Transcript
Fiber taper rig using a simplified heating source and the flame-brush technique Jean C. Graf∗ , Silvio A. Teston∗ , Pedro V. de Barba∗ , Jeferson Dallmann‡ , Jos´e A. S. Lima‡ , Hypolito J. Kalinowski† and Aleksander S. Paterno∗§ Department of Electrical Engineering∗ – Department of Mechanical Engineering‡ Santa Catarina State University, Joinville, Santa Catarina, Brazil 89223–100 § Email:
[email protected] † Federal University of Technology - Paran´a, Curitiba, Paran´a, Brazil 80230–901
Abstract—This paper presents the implementation of an automated fiber optic taper rig capable of manufacturing fiber optic tapers with diameters in the micron and sub-micron range. The implemented taper rig uses the flame-brush technique to taper the fiber optic with a low cost commercial heat source made by an air aspirated/butane/propane refillable and controllable micro-torch. Adiabatic tapers with exponential waist profiles and tapering excess loss of less than 1% are reported and the feasibility of such a simple heating source is demonstrated for the fabrication of fiber optic tapers.
I. I NTRODUCTION Bi-conical fiber optic tapers have been fabricated by several methods which are capable of producing taper waists with diameters in the micron and sub-micron range. Fiber optic tapers are attracting attention due to a myriad of emerging fiber optic applications, from fiber optic ordinary coupler fabrication to the implementation of new evanescent field sensors and its potential to nanotechnology [1]. The well known flame-brush technique [2] is one of the most used methods to fabricate bi-conical fiber optic tapers and uses a traveling flame-brush to heat the fiber optic while two traveling stages pull the taper ends. By configuring the flame-brush movement, several types of taper profiles can be obtained with a high degree of accuracy [3]. Other methods to fabricate tapers use different heating sources, as CO2 lasers [4] or electrical resistances depending on the type of the fiber optic to be tapered [1]. It is natural to think that the performance of the technique is dependent on the uniformity and stability of the process used during the fabrication. This may be evidenced in the case of the vibration produced in the translation stages during the stretching procedure or due to an instability of the flamebrush temperature, which may cause large excess losses (loss of power from the fundamental mode as it propagates from the input to the output of taper) [5]. For this reason, heat sources using high purity oxygen/butane flames controlled by expensive mass-flow controllers are usually used [6] and produce highly stable heat sources. The magnitude of the excess loss in the taper fabrication and the designed taper profile are the parameters that determine if a taper rig is properly fabricating the device. If one considers a bi-conical taper with a specific excess loss (less than 10%), tapers may be considered adiabatic as described in the literature considering
adiabaticity criteria for fiber optic tapers [7]. For this reason, any heat source may be used in the process if it does not cause the taper to have higher excess losses and produces the desired taper diameter profile. For the purpose of manufacturing long tapers, with tens of centimeters, which can reach nanometers in diameter, the stretching translation stages must have lengths of tens of centimeters and travel at very low speeds, while the traveling flame-brush must have its scanning length adjusted as the stretching translation stages progress [3]. In this paper a fiber optic taper rig is described that allows the fabrication of long tapers with the potential to fabricate tapers with lengths up to 1 m and uses a heat source provided by a commercial micro-torch which reveals itself to be sufficiently stable to produce adiabatic tapers. A. Methods and Taper rig description The fiber taper rig was devised and mounted in the Laboratory of Electronic Instrumentation at the Santa Catarina State University. It uses three custom made high precision traveling stages mechanically coupled to stepping motors. A flame-brush