Generating and Characterizing Cylindrical Vector beams

29 MAY 2018 Research article INFORMATION TECHNOLOGY Generating and Characterizing Cylindrical Vector Beams The following text is from one of the nalists in the 2017 SARA Abstract Contest. The writer was awarded First Place for the clarity and quality of the research project abstract. The other texts submitted to the SARA Contest are also available. Prabin Pradhan Bora Ung https://substance.etsmtl.ca/en/generation-caracterisation-faisceaux-cylindriques-vectoriels 1/5 The featured image was bought from Istock.com. Copyrights. SUMMARY Fuelled by the ever-increasing demand for high-bandwidth multimedia applications, the ultimate data transmission capacity of a standard single-mode ber (SMF)—the backbone of the Internet—is predicted to be reached within the next decade. Space division multiplexing (SDM) aims to harness the rich spatial mode diversities available in multimode bers (MMF) and few-mode bers (FMF) towards increasing the transmission capacity within a single strand of ber [1]. Moreover, it has been suggested that the same SDM techniques developed for generating and controlling modal transmission in MMF and FMF, could be utilized towards multi-parameter ber-optic sensing [2]. Research Objectives The rst objective of this project is to investigate specially designed FMFs as well as develop novel methods for the excitation and control of the elusive higher-order vector modes that can propagate in such specialty FMFs. Contrary to the classical scalar modes of FMF, the vector modes possess special polarization properties as well as the unique ability to carry quantum states of orbital angular momentum (OAM) [3] that provide new degrees of freedom for both optical communications and sensing. The second objective is to apply the developed techniques for the characterization of next-generation SDM communication links towards the multi-parameter distributed ber-optic sensing (of temperature and strain) in civil engineering structures and for niche biomedical sensors such as spinal cord trauma monitoring. Methodology The rst task is to design an ef cient mode converter to selectively excite the desired vector mode(s). This will be achieved via a tunable mechanical long-period ber grating (LPFG) [4]. An alternate method of generating the vector modes using a spatial light modulator (SLM) will be investigated and compared with the LPFG. ii) The second task will be to transmit and independently detect at the FMF output the co-propagating modes by means of a de-multiplexing device in the form of a SLM. iii) Next, the accurate characterization of these vector modes will be accomplished via the generation of a dynamic Brillouin grating (DBG) in the ber, an advanced metrological technique with promising applications in ber-optic communications [5] and distributed ber sensing [6]. iv) The nal phase will be to harness the vector modes in the development of novel SDM characterization techniques and in multi-parameter distributed ber-optic sensors. Results So far, we have demonstrated the selective excitation of cylindrical vector modes in FMF using a long period ber grating [4] and studied their non-linear Brillouin properties towards the fully distributed characterization of telecom links with potential applications in distributed ber sensing [4, 7]. In this paper, we reported the measurement of the Brillouin gain spectra (as shown in Figure 1) of vector modes in a few-mode ber for the rst time using a simple heterodyne detection technique. A tunable LPFG is used to selectively excite the vector modes supported by the fewmode ber. Further, we demonstrate the non-destructive measurement of the absolute effective refractive indices 2/5 (neff ) of vector modes with ~10−4 accuracy based on the acquired Brillouin frequency shifts of the modes. The proposed technique represents a new tool for probing and controlling vector modes as well as modes carrying orbital angular momentum in optical bers with potential applications in advanced optical communications and multiparameter ber-optic sensing. Fig. 1. Measured Brillouin gain spectra for fundamental and high-order vector modes, and corresponding Gaussian t curves [7]. Recently, we have been working on the generation of perfect cylindrical vector beams (PCVB) using the Fourier transformation of Bessel-Gauss vector beams [8]. Precise control over the ring diameter and ring width of vortex beams (i.e. beams carrying OAM) was recently achieved through the generation of perfect vortex beams, as demonstrated by a number of research groups [9, 10]. These recent works report the ability to maintain the dimensions of the beam intensity pattern irrespective of their topological charge. Furthermore, the reported new class of perfect vector vortex beams has been demonstrated with tailorable ring diameter irrespective of the polarization order. Though to the best of our knowledge, the independent control of the ring diameter and ring width for PCVB has not yet been demonstrated in its entirety. So far, efforts towards the generation of PCVBs that would enable one to fully tailor the intensity pro le of the CVBs of interest have yielded PCVBs with tunable ring diameter only that were of limited purity [11] (showing residual light intensity in the beams’ centre) via a method of limited exibility due to the use of static (Pancharatnam-Berry phase) optical element [11]. In this work, we demonstrate the generation of arbitrary PCVBs whose transverse intensity pro le (i.e. ring width and ring diameter) can be independently and easily controlled via an iris and a diffractive phase mask implemented on a programmable SLM as shown in Figure 2. 