JP2997557B2 - Frequency stabilized light source with narrow linewidth oscillation frequency spectrum - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency-stabilized light source (hereinafter, referred to as a frequency-stabilized light source) having a stable oscillation frequency and an oscillation frequency spectrum having a narrow line width.
More specifically, the laser beam from the semiconductor laser is passed through an optical path means configured using an absorption cell, an optical frequency shifter, and a semi-transparent mirror, and a saturated absorption line is formed on a slope of a linear absorption spectrum line. Is performed, and the laser frequency is stabilized at the center frequency of the saturated absorption line, and the slope of the linear absorption spectrum line is set to FM.
The present invention relates to a frequency stabilized light source which is used as an / AM converter and has a narrow laser line width. INDUSTRIAL APPLICABILITY The present invention is applied to various light measurement light sources, high-precision optical frequency reference light sources, stabilized light sources for physical and chemical measurements, light sources for coherent optical communication, and the like.

[0002]

2. Description of the Related Art As a prior art, a frequency-stabilized light source (Japanese Patent Application No. 1-53722;
No. 234484). In this prior art, the laser frequency of a semiconductor laser is stabilized by using a slope of a linear absorption spectrum line of an atom or a molecule. More specifically, after transmitting the laser light into the absorption cell, the laser light is received by a detector, and the light intensity of the received light is converted into an electric signal, whereby the laser frequency and the electric signal value are converted. The laser frequency is locked to the reference frequency by comparing the reference value corresponding to the reference frequency with the actual electric signal value, and electrically performing negative feedback so as to make the difference zero. This method is called a differential frequency stabilization method, and the laser frequency is stabilized using this method. However, a linear absorption spectrum line of an atom or a molecule changes its line width and intensity due to a change in temperature, so that the stabilized reference frequency changes, so that the oscillation frequency of the semiconductor laser also changes. There was a disadvantage that it would. To compensate for the disadvantage, a frequency-stabilized laser light source by the same applicant (Japanese Patent Application No.
This is because the oscillation frequency of the semiconductor laser light is expressed by the saturation absorption line of an atom whose saturation absorption line due to the hyperfine level stands on the slope of the linear absorption spectrum line. , And at the same time, narrow the laser line width by using the slope of the linear absorption spectrum line. However, according to this conventional technique, in an atom or a molecule having no hyperfine level, the saturation absorption line stands at the bottom of the linear absorption spectrum line, so that the laser line width cannot be reduced using the linear absorption spectrum line. There was a disadvantage.

[0003]

SUMMARY OF THE INVENTION An object of the present invention is to provide a frequency stabilized light source which solves the following problems in the prior art. (1) When stabilizing the laser frequency by the differential type,
Since the line width and intensity of the linear absorption spectrum line change due to the temperature change of the absorption cell containing atoms or molecules, the specific frequency for stabilizing the laser frequency of the semiconductor laser changes, and the stabilization occurs accordingly. The laser frequency of the semiconductor laser to be changed. (2) Further, in the case of stabilizing to a saturated absorption line of an atom or a molecule having no ultrafine level, the saturation absorption line stands at the bottom of the linear absorption spectrum line, so that the laser line width cannot be narrowed at the same time. .

[0004]

Therefore, in the present invention, the following technical means can be applied to the above-mentioned problems (1) and (2) to achieve a specific operation. (1) By adding an optical frequency shifter in the optical path, a saturated absorption line is made to stand on the slope of the linear absorption spectrum line, and the laser frequency is stabilized at the center frequency of the saturated absorption line, so that linear absorption caused by a temperature change is obtained. It is possible to prevent a change in a specific frequency to be stabilized due to a change in the line width or intensity of the spectral line. (2) (a) After transmitting the semiconductor laser light through an absorption cell in which atoms or molecules having no ultrafine structure are enclosed, the laser frequency of the transmitted semiconductor laser light is shifted by an optical frequency shifter. By transmitting the same light path again through the absorption cell, the saturated absorption line can be made to stand on the slope of the linear absorption spectrum line, so that the same effect as the saturation absorption spectrum line of the atom or molecule having the hyperfine structure can be obtained. Then, as shown in (1), the laser frequency was stabilized at the center frequency of the saturated absorption line. (B) At the same time, by efficiently converting the FM / AM signal using the slope of the linear absorption spectrum line, the laser line width can be narrowed while the laser frequency is stabilized.

[0005]

FIG. 1 shows an embodiment of a frequency stabilized light source according to the present invention. The laser light oscillated by the semiconductor laser 1 is
After passing through the half-wave plate 2, the polarization beam splitter 3
Thereby, the horizontally polarized light is branched in the direction b, and the vertically polarized light is branched in the direction a. The laser light from the semiconductor laser 1 is linearly polarized light, but by adjusting the half-wave plate 2, the branching ratio in the a-direction and the b-direction can be determined. In FIG. 1, the path of the laser beam is indicated by two solid arrows.
The laser light (vertically polarized light) branched in the direction a is used as output light, and the laser light (horizontal polarized light) branched in the direction b is transmitted through the quarter-wave plate 4. Since the laser light branched in the direction b is linearly polarized light, it is converted into circularly polarized light by passing through the quarter-wave plate 4. After being reflected by the semi-transparent mirror 5, the circularly-polarized light returns to the linearly-polarized light again by passing through the quarter-wave plate 4 from the opposite direction, but the polarization direction becomes vertical polarization. The light is emitted from the polarization beam splitter 3 only in the direction c.

In FIG. 1, the laser light incident from the direction d is a due to the combination of the half-wave plate 2, the polarizing beam splitter 3, and the quarter-wave plate 4 (optical path means 8).
Although the laser beam is branched in the directions b and b, the laser beam returning from the direction b is emitted only in the direction c. The laser light emitted in the direction b is transmitted through the quarter-wave plate 4 and then transmitted through the absorption cell 6.

In the absorption cell 6, atoms or molecules having no ultrafine level such as acetylene are sealed. The transmitted laser light transmitted through the absorption cell 6 is frequency-shifted by the optical frequency shifter 7 and then partially reflected on the same optical path by the semi-transparent mirror 5.

The amount of frequency shift in the optical frequency shifter 7 is Δf, the center frequency of linear absorption of atoms or molecules having no ultrafine level is fp, the speed of light is C, and the optical axis component of the speed of atoms or molecules is V. . Laser frequency Δf (= fp−Δ
When the laser light of f) is transmitted through the absorption cell 6, the laser light has a velocity component V in the optical axis direction of Δf = −fp (V / C).
It reacts with atoms or molecules that satisfy the following relational expression to be absorbed. In this case, the amount of light transmitted into the absorption cell 6 is made sufficiently large to cause saturation absorption.

The transmitted laser light transmitted through the absorption cell 6 is
By passing through the optical frequency shifter 7, the frequency shift amount Δ
The frequency is shifted by f (= −fp (V / C)), then a part of the transmitted laser light is reflected by the semi-transparent mirror 5, and the reflected laser light is transmitted through the optical frequency shifter 7 again, so that 2Δf ( = −2 fp (V / C)), and the reflected laser light of the laser frequency fp (1−V / C) is transmitted through the absorption cell 6 again. Figure 2
Shown in

That is, the laser beam having the laser frequency fp (1 + V / C) is transmitted through the absorption cell 6 for the first time, and the atoms or molecules having the velocity component V in the direction of the optical axis undergo saturated absorption. In the second time, the reflected laser light having the laser frequency fp (1-V / C) is transmitted in the opposite direction on the same optical path to react with the atoms or molecules having the velocity component V in the optical axis direction as in the first time. Let it. However, since the saturated absorption occurs at the first time, the reflected laser light transmitted at the second time is transmitted without being absorbed. For this reason, in the saturated absorption amount detector 9 which receives the reflected laser light branched in the c direction by the optical path means 8, fp-Δf (= fp) on the slope of the linear absorption spectrum line as shown in FIG.
A sharp protrusion A is seen at the optical frequency position of (1 + V / C). This is the saturated absorption line.

[0011] If the optical frequency shifter 7 is controlled so as to Delta] f ₁ frequency shift, it is possible to stand the saturated absorption lines in the optical frequency positions of fp-Δf _1. Further, when the optical frequency shifter 7 is controlled so as to Delta] f ₂ frequency shift, it is possible to stand the saturated absorption lines in the optical frequency positions of fp-Δf _2. This situation is shown in FIG.
(B). In FIG. 4, | Δf ₁ | <| Δf ₂ |. As described above, by controlling the optical frequency shifter 7, it is possible to change the positions of the saturated absorption lines output on the slope of the linear absorption spectrum line, that is, the positions of A and B. Usually, the width that can be frequency-shifted by the optical frequency shifter 7 is about ± (80 MHz to 120 MHz).

