Picosecond ultrasonics with a free-running dual-comb laser

We present a free-running 80-MHz dual-comb polarization-multiplexed solid-state laser which delivers 1.8 W of average power with 110-fs pulse duration per comb. With a high-sensitivity pump-probe setup, we apply this free-running dual-comb laser to picosecond ultrasonic measurements. The ultrasonic signatures in a semiconductor multi-quantum-well structure originating from the quantum wells and superlattice regions are revealed and discussed. We further demonstrate ultrasonic measurements on a thin-film metalized sample and compare these measurements to ones obtained with a pair of locked femtosecond lasers. Our data show that a free-running dual-comb laser is well-suited for picosecond ultrasonic measurements and thus it offers a significant reduction in complexity and cost for this widely adopted non-destructive testing techniqu

Supercontinuum-based hyperspectral LiDAR for precision laser scanning

Hyperspectral LiDAR enables non-contact mapping of the 3D surface geometry of an object along with its spectral reflectance signature and has proved to be effective for automated point cloud segmentation in various remote sensing applications. The established hyperspectral LiDAR methods offer a range precision of a few mm to a few cm for distances exceeding several meters. We propose a novel approach to hyperspectral LiDAR scanning based on a supercontinuum (SC) coherently broadened from a 780 nm frequency comb. It provides high precision distance measurements along with target reflectance over the 570–970 nm range of the SC output. The distance measurements are carried out by monitoring the differential phase delay of intermode beat notes generated by direct photodetection, while the backscattered light spectrum is acquired using a commercial CCD spectrometer with 0.16 nm resolution across the 400 nm bandwidth of the SC output. We demonstrate a measurement precision below 0.1 mm for a stand-off range up to 50 m on a diffuse target with around 89% reflectance. The measured relative accuracy as compared to a reference interferometer is on the order of 10−5 for distances up to 50 m. Initial results also indicate spectrum-based material classification within a 3D point cloud using a linear support vector machine. The results highlight the potential of this approach for joint high-precision laser scanning and automated material classification.ISSN:1094-408

Dual-comb optical parametric oscillator in the mid-infrared based on a single free-running cavity

We demonstrate a free-running single-cavity dual-comb optical parametric oscillator (OPO) pumped by a single-cavity dual-comb solid-state laser. The OPO ring cavity contains a single periodically-poled MgO-doped LiNbO3 (PPLN) crystal. Each idler beam has more than 245-mW average power at 3550 nm and 3579 nm center wavelengths (bandwidth 130 nm). The signal beams are simultaneously outcoupled with more than 220 mW per beam at 1499 nm and 1496 nm center wavelength. The nominal repetition rate is 80 MHz, while the repetition rate difference is tunable and set to 34 Hz. To evaluate the feasibility of using this type of source for dual-comb applications, we characterize the noise and coherence properties of the OPO signal beams. We find ultra-low relative intensity noise (RIN) below -158 dBc/Hz at offset frequencies above 1 MHz. A heterodyne beat note measurement with a continuous wave (cw) laser is performed to determine the linewidth of a radio-frequency (RF) comb line. We find a full-width half-maximum (FWHM) linewidth of around 400 Hz. Moreover, the interferometric measurement between the two signal beams reveals a surprising property: the center of the corresponding RF spectrum is always near zero frequency, even when tuning the pump repetition rate difference or the OPO cavity length. We explain this effect theoretically and discuss its implications for generating stable low-noise idler combs suitable for high-sensitivity mid-infrared dual-comb spectroscopy (DCS).ISSN:1094-408

Spatially multiplexed single-cavity dual-comb laser

Single-cavity dual-comb lasers are a new class of ultrafast lasers that have a wide possible application space including pump–probe sampling, optical ranging, and gas absorption spectroscopy. However, to date, laser cavity multiplexing has usually come with a trade-off in laser performance or relative timing noise suppression. We present a method for multiplexing a single laser cavity to support a pair of noise-correlated modes. These modes share all intracavity components and take a near-common path, but do not overlap on any active elements. We implement the method with an 80-MHz laser delivering more than 2.4 W of average power per comb with sub-140-fs pulses. We reach sub-cycle relative timing jitter of 2.2 fs [20 Hz, 100 kHz]. With this multiplexing technique, we can implement slow feedback on the repetition rate difference Δ��rep, enabling this quantity to be drift-free, have low jitter, and be adjustable—a key combination for practical applications that was lacking in prior single-cavity dual-comb systems.ISSN:2334-253

