Presentation is loading. Please wait.

Presentation is loading. Please wait.

Dr. Arun K. Majumdar 105 W. Mojave Rose Ave.

Similar presentations


Presentation on theme: "Dr. Arun K. Majumdar 105 W. Mojave Rose Ave."— Presentation transcript:

1 Free-Space Laser Communications: Fundamentals, System Design, Analysis and Applications
Dr. Arun K. Majumdar 105 W. Mojave Rose Ave. Ridgecrest, California 93555, USA Lecture Series:1 Brno University of Technology, Brno, Czech Republic December 1-6, 2009 Copyright © Arun K. Majumdar

2 Copyright © 2009 Arun K. Majumdar
Course Outline Introduction Definition of free-space laser communications Why optical communications? Optical / RF comparison Basic block diagram Applications overview Major sub-systems for laser communications systems and Link Analysis Laser Transmitter Modulation methods Transmitting optics Copyright © Arun K. Majumdar

3 Copyright © 2009 Arun K. Majumdar
Course Outline Optical Receiver Photo-detectors Pre-amplifier Optics, Fiber Optics Acquisition, Pointing, and Tracking 3. Optical Signal Detection Direct Detection: Detection statistics SNR Bit-Error-Rate (BER) probability Coherent Detection 4. Atmospheric Channel Effects Attenuation Beam Wonder Turbulence (Scintillation/ Fading) Turbid (rain, fog, snow) Cloud-free line of sight Copyright © Arun K. Majumdar

4 Copyright © 2009 Arun K. Majumdar
Course Outline Received Power Link Margin Data Rate Reliability Copyright © Arun K. Majumdar

5 Copyright © 2009 Arun K. Majumdar
Course Outline 5. Basic Free-Space Laser Communications System Wavelength Selection Free-Space Lasercom Subsystems Copyright © Arun K. Majumdar

6 Copyright © 2009 Arun K. Majumdar
Course Outline 6. Free-Space Laser Communications Systems Performance Metrics for evaluating the performance SNR and BER in presence of atmospheric turbulence Probability of Fade Examples Terrestrial (Horizontal Link) Uplink Downlink Copyright © Arun K. Majumdar

7 Copyright © 2009 Arun K. Majumdar
Course Outline Mitigating Turbulence Effects Multiple Transmitters Adaptive Optics Animation Show 9. Summary: Improvement of Lasercom Performance REFERENCES Copyright © Arun K. Majumdar

8 Copyright © 2009 Arun K. Majumdar
Objectives At the end of the course participants will be able to: Understand basic operational principles of free-space laser communications Describe lasercom systems using fundamental design concepts Describe atmospheric propagation effects on lasercom performance Quantitatively evaluate degradation in system performance as a function of various atmospheric parameters Perform link budget analysis and calculate Bit Error Rate (BER) Copyright © Arun K. Majumdar

9 WHAT IS THE BIG PICTURE OF FREE-SPACE LASER COMMUNICATIONS?
Air-to-Air Air-to-Ground Ground-to-Air Ground-to-Ground Copyright © Arun K. Majumdar

10 Why Optical Communications?
The main reason is the potential increase in information and power that can be transmitted Note: For a circular lens antenna of diameter d, transmitting an electromagnetic wave of wavelength λ, the antenna transmitter gain: Gain, Ga=16/ӨT2 ӨT = transmitting divergent angle ≈ λ/d, so that Ga = 16 d 2/ λ2 Example: 6 in lens antenna at 6x10^14 Hz has 122 dB Gain, compared to an improvement over an RF antenna of 210 ft (~ 64 m) generating gain of 60 dB ! Copyright © Arun K. Majumdar

11 Optical and RF comparison
Antenna Gain Comparison for Optical and RF Copyright © Arun K. Majumdar

12 Major sub-systems for laser communications systems and Link Analysis
Laser Transmitter Transmitter Optics Beam Propagation Optical Receiver Receiver Optics Acquisition, Pointing and Tracking Copyright © Arun K. Majumdar

13 Copyright © 2009 Arun K. Majumdar
Modulation Method Figure. Selected Modulation Formats Copyright © Arun K. Majumdar

14 Copyright © 2009 Arun K. Majumdar
Optical Receivers The purpose of the receiver is: To convert the optical signal to electrical Recover data DIRECT DETECTION Figure. Typical direct detection digital optical receiver Copyright © Arun K. Majumdar

15 Copyright © 2009 Arun K. Majumdar
Coherent Detection For detecting weak signal, coherent detection scheme is applied where the signal is mixed with a single-frequency strong local oscillator signal. The mixing process converts the weak signal to an intermediate frequency (IF) in the RF for improved detection and processing. Copyright © Arun K. Majumdar

