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, as seen in the Ossila Optical Spectrometer.
The blazed diffraction grating is a type of grating that 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, θB.
Blazed diffraction gratings will maximise the grating efficiency in one desired diffraction order at a specific wavelength, while other orders are minimised. 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 Gratings Conventions
In a spectrometer, the incoming wavefront approaches the grating at a fixed angle, θi. Depending on the wavelength, the grating diffracts incoming light by angle θm. 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 θi is positive and θd is negative. However, if the incident and diffracted light is on the same side of the surface normal, both angles are considered positive.
The Littrow configuration is an important concept for understanding blazed diffraction gratings in spectrometers. It involves the incident light striking the grating at θB 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.
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 θB 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 of the spectrometer 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 spectrometer performance.
The Ossila Optical Spectrometer has a grating blaze wavelength of 500 nm to achieve maximum sensitivity over the visible light region.
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