INTERFEROMETER UTILIZING SUPERFLUORESCENT OPTICAL SOURCE

Abstract: An improved broadband light source for a Sagnac interferometer includes a waveguide (110), such as a fluorescent optical fiber, that is pumped by a pump source (120) with a sufficient intensity to generate temporally incoherent light. The fluorescent optical fiber (110) has first and second ends, one end being an input end of the fiber (110). The broadband light is provided at an output of the fluorescent optical fiber (110) and is input to the interferometer. In order to prevent laser oscillations between the light source and the interferometer, one end of the fluorescent optical fiber (110) is formed so as to prevent reflections. The light output from the fluorescent fiber (110) to the interferometer comprises only that light that initially propagates toward the output of the optical fiber (110). In one embodiment of the light source the pump light from the pump source (120) is coupled into the fluorescent optical fiber (110) in a direction so that it travels away from the output of the fluorescent optical fiber (110) towards that first end. In an alternative embodiment, the fluorescent optical fiber (110) is a doubly clad optical fiber having a first acceptance region and a second acceptance region at the first end. The pump light is introduced into an aperture window between the two acceptance regions so that the pump light can be introduced into the first end at an angle without requiring a reflective surface at the first end.

Description: INTERFEROMETER UTILIZING SUPERFLUORESCENT OPTICAL SOURCE Background of the Invention Field of the Invention The present invention is in the field of optical interferometers and components, and, in particular, is in the field of broadband light sources for interferometers, rotatio sensors, and the like. Description of the Related Art A Sagnac interferometer comprises an optical loop, typically of optical fiber, that is used to sense rotation o an object onto which the loop is mounted. Briefly, suc interferometers operate by dividing the optical energy from light source into two substantially equal beams of light an causing the two beams of light to propagate around the loop i opposite directions. The two beams of light are combine after passing through the loop and the changes in intensity o the combined light caused by interference of the two beams i detected. In accordance with the well-known Sagnac effect rotation of the object and thus of the loop of fiber cause changes in the relative phase between light propagating in th two directions which in turn causes the detected intensity t change. The rotation rate of the loop can be determined fro the detected changes in the intensity. See, for example, U.S Patent Nos. 4,410,275; 4,529,312; 4,637,722; 4,671,658 4,687,330 and 4,836,676. With respect to Sagnac interferometers, it has been foun to be advantageous to use a broadband light source to provid the light introduced into the loop of optical fiber. See, fo example, U.S. Patent No. 4,637,025 wherein a super radiant light source iε described. The light source in U.S. Patent No. 4,637,025 operates by introducing a pump signal into a single-mode optical fiber having a core doped with an active fluorescent material such as neodymium or other rare earths. The pump light has a sufficient intensity to cause amplification of spontaneous emission of photons by the fluorescent material. In one embodiment (Figure 1), pump light is input into the optical fiber via a lens. In the second of the two embodiments, the pump light is introduced via a dichroic lens that is transparent to the pump light and highly reflective of emitted light. The pump light is absorbed by the fluorescent material and excites the electrons therein to higher energy states resulting in the emission light when the electrons transition to lower states. Because of the random manner in which the spontaneous emissions occur, the amplified emitted light is effectively spontaneous fluorescence and temporally incoherent. The two embodiments in U.S. Patent No.4,637,025 generate light that is emitted in all directions in the core of the fiber. A portion of the light generated in the core propagates directly out the output end of the fiber. A second portion of the light propagates toward the input end of the fiber. In the first embodiment, the light reaches the input end of the fiber. Although the input end does not have a reflector, as in the second embodiment, surfaces in the optical path cause a portion of the generated light to be reflected back into the fiber. In the second embodiment, the dichroic reflector is included to specifically reflect the generated light that propagates toward the input end portion back toward the output end portion. No reflector is provided at the output end of the fibe in U.S. Patent No. 4,637,025 so that laser oscillations ar purportedly prevented. See, for example. Column 5, lines 1-5, of the patent. Although there is no intent to support lase oscillations in such a light source, it has been found tha when such light sources are used in combination with Sagna interferometers, for example, the optical fiber loop of th interferometer acts like a mirror. That is, the ligh entering the fiber loop propagates around the loop and exit propagating in the opposite direction to the entry directio (i.e., back towards the light source). A portion of th exiting light re-enters the fluorescent optical fiber. Th dichroic reflector (Figure 2) at the input end of the optica fiber (Figure 1) reflects this return light. Thus, it can b seen that an unintentional laser oscillation can occur becaus of the interaction of the reflection at the input end of th fluorescent fiber and the "reflection" caused by the optica fiber loop. The laser oscillations are unacceptable for man applications. Summary of the Present Invention The present invention comprises an interferometer havin a light source and an optical loop. The light sourc comprises an optical waveguide formed of a material whi emits a broadband optical signal in response to pumping wi pump radiation. The light source introduces the emitt optical signal to the optical loop along a connecting optic path extending between the optical loop and the light sourc The loop returns at least a portion of light comprising t emitted optical signal back to the light source along t connecting optical path to provide a returning broadba optical signal. The light source is non-reflective for t returning optical signal, to prevent the returning optic signal from being reflected back to the loop. By preventi such reflection, the interferometer avoids