JPS62290190A - Two-wavelength oscillation laser device - Google Patents

Abstract

PURPOSE:To obtain the two-wavelength oscillation laser of simple constitution and control by using a laser which generate laser beams having corresponding wavelengths and output to a drive current and generating the laser beams of two different wavelengths by use of two different drive currents and applying feedback to the drive currents so as to make a modulation frequency component of the laser beams zero. CONSTITUTION:When a semiconductor laser 1 is driven by a current I1 and I2 (I2 > I1) with a current 10 as center, laser beams of wavelengths lambda1 and lambda2 are generated from the semiconductor laser 1. The wavelength lambda2 component is absorbed by methane in a methane cell 3 and by selecting a methane pressure properly, it can be made nearly the same as an output of the wavelength lambda1 component. A part of the laser beam output is detected by an optical sensor 5 and a modulation frequency component of that is detected by a lock-in amplifier 6 and is integrated by an integrator 8. A current control circuit 9 modulates the drive current between I1 and l2 at an oscillation frequency of an oscillator 7. When the modulation frequency component becomes zero by a feedback effect, a bias current I0 of a laser drive circuit 10 is retained and the oscillation is continued.

Description

Detailed Description of the Invention 3. Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a dual wavelength oscillation laser device that can generate 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 that applies this principle may be used in industrial measurement, pollution monitoring, etc. Widely used. - As an example, He-N
The 139pm band of laser light generated by the e-laser has vacuum wavelengths: 1.3922μm (in) and 3.3912μm.
There are two oscillation lines of μm (λ2): the ray is strongly absorbed by methane, and the ray is only slightly absorbed by methane.

Therefore, it is necessary to detect the presence or absence of methane with high sensitivity using a laser beam containing these two wavelength components. Since methane is the main component of city gas, leakage of city gas can be detected by detecting methane gas.

This type of methane leak sensor is described in U.S. Patent No. 4,489゜2.
No. 39, this U.S. patent's ff'
The s1 diagram shows the vacuum wavelength of 3.3922 p-m and 3.3922 p-m. :19
A sensor system using two 1ie-Ne lasers of 12 JLm is disclosed. In this sensor system, two mechanical choppers are used to modulate and emit two-wavelength laser beams from two 1le-Ne lasers at different frequencies, and after passing through the atmosphere, the laser beams are received. Among the sensor outputs, components synchronized with each frequency are detected by a lock-in amplifier, and the presence or absence of methane is detected from the ratio of the outputs.

Another method is to use one mechanical chopper and adjust the output of the two wavelengths to be equal by changing the transmission and reflection or by shifting the beams of the two wavelengths by the diameter of the chopper hole. By detecting the frequency component of the sensor output that is synchronized with the chopper using a lock-in amplifier, the presence or absence of methane can be detected based on the difference between the outputs of two wavelengths.

Since a methane detection system having such a configuration uses a large number of mirrors or half mirrors, the optical system is complicated and requires a large volume, making it extremely difficult to adjust the optical axis and causing a large loss of laser light. Also, it is possible that the optical axis may shift over time due to vibrations, temperature changes, or the like. In addition, signal processing is complicated, and high frequency modulation is not possible due to operational limitations of mechanical choppers, which is disadvantageous in terms of S/N ratio. Furthermore, in the former case, in addition to the two lasers, two choppers and two lock-in amplifiers are required, resulting in a large-scale equipment.

In the latter case, it is difficult to adjust the outputs of the two wavelengths to be equal, and there is a risk that the outputs of the two wavelengths may become unbalanced over time due to temperature changes or the like.

A sensor system that overcomes some of the drawbacks of the sensor system using the two He-Ne lasers described above is proposed in FIG. 4 of the US patent. This sensor system places a He-Ne plasma tube inside a resonant cavity made up of three mirrors, and uses a Fabry-Perotinterfer
A piezoelectric element is attached to one of the two mirrors that make up the intestine, and the piezoelectric element is vibrated at about 1 gm to alternately switch and generate two wavelengths.