16 Copyright © 2009 Arun K. Majumdar
Optical Receivers Receiver performance The Signal-to-Noise-Ratio for an optical receiver containing a p-i-n diode preceded by an EDFA of the receiver can be calculated as: SNR =Ip2 / (σ2T + σ2s+ σ2sig-sp+ σ2sp-sp) The Bit-Error-Rate (BER), is the probability of incorrect bit identification by the decision circuit of the receiver. With equal occurrence probabilities of logical “1” s and “0”s , and Gaussian noise, the BER is given by: BER = (1/4)· [erfc{(I1 –ID) / σ121/2} + erfc{(ID –I0) / σ021/2}] Where I1 and I0 are the average signal currents at the input of the decision circuit for a “1” and “0”, respectively σ1 and σ0 are the rms noise currents for a “1” and “0”. ID is the threshold current value of the decision circuit. An adequate choice of ID is: ID = (σ0 I1 + σ1 I0) / (σ1+ σ0) Thus, BER = (1/2) erfc(Q/21/2), where Q = (I1- I0) / (σ1+ σ0) Copyright © Arun K. Majumdar

17 Free-Space Laser Communication: the Atmospheric Channel
Laser power reduction due to atmospheric channel effects Potential atmospheric effects: Physical obstructions – birds, bugs, tree limbs, other Absorption – primarily due to water vapor and carbon dioxide Scattering – dust particles, water droplets (fog, rain, snow) Building sway – wind, differential heating/cooling, ground motion Scintillation – atmospheric turbulence Copyright © Arun K. Majumdar 3

18 Copyright © 2009 Arun K. Majumdar
Various atmospheric effects relevant to free-space laser communications Copyright © Arun K. Majumdar

19 The Atmospheric Channel: Absorption
Absorption depends on water vapor and carbon dioxide content of the atmospheric channel, which in turn depends on humidity and altitude Transmission “windows” occur at visible wavelengths and in the ranges m, 3-4 m, and 8-14 m. Copyright © Arun K. Majumdar 3

20 The Atmospheric Channel: Scattering
caused when wavelength collides with scattering particle no loss of energy, only directional redistribution physical size of particle determines type of scattering: particle    Rayleigh scattering (symmetric) particle    Mie scattering (forward direction) particle    extreme forward scattering Atoms & molecules Aerosols & droplets Transmittance (scattering + absorption): No smoke BER 10-8 Weak smoke BER 10-4 Heavy smoke BER 10-3 Communication Transmitter (155Mb/s) Transmitter Copyright © Arun K. Majumdar 3

21 Copyright © 2009 Arun K. Majumdar

22 Atmospheric Turbulence Effects on Propagation
The sun heats the ground, resulting in moisture and temperature gradients. The wind mixes these gradients causing the atmosphere to contain random pockets of air with differing refractive indices called “turbules.” These turbules have a statistical structure which is obtained from the mechanical turbulence caused by the wind. Fluctuations of the refractive index are locally homogeneous and isotropic: Copyright © Arun K. Majumdar 3

23 Turbulence-Induced Refractive Index Fluctuations
December 15, 2002 December 16, 2002 December 17, 2002 February 8, 2003 February 12, 2003 February 13, 2003 Copyright © Arun K. Majumdar

24 Copyright © 2009 Arun K. Majumdar
Atmospheric Models Copyright © Arun K. Majumdar

25 Copyright © 2009 Arun K. Majumdar

26 Copyright © 2009 Arun K. Majumdar
CLEAR1 Model where A= , B= , C= D= , E= , F= Copyright © Arun K. Majumdar

27 Propagation of a Gaussian Laser Beam in Free Space
Receiver beam size: Transmitter focusing parameter: Normalized diffractive distance: Receiver radius of curvature: Copyright © Arun K. Majumdar 3

28 Copyright © 2009 Arun K. Majumdar
Goal: Maximization of Intensity on Receiver Copyright © Arun K. Majumdar

29 Copyright © 2009 Arun K. Majumdar

30 Copyright © 2009 Arun K. Majumdar

31 Copyright © 2009 Arun K. Majumdar
What is Lens Aperture Averaging? Aperture-Averaging Factor A: describes the percent decrease in intensity fluctuations due to having a receiver that is larger than a point. Example: Log-Irradiance Variance = A = 0.75 Aperture-Averaged Log-Irradiance = (1.0)(.75) =0.75 25% reduction in scintillation Fluctuations in intensity are “averaged” over receiving aperture of diameter D: Aperture Averaging Model*: Copyright © Arun K. Majumdar *Ricklin and Davidson, JOSA A 20(5), 856, 2003. 3

