Monitoring of the RF excitation allows compensation of B₁ field errors, power estimation and detection of potential hardware failures that could compromise patient safety [1-4]. Using the integrated current sensor in a current-source switch-mode on-coil amplifier [5,6], B₁ amplitude and phase information can be recovered on the basis of three optical signals (one voltage for amplitude and two optical quadrature voltages for phase detection) [7]. We propose an improved amplifier design where amplitude, phase and frequency information are recovered in a remote controller from a single optical digital down-converted version of the sensed RF current. This prototype is a first step toward a practical implementation of real-time optical monitoring of the B₁ field per channel to ensure safe operation of a pTx system employing on-coil current source amplification. An overview of our implementation of on-coil amplification for RF transmission is show in Figure 1, highlighting the main aspects B₁ monitoring and integration with the MRI scanner. The on-coil current source amplifier has a loop integrated in an inner layer of the printed circuit board to sense output current [6]. Feedback of the detected current envelope is performed on the board to minimize load sensitivity [5,6]. In this new prototype, before envelope detection, the sensed current was split through a LC 2-way splitter to send part of the signal to an active mixer (AD8342) with local oscillator (LO) input of 297.0 MHz, 200 kHz away from the scanner frequency (297.2 MHz). An optical receiver and fanout buffer (output skew < 50 ps), located next to the amplifier board, generated multiple LO signals from a single optically received LO for future multi-channel implementation. After low-pass filtering, the output of the RF mixer (IF) was converted to 10-bit serial digital signal through a low power analog-to-digital converter (ADC AD7277) clocked at 25 MHz (optically transmitted and buffered on the amplifier side as all control signals). This digital signal was then optically transmitted to a remote computer that hosts the PCI board that controls the vector modulators in the custom-made pTx interface [6]. This interface generates multiple optical carriers and envelope signals from the single RF pulse transmitted by the scanner control to drive multiple on-coil switch-mode amplifiers[6]. In the current implementation, digital optical communication from the amplifier to the remote computer was possible through a remote optical receiver connected to a commercial adapter board (Nano River Technologies, Denmark) that converted 3-wire serial peripheral interface (SPI) to full speed universal serial bus protocol (USB2). In a benchtop demonstration, preliminary data was obtained with a single amplifier connected to a 6 cm diameter transmit coil. B1 field was measured with a calibrated probe coupled to the coil, and the down converted sensed signal was simultaneously measured with an oscilloscope probe at the input of the ADC (located on the amplifier), and monitored on the computer screen after down-converted RF waveform reconstruction was preformed in software. Figure 2 shows the RF output measured with the calibrated probe on the coil side (B1 = 25 µT at the center of the coil), the corresponding down converted signal at the input of the ADC and the signal remotely recovered at the controller side. We can see that the down-converted signal tracked accurately the B1 field measured with the probe and was successfully recovered on the remote computer. The modulation on the recovered amplitude is caused from the implemented sampling rate (~781k Hz). Figure 3 shows the recovered RF spectrum (for four iterations) while the RF carrier frequency of a sine pulse transmitted from the pTx interface to the amplifier was changed from 297.175 MHz to 297.225 MHz. The monitored down converted frequencies, indicated with different colors, were from 175 kHz to 225 kHz. We have presented preliminary data with a new on-coil amplifier prototype that performs down conversion and digital optical transmission of the sensed RF signal. The amplifier size and performance compared to a previous prototype [6] was not affected by the additional electronics. We successfully recovered the signal in a remote computer that hosts the pTx interface controlling the on-coil amplifiers. In addition to safety monitoring, phase and amplitude information carried in the recovered signal could be processed for automatic adjustments of the control inputs of the vector modulators located in the pTx interface [6]. This closed loop operation will allow fast and simple initial system calibration.