We propose and demonstrate a four-wavelength-switchable erbium-doped fiber laser (4WS-EDFL) with a four-channel superimposed high-birefringence fiber Bragg grating (SI-HBFBG) and a dual-coupler ring based compound-cavity (DCR-CC) filter. Both for the first time, a SI-HBFBG as a four-channel reflective filter is used in a multi-wavelength switchable fiber laser to define wavelength channels and a DCR-CC filter is used to select a single mode from dense longitudinal-modes in a fiber laser. We present in detail how to design, fabricate, and characterize the DCR-CC filter with both theoretical analysis and experimental results, which we believe is the first systematic approach for making a compound-cavity based filter used for selecting single-longitudinal mode (SLM) in a fiber laser. The enhanced polarization hole burning effect in a 2.9 m long erbium-doped fiber, coiled inside a three-loop polarization controller, and the polarization-mismatch-induced losses are introduced into the laser cavity to achieve wavelength-switching operations. We show that the 4WS-EDFL can be switched among fifteen lasing states, including four single-wavelength operations, six dual-wavelength operations, four three-wavelength operations and one four-wavelength operation, all with high stability. For demonstration, in switchable single-wavelength operations, the four SLM lasing outputs measured are all with an optical signal to noise ratio of >80 dB, a linewidth of <700 Hz, a relative intensity noise of ≤−156.7 dB/Hz at frequencies over 3 MHz, an output power fluctuation of ≤0.555 dB and excellent polarization characteristics.

We propose and demonstrate a high-performance wavelength-switchable erbium-doped fiber laser (EDFL), enabled by a figure-8 compound-ring-cavity (F8-CRC) filter for single-longitudinal-mode (SLM) selection and a polarization-managed four-channel filter (PM-FCF) for defining four lasing wavelengths. We introduce a novel methodology utilizing signal-flow graph combined with Mason’s rule to analyze a CRC filter in general and apply it to obtain the important design parameters for the F8-CRC used in this paper. By combining the functions of the F8-CRC filter and the PM-FCF assisted by the enhanced polarization hole-burning and polarization dependent loss, we achieve the EDFL with fifteen lasing states, including four single-, six dual-, four tri- and one quad-wavelength lasing operations. In particular, all the four single-wavelength operations are in stable SLM oscillation, typically with a linewidth of <600 Hz, a RIN of ≤−154.58 dB/Hz@≥3 MHz and an output power fluctuation of ≤±3.45%. In addition, all the six dual-wavelength operations have very similar performances, with the performance parameters close to those of the four single-wavelength operations, superior to our previous work and others’ similar work significantly. Finally, we achieve the wavelength-spacing tuning of dual-wavelength operations for photonic generation of tunable microwave signals, and successfully obtain a signal at 23.10 GHz as a demonstration.

We describe a first distributed polarization analysis (DPA) system using binary polarization rotators in an optical frequency domain reflectormeter (OFDR) capable of measuring the variations of polarization states along a single-mode fiber (SMF). We demonstrate using such a DPA system to accurately measure the distance-resolved birefringence with 12 fiber loops of different radii with different birefringence values along a length of SMF and obtain a bending-induced birefringence coefficient (BBC) of 6.601 × 10⁻¹⁰ m², agreeing well with the theoretically estimated value of 5.334 × 10⁻¹⁰ m². To further verify the measurement accuracy, we obtain the birefringence values of the 12 fiber loops of different radii one at a time using a non-distributed polarization analysis system with an accuracy traceable to a birefringence standard made with a quartz crystal, and obtain a BBC value of 6.490 × 10⁻¹⁰ m², agreeing well with our distributed measurement with a relative error of only 1.68%. In addition, we measure the residual birefringence of the SMF with both distributed and non-distributed polarization analysis systems and obtain similar results with a relative error of only 0.59%. Our experiments not only validate the performances of our DPA system, but also the first to use DPA to experimentally obtain the accurate birefringence values along the SMF and verify the theory of bending-induced birefringence. Our work further proves that such an OFDR-based DPA system is a practical tool for optical component characterization, nondestructive optical material inspection, and distributed fiber optic transversal stress sensing.

Mode filters are fundamental elements in a mode-division multiplexing (MDM) system for reducing modal cross-talk or realizing modal routing. However, the previously reported silicon mode filters can only filter one specific mode at a time and multiple modes filtering usually needs a cascade of several filters, which is adverse to highly integrated MDM systems. Here, we propose a unique concept to realize compact, scalable and flexible mode filters based on backward mode conversion gratings elaborately embedded in a multimode waveguide. Our proposed method is highly scalable for realizing a higher-order-mode-pass or band-mode-pass filter of any order and capable of flexibly filtering one or multiple modes simultaneously. We have demonstrated the concept through the design of four filters for different order of mode(s) and one mode demultiplexer based on such a filter, and the measurement of two fabricated 11μm length filters (TE1-pass/TE2-pass) show that an excellent performance of insertion loss 29dB/28.5dB is achieved over a bandwidth of 51.2nm/48.3nm, which are competitive with the state-of-the-art.

Bragg-gratings have been frequently used to design compact and high extinction ratio (ER) on-chip polarizers. However, the strong reflection of the unwanted polarization may deteriorate the performance of the light source or cause unwanted interferences. In this paper, we propose a Bragg-grating-based all-silicon TM-pass polarizer with low reflection, low insertion loss (IL) and high ER. Unlike previously reported polarizers based on single mode waveguides, we construct the Bragg grating with a multimode waveguide, which not only acts as a Bragg reflector, but also a mode-order converter to convert the reflected TE light into higher order modes to be eventually filtered out by utilizing a tapered transition. On the other hand, the grating has little adverse influence on the TM input light since it works at sub-wavelength-guided wave propagation regime. Finally, the polarizer obtained has a length of 30µm, an ER of 51.83dB, an IL of 0.08dB, and an operating bandwidth of ∼61nm for ER > 30dB at the wavelength of 1.55µm. More importantly, the reflection of the unwanted polarization is suppressed to −12.6dB, which can be further lowered via additional design optimization. Our work points to a new direction for making better on-chip polarizers.

An ultracompact and highly flexible mode-order conversion scheme by exploiting reciprocal mode transitions is proposed and investigated in detail, where an inverse/asymmetric input taper and an asymmetric output taper are introduced at both ends of a subwavelength gratings (SWGs) waveguide, respectively, to form two mode transition configurations. For given input TEp mode, it is gradually transited into a single-mode-like (SML) component from the input taper to the SWGs waveguide and then this SML component is inversely transited into the desired TEq mode from the SWGs waveguide to the output taper, where p < q and effective indices of given and desired modes are matched. Thus, such a transited SML component can serve as an exchanging light between given TEp and target TEq modes with matched effective indices so that arbitrary and flexible mode conversions can be realized by only choosing appropriate dimension parameters for specific mode transitions, significantly simplifying the device design. Results show that TEp-to-TEq (p= 0, 1, 2, 3&q= 1, 2, 3, 4) converters can achieve a high conversion efficiency of 83.2 % in the worst-case at 1.55 m, where total device lengths are ranged from 4.1 to 14.9 m. Moreover, conversion efficiency of > 80 % and modal crosstalk of < -10 dB can be achieved over a broad bandwidth of 80 nm in the worst-case. Additionally, fabrication tolerances are analyzed and light propagation profiles are also demonstrated.