DOI: 10.22184/1993-7296.FRos.2023.17.5.366.370
The design of an optical system for focusing a laser beam with an extremely small spot size in a wide range of distances from 50 m to 2 km is presented. Difficulties with controlling the laser beam and focusing it are reduced by using a circuit with a laser beam expander.
The design of an optical system for focusing a laser beam with an extremely small spot size in a wide range of distances from 50 m to 2 km is presented. Difficulties with controlling the laser beam and focusing it are reduced by using a circuit with a laser beam expander.
Теги: collimator freeform laser beam expander laser beam focusing коллиматор расширитель лазерного пучка фокусировка лазерного пучка фриформ
Laser Focusing System
I. P. Shishkin,, A. P. Schkadarevich, STC “LEMT” BELOMO, Minsk, Republic of Belarus
The design of an optical system for focusing a laser beam with an extremely small spot size in a wide range of distances from 50 m to 2 km is presented. Difficulties with controlling the laser beam and focusing it are reduced by using a circuit with a laser beam expander.
Keywords: laser beam focusing, collimator, laser beam expander, freeform
Article received on May 10, 2023
Article accepted on July 24, 2023
Introduction
Optical systems for focusing a laser beam ensure the efficiency of the use of optical-electronic systems in actual communication and control loops. The optical target designation system should create an extremely small laser spot on the object of observation with a satisfactory correction of aberrations, the magnitude of which is especially important at long distances.
The value of the laser spot in the first approximation can be represented as the sum of geometric and aberration spots:
∅ = ∅a + ∅г.
The geometric spot ∅г is calculated by the formula:
∅г = ∅в · Д / fк,
where ∅в – is the diameter of the laser fiber; Д – is the distance; fк – is the focal length of the collimator.
For the selected laser and a given range of distances, the size of the geometric spot will depend only on the focal length of the collimator, the value of which, in turn, is limited by the possibility of manufacturing large-diameter lenses, the dimensions of the mechanics and the focusing mechanism design.
The aberration spot will be determined by the level of aberration correction in the optical system. The optimal solution for creating a focusing laser system is a 2‑lens collimator circuit in which the lenses are separated by an air gap. By optimizing the 4 lens radii, it is possible to achieve satisfactory aberration correction and obtain an extremely small aberration spot, the size of which is especially important at long distances when the geometric spot is maximum. Focusing in the collimator is carried out by moving the output end of the laser fiber.
Two-lens collimator
In a two-lens collimator, the shape and relative position of the lenses have a significant impact on the correction of aberrations. Fig.1 shows the design options of a collimator with a focal length of 1 m for a laser with a divergence angle θ = 100 Mrad (numerical aperture Na = 0.1).
Fig. 2 shows ray aberrations plots of collimators for the axial point at infinity. The plots comparison demonstrate that the ray aberrations in the second variant are 10 times less than in the first one.
Spot diameters in mm for a laser with a fiber diameter of 25 um and a collimator with a focal length of 1m at various distances are shown in Table 1.
Beam expander
The larger focus of the collimator, the smaller geometric spot, but at the same time the dimensions of the structure grow, which means that difficulties arise with controlling the laser beam and focusing it. The scheme with a laser beam expander, which is shown in Fig. 3, allows to significantly reduce the length of the collimator [1]. At the same time, the focusing mechanism is simplified by moving the negative lens in the beam expander, which is essentially a Galileo telescope.
The design of the collimator in Fig.3 includes a collimating lens with a focus of 200 mm and a telescope with 5× magnification. Thus, the equivalent focus of the collimator will be 1m, and the length will be reduced by almost half. To reduce the number of lenses, the output lens of the telescope is made aspherical. If a breaking mirror is installed in parallel, then this solution can also be used to combine beams of several lasers (at least two).
MIRROR COLLIMATOR
If you use mirrors in the design of the collimator, then you can reduce its size as much as possible. Fig. 4 shows a collimator constructed according to the well–known classical scheme of a two-mirror astronomical lens, in which a large mirror is parabolic (Newton’s type), hyperbolic (Cassagrain’s type) or ellipsoidal (Gregory’s type). The small mirror in all schemes is flat. In the proposed scheme, the large mirror is spherical, and the small mirror is parabolic.
