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Low-mode-crosstalk coupler using multistage mode conversion
Optics Communications 450 (2019) 34–38
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Low-mode-crosstalk coupler using multistage mode conversion Junji Sakamoto a ,∗, Atsushi Nakamura b , Keiji Okamoto b , Toshikazu Hashimoto a a b
NTT Device Technology Labs., NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi, Kanagawa, Japan NTT Access Network Service Systems Labs, NTT Corporation, 1-7-1 Hanabatake, Tsukuba, Ibaraki, Japan
ARTICLE Keywords: Multimode OTDR Mode detection Optical waveguides 1 μm-band Mode converter
INFO
ABSTRACT We developed a low-mode-crosstalk coupler using silica planar lightwave circuit (PLC) technology. The coupler has two mode conversion stages. One is a mode converter for converting the fundamental mode to the secondorder mode, and the other is for converting the second-order mode to the first-order mode, which is the target mode. Isolation between fundamental mode and first-order mode can be enhanced by using indirect conversion with the second-order mode instead of direct conversion. In the fabricated coupler, the mode crosstalk between the fundamental mode and first-order mode was under -30 dB, and the conversion ratio was -0.8 dB at the wavelength of 1050 nm. The coupler can be used for sensitive 1 μm-band mode-detection optical time-domain reflectometry.
1. Introduction Optical waveguide circuits with multiple modes have been used in various optical systems. The multiple modes provide various benefits such as high-capacity transmission [1–5], sensitive optical sensing [6– 8], and optical circuit miniaturization [9–12]. One system using multiple modes is sensitive 1 μm-band mode-detection optical time domain reflectometry (OTDR) [6–8] which is used to diagnose optical fiber links. The OTDR system uses 1 μm-band probe pulses to generate not only the fundamental (LP01) mode but also the first-order (LP11a and LP11b) mode as the backscattered light in typical single-mode fibers (SMFs) compliant with ITU-T G.652. It also uses a mode coupler to separate the backscattered light into individual modes as well as to select the mode of the probe pulses. The first-order mode at the wavelength in the 1 μm band is more sensitive to trouble spots in optical fiber links, such as macro/micro-bends, than the fundamental mode in the communications wavelength band. Therefore 1 μm OTDR has a great potential for finding a potential trouble point before it has a serious impact on an optical communications system. The details of the 1 μm OTDR system are described in references [6–8]. The mode coupler, the key component of 1 μm OTDR, is fabricated by using a silica planar lightwave circuit (PLC) [13–16]. PLCs, silicabased optical waveguide circuits on a silicon substrate, were developed to construct various optical devices for optical communications, including beam splitters, wavelength filters, and optical switches [13–16]. Low loss and high mode conversion efficiency can be obtained by a mode coupler with a PLC. In this paper, we present a mode coupler with low mode crosstalk and high conversion efficiency by a multistage mode conversion ∗
method. Section 2 covers the operation of a conventional mode coupler and its problems. Section 3 describes our mode coupler, and Section 4 presents numerical simulation results. Section 5 describes and discuss the actual performance of a fabricated mode coupler. 2. Conventional mode coupler Fig. 1(a) shows a schematic of the 1 μm OTDR system. Input probe pulse selects probe mode LP01 or LP11 at the mode division multiplexing (MDM). Since the wavelength of the input light is 1.0 μm, LP01 and LP11 are induced as the modes of backscattered light at the measuring fiber. The backscattered light is separated into LP01 and LP11 components by the MDM. Then each component is detected by a photodiode. Therefore, OTDR using first-order mode can be used. To obtain the above functions, we use mode coupler with a PLC for MDM. Fig. 1(b) shows structure and operation of a conventional mode coupler. The gray line represents the core of the waveguide. The mode coupler consists of two waveguides with different widths. The fundamental mode propagating in the thin waveguide is converted to the LP11a mode by coupling to the thick waveguide. Complete mode conversion is induced by setting the waveguide widths so that the effective refractive index of each mode becomes equal. Almost all the LP01 and LP11b mode propagating in the thick waveguide passes through the coupler. The SMF at wavelength of 1.55 μm which is measured by OTDR is connected to port 4, and SMF at wavelength of 1.0 μm is connected to port 1 and 2. At the wavelength of 1.0 μm, the core diameter, refractive index difference, and cut-off wavelength of the SMF are, respectively, 5.3 μm, 0.48%, and 920 nm from the
Corresponding author. E-mail address: [email protected] (J. Sakamoto).
https://doi.org/10.1016/j.optcom.2019.05.049 Received 19 February 2019; Received in revised form 15 May 2019; Accepted 21 May 2019 Available online 23 May 2019 0030-4018/© 2019 Elsevier B.V. All rights reserved.
