J. Eur. Opt. Society-Rapid Publ. 22, 54( 2026) 533
Table 1. Switching results for different configurations and biasing currents.
Peak power( dBm) OSNR( dB) Peak power diff.( dB) k 1 k 2 k 1 k 2 k 1 � k 2
250 mA |
No-lasing |
�52.2 |
�52.4 |
18.8 |
18.6 |
0.2 |
|
k 1 lasing |
�10.1 |
�51.9 |
61.8 |
20.1 |
41.8 |
|
k 2 lasing |
�48.8 |
�11.9 |
23.2 |
60.0 |
36.9 |
|
k 1 þ k 2 lasing |
�12.7 |
�13.1 |
59.4 |
58.8 |
0.5 |
300 mA |
No-lasing |
�43.5 |
�45.6 |
25.5 |
23.4 |
2.0 |
|
k 1 lasing |
�9.7 |
�48.3 |
61.0 |
22.0 |
38.9 |
|
k 2 lasing |
�48.1 |
�10.8 |
22.7 |
60.0 |
37.3 |
|
k 1 þ k 2 lasing |
�11.8 |
�11.8 |
59.5 |
59.5 |
0.0 |
350 mA |
No-lasing |
�49.4 |
�50.0 |
21.1 |
20.5 |
0.6 |
|
k 1 lasing |
�8.3 |
�50.7 |
62.2 |
19.8 |
42.4 |
|
k 2 lasing |
�54.0 |
�14.4 |
16.9 |
56.6 |
39.6 |
|
k 1 þ k 2 lasing |
�10.7 |
�10.8 |
60.0 |
60.0 |
0.1 |
setup in optical sensing applications requiring stable longterm dual wavelength operation.
Since the cause of the undesired variations of the generated spectrum are the unavoidable random fluctuations of the polarization state of the light propagating within the ring, an automatic control loop was included to respond and recover from these oscillations. The operational procedure of the control loop begins with an initialization phase to identify a reference angular configuration that achieves the desired spectral configuration. Once this configuration is achieved, the system continuously monitors the optical power spectrum and triggers a local search algorithm if the difference between the peak powers of the desired spectrum and the measured spectrum exceeds a 1 dB threshold. This search prioritizes the HWP due to its critical impact on equalization, testing positive and negative angular variations of increasing magnitude until an error below 0.5 dB is achieved again. Should no valid equalization be found by rotating only the HWP, the algorithm selects the best-performing angle and subsequently adjusts the QWP rotation to identify the optimal combination within the search interval by following a similar procedure. While this mechanism significantly improves the management of slow drifts, the search process itself introduces transient fluctuations, such as momentary drops in emission lines, indicating that the current implementation is not yet fully optimized for all operating conditions. However, as undesired polarization fluctuations typically occur at slower rates than the time required for the control loop to reoptimize the PC positions, this scheme correctly anticipates and prevents significant deviations in the produced spectrum, resulting in an important improvement on the stability over long periods of continuous operation of the laser. When the control loop is enabled, the output spectrum remains stable, as demonstrated in Figures 5b – 5d, over an extended 5 h recording period. In Figure 5b the mean value of the peak powers for both wavelengths is presented, where the maximum fluctuation of the signal is 1.34 dB for a 95 % confidence level( CL). Additionally, Figures 5c – 5d show the peak powers of each wavelength, which present a maximum fluctuation of 2.13 and 1.71 dB respectively for a 95 % CL.
These fluctuations could potentially be further reduced, since the algorithm currently adjusts the PC paddles only when the power difference between the two lines exceeds a predefined threshold, without accounting for the individual variation trends. Additionally, as previously noted, the resolution of the control is constrained by the minimum rotation step of the paddles in our device, which is 0.2 °.
4 Conclusion
A multiwavelength switchable fiber ring laser operating in the L-band for sensing applications has been experimentally demonstrated. The system is based on a polarization-sensitive SOA as the gain medium and two FBGs that define the emitted wavelengths and that can simultaneously function as remote sensing elements. The FBGs were deployed 25 km from the system header via standard single-mode fiber, thereby enabling long-range remote sensing applications. The laser can be electronically switched at the header between all emission configurations – no lasing, single-wavelength, or dual-wavelength operation – with OSNR consistently exceeding 55 dB, MSSR of at least 37 dB, and power differences between lasing lines in the dual-wavelength configuration below 0.5 dB. Wavelength switching is enabled by a simplified motorized polarization controller composed of only two paddles( a QWP followed by a HWP), without the need for additional polarization-dependent elements. This configuration supports automatic control, enabling stable long-term operation without the need for environmental isolation or polarization-maintaining fiber thanks to a closed loop algorithm that self-stabilizes the output by fine-tuning the paddles rotations.
Funding
This work was supported in part by projects PID2022-137269OB, funded by MCIN / AEI / 10.13039 / 501100011033 and FEDER“ AwaytomakeEurope” and PID2019-104426GB-100 funded by MCIN / AEI, and the Public University of Navarre collaboration grants.