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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 12, Iss. 4 — Feb. 23, 2004
  • pp: 717–723
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High energy nanosecond laser pulses delivered single-mode through hollow-core PBG fibers

J. D. Shephard, J. D. C. Jones, D. P. Hand, G. Bouwmans, J.C. Knight, P. St.J. Russell, and B. J. Mangan  »View Author Affiliations


Optics Express, Vol. 12, Issue 4, pp. 717-723 (2004)
http://dx.doi.org/10.1364/OPEX.12.000717


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Abstract

We report on the development of hollow-core photonic bandgap fibers for the delivery of high energy pulses for precision micro-machining applications. Short pulses of (65ns pulse width) and energies of the order of 0.37mJ have been delivered in a single spatial mode through hollow-core photonic bandgap fibers at 1064nm using a high repetition rate (15kHz) Nd:YAG laser. The ultimate laser-induced damage threshold and practical limitations of current hollow-core fibers for the delivery of short optical pulses are discussed.

© 2004 Optical Society of America

1. Introduction

Optical damage currently limits the ability of conventional silica fiber optics to deliver the high beam quality, high peak power pulses required for micro-machining applications. High beam quality is particularly important for micro-machining, as it allows a small intense focused spot which influences the quality of the features being machined [1

1. D.P. Hand and J.D.C. Jones, “Single-mode delivery of Nd:YAG light for precision machining applications,” Appl. Opt. 37, 1602–1606 (1998) [CrossRef]

]. Laser systems are now commercially available which generate short (nanosecond) pulses at the high repetition rates (tens of kHz) and high peak powers suited to micro-machining. This is currently carried out using motorized stages to move the workpiece beneath the laser and hence describe the machining pattern. However, greater flexibility in the design of the machining systems would be possible if pulses could be delivered through a lightweight optical fiber system, in particular when considering the processing of more complex non-planar workpieces.

Single mode pulse delivery at 1064nm has been achieved previously [1

1. D.P. Hand and J.D.C. Jones, “Single-mode delivery of Nd:YAG light for precision machining applications,” Appl. Opt. 37, 1602–1606 (1998) [CrossRef]

] with peak powers of the order of 250W. However, the regime investigated in that report involved much longer pulses (0.125ms) than studied here, and they are not ideal for high precision micro-machining. Other work for fiber delivery of Nd:YAG laser light for machining purposes has concentrated on large core or multimode delivery [2

2. A. Kuhn, I.J. Blewett, D.P. Hand, P. French, M. Richmond, and J.D.C. Jones, “Optical fibre beam delivery of high-energy laser pulses: beam quality preservation and fibre end-preparation,” Opt. Lasers Eng. 34, 273–288 (2000) [CrossRef]

4

4. D. Su, A.A.B. Boechat, and J.D.C. Jones, “Optimum beam launching conditions for graded index optical fibres: theory and practice,” IEE Proceedings-J , 140, 221–226 (1993)

]. This enables high energy pulses (10–30J) to be delivered through the silica fiber without exceeding the laser induced damage threshold (LIDT) of silica. In this situation, however, the beam quality is much lower (M2~20) than that required for high precision machining [2

2. A. Kuhn, I.J. Blewett, D.P. Hand, P. French, M. Richmond, and J.D.C. Jones, “Optical fibre beam delivery of high-energy laser pulses: beam quality preservation and fibre end-preparation,” Opt. Lasers Eng. 34, 273–288 (2000) [CrossRef]

].

Fig. 1. Scanning electron micrograph image of hollow-core photonic bandgap fiber designed for use at 1064nm wavelength.