J. Sakamoto, A. Nakamura, K. Okamoto et al.
Optics Communications 450 (2019) 34–38
Fig. 2. Schematic of structure and operation of multi-stage mode coupler.
Fig. 3. Relationship and schematic operation of multi-stage mode coupler.
Fig. 1. (a) Schematic of 1 μm OTDR system. (b) Structure and operation of conventional mode coupler.
the first- and second-order mode curves indicate the cut-off width of the modes. As width W0, we selected 3.5 μm, where the waveguide stays single mode and W1 and W2 do not become too wide. W1 and W2 are automatically derived from W0 to match the effective refractive indexes between each mode. W1 and W2 are 10.1 and 16.4 μm in this case.
specification sheet. At 1.55 μm, they are 8.2 μm, 0.36%, and 1260 nm. The 1 μm-band probe pulse is input from either port 1 or port 2 to select the input probe mode. The input mode is selectively used depending on the length of the fiber to be measured and the necessary sensitivity. When port 1 is selected, the first-order mode probe light is generated. On the other hand, fundamental mode probe light is generated from port 2. Three modes – LP01, LP11a, and LP11b – are generated as the backscattered light. Since the higher order mode output to port 2 can be removed by adding a mode filter that narrows the waveguide, the fundamental mode and LP11a can be detected separately from the mode mixed light at port 1 and 2. Therefore the mode coupler can be used for 1 μm OTDR. The dotted lines from port 4 to port in Fig. 1(b) are mode crosstalk paths. A part of fundamental mode from port 4 is coupled to the thin waveguide with the fundamental mode and output from port 1 as a noise component. In this paper, the mode crosstalk is defined as log10 (𝑃LP01noise ∕𝑃LP01signal ), where 𝑃LP01signal is the fundamental mode component output from port 2 and 𝑃LP01noise is the noise component output from port 1. Since the noise component caused by the mode crosstalk obscures the signal component that enables us to detect macro/microbends sensitively, a mode coupler with low mode crosstalk is required.
4. Validation by numerical simulation As shown in Fig. 4(a), simulation model consists of mode coupler 1, 2, and 3. For simplicity, the simulation was conducted only for the coupler part without a fan-in and fan-out. Coupler 3 is added for the measurement because measurement system can evaluate only fundamental mode. It is omitted when the coupler is used in a 1 μm OTDR. The waveguide height, width, and 𝛥 were the same in the previous section. The gap and the length of L02 , L21 , and L10 were 3.0, 1900, 2450, 1400 μm, respectively. Fig. 4(b) shows the simulated optical propagation for the coupler. We used an three-dimensional beam propagation method. The wavelength of the input light was 1050 nm. Input light with the fundamental mode is converted to the second-order mode at coupler 1. Then, the light is converted to the first-order mode at coupler 2. Finally, the light is converted to the fundamental mode again. The coupler works as intended. Fig. 4(c) shows spectra of the coupler with coupler 3 included. The transmittance is −0.28 dB at a wavelength of 1050 nm, indicating that extra loss due to adding the waveguide hardly occurred. Next, we simulated the mode crosstalk. Fig. 5 shows the simulated optical propagation about mode crosstalk with a conventional mode coupler and the multi-stage mode coupler. The direction of optical propagation in Fig. 5 is opposite that in Fig. 4. In this simulation, a fan-out was added to calculate the output power. For the conventional mode coupler, a little light coupled to the output port is observed. For the multi-stage mode coupler, a little light coupled to the middle waveguide as the first-order mode is observed. However, the coupled light from the middle waveguide to output waveguide is not observed. This is because the effective refractive indexes between the input waveguide with the fundamental mode and the middle waveguide with first-order mode do not match and those between the middle waveguide with the first-order mode and output waveguide with the fundamental mode do not match. Since the coupler does not satisfy the mode conversion conditions, light passes through the two stages and
3. Design of multi-stage mode coupler Fig. 2 shows our mode coupler. An additional mode conversion waveguide placed between single-mode waveguide and multimode waveguide of the mode coupler and indirectly converts the mode into a desired one via intermediate mode. In this work, we choose the secondorder mode as the intermediate mode. By adding second-order mode, mode isolation is enhanced and mode crosstalk is reduced. By setting the waveguide width correctly, extra loss due to adding the waveguide does not occur owing to the all-optical power transitions between the modes. The refractive index difference (𝛥) and the core height were designed to be 0.42% and 5.6 μm. These values were decided to obtain good fiber coupling both SMFs at wavelengths of 1.0 and 1.55 μm band. Fig. 3 shows the relationship between the effective refractive index and waveguide width at a wavelength of 1050 nm. The starting points of 35
