Frequency comb-based four-wave-mixing spectroscopy.

The research work carried out within the framework of this PhD has focused on the generation of Optical Frequency Combs (OFC) in semiconductor lasers using the gain-switching (GS) technique. The work has been mainly experimental, although simulations have been used to contribute to a better understanding of the internal physics of the OFCs generation process. Various types of lasers have been used, specially distributed feedback (DFB) lasers and discrete mode lasers (DML). An exhaustive experimental characterisation of the properties of the OFCs generated by GS in a full range of operating conditions has been performed. Two ranges of repetition frequencies have been explored: the GHz range with applications in optical communications, and the range of hundreds of MHz for application in spectroscopy. The effect of the optical injection (OI) on the properties of the OFCs has been investigated, and interesting results have been obtained, such as the improvement or deterioration of the quality of the spectra depending on the GS and OI conditions. The optimal operating conditions to generate high quality OFCs at the different frequencies considered have been found. In order to improve the understanding of the different physical processes involved in the generation of the OFCs generated by GS, a simulation tool based on the rate equation model for semiconductor lasers has been developed, with detailed treatment of the noise sources. For a correct simulation of the OFCs the values of a set of internal laser parameters is are required. They were obtained through an original procedure based on the measurement of the relative intensity noise (RIN) spectra. Using this procedure, the uncertainty in the simulated results has been reduced, since it allows to extract more parameters than other procedures due to an adequate analytical treatment of the RIN equations. The simulations have completely reproduced the temporal and spectral response of different lasers in a wide range of electrical and optical injection conditions. From the simulations, the predominant physical processes that contribute to the generation of OFCs, dynamic chirp at high repetition frequencies, and adiabatic chirp at low frequencies, have been identified. It has been shown that it is possible to improve the characteristics of low frequency OFCs by driving the laser with electric pulses, instead of using sinusoidal driving. High quality 100 MHz OFCs (up to 866 tones in 10 dB) with good carrier-to-noise (CNR) ratio have been generated. It has also been shown that step-recovery diodes (SRDs) can be used as a source of electrical pulses to generate OFCs, thus reducing the cost and size of the equipment involved, while maintaining the quality of the OFCs. The role of the “jitter” of the electrical source in the quality of the OFCs has been studied theoretically and experimentally. Finally, the OFCs generated by gain-switching and optical injection have been used to measure the absorption spectra of different elements, using three different architectures: heterodyne detection, and dual-comb spectroscopy and a new implementation of this technique. In this implementation the dual-comb is generated from a single GS laser by alternating the repetition frequency. Promising results for the application of these OFCs in gas spectroscopy have been obtained.

Precise phase control of ultra-wide-bandwidth optical frequency combs has produced remarkable and unexpected progress in precision metrology and ultrafast science. The combination of the ability to do completely arbitrary, optical, waveform synthesis with recently developed optical pulse measurement techniques is analogous to the development of oscilloscopes and waveform generators in the early 20th century. The development of ultra-stable optical frequency standards into optical atomic clocks and optical frequency synthesizers again complement and rival the similar technologies developed in the radio frequency domain. I will cover a range of key advances that have been enabled by this revolutionary merge between the ultrafast and ultra-precision fields, including direct optical frequency measurement, carrier-envelope phase control, alloptical atomic clocks, optical frequency synthesizers, coherent pulse synthesis and distribution, and nonlinear spectroscopy.

