The ISS transfer function requirement is not complete without giving the plant transfer function, i.e., the conversion factors that take volts to watts at the EAOM, watts to volts at the PD, and everything in between.
The attachment shows the physical topology of the CTN ISS. The EAOM is a New Focus 4104, and the PD is a ThorLabs PDA10CS.
Looking at the EAOM manual, small-signal power modulation *δW* in response to a voltage *δV* is
with *V*_{π} no more than 300 V. From talking to Tara, it sounds like the input power *W* can be somewhere between 2 mW and 10 mW, but the power after the EAOM is going to be attenuated to 1 mW. So *W* is effectively 1 mW.
Also from talking to Tara, with 1 mW at the input he expects to get something like 0.6 mW out of the transmission. Half of this will go to the beat breadboard, and half to the ISS breadboard, so that's 0.3 mW incident on the ISS PD (so we have an optical throughput *a* = 0.3). The quantum efficiency *η* of the diode is something like 0.6 A/W. The PDA10CS has an internal preamp with different gain settings; the one to use here is probably *g* = 1.5 × 10^{4} V/A, since then we get something like 3 V dc coming out of the PD.
With these quantities, the plant transfer function (from volts at the EAOM to volts at the PD) is
which, with the above numerical values, is *P* =0.014 V/V, independent of frequency. So there's an attenuation of 70 or so that needs to be compensated for in the electronic part of the loop. **But before anyone solders in the relevant resistors and capacitors, the plant transfer function should actually be measured.** |