High-power optical continuous-wave waveguiding in a silica micro/nanofibre

We demonstrate low-loss continuous-wave optical waveguiding in a silica micro/nanofibre with power up to 13 W, making it possible for high-speed optomechanical driving of microparticles, and high-efficiency second/third harmonic generation.
High-power optical continuous-wave waveguiding in a silica micro/nanofibre
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Since its first experimental demonstration in 2003 [1], low-loss optical micro/nanofibre (MNF) with a diameter below or close to the wavelength of the light, has been emerging as a versatile fibre-optic platform in nanophotonics with a wide range of applications from optical sensors, atom optics, nonlinear optics to optomechanics [2]. Generally, increasing the waveguiding mode power is the most effective approach to enhance light-matter interaction, and explore new opportunities for both scientific research and technological applications. However, so far, the highest continuous-wave (CW) power waveguided in a silica MNF is ~0.4 W [3], with typical waveguiding power below 0.1 W (in CW or averaged power). Also, the optical damage threshold (or equivalently, how much power can a MNF transmit?) remains unknown, which drives us to carry out this investigation.

High-power CW optical waveguiding characteristics

In this work, we demonstrate high-power CW optical waveguiding in an optical MNF around 1550-nm wavelength with power up to 13 W. Experimentally, we fabricated a high-quality MNF by taper drawing a standard single-mode silica fibre and sealed it inside an airtight box filled with high-purity nitrogen gas to keep its pristine surface isolated from possible contamination, as schematically illustrated in Fig. 1a. The CW input light comes from a 1552-nm-wavelength CW fibre laser amplified by a low-noise erbium-doped fibre amplifier (EDFA), which can offer a CW output up to 13 W. Figure 1b shows that the measured output power changes quite linearly with the input power and the MNF maintains a transmittance >95% with waveguided power up to 13 W. Also, we show that the surface scattering intensity of a high-power waveguiding MNF increases with the waveguided power, without predominant single scattering spots (Fig. 1c). What’s exciting is that a MNF can safely handle a CW power >10 W even with a diameter of 410 nm (~1/4 of the vacuum wavelength!) and without observable degradation with accumulated operation time of more than 10 hours in a 2-month test!

Fig. 1 High-power CW optical waveguiding in a MNF around 1550-nm wavelength. (a) Schematic diagram of the experimental setup. SMF, single-mode fibre. EDFA, erbium-doped fibre amplifier. OSA, optical spectrum analyzer. (b) Measured optical transmittance of a 1.1-μm-diameter MNF with a CW waveguided power from 0 to 13 W. (c) Optical microscope images showing surface scattering in the same 1.1-μm-diameter MNF with CW waveguided power of 1 W, 5 W and 12 W, respectively. The exposure time is 3 ms in capturing all three images.

Optical damage threshold

As the MNF can transmit a CW power higher than 10 W without detectable degradation, the optical damage threshold should be much higher than the highest available power (~13 W) used in this work. To estimate the optical damage threshold, we tried many approaches and finally figured out an appropriate scheme. In short, we assembled a MNF into a knot resonator (Fig. 2a) and measured the power-dependent resonant peak shift of the knot resonator to retrieve the temperature rise of the MNF (Fig. 2b). Based on a thermal dissipation model, we successfully extrapolated an optical damage threshold of ~70 W at the annealing temperature (~1100 ℃) of the silica, as shown in Fig. 2c.

Fig. 2 Optical damage threshold of a MNF around 1550-nm wavelength. (a) Schematic diagram of a silica MNF knot resonator for measuring temperature rise in the MNF. (b) Transmission spectra of the knot with waveguided power increasing from 0.01 W to 7.0 W. Inset, power-dependent spectral shift of the resonance peak at resonant wavelength λres marked by an arrow. (c) Measured (blue triangle) and calculated (black line) power-dependent temperature of the MNF.

