JPH0644656B2 - Frequency stabilized semiconductor laser device - Google Patents

Description

The present invention relates to a semiconductor laser device that outputs laser light, and more particularly to a frequency-stabilized semiconductor laser device that outputs laser light having a highly stabilized frequency.

[Prior Art] Generally, the oscillation frequency of a semiconductor laser greatly changes depending on the current value injected (applied) to the semiconductor laser and the ambient temperature. Therefore, the frequency of the laser light indicated by the oscillation frequency output from this semiconductor laser also changes greatly. Furthermore, the laser light output from this semiconductor laser is not a coherent light having a perfect single frequency, and since many sideband frequencies and frequency noise are included in the vicinity of the oscillation frequency, the actual laser light Frequency + number
Has a spread of MHZ. With such a wide line width, when performing a performance test of optical communication equipment using this laser light,
The resolution of the measuring device is limited by the line width of the reference measuring light.

That is, in order to adopt such laser light as the reference light of the measuring instrument, it is necessary to solve the problems such as "poor frequency stability" and "wide line width" of the laser light.

As a method for stabilizing the first frequency, it has been proposed to use a Fabry-Perot resonator and control the frequency of laser light with reference to the resonance frequency of the Fabry-Perot resonator. Further, as a method of stabilizing the frequency to a higher degree, it has been proposed to control the frequency with reference to the characteristic spectral line of atoms or molecules.

Further, there are an electric method and an optical method for narrowing the second line width. In the electrical method, the laser light output from the semiconductor laser is passed through the Fabry-Perot resonator described above, and the steep slope of its transmission characteristics is used for frequency discrimination to provide broadband electrical feedback to the injection current of the semiconductor laser. Narrowing is performed by adding.

Further, in the optical method, the laser light output from the semiconductor laser is passed through a narrow band optical filter such as the Fabry-Perot resonator, and the transmitted light or reflected light is returned to the semiconductor laser, so-called optical locking is performed to narrow the light. To do.

By adopting these methods, the frequency of the laser light output from the semiconductor laser can be highly stabilized, and the line width can be reduced to several hundredths as compared with the case where band control is not executed. It is possible to narrow the area to tens of thousands.

A frequency-stabilized semiconductor laser device for frequency stabilization and line width narrowing is configured, for example, as shown in FIG.

A laser beam 2 output from the semiconductor laser 1 is branched by a beam splitter 3, and a part of the branched laser beam 2 passes through a lens 4 and is further reflected by a reflecting mirror 5 so as to be opposed to each other. The light is incident on the Fabry-Perot resonator 6 composed of 6a and 6b. The laser light 2 reflected by the Fabry-Perot resonator 6 is returned to the semiconductor laser 1 via the same path as the incident light, that is, the reflecting mirror 5, the lens 4 and the beam splitter 3, and so-called optical locking is performed.

On the other hand, the laser light transmitted through the Fabry-Perot resonator 6 is converted by the photoelectric detector 7 into a light intensity signal having a signal level corresponding to the light intensity. Further, an electrostrictive element 8 is attached to the back surface of the reflecting mirror 5. And the phase controller 9,
The light intensity signal output from the photoelectric detector 7 is processed and applied as a control signal to the electrostrictive element 8 to automatically adjust the phase of the laser light returning from the Fabry-Perot resonator 6 to the semiconductor laser 1 to always obtain the optimum phase condition. The above-mentioned optical lock is controlled so as to be executed.

Further, the laser beam 2 that has passed through the beam splitter 3 is further branched by another beam splitter 10. The branched laser light is incident on the absorption cell 11 in which atoms or molecules having a saturated absorption line serving as a reference frequency are enclosed.
The laser light that has passed through the absorption cell 11 is reflected by the reflecting mirror 12 and passes through the absorption cell 11 again,
Creates a saturated absorption line for an atom or molecule.

