aLIGO EOM test: Setup

• The modulation signal was supplied from an aux Marconi.
• Between the Marconi and the EOM, a 20dB coupler (ZFDC-20-5) was inserted. There the Marconi was connected to the output port, while the EOM was to the input port. This way, we can observe how much of the RF power is reflected back to Marconi.
• The beat setup (40m ELOG 13567) was used for the measurement. The EOM was placed in the beam path of the beat setup in the PSL side.
• To eliminate the modulation sidebands of 11MHz and 55MHz, the 40m Marconi and the freq generator were turned off (in this order).
• The nominal amplitude of the carrier beat note was -15dBm ~ -16dBm.
• The cable from the source to the EOM was ~3m. And the loss of this cable was ~0.4dB.

Measurement

• The EOM had three input ports. 

1. 9MHz input - In reality, there was no matching circuit.
2. Center port - matched at 24.1MHz and 118.3MHz. 24.1MHz port has no amplification (just matching), and 118.3MHz is resonant.
3. 45.5MHz port - resonantly matched at 45.5MHz

• The Marconi output power was set to be +13dBm. For the 45MHz measurement, 20dB attenuator is inserted right next to the Marconi so that the VSWR seen from the Marconi was improbed. (Marconi did not like the full reflection of unmatched circuit and shutdown due to the protection function.)
• The amplitude ratios between the sidebands and the carrier were multiplied by a factor of 2, to obtain the modulaiton depths. ( BesselJ(1,m)/BesselJ(0,m) ~ m/2 )
   
• The result is found in Attachment 2.
  • The center port showed the modulation response of 0.7mrad/V and 15mrad/V for 24.1MHz and 118.3MHz, respectively. This suggests that the amplification factor for 118.3MHz is ~x21.
  • The VSWR of the center port is below 1.5 at the target frequencies. That's as tuned in Downs and has not been changed by the crystal replace.
  • The 45MHz port has the modulation response of 0.034mrad/V. This later tuned out that the amplification of ~x19. The circuit is well matched at the resonant frequency.
     
• The linearity was checked with the 45MHz port (Attachment 3). The input power (idrectly connected to the EOM without 20dB attn) was varied between -17dBm to +13dBm. There was no sign of non linearity.
   
• The modulation response at 24MHz was compared at various input ports. (Attachment 4)
  • The input signal was amplified tobe 23dBm by ZHL-3A for better sideband visibility. The actual amplifier output was ~30dBm, and a 6dB ATTN was used to improve the VSWR to protect the amplifier.
  • The 9MHz port showed 3.6mrad/V and 1.8mrad/V with the port unterminated and terminated, respectively. This factor of two difference is as expected.
    This 1.8mrad/V is roughly x2.6 higher compared to the one of the matched 24/118MHz port. This is close number to the ratio of the plate sizes (14mm/5mm = 2.8).
  • With the current condition, the 9MHz (unterminated), 9MHz (terminated), 24/118MHz, and 45MHz ports requires 22dBm, 27dBm, 36dBm, and 21dBm to realize the current modulation depth of 0.014 at 24MHz.
  • Comparing this matched 9MHz performance, the amplification of the 45MHz port at 45MHz was determined to be ~x19.
     
• Considering these results, the modulation response of the center port at 24MHz seems too low. We don't want to supply 36dBm for the 0.014rad modulation (nominal number for H1).
  Here are some thoughts:
  • Use the 45MHz or 9MHz port for 24MHz modulation. Probably the unit is unmatched but, we can come up with the idea to improve the VSWR at 24MHz somehow?
  • Redistribute the plate length to have better modulation at 24MHz. Can we achieve sufficient modulation capability with the frequency of the long and short ports swapped? We hope that we don't need to start over the matching of the 24/118MHz again because the capacitances of the ports are almost the same.