Optical communication system, optical communication method, receiver, optical waveguide, and transmitter

Abstract

This invention enables reduction of power consumption while ensuring the waveform quality of a received signal.　Provided is an optical communication system wherein a transmitter and a receiver are connected by an optical waveguide and communicate with each other using light of a second wavelength. Herein, the optical waveguide only propagates a fundamental mode at a first wavelength, and the second wavelength is a wavelength at which the optical waveguide can propagate at least a primary mode together with the fundamental mode. An inter-mode propagation delay difference adjusting unit makes an adjustment such that the primary mode lags by one unit interval relative to the fundamental mode in an optical communication path for the light of the second wavelength including the optical waveguide. Furthermore, for example, a mode ratio adjusting unit adjusts the ratio between the fundamental mode and the primary mode in the light of the second wavelength incident on the optical waveguide from the transmitter.

Description

[Technical Field]

[0001] The present technology relates to an optical communication system, an optical communication method, a receiver, an optical waveguide, and a transmitter, and more particularly to an optical communication system that enables reduction in power consumption while ensuring waveform quality. [Background technology]

[0002] Optical communication using spatial coupling has been known for some time. In this type of optical communication, misalignment, especially in single-mode fibers, can cause significant loss of optical power. Therefore, in the past, high precision was required for components to minimize misalignment, leading to increased costs.

[0003] The present applicant has previously proposed an optical communication device that can reduce costs by relaxing the precision of positional deviation, a so-called double-mode optical communication device (see Patent Document 1). This optical communication device has an optical waveguide that propagates only fundamental mode light at a first wavelength, and communicates using light of a second wavelength. Here, the second wavelength is a wavelength that allows the optical waveguide to propagate at least the first-order mode as well as the fundamental mode.

[0004] Furthermore, it has been known that when driving a light emitting element such as a laser diode on the transmitting side of an optical communication system, it is necessary to pass a certain amount of bias current in order to ensure waveform quality. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2020 / 153236 Summary of the Invention [Problem to be solved by the invention]

[0006] The purpose of this technology is to reduce power consumption while ensuring the waveform quality of the received signal. [Means for solving the problem]

[0007] The concept of this technology is: An optical communication system in which a transmitter and a receiver are connected by an optical waveguide and communicate using light of a second wavelength, the optical waveguide propagates only a fundamental mode at a first wavelength; the second wavelength is a wavelength at which the optical waveguide can propagate at least a first-order mode along with the fundamental mode; an inter-mode propagation delay difference adjusting unit that adjusts one of the fundamental mode and the first-order mode so that it is delayed by one unit interval relative to the other in an optical communication path for light of the second wavelength that includes the optical waveguide; in optical communication systems.

[0008] This technology is an optical communication system in which a transmitter and a receiver are connected by an optical waveguide and communication is performed using light of a second wavelength. Here, the optical waveguide propagates only the fundamental mode at the first wavelength, and the second wavelength is a wavelength at which the optical waveguide can propagate at least the first-order mode as well as the fundamental mode. An inter-modal propagation delay difference adjustment unit adjusts one of the fundamental mode and the first-order mode to be delayed by one unit interval relative to the other in the optical communication path of light of the second wavelength, which includes the optical waveguide.

[0009] For example, the modal propagation delay difference adjusting unit may be configured with an optical waveguide. In this case, for example, the length and the refractive index distribution of the core and cladding of the optical waveguide are set so that one of the fundamental mode and the first-order mode is delayed by one unit interval relative to the other when light of the second wavelength propagates. When the modal propagation delay difference adjusting unit is configured with an optical waveguide in this way, it becomes possible to easily and reliably adjust one of the fundamental mode and the first-order mode so that it is delayed by one unit interval relative to the other in the optical communication path of light of the second wavelength.

[0010] Furthermore, for example, the modal propagation delay difference adjustment unit may be configured with an optical waveguide and a variable phase shifter in the receiver. In this way, when the modal propagation delay difference adjustment unit is configured with an optical waveguide and a variable phase shifter in the receiver, it becomes possible to use a general-purpose optical waveguide that is not adjusted so that one of the fundamental mode and the first-order mode is delayed by one unit interval relative to the other when light of the second wavelength propagates.

[0011] In this case, for example, the inter-modal propagation delay difference between the fundamental mode and the first-order mode in the variable phase shifter may be adjusted based on waveform quality information of a received signal obtained corresponding to light of the second wavelength via the optical communication path. For example, the inter-modal propagation delay difference between the fundamental mode and the first-order mode in the variable phase shifter is adjusted in a direction that reduces the level of overshoot or undershoot appearing in the received signal, or in a direction that reduces the bit error rate of the received signal. In this way, by adjusting the inter-modal propagation delay difference between the fundamental mode and the first-order mode in the variable phase shifter based on waveform quality information of a received signal obtained corresponding to light of the second wavelength via the optical communication path, it is possible to accurately adjust the waveform quality of the received signal in a direction that improves it.

[0012] In this case, the inter-modal propagation delay difference between the fundamental mode and the first-order mode in the variable phase shifter may be adjusted based on information about the inter-modal propagation delay difference between the fundamental mode and the first-order mode generated in the optical waveguide. By adjusting the inter-modal propagation delay difference between the fundamental mode and the first-order mode in the variable phase shifter in this way based on information about the inter-modal propagation delay difference between the fundamental mode and the first-order mode generated in the optical waveguide, it becomes possible to simply and accurately adjust one of the fundamental mode and the first-order mode to be delayed by one unit interval relative to the other in the optical communication path of light of the second wavelength.

[0013] In this way, this technology adjusts one of the fundamental mode and the first-order mode in the optical communication path of light of the second wavelength, including the optical waveguide, so that it is delayed by one unit interval relative to the other.Even if the bias current is kept low when driving a light-emitting element such as a laser diode on the transmitting side, deterioration of the waveform quality of the received signal can be suppressed, and therefore power consumption can be reduced while maintaining the waveform quality of the received signal.

[0014] In addition, the present technology may further include, for example, a mode ratio adjusting unit that adjusts the ratio between the fundamental mode and the first-order mode in the light of the second wavelength that is input from the transmitter to the optical waveguide. By adjusting the ratio between the fundamental mode and the first-order mode in the light of the second wavelength that is input from the transmitter to the optical waveguide in this manner, it is possible to further improve the waveform quality of the received signal.

[0015] For example, the mode ratio adjuster may adjust the amount of deviation of the core position of the optical fiber from the optical axis of a receptacle for connecting the optical waveguide of the transmitter, thereby making it possible to easily adjust the ratio of the fundamental mode and the first-order mode in the light of the second wavelength input from the transmitter to the optical waveguide.

[0016] In this case, for example, the adjustment of the deviation of the core position may be performed based on a control signal sent from the receiver, which makes it possible to easily adjust the deviation of the core position from the receiver side.

[0017] For example, the receiver may generate a control signal based on waveform quality information of a received signal obtained corresponding to light of the second wavelength via the optical communication path. Here, for example, the amount of core position shift is adjusted in a direction to reduce the level of overshoot or undershoot appearing in the received signal, or in a direction to reduce the bit error rate of the received signal. In this way, by generating a control signal based on waveform quality information of a received signal obtained corresponding to light of the second wavelength via the optical communication path, it is possible to accurately adjust the waveform quality of the received signal to improve it.

[0018] Another concept of the present technology is An optical communication method for communicating using light of a second wavelength in an optical communication system in which a transmitter and a receiver are connected by an optical waveguide, comprising: the optical waveguide propagates only a fundamental mode at a first wavelength; the second wavelength is a wavelength at which the optical waveguide can propagate at least a first-order mode along with the fundamental mode; In an optical communication path for light of the second wavelength including the optical waveguide, one of the fundamental mode and the first-order mode is adjusted to be delayed by one unit interval relative to the other. It is an optical communication method.

[0019] Another concept of the present technology is an optical input unit for inputting light of a second wavelength transmitted from a transmitter via an optical waveguide; the optical waveguide propagates only a fundamental mode at a first wavelength; the second wavelength is a wavelength at which the optical waveguide can propagate at least a first-order mode along with the fundamental mode; a control unit that generates a control signal for adjusting a ratio of the fundamental mode and the first-order mode of the second light input from the transmitter to the optical waveguide. It's in the receiver.

[0020] Another concept of the present technology is an optical input unit for inputting light of a second wavelength transmitted from a transmitter via an optical waveguide; the optical waveguide propagates only a fundamental mode at a first wavelength; the second wavelength is a wavelength at which the optical waveguide can propagate at least a first-order mode along with the fundamental mode; a variable phase shifter for adjusting the inter-mode propagation delay difference between a fundamental mode and a first-order mode in the light of the second wavelength input to the optical input unit; a control unit that generates a control signal for controlling the variable phase shifter; It's in the receiver.