1. Introduction Frequency combs have been investigated in the laboratory over the course of the last 25 years in a wide range of implementations and applications including but not limited to precision metrology, optical atomic clocks, LIDAR, low-phase-noise RF generation, and precision spectroscopy [1]. While the Nobel prize winning technology of frequency combs have shown their usefulness in a variety of applications, there have been few demonstrations of this technology in real-world applications [2, 3]. Here we present a mode-locked oscillator that has been designed to be environmentally robust and low cost, while maintaining suitability for use in frequency comb applications. Vescent Photonics is designing and manufacturing fiber mode-locked oscillators and frequency combs for government customers whose application deployments include satellites and terrestrial moving platforms. These designs allow for repetition rate matching at the time of manufacture, which is an important consideration for integration of this technology into optical atomic clocks, dual comb spectroscopy, and low-phase-noise radio frequency generation. Vescent currently is manufacturing oscillators at 100 and 200 MHz repetition rates, and these designs could be modified to give repetition rates between 80 and 250 MHz. While performance and environmental robustness are necessary metrics for real-world applications of frequency combs, the cost of these devices can also be a barrier to wide-spread deployment. Vescent Photonics is diligently working on optimizing all three parameters (performance, environmental robustness, and cost) to bring a deployable mode-locked laser and frequency comb solution to market. 2. Mode-locked oscillator performance The compact mode-locked oscillators presented here have been designed to be low-cost, low-volume, highperformance, and environmentally robust for field-deployed applications. Figure 1a shows the mechanical enclosure for the fiber oscillator measuring 0.12 L (10.75 cm x 6.35 cm x 1.72 cm) in volume; it should be noted that pump diodes have not been included in this packaging due to thermal considerations. An enclosure for a frequency comb system (optic only) has also been designed with a volume of 0.62 L (19.05 cm x 11.30 cm x 2.86 cm). Two length actuators are included in this design; a fast PZT with >200 kHz bandwidth that allows 150 Hz of tuning on a 100 MHz repetition rate, and slow temperature control of the fiber with a ~10 s thermal time constant and >40 kHz of tuning on a 100 MHz repetition rate. The oscillator optical spectrum is included in Figure 1b. Average optical powers out of the oscillators are on the order of a few milliwatts, and pulses are close to transform limited (~250 fs). Erbium-doped fiber amplifiers and pulse compression stages are routinely added to the oscillator outputs to achieve average powers >200 mW and pulse durations <100 fs. While these parameters are useful for a wide range of mode-locked laser applications, it also allows the detection of the carrier envelope offset frequency ( , a fundamental comb parameter) in an f-2f interferometer with greater than 40 dB SNR in 300 kHz resolution bandwidth (Figure 1c). To show suitability of these fiber mode-locked oscillators in frequency comb applications, fractional Allan Deviations of the in-loop stabilized signals have been included in Figure 1d where the absolute frequency deviations have been divided by the distance between the frequency comb lock points. Typical optical clock performance has also been plotted, where the performance of the locks is more than suitable for this application. 3. Summary A compact, low-cost, environmentally robust mode-locked oscillator has been shown to be suitable for frequency comb experiments. Detection and stabilization of the fundamental comb parameters is suitable for optical atomic clocks, as well as a host of real-world frequency comb applications including dual comb spectroscopy, low phase noise RF generation, and optical metrology.

Considering its different interaction forms with cellulose, water is very problematic as we consider high frequencies electromagnetic applications of papers. Thereby, cellulosic materials exhibit high losses in the radiofrequency (RF) domain. In this paper, the RF dielectric behavior of different papers versus their moisture content is described and the huge impact of water upon losses is shown. We also present a three components effective medium theory that is able to describe and predict the impact of this water content.

Introduction/purpose: This article analyzes the publicly available literature on drone classification in the radio frequency domain, focusing on detection and identification. Drones are increasingly used for illegal purposes, making classification techniques crucial. This review paper covers passive radio frequency sensors, classification techniques, and datasets that highlight the challenges. Methods: Researchers are developing antidrone solutions because drones have become valuable tools and targets for illegal activities. Due to the scope of the subject matter, the review included only the classification of drones via passive radio frequency sensors with a description of the classification techniques (set of algorithms, methods, and procedures) and the datasets used for performance testing. This study introduces a new categorization and offers deeper insights into publicly available drone classification techniques. Results: Based on the results of this study, it is apparent that deep learning algorithms are presently the most effective approach to addressing the challenge of drone classification within the radio frequency domain. One of the primary obstacles is the absence of a comprehensive standard for classifying drones in the radio frequency domain, which should be based on end-user requirements. Additionally, the results of two ablative experiments highlight the preprocessing of raw I/Q radio signals as an essential step in drone classification. Conclusion: In summary, the proposed categorization provides a valuable tool for literature review. Deep learning is the most effective technique for drone classification, but publicly available datasets with drone radio signals are limited. The key strength of this study is that it represents the first review of publicly available datasets with drone radio signals.

Length is one of the fundamental physical quantities and its precise measurement is very important for science and technology. Laser-based distance metrology technique is a powerful tool, which is widely applied in geodetic monitoring, environmental monitoring and precision measurement engineering including in space applications. Since the invention of the optical frequency comb (OFC) at the beginning of this century, it has been used as a versatile tool for many applications, such as frequency metrology, spectroscopy, etc. OFC has played an important role in distance metrology enabling the developed techniques to achieve high accuracy in long distance measurement. In the time domain, OFC is a pulse train, which can measure distance with time of flight method or correlation detection method. In the frequency domain, spectrum of OFC consists of a series of discrete lines with equal frequency difference. Methods of dispersive interferometry, multi-heterodyne interferometry, and multi-wavelength interferometry are the techniques proposed and demonstrated in spectral domain distance metrology. Though distance metrology techniques based on OFC have been developed for tens of years, there are still challenges that need to be overcome. In most applications the measurements are completed in the air, where the results are influenced by refractive index of air. As the OFC is a multi-wavelength laser source, refractive index of air is difficult to determine. The other challenge is complexity and high cost of measurement system. Until now, to my knowledge, there is no commercial rangefinder which is based on OFC. The large size, complex configuration and high price are the bottlenecks restricting OFC’s entrance into industrial applications.

