Polarisation-Multiplexing Ring-Cavity Fibre Laser

• Polarisation-multiplexing ring-cavity fibre lasers are fibre lasers that leverage dual polarization axes to generate two coherent lasing channels from a single gain medium.
• They employ birefringence and polarization control techniques, such as PBS, PCs, and nonlinear polarization rotation, to manage channel separation and wavelength selectivity.
• These architectures enable precise dual-comb spectroscopy, high-resolution LIDAR, and controllable multi-wavelength emission while reducing common-mode noise and environmental perturbations.

Searching arXiv for recent and foundational papers on polarization-multiplexing ring-cavity fiber lasers and related architectures. A polarisation-multiplexing ring-cavity fibre laser is a fibre-ring laser in which polarization is not merely a nuisance variable but an explicit intracavity degree of freedom used to define, separate, or control multiple lasing channels. In the most direct implementations, two pulse trains or combs occupy the fast and slow axes of a birefringent fibre segment inside a single ring cavity and therefore acquire slightly different group delays, repetition rates, and carrier-envelope offsets while still sharing the same gain medium and most of the optical path. In broader usage, related ring architectures employ polarization-dependent loss, birefringent filtering, nonlinear polarization rotation, or polarization-managed spectral filters to realize switchable wavelength sets, dual-comb generation, or polarization-selective higher-order-mode and orbital-angular-momentum output. A key distinction is that not every polarization-controlled ring is polarization-multiplexed in the strict simultaneous two-channel sense; some cavities use polarization only to select one lasing state at a time (Aldia et al., 2024, Cuevas et al., 2022, Cuevas, 4 Aug 2025, Wang et al., 2020).

1. Concept and formal description

In a ring cavity of geometric length $L$, the repetition rate of a pulse train is set by the round-trip time and the effective or group index of the propagating eigenmode. For polarization-multiplexed operation, the two orthogonal eigen-axes experience different indices, so the two intracavity channels satisfy

$f_{\mathrm{rep}} = \frac{c}{n_g L}$

or, equivalently in the dual-comb description,

$$f_{\mathrm{rep}}^{(i)} = \frac{c}{n_{\mathrm{eff}}^{(i)} L}.$$

The corresponding comb teeth are

$$f_n^{(i)} = f_{\mathrm{CEO}}^{(i)} + n f_{\mathrm{rep}}^{(i)},$$

and the multi-heterodyne radio-frequency mapping between two combs is

$$f_n^{(\mathrm{RF})} = n\,\Delta f_{\mathrm{rep}} + \Delta f_{\mathrm{CEO}}.$$

The repetition-rate difference arises from birefringence, with

$$\Delta f_{\mathrm{rep}} \approx \frac{c}{L}\left(\frac{1}{n_{\mathrm{eff}}^{(1)}}-\frac{1}{n_{\mathrm{eff}}^{(2)}}\right),$$

or, for small differences, from the combined effects of $\Delta n_g$ and any residual path-length mismatch $\Delta L$ (Cuevas et al., 2022, Aldia et al., 2024, Cuevas, 4 Aug 2025).

This framework explains why a single-cavity source can generate two mutually coherent outputs without the full phase-locking overhead of two independent lasers. Because both channels share the same gain, most of the same cavity, and much of the same environmental perturbation, common-mode fluctuations largely cancel in $\Delta f_{\mathrm{rep}}$ and $\Delta f_{\mathrm{CEO}}$. In the literature considered here, this principle underpins dual-comb operation, polarization-resolved sensing, and precision ranging. A closely related but not identical use of polarization occurs when birefringence and a polarizer are used to impose mode-selective loss, so that only one polarization-defined branch or transverse state survives threshold (Cuevas et al., 2022, Aldia et al., 2024, Cuevas, 4 Aug 2025, Wang et al., 2020).

2. Representative cavity architectures

The reported architectures span all-PM dual-comb rings, partially PM erbium rings with intracavity birefringent sections, polarization-filtered multi-wavelength rings, and all-PM ytterbium rings for higher-order-mode and OAM generation. Despite their differences, they repeatedly combine a doped-fibre gain section, pump injection through a WDM, an element that enforces unidirectionality, and one or more polarization-selective or birefringent subsystems (Aldia et al., 2024, Cuevas et al., 2022, Luo et al., 2010, Manna et al., 11 Apr 2026, Feng et al., 2021, Wang et al., 2020, Cuevas, 4 Aug 2025).

