Method for 10 Gbps near-ground quasi-static free-space laser transmission by nutation mutual coupling

Lei Zhang; Xiaonan Yu; Xiaonan Yu; Baiqiu Zhao; Tong Wang; Shoufeng Tong; Shoufeng Tong

doi:10.1364/OE.463051

1. Introduction

The field of free-space laser communication (FSO) has progressed steadily from proof-of-concept to commercialization phase. In comparison to microwave communication, FSO delivers a higher capacity and pace of communication, and it is not constrained by electromagnetic interference or spectrum. Compared to optical fiber communication, FSO offers flexible erection and mobile transmission to fulfill the wide-area dynamic coverage requirements of the 5G base station. Moreover, it provides an effective solution to realize high-speed data transmission between 5G base stations [1]. However, the FSO is limited by the rapid acquisition and alignment of communication light with a small beam divergence angle (µrad order) and an efficient coupling of the spatial light to the single-mode fiber. Nonetheless, an acquisition and tracking technology has been developed, and the technique often uses a coarse–fine composite tracking structure [2]. Specifically, low-bandwidth, large-execution-range actuators (such as single pendulum mirrors) have been employed for scanning a wide range and suppressing large-scale low-frequency jitters, whereas high-bandwidth, small-execution-range actuators (e.g., electromagnetic galvanometers) have been utilized to suppress small-amplitude high-frequency vibration [3]. The Artolink laser terminal of Russia's Mostcom company can achieve a link performance of 10 Gbps@1500 m, representing a high level of near-ground FSO [4]. However, it employs dual 90 mm aperture optical antennas to maximize the receiving efficiency.

Since Swanson proposed a solution for the latter based on fiber nutation [5], numerous researchers and research institutions have demonstrated that fiber nutation technology can significantly improve the coupling efficiency through analysis, demonstration, and practical exploration. Using a large aperture telescope as the receiving optical antenna, Ruilier et al. analyzed the coupling of space lasers into a single-mode fiber and studied the effect of beam distortion caused by turbulence [6]. Takenaka et al. established a mathematical model of the satellite-ground link based on atmospheric turbulence, and analyzed the effect of beam jitter and satellite platform jitter on the space laser-to-single-mode fiber coupling efficiency. The satellite-to-ground laser communication experiments were conducted using the OICETS satellite, and the results indicate that the fiber coupling loss of the satellite-to-ground downlink of laser communication is about 10-19 dB [7].

However, majority of the current research on fiber nutation technology focuses on the experimental assumption that the space beam is perfectly aligned with the receiving end face of the single-mode fiber, i.e., only the receiving field of view's coupling is completed. In practice, FSO is typically formed by a long-distance bidirectional point-to-point link or a point-to-multipoint communication network [8], including platform vibration, turbulence, and pointing errors. The nutation at the communication receiving end can only couple the space beam with the receiving fiber (receiving field of view), and fails to compensate for the energy loss caused by the beam pointing error at the transmission end [9–11].

This research proposes a method for near-ground quasistatic space laser communication based on two-way nutation mutual coupling. Specifically, acquisition tracking and nutation coupling were structurally realized using a single detector and actuator. In the two-way nutation, the receiver nutates to converge the receiving field of view and the transmitter nutates to further converge the beam point of the transmitter, thereby coinciding the transmitting optical axis and the receiving boresight. Furthermore, the peak value of the transmitted beam and the received mode field can be coincided for maximizing the efficiency of the communication link. The remainder of this article is arranged as follows: the nutation coupling algorithm model is presented in Chapter 2 with the derivation of the quantitative expression to improve the system receiving efficiency using the optical fiber coupling mathematical model. Thereafter, the architecture and operation of a near-ground space laser communication system based on nutation and mutual coupling is proposed in Chapter 3. In Chapter 4, the method of two-way nutation is discussed. The results of the indoor experiment and the 1 km field test were comparatively analyzed to verify the improvement in the overall system performance using the proposed method.

