Apparatus and method for eliminating polarization sensitivity in transducers

#This invention is concerned with altering signals, such as an optical carrier, according to various other signals and measurands of interest so that the signal or measurand of interest can be transmitted or measured.

#In many sensing systems, most notably fiber optic ones, a sensing element modifies some characteristic of input optical signal by changing some parameter of the optical system. Sensing elements can either be constructed of fiber (intrinsic), or can be some external fiber-coupled transduction element (extrinsic). The performance of most sensing elements (e.g. transducers, modulators, etc.) depends on an appropriate state of polarization of the input light. The signal carried by the light from the sensor can be degraded or completely eliminated by an inappropriate input state of polarization, this fading being commonly known as polarization induced signal fading. Signal fading is encountered when a conventional low-birefringence single mode optical fiber is used to deliver the optical input from a source to a sensor. The state of polarization at the input to the sensor is unpredictably varying due to environmental perturbations, and because of randomly distributed birefringence along the fiber.

#Several schemes have been devised to overcome the effects of polarization fading, including automatic polarization tracking, polarization diversity detection schemes based on output state of polarization selection, and use of polarization maintaining fiber. However, except for use of expensive polarization maintaining fiber, these techniques all rely on active control.

#FIG. 1 is a schematic diagram of one embodiment of the invention.

#FIG. 2 is a schematic diagram of another embodiment of the invention.

#FIG. 3 is a schematic diagram of an embodiment of the invention using a microwave transducer.

#Referring to the drawing figures, wherein like references correspond to like structure throughout the several views, FIG. 1 shows light from a polarized or unpolarized source 12 launched into a standard single mode optical fiber 10. The light traveling in fiber 10 toward module 13 assumes some state of polarization which is not known a priori. The light is preferably collimated, for example by GRIN lens 14, and split by polarization beam splitter 16; the horizontal state of polarization is directed to port 26 of beamsplitter 18, and the vertical state of polarization is directed through to the left of the figure to port 24. Faraday rotators 16, 20 rotate the states of polarization 45.degree. clockwise (as observed looking in the direction of travel), and launch the signals in opposite directions on a principal axis of polarization maintaining fiber 22. Fiber 22 guides the light through any polarization sensitive transducer (modulator, detector, etc.) that might be present in fiber 22. The light in fiber 22 is "on axis" independent of the state of polarization that initially arrives at the beam splitter 14 because, by rotating the polarization, both signals exiting ports 24, 26 are rotated by 45.degree., and thus both signals are linearly polarized along the same major axis of polarization maintaining fiber 22 as they counterpropagate between ports 24, 26. This state of polarization will typically be a preferred one for the transducer 28.

#Each of the respective outputs signals 24, 26 counterpropagates around 22 are rotated 45.degree. counterclockwise, and thus return to beam splitter 16 in the same state of polarization that it initially arrived. Therefore, all of the light exits beamsplitter 16 via fiber 20, and travels to detector 30, where the influence of transducer 28 can be detected. That is, any horizontal component initially arriving at beamsplitter 16 is reflected down the page into 26, 18, 22(a), travels clockwise around the loop, arrives at beamsplitter 16 in the horizontal state of polarization, and is reflected up the page into fiber 20. Likewise, any vertical component initially arriving at beamsplitter 16 is transmitted through into loop 24, 20, 22(b), travels counterclockwise around the loop, arrives at beamsplitter 16 in the vertical state of polarization, and is transmitted up into fiber 20. Thus, a polarization insensitive modulator module 13 is formed by using a polarization sensitive modulator, two Faraday rotators, a polarization beamsplitter, optional collimating lenses, and a short length of polarization maintaining fiber. Additionally, the signal returning from sensing element 28 is not co-propagating with (Rayleigh) backscattered light from source 12. Thus the signal to noise ratio of the apparatus of FIG. 1 is not diminished.

#If, however, one wishes to replace the double lengths of fibers 10, 20 with a single fiber, the embodiment of FIG. 2 permits this. Light from polarized source 12 is launched into a short length of polarization maintaining fiber 34. Fiber 34 is oriented so that the source light is transmitted through polarization beam splitter 37 so as to be launched into single mode fiber 11. In practice, fiber 11 may be very long (e.g. 10 km or more). The light traveling toward beamsplitter 16 converts into one state of polarization which is not known a priori. The light is split by polarization beam splitter 16 in the same manner as beamsplitter 16 of FIG. 1, the horizontal component being directed down to 19, and the vertical state of polarization being directed to the left of the figure. Faraday rotator 19 rotates the horizontal component by 90.degree., and launches it on a principal axis of portion 22a of fiber 22. Fiber 22 guides the light through any polarization sensitive transducer, etc., 28 which may be present. The light in fiber 22 is "on axis" regardless of the state of polarization that initially arrives at beamsplitter 16: the components launched into fiber portions 22a, b from output ports 24, 26 initially have polarizations 90.degree. apart; rotator 19 puts both components on the same axis in fiber 22. After exiting transducer 28, the originally vertical state of polarization component is rotated 90.degree. by rotator 19, and returns to beamsplitter 16 in the horizontal state of polarization. However, after exiting transducer 28, the originally horizontal state of polarization is on the same axis as the vertical component and so returns to beamsplitter 16 in the vertical state of polarization. Light traveling in both directions around the loop are rotated 90.degree. , and so returns to beamsplitter 16 in the opposite state of polarization from that in which it initially arrived. Therefore, all of the light returning to beamsplitter 16 is in the opposite state of polarization. It is readily seen that the light returning to fiber 11 has a state of polarization orthoconjugate to that which originally entered. That is at any point in fiber 11 the returning light signal is in the orthogonal state of polarization at every point in fiber 11, including at the point of return to beamsplitter 37--similar to a Faraday rotator and mirror combination. Hence, the light arriving back at beamsplitter 37 is reflected down to photodetector 32. Thus, a polarization insensitive remote sensing module 13' is formed by using a polarization sensitive modulator, a Faraday rotator, two polarization beam splitters with optional collimating lenses, and short lengths of polarization maintaining fiber.