OPTICAL WAVEGUIDE, OPTICAL COMMUNICATION DEVICE, OPTICAL COMMUNICATION METHOD, AND OPTICAL COMMUNICATION SYSTEM

To mitigate the accuracy of misalignment to reduce costs, and to suppress an inter-mode propagation delay difference to enable high-quality transmission of signals. An optical waveguide is configured to propagate only a fundamental mode at a first wavelength and propagate at least a first-order mode as well as the fundamental mode at a second wavelength. The optical waveguide is configured such that a refractive index distribution of a core and a cladding is controlled so that the inter-mode propagation delay difference is within a predetermined threshold, for example, the inter-mode propagation delay difference is zero, when communication is performed using light of the second wavelength. For example, the first wavelength is in a 1310-nm band and the second wavelength is in an 850-nm band.

TECHNICAL FIELD

The present technology relates to an optical waveguide, an optical communication device, an optical communication method, and an optical communication system, and more particularly, to an optical waveguide and the like suitable for use to reduce the accuracy of misalignment.

BACKGROUND ART

Conventionally, optical communication using spatial coupling (see, for example, PTL 1) is known. In the case of such optical communication, especially in a single-mode fiber, a large optical power loss occurs due to misalignment. For this reason, conventionally, high precision is required for parts in order to suppress misalignment, leading to an increase in cost.

CITATION LIST

Patent Literature

SUMMARY

Technical Problem

An object of the present technology is to mitigate the accuracy of misalignment to reduce costs, and to suppress the inter-mode propagation delay difference to enable high-quality transmission of signals.

Solution to Problem

A concept of the present technology is an optical waveguide configured to: propagate only a fundamental mode at a first wavelength; propagate at least a first-order mode as well as the fundamental mode at a second wavelength; and a refractive index distribution of a core and a cladding is controlled so that an inter-mode propagation delay difference is within a predetermined threshold when communication is performed using light of the second wavelength.

The optical waveguide of the present technology is configured to propagate only the fundamental mode at a first wavelength and propagate at least the first-order mode as well as the fundamental mode at a second wavelength. In the optical waveguide according to the present technology, the refractive index distribution of the core and the cladding is controlled so that the inter-mode propagation delay difference is within a predetermined threshold when communication is performed using light of the second wavelength.

For example, the first wavelength may be a wavelength at which chromatic dispersion is zero. Also, for example, the first wavelength may be between 300 nm and 5 μm. In this case, for example, the first wavelength may be a wavelength in the 1310-nm band or the 1550-nm band. Also, for example, the second wavelength may be a wavelength in the 850-nm band.

Thus, in the present technology, the optical waveguide propagates only the fundamental mode at the first wavelength, and propagates at least the first-order mode as well as the fundamental mode at the second wavelength. When communication is performed using light of the second wavelength, since at least the first-order mode component generated due to optical axis misalignment propagates together with the fundamental mode component, it is possible to reduce the coupling loss of optical power due to optical axis misalignment. In addition, in the present technology, the optical fiber is configured such that the refractive index distribution of the core and the cladding is controlled so that the inter-mode propagation delay difference is within a predetermined threshold when communication is performed using the light of the second wavelength. Thus, it is possible to keep the inter-mode propagation delay difference within a predetermined threshold when communication is performed using light of the second wavelength. Further, high-quality signal transmission can be realized without increasing the cost and power consumption due to the provision of a waveform distortion correction circuit.

In addition, in the present technology, for example, the refractive index distribution may include a distribution of refractive indices of a first region from a center to a first diameter, a second region to a second diameter outside the first region, a third region to a third diameter outside the second region, and a fourth region outside the third region.

In this case, for example, the refractive index of the third region may be higher than that of the fourth region, the refractive index of the second region may be equal to that of the fourth region, and the refractive index of the first region may be higher than that of the third region. The refractive index distribution of this case is a so-called segmented-core type distribution.

In the segmented-core refractive index distribution, for example, the first wavelength may be in a 1310-nm band and the second wavelength may be in an 850-nm band, the first diameter may be 7 μm, the second diameter may be 9 μm, the third diameter may be 11 μm, the refractive index of the fourth region may be 1.4524, a refractive index change amount of the third region with respect to the refractive index of the fourth region may be in a range of 0 to +0.0024, and the refractive index change amount of the first region with respect to the refractive index of the fourth region may be in a range of +0.00467 to +0.00541. In this case, for example, a refractive index change amount of the third region with respect to the refractive index of the fourth region may be +0.000827, and a refractive index change amount of the first region with respect to the refractive index of the fourth region may be +0.004882.

Also, in this case, for example, the refractive index of the third region may be equal to that of the fourth region, the refractive index of the second region may be higher than that of the fourth region, and the refractive index of the first region may be higher than that of the second region. The refractive index distribution of this case is a so-called stepped-type distribution.

In the stepped refractive index distribution, for example, the first wavelength may be in a 1310-nm band and the second wavelength may be in an 850-nm band, the first diameter may be 7 μm and the second diameter may be 13 μm, the refractive index of the fourth region may be 1.4524, a refractive index change amount of the second region with respect to the refractive index of the fourth region may be in a range of 0 to +0.0012, and a refractive index change amount of the first region with respect to the refractive index of the fourth region may be in a range of +0.00467 to +0.00526. In this case, a refractive index change amount of the second region with respect to the refractive index of the fourth region may be +0.000811, and a refractive index change amount of the first region with respect to the refractive index of the fourth region may be +0.005053.

Also, in this case, for example, the refractive index of the third region may be equal to that of the fourth region, the refractive index of the second region may be lower than that of the fourth region, and the refractive index of the first region may be higher than that of the fourth region. The refractive index distribution of this case is a so-called W-type distribution.

In the W-type refractive index distribution, for example, the first wavelength may be in a 1310-nm band and the second wavelength may be in an 850-nm band, the first diameter is 7 μm and the second diameter may be 9 μm, the refractive index of the fourth region may be 1.4524, a refractive index change amount of the first region with respect to the refractive index of the fourth region may be in a range of −0.0055 to 0, and a refractive index change amount of the first region with respect to the refractive index of the fourth region may be in a range of +0.00486 to +0.00467. In this case, a refractive index change amount of the first region with respect to the refractive index of the fourth region may be −0.002245, and a refractive index change amount of the first region with respect to the refractive index of the fourth region may be +0.004778.