The analogy between optical frequencies and RFs leads to a novel technique for RF photonic signal processing using a femtosecond laser comb modulated by a traveling-wave tunable filter, such as an acoustooptic tunable filter (AOTF) or a novel electrooptic tunable filter (EOTF). In this new scheme, the recent history of an applied RF waveform to-be-processed slides into the tunable filter and a femtosecond pulse train diffracts off the moving acoustooptically or electrooptically induced dielectric grating, producing a shaped optical pulse train with each shaped pulse as a compressed replica of the RF waveform contained within the device aperture that is sped up by the ratio between the center optical and RF frequency. For a CW RF tone input, only a narrowband group of the frequency comb lines in the incoming laser comb is spectrally filtered and modulated due to the phase-matching condition in the filter and is simultaneously Doppler shifted by that RF due to the traveling-wave grating obeying the conservation of energy. This allows us to use the traveling-wave tunable filter as a spectrally mapped Doppler-shifted modulator that encodes different RF components onto the corresponding optical frequency comb lines. RF signal processing can then be performed by using optical techniques to manipulate the spectrally modulated laser comb. To reconstruct the processed RF signal, the Doppler-shifted and optically processed pulse train is heterodyne detected by beating with a reference femtosecond pulse train from the same laser source. A high repetition rate femtosecond laser comb is modulated by an AOTF to experimentally demonstrate this novel RF photonic signal processing technique. We demonstrate an RF tunable bandpass/notch filter, an RF down-converter, and an RF jammer nuller as novel applications of ultrafast lasers.

Recently, a novel type of molecular THz laser with ∼mW-level output power was demonstrated [1,2]. This laser is optically pumped by a mid-IR Quantum Cascade Laser (QCL), therefore resulting in a much more compact, frequency agile and deployable source compared to standard, CO 2 -laser pumped, far-infrared gas lasers. This achievement has revived the interest for molecular THz lasers, in particular for their exploitation as low-phase noise THz local oscillators (LOs) operating at room temperature. Compared to CO 2 -laser pumping, a QCL pump offers an unrivalled frequency agility that allows accessing an unprecedented number of molecular transitions. On the other hand, its lower frequency stability can potentially impact the frequency noise of the THz laser source [3–5]. Here we report the first measurement of the frequency noise power spectral density (PSD) of a 1.073THz ammonia laser pumped by a 10.3μm QCL. This is achieved by using a commercial harmonic mixer to generate a beating between the ammonia laser frequency and the 1080th harmonic of the 1GHz repetition rate of a custom-made, 1560nm frequency comb (FC). We then exploit a voltage-controlled oscillator to track the beat-note signal and realize the frequency to amplitude conversion [6]. The measured frequency noise PSD is shown in Fig.1. In the range ∼30–100kHz we find a frequency noise 2 -pumped, 2.5THz methanol laser, reaching 0.1Hz2/Hz (−110dBc) at 40kHz from the carrier, limited by the frequency noise of the FC harmonic. Finally, we will show that it is possible to actively phase-lock the QCL-pumped ammonia laser to the FC repetition rate harmonic by controlling the QCL drive current, demonstrating a sub-Hz linewidth. These results pave the way to the development of QCL-pumped molecular lasers as compact, low-phase noise THz sources operating a room temperature.

Under the conventional scheme to generate an optical frequency comb (OFC) using an electro-optic modulator (EOM), the frequency interval of the OFC is determined via the frequency of the fundamental tone of the radio frequency (RF) driving signals. In this work, we use two harmonics without the fundamental tone to drive two EOMs, where the frequency interval of the generated flat OFC is the frequency of the fundamental tone. The orders of the two harmonics are coprime. Specifically, one harmonic drives the first branch of the dual-parallel Mach–Zehnder modulator (DPMZM) only, and the other harmonic drives the phase modulator (PM). The flatness of the OFC is achieved by adjusting the amplitude and phase of the RF driving harmonics as well as the bias of the EOM. Both a simulation and an experiment were carried out to verify the effectiveness of the proposed scheme. When the second harmonic drives the DPMZM and the third harmonic drives the PM, an 11-comb line OFC is generated, where the flatness of the OFC was 0.63 dB and 0.65 dB under the simulation and experiment, respectively. When the third harmonic drives the DPMZM and the second harmonic drives the PM, a 13-comb line OFC is generated, where the flatness of the OFC was 0.62 dB and 0.95 dB under the simulation and experiment, respectively. We also investigate the performance of the generated OFC when one harmonic drives two branches of the DPMZM and the other harmonic drives the PM. The comparison of the OFCs’ performance demonstrates the effectiveness of the proposed scheme.

