Guide to SPEX Optical Spectrometer

Transcription

1 Guide to SPEX Optical Spectrometer GENERAL DESCRIPTION A spectrometer is a device for analyzing an input light beam into its constituent wavelengths. The SPEX model 1704 spectrometer covers a range from approximately 175 nm in the near-uv to 1500 nm in the near IR with a resolution of about 0.01 nm. The wavelength transmitted is indicated on a digital counter, and controls are provided to either set the wavelength or scan at a specified rate. Precision adjustable input and output slits allow the operator to pick the best combination of spectral resolution and transmission for the desired purpose. The dispersive element is a plane reflection grating in a Czerny-Turner mounting, as sketched in Fig. 1. The entrance slit is placed at the focal point of a spherical mirror. The mirror reflects the incoming light as a parallel beam and directs it toward the grating. The grating reflects the light like a plane mirror, but different wavelengths are reflected at different angles according to the usual grating equation. A portion of the reflected light, representing a range of wavelengths, is incident on the second spherical mirror, which forms an image of the entrance slit on the plane of the exit slit, completing the wavelength selection. The grating is mounted on a rotating table so that different wavelengths can be directed toward the second mirror. The widths of the input and output slits are independently adjustable with precision micrometer drives. In addition, the height or length of the input slit can be changed in steps with exit slit grating on turntable spherical mirrors entrance slit Fig. 1 General layout of a Czerny-Turner grating spectrometer. [Type here] [Type here] [Type here]

2 a movable mask. Wider or longer slits transmit more of the incident light, but at the expense of wavelength resolution, so it is important for the operator to understand the effects of changing slit parameters. Consider first the case of a monochromatic source imaged onto the input slit. If the grating is rotated to reflect that wavelength onto the output slit, the effect of the optical system is simply to form a single image of the entrance slit on the exit slit. The magnification is nominally one, so the output image would be the same size as the input slit, although aberrations will make it slightly larger. Given this symmetry, it is reasonable to make the input and output widths the same. If we now imagine rotating the grating to scan a wavelength range the monochromatic image will move across the exit slit, and we will detect light as long as any part of the image falls within any part of the exit slit. By narrowing the slits we limit the angle of rotation over which we detect light, and therefore improve the resolution. At the same time, of course, we admit less light from the source through the smaller slit, and therefore lose output intensity. Although it is less obvious, the height of the slits also affects the resolution. This occurs because the optical aberrations of the spherical mirrors cause the image of a straight slit to be slightly curved. Light from the top or bottom of the entrance slit therefore passes through the exit slit at a slightly different grating angle than light from the center of the entrance slit. Reducing the height of the input slit therefore increases the resolution, but again at the expense of intensity. A source which is not monochromatic can be considered a mixture of many monochromatic sources. Each wavelength interval will produce an image of the entrance slit on the exit slit, and rotation of the grating will sweep the whole pattern across the exit slit. The trade-off between resolution and intensity then works as in the monochromatic case, for both slit width and slit height. The description above assumes that light entering the spectrometer diverges sufficiently to illuminate the whole surface of the first mirror. The parallel beam emerging from the mirror then illuminates the entire surface of the grating, resulting in minimal spectral width of the reflected light. The input optics must be arranged so that this condition is met. Guide to SPEX Optical Spectrometer 2

3 OPERATION 1. Input Optics The light source to be analyzed might be a gas discharge, a glowing solid, or even an arc. All of these produce diverging light rays over a substantial angle, so placing the source at the entrance slit would fully illuminate the first mirror. However, that is not usually feasible because the source might be enclosed in a large glass envelope (discharge tube, light bulb) or might damage the spectrometer (arc lamp). Increasing separation would help, but would drastically reduce the amount of light entering the spectrometer. Fig. 2 shows a better solution. A simple lens is used to focus an image of the source on the entrance slit. The input intensity is maximized if the angular range of the rays converging on the slit matches the input range of the spectrometer. Rays outside that angular range should be removed because they will miss the first mirror and then scatter internally, blurring the desired image. As suggested in Fig. 2, both goals are met by forming a 1:1 image of the source onto the slit. The focal length and clear diameter of the lens are chosen so that the input angle matches the spectrometer, as shown. 2. Slits The micrometer drives for the entrance and exit slits protrude from the top of the slit housings. The scale reads 200 divisions per turn, with each division indicating 2 µm of slit width. If the knob is turned below zero width, the slit jaws close against springs but are not damaged. The height of the entrance slit can be adjusted with a multi-position shutter whose control slides out of the side of the entrance slit housing. Detents and markings indicate closed (s), circular apertures at three different heights (dots), and rectangular apertures of 0.2, 1 and 2 cm source lens d o d i Fig. 2 Optical layout to match spectrometer to extended source. For unit magnification, d i = d o = 2f, where f is the focal length of the lens. Guide to SPEX Optical Spectrometer 3

4 height (numbers). The shutter can be seen behind the slit jaws when they are opened to several millimeter width. 3. Wavelength selection The wavelength passed by the spectrometer is chosen by rotating the grating. The rotation is accomplished with an electric stepper motor controlled by external electronics. A mechanical counter on the side of the spectrometer reads the selected wavelength in Ångstroms (1 Å = m). The switches on the controller have the following functions: POWER/SPEED (red): Move above center position to turn on controller power. Move below center to turn on controller power and decrease indicated sweep speeds by a factor of 5. MARKER (yellow and orange): Move yellow below center to turn on marker at rear panel output jacks. Set orange up for a marker pulse every 10 Å and down for marker every 100 Å. EXT/HALT/ADJ (green): Set above center for external sweep control. Set on center to sweep. Set below center to halt or use adjustment wheel. REV/FWD SLEW (blue): Used to rapidly change ('slew') the wavelength in the forward (increasing wavelength) or reverse direction. Green switch must be in center position to slew. SPEED (gray and white): Used to choose the wavelength sweep speed. Set the four white switches in the pattern for the desired speed in Å/sec, as shown on the upper part of the panel. Push the gray switch down to verify the setting, shown by the blinking light. Push the gray switch up to enter the new sweep speed. A light on the upper panel will indicated the selected speed. ADJUST (wheel at bottom of panel): Used to change wavelength manually. Green switch must be down to operate. To sweep through a chosen range of wavelengths first set the desired sweep speed. Use a combination of slewing and the adjustment wheel to reach the starting point for the sweep. Since the sweep is always in the direction of increasing wavelength, this must be at the short- Guide to SPEX Optical Spectrometer 4

5 wavelength end of the desired range. It is important to approach the starting point from at least 50 Å shorter wavelength because of mechanical backlash in the drive mechanism. Once at the starting wavelength, move the green switch to the center position to start the sweep, and move it below center to halt the sweep at the end of the range. 3. Detector Light emerging from the exit slit hits a detector sensitive to the appropriate range of wavelengths. For the visible region, a Hamamatsu type 1P28A phototube is mounted in a lightproof housing over the exit slit. When supplied with high voltage, the detector produces a current directly proportional to light intensity with very little current noise. Sensitivity is high from about 250 nm to 550 nm, with useful response to about 650 nm. Guide to SPEX Optical Spectrometer 5