CA1087690A - Optical communication system utilizing light emitting diode - Google Patents
Optical communication system utilizing light emitting diodeInfo
- Publication number
- CA1087690A CA1087690A CA286,507A CA286507A CA1087690A CA 1087690 A CA1087690 A CA 1087690A CA 286507 A CA286507 A CA 286507A CA 1087690 A CA1087690 A CA 1087690A
- Authority
- CA
- Canada
- Prior art keywords
- frequency
- circuit means
- drive signal
- light emitting
- emitting diode
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/516—Details of coding or modulation
- H04B10/54—Intensity modulation
- H04B10/541—Digital intensity or amplitude modulation
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/501—Structural aspects
- H04B10/502—LED transmitters
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/564—Power control
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/58—Compensation for non-linear transmitter output
Abstract
ABSTRACT OF THE DISCLOSURE
An optical communication system wherein light intensity modu-lation with no carrier is performed using a light emitting diode of which the characteristic curve of the drive signal current vs. optical output power changes with changes in the frequency of the drive signal. The modulation degree at frequencies higher than a predetermined frequency exceeding the thermal response frequency of the light emitting diode is kept larger than the modulation degree at frequencies lower than the thermal response frequency. This is done by using an emphasis circuit during modulation and a de-emphasis circuit during demodulation.
An optical communication system wherein light intensity modu-lation with no carrier is performed using a light emitting diode of which the characteristic curve of the drive signal current vs. optical output power changes with changes in the frequency of the drive signal. The modulation degree at frequencies higher than a predetermined frequency exceeding the thermal response frequency of the light emitting diode is kept larger than the modulation degree at frequencies lower than the thermal response frequency. This is done by using an emphasis circuit during modulation and a de-emphasis circuit during demodulation.
Description
This invention relates to an optical communication system, particularly to an improvement of modulation degree in an optical communication system utilizing a light emitting diode which has a linear characteristic of drive signal current vs. optical output power which changes in accordance with the frequency of the drive signal.
It is already known that the drive signal current vs~
optical output power characteristic of a light emitting diode is non-linear. When executing light intensity modulation in an 10 optical communication system utilizing such a light emitting diode, this non-linear characteristic does not give any problems with a digital signal, but has a disadvantage for analog signals in that the output signal waveform is distorted and sufficient modulation degree cannot be obtained in order to suppress such distortion.
It is a purpose of this invention to offer a modulation system in the light intensity modulation of a light emitting diode in an optical communication system using the abovementioned light emitting diode which ensures a large modulation degree and low distortion of the output signal.
It is another purpose of this invention to offer a simple circuit configuration of a modulation system which per-forms light intensity modulation of a light emitting diode in an optical communication system utilizing a light emitting diode.
An optical communication system of this invention is characterized by having a modulation system comprising an emphasis circuit which has a characteristic such that the ampli-fication degree at frequencies higher than the desired one ;
exceeding the thermal response frequency of the light emitting -~
30 diode is larger than that at frequencies lower than the desired frequency.
Thus, in accordance with a broad aspect of the --1-- ~
1~'76~0 invention, there is provided an optical communication system wherein light intensity modulation is performed using a light emitting diode having a characteristic curve of drive signal current vs. optical output power which changes according to the frequency of a drive signal, comprising a modulation system consisting of: emphasis circuit means having a degree of ampli-fication which is larger when the frequency of the drive signal is higher than a thermal response frequency of the light emit-ting diode than when the frequency of the drive signal is lower than the thermal response frequency, amplifier circuit means connected to said emphasis circuit means to amplify the output of said emphasis circuit means, driving circuit means connected to said amplifier circuit means to drive the light emitting diode according to the output of said amplifier circuit means, and a light emitting diode connected with said driving circuit means and adapted to be driven by the output of said driving cir-cuit means and a demodulation system consisting of: a light -receiving element which receives light emitted from the light emitting diode, preamplifier circuit means which is connected to said light receiving element to preamplify the output of said light receiving element, de-emphasis circuit means which is con- ~ ~
nected to said preamplifier circuit means and having an amplifi- . ~ .
cation degree which is smaller at frequencies of the drive sign-al higher than the thermal response frequency than at frequencies : of the drive signal lower than the thermal response frequency, and amplifier circuit means which is connected to said de-: emphasis circuit means to amplify the output of said de-emphasis circuit means.
Figure 1 is a graph showing the relation between the drive signal current and optical output power of a light emitting diode.
It is already known that the drive signal current vs~
optical output power characteristic of a light emitting diode is non-linear. When executing light intensity modulation in an 10 optical communication system utilizing such a light emitting diode, this non-linear characteristic does not give any problems with a digital signal, but has a disadvantage for analog signals in that the output signal waveform is distorted and sufficient modulation degree cannot be obtained in order to suppress such distortion.
It is a purpose of this invention to offer a modulation system in the light intensity modulation of a light emitting diode in an optical communication system using the abovementioned light emitting diode which ensures a large modulation degree and low distortion of the output signal.
