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Model 101/102 Monochromator Operation Manual

 

CONTENTS

1

DESCRIPTION

Introduction

About Gratings

2

INSTALLATION

Unpacking

Mounting on an Optical Rail

3

OPERATION

Slits

Gratings

Flippers

4

SERVICE

Changing Gratings

Adding Accessories

 

 

 

 

DESCRIPTION

Introduction

The PTI Model 101 Monochromator is a quarter-meter Czerny-Turner configuration with an f-number of 4. The PTI Model 102 Monochromator is a dual-grating version of the Model 101. The standard 1200 line/mm ruled grating disperses the white light spectrum (UV to IR) across the exit slit with a mechanical scanning range from zero order to 1,000 nanometers. The wavelength display dial reads the actual operating wavelength of the instrument. The base Stray light level is unsurpassed (less than 0.02%) at two bandwidths from the 365 nm Hg line. The slits are continuously adjustable from 0 to 6 mm.

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About Gratings

The monochromators can accommodate gratings other than 1200 line/mm for which the sine drive was designed. PTI offers an extensive selection of gratings.

NOTE: The use of different gratings will result in the need to correct the wavelength display by the factor indicated in the table below.

Grating Category (lines/mm) Mechanical Scanning Range(nm) Wavelength Display
Correction Factor
(X Dial Reading)
2400 0 to 500 0.5
1200 0 to 1,000 1
600 0 to 2,000 2
300 0 to 4,000 4
150 0 to 8,000 8
75 0 to 16,000 16

 

The table also lists the mechanical scanning range for each grating category when used in the first order, which is recommended for maximum light throughput. Although there is no such thing as zero nanometer radiation, the zero setting represents the placement of the grating for zero order. At zero order the grating acts like an ordinary mirror and white light exits from the monochromator. This is useful when broad band excitation and maximum light throughput is required, and also for verifying accuracy.

General Grating Properties

Maximum throughput generally increases as the number of grooves per millimeter (lines/mm) decreases. Optimum wavelength resolution always increases as lines/mm increases, i.e., reciprocal linear dispersion decreases. Standard ruled gratings usually offer the highest efficiency throughput (sometimes over 80%) of any grating when used near their blaze wavelength. However, they are prone to imperfections such as ghosts, which arise from periodic misrulings. The ghosts may cause this type of grating to produce stray light. If this problem exists, it can often be remedied using bandpass or cutoff filters.

Holographic gratings offer very low stray light levels because of fewer imperfections in their manufacture. Therefore, they may be more suited to applications which are very susceptible to stray light interference. Holographic gratings generally have lower throughput efficiency (typically 20%) but it is usually relatively constant over its complete useful range. But they also tend to introduce more polarization anomalies than ruled gratings. This should be considered when making polarization/anisotropy measurements. Polarization effects can be practically eliminated by passing the illumination beam through a depolarizer, certain types of diffusers, and fiber optics.

Determining Wavelength Bandpass

The bandpass is the range of wavelengths which the monochromator emits about a central wavelength setting. This depends on the grating lines/mm, slit widths, and somewhat upon the wavelength. For most applications the variation with wavelength can be ignored and the bandpass (BP) is simply the product of the slit width (W) times the reciprocal linear dispersion (Rld):

Rld = 106/(n x L x F)

BP = Rld x W

where:
Rld = Reciprocal Linear Dispersion (nm/mm)
BP = Bandpass (nm)
W = Slit Width (mm)
L = Grating (lines/mm)
F = Focal Length of Monochromator (200 mm)
n = Order Integer (1,2,3, ... ; use n=1 for highest throughput)

Therefore, the calculated reciprocal linear dispersion values for the most common grating types (lines/mm) are 2 nm/mm at 2400 lines/mm, 4 nm/mm at 1200 lines/mm, 8 nm/mm at 600 lines/mm, etc. When the bandpass must be precisely known, refer to the article, "Optimizing Grating Based Systems" by J.M. Lerner and A. Thevenon in Lasers and Applications, Jan. 1984.

The optimum resolution of the monochromators is, in practical terms, the minimum useful bandpass. This depends on the dispersion, slit width, and optical image quality. The resolution using 2400 lines/mm gratings is approximately 0.25 nm (slit width = 125 mm). The optimum resolution at 1200 lines/mm is twice as large, and so on, for other gratings.

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INSTALLATION

Unpacking

IMPORTANT: PTI monochromators are shipped with the wavelength dial set to the lowest limit to prevent grating movement during transport. Turn the dial clockwise to obtain operating wavelengths. No other preparation is necessary.

