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Optical Recirculaton Depolarizer and Method of Depolarizing Light

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Optical Recirculaton Depolarizer and Method of Depolarizing Light Patentamt Europaisches ||| || 1 1| || || || || || || || || ||| || (19) J European Patent Office Office europeen des brevets (11) EP 0 877 265 A1 (12) EUROPEAN PATENT APPLICATION (43) Date of publication:ation: (51) |nt. CI.6: G02B 6/34 11 .11 .1 998 Bulletin 1 998/46 (21) Application number: 98201367.4 (22) Date of filing: 28.04.1998 (84) Designated Contracting States: (72) Inventor: Paisheng, Shen AT BE CH CY DE DK ES Fl FR GB GR IE IT LI LU Fremont, CA 94539 (US) MCNLPTSE Designated Extension States: (74) Representative: AL LT LV MK RO SI Wharton, Peter Robert Urquhart-Dykes & Lord (30) Priority: 01.05.1997 US 847177 Inventions House Valley Court, Canal Road (71) Applicant: Bradford BD1 4SP (GB) Alliance Fiber Optics Products Inc. Sunnyvale, CA 94086 (US) (54) Optical recirculaton depolarizer and method of depolarizing light (57) An optical depolarizer (82) and method of depolarizing light is described. An input light beam is split into two beams. One split beam is recirculated 92 94-. through a birefringent medium (12) and looped back to ~\ ._ i — , f* be recombined with the input light. This allows a ( 7\ 96 weighted averaging of the different polarization states \ that result from birefringence in the recirculation path of ^"^^^ ^-uh the recirculated beam. The depolarizer (82) is formed as-. / from single mode fiber optic cables and fused single — v^c mode fiber couplers (84,86,88,90). Each fiber coupler f? ^"^-us (84,86,88,90) has an input pair of fibers and an output \l pair of fibers. One of the output fibers is coupled to one ^ — of the input fibers to form a recirculation loop. Addition- 8 ally, polarization controllers provided in the input fiber FigureFi and recirculation loop allow the degree of polarization of the output beam to be varied across a wide spectrum of values. CO CM r»- co Figure 1 o Q_ LU Printed by Xerox (UK) Business Services 2.16.3/3.4 1 EP 0 877 265 A1 2 Description depolarization of light propagating along the fiber. Thus, if depolarized light is used, birefringence no longer pro- BACKGROUND duces a degradation in system performance. One problem associated with utilizing depolarized Field of the Invention 5 light is that light sources used in fiber optic systems have a high degree of polarization (hereinafter DOP). The present invention relates to the field of optics in The DOP is defined as the fraction of optical power that general. More particularly, the present invention relates is polarized. To utilize polarized light sources, a depolar- to the field of depolarizers which have applications in izer must be employed to remove the DOP. Currently communications, sensors, optical instruments and other 10 available depolarizers have significant limitations which areas. reduce their practical applicability in both fiber optic communication systems and fiber optic instrumentation. Description of the Related Art One type of depolarizer is the electro-optic pseudo- depolarizer, which utilizes electrodes positioned on Many optical devices used in communications and 15 either side of a waveguide to change the refractive index instrumentation such as switches, couplers and modu- within the waveguide. The varying refractive index in lators, are highly sensitive to the state of polarization of turn varies the SOP of the light passing through the light. The performance of communication systems and waveguide. Although varying the refractive index of the instruments which utilize such optical devices varies as waveguide varies the SOP, the measured effective DOP the state of polarization (hereinafter SOP) varies. Fluc- 20 depends on detector speed. Over several cycles of var- tuations in the SOP can result in reduced signal to noise ying the refractive index of the waveguide, the time- ratios in fiber optic communication systems or averaged output light appears depolarized in that no decreased sensitivity and accuracy in fiber optic instru- one SOP is preferred during the averaging time. This ments. form of depolarization is called pseudo-depolarization When light passes through a fiber optic cable (here- 25 or time-averaged depolarization, and has the disadvan- inafter fiber), the initial polarization of the light, whether tage that light exiting the depolarizer within a narrow polarized elliptically or linearly, can be changed due to time interval has a high DOP. High speed detectors, varying environmental factors affecting the fiber. These however, detect light in a narrow time interval. Thus, a environmental factors produce changes in the index of high speed detector would capture light with a high DOP reflection of the fiber. Light propagating along the fiber 30 when the light is time averaged over the narrow time will pass through these regions having differing indexes interval. Additionally, the electro-optic pseudo-depolar- of refraction, thereby