JP2970700B2 - Fourier transform spectroscopy using a pulsed light source - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Fourier transform spectroscopy that obtains an analysis spectrum of a sample by taking out an interferogram using an interferometer and, more particularly, to a Fourier transform spectroscopy using a pulse light source as a light source. .

[0002]

2. Description of the Related Art FIG. 7 is a diagram for explaining a conventional example of Fourier transform light spectroscopy, and FIG. 8 is a diagram showing signal waveforms of the circuit shown in FIG.

In a rapid scan type Fourier transform optical spectroscopy represented by a Michelson interferometer, as shown in FIG. 7, a two-beam interferometer including a semi-transmissive mirror 22, a fixed mirror 23, and a movable mirror 24, a light source 21, and A detector 25 is used.
Then, the light from the light source 21 is split into two light beams by the semi-transmissive mirror 22 of the two-beam interferometer, and one is guided to the fixed mirror 23 side and the other is directed to the moving mirror 24 moving at a constant speed. An interferogram (FIG. 8B) based on the difference is detected. The interferogram is sampled by the AD converter 28 using a trigger signal having a period τ ₀ generated by an interferometer as shown in FIG. 8C, as shown in FIG. Converted to obtain an analytical spectrum. Since the sample is not directly related here, the description is omitted.

In the above-described Fourier transform optical spectroscopy of the rapid scan method, it has been required that the intensity of the light source is constant and the light is continuous in time. That is, as shown in FIG. 8A, the light source 21 emits light having a constant intensity and is continuous over time, and the light incident on the interferometer is such that the movable mirror 24 moves at a constant speed. Is modulated to a frequency proportional to the wave number of light, and is received by a detector.

[0005] Therefore, there is a problem that the light source to be used is restricted, and since the light is continuously irradiated, the target is limited to a sample which is not affected by the continuous irradiation.

Under such circumstances, the present applicant has already used a pulsed light source as a light source and a sampling method in Fourier transform spectroscopy in which an interferogram is taken out using an interferometer and Fourier transform is performed to obtain an analysis spectrum of the sample. A Fourier transform using a pulsed light source characterized by obtaining an analysis spectrum of a sample by emitting a pulse light having a shorter cycle than the cycle, extracting an interferogram of a low frequency component from a detector output, sampling and performing a Fourier transform. A spectroscopic method was proposed (Japanese Patent Application No. 2-82126).
issue). Since this method uses a pulsed light source as the light source,
A SOR light source or a pulsed laser-excited Raman sample can be used as a light source. In addition, the time for irradiating the measurement target with light can be reduced and the amount of light to be irradiated can be reduced. The Fourier transform spectroscopy can be applied to a measurement object where continuous irradiation or the like is not preferable, and the application object is expanded.

In addition, the present applicant has proposed a background-removed Fourier transform spectroscopy in which strong background light or fluorescence in Raman spectroscopy is removed from the Fourier transform spectroscopy using a pulsed light source as described above (Japanese Patent Application No. 2002-214,878). Hei 3-95115). This method extracts a signal only while the pulsed light source or the Raman excitation pulsed laser is emitting light, extracts a component of the signal in a predetermined frequency range, performs sampling and Fourier transform, thereby converting the spectrum of the measurement light. Therefore, strong background light and fluorescence in Raman spectroscopy can be removed, and weak Raman light and the like can be accurately measured.

[0008]

However, the above-mentioned proposal of Japanese Patent Application No. 2-82126 proposed by the present applicant is that the light emission cycle of the pulse light source is shorter than the sampling cycle of the interferogram, that is, the pulse theorem is determined by the sampling theorem. It is assumed that the light emission cycle of the light source is shorter than the reciprocal of twice the maximum frequency f _max of the interferogram signal (this is equal to or longer than the sampling cycle of the interferogram). Assuming that the light emission period of the pulse light source is τ, the above is τ <1 / 2f _max ,
Alternatively, it means that f _max <1 / 2τ was assumed.

The present invention has been made in view of such circumstances, and an object of the present invention is to perform the above-described Fourier transform spectroscopy using a pulse light source according to the present applicant's proposal, and to determine whether the light emission cycle of the pulse light source is an interface. If the maximum frequency f _max of the program signal is longer than the reciprocal of twice, that is,
An object of the present invention is to make it applicable even when the light emission cycle of the pulse light source is longer.

