Radiation Effects on Fiber Bragg Gratings: Vulnerability and Hardening Studies.

Fiber Bragg gratings (FBGs) are point optical fiber sensors that allow the monitoring of a diversity of environmental parameters, e.g., temperature or strain. Several research groups have studied radiation effects on the grating response, as they are implemented in harsh environments: high energy physics, space, and nuclear facilities. We report here the advances made to date in studies regarding the vulnerability and hardening of this sensor under radiation. First, we introduce its principle of operation. Second, the different grating inscription techniques are briefly illustrated as well as the differences among the various types. Then, we focus on the radiation effects induced on different FBGs. Radiation induces a shift in their Bragg wavelengths, which is a property serving to measure environmental parameters. This radiation-induced Bragg wavelength shift (RI-BWS) leads to a measurement error, whose amplitude and kinetics depend on many parameters: inscription conditions, fiber type, pre- or post-treatments, and irradiation conditions (nature, dose, dose rate, and temperature). Indeed, the radiation hardness of an FBG is not directly related to that of the fiber where it has been photo-inscribed by a laser. We review the influence of all these parameters and discuss how it is possible to manufacture FBGs with limited RI-BWS, opening the way to their implementation in radiation-rich environments.

Narrow linewidth near-UV InGaN laser diode based on external cavity fiber Bragg grating.

We realize a fiber Bragg grating InGaN-based laser diode emitting at 400 nm and demonstrate its high coherency. Thanks to the fabrication of a narrowband fiber Bragg grating in the near-UV, we can reach single-mode and single-frequency regimes for the self-injection locked diode. The device exhibits 44 dB side-mode suppression ratio and mW output power. Detailed frequency noise analysis reveals sub-MHz integrated linewidth and 16 kHz intrinsic linewidth. Such a narrow linewidth laser diode in the near-UV domain with a compact and low-cost design could find applications whenever coherency and interferometric resolutions are needed.

Influence of photo-inscription conditions on the radiation-response of fiber Bragg gratings.

We compared the sensitivity to X-rays of several fiber Bragg gratings (FBGs) written in the standard telecommunication fiber Corning SMF28 with different techniques. Standard gratings were manufactured with phase-mask and UV lasers, continuum wave (cw) at 244 nm or pulsed in the nanosecond domain at 248 nm, in a pre-hydrogenated fiber. Others gratings were written by exposures to a femtosecond IR-laser (800 nm), with both phase-mask and point by point techniques. The response of these FBGs was studied under X-rays at room temperature and 100°C, by highlighting their similarities and differences. Independently of the inscription technique, the two types of fs-FBGs have showed no big difference up to 1 MGy(SiO(2)) dose. A discussion on the causes of the radiation-induced peak change is also reported.

Mitigation of photodarkening phenomenon in fiber lasers by 633 nm light exposure.

In this work, the influence of photodarkening (PD) and photobleaching (PB) on the lasing features of the ytterbium-doped aluminosilicate fiber lasers is examined. Simultaneous PD and PB with 633 nm irradiation was monitored at the lasing wavelength of 1070 nm and compared with individually caused PD and PB effects. The variation of laser threshold and slope efficiency was reported. By analyzing the laser performances it was found that the ratio of excess loss at 633 and 1070 nm is expected to be less than 20. In addition, considerable mitigation of the PD with 633 nm light irradiation is demonstrated.