It is another purpose of this invention to offer a simple circuit configuration of a modulation system which per-forms light intensity modulation of a light emitting diode in an optical communication system utilizing a light emitting diode.
An optical communication system of this invention is characterized by having a modulation system comprising an emphasis circuit which has a characteristic such that the ampli-fication degree at frequencies higher than the desired one ;
exceeding the thermal response frequency of the light emitting -~
30 diode is larger than that at frequencies lower than the desired frequency.
Thus, in accordance with a broad aspect of the --1-- ~
1~'76~0 invention, there is provided an optical communication system wherein light intensity modulation is performed using a light emitting diode having a characteristic curve of drive signal current vs. optical output power which changes according to the frequency of a drive signal, comprising a modulation system consisting of: emphasis circuit means having a degree of ampli-fication which is larger when the frequency of the drive signal is higher than a thermal response frequency of the light emit-ting diode than when the frequency of the drive signal is lower than the thermal response frequency, amplifier circuit means connected to said emphasis circuit means to amplify the output of said emphasis circuit means, driving circuit means connected to said amplifier circuit means to drive the light emitting diode according to the output of said amplifier circuit means, and a light emitting diode connected with said driving circuit means and adapted to be driven by the output of said driving cir-cuit means and a demodulation system consisting of: a light -receiving element which receives light emitted from the light emitting diode, preamplifier circuit means which is connected to said light receiving element to preamplify the output of said light receiving element, de-emphasis circuit means which is con- ~ ~
nected to said preamplifier circuit means and having an amplifi- . ~ .
cation degree which is smaller at frequencies of the drive sign-al higher than the thermal response frequency than at frequencies : of the drive signal lower than the thermal response frequency, and amplifier circuit means which is connected to said de-: emphasis circuit means to amplify the output of said de-emphasis circuit means.
Figure 1 is a graph showing the relation between the drive signal current and optical output power of a light emitting diode.
-2-- , . .
:'.' ..
1087f~5~0 Figure 2 comprises graphs showing the influence of :
drive signal frequency of a light emitting diode on the relation :
beLween the drive signal current vs~ optical output power.
Figure 3 is a block diagram indicating an embodiment of a modu-., ' .
~::
-2a-: -.- ,.............. . :
.
9~
lation system and demodulation system of this invention.
Figure 4 is a circuit diagram of an embodiment of a modulation system according to this invention.
Figure 5 is a circuit diagram of an embodiment of a demodulation system according to this invention.
- Figure 6 is a graph showing a relation between drive signal frequency and output level of the emphasis circuit of this invention.
Figure 7 is a graph showing a relation between drive signal frequency and output level of the de-emphasis circuit of this invention.
Figure 1 shows a relation between drive signal current and optical output power of a light emitting diodeO
As indicated in the figure, the optical output power P of the light emitting diode gradually increases with an increase of the drive signal current I, and the relation is non-linear.
me region A in the non-linear region results from the non-linear charac*eristic of the voltage vs. current characteristic of a light emitting diode and the region B comes from deterioration of light emitting efficiency by temperature rise at the junction area of a light emitting diode due to the drive signal current.
mis invention relates to the non-linear characteristic in the region B. me area sandwiched by regions A and B shows almost linear characteristicO In order to obtain the signal with less distortion by executing light analog intensity modulation~ it is necessary to set the fixed bias to almost the center of this linear area and perform the amplitude modulation within this area. m erefore~ the modulation degree cannot be made large Figure 2 shows an influence of the drive signal frequency of a light emitting diode on the relation between the drive signal current and optical output power As shown in this figure~ when amplitude nodulation lS carried ;
_ 3 _ - , . ... .
1()87~i90 out by superimposing an AC signal to the fixed bias current IB applied to the light emitting diode, the relation between the drive signal current and optical output power, with the AC signal frequency fm changed as the parameter~ is improved in linearity in the region B of Figure 1 together with increases of frequency fm~ namely~ as the frequency changes to fl~ f2~ f3~000000~expanding the area showing a linear characteristicO
This phenomenon is caused by the thermal response of a light emitting diodeO Namely, in case of a light emitting diode, modulation is performed centering on the bias current IB and therefore the average power consumption of the light emitting diode is independent of the fre-quency. However~ since a light emitting diode has a thermal response timeconstant ~ , at the frequency near the thermal response frequency frc corresponding to this thermal response time constant ~ (here~ f~ =l/(2~r~
or higher than it~ temperature rise at the junction area of light emitting diode is small and deterioration of light emitting efficiency is small, thus improving non-linearity in the region B in Figure lo On the other hand~ at a frequency lower than the thermal response frequency~ temperature rise at the junction area of the light emitting diode becomes distinctive as the frequency becomes low and thereby linearity in the region B in Figure 1 is deteriorated due to decrease of light emitting efficiency.
