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Abstract
We present a reconfigurable ultra-broadband mode converter, which consists of a two-mode fiber (TMF) and pressure-loaded phase-shifted long-period alloyed waveguide grating. We design and fabricate the long-period alloyed waveguide gratings (LPAWG) with SU-8, chromium, and titanium via the photo-lithography and electric beam evaporation technique. With the help of the pressure loaded or released from the LPAWG onto the TMF, the device can realize reconfigurable mode conversion between the LP01 mode and the LP11 mode in the TMF, which is weak sensitive to the state of polarization. The mode conversion efficiency larger than 10 dB can be achieved with operation wavelength range of about 105 nm, which ranges from 1501.9 nm to 1606.7 nm. The proposed device can be further used in the large bandwidth mode division multiplexing (MDM) transmission and optical fiber sensing system based on few-mode fibers.
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1. Introduction
Mode division multiplexing (MDM) technology has attracted much interest in recent years because of its ability to increase the transmission capacity by exploiting the spatial modes of a few-mode fiber (FMF) [1–4]. A great number of optical devices have been used to manipulate various spatial modes that are applied in an MDM system, such as mode converters, mode (de)multiplexers, mode selective switches, and mode filters [5–13]. Mode converter, which can achieve the mode conversion between two spatial modes, is a key component in an MDM system. Various types of mode converters based on bulk-optic components, optical waveguides, and optical fibers have been reported. Bulk-optic-based mode converters can be implemented by the use of commercial components, such as spatial light modulators [5] and phase plates [6]. Waveguide-based mode converters can be realized by multimode interferometers [7], Mach-Zehnder interferometers [8], Y-junctions [9], directional couplers (DCs) [10], and long-period waveguide gratings (LPWGs) [11–13], etc. Fiber-based mode converters can be achieved by photonic lanterns [14], fiber couplers [15], fiber Bragg gratings (FBGs) [16], and long-period fiber gratings (LPFGs) [17–23].
Long-period gratings (LPGs), which have periodic structures, can be formed both in optical waveguides and optical fibers for achieving the mode conversion between two spatial modes. What is more, on-chip or waveguide-based platforms that have precise control of the device parameters, make the integrated device more compact and efficient. For example, the embedded long-period waveguide grating is reported to achieve both ultra-short length and wide operation bandwidth [11]. And mode (de)multiplexer is also reported to integrate a thermally induced LPG and an asymmetric Y-junction that serves to make the mode conversion reconfigurable [12]. By the use of phase-shifted LPWGs, a mode filter with a 10-dB bandwidth of about 190 nm is demonstrated [13]. However, the grating in a chip based on the waveguide platform is unlucky to introduce the additional loss during the butt coupling somehow. The fiber-based mode converter can be realized by inscribing the LPGs in an FMF, using a femtosecond laser or CO2 laser [17–20]. For example, the line-by-line femtosecond laser direct written technique is used in the grating inscription, which is reported to achieve the mode conversion between LP01 mode and LP11 mode over the C + L band [17]. The CO2-laser inscribed helical LPGs in a two-mode fiber is reported to provide a 10-dB bandwidth over 297 nm [18]. By the use of the single-side CO2-laser exposure technique, the apodised phase-shifted LPFGs are realized to achieve the mode conversion with 10-dB bandwidth of 182 nm [19]. And in our recent work, by integrating two shunt-wound LPFGs and an adiabatic Y-junction waveguide, the mode converter can be operated over the S + C + L band [20]. By the use of mechanical grating, a controllable all-fiber orbital angular momentum mode converter is demonstrated [24]. By the use of two lateral stress points in a TMF function as in-line fiber mode couplers to construct the modal interferometer, multi-channel mode converter has been presented [25]. The mode converters based on LPWGs provide flexible design ability and high precision in the control of the device parameters. The challenge is resisting in the alignment of the free-space light that couples from the fiber to the waveguide, and vice versa, which causes additional insertion loss. The fiber-based mode converters are compatible to connect with FMFs-based transmission systems. However, the fiber-grating-based mode converter cannot be easily reconfigurable. Therefore, it is necessary to achieve a mode conversion with the reconfigurable ability, ultra-broadband wavelength ranges and fiber compatibility for fiber-based MDM transmission systems in the future.
