58 SOUTH AFRICAN INSTITUTE OF ELECTRICAL ENGINEERS Vol.102(3) September 2011 MICROMACHINING OF OPTICAL FIBRES WITH A NANOSECOND LASER FOR OPTICAL COMMUNICATION AND SENSOR APPLICATIONS D. Schmieder, R. Samaradiwakera and J. Meyer Photonics Research Group, Faculty of Engineering and the Built Environment, University of Johannesburg, P.O. Box 524, Auckland Park 2006, Johannesburg, South Africa E-mail: dschmieder@uj.ac.za phone +27 11 5589 2648; fax +27 11 559 2344 Abstract: Micromachining of single-mode telecommunication fibres (SMF28) was accomplished with a Nd:YAG laser at a wavelength of 355 nm. Micromachining is important for the manufacturing of Bragg gratings and long period gratings which are used in add-drop filters and wavelength division multiplexers. Manufacturing of miniature Fabry-Perot interferometers used for temperature sensors is also possible. A short overview of micromachining concepts is presented. The experimental setup, as well as the equipment used, is described. Alignment processes, focal point determination and centering of the laser beam onto the optical fibre are outlined. Micromachining results are presented. Keywords: Micromachining optical fibres, light feeder, beam profile displays, focal point determination, micro Fabry-Perot interferometers. 1. INTRODUCTION To drill very small holes or slots in materials such as quartz, diamond, silicon or sapphire, short pulse lasers operating in the nanosecond, picosecond or femtosecond region are required. To produce micro features by direct machining, the laser beam is focused to spot sizes of below 30 μm. Small spot sizes are achieved using a TEM oo laser beam with a M 2 value not much larger than one. The M 2 factor, also called the beam quality factor or beam propagation factor, is a common measure of the beam quality of a laser beam. The M 2 factor of a laser beam limits the degree to which the beam can be focused for a given beam divergence angle, which is often limited by the numerical aperture of the focusing lens. The wavelength of the laser is not so much of a concern due to the high beam intensities involved, which leads to electron plasmas and ablation of the material. It has been found that UV laser light at a wavelength of 355 nm produces significantly less thermal damage and smaller holes than longer wavelength IR lasers [1]. The research described in this paper is focused on the ablation of single-mode communication fibre (SMF) with a Nd:YAG nanosecond laser with a wavelength of 355 nm. The purpose of the ablation is to manufacture miniature holes inside a single-mode optical fibre to create in fibre Fabry-Perot interferometers. The technique can also be used for the manufacturing of Bragg gratings, long period gratings for add-drop filters and wavelength division multiplexers used in optical communication systems. The technique is also ideally suited for the manufacturing of fast response temperature sensors. The following experimental features, which are under investigation, have a significant impact on the quality and size of the holes and cavity structures. Firstly, the selection of the machining lens, e.g. normal lenses with a focal length of 24.5, 35 or 50 mm; microscope objective lenses of 20 times magnification (NA = 0.45), 60 times magnification (NA = 0.65); or oil immersed microscope objective lenses of 100 times magnification (NA = 1.3). Secondly, the quality and kind of laser beams, such as Gaussian beams, flat top beams or Bessel beams. Thirdly, the duration and energy of the laser pulses and fourthly, the ablation speed, repetition frequency, and burst mode of the laser beam. Optical fibres are sheltered with a protective coating. For applications in optical communication and as sensors it is important to drill the holes and features right through the fibre. This is difficult because optical fibres display small hair cracks on the surface between core and cladding, which are formed during the fibre manufacturing process. A compromise has to be found between machining optical fibres with and without protective coating. The purpose of this paper is to present an approach to solve the problem of micromachining fibres with and without protective coating. The rest of the paper describes the micromachining concept, the micromachining experimental setup, laser beam profile, beam alignment process, focal point determination, laser beam positioning, and the micromachining of the optical fibres. 2. MICROMACHINING CONCEPT When intense nanosecond pulses are tightly focused, the intensity in the focal volume can become high enough to initiate absorption through nonlinear field ionization.
