Supermode Interference Research Articles

Tuning of optical fiber laser based on super-mode interference in a seven-core fiber

Tuning of optical fiber laser emissions based on super-mode interference in a seven core fiber (SCF) is presented. This super-mode interference is generated by a super-mode Mach–Zehnder interferometer constructed with a few millimeters of SCF spliced between two single mode fibers (SMFs). In the SCF, two super-modes are exited via the fundamental mode of the SMF. Then, an interference pattern caused to the optical path difference between super-modes is generated at the final of the SCF, which is collected with the spliced SMF. This interference pattern induces and tunes laser line emissions by adjusting the laser cavity losses in an optical fiber laser. By bending the Mach–Zehnder interferometer, a wavelength shift of the interference pattern is induced, which in turn causes a wavelength shift of the laser cavity losses and, therefore, a displacement of the laser wavelength emission. Laser line emissions were tuned by linear factors of around 0.91 nm mm−1, and the switching of simultaneous emissions was also obtained. The stability and reliability of the laser emission on the proposed method are also discussed.

Temperature Sensing Characteristics of a Four-Core Sapphire Derived Fiber Based on Supermode Interference

A Mach–Zehnder interferometer (MZI) sensor based on a coupled four-core sapphire-derived fiber (FSDF) for temperature sensing is proposed and demonstrated. The coupled FSDF with a high refractive index (RI) difference between core and cladding can support four LP01-like supermodes (LP01s modes) and eight LP11-like supermodes (LP11s modes). We have fabricated and investigated two sensor samples with a sensing length of 3.1 cm (Sensor I) based on LP01s-LP11s mode interference and with a sensing length of 9.0 cm (Sensor II) based on LP01s-LP01s mode and LP11s-LP11s mode interference. The high-temperature behaviors of both sensors were studied after annealing at temperatures up to 900 °C. The experimental results show that the temperature sensitivity of Sensor I is around 70 pm/°C. For Sensor II, multiple supermode interferences are involved providing different sensitivities. The sensitivity based on the first-order LP01s and second-order LP01s interference is about 136 pm/°C, and the sensitivity based on the first-order LP01s and fourth-order LP01s mode interference is about 36 pm/°C. The sensitivity based on two LP11s mode interference achieves about 64 pm/°C. The experimental results are compared to theoretical simulation. The theoretical simulations give additional information on how the geometry of the coupled multicore fiber affects its temperature properties. Adjusting the core radius and the core pitch is shown to have a considerable influence on the achievable sensitivity and may even change the sign of the sensor sensitivity to temperature. Therefore, designing a specific geometric structure of the coupled fiber is beneficial to optimize the sensing characteristic of this type of sensor.

Seven-core fiber based in-fiber Mach–Zehnder interferometer for temperature-immune curvature sensing

An in-fiber Mach–Zehnder interferometer (MZI) based on a brief coreless fiber (CLF) and a section of seven-core fiber (SCF) sandwiched in single mode fiber (SMF) is proposed and demonstrated. A supermode interference is created in the SCF that forms interference spectrum to perceive external curvature changes. A linear enhanced sensitivity of -26.5517 dB/m−1 for temperature-immune curvature measurement within the curvature range of 0.527–1.395 m−1 is experimentally achieved. Moreover, the strain response of the MZI is experimentally investigated. This MZI exhibits accurate and high sensitivity for curvature detection by economic intensity detection, which indicates its potential applications in the engineering field.

Hybrid optical fiber Fabry-Perot interferometer for nano-displacement sensing

Nano-displacement sensing based on an extrinsic hybrid fiber Fabry-Perot interferometer is proposed and demonstrated. The lead-in fiber tip of such an interferometer consists of a strongly-coupled multicore fiber section fusion spliced to a single-mode fiber. The referred lead-in fiber tip is placed in front of a microscope slide, whose rear surface is coated with a high reflecting layer. The gap between the end-face of the fiber tip and the layer is composed of an air cavity in series with a glass one. Light exiting from the lead-in fiber tip is partially reflected at the fiber-air and air-glass interfaces and the reflecting layer generating three beams that are recoupled into the multicore fiber and combined with supermode interference. By making the optical path length of the air cavity slightly different from the glass one, it is possible to generate an envelope in the interference spectra with a larger period. Thus, by tracking the shift of such an envelope, displacements of 0.47 nm can be resolved. The nano-displacement sensing approach reported here is easy to implement; moreover, the sensitivity, resolution, and dynamic range can be reconfigured by an appropriate selection of the glass cavity.

