Flat Fibers: Fabrication and Modal Characterization
This chapter presents the fabrication and modal characterization of flat fibers, a specialty optical fiber which has the advantages of both planar waveguides and standard optical fibers. In the introduction section, the demand for flat fibers is discussed including its applications. The main text of this chapter discusses the drawing process of flat fibers including issues such as flat fiber drawing repeatability and drawing of flat fibers with different dimensions. Next, the modal characterization of flat fibers is discussed, with a focus on the light propagation in flat fibers. Both simulation and experimental results show that the light propagation in flat fibers is inherently multimode propagation. However, it has been shown experimentally that adding two defect eyes at the sides of the core could produce flat fibers with single-mode propagation.
KeywordsFlat fibers Optical fiber fabrication Optical fibers characterization Multifunctional sensing fiber Single mode fiber
Optical fiber sensors can be used to detect physical changes in the environment surrounding the fiber. For example, a fiber biosensor can be used to detect the concentration of specific biological or chemical substance. However, the fiber sensor detection capability is limited as it can only detect a single specific target of sensing. Therefore, a combination of fiber sensors is often required to combine the optical signals (Zhang et al. 2013) and to allow multiple substances to be detected. The multifunctional property of planar waveguides gives rise to multipurpose usage in the same chip. In planar waveguides, light is guided on defined paths on a planar substrate. Planar waveguide is the basic component in integrated optic devices such as optical splitters, optical couplers, interferometers, and gratings (Buck 1995; Saleh and Teich 2007). Optical components such as couplers, filters, modulators, amplifiers, detectors, or even breakthrough diffraction limit plasmonics can be built on the same chip (Bian and Gong 2013). These optical devices can be fabricated on the flat surface of materials such as silica (Li and Henry 1996), silicon, semiconductor crystal, or plastic (Dutton 1998). However, the optical components often require advanced fabrication technology such as photolithography, UV writing, nonlinear resonators, and super-resolution imaging, quantum information, and light harvesting (Ferretti et al. 2013; Kildishev et al. 2013). Other fabrication methods for planar waveguides include etching and epitaxial growth (Singh 1996; Hunsperger 2009), but these methods are often costly and require a high precision (Dutton 1998). The limitations of planar waveguides are high propagation and coupling loss, rigid substrate and can be fabricated for only a short length (maximum of 10–15 cm).
An alternative solution to planar waveguides with added advantage of low propagation loss and high flexibility is fulfilled by a specialty optical fiber known as flat fiber (FF). FF was introduced in 2007 and was initially developed as alternative to planar waveguides due to its low-cost fabrication technique (Webb et al. 2007). The FF structure is almost similar to a buried channel planar waveguide, with a higher refractive index core layer and a lower refractive index at the cladding. The higher core refractive index allows light to continuously propagate inside the FF’s core via total internal reflection phenomena (Buck 1995; Kasap 2001). This unique fiber design fills the gap in integrated optic devices as a transport and sensing medium. FF can be described as a planar waveguide fiber with the advantages of having both optical fibers and planar waveguide’s properties, such as mechanically flexible, multifunctionality, long length fabrication, low coupling and propagation loss, etc. (Adikan et al. 2012). In addition, flat fibers also have the advantages of cylindrical optical fiber such as chemically inert, ability to withstand high ambient temperatures, and at the same time having a large flat surface area for material processing at low manufacturing costs (Kalli et al. 2015).
Due to the interesting properties of FF, there is a research demand and interest to develop technology for smart structures, integrated photonic circuits, and devices for optofluidic applications (Riziotis et al. 2014). Applications have been demonstrated via femtosecond laser micromachining, such as fabrication of optical structures, including ring and disk resonators and Mach-Zehnder interferometer. FFs solve the difficulties in producing substrate with such properties, where multistage process are required and the cost of manufacturing equipment limits the research area to well-funded groups (Duncan 2002; Scifres 1996).
Inscription of Bragg gratings, ring resonator, disc resonator, Mach-Zehnder interferometer, and microfluidic channel has been demonstrated on a FF using femtosecond laser inscription (Kalli et al. 2015). The inscription is done by having a focused beam from a femtosecond laser translated into a bulk transparent material which caused the refractive index of the core material to increase due to the nonlinear absorption process (Kalli et al. 2015).
