Diffraction gratings

Double monochromator with zero dispersion MZDD350i

The main limitation in research of weak spectral signals is a level of stray light. Stray light is mostly caused by quality of optical elements: mirrors and gratings. Besides, the stray light appears in the result of reflection of a spectrum from an entrance slit, a detector and from other structural units. The stray light can be minimized by using light absorbing partitions, by inclined installation of detectors, and by using holographic gratings.

Long-focus devices are characterized by lower level of stray light. Monochromators have lower stray light in comparison with spectrographs because the last named are used without any exit slit. Double monochromator is the best choice when the value of stray light is very important for research purposes. With a double monochromator – maximum suppression of stray light can be achieved.

The MZDD350i instrument is a double monochromator with zero dispersion consisting of two imaging monochromators MS3504i configured in one construction and the exit slit of the first monochromator is the entrance slit of the second monochromator. The device is fully automated. A set of four gratings and smoothly adjustable slits allow to operate in a wide spectral range with a required bandpass.

Zero dispersion is reached by exact correspondence of beam pass through the first monochromator to a beam return through the second monochromator. Optical parameters of the double monochromator with zero dispersion are determined by the size of the entrance slit and intermediate (middle) slit and by aberrations of the first monochromator. The size of the exit slit should be selected to avoid vignetting of the entrance slit image. The exit slit would be equal to the entrance slit, if no aberration.

Table 1 shows the design parameters of exit slit depending on size of entrance and intermediate slits for MZDD350i (diffraction grating 1200 G/mm, wavelength 546 nm).

Light at output in a single monochromator is a part of spectrum. The light leaving the double monochromator with zero dispersion is spectrally uniform at the output. This main feature, alongside with capability of maximum suppression of stray light, determines the fields of application of MZDD350i.

Brochure of double monochromator MZDD350i
Double monochromator MZDD350i

Table 1: size of exit slit

Entrance slit, mmIntermediate slit, mmExit slit, mm
  • Exit slit 0,031 mm = entrance slit 0,025 mm + intermediate slit 2 mm.

  • Exit slit 0,52 mm = entrance slit 0,5 mm + intermediate slit 2 mm.
  • Exit slit 1,05 mm = entrance slit 1 mm + intermediate slit 2 mm.
  • Exit slit 1,04 mm = entrance slit 1 mm + intermediate slit 1 mm.

Applications of MZDD350i

  • Spectral measuring instruments for UV, Visible and IR regions where tunable monochromatic light is wanted: The MZDD350i can operate as a tunable filter with adjustable bandpass and extremely low straight light.
  • Raman spectroscopy: Extremely low stray light allows to performs measurements close to the excitation laser line without using any Notch or Edge filter.
  • Spectroscopy of pulsed light sources: There is almost zero broadening of the light pulses after passing through the double monochromator in consequence of the exact inverse beam path in the second monochromator in regard to the first monochromator. There is an identical pass length for all wavelengths.
  • Measurements of the CCD detectors quantum efficiency: Light from the exit slit falls on the imaging detector and the light is measured with a calibrated detector simultaneously.

Features of MZDD350i

  • Wide spectral range: ultraviolet, visible and infrared spectral regions
  • Low stray light and high aperture
  • High wavelength accuracy and repeatability
  • Fully computer controlled

SOL instruments methodical materials:

Optical layout and components of the MZDD350i device

Optical layout of the double monochromator with dispersion subtraction based on MS3504i is shown in Fig.1.

Optical components 1-6 and 14 belong to the first monochromator, optical elements 8-12 belong to the second monochromator. Spectral slit 7 is the exit slit of the first monochromator and in the same time it is an entrance slit of the second monochromator. This slit is called the intermediate.

Optionally, the first monochromator can be supplied with the motorized flip mirror 6. It allows to use, additionally, the axial output port of the first monochromator. In this case, the first monochromator will operate as an imaging spectrograph.

Оптическая схема монохроматора MZDD350i
Fig.1. Optical scheme of MZDD350i

1 – entrance slit
2 – entrance flip mirror
3 – collimating mirror
4 – grating of 1st monochromator
5 – camera mirror
6 – motorized flip mirror
7 – intermediate slit
8 – motorized flip mirror
9 – collimating mirror
10 – grating of 2nd monochromator
11 – camera mirror
12 – exit flip mirror
13 – exit slit
14 – shutter