Laser processing method and laser processing apparatus
Ultrashort pulse laser processing bores, welds or cuts objects (work pieces) by converging ultrashort laser pulses by a lens on the objects (work pieces) positioned at the focus and heating small spots or narrow lines on the objects (work pieces). Shortage of a focal depth of the lens prevents the ultrashort pulse laser processing from positioning the object (a work piece) and forming a deep, constant-diameter cylindrical hole. Z-parameter is defined to be Z=2fcΔt/Δi 2 , where Δt is a FWHM pulse width of the ultrashort pulse laser, Δi is a FWHM beam diameter of the ultrashort pulse, f is a focal length of the lens and c is the light velocity in vacuum. Selection of an optical system including a diffraction-type lens which gives the Z-parameter less than 1 (Z<1) prolongs the focal depth. Expansion of the focal depth facilitates the positioning of objects (work pieces) and enables the ultrashort pulse laser apparatus to bore a deep, constant-diameter cylindrical hole.
1. A laser processing method comprising the steps of:
emitting ultrashort light pulses having a wavelength λ, an FWHM diameter Δi and an FWHM pulse width Δt from a pulse laser, wherein the FWHM diameter Δi is a full width of a peak of laser power distribution at a height of half of the maximum and the FWHM width Δt is a full width of a peak of pulse width distribution at a height of half of the maximum; and
converging the ultrashort light pulses by a diffractive lens having a focal length f to a work piece placed at the focal point of the diffractive lens,
wherein the focal length f, the FWHM diameter Δi and the FWHM pulse width Δt are adjusted so that a Z-parameter defined by Z p =2fcΔt/Δi 2 satisfies an inequality Z p =2fcΔt/Δi 2 ≦1, where c is the light velocity in vacuum.
2. The laser processing method according to claim 1 , wherein the FWHM pulse width Δt is less than 20 ps.
3. The laser processing method according to claim 1 , wherein the diffractive lens is a thickness modulating lens having thickness distribution defined by a function h g (r):
h
g
(
r
)
=
λ
g
(
n
f
-
1
)
·
int
[
g
·
mod
{
(
n
-
1
)
d
(
r
)
λ
,
1
}
]
+
h
0
,
where g is a division number, d(r) is a thickness function of a reference refractive lens having a focal length f, n is a refractive index of the reference refractive lens, n f is a refractive index of the diffractive lens, h 0 is a base thickness and r is a radial coordinate.
4. The laser processing method according to claim 1 , wherein the diffractive lens is a thickness modulating lens having thickness distribution defined by a function h 2 (r):
h
2
(
r
)
=
λ
2
(
n
f
-
1
)
·
int
[
2
mod
{
(
n
-
1
)
d
(
r
)
λ
,
1
}
]
+
h
0
,
where a division number is 2, d(r) is a thickness function of a reference refractive lens having a focal length f, n is a refractive index of the reference refractive lens, n f is a refractive index of the diffractive lens, h 0 is a base thickness and r is a radial coordinate.
5. The laser processing method according to claim 1 , wherein the diffractive lens is a refractive index modulating lens having refractive distribution defined by a function n g (r):
n
g
(
r
)
=
λ
g
·
d
f
·
int
[
g
·
mod
{
(
n
-
1
)
d
(
r
)
λ
,
1
}
]
+
n
0
,
where g is a division number, d(r) is a thickness function of a reference refractive lens having a focal length f, n is a refractive index of the reference refractive lens, d f is a thickness of the diffractive lens, n 0 is a base refractive index and r is a radial coordinate.
6. The laser processing method according to claim 1 , wherein the diffractive lens is a refractive index modulating lens having refractive distribution defined by a function n 2 (r):
n
2
(
r
)
=
λ
2
d
f
·
int
[
2
mod
{
(
n
-
1
)
d
(
r
)
λ
,
1
}
]
+
n
0
,
where a division number is 2, d(r) is a thickness function of a reference refractive lens having a focal length f, n is a refractive index of the reference refractive lens, d f is a thickness of the diffractive lens, n 0 is a base refractive index and r is a radial coordinate.
7. The laser processing method according to claim 1 , wherein the diffractive lens is a thickness modulating lens having thickness distribution defined by a function h g (r):
h
g
(
r
)
=
1
(
n
f
-
1
)
mod
{
(
n
-
1
)
d
(
r
)
,
λ
}
+
h
0
,
where g is a division number, d(r) is a thickness function of a reference refractive lens having a focal length f, n is a refractive index of the reference refractive lens, n f is a refractive index of the diffractive lens, h 0 is a base thickness and r is a radial coordinate.
