Real-time Focusing of Light through Multimode Fibers Fanting Kong, Nicholas V. Proscia, Yin Cen, and Y.C. Chen Department of Physics and Astronomy, Hunter College of the City University. of New York, New York, NY 10065
[email protected]
Abstract: We demonstrate real-time focusing of light through a 25-m-core, 0.1- NA high-power multimode fiber by wavefront shaping, which results nearly 50% of the energy converged to a spot within 0.3 seconds. OCIS codes: (110.0113) Imaging through turbid media; ( 110.1080) Active or adaptive optics; (060. 2350) Fiber 0ptics imaging.
Multimode fibers are used for the transport of light from a laser source to the place where it is needed, particularly when the higher optical power requires a larger area of the fiber core. However, many transverse modes interfere resulting in a random interference pattern with reduced spatial coherence. The interference pattern is extremely sensitive to any external disturbance to the fiber. In this paper, we demonstrate real-time focusing of light through multimode fibers to a diffraction limited spot by wavefront shaping. Our work is motivated by recent studies which showed that coherent light can be focused through optically diffusive media when the scattering channels interfere constructively at a designated location[1-3]. Recently, this principle has been applied to optical fibers to transmit structured intensity pattern through multimode fibers[4-6] which are essentially a closed system of diffused reflectors with a fixed number of modes (channels). The control of the interference condition is made possible by the use of spatial light modulators or deformable mirrors with hundreds to thousands of elements. The focusing process is usually slow due to the large number of steps and the slowness of the liquid crystal light modulators. In this paper, we report real-time phase correction with a MEMS deformable mirror to achieve realt-time re-focuing of light through a multimode fiber. The schematic of the experimental setup is shown in Fig. 1. 800nm laser
800nm laser
Multimode fiber
Multimode fiber 25m core , NA =0.1
PD
BS 12x12 DM
BS
PD
CCD
Pinhole CCD 12x12 DM
(a)
(b)
Fig. 1. Schematic of experimental setup. DM: deformable mirror, PD: photodiode, BS: beam splitter. The deformable mirror is placed (a) before and (b) after the fiber transmission.
We have tested two configurations of wavefront shaping, before and after the fiber transmission. In the first configuration, as shown in Fig. 1(a), the laser beam is expanded to fill the aperture of the deformable mirror and the reflected laser beam is coupled into a 3-meter-long high- power multimode fiber, Thorlabs HPSC25, with a core diameter of 25 microns and a numerical aperture of 0.1. A pinhole whose diameter matches the sizes of a typical grain is placed in front of a photodiode in the far field. A camera is in line to monitor the beam profile. In the second configuration, as shown in Fig. 1(b), the deformable mirror is used to shape the random output field from the optical fiber. The magnified image of the end surface of the fiber core is projected to the deformable mirror. The reflected light from the deformable mirror is focused by a converging lens. A pinhole is placed in front of a photodiode which is placed at the focal plane of the converging lens. A single-frequency diode laser emitting at 820 nm is used as the light source. The deformable mirror consists of 140 (12x12 without corner elements) micro-electro-
mechanical mirrors (Boston Micromachines Corporation) with a full aperture of 3.3 mm. In both cases, measurements of the photodiode signal are performed for axial mirror displacements of a single pixel to create round-trip phase delays of reflected light from 0 to 2𝜋 in 20 steps. After each scan, the mirror displacement was set to the value that gives rise to the maximum photodiode signal, before advancing to the next pixel. The two configurations yield the same results in the enhancement of intensity at a spot at the output end. However, the process of wavefront shaping before the fiber transmission has the effect of making the input beam spatially incoherent and thus significantly reduced the coupling efficiency into the fiber to less than 10%. Fig. 2(a) shows the photo of a typical speckle pattern at the end of the fiber before the phase optimization. The pattern consists of random bright spots spreading over the entire surface. After the phase optimization process, the intensity of a speckle grain near the center of the core is enhanced by 50 times while the brightness of the surrounding grains is reduced. The photo of the enhanced speckle grain is shown in Fig. 2(b). The energy falling in the central spot is 48% of the total energy.
25 m Fig.2 Beam profile at the output before (left) and after (right) wavefront optimization
Fig. 3 Recovery of the intensity of the focused spot as a function of time after the fiber is mechanically disturbed.
The time to complete one step of pixel displacement is 0.12 millisecond. The entire process, involving 140 elements and 20 steps of displacements for each pixel to change the phase from 0 to 2π is completed within 0.36 seconds. The focused spot is highly sensitive to external perturbations to the fiber and a small mechanical disturbance can totally alter intensity and location of the bright spot. Fig. 3 shows the intensity of the focused spot as a function of time when the fiber is deliberately disturbed while the deformable mirror is continuously running to restore the optimized phase. The bright spot is recovered within 0.3 seconds.
References: [1] I.M. Vellekoop, and A.P. Mosk, Opt. Lett.,”Focusing coherent light through opaque strongly scattering media”, 32, 2309 -2311 (2007). [2] I. M. Vellekoop, A. Lagendijk, and A. P. Mosk Nat. Photonics, “Exploiting disorder for perfect focusing”, 4, 320-322 (2010). [3] Fanting Kong, R.H. Silverman, Liping Liu, Parag Chitnis, Kotik K. Lee, and Y.C. Chen, “Photoacoustic-guided convergence of light through scattering media”, Opt. Lett., 36, 2053-2055 (2011). [4] Roberto Di Leonardo and Silvio Bianchi, “Hologram transmission through multi-mode optical fibers” ,Opt. Expr. 19, 247-254 (2011). [5] Tomas Cizmar and Kishan Dholakia , “ Shaping the light transmission through amultimode optical fibre: complex transformation analysis and applications in biophotonics”, Opt. Expr. 19, 18871-18884 (2011). [6] F. Kong, N. V. Proscia, K. K. Lee, and Y. Chen, "Controlling Spatial Coherence in Multimode Fibers," in Adaptive Optics: Methods, Analysis and Applications, OSA Technical Digest (CD) (Optical Society of America, 2011), paper AMC4.
