ABSTRACT
Extreme lasers capable of short, high-energy pulses are probing the frontiers of science and advancing practical technology. The utility of such lasers increases with their average power delivery, which enables faster data acquisition, higher flux of laser-driven particle and radiation sources and more efficient material processing. However, the same extreme energies and electric field strengths of these lasers are currently preventing their direct and high accuracy measurement for these experimental applications. To overcome this limitation, we use the momentum of the laser pulses as a measurement proxy for their energy. When light reflects from an ideal mirror, its momentum is transferred to the mirror, but its energy is reflected. We demonstrate here a force-sensing mirror configuration to measure laser pulse energies up to 100 J/pulse (10 ns duration, 10 Hz repetition rate) from a kilowatt-level average power multi-slab laser operated at the HiLASE facility of the Czech Academy of Sciences. We combine a radiation-pressure power meter with a charge integrator photodiode to form what we refer to as a Radiation Pressure Energy Meter. To our knowledge, this is the first demonstration of a high-accuracy, non-absorbing, SI traceable primary standard measurement of both single and average pulse energies of a 1-kW-average-power pulsed laser source. With this, we demonstrate a practical method for in-situ calibration of the traditional thermal instruments (pyroelectric detectors) currently used for indirect measurements of energy and power of such extreme lasers.
ABSTRACT
One major objective of the European Joint Research Project "Traceability for surface spectral solar ultraviolet (UV) radiation" was to reduce the uncertainty of spectral UV measurements. The measurement instrument used for this work was the portable UV European reference spectroradiometer Qasume. The calibration uncertainty of this instrument was decreased and validated by a comparison of direct calibrations against a primary standard for spectral irradiance, a high temperature blackbody radiator, and against a reference detector using a spectrally tunable laser as a monochromatic source. The spectral irradiance responsivity of the reference detector is traceable to the primary standard of optical power, realized through a cryogenic radiometer, and to the SI unit of meter. The measuring technique was improved by the construction of a new reference spectroradiometer, QasumeII. An improved input optics removes the dependences of the measured solar irradiance on the angle of incident for solar zenith angle smaller than 75 deg. Moreover, a hybrid photon detection system enables continuous tracking of the instrument's responsivity changes. For both spectroradiometer systems an uncertainty budget was calculated. The improvements have reduced the measurement uncertainties of solar spectral UV irradiance measurements from 4.8% in 2005 to 2.0% (k=2) in the spectral region above 310 nm. The largest sources of uncertainty were the absolute spectral irradiance responsivity calibration, the angular response uncertainty, and the instrument stability using the hybrid detector, which were reduced from 3.6% to 1.1%, from 1.2% to 0.6%, and from 0.65% to 0.4%, with respect to the situation prior to the project. The new instrument was validated during a four month intercomparison relative to the Qasume reference. The mean ratio of the solar irradiance scans between the two reference spectroradiometers has an offset of +0.7% and a standard deviation of ±1.5% for a wavelength greater than 305 nm, which is well within the combined uncertainty of 3.7% calculated from the uncertainties of the two systems.
ABSTRACT
We introduce a technique for measuring detection efficiency that is traceable to the primary standard, the cryogenic radiometer, through a reference silicon photodiode trap detector. The trap detector, used in conjunction with a switched integrator amplifier, can measure signals down to the 0.1 pW (3 x 105 photons second-1) level with 0.1% uncertainty in a total integration time of 300 seconds. This provides a convenient calibration standard for measurements at these levels across the optical spectrum (UV - near IR). A second technique is also described, based on correlated photons produced via parametric down-conversion. This can be used to directly measure detection efficiency in the photon counting regime, and provides a route for expanding the formulation of the candela in terms of photon flux to enable it to address the needs of emerging quantum optical technologies and applications. The two independent techniques were cross-validated by a comparison carried out at 702.2 nm, which showed agreement to within 0.2%.
ABSTRACT
In the field of low flux optical measurements, the development and use of large area silicon detectors is becoming more frequent. The current/voltage conversion of their photocurrent presents a set of problems for traditional transimpedance amplifiers. The switched integration principle overcomes these limitations. We describe the development of a fully characterized current-voltage amplifier using the switched integrator technique. Two distinct systems have been developed in parallel at the United Kingdom's National Physical Laboratory (NPL) and Czech Metrology Institute (CMI) laboratories. We present the circuit theory and best practice in the design and construction of switched integrators. In conclusion the results achieved and future developments are discussed.
