Comparisons of Cn 2 measurements and power-in-fiber data from two long-path free-space optical communication experiments

Comparisons of Cn 2 measurements and power-in-fiber data from

two long-path free-space optical communication experiments

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Ronald R. Parenti, Steven Michael, Jeffrey M. Roth and

Timothy M. Yarnall, "Comparisons of Cn2 measurements and

power-in-fiber data from two long-path free-space optical

communication experiments", Proc. SPIE 7814, 78140Z (2010);

doi:10.1117/12.864256 © 2010 SPIE

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http://dx.doi.org/10.1117/12.864256

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http://hdl.handle.net/1721.1/61641

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Comparisons of C

Measurements and Power-in-Fiber Data from

Two Long-Path Free-Space Optical Communication Experiments

Ronald R. Parenti, Steven Michael, Jeffrey M. Roth, and Timothy M. Yarnall

Massachusetts Institute of Technology Lincoln Laboratory, Lexington, MA 02420-9108

ABSTRACT

Over a two-year period beginning in early 2008, MIT Lincoln Laboratory conducted two free-space optical communication experiments designed to test the ability of spatial beam diversity, symbol encoding, and interleaving to reduce the effects of turbulence-induced scintillation. The first of these exercises demonstrated a 2.7 Gb/s link over a ground-level 5.4 km horizontal path. Signal detection was accomplished through the use of four spatially-separated 12 mm apertures that coupled the received light into pre-amplified single-mode fiber detectors. Similar equipment was used in a second experiment performed in the fall of 2009, which demonstrated an error-free air-to-ground link at propagation ranges up to 60 km. In both of these tests power levels at all fiber outputs were sampled at 1 msec intervals, which enabled a high-rate characterization of the received signal fluctuations.

The database developed from these experiments encompasses a wide range of propagation geometries and turbulence conditions. This information has subsequently been analyzed in an attempt to correlate estimates of the turbulence profile with measurements of the scintillation index, characteristic fading time constant, scintillation patch size, and the shape parameters of the statistical distributions of the received signals. Significant findings include observations of rapid changes in the scintillation index driven by solar flux variations, consistent similarities in the values of the alpha and beta shape parameters of the gamma-gamma distribution function, and strong evidence of channel reciprocity.

This work was sponsored by the Department of Defense, RRCO DDR&E, under Air Force Contract FA8721-05-C-0002. Opinions, interpretations, conclusions and recommendations are those of the authors and are not necessarily endorsed by the United States Government.

Keywords: free-space optical communications, laser communications, turbulence models, optical scintillation

1. INTRODUCTION

In a five-month rapid development effort that began in the spring of 2008, the Advanced Lasercom Systems and Operations Group at MIT Lincoln Laboratory built and fielded two custom 2.7 gigabaud lasercom terminals at field sites located in the western suburbs of Boston. The 1.55 µm transmitter was situated in a shed attached to a fire tower, while the receiver was housed in the dome of the Laboratory's Firepond telescope facility in Westford, Massachusetts. The topology of the 5.4 km, near-horizontal propagation path is shown in Figure 1.

Figure 1. Vertical profile of the propagation path for the Firepond communication link experiment. The total propagation distance was 5.4 km.

This initial field effort, which was known as the MASIVS test, was designed to transmit light from a single actively-tracked aperture; at the opposite end of the communication link, signals from four independently-actively-tracked and temporally-synchronized receivers were summed to reduce the effects of scintillation. To stabilize the pointing of the transmit beam, low-power laser beams were sent from each of the four receiver apertures back to the transmitter. Temporal diversity was achieved using symbol interleaving combined with forward error correction coding (FEC), which was capable of imposing interleaver spans as long as 2 seconds. Data were transmitted via standard terrestrial network protocols, specifically, Ethernet over SONET. This field effort culminated in a series of data collects throughout much of September 2008. These results verified the ability of the system to provide high-reliability, error-free communications under widely varying turbulence conditions using small (12 mm diameter) transceiver optics and a communications laser that supplied less than 50 mW of optical power1.