Finally, when the AC signal frequency f satisfies the following equation (1), the region B in Figure 1 shows almost linear characteristic.
f ~ f~=l/(21r~) ........... (1) This invention utilizes this characteristic, namely, modulation degree is sufficiently large within the range where the modulation signal frequency f satisfies the equation (1), while it is small in the range where modulation frequency f is near to the thermal response frequency f~
or lower than it. Thereby the modulation is performed within the range ;
where excellent linearity is ensured. Thus, as a whole, the modulation ~ -- ' .
108~69V
degree can be made large with less distortion factor.
Figure 3 shows a block diagram of the configuration of the modu-lation system (a) and demodulation system (b) by this invention.
The modulation system will be described first.
When a-~ive signal is applied to the drive terminal 1, the emphasis circuit 2 emphasizes the input signal in such a manner that if the drive signal is higher than the predetermined frequency, the amplification degree is made larger than that when the drive signal is lower than the pre-determined frequency. The emphasized signal is then applied to the amplifier
:'.' ..
1087f~5~0 Figure 2 comprises graphs showing the influence of :
drive signal frequency of a light emitting diode on the relation :
beLween the drive signal current vs~ optical output power.
Figure 3 is a block diagram indicating an embodiment of a modu-., ' .
~::
-2a-: -.- ,.............. . :
.
9~
lation system and demodulation system of this invention.
Figure 4 is a circuit diagram of an embodiment of a modulation system according to this invention.
Figure 5 is a circuit diagram of an embodiment of a demodulation system according to this invention.
- Figure 6 is a graph showing a relation between drive signal frequency and output level of the emphasis circuit of this invention.
Figure 7 is a graph showing a relation between drive signal frequency and output level of the de-emphasis circuit of this invention.
Figure 1 shows a relation between drive signal current and optical output power of a light emitting diodeO
As indicated in the figure, the optical output power P of the light emitting diode gradually increases with an increase of the drive signal current I, and the relation is non-linear.
me region A in the non-linear region results from the non-linear charac*eristic of the voltage vs. current characteristic of a light emitting diode and the region B comes from deterioration of light emitting efficiency by temperature rise at the junction area of a light emitting diode due to the drive signal current.
mis invention relates to the non-linear characteristic in the region B. me area sandwiched by regions A and B shows almost linear characteristicO In order to obtain the signal with less distortion by executing light analog intensity modulation~ it is necessary to set the fixed bias to almost the center of this linear area and perform the amplitude modulation within this area. m erefore~ the modulation degree cannot be made large Figure 2 shows an influence of the drive signal frequency of a light emitting diode on the relation between the drive signal current and optical output power As shown in this figure~ when amplitude nodulation lS carried ;
_ 3 _ - , . ... .
1()87~i90 out by superimposing an AC signal to the fixed bias current IB applied to the light emitting diode, the relation between the drive signal current and optical output power, with the AC signal frequency fm changed as the parameter~ is improved in linearity in the region B of Figure 1 together with increases of frequency fm~ namely~ as the frequency changes to fl~ f2~ f3~000000~expanding the area showing a linear characteristicO
This phenomenon is caused by the thermal response of a light emitting diodeO Namely, in case of a light emitting diode, modulation is performed centering on the bias current IB and therefore the average power consumption of the light emitting diode is independent of the fre-quency. However~ since a light emitting diode has a thermal response timeconstant ~ , at the frequency near the thermal response frequency frc corresponding to this thermal response time constant ~ (here~ f~ =l/(2~r~
or higher than it~ temperature rise at the junction area of light emitting diode is small and deterioration of light emitting efficiency is small, thus improving non-linearity in the region B in Figure lo On the other hand~ at a frequency lower than the thermal response frequency~ temperature rise at the junction area of the light emitting diode becomes distinctive as the frequency becomes low and thereby linearity in the region B in Figure 1 is deteriorated due to decrease of light emitting efficiency.
Finally, when the AC signal frequency f satisfies the following equation (1), the region B in Figure 1 shows almost linear characteristic.
f ~ f~=l/(21r~) ........... (1) This invention utilizes this characteristic, namely, modulation degree is sufficiently large within the range where the modulation signal frequency f satisfies the equation (1), while it is small in the range where modulation frequency f is near to the thermal response frequency f~
or lower than it. Thereby the modulation is performed within the range ;
where excellent linearity is ensured. Thus, as a whole, the modulation ~ -- ' .
108~69V
degree can be made large with less distortion factor.
Figure 3 shows a block diagram of the configuration of the modu-lation system (a) and demodulation system (b) by this invention.
The modulation system will be described first.
When a-~ive signal is applied to the drive terminal 1, the emphasis circuit 2 emphasizes the input signal in such a manner that if the drive signal is higher than the predetermined frequency, the amplification degree is made larger than that when the drive signal is lower than the pre-determined frequency. The emphasized signal is then applied to the amplifier
3 and the amplified signal is applied to the light emitting diode driving circuit 4 in order to drive the light emitting diode 5.
Thereby, a light emitting diode 5 emits light having an intensity in accordance with the amplitude of the driving signal.
Next, the demodulation system will be explained.
As explained above, when the light emitting diode emits light having an intensity in accordance with the signal amplitude, the light receiving element 6 receives this light. A minute output of the light receiving element 6 is pre-amplified by the pre-amplifier circuit 7 and the amplified signal is then applied to the de-emphasis circuit 8 which has the opposite characteristic of the abovementioned emphasis circuit, that is, when the output signal of the pre-amplifier circuit 7 is higher than the predetermined frequency, the amplification factor is set samller than that when the frequency is lower than the predetermined frequency. Thereby, the emphasized signal is de-emphasized and then it is amplified again by the amplifier circuit 9. Thus, a signal similar to the drive signal to the input terminal 1 of the modulation system appears at the output terminal-10.
Figure 4 shows a more specific circuit configuration of the modulation system. In this figure, 1 represents the input terminal; 2, emphasis circuit; 3, amplifier circuit; 4, driving circuit; 5, light emitting _ 5 _ . , .
diode. These are already shown in Figure 3.
The input terminal 1 is connected to the emphasis circuit 2. The circuit configuration of emphasis circuit 2 is as follows; namely, the resis-tor Rl and capacitor Cl are respectively connected in parallel to the resis-tors R2 and R3 having the same resistance value, and in addition, resistor R4 and inductance element Ll are connected in series between the~connecting point of the resistors R2 and R3 and ground.
In this circuit configuration, if the constant of each element is determined precedingly, capacitor Cl becomes low impedance and inductance element Ll becomes high impedance for an input signal of higher frequency than the higher frequency. Thereby, a drive signal whose frequency is higher than the determined frequency is changed into a high level output signal. On the other hand, if the drive signal is lower than the determined frequency, capacitor Cl becomes high impedance and inductance element becomes low impedance. Thereby, the characteristic is such that the input signal is i suppressed and a low level output is obtained.
Therefore~ this emphasis circuit has the characteristic equivalent that the amplification factor is larger when the drive signal frequency is higher than the predeterm~ned one as compared with that when the frequency is lower than the determined frequency. This characteristic will be explained in~lmore detail referring to Figure 6.
Figure 6 shows an example of the frequency characteristic of the emphasis circuit giving the output to the light emitting diode. The f-axis indicates frequency of drive signal, while the L-axis indicates the~output level (dB) of the emphasis circuit. ~-In this figure, in the low frequency area, namely, at the fre-quency fl, for example, 30 kH~ which is slightly higher than the thermal response frequency f~ of the light emitting diode, for example, 20 to 25 kHz, a certain loss is given to the drive signal of the emphasis circuit 2, 7~9(11 whereby a low level output 11 is obtained. Meanwhile, in the area between frequency fl and the frequency f2 which is sufficiently higher than the thermal response frequency f1~ for example, at 100 kHz, the attenuation of the drive signal by the emphasis circuit 2 is gradually decreased. More-over, for a frequency higher than f2, there is no attenuation so that a high level output 12 can be obtained. This can be understood from the frequency characteristic shown in Figure 6.
The emphasis circuit having such characteristic is connected to the amplifier circuit 3 via the resistor R5 shown in Figure 4.
To be more specific, the amplifier circuit 3 is a differential amplifier.
The resistance value of the resistor R5 can be changed and it is provided in order to adjust the impedance matching between the emphasis circuit and amplifier circuit.
The amplifier circuit 3 is connected to the driving circuit 4 of light emitting diode 5. The driving circuit 4 has such a configuration that the emitter of transistor Tr 1 is grounded via the resistor R6 and the output of amplifier circuit 3 is connected to the base of transistor Tr l.
The transistor Tr 1 forms a class A amplifier and a current corresponding to the signal applied to the base of transistor Tr 1 flows between the collector and emitter of transistor Tr 1.
Moreover, a negative feed back loop is formed by connecting the emitter of transistor Tr 1 and the negative terminal of the amplifier 3, in ~ ~
order to compensate for non-linearity of the driving circuit. -A light emitting diode 5 is connected to the driving circuit 4~
; more specificly between the collector of transistor Tr 1 and the power source +E.
Therefore, the bias current IB of light emitting diode 5 is determined mainly by the resistor R6.
~, .
, -- .
'.: - ~ ~:
1087~0 Figure 5 shows a more specific circuit configuration of the demodulation system.
In this figure, 6 represents the light receiving element; 7, preamplifier circuit; 8, de-emphasis circuit; 9, amplifier circuit; 10, output terminal. These are also indicated in Figure 3.
Practically, the light receiving element 6 may consist of a PIN
diode, avalanche photo-diode (APD) or other element and it is connected to the input of the~pre~amplifier circuit 7. Moreover, the output of the amplifier circuit 7 is connected to the input of de-emphasis circuit 8.
In the de-emphasis circuit 8 the resistor ~ 1 and inductance element Lll are respectively connected in series to resistors ~ 2 and ~ 3 ~ -having the same resistance value. Simultaneously, the resistor ~ 4 and capacitor Cll are connected in series between the connecting point of resistors ~2 and ~ 3 and ground.
In this circuit configuration, when the constant(value) of each element is determined so that desired conditions are satisfied, if the out-put signal frequency of the pre-amplifier 7 is higher than the predeter-mined frequency, the inductance element Lll becomes high impedance while the capacitor Cll becomes low impedance. Thereby the output current of pre-amplifier circuit 7 flows mainly to ground via the capacitor Cll. Thus, the output signal of the pre-amplifier circuit 7 is applied to the amplifier circuit 9 at a low level. On the other hand, when the output signal fre- ;~
quency of pre-amplifier circuit 7 is lower than the determined frequency, the inductance element Lll becomes low impedance, while the capacitor C
becomes high impedance. Therefore, the output current of pre-amplifier circuit 7 flows mainly through the inductance element Lll. Thus, the output signal of the pre-amplifier circuit 7 is applied to the amplifier circuit 9 at a high level.
For this reason, when the output signal of the pre-amplifier .:
1()~376910 circuit 7 is higher than the determined frequency, this de-emphasis circuit has a characteristic equivalent to an amplification factor which is lower than that in the case when such frequency is lower than the determined frequency.
This characteristic will be explained in more detail by referring to Figure 7.
Figure 7 shows an example of the frequency characteristic of the de-emphasis circuit. The f-axis indicates the signal frequency, while the L-axis indicates the output level (dB) of the de-emphasis circuit.
lo In this figure, the frequency characteristic is such that in the low frequency area, namely, at the frequency fl, for example, 30 kHz, which is slightly higher than the thermal response frequency f1~of the light emitting diode, for example, 20 to 25 kHz, a high level output 14 can be obtained. Meanwhile, in the area between the frequency fl and frequency f2 which is sufficiently higher than the thermal response frequency f,~, for example, 100 kHz, the level is gradually decreased. At a frequency higher than the frequency f2, a low level signal 13 is present at the output.
The de-emphasis circuit having such a characteristic is connected to the output terminal 10 via the amplifier circuit 9 as shown in Figure 5.
According to the circuit mentioned above, non-linearity of region B in Figure 1 is improved and the linear area is effectively expanded. There-fore, as compared with a system not depending on this invention, the bias current can be set large and modulation degree can also be made large.
For example, in case of executing modulation with low distortion factor without pre-emphasis, modulation degree was at most 40% when the bias current was selected to be about 80 mA. But, in the modulation system with ~ ;
pre-emphasis in accordance with this invention, the bias current can be raised up to 100 mA or so and the modulation degree can be raised up to about 80% even with the same distortion factor.
g .:,:
: .: - . ,.. - . : ~ -~0~7~90 Here, it is obvious that a non-linear characteristic due to the thermal response of light emitting diode gives no problem when introducing the modulation method utilizing the carrier. Namely, there is no problem in the non-linearity characteristic so long as a carrier having a very much higher frequency than the thermal response frequency of the light emitting diode is modulated with a signal and the light emitting diode is driven by such modulated signal. However, for example, in case of amplitude-modulating (AM) the carrier, available frequency is as high as 30 MHz due to the fre-quency characteristic of the light emitting diode. Therefore, the available carrier frequency is as high as about 15 MHz and available frequency band is not wide enough. On the other hand, when single side band modulation -method or vestigial side band modulation method is used, available band-width can be widened but the system itself becomes complicated and uneconomi-- cal. The optical communication system not using the carrier disclosed by this invention ensures simple configuration and allows the available band-width of light emitting diode to be directly used as the baseband signal bandwidth.
Therefore, according to the optical communication system of this invention, a problem of non-linearity of light emitting diode can be solved and modulation degree can also be made large. It is a distinctive advantage of this inven~ion.
.; ''.
~ . . .: ~
Thereby, a light emitting diode 5 emits light having an intensity in accordance with the amplitude of the driving signal.
Next, the demodulation system will be explained.
As explained above, when the light emitting diode emits light having an intensity in accordance with the signal amplitude, the light receiving element 6 receives this light. A minute output of the light receiving element 6 is pre-amplified by the pre-amplifier circuit 7 and the amplified signal is then applied to the de-emphasis circuit 8 which has the opposite characteristic of the abovementioned emphasis circuit, that is, when the output signal of the pre-amplifier circuit 7 is higher than the predetermined frequency, the amplification factor is set samller than that when the frequency is lower than the predetermined frequency. Thereby, the emphasized signal is de-emphasized and then it is amplified again by the amplifier circuit 9. Thus, a signal similar to the drive signal to the input terminal 1 of the modulation system appears at the output terminal-10.
