3.1.

Structure of the Receiver and Tests of Compliance with SKA-low Specifications

The analogue receiver is divided into three main parts: the RF/optical transmitter, called front end module (FEM), the optical fiber, and the optical/RF receiver, called preanalogue to digital unit (PREADU). The RFoF link is completely embedded in the RF analogue receiver chain since (see Fig. 5) its active components, the WDM laser sources and the WDM double PD, are parts of the FEM and PREADU circuits, respectively.

Fig. 5

SKA-low receiver chain.

Two FEM installation options have been investigated and tested in the field thanks to the demonstrators. For AAVS1 [see Fig. 6(a)], FEMs are located within the antennas apex, just after the low-noise amplifier (LNA) and connected by a hybrid fiber-optic and copper power cable to a central antenna power interface unit (APIU). For AAVS2 [see Fig. 6(b)], the FEMs for a group of antennas are installed inside a box called small modular aggregation and RFoF trunk box (SMARTbox) and their connection with the antennas is by short ($\sim 10\mathrm{m}$) coaxial cables. The current design with SMARTbox for SKA1, implemented and tested with AAVS2, offers advantages for maintainability and accessibility with respect to the original concept adopted in AAVS1.

Fig. 6

FEM installation options: at the SKALA2 antenna apex for (a) AAVS1 and grouped by 16 inside the SMART boxes for (b) AAVS2.

The FEM contains a high-pass filter, with a 3-dB cut-off frequency around 35 MHz, which helps to reject strong radio frequency interferers (RFI) located within its stop band, the in-line bias-T to feed the LNA power, an RF amplifier, and the laser with its automatic optical power control circuit. As depicted in the general scheme of Fig. 5, the 2 polarization signals directly modulate the laser currents operating at 1270 and 1330 nm, which are multiplexed onto a single optical fiber for each antenna and then routed from the field node to the remote acquisition systems. Figure 7 allows to appreciate the actual realization of the FEM and of the PREADU.

Fig. 7

FEM PCB and its enclosure on the left and a portion of a PREADU board with the metallic shield removed on the right.

At the processing facility, the PREADU board converts the optical signal back to RF using a WDM double PIN photodiode receiver, which separates the two RF polarizations, performs low-pass filtering for antialiasing, and provides further amplification prior to the ADC. A digital step-attenuator (DSA) and a final amplification stage set the correct levels to the acquisition system (see again Fig. 7 on the right).

In the current design, 32 signal RF chains will be assembled in two “receiver module” (PREADU) each carrying 16 channels, the two RF polarizations of 8 antennas [see Fig. 8(a)]. Receiver modules are mounted on the signal processing system processing boards (ADU board), containing the ADCs and the digital components. One ADU board and two PREADU boards compose a tile processing module (TPM), which is shown in Fig. 8(b). Each station includes 16 TPM modules and supporting systems, hosted in a temperature controlled and screened facility: the CPF, for all the core stations, or the RPFs, for stations in the array arms, to limit maximum fiber length.

Fig. 8

(a) One PREADU board and (b) an assembled TPM where two PREADU boards are connected to an ADU board.

From the analysis point of view, the derivation of the main receiver specifications from SKA’s level 2 (L2) requirements was found to be a crucial aspect. Some boundary conditions have been assumed, such as the performances of the antenna/LNA, the main ADC specs (i.e., number of bits, Vpp,…), and the RFI measured at the MRO site. With regards to the data acquisition part and to the evaluation of the optimal RF level at the input of the sampler delivered from the analogue receiver, the analysis carried out also considers the correlation systems that exist in the digital section, in particular, the requirements for:

The conclusions of the analysis are the SKA-low receiver requirements reported in Table 2, where only the ambient (air) temperature range at MRO has been specified as $-10°\mathrm{C}$ to $+50°\mathrm{C}$. Note that the real operative temperature of the electronics composing the receiver chain and, in particular, of the FEM, strongly depends on its location. Measurements carried on with SKALA2/AAVS1 system showed that the maximum operative temperature experienced by the FEM is about $+70°\mathrm{C}$, similar situation with the first release of the AAVS2 SMART boxes. In both cases, the actual temperature reached by the FEM printed circuit board (PCB) is even in the excess of $+80°\mathrm{C}$. Such high temperatures cannot be tolerated, both for the detrimental effects on the behavior of the electronics and for the expected reduced lifetime of the devices, especially the laser sources. For that reason, a particular attention has been devoted to optimize the thermal aspects of the FEM installation, which has led to the last SMARTbox design where the maximum operative temperature has been reduced to $+65°\mathrm{C}$ (see Fig. 9).

Table 2

SKA-low receiver specifications (second column) and the correspondent FAT requirements (third column).

