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Picosecond and nanosecond pulse delivery through a hollow-core Negative Curvature
Fiber for micro-machining applications
Jaworski, Piotr; Yu, Fei; Maier, Robert R J; Wadsworth, William J; Knight, Jonathan C;
Shephard, Jonathan D.; Hand, Duncan Paul
Document Version Publisher's PDF, also known as Version of record Link to publication in Heriot-Watt Research Gateway
Citation for published version (APA):
Jaworski, P., Yu, F., Maier, R. R. J., Wadsworth, W. J., Knight, J. C., Shephard, J. D., & Hand, D. P. (2013).
Picosecond and nanosecond pulse delivery through a hollow-core Negative Curvature Fiber for micro-machining applications. Optics Express, 21(19), 22742-22753. DOI: 10.1364/OE.21.022742 Picosecond and nanosecond pulse delivery through a hollow-core Negative Curvature Fiber for micro-machining applications Piotr Jaworski,1* Fei Yu,2 Robert R.J. Maier,1 William J. Wadsworth,2 Jonathan C. Knight,2 Jonathan D. Shephard,1 and Duncan P. Hand1 Applied Optics and Photonics Group, Institute of Photonics and Quantum Sciences, Heriot-Watt University, Edinburg, EH14 4AS, UK Centre for Photonics and Photonic Materials, Department of Physics, University of Bath, Bath, BA2 7AY, UK *email@example.com Abstract: We present high average power picosecond and nanosecond pulse delivery at 1030 nm and 1064 nm wavelengths respectively through a novel hollow-core Negative Curvature Fiber (NCF) for high-precision micro-machining applications. Picosecond pulses with an average power above 36 W and energies of 92 µJ, corresponding to a peak power density of 1.5 TWcm−2 have been transmitted through the fiber without introducing any damage to the input and output fiber end-faces. High-energy nanosecond pulses (1 mJ), which are ideal for micro-machining have been successfully delivered through the NCF with a coupling efficiency of 92%.
Picosecond and nanosecond pulse delivery have been demonstrated in fiberbased laser micro-machining of fused silica, aluminum and titanium.
©2013 Optical Society of America OCIS codes: (060.0060) Fiber optics and optical communications; (060.2270) Fiber characterization; (060.5295) Photonic crystal fibers; (350.3390) Laser materials processing.
References and Links
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#191496 - $15.00 USD Received 12 Jun 2013; revised 7 Sep 2013; accepted 9 Sep 2013; published 19 Sep 2013 (C) 2013 OSA 23 September 2013 | Vol. 21, No. 19 | DOI:10.1364/OE.21.022742 | OPTICS EXPRESS 22742
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1. Introduction High average power short pulsed lasers are increasingly used for micromachining. However flexible fiber beam delivery systems for such lasers are limited in terms of pulse energy, restricting the range of potential applications. As a result, these lasers are typically used to process flat parts placed under a galvo scan head. A suitable fiber delivery system is hence important to extend applications to the processing of complex 3D components.
Results obtained over the last couple of years have demonstrated the suitability of hollowcore photonic crystal fibers (HC-PCFs), hollow waveguides and large mode area solid core fibers for high-energy pulse delivery [1–6]. These fibers overcome limitations due to the low damage threshold and nonlinear effects dominant in conventional single-mode solid core silica fibers [7,8] and can also minimize attenuation imposed by material absorption .
However their energy handling capability is still limited to approximately 1 mJ in the ns pulse regime and efficient delivery of high energy ps pulses was not reported.
In 2012 Benabid et al reported a new design of kagome-type hollow-core microstructured fiber  with a hypocycloid shaped core (negative curvature), capable of delivering ns pulses with energies in the range of 10 mJ. Moreover their latest work presents delivery of
10.5 ps pulses with an energy of approximately 97 µJ and an average power of 5 W, corresponding to a peak power of 8 MW through 10 cm long kagome fiber . These reported fibers however have a complicated structure, which requires stacking many layers of capillaries during preform fabrication. Consequently a less complex design which still provided the negative curvature of the core wall, the so called Negative Curvature Fiber (NCF), was presented in [12,13]. This fiber proved to be a great candidate for high-power delivery of Er:YAG laser pulses at a wavelength of 2.94 µm for medical applications .
