Tunable resolution terahertz dual frequency comb spectrometer

Triggerless data acquisition in asynchronous optical-sampling terahertz time-domain spectroscopy based on a dual-comb system

By using two mutually phase-locked optical frequency combs with slightly different repetition rates, we demonstrate asynchronous optical-sampling terahertz time-domain spectroscopy (ASOPS THz-TDS) without using any trigger signals or optical delay lines. Due to a tight stabilization of the repetition frequencies, it was possible to accumulate the data over 48 minutes in a triggerless manner without signal degradation. The fractional frequency stability of the measured terahertz signal is evaluated to be ∼8.0 × 10^-17 after 730 s. The frequency accuracy of the obtained terahertz spectrum is ensured by phase-locking the two frequency combs to a frequency standard. To clarify the performance of our system, we characterized the absorption line of water vapor around 0.557 THz. The good agreement of the measured center frequency and linewidth with the values predicted from the HITRAN database verifies the suitability of our ASOPS THz-TDS system for precise measurements.

Flexible terahertz opto-electronic frequency comb light source tunable over 3.5 THz

We demonstrate a terahertz (THz) frequency comb that is flexible in terms of its frequency range and the number and spacing of comb lines. We use a combination of near-infrared laser diodes, phase modulation, and opto-electronic frequency conversion. The THz comb lines are characterized to be <10MHz by resolving the pressure-dependent collisional broadening of an ammonia molecule rotational mode.

Interleaved electro-optic dual comb generation to expand bandwidth and scan rate for molecular spectroscopy and dynamics studies near 1.6 µm

A chirped-pulse interleaving method is reported for generation of dual optical frequency combs based on electro-optic phase modulators (EOM) in a free-running all-fiber based system. Methods are discussed to easily modify the linear scan rate and comb resolution by more than three orders of magnitude and to significantly increase the spectral bandwidth coverage. The agility of the technique is shown to both capture complex line shapes and to magnify rapid passage effects in spectroscopic and molecular dynamics studies of CO₂. These methods are well-suited for applications in the areas of remote sensing of greenhouse gas emissions, molecular reaction dynamics, and sub-Doppler studies across the wide spectral regions accessible to EOMs.

Enhanced electro-optic beam deflection of relaxor ferroelectric KTN crystals by electric-field-induced high permittivity

Most applications of a ferroelectric-based electro-optic (EO) beam deflector have been limited by the high applied voltage. In this Letter, we report a dramatically increased EO beam deflection in relaxor ferroelectric potassium tantalate niobate (KTN) crystals by using the electric-field-enhanced permittivity. Due to the existence of the electric-field-induced phase transition in relaxor ferroelectric materials, the dielectric permittivity can be substantially increased by the applied electric field at a certain temperature. Both the theoretical study and the experimental verifications on the enhanced beam deflection and EO effect in the case with the electric-field-induced high permittivity were conducted. The experimental results confirmed that there was a three-fold increase in the deflection angle, which represented a dramatic increase in the deflection angle. By offering a wider deflection range and a lower driving voltage, such a largely enhanced beam deflection is of great benefit to the KTN deflector.

Broadband and high-resolution electro-optic dual-comb interferometer with frequency agility

We propose a broadband and high-resolution dual-comb interferometer (DCI) realized with dense electro-optic frequency combs (EOFCs), which are generated with two-stage electro-optic modulation and nonlinear fiber spectral broadening. The broadband coarse comb is generated in the first-stage modulation and the space between neighboring combs is filled with a dense electrical comb in the second-stage modulation. The spectral resolution of the DCI can be flexibly changed from 10 MHz to 1 GHz easily as required, and electro-optic DCIs with 10-MHz, 100-MHz, and 1-GHz frequency resolutions are experimentally realized. Meanwhile, DCI working in the quasi-integer-ratio mode is easily implemented in this system for the increased refresh rate and improved figure of merit especially for high resolution. As a demonstration, 150000 comb lines with 10 MHz frequency interval are resolved in the experiment.

Electro-optic THz dual-comb architecture for high-resolution, absolute spectroscopy

An absolute-frequency terahertz (THz) dual-frequency comb spectrometer based on electro-optic modulators for tunable, high-resolution, and real-time rapid acquisition is presented. An optical line of a master frequency comb (filtered via optical injection locking) serves as the seed to electro-optically generate a pair of new frequency combs (probe and local oscillator). Photomixing both combs with another coherent line from the same original master comb generates a narrow linewidth THz dual-comb with teeth frequencies that can be referenced to a radio-frequency standard. The system is validated with a proof-of-principle measurement of a microwave filter in the W-band.

Fast MHz spectral-resolution dual-comb spectroscopy with electro-optic modulators

Frequency-agile dual-comb spectroscopy (DCS) with electro-optic modulators (EOMs) promises to facilitate the implementation of DCS in many environmental applications. In this Letter, we demonstrate a 1 MHz resolution electro-optic dual-comb system with a measurement speed of 20 kHz. We generate an electro-optic frequency comb (EOFC) consisting of 50000 teeth with 1 MHz line spacing. Each comb tooth is well resolved by another EOFC with 2.5 GHz line spacing. We record transmittances of high Q-factor resonators within milliseconds. Our figure of merit is shown to be 36 times more sensitive than a configuration that would beat two combs of 1 MHz spacing.

Dual terahertz comb spectroscopy with a single free-running fibre laser

Dual terahertz (THz) comb spectroscopy enables high spectral resolution, high spectral accuracy, and broad spectral coverage; however, the requirement for dual stabilized femtosecond lasers hampers its versatility. We here report the first demonstration of dual THz comb spectroscopy using a single free-running fibre laser. By tuning the cavity-loss-dependent gain profile with an intracavity Lyot filter together with precise management of the cavity length and dispersion, dual-wavelength comb light beams with slightly detuned repetition frequencies are generated in a single laser cavity. Due to sharing of the same cavity, such comb light beams suffer from common-mode fluctuation of the repetition frequency, and hence the corresponding frequency difference between them is passively stable around a few hundred hertz within millihertz fluctuation. While greatly reducing the size, complexity, and cost of the laser source by use of a single free-running fibre laser, the dual THz comb spectroscopy system maintains a spectral bandwidth and dynamic range of spectral power comparable to a system equipped with dual stabilized fibre lasers, and can be effectively applied to high-precision spectroscopy of acetonitrile gas at atmospheric pressure. The demonstrated results indicate that this system is an attractive solution for practical applications of THz spectroscopy and other applications.

Dense electro-optic frequency comb generated by two-stage modulation for dual-comb spectroscopy

An electro-optic frequency comb enables frequency-agile comb-based spectroscopy without using sophisticated phase-locking electronics. Nevertheless, dense electro-optic frequency combs over broad spans have yet to be developed. In this Letter, we propose a straightforward and efficient method for electro-optic frequency comb generation with a small line spacing and a large span. This method is based on two-stage modulation: generating an 18 GHz line-spacing comb at the first stage and a 250 MHz line-spacing comb at the second stage. After generating an electro-optic frequency comb covering 1500 lines, we set up an easily established mutually coherent hybrid dual-comb interferometer, which combines the generated electro-optic frequency comb and a free-running mode-locked laser. As a proof of concept, this hybrid dual-comb interferometer is used to measure the absorption and dispersion profiles of the molecular transition of H¹³CN with a spectral resolution of 250 MHz.