Carbon dioxide laser fabrication of fused-fiber couplers and tapers Timothy E. Dimmick, George Kakarantzas, Timothy A. Birks, and Philip St. J. Russell
We report the development of a fiber taper and fused-fiber coupler fabrication rig that uses a scanning, focused, CO2 laser beam as the heat source. As a result of the pointlike heat source and the versatility associated with scanning, tapers of any transition shape and uniform taper waist can be produced. Tapers with both a linear shape and an exponential transition shape were measured. The taper waist uniformity was measured and shown to be better than ⫾1.2%. The rig was also used to make fused-fiber couplers. Couplers with excess loss below ⫺0.1 dB were routinely produced. © 1999 Optical Society of America OCIS codes: 060.2310, 060.1810, 060.2340, 220.0220, 220.4610.
1. Introduction
The fused-fiber directional coupler has found wide acceptance as a basic building block within the telecommunications industry. The device is widely used as an optical power splitter but has also found more exotic applications within acousto-optically controlled optical switches and filters.1 In all applications, control of the fiber taper shape is important for producing small, low-loss devices, but in the latter case precise control of the coupler waist profile is essential to tailor the spectral response of the device. Although fabrication of tapered fiber couplers is commonplace, the precise control over fiber taper shape that is necessary for such devices has only recently been achieved. Traditional means of fabricating fused tapered couplers involves heating the fibers in a flame and pulling them while monitoring the coupling ratio with photodiodes. Pulling is halted and the flame removed when the desired coupling ratio is achieved. Fabrication with a flame can lead to contamination of the coupler with combustion by-products, and variations in the temperature of the burner may lead to
The authors are with the Department of Physics, University of Bath, Claverton Down, Bath BA2 7AY, UK. T. E. Dimmick is on leave from the Laboratory for Physical Sciences, 8050 Greenmead Drive, College Park, Maryland 20740. His e-mail address is
[email protected]. Received 29 April 1999; revised manuscript received 2 August 1999. 0003-6935兾99兾336845-04$15.00兾0 © 1999 Optical Society of America
nonuniformities in the taper waist radius. Theoretical analysis has shown that precise control of the taper shape and waist radius is possible by use of a pointlike heat source that is scanned along the fibers in a controlled fashion as the fibers are pulled.2 Such a system has been demonstrated with a small oxybutane burner that was moved to and fro under computer control.3 Control over the taper shape was limited in this system because of the finite size of the flame, the inertia of the burner 共which prevented rapid changes in direction兲, and fluctuations in the burner temperature that were due to air currents and other environmental variations. Resistive electrical heaters can overcome some of these drawbacks, but heat transfer from them to the fiber is too slow for the moving heat source technique to be practical. One method of overcoming the liabilities associated with moving burners is to employ laser heating. A CO2 laser beam is clean, controllable, fast acting, and free of inertia. CO2 lasers have been used in the production of tapered fiber tips for scanning nearfield optical microscopy,4 and several researchers have carried out theoretical studies.5,6 Recently, Yokota et al.7 have demonstrated a method of producing fused-fiber couplers using CO2 laser-induced heating. However, their method did not employ a moving pointlike heat source and therefore did not provide precise control over the fiber taper shape. Here we report the development of a fiber taper and fused-fiber coupler fabrication system that is capable of producing low-loss fiber tapers and couplers of precise taper shape and waist dimensions. Such precise control is required for the production of reso20 November 1999 兾 Vol. 38, No. 33 兾 APPLIED OPTICS
6845
Fig. 1. Schematic diagram of the taper rig.
