JP2008166494A - Wavelength stabilized laser and interferometer - Google Patents

Abstract

A wavelength-stabilized laser that can output a laser beam with a very narrow bandwidth and can suppress fluctuations in oscillation wavelength and output is obtained.
A semiconductor laser 10 that is driven to be superposed at a high frequency; a feedback means 14 such as a half mirror that returns a part of a laser beam 11 emitted from the semiconductor laser 10 to the semiconductor laser; and the feedback means 14 and the semiconductor laser 10 In a wavelength stabilized laser comprising a wavelength selecting means 21 such as a band pass filter that passes only light of a specific wavelength that is disposed between and in the optical path of the laser beam 11 other than that returned to the semiconductor laser 10 Narrow band narrowing means 21 and 22 including a band pass filter or the like for further narrowing the wavelength of the laser beam 11 selected by the wavelength selecting means 21 are provided.
[Selection] Figure 1

Description

  The present invention relates to a wavelength stabilized laser that stabilizes the oscillation wavelength of a laser beam emitted from a semiconductor laser.

  The present invention also relates to an interferometer using the wavelength stabilized laser as described above as a light source.

  Conventionally, for example, as shown in Patent Document 1, a semiconductor laser, a feedback means such as a half mirror that returns a part of a laser beam emitted from the semiconductor laser to the semiconductor laser, and the feedback means and the semiconductor laser A wavelength-stabilized laser comprising a wavelength selection means such as a band-pass filter that is arranged between the two and transmits only light of a specific wavelength is known. In the wavelength stabilized laser having such a configuration, the laser beam selected by the wavelength selecting unit is returned to the semiconductor laser, so that the oscillation wavelength of the semiconductor laser is stabilized.

Also, in this type of wavelength stabilized laser, as shown in the above-mentioned Patent Document 1, it is possible to further suppress the longitudinal mode competition by driving the semiconductor laser with high frequency superposition to further stabilize the oscillation wavelength. Proposed.
JP 2001-223431 A

  The wavelength-stabilized laser disclosed in Patent Document 1 can achieve the intended purpose. However, when it is used as a light source for an interferometer, for example, the coherent length is not sufficient, which causes a reduction in resolution. There is also a problem that leads to Since this coherent length is inversely proportional to the wavelength width of the light, it is desired to further narrow the output laser beam in the wavelength stabilized laser as described above.

  In order to meet the demand for further narrowing of the wavelength-stabilized laser disclosed in Patent Document 1, it is conceivable to apply a narrower pass band as wavelength selection means such as a band-pass filter. In fact, by doing so, it can be confirmed that a laser beam with a narrower wavelength can be obtained.

  However, when such a bandpass filter having a narrower pass band is used, a new problem has been recognized that the oscillation wavelength and output are likely to fluctuate.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a wavelength-stabilized laser that can output a laser beam with a very narrow bandwidth and can suppress fluctuations in oscillation wavelength and output. .

  Another object of the present invention is to provide an interferometer that can obtain a high resolution by using a laser beam having a sufficiently large coherent length.

The wavelength-stabilized laser according to the present invention, as described above,
A semiconductor laser driven by high frequency superposition;
Feedback means for returning a part of the laser beam emitted from the semiconductor laser to the semiconductor laser;
In the wavelength stabilization laser comprising wavelength selection means arranged between the feedback means and the semiconductor laser and passing only light of a specific wavelength,
The optical path of the laser beam other than the laser beam returned to the semiconductor laser is provided with narrow band narrowing means for further narrowing the wavelength of the laser beam wavelength selected by the wavelength selecting means. is there.

  In addition, as the above-described wavelength selecting means and narrowing band means, those composed of bandpass filters can be suitably used. In that case, two band pass filters having the same characteristics as the band selection filter as the wavelength selection unit are used as the band narrowing unit, and these two band pass filters serve as the band pass filter as the wavelength selection unit. It is desirable to set the inclination so that the passbands are on the short wavelength side and the long wavelength side, respectively, with respect to the passband of the filter.

  On the other hand, the interferometer according to the present invention is characterized in that the wavelength stabilized laser according to the present invention described above is used as a light source.

