 1.55 µm were used [12]. The relationship between Aeff and the core diameter is shown in Fig.5.2:
Fig.5.2 Relationship of Aeff end the core diameter (left) and cross section of Bi-NLF(right).
To make Aeff smaller and to satisfy the single mode condition at 1.5µm, the core diameter is
necessary within the range from 1.9 to 1.4µm. Fig.5.2 also shows a cross section of the fiber which was drawn from a preform. The core diameter is 1.72µm and the fiber diameter is 125.4µm. Therefore, the effective core area Aeff is estimated to be 3.3µm2. This fiber can be
fusion-spliced to silica fibers, as shown in Fig.5.3, using an UHNA (Ultra High Numerical Aperture) fiber as a tapering fiber between Bi-NLF and SMF.
5.3 Polarization Maintaining Fibers
Single-mode propagation is only an ideal condition, since it would require a fiber with a perfectly cylindrical core of uniform diameter and isotropic material. Real single-mode fibers are not truly single-mode since they can support two degenerate modes that are dominantly polarized in two orthogonal directions. Only for an ideal fiber a mode excited with its polarization in the x direction would not couple to the mode with the orthogonal polarization state. In practise, small variations in geometry or anisotropy result in a mixing of the two polarization states thus breaking the mode degeneracy. The mode-propagation constant β becomes slightly different for the modes polarized in x and y directions, and this is referred to as modal birefringence [2]. The entity of modal birefringence is given by:
x y 0 | | B = | | k β −β = nx−ny
where nx and ny are the effective mode indices in the two orthogonal polarization states. The
axis along which the effective mode index is smaller is called the fast axis as the group velocity is larger for light propagating in that direction. Accordingly the axis with the larger mode index is called the slow axis. In conventional single-mode fibers B changes randomly along the fiber because of fluctuations in the core shape and stress-induced anisotropy. As a result, light launched into the fiber with a precise polarization quickly reaches a state of arbitrary polarization. For some applications it is desirable that fiber transmits light without changing its polarization state, thus appropriate fibers have been realized. Such fibers are called polarization-maintaining or polarization-preserving. They are obtained by intentionally introducing a large amount of birefringence through design modifications so that small, random birefringence fluctuations don’t effect significantly light polarization. One scheme consists of breaking the fiber cylindrical symmetry by making either the core or the cladding elliptical in shape. The degree of birefringence achieved by this technique is rather small (B~10-6). An alternative scheme makes use of stress-induced birefringence and can reach B~10-4. In a widely adopted design, two rods of borosilicate glass are inserted on the opposite sides of the core at the preform stage. The modal birefringence depends on the location and the thickness of the stress-applying elements.
Fig.5.4 Schematic illustration of different PM fibers.
Fibers of this type are “PANDA” and “BOW-TIE”. The use of a polarization maintaining fiber requires an identification of the slow and fast axes before the linearly polarized light is launched into the fiber. If the polarization axis of the incident light coincides with the slow or the fast axis, the polarization remains unchanged during propagation.
5.3.1 OFS PM Highly Nonlinear Fiber
Fig.5.5 Illustration of fiber cross section.
This fiber maintains almost the same nonlinear characteristics of the Not-PM version. It utilizes industry-standard, stress-applying parts (SAPs) to create two axes in the core, each of which guides light at a different velocity.
As a drawback, the fiber exhibits some problems with insertion loss due to internal splices.
5.3.2 Photonic Crystal Fiber
Photonic crystal fibers guide light by confining it within an array of microscopic airholes along the fiber. The cladding is constituted of air holes arranged as periodically as the wavelength of the light. Air in the cladding leads to a huge index step because of the large difference in the refractive index n between air (n = 1) and silica (n = 1.45).
Fig.5.6 Illustration of PCF [14].
Fabrication Techniques:
The typical starting point is to realize a preform using an array of hollow capillary silica tubes bundled around a pure silica rod replacing the center capillary. In a fiber draw tower, the manufacturer heats the preform to around 2000°C and carefully pulls the preform, using gravity and pressure, into a fiber typically 125μm in diameter [15].
Fig.5.7 Fabrication of a PCF: Preform comprises a stack of tubes and a single solid rod to form core; Preform is mounted in furnace and heated to ~2000°C; The preform is drawn to fiber; The fiber is coated and coiled on spools [14].
This microscale fiber maintains the structure of the preform. By altering the pattern of airholes or the materials used, it is possible to manipulate many characteristics of PCFs, such as the singlemode cut-off wavelength, the NA, and the nonlinear coefficient. This allows the availability of various classes of PCFs, each one presenting a particular design:
Fig.5.8
Large-mode-area fibers
Fig.5.10
High Numerical Aperture fibre Fig.5.9 Polarization-Maintaining PCF Fig.5.11 Large-Mode-Area Air-Clad fibre Fig.5.12
Nonlinear Photonic Crystal Fibres Very high index contrast
Fig.5.13 Main parameters of the PCF we used in our measurements. To notice the values of nonlinear coefficient and GVD [16].