Recently developed hollow-core photonic bandgap (PBG) fibers [5

5. R.F. Cregan, B.J. Mangan, J.C. Knight, T.A. Birks, P.S. Russell, P.J. Roberts, and D.C. Allan “Single-mode photonic bandgap guidance of light in air,” Science 285, 1537–1539 (1999) [CrossRef] [PubMed]

] have the potential to overcome the limitations discussed above as the majority of the power is contained within a hollow-core and is delivered in a single mode. These fibers are currently under intensive investigation for high power amplification applications [6

6. J. Limpert, T. Schreiber, S. Nolte, H. Zellmer, and A. Tünnermann “All fiber chirped-pulse amplification system based on compression in air-guiding photonic bandgap fiber,” Opt. Express 11, 3332–3337 (2003) http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-24-3332 [CrossRef] [PubMed]

] and also for transmission of femtosecond solitons with very high (5.5 MW) peak powers [7

7. D. G. Ouzounov, F. R. Ahmad, D. Muller, N. Venkataraman, M. T. Gallagher, M. G. Thomas, J. Silcox, K. W. Koch, and A. L. Gaeta1 “Generation of Megawatt Optical Solitons in Hollow-Core Photonic Band-Gap Fibers,” Science 301, 1702–1704 (2003) [CrossRef] [PubMed]

], the pulse energies, however, remaining low. In this report the focus is on delivering the nanosecond pulses frequently used for machining. We study the delivery of high power pulses with low M2 which, when delivered at high repetition rates, can be used for precision machining. The damage limitations of the hollow-core fiber are also investigated. The PBG fiber used in this investigation (see Fig. 1) has a hollow core approximately 8.2µm in diameter, surrounded by a 7-layer photonic bandgap structure. A measured attenuation curve is given in Fig. 2. The low-loss PBG region spans 180nm centered around 1060nm wavelength. Within the bandgap, attenuation drops to 60dB/km, while on the band edges it oscillates around 1000dB/km. The fiber operates as if single mode at 1064nm with an NA of approximately 0.12 and a mode field diameter of approximately 6.5µm. (In principle, several modes are guided, but these are not observed in our experiments. [8

8. G. Bouwmans, F. Luan, Jonathan Cave Knight, P. St. J. Russell, L. Farr, B. J. Mangan, and H. Sabert “Properties of a hollow-core photonic bandgap fiber at 850 nm wavelength,” Opt. Express 11, 1613–1620 (2003) http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-14-1613 [CrossRef] [PubMed]

])

Fig. 2. Attenuation in the hollow-core PBG fiber. The minimum attenuation is around 60dB/km, and the fiber guides over a band of roughly 180nm width. The inset shows the low-loss region in greater detail.

2. Damage limitations of the PBG fiber

As the fibers used in this study have a hollow core, their pulse delivery capability will be limited by the LIDT of the silica/air PBG cladding structure surrounding the core. Therefore to determine this limit Q-switched pulses at 1064nm wavelength were deliberately focussed onto the PBG cladding structure (as opposed to guiding through the hollow core) to test the damage threshold of the PBG web structure (see Fig. 1). A pulse width of 8ns (M2~5–6) at repetition rate of 10Hz was used for the tests.

Fig. 3. Optical micrograph showing damage to hollow-core fiber. The PBG cladding structure is completely ablated by 450µJ pulses, guidelines represent 50µm spacing.
Fig. 4. Temporal pulse profile delivered from Q-switched Nd:YAG at 1064nm and repetition rate of 15kHz

3. Pulse delivery

For the pulse delivery investigation, a diode-pumped Nd:YAG laser was used with a high-quality, low divergence beam. This operated at 1064nm in TEM00 (M2~1.2), delivering approximately 65ns pulses (see figure 4) at a repetition rate of 15kHz. The laser output was expanded and launched into 1m of PBG fiber using a 10× microscope objective. The objective lens was not completely filled with resulting in an effective launch NA of 0.21 and a corresponding spot size of ~4.2µm. It should be pointed out that the spot size was deliberately chosen to be smaller than the core size of the PBG fiber to avoid significant energy being incident on the PBG cladding. Naturally, this resulted in the NA being overfilled and hence a loss of efficiency on launch, the best efficiency achieved being approximately 30%. We would expect this to be improved through the use of more careful beam conditioning optics. Optimum launch conditions were achieved by a combination of viewing the launch end of the fiber via CCD camera to ensure the spot was centred on the air core and, optimising output with a power meter. Figure 5 shows the observed near-field pattern recorded using a low-power cw source through a long length of fiber (20m) and a far-field pattern observed using the ns pulse train and one meter of fiber. In both cases, the output shows a high-quality guided mode.

