LASER INSTRUMENTATION
Interferometer calibrates laser wavelengths
A novel wideband wavelength meter capable of accurately calibrating gas, solid-state, and semiconductor CW laser sources across a wavelength range from 0.4 to 1.8 µm has been introduced by Campagnie des Senseurs Optiques (CSO, Grenoble, France); the first unit will soon be delivered to the Centre National d`Etudes Spatiales (CNES, Toulouse, France).
Conventionally, precision wavelength measurement has been done with a heterodyne-based device. This me thod, however, requires a slight difference between the frequency of the laser being calibrated and that of the reference laser. Hence, devices based on heterodyning are used for measuring wavelengths very close to that of a conventional reference gas laser such as a HeNe or other well-characterized device.
The advent of semiconductor lasers and their increasing use as long-term stable reference sources (with long-term stability around 1 part in 108) has meant that some other method of measuring wavelength is essential. The CSO wavelength meter is a dual Michelson interferometer in which the interference fringes from the laser being calibrated and from a reference laser are counted for a certain length variation of the measurement beam. The reference laser has a known frequency that is stabilized to 1 part in 109.
The interferometer is a Fresnel prism (see figure). Polarizers allow the device to generate separately two sets of fringes, which are then sent to their respective detectors. Each beam is previously polarized and phase shifted by the Fresnel prism. When a corner cube is moved, a whole (integral) number of fringes generated by the calibration and reference beams are counted; fractions of fringes are also counted by fringe interpolation. The translation motion of the corner cube is about 10 cm so for a wavelength of 0.5 µm, 4 ¥ 105 fringes are counted; one fringe corresponds to l/2.
Signal processing is a major component of the system. It allows fringe interpolation with an accuracy better than 10-3 (one fringe divided into 1000 points), which provides wavelength measurement with a relative uncertainty (Dl/l) of 3 ¥ 10-8. Operation in a vacuum eliminates errors of 10-8 magnitude caused by the change of refractive index of air with wavelength.
The overall dimensions of the complete system including computer are 550 ¥ 350 ¥ 170 mm. According to CSO, the device will open up many applications in the space and telecommunications industries and wherever lasers have to be precisely calibrated, particularly the newest highly stable semiconductor lasers.
Roland Roux