A kind of broadband mode converter based on the asymmetric double core photonic crystal fiber
Technical field
The present invention relates to technical fields such as optical fiber communication and sensing, refer in particular to a kind of optical fibre mode converter.
Background technology
Optical fibre mode converter be a kind of realize in the optical fiber not isotype between the device of conversion, in fields such as optical communication net and sensings important application is arranged.At present, multiple novel optical fiber structure is: the one, and the Bragg type optical fiber of employing photon band gap principle, the pattern that its loss is minimum is not the LP in the ordinary optic fibre
01Mould, but TE
01Mould.The 2nd, adopt high-order mode transmission realization dispersion compensation technology and convert basic mode to high-order mode, thereby realize big mould field, low-loss light transmission.Wherein, the modal mode converter that is based on fiber grating, but when adopting fiber grating, basic mode and the high-order mode that is converted to all are to transmit in same optical fiber, therefore, if fiber grating can not be realized basic mode absolutely is converted to high-order mode, then in optical fiber, will there be remaining basic mode energy in optical fiber, to transmit, these dump energies will be disturbed the light signal generating of transmission, thereby influence the performance of device.In addition, though higher conversion efficiency greater than 99% is arranged between this fiber grating pattern, its bandwidth of operation is very narrow, is about several nanometers.And draw the full optical fibre mode converter form can be realized High Extinction Ratio in the very wide scope of bandwidth mode switch based on the photonic crystal fiber fusion technology, but that this device requires aspect manufacture craft is very high, and be difficult to the realization batch process.
Application number is 200810021652.8, and name is called " a kind of optical fibre mode converter " and discloses a kind of mode converter based on asymmetric double-core photonic crystal fiber, and this mode converter is by making the basic mode (LP of a fibre core
01) and the high-order mode (LP of another fibre core
02) the method that equates of effective refractive index realize efficient coupling between basic mode and the high-order mode, promptly utilize the conversion of coupling implementation pattern, the Polarization Dependent Loss of this mode converter is little and can realize batch process, but its defective is: the effective refractive index curve of two patterns can only intersect on a Frequency point in this structure, at other Frequency point place, because the effective refractive index of two patterns is unequal, be that both phase places do not match, according to coupled wave theory as can be known, can only realize the coupling of part, this has just directly caused the decline of conversion efficiency, owing to the effective refractive index between two patterns can only mate in a very narrow band frequency scope, therefore its bandwidth of operation is narrower, has only 14nm.
Summary of the invention
The objective of the invention is at the deficiencies in the prior art, propose a kind of novel broadband mode converter based on the asymmetric double core photonic crystal fiber, can realization work in very wide bandwidth range.
The technical solution used in the present invention is: be made up of host material and distribution hole thereon, every three adjacent holes constitute an equilateral triangle grid, and the center in each hole lays respectively on the node of equilateral triangle grid, the airport that lacks in equilateral triangle grid forms fibre core, is distributed in the dielectric rod that seven refractive indexes on the equilateral triangle grid node are lower than host material and forms fibre core; Airport around the fibre core innermost layer is three macropores and three apertures, and the diameter of macropore is greater than the diameter of aperture, and these three macropores and three apertures are spaced and are distributed in successively on each node of triangular mesh, and the inner air hole of next-door neighbour's fibre core is an aperture.
The ratio of the spacing between the diameter of dielectric rod and adjacent two nodes is less than 0.5.
The diameter ratio of macropore and aperture is 3: 1~2:!
The refractive index of the refractive index ratio host material of dielectric rod low 0~0.01.
The invention has the beneficial effects as follows: realized the conversion between the basic mode and high-order mode in the asymmetric fibre core, adjust the effective refractive index of basic mode and high-order mode effectively by the refringence that changes doped dielectric rod in two fibre core covering airport diameters and the fibre core, in two fibre cores, do not have birefringence.The present invention can realize the mode switch in broadband, and its operating wavelength range is 1.45~1.61 μ m, and promptly bandwidth of operation can reach 160nm.
Description of drawings
The present invention is further described below in conjunction with the drawings and specific embodiments.
Fig. 1 is the cross section of optic fibre synoptic diagram of one embodiment of the invention.
Fig. 2 is the LP of the little fibre core in the structure shown in Figure 1
01The LP of mould and big fibre core
02The effective refractive index of mould and wavelength concern synoptic diagram.