In this paper, we present a reconfigurable ultra-broadband mode converter based on a two-mode fiber (TMF) and pressure-loaded phase-shifted long-period alloyed waveguide grating (LPAWG). We design and fabricate the LPAWG with SU-8, chromium and titanium material via the photo-lithography and electric beam evaporation technique. By the use of pressure-loaded setup, the proposed device can realize the reconfigurable mode conversion between the LP01 mode and the LP11 mode in a TMF. The mode conversion efficiency of larger than 10 dB can be achieved with an operation wavelength range of about 105 nm from 1501.9 nm to 1606.7 nm. The proposed mode converter can be further used as an ultra-broadband tunable mode converter, which could find applications in the MDM transmission and optical fiber sensing systems based on few-mode fibers.
2. Operation principle, design, and fabrication
Figure 1 shows the schematic diagram of the proposed device, which consists of a TMF sandwiched in an LPAWG and a planar slab. The signal light (LP01 mode) is launched in the TMF from the input end, and then obtain the LP11 mode of the signal light from the output end. In our design, the fiber utilized in the design is a two-mode step-index fiber, which supports the LP01 mode and the LP11 mode. The numerical aperture (NA) and the cladding refractive indices (RI) are about 0.2 and 1.44681, respectively. And the diameters of the core and cladding are 19 and 125 µm. The metal slab is placed above the TMF with tunable pressure loaded via the usage of various weights. Figure 2 shows the profile view of the LPAWG. The LPAWG has a grating period (Λ) of 1154 µm and three-section gratings with lengths of 4Λ, 10Λ, and 16Λ. The LPAWG is placed under the TMF with the pressure is loaded from the upper slab, which makes the RI of the TMF change periodically according to the perturbations caused by the LPAWG profiles. Thus, the long-period gratings can be formed by loading the stress on the TMF via mechanical gratings. As a result, the launched LP01 mode is converted to the LP11 mode. When the pressure is released, the mode conversion will deactivate and the signal light keeps as the LP01 mode in the output end. By the use of the pressure that is loaded or released from the slab, the device can realize the function of reconfigurable mode conversion.
The LPG operates via the function of the periodic change of the RI both in the fiber and the waveguide platforms. For a grating, the grating period and cycle number are key parameters, which can determine the resonant wavelength and operation bandwidth. The operation principle of an LPG is based on the phase-match condition between the modes, and the resonant wavelength (λ0) can be expressed as [20]
where n01 and n11 are the effective indices of the LP01 and the LP11 modes, respectively, and Λ is the grating period. In this work, the pressure-loaded grating structure is composed of an apodised grating formed by alloy and SU-8 to serve to convert the LP01 mode and the LP11 mode. According to Eq. (1), the fundamental core mode can be converted to any order of the higher-order core mode by changing the grating period.Figure 3 shows the transmission spectra of the uniform LPG and phase-shifted LPG by the transfer matrix method. The resonance dip is set at the wavelength of 1550 nm, where the mode conversion between the LP01 and the LP11 mode at 1550 nm is of the highest conversion efficiency. We fix the total length of the grating to be L = 30Λ with period number of 30. When the grating period is set at Λ = 1154 µm with the length of each section is given by 10Λ (i.e., L1 = L2 = L3 = 10Λ), the 10-dB bandwidth (i.e., 90% conversion) of the uniform LPG is only 24 nm (from 1538 nm to 1562 nm). To extend the operation bandwidth of the converted LP11 mode, phase-shifted gratings are applied in the design. Here, we fix L2 = 10Λ and optimize the operation performance with different length combinations of L1 and L3. As shown in Fig. 3, the length combination, L1 = 4Λ, L2 = 10Λ, and L3 = 16Λ offers the largest bandwidth. The π phase-shift can extend the operation bandwidth and improve the mode conversion efficiency, where the fabrication processing is the same as the normal grating. By introducing two π-phase shift points and dividing the grating into three sections with lengths of 4Λ, 10Λ, and 16Λ, the grating bandwidth, such as the 10-dB bandwidth, can be obtained over 195 nm (from 1459 nm to 1654 nm). In this work, we use a three-section phase shifted grating to realize the ultra-broadband operation function.
Fig. 3. The normalized transmission spectra of the three-section LPGs calculated for different length combinations with L = 30Λ, L2 = 10Λ.
