Yabai He

Ringdown and cavity-enhanced absorption spectroscopy using a continuous-wave tunable diode laser and a rapidly swept optical cavity

Abstract We use a rapidly swept optical cavity in a novel approach to cavity ringdown spectroscopy (CRDS) with a continuous-wave (cw) laser. This simplifies cw-CRDS by eliminating the need for a fast optical switch and is demonstrated by recording weak combination-band spectra of carbon dioxide diluted in nitrogen, using a cw single-mode tunable diode laser (TDL) in the 1.55 μm near-infrared region. We employ a simultaneous combination of CRDS and cavity-enhanced absorption spectroscopy, to compensate for nonlinear CRDS response that is observed for stronger absorption features. Experimental results and performance characteristics are presented, together with numerical simulations.

Optical heterodyne signal generation and detection in cavity ringdown spectroscopy based on a rapidly swept cavity

Abstract A novel approach to cavity ringdown spectroscopy uses a continuous-wave laser and a rapidly swept optical cavity to shift the frequency of optical radiation stored in the cavity. This frequency-shifted radiation from the ringdown cavity is then combined with incident laser radiation to generate optical heterodyne signals, simply and efficiently. A noise-limited absorbance sensitivity of 3×10 −9 cm −1 is realised, using ∼35 μW of single-mode radiation from a 1.53 μm tunable diode laser. The resonance properties of a swept optical cavity simplify this heterodyne-detected technique by avoiding the customary need for a fast optical switch or for wavelength-locking of cavity length.

Continuous-wave cavity ringdown absorption spectroscopy with a swept-frequency laser : rapid spectral sensing of gas-phase molecules

A cavity ringdown spectrometer, based on a continuous-wave swept-frequency laser, enables efficient, rapid recording of wide-ranging absorption spectra as characteristic spectral signatures of airborne molecules. The rapidly swept laser frequency resonates with the longitudinal modes of the ringdown cavity, effectively sampling the absorption spectrum of an intracavity gas at intervals defined by the cavitys free spectral range and generating a full absorption spectrum within a single rapid sweep of the widely tunable laser frequency. We report a new analog detection scheme that registers a single data point for each buildup and ringdown decay event without logging details of the full signal waveform; this minimizes demand on digitizer speed and memory depth, reducing the time scale of data processing. This results in a compact, robust, easy-to-use instrument that offers fresh prospects for spectroscopic sensing of trace species in the atmosphere.

Pulsed injection-seeded optical parametric oscillator with low frequency chirp for high-resolution spectroscopy

An injection-seeded optical parametric oscillator (OPO), based on periodically poled KTiOPO4 and pumped by a frequency-doubled, nanosecond-pulsed Nd:YAG laser, generates continuously tunable, single-longitudinal-mode, pulsed output at approximately 842 nm for high-resolution spectroscopy. Optical-heterodyne measurements show that the OPO frequency chirp increases linearly with detuning from the free-running (unseeded) OPO frequency and can be maintained as low as 10 MHz. Other factors affecting chirp are identified.

Tunable single-mode operation of a pulsed optical parametric oscillator pumped by a multimode laser

High-quality single-longitudinal-mode (SLM) tunable signal radiation is generated by a pulsed optical parametric oscillator (OPO) pumped by a compact, inexpensive multimode laser. The OPO is based on periodically poled lithium niobate (PPLN) in a ring cavity that is injection seeded at its resonated signal wavelength by a single-mode tunable diode laser. Accurate control of the OPO cavity length and crystal temperature ensures a continuously tunable SLM signal output frequency range of >7.5 THz (>250 cm(-1)); the corresponding idler output remains multimode. High-resolution molecular spectra are recorded to verify OPO performance at wavelengths of ~1.55 mum. The observed signal optical bandwidth of </=120 MHz (</=0.0040 cm(-1)) compares favorably with that of a more elaborate PPLN OPO system pumped by a pulsed single-mode laser.

Locking the cavity of a pulsed periodically poled lithium niobate optical parametric oscillator to the wavelength of a continuous-wave injection seeder by an “intensity-dip” method

Injection seeding by a single-mode continuous-wave (cw) laser provides a convenient way to achieve narrowband tunable operation of a laser with a broad spectral gain profile, or of an optical parametric oscillator (OPO). Continuous single-mode tunability of the laser or OPO output usually requires the length of the optical cavity to be controlled as the injection-seeding wavelength is scanned. We report a novel variant on established methods of locking the optical cavity length to the seed wavelength. Our approach takes advantage of the resonance properties of an optical cavity. When the cavity is in resonance with the cw seed radiation, the total intensity of that radiation reflected off the cavity displays a pronounced dip; this intensity dip can be used as a locking signal to reset the cavity length piezoelectrically during each interval between the pump pulses that excite the laser or OPO. Our active cavity-locking scheme is realized in the case of a ring-cavity OPO, incorporating periodically poled l...

Transition from single-mode to multimode operation of an injection-seeded pulsed optical parametric oscillator

Optical-heterodyne measurements are made on ~842-nm signal output of an injection-seeded optical parametric oscillator (OPO) based on periodically poled KTiOPO4 pumped at 532 nm by long (~27-ns) pulses from a Nd:YAG laser. At low pump energies (</= 2.5 times the free-running threshold), the narrowband tunable OPO output is single-longitudinal-mode (SLM) and frequency chirp can be <10 MHz, much less than the transform-limited optical bandwidth (~17.5 MHz). We explore the transition from SLM operation to multimode operation as pump energy or phase mismatch are increased, causing unseeded cavity modes to build up later in the pulse.

Spectroscopic Applications of Optical Parametric Oscillators

This article focuses on the spectroscopic applications of optical parametric oscillators in the sensing of chemical processes, in industrial or environmental diagnostics, and in basic optical physics.

Frequency-Tunable THz Source Based on Stimulated Polariton Scattering in

We report a very compact, laser diode end-pumped THz laser source, which utilizes two resonators to generate frequency-tunable radiation across the range 1.53-2.82 THz via stimulated polariton scattering (SPS) in a Mg:LiNbO3 crystal. To the best of our knowledge, this THz system operates with the lowest pump power threshold (2.4 W), and highest conversion efficiency (6.45ţW average output power at 1.82 THz for 5 W input diode pump power) ever reported for a THz source based on SPS.

{\rm Mg{:}LiNbO}_{3}

We present a radio-frequency (RF) signal distribution system via optical fibers. We utilize an all-digital platform that encompasses a phase-locked loop, numerically-controlled oscillator, and fiber phase noise cancellation system. Our system achieves a fractional frequency transfer stability of 4 × 10-13 at 1 s and 6 × 10-17 at one day for the distribution of RF signals over 70 km of optical fiber. We demonstrate that this performance can be achieved with standard crystal oscillators. Our system is scalable, configurable, and flexible, allowing distribution of signals at different frequencies while maintaining over two orders of magnitude of the fiber phase noise suppression.