Radiation characteristic measuring apparatus for laser diode

An apparatus for measuring a radiation characteristic of a laser diode. An AC signal and a step-like DC voltage are applied to the laser diode and the radiant power output thereof is received by a photodiode having an output to which a first amplifier, a transformer, a capacitor and an AC amplifier for extracting the AC signal. The apparatus further comprises a first switch connected between the input of the first amplifier and the capacitor, and a second switch connected between both ends of the transformer. Every time the step-like voltage is stepped up, the first and second switches are closed for a short time as compared with the duration of a step period to protect the measurement from the influence of the change in the DC voltage.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an apparatus for measuring radiation 
characteristic of a laser diode. More particularly, the invention concerns 
a radiation characteristic measuring apparatus which is protected from the 
influence of change in a DC (direct current) voltage upon measurement of 
differentiated characteristic of a laser diode by applying thereto an AC 
(alternating current) voltage in superposition to a step-like DC (direct 
current) voltage. 
2. Description of the Prior Art 
FIG. 2 of the accompanying drawings shows a radiant-power or photo-output 
versus excitation- or drive-current characteristic of a laser diode. This 
characteristic curve provides a criterion for deciding whether a laser 
diode in concern is to be satisfactory or not in respect to the 
characteristic. More specifically, when the characteristic curve as 
measured is of a form fluctuating unnaturally, it may be decided that the 
laser diode in concern has poor characteristic. Accordingly, decision as 
to whether a laser diode is to be satisfactory or not in respect to the 
characteristic may be realized by differentiating a signal representative 
of the characteristic curve illustrated in FIG. 2. In a practical 
application, the differentiation may be carried out by applying a stepwise 
increasing DC voltage to the laser diode and measuring the radiant power 
output thereof. When the signal derived through the differentiation 
represents a uniformly increasing curve, it may be decided that the diode 
as tested is of a good quality, while fluctuation present in the curve 
resulting from the differentiation will means that the laser diode in 
concern is not to be satisfactory in respect to the characteristic in 
concern. It is however noticed that a problem is often encountered in 
making the decision mentioned above because the data obtained through 
differentiation is often of so small magnitude that discrimination is 
rendered very difficult. In an attempt to avoid such difficulty, it has 
been proposed to apply an AC voltage to the laser diode under test in 
super-position to the DC voltage, wherein the differential measurement is 
performed by amplifying the AC voltage from the laser diode. In that case, 
however, when a step-like DC voltage is employed, data as obtained 
undergoes a change every time the DC voltage changes stepwise, making it 
difficult to discriminate whether the detected data increases uniformly or 
suffers fluctuation. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an apparatus 
for measuring a radiant-power-output versus excitation current 
characteristic of a laser diode by employing an AC voltage in 
superposition to a step-like DC voltage, which apparatus is substantially 
insusceptible to the influence of the step-like DC voltage. 
In view of the above object, there is provided according to an aspect of 
the present invention an apparatus for measuring a radiation 
characteristic of a laser diode, in which an AC voltage and a step-like DC 
voltage are applied to the laser diode whose radiant power output is 
received by a photodiode having an output to which a first amplifier, a 
transformer, a capacitor and an AC amplifier for extracing the AC signal 
are connected, which apparatus further comprises a first switch connected 
between the input of the first amplifier and the capacitor, and a second 
switch connected between both ends of the transformer, wherein every time 
the step-like voltage is stepped up, the first and second switches are 
closed for a short time as compared with the duration of a step period of 
the step-like DC voltage. With the arrangement of the invention described 
above, the measurement can be protected from adverse influence of the 
change in the DC voltage and can be performed with an improved 
reliability. 
The above and other objects, features and advantages of the present 
invention will become apparent upon reading the following description of 
the preferred embodiments of the invention taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Now, the invention will be described in detail by referring to FIG. 1 which 
shows an exemplary embodiment of the invention. 
In FIG. 1, a reference numeral 1 denotes a laser diode whose radiation 
characteristic is to be measured, a numeral 2 denotes a power supply 
source, 3 denotes a photodiode, 4 denotes an amplifier, 5 denotes a 
transformer, 6 denotes a capacitor, 7 denotes an AC amplifier, 8 denotes a 
detector, 9 denotes an indicator, 10 denotes a controller, and numerals 11 
and 12 denotes switches, respectively. Each of the switches 11 and 12 may 
be constituted by an analogue switch. 
The power supply source 2 is so arranged as to apply an AC voltage and a 
step-like DC voltage to the laser diode 1 under test. The radiant power 
output of this laser diode is detected by the photodiode 3. The output 
signal of the photodiode 3 is sent to the transformer 5 after having been 
amplified through the amplifier 4. The capacitor 6 serves to remove the DC 
component so that only the AC signal is derived from the secondary of the 
transformer 5. The AC signal thus derived is then amplified through the AC 
amplifier 7 to be subsequently detected by the detector 8. Since the 
indicator 9 is supplied with the AC signal corresponding to the AC voltage 
applied to the laser diode 1, the differential characteristic in concern 
of the laser diode 1 can be determined on the basis of the values 
indicated by the indicator 9. 
FIG. 3 graphically shows the output characteristic of the power supply 
source 2. As will be seen in the figure, a step-like DC voltage 13 is 
superposed with an AC voltage 14. It will be understood that the power 
supply source 2 is adapted to apply to the laser diode 1 the combined 
voltage which varies as a function of time lapse in the manner illustrated 
in FIG. 3 under the control of the controller 10. 
The switches 11 and 12 are interlocked with each other and adapted to be 
opened (off) and closed (on) under the command of the controller 10. 
Turning to FIG. 1, the switch 11 is disposed to connect one input of the 
amplifier 4 to the capacitor 6, while the switch 12 is inserted between 
both ends of the primary winding of the transformer 5. 
Operations of the switches 11 and 12 are illustrated in FIG. 4. In the 
figure, reference symbols T1, T2 and T3 represent clock pulses generated 
by the controller 10. Under the timing of this clock signal, the DC 
voltage is increased stepwise as illustrated in FIG. 3. More specifically, 
a time interval between the clock pulses T1 and T2 corresponds to a step 
period of the DC voltage 13 shown in FIG. 3. Both the switches 11 and 12 
are simultaneously closed in response to each of the clock pulses T1, T2, 
and T3 and remained in the closed or on-state for a time shorter than the 
one step period and again opened. By way of example, assuming that the 
step period of the clock signal is 1 ms, both the switch 11 and 12 are 
closed for 40 .mu.s. 
By closing the switch 11 for a period represented by T11, T21, the DC 
voltage is applied to the capacitor 6 which is thus charged with the DC 
voltage. Since the switch 12 is closed whenever the capacitor 6 is charged 
through the closed switch 11, a short-circuit is formed between both ends 
of the primary of the transformer 5 by the switch 12, resulting in that 
the AC signal is interrupted. 
The time intervals T12 and T22 in FIG. 4 correspond to the measuring 
periods, respectively, during which the DC voltage is applied to the 
capacitor 6 so that no charging current flows therein. 
In this manner, only the AC voltage component 14 superposed on the DC 
voltage 13 as illustrated in FIG. 3 can be extracted in a stabilized state 
during the measuring periods T12 and T22 to be utilized in the measurement 
of the differentiated radiation characteristic of the laser diode 1. 
As will now be appreciated from the foregoing description by virtue of such 
arrangement that the DC voltage is applied to the DC cut-off capacitor 6 
for a short time through the switch 11 every time the DC voltage changes 
while a shortcircuit is formed between both ends of the primary circuit of 
the transformer 5 by means of the switch 12 for the same time, the AC 
signal superposed on the DC voltage can be detected without undergoing 
influence of the step-like change in the DC voltage, whereby the 
differentiated characteristic in concern of the laser diode can be 
measured in the stabilized state. 
In the foregoing, the present invention has been described in conjunction 
with an embodiment thereof. It should however be understood that the 
invention is never restricted exactly to the disclosure made herein. Many 
modifications and versions will readily occur to those skilled in the art 
without departing from the spirit and scope of the invention set forth in 
claims.