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Three-dimensional printing of a beam expander to enable the combination of hundred-micron optical elements and a single-mode fiber

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Optics Letters
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Haodong Zhu
Haodong Zhu
Haodong Zhu
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Minglong Li
Minglong Li
Minglong Li
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Tie Hu at Huazhong University of Science and Technology
Tie Hu
Ming Zhao
Ming Zhao
Ming Zhao
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Abstract and Figures

We use a flexible two-photon photopolymerization direct laser writing to fabricate an integrated diffractive lens system on a fiber tip to expand the output beam of the fiber. The results show that the micro-integrated beam expander based on double lenses (axial size of about 100 μm) has a magnification of 5.9 and a loss of 0.062 dB. Subsequently, we demonstrate the fabrication of a spiral phase plate (diffractive optical elements) and micro-lens arrays (refractive optical elements) on an integrated beam expander, and their optical properties are measured and analyzed, respectively. This Letter is an exploration of the future integrated micro-optical systems on an optical fiber tip.
Integrated beam expander fabricated by a DLW. (a) Beam expander consisting of two diffractive lenses and a stable mechanical support structure. (b) and (c) Spiral phase plate and micro-lens array to be integrated. (d) Schematic diagram of the expander. (e) FDTD simulations of the propagation of a Gaussian beam without lens group and with lens group, respectively, the amplitude and phase are taken from the light blue dashed line. (f) SEM image of the expander. The scale bar in the image is 30 μm.
Integrated beam expander fabricated by a DLW. (a) Beam expander consisting of two diffractive lenses and a stable mechanical support structure. (b) and (c) Spiral phase plate and micro-lens array to be integrated. (d) Schematic diagram of the expander. (e) FDTD simulations of the propagation of a Gaussian beam without lens group and with lens group, respectively, the amplitude and phase are taken from the light blue dashed line. (f) SEM image of the expander. The scale bar in the image is 30 μm.
… 
Measurement results of divergence angle, the distance between the adjacent beam cross sections is 3 mm, position 0 is the start point of the measurement. (a) Series of beam cross sections of a SMF along the propagation direction. (b) Linear fitting of the radius of the SMF with the change of position, slope k = 0.104. (c) Series of beam cross sections of a fiber with an expander, and the variation is significantly smaller than that of the SMF. (d) Linear fitting of the radius of fiber with an expander with the change of position, slope k = 0.018. The scale bars are 1 mm.
Measurement results of divergence angle, the distance between the adjacent beam cross sections is 3 mm, position 0 is the start point of the measurement. (a) Series of beam cross sections of a SMF along the propagation direction. (b) Linear fitting of the radius of the SMF with the change of position, slope k = 0.104. (c) Series of beam cross sections of a fiber with an expander, and the variation is significantly smaller than that of the SMF. (d) Linear fitting of the radius of fiber with an expander with the change of position, slope k = 0.018. The scale bars are 1 mm.
… 
Integrated vortex beam generator on a fiber tip. From top to bottom are the SPPs with topological charges of 1, 2, and 3, respectively. (a)-(c) Phase distribution of SPPs. (d)-(f) Top views of the integrated VB generators under an SEM. Scale bars, 20 μm. (g)-(i) Oblique views of the integrated VB generators under an SEM. Scale bars, 30 μm. (j)-(l) Front view of the VB generators observed under the optical microscope (illuminated light λ = 595 nm). Scale bars, 30 μm.
Integrated vortex beam generator on a fiber tip. From top to bottom are the SPPs with topological charges of 1, 2, and 3, respectively. (a)-(c) Phase distribution of SPPs. (d)-(f) Top views of the integrated VB generators under an SEM. Scale bars, 20 μm. (g)-(i) Oblique views of the integrated VB generators under an SEM. Scale bars, 30 μm. (j)-(l) Front view of the VB generators observed under the optical microscope (illuminated light λ = 595 nm). Scale bars, 30 μm.
… 
Experimental results of the VB generator. From left to right are the SPPs with topological charges of 1, 2, and 3, respectively. (a) Optical system used to observe the far-field diffraction and interference patterns. (b)-(d) Far-field diffraction patterns. The bright ring size increases with the increase of the topological charge. (e)-(g) Interference pattern of the VB and plane wave. (h)-(j) Interference pattern of the VB and spherical wave. The scale bars in all images are 200 μm.
Experimental results of the VB generator. From left to right are the SPPs with topological charges of 1, 2, and 3, respectively. (a) Optical system used to observe the far-field diffraction and interference patterns. (b)-(d) Far-field diffraction patterns. The bright ring size increases with the increase of the topological charge. (e)-(g) Interference pattern of the VB and plane wave. (h)-(j) Interference pattern of the VB and spherical wave. The scale bars in all images are 200 μm.
… 
Integrated micro-lens array on a fiber tip. (a) Optical system used to observe the focal spot array. (b) Model of the micro-lens array. (c) Oblique view of the integrated MLA under an SEM. Scale bars, 30 μm. (d) Focal spot arrays at an incident wavelength λ = 1550 nm. Scale bars, 300 μm.
Integrated micro-lens array on a fiber tip. (a) Optical system used to observe the focal spot array. (b) Model of the micro-lens array. (c) Oblique view of the integrated MLA under an SEM. Scale bars, 30 μm. (d) Focal spot arrays at an incident wavelength λ = 1550 nm. Scale bars, 300 μm.
