Optical layout of a Fabry-Perot Michelson laser interferometer, showing primary components. The four test masses, beam splitter, and power recycling mirror are physically located in an ultrahigh vacuum system and are seismically isolated. A photodiode at the anti-symmetric port detects differential arm length changes.
Block diagram of the Input Optics subsystem. The IO is located between the pre-stabilized laser and the recycling mirror and consists of four principle components: electro-optic modulator, mode cleaner, Farday isolator, and mode-matching telescope. The electro-optic modulator is the only IO component outside of the vacuum system. Diagram is not to scale.
Enhanced LIGO Input Optics optical and sensing configuration. The HAM1 (horizontal access module) vacuum chamber is featured in the center, with locations of all major optics superimposed. HAM2 is shown on the right, with its components. These tables are separated by 12 m. The primary beam path, beginning at the pre-stabilized laser and going to the power recycling mirror, is shown in red as a solid line, and auxiliary beams are different colors and dotted. The MMTs, MCs, and steering mirror (SM) are suspended; all other optics are fixed to the seismically isolated table. The laser and sensing and diagnostic photodiodes are on in-air tables.
Electro-optic modulator design. (a) The single RTP crystal is sandwiched between three sets of electrodes that apply three different modulation frequencies. The wedged ends of the crystal separate the polarizations of the light. The p-polarized light is used in the interferometer. (b) A schematic for each of the three impedance matching circuits of the EOM. For the three sets of electrodes, each of which creates its own C crystal , a capacitor is placed parallel to the LC circuit formed by the crystal and a hand-wound inductor. The circuits provide 50 Ω input impedance on resonance and are housed in a separate box from the crystal.
Faraday isolator photograph and schematic. The FI preserves the polarization of the light in the forward-going direction and rotates it by 90° in the reverse direction. Light from the MC enters from the left and exits at the right towards the interferometer. It is ideally p-polarized, but any s-polarization contamination is promptly diverted ∼10 mrad by the CWP and then reflected by the TFP and dumped. The p-polarized reflected beam from the interferometer enters from the right and is rotated to s-polarized light which is picked-off by the TFP and sent to the Interferometer Sensing and Control (ISC) table. Any imperfections in the Faraday rotation of the interferometer return beam results in p-polarized light traveling backwards along the original input path.
Data from the MC absorption measurement post drag-wiping. Power into the MC was cycled between 0.9 W and 5.1 W at 3-h intervals (bottom frame) and the change in frequency of the drumhead mode of each mirror was recorded (top frame). The ambient temperature (middle frame) was also recorded in order to correct for its effects.
Faraday isolator isolation ratio as measured in air prior to installation and in situ in vacuum. The isolation worsens by a factor of 6 upon placement of the FI in vacuum. The linear fits to the data show a constant in-air isolation ratio and an in-vacuum isolation ratio degradation of 0.02 dB/W.
Mode cleaner and Faraday isolator thermal drift data. (a) Angular motion of the beam at the MC waist and FI rotator as the input power is stepped. The beam is double-passed through the Faraday isolator, so it experiences twice the input power. (b) Average beam angle per power level in the MC and FI. Linear fits to the data are also shown. The slopes for MC yaw, MC pitch, FI yaw, and FI pitch, respectively, are 0.0047, 0.44, 1.8, and 3.2 μrad/W.
Profile at high and low powers of a pick-off of the beam transmitted through the MC. The precision of the beam profiler is ±5%. Within the error of the measurement, there are no obvious degradations.
Faraday isolator thermal lensing data. With 25 W into the Faraday isolator (corresponding to 50 W in double pass), the beam has a steeper divergence than a pure TEM00 beam, indicating the presence of higher order modes. Errors are ±5.0% for each data point.
EOM thermal lensing data. The x- and y-direction beam profiles with 160 W through the EOM (closed circles and squares) place a lower limit of 4 m on the induced thermal lens when compared to the beam profiles without the EOM (open circles and squares).
Comparison of selected properties of the Initial and Enhanced LIGO EOM crystals, LiNbO3, and RTP, respectively. RTP was preferred for Enhanced LIGO because of its lower absorption, superior thermal properties, and similar electro-optic properties.19
Enhanced LIGO IO power budget. Errors are ±1%, except for the TFP loss whose error is ±0.1%. The composite MC transmission is the percentage of power after the MC to before the MC and is the product of the MC visibility and transmission. Initial LIGO values, where known, are included in parentheses and have errors of several percent.
Absorption values for the Livingston and Hanford mode cleaner mirrors before (in parentheses) and after drag wiping. The precision is ±10%.
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