High-Sensitivity Bending Sensor Based on Supermode Interference in Coupled Four-Core Sapphire-Derived Fiber

A Mach-Zehnder interferometer (MZI) sensor based on a four-core sapphire-derived fiber (FSDF) for bending sensing is proposed and demonstrated. The MZI is formed by splicing a coupled FSDF between two single-mode fibers, delivering supermode interference among four LP01-like supermodes (LP01s modes) and eight LP11-like supermodes (LP11s modes). Benefiting from the different response of the field distribution of each supermode to curvature changes, MZI sensors with a variety of bending properties can be fabricated based on the interference of different supermodes in the FSDF. The experimental results show that the sensitivity of the FSDF-MZI sensor I based on the interference between the LP01s and the LP11s modes is about 4.5 dB/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> in the curvature range from 0.4 to 2.5 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . The FSDF-MZI sensor II based on the interference of two LP11s modes can realize bending sensing in ± Y directions. The sensitivity of FSDF-MZI sensor II in the curvature range of -10.0-5.0 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> is about -24.526 nm/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . It suggests that the proposed FSDF-MZI sensor has potential application in the field of structure health detection.

Long-range multicore optical fiber displacement sensor.

In this Letter, a long-range optical fiber displacement sensor based on an extrinsic Fabry-Perot interferometer (EFPI) built with a strongly coupled multicore fiber (SCMCF) is proposed and demonstrated. To fabricate the device, 9.2 mm of SCMCF was spliced to a conventional single-mode fiber (SMF). The sensor reflection spectrum is affected by super-mode interference in the SCMCF and the interference produced by the EFPI. Displacement of the SMF-SCMCF tip with respect to a reflecting surface produces quantifiable changes in the amplitude and period of the interference pattern in the reflection spectrum. Since the multicore fiber is an efficient light collecting area, sufficient signal intensity can be obtained for displacements of several centimeters. By analyzing the interference pattern in the Fourier domain, it was possible to measure displacements up to 50 mm with a resolution of approximately 500 nm. To our knowledge, this is the first time that a multicore fiber has been used to build a displacement sensor. The dynamic measurement range is at least seven times larger than that achieved with an EFPI built with a conventional SMF. Moreover, the SMF-SCMCF tip is robust and easy to fabricate and replicate.

Observation of split evanescent field distributions in tapered multicore fibers for multiline nanoparticle trapping and microsensing.

The optical attractive force in tapered single-mode fibers (SMFs) is usually uniformly distributed around the tapered section and has been found to be important for trapping and manipulating targeted atoms and nanoparticles. In contrast, a peculiar phenomenon of the evanescent field splitting along the azimuth axis can be experimentally observed by tapering a weakly-coupled MCF into a strongly-coupled MCF to generate supermode interference. Moreover, the supermode interference produces a hexagonally distributed evanescent field and its six vertices give rise to the multiline optical attractive force. For such spectral resonances, the optimum extinction ratio for the transmission dips is given by 47.4 dB, this being determined using an index liquid to cover the tapered MCF. The resonant dips move to a greater extent at longer wavelengths, with the optimum tuning efficiency of 392 nm/RIU for index sensing. The split evanescent fields respectively attract the excited upconversion nanoparticles in the liquid to be linearly aligned and running down the tapered region over the fiber surface, emitting green light with 60° symmetry. The charged nanoparticles were periodically self-organized, with a period of around 1.53 µm. The parallel lines, with 60° rotational symmetry, can be useful for (1) indicating the exact locations of the side-cores or orientations of the tapered MCF; (2) as precision alignment keys for micro-optical manipulation; and (3) enhancing the upconversion light, or for use in lasers, coupling back to the MCF. The split evanescent fields can be promising for developing new evanescent field-based active and passive fiber components with nano-structures.

Room-Temperature Power-Stabilized Narrow-Linewidth Tunable Erbium-Doped Fiber Ring Laser Based on Cascaded Mach-Zehnder Interferometers With Different Free Spectral Range for Strain Sensing

An automatically power-stabilized (with power fluctuation <0.155 dB), narrow-linewidth (0.0171 nm), wavelength-tunable (10.69 nm) erbium-doped fiber laser has been proposed by cascading two fiber Mach–Zehnder interferometers (MZI) without using any temperature controlling device. One of the MZIs (here called the 1st MZI) is composed of two 3 dB couplers to form interference patterns while the other MZI (here termed the 2nd MZI) is constructed with a tapered seven-core fiber (SCF) and based on the principle of supermode interference. For the two MZIs, the free spectral range (FSR), the passband bandwidth and the extinction ratio (ER) at 1560 nm are 0.37 nm, 0.19 nm, 16.6 dB and 13.93 nm, 7.93 nm, 10.1 dB, respectively. Due to the major difference between the two FSR values, the 1st MZI and the 2nd MZI respectively play a role in controlling the laser linewidth and suppressing the homogeneous broadening effect to reach to a satisfactory level of power stability. The 2nd MZI is also used to fine tune the laser wavelength by applying strain to the tapered SCF (TSCF) over the spectral range of 1570.22–1559.33 nm, with an incremental step of 0.37 nm being used. The side-mode suppression ratio (SMSR) of the tunable fiber laser can be up to 45 dB. By appropriately adjusting the polarization controller, dual wavelength lasing can also be achieved. For single wavelength lasing, the 3 dB laser linewidth is 0.0171 nm. The power fluctuation, without a temperature controlling device being used and operating at room temperature, is found to be less than 0.155 dB over 1 hour while the central wavelength drift is less than 0.19 nm.

