|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 stray light. Monochromators have lower stray light in comparison with spectrographsbecause 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. With a double monochromator maximum suppression of stray light can be achieved.
MZDD350i 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 double monochromator with zero dispersion are determined by size of the entrance and intermediate (middle) slits and by aberrations of the first monochromator. The size of exit slit should be selected to avoid vignetting of the entrance slit image. The exit slit would be equal to 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.
- 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 speсtroscopy
Extremely low stray light allows to performs measurements close to the excitation laser line without using any Notch or Edge filter
- Spectrosсopy of pulsed light sources
There is almost zero broadening of light pulses after passing through the double monochromator thanks to exact inverse beam path in the second monochromator in regard to the first one. There is 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
- Wide spectral range: UV, VIS, IR
- Low stray light
- High aperture
- High wavelength accuracy and repeatability
- Fully computer controlled
Optical layout and components
Optical layout of Double monochromator with dispersion subtraction based on MS3504i is shown in Fig.1. Optical components 1-6 and 14 belong to the 1st monochromator, optical elements 8 -12 belong to the 2nd monochromator. Spectral slit 7 is exit slit of the 1stmonochromator and in the same time it is an entrance slit of the 2nd monochromator. Thit slit is called intermediate.
The 1st monochromator can be supplied optionally with motorized flip mirror 6. It allows to use axial output port of the 1stmonochromator. The 1st monochromator will operate as imaging spectrograph in this case.
Fig.1. MZDD350i Optical Layout
1 – entrance slit, 2 – 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 – flip mirror, 13 – exit slit, 14 – shutter
|Configuration:||Two imaging monochromators with Cherny-Turner optical scheme.
Monochromators are cascade situated for dispersion subtraction
|Ports*:||1 input and 1 output|
|Wavelength range:||185 nm – 60 μm (depends on the type of grating)|
|Mirror focal lengths:||300 mm and 350 mm|
|Scanning range, limited by grating rotation angle:||0 – 1270 nm (for 1200 l/mm grating)|
|Stray light:||5х10-10 (nm from laser line 632.8 nm)|
*Axial exit port of the1st monochromator MS3504i can be used additionally.
|Motor:||Stepper, with fractional steps|
|Step size:||1.62 arc seconds|
|Precision:||± 1 step|
|Max.speed:||10 000 steps/s|
|Spectral resolution:||0.07 nm|
|Repeatability:||± 0.03 nm|
|Wavelength accuracy:||± 0.06 nm|
|Average scanning step:||0.01 nm|
*For grating 1200 l/mm, slit width 15 μm, wavelength – 546 nm
|Size:||70 х 70 х 10 mm|
|Rotation:||About the center of grating working surface|
|Mounting:||Automated 4-grating turret in each monochromator|
|Grating repeatability of monochromators|
|– *wavelength:||± 0.03 nm|
|– vertical image:||± 0.050 nm|
* For grating 1200 l/mm, slit width 15 μm, wavelength – 546 nm
|Type of spectral slit:||Automated (combined)||Manual|
|Control:||Automated (drive) or
|Manual by micrometer|
|Width:||Regulated from 0 to 2.0 mm|
|Parallelism:||± 1 μm|
|Accuracy (slit 1 mm):||± 10 μm|
|Repeatability:||± 1 μm||± 1.5 μm|
|Reading accuracy (micrometer):||2 μm|
|Step size:||0.5 μm||–|
|Height:||Diaphragm-regulated from 0 to 10 mm|
|Shut time:||~100 ms|
|Max. Frequency:||1 Hz|
|Control:||On-board CPU or TTL-signals from the external device|
|External interface:||Ethernet, USB|
|Supply voltage:||(100…220) V, 50/60 Hz|
|Power consumption:||Not more than 70 W|
|While choosing a device configuration a proper selection of diffraction gratings is important. A proper selection of diffraction grating allows one to obtain the best combination of high energetic efficiency and spectral resolution.
The basic grating parameters which determine their right choice you can find in the methodical material “Spectral Instrument. Basic Concepts and Characteristics.”
We propose a wide range of diffraction gratings to be used in MZDD350i.
Basic parameters for correct selection of diffraction gratings:
In the Table “Selection of diffraction gratings” groove density and blaze wavelength belong to a diffraction grating parameters. Other parameters characterize a spectral device together with a selected grating.
- Groove density. The diffraction grating has a periodic structure. Such parameters as resolving power and free spectral range are determined by the periodic structure properties of a diffraction grating. The period of grating is the distance through which grooves are repeated. The reciprocal value of a grating period is called the groove density and displays the number of grooves in 1 mm.
- Blaze wavelength. The reflectivity of a grating depends on the grooves geometry, because the direction to the center of the diffraction maximum is determined by the mirror reflection of the incident beam from the edge of a groove. In spite of rather flat profile of diffraction maximum a blaze wavelength that corresponds to the maximum efficiency of the grating has been adopted as a feature for convenience of calculation.
When selecting a grating it is useful to determine a blaze wavelength for a required grating on the spectral range of limited wavelengths WL1 and WL2. The blaze wavelength is determined by the ratio:
WLBlaze = 2 ∙ WL1 ∙WL2 / (WL1 + WL2).
