Concave Gratings

Spectrum Scientific specializes in designing and manufacturing holographic concave gratings. SSI concave gratings can be supplied with blazed or sinusoidal profiles and can be aberration corrected if required.

Concave Grating types manufactured by SSI:

• Flat field (aberration corrected) concave grating (see list)
• Constant deviation monochromator concave grating
• Rowland type concave grating

Blazed Holographic Concave Gratings

SSI is able to blaze concave holographic diffraction gratings without the need for ion etching. This greatly reduces the stray light while maximizing energy at a specific spectral region. Unlike other concave grating blazing techniques, our process yields a blaze profile that varies across the grating surface, keeping the energy where the application requires. We have successfully produced blazed concave gratings with >80 % efficiency at the peak wavelength.

For a list of our most popular concave gratings for Flat Field Spectrographs, please click here

Advantages of SSI Proprietary Blazing Technique:

• Significant reduction in stray light compared
  to ruled or ion-etched gratings
• Energy maximized at blaze wavelength
• Blaze profile varies across the grating surface
• No loss of energy

The main advantage of a concave grating is that it can be used as the primary dispersive and focusing element in a spectroscopic instrument. A concave grating reduces the number of optical elements required, increasing throughput and instrument efficiency.

The different types of Concave Grating

Aberration Corrected (Flat Field Imaging) Concave Grating:

An aberration corrected concave grating (or flat field imaging concave grating) has grooves that are neither parallel nor equidistant.

This type of concave grating is designed to eliminate astigmatism and allow the required spectral range to be imaged on a plane. This makes the aberration corrected concave grating ideal for use with planar array detectors such as photo diode arrays (PDA) or charge coupled device (CCD) detectors.

SSI offers a range of concave gratings optimised for flat field spectrographs (for details click here)

Constant Deviation Monochromator Concave Grating:

The constant deviation monochromator grating is used in a scanning monochromator where the grating is rotated and scans the signal from the entrance slit across the exit slit. The deviation angle between the incident signal and diffracted signal remains constant.

The main advantage of a constant deviation concave grating over a plane grating is that it removes the need for collimating and focusing optics, reducing the number of optical elements and increasing throughput. It also allows more compact instrument design

Flat Field Imaging Grating Layout

Constant Deviation Monochromator
Grating Layout

Rowland Type Concave Grating:

The Rowland type concave grating has grooves that are straight and equidistant.

This type of concave grating diffracts the spectrum onto a Rowland circle which is defined as a circle where the diameter of the circle is equal to the radius of curvature of the concave grating.

Rowland type concave gratings suffer from astigmatism but other aberrations are small.

Advantages of SSI Concave Holographic Diffraction Gratings

• High efficiency
• Extremely low stray light
• Reliable and proven replication method
• Repeatable performance from grating to grating

All SSI holographic diffraction gratings are precision replicas that have very low stray light and optical performance indistinguishable from master gratings.