1. Introduction

High power ultrashort pulsed lasers are widely used in the fields of biophotonics, micromachining, surface texturing and sensing [1–5]. Flexible and robust means for beam delivery over a meter-long optical path are desirable. Compared to free-space beam guiding, which is prone to misalignment, drift, vibration, air turbulence, dust, aerosols etc., fiber delivery offers superior robustness [6]. It also allows to separate the laser from the work station and thus to reduce the size and the required mechanical stability of the processing machine. Due to fiber flexibility, light can be easily delivered to different areas of the workpiece, which would require cumbersome procedure of moving optics in case of free-space beam guiding. Additionally, the low loss fiber delivery of the femtosecond oscillator pulses could simplify the amplified system architectures and enable more compact footprint.

For efficient applications, preservation of optical pulse properties is necessary. Single-mode fibers preserve a spatial beam profile suitable for diffraction-limited focusing. However, near-infrared femtosecond pulses are severely broadened in standard single-mode fibers due to chromatic dispersion and power-dependent nonlinear effects, which become significant at pulse energies as low as 0.1 nJ [7,8]. Large-core fibers show reduced nonlinear effects, but broadening still occurs at high intensities, and typically several modes are supported [8,9]. Alternative methods of shaping the input pulse or recompression of the output pulse have been developed [10–12], but these approaches increase the complexity of the whole system.

It is therefore desirable to propagate the pulse in a hollow air core to minimize dispersion and nonlinear effects. As all conventional optical fibers guide light by total internal reflection, which requires that the core have a higher refractive index than the cladding [13], their drawback is that no solid cladding material exists with a refractive index lower than that of air. Alternative optical waveguides have been developed in which light can be guided in an air core by so-called Bragg photonic bandgap guidance [14]. Commercialization of these fibers has opened wider range of operating wavelengths and potential applications. Photonic bandgap hollow-core photonic crystal fibers (HC PCF) were proven beneficial for high power delivery [15–17]. However, these fibers have a limited bandwidth (∼70 THz) and a small core size (∼10 μm). Additionally, its guided core mode has a large spatial overlap with the silica core wall, thus limiting the coupled power level to be within several microjoules [18]. The cladding structure consisting of fine silica webs arranged in a Kagome lattice surrounded by air [19] showed a wide transmission bandwidth, low spatial overlap with silica, and low group velocity dispersion [20]. Microjoule level ultrashort pulses have already been delivered using a kagome fiber [21,22]. Recently, a hypocycloid-core kagome HC PCF was demonstrated showing attenuation figure as low as 180 dB ∕ km at near IR over optical transmission bandwidth of 200 THz, and lower optical power overlap with the core silica surround [18,23]. Furthermore, recently it was demonstrated only 7.5 dB/km transmission losses at around 1600 nm [24].

As mentioned earlier, ability to move the fiber is advantageous for many applications. In this paper, we investigate ultrashort pulse delivery by a commercial HC PCF in dynamic situations. We first present coupling efficiency and changes in spectrum and pulse duration at the fiber output, and then depolarization induced by bending, coiling and torsion of the fiber.

2. Fiber coupling measurements

For all experiments, we used a commercial Yb-based oscillator Ybix (from Lumentum Switzerland AG) delivering 3.4 W average power at 84 MHz repetition rate, pulse duration of 160 fs at 1033 nm central wavelength. The delivery fiber is a 1 meter long commercial HC PCF from Thorlabs, model HC-1060, connectorized with standard Fiber Connector Angled Physical Contact (FC/APC), with 10 μm core, mode field diameter (MFD) of 7.5 μm and cladding diameter of 123 μm. Figure 1 shows the experimental setup scheme. The laser light is coupled into the fiber using an achromatic lens with 25 mm focus length (L1). To attenuate light on the beginning of the coupling process and not to burn the fiber facet, a half waveplate (λ\2) and a polarizer (P1) are installed in front of the focusing lens. Two high reflectivity (HR) mirrors (M1 and M2) are also used for the coupling optimization. The fiber output beam is collimated using 25 mm achromatic lens (L2), and then sent through additional polarizer (P2) for depolarization measurements described in the following sections, or directly to diagnostics instruments, i.e. the spectrometer and autocorrelator.

 

Fig. 1 Experimental setup scheme (M1 and M2: HR AOI45° alignment mirrors, λ\2: half-waveplate, P1: polarizer, L1: achromatic 25 mm focusing lens, L2: achromatic 25 mm collimating lens, P2: polarizer).

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We achieved 73% coupling efficiency (26% losses include facet reflections and 0.1 dB transmission losses) with 2 W output average power and 24 nJ pulse energy. The fiber output beam quality is excellent, as the fiber is single mode.

The left panel of Fig. 2 shows fiber input and output spectra measured with OceanOptics spectrometer, model USB2000 + . There are clearly no significant spectral distortions and no non-linear spectral effects are observed. Input and output spectra practically overlap.

