JPS6237827B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a frequency modulation method of a laser light frequency in a system for detecting air pollutants and the like using a laser method.

Laser-based air pollution monitoring systems are usually the first
As shown in the figure, for example, several
A retroreflector r is installed on a long optical path with a gap of 100 m, and radiation infrared rays are absorbed as they travel back and forth through pollutant gases such as sulfur dioxide gas (SO ₂ ) and carbon monoxide (CO) floating on this long optical path. The concentration of the pollutant gas (hereinafter simply referred to as gas unless otherwise specified) is determined from its characteristics, and its outline will be briefly explained. Observation device 20 in Figure 1
Infrared radiation to the outside comes from a wavelength-tunable semiconductor diode laser (hereinafter simply referred to as a laser) DL.
Light having a power of Po, such as infrared light, is reflected by the front surface 2 of the internal mirror of the Cassegrain lens 1 to radiate it in the optical path, and the incident light is reflected by a retroreflector r placed far away and returned. The light is reflected by the front surface 3 of the outer mirror and the rear surface 4 of the inner mirror of the same Cassegrain lens 1 and is incident on the light receiving element D, and the result of light absorption by the gas that occurs when the light travels back and forth at this time is recorded by the recorder 5. However, in this case, the light reception power Pr by the light receiving element D is given by Pr=Po exp {−α(ν) CL}f(t) (1). Here, α(ν) is the absorption coefficient of the gas as a function of the laser beam frequency ν, C is the concentration of the gas, and L is the round trip optical path length of the laser beam. In addition, the optical chopper CH in Fig. 1 is the light receiving element D.
The electrical output signal from the
In order to process the infrared light emitted from the laser DL in advance by the signal processing system 21 consisting of the second lock-in amplifiers LA ₁ and LA ₂ and the divider 7,
It is intended to continue at a frequency of Hz. The dashed line A indicates the first lock-in amplifier from the chipper CH.
A reference signal transmission path to LA ₁ is formed, and therefore, a voltage Er corresponding to the received light power Pr is output from the first lock-in amplifier LA ₁ . By the way, the optical absorption spectrum of a gas is the absorption curve shown in Figure 2a, with the optical frequency ν on the horizontal axis and the received light power Pr on the vertical axis. Due to the disturbance, the absorption value m of the curve fluctuates, making observation difficult. Therefore, by differentiating both sides of equation (1) with respect to ν and normalizing it using equation (1) above, we get Pr'/Pr=Po'/Po-CLα'...(2) and eliminate the disturbance coefficient f(t). can. This is the differential measurement method mentioned above, but since α' has both positive and negative values, the differential result of the curve in Figure 2a, ie (Pr'/Pr), exhibits two peaks as shown in Figure 2b.N Since the curve is in the shape of a letter, hereinafter, Fig. 2 a will be referred to as a simple absorption curve, and Fig. 2 c will be referred to as a differential absorption curve. Note that Pr', Po', and α' in equation (2) are the differential values of Pr, Po, and α in equation (1), respectively, with respect to the optical frequency ν.

In order to perform such differential measurement, conventionally, Figure 2a
When driving the laser DL with a direct current shown as I ₀ in the figure in order to emit light with a frequency ν ₀ corresponding to the inflection point O of the absorption curve from the laser DL, the source of the current, i.e. Power supply 8 in Figure 1
Inside, an alternating current I _AC of minute amplitude having peak values _{I 1 and} I ₂ is superimposed on the current I 0, thereby changing the light emission frequency ν from the laser DL from ν ₁ to ν ₂ .
It changed the frequency to a certain level, creating a kind of frequency modulation. The second lock-in amplifier _LA2 in _FIG . ₂ operates in a differential mode, and therefore a differential output voltage E'r appears at the output of the amplifier _LA2 , which corresponds to the differential value Pr' of the received light power Pr. This differential output voltage E'r and the first lock-in amplifier mentioned above
Since the output voltage Er (corresponding to the received light power Pr) from LA ₁ is subtracted by the subtracter 7, the value of (Pr'/Pr) in Fig. 2b, which corresponds to Er'/Er, is obtained by the recorder 5. recorded. Note that in FIG. 1, the diode laser is simply indicated by the symbol DL for convenience of explanation, but the actual diode laser element is part of a circulating refrigerator that forms the inner cylinder of the dewar (not shown). It is necessary to supply and circulate high pressure helium gas into the dewar from a compressor, also not shown, to cool the diode laser element.

