The damage caused by cleaving holey fibers is investigated as a function of cleaving force. Comparisons are made with standard optical fibers and holey fibers. Optimum cleaving forces are determined for a number of holey fiber air fractions and fiber diameters. A simple technique for removing cleave damage is also presented.
©2003 Optical Society of America
During the past few years, there has been extensive investigation into the fabrication, properties and application of holey fibers [1–3]. These fibers contain air holes that run the length of the fiber and are generally created by stacking and drawing high quality capillary tubes. Guidance in these fibers is a result of either a difference in index between the solid fiber core and the holey cladding, or via the existence of a confining photonic band gap. Holey fibers offer a range of potential advantages over standard fibers such as extended single mode operation, and the ability to control group velocity dispersion and mode-field diameter.
In this paper we investigate the effect that the holey structure has on the precision cleaving of these fibers. For normal fibers, specific forces are required in order to optimize the quality of the cleave produced. It is not surprising that for holey fibers, standard fiber cleaving conditions are inadequate, and in fact cause significant damage. Even in the highest quality fibers produced by numerous authors, damage is apparent proximate to the holes in the fiber end faces [4–8]. When fibers are cleaved, a crack propagates through the material from one side of the fiber to the other. Modifications in the material composition, change the local propagation, often causing interference and damage. One previous publication deals with this type of structural damage for normal fibers with strongly doped cores . For the case of holey fibers, the material differences are maximized in the “hole” regions. The crack in this case is forced to propagate around the hole.
In order to accurately characterize the damage caused by cleaving for holey fibers, we have used a standard atomic force microscope to profile the fiber end face surface quality. This local probe technique has been used numerous times for fiber profiling in the past, and has been demonstrated to be an excellent tool for high resolution fiber characterization [10–12].
The issue of damage formed during processes such as cleaving [13–15] is a significant one as it can form the basis for accelerated degradation of the fiber. For example, these regions can serve as convenient seeds for water-based crack growth in the glass as well as cause potential mechanical fatigue problems when spliced onto other fibers or inserted into precision aligned components. The long-term performance of these fibers as serious alternatives to conventional fibers, therefore, is brought into question. Consequently, there is considerable interest in overcoming the problems associated with cleaving holey fibers. In this article we have considered two alternative solutions. The first is related to the specific force used during the cleaving process. By carefully taking note of the fiber size and air fraction, adequate cleaving forces can be determined to minimize fracture damage. The second methodology is more time consuming and involves the use of chemical etchants to remove the damage from the fiber end face.
2. Experiment and results
To investigate the effect of cleaving on holey fibers, two fiber samples were created with different air fractions. In addition, fibers were fabricated both via capillary stacking and hole milling to distinguish any possible effect caused by the stacking process. A standard solid silica fiber was also cleaved for use as a control. Standard MCVD optical fibers are cleaved under 200–250g force, producing optimum results. Throughout this paper we will use the unit of ‘g’ to represent forces, as found on the commercial cleaving tool. This refers to grams and as such a force of 200g is equivalent to 0.2×9.80665=1.96 N.
The York FK11 cleaving tool was utilized in these experiments to produce forces with ±2g accuracy. For each tension used the fiber was cleaved three times to confirm the quality of the end face. On occasion, due to problems with the fiber or cleaving there was a delay in the breaking of the fiber. In these cases, the fiber was re-cleaved. The resulting fiber end faces were measured using a commercial atomic force microscope (AFM) operating in contact mode. The AFM is capable of measuring the fiber end face topography with nanometer lateral precision and 0.1nm vertical precision. The fibers utilized and their parameters are listed in Table 1.
Figure 1 shows an AFM image of the end face of holey fiber ASM001_A2 cleaved using the York FK11 with a force of 220g. The damage to the fiber end face caused by the cleaving process in this case is clearly evident.
The fracture origin in this case is to the right of the bottom left corner of the image. Each line of damage is observed to extend away from this origin, with the damage increasing as the fracture propagates through the fiber. In this case the damage caused by cleaving behind each hole is of the order 200nm in height. The increase in damage with distance from the fracture origin observed is due to the increase in speed of the propagating crack front through the fiber. The applied force in the system is kept constant, but as the fiber fractures, the stress is distributed over the region of fiber left unaffected, which is reduced in size as the fracture propagates. As a result of constant force and reduced fiber area, the speed of fracture propagation increases as the force per unit area increases (increased stress), causing greater damage.
During our initial investigation, an interesting question arose with regards to the location of the damage. It was suggested that the damage might occur along the interfaces of the original capillaries. In order to discount this idea, both a milled preform fiber (i.e., no capillaries) and a capillary fiber with the interstitial holes open were examined. The milled preform fiber showed the same evidence of damage as the capillary fiber (Fig. 1). The capillary fiber with the interstitial holes open clearly demonstrated that the damage lines were not directed to the interstitial holes and that in fact the interstitial holes themselves had the same structural damage (Fig. 2 - Left).
Also noteworthy, is that the damage caused on the far side of the hole is always located at a point such that the two paths around the hole are of equal distance. Consider Fig. 2 (right) in which asymmetric holes are presented. The fracture propagation in this case begins in the bottom right hand corner and is represented by a solid line. As it approaches the first hole, it is diverted to the left and right of the hole around paths A and B respectively. The crack meets up again on the far side of the hole and the overlap causes the expected damaged to the end face. If we now consider holes 2 & 6 and 3 & 5 its clear that the paths A and B around these two pairs does not place the damage on the far side of the hole, but at a point equidistant along paths A and B from the first point of contact with the hole.