32 Copyright © 2009 Arun K. Majumdar
Behavior of the Aperture Averaging Factor A Aperture averaging can significantly reduce intensity scintillations Scintillations increase with path length For smaller aperture sizes in stronger turbulence, scintillations can be severe Doubling the receiver aperture size decreases scintillations by about a factor of two Doubling the wavelength roughly doubles the aperture size required to “average” scintillations Degree of beam divergence does not play a significant role Copyright © Arun K. Majumdar 3

33 Copyright © 2009 Arun K. Majumdar
Coherence-Induced “Artificial” Aperture Averaging Aperture Averaging: Fluctuations in intensity are “averaged” over the receiving aperture of diameter D “Artificial” Aperture Averaging: reduce the beam coherence length rather than increase the receiving lens diameter Copyright © Arun K. Majumdar 3

34 Copyright © 2009 Arun K. Majumdar
Aperture-Averaged Log-Intensity Variance = log-intensity variance averaged over 10 cm diameter aperture log-intensity variance showing off-axis fluctuations (point receiver) Copyright © Arun K. Majumdar 3

35 Copyright © 2009 Arun K. Majumdar

36 Optical Communication Link
Figure 1 illustrates the major subsystems in a complete free-space laser communications system. Transmitter Channel Receiver Data Data Source Laser Free-Space Absorption Scattering Turbulence Background radiance Detection Direct Detection Optical Preamplified Heterodyne Modulator Internal External Demodulation Incoherent/Coherent Optical/Electrical Bit Rate Detector p-i-n PD APD Coding Amplifier Decoding Copyright © Arun K. Majumdar Bit-Error Rate (BER)

37 Basic Free-Space Laser Communications System
Copyright © Arun K. Majumdar

38 Wavelength selection criteria
Choice of the transmitting laser wavelength will depend upon: Atmospheric propagation characteristics Optical background noise Technologies developed for lasers, detectors, and spectral filters (wind velocity of 30 m/s, and a 45º zenith angle for propagation using Hufnagel approximation were assumed) Copyright © Arun K. Majumdar

39 Free-Space Laser Communications Link Analysis
Consider a transmitter antenna with gain GT transmitting a total power PT Watts for a communication range, L. Copyright © Arun K. Majumdar

40 Copyright © 2009 Arun K. Majumdar
Free-Space Laser Communication Link Equation, Link Margin and Data Rate Received Power Link equation combines attenuation and geometrical aspects to calculate the received optical power as a function of range, telescope aperture sizes and atmospheric transmissions. The link equation can be used to generate power detection curves as a function of range. Figure shows the calculated received power as a function of range for the case of a 10 Mbit/s bandwidth, using a LED operating at μm wavelength, 40 mW power, 13-cm receiver, atmospheric transmission r3eceiver4 optical efficiency of 0.2, transmitter divergence angle of 1 degree = radians, and NEP (noise equivalent power) of the Si detector of 300 nW for daytime operation. (Ref. Dennis Killinger, “Free space optics for laser communication through air,” Optics & Photonics News, October 2002) Light Haze: low attenuation (10-4/m or 0.2 dB/Km) Clouds similar to modertae fog- Modertae attaenuation ( 10-2/m or dB/Km) Copyright © Arun K. Majumdar

41 Copyright © 2009 Arun K. Majumdar
Link Margin Link margin describes how much margin a given system has at a given range to compensate for scattering, absorption and turbulence losses. The link margin is defined as: M = (Received Power Available)/ (Required Received Power) Required Received power for a given data rate and receiver sensitivity is: Preq = Nb.r.(hc/λ) where Nb is the receiver sensitivity (Photons/Bit), r is the data rate, h = Planck’s constant, c = velocity of light The Margin, M is then given by: M = PT/[r.(hc/λ) ].(dR2/θT2L2)τatm τ TτR.(1/ Nb) Data Rate The data rate is given by: r = (PT τatm τ TτR..A)[π(θT/2)2L2.Ep. Nb.] where Ep is the laser photon energy=hc/ λ. Example: For a 10 cm telescope, diffraction limited divergence = 14 μrad, transmitter peak power =200 mW, transmitter efficiency =o.5, receiver efficiency = 0.5, and using an avalanche photo-detector with sensitivity of 60 photons/bit for 10-8 BER , the Figure shows the data rate as a function of range, L. Copyright © Arun K. Majumdar

