Apr 25, 2013 - â¢Wavelength dependence of Tx and Rx optics o Lens focal length o Propagation properties o Coupling to/from fiber optics. ⢠Combining at the ...
Wavelength Agile FSO Receiver Wei Yi, Peter G. LoPresti Electrical Engineering Department University of Tulsa
Asaad Kaadan, Hazem H. Refai Electrical and Computer Engineering Department University of Oklahoma
Supported by NSF Grant #ECCS-0725801
April 25, 2013
ICNS 2013
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Abstract The design of a free-space optical (FSO) transceiver suitable for the demands of establishing reliable communication between mobile nodes such as UAVs requires the solution of difficult problems related to pointing, acquisition and tracking. To accommodate the inevitably larger misalignment conditions encountered in mobile applications, the receiver must possess a large field of view (for angular misalignments), be tolerant of translations away from the optical axis of the receiver antenna, and maintain a large collecting area to realize a workable power budget for the overall link. The transmitter must be able to deliver the best quality beam, in terms of peak power and power distribution, in the direction of the receiver as possible. Ideally, the ability to tolerate misalignments due to mobility at the receiver and proper choice of parameters at the transmitter allows the system to also tolerate the effects of atmospheric turbulence and weather on the transmitted beam, which normally can cause signal fade or complete signal loss even in well-designed FSO systems between stationary platforms. One approach to turbulence mitigation is to use a wavelength diversity scheme. The optimum wavelength for transmission varies as such factors as turbulence strength, absorption, and weather effects such as rain and fog vary. An FSO system for mobile communication that is wavelength agile will perform better than one that uses a single wavelength. We have constructed a FSO transmitter and receiver based on optical fiber bundles and adapted the transmitter to be capable of wavelength diverse transmission. In this paper, we present an experimental investigation of the performance of the system as a function of transmission misalignment, turbulence, and weather for a wavelength diversity scheme, which consists of switching between multiple transmission wavelengths, for reducing the impact of turbulence. Three wavelengths, 850nm, 1310nm, and 1550nm, are emitted by one or more transmitting fibers, and the effects of turbulence and misalignment experimentally evaluated in an indoor environment. A system is designed to detect changes in transmission and switch the transmitter to the appropriate wavelength. The receiver retained the link for a reduced range of misalignment at all wavelengths without adjustments, indicating that adjustment of the receiver immediately after a wavelength changes was not necessarily required.
April 25, 2013
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OUTLINE I.
Introduction • Goals of the research program • Wavelength Diversity
II.
System Description • Transmitter and Receiver design • Experimental setup
III. Experiments • Misalignment tolerance • Wavelength dependence • Turbulence sensitivity IV. Summary
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I.
INTRODUCTION Goals: • FSO transmitter and receiver capable of mobile operation • Maintain link connectivity for large o Angular misalignment oTranslational misalignment • Reduce fades due to turbulence and weather effects o Spatial diversity demonstrated o Wavelength Diversity possible?
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I.
INTRODUCTION Wavelength Diversity: • Use widely different wavelengths for transmission • Absorption, scattering different at each wavelength • Determining Factors: o Atmospheric absorption (O2, CO2, CO, N2, H2O etc.) o Weather absorption/scattering (rain, fog, etc.) o Turbulence – refractive index eddy size • Match wavelength used to channel conditions o Example: Use longer wavelengths as eddy size increases
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I.
INTRODUCTION Wavelength Diversity Concerns/Challenges: •Wavelength dependence of Tx and Rx optics o Lens focal length o Propagation properties o Coupling to/from fiber optics • Combining at the transmitter – one Tx, not many • Timing and effects of changing wavelengths Purpose of Current Study • Wavelength sensitivity of fiber-bundle transceiver design o How far do properties degrade when switching ɉ o How much adjustment needed to recover best link • Initial studies into turbulence effects/sensitivity
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II.
SYSTEM DESCRIPTION A. General System Description • Transmitter with multiple output fibers • Multiple sources can be coupled to Tx fibers • Fiber-bundle based receiver • Tx and/or Rx movable perpendicular to link axis • Turbulence chamber can be added
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II.
SYSTEM DESCRIPTION B. Transmitter Details • Sources: o 1310 nm: BERT, OC-3 (155 Mb/s), SONET o 1550 nm, 850 nm: E/O converter, 1MHz square wave – amp on 1550 nm available • 1 x 4 power splitter coupled to linear array of fibers • 3-lens telescope – control divergence and deflection
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II.
SYSTEM DESCRIPTION C. Receiver Details • Collecting lens array – 0.09 in diam, f =3 mm, hex pattern • One fiber per lens – N.A. = 0.37, 400 μm core • Summing optics – direct all light to photodetector • InGaAs, 150 MHz bandwidth (PDA10CF ThorLabs)
•
Analyzers: Communication analyzer, oscilloscope
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III.
MISALIGNMENT TOLERANCE EXPERIMENTS A. Basic Procedure 1. Align axes of transmitter and receiver 2.