changing the initial SOP of the izer is an active system requiring both driving circuitry light as it propagates along the fiber. This effect of alter- and a power supply. Failure of any of these active com- ing the SOP of light as it passes through a medium is ponents would result in the light exiting the waveguide called birefringence. The polarization received at the 35 with a high DOP. Another drawback of the electro-optic output end of the fiber may thus change radically from pseudo-depolarizer is its high cost. An electro-optic the initial SOP at the input end of the fiber. Because pseudo-depolarizer costs approximately $1000.00. birefringence is affected by varying environmental fac- Another type of currently available depolarizer is tors, the output SOP will not have a predictable relation the acoustic depolarizer. A driving speaker vibrates a to the input SOP. Instruments and devices sensitive to 40 segment of fiber within the depolarizer, thereby altering SOP will therefore have their performance degraded in the index of refraction within the fiber as the fiber bends a manner which cannot easily be predicted or corrected. and vibrates. The index of refraction within the fiber var- One solution to the problem of birefringence is to ies at the frequency of the speaker. Polarized light pass- replace common single mode fiber with polarization ing through the vibrating fiber has its SOP altered at the maintaining fiber (hereinafter PMF), which is not sensi- 45 frequency of the speaker. As with the electro-optic tive to environmental factors and therefore preserves pseudo-depolarizer, the acoustic depolarizer depolar- the initial SOP as light propagates along the fiber. While izes light on a time averaged basis. The DOP then var- PMF has advantages over standard single mode fiber, it ies at the frequency of the driving speaker. For is also very expensive to use. One meter of PMF costs detectors and instruments which detect polarization approximately $10.00, roughly 100 times the cost of sin- so states at a time interval narrower than the time interval gle mode fiber. for depolarization, which is dependent on the frequency While single mode fiber has the drawback of bire- of the speaker, light exiting the acoustic depolarizer will fringence, if the incident light propagating within the have a noticeable DOP. Another disadvantage of the fiber is depolarized, then the birefringent effect will not driving speaker depolarizer is that it is an active system alter the SOP. Depolarized light is the combination of 55 which relies on the performance of the driving speaker. light of all polarization states in equal proportion. Bire- In addition to the costs associated with the driving fringence in single mode fiber alters all of the polariza- speaker and associated circuitry, such a system is tion states equally, thereby preserving the prone to failure if any of the many components of the 2 3 EP 0 877 265 A1 4 speaker or driving circuitry fail. Thus, although the driv- Accordingly, it is desired that the present invention ing speaker depolarizer reduces the DOP, the output overcome the limitations of current optical depolarizers. light still retains a significant DOP within a narrow time interval. SUMMARY OF THE INVENTION Another known depolarizer is the Lyot depolarizer. 5 The Lyot depolarizer consists of two plates of quartz The present invention provides an optical depolar- crystal having large retardances. The light source uti- izer and method of depolarizing light, wherein light is lized with a Lyot depolarizer is a broad band source, for depolarized by splitting the beam into an output beam example a superluminescent diode. The crystals are and a recirculation beam. The recirculation beam prop- arranged such that the incident light passes through the 10 agates along a birefringent path and is then recombined first crystal and into a second crystal adjacent to the first with the input beam before the input beam is split into crystal. The ratio of thickness of the two crystals is 2:1 . the output beam and recirculation beam. The combined While the light exiting the second crystal is depolarized input beam and recirculation beam is then split into an over a large wavelength region, light in a small wave- output beam and a recirculation beam. This process of length region is not depolarized. Thus, the Lyot depolar- 15 splitting, recirculating along a birefringent path and izer is ineffective for depolarizing monochromatic or recombining with the input beam averages beams with narrow wavelength light sources. Another drawback of different states of polarization such that the output the Lyot depolarizer is the high cost associated with beam is the average of many light beams with differing using a broad band light source. Broad band light states of polarization. sources have the additional disadvantage of having 20 In one embodiment of the present invention a bire- lower output power than is possible with narrow band fringent element is used with a plurality of mirrors.
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