Core Technology

BRISTOL INSTRUMENTS is a world leader in laser wavelength measurement technology. It earned this leadership role by bringing together many of the experts from the now-defunct Burleigh Instruments, a company that was the pioneer of commercially available laser wavelength meters.

The primary technology utilized by Bristol Instruments is optical interferometry. Detailed analysis of the interference phenomena, a consequence of the wave properties of light, can result in a variety of precise measurements. In particular, Bristol Instruments employs Michelson and Fizeau interferometer-based technology in its products to accurately characterize the spectral properties of lasers.

Such analysis is important for applications such as high-resolution laser spectroscopy, photochemistry, optical remote sensing, and optical fiber communications. Bristol Instruments also uses its Michelson interferometer technology to accurately measure the thickness of transparent and semi-transparent materials. This information is useful in the development and production of specialty plastic films, medical tubing, and ophthalmic products.

The wavelength meters offered by Bristol Instruments use optical interferometry to measure a laser’s absolute wavelength very accurately. Measurements are made in real-time resulting in the ability to automatically report and control laser wavelength. Two types of interferometers are used. These are the Michelson interferometer and the Fizeau etalon.

Michelson Interferometer
A scanning-mirror Michelson interferometer is used to generate information from the interference of two beams that originate from the same source. The optical input is split between a fixed path and a path that is smoothly changing in length. Both beams are reflected and recombined to produce an interference pattern that varies in time as a consequence of the changing phase relationship between the beams. Interference patterns are generated simultaneously from the laser under test and from a built-in HeNe reference laser. By comparing the number of interference fringes generated during the interferometer’s scan, the absolute wavelength of the laser under test can be determined to an accuracy as high as ± 0.0001 nm.

Wavelength meters based on the scanning-mirror Michelson interferometer provide the greatest reliability because this technique lends itself to continuous calibration with a built-in wavelength standard. However, its operation is limited to CW and quasi-CW lasers because the generation of a temporal interference pattern is incompatible with pulsed lasers. Another advantage of the Michelson interferometer technique is its versatility, resulting in operation from 375 nm to 12 µm.

Fizeau Etalon
This type of wavelength meter uses multiple fixed-spaced Fizeau etalons to generate spatial interference patterns that are compared to those produced with a built-in reference laser. Data from the etalons are collected by imaging the interference patterns onto a photodetector array. The spacing between the interference fringes is used to calculate absolute laser wavelength in a stepwise fashion. The results from the low-resolution etalons are used to obtain the high-accuracy measurement from the highest resolution etalon.

The Fizeau etalon technique is used for wavelength meters that are intended to measure the absolute wavelength of pulsed lasers. The photodetector array can capture the spatial interference patterns of virtually every laser pulse to a repetition rate as high as 1 kHz. However, operation is limited to lasers that operate at wavelengths less than 1700 nm because longer-wavelength photodetector arrays currently are not cost effective.

Bristol Instruments also uses optical interferometry to provide a more complete spectral characterization of CW, quasi-CW, and high-repetition rate pulsed lasers. The spectrum analyzers offered by Bristol Instruments generate a power versus wavelength spectrum so that laser parameters such as absolute wavelength, linewidth, and longitudinal mode structure can be measured.

The spectrum analyzer uses a technique that is similar to a Michelson interferometer-based wavelength meter. However, instead of simply counting interference fringes to calculate wavelength, it uses a fast Fourier transform (FFT) to analyze the interference pattern. That is, the Michelson interferometer generates a temporal interference signal with a waveform that is representative of all the frequency (wavelength) components of the laser under test. By computing an FFT, a conventional optical frequency (wavelength) spectrum can be derived resulting in the desired spectral information.

By using this technique, the Bristol Instruments’ spectrum analyzer becomes a high-accuracy wavelength meter and high-resolution spectrum analyzer in one instrument. Absolute wavelength is measured to an accuracy as high as
± 0.0001 nm and the laser’s power vs wavelength spectrum is determined to a resolution as high as 2 GHz.

Bristol Instruments uses its proven Michelson interferometer-based wavelength meter technology to measure the absolute thickness of a variety of materials. Light from a superluminescent LED is sent to the material under test through an optical probe. Reflections from every surface (top, bottom, and internal) are collected and returned to the Michelson interferometer. An analysis of the reflected optical signals is then done using the interferometer in a way similar to the analysis done by a wavelength meter.

Absolute thickness is measured to an accuracy as high as ± 0.1 μm and with a repeatability as high as ± 0.02 μm. As with the wavelength meters, the thickness measurement is guaranteed over long periods of time by continuously referencing to a built-in intrinsic standard of length.