Markku Vainio

Frequency comb generation by a continuous-wave-pumped optical parametric oscillator based on cascading quadratic nonlinearities

We report optical frequency comb generation by a continuous-wave pumped optical parametric oscillator (OPO) without any active modulation. The OPO is configured as singly resonant with an additional nonlinear crystal (periodically poled MgO:LiNbO3) placed inside the OPO for phase mismatched second harmonic generation (SHG) of the resonating signal beam. The phase mismatched SHG causes cascading χ(2) nonlinearities, which can substantially increase the effective χ(3) nonlinearity in MgO:LiNbO3, leading to spectral broadening of the OPO signal beam via self-phase modulation. The OPO generates a stable 4 THz wide (-30 dB) frequency comb centered at 1.56 μm.

An AC Josephson Voltage Standard for AC–DC Transfer-Standard Measurements

We describe the traceability chain of length measurements at Centre for Metrology and Accreditation (MIKES) from atomic clocks to the lasers of primary interferometers. Crucial part of the traceability chain, an optical frequency comb generator linking optical frequencies to atomic clocks, is described in detail. The frequency comb generator is used in frequency calibrations of iodine-stabilized lasers, which are operated in compliance with the recommendations of the practical realization of the definition of the meter. Measured absolute frequencies of iodine-stabilized lasers, time records of the measurements, and the respective Allan deviations demonstrate the solid performance of the MIKES laser frequency standards. The results are in good agreement with the recommended values, as well as with the independent characterizations of the measured lasers

Traceability of Laser Frequency Calibrations at MIKES

We describe the traceability chain of length measurements at Centre for Metrology and Accreditation (MIKES) from atomic clocks to the lasers of primary interferometers. Crucial part of the traceability chain, an optical frequency comb generator linking optical frequencies to atomic clocks, is described in detail. The frequency comb generator is used in frequency calibrations of iodine-stabilized lasers, which are operated in compliance with the recommendations of the Practical Realization of the Definition of the Meter. Measured absolute frequencies of iodine-stabilized lasers, time records of the measurements, and the respective Allan deviations demonstrate the solid performance of the MIKES laser frequency standards. The results are in good agreement with the recommended values, as well as with the independent characterizations of the measured lasers.

Radiocarbon dioxide detection based on cavity ring-down spectroscopy and a quantum cascade laser

Monitoring of radiocarbon (C14) in carbon dioxide is demonstrated using mid-infrared spectroscopy and a quantum cascade laser. The measurement is based on cavity ring-down spectroscopy, and a high sensitivity is achieved with a simple setup. The instrument was tested using a standardized sample containing elevated levels of radiocarbon. Radiocarbon dioxide could be detected from samples with an isotopic ratio C14/C as low as 50 parts-per-trillion, corresponding to an activity of 5  kBq/m(3) in pure CO(2), or 2  Bq/m(3) in air after extraction of the CO(2) from an air sample. The instrument is simple, compact, and robust, making it the ideal tool for on-site measurements. It is aimed for monitoring radioactive gaseous emissions in a nuclear power environment, during the operation and decommissioning of nuclear power plants. Its high sensitivity also makes it the ideal tool for the detection of leaks in radioactive waste repositories.

Grating-cavity continuous-wave optical parametric oscillators for high-resolution mid-infrared spectroscopy

The use of grating as a spectral filter provides a simple way of improving wavelength tuning and stability of continuous-wave optical parametric oscillators (cw OPOs). In this paper, we discuss how to design and use such grating-cavity cw OPOs for high-resolution spectroscopy in the molecular fingerprint region at ∼3μm. The first design presented in the paper is based on a metal-coated diffraction grating, which produces fast and broad wavelength tuning and high wavelength stability. The second design uses a bulk Bragg grating for high optical power and good spectral purity. We report a new Bragg-grating OPO and demonstrate its use in a Doppler-free absorption spectroscopy of CH4 at ∼3.22μm. In addition, we describe a new balanced detection scheme, which can be used to improve the signal-to-noise ratio of absorption measurements if the measurement noise is limited by the intensity noise of the mid-infrared OPO.

