Small core-diameter graded-index optical fiber

Abstract

A small core-diameter graded-index optical fiber include a core layer and a cladding having an inner cladding layer, a depressed cladding layer, and an outer cladding layer from inside to outside thereof. The core layer has a parabolic refractive index profile with a distribution index in a range of 1.9-2.1, a radius in a range of 10-21 m, and a 1 max in a range of 0.7-1.7% at a core layer center, and is a silica glass layer co-doped with germanium, phosphorus, and fluoride. The inner cladding layer is a pure silica layer or an F-doped silica glass layer, and has a unilateral width in a range of 0.5-5 m and a 2 in a range of 0.4-0%. The depressed cladding layer has a unilateral width in a range of 2-10 m and a 3 in a range from 0.8% to 0.2%. The outer cladding layer is a pure silica glass layer.

Claims

1. A small core-diameter graded-index optical fiber, comprising: a core layer and a cladding which includes an inner cladding layer, a depressed cladding layer, and an outer cladding layer from inside to outside thereof, wherein, the core layer has a parabolic refractive index profile with a distribution index in a range from 1.9 to 2.1, a radius R1 in a range from 10 to 21 m, and a maximum relative refractive index difference 1max in a range from 0.7% to 1.7% at a core layer center, and is a silica glass layer co-doped with germanium Ge, phosphorus P, and fluoride F; the inner cladding layer is a pure silica layer or an F-doped silica glass layer, and has a unilateral width R2R1 in a range from 0.5 to 5 m and a relative refractive index difference 2 in a range from 0.4% to 0%; the depressed cladding layer has a unilateral width R3R2 in a range from 2 to 10 m and a relative refractive index difference 3 in a range from 0.8% to 0.2%; and the outer cladding layer is a pure silica glass layer, wherein P and Ge are used as positive dopants in the core layer, and a concentration of P in the core layer changes to form, from inside to outside, a flat region in which the concentration of P remains substantially unchanged and a graded region in which the concentration of P gradually decreases, and wherein the flat region has a width T1 in a range from 1 to 19.5 m, and the graded region has a width T2=R1T1, and T2>1.5 m.

2. The small core-diameter graded-index optical fiber according to claim 1, wherein the contribution P0 of P at the core layer center is in a range from 0.01% to 0.30%; the contribution of P at a boundary between the flat region and the graded region is P1; a contribution fluctuation P10 of P at a boundary between the core layer center and the flat region is $P10=2.Math.\frac{P1-P0}{P1+P0}.Math.,$ P10 being less than or equal to 5%; the contribution P2 of P at an outer edge of the core layer is in a range from 0% to 0.15%; and a contribution difference P21 of P between the flat region and an outer edge of the graded region is P21=P2P1, P21 being in a range from 0.3% to 0.01%.

3. The small core-diameter graded-index optical fiber according to claim 1, wherein F is used as a negative dopant in the core layer, and has a doping amount increasing from the core layer center to an edge of the core layer, a contribution F0 of F at the core layer center being in a range from 0.0% to 0.1%, and a contribution F1 of F at the edge of the core layer being in a range from 0.45% to 0.1%.

4. The small core-diameter graded-index optical fiber according to claim 1, wherein the inner cladding layer has a relative refractive index difference 2 less than or equal to a relative refractive index difference 1 min at the edge of the core layer, i.e., 21 min.

5. The small core-diameter graded-index optical fiber according to claim 1, wherein the fiber has a bandwidth of 3500 MHz-km or more than 3500 MHz-km at a wavelength of 850 nm, a bandwidth of 2000 MHz-km or more than 2000 MHz-km at a wavelength of 950 nm, and a bandwidth of 500 MHz-km or more than 500 MHz-km at a wavelength of 1300 nm.

6. The small core-diameter graded-index optical fiber according to claim 1, wherein the fiber has a bandwidth of 5000 MHz-km or more than 5000 MHz-km at a wavelength of 850 nm, a bandwidth of 3300 MHz-km or more than 3300 MHz-km at a wavelength of 950 nm, and a bandwidth of 600 MHz-km or more than 600 MHz-km at a wavelength of 1300 nm.

7. The small core-diameter graded-index optical fiber according to claim 1, wherein a fundamental mode LP01 of the fiber at 1310 nm or 1550 nm has a mode field diameter in a range from 8 to 12 m.

