Application of the cross-gain modulation in erbium-doped fiber to increase the effective spectral bandwidth of an interrogator

This study investigates the impact of Cross-Gain Modulation (XGM) in erbium-doped fiber on the effective spectral bandwidth of a fiber-optic sensor interrogation system employing Fiber Bragg Gratings (FBGs) and a Distributed Feedback (DFB) laser diode. DFB-laser-based interrogators offer high scanning speeds (up to 33 pm/ns) and broad wavelength tuning ranges (up to 10 nm). However, wavelength tuning in such lasers often introduces significant fluctuations in the instantaneous power of the probing pulse — up to 20 dB, which can lead to measurement errors when interrogating FBG-based sensors. To mitigate this, only the portion of the pulse with relatively stable power (within 1 dB) is typically used for analysis. This approach, however, reduces the effective spectral bandwidth of the interrogator by up to 20 %. To solve this problem, we propose, for the first time, the use of XGM in erbium-doped fiber to enhance performance. To evaluate the potential of XGM for increasing the effective spectral bandwidth, we conducted a theoretical analysis of the interaction between two optical signals in erbium-doped fiber: the interrogator probe signal and an additional control signal. The influence of XGM on power stability and effective spectral bandwidth was assessed through numerical modeling using the OptiSystem software. We also examined how the shape of the control signal and the timing of its initiation affect the interrogator spectral bandwidth. This approach enables not only the optimization of the temporal profile of sub-microsecond optical pulses but also their amplitude enhancement through fiber amplification. The results show that XGM can effectively modulate the instantaneous power of the probe signal with a modulation depth of up to 30 dB — sufficient to stabilize the output of DFB laser pulses. Simulations confirm that appropriate shaping and timing of the control signal can significantly reduce power fluctuations. Specifically, using rectangular control pulses decreases the power variation from 20 dB to 7 dB. Furthermore, the duration of stable pulse modulation (within 1 dB of the peak power) increases from 62 ns to 267 ns, leading to a 4.3-fold expansion of the interrogator effective spectral bandwidth. The application of XGM in erbium-doped fiber offers a promising solution for improving the stability of DFB-laser-based interrogators without relying on high-frequency attenuators. This enhancement extends the operational range of the system and relaxes the reflectivity requirements for the FBGs used in the sensor network.