[0010]

For this purpose, Fourier transform spectroscopy using a pulse light source according to the present invention obtains an interferogram from a detector output using a rapid scan interferometer, and performs Fourier transform to obtain an interferogram. Fourier transform spectroscopy for obtaining a spectrum, where a pulse light source is used as a light source, and a modulation frequency by an interferometer is f and a frequency of the pulse light source is 1 / τ, m / 2τ <f <(m + 1) / 2
In Fourier transform spectroscopy applied when a detector output signal exists only at τ (m is a positive integer), the detector output signal is multiplied by a sine waveform signal having a frequency 1 / τ synchronized with the emission of the pulse light source. After that, a spectrum of the measurement light is obtained by performing a Fourier transform.

[0011]

In the Fourier transform spectroscopy using a pulse light source according to the present invention, when a pulse light source is used as the light source and the modulation frequency of the interferometer is f and the frequency of the pulse light source is 1 / τ, m / 2τ <f < (M + 1) / 2τ (m is a positive integer)
, A pulsed light source whose emission cycle is longer than the reciprocal of twice the maximum frequency f _max of the interferogram signal,
That is, spectrum spectrometry can be performed using a pulse light source having a longer light emission cycle. Therefore, more light sources can be used as pulse light sources.

Further, as in the premise method of the present invention, since a pulse light source is used as a light source, an SOR light source, a pulsed laser-excited Raman sample, or the like can be used as a light source, and the object to be measured is irradiated with light. Since the time can be shortened and the amount of light to be irradiated can be reduced, Fourier transform spectroscopy can be applied to measurement objects affected by light irradiation or measurement objects where continuous irradiation is not preferable. It spreads.

In addition, the method of the present invention can be easily implemented without significantly changing the configuration of the measuring device, as compared with the method presupposed by the present invention.

[0014]

As described above, in the Fourier transform spectroscopy using the pulse light source previously proposed by the present applicant, the modulation frequency f of the spectrometer is such that the light emission frequency of the pulse light source is 1 / τ (τ: pulse light emission). (Period) has been handled only in a range smaller than half. On the other hand, the present invention deals with a case where a signal exists only in the range of m / 2τ <f <(m + 1) / 2τ (m is a positive integer).

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a basic configuration of a Fourier transform spectrometer for performing one embodiment of Fourier transform spectroscopy using a pulse light source according to the present invention. In FIG. 1, 1
Is a pulse light source, 2 is an interferometer, 3 is a sample (in this case, the sample 3 is a transmission sample, but may be a reflection sample, a scattering sample, etc.), 4 is a detector, 5 is a preamplifier, and 6 is 7 is a lock-in amplifier, 8 is a timer, 9 is a light source power supply, 10 is a main amplifier, 11 is an AD converter, 1
2 indicates a computer. The timer 8 includes an AD converter 11
A signal having a constant period τ is supplied to the light source power supply 9 asynchronously with a trigger signal (a sampling signal for A / D-converting an interferogram and taking it into a computer to perform a Fourier transform), and a cosine signal synchronized with the signal is used as a synchronization signal. This is given to the lock-in amplifier 7. The pulse light source 1 is a pulse laser or the like that emits a pulse light having the same intensity each time a signal from the timer 8 is excited by the light source power supply 9.
adiation) A light source or a pulsed laser-excited Raman sample may be used. When the intensity of the light source fluctuates, the intensity may be monitored to standardize the detector output. The low-pass filter 6 extracts a low-frequency component (f <1 / 2τ, as described later) of the spectrum obtained from the preamplifier 5 and converts it into an analog signal. Further, the lock-in amplifier 7 synchronizes the analog signal extracted by the low-pass filter 6 with the cosine signal of the period τ from the timer 8, thereby canceling the phase component of the analog signal and changing the frequency of the analog signal. The signal is converted into an inherent frequency and an original analog interferogram signal is output. The AD converter 11 is for sampling an analog interferogram obtained through the lock-in amplifier 7 by a trigger signal having a period τ ₀ .

Therefore, the output from the detector 4 is input to the main amplifier 10 via the low-pass filter 6 and the lock-in amplifier 7, and is sampled by the AD converter 11 for Fourier transform. By the way, if the minimum and maximum of the frequency (interferogram frequency) at which the spectrum of the measurement light is modulated by the interferometer 2 are fmin and fmax, τ> m / 2fmin (a) τ <(m + 1) / 2fmax The light emission of the pulse light source 1 is repeated at a period τ that satisfies the condition (b). Here, m is a positive integer. That is, this is a case where the spectrum of the measurement light exists only in the range of m / 2τ <f <(m + 1) / 2τ. FIG. 2A shows a case where m = 1. This modulation frequency f is represented by f = 2vσ, where σ is the wave number of the spectrum (1 / λ when the wavelength is λ) and v is the moving speed of the movable mirror of the interferometer 2, so that the above condition is satisfied. As described above, the emission spectrum region (wavelength range) from the light source 1 may be limited by an optical filter, or the moving speed of the movable mirror may be adjusted to satisfy the above condition. One of the features of this spectrometry is that it is not necessary to synchronize the repetition frequency 1 / τ of the light source 1 with the movement of the movable mirror, as can be seen from the above-mentioned conditions.

