I. Z-Block

1. Definition and Structure of the Z-Block

The Z-Block is a core optical component used in wavelength division multiplexing/demultiplexing (WDM) systems. Structurally, it is typically composed of several integrated optical elements, including collimating lenses, rhomboid prisms, and specially designed optical mirrors. These components are precisely aligned and combined through an intricate optical path design to form a compact and efficient optical module.

For example, in a common four-channel Z-Block design, each channel’s collimator converts the divergent light from the fiber into parallel light. Through the coordinated use of lenses and mirrors, the module achieves wavelength-based splitting or combining of optical signals.

2. Working Principle of the Z-Block

The operating principle of the Z-Block is based on optical refraction, reflection, and interference. In a WDM process, light signals of different wavelengths enter the Z-Block from the input fiber and are first collimated into parallel beams. These beams then pass through specially designed optical elements—such as thin-film filters utilizing interference principles—which separate or combine light according to its wavelength.

In demultiplexing, the composite light signal is similarly collimated and then directed, through the optical structure, to corresponding output ports according to its wavelength order. This precision optical design enables the Z-Block to accurately perform multiplexing and demultiplexing functions in optical communication systems, ensuring high-efficiency transmission and processing of multi-wavelength signals.

3. Key Considerations in Z-Block Coating

(1) Strict Control of the Coating Environment
Z-Block coating requires an exceptionally clean environment. Even microscopic dust particles can cause defects in the coating layer, leading to increased light scattering, reduced optical performance, and higher signal loss. In production, coating rooms are equipped with advanced cleanroom systems to maintain ultra-low particulate levels and ensure optimal coating quality.

(2) Precise Selection of Coating Materials
The choice of coating materials depends on the application scenario and optical performance requirements. Different materials have unique optical properties such as refractive index and absorption rate. For example, coatings for near-infrared applications often use materials like titanium dioxide (TiO₂) and silicon dioxide (SiO₂), which offer low absorption and high transmission in this wavelength range. Proper combination and deposition of these materials ensure efficient Z-Block performance.

(3) Accurate Process Parameter Control
Coating parameters—such as temperature, vacuum level, and deposition rate—directly impact film quality. Excessive temperature can cause thermal stress and cracking, while low temperature may reduce crystallinity and uniformity. Therefore, advanced temperature control, stable vacuum systems, and precise deposition control are essential to ensure coating quality and performance.

4. Advantages of the Z-Block

High Isolation: The unique optical design provides excellent wavelength isolation, minimizing crosstalk and improving signal quality.

Temperature Stability: Z-Blocks are less sensitive to temperature fluctuations and maintain stable performance in diverse environments.

Flexible Manufacturing: Mature fabrication processes and material options allow customization for various applications.

5. Disadvantages of the Z-Block

High Cost: Complex materials and manufacturing processes increase production costs, limiting use in cost-sensitive applications.

Large Size: The relatively large structure requires more installation space, making it less suitable for compact systems.

Optical Path Deviation: The Z-type optical design relies on two parallel glass surfaces. However, real-world tolerances, coating-induced warpage, and assembly errors can cause multi-reflection distortion and alignment issues, making optical path consistency difficult to maintain.

II. TFF Prism

1. Definition and Structure of the TFF Prism

A TFF (Thin Film Filter) prism is an optical element based on thin-film interference technology. It primarily consists of a prism substrate coated with multiple layers of thin-film filters. The substrate—typically made from high-quality optical materials such as quartz or K9 glass—provides mechanical support.

The thin-film filter is formed by sequentially depositing multiple layers of materials with varying optical properties, each with precisely controlled thickness and refractive index. This multilayer design enables wavelength-specific filtering, reflection, or transmission.

By combining thin-film filter technology with prism optics, the TFF prism precisely manipulates light within the prism through interference and refraction, achieving wavelength selection functions such as filtering, reflection, and transmission. In optical modules, TFF prisms often use the Z-Block configuration to realize WDM and demultiplexing functions through accurate control of light paths and optical behavior.

2. Working Principle of the TFF Prism

The TFF prism operates based on thin-film interference. When light enters the prism, it undergoes multiple reflections and refractions at the interfaces of the thin-film layers. Due to the wavelength-dependent propagation characteristics, carefully designed film thickness and refractive indices can cause constructive interference (high transmission) for specific wavelengths, and destructive interference (high reflection or low transmission) for others.

For instance, in a WDM application, a TFF prism can separate or combine optical signals of different wavelengths through its multilayer thin-film structure, achieving precise wavelength selection and signal processing.

3. Key Considerations in TFF Prism Coating

(1) Fine Treatment of the Substrate Surface
Before coating, the prism surface must be thoroughly cleaned to remove oils, dust, and other contaminants, which can weaken coating adhesion or cause defects. Cleaning typically involves chemical and ultrasonic methods followed by surface activation to enhance film bonding.

(2) Precision in Film Design and Thickness Control
The optical performance of TFF prisms relies heavily on film structure and thickness. Even slight deviations can shift filter characteristics. Therefore, advanced monitoring techniques—such as optical interference monitoring—are used to control thickness and refractive index in real time during deposition.

(3) Post-Coating Treatment and Quality Inspection
After deposition, annealing is often performed to release internal stress and improve stability. Comprehensive testing follows, including evaluations of coating uniformity, adhesion, and optical performance using microscopy, tape tests, and spectral analysis to verify that the prism meets design specifications.