laser assisted jacket removal and writing of fiber bragg gratings using a single laser source

Laser assisted jacket removal and writing offiber Bragg gratings using a single laser source

Benjamin F. Johnston, Andrew J. Lee, Michael J. Withford and James A. PiperCentre for Lasers and Applications,

Division of Information and Communication SciencesMacquarie University, 2109, Australia.

[email protected]

Abstract: The 289 and 255nm outputs of a frequency doubled copper laserare used to remove the polymer jacket of optic fiber and write fiber Bragggratings respectively. Our studies show that 289nm laser light is ideal forstripping the polymer jacket in a well defined manner without interactingsignificantly with the core of photosensitive fibers.©2002 Optical Society of AmericaOCIS codes: (060.2340) Fiber optics components; (140.3610) Laser, ultraviolet

References and Links1. A. Othonos and K. Kalli, Fiber Bragg gratings: fundamentals and applications in telecommunications and

sensing (Artech House, Norwood, 1999) Chap. 2.2. J. Albert, B. Malo, F. Bilodeau, D. C. Johnson, K. O. Hill, Y. Hibino and M. Kawachi, “Photosensitivity in

Ge-doped silica optical waveguides and fibers with 193nm light from a ArF excimer laser,” Opt. Lett. 19,387-389 (1994).

3. C. G. Askins, T. -E. Tsai, G. M. Williams, M. A. Putnam, M. Bashkansky and E. J. Friebele, “Fiber Braggreflectors prepared by a single excimer pulse,” Opt. Lett. 17, 833-836 (1992).

4. G. Meltz, W. W. Morey and W. H. Glenn, “Formation of Bragg gratings in optical fibers by a transverseholographic method,” Opt. Lett. 14, 823-825 (1989).

5. C. J. Paddison, J. M. Dawes, D. J. W. Brown, M. J. Withford, R. I. Trickett and P. A. Krug, “Multiple fibergratings fabricated using frequency doubled copper vapour lasers,” Electron. Lett. 34, 2407-2408 (1998).

6. J. R. Armitage, “Fiber Bragg Reflectors written at 262nm using a frequency quadrupled diode pumpedNd3+:YLF laser,” Electron. Lett. 29, 1181-1183 (1993).

7. S. E. Kanellopoulos, V. A. Handerek and A. J. Rogers, “Photoinduced polarisation couplers in ellipticalcore optical fibers written using 535 and 266nm sources,” Electron. Lett. 28, 1558-1560 (1992).

8. W. Griffioen, “Strippability of optical fibers,” in Proceedings of 11th Annual Conference on EuropeanFiber Optic Communications and Networks, (European Institute of Communications and Networks,Geneva) 239-244 (1993).

9. D. S. Starodubov, V. Grubsky and J. Feinberg, “Efficient Bragg grating fabrication in a fiber through itspolymer jacket using near-UV light,” Electron. Lett. 33, 1331-1333 (1997).

10. D. S. Starodubov, V. Grubsky and J. Feinberg, “Ultrastrong fiber gratings and their applications,” inOptical Fiber Reliability and Testing, Proc. SPIE 3848, 178-185 (1999).

11. R. P. Espindola, R. M. Atkins, N. P. Wang, D. A. Simoff, M. A. Paczkowski, R. S. Windeler, D. L.Brownlow, D. S. Shenk, P. A. Glodis, T. A. Strasser, J. J. DeMarco and P. J. Chandonnet, “Highlyreflective fiber Bragg gratings written through a vinyl ether coating,” IEEE Photon. Tech. Lett. 11, 833-835(1999).

12. D. C. Psaila and H. G. Inglis, “Packaging of optical fiber Bragg gratings,” in Proceedings of 51st ElectronicComponents and Technology Conference, (Institute of Electrical and Electronics Engineers, New York,2001), pp. 439-443.

13. F. Barnier, P. E. Dyer, P. Monk, H. V. Snelling and H. Rourke, “Fiber optic jacket removal by pulsed laserablation,” J. Phys. D: Appl. Phys. 33, 757-759 (2000).

14. T. E. Dimmick, G. Kakarantzas, T. A. Birks and P. St. J. Russell, “Carbon dioxide laser fabrication of fusedfiber couplers and tapers,” Appl. Opt. 38, 6845-6848 (1999).

15. D. W. Coutts and J. A. Piper, “One watt average power by second harmonic and sum frequency generationfrom a single medium scale copper vapour laser,” IEEE. Quantum Electron. 28, 1761-1764 (1992).

