Measurement of absolute frequency of continuous-wave terahertz radiation in real time using a free-running, dual-wavelength mode-locked, erbium-doped fibre laser

Computationally image-corrected dual-comb microscopy with a free-running single-cavity dual-comb fiber laser

Dual-comb microscopy (DCM), an interesting imaging modality based on the optical-frequency-comb (OFC) mode and image pixel one-to-one correspondence, benefits from scan-less full-field imaging and simultaneous confocal amplitude and phase imaging. However, the two fully frequency-stabilized OFC sources requirement hampers DCM practicality due to the complexity and costs. Here, a bidirectional single-cavity dual-comb fiber laser (SCDCFL) is adopted as a DCM low-complexity OFC source. Although the residual timing jitter in the SCDCFL blurs the image of a static object acquired by DCM, computational image correction significantly suppresses the image blur. Nanometer-order step surface profilometry with a 14.0 nm uncertainty highlights the computationally image-corrected DCM effectiveness. We further discuss a possibility to expand the computational image correction to a dynamic object and demonstrate its preliminary experiment. The proposed method enhances the DCM generality and practicality due to low-complexity OFC source.

Broadband high-resolution microwave frequency measurement based on photonic undersampling via using three cavity-less optical pulse sources with coprime repetition rates

A photonic-assisted broadband and high-resolution microwave frequency measurement scheme is proposed and demonstrated based on undersampling via using three cavity-less optical pulse sources with coprime repetition rates. After undersampling by three ultrashort pulse trains with repetition rates in the order of gigahertz, input microwave signal is down-converted to three intermediate-frequency (IF) signals located in the first Nyquist frequency range. Through measuring the frequencies of the IF signals via fast Fourier transform after digitization by the commercially available analog-to-digital convertors, the input microwave signal frequency can be retrieved based on the frequency identification algorithm. In the proof-of-concept experiment, three ultrashort pulse trains with repetition rates of 2.99, 3.07, and 3.10 GHz are generated by a cavity-less optical pulse source, where the pulse widths are 9.5, 9.6, and 9.8 ps, respectively. Through using these three ultrashort optical pulse trains, a frequency measurement range up to 40 GHz is realized, where the frequency measurement error is less than ±5kHz, and the spurious-free dynamic range is 91.25dBcHz^2/3.

Ultrafast Fiber Lasers: An Expanding Versatile Toolbox

Ultrafast fiber lasers have gained rapid advances in last decades for their intrinsic merits such as potential of all-fiber format, excellent beam quality, superior power scalability, and high single-pass gain, which opened widespread applications in high-field science, laser machining, precision metrology, optical communication, microscopy and spectroscopy, and modern ophthalmology, to name a few. Performance of an ultrafast fiber laser is well defined by the laser parameters including repetition rate, spectral bandwidth, pulse duration, pulse energy, wavelength tuning range, and average power. During past years, these parameters have been pushed to an unprecedented level. In this paper, we review these enabling technologies and explicitly show that the nonlinear interaction between ultrafast pulses and optical fibers plays the essential role. As a result of rapid development in both active and passive fibers, the toolbox of ultrafast fiber lasers will continue to expand and provide solutions to scientific and industrial problems.

Broad bandwidth dual-wavelength fiber laser simultaneously delivering stretched pulse and dissipative soliton

We numerically and experimentally demonstrate the generation of broad bandwidth mode-locked dual-wavelength pulses with diverse-pattern from a dispersion managed erbium-doped (Er-doped) fiber laser. The two-peak gain profile of the Er-doped fiber is shown to have advantages in achieving broadband dual-wavelength pulses compared to a comb filter in our cavity. Our obtained bandwidths of 24 nm and 11.5 nm represent the broadest achieved in an Er-doped dual-wavelength fiber laser to date. In addition, the weak third-order dispersion (TOD) of the fibers facilitates two dispersion-pattern pulses (one stretched pulse and one dissipative soliton) generated in the near zero dispersion regime. Our results provide a convenient, effective way to obtain such sources for potential applications, such as in dual-comb metrology and multicolor pulses in nonlinear microscopy.