Architecture	Main elements	Reported function
All-PM Er dual-comb ring	78 cm Er-doped PM fibre, PM WDM, two PBS, two PBCC, NALM, shared 116 cm PM1550 fibre	Free-running single-cavity dual-comb spectroscopy
Er ring with PM section and CNT SA	0.85 m EDF, 1 m PM-1550 fibre, CNT film SA, 90/10 coupler, isolator, intracavity PC	Two orthogonal combs with tunable $f_{\mathrm{rep}} = \frac{c}{n_g L}$0
Compact optimized dual-comb ring	0.45 m EDF, 0.5 m PM-1550 fibre, CNT SA, 25/75 coupler, isolator, 5.25 m cavity	Stable dual-comb generation for LIDAR
All-PM Yb ring with free-space polarization section	70 cm PMYDF, PMWDM, PM980–PM1550 offset/cross splice, HW1, HW2, QWP, PBS	LP$f_{\mathrm{rep}} = \frac{c}{n_g L}$1 selection and OAM generation

In the all-PM erbium dual-comb laser, polarization is split at a fibre-PBS into the fast and slow axes, and two independent mode-locked pulse trains are formed on orthogonal PM axes while sharing the Er gain and a common PM1550 segment. The free-space NALM section in that system uses FRs, QWPs, PBS optics, and matched arms of about 13.5 cm, with one end mirror mounted on a translation stage to tune $f_{\mathrm{rep}} = \frac{c}{n_g L}$2 (Aldia et al., 2024).

In the 17.3 m erbium ring reported for polarization dynamics and tunability, a 1 m PM-1550 segment provides strong intracavity birefringence, a CNT film supplies passive mode locking, and an intracavity 3-paddle PC adjusts the state of polarization and birefringent filtering conditions. An external PC followed by a PBS is used for polarization separation and measurement rather than for cavity closure (Cuevas et al., 2022).

The 5.25 m optimized compact ring retains the same basic single-cavity dual-comb principle but uses a shorter, higher-doped gain fibre, a shorter PM section, a 25/75 output coupler, and packaged fibre routing to improve robustness and reduce intracavity peak power. This design was explicitly optimized for long-duration operation and dual-comb LIDAR (Cuevas, 4 Aug 2025).

Other ring architectures use polarization in different ways. The modified dual-pass Mach–Zehnder interferometer ring laser uses 4.5 m of EDF, a 980/1550 WDM, a PD-ISO, three PCs, a 50:50 coupler, a 30:70 coupler, a 2 mm arm-length difference, and a 4 m PZT phase modulator to generate interleaved spectral combs. The all-fibre NPR ring uses 3 m EDF, 17 m SMF, two PCs, a PD-ISO, and a 10:90 output coupler in a 25 m cavity. The figure-8 compound-ring-cavity laser combines an 18.25 m erbium ring with a three-coupler F8-CRC filter and a polarization-managed four-channel filter composed of two HB-FBGs spliced with a 45° axis offset (Luo et al., 2010, Manna et al., 11 Apr 2026, Feng et al., 2021).

3. Polarization control, multiplexing, and mode-selection mechanisms

The most literal polarization-multiplexing mechanism is axis multiplexing in a birefringent cavity. The fast and slow axes have different group indices and therefore different round-trip times, so two pulse trains can co-propagate in one ring while remaining separable after a PBS. In the PM-segment erbium ring, the PM fibre and intracavity PC act as an adjustable Lyot-like filter that supports two orthogonal SOP eigenmodes aligned to the PM axes; each eigenmode forms its own stable pulse train with a slightly different repetition rate. In the all-PM gain-sharing dual-comb laser, the fibre-PBS launches the two channels directly into the fast and slow axes, and balancing the linear losses in the two NALM arms enables simultaneous stable single-pulse operation on both axes (Cuevas et al., 2022, Aldia et al., 2024).

A second mechanism is wavelength-dependent polarization rotation, often implemented with an isolator and PCs or with an interferometric comb filter. In the modified dual-pass MZI laser, PC1 and PC3 control whether the filter behaves effectively as single-pass or dual-pass and whether the cavity selects one or the other of two complementary interleaved combs. The PD-ISO converts polarization changes into wavelength-selective loss, and the design description states that if the two interleaved combs are made orthogonal in polarization at the output, a PBS can demultiplex them into two polarization channels (Luo et al., 2010).

A third mechanism is nonlinear polarization rotation. In the 25 m all-fibre NPR ring, the PD-ISO acts as both polarizer and analyzer, while PC-1 and PC-2 set the projections that determine both saturable absorption and a Lyot-like birefringent comb filter. The normalized transmission is written as

$f_{\mathrm{rep}} = \frac{c}{n_g L}$3

with

$f_{\mathrm{rep}} = \frac{c}{n_g L}$4

This transmission simultaneously supplies intensity-dependent loss and periodic wavelength selectivity, so the cavity can move between single- and multi-wavelength mode-locking without changing hardware (Manna et al., 11 Apr 2026).

A fourth mechanism is polarization rotation technique in all-PM higher-order-mode lasers. In the ytterbium PM ring, offset splicing between PM980 and PM1550 excites LP$f_{\mathrm{rep}} = \frac{c}{n_g L}$5, and cross splicing provides birefringence-induced mode filtering. The PMF beat length for mode $f_{\mathrm{rep}} = \frac{c}{n_g L}$6 is

$f_{\mathrm{rep}} = \frac{c}{n_g L}$7

with $f_{\mathrm{rep}} = \frac{c}{n_g L}$8. Jones matrices describe the PMF and waveplates, and a PBS imposes polarization-dependent round-trip loss. For OAM conversion, the LP$f_{\mathrm{rep}} = \frac{c}{n_g L}$9 even and odd components must acquire a relative phase

$$f_{\mathrm{rep}}^{(i)} = \frac{c}{n_{\mathrm{eff}}^{(i)} L}.$$0

so that

$$f_{\mathrm{rep}}^{(i)} = \frac{c}{n_{\mathrm{eff}}^{(i)} L}.$$1

Crucially, that ring is polarization-selective rather than polarization-multiplexed as demonstrated: lasing occurs along one polarization-defined path selected by PRT, and simultaneous orthogonal polarization channels were not reported (Wang et al., 2020).

4. Operating regimes and measured performance

Dual-comb polarization-multiplexed ring lasers have been demonstrated in several parameter ranges. In the 17.3 m erbium ring, a stable operating point yielded $$f_{\mathrm{rep}}^{(i)} = \frac{c}{n_{\mathrm{eff}}^{(i)} L}.$$2 MHz, $$f_{\mathrm{rep}}^{(i)} = \frac{c}{n_{\mathrm{eff}}^{(i)} L}.$$3 MHz, and $$f_{\mathrm{rep}}^{(i)} = \frac{c}{n_{\mathrm{eff}}^{(i)} L}.$$4 Hz. The polarization-resolved spectra were centered at 1559.45 nm and 1559.62 nm with FWHM values of about 1.85 nm and 1.91 nm, the RF peaks had SNR $$f_{\mathrm{rep}}^{(i)} = \frac{c}{n_{\mathrm{eff}}^{(i)} L}.$$5 dB and linewidths of 1.4 Hz and 1.5 Hz at RBW = 1 Hz, and the measured extinction ratio after PBS separation was about 16.5 dB. Long-term drift of $$f_{\mathrm{rep}}^{(i)} = \frac{c}{n_{\mathrm{eff}}^{(i)} L}.$$6 was about 6 Hz over 6 hours, and the separated channels showed $$f_{\mathrm{rep}}^{(i)} = \frac{c}{n_{\mathrm{eff}}^{(i)} L}.$$7 and $$f_{\mathrm{rep}}^{(i)} = \frac{c}{n_{\mathrm{eff}}^{(i)} L}.$$8 (Cuevas et al., 2022).