As described above, the contaminant gas concentration C can be easily determined from the size of the peak n of the differential absorption value (Pr'/Pr) curve in Figure 2b. In addition to the need to extremely stabilize the amplitude of the alternating current I _AC to be superimposed on the drive current I ₀ , it is also necessary to provide such a highly accurate alternating current in the power supply 8 in FIG. Such a high-precision AC current generator is extremely expensive, and also requires a type of addition circuit to superimpose such a minute current I _AC on the DC current I ₀ , which makes the inside of the laser drive power supply 8 extremely complicated. There was a drawback of doing so.

The present invention has been made in view of these drawbacks, and it does not superimpose a highly accurate sinusoidal current I _AC on the diode laser drive current I ₀ , but instead allows the compression and expansion of a cryogen such as high-pressure helium gas supplied from a compressor. Based on the thermal cycle, the laser
The temperature of the DL is changed periodically, thereby frequency modulating the emission frequency ν from the laser DL, which will be described in detail below with reference to the drawings.

FIG. 2 shows a frequency modulation method of the laser light frequency ν according to the present invention, and the same symbols are attached to the same parts as in FIG. 1.

First, the diode/laser element DL is installed in the cold head H in the dewar DU, which constitutes a part of the circulating refrigerator, so as to have good thermal conductivity. In this way, a refrigerant such as high-pressure helium gas is supplied and circulated from the compressor CO. Compressor of high pressure helium gas in the above dewar
When compressed by CO, the temperature of the cold head increases; conversely, when the gas expands, the temperature decreases.
These temperature increases and decreases in the cold head are periodically brought about by the extremely regular operation of the compressor CO, and these temperature increases and decreases are directly transmitted to the laser element DL. The emission frequency ν of the laser element DL is the same as that of the diode element.
Since it directly depends on the temperature of DL, if the diode element DL is supplied in advance with a direct current I ₀ that provides a reference optical frequency ν ₀ from the power supply 8,
The light emission frequency ν of the diode element DL changes accurately from ν ₁ to ν ₂ with this optical frequency ν ₀ as the center, and therefore accurate frequency modulation can be performed on the optical frequency ν ₀ . This laser diode
The light (infrared rays) from the DL is emitted in the direction of the arrow, but Chiyotsupa CH plays the role of intermittent this light. Signal processing system 21 from the chipotop channel
The transmission path of the first reference signal to the first lock-in amplifier LA ₁ (not shown) is indicated by arrow A, but the path to the second lock-in amplifier LA ₂ (not shown) in the same signal processing system 21 is indicated by arrow B. The second reference signal to be transmitted is thus extracted using a pressure transducer T attached to the compressor CO. In this way, the first lock-in amplifier LA ₁ outputs an output voltage Er corresponding to the received light power Pr of the light incident on the light receiving element D via the optical path Y, while the second lock-in amplifier
A differential output voltage Er' corresponding to the differential value Pr of the received light power Pr is extracted from LA ₂ , and this is output from the signal processing system 21 and recorded by the recorder 5, so that it is shown in FIG. 2b above. The same differential absorption curve will be obtained.

If the optical frequency modulation method according to the present invention described above is used, there is no need for a highly accurate minute alternating current generator to be superimposed on the driving direct current of the diode laser corresponding to the reference optical frequency. Since the circuitry surrounding the light projector in the observation device is greatly simplified, great practical effects can be expected.

[Brief explanation of the drawing]

Fig. 1 is a system diagram showing an example of a measuring device for atmospheric pollutant gas concentration, Fig. 2 a and b are diagrams showing absorption curves and differential absorption curves of a light absorption spectrum obtained by the measuring device, and Fig. 3 1 is a system diagram showing the relationship between an optical frequency modulation device based on an optical frequency modulation method according to the present invention and devices related thereto; FIG. CH...Chiyotsupa, CO...Compressor, DL...Diode laser, D...Photodetector, H...Cold head, T...Pressure transducer, W...Cryogen supply circulation path, F...Radiation optical path, Y...Incoming optical path, 5
...Recorder, 8...DC power supply, 21...Signal processing system.

Claims (1)

[Claims] 1. In a frequency modulation type light emitting system that provides a predetermined frequency deviation with respect to a reference light frequency emitted from a diode/laser element installed in the cold head of a circulating refrigerator, the reference light is applied to the diode/laser element. While supplying a direct current corresponding to the frequency, a temperature change cycle based on compression and expansion of the refrigerator is made to correspond to a frequency deviation to be given with respect to the reference optical frequency, and a frequency deviation is given by the temperature change. An optical frequency modulation method characterized by: