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Abstract
We report an ultra-broadband LP11 mode converter with high purity based on integrated two shunt-wound long-period fiber gratings (LPFGs) and an adiabatic Y-junction, together with a high-order-mode bandpass filter. Two shunt-wound LPFGs are inscribed by CO2 laser in a two-mode fiber to achieve a 10 dB bandwidth of 50 nm and 51 nm at resonance wavelengths of 1530 nm and 1570 nm, respectively. Meanwhile, the Y-junction fabricated by lithography can be operated over S + C+L band to combine the converted LP11 mode. The presented ultra-broadband mode converter is able to achieve a mode conversion efficiency of 95%, together with a wavelength-dependent loss of less than 3 dB over the S + C+L band. This device has low modal crosstalk of 17 dB between the LP01 and LP11 modes, because most of the residual LP01 mode is further filtered by a high-order-mode bandpass filter at the output port of the Y-junction. The insertion loss of mode converter is estimated to be lower than 2.7 dB, due to the use of low loss polymer material during the fabrication. The proposed ultra-broadband LP11 mode converter with high purity is promising for the application of ultra-broadband mode-division-multiplexing transmission systems.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Mode-division multiplexing (MDM) that allows different spatial channels to carry independent information over a few-mode fiber (FMF) is a promising technique to improve the transmission capacity of single optical fiber [1–3]. In an MDM system, a mode converter is necessary to convert the fundamental mode to a high-order mode to distribute different signals into corresponding mode channels of an FMF. Such a mode converter should be designed to operate over C-band to realize the function of mode division (de)multiplexing with the capability of wavelength-division multiplexing compatibility. Mode converters have been reported with bulk-optic components, optical fibers, and optical waveguides [4–17]. Mode converters realized by the use of commercial optical components such as phase-plate can easily realize the conversion of any fundamental mode to high-order modes [4]. However, free-space light alignment has challenges to couple the spatial mode into an FMF, leading to an additional insertion loss. Mode converter designed via a photonic crystal fiber is a funny finding, which realizes a broadband conversion of three modes [5]. Mode converters fabricated by femtosecond laser and CO2 laser in the FMF are continually reported, where both of them are wavelength-tunable [6–9]. Mode converters based on the waveguide platform are reported in the form of directional couplers [10], Mach-Zehnder interferometer [11], multimode interferometers [12], long-period fiber gratings (LPFGs) [13,14], subwavelength gratings [15,16], and subwavelength Y-junctions [17]. However, most of the reported works operate over the C-band or C + L band. In the future, the operation bandwidth of the high-order mode is in need to cover the S + C+L band or even in 2 µm [9], where mode converter with high purity is therefore in the future developing. Adiabatic Y-junction based on the principle of mode evolution can operate over C + L band. By the use of this mechanism, an ultra-broadband mode multiplexer that serves to (de)multiplex four waveguide modes are reported, where the device can also convert the fundamental mode to any four waveguide modes with the crosstalk talk of lower than −11.6 dB over C + L band [18]. By the use of such an adiabatic Y-junction, we can further extend the device bandwidth. Furthermore, an ultra-broadband mode filter can be integrated in order to increase the mode purity, and such a mode filter can be carried out by the use of graphene embedded waveguide structure [19,20]. Such reported graphene embedded mode filter can serve as a high-order-mode bandpass filter over the S + C+L band with a mode suppression of the fundamental mode of 19 dB [20].
In this paper, we present a study of an ultra-broadband LP11 mode converter with high purity by two shunt-wound LPFGs and an adiabatic Y-junction, together with a high-order-mode bandpass filter. The study separates into two parts, the first part introduces the generation of LP11 mode in an FMF-based LPFG, and the second part introduces the combination of LP11 mode via an adiabatic Y-junction over the S + C+L band and further filtering out the residual fundamental mode with a high-order-mode bandpass filter. The reported device can serve as an ultra-broadband LP11 mode converter with high purity in future ultra-broadband MDM transmission system.