8. The small core-diameter graded-index optical fiber according to claim 1, wherein the fiber has a macrobending loss of less than 0.2 dB with a 7.5 mm-bending radius and two turns at a wavelength of 850 nm, and a macrobending loss of less than 0.5 dB with a 7.5 mm-bending radius and two turns at a wavelength of 1300 nm.

9. The small core-diameter graded-index optical fiber according to claim 1, wherein the core layer has a radius R1 in a range from 12 to 20 m.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 schematically shows a refractive index profile of one embodiment of the present invention;

(2) FIG. 2 schematically shows a refractive index profile of another embodiment of the present invention;

(3) FIG. 3 schematically shows a relationship between a mode field diameter of a fundamental mode LP.sub.01 at 1310 nm and a core diameter R1 and (1 max2) of the present invention;

(4) FIG. 4 schematically shows a relationship between a mode field diameter of a fundamental mode LP.sub.01 at 1550 nm and a core diameter R1 and (1 max2) of the present invention; and

(5) FIG. 5 schematically shows a doping amount profile of one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(6) Specific embodiments will be given below to further illustrate the present invention.

(7) A fiber comprises a core layer and a cladding. The core layer has a parabolic refractive index profile with in a range from 1.9 to 2.1, a radius R1, and a maximum relative refractive index difference 1 max in a range from 0.7% to 1.7% at a core layer center. The contribution of P at the core layer center is P0; the contribution of P at a boundary between a flat region and a graded region is P1; the contribution fluctuation of P at a boundary of the core layer center and the flat region is

(8) $P10=2.Math.\frac{P1-P0}{P1+P0}.Math..$
The flat region has a width T1, and the graded region has a width T2=R1T1. The contribution of P at an outer edge of the core layer is P2, and the contribution difference of P between the flat region and an outer edge of the graded region is P21=P2P1. The contribution of F at the core layer center is F0, and the contribution of F at an edge of the core layer is F1. The cladding has an inner cladding layer, a depressed cladding layer, and an outer cladding layer from inside to outside thereof. The inner cladding layer has a radius R2 and a relative refractive index difference 2; the depressed cladding layer has radius R3 and a relative refractive index difference 3; and the outer cladding layer is a pure silica glass layer with a radius of 62.5 m.

(9) According to the present invention, a set of preforms are prepared, drawn, and coated with double layers. The structure and main performance parameters of the fiber are shown in Table 1.

(10) Macrobending loss is tested as follows. The fiber under test is wound one turn to form a circle with a certain diameter (for example, 10 mm, 15 mm, 20 mm, 30 mm, etc.), and the circle is then released. A change in optical power after the winding relative to that before the winding is tested as the macrobending loss of the fiber.

(11) The overfilled bandwidth is measured according to the FOTP-204 method, and the test adopts the overfilled condition.

(12) TABLE-US-00001 TABLE 1 Main structure parameters and performance parameters of the fiber Examples Main parameters of the fiber 1 2 3 4 R1 (m) 15.2 14.3 18.5 12.4 1max (%) 1.19 1.10 1.48 0.98 Core layer 2.070 2.052 2.028 2.042 F0 (%) 0.02 0.04 0.08 0.04 F1 (%) 0.23 0.32 0.38 0.25 T1 (m) 2.6 11.8 6.8 7.9 T2 (m) 12.6 2.5 11.7 4.5 P0 (%) 0.08 0.17 0.13 0.18 P10 (%) 1.7 2.8 4.3 4.4 P21 (%) 0.07 0.11 0.08 0.15 P2 (%) 0.01 0.06 0.05 0.03 R2 (m) 16.2 17.4 21.1 14.1 2 (%) 0.05 0.18 0.01 0.29 R3 (m) 22.0 22.3 28.4 17.7 3 (%) 0.35 0.48 0.56 0.60 overfilled bandwidth at 850 nm (MHz-km) 5181 6300 3872 8394 overfilled bandwidth at 950 nm (MHz-km) 2740 3427 2303 3377 overfilled bandwidth at 1300 nm (MHz-km) 799 650 550 792 macrobending loss with a 7.5 mm-bending 0.05 0.03 0.01 0.02 radius and two turns at 850 nm (dB) macrobending loss with a 7.5 mm-bending 0.10 0.08 0.02 0.05 radius and two turns at 1300 nm (dB) LP.sub.01 MFD at 1310 nm (m) 10.6 10.3 11.0 9.4 LP.sub.01 MFD at 1550 nm (m) 11.5 11.2 11.9 10.2