If this dual-wavelength oscillation laser is used, a chopper is not required, making the configuration simpler, or a methane cell may be inserted into the resonator to equalize the output of the two wavelengths, but since there is no feedback mechanism, Since the wavelength output is completely equal, there is a risk that the balance will be lost over time due to temperature changes, etc. Another problem is 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 U.S. patent, the paper "I+*proved Use of Gran" by J. E. Bjorkhol-et al.
tings 1nTunable La5ers", does it mention a 3-mirror resonator that improves the reflectance of the gratings by installing a half mirror just before the gratings, increases the efficiency of the resonator, and increases the output? F with half mirror and Grating S
It is similar to arby-PeroL, and the reflectance cannot be increased unless the position of the half mirror is controlled on the wavelength order. Conversely, it is also possible that the reflectance becomes O.

By the way, the wavelengths that are easily absorbed by methane are the above 3:1.
In addition to the 9JLm band, there are also the 113ALm band and the 1.67gm band. On the other hand, there are 1. : It is known that it generates laser light in the lpm band. Considering the use of this semiconductor laser to detect methane leakage, it is known that the laser light output and oscillation wavelength of the semiconductor laser change depending on the driving current as shown in Figures 4 and 5. ing. Therefore, by alternately changing the driving current of the semiconductor laser as 11 and I2, it is possible to oscillate laser beams of two different wavelengths.

(Objective 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 compact, high-output, high-modulation-frequency dual-wavelength oscillation semiconductor laser, and to achieve this object. To,
It uses a laser that oscillates laser light with a wavelength and output that corresponds to the driving current, and modulates the laser with two different driving currents around a predetermined current to oscillate laser light with two different wavelengths. The structure is such that feedback is applied to the drive current using an absorber or a reflector so that the modulation frequency component of the laser output becomes zero.

(Example) The present invention will be described below based on the drawings. Note that although the two-wavelength oscillation laser device illustrated below will be explained as being for methane testing, it goes without saying that the present invention is not limited thereto.

FIG. 1 shows an embodiment of a two-wavelength oscillation semiconductor laser device according to the present invention aimed at detecting methane, where 1 is a GaAs 1nP semiconductor laser, and 2 is a semiconductor laser ln.
a collimating lens that condenses the laser light generated from the 3
is a methane cell containing methane gas, 4 is a beam splitter placed in the optical axis of the laser beam and splits the laser beam that has passed through the methane cell 3, 5 is an optical sensor such as a PIN photodiode, and 6 changes in synchronization with the frequency of the oscillator 7. 8 is an integrator that integrates the output of the lock-in amplifier 6; 9 is a semiconductor laser l based on the output of the integrator 8 and the output of the oscillator 7;
10 is a laser drive circuit that drives the semiconductor laser l with two different currents around the modulation center controlled by the current control circuit 9;
1 is a well-known temperature control circuit that controls the temperature of the semiconductor laser 1 based on the output of the integrator 8.

Now, the semiconductor laser l is connected to a current I by the laser drive circuit lO.
. When driven by 11 and I 2 (I 2) l I), the wavelength enters and enters from the semiconductor laser l. A laser beam is generated. After this laser light is focused by the collimating lens 2, it passes through the methane cell 3 or the wavelength input 2.
The component is absorbed by the methane cell in the methane cell 3, reducing the total output.If the methane pressure is appropriately selected, the input wavelength can be made almost equal to the output of the component.

Now, the beam light output P(1) of the input wavelength component and the wavelength λ2 component whose outputs have become almost equal is split into PA(1) and PA(i) by the beam splitter 4, and the split laser light The output component P A(+) is calculated using the following formula% formula% where A is a constant, G(1) is the laser oscillation output, α(1
) is the absorption coefficient of methane, C is the methane pressure in the methane cell 3, and the intersection is the length of the methane cell 3. Therefore, cl corresponds to the amount of methane in the methane cell), and the absorption coefficient α is determined by driving as shown in Figure 2. It is a function of the current ■.

Now set the current I to an appropriate value I. ΔI centered on .

When micro-vibrated at frequency f, the divided optical output component P
A(1) can be expressed by the following formula.

exp(−(X(+)Cl) −△I 5in2π
ft + higher-order component Here, only the modulation frequency component (that is, f component) of the optical output component PA (1) is detected by the lock-in amplifier 6, and the output is used as an error signal to adjust the center of the current I so that it becomes 0. . Apply feedback to change the In other words, the current center should satisfy ■. control.