The laser light that has passed through the absorption cell 11 again passes through the beam splitter 10 as it is, and is converted into a light intensity signal by the photoelectric detector 13. The light intensity signal output from the photoelectric detector 13 is input to the Fabry-Perot controller 14.
An electrostrictive element 15 is attached to the back surface of the reflecting mirror 6a on the photoelectric detector 7 side of the Fabry-Perot resonator 6. Then, the Fabry-Perot controller 14 processes the light intensity signal output from the photoelectric detector 13 and applies it as a control signal to the electrostrictive element 15, and the frequency of the laser beam 2 returning from the Fabry-Perot resonator 6 to the semiconductor laser 1. The resonance frequency of the Fabry-Perot resonator 6 is controlled by moving the position of the reflecting mirror 6a so as to match the center frequency f ₀ of the saturated absorption line of the atom or molecule of the absorption cell 11.

In the frequency-stabilized semiconductor laser device having such a configuration, as is well known, the resonance frequency of the Fabry-Perot resonator 6 depends on the distance between the pair of reflecting mirrors 6a and 6b facing each other. Then, as shown in FIG. 6, when the frequency of the input laser light is continuously changed, the laser light output from the Fabry-Perot resonator 6 has a frequency characteristic including a plurality of peak waveforms 16 shown in FIG. Have. The finesse F (= f _SR / Δf) indicated by the ratio of the free spectral range f _SR indicating the frequency width between the peak waveforms 16 and the half width Δf of the peak waveform 16 is extremely high.
As a result, the line width of the laser light 2 output from the optically locked semiconductor laser 1 is narrowed. At the same time, its frequency is stabilized at the resonance frequency of the Fabry-Perot resonator 6.

Further, the phase of the laser light 2 is controlled by the phase controller 9 so that the optical lock is in the optimum state. In order to detect the optimum phase condition, the electrostrictive element 8 mounted on the reflecting mirror 5 is frequency-modulated at a frequency of about 10 kHz to vibrate the reflecting mirror 5. As a result, the laser light transmitted from the semiconductor laser 1 and transmitted through the beam splitters 3 and 10 is frequency-modulated, but this modulation width is very small because it only controls the phase and can be almost ignored.

Further, the Fabry-Perot controller 14 controls the resonance frequency of the Fabry-Perot resonator 6 so as to match the center frequency f ₀ of the saturated absorption line 17 of the atom or molecule shown in FIG. The saturated absorption line 17 has a specific frequency value determined by the type of atom or molecule contained in the absorption cell 11, and this frequency value is not affected by mechanical vibration or ambient temperature. Therefore, the frequency of the laser beam 2 output from the semiconductor laser 1 is further stabilized and the reproducibility is further improved.

In order to detect the center frequency f ₀ of the saturated absorption line 17 of the absorption cell 11, the electrostrictive element 15 mounted on the reflecting mirror 6a of the Fabry-Perot resonator 6 is frequency-modulated at a frequency of about 1 kHz and reflected. The mirror 6a is vibrating.

As described above, by using the Fabry-Perot resonator 6, the absorption cell 11, the phase controller 9, and the Fabry-Perot controller 14, the frequency of the laser light 2 output from the semiconductor laser 1 can be highly stabilized and the line width can be further increased. Can be significantly narrowed.

The details of the above-mentioned technique are described in the following documents, for example.

B. Dahmani et al "Optical Lock Frequency Stabilization of Semiconductor Lasers" Optic Lett., Vol.12.No.11 (1987) [Problems to be solved by the invention] However, the frequency stabilization shown in FIG. The semiconductor laser device also has the following problems to be solved.

That is, the Fabry-Perot resonator 6 is composed of two reflecting mirrors 6a and 6b having concave lens-shaped cross sections facing each other. The resonance frequency of the two reflecting mirrors 6a,
Since it depends on the mutual distance between 6b, it greatly fluctuates due to ambient temperature and mechanical vibration. As described above, in order for the Fabry-Perot controller 14 to control the resonance frequency to the center frequency f ₀ of the saturated absorption line 17 of atoms or molecules of the absorption cell 11, it is necessary to vibrate the reflecting mirror 6a. But,
The reflecting mirror 6a having a concave lens-shaped cross section has a considerably larger shape and a heavier weight than the reflecting mirror 5 having another plane, and it is difficult to control the vibration of the reflecting mirror 6a with a large amplitude and at a high speed. Met. For example, the vibration frequency is 1 at most
It is kHz. Therefore, there is a problem that the response speed in frequency control is reduced.

Further, the frequency width of the frequency modulation applied to the reflecting mirror 6a of the Fabry-Perot resonator 6 described above requires at least the line width (about 2Δf) of the peak waveform 16 shown in FIG. There is a problem that the frequency modulation component included in is large.