2. Mutual coupling mechanism and nutation algorithm

2.1 Efficiencies of system reception

After long-distance transmission, the Gaussian beam approximated into an ideal plane wave as it reached the receiving antenna port. The optical system converges the light field, which diffracts on the focal plane and distributes in an Airy disk pattern. Accordingly, the light intensity distribution on the focal plane can be expressed as follows:

(1)$${E_O}({{r_o}} )= \pi {R^2}\frac{{\exp ({jkf} )}}{{j\lambda f}}\exp \left( {jk\frac{{r_o^2}}{{2f}}} \right)\left[ {2{J_1}\left( {\frac{{2\pi R{r_o}}}{{\lambda f}}} \right)/\frac{{2\pi R{r_o}}}{{\lambda f}}} \right], $$

where $\lambda $ denotes the wavelength, $2R$ represents the coupling lens’ aperture stop, f denotes the coupling lens’ focal length, k indicates the wavenumber, and ${J_1}$ represents the first-order Bessel function [12].

The principle of the fiber coupling is illustrated in Fig. 1. As the single-mode fiber's boundary conditions satisfies only the transmission conditions of the light field's fundamental mode [11,13], the process of coupling the plane wave into the single-mode fiber can be understood by placing the spatial filter ${F_O}$ on the focal plane O, and thereafter, spatially filtering the focused light field ${E_O}$[14]. Although the fundamental mode field distribution ${F_O}({{r_o}} )$ of a single-mode fiber constituted a zero-order Bessel function, it was usually approximated using a Gaussian distribution to facilitate analysis and calculation:

(2)$${F_O}({{r_o}} )= \sqrt {\frac{2}{{\pi \omega _o^2}}} \exp \left( { - \frac{{r_o^2}}{{\omega_o^2}}} \right), $$

where ${\omega _o}$ denotes the mode field radius of the single-mode fiber.

Fig. 1. Schematic of fiber coupling.

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The coupling efficiency $\eta $ of the space light with a single-mode fiber can be defined as follows:

(3)$$\eta = \frac{{{{\left|{\int\!\!\!\int {E_O^\ast ({{r_o}} ){F_O}({{r_o}} )ds} } \right|}^2}}}{{{{\int\!\!\!\int {|{{E_O}({{r_o}} )} |} }^2}ds \cdot {{\int\!\!\!\int {|{{F_O}({{r_o}} )} |} }^2}ds}}. $$

This represents the ratio of the optical power entering the single-mode fiber with respect to the optical power received by the coupling lens [15]. In the expression, $E_O^\ast ({{r_o}} )$ and ${E_O}({{r_o}} )$ form complex conjugates of each other, which were integrated over the entire focal plane. Comprehensively, the coupling efficiency is a critical parameter for evaluating the performance of a coupled system. Accordingly, the calculation of the correlation between the received light field ${E_O}({{r_o}} )$ and the fundamental mode transmission mode field ${F_O}({{r_o}} )$ forms its essence. Thus, a strong correlation between ${E_O}({{r_o}} )$ and ${F_O}({{r_o}} )$ indicates a greater coupling efficiency; conversely, a weaker correlation signifies a lower coupling efficiency.

According to Parseval's theorem, the coupling efficiency calculated at the focal plane O is identical to the coupling efficiency computed at the entrance pupil plane A. More specifically, the light field at the entrance pupil can be expressed as a plane wave, which simplifies the coupling efficiency analysis. At the entrance pupil, the coupling efficiency can be expressed as follows:

(4)$$\eta = \frac{{{{\left|{\int\!\!\!\int {E_A^\ast ({{r_a}} ){F_A}({{r_a}} )ds} } \right|}^2}}}{{{{\int\!\!\!\int {|{{E_A}({{r_a}} )} |} }^2}ds \cdot {{\int\!\!\!\int {|{{F_A}({{r_a}} )} |} }^2}ds}}. $$

All the above-mentioned expressions define the space-light coupling efficiency with respect to the fiber under ideal conditions. However, various deviations between the central axis of the receiving optical system and the incident light are unavoidable in practical applications. In case the incident beam deviates from the optical axis of the coupling lens, the optical field at the focal plane of the coupling lens is displaced radially away from the central optical axis, thereby affecting the transmission mode field of the single-mode fiber [16,17]. The analysis of coupling efficiency was performed herein.

As the incident beam intersects the receiving optical axis at an included angle ${\theta _b}$, the light field distribution at the entrance pupil follows

(5)$${E_A}({{r_a}} )= {e^{ - ik{r_a}\sin {\theta _b}}}P({{r_a}} ). $$

This formula can be converted to a Cartesian coordinate system based on the following relation:

(6)$${E_A}({{x_a},{y_a}} )= {e^{ - ik({{x_a}\sin {\theta_{bx}} + {y_a}\cos {\theta_{by}}} )}}P({{x_a},{y_a}} ), $$

where ${x_a}$ denotes the component of ${r_a}$ in the ${X_a}$ direction, ${y_a}$ denotes the component of ${r_a}$ in the ${Y_a}$ direction, ${\theta _{bx}}$ indicates the angle between the incident light and ${X_a}Z$ plane, and ${\theta _{by}}$ denotes the angle between the incident light and ${Y_a}Z$ plane.

