Title: GaN green LED drive device and optical transmission device
Abstract: The invention provides a GaN green LED drive device comprising: a bias current output unit for outputting a pulse-like bias current having a low level higher by a predetermined value than a zero level; and a peaking unit for peaking the bias current outputted from the bias current output unit to make rise of the bias current higher than a high level and make fall of the bias current lower than the low level to thereby obtain a peaked bias current supplied to a GaN green LED. The bias current is peaked while the low level of the bias current is set at a higher value (10 mA) than that in the related art. Hence, the fall time tf becomes 6.6 nsec. The fall time tf can be shortened greatly compared with that in the related art.
Patent Number: 6,961,033 Issued on 11/01/2005 to Fukumoto, et al.
| Inventors:
|
Fukumoto; Shigeru (Aichi, JP);
Kato; Satoru (Aichi, JP);
Ito; Hiroshi (Aichi, JP)
|
| Assignee:
|
Toyoda Gosei Co., Ltd. (Aichi, JP)
|
| Appl. No.:
|
303854 |
| Filed:
|
November 26, 2002 |
Foreign Application Priority Data
| Nov 27, 2001[JP] | P2001-361168 |
| Current U.S. Class: |
345/82 |
| Intern'l Class: |
G09G 003/32 |
| Field of Search: |
372/29011- 29014
345/82
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Hjerpe; Richard
Assistant Examiner: Amadiz; Rodney
Attorney, Agent or Firm: McGinn & Gibb, PLLC
Claims
1. A GaN green LED drive device comprising:
a bias current output unit for outputting a pulse-like bias current having a
predetermined low level value higher than a zero value; and
a peaking unit for peaking said bias current outputted from said bias current
output unit to make rise of said bias current higher than a high level value and
make fall of said bias current lower than said low level value to thereby obtain
a peaked bias current supplied to a GaN green LED,
wherein said bias current output unit selectively sets said low level value of
said bias current based on a predetermined correlation between a set acting speed
of the GaN green LED and the bias current.
2. A GaN green LED drive device comprising:
a bias current output unit for outputting a pulse-like bias current having a
predetermined low level value higher than a zero value; and
a peaking unit for peaking said bias current outputted from said bias current
output unit to make rise of said bias current higher than a high level value and
make fall of said bias current lower than said low level value to thereby obtain
a peaked bias current supplied to a GaN green LED,
wherein said bias current output unit selects said low level value of said bias
current to be higher as a set acting speed of said GaN green LED increases.
3. A GaN green LED drive device according to claim 1, wherein said bias current
output unit outputs a pulse-like bias current having a low level value of not lower
than 4 mA.
4. An optical transmission device having a GaN green LED drive device defined
in claim 1.
5. A GaN green LED drive device according to claim 2, wherein said bias current
output unit outputs a pulse-like bias current having a low level value of not lower
than 4 mA.
6. A GaN green LED drive device according to claim 1, wherein said bias current
output unit selects said low level value of said bias current to be higher as the
set acting speed of said GaN green LED increases.
7. A GaN green LED drive device according to claim 2, wherein said bias current
output unit selectively sets said low level value of said bias current based on
a predetermined correlation between the set acting speed of the GaN green LED and
the bias current.
8. A GaN green LED drive device according to claim 1, wherein said predetermined
correlation between said set acting speed of the GaN green LED and the bias current
is based on a fall time and a rise time of said GaN green LED.
9. A GaN green LED drive device according to claim 1, wherein said predetermined
correlation between said set acting speed of the GaN green LED and the bias current
is based on a fall time and not a rise time of said GaN green LED.
10. A GaN green LED drive device according to claim 1, wherein said bias current
output unit outputs a pulse-like bias current having a low level value of not lower
than 5 mA.
11. A GaN green LED drive device according to claim 1, wherein said bias current
output unit outputs a pulse-like bias current having a low level value of not lower
than 6 mA.
12. A GaN green LED drive device according to claim 1, wherein said bias current
output unit outputs a pulse-like bias current having a low level value of not lower
than 7 mA.
13. A GaN green LED drive device according to claim 1, wherein said bias current
output unit outputs a pulse-like bias current having a low level value of not lower
than 8.5 mA.
