The Importance Of Photometry In The Laser Interferometer

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1 Introduction Interferometry is a measurement technique which utilizes the interference of waves in a medium to cre-ate interference patterns [5]. In general, the waves can be light, sound or radio waves. The interference pattern is detected by a photodetector and it provides some useful information about the properties of the object or phenomenon under test. Interferometers are used to make very small measurements that cannot be achieved with any other method. An important application of interferometry is in the Laser Interferometer Gravitation wave Observatory(LIGO) which is used to detect gravitational waves predicted by Einstein.1.1 The LIGO interferometer is like the Michelson Interferometer as shown in …show more content…
As they travel back towards the beam splitter, they merge together to form a single beam. As the beams merge, they interfere with each other. The merged beam passes to the photodetector which measures the intensity. If the length of the arms are the same, then the beams will travel the same distance before interfering. Depending on how the mirrors are placed, the photodetector will measure either constructive interference i.e.same intensity as incident beam or destructive interference i.e. zero intensity.The LIGO interferometer is set up in such a way that the photodetectors measure zero intensity if the arms are of same length. The LIGO interferometer is a L-shaped interferometer, with each arm 4km in length. The longer the arms, the farther the laser has to travel which increases the sensitivity of the instrument. In order to measure a change in length that is 10000 times smaller than a proton,even an arm length of 4 km is not enough. To increase the length of the arms, without physically increasing the length, Fabry-Perot cavities were introduced to the Michelson configuration. Each arm was equipped with a Fabry-Perot cavity by adding mirrors …show more content…
This effectively increases the length of the arms to 1120 km[2].In addition to the length of the arms, another aspect that greatly influences the LIGO interferometer is the output power of the laser. Sensitivity depends on the arm length and the resolution depends on the laser power. This is because as the power of the laser increases, more photons are generated. When higher number of photons merge at the beam splitter, sharper interference pattern is generated. According to theoretical calculations, a full sensitivity for LIGO can be achieved if the laser operates at 750kW. But the laser used in the LIGO can produce only 200 W of power. Building a laser than can produce 750kW power is also practically impossible. To address this issue, the LIGO uses power recycling mirrors which are placed in between the laser source and the beam splitter. The power recycling mirrors are made to be partially reflective. The laser light is made to pass through the power recycling mirrors before it enters the beam splitter which splits it into two. The instrument is constructed such that all the light coming back from the arms is sent back to power recycling mirrors where they combine with the laser

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