Dynamic and precise long-distance ranging using a free-running dual-comb laser

Long-distance ranging is a crucial tool for both industrial and scientific applications. Laser-based distance metrology offers unprecedented precision making it the ideal approach for many deployments. In particular, dual-comb ranging is favorable due to its inherently high precision and sampling rate. To make high-performance long-range dual-comb LiDAR more accessible by reducing both cost and complexity, here we demonstrate a fiber-based dual-comb LiDAR frontend combined with a free-running diode-pumped solid-state dual-comb laser that allows for sub-µm measurement precision while offering a theoretical ambiguity range of more than 200 km. Our system simultaneously measures distance with the role of each comb interchanged, thereby enabling Vernier-based determination of the number of ambiguity ranges. As a proof-of-principle experiment, we measure the distance to a moving target over more than 10 m with sub-µm precision and high update rate, corresponding to a relative precision of 1e−7. For a static target at a similar distance, we achieve an instantaneous precision of 0.29 µm with an update time of 1.50 ms. With a longer averaging time of 200 ms, we reach a precision of around 33 nm, which corresponds to a relative precision of about 3·1e−9 with a time-of-flight-based approach.ISSN:1094-408

Free-running Yb:KYW dual-comb oscillator in a MOPA architecture

Single-cavity dual-combs comprise a rapidly emerging technology platform suitable for a wide range of applications like optical ranging, equivalent time sampling, and spectroscopy. However, it remains a challenging task to develop a dual-comb system that exhibits low relative frequency fluctuations to allow for comb line resolved measurements, while simultaneously offering high average power and short pulse durations. Here we combine a passively cooled and compact dual-comb solid-state oscillator with a pair of core-pumped Yb-fiber-based amplifiers in a master-oscillator power-amplifier (MOPA) architecture. The Yb:KYW oscillator operates at 250 MHz and uses polarization multiplexing for dual-comb generation. To the best of our knowledge, this is the first demonstration of a single-cavity dual-comb based on this gain material. As the pulse timing characteristics inherent to the oscillator are preserved in the amplification process, the proposed hybrid approach leverages the benefit of both the ultra-low noise solid-state laser and the advantages inherent to fiber amplifier systems such as straight-forward power scaling. The amplifier is optimized for minimal pulse broadening while still providing significant amplification and spectral broadening. We obtain around 1 W of power per output beam with pulses then compressed down to sub-90 fs using a simple grating compressor, while no pre-chirping or other dispersion management is needed. The full-width half-maximum (FWHM) of the radio-frequency comb teeth is 700 Hz for a measurement duration of 100 ms, which is much less than the typical repetition rate difference, making this passively stable source well-suited for indefinite coherent signal averaging via computational phase tracking.ISSN:1094-408

Coherently averaged dual-comb spectroscopy with a low-noise and high-power free-running gigahertz dual-comb laser

We present a new type of dual optical frequency comb source capable of scaling applications to high measurement speeds while combining high average power, ultra-low noise operation, and a compact setup. Our approach is based on a diode-pumped solid-state laser cavity which includes an intracavity biprism operated at Brewster angle to generate two spatially-separated modes with highly correlated properties. The 15-cm-long cavity uses an Yb:CALGO crystal and a semiconductor saturable absorber mirror as an end mirror to generate more than 3 W average power per comb, below 80 fs pulse duration, a repetition rate of 1.03 GHz, and a continuously tunable repetition rate difference up to 27 kHz. We carefully investigate the coherence properties of the dual-comb by a series of heterodyne measurements, revealing several important features: (1) ultra-low jitter on the uncorrelated part of the timing noise; (2) the radio frequency comb lines of the interferograms are fully resolved in free-running operation; (3) we validate that through a simple measurement of the interferograms we can determine the fluctuations of the phase of all the radio frequency comb lines; (4) this phase information is used in a post-processing routine to perform coherently averaged dual-comb spectroscopy of acetylene (C2H2) over long timescales. Our results represent a powerful and general approach to dual-comb applications by combining low noise and high power operation directly from a highly compact laser oscillator.ISSN:1094-408

Low-Noise Femtosecond SESAM Modelocked Diode-Pumped Cr:ZnS Oscillator

Cr-doped ZnS and ZnSe are excellent gain mediums for high power and broadband ultrashort pulse generation in the 2 – 3 μm wavelength range. SESAM modelocked Cr:ZnS oscillators have the advantage of reliable, self-starting passive modelocking. We present a diode-pumped, SESAM modelocked Cr:ZnS oscillator delivering ultrashort pulses of 189 fs at 550 mW average output power with a repetition rate of 435 MHz with low relative intensity noise (RIN) and timing jitter. We measured an integrated RIN of 0.05% within a frequency span of [10 Hz, 5 MHz] dominated by the 1560-nm pump diode, and a very low integrated timing jitter of 10.9 fs [2 kHz, 10 MHz]. This type of laser source benefits not only from very low noise but also from reduced complexity and cost due to direct diode pumping, which is suitable for many applications such as spectroscopy, ranging, and frequency conversion.ISSN:0018-9197ISSN:1558-171