Erkki Ikonen

Passive frequency and intensity stabilization of extended-cavity diode lasers

Studies are reported on frequency drifts in extended-cavity diode lasers caused by external effects, such as changes in temperature and air pressure. A laser system operating at 780 nm has been constructed utilizing low expansion materials and such mechanical structures that compensate for the external effects. By placing the laser system in a pressure-proof and temperature-controlled housing, a relative frequency stability of better than 10−10 (40 kHz) is obtained for integration times of 10 μs to 10 s. The drift of the laser frequency caused by spectral aging of the diode laser is about 3 MHz/h. As a consequence of high passive stability, the variations of the laser intensity are also greatly reduced to a relative drift value of 4×10−5/h.

Quantum interference of tunably indistinguishable photons from remote organic molecules

We demonstrate two-photon interference using two remote organic molecules as bright solid-state sources of indistinguishable photons. By varying the transition frequency and spectral width of one molecule, we explore the effect of photon distinguishability.

Photometry, radiometry and 'the candela': evolution in the classical and quantum world

The metrological fields of photometry and radiometry and their associated units are closely linked through the current definition of the base unit of luminous intensity—the candela. These fields are important to a wide range of applications requiring precise and accurate measurements of electromagnetic radiation and, in particular, the amount of radiant energy (light) that is perceived by the human eye. The candela has been one of the base units since the inception of the International System of Units (SI) and is the only base unit that quantifies a fundamental biological process—human vision. This photobiological process spans an enormous dynamic range of light levels from a few-photon interaction involved in triggering the vision mechanism to a level of more than 1015 photons per second that is accommodated by the visual response under bright daylight conditions. This position paper, prepared by members of the Task Group on the SI of the Consultative Committee for Photometry and Radiometry Strategic Planning Working Group (CCPR WG-SP), reviews the evolution of these fields of optical radiation measurements and their consequent impact on definitions and realization of the candela. Over the past several decades, there have been significant developments in sources, detectors, measuring instruments and techniques, that have improved the measurement of photometric and radiometric quantities for classical applications in lighting design, manufacturing and quality control processes involving optical sources, detectors and materials. These improved realizations largely underpin the present (1979) definition of the candela. There is no consensus on whether this radiant-based definition fully satisfies the current and projected needs of the optical radiation community. There is also no consensus on whether a reformulation of the definition of the candela in terms of photon flux will be applicable to the lighting community. However, there have been significant recent advances in radiometry in the development of single-photon sources and single-photon detectors and the growth of associated technologies, such as quantum computing and quantum cryptography. The international acceptance of these new quantum-based technologies requires improved traceability and reliability of measurements at the level of a few photons. This review of the evolution of the candela and the impact of its possible reformulation might lead, in the future, to a reformulation in terms of quantum units (photons). This discussion is timely since redefinitions of four of the other SI base units are being considered now in terms of fundamental constants to provide a more universally realizable quantum-based SI system. This paper also introduces for the first time a fundamental constant for photometry.

High-accuracy spectrometer for measurement of regular spectral transmittance

A high-accuracy spectrometer has been developed for measuring regular spectral transmittance. The spectrometer is an automated, single-beam instrument that is based on a grating monochromator, reflecting optics, and an averaging sphere detector unit with a silicon photodiode. The uncertainties related to wavelength calibration, detector nonlinearity, system instability, beam displacement, polarization, stray light, interreflections, and beam uniformity are determined for the visible spectral range from 380 to 780 nm. A total uncertainty of 3 × 10(-4) (1σ) is estimated for transmittance measurements of homogeneous neutral-density filters. The uncertainty of the wavelength scale is 0.06 nm. As a specific application, calibration of V(λ)-correction filters is studied. To verify the accuracy of the transmittance measurements, a comparison of the measured and predicted transmittances of a sample of high-purity fused silica is made, revealing agreement at the 5 × 10(-4) level.

Gonioreflectometer for measuring spectral diffuse reflectance

Gonioreflectometric determination of reflectance factors that involves hemispherical collection of reflected flux, which is an alternative to integrating sphere-based methods, is discussed. A detailed description of a gonioreflectometer built at the Helsinki University of Technology is presented. The instrument is used to establish an absolute scale of total diffuse reflectance factors throughout the spectral range 360-830 nm. The hemispherical reflectance factors are obtained through integration of the gonioreflectometric measurement results. The reflectance factors of white high-quality artifacts can be determined with a combined standard uncertainty of 0.20%. Results of test measurements were found to be in agreement with values traceable to other absolute scales based on integrating-sphere methods.