Also, in this case, for example, the refractive indices of the third region and the second region may be equal to that of the fourth region, and the refractive index of the first region may be higher than that of the fourth region. The refractive index distribution of this case is a so-called SI (step index)-type distribution. In the SI-type refractive index distribution, for example, the first wavelength may be in a 1310-nm band and the second wavelength may be in an 850-nm band, the first diameter may be 7 μm, the refractive index of the fourth region may be 1.4524, and a refractive index change amount of the first region with respect to the refractive index of the fourth region may be +0.00467.

Additionally, another concept of the present technology is an optical communication device comprising: an optical waveguide configured to propagate only a fundamental mode at a first wavelength and propagate at least a first-order mode as well as the fundamental mode at a second wavelength, the optical waveguide is configured such that a refractive index distribution of a core and a cladding is controlled so that an inter-mode propagation delay difference is within a predetermined threshold when communication is performed using light of the second wavelength, and the optical communication device performs communication using light of the second wavelength.

Additionally, another concept of the present technology is an optical communication method for performing communication using light of a second wavelength in an optical waveguide configured to propagate only a fundamental mode at a first wavelength and propagate at least a first-order mode as well as the fundamental mode at the second wavelength and configured such that a refractive index distribution of a core and a cladding is controlled so that an inter-mode propagation delay difference is within a predetermined threshold when communication is performed using light of the second wavelength.

Additionally, another concept of the present technology is an optical communication system in which a transmitter and a receiver are connected by an optical waveguide, wherein the optical waveguide is configured to propagate only a fundamental mode at a first wavelength and propagate at least a first-order mode as well as the fundamental mode at a second wavelength and configured such that a refractive index distribution of a core and a cladding is controlled so that an inter-mode propagation delay difference is within a predetermined threshold when communication is performed using light of the second wavelength, and the transmitter and the receiver perform communication using light of the second wavelength in the optical waveguide.

DESCRIPTION OF EMBODIMENTS

Modes for carrying out the present invention (hereinafter referred to as “embodiments”) will be described hereinafter. The descriptions will be given in the following order.1. Embodiment2. Modification example

Basic Explanation about Present Technology

First, the technology related to the present technology will be described.FIG.1shows an outline of optical communication using spatial coupling. In this case, the light emitted from an optical fiber10T on the transmitting side is shaped into collimated light by a lens11T and the collimated light is emitted. Then, this collimated light is condensed by a lens11R on the receiving side and is incident on an optical fiber10R. In the case of this optical communication, especially in a single-mode fiber, a large optical power loss occurs due to misalignment. The optical fibers10T and10R have a double structure including a central core10aserving as an optical path and a cladding10bsurrounding the central core10a.

Next, the basic concept of modes will be explained. When light is caused to propagate in a single mode through an optical fiber, it is necessary to determine parameters such as the refractive index and the core diameter of the fiber so that only one mode exists.

FIG.2(a)shows the basic structure of an optical fiber. An optical fiber has a structure in which a central portion called a core is covered with a layer called a cladding. In this case, the core has a high refractive index n1 and the cladding has a low refractive index n2, so that light propagates while being confined in the core.

FIG.2(b)shows the LPml (Linearly Polarized) mode of a stepped optical fiber and the normalized propagation constant b as a function of the normalized frequency V. The vertical axis the normalized propagation constant b, in which b is 0 when a certain mode does not pass through the fiber (light is blocked) and b approaches 1 as the optical power is confined in the core (light can propagate through the fiber). The horizontal axis the normalized frequency V, which can be expressed by the following formula (1). Here, d is the core diameter, NA is the numerical aperture, and λ is the wavelength of light.

For example, when V is 2.405, the LP11 mode is blocked, so only the LP01 mode exists. Therefore, the state of V=2.405 or less is the single mode. Here, LP01 is the fundamental mode (zeroth-order mode), and LP11, LP21, . . . , and so on are the first-order mode, the second-order mode, . . . , and so on.

For example, as shown inFIG.3(a), the normalized frequency V will be considered in the case of 1310 nm, which is typical for a single mode. Here, assuming that the core diameter d and the numerical aperture NA are d=8 μm and NA=0.1, which are general parameters for a 1310-nm optical fiber, and the wavelength of light propagating through the fiber is 1310 nm, V is 1.92 from the formula (1).

Therefore, as shown inFIG.3(b), the normalized frequency V is 2.405 or less, so that only the fundamental mode of LP01 propagates, resulting in a single mode. Here, when the core diameter increases, the number of propagation modes increases. Incidentally, for example, in a general multi-mode fiber, the core diameter is set to 50 μm, so that several hundred modes propagate.

Considering optical communication using spatial coupling as shown inFIG.1, in a single mode, since the core diameter is small, there is a problem that the alignment of the optical coupling portion on the transmitting/receiving sides becomes severe, and the precision requirement for accurately aligning the optical axis increases.

In order to solve this problem, it is common to use high precision parts or to machine a light input portion into an optical fiber to facilitate the insertion of the light into the fiber core. However, high-precision parts are expensive, and those that require machining are expensive, so connectors and systems for single-mode communication are generally expensive.

FIGS.4and5show an example of factors that degrade the accuracy of optical axis alignment. For example, as shown inFIG.4(a), optical axis misalignment occurs due to uneven amounts of fixing materials16T and16R for fixing ferrules15T and15R and optical fibers10T and10R. Further, for example, as shown inFIG.4(b), optical axis misalignment occurs due to insufficient shaping accuracy of lenses11T and11R.

Further, as shown inFIGS.5(a) and5(b), optical axis misalignment occurs due to insufficient precision of the positioning mechanisms (recessed portion17T and protruded portion17R) provided in the ferrules15T and15R. A convex portion17R shown inFIGS.5(a) and5(b)may be a pin.

According to a first aspect of the present technology, the optical fiber is configured to propagate only the fundamental mode at a first wavelength and propagates at least the first-order mode as well as the fundamental mode at a second wavelength. Here, the optical fiber is configured to have zero chromatic dispersion at the first wavelength. For example, the first wavelength is 1310 nm. In this case, under the conditions ofFIG.3(a), the normalized frequency V is 1.92 as shown inFIG.3(b), and the optical fiber functions as a single-mode fiber.

