JPH0815225B2 - Dual wavelength oscillation laser device - Google Patents

Description

The present invention relates to a two-wavelength oscillation laser device capable of generating laser light containing two wavelength components.

(Prior Art) It is known that the presence or absence of gas can be detected by utilizing the fact that a laser beam of a specific wavelength is easily absorbed by a certain type of gas, and sensing technology applying this principle is used for industrial measurement, pollution monitoring, etc. Widely used. As an example, He-Ne
In the 3.39 μm band of the laser light generated by the laser, there are two oscillation lines with vacuum wavelengths of 3.3922 μm (λ ₁ ) and 3.3912 μm (λ ₂ ), λ ₁ is strongly absorbed by methane, and λ ₂ is methane. Is absorbed only slightly. Therefore, it is possible to detect the presence or absence of methane with high sensitivity by using laser light including these two wavelength components. Since methane is the main component of city gas, the leak of city gas can be detected by detecting methane gas.

A methane leak sensor of this type has already been proposed in US Pat. No. 4,489,239, which is shown in FIG.
A sensor system using two He-Ne lasers with vacuum wavelengths of 3.3922 μm and 3.3912 μm is disclosed. In this sensor system, two wavelengths of laser light from two He-Ne lasers are modulated with different frequencies by using two mechanical choppers and emitted. After passing through the atmosphere,
Of the sensor output that receives the laser light, the component synchronized with each frequency is detected by the lock-in amplifier,
The presence or absence of methane is detected from the ratio of the outputs.

As another method, if one mechanical chopper is used and the two wavelengths are adjusted to the same output by repeating passing and reflection or shifting the two wavelength beams by the diameter of the hole of the chopper, the sensor By detecting the frequency component of the output synchronized with the chopper with the lock-in amplifier, it is possible to detect the presence or absence of methane from the difference between the outputs of the two wavelengths.

Since the methane detection system having such a configuration uses a large number of mirrors and half mirrors, not only the optical system is complicated and requires a large volume, but also the optical axis adjustment is troublesome, and the loss of laser light is large. In addition, the optical axis may be deviated over time due to vibration, temperature change, or the like. In addition, since the signal processing is complicated, high-frequency modulation cannot be performed due to the operational limit of the mechanical chopper, which is disadvantageous in terms of SN ratio. Further, in the former case, in addition to the two lasers, two choppers and two lock-in amplifiers are required, so that the device becomes large-scale. In the latter case, it is difficult to adjust the two wavelengths to the same output, and there is a possibility that the output of the two wavelengths may be unbalanced over time due to temperature changes and the like.

A sensor system which eliminates some of the drawbacks of the sensor system using the two He-Ne lasers is proposed in FIG. In this sensor system, a He-Ne plasma tube is placed in a resonance cavity composed of three mirrors, and a piezoelectric element is attached to one of the two mirrors forming the Fabry-Perot interferometer.
Two wavelengths are alternately switched and generated by oscillating at .mu.m.

If this two-wavelength oscillation laser is used, the chopper becomes unnecessary and the configuration is simple. However, it is conceivable to insert a methane cell in the resonator to make the outputs of the two wavelengths equal, but since there is no feedback mechanism, The wavelength outputs cannot be completely equalized, and the balance may be lost over time due to temperature changes and the like. There is also a problem that the output cannot be increased because the resonator length is limited.

On the other hand, as mentioned in column 9, lines 56 to 62 of the above US patent, JEBjorkholm et al., “Improve
d Use of Grantings in Tunable Lasers "
Gratings by installing a half mirror just before s
The reflectivity of the resonator is improved, the efficiency of the resonator is increased, and the output is increased.
Mirror resonators are mentioned, but half mirrors and Grat
It is like a Farby-Perot with ings, and the reflectance does not increase unless the position of the half mirror is controlled on the wavelength order. On the contrary, it is possible that the reflectance becomes zero.

By the way, the above wavelength of 3.39μ is easily absorbed by methane.
In addition to the m band, there are 1.33 μm band and 1.67 μm band. on the other hand,
It is known that a semiconductor laser used for communication emits a laser beam of 1.3 μm band. Therefore, considering the use of this semiconductor laser for methane leakage detection, it is known that the laser light output and oscillation wavelength of the semiconductor laser change as shown in FIGS. 4 and 5 depending on the drive current. There is. Therefore, the drive current of the semiconductor laser
By alternately changing I ₁ and I ₂ , two different wavelength laser lights can be oscillated.

