Fiber Comb Research Articles

Recent Advances, Applications, and Perspectives in Erbium-Doped Fiber Combs

Optical frequency combs have emerged as a new generation of metrological tools, driving advancements in various fields such as free-space two-way time–frequency transfer, low-noise microwave source generation, and gas molecule detection. Among them, fiber combs based on erbium-doped fiber mode-locked lasers have garnered significant attention due to their numerous advantages, including low noise, high system integration, and cost-effectiveness. In this review, we discuss recent developments in erbium-doped fiber combs and analyze the advantages and disadvantages of constructing fiber combs utilizing different erbium-doped mode-locked fiber lasers. First, we provide a brief introduction to the basic principles of optical frequency combs. Then, we explore erbium-doped fiber combs implemented utilizing various mode-locking techniques, such as nonlinear polarization rotation (NPR), real saturable absorber (SA), and nonlinear amplifying loop mirror (NALM). Finally, we present an outlook on the future perspectives of erbium-doped fiber combs.

Broadband, high-power optical frequency combs covering visible to near-infrared spectral range.

Optical frequency combs (OFCs) have become essential tools in a wide range of metrological and scientific research fields. However, in the reported literature, OFCs that cover the visible spectral range have a limited bandwidth and pulse energy. These drawbacks limit their potential applications, such as high-signal-to-noise ratio spectroscopic measurements. In this work, we demonstrate a broadband, high-power optical frequency comb covering the visible to near-infrared range (550 nm to 900 nm) with a high average power of approximately 300 mW. This is accomplished by the power scaling of optical pulses from a fully stabilized Er:fiber comb, coherent spectral broadening and finally the utilization of a PPLN's χ(2) nonlinearity. The broadband, high-power, fully stabilized visible OFCs showcased in this work offer reliable laser sources for high-precision spectroscopic measurements, imaging, and comparisons of optical clocks.

Toward a compact fiber comb with 1.66 × 10−12 instability based on acetylene-filled photonic microcells

Toward a compact fiber comb with 1.66 × 10−12 instability based on acetylene-filled photonic microcells

Wavelength-tunability of arbitrary transmittance functions in first-order fiber comb filters based on heterogeneous waveplate combination

We investigated the wavelength tunability of arbitrarily chosen transmittance functions (ACTFs) in first-order fiber comb filters using heterogeneous waveplate combinations: specifically, a combination of dual quarter-wave plates (QWPs) and a set of a half-wave plate and a QWP in a polarization-diversified fiber loop (PDFL). PDFL comb filters studied here create polarization interference using two polarization-maintaining fiber (PMF) segments of equal length and a polarization beam splitter. We considered two comb filters with different waveplate combinations to scrutinize the effect of waveplate combination order. By analyzing the effect of the waveplates and PMF on the output polarization state (OPS) of the filter, we identified the orientation angles of the waveplates that resulted in a redshift of the ACTF by one free spectral range (λS) by making all polarization states on the OPS trace of the second PMF move in the direction of wavelength decrease. For each filter, we theoretically derived 360 sets of waveplate angles that enabled the tuning of its ACTF by λS. Among these sets, we picked eight sets inducing a wavelength shift of λS/8 for each increase in set order and displayed wavelength-shifted comb spectra at these eight sets, which corroborate the wavelength tunability of the ACTFs of the filters.

Analysis of Gas Mixtures with Broadband Dual Frequency Comb Spectroscopy and Unsupervised Learning Neural Network

Broadband mid‐infrared spectroscopy not only offers supreme sensitivity for the massively parallel detection of trace gases but also presents many challenges. Herein, a new platform combining the advantages of a mid‐infrared dual‐comb spectrometer based on two difference‐frequency generation combs pumped by femtosecond Er‐doped fiber comb oscillators and an unsupervised deep learning neural network consisting of information extraction and information mapping blocks is presented. The scarce data problem, the uncertainties of apparatus, and manual operations intrinsic to multicomponent gas mixture analysis are overcome by coupling an unsupervised leaning approach with a model‐agnostic, physics‐informed data augmentation strategy using simulated data from spectral databases. The system provides reliable simultaneous identification of gas species, concentration retrieval, as well as ambient pressure prediction and eliminates the negative impacts on the measurement, such as model error, baseline fluctuation, and unknown absorbers. Parallel optical detection of 31 different mixtures of 5 gas species over a 2900–3100 cm−1 spectral range with a sub part‐per‐billion sensitivity is demonstrated showing the potential in various applications such as atmospheric monitoring, diagnostics with breath biomarkers, and capturing rapid chemical reaction kinetics.