The refractive index of the fiber varies with the lateral pressure or axial strain due to the photo-elastic effect. The variation of the RI (Δn) induced by the lateral pressure can be calculated by [26]
where k is the strain-optic coefficient, F is the lateral pressure applied on the fiber, r and l are the outer radius and length of the fiber, E is Young's modulus of the fiber material. We further calculate the pressure-induced variation for the transmission spectrum. The lateral pressure applied on the fiber will change the effective refractive index of the fiber periodically according to the LPAWG profile, and the coupling coefficient can be manipulated by applying different pressures. The transmission spectra of the LP01 mode are shown in Fig. 4, where Fig. 4(a) is the transmission spectra of a uniform LPG and Fig. 4(b) is the transmission spectra of a phase-shifted LPG. The resonant dip is deepening with the pressure increasing for both the uniform LPG and phase-shifted LPG.The microfabrication process of the LPAWG is shown in Fig. 5. We follow the design parameters as closely as possible in the fabrication of the alloyed waveguide device. First, the wafer surface is cleaned with acetone, propanol, and deionized water in sequence. Then the plasma cleaning process is used to stimulate the surface energy of materials effectively, which can make the coating process firm. After that, the polymer material (SU-8) is spin-coated on the silicon wafer uniformly by a spinner and the thickness is about 10.0 µm. A hotplate is used to solidify the thin film through two steps: pre-bake and hard-bake. The thin film is first pre-baked after the spin coating process because it remains in the solvent state. A mask is required to define the patterns and the thin film is shaped into the grating array by photo-lithography. The hard bake is applied after the photo-lithography process. Finally, by the use of the electric beam evaporation method, chromium-titanium alloy is deposited on the grating array surface, which can make the device more robust. The thickness of the chromium-titanium alloy layer is about 0.1 µm. The top views of the fabricated LPAWGs are also presented in Fig. 6 captured by an optical microscope. The length between two notches is the grating period pitch Λ, which is shown in Fig. 6(a). Figure 6(b) and 6(c) show the images of the two π-phase shift sections of the grating.
Fig. 4. The simulation results of the transmission spectra for (a) a uniform LPG and (b) a phase-shifted LPG when different pressures are loaded.
Fig. 6. The microscopic images of the (a) uniform section and (b), (c) two π-phase shift sections of LPAWG.
3. Experiment results and discussions
The experimental setup is shown in Fig. 7(a) for the device measurement. We select the broadband source (BBS, Hoyatek) and tunable light source (TLS, Ovlink) as the light source, respectively, for the measurements of the near filed patterns and transmission spectra. The polarization controller (PC) is applied to control the polarization state of the input light. The charge-coupled device (CCD, Ophir-Spiricon) and the optical spectrum analyzer (OSA, Yokogawa, AQ6370D) are applied to capture the near-field patterns and monitor the transmission spectra, respectively. As shown in Fig. 7(b), the pressure-loaded setup includes a TMF, an LPAWG, a slab, and some weights with different sizes. The experimental setup is placed in a stable holder, which promises that the pressure is loaded on the TMF uniformly. The mass block is placed on the metal slab and all the forces of the weights can be transferred to the metal slab. The metal slab is fixed horizontally with the help of the holder, which is only attached on the surface of the TMF to ensure the pressure is loaded on the TMF. The TMF is sandwiched between the LPAWG and the slab. The various sizes of weights are selected and placed on the slab, which makes the loaded pressure transfer onto the TMF controllable. Additionally, the loaded pressure can be adjusted by changing the sizes of weights.
Fig. 7. The measurement setup of the proposed mode multiplexer, which includes (a) the experimental setup and (b) the pressure-loaded setup.
By launching the Gaussian beam into the input end of the TMF with a broadband source (BBS) and a tunable light source (TLS), respectively, the output near field pattern of the LP01 mode or the LP11 mode or their mixture form is captured with an infrared CCD from the output end of the TMF. The mode field pattern varies with the loaded pressure changing, as shown in Fig. 8, which presents the evolution of the mode pattern under different pressures. When there is no pressure loaded on the slab, the clear LP01 mode pattern can be captured without mode conversion. With the loaded pressure changing, the mode patterns show a mixture form between the LP01 and the LP11 mode, and the LP01 mode is converted to the LP11 mode gradually. The mode field pattern with the highest mode conversion efficiency is achieved when the value of the loaded pressure is set at 2.1 MPa. As a result, the device can realize a reconfigurable fiber mode conversion function with tunable pressure loading. The mode field patterns under different wavelengths (i.e., 1530, 1550, 1565 nm) and the whole C + L band (i.e., wavelength range from 1530 nm to 1610 nm) are captured and shown in Fig. 8, which confirm the capability of the mode conversion of this device.
Fig. 8. The output near field patterns of the LP01 mode, the LP11 mode and their mixture captured with an infrared CCD under different loaded pressures.
We evaluate the operation bandwidth of the proposed fiber mode converter by measuring the transmission spectra. The light from the BBS is adjusted by a polarization controller and then launched into the TMF from the input end. Subsequently, the output light is collected from the output end of the TMF and analyzed by an OSA. Two samples are demonstrated in our experiment as a comparison, which includes a uniform LPAWG and a π-phase shifted LPAWG. Figure 9(a) shows the transmission spectra of the LP01 mode with the uniform LPAWG in the measurement setup. The depth of the resonant dip increases with the loaded pressure. The maximum mode conversion efficiency obtained in the experiment is about 33.7 dB with the loaded pressure of 2.1 MPa. For the uniform LPAWG, the 10-dB bandwidth of 40 nm (from 1537.6 nm to 1577.9 nm) is obtained, displayed in Fig. 9(b). Figure 9(b) also shows that the device is insensitive to the state of polarization. The relationship between the loaded pressure and the resonant dip transmission is shown in Fig. 9(c) at 1550nm, which is fitted with a quadratic equation.
Fig. 9. (a) The transmission spectra under different pressures, (b) transmission spectra with two polarization states and 10-dB operation bandwidth, (c) the fitting of the loaded pressure and resonant dip transmission for the uniform LPAWG.
Similarly, Fig. 10(a) shows the transmission spectra of the LP01 mode with the π-phase shifted LPAWG applied in the measurement. The depth of the resonant dip increases with the loaded pressure. The mode conversion efficiency of the LP01 mode to the LP11 mode with conversion efficiency larger than 90% is defined by the transmission value below −10 dB. With the loaded pressure of 2.1 MPa, the maximum mode conversion efficiency is about 24.9 dB measured in the experiment. For the π-phase shifted LPAWG, the 10-dB operation bandwidth of 105 nm (from 1501.9 nm to 1606.7 nm) is obtained, displayed in Fig. 10(b). Figure 10(b) also shows that the device is insensitive to the signal polarization states. Slight interference occurs in the experiment due to the unfortunate nonuniform loaded pressure, however, the introduced interference caused by the limited experimental setup does not affect the performance of the device much. The relationship between the loaded pressure and the resonant dip transmission is shown in Fig. 10(c), which is also fitted with a quadratic equation in one variable. By the use of phase-shifted LPAWG, the operation bandwidth can be extended significantly. Although the uncertainties exist in controlling the grating parameters during the fabrication process that lead to the modal interference in the pressure loading process occurred, the experimental results and the theoretical results present good agreement for both the uniform LPAWG and the π-phase shifted LPAWG.
Fig. 10. (a) The transmission spectra under different pressures, (b) transmission spectra with two polarization states and 10-dB operation bandwidth, (c) the fitting of the loaded pressure and resonant dip transmission for the phase-shifted LPAWG.
4. Conclusion
We have proposed and demonstrated a reconfigurable ultra-broadband mode converter based on TMF and pressure-loaded phase-shifted LPAWG. We have designed and fabricated the LPAWG with SU-8, chromium and titanium material via the photo-lithography and electric beam evaporation technique. By loading or releasing the pressure through a slab, the device can realize reconfigurable mode conversion between the LP01 mode and the LP11 mode in a TMF. The device is demonstrated to achieve the mode conversion with a large operation bandwidth of 105 nm from 1501.9 nm to 1606.7 nm, together with the characteristic of insensitivity to the state of polarization. The proposed mode converter can be further applied in the MDM transmission system, where ultra-broadband tunable mode conversion is required based on few-mode fibers.
Funding
National Key Research and Development Program of China (2018YFB1800903); National Natural Science Foundation of China (62205067).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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