Wavelength Diversity Assessment of Fiber Bundle Receiver Under Misalignment and Turbulence

Peter G. LoPresti*a, Wei Yia, Eric Rohlmana, Hazem Refaib

aDept. of Electrical Engineering, Univ. of Tulsa, 800 South Tucker Drive, Tulsa, OK USA 74104 bDept. of Electrical and Computer Engineering, University of Oklahoma, Tulsa, OK, USA 74135;

ABSTRACT

We have constructed a fiber-bundle based FSO receiver and investigated its performance as a function of transmission misalignment, turbulence, and weather. We also investigate 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. The receiver retained the link for a reduced range of misalignment at all wavelengths without adjustments. Initial outdoor experiments were conducted using 1550nm under various lighting, wind, and turbulence conditions. Keywords: optical wireless, control, mobile, coverage area, wavelength diversity

1. INTRODUCTION The design of a free-space optical (FSO) transceiver for communicating between mobile nodes requires pointing and tracking solutions that overcome the presence of atmospheric turbulence and weather, which can cause signal fade or complete signal loss even in well-designed systems. Solutions such as increasing the transmitter power and increasing the collection area of the receiver have limited utility for mobile platforms where size, weight, and power consumption present significant constraints.1-3 Several solutions to the tracking problem have reduced the severity of signal fades due to various sources of turbulence but there remains ample room for further improvement.

Wavelength diversity has shown promise as a complementary solution for counteracting the effects of turbulence and weather.4,5 For a given turbulence condition, characterized by its inner and outer scale for turbulent eddies, the strength of diffraction and refraction of the optical beam is strongly dependent on wavelength. Absorption and scattering by rain, fog, and clouds is also wavelength dependent, and no single wavelength choice performs well in all combinations of weather and turbulence.4 For a wavelength diversity solution, knowledge of the existing conditions will dictate a switching of the transmitter wavelength to maximize the quality and power of the signal collected by the receiver.

A potential concern at the receiver is the wavelength dependence of the optical components used to collect the light and deliver it to the optical-to-electrical output device (photodiode or the like), including potentially the coupling of light to an optical fiber. Even small variations in the imaging properties of these components can have a strong impact on the optical power throughput of the link, and this impact is almost always negative. If the negative impact is too severe, the link may be lost for a longer period of time, beyond the loss due to fading, until the receiver is adapted for operation at the new wavelength. To use wavelength diversity, the design of the transceivers must properly address this concern.

Recently, the effectiveness of designs that use fiber optic bundles and lens arrays to improve the misalignment tolerance of an FSO receiver has been demonstrated for single wavelength operation.6-9 The resulting receiver has a greatly enhanced field of view and is significantly more tolerant of both angular and translational misalignment between transceivers, making it a potentially excellent option for mobile FSO applications. When such a receiver is combined with a transmitter that uses a fiber optic bundle to control the transmitting beam area and pointing angle,1,6 the potential exists for producing a compact, lightweight FSO communication system that can maintain a quality link under adverse operation conditions.10,11 However, the tolerance of these new designs with respect to changes in the operating wavelength have not been studied, especially experimentally. Ideally, the power budget and misalignment tolerance for the link would vary only slightly with a variation in wavelength.

*peter-lopresti@utulsa.edu; phone 1 918 631-3274; fax 1 918 631-3344; www.utulsa.com

Atmospheric Propagation X, edited by Linda M. Wasiczko Thomas, Earl J. Spillar, Proc. of SPIE Vol. 8732, 87320C · © 2013 SPIE · CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2018239

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In this paper, we investigate experimentally the operation of typical transceiver designs as a function of the operating wavelength. The investigation builds upon a previously developed theoretical simulation of a link constructed from transceivers that use fiber bundles at both the transmitter and receiver.12. The investigation focuses on the ability of the transceivers to maintain a viable link under different misalignment conditions, and looks at how well the receiver, in particular, performs at the moment the wavelength is switched, to determine what effect the switching has on the link prior to adjusting the receiver to its optimal settings for the new wavelength.