8. The laser processing method according to claim 1 , wherein the Z parameter Z p satisfies an inequality of 0.25≦Z p ≦1.
9. The laser processing method according to claim 1 , wherein the work piece is a material transparent to the laser light having the wavelength λ.
10. The laser processing method according to claim 9 , wherein the transparent material is a resin material.
11. A laser processing apparatus comprising:
a pulse laser for emitting ultrashort light pulses having a wavelength λ, an FWHM diameter Δi and an FWHM pulse width Δt, wherein the FWHM diameter Δi is a full width of a peak of laser power distribution at a height of half of the maximum and the FWHM width Δt is a full width of a peak of pulse width distribution at a height of half of the maximum, and
a diffractive lens having a focal length f for converging the ultrashort light pulses to a work piece placed at the focal point of the diffractive lens,
wherein the focal length f, the FWHM diameter Δi and the FWHM pulse width Δt are adjusted so that a Z-parameter defined by Z p =2fcΔt/Δi 2 satisfies an inequality Z p =2fcΔt/Δi 2 ≦1, where c is the light velocity in vacuum.
12. The laser processing apparatus according to claim 11 , wherein the diffractive lens is a thickness modulating lens having thickness distribution defined by a function h g (r):
h
g
(
r
)
=
λ
g
(
n
f
-
1
)
·
int
[
g
·
mod
{
(
n
-
1
)
d
(
r
)
λ
,
1
}
]
+
h
0
,
where g is a division number, d(r) is a thickness function of a reference refractive lens having a focal length f, n is a refractive index of the reference refractive lens, n f is a refractive index of the diffractive lens, h 0 is a base thickness and r is a radial coordinate.
13. The laser processing apparatus according to claim 11 , wherein the diffractive lens is a thickness modulating lens having thickness distribution defined by a function h 2 (r):
h
g
(
r
)
=
λ
2
(
n
f
-
1
)
·
int
[
2
mod
{
(
n
-
1
)
d
(
r
)
λ
,
1
}
]
+
h
0
,
where a division number is 2, d(r) is a thickness function of a reference refractive lens having a focal length f, n is a refractive index of the reference refractive lens, n f is a refractive index of the diffractive lens, h 0 is a base thickness and r is a radial coordinate.
14. The laser processing apparatus according to claim 11 , wherein the diffractive lens is a refractive index modulating lens having refractive distribution defined by a function n g (r):
n
g
(
r
)
=
λ
g
·
d
f
·
int
[
g
·
mod
{
(
n
-
1
)
d
(
r
)
λ
,
1
}
]
+
n
0
,
where g is a division number, d(r) is a thickness function of a reference refractive lens having a focal length f, n is a refractive index of the reference refractive lens, d f is a thickness of the diffractive lens, n 0 is a base refractive index and r is a radial coordinate.
15. The laser processing apparatus according to claim 11 , wherein the diffractive lens is a refractive index modulating lens having refractive distribution defined by a function n 2 (r):
n
2
(
r
)
=
λ
2
d
f
·
int
[
2
mod
{
(
n
-
1
)
d
(
r
)
λ
,
1
}
]
+
n
0
,
where the division number is 2, d(r) is a thickness function of a reference refractive lens having a focal length f, n is a refractive index of the reference refractive lens, d f is a thickness of the diffractive lens, n 0 is a base refractive index and r is a radial coordinate.
16. The laser processing method according to claim 11 , wherein the diffractive lens is a thickness modulating lens having thickness distribution defined by a function h g (r):
h
g
(
r
)
=
1
(
n
f
-
1
)
mod
{
(
n
-
1
)
d
(
r
)
,
λ
}
+
h
0
,
where g is a division number d(r) is a thickness function of a reference refractive lens having a focal length f, n is a refractive index of the reference refractive lens, n f is a refractive index of the diffractive lens, h 0 is a base thickness and r is a radial coordinate.
17. A laser processing method comprising the steps of:
emitting light pulses having a wavelength λ, an FWHM diameter Δi and an FWHM pulse width Δt from a pulse laser, wherein the FWHM diameter Δi is a full width of a peak of laser power distribution at a height of half of the maximum and the FWHM width Δt is a full width of a peak of pulse width distribution at a height of half of the maximum;
converging the light pulses by a diffractive lens having a focal length f to a work piece placed at or near the focal point of the diffractive lens; and
adjusting at least one of the focal length f, the FWHM diameter Δi and the FWHM pulse width Δt, so that a Z-parameter defined by Z p =2fcΔt/Δi 2 satisfies an inequality Z p =2fcΔt/Δi 2 ≦1, where c is the light velocity in vacuum.