However, the space-time distortion caused by gravitational waves is very small. For example, when GW150914 reaches the earth, the strain is only 10-21. LIGO is an L-shaped modified Michelson interferometer, and its structure is shown in Figure 1. Two 4 km long interference arms are perpendicular to each other, and the arm length change produced by GW150914 is only on the order of 10-18m.
In order to obtain sufficient sensitivity, the high-tech LIGO detector has adopted several optical enhancement measures. First, each interference arm itself is a Fabry-Perot cavity. The laser pointer entering the interference arm, unlike the general Michelson interferometer, only goes back and forth and then returns to the beam splitter to interfere with the beam of the other arm, but is “saved” in the interference arm for a period of time. The laser oscillates back and forth about 300 times in the Fabry-Perot cavity before returning to the beam splitter, which means that the sensitivity of the phase of the light field to changes in length has been enhanced 300 times.
Or in other words, the effective arm length of the interferometer is increased by 300 times. At the same time, the laser power in the cavity is also increased by 300 times due to resonance enhancement. Secondly, the interferometer works near the dark state, most of the laser light is reflected back to the original laser direction. Therefore, after the laser and before the beam splitter, a partially transparent mirror is placed to resonate and enhance the laser in the interferometer, and the enhancement effect is about 35 times. Therefore, the 20W laser input is enhanced to about 700 W at the beam splitter and 100 kW in each arm. In addition, a partially transparent mirror is placed at the output of the interferometer to enhance signal extraction and detector bandwidth.
All these interferometer technical measures are to maximize the optical signal generated by the gravitational wave strain, thereby reducing the photon shot noise. Photon shot noise is caused by the statistical fluctuations of the light quanta reaching the photoelectric probe. It is the main noise source at the high-frequency end of the blue laser pointer interference gravitational wave detector. Its influence on the signal-to-noise ratio is proportional to the square root of the laser power in the interferometer Inversely. In the next few years, the high-tech LIGO detector plans to increase the incident laser power to increase the circulating laser power in the interferometer to 750 kW to further reduce noise.
To achieve these enhancements, all these coupled optical cavities need to be locked by a servo control system. The cavity length of the Fabry-Perot cavity in the interference arm is stable to less than 100 fm, while the other optical cavities are kept within 1 to 10 pm. At the same time, each mirror needs to be adjusted and kept perfectly coincident with the optical axis, and the deviation is within tens of nanoradians.
The high-tech LIGO detector can detect extremely weak gravitational wave strain, and is bound to be very sensitive to tiny vibrations in the environment. It is essentially a huge seismograph that can sense vehicles on nearby roads, distant waves hitting the shore, and almost all important earthquakes on the earth. These mechanical vibrations are the main noise source at the low frequency end. In order to reduce vibration and noise, each mirror is suspended on a complex quadruple pendulum system, as shown in Figure 2; and this quadruple pendulum system is fixed on an active vibration isolation platform. In this way, most of the mechanical waves are filtered out and external interference is isolated.