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

Ultrafine electro-optical frequency comb based on cascade phase modulation with cyclic frequency shifting

An ultrafine electro-optical frequency comb (EOFC) with plentiful comb teeth is demonstrated. Adopting a single-frequency fiber laser as a light source, cascade phase modulation based on a sinusoidal signal and a frequency-time transformation (FTT) signal is executed to generate the EOFC with high fineness. Meanwhile, a cyclic fast frequency shifting strategy is introduced to boost the number of comb teeth and the bandwidth of the EOFC. As a result, an EOFC with 12600 comb lines covering a broad bandwidth from -6.3 GHz to 6.3 GHz is established, corresponding to an ultrafine comb space of 1 MHz. Moreover, the power fluctuation of a comb tooth is less than 0.5 dBm. This state-of-the-art EOFC has significant potential in the field of precision spectroscopy.

Time-expanded φOTDR using low-frequency electronics

Time expanded phase-sensitive optical time-domain reflectometry (TE-φOTDR) is a recently reported technique for distributed optical fiber sensing based on the interference of two mutually coherent optical frequency combs. This approach enables distributed acoustic sensing with centimeter resolution while keeping the detection bandwidth in the megahertz range. In this paper, we demonstrate that TE-φOTDR can be realized with low-frequency electronics for both signal generation and detection. This achievement is possible thanks to the use of a couple of electro-optic comb generators driven by commercially available step recovery diodes. These components are fed by radio frequencies that are orders of magnitude lower than those involved in the signals so far originated by ultrafast waveform generation. The result is a simple, compact, low-cost and potentially field-deployable sensor that works without resorting to any decoding algorithm. Besides, high-resolution distributed sensing is carried out with no need of coding strategies or enhanced backscatter fibers. To check the capabilities of our system, we perform distributed strain sensing over a range of 20 m. The spatial resolution is 3 cm and the acoustic sampling rate can be increased up to 200 Hz. This performance reveals the prospective of the proposed approach for field applications, including structural health monitoring.

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.

Phase-stable repetition rate multiplication of dual-comb spectroscopy based on a cascaded Mach-Zehnder interferometer

In this Letter, we demonstrate a passive all-fiber pulse delay method for repetition rate multiplication of dual-comb spectroscopy. By combining a cascaded Mach-Zehnder interferometer and digital error correction, a mode-resolved spectrum with improved acquisition speed and sensitivity can be obtained. This technique has the strengths of compact, broadband, high energetic efficiency, and low complexity. Due to the use of an adaptive post-processing algorithm, sophisticated closed-loop feedback electronics are not required, which provides a simple and effective scheme to break through the physical limitation of the repetition frequency of the frequency comb for phase-stable dual-comb applications.

Rapid optical tomographic vibrometry using a swept multi-gigahertz comb

We propose a rapid tomographic vibrometer technique using an optical comb to measure internal vibrations, transient phenomena, and tomographic distributions in biological tissue and microelectromechanical system devices at high frequencies. This method allows phase-sensitive tomographic measurement in the depth direction at a multi-MHz scan rate using a frequency-modulated broadband electrooptic multi-GHz supercontinuum comb. The frequency spacing was swept instantaneously in time and axisymmetrically about the center wavelength via a dual-drive Mach-Zehnder modulator driven by a variable radio frequency signal. This unique sweeping method permits direct measurement of fringe-free interferometric amplitude and phase with arbitrarily changeable measurement range and scan rate. Therefore, a compressive measurement can be made in only the depth region where the vibration exists, reducing the number of measurement points. In a proof-of-principle experiment, the interferometric amplitude and phase were investigated for in-phase and quadrature phase-shifted interferograms obtained by a polarization demodulator. Tomographic transient displacement measurements were performed using a 0.12 mm thick glass film and piezo-electric transducer oscillating at 10-100 kHz with scan rates in the range 1-20 MHz. The depth resolution and precision of the vibrometer were estimated to be approximately 25 µm and 1.0 nm, respectively.

