Comb Teeth Research Articles

Optical frequency linked dual-comb absorption spectrum measurement

Dual-comb spectroscopy is becoming a highlighted topic in broadband spectrum measurement techniques because of two outstanding advantages. One is its highly stable output frequency, which leads to an appealing resolution, and the other is the omitting of moving parts, which helps achieve extreme fast sampling rate. Utilizing the traditional radio frequency linked combs, however, obstructs the dual-comb spectroscopy reaching satisfied performance because the phase noise of the radio frequency standard causes the dual-comb mutual coherence to severely degrade. Specifically, traditional frequency comb stabilizes the carrier envelope offset at a radio frequency by a self-reference system, and the order number of each output comb tooth is over a hundred thousand. Thus, the phase noise of the radio frequency reference is significantly multiplied in output optical frequency by the same order of magnitude as the tooth order number. In this paper, we demonstrate an optical frequency linked dual-comb spectrometer where the two combs are locked to a common narrow linewidth laser. In this configuration, the two combs are synchronized at an identical optical frequency, which means that the carrier envelope offset of the two combs are changed to an optical frequency and the order number of the output comb teeth are reduced by two orders of magnitude. Therefore, not only the complex and costly self-reference system can be removed but also the phase noise of the optical frequency of each comb tooth is effectively reduced, which leads to lower mutual frequency jitters and better mutual coherence. To prove the performance, we measure the 1+3 P branch of 13C2H2 molecular and the results accord well with the reported line positions and reveals a spectral resolution of 0.086 cm-1. The average signal-to-noise ratio exceeds 200:1 (62.5 ms, 100 times on average) and the noise equivalent coefficient is 6.0106 cm-1Hz-1/2. This work provides a solution for pragmatic dual-comb spectroscopy with high resolution and low-cost configuration.

Electromagnetically induced transparency in vacuum and buffer gas potassium cells probed via electro-optic frequency combs.

Electromagnetically induced transparency (EIT) in K39 and K41 was probed using electro-optic frequency combs generated by applying chirped waveforms to a phase modulator. The carrier tone of the frequency comb served as the pump beam and induced the necessary optical cycling. Comb tooth spacings as narrow as 20kHz were used to probe potassium in both buffer gas and evacuated cells at elevated temperatures. Atomic absorption features as narrow as 33(5)kHz were observed, allowing for the K39 lower-state hyperfine splitting to be optically measured with a fit uncertainty of 2kHz. Due to the ultranarrow width of the EIT features, long-lived optical free induction decays were also observed which allowed for background-free detection.

Design of a treatment chamber for low-voltage pulsed electric field sterilization

Abstract In this paper, different microchips were simulated by the finite element method (FEM) with the aim to clarify the effect of topological parameter on inactivating microorganisms vaccinated in blueberry juice under low voltage for 50–250 μs and 20–100 pulses. For the planar comb teeth microchips in the first generation, only a 1.01 log 10 of Escherichia coli ( E. coli ) was achieved at 25 °C in all the solutions, then, with the aim to obtain a more uniform electric field distribution, an interdigitated electrode structure are used in the second-generation microchip which improved the inactivation effect of E. coli up to 2.85 log 10 reduction. At the end, by considering the problems presented by the two aforementioned microchips, the structure of a parallel plate as the third-generation microchip was designed and the best experimental platform was built. This microtreatment chamber exhibited excellent inactivation effect on E. coli , Saccharomyces cerevisiae , and Staphylococcus aureus of 5.32, 5.42, and 4.77 log 10 cycles respectively. These results will provide a meaningful guidance in inactivate microorganisms by low-voltage PEF.

Enhancement of the performance of a fiber-based frequency comb by referencing to an acetylene-stabilized fiber laser.

We demonstrate a significant improvement in the performance of a fiber-based frequency comb when a GPS-disciplined Rb clock is replaced with an acetylene-stabilized laser as the frequency reference. We have developed a compact, maintenance-free acetylene-stabilized fiber laser with a sub-kHz short-term linewidth and an Allan deviation below 3×10-13 for integration times above 1 s. Switching the comb reference from the Rb clock to the acetylene-stabilized laser improves both comb tooth linewidth and Allan deviation by about two orders of magnitude. Furthermore, long-term measurements of the acetylene-stabilized laser frequency with reference to the GPS-disciplined clock indicate a potential relative frequency uncertainty of 2 × 10-12.