attached to another traveling stage, as shown in the 3-D layout in fig. 1, is used as a heat source. The translation stages where the fiber is positioned operate at a maximum speed of 20 mm/min and are capable of stretching the fiber up to 1000 mm. There are two possible heat sources for the taper rig. The one shown in fig. 1 uses a high purity oxygen-butane torch with gas flow controlled by rotameters. The tested configuration in this work uses another heat source which is a commercial air aspirated/butane/propane mini-torch (Jackwal HT6020) installed in the modified taper rig as shown in fig. 2. While the two slow stages stretch the fiber, another traveling stage moves the flame-brush in a zigzag movement configuration that allows a fiber optic length to be heated according to the flame-brush technique theory [3]. A schematic illustration of a fiber taper is shown in fig. 3, where the transition region of the taper is shown as a function r(z) of the position z in the fiber axis. This transition region of length z0 can be produced with various profiles. For the fabrication of an exponential taper, the hot-zone must be kept constant, L0 , during the whole process, meaning that the scanning length of the flame-brush is constant and the taper waist Lw = L0 [3]. In this case, the α parameter that causes the hot-zone to vary
during the process is set to α = 0, since the hot-zone length L(x) = L0 + αx, where x is the stretching length. The motors in the traveling stages are controlled by microstep resolution controllers and are programmed via a userfriendly interface written in C++ and compiled with the GNUGCC compiler. The interface software uses an open-source application programming interface for writing graphical user interfaces (wxWidget). The specification of the fiber taper to be produced that serves as input to the Taper Rig Automation Software (TAPERAS) is shown in fig. 4. During the process the light from a constant intensity LED with center wavelength in 1300 nm is launched into one of the fiber ends and the signal is monitored at the other end by a photo-detector. The attenuation in the detected light indicates if the process is producing an adiabatic taper. In the software, the threshold attenuation that may stop the taper rig can be configured and is chosen to be at 10% of the LED intensity in the beginning of the process with this commercial micro-torch. In fig. 5 a photo shows the prototype of the taper rig and its control panel. The control panel is shown in its final version and contains the controllers, the hub that distributes the signal from the computer serial port and the power circuits that energize the system. The table with the traveling stages is a prototype in a preliminary version which in the near future is going to be mounted on an optical breadboard table. The fiber optic used in the process was an enhanced singlemode fiber (Draktel G.652.C/D) which had its acrylate coating removed by a fiber coating stripper and after that it was repeatedly wiped with acetone. Two fiber splicers were used to connect the fiber optic to the LED and the photo-detector. In the experimental process demonstrated here an exponential transition is produced for a taper with a waist diameter of 20 µm. The tapers were measured in a stereo-microscope and photos were taken to illustrate the waist diameter and sections of the transition region in the taper.
Fig. 2. Flame-brush 3D layout using a commercial micro-torch in the modified taper rig.
Transition
zo
2r1
r(z)
Waist length
Transition
Lw
zo
2rw
z
Fig. 3.
Schematic diagram of a taper with linear transition region.
Fig. 4. Taper Rig Automation Software input screen during the manufacturing process.
Fig. 1.
Taper rig 3D layout with its traveling stages and the flame-brush.
B. Results and discussion The tapering process must be monitored at least by observing the output intensity of light at the taper end. If possible, the taper profile should be observed with a videomicroscope, however in this work only the final taper results
Fig. 5. Prototype of the taper rig and control panel connected to a personal computer.