nant acousto-optic devices in fiber tapers and couplers, for example. 2. Taper Rig Design
A schematic diagram of the taper rig is shown in Fig. 1. A 20-W Synrad Invar stabilized CO2 laser is used as the heat source. Light from the laser is directed through a shutter and variable attenuator. Following the attenuator, the CO2 laser light is combined with a He–Ne laser beam. The He–Ne beam is aligned to coincide with the CO2 beam and is used as a guide to direct the CO2 laser radiation onto the fiber. A thin-film polarizer following the beam combiner polarizes the CO2 light perpendicular to the fiber. A partial reflector transmits 5% of the CO2 laser light to a detector for power monitoring purposes. Finally the light is focused onto the fiber with a 300-mm focal length ZnSe lens and a galvanometerdriven scanning mirror. The scanning mirror is capable of rapidly redirecting the beam anywhere along a 7-cm length of fiber. Two Aerotech motor-driven linear translation stages are used to pull the fibers. The stages, scanning mirror, attenuator, powermeter, and shutter are controlled by a computer running LabView software. The entire tapering process proceeds under computer control. Laser heating of optical fibers is qualitatively quite different from heating by means of a flame. In the latter case the condition of the fiber 共its presence or its absence in the flame兲 has little or no effect on the temperature of the heat source. This means that the characteristics of the burner need not change during the fabrication process. With laser heating the heat is generated within the fiber, and so the temperature of the fiber depends not only on the laser power but also on the absorption coefficient 共which varies with temperature and wavelength兲,8 cooling rate 共which is a function of temperature and diameter兲,9 the overlap of the fiber cross section with the focused laser spot, and even the size of the fiber as a Mie resonator.4,6 As a consequence, the laser power must be adjusted throughout the tapering process to maintain the ideal softening temperature. Although several papers5,6 have been published that provide the basis for theoretical predictions of the required power as a function of taper diameter, their results depend heavily on empirically determined pa6846
APPLIED OPTICS 兾 Vol. 38, No. 33 兾 20 November 1999
Fig. 2. Calibration curve showing the desired laser power as a function of taper diameter.
rameters, many of which vary with temperature. Consequently, we developed an experimental procedure wherein we visually observe the incandescent light emitted from the hot taper and manually increase the laser power throughout the tapering process to maintain constant emission. This procedure was found to be quite sensitive because small changes in the laser power result in significant changes in the brightness of the incandescent light. Using this procedure we obtained the desired laser power as a function of the taper diameter and incorporated this result into the control software. A calibration curve used for producing single fiber tapers is shown in Fig. 2. This curve was obtained with a focused laser spot size of 820 m full width at half-maximum 共FWHM兲. It can be seen that more laser power is needed to heat smaller-diameter fibers, because convective cooling dominates heat transfer at small diameters and limits the minimum diameter that can be reached with a given focused spot size.7 Fiber tapers of arbitrary shape can be obtained by use of this rig when the method prescribed in the published theory is followed.2 Because the theory predicts the taper diameter at any point along the taper throughout the fabrication process, the computer was programmed to adjust the laser power to provide the optimum temperature at each point. In practice, for tapers of uniform waist diameter a power adjustment is required at the start of each sweep of the laser spot. Because of the small size of the heated spot on the fiber it is important that the fibers are held perfectly straight so that shear stresses are minimized. Otherwise, when the fibers are initially heated to the softening point, they will shear transversely over a short length causing optical loss. For cases in which precise control of the taper shape is not required, it is advantageous to increase the laser spot size, thus reducing the potential for loss by shearing. This is accomplished through the software which includes an option for setting the amplitude of a fast dither signal that is added to the scanner drive voltage. The dither signal 共which is fast compared to the thermal response time of the fiber兲 spreads the spot along the fiber axis, resulting in an increase in the length of the instantaneous hot zone. Because shear stresses are
Fig. 3. Plot of fiber taper diameter as a function of distance along the taper. The taper was made with a constant sweep length of 4.75 mm resulting in exponential transitions. Measurements are shown as circles. The solid curve is the taper shape predicted by theory.2
distributed along a longer length of soft fiber, losses are reduced. 3. Experimental Results
The taper rig was used to manufacture both singlefiber tapers and fused tapered couplers. Tapers as narrow as 4.6 m in diameter were obtained with a laser power of 13 W and a focused spot size of 820 m FWHM. It is possible to obtain smaller diameters by reducing the focused spot size. Control over the shape of the taper transition was achieved by varying the extent of the laser sweep throughout the course of the taper manufacture.2 Figures 3 and 4 depict measured taper transition profiles for two single-fiber tapers. In both cases, the laser was swept at a constant velocity of 2 mm兾s and the fiber was stretched at a rate of 0.08 mm兾s. Figure 3 shows an exponential profile that was obtained by sweeping the CO2 laser spot over a constant distance of 4.75 mm. The final taper diameter was 38 m and was obtained with an extension of 11.4 mm. Figure 4 depicts a linear profile that was obtained by starting with a sweep width of 15.6 mm and reducing the extent of the laser sweep at a rate equal to half of the rate of extension. In both cases the taper waist was uniform and equal to the laser sweep width at the com-
Fig. 4. Plot of fiber taper diameter as a function of distance along the taper. The taper was made with an initial sweep length of 15.6 mm that was decreased at a rate of half of the rate at which the fibers were pulled. The resulting taper transitions were linear.