High-power optical applications

It has been reported that the evanescent field waveguided along the MNF offers a flexible and precise platform for optomechanically manipulating (e.g., trapping or propelling) microparticles [4]. We assume that with much higher optical power and thus much larger optical force, it is possible to drive microparticles in air or vacuum with high speed. Figure 3a shows optomechanically driving a silicone oil droplet using a high-power CW waveguiding MNF. The ellipsoid-shaped oil droplet (11 μm in the major axis and 10 μm in the minor axis) is wrapped around a 1-μm-diameter MNF in air. A 1552-nm-wavelength CW light, used as the propelling light, is coupled into and waveguided along the MNF from left to right to drive the droplet. Interestingly, the measured power-dependent droplet velocity vo (e.g., vo = 2.1 mm/s at a waveguided power of 2.2 W, see Fig. 3b) confirms that the droplet can be driven more than 10 times faster than those reported in previous MNF-based optomechanics systems [5]. Considering optomechanical manipulation in a non-liquid environment is critical to optical precision metrology and quantum optics, we foresee that the high-power MNF manipulation of microscale objects in air may pave a step toward a fibre-based platform for high-speed optomechanical manipulation in non-liquid atmosphere or vacuum.

Fig. 3 High-power optical applications using silica MNFs. (a) Time-sequential optical microscope images of driving an oil droplet (11 μm × 10 μm ellipsoid) along a 1-μm-diameter MNF. A 0.7-W-power light is waveguided along the MNF from left to right. (b) Measured (blue squares) and fitted (black line) power-dependent velocity of the oil droplet driven by the waveguiding light in the MNF. Inset, a close-up of the droplet velocity with waveguided power below 0.5 W. (c) Dependence of the output power (PHG) of THG/SHG on the waveguided power (Pin) of a MNF. ξ, conversion efficiency of harmonic generation.

On the other hand, MNF-based nonlinear optical effects have been attracting increasing interest in recent years [6]. Due to the low optical nonlinearity of silica, usually these effects are generated by short pulses, although CW nonlinear effects are also desired in MNFs. Since the nonlinear optical effect is generally proportional to pump power, we investigate the possibility of the third harmonic generation (THG) and second harmonic generation (SHG) in MNFs with high-power CW waveguiding. From the theoretical calculation, we obtained the appropriate MNF diameter to achieve phase matching and maximum mode overlapping. In the experiment, we fabricated the MNF with high-precision diameter control using direct mode cut-off feedback technology [7] and used a high-wavelength-accuracy tunable laser as the seed source to achieve perfect intermodal phase matching. We demonstrate, for the first time, intermodal-phase-matched SHG with efficiency higher than those pumped by short pulses and THG with efficiency that falling in the range of typical results reported previously using short pulses (Fig. 3c). The extraordinary CW conversion efficiency comes from combined factors of high waveguiding power, perfectly matched phase and relatively large interaction length. As mentioned previously, the power we used here is lower than the damage threshold (70 W), indicating that the nonlinear frequency conversion efficiency could be even higher when higher CW waveguiding power is available.

Perspectives

As CW waveguiding is desired in a variety of MNF-based applications noted above, we foresee that our results on high-power waveguiding MNFs may extend MNF optics into the high-power region, and open up new opportunities for MNF-based technology ranging from fibre laser, nonlinear frequency conversion, optomechanics to biophotonics and atom optics.

References

[1] Tong, L. M. et al. Subwavelength-diameter silica wires for low-loss optical wave guiding. Nature 426, 816-819 (2003).

[2] Wu, X. Q. & Tong, L. M. Optical microfibers and nanofibers. Nanophotonics 2, 407-428 (2013).

[3] Hoffman, J. E. et al. Ultrahigh transmission optical nanofibers. AIP Advances 4, 067124 (2014).

[4] Li, Y. C. et al. Optical fiber technologies for nanomanipulation and biodetection: a review. Journal of Lightwave Technology 39, 251-262 (2021).

[5] Maimaiti, A. et al. Higher order microfibre modes for dielectric particle trapping and propulsion. Scientific Reports 5, 9077 (2015).

[6] Xu, F., Wu, Z. X. & Lu, Y. Q. Nonlinear optics in optical-fiber nanowires and their applications. Progress in Quantum Electronics 55, 35-51 (2017).

[7] Kang, Y. et al. Ultrahigh-precision diameter control of nanofiber using direct mode cutoff feedback. IEEE Photonics Technology Letters 32, 219-222 (2020).

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