JPH0642573B2 - Laser wavelength stabilization method - Google Patents

Description

The present invention relates to a method for stabilizing the wavelength of light from a light source in a spectroscopic measurement device using a laser light source.

In a spectroscopic measurement device that uses a wavelength tunable laser as a light source,
It is desirable to use narrow wavelength spectrally controlled light.

In order to control the wavelength, a wavelength control element may be incorporated in the laser oscillator itself, but normally, the laser oscillator is provided with a single mode oscillation function, and the wavelength is controlled outside the resonator. In this case, the laser beam is often divided into two, one for measuring the sample and the other for controlling the wavelength.

On the other hand, in the present invention, without splitting the laser beam,
This is a method of obtaining a stabilized laser beam having a narrow spectral width by using the dispersion element itself of the light receiving section used for eliminating noise light for wavelength control.

A stabilization method for splitting a normal beam will be described with reference to FIG. 1 for comparison with the present invention.

In FIG. 1, the light from the laser light source 1 having a variable wavelength is divided into two by a beam splitter 2. The measuring light is modulated by the measuring signal modulator 3, passes through the sample 4, and is detected by the signal detector 7 to give a signal output 8.
At this time, the dispersive element 6 is often placed in front of the signal detector 7 in order to eliminate noise light such as stray light or background light. The dispersive element 6 and the signal detector 7 constitute the measurement signal light receiving section 5. Further, the measurement signal modulator 3 is amplitude-modulated by a signal synchronized with the frequency ν _s given by the measurement signal modulation frequency generator 9, and this frequency ν _s is used as a synchronous detection frequency of the signal detector 7. (In addition to this, there are various methods such as modulating the laser light source).

The reference laser light split by the beam splitter 2 passes through the wavelength reference dispersion element 11. The wavelength reference dispersive element 11 is configured to have a property that the transmittance changes with a change in wavelength.

FIG. 1 considers a case where the oscillation wavelength changes depending on the drive current value like a semiconductor laser. The oscillation wavelength is determined by the sum of the bias current from the bias power supply 16 and the modulation current from the modulation power supply 17 (the modulation for the measurement signal is amplitude modulation such as on / off of the current and does not contribute to the frequency modulation). . The modulation frequency ν _M of the modulation power supply 17 is given by the reference signal modulation frequency generator 18.

The laser light frequency-modulated at the modulation frequency ν _M passes through the wavelength reference dispersion element 11 and is then synchronously detected at the modulation frequency ν _M in the reference light detector 12. The wavelength reference dispersive element 11 and the reference light detector 12 serve as a reference signal light receiving unit 10.
Make up. The reference signal output 13 which is the output of the reference light detector 12 is compared with the signal from the standard device 14 by the comparator 15, and the bias power supply 16 is set so that the difference becomes 0 (zero).
And the wavelength is stabilized.

The present invention does not divide the laser light for measurement and reference, but uses the same laser light in common. The embodiment will be described below with reference to FIG.

In FIG. 2, the light from the laser light source 21 driven by the sum current of the modulation power supply 22 and the bias power supply 32 passes through the measurement sample 23 without being split, and is incident on the dispersion element 26 together with the external noise light 25. To do. The dispersive element 26 and the photodetector 27 form a light receiving unit 33. Here, the dispersive element 26 is the dispersive element 6 of the measurement signal photoreceptive section 5 of FIG. 1 and the wavelength reference dispersive element 11 of the reference signal photoreceptive section 10 of FIG.
Both of them have the characteristics, and have the property of changing the transmittance with respect to the change of the wavelength. However, since it is also used for measurement signals, it needs to have good transmittance, and absorption type such as absorption lines of atoms and molecules cannot be used.

The laser light has a frequency ν given by the modulation frequency generator 24.
Since the light is frequency-modulated by _M , the light passing through the dispersive element 26 can take out the modulation component output 29 modulated by ν _M from the detection signal of the photodetector 27 separately from the measurement signal output 28.

