Magnetic fluid (MF), also called ferrofluid, is a kind of stable colloid which consists of magnetic nanoparticles decorated with surfactant and evenly dispersed in the carrier liquid. When the magnetic field is applied onto the MF, the magnetic nanoparticles will form into chain-like clusters along the applied magnetic field, which could be controlled by the magnetic field intensity. Hence, the MF would exhibit a good variety of magneto-optical properties including tunable refractive index, tunable transmittance, tunable birefringence, dichroism and tunable photonic crystal structure. By utilizing these intriguing properties, some previous work have been proposed on novel magneto-optical devices with high performances:
Thanks to the Ph.D. Candidate Research Innovation Fund of Nankai University, we continue our research on the MF based tunable devices, based on the above works. Firstly, we investigated the tuning mechanism of MF based devices including the directly and indirectly tuning mechanism. For the directly tuning mechanism, the light is directly injected into the MF. The tuning properties can be outlined by the Transfer matrix method which can be expressed as:
This mechanism is suitable to analyze the MF thin film based devices such as a MF filled fiber F-P cavity. For the indirectly aspect, the fiber is often immersed in the MF. In this case, the magneto-optical properties of the MF would influence the coupling between the propagating modes in the fiber and their phase. Hence, the spectral properties would change accordingly. The coupling efficient and the phase can be respectively expressed as
. By tuning the RI of the MF, the mode coupling and mode phases would affected. Then, the magneto-optical devices is achieved.
Based on the above analysis, we have proposed some MF based fiber devices with high performance by utilizing the tunable RI property of the MF:
Besides these magneto-optical properties, the MF has the properties of high absorption and thermo-optical effect in the meanwhile. Due to the locality of the laser power, the laser-heated region of the MF will exhibit large refractive index variation. This laser-induced thermal effect of the MF could be utilized to develop all-optically controlled functional devices for applications in future all-optical networks. Some previous works on this project have also been conducted.
In this section, I will introduce the works accomplished in the funding project in detail and give some brief introduction about our follow-up works.
In this work, a magnetically controllable wavelength-selective fiber coupler for WDM applications has been proposed. Its wavelength tunability is achieved by integrating the fiber coupler with the MF. Figure 1 shows the Schematic diagram of the proposed magnetically controllable WDM coupler.
Fig. 1 Schematic diagram of the proposed magnetically controllable WDM coupler
It is fabricated by a homemade fiber coupler immersed into the MF (EMG605, Ferrotec) though a capillary. The fiber coupler is fabricated using a fiber taper machine (produced by E-Otron, Shanghai, China). The coupler is held straightly to avoid the leakage of the MF and then the capillary is sealed with the fiber coupler at both ends using paraffin. Figure 2 shows the Wavelength-dependent splitting ratio of the magnetically controllable WDM which indicates that the band-rejection channel and band-pass channel have move towards the longer wavelength.
Fig. 2 (a), (b) Wavelength-dependent splitting ratio of the magnetically controllable WDM
Fig. 3 Wavelength and Splitting ratio as functions of the applied magnetic field intensity for the four selected channels
The dependences of the selected channel wavelengths on the applied magnetic field intensity have been experimentally investigated, as illustrated in Fig. 3. For band-rejection channels, as the applied magnetic intensity increases from 0 Oe to 500 Oe, the channel wavelength moves from 1069.9 nm to 1080.3 nm for Channel 1and its SR varies from 0.349% to 0.766%. And for Channel 3, the channel wavelength increases from 1400.5 nm to 1417.9nm and its SR increases from 0.283% to 5.6%. The respectively wavelength of the band-pass channels increase by 13.6 nm and 24 nm within an applied magnetic field intensity range from 0 Oe to 500 Oe for Channel 2 and 4.
Our proposed magnetically controllable WDM coupler has high channel wavelength tunability and splitting ratio with high isolation, which ensures its applicability for potential applications in fiber laser and fiber-optic communications systems, as well as fiber sensing occasions. Furthermore, our proposed schemes also support the magnetically controllable mode division multiplexing by using the MF-integrated fiber coupler.
