Holographic Gratings

Angle of Incidence, AOI
The angle (α°) between incident light and the grating normal.

Laser Induced Damage Threshold (LIDT)
Defined as permanent surface change and expressed in J/cm² or W/cm² and depends on factors like pulse energy, pulse duration and repetition rate, wavelength and beam profile (intensity profile). These factors vary between setups, and Spectrogon can not state a customer specific LIDT.

For ns pulses in the 800-1100 nm range, expected damage threshold is >2.3 J/cm² (50 Hz, 10 ns)* . For fs pulses in the 800-1100 nm range, expected damage threshold is >0.3 J/cm² (50 Hz, 100 fs)*.

*Laser Induced Damage Threshold results have been measured according to ISO 21254-2: 1000-on-1 test procedure.

Deviation angle
The angle θ between incident light α and -1 order diffracted light β. (θ° = |α°- β°| ),  β° being the diffraction angle. Also called Include angle. When the deviation angle is 0°, the grating is in so called Littrow configuration, and the AOI is called the Littrow angle.

Diffraction order
For most applications only one diffraction order is used, and one would like all the diffracted light to go into that order giving an absolute efficiency of 100 % for all wavelengths.The diffracted order with m = -1 is the order normally used in monochromators, spectrographs, and spectrometers. Please use our Grating Design Tool to minimize diffraction orders and energy losses.

Dispersion
The angular dispersion is the amount of change of diffraction angle per unit change of the wavelength. It is a measure of the angular separation between beams of adjacent wavelengths. High dispersion can be achieved either by choosing a diffraction grating with a high groove frequency, or by using a coarse diffraction grating in high diffraction order. Generally a fine pitch diffraction grating would be preferred because of the larger free spectral range.

Efficiency
The absolute efficiency is defined as the amount of the incident flux that is diffracted into a given diffraction order, in most cases the -1 order. Efficiency is usually expressed as percent absolute efficiency for a certain polarisation state and diffraction order;

• TM/-1 meaning percent efficiency for TM-polarised light in -1 order diffraction.
• TE/-1 meaning percent efficiency for TE-polarised light in -1 order diffraction.
• (TM+TE)/2, -1 meaning percent efficiency for unpolarised (avg polarisation) light in -1 order diffraction.

The relative efficiency is related to the reflectance of a mirror, coated with the same material as the grating, and it should be noted that the relative efficiency is always higher than the absolute efficiency. Spectrogon use absolute efficiency.

Littrow configuration
The grating is mounted so that light of the desired wavelength is diffracted back along the incident beam, and the wavelength is scanned by rotating the grating. Generally, an intracavity achromatic lens is used, which expands the laser beam to fill a relatively large area of the grating. The zero order diffracted beam can be used as the output laser beam; however, a disadvantage is that the beam will have different directions as the grating is rotated.

Grazing incidence
The grating is kept fixed at an high angle of incidence, and the wavelength is tuned by rotating a special tuning mirror. No beam expanding lens is needed, and therefore a smaller grating can be used. The large incidence angle implies, however, that the ruled width of the grating has to be considerably greater than the groove length.

Polarisation
The efficiency of a diffraction grating usually varies with the polarisation of interest. Therefore the polarisation state must be determined. TM-polarisation meaning the electrical vector is perpendicular to the grating grooves corresponds to P-polarisation. TE-polarisation meaning the electrical vector is parallel to the grating grooves corresponds to S-polarisation. (TM+TE)/2 meaning unpolarised light.

Sinusoidal gratings
Holographically manufactured diffraction gratings of standard type have a sinusoidal groove profile. The efficiency curve is rather smooth and flatter than for ruled gratings. The efficiency is optimized for specific spectral regions by varying the groove depth, and it may still be high, especially for gratings with high frequency. When the groove spacing is less than about 1.25 times the wavelength, only the -1 and 0 orders exist, and if the grating has an appropriate groove depth, most of the diffracted light goes into the -1 order. In this region, holographically recorded gratings give well over 50 % absolute efficiency.

Stray Light
Working at the detection limit of an optical instrument, the stray light level from the grating and other optics will set the ultimate limit of detection. Holographic diffraction gratings are known for their low level of stray light and the total absence of “ghosts” in the spectral image. This is due to the very precise spacing between grooves which is achieved in the interference pattern exposure. However there are sources of stray light also from holographically recorded diffraction gratings, and the stray light levels may vary considerably between gratings due to differences in the manufacturing processes used. It is widely known that holographic diffraction gratings have much lower stray light levels than ruled ones; nevertheless, Spectrogon has been able to improve the holographic process even further. The stray light levels are about ten times lower from a Spectrogon grating compared to a ordinary holographic diffraction grating, which in a double spectrometer, implies an improvement by a factor of 100 of the spectral purity.

Thermal expansion coefficient (α) of grating substrate material

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