14. A GaN green LED drive device according to claim 1, wherein said bias current
output unit outputs a pulse-like bias current having a low level value of not lower
than 10 mA.
15. An optical transmission device having a GaN green LED drive device defined
in claim 2.
16. A GaN green LED drive device comprising:
a GaN green LED;
means for outputting a pulse-like bias current having a predetermined low level
value higher than a zero value; and
means for peaking said bias current outputted from said means for outputting
to make rise of said bias current higher than a high level value and make fall
of said bias current lower than said low level value to thereby obtain a peaked
bias current supplied to the GaN green LED,
wherein said means for outputting selectively sets said low level value of a
bias current based on a predetermined correlation between a set acting speed of
the GaN green LED and the bias current.
17. The GaN green LED drive device according to claim 16, wherein said means
for outputting selectively sets said low level value of said bias current to be
higher as the set acting speed of said GaN green LED increases.
Description
The present application is based on Japanese Patent Application No. 2001-361168,
the entire contents of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a GaN green LED drive device and an optical
transmission device and particularly to a GaN green LED drive device for driving
a GaN green LED (light-emitting diode) at a high speed and an optical transmission
device using the GaN green LED drive device.
2. Description of the Related Art
An optical transmission device for transmitting and receiving an optical signal
through a plastic optical fiber (hereinafter referred to as "POF") which is an
optical transmission medium is used in relatively short-distance (not longer than
100 m) optical communication such as inter-device optical communication. The optical
transmission device includes a light-emitting device for generating an optical
signal, and a light-receiving device for receiving an optical signal from another
optical transmission device.
Generally, an AlGaInP red LED is used as the light-emitting device. The
AlGaInP red LED can make high-speed response. The AlGaInP red LED, however, has
a problem that the emitted light output is reduced greatly in accordance with the
temperature change. For example, the wavelength of the emitted light output is
reduced by about 20 nm when the temperature change is 100° C. For this reason,
transmission loss in the POF varies largely, so that the transmission distance
is limited to about 50 m.
On the other hand, a GaN green LED (wavelength: 520 nm) for emitting green light
in a low-loss wavelength range in the POP has come onto the market for the display
purpose in recent years. It was conceived that the GaN green LED could transmit
light by a distance of 100 m or longer, because green light in a low-loss wavelength
range in the POF is emitted as well as because the GaN green LED has such characteristic
that lowering of the emitted light output and fluctuation of the wavelength due
to the temperature change are smaller as compared with the red LED. The GaN green
LED, however, has a problem of being bad in trailing edge characteristic.
FIG. 7A is a waveform graph of a conventional bias current supplied to the GaN
green LED. The bias current is shaped like a pulse to obtain a digital optical
signal. The low level of the bias current is about 0 mA whereas the high level
of the bias current is 20 mA. FIG. 7B is a waveform graph of a peaked bias current.
FIG. 7C is a waveform graph of a light output emitted from the GaN green LED. As
shown in FIG. 7C, the trailing edge of the emitted light output is sharp just after
the beginning of the fall but becomes slower with the passage of time after the
beginning of the fall. Hence, the trailing edge characteristic of the GaN green
LED is very bad.
FIG. 8 is a graph showing waveforms for measuring the fall time of the GaN green
LED. In FIG. 8, the upper trace part shows a waveform of a bias current supplied
to the GaN green LED, and the lower trace part shows a waveform of a light output
emitted from the GaN green LED. As shown in FIG. 8, the trailing edge characteristic
of the light output in a digital operation of the GaN green LED is about 38 nsec
in terms of the fall time t, required for changing the output from 90% to 10%.
In this case, the transmission speed of an optical signal outputted from the GaN
green LED is limited to about 20 Mbps. Hence, the GaN green LED cannot be applied
to a high-speed optical communication device;
SUMMARY OF THE INVENTION
The invention is proposed to solve the problems and an object of the invention
is to provide a GaN green LED drive device for shortening the fall time of a GaN
green LED to drive the GaN green LED at a high speed, and an optical transmission
device using the GaN green LED drive device.