Figure 4 shows a more specific circuit configuration of the modulation system. In this figure, 1 represents the input terminal; 2, emphasis circuit; 3, amplifier circuit; 4, driving circuit; 5, light emitting _ 5 _ . , .
diode. These are already shown in Figure 3.
The input terminal 1 is connected to the emphasis circuit 2. The circuit configuration of emphasis circuit 2 is as follows; namely, the resis-tor Rl and capacitor Cl are respectively connected in parallel to the resis-tors R2 and R3 having the same resistance value, and in addition, resistor R4 and inductance element Ll are connected in series between the~connecting point of the resistors R2 and R3 and ground.
In this circuit configuration, if the constant of each element is determined precedingly, capacitor Cl becomes low impedance and inductance element Ll becomes high impedance for an input signal of higher frequency than the higher frequency. Thereby, a drive signal whose frequency is higher than the determined frequency is changed into a high level output signal. On the other hand, if the drive signal is lower than the determined frequency, capacitor Cl becomes high impedance and inductance element becomes low impedance. Thereby, the characteristic is such that the input signal is i suppressed and a low level output is obtained.
Therefore~ this emphasis circuit has the characteristic equivalent that the amplification factor is larger when the drive signal frequency is higher than the predeterm~ned one as compared with that when the frequency is lower than the determined frequency. This characteristic will be explained in~lmore detail referring to Figure 6.
Figure 6 shows an example of the frequency characteristic of the emphasis circuit giving the output to the light emitting diode. The f-axis indicates frequency of drive signal, while the L-axis indicates the~output level (dB) of the emphasis circuit. ~-In this figure, in the low frequency area, namely, at the fre-quency fl, for example, 30 kH~ which is slightly higher than the thermal response frequency f~ of the light emitting diode, for example, 20 to 25 kHz, a certain loss is given to the drive signal of the emphasis circuit 2, 7~9(11 whereby a low level output 11 is obtained. Meanwhile, in the area between frequency fl and the frequency f2 which is sufficiently higher than the thermal response frequency f1~ for example, at 100 kHz, the attenuation of the drive signal by the emphasis circuit 2 is gradually decreased. More-over, for a frequency higher than f2, there is no attenuation so that a high level output 12 can be obtained. This can be understood from the frequency characteristic shown in Figure 6.
The emphasis circuit having such characteristic is connected to the amplifier circuit 3 via the resistor R5 shown in Figure 4.
To be more specific, the amplifier circuit 3 is a differential amplifier.
The resistance value of the resistor R5 can be changed and it is provided in order to adjust the impedance matching between the emphasis circuit and amplifier circuit.
The amplifier circuit 3 is connected to the driving circuit 4 of light emitting diode 5. The driving circuit 4 has such a configuration that the emitter of transistor Tr 1 is grounded via the resistor R6 and the output of amplifier circuit 3 is connected to the base of transistor Tr l.
The transistor Tr 1 forms a class A amplifier and a current corresponding to the signal applied to the base of transistor Tr 1 flows between the collector and emitter of transistor Tr 1.
Moreover, a negative feed back loop is formed by connecting the emitter of transistor Tr 1 and the negative terminal of the amplifier 3, in ~ ~
order to compensate for non-linearity of the driving circuit. -A light emitting diode 5 is connected to the driving circuit 4~
; more specificly between the collector of transistor Tr 1 and the power source +E.
Therefore, the bias current IB of light emitting diode 5 is determined mainly by the resistor R6.
~, .
, -- .
'.: - ~ ~:
1087~0 Figure 5 shows a more specific circuit configuration of the demodulation system.
In this figure, 6 represents the light receiving element; 7, preamplifier circuit; 8, de-emphasis circuit; 9, amplifier circuit; 10, output terminal. These are also indicated in Figure 3.
Practically, the light receiving element 6 may consist of a PIN
diode, avalanche photo-diode (APD) or other element and it is connected to the input of the~pre~amplifier circuit 7. Moreover, the output of the amplifier circuit 7 is connected to the input of de-emphasis circuit 8.
In the de-emphasis circuit 8 the resistor ~ 1 and inductance element Lll are respectively connected in series to resistors ~ 2 and ~ 3 ~ -having the same resistance value. Simultaneously, the resistor ~ 4 and capacitor Cll are connected in series between the connecting point of resistors ~2 and ~ 3 and ground.
In this circuit configuration, when the constant(value) of each element is determined so that desired conditions are satisfied, if the out-put signal frequency of the pre-amplifier 7 is higher than the predeter-mined frequency, the inductance element Lll becomes high impedance while the capacitor Cll becomes low impedance. Thereby the output current of pre-amplifier circuit 7 flows mainly to ground via the capacitor Cll. Thus, the output signal of the pre-amplifier circuit 7 is applied to the amplifier circuit 9 at a low level. On the other hand, when the output signal fre- ;~
quency of pre-amplifier circuit 7 is lower than the determined frequency, the inductance element Lll becomes low impedance, while the capacitor C
becomes high impedance. Therefore, the output current of pre-amplifier circuit 7 flows mainly through the inductance element Lll. Thus, the output signal of the pre-amplifier circuit 7 is applied to the amplifier circuit 9 at a high level.