Gauge-block Interferometer Based on One Stabilized Laser and a White-light Source

An automated interferometer equipment is described which can be used for calibration of gauge blocks with lengths up to 1 000 mm. The main advantage of the new type of interferometer is that no prior knowledge on the length of the gauge block is required. The measurement method is based on scanning and it utilizes a white-light source and one stabilized 633 nm laser. White-light interference signals from the reference flat and from the front surface of the gauge block indicate the distance to be measured with the laser. The final length is determined from the average laser-signal phase shift between the gauge and the reference flat. Environmental conditions are measured and the necessary corrections are made automatically. An integrating sphere used for evaluation of the surface roughness correction is described. Results of the various test measurements with both short and long gauge blocks are given. The estimated overall uncertainty (2 σ) is 30 nm for a 100 mm gauge block and 160 nm for a 1 000 mm gauge block. Repeatability of wringing is the main source of uncertainty with short gauge blocks. With long gauge blocks, the accuracy is limited by uncertainty in the temperature of the gauge and in the air refractive index.

Spectral reflectance of silicon photodiodes

A precision spectrometer was used to measure the spectral reflectance of a silicon photodiode over the wavelength range from 250 to 850 nm. The results were compared with the corresponding values predicted by a model based on thin-film Fresnel formulas and the known refractive indices of silicon and silicon dioxide. The good agreement at the level of 2 x 10(-3) in the visible wavelength range verifies that the reflection model can be used for accurate extrapolation of the spectral reflectance and responsivity of silicon photodiode devices. In addition, characterization of the photodiode reflectance in the ultraviolet region improves the accuracy of the spectral irradiance measurements when filter radiometers based on trap detectors are used.

Frequency stabilization of a diode laser to Doppler-free spectrum of molecular iodine at 633 nm

Abstract We report on frequency stabilization of a diode laser to the hyperfine components of the P(33) 6-3 and R(127) 11-5 transitions of molecular iodine at the He-Ne laser wavelength of 633 nm. Single frequency operation and wavelength control of the diode laser in a compact form is obtained by employing weak optical feedback from an integrated microlens. The diode laser driven by an ultra low noise current supply provides nearly shot noise limited detection. A relative frequency stability of 5×10 −12 is achieved at an integration time of 100 s. Harmonic distortion of the modulated output of the diode laser due to spurious optical feedback is considered to be the main effect limiting the day-to-day frequency reproducibility of 5×10 −11 .

Development of a detector-based absolute spectral irradiance scale in the 380–900-nm spectral range

A detector-based absolute scale for spectral irradiance in the 380-900-nm wavelength region has been developed and tested at the Helsinki University of Technology (HUT). Derivation of the scale and its use for photometric and colorimetric measurements are described. A thorough characterization of a filter radiometer, constructed from a reflection trap detector, a precision aperture, and a set of seven temperature-controlled bandpass filters, is presented. A detailed uncertainty analysis of the scale indicates a relative standard uncertainty of approximately 0.2% throughout most of the wavelength region. The standard uncertainties obtained in measurements of correlated color temperature and luminous intensity of three Osram Wi41/G tungsten-halogen lamps are 2 K and 0.3%, respectively. The spectral irradiance scale is compared with the HUT luminous intensity scale. The agreement of the results at the 0.1% level is well within the combined standard uncertainty of the two scales.

Spectral irradiance measurements of tungsten lamps with filter radiometers in the spectral range 290 nm to 900 nm

A method of measuring the absolute spectral irradiance of quartz-halogen-tungsten lamps is described, based on the known responsivity of a filter radiometer, the components of which are separately characterized. The characterization is described for the wide wavelength range essential for deriving the spectrum of a lamp, from 260 nm to 950 nm. Novel methods of interpolation and measurement are implemented for the spectral responsivity of the filter radiometer. The combined standard uncertainty of spectral irradiance measurements is less than 1.4 parts in 102 from 290 nm to 320 nm (ultraviolet B) and 4 parts in 103 from 440 nm to 900 nm (visible to near-infrared). As an example, the derived spectral irradiances of two lamps measured at the Helsinki University of Technology (HUT, Finland) are presented and compared with the measurement results of the National Institute of Standards and Technology (NIST, USA) and the Physikalisch-Technische Bundesanstalt (PTB, Germany). The comparisons indicate that the HUT spectral irradiance scale is between those of the NIST and the PTB in the wavelength range 290 nm to 900 nm. The long-term reproducibility of the spectral irradiance measurements is also presented. Over a period of two years, the reproducibility appears to be better than 1 part in 102.