Besides femtosecond laser, a more popular approach for developing optical/photonic devices on FFs is direct UV writing (Holmes et al. 2008; Adikan 2007). Studies were done to characterize UV-written (λUV = 244 nm) chips by studying the effective index value obtained from the grating experiment. Y-splitters/combiner, Bragg gratings, ring resonator, and straight channel are examples of devices that can be fabricated on FFs by using UV-written method (Adikan et al. 2012). Single beam direct UV writing can also be used to define waveguide channels in the core of FF (Holmes et al. 2008; Adikan 2007). But the core needs to be photosensitive and this can be done by doping the core with germanium and/or boron. Using direct grating writing where Bragg gratings are written on the core of the fiber and applying the principle of shifting the Bragg wavelength, evanescent field sensors have been demonstrated on FFs (Holmes et al. 2008).
The losses in FFs are not as low as cylindrical optical fibers and are typically in the region of 0.1 dB/m. However, this depends on its fabrication methods. Over the years, improvements have been made to realize this technology (Kalli et al. 2015). Flat fibers can also be cleaved quite efficiently, unlike its predecessor of planar silica on silicon samples which needs to be polished for laser inscription.
It is often desirable for lab-on-a-chip, optofluidics, and biosensor to “access” the guiding light so that the evanescent field can interact with an external fluid sample, typically for refractive index measurements. To improve the evanescent field in conventional fibers, in most of the cases, polishing of optical fibers to the core-cladding boundary is favored, a method which is very time consuming and inefficient. On the contrary, the potential of connecting the waveguide core with the surface using a femtosecond laser waveguide offers an alternative method that is less time consuming. In this case the “connection” distance between the core and surface is short, perhaps only 50 μm, and waveguide losses will not be significant. FF offers stronger evanescent field inherently due to the thinner cladding and much wider guiding area compared to conventional fiber and has the potential for fusion splicing with standard optical fiber as used in high-power fiber applications (Adikan et al. 2012).
Flat Fiber Fabrication
The fabrication of FFs begins with the fabrication of fiber preform. Nowadays, most preforms are fabricated using vapor deposition methods such as modified chemical vapor deposition (MCVD), outer vapor deposition (OVD), plasma-activated chemical vapor deposition (PAVD), and vapor axial deposition. These methods ensure quality and pureness of the preform during the dopant deposition process. Maintaining the purity of the preform minimizes the fiber attenuation due to absorption and scattering.
For fabrication of FF, a glass tube or hollow preform is used. The next stage of fabrication is to draw the hollow preform using a standard drawing tower. The drawing of FF is similar to other conventional optical fibers where the top end of the preform is clamped to the preform holder or feeder and the tip of the other end is placed inside the furnace. The motorized preform stage will slowly feed the preform down at a specified preform feeding speed into a furnace. The furnace typically uses a graphite heating element due to its thermal and mechanical properties (Payne and Gambling 1976). The condition inside the furnace should be maintained to be free from impurities and oxides built up since graphite has a high oxidation rate at high temperature. This can be done by purging the furnace with argon gas.
The neck-down profile is a function of the fiber drawing conditions such as furnace temperature, draw speed, feed speed and vacuum pressure, physical and material properties of the preform, and the type of heating element used in the furnace (Paek and Runk 1978; Choudhury and Jaluria 1998; Cheng and Jaluria 2002; Mawardi and Pitchumani 2010). It also determines the mechanical and optical characteristics of the optical fiber.
For the case of FF, since the preform is in a hollow form, the output fiber is initially in capillary form. Fabrication of FF begins when vacuum pressure is applied at the top of the hollow preform. As soon as vacuum pressure is applied, the preform neck-down region experiences vacuum force, causing the region to collapse or flatten. In this process, the circular capillary shape converts into flat shape, which is called flat fiber.
FF dimensions can be approximated (without considering the temperature effect) from the capillary dimensions. The thickness of the fabricated FF will be equal to the outer diameter of the capillary minus the inner diameter of the capillary. The width of the core is the inner radius of the capillary multiply with π and the width of the cladding will be equal to the width of the core and the thickness of the fiber.