As an example, when the first-order differential component of the phase detection of the modulation frequency ν _M is used as the modulation component output 29 (see FIG. 4), the modulation component output 29 of the light passing through the dispersive element 26 is an output proportional to the frequency deviation. Since it indicates the value, the bias power supply 3 is compared by the comparator 31 with the reference value given by the reference device 30.
A feedback signal to 2 is applied to stabilize the oscillation frequency.

Normally, the light from the laser light source 21 is amplitude-modulated by the measurement signal modulator 34 by the frequency signal ν _S given by the measurement signal modulation frequency generator 35, and the photodetector 27
Then, the synchronization component of the frequency ν _S is detected to improve the sensitivity. Further, in this embodiment, the laser light source 21 is such that the oscillation wavelength monotonously changes with respect to the laser driving (excitation) current as shown in FIG. Further, the dispersive element 26 shown in FIG.
It is assumed that it has a narrow band spectral transmission characteristic as shown in FIG.

In FIG. 2, when the light that has been frequency-modulated by the modulation power source 22 at the frequency ν _M enters the dispersive element 26 having the transmission characteristics shown in FIG. 4 (a), the first-order derivative of the frequency ν _M of the transmitted light. The signal is the fourth signal corresponding to the wavelength determined by the bias current value.
The dispersion curve as shown in Figure (b) is given. The difference between the primary differential signal value and the reference value is the deviation. By providing a feedback signal to the bias power supply 32 so that this deviation becomes 0 (zero), the laser light source wavelength is stabilized at a wavelength determined by the reference value.

The present invention has the following advantages.

1) Since the beam is not divided into two, the light quantity can be used 100% effectively.

2) Since there is no distinction between measurement and reference, a single dispersive element can be used for both measurement and wavelength stabilization. As a result, it is not necessary to worry about the deviation of the center wavelengths of the measurement and reference dispersive elements, and a narrow band transmission type dispersive element can be used. As the dispersive element, a prism, a diffraction grating, an interference filter, or a Fabry-Perot etalon can be arbitrarily selected in the range of several 100 cm ^-1 to several KHz depending on the required spectral width.

3) Since a dispersion element having a narrow spectrum width can be used, external scattered light can be easily eliminated.

4) The single light-receiving unit simplifies the device configuration. In addition,
When the wavelength is swept, the central wavelength of the dispersive element (FIG. 26) can be swept by some method.

[Brief description of drawings]

FIG. 1 is a system configuration diagram for explaining a conventional wavelength stabilization method, FIG. 2 is a system configuration diagram for explaining the wavelength stabilization method of the present invention, and FIG. 3 is a driving (excitation) of a laser light source semiconductor element in the present invention. ) A characteristic diagram showing the relationship when the oscillation wavelength is monotonic with respect to the current value. FIGS. 4 (a) and 4 (b) show the wavelength characteristic (a) diagram of the dispersive element and the reference signal for the modulation signal in the present invention. FIG. 7 is a characteristic diagram showing a relationship (b) with a shift.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Masahiro Tsubaki, Inventor, Masahiro Tsubaki, Moriya, Moriya-machi, Kita-Soma-gun, Ibaraki 1 249 Moriya Ko, Meisei Electric Co., Ltd. Moriya Plant (56) Reference Japanese Patent Laid-Open No. 61-30088 (JP, A)

Claims (1)

[Claims] 1. A laser light source with wavelength modulation, a light receiving section having a narrow band dispersion element whose transmittance changes with respect to a change in wavelength of a laser beam passing through a sample space, and an output from the light receiving section. The detection value of the modulated signal is compared with a reference value and consists of a feedback unit that changes the laser operating point by the amount of deviation from the reference value of the detection value, from the laser light source that has passed through the sample space. A laser that fixes the laser oscillation wavelength to a wavelength unique to the narrow band dispersion element of the light receiving section by making all of the laser light incident on the narrow band dispersion element of the light receiving section and eliminates noise light from the outside. Wavelength stabilization method.