Fig. 9

(a) Thermal simulation of the first and (b) the optimized AAVS2 SMARTbox model. The following boundary conditions have been considered: external temperature: 42°C, solar thermal flux: 2019/12/10, 02:30 pm, latitude: $-26°$ 42′, longitude: 116° 40′. Temperature scale is from $+50°\mathrm{C}$ to $+80°\mathrm{C}$.

Considering the large numbers of receiving chains (Fig. 10) and the impracticality to test all the electronics for the whole set of the specifications (for the full range of FEM temperatures and different lengths of fiber), the correspondent factory acceptance test (FAT) performed at ambient temperature and FEM-PREADU direct connection has been derived.

Fig. 10

Pictures taken during the AAVS1 FEM and PREADU productions.

Below are reported both some test examples of receiver specs verifications (see Fig. 11) and an example of the results of the FAT measurements that were performed for the entire production for AAVS1, EDA2, and AAVS2 (see Figs. 12 and 13).

Fig. 11

Examples of receiver verification measurements versus FEM temperature: (a) noise figure (NF) at 160 MHz (the measurements include 6 km of SMF fiber between FEM and PREADU at ambient temperature) and (b) FEM power consumption. The causes which lead to the higher NF in the 1270 nm link are discussed in Sec. 3.2.1.

Fig. 12

Measurement setup to characterize the SKA-low receiver versus the FEM operative temperature. The Keysight PNA-X has been programmed in order to measure not only the four ports $S$-parameters but also the NF, the 1-dB compression point and the second- and third-order intercept points. A MATLAB code has been written in order to control all instrumentation, acquire, and process the data.

Fig. 13

(a)–(f) Example of FAT measurements of one FEM produced for AAVS2. The measurements are performed automatically with the setup as shown in Fig. 12 but with the FEM at ambient temperature and without any fiber reel between FEM and PREADU.

3.2.

Gain and Phase Response of the Receiver Under Temperature Variations

Science data processing and analysis techniques require the signal chains to be stable across all of the data samples collected in an observation so that resulting variations in the data output can be attributed to the observed sky and not to the measurement system. Calibration of the signal chains is the adjustment of the signal chains to standardize and hopefully optimize their behavior at a point in time, and the system is required to maintain this behavior between reference measurements, optimally for long periods to ensure that data are consistent and to minimize the overhead in capturing and processing calibration data.

Practical calibration strategies basically compare the response of all signal chains in a system to a common reference signal and adjust or offset their actual complex transfer functions to achieve a desired, or at least similar, gain and phase behavior at all frequencies across the band. Such reference measurements can only be done when suitable input signals are available. Because of the large number of independent signal inputs, local reference signal generation was deemed too expensive to implement at the required level of accuracy. However, the sky itself presents a common input signal to all antennas that can be used as the calibration reference with appropriate processing. Decomposing the transfer function into terms related to individual driving functions allows basing these adjustments on a combination of end-to-end measurements and modeled effects, reducing the dependence on real-time sky-based measurements, as well as allowing statistical analysis to improve the long-term behavior of the system.

The period over which the system must remain stable depends on both the science requirements for precision and the time scales of both the physical environmental variations and measurement techniques. Several different time scales emerge from these considerations: typical environmental effects show periodicity of 12 and 24 h, local thermal time constants of electronic components are typically of order minutes, and on-sky measurements can be affected by ionospheric effects that suggest recalculation epochs of order 10 min (600 s). A further complication results from the wide field of view of individual antennas, which effectively see the entire sky hemisphere and are thus dominated by large-scale features such as the galaxy and Sun but cannot resolve individual point sources. This means that in practice, direct interferometric calibration of individual antennas cannot be done to a sufficient level of accuracy if neither the galaxy or the Sun is visible, limiting the time periods when such measurements can be made. The implications of these timescales are profound, as it is clear that a parameter that varies on 24-h timescales need not be recalculated frequently, while those that vary at the level of minutes cannot be corrected by measurements on a 10-min cadence. Therefore, a combination of calibration techniques including direct but infrequent on-sky measurement of signal chain gains with forward modeling of locally faster variations will be necessary to achieve the SKA requirements.

Analysis of the signal chain confirms that the dominant effect causing changes in the signal chain transfer functions (phase and gain) is driven by the environmental temperature of the components. The SKA system architecture locates most of these electronic components in the relatively stable environment of the processing facilities thus limiting this issue to the LNA, the FEM, and the optical fiber itself. In the following sections, the contributions of the FEM and optical fiber will be shown. The influence of the temperature variation on the LNA will not be considered as it is not part of the receiver. However, it has been verified that its contribution is not significant with the respect to the ones of the other components.

3.2.1.

Amplitude and phase stability due to the FEM temperature only

To investigate the impact of the only temperature exposition of the FEM on overall stability, a characterization has been performed in the laboratory using a climatic chamber, where the temperature was varied from $-10°\mathrm{C}$ to $+80°\mathrm{C}$.

