Turn application requirements into laser parameter
To find the right laser diode for your case, you probably start with a set of parameters given by your application. We will do this here with the help of an example. Let’s assume we want to build a decent laser interferometer for surface profile or velocity measurement.
For this device we need a laser diode with a coherence length of 1- 10 m, the interferometric pattern should stay stable through temperature change (<0.1 nm/K). I need a collimated Gaussian beam and the power should be higher than 80 mW. The detector I am using is based on Si, which works only for wavelength < 1100 nm. The center wavelength itself and polarization are less important in this case. We have no idea about package or pinning at that point.
Table 1: from application specs to laser parameters. A reference table. Example data in bold
|Application requirements ||Laser parameters |
|Coherence length L = 1 – 10 m |
Spectral resolution Bandpass of the filters etc.
Linewidth Δν = 10 – 100 MHz
Wavelength stability < 0.1 nm/K
Wavelength λ< 1100 nm
Beam quality, divergence, beam spot profile and size etc.
Transverse Mode, M² M² < 1.1
Intensity, brilliance etc.
|Power P > 80 mW |
Table 1 shows you the data we have so far. Pure application requirements are on the left, laser parameters are on the right side. From the coherence length I can calculate the linewidth using Δν =c/πL= 9.6-95.5 MHz.
For those new in the field we will now explain the parameters in more detail. Most of the details are based on Rüdiger Paschottta’s RP Photonics Encyclopedia, which is an excellent resource for all kind of background knowledge.
Coherence length: Distance over which the coherence significantly decays. Actually, it even relates to the temporal coherence length, but for our purposes, the definition above is sufficient. For more details and a calculator, you may refer to www.rp-photonics.com/coherence_length.html . We used the following formula later in this tutorial: Δν =c/πL where Δν is the bandwidth (or linewidth), c the speed of light and L the coherence length
Spectral resolution: The spectral resolution denotes the relation between the bandwidth (in nm) and the wavelength: R= λ/ Δ λ. In case of a spectrograph, or, more generally, of a frequency spectrum, is a measure of its ability to resolve features in the electromagnetic spectrum.
If you want to calculate the bandwidth in MHz from a nm value, you may use the following formula: Δν= Δλ*c/λ² . Or you use one of the internet calculators such as the one at www.photonicsolutions.co.uk/wavelengths.php which even offers the conversion into cm-1 and four other units of bandwidth.
Bandpass Some sensors for the detection of a laser signal use interference filters in order to block disturbing ambient light. Thus the wavelength of the laser source has to be kept within the small transmission range of the filter. For the vendor that’s important information, but in our example a limited center wavelength tolerance can be neglected.
Beam quality can be defined in several ways. One is the M² factor which says how close a beam is to its ideal Gaussian shape. So 1.0 denotes a perfect Gaussian beam. Another one is the Beam Parameter Product BPP, for which we have to multiply the beam waist at focus with the far field divergence. For more details see: www.rp-photonics.com/beam_quality.html .
Intensity denotes the laser power in the beam area, preferably the focus. Accordingly, its unit is Watt/cm². The question here is what you take as beam area. A detailed discussion can be found at www.rp-photonics.com/optical_intensity.html .
Beam profile is a name for the intensity distribution in the laser beam. Depending on that distribution it might be flat top (rectangular distribution) or Gaussian. A single mode beam is usually (close to) Gaussian, where as a multi-mode beam is usually not Gaussian. It may have a variety of shapes depending on the number and intensity distribution of the mixed modes.
Brilliance or brightness of a laser source gives a measure for its output power and beam quality in one number. Essentially it is the laser power divided by the beam parameter product. Therefore, its unit is Watt/cm²*steradian. A more detailed discussion is here www.rp-photonics.com/brightness.html .