Singlemode VCSEL for Spectroscopic Applications
Single-mode VCSEL at 760 and 948 nm
VCSEL
Absorption spectroscopy by means of tunable lasers is a prime application for single mode VCSELs. Some real pioneer work was done here using VCSEL-based systems on the first industrial diode laser spectrometers, which were built in economically significant numbers.
Among its uses, optical measurement of oxygen has to be highlighted. This occurs at the band around 760 nm. In addition to this wavelength we also offer single mode VCSELs at 948 nm, which can be used for humidity measurement.
By nature, single-mode VCSELs are free of mode hops. Nevertheless, for spectroscopic applications, versions with an integrated Peltier cooler are recommended. Only in this way can the working point (and hence the calibration) be precisely maintained, even with temperature conditions that fluctuate.
The principle behind this type of measurement is such that the diode laser serves as a selective light source and sweeps a characteristic part of the spectrum. An additional spectral device is not required. Spectral tuning is usually done by changes in the current while the temperature remains constant. In this way, high measurement frequencies and resolutions of approximately 0.000004 nm can be achieved. The transmitted radiation is measured and the concentration of the substance in question is calculated by the Lambert Beer law. Single-mode VCSELs of this kind are 100% free of pole flips.
A good single-mode (SM) VCSEL has a bandwidth of less than 30 MHz (or a coherence length of several tens of meters). However, from a practical point of view, it is rather difficult to implement this type of single-mode VCSEL. The challenge lies in the high current tuning rate of 0.4 nm/mA, which is approximately thirty times greater than that of DFB lasers. Each fluctuation of the operating current leads to a strong shift in the wavelength. In other words, the spectral width of the single-mode VCSEL (and with it the coherence length) is a very sensitive measurement of the noise of power supplies.
Single-mode VCSEL Array at 763 nm
An 8-element single-mode laser diode array has recently become available at LASER COMPONENTS under the name SPECDILAS-V-763-MTE-2x4L. Separated by a distance of 500 µm, two rows of four diodes each are equipped with a pitch of 250 µm and mounted on a Peltier cooler. This helps to stabilize the temperature in challenging experiments. With this array it is possible to measure two dimensionally, for example oxygen concentration using the change in absorption rate or the speed of an object using a laser Doppler method, unlike in the past when it was only possible to measure in one spot. As a result, even fluctuations or discrepancies in distribution (flames) can be more realistically recorded.
