Blazed Gratings

Diffraction Grating Conventions | Littrow Configuration | Blaze Wavelength

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Spectrometers use either a diffraction grating or a prism to split incoming light into its constituent wavelengths. There are various types of diffraction gratings available, such as reflective or transmission, ruled or holographic. One commonly used type is the blazed diffraction grating, which is used across our range of spectroscopy equipment:

The blazed diffraction grating has a "sawtooth" profile. In this case, the grating spacing, d, is defined by the width of each triangular 'tooth' (step). The tilt angle of each step relative to the grating surface is known as the blaze angle, θ_B. Blazed diffraction gratings maximize the grating efficiency in one desired diffraction order at a specific wavelength, while other orders are minimized. This means maximum efficiency of the grating will be at the blaze wavelength. Light that deviates from this wavelength is diffracted with lesser intensity.

Diffraction Grating Conventions

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In a spectrometer, the incoming wavefront approaches the grating at a fixed angle, θ_i. Depending on the wavelength, the grating diffracts incoming light by angle θ_m. Both angles are defined with respect to the grating surface normal. Constructive interference will occur when the following equation is true.

where λ is the wavelength of incoming light, and m is an integer that represents the diffraction mode.

Be careful when defining the signs in this equation. The convention for reflective gratings is that if the incident and diffractive angles are on opposite sides of the grating surface normal, then θ_i is positive and θ_d is negative. However, if the incident and diffracted light is on the same side of the surface normal, both angles are considered positive.

Littrow Configuration

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The Littrow configuration is an important concept for understanding blazed diffraction gratings in spectrometers. It involves the incident light striking the grating at θ_B relative to the grating's surface normal. This angle is equal to the angle between the grating and the "flat surface" (see previous diagram). At this angle, the incident light is reflected directly back along its incoming path, with no intensity lost due to diffraction.

Blaze Wavelength

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The diffraction angle of a beam is wavelength dependent, so the Littrow Configuration can only be achieved at one wavelength. This is known as the blaze wavelength. The blaze wavelength (λ) relates to the grating period (d) and θ_B through the following equation:

Consequently, a spectrometer equipped with a blazed grating will be most sensitive to light at the blaze wavelength. As the light deviates from the blaze wavelength, the sensitivity to that particular light diminishes. Therefore, it is crucial to select the blaze wavelength at the optimum point within a desired spectral range to ensure optimum performance. The Ossila USB Spectrometer has a grating blaze wavelength of 500 nm to achieve maximum sensitivity over the visible light region.

Second Order Effects

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Blazed gratings can introduce second order effects in optical devices such as monochromators. The above relationship shows that the first order of diffraction of 800 nm light will have the diffraction ngle as the second order of 400 nm light (as diffraction is determined by mλ).

Therefore, it is important to account for this when using devices with blazed gratings. When measuring optical properties across a broad spectral range, it is good practise to only scan up to double the lowest wavelength measured. For example, to measure a 400-1000 nm absorbance spectrum, you should measure 400-750 nm in one measurement, then measure 750-1000 nm in a second measurement with the 500 nm longpass filter in place.

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