2x2 Fiber Optic Coupler
A comprehensive technical overview of ont fiber optic couplers, their manufacturing processes, coupling mechanisms, performance parameters, and applications in modern optical communication systems.
Manufacturing Processes of All-Fiber Couplers
The ont fiber optic coupler manufacturing involves several precision techniques, each with distinct advantages and limitations. The primary methods include polishing, etching, and fused biconical tapering, each producing couplers with different characteristics suitable for specific applications.
Polishing Method
In the polishing method, bare optical fibers are fixed on a quartz substrate with a specific curvature, and the fiber sides are polished to remove part of the cladding. Two such polished bare fibers are then joined together, creating a directional coupler using the evanescent field that penetrates the core-cladding interface, as shown in Figure 3-11(a).
Disadvantage: Devices produced using this method exhibit poor thermal stability and mechanical stability, making them less suitable for harsh environment applications where ont fiber optic components must maintain performance under varying conditions.
Etching Method
The etching method uses chemical processes to remove the cladding from a section of bare fiber. Two etched fibers are then twisted together to form the fiber coupler. This method offers simplicity in production but comes with significant drawbacks.
Disadvantages: Poor process consistency, inadequate thermal stability, and high insertion loss limit the applications of etch-based ont fiber optic couplers in high-performance communication systems.
Fused Biconical Taper Method
The fused biconical taper method involves bringing two (or more) stripped optical fibers together, heating them to their melting point, and simultaneously stretching them from both sides. This process forms a special waveguide structure in the heating zone with a biconical shape, as illustrated in Figure 3-11(b).
By controlling the fiber twist angle and stretching length, different splitting ratios can be achieved. Finally, the tapered region is fixed on a quartz substrate with curing adhesive and inserted into a stainless steel tube for protection.
Advantage: Compared with the other two methods, ont fiber optic couplers manufactured using fused biconical taper technology offer superior practicality, better performance consistency, and higher reliability in operational environments.
Figure 3-11: Coupling regions and formation of ont fiber optic couplers (a) Polished splice type (b) Fused taper type
Coupling Mechanism
Numerous scholars have analyzed the mechanism of ont fiber optic couplers from different perspectives, proposing various approximation models. The following introduces a coupling mechanism for single-mode fiber couplers using a 2x2 coupler as an example. When the cores of two fibers with identical structural characteristics are brought close together, the coupling of optical signals in the two cores can be analyzed using coupled mode equations:
dP₁/dz = -jβ₁P₁ + C₁₂P₂ (3-1)
dP₂/dz = -jβ₂P₂ + C₂₁P₁ (3-2)
Where:
- P₁ – Transmission power in the through arm
- P₂ – Power entering the coupling arm
- β₁, β₂ – Propagation constants of the two fibers
- C₁₂ and C₂₁ – Coupling coefficients from the through arm to the coupling arm and vice versa, generally C₁₂ = C₂₁ = C
C represents the effectiveness with which the evanescent field of the guided mode in one fiber excites an optical guided mode in the other fiber through the coupling region. This parameter is critical in determining the performance characteristics of ont fiber optic couplers across different operating wavelengths and environmental conditions.
For fused step-index weakly guiding tapered fiber couplers, the coupling coefficient C can be approximately expressed as:
C = (K₀(wd/a)) / (2n₁²aV K₁(w)) (3-3)
Where:
- λ – Optical wavelength
- n₁ – Core refractive index
- d – Fiber core spacing
- a – Core radius
- V – Normalized frequency
- u and w – HE₁₁ mode transverse propagation constants
- K₀ and K₁ – Zero-order and first-order Bessel functions
By integrating equations (3-1) and (3-2), the optical power distribution in the two fibers can be obtained:
P₁ = P₀ cos²(Cz) (3-4)
P₂ = P₀ sin²(Cz) (3-5)
Where P₀ is the input optical power at z=0 into the input fiber. This power distribution forms the basis for designing ont fiber optic couplers with specific splitting ratios for various communication system requirements.
Figure 3-12: Periodic coupling process of optical power in through arm and coupling arm of an ont fiber optic coupler
Figure 3-12 illustrates the coupling and exchange规律 of optical power in the two cores of the coupling region as a function of the coupling region length. Fiber couplers can achieve the required coupling ratio by adjusting the length of the coupling region. However, if the stretching length is too long, the core becomes excessively thin, causing energy radiation and significantly increasing insertion loss – a critical consideration in optimizing ont fiber optic coupler design.