Vol.102(3) September 2011 SOUTH AFRICAN INSTITUTE OF ELECTRICAL ENGINEERS 59 The nonlinear absorption of the laser radiation results in the creation of an electron-ion plasma in the focal volume of the beam [2]. In nonlinear media such as air or water a white-light super-continuum is generated in the focal volume. For optical fibres, exposed to the laser radiation, ablation takes place on the surface of the fibre. In addition to ablation with nanosecond laser pulses, thermal and shock wave effects are observed. The high intensity in the focal realm allows the machining of cavities for Fabry-Perot interferometers in the optical fibres. 3. EXPERIMENTAL MICROMACHINING SETUP WITH A HORIZONTAL LASER BEAM A similar experimental setup as described by Rao et al. [2], Wei et al. [3], and Marshall et al. [4] is presented. For the machining process a Surelite Nd:YAG laser from Continuum was used. The laser operated in the frequency-doubled and frequency-tripled mode. The frequency-tripled wavelength at 355 nm was used to implement the laser micromachining of the optical fibres. The wavelength for micromachining is not so important, because ablation occurs when a highly, ionized plasma is formed in the focal region. However, the shorter the wavelength is, the smaller is the size of the holes. This is because the beam size in the focal region depends on the wavelength. Thermal distortions are suppressed when the laser pulses are faster than the development of the thermal effects. The output power of the Nd:YAG laser is selected by varying the Q-switch delay, the voltage on the flashlamps and the pulse repetition frequency. For a typical setting of Q-switch delay: 130 μs, voltage on the flashlamps: 1.2 kv and pulse repetition frequency: 10 Hz, the output power was 54 mw, and the energy/pulse 5.4 mj/pulse. Apart from the Nd:YAG laser and the frequency doubling and tripling crystals, the system consisted of a filter and a dichroic mirror used to eliminate the frequency doubled 532 nm radiation, a 1 3 mm round aperture, a 50 mm focusing lens, a translation stage with fibre holding clamps and a setup for a camera. The camera was computer controlled and operated with the software ProScope HR. The magnification of the camera is 400 times. The schematic drawing of the micromachining setup is shown in figure 1. The three axis translation stage shown in figure 1 can be shifted back and forth between the position where the machining takes place and the position where the camera is located. The whole experimental micromachining setup is shown in picture 1. On the left side the filter (A) and the dichroic mirror (B), which reject the 532 nm laser radiation, and transmit the 355 nm laser light, are placed. In front of the dichroic mirror the head (C) of a power meter can be seen, which can be shifted into and out of the laser beam. The power meter (D) can be seen in the background. The next items in the laser light pass are an aperture (round, 1-3 mm diameter) (E) and the machining lens (F) with a focal length of 50 mm. Next to the lens at the edge of the breadboard the light feeder (G) is visible. Behind the lens the three dimensional translation stage (H) is placed on a rail. In the background on the left side is the Nd:YAG laser (K) and on the right side the computer screen (L), displaying the image from the camera using ProScope HR software. Dichroic Mirror Reflects 355 nm Transmits 532 nm 50 mm Lens Fiber Light Beam Three Axis Translation Stage Dichroic Mirror Box Filter removes 532 nm Circular Aperture 1, 2 and 3 mm Clamps Frequency doubling and tripling crystals (2 KDPs) Camera Fiber Clamps Three Axis Translation Stage Nd:YAG Laser Computer Figure 1: Schematic drawing of the micromachining facility setup. A K B C Picture 1: Experimental micromachining setup D An alignment screen with two clamps to mount the optical fibre is attached to the translation stage and displayed in detail in picture 2. The translation stage can be shifted towards the camera which captures the images from the machined fibre. Picture 3 displays the camera (A), pointing at the mounted optical fibre. Inside the camera is a white light LED ring, which illuminates the optical fibre. The optical fibre is inserted vertically to allow the positioning of the E I F L H G