Optimize power throughput for 1310 nm • Acts as reference for entire experiment • Adjust positioning of optical elements • One transmitting fiber at this point
3.
Adjust angular or translational misalignment of Tx at 1310 nm
4.
Measure collected power vs. misalignment
5.
Change wavelength – DO NOT ADJUST optics!
6.
Measure and compare with 1310 nm behavior. April 25, 2013
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III.
MISALIGNMENT TOLERANCE EXPERIMENTS A. Translational Misalignment Experiment • Move Tx perpendicular to optical axis • Record collected power vs. translation
ȟx
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MISALIGNMENT TOLERANCE EXPERIMENTS A. Translational Misalignment Experiment • Power adjusted for Tx power, Rx sensitivity vs. ɉ • Relative power loss at non-optimal wavelengths • Max ȟx increase 1550 nm, slight decrease 850 nm 4500
Normarlized Collected Average Power (mV)
III.
4000 3500 3000 2500 1550nm
2000
850nm
1500
1310nm
1000 500 0 -1
-0.5
0
0.5
1
• X (cm)
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III.
MISALIGNMENT TOLERANCE EXPERIMENTS B. Angular Misalignment Experiment • Move Tx perpendicular to optical axis • Tilt Tx to target center of Rx lens array • Record collected power vs. angle
Ʌ
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MISALIGNMENT TOLERANCE EXPERIMENTS B. Angular Misalignment Experiment • Power normalized for comparison • Misalignment tolerance maintained, at least in part • Max Ʌ increase 1550 nm, decrease 850 nm 1 0.9 0.8
Normalized Power Collected
III.
0.7 0.6 0.5
850nm 1310nm
0.4
1550nm
0.3 0.2 0.1 0 0
0.5
1
1.5
2
2.5
3
3.5
Delfection Angle (degree)
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III.
MISALIGNMENT TOLERANCE EXPERIMENTS C.
Magnitude of Misalignment • How much adjustment needed to re-align the receiver? • No adjustment made to the transmitter
Example for 1310 nm 1310 nm eye, no adjust
• •
1310 nm eye, adjusted Rx
OC-3; 20 mV/div 26 mV (no adjust); 32 mV (0.25 mm adjust) – 23% increase April 25, 2013
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III.
MISALIGNMENT TOLERANCE EXPERIMENTS C.
Magnitude of Misalignment • Any different for 1550 nm?
1550 square, no adjust
• •
1550 nm square, adjusted
1 MHz, 100 mV/div 350 mV (no adjust); 480 mV (0.27 mm adjust) – 37% increase
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III.
MISALIGNMENT TOLERANCE EXPERIMENTS D. General Conclusions • Extent of misalignment tolerance impacted by • change •
Improvement of tolerance over standard FSO receivers maintained at all • , even before adjustments
•
Minor adjustments to optical alignment needed to return full operational capabilities
•
Must electronically compensate for differences in wavelength sensitivity of detector, transmitter power
•
Strong chance of the link remaining viable after a wavelength change prior to transceiver optimization
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III.
PRELIMINARY TURBULENCE EXPERIMENTS A. General Procedures • Align transmitter and receiver axes, optimal alignment •
Measure signal prior to introducing turbulence
•
Induce atmospheric disturbance within “turbulence box” o Hot plate – heating-related turbulence o Fans – wind-related disturbance o Can also emulate fog, rain
•
Compare clear atmosphere signal vs. turbulent signal
•
Sweep Rx to analyze coverage area effects
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III.
PRELIMINARY TURBULENCE EXPERIMENTS B. Sample Results • 1550 nm signal, with and without turbulence • 100 mV/div (original), 200 mV/div (with turbulence)
• •
Open range decrease 350 mV to 200 mV (unadjusted Rx) Range increase to 250 mV when Rx alignment adjusted
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III.
PRELIMINARY TURBULENCE EXPERIMENTS B. Sample Results • 1550 nm signal, with turbulence and translation • Translation stage is motor driven, outdoor version shown
• • •
Upper level decreased in power Turbulence level clearly visible – reduces usable range (• x) Still improvements to be made
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III.
PRELIMINARY TURBULENCE EXPERIMENTS C. Wavelength Diversity Switching System • Turbulence detector o Currently Shack-Hartmann sensor + guide laser o Computer processing
• •
Choose signal source based on turbulence/other statistics Eventually measurement to be made directly from Rx o Channel estimation from parallel paths o Theoretical treatment shows promise
Moradi, H., H. H. Refai, P. G. LoPresti, 2012, IEEE Trans. Vehicular Tech., vol. 61, n. 3, 1174 – 1181. April 25, 2013
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IV.
SUMMARY •
Wavelength diversity has potential for mitigating atmospheric effects on FSO systems
•
Fiber bundle-based receiver retains misalignment tolerance through a wavelength change
•
Adjustments required to restore optimum operation are reasonably small
•
Initial studies with turbulence show promise of the method
•
More extensive turbulence studies in process
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Questions? April 25, 2013
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