16. P. Niay, P. Bernage, S. Legoubin, M. Douay, W. X. Xie, J. F. Bayon, T. Georges, M. Monerie and B.Poumellec, “Behaviour of spectral transmissions of Bragg gratings written in germania-doped fibers:writing and erasing experiments using pulsed or cw uv exposure,” Opt. Commun. 113, 176-192 (1994).

17. A. Lee, M. J. Withford, J. M. Dawes, “Optical fiber photosensitivity and the dynamics of fiber Bragggrating growth,” in Proceedings of Australasian Conference on Optics and Laser Spectroscopy, (AustralianOptical Society) 87 (2001).

18. E. K. Illy, D. J. W. Brown, M. J. Withford and J. A. Piper, “Optimisation of trepanning strategies formicromachining polymers with high pulse rate UV lasers,” Proc. Of SPIE: High power lasers inmanufacturing, X. Chen et al (eds) 3888, 608-616 (2000).

(C) 2002 OSA 12 August 2002 / Vol. 10, No. 16 / OPTICS EXPRESS 818#1386 - $15.00 US Received June 20, 2002; Revised August 02, 2002

19. G. Ogura, “Laser stripping of optical fibers opens up new applications,” Laser Focus World (PenwellPublishing) 37, 169-176 (2001).

20. See for example Resostrip at http://www.resonetics.com/Telecom/reso.htm.

1. Introduction

Fiber Bragg gratings are key elements in high speed optical networks and fiber sensors. Thesedevices are fabricated by photo-inducing periodic variations in the refractive index of the coreof optic fiber via exposure to an UV interferometric pattern. The fibers used for thisapplication are photosensitive over most of the UV spectrum, although they are generally onlyweakly photosensitive at wavelengths >270nm [1]. Consequently, ArF (193nm) [2] and KrFexcimers (248nm) [3], frequency doubled argon ion (244nm) [4] and copper lasers (255nm)[5], and frequency quadrupled Nd:YLF (262nm) [6] and Nd:YAG lasers (266nm) [7] are alleffective sources for fabricating fiber Bragg gratings. Unfortunately, the protective polymerjacket of standard fiber is generally highly absorbing at wavelengths <300nm and therefore,must be removed.

There are two main techniques for removing the fiber jacket. The most common of theseis mechanical stripping, however, process repeatability can be a problem as the blade in thesesystems wears with time and commercial devices are not suited to removing the jacket fromthe centre of a length of fiber. Another commonly used method for jacket removal is chemicalstripping using solutions of dichloromethane, however, volatile chemicals such as this areundesirable for clean room environments. Perhaps the predominant problem associated withthese methods is that they both depend on a degree of physical contact with the fiber, whichultimately weakens the fiber and reduces its longevity [8]. In the former case the fiber must beclamped in position while the jacket is forcibly removed, while in the latter the fiber must bephysically cleaned in a secondary process to remove any chemical residue.

Although novel fiber Bragg grating writing techniques have been demonstrated whichobviate the need to remove the outer coating, such as writing at near UV wavelengths [9,10]or in fibers with UV transmissible outer coatings [11], it is still necessary to remove smallsections of the coating when packaging these devices into athermal protective cases. This isbecause mounting via the soft outer polymer jacket causes excessive drift of the centralwavelength. As a result, there is significant interest in developing non-contact methods ofpolymer jacket removal. One example is chemical stripping using hot sulphuric acid, whichhas been shown to result in a similar tensile strength to pristine fiber [12]. The disadvantage ofthis method (shared by other “wet” processing methods) is that it is difficult to accuratelycontrol the region to be stripped. Another method generating significant interest is polymerjacket removal via laser ablation, which as well as being a “dry” processing technique alsopermits better control of the region of jacket to be removed. Fiber jacket removal has alreadybeen successfully demonstrated using short wavelength ArF and KrF excimer lasers, and longwavelength CO2 lasers (10.6µm) [13], however, there are a number of problems associatedwith using these wavelengths for this purpose. Firstly, laser ablation using CO2 laser radiationis a pyrolytic process thus it is difficult to produce well defined features within the polymerjacket using this source. Furthermore, glass is highly absorbing at 10.6µm [14], therefore,exposure to CO2 radiation causes heating of the fiber which increases the potential fordestructive damage, and accelerates outgassing of the hydrogen load resulting in a loss offiber photosensitivity. UV laser radiation is better suited to polymer ablation because materialremoval occurs largely via photolytic mechanisms. In particular, polymers are highlyabsorbing in the UV and UV photons often have sufficient energy to directly break molecularbonds, resulting in less thermal loading of the substrate and cleaner ablation. The down side inthis case is that the standard fibers used in the fabrication of fiber Bragg gratings are veryphotosensitive to the ArF and KrF excimer wavelengths of 193 and 248nm respectively. Pre-


http://www.resonetics.com/Telecom/reso.htm

exposure to intense light at these wavelengths can result in bleaching of the fiber core makingthe fabrication of fiber Bragg gratings more difficult.