could be recorded by the observations of short fiber sections. An important point to be considered during the fiber optic preparation before the tapering is the cleanness of the fiber surface subject to the heat source. Dirt or acrylate on the surface of the fiber while it is being heated results in an abrupt drop in the monitored light intensity, causing the taper to have non-uniformities in its profile and be non-adiabatic. Other issues result in non-adiabaticity, like the instability of the flame or even if the flame has a high mass flow at the torch output. With a high flow of gas at the torch output, for a given small taper diameter, the mass flow of gas may break it. For this reason, the mass flow must be minimal to produce the required temperature and more importantly to manufacture tapers with sub-micrometric diameters. For the fabricated taper with final waist diameter 2rw = 20 µm, a section of the beginning of the waist is shown in fig. 6. The initial oscillation amplitude that defines the hotzone in an exponential taper was set to 20 mm. The stretching speed was set to 3 mm/min and the burner speed, to 4 mm/s, which are typical speeds usually reported in the literature for this technique [2]. The transition length for this taper was 36.5 mm. Other section of the transition regions is also shown in fig. 7. The waist of a 2rw = 20 µm diameter taper is compared to a non processed fiber having 2rw = 125 µm in fig. 8. With respect to the taper rig performance, if one wants to fabricate a taper with a given rw , and an initial fiber diameter of 125 µm and having an exponential profile (α = 0), the rig must stretch the fiber up to a distance given by [3]: Ltotal = 2L0 ln(
125 ) 2rw
(1)
where rw is in micrometers. For an exponential taper with nanometric diameter of 50 nm and hot-zone L0 = 70 mm, the taper rig must stretch the fiber up to Ltotal ≈ 1000 mm. The mounted rig is therefore capable of reaching such nanometric diameters. However, there is still the problem of evaluating the taper profile uniformity and develop a method for its transportation, which at the present moment is to be developed by the authors.
Fig. 6.
Fig. 7.
Waist of exponential taper with diameter 2rw = 20 µm.
Section of transition region of taper with diameter 2rw = 20 µm.
II. C ONCLUSION A fiber optic taper rig having a simplified heat source and using the flame-brush technique was implemented. To the authors’ knowledge, this is the first time such a simplified
Fig. 8. Non processed fiber and section of waist region of taper with diameter 2rw = 20 µm.
and low cost heat source is used to fabricate adiabatic biconical fiber optic tapers. In addition, the structure of the taper rig allows the fabrication of long tapers in the range of 1000 mm with the potential to reach waists with submicrometric diameters and different types of profiles. The rig supports also the use of other more sophisticated heat sources, as an oxy-butane flame-brush with controlled gas flow, and this is the next step in the development of the rig together with its installation in an optical breadboard table. The system is automated and provides a means to fabricate micro- and nanowires for the research of new applications in optoelectronics, fiber optic communications and sensing. ACKNOWLEDGMENT The authors gratefully acknowledge the support of The National Council of Technological and Scientific Development (CNPq) under grant process number 576114/2008-1. R EFERENCES [1] G. Brambilla, F. Xu, P. Horak, Y. Jung, F. Koizumi, N. P. Sessions, E. Koukharenko, X. Feng, G. S. Murugan, J. S. Wilkinson, and D. J. Richardson, “Optical fiber nanowires and microwires: fabrication and applications,” Advances in Optics and Photonics, vol. 1, p. 107–161, 2009. [2] R. P. Kenny, T. A. Birks, and K. P. Oakley, “Control of optical fibre taper shape,” Electronics Letters, vol. 27, pp. 1654–1656, 1991. [3] T. A. Birks and Y. W. Li, “The shape of fiber tapers,” Journal of Lightwave Technology, vol. 10, no. 4, pp. 432–438, 1992. [4] G. Kakarantzas, T. E. Dimmick, T. A. Dirks, and P. S. J. Russell, “Fabrication of high performance fibre tapers and couplers using a CO2 laser rig,” CLEO/Pacific Rim - Paper WB1, pp. 127–128, 1999. [5] G. Brambilla, V. Finazzi, and D. J. Richardson, “Ultra-low-loss optical fiber nanotapers,” Optics Express, vol. 12, no. 10, p. 2258–2263, 2004. [6] N. Vukovic, N. G. R. Broderick, M. Petrovich, and G. Brambilla, “Novel method for the fabrication of long optical fiber tapers,” IEEE Photonics Technology Letters, vol. 20, no. 14, pp. 1264–1266, 2004. [7] R. J. Black, S. Lacroix, F. Gonthier, and J. D. Love, “Tapered single-mode fibres and devices part 2: Experimental and theoretical quantification,” IEE Proceedings Journal, vol. 138, no. 5, p. 355–364, 1991.