[0021] In this case, for example, the control unit may further generate a control signal for adjusting the ratio between the fundamental mode and the first-order mode of the light of the second wavelength input from the transmitter to the optical waveguide.

[0022] Another concept of the present technology is At the first wavelength, only the fundamental mode propagates, propagating at least a first order mode along with the fundamental mode at a second wavelength; The length and the refractive index profiles of the core and cladding are set so that one of the fundamental mode and the first-order mode is delayed by one unit interval relative to the other when light of the second wavelength propagates. in the optical waveguide.

[0023] Another concept of the present technology is an optical output unit for outputting light of the second wavelength to a receiver via an optical waveguide; the optical waveguide propagates only a fundamental mode at a first wavelength; the second wavelength is a wavelength at which the optical waveguide can propagate at least a first-order mode along with the fundamental mode; The optical output section is configured to be able to adjust the ratio of the fundamental mode and the first-order mode in the light of the second wavelength input to the optical waveguide. It's in the transmitter. [Brief explanation of the drawings]

[0024] [Figure 1] FIG. 1 is a diagram illustrating an overview of optical communication using spatial coupling. [Figure 2] 1 is a diagram showing the basic structure of an optical fiber and the LPml mode of a step index optical fiber. [Figure 3] This is a diagram considering the normalized frequency V in the case of a typical single-mode fiber with a wavelength of 1310 nm. [Figure 4] 10A and 10B are diagrams illustrating an example of factors that cause deterioration in accuracy of optical axis alignment. [Figure 5] 10A and 10B are diagrams illustrating an example of factors that cause deterioration in accuracy of optical axis alignment. [Figure 6] FIG. 1 is a diagram for explaining that when light with a wavelength of 850 nm is input to a 1310 nm single-mode fiber, a fundamental mode of LP01 and a first-order mode of LP11 can exist. [Figure 7] This is a diagram for considering the case where an optical axis misalignment occurs under the condition that only the fundamental mode of LP01 exists in the input light. [Figure 8] 10 is a graph showing the results of a simulation of the amount of loss when the wavelength of input light is 1310 nm and 850 nm. [Figure 9] FIG. 10 is a diagram showing that when there is no optical axis misalignment, only the fundamental mode exists in the input light, but when there is an optical axis misalignment, part of the fundamental mode is converted into the first-order mode. [Figure 10] 10 is a graph for explaining that the fundamental mode is converted into the first-order mode depending on the deviation. [Figure 11] FIG. 1 is a block diagram showing an example of the configuration of a double-mode optical communication system. [Figure 12] FIG. 2 is a diagram illustrating an example of the configuration of a driver IC and a light-emitting unit of a transmitter. [Figure 13] FIG. 10 is a diagram illustrating an example of a VCSEL output frequency characteristic. [Figure 14] FIG. 10 is a diagram illustrating an example of an eye pattern of an optical waveform. [Figure 15] This is a diagram showing an example of the propagation of light consisting of the fundamental mode and first-order mode at a wavelength of 850 nm from an 850 nm light source through a conventional 1310 nm fiber (a single-mode optical fiber that propagates only the fundamental mode (zeroth-order mode) at a wavelength of 1310 nm). [Figure 16] FIG. 10 is a diagram showing an example of the fundamental mode, first-order mode, and combined waveform at the input end (point A) and output end (point B) of a 1310 nm fiber when the bias current applied to the light emitting element of the 850 nm light source is small. [Figure 17] This is a diagram showing the combined waveforms of the fundamental mode and the first-order mode at the input end (point A) and the output end (point B) in the form of eye patterns. [Figure 18]1 is a block diagram showing an example of the configuration of an optical communication system according to a first embodiment; [Figure 19] 10A and 10B are diagrams for explaining an example of control of the refractive index distribution of the core and cladding. [Figure 20] FIG. 10 is a diagram illustrating an example of a state in which a receptacle of a transmitter and a plug of a cable are connected. [Figure 21] FIG. 2 is a perspective view showing a schematic configuration of a transmitter receptacle and a cable plug. [Figure 22] 10A and 10B are diagrams illustrating an example of a circuit configuration for acquiring the level of an overshoot that appears in a received signal. [Figure 23] 10 is a block diagram showing another example of the configuration of an optical communication system. [Figure 24] FIG. 10 is a block diagram showing a configuration example of an optical communication system according to a second embodiment. [Figure 25] 1A and 1B are diagrams illustrating an example of the configuration of a variable phase shifter. DETAILED DESCRIPTION OF THE INVENTION

[0025] The following describes modes for carrying out the invention (hereinafter referred to as "embodiments") in the following order: 1. Embodiment 2. Variations

[0026] <1. Embodiment> [Explanation of technologies related to this technology] First, we will explain the technology related to this technique. Figure 1 shows an overview of optical communication using spatial coupling. In this case, light emitted from an optical fiber 10T on the transmitting side is shaped into collimated light by a lens 11T and then emitted. This collimated light is then focused by a lens 11R on the receiving side and input to an optical fiber 10R. In this optical communication, misalignment can cause a large loss of optical power, especially in single-mode fibers. Note that the optical fibers 10T and 10R have a double structure consisting of a central core 10a, which serves as the optical path, and a cladding 10b that surrounds it.

[0027] Next, we will explain the basic concept of modes. When attempting to propagate in a single mode through an optical fiber, parameters such as the refractive index and core diameter of the fiber must be determined so that only one mode exists.

[0028] Figure 2(a) shows the basic structure of an optical fiber. An optical fiber has a central part called the core covered with a layer called the cladding. In this case, the refractive index of the core, n1, is high and the refractive index of the cladding, n2, is low, so light is confined within the core and propagates.

[0029] Figure 2(b) shows the LPml (Linearly Polarized) mode of a step-index optical fiber, showing the normalized propagation constant b as a function of normalized frequency V. The vertical axis is the normalized propagation constant b, where b = 0 when a certain mode does not pass (is blocked), and as more optical power is confined within the core (the more it can propagate), b approaches 1. The horizontal axis is the normalized frequency V, which can be expressed by the following equation (1): where d is the core diameter, NA is the numerical aperture, and λ is the wavelength of light. V=πdNA / λ (1)

[0030] For example, when V = 2.405, LP11 is cut off, and only LP01 mode exists. Therefore, the state below V = 2.405 is a single mode. Here, LP01 is the fundamental mode (zeroth-order mode), and LP11, LP21, etc. are the first-order mode, second-order mode, etc., respectively.

[0031] For example, consider the normalized frequency V in the case of a typical single-mode fiber with a wavelength of 1310 nm, as shown in Figure 3(a). If the core diameter d and numerical aperture NA are set to 8 μm and 0.1, respectively, which are typical parameters for a 1310 nm optical fiber, and the wavelength of the light propagating through the fiber is 1310 nm, then V = 1.92, as determined from equation (1).

[0032] Therefore, as shown in Figure 3(b), the normalized frequency V is 2.405 or less, so only the fundamental mode of LPO1 propagates, resulting in a single mode. Increasing the core diameter here increases the number of modes that can propagate. Incidentally, for example, a typical multimode fiber can propagate several hundred modes by setting the core diameter to a value of 50 μm.

[0033] When considering optical communications using spatial coupling as shown in Figure 1, the small core diameter of a single-mode fiber makes it difficult to align the optical coupling parts on the transmitting and receiving sides, posing a problem of high precision requirements for accurately aligning the optical axes.

[0034] To solve this problem, high-precision parts are generally used, or the optical input part of the optical fiber is processed to make it easier to insert light into the fiber core. However, high-precision parts are expensive, and parts that require processing are expensive to process, so connectors and systems for single-mode communication are generally expensive.

[0035] 4 and 5 show examples of factors that can cause deterioration in the accuracy of optical axis alignment. For example, as shown in Fig. 4(a), optical axis misalignment occurs due to uneven amounts of fixing materials 16T and 16R that fix ferrules 15T and 15R to optical fibers 10T and 10R. Also, as shown in Fig. 4(b), optical axis misalignment occurs due to insufficient shaping accuracy of lenses 11T and 11R.

[0036] 5(a) and (b), the optical axis misalignment occurs due to insufficient precision of the alignment mechanisms (concave portion 17T, convex portion 17R) provided on the ferrules 15T and 15R. Note that the convex portion 17R shown in FIGS. 5(a) and (b) may be a pin.

[0037] This technology enables reduction in power consumption while maintaining the waveform quality of received signals in, for example, a double-mode optical communication system in which cost reductions can be achieved by relaxing the precision of positional deviations.

[0038] Here, a double-mode optical communication system has an optical waveguide that propagates only the fundamental mode at a first wavelength, and communicates using light of a second wavelength, where the second wavelength is a wavelength at which the optical waveguide can propagate at least the first-order mode as well as the fundamental mode.