resonance betwe the source and the loop, and prevents substantial narrowing the spectral line width. In a preferred embodiment, the light source additional comprises a source of pump light coupled to optically pump t light-emitting material such that the pump light propagates in the waveguide in a direction opposite to that of the emitted optical signal. The waveguide comprises an optical fiber having a core surrounded by a cladding, and the pump light source couples the pump light into the cladding. The optical fiber has a first numerical aperture corresponding to a first acceptance region, and the cladding has a second numerical aperture corresponding to a second acceptance region. The acceptance regions define an aperture window therebetween, and the pump source introduces the pump light into the aperture window at one end of the fiber. In one embodiment, the core of the optical fiber is circular and single mode, while the cladding is rectangular and multi-mode. The fiber is doped with neodymium or other light-emitting material. In accordance with a further aspect of the invention, the returning optical signal propagates through the waveguide and exits the waveguide at an end thereof. A photodetector is positioned at such end to detect the returning optical signal, and is arranged to prevent light incident thereon from being reflected back into the loop. In the preferred embodiment, the optical loop comprises an optical waveguide having two polarization modes, and the emitted optical signal has a coherence time which is significantly less than the propagation time difference between the modes after traversing the length of the loop. The invention also encompasses a method of operating an interferometer having a light source and an optical sensing loop. In accordance with this method, pump radiation is input to an optical waveguide to pump the optical waveguide to emit broadband light. Emitted light from the light source propagates towards the optical sensing loop, and light from the optical sensing loop is returned towards the light source without returning to the loop to prevent optical resonance of the emitted light in the interferometer. Preferably, the emitted light is emitted from a first end of the waveguide and the pump radiation is coupled to the waveguide in the form of optical energy at a location between the sensing loop and a second end of the waveguide. A further preferred aspect of the method involves passing the emitted light propagating from the loop to the source through the waveguide to provide amplified light and detecting the amplified light. Brief Description of the Drawings Figure 1 is a pictorial representation of a conventional broadband light source. Figure 2 is a pictorial representation of a conventional Sagnac interferometer. Figure 3 is a pictorial representation of a conventional Sagnac interferometer and a broadband light source in accordance with one aspect of the present invention. Figure 4 is a pictorial representation of a conventional Sagnac interferometer and a broadband light source in accordance with a second aspect of the present invention. Figure 5 is a pictorial representation of a conventional Sagnac interferometer and a broadband light source in accordance with an additional aspect of the present invention in which the broadband light source comprises a double-clad neodymium-doped optical fiber. Figure 6 is a cross-sectional view of the double-clad neodymium-doped optical fiber taken along the lines 6-6 in Figure 5. Figure 7 is cross-sectional representation of the juxtaposition of the double-clad neodymium-doped optical fibe with the multimode optical fiber of Figure 5. Figure 8 is a pictorial representation of a conventiona Sagnac interferometer and the broadband light source i accordance with the present invention, illustrating a alternative placement of the pump source with respect to the double-clad neodymium doped optical fiber. Figure 9 is a pictorial representation of a conventional Sagnac interferometer and the broadband light source in accordance with the present invention, illustrating a further alternative placement of the pump source with respect to the double-clad neodymium doped optical fiber. Figure 10 is a pictorial representation of a conventional Sagnac interferometer in combination with a further embodiment of a broadband light source in accordance with the present invention, wherein the pump light is coupled to a doped optical fiber via a dichroic mirror. Figure 11 is a pictorial representation of the Sagnac interferometer and the broadband light source of Figure 10, wherein the photodetector is positioned to receive amplified light from the doped optical fiber. Figure 12 is a pictorial representation of the Sagnac interferometer and the broadband light source of Figure 3, wherein the photodetector is positioned to receive amplified light from the doped optical fiber. Figure 13 is a pictorial representation of the Sagnac interferometer and the broadband light source of Figure 9, wherein the photodetector is positioned to receive amplified light from the doped optical fiber. Detailed Description of the Preferred Embodiments Figure 1 illustrates an exemplary broadband light source 100. As illustrated, the light source 100 includes a waveguide comprising an optical fiber 110 having a first end 112 and a second end 114. The optical fiber 110 is a fluorescent optical fiber. That is, when the optical fiber 110 is pumped with optical energy within specified ranges of optical wavelengths (referred to as the absorption bands of the optical fiber) , the optical fiber 110 generates output light having a wavelength responsive to the wavelength of the pump optical energy. The optical fiber 110 comprises a core of a host glass that is doped with an active fluorescent material such as neodymium which absorbs light having wavelengths on the order of 0.82 microns, for example. The absorbed photons from the pump optical energy excite the electrons in the active material to higher energy electron energy states, and, when the electrons transition to lower energy levels, photons are emitted at characteristic emissio bands, or fluorescing wavelengths. For example, in the cas of neodymium, the emission bands are 1.06 microns and 1.35 microns. The transitions through the lower energy levels bac to the ground state for spontaneous emission occur in a rando manner to cause the photon emissions caused by the pump ligh to be amplified spontaneous fluorescence, thus causing th emitted output light to be temporally incoherent. The broadband light source 100 further includes a pum light source 120 which may be, for example, a