CN204720772U - A kind of laser coherence length conditioning equipment based on spectral dispersion principle - Google Patents

Abstract

The utility model discloses a laser coherent length adjustment device based on the principle of spectral dispersion. The device includes a laser light source unit (1), a spectral dispersion unit (2) and a spectral control unit (3), and may also include a spectral detection unit (4). , the laser light (S1) emitted by the laser light source unit (1) is incident on the spectral dispersion unit (2), and the spectral dispersion unit (2) disperses and splits the laser light (S1) to make it incident on the spectrum control unit (3), The spectrum control unit (3) is used to adjust the spectral components of the dispersed laser light, so as to control the coherence length of the output laser light. The utility model adjusts the laser coherence length outside the laser cavity, and the operation is simple and convenient, and does not affect the stability of the laser system itself. The operation is simple, convenient, and reliable, and can be well applied to various interferometric measurements field.

Description

A laser coherence length adjustment device based on the principle of spectral dispersion

technical field

The utility model relates to the field of laser technology, in particular to a laser coherent length adjustment device based on the principle of spectral dispersion.

Background technique

The coherence length is defined for the time coherence of the light source. It is a physical quantity to measure the time coherence performance of the light source. It is defined as the maximum optical path difference that the light source light can achieve coherence; the time coherence of the light source is also reflected in its monochromaticity. The specific value The index is spectral line width (referred to as line width). It can be seen that the laser coherence length (L _c ) and the laser linewidth (Δλ) are two closely related physical quantities, and they approximately satisfy the following formula: L _c ≈λ ² /Δλ. Therefore, adjusting the laser coherence length is inversely proportional to adjusting the laser line width.

Since the American scientist T.H. Maiman used ruby crystals to realize the world's first laser in 1960, lasers have been widely used in various optical measurement technologies due to their good coherence, such as the well-known coherent optical fiber communication, laser radar, Quantum frequency standard, holography and other fields. On the other hand, wide-spectrum, low-coherence and high-brightness light sources also have a lot of application space, such as coherence tomography, rainbow measurement, tiny object scanning and other precision measurement technology applications.

The coherence length of the commonly used laser is fixed. If some measures are taken to realize the continuous adjustment of the laser coherence length, its application range can be greatly expanded, the measurement accuracy can be improved and the installation and adjustment can be facilitated. Taking the high-precision surface shape detection as an example, when using point diffraction interferometer to detect the surface shape error of the reflective element, increase the coherence length of the light source (such as ~cm level) in the rough adjustment stage, so as to facilitate the rough determination of the equal optical path position; In the fine-tuning stage, the coherent length of the light source (such as ~ mm order) is reduced to facilitate more accurate determination of the equi-optical path position, reduce the impact of environmental factors and light source frequency stability on the measurement results, eliminate coherent noise, and improve measurement accuracy.

Most of the existing technologies for adjusting the laser coherence length are to adjust the parameters of the laser cavity itself (such as: cavity length, loss, etc.) to realize the adjustment of the output laser coherence length, which belongs to intracavity adjustment. For example: the technology of adjusting the cavity length of the resonator, which is to adjust the coherence length of the laser by controlling the number of longitudinal modes oscillating simultaneously in the laser cavity. This method is easy to cause detuning of the laser cavity during the stretching process of the cavity length, and the excessively long cavity length will lead to an excessively large system volume.

Utility model content

(1) Technical problems to be solved

The utility model aims to solve the problems that the existing technology for adjusting laser coherence length easily causes laser cavity detuning and system volume is too large.

(2) Technical solution

In order to solve the above technical problems, the utility model proposes a laser coherence length adjustment device, including a laser light source unit, a spectrum dispersion unit and a spectrum control unit, wherein the laser light emitted by the laser light source unit is incident on the spectrum dispersion unit; The dispersion unit disperses and splits the laser light and makes it incident on the spectrum control unit; the spectrum control unit is used to adjust the spectral components of the laser light after dispersion and split, so as to control the coherence length of the output laser light.

According to a specific embodiment of the present invention, the device further includes a spectrum detection unit, which is used to measure the coherence length of the output laser light after the spectral components have been adjusted.

According to a specific embodiment of the present invention, the spectral dispersion unit includes a grating, a prism, and an F-P etalon.

According to a specific embodiment of the present invention, the spectrum control unit is a combination of a beam focusing element and a slit, or a combination of a beam focusing element and an aperture.

According to a specific embodiment of the present utility model, the slit and the aperture are fixed in size or adjustable in size.

According to a specific embodiment of the present invention, the spectrum detection unit includes a Michelson interferometer, a Fabry-Perot interferometer, a spectrometer, and the like.

(3) Beneficial effects

The utility model utilizes the principle of spectral dispersion to adjust the coherent length of the laser. All adjustments are carried out outside the laser cavity. The operation is simple and convenient, and will not affect the stability of the laser system itself. Applied in various interferometry and other related fields.

Description of drawings

Fig. 1 is the schematic diagram of the optical path structure of the laser coherence length adjustment device based on the principle of spectral dispersion of the present invention;

2 is a schematic diagram of the optical path of the first embodiment of the laser coherence length adjustment device based on the principle of spectral dispersion of the present invention;

Fig. 3 is a schematic diagram of the optical path of the second embodiment of the laser coherence length adjustment device based on the principle of spectral dispersion of the present invention.

Detailed ways

Fig. 1 is a schematic diagram of the optical path structure of the laser coherence length adjustment device based on the principle of spectral dispersion of the present invention. As shown in FIG. 1 , the device includes a laser light source unit 1 , a spectrum dispersion unit 2 , and a spectrum control unit 3 .

Wherein, the laser light S1 emitted by the laser light source unit 1 is incident to the spectrum dispersion unit 2 , and then incident to the spectrum control unit 3 after being dispersed and split. The spectrum control unit 3 can adjust the spectral components of the output laser light after dispersion and splitting, so as to control the coherence length of the output laser light.

The device may also include a spectrum detection unit 4, the adjusted laser S2 is used as output laser light, and enters the spectrum detection unit 4 to measure its coherence length.

The laser light source unit 1 may include a laser light source of any wavelength, and may also include a pulsed laser or a continuous laser light source. The spectral dispersion unit 2 may include gratings, prisms, F-P etalons, etc. or various combinations thereof. The spectrum control unit 3 may be a combination of various beam focusing elements, slits, diaphragms and the like. The slits and diaphragms can be fixed or adjustable. The spectrum detection unit 4 may include spectrometers, Michelson interferometers, Fabry-Perot interferometers and other spectral detection instruments.

In order to make the purpose, technical solutions and advantages of the utility model clearer, the utility model will be further described in detail below in conjunction with specific embodiments.

Fig. 2 is a schematic diagram of the optical path of the first embodiment of the laser coherence length adjustment device based on the principle of spectral dispersion of the present invention. As shown in FIG. 2 , the device of this embodiment includes: a laser light source 11 , a collimating mirror 12 , a diffraction grating 21 , a focusing mirror 31 , an exit slit 32 , and a Michelson interferometer 41 . Wherein, the laser light source 11 and the collimating mirror 12 constitute the laser light source unit 1, the diffraction grating 21 is a spectral dispersion element, constitute the spectral dispersion unit 2, the focusing mirror 31 and the exit slit 32 constitute the spectral control unit 3, and the Michelson interferometer 41 constitutes Spectrum detection unit 4.

The laser light emitted by the laser source 11 is collimated by the collimating mirror 12 and then incident on the diffraction grating 21 at a certain angle. Diffraction grating 21 will disperse and split the incident laser light. According to the grating equation (d(sinθi± _sinθo )= _mλ , m=0, ±1, ±2,...), it will form many diffraction orders of outgoing laser light, and Each diffraction order (m≠0) laser corresponds to a series of corresponding wavelengths of laser light with different exit angles. Take the higher-energy diffraction splitting order and make it incident on the focusing lens 31. Since each incident wavelength of laser light corresponds to a different incident angle, at the focal plane of the focusing lens 31, each wavelength of laser light will be arranged separately in space. The light transmission size of the exit slit 32 can be continuously adjusted mechanically, placed on the focal plane of the focusing lens 31, and the center of the slit is located at the center of the focused spot. By adjusting the size of the exit slit 32 , the spectral components of the output laser can be controlled, thereby controlling the line width and coherence length of the output laser. Finally, a Michelson interferometer 41 is used to measure the coherence length of the regulated laser light.

The diffraction grating has relatively high diffraction efficiency for the laser wave band of the corresponding light source.

The principle of adjusting the laser linewidth and coherence length will be described in detail below in terms of the parallel light beam splitting by the grating and focusing by the focusing mirror.

From the knowledge of geometrical optics, we know that: a. Any light incident on the center of the lens, the direction of propagation after passing through the lens remains unchanged; b. After a beam of parallel light of a specific wavelength is incident on the lens, the focal point is located on the focal plane behind the lens. Therefore, if a beam of parallel light with the same wavelength is incident on the lens, the intersection point of the light passing through the center of the lens and the focal plane of the lens corresponds to the converging point of the incident parallel light after passing through the lens.

As shown in FIG. 2 , suppose a beam of parallel light S1 with a linewidth of Δλ=λ ₁ -λ ₂ is incident on the grating 21, and after being diffracted and split by the grating 21, it will be divided into multiple beams of parallel light with different exit angles. According to the grating _formula , each exit angle corresponds to a specific wavelength within the corresponding order λ1 to _λ2 . Take m=first-order diffracted light to calculate the distance Δh between two corresponding converging points on the focal plane of two beams of parallel light with wavelengths λ ₁ and λ ₂ .

Assuming that the incident angle of the parallel light beam on the grating is θ _i , the diffraction angle of the wavelength λ ₁ light in the m=1 order diffracted light is θ _o1 , and the diffraction angle of the wavelength λ ₂ light in the m=1 order diffracted light is θ _o2 ; the grating The distance between the focusing mirror 31 and the focusing mirror 31 is L ₁ , the distance between the focusing mirror 31 and the focal plane is L ₂ =f; the grating constant is d, then by the grating equation:

d(sinθ _i -sinθ _o1 )＝λ ₁ (1)

d(sinθ _i -sinθ _o2 )＝λ ₂ (2)

Then the angle difference between the laser beams with two wavelengths is:

Δθ＝θ _o2 -θ _o1 (3)

＝arc sin[sinθ _i -(λ ₁ -Δλ)/d]-arcsin[sinθ _i -λ ₁ /d]

Therefore, at the focal plane of the lens, the distance between the focal points of the two wavelength lasers is:

Δh≈Δθ·L ₂ (4)

＝[arc sin(sinη _i -(λ ₁ -Δλ)/d)-arcsin(sinθ _i -λ ₁ /d)]·f

It can be seen from the above formula that on the focal plane of the focusing mirror 31, the laser beams of the _two wavelengths λ1 and _λ2 are spatially separated by a distance Δh, and the distance Δh is also different with the difference of the incident laser line width Δλ. Therefore, by adding a size-adjustable slit 32 at the focal point of the focusing mirror 31, it is possible to control how many spectral components pass through the slit 32, thereby adjusting the output laser line width and coherence length.

Fig. 3 is a schematic diagram of the optical path of the second embodiment of the laser coherence length adjustment device based on the principle of spectral dispersion of the present invention. As shown in FIG. 3 , the device of this embodiment includes a collimated laser source 13 , a prism 22 , a focusing mirror 31 , an exit slit 32 , and a Michelson interferometer 41 . Wherein, the prism 22 is a spectral dispersion element, the focusing mirror 31 and the exit slit 32 constitute a spectral control element, and the Michelson interferometer 41 is a spectral detection element. Described collimated laser source 13 sends out the collimated laser light that line width is Δλ=λ ₁ -λ ₂ , and laser light is incident on the prism 22, and after its dispersion splits light, on the exit surface of prism 22 from the direction of apex angle to bottom angle A series of parallel lights with wavelengths from large to small will be formed on the surface, and these parallel lights are incident on the focusing mirror 31 . According to the above analysis, since each wavelength of laser light corresponds to a different incident angle, at the focal plane of the focusing lens 31 , each wavelength component will be arranged separately in space. The light passing size of the exit slit 32 is fixed, placed on the focal plane of the focusing lens 31 , the center of the slit is located on the optical axis of the focused beam, and can move forward or backward continuously along the optical axis. By adjusting the position of the exit slit 32 on the optical axis, the number of spectral components passing through the slit can be controlled, thereby controlling the line width and coherence length of the output laser. Finally, a Michelson interferometer 41 is used to measure the coherence length of the regulated laser light.

The above analysis is the ideal adjustment situation of the utility model. Many factors affecting the coherence length of the output laser may be introduced in the actual experiment, such as: the beam quality and collimation of the laser source itself, the chromatic aberration and aberration of the lens itself, grating, prism Or the manufacturing error introduced by the etalon during the processing and so on.

In summary, the laser coherence length adjustment device based on the principle of spectral dispersion proposed by the utility model is based on the principle of spectral dispersion and uses dispersion elements and control elements to control the spectrum of the light source without any impact on the laser cavity itself. All operations are in the The laser is carried out outside the cavity, the operation is simple and convenient, and it is suitable for most output lasers.

The specific embodiments described above have further described the purpose, technical solutions and beneficial effects of the utility model in detail. It should be understood that the above descriptions are only specific embodiments of the utility model and are not intended to limit the utility model. For new models, any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present utility model shall be included within the protection scope of the present utility model.

Claims (6)

1. A laser coherence length adjustment device, comprising a laser light source unit (1), a spectral dispersion unit (2) and a spectral control unit (3), wherein, The laser light (S1) emitted by the laser light source unit (1) is incident on the spectral dispersion unit (2); The spectral dispersion unit (2) disperses and splits the laser light (S1) to make it incident on the spectral control unit (3); The spectrum control unit (3) is used to adjust the spectral components of the dispersed laser light, so as to control the coherence length of the output laser light. 2. The laser coherence length adjustment device according to claim 1, further comprising a spectrum detection unit (4), which is used to measure the coherence length of the output laser light after the spectral components have been adjusted. 3. The laser coherence length adjustment device according to claim 1 or 2, characterized in that the spectral dispersion unit (2) comprises a grating, a prism or an F-P etalon. 4. The laser coherence length adjustment device according to claim 1 or 2, characterized in that the spectrum control unit (3) is a combination of a beam focusing element and a slit, or a combination of a beam focusing element and an aperture. 5 . The laser coherence length adjustment device according to claim 4 , wherein the slit and the aperture are fixed in size or adjustable in size. 5 . 6. The laser coherence length adjustment device according to claim 2, characterized in that, the spectral detection unit (4) comprises a Michelson interferometer, a Fabry-Perot interferometer, and a spectrometer.