Figure 14 represents a measurement of a sample of six FEMs taken from the AAVS2 production, showing the variation of the absolute gain of both 1270- and 1330-nm channels. The curves decrease monotonically with temperature and justify, as shown in the previous section, why it was necessary to reduce as much as possible the FEM maximum operative temperature to $+65°\mathrm{C}$ during the day. From the same figure, it is also possible to gather that by setting the correct value of the DSA, it is possible to obtain the gain requirement (41 dB) over the entire temperature range. The gain drop of the 1270-nm channel has been found to always be worse than the 1330-nm one for all measured devices also from different suppliers. The reason lies in the fact that the 1270-nm optical carrier is at 90 deg with respect to the fiber optic and has to be reflected by the WDM thin film filter and not be transmitted as for the 1330-nm carrier, making it more sensitive to mechanical stress on the package assembly due to temperature variations. A study to reduce such difference between the two channels is undergoing by the suppliers of the optical components and new samples will be tested in the coming months.

Fig. 14

RF gain at 160 MHz versus FEM operative temperature (with 6 km of G.652.D fiber and PREADU, with DSA set at 0 dB both at ambient temperature).

Regarding the phase stability, Fig. 15 shows the impact of the RF electronics of the FEM when subjected to temperature variation. This figure shows the absolute phase variation of both wavelengths. These experimental results can be compared with the expected (simulated) ones generated by the combined effect of temperature-induced wavelength shift of the laser and fiber chromatic dispersion. By comparison, it is possible to appreciate the same order of magnitude of the absolute phase variation, which furthermore justifies the choice of the two wavelengths.

Fig. 15

Normalized absolute phase variation with respect to FEM operative temperature $\delta T_{\mathrm{TX}}$ normalized (to their max values) at 160 MHz for both RF1 and RF2 considering only the analogue RF electronics (solid lines) and the one expected due to temperature-induced wavelength-shift and fiber dispersion (dashed lines).

Considering that in SKA-low the temperature variations on the FEMs can be easily different one from the other due to the high dislocation of the antennas and that those cannot be thermalized since this would require a major increase of cost and power consumption, the phase error must be minimized as much as possible acting on the choice of the components.

Roughly speaking, it is possible to show that, neglecting the variation of the refractive index of the fiber with the temperature, the phase error among two optical links of length $L$, carrying an RF signal of frequency $f_{\mathrm{RF}}$ and working at same nominal wavelength $\lambda$, produced by the temperature variations on two separate transmitters TX1 and TX2 can be written as

Eq. (1)

$$\delta {\varphi }_1-\delta {\varphi }_2\simeq 2\pi ·f_{\mathrm{RF}}·D·C_{\lambda }·L·(\delta T_{\mathrm{TX}1}-\delta T_{\mathrm{TX}2}),$$

where $\delta {\varphi }_1,\delta {\varphi }_2$ are the absolute values of phase drift due to the absolute temperature variation $\delta T_{\mathrm{TX}1,}\delta T_{\mathrm{TX}2}$ on the transmitters, respectively. Moreover, $D$ is the fiber chromatic dispersion coefficient and $C_{\lambda }$ is the wavelength–temperature slope coefficient. This simple equation shows that the phase error is proportional to these last two quantities, which address the choice of the wavelengths to the ones that minimize chromatic dispersion and wavelength–temperature slope coefficient.

Among the dual-wavelength laser technologies available on the market, which requires at least 60 nm of spacing to let the C-WDM diplexer work fine, the selected optical wavelengths pair has been 1270/1330 nm, which was preferred with respect to other options such as the one more often used in commercial applications, 1310/1550 nm.

Although the value of $C_{\lambda }\simeq 0.1({\mathrm{nm}\mathrm{K}}^{-1})$ is almost similar for the mentioned wavelengths, the chosen ones present values of chromatic dispersion around $D(\lambda =1270\mathrm{nm})\simeq -3({\mathrm{ps}\mathrm{nm}}^{-1}{\mathrm{Km}}^{-1})$ and $D(\lambda =1330\mathrm{nm})\simeq 1.5({\mathrm{ps}\mathrm{nm}}^{-1}{\mathrm{Km}}^{-1})$. Instead, although at 1310 nm the dispersion is practically null, for 1550 nm the value of dispersion is about $D(\lambda =1550\mathrm{nm})\simeq 17({\mathrm{ps}\mathrm{nm}}^{-1}{\mathrm{Km}}^{-1})$.

Figure 16 shows a simulation of the maximum phase variation obtained when two transmitters are subjected to a slight temperature difference for 6 km of G.652.D fiber. This figure compares 1310/1550 nm technology with the one employed at 1270/1330 nm.

Fig. 16

Simulation of the phase error due to a different temperature behavior on the transmitters for 6-km RFoF link. In this figure, $\delta T_{\mathrm{TX}1}$ and $\delta T_{\mathrm{TX}2}$ represent the temperature of two different transmitters, and the curves are shown as a function of the difference of such temperatures.

