ADF structure and basic optical principles

ADF Structure and Principles

ADF Structure Overview

  • Components: 13 main components including:

    • RP-Cap, Steel Tube, PP-Cap: Seals products for reliability; ensures long-term usage of internal components.

    • Pigtail: Fiber holder, functions similarly to a 1X2 Fiber Array.

    • C-lens/G-lens: Used for focusing light.

    • Filter: Filters specific wavelengths; allows designated wavelengths to pass through, while others reflect.

Optical Principles

Add Drop Filter

  • Wavelength Representation:

    • λ1, λ2: Used to indicate different wavelength paths.

    • Red Line: Represents the Drop (分波).

    • Blue Line: Represents the Add (合波).

    • Ports include:

      • Common Port

      • Reflect Port

      • Pass Port

Optical Terms

  • 子午面 (Meridian Plane): A plane through the central axis of a lens.

  • 子午光線 (Meridian Rays): Rays located within the meridian plane.

  • 焦平面 (Focal Plane)

  • 焦距 (Focal Distance)

  • 節距 (Pitch)

  • 光軸 (Optical Axis)

  • 凸透鏡 (Convex Lens): A type of lens that converges light.

Lens Design Considerations

Why Filter Reflection Surface should be on Lens Focal Plane

  1. Better Insertion Loss: Placing filter surface at the lens's focus improves insertion loss performance.

Refractive Index

  • Law of Refraction: n1sin(θ1)=n2sin(θ2)

  • Values:

    • Vacuum refractive index: 1; n2 at 1550nm = 1.744 (example of C-lens)

    • Wavelength Impact: Different wavelengths have different refractive indices; shorter wavelengths (higher frequencies) have greater refractive indices.

      • Example: Focal length of 1260nm is smaller than that of 1610nm.

Optical Calculation for C-lens

Related Calculations:

  • Equations:

    • S1 = f = n1*R/(n2-n1)

    • h2 = n1*L/n2

    • S2 = f - h2 = n1R/(n2-n1)-n1L/n2

  • Design Considerations:

    • Radius of curvature (R)

    • Focal length (f)

    • Impact of R on Beam Size and Exit Angle

    • Relationship between Pigtail Size and Exit Angle

Filter Relationships

Incident Angle and Wavelength

  • Filter Formula:

    • ( \lambda_i= \lambda_0 \sqrt{1-(\frac{n_1}{n_2})^2 \sin^2(i_1)} )

    • Variables:

      • λi: central wavelength of emission at angle i

      • λ0: central wavelength of emission at 0 degrees

      • n2: Equivalent refractive index of Filter WDM Coating

      • (n1/n2)²: Filter constant

  • Application: As the filter's incident angle increases, its central wavelength decreases.

Power Units

Definitions and Calculations

  • Power Units:

    • Power is measured in milliwatts (mw) where 1W = 1000mw.

  • Relative and Absolute Values:

    • dBm: Defined as milliwatt, with 0 dBm = 1mw.

    • dB: Represents a relative value.

    • Calculation Example: If incident light energy is 100% and outgoing is 50%, then relative comparison yields +3dB.

  • Loss Calculation:

    • Loss calculated as Loss = -10 log A/B; where PPIL indicates actual loss in dB.

Pigtail Symmetrical Structure

  • Two Optical Paths:

    • Axial symmetry (green): best for concentricity.

    • Centrally symmetric (pink): alternative design.

Product Alignment Assessment

  • Judging Tilt and Misalignment:

    • Evaluations include angular misalignment, tilt, and other spatial misalignments.

ADF Specification Definitions

  • Definitions Related to ADF

    1. CW, CS, CP: Definitions for channels and spacing.

    2. Add/Drop Channel Max Insertion Loss

    3. In-Band Loss Variation (Ripple)

    4. Remaining Channel Max Insertion Loss

    5. Remaining Channel Isolation

    6. Adjacent Channel Isolation

    7. Nonadjacent Channel Isolation

    8. Insertion Loss Temperature Sensitivity

    9. Wavelength Temperature Sensitivity

    10. Polarization Dependent Loss (PDL)

    11. Directivity (Dir)

    12. Return Loss

Channel Spacing and Definitions

  • Channel Spacing: The distance between Center Wavelength and adjacent channel passband.

  • 0dB Definitions: Various metrics relative to pass port losses and isolations defined within temperature and wavelength ranges.