The Filter Design wizard builds a multi-cavity bandpass prototype filter — DWDM, LWDM, or a narrow notch — from a handful of physical parameters. It walks you through choosing materials and a target shape, suggests a cavity count and a matching prototype, and hands back a ready-to-refine design with the merit operands already filled in.

The structure it generates is a symmetric stack of quarter-wave mirror blocks separated by Fabry-Perot spacer cavities:

Substrate | M₁ S₁ M₂ S₂ … Mq Sq M(q+1) | (optional AR)

Each Mᵢ is a quarter-wave high-reflector block and each Sᵢ is a half-wave spacer cavity. The outer and inner mirror blocks use slightly different layer counts so that neither spacer boundary collapses into an inert full-wave layer.

Steps

The wizard runs in six steps:

1. Materials — pick the high-index (H) and low-index (L) coating materials, and optionally the substrate, incident medium, and an oblique angle of incidence. Both materials should be effectively lossless at the design wavelength (the wizard warns if absorption is significant), because loss in a high-Q cavity badly reduces peak transmittance.

2. Parameters — set the centre wavelength λ₀, the passband half-width, and the stopband half-width. The shape factor (the ratio of the two) is shown live as you type, along with an ideal-target preview.

3. Cavities — the wizard recommends a cavity count from your shape factor; you can override it.

4. Prototype — choose from a table of candidate prototypes, each labelled by its mirror and spacer orders with an estimated bandwidth, and pick the row closest to your target.

5. Refine the candidate — a short integer search settles the chosen prototype’s layer arrangement.

6. Anti-reflection — the design moves from the simplified embedded case to the real incident medium, adding an anti-reflection coating so the finished filter performs in air.

How to read it

The preview plots transmittance on a finely sampled wavelength grid around λ₀ so that the narrow passband renders clearly. Before refinement, a symmetric N-cavity prototype shows N small ripples straddling λ₀ — this is the expected Chebyshev-style response, not a fault; Refinement merges them into a flat top.

When you confirm, a new design is committed with the calculated stack and the merit operands pre-filled in the Merit Function Editor: an averaged transmittance target in the passband and averaged reflectance targets in the stopbands. Run Refinement next to finish the design.

References

• H. A. Macleod, Thin-Film Optical Filters, 5th ed., Ch. 7 (Eq. 7.27) and §8.2.
• Tikhonravov & Trubetskov, Appl. Opt. 41, 3036 (2002), §3.