The Effect of Variation of Corn Comb Fiber Composition on The Physical Properties of Biofoam with The Addition of Aerogel Silica

Styrofoam has many negative impacts because it harms health, pollutes the environment, and cannot decompose in nature. Efforts have been made to develop biofoam derived from natural kinds of stuff to reduce the harmful effects of styrofoam. So that they are safer and do not pollute the environment. Biofoam in this study was made from tapioca starch, corncob fiber, a binding polymer in the form of Polyvinyl Alcohol (PVA), and a superhydrophobic agent in the form of silica aerogel. This research was conducted to determine the effect of variations in the composition of corncob fiber on the physical properties of biofoam with the addition of silica aerogel. The production of corncob fiber was carried out in two stages, namely using 10% NaOH to remove lignin and bleaching using 10% H2O2 to bleach the fibers, which were then combined into the biofoam mixture. The production of biofoam used the thermopressing method with a temperature of 150° and pressed for 4 minutes with fiber variations of 13.75g, 16.25g, 18.75g, and 21.25g. Add conclusion of the research is 18.75 g of fiber with a water absorption value of (1.2 ± 0.2)%, a density of (0.5 ± 0.1)g/cm3, and a compressive strength of 3.60 MPa.

광섬유 빗살 필터의 사분파장 지연기 연속 조합을 이용한 협대역 다파장 스펙트럼의 연속 주파수 조정에 관한 연구

We propose an optical fiber comb filter based on a polarization-diversity loop structure, which can continuously adjust the narrowband multiwavelength spectrum. The proposed comb filter consists of a polarization beam splitter(PBS), a successive combination of quarter-wave retarders(QWRs), a set of a half-wave retarder and a QWR, and two polarization-maintaining fiber(PMF) segments of equal length. For the convenient adjustment of the narrowband multiwavelength spectrum, the successive combination of QWRs was located between the PBS and the first PMF segment. The transmittance function of the proposed filter was theoretically derived using the Jones calculus. Based on the filter transmittance, we theoretically predicted the orientation angle sets of four wave retarders, which allow the extra phase difference φ of the transmittance function to vary from 0˚ to 315˚ with increments of 45˚. Then, the proposed filter successfully demonstrated the predicted results by obtaining ∼0.0915 ㎚ average wavelength shift and a high linearity of 0.99878.

Precision dual-comb spectroscopy using wavelength-converted frequency combs with low repetition rates

Precision spectroscopy contributed significantly to the development of quantum mechanics in its early stages. In the twenty-first century, precision spectroscopy has played an important role in several fields, including fundamental physics, precision measurement, environmental monitoring, and medical diagnostics. An optical frequency comb is indispensable in determining the frequency axis in precision spectroscopy and it is useful as a light source for spectroscopy. Dual-comb spectroscopy uses two frequency combs with slightly different repetition rates and has the potential to surpass conventional Fourier-transform infrared spectrometers. The resolution of dual-comb spectroscopy is limited by the frequency spacing of the comb components, that is, the repetition rate of the comb. We demonstrate dual-comb spectroscopy in the visible-wavelength region using wavelength-converted frequency combs from Er-doped fiber combs. The repetition rates of the combs are relatively low at 19.8 MHz, resulting in relatively high resolution in the dual-comb spectroscopy. The observed spectral shape in dual-comb spectroscopy agrees well with the fitting result based on the hyperfine structure of molecular iodine. The realized dual-comb spectroscopy using wavelength-converted Er-doped fiber combs is reliable (maintenance free) and applicable in other experiments at visible wavelengths.

An Investigation of All Fiber Free-Running Dual-Comb Spectroscopy.