2. EXPERIMENTAL SYSTEM The transceiver system used in the experimental evaluation is shown in Figure 1. The optical power from the signal source is coupled to a fiber, which is then connected to a 1-by-4 splitter. Three of the splitter outputs are connected to three adjacent fibers within a fiber ribbon cable. The output side of the bundle cable, shown in Figure 2, is mounted on an optical bench and serves as the input to the transmitter telescope. The middle fiber of the three adjacent fibers is aligned with the optical axis of the telescope. For some investigations, the splitter is omitted, and the signal source is connected directly to the fiber aligned with the telescope’s optical axis. The telescope consists of three lenses: an input convex lens, a concave lens for adjusting the effective location of the fiber output spots with respect to the final lens, and a convex lens to control the output beam divergence. By illuminating different fibers in the array, the beam can steered towards different parts of the output plane (single fiber illumination) or a large area at the output plane can be illuminated in a more uniform manner than that achieved by increasing the divergence (multi-fiber illumination). The transmitter is mounted on a platform that allows the transmitter to be translated perpendicular to the optical axis of the link and to be rotated with respect to the optical axis, allowing translational and angular misalignment, respectively.

Figure 1: Experimental Setup

At the receiver, an array of small (0.09 in diameter) lenses is used to collect the light and couple it to an array of optical fibers. Each fiber has a core of 400 µm in diameter and a numerical aperture of 0.37. The output from the fiber array is coupled through a series of lenses to the collecting area of an amplified InGaAs photodetector (PDA10CF from ThorLabs) with 150 MHz bandwidth. The output of the detector is coupled to one of two oscilloscopes, depending on the data to be collected in a given experiment.

For indoor experiments in the laboratory, the length of the laboratory bench limited the link distance to 1.5 meters. The receiver was affixed to the bench and the transmitter was placed on a rail that allowed motion in the direction perpendicular to the optical axis of the transmitter, referred to hereafter as the x direction. The output lens of the telescope had a focal length of 75 mm, and only the center fiber of the transmitting array was employed.

For outdoor experiments, the experimental conditions and apparatus were modified. The entire receiver system was constructed on a plate that was subsequently mounted on a moving platform to conduct coverage range experiments. The completed motorized translating unit is shown in Figure 2. The platform rode on parallel rails and was driven by a screw mechanism and stepper motor. The speed and step size of the motor were selected using a microprocessor based controller designed specifically for the experimental apparatus. To stabilize the receiver components further, metal rods were used to make the fiber bundle section more rigid. The transmission length was extended to 3.96 meters to allow for later transmission experiments over water features so that all outdoor experiments might be of the same length. For the results presented here, all transmission was over a brick walkway that is visible in Figure 2. All three of the transmitting fibers were used for some of these experiments.

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Figure 2: Physical construction of the outdoor experiment

3. INDOOR EXPERIMENTS The indoor experiments investigated the effect of changing the transmitter wavelength on the he angular and

translational misalignment tolerances achieved by the reciever. The purpose behind receiver design is to broaden the range of misalignments for which the link continues to be maintained, so that the link continues to function as the transmitter and/or the receiver is moving. As the optical components in both the transmitter and the receiver systems have properties that are functions of wavelength, it was important to assess whether or not the broadened misalignment tolerance could be retained when the transmitting wavelength is switched in response to changes in the atmospheric conditions between the transmitter and receiver.

For the experimental investigations, the first step was to align the transmitter and receiver to achieve an optimal condition to serve as the reference case for all measurements. The optical axes of the transmitter and receiver were aligned using only the center transmitting fiber and a wavelength of 1310 nm. The axes were considered aligned when the maximum signal was observed at the output of the receiver, as measured by the maximum peak-to-peak voltage output by the photodetector module. The positioning of the lenses and detector were then adjusted to optimize the signal collected by the receiver for 1310 nm. The transmitter lenses were adjusted to produce a beam diameter at the receiver that covered approximately all of the lenses that were coupled to a fiber in the bundle of the receiver. All misalignments and changes to the operation of the system were made from this initial condition. 3.1 Transverse Misalignment

The first investigation examined the transverse misalignment tolerance of the link for each of three wavelengths – 1310 nm, 850 nm, and 1550 nm. The transmitter was moved along an optical rail running perpendicular to the optical axis in 0.1 cm steps, with no adjustments made to the angle of the transmitted beam or the angle of the transmitter node. The average collected power was monitored as a function of the lateral translation by measuring the average output voltage from the detector. After each measurement cycle, the transmitter was returned to the starting position and the alignment confirmed by a return to the original measured average voltage. The measurements were repeated for each wavelength of interest. Again, no adjustment of the receiver optics was made at any point in the experiment once the original alignment was established.