According to the diffraction theory, the light field distribution at the focal plane of the coupling lens can be defined based on the Fraunhofer diffraction of the light field at the entrance pupil:

(7)$$\begin{array}{l} {E_o}({{x_o},{y_o},{\theta_{bx}},{\theta_{by}}} )={-} \frac{i}{{\lambda f}}{e^{ikf}}{e^{ik\frac{{x_o^2 + y_o^2}}{{2f}}}}\int\!\!\!\int {{E_A}} ({{x_a},{y_a},{\theta_{bx}},{\theta_{by}}} ){e^{ - ik\frac{{{x_o}{x_a} + {y_o}{y_a}}}{f}}}d{x_a}d{y_a}\\ ={-} \frac{i}{{\lambda f}}{e^{ikf}}{e^{ik\frac{{x_o^2 + y_o^2}}{{2f}}}}\int\!\!\!\int P ({{x_a},{y_a}} ){e^{ - ik\frac{{{x_a}({{x_a} + f\sin {\theta_{bx}}} )+ {y_a}({{y_a} + f\cos {\theta_{by}}} )}}{f}}}d{x_a}d{y_a} \end{array}. $$

The coupling efficiency of the space light with respect to the single-mode fiber can be defined using an angle ${\theta _b}$ between the incident beam and optical axis as follows:

(8)$${\eta _{{\theta _b}}} = \frac{{{{\left|{\int\!\!\!\int {E_O^\ast ({{x_o},{y_o},{\theta_{bx}},{\theta_{by}}} ){F_O}({{x_o},{y_o}} )ds} } \right|}^2}}}{{{{\int\!\!\!\int {|{{E_O}({{x_o},{y_o},{\theta_{bx}},{\theta_{by}}} )} |} }^2}ds \cdot {{\int\!\!\!\int {|{{F_O}({{x_o},{y_o}} )} |} }^2}ds}}. $$

As the coupling efficiencies at the focal plane and entrance pupil are equal, that at the entrance pupil can be calculated as follows to simplify the derivation:

(9)$$\begin{aligned} {\eta _{{\theta _b}}} &= \frac{{{{\left|{\int\!\!\!\int {E_A^\ast ({{x_a},{y_a},{\theta_{bx}},{\theta_{by}}} ){F_A}({{x_a},{y_a}} )ds} } \right|}^2}}}{{{{\int\!\!\!\int {|{{E_A}({{x_a},{y_a},{\theta_{bx}},{\theta_{by}}} )} |} }^2}ds \cdot {{\int\!\!\!\int {|{{F_A}({{x_a},{y_a}} )} |} }^2}ds}}\\ & = \frac{{{{\left|{\int_0^{2\pi } {\int_0^R {\sqrt {\frac{2}{{\pi \omega_a^2}}} \exp ({ - ik{r_a}\sin {\theta_b}} )\exp \left( { - \frac{{r_a^2}}{{\omega_a^2}}} \right)} {r_a}d{r_a}d\theta } } \right|}^2}}}{{\pi {R^2}}} \end{aligned}. $$

The receiving loss of the bidirectional space laser communication link is dependent on the incident light angle as well as the light field distribution at the entrance pupil. According to the Gaussian beam characteristics, the light-field distribution at the entrance pupil attained its maximum value ${E_A}(0 )$ at the center point. In case the fiber coupling efficiency satisfied the system performance boundary, ${\theta _b} = 0^\circ $, the offset ${r_a}$ of the optical field center with respect to the receiving optical axis emerges as the determining factor influencing the efficiency of the receiving system.

2.2 Nutation coupling algorithm

According to the mode field matching theory, the coupling efficiency decreased if the focus of the incident light was not perfectly aligned with the fiber. During the nutation process, the receiving end face of the fiber scanned the focal plane of the incident light, and the coupling efficiency remained stable if the focus was centered on the fiber [18–21].

After passing through the optical system at the receiving end, the space light converged at the end face of the fiber, and subsequently, it coupled into the single-mode fiber. In addition, the received optical power was detected by the access optical module, which served as the feedback unit’s input to complete the energy closed loop. The basic concept of the nutation coupling is illustrated in Fig. 2. At the receiving end of the fiber, the light spot center was used as the origin to establish a Cartesian coordinate system, and two sinusoidal signals with a phase difference of 90° were independently used to drive the azimuth and pitch actuators. The nutation center is denoted as O, the radius of nutation by R, the convergence step size is denoted as ${K_p}$, and the number of sampling points in a nutation cycle is denoted by N. At this instant, the motion of the light spot can be stated as follows:

(10)$$\left\{ {\begin{array}{l} {x = {X_O} + R\cos \left( {2\pi \frac{i}{N}} \right)}\\ {y = {Y_O} + R\sin \left( {2\pi \frac{i}{N}} \right)} \end{array}} \right.. $$

In the above expression, $({{X_O},{Y_O}} )$ denotes the initial nutation center, and $i$ denotes the i-th sampling point in a single cycle.

Fig. 2. Principle of nutation coupling algorithm. (a) Convergence by circular scan. (b) Light beam forms an Airy disk on fiber’s end face.

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The received power at N points were compared during a nutation cycle to determine the polar coordinates of the energy distribution. In particular, the nutation convergence direction was determined using the polar coordinates corresponding to the maximum power point. Thereafter, the nutation center trajected in this direction and initiated the subsequent cycle of the circular scanning. After a single iteration, the scanning trajectory can be expressed as follows:

(11)$$\left\{ {\begin{array}{l} {x = {X_O} + {K_p}\cos (2\pi \frac{{{i_{\max }}}}{N}) + R\cos \left( {2\pi \frac{i}{N}} \right)}\\ {y = {Y_O} + {K_p}\sin (2\pi \frac{{{i_{\max }}}}{N}) + R\sin \left( {2\pi \frac{i}{N}} \right)} \end{array}} \right., $$

where ${i_{\max }}$ denotes the index of the energy maximum point of a single cycle in the formula. The execution principle is illustrated in Fig. 2.

Moreover, we continued the iteration to obtain the execution function for nutation coupling:

(12)$$\left\{ {\begin{array}{l} {x = {X_O} + \sum\limits_{j = 1}^M {\left[ {{K_p}\cos (2\pi \frac{{i_{\max }^{j - 1}}}{N})} \right] + R\cos \left( {2\pi \frac{{{i^j}}}{N}} \right)} }\\ {y = {Y_O} + \sum\limits_{j = 1}^M {\left[ {{K_p}\sin (2\pi \frac{{i_{\max }^{j - 1}}}{N})} \right]} + R\sin \left( {2\pi \frac{{{i^j}}}{N}} \right)} \end{array}} \right., $$

where j denotes the number of nutation cycles.

After a finite number of iterations, it eventually converged to the maximum received optical power.

3. Configuration for near-ground space laser communication

In this study, we created a two-way space laser communication system with a completely symmetrical double-ended structure to test the performance of the nutation mutual coupling approach. The structure of one end is depicted in Fig. 3, comprising three modules: an optical–mechanical structure module, an acquisition tracking and nutation control module, and a communication transmitting and receiving module. The optical–mechanical construction employed an optical fiber circulator to segregate the communication transmission and reception phases. In addition, it used a transmission-type converging optical channel with a shared aperture for both transmission and reception. The optical channel housed beacon light of 850 nm and communication light of 1550 nm. As shown in Fig. 3, the space laser communication system is enhanced for applying the two-way nutation approach in near-ground space laser communication. The baseband information, such as the received optical power and off-target amount of the laser communication terminal, is modulated into the beacon light that enables the transmitter to receive the power feedback without using radio frequency. Thus, the nutation closed-loop of the bidirectional laser communication link is realized. The divergence angle of the beacon and communications light were 3 mrad and 150 µrad, and their respective receiving field of view was 1.5° and 150 µrad. The modulation information of the beacon light and the position information of the light spot were solved by the analog signal acquired from a position sensitive detector (PSD) with a target surface size of 10 mm × 10 mm.

Fig. 3. Near-ground space laser communication terminal structure.

