Cascaded Sagnac Loop Research Articles

High Extinction Ratio and Low Temperature Cross-Sensitivity Strain Sensor Based on Vernier Effect Through Hybrid Cascaded Sagnac Loop

Low extinction ratio (ER) and high temperature cross-sensitivity are serious but common problems for most strain sensors based on Vernier effect. In our work, we propose a strain sensor based on Vernier effect. It consists of hybrid cascaded Sagnac loop, one part is a microfiber coupler Sagnac loop (MFC–SL), and the other is a Sagnac loop connected to a Section of polarization-maintaining photonic crystal fiber (PMPCF–SL). Different from the traditional Vernier effect, in the proposed sensor, the spectra of two interferometers shift to opposite directions when the strain changes, which belongs to the enhanced Vernier effect. When ambient temperature varies, the spectra of the two interferometers shift to the same direction, which belongs to the reduced Vernier effect. Thus, its strain sensitivity is improved and its temperature cross sensitivity is reduced. In addition, the MFC–SL and PMPCF–SL exhibit a high fringe ER due to their good transmission of two orthogonal modes. Experimental show that the sensitivity of strain is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- 124.17\,\,\text {pm}/\mu \varepsilon $ </tex-math></inline-formula> , while the temperature sensitivity is only <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$6.13~\text {pm}/^{\circ }\text{C}$ </tex-math></inline-formula> , and the ER of envelope is about 9 dB. Its robustness can be enhanced by the method of packaging with silicone adhesive. It will have a potential application in the field of strain sensing.

Tunable and switchable multi-wavelength random distributed feedback fiber laser based on SMF and a cascaded Sagnac ring filter.

A tunable and switchable multi-wavelength random distributed feedback fiber laser based on cascaded Sagnac loops is proposed and experimentally demonstrated. The random distribution feedback of the laser is provided by the Rayleigh scattering generated by the single-mode fiber (SMF). The cascaded Sagnac loops act as a filter and a reflector in the half-open cavity laser. The single-, dual-, three-, and four-wavelength channels can be realized by adjusting the angle of the polarization controller at the pump power of 300 mW. In the single-, dual-, and three-wavelength channels, the wavelength spacing can be maintained, and the laser wavelength position can be changed at the same time. The maximum wavelength tuning ranges of single-, dual-, and three-wavelength outputs are about 4.5 nm, 2.6 nm, and 1 nm, respectively. The proposed multi-wavelength random fiber laser has the advantages of simple structure and low threshold, and it has good application prospects in remote sensing and imaging systems.

Switchable Multi-Wavelength Thulium-Doped Fiber Laser Using a Cascaded or Two-Segment Sagnac Loop Filter

A switchable multi-wavelength thulium-doped fiber laser (TDFL) using either two-stage cascaded or two-segment Sagnac loop filters are proposed and analyzed. Both filters incorporate a 3-m and 1.7-m long polarization-maintaining fiber, which acts as a comb filter. By adjusting two polarization controllers, when the TDFL used a cascaded Sagnac loop filter, eight stable single-wavelength operations were obtained with easy switching among them. The wavelength range was 16.36 nm during the single-wavelength switchable operation, with the optical signal-to-noise ratio up to 55 dB. This TDFL can achieve at most quintuple-wavelength operation. When the TDFL used a two-segment Sagnac loop filter, we achieved single-wavelength and dual-wavelength switchable. This TDFL can achieve at most stable quadruple-wavelength operation. In dual-wavelength switchable operation, the maximum and minimum wavelength spacings were 21.78 nm and 1.50 nm. Due to its abundant range of wavelengths, the proposed TDFLs has great potential in wavelength-division multiplexing.

Sensitivity enhanced temperature sensor with cascaded Sagnac loops based on harmonic Vernier effect

A sensitivity enhanced temperature sensor with cascaded Sagnac loops based on harmonic Vernier effect has been proposed and verified by experiments. The harmonic Vernier effect can be achieved, through controlling the polarization-maintaining fiber (PMF) length of one Sagnac loop to be an integral multiple of the other with a small deviation. The experimental results show that in temperature range of 30–70 °C, the temperature measurement sensitivity of the proposed sensor device based on the harmonic Vernier effect is 3.66 nm/°C, which is 22.5 times that of the single Sagnac loop (0.163 nm/°C), and 1.7 times that of the sensor based on the basic Vernier effect (2.12 nm/°C). Thanks to the advantages of the proposed sensing structure’s high sensitivity and full fiber configuration, it can be used in various specific fields, such as power grids and mines.