Also, for example, the second wavelength is 850 nm. When light with a wavelength of 850 nm instead of 1310 nm is input to the optical fiber under the same conditions as inFIG.3(a), the normalized frequency V is 2.96 as shown inFIG.6(b). Therefore, as shown inFIG.6(a), a fundamental mode of LP01 and a first-order mode of LP11 can exist.

A case where when an optical system as shown inFIG.7(a)is assembled, the position of the optical fiber on the receiving side is misaligned in the direction perpendicular to the optical axis (see the arrows inFIGS.7(a) and7(b)), that is, optical axis misalignment occurs under the condition that only the fundamental mode of LP01 exists in the input light will be considered.

FIG.8is a graph showing simulation results of optical power coupling efficiency in that case. The horizontal axis represents the amount of optical axis misalignment, and the vertical axis represents the coupling efficiency. With no misalignment, 100% of the power propagates through the optical fiber and the coupling efficiency is 1. Then, for example, if only 50% of the power of the input light propagates through the optical fiber, the coupling efficiency is 0.5.

Comparing by the wavelengths 1310 nm and 850 nm of input light, it can be seen that the characteristics of the 850 nm case are better. The reason for this that only the fundamental mode can propagate in the case of 1310 nm, whereas the first-order mode as well as the fundamental mode can propagate in the case of 850 nm (seeFIG.6(a)).

That is, when there is no optical axis misalignment, only the fundamental mode exists in the input light as shown inFIG.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 utilizing the phase difference caused by the refractive index difference between the cladding and the core, as shown inFIG.9(b). This first-order mode cannot propagate in the case of 1310 nm, but this first-order mode can also propagate in the case of 850 nm, so the characteristics in the case of 850 nm are improved.

In the graph ofFIG.10, the fundamental mode (zeroth-order mode) component and the first-order mode component are shown separately, and the sum of them is the total curve. Since the input light exists only in the fundamental mode, it can be seen that the fundamental mode is converted to the first-order mode according to the misalignment. On the other hand, in the case of 1310 nm, only the fundamental mode can propagate as shown inFIG.3(a), so the fundamental mode is purely reduced as shown inFIG.8.

InFIG.8, comparing by the coupling efficiency for the cases of 1310 nm and 850 nm, the accuracy of misalignment can be mitigated by about 1.8 times at the coupling efficiency of 0.8 (about −1 dB) and by 2.35 times at the coupling efficiency of 0.9 (about −0.5 dB).

As described above, if the optical fiber is configured to propagate only the fundamental mode at a first wavelength (for example, 1310 nm) and propagate at least the first-order mode as well as the fundamental mode at a second wavelength (for example, 850 nm), when communication is performed using light of the second wavelength, since at least the first-order mode component generated due to optical axis misalignment propagates together with the fundamental mode component, it is possible to reduce the coupling loss of optical power due to optical axis misalignment.

According to a second aspect of the present technology, the optical fiber is configured such that a refractive index distribution of the core and the cladding is controlled so that an inter-mode propagation delay difference is within a predetermined threshold, for example, the inter-mode propagation delay difference is zero, when communication is performed using light of the second wavelength.

FIG.11shows an example of a case where a conventional 1310-nm fiber (single-mode optical fiber propagating only the zeroth-order mode (fundamental mode) at a wavelength of 1310 nm) transmits light (optical signal) from an 850-nm light source, composed of the zeroth-order mode component and the first-order mode component of the wavelength of 850 nm.

In this case, a propagation delay difference occurs between the zeroth-order mode and the first-order mode at the output end of the optical fiber. Such an inter-mode propagation delay difference is caused by a difference in reflection angles of light components of each mode within the optical fiber. In this case, the higher the order, the steeper the angle of reflection. That is, an inter-mode propagation delay difference occurs due to the change in the optical path length depending on the mode.

As shown inFIG.11, when “1” is expressed by the sum of the power of the zeroth-order mode and the first-order mode at the input end of an optical fiber, if an inter-mode propagation delay difference occurs at the output end of the optical fiber, a step occurs in the rising waveform from “0” to “1” or the falling waveform from “1” to “0”. This phenomenon causes waveform distortion in data transmission, resulting in deterioration of signal quality. This deterioration in signal quality becomes more prominent as the length of the optical fiber increases and as the data rate increases.

When waveform distortion that causes signal quality deterioration occurs in this way, although it is conceivable to correct the waveform distortion to suppress signal quality deterioration, a waveform distortion correction circuit is required on the transmitting and receiving sides, leading to an increase in cost and power consumption.

In the present technology, as described above, the refractive index distribution of the core and cladding of the optical fiber is controlled so that the inter-mode propagation delay difference is within a predetermined threshold when communication is performed using light of the second wavelength. As a result, high-quality signal transmission can be realized without increasing the cost and power consumption due to the provision of a waveform distortion correction circuit.

When propagating through the optical fiber, each mode of light advances while its intensity distribution does not fit within the core but spreads into the cladding.

FIG.12shows an example of the intensity distribution when the fundamental mode (zeroth-order mode) and the first-order mode propagate through the optical fiber. As shown in the figure, it can be seen that the intensity distribution of both the fundamental mode and the first-order mode penetrates into the cladding.

The core has a higher refractive index than the cladding, which means that the propagation speed of light in the core is slower than that of the cladding. In general, the higher the order of the mode, the steeper the angle of total reflection of light propagating in the optical fiber and the longer becomes the propagation path. However, since the intensity distribution of light passing through the cladding side increases, it is possible to control the propagation speed of each mode uniformly by appropriately controlling the refractive index distribution of the core and cladding.

FIG.13(a)shows a cross-section of an optical fiber. Also,FIGS.13(b) to13(e)show examples of a refractive index distribution of the core and the cladding. This refractive index distribution shows the refractive index distribution near the core on the line A-B inFIG.13(a), where the vertical axis indicates the refractive index and the horizontal axis indicates the physical distance. In the illustrated example, the diameter of the core is a, but the diameter is not necessarily limited to this, and the diameter of the core may be defined as smaller or larger than a.