Now, the principle of spectral separation using an apparatus having such a configuration will be described below. The light periodically emitted from the light source 1 as shown in FIG.
(T) is represented by a comb function Шτ (t) indicating a repetitive operation arranged at equal intervals of time τ, and the transmission spectrum of sample 3 is represented by T
Assuming that (σ), the output signal F (x, t) from the detector 4 is as shown in equation (1).

F (x, t) = Шτ (t) ∫T (σ) B (σ) cos2πx σ dσ (1) where x is the optical path difference of the interferometer 2, σ is the wave number of the spectrum, B (σ) represents a background spectrum when the sample 3 is removed from the sample chamber of the FT-IR / spectrometer section. FIG. 3B shows an approximate state of the output signal of the interferometer by a solid line. Note that x is a time variable t ′ and x =
Although there is a relationship of 2vt ', since the light emission of the light source 1 is not synchronized with the movement of the movable mirror of the interferometer 2, t and t' have no phase correlation. That is, the phase amount Δt = t′−t between t and t ′ takes a different value each time the movable mirror scans. Therefore, the expression (1) indicates that the interferogram shows a different value each time the movable mirror scans.

The integral part of the equation (1) is an interferogram (analog interferogram) of the spectrum T (σ) B (σ) to be measured. In the entire equation (1), the analog signal is Δτ (t) ) Is sampled discrete data (digital interferogram).
Since the variables t and x are asynchronous in the sampling, the position where the interferogram is sampled is different each time the movable mirror scans.

In order to know the output from the detector 4 in detail, it is necessary to perform a Fourier transform on Шτ (t) in the equation (1) at a time t ′ correlated with the movement of the movable mirror, and to examine the spectrum of the signal. To

∫Шτ (t) exp (−i2πft ′) dt ′ = (1 / τ) exp (−i2πΔtf) × Ш1 / τ (f) · (2) = (1 / τ) ｛δ (f) + exp (-i2πΔt / τ) δ (f-1 / τ) + ・ ・ ・ ・ + exp (i2πΔt / τ) δ (f + 1 / τ) + ・ ・ ・｝ (2 ') (2) Is a comb function with a phase term. The spectrum obtained when the Fourier transform of the integral part of the equation (1) is performed with respect to time is obtained by using the modulation frequency f as a variable, excluding the coefficients.
(f) B (f). Therefore, the entire expression (1), which is the output signal from the detector 4, is the zero-order term δ of the expression (2 ′).
In addition to the measurement target spectrum T (f) B (f) obtained from (f), exp (-i2πΔt / τ) δ (f
-1 / τ), exp (i2πΔt / τ) δ (f + 1 / τ)
Are obtained from the carrier frequency n / τ (n
(An integer indicating the order). Here, m / 2τ <f <(m +
From the condition of 1) / 2τ, the spectra of the side bands do not overlap. This is shown in FIG. The numbers written in the respective spectra in this figure indicate that they are sidebands of the carrier frequency of the same number.

In the case of the present invention, the output from the detector 4 is a low-pass filter 6 having the characteristics shown in FIG.
As shown in FIG. 4B, when m = 1, only the spectra of numbers 1 and -1 are extracted.
At this time, the low-pass filter 6 outputs an analog signal having a spectral component to which the +1 order term and the −1 order term of the equation (2 ′) contribute. Then, in the lock-in amplifier 7, this signal is multiplied by cos2πt / τ. In order to check the spectrum of the signal after the multiplication, Fourier transform is performed on cos2πt / τ at a time t ′ correlated with the movement of the movable mirror.

