Fabrication of molded chalcogenide-glass lens for thermal imaging applications

Du Hwan Cha; Hye-Jeong Kim; Yeon Hwang; Jae Cheol Jeong; Jeong-Ho Kim

doi:10.1364/AO.51.005649

Fabrication of molded chalcogenide-glass lens for thermal imaging applications

Du Hwan Cha, Hye-Jeong Kim, Yeon Hwang, Jae Cheol Jeong, and Jeong-Ho Kim

Applied Optics
Vol. 51,
Issue 23,
pp. 5649-5656
(2012)
•doi: 10.1364/AO.51.005649

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Du Hwan Cha, Hye-Jeong Kim, Yeon Hwang, Jae Cheol Jeong, and Jeong-Ho Kim, "Fabrication of molded chalcogenide-glass lens for thermal imaging applications," Appl. Opt. 51, 5649-5656 (2012)

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Abstract

With the recent development of less costly uncooled detector technology, expensive optics are among the remaining significant cost drivers. As a potential solution to this problem, the fabrication of IR lenses using chalcogenide glass has been studied in recent years. We report on the fabrication of a molded chalcogenide-glass lens for car night vision and on the evaluation of the lens. The moldability of chalcogenide glass was characterized through transcription properties of the mold’s surface. In addition, both IR transmittance and x-ray diffraction patterns of the molded chalcogenide-glass lens were evaluated to verify the compositional and structural stability of the glass material under the given molding conditions.

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Thermal properties	Thermal conductivity ( $W / m \cdot K$ )		0.23
	Thermal expansion coefficient ( $10^{- 6} / K$ )		14.1
	Transition temperature ( $T_{g}$ ) (°C)		285
	Softening point ( $T_{s}$ ) (°C)		309
Optical properties	Refractive index at 10 μm	25 °C	2.58609
		80 °C	2.58928
	Thermo-optic coefficient at 10 μm ( $10^{- 6} / K$ )		58
Mechanical properties	Young’s modulus (GPa)		19.11

Term	Form error (PV)	Roughness (Ra)	Tilt	Decenter	Transmittance (Wavelength 8–12 μm)
Fabrication tolerance	$< 1.0 μm$	$< 20 nm$	$< 3 arcmin$	$< 20 μm$	$> 90 %$

Grinding process
Wheel configuration	SD 2000, Resin bond
Workpiece speed	350 rpm
Grinding wheel speed	37,000 rpm
Feed rate	$1.0 mm / \min$
Depth of cut	0.25 μm
Polishing process
Diamond paste (grain size)	Rough: 1.0 μm, Finish: 0.25 μm
Polishing medium	Nylon

		L1		L2
		Surface A	Surface B	Surface A	Surface B
Form error (PV; μm)	Before	0.71	0.45	0.56	0.55
Form error (PV; μm)	After	0.65	0.53	0.70	0.43
Surface roughness (Ra; nm)	Before	13.0	8.3	10.7	12.5
Surface roughness (Ra; nm)	After	17.5	11.0	14.2	9.8

		Heating stage
		First	Second	Pressing stage	Slow cooling stage
Temperature (°C)	Upper plate	—	—	320	200
Temperature (°C)	Lower plate	250	320	320	200
Pressing force (N)		—	—	500	200
Unit-process time (s)		800

Aspheric coef.		C	K	A4	A6	A8	A10
Surface A	Before	$4.94 e^{- 2}$	$- 1.58$	$- 1.98 e^{- 5}$	$- 3.62 e^{- 7}$	$- 1.31 e^{- 10}$	$- 2.41 e^{- 11}$
Surface A	After	$4.89 e^{- 2}$	$- 1.58$	$- 1.48 e^{- 5}$	$- 7.54 e^{- 7}$	$1.35 e^{- 8}$	$- 2.46 e^{- 10}$
Surface B	Before	$4.35 e^{- 2}$	$- 4.43$	$- 5.87 e^{- 5}$	$- 1.00 e^{- 6}$	$5.04 e^{- 10}$	$2.42 e^{- 11}$
Surface B	After	$4.28 e^{- 2}$	$- 4.43$	$- 5.96 e^{- 5}$	$- 9.02 e^{- 7}$	$- 3.08 e^{- 9}$	$1.16 e^{- 10}$

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