Fig. 5. Plot of the measured taper waist uniformity as a function of distance along the taper waist.
pletion of the pull. Unlike Ref. 3, the small size and fast response of the laser spot lead to taper profiles with sharp 共not rounded兲 corners at the ends of the taper transitions. We prepared a nominally 20-m-diameter taper using a constant sweep length of 8 mm. The uniformity of the taper waist was measured by use of the whispering gallery mode technique described by Birks and Knight.10 The relative waist diameter was measured at 125-m intervals along the taper waist. The results are depicted in Fig. 5, where it is evident that the taper is uniform to within ⫾1.2%. We believe that the ripple in the waist diameter is a result of fluctuations in the CO2 laser power. The laser, which was not actively stabilized, exhibited short-term power fluctuations of ⫾2%. Low-loss fused-fiber couplers were also fabricated with the taper rig. Fused-fiber couplers of specified coupling ratio were obtained by monitoring the output of the fibers and aborting the pull when the desired ratio was obtained. Figure 6 shows the coupler output powers of a 50:50 coupler measured throughout the pull. Low-loss couplers were more difficult to obtain than tapers, requiring greater control over the laser power and higher stability. Nevertheless we were repeatably able to obtain couplers with loss as low as ⫺0.02 dB. A histogram of excess loss of ten sequentially manufactured 50:50 couplers with exponential taper transitions is shown in Fig. 7.
Fig. 6. Coupler output power plotted as a function of pulling time for a 50:50 fused-fiber coupler. 20 November 1999 兾 Vol. 38, No. 33 兾 APPLIED OPTICS
6847
References
Fig. 7. Histogram showing excess loss of ten sequentially manufactured fused-fiber couplers.
4. Conclusion
We have reported the development of a fiber taper and fused-fiber coupler fabrication system utilizing a scanning, focused, CO2 laser beam as the heat source. We have used the system to produce low-loss tapers of precise taper profile and uniform waist diameter. Both exponential and linear taper shapes were produced and measured. The uniformity of the taper waist was measured by use of a whispering gallery mode technique and was shown to be better than ⫾1.2%. We believe that better uniformity is possible when the laser power is actively stabilized.
6848
APPLIED OPTICS 兾 Vol. 38, No. 33 兾 20 November 1999
1. T. A. Birks, P. St. J. Russell, and D. O. Culverhouse, “The acousto-optic effect in single-mode fiber tapers and couplers,” J. Lightwave Technol. 14, 2519 –2529 共1996兲. 2. T. A. Birks and Y. W. Li, “The shape of fiber tapers,” J. Lightwave Technol. 10, 432– 438 共1992兲. 3. R. P. Kenny, T. A. Birks, and K. P. Oakley, “Control of optical fibre taper shape,” Electron. Lett. 27, 1654 –1656 共1991兲. 4. R. L. Williamson and M. J. Miles, “Melt-drawn scanning nearfield optical microscopy probe profiles,” J. Appl. Phys. 80, 4804 – 4812 共1996兲. 5. D. R. Fairbanks, “Thermal visco-elastic simulation model for tapering of laser-heated fused silica fiber,” in Fiber Optic Components and Reliability, P. M. Kopera and D. K. Paul, eds., Proc. SPIE 1580, 188 –196 共1991兲. 6. A. J. C. Grellier, N. K. Zayer, and C. N. Pannell, “Heat transfer modeling in CO2 laser processing of optical fibres,” Opt. Commun. 152, 324 –328 共1998兲. 7. H. Yokota, E. Sugai, and Y. Saaki, “Optical irradiation method for fiber coupler fabrications,” Opt. Rev. 4, 104 –107 共1997兲. 8. A. D. McLachlan and F. P. Meyer, “Temperature dependence of the extinction coefficient of fused silica for CO2 laser wavelengths,” Appl. Opt. 26, 1728 –1731 共1987兲. 9. M. N. Ozisic, Basic Heat Transfer 共McGraw-Hill, New York, 1977兲, p. 307. 10. T. A. Birks and J. C. Knight, “Excitation of whispering gallery modes in fibres by fibres,” in Conference on Lasers and ElectroOptics 共CLEO兾Europe兲 共Optical Society of America, Washington, D.C., 1998兲, paper CThI2.