[0039] Let us now explain a double-mode optical communication system. For example, if light with a wavelength of 850 nm, rather than 1310 nm, is input into an optical fiber under the same conditions as in Figure 3(a), the normalized frequency V will be 2.96, as shown in Figure 6(b). Therefore, as shown in Figure 6(a), the fundamental mode of LP01 and the first-order mode of LP11 can exist.

[0040] Consider the case where an optical system such as that shown in Figure 7(a) is constructed, and only the fundamental mode of LPO1 exists in the input light, and the position of the optical fiber on the receiving side is shifted in the direction perpendicular to the optical axis (see the arrows in Figures 7(a) and (b)), i.e., optical axis misalignment occurs.

[0041] Figure 8 is a graph showing the results of a simulation of the optical power coupling efficiency in this case. The horizontal axis represents the amount of optical axis misalignment, and the vertical axis represents the coupling efficiency. When there is no misalignment, 100% of the power propagates into the optical fiber, resulting in a coupling efficiency of 1. If, for example, only 50% of the power of the input light propagates into the optical fiber, the coupling efficiency is 0.5.

[0042] Comparing the input light wavelengths of 1310 nm and 850 nm, it can be seen that the characteristics are better at 850 nm. This is because at 1310 nm, only the fundamental mode can propagate, whereas at 850 nm, both the fundamental mode and the first-order mode can propagate (see Figure 6(a)).

[0043] In other words, when there is no optical axis misalignment, only the fundamental mode exists in the input light, as shown in Figure 9(a). On the other hand, when there is optical axis misalignment, part of the fundamental mode is converted to the first-order mode by taking advantage of the phase difference caused by the difference in refractive index between the cladding and the core, as shown in Figure 9(b). At 1310 nm, this first-order mode cannot propagate, but at 850 nm, this first-order mode can also propagate, resulting in improved characteristics at 850 nm.

[0044] The graph in Figure 10 shows the fundamental mode (0th-order mode) component and the 1st-order mode component separately, and the sum of these forms the total curve. Since only the fundamental mode exists in the input light, it can be seen that the fundamental mode is converted to the 1st-order mode depending on the deviation. On the other hand, in the case of 1310 nm, only the fundamental mode can propagate, as shown in Figure 3(a), so the fundamental mode is purely reduced, as shown in Figure 8.

[0045] In FIG. 8, for 1310 nm and 850 nm, the precision requirement for positional deviation can be relaxed by approximately 1.8 times when the coupling efficiency is 0.8 (approximately -1 dB), and by approximately 2.35 times when the coupling efficiency is 0.9 (approximately -0.5 dB).

[0046] In this way, by configuring an optical fiber to be capable of propagating only the fundamental mode at a first wavelength (e.g., 1310 nm), and by configuring the optical fiber to communicate using light of a second wavelength (e.g., 850 nm) that can propagate the first-order mode as well as the fundamental mode, it is possible to increase the optical power coupling efficiency.

[0047] 11 shows an example of the configuration of a double-mode optical communication system 10. This optical communication system 10 has a transmitter 100, a receiver 200, and a cable 300. The transmitter 100 and the receiver 200 are connected via the cable 300.

[0048] The transmitter 100 is, for example, an AV source such as a personal computer, a game console, a disc player, a set-top box, a digital camera, a mobile phone, etc. The receiver 200 is, for example, a television receiver, a projector, a head-mounted display, etc.

[0049] Transmitter 100 has a transmission processing unit 104, a driver IC 105, a light emitting unit 101, an optical fiber 103, and a receptacle 102. Light emitting unit 101 is equipped with a light emitting element such as a laser diode (VCSEL) or an LED (light emitting diode). Light emitting unit 101 is driven by driver IC 105 based on transmission data supplied from transmission processing unit 104, and outputs light (optical signal) corresponding to the transmission data. Optical fiber 103 propagates the optical signal output from light emitting unit 101 to receptacle 102, which serves as an optical output unit.

[0050] Receiver 200 has receptacle 201, light receiving unit 202, optical fiber 203, amplifier unit 204, and reception processing unit 205. Light receiving unit 202 is equipped with a light receiving element such as a photodiode. Light receiving unit 202 converts an optical signal sent from receptacle 201, which serves as an optical input unit, via optical fiber 203 into an electrical signal. The electrical signal output from light receiving unit 202 is amplified by amplifier unit 204 and supplied to reception processing unit 205 as a received signal. Reception processing unit 205 performs processing such as data sampling and demodulation on the received signal to obtain received data.

[0051] The cable 300 has plugs 302 and 303 at one end and the other end of an optical fiber 301 serving as an optical waveguide. The plug 302 at one end of the optical fiber 301 is connected to a receptacle 102 of the transmitter 100, and the plug 303 at the other end of the optical fiber 301 is connected to a receptacle 201 of the receiver 200.

[0052] The optical fiber 103 of the transmitter 100, the optical fiber 203 of the receiver 200, and the optical fiber 301 of the cable 300 are configured to propagate only fundamental mode components at a first wavelength. These optical fibers are also configured to have zero chromatic dispersion at the first wavelength. For example, the first wavelength is 1310 nm, and the core diameter d and numerical aperture NA are set to d=8 μm and NA=0.1, which are typical parameters of a 1310 nm optical fiber, respectively, and the normalized frequency V is set to 1.92. As a result, these optical fibers function as single-mode fibers at the wavelength of 1310 nm (see FIG. 3).

[0053] In the optical communication system 10, communication is performed using light of a second wavelength. Here, the second wavelength is a wavelength at which each of the above-mentioned optical fibers can propagate not only the fundamental mode but also the first mode. Specifically, for example, the second wavelength is 850 nm. When light of 850 nm is used, these optical fibers have a normalized frequency V=2.96, and therefore can propagate not only the fundamental mode but also the first mode, and function as a double-mode fiber (see FIG. 6).

[0054] 12 shows an example of the configuration of the driver IC 105 and light-emitting unit 101 of the transmitter 100. In this example, the light-emitting unit 101 is configured as a laser diode (LD) serving as a light-emitting element, and the driver IC 105 constitutes a laser diode driver (LDD). The driver IC 105 is configured to pass a bias current Ib from a power supply VDD1 to the laser diode, and to pass a modulation current Im corresponding to transmission data from a power supply VDD2 to the laser diode. This causes the laser diode to output light (optical signal) corresponding to the transmission data.

[0055] It is known that the frequency characteristics of a laser diode change depending on the bias current Ib, as shown in Figure 13, where the VCSEL output frequency characteristics are generally higher, and the smaller the bias current Ib, the more peaking the characteristics become. In this case, as shown in Figure 14, the optical waveform eye pattern, the smaller the bias current Ib, that is, the more peaking the frequency characteristics become, the more degraded the optical waveform becomes.

[0056] This technology takes advantage of the double-mode characteristic that when light propagates through an optical waveguide (e.g., optical fiber), at least the first-order mode can be propagated along with the fundamental mode (zeroth-order mode). This ensures the waveform quality of the received signal and reduces power consumption even when the bias current flowing through the light-emitting element is reduced.

[0057] Figure 15 shows an example of the propagation of light consisting of the fundamental mode and first-order mode at a wavelength of 850 nm from an 850 nm light source through a conventional 1310 nm fiber (a single-mode optical fiber that propagates only the fundamental mode (zeroth-order mode) at a wavelength of 1310 nm).

[0058] In this case, a difference in inter-mode propagation delay occurs between the fundamental mode and the first-order mode at the output end of the optical fiber. This difference in inter-mode propagation delay occurs due to the difference in the reflection angle of the light components of each mode within the optical fiber. In this case, the higher the reflection angle, the steeper it becomes, so the higher the order, the greater the delay.

[0059] This technology utilizes this inter-mode propagation delay difference to delay the first-order mode by one unit interval relative to the fundamental mode, thereby improving the waveform quality of the received signal when the bias current flowing through the light-emitting element is reduced to reduce power consumption.

[0060] FIG. 16 shows an example of the fundamental mode, first-order mode, and combined waveform at the input end (point A) and output end (point B) of a 1310 nm fiber when the bias current flowing through the light emitting element of the 850 nm light source is small.

[0061] As shown in Figure 16(a), at the input end (point A), there is no difference in propagation delay between modes, so the fundamental mode and first-order mode are in phase. In this case, the fundamental mode and first-order mode have the same waveform, so the combined waveform will have overshoot and undershoot at the rising and falling edges. If this combined waveform is shown as an eye pattern, it will look like the waveform when the bias current Ib in Figure 14 above is small.

[0062] In a 1310 nm fiber, when the first-order mode is delayed by one unit interval (1 UI) relative to the fundamental mode, the first-order mode acts to cancel the overshoot and undershoot waveforms of the fundamental mode at the output end (point B), as shown in Figure 16(b), improving the quality of the combined waveform. In this case, the amount of cancellation can be controlled by changing the ratio of the first-order mode to the fundamental mode, making it possible to further improve the quality of the combined waveform.