 

Fig. 2 Left: Optical spectra of the laser (red) and fiber (blue) output beams. Right: Autocorrelation traces of the laser (red solid line) and fiber (blue solid line) output beams, and the corresponding sech² fit curves (dashed and dotted line, for the laser and fiber output, respectively).

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Pulse duration is measured with a commercial second-harmonic generation (SHG) intensity autocorrelator from Femtochrome Research Inc. Polarizators are used in the setup to measure the depolarization and are removed during the pulse duration measurements. The right panel of Fig. 2 shows the autocorrelation traces of the laser and fiber outputs. The temporal profile of the pulse after the fiber is not distorted but it is broadened. Assuming the sech² shape, we calculated fiber output pulse duration to be 460 fs, almost 3 times longer than the input pulse duration of 160 fs.

3. Fiber bending and coiling measurements

The experimental setup for the bending experiments is shown in Fig. 3. The fiber input and output connectors are kept in the same plane, while the rest of the bent fiber was rotated to three different planes: in plane with the input and output (Fig. 3, left), at 45° (Fig. 3, middle), and at 90° (Fig. 3, right) with respect to the input-output plane. The fiber was bent from 150 mm to 50 mm radius in steps of 25 mm.

 

Fig. 3 The setup for the fiber bending experiments with three bending planes: in plane (left), at 45° (middle), and at 90° (right) with respect to the input and output plane. The bending radius was varied from 150 mm to 50 mm.

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Figure 4 shows the depolarization dependence on the bending radius and the additional bending plane rotation. We defined the depolarization as the ratio of the minimum and maximum power transmitted through the polarizer after the fiber output. The input laser depolarization measured in the same way is less than 1%, which is the noise level of the measurement. No significant difference was observed when fiber is bent in clockwise (CW) or counterclockwise (CCW) direction, in or out of input-output plane. Depolarization varies 1%, for radius variation from 150 mm down to 50 mm in the same plane, and from 11% to 13-14% for different perpendicular bending planes. By inserting an additional half-wave plate prior to fiber input, the laser input polarization is fine-tuned for the minimum output depolarization. The minimum depolarization measured by using this additional waveplate was 6.4%.

 

Fig. 4 Depolarization as a function of bending radius, bending plane and direction (CW: clockwise, CCW: counter clockwise). Depolarization varies few percent when fiber is bent in clockwise and counterclockwise direction, and in addition, in or out of plane, for radius change from 150 mm down to 50 mm.

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Figure 5 shows the setup used for the coiling measurements. The fiber is coiled into a spiral with difference in height within the spiral of 20 mm per round. Difference in height from fiber input to output was varied from 120 mm to 190 mm. Depolarization induced by coiling is considerably low. Namely, when coiling in CW direction, depolarization remains 6.4% for both 120 mm and 190 mm height difference. When coiling in CCW direction, depolarization increases to 10.9% and 10.7% for 120 mm and 190 mm height difference, respectively.

 

Fig. 5 Experimental setup for coiling measurements.

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4. Fiber torsion measurements

For the torsion experiments, the fiber remained in one plane, straight, and torsion was applied in 22.5° steps in CW and CCW directions (see Fig. 6). Figure 7 shows depolarization dependence on the torsion angle and direction. Depolarization dependence seems somewhat periodic with 180-degree torsion angle period. There are torsion angle ranges in which depolarization remains low, between 5 and 8%, and ranges in which strong depolarization up to 52% is observed. For torsion in CW direction, depolarization is high in the range from 0° (which is arbitrary chosen) to 60°, remains then low up to about 140°, increases again in the range up to 240° with a maximum at about 180°, and stays low from 240 to 360°. The same trend is observed when rotating in CCW direction with an offset of ~50 deg regarding the maxima positions. We believe that this depolarization behavior with torsion is due to strongly changing stress in the fiber and periodically changing the waveguiding properties of the PCF. As the PCF is rotationally symmetric, the offset of depolarization for CCW torsion is most likely due to holey microstructure manufacturing imperfections. Rigorous waveguiding simulations are under way and extensive results including depolarization are to be published elsewhere.

 

Fig. 6 Experimental setup for torsion measurements.

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Fig. 7 Depolarization dependence on torsion angle and direction.

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5. Summary

We achieved fiber delivery of high power femtosecond pulses with a simple FC/APC connectorized hollow core fiber patchcord and to the best of our knowledge for the first time characterized its power, spectrum, autocorrelation and polarization with various movements.

Coupling efficiency of up to 73% is achieved with output of 2 W and 24 nJ. Minimum depolarization achieved at this average power and pulse energy is 6.4%. Depolarization increases up to 14% with various bending movements. Output is relatively insensitive on coiling and bending down to 5 cm radius. There is a large depolarization of about 50% at certain torsion angles, probably due to strongly changing waveguiding properties of the PCF with torsion. These properties are to be further simulated.

Output spectrum did not change significantly. The input pulses of 160 fs are broadened to 460 fs. The next step will be to compress pulses towards the input pulse duration. We also plan to test fiber delivery of pulse energies towards μJ level with this simple fiber patchcord.