Furthermore, in the frequency-stabilized semiconductor laser device of FIG. 5, first, the phase controller 9 is used to optically lock the frequency of the laser light 2 output from the semiconductor laser 1 to the resonance frequency of the Fabry-Perot resonator 6. Next, the resonance frequency of the Fabry-Perot resonator 6 is controlled so as to match the center frequency f ₀ of the saturated absorption line 17 of the atom or molecule. That is, two-step control is required to obtain the stabilization of the frequency of the output laser light 2 and the constriction of the line width, which not only makes the entire device complicated and large, but also makes the adjustment very complicated. Become. Further, the manufacturing cost is significantly increased.

The present invention has been made in view of such circumstances,
By optically locking the frequency of the laser light using the absorption line by sub-Doppler spectroscopy of the optical filter and further the center frequency of the saturated absorption line of atoms and molecules,
With a simple configuration, frequency stabilization of the output laser light,
It is an object of the present invention to provide a frequency-stabilized semiconductor laser device capable of simultaneously narrowing the line width.

[Means for Solving the Problems] In order to solve the above problems, in the frequency-stabilized semiconductor laser device of the first aspect of the present invention, a semiconductor laser that outputs laser light and a laser light output from the semiconductor laser are provided. An optical branching device that is inserted in the output path and branches a part of the laser light, an optical filter that uses a saturated absorption line of an atom or a molecule that transmits the laser light branched by this optical branching device, and this optical A reflection mirror that reflects the laser light that has passed through the filter; the laser light reflected by the reflection mirror is returned to the semiconductor laser through an optical branching device, and the frequency of the laser light output from the semiconductor laser is an optical filter. Optical locking is performed at a constant frequency and line width determined by the characteristics of.

In addition, in the second invention, in addition to the respective means in the above-mentioned first invention, a photoelectric detector for detecting the light intensity of the laser beam returning to the semiconductor laser, and a reflector are mounted,
An electrostrictive element for changing the position of the reflecting mirror in the optical path direction and a phase controller for controlling the position of the reflecting mirror by the electrostrictive element so that the light intensity signal detected by the photoelectric detector has a maximum value are provided. ing.

Furthermore, in the frequency-stabilized semiconductor laser device of the third invention, a semiconductor laser that outputs laser light and an output path of the laser light output from this semiconductor laser are inserted to branch a part of the laser light. An optical branching device, an optical filter using an absorption line by sub-Doppler spectroscopy for transmitting the laser light branched by the optical branching device, and a reflecting mirror for reflecting the laser light passing through the optical filter, The laser light reflected by the reflecting mirror is returned to the semiconductor laser, and the frequency of the laser light output from the semiconductor laser is optically locked at a constant frequency and line width determined by the characteristics of the optical filter.

Further, in a fourth aspect based on the third aspect, the photoelectric detector for detecting the light intensity of the laser beam returning to the semiconductor laser and the reflecting mirror are mounted, and the position of the reflecting mirror in the optical path direction is varied. An electrostrictive element and a phase controller that controls the position of the reflecting mirror by the electrostrictive element so that the light intensity signal detected by the photoelectric detector has a maximum value are provided.

[Operation] According to the frequency-stabilized semiconductor laser device according to the first aspect of the invention thus configured, as shown in FIG. 1, the laser beam b output from the semiconductor laser a is reflected by the optical branching device c. The part is branched, and the branched laser light is incident on the optical filter d using the saturated absorption line of atoms or molecules. The laser beam that has passed through the optical filter d is reflected by the reflecting mirror e, passes through the optical filter d again, and is returned to the semiconductor laser a through the optical branching device c. That is, the laser beam b output from the semiconductor laser a is optically locked by the reciprocating path of the laser beam formed by the reflecting mirror e, the optical filter d and the optical branching device c, and the frequency of the laser beam b is saturated by the optical filter d. Controlled to the center frequency of the absorption line.

The center frequency of the saturated absorption line of the atom or molecule has a specific frequency value determined by the type of atom or molecule contained in the optical filter, and its frequency bandwidth is very narrow. The frequency of the laser light b to be generated is stabilized, and the line width is significantly narrowed.