FF can be drawn in two methods, either in single-stage or two-stage drawing. In single-stage drawing, a bulky hollow preform is directly drawn into a desired FF diameter. In this method, the diameter conversion rate from preform diameter to the output capillary diameter (or FF dimension) is very high, for example, drawing a 1 mm diameter capillary directly from a 25 mm diameter tube. In a two-stage method, a bulky size hollow tube preform is first drawn into a 2–3 mm diameter capillary and the same capillary is redrawn into the desired FF diameter, where the conversion ratio from the capillary to the final fiber is much smaller than that of single-stage FF fabrication.
Flat Fiber Drawing Repeatability
In general, the fabrication of FF is repeatable if all the fabrication parameters and the fiber preform used are similar. Fabrication of a FF with dimension of 0.385 × 0.107 mm is repeated over three times using a pure silica preform with an OD/ID of 25 mm/19 mm. The drawing parameters including furnace temperature, tension, vacuum pressure, feed rate, and draw speed were kept constant in all three experimental trials. The drawing parameters were first specified based on the drawing of a capillary with a 0.3 mm diameter. When the drawing of the capillary at this diameter has stabilized, vacuum pressure is then applied to ensure a consistent FF dimension. In each of the three experimental trials, seven samples were randomly selected from the drawn FFs. From a total of 21 samples, a maximum variation of around 5.5% (or ±2.7%) is observed in the FF dimensions. It should be noted that this variation is observed in the condition that the dimension of the fiber was not on the auto-control mode; instead, the system parameters were set once at the beginning of the fabrication till the end for all the three fabrication trials.
Flat Fibers with Different Dimensions
Characterization of Flat Fibers: Mode Propagation
Multimode Propagation in Flat Fibers
Tables 1 and 2 show the neff and power distribution per mode in the core and eye area in the 5 μm and 1.6 μm core thickness FFs, respectively. The results are calculated in power flow, time average (W/m2).
Referring to Table 1, for the FF core thickness of 5 μm as the power values imply, all modes from the fundamental to higher-order modes (up to 12) have high power in the FF core area. Referring to Table 2, for the FF core thickness of 1.6 μm, there is still a significant amount of power in the flat fiber core area from the fundamental to higher-order modes (up to 12) as well. These simulation results confirm that both FFs with thin or thick core are highly multimode fiber. This also signifies that even by reducing the core thickness, the multimode properties still exist in the slow axis, due to the wide dimensional area.
For validation, a simple test is performed on one of the FF used for the simulation above, i.e., the flat fiber with a thick core thickness of 5 μm. Figure 6 shows the mode operation in the FF captured by charged-coupled device at three different launching conditions, i.e., (a) incident light applied into the flat fiber’s left eye, (b) light applied into the center core, and (c) light applied into the right eye. It can be observed experimentally that the core and eyes were being guided in all three launching conditions. The fiber shows multimode operation in all launching conditions, as observed in the simulations shown in Fig. 5. By applying light into the FFs’ eye, light travels into the central core area as well as into the other eye area and vice versa. Even though light is applied with different launching positions, the mode operation of the FF always supports multimode propagation and does not reduce into single-mode operation.
Single-Mode Propagation in Flat Fibers
neff and power distribution per mode in the FF core and eye area for the 5 μm core thickness
neff and power distribution per mode in the FF core and eye area for the 1.6 μm core thickness
The simulation results in Fig. 7 seems to suggest that the higher-order modes after leaking into the eyes were trapped there and not returning back into the central core area. This phenomenon that exists in this FF has never been observed in the ideal FFs. This phenomenon is further clarified by experimental results as seen in Fig. 8.
Figure 8 depicts three different launching conditions of SMF coupled into the FF with defect eye holes (without filling the index matching oil inside the defect holes) (Poh et al. 2017). In Fig. 8a, c, light was launched into the fiber’s left and right eyes, respectively. It was observed that the light propagating from the eye area does not propagate into the FF central core area. This was unlike what was observed in the ideal FF without defect eye holes presented in Fig. 6. Figure 8b shows the launching condition into the FF central core area. In this experiment, the incident light from the SMF is being moved slightly from one end to another end of the central core. In all swiping positions, the light propagating from the central core area never travelled into the eyes unlike as observed in the FF without defect eye holes shown in Fig. 6. However, in all conditions, the resulting mode guidance in the central core was always multimode as shown in Fig. 8f.
