研究目的
To compare different optical coupler technologies with respect to their bit-error rate (BER) performance as multiplex devices in a (2 × 2) multiple-input multiple-output (MIMO) environment and to determine power coupling coefficients between mode groups using impulse response measurements at 1327 nm.
研究成果
The refined method for determining mode group power coupling matrices shows improved accuracy. The customized fusion coupler (CFC) provides the best BER performance in a (2 × 2) MIMO system at 1327 nm, with optimal SMF to MMF launch eccentricities varying by coupler type. Small deviations (2 μm) from optimal launch can significantly increase BER, emphasizing the need for careful setup optimization.
研究不足
The method does not work in the 850 nm optical window due to high chromatic dispersion preventing modal separation. Coupling results may vary at different wavelengths, though basic structure is similar at 1327 nm and 1550 nm. The measurement accuracy depends on the alignment device (e.g., hexapod for higher accuracy). Some coupler types show unexpected coupling behaviors that need further investigation.
1：Experimental Design and Method Selection:
The method is based on impulse response measurements to determine power coupling coefficients between mode groups in optical devices. A constrained least squares optimization with interior-point algorithm is used to estimate coupling coefficients.
2：Sample Selection and Data Sources:
The DUTs include a 1 km OM4 grade MMF and various coupler types (SFC, AFC, PC, MC, CFC). Measurements are conducted with different excitation conditions by varying SMF to MMF radial eccentricity from 0 to 20 μm in 1 μm increments.
3：List of Experimental Equipment and Materials:
Equipment includes a pulse generator, laser diode with nonlinear absorption layer, SMF, MMF (OM4 grade), hexapod alignment device, sampling oscilloscope with 36 GHz detector, and various optical couplers (SFC, AFC, PC, MC, CFC).
4：Experimental Procedures and Operational Workflow:
A short optical pulse (25 ps FWHM) is generated and launched into an MMF with specific radial eccentricity to excite mode groups. Impulse responses are measured without and with the DUT. Signal deconvolution is applied to eliminate input pulse spectrum influence. Multiple measurements with different eccentricities are performed to overdetermine the system.
5：Data Analysis Methods:
The power coupling coefficients are determined using a constrained least squares optimization. The normalized mean absolute error (NMAE) is computed to evaluate the quality of estimated coupling matrices. BER performance is simulated based on the obtained coupling matrices for different launch parameters.
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pulse generator
Generates short optical pulses for impulse response measurements.
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laser diode
Driven by the pulse generator to produce optical pulses.
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single-mode fiber
SMF
Used for launching optical signals into multi-mode fiber.
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multi-mode fiber
OM4 grade MMF
Transmission medium for mode group excitation and separation.
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hexapod alignment device
Aligns SMF to MMF with radial eccentricity for precise mode group excitation.
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sampling oscilloscope
Measures electrical impulse responses at the receiver side.
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symmetric fusion coupler
SFC
Optical coupler for mode multiplexing in MIMO systems.
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asymmetric fusion coupler
AFC
Optical coupler for mode multiplexing with asymmetric coupling.
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polished coupler
PC
Optical coupler technology compared for performance.
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mirror coupler
MC
Optical coupler technology compared for performance.
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customized fusion coupler
CFC
Optical coupler customized for specific mode group coupling, used in demultiplexing.
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