Fig. 5. Near-field (left, recorded with a low-power cw source) and false-color far field image (recorded using ns pulses) at 1064nm after transmission through 20m (left) and 1m (right) of PBG fiber. Excellent beam quality is observed in both cases.

The maximum pulse energy delivered was measured as 380µJ, corresponding to an average power of 5.6W and a peak power of around 6kW. However, this did not represent the power at which the fiber was damaged but rather the limit of the power available from the Q-switched laser due to the beam conditioning optics and the efficiency of the launch (roughly 30%).

The pulse energy delivered by the fiber remained constant (within a few percent) for the duration of the test (roughly 2 mins or 1,800,000 pulses) suggesting that no significant damage was occurring at the launch or delivery end. Although 2 minutes does not represent a practical time scale for micro machining, due to the high laser repetition rate (15kHz), it was regarded that the number of pulses was sufficient for this initial investigation. As the PBG fiber did not fail catastrophically during this period longer test times combined with actual material processing are planned in the future. On inspection however, there was a noticeable difference between the fiber before and after the tests (Fig. 6.) We believe that non-catastrophic damage may be occurring to the PBG fiber cladding. Damage can also be seen on the unstructured silica cladding surrounding the PBG structure. However, these effects did not measurably degrade the ability of the fiber to deliver light. It is also worth observing that our experiments were performed in an atmospheric environment, and the results might well be different in an inert gas.

The heat dissipation in a microstructured fiber has previously been modelled [11

11. J. Limpert, T. Schreiber, A. Liem, S. Nolte, H. Zellmer, T. Peschel, V. Guyenot, and A. Tünnermann “Thermo-optical properties of air-clad photonic crystal fiber lasers in high power operation,” Opt. Express 11, 2982–2990 (2003) http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-22-2982 [CrossRef] [PubMed]

], and although those workers did not consider an air-core PBG fiber the principles appear to apply. It is suggested that the properties of the PBG fiber cladding have a large effect on the heat dissipation and temperatures in the fiber. In particular thinner silica webs might lead to higher thermal loads and stresses within the PBG fiber cladding because the conductivity is decreased (in the case where conduction is the main heat transfer mechanism). In the fiber tested in this investigation the PBG fiber cladding is relatively large and contains very thin silica webs (of the order of a few hundred nanometers). This structure might not be ideally suited to removal of heat from the central region although the heat absorbed is unlikely to cause significant damage to the fiber. A further investigation of this phenomenon will require both experimental and modeling work.

Fig. 6. Optical micrographs of fiber endfaces before (left) and after (right) pulse delivery. No noticeable change in the pulse delivery was observed.

The PBG fiber was also observed using an Environmental Scanning Electron Microscope (ESEM), in which non-conducting samples do not have to be coated as static charge is removed using a gas flow. This allows direct imaging of the silica PBG fiber cladding to inspect for damage on a sub-micron scale. In Fig. 7 we see a PBG fiber as cleaved and after pulsed delivery. Although not conclusive (further statistical damage studies with subsequent ESEM analysis are planned in the near future) it would appear that after pulse delivery small-scale damage may be occurring. The end faces of the silica capillaries used in the PBG fiber cladding structure appear to become detached and thin layers or rings are splaying off the surface. Although similar effects can be seen in freshly cleaved samples (Fig. 1) the frequency of these sites appears to increase after laser pulse delivery.

Fig. 7. Environmental scanning electron micrographs of fiber end-faces freshly-cleaved (left) and after being used for pulse delivery (right).