42 Copyright © 2009 Arun K. Majumdar
Ref. Scott Bloom, Eric Korevaar, John Schuster and Hienz Willebrand, “Understanding the performance of free-spaceoptics,” JON (OSA), Vol.2, No.6, ((2003). Ref. E. Korevaar, S. Bloom, K. Slatnick, V. Chan, I.Chen, M.Rivers, C. Foster, K. Choi and C.S. Liu, “Status of SDIO/IS&T Lasercom Testbed Program,” SPIE. Vol.1866 (1993). Copyright © Arun K. Majumdar

43 Copyright © 2009 Arun K. Majumdar
Table 1. Link Analysis Example of a Satellite-to-Ground Laser Communication System Copyright © Arun K. Majumdar

44 Copyright © 2009 Arun K. Majumdar
Example 2. Link Budget for 10 Gbps Laser Communication between Satellite and Ground Station Parameter/Item Downlink (satellite -to- ground) Uplink (ground-to-satellite) Wavelength Laser Power Transmitting Antenna (efficiency= 50%) Antenna Gain Range Free-Space Loss Receiving Antenna (efficiency=50%) Atmospheric Loss , etc.(Absorption loss: 3.0 dB, Strehl ratio due to the atmospheric turbulence: 0.27 dB, coupling loss for wavefront sensing:0.5 db) Receiving Power Sensitivity REQUIRED POWER MARGIN 1.55 micrometer 1 Watt 20 cm dB 38,000 km dB 100 cm dB –10.1 dB dBm 70 photons/bit 40.47 dBm 2.9 dB 38,000 km dB -9.6 dB dB 4.6 dB Copyright © Arun K. Majumdar

45 Copyright © 2009 Arun K. Majumdar
EXAMPLE 3. Very Short Range through Low Visibility Atmospheric Laser Communication Link Parameter/Item Factor Atmospheric Loss Wavelength Range Data Rate Peak Laser Power Transmit Aperture Transmit Divergence (at 1/e2 point) Receiver Aperture Receiver Sensitivity Peak Laser Transmit Power Extinction ratio degradation Pointing Loss Geometric Range Loss Atmospheric Scintillation Fade Receive Optics Attenuation Bandpass Filter Loss RECEIVED PEAK POWER AT DETECTOR REQUIRED PEAK POWER AT DETECTOR LINK MARGIN AT RANGE -200 dB/Km 785 nm 200 meter 1250 Mbit/s mW 5 cm 0.5 mradian 7.5 cm 800 nWatt dBW -0.2 dB -1 dB -2.50 dB -40 dB -1.4 dB -0.7 dB dB dB -0.27 dB Copyright © Arun K. Majumdar

46 RELIABILITY OF LASER COMMUNICATION LINKS
Consider the link power budget. It includes all average losses of optical power P [dBm], which arise between the laser source and the receiving photo-detector. Pt [dBm] = transmitter power, Prec [dBm] = received power, P0 [dBm] = receiver sensitivity and Lp [dBm] = propagation loss. LM is an initial link parameter that serves to express the reliability of the lasercom system. LM = Pt - Lp - P0 The link availability is a percentage of time Tav[%], when the data transmission bit error rate is less than its defined value. The link availability can be expressed as by a probability that additional optical power losses LA [dB] caused by atmospheric effects are less than link margin LM. The attenuation of radiation in the atmosphere has a dominant share among all losses. The link availability can be expressed by means of a probability density p(A) of an attenuation coefficient A [dB/km] from the following equation: where A is the limiting attenuation coefficient value, which is given by A = [LM(D)/D].1000, D being the range. Copyright © Arun K. Majumdar

47 Copyright © 2009 Arun K. Majumdar

48 Copyright © 2009 Arun K. Majumdar

49 The Probability of Error, Bit Error Rate (BER)
pI(s) = probability distribution of irradiance Is= instantaneous signal current with mean value <Ps> = mean signal value <SNR> is the mean SNR in presence of turbulence Copyright © Arun K. Majumdar

50 Copyright © 2009 Arun K. Majumdar

51 Copyright © 2009 Arun K. Majumdar

52 Copyright © 2009 Arun K. Majumdar

53 Copyright © 2009 Arun K. Majumdar

54 Copyright © 2009 Arun K. Majumdar

55 Copyright © 2009 Arun K. Majumdar

56 Copyright © 2009 Arun K. Majumdar

57 Copyright © 2009 Arun K. Majumdar

58 Copyright © 2009 Arun K. Majumdar

59 Copyright © 2009 Arun K. Majumdar
Effect of Atmospheric Turbulence on Bit Error Rate Atmospheric turbulence significantly impacts BER Even with aperture averaging, reduction in BER is several orders of magnitude As atmospheric turbulence strength and path lengths increase, so does the BER What to do? Adaptive Optics Copyright © Arun K. Majumdar 3

