Multi-plane light conversion is a method of performing spatial basis transformations using cascaded phase plates separated by Fourier transforms or free-space propagation. In general, the number of phase plates required scales with the dimensionality (total number of modes) in the transformation. This is a practical limitation of the technique as it relates to scaling to large mode counts. Firstly, requiring many planes increases the complexity of the optical system itself making it difficult to implement, but also because even a very small loss per plane will grow exponentially as more and more planes are added, causing a theoretically lossless optical system, to be far from lossless in practice. Spatial basis transformations of particular interest are those which take a set of spatial modes which exist in the same or similar space, and transform them into an array of spatially separated spots. Analogous to the operation performed by a diffraction grating in the wavelength domain, or a polarizing beamsplitting in the polarization domain. Decomposing the Laguerre-Gaussian, Hermite-Gaussian or related bases to an array of spots are examples of this and are relevant to many areas of light propagation in free-space and optical fibre. In this paper we present our work on designing multi-plane light conversion devices capable or operating on large numbers of spatial modes in a scalable fashion.
As the nonlinear capacity limit of single mode fiber (SMF) transmission systems is being approached, space-division multiplexing (SDM) in multicore fibers (MCFs) or few-mode fibers (FMFs) is currently under intense investigations to achieve ultrahigh spectral efficiency per fiber. Meanwhile, a key advantage of SDM over simply increasing the number of SMFs, is its inherent device integration and resource sharing capability. This can potentially provide significant benefits in terms of the cost per bit in future optical networks. In order to efficiently address capacity scaling in a single optical fiber, few-mode and multicore erbium-doped fiber amplifiers are being developed. Critical for the implementation of SDM amplifiers is to achieve almost the same amount of gain for all spatial channels. In this respect, we have recently demonstrated multimode fiber amplifiers, supporting >15 modes, with a maximum differential modal gain of 2 dB and negligible mode mixing.
Integrated space-division multiplexed (SDM) erbium-doped fiber amplifiers (EDFAs) are not only inevitable for SDM systems, but can be an alternative solution to nowadays EDFA array for parallel amplification. SDM EDFAs are expected to provide substantial complexity and cost savings through spatial-integration compared to duplicating single-mode fiber amplifiers. High output power and low noise figure can be achieved by cladding-pumped SDM EDFAs. In this paper, different cladding pumping solutions, cladding-pumped single-mode and multimode multi-core EDFAs will be discussed.
The capacity of optical transmission systems has increased dramatically since their first deployments in the mid
1970s . However, studies show that the theoretical capacity limit of single-mode fiber is about to be reached, and
space-division multiplexing has been proposed to overcome this limit. With the high levels of integration needed
for economic deployment, space-division multiplexing may exhibit large crosstalk between the supported fiber
modes. We propose to use coherent multiple-input multiple-output (MIMO) digital signal processing (DSP), a
technique widely used in wireless communication, to compensate crosstalk present in spatial multiplexing over
fibers. According to MIMO theory, crosstalk in multi-mode transmission systems can be completely reversed
if the crosstalk is described by a unitary transformation. For optical fibers this is fulfilled if all available fiber
modes can be selectively excited and if all the modes are coherently detected at the end of the fiber, provided
that mode-dependent loss is negligible. We successfully applied the technique to demonstrate the transmission
of six independent mode-multiplexed 20-Gbaud QPSK signals over a single, optically amplified span of 137-km few-mode fiber (FMF). Further, in a multi-span experiment, we reach 1200 km by transmitting over a
3-core coupled-core fiber (CCF). Details for both experiments will be presented, including the description of the
supported polarization- and spatial modes of the fiber, the mode multiplexers used to launch and detect the
modes, and the MIMO DSP algorithm used to recover the channels.
A MEMS SLM with an array of 64×64 pixels, each 120 μm ×120 μm in size, with 98% fill-factor, has been developed.