∫cos2πt / τ exp (-i2πft ') dt' = ∫ ｛exp (i2πt / τ) / 2 + exp (-i2πt / τ) / 2｝ exp (-i2πft ') dt' = 1/2 ｛ exp (−i2πΔt / τ) δ (f−1 / τ) + exp (i2πΔt / τ) δ (f + 1 / τ)｝ (3) The output of the lock-in amplifier 7 is calculated based on the convolution theorem. The spectrum is expressed by the +1 order term and -1 of the equation (2 ′).
Spectral components contributed by the next term (Fig. 4 (B)) and (3)
Is the convolution of the equation, but the Delak function δ
Since (t) is simply an action of shifting the coordinates, the positive region of the spectrum contains the +1 order term (exp (−i2πΔt / τ) δ (f−1 / τ)) of the equation (2 ′). -1 order term (exp
(i2πΔt / τ) δ (f + 1 / τ)) contributes to the phase component (exp) of the first term of the equation (3).
(-i2πΔt / τ)) and a spectrum as shown in FIG. 4C having a phase component obtained by multiplying the frequency by 1 / τ on the positive side. Looking at the phase after multiplication, a component having a frequency higher than 1 / τ based on the +1 order term has a phase component of e
xp (-i4πΔt / τ), which still has a phase component, and takes a different value each time the moving mirror of the interferometer 2 scans. On the other hand, the component whose frequency is lower than 1 / τ based on the -1 order term has a constant value irrelevant to the scanning of the movable mirror since the phase component is canceled and no longer has the phase component.

The spectrum of the component based on the -1 order term is given by the following equation (4).

F (x) = (1 / τ) ∫T (σ) B (σ) cos2πx σ dσ (4) Equation (4) is an interface of an analog shape of the spectrum of the measurement light transmitted through the sample 3. This is a program independent of time t. That is, a digital signal as shown in FIG. 3B sampled by the light source 1 is restored to an analog signal as shown in FIG. 3C through the low-pass filter 6 and the lock-in amplifier 7. Since this has the same form as the interferogram obtained by the normal FT-IR spectrometer measurement, the period τ ₀ of the interferometer 2 provided by the AD converter 11 is the same as the processing in the normal FT-IR spectrometer. Is sampled with the reference signal (FIG. 3 (D)) and Fourier-transformed by the computer 12, thereby obtaining a spectrum T (σ) B (σ). In this case, tau ₀ is the minimum and maximum modulation frequency with the interferogram signal output from the lock-in amplifier 7 f _min, when the _{f max, τ 0> m '} / 2f min, τ 0 <(m' +1) / 2f _max must be satisfied. Here, m 'is zero or a positive integer.

From the above, when m = 1, the spectrum T
Since (σ) B (σ) is obtained, the transmittance spectrum T (σ) can be obtained by taking the ratio with the background spectrum B (σ) obtained in the same manner as above.

In the above description, the component based on the +1 order term in FIG. 4C can be eliminated by appropriately selecting the sampling frequency for the Fourier transform, and the lock-in amplifier 7 in FIG. A low-pass filter for removing this component may be inserted between the amplifiers 10 to remove it.

As described above, according to the Fourier transform spectroscopy using the pulse light source of the present invention, a periodic pulse light source whose emission cycle is longer than the reciprocal of twice the maximum frequency f _max of the interferogram signal, that is, In addition, spectral spectrometry can be performed using a pulsed light source having a longer light emission cycle.

In the above method, the low-pass filter 6 is used to restrict the spectral components shown in FIG. 2B obtained from the detector 4 as shown in FIG. Although the interferogram is restored by inputting the signal to the low-pass filter 7, the interferogram can be restored by synchronizing with the lock-in amplifier 7 even if the low-pass filter 6 is omitted.
In this case, the mathematical description is omitted, but when synchronization is achieved by the lock-in amplifier 7, the spectrum of the distribution as shown in FIG. 2B obtained from the detector 4 is shifted to the positive side by 1 / τ. The result obtained by adding the value shifted by 1 / τ to the negative side is output from the lock-in amplifier 7. And 1
The components shifted between / 2τ and 1 / τ are a -1st order low frequency side component (right side) and a + 1st order high frequency side component (right side). It has the same form as the original interferogram having no phase component. Further, a spectrum having a phase component below 1 / 2τ and above 1 / τ appears. For example, a band-pass filter excluding these components is inserted between the lock-in amplifier 7 and the main amplifier 10 to remove the spectrum. .

Next, when the Fourier spectroscopy of the present invention is applied to Raman spectroscopy, as shown in FIG.
The sample 3 is excited by the pulse laser 13 emitting light at the period τ, and Raman light from the sample 3 is detected by the detector 4 via the interferometer 2.
The output is input to the main amplifier 10 via the preamplifier 5, the low-pass filter 6, and the lock-in amplifier 7 in the same manner as in FIG. 1, and is sampled by the AD converter 11 for Fourier transform. . Also in this case, the emission cycle τ of the excitation pulse laser 13 is fmin, which is the minimum and maximum of the frequency (interferogram frequency) at which the spectrum of the Raman light from the sample 3 is modulated by the interferometer 2.
, Fmax, emission is repeated while satisfying the following condition: τ> m / 2fmin (a) τ <(m + 1) / 2fmax (b) Here, m is a positive integer. That is, the spectrum of the measured Raman light is m
/ 2τ <f <(m + 1) / 2τ.

