JPH06265468A - Spectroscopic method and spectroscopic device - Google Patents

Abstract

(57) [Summary] [Object] To obtain a high-sensitivity spectroscopic device that operates below the shot noise limit for the spectroscopic device by FM spectroscopy that measures the absorption of atoms or molecules. [Configuration] A polarization beam splitter is installed so that the light from the first light source does not enter the sample, and a second light source that oscillates light in synchronization with the reflected light from the polarization beam splitter upon incidence is installed. The light of the second light source is completely incident on the sample. A laser that oscillates in an amplitude squeezed state is used as the injection-locked second light source.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spectroscopic device for measuring absorption of atoms or molecules using laser light.

[0002]

2. Description of the Related Art FIG. 6 is a block diagram showing a schematic configuration of a conventional FM spectroscopic device. 1 is a light source for FM modulation (hereinafter referred to as a master laser), 2 is a DC power source for the light source, and 3 is
Is an AC power source for FM modulation, 4 is a spectroscopic sample, 5 is a photodetector, 6 is an amplifier, 7 is a frequency selective detector, and 8 is a recorder.

A semiconductor laser is used as the light source 1, and a frequency modulation (FM modulation) is performed by superimposing an AC current from the FM modulation AC power supply 3 on an injection current from the light source DC power supply 2 to the light source 1.
Then, the master laser 1 emits FM-modulated beam light. When this beam light passes through the spectroscopic sample 4, its FM modulation component is converted into amplitude modulation (AM modulation) according to the absorption characteristics of the spectroscopic sample 4. The beam light subjected to the AM modulation enters the photodetector 5, is detected, and is converted into a detection current. This detected current is amplified by the amplifier 6,
The amplified detection current is measured as a signal component by the frequency selective detector 7 and recorded in the recorder 8.

As described above, the FM spectroscopy is a method in which the AM signal component FM-AM converted according to the absorption characteristics of the atoms or molecules constituting the sample is detected by the photodetector.

In the conventional FM spectroscopy, an FM-modulated laser beam is used as a light source. Generally, when the laser beam is FM-modulated, its amplitude is also slightly AM-modulated. That is, when performing the FM modulation by superimposing the AC current from the FM modulation AC power supply 3 on the injection current from the light source DC power supply 2, the AM modulation is performed by the amplitude component of the superimposed high frequency current in addition to the FM modulation component. Ingredients are also generated. This AM-modulated component is FM-AM-converted A according to the absorption characteristics of the sample.
Since the frequency is the same as the M signal component, it was observed as a background component regardless of absorption or non-absorption (IE
EE. J. Quantum Electron. QE-18.4. Pp. 582-
(See page 595).

[0006]

However, in the conventional FM spectroscope described above, the unnecessary AM modulation component generated at the time of the FM modulation of the light source has the same frequency as the detected AM signal component, and is therefore measured. Since the background of the AM signal component is pushed up, the shot noise limit, which is the standard quantum limit, cannot be detected, and there is a problem that the detection sensitivity decreases.

The present invention has been made to solve the above problems, and an object of the present invention is to limit the shot noise to or below the shot noise limit in a spectroscopic apparatus by FM spectroscopy for measuring the absorption characteristics of atoms or molecules. An object of the present invention is to provide a highly sensitive spectroscopic device capable of performing measurement at room temperature.

The other objects and novel features of the present invention will become apparent from the description of this specification and the accompanying drawings.

[0009]

In order to achieve the above-mentioned object, the spectroscopic method of the present invention is such that an optical signal FM-modulated by a first light source is incident on a spectroscopic measurement sample and transmitted from the spectroscopic measurement sample. In the spectroscopic method of detecting the AM optical signal and measuring the absorption spectrum of atoms or molecules of the spectroscopic measurement sample, the optical signal incident on the spectroscopic measurement sample is an FM-modulated optical signal of the first light source. It is characterized in that the AM modulation component at the time of the FM modulation of the first light source is suppressed by using the FM optical signal oscillated by injection locking the second light source.

The spectroscopic device of the present invention comprises a frequency-modulated first light source, a photodetector for detecting an output from the first light source, and an amplifier to which an output current from the photodetector is input.
A frequency selective detector to which the output current from the amplifier is input, and a recorder to which the signal component selected by the frequency selective detector is input, are provided between the first light source and the photodetector. A second light source in a spectroscopic apparatus for arranging a sample in the optical path of the sample, arranging the sample in the optical path between the first light source and the photodetector, and measuring the absorption of atoms or molecules of the sample. The output light from the first light source is input to the second light source, and the sample is placed in the optical path between the second light source and the photodetector.

