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Gratings in two-mode fiber for Efficient Optical Angular. Momentum Generation. Yunhe Zhao1,2, Yunqi Liu1,*, Chengbo Mou1,*, Neil Gordon2, Kaiming Zhou2, ...
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Femtosecond Laser Inscribed Axial Long-period Fiber Gratings in two-mode fiber for Efficient Optical Angular Momentum Generation Yunhe Zhao1,2, Yunqi Liu1,*, Chengbo Mou1,*, Neil Gordon2, Kaiming Zhou2, Lin Zhang2, Tingyun Wang1 1

Key Lab of Specialty Fiber Optics and Optical Access Networks, School of Communication and Information Engineering, Shanghai University, 149 Yanchang Road, Shanghai 200072, China. 2 Aston Institute of Photonic Technologies, Aston University, Birmingham B4 7ET, UK. *Corresponding Author: [email protected], [email protected]

Abstract: We demonstrate a novel all-fiber mode converter based on an axial long-period fiber grating which was inscribed in two-mode fiber using a femtosecond laser. The OAM±1,1 modes can be effectively generated using this mode converter. OCIS codes: (050.2770) Gratings; (320.2250) Femtosecond phenomena; (050.4865) Optical vortices

1. Introduction Mode-division multiplexing (MDM), which utilizes spatial linear polarization (LP) modes or orbital angular momentum (OAM) in few-mode fiber (FMF) carrying independent data, is a potential approach to increase the transmission capacity in optical communication systems [1, 2]. Mode converters play key roles in these MDM systems. Recently, several types of mode converters have been proposed and demonstrated, and these can be divided into three categories: bulk-optic mode converters [1, 3], waveguide mode converters [4] and all-fiber mode converters. Compared with the first two kinds of mode converters, the all-fiber one is especially suited for transmission converters due to its inherent compatibility with FMF. All-fiber mode converters such as mode selective couplers [5], FMF Bragg gratings [6], and long-period fiber gratings (LPFGs) [7-9] have been studied extensively to achieve efficient conversion between the LP01 mode and higher-order modes. Among these, mechanical [8] and CO2-laser inscribed [9] LPFGs as all-fiber transmission mode converters have been reported to achieve OAM states with topological charges of ±1. However, the mechanically produced LPFG is less stable, although it can be quite flexible. Also, the CO2 inscribed LPFG with large beam spot size has difficulties in higherorder mode conversion. Femtosecond laser inscription is an efficient technology that has been applied to fabricate all-fiber devices such as resonators, micro-channels and grating devices [10-12]. Due to the characteristics of femtosecond lasers, precisely controlled fiber core inscription can be achieved. In this paper, we demonstrate an all-fiber mode converter based on axial LPFGs (ALPFGs) inscribed in twomode fiber (TMF, supplied by OFS) with a femtosecond laser. The femtosecond laser inscription along the fiber axis can induce asymmetric effective index modification of the fiber, which can easily realize the conversion between LP01 mode and LP11 mode. The maximum polarization dependent loss (PDL) of the fabricated ALPFG was measured to be 3.9 dB at the resonance wavelength. A conversion efficiency of ~80% has been achieved. Efficient OAM±1,1 modes can be generated by the as-fabricated ALPFG mode converter at the selected resonance wavelength. This proposed all-fiber mode converter may facilitate efficient application in the MDM systems. 2. Experiments

The TMF used in our experiment has a core/cladding diameter of 19/125 μm and a core/cladding index of 1.449/1.444, which supports the propagation of both LP01 and LP11 modes. The TMF was mounted on a standard microscopic glass slide. The ALPFG with a period of 1.18 mm and a total of 10 periods was inscribed in TMF by a tightly focused femtosecond laser (1064 nm wavelength) with a pulse rate of 100 kHz, and output power of 96.2 mW. A scanning speed of 0.2 mm/s was adopted to perform the inscription. Figure 1 shows a typical microscopic image of an ALPFG inscribed in a TMF with an inscription length of 590 µm separated by 590 µm. (a)

(b) 590 μm

590 μm

Fig. 1 Microscopic images of femtosecond laser inscribed ALPFG with the inscription length of 590 µm (a) separated by 590 µm (b).

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To characterize the ALPFG introduced by femtosecond laser inscription, a commercial tunable laser (81600B, Agilent) and an optical component analyzer (N7788BD, Agilent) were used. The ALPFG was spliced to standard single-mode fiber (SMF) at both ends for the ease of measurement. The measured transmission spectrum of ALPFG is shown in Fig. 2(a). The spectral oscillation is due to the intermodal interference from the SMF-TMF-SMF structure. The ALPFG has a maximum contrast of ~7 dB at resonance wavelength of 1513.6 nm. The transmission spectra of the ALPFG for linearly polarized radiation are illustrated in Fig. 2(b) together with the corresponding spectral PDL. The maximum PDL was measured to be 3.9 dB at 1515.2 nm. -10

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Fig. 2 (a) Measured transmission spectrum of the ALPFG, (b) measured polarization resolved transmission spectra of the ALPFG and PDL.

(a) Mode Converter Tunable Laser

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Fig. 3 (a) Schematic diagram of the experimental setup used to monitor intensity profiles of the generated modes from the ALPFG mode converter, (b) monitored intensity profiles at different wavelengths.