The large X-effect on secondary sexual characters and the genetics of variation in sex comb tooth number in Drosophila subobscura.

Genetic studies of secondary sexual traits provide insights into whether and how selection drove their divergence among populations, and these studies often focus on the fraction of variation attributable to genes on the X‐chromosome. However, such studies may sometimes misinterpret the amount of variation attributable to the X‐chromosome if using only simple reciprocal F1 crosses, or they may presume sexual selection has affected the observed phenotypic variation. We examined the genetics of a secondary sexual trait, male sex comb size, in Drosophila subobscura. This species bears unusually large sex combs for its species group, and therefore, this trait may be a good candidate for having been affected by natural or sexual selection. We observed significant heritable variation in number of teeth of the distal sex comb across strains. While reciprocal F1 crosses seemed to implicate a disproportionate X‐chromosome effect, further examination in the F2 progeny showed that transgressive autosomal effects inflated the estimate of variation associated with the X‐chromosome in the F1. Instead, the X‐chromosome appears to confer the smallest contribution of all major chromosomes to the observed phenotypic variation. Further, we failed to detect effects on copulation latency or duration associated with the observed phenotypic variation. Overall, this study presents an examination of the genetics underlying segregating phenotypic variation within species and illustrates two common pitfalls associated with some past studies of the genetic basis of secondary sexual traits.

Polarization-sensitive dual-comb spectroscopy

We introduce a method of polarization-sensitive dual-comb spectroscopy. This technique is a combination of dual-comb spectroscopy and rotating compensator polarimetry, in which the rotation frequency of the compensator is optimized to establish the frequency sidebands of each comb tooth. Thus, we can determine the polarization state of each comb tooth by analyzing the amplitudes and phases of the sidebands. For a frequency resolution of 48 MHz and a measurement time of 726.5 s, the precisions of the orientation and the ellipticity angles of the individual comb teeth are ∼0.01 rad.

Multiplexed sub-Doppler spectroscopy with an optical frequency comb.

An optical frequency comb generated with an electro-optic phase modulator and a chirped radiofrequency waveform is used to perform pump-probe spectroscopy on the D1 and D2 transitions of atomic potassium at 770.1 nm and 766.7 nm, respectively. With a comb tooth spacing of 200 kHz and an optical bandwidth of 2 GHz the hyperfine transitions can be simultaneously observed. Interferograms are recorded in as little as 5 μs (a timescale corresponding to the inverse of the comb tooth spacing). Importantly, the sub-Doppler features can be measured as long as the laser carrier frequency lies within the Doppler profile, thus removing the need for slow scanning or a priori knowledge of the frequencies of the sub-Doppler features. Sub-Doppler optical frequency comb spectroscopy has the potential to dramatically reduce acquisition times and allow for rapid and accurate assignment of complex molecular and atomic spectra which are presently intractable.

Carrier-envelope offset frequency linewidth narrowing in a Cr:forsterite laser-based frequency comb.

Cr:forsterite laser-based frequency combs are useful for spectroscopic purposes in the near-IR wavelength region. However, self-referenced Cr:forsterite combs tend to exhibit wide carrier-envelope offset frequency (f0) linewidths, which result in broad comb teeth. This can be attributed to significant frequency noise across the comb's spectral bandwidth. We have stabilized a prism-based Cr:forsterite laser comb and observed narrowing of the f0 linewidth from ∼1.4 MHz down to ∼100 kHz by changing only the prism insertion, and to 23kHz by inserting a knife edge into the intracavity beam while keeping the same prism insertion. The noise dynamics of the Cr:forsterite laser frequency comb are investigated with the goal of explaining this f0 narrowing phenomenon.

