JPS5845195B2 - Diode laser emission wavelength control method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for controlling the wavelength variable range to a constant level in an infrared ray gas analyzer, particularly a gas analyzer using a wavelength tunable diode laser as a light source. Using a laser diode, change the emission wavelength by externally controlling the diode current, measure the infrared absorption spectrum in the gas under test that corresponds to the infrared wavelength, and measure the wavelength and absolute value of the absorption peak of that spectrum. It is similar to performing gas analysis using

However, the wavelength range of light emitted from the laser diode fluctuates due to fluctuations in the external current supply source applied to the laser diode, external temperature, and other fluctuation factors, making it impossible to perform accurate gas analysis.

The present invention eliminates the above-mentioned drawbacks by making the diode current waveform in the adjustment cycle for controlling the oscillation wavelength range of the laser diode at a constant Q and the diode current waveform in the measurement cycle used for wavelength scanning during spectroscopic analysis to be the same. It uses a sawtooth waveform to maintain a constant tracking characteristic of current changes (in contrast to changes in light wavelength) and to maintain stable high-precision spectroscopic analysis within a specific wavelength range of the laser beam to be scanned. , the light emission output from the laser diode is introduced into a laser emission wavelength detection device, the detection signal detected by the detection device is introduced into a control device that stores a reference wavelength signal, that is, a reference wavelength characteristic, and the control device,
A deviation between the detected wavelength signal and a reference wavelength signal stored in advance is detected, a diode current regulator is controlled based on the deviation signal, and the current controlled by the current regulator is applied to the diode. By supplying the diode current, the variation range of the diode current is controlled to be constant, and the laser emission wavelength range is kept constant.

Embodiments of the present invention will be described below with reference to the drawings.

In each explanatory diagram, the same functional parts are given the same reference numerals and explained.

FIG. 1 is a block diagram of a diode laser emission wavelength control device according to the present invention, which is composed of a laser diode A, a laser oscillation wavelength detection device B, a control device C, and a current adjustment device.

In such a configuration, the laser beam emitted from the diode A emits most of the light 26 through the beam subrick 25 as a light source for spectroscopic measurement of gas containing contaminants, and a part of the light 27 will be explained in detail later. The light is incident on the wavelength detection device B.

The differential output voltage output from this wavelength detection device B is fed back to the diode current regulator through the control device C, and this regulator outputs a diode current that controls the emission wavelength of the laser diode within the required scanning range. , is supplied to the diode A.

Next, the above laser oscillation wavelength detection device will be explained using FIG. 2 and FIGS. 3A to 3C.

In FIG. 2, 27 is a light beam obtained by branching a part of the light emitted from the laser diode A.

Reference numeral 2 denotes a band-pass type p-wave device for removing stray light from the outside, and a light beam 3 passing through the p-wave device 2 is split by an optical splitter 4 into two light beams 5 and 6 of appropriate intensity.

One beam 5 passes through a wave detector 7 to become a light beam 8 and enters a photodetector 9.

The other light beam 6 passes through the wave generator 10 and becomes the light beam 11.
The light beam 11 further passes through a variable optical attenuator 12 to become a light beam 13 and enters a photodetector 14.

The outputs of photodetectors 9 and 14 are connected to output wiring 15, 16°1
It will be taken out on 7th.

15 is a common wiring, and two outputs appear at 16 and 17.

These outputs are amplified by logarithmic amplifier 18 and output on lines 19 and 20.

The light beam 27 that is extracted by branching a part of the light emitted from the laser diode A described above is one mode selected from several light emission modes of the laser diode A, as shown in FIG. 3a. It has an emission wavelength and an emission intensity, and this relationship is shown in a curve 21.

In the figure, the horizontal axis indicates the emission wavelength, and the vertical line indicates the emission intensity.

In FIG. 3, the transmission i14% curves for the optical wavelength I of the single-wavelength filters 7 and 10 are shown at 22 and 23, respectively.

This filter can be implemented using an interference filter or an Efron.

In addition, these filters have a wavelength range in which the transmittance changes almost linearly with respect to wavelength, and the wavelength ranges widely intersect, so that the wavelength range of curve 21 shown in FIG. 3a is It is selected so that it overlaps with

The wavelength marked at the intersection of the curves 22 and 23 described above is λ.

shall be. The wavelength of the laser beam is now λ. The optical attenuator 12 is adjusted so that the logarithm of the output ratio becomes 0 when .

FIG. 3C shows a calibration characteristic curve 24 in which the laser light wavelength and output ratio are expressed as logarithmic output.

In the figure, the vertical axis represents the logarithm of the output ratio, and the horizontal axis represents the wavelength of light.

Therefore, the output ratio to be measured is the laser emission wavelength λ.

You can see how much it deviates from.

This method is not easily affected by the magnitude of the laser light intensity and can measure the deviation amount of the light wavelength with high reliability.

Next, the operation of the system of the present invention using the laser emission wavelength detection method described in section L below will be explained.

In FIG. 1, the current in laser diode A is directly regulated by diode current regulator D.

The control device C supplies a bias voltage to a diode current controller that controls the laser diode current according to the program shown in FIG. (iii) A time reading circuit that generates a laser beam with a specified wavelength; (Gv) an AD-DA conversion circuit that stores and holds the bias voltage; It includes a combination of known electronic circuits such as a sequential switching system controlled at timings according to a set program.