That is, in the spectroscopic method and the spectroscopic device of the present invention, the output light of the FM-modulated first light source is input and oscillates in phase synchronization with the output light of the first light source. The most important feature is the installation of a light source.

A light source that oscillates in an amplitude squeezed state is used as the phase-synchronized second light source.

[0013]

According to the above-mentioned means, in the spectroscopic method and apparatus by the FM spectroscopic method, the FM containing unnecessary AM modulation component is included.
By injecting the modulated light from the first light source into the second light source, the output light from the second light source is phase-locked with the light from the first light source, and the FM modulation component is preserved. But A
The M modulation component is suppressed and disappears. Therefore, since the light of the second light source incident on the spectroscopic sample does not include an unnecessary AM modulation component, only the AM signal generated by the spectroscopic sample is detected by the photodetector. The detection limit is determined only by the amplitude noise characteristic of the oscillation light from the second light source, the shot noise limit can be measured, and a highly sensitive spectroscopic device can be obtained.

Here, when the amplitude noise characteristic of the oscillation light of the second light source is in the amplitude squeezed state below the shot noise level, it is possible to measure below the shot noise level.

[0015]

Embodiments of the present invention will now be described in detail with reference to the drawings.

In all the drawings for explaining the embodiments, those having the same function are designated by the same name and the same reference numeral, and the repeated description thereof will be omitted.

FIG. 1 is a block diagram showing a schematic configuration of a first embodiment of a spectroscopic device according to the present invention. As shown in FIG. 1, the spectroscopic device of the first embodiment includes a master laser (FM modulation light source) 1, a light source DC power supply 2, an FM modulation AC power supply 3, a spectroscopic sample 4, an optical detector 5, Amplifier 6, frequency selective detector 7, recorder 8, optical isolator 9, 10
0% reflector 10, injection locking laser light source (second light source)
11, a laser DC power supply 12 for injection locking, a Faraday rotator 13, and a polarization beam splitter 14.

A polarization beam splitter 14 is installed so as to completely prevent the light from the FM modulation light source 1 from entering the sample, and the light from the light source 1 reflected by the polarization beam splitter 14 is An injection locking laser light source (second light source) 11 that oscillates light in synchronization with the incident light by being incident is installed, and the light from the injection locking laser light source (second light source) 11 is It is designed to be completely incident on the sample.

Next, the operation of the spectroscopic device of the first embodiment will be described with reference to FIG.

A semiconductor laser is used as the master laser 1, and an AC current from the FM modulation AC power supply 3 is superposed on an injection current from the light source DC power supply 2 to the master laser 1 so that F
M-modulation is performed, and a beam light subjected to FM modulation is emitted from the master laser 1. The output light of the master laser 1 (hereinafter, referred to as a master laser light) contains a slight AM modulation component in addition to the FM modulation component. The master laser light passes through the optical isolator 9, and is 100% reflecting mirror 10,
The light is sequentially reflected by the polarization beam splitter 14 by 100%, passes through the Faraday rotator 13, and is injected into an injection locking laser light source (hereinafter referred to as a slave laser) 11.

By this injection, the slave laser 11 is
The output light is oscillated in synchronization with the phase of the master laser light.
The output light of the slave laser 11 (hereinafter referred to as slave laser light) does not include the AM modulation component included in the master laser light. The slave laser light passes through the Faraday rotator 13 and reaches the polarization beam splitter 14.

Here, if the slave laser light is oscillated with polarization in the same direction as the input light, the master laser light is polarized by the Faraday rotator 13 in the polarization direction of the slave laser light.
Is rotated by 45 degrees, and the slave laser light is rotated by 45 degrees in the same direction. Therefore, the slave laser light is rotated by 90 degrees in total and passes through the polarization beam splitter 14 by 100%. When the slave laser light that has passed through the polarization beam splitter 14 passes through the spectroscopic sample 4, its FM modulation component is converted into AM modulation according to the absorption characteristics of the spectroscopic sample 4. The slave laser light subjected to AM modulation enters the photodetector 5, is detected, and is converted into a detection current. This detected current is amplified by the amplifier 6, and the amplified detected current is measured as a characteristic value for each frequency by the frequency selection detector 7, and the recorder 8
Recorded in.

As can be seen from the above description, the following effects can be obtained with the spectroscopic device of the first embodiment.

