Photonic Generation of Microwave Waveforms Based on a Polarization Modulator in a Sagnac Loop

Abstract

An optical microwave waveform generator using a polarization modulator (PolM) in a Sagnac loop is proposed and experimentally demonstrated. Microwave waveforms including a triangular waveform, a sawtooth waveform, and a square waveform, can be generated using a sinusoidal signal to modulate a light wave at a PolM in a Sagnac loop. In the proposed microwave waveform generator, a sinusoidal microwave signal is applied to the PolM in the Sagnac loop. Due to the velocity mismatch, only the clockwise light wave in the Sagnac loop is effectively modulated by the sinusoidal microwave signal at the PolM, and the counter-clockwise light wave is not modulated. Along the clockwise direction, the powers of the odd-and even-order sidebands can be controlled separately by tuning a polarization controller after the PolM. In addition, the output power of the optical carrier can be independently controlled by combining the counter-clockwise and clockwise optical carriers at the output of a polarization beam splitter. As a result, a modulated signal with controllable odd-and even-order sidebands is generated. By applying the modulated signal to a photodetector, a microwave signal with fully controllable odd-and even-order harmonics is generated, which corresponds to a desired microwave waveform. A theoretical analysis is developed, which is validated by an experiment. A triangular, sawtooth, and square waveform with a repetition rate tunable from 2 to 4 GHz is experimentally generated.

Photonic Generation of Triangular Waveform Utilizing Time-Domain Synthesis

“…In the reported schemes, an interleaver [11], a microwave photonic filter (MPF) [12], or stimulated Brillouin scattering (SBS) [13] need to be introduced to remove unwanted frequency components or recover optical carrier, and a additional optical filter or phase shifter is necessary to achieve the phase condition. Furthermore, in [14] a configuration of a polarization modulator (PolM) in a Sagnac Loop has been reported. Through the bidirectional modulation of the PolM, the optical carrier and the odd-order and even-order sidebands can be independently controlled, which may generate triangular pulses by manipulating microwave harmonics.…”

Generation of triangular waveforms based on a microwave photonic filter with negative coefficient

“…External modulation of a continuous wave optical signal is a promising solution to generate triangular waveforms with full-duty-cycle [7][8][9][10][11][12][13]. The modulated optical sidebands are manipulated in the optical domain.…”

“…However, it is noted that the methods reported in [7][8][9] were only theoretically analyzed and simulated without providing experimental supports. Recently, triangular waveforms was experimentally generated in [10][11][12][13]. In [10], a DPMZM and a 90° hybrid electrical coupler were used to generate triangular waveforms.…”

“…Sophisticated bias control is required since there are three biases to be controlled in a DPMZM. Liu et al reported a method to generate triangular waveforms using a polarization modulator incorporated in a Sagnac loop [12]. However, the system might suffer from stability problem due to the use of Sagnac interferometer and complicated polarization controls.…”

Recent progresses on optical arbitrary waveform generation

“…Among them, various electro-optic modulators have been designed and fabricated, such as phase modulator, intensity modulator, polarization modulator, dual-parallel/ dual-drive Mach-Zehnder modulator and quadrature amplitude modulation (QAM) modulator, etc [82][83][84][85][86][87][88][89][90][91][92][93][94][95]. Advanced format modulators allow us to control the magnitude and phase of an input optical signals by adjusting the modulation strategy.…”