AdV INJ System prepares the laser beam for the injection in the interferometer.

To accomplish this goal, it needs to monitor with high accuracy the power and the position of the laser beam at various points before it is made available to the interferometer. This goal is accomplished using PhotoDiode (PD) and Quadrant PhotoDiode (QPD) Sensors respectively, and their associated electronics.

PDs

There are two versions of the photodiode electronics that INJ will use:

1. DCPD

this low-noise electronics is usually used for beam power monitoring purpose. Its bandwidth starts from DC and goes up wherever the sensor used allows it to go

2.WBPD

The main goal this units aims to achieve is to have a bandwidth sufficient to respond at the frequencies used to phase-modulate the beam (ideally, up to 131 MHz)

QPDs

There are three versions of this kind of electronics for the INJ system.

In all cases, the processing part that allows to calculate the asymmetries (i.e., the actual position of the beam) is computed in separate units installed away from the optical benches. The board which does the math is known as QPD Algebra Board

1. DCQPD

Main goal of this sensors and their associated electronics is to monitor the beam position at low frequency. High accuracy and negligible offset are among its requirements

2. RFQPD

These units are demodulated quadrant electronics. Their purpose is to extract the information contained at the modulation frequency. As a bonus, thy make available also the DC power

3. WBQPD

These are WideBand sensors and, as the name suggests, they cover the bandwidth going from the DC all the way up to ten of MHz