[0063] In Figure 16(b), it is shown that no overshoot or undershoot remains at the rising and falling edges of the combined waveform, but it is possible that overshoot or undershoot remains at the rising and falling edges of the combined waveform depending on the ratio of the first-order mode to the fundamental mode. However, by controlling the amount of cancellation by changing the ratio of the first-order mode to the fundamental mode, it is possible to eliminate overshoot or undershoot at the rising and falling edges of the combined waveform, making it possible to further improve the quality of the combined waveform.

[0064] Figure 17(a) is an example of an eye pattern showing the combined waveform at the input end (point A), and Figure 17(b) is an example of an eye pattern showing the combined waveform at the output end (point B), and it can be seen that the waveform quality at the output end (point B) has improved.

[0065] Note that the example shown in FIG. 16 is an example in which the transmission data is binary data such as NRZ data, but the present technology may also be applicable to cases in which the transmission data is multi-level data such as PAM4 data or PAM8 data, although detailed description will be omitted.

[0066] "Configuration example of optical communication system as first embodiment" Fig. 18 shows an example of the configuration of an optical communication system 10A as the first embodiment. In Fig. 18, parts corresponding to those in Fig. 11 are given the same reference numerals, and detailed descriptions thereof will be omitted where appropriate.

[0067] This optical communication system 10A includes a transmitter 100A, a receiver 200A, and a cable 300A. The transmitter 100A and the receiver 200A are connected via the cable 300A. The cable 300A has plugs 302 and 303 at one and the other ends of an optical fiber 301A serving as an optical waveguide. The plug 302 is connected to a receptacle 102A of the transmitter 100A, and the plug 303 is connected to a receptacle 201A of the receiver 200A.

[0068] In this optical communication system 10A, as in the optical communication system 10 shown in Figure 11, the optical fibers of the transmitter 100A, cable 300A, and receiver 200A that constitute the optical communication path are assumed to propagate only fundamental mode components at a first wavelength (e.g., 1310 nm), and communication is carried out using light at a second wavelength (e.g., 850 nm).

[0069] This optical communication system 10A is an example in which an intermodal delay difference adjustment unit that adjusts the first-order mode to be delayed by one unit interval relative to the fundamental mode in an optical communication path for light of a second wavelength that includes optical fiber 301A is configured by optical fiber 301A of cable 300A. Here, the length and the refractive index distribution of the core and cladding of optical fiber 301A are set so that the first-order mode is delayed by one unit interval relative to the fundamental mode when light of the second wavelength propagates.

[0070] Fig. 19(a) shows a cross section of an optical fiber. Fig. 19(b) to (e) show examples of the refractive index distribution of the core and cladding. This refractive index distribution shows the refractive index distribution near the core on line AB in Fig. 19(a), with the vertical axis representing the refractive index and the horizontal axis representing the physical distance. Note that in the illustrated example, the core diameter is a, but this is not necessarily limited to this, and the core diameter may be defined as being smaller or larger than a.

[0071] Figure 19(b) shows a so-called segmented core type refractive index profile, Figure 19(c) shows a so-called stepped type refractive index profile, Figure 19(d) shows a so-called W-type refractive index profile, and Figure 19(e) shows a so-called SI (step index) type refractive index profile.

[0072] 19(b) to 19(e), the refractive index profile of the optical fiber includes a first region from the center to a first diameter a, a second region from the center to a second diameter b, a third region from the center to a third diameter c, and a fourth region from the center to a third region, as shown in Fig. 19(b) to (e). Here, the amounts of change in refractive index of the first region, second region, and third region relative to the refractive index of the fourth region, i.e., when the refractive index of the fourth region is used as the reference, are denoted by A, x, and y, respectively.

[0073] In the segmented core type, the refractive index of the third region is higher than that of the fourth region, the refractive index of the second region is equal to that of the fourth region, and the refractive index of the first region is higher than that of the third region. In the stepped core type, the refractive index of the third region is equal to that of the fourth region, the refractive index of the second region is higher than that of the fourth region, and the refractive index of the first region is higher than that of the second region.

[0074] In the case of a W-type, the refractive index of the third region is equal to the refractive index of the fourth region, the refractive index of the second region is lower than the refractive index of the fourth region, and the refractive index of the first region is higher than the refractive index of the fourth region. In the case of an SI-type, the refractive indexes of the third region and the second region are equal to the refractive index of the fourth region, and the refractive index of the first region is higher than the refractive index of the fourth region.

[0075] The transmitter 100A has a control unit 106, a transmission processing unit 104, a driver IC 105, a light emitting unit 101, an optical fiber 103, and a receptacle 102A. The control unit 106 controls the operation of each unit of the transmitter 100A. The control unit 106 is capable of exchanging information such as capability information between the devices with the control unit 206 of the receiver 200A via a signal line (not shown) included in the cable 300A.

[0076] Light emitting unit 101 is driven by driver IC 105 based on transmission data supplied from transmission processing unit 104, and outputs light (optical signal) corresponding to the transmission data. Optical fiber 103 propagates the optical signal output from light emitting unit 101 to receptacle 102A, which serves as an optical output unit.

[0077] Receptacle 102A is configured to be able to adjust the ratio of the fundamental mode and the first-order mode in the light input to optical fiber 301A constituting cable 300A. Specifically, receptacle 102A is configured to be able to adjust the amount of deviation of the core position of optical fiber 103 from the optical axis. In this case, the light (optical signal) output from light emitting unit 101 may contain only the fundamental mode, or may contain both the fundamental mode and the first-order mode.

[0078] FIG. 20 shows an example of a state in which the receptacle 102A of the transmitter 100A and the plug 302 of the cable 300A are connected.

[0079] Receptacle 102A includes receptacle body 111 configured by connecting first optical section 112 and second optical section 113.

[0080] The first optical section 112 is made of a light-transmitting material such as synthetic resin or glass, or a material such as silicon that transmits specific wavelengths. A concave light emitting section (light transmitting space) 121 is formed on the front side of the first optical section 112. Lenses 122 corresponding to each channel are integrally formed on the first optical section 112 and aligned horizontally at the bottom of the light emitting section 121. By integrally forming the lenses 122 on the first optical section 112 in this manner, the positional accuracy of the lenses 122 relative to the first optical section 112 can be improved.

[0081] The second optical section 113 has a configuration in which a fiber ferrule 132 is disposed inside a fiber ferrule positioning member 131 having a square cylindrical shape and fixed by adhesive or the like to the back side of the first optical section 112. The fiber ferrule positioning member 131 may be integral with the first optical section 112.

[0082] The upper and lower two surfaces of the fiber ferrule 132 are fixed in a floating structure to the inner surface of the fiber ferrule positioning member 131 via a serially connected structure of a shape-changing member 133 formed of, for example, a piezoelectric element and a spring 134. A light-transmitting material 135 is inserted between the rear side of the first optical section 112 and the front side of the fiber ferrule 132.

[0083] Like the above-described first optical section 112, the fiber ferrule 132 is made of a light-transmitting material such as synthetic resin or glass, or a material such as silicon that transmits specific wavelengths. This fiber ferrule 132 has a plurality of optical fiber insertion holes 136 extending forward from the rear side and aligned horizontally in correspondence with the lenses 122 of each channel of the first optical section 112. The optical fiber 103 has a double structure consisting of a central core 103a that forms the optical path and a cladding 103b that surrounds the core 103a.

[0084] The optical fiber insertion hole 136 of each channel is formed so that its bottom position, that is, the contact position of the tip (incident end) of the optical fiber 103 when inserted, coincides with the focal position of the lens 122.

[0085] The fiber ferrule 132 is also formed with an adhesive injection hole 137 extending downward from the upper surface side so as to communicate with the vicinity of the bottom positions of the multiple optical fiber insertion holes 136 that are aligned horizontally. After the optical fiber 103 is inserted into the optical fiber insertion hole 136, adhesive 138 is injected from the adhesive injection hole 137 around the optical fiber 103, thereby fixing the optical fiber 103 to the fiber ferrule 132.

[0086] In receptacle 102A of transmitter 100A, lens 122 has a function of shaping light emitted from optical fiber 103 into collimated light and emitting the collimated light. As a result, light emitted from the output end of optical fiber 103 with a predetermined NA is incident on lens 122, shaped into collimated light, and emitted (output).

[0087] In addition, in receptacle 102A of transmitter 100A, the amount of deviation of the core position of optical fiber 103 from the optical axis is controlled (adjusted) by supplying a control signal to shape-changing members 133 arranged above and below fiber ferrule 132. This control signal is supplied from control unit 106 of transmitter 100A based on a control signal supplied from control unit 206 of receiver 200A to control unit 106 of transmitter 100A.