It is worth emphasizing that all the considerations and results above, which led to the choice of the transmitters’ wavelengths, have been made neglecting the possible differences in the variation of the refractive index of the fibers due to external temperature, condition that can be easily achieved by burying the fiber cables. If this is not the case, and fibers are laid over the ground, the phase error is more influenced by the nonhomogenous temperature and solar radiation affecting the optical fiber, as will be shown later.

3.2.2.

Amplitude and phase stability due to the fiber temperature only

A series of RFoF link measurements have been performed directly in the field at the MRO. Between 2014 and 2016, there had been several measurements on different fiber-optic cables. The measurements include several kilometer-length cables that are routed on the ground (and exposed to the temperature fluctuation), as well as those buried underground. We also performed measurements on two cables that are subsequently used for AAVS1 [the AFL spider web ribbon cables (SWR)] and for AAVS2 projects (the regular loose-tube fiber cable). Both AAVS cables are routed above ground in the field, as the MRO has shallow bedrock and it is challenging to dig a trench to bury the cable. The cables are routed along a 6-km distance between the AAVS site and the ASKAP control building, where the TPM are located.

We performed measurements to record the magnitude and phase fluctuations of the RF-modulated optical signal transported on the cables, to quantify the effect of the environmental exposure on the signal stability. The experimental setup is depicted in Fig. 17, utilizing the multiport vector network analyzer (VNA), the actual AAVS WDM RFoF modules, and different cables. In the experiments, we recorded the $S$-parameter (both magnitude and phase) of the RF signal from a pair of fiber-optic cores, selected from opposite extremities inside the cross section of the cable assembly. Each fiber carries the two wavelengths (1270 and 1330 nm), so in total, there are four sets of $S$-parameter from a pair of RFoF modules. We performed a full 24-h measurement each day, with an interval of 10 s between consecutive measurements, across four VNA channels covering SKA-low frequency band (50 to 350 MHz) whenever possible, depending on the aperture settings of the VNA. This is to ensure accurate phase slope measurement across the frequency and that the phase difference between two adjacent measurement points is $<180\mathrm{deg}$. The VNA and RFoF module were placed inside a temperature-controlled server rack at the control building to ensure stable operating temperature and minimal fluctuation contribution from the instruments and from the RFoF optical transmitter and receiver (OTX and ORX, respectively).

Fig. 17

(a) Experimental setup to measure the magnitude and (b) phase stability of RF-modulated signal transported through fiber-optic cable in the field, utilizing multiport VNA and AAVS RFoF modules.

The results of the field test indicated that outdoor exposure mainly affects the phase stability of the RF signal transmitted over the FO cable. Indeed, for a pair of fibers, assuming that the linear thermal expansion coefficient and the temperature coefficient of refractive index of each fiber are similar, the phase error due to temperature fluctuation of signal transported in a pair of fiber cores ($F_1$ and $F_2$) can be estimated as³³

Eq. (2)

$$\delta {\varphi }_1-\delta {\varphi }_2=2\pi ·f·|\delta {\tau }_1-\delta {\tau }_2|=2\pi ·f·|[(n·{\alpha }_L)+{\beta }_n]·\frac{1}{c}·[(L_{0,F_1}·\delta T_{F_1})-(L_{0,F_2}·\delta T_{F_2})]|,$$

where $\delta {\varphi }_1,\delta {\varphi }_2$ are the absolute phase variations of the two links of initial lengths $L_{0,F_1},L_{0,F_2}$, respectively, whereas $\delta {\tau }_1,\delta {\tau }_2$ are their correspondent group delay variations. Moreover, ${\alpha }_L=\frac{1}{L}\frac{dL}{dT}$ and ${\beta }_n=\frac{dn}{dT}$ are, respectively, the linear thermal expansion coefficient and the refractive index temperature coefficient, whereas $\delta T_{F_1},\delta T_{F_2}$ are the temperature variations of fibers 1 and 2, respectively, $n=1.47$ is the refractive index of the fibers core, and $c$ is the speed of light in vacuum. Figure 18 shows an example of absolute phase variation (i.e., $\delta {\varphi }_1$ or $\delta {\varphi }_2$) for 6 km of fiber length and considering typical value for $\alpha \sim 0.56\times {10}^{-6}(°{\mathrm{C}}^{-1})$ and for $\beta \sim 1.2\times {10}^{-5}(°{\mathrm{C}}^{-1})$. This figure compares then Eqs. (1) and (2), showing that the main contribution to the absolute phase variation, and consequently on the phase error, is given by the variation of the refractive index with temperature where the latter produces a phase error higher by about two orders of magnitude.

Fig. 18

Normalized absolute phase simulated considering the temperature-induced wavelength-shift on the FEM only (solid lines) and considering only temperature-induced fiber refractive index change (dashed lines).

