The present disclosure relates to a mode conversion device in which a long period grating (LPG) is formed in an optical fiber and a design method therefor.
Non Patent Literature 1 discloses an LPG in which only light with a certain wavelength is selectively coupled to a cladding mode of an optical fiber.
In an LPG, a relationship between coupling efficiency and a full width at half maximum is uniquely obtained. However, when any value of a complete coupling length, a fiber structure, and a center wavelength changes, the relationship between the coupling efficiency and the full width at half maximum also changes. Therefore, in a mode conversion device that has an LPG, there is a problem that it is difficult to arbitrarily design coupling efficiency and a full width at half maximum. “Full width at half maximum” means a wavelength band in which coupling efficiency is a half of coupling efficiency of mode conversion at a center wavelength, and is a wavelength range in which mode conversion of the mode conversion device is possible.
In order to solve the foregoing problems, an objective of the present invention is to provide a mode conversion device capable of designing any coupling efficiency and a full width at half maximum, and a design method therefor.
In order to achieve the foregoing objective, a mode conversion device according to the present invention adjusts parameters based on a relationship between a wavelength differential of a propagation constant difference Δβ between two modes to be converted and coupling efficiency and a full width at half maximum.
Specifically, the mode conversion device according to the present invention includes a long period grating at a core of an optical fiber through which light is able to propagate in at least two propagation modes. The long period grating satisfies a relationship of Expression C1.
where
a full width at half maximum FWHM is a wavelength band in which the coupling efficiency is a half of coupling efficiency of mode conversion at a center wavelength, C is coupling efficiency, Lc is a complete coupling length, Lg is a grating length, Λ is a grating pitch, and Δβ is a propagation constant difference between the two propagation modes at the center wavelength of a mode conversion target.
By forming the LPG, having the grating pitch A and the grating length Lg and satisfying the relationship of Expression C1, on the core of the optical fiber having the core radius a and the relative refractive index difference Δ of the core, it is possible to obtain the mode conversion device that has a desired full width at half maximum FWHM and the coupling efficiency C at a desired wavelength.
A specific design method is a design method of determining a design parameter of a long period grating installed at a core of an optical fiber through which light is able to propagate in at least two propagation modes. The method includes:
where, the full width at half maximum FWHM is a wavelength band in which the coupling efficiency is a half of coupling efficiency of mode conversion at the center wavelength λ0.
Accordingly, the present invention can provide the mode conversion device capable of designing any coupling amount and a full width at half maximum, and a design method therefor.
The mode conversion device according to the present invention further includes a tap waveguide that is at a rear stage of the long period grating in a propagation direction of light and outputs, from a side surface of the optical fiber, light with a desired wavelength converted from one mode to another mode by the long period grating in light propagating through the core of the optical fiber.
The mode conversion device can extract light with a desired wavelength from the light propagating through the optical fiber at a desired power.
Here, in the mode conversion device according to the present invention, a set of the long period grating and the tap waveguide may be vertically aligned in the optical fiber.
In this situation, the long period grating has different design parameters described in Expression C1 so that wavelengths of light converted from the one mode to the other mode are different from each other. Light with a different wavelength can be extracted from each tap waveguide.
In this situation, the grating lengths Lg of the long period grating may be different so that a wavelength of light to be converted from the one mode to the other mode is identical and the coupling efficiency is different. It is possible to extract light with different desired power from each tap waveguide.
The foregoing inventions can be combined where possible.
The present invention can provide a mode conversion device capable of designing any coupling efficiency and a full width at half maximum and a design method therefor.
Embodiments of the present invention will be described below with reference to the accompanying drawings. The embodiments to be described below are examples of the present invention, and the present invention is not limited to the following embodiments. Like components are denoted by like reference numerals in this specification and the drawings.
Here, a length at which incident mode 1 is completely coupled to mode 2 is referred to as a complete coupling length (Lc). Lc is determined in accordance with a mode conversion amount per perturbation. When the mode conversion amount per perturbation is large, the complete coupling length becomes short. On the other hand, when the mode conversion amount per perturbation is large, a loss due to mode mismatch increases. The grating length is defined as Lg. When Lg=Lc, as described above, mode 1 is completely converted into mode 2, as illustrated in
At this time, coupling efficiency C at a wavelength λ0 is expressed in Expression (2).
When Lg=Lc, mode 1 is completely coupled to mode 2, and C=1.
As illustrated in
Furthermore, by calculating a product of the FWHM and Lc, the relationship between the FWHM and dΔβ/dλ is uniquely determined for the coupling efficiency regardless of Lc.
As expressed in Expressions (3) to (6), it can be understood that the relationship between the product of the FWHM and Lc and dΔβ/dλ can be approximated to an inversely proportional expression expressed in Expression (7).
Here, a relationship between the coupling efficiency and a coefficient b of inverse proportion is illustrated in
[Math. 8]
b=−1169.3C+1705.6 (8)
Here, C is the combining efficiency of linear display.
Accordingly, by providing a grating structure satisfying Expressions (1), (2), (7), and (8) in the optical fiber, a mode conversion device that has any coupling efficiency and bandwidth can be configured. That is, a mode conversion device according to the present invention includes a long period grating at a core of an optical fiber through which light is able to propagate in at least two propagation modes. The long period grating satisfies a relationship of Expression C1.
where
a full width at half maximum FWHM is a wavelength band in which the coupling efficiency is a half of coupling efficiency of mode conversion at a center wavelength, C is coupling efficiency, Lc is a complete coupling length, Lg is a grating length, Λ is a grating pitch, and Δβ is a propagation constant difference between the two propagation modes at the center wavelength of a mode conversion target.