Fig. 3 is light signal transmission range in optical fiber shown in Figure 1 be respectively 0 (Fig. 3 a), L/3 (Fig. 3 b), 2/3L (Fig. 3 c), field distribution synoptic diagram during L (Fig. 3 d).
Embodiment
As Fig. 1, the present invention is made of pure quartz substrate material 1 and the hole 2, macropore 5, aperture 6 and the dielectric rod 7 that are distributed on the host material 1.Hole 2 is periodic regular in the optical fiber background material arranges, and every three adjacent holes 2 constitute an equilateral triangle grid, and the center in each hole 2 lays respectively on the node of equilateral triangle grid, and the spacing between two adjacent nodes is defined as one-period Λ.An airport that lacks in equilateral triangle grid forms fibre core 3, be distributed in the dielectric rod 7 that seven refractive indexes on the equilateral triangle grid node are lower than host material 1 and form fibre cores 4, it is doped dielectric rod 7 in the fibre core 4, the ratio of the diameter of this dielectric rod 7 and periods lambda is less than 0.5, be that the diameter of dielectric rod 7 requires 0.5 times less than periods lambda, the refractive index of the refractive index ratio host material 1 of this dielectric rod 7 is low by 0~0.01, and dielectric rod 7 is oversize or cross big city with host material 1 refringence and cause two fibre cores, 3,4 patterns to be difficult to realize the broadband coupling.Fibre core 4 is made the less relatively dielectric rod 7 of this seven diameters rather than the method for a big dielectric rod, can reach the effective refractive index of adjusting the high-order mode in the fibre core 4 and the purpose that can not cause the mode field generation seriously to distort simultaneously.Airport around fibre core 3 innermost layers is three macropores 5 and three apertures 6, these three macropores 5 and three apertures 6 are spaced and are distributed in successively on each node of triangular mesh, wherein, airport between fibre core 3 and the fibre core 4 is an aperture 6, the inner air hole that promptly is close to fibre core 4 is an aperture 6, fibre core 4 covering holes are taken as aperture 6, make fibre core 4 not have birefringence.The diameter of the macropore 5 of fibre core 3 coverings is always greater than the diameter of aperture 6, and preferred macropore 5 is 3: 1~2 with the diameter ratio of aperture 6:! This arranging of macropore 5 and aperture 6 makes fibre core 3 have three rank symmetry, two linear polarization basic mode is a degeneracy, there is not birefringent phenomenon, help the coupling of two kinds of patterns in asymmetrical fibre core 3 and the fibre core 4, thus when having avoided Mode Coupling because occur strong polarization correlated and influence the performance of device.
One embodiment of the present of invention below are provided:
Embodiment
As shown in Figure 1, the host material 1 of twin-core fiber is pure quartz, on the node that is centered close to equilateral triangle grid of optical fiber mesopore 2, periods lambda is 4.5 μ m, the diameter of macropore 5 is 3.38 μ m, the diameter of aperture 6 and dielectric rod 7 is 1.35 μ m, the refractive index of the refractive index ratio host material 1 of dielectric rod 7 low 0.003.LP in the little fibre core 3
01LP in mould and the big fibre core 4
02The effective refractive index of mould and the relation of wavelength are as shown in Figure 2.As can be seen from Figure 2, both can in very wide frequency range, realize the coupling, shown in frequency range in, the effective refractive index difference of two-mode is all less than 1.5 * 10
-5The length of getting optical fiber is L=5.9mm, and all under the prerequisite less than 0.5dB, its operating wavelength range is 1.45~1.61 μ m in the loss of two polarization states, and promptly its bandwidth of operation can reach 160nm and Polarization Dependent Loss less than 0.28dB.In centre wavelength (λ=1.55 μ m) position, its Polarization Dependent Loss less than 0.04dB and loss less than 0.27dB.Fig. 3 has provided the transmission course of light signal in optical fiber, passes through Fig. 3 (a) and (b), (c), (d) back optical fiber successively with the basic mode light (LP that imports
01Mould) converted high-order mode (LP to
02) mould.
That present embodiment is realized is the basic mode (LP of fibre core 3
01) and the high-order mode (LP of fibre core 4
02) between conversion, in like manner also can realize the conversion between other high-order mode in the basic mode and fibre core 4 in the fibre core 3; According to optics reciprocity principle, this mode-conversion device can realize that equally the high-order mode that will import in the fibre core 4 is converted to the basic mode in the fibre core 3.