Equation (2) gives us an estimate of the relative phase variation in a pair of fibers. Based on the measurement result in the field, this equation also gives us an estimate of the phase variation with respect to thermal expansion as we scale the length of the cable, assuming that the cable has the same property with the one used in the measurement.

Figure 19 shows the maximum relative magnitude variation (a) and relative phase variation (b) processed for each 600-s window in 24-h from two different RFoF modules, the first module operates at 1270 nm and the second module operates at 1330 nm. It can be seen that thermal effects affect the phase stability more than the magnitude stability. Note that although the measurements were performed on a fiber link of 10.6 km length, the phase plots shown in Fig. 19(b) are scaled to 6 km to reflect the SKA-low transmission. Moreover, the relative phase variation at a particular frequency not covered by the measurement can be estimated by linear extrapolation of the data taken between 100 and 190 MHz.

Fig. 19

(a) The 600-s relative magnitude variation statistics and (b) the 600-s relative phase variation statistics of RF signals (between 100 and 190 MHz) transmitted over a pair fiber-optic link over a 24-h period. The first module is operating at 1330 nm and the second module is operating at 1270 nm, transmitted across two different fiber cores (worst-case scenario). The cable is a $\sim 5.3\text{-}\mathrm{km}$ loose tube cable used in AAVS2, and it is a loop-back transmission forming a total length of $\sim 10.6\text{-}\mathrm{km}$ transmission. The plot shown for relative phase variation statistics is scaled to 6 km to reflect the SKA-low transmission distance.

The magnitude drift and relative phase drift over 24-h period are depicted in Figs. 20(a) and 20(b), where the maximum phase difference between two different fibers can reach up to $\sim 40\mathrm{deg}$ (phase, on a cable with a total length of 6 km), for RF signal transmitted at 160 MHz across the cable. The maximum drift, relative to the start of the measurement taken at midnight, occurs around noon time, which corresponds with the maximum ambient temperature on site at the MRO. By analyzing the weather data [see Fig. 20(c)], we have established that the phase stability is sensitive to thermal fluctuation on the cable, which is correlated with the solar irradiation, ambient temperature data, and wind.

Fig. 20

(a) The 24-h magnitude drift and (b) the relative phase drift of RF signals (at 160 MHz) transmitted over a pair fiber-optic link where the first module is operating at 1270 nm and the second module is operating at 1330 nm, the RF signal from each module is transmitted across two different fiber cores. The cable is a 5.5-km loose tube cable used in AAVS2, and it is a loop-back transmission forming a total length of 11-km transmission. The relative phase drift plot shown here is scaled for a 6-km transmission to reflect the SKA-low transmission distance. (c) The solar irradiation, ambient temperature, and wind speed are taken from the weather station data at the MRO.

One possible option to minimize the temperature variation of the cable is by underground burial. We performed measurements of buried cable versus surface laid cable, where it was found from the measurement results that buried cable has very good gain and phase stability compared to the surface laid cable; ambient temperature, solar irradiation, or wind changes have negligible effect on the stability of the phase of the RF signal transmitted through the cable in the 600-s window throughout a 24-h period (Fig. 21). Moreover, as predicted by the simulations, the phase variations induced by the change with temperature of the fiber refractive index (surface laid cable) are of two orders of magnitude higher compared to the ones produced by the only temperature-induced wavelength shift on the FEM (buried cable).

Fig. 21

The 5-day 600-s relative phase variations statistics of the 6-km surface laid cable (dashed line) versus 6-km buried cable (solid line) at 160 MHz.

As a result of the investigations performed on the AAVS and EDA demonstrators, which with a 5-km length of exposed optical fiber cable can be considered as worst-case examples, the fiber cables will largely be buried in the SKA-low system. The full range of temperature and solar irradiation changes will therefore be undergone only by short-fiber segments, up to $\sim 30\mathrm{m}$ long, within the antenna fields.

3.3.

Mitigation of the Detrimental Effects due to Rayleigh Backscattering

During a test driven site trip on November 2018 (so called phase 0), a SKALA4-AL antenna and an EDA2 dipole were installed at MRO. The antennas were connected to a dedicated TPM hosted in the ASKAP control building through the fiber cable deployed for AAVS1. Since the first acquisitions, two problems have been encountered, related, respectively, to the presence of noise and to the presence of distortion terms.

The cause of both problems has been identified in the Rayleigh optical backscattering.³⁴ This effect is originated by the interaction of the emitted light by the laser and the back-reflected one by the fiber into the laser cavity itself, so basically it is strongly dependent from the optical isolator integrated in the optical source. The detriment is equally present on both the RFoF links used for $X$- and $Y$-polarization signals because of the vicinity of the two wavelengths utilized (1270 and 1330 nm, respectively).