[0088] The plug 302 includes a plug body 311. The plug body 311 is made of a light-transmitting material such as synthetic resin or glass, or a material such as silicon that transmits light of a specific wavelength, and has a lensed ferrule configuration.

[0089] By configuring plug body 311 as a ferrule with a lens, it is possible to easily align the optical axes of the optical fiber and the lens. Also, by configuring plug body 311 as a ferrule with a lens, it is possible to easily achieve multi-channel communication even in the case of multiple channels by simply inserting the optical fiber into the ferrule.

[0090] A concave light incident portion (light transmission space) 313 is formed on the front side of the plug body 311. A plurality of lenses (convex lenses) 314 corresponding to each channel are formed integrally with the plug body 311 and aligned horizontally at the bottom of the light incident portion 313.

[0091] The plug body 311 is also provided with a plurality of optical fiber insertion holes 316 extending forward from the rear side and aligned horizontally in accordance with the lenses 314 of each channel. The optical fiber 301A has a double structure consisting of a central core 301Aa which serves as an optical path and a cladding 301Ab which surrounds the core 301Aa.

[0092] The optical fiber insertion hole 316 of each channel is shaped so that the core 301Aa of the optical fiber 301A to be inserted therein coincides with the optical axis of the corresponding lens 314. Furthermore, the optical fiber insertion hole 316 of each channel is shaped so that its bottom position, that is, the contact position of the tip (emission end) of the optical fiber 301A when inserted, coincides with the focal position of the lens 314.

[0093] The plug body 311 is also formed with an adhesive injection hole 312 extending downward from the upper surface side so as to communicate with the vicinity of the bottom positions of the plurality of horizontally aligned optical fiber insertion holes 316. After the optical fiber 301A is inserted into the optical fiber insertion hole 316, adhesive 317 is injected from the adhesive injection hole 312 around the optical fiber 301A, thereby fixing the optical fiber 301A to the plug body 311.

[0094] In plug 302 of cable 300A, lens 314 has the function of focusing the collimated light that is incident on it. In this case, the collimated light is incident on lens 314 and focused, and this focused light is incident on the input end of optical fiber 301A.

[0095] Fig. 21 is a perspective view that schematically shows the configuration of receptacle 102A of transmitter 100A and plug 302 of cable 300A. Although detailed explanation will be omitted, in Fig. 21, parts that correspond to those in Fig. 20 are given the same reference numerals, and detailed explanations thereof will be omitted as appropriate.

[0096] In FIG. 21, the fiber ferrule positioning member 131 constituting the receptacle 102A and the spring 134 disposed between the fiber ferrule positioning member 131 and the shape-changing member 133 are removed.

[0097] 20, a convex or concave position restricting section 115, which is concave in the illustrated example, is integrally formed on the front side of first optical section 112 of receptacle 102A for aligning with plug body 311 of plug 302. Also, although not shown in FIG. 20, a convex or concave position restricting section 315, which is convex in the illustrated example, is integrally formed on the front side of plug body 311 of plug 302 for aligning with first optical section 112 of receptacle 102A.

[0098] In this way, position control section 115 is formed on the front side of first optical section 112 of receptacle 102A, and position control section 315 is formed on the front side of plug body 311 of plug 302, so that when receptacle 102A and plug 302 are connected, they fit together, making it easy to align the optical axes of receptacle 102A and plug 302.

[0099] 18, receiver 200A has control unit 206, receptacle 201, light receiving unit 202, optical fiber 203, amplifier 204, and reception processing unit 205A. Control unit 206 controls the operation of each unit of receiver 200A. This control unit 206 is capable of exchanging information with control unit 106 of transmitter 100A via a signal line (not shown) included in cable 300A.

[0100] Light receiving unit 202 converts light (optical signal) sent from receptacle 201, which serves as an optical input unit, via optical fiber 203 into an electrical signal. The electrical signal output from light receiving unit 202 is amplified by amplifier 204 and supplied to reception processing unit 205A as a received signal. Reception processing unit 205A performs processing such as data sampling and demodulation on the received signal to obtain received data.

[0101] Furthermore, reception processing unit 205A acquires waveform quality information of the received signal. In this embodiment, reception processing unit 205A acquires (1) the level of overshoot or undershoot appearing in the received signal, or (2) the bit error rate of the received signal as waveform quality information. Here, if the waveform quality of the received signal is good, the level of overshoot or undershoot will be small and the bit error rate of the received signal will also be small. Conversely, if the waveform quality of the received signal is poor, the level of overshoot or undershoot will be large and the bit error rate of the received signal will also be large.

[0102] 22(a) shows an example of the configuration of a circuit for acquiring the level of an overshoot that appears in a received signal. The received signal is input to sample-and-hold circuits 251 and 252. A sampling clock is supplied to sample-and-hold circuit 251 at time T1, and is also supplied to sample-and-hold circuit 252 at time T2 via delay circuit 253.

[0103] In this case, as shown in FIG. 22(b), sample-and-hold circuit 251 samples and holds level V1 of the portion of the received signal where an overshoot appears at time T1, and sample-and-hold circuit 252 samples and holds level V2 of the portion where the effect of the overshoot has disappeared and stabilized at time T2.

[0104] For example, at a point where a transition occurs from "0" to "1," time points T1 and T2 are set to the first and second half positions of 1 UI corresponding to this "1." Also, at a point where a transition occurs from "0" to "1" and multiple "1"s occur consecutively, time points T1 and T2 are set to the position of the first "1" and the position of any subsequent "1," for example, the position of the last "1."

[0105] 22(a), the levels V1 and V2 sampled and held by the sample-and-hold circuits 251 and 252 are input to a comparator 254, and the overshoot level V1-V2 is obtained from the comparator 254. In this case, V1>V2.

[0106] The undershoot level can also be obtained using the circuit for obtaining the overshoot level shown in FIG. 22(a). In this case, the time points T1 and T2 are set, for example, at the point where "1" transitions to "0", to the first and second half positions of 1 UI corresponding to this "0". Also, for example, the time points T1 and T2 are set, for example, at the point where "1" transitions to "0" and multiple "0"s are consecutive, to the position of the first "0" and the position of any of the subsequent "0", for example, the position of the last "0". The undershoot level V1-V2 is obtained from the comparator 254. In this case, V1 <V2である。

[0107] Returning to FIG. 18, the control unit 206 generates a control signal for adjusting the amount of deviation of the core position of the receptacle 102A of the transmitter 100A based on the waveform quality information (overshoot or undershoot level, bit error rate) of the received signal acquired by the receiving processing unit 205A.

[0108] In this case, the control unit 206 sequentially changes the control signal so as to adjust the deviation of the core position of the optical fiber 103 from the optical axis in the receptacle 102A of the transmitter 100A in a direction that improves the waveform quality of the received signal, i.e., in a direction that reduces the level of overshoot or undershoot that appears in the received signal, or in a direction that reduces the bit error rate of the received signal.

[0109] Control unit 206 sends this control signal to control unit 106 of transmitter 100A via a signal line (not shown) of cable 300A. Based on the control signal sent from control unit 206 of receiver 200A in this manner, control unit 106 of transmitter 100A adjusts the amount of deviation of the core position of optical fiber 103 from the optical axis in receptacle 102A, and therefore the proportion of the fundamental mode and the first-order mode in the light input to optical fiber 301A that constitutes cable 300A.

[0110] In the control unit 206 of the receiver 200A, the control signal for adjusting the amount of deviation of the core position of the receptacle 102A of the transmitter 100A based on the waveform quality information of the received signal may be changed, for example, only during a training period provided before the data transmission period in which the transmission data is actually transmitted, or may be changed during the data transmission period in addition to the training period.

[0111] When this is performed only during the training period, the control signal finally determined by the control unit 206 of the receiver 200A during the training period is used during the data transmission period, and the amount of deviation of the core position of the optical fiber 103 relative to the optical axis in the receptacle 102A is fixedly controlled.

[0112] As described above, in the optical communication system 10A shown in FIG. 18, the first-order mode is adjusted to be delayed by one unit interval relative to the fundamental mode in the optical fiber 301A of the cable 300A that constitutes the optical communication path for light of the second wavelength. Even if the bias current is kept low when driving a light-emitting element such as a laser diode in the transmitter 100A, deterioration in the waveform quality of the received signal at the receiver 200A can be suppressed. Therefore, it is possible to reduce power consumption while ensuring the waveform quality of the received signal at the receiver 200A.

[0113] In addition, in the optical communication system 10A shown in Figure 18, the first mode is adjusted to be delayed by one unit interval from the fundamental mode in the optical fiber 301A of the cable 300A that constitutes the optical communication path of light of the second wavelength, and it is possible to easily and reliably adjust the first mode to be delayed by one unit interval from the fundamental mode in the optical communication path of light of the second wavelength.

[0114] In addition, in the optical communication system 10A shown in FIG. 18, the amount of deviation of the core position of the optical fiber 103 from the optical axis in the receptacle 102A of the transmitter 100A, and therefore the ratio of the fundamental mode and the first-order mode in the light input to the optical fiber 301A constituting the cable 300A, is adjusted in a direction that improves the waveform quality of the received signal at the receiver 200A, thereby making it possible to further improve the waveform quality of the received signal at the receiver 200A.

[0115] In addition, in the optical communication system 10A shown in FIG. 18, a control signal for adjusting the amount of deviation of the core position of the receptacle 102A of the transmitter 100A, which is generated by the control unit 206 of the receiver 200A, is sent once to the control unit 106 of the transmitter 100A via a signal line (not shown) of the cable 300A, and the control unit 106 adjusts the amount of deviation of the core position of the optical fiber 103 relative to the optical axis in the receptacle 102A based on the control signal. The control signal sent from the control unit 206 of the receiver 200A to the control unit 106 of the transmitter 100A can be included as one parameter of the information group exchanged between the control unit 206 of the receiver 200A and the control unit 106 of the transmitter 100A, resulting in a versatile configuration.

[0116] If the only information exchanged between transmitter 100A and receiver 200A is a control signal for adjusting the amount of misalignment of the core position of receptacle 102A of transmitter 100A, it is also possible to directly adjust the amount of misalignment of the core position of receptacle 102A using the control signal sent from receiver 200A via a signal line (not shown) of cable 300A, as shown in Fig. 23. In this case, the control signal is supplied directly to shape-changing member 133 of receptacle 102A, and its shape is controlled (adjusted).

[0117] In addition, in the optical communication system 10A shown in Figure 18, an example is shown in which a control signal from the control unit 206 of the receiver 200A is sent to the receiver 100A side via a signal line (not shown) included in the cable 300A, but a configuration in which it is sent via a signal line not included in the cable 300A is also possible.

[0118] "Configuration example of optical communication system as second embodiment" Fig. 24 shows an example of the configuration of an optical communication system 10B as the second embodiment. In Fig. 24, parts corresponding to those in Fig. 11 and Fig. 18 are given the same reference numerals, and detailed description thereof will be omitted as appropriate.

[0119] This optical communication system 10B has a transmitter 100A, a receiver 200B, and a cable 300. The transmitter 100A and the receiver 200B are connected via the cable 300. The cable 300 has plugs 302 and 303 at one end and the other end of an optical fiber 301 serving as an optical waveguide. The plug 302 is connected to a receptacle 102A of the transmitter 100A, and the plug 303 is connected to a receptacle 201 of the receiver 200B.

[0120] In this optical communication system 10B, similarly to the optical communication systems 10 and 10A shown in Figures 11 and 18, the optical fibers of the transmitter 100A, cable 300, and receiver 200B that constitute the optical communication path are assumed to propagate only fundamental mode components at a first wavelength (e.g., 1310 nm), and communication is carried out using light at a second wavelength (e.g., 850 nm).

[0121] This optical communication system 10B is an example in which an intermodal delay difference adjustment unit that adjusts the first-order mode to be delayed by one unit interval relative to the fundamental mode in an optical communication path of light of a second wavelength that includes an optical fiber 301 is configured by the optical fiber 301 of the cable 300 and a variable phase shifter 207 in the receiver 200B.

[0122] Here, the delay of the first-order mode relative to the fundamental mode when light of the second wavelength propagates is set to be shorter than one unit interval in optical fiber 301. Then, the delay of the first-order mode relative to the fundamental mode generated in variable phase shifter 207 is added, and the first-order mode is adjusted to be delayed by one unit interval relative to the fundamental mode in the optical communication path of light of the second wavelength.

[0123] Although detailed description of the transmitter 100A will be omitted, the transmitter 100A has the same configuration as the transmitter 100A in the optical communication system 10A of FIG.

[0124] Receiver 200B has control unit 206B, receptacle 201, optical fiber 203, variable phase shifter 207, light receiving unit 202, amplifier 204, and reception processing unit 205A. Control unit 206B controls the operation of each unit of receiver 200B. This control unit 206B is capable of exchanging information with control unit 106 of transmitter 100A via a signal line (not shown) included in cable 300A.

[0125] Variable phase shifter 207 adjusts the inter-modal propagation delay difference between the fundamental mode and the first mode of light (optical signal) of the second wavelength transmitted from receptacle 201 as an optical input unit via optical fiber 203, based on a control signal transmitted from control unit 206B. In this case, in combination with the inter-modal propagation delay difference in optical fiber 301 of cable 300, the first mode is adjusted to be delayed by one unit interval relative to the fundamental mode in the optical communication path of the light of the second wavelength.

[0126] 25(a) and (b) show an example configuration of variable phase shifter 207. Fig. 25(a) shows a top view, and Fig. 25(b) shows a side view. Variable phase shifter 207 has a configuration in which copper (Cu) wiring 272 is provided on an optical waveguide, for example, a polymer waveguide 271. When a current flows from point A to point B, copper wiring 272 generates heat due to resistance, which in turn changes the refractive index of polymer waveguide 271 and changes the inter-modal propagation delay difference between the fundamental mode and the first-order mode.

[0127] In this case, if the amount of current flowing through the copper wiring 272 is changed, the amount of heat generated by this copper wiring 272 also changes, making it possible to control the refractive index of the polymer waveguide 271 and therefore the inter-mode propagation delay difference between the fundamental mode and the first-order mode. Figure 25(c) shows an example of the correspondence between the amount of current and the amount of phase shift in the fundamental mode and the first-order mode, and it can be seen that as the amount of current increases, the inter-mode propagation delay difference between the fundamental mode and the first-order mode increases.

[0128] The light receiving unit 202 converts the light (optical signal) output from the variable phase shifter 207 into an electrical signal. The electrical signal output from the light receiving unit 202 is amplified by the amplifier 204 and supplied to the reception processing unit 205A as a reception signal. The reception processing unit 205A performs processing such as data sampling and demodulation on the reception signal to obtain reception data.

[0129] Furthermore, reception processing unit 205A acquires waveform quality information (overshoot or undershoot level, bit error rate) of the received signal and sends it to control unit 206B. Based on this waveform quality information, control unit 206B generates a control signal for adjusting the inter-mode propagation delay difference between the fundamental mode and the first-order mode of the light (optical signal) of the second wavelength in variable phase shifter 207.

[0130] In this case, the control unit 206B sequentially changes the control signal so that the inter-mode propagation delay difference between the fundamental mode and the first-order mode of the light (optical signal) of the second wavelength in the variable phase shifter 207 is adjusted in a direction that improves the waveform quality of the received signal, that is, in a direction that reduces the level of overshoot or undershoot appearing in the received signal, or in a direction that reduces the bit error rate of the received signal.

[0131] The control unit 206B sends this control signal to the variable phase shifter 207. Based on the control signal sent from the control unit 206B in this manner, the variable phase shifter 207 adjusts the inter-modal propagation delay difference between the fundamental mode and the first mode of the light (optical signal) at the second wavelength, combined with the inter-modal propagation delay difference in the optical fiber 301 of the cable 300, so that the first mode is delayed by one unit interval relative to the fundamental mode in the optical communication path of the light at the second wavelength.

[0132] In addition, the control unit 206B generates a control signal for adjusting the amount of deviation of the core position of the receptacle 102A of the transmitter 100A based on waveform quality information (overshoot or undershoot level, bit error rate) of the received signal acquired by the receiving processing unit 205A.

[0133] In this case, the control unit 206B sequentially changes the control signal so as to adjust the deviation of the core position of the optical fiber 103 from the optical axis in the receptacle 102A of the transmitter 100A in a direction that improves the waveform quality of the received signal, i.e., in a direction that reduces the level of overshoot or undershoot that appears in the received signal, or in a direction that reduces the bit error rate of the received signal.

[0134] Control unit 206B sends this control signal to control unit 106 of transmitter 100A via a signal line (not shown) of cable 300. Based on the control signal sent from control unit 206B of receiver 200B in this manner, control unit 106 of transmitter 100A adjusts the amount of deviation of the core position of optical fiber 103 from the optical axis in receptacle 102A, and therefore the proportion of the fundamental mode and the first-order mode in the light input to optical fiber 301 that constitutes cable 300.

[0135] In control unit 206B of receiver 200B, the control signal for adjusting the inter-mode propagation delay difference between the fundamental mode and the first mode of the second wavelength light (optical signal) in variable phase shifter 207 based on waveform quality information of the received signal, and the control signal for adjusting the amount of deviation in the core position of receptacle 102A of transmitter 100A may be changed, for example, only during a training period provided before the data transmission period, or may be changed during the data transmission period in addition to the training period.

