Fused 3×3 single-mode fiber couplers have been fabricatedwith low excess loss, low uniformity error in splitting ratio, and ahigh degree of stability with regard to changes in temperature and inputpolarization. A special alignment system was utilized to tightly controlthe process parameters and to keep the fiber cores in the fusion regionin a perfect equilateral triangle shape before, during, and afterdrawing the 3×3 coupler. The 3×3 couplers which weremanufactured by using the special alignment system have not only goodperformance characteristics but also excellent resistance to thermalvariation, hot humidity, vibration, and shock. The 3×3 couplersmeet all the requirements of optical-fiber interferometric sensors andcoherent communications with regard to performance characteristics andresistance to severe environment
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