In the spring and summer of 2009, an airborne link demonstration referred to as the FOCAL (Free-Space Optical Communications Airborne Link) experiment was assembled using essentially the same communication hardware employed in the horizontal-path test. During this exercise, a 2.7 gigabaud link was established between a single 25 mm transmitter in a Twin Otter aircraft and four 12 mm receivers situated in a small optical dome on the roof of Lincoln Laboratory's main facility. Owing to line-of-sight limitations of the ground facility the flight profiles were restricted to arcs that covered about 180°, as illustrated in Figure 2. Although this hardware was initially designed to achieve a maximum range of 25 km with a 200 mW source, in the final set of flights of the 13-flight sequence reliable links were maintained out to a range of 60 km by increasing the transmit power to 700 mW. This test successfully demonstrated file transfers of frames containing random data fields as well as high definition video recorded by the MASIVS (Multi-Aperture Sparse-Imager Video System) sensor constructed by the Advanced Electro-Optical Systems Group at Lincoln Laboratory.

During both of the experimental efforts just described, ancillary measurements of channel conditions were provided by portable weather stations and a Scintec BLS900 scintillometer; in addition, a Goodrich infrared camera was used to obtain 2-dimensional imagery of receiver-plane scintillation patterns. A description of the data generated by this instrument suite is the primary subject of this article.

Figure 2. Example of a flight path used for the FOCAL air-to-ground communication experiment. A total of 13 flights were performed, and the radii of the flight arcs ranged from 25 to 60 km.

2. TRANSCEIVER ARCHITECTURE OVERVIEW

The suppression of turbulence-induced scintillation is essential for achieving good performance in free-space links that operate at optical wavelengths. The system constructed by Lincoln Laboratory incorporates two separate approaches to mitigate the impact of scintillation. Spatial diversity is exploited through the use of multiple small-aperture receivers, which are specifically designed to exhibit uncorrelated fluctuation statistics. In the limit of zero mutual correlation, the variance of the sum of N receiver outputs will be reduced by a factor of 1/N. The likelihood of data loss is further reduced through byte-level interleaving combined with forward error correction. A more detailed discussion of the theoretical performance of this encoding and interleaving scheme has been published2_.

A schematic description of the transmit and receive optics used in the MASIVS and FOCAL experiments is shown in Figure 3. The 1.55 µm downlink laser source propagates through a single-aperture transmitter, as indicated by the arrow that projects from the beam-tracking module on the left side of Figure 3. Transmitter beam stabilization is maintained using centroid information derived from four mutually-incoherent 1.55 µm tracking lasers propagated from the ground terminal (signified by the arrows emanating from the trackers on the right side of Figure 3). The ground terminal incorporates four receivers, each of which is independently pointed to ensure that the collected light is efficiently coupled into pre-amplified single-mode fiber detectors. The electrical signals from these four detectors are incoherently added, and the summation signal is subsequently sent to the symbol decoding hardware.

Figure 3. Downlink optical diagram showing the data-transmission beam (indicated by the arrows projecting from the left side of this figure) and upward-propagating tracking beams (indicated by the arrows projecting from the right side of this figure). Separate high-bandwidth trackers are used to stabilize the pointing direction of each of the laser sources and correct angle-of-arrival errors at the detectors.

The data encoding and interleaving hardware constructed for these experiments is beyond the scope of this paper, but a detailed description can be found elsewhere3. A key aspect of this architecture is that network-compatible interfaces were incorporated in both transceiver nodes, which facilitated the transmission of Gigabit Ethernet (GbE) and SONET (OC-12, OC-48) for the 2.5G rate families. Additionally, client interfaces of OC-192 and 10GE-WAN with interleaved OTU2 on the free space interface for the 10G rate families can be supported by this hardware. These capabilities allow terrestrial fiber-optic network traffic to be transparently transported over the free-space optical link. It should be emphasized that this implementation of data interleaving does not reduce the overall data rate, but increased network latency is an inherent side effect of temporal interleaving.

