JPH0417366B2 - - Google Patents

Description

[Detailed description of the invention] B Industrial application field The present invention relates to Fourier transform spectrophotometers.

B. Prior Art A Fourier transform spectrophotometer uses a sampling signal for sampling the photometric output of the main beam and an interference output of a laser beam incident parallel to the main beam to control the moving speed of the scanning mirror. However, the interference output of this laser beam contains noise due to various causes, and is particularly sensitive to the influence of slight external vibrations.

The figure shows one embodiment of the present invention, and the above-mentioned points will be specifically explained with reference to this figure. In this example, the Fourier transform spectrophotometer is constructed by a Michelson interferometer. 1 is a beam splitter, 2 is a fixed mirror 3 is a scanning mirror, 4 is a main beam source, and 5 is a laser. The principal beam F and the laser beam L are parallel. D1 is a light-receiving element that detects changes due to interference in the principal beam, and D2 is a light-receiving element that detects interference between laser beams. The main beam is infrared light, and the laser beam is visible light whose wavelength is shorter than the wavelength of the main beam. The interference output of the laser beam is waveform-shaped and used as a sampling signal of the interference output of the principal beam. Furthermore, since the interference output of the laser beam is also a scale of the amount of movement of the scanning mirror in units of 1/2 of the wavelength of the laser beam, it is used to control the movement speed of the scanning mirror. The interference output of this laser beam includes not only noise caused by electrical causes but also noise caused by mechanical vibration caused by the movement of the mechanism when driving the scanning mirror and noise caused by external vibration. Noise due to mechanical causes is caused by mechanical disturbances that induce free vibrations of the scanning mirror, fixed mirror, frame of the interferometer, etc., and this vibration changes the interference state. The one with the frequency is dominant.

The noise of the interference output of the laser beam described above shifts the phase of the sampling signal, causing measurement errors. Conventionally, countermeasures against these noises include making the interferometer structurally more robust and improving the environment in which the device is installed (vibration isolation, blocking electrical influences, etc.)
As a result, both the equipment price and the equipment installation cost were high.

C. Purpose The present invention aims to reduce noise in laser beam interference output from a different perspective from conventional noise countermeasures, and to improve the S/N ratio of laser beam interference output without causing an increase in price compared to conventional devices. With the goal.

D. Configuration The present invention extracts laser beam interference output only when the scanning mirror is moving at a speed near a specific speed, and a band whose center frequency is the frequency of the interference output at the specific moving speed of the scanning mirror. This is a Fourier transform spectrophotometer that extracts the laser beam interference output through a pass filter.

E. Example The figure shows an example of the present invention. Since I have already explained some of it, I will omit that part of the explanation. The output of the laser beam interference detection light receiving element D2 is amplified by a preamplifier A1, passed through a bandpass filter A, then amplified by a main amplifier A3, and converted into a pulse signal by a waveform shaping circuit A4. After the output of the light-receiving element D1 for detecting interference of the main beam is amplified by the amplifier A5,
A sample hold circuit to which the pulse signal obtained by A4 is applied as a sampling pulse.
The sampled signal input to SH is A/
The signal is digitized by the D converter AD and input to the data processing circuit C. The pulse period of the pulse signal obtained by A4 is detected by a pulse period detection circuit K, the pulse period detected by a comparator circuit Cp is compared with a reference period, and the motor is driven so that the difference between the two becomes 0. The control circuit DR is controlled, and the motor M that drives the scanning mirror 3 is driven by the DR. The difference output of the other comparator circuit Cp is the other comparator circuit.
Cp' is compared with a reference value, and only when the difference output of Cp is less than this reference value, the gate G is opened and the sampling pulse is sent to the sample-and-hold circuit SH. With this configuration, the sampling operation of the interference output of the main beam is prohibited before the scanning mirror starts the scanning movement and has not yet reached a predetermined speed.

The center frequency of the bandpass filter A2 is matched to the frequency of the laser beam interference output at a normal moving speed of the scanning mirror 3. If the interferometer is made to have a certain degree of structural strength, even if it is affected by normal external vibrations when the scanning mirror is stopped, the optical path difference between the two beams of the interferometer due to the vibrational deflection of the interferometer will be reduced. The change in can be less than half the wavelength of the laser beam. Therefore, in such a case, the output of the light-receiving element D2 shows a waveform change in which the free vibration frequencies of various modes of the structure of the interferometer are superimposed. Since this waveform is considered to be an important component of noise due to mechanical causes, the frequency of the laser beam interference output obtained by it is determined by the normal moving speed of the scanning mirror, which is caused by the external action when the interferometer is stationary. If the frequency component of the change in beam interference output can be selected to be different from any of the frequency components (this is a design problem), noise due to mechanical causes can be almost completely removed by the action of filter A2. .

Effect The Fourier transform spectrophotometer of the present invention has the above-described configuration, and the scanning mirror is controlled so that the laser beam interference output has a constant frequency, and the laser beam interference output is extracted through a filter of a specific frequency. Only the noise at frequencies near that frequency will be affected as noise, reducing the total amount of noise and enabling highly accurate measurements.
Moreover, since it is realized by a configuration on an electric circuit, there is almost no increase in the price of the device.

[Brief explanation of drawings]

The drawing is a block diagram showing the configuration of an apparatus according to an embodiment of the present invention. 1...Beam splitter, 2...Fixed mirror, 3...
...Scanning mirror, 4...Primary flux light source, 5...Laser,
D1... Light receiving element for main beam interference detection, D2... Light receiving element for laser beam interference detection, A2... Bandpass filter, A4... Waveform shaping circuit, SH...
...Sample hold circuit, C...Data processing circuit, K...Pulse period detection circuit, Cp, Cp'...Comparison circuit, G...Gate, DR...Motor drive control circuit, M...Scanning mirror drive motor .

Claims (1)

[Claims] 1 In a configuration in which the interference of a laser beam parallel to the main beam is detected and used as a sampling pulse of the interference output of the main beam, there is a control means for driving the scanning mirror at a constant speed, and the interference output of the laser beam at the scanning speed is A Fourier transform spectrophotometer that extracts the laser beam interference output through a bandpass filter whose center frequency is .