The Fiber-Optic Link

Figure 21.5: Schematic block diagram of the Fiber Optic Link. See the text for more information on each block.

Figure 21.5 shows the schematic block diagram and the nominal powers at different stages of the the fiber-optic return link^21.14. The link consists of a laser diode (which converts the input electrical signal into an optical signal), the optical fiber itself, a photo diode (which converts the optical signal back into an electrical signal) followed by an amplifier and a 5 way divider which separates out the monitoring data as well as the two polarizations of the astronomical signal.

The Fiber optic link is designed to provide a net gain of 0 dB from the input (P9 in Figure 21.4) to the output (P14 in Figure 21.5) which is also the input to the baseband system discussed in Chapter 23. The link is meant to have 0 dB gain irrespective of the length of the fiber optic cable linking the antenna to the CEB. The attenuator (ATT3 in the Figure 21.5) can be varied in accordance with link optical loss to provide this no loss/gain configuration. The level diagram shows the attenuator settings for 0, 5, 10 and 11 dB of optical loss (L$_{\rm opt}$).

The fiber-optic receiver also contains 32 MHz SAW filters centered at 130 and 175 MHz to separate out the 130 and the 175 MHz IF signals for routing to the base band converter subsystem. The level of the signal at this point (P15) is nominally $-49$ dBm^21.15

Figure 21.6: Values or expressions for the gain/loss at in the various stages of the fiber optic link. See also Figure 21.7.

Figure 21.7: Equivalent system noise temperatures at various stages of the GMRT fiber optic link. The link has been designed to have a net gain of $0$ dB and to increase the system temperature by less than 1%.

An ideal communications link would transfer signals unaltered from the input to the output. Any real link however introduces both additional noise as well as distortions into the signal it transports. In the GMRT fiber-optic link, these non idealities include the laser intensity noise, shot noise and thermal noise of the laser diode, loss and reflections in the optical fiber, as well as shot noise and thermal noise in the photo-diode. Figure 21.6 gives expressions for these various noise terms, and Figure 21.7 and Table 21.7 give the expected values for the various noise terms for the GMRT fiber optical link. The largest loss is for the most distant antennas, and turns out to be $\sim 11$ dB. From Table 21.2 (or Fig 21.8) the corresponding equivalent input noise (EIN) is $\sim -41$ dBm. The nominal input power level (P9) of $-20$ dBm would hence give a signal to noise ratio of $\sim 20$ dB, i.e. $100$. In this case, the system temperature is degraded by 1% due to noise added by the link.

Figure 21.8: Equivalent input noise as a function of optical loss for the GMRT fiber optic link. The maximum optical loss (which occurs for the most distant antennas) is $\sim 11$ dB.

Table 21.2: Equivalent system noise temperatures as a function of link loss at various stages of the GMRT fiber optic link. See also Figure 21.7.

Footnotes

... link^21.14

As discussed in Chapter 22 each antenna has two fibers connecting it to the CEB. One fiber is used to send control signals to the antenna, and is referred to as the forward link, while the other fiber is used to bring back the astronomical signal and monitoring data to the CEB and is called the return link.

... dBm^21.15

The fiber-optic receiver has a monitor point at the front panel in order to allow measurements of the IF signals and other carriers using a Spectrum Analyzer.