Femtosecond laser writing has become one of the most powerful fabrication tools in modern photonics. It enables engineers to create precise, three‑dimensional optical circuits directly inside transparent materials, making it ideal for building custom photonic chips. This technology is central to Modular Photonics’ ability to design and manufacture bespoke optical components, including the monolithic structures used in advanced multicore fibre fanouts.

This article explains how femtosecond laser writing works, why it is uniquely suited to custom chip fabrication, and how it is transforming the way integrated photonic devices are designed.

What Is Femtosecond Laser Writing?

Femtosecond laser writing is a technique that uses ultrafast laser pulses—lasting only a few quadrillionths of a second—to permanently modify the refractive index of a transparent material. When the laser is focused inside glass, it creates a smooth, localised change in the material’s structure. This modified region acts as a waveguide, capable of guiding light just like an optical fibre.

Because the laser can be scanned in three dimensions, engineers can “write” complex optical circuits directly inside a solid substrate, forming a custom photonic chip without masks, etching, or lithography.

Why Femtosecond Laser Writing Is Ideal for Custom Photonic Chips

Custom photonic chips often require routing geometries, coupling structures, and optical functions that cannot be achieved with traditional planar fabrication. Femtosecond laser writing overcomes these limitations through several key advantages.

1. True 3D Optical Circuitry

Unlike lithographic processes, which are restricted to two‑dimensional surfaces, femtosecond laser writing allows waveguides to be placed anywhere inside the volume of the glass. This enables:

  • Three‑dimensional routing
  • Compact crossover structures
  • Vertical and horizontal coupling paths
  • Dense optical networks in a small footprint

This flexibility is essential for custom chip designs where space, geometry, and optical alignment must be optimised.

2. Low‑Loss, Low‑Stress Waveguides

The ultrafast nature of the laser pulses ensures that energy is deposited faster than the material can heat up. This prevents cracking, melting, or stress, resulting in:

  • Smooth waveguide profiles
  • Low insertion loss
  • Minimal birefringence
  • High long‑term stability

These characteristics are critical for high‑performance custom photonic chips.

3. Rapid Prototyping and Design Freedom

Femtosecond laser writing does not require masks, cleanrooms, or complex processing steps. Designs can be changed quickly, allowing:

  • Fast iteration
  • Custom geometries
  • One‑off prototypes
  • Small or large batch production

This makes the technology ideal for research, development, and specialised applications where traditional fabrication would be too slow or expensive. It is also used by Modular Photonics for large-scale, high volume chip production.

4. Compatibility with Standard Materials

The technique works in a wide range of transparent materials, including:

  • Fused silica
  • Borosilicate glass
  • Specialty glasses for photonics

This allows custom chips to be fabricated in materials that match the optical properties of the systems they will be integrated into.

Applications of Femtosecond‑Written Custom Photonic Chips

Custom photonic chips created with femtosecond laser writing are used in many advanced optical systems, including:

  • Multicore fibre fanouts and mode multiplexers
  • Fibre‑to‑chip coupling interfaces
  • Quantum photonic circuits
  • Beam splitters and interferometers
  • 3D waveguide networks
  • Optical signal processing
  • Sensing and metrology systems

The ability to create stable, low‑loss optical pathways inside a monolithic substrate makes these chips ideal for demanding environments.

Why Modular Photonics Uses Femtosecond Laser Writing

Modular Photonics employs femtosecond laser writing to fabricate custom photonic chips with exceptional performance and reliability. This technology enables:

  • Precise core‑to‑core mapping
  • Low‑loss transitions between fibre and chip
  • Stable, monolithic optical structures
  • Custom geometries tailored
  • High repeatability and long‑term alignment stability

These capabilities are essential for producing devices such as the MCMUX multicore fibre fanout, where optical precision and mechanical robustness are critical.

The Future of Custom Photonic Chip Fabrication

As integrated photonics continues to expand into telecommunications, sensing, quantum technologies, and data‑centre interconnects, femtosecond laser writing is becoming increasingly important. Its ability to create complex, three‑dimensional optical circuits quickly and reliably makes it a key technology for the next generation of custom photonic devices.

Whether used for prototyping, specialised components, or full‑scale production, femtosecond laser writing offers unmatched flexibility and performance for custom chip fabrication.

 

For more information on Custom PICs click here.