What is a neutral density filter for in laser applications?
Wednesday, May 27, 2020
When profiling or performing measurements on a laser beam, it is often necessary to manipulate the beam to a state where it can be accurately measured. This can involve either increasing beam size if it is too small, or decreasing it if it is too large.
However, most times, the laser’s power needs to be reduced to a certain level before it can be measured. While there are several accessories that can be used to achieve this, we will focus on neutral density (ND) filters in this article.
Simply put, an ND filter is a laser beam attenuator. In other words, it is designed to reduce the intensity of the beam without changing its other properties. These relatively small, simple devices work much like sunglasses do – by absorbing the laser light before it reaches the measuring device.
These filters are essential since most beam profilers and other measuring devices are rated with a damage threshold. This threshold, which can be found in your device’s manual, specifies the maximum intensity that your measuring sensor can take before damage occurs.
Regardless of the laser sensor used (CMOS, CCD, or scanning slit), attenuation may be needed to obtain a usable signal that is not saturating or damaging to the sensor. Therefore, by attenuating your beam before it reaches your measuring device, you can ensure the accuracy, reliability and longevity of your equipment.
While ND filters are effective at regulating laser beam intensities, you should, however, keep in mind that these devices are not high-powered attenuators. Applying excessive optical powers can eventually lead to thermal beam distortions, thermal lensing, and other types of laser-induced damage to the filter.
Typically, especially for high-powered lasers, beam splitters and beam expander/reducers are at the front line. ND filters are usually used as a last line of defense before the beam hits your measurement sensor.
One of the biggest advantages of ND filters is that they can be placed in series (in line with each other) to achieve various degrees of total attenuation. When used in this fashion, it is recommended to place the weakest (less attenuating) filters first and the strongest last (or closest to the measuring sensor).
Additionally, when using ND filters in series, you should also ensure that the laser beam impacts the filter at a 90-degree angle (perpendicular). If this is not achieved, reflectivity can result in the formation of optical resonance, which can adversely impact the quality of the overall transmission. In extreme cases, this modified transmission can even give rise to high resonant power, which can damage the filters.
ND filters are a must for most laser calibration, testing, and measuring applications. Regardless of the type of laser beam system or measuring equipment you are using, attenuation should be a top priority to ensure that you are receiving accurate readings, and most importantly, not exceeding the damage threshold of your equipment.