According to the invention, the low level of a pulse-like bias current
supplied to a GaN green LED is set at a higher value than that in the related art
to thereby solve the problems.
FIG. 1 is a graph showing waveforms for measuring the fall time t
f
of the GaN green LED in the case where the pulse-like bias current is peaked while
the low level of the pulse-like bias current is set at a higher value (10 mA) than
that in the related art. In FIG. 1, the upper trace part shows a waveform of the
bias current supplied to the GaN green LED, and the lower trace part shows a waveform
of the light output emitted from the GaN green LED. As shown in FIG. 1, the fall
time t
f is 6.6 nsec. Hence, the fall time t
f can be shortened
greatly compared with that in the related art.
(1) According to the invention, there is provided a GaN green LED drive device
having: a bias current output unit for outputting a pulse-like bias current having
a low level higher by a predetermined value than a zero level; and a peaking unit
for peaking the bias current outputted from the bias current output unit to make
the rise of the bias current higher than a high level and make the fall of the
bias current lower than the low level to thereby obtain a peaked bias current supplied
to a GaN green LED.
In the invention, the bias current output unit operates so that the low level
of the bias current supplied to the GaN green LED is selected to be higher than
a zero level. Hence, only the sharp region of the trailing edge of the light output
emitted from the GaN green LED is used so that the fall time can be shortened.
In the leading edge of the bias current, the peaking unit makes the bias current
higher than the high level of the bias current to thereby quicken the rise of the
light output emitted from the GaN green LED. In the trailing edge of the bias current,
the peaking unit absorbs electric charge to make the bias current lower than the
low level of the bias current to thereby quicken the fall of the emitted light output.
In this manner, the bias current is peaked while the low level of the bias current
is selected to be higher than the zero level. Hence, particularly the fall time
of the GaN green LED can be shortened greatly to drive the GaN green LED at a high speed.
(2) According to the invention, in the GaN green LED drive device as in (1),
the bias current output unit selects the low level of the bias current to be higher
as the set acting speed of the GaN green LED increases.
The acting speed of the GaN green LED is decided on the basis of the rise time
and the fall time. The rise time is substantially constant but the fall time becomes
shorter as the low level of the bias current becomes higher. Hence, when the low
level of the bias current is selected to be high, the set acting speed of the GaN
green LED can be quickened.
(3) According to the invention, in the GaN green LED drive device as in (1) or
(2), the bias current output unit outputs a pulse-like bias current having a low
level of not lower than 4 mA.
When the acting speed of the GaN green LED is selected so that an optical signal
is outputted at the rate of 50 Mbps or higher, it is obvious from the correlation
between the acting speed of the GaN green LED and the low level of the bias current
that the low level of the bias current must be set to be not lower than 4 mA.
(4) According to the invention, there is provided an optical transmission device
having a GaN green LED drive device defined in any one of (1) through (3).
Hence, because the GaN green LED can be driven at a high speed, a high-speed
long-distance optical data link system can be achieved.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph showing waveforms for measuring the fall time of a GaN green
LED in the case where a bias current is peaked while the low level of the bias
current is set at a higher value (10 mA) than that in the related art;
FIG. 2 is a diagram showing the schematic configuration of an optical transmission
device according to an embodiment of the invention;
FIG. 3 is a diagram showing a specific example of the circuit configuration
of an LED drive circuit;
FIG. 4A is a waveform graph of a bias current output from a bias current source
21; FIG. 4B is a waveform graph of a peaked bias current obtained by peaking
the bias current; and FIG. 4C is a waveform graph of a light output emitted
from the GaN green LED;
FIG. 5 is a graph showing the relation between rise time t
r/fall
time t
f and theoretical acting speed BW with respect to a low-level
bias current;
FIG. 6 is a diagram showing another example of the circuit configuration of
the LED drive circuit;
FIG. 7A is a waveform graph of a conventional bias current supplied to the GaN
green LED; FIG. 7B is a waveform graph of a peaked bias current obtained by peaking
the bias current; and FIG. 7C is a waveform graph of a light output emitted from
the GaN green LED; and
FIG. 8 is a graph showing waveforms for measuring the fall time of the GaN green LED.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A preferred embodiment of the invention will be described below in detail with
reference to the drawings.