Each reflector in the array is capable of 5 μm of stroke, and ±4° tip and tilt. From a prototype array, 14 contiguous pixels
have been independently wired-out to off-chip drive electronics. These 14 pixels have been demonstrated to be effective in
an off-the-shelf AO system (with requisite modifications to suit the SLM). For a low-order static aberration, the measured
Strehl ratio has been improved from 0.069 to 0.861, a factor of 12 improvement.
We present an analytic evaluation of the optical performance of a tip/tilt and piston Spatial Light Modulator, for use in adaptive optic and beam steering applications. The impact of array fill factor, pixel curvature, position and tilt accuracy, and pixel yield are discussed in detail. Two arrangements for beam steering are presented and analyzed; in the first the SLM is used as a programmable higher-order diffraction grating, and in the second the SLM acts as a programmable Fresnel lens inside a telescope.
As telecom networks increase in complexity there is a need for systems capable of manage numerous optical signals. Many of the channel-manipulation functions can be done more effectively in the optical domain. MEMS devices are especially well suited for this functions since they can offer large number of degrees of freedom in a limited space, thus providing high levels of optical integration.
We have designed, fabricated and tested optical MEMS devices at the core of Optical Cross Connects, WDM spectrum equalizers and Optical Add-Drop multiplexors based on different fabrication technologies such as polySi surface micromachining, single crystal SOI and combination of both. We show specific examples of these devices, discussing design trade-offs, fabrication requirements and optical performance in each case.
Optical crossconnect switches with large port counts are the key components for the management of upcoming optical networks. Most technologies proposed for optical switches are essentially planar in geometry, which leads to switch dimensions scaling with the square of the port number. Planar technologies will not scale beyond 64 X 64 ports. To achieve larger (256 X 256, 1296 X 1296) switches, a three dimensional switch geometry is required. Micro electro-mechanical systems (MEMS) are the key technology to implement array of small two tilt axis beam steering mirrors. The presented systems consist of 2D arrays of MEMS mirrors and 2D fiber arrays each with a collimating microlens array. A cross connect path consist of light leaving one fiber and being collimated and projected onto a MEMS micro-mirror by a microlens. The first micro-mirror tilts so as to direct the beam onto a second micro-mirror, and the second micro-mirror tilts so as to direct the light towards a microlens where it is coupled into the output fiber. In this configuration the length of the switch scales linearly with number of ports, and the maximum port number is determined by used the micro mirror technology. Two switches are presented: the first with 256 port and a mean insertion loss of 7 dB and the second with 1296 ports and an insertion loss of 5.1 dB. Both switch show a crosstalk smaller than -50 dB. The optical performance has been verified with input optical signals ranging from 40 DWDM 40 Gb/s and 320 Gb/s TDM data. On the switch with 1296 ports a potential aggregate switch capacity of 2.08 Petabit/s has been demonstrated.
Interband photorefractive gratings can be generated by illumination of an electro-optic crystal in the high absorption spectral region. In contrast to conventional photorefractive effects that rely on mid-gap doping levels, the gratings induced by such interband phototransitions exhibit a much faster response time (approximately equals 10 microsecond(s) at 1 W/cm<SUP>2</SUP> in KnbO<SUB>3</SUB>) and are very robust with respect to intense illumination at sub-bandgap wavelengths. Due to the large light absorption, the interband gratings are limited to a relatively thin layer (typically 100-200 micrometers ) at the crystal surface. We discuss the main differences existing between conventional and interband photorefractive gratings. Besides by the electro-optic coefficients and the effective carrier mobilities, the strength of the gratings in a given crystal is controlled by the light intensity and the laser wavelength. Unlike in the conventional case, the photoconductivity and response speed depend here on the square-root of the light intensity. Interband photorefractive effects are interesting for applications of an optical incoherent-to-coherent converter and of an optical Joint Fourier Transform correlators based on interband dynamic holography. Finally we describe the use of this effect for creating robust and fast reconfigurable light induced waveguides in KnbO<SUB>3</SUB>.