Wideband and high-resolution spectroscopy based on an ultra-fine electro-optic frequency comb with seed lightwave selection via injection locking

The digital ultra-fine electro-optic frequency comb (UFEOFC) enables high-resolution spectroscopy in various applications with a limited bandwidth. In this Letter, we propose a novel, to the best of our knowledge, UFEOFC-based dual-comb spectroscopy to realize megahertz resolution and broadened bandwidth. An UFEOFC with 1 MHz line-spacing and 18 GHz bandwidth is generated and resolved in a dual-comb interferometer working in quasi-integer-ratio mode. One line selected from a master EOFC with 18 GHz line-spacing via optical injection locking serves as the seed lightwave. Successive selection of 20 lines realizes wideband measurement covering 360 GHz, and a reflectance spectrum of a phase-shift fiber Bragg grating is obtained in the demonstration. A spectrum with 360,000 lines is demodulated in 26 ms due to the low injection lock dead time set to be 300µs between adjacent lines, by which the figure of merit reaches 3.39×10⁷. The system promotes more prospects in practical spectroscopic and sensing applications.

Rayleigh speckle-based wavemeter with high dynamic range and fast reference speckle establishment process assisted by optical frequency combs

Compact speckle-based spectrometers that acquire lightwave wavelength from the speckle generated by turbid media are promising for high-resolution spectral analysis. For these devices, the reference establishment process is time consuming, and it is very difficult to obtain reference speckles covering a wide bandwidth with high resolution, which restricts the dynamic range in frequency (the ratio of bandwidth to resolution). In this Letter, we introduce optical frequency combs (OFCs) to the system to overcome these problems, which exist in the wavemeter based on Rayleigh speckle obtained from a single-mode fiber. In the experiment, the proposed wavemeter has a 1.5 nm bandwidth with 60 am resolution, covering a dynamic range in the frequency of 2×10⁷, and a fast reference speckle establishment process that takes only 0.9 ms. The proposed method assisted by OFCs is a good prospect for a more practical speckle-based wavemeter with higher dynamic ranges.

All-fiber phase modulator and switch based on local surface plasmon resonance effect of the gold nanoparticles embedded in gel membrane

All-fiber modulators and switches have drawn great interest in the photonics domain, and they are applied in viable photonic and optoelectronic devices. In this work, with the assistance of an agarose membrane, aspherical gold nanoparticles are embedded on the surface of the microfiber treated with the piranha solution. An all-fiber Mach-Zehnder interferometer was used to realize a low-cost, low-loss, and conveniently prepared all-fiber phase modulator. By taking advantage of the local surface plasmon resonance effect of gold nanoparticles embedded in the agarose membrane, under the excitation of near-infrared region light, the gold nanoparticles were excited to change the effective refractive index of one arm of the Mach-Zehnder interferometer. A maximum phase shift of ∼6π at 1550 nm was obtained from the device. In addition, an all-optical switch was achieved with a rising edge time of 47 ms and falling edge time of 14 ms. The proposed all-fiber modulator and switch based on the local surface plasmon resonance effect of gold nanoparticles embedded in agarose membrane will provide great potential in all-optical fiber systems.

Gain-switched semiconductor lasers with pulsed excitation and optical injection for dual-comb spectroscopy

In this work we demonstrate the capability of two gain-switched optically injected semiconductor lasers to perform high-resolution dual-comb spectroscopy. The use of low duty cycle pulse trains to gain switch the lasers, combined with optical injection, allows us to obtain flat-topped optical frequency combs with 350 optical lines (within 10 dB) spaced by 100 MHz. These frequency combs significantly improve the spectral resolution reported so far on dual-comb spectroscopy with gain-switched laser diodes. We evaluate the performance of our system by measuring the transmission profile of an absorption line of H¹³CN at the C-band, analyzing the attainable signal-to-noise ratio for a range of averaging times.

Dual electro-optic frequency comb spectroscopy using pseudo-random modulation

An approach for dual-comb spectroscopy using electro-optic (EO) phase modulation is reported. Maximum-length pseudo-random binary sequences allow for energy-efficient and flexible comb generation. Self-correction of interferograms is shown to remove relative comb drifts and improve mutual coherence, even for EO combs derived from the same laser source. Methane spectroscopy is reported over a ∼10  GHz spectral range, limited by the modulators' bandwidth. The potential of a simple EO comb instrument is demonstrated to rapidly quantify atmospheric methane emissions with ppb precision over 1 km.