Transmission of Multiple Signals through an Optical Fiber Using Wavefront Shaping

Summary

We demonstrate the transmission of multiple independent signals through a multimode fiber using wavefront shaping employing a single spatial light modulator. By modulating the wavefront for each signal individually, spatially separated foci are transmitted. Potential applications are multiplexed data transfer in communications engineering and endoscopic light delivery in biophotonics.

Abstract

The transmission of multiple independent optical signals through a multimode fiber is accomplished using wavefront shaping in order to compensate for the light distortion during the propagation within the fiber. Our methodology is based on digital optical phase conjugation employing only a single spatial light modulator, where the optical wavefront is individually modulated at different regions of the modulator, one region per light signal. Digital optical phase conjugation approaches are considered to be faster than other wavefront shaping approaches, where (for example) a complete determination of the wave propagation behavior of the fiber is performed. In contrast, the presented approach is time-efficient since it only requires one calibration per light signal. The proposed method is potentially appropriate for spatial division multiplexing in communications engineering. Further application fields are endoscopic light delivery in biophotonics, especially in optogenetics, where single cells in biological tissue have to be selectively illuminated with high spatial and temporal resolution.

Introduction

The transmission of multiple light signals through a multimode fiber (MMF) is evident in communications engineering¹ and biophotonics². In communications engineering, space-division multiplexing (SDM) is believed to be a viable solution in order to enhance the transmission capacity of optical fibers for future data transfer applications benefiting from a higher utilization of the limited space, compared to multiple single-mode fibers³. In biophotonics, biological samples are manipulated by light transmitting through an MMF endoscope⁴. For example, the independent optical co....

Protocol

1. Assembling the Experimental Setup

1. Preparing the proximal side
   1. Place and fix the laser providing a collimated light beam – or use a fiber-coupled laser with collimation optics at the exit facet of the fiber.
   2. Put the polarizing beam splitter (PBS) to split the laser beam into reference and object beam. Turn the orientation of the half wave plates (HWP) by rotating the HWP in its rotation mount until the power of reference beam and object beam (at the distal side) is roughly the same. Check this by putting a screen into both reference and object beam. Choose the orientation of the PBS so that the polarization o....

Results

Typical output signals at the distal side of the 2 m long fiber are depicted in Figure 2. Note that the desired focal spot (peak) is accompanied by an undesired speckle pattern (background), which is due to imperfection of the DOPC as a matter of principle. The corresponding peak-to-background ratio (PBR) amounts to 53 (solely signal 1 is 'on'), 36 (solely signal 2 is 'on') and 20 (both signals 1 and 2 are 'on') here, respectively. The PBR can be increased when a fiber th.......

Discussion

The assembling of the experimental setup (step 1 in the protocol) requires a thorough alignment of the optical components with respect to each other. The most important aspect is the rectangular incidence of the reference beams onto the SLM in order to ensure a high PBR.

In order to enhance the setup to more than two transmitted signals, additional beam splitters could be used. As an alternative, a fiber-based implementation would be more compact and robust allowing the system to be portable f.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
spatial light modulator	Holoeye	PLUTO-VIS-016
CMOS camera	Mikrotron	MC4082
diode-pumped solid state laser	Laser Quantum	torus 532
CCD camera	IDS	U3-3482LE-M CMOS	camera; suitable as well
lens 1	Qioptiq	G063204000
lens 2	Qioptiq	G063203000
lens 3	Thorlabs	AC508-180-A-ML
multimode fiber	Thorlabs	M14L02
beam splitters	Thorlabs	BS013	9x
polarizing beam splitters	Thorlabs	PBS251
mirrors	Thorlabs	PF10-03-P01	5x
microscope objectives	Thorlabs	RMS20X	2x
half wave plates	Thorlabs	WPH10M-532	2x
linear polarizer	Thorlabs	LPVISB050-MP2
optical modulators	Thorlabs	MC2000B-EC	2x
linear and rotation stage for CMOS camera	Thorlabs	XYR1/M
fiber connector	Thorlabs	S120-SMA	2x
reducing ring for microscope objectives	Qioptiq	G061621000	2x
xy adjustment for objective adapters	Qioptiq	G061025000	2x
z translation mount for fiber adapter	Thorlabs	SM1Z	2x
rods for fiber alignment to objectives	Qioptiq	G061210000	8x
mounts for lenses 1 and 2 plus two phantom mounts	Qioptiq	G061047000	4x
rail carriers for objective and lens mounts	Qioptiq	G061372000	6x
rail for rail carriers	Qioptiq	G061359000	2x
adapter for CCD camera to 1 post			in-house
adapter for laser to 4 posts			in-house
mount for lens 3	Thorlabs	LMR2/M
mounts for half wave plates	Thorlabs	RSP1D/M	2
mounts for mirrors	Thorlabs	KM100	5x
mount for linear polarizer	Thorlabs	RSP05/M
mounts for beam splitters and SLM	Thorlabs	KM100PM/M	11x
clamping arms for beam splitters and SLM	Thorlabs	PM4/M	11x
posts for mounts, rail carriers and adapters	Thorlabs	TR75/M	29x
holders for posts	Thorlabs	PH50/M	29x
pedestals for holders	Thorlabs	BE1/M	29x
clamping forks for pedestals	Thorlabs	CF125	29x