(Object and Structure of the Invention) The present invention has been made in view of the above points, and an object of the present invention is to propose a small-sized, high-output, high-modulation-frequency, two-wavelength oscillation semiconductor laser, and to achieve this object. , A laser that oscillates a laser beam having a wavelength and an output according to a drive current is used, and the laser is modulated with two different drive currents around a predetermined current to oscillate a laser beam having two different wavelengths. A certain absorber or reflector is used to feed back the drive current so that the modulation frequency component of the laser output becomes zero.

(Example) The present invention will be described below with reference to the drawings. The two-wavelength oscillation laser device exemplified below is described for methane detection, but it goes without saying that the present invention is not limited to this.

FIG. 1 shows an embodiment of a two-wavelength oscillation semiconductor laser device according to the present invention for the purpose of detecting methane. 1 is a GaYsInP semiconductor laser, and 2 is a collimator for condensing laser light generated from the semiconductor laser 1. A lens 3 is a methane cell containing methane gas, 4 is a beam splitter which is arranged in the optical axis of the laser beam and splits the laser beam that has passed through the methane cell 5, 5 is an optical sensor such as a PIN photodiode, 6 is synchronized with the frequency of the oscillator 7. The lock-in amplifier for detecting the changing output component of the optical sensor 5, 8 is an integrator for integrating the output of the lock-in amplifier 6, and 9 is the semiconductor laser 1 based on the output of the integrator 8 and the output of the oscillator 7. A current control circuit for controlling the driving current of the semiconductor laser 1, 10 drives the semiconductor laser 1 with two different currents centering on the modulation center controlled by the current control circuit 9. Chromatography The drive circuit, 11 is a well-known temperature control circuit for controlling the temperature of the semiconductor laser 1 based also on the output of the integrator 8.

Now, the semiconductor laser 1 is supplied with a current I ₀ by the laser drive circuit 10.
When driven by I ₁ and I ₂ (I ₂ > I ₁ ) centering on, the semiconductor laser 1 emits laser light of wavelengths λ ₁ and λ ₂ .
This laser light is condensed by the collimator lens 2 and then passes through the methane cell 3. However, the wavelength λ ₂ component is absorbed by the methane cell in the methane cell 3 and the total output decreases, and if the methane pressure is properly selected, the wavelength λ ₁ component Can be approximately equal to the output of.

Now, the beam light outputs P (I) of the wavelength λ ₁ component and the wavelength λ ₂ component whose outputs are almost equal in this way are split into P _A (I) and P _B (I) by the beam splitter 4 and split. The laser light output component P _A (I) can be expressed by the following equation.

P _A (I) = A · G (I) exp {−α (I) cl} where A is a constant, G (I) is the laser oscillation output, α
(I) The absorption coefficient of methane, c is the methane pressure in the methane cell 3, l is the length of the methane cell 3 (hence cl is equivalent to the amount of methane in the methane cell), and the absorption coefficient α is as shown in FIG. Is a function of the drive current I.

Now, the current I is centered around an appropriate value I ₀ , ΔI, and frequency f
When the light is vibrated slightly, the divided light output component P _A (I) can be expressed by the following equation.

Here, only the modulation frequency component (that is, the f component) of the optical output component P _A (I) is detected by the lock-in amplifier 6, and the output is used as an error signal to change the center I ₀ of the current so that it becomes ₀ . Give feedback to let them know. That is, The current center I ₀ is controlled so that

The optical sensor 5, the lock-in amplifier 6, the integrator 8, and the current control circuit 9 shown in FIG. 1 constitute a feedback circuit. A part of the laser light output is detected by the optical sensor 5, and the modulation frequency component among them is detected. Is detected by the lock-in amplifier 6, integrated by the integrator 8, and the current control circuit 9 causes the drive current to flow between I ₁ and I ₂ at the oscillation frequency of the oscillator 7 centered on the bias current I ₀ of the laser drive circuit 10. Modulate with. When the modulation frequency component disappears due to the feedback effect, the integrator 8 holds the bias current I ₀ of the laser drive circuit 10 by the integrated value held by the integrator 8 at that time, and oscillation is continued.

