为了研究金属镀层对光纤宏弯损耗性能的影响，建立了带金属镀层的光纤环所受热应力及热应力引起的弹光效应的数学模型。计算了金属镀层光纤环的热应力系数和折射率热应力系数。仿真分析了光纤环向热应力系数Kθt、径向热应力系数Krt和折射率热应力系数Kn的主要影响因素。结果表明：Kθt远大于Krt，光纤主要受到环向热应力，径向热应力可忽略；热应力及其引起的折射率变化与径向位置和镀层厚度有关，与光纤环的弯曲半径基本无关；镀层厚度在0~2000 μm范围内，随着厚度增加，Kθt和Kn均会先快速增大，再缓慢增大并趋于稳定；Kn为负值，随着温度增加，热应力将引起光纤折射率逐渐减小。该模型从理论上解释了金属镀层光纤环的热应力会引起光纤折射率变小，从而改变光纤的宏弯损耗。

In order to study effect of the metal coating on the macrobending loss properties of a fiber loop, two mathematical models of the thermal stress and the associated photoelastic effect of the metal coated fiber loop are established. The thermal stress coefficients and thermal stress dependent refractive index coefficients of the fiber loop with metal coating are calculated. Impact factors of the thermal stress coefficients Kθt, Krt and the thermal stress dependent refractive index coefficient Kn are simulated. The results indicate that Kθt is much greater than Krt at the same circumstance. Consequently, the metal coated fiber loop mainly suffers the annular thermal stress and the radial thermal stress can be ignored. The thermal stress and its induced refractive index change alter with the reference radial position and thickness of the coating rather than the bend radius of the loop. When the thickness of the metal coating increases from 0 to 2000 μm, Kθt and Kn both undergo a process of a sharp increase, and than transfer to a slow growth and tend to a stable value. Since Kn is negative, refractive index decreases as the temperature increases. The presented models are feasible to explain that refractive index of the fiber decreases with the increase of temperature, thus altering the macrobending loss of the metal coated fiber loop.