FBT coupler

Basic principle of fused tapered coupler

Figure 1. Fiber fusion tapering process

During the manufacturing process, port P0 continuously inputs light waves and then monitors the output power of each output port in real time. When the designed coupling ratio is reached, the fully automatic manufacturing process stops drawing, and this process is known as the melt tapering process.

v mode coupling

The most basic principle of FBT devices is the theory of mode coupling. If multiple modes can be transmitted in a waveguide, when there are perturbations, such as the change of waveguide diameter, energy exchange will occur between these modes, that is, the modes will be coupled. The coupling coefficient between modes is related to the difference of propagation constants between modes. The closer the propagation constant is, the larger the coupling coefficient will be.

v Taper limitation

The diameter of optical fiber melt-cone region decreases gradually. If the diameter changes too fast, there will be high order mode excitation and loss. The limitation of taper corresponds to the drawing speed during tapering.

v Single fiber coning zone

The cone-melting region of single optical fiber can be divided into A, B and C, and the dividing points are P and Q. When the fiber diameter decreases gradually, the normalized cutoff frequency decreases simultaneously, and the mode field in the fiber core expands gradually. When the normalized frequency of P and Q points is reduced to the critical value, the mode field is no longer limited to the transmission in the fiber core, but transferred to the waveguide composed of cladding/air, and the function of the fiber core can be ignored.

Ø cladding/air refractive index difference greater than the fiber core/cladding refractive index difference, while the B area diameter of the fiber core diameter is big, also so B area cladding/air duct of the normalized frequency is greater than A area, multiple mode waveguide.

Ø In area B starting point P is only the lowest order mode HE11 motivated, then with the energy coupling of the other models, is one of the most important and adjacent mode coupling between HE12, because both the propagation constant of the closest.

Ø At the point Q, only the HE11 mode can be re-captured by the output fiber into the core/cladding mode. The energy coupled to the HE12 mode but not returned to the HE11 mode will be lost as the high-order mode in the C region and the fiber after it.

FIG. 3 shows HE11 mode and HE12 mode in cone-melting zone of single fiber

HE11 mode and HE12 mode coupling in B area is circular, depends on the coupling coefficient and B zone length, so the single fiber transmittance of cone area and cone length related, in a long cycle.

Ø 
Ø coupling between two patterns take long associated with the wavelength, so the single fiber melt cone area is wavelength related to transmittance.
The coupling between the two modes varies with the external medium, such as soaking in the matching liquid to form a cladding/matching liquid waveguide instead of the cladding/air waveguide, and the transmittance will also change

v Double fiber fusion cone region

Figure 4. Dual fiber fusion cone area

Similar to the cone-melting region of single fiber, the double-fiber cone-melting region can also be divided into three parts. At point P, the fiber core/cladding mode is converted into cladding/air mode, and at point Q, it is re-captured into the fiber core/cladding mode.

Figure 5. Even mode and odd mode in the cone-melting region of double optical fiber

Different from the cone-melting region of single fiber, region B of double-fiber cone-melting region is elliptical, and mode coupling occurs in even mode and odd mode. The distribution of mode field in elliptic section is shown in FIG. 5.

v Approximate model of dual fiber fusion cone region 

Figure 6 shows the change of fiber diameter in the melting cone area

The dual fiber fusion cone region can be approximated by the model in the figure above, where W is the width of the heating source such as the fire head (equivalent to a certain width for the heating source sweeping back and forth), and L is the unilateral tensile length. It is assumed that the diameter of the cone region within the width of W is a constant, and it increases exponentially on both sides.

W=2mm and D0=50um were taken to obtain the relationship between coupling ratio and tensile length, as shown in FIG. 7.

FIG. 7 shows the variation of coupling ratio with tensile length

Coupling occurs at 1550nm before 1310nm, and the coupling ratio circulates between 0 and 100% with increasing tensile length, and the cycle frequency becomes faster and faster. The cross between 1550nm and 1310nm occurs, that is, the NTH + 1st cycle at 1550nm precedes the NTH cycle at 1310nm.

v Single window narrow band coupler

As shown in Figure 7, when making the Coupler, stop stretching and heating at point A to get a 3dB coupler at 1550nm and stop at point B to get a 3dB coupler at 1310nm.