In addition, in the second invention, in addition to the above-described actions, the position of the reflecting mirror is controlled by the electrostrictive element so that the light intensity of the laser beam returned to the semiconductor laser is maximized,
The optical lock is executed under the optimum phase condition.

Further, according to the frequency-stabilized semiconductor laser device of the third invention, as shown in FIG. 2, instead of the optical filter d using the saturated absorption line of atoms or molecules in the semiconductor laser device of FIG. 1, The optical filter d using the saturated absorption line of atoms or molecules also uses the optical filter f using the absorption line by sub-Doppler spectroscopy included in the embodiment.

Then, the laser light b output from the semiconductor laser a is branched by the optical branching device c and is incident on the optical filter f using the absorption line by sub-Doppler spectroscopy. The laser beam that has passed through the optical filter f has two reflecting mirrors g as shown in the drawing.
It is reflected by ₁ and g ₁ and returned to the semiconductor laser a through another optical branching device h. That is, the laser light is optically locked as in the case of FIG.

In addition, one reflecting mirror g is arranged at a right angle to the optical path,
As in the case of FIG. 1, it is possible to return the laser light transmitted through the optical filter f to the semiconductor laser a again via the optical filter f and the optical branching device c.

Since the optical filter f using the absorption line by the sub-Doppler spectroscopy has a very narrow frequency bandwidth,
Similar to the figure, the laser light b output from the semiconductor laser a
The frequency is stabilized and the line width is significantly narrowed.

Further, in the fourth invention, like the second invention,
The position of the reflecting mirror g is controlled by an electrostrictive element so that the light intensity of the laser beam returning to the semiconductor laser a is maximized.
The optical lock is executed under the optimum phase condition.

[Embodiment] An embodiment of the present invention will be described below with reference to the drawings.

FIG. 3 is a schematic configuration diagram of the frequency-stabilized semiconductor laser device of the embodiment. The laser light 22 output from the semiconductor laser 21 is split by a beam splitter 23 as an optical splitter. A part of the branched laser light 22 is incident on the absorption cell 24 as an optical filter.

In this absorption cell 24, atoms or molecules having a saturated absorption line 17 shown in FIG. Specifically, for example, Rb having a center wavelength of 780 nm of absorption line, alkali metal such as Cs having a center wavelength of 852 nm, C ₂ H ₂ having a center wavelength of 1.5 μm band,
Gas molecules such as HCN are enclosed. If the light intensity of the laser light incident on the absorption cell 24 is sufficiently high, the above-mentioned absorption of atoms or molecules is saturated and the saturated absorption line 17 is obtained.

The laser beam 22 that has passed through the absorption cell 24 passes through the lens 25, is reflected by the reflecting mirror 26, and passes through the absorption cell 24 again to generate a sharp saturated absorption line of atoms or molecules. The saturated absorption line 17 acts as an optical filter.

Part of the laser light that has passed through the absorption cell 24 again is reflected by the beam splitter 23 and returned to the semiconductor laser 21. That is, the laser light reflected by the reflector 26 passes through the same path as the incident light, that is, the reflecting mirror 26, the lens 25, the absorption cell 24, and the beam splitter 23, and the semiconductor laser 2
1, and the optical lock is performed.

On the other hand, part of the laser light that has passed through the absorption cell 24 again passes through the beam splitter 23 as it is, and is converted into a light intensity signal by the photoelectric detector 27. Photoelectric detector 27
The light intensity signal output from is input to the phase controller 28. An electrostrictive element 29 is attached to the back surface of the reflecting mirror 26. Then, the phase controller 28 processes the light intensity signal output from the photoelectric detector 27 to generate the electrostrictive element 29.
Applied as a control signal to the reflection mirror 26 and the absorption cell 24.
The position of the reflecting mirror 29 is moved so that the light intensity of the laser beam returning to the semiconductor laser 21 via the beam splitter 23 becomes the maximum value, that is, the optical lock described above is executed under the optimum phase condition. The phase of the laser light returning to the semiconductor laser 21 is controlled.

In order to detect the optimum phase condition, the electrostrictive element 29 mounted on the reflecting mirror 26 is frequency-modulated to vibrate the reflecting mirror 26.