Effects of Grinding Fluid Excited by Ultrasonic Vibration

Ultrasonic excited fluid has been researched for machining of hard-to-grind materials. Ultrasonic vibration is applied to grinding fluid by an ultrasonic oscillating comb-shape effecter with integrated nozzle. Grinding fluid discharges from a nozzle placed between the comb’s feet and passes through the vacant space between comb teeth. By this setup, flowing grinding fluid can be continuously excited by ultrasonic vibration. Based on the principle of an ultrasonic washing machine, impulsive force caused by cavitation bubble will reduce the adhesion of chips on the cutting face of grain and chip pockets. Some effects of ultrasonic excited grinding fluid have been recorded such as reducing grinding heat in the case of grinding for Titanium alloy and decreasing in grinding force, improving surface roughness in the case of grinding for Aluminum and stainless alloy. However, the reason of better grinding performance is still unknown. Therefore, experiments conducted with different type of grinding fluids with and without ultrasonic vibration are needed. Pure Titanium, which considered a hard-to-cut material, is chosen as work material. Grinding forces and grinding heat during grinding will be measured and evaluated to clarify the mechanism of ultrasonic excited grinding fluid.

Picometer-resolution dual-comb spectroscopy with a free-running fiber laser.

Dual-comb spectroscopy holds the promise as real-time, high-resolution spectroscopy tools. However, in its conventional schemes, the stringent requirement on the coherence between two lasers requires sophisticated control systems. By replacing control electronics with an all-optical dual-comb lasing scheme, a simplified dual-comb spectroscopy scheme is demonstrated using one dual-wavelength, passively mode-locked fiber laser. Pulses with a intracavity-dispersion-determined repetition-frequency difference are shown to have good mutual coherence and stability. Capability to resolve the comb teeth and a picometer-wide optical spectral resolution are demonstrated using a simple data acquisition system. Energy-efficient, free-running fiber lasers with a small comb-tooth-spacing could enable low-cost dual-comb systems.

Genetic dissection of intraspecific variation in a male-specific sexual trait in Drosophila melanogaster.

An open question in evolutionary biology is the relationship between standing variation for a trait and the variation that leads to interspecific divergence. By identifying loci underlying phenotypic variation in intra- and interspecific crosses we can determine the extent to which polymorphism and divergence are controlled by the same genomic regions. Sexual traits provide abundant examples of morphological and behavioral diversity within and among species, and here we leverage variation in the Drosophila sex comb to address this question. The sex comb is an array of modified bristles or ‘teeth' present on the male forelegs of several Drosophilid species. Males use the comb to grasp females during copulation, and ablation experiments have shown that males lacking comb teeth typically fail to mate. We measured tooth number in >700 genotypes derived from a multiparental advanced-intercross population, mapping three moderate-effect loci contributing to trait heritability. Two quantitative trait loci (QTLs) coincide with previously identified intra- and interspecific sex comb QTL, but such overlap can be explained by chance alone, in part because of the broad swathes of the genome implicated by earlier, low-resolution QTL scans. Our mapped QTL regions encompass 70–124 genes, but do not include those genes known to be involved in developmental specification of the comb. Nonetheless, we identified plausible candidates within all QTL intervals, and used RNA interference to validate effects at four loci. Notably, TweedleS expression knockdown substantially reduces tooth number. The genes we highlight are strong candidates to harbor segregating, functional variants contributing to sex comb tooth number.

Growth and characterization of ultra-long ZnO nanocombs

ZnO nanocombs with 25 μm comb teeth were synthesized by chemical vapor deposition (CVD) method. Experiments were carried out to investigate the influence of carrier gas flow rate and temperature on ZnO comb teeth growth. The growth mechanism was demonstrated according to the morphology of prepared nanocombs under different growth parameters. The experimental results showed that the intensity of green emission significantly increased when the ZnO nanocombs became thinner and longer. It attributed to much more hanging bonds and oxygen vacancy on the surfaces of comb teeth.

Focus on lasers and imaging

Focus on lasers and imaging

Frequency-noise measurements of optical frequency combs by multiple fringe-side discriminator.

The frequency noise of an optical frequency comb is routinely measured through the hetherodyne beat of one comb tooth against a stable continuous-wave laser. After frequency-to-voltage conversion, the beatnote is sent to a spectrum analyzer to retrive the power spectral density of the frequency noise. Because narrow-linewidth continuous-wave lasers are available only at certain wavelengths, heterodyning the comb tooth can be challenging. We present a new technique for direct characterization of the frequency noise of an optical frequency comb, requiring no supplementary reference lasers and easily applicable in all spectral regions from the terahertz to the ultraviolet. The technique is based on the combination of a low finesse Fabry-Perot resonator and the so-called “fringe-side locking” method, usually adopted to characterize the spectral purity of single-frequency lasers, here generalized to optical frequency combs. The effectiveness of this technique is demonstrated with an Er-fiber comb source across the wavelength range from 1 to 2 μm.