60 SOUTH AFRICAN INSTITUTE OF ELECTRICAL ENGINEERS Vol.102(3) September 2011 camera very close to the optical fibre while preventing the image being obstructed by the fibre clamps (B). The camera can be focused onto the side surface of the fibre or onto the end surface of the fibre. During the machining process the camera is focused onto the end of the fibre. Red light from the light feeder is coupled into the fibre when the camera is focused on the fibre end surface. the red LED light only into the core of the fibre or into the core and the cladding of the fibre. Picture 4 shows the camera image of the end surface of the fibre. The red light from the light feeder is clearly visible in the core of the fibre. Picture 4: Camera display showing the core of the fibre illuminated by the light from the LED light source. Picture 2: Alignment screen with fibre clamps B A Figure 2: Light feeder schematic 4. LASER BEAM PROFILE The laser beam was imaged with a 25.4 mm focal lens onto a white screen where the beam profile was observed. Picture 3: Camera setup focused onto the fibre 3.1 THE LIGHT FEEDER A prototype model of a LED/Laser Light Feeder was fabricated to illuminate the fibre with visible laser light for monitoring purposes during the micromachining process. Figure 2 shows the light feeder construction with a convex lens and a built-in rechargeable battery pack. The LED housing on the left side can be adjusted to couple Picture 5: Laser beam profile behind a 1 mm aperture
Vol.102(3) September 2011 SOUTH AFRICAN INSTITUTE OF ELECTRICAL ENGINEERS 61 The laser setting was: Q-switch delay 120 μs, voltage on the flashlamps 1.2 kv and the pulse repetition frequency 10 Hz. Picture 5 shows the beam profile of the laser beam behind a 1 mm aperture. The beam profile behind the aperture displays the interference rings. With an aperture the beam profile is not as good as without aperture, but the advantage is, smaller holes can be machined, because the laser beam is curtailed. 5. THE ALIGNMENT PROCESS To establish the optical axis the laser beam was aligned in the vertical and horizontal directions from the dichroic mirror to the three axis translation stage, where the optical fibre was placed. The vertical alignment of the laser beam was achieved by measuring the height of the laser beam near the dichroic mirror and near the translation stage. The dichroic mirror was adjusted until the height of the laser beam was parallel to the breadboard. The horizontal alignment was accomplished by following the holes on the breadboard and placing holders with apertures of 1 mm diameter in the holes. After having set up the optical axis, the optical components were inserted. The following step is to place the machining lens at a right angle to the laser beam. For this purpose an observation screen was mounted in place of the optical fibre. The lens was removed and the laser beam directed through the centre of the lens holder onto the screen. The position of the laser beam on the screen was marked. The lens was then placed back into the lens holder and aligned until the laser beam was back at the marked position on the screen. frequency. Air breakdown caused by the laser beam occurs at the position of the focal point. When air breakdown occurs a white light supercontinuum is generated, because air is a nonlinear medium. Air breakdown was used to determine the focal point of the laser beam. Air breakdown was obtained by setting the laser to: Q-switch delay 130 μs, voltage on the flashlamps 1.2 kv and pulse repetition frequency 10 Hz. There was no aperture inserted. In picture 6 the white spot where air breakdown occurs can be seen. The focal point of the machining lens is situated at the air breakdown point. The surfaces of the two clamps were brought in line with the air breakdown spot by turning the micrometer screw of the three axis translation stage, which moves the clamps towards or away from the machining lens. The fibre was installed at this position and the micrometer screw reading was taken. A further improvement of the location of the focal point can be achieved by machining holes at different positions near the focal point spot into the fibre. The focal point is situated at the location where the smallest hole is machined. 7. POSITIONING OF THE FOCUSED LASER BEAM IN THE MIDDLE OF THE OPTICAL FIBRE As long as the optical fibre is not in the path of the laser beam a small blue spot is observed on the screen behind the optical fibre. The blue spot is the fluorescence light of the ultra violet laser beam which can t be seen with the naked eye. When the optical fibre is in the path of the laser beam a blue line appears on the screen. The blue line is vertical to the optical fibre. The line is the diffraction pattern created by the optical fibre. A method was discovered to place the focused laser beam in the middle of the optical fibre. Positioning of the fibre is done on very low power settings of the laser beam to avoid ablation or marking of the fibre. With a laser setting of: Q-switch delay 116 μs, voltage on the flashlamps 1.14 kv and pulse repetition frequency of 10 Hz the positioning was accomplished. Method Picture 6: Air breakdown white light super-continuum indicated by the arrow 6. DETERMINATION OF THE FOCAL POINT USING THE AIR BREAKDOWN The challenge is to determine the exact focal point, the correct ablation energy and the right pulse repetition By turning the micrometer screw the optical fibre is shifted towards the focused laser beam. When the focused laser beam is reached, instead of the small blue spot a blue line appears on the screen behind the fibre, as shown in figure 3. At this position the micrometer screw reading is taken. The fibre is moved through the laser beam until the blue line disappears and the small blue spot reappears. Another reading is taken at this position. The middle of these two positions is the centre position of the fibre. To confirm, the distance between the two positions must be the width of the fibre, which is 250 μm for a fibre with
