The growing demand for fast wireless communications has resulted in an overpopulated radio frequency (RF) spectrum that lacks bandwidth to meet demand. The wireless community has employed two tactics to overcome this problem: 1) put more data in a given bandwidth, i.e., higher spectral efficiency, and 2) use the available bandwidth more efficiently by tuning the user's channel, i.e. carrier frequency, to an unused part of spectrum. The first tactic, higher spectral efficiency, is achieved by using complex signals that carry information on both the in-phase (I) carrier and quadrature-phase (Q) carrier—quadrature amplitude modulation (QAM)—and increasing the number of levels on each carrier—constellation size (4QAM, 16QAM, etc.). However, higher constellation sizes and higher bandwidth require higher fidelity RF signals.
Our feasibility study examined a photonic-based high-fidelity RF amplitude and phase modulator that modulates the amplitude and phase of femtosecond laser pulses to generate relatively high power, complex microwave waveforms at carrier frequencies up to 70 GHz. We built a proof of concept system in a laboratory setting that demonstrated the photonic generation of QAM signals at electronically tunable RF carrier frequencies. Our lab prototype achieved a signal-to-noise-and-distortion (SINAD) ratio of 39 dB over a 500 MHz bandwidth. The signal-to-noise ratio of the system was measured at 42 dB over a 500 MHz bandwidth, on par with state-of-the-art. Inadequate pre-compensation of the electronic digital-to-analog converter (eDAC), amplifier frequency response, and the noise of the measurement equipment were key limitations to the SINAD. Simulations showed that, with perfect pre-compensation and ideal measurement equipment, a noise-limited SINAD of 47dB could be achieved. Our simulations demonstrated that multiple eDACs could be "optically stacked" to achieve 54 dB SINAD. The feasibility study demonstrated higher fidelity RF signal generation to increase the spectral efficiency and information capacity of signals transmitted by land, sea, air, and space platforms.
Our research supports Lawrence Livermore National Laboratory's mission-related applications that require exquisite control of RF waveform characteristics with high fidelity such as next generation communications, quantum computing, sensing, and radar. The study leveraged the Laboratory's core capabilities in laser and optical science and technologies and advanced the Laboratory Director's initiative in space science and security.
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