Midinfrared frequency comb from self-stable degenerate GaAs optical parametric oscillator.

We pump a degenerate frequency-divide-by-two optical parametric oscillator (OPO) based on orientation-patterned GaAs with a stable Tm frequency comb at 2 micrometer wavelength and measure the OPO comb offset frequency and linewidth. We show frequency division by two with sub-Hz relative linewidth of the comb teeth. The OPO thermally self-stabilizes and oscillates for nearly an hour without any active control.

Acoustics in between: Perception of sound in rooms beyond standard criteria.

This article is divided into six parts: Between Time (how early reflections are distributed in time and the possibility of comb filter coloration), Between Musicians (how musicians on stage influence the acoustics, and how distinct, strong reflections on stage create comb filters), Between Mouth and Ears (how persons who are blind see with their ears: echolocation in both time domain and frequency domain), Between Measurements (how musicians and actors perceive reverberation and room acoustic details not covered by the standardized criteria, and how one can investigate this by one's own handclaps and tongue drops recorded with in-ear microphones), Between Echoes (how An echo is not an echo, but is highly dependent on the type and duration of the signal, masking by other sounds [music and noise], and masking by the signal itself), and finally, Between Walls (how details like room resonances and shimmering treble are more important for the perceived acoustics of [too] small rehearsal rooms than common reverberation time and SPL criteria).Between TimeSound travels slowly, just 343 m/s at common room temperature, only 1235 km/h. This is actually extremely slow compared to other wave phenomena like light and makes sound much more interesting. When you turn on the light, the whole room is illuminated immediately. Your clap, however, sneaks slowly around: searching, tasting the walls, making advanced combinations with its own delayed reflections. Because sound is so slow, and because we can hear such a huge span of frequencies, we must treat bass sounds and high frequency treble in totally different ways. The lowest bass frequency we perceive as sound (some 20 Hz) has a wavelength of 17.2 m. That is longer than typical room dimensions. On the other side, high pitched brilliance at 15,000 Hz has a wavelength of just a couple of centimeters, and is easily disturbed/scattered by just small objects and irregularities.When the sound radiates from a source, the simplest obstruction it can meet on its way is a single surface, which will give a reflection (if the surface is hard and smooth and has a dimension on the order of or larger than the wavelength of the sound). When a sound is combined with a reflection of itself, the result depends on the excess distance the reflection travels, compared with the direct sound path: the time delay. We all know that if a single reflecting surface is far away, we perceive the reflection as a distinct echo. When the time delay is shorter than some 50 ms (the sound has traveled less than some 17 m), we might not perceive the reflection as an echo for speech because the direct sound and the reflection more or less combines into one sound; but, the frequency content (timbre/klangfarbe) of the sound will change. Figure 1 a-c shows typical frequency responses when a (broadband) direct sound is combined with a reflection. The short reflection in Figure 1a gives a very broad comb filter with large spacing between the teeth of the comb (a large comb-between-teeth bandwidth [CBTB], almost like a simple bass control on a Hi-Fi-system). In Figure 1b, the long delay between the direct sound and the reflected sound gives very narrow spacing between comb teeth, no noticeable coloration, and the reflection is perceived as a distinct echo in the time domain. Figure 1c shows something in between: The extra sound path for the reflected sound is 3.43 m, which gives a delay of 10 ms and a CBTB of 1000/10 100 Hz. This could be the reflection coming back to a person standing 1.715 m in front of a wall.This means that the delay between the direct and the reflected sound from surfaces at distances typical in a medium-sized room might produce an audible comb filter. The average person perhaps does not notice such coloration of the sound because often there are several surfaces at different distances that more or less randomize the comb filters, or perhaps the person has not experienced the original/direct sound by itself, unmodified by the reflection. …

Broadband Phase Spectroscopy over Turbulent Air Paths.