3. MEASUREMENTS OF CHANNEL BEHAVIOR

Previous comparisons of scintillometer and power-in-fiber output generated during the MASIVS horizontal-path experiment indicate that the magnitude of the signal fluctuations under daytime conditions are consistent with those predicted by the Hufnagel-Valley (HV5/7) model4. The anticipated diurnal cycles were seen during those measurements;

the daily channel patterns usually included brief periods of very low Cn2 readings during the morning and evening

transition periods. Rapid changes in the channel characteristics were also discovered during daytime operation on partly-cloudy days, which were well correlated with the fluctuations in the solar radiance along the propagation path; the time constants of these Cn2 fluctuations were typically of the order of a few minutes. The propagation path described

previously in Figure 1 was about 50 m above terrain that was covered primarily by natural vegetation.

During the FOCAL air-to-ground communication link tests, ground-level measurements of the scintillation strength were made over a 1.6 km horizontal path between two building roof-tops on Hanscom field. The data illustrated in Figure 4 show a somewhat larger day/night variation than that seen in the prior year for the vegetation-covered terrain, but rapid changes in the Cn2 conditions were again observed when the strength of the solar radiance varied. On average, the

Figure 4. Scintillometer results from one week of channel measurements during the FOCAL experiment.

The two-dimensional scintillation imagery, which was collected at the focal plane of the receiver in both sets of experiments, provided another useful characterization of channel behavior. As suggested in Figure 5, the spatial structure of the far-field intensity pattern was found to be inversely related to the turbulence strength. Several published theories predict that this correlation parameter will be independent of Cn2 for weak turbulence and approximately equal

to the Fresnel distance for a propagation path of length L

ρI ≈ λ L , (1)

whereas for strong turbulence the functional relationship is expected to be5

ρI ≈

0.4 λ L

σ_I2

( )

Moderate Turbulence

Strong Turbulence

Figure 5. Scintillometer imagery collected during moderate and strong turbulence conditions over a 5.4 km propagation path. The characteristic scale size of the intensity fluctuations is found to be a function of the turbulence strength, and becomes smaller as Cn2 becomes large.

Equations (1) and (2) have a direct impact on the design of multiple small-aperture transceivers, since the chosen aperture diameter must be smaller than the expected r0 value to avoid the need for high-order phase correction and have

an aperture spacing greater than ρI to ensure that the scintillation-induced signal fluctuations will be uncorrelated. The

relationship given in Equation (2) was qualitatively verified during the horizontal-path experiments, for which the average receiver separation was about 15 cm. As indicated in Figure 6, the ratio between the signal variation measured by a single receiver and the incoherent sum of four receivers has a strong dependence on the measured turbulence strength, and the ratio approaches 4 as Cn2 value becomes large. While the full benefit of multiple receivers is not

realized under weak turbulence conditions, scintillation mitigation is less important in this regime since the absolute magnitude of channel-induced fluctuations is much smaller.

Figure 6. Ratio of single-receiver signal fluctuations and the average of four incoherent signals as a function of Cn2. These results are consistent with a correlation distance that is inversely proportional to the strength of the turbulence along the propagation channel. These data were derived from the MASIVS horizontal-path experiment.

4. ANALYSIS OF AIR-TO-GROUND POWER-IN-FIBER SIGNAL FLUCTUATIONS

The air-to-ground link geometry used in the series of tests performed in 2009 provided a good opportunity to compare measurements of the scintillation index with predictions derived from several turbulence profile models. Estimates were made using the spherical-wave expression for Rytov scintillation for propagation from the slant range L to the ground

σR2 ≈ 0.561 k7 6 Cn2

( )

∫

The data shown in Figure 7 were taken from a single day during which the Twin Otter aircraft was flown at approximately 12 kft at slant ranges from 20 km to 60 km. As indicated in this figure, the collected data were most accurately predicted by calculations based on the HV5/7 daytime turbulence profile.