A dual-comb spectroscopy (DCS) system uses two phase-locked optical frequency combs with a slight difference in the repetition frequency. The spectrum can be sampled in the optical frequency (OF) domain and reproduces the characteristics in the radio frequency (RF) domain through asynchronous optical sampling. Therefore, the DCS system shows great advantages in achieving precision spectral measurement. During application, the question of how to reserve the mutual coherence between the two combs is the key issue affecting the application of the DCS system. This paper focuses on a software algorithm used to realize the mutual coherence of the two combs. Therefore, a pair of free-running large anomalous dispersion fiber combs, with a center wavelength of approximately 1064 nm, was used. After the signal process, the absorption spectra of multiple species were simultaneously obtained (simulated using the reflective spectra of narrow-bandwidth fiber Bragg gratings, abbreviated as FBG). The signal-to-noise ratio (SNR) could reach 13.97 dB (25) during the 100 ms sampling time. In this study, the feasibility of the system was first verified through the simulation system; then, a principal demonstration experiment was successfully executed. The whole system was connected by the optical fiber without additional phase-locking equipment, showing promise as a potential solution for the low-cost and practical application of DCS systems.

Single-cavity dual-comb fiber lasers and their applications

Single-cavity, dual-comb lasers are those specially designed mode-locked lasers that can emit more than one, asynchronous ultrashort pulse trains with stable repetition frequency difference between them. Unlike the long-studied, widely-used femtosecond lasers generating one stable pulse train, systematic investigation on them and their potential dual-comb applications only began, based on the fiber laser platform, around a decade ago, despite sporadic and limited reports of similar lasing phenomena since the beginning of the mode-locked laser studies. From a historic perspective, the birth of this novel technology is the lucky outcome of the timely collision of perpetual search for novel pulsing laser dynamics and concerted pursuit of open-minded solutions for out-of-lab dual-comb systems in the 2010s. In this review article, first, the current schemes to implement single-cavity dual optical frequency comb fiber lasers and their applications are summarized, based on the concept of multiplexed mode-locked lasers. The characteristics of reported single-cavity, dual-comb fiber lasers are discussed as well as their applications in spectroscopy, ranging, Terahertz (THz) spectroscopy, and asynchronous optical sampling (ASOPS). Finally, the more recent development of single-cavity, multi-comb lasers is presented.

Study of the relations of interspecies hybridization of the parental forms with plant height in f1, fb and f2, f3 compounds of per branch of the first harvest (hs)

Particular attention is paid to their genetic mechanisms during the interspecific hybridization using of G.hirsutum L. and G.barbadense L. species in the cotton plants. The study of the genetic foundations of chasmogamous and cleistogamous flower traits in solving the urgent problems facing the science of genetics and cotton breeding includes its technological processes, adaptation to the world standards, the actual problem of seed science, and basic solutions. In the interspecific hybridization of the parental forms, F1, Fb and F2, F3 compounds, the plant height and first branch of the crop (hs), fertile, resistant to various diseases, dominant specimens with a high predominance of fiber yield and fiber length, comb, gene collections and varieties are paid the main attention.

All-fiber acetylene-referenced optical frequency comb

We demonstrated an all-fiber compact Erbium-doped optical frequency comb based on optical reference. Compared with the traditional RF reference, the whole system adopts the all-fiber design of optical reference. Comb stabilization is achieved by simultaneously locking the repetition rate (fr) to an RF reference and an optical comb tooth to a CW reference. The standard deviation of the fr frequency in 4.5 hours was recorded as 0.37 mHz, leading to a fraction instability of 4.3 ×10−12 at 1 s gate time and 2.8 ×10−13 at 10 s gate time. The locking of single comb tooth is achieved by locking the optical comb and the beat note of the CW laser. We used the P(16) overtone transition line of 13C2H2 as the CW reference and optimized the stabilization by choosing appropriate lock point on the Doppler-limited absorption transition. The comb was able to achieve stable locking for almost five hours. The short-term fraction instability of the optical frequency was measured to be 6.56 ×10−10 at 1 s gate time, which is mostly from the CW reference. Our approach of acetylene-stabilized fiber comb offers a feasible solution to help fiber combs move from laboratory to outdoors applications based on fiber sensing, laser stabilization and spectrum analysis.