There are two key issues that need to be considered with regards to the damage demonstrated for holey fiber end faces. Firstly, the fiber diameter needs to be taken into account, and secondly the air fraction of that fiber needs to be considered. In order to investigate these two parameters, we first compare a 125µm solid fiber with a 125µm holey fiber (compares air fraction) and then we compare a 125µm holey fiber with an 85µm holey fiber (same air fraction, different size).
Each fiber was cleaved with a variety of forces and the resulting topography mapped with the atomic force microscope. To gauge the damage caused by the cleaving process, the damage height behind the hole in the first ring, and furthest from the origin of the cleave was measured. In Fig. 2 this hole is labeled #4. Figure 3 shows a plot of the damage caused versus the force used for cleaving. The solid fiber data is shown as a single point with ~2nm RMS roughness for a nominal range of cleaving forces between 200–250g.
In Fig. 3 the red point indicates the optimum force value for a standard solid fiber. In reality there is very little change in the quality of the end face for this fiber over quite a range of forces. In comparison, Fiber ASM001_A2, the milled holey fiber with 125µm outer diameter, has an extensive amount of damage at higher cleave forces. From the plot it is evident that this fiber has an optimum cleaving force of about 110g, well below that used for standard fibers. It is important to note that this “optimum” value is determined for minimization of damage around the holes. The rest of the fiber, even at higher forces has a good quality end face. Thus the increased air fraction in this case has resulted in significant damage. As expected, if the fiber diameter is reduced, but the air fraction maintained, the required optimum force is also reduced. Fiber ASC010_01_A4 has an optimum cleaving force of approximately 75g, as indicated by Fig. 3. Figure 4 shows the difference between the end face topography for fiber ASM001_A2 for cleave forces of 220g and 110g. The image on the left, from the 220g cleave has substantially more damage than the 110g image.
Even under optimized conditions we find that the holey fibers show a small amount of damage on the end face, proximate to the holes, after cleaving. In general this was represented by 20–60nm structural flaws. This could only be avoided if the force applied to the fiber could be changed during the cleaving process, so as to keep the tension constant as the fiber area changes. To achieve this, we propose a variable force cleaving tool design that would vary the applied force as the fracture propagates across the fiber.
In order to further reduce the damage to the fiber end face, holey fibers were etched in 48% hydrofluoric acid for approximately 2 seconds. This effectively eliminated the structural flaws caused by cleaving, as show in Fig. 5. Fibers with more substantial damage could be etched for more substantial times to further improve end face quality. The general surface quality of the fiber end face was unaffected by this process, with RMS roughness below 2nm before and after etching. The sharpness of hole edges is slightly reduced by this process.
The damage caused to optical fibers during the cleaving process is of particular interest, raising questions about long term stability of the glass system and as well resulting in undesirable scattering losses. Holey fibers are particularly susceptible to micro-damage due to the inability of fractures to propagate across the air regions. Cleaving forces for these fibers need to be carefully controlled to achieve optimum results. Ideally, cleaving should be performed with a variable force system so that the tension in the fiber can be kept constant as the fracture propagates. In the absence of such a device being available, a combination of force optimization and chemical etching produces high quality fiber end faces.
S.T. Huntington acknowledges the financial support of the Australian Research Council.
References and Links
4. P.J. Bennett, T.M. Monro, and D.J. Richardson, “Toward practical holey fiber technology: fabrication, plicing, modeling, and characterization,” Opt. Lett. 24, 1203–1205 (1999). [CrossRef]
6. W.J. Wadsworth, R.M. Percival, G. Bouwmans, J.C. Knight, and P.St.J. Russell, “High power air-clad photonic crystal fiber laser,” Opt. Express 11, 48–52 (2003). http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-1-48 [CrossRef] [PubMed]
7. T.M. Monro, W. Belardi, K. Furusawa, J.C. Baggett, N.G.R. Broderick, and D.J. Richardson, “Sensing with microstructured optical fibers,” Meas. Sci. Technol. 12, 854–858 (2001). [CrossRef]
8. S.T. Huntington, J. Katsifolis, B.C. Gibson, J. Canning, K. Lyytikainen, J. Zagari, L.W. Cahill, and J.D. Love, “Retaining and characterizing nano-structure within tapered air-silica structured optical fibers,” Opt. Express 11, 98–104 (2003). http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-2-98 [CrossRef] [PubMed]
9. B. Poumellec, Ph. Guenot, R. Nadjo, B. Keita, and M. Nicolardot, “Information obtained from the surface profile of a cut single mode fiber,” J.Lightwave Technol.17 (1999). [CrossRef]
10. Q. Zhong and D. Inniss, “Characterization of lightguiding structure of optical fibers by atomic force microscopy,” J. Lightwave Technol. 12, 1517–1523 (1994). [CrossRef]
11. S.T. Huntington, P. Mulvaney, A. Roberts, K.A. Nugent, and M. Bazylenko, “Atomic force microscopy for the determination of refractive index profiles of optical fibers and waveguides: A Quantitative study,” J. Appl. Phys. 82, 2730–2734 (1997). [CrossRef]
12. C.W.J. Hillman, W.S. Brocklesby, T.M. Monro, W. Belardi, and D.J. Richardson, “Structural and optical characterisation of holey fibers using scanning probe microscopy,” Elec.Lett. 37, 1283–1284 (2001). [CrossRef]
13. S.L. Semjonov and C.R. Kurkjian, “Strength of silica fibers with micron size flaws,” J. Non-Crystalline Solids 283, 220–224 (2001). [CrossRef]
14. C.P. Chen and T.H. Chang, “Fracture mechanics evaluation of optical fibers,” Mat. Chem. and Phys. 77, 110–116 (2002). [CrossRef]
15. C.D. Rabii and J.A. Harrington, “Mechanical properties of hollow glass waveguides,” Opt.Eng. 38, 1490–1499 (1999). [CrossRef]