As shown inFIGS.13(b) to13(e), the refractive index distribution of the optical fiber includes the distribution of refractive indices of a first region from the center to a first diameter a, a second region to a second diameter b outside this first region, a third region to a third diameter c outside the second region, and a fourth region outside the third region. Here, the refractive index change amounts of the first, second, and third regions with respect to the refractive index of the fourth region, that is, when the refractive index of the fourth region is used as a reference, are defined as A, x, and y, respectively.

In the case of 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 case of the stepped-type, the refractive index of the third region is equal to than in 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.

In the case of the W-type, the refractive index of the third region is equal to that of the fourth region, the refractive index of the second region is lower than that of the fourth region, and the refractive index of the first region is higher than that of the fourth region. In the case of the SI-type, the refractive indices of the third region and the second region are equal to that of the fourth region, and the refractive index of the first region is higher than that of the fourth region.

The refractive index distribution of each type is controlled as follows, for example, so that the inter-mode propagation delay difference is zero under double-mode conditions, that is, under the combination of an 850-nm light source and a 1310-nm fiber. Note that the refractive index of the fourth region is 1.4524 according to the Sellmeier's dispersion formula. That is, if the parameters of general quartz are applied to the Sellmeier's dispersion formula, the refractive index for light with a wavelength of 850 nm is 1.4524.

In the case of the segmented-core type, the first diameter a is 7 μm, the second diameter b is 9 μm, the third diameter c is 11 μm, the refractive index change amount y of the third region with respect to the refractive index of the fourth region is y+0.000827, and the refractive index change amount A of the first region with respect to the refractive index of the fourth region is +0.004882.

FIG.14shows simulation results of the inter-mode propagation delay difference (propagation delay difference between the fundamental mode and the first-order mode) when using a segmented-core optical fiber whose refractive index distribution is controlled as described above. Here, the horizontal axis the wavelength of the light source, and the vertical axis the inter-mode propagation delay difference. From this simulation result, it can be seen that the inter-mode propagation delay difference (propagation delay difference between the fundamental mode and the first-order mode) is zero or substantially zero at 850 nm (=0.85 μm).

In the case of the stepped-type, the first diameter a is 7 μm, the second diameter b is 13 μm, the refractive index change amount x of the second region with respect to the refractive index of the fourth region is +0.000811, and the refractive index change amount A of the first region with respect to the refractive index of the fourth region is +0.005053.

FIG.15shows simulation results of the inter-mode propagation delay difference (propagation delay difference between the fundamental mode and the first-order mode) when using a stepped optical fiber whose refractive index distribution is controlled as described above. Here, the horizontal axis the wavelength of the light source, and the vertical axis the inter-mode propagation delay difference. From this simulation result, it can be seen that the inter-mode propagation delay difference (propagation delay difference between the fundamental mode and the first-order mode) is zero or substantially zero at 850 nm (=0.85 μm).

In the case of the W-type, the first diameter a is 7 μm, the second diameter b is 9 μm, the refractive index change amount x of the first region with respect to the refractive index of the fourth region is −0.002245, and the refractive index change amount A of the first region with respect to the refractive index of the fourth region is +0.004778.

FIG.16shows simulation results of the inter-mode propagation delay difference (propagation delay difference between the fundamental mode and the first-order mode) when using a W-type optical fiber whose refractive index distribution is controlled as described above. Here, the horizontal axis the wavelength of the light source, and the vertical axis the inter-mode propagation delay difference. From this simulation result, it can be seen that the inter-mode propagation delay difference (propagation delay difference between the fundamental mode and the first-order mode) is zero or substantially zero at 850 nm (=0.85 μm).

In the case of the SI-type, the first diameter is a, and the refractive index change amount A of the first region with respect to the refractive index of the fourth region is +0.00467.

FIG.17shows simulation results of the inter-mode propagation delay difference (propagation delay difference between the fundamental mode and the first-order mode) when using an SI-type optical fiber whose refractive index distribution is controlled as described above. Here, the horizontal axis the wavelength of the light source, and the vertical axis the inter-mode propagation delay difference. From this simulation result, it can be seen that the inter-mode propagation delay difference (propagation delay difference between the fundamental mode and the first-order mode) is zero or substantially zero at 850 nm (=0.85 μm).

FIG.18shows simulation results of the relationship of refractive indices in which the inter-mode propagation delay difference (propagation delay difference between the fundamental mode and the first-order mode) is zero in the case of the segmented-core type (seeFIG.13(b)) under the double-mode condition, that is, a combination of an 850-nm light source and a 1310-nm fiber when the refractive index of the fourth region is 1.4524, the first diameter a is 7 μm, the second diameter b is 9 μm, and the third diameter c is 11 μm.

Here, the horizontal axis the refractive index change amount y of the third region with respect to the refractive index of the fourth region, and the vertical axis the refractive index change amount A of the first region with respect to the refractive index of the fourth region. The refractive index change amount A when the refractive index change amount y is zero is +0.00467 because it is equal to the case where the inter-mode propagation delay difference is zero in the case of the SI-type.

From this simulation result, it can be seen that when the inter-mode propagation delay difference is made zero, the refractive index change amount y of the third region with respect to the refractive index of the fourth region can take a value within the range of 0 to +0.0024, and the refractive index change amount A of the first region with respect to the refractive index of the fourth region can take a value within the range of +0.00467 to +0.00541.

This simulation result shows the refractive index relationship under the condition that only the fundamental mode and the first-order mode exist, and is the refractive index relationship under the condition that higher-order modes do not exist.

If the inter-mode propagation delay difference between the fundamental mode and the first-order mode is made zero in the range of the refractive index change amount y exceeding +0.0024 and a refractive index conversion amount exceeding +0.00541, the refractive index difference between the core and the cladding becomes large. As a result, the angle at which the light can be totally reflected becomes steeper, and the conditions for transmitting the second-order mode are satisfied. Thus, the second-order mode occurring due to optical axis misalignment, for example, also propagates through the optical fiber. In that case, the inter-mode propagation delay difference cannot be adjusted with respect to the second-order mode, leading to the occurrence of waveform distortion.

Therefore, under the condition that only the fundamental mode and the first-order mode exist, as described above, the refractive index change amount y is within the range of 0 to +0.0024, and the refractive index change amount A is within the range of +0.00467 to +0.00541. Although detailed explanation is omitted, the simulation results for the stepped-type and the W-type shown below also show the relationship of the refractive indices under the condition that only the fundamental mode and the first-order mode exist.