[0136] When this is performed only during the training period, during the data transmission period, the inter-mode propagation delay difference between the fundamental mode and the first mode of the second wavelength light (optical signal) in the variable phase shifter 207 and the deviation amount of the core position of the optical fiber 103 from the optical axis in the receptacle 102A of the transmitter 100A are fixedly controlled using the control signal that is finally changed by the control unit 206B of the receiver 200B during the training period.

[0137] As described above, in the optical communication system 10B shown in FIG. 24, the optical fiber 301 of the cable 300 constituting the optical communication path for light of the second wavelength and the variable phase shifter in the receiver 200B are adjusted so that the first-order mode is delayed by one unit interval relative to the fundamental mode. Even if the bias current is kept low when driving a light-emitting element such as a laser diode in the transmitter 100A, deterioration in the waveform quality of the received signal in the receiver 200B can be suppressed. Therefore, it is possible to reduce power consumption while ensuring the waveform quality of the received signal in the receiver 200A.

[0138] In addition, in the optical communication system 10B shown in Figure 24, the optical fiber 301 of the cable 300 that constitutes the optical communication path for light of the second wavelength and the variable phase shifter in the receiver 200B are adjusted so that the first-order mode is delayed by one unit interval relative to the fundamental mode, making it possible to use a general-purpose cable 300 (optical fiber 301) that is not adjusted so that the first-order mode is delayed by one unit interval relative to the fundamental mode when light of the second wavelength propagates.

[0139] Furthermore, in the optical communication system 10B shown in FIG. 24, similarly to the optical communication system 10A shown in FIG. 18, the amount of deviation of the core position of the optical fiber 103 from the optical axis in the receptacle 102A of the transmitter 100A, and therefore the ratio of the fundamental mode and the first-order mode in the light input to the optical fiber 301 constituting the cable 300, is adjusted in a direction that improves the waveform quality of the received signal at the receiver 200B, thereby making it possible to further improve the waveform quality of the received signal at the receiver 200B.

[0140] Furthermore, in the optical communication system 10B shown in FIG. 24, similarly to the optical communication system 10A shown in FIG. 18, a control signal for adjusting the amount of deviation of the core position of the receptacle 102A of the transmitter 100A, which is generated by the control unit 206B of the receiver 200B, is sent once to the control unit 106 of the transmitter 100A via a signal line (not shown) of the cable 300, and the control unit 106 adjusts the amount of deviation of the core position of the optical fiber 103 relative to the optical axis in the receptacle 102A based on the control signal. It is possible to include the control signal sent from the control unit 206B of the receiver 200B to the control unit 106 of the transmitter 100A as one parameter of the information group exchanged between the control unit 206B of the receiver 200B and the control unit 106 of the transmitter 100A, resulting in a versatile configuration.

[0141] In addition, in the optical communication system 10B shown in FIG. 24, the inter-mode propagation delay difference between the fundamental mode and the first-order mode in the variable phase shifter 207 of the receiver 200B is adjusted based on the waveform quality information of the received signal, and it becomes possible to perform an adjustment with high precision in a direction that improves the waveform quality of the received signal.

[0142] It is also possible to configure the inter-mode propagation delay difference between the fundamental mode and the first-order mode in variable phase shifter 207 of receiver 200B to be adjusted based on information about the inter-mode propagation delay difference between the fundamental mode and the first-order mode generated in optical fiber 301 of cable 300. This makes it possible to simply and accurately adjust the first-order mode so that it is delayed by one unit interval relative to the fundamental mode in the optical communication path of light of the second wavelength.

[0143] In this case, it is conceivable that the control unit 206B acquires information about the inter-modal propagation delay difference between the fundamental mode and the first mode generated in the optical fiber 301 of the cable 300, for example, from an IC tag embedded in, for example, the plug 303 of the cable 300. In this case, it is also conceivable that the control unit 206B acquires information about the inter-modal propagation delay difference between the fundamental mode and the first mode generated in the optical fiber 301 of the cable 300, based on a user's input operation from a user operation unit (not shown).

[0144] <2. Modifications> In the above-described embodiment, an example has been shown in which the first-order mode is adjusted to be delayed by one unit interval from the fundamental mode. However, it is not necessarily necessary to delay the first-order mode by one unit interval from the fundamental mode, and in some cases, a configuration in which the fundamental mode is delayed by one unit interval from the first-order mode can be considered to achieve the same effect. In this case, in an optical fiber, it is possible to delay the fundamental mode from the first-order mode by changing the refractive index parameters. Also, in a variable phase shifter, it is possible to delay the fundamental mode from the first-order mode by using a material.

[0145] In the above embodiment, the first wavelength is described as 1310 nm, but since a laser light source or an LED light source may be used as the light source, the first wavelength may be, for example, between 300 nm and 5 μm.

[0146] Furthermore, in the above-described embodiment, the first wavelength is described as 1310 nm, but this first wavelength may be a wavelength in the 1310 nm band that includes 1310 nm. Furthermore, in the above-described embodiment, the first wavelength is described as 1310 nm, but this first wavelength may be 1550 nm or a wavelength in the 1550 nm band that includes 1550 nm. Furthermore, in the above-described embodiment, the second wavelength is described as 850 nm, but this second wavelength may be a wavelength in the 850 nm band that includes 850 nm.

[0147] Furthermore, in the above-described embodiment, an example has been described in which the optical waveguide is an optical fiber, but it goes without saying that the present technology can also be applied to optical waveguides other than optical fibers, such as silicon optical waveguides.

[0148] Although the preferred embodiments of the present disclosure have been described in detail above with reference to the accompanying drawings, the technical scope of the present disclosure is not limited to such examples. It is clear that a person skilled in the art of the present disclosure can conceive of various modified or altered examples within the scope of the technical idea described in the claims, and it is understood that these also naturally fall within the technical scope of the present disclosure.

[0149] Furthermore, the effects described herein are merely descriptive or exemplary and are not limiting. In other words, the technology according to the present disclosure may achieve other effects that are apparent to those skilled in the art from the description of this specification, in addition to or in place of the above-described effects.

[0150] The present technology can also be configured as follows. (1) An optical communication system in which a transmitter and a receiver are connected by an optical waveguide and communicate using light of a second wavelength, the optical waveguide propagates only a fundamental mode at a first wavelength; the second wavelength is a wavelength at which the optical waveguide can propagate at least a first-order mode along with the fundamental mode; an inter-mode propagation delay difference adjusting unit that adjusts one of the fundamental mode and the first-order mode so that it is delayed by one unit interval relative to the other in an optical communication path for light of the second wavelength that includes the optical waveguide; Optical communication system. (2) The inter-mode propagation delay difference adjusting section is configured by the optical waveguide. The optical communication system according to (1) above. (3) The optical waveguide has a length and a refractive index profile of the core and clad set so that one of the fundamental mode and the first-order mode is delayed by one unit interval relative to the other when light of the second wavelength propagates. The optical communication system according to (2) above. (4) The intermodal propagation delay difference adjusting unit is composed of the optical waveguide and a variable phase shifter in the receiver. The optical communication system according to (1) above. (5) The inter-mode propagation delay difference between the fundamental mode and the first-order mode in the variable phase shifter is adjusted based on waveform quality information of a received signal obtained corresponding to the light of the second wavelength via the optical communication path. The optical communication system according to (4) above. (6) The inter-mode propagation delay difference between the fundamental mode and the first mode in the variable phase shifter is adjusted in a direction that reduces the level of overshoot or undershoot appearing in the received signal. The optical communication system according to (5) above. (7) The inter-mode propagation delay difference between the fundamental mode and the first mode in the variable phase shifter is adjusted in a direction that reduces the bit error rate of the received signal. The optical communication system according to (5) above. (8) The intermodal propagation delay difference between the fundamental mode and the first-order mode in the variable phase shifter is adjusted based on information about the intermodal propagation delay difference between the fundamental mode and the first-order mode generated in the optical waveguide. The optical communication system according to (4) above. (9) A mode ratio adjusting unit is further provided that adjusts the ratio of the fundamental mode and the first-order mode in the light of the second wavelength input from the transmitter to the optical waveguide. The optical communication system according to any one of (1) to (8). (10) The mode ratio adjusting unit adjusts the amount of deviation of the core position of the optical fiber with respect to the optical axis in a receptacle for connecting the optical waveguide of the transmitter. The optical communication system according to (9) above. (11) The adjustment of the deviation of the core position is performed based on a control signal sent from the receiver. The optical communication system according to (10) above. (12) The receiver generates the control signal based on waveform quality information of a received signal obtained corresponding to the light of the second wavelength via the optical communication path. The optical communication system according to (11) above. (13) The amount of deviation of the core position is adjusted in a direction to reduce the level of overshoot or undershoot appearing in the received signal. The optical communication system according to (12) above. (14) The amount of deviation of the core position is adjusted in a direction that reduces the bit error rate of the received signal. The optical communication system according to (12) above. (15) An optical communication method for communicating using light of a second wavelength in an optical communication system in which a transmitter and a receiver are connected by an optical waveguide, comprising: the optical waveguide propagates only a fundamental mode at a first wavelength; the second wavelength is a wavelength at which the optical waveguide can propagate at least a first-order mode along with the fundamental mode; In an optical communication path for light of the second wavelength including the optical waveguide, one of the fundamental mode and the first-order mode is adjusted to be delayed by one unit interval relative to the other. Optical communication method. (16) An optical input unit for inputting light of a second wavelength transmitted from a transmitter via an optical waveguide, the optical waveguide propagates only a fundamental mode at a first wavelength; the second wavelength is a wavelength at which the optical waveguide can propagate at least a first-order mode along with the fundamental mode; a control unit that generates a control signal for adjusting a ratio of the fundamental mode and the first-order mode of the second light input from the transmitter to the optical waveguide. Receiver. (17) An optical input unit for inputting light of a second wavelength transmitted from a transmitter via an optical waveguide, the optical waveguide propagates only a fundamental mode at a first wavelength; the second wavelength is a wavelength at which the optical waveguide can propagate at least a first-order mode along with the fundamental mode; a variable phase shifter for adjusting the inter-mode propagation delay difference between a fundamental mode and a first-order mode in the light of the second wavelength input to the optical input section; a control unit that generates a control signal for controlling the variable phase shifter; Receiver. (18) The control unit further generates a control signal for adjusting a ratio between the fundamental mode and the first-order mode of the light of the second wavelength input from the transmitter to the optical waveguide. The receiver according to (17) above. (19) At the first wavelength, only the fundamental mode propagates, propagating at least a first order mode along with the fundamental mode at a second wavelength; The length and the refractive index profiles of the core and cladding are set so that one of the fundamental mode and the first-order mode is delayed by one unit interval relative to the other when light of the second wavelength propagates. optical waveguide. (20) An optical output unit for outputting light of the second wavelength to a receiver through an optical waveguide; the optical waveguide propagates only a fundamental mode at a first wavelength; the second wavelength is a wavelength at which the optical waveguide can propagate at least a first-order mode along with the fundamental mode; The optical output section is configured to be able to adjust the ratio of the fundamental mode and the first-order mode in the light of the second wavelength input to the optical waveguide. Transmitter. [Explanation of symbols]