Laser ablation of fiber polymer jackets is best achieved using high peak power, UV lasersources with wavelengths between 270-300nm for which these fibers are only weaklyphotosensitive yet polymer absorption is high. One such source is the frequency doubledcopper laser or UV-CVL that produces 255, 271 or 289nm pulsed output. In this paper wepresent results of a study demonstrating the efficacy of UV-CVLs as sources for bothremoving the polymer jacket and writing fiber Bragg gratings. Moreover, both steps areachieved without extra handling of the fiber.

2. Experiment

The copper vapour laser used in these studies was nominally a 20W device (tube diameter25mm, length 1m) operating at a pulse rate of 10kHz. The output beam was reduced in size to~4mm using a mirror telescope and a 1mm diameter aperture placed at the focus of telescoperemoved the low beam quality amplified spontaneous emission. The remaining 10W of HBQ(~6W of green and ~4W of yellow) was line focussed (using an f=63mm cylindrical lens) intoa non-linear crystal, a technique that has been shown to reduce the chance of crystal damageand minimise the effects of walk-off [15]. The non-linear crystal was beta barium borate(BBO) with dimensions 6mm x 4mm x 8mm and was cut for type 1 second harmonicgeneration (θpm = 47o, φ=90o). For these experiments we varied the tuning angle to frequencydouble either the 511 or 578nm fundamental, producing 255 and 289nm light respectively;note however, that two non-linear crystals could be used to generate 255 and 289nm outputsimultaneously. The UV output was separated from the fundamental using a Pellin Brocaprism. Output powers of ~500mW were available at both UV wavelengths.

Nufern photosensitive fiber (GF1) was used throughout these experiments. The fiber wasalso hydrogenated to increase its photosensitivity. The fiber was fixed in position on top of ay-z-tilt positioning stage, which in turn was mounted upon a motorised translation stage(Physik Instrumente) allowing computer controlled movement in the x direction (parallel tothe fiber axis/perpendicular to the beam path). During polymer jacket removal the UV beamwas focussed to a spot ~10um in diameter using a 10x microscope objective. Alignment of thefocal spot with the centre of the fiber was facilitated by an on-line imaging system. FiberBragg gratings were written using standard phase mask techniques which in our case includeda cylindrical lens (f=75mm) to focus the light onto the fiber via the phase mask (alignedparallel to and approximately 0.5mm in front of the fiber). Note that the phase mask used inthese experiments was optimised for a frequency doubled argon ion laser (244nm) and had asignificant zero order component, effects which contribute to an overall reduction in the peakrejection levels of the gratings written with this system. Both the cylindrical lens/phase maskpair used during grating writing, and the microscope objective used during jacket removalwere mounted upon standard kinematic plates, which allowed for easy interchange betweenthe two processes. The fiber Bragg gratings written with this system were monitored using alaboratory built broadband light source and an Anritsu Optical Spectrum Analyser(MS9710c).

3. Results and Discussion

In order to gauge the response of photosensitive fiber to 289nm radiation we first investigatedthe growth rates of fiber Bragg gratings written with both 255nm and 289nm light, as shownin Figure 1. Note that in this experiment the polymer jackets were removed usingmechanically strippers. Other than the differing wavelengths each grating was written underidentical conditions, namely, exposure of a 3-4mm length of fiber to 50mW of UV focussedonto the fiber with the cylindrical lens (fluence = 60W/cm2). Figure 2 shows thecorresponding shift in the Bragg wavelength with time for these two gratings. The gratingwritten with 255nm light took ~5 minutes to grow to a maximum depth of 11dB after whichthe grating started to erase [16,17]. The Bragg wavelength shifted by ~2 nm during this time.


By comparison, even after 20 minutes exposure the grating written with 289nm light stillmeasured <2dB and showed no signs of erasure. Furthermore, the Bragg wavelength shiftedby <1nm during this period. These results highlight the low photosensitivity of this fiber to289nm light. In particular, the average refractive index increased by 2.4 x 10-3 after 5 minutesexposure to 255nm light, and by only 4.8 x 10-4 after 20 minutes exposure to 289nm light. Ineffect the photosensitivity of this fiber to 289nm light is a factor of 20 smaller than that to255nm light at the same power densities.

