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Abstract
We propose and investigate an all-fiber thermo-optic modulator based on a side-polished twin-core fiber (TCF) Michelson interferometer (MI) coated with NaNdF4 nanoparticles. The MI was fabricated by tapering the splicing point between the TCF and a single mode fiber (SMF). A short suspended core fiber (SCF) is spliced to one core of the TCF to introduce a fixed optical phase difference (OPD). The side-polished core is coated with photo-thermal material NaNdF4. Owing to the ohmic heating of NaNdF4 nanoparticles under 808 nm pump laser, the effective refractive index of the polished core is changed, resulting in a phase shift of the MI. The MI has a significant modulation phase shift with 2.9 π near the wavelength of 1260 nm and can obtain an optical switching with a rise (fall) time of 152 (50) ms. The proposed device will have a great application potential in optical modulators due to compact structure and strong robustness.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical modulators play a significant role in modifying the frequency, phase, amplitude and polarization of the propagating beam [1–4]. They are crucial in communication systems, interference sensing devices and photodetectors. Specifically, all-fiber modulators, utilizing optical nonlinear effects such as optical Kerr effect, stimulated Brillouin scattering and self-phase modulation [5–7], are more desirable in optical systems due to the advantages of large capacity, low loss and anti-electromagnetic interference. However, weak nonlinear effect of optical fiber limits the modulation depth [8,9], thus a variety of high nonlinear optical materials are exploited in optical modulators. According to unique optical characteristics of nonlinear materials, modulators can be divided into electro-optic, magneto-optic, thermo-optic and acousto-optic modulators [10–13]. Especially, thermal-optic modulators are widely used due to convenient integration of thermal-optic materials and easy control of thermal effect. According to the literatures, thermal-optic materials include transition metal dichalcogenides (TMDs) [14,15], graphene [16], Au nanorods [17,18] and so on. Recently, as the development of nanotechnology, rare-earth nanoparticles have attracted widespread attention. Owning to abundant energy levels structure of rare earth ions, they have excellent luminescence properties, which make the rare-earth nanoparticles have a broad application prospect ranging from biological imaging, drug delivery, sensors to data storage [19,20]. Compared to other materials mentioned above, NdNaF4 [21,22], a kind of rare earth doped nanoparticles, has an extremely high photo-thermal conversion efficiency, with a size of approximate 10 nm and simple structure, thus it is promising as a new photo-thermal phase modulation material.
In order to optimize the performance of the modulators via improving the slope of the modulation response, various structural fibers integrated with special photo-thermal materials were proposed, for instance, D-shaped fiber, microfiber and tapered fiber [23–25]. Recently, Gan et al. demonstrated an all-fiber phase shifter based on Mach-Zehnder interferometer (MZI), in which a graphene-coated microfiber was used as the sensing arm, and obtained a phase shift exceeding 21π with a nearly linear slope of 0.091 π/mW [26]. Wu et al. proposed an all-optical phase shifter in WS2 covered tapered fiber and achieved the maximum phase tuning range 6.1 π with a slope efficiency of 0.0174 π/mW [27]. Wang et al. proposed an efficient graphene assisted microfiber resonator with a resonant wavelength shift rate of 71 pm/mW [28]. Since all of the aforementioned structures resort to two separate optical fibers, the sizes of the devices are large. In addition, microfiber or tapered fiber based devices have high sensitivities, however, they have poor mechanical strength and weak robustness due to their small diameters, which limits their practical applications.
In this work, we present an ultra-compact all-fiber thermo-optic modulator based on a side-polished twin core fiber (TCF) Michelson interferometer (MI) coated with NdNaF4 nanoparticles. Two cores of the TCF act as the reference and sensing arms, respectively. A short suspended core fiber (SCF) is spliced to one core of the TCF to introduce a fixed optical phase difference (OPD). Two reflected beams from the fiber end form the MI. Due to the high photo-thermal effect of NdNaF4 nanoparticles coated on the side-polished core, the effective refractive index difference between two cores will change under 808 nm pumping, as a result it causes the shift of MI peak. The experimental results demonstrate that a maximum phase shift exceeding 2.9 π with a slope of 0.00568 π/mW is achieved. The size of the modulator can be greatly reduced due to its stable OPD from a short piece of the SCF. Besides, the proposed structure is robust enough to accomplish all-fiber thermo-optic modulation.
