The performance of the tapered velocity coupler for single mode fibers is investigated theoretically and experimentally. An improved analytical solution convenient for analyzing and designing the tapered velocity couplers is derived. The mode cross-talk in the tapered region and the mode conversion loss due to the discontinuity at the ends of the fibers are measured and are compared with the theoretical results. Simultaneous shaping of the multiple single mode fibers into tapered structure is successfully realized and a low loss splicing (less than 0.2 dB) is achieved either by using adhesives or by the thermal fusion technique.

The standard quantum limit (SQL) for optical communication is imposed by the quantum noise of a laser light (coherent state) and by the quantum noise of a photon absorbing detector (vacuum field fluctuation). In this paper, we will review the origins of the SQL for optical communications and emerging new technologies for surpassing the SQL. An amplitude squeezed state of light has smaller photon counting noise than a coherent state of light. The squeezed amplitude noise of more than 10 dB below the shot noise level was observed in a semiconductor laser. A quantum nondemolition (QND) measurement makes it possible to read signal information without absorbing signal photons, i.e. without destroying signal information. Experimental efforts to realize the QND measurement of photon number are proceeding in several laboratories.