The solution adopted to counteract the mentioned effects of Rayleigh backscattering (RBS) consisted in the direct modulation of the laser source through a sinusoidal tone of appropriate amplitude $I_{\mathrm{dith}}$ and frequency $f_{\mathrm{dith}}$ (see Fig. 22).

Fig. 22

Simplified scheme depicting the introduction of the dithering tones in each of the two RFoF transmitters utilized in the receiving chain of AAVS2.

This solution (often indicated as dithering tone technique) was already proposed in the past, with the aim to reduce interferometric noise in optical fiber systems, including the RBS induced noise.³⁵ Its beneficial effect in attaining the reduction of the noise level at low frequencies was then expected.

In addition to this, a comprehensive theoretical and experimental investigation was performed with reference to the counterintuitive behavior observed in the spurious frequencies generated at the receiving end of the RFoF system by a sinusoidal modulating tone. The reason of this behavior could then be explained, and thanks to this theoretical result it was possible to demonstrate that the same dithering tone technique could constitute an appropriate countermeasure also in reducing the global impact of the system nonlinearities.

Figure 23 visualizes the behavior of the RFoF system in terms of generation of ${\mathrm{HD}}_2$, which consists in the amount of power received at the double $(2f_{\mathrm{RF}})$ of the modulating tone frequency.

Fig. 23

Behavior of the power received by the RFoF system at the frequency $2f_{\mathrm{RF}}$ where both the “intuitive” behavior ($P_{\mathrm{RF}}\ge 0\mathrm{dBm}$) and the “counterintuitive” one ($P_{\mathrm{RF}}<0\mathrm{dBm}$) can be appreciated.

For values of $P_{\mathrm{RF}}$ higher than a certain threshold value (in the example of the figure around 0 dBm), the laser nonlinearity is the prevailing source of ${\mathrm{HD}}_2$, regardless of the frequency $f_{\mathrm{RF}}$ of the modulating tone. This gives rise to an “intuitive” behavior of the nonlinearities, since increasing values of the amplitude of the modulating current determine increasing values of ${\mathrm{HD}}_2$.

On the contrary, it can be appreciated that for a given value of modulating frequency $f_{\mathrm{RF}}$ belonging to the interval between $f_{\mathrm{RF}}=10\mathrm{MHz}$ and $f_{\mathrm{RF}}$ a little $>350\mathrm{MHz}$ (interval that includes the SKA-low band 50 to 350 MHz, when $P_{\mathrm{RF}}$ is progressively reduced from 0 dBm to lower values, ${\mathrm{HD}}_2$ does not exhibit a monotonic behavior. Indeed, it initially decreases, then it “counterintuitively” increases again, reaching a maximum value (correspondent to the yellow region in Fig. 23), after which a final decrease of ${\mathrm{HD}}_2$ takes place.

The reason of this behavior lies in a combined effect of RBS and laser frequency chirp. As a consequence of that, the behavior of ${\mathrm{HD}}_2$ can be shown to be dependent on the phase modulation index $M_{\mathrm{RF}}\propto K_f\frac{\sqrt{P_{\mathrm{RF}}}}{f_{\mathrm{RF}}}$, where $K_f$ represents the adiabatic chirp coefficient of the laser utilized.³⁶ Note that this phenomenon regards all the possible spurious frequency terms that can be received by the RFoF link.

As mentioned above, the dithering tone technique constitutes an effective countermeasure to the nonlinear spurious frequencies generation just described, as well as to the undesired presence of noise peaks and noise floor jumps that were observed mostly for $f_{\mathrm{RF}}<50\mathrm{MHz}$ but were present also within the SKA-low bandwidth.

It is important to point out that to solve these issues, it was not possible to act on other causes (such as the insertion of a double-stage isolator to reduce the amount of RBS feedback and the use of an external modulator to reduce the chirp coefficient $K_f$) since it would have increased dramatically the cost of the link.

Figures 24(a) and 24(b) allow to appreciate the beneficial effects to the quality of the signal received by the RFoF link in terms of reduction of the noise effect. The bandwidth 5 to 45 MHz is represented, which is the one where it is possible to better appreciate the improvement coming from the dithering tone technique adoption. The spikes are clearly recognizable in the live spectrum yellow trace on the top of Fig. 24(a), whereas the jumps of the noise floors appear as horizontal stripes in the spectrogram, represented in the bottom part of this figure.

Fig. 24

Captured live image of the measured spectrum of the power received by the RFoF system in the interval 5 to 45 MHz (a) with and (b) without the dithering technique applied.

In Fig. 24(b), the effect of the introduction of the dithering tone in the system can be clearly appreciated, since the spikes and the horizontal stripes have disappeared, respectively, in the live spectrum and in the spectrogram.

In addition to the impact on spikes and noise, the effect of the dithering tone is shown in Fig. 25 for which regards the level of the output intercept point of second order (OIP2), where it can be appreciated the strong improvement at low frequency.

Fig. 25

Example of behavior of the OIP2 for the 1330-nm wavelength of the RFoF link employing 6 km of fiber span.