laser diode, o the like, that provides an optical pump signal having wavelength within one of the absorption bands of th fluorescent optical fiber 110, for example, 0.82 microns. Th pump light provided by the pump light source 120 is introduce into the first end 112 of the optical fiber 110 via a len 122, or the like, so that the pump light propagates in th fluorescent material in the core of the optical fiber 110 The intensity of the pump light provided by the pump ligh source 120 is selected to be sufficiently great to cause population inversion of the electrons in the fluorescen material, thereby supporting amplified spontaneous emission o light from the fluorescent material. The length of th fluorescent optical fiber 110 is selected to be sufficientl long so that substantially all the pump optical energy i absorbed by the fluorescent material and little, if any, pum optical energy is emitted from the second end 114 of the optical fiber 110. The emitted light energy has a high radiant intensity relative to the light produced by a so-called super radiant light-emitting diode (LED) . In addition, the emitted light has a wavelength distribution that is broader than the characteristic spectral line output of a laser diode, has a low temporal coherence, and has a principal wavelength that is generally temperature independent. The emitted light is generated in all directions within the fluorescent material in the core of the fluorescent optical fiber 110. The portion of the light initially propagating in the general direction of the second end 114 (referred to herein as the forward propagating light) will be emitted therefrom as a broadband output signal. Figure 2 illustrates a conventional Sagnac interferometer 102 having a light source 123 (typically a superluminescent diode) coupled to an optical fiber 125. Light from the source 123 is introduced into the Sagnac interferometer 102 by, for example, forming the optical fiber 125 into one-half of a first directional coupler 126, which is preferably constructed in accordance with U.S. Patent No. 4,536,058, or the like. The second half of the first directional coupler is formed on an optical fiber 130 which has a first end 132 and a second end 134. ' The first directional coupler 126 is preferabl constructed to couple approximately 50 percent of the light from the fluorescent optical fiber 110 to the optical fibe 130 in the Sagnac interferometer 102. In addition to the coupler 126, the Sagnac interferomete 102 further comprises a second directional coupler 140 tha forms a portion of the optical fiber 130 into a loop 142 between the two ends 132 and 134. The loop 142 operates a the sensing portion of the interferometer 102. The secon directional coupler 140 is preferably constructed in the sam manner as the first directional coupler 126 and is als preferably a 50 percent coupler such that approximately 50 percent of the light coupled to the optical fiber 130 from the light source 123 propagates around the loop 142 in a first direction (clockwise in Figure 2) and approximately 50 percent propagates around the loop 142 in a second opposite direction (counterclockwise in Figure 2) . The light propagating around the loop 142 is recombined by the second directional coupler 140, and the recombined light signal propagates back toward the first directional coupler 126. Approximately 50 percent of the light is provided as an output signal via the first end 132 of the optical fiber 130 with the other 50 percent being coupled back to the light source 123. The output signal from the first end 132 is detected by a detector 150 which provides an electrical output signal on a line 152 that is provided to a processor 154. The processor 154 processes the electrical output signal and provides a calculated output signal Ω on a bus 156 responsive to the direction and rate at which the loop 142 is rotated. The operation of Sagnac interferometers is well-known and will no be discussed in detail herein. One skilled in the art will recognize that additional components are frequently used t improve the operation of such interferometers. For example, the Sagnac interferometer 102 of Figure 2 further includes phase modulator 158 driven by the processor 154 tha introduces a phase modulation into the counterpropagatin light signals within the loop 142 to enable the electrica output signal to be synchronously demodulated. An example o a Sagnac interferometer of this type is disclosed in U.S. Patent Nos. 4,671,658 and 4,881,817. Other components, suc as a polarizer 160 positioned on the optical fiber 130 betwee the first directional coupler 126 and the second directiona coupler 140, are also advantageously used in man applications. One skilled in the art will also recognize tha portions of the Sagnac interferometer 102 can b advantageously constructed using integrated optic components or bulk optic components. The broadband light source 110 of Figure 1 may be substituted for the light source 123 of the interferometer 102 to supply broadband light thereto. This may be accomplished either by coupling the second end 114 of the fiber 110 to the input end of the fiber 125 or by forming the fiber 110 into one-half of the first directional coupler 126. The light generated in the fluorescent optical fiber 110 that propagates toward the first end 112 (referred to herein as the backward propagating light) will be generally emitted from the first end 112 toward the pump source 120. However, since the first end 112 will generally be either a smooth flat end or a smooth spherical end (as described in U.S. Patent No.4,637,025), the first end 112 will act as a partial reflector of the backward propagating light and will cause the backward propagating light to be reflected back toward the second end 114 of the optical fiber 110. Further, the lens and pump source have surfaces which reflect light. The reflected light will also be introduced into Sagnac interferometer 102. In U.S. Patent No. 4,637,025, it was considered desirable to reflect the backward propagating light. For example, in Figure 2 of that patent, an embodiment is disclosed in which a dichroic reflector is formed on the first end 112 so that substantially all the backward propagating emitted light is reflected toward the second end 114. Applicants have discovered that the reflection of the backward propagating light is undesirable in man applications. In particular, the loop 142 provides the same effect as a mirror, and up to 50 percent of the light introduced into the Sagnac interferometer 102 and propagatin around the loop 142 can be coupled back to the light source. If the light source of