3/5 Fig 2. Experimental and simulated generation of PCVBs. The type of PCVB generated (TM01, TE01, even and odd HE21) depends on the speci c topological charges and phase differences ascribed to each interfering beam [8]. The demonstration of PCVBs is implemented via an interferometric method employing a spatial light modulator that allows the independent control of the ring diameter and ring width of the PCVB. The proposed scheme enables to generate different types of CVB with precise user-de ned transverse dimensions. The dynamic control of the ring width, ring diameter, and the speci c type of PCVBs desired (in this work: TM01, TE01, odd and even HE21 modes) is theoretically as well as experimentally demonstrated. The ability to generate perfect cylindrical vector beams has implications for the ef cient launch of exotic optical modes in specialty bers, in the eld of optical trapping as well as for super-resolution microscopy. IMPACT The proposed research will contribute to revealing the underlying physics that guide the rich light-matter interactions inside few-mode bers and develop new scienti c methods to generate, transmit, shape and characterize the vector modes that constitute the fundamental basis set of light propagation in these important waveguides. In doing so, the research outcomes will have a direct impact on the development of novel metrological techniques for next-generation communication networks, a strategic area of contemporary socioeconomic development, as well as in the eld of distributed ber-optic sensing that promises new practical advances for the live remote monitoring of civil engineering structures and in biomedical research. Additional information For more information on this research, please read the following articles: Pradhan, P., et al., The Brillouin gain of vector modes in a few-mode ber. Scienti c Reports, 2017. 7. Pradhan, P., Sharma, M., & Ung, B. (2018). Generation of perfect cylindrical vector beams with complete control over the ring width and ring diameter. IEEE Photonics Journal, 10(1). 4/5 Prabin Pradhan Author's pro le Prabin Pradhan is a Ph.D. Student in the Electrical Engineering Department at ÉTS. His current research involves generation and characterization of cylindrical vector beams in a few-mode ber. Program : Electrical Engineering Research laboratories : PHotonic Innovations lab (PHI_lab) Bora Ung Author's pro le Bora Ung is a professor in the Department of Electrical Engineering at the ÉTS and a member of the Strategic Center for Optics, Photonics and Laser (COPL). Program : Electrical Engineering Research laboratories : PHotonic Innovations lab (PHI_lab) Research laboratories : PHotonic Innovations lab (PHI_lab) Field(s) of expertise : Photonic devices Optical communications Doped optical bers References 1. Richardson, D., J. Fini, and L. Nelson, Space-division multiplexing in optical fibres. Nature Photonics, 2013. 7(5): p. 354-362. 2. Li, A., et al., Few-mode fiber based optical sensors. Optics express, 2015. 23(2): p. 1139-1150. 3. Ung, B., et al., Few-mode fiber with inverse-parabolic graded-index profile for transmission of OAM-carrying modes. Optics express, 2014. 22(15): p. 18044-18055. 4. Pradhan, P., et al. Excitation of vector modes in few-mode fiber using wire-based mechanical long period fiber grating. in Photonics North, 2015. 2015. IEEE. 5. Santagiustina, M., et al., All-optical signal processing using dynamic Brillouin gratings. Scientific reports, 2013. 3. 6. Mizuno, Y., et al., Brillouin scattering in multi-core optical fibers for sensing applications. Scientific reports, 2015. 5. 7. Pradhan, P., et al., The Brillouin gain of vector modes in a few-mode fiber. Scientific Reports, 2017. 7. 8. Pradhan, P., Sharma, M., & Ung, B. (2018). Generation of perfect cylindrical vector beams with complete control over the ring width and ring diameter. IEEE Photonics Journal, 10(1). 9. Li, P., et al., Generation of perfect vectorial vortex beams. Optics Letters, 2016. 41(10): p. 2205-2208. 5/5 10. Ostrovsky, A.S., C. Rickenstorff-Parrao, and V. Arrizón, Generation of the “perfect” optical vortex using a liquid-crystal spatial light modulator. Optics letters, 2013. 38(4): p. 534-536. 11. Liu, Y., et al., Generation of perfect vortex and vector beams based on Pancharatnam-Berry phase elements. Scientific Reports, 2017. 7. Images references All images are from the authors. Substance CC license applies to them.