J. Sakamoto, A. Nakamura, K. Okamoto et al.
Optics Communications 450 (2019) 34–38
Fig. 5. Simulation of optical propagation with conventional mode coupler and multi-stage mode coupler.
Fig. 6. Cross section of the fabricated waveguide.
Fig. 4. (a) Layout of optical circuit for simulation. Simulated (b) optical propagation and (c) relationship between wavelength and transmittance.
the isolation between the modes is enhanced. The transmittances when the fundamental mode was input to a conventional mode coupler and the multi-stage mode coupler were −31.8 and −49.0 dB, respectively. The multi-stage mode coupler can suppress mode crosstalk by about 20 dB compared to the conventional mode coupler.
Fig. 7. Experimental set up for the measurement and fabricated mode coupler.
5. Experimental results and discussion good. From the measurement results for port 1 to 4 and port 3 to 6, the conversion efficiency of the fundamental mode to second-order mode and the first-order mode to fundamental mode were −0.36 and −0.22 dB, respectively. Since the total transmittance of the coupler is −0.86 dB, the conversion efficiency of second-order mode and first-order mode can be estimated as −0.28 dB. Fig. 9 shows the transmittance at a wavelength of 1050 nm from port 5 to 1 when the input position was changed from the alignment position in the lateral direction after coupler 3 had been omitted. The measurement indicates the mode crosstalk due to the fundamental mode at the port 5 to the fundamental mode at the port 1. The values on the x axis are the display values of the alignment machine. The transmittance at the alignment point is the minimum. When the position shifts, the transmittance increases. It means mode crosstalk is increase. This is because when the position deviates from the alignment
We fabricated the multi-stage mode coupler using a PLC. A cross section of the waveguide is shown in Fig. 6. The conventional PLC fabrication process was used [16]. Fig. 7 shows the experimental setup and fabricated mode coupler. The coupler was evaluated by using a SMF butt-coupled through its input and output port. In practice, a SMF for the wavelength of 1.55 μm will be connected to the output port of the first-order mode. To evaluate the coupler characteristics accurately at the wavelength of 1.0 μm, a SMF for 1.0 μm was used in this experiment. A non-polarized broad band light source, tunable wavelength filter, and optical power meter were used. Fig. 8 shows the measurement result for the fabricated mode coupler along with the simulation result and the optical circuit layout. The fiber coupling loss of about 0.1 dB/point does not include in the spectra. The transmittance from port 1 to 6 is −0.82 dB. Although the loss is slightly larger than the simulation, the agreement with the simulation result is 36
J. Sakamoto, A. Nakamura, K. Okamoto et al.
Optics Communications 450 (2019) 34–38
Fig. 10. Overlap integral between 𝜑0 and 𝜑1 when changing the distance of field centers. Fig. 8. Spectra from port 1 to 6 of fabricated mode coupler using silica PLC.
Fig. 11. Transmittance at a wavelength of 1050 nm from port 5 to 1when the input position was changed from the alignment position before and after omitting coupler 3.
6. Conclusion Fig. 9. Transmittance at a wavelength of 1050 nm from port 5 to 1 when the input position was changed from the alignment position after omitting coupler 3.
