Coherent light generators – Particular beam control device – Q-switch
Reexamination Certificate
2001-06-05
2004-08-24
Jackson, Jerome (Department: 2815)
Coherent light generators
Particular beam control device
Q-switch
C372S025000, C372S030000, C372S022000
Reexamination Certificate
active
06782012
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a laser device with a Q switch for generating a higher harmonics, a method of controlling the device, and a laser processing machine comprising the device.
BACKGROUND OF THE INVENTION
A conventional Q switch laser device will be explained.
FIG. 7
shows a configuration of a conventional a harmonic-generating Q switch laser device. The device comprises a laser head
11
, a Q switch
12
, an excitation light source
13
, a radio frequency (RF) driver
14
for the Q switch, a controller
15
, an interface
16
, a power source
17
, and an operation unit
18
.
The operation of the device will be explained with referring to FIG.
7
. An oscillation condition instruction set through the operation unit
18
such as a personal computer (PC) is sent to the control circuit
15
through the interface
16
. The controller
15
interprets the transmitted oscillation condition instruction, generates a control signal according to the instruction, transmits the condition to the excitation light source
13
and RF driver
14
for the Q switch, and determines the oscillation condition of the laser head. At the same time, the controller
15
also controls the alarm from the laser head
11
and a temperature of a nonlinear optical crystal.
An optical operation of the harmonic-generating Q switch laser
An optical operation of the harmonic-generating Q switch laser device will be explained with referring to FIG.
13
.
FIG. 13
shows an internal structure of the laser head
11
of the harmonic-generating Q switch laser device shown in FIG.
7
. The head
11
comprises a reflecting mirror
21
, a Q switch element
42
, a gain medium
23
, an output mirror
24
, a condenser lens
25
, a nonlinear optical crystal
26
, an optical lens
27
, a narrow band filter or dichroic mirror
28
, and two lenses
25
and
27
. The lenses
25
,
27
function as a collimator. The device where the nonlinear optical crystal
26
is disposed outside of the mirrors
21
,
24
is called an extra cavity system.
The optical operation of this harmonic-generating Q switch laser head will be explained with referring to FIG.
13
. When an excitation light enters the gain medium
23
, an optical resonation occurs between the reflector mirror
21
and output mirror
24
. In this case, when the Q switch element
42
inserted between the mirrors
21
and
24
is turned on, the optical path opens, and the laser oscillates. When the element is turned off, the optical path closes, and the oscillation stops. Thus, a pulse laser oscillation is enabled. The Q switch element
42
is turned on and off by the RF driver
14
for the Q switch, and enables the Q switch laser head to pulse-oscillate. The laser light issued from the output mirror
24
is condensed by the condenser lens
25
, and emitted to the nonlinear optical crystal
26
. A harmonic laser is generated by the nonlinear optical crystal
26
, is collimated by the optical lens
27
, and is separated into an IR laser, a fundamental wave and harmonic laser by the narrow band filter or dichroic mirror
28
. The harmonic laser is used for a processing machine.
For example, the power P
SHG
of a harmonic laser of second harmonic generation (SHG) is expressed in the following formula.
P
SHG
=d
eff
·I
eff
·(
P
IR
)
2
/A
where d
eff
is a nonlinear constant, a constant determined by the type of the nonlinear optical crystal
26
, the larger constant, a higher conversion efficiency; l
eff
is an effective length, a length in which the nonlinear optical crystal
26
has a nonlinear effect; P
IR
is the power of the fundamental wave; and A is a laser beam area.
Under this relationship, reducing the beam area A and reinforcing the fundamental wave power P
IR
into the nonlinear optical crystal
26
increases the harmonic laser output. In order to increase the power, the beam put into the nonlinear optical crystal
26
is condensed.
However, the nonlinear optical crystal
26
has a limited light strength, i.e., the crystal is broken when a light exceeding a damage threshold is input. Even if the light is not reaching the damage threshold, a light approaching the damage threshold affects a life of the nonlinear optical crystal
26
.
The optical characteristic of the harmonic-generating Q switch laser and a breakdown of the nonlinear optical crystal will be explained with referring to FIG.
9
.
FIG. 9
shows the oscillation characteristic of the Q switch laser. In a simplest method of operating the Q switch laser device, an arc current of a lamp or current of a laser diode (LD) is maintained in constant, and accordingly, the excitation light power is kept constant, and the Q switch is turned on and off. However, since the excitation light enters the gain medium
23
for a long period while the Q switch closes before an oscillation is started, a gain accumulated in the gain medium
23
excesses a specified value. Therefore, when a gate signal for starting an oscillation is turned on, a giant pulse is generated at a first shot or several shots depending on cases. This giant pulse may destroy the nonlinear optical crystal
26
or shorten the life of the crystal.
To avoid generating such a giant pulse, the controller of the Q switch laser device may have a first pulse suppression (FPS) function.
Referring to
FIG. 10
, the FPS function of the Q switch laser device will be explained. In order to suppress the giant pulse in the first shot or several shots when the gate signal is turned on upon starting the oscillation, the excitation light into the gain medium
23
is weakened to such an extent as to maintain a specified gain while the Q switch is closed for a long time.
In this method, the laser gain accumulated in the gain medium
23
is prevented from getting excessive. Since the maximum excitation light power is limited, the gradient of building-up the laser gain is limited. The period until the specified laser gain is accumulated depends on the interval of the pulse train following the first pulse, that is, a pulse frequency. Therefore, whenever the pulse frequency changes, the condition of the FPS function such as the laser gain retention current &Dgr;I and retention period &Dgr;T must be adjusted.
In other method of realizing the FPS function, the rise time of the Q switch may be delayed, but it is similarly needed to set the condition of the FPS function for each pulse frequency.
FIG. 11
shows an optical system of a laser processing machine having the harmonic-generating Q switch laser device. The processing machine comprises a harmonic-generating Q switch laser device
31
, a collimator lens
32
, a mask changer
33
, a bend mirror
34
, a galvanoscanner
35
, a scanner lens
36
, and a working table
37
.
The laser emitted from the harmonic-generating Q switch laser device
31
have the beam diameter optimized by the collimator
32
, and is emitted to the mask on the mask changer
33
. A portion of the emitted laser passes through the mask, and is condensed at a specified position through the scanner lens
36
by the galvanoscanner
35
through the bend mirror
34
, and processes the work fixed on the table
37
.
While processing the work, generally, a pulse train of a specified frequency is needed, a long pause period is necessary in order to convey the work, and the pulse oscillation and pause period are repeated.
FIG. 12
shows an output characteristic of the harmonic-generating Q switch laser device. A low pulse frequency increases the pulse energy, and increases a possibility of damaging the nonlinear optical crystal
26
or shortening of the life of the crystal.
Therefore, a long pause period longer in order to convey the work makes the gain be accumulated excessively in the gain medium
23
, and generates the giant pulse, which damages the nonlinear optical crystal
26
or shorten the life of the crystal, and thereby reduces the reliability.
To avoid such circumstances, the harmonic-generating Q switch laser device
31
generally incorporated in the laser processing machine has the FPS function as mentioned a
Karasaki Hidehiko
Ukita Katsuichi
Jackson Jerome
Landau Matthew
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