2. Design
Figure 1 shows the schematic diagram of the proposed ultra-broadband LP11 mode converter with high purity, including two shunt-wound LPFGs, an adiabatic Y-junction, and a high-order-mode bandpass filter. The LPFG serves to convert a fundamental mode (LP01 mode) to a LP11 mode, where the resonant wavelength and the operation bandwidth of LPFGs are determined by the grating period and the number of grating period. An adiabatic Y-junction serves to combine the power of LP11 mode into one few-mode waveguide. At last, the high-order mode bandpass filter serves to filter out the residual fundamental mode and only allows the designated LP11 mode to output.
LPFG acting as a mode converter has been reported at the platform of two-mode fiber (TMF) and waveguide structure. In this work, we use the TMF in the design, where the fiber core diameter and the fiber cladding diameter are 19 µm and 125 µm, respectively. The refractive index (RI) of fiber cladding and the numerical aperture (NA) are 1.44681 and 0.12, respectively. For a grating, the resonant wavelength and the operation bandwidth of LPFGs depend on the grating period and the number of grating period [21]. By the use of transfer matrix method, for example, as shown in Fig. 2, the resonant wavelength can be determined by
where ${n_{01}} = 1.448099$ and ${n_{11}} = 1.446756$ are the effective RIs of the LP01 and LP11 modes, respectively, and $\mathrm{\Lambda }$ is the grating period. When the grating period set at $\mathrm{\Lambda } = 1150{\;\ \mathrm{\mu} \mathrm{m}}$ and $\mathrm{\Lambda } = 1240{\;\ \mathrm{\mu} \mathrm{m}}$, the input LP01 mode converted to the LP11 with the highest conversion efficiency are at 1530 nm and 1570 nm, respectively. With the number of grating period is fixed at 20, the coupling coefficient is achieved to be 206.8 m-1 and 201.6 m-1 at the resonant dip of 1530 nm and 1570 nm, respectively. A 10-dB bandwidth of 65 nm can be obtained at the resonant dip of 1530 nm. The results are the same when the resonant dip shifts to the wavelength of 1570 nm. As shown in Fig. 2, the results are numerically evaluated to be polarization insensitive. The bandwidth of a normal LPG is narrow; however, the bandwidth can be extended if we combine two pairs of LPFG together by the use of shunt-wound method to recompose their spectra.
Fig. 2. Transmission spectra of the LPFGs with different grating periods and input state of polarization (SOP).
To extend the operation bandwidth of the converted LP11 mode, an adiabatic Y-junction is applied to combine the LP11 mode, as soon as the mode conversions are completed from the output ends of LPFGs, as shown in Fig. 3(a). As for the adiabatic Y-junction, the RIs of both core and cladding are fixed at nco = 1.52 and ncl = 1.51, respectively. The width and height of the core are 12 µm and 13 µm, respectively. This core dimension can have a low coupling loss with the TMF, according to the calculation [22]. Both LP11 modes are launched into the Y-junction from the port of Input 1 and Input 2, respectively, and then are combined as a new LP11 mode at the output port of the adiabatic Y-junction. We define that the power of the LP11 mode launches into the port of Input 1 and Input 2 as PIn1 and PIn2, respectively. Different combination ratios of the input power are taken into discussions, as shown in Fig. 3(b), when the PIn1/PIn2 = 1 (PIn1 = 0.5 and PIn2 = 0.5), the PIn1/PIn2 = 0.43 (PIn1 = 0.3 and PIn2 = 0.7), and the PIn1/PIn2 = 0 (PIn1 = 0 and PIn2 = 1) are set. Assuming the input power is 1 mW (0 dBm), the normalized output power is shown in Fig. 3(c). The insertion loss is 0.01 dB, when PIn1 > 0.4 mW and PIn2 < 0.6 mW are satisfied. It is further increased to 0.22 dB, when PIn1 > 0.3 mW and PIn2 < 0.7 mW are considered. The normalized output power is insensitive to the operation wavelength over the S + C+L band, as shown in Fig. 3(d). The normalized output power is weakly dependent on SOP. Next, a high-order-mode bandpass filter serves to attenuate the fundamental mode without affecting the LP11 mode. Our previous work proves that an L-shape 3D graphene embedded waveguide with graphene length of 11 mm can filter out ∼ 19 dB of the LP01 mode and the insertion loss of the LP11 mode is lower than 1.6 (1.0) dB for the x-polarization (y-polarization) polarization over the S + C+L band [20].
Fig. 3. (a) Diagram of an adiabatic Y-junction for combining the LP11 modes from input 1 and input 2 to the output port. (b) Transmission paths of the LP11 modes with different power ratios launched from input 1 and input 2 and the normalized output power from the output port. (c) Simulated normalized output power against the input power ratios. (d) Normalized output power with the power ratios (PIn1/PIn2) against the operation wavelengths.
By cascading these optical components together, an effective ultra-broadband mode conversion for the LP01-LP11 mode conversion is achieved. As shown in Fig. 4, the wavelength-dependent loss is within 3 dB over the S + C+L band for both x- and y-polarization. The insertion loss and the unbalance loss for the x- and y-polarization are mainly caused by the high-order-mode bandpass filter, which can be further optimized.
Fig. 4. Output power of the proposed ultra-broadband LP11 mode convertor for x- and y-polarization in simulation.
3. Results and discussions
We fabricated the LPFGs in the TMF by the CO2 laser direct inscription method, where repeat scanning being equals to the grating period is necessary to obtain enough coupling coefficient for the LPFG. The experimental setup is shown in Fig. 5(a), where a supercontinuum source and an optical spectrum analyzer are applied to monitor the fabrication process, two linear tapered mode filters are applied to ensure the purity of the input LP01 mode and to filter out the converted LP11 mode. The CO2 laser makes the RI change of the TMF only on one side, which makes it possible to convert the LP01 mode to the LP11 mode, due to the asymmetric RI change distribution arising in the TMF. The fabricated LPFGs are shown in Fig. 5(b), where the length between nicks is grating period pitch Λ. The output near field pattern of the LP11 mode is captured with an infrared CCD (Ophir-Spiricon) by launching the LP01 mode from one side of the LPFGs with the tunable wavelengths from 1530 nm to 1565 nm, which confirms the LPFG capability as mode converter.
Fig. 5. (a) Experimental setup for the fabrication of LPFGs, and (b) output near field patterns of the LP11 mode with the LP01 mode input over the C-band.
As shown in Fig. 6 by filtering out the converted LP11 mode, we can obtain the transmission spectrum of the LP01 mode, where the mode filter is an adiabatic linear taper and the filtered power comes from the converted LP11 mode. We fabricated two pairs of LPFGs with grating periods of $\mathrm{\Lambda } = 1150{\;\ \mathrm{\mu} \mathrm{m}}$ and $\mathrm{\Lambda } = 1240{\;\ \mathrm{\mu} \mathrm{m}}$, and the highest conversion efficiency at the wavelength of ∼1530 nm and ∼1570 nm, respectively. The fabricated LPFGs are insensitive to the SOP of input light, as shown in Fig. 6. Therefore, we can fabricate an adiabatic Y-junction to combine the converted LP11 mode and filter out any residual LP01 mode, with the help of a high-order-mode bandpass filter.
Fig. 6. Transmission spectra of the fabricated LPFG with a grating period of $\mathrm{\Lambda } = 1150{\;\ \mathrm{\mu} \mathrm{m}}$ and $\mathrm{\Lambda } = 1240{\;\ \mathrm{\mu} \mathrm{m}}$ for (a) x- and (b) y-polarization.
We fabricated the Y-junction by using standard optical lithography, where the device dimensions are the same as the design. We used polymer material OrmoCore and OrmoClad (Micro Resist Technology) as core and cladding material, respectively, which can be mixed in different proportions to optimize the RI. To fabricate the adiabatic Y-junction, we first spin-coated a thick layer of OrmoClad polymer as lower cladding on the silicon surface, which was cured by the UV exposure. Then we spin-coated a mixed OrmoCore and OrmoClad (with a mixture ratio of 1) thin film on the cladding as the core layer with the desired thickness. After that, we applied the UV-direct-written method to transfer the designed patterns on the core layer. The core height was further trimmed by the RIE etching after the developing, which aims to be the exactly height as the design parameter. Finally, we transferred a thick layer of cladding (OrmoClad) onto the core as the upper cladding, which is cured by UV exposure. The top view of the fabricated waveguide is shown in Fig. 7(a) during the fabrication, where the gap of the Y-junction in the parallel waveguide section is about 330 µm and the length of the S-bend is about 5 mm. The end face of the fabricated device is shown in Fig. 7(b), which is obtained by directly cutting the silicon wafer and then captured in the parallel waveguide section. The core width (up), the core width (down), and the core height are about 14 µm, 13 µm, and 12 µm, respectively, due to the fabrication error by the lithography. However, it doesn’t not degrade the performance of this device. The experimental results show that this device has large fabrication tolerance. The top view of the fabricated device with the upper cladding coating is shown in Fig. 7(c), where P1 and P2 are two ports of Y-junction for merging the power and P3 serves as an output port.
Fig. 7. (a) Top view of the waveguides during the fabrication process, (b) end face of the fabricated device, and (c) top view of the device coated with upper cladding.
To characterize the performance of the adiabatic Y-junction, we launched LP11 mode that converted by the LPFGs into the input ports of P1 and P2, respectively, by the butt-coupling, and captured the output near-field pattern from P3 at 1550 nm, as shown in Fig. 8(a). Figure 8(b) shows the output near-field patterns of the LP11 mode from output ports of P1 and P2 at different wavelengths over the C-band with the input LP11 mode in P3. Those results confirm the correct function of the mode combination of the LP11 mode. The proportion of the input power of the LP11 mode for the P1 and P2 over the S + C+L band, respectively, are shown in Fig. 9. The normalized input powers are within a region from 0.3 to 0.7 over the S + C+L band, which confirms that the conversion efficiency can be higher than 95% over the S + C+L band. The insertion loss is measured by direct launching the LP11 mode into a straight waveguide with a length of 20 mm. The insertion loss, which includes the coupling loss and material loss, is measured to be about 0.5 dB. A high-order-mode bandpass filter is utilized to filter out the residual fundamental mode, where the mode extinction ratio against the LP01 mode is ∼17 dB over the S + C+L band and the insertion losses of the LP11 mode are about 2.2 dB and 2.0 dB for x- and y-polarization, respectively. As the result, normalized powers of the converted LP11 mode are shown in Fig. 10 for x- and y-polarization, where the wavelength-dependent loss is within 3 dB in the S + C+L band. The key point to achieve the power combination with an insertion loss of less than 3 dB is to reduce the insertion loss of input mode and maintain the normalized input powers with a power ratio of around 1. The polarization-depended loss is only 0.2 dB due to the fabrication error, which shows better performance than the numerical simulation.
Fig. 8. (a) Near field patterns of the LP11 mode from output port (P3) with the LP11 mode launched from the input port of P1 and P2, respectively. (b) Near field patterns of the LP11 mode from the output port (P1) and the output port (P2) with the LP11 mode launched from the input port of P3 in the C-band.
Fig. 10. Output power of the proposed ultra-broadband LP11 mode converter for x- and y-polarization.
4. Conclusion
In summary, we present an ultra-broadband LP11 mode converter with high purity based on two shunt-wound LPFGs and an adiabatic Y-junction, together with a high-order-mode bandpass filter. The presented ultra-broadband LP11 mode converter is measured to achieve mode conversion efficiency with wavelength-dependent loss within 3 dB over the S + C+L band, and the transmission characteristics of this device are insensitive to polarization. This device has low modal crosstalk of the LP01 mode (< 17 dB), as most of the residual LP01 mode is filtered by the high-order-mode bandpass filter. The major insertion loss is mainly caused by the high-order-mode bandpass filter, which can be further optimized during the design and fabrication. The design principle can be further applied to waveguide-based mode splitting and combining devices for a range of mode manipulating, such as power splitter for the high-order modes and mode-dependent loss compensator as well.
Funding
Research and Development Plan in Key Areas of Guangdong Province (2018B010114002); The Pearl River Talent Recruitment Program (2019ZT08X340); National Natural Science Foundation of China (61875069, 62175038).
Acknowledgments
The authors would like to thank Prof. K. S. Chiang from CityU HK for the support of TMF and Prof. K. X. Chen from UESTC for the support of Micro Resist polymer.
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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