Optical signal measuring device and optical signal measuring method
The optical signal measuring device employs a two-dimensional photodetector array to separate and observe orthogonal polarization components of interference light, addressing the challenge of high-speed waveform capture in multimode optical fibers using low-cost electronics.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Patents
- Current Assignee / Owner
- NIPPON TELEGRAPH & TELEPHONE CORP
- Filing Date
- 2023-02-27
- Publication Date
- 2026-07-23
AI Technical Summary
Existing techniques fail to capture high-speed waveform changes in optical communication due to the limitations of infrared cameras and the interference between photodetector elements, making it difficult to observe the spatial distribution of complex electric fields in multimode optical fibers at speeds of 10 to 100 Gbaud/s.
An optical signal measuring device and method that uses a two-dimensional photodetector array to separate and observe orthogonal polarization components of interference light generated by combining signal and sampling pulse lights with a tilted polarization direction, employing low-speed and inexpensive electronic components.
Enables measurement of ultra-high-speed multimode communication signals using low-speed and cost-effective components, capable of handling repetitive and pseudo-random bit sequence signals.
Smart Images
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Abstract
Description
Technical Field
[0001] The present disclosure relates to a two-dimensional optical signal measurement technique for analyzing the propagation characteristics of a multimode optical fiber itself.
Background Art
[0002] Techniques for multiplexing signals into multiple spatial modes of a multimode optical fiber (spatial multiplexing communication) have been developed. In a multimode optical fiber, the electric field vectors of the light in each mode have a spatial distribution that is orthogonal to each other. FIG. 1 sketches the electric field vector distributions of two lowest-order mode groups (LP01 mode, LP11 mode) of a multimode optical fiber. The arrows in FIG. 1 represent the electric field vectors. In a spatial multiplexing communication system, different signals are multiplexed by these independent spatial vector modes. Each spatial mode is separated by a "mode demultiplexer" and coupled to a separate single-mode fiber, after which its complex amplitude is "received".
[0003] When analyzing the propagation characteristics of a multimode optical fiber itself, it is desired to directly observe the change in the spatial distribution of the complex electric field of the core cross-section of the optical fiber (the time waveform for each mode). That is, a "two-dimensional optical signal measurement technique" for observing the two-dimensional spatial complex amplitude distribution while following the signal speed of optical communication (10 to 100 Gbaud / s) is required, but such a technique does not exist. One of the reasons is that the realization of a photodetector array operating in such a high-speed region is almost impossible due to interference between elements.
Prior Art Documents
Non-Patent Documents
[0004]
Non-Patent Document ۱
[0005] Non-patent document 1 discloses a method for directly observing changes in the spatial distribution of complex electric fields (time waveforms for each mode) in the core cross-section of an optical fiber using an infrared camera. However, the response characteristics of typical infrared cameras are not fast enough to keep up with the signal speed of optical communication (10 to 100 Gbaud / s). In other words, currently there is a challenge in capturing high-speed waveform changes such as those at the signal speed of optical communication.
[0006] Therefore, the present invention aims to provide an optical signal measuring device and an optical signal measuring method that can measure the complex amplitude of ultra-high-speed multimode communication using low-speed and inexpensive electronic components, in order to solve the aforementioned problems. [Means for solving the problem]
[0007] To achieve the above objective, the optical signal measuring device according to the present invention extracts a repeatedly modulated signal light to be measured and a sampling pulse light having the same wavelength as the signal light to be measured but a different repetition period into space and interferes with them, separates the interference signal into orthogonal polarization components, and observes the spatial distribution of each polarization component with a two-dimensional photodetector array.
[0008] Specifically, the optical signal measuring device according to the present invention is an optical signal measuring device that observes the time waveform of each mode of a signal light to be measured, which is modulated with a repetition period T and propagates through a multimode optical fiber in two lowest-order mode groups, A pulse generator that generates sampling pulse light having the same wavelength as the signal light under measurement and a repetition period T+ΔT, An interference unit that outputs the signal light to be measured into space from the multimode optical fiber, outputs the sampling pulse light into space with its polarization direction tilted 45° relative to the signal light to be measured, and generates interference light by interfering the signal light to be measured and the sampling pulse light in space, A two-dimensional photodetector array separates the aforementioned interference light into orthogonal polarization components and observes the spatial distribution of each polarization component, It is characterized by having the following features.