(13) The small core-diameter graded-index optical fiber has a smaller core diameter, less guided modes, and theoretically a higher bandwidth than a conventional multimode fiber. In order to satisfy the conditions of multimode transmission and reduce inter-mode dispersion in fibers, the core layer refractive index profile of the small core-diameter graded-index optical fiber adopts a design of an profile similar to that of the conventional multimode fiber. In order to perform single-mode transmission, the small core-diameter graded-index optical fiber is enabled to have a mode field diameter (MFD) of a fundamental mode LP.sub.01 in a single-mode transmission window matched with the MFD of the conventional single-mode fiber by means of an appropriate refractive index profile design. When applied to integrated systems such as narrow cabinets, wiring boxes, etc., the fiber will undergo a very small bending radius. High-order modes transmitted near an edge of the fiber core easily suffer from leakage, thus causing signal loss. The small core-diameter graded-index optical fiber limits leakage of higher-order modes by adding a low refractive index region in the fiber cladding, thus minimizing signal loss.

(14) When used for single-mode transmission, the small core-diameter graded-index optical fiber is in quasi-single-mode transmission, and its coupling with the single-mode fiber is related to the matching degree of the mode field diameters of the fundamental modes LP01 therebetween. The tolerance of the mode field diameters directly influences the splice loss of the fibers. Studies have shown that the splice loss of two single-mode fibers with mode field diameters of d.sub.1 and d.sub.2 respectively can be expressed as follows:

(15) ${}_s=20\log \frac{d_1^2+d_2^2}{2d_1{d}_2}.$
ideally, when d.sub.1=d.sub.2, i.e., the two fibers have the same the mode field diameter, the splice loss .sub.s=0.

(16) The center value of the mode field diameter specified under the ITU-T G.652.D standard is in a range from 8.6 m to 9.5 m0.6 m. Therefore, for G.652 fibers with MFDs of 8.6 m and 9.5 m, respectively, at 1310 nm, if the coupling loss is to be controlled within 0.1 dB, the MFDs of fundamental modes LP01 of the small core-diameter graded-index optical fibers at 1310 nm have to be in a range from 7.4 to 10 m and in a range from 8.2 to 11 m, respectively. For single-mode fibers with MFDs of 10 m and 11 m, respectively, at 1550 nm, if the coupling loss is to be controlled within 0.1 dB, the MFDs of the fundamental modes LP01 of the small core-diameter graded-index optical fibers at 1550 nm have to be in a range from 8.6 to 11.6 m and in a range from 9.5 to 12.8 m, respectively.

(17) In sum, the invention relates to a small core-diameter graded-index optical fiber including a core layer and a cladding which includes an inner cladding layer, a depressed cladding layer, and an outer cladding layer from inside to outside thereof. The core layer has a parabolic refractive index profile with a distribution index in a range from 1.9 to 2.1, a radius R1 in a range from 10 to 21 m, and a maximum relative refractive index difference 1 max in a range from 0.7% to 1.7% at a core layer center, and is a silica glass layer co-doped with germanium Ge, phosphorus P, and fluoride F. The inner cladding layer is a pure silica layer or an F-doped silica glass layer, and has a unilateral width in a range from 0.5 to 5 m and a 2 in a range from 0.4% to 0%. The depressed cladding layer has a unilateral width in a range from 2 to 10 m and a 3 in a range from 0.8% to 0.2%. The outer cladding layer is a pure silica glass layer. The fiber not only is compatible with existing OM3/OM4 multimode fibers, but also can support the wavelength-division multiplexing technology in a wavelength range from 850 nm to 950 nm. The fiber is also compatible with a single-mode fiber, and can support 1310 nm or 1550 nm single-mode transmission. The fiber has excellent bending resistance and is suitable for use in an access network or a miniaturized optical device.

(18) The foregoing description of the exemplary embodiments of the invention has been presented only for the purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in light of the above teaching.

(19) While there has been shown several and alternate embodiments of the present invention, it is to be understood that certain changes can be made as would be known to one skilled in the art without departing from the underlying scope of the invention as is discussed and set forth above and below including claims and drawings. Furthermore, the embodiments described above are only intended to illustrate the principles of the present invention and are not intended to limit the scope of the invention to the disclosed elements.