2. Structure and preparation of modulator
The schematic of the proposed phase modulator is shown in Fig. 1(a), and the MI consists of SMF, side-polished TCF and SCF. The cross sections of the TCF and SCF are shown in Figs. 1(b) and 1(e). The TCF contains two homogeneous cores that are symmetrically located in the cladding. The cladding diameter of the TCF is 125 μm, the diameters of both the two cores approach 8.9 μm and the distance between the two cores is 64 μm. The effective refractive index of the core is 1.448 at the wavelength of 1550 nm, and the index difference between the core and the cladding is 0.004. The diameter of the air hole, cladding and the SCF core are 88, 125, and 9 μm, respectively. The core of the SCF suspends on the cladding wall.
Fig. 1. (a) The 3D schematic of the phase modulator. (b) The cross section of the TCF. (c) The tapered area. (d) Cross section of the polished TCF. (e) The cross section of the SCF. (f) The monitored intensities of the two cores.
The SMF and TCF are spliced without any lateral offset between the claddings by using a fusion splicer in auto-alignment mode. The optical fiber is tapered near the splicing point by adjusting the splicing current appropriately. The image of the tapered area is shown in Fig. 1(c). The tapered area acts as a beam splitter, and light from the SMF can be coupled into two cores of the TCF with 1:1 power ratio because the two cores have the same size and are also symmetrically distributed in the cladding [29]. Two reflected beams from the end faces of the TCF interference again through the tapered area to form the MI. The discharge duration time, the current of the fusion splicer and the overlap length are set to be 30 ms, 45 bits and 5 μm, respectively. In order to obtain a high extinction ratio in the interference spectrum, the infrared camera (SU320CSX) is used to real-time monitor the power distribution in two cores during the tapering process as shown in Fig. 1(f). The tapering process is stopped until the light intensities in the two cores reach the maximum values and are approximately identical with each other.
In order to enhance the evanescent field of the fiber core to improve the sensitivity to the external environment, the TCF is side-polished and the polished core is exposed as the modulation arm. During the polishing process, the plane with the two cores is adjusted under the microscope until it is perpendicular to the axial direction of the rotating wheel of the side-polishing machine. Meanwhile, the pine oil is dripped onto the TCF for achieving a clear view of the core position. In order to avoid the fiber broken by the roller, two weights are hung on the fiber next to the supporting clamps to buffer the stress during the polishing. A CCD is used to monitor the diameter of the optical fiber in real time. The cross section of the polished TCF is displayed in Fig. 1(d). The polishing depth d is defined as 0 μm when the core is exactly polished. In our experiment, the polishing length L1 is 2 mm, and the polishing depth d is -4.5 μm, that is, the fiber core is polished to only half of the original diameter. Firstly, rough sandpaper (800-mesh) is used for pre-polishing, before the desired polishing depth is approaching, fine sandpaper (2000-mesh) will be adopted in the rest of the process to make the polished area smoother. The loss is about 3 dB when the polishing depth d is -4.5 μm and the polishing length L1 is 2 mm.
The OPD of the MI can be obtained by bending the TCF. However, in order to obtain a larger OPD as well as small free spectral range (FSR), longer TCF and smaller bending radius are required. Furthermore, a slight perturbation in the bent fiber will lead to the change of the interference spectrum of the MI, correspondingly it will bring a large error to the accuracy of the phase modulation. In order to introduce a stable OPD, a short piece of SCF (L2) is spliced to one core of the TCF by splicer machine (Fujikura, ARC master FSM-100P+). The two cores of the TCF are named as core 1 and core 2, respectively. Adjusting the relative space position between the TCF and the SCF, core 1 of the TCF is precisely aligned and welded with the suspended core of the SCF, while core 2 is placed in the air hole of the SCF. Therefore, the optical path of core 1 is extended and longer than that of core 2, resulting in a large and stable OPD. The lateral view of the SCF spliced with the TCF is shown in Fig. 2(a), where the air hole of SCF did not have any collapse and deformation. Figure 2(b) shows the light intensity on the end of the SCF, only a bright spot from the core of the SCF was observed. The core 2 is in the air hole of SCF and cannot be captured due to the defocus.
Fig. 2. (a) The lateral microscope image of the SCF spliced with TCF. (b) The measured optical intensity map on the end face of the SCF. (c) The reflection spectra of the MIs with different lengths of SCF.
The fabricated MI was connected with the light source and the circulator, and the reflection spectrum was measured by an optical spectrum analyzer (OSA, AQ-6317B). During the experiment, the optical fiber was kept straight to avoid the OPD caused by bending TCF. The reflection spectra of the MI with different long SCFs are shown in Fig. 2(c). The FSRs of the MI are 0.7 and 1.2 nm when the lengths of the SCFs are 820 and 580 μm, respectively. To make the phase modulator more compact, the length of the TCF can be further shortened. The OPD mainly depends on the length of SCF.
The NaNdF4 nanoparticles can be deposited onto the polished fiber, which is the key factor in the all-fiber phase modulator. NaNdF4 nanoparticles were prepared by thermal decomposition and then were poured into cyclohexane. The mixture of 0.1 mmol/mL was placed in an oscillating machine so that the nanoparticles could be completely dissolved in cyclohexane. After the precipitate fully disappears, the liquid became transparent. The transmission electron microscope (TEM) of the prepared NdNaF4 is shown in Fig. 3(a). The size of NdNaF4 nanoparticles is about 12 nm, and the fluorescence spectrometry is displayed in Fig. 3(b). Under the excitation of 808 nm laser, its emission spectrum is in the near infrared wavelength, the main emission peaks locate at 865 and 895 nm while the maximum light intensity occurs at 865 nm. Among NaNdF4 nanoparticles, Nd3+ is the main element to absorb 808 nm light. One heating/cooling cycle of NdNaF4 nanoparticles is shown in Fig. 3(c), where ΔT is the temperature difference. The photo-thermal conversion efficiency η of NdNaF4 nanoparticles can be calculated through ΔT [19] and is up to 85%, which is higher than previously reported thermo-optic materials. In order to deposit the NdNaF4 nanoparticles onto the side-polished surface of the TCF, the NdNaF4/cyclohexane solution was dropped on the polished area and was dried over 1 hour under the room temperature to volatilize fully the cyclohexane.
Fig. 3. (a) The TEM image and (b) the fluorescence spectrometry of NaNdF4 nanoparticles. (c) One heating/cooling cycle of NaNdF4 nanoparticles.
The SEM image of the side-polished TCF coated with NaNdF4 nanoparticles is shown in Fig. 4. It can be clearly observed that the polished surface of the TCF is uniformly covered by NaNdF4 nanoparticles. After the cyclohexane is completely evaporated, the agglomeration phenomenon of NaNdF4 nanoparticles occurs, and the diameter of the agglomerated pellets is observed to be about 40 nm. The loss of the polished core after coating NaNdF4 nanoparticles is ∼7 dB.
Fig. 4. (a) The SEM image of the side-polished TCF coated NaNdF4 nanoparticles. (b) Zoom-in view of (a).
3. Phase modulator principle
The MI was fabricated by tapering the splicing point between the TCF and the SMF. The other end of the TCF was spliced with a piece of SCF to introduce a stable OPD. The end face of the SCF was carefully cleaved to obtain 4% reflection at the silica-air interface. The light from the SMF is equally coupled into the two cores of the TCF through the tapered splicing point. Then the light in core 1 propagates forward through the SCF and is reflected back at the end face of the suspended core, while the light in core 2 is reflected at the interface between the TCF and the air. The output intensity of the MI is given by:
Here, I1 and I2 are the light intensities in the core 1 and core 2 of the TCF, respectively. L1 is the polished length of the TCF, i.e., the effective working length of the phase modulator. L2 is the length of the SCF, λ is the wavelength, n2 is the effective refractive index of the suspended core. During the experiment, the sample remains straight, so the OPD from the SCF is constant. The change of the phase difference ${\Delta \Phi }$ is only influenced by effective refractive index difference $\Delta {n_\textrm{1}}$ between the two cores in the polished area. Correspondingly, the FSR can be calculated by Eq. (3).
When the pump light irradiates the polished core coated with NaNdF4 nanoparticles, the temperature of the NaNdF4 will increase due to its high photo-thermal coefficient. As the pump power increases, the NaNdF4 nanoparticles generate the ohmic heating to heat the polished core gradually, thus the effective refractive index of the polished TCF core will change. Therefore, a temperature difference between the polished core and unpolished core is introduced. The phase change ${\Delta \Phi }$ at wavelength λ is determined by the temperature change $\mathrm{\Delta }T$ and is shown below:
4. Experimental results and discussions
The schematic of the experimental setup for measuring the phase shift in NaNdF4 coated side-polished TCF is shown in Fig. 5. The signal light from the super-continuum (YSL, SC-5) fiber laser was launched into the SMF connected with the circulator, and then was coupled into the two cores of NaNdF4 coated side-polished TCF by the tapered region. The forwardly propagating beams were reflected at the interfaces of the TCF-air and SCF-air, respectively. Finally, the two reflected beams interfered at the tapered zone and the interference spectrum from the output port of the circulator was monitored by the OSA with 0.2 nm spectral resolution. The 808 nm laser with the spot size of ∼ 1 mm2 was used as the pump light and directly illuminates the photo-thermal material from the lateral face of the TCF to control the phase shift of the MI.