62 SOUTH AFRICAN INSTITUTE OF ELECTRICAL ENGINEERS Vol.102(3) September 2011 protective coating or 125 μm for a fibre without protective coating. approximately 40 μm. The red LED light from the light feeder is scatterd from the position of the hole. Focused Laser Beam Fiber Diffracted Laser Beam With the same laser setting holes were machined with 1 shot, 30 shots, 40 shots and 50 shots. The diameters of the holes became larger with increased number of shots. The diagram in figure 4 shows the hole-diameters as a function of the number of shots. Figure 3: Diffracted laser beam for determining the machining point 1 shot hole size: 31 μm 30 shots hole size: 56 μm 40 shots hole size: 68 μm 50 shots hole size: 81 μm 90 Hole Diameters versus Number of Shots Diameter of the Holes [micro meter] 80 70 60 50 40 30 20-10 0 10 20 30 40 50 60 Number of Shots Picture 7: Blue line orthogonal to the optical fibre Figure 4: Hole-diameters versus number of shots Picture 7 displays the image of the blue line on the screen behind the fibre. The blue line can be seen as long as the focused laser beam is on the fibre. 8. MICROMACHINING OF OPTICAL FIBRES The micromachining of the fibre was done using a lens with a focal length of 50 mm. After machining, the fibre was shifted with the translation stage towards the camera to view the results from the micromachining. 8.1 MICROMACHINING OF OPTICAL FIBRES WITH PROTECTIVE COATING Micromachining was started on an optical fibre with protective coating. The diameter of the fibre was 250 μm. The laser was operated at: Q-switch delay 120 μs, voltage on the flashlamps 1.15 kv, pulse repetition frequency 1 Hz, output power 1 mw, energy/pulse 1 mj and aperture 3 mm. Red light from the light feeder was coupled into the fibre. The red light in the fibre enables visualisation of the scattered light at the machined spot. Four holes were machined. One hole is shown indicated by the arrow in picture 8. The diameter of the holes was Picture 8: The camera image of the 40 μm hole machined into the fibre with illumination from the camera switched on The red light from the light feeder is coupled into the optical fibre and the output at the end face of the optical fibre is viewed with the camera. The core diameter of the single mode fibre (SMF-28) used is 8 μm, the cladding diameter is 125 μm and the total fibre diameter with protective coating is 250 μm. When the protective
Vol.102(3) September 2011 SOUTH AFRICAN INSTITUTE OF ELECTRICAL ENGINEERS 63 coating of the fibre is removed at the coupling end in the light feeder, the light is coupled into the cladding and core as seen in picture 9. A circular aperture of 1.5 mm was inserted into the laser beam and a new machining attempt was started with a laser setting of: Q-switch delay 120 μs, voltage on the flashlamps 1.2 kv and pulse repetition frequency 1 Hz. Holes were machined into the fibre at different locations with one shot for each hole. The holes are all the same and about 25 μm in diameter, one hole is shown in picture 10. Picture 11: Hole machined when laser operated at 1 Hz pulse repetition frequency for 15 minutes Another experiment was executed with the same laser settings. One hole was machined with 26 laser pulses, each laser pulse at an interval of 20 seconds. The holesize achieved was about 20 μm. 8.2 MICROMACHINING OF OPTICAL FIBRES WITHOUT PROTECTIVE COATING Picture 9: Camera image of the light coupled into the core as seen from the fibre end face Optical fibres without protective coating were machined. The diameter of the fibres without protective coating is 125 μm. The setting of the laser was: Q-switch delay 115 μs, voltage on the flashlamps 1.2 kv and the pulse repetition frequency 1 Hz. An aperture of 2 mm and a 0.4 ND filter (39.41 % transmission) were inserted. Holes were machined at different positions in the optical fibre. Each hole was machined with one laser pulse. The diameter of the holes is 8 μm. Picture 12 shows a single hole. Picture 10: A 25 μm hole machined into the fibre using one shot While monitoring the light from the light feeder at the end face of the fibre one hole was machined running the laser at a pulse repetition rate of 1 Hz for 15 minutes. When machining started, flickering of the light from the fibre end face could be observed, probably because of vibrations of the fibre. The hole became bigger because of thermal effects. The result is shown in picture 11. Picture 12: Single hole machined with 1 laser pulse, the hole-diameter is 8 μm.