Figure 3 (a) shows a schematic diagram of the experimental setup used to monitor the transmission intensity profiles of the converted modes from the ALPFG mode converter. The standard LP01 mode from a tunable laser was launched into the TMF which contains the ALPFG. The converted LP11 mode is then propagated through a further ~10 cm of TMF. A focusing lens (focal length: 13.8 mm) was used to adjust the beam size. The intensity profile of the LP11 mode was then recorded by a CCD camera (Model C10633-23, Hamamtsu Photonics). The monitored intensity profiles at different wavelengths in the rejection band are illustrated in Fig. 3(b). It is obvious that all the monitored intensity profiles are LP11-like intensity profiles and the polarization orientations are the same. However, the conversion efficiency varies with wavelength and is expected to be highest near the resonance wavelength. The intensity profile at the wavelength of 1507 nm appears closest to a pure LP11 mode. This selected wavelength is slightly different from the resonance wavelength of the ALPFG transmission dip, which is probably due to the fiber

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tension induced spectral shift. (a)

(b)

OAM+1,1

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OAM−1,1

Fig. 4 Measured intensity profile of OAM+1,1 (a) and OAM−1,1 (c) at the wavelength of 1507 nm after propagating ~3 m TMF. Measured interference pattern of OAM+1,1 (b) and OAM−1,1 (d) at the wavelength of 1507 nm with a reference Gaussian beam.

We next measured the intensity profiles of the generated vector modes by using the interference method [9]. Figure 4 (a) and (c) show the intensity profiles of OAM+1,1 and OAM−1,1 at the wavelength of 1507 nm after propagating through ~3 m TMF. The interference patterns of the two OAM modes with the reference Gaussian beam are depicted in Fig. 4 (b) and (d), respectively. The detected OAM±1,1 modes originate from the π/2-phaseshifted linear combinations of the vector HE21 modes [9]. The results confirm that the ALPFG mode converter has high mode purity. According to the contrast of the resonance dips, the conversion efficiency of the mode converter is 80%. 3. Conclusion In conclusion, we have proposed a novel all-fiber mode converter based on the ALPFG inscribed in TMF with a femtosecond laser. Mode conversion between LP01 mode and LP11 mode with efficiency of ~80% has been achieved. Efficient OAM±1,1 modes can be generated by the ALPFG mode converter around the resonance wavelength. The proposed all-fiber mode converter is a candidate component for exploitation in future MDM systems. 4. Acknowledgements

The research was supported by the National Natural Science Foundation of China (61377083, 61077065). Yunhe Zhao acknowledges the China Scholarship Council for financial support. Chengbo Mou would like to acknowledge support from the Young Eastern Scholar program (QD2015027) at Shanghai Institutions of Higher Learning and the “Young 1000 Talent Plan” program of China. 5. References [1] R. Ryf, S. Randel, A. H. Gnauck, C. Bolle, A. Sierra, S. Mumtaz, M. Esmaeelpour, E. C. Burrows, R. J. Essiambre, P. J. Winzer, D. W. Peckham, A. H. McCurdy, and R. Lingle, “Mode-division multiplexing over 96 km of few-mode fiber using coherent 6 x 6 MIMO processing,” J. Lightwave Technol. 30, 521-531 (2012). [2] N. Bozinovic, Y. Yue, Y. Ren, M. Tur, P. Kristensen, H. Huang, A. E. Willner, and S. Ramachandran, “Terabit-scale orbital angular momentum mode division multiplexing in fibers,” Science 340, 1545-1548 (2013). [3] M. Salsi, C. Koebele, D. Sperti, P. Tran, H. Mardoyan, P. Brindel, S. Bigo, A. Boutin, F. Verluise, P. Sillard, M. Bigot-Astruc, L. Provost, and G. Charlet, “Mode-division multiplexing of 2 x 100 Gb/s channels using an LCOS-based spatial modulator,” J. Lightwave Technol. 30, 618623 (2012). [4] K. Saitoh, T. Uematsu, N. Hanzawa, Y. Ishizaka, K. Masumoto, T. Sakamoto, T. Matsui, K. Tsujikawa, and F. Yamamoto, “PLC-based LP11 mode rotator for mode-division multiplexing transmission,” Opt. Express 22, 19117-19130 (2014). [5] K. J. Park, K. Y. Song, Y. K. Kim, J. H. Lee, and B. Y. Kim, “Broadband mode division multiplexer using all-fiber mode selective couplers,” Opt. Express 24, 3543-3549 (2016). [6] Y. Gao, J. Sun, G. Chen, and C. Sima, “Demonstration of simultaneous mode conversion and demultiplexing for mode and wavelength division multiplexing systems based on tilted few-mode fiber Bragg gratings,” Opt. Express, 23, 9959-9967 (2015). [7] I. Giles, A. Obeysekara, R. Chen, D. Giles, F. Poletti, and D. Richardson, “Fiber LPG mode converters and mode selection technique for multimode SDM,” IEEE Photon. Technol. Lett. 24, 1922-1925 (2012). [8] S. Li, Q. Mo, X. Hu, C. Du, and J. Wang, “Controllable all-fiber orbital angular momentum mode converter,” Opt. Lett. 40, 4376 (2015). [9] Y. Zhao, Y. Liu, L. Zhang, C. Zhang, J. Wen, and T. Wang, “Mode converter based on the long-period fiber gratings written in the two-mode fiber,” Opt. Express 24, 6186-6195 (2016). [10] J. R. Grenier, L. A. Fernandes, and P. R. Herman, “Femtosecond laser inscription of asymmetric directional couplers for in-fiber optical taps and fiber cladding photonics,” Opt. Express 23, 16760-16771 (2015). [11] G. C. B. Lee, C. Mou, K. Zhou, and K. Sugden, “Optimization and characterization of femtosecond laser inscribed in-fiber microchannels for liquid sensing,” J. Lightwave Technol. 33, 2561-2565 (2015). [12] J. Thomas, C. Voigtlaender, R. G. Becker, D. Richter, A. Tuennermann, and S. Nolte, “Femtosecond pulse written fiber gratings: a new avenue to integrated fiber technology,” Laser Photon. Rev. 6, 709-723 (2012).