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Mounting on an Optical Rail

The monochromator may be mounted on an optical rail. The base has two M-5 tapped holes situated along the optical axis of the monochromator.

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OPERATION

Slits

The slits are continuously adjustable from 0 to 6 mm with the slit micrometers. When used with the standard 1200 groove per mm grating, the reciprocal dispersion is 4 nm per mm, or 2 nm per revolution of the slit micrometer. Turning the micrometer clockwise closes the slit; counterclockwise rotation opens it.

The rotating element of the slit micrometer is calibrated to display the slit width in 0.01 mm increments, and the stationary portion is marked every 0.5 mm. The slits will open 1 mm for every two complete turns of the micrometer. When it is not possible to view the calibrations directly, the top of the micrometer knob has a screwdriver slot which may be used to keep track of slit width. The closed position can be felt when clockwise rotation is met with resistance.

WARNING: Never force the slits clockwise upon closing. Stop closing the slits when resistance is felt. Otherwise, calibration will be lost and the slits may be damaged.

 

Setting Wavelength Bandpass

The bandpass is set by adjusting the slit width. When increased light intensity is required, the slits should be opened wider. This is only appropriate if wavelength resolution can be sacrificed.

When optimum and true bandpass readings are needed, both the entrance and exit slits of a particular monochromator must have the same setting. This will also reduce stray light. Note that the slit height adjustment sliders (if present) should be fully opened when maximum throughput is required. However, partly closing them can sometimes help to increase resolution.

At zero order, the grating acts like an ordinary mirror and white light exits from the monochromator. Zero order is selected by specifying a wavelength of 0 nm. The zero order setting of a monochromator is useful when broadband excitation is required for maximum light throughput. Setting a monochromator to 0 nm is also a quick way to confirm that it is calibrated.

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Gratings

The Model 102 Monochromator has a dual grating turret with a selector knob. Rotate the knob in the direction of the arrow to switch gratings.

If your monochromator was ordered with optional flipping mirror(s), there will be control knobs for them on the top of the unit. Rotate the knob(s) in one direction or the other to select the device attached to the entrance and/or exit slit(s).

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SERVICE

Changing Gratings

Changing gratings requires opening the monochromator and disassembling the grating holder. It is recommended that grating changes be performed by PTI.

NOTE: If the monochromator is already installed as part of a PTI system (such as a QuantaMaster™ or RatioMaster™), the unit is not user-serviceable. Servicing should be performed by PTI personnel. User adjustment of the unit will void the Instrument Warranty.

NOTE: It is not possible to change gratings in the Model 102 dual-grating monochromator without damaging them. All grating changes for Model 102 must be performed by PTI.

Changing gratings in a Model 101 Monochromator

  1. If the unit has flipping mirror(s), remove the control knob(s) from the shaft(s). Then remove the six cover screws and remove the cover (see the figure below).

  2. Remove the T-bar from above the grating holder (see the figure above).

  3. Remove the upper grating holder plate and the retaining spring.

  4. WARNING: Never turn the sine drive pin screw on the grating holder base. This would change the wavelength calibration of the monochromator.

  5. Slide the old grating out of the holder.

    WARNING: Never touch the surface of the grating or the mirrors inside the unit. Immediate and permanent damage will result. Wear soft cotton gloves when working with gratings.

  6. Slide the new grating into the holder. Note that on top of the grating is an arrow which should be pointing towards the entrance slit when the grating is mounted.

  7. Replace the retaining spring and upper holder plate.

  8. Replace the T-bar and monochromator cover. No additional alignment is necessary.

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Adding Accessories

Installing a Stepping Motor Drive

PTI monochromators are designed to accept stepping motor drives with no need for modification. Installation is straightforward.

  1. Remove flipping mirror and/or grating selector knob(s) as necessary and remove the monochromator cover.

  2. Remove the large plug on the back of the monochromator, as well as the two smaller ones on either side of the large plug.

  3. Install the shaft extension, PTI part number 780006, and tighten.

  4. Attach the motor and mount assembly to the monochromator base with two screws.

  5. Slide the shaft extension over the motor shaft and tighten.

  6. Replace the cover.

Installing a Nitrogen Inlet Valve

The nitrogen inlet valve is installed by simply removing one of the threaded plugs located at various points on the monochromator. The inlet valve is then threaded into this hole. Ensure that the threads are properly matched.

Other Optical Accessories

Each of the slit assemblies has two sets of four flat-head machine screws. Optical accessories may be attached by removing the appropriate set of screws and threading the accessory mounting screws into the holes.

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