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The experimental system core involved an acquisition tracking and nutation control module, which included a PSD, a master control board, and an actuator to form a closed-loop system. The actuator was composed of two linear stepper actuators that were mutually orthogonal. Through convergence and wavelength splitting, the beacon light was shined on the PSD's surface, and the PSD's photoelectric effect turned it into four analog currents, which were sampled using the ADC and the input to the master control board's signal processing unit at a sampling rate of 200 ksps. In particular, the signal was segmented into two components for processing. First, the high-frequency component of the handshake information exchange was demodulated, and thereafter, the background light-suppression algorithm and low-pass filtering were utilized to complete the spot location solution. Upon receiving the feedback from these two pieces of information and communication, the integrated control unit generated driving pulses and completed the acquisition tracking and nutation coupling via the stepping actuator's reaction. Overall, only a single detector and execution structure were required to complete the operation.

The communication transmission and receiving module adopted the OOK communication system, and specifically, the transmitting section used the FPGA GTX interface as an external data source interface. Furthermore, it activated the high-speed transceiver core to generate a 10 Gbps pseudorandom code, which was amplified and transmitted through EDFA after modulation. Subsequently, the receiving section recovered the baseband signal with CDR, detected the communication bit error rate in real time, and yielded a detection sensitivity of -20 dBm@10Gbps. Simultaneously, the received optical power was reverted to the main control board to complete the energy loop. The actual test scene is shown in Fig. 4.

Fig. 4. Field 1 km two-way nutation mutual coupling test.

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4. Nutation mutual coupling test results

4.1 Single-ended nutation coupling

In this study, we utilized a collimator of 10 m focal length to establish a desktop experimental system for the initial evaluation of the nutation coupling algorithm. The communication optical power facilely converged for the complete peak power correspondence using nutation coupling, as depicted in Fig. 5.

Fig. 5. Desktop test results of nutation coupling algorithm. (a) Received optical power curve. (b) Nutation coupled polar curve.

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The entire process of the nutation convergence can be visualized by adjusting the polar coordinates, as illustrated in Fig. 5(b), where each curve represents a complete nutation cycle and the polar coordinate angle corresponding to the peak of the curve represents the direction of convergence. If the polar coordinate curve is proximate to the water quality, it implies that the optical fiber mode field at the receiving end has completed the most suitable match with the current optical field.

To simplify the description, the receiving and transmitting ends are denoted by A and B, respectively. However, the bidirectionality of the experiment’s communication channel is retained despite using a transmitter and receiver for analytical convenience.

The transmission power was set as 15 dBm in the field test involving the 1 km link to reduce the acquisition period and enhance the acquisition probability. In addition, the skip-raster helical scanning algorithm was utilized to execute the communication light acquisition after the beacon light has been tracked by both ends, and the nutation coupling is activated. Through nutation convergence, the peak power estimated as approximately 100 µW. More importantly, the nutation coupling satisfied the peak correspondence at point A, as illustrated in Fig. 6. Combining the link distance of 1 km, divergence angle of 150 µrad communication light, and receiving aperture of 50 mm, the spatial geometric loss can be evaluated as -9 dB (ratio of the spot area at receiving port to the area of receiving primary mirror). Additionally, the insertion loss and SFP optical power read error generate a total loss of -4 dB. Furthermore, power losses are generated by optical transmittance and fiber circulators. The transmittance of the optical antenna of the system is 80%, and the total loss from optical transmission and reception efficiency at both ends is ${\eta _t} + {\eta _r} = 10\lg (0.8 \times 0.8) \approx{-} 1.9dB$, where ${\eta _t},{\eta _r}$ are the losses of the transmitting and receiving optical antennas, respectively. To achieve the transmit-receive isolation under the condition that transmit and receive co-axial, we use a multimode-mode fiber circulator. Because the circulator fiber interface is an SC interface, a single-mode fiber patch cord is used at one end to connect it to the SFP. A theoretical power loss of approximately 10 dB is generated from multimode to single-mode fiber; however, the proposed fiber circulator uses graded index fiber. The experiments indicate that the graded index fiber induces beam-shrinking in the laser, and the loss from the multimode-mode fiber circulator to single-mode jumper is approximately -4 dB. Therefore, the loss from the entire laser communication system is approximately -19 dB. If the power is transmitted at 15dBm, the optical power at the receiving end should be -4dBm. The spatial geometric loss of -9 dB is simultaneously calculated based on the uniform distribution model. Considering that the optical field is approximately a Gaussian distribution, the actual received optical power should be about 3 dB higher than -4dBm, i.e., -1dBm.

Fig. 6. Single-ended nutation coupling curve.

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At this instant, the peak power can approach 400 µW based on the stated conditions, assuming the uniform distribution of the spot. Accordingly, a higher peak power should be obtained for the actual Gaussian beam.

According to the theoretical analysis presented in Section 2, as the fiber coupling efficiency achieved the system performance boundary for the bidirectional space laser communication link, the offset between the receiving optical axis and the optical field center at the entrance pupil emerges as the critical factor influencing the receiving efficiency of the system. However, the alignment of the communication receiving field of view, i.e., the coupling of space light to the receiving fiber, was completed after the nutation of end A to the limit. The peak of the Gaussian beam did not enter the receiving interface, and the unilateral nutation at the receiving end could converge to yield the highest communication optical power in a restricted range, because the sending direction and receiving field of vision were not completely coaxial.

To validate this analysis, we performed a full-field skip-raster helical scan and determined the light field distribution of the receiving port at various pointing angles. The light-field distribution for the deviation in the B-end pointing is illustrated in Fig. 7(a), and that for the direction of end B coaxially corresponding to the receiving field of view of end A is projected in Fig. 7(b). As observed, the transmission direction significantly affected the received peak power.

Fig. 7. Light field distribution on focal plane of coupling lens. (a) Pointing error. (b) Most suitable light-field distribution across entire field of view.

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4.2 Two-way nutation mutual coupling

In particular, a lower energy threshold is used in long-distance link conditions to increase the probability of acquisition. Thus, even if the primary lobe of the communication beam fails to completely cover the receiving surface, the energy from the side lobes can trigger the acquisition threshold. Thus, two-way nutation mutual coupling can be used to effectively resolve this limitation.

The entire process of two-way nutation is described as follows. First, the two ends are positioned using the optical sight such that the beacon light directly enters the PSD receiving field of view. Second, the double-end tracking is controlled for calibrating the PSD coordinate origin, and small-range skip-scanning is implemented on the communication beam; this step is called acquisition. Third, the process of nutation coupling commences, in which the transmitter keeps the tracking coordinates unchanged and the receiver implements nutation coupling to converge the receiving field of view. The receiving end maintains a staring state after completing its nutation, and the transmitting end implements nutation convergence to the transmitting direction until the optical power of the receiving end attains maxima. Hence, the process of two-way nutation coupling is completed, and the communication link is converged to the best state.

In terms of time sequences, the two-way nutation of the two communication terminals is not implemented simultaneously. In fiber nutation, the iteration of the nutation center is implemented by recovering the power peaks from the preceding scan cycle. In two-way nutation, the receiving end consists of a circular scan of the fiber's receiving field of vision, whereas the transmitting end consists of a circular scan of the transmitted beam relative to the receiving end. The single-ended nutation must be based on the assumption that the other end is static, such that the power distribution at the iteration moment of the nutation center corresponds to that recorded in the array. If both terminals are nutated simultaneously, the timing sequence is distorted, thereby causing the divergence of two-way nutation or severance of link.

The two-way nutation energy curve is plotted in Fig. 8(a). After approximately 10 nutation cycles, the power peak was achieved at point B, which was consistent with the results portrayed in Fig. 6. At this point, the activation of B-side nutation converged the communication transmitting optical axis on the communication receiving optical axis, thereby resulting in a peak optical power of 600 µW at point C. Compared to that at point A, the received power was increased by approximately 8 dB, and at this instant, the light-field distribution on the end face of the receiving fiber corresponded to that in Fig. 7(b).

Fig. 8. Two-way nutation mutual coupling results. (a) Point A corresponds to nutation convergence peak of receiving end, whereas point B corresponds to optimal correspondence of optical field in bidirectional nutation and receiving mode field of fiber. (b) Actual link status of each node.

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Figure 8(b) shows the link status corresponding to each stage of the two-way nutation procedure. The first step is acquisition, in which neither the transmitter points nor the receiver’s field of view converges. The second step is the nutation of the receiving end, in which the direction of the transmitted beam does not change and the receiving field of view converges to the optimal angle. The third step is the nutation of the transmitting end, in which the direction of the transmitting beam gradually converges to the center of the receiving field of view. The center of the optical field is aligned with the receiving optical antenna, and the link attains an optimum state.