FIG. 2 is a diagram showing the schematic configuration of an optical transmission
device
1 according to an embodiment of the invention. The optical transmission
device
1 transmits and receives an optical signal through a POF
2
which is an optical transmission medium. The optical transmission device
1
includes a GaN green LED (hereinafter referred to as "GaN green LED")
10
for emitting green light, an LED drive circuit
20 for driving the GaN green
LED
10, and a photodiode
30 for receiving light from another light
transmission device through the POF
2.
FIG. 3 is a diagram showing a specific example of the circuit configuration
of the LED drive circuit
20. The LED drive circuit
20 has a bias
current source
21 for generating a bias current to be supplied to the GaN
green LED
10, and a peaking circuit
22 for peaking the bias current.
The bias current source
21 is connected to an anode of the GaN green LED
10 through a resistor R
1. The anode of the GaN green LED
10
is supplied with a DC constant voltage of 5 V through a resistor R
2. On
the other hand, a cathode of the GaN green LED
10 is grounded.
The peaking circuit
22 is connected between the bias current source
21
and the anode of the GaN green LED
10. The peaking circuit
22 is
constituted by a series circuit having a capacitor C and a resistor R
3.
The time constant of the peaking circuit
22 is decided on the basis of the
product of the capacitor C and the resistor R
3. Specifically, the time constant
is preferably selected to be substantially equal to the life time of a minority
carrier of the GaN green LED
10.
Incidentally, in the LED drive circuit
20 in this embodiment,
the values of the resistors R
1, R
2 and R
3 and the capacitor
C are selected to be 308 [Ω], 101 [Ω], 4.7 [Ω] and 68 [pF] respectively.
Upon reception of a TLL input signal from the LED drive circuit
20 configured
as described above, the GaN green LED
10 emits an in-phase light output
as follows.
FIG. 4A is a waveform graph of a bias current outputted from the bias current
source
21. The bias current has a low level of 10 mA and a high level of
20 mA. FIG. 4B is a waveform graph of a peaked bias current obtained by peaking
the bias current. FIG. 4C is a waveform graph of a light output emitted from the
GaN green LED
10.
In the related art, as shown in FIG. 7C, the trailing edge of a light output
emitted
from the GaN green LED was sharp just after the beginning of the fall but became
slower with the passage of time after the beginning of the fall. Therefore, when
the LED drive circuit
20 in the embodiment makes the low level of the bias
current higher than the zero level, only the sharp portion of the trailing edge
of the emitted light output is used so that the fall time which was t, in the related
art can be reduced to t
2, as shown in FIG. 4C When the bias current
source
21 outputs a bias current of 10 mA, the light output emitted from
the GaN green LED
10 is small because the voltage applied to the anode of
the GaN green LED
10 is low. When the bias current source
21 outputs
a bias current of 20 mA, the light output emitted from the GaN green LED
10
becomes large because the voltage applied to the anode of the GaN green LED
10
is high. In this manner, the GaN green LED
10 emits an in-phase light output
equal in phase to the bias current.
FIG. 5 is a graph showing the relation between rise time t
r/fall
time t
f and theoretical acting speed BW with respect to a low-level
bias current. The rise time t
r is substantially constant regardless
of the low level of the bias current. The fall time t
f becomes shorter
as the low level of the bias current becomes higher. Hence, the GaN green LED
10
can be operated at a higher speed when the low level of the bias current is set
to be higher.
In an NRZ (Non-Return-to-Zero) modulation method, the theoretical acting speed
BW is calculated by the following expression (1).
##EQU1##
To operate the GaN green LET)
10 at the rate of 100 Mbps or higher, both
the rise time t
r and the fall time t
f must be selected to
be not longer than 7 nsec. Since the rise time t
r is constant and about
4 nsec, it is necessary to select the low level of the bias current to be not lower
than 10 mA. Incidentally, when the GaN green LED
10 is to be operated at
the rate of about 50 Mbps, it is necessary to select the low level of the bias
current to be about 4 mA.