60 Partial Coherence: Poor Man’s Adaptive Optics
Weak turbulence: PCB reduces BER by 3 orders of magnitude Moderate turbulence: PCB reduces BER by only 1 order of magnitude Copyright © Arun K. Majumdar 3

61 Copyright © 2009 Arun K. Majumdar
*Laser Beam Scintillation with Applications, L.C. Andrews, R.L. Phillips, and C.Y. Hopen (SPIE Press, Bellingham, 2001). Copyright © Arun K. Majumdar

62 Copyright © 2009 Arun K. Majumdar
Laser Beam Scintillation with Applications, L.C. Andrews, R.L. Phillips, and C.Y. Hopen (SPIE Press, Bellingham, 2001). Copyright © Arun K. Majumdar

63 Copyright © 2009 Arun K. Majumdar

64 Copyright © 2009 Arun K. Majumdar
Probability of Fade for Uplink and Downlink * Laser Beam Scintillation with Applications, L.C. Andrews, R.L. Phillips, and C.Y. Hopen (SPIE Press, Bellingham, 2001). Copyright © Arun K. Majumdar

65 Mitigating Turbulence Effects
Multiple Transmitters Approach (Courtesy Jaime Anguita: Ref. Jai Anguita, Mark A. Neifeld and Bane Vasic, “Multi-Beam Space-Time Coded Communication Systems for Optical Atmospheric Channels,” Proc. SPIE, Free-Space Laser Communications VI, Vol. 6304, Paper # 50, 2006) Aperture averaging and multiple beams is effective in reducing scintillation, improving performance Adaptive Optics approach can be incorporated to mitigate turbulence effects for achieving free space laser communications Copyright © Arun K. Majumdar

66 Copyright © 2009 Arun K. Majumdar

67 Copyright © 2009 Arun K. Majumdar
REFERENCES  1.      Free-Space Laser Communications: Principles and Advances, A. K. Majumdar and J. C. Ricklin, Eds. (Springer, 2008) 1a. A.K. Majumdar and J.C. Ricklin, “Effects of the atmospheric channel on free-space laser communications”, Proc. of SPIE Vol. 5892, 2005. 2. J. C. Ricklin and F. M. Davidson, “Atmospheric optical communication with a Gaussian Schell beam,” J. Opt. Soc. Am. A 20(5), (2003). 3. J. C. Ricklin and F. M. Davidson, “Atmospheric turbulence effects on a partially coherent Gaussian beam: implications for free-space laser communication,” J. Opt. Soc. Am. A 19(9), (2002). 4. W. B. Miller, J. C. Ricklin and L. C. Andrews, “Log-amplitude variance and wave structure function: a new perspective for Gaussian beams,” J. Opt. Soc. Am. A 10(4), (1993). 5. L. C. Andrews, W. B. Miller and J. C. Ricklin, “Geometrical representation of Gaussian beams propagating through complex optical systems,” Appl. Opt. 32(30), (1993). 6. Laser Beam Propagation Through Random Media, L. C. Andrews and R. L. Phillips (SPIE Press, Bellingham, 1998). 7. Laser Beam Scintillation with Applications, L.C. Andrews, R.L. Phillips, and C.Y. Hopen (SPIE Press, Bellingham, 2001). 8. Optical Communications, R.M. Gagliardi and S. Karp (R.E. Krieger Publishing Company, 1988). 9. Optical Channels, S. Karp, R.M. Gagliardi, S.E. Moran and L. B. Stotts ( Plenum Press, New York, 1988). Copyright © Arun K. Majumdar

68 Copyright © 2009 Arun K. Majumdar
REFERENCES  10. I.I. Kim, H.Hakakha, P. Adhikari, E. Korevaar and A.K. Majumdar, “Scintillation reduction using multiple transmitters” in  Free-Space Laser Communication Technologies IX, Proc. SPIE, 2990, (1997). 11. A.K. Majumdar, “Optical communication between aircraft in low-visibility atmosphere using diode lasers,” Appl. Opt. 24, (1985). 12. A.K. Majumdar and W.C. Brown, “Atmospheric turbulence effects on the performance of multi-gigabit downlink PPM laser communications,” SPIE Vol.1218 Free-Space Laser Communication Technologies II , (1990). Copyright © Arun K. Majumdar


Download ppt "Dr. Arun K. Majumdar 105 W. Mojave Rose Ave."

Similar presentations


Ads by Google