For this reason, when the output signal of the pre-amplifier .:
1()~376910 circuit 7 is higher than the determined frequency, this de-emphasis circuit has a characteristic equivalent to an amplification factor which is lower than that in the case when such frequency is lower than the determined frequency.
This characteristic will be explained in more detail by referring to Figure 7.
Figure 7 shows an example of the frequency characteristic of the de-emphasis circuit. The f-axis indicates the signal frequency, while the L-axis indicates the output level (dB) of the de-emphasis circuit.
lo In this figure, the frequency characteristic is such that in the low frequency area, namely, at the frequency fl, for example, 30 kHz, which is slightly higher than the thermal response frequency f1~of the light emitting diode, for example, 20 to 25 kHz, a high level output 14 can be obtained. Meanwhile, in the area between the frequency fl and frequency f2 which is sufficiently higher than the thermal response frequency f,~, for example, 100 kHz, the level is gradually decreased. At a frequency higher than the frequency f2, a low level signal 13 is present at the output.
The de-emphasis circuit having such a characteristic is connected to the output terminal 10 via the amplifier circuit 9 as shown in Figure 5.
According to the circuit mentioned above, non-linearity of region B in Figure 1 is improved and the linear area is effectively expanded. There-fore, as compared with a system not depending on this invention, the bias current can be set large and modulation degree can also be made large.
For example, in case of executing modulation with low distortion factor without pre-emphasis, modulation degree was at most 40% when the bias current was selected to be about 80 mA. But, in the modulation system with ~ ;
pre-emphasis in accordance with this invention, the bias current can be raised up to 100 mA or so and the modulation degree can be raised up to about 80% even with the same distortion factor.
g .:,:
: .: - . ,.. - . : ~ -~0~7~90 Here, it is obvious that a non-linear characteristic due to the thermal response of light emitting diode gives no problem when introducing the modulation method utilizing the carrier. Namely, there is no problem in the non-linearity characteristic so long as a carrier having a very much higher frequency than the thermal response frequency of the light emitting diode is modulated with a signal and the light emitting diode is driven by such modulated signal. However, for example, in case of amplitude-modulating (AM) the carrier, available frequency is as high as 30 MHz due to the fre-quency characteristic of the light emitting diode. Therefore, the available carrier frequency is as high as about 15 MHz and available frequency band is not wide enough. On the other hand, when single side band modulation -method or vestigial side band modulation method is used, available band-width can be widened but the system itself becomes complicated and uneconomi-- cal. The optical communication system not using the carrier disclosed by this invention ensures simple configuration and allows the available band-width of light emitting diode to be directly used as the baseband signal bandwidth.
Therefore, according to the optical communication system of this invention, a problem of non-linearity of light emitting diode can be solved and modulation degree can also be made large. It is a distinctive advantage of this inven~ion.
.; ''.
~ . . .: ~
Claims (6)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. An optical communication system wherein light intensity modulation is performed using a light emitting diode having a characteristic curve of drive signal current vs. optical output power which changes according to the frequency of a drive signal, comprising a modulation system consisting of: emphasis circuit means having a degree of amplification which is larger when the frequency of the drive signal is higher than a thermal response frequency of the light emitting diode than when the frequency of the drive signal is lower than the thermal response frequency, amplifier circuit means connected to said emphasis circuit means to amplify the output of said emphasis circuit means, driving circuit means connected to said amplifier circuit means to drive the light emitting diode according to the output of said ampli-fier circuit means, and a light emitting diode connected with said driving circuit means and adapted to be driven by the output of said driving circuit means and a demodulation system consisting of: a light receiving element which receives light emitted from the light emitting diode, preamplifier circuit means which is connected to said light receiving element to preamplify the output of said light receiving element, de-empha-sis circuit means which is connected to said preamplifier cir-cuit means and having an amplification degree which is smaller at frequencies of the drive signal higher than the thermal re-sponse frequency than at frequencies of the drive signal lower than the thermal response frequency, and amplifier circuit means which is connected to said de-emphasis circuit means to amplify the output of said de-emphasis circuit means.
2. An optical communication system according to claim 1, said emphasis circuit means comprising a first resistor and capacitor respectively connected in parallel to a second re-sistor and a third resistor having the same resistance value and in addition a fourth resistor and inductance element being con-nected in series between a connecting point of said second and said third resistors and ground.
3. An optical communication system according to claim 1, said emphasis circuit means having an amplification degree of a first constant value at frequencies of the drive signal lower than the first predetermined frequency which is higher than the thermal response frequency of the light emitting diode, increases gradually with increases of the frequency of said drive signal above said first predetermined frequency but below a second predetermined frequency, and has the second constant value larger than the said first constant value when the frequency of the drive signal is higher than said second predetermined frequency.
