Single mode fiber optic single sideband modulator and method of frequency shifting using same

#The invention relates to the field of fiber optic sensors and modulators. More particularly, the invention relates to the field of frequency shifters for fiber optic systems.

#It has long been known that light from a moving source will be shifted in frequency because of the Doppler effect. The change in frequency or wavelength is a function of the relative velocity of the source and observer. The wavelength will become shorter and the frequency high when the source is moving toward the observer, and vice versa.

#It has also been known that the wavelength of light will be changed upon reflection from a moving mirror. The moving mirror adds to the energy content of the impinging photon thereby increasing the frequency of the light. This phenomenon is discussed by Meyer-Arendt in "Introduction to Classical and Modern Optics", Prentice Hall (1972) at pages 539-540.

#FIG. 1 is an exploded, perspective view of a narrowband directional coupler.

#FIG. 2 is a cross section along the section line 2--2 in FIG. 1 showing the shape of the ridges.

#FIGS. 3(a)-3(g) show a diagram of the stressed regions in the fiber and the amount of power in the various polarization modes at various points along the fiber.

#FIG. 1 shows the apparatus of a narrowband polarization coupler in which a polished, flat surface 10 is machined on a metal or plastic block 11. The surface 10 should be smooth and flat to within a few microns. The surface 10 serves as the first of two surfaces between which an optical fiber waveguide is squeezed.

#The second surface is a multiple ridge region 12 machined on the under surface of a second block 14. As discussed hereinafter, the ridges 12 provide coupling elements which, when pressed against the fiber, stress the fiber to cause light to be coupled between the modes. Referring momentarily to FIG. 2 there is shown a cross section of the ridged region 12 in which a plurality of ridges 16 are formed. The ridges 16 are formed by machining the block 14 to provide spaced, parallel notches or grooves 17, such that there is formed a plurality of polished ridge surfaces 18 having a width W and a separation S between the edges of adjacent ridges. To obtain maximum coupling for a given force exerted on the fiber, the width W between the edges of each ridge should be one half the beat length of the fiber for light at the particular frequency which is utilized.

#Beat length, L, is related to the fact that, in optical fibers, light travels at different velocities in different polarization modes, yielding an optical path length difference between the modes. This causes the phase of light in one mode to shift continuously relative to the phase of light travelling in the other mode. Thus, two components of a light wave which start in phase but which travel in different polarization modes in monomode birefringent fiber will slowly separate in phase. The "beat length" of the fiber is the distance required for the light component in one mode to separate in phase relative to the light component in the other mode by 360 degrees, at which time the two components will again be in phase. Thus, one beat length will have been travelled when the two components have shifted in phase by 360.degree.. After travelling one half of a beat length, the two components will be separated by 180 degrees or .pi./2 radians.