[19]
`United States Patent
`5,835,250
`[11] Patent Number:
`[45] Date of Patent: Nov. 10, 1998
`Kanesaka
`
`
`
`U8005835250A
`
`[54] DEVICE FOR DRIVING LIGHT EMITTING
`ELEMENT
`
`[75]
`
`Inventor: Hiroki Kanesaka, Kawasaki, Japan
`
`[73] Assignee: Fujitsu Limited, Kawasaki, Japan
`
`[21] Appl. No.: 606,820
`
`[22]
`
`Filed:
`
`Feb. 26, 1996
`
`[30]
`
`Foreign Application Priority Data
`
`Aug. 23, 1995
`
`[JP]
`
`Japan .................................... 7—214976
`
`Int. Cl.6 ..................................................... H04B 10/04
`[51]
`[52] US. Cl.
`............................................. 359/183; 357/186
`[58] Field of Search ..................................... 359/180, 181,
`359/184, 185, 186, 187, 183; 372/25, 26,
`30, 31, 50
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`4,523,089
`4,803,384
`4,813,048
`4,816,699
`5,146,464
`5,287,375
`
`........................... 250/205
`.
`.
`
`6/1985 Maeda et a1.
`2/1989 Yamane et a1.
`3/1989 Yamane et a1.
`3/1989 Mori et a1.
`.
`9/1992 Uemura ..................................... 372/38
`2/1994 Fujimoto ................................... 372/38
`OTHER PUBLICATIONS
`
`Japanese Patent Laid Open No. 57—208738, Dec. 21, 1982
`(Japanese Patent Appln. No. 56—94959, filed Jun. 18, 1981)
`& English translation of Title page and Claim 1.
`
`Japanese Patent Laid Open No. 61—265884, Nov. 25, 1986
`(Japanese Patent Appln. No. 60—108507, filed May 20,
`1985) & English translation.
`Japanese Patent Laid Open No. 62—286292, Dec. 12, 1987
`(Japanese Patent Appln. No. 61—129639, filed Jun. 4, 1986)
`& English translation.
`Japanese Patent Laid Open No. 63—828, Jan. 5, 1988 (Japa-
`nese Patent Appln. No. 61—144038, filed Jun. 20, 1986) &
`English translation of Title page and Claims 1—6.
`Japanese Patent Laid Open No. 63—35032, Feb. 15, 1988
`(Japanese Patent Appln. No. 61—179495, filed Jul. 30, 1986)
`& English translation of Title page and Claims 1—4.
`
`Primary Examiner—Rafael Bacares
`Attorney, Agent, or Firm—Staas & Halsey
`
`[57]
`
`ABSTRACT
`
`AdeVice for driving a light emitting element, including a DC
`power source for generating a DC current, a signal driving
`unit for controlling, in an on-off manner, the DC current
`generated in the DC power source on the basis of signal data
`with timing in accordance with the signal data, a light
`emitting element which is driven by the DC current con-
`trolled in the on-off manner by the signal driving unit and
`emits light when the DC current flows thereto, a high-speed
`pulse controlling unit for controlling, in an on-off manner,
`the DC current which drives the light emitting element at a
`sufficiently short period in comparison with the signal data
`to pulse the DC current, and a driving control unit for
`controlling the timing of on-off control of the high-speed
`pulse controlling unit in accordance with a required mean
`optical output.
`
`21 Claims, 11 Drawing Sheets
`
`
`
` COMPARATOR
`
`
`
`|6~
`
`
`
`
`20
`
`PULSE WIDTH
`CONTROLUNG UNIT
`
`
`
`DRIVING UNIT
`
`l3
`
`
` SIGNAL
`
`
`BIAS CURRENT
`DRIVING UNIT
`
`TEMPERATURE
`COMPENSATION
`
`DATA IN
`
`RIGID-1002 page 1
`
`RIGID-1002 page 1
`
`

`

`US. Patent
`
`Nov. 10, 1998
`
`Sheet 1 0f 11
`
`5,835,250
`
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`RIGID-1002 page 2
`
`RIGID-1002 page 2
`
`
`
`
`

