RESUMO
The ability to measure the orbital angular momentum (OAM) distribution of vortex light is essential for OAM applications. Although there have been many studies on the measurement of OAM modes, it is difficult to quantitatively and instantaneously measure the power distribution among different OAM modes, let alone measure the phase distribution among them. In this work, we propose an OAM complex spectrum analyzer that enables simultaneous measurements of the power and phase distributions of OAM modes by employing the rotational Doppler effect. The original OAM mode distribution is mapped to an electrical spectrum of beat signals using a photodetector. The power and phase distributions of superimposed OAM beams are successfully retrieved by analyzing the electrical spectrum. We also extend the measurement technique to other spatial modes, such as linear polarization modes. These results represent a new landmark in spatial mode analysis and show great potential for applications in OAM-based systems and optical communication systems with mode-division multiplexing.
RESUMO
We experimentally demonstrate a terahertz-bandwidth photonic differentiator employing a silicon-on-insulator directional coupler. The integrated waveguide coupler with two identical paralleled strip waveguides achieves a first-order differentiator when full energy coupling is met from one waveguide to another. The integrated waveguide coupler can offer different operation bandwidths by changing the length and gap of the strip waveguides. Due to the large 3 dB bandwidth of the directional coupler, we implement the first differentiator with an operation bandwidth of 1.25 THz. The performance of this photonic differentiator is tested using Gaussian-like pulses with a pulsewidth of 2.8 ps, 4 ps, 6 ps, 8 ps, and 10 ps, respectively. The differentiation processing errors and relative energy efficiency are also discussed. This silicon chip may have potential applications in integrated photonic computing circuits with sub-picosecond pulses.