By applying a higher index matching oil into the FF’s eye holes, the multimode propagation in the FF reduces to single-mode operation as illustrated in Fig. 9. The dotted red curves in Fig. 9 depicts a multimode behavior measured in the FF before oil infiltration in the eye holes. In this condition, light is strongly confined in both the fast and slow axis of the FF. As soon as the FF eye holes were filled with refractive index matching oil, the higher-order modes were successfully reduced as shown by the solid green lines. It could be possible that the higher-order modes are attenuated through leaky mode mechanism and reduced to single-mode operation.
Fabrication of flat fiber is shown to be practically feasible using the conventional fiber drawing tower. Fabrication of this fiber is shown to be repeatable with a variation of around ±2.7% by employing the same drawing parameters and the preform material. FFs with different dimensions but almost similar width-to-thickness ratio (±.56% variation) can be fabricated by keeping the drawing tension constant. FFs with variety of dimensions can be fabricated when the drawing conversion from preform diameter to the fiber dimension is large and by varying the drawing tension or furnace temperature. FFs have inherently multimode propagation due to large core area or core width. However, single-mode FF is shown to be possible by applying a suitable refractive index material in the FF eye holes. This material would be either liquid form for short fiber length or would be a glass rod (with slightly higher refractive index than the FF central core) placed in the fiber during the FF fabrication. FFs would be a good alternative to planar waveguide with the advantages of flexibility, long fabrication feasibility, lower loss and ease of cleaving.
- F.R.M. Adikan, Direct UV-Written Waveguide Devices (University of Southampton, Southampton, 2007)Google Scholar
- J.A. Buck, Fundamentals of Optical Fibers, (Wiley, 1995)Google Scholar
- H. Duncan, Fabrication of substrates for planar waveguide devices and planarwaveguide devices, Google Patents, (2002)Google Scholar
- H.J. Dutton, Understanding Optical Communications (Prentice Hall PTR, New Jersey, 1998)Google Scholar
- C. Holmes, F.R.M. Adikan, A.S. Webb, J.C. Gates, C.B. Gawith, J.K. Sahu, P.G. Smith, D.N. Payne, Evanescent Field Sensing in Novel Flat Fiber (Optical Society of America, 2008), pp. CMJJ3Google Scholar
- R.G. Hunsperger, Integrated Optics Theory and Technology (Springer, Berlin, 2009)Google Scholar
- K. Kalli, C. Riziotis, A. Posporis, C. Markos, C. Koutsides, S. Ambran, A.S. Webb, C. Holmes, J.C. Gates, J.K. Sahu, P.G.R. Smith, Flat fibre and femtosecond laser technology as a novel photonic integration platform for optofluidic based biosensing devices and lab-on-chip applications: Current results and future perspectives. Sensors Actuators B Chem. 209, 1030–1040 (2015)CrossRefGoogle Scholar
- S.O. Kasap, Optoelectronics and Photonics: Principles and Practices (Pearson, Boston, 2001)Google Scholar
- D.N. Payne, W.A. Gambling, A resistance-heated high temperature furnace for drawing silica-based fibres for optical communications. Am. Ceram. Soc. Bull. 55, 195–197 (1976)Google Scholar
- C. Riziotis, K. Kalli, C. Markos, A. Posporis, C. Koutsides, A.S. Webb, C. Holmes, J.C. Gates, J.K. Sahu, P.G.R. Smith, Flexible glass flat-fibre chips and femtosecond laser inscription as enabling technologies for photonic devices. At Photonics West (2014), United States, pp. 89820G–89820G-89828Google Scholar
- B.E. Saleh, M.C. Teich, Semiconductor photon detectors. Fundam. Photonics, 644–695 (2007)Google Scholar
- D.R. Scifres, C.A. San Jose, Multiple core fiber laser and optical amplifier, Google Patents, United States Patent 5566196, Application Number: 08/330262 (1996)Google Scholar
- J. Singh, Optoelectronics: An Introduction to Materials and Devices (McGraw-Hill College, New York, 1996)Google Scholar