Temperature Sensor Based on Fiber Ring Laser With Cascaded Fiber Optic Sagnac Interferometers

A new temperature fiber ring laser (FRL) sensor based on a cascaded Sagnac loops fiber structure is proposed and experimentally demonstrated. The optical FRL sensor consists of cascaded Sagnac loops by inserting two polarization-maintaining optical fibers (PMF) with slightly different lengths. PMF with length of 56 cm and 75 cm are used in Sagnac loops as a filter and sensing unit in laser cavity. The working principle of the sensor is based on the phase shift (θ) caused by birefringence between two principal polarization modes and enhance the sensitivity by constructing a Vernier-scale. In an appropriate temperature range (25 °C - 31 °C), the detection sensitivity of FRL sensor based on a cascaded Sagnac structure is significantly higher than other FRL sensors. Thanks to the laser sensing system, the sensitivity can be modulated by changing the free spectral range (FSR). The experimental results show that the temperature sensitivity of the cascade Sagnac structure sensor is - 4.031 nm / °C, which is five times higher than that of FRL sensor base on the single Sagnac structure.

Reconfigurable silicon bandpass filters based on cascaded Sagnac loop mirrors.

We demonstrate a high-performance reconfigurable bandpass filter implemented by cascaded Sagnac loop mirror (SLM)-based coupled resonator optical waveguides (CROWs) on the silicon-on-insulator platform. By dynamic thermal tuning of the reflectivity in each SLM, the proposed filter can achieve simultaneous 3 dB bandwidth tuning from 8.50 to 20.25 GHz and a central wavelength tuning range of 216.25 GHz. A box-like filtering response with an ultra-high extinction ratio up to 70 dB and an ultra-sharp roll-off of 0.61 are observed in a 6th-order SLM-coupled resonator optical waveguide (SLM-CROW). The proposed reconfigurable SLM-CROW filter can satisfy the demand for next-generation flexible-grid WDM networks.

Cascaded Sagnac Loops Embedded With Two Polarization Maintaining Photonic Crystal Fibers for Highly Sensitive Strain Measurement

A strain sensor based on cascaded Sagnac loops with two polarization-maintaining photonic crystal fibers (PMPCFs) has been experimentally demonstrated. Air holes distributed on the cross section of PMPCF go through the whole fiber, which makes the special fiber more sensitive to ambient force compared with conventional optical fiber. The lengths of the two PMPCFs are close, but not the same, to form envelopes on the total output spectrum and then obtain high resolution to the detected parameter by the Vernier effect. The influences of the difference in length of the two PMPCFS on spectral envelopes are discussed. As the length of the PMPCF in reference loop increases, free spectrum range of the cascaded Sagnac loops (or envelope period) and magnification decrease. The sensitivities of this strain sensor are calculated by tracking upper and lower envelopes. Results reveal that its average sensitivity is up to 45.15 pm/με and the corresponding resolution is 0.44 με as strain varies from 0 to 2000 με. Compared with a single Sagnac loop, the sensitivity magnification is about 28.94. The sensing characteristics of a conventional polarization-maintaining fiber (PMF) are studied to compare with the PMPCF. Moreover, reversibility has also been proved to possess low error rate. In short, the fiber strain sensor possesses high sensitivity and great reversibility.

Tunable and Switchable Multi-Wavelength Random Distributed Feedback Fiber Laser Based on Smf and Cascaded Sagnac Loops Filter

Tunable and Switchable Multi-Wavelength Random Distributed Feedback Fiber Laser Based on Smf and Cascaded Sagnac Loops Filter

Scale factor improvement for angular velocity measurement based on an optoelectronic oscillator

A scheme of angular velocity measurement with an improved scale factor is proposed and experimentally demonstrated based on the joint operation of a Sagnac loop and an optoelectronic oscillator (OEO). In this scheme, the Sagnac-induced phase difference is mapped into the oscillating frequency shift of the OEO with a large scale factor by making one of the two counter-propagating signals in the Sagnac loop carrier-suppressed modulated and the other just travel through the modulator. The cascaded Sagnac loop and OEO structure can further improve the scale factor by setting a long Sagnac loop and a short OEO loop independently. A sensitivity scale factor as high as 742kHz/(rad/s) is obtained in our experiment.