The results of the experiments are shown in Figures 3. The collected power was normalized to the power collected at zero translation, which was also the peak power collected for each wavelength, to focus on the width of each curve. For the reference case of 1310 nm, a non-zero signal power was collected for a range of 0.8 cm, which is slightly less than the radius of the useful collecting lens array which was 1.143 cm. Thus, for the most part, the allowable transmitter translation was limited by the size of the beam. For the 1550 nm source, both a larger transmitted power and an increase in the divergence of the transmitted beam caused an increase in the range over which measurable signal was recovered to 1.3 cm. When the 850 nm source was now employed at the transmitter. Due to the lower power available from this source and a much larger divergence observed in the transmitted beam, both the peak collected power and the translational range are reduced. However, the receiver was still able to collect an appreciable signal over 0.6 cm or only 0.2 cm less than the better aligned 1310 nm case. Therefore, the transmission link was able to retain some of the

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misalignment tolerant behavior it was designed for when the wavelength was changed and prior to adjustment of the optical systems. It is certainly feasible that the telescope of the transmitter could be designed to allow a more consistent beam size at the receiver for the different wavelengths, at least for some range of operating distance, to further reduce changes in the operating performance.

Figure 3: Normalized power collected vs. translation of transmitter

3.2 Angular Misalignment The second investigation examined the angular misalignment tolerance of the link as a function of wavelength. In this investigation, the transmitter was again moved along the rail in a direction perpendicular to the optical axis. At each position, the transmitter was rotated to point back toward the center of the receiver’s collecting lens array. The alignment of the transmitter was adjusted until the maximum average signal power was measured from the output of the photodetector. This modeled a beam striking the receiver with angular misalignment but little to zero translational misalignment of the beam center with respect to the center of the receiver. The process was repeated for each translation position until power was no longer collected by the receiver. The angular misalignment was calculated from simple geometry by θ = tan-1 (Δx / L), where Δx is the lateral translation of the transmitter and L is the distance between the transmitter and receiver along the optical axis of the receiver.

The results of the experiments are shown in Figure 4. For the reference 1310 nms wavelength, the angular misalignment tolerated by the receiver was slightly larger than two degrees. This is consistent with earlier investigations on the fiber bundle receiver.10-12 A notable aspect of the angular response is that the reduction in collected power is not monotonic, but occurs effectively in stages, with drops in power followed by plateaus. The same behavior is observed in the response for the 1550 nm and 850 nm wavelengths, though somewhat less pronounced. The collected power falls to 20% of the maximum at approximately 2.75 degrees for 1550 nm and 1.2 degrees for 850 nm. Thus, regardless of the transmitted power, the dependence of the collected power on the angular misalignment is similar for all of the wavelengths, even though the receiver has not been optimized for the other wavelengths.

Another approach used to evaluate the wavelength dependence of the link was to determine just how far the alignment of the receiver needed to be adjusted to recover the alignment and produce the largest possible collected power. For this experiment, the receiver was first optimized for the 850 nm wavelength, and the wavelength changed to 1310 nm and 1550 nm wavelengths. An initial reading of the collected power was made without adjusting the receiver optics, and a second reading was made after adjustments to the receiver optics were made. For 1310 nm, an OC-3 (155 Mb/s) SONET signal was used, and the average power improved from 90.8 mV to 114.8 mV after an adjustment of only 0.25 mm, with the eye height improving from 26 mV to 32 mV. For the 1550 nm experiment, a BERT output was not available, so the amplified output of an electrical-to-optical converter driven by a 1 MHz square wave from a function generator was used as the signal transmitted by the link. The average power improved from 0.8 V to 1.00 V after an

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adjustment of only 0.27 mm, and the peak-to-peak signal improved from 350 mV to 480 mV. Therefore, only small adjustments to the design or alignment of the receiver can reduce the wavelength dependence of the receiver even further.