Figure 1 is used in the interferomete of Figure 2, the source will reflect a portion of this ligh so that it again propagates back towards the interferomete loop. It can be seen that the source of Figure 1 and the loo of Figure 2 act as the two mirrors of a resonant cavity Thus, resonant lasing can occur to cause the generation o undesirable temporally coherent laser light. Figure 3 illustrates a first embodiment of a broadban light source which prevents the resonant lasing from occurrin by eliminating the reflection of light at one end of fluorescent optical fiber. This embodiment uses the sam basic configuration as the Sagnac interferometer of Figure 2 and includes the optical fiber 130 having its first end 13 and its second end 134. The basic operation of the Sagna interferometer of Figure 3 iε substantially aε describe above. As shown in Figure 3, a light source 200 comprises fluorescent optical fiber 210 which advantageously has neodymium-doped core such as described above, or other ra earth-doped core such as erbium. The fluorescent optic fiber has a first end 212 and a second end 214. The lig source 200 further includes the pump source 120 which coupled to the second 214 of the fluorescent optical fib using the lens 122, for example. Thus, the pump lig introduced into the fluorescent optical fiber 210 propagat from the second end 214 back towards the first end 212. T Sagnac interferometer is coupled to the light source 200 v a coupler 220 which is formed onto the fluorescent optic fiber 210 proximate to its second end 214 and iε formed on the optical fiber 130 proximate to its first end 132. Unli the coupler 126 of Figure 2, the coupler 220 of Figure 3 iε multiplexing coupler. As described, for example, in U. Patent No. 4,556,279, the multiplexing coupler 220 constructed so that it couples different percentages of lig between the two coupler halves in accordance with t wavelength of the light. For example, the multiplexi coupler 220 is constructed so that substantially all the lig introduced into the optical fiber 230 at the wavelength of t pu p signal (e.g., 0.82 microns) is not coupled and remains in the fluorescent optical fiber 210 to cause the fluorescent effect deεcribed above. On the other hand, the multiplexing coupler 220 causeε approximately 50 percent of the fluoreεcent light generated within the fluoreεcent optical fiber 210 and propagating in the forward direction toward the second end 214 to be coupled from the fluorescent optical fiber 210 to the optical fiber 130. The coupled light propagates in the optical fiber 130 to the directional coupler 140 and thus to the loop 142 of the Sagnac interferometer. In the preferred embodiment, the loop 142 compriseε a waveguide (εuch aε an optical fiber) having two polarization modeε which propagate light at different velocities, εuch that the light traverεing the loop in one mode yields a propagation time difference with respect to light traversing the loop in the other mode. Preferably, the propagation time difference is significantly greater than the coherence time of the light input to the loop. The first end 212 of the fluorescent optical fiber 210 is specifically formed to preclude reflections at the first end 212. For example, in Figure 3, a slashed line acrosε the first end 212 indicates that the first end 212 has been cut at an angle (e.g., 15 degrees) so that light propagating to the first end will be non-reflectively emitted from the first end 212. Thus, subεtantially no light propagating toward the firεt end 212 will be reflected back toward the second end 214. The detector 150 iε poεitioned proximate to the first end 132 of the optical fiber 130 to detect light emitted therefrom. A filter 240 iε advantageously poεitioned between the first end 132 and the detector 240. The purpose for the filter 240 will be briefly discussed below. The pump light coupled to the fluorescent optical fiber 210 stimulateε the emiεsion of broadband light as discuεεed above in connection with Figure 1. The intensity of the pump light is selected to be sufficiently great to cause a population inversion of the electrons in the fluorescent material, thereby εupporting amplified εpontaneouε emission of light from the fluorescent material. The length of the fluorescent optical fiber 210 is εelected to be εufficiently long εo that εubstantially all the pump optical energy is absorbed by the fluorescent material and little, if any, pump optical energy is emitted from the first end 212 of the optical fiber 210; however, it should be underεtood that εince the firεt end 212 is non-reflectively terminated, this is not a stringent requirement. The re-combined light returning from the Sagnac interferometer returns to the multiplexing coupler 220 where 50 percent of the re-combined light is coupled to the fluorescent optical fiber 210 and propagates to the first end 212 and is non-reflectively emitted therefrom. The other 50 percent of the re-combined light remains in the optical fiber 130 and propagates to the first end 132 where it is emitted. The emitted light paεεeε through the filter 240 to the detector 150 where it is detected to generate a responεive electrical signal that is processed as discussed above. Although the multiplexing coupler 220 is preferably constructed so that subεtantially none of the light from the pump εource 120 is coupled to the optical fiber 130, the filter 240 is included to filter out any pump light that may be coupled. The filter 240 is selected to have optical characteristicε such that subεtantially all the light at the pump light wavelength (e.g., 0.82 microns) is blocked and such that substantially all the light in the emission band (e.g., 1.06 microns) is tranεmitted through the filter 240 to the detector 150. It can be εeen that by non-reflectively terminating the firεt end 212 of the fluoreεcent optical fiber 210, there is no posεibility of creating a resonant cavity to support laser oscillations. This is particularly advantageous in maintaining the broadband, temporally incoherent characteristicε of the light introduced into the Sagnac interferometer from the broadband light source 200, and in preventing spectral narrowing of the light propagating in the interferometer. In the embodiment of Figure 3, a portion of the light emitted by the pump source 120 may undesirably couple through the multiplexing coupler 220 to the optical fiber 130. There is a posεibility that the pump light could interfere with the operation of the detector 150. Figure 4 illustrates an alternative embodiment of the present invention in which the detector 150 is effectivel isolated from the pump light. As in Figure 