The LED drive circuit
20 may be also configured as follows. FIG. 6 is
a diagram showing another example of the circuit configuration of the LED drive
circuit
20. The LED drive circuit
20 has a bias current source
21
for generating a bias current to be supplied to the GaN green LED
10, and
a peaking circuit
22 for peaking the bias current.
The bias current source
21 is connected to a cathode of the GaN green
LED
10 through a resistor R
4. The cathode of the GaN green LED
10
is grounded through a resistor R
5. An anode of the GaN green LED
10
is supplied with a DC constant voltage of 5 V.
The peaking circuit
22 is connected between the bias current source
21
and the cathode of the GaN green LED
10. The peaking circuit
22 is
constituted by a series circuit having a capacitor C and a resistor R
3.
Incidentally, in the LED drive circuit
20, the values of the resistors R
4,
R
5 and R
3 and the capacitor C are selected to be 172 [Ω], 78
[Ω], 4.7 [Ω] and 68 [pF] respectively.
Upon reception of a TLL input signal from the LED drive circuit
20 configured
as described above, the GaN green LED
10 emits a reversed-phase light output
as follows. When the bias current source
21 outputs a bias current of 10
mA, the light output emitted from the GaN green LED
10 becomes large because
the voltage applied to the cathode of the GaN green LED
10 is low. When
the bias current source
21 outputs a bias current of 20 mA, the light output
emitted from the GaN green LED
10 becomes small because the voltage applied
to the cathode of the GaN green LED
10 is high. Incidentally, the relation
between rise time t
r/fall time t
f and theoretical acting
speed BW with respect to the low-level bias current is substantially the same as
that in the in-phase case.
As described above, the LED drive circuit
20 increases the low level of
the bias current by a predetermined value to thereby shorten the fall time t
f
of the GaN green LED
10 greatly. As a result, the GaN green LED
10
can be driven at a high speed.
Hence, the optical transmission device
1 having the LED drive circuit
20 can output an optical signal at a wavelength capable of being transmitted
through the POF
2 with low loss. In addition, even if the temperature changes,
the optical transmission device
1 can suppress both lowering of the emitted
light output and fluctuation of the wavelength compared with use of a red LED.
Hence, an optical data link system capable of transmitting optical data by a distance
of 100 m or longer can be provided inexpensively though the optical data link system
could not be provided by use of a red LED in the related art.
Incidentally, the invention is not limited to the embodiment and various
changes may be made on design without departing from the scope of claim.
For example, the low level of the bias current is not limited to 4 mA or 10 mA
but may be selected to be an optimum value in accordance with the acting speed
of the GaN green LED
10. Specifically, the low level of the bias current
may be decided in accordance with the fall time t, calculated on the basis of the
theoretical acting speed BW substituted in the expression (1). That is, the bias
current may be decided on the basis of the theoretical acting speed BW by use of
the correlation shown in FIG.
5. According to the correlation shown in FIG.
5, the preferred values of the low level of the bias current are 5 mA, 6 mA, 7
mA and 8.5 mA respectively when, for example, the values of the theoretical acting
speed BW are 60 Mbps, 70 Mbps, 80 Mbps and 90 Mbps.
The values of the resistors RI to RS and the capacitor C are not limited to these
values. It is a matter of course that the resistors R
1 to R
5 and
the capacitor C may have other values if the operation and effect of the invention
can be fulfilled.
In the GaN green LED drive device according to the invention, a pulse-like bias
current having a low level larger by a predetermined value than the zero level
is outputted. The outputted bias current is peaked so that the rise of the bias
current becomes higher than a high level while the fall of the bias current becomes
lower than the low level. The peaked bias current is supplied to the GaN green
LED. Hence, the fall time of the GaN green LED is shortened greatly so that the
GaN green LED can be driven at a high speed.
Moreover, in the GaN green LED drive device according to the invention,
the low level of the bias current is selected to be higher as the set acting speed
of the GaN green LED becomes higher. Hence, the GaN green LED can be driven at
a higher speed.
The optical transmission device according to the invention has the GaN green
LED drive device described above. Hence, a high-speed long-distance optical data
link system can be achieved, because the GaN green LED can be driven at a high speed.
*