Dead-zone free single-shot three-dimensional measurement using a high-repetition-rate Yb:fiber comb

Using spectral interference from a highly chirped and high-repetition-rate optical frequency comb, a technique for dead-zone free one-shot three-dimensional (3D) imaging was developed. We generated continuously emitted pulse train (dead-zone-free pulse) using a highly chirped pulse from a 750-MHz repetition mode-locked Yb fiber comb. The measurement performances of the system were evaluated using an optical spectrum analyzer, and a longitudinal delay length resolution of 31 µm was obtained at arbitrary position for more than 400 mm range. Using an imaging spectrometer capable of multipoint spectroscopy, a delay length resolution of 81 µm was demonstrated. Finally, for a proof-of-concept measurement of 3D imaging, line profiles of three mirrors located in the middle of 200-mm-distance, corresponding to pulse-to-pulse interval, were measured without any scanning with an uncertainty of 0.7 mm. The uncertainty could be further improved by optical alignment and analysis to the level of 10 ∼ 100 µm at arbitrary position over the range not limited to pulse-to-pulse interval.

Precision frequency transfer with fiber frequency combs

We review methods for precision transfer of frequencies across broad optical wavelength ranges. Single-branch supercontinuum generation allows for a frequency transfer stability of < 1 × 10−17in 1 s across an octave. With supercontinuum stitching, highly coherent supercontinuum spectra spanning across more than two octaves are generated. With noise cancellation techniques a relative frequency transfer stability of ≈ 2 × 10−18in 1 s can be achieved. Highly stable frequency transfer along with a maximization of power per mode at multiple freely selectable frequency bands is further enabledviapulse shaping techniques. We also include a brief review of general fiber combs and research aimed at frequency extension of frequency combs covering the whole spectral range from the XUV to the mid IR, power scaling of frequency combs as well as low noise microwave and mmwave technology enabled with frequency combs.

High-resolution time-resolved spectroscopy based on hybrid asynchronous optical sampling

The capability of characterizing arbitrary and non-repetitive emission spectra with a high resolution in real-time is of great merit in various research fields. Optical frequency combs provide precise and stable frequency grids for the measurement of a single spectral line with high accuracy. Particularly, dual-comb spectroscopy enables spectral measurement with a large bandwidth spanning tens of nanometers, but it is limited to measuring absorption spectra and has to trade-off spectral resolution vs the acquisition frame rate set by the repetition rate. Here, to alleviate these restrictions, we propose and demonstrate time-resolved spectroscopy for an emission spectrum based on hybrid asynchronous optical sampling, which features a spectral resolution of 0.63 pm, a frame rate of 1 MHz, and a measurement bandwidth of 13.6 nm, simultaneously. A mode-locked fiber comb with a repetition frequency of f1 is harnessed to interrogate emission spectral features with high resolution via optical Fourier transform achieved using a time-lens. Subsequently, a soliton microcomb of repetition frequency f2s ≈ 1000f1 serving as a probe pulse implements hybrid asynchronous optical sampling, thus significantly increasing the acquisition rate by nearly 3 orders of magnitude. As a proof-of-concept demonstration, the frequency trajectory of a rapidly scanning laser with a linear chirp of 6.2 THz/s is tracked. We believe that chip-scale microcombs will make the fast and high-resolution emission spectroscopy presented here a powerful tool for widespread applications.

Design and analysis of polarization-beam-splitter-based fiber optical comb filter

In this paper, we proposed and demonstrated a novel switchable fiber comb filter based on polarization beam splitter (PBS). The proposed filter transmission characteristics is theoretically analyzed in detail by using the transmission matrix theory. The simulated results show that the filter can generate two different output channels selected by adjusting the polarization controllers (PC) in front of the PBS, and the two different channels’ wavelength intervals can be changed with polarization-maintaining fiber’s (PMF’s) characteristics. Different PMFs give a new switchable combination of channel intervals. A multi-wavelength fiber laser (MWFL) based on the proposed filter is experimentally demonstrated. Its wavelength interval can be switched among 0.85 nm, 0.40 nm, which is consistent with the simulated results of the filter. The power fluctuation of the two channel interval outputs in one hour is less than 0.88 dB and 1.37 dB, and the wavelength fluctuation is less than 0.06 nm and 0.18 nm, respectively.

Quasi-real-time dual-comb spectroscopy with 750-MHz Yb:fiber combs.