`

`US. Patent
`
`Nov. 10,1998
`
`Sheet 2 0f 11
`
`5,835,250
`
`FIG. 2
`
`DATA Jm
`
`
`
`
`
`
`
`
`
`OPTICAL OUTPUT
`WAVEFORM
`
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`
`MEAN OPTchL
`OUTPUT POWER
`
`
`
`
`
`
`
`
`
`
`
`
`
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`
`
`OPTICAL OUTPUT
`WAVEFORM
`
`MEAN OPTICAL
`OUTPUT POWER
`
`RIGID-1002 page 3
`
`RIGID-1002 page 3
`
`

`

`US. Patent
`
`Nov. 10, 1998
`
`Sheet 3 0f 11
`
`5,835,250
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`RIGID-1002 page 4
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`

`

`US. Patent
`
`Nov. 10,1998
`
`Sheet 4 0f 11
`
`5,835,250
`
`
`
`
`
`
`
`OPTICAL QUTPUT
`WAVEFORM
`
`MEAN OPTICAL
`OUTPUT POWER
`
`
`
`OPTICAL OUTPUT
`WAVEFORM
`
`MEAN OPTICAL
`OUTPUT POWER
`
`RIGID-1002 page 5
`
`RIGID-1002 page 5
`
`

`

`US. Patent
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`RIGID-1002 page 6
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`RIGID-1002 page 6
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`
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`

`

`US. Patent
`
`Nov. 10, 1998
`
`Sheet 6 0f 11
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`5,835,250
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`RIGID-1002 page 7
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`
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`

`

`US. Patent
`
`Nov. 10,1998
`
`Sheet 7 0f 11
`
`5,835,250
`
`COMPARATOR
`
`BIASCURRENTDRIVINGUNIT
`
`
`
`
`
`DRIVINGUNIT
`FIG.9 l9
`SIGNAL
`
`
`
`
`
`37\
`
`
`COMPARATOR
`
`RIGID-1002 page 8
`
`RIGID-1002 page 8
`
`

`

`US. Patent
`
`Nov. 10,1998
`
`Sheet 8 0f 11
`
`5,835,250
`
`I-LCURVE
`
`FIG.10
`
`CURRENT
`
`DRIVING
`
` NONLINEAR
`PART'
`
`EQVLWOA indJJ‘IO ad HOLINOW
`
`RIGID-1002 page 9
`
`RIGID-1002 page 9
`
`

`

`US. Patent
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`RIGID-1002 page 10
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`
`
`
`
`
`

`

`US. Patent
`
`Nov. 10, 1998
`
`Sheet 10 0f 11
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`5,835,250
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`US. Patent
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`RIGID-1002 page 12
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`RIGID-1002 page 12
`
`
`