In this paper we investigate a similar hollow-core Negative Curvature Fiber optimized for the delivery of high-power, high-energy ns and ps pulses in the 1 µm wavelength region for precision micro-machining applications.
2. Negative Curvature Fiber (NCF)
2.1 Fiber structure and light guidance mechanism The NCF was fabricated by the commonly used stack and draw technique, described in .
Eight identical circular capillaries were used to form a preform, which was drawn down to produce a final fiber with 43 µm diameter hollow air core and NA of 0.03 (measured) as shown in Fig. 1(a). To shape the negative curvature structure different pressures were applied to the core and the cladding during the fiber drawing process (details of fiber fabrication are given in ). The crucial points of the fiber are cladding nodes between adjacent capillaries, marked with red circles in Fig. 1(b), which introduce high losses. These nodes behave as independent waveguides supporting their own lossy modes. Therefore, the curvature of the cladding (core wall) is directed in the opposite direction (in comparison with a typical cylindrical core such as in the HC-PCF structure) in order to physically separate the fiber guided mode from the cladding nodes, significantly reducing coupling between them.
#191496 - $15.00 USD Received 12 Jun 2013; revised 7 Sep 2013; accepted 9 Sep 2013; published 19 Sep 2013 (C) 2013 OSA 23 September 2013 | Vol. 21, No. 19 | DOI:10.1364/OE.21.022742 | OPTICS EXPRESS 22743 The light guidance mechanism in the NCF is based on the Anti-Resonant Reflecting Optical Waveguiding (ARROW) phenomenon, which means that its structure can behave as a Fabry-Perot resonant cavity . Therefore all wavelengths of light which are not in resonance with the core wall are reflected back into the core and propagate with low loss as a result of destructive interference in the Fabry-Perot resonator. The resonant frequencies, meanwhile, cannot be confined in the core and leak away to the cladding region where they are highly attenuated. The wavelengths that are guided are hence strongly dependent on the thickness of the core wall (capillaries); for guidance at 1030 nm and 1064 nm a suitable wall thickness is in the range of 910 ± 50 nm.
Fig. 1. (a) SEM image of the NCF used in the experiments. (b) SEM picture of the capillary forming the negative curvature of the fiber core wall.
2.2 Fiber attenuation To measure the attenuation of the NCF at 1030 nm and 1064 nm wavelengths a cutback technique was used. With this fiber a high bending sensitivity was observed (further investigation of this phenomenon is described in section 2.3), therefore, to measure attenuation independently of bend loss a 1 m long, straight fiber was used. Any additional micro-bends, which could potentially affect the results, were minimized by maintaining the fiber within the same horizontal position during the entire experiment.
A TRUMPF TruMicro ps laser (M2~1.3) and a JDS Uniphase microchip ns laser (M2~1.2) were used as coherent light sources for 1030 nm and 1064 nm wavelengths respectively. In both cases the fiber was cut from 1 m to 10 cm while maintaining the same coupling conditions. The fiber attenuation was measured to be at the level of 0.23 dB/m and 0.16 dB/m for 1030 nm and 1064 nm respectively.
To obtain a loss spectrum over a wide spectral range from 1000 nm to 1400 nm (in this case with a coiled fiber) and determine the antiresonant bandwidth position, an additional cutback measurement was carried out with a broadband light source (a tungsten halogen bulb). The FC-connectorized fiber was connected to an Ando Optical Spectrum Analyzer AQ-6315B. In this case the fiber was cutback from 87 m to 3 m. The low-loss region covers over 300 nm (1000 nm −1330 nm) of the IR spectral bandwidth. The attenuation spectrum of the NCF is presented in Fig. 2.
2.3 Bending losses The fabricated NCF is sensitive to bending or any applied physical force e.g. point load, which introduce additional, significant losses. To fully understand the bending loss mechanism of the fiber, a series of different tests were performed. The measurements were conducted at 1064 nm with a JDS Uniphase microchip ns laser.
The bending losses were measured for both a coiled fiber (4.1 m long) and with a 180° bend (3.1 m long) fiber. In both cases, the launching side of the fiber and its bent part were kept in the same horizontal plane to avoid micro-bends. Moreover, to maintain the same coupling conditions for each measurement the input end-face of the fiber was not moved during the entire experiment.