DE102005059986B4 - chopper disc - Google Patents

Abstract

Arrangement for recording emission spectra by means of laser-induced plasma spectroscopy (LIPS) containing
(a) a laser (24) for generating a laser-induced plasma in a sample (26) which emits light (30) of different wavelengths,
(b) means (36) for spectral decomposition of the light emitted by the plasma,
(c) a detector (38) for receiving the light emitted by the plasma, and
(D) closure means (44) for interrupting the beam path of the emitted light, wherein
(e) the closure means (44) are formed by a rotating chopper disc having an opening (54) through which the light emitted by the plasma is directed, and an aperture (70) through which an auxiliary photosensitive detector (72) for generating a trigger signal (40) for triggering the laser can be illuminated
characterized in that
(F) further openings (56; 58; 60; 62) are provided in the direction of movement (52) adjacent to the first opening (54) of the chopper disk (44), and
(g) the openings (54, 56, ...

Description

Technical area

• (a) einen Laser zur Erzeugung eines laserinduzierten Plasmas in einer Probe, welches Licht unterschiedlicher Wellenlängen emittiert,
• (b) Mittel zur spektralen Zerlegung des von dem Plasma emittierten Lichtes,
• (c) einen Detektor zur Aufnahme des von dem Plasma emittierten Lichtes, und
• (d) Verschlußmittel zum Unterbrechen des Strahlengangs des emittierten Lichtes, wobei
• (e) die Verschlußmittel von einer rotierenden Chopperscheibe gebildet sind, welche eine Öffnung aufweist, durch welche das von dem Plasma emittierte Licht geleitet wird und eine Öffnung aufweist, durch welche ein lichtempfindlicher Hilfsdetektor zur Erzeugung eines Triggersignals zum Triggern des Lasers beleuchtbar ist.

The invention relates to an arrangement for recording emission spectra by means of laser-induced plasma spectroscopy (LIPS)

• (a) a laser for generating a laser-induced plasma in a sample which emits light of different wavelengths,
• (b) means for spectral decomposition of the light emitted by the plasma,
• (c) a detector for receiving the light emitted by the plasma, and
• (D) closure means for interrupting the beam path of the emitted light, wherein
• (E) the closure means are formed by a rotating chopper disc having an opening through which the light emitted from the plasma is passed and having an opening through which an auxiliary photosensitive detector for generating a trigger signal for triggering the laser is illuminated.

at The laser-induced plasma spectroscopy becomes a high-intensity Laser beam blasted onto a sample. The resulting heat dissolves low Quantities of material and produces a plasma from it. This plasma emits light. The light can be detected by spectroscopic methods to be analyzed. From the wavelength spectra can draw conclusions the composition of the sample are drawn. Due to the procedure For plasma generation, the emitted light is very different. Good heat-conducting Samples lead a big one Proportion of heat so that the Plasma and thus the spectrum is formed differently than at less thermally conductive Rehearse. The same applies to Samples with different degrees of absorption.

The Accuracy of the analysis generally improves with the quality of the recorded spectrum. The better the spectral resolution and the signal-to-noise ratio of the Spectrum, the more precise Statements can to be hit.

The Spectrum generated with LIPS varies with time. In a short Laser pulse of less than 1 μs Duration, a plasma is generated that emits light for about 10 microseconds. It will immediately after the laser pulse initially generates a spectrum, the due to the high plasma temperature essentially from a recombination continuum with few, strongly broadened lines exists. After about 1-2μs are located the atoms are in excited states, those under radiation emission return to the ground state or less excited states. This comparatively sharp lines are emitted.

The with a detector, e.g. a CCD line with a time resolution of 1-100ms, Detects detected radiation these temporal changes of the spectrum. Accordingly, an integrated spectrum in which both the spectral continuum and the one on it lying individual lines are to be found. The lines generally have a lower intensity as the continuum.

State of the art

Arrangements with an intensified camera are known. Such a camera includes, for example, a CCD detector with electronics, which is preceded by an electrical amplifier. The amplifier is formed by a so-called multichannel plate (MCP). This is essentially a glass plate having a plurality of tubular channels in which the photoelectrons generated in a transmitted light photocathode arranged in front are accelerated. Similar to a secondary electron multiplier, each electron is amplified up to 10 ⁶ times by applying a voltage. These multiplied electrons are directed onto a phosphorescent coated plate in front of the detector. This in turn generates photons, which arrive at the detector via an optical system, eg an optical fiber plate.