That is, the master laser light containing the unnecessary AM modulation component is prevented from entering the spectroscopic sample 4 side by the polarization beam splitter 14, while the slave laser light is injected by the master laser light. When oscillating in phase synchronization, unnecessary AM modulation components of the master laser light disappear and slave laser light incident on the spectroscopic sample 4 is
Since no unnecessary AM modulation component is included, only the AM signal component generated in the spectroscopic sample 4 is detected by the photodetector 5. Therefore, the detection limit is determined only by the noise characteristics of the slave laser light, and when a normal laser is used as the slave laser, measurement at the shot noise limit becomes possible.

Furthermore, if a semiconductor laser operated at a constant current is used as the slave laser, its amplitude noise becomes an amplitude squeezed state smaller than the shot noise level,
It is possible to measure below the shot noise level. When the slave laser light is used in the amplitude squeezed state, the absorption characteristic causes a loss, but the amplitude squeezed state is slightly destroyed at a very small absorption loss such as that caused when using this spectroscopy. do not become.

In the following, it will be explained that the above-mentioned effects were quantitatively confirmed by experiments.

FIG. 2 is a block diagram showing the schematic arrangement of an apparatus for determining the presence / absence of the effect of the spectroscopic apparatus of the first embodiment. Reference numeral 15 denotes a 50% -50% beam splitter, 5
Reference numerals 1 and 52 are optical detectors, 16 is a delay line, 17 is a differential amplifier, and 18 is a spectrum analyzer. The detection circuit composed of the beam splitter 15, the two optical detectors 51 and 52, the delay line 16, and the differential amplifier 17 is called a delay type balanced mixer.

Next, the measuring method will be briefly described. The slave laser light that has passed through the polarization beam splitter 14
It is equally divided by the 0% -50% beam splitter 15, and detected by the photodetectors 51 and 52. One of the two detected currents is connected to the differential amplifier 17 through the delay line 16 and the other is directly connected to the differential amplifier 17, and the output signal thereof is measured by the spectrum analyzer 18.

Now, assuming that the delay time of the delay line 16 is τ, when the frequency ωim = πmτ (where m is 0 or a natural number), the two input signals of the differential amplifier 17 are in phase, so that At the output of the differential amplifier 17, a component of the difference between the two input detection currents is output, and shows a shot noise level regardless of the AM noise component of the input signal light of the delay type balanced mixer. On the other hand, the frequency ωon is

[0030]

When ωon = π (2n + 1) τ / 2 (where n is 0 or a natural number), the two input signals have opposite phases, so the differential amplifier 17 outputs two inputs. The sum component of the detection currents is output and indicates the AM noise component of the input signal light of the delay type balanced mixer.

FIG. 3 shows an experimental result of noise measured by the measuring apparatus including the delay type balanced mixer and the spectrum analyzer 18 having the configuration of FIG. In FIG.
The horizontal axis represents the measurement frequency and the vertical axis represents the noise level. trace
(A) is the case where the light of the master laser is directly detected, and shows the shot noise level at the frequency ωim.
On indicates the amplitude noise of the master laser.

As can be seen from FIG. 3, since the noise level of the frequency ωon is higher than the noise level of the frequency ωim, the amplitude noise of the master laser is in the excessive noise state.
Trace (B) shows the slave laser 11 without injection locking.
This shows a single amplitude noise. At this time, since the slave laser 11 is operating at a constant current, its amplitude noise is in the amplitude squeezed state (the amplitude noise is smaller than the shot noise level), and the frequency of the frequency ωon is higher than that of the frequency ωim. The noise level is lower.

FIG. 4 shows a characteristic in which the amplitude noise of the slave laser light in the injection locked state is standardized by the shot noise level. 4, the horizontal axis represents the detuning frequency, which is the difference between the frequency of the master laser light and the free-running frequency of the slave laser light, and 0 on the vertical axis is the shot noise level. Trace (C) is the noise level of the free-running state of the slave laser light,
Amplitude squeezed.

Trace (D) is the noise level in the injection locked state with the master laser light of trace (A) of FIG.
At least within the lock range of FIG. 4 in which the slave laser light is phase-locked with the master laser light, the amplitude squeezed state is maintained and the amplitude noise does not increase. This means that by injection locking, the frequency and phase components of the slave laser light are phase locked to the master laser light, and
It has been shown that the unnecessary AM modulation component contained in the master laser light can be completely suppressed, and the detection limit thereof is determined only by the noise characteristic of the slave laser light.

