Introduction

The Giant Metrewave Radio Telescope (GMRT) consists of a distributed array of antennas all connected to a Central Electronics Building (CEB) via optical fiber links. Optical fibers are superior to the more traditional co-axial cables or waveguides in a variety of respects. Optical fibers have lower transmission losses, higher bandwidth and have better isolation against radio frequency interference. More quantitatively, while the loss in co-axial cables are several 10 s of dB/km, the loss in optical fibers is as low as 0.2 dB/km. Further 100 GHz-km bandwidths are routinely achievable in single mode optical fibers, while the achievable bandwidth for co-axial cables is only $\sim 20$ MHz-km.

The optical fiber link between the CEB and a given antenna has two major functions:

1. Transmission of local oscillator (LO) as well as control signals from the CEB to the antenna, and
2. Transmission of the astronomical signal as well as monitoring data from the antenna to the CEB.

Figure 22.1: Schematic of the optical link at the GMRT. Each antenna is connected to the central electronics building by two fibers, one for the forward link, and the other for the return link.

As shown in Figure 22.1 there are two fibers between each antenna and the CEB, one of which forms part of the forward link and carries the control and LO signals to the antenna, and the other of which forms part of the return link and carries the astronomical signal (at the IF frequency, see Chapters 21, 23) and the monitoring data (also referred to as telemetry data) and the return LO^22.1 back to the CEB. Each link consists an optical transmitter (a laser diode) the fiber itself (which is a single mode glass fiber), and a receiver (a photo diode). A block diagram of the GMRT optical Link is show in Figure 22.2 and the frequencies of the different signals that are transported by the link are also indicated. We now discuss the various elements of the GMRT optical link in some more detail.

Figure 22.2: Configuration of the GMRT optical communications link. The upper panel shows the forward link that takes control signal and LO signals from the CEB to the antenna. The lower panel shows the return link that brings the astronomical signal (at the IF frequency) as well as the telemetry and return LO signals from the antenna to the CEB. The frequencies of the various signals transported by the link are also indicated.

Footnotes

... LO^22.1

The return LO is useful in measuring the phase stability of the system as well as in correction for the phase introduced during the LO transmission process.