4. Conclusion

Although the laser induced damage threshold of the PBG fiber cladding structure is not particularly high (around 120 Jcm-2 using 8ns pulses and low beam quality), the low overlap of the guided mode with the glass enables high-power nanosecond pulse transmission in a single mode through a hollow core. The high energies and high beam quality delivered through the PBG fibers are of the level required for precision machining applications, in particular when combined with the high repetition rates used here. They are more than an order of magnitude higher than has been reported with conventional step-index single-mode fibers. The damage limit of the PBG fibers has not been reached and it is expected that higher peak powers/energies will be delivered in the near future. With further optimization of the fiber design for this specific application, we expect a further order of magnitude increase in the delivered pulse energy. This promises a new generation of single-mode fibers for high-precision laser machining applications.

Acknowledgments

We would like to thank Dr Jim Buckman, School of Petroleum Engineering, Heriot-Watt University, UK, for his help with the ESEM imaging.

References and Links

1.

D.P. Hand and J.D.C. Jones, “Single-mode delivery of Nd:YAG light for precision machining applications,” Appl. Opt. 37, 1602–1606 (1998) [CrossRef]

2.

A. Kuhn, I.J. Blewett, D.P. Hand, P. French, M. Richmond, and J.D.C. Jones, “Optical fibre beam delivery of high-energy laser pulses: beam quality preservation and fibre end-preparation,” Opt. Lasers Eng. 34, 273–288 (2000) [CrossRef]

3.

Andreas Kuhn, Paul French, Duncan P. Hand, Ian J. Blewett, Mark Richmond, and Julian D. C. Jones, “Preparation of fiber optics for the delivery of high-energy high-beam-quality Nd:YAG laser pulses” Appl. Opt. 39, 6136–6143 (2000) [CrossRef]

4.

D. Su, A.A.B. Boechat, and J.D.C. Jones, “Optimum beam launching conditions for graded index optical fibres: theory and practice,” IEE Proceedings-J , 140, 221–226 (1993)

5.

R.F. Cregan, B.J. Mangan, J.C. Knight, T.A. Birks, P.S. Russell, P.J. Roberts, and D.C. Allan “Single-mode photonic bandgap guidance of light in air,” Science 285, 1537–1539 (1999) [CrossRef] [PubMed]

6.

J. Limpert, T. Schreiber, S. Nolte, H. Zellmer, and A. Tünnermann “All fiber chirped-pulse amplification system based on compression in air-guiding photonic bandgap fiber,” Opt. Express 11, 3332–3337 (2003) http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-24-3332 [CrossRef] [PubMed]

7.

D. G. Ouzounov, F. R. Ahmad, D. Muller, N. Venkataraman, M. T. Gallagher, M. G. Thomas, J. Silcox, K. W. Koch, and A. L. Gaeta1 “Generation of Megawatt Optical Solitons in Hollow-Core Photonic Band-Gap Fibers,” Science 301, 1702–1704 (2003) [CrossRef] [PubMed]

8.

G. Bouwmans, F. Luan, Jonathan Cave Knight, P. St. J. Russell, L. Farr, B. J. Mangan, and H. Sabert “Properties of a hollow-core photonic bandgap fiber at 850 nm wavelength,” Opt. Express 11, 1613–1620 (2003) http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-14-1613 [CrossRef] [PubMed]

9.

F. Rainer, L. J. Atherton, J. H. Campbell, F. D. DeMarco, M. R. Kozolowski, A. J. Morgan, and M. C. Staggs, “Four-harmonic database of laser-damage testing,” Proc. SPIE 1624116 (1992) [CrossRef]

10.

T.J. Stephens, “Fibre-optic Delivery of High Peak Power Laser Pulses for Flow Measurement,” PhD Thesis, Heriot-Watt University, UK (2003)

11.