FIG.19shows simulation results of the relationship of refractive indices in which the inter-mode propagation delay difference (propagation delay difference between the fundamental mode and the first-order mode) is zero in the case of the stepped-type (seeFIG.13(c)) under the double-mode condition, that is, a combination of an 850-nm light source and a 1310-nm fiber when the refractive index of the fourth region is 1.4524, the first diameter a is 7 μm, and the second diameter b is 13 μm.

Here, the horizontal axis the refractive index change amount x of the second region with respect to the refractive index of the fourth region, and the vertical axis the refractive index change amount A of the first region with respect to the refractive index of the fourth region. The refractive index change amount A when the refractive index change amount x is zero is +0.00467 because it is equal to the case where the inter-mode propagation delay difference is zero in the SI-type.

From this simulation result, it can be seen that when the inter-mode propagation delay difference is made zero, the refractive index change amount x of the second region with respect to the refractive index of the fourth region can take a value within the range of 0 to +0.0012, and the refractive index change amount A of the first region with respect to the refractive index of the fourth region can take a value within the range of +0.00467 to +0.00526.

FIG.20shows simulation results of the relationship of refractive indices in which the inter-mode propagation delay difference (propagation delay difference between the fundamental mode and the first-order mode) is zero in the case of the W-type (seeFIG.13(d)) under the double-mode condition, that is, a combination of an 850-nm light source and a 1310-nm fiber when the refractive index of the fourth region is 1.4524, the first diameter a is 7 μm, and the second diameter b is 9 μm.

Here, the horizontal axis the refractive index change amount x of the second region with respect to the refractive index of the fourth region, and the vertical axis the refractive index change amount A of the first region with respect to the refractive index of the fourth region. The refractive index change amount A when the refractive index change amount x is zero is +0.00467 because it is equal to the case where the inter-mode propagation delay difference is zero in the SI-type.

From this simulation result, it can be seen that when the inter-mode propagation delay difference is made zero, the refractive index change amount x of the second region with respect to the refractive index of the fourth region can take a value within the range of −0.0055 to 0, and the refractive index change amount A of the first region with respect to the refractive index of the fourth region can take a value within the range of +0.00486 to +0.00467.

In this way, the optical fiber is configured such that the refractive index distribution of the core and the cladding is controlled so that the inter-mode propagation delay difference is within a predetermined threshold, for example, the inter-mode propagation delay difference is zero, when communication is performed using the light of the second wavelength. Thus, it is possible to keep the inter-mode propagation delay difference within a predetermined threshold when communication is performed using light of the second wavelength. Further, high-quality signal transmission can be realized without increasing the cost and power consumption due to the provision of a waveform distortion correction circuit.

FIG.21shows a configuration example of a transmission/reception system100. This transmission/reception system100has a transmitter200, a receiver300and a cable400. The transmitter200is, for example, an AV source such as a personal computer (PC), a game console, a disc player, a set-top box, a digital camera, a mobile phone, and the like. The receiver300is, for example, a television receiver, a projector, a PC monitor, or the like. The transmitter200and the receiver300are connected via the cable400.

The transmitter200has a light-emitting portion201, a connector202as a receptacle, and an optical fiber203that propagates the light emitted by the light-emitting portion201to the connector202. The light-emitting portion201includes a laser element such as a VCSEL, or a light-emitting element (light source) such as an LED (light-emitting diode). The light-emitting portion201converts an electrical signal (transmission signal) generated by a transmission circuit (not shown) to an optical signal. The light (optical signal) emitted by the light-emitting portion201propagates to the connector202through the optical fiber203.

The receiver300also has a connector301as a receptacle, a light-receiving portion302, and an optical fiber303for propagating the light obtained at the connector301to the light-receiving portion302. The light-receiving portion302includes a light-receiving element such as a photodiode. The light-receiving portion302converts an optical signal transmitted from the connector301to an electric signal (receiving signal) and supplies the electric signal to a receiving circuit (not shown).

The cable400is configured to have connectors402and403as plugs at one end and the other end of the optical fiber401. The connector402at one end of the optical fiber401is connected to the connector202of the transmitter200, and the connector403at the other end of the optical fiber401is connected to the connector301of the receiver300.

In this transmission/reception system100, the optical fiber of the present technology is applied to the optical fiber203of the transmitter200, the optical fiber303of the receiver300, and the optical fiber401of the cable400. That is, these optical fibers are configured to propagate only the fundamental mode at a first wavelength and propagate at least the first-order mode as well as the fundamental mode at a second wavelength.

In addition, these optical fibers are configured such that a refractive index distribution of the core and the cladding is controlled so that an inter-mode propagation delay difference is within a predetermined threshold, for example, the inter-mode propagation delay difference is zero, when communication is performed using light of the second wavelength (seeFIGS.13(b) to13(e)). In this transmission/reception system100, communication is performed using light of the second wavelength.

In other words, communication is performed under double-mode conditions, for example, that a combination of an 850-nm light source and a 1310-nm fiber (whose refractive index distribution of the core and cladding is controlled so that the inter-mode propagation delay difference is zero) is applied.

FIG.22shows a configuration example of the light-emitting portion201and the connector202in the transmitter200. This configuration example is an example, and the configuration of the transmitter200is not limited to this.

The light-emitting portion201has a ferrule211. The ferrule211is made of a light-transmissive material such as a synthetic resin or glass, or a material such as silicon that transmits a specific wavelength.

The ferrule211is provided with an optical fiber insertion hole216extending rearward from the front side. The optical fiber203is fixed to the ferrule211with an adhesive217after being inserted into the optical fiber insertion hole216. The optical fiber203has a double structure including a central core203aserving as an optical path and a cladding203bsurrounding the central core203a.

A substrate212on which the light-emitting element213and a light-emitting element driver218are mounted is fixed to the lower surface side of the ferrule211. In this case, the light-emitting element213is mounted on the substrate212in alignment with the optical fiber203. Here, the position of the substrate212is adjusted and fixed so that the exit portion of the light-emitting element213is aligned with the optical axis of the optical fiber203.

Further, the ferrule211is formed with an arrangement hole214extending upward from the lower surface side. In order to change the optical path of the light from the light-emitting element213to the direction of the optical fiber203, the bottom portion of the arrangement hole214is formed as an inclined surface, and a mirror (optical path changing portion)215is arranged on this inclined surface. Regarding the mirror215, it is conceivable not only to fix the mirror215which is separately produced to the inclined surface, but also to form the mirror215on the inclined surface by vapor deposition or the like.

The connector202has a connector main body221. The connector main body221is made of a light-transmissive material such as a synthetic resin or glass, or a material such as silicon that transmits a specific wavelength, and is configured as a ferrule with a lens.

By configuring the connector main body221as a ferrule with a lens in this way, it is possible to easily achieve the optical axis alignment between the optical fiber and the lens. Since the connector main body221is configured as a ferrule with a lens in this way, multi-channel communication can be easily realized simply by inserting an optical fiber into the ferrule even in the case of multi-channel.

The connector main body221has a concave light exit portion (light transmission space)223formed on the front side thereof. A lens (convex lens)224is formed integrally with the connector main body221so as to be positioned at the bottom of the light exit portion223.

Further, an optical fiber insertion hole226extending forward from the back side is provided in the connector main body221so as to be aligned with the lens224. The optical fiber insertion hole226is formed so that the optical axis of the lens224is aligned with the core203aof the optical fiber203inserted therein. The optical fiber insertion hole226is shaped so that its bottom position, that is, the contact position of the tip (incident end) when the optical fiber203is inserted, coincides with the focal position of the lens224.

Further, an adhesive injection hole222extending downward from the upper surface side is formed in the connector main body221so as to communicate with the vicinity of the bottom position of the optical fiber insertion hole226. After the optical fiber203is inserted into the optical fiber insertion hole226, an adhesive227is injected around the optical fiber203from the adhesive injection hole222, whereby the optical fiber203is fixed to the connector main body221.

In the connector202, the lens224has a function of shaping the light emitted from the optical fiber203into collimated light and emitting the collimated light. As a result, the light emitted from the output end of the optical fiber203with a predetermined NA is incident on the lens224, shaped into collimated light, and emitted.

FIG.23shows a configuration example of the connector301and the light-receiving portion302in the receiver300. This configuration example is an example, and the configuration of the receiver300is not limited to this.

The connector301has a connector main body311. The connector main body311is made of, for example, a light-transmissive material such as a synthetic resin or glass, or a material such as silicon that transmits a specific wavelength, and is configured as a ferrule with a lens.

By configuring the connector main body311as a ferrule with a lens in this way, it is possible to easily achieve the optical axis alignment between the optical fiber and the lens. Since the connector main body311is configured as a ferrule with a lens in this way, multi-channel communication can be easily realized by simply inserting an optical fiber into the ferrule even in the case of multi-channel.

The connector main body311has a concave light incident portion (light transmission space)313formed on the front side thereof. A lens (convex lens)314is formed integrally with the connector main body311so as to be positioned at the bottom of the light incident portion313.

Further, an optical fiber insertion hole316extending forward from the back side is provided in the connector main body311so as to be aligned with the lens314. The optical fiber303has a double structure including a central core303aserving as an optical path and a cladding303bsurrounding the core303a.

The optical fiber insertion hole316is shaped so that the optical axis of the lens314is aligned with the core303aof the optical fiber303inserted therein. The optical fiber insertion hole316is shaped so that its bottom position, that is, the contact position of the tip (incident end) when the optical fiber303is inserted, coincides with the focal position of the lens314.

Further, an adhesive injection hole312extending downward from the upper surface side is formed in the connector main body311so as to communicate with the vicinity of the bottom position of the optical fiber insertion hole316. After the optical fiber303is inserted into the optical fiber insertion hole316, an adhesive317is injected around the optical fiber303from the adhesive injection hole312, whereby the optical fiber303is fixed to the connector main body311.

In the connector301, the lens314has a function of condensing incident collimated light. In this case, the collimated light is incident on the lens314and condensed, and this condensed light is incident on the incident end of the optical fiber303with a predetermined NA.

Further, the light-receiving portion302has a ferrule321. The ferrule321is made of a light-transmissive material such as a synthetic resin or glass, or a material such as silicon that transmits a specific wavelength.

The ferrule321is provided with an optical fiber insertion hole326extending rearward from the front side. The optical fiber303is fixed to the ferrule321with an adhesive327after being inserted into the optical fiber insertion hole326. The ferrule321is formed with an arrangement hole324extending upward from the lower surface side.

A substrate322on which a light-receiving element323and a processor328are mounted is fixed to the lower surface side of the ferrule321. The position of the substrate322is adjusted and fixed so that the incident portion of the light-receiving element323is aligned with the optical axis of the optical fiber303.

Further, in order to change the optical path of light from the optical fiber303to the direction of the light-receiving element323, the bottom portion of the arrangement hole324is formed as an inclined surface, and a mirror (optical path changing portion)325is arranged on this inclined surface. Regarding the mirror325, it is conceivable not only to fix the mirror325which is separately produced to the inclined surface, but also to form the mirror325on the inclined surface by vapor deposition or the like.

FIG.24shows a configuration example of the connectors402and403in the cable400. This configuration example is an example, and the configuration of the cable400is not limited to this.

The connector402has a connector main body421. The connector main body421is made of a light-transmissive material such as a synthetic resin or glass, or a material such as silicon that transmits a specific wavelength, and is configured as a ferrule with a lens.

By configuring the connector main body421as a ferrule with a lens in this way, it is possible to easily achieve the optical axis alignment between the optical fiber and the lens. Since the connector main body421is configured as a ferrule with a lens in this way, multi-channel communication can be easily realized by simply inserting an optical fiber into the ferrule even in the case of multi-channel.

The connector main body421has a concave light incident portion (light transmission space)423formed on the front side thereof. A lens (convex lens)424is formed integrally with the connector main body421so as to be positioned at the bottom of the light incident portion423.

In addition, an optical fiber insertion hole426extending forward from the rear side is provided in the connector main body421so as to be aligned with the lens424. The optical fiber401has a double structure including a central core401aserving as an optical path and a cladding401bsurrounding the core401a.

The optical fiber insertion hole426is formed so that the optical axis of the lens424is aligned with the core401aof the optical fiber401inserted therein. The optical fiber insertion hole426is shaped so that its bottom position, that is, the contact position of the tip (incident end) when the optical fiber401is inserted, coincides with the focal position of the lens424.