Multicore fiber temperature sensor with fast response times

A multicore fiber (MCF) based temperature sensor was fabricated and packaged into a stainless-steel tube to protect the bare-fiber, and it offers a commercial friendly form factor. The sensor displayed a periodic supermode interference pattern that red-shifts with increasing temperature, which was used to calibrate the sensor. Additionally, an economical photodiode-based interrogator was fabricated from off-the-shelf parts and used to calibrate the sensor in both liquid and gas mediums. The response time to changes in temperature for the MCF sensor was found to be an order of magnitude faster than a thermocouple of identical diameter, 0.09 s compared to >1.0 s, respectively.

Wavelength-tunable all-fiber mode-locked laser based on supermode interference in a seven-core fiber.

A wavelength-tunable all-fiber mode-locked erbium-doped fiber laser has been proposed and realized by using a supermode interference filter (SMIF). The SMIF is fabricated by splicing a segment of seven-core fiber (SCF) to two standard single-mode fibers. Since two supermodes of the propagating light are excited in the SCF, the transmission spectrum of the SMIF shows a clean broadband comb-shape characteristic. By bending the SMIF in the proposed mode-locked laser, the output spectrum can be continuously tuned in a wavelength range up to 22nm while keeping mode-locking operation. The self-starting laser produces 230fs pulses with a spectral width of 14nm.

Accurate strain sensing based on super-mode interference in strongly coupled multi-core optical fibres

We report on the use of a multi-core fibre (MCF) comprising strongly-coupled cores for accurate strain sensing. Our MCF is designed to mode match a standard single mode optical fibre. This allows us to fabricate simple MCF interferometers whose interrogation is carried out with light sources, detectors and fibre components readily available from the optical communications tool box. Our MCF interferometers were used for sensing strain. The sensor calibration was carried out in a high-fidelity aerospace test laboratory. In addition, a packaged MCF interferometer was transferred into field trials to validate its performance under deployment conditions, specifically the sensors were installed in a historical iron bridge. Our results suggest that the MCF strain sensors here proposed are likely to reach the readiness level to compete with other mature sensor technologies, hence to find commercial application. An important advantage of our MCF interferometers is their capability to operate at very high temperatures.

Compact fiber-optic curvature sensor based on super-mode interference in a seven-core fiber.

A compact, low loss, and highly sensitive optical fiber curvature sensor is presented. The device consists of a few-millimeter-long piece of seven-core fiber spliced between two single-mode fibers. When the optical fiber device is kept straight, a pronounced interference pattern appears in the transmission spectrum. However, when the device is bent, a spectral shift of the interference pattern is produced, and the visibility of the interference notches changes. This allows for using either visibility or spectral shift for sensor interrogation. The dynamic range of the device can be tailored through the proper selection of the length of the seven-core fiber. The effects of temperature and refractive index of the external medium on the response of the curvature sensor are also discussed. Linear sensitivity of about 3000 nm/mm(-1) for bending was observed experimentally.

Optimization of multicore fiber for high-temperature sensing.

We demonstrate a novel high-temperature sensor using multicore fiber (MCF) spliced between two single-mode fibers. Launching light into such fiber chains creates a supermode interference pattern in the MCF that translates into a periodic modulation in the transmission spectrum. The spectrum shifts with changes in temperature and can be easily monitored in real time. This device is simple to fabricate and has been experimentally shown to operate at temperatures up to 1000°C in a very stable manner. Through simulation, we have optimized the multicore fiber design for sharp spectral features and high overall transmission in the optical communications window. Comparison between the experiment and the simulation has also allowed determination of the thermo-optic coefficient of the MCF as a function of temperature.

Vertical coupling between gap plasmon waveguides

This work examines vertical coupling between gap plasmon waveguides for use in high confinement power transfer and power splitting applications at 1.55 microm free space wavelength. The supermode interference method is used to obtain key coupler performance parameters such as coupling length, extinction ratio, net coupled output power, radiated power, and reflected power as a function of waveguide center-to-center spacing, core refractive index, and gap width. The initial power distribution among the two coupler supermodes is obtained via the mode matching method for a single input waveguide feed. Excellent agreement with three-dimensional finite difference time domain simulations is observed for the case of square 50 nm gaps with core refractive indices of 2.50 and a center-to-center spacing of 112 nm. Local maxima in the net coupled output power are found to coincide with local minima in the coupling length. An increase in the core refractive index from 1.00 to 2.5 increases the local maximum net coupled output power from 6.4% to 49% but decreases the extinction ratio from 12.7 to 6.94. A sweep of the width of the core from 25 to 100 nm increases the net coupled output power from 43.7% to 52.0%, but increases the coupling length from 1.58 to 3.19 ???m and decreases the extinction ratio from 7.39 to 6.57.