JPS60146201A - Laser light absorbing device - Google Patents

Abstract

PURPOSE:To prevent a local temperature rise, and to prevent a damage of an absorbing plate by constituting the titled device so that a coefficient of linear absorption of the absorbing plate increases toward an emitting side from an incident side, and an energy absorption rate in the absorbing plate becomes uniform. CONSTITUTION:This device is constituted of a laser light 1 polarized in parallel to an incident surface, an auxiliary absorbing plate 6 consisting of a color glass filter whose transmittivity is <=1/10<6>%, and an absorbing plate 7. Until the incident light is attenuated to 1%, a coefficient of linear absorption is increased in accordance with lambda=lambda0/(1- lambda0X) (in the expression, lambda, lambda0 and X denote a coefficient of linear absorption, a coefficient of linear absorption of an incident end, and a distance separated from the incident end, respectively), and thereafter, it is kept constant. In case when an incident energy of the laser light is 10J, when a path length in the absorbing plate is 0.99/lambda0, its energy is attenuated to 0.1J. A range for increasing the coefficient of linear absorption is changed by an intensity of the incident light. As for the laser light which has been made incident at a Brewster's angle with a normal of the plate 7, the whole energy is absorbed in the plate 7. The reflected light generated by a fact that the laser light is not a complete parallel is absorbed by the auxiliary absorbing plate 6.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] This invention relates to a laser light absorption device used in laser scattering measurement and the like.

[Background of the invention]

In laser scattering measurement, which irradiates atoms and molecules with laser light and measures the state of the atoms and molecules from the wavelength and intensity of the scattered light or fluorescent light, the laser light scattered by surrounding objects and walls enters the detector. It becomes the background. Therefore, the laser beam that has passed through the measurement area needs to be completely absorbed.

Brewster's law is usually used to absorb laser light. As shown in FIG. 1, when laser light l polarized parallel to the incident plane is incident at Brewster's angle θB, the laser light is completely transmitted without being reflected. Assuming that the refraction weight of the transmitting material 4 is n, the Brewster angle θB is given by tan θB=n. A conventional laser light absorption device is shown in FIG. Laser beam 1 polarized parallel to the plane of incidence
When the laser beam is incident on the absorption plate 5 with a Brewster angle θnk, all the laser light enters the absorption plate 5 and is absorbed therein. As the absorption plate 5, a normally colored glass filter is used. Actual laser light is not perfectly parallel light, but has a diffusion angle of about 1 milliradian. Therefore, the laser source does not enter the absorption plate 5 with the Brewster angle θBf, and a portion of the light is reflected. An auxiliary absorption plate 6 is provided to absorb this reflected light.

The auxiliary absorption plate 6 also uses a glass filter of the same color as the absorption plate 5. The problem with this conventional method is that since the linear absorption coefficient of the absorption plate 5 in the direction of incident light is constant, more energy is absorbed on the incident side than on the output side. Therefore, a rapid temperature rise occurs at the incident end, and the absorber 5 may be damaged due to thermal expansion. Furthermore, since the reflected light from the absorption plate 5 forms a Brewster-angle and does not enter the auxiliary absorption plate 6, the reflected light from the auxiliary absorption plate 6 is the main cause of background.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to overcome the drawbacks of the prior art described above and provide a highly reliable laser light absorption device.

[Summary of the invention]

In order to achieve the above object, for the former, in the laser light absorption device of the present invention, the linear absorption coefficient is changed so that the absorption rate of laser light becomes uniform within the absorption plate 5. or,
For the latter, an antireflection film with a thickness of one wavelength is deposited on the incident surface of the auxiliary absorption plate. The absorption rate of the laser light intensity within the absorption plate 5 is dl=-λiax (1). Here, λ is the linear absorption coefficient, and ■ is the laser light intensity. The laser light intensity at a position X away from the laser light incident end is I == I e 6- fo "λd x (2). Here, ■ is the incident light intensity. Expression (2) is (
1) Substituting into the equation yields the following equation.

d■=-,I. λe−/, xλdXdx (3) Therefore,
In order to make the energy absorption rate of laser light uniform, (3
) as long as it is constant. That is, λe−,ノ”o””
It is sufficient if x = Co (const l (4) holds. When formula (4) is solved, λ = λo/(1 - λ, xl
(5) It becomes. Here λ. is the linear absorption coefficient at the input end. Therefore, by increasing the linear absorption coefficient of the absorption plate 5 from the incident side to the output side so as to satisfy equation (5), the energy absorption ratio within the absorption plate 5 can be made uniform. .

Thereby, local thermal expansion can be prevented, and damage to the absorption plate 5 can be prevented. Furthermore, as is clear from equation (5)', the thickness of the absorption plate 5 required to completely absorb the laser beam depends on the linear absorption coefficient at the incident end, and is 1/
λ0.

[Embodiments of the invention]

Hereinafter, the present invention will be explained in detail with reference to Examples.

FIG. 3 shows one embodiment of the present invention. In the figure, ■ is
Laser light polarized parallel to the plane of incidence, 6, has a transmittance of 10
- an auxiliary absorption plate consisting of a colored glass filter of not more than 6%;
7V'i As mentioned above, this is an absorption plate made of a colored glass filter whose linear absorption coefficient increases from the incident side to the output side. In Equation (5), the linear absorption coefficient at the output end must be infinite, and such a colored glass filter cannot be manufactured. Therefore, in the present invention, as shown in FIG.
The absorption coefficient is increased according to equation (5) until it is attenuated to
Then, the energy is 0. It is attenuated to l J.

Therefore, even if the linear absorption coefficient is kept constant within a range where the path length is greater than 099/λ0, no damage will occur due to thermal expansion. The thickness of the absorber 7 required to attenuate the intensity of the laser beam by 1 (1''6 times) is 1.09/λ0.

According to equation (5), the range in which the linear absorption coefficient is increased must be changed depending on the intensity of the incident light. The laser beam 1 that enters at a Brewster's angle with the normal line of the absorption plate 7 has all its energy absorbed within the absorption plate 7. The reflected light generated from the fact that the laser beam is not completely parallel light is absorbed by the auxiliary absorption plate 6.
It is absorbed by.

FIG. 5 shows another embodiment of the present invention. The auxiliary absorption plate 6 absorbs the laser beam reflected by the absorption plate 7, but since the reflected light from the absorption plate 7 and the auxiliary absorption plate 6 do not form a Brewster angle, the auxiliary absorption plate 6 absorbs the laser beam reflected by the absorption plate 7. There is light that is reflected. Therefore, an antireflection film 8f having an optical thickness of one wavelength is provided on the incident surface of the auxiliary absorption plate 6. As a result, the reflection from the auxiliary absorber 6'! In other words, the background caused by laser light can be reduced. The energy density of the laser beam incident on the absorption plate 7 is high, so if an anti-reflection film is provided on top of it, it will be damaged. However, the energy incident on the auxiliary absorption plate 6 is The rate is more than two orders of magnitude higher than that in comparison, so it is possible to deposit an antireflection film.

〔Effect of the invention〕

As explained above, according to the present invention, the linear absorption coefficient of the absorption plate is increased from the incident side to the output side, and the energy absorption rate within the absorption plate is made uniform, thereby suppressing local temperature rise. It can be prevented. This prevents damage to the absorption plate.

Furthermore, by depositing an antireflection film on the auxiliary absorption plate, it is possible to reduce the incidence of laser light on the detection device.

These improvements can improve the reliability and performance of the laser light absorption device, and greatly contribute to the measurement accuracy in laser scattering measurements.

[Brief explanation of the drawing]

Fig. 1 is a principle diagram showing Brewster's law, Fig. 2 is an explanatory diagram showing an embodiment of the conventional technique, Fig. 3 is a perspective view showing an embodiment of the present invention, and Fig. 4 is a diagram showing the principle of Brewster's law. FIG. 5 is an explanatory diagram showing the relationship between the change in the linear absorption coefficient of the absorption plate and the attenuation of laser light, and FIG. 5 is a perspective view showing another embodiment of the present invention. $1 (] $20 $3rlt '$4 Closed p and umbrella flea in the forest

Claims (1)

[Claims] 1. Install an absorption plate whose linear absorption coefficient increases in the direction of the incident light so that its normal line forms a Brewster angle with the direction of the incident light, and make an angle with the absorption plate perpendicular to the plane of incidence that is less than the Brewster angle. 1. A laser light absorption device characterized in that an auxiliary absorption plate is provided at a position sandwiching the incident light.