Tuomas Hieta

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).

High-precision diode-laser-based temperature measurement for air refractive index compensation

We present a laser-based system to measure the refractive index of air over a long path length. In optical distance measurements, it is essential to know the refractive index of air with high accuracy. Commonly, the refractive index of air is calculated from the properties of the ambient air using either Ciddor or Edlén equations, where the dominant uncertainty component is in most cases the air temperature. The method developed in this work utilizes direct absorption spectroscopy of oxygen to measure the average temperature of air and of water vapor to measure relative humidity. The method allows measurement of temperature and humidity over the same beam path as in optical distance measurement, providing spatially well-matching data. Indoor and outdoor measurements demonstrate the effectiveness of the method. In particular, we demonstrate an effective compensation of the refractive index of air in an interferometric length measurement at a time-variant and spatially nonhomogeneous temperature over a long time period. Further, we were able to demonstrate 7 mK RMS noise over a 67 m path length using a 120 s sample time. To our knowledge, this is the best temperature precision reported for a spectroscopic temperature measurement.

Continuous-wave optical parametric oscillator based on a Bragg grating

A single-mode cw optical parametric oscillator (OPO) based on a bulk Bragg grating is reported. The OPO is capable of producing multiwatt output power both at the signal (1535 nm) and idler (3468 nm) wavelengths with a side-mode suppression ratio better than 29 dB. Coarse frequency tuning of 120 GHz(4 cm(-1)) and fast mode-hop-free tuning of 40 GHz(1.3 cm(-1)) have been achieved by scanning the grating temperature and the OPO pump laser, respectively.

Frequency-comb-referenced mid-infrared source for high-precision spectroscopy

We report on a tunable continuous-wave mid-infrared optical parametric oscillator (OPO), which is locked to a fully stabilized near-infrared optical frequency comb using a frequency doubling scheme. The OPO is used for 40 GHz mode-hop-free, frequency-comb-locked scans in the wavelength region between 2.7 and 3.4 μm. We demonstrate the applicability of the method to high-precision cavity-ring-down spectroscopy of nitrous oxide (N2O) and water (H2O) at 2.85 µm and of methane (CH4) at 3.2 μm.

Reliable Determination of Optical Fiber Nonlinearity Using Dispersion Simulations and Improved Power Measurements

We have improved the accuracy of the continuous-wave self-phase modulation method for measuring the nonlinearity of optical fibers. Evaluation of the measurement uncertainty shows that the most significant source of uncertainty is the measurement of fiber-optic power. However, chromatic dispersion can also have a significant effect on the apparent results if it is not taken properly into account. We demonstrate means to achieve an expanded uncertainty of 2% (coverage factor k=2) for the measurement of the nonlinear coefficient n2 /Aeff. Also, the metrological aspects related to the determination of the nonlinear coefficient and the measurement uncertainty are discussed

Effective humidity in length measurements: comparison of three approaches

Humidity is one of the key atmospheric parameters influencing the refractive index of air. Consequently, humidity influences all length measurements whose scale is derived from the speed of light. In this work, we present two laser spectrometers developed for determining the average humidity of air over a long measurement path where local variations may be difficult to measure using conventional sensors. Further, these laser-based systems allow, in principle, good spatial overlap with the beams used in dimensional measurements. The developed systems were compared to each other and to traceable reference sensors during a 65 h measurement campaign. The performance of the systems was investigated under three different conditions: steady state, humidity transient and temperature transient. Both systems were separately tested in outdoor environment at distances up to several hundreds of metres. The measurement results demonstrate that the systems are able to measure the relative humidity below the 4% uncertainty level both in indoor and in outdoor environment.

Sub-parts-per-trillion level sensitivity in trace gas detection by cantilever-enhanced photo-acoustic spectroscopy

An exceptional property of photo-acoustic spectroscopy is the zero-background in wavelength modulation configuration while the signal varies linearly as a function of absorbed laser power. Here, we make use of this property by combining a highly sensitive cantilever-enhanced photo-acoustic detector, a particularly stable high-power narrow-linewidth mid-infrared continuous-wave optical parametric oscillator, and a strong absorption cross-section of hydrogen fluoride to demonstrate the ability of cantilever-enhanced photo-acoustic spectroscopy to reach sub-parts-per-trillion level sensitivity in trace gas detection. The high stability of the experimental setup allows long averaging times. A noise equivalent concentration of 650 parts-per-quadrillion is reached in 32 minutes.

Frequency Comb-Linked Mid-Infrared Continuous-Wave Optical Parametric Oscillator

We report on a frequency comb-referenced mid-infrared continuous-wave optical parametric oscillator. The frequency of the mid-infrared idler beam is locked to a near-infrared frequency comb after frequency doubling the idler beam.

Measurement of Er-Doped Fiber Nonlinearity Using Continuous-Wave Self-Phase Modulation Method

The intensity-dependent nonlinear coefficient n 2/A eff of a short Er-doped, amplifying fiber is determined using a method thoroughly studied for measurements of passive fibers. Measurement uncertainty of 3.0% is achieved using modified continuous-wave self-phase modulation method. In addition to metrological aspects, we also discuss limitations and conditions imposed by the Er-doped fiber as compared with the measurement of a pure silica core fiber. The used measurement method is advantageous as compared with other techniques due to its reliability and ease of implementation.