We developed a multi-stage mode coupler, that can reduce mode crosstalk. The coupler has two stages of mode conversion. The first stage is mode conversion from the fundamental mode to second-order mode. The second stage is that from the second-order mode to firstorder mode. By adding the second-order mode, the mode isolation became higher than that for direct conversion from the fundamental mode to first-order mode. The total conversion efficiency and mode crosstalk of the fabricated coupler were −0.81 dB and less than −30 dB, respectively We successfully obtained low mode crosstalk coupler with high conversion efficiency. The coupler will be helpful for sensitive measurement by 1 μm-band mode-detection OTDR.
point, the first-order mode is induced. The minimum transmittance of −24 dB is higher than the simulation result in Section 3. To understand why the transmittance from port 5 to 1 is high, we consider the first-order mode coupling ratio at the fiber and waveguide coupling point. Coupling ratio 𝐶 can be calculated by the overlap integral between the field pattern of fundamental mode, 𝜑0 , at the fiber and the field pattern of first-order mode, 𝜑1 , at the waveguide: 𝐶=
∬
𝜑∗0 (𝑥, 𝑦)𝜑1 (𝑥, 𝑦)𝑑𝑥𝑑𝑦
References
Fig. 10 shows the calculated overlap integral. The 𝜑0 and 𝜑1 were calculated by the three-dimensional mode solver. Even when the mismatch distance becomes 0.1 μm, about −30 dB of first-order mode is induced in the waveguide. Considering the machine feed accuracy and the angle deviation of the fiber in the measurement system, it is estimated that the total deviation becomes about 0.1 μm. Furthermore, it is considered that the roughness of the input face and the side wall of the waveguide induces an extra first-order mode. To confirm the influence of the first-order mode induced at the input surface, we measured the transmittance from port 5 to 1 before omitting coupler 3. Fig. 11 shows the measurement result. The transmittance decreased to under −30 dB because the first-order mode induced by the deviation between the fiber and waveguide is coupled and output to port 3. This means that the mode crosstalk bottleneck is not the coupler performance but the accuracy of fiber coupling to the waveguide. Therefore, we successfully fabricated a multi-stage mode coupler with mode crosstalk from the fundamental mode suppressed to under −30 dB. On the other hand, to derive the performance of the optical circuit, a high-precision fiber connection is required. For example, to obtain under −30 dB mode crosstalk, it is necessary to fix the fiber with an accuracy of 0.1 μm or less because coupler 3 will be omitted in practical applications.
[1] T. Mizuno, T. Kobayashi, H. Takara, A. Sano, H. Kawakami, T. Nakagawa, Y. Miyamoto, Y. Abe, T. Goh, M. Oguma, T. Sakamoto, Y. Sasaki, I. Ishida, K. Takenaga, S. Matsuo, K. Saitoh, T. Morioka, 12-core× 3 –mode dense space division multiplexed transmission over 40 km employing multi-carrier signals with parallel MIMO equalization, in: Proc. OFC, Th5B. 2, San Francisco, 2014. [2] T. Sakamoto, T. Matsui, K. Saitoh, S. Saitoh, K. Takenaga, T. Mizuno, Y. Abe, K. Shibahara, Y. Tobita, S. Matsuo, K. Aikawa, S. Aozasa, K. Nakajima, Y. Miyamoto, Low-Loss and Low-DMD 6-Mode 19-Core fiber with cladding diameter of less than 250 μm, J. Lightwave Technol. 35 (3) (2017) 443–449. [3] J.V. Weerdenburg, A.V. Benitez, R.V. Uden, P. Sillard, D. Molin, A.A. Correa, E. A Lopez, M. Kuschnerov, F. Huijskens, H.D. Waardt, T. Koonen, R.A. Correa, C. Okonkwo, 10 spatial mode transmission using low differential mode delay 6-LP fiber using all-fiber photonic lanterns, Opt. Express 23 (19) (2015) 24759–24769. [4] R.G.H. van Uden, R. Amezcua Correa, E. Antonio Lopez, F.M. Huijskens, C. Xia, G. Li, A. Schülzgen, H. de Waardt, A.M.J. Koonen, C.M. Okonkwo, Ultrahigh-density spatial division multiplexing with a few-mode multicore fibre, Nat. Photonics 8 (2014) 865–870. [5] T. Mori, T. Sakamoto, M. Wada, T. Yamamoto, F. Yamamoto, Few-mode fibers supporting more than two LP modes for mode-division-multiplexed transmission with MIMO DSP, J. Lightwave Technol. 32 (14) (2014) 2468–2479. [6] A. Nakamura, K. Okamoto, Y. Koshikiya, T. Manabe, M. Oguma, T. Hashimoto, M. Itoh, High-sensitivity detection of fiber bends: 1 μm-Band mode-detection OTDR, J. Lightwave Technol. 33 (23) (2015) 4862–4869. 37