We present quasi-real-time dual-comb spectroscopy (DCS) using two Yb:fiber combs with ∼750 MHz repetition rates. A computational coherent averaging technique is employed to correct timing and phase fluctuations of the measured dual-comb interferogram (IGM). Quasi-real-time phase correction of 1-ms long acquisitions occurs every 1.5 seconds and is assisted by coarse radio frequency (RF) phase-locking of an isolated RF comb mode. After resampling and global offset phase correction, the RF comb linewidth is reduced from 200 kHz to ∼1 kHz, while the line-to-floor ratio increases 13 dB in power in 1 ms. Using simultaneous offset frequency correction in opposite phases, we correct the aliased RF spectrum spanning three Nyquist zones, which yields an optical coverage of ∼180 GHz around 1.035 µm probed on a sub-microsecond timescale. The absorption profile of gaseous acetylene is observed to validate the presented technique.

Slow saturable absorption for optimal operation in a soliton comb laser.

In this article, we study how the choice of parameters of a slow saturable absorber (SSA) affects the stable operation of a soliton fiber comb laser. We show that a shorter recovery time for the SSA does not always lead to shorter modelocked pulses. Instead, increasing the cavity gain plays a critical role in generating stable modelocked pulses with higher energy and shorter durations. We find that more stable, shorter, and more energetic output pulses can be achieved with lower saturation energies of the SSA and/or higher anomalous dispersion within the cavity.

Polarization-independent wavelength-tunable flat-top comb filter based on quarter-wave polarization transformation

By harnessing quarter-wave polarization transformation, we propose a polarization-independent wavelength-tunable flat-top fiber comb filter composed of a polarization beam splitter, two polarization-maintaining fiber segments of equal length, a set of a half-wave plate and a quarter-wave plate (QWP), and dual QWPs, the simplest polarization controller. Using the Jones calculus, we derived the polarization-independent filter transmittance and sought the possible loci of the orientation angles (OAs) of the four waveplates as the functions of the transmittance phase (ϕ). With these OA loci, flat-top transmission spectra were calculated for eight ϕ values from 0° to 315° (increment: 45°), which manifests the frequency tunability of the filter. This theoretical prediction was also experimentally verified by using a broadband light source and optical spectrum analyzer. Eight measured flat-top transmission spectra were spaced 0.1 nm apart in wavelength as predicted, and the wavelength-tuning linearity was high enough to exhibit an adjusted R 2 value of ∼0.99925.

Delay-Spectral Focusing Dual-Comb Coherent Raman Spectroscopy for Rapid Detection in the High-Wavenumber Region

Rapid multiplex coherent anti-Stokes Raman scattering (CARS) spectroscopy in the high-wavenumber (HW) region shows great advantages in real-time dynamic process visualizations, clinical diagnosis, abundant microplastic assessment, etc. Fourier transform CARS (FT-CARS) improves the acquisition speed of multiplex CARS to the order of tens of kHz, yet typical approaches to utilize the intrinsic coherence of ultrabroadband pulses notably impede the attainable Raman vibrations in the HW region. Here, a novel delay-spectral focusing dual-comb (DC) CARS scheme is proposed for rapid HW Raman detection based on two fiber combs with 100 MHz repetition rates. By combining the particular advantages of DC asynchronous optical sampling and spectral focusing instantaneous single molecular vibration excitation, the Raman spectrum directly maps to DC relative delay, releasing the coherence constraint of excitation sources. Thus, an Er comb and a Yb comb, as pump and Stokes excitation pulses, respectively, flexibly match the Raman vibration in the HW region. Further, a rapid delay focusing method with intracavity electro-optic modulation is applied to actively control the DC relative delay scanning in the Raman shift region of interest. With these efforts, the spectral acquisition rate is improved more than 1000-fold up to 40 000 spectra/s, while keeping spectral resolution (∼10 cm–1) and the signal-to-noise ratio (∼260) stable along with active acquisition rate tuning. Combined with a microfluidic device, high-speed measurement in the HW region of 15 μm microbeads in flow is demonstrated by the system, reaching a high throughput of around 2150 events/s and more than 99% classification consistency. The prospect of rapid acquiring multiplex CARS spectra without the sacrifice of spectral resolution or the need for the coherence of the pulse sources offers huge potential in rapid monitoring circumstances, such as flow cytometry and microspectroscopic imaging in biomedicine.