Citation: W. Lin, H. Zhang, B. Song, Y. Miao, B. Liu, D. Yan, and Y. Liu, “Magnetically controllable wavelength-division-multiplexing fiber coupler”, Optics Express 23(9), 11123-11134, 2015. (SCI: CH9EB; EI: )
In this work, we have demonstrated a magnetic field sensor based on the fiber taper coupler coated with Magnetic fluid. The proposed sensor is fabricated by immersing a fiber taper coupler into the Magnetic fluid and then sealing it with the paraffin. Figure 4 shows the experimental set of the proposed sensor.
Fig. 4 Schematic diagram of the proposed magnetic field sensor
Figure 5 and 6 show the responses to the magnetic field intensity, which indicates that the sensor exhibits high response as a function of the magnetic field with sensitivities of 0.154 nm/Oe with measurement range from 50 Oe to 200 Oe and -0.06301 dB/ Oe from 75 Oe to 200 Oe.
Fig.5 Transmission spectra of the Port D as a function of the magnetic field intensity
Fig.6 (a) Wavelength response of the 4 selected dips and (b) transmission response of the 4 peaks versus the magnetic field intensity
Owing to the advantages of high sensitivity, small footprint, and ease of fabrication, the proposed sensor would find potential applications in magnetic field sensing field.
Citation: W. Lin, H. Zhang, B. Song, B. Liu, Y. Miao and Y. Liu, “Magnetic field sensor based on fiber taper coupler coated with Magnetic fluid”, OFS2015 24th International Conference on Optical Fiber Sensors, Proc. of SPIE, 9634 96347U-1~4, 2015.
In this work, we propose and experimentally demonstrate an all-optically tunable filter based on a fiber taper coupler immersed in the MFs by exploiting the laser-induced thermal effect. Figure 7 shows the schematic illustration of the optically tunable fiber coupler.
Fig. 7. A schematic illustration of the optically tunable fiber coupler; SBS: super-continuum broadband source.
The central operation wavelength of the filter could be tuned by controlling the pump laser power. The Output transmission spectra under different pump current is shown in fig. 8.
Fig.8. Output transmission spectra under different pump currents; figure (a) and (b) correspond to current increase and decrease cases, respectively.
Fig. 9. Spectral notch wavelength as a function of input pump current
Figure 9 shows the spectral notch wavelength as a function of input pump current. It is obvious that the notch wavelength shift behavior is reversible after a full measurement cycle. And the central wavelength of the spectral notch exhibits the nonlinear behavior as function as the pump power.
Owing to its several advantages such as compactness, ease of fabrication, and high tunability, the proposed tunable filter is anticipated to find potential applications in fiber laser and amplification, fiber-optic sensing as well as optical fiber communications applications. And moreover, the all-optical manipulation approach of our proposed fiber filter makes it a promising functional component for future all-optical networks.
Citation: W. Lin, B. Liu , H. Zhang , B. Song , D. Yan , Y. Miao , H. Liu, and Y. Liu, “Laser-induced thermal effect for tunable filter employing Ferrofluid and fiber taper coupler”, IEEE Photonics Technology Letters, 27(22), 2339-2342, 2015. (SCI: EI:)
The MF coated fiber fused coupler can act as an energy allocator which can control the output of the fiber ring laser and the spectral response of the Sagnac interferometer filter. The following figure gives the magnetic field intensity dependence of the output power.
Fig. 10 (a) Central wavelength of the two peaks under different magnetic field intensity and (b) their wavelength spacing
Figure 11 shows the spectral response of the MF-coated-fiber-coupler-based Sagnac interferometer filter.
Fig. 11 Experimental setup and spectral response of the proposed Sagnac interferometer filter
These works demonstrate the good performance in system energy administration as well as signal processing and indicates that the MF coated fiber coupler would have potential applications in all fiber optical system.
After this project is completed, I also continue doing research on the MF based fiber devices. The previous works on theoretical analysis and preparation of the MOF filled with liquid having refractive index close to the silica background have been also conducted. The work have been published in Optics Communications. And its application in simultaneous measurement of temperature and axial force has also been demonstrated, which is published in Applied Optics. Recently, we utilized the laser absorption of the MF filled in the MOF to control the whispering gallery modes in MOF. The wavelength response of the resonance dips exhibit linear power dependence, and owing to such desirable merits as ease of fabrication, high sensitivity and laser-assisted tunability, the proposed optical tuning approach of WGMs in the MFIMOF would find promising applications in the areas of optical filtering, sensing, and signal processing, as well as future all-optical networking systems. The abovementioned work has been published in Scientific Reports.