Coherent light generators – Particular component circuitry – For driving or controlling laser
Reexamination Certificate
1998-09-03
2001-07-10
Kim, Robert H. (Department: 2877)
Coherent light generators
Particular component circuitry
For driving or controlling laser
C372S038010, C372S038070, C372S038090, C372S008000
Reexamination Certificate
active
06259714
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a power source control apparatus for controlling a current supply to a laser diode in a semiconductor laser. The semiconductor laser oscillates laser by supplying current to a p-n junction.
2. Description of Related Art
FIG. 10
shows an overall circuit of a conventional power source control apparatus for laser diodes.
Referring to
FIG. 10
, a power source
1
is composed of a constant voltage source like a D.C. power source and so on. A current control switching element
2
is connected to the power source
1
so as to perform a switching operation by an external signal, i.e. an output command signal PL. The power source
1
is connected to a series circuit of the switching element
2
and a reverse-biased diode
3
. A reactor
4
is connected to a junction point “a”, where the switching element
2
and the diode
3
are connected in series with each other. The other terminal of the reactor
4
is connected at a junction point “b” to an anode side of laser diodes LD
1
-LDn, which are connected in series. On the other hand, a cathode side of the laser diodes LD
1
-LDn is connected at a junction point “c” to a junction point between the power source
1
and the diode
3
. Power is supplied to the laser diodes LD
1
-LDn by switching control of the switching element
2
through the reactor
4
.
The laser diodes LD
1
-LDn respectively emit laser beams to a solid state laser medium or rod
5
. Such laser beams are excited and oscillated between a reflecting mirror
7
(100% reflectance) and a semi-transparent mirror
6
, so that a laser beam
8
goes out from the semi-transparent mirror
6
. In order to supply current to the laser diodes LD
1
-LDn, a current detector
9
detects a current of the reactor
4
. Then, an output of the detector
9
is fed back to an error comparator
10
and compared with a command value ITH of a commander
11
. The switching element
2
performs switching control according to a comparison result so as to feed a current of an equal value to the command value ITH. The command value signal ITH is turned on and off by an analog gate
12
, which is operated by an output drive signal PL. Consequently, the current supplied to the laser diodes LD
1
-LDn is on-off controlled so that a constant current set as the command value ITH is supplied to the laser diodes LD
1
-LDn.
FIGS. 11
a
-
11
c
show waveforms and a time chart explaining an operation of the control apparatus of FIG.
10
.
FIG. 11
a
shows a waveform of the drive signal PL for turning on and off the analog gate
10
.
FIG. 11
b
shows a waveform of a laser diode input current I that is a current supplied to the laser diodes LD
1
-LDn.
FIG. 11
c
is a waveform of a laser diode applied voltage V that is a voltage applied to the laser diodes LD
1
-LDn.
When the signal PL becomes on at a timing T
1
, the switching element
2
turns on. Then, the current I increases at a fixed time constant by an inductance of the reactor
4
. When the current I of the reactor
4
reaches the command value ITH at a timing T
2
, the switching element
2
is on-off controlled to keep the current I at such constant value. At this time, the output of the detector
9
is input into the comparator
10
and compared with the command value ITH, so that the current of the same value as the command value ITH is supplied by the switching control. When the signal PL is switched off at a timing T
3
, the switching element
2
turns off. Then, the current I of the reactor
4
begins decreasing at a fixed time constant. Since the reactor
4
stores energy, which is decided by the inductance and the current value, the current I is not turned off until a timing T
4
. The inductance of the reactor
4
stores the energy between the timings T
1
and T
2
when the current I increases. Therefore, in case the inductance is large, it takes a long time. Especially, when a short width pulse is output between the timings T
5
-T
6
, the current I cannot be a rectangular wave as shown by the output current waveform Ill between the timings T
5
-T
7
.
Namely, the conventional control apparatus cannot supply an input current of rectangular wave to the laser diodes LD
1
-LDn in response to a short width pulse. Thus, the output current has a slow response speed, and it is impossible to output a correct pulse current waveform.
BRIEF SUMMARY OF THE INVENTION
An object of the present invention is to provide a power source control apparatus for a laser diode that has a rapid responsibility in supplying current to a laser diode.
According to a first aspect of the invention, a power source control apparatus for a laser diode comprises a laser diode. A constant current source is connected to the laser diode. A current switching circuit forms a circulating circuit to circulate a current output from the constant current source when a laser output command signal for making the laser diode output a laser beam is off. The current switching circuit forms a circuit to supply power from the constant power source to the laser diode from the constant power source when the laser output command signal is on.
The current switching circuit may include a first switching element connected in parallel with the laser diode and a second switching element connected in parallel with the laser diode. The current switching circuit turns on the first switching element while turning off the second switching element when the laser output command signal is off, thereby circulating a current from the constant current source within the current switching circuit through the first switching element. The current switching circuit turning on the second switching element while turning off the first switching element when the laser output command signal is on, thereby outputting the current from the constant current source to the laser diode through the second switching element.
The current switching circuit may include a switching element connected in parallel with the laser diode. The current switching circuit turns on the switching element when the laser output command signal is off, thereby circulating a current from the constant current source within the current switching circuit through the switching element. The current switching circuit turns off the switching element when the laser output command signal is on, thereby outputting the current from the constant current source to the laser diode.
With such features, current change (di/dt) of the current output from the constant current source can be substantially ignored, so that influence due to inductance contained in the constant current source can be ignored, too. Thus, constant current pulses can be supplied with steep rise and fall according to a capacity of the current switching circuit. Consequently, the current supplied to the laser diode has a rapid response.
The constant current source may comprise a reactor connected in series with the laser diode. A current controlling switching element may be further connected in series with the laser diode and the reactor. The current controlling switching element performs on-off control so as to supply a constant current to the laser diode. A power source may be further provided to supply the power to the laser diode via the current controlling switching element and the reactor connected in series. The current switching circuit as the circulating circuit supplies current to the current controlling switching element and the reactor.
With such features, current change (di/dt) flowing in the reactor can be substantially ignored, so that influence due to inductance thereof can be ignored, too. Thus, constant current pulses can be supplied with steep rise and fall according to a capacity of the current switching circuit. Consequently, the current supplied to the laser diode has a rapid response.
The current controlling switching element may be turned on in advance when the laser output command signal is off so that the current value of the reactor becomes equal to a command value. The laser output command signal is
Kim Robert H.
Mitsubishi Denki & Kabushiki Kaisha
Rodriguez Armando
Sughrue Mion Zinn Macpeak & Seas, PLLC
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