3, the Sagna interferometer is coupled to a broadband light source 300. The broadband light source 300 includes a fluorescent optical fiber 310 having a first end 312 and a second end 314. Th fluoreεcent optical fiber 310 haε the characteriεtic discussed above. A first multiplexing coupler 320 is forme on the fluorescent optical fiber 310 to couple the fluorescen optical fiber to an optical fiber 330. The optical fiber 33 has a first end 332 and a second end 334. The pump ligh source 120 is positioned to input light to the second end 31 of the fluorescent optical fiber 310 via the lens 122, a discuεεed above. The multiplexing coupler 320 is constructe so that s ' ubstantially none of the pump light is coupled fro the fluorescent optical fiber 310 to the optical fiber 330. Thus, substantially all the pump light propagates toward th first end 312 and is absorbed by the fluorescent material i the fluorescent optical fiber 310. The first end of th fluorescent optical fiber 310 iε non-reflectively terminate (e.g., by cutting it at an angle of, for example, 15 degrees so that substantially all the pump light and substantially al of the emitted light propagating toward the first end 312 ar discharged therefrom. The multiplexing coupler 320 is furthe constructed to provide substantially 100 percent coupling a the emiεsion wavelength (e.g., l.oβ microns) of t fluorescent optical fiber 310 so that the emitted lig propagating toward the second end 314 is coupled from t fluorescent optical fiber 310 to the optical fiber 330. Thu substantially none of the light in the emisεion band wi propagate to the pump source 120. The first half of the first directional coupler 126 formed on the optical fiber 330. The second half of the fir directional coupler is formed on the optical fiber 130. T first directional coupler 126 iε conεtructed to coup approximately 50 percent of the light in the optical fiber 3 to the optical fiber 130 in the Sagnac interferometer. T 50-percent portion of the light coupled to the optical fib 130 propagates to the second directional coupler 140 and thus coupled to the loop 142 of the Sagnac interferometer. The re-combined light from the coupler 140 of the Sagn interferometer propagates back to the first direction coupler 126. Approximately 50 percent of the light remains the optical fiber 130 and is emitted from the first end 132 the detector 150. The other 50 percent of the re-combin light iε coupled to the optical fiber 330 and propagates ba to the multiplexing coupler 320 where it is coupled to t fluorescent optical fiber 310 to propagate to the no reflective first end 312 and be discharged therefrom. Agai the first end 312 iε non-reflectively terminated to preve the formation of a resonant cavity. As illustrated, the detector 150 is positioned proxima to the first end 132 of the optical fiber 130. It can be se that there is subεtantially no probability of any of the pu light reaching the detector 150 with the pump εource 1 positioned as shown. Thus, there is no need for a filt between the first end 132 and the detector 150. Figure 5 illustrates a further embodiment of the prese invention. The interferometer is constructed substantially described above. The interferometer is connected to -16- broadband light εource 400 via the first directional coupler 126. The directional coupler 126 coupleε the optical fiber 130 to an optical fiber 402 that haε a firεt end 404 and a second end 406. The broadband light source 400 iε constructed using a double-clad fluoreεcent optical fiber 410 εuch aε iε available from Polaroid Corporation. The double-clad optical fiber 400 iε εhown in more detail in a cross sectional view in Figure 6. As illustrated, the double-clad optical fiber 410 includes an inner core 420 comprising silica glass doped with approximately 0.5 percent by weight of Nd 2 0 3 and 3.8 percent by weight of A1 2 0 3 . The core 420 has a numerical aperture of 0.16. The core 420 has a diameter of approximately 4.8 microns and is surrounded by a first cladding 422 having an approximately rectangular shape (e.g., having two substantially parallel sideε connected by slightly rounded ends, as shown) . The first cladding 422 has approximate rectangular dimensionε of 110 microns by 45 microns to provide a ratio of first cladding area to core area of approximately 274. The first cladding 422 comprises mainly silica (Si0 2 ) . The first cladding 422 iε εurrounded by a second cladding 424 which iε a firεt buffer coating. The εecond cladding 424 co priεes a soft fluro-polymer with a refractive index of approximately 1.39. The numerical aperture between the first cladding 422 and the second cladding 424 is approximately 0.4. The εecond cladding 424 iε εurrounded by a εecond or outer buffer coating 426 which co priεeε a commercial hard polymer for protecting the double-clad optical fiber 410. The firεt cladding 422 functionε aε a multimode core of the double-clad optical fiber 410. Aε will be discussed below, the multimode core (i.e., the first cladding 422) will accept light that is introduced at such an angle that it will not be accepted by the inner core 420 of the double-clad optical fiber 410. Similarly, light can be introduced into the firεt cladding 422 at a position such that it does no enter the inner core 420. Returning to Figure 5, the double-clad optical fiber 41 has a first end 430 and a second end 432, each of which i non-reflectively terminated by cutting the two endε at angle (e.g., the firεt end 430 and the εecond end 432 are cut a approximately 15 degreeε) . The εecond end 432 is positione proximate to the first end 404 of the optical fiber 402 in th interferometer. A lens (not shown) can advantageously be use to direct light from the second end 432 of the double-cla optical fiber 410 into the firεt end 404 of the optical fibe 402. Thus, approximately 50 percent of the light generated b the broadband light source 400 is coupled to the Sagna interferometer. The first end 430 of the double-clad optical fiber 410 i positioned to receive pump light from a pump source 440. I the embodiment of Figure 5, the pump source 440 comprises diode array 442, such as a GaAlAs phased array, and multimode fiber 444. The multimode fiber 444 has a first en 446 and a second 448. The diode array 442 introduces ligh into the first end 446 and it propagates to the second en 448. The second end 448 of the multimode fiber 444 i pigtailed to the double-clad optical fiber 410 so that th light is coupled into the double-clad optical fiber 410. Thi is illustrated more clearly in Figure 7 which is a crosε εectional view of the pigtail εplice between the two fiber 410, 444. Aε