The reinforcement here has two functions: on the one hand, it serves to increase the intensity and for another, she can over the applied voltage very fast down to the ns range be switched on and off.

at Such an arrangement can be a trigger signal, which with the Laser pulse is correlated to be delayed. Then turns on the gain only after this predetermined delay. Until then, the leaves amplifier no photoelectrons. Only then will light become that way directed to the detector when e.g. 1-2μs have passed and from the laser plasma no annoying Continuum is emitted more. The detector can work accordingly work in different sensitivity ranges. Also is it possible make a selection of the time window in which to measure.

The known arrangement is complex and the components used are expensive. Furthermore, reduction of quantum efficiency and the spatial resolution disadvantageous when using an MCP in front of the CCD receiver.

From the LIDAR technique, experimental, non-commercialized arrangements are known in which a chopper disc is used as a shutter to forward the sensitive detector to backscattering radiation from the outgoing laser beam protect. These arrangements are expensive and not suitable for mass production.

The not pre-published DE 10 2004 028 001 A1 discloses an arrangement according to the preamble of claim 1, wherein a chopper disc is used with a rotating opening.

Disclosure of the invention

It is the object of the invention, an arrangement of the aforementioned Kind of creating, with which a fast switching of the measured light allows that is cost effective and can be produced in series.

According to the invention Task solved by that more openings provided in the direction of movement adjacent to the first opening of the chopper disc are and the openings different dimensions in the direction of movement of the chopper disc to have.

By the opening the measuring light beam is off directed to the plasma. Every opening can depend on from the rotation speed of the chopper disk and its size a time window be assigned for that the light beam is transmitted. A large opening lets the light beam for a longer time through, a smaller opening only for a short time Time. At high rotational speeds in the range of several Ten thousand revolutions per minute can be done in this way very much realize short time windows in the μs range.

to Definition of a time window defined relative to the laser pulse Furthermore, a trigger signal is generated. With this trigger signal the laser is triggerable. Light that falls through the opening arrives directly or via optical Means, e.g. Optical fibers applied to a photodetector, e.g. a Photocell. This forms a photoelectric barrier. The photocell generates a electrical signal, which is connected to the electronic circuit of the Lasers is directed and triggers this.

Preferably is the opening, through which the light emitted by the plasma is passed, a other opening, as that through which the auxiliary photosensitive detector can be illuminated is. But it is also possible only one opening to use and to position the auxiliary detector suitable, or a set up appropriate electronic circuit to detect the signal. Also it is possible the openings at the same distance from the axis of rotation to position. It But depending on the construction advantageous, a first opening to Detection of the measuring light beam at a first distance and a second opening to open to arrange the photocell at a second distance. The second opening is located either further in or further out.

On this way will be over the second opening of a defined position of the chopper disc even at very high rotational speeds assigned a defined time. This time scale applies over the Trigger signal for the laser. By suitable positioning of the openings, as well as the positioning of the light barrier to each other, the measurement in a desired Time window done.

In A preferred embodiment of the invention is a delay circuit provided over which the trigger signal for the laser is delayable is. Preferably, the delay is the delay circuit thereby formed adjustable. Then the laser can be earlier or later be triggered. The first opening always happens at the same time as the measuring light beam the common time scale. On the Time at which the laser is triggered then becomes a time window for the Measurement selected.

at the invention are more openings next to the first opening provided in the direction of movement of the chopper disc. The openings have different dimensions in the direction of movement of the chopper disc. So can for example, five openings be provided along a circular arc about the axis of rotation, of which the second opening twice as wide in the direction of movement, the third opening the four times, the fourth opening the eightfold, and the fifth opening the 16 times the width of the first opening having.

at In this embodiment, the laser can always be triggered, that this measuring light is generated exactly at a time at which the desired opening the Beam path opens. In this way, in addition to the measurement time, the measurement duration in Frame of the available standing openings are selected. It is understood that for different measuring tasks also different chopper discs or different rotational speeds can be used.

Preferably are the openings spaced in the direction of movement, wherein the distances are selected so that the of a light pulse generated at a selected rotational speed the chopper disc in any case only passes through an opening.

For the application described, a motor with a stabilized rotational speed in the range above 10 000, preferably above 40 000, revolutions per minute is preferably provided for driving the chopper disc. Such a motor works after a start-up phase se with a very constant rotational speed, so that the selected time windows have small time errors.

In In a further embodiment of the invention, the chopper disc rotates in a narrow air gap. This will cause excessive noise due to the Air friction avoided. An elaborate one Vacuum arrangement can be omitted.

Preferably is the chopper disc to achieve steep switching edges on one Place positioned in the beam path where the Meßlichtbündel is particularly tight constricted, e.g. at the location of the entrance slit of the spectrometer.

refinements The invention are the subject of the dependent claims. An embodiment is explained in more detail below with reference to the accompanying drawings.

Brief description of the drawings

1 shows the time course of a LIPS signal, which is integrated over all wavelengths.

2 shows the spectral course of a LIPS signal at an early measurement time.

3 shows the spectral course of a LIPS signal at a late measurement time.

4 shows a schematic LIPS measurement setup.

5 shows a chopper disc with multiple openings for measuring a LIPS signal over different time windows.

6 illustrates the time course of the measuring phases with a chopper disc 5

7 shows an alternative chopper disc, in which the opening for the measuring beam also represents the trigger window.

Description of the embodiment

In 1 is the wavelength-integrated, time course of a typical LIPS signal shown. The intensity has one 10 designated curve. In the first 1 to 2 μs a high intensity is emitted. After that, the intensity decreases sharply.

The spectral distribution of the intensity is temporally variable. An excerpt from a typical spectrum at an early stage 12 and later 14 is in 2 and 3 shown. Most of the light is from a recombination continuum 16 educated. Furthermore, there are spectral lines 17 . 18 . 20 and 22 which, however, are much weaker than the continuum.

In contrast, the spectrum shows at a later date 14 virtually no continuous radiation, but includes predominantly lines. The analytically interesting part is often seen in the spectral lines. It is therefore desirable to blank out the continuous radiating portion of the first time after plasma formation.

The has the advantage that the Dynamics of the detector better for the lines can be exploited.

The LIPS arrangement in 4 includes a laser 24 for generating a strong laser pulse 28 , This laser pulse is applied to the sample to be analyzed 26 directed and produces a plasma. The light emitted by the plasma 30 is using appropriate optical means 32 . 34 in a spectrometer 36 , for example, a high-resolution echelle spectrometer, passed and then with a CCD matrix 38 detected as a spatially resolving detector.

The laser pulse is triggered by a trigger signal 40 triggered. The trigger signal 40 is from a controller 42 via a delay circuit 46 controlled with adjustable delay. Between the entrance gap 35 of the spectrometer 36 and the imaging optical system 34 is a rotating chopper disc 44 arranged. The chopper disc 44 is in 5 shown in detail.

The chopper disc 44 is driven by a motor with particularly high smoothness and stability even at high speeds. Such an engine is known, for example, from Maxxon Motors. It is small and can reach speeds in the range of up to 60,000 revolutions per minute. The chopper disc rotates in the direction of the arrow 52 around an axis 50 ,

In the chopper disk 44 are openings 54 . 56 . 58 . 60 and 62 with different width 64 provided in the direction of movement. The openings are the same height 66 and the same distance 68 to the axis of rotation 50 , The opening 54 is twice as wide as the opening 56 , The opening 56 is twice as wide as the opening 58 , The opening 58 is twice as wide as the opening 60 , The opening 60 is twice as wide as the opening 62 , At a speed of 40,000 revolutions per minute is the time window for the radiation through the opening 62 is passed through, 1 μs. The openings 54 . 56 . 58 . 60 and 62 are at the height of entry gap 35 arranged. Accordingly, the beam path in the spectrometer 36 released or blocked by the openings.

Another opening 70 is provided further inside in the chopper disc. light 69 from the light source 71 the photocell, which falls through this opening, is placed on the photodetector 72 the photocell is routed ( 4 ). In the figure, the light barrier is arranged directly on the chopper disc. However, it is also possible to direct the light via optical fibers to another location.

The photodetector 72 the photocell generates an electrical signal 74 that is connected to the delay circuit 46 is forwarded. The delay circuit generates a trigger signal 40 with a selected delay for the laser pulse. The radiation emitted by the plasma generated by the laser pulse 30 falls onto the chopper disk. Radiation from the plasma (measuring radiation) that falls on the chopper disk during a period of time to which an opening in front of the entrance slit is located is detected.

The described arrangement can be adjusted by adjusting the delay of the trigger signal 40 as to the signal 74 Select when the laser pulse is generated and the plasma is generated. In other words, it is possible to select when the desired measuring radiation 30 to arrive at the chopper disc. In this way, the opening can be selected, through which the measuring radiation in the spectrometer 36 should arrive. This selects the time window and the time of the measurement.

The selectable time window is based on 6 described in more detail. The measured intensity is plotted over time. The periods 76 . 78 . 80 . 82 and 84 correspond to the periods during which the openings in the chopper disc at the entrance slit 35 pass the spectrometer once each. The signals shown would be obtained with continuous illumination and one revolution.

The measuring signal is not continuous, but has a time course 86 , This is in 6 shown. By selecting a suitable delay 90 For example, the signal may be generated to coincide with the chopper disk at a precise time corresponding to a desired time window represented by one of the openings. With 88 the case for a delay is shown in which the last part of the measurement signal is being measured. The intensive first part 96 does not fall in the time window 78 and will not be measured. By setting a slightly larger delay 90 would the time window 78 chosen so that the middle range of the signal is detected.

The Arrangement allows the selection of the duration of the measurement, as well as the beginning of the measurement.

In 7 is the simplest case of a chopper disc 100 shown. The chopper disc 100 has only one opening 102 on. In this embodiment, the opening initially passes the light barrier and provides a signal to the delay circuit. This calculates the time at which the laser must be triggered. Subsequently, it supplies a trigger signal such that the measuring light beam can be detected with the same opening. It is understood that the light barrier is arranged at the same radius as the entrance slit of the spectrometer.

Claims (7)

Arrangement for recording emission spectra by laser-induced plasma spectroscopy (LIPS) comprising (a) a laser ( 24 ) for generating a laser-induced plasma in a sample ( 26 ), which light ( 30 ) of different wavelengths, (b) means ( 36 ) for the spectral decomposition of the light emitted by the plasma, (c) a detector ( 38 ) for receiving the light emitted by the plasma, and (d) sealing means ( 44 ) for interrupting the beam path of the emitted light, wherein (e) the closure means ( 44 ) are formed by a rotating chopper disc which has an opening ( 54 ), through which the light emitted by the plasma is passed, and an opening (FIG. 70 ), by which an auxiliary photosensitive detector ( 72 ) for generating a trigger signal ( 40 ) is illuminated for triggering the laser, characterized in that (f) further openings ( 56 ; 58 ; 60 ; 62 ) in the direction of movement ( 52 ) next to the first opening ( 54 ) of the chopper disc ( 44 ) and (g) the openings ( 54 ; 56 ; 58 ; 60 ; 62 ) different dimensions ( 64 ) in the direction of movement ( 52 ) have the chopper disc. Arrangement according to claim 1, characterized that the opening, through which the light emitted by the plasma is conducted, another opening than that through which the auxiliary photosensitive detector can be illuminated is. Arrangement according to Claim 1 or 2, characterized in that a delay circuit ( 46 ) is provided, via which the trigger signal ( 40 ) is delayable for the laser. Arrangement according to claim 3, characterized in that the delay of the delay circuit ( 46 ) is adjustable. Arrangement according to one of the preceding claims, characterized in that the openings ( 54 ; 56 ; 58 ; 60 ; 62 ) are spaced in the direction of movement, wherein the distances are chosen so that the light emission generated by a laser pulse at a selected rotational speed of the chopper disc in each case only passes through an opening. Arrangement according to one of the preceding claims, characterized characterized in that a Motor with stabilized rotational speed in the range above from 10,000, preferably above 40,000 revolutions per minute is provided for driving the chopper disc. Arrangement according to claim 6, characterized that the chopper disc rotates in a narrow air gap.