The aforementioned fiber nutation methods are unidirectional at the receiving end. The one-way nutation of the receiving end can only correct the receiving boresight; however, there is a deviation between the transmitting optical axis and the receiving boresight. The maximum optical power coupled into the fiber depends on the off-axis amounts corresponding to the Airy disk at the end face of the receiving fiber and the center of the transmitted optical field relative to the central axis of the receiving field of view. In the two-way nutation, the nutation of the receiver converges the receiving field of view and that of the transmitter further converges the beam point of the transmitter, thereby coinciding the transmitting optical axis and the receiving boresight. Using two-way nutation, the peak value of the transmitted beam and the received mode field can be coincided for maximizing the efficiency of the communication link.

Furthermore, the two-way nutation method optimizes the acquisition time. Experiments involving distances of 500 m and 1 km have been performed multiple times, and the testing is done at every 50 m from 10 m to 500 m. The resulting acquisition time is less than 2s.

4.3 10 Gbps laser transmission

We obtained the light intensity scintillation distribution for the 1 km laser link by continuously observing the fluctuation of the received optical power over a period. Figure 9 shows that the optical power is concentrated between 25 uW and 100 uW due to the impact of atmospheric turbulence disturbance, and the scintillation index is 0.07.

Fig. 9. Flicker variance distribution of 10 Gbps laser links.

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We compared the communication quality of single-ended nutation with two-way nutation for the above link conditions, using communication bit error rate and video transmission tests. In the bit error rate test, the high-speed transceiver generates a 10 Gbps pseudo-random code. The receiving end uses IBERT to detect the communication data. The communication link information for single-ended nutation is shown in Fig. 10. The transmission rate is 10.3125 Gbps, with a bit error rate of 5.395E-6. However, video transmission is frequently interrupted by the atmospheric turbulence disturbances. This indicates that, while the communication bit error rate has met the FSO standard, the link state is near the detection threshold.

Fig. 10. Communication bit error rate and digital eye diagram for single-ended nutation.

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The link bit error rate significantly reduces following two-way nutation. As illustrated in Fig. 11, the laser link's average bit error rate is 2.478E-9. By comparing the digital eye diagrams for the two scenarios. two-way nutation produces a substantially higher-quality communication signal than single-ended nutation, and the video signal can be sent reliably. As illustrated in Fig. 4, the B end's control interface is transmitted through the laser link and displayed on the A end's screen.

Fig. 11. Communication bit error rate and digital eye diagram for two-way nutation.

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5. Conclusion

Through two-way convergence, the receiving efficiency of the laser communication system was effectively improved with the a simple structure and control method. With a 50 mm aperture, 2.478E-9 bit error rate for 10 Gbps laser transmission at a distance of 1 km is achieved. According to theoretical analysis, the nutation coupling at the receiving end could complete the coupling of the space light to the optical fiber, while the nutation at the transmission end converged the emitted light axis with the center of the receiving field of view. As verified from the 1 km field test, the two-way nutation mutual coupling increased the communication link margin by approximately 8 dB in comparison to single-ended nutation tracking. Using the bidirectional nutation method, the efficiency of the communication system was maximized, which was crucial for enhancing the reliability and stability of near-ground space laser communication system. The introduction of beacon communication facilitated the establishment of a link without using radio frequency. Furthermore, two orthogonal stepper actuators were used to replace the traditional nutation actuator FSM. The cost of actuators is approximately one-tenth of that of FSM and only a single stage of actuators is required for complete capture tracking and nutation coupling, which greatly simplifies the structure of the laser terminal. This has great significance in the large-scale production and usage of light, small, and high-speed space laser communication terminals.

Funding

Education Department of Jilin Province (JJKH20220745KJ); Department of Science and Technology of Jilin Province (QT202102).

Disclosures

The authors declare no conflicts of interest.

Data availability

At present, the data underlying the results presented in this study are not publicly available, but may be obtained from the authors upon reasonable request.

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Method for 10 Gbps near-ground quasi-static free-space laser transmission by nutation mutual coupling

Abstract

1. Introduction

2. Mutual coupling mechanism and nutation algorithm

2.1 Efficiencies of system reception

2.2 Nutation coupling algorithm

3. Configuration for near-ground space laser communication

4. Nutation mutual coupling test results

4.1 Single-ended nutation coupling

4.2 Two-way nutation mutual coupling

4.3 10 Gbps laser transmission

5. Conclusion

Funding

Disclosures

Data availability

References

Data availability

Cited By

Figures (11)

Equations (12)

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