3.1

Fiber design

For the broadband wavefront filtering application in DARWIN, the waveguide must satisfy strict requirements concerning operational wavelength range, diameter and profile of the fundamental optical mode, etc. Various waveguide technologies, including hollow fibers, planar waveguides, index guided photonic crystal fibers, photonic band gap fibers and the conventional step index single mode fiber were compared taking into account these requirements. On the basis of this comparison, the conventional single mode fiber was selected as most suitable technology for meeting all essential requirements for DARWIN.

For a conventional step index single mode fiber, the coupling efficiency will drop dramatically with longer wavelength. Hence, splitting the entire DARWIN wavelength range of 4 – 20 μm into several bands, involving different fiber designs, is the most favourable solution. As compromise, a dual bands system consisting of two different single mode fibers for each wavelength band is proposed: the short wavelength (SW) fiber and the long wavelength (LW) fiber.

Using the refractive index and dispersion data achieved for the bulk TAS glasses, waveguide modelling calculations resulted in the following parameters for the dual band configuration (see table 1).

Table 1:

Dual wavelength band system

For the short wavelength (SW) fiber the relation between the Mode Field Diameter of the fundamental mode and the wavelength has been modeled, using a commercial fiber design software package. The calculation results are presented in figure 3.

Fig. 3:

Calculated Mode Field Diameter (MFD) for the Short Wavelength TAS fiber as a function of the wavelength (blue line with markers). The red dashed line presents the desired linear relation between the MFD and the wavelength for optimized coupling @ 8 μm.

It is important to note that the cut-off wavelength of the TAS SW fiber is calculated to be about 3.72 μm. Furthermore, the coupling efficiency over the operational wavelength is found to be higher than 80%. In the following the fiber manufacturing and characterization of an SW step index TAS fiber configuration are described.

3.2

Fiber manufacturing

Step index core-clad fibers were manufactured using different preform techniques, basically modifications of the well-known rod-in-tube prefom method.

For all techniques cladding tubes must be fabricated.

The cladding tubes have been made by the rotational casting method. The molten cladding glass contained in a quartz ampoule is subjected to fast horizontal rotation in a centrifugal machine with speeds between 3000 and 5000 rounds per minutes. Subsequently, the silica tube is annealed and cooled to room temperature. The dimensions are typically 4 mm internal diameter, 10 mm external diameter and 10 cm length. The internal diameter of the tube is defined by the weight of the glass introduced in the silica ampoule.

Core glass rods of 4 mm diameter are obtained by stretching a 10 mm rod in core glass composition with the help of a drawing tower. When using the common rod-tube method, a 4 mm core rod is inserted in a clad tube with an external diameter of 10 mm and an internal diameter of 4 mm. After stretching this core/clad1 preform into a new rod of 4 mm diameter, the rod-in-tube method can be repeated a few times in order to obtain a final core/clad1/clad2/clad3 preform with the required core/clad diameter ratio as in the final fiber. The fiber is drawn from the final preform. However, it has been found that, especially the critical interface for waveguiding, i.e. the interface between core and clad1, is far from perfect when the above common rod-in-tube method is applied. Therefore, alternative methods have been developed and tested. The modified methods are denominated as Internal Rotational Casting Method (IRCM), Fiber in Tube Vacuum (FTVM) and Rod in Tube Vacuum Method (RTVM). These different preform methods are described in separate papers [3,4].

TAS fibers with a core diameter of 22 μm and a cladding diameter of about 500 μm have been drawn from preforms, using a set-up, schematically shown in figure 4. The diameter is controlled by the rotational speed of the drum and the vertical speed of the preform in the furnace. Typically, fibers of 5 - 10 m length have been drawn without interruption or breakage.

Fig. 4:

Scheme of fiber drawing of preform by FTVM

Several fiber samples have been coated with an absorbing Gallium coating to eliminate the propagation of higher order optical modes in the cladding (cladding modes). For this, Gallium is melted in a holder at a temperature of 55 °C. Then the TAS fiber is pulled through the molten Gallium several times to obtain a homogeneous coating. The thickness of the coating varies typically between 15 and 45 μm. TAS fiber samples up to a length of 50 cm have been treated successfully using this method.

4.1

Set-up for far field intensity measurements

To characterize the mode profile of transmitted light through the fiber samples, 2D far field intensity (FFI) distribution measurements have been carried out.

The set-up used for FFI measurements at 10.6 μm wavelength basically consists of a CO2 laser and a 2D-array microbolometer IR CCD camera and is shown schematically in fig 5.

Fig. 5:

Set-up for FFI distribution measurements with step index TAS fibers

A pinhole of 18.3 μm diameter in front of the fiber sample ensures that the laser light is coupled mainly into the core of the fiber. The light transmitted by the fiber is analyzed by the IR camera.

With this set-up, measurements have been conducted on different, step index TAS fiber samples to evaluate the effect of different parameters on the performance of the fibers, including:

4.2

Results of optical characterization

A typical far field intensity distribution of an uncoated step index TAS fiber is shown in figure 6. The large number of side lobes visible in this figure indicates the presence of many higher order modes in the transmitted signal.

Fig. 6:

Typical 2D far field intensity distribution of an uncoated step index TAS fiber, showing many side lobes

In order to suppress the propagation of cladding modes, a 23 cm long piece of a step index TAS fiber has been coated by an absorbing Ga coating. The measured far field intensity distribution of this 23 cm long Ga-coated TAS fiber is visualized in a 2D and 3D perspective in figure 7.

Fig. 7:

Representation in 2D and 3D of far field intensity distribution of a 23 cm long TAS step index fiber sample, coated with an absorbing Ga-layer

Apparently, the transmitted signal has only one remaining peak with a Gaussian intensity distribution, indicating single mode operation of the fiber. However, as can be seen in figure 7, the output intensity is not circular symmetric, but elliptical. This is most probably caused by non-rotation-symmetric stresses, introduced during the fiber drawing process (IRCM).

It has been found that the Rod in Tube Vacuum Method (RTVM) provides improved control of the final fiber quality parameters, like a smooth interface between core and cladding and a good circularity of the core, as can be seen in figure 8 and 9. Figure 8 shows a cross section of a TAS fiber with a high refractive index contrast between core and cladding, prepared by RTVM. Figure 9 shows the far field intensity distribution of a 36 cm long TAS step index fiber, prepared by RTVM and coated with a Ga-layer. It can be seen in this figure that this fiber has a circular intensity distribution.

Fig. 8:

Picture of a cross section of a high contrast step index TAS fiber prepared by RTVM

Fig. 9:

FFI measurement @ 10.6 μm measured at a distance of about 10 mm from a 36 cm Ga-coated TAS fiber, prepared by RTVM

The attenuation of the single mode fiber samples has been measured by the cut-back method using the CO2 laser at 10.6 μm. The attenuation at this wavelength is found to be about 0.1 dB/cm. Thus, it may be assumed that the minimum loss in the region 4 – 9 μm will be lower than 0.1 dB/cm.