JPH0718762B2 - Absorption spectroscopy analyzer using tunable laser - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention performs optical path noise suppression modulation that removes the influence of a parasitic spectrum when measuring an absorption spectrum using a wavelength tunable semiconductor laser in a gas analyzer or the like. An absorption spectroscopy analyzer.

[Background of the Technology] A spectroscopic analyzer such as a gas analyzer that measures an absorption spectrum to determine a component concentration of a substance requires a variable wavelength light source for sweeping a certain spectral region.

By the way, lead salt compound semiconductor lasers, GaAlAs lasers, GaAl
With AsP lasers, the oscillation wavelength changes to some extent when the applied current is changed, and the oscillation output is remarkably monochromatic among many types of lasers, and its half-value width is the Doppler width of a normal temperature gas. It is narrower than.

Therefore, when these semiconductor lasers are used as the variable wavelength light source, the absorption spectrum of gas can be measured with extremely high resolution. However, in the spectroscopic analyzer using the semiconductor laser as the variable wavelength light source, as shown in FIG. 1, the semiconductor laser 1 is combined with a very small amount of light that partially returns from the optical elements 3 to 5, and The intensity depends on the wavelength.

Therefore, when the laser wavelength is swept, a parasitic spectrum of a periodic wave shape generated based on light of different wavelengths output at the same time is superimposed on the true absorption spectrum, and it is attempted to measure the size of a particularly weak absorption line. When this happens, it becomes a serious obstacle that is almost undetectable. The ratio of magnitude between the amplitude of the parasitic spectrum of this periodic wave shape and the transmitted laser power may be 10 ^{-2 to} 10 ^-7 or less, but it is difficult to predict parameters other than the period, and It is also difficult to suppress by statistical processing.

There are various causes of the generation of the parasitic spectrum, but as shown in FIG. 1, in the absorption spectroscopic analyzer, in order to make the most effective use of the laser output, the lens systems 3 and 4 are used for the photoelectric converter 5 The laser light is focused on the laser beam, and what is generated by the cavity formed between the laser and the condenser of the lens 4 has the largest amplitude and is serious. That is, for example, if the length L of the cavity is 1 m, the period of the parasitic spectrum is 0.005 cm ^{-1 in} wave number, and the Doppler width of methane gas at a temperature of 300 K is 0.00
There is a problem that it is difficult to distinguish it from 2 cm ^-1 .

By the way, since the absorption line width of liquid, solid, atmospheric pressure or high-pressure gas which is generally treated by spectroscopic analysis is wider than about 0.1 cm ^-1 , it is possible to suppress the above parasitic spectrum.

Such a problem is peculiar when a coherent light source such as a laser is used, and hardly occurs in a spectroscopic analyzer using a normal white light source.

Next, the influence of the above-mentioned periodic parasitic spectrum when the derivative spectroscopy is further used will be described. FIG. 2 is an explanatory diagram thereof. Driving current I (t) of the semiconductor laser 6
Is modulated by the modulator 8 by the oscillator 7 having the frequency f, and is represented by I (t) = I _o + I _a η (t) (1) Where I _o is the DC component, I _a is the current modulation width, and η
(T) is a modulation waveform, and a periodic function of a sine wave or a cosine wave is generally used. Therefore, the light output from the semiconductor laser 6 is modulated and its frequency ν (t)
Is expressed by ν (t) = ν _o + Δ · η (t) (2) as shown in the following equation and FIG. Here, ν ₀ is a reference frequency and Δ is a frequency modulation width.

The modulated laser light passes through the gas to be measured in the cavity 9, is detected by the photoelectric converter 10, and is converted into an electric signal.

The phase-locking detector 11 detects the n-th harmonic component of f out of the signal output from the photoelectric converter 10 in a phase-locked manner. In the illustrated example, the 2f signal extracted from the oscillator 7 is 2f.
The second harmonic component is synchronously detected. 2 obtained at this time
The spectrum based on the second harmonic component is the original spectrum,
That is, it represents a changed portion in the zero-order spectrum, for example, a sharply pointed spectrum shape such as an absorption spectrum of gas is emphasized. Since this uses the derivative of the zero-order spectrum approximately, that is, the derivative,
Called derivative spectroscopy.

When such a derivative spectroscopy method is used, the presence of the above-mentioned periodic parasitic spectrum is also emphasized, which makes spectroscopic measurement more difficult.

[Object and Features of the Invention]

An object of the present invention is to provide a means for suppressing a periodic shape parasitic spectrum having a spectrum period shorter than the width of an absorption line generated as optical path noise in a spectroscopic analysis using a wavelength tunable semiconductor laser and a derivative spectroscopy. Therefore, in the absorption spectroscopy analyzer using the wavelength tunable laser of the present invention, the signal of the first waveform that changes with a relatively large amplitude and a long period changes to a signal with a relatively small amplitude and a short period or changes randomly. It is characterized in that it is provided with a modulation means for performing wavelength modulation of the laser light by using a modulation signal in which a signal of the second waveform is superposed, and suppresses optical path noise.