When the oscillation wavelength of the semiconductor laser 1 changes due to temperature change or the like during operation, the drive current can be slightly changed and corrected by the current control circuit 9 based on the integrated value of the integrator 8, but based on the integrated value. The temperature control circuit 11 can make a large correction. However, since the light output control by the temperature control is usually performed and is not the gist of the present invention, further description will be omitted.

Semiconductor laser 1 feedback-controlled in this way
Laser light P _B (I) of wavelengths λ ₁ and λ ₂ generated from
As shown in the figure, the light is passed through the region K to be inspected, received by the same optical sensor 12 as the optical sensor 5, and the lock-in amplifier 13 detects the modulation frequency component (that is, the f component). The area to be inspected K
If there is no methane gas, lock-in amplifier 13
Output is 0, but when methane gas MG (enclosed by a broken line) exists, the methane pressure of the leaked methane in that region is C, and the leak length is L, the light output after passing through the region K. P _B (I) can be expressed by the following equation.

Therefore, considering the above equation (1), the modulation frequency component of P _B (I) is Therefore, in the low methane amount region of αCL << 1, the light output is proportional to CL. If this modulation frequency component is recorded by the recorder 14, the presence or absence of leakage methane and the leakage amount can be obtained.

In the above embodiment, the methane cell was used to equalize the outputs of the two wavelength laser beams output from the semiconductor laser.
Instead of the methane cell, a dielectric coating having a wavelength characteristic may be applied to both end faces of the laser constituting the resonator of the semiconductor laser.

Although the present invention has been illustrated with respect to the semiconductor laser, the present invention is not limited to this, and can be applied to a laser whose oscillation wavelength changes in an analog manner, such as a dye laser.

(Effects of the Invention) As described above, according to the present invention, a laser that oscillates a laser beam having a wavelength and an output according to a driving current is used, and the laser is modulated by two different driving currents around a predetermined current. And oscillate two different wavelengths of laser light,
Since the driving current is fed back so that the modulation frequency component of the laser light becomes zero, a dual wavelength oscillation laser can be realized with a simple configuration and control.

If a gas detection system is constructed by using the dual wavelength oscillation laser device according to the present invention, since only one laser is required, the number of mirrors and half mirrors used is reduced, the optical system is simplified, the optical loss is reduced, and the optical axis is reduced. The trouble of adjustment is reduced. In addition, since the drive center current of the laser is electrically modulated, it is possible to perform modulation at a higher frequency than that of the conventional mechanical chopper and improve the SN ratio. Further, since it is not necessary to separate and detect two wavelengths, the measurement system (for example, a signal processing system including a lock-in amplifier) becomes extremely simple.

Furthermore, if the laser device according to the present invention is used to guide an oscillated laser beam through an optical fiber, it is possible to detect gas remotely and in a wide area, and its application fields such as gas leak detection and industrial measurement and pollution monitoring are considered to be extremely wide. .

[Brief description of drawings]

FIG. 1 is a schematic diagram of an embodiment of a two-wavelength oscillation laser device according to the present invention, FIG. 2 is a characteristic diagram showing a relationship between a driving current of a semiconductor laser and a methane absorption coefficient α, and FIG. 3 is according to the present invention. FIG. 5 is a schematic diagram of a system for detecting leaked methane using laser light generated by a two-wavelength oscillation laser device, FIG. 4 is a characteristic diagram showing a relationship between a driving current of a semiconductor laser and laser light output, and FIG. FIG. 3 is a characteristic diagram showing the relationship between the drive current of a semiconductor laser and the wavelength of oscillated laser light. 1 ... Semiconductor laser, 3 ... Methane cell, 4 ... Beam splitter, 5 ... Optical sensor, 6 ... Lock enamp,
7 ... Oscillator, 8 ... Integrator, 9 ... Current control circuit

Claims (1)

[Claims] 1. A laser that oscillates a laser beam having a wavelength and an output according to a drive current, a laser drive circuit that modulates and drives the laser with two different current values centered around a predetermined current value, and Output adjusting means for adjusting the outputs of the two wavelength components oscillated by the laser to be substantially equal to each other, and a current control circuit for controlling the predetermined current value of the laser drive circuit so that the modulation frequency component of the laser output becomes almost zero. A two-wavelength oscillating laser device having.