Figure 8. Wavelength dependence of a 1310nm narrow-band 3dB coupler

FIG. 8 shows the wavelength correlation of a 3dB coupler of 1310nm (corresponding to the stretch length of point B of 3.78mm in FIG.7). It can be seen that the coupling ratio varies approximately linearly with the wavelength, and the bandwidth satisfying the coupling ratio of 50%±3% is only 1310nm±20nm, which is called a single-window narrow-band coupler

v Single window broadband coupler

If stretching and heating are stopped at point C in FIG. 7, a coupler with a coupling ratio of 100% at 1550nm is obtained. Of course, couplers with a coupling ratio of 100% have no practical use, but we observe that point C is at the turning point of the curve and its wavelength dependence is good.

Figure 9. Cease-fire point selection of a 1310nm narrowband 3dB coupler

If two asymmetric fibers are used for coupling, the maximum coupling ratio is reduced to around 50%, as shown in Figure 9, where the difference between the radii of the two fibers is Δr/r=0.0158. If cease-fire is selected at point P, a wideband 3dB coupler of 1310nm is obtained, and its wavelength correlation is shown in Figure 10. We see that the wavelength correlation is much improved compared with the narrowband coupler, and the coupling ratio meets 50%±3% of the bandwidth up to 1310nm±90nm.

Figure 10. Wavelength dependence of 1310nm wideband 3dB coupler

v Double-window wideband coupler

At point D in FIG. 7, 1310nm and 1550nm have the same coupling ratio of 90%. If two asymmetric fibers are used for coupling, point D is reduced to around 50%, as shown in FIG. 11, where the radius difference between the two fibers is Δr/r=0.015. Choose to cease fire at point D, then 1310nm/1550nm double-window wideband 3dB coupler is obtained, its wavelength correlation is shown in FIG. 12, and the bandwidth reaches 1310nm±40nm and 1550nm±40nm if the coupling ratio meets 50%±3%.

Figure 11. Ceasefire point selection of a wideband 3dB coupler with double Windows at 1310nm/1550nm

Figure 12. Wavelength dependence of 1310nm/1550nm dual-window wideband 3dB coupler

The coupling ratio of this coupler is 50%±6% within the 350nm bandwidth of 1250nm to 1600nm, so it is called wavelength independent coupler

v FBT wavelength division multiplexer

If the cease-fire is selected at point E in FIG. 7, all 1550nm light is coupled to another fiber, while 1310nm light is left in the original fiber, thus a 1310/1550nm wavelength division multiplexer is obtained, as shown in FIG. 13

Figure 13. Spectral lines of FBT type WDM

The isolation degree of FBT WDM is shown in FIG. 14. Considering that the coupling ratio cannot reach 100% in the actual process, it is set as 99.5%, which is generally achievable. The isolation decreases rapidly near the central wavelength, and two-stage WDM is often used in series to further improve the isolation. When two WDM stages are connected in series, their central wavelength is reversely offset by the same wavelength. For example, 1290nm/1530nm and 1330nm/1570nm are respectively taken, and relatively flat isolation degree can be obtained. The isolation degree is greater than 30dB within 50nm bandwidth.

Figure 14. Isolation degree of FBT type WDM

v Multimode optical fiber coupler

The coupling mechanism in the cone-melting region of multi-mode fiber is similar to that of single-mode fiber, but the difference is that there are more modes involved in the coupling. The relationship between the coupling ratio and the tensile length is shown in FIG. 15.

FIG. 15 shows the relationship between coupling ratio and tensile length of multimode fiber coupler

At first, the coupling ratio increases approximately linearly with the stretch length, but no longer increases when it reaches 50%, but stabilizes around 50% with a slight fluctuation. The reason is that many modes are involved in coupling, and the beat length of their coupling is different. The statistical result of a large number of modes coupling ratio is that the total coupling ratio is stable around 50%.
The multi-mode fiber coupler shows good bandwidth characteristics. It can achieve a coupling ratio of 50%±5% in the bandwidth of 800nm~1700nm and 900nm. It is also because the statistical properties of a large number of mode coupling ratios cancel out the wavelength dependence of a single mode coupling ratio.

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