The all-PM gain-sharing dual-comb ring operated near 74.74 MHz and offered tunability of $$f_{\mathrm{rep}}^{(i)} = \frac{c}{n_{\mathrm{eff}}^{(i)} L}.$$9 from 500 Hz to 200 kHz. In the spectroscopy demonstration, $$f_n^{(i)} = f_{\mathrm{CEO}}^{(i)} + n f_{\mathrm{rep}}^{(i)},$$0 kHz was used. The fast- and slow-axis outputs were centered at 1571.8 nm and 1564.6 nm with FWHM values of 20.09 nm and 23.55 nm, and the output powers were 14 mW and 11 mW. Over one hour of free-running operation, the measured standard deviations were $$f_n^{(i)} = f_{\mathrm{CEO}}^{(i)} + n f_{\mathrm{rep}}^{(i)},$$1 Hz, $$f_n^{(i)} = f_{\mathrm{CEO}}^{(i)} + n f_{\mathrm{rep}}^{(i)},$$2 Hz, and $$f_n^{(i)} = f_{\mathrm{CEO}}^{(i)} + n f_{\mathrm{rep}}^{(i)},$$3 Hz. Stable dual-comb operation was reported over months (Aldia et al., 2024).

The optimized compact dual-comb cavity operated at a fundamental repetition rate of 39.25 MHz and a typical $$f_n^{(i)} = f_{\mathrm{CEO}}^{(i)} + n f_{\mathrm{rep}}^{(i)},$$4 range of about 820–1142 Hz. The thesis reports minimal drift of 1 Hz per hour, operation over 250 hours, and sub-millimetre precision in 5-metre ambiguity ranges for dual-comb LIDAR; one proof-of-concept measurement reached about 0.34 mm precision with 10 ms averaging and electrical low-pass filtering. The same work also reported a 5.25 m cavity, central wavelengths near 1559.78 nm and 1559.455 nm, spectral FWHM values near 1.83–1.84 nm, and RF phase noise near $$f_n^{(i)} = f_{\mathrm{CEO}}^{(i)} + n f_{\mathrm{rep}}^{(i)},$$5 dBc/Hz at 100 Hz offset (Cuevas, 4 Aug 2025).

Polarization-engineered multi-wavelength rings show a different performance profile. The modified dual-pass MZI laser produced up to 29 stable lasing lines with 0.4 nm spacing and 14 lasing wavelengths with 0.8 nm spacing within a 3 dB bandwidth, with maximum power fluctuation below 1.0 dB and maximum wavelength drift about 0.05 nm over 30 minutes at room temperature. The all-fibre NPR ring supported single- to seven-wavelength mode locking, with threshold around 18 mW, seven-wavelength operation around 50 mW, and a repetition rate of 8.014 MHz; the single-wavelength state at 1562.40 nm had OSNR $$f_n^{(i)} = f_{\mathrm{CEO}}^{(i)} + n f_{\mathrm{rep}}^{(i)},$$6 dB, SMSR $$f_n^{(i)} = f_{\mathrm{CEO}}^{(i)} + n f_{\mathrm{rep}}^{(i)},$$7 dB, and linewidth about 0.1 nm, while the seven-wavelength state showed RF SNR $$f_n^{(i)} = f_{\mathrm{CEO}}^{(i)} + n f_{\mathrm{rep}}^{(i)},$$8 dB and negligible wavelength drift over 1 hour. The figure-8 compound-ring-cavity laser realized 15 lasing states, including four single-, six dual-, four tri-, and one quad-wavelength state; all four single-wavelength states were stable SLM oscillations with linewidths below 600 Hz, RIN values no higher than $$f_n^{(i)} = f_{\mathrm{CEO}}^{(i)} + n f_{\mathrm{rep}}^{(i)},$$9 dB/Hz at frequencies of at least 3 MHz, and output-power fluctuation no higher than $$f_n^{(\mathrm{RF})} = n\,\Delta f_{\mathrm{rep}} + \Delta f_{\mathrm{CEO}}.$$0 (Luo et al., 2010, Manna et al., 11 Apr 2026, Feng et al., 2021).

Polarization-controlled ring lasers have also been used for transverse-mode and OAM control. In the all-PM ytterbium ring, clockwise and anticlockwise spiral fringes in a Mach–Zehnder interferometer confirmed $$f_n^{(\mathrm{RF})} = n\,\Delta f_{\mathrm{rep}} + \Delta f_{\mathrm{CEO}}.$$1 and $$f_n^{(\mathrm{RF})} = n\,\Delta f_{\mathrm{rep}} + \Delta f_{\mathrm{CEO}}.$$2 vortex beams, and Fourier analysis of the azimuthal intensity ring gave reported OAM purities of 91.1% for $$f_n^{(\mathrm{RF})} = n\,\Delta f_{\mathrm{rep}} + \Delta f_{\mathrm{CEO}}.$$3 and 96.55% for $$f_n^{(\mathrm{RF})} = n\,\Delta f_{\mathrm{rep}} + \Delta f_{\mathrm{CEO}}.$$4 (Wang et al., 2020).

5. Dynamics, diagnostics, and applications

The experimental literature treats polarization multiplexing not only as a static cavity property but also as a dynamic process. In the dual-comb erbium ring studied with fast polarimetry, the normalized Stokes parameters and the degree of polarization oscillated in time because the two pulse trains overlapped with slightly different repetition rates; on the Poincaré sphere, the SOP trajectory formed arcs. After polarization separation by a PBS, the two channels exhibited high DOP values, supporting the interpretation that the cavity sustains two well-defined polarization eigenmodes. The same system was proposed for polarization spectroscopy and dual-comb-based polarimetry, with the RF mapping

$$f_n^{(\mathrm{RF})} = n\,\Delta f_{\mathrm{rep}} + \Delta f_{\mathrm{CEO}}.$$5

enabling high-speed spectral retrieval in the radio-frequency domain (Cuevas et al., 2022).

In the optimized LIDAR-oriented single-cavity comb, dispersive Fourier transform over an 11 km SMF-28 link was used to visualize build-up and propagation dynamics. The reported dual-comb onset followed a two-stage process: spontaneous spikes generated by the saturable absorber could broaden spectrally, split into two orthogonal SOPs, and stabilize only if their energy crossed appropriate thresholds. Once established, the two combs propagated stably, and observed collisions did not alter pulse shape or velocity. For ranging, the one-way distance follows

$$f_n^{(\mathrm{RF})} = n\,\Delta f_{\mathrm{rep}} + \Delta f_{\mathrm{CEO}}.$$6

with the ambiguity range

$$f_n^{(\mathrm{RF})} = n\,\Delta f_{\mathrm{rep}} + \Delta f_{\mathrm{CEO}}.$$7

This architecture was explicitly developed for dual-comb LIDAR, where the reported robustness and low drift are operationally important (Cuevas, 4 Aug 2025).

The all-PM dual-comb laser was used for proof-of-principle carbon-monoxide dual-comb spectroscopy without active stabilization. A CO cell at 296 K, 790 Torr, and 30 cm path length was measured with $$f_n^{(\mathrm{RF})} = n\,\Delta f_{\mathrm{rep}} + \Delta f_{\mathrm{CEO}}.$$8 kHz, 20 $$f_n^{(\mathrm{RF})} = n\,\Delta f_{\mathrm{rep}} + \Delta f_{\mathrm{CEO}}.$$9s interferograms, and 250 sequentially recorded traces averaged after phase correction. The resulting absorption features agreed with HITRAN calculations within the measurement noise floor. This directly links polarization multiplexing with free-running dual-comb spectroscopy in a compact PM platform (Aldia et al., 2024).

Multi-wavelength polarization-managed rings support other application classes. The modified dual-pass MZI architecture targets tunable and switchable multi-wavelength emission, while the NPR cavity emphasizes reconfigurable DWDM-compatible ultrafast sources and maps selective channel activation or suppression to binary logical states. The figure-8 compound-ring-cavity laser demonstrates tunable microwave generation through dual-wavelength beating, including a reported 23.10 GHz signal obtained from a tuned wavelength pair. The all-PM Yb ring adds a spatial-mode dimension by generating LP$$\Delta f_{\mathrm{rep}} \approx \frac{c}{L}\left(\frac{1}{n_{\mathrm{eff}}^{(1)}}-\frac{1}{n_{\mathrm{eff}}^{(2)}}\right),$$0 and linearly polarized OAM beams. Taken together, these results suggest that the field spans at least four use cases: dual-comb metrology, spectroscopy, microwave photonics, and structured-light generation (Luo et al., 2010, Manna et al., 11 Apr 2026, Feng et al., 2021, Wang et al., 2020).

6. Limitations, misconceptions, and design trade-offs

A recurrent misconception is that any ring laser with PBSs, PCs, or birefringent fibres is automatically polarization-multiplexed. The all-PM Yb ring is an explicit counterexample: the cavity uses a PBS as an intra-cavity polarization filter and output coupler, but simultaneous lasing of two orthogonal polarization channels was not reported, and the output OAM beams were linearly polarized. That system is better described as polarization-selective unless extended with duplicated polarization paths, PM PBS/PBC elements, and separate phase-and-loss control in each path (Wang et al., 2020).

Another recurring limitation is incomplete reporting of polarization metrics. In the Yb OAM ring, neither PER nor polarization-channel crosstalk was reported. In the all-PM dual-comb spectroscopy laser, polarization extinction ratios and quantitative crosstalk were likewise not reported. In the dual-comb erbium ring with external PBS separation, the measured extinction ratio of about 16.5 dB was below the PBS nominal value and was explicitly limited by imperfect SOP alignment and finite PBS extinction. These observations make clear that polarization multiplexing depends not only on birefringence but also on alignment, component quality, and residual coupling (Aldia et al., 2024, Cuevas et al., 2022, Wang et al., 2020).

The stability envelope is also architecture-dependent. In the 17.3 m dual-comb erbium ring, stable $$\Delta f_{\mathrm{rep}} \approx \frac{c}{L}\left(\frac{1}{n_{\mathrm{eff}}^{(1)}}-\frac{1}{n_{\mathrm{eff}}^{(2)}}\right),$$1 values were typically in the range of about 110–250 Hz, while regimes below about 100 Hz tended to collapse. In the all-PM gain-sharing source, the lower end of $$\Delta f_{\mathrm{rep}} \approx \frac{c}{L}\left(\frac{1}{n_{\mathrm{eff}}^{(1)}}-\frac{1}{n_{\mathrm{eff}}^{(2)}}\right),$$2 was limited by injection locking and environmental stability, whereas larger $$\Delta f_{\mathrm{rep}} \approx \frac{c}{L}\left(\frac{1}{n_{\mathrm{eff}}^{(1)}}-\frac{1}{n_{\mathrm{eff}}^{(2)}}\right),$$3 reduced non-aliasing bandwidth and mutual coherence. In OAM generation, LP$$\Delta f_{\mathrm{rep}} \approx \frac{c}{L}\left(\frac{1}{n_{\mathrm{eff}}^{(1)}}-\frac{1}{n_{\mathrm{eff}}^{(2)}}\right),$$4 operation was robust against bending down to 5 mm diameter, but the OAM state was more sensitive because the even/odd phase difference had to stay near $$\Delta f_{\mathrm{rep}} \approx \frac{c}{L}\left(\frac{1}{n_{\mathrm{eff}}^{(1)}}-\frac{1}{n_{\mathrm{eff}}^{(2)}}\right),$$5 (Cuevas et al., 2022, Aldia et al., 2024, Wang et al., 2020).

Spectral purity and line selectivity introduce further trade-offs. The modified dual-pass MZI ring achieved dense multi-line spectra, but OSNR and linewidth were not reported and the cavity modes within each comb passband were expected to be multi-longitudinal-mode unless additional narrowband elements were added. By contrast, the figure-8 compound-ring-cavity design deliberately imposed an FSR near 10.2 GHz and an 8.0–8.75 MHz passband so that only one CRC peak would fall within each PM-FCF channel and only one longitudinal mode would pass per comb tooth, at the cost of a more elaborate filter subsystem (Luo et al., 2010, Feng et al., 2021).

The available papers also describe several concrete extension paths. For strict simultaneous polarization multiplexing in all-PM mode-selective rings, the proposed route is to replace bulk polarization optics with PM PBS/PBC components, add an in-loop isolator for enforced unidirectionality, and provide independent phase control and variable attenuation on each polarization path. For long-term field use, enclosure, strain relief, PM components, and tighter thermal control repeatedly appear as stabilizing measures. These proposals suggest that future work is likely to move toward more completely PM, more actively controlled, and more application-specific implementations rather than toward a single universal cavity template (Wang et al., 2020, Cuevas, 4 Aug 2025, Aldia et al., 2024).