Types of Diffraction Grating

 

Diffraction Gratings

A diffraction grating is an optical component that disperses light (or other electromagnetic energy) into its individual wavelengths. This property makes diffraction gratings essential in instruments such as spectrometers, monochromators, and optical analyzers used across industries from analytical chemistry and astronomy to telecommunications and laser systems.

How Diffraction Gratings Work

When light strikes a diffraction grating, it is separated into its spectral components by a process known as interference.

The behavior of the diffracted light – its efficiency, wavelength range, and resolving power – depends on several design factors:

• Groove Density (lines/mm): Determines dispersion and resolution.
• Groove Depth and Profile: Affects diffraction efficiency and blaze wavelength.
• Coating Material: Optimized for the spectral range (UV, visible, IR)

Types of Diffraction Gratings

There are two primary manufacturing methods used to produce diffraction gratings: ruled and holographic.

Ruled Diffraction Gratings

Ruled gratings are created using a ruling engine, a precision instrument that mechanically cuts fine grooves into a coated substrate (typically a glass blank with a reflective metallic layer). A diamond-tipped tool carefully engraves each groove in a controlled pattern, allowing precise definition of groove density and blaze angle.

Ruled gratings generally provide:

• High diffraction efficiency, particularly at the blaze wavelength
• Strong signal intensity for applications like laser systems and spectroscopy
• However, they may exhibit periodic errors or stray light due to mechanical imperfections in the ruling process

Holographic Diffraction Gratings

Holographic gratings are fabricated using interference lithography, which forms grooves through the interference pattern of two coherent laser beams. This optical recording process eliminates mechanical ruling errors, resulting in:

• Exceptionally smooth groove surfaces
• Low stray light and excellent wavefront quality
• Ideal for broadband or low-noise applications, such as Raman and fluorescence spectroscopy
• Can be blazed to increase efficiency at specific wavelengths

Grating Surface Profiles

The groove profile plays a key role in determining the efficiency and spectral response of the grating:

• Blazed Gratings: Have a “saw-tooth” groove shape designed to direct most diffracted energy into a specific order or wavelength range, maximizing efficiency. SSI uses a proprietary optical blazing method to provide high UV efficiency with very low stray light compared to conventional ion etched blazing
• Sinusoidal Gratings: Offer smoother, wave-like grooves that produce broader spectral coverage but typically lower peak efficiency

Master and Replica Gratings

Creating a master diffraction grating is a complex and costly process involving extreme precision.

To make high-performance gratings more accessible, replica gratings are produced by optical replication – transferring the surface structure of a master or sub-master onto a new substrate using specialized resins and coatings.

A single master can generate thousands of high-fidelity replicas, offering nearly identical optical performance while dramatically reducing unit cost. This replication process is widely used across the industry for both plane and concave gratings.