Figure 4: Power collected vs. transmitter angle

4. OUTDOOR EXPERIMENTS 4.1 Test Procedure

For the outdoor experiments, the primary wavelength used was 1550 nm. The availability of an optical amplifier at this wavelength permitted ready extension of the operating distance needed for the outdoor experiments. The signal from the amplifier was passed through a 1 x 4 splitter, and three of the splitter outputs used to drive the three central fibers of the transmitting fiber array, as shown in Figure 5. The output convex lens of the transmitter telescope was changed to one with a focal length of 40 mm. Based on theoretical calculations, this choice of focal length would readily allow the beams generated by each of the fibers to overlap only slightly at the input plane of the receiver. This arrangement would allow a large coverage range without producing dead zones of no received signal from the receiver output.

Figure 5: Experimental Configuration for outdoor experiment

Amplifier

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For each experiment, the first step was to record data related to wind speed, temperature, and the sunlight level in the location of the experiment. Experiments were conducted under four primary conditions: high sun, high wind; high sun, low wind; low sun, high wind; and low sun, low wind. Experimental data was collected over multiple runs by the oscilloscope and a digital multimeter and then processed off-line.

For analysis purposes, three signal parameters were measured. For all experiments, the 1550 nm source was modulated by a 1 MHz square wave. The first parameter of interest for the recovered square wave was the maximum power recorded by the multimeter, referred to as Max Vpower. The second parameter was the peak-to-peak voltage of the square wave as measured on the oscilloscope, referred to as Max ΔV. The third parameter was the mean noise amplitude measured on the “1” or upper level of the square wave, referred to as Vnoise.

4.2 Experimental results

The first experimental runs were performed to adjust the divergence of the transmitted beams so that they just overlapped at the receiver. In this case, a constant (unmodulated) signal power was used and the detector output recorded as the receiver was translated by the motor. The results obtained for the final alignment conditions are shown in Figure 6. The measured response for a single output fiber is shown in Figure 6(a), and the response for three output fibers is shown in Figure 6(b). The response for the three-fiber case clearly indicated that the contributions of the three fibers overlap in space and that the collected power by the receiver is very closely equal from all three fibers. All further data reported here was for the case of three transmitting fibers.

Figure 6(a): Response for one transmitting fiber Figure 6(b): Response for two transmitting fibers

Sample signals collected from the fast oscilloscope are shown in Figure 7 for each of the four general cases for the operating environment. The related anaylsis data is shown in Figure 8. From Figures 7(a) and (b), when the sunlight is very high, the mean electrical noise appearing in the oscilloscope becomes larger, although by only a small amount compared to the low sunlight case. For instance, the mean amplitude of the noise is 6 mV and 4 mV respectively in the high and low sunlight cases as quoted in Figure 8. The higher temperature and the resultant turbulence is the most likely reason for the increase in electrical noise for the high sunlight case. It is encouraging, however, that the increase in the noise is relatively small, which suggests a strong ability of the receiver to resist changes in the signal due to increasing turbulence. Similarly, the magnitude of the electrical noise is observed on the oscilloscope to increase as the wind gets stronger, with a mean noise amplitude of 4 mV to 5 mV respectively for the low sunlight case, and a mean amplitude of 6 mV to 7 mV respectively in the high sunlight case as shown in Figure 8. The associated signals for these cases are shown in Figure 7(c) and (d), where the shapes of the signal in the oscilloscope are observed to have slightly greater noise signal with increasing wind speed. It is important to note that the increase in the noise is expected to have two components, these being the increased turbulence and the receiver vibration caused by the strong wind.

The impact of the electrical noise as an undesirable disturbance on the useful information of the signal is measured by the quantity Max ΔV. Therefore, the quality of the collected signal increases as the difference measured by Max ΔV is larger. Based on the data in Figure 8, Max ΔV decreases in value to either 32mV and 25mV as either the sunlight or the wind gets higher, while the value of Max ΔV is 50mv in the case of the low sunlight and low wind. From this data, the effects of the sunlight and wind on the quality of the collected signal are similar due to the similar results obtained for the Max ΔV decline under either the high sunlight or the high wind case. Hence, it is also deduced that the

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signal in the transmission will suffer more severely under the double effects of high sunlight and high wind. The data obtained for the case of high sunlight and high wind demonstrates this presumption, as the result for Max ΔV is only 1mV, which means that it is difficult to discern the high voltage level for a “1” from low voltage level for the a “0” order. This is easily observed from the signal collected by the oscilloscope, since the received signal is almost completely obscured even if the receiver is aligned to the transmitter.