illuεtrated in Figure 7, the multimode fiber 444 ha a core 450 and an outer cladding 452. The multimode fiber 44 iε poεitioned on the angled cut firεt end 430 of the double clad optical fiber 410 εuch that the core 450 is juxtapos with the first cladding 422 of the double-clad optical fib 410. Thus, the light discharged from the multimode fiber 44 enterε into the firεt cladding 422 of the double-clad optic fiber 410 and begins propagating therein. Since the lig enters the double-clad optical fiber 410 at an angle, the light is not guided by the core 420 of the double-clad optical fiber 410, but rather repeatedly traverseε the core 420. As the light traverses the core 420, it is absorbed by the neodymium doping to cause the excitation of the electrons therein, as discussed above. This resultε in εuperfluoreεcing and the emiεεion of broadband light into the core 420. The emitted broadband light propagates to the εeσond end 432 of the double-clad optical fiber 410 where it is coupled to the Sagnac interferometer. The angled cut of the second end 432 prevents reflection of any of the emitted light back toward the first end 430. Similarly, the angled cut of the first end 430 prevents any of the backward propagating light from being reflected toward the second end 432. Thus, there is substantially no likelihood of creating a reεonant cavity that would εupport laεer oscillations, and the broadband characteristics of the superfluorescent output εignal are thereby preεerved. In Figures 5 and 7, the pump light from the multimode fiber 444 is introduced into the double-clad optical fiber 410 at an angle εuch that the light iε introduced into an aperture window between the numerical aperture of the core and the numerical aperture of the cladding. That iε, the angle of the multimode fiber 444 with reεpect to the double-clad optical fiber 410 iε εufficiently greater than the acceptance angle of the core 420 that the light is not guided in the core 420. On the other hand, the angle of the multimode fiber 444 with respect to the double-clad optical fiber 410 is sufficiently leεε than the acceptance angle of the first cladding 422 εo that the light is guided within the first cladding 422 and traverseε the core 420, aε diεcuεsed above. In other words, the light is introduced into the double-clad optical fiber 410 outside the numerical aperture of the core 420 and within the numerical aperture of the firεt cladding 422. In the exemplary double-clad optical fiber 410 having a core numerical aperture of approximately 0.16 and a first claddi numerical aperture of approximately 0.40, the aperture wind or acceptance window corresponds to a range of angles great than approximately 8 degrees and less than approximately degrees. For example, in one particular embodiment of t invention, light is introduced at an angle of 15 degrees to well within the acceptance window for the first cladding 42 In Figure 7, the core 450 of the multimode optical fib 444 is offset from the inner core 420 of the double-cl optical fiber 410 so that the light enterε only the multimo core (i.e., the first cladding 422) of the double-clad optic fiber 410. In addition, the two fibers are positioned so th the longitudinal axes of the two fibers are at an angle approximately 15 degrees, for example, so that the light only within the acceptance window of the multimode core (i.e the first cladding 422) of the double-clad optical fiber 41 Thus, the pump light from the multimode fiber 444 is n accepted by the inner core 420. These two methods precluding light from entering the inner core 420 (i.e positioning the two fibers at an angle and offsetting the t inner cores) can be used together as shown or separately. T offset of the two inner cores has the further advantage th the fluorescent light generated within the inner core 420 the double-clad optical fiber 410 does not couple to the inn core 450 of the multimode optical fiber 444. This preclud any light from being reflected at the first end 448 of t multimode optical fiber 444 and re-entering the double-cl optical fiber 410. Figure 8 illustrates an alternative embodiment in whi the pump source 440 is positioned proximate to the first e 430 of the double-clad optical fiber 410. However, the p source 440 is not pigtailed to the first end 430. Rather, t light is directed toward the first end 430 as a beam 500. beam can be focused onto the firεt end 430 with a lenε ( shown) . Again, the pump source 440 is positioned at an an -zo- with reεpect to the double-clad optical fiber 410 so that the beam iε within an aperture window defined between the numerical apertures of the core 420 and the first cladding 422. Figure 9 illustrates a still further embodiment of the present invention wherein the pump source 440 is positioned at an angle with respect to the second end 432 of the double-clad optical fiber 410. Since the pump source 440 is at an angle it can be positioned away from the centerline of the double- clad optical fiber 410 so as not to interfere with the light emitted from the second end 432 while remaining with the acceptance window of the first cladding 422. In this embodiment, the pump light propagates away from the Sagnac interferometer so that there is substantially no possibility of the pump light entering the interferometer and interfering with its operation. Further, the light returning from the Sagnac interferometer propagates from the second end 432 toward the first end 430 of the double-clad optical fiber 410 where it is non-reflectively discharged. Thus, there iε subεtantially no probability of the light from the interferometer entering the pump source 440. Figure 10 illustrateε a further embodiment of the present invention in which a broadband light source 600 comprises a double-clad fluorescent optical fiber 610 having a first end 612 and a second end 614. The double-clad fiber 610 is advantageously the same as the fiber 400 described above available from Polaroid Corporation. Optical pump light is provided by a diode array pump 620 which is advantageously a 500 milliwatt Spectra Diode Labs 815-nanometer laser diode array. In the preferred embodiment, the diode array pump 620 is operated at approximately 350 milliwatts with a current of 650 Ha and provides an optical output signal having a 3 Db bandwidth of 2.75 nanometers. The output of the diode array pump 620 iε collimated by a first microscope objective lens 624 and directed onto a narrowband dichroic mirror 630. The dichroic mirror 630 is selected to reflect εubstantially all light having a wavelength of 815 nanometerε, the wavelength of the pump light provided by the diode array pump 620. The dichroic