Fig. 5. The schematic of the experimental setup for measuring the phase shift in NaNdF4 coated side-polished TCF.
The reflected interference spectra for different pump powers are shown in Fig. 6(a), in which the length of the SCF is 390 μm. The dependences of the interference peak wavelength and phase shift on the pump power are shown in Figs. 6(b) and 6(c). When the pump power increases from 0 to 520 mW, the interference spectrum has a red shift of 2.24 nm. Obviously, the drift of the interference spectrum is more than one period. The modulation efficiencies of the wavelength and phase shift are 4.43 pm/mW and 0.00568 π/mW, respectively. The spectral shift is linear with the increase of the pump power with R2 = 0.998. The maximum phase shift can reach about 2.9 π at a pump energy of 520 mW.
Fig. 6. (a) The shift of the interference spectrum with the pump power. The change of (b) interference peak wavelength and (c) phase shift with the pump power.
To further study the response time of all-fiber phase modulator and switch, a tunable laser is used to replace the super-continuum laser and its wavelength is tuned to the valley of the interference spectrum. The output optical signal is converted into an electrical signal by the photoelectric converter and is collected by the acquisition card. The reflection without pump light irradiation is minimal at the set wavelength, while it reaches the maximum when the 190 mW pump light irradiates NaNdF4 nanoparticles. Figure 7 shows the switching time response of the NaNdF4 coated modulator with the pump light on and off. The temporal responses with a rise time of 152 ms and a fall time of 50 ms are obtained, following the 10%-90% rule. The rising time is longer than the falling time. It is because the heat is dissipated between the NaNdF4 nanoparticles and air, the thermal diffusion process between the material and the fiber core needs more time in the rising time.
The humidity and temperature responses of the modulator were measured. The sample is placed in a humidity-controlled chamber with 25 °C constant temperature. The relative humidity varies from 55% to 80%, the measured results are shown in Fig. 8(a). When the relative humidity is low than 65%, the interference peak is relatively stable and has a slight wavelength shift. Therefore, the modulator has a small humidity crosstalk under low humidity condition. Similarly, the temperature responses during the heating and cooling processes are shown in Fig. 8(b) under 40% constant relative humidity. As the temperature increases, the effective refractive index of the NaNdF4 nanoparticles changes, resulting in a change in the effective refractive index difference between the two cores, and the interference peak has a blue shift. The drift of interference peak is only ∼0.3 nm in the temperature range of 30-80 °C. The modulator is not sensitive to the temperature, especially high temperature.
5. Conclusion
In conclusion, an all-fiber compact thermo-optic modulator has been demonstrated based on NaNdF4 nanoparticles coated side-polished TCF. A piece of SCF is spliced to the TCF to produce a fixed OPD, which effectively reduces the length of the device and greatly improves the stability of the modulator. Owning to high photo-thermal coefficient of the NaNdF4 nanoparticles, the measured results show that a phase shift of 2.9 π can be obtained and modulation efficiency has a slope of 0.00568 π/mW with an excellent linear correlation coefficient of 0.998. The interaction length of the polished modulation area is less than 2 mm, shorter than those of previously reported thermo-optic modulators [17,18,26]. In addition, the device shows a response with a rise (fall) time of 152/50 ms, following the 10%-90% rule. Benefiting from the advantages of compact structure, strong robustness and low cost, the proposed all-fiber thermos-optic modulator will have great potential applications in on-line optical devices.
Funding
National Natural Science Foundation of China (U1931121, 91750107, 61675054); Natural Science Foundation of Heilongjiang Province (ZD2018015, ZD2020F002); Higher Education Discipline Innovation Project (B13015); Fundamental Research Funds for the Central Universities (3072020CFT2501, 3072020CFT2504).
Disclosures
The authors declare no conflicts of interest.
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