We utilize aperture diversity combined with multiple-mode receivers and multiple-input-multiple-output (MIMO) digital signal processing (DSP) to demonstrate enhanced tolerance to atmospheric turbulence and spatial misalignment in a 10 Gbit/s quadrature-phase-shift-keyed (QPSK) free-space optical (FSO) link. Turbulence and misalignment could cause power coupling from the fundamental Gaussian mode into higher-order modes. Therefore, we detect power from multiple modes and use MIMO DSP to enhance the recovery of the original data. In our approach, (a) each of multiple transmitter apertures transmits a single fundamental Gaussian beam carrying the same data stream, (b) each of multiple receiver apertures detects the signals that are coupled from the fundamental Gaussian beams to multiple orbital angular momentum (OAM) modes, and (c) MIMO DSP is used to recover the data over multiple modes and receivers. Our simulation shows that the outage probability could be reduced from
We propose a Hermite–Gaussian (HG) mode-demulti-plexing hybrid (MDH) for coherent detection of mode-division multiplexed signals. The MDH, which performs multiple functionalities, including demultiplexing, local oscillator splitting, and optical 90-deg mixing, is realized based on the multi-plane light conversion technique. An isosceles right triangle output layout is employed to reduce the number of phase masks to fewer than the number of modes, significantly simplifying the construction of the MDH. A 10-Hermite–Gaussian (HG) mode MDH with only five phase masks is demonstrated by numerical simulation, achieving an insertion loss (IL) and mode dependent loss as low as
- NSF-PAR ID:
- 10143304
- Publisher / Repository:
- Optical Society of America
- Date Published:
- Journal Name:
- Optics Letters
- Volume:
- 45
- Issue:
- 8
- ISSN:
- 0146-9592; OPLEDP
- Page Range / eLocation ID:
- Article No. 2219
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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to . Moreover, we experimentally demonstrate the scheme by transmitting two fundamental Gaussian beams carrying the same data stream and recovering the signals on OAM modes 0 and at each receiver aperture. We measure an up to power-penalty reduction for a bit error rate (BER) at the 7% forward error correction limit for a 10 Gbit/s QPSK signal. -
This paper analytically and numerically investigates misalignment and mode-mismatch-induced power coupling coefficients and losses as a function of Hermite–Gauss (HG) mode order. We show that higher-order HG modes are more susceptible to beam perturbations when, for example, coupling into optical cavities: the misalignment and mode-mismatch-induced power coupling losses scale linearly and quadratically with respect to the mode indices, respectively. As a result, the mode-mismatch tolerance for the
mode is reduced to a factor of 0.28 relative to the currently used mode. This is a potential hurdle to using higher-order modes to reduce thermal noise in future gravitational-wave detectors. -
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