Broadband atmospheric phase spectra are acquired with a phase-sensitive dual-frequency-comb spectrometer by implementing adaptive compensation for the strong decoherence from atmospheric turbulence. The compensation is possible due to the pistonlike behavior of turbulence across a single spatial-mode path combined with the intrinsic frequency stability and high sampling speed associated with dual-comb spectroscopy. The atmospheric phase spectrum is measured across 2 km of air at each of the 70,000 comb teeth spanning 233 cm(-1) across hundreds of near-infrared rovibrational resonances of CO(2), CH(4), and H(2)O with submilliradian uncertainty, corresponding to a 10(-13) refractive index sensitivity. Trace gas concentrations extracted directly from the phase spectrum reach 0.7 ppm uncertainty, demonstrated here for CO(2). While conventional broadband spectroscopy only measures intensity absorption, this approach enables measurement of the full complex susceptibility even in practical open path sensing.

Growth of Single-sided ZnO nanocombs/ML graphene Heterostructures

We report catalyst-free growth of high-density single-sided ZnO nanocombs for the first time on a multi-layer graphene (MLG). Structural analysis based on scanning electron microscope reveals the nanocomb ribbon average diameter and length are about 90–600nm and 5–60μm, respectively, while the diameter and length of the comb tooth are about 30–100nm and 100–700nm respectively. In general, the length of the teeth decreases gradually from one end of the nanocomb ribbon to another. ZnO crystal growth seems to involve two steps which are the formation of Zn buffer layer/graphene, which works as growth nucleation sites and long nanowires ends with nanocombs structure. Raman and PL optical transitions prove the well-faceted hexagonal structure of ZnO nanocombs as well as the existence of defects such as O vacancies and Zn interstitials. Graphene-based inorganic hybrid nanostructures provide several potential applications in optoelectronics and nanoscale electronics such as nanogenerators, photovoltaic devices, optical devices, and photodetectors.

Self-Referenced <inline-formula> <tex-math notation="LaTeX">$f$ </tex-math></inline-formula>-to-<inline-formula> <tex-math notation="LaTeX">$2f$ </tex-math></inline-formula> Beat Note of a Period-Doubling Mode-Locked Yb-Fiber Laser

We demonstrated a period-doubling modelocked (PD-ML) state with the pulse train locked to the twice of cavity roundtrip time in the Yb-fiber laser. The overall comb tooth of such PD-ML laser is characterized through self-referenced f-to-2f beat note. In contrast to normally fundamental mode-locking or harmonic mode-locking, the carrier-envelope offset frequency ( f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ) was observed to shrink into half of the corresponding fundamentally mode-locked laser in the frequency domain. Meanwhile, the regenerated half harmonics of a repetition frequency exhibited an amplitude tunable property. Although the f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> signal slightly deteriorated, a modulation rate of 4.65 MHz/mA within a linear tuning range of 100 MHz was achieved in our PD-ML laser.

Theoretical study of the periodic quantum phase modulation of the dipole response in atomic He

Based on the laser-induced-phase model, periodic quantum phase modulation of the dipole response in atomic He is studied theoretically. The two-level system of the transition 1s2→1s2p with a delay width of 1.8 × 109 s-1 and an energy difference of 21.2 eV between the excited state and the ground state is used in the calculation. The system is excited by attosecond laser pulse from high harmonic generator, and the spectral response of the system is of single isolated symmetric Lorentzian absorption line. After the excitation, near infrared (NIR) femtosecond laser pulse train with a repetition rate of 5 GHz, central frequency 780 nm, and pulse duration of 100 fs, is utilized to periodically modify the spontaneous decay of the excited 1s2p level. The incremental phase step Δφ depends on the intensity of the NIR laser pulse, while the initial offset phase φ can be controlled independently by partially overlapping the first NIR pulse with the excitation. Simulated results show that the Lorentzian absorption line is transformed into comb-like spectral structure with equal gap depending on the repetition rate of the NIR pulse train. The line shape of each comb tooth is symmetric Lorentzian line by setting φ = Δφ/2 = π/2, while it is Fano line by setting φ = Δφ = π. The location of the comb structure is mainly dependent on the energy difference between the excited state and the ground state, while it can be slightly tuned by controlling the incremental phase step Δφ. We develop an analytic description of the comb-like spectral structure by Fourier analysis, depending on both the atomic and the phase-control properties. The analytical expressions can be readily used to estimate the exact experimental parameters. The universality of this mechanism allows the spectral modulation in arbitrary atomic system at arbitrary frequency, including the hard X-ray regime, by using reference transitions in highly charged ions. The generalization of this approach should thus not only enable relative frequency measurement and relevant applications at extremely high frequencies, but also open the way for pulse shaping at arbitrary frequencies.