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Optics Communications 450 (2019) 34–38 [11] J. Sakamoto, T. Goh, S. Katayose, K. Watanabe, M. Itoh, T. Hashimoto, Compact and low-loss RGB coupler using mode-conversion waveguides, Opt. Commun. 420 (2018) 46–51. [12] J. Sakamoto, T. Goh, S. Katayose, R. Kasahara, T. Hashimoto, Shape-optimized multi-mode interference for a wideband visible light coupler, Opt. Commun. 433 (2019) 221–225. [13] M. Kawachi, Recent progress in silica-based planar lightwave circuits on silicon, IEEE Proc.- Optoelectron. 143 (5) (1996) 257–262. [14] R. Nagase, A. Himeno, M. Okuno, K. Kato, K. Yukimatsu, M. Kawachi, Silicabased 8 × 8 optical matrix switch module with hybrid integrated driving circuits and its system application, J. Lightwave Technol. 12 (9) (1994) 1631–1639. [15] M. Harumoto, O. Shimakawa, K. Takahashi, M. Tamura, Ultra-compact 3-port WDM filter arrays based on novel planar lightwave circuit assembly technique, in: Proc. OFC06, 2006, OWV4. [16] A. Himeno, K. Kato, T. Miya, Silica-based planar lightwave circuits, J. Sel. Top. Quantum Electron. 4 (1998) 913–924.
[7] A. Nakamura, K. Okamoto, Y. Koshikiya, T. Manabe, M. Oguma, T. Hashimoto, M. Itoh, Loss cause identification by evaluating backscattered modal loss ratio obtained with 1 μm-Band mode-detection OTDR, J. Lightwave Technol. 34 (15) (2016) 3568–3576. [8] A. Nakamura, K. Okamoto, Y. Koshikiya, T. Manabe, Highly sensitive detection of microbending in single-mode fibers and its applications, Opt. Express 25 (5) (2017) 5742–5748. [9] Y. Doi, M. Oguma, T. Yoshimatsu, T. Ohno, I. Ogawa, E. Yoshida, T. Hashimoto, H. Sanjo, Compact high-responsivity receiver optical subassembly with a multimode-output-arrayed waveguide grating for 100-Gb/s ethernet, J. Lightwave Technol. 33 (15) (2015) 3286–3292. [10] J. Sakamoto, S. Katayose, K. Watanabe, M. Itoh, T. Hashimoto, High-efficiency multiple-light-source red-green-blue power combiner with optical waveguide mode coupling technique, Proc. SPIE 10126 (2017) 101260M-1–101260M-7.
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Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Low-mode-crosstalk coupler using multistage mode conversion Junji Sakamoto a ,∗, Atsushi Nakamura b , Keiji Okamoto b , Toshikazu Hashimoto a a b
NTT Device Technology Labs., NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi, Kanagawa, Japan NTT Access Network Service Systems Labs, NTT Corporation, 1-7-1 Hanabatake, Tsukuba, Ibaraki, Japan
ARTICLE Keywords: Multimode OTDR Mode detection Optical waveguides 1 μm-band Mode converter
INFO
ABSTRACT We developed a low-mode-crosstalk coupler using silica planar lightwave circuit (PLC) technology. The coupler has two mode conversion stages. One is a mode converter for converting the fundamental mode to the secondorder mode, and the other is for converting the second-order mode to the first-order mode, which is the target mode. Isolation between fundamental mode and first-order mode can be enhanced by using indirect conversion with the second-order mode instead of direct conversion. In the fabricated coupler, the mode crosstalk between the fundamental mode and first-order mode was under -30 dB, and the conversion ratio was -0.8 dB at the wavelength of 1050 nm. The coupler can be used for sensitive 1 μm-band mode-detection optical time-domain reflectometry.
