Coherent couplers

S. M. Jensen, The nonlinear coherent coupler, IEEE Journal Quantum Electronics, QE-18 1579-1583 (1982)... 

A strict derivation of the comb properties is not feasible as it depends on the special dispersion characteristics of the laser cavity and these data are not accessible with the desired degree of accuracy. Instead we only assume that the laser emits a stable coherent pulse train without any detailed consideration of how this is possible. Further we assume that the electric field E(t), measured for example at the output coupler, can be written as the product of a periodic envelope function A ) and a carrier wave C(t) ... 

So we see that spectral broadening of the comb [29,30] is achieved by imposing a large frequency chirp on each of the pulses. Provided that the coupling efficiency into the fiber is stable, the periodicity of the pulse train is maintained. The discussion of section 3 is thus equally valid if the electric field E(t) as measured for example at the fiber output facet instead of the laser output coupler. As described below we have used a frequency comb widened to more than 45 THz by a conventional single mode fiber to perform the first phase coherent vacuum UV to radio frequency comparison in our Garching laboratory [16,31], In recent experiments we have confirmed that the fiber does not affect the mode spacing constancy within our experimental uncertainty of a few parts in 1018 [13]. 

In optical sensing system, it is necessary to employ optical couplers and lenses in order to focus the light beam to the optical fiber and direct radiation from the return fiber to the photodetector. Injecting light from a source into a fiber is a more tedious operation. Laser sources produce a coherent beam of light with a cross-section almost equal to the cross-sectional area of the optical fiber, and, therefore, can be coupled very efficiently to the fiber. On the... 

Figure 16. Principle of OCT imaging showing the interference signal from a laser source and a short coherence-length (broad band) source as the reference mirror is scanned (Left) and fiber-optic implementation (Right) in which a 3dB fiber coupler serves the same purpose as the 50 50 beam splitter in...

---