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Study Proposes Fiber Laser Sensor Capable of Measuring Strain and Temperatures

In an article published in the open-access journal Sensors, researchers presented a novel C-band dual-wavelength erbium-doped fiber laser aided by an artificial backscatter reflector (ABR) based on femtosecond laser direct writing.

Study: A Dual- Wavelength Fiber Laser Sensor with Temperature and Strain Discrimination. Image Credit: Micha Weber/Shutterstock.com

The artificial backscatter reflector was fabricated into a 32 mm-long single-mode fiber (SMF) using femtosecond laser direct writing. The resulting C-band erbium-doped fiber laser with wavelengths of 1530.81 nm and 1527.7 nm as its centers displayed an optical signal-to-noise ratio of 46 dB at 150 mW.

The power disparity between the two channels of the unique C-band erbium-doped fiber laser was only 0.02 dB, irrespective of the pump power, creating a dual-emission laser with excellent equalization. Simultaneous measurements of the central wavelength instability and output power levels were 0.01 nm and 0.3 dB for both channels. The threshold pumping power was 40 mW.

Finally, the capacity of the artificial backscatter reflector enhanced dual-wavelength C-band erbium-doped fiber laser for sensing applications was investigated. Simultaneous sensitivity measurement showed that strain and temperature could be measured simultaneously with sensitivity values of around 1 pm/ìå and 9.29 pm/°C, respectively.

Understanding the Multiwavelength Fiber Lasers

Applications for multiwavelength fiber lasers include telecommunications, terahertz wave generation, sensing, fiber-optic tests, and simultaneous measurement. The most common and effective amplification medium for this type of laser is erbium-doped fibers.

At room temperature, the C-band erbium-doped fiber laser has a homogeneous gain medium, which causes some additional issues, such as significant mode competition. In some cases, liquid nitrogen cooling may be required to reduce the homogenous gain broadening caused by such competition, resulting in power fluctuations greater than 1.5 dB and power flatness worse than 16 dB.

For various uses, femtosecond laser direct-write optical fiber topologies are interesting. Different applications in optical fiber sensors have been made possible by the invention of fiber-optic microstructure based on refractive index alteration under femtosecond laser irradiation. Potential femtosecond laser technology applications include multiparameter, curvature, and strain sensors.

In this paper, researchers introduced a novel artificial backscatter reflector made of a single-mode fiber. The artificial backscatter reflector was explicitly created to enhance the performance of the dual-wavelength lasers. Compared to earlier C-band erbium-doped fiber lasers, the artificial backscatter reflector enabled low threshold pump power, high-power stability, and remarkable equalization, resulting in a significant optical signal-to-noise ratio (OSNR). With sensitivities comparable to those of other traditional reflectors, the newly designed artificial backscatter reflector fiber structure ensured simultaneous measurement of temperature and strain. 

Experimental Demonstrations

A femtosecond laser from CALMAR lasers, functioning at 1030 nm wavelength, a pulse duration of 370 fs, and a fluctuating pulse repetition rate accessible up to 120 kHz, was used to produce the artificial backscatter reflector. The femtosecond laser processing improved the distributed dispersion because it increased the refractive index inhomogeneity of the fiber.

First, an optical spectrum analyzer and a broad C-band light source were used to determine the reflectance of the artificial backscatter reflector. Second, the backscattered optical power was measured as the function of the length of the fiber-based reflector inscribed into an SMF. Finally, this optical power was obtained using a spatial resolution optical backscattered reflectometer, frequently used for fiber testing and as a multiparameter sensor.

In the experimental setup, the light injected into the cavity exited through ports one and two of the four-port optical circulator, ending at a fiber loop mirror (FLM). The FLM had a three-port optical circulator, and ports one and three were linked to a new variable optical attenuator (VOA) to regulate how much light was reflected into the remaining cavity. The random fiber grating reflector's reflected light eventually circled back around via ports three and four of the four-port circulator, hitting the C-band erbium-doped fiber laser amplifier at its entrance port and making the complete circuit around the cavity.

A high-performance optical spectrum analyzer was used to assess the longitudinal mode behavior of the C-band erbium-doped fiber laser. With a dynamic range of more than 80 dB, this instrument delivered a spectral resolution of 0.08 pm. The findings revealed that it was possible to distinguish between longitudinal mode behavior with the instrument's resolution, removing the need to examine the electric beat by a tunable laser source (TLS).

A narrowband (1.5 nm) bandpass tunable optical filter was inserted between the optical coupler and the polarization controller, centering the transmission band on every lasing wavelength one at a time to examine the behavior of each emission line in the absence of the other. The fact that single longitudinal mode behavior persisted in all scenarios disproved the theory that the single longitudinal mode behavior was driven by inter-channel seed light interaction, mode competition, or mode hopping annihilation. Instead, the reflector's random fiber grating appeared to be the only cause of the single longitudinal mode behavior. 

An analysis was conducted on the output power stability with time. The power instability was discovered to be less than 0.5 dB in each case. The stability values were higher than those for stable lasers because of the proposed laser's single longitudinal mode behavior. In addition, single longitudinal mode behavior operation was accomplished without saturable absorbers.

Artificial Backscatter Reflectors Improve C-band Erbium-Doped Fiber Lasers

This work experimentally demonstrated an improved artificial backscatter fiber reflector-based dual-wavelength C-band erbium-doped fiber laser. With a length of 32 mm, the backscatter reflector was directly written using a femtosecond laser into an SMF. When pumped at 150 mW, dual-wavelengths were centered at 1530.81 nm and 1527.7 nm and had an optical signal-to-noise ratio of 47 dB. Independent of pump power, the difference between the two laser emission lines was as low as 0.02 dB, indicating a very high equalization level. Moreover, single longitudinal mode behavior was demonstrated by both emission lines.

When pumped at 400 mW, these parameters led to high stability in the emission wavelength and output power levels, which varied by only 0.01 nm and 0.3 dB, respectively, over one hour with a confidence level of 95%. In addition, the experimental evidence showed that the artificial backscatter reflectors could serve as mirrors and multiparameter sensors, with a sensitivity of 9.29 pm/°C for temperature and roughly 1 pm/ìå for strain.

Reference

Sanchez-Gonzalez, A., et al. (2022). A Dual- Wavelength Fiber Laser Sensor with Temperature and Strain Discrimination. Sensors, 22(18), 6888. https://www.mdpi.com/1424-8220/22/18/6888/htm

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Pritam Roy

Written by

Pritam Roy

Pritam Roy is a science writer based in Guwahati, India. He has his B. E in Electrical Engineering from Assam Engineering College, Guwahati, and his M. Tech in Electrical & Electronics Engineering from IIT Guwahati, with a specialization in RF & Photonics. Pritam’s master's research project was based on wireless power transfer (WPT) over the far field. The research project included simulations and fabrications of RF rectifiers for transferring power wirelessly.

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