4. An optical communication system according to claim 1, wherein a negative feed back loop is formed between said driving circuit means and said amplifier circuit means.
5. An optical communication system according to claim 1, said de-emphasis circuit means comprising a first resistor and inductance element respectively connected in parallel to a second resistor and a third resistor having the same resistance value and in addition a fourth resistor and capacitor being connected in series between a connecting point of said second and said third resistors and ground.
6. An optical communication system according to claim 1, said de-emphasis circuit means having an amplification degree of a first constant value at frequencies of the drive signal lower than the first predetermined frequency which is higher than the thermal response frequency of the light emitting diode, decreases gradually as the increase of the frequency of said drive signal exceeds said first predetermined frequency while remaining lower than the second predetermined frequency, and has a second con-stant value which is smaller than said first constant value at frequencies of the drive signal higher than said second pre-determined frequency.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP51-109702 | 1976-09-13 | ||
| JP51109702A JPS5813038B2 (en) | 1976-09-13 | 1976-09-13 | LED modulation method |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1087690A true CA1087690A (en) | 1980-10-14 |
Family
ID=14517034
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA286,507A Expired CA1087690A (en) | 1976-09-13 | 1977-09-12 | Optical communication system utilizing light emitting diode |
Country Status (6)
| Country | Link |
|---|---|
| US (1) | US4207459A (en) |
| JP (1) | JPS5813038B2 (en) |
| CA (1) | CA1087690A (en) |
| DE (1) | DE2740343A1 (en) |
| FR (1) | FR2364572A1 (en) |
| GB (1) | GB1581238A (en) |
Families Citing this family (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2495412A1 (en) * | 1980-12-02 | 1982-06-04 | Thomson Csf | DIRECTLY MODULATED INFORMATION TRANSMISSION SYSTEM FOR OPTICALLY BANDWIDTH OPTICALLY LINKED LIGHT EXTENDED TO LOW FREQUENCIES AND CONTINUOUS |
| SE426345B (en) * | 1981-05-18 | 1982-12-27 | Asea Ab | FIBEROPTICAL METHOD FOR SATURING PHYSICAL AND / OR CHEMICAL SIZES, BASED ON SENSOR MATERIAL WITH A LINEAR LIGHT IN / LIGHT OUT CHARACTERISTICS |
| FR2556105B1 (en) * | 1983-12-02 | 1986-08-29 | Thomson Csf | OPTICAL TRANSMISSION CHANNEL WITH ELECTRICAL CONNECTORS |
| FR2630279B1 (en) * | 1988-04-15 | 1990-07-13 | Thomson Csf | METHOD AND DEVICE FOR CONTROLLING A DIGITAL LINK |
| US5202783A (en) * | 1991-06-24 | 1993-04-13 | The United States Of America As Represented By The Secretary Of The Navy | Secure and programmable friendly target recognition system |
| US5941989A (en) * | 1997-01-07 | 1999-08-24 | Micron Electronics, Inc. | Apparatus for indicating power-consumption status in a computer system |
| DE19815491C2 (en) * | 1998-04-07 | 2000-04-27 | Vishay Semiconductor Gmbh | Method and circuit arrangement for driving a luminous diode |
| JP2003134051A (en) * | 2001-10-25 | 2003-05-09 | Opnext Japan Inc | Optical receiving module, optical receiver and optical fiber communication equipment |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3814929A (en) * | 1972-05-05 | 1974-06-04 | Sperry Rand Corp | Laser data transmitter with wide frequency bandwidth |
| US3845242A (en) * | 1972-11-21 | 1974-10-29 | Minnesota Mining & Mfg | Video signal processing system for facsimile transmission |
| US3927316A (en) * | 1974-06-07 | 1975-12-16 | Zenith Radio Corp | Wireless speaker system using infra-red link |
| US3984824A (en) * | 1975-07-25 | 1976-10-05 | The United States Of America As Represented By The Secretary Of The Army | Wide-band optical analog signal link using fiber optics |
-
1976
- 1976-09-13 JP JP51109702A patent/JPS5813038B2/en not_active Expired
-
1977
- 1977-09-05 GB GB37046/77A patent/GB1581238A/en not_active Expired
- 1977-09-07 DE DE19772740343 patent/DE2740343A1/en not_active Ceased
- 1977-09-09 FR FR7727324A patent/FR2364572A1/en active Granted
- 1977-09-09 US US05/831,851 patent/US4207459A/en not_active Expired - Lifetime
- 1977-09-12 CA CA286,507A patent/CA1087690A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| DE2740343A1 (en) | 1978-04-06 |
| GB1581238A (en) | 1980-12-10 |
| US4207459A (en) | 1980-06-10 |
| FR2364572A1 (en) | 1978-04-07 |
| JPS5813038B2 (en) | 1983-03-11 |
| JPS5335355A (en) | 1978-04-01 |
| FR2364572B1 (en) | 1982-11-19 |
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