Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
1999-02-22
2001-07-10
Le, N. (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S247000
Reexamination Certificate
active
06259466
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image formation apparatus, and more particularly to an image formation apparatus such as a copy machine, a printer, a facsimile apparatus or the like which exposes a photosensitive body by driving a laser beam source on the basis of a detection signal obtained by detecting a laser beam from the laser beam source to form a latent image on a photosensitive face of the photosensitive body.
2. Related Background Art
In an image formation apparatus or the like which performs digital optical communication or an electrophotographic process, a laser diode is used as a light emission element to convert an electrical pulse signal into an optical pulse. For this laser diode, it is required to be able to obtain the desired light emission quantity even if the operation temperature of the element changes. However, since the light emission characteristic of the laser diode highly depends on the operation temperature, it is necessary to control a laser diode drive current by a light emission element drive circuit so as to obtain the desired light quantity even if the operation temperature changes.
As a first conventional example,
FIG. 30
shows the structure of the laser diode drive circuit which performs the pulse current control on a laser of cathode drive type.
In
FIG. 30
, numeral
3001
denotes a comparator, numerals
3002
and
3006
denote reference voltage sources, numeral
3003
denotes a sample-and-hold (S/H) circuit, numeral
3004
denotes a hold capacitor (CH), numeral
3005
denotes a current amplification circuit, numeral
3008
denotes a reference current source (IO), numeral
3007
denotes a switching circuit (SW), numeral
3011
denotes a laser diode (LD), numeral
3012
denotes a photodiode (PD), and numeral
3010
denotes a monitor resistor (RM).
In the conventional example shown in
FIG. 30
, for the sampling state of the S/H circuit
3003
(referred as APC (automatic power control) operation hereinafter), the switching circuit
3007
is ON, and input data (DATA) is set such that the laser diode
3011
is in its entire-face light emission state. In the APC operation, the light quantity from the laser diode
3011
is monitored at the photodiode
3012
such that the light emission quantity of the diode
3011
becomes the desired quantity. Then, if a monitor current IM produced at the photodiode
3012
flows in the monitor resistor
3010
, a monitor voltage VM is produced at the end of the monitor resistor
3010
. Further, the laser diode drive current is controlled by the current amplification circuit
3005
on the basis of the reference current source
3008
such that the monitor voltage VM becomes constant (i.e., light emission quantity becomes constant).
Further, during the hold of the S/H circuit
3003
, the laser diode drive current is set to be ON/OFF by the switching circuit
3007
according to the input data, whereby the pulse modulation signal is given to the laser diode
3011
.
However, in the structure shown in
FIG. 30
, if the operation frequency in the optical pulse modulation becomes high, the problem of light emission delay which is peculiar to the laser diode occurs, whereby the transition characteristic of the modulated optical pulse deteriorates.
FIG. 31
shows a second conventional example relating to one method to solve the above problem of the first conventional example. In the second conventional example, a DC bias current is added to the laser diode drive current to improve the transition characteristic of the optical pulse which has been deteriorated by the light emission delay of the laser diode. Since the basic structure of the second conventional example is the same as that of the first conventional example shown in
FIG. 30
, the detailed explanation thereof is omitted. In
FIG. 31
, numeral
3009
denotes a current source (IB) which produces the bias current, and numeral
3015
denotes a reference pulse current source (IPO).
Also, in the second conventional example of
FIG. 31
, during the APC operation, the switching circuit
3007
is ON, and the input data is set such that the laser diode
3011
is in its entire-face light emission state. A pulse current IP is controlled by the current amplification circuit
3005
according to the reference pulse current IPO, on the basis of the monitor voltage VM obtained by the structure consisting of the photodiode
3012
and the monitor resistor
3010
, so that the light emission quantity of the laser diode
3011
becomes constant in the entire-face light emission state. Then, a laser diode drive current ILD is determined by superimposing the pulse current IP on the bias current IB.
Further, during the hold of the S/H circuit
3003
, the pulse current IP is set to be ON/OFF by the switching circuit
3007
according to the input data, whereby the pulse modulation signal is given to the laser diode drive current ILD.
In the second conventional example of
FIG. 31
, if the bias current IB is not added nearly up to the threshold current from which the laser diode
3011
starts light emission, it is impossible to effectively lower the light emission delay of the diode
3011
.
However, in the second conventional example, as described above, the oscillation threshold current of the diode
3011
changes due to the operation temperature. Also, such the current changes according to respective elements. For these reasons, since the optical pulse does not completely come to be OFF, there is every possibility that a sufficient quenching ratio can not be obtained. Therefore, it is difficult in the practical use to set the bias current to be the fixed value nearby the threshold current.
FIG. 32
shows a third conventional example. Like the second conventional example, the third conventional example relates to the method in which the bias current is added to the drive current. However, the current to be controlled is the bias current while the pulse current is given as the fixed current. In
FIG. 32
, it should be noted that the detailed explanations of the parts added with the same reference numerals as those in
FIG. 30
are omitted. In the drawing, numeral
3013
denotes a reference bias current source (IB
0
) which determines a bias current IB, and numeral
3014
denotes a pulse current source (IP) which produces the pulse current IP.
Also, in the third conventional example of
FIG. 32
, during the APC operation, the switching circuit
3007
is ON, and the input data is set such that the laser diode
3011
is in its entire-face light emission state. In order that the light emission quantity of the laser diode
3011
reaches the desired value in the light emission state, the bias current IB is controlled by the current amplification circuit
3005
according to the reference bias current IB
0
, on the basis of the error voltage (i.e., difference voltage) between the monitor voltage VM obtained by the structure consisting of the photodiode
3012
and the resistor
3010
and a reference voltage Vref
1
corresponding to the desired light quantity, thereby controlling the laser diode drive current ILD. Further, during the hold of the S/H circuit
3003
, the pulse current IP is set to be ON/OFF by the switching circuit
3007
according to the input data, whereby the pulse data is given to the laser diode drive current ILD to perform the optical pulse modulation.
However, in the third conventional example of
FIG. 32
, e.g., when the laser diode drive current may be small in such the case as the laser diode
3011
operates at low temperature, there is some possibility that the bias current becomes unnecessary and thus the operation becomes uncontrollable.
Hereinafter, the case where the operation becomes uncontrollable will be explained in detail.
FIG. 33
shows the relation between the laser drive diode current ILD and a light output P based on the change in the operation temperature of the general laser diode.
If the operation temperature rises, the threshold current increases, whereby the laser diode drive current ILD increases. In
Kawasaki Somei
Oomura Masanobu
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Le N.
Pham Hai C.
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