The mean signal power transmitted through the optical channel and the scintillation index provide a rough estimate of the static and dynamic propagation losses due to the effects of atmospheric attenuation and turbulence, but it is well known that the detailed structure of the low-signal tail of the signal distribution function has a strong impact on the performance of free-space communications. In recent years the 2-parameter gamma-gamma function proposed by Andrews and Phillips6 _{has been widely applied to this problem, and several experimental groups have reported good}

success in applying this model. The Lincoln Laboratory link experiments performed in 2008 and 2009 also show good agreement with the gamma-gamma model, but the analysis of these data consistently show that a 1-parameter variation of this model that incorporates the constraint α=β yields adequate accuracy. This result is illustrated in Figure 8, which compares the 1-parameter model with signal distributions derived from receiver data generated by both the ground-based

Scintillation Reduction Factor

10 -45 Four-Laser Sum Output x= 3.6 Receiver Power - r'-rModeIwithu= -40 -55 -50 -.45 -40 Power-In-FIber (dBmW) 45 Receiver Power - .- - r-r Model with a = -40 -35 -30 -25 -20 -15 Powr-Th-FIbe.r dBmW) -45 -10

and airborne receivers during a typical FOCAL communication test. An analysis of data from the MASIVS experiment performed in the previous year produced similar conclusions2.

Figure 7. Comparison of measured power-in-fiber scintillation index with predictions derived using standard turbulence profile models. Best agreement was usually obtained with the Hufnagel-Valley daytime model.

Airborne Receiver

Ground Receiver

Figure 8. Results of optimal curve fitting to received-signal probability density data collected by the ground and airborne receivers in the FOCAL system. Calculations of this type consistently found good fits to gamma-gamma functions with the constraint α=β.

A possible explanation for the observed degeneracy of the distribution shape parameters is the incorporation of high-precision tracking in the transceivers at both ends of the communication link. This premise was tested analytically by first assuming a degenerate form of the gamma-gamma distribution to represent the scintillation function

p_scint

( )

I

( )

I

(

I

)

and constructing a power-law function to describe the effect of Gaussian pointing errors applied to a two-dimensional Gaussian beam7

p_tilt

(

I I

)

I

(

I I

)

n−1

. (5)

The marginal distribution shown below was then computed numerically for a range of parametric values

p_net

( )

I

(

I I

)

( )

I

0 ∞

∫

I

The resulting functions were found to closely resemble the 2-parameter gamma-gamma model, and curve fits to the gamma-gamma function yielded α and β parameters that were identical until the tracking errors become comparable to the diffraction-limited angular resolution of the receiver. Once the tracking errors reach this threshold the values of the two shape parameters rapidly diverge, with one growing much larger than the zero-jitter value. The results of this study, which are briefly summarized in Figure 9, are consistent with measurements made during tests when the track stabilization was intentionally disabled or suffered a mechanical malfunction.

Figure 9. Dependence of the gamma-gamma fit parameters on the severity of the receiver tracking error. The values of the parameters are found to diverge when the tracking errors becomes comparable to the receiver's diffraction-limited angular resolution.

5. CHANNEL RECIPROCITY

As discussed in Section 2 of this article and illustrated in Figure 3, the high-bandwidth data link demonstrated in these tests was established between the single transmitter on the airborne transceiver and four ground-based receivers. However, the four upward-propagating tracking beams generated by the ground node are incoherently collected by a single receiver and the received power is sampled at 1 kHz by a single-mode fiber detector. In this manner, simultaneous measurements of channel transmission were obtained, which enabled a variety of interesting cross-correlation statistics to be computed. Of particular interest is the evidence of channel reciprocity. Point-to-point channel reciprocity was rigorously proven by Shapiro8 _{in 1971 and subsequently addressed by Fried and Yura}9_{, but to our}

knowledge has never been validated outside of the laboratory. Time-correlated analysis of data from both the horizontal-path experiments and air-to-ground experiments conducted by Lincoln Laboratory has uncovered strong evidence of the reciprocity effect in an optical link that incorporates transmit and receive apertures of finite dimensions. Figure 10 shows an example of a cross-correlation function developed from optical power time series collected simultaneously by the two receiver nodes. The cross-correlation peak at zero time shift is seen to be very strong, and the width of the peak is only slightly larger than one sampling time period. (The slight offset seen in this chart is due to a misalignment of the two computer clocks.) The insert shows a short time sequence comparison for zero-time displacement.

o .6 0.5 0.4 O3 0.2 0.1 -01 -10 I

Time Series Comparison

5

Displacement (sec)

10

Figure 10. Cross-correlation function for the time-series outputs of receivers of the two nodes of the optical link. The peak offset is due to a time misalignment of the two computers used in the data recording.