mirror 630 is further εelected to be εubεtantially tranεparent to light having a wavelength of 1060 nanometerε. The dichroic mirror is preferably oriented at an angle of 45° with respect to the direction of propagation of the pump light so that the pump light is reflected at an angle of 90" toward a εecond microscope objective lens 634. The second microscope objective lens 634 focuses the pump light into the multimode core of the double-clad fiber 610. The overall coupling efficiency from the diode array pump 620 to the multimode core of the fiber 610 iε approximately 50%. As discussed above, the pump light propagating in the fiber 610 causes fluorescence which generates an optical output signal having a wavelength of 1060 nanometers. The first end 612 of the fiber 610 is cut at an angle so that the portion of the signal propagating toward the first end 612 is non-reflectively coupled from the fiber 610. The portion of the optical signal propagating toward the second end 614 iε coupled from the fiber 610 and paεεeε through the εecond microscope objective lens 634 to the dichroic mirror 630. Since the dichroic mirror 630 is transparent at 1060 nanometers, the 1060-nanometer optical signal passes through the dichroic mirror to a third microscope objective lens 640. The third microscope objective lens focuseε the optical signal onto a first end 650 of an optical fiber 652 which has a second end 654. The optical fiber 652 iε formed into one-half of the directional coupler 126 discussed above. The directional coupler 126 couples the light to the optical fiber 130 so that it propagates in the rotation sensor loop 142, as discusεed above. The light returning from the interferomete loop 142 propagateε to the first end 132 of the optical fiber 130 where it is emitted onto the photodetector 150. The operation of the photodetector 150 and the processor 154 i detecting and processing the optical output signal has been discussed above. Figure 11 illustrates a further embodiment of the invention derived from the embodiment of Figure 10 wherein the coupler 126 is eliminated and the interferometer output signal is coupled directly from the optical fiber 130 to the broadband light source 600. The output signal from the interferometer passes through the third microscope objective lens 640, through the dichroic mirror 630, through the second microscope objective lens 634 to the second end 614 of the double-clad fiber 610. The photodetector 150 is poεitioned proximate to the firεt end 612 of the double-clad optical fiber 610 and receives the light after it haε propagated through the optical fiber 610. However, εince the double-clad optical fiber 610 is pumped by the pump light from the diode array pump 620, the optical output signal from the interferometer is amplified within the double-clad optical fiber 610. Thus, the light incident on the photodetector 150 in the embodiment of Figure 11 has a greater power than the light incident on the photodetector 150 in the embodiment of Figure 10. As diεcussed in more detail in co-pending Application Serial No. (Attorney Docket No. STANF.94A), filed on the same date as the present application, to prevent gain modulation in fluorescent fibers, the frequency of the phase modulation in the interferometer loop should be above a threshold frequency. For an erbium-doped fiber, the gain modulation is quite high for modulator frequencies up to about 500 Hz to 1 KHz, and then decreaseε rapidly. The fluorescence lifetime of neodymium is on the order of 40 μsec, less than the lifetime of erbium, which iε about 10-15 mε. Thuε, the threshold frequency for erbium doped fibers is less than for neodymium doped fibers. In Sagnac interferometers, the modulation frequency is linked to the length of the interferometer loop (see, for example, U.S. Patent Noε. 4,410,275 and 4,671,658). For fiber loops 1 km i length, the modulation frequency is about 200 KHz. At suc frequency, the gain modulation iε almost zero for erbium dope fibers and negligible for neodymium doped fibers. Preferably, in the embodiment of Figure 11, th photodetector 150 is oriented in the optical path so that th light receiving surface on the photodetector 150 iε at a angle with the optical path of light exiting the fluorescen fiber. Thus, any light reflected by this surface of th photodetector 150 will not re-enter the first end 612 of th optical fiber 610. If such light were to re-enter the optica fiber 610, it could create a resonant cavity between th photodetector 150 and the loop 142, an effect which th embodiments of the present invention otherwise avoid. Th angle at which the photodetector 150 is oriented is selecte so that any reflected light will be outεide the numerica aperture of the optical fiber 610. For example, for a optical fiber 610 having a numerical aperture of about 1.1 t 1.2, the angle at which the reflective εurface iε oriente εhould be in the range of at leaεt 6 to 7 degreeε. I preferred embodimentε, an angle of 10 degrees is utilized t further decrease the likelihood of any reflected ligh entering the optical fiber 610. Figures 12 and 13 correspond to Figures 3 and 9 respectively, with the photodetector 150 moved to t respective first ends of the fluorescing optical fibers 21 and 410. In each of the embodiments of Figures 12 and 13, t optical output signal from the interferometer is amplifi within the fluorescing optical fiber to provide an amplifi optical output signal to be detected by the photodetector 15 One skilled in the art will appreciate that t embodiments of Figures 4, 5 and 8 can be similarly modified positioning the photodetector 150 to receive the optic output signal after it has propagated through the double-cl optical fiber 400 in each embodiment. In addition, althou the preferred embodiments were described in terms of a closed loop interferometer in which the detector output is used to drive the phase modulator, it will be recognized that the invention may be implemented in an open loop interferometer such as discloεed in U.S. Patent Noε. 4,779,975 and 4,410,275. It can be εeen that the foregoing embodimentε deεcribe improved broadband light εourceε in which the poεεibilities of undesirable laεer oscillation are subεtantially eliminated. Thus, desired broadband and temporally incoherent characteristicε of the εuperfluoreεcent light generated by the light εource are maintained. Although described above in connection with the preferred embodiments, it should be understood that modifications within the scope of the invention may be apparent to those skilled in the art, and all εuch modifications are intended to be within the scope of the appended claims.