… 
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Letter Vol. 48,No. 20 /15 October 2023 / Optics Letters 5379
Three-dimensional printing of a beam expander to
enable the combination of hundred-micron optical
elements and a single-mode fiber
Haodong Zhu,†Minglong Li,†Tie Hu, Ming Zhao,∗AND ZhenYu Yang
Nanophotonics Laboratory, School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074,
China
†These authors contributed equally to this Letter.
*zhaoming@hust.edu.cn
Received 27 June 2023; revised 13 September 2023; accepted 14 September 2023; posted 14 September 2023; published 10 October 2023
We use a flexible two-photon photopolymerization direct
laser writing to fabricate an integrated diffractive lens sys-
tem on a fiber tip to expand the output beam of the fiber.
The results show that the micro-integrated beam expander
based on double lenses (axial size of about 100 µm) has a
magnification of 5.9 and a loss of 0.062 dB. Subsequently,
we demonstrate the fabrication of a spiral phase plate
(diffractive optical elements) and micro-lens arrays (refrac-
tive optical elements) on an integrated beam expander, and
their optical properties are measured and analyzed, respec-
tively. This Letter is an exploration of the future integrated
micro-optical systems on an optical fiber tip. © 2023 Optica
Publishing Group
https://doi.org/10.1364/OL.499114
Compared with traditional optical elements, micro-optical ele-
ments have the advantages of compact structure, small volume
and integration. They have important application prospects in
the fields of optical imaging [1,2], biomedicine [3], information
storage [4], and laser technology [5,6]. Integrating fiber micro-
optical systems (FMOS) on fiber tips is a hot and challenging
field of scientific research [3,7–18]. Optical fibers provide a low-
cost, flexible, and remotely operable platform for micro-optical
systems; integrated micro-optical systems enable flexible control
of the output beam of the fiber.
The output Gaussian beam of the single-mode fiber (SMF)
has a small cross section and a large divergence angle, which
makes it difficult to adapt to complex diffractive optical ele-
ments (DOEs) or refractive optical elements (ROEs) with a
relatively large size. For most applications, the bulky com-
mercial fiber collimators are often used to provide beam
collimation. At present, researchers have achieved beam
expansion by fusing a piece of multi-mode fiber (MMF)
at the end of the SMF [13,14], but it requires precise
cutting of special MMF to obtain the calculated wave-
front.
Recently, various methods have been reported to fabricate
micro-structures on the fiber tips, including electron-beam
lithography [7], chemical etching [8], focused ion beam [9,10],
and two-photon photopolymerization direct laser writing (DLW)
[11–18]. Among them, DLW is a maskless lithography technol-
ogy with a simple process and a machining accuracy beyond
the diffraction limit, and it can manufacture complex three-
dimensional micro-structures on any surface, including complex
DOEs and ROEs on the fiber tips.
In this Letter, we demonstrate a new method for expand-
ing the output beam of the SMF. Spiral phase plate (SPP) and
micro-lens array (MLA) are fabricated on designed expanders,
and their optical properties are measured and analyzed, respec-
tively. The integrated beam expander on the fiber tip is shown in
Fig. 1(a), which consists of two diffractive lenses with different
focal lengths and a stable mechanical support structure. SPP and
MLA shown in Figs. 1(b) and 1(c) are fabricated directly on the
upper surface of the beam expander. All the micro-structures
in this Letter are manufactured by a self-built DLW system.
Simulations and experiments prove that this double-lens-based
expander can effectively modulate the output beam to obtain
a collimating beam, which matches the traditional DOEs and
ROEs. This Letter proves the possibility of fabricating traditional
micro-optical elements on the fiber tips, which is a beneficial
exploration for the integration of complex micro-optical systems
on the fiber tips in the future.
For a standard SMF, when λ=1550 nm, the waist and the
divergence angle of the output Gaussian beam are ω0≈5µm
and θ≈5.7◦, respectively. The waist of the Gaussian beam is
on the tip of the fiber. The propagation of the Gaussian beam
through a thin lens is given by
⎧
⎪
⎪
⎪
⎪
⎪⎨
⎪
⎪
⎪
⎪
⎪
⎩
ω′
0=ω0
(1−l/F)2+(f/F)2
l′=F+F2(l−F)
(F−l)2+f2
,(1)
where ω′
0and l′are, respectively, the waist radius and the image
distance of the Gaussian beam in the image space; ω0and l
are, respectively, the waist radius and the object distance of the
Gaussian beam in the object space; Fis the focal length of
the lens; and f=πω2
0/λis the Rayleigh range. From Eq. (1),
when l=F,ω′
0=(λ/πω0)F≈0.1F(λ=1550 nm) reaches a
maximum value. It can be seen that it would theoretically require
F≈500 µmto expand the waist radius of the output Gaussian
0146-9592/23/205379-04 Journal ©2023 Optica Publishing Group
... Two-photon direct laser writing (DLW) technology has the advantages of flexible 3D fabrication, high precision, and easy operation, making it suitable for the fabrication of diffraction devices with arbitrary surface profiles on the facet of a single mode fiber (SMF) [29][30][31]. This makes it possible to modulate the phase to increase the NA of the FOTs. ...
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