However, it is important to point out that the insertion of the dithering tone, which directly modulates the laser, can generate itself intermodulation distortion that must be controlled properly. To derive a specification of the maximum acceptable level of the intermodulation that can be tolerated, it is considered that its level should be below the noise at the output of the receiver considering the finest bandwidth of SKA-low (266 Hz). For example, considering the most powerful RFI (generated by the Orbcomm satellite) at 160 MHz and considering the use of the SKALA4 antenna, an intermodulation product lower than at least 50 dB is required to make it not recognizable by the system.

As a consequence of this theoretical and experimental investigation, the 256 RFoF links of both EDA2 and AAVS2 were all equipped with a dithering tone generator at 8.5 kHz and amplitude 1.25 mA, which allows the receiving end to reach acceptable values of the noise floor and of the undesired spurious frequency terms generated by both RBS and the dithering tone itself. A comprehensive explanation of the theory that is behind the improvements obtained can be found in Refs. 37 and 38.

4.1.

Minimization of the Detriments Due to the Polarization-Dependent Loss

Unexpected fluctuations of the optical received powers at both wavelengths were observed and investigated on the deployed RFoF links of AAVS2 in March 2020 (see Fig. 26). The behavior appeared to be connected to the exposure of the optical fiber cable to external environmental quantities (e.g., temperature). An investigation was then carried out at laboratory level, with the aid of a calibrated thermal chamber, to identify the reason of such behavior and envisage possible countermeasures.

Fig. 26

Three days of on-site measurements of the total RF power at 160 MHz (left axis) of both $X$ and $Y$ polarization (normalized at their respective first value) captured from one AAVS2 antenna, carried to the TPM by the designed RFoF link of about 6 km of fiber laid on the ground. Measurements of solar radiation and temperature taken from the Commonwealth Scientific and Industrial Research Organization (CSIRO) weather station are also shown in the right axes in order to correlate the behavior of the total RF power with the climatic conditions.

The described undesired behavior resulted due to a combined effect of polarization mode dispersion (PMD) of the optical fiber and the polarization-dependent loss (PDL) of the WDM dichroic filter located at the receiver site, given by the polarization-dependent values of its reflection and transmission coefficients. The representation reported in Fig. 27 will be utilized to illustrate the mechanism causing the received RF power fluctuations.

Fig. 27

Simplified representation of the mechanism determining the fluctuations of both the received RF powers carried by the optical fields at 1330 and 1270 nm. PMD, polarization mode dispersion; Tx’s, optical transmitters; and Rx’s, optical receivers.

As mentioned above, the two RF currents that come from the antenna downlink and carry the $Y$- and $X$-polarized sky signals modulate, respectively, lasers emitting at wavelengths 1330 and 1270 nm. The two resulting optical signals are multiplexed through a parallelepiped-like dichroic WDM combiner, which is hit by both fields with an incident angle of about 45 deg with respect to the vectors perpendicular to the respective incidence surfaces.

At the optical transmitters’ site, the unavoidable temperature excursions undergone by the be transmitters can cause variations of parameters like the conversion efficiency of the lasers or the precise value of the wavelength of the emitted fields, whereas the polarization of the optical signals exiting from each of the two lasers (TE or TM or a combination of the two) can be regarded as stable in time. This fact brings as a consequence that the unavoidable difference exhibited by any dichroic filter presents in its reflection and transmission coefficients in dependence on the fact that the incident field exhibits a TE or a TM polarization does not determine any undesired fluctuation on the optical powers entering the optical fiber. Indeed, in passing through the dichroic WDM filter located at the transmitters’ site, the optical field at 1330 nm (green dashed arrow in Fig. 27) will see an equivalent transmission coefficient stable in time, and stable in time as well will be the equivalent reflection coefficient seen by the optical field at 1270 nm (blue dashed arrow in Fig. 27).

Unlike what has just been described, each one of the two optical fields (the one at 1330 nm and the one at 1270 nm) that hit the parallelepiped-like dichroic WDM splitter at the receiver side (see again Fig. 27) presents a polarization that changes in time. The reason for this behavior comes from the time variations of environmental quantities along the fiber path, which in turn induce time variations of the refractive index in different portions of the optical cable. The polarization dependence of the reflection and transmission coefficients of the dichroic filter determines this time undesired fluctuations of the optical powers detected by the two photodiodes.

In order to quantify the impact of the phenomenon, a laboratory test was performed on the RFoF system. The corresponding experimental setup is depicted in Fig. 28.

Fig. 28

Experimental setup utilized to determine the impact of the PDL caused by the characteristics of the optical receiver utilized. WDM RX “A:” optical receiver presently utilized in the RFoF system. WDM RX “B:” optical receiver where an external WDM filter with low polarization dependence separates the two wavelengths, which subsequently reach two separate PIN-based optical receivers.