J. Limpert, T. Schreiber, A. Liem, S. Nolte, H. Zellmer, T. Peschel, V. Guyenot, and A. Tünnermann “Thermo-optical properties of air-clad photonic crystal fiber lasers in high power operation,” Opt. Express 11, 2982–2990 (2003) http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-22-2982 [CrossRef] [PubMed]

OCIS Codes
(060.0060) Fiber optics and optical communications : Fiber optics and optical communications
(060.2430) Fiber optics and optical communications : Fibers, single-mode
(350.3390) Other areas of optics : Laser materials processing

ToC Category:
Research Papers

History
Original Manuscript: December 19, 2003
Revised Manuscript: February 18, 2004
Published: February 23, 2004

Citation
Jonathan Shephard, J. Jones, D. Hand, G. Bouwmans, J. Knight, P. Russell, and B. Mangan, "High energy nanosecond laser pulses delivered single-mode through hollow-core PBG fibers," Opt. Express 12, 717-723 (2004)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-12-4-717


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References

  1. D.P. Hand and J.D.C. Jones, �??Single-mode delivery of Nd:YAG light for precision machining applications,�?? Appl. Opt. 37, 1602-1606 (1998) [CrossRef]
  2. A. Kuhn, I.J. Blewett, D.P. Hand, P. French, M. Richmond, and J.D.C. Jones, �??Optical fibre beam delivery of high-energy laser pulses: beam quality preservation and fibre end-preparation,�?? Opt. Lasers Eng. 34, 273-288 (2000) [CrossRef]
  3. Andreas Kuhn, Paul French, Duncan P. Hand, Ian J. Blewett, Mark Richmond, and Julian D. C. Jones, �??Preparation of fiber optics for the delivery of high-energy high-beam-quality Nd:YAG laser pulses,�?? Appl. Opt. 39, 6136-6143 (2000) [CrossRef]
  4. D. Su, A.A.B. Boechat and J.D.C. Jones, �??Optimum beam launching conditions for graded index optical fibres: theory and practice,�?? IEE Proceedings-J, 140, 221-226 (1993)
  5. R.F. Cregan, B.J. Mangan, J.C. Knight, T.A. Birks, P.S. Russell, P.J. Roberts, and D.C. Allan �??Single-mode photonic bandgap guidance of light in air,�?? Science 285, 1537-1539 (1999) [CrossRef] [PubMed]
  6. J. Limpert, T. Schreiber, S. Nolte, H. Zellmer, A. Tünnermann �??All fiber chirped-pulse amplification system based on compression in air-guiding photonic bandgap fiber,�?? Opt. Express 11, 3332-3337 (2003) <a href="http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-24-3332">http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-24-3332</a> [CrossRef] [PubMed]
  7. D. G. Ouzounov, F. R. Ahmad, D. Muller, N. Venkataraman, M. T. Gallagher M. G. Thomas, J. Silcox, K. W. Koch and A. L. Gaeta1 �??Generation of Megawatt Optical Solitons in Hollow-Core Photonic Band-Gap Fibers,�?? Science 301, 1702-1704 (2003) [CrossRef] [PubMed]
  8. G. Bouwmans, F. Luan, Jonathan Cave Knight, P. St. J. Russell, L. Farr, B. J. Mangan, and H. Sabert �??Properties of a hollow-core photonic bandgap fiber at 850 nm wavelength,�?? Opt. Express 11, 1613-1620 (2003) <a href="http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-14-1613">http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-14-1613</a> [CrossRef] [PubMed]
  9. F. Rainer, L. J. Atherton, J. H. Campbell, F. D. DeMarco, M. R. Kozolowski, A. J. Morgan, and M. C. Staggs, �??Four-harmonic database of laser-damage testing,�?? Proc. SPIE 1624 116 (1992) [CrossRef]
  10. T.J. Stephens, �??Fibre-optic Delivery of High Peak Power Laser Pulses for Flow Measurement,�?? PhD Thesis, Heriot-Watt University, UK (2003)
  11. J. Limpert, T. Schreiber, A. Liem, S. Nolte, H. Zellmer, T. Peschel, V. Guyenot and A. Tünnermann �??Thermo-optical properties of air-clad photonic crystal fiber lasers in high power operation,�?? Opt. Express 11, 2982-2990 (2003) <a href="http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-22-2982 ">http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-22-2982</a> [CrossRef] [PubMed]

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