[0009] Furthermore, the optical signal measurement method according to the present invention is The measurement signal, modulated with a repetition period T, is propagated in two lowest-order mode groups using a multimode optical fiber. To generate sampling pulse light with the same wavelength as the signal light to be measured and a repetition period T+ΔT, To output the signal light to be measured into space from the multimode optical fiber, The polarization direction is tilted 45° with respect to the signal light to be measured and the sampled pulse light is output into the space. In the aforementioned space, interference is generated by interfering the signal light to be measured with the sampling pulse light, and The interference light is separated into orthogonal polarization components, and the spatial distribution of each polarization component is received by a two-dimensional photodetector array. The time waveforms of each mode of the signal light under measurement propagating through the multimode optical fiber are observed.
[0010] This optical signal measurement device (method) employs a linear sampling method. The linear sampling method is a technique for measuring the instantaneous complex amplitude of a signal light by detecting the coherent correlation between the signal light and a short optical pulse (sampling pulse light) having the same wavelength as the signal light. As shown in Figure 2, this method slightly detunes (ΔT) the repetition period T of the signal light 21 and the period of the sampling pulse light 22, and by detecting the instantaneous amplitude 23 while shifting the interference point, the waveform 24 of the entire signal light can be observed.
[0011] The speed of the observable signal is determined solely by the width (T+ΔT) of the sampling pulse light 22, making it capable of handling ultrafast signals. The electronic components used only need to be able to keep up with the sampling pulse period, and those with a bandwidth of several MHz can be used. The observable signal light is limited to repetitive signals, but it can also handle pseudo-random bit sequence (PRBS) signals used for verifying communication systems.
[0012] Therefore, the present invention can provide an optical signal measuring device and an optical signal measuring method that can measure the complex amplitude of ultra-high-speed multimode communication using low-speed and inexpensive electronic components. [Effects of the Invention]
[0013] The present invention provides an optical signal measuring device and optical signal measuring method that can measure the complex amplitude of ultra-high-speed multimode communication using low-speed and inexpensive electronic components. [Brief explanation of the drawing]
[0014] [Figure 1] This is a diagram illustrating the spatial modes of optical fibers. [Figure 2] This is a diagram illustrating the linear optical sampling method. [Figure 3] This is a diagram illustrating the optical signal measuring device according to the present invention. [Modes for carrying out the invention]
[0015] Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, components having the same reference numerals are assumed to be the same components.
[0016] FIG. 3 is a diagram for explaining the optical signal measurement device 301 of the present embodiment. The optical signal measurement device 301 is a device that observes the time waveform for each mode of the measurement signal light modulated at the repetition period T, which propagates in two lowest-order mode groups in a multimode optical fiber. A pulse generator that generates sampling pulse light having the same wavelength as the measurement signal light and a repetition period T + ΔT. An interference unit that outputs the measurement signal light from the multimode optical fiber into space, outputs the sampling pulse light into the space with the polarization direction inclined by 45° with respect to the measurement signal light, and generates interference light by interfering the measurement signal light and the sampling pulse light in the space. A two-dimensional photodetector array that separates the interference light into orthogonal polarization components and observes the spatial distribution of each polarization component. It includes.
[0017] The optical signal measurement device 301 propagates the measurement signal light 21 modulated at the repetition period T in a multimode optical fiber 31, which is a measurement optical fiber, in two lowest-order mode groups (LP 01 mode and LP 11 mode, see FIG. 1). The optical signal measurement device 301 generates sampling pulse light 22 having the same wavelength as the measurement signal light 21 and a repetition period T + ΔT (frequency f Hz). In the optical signal measurement device 301 of FIG. 3, the pulse generator that generates the sampling pulse light 22 is, for example, a mode-locked laser 33 that oscillates an ultrashort pulse (pulse width of about 1 picosecond) having the same wavelength as the modulation signal light at a period of f Hz (about 10 MHz).
