29 May 2018 |
Research article |
Sensors, Networks and Connectivity
Generating and Characterizing Cylindrical Vector Beams
The following text is from one of the finalists 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.
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Fuelled by the ever-increasing demand for high-bandwidth multimedia applications, the ultimate data transmission capacity of a standard single-mode fiber (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 fibers (MMF) and few-mode fibers (FMF) towards increasing the transmission capacity within a single strand of fiber . 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 fiber-optic sensing .
The first 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)  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 fiber-optic sensing (of temperature and strain) in civil engineering structures and for niche biomedical sensors such as spinal cord trauma monitoring.
The first task is to design an efficient mode converter to selectively excite the desired vector mode(s). This will be achieved via a tunable mechanical long-period fiber grating (LPFG) . 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 fiber, an advanced metrological technique with promising applications in fiber-optic communications  and distributed fiber sensing . iv) The final phase will be to harness the vector modes in the development of novel SDM characterization techniques and in multi-parameter distributed fiber-optic sensors.
So far, we have demonstrated the selective excitation of cylindrical vector modes in FMF using a long period fiber grating  and studied their non-linear Brillouin properties towards the fully distributed characterization of telecom links with potential applications in distributed fiber 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 fiber for the first time using a simple heterodyne detection technique. A tunable LPFG is used to selectively excite the vector modes supported by the few-mode fiber. Further, we demonstrate the non-destructive measurement of the absolute effective refractive indices (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 fibers with potential applications in advanced optical communications and multi-parameter fiber-optic sensing.
The proposed research will contribute to revealing the underlying physics that guide the rich light-matter interactions inside few-mode fibers and develop new scientific 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 field of distributed fiber-optic sensing that promises new practical advances for the live remote monitoring of civil engineering structures and in biomedical research.
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 fiber. Scientific 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).
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 fiber.
Program : Electrical Engineering
Research laboratories : PHI_lab - PHotonic Innovations lab
Bora Ung is a professor in the Department of Electrical Engineering at ÉTS and a member of the Strategic Center for Optics, Photonics and Laser (COPL).
Program : Electrical Engineering
Research laboratories : LACIME – Communications and Microelectronic Integration Laboratory PHI_lab - PHotonic Innovations lab CÉRIÉC – Centre for Intersectoral Study and Research into the Circular Economy