If an optical frequency shifter 7 is not used for saturation spectroscopy in an atom or a molecule having no hyperfine level, the following will occur. First, as shown in FIG. 5, a laser beam having a laser frequency fLD (= fp (1 + V / C)) is transmitted into the absorption cell 6, and then reflected by the semi-transparent mirror 5 or the like, and again inside the absorption cell 6. Consider the case where light is transmitted through In this case, the transmission of the first laser light into the absorption cell 6, but to saturate the atoms or molecules having a velocity component V in the optical axis direction, the second reflected laser beam (f p (1 + V /
In the transmission of C)), a saturated absorption line is not output because atoms or molecules having a velocity component (-V) react in the optical axis direction.

Next, a laser beam having the same laser frequency fLD as the center frequency fp of the linear absorption of atoms or molecules is applied to the absorption cell 6.
After transmission within, consider the case where re transmitting the reflected laser beam reflected f p in the absorption cell by half mirror 5. FIG. 6 shows a schematic diagram of this case. in this case,
In order to react atoms or molecules having zero velocity component in the optical axis direction, the saturated absorption line A stands at the bottom of the linear absorption spectrum line,
Since the laser frequency of the semiconductor laser 1 is stabilized at the bottom of the linear absorption spectrum line as shown in FIG. 7, the laser line width cannot be reduced at the same time.

The laser frequency fLD of the semiconductor laser 1 is adjusted to be close to the center frequency fRP of the saturated absorption line A, and a modulation signal from a modulation signal source 10 is input to a semiconductor laser controller 11 as shown in FIG. When the laser frequency is directly modulated, an output as shown in FIG. This output is output to the phase detector 12 shown in FIG.
, The laser frequency fLD of the semiconductor laser 1
Is obtained as the differential value output of the saturated absorption line with respect to. FIG. 9 shows a phase detector output Vd which is a differential value output of the saturated absorption line.

When the laser frequency fLD of the semiconductor laser 1 is near the center frequency fRP of the saturated absorption line, there is a difference between the difference frequency between the laser frequency fLD and the center frequency fRP of the saturated absorption line and the output Vd of the phase detector. If the phase detector output Vd is electrically negatively fed back to the semiconductor laser controller 11 shown in FIG. 1 so that the phase detector output Vd always becomes zero, the semiconductor laser shown in FIG. The one laser frequency fLD is stabilized at the center frequency fRP of the saturated absorption line. The frequency control band of the feedback loop for stabilizing the laser frequency fLD of the semiconductor laser 1 is DC to 1 k
HZ.

The laser light transmitted through the semi-transparent mirror 5 shown in FIG. 1 is converted into an electric signal by the linear absorption detector 13 and is converted into an electric signal.
0 is output as a linear absorption spectrum line. The laser frequency fLD of the semiconductor laser 1 shown in FIG.
Since it is stabilized by the saturated absorption line at the slope position of the linear absorption spectrum line (the position where the slope is not at the top of a mountain or at the bottom of a valley), optical frequency noise can be expressed with high sensitivity. The laser output can be converted into light intensity noise having the following characteristics. This output is subjected to electrical negative feedback to the semiconductor laser 1, thereby stabilizing the laser frequency and narrowing the laser line width using the linear absorption spectrum line. it can. The frequency control band of the feedback loop for narrowing the laser line width is 1 kHz to 100 MHz. In this way, by controlling using the optical frequency shifter 7, the saturated absorption line is made to stand at the slope position of the linear absorption spectrum line, and the laser frequency of the semiconductor laser 1 is stabilized at the center frequency fRP of the saturated absorption line, At the same time, a frequency-stabilized light source with a narrow laser linewidth using the slope of the linear absorption spectrum line was realized.

