Optical Frequency Standards - Calibration Services

Calibration of Helium-Neon Lasers at 633 nm

The Optical Frequency Standards (OFS) project can offer routine frequency and vacuum wavelength calibration of client helium-neon lasers at 633 nm with an uncertainty of less than 1x10-10, normally limited by the reproducibility and stability of the client laser.
The calibration method involves comparing the frequency of the client's laser with that of the National Standard (a known frequency) and measuring the difference frequency. This is done by overlapping the output beams of the two lasers so that the combined beams illuminate a fast-response photodiode. A beat frequency equal to the relatively small difference (typically less than a few hundred megahertz) between the two laser frequencies is generated at the photodiode and this signal is measured with a calibrated frequency counter.

Frequency measurements are performed with the NRC laser stabilized on a series of four hyperfine lines in the R(127) line of the 11-5 band of the B-X 127I2 (iodine) transition to obtain an unambiguous determination of the client's laser frequency. During a typical calibration, the client's laser frequency is checked on over five different occasions over a period of at least three days in order to determine the overall stability and frequency reproducibility. Estimates of the client's laser warm-up period and sensitivity to optical feedback are also provided.

In most requested calibrations, the client's 633-nm helium-neon laser system employs a frequency stabilization method that is inferior to locking on an iodine resonance. For example, polarization stabilization, Lamb-dip stabilization, Zeeman stabilization, etc. are used. For these types of lasers, the frequency is calibrated to an uncertainty commensurate with its stability: generally, the uncertainty will not be worse than 5 parts in 109. This is more than sufficient for most industrial applications.

Client helium-neon lasers that are iodine-stabilized can be calibrated to uncertainties approaching the limits of the NRC laser which has been calibrated through comparison with a laser at the Bureau International des Poids et Mesures (BIPM) and directly through the NRC optical frequency comb. Under optimal operating conditions, the NRC laser has an uncertainty of less than 2 10-11. More precautions and additional measurements must be taken to realize this level of accuracy and the fee is accordingly higher.

Calibration of Lasers at 1511 to 1552 nm

These calibrations use one of two similar NRC built laser systems which are frequency stabilized on saturated absorption overtone lines in 12C2H2 and 13C2H2 (acetylene). Our acetylenestabilized lasers have been calibrated at over one hundred different lines with traceability to the SI second by means of the NRC optical frequency comb and through frequency difference measurements. These lasers are calibrated periodically with the NRC optical frequency comb. The large number of absorption lines between 1511 nm and 1552 nm permits client lasers to be calibrated over much of this region through heterodyne beat measurements. The calibration procedure is similar to that described previously for helium-neon lasers at 633 nm.

Custom Calibration of Lasers at 550 to 1150 nm and 1530 to 1560 nm

Custom calibrations of laser frequency/wavelength with traceability to the SI second through the NRC optical frequency comb are offered for lasers in the ranges 550 nm to 1150 nm and 1530 nm to 1560 nm.. Laser frequency is determined by measuring the heterodyne beat between the client laser and a single element of the optical comb. The frequencies of the comb elements are fixed by locking both the comb's repetition frequency and its offset to a signal from a calibrated hydrogen maser. In this way, an uncertainty of less than 10 Hz is achievable from 193 to 550 THz. In practice, the uncertainty is determined by the reproducibility and stability of the client laser.

Contact:
Dr. John Bernard
Phone: 1-613-993-2181
Email: john.bernard@nrc-cnrc.gc.ca