Parts-per-trillion-level detection of nitrogen dioxide by cantilever-enhanced photo-acoustic spectroscopy.

We present a simple and highly sensitive cantilever-enhanced photo-acoustic sensor for detection of nitrogen dioxide. A noise equivalent detection limit of 50 parts-per-trillion in 1 s is demonstrated. The limit was reached with an average optical power of 4.7 W using a continuous-wave laser at 532 nm. The achieved normalized noise equivalent absorption coefficient was 2.6×10(-10)  W cm(-1) Hz(-1/2).

High sensitivity trace gas detection by cantilever-enhanced photoacoustic spectroscopy using a mid-infrared continuous-wave optical parametric oscillator

Highly sensitive cantilever-enhanced photoacoustic detection of hydrogen cyanide and methane in the mid-infrared region is demonstrated. A mid-infrared continuous-wave frequency tunable optical parametric oscillator was used as a light source in the experimental setup. Noise equivalent detection limits of 190 ppt (1 s) and 65 ppt (30 s) were achieved for HCN and CH(4), respectively. The normalized noise equivalent absorption coefficient is 1.8 × 10(-9) W cm(-1) Hz(-1/2).

Stable operation of a cw optical parametric oscillator near the signal–idler degeneracy

The frequency stability of a cw optical parametric oscillator (cw OPO) near the signal-idler degeneracy has been studied. The strong tendency of a near-degenerate OPO to mode hop has been suppressed by using a bulk Bragg grating as a spectral filter in the OPO cavity. An experimental demonstration of stable parametric oscillation in a single longitudinal mode of the OPO cavity is reported, together with the capability of tuning the signal-idler difference frequency from 1 to 4 THz. The OPO has potential use in cw terahertz generation.

High-power mid-infrared frequency comb from a continuous-wave-pumped bulk optical parametric oscillator

We demonstrate that it is possible to obtain a mid-infrared optical frequency comb (OFC) experimentally by using a continuous-wave-pumped optical parametric oscillator (OPO). The comb is generated without any active modulation. It is based on cascading quadratic nonlinearities that arise from intra-cavity phase mismatched second harmonic generation of the signal wave that resonates in the OPO. The generated OFC is transferred from the signal wavelength (near-infrared) to the idler wavelength (mid-infrared) by intracavity difference frequency generation between the OPO pump wave and the signal comb. We have produced a mid-infrared frequency comb which is tunable from 3.0 to 3.4 µm with an average output power of up to 3.1 W.

Experimental observation and analysis of the 3ν1(Σg) stretching vibrational state of acetylene using continuous-wave infrared stimulated emission

We present a sensitive experimental method for molecular spectroscopy that can be used to determine ro-vibrational states using mid-infrared stimulated emission. Our infrared stimulated emission probing (IRSEP) experiment is based on using a narrow-line, continuous-wave Ti:sapphire laser beam (pump) to excite the molecules to an upper vibrational state and a continuous-wave, mid-infrared beam from an optical parametric oscillator (probe) to detect the stimulated emission by the excited molecules. Spectroscopic data are gathered by tuning the wavelengths of the beams. The molecules are probed before their velocity distribution is disturbed by collisions, which leads to a sub-Doppler resolution. The full width at half maximum of the emission peaks is below 10 MHz. The stimulated emission lines are measured with an accuracy of at least 0.005 cm(-1). We use the IRSEP experiment to observe and analyze the symmetric ro-vibrational state [21+] (3ν1(Σg)) of acetylene (C2H2). This state is not accessible via one photon transitions from the ground vibrational state. We use the least-squares method to determine that the band center is at 9991.9725 (12) cm(-1) and the rotational parameters are B = 1.156145 (22) and D = 1.608 (87) × 10(-6) cm(-1), where the uncertainties in parentheses are one-standard errors in the least significant digit.