Experimental research on an L-band multi-wavelength erbium-doped fiber laser based on a cascaded Sagnac loop and M–Z filters

In this paper, we proposed and experimentally demonstrated an L-band multi-wavelength erbium-doped fiber (EDF) laser with a cascaded Sagnac loop, M–Z interferometer filters and a non-linear optical loop mirror (NOLM). The NOLM acted as the intensity-dependent transmissivity; it can effectively alleviate the mode competition and beating caused by homogeneous broadening. The transfer characteristic of the Sagnac loop was investigated firstly and got the optimized parameters. Then the laser output spectrum with different kinds of filters in the cavity was experimentally tested when the pump power was fixed at 450 mW. The experimental results showed that the EDF laser with the cascaded filters in the cavity had a better performance, such as a better flat lasing bandwidth within 3 dB, higher side-mode-suppression ratio and more comb lines generated. The peak power fluctuation was less than 0.35 dB within a 30 min monitor time.

Fabry–Perot cavity cascaded sagnac loops for temperature and strain measurements (Erratum)

Fabry–Perot cavity cascaded sagnac loops for temperature and strain measurements (Erratum)

Fabry–Perot cavity cascaded sagnac loops for temperature and strain measurements

The fabrication process and temperature and strain characterizations of an all-fiber sensor are presented. The sensing structure based on a Fabry–Perot cavity (FPC) and sagnac loops was proposed and experimentally demonstrated for measurements of temperature and strain. The FPC consists of a micropiece of chemical etched multimode fiber end face, welded with another single mode fiber. Then, the sagnac loops composed of polarization maintaining fiber was connected to the FPC. The sensor was fabricated and tested for temperature and strain. Experimental results show that sensitivity of temperature and strain is 0.71 ± 0.03 nm / ° C and 1.30 ± 0.01 pm / μϵ, respectively; the linearity are 0.9970 and 0.9996, respectively.

Advanced photonic filters based on cascaded Sagnac loop reflector resonators in silicon-on-insulator nanowires

We demonstrate advanced integrated photonic filters in silicon-on-insulator (SOI) nanowires implemented by cascaded Sagnac loop reflector (CSLR) resonators. We investigate mode splitting in these standing-wave (SW) resonators and demonstrate its use for engineering the spectral profile of on-chip photonic filters. By changing the reflectivity of the Sagnac loop reflectors (SLRs) and the phase shifts along the connecting waveguides, we tailor mode splitting in the CSLR resonators to achieve a wide range of filter shapes for diverse applications including enhanced light trapping, flat-top filtering, Q factor enhancement, and signal reshaping. We present the theoretical designs and compare the CSLR resonators with three, four, and eight SLRs fabricated in SOI. We achieve versatile filter shapes in the measured transmission spectra via diverse mode splitting that agree well with theory. This work confirms the effectiveness of using CSLR resonators as integrated multi-functional SW filters for flexible spectral engineering.

Compact tunable photonic comb filter on a silicon platform.

We propose and demonstrate a compact wavelength- and bandwidth-tunable photonic comb filter on a silicon platform. It is a Fabry-Perot (FP) cavity fabricated on silicon-on-insulator, which is composed of two cascaded Sagnac loop mirrors (SLMs) with multimode-interferometer-assisted Mach-Zehnder interferometer (MZI) couplers. The effective length of the FP cavity and the reflectivity of the SLMs can be dynamically changed based on thermal-optical effects by tuning three phase shifters along the cavity length and MZI arms, leading to center wavelength tuning and bandwidth tuning of the comb filter. Three tuning cases are investigated. By independently or simultaneously tuning three microheaters, center wavelength tuning with a tuning efficiency of ∼0.017 nm/mW and bandwidth tuning from 4.37 to 27.6GHz are achieved in the experiment.