[0017]

As described above, the frequency-stabilized light source according to the present invention controls the frequency shift amount of the optical frequency shifter inserted in the optical path so that the saturated absorption line is shifted to the position of the slope of the linear absorption spectrum line. By stabilizing the frequency of the semiconductor laser light at the peak frequency of the saturated absorption line and narrowing the laser line width using the linear absorption spectrum line, the following specific effects are obtained. (1) Since the laser frequency of the semiconductor laser is stabilized at the saturated absorption line where the optical frequency position does not change even if the temperature changes, the oscillation frequency of the semiconductor laser does not change even if the temperature changes. (2) Further, since the saturated absorption line is made to stand at the slope position of the saturated absorption spectrum line by using the optical frequency shifter, and the laser frequency of the semiconductor laser is stabilized there,
At the same time as the stabilization of the laser frequency, the laser line width using the slope of the linear absorption spectrum line could be narrowed.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an embodiment of a frequency stabilized light source according to the present invention.

FIG. 2 is a diagram showing a relationship between transmission and reflection of laser light in an absorption cell.

FIG. 3 is a view showing a saturated absorption line observed in FIG. 2;

FIG. 4A is a diagram illustrating a saturated absorption line when Δf ₁ frequency shift control is performed on an optical frequency shifter, and FIG. 4B is a diagram illustrating a saturated absorption line when Δf ₂ frequency shift control is performed on an optical frequency shifter. b).

FIG. 5 is a diagram showing a state in which laser light is transmitted through an absorption cell, then reflected by a semi-transparent mirror, and transmitted again through the absorption cell.

FIG. 6 shows a state in which laser light having the same laser frequency as the center frequency of linear absorption of atoms or molecules having no ultrafine level is transmitted through the absorption cell, reflected by the semi-transparent mirror, and transmitted again through the absorption cell. FIG.

FIG. 7 is a diagram showing a saturated absorption line observed in FIG. 6 and a linear absorption spectrum line of an atom or a molecule.

FIG. 8 is a diagram showing a waveform output from a saturated absorption amount detector when a modulation signal is output to a semiconductor laser controller to directly modulate a laser frequency.

9 is a diagram showing a differential value output Vd of a saturated absorption line obtained by phase detection of the output shown in FIG. 8 by a phase detector.

FIG. 10 is a diagram showing a linear absorption spectrum line.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Semiconductor laser 2 1/2 wavelength plate 3 Polarization beam splitter 4 1/4 wavelength plate 5 Semi-reflection mirror 6 Absorption cell 7 Optical frequency shifter 8 Optical path means 9 Saturation absorption amount detector 10 Modulation signal source 11 Semiconductor laser controller 12 Phase detection 13 Linear absorption detector

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) H01S 3/18 H01S 3/133 JICST file (JOIS)

Claims (1)

(57) [Claims] 1. A semiconductor laser (1), an absorption cell (6) in which atoms or molecules are sealed for transmitting a laser beam emitted from the semiconductor laser, and a frequency of the laser beam transmitted through the absorption cell is arbitrarily set. An optical frequency shifter (7) that shifts the laser light by the frequency described above, a semi-transparent mirror (5) that reflects a part of the laser light frequency-shifted by the optical frequency shifter on the same optical path, and a laser light transmitted through the semi-transparent mirror. A linear absorption detector (13) for receiving the light and converting the linear absorption by the absorption cell into an electric signal; and transmitting the reflected laser light reflected by the semi-transparent mirror again to the optical frequency shifter and the absorption cell in the same optical path. An optical path means (8) for generating a saturated absorption line on a slope within the linear absorption spectrum line by transmitting the laser beam in a direction different from the laser beam emitted from the semiconductor laser; A saturated absorption amount detector (9) for receiving the reflected laser light emitted from the optical path means and converting the saturated absorption amount into an electric signal; and a semiconductor laser controller (11) for controlling an injection current to the semiconductor laser. A modulation signal source (10) for outputting a modulation signal for modulating the frequency of laser light emitted from the semiconductor laser through the semiconductor laser controller; a modulation signal output from the modulation signal source; A phase detector (12) that performs phase detection based on the electric signal output from the quantity detector and outputs a signal that depends on the difference between the frequency of the laser light emitted from the semiconductor laser and the center frequency of the saturated absorption line. ), The signal output from the phase detector is electrically negatively fed back to the semiconductor laser controller, so that the laser frequency of the semiconductor laser is set at the center of the saturated absorption line. The laser line width of the semiconductor laser is reduced by stabilizing the wave number and electrically negatively feeding back the signal output from the linear absorption detector as a frequency noise signal to the semiconductor laser controller. A frequency-stabilized light source having a narrow linewidth oscillation frequency spectrum.