Here, the calculation is performed by exemplifying conversion efficiency of the LP01 mode and the LP11 mode. However, for example, coupling between other modes, such as the LP01 mode and a cladding mode, the LP01 mode and an LP02 mode, or the LP11 mode and an LP21 mode, can be similarly considered to be applied. Regardless of a step index structure of the optical fiber, another structure such as a graded index structure can be similarly considered.
The design method includes:
where, the full width at half maximum FWHM is a wavelength band in which the coupling efficiency is a half of coupling efficiency of mode conversion at the center wavelength λ0.
Design parameters are dΔβ/dλ, Lc, Lg, λ, λ0, C, and FWHM indicating an optical fiber structure. First, in step S01, the core radius a (μm) of the optical fiber, the relative refractive index difference Δ (%), the center wavelength λ0 (nm) of light subjected to mode conversion, the coupling efficiency C, and the full width at half maximum FWHM (nm) are granted as specification values.
In step S02, from the core radius a (μm) and the relative refractive index difference Δ (%) of the optical fiber structure, the propagation constant difference Δβ at the center wavelength λ0 (nm) and the wavelength differential dΔβ/dλ are obtained through mode analysis.
In step S03, the propagation constant difference Δβ is substituted into Expression (C2) to calculate the grating pitch Λ (μm).
In step S04, the coupling efficiency C which is a specification value is substituted into Expression (C3) to calculate the coefficient b.
In step S05, the coefficient b, the full width at half maximum FWHM of the specification value, and the wavelength differential dΔβ/dλ are substituted into Expression (C4) to calculate the complete coupling length Lc.
In step S06, the grating length Lg is calculated by substituting the complete coupling length Lc and the coupling efficiency C as a specification value into Expression C5.
Step S03 and steps (S04 to S06) may be performed simultaneously, or the step S03 or the steps (S04 to S06) may be first performed.
A mode-convertible wavelength range (FWHM) differs depending on a device that uses LPG. For example, when a wide band is used in mode multiplex transmission or the like, the band is preferably wide. On the other hand, in the case of a tap device to be described in a third embodiment, the band is preferably narrow. The design method according to the present embodiment is a point that the grating pitch A and the grating length Lg can be derived according to a purpose (specification) of the device.
In the present embodiment, one mode is assumed to be a fundamental mode, and another mode is a higher-order mode in description.
The mode conversion device 301 includes:
The optical fiber 50 is assumed to be a step index fiber. In the optical fiber 50, the grating unit 20 and the tap unit 10 are sequentially formed in the longitudinal direction. A direction in which light can be incident on the tap waveguide 53 is defined as an optical waveguide direction. In
The grating unit 20 causes the long period grating 21 to convert light with a wavelength to be extracted in light propagating through the core 51 of the optical fiber 50 by a desired amount from the LP01 mode to the LP11 mode. The grating structure can be implemented, for example, by femtosecond laser machining, CO2 laser machining, or grating pressing.
The tap unit 10 includes the tap waveguide 53 extending from the center of the core 51 to the side surface of the optical fiber 50 (an interface of the cladding 52) at an angle α. The tap unit 10 selectively extracts only the LP11 mode from the core 51 by controlling the angle α between the tap waveguide 53 and the core 51, a diameter dt of the tap waveguide 53, and a refractive index of the tap waveguide 53.
Here, light coupled from the core 51 to the tap waveguide 53 is defined as tap light, and light directly propagating through the core 51 is defined as transmitted light. For example, by connecting a light receiver 54 to an output end (a side surface of the optical fiber 50) of the tap unit 10, it is possible to extract and receive only the tap light from the optical fiber 50.
In the tap unit 10, the coupling efficiency from the core 51 to the tap waveguide 53 strongly depends on the propagation mode of light propagating through the core 51. This is because as the mode is higher, confinement is smaller and coupling to the tap waveguide 53 is easier. Therefore, only the higher-order mode can be transitioned to the tap waveguide.
Here, in order to couple only the higher-order mode to the tap waveguide 53, the refractive index of the tap waveguide 53 and the value of the diameter dt are important. If these values are too large, an NA of the tap waveguide 53 increases, and the LP01 mode is also easily coupled. Therefore, a loss of the transmitted light increases. Conversely, if these values are too small, the NA of the tap waveguide 53 becomes small, and the high-order mode is hardly coupled. Therefore, coupling efficiency of the tap light to the tap waveguide 53 decreases. That is, it is necessary to appropriately determine the refractive index of the tap waveguide 53 and the value of the diameter dt.
In order to couple the light in the higher-order mode to the tap waveguide 53 with high efficiency and propagate the light in the fundamental mode while being confined in the core 51, it is necessary to make a sufficiently small and transition the mode adiabatically. When a is large, the LP01 mode is also coupled to the radiation mode under an influence of the tap waveguide 51, and a loss occurs. Therefore, an upper limit value of a is determined from the viewpoint of the loss of the LP01 mode. Conversely, a can take any value larger than 0. However, an entire length Ltap of the tap unit 10 is settled in accordance with α. Therefore, a lower limit value of a is determined from the viewpoint of a required condition for a propagation loss of the tap waveguide 53 and the entire length of the device.
In a general single-mode fiber, a diameter df of the optical fiber 50 is 125 μm. For example, to set the tap unit Ltap to be equal to or less than 5 cm, α is required to be set to equal to or more than 0.07°.
The grating unit 20 includes the grating 21 with the pitch Λ. For example, the grating 21 is a long period fiber grating (LPG). In order for the grating unit 20 to convert light with any wavelength λ and a desired light amount from the LP01 mode to the LP 11 mode, the grating 21 is molded with the design parameters described in the first and second embodiments.
The long period grating 21 of the mode conversion device 302, in
The mode conversion device 302 in
In the mode conversion device 302 in
The mode conversion device 302 in
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/JP2021/005851 | 2/17/2021 | WO |