Cross-referenced deadband-free microwave frequency measurement with cascaded-four-wave-mixing-based photonic harmonic down-conversion

A cross-referenced and deadband-free method with photonic harmonic down-conversion is proposed for microwave frequency measurement based on cascaded-four-wave-mixing (CFWM) in semiconductor optical amplifiers. The proposed method enables ultra-wide and accurate frequency measurement with low-frequency spectrum detection, and at the same time achieves deadband-free and multi-tone frequency measurement by cross-referenced frequency discrimination. For a proof of concept, microwave signal measurement is experimentally demonstrated up to the 40 GHz frequency range with an 0.2-MHz measurement error. The frequency measurement features ease of configuration by only changing the low-frequency electrical local oscillators of the CFWM-based photonic harmonic down-converter.

Self-triggered Asynchronous Optical Sampling Terahertz Spectroscopy using a Bidirectional Mode-locked Fiber Laser

We report a self-triggered asynchronous optical sampling terahertz spectroscopy system based on a single bidirectional mode-locked fiber laser and plasmonics-enhanced photoconductive nanoantennas. The fiber laser generates two optical mutually coherent pulse trains with a stable repetition rate difference, enabling time-domain terahertz spectroscopy without using any mechanical delay line, stabilization electronics, or external trigger. The resolved terahertz spectra over a 0.1-2 THz frequency range and a 30-second measurement time show more than a 70-dB dynamic range, revealing water absorption lines matching the HITRAN database, through a light-weight and compact spectroscopy setup.

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.

Frequency accurate coherent electro-optic dual-comb spectroscopy in real-time

Electro-optic dual-comb spectrometers have proved to be a promising technology for sensitive, high-resolution and rapid spectral measurements. Electro-optic combs possess very attractive features like simplicity, reliability, bright optical teeth, and typically moderate but quickly tunable optical spans. Furthermore, in a dual-comb arrangement, narrowband electro-optic combs are generated with a level of mutual coherence that is sufficiently high to enable optical multiheterodyning without inter-comb stabilization or signal processing systems. However, this valuable tool still presents several limitations; for instance, on most systems, absolute frequency accuracy and long-term stability cannot be guaranteed; likewise, interferometer-induced phase noise restricts coherence time and limits the attainable signal-to-noise ratio. In this paper, we address these drawbacks and demonstrate a cost-efficient absolute electro-optic dual-comb instrument based on a frequency stabilization mechanism and a novel adaptive interferogram acquisition approach devised for electro-optic dual-combs capable of operating in real-time. The spectrometer, completely built from commercial components, provides sub-ppm frequency uncertainties and enables a signal-to-noise ratio of 10000 (intensity noise) in 30 seconds of integration time.

Asynchronous and synchronous dual-wavelength pulse generation in a passively mode-locked fiber laser with a mode-locker

The evolution from asynchronous to synchronous dual-wavelength pulse generation in a passively mode-locked fiber laser is experimentally investigated by tailoring the intracavity dispersion. Through tuning the intracavity-loss-dependent gain profile and the birefringence-induced filter effect, asynchronous dual-wavelength soliton pulses can be generated until the intracavity anomalous dispersion is reduced to ∼8  fs/nm. The transition from asynchronous to synchronous pulse generation is then observed at an elevated pump power in the presence of residual anomalous dispersion, and it is shown that pulses are temporally synchronized at the mode-locker in the cavity. Spectral sidelobes are observed and could be attributed to the four-wave-mixing effect between dual-wavelength pulses at the carbon nanotube mode-locker. These results could provide further insight into the design and realization of such dual-wavelength ultrafast lasers for different applications such as dual-comb metrology as well as better understanding of the inter-pulse interactions in such dual-comb lasers.