  According to the research of the present inventor, the problem that the oscillation wavelength and the output are likely to fluctuate when the one having a very narrow pass band is applied as the wavelength selection means in the configuration described in Patent Document 1 is caused by the following points. Turned out to be. In other words, when wavelength selection means capable of realizing a narrow wavelength band required for a high-performance interferometer is applied, the longitudinal mode is unified and mode competition does not occur, but driving such as environmental temperature When the condition fluctuates, the unified longitudinal mode itself changes (it is considered to jump to the adjacent mode that can originally oscillate) and causes the above-mentioned problem. That is, it is considered that the effect of high-frequency superposition performed for stabilizing the output is impaired when wavelength selecting means such as a bandpass filter having a very narrow pass band is applied.

  The wavelength-stabilized laser of the present invention has been obtained based on this new knowledge, and the wavelength of the laser beam wavelength-selected by the wavelength selecting means is further added to the optical path of the laser beam other than that returned to the semiconductor laser. By providing the narrowing means for narrowing the band, it is possible to obtain an extremely narrow band laser beam that has passed through the narrowing means.

  On the other hand, if the laser beam to be used is narrowed in this way, the wavelength selection means arranged between the feedback means and the semiconductor laser is a comparison that can include several longitudinal modes. Those having a wide pass band can be applied. Therefore, for example, a laser beam consisting of several longitudinal modes can be fed back to the semiconductor laser, so that the effect of high frequency superposition is left, that is, a single longitudinal mode is prevented from jumping depending on driving conditions, Oscillation wavelength and output fluctuation can be suppressed.

  In the wavelength-stabilized laser of the present invention, a bandpass filter that allows a desired wavelength to pass can be used as the band narrowing unit. In particular, the band narrowing unit is the same as the bandpass filter as the wavelength selecting unit. Two characteristic bandpass filters are used, and these two bandpass filters are inclined so that the passbands are on the short wavelength side and the long wavelength side, respectively, from the passband of the bandpass filter as the wavelength selection means. If set, the three band-pass filters can be formed from the same process using the same substrate, so that the cost of the apparatus can be reduced.

  That is, this type of band-pass filter is usually manufactured by controlling the conditions such as the film thickness of the multilayer coating each time so as to obtain desired characteristics. If the characteristics of the filter are the same, the three band-pass filters can be manufactured at the same time by cutting a relatively large substrate coated with a multilayer film, etc., and the cost can be reduced. In addition, it is possible to simultaneously produce more, for example, 30 band-pass filters for 30 wavelength stabilizing lasers, and the cost reduction effect in that case is greater. If the cost reduction of the bandpass filters is achieved in this way, naturally, the cost reduction of the wavelength stabilized laser using those bandpass filters can be realized.

  In addition, when a band pass filter is used as both the wavelength selection unit and the band narrowing unit, use a band pass filter that is narrower than the band pass filter that is the wavelength selection unit. Also good.

  On the other hand, since the wavelength stabilized laser according to the present invention described above is used as a light source, the interferometer according to the present invention uses a laser beam with a narrowed wavelength, that is, a sufficiently large coherent length. Thus, high resolution can be obtained.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows a wavelength-stabilized laser according to an embodiment of the present invention, and FIG. 2 shows a semiconductor laser driving circuit thereof. As shown in FIG. 1, the wavelength stabilized laser 1 is converted into a collimated light by a semiconductor laser 10, a collimator lens 12 for collimating a laser beam 11 emitted from the semiconductor laser 10 in a divergent light state. A condensing lens 13 for converging the laser beam 11, a half mirror 14 disposed at the converging position of the laser beam 11 by the condensing lens 13, and a narrow band first band disposed between the lenses 12 and 13. A pass filter (hereinafter referred to as BPF) 21, a collimator lens 15 that collimates the laser beam 11 that has passed through the half mirror 14 and then diverges, and a narrow path disposed in the optical path of the collimated laser beam 11 A second BPF 22 in the band and a third BPF 23 in the narrow band disposed in the optical path are also included.

  The first BPF 21 constitutes the wavelength selecting means in the present invention, while the second and third BPFs 22 and 23 constitute the band narrowing means in the present invention, but have the same characteristics. Is used. Such BPFs 21, 22 and 23 having the same characteristics are formed by, for example, forming a predetermined dielectric multilayer film corresponding to a desired pass band on a substrate sufficiently larger than each size, and then cutting the substrate small. Produced. Thus, if the three BPFs 21, 22 and 23 are produced by a common process other than the last cutting process, they can be obtained at a relatively low cost.

  These BPFs 21, 22 and 23, which are dielectric multilayer filters, have a characteristic that the pass band becomes lower as the inclination is larger within a predetermined inclination range with respect to the optical path of light passing through (in this case, transmitted). In the present embodiment, the inclinations are set so that BPF22 is the maximum, BPF23 is the minimum, and BPF21 is an intermediate value. Thereby, the passbands of these BPFs 21, 22 and 23 are as shown by 31, 32 and 33 in the schematic diagram 2 respectively.

  On the other hand, a semiconductor laser 10 that emits a laser beam 11 having a central wavelength of 405 nm is used. Here, if there are a plurality of Fabry-Perot modes between the cleavage planes of the semiconductor laser 10 in the passband of the BPF 21, longitudinal mode competition may occur. In order to suppress this longitudinal mode competition, the semiconductor laser 10 is driven by a semiconductor laser driving circuit 40 whose detailed configuration is shown in FIG.

  That is, in this drive circuit 40, a high frequency wave emitted from the AC power supply 43 and passed through the capacitor 44 is superimposed on a DC current component emitted from the DC power supply 41 and passed through the coil. This high frequency superimposed current is applied to the semiconductor laser 10. As described above, the modulation driving is performed by superimposing the high frequency on the driving current of the semiconductor laser 10, so that the driving current does not stay in the region causing the longitudinal mode competition, and the longitudinal mode competition is suppressed. In this example, the frequency of the high frequency is 350 MHz, and the amplitude is about 50% of the peak value of the driving current of the semiconductor laser 10.

  The passband (half-value width) of the first BPF 21 is 1.0 nm, and the laser beam 11 having a center wavelength of 405 nm is wavelength-selected by this BPF 21. The laser beam 11 thus selected for wavelength is reflected by the mirror 14 and returned to the semiconductor laser 10, so that so-called optical feedback is performed, so that the oscillation wavelength of the semiconductor laser 10 is stabilized. The laser beam 11 whose wavelength is stabilized in this way is partially transmitted through the half mirror 14 and further passes through the BPFs 22 and 23 and used for a predetermined application.

  In this wavelength stabilized laser, the coherent length of the semiconductor laser 10 is about 100 mm. The optical length between the light emitting end face of the semiconductor laser 10 and the mirror 20 is 150 mm, which is larger than the coherent length.

  Further, as described above, when the passbands of the three BPFs 21, 22, and 23 are as shown by 31, 32, and 33 in FIG. 2, respectively, lasers that pass through the BPFs 22 and 23 and become used light The beam 11 is only in the wavelength band indicated by hatching in FIG. That is, since both skirt portions of the pass band of BPF 21 are cut by BPF 22 and 23, even if there are several oscillation longitudinal modes defined by the pass band of BPF 21, the laser beam passing through BPF 22 and 23 is present. 11 is a single vertical mode, or the vertical mode is less than the above several. Thus, in this embodiment, a laser beam 11 having a very narrow band can be obtained as the used light.

  On the other hand, as the first BPF 21, one having a pass band wider than the pass bands of both the BPFs 22 and 23 is applied, so that the semiconductor laser 10 has, for example, several longitudinal modes as described above. Since the laser beam 11 is fed back, fluctuations in the oscillation wavelength and output can be suppressed while leaving the effect of high frequency superimposition as it is.

  In particular, in the present embodiment, as described above, the three BPFs 21, 22 and 23 have the same characteristics and can be manufactured from the same process using the same substrate, so that they can be obtained at a low cost. As a result, cost reduction of the wavelength stabilization laser 1 is achieved.

  However, it is not always necessary to configure the wavelength selection unit and the narrowband unit in the present invention from the BPF having the same characteristics as described above. For example, the narrowband unit has a desired wavelength whose passband is narrower than that of the wavelength selection unit. A BPF made of a dielectric multilayer film that transmits only the band may be used. When a BPF that transmits only a desired wavelength band is used, the BPF can be used without being inclined.

  Further, when using the three BPFs 21, 22 and 23 as described above, the wavelength holding light source can be changed by configuring the filter holding means so that the inclination of the three BPFs 21, 22 and 23 can be changed. Can be formed. In addition, while the inclination of BPF 21 is fixed, by configuring the filter holding means so that the inclinations of BPF 22 and 23 can be changed, it is possible to form a spectrally variable light source having a constant center wavelength.

  Next, an embodiment of an interferometer according to the present invention will be described with reference to FIG. The interferometer of this embodiment is a Michelson interferometer as an example, and the wavelength stabilized laser 1 according to the present invention is used as a light source.

  In this Michelson interferometer, a laser beam 11 emitted from a light source (wavelength-stabilized laser) 1 is collimated by a collimator lens 50 and divided into two optical paths (amplitude division) by a half mirror 51. The two divided laser beams 11 are reflected by mirrors 52 and 53, respectively, are returned to their original optical paths, overlapped by a half mirror 51, condensed by a condensing lens 54, and then converged by an imaging lens 55. Sent to camera 56.

  In the TV camera 56, an image of interference fringes is captured by the laser beam 11 that is superimposed after being divided into two. Therefore, if one mirror 52 is a plane (reference surface) polished with high precision and the other mirror 53 is the test surface, the shape of the test surface can be measured based on the interference fringes. It becomes.

  As described above, this interferometer using the wavelength stabilized laser 1 according to the present invention as a light source has a high resolution by using the laser beam 11 whose wavelength is sufficiently narrowed, that is, having a sufficiently large coherent length. Is obtained. Such an effect is not limited to the Michelson interferometer, but can be generally obtained when the wavelength stabilized laser 1 according to the present invention is applied to other types of interferometers.

  The wavelength stabilized laser according to the present invention is not limited to the above-described interferometer such as a Michelson interferometer, but can be applied to other devices that require a narrow-band laser beam. Of course.

1 is a schematic side view showing a wavelength-stabilized laser according to an embodiment of the present invention. Circuit diagram showing the drive circuit of the wavelength stabilized laser Schematic showing the characteristics of wavelength selection means and narrowband means in the wavelength stabilized laser 1 is a schematic plan view showing an interferometer according to an embodiment of the present invention.

Explanation of symbols

1 wavelength stabilized laser
10 Semiconductor laser
11 Laser beam
12, 50 Collimator lens
13, 54 condenser lens
14, 51 half mirror
15 Collimator lens
21 First bandpass filter (wavelength selection means)
22 Second bandpass filter (band narrowing means)
23 Third bandpass filter (band narrowing means)
40 Semiconductor laser drive circuit
52, 53 mirror
55 Imaging lens
56 TV camera

Claims (5)

A semiconductor laser driven by high frequency superposition;
Feedback means for returning a part of the laser beam emitted from the semiconductor laser to the semiconductor laser;
In the wavelength stabilization laser comprising wavelength selection means arranged between the feedback means and the semiconductor laser and passing only light of a specific wavelength,
Wavelength stabilization characterized in that narrowing means for further narrowing the wavelength of the laser beam wavelength selected by the wavelength selecting means is provided in the optical path of the laser beam other than that returned to the semiconductor laser. Laser.   2. A wavelength stabilized laser according to claim 1, wherein said wavelength selecting means and said narrow band narrowing means comprise band pass filters. The bandpass filter as the band narrowing means comprises two bandpass filters having the same characteristics as the bandpass filter as the wavelength selection means,
The two band-pass filters are set so that the inclination is set so that the pass band is on the short wavelength side and the long wavelength side, respectively, from the pass band of the band-pass filter as the wavelength selecting means. Item 3. A wavelength-stabilized laser according to Item 2.   The wavelength-stabilized laser according to claim 2, wherein the band-pass filter as the narrow band narrowing means has a narrower pass band than the band pass filter as the wavelength selecting means.   5. An interferometer, wherein the wavelength stabilized laser according to claim 1 is used as a light source.

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