FreeSpace Optical Communications for Tactical Applications

1
Atmospheric aberrations in coherent laser
systems Snowmass, July 12, 2007 Aniceto
Belmonte belmonte_at_tsc.upc.edu
2
Atmospheric Optical Systems
3
Index

• Simulated Experiments on Atmospheric
  Propagation
• Compensation Methods on Coherent Measurements
• Beam Projection on Coherent Lidars
• Conclusions

4
Work Basis

• Optical phase perturbations destroy the spatial
  coherence of a laser beam as it propagates
  through the atmosphere. It restricts the received
  power levels in optical coherent systems.
• Temporal fading associate with optical amplitude
  fluctuations increases the uncertainty in the
  measurements.
• Performance limitations imposed by atmospheric
  turbulence on specific coherent systems need to
  be quantify.
• Main task is the quantification of the
  performance achievable in coherent optical
  systems using atmospheric compensation techniques.

5
Atmospheric Effects on Received Signal
6
Available Techniques
!?
Rytov
Simulations
Asymptotic
Heuristic ?
7
Split-Step Solution

• Based on the Fresnel approximation to the wave
  equation
• Atmosphere is modeled as a set of two-dimensional
  random phase screens
• All simulations use the Hill turbulence spectrum
  (1-mm to 5-m scales)
• Uniform and Non-Uniform (Hufnagel-Valley model)
  turbulence profiles
• Temporal and spatial analysis

8
Receiver Plane Formulation
9
Target Plane Formulation
10
Simulated Performance Monostatic
11
Simulated Performance Bistatic
T
BPLO
12
Misalignment Effects
13
Coherent Power Fluctuations
5000
5000
Strong Cn2
Moderate Cn2
4000
4000
3000
3000
? 2 ?m
Altitude m
30
2000
2000
60
90 (Zenith)
1000
1000
0
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0
0.1
0.2
0.3
0.4
0.5
Coherent Power Standard Deviation
Coherent Power Standard Deviation
14
Uncertainty Temporal Averaging
15
Free-Space Optical Communication Systems

• Optical phase perturbations restricts the
  received power levels in optical communications.
• Temporal fading associate with optical amplitude
  fluctuations increases the error in the
  communication link.

16
Index

• Simulated Experiments on Atmospheric
  Propagation
• Compensation Methods on Coherent Measurements
• Beam Projection on Coherent Lidars
• Conclusions

17
Atmospheric Compensation Techniques
ATMOSPHERIC EFFECTS ON RECEIVED SIGNAL
PHASE DISTORTION
BEAM WANDER
BEAM SPREADING
SCINTILLATION
ATMOSPHERIC COMPENSATION TECHNIQUES
PHASE COMPENSATED RECEIVERS
APERTURE INTEGRATOR/ARRAYS
RECIPROCITYPOINTING
DIRECT DETECTION GROUND, DOWNLINK
DIRECT, HETERODYNE GROUND, DOWNLINK
DIRECT, HETERODYNE GROUND, DOWN/UP LINKS
18
Phase Compensation on Coherent FSO

• In communication with optical heterodyne
  detection, as in imaging systems, the aim of
  phase compensation is to restore
  diffraction-limited resolution. Technology of
  adaptive optics communications is identical to
  that of adaptive optics imaging Measurement,
  reconstruction, and conjugation of the wavefront
  (spatial phase conjugation of Zernike modes).

19
Atmospheric Compensation Needs in FSO
Detector-plane Intensity Distributions
20
Adaptive Optics in Direct-Detection FSO
21
FSO Coherent Power Gain
22
Speckle in Coherent Lidar

• The target is a distributed aerosol, which
  creates target speckle with decorrelation times
  in the order of 1 µs.
• Mirror segments response times are about 0.1?1ms,
  hence compensation system allows system
  bandwidths of about 1 kHz. Any phase conjugation
  system will be too slow to compensate for target
  speckle.

23
The Optimization Problem

• We need to consider the speckle averaged coherent
  signal. Consequently, a rapid pulse repetition
  rate is required from the laser. Nowadays systems
  have the required specifications.
• The power level reaching the receiver is
  extremely low and wavefront sensor should use
  coherent detection. Also, wavefront conjugation
  technique has problems related to the presence of
  intensity scintillation.
• Wavefront correctors based on MEM systems have
  large bandwidth and a reduced tag price. The
  wavefront sensor and the phase reconstruction
  hardware are the major obstacles to achieving
  fast, inexpensive adaptive systems.

24
Non-Conjugated Adaptive Optics

• There is another wavefront control paradigm.
  Instead of considering the wavefront conjugation
  based on the reciprocity principle, it is
  possible to compensate wavefront distortion using
  direct system performance metric optimization.
• We analyze a system implementing a non-conjugate
  adaptive optics with use efficient parallel
  model-free optimization algorithms (Gradient
  descent optimization).
• The metric can be considered as a functional that
  depends on the phase aberrations introduced by
  atmospheric turbulence.

25
Blind (Free-Model) Compensation
26
Blind (Free-Model) Algorithms

• The algorithm choose the mirror shape to maximize
  the speckle averaged coherent signal power.
  Compensation can consider either the transmitted
  beam or the local oscillator beam.
• Compensation algorithms can be associated with a
  metric defined in terms of the overlap integral
  of the transmitted and BPLO irradiances at the
  target plane. The speckle averaged coherent
  signal power P is defined through the overlap
  integral

27
LO Atmospheric Beam Projection

• The problem of adaptive laser beam projection
  onto an extended aerosol target in the atmosphere
  needs to be considered. Beam compensation is
  considered through conjugation of the wave
  phase.
• Using the target-plane formulation and our
  simulation techniques, it is straightforward to
  estimate the phase-correction system reliability
  and its effects on the coherent lidar
  performance.

28
Coherent Power as Quality Metric
29
LO Control Wavefront
30
Beam Projection
31
Index

• Simulated Experiments on Atmospheric
  Propagation
• Compensation Methods on Coherent Measurements
• Beam Projection on Coherent Lidars
• Conclusions

32
Coherent Power Gain vs Elevation Angle
33
Coherent Power Gain
34
Coherent Power Gain
35
Coherent Power Gain vs Aperture Size
36
Coherent Power Gain
5000
5000
D 10 cm
D 20 cm
4000
4000
D 40 cm
? 45
3000
3000
Altitude m
? 1 ?m
2000
2000
1000
1000
Strong Cn2
Moderate Cn2
0
0
0
10
20
30
40
50
0
10
20
30
40
50
Coherent Power Gain
Coherent Power Gain
37
Misalignment Compensation
38
Misalignment Compensation
39
Index

• Simulated Experiments on Atmospheric
  Propagation
• Compensation Methods on Coherent Measurements
• Beam Projection on Coherent Lidars
• Conclusions

40
Technique Summary

• Feasibility of Beam Propagation
  Technique Well-known Limits of Applicability
• Simulation of Coherent Laser System
  Performance Practical Systems Analysis
• Results are encouraging Compensation techniques
  may extend the deployment distance and/or
  quality of atmospheric optical systems.
• Room for improvement New algorithms and Full
  Field Compensation
• Results must be viewed as benchmarks whose
  achievements may require the development of
  devices.