The results of the measurement of the total RF power performed utilizing the optical receiver presently adopted in the RFoF system are shown in Fig. 29(a), whereas the results of the measurement performed utilizing the optical receiver identified as WDM RX “B” in Fig. 28 are instead shown in Fig. 29(b).

Fig. 29

Measured impact of the PDL on the RF total power caused by the (a) characteristics of the optical receiver presently adopted in the AAVS2 RFoF system and (b) characteristics of the optical receiver where an external WDM filter with low polarization dependence separates the two wavelengths, which subsequently reach two separate PIN-based optical receivers.

From Figs. 29(a) and 29(b), the PDL of the two WDM filters can be computed from its relation with the variation of the RF power as follows:

In case of the internal WDM filter, the PDL is about 0.2 and 0.07 dB for 1270 and 1330 nm, respectively, while those values are greatly reduced with the external WDM filter to 0.04 dB for both 1270 and 1330 nm.

With regard to this point, however, it must be noted that besides the possible improvements connected to a change in the receiving WDM filter characteristics, a significant mitigation of the PDL impact is expected to come from the operation of burying the optical cable, thanks to the resulting important reduction of the temperature fluctuations that would be undergone by the optical fiber in this case. The investigation activity is then proceeding in different directions, with the aim to identify the most appropriate measure to take in order to reach a satisfactory PDL mitigation.

4.2.

Monitoring of Fiber-Induced Delay on the RFoF Channel

Ensuring the stability of the analogue signal transport from the antennas to the digitizers is essential to the performance of the SKA system. A major element of this is contributed by variability in the time taken by the signal to cover the distance to the digitizer. To address this issue, work has explored providing continuous monitoring of the fiber-induced delay in order to perform, when necessary, appropriate recalibrations of the phase differences among the contributions to the received signal coming from the different antennas. Factors that may affect the value of the fiber-induced delay include environmental variations of physical parameters (like temperature), which in turn present an impact on the behaviors of optical devices and components, such as the laser source and the optical fibers.

Within this context, a technique is presently investigated to monitor the variations in the time taken by the signal to cover the fiber length within the RFoF system. The corresponding system is represented in Fig. 30.

Fig. 30

Sketch of the proposed system in its possible implementation within the actual receiver of AAVS2, (PD, photodiode; Isol., optical isolator; and Amp., electrical amplifier).

As can be appreciated, the system monitors the variation of the frequency amplitude response $(|S_{21}{|}^2)$ produced by a Michelson-type interferometer. A $|S_{21}{|}^2$ evaluator (which can either consist in a VNA or in a simpler device of much lower cost) modulates a 10-mW 1550 nm DFB laser with a sinusoidal RF tone of frequency $f_{\mathrm{RF}}$. The resulting optical signal then enters the mentioned interferometer, which is realized by exploiting a 50:50 fiber coupler. Still referring to Fig. 30, the left-hand side of the coupler is fed by the DFB laser, then one output of the coupler is connected to a reference thermalized fiber of length $L_1$, whereas the other output is connected to the fiber under test (FUT) of length $L_2$. Both fibers are terminated with a PC connector and the relative length between them should stay in the order of a few meters. The two reflected signals $\overrightarrow{E_{1,r}}$, $\overrightarrow{E_{2,r}}$ are then combined by the coupler and sent to a PIN photodiode, followed by an LNA of 42 dB of gain, where the observed behavior of ${|S_{21}|}^2$ is of the following type:

Note that while the proposed monitoring system is envisaged for application within SKA-low, its realizations have so far been limited to laboratory prototypes. In Figs. 31(a) and 31(b), the results of one of the experiments are shown where two fibers with nominal length of around 6 km have been employed, which is considered, up to now, the maximum fiber length deployable in SKA-low.

Fig. 31

(a) Shift of the minimum (blue region) of ${|S_{21}|}^2$ due to temperature variation inside the chamber. (b) Resulting measurement of the variation of ${\tau }_2$, named $\delta {\tau }_2$.

In particular, with a commercial optical time domain reflectometer system, the length difference between the two fibers resulted to be around 150 m.

The chamber has been set to apply a temperature variation from $-10°\mathrm{C}$ to $+50°\mathrm{C}$ with a slope of $\frac{dT}{dt}=0.2°\mathrm{C}/\min$. The temperature was measured with a resistance temperature detector placed within the chamber. The slope of the curve, normalized to the nominal length $L_2$, is about $39\mathrm{ps}(\mathrm{km}/°\mathrm{C})$, in agreement with the theoretical expected value of $40\mathrm{ps}(\mathrm{km}/°\mathrm{C})$ (see Ref. 33), showing a standard deviation about 60 ps, representing the precision of the system.

Further upgrades of the system are under study; for example, the possibility of using commercial reflectors to enhance the reflected power in both fibers and the possible replacement of the VNA with lower-cost devices.