As can be seen from the above description, it has been quantitatively confirmed that the spectroscopic device of the first embodiment can completely suppress the unnecessary AM modulation component generated during the FM modulation of the light source.

Needless to say, a lock-in detector that detects only the AM signal component in synchronization with the AC signal for FM modulation may be used as the detector that detects the AM signal component, and the same operation is performed.

FIG. 5 is a block diagram showing a schematic configuration of the second embodiment of the spectroscopic device according to the present invention.

As shown in FIG. 5, the spectroscopic device of the second embodiment has an arrangement for injecting the light of the first light source from the side opposite to the output light direction of the second light source. The first
Is the same as the embodiment described above.

In the above embodiment, an optical isolator is installed between the master laser 1 and the slave laser 11. This is because the master laser light is reflected by the slave laser 11 and returns to the master laser 1. The purpose is to prevent the slave laser light from entering the master laser 1 and becoming unstable, and is not a main component of the present invention.

Although the present invention has been specifically described based on the embodiments, the present invention is not limited to the above embodiments, and it is needless to say that various modifications can be made without departing from the scope of the invention. .

[0041]

As described above, according to the spectroscopic device of the present invention, the unnecessary AM modulation component generated when the light source is FM-modulated is completely suppressed, and the shot noise limit or less is achieved. It operates and can obtain highly sensitive spectrum.

[Brief description of drawings]

FIG. 1 is a block configuration diagram showing a schematic configuration of a first embodiment of a spectroscopic device according to the present invention,

FIG. 2 is a block configuration diagram showing a schematic configuration of an apparatus for determining the effect of the first embodiment,

FIG. 3 is a diagram showing noise characteristics measured without injection locking according to the first embodiment;

FIG. 4 is a diagram showing characteristics of amplitude noise when normalized by the shot noise level of the first embodiment;

FIG. 5 is a block configuration diagram showing a schematic configuration of a second embodiment of the spectroscopic device according to the present invention,

FIG. 6 is a block configuration diagram showing a schematic configuration of a conventional spectroscopic device.

[Explanation of symbols]

1 ... FM modulation light source (master laser), 2 ... Light source DC power supply, 3 ... FM modulation AC power supply, 4 ... Spectroscopic sample, 5,
51, 52 ... Photodetector, 6 ... Amplifier, 7 ... Frequency selection detector, 8 ... Recorder, 9 ... Optical isolator, 10 ... 100
% Reflection mirror, 11 ... Laser light source for injection locking (second light source), 12 ... Laser DC power supply for injection locking, 13 ... Faraday rotator, 14 ... Polarization beam splitter, 15
... 50% -50% beam splitter, 16 ... delay line,
17 ... Differential amplifier, 18 ... Spectrum analyzer,
(A) ... Noise characteristics of master laser light, (B) ... Noise characteristics of slave laser light, (C) ... Noise characteristics of slave laser free-running state, (D) ... Noise characteristics of slave laser injected state.

Claims (4)

[Claims] 1. The absorption of atoms or molecules of the spectroscopic measurement sample by detecting an AM optical signal transmitted from the spectroscopic measurement sample by injecting an FM-modulated optical signal from a first light source into the spectroscopic measurement sample. In the spectroscopic method for measuring the spectrum,
As an optical signal incident on the measurement sample for spectroscopy, an FM optical signal obtained by oscillating the second light source in injection locking with an optical signal obtained by FM-modulating the first light source is used to perform FM modulation of the first light source. A spectroscopic method characterized by suppressing the AM modulation component at the time. 2. The spectroscopic method according to claim 1, wherein a second optical signal is generated by FM-modulating the first light source.
The FM optical signal generated by oscillating the above light source in injection locking is in the amplitude squeezed state. 3. A frequency-modulated first light source, and the first light source.
A photodetector that detects the output from the light source, an amplifier to which the output current from the photodetector is input, a frequency selection detector to which the output current from the amplifier is input, and a selection from the frequency selection detector An optical path between the first light source and the photodetector, wherein a sample is placed in the optical path between the first light source and the photodetector. In the spectroscopic device for arranging a sample in the sample and measuring the absorption of atoms or molecules of the sample, a second light source is installed, and
The light output from the light source is input to the second light source, and the sample is placed in the optical path between the second light source and the photodetector. 4. The spectroscopic device according to claim 3, wherein a light source that oscillates in an amplitude squeezed state is used as the second light source.