[0018] The optical signal measuring device 301 outputs the signal light 21 to be measured from the multimode optical fiber 31 into space 30. Furthermore, the optical signal measuring device 301 outputs the sampling pulse light 22 into space 30 with its polarization direction tilted by 45° relative to the signal light 21 to be measured. Then, in space 30, the signal light 21 to be measured and the sampling pulse light 22 interfere to generate interference light 27. The signal light under measurement 21 and the sampling pulse light 22 transmitted through the multimode optical fiber 31 are output into space 30 by a lens system (green lens 32 and lens 39), and are combined and interfered with by, for example, a beam splitter 35 to become interference light 27. In the optical signal measuring device 301 of Figure 3, the interference part is the green lens 32, lens 39, and beam splitter 35.
[0019] The optical signal measuring device 301 separates the interfering light 27 into orthogonal polarization components and receives the spatial distribution of each polarization component with a two-dimensional photodetector array 37. The interfering light 27 is separated into x-polarized and y-polarized components by the polarizing beam splitter 36. The separated light is then received by photodetector arrays (2×2PDA) (37-x and 37-y) to obtain the x and y components (4 channels each for the x and y components) of the vector mode electric field distribution. Using these signals (f 8-channel signals per second), the complex amplitudes of the orthogonal spatial modes shown in Figure 1 can be identified.
[0020] The PDA37 only needs to be able to keep up with the repetition frequency f of the mode-locked laser 33, so a low-speed and inexpensive device can be used.
[0021] In the optical signal measuring device 301 in Figure 3, the PDA37 is configured in a 2x2 configuration, LP 01 Mode, LP 11 It is possible to observe modes. In the optical signal measuring device 301, the number of modes that can be observed can be expanded by increasing the number of elements in the PDA 37. [Explanation of symbols]
[0022] 21: Signal light under test 22: Sampling pulse light 23: Instantaneous Amplitude 24: Waveform 25: Sampling Clock 27: Interferometry 30: Space 31: Optical fiber under test (multimode optical fiber) 32: Green Lens 33: Mode-locked laser 34: Polarization-maintaining fiber 35: Beam Splitter 36: Polarizing Beam Splitter 37:2×2-PDA 38:8ch-A / D conversion 39: Lens 301: Optical signal measuring device
Claims
1. An optical signal measuring device for observing the time waveform of each mode of a signal light under test, modulated with a repetition period T, which propagates through a multimode optical fiber in multiple linearly polarized modes including the fundamental mode and higher-order modes, A pulse generator that generates sampling pulse light having the same wavelength as the signal light to be measured and a repetition period T + ΔT, An interference unit that outputs the signal light to be measured into space from the multimode optical fiber, outputs the sampling pulse light into space with its polarization direction tilted 45° relative to the signal light to be measured, and generates interference light by interfering the signal light to be measured and the sampling pulse light in space, A two-dimensional photodetector array separates the aforementioned interference light into orthogonal polarization components and observes the spatial distribution of each polarization component, An optical signal measuring device characterized by comprising the following features.
2. The measurement signal light, modulated with a repetition period T, is propagated in a multimode optical fiber using multiple linearly polarized modes, including the fundamental mode and higher-order modes. To generate sampling pulse light with the same wavelength as the signal light under measurement and a repetition period T + ΔT, To output the signal light to be measured into space from the multimode optical fiber, The polarization direction is tilted 45° with respect to the signal light to be measured and the sampled pulse light is output into the space. In the aforementioned space, interference is generated by interfering the signal light to be measured with the sampling pulse light, and The aforementioned interference light is separated into orthogonal polarization components, and the spatial distribution of each polarization component is received by a two-dimensional photodetector array. An optical signal measurement method for observing the time waveform of each mode of the signal light to be measured propagating through the multimode optical fiber.