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 control of individual neurons using MMF endoscopes is of interest for optogenetics in order to study neuronal networks in the brain⁵. However, the light projected onto the MMF input facet is subject to distortion due to mode mixing and dispersion during propagation to the output facet of the MMF. As a result, the light propagation is altered, which makes signal transmission challenging.

Wavefront shaping methods^6,7 are applied in scattering media using spatial light modulators (SLM) and enable the compensation for the distortion due to scattering during light propagation⁸. There are iterative approaches that optimize the output using an optical feedback⁹. These approaches are rather time consuming because of the necessity for numerous iterations and the high degree of freedom, corresponding to a large number of modulator elements. Another approach is to completely determine the distortion within the MMF described by its transmission matrix¹⁰. If the number of modes to be transmitted is large, this will be time consuming as well. In contrast, digital optical phase conjugation (DOPC) is considered to be fast and advantageous here, since only few focal spots have to be generated at the output facet of the MMF. Phase conjugation approaches have also been demonstrated for focusing or imaging through biological tissue^12,13,14.

So far, DOPC was employed for a single time signal only^15,16, and was applied for the transmission of light through an MMF¹⁷. A DOPC approach for multiple independent signals has not been accomplished. We have developed an enhanced DOPC method providing the independent transmission of multiple light signals using individual wavefront shaping for each signal employing a single phase-only SLM¹⁸. To this aim, the SLM is segmented into regions, one for each signal to be transmitted. The proposed experimental setup is depicted in Figure 1, where a calibration is performed in a) before the actual transmission happens in b).

Figure 1: Experimental setup. BS = beam splitter, CCD = charge-coupled device, OM = optical modulator, CMOS = complementary metal-oxide semiconductor, HWP = half wave plate, L = lens, LP = linear polarizer, MMF = multimode fiber, OBJ = microscope objective, PBS = polarizing beam splitter, SLM = spatial light modulator (phase only) — only relevant beams for (a) the calibration and (b) the transmission are depicted Please click here to view a larger version of this figure.

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 of the reference beam fits the polarization-sensitive spatial light modulator (SLM).
   3. Put a beam splitter (BS) into the reference beam to split the reference beam into two beams. Place the optical modulators (OM) such that these two beams coming from BS1 can pass OM1 and OM2, respectively.
   4. Combine the two beams passing OM1 and OM2 at BS2 employing two mirrors. Adjust the beam splitters and mirrors so that both beams are spatially separated.
   5. Carefully align BS5 to ensure that the direction of incidence of both beams is perpendicular to the pixel plane of the SLM, ignoring BS3 and BS4 at first. At first, nothing is displayed on the SLM, i.e., it acts like a mirror until the end of the calibration (throughout step 2).
   6. Adjust the position and the distance between the two lenses (L) constituting a Keplerian telescope in order to get a sharp image of the SLM plane on the complementary metal-oxide semiconductor (CMOS) camera. Watch the correct orientation of L1 and L2 (flat sides are facing each other) to minimize aberrations.
2. Preparing the distal side
   1. Use BS7 to split the object beam into two beams and combine them at BS8 employing two mirrors. Again, adjust the beam splitters and mirrors so that both beams are spatially separated.
   2. Deflect both beams using BS9 to aim them to the microscope objective (OBJ). Focus OBJ2 on the distal end of the multimode fiber (MMF). Check the focus by observing the back reflection from the MMF employing L3 and a charge-coupled device (CCD) camera.
3. Connecting proximal and distal side
   1. Collimate the light from the object beam exiting the MMF employing OBJ1.
   2. Split the object beam using BS6, ignore the linear polarizer (LP) at first. Combine both object beams with both reference beams at BS3 and BS4 employing a mirror. Adjust the beam splitters and mirrors so that each pair of reference and object beam overlap at the SLM, intersecting with a small angle (less than 1°).
   3. Ensure that the power of the reference and the object beam are approximately equal by turning the orientation of the HWP, according to step 1.1.2.
   4. Check the interference pattern (off-axis hologram) at the CMOS camera and adjust the intersection angle accordingly. Increase the angle, until the interference fringe spacing roughly equals the size of two pixels on the CMOS camera.
   5. Adjust the orientation of the LP to match the polarization of object and reference beam in order to get a maximum contrast of the interference pattern in the CMOS camera image, so that the camera image shows distinct fringes.

2. Calibrating the System

1. Calibrating the pixel relation between SLM and CMOS
   1. Illuminate the whole SLM using only one of the reference beams and block the other reference and objects beams.
   2. Capture an image of the SLM with the CMOS camera.
   3. Get the coordinates of the upper left corner of the SLM in the CMOS camera image, e.g. using graphics software and the mouse cursor at the PC. Use these pixel coordinates as the point of origin regarding the SLM.
   4. Remove all beam blocks.
2. Calibrating the signal paths
   1. Block both reference beam 2 and object beam 2.
   2. Capture an image of the hologram with the CMOS camera. Evaluate the phase in the recorded hologram using angular spectrum method¹⁹. Calculate the inverted phase in the corresponding region of beam 1.
   3. Remove the former beam blocks and now block both reference beam 1 and object beam 1.
   4. Capture an image of the hologram with the CMOS camera. Measure the phase in the recorded hologram using angular spectrum method again. Calculate the inverted phase at the corresponding region of beam 2.
   5. Remove all beam blocks.

3. Transmitting the Signals

1. Block the object beam.
2. Stitch the calculated inverted phase images at the corresponding regions of beam 1 and 2 together and display the entire image on the SLM, typically using the computer graphics port.
3. Start the modulation of the input signals 1 and 2 by activating OM1 and OM2.
4. Observe the output signals 1 and 2 on the CCD camera.

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