Multimode vs. Single-mode Fiber Couplers
Multimode fiber couplers differ significantly from single-mode fiber couplers in their operation. In multimode ont fiber optic couplers, when guided modes in the core reach the tapered coupling region, higher-order modes with incident angles exceeding the core-cladding boundary angle spill over into the cladding, becoming cladding modes that propagate within the cladding. Lower-order modes, however, remain in their original core.
When the tapered region thickens again, higher-order modes are once again confined to the core as guided modes. Since the fused tapered coupling region shares the same cladding, the higher-order mode power entering the core is common to both fibers and splits equally between the two fiber outputs. The total power splitting ratio depends on the length of the tapered coupling region and the cladding thickness, making these parameters critical in multimode ont fiber optic coupler design and manufacturing.
Performance Indicators of Fiber Optic Couplers
The main performance parameters characterizing ont fiber optic couplers include splitting ratio or coupling ratio, channel insertion loss, excess loss, and crosstalk. These parameters determine the suitability of a coupler for specific applications and directly impact system performance.
1) Splitting Ratio or Coupling Ratio
The splitting ratio S represents the ratio of the optical power Pj at a particular output port (j) to the total output power of all output ports:
Sj = (Pj / ΣPi) × 100% (3-6)
The required splitting ratio can be achieved by adjusting the length of the coupling region in ont fiber optic couplers, allowing for customization to meet specific system requirements.
2) Channel Insertion Loss
Channel insertion loss Li-j represents the loss from input channel (i) to the specified output channel (j), defined as:
Li-j = 10lg(Pi / Pj) (dB) (3-7)
This parameter is crucial in determining signal strength through the ont fiber optic coupler and directly affects overall system link budget calculations.
3) Excess Loss
Excess loss Le represents the total loss introduced by the coupler, defined as the ratio of the sum of output signal powers to the input power:
Le = 10lg(Pi / ΣPj) (dB) (3-8)
High-performance directional couplers, particularly premium ont fiber optic components, should have excess loss less than 1dB to minimize signal degradation in critical applications.
4) Crosstalk
Crosstalk Lc represents the logarithmic ratio of the input signal at one port to the optical power scattered or reflected back to another input port. The logarithm of the reciprocal ratio is called isolation.
For the 2x2 fiber coupler shown in Figure 3-11(b), crosstalk Lc can be expressed as:
Lc = 10lg(P3 / P1) (dB) (3-9)
An ideal coupler would have zero crosstalk (negative infinity in decibels) and infinite isolation. Practical ont fiber optic couplers cannot achieve zero crosstalk, but high-quality directional couplers should have isolation greater than 40dB.
Performance testing of ont fiber optic couplers in laboratory conditions
Fiber Optic Star Couplers
Manufacturing multimode fiber star couplers using fused biconical taper technology is relatively straightforward. The coupling characteristics of fused tapered tree and star multimode fiber couplers are sensitive to modes, resulting in significant power variations at the output.
For single-mode fibers, however, manufacturing multi-core fused tapered star couplers requires precise adjustment of the coupling between the evanescent fields of multiple fibers, making implementation more challenging. Consequently, N×N star couplers are typically constructed by cascading multiple 2×2 single-mode ont fiber optic couplers.
As shown in Figure 3-13, a 4×4 coupler can be constructed by cascading 4 2×2 fiber couplers, while an 8×8 coupler requires 12 2×2 couplers. Using similar methods, 1×2 or 2×2 couplers can be connected in stages to form 2×N or N×N star couplers, providing flexible solutions for various ont fiber optic network topologies.
Figure 3-13: Construction of N×N star couplers using cascaded 2×2 ont fiber optic couplers
Star couplers play a crucial role in passive optical networks (PONs), enabling point-to-multipoint communication by distributing signals from a central office to multiple subscribers. The use of cascaded 2×2 ont fiber optic couplers allows for scalable, cost-effective network architectures that can be expanded as demand grows.
The performance of star couplers depends heavily on the quality of the individual 2×2 couplers used in their construction. Low excess loss, consistent splitting ratios, and high isolation are essential characteristics for ont fiber optic components used in star coupler configurations to ensure reliable signal distribution across all network branches.
Modern manufacturing techniques have significantly improved the performance and reliability of both individual couplers and star coupler assemblies. Advanced process control in the fused biconical taper method has enabled the production of ont fiber optic couplers with tightly controlled parameters, making them suitable for high-speed, high-bandwidth communication systems.