Further, an adhesive injection hole422extending downward from the upper surface side is formed in the connector main body421so as to communicate with the vicinity of the bottom position of the optical fiber insertion hole426. After the optical fiber401is inserted into the optical fiber insertion hole426, an adhesive427is injected around the optical fiber401from the adhesive injection hole422, whereby the optical fiber401is fixed to the connector main body421.

In the connector402, the lens424has a function of condensing incident collimated light. In this case, the collimated light is incident on the lens424and condensed, and this condensed light is incident on the incident end of the optical fiber401with a predetermined NA.

The connector403has a connector main body431. The connector main body431is made of a light-transmissive material such as a synthetic resin or glass, or a material such as silicon that transmits a specific wavelength, and is configured as a ferrule with a lens.

By configuring the connector main body431as a ferrule with a lens in this way, it is possible to easily achieve the optical axis alignment between the optical fiber and the lens. Since the connector main body431is configured as a ferrule with a lens in this way, multi-channel communication can be easily realized by simply inserting an optical fiber into the ferrule even in the case of multi-channel.

The connector main body431has a concave light-emitting portion (light transmission space)433formed on the front side thereof. A lens (convex lens)434is formed integrally with the connector main body431so as to be positioned at the bottom of the light-emitting portion433.

In addition, an optical fiber insertion hole436extending forward from the rear side is provided in the connector main body431so as to be aligned with the lens434.

The optical fiber insertion hole436is shaped so that the optical axis of the lens434is aligned with the core401aof the optical fiber401inserted therein. The optical fiber insertion hole436is formed so that its bottom position, that is, the contact position of the tip (incident end) when the optical fiber401is inserted, coincides with the focal position of the lens434.

Further, an adhesive injection hole432extending downward from the upper surface side is formed in the connector main body431so as to communicate with the vicinity of the bottom position of the optical fiber insertion hole436. After the optical fiber401is inserted into the optical fiber insertion hole436, an adhesive437is injected around the optical fiber401from the adhesive injection hole432, whereby the optical fiber401is fixed to the connector main body431.

In the connector403, the lens434has a function of shaping the light emitted from the optical fiber401into collimated light and emitting the collimated light. As a result, the light emitted from the output end of the optical fiber401with a predetermined NA is incident on the lens434, shaped into collimated light, and emitted.

In the transmission/reception system100shown inFIG.21, the optical fibers203,303, and401are configured to propagate only the fundamental mode at a first wavelength (for example, 1310 nm) and propagate at least the first-order mode as well as the fundamental mode at a second wavelength (for example, 850 nm), and communication is performed using light of the second wavelength. Therefore, since at least the first-order mode component generated due to optical axis misalignment propagates together with the fundamental mode component, it is possible to reduce the coupling loss of optical power due to optical axis misalignment.

In the transmission/reception system100shown inFIG.21, the optical fibers203,303, and401are configured such that a refractive index distribution of the core and the cladding is controlled so that an inter-mode propagation delay difference is within a predetermined threshold, for example, the inter-mode propagation delay difference is zero, when communication is performed using light of the second wavelength. Therefore, since the inter-mode propagation delay difference is within a predetermined threshold when communication is performed using light of the second wavelength, high-quality signal transmission can be realized without increasing the cost and power consumption due to the provision of a waveform distortion correction circuit.

FIG.25shows a configuration example of a transmission/reception system100A. InFIG.25, the parts corresponding to those inFIG.21are designated by the same reference signs, and detailed descriptions thereof will be omitted as appropriate. This transmission/reception system100A has a so-called pigtail-type transmitter in which a transmitter200and a cable400are integrally formed. In this case, the transmitting side of the cable400is fixedly connected to the transmitter200, and the light emitted by the light-emitting portion201is directly incident on the optical fiber401of the cable400. Others of this transmission/reception system100A are configured in the same manner as the transmission/reception system100ofFIG.21.

In the transmission/reception system100A, the optical fiber of the present technology is applied to the optical fiber401of the cable400and the optical fiber303of the receiver300. That is, these optical fibers are configured to propagate only the fundamental mode at a first wavelength and propagate at least the first-order mode as well as the fundamental mode at a second wavelength.

In addition, the optical fibers are configured such that a refractive index distribution of the core and the cladding is controlled so that an inter-mode propagation delay difference is within a predetermined threshold, for example, the inter-mode propagation delay difference is zero, when communication is performed using light of the second wavelength (seeFIGS.13(b) to13(e)). In this transmission/reception system100A, communication is performed using light of the second wavelength.

In other words, communication is performed under double-mode conditions, for example, that a combination of an 850-nm light source and a 1310-nm fiber (whose refractive index distribution of the core and cladding is controlled so that the inter-mode propagation delay difference is zero) is applied.

FIG.26shows a configuration example of the light-emitting portion201in the transmitter200and the connector403in the cable400. The light-emitting portion201is the same as that described above with reference toFIG.22, so description thereof will be omitted. Also, the connector403is the same as that described above with reference toFIG.24, so description thereof will be omitted.

The transmission/reception system100A shown inFIG.25can also obtain the same effects as the transmission/reception system100shown inFIG.21.

2. Modification Example

In the above-described embodiment, the present technology has been described with an example in which the refractive index distribution of the core and the cladding is controlled so that the inter-mode propagation delay difference is zero, but the inter-mode propagation delay difference is not necessarily zero. It is also conceivable to control the refractive index distribution of the core and the cladding so that the inter-mode propagation delay difference is within a predetermined threshold determined according to distance, transmission rate, and the like.

In the above-described embodiments, the segmented-core type, stepped-type, W-type, and SI-type are shown as the types of the refractive index distribution of the core and the cladding, but the types are not limited thereto, and the refractive index distribution may be realized in other types. Parameters such as the values of the first diameter a, the second diameter b, and the third diameter c are not limited to those mentioned in the above embodiment.