Claims: CLAIMS : 1. An interferometer, comprising: an optical loop (130) ; and a light εource (123) comprising an optic waveguide (110), said optical waveguide (11 comprising a material which emits an emitted broadba optical signal in response to pumping with pu radiation, said light source (123) introducing sa emitted optical signal to said optical loop (130) alo a connecting optical path extending between said lig source (123) and said optical loop (130) , said interferometer being characterized in that said l (130) returns at least a portion of light comprising s emitted optical signal back to said light source (123) al said connecting optical path to provide a return broadband optical signal, said light source (123) being n reflective for said returning optical signal to prevent s returning optical signal from being reflected back to s loop (130) . 2. The interferometer as defined in Claim 1, wher said light source (123) compriseε a εource of pump li (120) coupled to optically pump εaid material εuch that s pump light propagates in said waveguide (110) in a direct opposite to that of said emitted optical signal. 3. The interferometer as defined in Claim 1 or wherein said waveguide (110) comprises an optical fi having a core surrounded by a cladding and said li source (123) comprises an optical source of pump li (120) coupled to introduce light into said cladding. 4. The interferometer as defined in Claim 3, wher said optical fiber has a first numerical apert corresponding to a first acceptance region and s cladding has a second numerical aperture corresponding t second acceptance region, said acceptance regions defin an aperture window therebetween, said pump source (1 introducing pump light into said aperture window. 5. The interferometer as defined in any one of the preceding claims, additionally comprising a polarizer (160) positioned in said connecting path such that both said emitted optical signal and said returning optical signal pass through said polarizer (160) . 6. The interferometer as defined in Claim 4, wherein εaid cladding has a non-circular cross section. 7. The interferometer as defined in Claim 6, wherein said cross section is rectangular. 8. The interferometer as defined in Claim 3, wherein said core of said optical fiber is single-mode and said cladding is multimode. 9. The interferometer as defined in any one of Claims 3-8, wherein said optical fiber (110) is doped with neodymium. 10. The interferometer as defined in Claim 1, wherein said optical loop (130) comprises an optical waveguide having two polarization modeε, εaid emitted optical εignal having a coherence time which iε significantly less than the propagation time difference between said modeε after traverεing the length of said loop (130) . 11. The interferometer as defined in Claim 1, wherein at least a portion of εaid returning optical εignal propagateε through εaid waveguide (110) and exits said waveguide (110) at an end thereof. 12. The interferometer as defined in Claim 1, additionally compriεing a photodetector (150) for detecting εaid portion of said returning optical signal, said photodetector (150) being dispoεed at said end of said waveguide (110) and arranged to prevent light incident thereon from being reflected back to said loop (130) . 13. The interferometer as defined in any one of the preceding claims, wherein said waveguide (110) comprises a fluorescent optical fiber. 14. The interferometer as defined in any one of the preceding claims, wherein said waveguide (110) comprises a longitudinal axis and at least one end and wherein said pu p radiation is input into said one end of said wavegui (110) at an angle to the longitudinal axis of said optic waveguide (110) . 15. A method of operating a Sagnac interferomet having a light source (123) and an optical sensing lo (130), said light source (123) comprising an optic waveguide (110) comprising a material which will e light, said sensing loop (130) being coupled to recei light from said optical waveguide (110) , said meth being characterized by the steps of: inputting pump radiation to the optical wavegui (110) to pump said optical waveguide (110) to e broadband light; propagating emitted light from the light sou (123) towards the optical sensing loop (130) ; and propagating emitted light from the opti sensing loop (130) towards the light source (1 without returning the emitted light to the loop (1 to prevent optical resonance of the emitted light εaid interferometer. 16. The method aε defined in Claim 15, wherein ε waveguide (110) compriεes a first end (112) and a second (114) , said emitted light being emitted from said first (112) , and wherein the step of inputting pump radiat comprises coupling optical pump energy to the wavegu (110) at locations between the senεing loop (130) and s second end (114) of the waveguide (110) . 17. The method as defined in any one of Claimε 15- additionally co priεing the εtep of passing emitted li propagating from the loop (130) to the light source (1 through the waveguide (110) to provide amplified light, further comprising the step of detecting the amplif light. 18. The method as defined in any one of Claims 15- wherein the step of inputting pump radiation comprises step of orienting said pump radiation so that the direct of propagation of the pump radiation at one end of s waveguide (110) is at an angle with respect to the direction of propagation of the light signal emitted from the end of said waveguide (110) .

Inventors: DIGONNET MICHEL J F, FESLER KENNETH A, FLING JOHN J, KIM BYOUNG Y, SHAW HERBERT J, DIGONNET, MICHEL, J., F., FESLER, KENNETH, A., FLING, JOHN, J., KIM, BYOUNG, Y., SHAW, HERBERT, J.
Applicants: UNIV LELAND STANFORD JUNIOR, THE BOARD OF TRUSTEES OF THE LELAND STANFORD JUNIO
Application year: 1990
Main classification: G01B9
IPC codes: G01B9, G01C19, G01P3