Coherent light generators – Particular beam control device – Having particular beam control circuit component
Reexamination Certificate
2000-09-26
2002-10-15
Ip, Paul (Department: 2828)
Coherent light generators
Particular beam control device
Having particular beam control circuit component
C372S030000, C372S038060, C347S246000
Reexamination Certificate
active
06466594
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a laser modulating device employed in, for example, a laser scanning device, and particularly to a laser modulating device equipped with for modulating the output power of the laser the laser diode.
In a laser scanning device having a laser diode as a light source, a laser modulating device is generally provided. The laser modulating device modulates the output power of the laser diode while the emitted beam is deflected to scan on a photoconductive surface. An image having a black density distribution corresponding to modulated level of the intensity of the laser beam is formed on the photoconductive surface.
In such a device, electrical currents corresponding to a minimum black density level, i.e., the white level and a maximum black density level are firstly determined. Then, the laser diode is driven such that an electrical current whose level is between the above-described white level and black level currents is supplied. In order to compensate for the intensity of the emitted beam due to individual characteristic errors of laser diodes and/or change of ambient temperature, part of light emitted by the laser diode is monitored, and a feed-back control which is known as an APC (automatic power control) is executed.
FIG. 6
shows a block diagram of an example of a conventional modulating circuit employing the APC system. A D/A (digital to analog) converter
21
outputs a data voltage Vdata corresponding to an input data signal Sdata representative of black density level of an image. The Sdata is generated based on the maximum level voltage Vmax corresponding to maximum output power Pmax of a laser diode LD, which is transmitted from a maximum level setting circuit
22
.
The data voltage Vdata is added to a minimum level voltage Vmin by an adder
24
. The minimum level voltage Vmin is a voltage for setting the minimum output power Pmin of the laser diode LD, which is transmitted from a minimum level setting circuit
23
. With this structure, the adder
24
outputs a reference voltage Vref which is a sum of the minimum level voltage Vmin and the data voltage Vdata. The reference voltage Vref is input to an input terminal of a differential amplifier
25
, and is compared with the monitor voltage Vmon, which is input to the other input terminal of the differential amplifier
25
. The differential amplifier outputs a voltage Vdef, which is an amplified voltage of a difference between the monitor voltage Vmon and the reference voltage Vref. The voltage Vdef is converted into an electrical current corresponding thereto by a V/I (voltage to current) conversion circuit
26
. The electrical current is supplied to the laser diode LD as a driving current Id. The laser beam emitted by the laser diode LD is received by a photo diode PD for monitoring, which receives part of the laser beam (generally a beam backwardly emitted by the laser diode LD) and outputs a monitoring current Imon. The monitoring current Imon is converted into the monitor voltage Vmon by an I/V (current to voltage) conversion circuit
27
.
In this modulating device, depending on a data value of the data signal Sdata, the data voltage Vdata output by the D/A conversion circuit
21
varies. Therefore, the reference voltage Vref varies in accordance with the data signal Sdata since the reference voltage Vref is generated based on the data voltage Vdata. If the intensity of the light beam emitted by the laser diode LD is smaller than the intensity corresponding to the value of the data signal Sdata, the monitoring current Imon is relatively small, and therefore, the voltage Vmon output by the I/V conversion circuit
27
is small. In this case, the differential amplifier
25
outputs a relatively large voltage Vdef, and therefore, the driving current Id output by the V/I conversion circuit
26
is increased, and the intensity of the light beam emitted by the laser diode LD is increased. If the intensity of the laser beam is greater than an intensity corresponding to the reference voltage Vref, the monitor voltage Vmon is greater than the reference voltage Vref. Then, the output voltage Vdef of the differential amplifier
25
is lowered. Thus, the driving current Id output by the V/I conversion circuit
26
is lowered, and the intensity of the light beam emitted by the laser diode LD is lowered.
With the above-described APC system, the intensity of the laser beam emitted by the laser diode LD is controlled to be an intensity corresponding to the reference voltage Vref. If the data voltage Vdata is changed as the data signal Sdata is changed, the reference voltage Vref is changed, and then the intensity of the laser beam output by the laser diode LD is changed accordingly. That is, the intensity of the laser beam emitted by the laser diode LD is modulated in accordance with the data signal Sdata. Thus, black density of an image formed on a photoconductive surface (not shown) is adjusted in accordance with the data signal Sdata.
In the above-described modulating device employing the APC system, when the data signal Sdata is changed and the reference voltage Vref is changed, the intensity of the laser beam emitted by the laser diode LD should be quickly changed responsive to the change of the reference voltage Vref. Therefore, the D/A conversion circuit
21
, the I/V conversion circuit
27
, the differential amplifier
25
and the V/I conversion circuit
26
should be constituted as quick-response (high-speed) circuits. However, in order to constitute a quick-response circuit, electronic elements and semi-conductor elements should be quick-responsive ones, which increases a manufacturing cost of each circuit. Further, even if the each circuit is constituted as a quick-responsive circuit, due to accumulated affects of potential delays of the I/V conversion circuit
27
, differential amplifier
25
and V/I conversion circuit
26
, it is still difficult to lower the delays sufficiently. Thus, it has been difficult to constitute a quick-response APC type modulating device.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide an improved laser modulating device having a quick-response so that the delay within an APC loop is relatively small.
For the above object, according to the present invention, there is provided a laser modulating device for driving a laser diode to emit a modulated laser beam in accordance with input data. The laser modulating device is provided with a monitoring system that detects output power of the laser diode and outputs a detection signal corresponding to the output power, a minimum output level setting system that outputs a minimum level signal defining a minimum output power of the laser diode, a modulating signal generating system that generates a modulating signal representing modulation information of the output power of the laser diode corresponding to input data, a first adder that outputs a sum of the minimum level signal and the modulating signal as a reference signal, a comparing system that compares the reference signal and the detection signal and outputs the driving signal so as to make the detection signal coincide with the reference signal, and a circuit that supplies the modulating signal to the laser diode, the modulating signal being added to the driving signal.
With this configuration, the modulating signal is directly supplied to the laser diode at a first stage, the output power is adjust quickly. Further, the APC functions after the output power was adjusted, and therefore, the output power is stabilized. Furthermore, regardless of the amplitude of the modulating signal, the driving signal has substantially the same amplitude, which also stabilizes the output power of the laser diode.
Optionally, the modulating signal generating system may include a maximum output level setting system that outputs a maximum level voltage required for setting a maximum output power of the laser diode, and a modulating voltage generating circuit that generates, in accordance with the maximum level voltage, a data
Al-Nazer Leith
Asahi Kogaku Kogyo Kabushiki Kaisha
Ip Paul
LandOfFree
Laser modulating device does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Laser modulating device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Laser modulating device will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2993237