Coherent light generators – Particular component circuitry – For driving or controlling laser
Reexamination Certificate
1999-08-04
2001-09-04
Scott, Jr., Leon (Department: 2881)
Coherent light generators
Particular component circuitry
For driving or controlling laser
Reexamination Certificate
active
06285692
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method of driving a laser diode which may be utilized in high rate data transmission or the like, for example, and a driving apparatus using the driving method.
FIG. 11
shows an example of a conventional laser diode driver arrangement in its simplest form. A pair of transistors Q
1
and Q
2
in differential connection have their emitters connected to a current source Isd in common. A fixed bias voltage Vb is connected to the base of one of the transistors, Q
2
. Accordingly, the transistors Q
1
and Q
2
operate as switching elements in a manner such that when one of them is turned on, the other is turned off. In the example shown, a laser diode LD is connected to the collector of the transistor Q
1
, the base of which is connected with an input terminal T
IN
which is fed with a pulse drive signal V
IN
of a positive polarity. Each time a pulse in the signal V
IN
rises to assume a high level, the transistor Q
1
. is turned on to cause a light emission from the laser diode LD, thus radiating radiation P
OUT
. When the pulse applied to the input terminal T
IN
falls, the transistors Q
1
is turned off, while the transistor Q2 is turned on. The pair of transistors Q
1
and Q
2
are turned on and off in a differential manner to achieve a high rate switching operation.
In
FIG. 12
, there is illustrated a relationship between the current If passing through the laser diode LD and an output radiation P
OUT
, which are depicted at A and B, respectively. A flow of bias current Ib which is close to an emission initiating current Ith is maintained through the laser diode LD by a current source Isb. A sophistication is made such that as soon as the transistors Q
1
is turned on, the drive current If immediately exceeds the emission initiating current Ith, thus minimizing a lag in the light emission. The drive current If is shown at A in
FIG. 12
while a response of radiation P
OUT
is shown at B in
FIG. 12. A
lag of radiation P
OUT
with respect to the rising and the falling edge of the drive current If is shown exaggerated at T
DLY(ON)
and T
DLY(OFF)
as will be noted in the graph B of FIG.
12
.
A lag in the termination of radiation P
OUT
is attributable to the fact that a carrier concentration which remains in the active layer of the laser diode LD fails to return to zero rapidly when the drive voltage is interrupted. A difficulty that is caused by the lag in reducing the carrier concentration is illustrated in
FIG. 13
For example, as illustrated in
FIG. 13A
, as the OFF interval T
1
of the drive current I
f
or the extinction interval becomes shortened, the drive current I
f
would begin to flow at a timing when the carrier concentration is not reduced sufficiently, as shown in
FIG. 13B
, the time interval T
DLY(ON)
until the next emission is reached would be shorter or T
DLY(ON1)
>, T
DLY(ON2)
, as illustrated in FIG.
13
C. In the example shown in
FIG. 2
, it is longer again in the OFF interval T
2
, whereby the delay of the emission with respect to the immediately following rising edge of the drive current, or T
DLY(ON3)
, is longer than T
DLY(ON2)
. This means that a jitter is produced depending on the length of the extinction period T
OFF
, and the occurrence of a jitter presents a fault to the high rate transmission in a disadvantageous manner. This problem is discussed in“Effect of Bit-Rate, Bias and Threshold Currents on Turn-on Timing Jitter in Lasers Modulated with Uncoded and Coded Waveforms,” IEEE Photonics Technology Letters, Vol. 8, No. 3, March 1996, pp 461-463. This literature indicates that the turn-on time T
ON
of the laser diode LD after extinction period T
OFF
is given as follows:
T
ON
=
τ
ln
{
1
+
Ith
-
Ib
Im
-
Ith
[
1
-
exp
(
-
T
OFF
/
τ
)
]
}
(1)
where &tgr; represents a residual carrier concentration (carrier lifetime) in the active layer of the laser diode LD.
FIG. 14
shows the relationship as represented by the equation (1) where turn-on time T
ON
and the extinction period T
OFF
are normalized with &tgr;, respectively. Curves
12
A,
12
B and
12
C depicted in
FIG. 12
correspond to values of Im which are equal to 2Ith, 3Ith and 4Ith, respectively, for Ib=0.5 Ith. It will be seen from
FIG. 14
that the turn-on time T
ON
varies with the extinction period T
OFF
for a range of T
OFF
from 0 to nearly three of four times &tgr;, but remains constant for any subsequent change in the extinction period T
OFF
. The lifetime &tgr; of residual carriers of a laser diode is usually on the order of 2 to 3 ns, and thus it is seen that if a time interval from a preceding pulse is less than 10 ns, a jitter occurs unavoidably. However, the jitter itself (or T
ON
) can be reduced if the drive current If is increased.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method of driving a laser diode which is capable of suppressing the occurrence of a jitter and a laser diode driver arrangement which utilizes the driving method.
An apparatus for driving a laser diode according to a first aspect of the invention comprises:
a laser diode;
power supply means for supplying a driving power to the laser diode;
switching means connected in series with the laser diode for controlling a drive current which passes through the laser diode in accordance with a drive signal which repeats an ON and an OFF interval to switch the laser diode between a light emission and an extinction;
time measuring means for measuring the OFF interval of the drive signal and delivering a time signal having a level which corresponds to the interval; and
current control means for controlling the drive current in accordance with a control signal which corresponds to the time signal such that the shorter the OFF interval, the longer the rising of the drive current is retarded.
An apparatus for driving a laser diode according to a second aspect of the invention comprises:
a laser diode;
power supply means for supplying a driving power to the laser diode;
switching means connected in series with the laser diode for controlling a drive current which passes through the laser diode in accordance with a drive signal to switch the laser diode between a light emission and an extinction; and
differential current generating means for generating a differential current from the drive signal and for superimposing the differential current upon the drive current through the laser diode.
A method of driving a laser diode according to the first aspect of the invention comprises the steps of:
(a) measuring an OFF interval in the drive signal and producing a time signal of a level which corresponds to the OFF interval; and
(b) controlling the drive current on the basis of the time signal such that the shorter the measured interval, the longer the rising time which the drive current is retarded.
A method of driving a laser diode according to the second aspect of the invention comprises the steps of:
(a) generating a differential current from the drive signal; and
(b) superimposing the differential current upon the drive current so that the differential current from the falling edge of the drive current is effective in reducing residual carriers.
With the method and apparatus for driving a laser diode according to the invention, the shorter the time interval since a preceding drive current has been interrupted, the greater the magnitude of a reverse current that is superimposed upon the drive current at the next time the drive current rises. In this manner, the next emission is controlled to be occurring in a retarded direction. Consequently, a rising of the emission is restricted, affording an advantage that the occurrence of a jitter in the data that is optically transmitted is suppressed.
REFERENCES:
patent: 4709370 (1987-11-01), Bednarz et al.
patent: 5140175 (1992-08-01), Yagi et al.
patent: 5163063 (1992-11-01), Yoshikawa et al.
patent: 5283794 (1994-02-01), Gibbs et al.
patent: 5315650 (1994-05-01), Tanaka
patent: 5430749 (1995-07-01), Horie
patent: 6072761 (2000-06-01), Tani
Advantest Corporation
Gallagher & Lathrop
Jr. Leon Scott
Lathrop David N.
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