In the above-described embodiment, although the first wavelength is described as 1310 nm, since a laser light source or an LED light source can be used as the light source, the first wavelength can be set between 300 nm and 5 μm, for example. In the above-described embodiment, although the first wavelength is described as 1310 nm, it is also conceivable that the first wavelength is a wavelength in the 1310-nm band including 1310 nm. In the above-described embodiment, although the first wavelength is described as 1310 nm, it is also conceivable that the first wavelength is 1550 nm or a wavelength in the 1550-nm band including 1550 nm. In the above-described embodiment, although the second wavelength is described as 850 nm, it is also conceivable that the second wavelength is a wavelength in the 850-nm band including 850 nm.

Further, in the above embodiments, an example in which the optical waveguide is an optical fiber has been described, but the present technology can naturally be similarly applied to an optical waveguide other than optical fiber, such as a silicon optical waveguide.

Although the preferred embodiments of the present disclosure have been described in detail with reference to the accompanying figures as described above, the technical scope of the present disclosure is not limited to such examples. It is apparent that those having ordinary knowledge in the technical field of the present disclosure could conceive various modification examples or revisions within the scope of the technical ideas set forth in the claims, and it should be understood that these also naturally fall within the technical scope of the present disclosure.

Further, the effects described in the present specification are merely explanatory or exemplary and are not intended as limiting. In other words, the technologies according to the present disclosure may exhibit other effects apparent to those skilled in the art from the description herein, in addition to or in place of the above effects.

Note that the present technology can also have the following configurations.(1) An optical waveguide configured to: propagate only a fundamental mode at a first wavelength; propagate at least a first-order mode as well as the fundamental mode at a second wavelength; and a refractive index distribution of a core and a cladding is controlled so that an inter-mode propagation delay difference is within a predetermined threshold when communication is performed using light of the second wavelength.(2) The optical waveguide according to (1), wherein the refractive index distribution includes a distribution of refractive indices of a first region from a center to a first diameter, a second region to a second diameter outside the first region, a third region to a third diameter outside the second region, and a fourth region outside the third region.(3) The optical waveguide according to (2), wherein 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.(4) The optical waveguide according to (3), wherein the first wavelength is in a 1310-nm band and the second wavelength is in an 850-nm band, the first diameter is 7 μm, the second diameter is 9 μm, the third diameter is 11 μm, the refractive index of the fourth region is 1.4524, a refractive index change amount of the third region with respect to the refractive index of the fourth region is in a range of 0 to +0.0024, and a refractive index change amount of the first region with respect to the refractive index of the fourth region is in a range of +0.00467 to +0.00541.(5) The optical waveguide according to (4), wherein a refractive index change amount of the third region with respect to the refractive index of the fourth region is +0.000827, and a refractive index change amount of the first region with respect to the refractive index of the fourth region is +0.004882.(6) The optical waveguide according to (2), wherein 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.(7) The optical waveguide according to (6), wherein the first wavelength is in a 1310-nm band and the second wavelength is in an 850-nm band, the first diameter is 7 μm and the second diameter is 13 μm, the refractive index of the fourth region is 1.4524, a refractive index change amount of the second region with respect to the refractive index of the fourth region is in a range of 0 to +0.0012, and a refractive index change amount of the first region with respect to the refractive index of the fourth region is in a range of +0.00467 to +0.00526.(8) The optical waveguide according to (7), wherein a refractive index change amount of the second region with respect to the refractive index of the fourth region is +0.000811, and a refractive index change amount of the first region with respect to the refractive index of the fourth region is +0.005053.(9) The optical waveguide according to (2), wherein the refractive index of the third region is equal to that of the fourth region, the refractive index of the second region is lower than that of the fourth region, and the refractive index of the first region is higher than that of the fourth region.(10) The optical waveguide according to (9), wherein the first wavelength is in a 1310-nm band and the second wavelength is in an 850-nm band, the first diameter is 7 μm and the second diameter is 9 μm, the refractive index of the fourth region is 1.4524, a refractive index change amount of the first region with respect to the refractive index of the fourth region is in a range of −0.0055 to 0, and a refractive index change amount of the first region with respect to the refractive index of the fourth region is in a range of +0.00486 to +0.00467.(11) The optical waveguide according to (10), wherein a refractive index change amount of the first region with respect to the refractive index of the fourth region is −0.002245, and a refractive index change amount of the first region with respect to the refractive index of the fourth region is +0.004778.(12) The optical waveguide according to (2), wherein the refractive indices of the third region and the second region are equal to that of the fourth region, and the refractive index of the first region is higher than that of the fourth region.(13) The optical waveguide according to (12), wherein the first wavelength is in a 1310-nm band and the second wavelength is in an 850-nm band, the first diameter is 7 μm, the refractive index of the fourth region is 1.4524, and a refractive index change amount of the first region with respect to the refractive index of the fourth region is +0.00467.(14) The optical waveguide according to (1), wherein the first wavelength is a wavelength at which chromatic dispersion is zero.(15) The optical waveguide according to (1) or (14), wherein the first wavelength is between 300 nm and 5 μm.(16) The optical waveguide according to (15), wherein the first wavelength is a wavelength in a 1310-nm band or a 1550-nm band.(17) The optical waveguide according to (1) or (2), wherein the second wavelength is a wavelength in an 850-nm band.(18) An optical communication device comprising: an optical waveguide configured to propagate only a fundamental mode at a first wavelength and propagate at least a first-order mode as well as the fundamental mode at a second wavelength, the optical waveguide is configured such that a refractive index distribution of a core and a cladding is controlled so that an inter-mode propagation delay difference is within a predetermined threshold when communication is performed using light of the second wavelength, and the optical communication device performs communication using light of the second wavelength.(19) An optical communication method for performing communication using light of a second wavelength in an optical waveguide configured to propagate only a fundamental mode at a first wavelength and propagate at least a first-order mode as well as the fundamental mode at the second wavelength and configured such that a refractive index distribution of a core and a cladding is controlled so that an inter-mode propagation delay difference is within a predetermined threshold when communication is performed using light of the second wavelength.(20) An optical communication system in which a transmitter and a receiver are connected by an optical waveguide, wherein the optical waveguide is configured to propagate only a fundamental mode at a first wavelength and propagate at least a first-order mode as well as the fundamental mode at a second wavelength and configured such that a refractive index distribution of a core and a cladding is controlled so that an inter-mode propagation delay difference is within a predetermined threshold when communication is performed using light of the second wavelength, and the transmitter and the receiver perform communication using light of the second wavelength in the optical waveguide.

REFERENCE SIGNS LIST