[Principle of Invention]

When the oscillation frequency of the wavelength tunable semiconductor laser is conventionally modulated by, for example, a sine wave, as shown in FIG. 3, the sine wave has a gradual wave near its apex (ν _o + Δ, ν _o −Δ). , And the amplitude density distribution of the frequency in that part is extremely large. The present invention was made by paying attention to the fact that, when the modulation waveform includes a section having a large amplitude density distribution, the responsivity to the periodic shape parasitic spectrum having a pitch shorter than the absorption spectral line width is enhanced. .

The present invention thereby eliminates the gradual changes in the modulation waveform of the semiconductor laser.

More specifically, in general, the absorption spectrum ρ
The spectrum obtained when (ν) is measured using a spectroscope Is Is represented. Where g (η) is called the device function,
Represents the spectral characteristics of the spectroscope. If the equation (3) is further transformed by Fourier transform, then (d) = S (d) G (d) (4). d is equal to 1 / ν, and S and G are respectively Is a Fourier transform of. Here, the periodic interference spectrum having a short pitch is represented as a line spectrum having a large d.

By changing the modulation waveform η (t), the shape of g (η), and hence the distribution of G (d), changes, but g (η)
Is the amplitude density distribution over one period of η (t), In the vicinity of, the absolute value becomes extremely large, and g (η) has a spike-like shape there. Therefore, the Fourier transform of this G (d) has amplitude even in the high d region,
It will respond to the short pitch jamming spectrum. In contrast, As small as possible in the time domain near
By sharpening the point near the extreme value of (t) as sharply as possible, G
The amplitude in the high d region of (d) is reduced, and sensitivity to such an interference spectrum is suppressed.

As an example, in FIG. 4, when the modulation waveform is changed from the sine waveform (a) to the triangular wave (b) and the folded hyperbolic sine (sinh) waveform (c) in the second harmonic spectroscopy (n = 2), The change of each g (η) is shown.

It can be seen that the infinitely extending spike at η = ± 1 in Fig. 4 (a) is within a finite value in Fig. 4 (b), and the height of the step is suppressed in Fig. 4 (c). . In the present invention, when the ramp signal δ (t) is further added to the equation (2), the shape of g (η) is divided into a plurality of spike portions, and further, this is made into a pseudo random signal, so that spikes and steps are generated. Both can approach a smooth shape. That is, the Fourier transform G (d) is suppressed in amplitude in the high region of d.

Example of Invention

Next, some modulation waveforms of an exemplary embodiment of the present invention are shown.

FIG. 5 (a) is a signal obtained by superimposing a signal having a ramp-like signal inclination with a small amplitude and a short cycle as an additional signal δ (t) on the modulation waveform represented by the equation (2). , The total number of modulation waveforms ν (t) is represented by ν (t) = ν _o + Δ · η (t) A + δ (t) (5). This method is used when the wavelength is swept by digital control, for example, when the central wavelength remains constant for each data point, that is, when the wavelength density distribution does not have a sharp shape near a specific wavelength. It is convenient.

FIG. 5 (b) shows a pseudo random signal (coarse change signal) having a small amplitude added to the waveform of FIG. 5 (c).
It is superimposed as (t), and good results can be obtained.

The methods shown in (a) and (b) of FIG. 5 can be carried out redundantly, and the effect is additively improved.

〔The invention's effect〕

As described above, according to the present invention, it is possible to suppress a parasitic spectrum having a pitch shorter than the absorption spectrum by simply devising the modulation waveform of the wavelength tunable laser, and to measure the absorption spectrum analyzer at low cost. The accuracy can be greatly improved.

[Brief description of drawings]

FIG. 1 is a schematic diagram of an absorption spectroscopy analyzer using a semiconductor laser, FIG. 2 is an explanatory diagram of derivative spectroscopy, FIG. 3 is an explanatory diagram of a modulation frequency of semiconductor laser output light, and FIG. 4 is the present invention. FIG. 5 is an explanatory view of the modulation waveform of the embodiment of the present invention. In the figure, 6 is a semiconductor laser, 7 is an oscillator, 8 is a modulator, and 9
Is a cavity, 10 is a photoelectric converter, 11 is a phase-locked detector,
ν _o is a reference frequency, Δ is a frequency modulation width, η (t) is a modulation waveform, and δ (t) is an additional signal.

Claims (1)

[Claims] 1. An absorption spectroscopic analyzer using a wavelength tunable laser, which changes with a relatively large amplitude and a long period.
Is provided with a modulation means for performing wavelength modulation of laser light by using a modulation signal obtained by superimposing a second waveform signal that changes with a relatively small amplitude and a short period or randomly changes on the signal of the waveform An absorption spectroscopic analyzer using a wavelength tunable laser.