`

`5,835,250
`
`1
`DEVICE FOR DRIVING LIGHT EMITTING
`ELEMENT
`
`BACKGROUND OF THE INVENTION
`
`The present invention relates to a device for driving a light
`emitting element which drives a light emitting element such
`as a laser diode, and specifically relates to a device for
`driving light emitting elements which is suitable as a signal
`source of an optical transmission system for carrying out a
`data transmission by light.
`In the optical transmission system for carrying out the
`data transmission by an optical signal, a light emitting
`element such as a laser diode (referred to as “LD”
`hereinafter) is driven on the basis of an electric signal to
`generate an optical signal, and the optical signal is trans-
`mitted to an optical transmission path such as an optical fiber
`cable.
`
`Up to the present, a LD driving device in the optical
`transmission system has been constructed as shown in FIG.
`13.
`
`The LD driving device shown in FIG. 13 comprises a LD
`(Laser Diode) 1, a photodiode (referred to as “PD”
`hereinafter) 2 for monitoring, a signal driving unit 3, a bias
`current driving unit 4, a mean value detection unit 5, a
`reference voltage generation unit 6, a comparator 7, a
`resistance 8 and a DC power source 9.
`When the LD 1 emits light, the light is detected in the PD
`2 for monitoring, an optical voltage corresponding to an
`optical intensity of the LD 1 is generated between terminals
`of the resistance 8, and a mean value of the optical voltage
`is obtained by the mean value detection unit 5. The reference
`voltage generation unit 6 generates a voltage which has been
`set up previously in accordance with a desirable mean output
`of optical output. The comparator 7 compares the mean
`value output of the mean value detection unit 5 with the
`output reference voltage of the reference voltage generation
`unit 6 to supply the signal driving unit 3 with a signal
`corresponding to the difference.
`The signal driving unit 3, in an on-off manner, multiply-
`controls a DC voltage which is supplied to the LD 1 from the
`DC power source 9 and makes it function as a signal pulse
`current in accordance with a signal input of binary data to be
`transmitted (DATA IN), and controls an amplitude of the
`current supplied to the LD 1 in the ON state in accordance
`with the signal supplied from the comparator 7. At this
`moment,
`the signal driving unit 3 controls the current
`supplied to the LD 1 by increasing it so as to enlarge the
`mean value output, when the mean value output of the mean
`value detection unit 5 is smaller than the output reference
`voltage of the reference voltage generation unit 6, and
`controls the current supplied to the LD 1 by decreasing it so
`as to reduce the mean value output, when the mean value
`output is larger than the output reference voltage.
`The bias current driving unit 4 lets a predetermined bias
`current flow into the LD 1 from the DC power source 9 and
`biases the current which drives the LD 1, even when the
`signal input is “0”, namely, the signal driving unit 3 is in a
`OFF state. A temperature compensation signal is supplied to
`the bias current driving unit 4, and the bias current driving
`unit 4 controls the value of the predetermined bias current in
`accordance with the temperature.
`In the LD driving device shown in FIG. 13, the LD 1 is
`driven by the current in which the signal pulse current is
`superimposed on the bias current, the signal pulse current is
`
`10
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`15
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`20
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`25
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`30
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`35
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`45
`
`50
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`60
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`65
`
`2
`controlled so that the mean value of the output voltage of the
`PD 2 for monitoring is equal to the reference voltage, and the
`bias current is controlled so that it compensates a change by
`temperature of on-off threshold (current) of the LD 1 which
`generates the optical signal by the signal pulse current and
`the signal level is changed in the vicinity of the threshold all
`the time.
`
`A level condition between the transmission and receiving
`in the optical
`transmission system varies depending on
`transmitting conditions such as a transmission distance. For
`instance, an optical output power of the LD of the trans-
`mission side is set up so as to meet the level condition
`between the transmission and receiving, when a dynamic
`range of the receiving side is restricted to the predetermined
`range. In a conventional LD driving device, the troubles
`such as deterioration of action waveform and deterioration
`
`of extinction ratio (light quantity ratio of lights-out/lighting-
`up) occur, because a small signal current
`is driven by
`relatively restricted amplitude in the vicinity of threshold
`current, if LD for high output is used in a low power without
`carrying out a temperature compensation by the bias current.
`For this reason, the LD and the driving circuit should be
`changed for each optical output power to be set up so as to
`make it possible to transmit a signal appropriately, and this
`fact causes an increase of the cost.
`
`the manhour are
`there is a problem in that
`Besides,
`increased when the LD driving device is manufactured and
`used, because the data concerning the change of temperature
`of the threshold temperature current in a working tempera-
`ture range is obtained beforehand for individual LD and the
`bias current is adjusted in accordance with the data, when
`the temperature compensation of the bias current is carried
`out. Further, divergence from the initial state due to a
`deterioration with the passage of time in the component/
`element of the LD driving device and the like is not
`compensated.
`The present inventor has found that the problems men-
`tioned above can be solved by controlling the DC current
`which drives the light emitting element in the high-speed
`pulse and controlling the timing of the on-off control of the
`pulse in accordance with the mean optical output, or coor-
`dinating the control of the bias current driving unit and the
`control of the signal driving unit, or by the both, and has thus
`accomplished the present invention.
`
`SUMMARY OF THE INVENTION
`
`An object of the present invention is to provide a device
`for driving a light emitting element which is capable of
`driving the light emitting element such as the LD appropri-
`ately for the various output powers and makes it possible to
`apply the light emitting element for high output to uses for
`low output.
`In a first aspect of the present invention, it is provided a
`device for driving a light emitting element, which com-
`prises:
`a DC power source for generating a DC current;
`a signal driving unit for controlling, in an on-off manner,
`the DC current generated in the DC power source on
`the basis of a signal data with timing in accordance with
`the signal data;
`a light emitting element which is driven by the DC current
`controlled in the on-off manner by the signal driving
`unit and emits light when the DC current flows thereto;
`a high-speed pulse controlling unit for controlling, in an
`on-off manner, the DC current which drives the light
`
`RIGID- 1002 page 13
`
`RIGID-1002 page 13
`
`

`

`5,835,250
`
`3
`emitting element at a sufficiently short period in com-
`parison with the signal data to pulse the DC current;
`and
`
`a driving control unit for controlling the timing of on-off
`control of said high-speed pulse controlling unit in
`accordance with a required mean optical output.
`The high-speed pulse controlling unit may pulse the DC
`current which drives the light emitting element by
`controlling, in an on-off manner, the DC current which is
`supplied to the signal driving unit from the DC power source
`at the sufficiently short period in comparison with the signal
`data to pulse the DC current.
`The high-speed pulse controlling unit may pulse the DC
`current which drives the light emitting elements by
`controlling, in an on-off manner, the DC current which is
`supplied to the light emitting element from the signal driving
`unit at the sufficiently short period in comparison with the
`signal data to pulse the DC current.
`The signal driving unit may control, in an on-off manner,
`the DC current on the basis of the signal data, at an
`amplitude and with timing in accordance with the signal
`data.
`The driving control unit may include a pulse width
`controlling unit for controlling variably a pulse width duty
`ratio of the high-speed pulse controlling unit to pulse the DC
`current.
`
`The driving control unit may include a pulse pattern
`controlling unit for controlling variably a pulse pattern of the
`high-speed pulse controlling unit to pulse the DC current.
`The pulse pattern controlling unit may include a pulse
`density controlling unit for selecting and extracting inter-
`mittently a pulse in a high frequency pulse line formed by
`the high-speed pulse controlling unit, and controlling vari-
`ably a pulse density of the pulse to be extracted to pulse the
`DC current.
`
`The pulse pattern controlling unit may include a pulse
`number controlling unit for selecting and extracting inter-
`mittently a pulse in a high frequency pulse line formed by
`the high-speed pulse controlling unit at a predetermined
`period, and changing variably a pulse number of the pulse to
`be extracted per period to pulse the DC current.
`The device of the present invention may further comprise
`a light receiving element for detecting an optical output of
`the light emitting element, a mean value detection unit for
`detecting a mean value of the output of the light receiving
`element and a comparison unit for comparing the mean
`value with a reference value, and the driving control unit
`may control the high-speed pulse controlling unit in accor-
`dance with a comparison result of the comparison unit.
`The device of the present invention may further comprise
`a light receiving element for detecting an optical output of
`the light emitting element, a peak value detection unit for
`detecting a peak value of the output of the light receiving
`element and a comparison unit for comparing the peak value
`with a reference value, and the signal driving unit may
`control the amplitude of the DC current which is supplied to
`the light emitting element in accordance with the compari-
`son result of the comparison unit.
`Further, a bias current driving unit for supplying a pre-
`determined bias current to the light emitting element from
`the DC power source regardless of the action of the signal
`driving unit and the driving control unit may be included in
`the device of the present invention.
`The bias current driving unit may control variably a bias
`current
`in accordance with a temperature compensation
`signal.
`The device of the present invention may further comprise
`a light receiving element for detecting an optical output of
`
`10
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`30
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`35
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`
`4
`the light emitting element, a mean value detection unit for
`detecting a mean value of the output of the light receiving
`element, a peak value detection unit for detecting a peak
`value of the output of the light receiving element, a first
`reference value generation unit for generating a predeter-
`mined first reference value, a first comparison unit for
`comparing the peak value with the first reference value, a
`bias current driving unit for supplying a predetermined bias
`current to the light emitting element from the DC power
`source regardless of the action of the signal driving unit and
`the driving control unit, in accordance with the comparison
`result of the first comparison unit, a 1/2 (half) calculation unit
`for calculating a 1/2 (half) peak value corresponding to the 1/2
`(half) of the peak value, a second reference value generation
`unit for adding a predetermined second reference value to
`the 1/2 (half) peak value and a second comparison unit for
`comparing the mean value with the 1/2 (half) peak value to
`which the second reference value has been added, and the
`signal driving unit may control the amplitude of the DC
`current which is supplied to the light emitting element in
`accordance with the comparison result of the second com-
`parison unit.
`The driving control unit may control the high-speed pulse
`controlling unit
`in accordance with a controlling signal
`supplied from the outside.
`In a second aspect of the present invention, it is provided
`a device for driving a light emitting element, which com-
`prises:
`a DC power source for generating a DC current;
`a signal driving unit for controlling, in an on-off manner,
`the DC current generated in the DC power source on
`the basis of a signal data, at an amplitude and with
`timing in accordance with the signal data;
`a light emitting element which is driven by the DC current
`controlled in the on-off manner by the signal driving
`unit and emits light when the DC current flows thereto;
`a light receiving element for detecting an optical output of
`the light emitting element;
`a mean value detection unit for detecting a mean value of
`the output of the light receiving element;
`a peak value detection unit for detecting a peak value of
`the output of the light receiving element;
`a first reference value generation unit for generating a
`predetermined first reference value;
`a first comparison unit for comparing the peak value with
`the first reference value;
`a bias current driving unit for supplying a predetermined
`bias current to the light emitting element from the DC
`power source regardless of the action of the signal
`driving unit, in accordance with the comparison result
`of the first comparison unit;
`a 1/2 (half) calculation unit for calculating a 1/2 (half) peak
`value corresponding to the 1/2 (half) of the peak value;
`a second reference value generation unit for adding a
`predetermined second reference value to the half peak
`value; and
`a second comparison unit for comparing the mean value
`with the half peak value to which the second reference
`value has been added and controlling the signal driving
`unit in accordance with the comparison result to make
`the amplitude of the DC current which is supplied to the
`light emitting element as a value corresponding to the
`comparison result.
`The light emitting element may be a laser diode.
`The light receiving element may be a photodiode.
`
`RIGID-1002 page 14
`
`RIGID-1002 page 14
`
`

`

`5,835,250
`
`5
`The device for driving the light emitting element of the
`present invention constructed in the above-mentioned way is
`capable of driving the light emitting element such as the LD
`appropriately for the various output powers without control-
`ling the peak value of the driving current, and the light
`emitting element for high output can be applied to uses for
`the low output.
`the mean optical output power can be
`For instance,
`changed without changing the peak value of the driving
`current by controlling,
`in an on-off manner,
`the driving
`current when the light emitting element such as the LD emits
`light, as the pulse of sufficiently quick period in comparison
`with the transmission speed, and by controlling variably the
`pulse width duty or the pulse number per unit time. At this
`moment, in the receiving side, the waveform in which the
`on-off output of the pulse is averaged as a receiving output
`can be obtained by setting up a frequency band of an
`amplifier of the receiving signal in the period less than the
`on-off period of the pulse. By these actions, the mean output
`power can be set up in a wide range by using one kind of LD
`and driving circuit, and the LD and the driving circuit can be
`generalized in the system of various conditions. Besides, the
`mean output power can be controlled from the outside by
`changing the pulse width or the pulse number according to
`the controlling signal supplied from the outside.
`When the driving current of the light emitting element
`such as the LD is larger than the threshold,
`the driving
`current is related almost linearly to the output voltage of the
`PD for monitoring. In many cases, the relationship between
`the driving current and the output voltage of the PD for
`monitoring is nonlinear, if the driving current is reduced to
`be brought close to the threshold current. Further, the output
`voltage of the PD for monitoring is almost fixed, if the
`driving current is not more than the threshold current. The
`bias current can be kept in the vicinity of the threshold
`current automatically even when the threshold current is
`changed by temperature, if the microscopic current state in
`the vicinity of the nonlinear part is detected.
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`FIG. 1 is a block diagram indicating a construction of a
`device for driving a light emitting element of the first
`embodiment of the present invention.
`FIG. 2 is a signal waveform diagram for explaining the
`action of the device shown in FIG. 1.
`
`FIG. 3 is a signal waveform diagram for explaining the
`action of the device shown in FIG. 1.
`
`FIG. 4 is a block diagram indicating a construction of a
`device for driving a light emitting element of the second
`embodiment of the present invention.
`FIG. 5 is a signal waveform diagram for explaining the
`action of the device shown in FIG. 4.
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`FIG. 6 is a signal waveform diagram for explaining the
`action of the device shown in FIG. 4.
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`FIG. 7 is a block diagram indicating a construction of a
`device for driving a light emitting element of the third
`embodiment of the present invention.
`FIG. 8 is a block diagram indicating a construction of a
`device for driving a light emitting element of the fourth
`embodiment of the present invention.
`FIG. 9 is a block diagram indicating a construction of a
`device for driving a light emitting element of the fifth
`embodiment of the present invention.
`FIG. 10 is a diagram for explaining the action of the
`device shown in FIG. 9 and indicating the relationship
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`between the driving current and the output voltage of the PD
`for monitoring.
`FIG. 11 is a block diagram indicating a construction of a
`device for driving a light emitting element of the sixth
`embodiment of the present invention.
`FIG. 12 is a block diagram indicating a construction of a
`device for driving a light emitting element of the seventh
`embodiment of the present invention.
`FIG. 13 is a block diagram indicating a construction of
`one example of a conventional device for driving a light
`emitting element.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`The embodiments of the device for driving the light
`emitting element of the present invention will be described
`by referring to the drawings as follows.
`[EMBODIMENT 1]
`FIG. 1 indicates the construction of the first embodiment
`
`of the LD driving device to which the device for driving the
`light emitting element according to the present invention has
`been applied.
`The LD driving device shown in FIG. 1 comprises a LD
`(Laser Diode) 11, PD (PhotoDiode) 12 for monitoring, a
`signal driving unit 13, a bias current driving unit 14, a mean
`value detection unit 15, a reference voltage generation unit
`16, a comparator 17, a resistance 18, a DC power source 19,
`a pulse width controlling unit 20 and a high-speed pulse
`controlling unit 21.
`The LD 11 is a LD which functions as a light emitting
`element driven by the current, and the PD 12 for monitoring
`is a PD which functions as a light receiving element for
`monitoring the emission of the LD 11.
`The signal driving unit 13 drives, in an on-off manner, the
`current which is supplied from the DC power source 19 so
`as to drive the LD 11 in accordance with the signal data input
`(DATA IN) supplied from the outside. The signal data is
`binary data to be transmitted. It also can be thought that the
`signal driving unit 13 multiplies the current which is sup-
`plied to the LD 11 from the DC power source 19 by the
`signal data of “1”/“0”, namely, “H”/“L”.
`The bias current driving unit 14 biases the current which
`is supplied to the LD 11 from the DC power source 19 by a
`predetermined bias value and controls variably the prede-
`termined bias value in accordance with the temperature
`compensation signal which is supplied from the outside so
`as to compensate a temperature characteristic. Specifically,
`the bias current driving unit 14 superimposes the bias
`current of the predetermined bias value on the current which
`is supplied to the LD 11 from the DC power source 19, and
`increases and decreases the bias current in accordance with
`
`the temperature compensation signal supplied from the
`outside.
`The mean value detection unit 15 detects a mean value of
`
`the voltage corresponding to the light emitting intensity of
`the LD 11 which is detected by the PD 12 for monitoring and
`appears between the terminals of the resistance 18, and
`supplies it to the comparator 17.
`The reference voltage generation unit 16 generates a
`reference voltage corresponding to the desired mean value.
`The comparator 17 compares the reference voltage obtained
`in the reference voltage generation unit 16 with the mean
`value obtained in the mean value detection unit 15 and
`
`supplies the comparison result to the pulse width controlling
`unit 20. The comparison unit is formed by the reference
`voltage generation unit 16 and the comparator 17.
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`RIGID- 1002 page 15
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`The pulse width controlling unit 20 is a driving control
`unit for controlling the high-speed pulse controlling unit 21
`in response to the output of the comparator 17.
`The high-speed pulse controlling unit 21 controls, in an
`on-off manner, the current which is supplied to the LD 11
`from the DC power source 19 at the sufficiently short period
`in comparison with the signal data so as to pulse the current.
`In the high-speed pulse controlling unit 21, the pulse width
`duty ratio of the pulse line is controlled variably by the pulse
`width controlling unit 20.
`The action of the LD driving device shown in FIG. 1 will
`be described concretely.
`When the LD 11 emits light, the light of the LD 11 is
`detected by the PD 12 for monitoring, and the optical
`voltage corresponding to the optical intensity of the LD 11
`occurs between the terminals of the resistance 18. The mean
`value of the optical voltage is calculated in the mean value
`detection unit 15. The reference voltage which has been set
`up previously in accordance with the desired mean output of
`the optical output
`is generated in the reference voltage
`generation unit 16. The mean value output of the mean value
`detection unit 15 is compared with the output reference
`voltage of the reference voltage generation unit 16 in the
`comparator 17, and the signal corresponding to the differ-
`ence is supplied to the pulse width controlling unit 20.
`The current which is supplied to the LD 11 from the DC
`power source 19 is controlled in an on-off manner to be
`pulsed by the high-speed pulse controlling unit 21 at the
`sufficiently short period in comparison with the signal data
`of the binary data to be transmitted. At the same time, the