Detailed characterisation of polymer jacket ablation at 289nm was undertaken to identifythe best set of operating parameters for coating removal. Briefly, it was found that damage tothe fiber occurred when output powers >200mW were used while slow translation speedsresulted in additional heat deposition within the polymer jacket and poor edge definition. Thebest results were achieved using output powers from 100 to 150mW, and several translationsat the fastest speed possible from our motorised stages (ie. 12.5mm/s) rather than a single passat a slow translation speed [18]. Figure 3 shows a scanning electron micrograph of a 20µmwide transverse scribe machined at an output power of 150mW, and 9 translations at a speed

Fig. 3. A 20 µm wide transverse scribe machined into the polymer jacket of an optical fiberusing 289nm laser radiation.

Figs. 1 and 2. Comparison of the growth dynamics and spectral shift of Bragg gratingswritten with 255 and 289nm laser light. Note that these fibers were prepared usingmechanical stripping.

Time (s)

0 200 400 600 800 1000 1200 1400

Gra

tin

gD

epth

(dB

)

0

2

4

6

8

10

12

255nm

289nm

Time (s)

0 200 400 600 800 1000 1200

Bra

ggW

avel

engt

h(n

m)

1527

1528

1529

1530

255nm

289nm


of 12.5mm/s. The edge definition of the scribe wall is high pointing to the photolytic nature ofthe material removal under these conditions. Indeed, upon close inspection the junctionbetween the inner and outer acrylate coatings used in this type of fiber can be seen. Mostimportantly, the surface of the fiber cladding is fully exposed and free of surface contaminantsand tensile strength tests of fibers machined under these conditions showed them to havecomparable strength to pristine fiber. The final point to make is that although the fiber core isexposed to very high UV power densities (eg ~200kW/cm2) during the stripping process thetotal amount of time any particular point is irradiated by UV light is only 1/100 of a second.This equates to a total energy per unit area of 2kJ/cm2, well below the value of 75kJ/cm2 thefiber experienced during 20 minutes of exposure to 289nm light in the photosensitivityexperiment discussed above.

Figure 4 shows a “window” machined through the polymer jacket and parallel to the fiberaxis using 150mW of 289nm laser light, and 10 translations at a speed of 12.5mm/s. Theripple seen in the walls of this scribe is the result of chatter in the motorised positioningstages. The transmission spectrum for a 20dB fiber Bragg grating written in 3 minutes throughone of these windows in the polymer jacket is shown in Figure 5. Although it is possible toengineer a system that rotates the laser beam around the fiber and completely removes thepolymer jacket [19,20] for simplicity’s sake we chose to only remove small longitudinalsections (60µm wide and 25mm long) of the polymer jacket via laser ablation. This did

Fig. 4. A longitudinal “window” machined into the polymer jacket prior to writing a fiberBragg grating.

Fig. 5. Fibre Bragg grating written with 255nm laser radiation after stripping of the polymerjacket using 289nm light.

Wavelength (nm)

1524 1526 1528 1530 1532 1534

Tra

nsm

issi

on

(dB

)

-30

-25

-20

-15

-10

-5

0


however compound the difficulties associated with aligning the line focus with the fiber coreand placed additional constraints on the operating parameters for writing fiber Bragg gratings.For example, tight focussing geometries had to be used to fit the UV beam within theboundaries set by the “window” machined in the side of the fiber jacket, and the output powerfrom the UV-CVL had to be reduced to 50mW to avoid ablating the remaining acrylatecoating and damaging the phase mask. Despite these constraints, the gratings written in thisfashion were similar to those written using the same 255nm source and mechanically strippedfiber in conjunction with conventional focussing geometries and standard UV output powers[5]. In particular, we saw no evidence that the fiber photosensitivity had been lessened whenremoving the polymer jacket with 289nm laser light. Indeed, the grating shown in Figure 5grew at a faster rate and to a greater depth that those gratings written with this system aftermechanical or chemical stripping. However, further studies are required to determine if thefaster growth rates in this case are due to increased photosensitivity, a cleaner fiber surface, areduction in the stresses induced in the fiber during jacket removal, or some other cause.

4. Summary

We have demonstrated that the 289nm and 255nm outputs from a frequency doubled copperlaser can be used to both strip the outer polymer jacket and write fiber Bragg gratingsrespectively. Tensile strength tests showed that fibers stripped in this fashion have comparablestrength to pristine fiber. The fibers used in study are only weakly photosensitive to 289nmlight, thus, strong gratings could still be fabricated after the stripping process.

Acknowledgment

This work was funded by Macquarie University and JDS Uniphase Pty. Ltd., North Ryde,Australia.


laser assisted jacket removal and writing of fiber bragg gratings using a single laser source

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