Figure 11 gives a more quantitative summary of the results derived in this mutual-correlation analysis. The most significant number in this table is the correlation between the single receiver on the airborne platform (which incoherently sums the four tracking beams) and the sum output of the four ground-based receivers. The numbers shown are typical for the range of test conditions covered; correlation values in the range of 50 to 80% were routinely obtained. The implication of this result is that it is possible for the transmitter to estimate the likelihood that a bit will be correctly received before it is sent. This information can potentially be used in a variety of ways to improve link reliability and reduce laser power requirements.

Figure 11. Representative correlation results for the horizontal-path and air-to-ground experiments. The correlation values in the last column represent to the closest match between the bi-directional path geometries defined by the propagation paths of the four tracking beams projected from the ground-based transceiver. The propagation ranges for these two tests were 5.4 km and 40 km, respectively.

6. CONCLUSIONS

In support of a comprehensive set of free-space communication link experiments conducted over a two-year period by Lincoln Laboratory, a variety of data analysis activities were performed that have added to our understanding of the optical channel. Measurements of the ground-level turbulence strength generally confirmed the predictions of the HV5/7

model, but rapid variations in the daytime Cn2 values were also observed and found to be well correlated with

fluctuations in solar illumination. Two-dimensional images of far-field scintillation patterns were recorded frequently during these experiments, and subsequent analysis of these data demonstrated that the characteristic spatial correlation radius is a function of both the Fresnel distance and the scintillation index. The validity of the gamma-gamma model was also investigated, and it was found that the two shape parameters were usually equivalent for the propagation geometries used in these measurements.

One of the more interesting observations made during these tests was the high correlation between the power-in-fiber signal fluctuations seen by the two transceiver nodes. The practical utility of this effect is still undetermined, but the potential exists for improved link reliability and more efficient use of source power.

REFERENCES

[1] Moores, J. D., Walther, F. G., Greco, J. A., Michael, S., Wilcox Jr., W. E., Volpicelli, A. M., Magliocco, R. J., and Henion, S. R., "Architecture Overview and Data Summary of a 5.4 km Free-Space Laser Communications Experiment," SPIE 7464, 746404 (2009).

[2] Michael, S., Parenti, R. R., Moores, J. D., Wilcox Jr., W. E., Yarnall, T. M., Volpicelli, A. M., and Taylor, J. A., "The Use of Statistical Channel Models, Full-Field Propagation Codes, and Field Data to Predict Link Availability," SPIE 7464, 74640O (2009).

[3] Greco, J. A., "Design of the High-Speed Framing, FEC, and Interleaving Hardware Used in a 5.4km Free-Space Optical Communication Experiment," SPIE 7464, 746409 (2009).

[4] Yarnall, T. M., Michael, S., Moores, J. D., Parenti, R. R., and Wilcox Jr., W. E., "Analysis of Rapid Cn2 Fluctuations Observed During a 5-km Communication Link Experiment," SPIE 7324, 73240B (2009).

[5] Fante, R. L., "Electromagnetic Beam Propagation in Turbulent Media," Pro. IEEE 63, 1669 (1975).

[6] Andrews, L. C., Phillips, R. L., and Hopen, C. Y., [Laser Beam Scintillation with Applications], SPIE Optical Engineering Press, Bellingham, 90 (2001).

[7] Fried, D. L., "Statistics of Laser Beam Fade Induced by Pointing Jitter," Appl. Opt. 12, 422 (1973). [8] Shapiro, J. H., "Reciprocity of the Turbulent Atmosphere," J. Opt. Soc. Am. 61, 492 (1971).

[9] Fried, D. L. and Yura, H. T., “Telescope-Performance Reciprocity for Propagation in a Turbulent Medium,” J. Opt. Soc. Am. 62, 600 (1972).