1. Introduction Optical waveguide circuits with multiple modes have been used in various optical systems. The multiple modes provide various benefits such as high-capacity transmission [1–5], sensitive optical sensing [6– 8], and optical circuit miniaturization [9–12]. One system using multiple modes is sensitive 1 μm-band mode-detection optical time domain reflectometry (OTDR) [6–8] which is used to diagnose optical fiber links. The OTDR system uses 1 μm-band probe pulses to generate not only the fundamental (LP01) mode but also the first-order (LP11a and LP11b) mode as the backscattered light in typical single-mode fibers (SMFs) compliant with ITU-T G.652. It also uses a mode coupler to separate the backscattered light into individual modes as well as to select the mode of the probe pulses. The first-order mode at the wavelength in the 1 μm band is more sensitive to trouble spots in optical fiber links, such as macro/micro-bends, than the fundamental mode in the communications wavelength band. Therefore 1 μm OTDR has a great potential for finding a potential trouble point before it has a serious impact on an optical communications system. The details of the 1 μm OTDR system are described in references [6–8]. The mode coupler, the key component of 1 μm OTDR, is fabricated by using a silica planar lightwave circuit (PLC) [13–16]. PLCs, silicabased optical waveguide circuits on a silicon substrate, were developed to construct various optical devices for optical communications, including beam splitters, wavelength filters, and optical switches [13–16]. Low loss and high mode conversion efficiency can be obtained by a mode coupler with a PLC. In this paper, we present a mode coupler with low mode crosstalk and high conversion efficiency by a multistage mode conversion ∗
method. Section 2 covers the operation of a conventional mode coupler and its problems. Section 3 describes our mode coupler, and Section 4 presents numerical simulation results. Section 5 describes and discuss the actual performance of a fabricated mode coupler. 2. Conventional mode coupler Fig. 1(a) shows a schematic of the 1 μm OTDR system. Input probe pulse selects probe mode LP01 or LP11 at the mode division multiplexing (MDM). Since the wavelength of the input light is 1.0 μm, LP01 and LP11 are induced as the modes of backscattered light at the measuring fiber. The backscattered light is separated into LP01 and LP11 components by the MDM. Then each component is detected by a photodiode. Therefore, OTDR using first-order mode can be used. To obtain the above functions, we use mode coupler with a PLC for MDM. Fig. 1(b) shows structure and operation of a conventional mode coupler. The gray line represents the core of the waveguide. The mode coupler consists of two waveguides with different widths. The fundamental mode propagating in the thin waveguide is converted to the LP11a mode by coupling to the thick waveguide. Complete mode conversion is induced by setting the waveguide widths so that the effective refractive index of each mode becomes equal. Almost all the LP01 and LP11b mode propagating in the thick waveguide passes through the coupler. The SMF at wavelength of 1.55 μm which is measured by OTDR is connected to port 4, and SMF at wavelength of 1.0 μm is connected to port 1 and 2. At the wavelength of 1.0 μm, the core diameter, refractive index difference, and cut-off wavelength of the SMF are, respectively, 5.3 μm, 0.48%, and 920 nm from the
Corresponding author. E-mail address: [email protected] (J. Sakamoto).
https://doi.org/10.1016/j.optcom.2019.05.049 Received 19 February 2019; Received in revised form 15 May 2019; Accepted 21 May 2019 Available online 23 May 2019 0030-4018/© 2019 Elsevier B.V. All rights reserved.
J. Sakamoto, A. Nakamura, K. Okamoto et al.
Optics Communications 450 (2019) 34–38
Fig. 2. Schematic of structure and operation of multi-stage mode coupler.
Fig. 3. Relationship and schematic operation of multi-stage mode coupler.
Fig. 1. (a) Schematic of 1 μm OTDR system. (b) Structure and operation of conventional mode coupler.
the first- and second-order mode curves indicate the cut-off width of the modes. As width W0, we selected 3.5 μm, where the waveguide stays single mode and W1 and W2 do not become too wide. W1 and W2 are automatically derived from W0 to match the effective refractive indexes between each mode. W1 and W2 are 10.1 and 16.4 μm in this case.
specification sheet. At 1.55 μm, they are 8.2 μm, 0.36%, and 1260 nm. The 1 μm-band probe pulse is input from either port 1 or port 2 to select the input probe mode. The input mode is selectively used depending on the length of the fiber to be measured and the necessary sensitivity. When port 1 is selected, the first-order mode probe light is generated. On the other hand, fundamental mode probe light is generated from port 2. Three modes – LP01, LP11a, and LP11b – are generated as the backscattered light. Since the higher order mode output to port 2 can be removed by adding a mode filter that narrows the waveguide, the fundamental mode and LP11a can be detected separately from the mode mixed light at port 1 and 2. Therefore the mode coupler can be used for 1 μm OTDR. The dotted lines from port 4 to port in Fig. 1(b) are mode crosstalk paths. A part of fundamental mode from port 4 is coupled to the thin waveguide with the fundamental mode and output from port 1 as a noise component. In this paper, the mode crosstalk is defined as log10 (𝑃LP01noise ∕𝑃LP01signal ), where 𝑃LP01signal is the fundamental mode component output from port 2 and 𝑃LP01noise is the noise component output from port 1. Since the noise component caused by the mode crosstalk obscures the signal component that enables us to detect macro/microbends sensitively, a mode coupler with low mode crosstalk is required.