According to the frequency-stabilized semiconductor laser device configured as described above, the laser light 22 output from the semiconductor laser 21
The optical path including the absorption cell 24 is optically locked to the frequency determined by the characteristics of the absorption cell 24. The value of the center frequency f ₀ of the saturated absorption line 17 determined by the type of atom or molecule in the absorption cell 24 is not affected by the ambient temperature or mechanical vibration. That is, the frequency of the laser beam 22 can be stabilized at a constant value only by the absorption cell 24.

Further, the frequency bandwidth of the saturated absorption line 17 shown in FIG. 7 determined by the type of atom or molecule of the absorption cell 24 is compared with the frequency bandwidth of the beak waveform 16 shown in FIG. 6 in the Fabry-Perot resonator 6 of the conventional device. And it is very narrow. Therefore, the line width of the laser beam 22 can be significantly narrowed.

In this way, the laser light 22 output from the semiconductor laser 21 is simultaneously stabilized by the optical filter including the absorption cell 24 to stabilize the frequency and narrow the line width. Therefore, the control loop of the Fabry-Perot resonator 6 can be omitted, as compared with the conventional semiconductor laser device in which desired frequency stabilization and line width narrowing are achieved through the two-step control shown in FIG. The configuration can be greatly simplified. Therefore, the adjustment work can be greatly simplified, and the manufacturing cost can be significantly reduced.

Further, as in the conventional device shown in FIG. 5, the Fabry-Perot resonator 6 is used to control the frequency of the output laser light.
Since it is not necessary to vibrate the reflecting mirror 6a having a large concave concave lens cross section with a large amplitude, the output laser light 22
The frequency is not frequency-modulated in a wide frequency range.

As described above, the electrostrictive element 29 mounted on the reflecting mirror 26 is frequency-modulated to detect the optimum phase condition, and the reflecting mirror 26 is vibrated. As a result, the frequency of the laser light transmitted from the semiconductor laser 21 through the beam splitter 23 and output to the outside is slightly frequency-modulated, but this modulation width is for detecting the optimum phase. Is much smaller than the frequency modulation width for controlling the frequency. Therefore, the frequency modulation component contained in the output laser light 22 is extremely small and can be almost ignored.

Further, since the weight of the reflecting mirror 26 formed of the plane mirror is significantly smaller than that of the reflecting mirror 6a of the Fabry-Perot resonator 6, it can be sufficiently controlled even if it is vibrated at a high frequency, and the response speed in frequency control is high. It never drops. Incidentally, this reflecting mirror 26 can vibrate at about 10 kHz, and the reflecting mirror 6 of the Fabry-Perot resonator 6 can be oscillated.
A response speed 10 times or more can be secured as compared with 1 kHz in a.

Since it is possible to obtain such excellent frequency characteristics, this semiconductor laser device can be used as, for example, a high-accuracy optical frequency reference, a high-resolution spectroscopic measurement light source, a reference light source for length measurement / ranging optical equipment, It can be applied as a reference light source for precision optical application measuring instruments and a light source for coherent optical communication.

FIG. 4 is a schematic configuration diagram of a frequency-stabilized semiconductor laser device according to another embodiment of the present invention. The same parts as those in FIG. 3 are designated by the same reference numerals. Therefore, detailed description of the overlapping portions will be omitted.

In this embodiment, the laser beam 22 output from the semiconductor laser 21 passes through one beam splitter 30 and is split by the other beam splitter 23. The branched laser light 22 is incident on the optical filter 31 using the absorption line by sub-Doppler spectroscopy. The laser beam 22 that has passed through the optical filter 31 receives the pair of reflecting mirrors 26.
They are reflected by a and 26b, respectively, and are returned to the semiconductor laser 21 via the one beam splitter 30.
That is, the laser light 2 output from the semiconductor laser 21
2 is optically locked by the above-mentioned route,
The optical filter 31 is controlled to have a constant frequency and a line width determined by the frequency of the absorption line by sub-Doppler spectroscopy.

Further, the light intensity of the laser beam from the reflecting mirror 26b that has passed through one beam splitter 30 is converted into an electric signal by the photoelectric detector 27 and input to the phase controller 28. The phase controller 28 processes the light intensity signal output from the photoelectric detector 27, applies it as a control signal to the electrostrictive element 29 mounted on one of the reflecting mirrors 26a, and returns the optimum laser light to the semiconductor laser 21. It is controlled so that it is optically locked under the phase condition.

Also in the frequency-stabilized semiconductor laser device configured as described above, the optical filter 31 using the absorption line by sub-Doppler spectroscopy has a very narrow frequency band width, and therefore, the same effect as the embodiment of FIG. 3 is obtained. Obtainable.

[Effects of the Invention] As described above, according to the frequency-stabilized semiconductor laser device of the present invention, a part of the laser light output from the semiconductor laser is absorbed by sub-Doppler spectroscopy, and further,
Passing through an optical filter that uses saturated absorption lines of atoms or molecules and returning a part of the transmitted laser light to a semiconductor laser to simultaneously stabilize the frequency of the laser light and narrow the line width. It is possible.
As a result, while maintaining the above-mentioned high performance, the configuration of the entire apparatus can be simplified, the adjustment work can be simplified, and the manufacturing cost can be significantly reduced.

[Brief description of drawings]

1 and 2 are functional block diagrams showing the configuration of the present invention, FIG. 3 is a schematic configuration diagram of a frequency-stabilized semiconductor laser device according to one embodiment of the present invention, and FIG. 4 is another example of the present invention. 5 is a schematic configuration diagram of a frequency-stabilized semiconductor laser device according to the embodiment of the present invention, FIG. 5 is a schematic configuration diagram of a conventional frequency-stabilized semiconductor laser device, and FIG. 6 is a frequency identification diagram of a Fabry-Perot resonator of the conventional device. FIG. 7 is a diagram showing saturated absorption lines of atoms or molecules in the absorption cell. 17 ... Saturation absorption line, 21 ... Semiconductor laser, 22 ... Laser light, 23 ... Beam splitter (optical branching device), 24 ... Absorption cell (optical filter), 25 ... Lens, 26, 26
a, 26b ... Reflecting mirror, 27 ... Photoelectric detector, 28 ... Phase controller, 29 ... Electrostrictive element, 31 ... Optical filter.

Claims (4)

[Claims] 1. A semiconductor laser (21) for outputting a laser beam,
An optical branching device (23) which is inserted in the output path of the laser light output from this semiconductor laser and branches a part of the laser light.
And an optical filter (2 that uses saturated absorption lines of atoms or molecules that transmits the laser light split by this optical splitter).
4) and a reflecting mirror (26) for reflecting the laser light transmitted through the optical filter, and the semiconductor laser by reflecting the laser light reflected by the reflecting mirror to the semiconductor laser through the optical branching device. A frequency-stabilized semiconductor laser device in which the frequency of laser light output from a laser is optically locked to a constant frequency and line width determined by the characteristics of the optical filter. 2. A photoelectric detector (27) for detecting the light intensity of laser light returning to the semiconductor laser, and an electrostrictive element (2) mounted on the reflecting mirror for varying the optical path direction position of the reflecting mirror.
9. A phase controller (28) comprising: 9) and a phase controller (28) for controlling the position of the reflecting mirror by the electrostrictive element so that the light intensity signal detected by the photoelectric detector has a maximum value. Frequency stabilized semiconductor laser device. 3. A semiconductor laser (21) for outputting a laser beam,
An optical branching device (23) which is inserted in the output path of the laser light output from this semiconductor laser and branches a part of the laser light.
And an optical filter (31) that utilizes absorption lines by sub-Doppler spectroscopy that transmits the laser light branched by this optical branching device, and a reflecting mirror that reflects the laser light that has passed through this optical filter (26a, 26b) ) Is provided, the laser light reflected by the reflecting mirror is returned to the semiconductor laser, and the frequency of the laser light output from the semiconductor laser is optically locked to a constant frequency and a line width determined by the characteristics of the optical filter. A frequency-stabilized semiconductor laser device characterized by the above. 4. A photoelectric detector for detecting the light intensity of laser light returning to the semiconductor laser, and a reflector mounted on the reflecting mirror,
An electrostrictive element for varying the position of the reflecting mirror in the optical path and a phase controller for controlling the position of the reflecting mirror by the electrostrictive element so that the light intensity signal detected by the photoelectric detector has a maximum value. The frequency-stabilized semiconductor laser device according to claim 3, further comprising: