LED stabilizing light source device

An LED stabilizing light source device comprises an LED, a drive circuit for driving the LED, a wavelength filter for filtering the light emitted from the LED, a light splitter for splitting the light passing through the filter, a photodiode for receiving the split light. The device further comprises a feedback loop for feeding an output of the photodiode to the drive circuit.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an LED (Light-Emitting Diode) stabilizing 
light source device suitable for measurement of light propagation 
characteristics of an optical fiber. 
2. Description of the Related Art 
When the ambient temperature of an LED serving as a light-emitting element 
changes, output light therefrom also changes. Therefore, conventionally, 
the ambient temperature of the LED is measured using a heat-sensitive 
element such as a thermistor, and an output from a drive circuit for 
driving the LED is controlled in accordance with the sensed temperature, 
thus stabilizing the output light from the LED. 
In the above conventional system, however, it generally takes several tens 
of minutes until the output light is stabilized after a power switch is 
turned on. 
Since, furthermore, the temperature characteristics of LEDs are generally 
different, each LED requires a thermistor suitable to the LED. However, 
the temperature characteristics of LEDs and thermistors are generally not 
the same, thus an error will occur between the temperature 
characteristics. Therefore, it is required to compensate for the error to 
control the driving of the LEDs with high precision. However, this is 
troublesome and lowers the operability of the device. 
SUMMARY OF THE INVENTION 
The present invention has been made in consideration of the above 
situations, and has as its object to provide an LED stabilizing light 
source device which has a high operability, and starts to operate in a 
short time from the power-on of the device. 
According to the present invention, there is provided an LED stabilizing 
light source device comprising an LED, a drive circuit for driving said 
LED, a wavelength filter having a predetermined passing band, for 
receiving output light from said LED, and passing therethrough a portion 
of the light whose wavelength is within said predetermined passing band, a 
light splitter for splitting a beam of the light passing through said 
wavelength filter, a beam guide member for guiding one split beam to the 
outside of the light source, a beam receiving member for receiving the 
other split beam, and outputting a signal corresponding to the intensity 
of said other split beam, and a feedback loop for feeding back an output 
from said beam receiving member to said drive circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1, an LED 1 receives a drive current from a drive circuit 2 and 
emits light having an intensity corresponding to the magnitude of the 
drive current. The emitted light is input to a band-pass wavelength filter 
or band-pass color filter 3 having a predetermined passing band of 
wavelength, and a portion of the light whose wavelength is within the 
passing band of the filter 3 passes via the filter 3. The light portion is 
input to an optical fiber F0 of a fiber coupler or light splitter 4 via an 
optical fiber F1 of the same type as the optical fiber F0, to split the 
light portion into two light beams. The optical fiber F0 is formed of two 
optical fibers f1 and f2 of the same type. One split beam is guided to the 
outside via an optical fiber F2 which is the same type as the optical 
fibers F0 and F1. The other split beam is input to a photodiode PD5 
serving as a light-receiving element via an optical fiber F3 which is the 
same type as the optical fibers F0, F1, and F2. A current corresponding to 
the intensity of the input beam flows through the photodiode PD5 into a 
pre-amplifier 6. A voltage corresponding to the current is output from the 
preamplifier 6. The output voltage is input to a comparator 7, as Vin, and 
is compared with a reference voltage Vref. The reference voltage Vref 
determines the intensity of the output beam from the fiber coupler 4. When 
the voltage Vin coincides with the reference voltage Vref, the intensity 
of the output beam from the coupler 4 is equal to the intensity determined 
by the reference voltage Vref. The comparator 7 supplies an output voltage 
corresponding to the comparison result to the drive circuit 2. The drive 
circuit 2 supplies a current corresponding to the output voltage from the 
comparator 7 to the LED 1 to control the output light from the LED 1. 
As described above, a light beam passing through the wavelength filter or 
color filter 3 is compared with the reference value by the comparator 7, 
and the drive current of the drive circuit 2 is controlled in accordance 
with the comparison result, so that the intensity of the output light from 
the LED is controlled to a predetermined intensity. In other words, a 
light beam passing through the filter 3 is fed back to the drive system of 
the LED 1, so that the voltage Vin coincides with the reference voltage 
Vref to set the intensity of the output light from the LED 1 to a 
predetermined intensity. 
According to the above system, therefore, even if ambient temperature is 
changed, the intensity of the output beam to the outside of the device can 
be stabilized. 
Also according to the above system, even if the wavelength characteristics 
of the output light of the LED 1 changes, the intensity of the output beam 
is kept stable. More specifically, assume that the passing band 
characteristics of the wavelength filter or color filter 3 is represented 
by a curve B as is shown in FIG. 2A, and the wavelength characteristic 
curve of light emitted from the LED 1 changes from a curve A1 to a curve 
A2 depending on a change in temperature. In this case, when the wavelength 
characteristics of the output light from the LED 1 are represented by the 
curve A1, the light beam passing through the wavelength filter 3 has the 
spectrum curve shown in FIG. 2B. In other words, the spectrum curve of the 
light beam passing through the filter 3 corresponds to the curve B of the 
passing band characteristics of the filter 3. However, when the wavelength 
characteristics of the output light of the LED 1 are represented by the 
curve A2, the light beam passing through the filter 3 has the spectrum 
curve shown in FIG. 2C. The spectrum curve of the light beam does not 
coincide with the curve B of the passing band characteristics of the 
filter 3, and the intensity of the light beam passing through the filter 3 
is reduced. For this reason, if the wavelength characteristics of the 
output light are not compensated, the intensity of the output beam to the 
outside of the device is reduced. However, in this system, a light beam 
passing through the wavelength filter 3 is fed back to the drive system of 
the LED 1 to control the drive power to be applied to the LED 1. 
Therefore, in this system, the drive current is increased by the magnitude 
corresponding to the difference between the intensity of the output beam 
and the predetermined intensity, and the spectrum of the output beam from 
the LED 1 is increased, as indicated by a curve A2' shown in FIG. 2A. As a 
result, the spectrum curve of the beam passing through the wavelength 
filter 3 is substantially the same as that shown in FIG. 2B, and the 
intensity of the output beam to the outside of the device can be 
stabilized. 
As described above, according to the system, since the output light 
characteristics and the wavelength characteristics are controlled by a 
feedback system, the output beam to the outside of the device can be 
stabilized within a short period of time after the power switch is turned 
on. 
Note that, as the passing band of the wavelength filter or color filter 3 
is smaller, the stability of the wavelength of the light beam passing 
through the filter 3 can be improved. Assume that the characteristic curve 
of the passing band of the wavelength filter 3 is represented by a curve B 
as is shown in FIG. 3A, and the passing band of the filter 3 is relatively 
broad. In this case, when the spectral characteristics of the light 
emitted from the LED 1 is represented by a curve A1 as is shown in FIG. 
3A, the light beam passing through the wavelength filter 3 has the 
spectral characteristics shown in FIG. 3B. The characteristics coincides 
with the characteristics of the wavelength filter 3 represented by the 
curve B shown in FIG. 3A. However, assume that the spectral 
characteristics of the light emitted from the LED 1 is offset from that 
represented by the curve A1 to that represented by the curve A2, and the 
spectral characteristic is corrected by a feedback control, as represented 
by a curve A2' as is shown in FIG. 3A. In this case, the spectrum of the 
light beam passed through the filter 3 is distorted as shown in FIG. 3C, 
and its central wavelength is offset. In this case, if the filter 3 having 
a small passing band is used, the offset of the wavelength can be 
prevented. Thus, the filter 3 having a small passing band is preferably 
used. 
The half-width of the emission spectrum of the LED normally falls within 
the range of about 100 nm to 150 nm, as far as the LED having a center 
wavelength of 1.3 .mu.m or 1.55 .mu.m is concerned. A change of the 
wavelength of the emission spectrum of the LED in the temperature range of 
-10.degree. C. to 50.degree. C. is about 50 nm in maximum, and about 15 nm 
in minimum. In this case, the half-width of the wavelength filter 3 is 
preferably 20 nm or less. 
When the device was driven under the temperature range of -10.degree. C. to 
50.degree. C. by the above-mentioned system using an LED having a center 
wavelength of 1.3 .mu.m and a filter having a half-width of 20 nm, 
respectively, the offset of the central wavelength of the spectrum of the 
light beam passing through the wavelength filter or color filter 3 was 1 
nm or less. 
In the above embodiment, the optical fiber F0 forming the fiber coupler 4, 
the optical fiber F1 for guiding a light beam to the fiber coupler 4, the 
optical fiber F2 for guiding a light beam from the fiber coupler 4 to the 
outside of the device, and the optical fiber F3 for guiding a light beam 
to the photodiode PD5, are all the same type. For example, these optical 
fiber F0-F3 are all of single-mode type. Therefore, the transmission 
losses of light beams through the fibers F0, F1, F2, and F3 are equal to 
each other, and the stability of the output beam of the device is 
enhanced. 
As described above, the fiber coupler 4 comprises the optical fiber F0. The 
optical fiber F0 is formed of two optical fibers f1 and f2 superposed each 
other at the light splitting portion where the claddings are thinner than 
the remaining portion of the claddings. The optical fiber F0 can be 
prepared by superposing two optical fibers each other, and drawing the 
superposed portion of the fibers in the longitudinal direction to make the 
claddings of the superposed portion thin so that a light passing through 
the fiber f1 enters the fiber f2 via the the claddings. The thinning of 
the claddings may alternatively be performed by scraping those portions of 
the claddings where the fibers are to be superposed. 