64 SOUTH AFRICAN INSTITUTE OF ELECTRICAL ENGINEERS Vol.102(3) September 2011 8.3 DRILLING HOLES RIGHT THROUGH AN OPTICAL FIBRE WITHOUT PROTECTIVE COATING Holes were drilled right through an optical fiber without protective coating using the following Nd:YAG laser settings: Q-switch delay 115 μs, voltage on the flashlamps 1.2 kv and a pulse repetition frequency of 10 Hz. With an aperture of 1 mm the output power behind the aperture was 1 mw. was drilled through the optical fibre with protective coating (diameter 250 μm). It took about 15 minutes. The hole was about 60 μm in diameter and is shown in picture 15. During drilling the red LED light output at the end face of the fibre was observed. When ablation occurred, flickering of the red LED light was observed. The red LED light from the light feeder is coupled into the uncoated optical fibre and monitored at the end of the optical fibre with the camera. When drilling starts one can observe fluctuations of the red light at the fibre end. This is probably due to the laser pulse shockwaves causing the fibre to vibrate. After about 20 minutes the drilling is complete and one observes the red light at the position of the hole, in front as well as at the back of the optical fibre. The hole-size is about 20 μm. Picture 13 shows the entrance of the hole. The red light is emerging from both sides of the fibre, the entrance and the exit. Picture 14: light A 12.5 μm hole with emerging red LED Picture 13: Entrance hole With the same laser setting another attempt was launched. The drilling took about 2 minutes, and the size of the spot was 12.5 μm as shown in picture 14. The red light emerging from the small hole can be seen. 8.4 DRILLING HOLES RIGHT THROUGH AN OPTICAL FIBRE WITH PROTECTIVE COATING An experiment was performed attempting to drill through an optical fibre without protective coating. The selected laser setting was: Q-switch delay 130 μs, voltage on the flashlamps 1.2 kv and the pulse repetition frequency 10 Hz. The output power was 16 mw. With this setting and with no aperture a hole Picture 15: 60 μm hole drilled through an optical fibre with protective coating 9. CONCLUSION Holes were micromachined in single-mode telecommunication fibres (SMF28) with a Nd:YAG laser at a wavelength of 355 nm. The experiments have shown, it is possible, to drill holes into optical fibres with high precision. Ways were found to determine the focal point and to centre the laser beam onto the optical fibre. The selection of the machining lens, the quality and the kind of laser beam, the duration and energy of the laser pulses, the ablation speed, repetition frequency and burst mode have a significant impact on the quality and size of
Vol.102(3) September 2011 SOUTH AFRICAN INSTITUTE OF ELECTRICAL ENGINEERS 65 the holes and cavity structures. They are under further investigation. Micro Fabry-Perot Interferometer in Optical Fibres Top View Fibre (250 μm) Fibrecore Front View Fabry-Perot Cavity Figure 4: Fabry-Perot cavity in optical fibres For applications in optical communications and as sensors it is important to drill the holes and features right through the fibre. The next step is to machine Fabry-Perot cavities into the fibres as shown in figure 4 and etch them with hydrofluoric acid to improve the surface quality. The manufactured structures have to be characterized before attempting to machine them in sapphire fibre. 10. REFERENCES [1] A. Ostendorf, K. Koerber, T. Nether, T. Temme: Material Processing Applications for Diode Pumped Solid State Lasers, In: Lambda Highlights, No. 57 (Lambda Physik, Göttingen 2000) pp. 1-2. [2] Yun-Jiang Rao, Ming deng, De-Wen Duan, Xiao- Chen Yang, Tau Zhu, Guang-Hua Cheng: Micro Fabry-Perot Interferometers in silica fibers machined by femtosecond laser, Optics Express 15(21), 14123-14128 (2007). [3] Tao Wei, Yukun Han, Hai-Lung Tsai, and Hai Xiao, Miniaturized fiber inline Fabry-Perot Interferometer fabricated with a femtosecond laser, Opt. Lett. 33(6), 536-538 (2008). [4] Graham D. Marshall, Martin Ams, and Michael J. Withford, Point by point femtosecond laser inscription of fibre and waveguide Bragg gratings for photonic device fabrication, Proc. PICALO, 360-362 (2006).