A New RF System For the Upgrade

The Upgraded 40 Meter will have an entirely new configuration of the RF system. That is mostly because of two fundamental reasons:

1. the length of the short cavities will change (getting longer);

2. the interferometer will be set in a broadband sensing configuration and a new locking scheme and signals will be necessary.

Main Features of the new RF Scheme

The change of the length of the recycling cavities will impose different sideband frequencies. The frequency f1 of the first sideband will be now about 11 MHz, ad the second one will be 5xf1=55 MHz.

Sideband generation

The modulations will be generated by a single broadband EOM, thus ending the current configuration with the two EOMs in parallel in the Mach-Zehnder.

Also there will be a single main oscillator to generate the main 11 MHz frequency. All the other signals, with frequencies multiple of the main one, 2x, 3x, 5x, 10x, 15x respectively, will be generated starting from it, rather than by independent oscillators.

f1 will be generated by a crystal oscillator; f2 will be obtained by f1 by means of a 5-time frequency multiplier. The two signals will then be combined into one which will modulate the EOM driver.

The designed modulation depth for the Upgrade will range somewhere in between 0.1 and 0.3. The New Focus KTP 4064 boradband EOM has an efficiency factor of 13mrad/V. That means that the driving voltage should range between 8 and 23 V, that is between 30 dBm and 40 dBm. The frequency multiplier will output 20 dBm, so 20 dBm will have to be gained by the signals before the EOM in order to match the requirements on the modulation depth. The step will be covered in part by the EOM driver in part by an low noise, high power amplifier placed in between.

The requirements on this amplifier depend on the gain achieved by the EOM driver, which is still unknown for sure at the moment. Although we can expect two possible scenarios:

1. (very) pessimistic: the driver won't be able to provide a positive gain
2. realistic: the driver will give us about 10 dB
3. optimistic: the driver will provide 20 dB as in theory

In either case the choice of the amplifier will be different. for instance, the following could be two possible candidates:

1. +20 dB gain by [http://www.ophirrf.com/files/products/5303055.pdf Ophir Solid State Broadband High Power RF Amplifier 5303055]

2. +10 dB gain: this will be hard, because we will need something that makes the step from +20 dBm to +30 dBm (gain of +10 dB); unless we ask for less +20 dBm at the output of the multiplier
3. no amplifier will be needed (?)

After the multiplier and before the driver, the f5 signal will be have to be amplified

RF photodiodes demodulation

The plan under study also uses the same signal from the main oscillator for the demodulation of the signals from the RF photodiodes.

A diagram representing the configuration of the new RF system is shown [https://nodus.ligo.caltech.edu:30889/UpgradeRFplan/yEd_RFplan.html here].

A diagram representing the current configuration of the 40m RF System is shown [https://nodus.ligo.caltech.edu:30889/Old40mRFsystem/yEd_RFsystem_Old40m.html here].

Tables and diagrams

[attachment:40mUpgrade_PDlists.pdf Table of old and new PDs plus diagram of signal extraction scheme]

[attachment:40mPDs.pdf Table of PDs and Demodulators]