J. Eur. Opt. Society-Rapid Publ. 22, 54( 2026) 531
Fig. 2.( a) ASE spectra at BOA’ s output for different driving currents;( b) Experimental setup employed for evaluating BOA’ s polarization dependence;( c) Output power spectra at the PBS output ports.
( SOP) can resonate within the cavity [ 20 – 22 ]. In conventional three-paddle PCs, formed by a cascade of three retarder plates in the form QWP – HWP – QWP( QWP: quarterwave plate, HWP: half-wave plate), precise adjustment of the orientations of the three paddles allows an arbitrary input polarization to be transformed into a well-defined output SOP [ 23 ], thereby enabling resonance at the wavelength associated with that polarization state. In contrast, in the present work, a reduced two-paddle PC, consisting of a QWP followed by an HWP, is employed. Although this configuration cannot generate an arbitrary SOP independently of the input, Jones-matrix analysis shows that it is sufficient to convert a general elliptical polarization into a linearly polarized state with a controllable orientation by proper adjustment of the plates’ angles [ 24 ]. When combined with the strong polarization-dependent gain of the BOA, which is shown in Figure 2c, this linear polarization rotation translates into distinct cavity gains for different SOPs, enabling polarization-selective amplification and wavelength switching without the need for a full three-paddle PC. Hence, with only two paddles to be adjusted instead of the usual three, the tuning process for selecting the desired lasing configuration becomes simpler and faster. The ring is closed with a 75:25 coupler that extracts part of the generated light for its measurement with an optical spectrum analyzer with a resolution of 0.03 nm,( OSA, model MS9740B-009, from Anritsu).
The filter used to define the emitted lines wavelengths is constructed by concatenation of two FBGs. Their central wavelengths, 1590 nm( FBG 1) and 1610 nm( FBG 2), are positioned within the BOA’ s L-band ASE spectrum. As illustrated in Figure 1, both FBGs exhibit a 0.22 nm bandwidth with reflectivities of �0.75 dB and �1.05 dB, respectively. Despite a minor mismatch in reflectivities, the
variation is insufficient to inhibit dual-wavelength operation. Besides their filtering function, these FBGs are separated from the rest of the setup by a 25 km spool of single mode fiber( SMF), serving also as the remotely controlled sensing mechanism, as demonstrated in [ 17 ], and enabling long distance sensing applications. All experimental measurements were conducted at room temperature, and no vibration isolation or temperature compensation techniques were employed.
3 Results
The switchable operation of the system is validated by driving the BOA at bias currents of 250, 300, and 350 mA, with all measurements performed at room temperature. The laser emission configuration is controlled by adjusting the rotation of both PC paddles. To explore the full range of possible operating modes under different biasing conditions, all configurations – namely no-lasing, single-wavelength, and dual-wavelength states – are systematically tuned using a search algorithm. This algorithm evaluates all possible paddle configurations of the motorized PC and optimizes different parameters depending on the target configuration. For single-wavelength and no-lasing states, the paddle positions are optimized to maximize suppression of undesired emission lines. For dual-wavelength operation, the optimization minimizes the optical power difference between the two lasing peaks, ensuring balanced output across both channels.
An example of the tunability of the system can be seen in Figure 3. In this example, the QWP is held fixed while the HWP is rotated to different positions with a 5-degree resolution. As shown, different combinations of optical