The optical sensor 5, lock-in amplifier 6, integrator 8, and current control circuit 9 shown in FIG. The bias current I of the integrator drive circuit 10 is detected by the lock-in amplifier 6. Drive current at the oscillation frequency of oscillator 7 centered on ■8,■
It modulates between 2. When the modulation frequency component disappears due to the feedback effect, the current control circuit 9 controls the bias current (2) of the laser drive circuit 10 based on the integral value held by the integrator 8 at that time. is maintained and oscillation continues.

If the oscillation wavelength of the semiconductor laser l changes due to a temperature change during operation, the drive current can be corrected by slightly changing the drive current using the current control circuit 9 based on the integral value of the integrator 8; Therefore, the temperature v control circuit 11 can make a significant correction. However, this optical output control by temperature control is normally performed and is not the gist of the present invention, so further explanation 11 will be omitted.

Laser beams PQ(1) of wavelengths 1 and 2 generated from the semiconductor laser l subjected to feedback control in this manner are passed through the test area K as shown in FIG. 12 receives the light, and a lock-in amplifier 13 detects the modulation frequency a component (ie, f component). If methane gas does not exist in the test region K, the output of the lock-in amplifier 13 is 0, or if methane gas MG (indicated by a broken line) exists, the methane pressure of leaked methane in that region is If C1 leakage length is L, area K
The optical output P a (1) after passing through can be expressed by the following equation.

Pa(1)= B −G(1)exp (−a(1)(
c l +CL) )exp(-Q(1) (C sentence
+CL)) -Δ I sin2w fL+higher-order component Then, considering the above equation (1), the modulation frequency component of pa(r) becomes (C sentence + CL))・△I, and αCL(low methane of 1) In the quantity domain, the optical output is proportional to CL.If this modulated frequency component is recorded by the recorder 14, the presence or absence of leaked methane and the amount of leakage can be obtained.

In the above embodiment, a methane cell was used to equalize the output of the laser light of two wavelengths output from the semiconductor laser, but instead of the methane cell, a dielectric material with wavelength characteristics is used on both end faces of the laser that constitutes the resonator of the semiconductor laser. A coating may be applied.

Although the embodiment 11 is exemplified with respect to a 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 explained above, in the present invention, a laser that oscillates a laser beam with a wavelength and output depending on a drive current is used, and the laser is modulated with two different drive currents around a predetermined current. Since the structure is configured such that laser beams of two different wavelengths are oscillated and feedback is applied to the drive current so that the modulation frequency component of the laser beam becomes 0, a two-wavelength oscillation laser can be realized with a simple configuration and control.

If a gas detection system is configured using the dual wavelength oscillation laser device according to the present invention, only one laser is required, so fewer mirrors and half mirrors are used, simplifying the optical system, reducing optical loss, and reducing the optical axis. The hassle of adjustment is reduced. Furthermore, since the laser drive center current is electrically modulated, it is possible to modulate at a higher frequency than in conventional mechanical choppers, thereby improving the S/N ratio. Furthermore, since there is no need to separate and detect two wavelengths, the measurement system (for example, a signal processing system including a lock-in amplifier) becomes extremely simple.

Furthermore, by using the laser device according to the present invention and guiding the oscillated laser beam through an optical fiber, it becomes possible to detect gases remotely and over a wide area, and its application fields are considered to be extremely wide, including gas leak detection, industrial measurement, and pollution monitoring. .

[Brief explanation of the drawing]

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 the relationship between the drive current of the semiconductor laser and the methane absorption coefficient α, and FIG. 3 is a diagram according to the present invention. A schematic diagram of a system for detecting leaked methane using laser light generated by a two-wavelength oscillation laser device. Figure 4 is a characteristic diagram showing the relationship between semiconductor laser drive current and laser light output. Figure 5 1 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-in amplifier, 7... Oscillator, 8... Integrator, 9...
Current control circuit patent applicant Tokyo Gas Co., Ltd. Agent Patent attorney Hiroshi Suzuki

Claims (1)

[Claims] a laser that oscillates laser light with a wavelength and output that corresponds 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;
an output adjustment means for adjusting substantially equal outputs of two wavelength components oscillated by the laser; 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 approximately 0. A dual wavelength oscillation laser device characterized by having: