Optical Microfiber Phase Modulator Directly Driven with Low-power Light
The refractive index of optical microfiber (OM) changes with the temperature of OM caused by intensity modulated pump light. The phase of the probe light will be modulated while it passes through this fraction of OM because of thermo-optic effect.
Optical phase modulator is an important component in optical systems. But at present most optical phase modulators based on electro-optic effect, thermo-optic effect or elastic-optic effect are driven by electrical signal and not suitable in compact optical system.
The researchers, led by associate professor Zhangqi Song, from National University of Defense Technology, presented an optical microfiber (OM) phase modulator directly driven by low-power light. It is reported in Chinese Optics Letters Volume 12, No.9, 2014 (/col/abstract.cfm?uri=col-12-9-090606).
OM is a kind of flexible optical waveguide between conventional optical fiber and integrated optical guide, and it is convenient to connect micro-components to optical fibers by OM. In the last decade, a variety of compact components and devices based on OMs have been reported, which makes it possible to construct tiny optical systems with OMs.
In Zhangqi Song' research, the main part of the OM phase modulator is a fraction of OM taped from a section of normal communication optical fiber with much higher extrinsic absorption at 980 nm than intrinsic absorption because of hydroxyls, dopants, and impurities. When exposed to 980 nm light, the OM absorbs a small part of pump light and turns it into heat. Because of the tiny volume and a large ratio between surface and volume, the OM can be heated up and cooled down quickly with low-power pump light, which changes the refractive index. So the phase of probe light is modulated because of thermo-optic effect caused by pump light.
Theoretical analysis shows that the bandwidth and phase response coefficient can be improved by reducing OM radius, increasing the length of OM and absorption coefficient of pump light. In this research, the OM phase modulator driven by low-power light has been experimentally demonstrated and preliminary theoretical analysis is given. Further works will be focused on increasing phase response and bandwidth of the OM phase modulator.
"The OM phase modulator makes it possible to construct advanced compact all-optic system and optical fiber sensor based on OM," said Professor Zhou Meng from this group. The OM phase modulator features simple structure, large bandwidth, low driven power, and can be driven by dislocated pump light. It has promising application in all fiber-optics sensing system and integrated optics.
低功率激光驱动微纳光纤相位调制器
微纳光纤在抽运光的作用下，其内部温度可以快速变化，从而导致折射率随之发生改变。当信号光通过时，其相位被调制。
相位调制器是光学系统中一种重要器件，但目前大部分基于电光效应、热光效应或弹光效应的相位调制器需要通过电信号驱动，不易实现小体积化。
国防科学技术大学宋章启副教授课题组提出了一种基于光驱动的微纳光纤相位调制器实现方案，相关研究成果发表在Chinese Optics Letters 2014年第9期上（http://www.opticsinfobase.org/col/abstract.cfm?uri=col-12-9-090606）。
微纳光纤是介于传统光纤和集成光波导之间的一种柔性光导纤维，可以方便地实现微观器件与传统光纤之间的连接。近年来，基于微纳光纤的微光学器件成为研究热点，使得构建基于微纳光纤的微光学系统成为可能。
在微纳光纤制备过程中，宋章启副教授课题组通过适当控制工艺提高其对980 nm波长光的非本征吸收。当以该波长的光抽运微纳光纤时，吸收引起的光能量损耗有一定比例转化为热。由于微纳光纤具有较小的体积和较大的表面积体积比，其内部温度可以随抽运光功率的变化快速升高或降低，从而导致折射率发生变化。当信号光通过时，由于热光效应，信号光的相位将被调制。
理论分析表明，通过减小微纳光纤半径，增大微纳光纤长度和对抽运光的吸收系数可以进一步提高微纳光纤相位调制器的带宽和调制系数。
该工作从实验上验证了光驱动微纳光纤相位调制器的可行性，并进行了初步理论解释。下一步工作将着重提高该相位调制器的调制系数和带宽。
该课题组的孟洲教授认为，光驱动的微纳光纤相位调制器使得构建更为复杂的全光微纳光纤或光纤传感系统成为可能。
基于光驱动的微纳光纤相位调制器具有结构简单、驱动光功率低、带宽大和可远程无源驱动的特点，在全光传感系统和集成光学的研究中具有潜在的应用价值。