[0151] 10A, 10B Optical communication system 100A...Transmitter 101 Light-emitting part 102A···Receptacle 103 Optical Fiber 104 Transmission processing unit 105 Driver IC 106 Control unit 111···Receptacle body 112···First Optical Department 113...Second Optical Department 131 Fiber ferrule positioning member 132 Fiber Ferrule 133....Shape-changing member 134 Spring 200A, 200B receiver 201···Receptacle 202... Light receiving section 203 Optical Fiber 204 Amplification section 205A Receiving processing unit 206, 206B...Control unit 207 Variable Phase Shifter 251, 252... Sample and hold circuit 253 Delay circuit 271···Polymer waveguide 272···Copper wiring 300,300A cable 301, 301A... Optical fiber 302, 303...Plug 311···Plug body

Claims

1. An optical communication system in which a transmitter and a receiver are connected by an optical waveguide and communication is performed using light of a second wavelength, the optical waveguide propagates only a fundamental mode at a first wavelength; the second wavelength is a wavelength at which the optical waveguide can propagate at least a first-order mode along with the fundamental mode; an inter-mode propagation delay difference adjusting unit that adjusts one of the fundamental mode and the first-order mode so that it is delayed by one unit interval relative to the other in an optical communication path for light of the second wavelength that includes the optical waveguide; Optical communication system.

2. The inter-mode propagation delay difference adjusting section is configured by the optical waveguide.

2. The optical communication system according to claim 1.

3. The optical waveguide has a length and a refractive index profile of the core and clad set so that one of the fundamental mode and the first-order mode is delayed by one unit interval relative to the other when light of the second wavelength propagates.

3. The optical communication system according to claim 2.

4. The intermodal propagation delay difference adjusting unit is composed of the optical waveguide and a variable phase shifter in the receiver.

2. The optical communication system according to claim 1.

5. The inter-mode propagation delay difference between the fundamental mode and the first-order mode in the variable phase shifter is adjusted based on waveform quality information of a received signal obtained corresponding to the light of the second wavelength that has passed through the optical communication path.

5. The optical communication system according to claim 4.

6. The inter-mode propagation delay difference between the fundamental mode and the first-order mode in the variable phase shifter is adjusted in a direction that reduces the level of overshoot or undershoot appearing in the received signal.

6. The optical communication system according to claim 5.

7. The inter-mode propagation delay difference between the fundamental mode and the first-order mode in the variable phase shifter is adjusted in a direction that reduces the bit error rate of the received signal.

6. The optical communication system according to claim 5.

8. The intermodal propagation delay difference between the fundamental mode and the first-order mode in the variable phase shifter is adjusted based on information about the intermodal propagation delay difference between the fundamental mode and the first-order mode generated in the optical waveguide.

5. The optical communication system according to claim 4.

9. a mode ratio adjusting unit that adjusts the ratio of the fundamental mode and the first-order mode in the light of the second wavelength input from the transmitter to the optical waveguide; 2. The optical communication system according to claim 1.

10. The mode ratio adjusting unit adjusts the amount of deviation of the core position of the optical fiber from the optical axis in a receptacle for connecting the optical waveguide of the transmitter.

10. The optical communication system according to claim 9.

11. The adjustment of the deviation of the core position is performed based on a control signal sent from the receiver.

11. The optical communication system according to claim 10.

12. The receiver generates the control signal based on waveform quality information of a received signal obtained corresponding to the light of the second wavelength via the optical communication path.

12. The optical communication system according to claim 11.

13. The amount of deviation of the core position is adjusted in a direction that reduces the level of overshoot or undershoot appearing in the received signal.

13. The optical communication system according to claim 12.

14. The amount of deviation of the core position is adjusted in a direction that reduces the bit error rate of the received signal.

13. The optical communication system according to claim 12.

15. An optical communication method for communicating using light of a second wavelength in an optical communication system in which a transmitter and a receiver are connected by an optical waveguide, comprising: the optical waveguide propagates only a fundamental mode at a first wavelength; the second wavelength is a wavelength at which the optical waveguide can propagate at least a first-order mode along with the fundamental mode; In an optical communication path for light of the second wavelength including the optical waveguide, one of the fundamental mode and the first-order mode is adjusted to be delayed by one unit interval relative to the other. Optical communication method.

16. an optical input unit for inputting light of a second wavelength transmitted from a transmitter via an optical waveguide; the optical waveguide propagates only a fundamental mode at a first wavelength; the second wavelength is a wavelength at which the optical waveguide can propagate at least a first-order mode along with the fundamental mode; a control unit that generates a control signal for adjusting a ratio of a fundamental mode and a first-order mode of the light of the second wavelength input from the transmitter to the optical waveguide. Receiver.

17. an optical input unit for inputting light of a second wavelength transmitted from a transmitter via an optical waveguide; the optical waveguide propagates only a fundamental mode at a first wavelength; the second wavelength is a wavelength at which the optical waveguide can propagate at least a first-order mode along with the fundamental mode; a variable phase shifter for adjusting the inter-mode propagation delay difference between a fundamental mode and a first-order mode in the light of the second wavelength input to the optical input section; a control unit that generates a control signal for controlling the variable phase shifter; The variable phase shifter, together with the optical waveguide, constitutes an inter-mode propagation delay difference adjusting unit that adjusts one of the fundamental mode and the first-order mode to be delayed by one unit interval relative to the other in an optical communication path of light of the second wavelength that includes the optical waveguide. Receiver.

18. The control unit further generates a control signal for adjusting the ratio of the fundamental mode and the first-order mode of the light of the second wavelength input from the transmitter to the optical waveguide.

18. The receiver of claim 17.

19. At the first wavelength, only the fundamental mode propagates; propagating at least a first order mode along with the fundamental mode at a second wavelength; The length and the refractive index profiles of the core and cladding are set so that one of the fundamental mode and the first-order mode is delayed by one unit interval relative to the other when light of the second wavelength propagates. optical waveguide.

20. an optical output unit for outputting light of the second wavelength to a receiver via an optical waveguide; the optical waveguide propagates only a fundamental mode at a first wavelength; the second wavelength is a wavelength at which the optical waveguide can propagate at least a first-order mode along with the fundamental mode; The optical output section is configured to be able to adjust the ratio of the fundamental mode and the first-order mode in the light of the second wavelength input to the optical waveguide. Transmitter.

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