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.

1. Soto V.D., López-Amo M. Truly remote fiber optic sensor networks. Journal of Physics: Photonics, 2019, vol. 1, no. 4, pp. 042002. doi: 10.1088/2515-7647/ab3f0e

2. Ma Z., Chen X. Fiber Bragg gratings sensors for aircraft wing shape measurement: Recent applications and technical analysis. Sensors, 2018, vol. 19, no. 1, pp. 55. doi: 10.3390/s19010055

3. Schena E.,Tosi D., Saccomandi P., Lewis E., Kim T. Fiber optic sensors for temperature monitoring during thermal treatments : an overview. Sensors, 2016, vol. 16, no. 7, pp. 1144. doi: 10.3390/s16071144

4. Qiao X., Shao Z.H., Bao W.J., Rong Q.Z. Fiber bragg grating sensors for the oil industry. Sensors, 2017, vol. 17, no. 3, pp. 429. doi: 10.3390/s17030429

5. Al-Fakih E., Osman N.A.A., Adikan F.R.M. The use of fiber Bragg grating sensors in biomechanics and rehabilitation applications: The state-of-the-art and ongoing research topics. Sensors, 2012, vol. 12, no. 10, pp. 12890–12926. doi: 10.3390/s121012890

6. Xia L., Cheng R., Li W., Liu D.M. Identical FBG-based quasidistributed sensing by monitoring the microwave responses. IEEE Photonics Technology Letters, 2015, vol. 27, no. 3, pp. 323–325. doi: 10.1109/lpt.2014.2370650

7. Margulis W., Lindberg R., Laurell F., Hedin G. Intracavity interrogation of an array of fiber Bragg gratings. Optics Express, 2021, vol. 29, no. 1, pp. 111–118. doi: 10.1364/OE.414094

8. Luo Z., Wen H., Guo H., Yang M. A time-and wavelength-division multiplexing sensor network with ultra-weak fiber Bragg gratings. Optics Express, 2013, vol. 21, no. 19. pp. 22799–22807. doi: 10.1364/OE.21.022799

9. Chen J., Liu B., Zhang H. Review of fiber Bragg grating sensor technology. Frontiers of Optoelectronics in China, 2011, vol. 4, no. 2, pp. 204–212. doi: 10.1007/s12200-011-0130-4

10. Darwich D., Youssef A., Zaraket H. Low-cost multiple FBG interrogation technique for static applications. Optics Letters, 2020, vol. 45, no. 5, pp. 1116–1119. doi: 10.1364/OL.386053

11. Wilson A., James S.W., Tatam R.P. Time-division-multiplexed interrogation of fibre Bragg grating sensors using laser diodes. Measurement Science and Technology, 2001, vol. 12, no. 2, pp. 181. doi: 10.1088/0957-0233/12/2/309

12. Cui J., Hu Y., Feng K., Li J., Tan J. FBG interrogation method with high resolution and response speed based on a reflective-matched FBG scheme. Sensors, 2015, vol. 15, no. 7, pp. 16516–16535. doi: 10.3390/s150716516

13. Yan H.T., Liu Q., Ming Y., Luo W., Chen Y., Lu Y.Q. Metallic grating on a D-shaped fiber for refractive index sensing. IEEE Photonics Journal, 2013, vol. 5, no. 5, pp. 4800706. doi: 10.1109/JPHOT.2013.2284244

14. Kouroussis G., Kinet D., Mendoza E., Dupuy J., Moeyaert V., Caucheteur C. Edge-filter technique and dominant frequency analysis for high-speed railway monitoring with fiber Bragg gratings. Smart Materials and Structures, 2016, vol. 25, no. 7, pp. 075029. doi: 10.1088/0964-1726/25/7/075029

15. Cheng R., Xia L., Zhou J., Liu D. Wavelength interrogation of fiber Bragg grating sensors based on crossed optical Gaussian filters. Optics Letters, 2015, vol. 40, no. 8, pp. 1760–1763. doi: 10.1364/OL.40.001760

16. Fernandez M.P., Bulus-Rossini L.A., Cruz J.L., Andres M.V., Costanzo-Caso P.A. High-speed and high-resolution interrogation of FBG sensors using wavelength-to-time mapping and Gaussian filters. Optics Express, 2019, vol. 27, no. 25, pp. 36815–36823. doi: 10.1364/OE.27.036815

17. Njegovec M., Donlagic D. Interrogation of FBGs and FBGs arrays using standard telecom DFB diode. Journal of Lightwave Technology, 2016, vol. 34, no. 22, pp. 5340–5348. doi: 10.1109/JLT.2016.2616725

18. Javernik A., Donlagic D. High-speed interrogation of low-finesse Fabry–Perot sensors using a telecom DFB laser diode. Journal of Lightwave Technology, 2017, vol. 35, no. 11, pp. 2280–2290. doi: 10.1109/JLT.2016.2642222

19. Mizunami T., Hirose S., Yoshinaga T., Yamamoto K. Power-stabilized tunable narrow-band source using a VCSEL and an EDFA for FBG sensor interrogation. Measurement Science and Technology, 2013, vol. 24, no. 9, pp. 094017. doi: 10.1088/0957-0233/24/9/094017

20. Tang Y., Siahmakoun A., Granieri S.C., Vlahovic B., Cheng C. Erbium doped fiber ring laser for optical wavelength conversion. Optik, 2011, vol. 122, no. 4, pp. 340–344. doi: 10.1016/j.ijleo.2010.01.007

21. Ahosan-ul-Karim M., Hossain M.I., Mehal M.T., Majumder S.P. All optical wavelength conversion based on cross gain modulation in Erbium Doped Fiber Amplifier. Proc. of the Asia-Pacific Microwave Conference, 2007, pp. 1–4. doi: 10.1109/APMC.2007.4554657

22. Giles C.R., Desurvire E. Modeling erbium-doped fiber amplifiers. Journal of Lightwave Technology, 1991, vol. 9, no. 2, pp. 271–283. doi: 10.1109/50.65886