It is important to note that the value of Max ΔV has a proportional relationship with the maximum power of the collected signal Max Vpower. As shown in Figure 8, the Max Vpower the receiver can collect is increasing as the Max ΔV is increasing under different turbulence conditions. The revelation of this finding is that the atmospheric turbulence has animpact not only on the quality of the collected signal but also on the power of the collected signal.

(a) (b)

(c) (d)

Figure 7: Typical recorded oscilloscope outputs for the four typical environmental cases. (a) High sunlight and high wind (5 mV/div, 500 ns/div). (b) High sunlight and low wind (10 mV/div, 500 ns/div). (c) Low sunlight and

low wind (50 mV/div, 500 ns/div). (d) Low sunlight and high wind (10 mV/div, 500 ns/div).

5. SUMMARY We have constructed a fiber-bundle based FSO receiver and investigated its performance as a function of transmission misalignment, turbulence, and weather as the transmitting wavelength is varied from 850 nm to 1310 nm and 1550 nm. The experimental results show the effects of wavelength on the misalignment tolerance performance of the link in an indoor environment, while impacting the range of misalignment tolerated somewhat, were not sufficiently large to disrupt the link, and the receiver retained the link for a reduced range of misalignment at all wavelengths without adjustments. Initial outdoor experiments were conducted using 1550nm under various lighting, wind, and turbulence conditions. The initial results show that environmental conditions has varying impact on the results, though the impact was tolerable for all but the most extreme conditions. Further development and evaluation of a more complete system based on these initial successes is underway.

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Max ΔV (mV)(VMAX-VMIN)

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Vnoise (mv)Max Vpower (mV)

strong sunshine and strong wind 1 7 11.1

strong sunshine and weak wind 32 6 70.9weak sunshine and weak wind 50 4 101.8weak sunshine and strong wind 25 5 39.3

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Figure 8: Collected signal analysis for different environmental conditions

REFERENCES

[1] Sevincer, A., M. Bilgi, M. Yuksel, N. Pala, 2010, “Prototyping multi-transceiver free-space-optical communication structures,” in IEEE International Conference on Communications, pp. 1-5 (2010).

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[4] Harris, A., J. J. Sluss, Jr., H. H. Refai, P. G. LoPresti, “Atmospheric turbulence effects on a wavelength diversified ground-to-UAV FSO link,” Proc. SPIE, vol. 6105 (2006).

[5] Harris, A., J. J. Sluss, Jr., H. H. Refai, P. G. LoPresti,“Free-space optical wavelength diversity scheme for fog mitigation in a ground-to-unmanned-aerial-vehicle communications link,” Optical Engineering, vol. 45, n. 8, p. 86001-1-12 (2006).

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[7] Zhou, D., P. G. LoPresti, N. Brooks, H. Refai, “Evaluation of free-space optical fiber bundle transmitter configurations for receiver tracking,” Proc. SPIE, vol. 7324, pp. 73240K-1 – 73240K-8 (2009).

[8] Hahn, D. V. , D. M. Brown, N. W. Rolander, J. E. Sluz, R. Venkat, “ Fiber optic bundle array wide field-of-view optical receiver for free space optical communications,” Optics Letters, vol. 35, no. 21, pp. 3559-3561 (2010).

[9] Zhou, D., P. G. LoPresti, H. H. Refai, “Enlargement of beam coverage in FSO mobile network, Journal of Lightwave Technology,” vol. 29, no. 10, pp. 1583-1589 (2011).

[10] LoPresti, P. G., D. Zhou, H. Refai, “Evaluation of the performance of a fiber-bundle-based optical wireless link,” Proc. SPIE, vol. 8038, pp. 80380I-1 – 80380I-7 (2011).

[11] Zhou, D., P. LoPresti, H. Refai, “Evaluation of fiber-bundle based transmitter configurations with alignment control algorithm for mobile FSO nodes,” IEEE Journal of Lightwave Technology, vol. 31, no. 2, p. 249 – 256 (2013).

[12] LoPresti, P. G., D. Zhou, Z. Shi, H. H. Refai, “Design simulation and analysis of a fiber-bundle based optical wireless link, Proc. SPIE vol. 8380, pp. 8380E-1-9 (2012).

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