4. Validation by numerical simulation As shown in Fig. 4(a), simulation model consists of mode coupler 1, 2, and 3. For simplicity, the simulation was conducted only for the coupler part without a fan-in and fan-out. Coupler 3 is added for the measurement because measurement system can evaluate only fundamental mode. It is omitted when the coupler is used in a 1 μm OTDR. The waveguide height, width, and 𝛥 were the same in the previous section. The gap and the length of L02 , L21 , and L10 were 3.0, 1900, 2450, 1400 μm, respectively. Fig. 4(b) shows the simulated optical propagation for the coupler. We used an three-dimensional beam propagation method. The wavelength of the input light was 1050 nm. Input light with the fundamental mode is converted to the second-order mode at coupler 1. Then, the light is converted to the first-order mode at coupler 2. Finally, the light is converted to the fundamental mode again. The coupler works as intended. Fig. 4(c) shows spectra of the coupler with coupler 3 included. The transmittance is −0.28 dB at a wavelength of 1050 nm, indicating that extra loss due to adding the waveguide hardly occurred. Next, we simulated the mode crosstalk. Fig. 5 shows the simulated optical propagation about mode crosstalk with a conventional mode coupler and the multi-stage mode coupler. The direction of optical propagation in Fig. 5 is opposite that in Fig. 4. In this simulation, a fan-out was added to calculate the output power. For the conventional mode coupler, a little light coupled to the output port is observed. For the multi-stage mode coupler, a little light coupled to the middle waveguide as the first-order mode is observed. However, the coupled light from the middle waveguide to output waveguide is not observed. This is because the effective refractive indexes between the input waveguide with the fundamental mode and the middle waveguide with first-order mode do not match and those between the middle waveguide with the first-order mode and output waveguide with the fundamental mode do not match. Since the coupler does not satisfy the mode conversion conditions, light passes through the two stages and
3. Design of multi-stage mode coupler Fig. 2 shows our mode coupler. An additional mode conversion waveguide placed between single-mode waveguide and multimode waveguide of the mode coupler and indirectly converts the mode into a desired one via intermediate mode. In this work, we choose the secondorder mode as the intermediate mode. By adding second-order mode, mode isolation is enhanced and mode crosstalk is reduced. By setting the waveguide width correctly, extra loss due to adding the waveguide does not occur owing to the all-optical power transitions between the modes. The refractive index difference (𝛥) and the core height were designed to be 0.42% and 5.6 μm. These values were decided to obtain good fiber coupling both SMFs at wavelengths of 1.0 and 1.55 μm band. Fig. 3 shows the relationship between the effective refractive index and waveguide width at a wavelength of 1050 nm. The starting points of 35
J. Sakamoto, A. Nakamura, K. Okamoto et al.
Optics Communications 450 (2019) 34–38
Fig. 5. Simulation of optical propagation with conventional mode coupler and multi-stage mode coupler.
Fig. 6. Cross section of the fabricated waveguide.
Fig. 4. (a) Layout of optical circuit for simulation. Simulated (b) optical propagation and (c) relationship between wavelength and transmittance.
the isolation between the modes is enhanced. The transmittances when the fundamental mode was input to a conventional mode coupler and the multi-stage mode coupler were −31.8 and −49.0 dB, respectively. The multi-stage mode coupler can suppress mode crosstalk by about 20 dB compared to the conventional mode coupler.
Fig. 7. Experimental set up for the measurement and fabricated mode coupler.