That is, FIG. 4a shows the DC bias current 11? appearing in the laser diode A. A current waveform in which sawtooth wave currents 28 are overlapped is shown.

It is assumed that the diode has now reached a steady state of operation.

FIG. 4 shows the tracking characteristics of the optical wavelength by the diode current.

Next, using Figure 4a and Figure 4A above, we will explain the operation of the diode current adjustment cycle and measurement cycle.
This will be explained with reference to the figures.

An arbitrary sawtooth wave 29 is selected by an electronic switch of a sequential switching circuit included in the control device C, and this is called an adjustment cycle.

The starting time of the pt cycle is detected as 1 = 0 from the fall of the end point of the preceding sawtooth wave during this adjustment cycle.

, On the other hand, λ after the fall time td of the red wavelength shown in Figure 4 has elapsed.

A circuit capable of detecting the time when tx is given, that is, a time reading circuit that generates a laser beam of a specified wavelength, is operated, this time is converted into a pulse frequency, and the time is counted to know the time tx.

By knowing the time tx as described above, the emission wavelength λ of the diode and the reference wavelength λ are determined by referring to the data of diode current vs. oscillation wavelength characteristics stored in the control device C.

Find the bias amount for adjusting the diode current, store it in the AD-DA conversion circuit, and supply this signal from the control m11 device C to the diode current controller to generate the sawtooth wave. The median of the maximum and minimum emission wavelengths is approximately λ.

(The diode's DC bias current is
.

adjusted to. In this way, the current waveform 30 is obtained.

When the adjustment cycle 29 is completed, the electronic switch is switched and the measurement cycle 31 begins.

After that, after a few cycles, Figure 4 shows 32.
Analytical measurements of contaminants (scanning of the required wavelength range are performed) as shown in FIG.

Meanwhile, the wavelength scanning range is λ.

or λ to set it to λ1.

An electronic circuit may be provided to operate the measurement system only between the time t1 when λ1 is given and the time t1 when λ1 is given.

In the method described below, since the same sawtooth waveform is used for the adjustment cycle and the measurement cycle, it is possible to keep the tracking characteristic of the change in emission wavelength with respect to the change in diode current almost constant, and the adjustment cycle is performed at appropriate time intervals. By electronically switching to and repeating the above-described adjustment of the scanning wavelength range, the wavelength scanning range can be maintained stably and with high accuracy at all times.

Note that since two photodetectors are used in the optical wavelength detection device B in the above method, measurement errors occur as their photoelectric sensitivities change over time.

In order to eliminate this measurement error, a part of the configuration shown in FIG. 2 described above may be improved as shown in FIG. 5.

In FIG. 5, a reflector 33 is used to reflect the light beam 13 into a light beam 34, which, together with the light beam 8, is incident on the same detector 9.

In front of this detector 9 a mechanical light chopper or an electrical light shack 35 and 36 are installed for the light beam 8 and the light beam 34.
By discontinuing each beam in an alternating manner, each beam is separately detected as an electrical pulse output, and further converted into a logarithm of the output ratio.

From now on, it is clear that the emission wavelength of the laser diode can be controlled in the same manner as explained with reference to FIG.

Using this method, it is possible to significantly reduce the influence of aging of the photoelectric characteristics of the photodetector on the accuracy of wavelength detection.

Through the above method, fluctuations in the laser light wavelength due to power supply fluctuations, secular changes in the diode's light emitting characteristics, and other causes are constantly corrected, and the laser diode is controlled to continuously vary the white light within the required range from the reference wavelength. By repeating and stably emitting laser light of the same wavelength, it is possible to perform instant and wide-area spectroscopic analysis of air pollutants.

The control characteristics obtained by this method are insensitive to mechanical vibrations and can provide practically stable operation.

[Brief explanation of drawings]

Figure 1 shows the change in emission wavelength of the diode laser of the present invention 41
Figure 2 is a detailed explanatory diagram of the laser emission wavelength detection device, Figures 3a to 3C are diagrams explaining the procedure for detecting diode laser light, and Figures 4a to 4 5 is a diagram illustrating the process of controlling the diode current and emission wavelength, and FIG. 5 is a diagram illustrating another configuration of the laser emission wavelength detection device. A: Elm diode, B: laser emission wavelength detection device,
C: Control device, D: Diode current regulator, 2, 10,
7: filter, 4, 25: beer, splitter, 9, 1
4: Detector, 18 dilogarithmic amplifier, 33: Reflector, 35.
36: Hikari Shack.

Claims (1)

[Claims] 1 A wavelength tunable diode laser is used as a light source, and a current regulator for wavelength control is connected to the diode laser so as to scan the emission wavelength over a predetermined range, while the emission output of the diode laser is detected and the wavelength is adjusted. An emission wavelength detection device for identifying the emission wavelength is provided, and while changing the emission wavelength by supplying a drive current that changes with a predetermined waveform from the current regulator to the diode laser, the emission wavelength is detected by an output signal from the emission wavelength detection device. The wavelength change range is maintained constant by detecting the point in time when the wavelength reaches a predetermined wavelength within the scanning range, and adjusting the current for wavelength scanning based on the driving current value for the diode laser at that time. A diode laser emission wavelength control method characterized by the following.