The disadvantage of a two-mirror collimator is the shielding of the central zone of the entrance pupil.
The effective area of the entrance pupil during the reverse movement can be calculated from the difference in the area of the mirrors:
,
where S1 и S2 – area of the mirrors, D1 и D2 – their diameters.
The area of a small mirror is determined both by the size of the concave radius of the large mirror R2 (the smaller the radius of the large mirror, the smaller the area of the small mirror and the larger the effective area of the entrance pupil) and the size of the aberration spot.
Table 2 shows examples of the ratios of the radius of the large mirror R2, the diameter of the small mirror D1, the dimensions and aberration spot of the collimator with a focal length of 1m and the diameter of the large mirror D2 = 200 mm. Fig.5 shows the dependence of the spot size on the mirror diameter D1.
Fig.6 shows an example of a 3‑mirror cascade collimator circuit. All mirrors have the shape of an off-axis aspheric (freeform), which makes it possible to ensure the minimum spot of the laser output without shielding the central zone of the entrance pupil. The working diameter of the output mirror 3 and the length of the collimator (in the plane of the figure) for an equivalent focus of 1m is ~200 mm.
The operation of a 3x-mirror collimator when focusing to infinity can be described as follows. A laser beam with 100mrad divergence angle of hits mirror 1 with a focal length of 200 mm, then a collimated beam of ø40 mm using a mirror telescope (mirror 2 and mirror 3) with 5× magnification is converted into a collimated beam of ø200 mm. Focusing at various distances (50 m - 2 km) is carried out by moving the output end of the laser fiber within 3–20 mm (depending on the specification).
CONCLUSION
An optical target designation system is proposed that creates an extremely small laser spot on the object of observation at distances up to 2 000 meters. Calculations have shown that the choice of the optimal collimator design and the using lenses or mirrors with freeform shape allows to create a compact focusing system that ensures the minimum laser spot size for use within a wide range of distances.
AUTHORS
Shishkin Igor Petrovich, Cand.of Sc. (Eng.), STC “LEMT” BelOMO, Minsk, Republic of Belarus.
ORCID ID: 0000-0002-4592-1060
Shkadarevich Aleksey Petrovich, Dr. of Sc.(Phys.&Math.), STC “LEMT”, BelOMO, Minsk, Republic of Belarus.
Conflict of interest
The authors declare no conflict of interest.
I. P. Shishkin,, A. P. Schkadarevich, STC “LEMT” BELOMO, Minsk, Republic of Belarus
The design of an optical system for focusing a laser beam with an extremely small spot size in a wide range of distances from 50 m to 2 km is presented. Difficulties with controlling the laser beam and focusing it are reduced by using a circuit with a laser beam expander.
Keywords: laser beam focusing, collimator, laser beam expander, freeform
Article received on May 10, 2023
Article accepted on July 24, 2023
Introduction
Optical systems for focusing a laser beam ensure the efficiency of the use of optical-electronic systems in actual communication and control loops. The optical target designation system should create an extremely small laser spot on the object of observation with a satisfactory correction of aberrations, the magnitude of which is especially important at long distances.
The value of the laser spot in the first approximation can be represented as the sum of geometric and aberration spots:
∅ = ∅a + ∅г.
The geometric spot ∅г is calculated by the formula:
∅г = ∅в · Д / fк,
where ∅в – is the diameter of the laser fiber; Д – is the distance; fк – is the focal length of the collimator.
For the selected laser and a given range of distances, the size of the geometric spot will depend only on the focal length of the collimator, the value of which, in turn, is limited by the possibility of manufacturing large-diameter lenses, the dimensions of the mechanics and the focusing mechanism design.
The aberration spot will be determined by the level of aberration correction in the optical system. The optimal solution for creating a focusing laser system is a 2‑lens collimator circuit in which the lenses are separated by an air gap. By optimizing the 4 lens radii, it is possible to achieve satisfactory aberration correction and obtain an extremely small aberration spot, the size of which is especially important at long distances when the geometric spot is maximum. Focusing in the collimator is carried out by moving the output end of the laser fiber.
Two-lens collimator
In a two-lens collimator, the shape and relative position of the lenses have a significant impact on the correction of aberrations. Fig.1 shows the design options of a collimator with a focal length of 1 m for a laser with a divergence angle θ = 100 Mrad (numerical aperture Na = 0.1).
Fig. 2 shows ray aberrations plots of collimators for the axial point at infinity. The plots comparison demonstrate that the ray aberrations in the second variant are 10 times less than in the first one.
Spot diameters in mm for a laser with a fiber diameter of 25 um and a collimator with a focal length of 1m at various distances are shown in Table 1.
Beam expander
The larger focus of the collimator, the smaller geometric spot, but at the same time the dimensions of the structure grow, which means that difficulties arise with controlling the laser beam and focusing it. The scheme with a laser beam expander, which is shown in Fig. 3, allows to significantly reduce the length of the collimator [1]. At the same time, the focusing mechanism is simplified by moving the negative lens in the beam expander, which is essentially a Galileo telescope.
The design of the collimator in Fig.3 includes a collimating lens with a focus of 200 mm and a telescope with 5× magnification. Thus, the equivalent focus of the collimator will be 1m, and the length will be reduced by almost half. To reduce the number of lenses, the output lens of the telescope is made aspherical. If a breaking mirror is installed in parallel, then this solution can also be used to combine beams of several lasers (at least two).
MIRROR COLLIMATOR
If you use mirrors in the design of the collimator, then you can reduce its size as much as possible. Fig. 4 shows a collimator constructed according to the well–known classical scheme of a two-mirror astronomical lens, in which a large mirror is parabolic (Newton’s type), hyperbolic (Cassagrain’s type) or ellipsoidal (Gregory’s type). The small mirror in all schemes is flat. In the proposed scheme, the large mirror is spherical, and the small mirror is parabolic.
The disadvantage of a two-mirror collimator is the shielding of the central zone of the entrance pupil.
The effective area of the entrance pupil during the reverse movement can be calculated from the difference in the area of the mirrors:
,
where S1 и S2 – area of the mirrors, D1 и D2 – their diameters.
The area of a small mirror is determined both by the size of the concave radius of the large mirror R2 (the smaller the radius of the large mirror, the smaller the area of the small mirror and the larger the effective area of the entrance pupil) and the size of the aberration spot.
Table 2 shows examples of the ratios of the radius of the large mirror R2, the diameter of the small mirror D1, the dimensions and aberration spot of the collimator with a focal length of 1m and the diameter of the large mirror D2 = 200 mm. Fig.5 shows the dependence of the spot size on the mirror diameter D1.
Fig.6 shows an example of a 3‑mirror cascade collimator circuit. All mirrors have the shape of an off-axis aspheric (freeform), which makes it possible to ensure the minimum spot of the laser output without shielding the central zone of the entrance pupil. The working diameter of the output mirror 3 and the length of the collimator (in the plane of the figure) for an equivalent focus of 1m is ~200 mm.
The operation of a 3x-mirror collimator when focusing to infinity can be described as follows. A laser beam with 100mrad divergence angle of hits mirror 1 with a focal length of 200 mm, then a collimated beam of ø40 mm using a mirror telescope (mirror 2 and mirror 3) with 5× magnification is converted into a collimated beam of ø200 mm. Focusing at various distances (50 m - 2 km) is carried out by moving the output end of the laser fiber within 3–20 mm (depending on the specification).
CONCLUSION
An optical target designation system is proposed that creates an extremely small laser spot on the object of observation at distances up to 2 000 meters. Calculations have shown that the choice of the optimal collimator design and the using lenses or mirrors with freeform shape allows to create a compact focusing system that ensures the minimum laser spot size for use within a wide range of distances.
AUTHORS
Shishkin Igor Petrovich, Cand.of Sc. (Eng.), STC “LEMT” BelOMO, Minsk, Republic of Belarus.
ORCID ID: 0000-0002-4592-1060
Shkadarevich Aleksey Petrovich, Dr. of Sc.(Phys.&Math.), STC “LEMT”, BelOMO, Minsk, Republic of Belarus.
Conflict of interest
The authors declare no conflict of interest.
Readers feedback