5. Experimental results and discussion good. From the measurement results for port 1 to 4 and port 3 to 6, the conversion efficiency of the fundamental mode to second-order mode and the first-order mode to fundamental mode were −0.36 and −0.22 dB, respectively. Since the total transmittance of the coupler is −0.86 dB, the conversion efficiency of second-order mode and first-order mode can be estimated as −0.28 dB. Fig. 9 shows the transmittance at a wavelength of 1050 nm from port 5 to 1 when the input position was changed from the alignment position in the lateral direction after coupler 3 had been omitted. The measurement indicates the mode crosstalk due to the fundamental mode at the port 5 to the fundamental mode at the port 1. The values on the x axis are the display values of the alignment machine. The transmittance at the alignment point is the minimum. When the position shifts, the transmittance increases. It means mode crosstalk is increase. This is because when the position deviates from the alignment
We fabricated the multi-stage mode coupler using a PLC. A cross section of the waveguide is shown in Fig. 6. The conventional PLC fabrication process was used [16]. Fig. 7 shows the experimental setup and fabricated mode coupler. The coupler was evaluated by using a SMF butt-coupled through its input and output port. In practice, a SMF for the wavelength of 1.55 μm will be connected to the output port of the first-order mode. To evaluate the coupler characteristics accurately at the wavelength of 1.0 μm, a SMF for 1.0 μm was used in this experiment. A non-polarized broad band light source, tunable wavelength filter, and optical power meter were used. Fig. 8 shows the measurement result for the fabricated mode coupler along with the simulation result and the optical circuit layout. The fiber coupling loss of about 0.1 dB/point does not include in the spectra. The transmittance from port 1 to 6 is −0.82 dB. Although the loss is slightly larger than the simulation, the agreement with the simulation result is 36
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Optics Communications 450 (2019) 34–38
Fig. 10. Overlap integral between 𝜑0 and 𝜑1 when changing the distance of field centers. Fig. 8. Spectra from port 1 to 6 of fabricated mode coupler using silica PLC.
Fig. 11. Transmittance at a wavelength of 1050 nm from port 5 to 1when the input position was changed from the alignment position before and after omitting coupler 3.
6. Conclusion Fig. 9. Transmittance at a wavelength of 1050 nm from port 5 to 1 when the input position was changed from the alignment position after omitting coupler 3.
We developed a multi-stage mode coupler, that can reduce mode crosstalk. The coupler has two stages of mode conversion. The first stage is mode conversion from the fundamental mode to second-order mode. The second stage is that from the second-order mode to firstorder mode. By adding the second-order mode, the mode isolation became higher than that for direct conversion from the fundamental mode to first-order mode. The total conversion efficiency and mode crosstalk of the fabricated coupler were −0.81 dB and less than −30 dB, respectively We successfully obtained low mode crosstalk coupler with high conversion efficiency. The coupler will be helpful for sensitive measurement by 1 μm-band mode-detection OTDR.
point, the first-order mode is induced. The minimum transmittance of −24 dB is higher than the simulation result in Section 3. To understand why the transmittance from port 5 to 1 is high, we consider the first-order mode coupling ratio at the fiber and waveguide coupling point. Coupling ratio 𝐶 can be calculated by the overlap integral between the field pattern of fundamental mode, 𝜑0 , at the fiber and the field pattern of first-order mode, 𝜑1 , at the waveguide: 𝐶=
∬
𝜑∗0 (𝑥, 𝑦)𝜑1 (𝑥, 𝑦)𝑑𝑥𝑑𝑦
References
Fig. 10 shows the calculated overlap integral. The 𝜑0 and 𝜑1 were calculated by the three-dimensional mode solver. Even when the mismatch distance becomes 0.1 μm, about −30 dB of first-order mode is induced in the waveguide. Considering the machine feed accuracy and the angle deviation of the fiber in the measurement system, it is estimated that the total deviation becomes about 0.1 μm. Furthermore, it is considered that the roughness of the input face and the side wall of the waveguide induces an extra first-order mode. To confirm the influence of the first-order mode induced at the input surface, we measured the transmittance from port 5 to 1 before omitting coupler 3. Fig. 11 shows the measurement result. The transmittance decreased to under −30 dB because the first-order mode induced by the deviation between the fiber and waveguide is coupled and output to port 3. This means that the mode crosstalk bottleneck is not the coupler performance but the accuracy of fiber coupling to the waveguide. Therefore, we successfully fabricated a multi-stage mode coupler with mode crosstalk from the fundamental mode suppressed to under −30 dB. On the other hand, to derive the performance of the optical circuit, a high-precision fiber connection is required. For example, to obtain under −30 dB mode crosstalk, it is necessary to fix the fiber with an accuracy of 0.1 μm or less because coupler 3 will be omitted in practical applications.
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