2.1.

Transmission of an Analog TV Signal in the Left-to-Right Direction

In the first step, the MLDs (operated by temperature controllers to guarantee the stability of their optical parameters to thermal fluctuations) used in this experiment are optically characterized by means of an optical spectrum analyzer.

For this case, MLD_1 (Thorlabs, model S1FC1550) at an optical power (OP) of 1.25 mW gives ${\lambda }_0=1536.05\mathrm{nm}$, $\mathrm{\Delta }\lambda =6.15\mathrm{nm}$, and $\delta \lambda =1.1\mathrm{nm}$. In order to avoid reflections to the source and to maximize the modulator output OP, it is necessary to use an optical isolator (OI) and a polarization controller (PC), respectively. The polarized light is injected to Mach–Zehnder intensity modulator 1 (MZ-IM_1) (Photline, model MXAN-LN-20, insertion loss of 2.7 dB, operating wavelength of 1530 to 1580 nm, and $V_{\text{bias}}=2.97\mathrm{V}$). The microwave signal generator MSG_1 (Anritsu, model MG3692, 0.01–20.0 GHz) provides an electrical signal of 2.31 GHz at a power of 5 dBm. This signal is divided through a splitter (power divider 1); part of this electrical signal is transmitted through cable to demodulate the signal, and the rest is mixed (mixer 1) with an amplified analog National Television System Committee (NTSC_1) TV signal of 67.25 MHz. The mixed electrical signal is applied to the RF port of MZ-IM_1 in order to modulate the light emitted by MLD_1. Then, the optical output of MZ-IM_1 is launched to Port1 of an optical circulator (OC_1) passing to Port2, where it is coupled into a coil of $L=25.24\mathrm{km}$ (available in our laboratory) of single-mode standard fiber (SM-SF), which exhibits $\alpha =0.2\mathrm{dB}/\mathrm{km}$ and $D=15.81\mathrm{ps}/\mathrm{nm}\text{-}\mathrm{km}$ at ${\lambda }_0=1550\mathrm{nm}$. At the end of the optical link, the light is injected to Port2 of OC_2 passing to Port3, which is connected to a fast PD (PD_1, MITEQ, model DR-125G-A, bandwidth of 13 GHz and $\mathfrak{R}=0.9\mathrm{A}/\mathrm{W}$). The electrical signal delivered by PD_1 is amplified and mixed (mixer 2) with the demodulated signal to suppress the carrier signal of 2.31 GHz. Finally, after a stage of amplification and low pass filtering (LPF), the recovered analog TV signal is passed through power divider 2 and is connected to an electrical spectrum analyzer (ESA_1, Anritsu, model MS2830A-044, 9 kHz to 26.5 GHz) or to an oscilloscope (Keysight DSA V084A, 8 GHz, $80\mathrm{Gsa}/\mathrm{s}$). Figure 4 shows the measured electrical spectrum of the transmitted and recovered TV signals, showing a SNR of 49.66 and 37.62 dB, respectively. The time-domain waveforms of standard NTSC corresponding to the transmitted (upper) and recovered (lower) signals measured with an oscilloscope are depicted in Fig. 5.

Fig. 4

Electrical spectrum for transmitted and recovered analog TV-signal-channel 4.

Fig. 5

Time-domain waveforms for (a) transmitted and (b) recovered analog TV-signal-channel 4.

It is important to note that the optical circulators used in this experiment have the following optical specifications: wavelength range 1525 to 1610 nm, isolation $>40\mathrm{dB}$, directivity $(\text{port}1\to \text{port}3)>50\mathrm{dB}$, and return loss $\ge 50\mathrm{dB}$.

2.2.

Transmission of an Analog TV Signal in the Right-to-Left Direction

The optical source used is MLD_2 (Thorlabs, model LPS-1550-FC), which at an $\mathrm{OP}=1.21\mathrm{mW}$ gives ${\lambda }_0=1547.2\mathrm{nm}$, $\mathrm{\Delta }\lambda =7.31\mathrm{nm}$, and $\delta \lambda =1.1\mathrm{nm}$. The description for this transmission direction is similar to the case previously described. The main differences are the use of MZ-IM_2 (JDSU, model AM-150, insertion loss of 5.0 dB, operating wavelength 1540 to 1560 nm, and $V_{\text{bias}}=4.13\mathrm{V}$) as well as MSG_2, whose main limitation is its frequency range of operation (Agilent, model E4425B, 250 kHz to 3.0 GHz). Thus, the light issued from MLD_2 passes through the OI and the PC and is injected to MZ-IM_2. The electrical signal at the output of MSG_2 is divided (power divider 3) in order to feed MZ-IM_2 and to generate the demodulated signal. The resulting mixed (mixer 3) electrical signal between MSG_2 of 2.08 GHz ($-10\mathrm{dBm}$) and NTCS_2 TV signal of 61.25 MHz is connected in the RF port of MZ-IM_2. The modulated optical signal at the output of MZ-IM_2 is launched to Port1 of OC_2 passing to Port2, where it is connected into the bobbin of SM-SF for its return in the left direction. Then the light is attached to Port2 of OC_1 passing to Port3 in order to be launched to PD_2 (MITEQ, model DR-125G-A, bandwidth of 13 GHz and $\mathfrak{R}=0.9\mathrm{A}/\mathrm{W}$). The recovered electrical signal at the output of PD_2 is amplified and passed onto an electrical mixer (mixer 4) in order to be added to an electrical carrier of 2.08 GHz that is generated by MSG_2 and transmitted through cable. Finally, after an amplifier and a LPF, the recovered analog TV signal is connected to power divider 4, where it can be attached to ESA_2 or to an oscilloscope. Figure 6 displays the spectrum of the TV signal of 61.25 MHz transmitted and recovered, with a power level of $-7.97\mathrm{dBm}$ ($\mathrm{SNR}=57.03\mathrm{dB}$) and $-20.23\mathrm{dBm}$ ($\mathrm{SNR}=44.77\mathrm{dB}$), respectively. Figure 7 depicts the time-domain waveforms of standard NTSC obtained through an oscilloscope, where upper and lower traces correspond to the transmitted and recovered signals.

Fig. 6

Electrical spectrum for transmitted and recovered analog TV-signal-channel 3.

Fig. 7

Time-domain waveforms for (a) transmitted and (b) recovered analog TV-signal-channel 3.