Dual Optical Combs Research Articles

Frequency Diverse Array Signal Generation and Beamforming Based on Dual Optical Combs

Frequency Diverse Array Signal Generation and Beamforming Based on Dual Optical Combs

Wideband Frequency and Angle-of-Arrival Measurement System Based on Optical Subsampling

We proposed a system that simultaneously supports measuring the frequency and the angle of arrival (AOA) of the radio frequency (RF) signal. In the proposed system, the RF signals from different antennas are loaded on different arms of a dual-polarization Mach-Zehnder modulator (Dpol-MZM) and are subsampled by dual optical combs, respectively. The frequency of the receiving RF signal can be obtained from two intermediate frequencies (IF), and AOA can be obtained from the phase difference between the upper- and lower-channel IF signals. In the experiment, 0.5–40 GHz frequency measurement range with an error of ±0.06 MHz and ±180 phase shift measurement range with an error of 1.8 can be achieved based on a low-frequency electronic analog-to-digital converter (ADC). The processing of both broadband signal and multitone signal and the system stability are also verified.

Gas Temperature Measurement by Aligning Absorption Spectroscopy of Dual- Phase -Unlocked Optical Combs

An accurate gas temperature measurement method with dual phase-unlocked comb absorption spectroscopy was presented. Firstly, the spectra of interferograms of the dual optical combs were aligned by full-spectrum cross-correlation. The frequency jittering of the spectra relative to the reference spectra was determined by cross-correlation and corrected by spectral shift. Secondly, a modified multispectral fitting approach was proposed to eliminate the wavelength-dependent intensity difference between the reference and measurement arms. The difference was eliminated to obtain the modified spectra by the differential operation without scaling between the two arms. Then, the gas temperature was calculated from the modified spectra. In the experiment, acetylene in a heatable airtight cell was employed as the target gas, and the spectra over 0.8 THz were obtained. In the range from room temperature to 180 °C, the measured temperature agreed linearly with the reference, and the standard deviation of the temperature obtained by averaging 800 spectra for one second was less than 1.2 °C. The deviation between the mean value of measured temperatures and reference temperature did not exceed 1.9 °C.

All-Fiber Bidirectional Mode-Locked Ultrafast Fiber Laser at 2 μm

In this paper, we demonstrated an all-fiber bidirectional ultrafast thulium-doped fiber laser at 2 μm with a single-wall carbon nanotube presented as saturable absorber. We successfully obtained bidirectional mode-locking operations with pulse repetition frequency adjustable from 35 to 122 MHz by shortening the cavity length. Meanwhile, with the reduction of the intracavity dispersion, the number of Kelly sidebands was significantly decreased and the output energy was more concentrated on the soliton pulse. Besides, by manipulating the pump power and polarization controller, the two results of bidirectional mode-locked pulses with the same repetition frequency and differential repetition frequency were observed. We found that the repetition rate difference between the clockwise and counterclockwise pulse trains was adjustable by changing the pump power or controlling the intracavity polarization. When the pulse repetition frequency was 35 MHz and 122 MHz, the adjustment range of repetition rate difference was 764–922 Hz and 540–2000 Hz, respectively. It is believed that this novel laser source can support free-running dual optical comb spectroscopy to detect important air gases like H2O or CO2 in the future.

Analysis of an innovative method for large-scale high-precision absolute distance measurement based on multi-heterodyne interference of dual optical frequency combs

Femtosecond optical frequency comb (FOFC) has been widely used in time-frequency technique and precision spectral measurement. The derivative technique for absolute distance measurement by FOFC, which has features of high-speed, large-scale and high-precision, has become a worldwide research hotspot and is promising to be directly applied in some precision ranging missions, such as large equipment manufacturing, satellites formation flying, laser radar and space gravitation measurement, etc. An innovative method for large-scale and high-precision absolute distance measurement based on multi-heterodyne of dual FOFCs, is proposed in this paper. This method combines the multi-heterodyne cross-correlation distance measurement of dual optical combs with the beat-frequency distance measurement based on repetition frequency of the comb, so that it achieves large-scale and high-precision absolute distance measurement without relying on the earlier judgment with time-of-flight measurement, scanning the repetition frequency or scanning the reference beam path. Based on the basic theory of FOFC and the ranging scheme, the theoretical model for large scale distance measurement chain based on dual FOFCs has been constructed; influence of the multi-heterodyne lowest spectral lines and the repetition frequency stability on the measurement results has been discussed, and lots of simulation calculations have been done. Simulation results show that the method has achieved measurement errors better than ± 50 pm on the premise of not considering the phase demodulation accuracy, and the impact caused by the deviation of the lowest multi-heterodyne spectrum is figured out to be far below the ranging resolution of the multi-heterodyne measurement, which has verified that the proposed method may be used to realize large-scale and high-precision absolute distance measurement.