Optical high repetition rate stabilization based an all-polarization-maintaining figure-of-nine Er-fiber laser

Stabilization of quantum-cascade-laser frequency combs by means of external optical feedback is presented. Experimental results suggest reduced phase-noise in the frequency comb, allowing comb operation in previously unstable regimes. © 2019 The Authors

Broadband optical frequency comb (OFC) generation based on a single electro-absorption modulator (EAM) is proposed. The EAM is driven by a radio frequency (RF) multi-frequency signal generated by a multiplication coupler composed of an electrical power splitter and an arithmetic circuit. Thus the number of comb-lines of the generated OFC can be increased. A complete theoretical model of OFC generation by an EAM driven by nth power of the RF source is established, and the performance of the OFC is analyzed by using OptiSystem software. The results show that, the number of comb-lines of the OFC is positively correlated with the number of multiplication of the RF source signal. The frequency spacing of the comb-lines is twice the frequency of the RF source signal and is tunable by adjusting the frequency of the RF source signal. Increasing chirp factor and modulation index of EAM could increase the number of comb-lines of the generated OFC. The amplitude of the RF source signal had little impact on the flatness of the OFC and the average OFC power. The scheme developed is not only simple and low-cost, but also can produce a large number of comb-lines.Graphical

Integer and fractional spectral self-imaging effects are induced on infinite-duration periodic frequency combs (probe signal) using cross-phase modulation (XPM) with a parabolic pulse train as pump signal. Free-spectral-range tuning (fractional effects) or wavelength-shifting (integer effects) of the frequency comb can be achieved by changing the parabolic pulse peak power or/and repetition rate without affecting the spectral envelope shape and bandwidth of the original comb. For design purposes, we derive the complete family of different pump signals that allow implementing a desired spectral self-imaging process. Numerical simulation results validate our theoretical analysis. We also investigate the detrimental influence of group-delay walk-off and deviations in the nominal temporal shape or power of the pump pulses on the generated output frequency combs.

A frequency comb generated by a 167 MHz repetition frequency erbium-doped fiber ring laser using a carbon nanotube saturable absorber is phase-stabilized for the first time. Measurements of the in-loop phase noise show an integrated phase error on the carrier envelope offset frequency of 0.35 radians. The carbon nanotube fiber laser comb is compared with a CW laser near 1533 nm stabilized to the nu(1) + nu(3) overtone transition in an acetylene-filled kagome photonic crystal fiber reference, while the CW laser is simultaneously compared to another frequency comb based on a Cr:Forsterite laser. These measurements demonstrate that the stability of a GPS-disciplined Rb clock is transferred to the comb, resulting in an upper limit on the locked comb's frequency instability of 1.2 x 10(-11) in 1 s, and a relative instability of <3 x 10(-12) in 1 s. The carbon nanotube laser frequency comb offers much promise as a robust and inexpensive all-fiber frequency comb with potential for scaling to higher repetition frequencies.

We present the design and capabilities of a high-resolution, decade-spanning ASynchronous OPtical Sampling (ASOPS)-based TeraHertz Time-Domain Spectroscopy (THz-TDS) instrument. Our system employs dual mode-locked femtosecond Ti:Sapphire oscillators with repetition rates offset locked at 100 Hz via a Phase-Locked Loop (PLL) operating at the 60th harmonic of the ∼80 MHz oscillator repetition rates. The respective time delays of the individual laser pulses are scanned across a 12.5 ns window in a laboratory scan time of 10 ms, supporting a time delay resolution as fine as 15.6 fs. The repetition rate of the pump oscillator is synchronized to a Rb frequency standard via a PLL operating at the 12th harmonic of the oscillator repetition rate, achieving milliHertz (mHz) stability. We characterize the timing jitter of the system using an air-spaced etalon, an optical cross correlator, and the phase noise spectrum of the PLL. Spectroscopic applications of ASOPS-THz-TDS are demonstrated by measuring water vapor absorption lines from 0.55 to 3.35 THz and acetonitrile absorption lines from 0.13 to 1.39 THz in a short pathlength gas cell. With 70 min of data acquisition, a 50 dB signal-to-noise ratio is achieved. The achieved root-mean-square deviation is 14.6 MHz, with a mean deviation of 11.6 MHz, for the measured water line center frequencies as compared to the JPL molecular spectroscopy database. Further, with the same instrument and data acquisition hardware, we use the ability to control the repetition rate of the pump oscillator to enable THz frequency comb spectroscopy (THz-FCS). Here, a frequency comb with a tooth width of 5 MHz is generated and used to fully resolve the pure rotational spectrum of acetonitrile with Doppler-limited precision. The oscillator repetition rate stability achieved by our PLL lock circuits enables sub-MHz tooth width generation, if desired. This instrument provides unprecedented decade-spanning, tunable resolution, from 80 MHz down to sub-MHz, and heralds a new generation of gas-phase spectroscopic tools in the THz region.