- Reciprocal linear dispersion. The light from a grating in the focal plane of a spectral instrument forms a spectrum. Linear dispersion is used for a spectrum characteristic, which is defined as a reciprocal value of the product of angular dispersion of a grating and focal length of a spectral instrument and shows the spectral range that falls on a single linear distance in a focal plane.
- The spectral resolution. Limiting resolution of a spectral instrument is equal to the minimum half-width of its instrument function. The instrument function is determined by finite sizes of entrance diaphragm, aberrations and also distortions caused by inaccuracy in manufacturing and adjustment of optical elements of instrument. In addition, instrument function, and thus spectral resolution are dependent on lighting method of entrance slit, used aperture and registration system parameters.
- Operating wavelength range. Energy efficiency range referred to the wavelength region in which grating reflection coefficient is not less than 0.405 of the maximum. In the first order this range is limited by the wavelengths: ⅔ ∙ WLBlazeand 2 ∙ WLBlaze.
Operating range usually corresponds to a grating energy efficiency. In some cases, for gratings with a high groove density and large blaze wavelength, the long-wave border of operating spectral range of instrument is limited by a maximal rotation angle of a grating which is defined by design of instrument.
Table for selection of diffraction gratings
| *Reciprocal linear dispersion,
|Operating wavelengths range, nm
limited by the grating rotation angle, nm
|773600||3600||hol||0.76**||0.025||185 – 425||425|
|772422||2400||225||1.17||0.035||185 – 450||640|
|772427||2400||270||1.16||0.035||185 – 540||640|
|772770||2400||400||1.11||0.035||270 – 640||640|
|771827||1800||270||1.57||0.04||185 – 540||860|
|771840||1800||400||1.54||0.04||265 – 800||860|
|771850||1800||500||1.49||0.04||330 – 860||860|
|771875||1800||750||1.29||0.04||500 – 860||860|
|771225||1200||250||2.36||0.07||185 – 500||1280|
|771228||1200||280||2.36||0.07||185 – 560||1280|
|771240||1200||400||2.35||0.07||265 – 800||1280|
|771250||1200||500||2.33||0.07||330 – 1000||1280|
|771260||1200||600||2.30||0.07||400 – 1200||1280|
|771275||1200||750||2.24||0.07||500 – 1280||1280|
|770623||600||230||4.72||0.14||185 – 460||2560|
|770630||600||300||4.72||0.14||200 – 600||2560|
|770640||600||400||4.73||0.14||265 – 800||2560|
|770650||600||500||4.73||0.14||330 – 1000||2560|
|770660||600||600||4.73||0.14||400 – 1200||2560|
|770675||600||750||4.71||0.14||500 – 1500||2560|
|7704008||400||800||7.09||0.21||540 – 1600||3840|
|7704012||400||1200||7.06||0.21||800 – 2400||3840|
|7704017||400||1700||6.94||0.21||1135 – 3400||3840|
|7704020||400||2000||6.83||0.21||1335 – 3840||3840|
|7704025||400||2500||6.57||0.21||1665 – 3840||3840|
|770330||300||300||9.42||0.28||200 – 600||5120|
|770335||300||350||9.42||0.28||235 – 700||5120|
|770350||300||500||9.44||0.28||335 – 1000||5120|
|770360||300||600||9.45||0.28||400 – 1200||5120|
|770370||300||700||9.45||0.28||465 – 1400||5120|
|7703010||300||1000||9.46||0.28||665 – 2000||5120|
|7703015||300||1500||9.42||0.28||1000 – 3000||5120|
|7703020||300||2000||9.33||0.28||1335 – 4000||5120|
|7703030||300||3000||8.95||0.28||2000 – 5120||5120|
|7703035||300||3500||8.65||0.28||2335 – 5120||5120|
|770235||200||350||14.11||0.42||235 – 700||7680|
|770240||200||400||14.12||0.42||265 – 800||7680|
|770250||200||500||14.13||0.42||335 – 1000||7680|
|770270||200||700||14.15||0.42||465 – 1400||7680|
|7702015||200||1500||14.19||0.42||1000 – 3000||7680|
|7702020||200||2000||14.16||0.42||1335 – 4000||7680|
|7702025||200||2500||14.10||0.42||1665 – 5000||7680|
|7702030||200||3000||14.00||0.42||2000 – 6000||7680|
|771547||150||475||18.81||0.56||315 – 950||10300|
|771570||150||700||18.85||0.56||465 – 1400||10300|
|7715030||150||3000||18.85||0.56||2000 – 6000||10300|
|7715045||150||4500||18.52||0.56||3000 – 9000||10300|
|770165||100||650||28.21||0.84||435 – 1300||15400|
|770170||100||700||28.22||0.84||465 – 1400||15400|
|7701010||100||1000||28.26||0.84||665 – 2000||15400|
|7701020||100||2000||28.36||0.84||1335 – 4000||15400|
|7701025||100||2500||28.38||0.84||1665 – 5000||15400|
|7701030||100||3000||28.38||0.84||2000 – 6000||15400|
|7701042||100||4160||28.31||0.84||2800 – 8300||15400|
|7701048||100||4840||28.23||0.84||3300 – 9600||15400|
* For the 1st monochromator at the blaze wavelength
** at the wavelength 225 nm