FIG. 4 shows an arrangement of the comparator 7 and the drive circuit 2 
used in the LED stabilizing light source device shown in FIG. 1. The 
comparator 7 includes npn transistors T1 to T4 and a current supply source 
CS, and the drive circuit 2 includes an npn transistor T5 and a load 
resistor R. A pair of npn transistors T1 and T2 constitute a differential 
amplifier. The emitter of each transistor is grounded via the current 
supply source CS made of, e.g., a resistor. A voltage Vin is input to the 
base of the transistor T1, and the base of the transistor T2 is connected 
to the reference voltage Vref. A pair of npn transistors T3 and T4 
constitute a current mirror circuit. The collector of each transistor is 
connected to a power supply potential Vcc, and the gates of the 
transistors are connected to each other. The emitter of the transistor T4 
is connected to its base and the collector of the transistor T2. The 
emitter of the transistor T3 is connected to the collector of the 
transistor T1. The collector of the transistor T1 serves as an output 
terminal of this amplifier, and is connected to the drive circuit 2 in the 
device shown in FIG. 1. 
The base of the transistor T2 is connected to the reference voltage Vref, 
so that its collector potential is constant. The collector of the 
transistor T5 in the drive circuit 2 is connected to the power supply 
potential Vcc via the LED 1, and the emitter of the transistor 5 is 
grounded via a load resistor R. The gate of the transistor 5 is connected 
to the collector of the transistor T1 serving as the output terminal of 
the comparator 7. 
An operation of the comparator and the drive circuit in FIG. 4 will be 
described below. 
Assume that the spectrum of the light beam passing through the wavelength 
filter or color filter 3 exceeds a predetermined value determined by the 
reference voltage Vref. In this case, the voltage Vin is higher than the 
reference voltage Vref. Therefore, the transistor T1 is turned on, and its 
collector potential, i.e., an output potential Vout of the comparator 7 is 
reduced in correspondence with the input voltage Vin. Thus, the base 
voltage of the transistor T5 in the drive circuit 2 is reduced. For this 
reason, the collector current of the transistor T5, i.e., the drive 
current of the drive circuit 2, is decreased, and the intensity of the 
emission spectrum of the LED 1 is reduced. Therefore, a current flowing 
through the photodiode PD5 is decreased, and the voltage Vin is reduced. 
This cycle is repeated, and the voltage Vin coincides with the reference 
voltage Vref. 
In contrast with the above case, assume that the spectrum of the light beam 
passing through the wavelength filter 3 is a predetermined value or less. 
In this case, the voltage Vin is lower than the reference voltage Vref. 
For this reason, the transistor T1 is turned off, and the output potential 
Vout is increased in correspondence with the input voltage Vin. Therefore, 
the base potential of the transistor T5 in the drive circuit 2 is 
increased. For this reason, the collector current of the transistor T5 is 
increased, and the intensity of the emission spectrum of the LED 1 is 
increased. Therefore, a current flowing through the photodiode PD5 is 
increased, and the voltage Vin is also increased. This cycle is repeated 
and the voltage Vin coincides wit the reference voltage Vref. 
FIG. 5 shows an LED stabilizing light source device according to another 
embodiment of the present invention. 
Those portions the same as in the device shown in FIG. 1 are designated by 
the same reference characters, the descriptions thereof being omitted. 
In this embodiment, the light splitter 14 is formed of rod lens 151, 152, 
153 and a half mirror 16. The rod lenses are focusing lenses. The light 
beam from wavelength filter 3 is impinged on the half mirror 16 via the 
rod lens 15.sub.1. The half mirror 16 splits the light beam into two light 
beams. One light beam is guided to the outside of the device via the rod 
lens 15.sub.2 and a milti-mode optical filter F11, and the other light 
beam is guided to the photodiode PD5 via the rod lens 15.sub.3 and a 
multi-mode optical filter F12. 
In this embodiment, the guiding optical fibers F11 and F12 are both of 
multi-mode type. Since, the optical fibers are of the same type, the 
transmission losses of light beams passing therethrough are equal, and the 
stability of the output beam of the device is enhanced. 
According to the present invention, a system wherein the light beam passing 
through the wavelength filter or color filter is split by the light 
splitter, and one split beam is fed back to the drive system of the LED to 
control the drive power of the LED is employed. Therefore, even if the 
wavelength characteristics of the output light from the LED changes, or 
even if the output light characteristics change, the intensity of the 
output beam output to the outside of the device can be kept stable. In 
addition, a stable output beam can be obtained within a short period of 
time by control using the feedback system after the power supply is turned 
on.