`signal driving unit 13 in an on-off manner, multiply-controls
`the DC current which is supplied to the LD 11 through the
`high-speed pulse controlling unit 12 and makes it function as
`a signal pulse current.
`As shown in FIG. 2, when the mean value output of the
`mean value detection unit 15 is larger than the output
`reference voltage of the reference voltage generation unit 16,
`the pulse width controlling unit 20 controls the high-speed
`pulse controlling unit 21 so that an on-duty of the pulse
`width is decreased, in order to decrease the mean value of
`the current supplied to the LD 11 to reduce the mean value
`output. As shown in FIG. 3, when the mean value output is
`smaller than the output reference voltage, the pulse width
`controlling unit 20 controls the high-speed pulse controlling
`unit 21 so that the on-duty of the pulse width is enlarged, in
`order to increase the mean value of the current supplied to
`the LD 11 to enlarge the mean value output.
`The bias current driving unit 14 lets the predetermined
`bias current flow into the LD 11 from the DC power source
`19 and biases the current which drives the LD 11, even when
`the signal input is “0”, namely, the signal driving unit 13 is
`in a OFF state. The temperature compensation signal is
`supplied to the bias current driving unit 14 so that the value
`of the predetermined bias current is controlled in accordance
`with the temperature.
`As mentioned hereinbefore, the LD driving device shown
`in FIG. 1 is provided with the high-speed pulse controlling
`unit 21 and the pulse width controlling unit 20 for control-
`ling the pulse width, and the mean optical output power of
`the LD 11 can be kept at the value which has been set up
`previously, without controlling the peak value of the driving
`current of the LD 11, by controlling, in an on-off manner, the
`light emitting element at
`the pulse of sufficiently quick
`period in comparison with the driving current based on the
`signal data input when the LD 11 emits light, and controlling
`the pulse width duty so that the mean value of the voltage
`based on the output of the PD 12 for monitoring is equal to
`the reference voltage.
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`Namely, the mean optical output power can be changed
`without changing the peak value of the driving current, by
`controlling, in an on-off manner, the driving current when
`the LD 11 emits light at the pulse of sufficiently quick period
`in comparison with the signal transmission speed and con-
`trolling variably the pulse width duty of the driving current.
`Further, in this case, in the receiving side, the waveform in
`which the on-off output of the pulse has been averaged as a
`receiving output can be obtained by setting up the frequency
`band of the amplifier of the receiving signal in the period
`less than the on-off period of the pulse. By these actions, the
`mean output power can be set up in a wide range by using
`the same kind of LD 11 and driving circuit, and the LD 11
`and the driving circuit can be generalized in the system of
`various conditions.
`FIG. 4 indicates a construction of the second embodiment
`
`of the LD driving device to which the device for driving the
`light emitting element according to the present invention has
`been applied.
`In FIG. 4, the detailed descriptions concerning the same
`parts as those of FIG. 1 will be omitted by giving the
`identical numbers to them. Namely, in the LD driving device
`shown in FIG. 4, a LD 11, a PD 12 for monitoring, a signal
`driving unit 13, a bias current driving unit 14, a mean value
`detection unit 15, a reference voltage generation unit 16, a
`comparator 17, a resistance 18 and a DC power source 19 are
`the same as those of FIG. 1. Further, the LD driving device
`shown in FIG. 4 is provided with a pulse pattern controlling
`unit 22 and a high-speed pulse controlling unit 23 respec-
`tively instead of the pulse width controlling unit 20 and the
`high-speed pulse controlling unit 21 shown in FIG. 1.
`The comparator 17 compares the reference voltage
`obtained in the reference voltage generation unit 16 with the
`mean value obtained in the mean value detection unit 15 and
`
`supplies the comparison result to the pulse pattern control-
`ling unit 22. The comparison unit is formed by the reference
`voltage generation unit 16 and the comparator 17.
`The pulse pattern controlling unit 22 is a driving control
`unit for controlling the high-speed pul