EXPERIMENTAL LINEWIDTH-INSENSITIVE COHERENT ANALOG OPTICAL LINK

We constructed an experimental linewidth-insensitive coherent analog o ptical link. The transmitter utilizes an external electro-optic amplit ude modulator and a semiconductor laser. The receiver consists of a he terodyne front-end, a wideband filter, square law detector and narrowb and lowpass filter. We performed experimental measurements and theoret ical analyses of the spurious-free dynamic range (SFDR), link gain and noise figure for both the coherent AM and the direct detection links; we investigated the dependencies of the foregoing parameters on the r eceived optical signal power, laser linewidth, IF bandwidth, and the l aser relative intensity noise (RIN). By selecting a wide enough bandpa ss filter, we made the coherent AM link insensitive to laser Linewidth . The coherent AM link exhibits a higher SFDR than the corresponding d irect detection link when the received optical signal power is less th an 85 mu W. The noise figure for the coherent link is greater than tha t for the direct detection link under all conditions investigated. For received optical signal powers greater than 4 mu W, the link gain for the direct detection link is greater than that for the coherent AM li nk. The following are the link parameters that have been achieved for the coherent AM link investigated: SFDR = 88 dB.Hz(2/3), link gain -25 dB and noise figure 78 dB; this performance has been obtained with a received optical signal power of 85 mu W, and a local oscillator power at the photodetector of 228 mu W. The link performance can be further improved by auxiliary subsystems such as a balanced receiver and impe dance matched transmitter and receiver ends; and/or by using better op tical and electrical devices like higher power lasers, linearized opti cal modulators, low-noise and high gain RF amplifiers, and optical amp lifiers.