Selective multimode excitation using volume holographic mode multiplexer

Abstract

We propose a mode multiplexer based on volume holograms to realize a simple and efficient mode-divisionmultiplexed transmission system that supports a large number of modes. Selective multiexcitation of three spatial modes into a conventional multimode fiber is experimentally demonstrated. This device could potentially multiplex 10 or more modes. Future perspectives of the mode multiplexer for application in mode-division multiplexing are also discussed. [3][4][5][6][7]. These works were mainly aimed at low-loss and long-distance MDM transmission, which is one of the most important performance indicators in the application of MDM to long-haul systems, and 1200 km transmission has been achieved in [6]. Another key factor is the number of modes that can be supported by the system. In [8], five-mode (LP 01 and two degenerate LP 11 and LP 21 modes) transmission was demonstrated, and challenges to attain a larger number of modes were discussed. For mode multiplexers, device simplicity and efficiency are the main issues associated with increasing the number of modes. Most proposed mode multiplexers become complex and lossy as the number of modes increases because one or more additional optical elements, such as beam splitters (BSs), are required in the addition of one mode. In our previous work, to show the possibility of selective multimode excitation that uses only one element, we demonstrated multiple and variable two-mode excitations using a single spatial light modulator (SLM) [9]. The two modes were simultaneously generated by dividing the SLM display into two regions, and these modes were multiplexed at the end of the multimode fiber input. However, the insertion loss was large (20 dB) because of the low efficiency of the SLM and the nonvertical incidence into the fiber.In this Letter, we propose a simpler and more efficient mode multiplexer based on volume holograms. The operating principle and the advantages of the volume holographic mode multiplexer are explained, and the experimental results show successful demonstration of a selective three-mode excitation into a conventional graded-index multimode fiber (GI-MMF) with a core diameter of 62.5 μm. Figure 1 shows the concept and the operating principle of the volume holographic mode multiplexer, which is very similar to that of volume holographic data storage with angular multiplexing [10,11]. During the recording process, a volume hologram is recorded in a holographic medium by interference between a mode beam and a reference beam. The mode beam is transformed into a desired spatial mode, which has intensity and phase fields at the SLM. The mode is changed when the recording has been completed and is ready for the next recording. The reference beam is a plane wave, and its incident angle θ is shifted corresponding to the change in the spatial mode for angular multiplexed recording. In this process, the spatial modes are recorded in multiple, and the volume holographic mode multiplexer is structured. During the transmission process, reference beams are repla...

Holographically encoded volume phase masks

“…Instead, a quasiachromatic element can be generated by encoding stepped phase mask profiles into transmitting volume Bragg gratings (TBGs), 26,27 which produce holographic phase masks (HPMs). Though this technique has been demonstrated for HPMs utilized at reconstruction wavelengths identical to the recording wavelength, 28,29 we show here that HPMs can produce identical diffracted phase profiles over a wide range of wavelengths as long as the Bragg condition of the volume grating is satisfied. This is in contrast to more complex holograms that, though they can be read at any wavelength satisfying the Bragg condition, cannot generally reconstruct the same phase profile at wavelengths different than the recording one.…”

Learning to see through multimode fibers

“…The transmission of images through a MMF was first presented in 1967 by Spitz and Wertz [3] who demonstrated experimentally that the distortion introduced by modal dispersion can be undone by phase conjugation. In the intervening years numerous publications have described methods for transmitting image information through MMFs or scattering media [4][5][6][7][8][9][10][11][12][13][14][15]. These methods rely on coherent (holographic) recording of the speckle pattern detected at the distal end of the fiber and they use phase conjugation or the transmission matrix method to compensate for the effects of modal dispersion and either focus the light at the distal end or project focused images.…”

Space-division multiplexing: the next frontier in optical communication

“…Another potential method is the use of holograms [114][115][116]. The holographic method is theoretically possible [117,118] but hard to implement because volume holographic materials at the telecommunication wavelengths are not currently available and phase-only holograms suffer from losses and crosstalk.…”