Micro-ring resonator quality factor enhancement via an integrated Fabry-Perot cavity

We propose and experimentally demonstrate the enhancement in the filtering quality (Q) factor of an integrated micro-ring resonator (MRR) by embedding it in an integrated Fabry-Perot (FP) cavity formed by cascaded Sagnac loop reflectors. By utilizing coherent interference within the FP cavity to reshape the transmission spectrum of the MRR, both the Q factor and the extinction ratio (ER) can be significantly improved. The device is theoretically analyzed and practically fabricated on a silicon-on-insulator wafer. Experimental results show that up to 11-times improvement in the Q factor, together with an 8-dB increase in the ER, can be achieved via our proposed method. The impact of varying structural parameters on the device performance is also investigated and verified by the measured spectra of the fabricated devices with different structural parameters.

Tunable multi-wavelength thulium-doped fiber laser incorporating two-stage cascaded Sagnac loop comb filter

A tunable multi-wavelength narrow-linewidth thulium-doped fiber laser employing two-stage cascaded Sagnac loop mirrors is proposed and experimentally demonstrated. The designed fiber laser is composed of a pump source, wavelength division multiplex, circulator, thulium-doped fiber, polarization controllers (PCs), couplers and polarization-maintaining fibers (PMFs). Two cascaded Sagnac loops are used as the cavity reflector and filter, and the proposed filter is fabricated using two sections of PMFs with 2-m and 1-m lengths, respectively. In the experiment, the laser threshold is 110 mW, and laser can emit single, double, triple, quadruple and quintuple wavelengths in the spectral range of 1873–1901 nm through the simultaneous adjustment of the two PCs. The power fluctuations and 3-dB linewidth are less than 2.1 dB and 0.2 nm, respectively, over 10 min at room temperature, and the side-mode suppression ratio is greater than 20 dB. The proposed laser will be useful in various fields, such as spectral analysis, fiber sensing and optical communication.

Tunable dual-wavelength fiber laser based on an MMI filter in a cascaded Sagnac loop interferometer

A widely tunable dual-wavelength erbium-doped fiber laser based on a cascaded Sagnac loop interferometer incorporating a multimode interference filter is proposed and experimentally demonstrated in this paper. The mode selection is implemented by using the cascaded Sagnac loop interferometer with two segments of polarization maintaining fibers, and the wavelength tuning was achieved by using the refractive index characteristic of multimode interference effects. The tunable dual-wavelength fiber laser has a wavelength tuning of about 40 nm with a signal-to-noise ratio of more than 50 dB.

High-Extinction-Ratio Multipassband Filter With Flat-Top and Low-Dispersion

This paper proposes a novel structure to realize a high-extinction-ratio multipassband filter (HERMPF) with flat-top, low dispersion, and large passband and stopband bandwidths. Spectrum and chromatic dispersion of the HERMPF are given under ideal conditions, which show that such a filter has excellent performance. Analysis on fabrication tolerances is given as well, focusing on factors such as the length mismatch and loss which degrade the spectral and chromatic dispersion performances of HERMPF in practice. A single-cavity ring resonator is inserted into either arm of a Mach-Zehnder interferometer to form a Mach-Zehnder ring resonator interferometer (MZRRI), which is an infinite-impulse- response half-band filter with two complementary channels (even and odd channels). MZRRI is the core element of HERMPF which determines the spectral performance. A set of multicavity Gires-Tournois etalon composed of cascaded Sagnac loop mirrors is proposed as the tunable dispersion compensator and fully reflective mirror in HERMPF. The third main part of HERMPF is a 1 × 2 switch, which is used to select the even/odd channel of MZRRI. Thus, HERMPF can be used as an even/odd channel switch.

Design of flat-top interleaver and tunable dispersion compensator using cascaded Sagnac loop mirrors and ring resonators

We propose multicavity reflective Gires-Tournois etalons (MCR-GTE) and generalized multicavity transmissive Gires-Tournois etalons (GMCT-GTE) composed of cascaded Sagnac loop mirrors and ring resonators, respectively. Michelson-Gires-Tournois interferometers (MGTI) based on two sets of MCR-GTEs are theoretically studied. As the focal point, we demonstrate the GMCT-GTE, which is a reciprocal and transmissive element as a tunable dispersion compensator (TDC) for the proposed interleaver. The reciprocal and transmissive TDC is superior to reflective ones in terms of saving the number of TDCs. Only one set of TDCs is sufficient for the two output ports of the MGTI proposed. Discussions on fabrication tolerances are given as well, focusing on two main factors that degrade the performance of the interleaver and its TDC in practice, i.e., the length mismatch and loss.