By the way, the background removal method proposed by the present applicant in Japanese Patent Application No. 3-95115 can also be applied to the method of the present invention. One example is shown in FIG. That is, in this example, in the Raman spectrometer of FIG. 5, a gate circuit 14 is provided between the preamplifier 5 and the low-pass filter 6, and this circuit 14 is opened only at the time of Raman excitation to remove fluorescence accompanying Raman excitation. Is what you do. As to the background removing method, various other modifications and applications are possible as in the case of Japanese Patent Application No. 3-95115, and reference is made to the specification.

Note that the present invention is not limited to the above-described embodiment, and various modifications are possible. For example, instead of the low-pass filter, a method of extracting only a folded spectrum of a high frequency using a band-pass filter having a frequency response characteristic of extracting a first-order or higher-order spectral component having a frequency of 1 / τ or more in FIG. It is possible.
In that case, the frequency of the signal to be synchronized by the lock-in amplifier must be an integral multiple of 1 / τ. Further, Fourier transform spectroscopy using the pulse light source of the present invention can also be applied to time-resolved spectroscopy.

[0033]

As is apparent from the above description, according to the Fourier transform spectroscopy using the pulse light source of the present invention, the pulse light source is used as the light source, the modulation frequency by the interferometer is f, and the frequency of the pulse light source is When 1 / τ, m / 2τ
<F <(m + 1) / 2τ (m is a positive integer), so that the present invention can be applied to the case where the detector output signal exists only at the maximum frequency f of the interferogram signal.
_A periodic pulsed light source longer than the reciprocal of twice _max , ie
Using a pulsed light source having a longer light emission cycle, spectral spectrometry can be performed. Therefore, more light sources can be used as pulse light sources.

Further, as in the premise method of the present invention, since a pulse light source is used as the light source, an SOR light source, a pulsed laser-excited Raman sample, or the like can be used as the light source, and the object to be measured is irradiated with light. Since the time can be shortened and the amount of light to be irradiated can be reduced, Fourier transform spectroscopy can be applied to measurement objects affected by light irradiation or measurement objects where continuous irradiation is not preferable. It spreads.

In addition, the method of the present invention can be easily performed without significantly changing the configuration of the measuring device, as compared with the method presupposed by the present invention.

[Brief description of the drawings]

FIG. 1 is a diagram showing a basic configuration of a Fourier transform spectrometer for performing Fourier transform spectroscopy using a pulse light source according to the present invention.

FIG. 2 is a diagram for explaining an analog interferogram and a digital interferogram and spectra included therein;

FIG. 3 is a waveform diagram for explaining the operation of Fourier transform spectroscopy using the pulse light source according to the present invention.

4 is a diagram showing a frequency response characteristic of a low-pass filter used in the device of FIG. 1, an output spectrum thereof, and an output spectrum of a lock-in amplifier.

FIG. 5 is a diagram showing a main part of a configuration of an embodiment in which the Fourier transform spectroscopy of the present invention is applied to Raman spectroscopy.

FIG. 6 is a diagram showing a configuration of an embodiment in which a background removal method is applied to the apparatus of FIG.

FIG. 7 is a diagram illustrating a configuration of a conventional Fourier transform spectrometer.

8 is a waveform chart for explaining the operation of the device of FIG. 7;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Periodic pulse light source 2 ... Interferometer 3 ... Sample 4 ... Detector 5 ... Preamplifier 6 ... Low-pass filter 7 ... Lock-in amplifier 8 ... Timer 9 ... Light source power supply 10 ... Main amplifier 11 ... AD converter 12 ... Computer 13 ... Raman Excitation pulse laser 14 ... Gate circuit

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G01J 3/00-3/52 G01N 21/65

Claims (1)

(57) [Claims] 1. A Fourier transform spectroscopy method for obtaining an interferogram from a detector output using a rapid scan interferometer and performing a Fourier transform to obtain a spectrum of measurement light, wherein a pulse light source is used as a light source and the interferometer is used. When the modulation frequency is f and the frequency of the pulse light source is 1 / τ, m
/ 2τ <f <(m + 1) / 2τ (m is a positive integer), in a Fourier transform spectroscopy applied when a detector output signal exists only at a frequency synchronized with the emission of the pulse light source to the detector output signal Fourier transform spectroscopy using a pulsed light source, wherein a spectrum of the measurement light is obtained by multiplying by a 1 / τ sine waveform signal and then performing Fourier transform.