Coherent light generators – Particular component circuitry – For driving or controlling laser
Reexamination Certificate
2003-01-17
2004-10-26
Leung, Quyen (Department: 2828)
Coherent light generators
Particular component circuitry
For driving or controlling laser
Reexamination Certificate
active
06810051
ABSTRACT:
This application claims the benefit of the filing date as provided by 35 U.S.C. 119 of European patent application number EP-02001096.3 filed on Jan. 22, 2002, the disclosure of which is incorporated herein by reference.
FIELD OF THE INVENTION
The present invention relates in general to laser drivers coupled to laser modules for modulating the laser output thereof. In particular, the present invention relates to solutions enhancing the operation of laser drivers being AC coupled to laser modules.
BACKGROUND OF THE INVENTION
The optical output power of laser diodes varies as a function of temperature and operating time. While temperature effects on the optical output power can be compensated for by cooling of laser diodes, for example by means of cooled laser modules including laser diodes, aging effects require some kind of regulation loop. To compensate the impact of aging it is known to employ a laser module including a monitor diode for monitoring the optical output power of a laser diode. Suitable monitor diodes are optical sensors providing a photo current being representative of the present optical output power of the laser diode. On the basis of the output of the monitor diode, the present optical laser output power is determined to adjust a current used as power supply for the laser module and the laser diode, respectively.
Such a measure for compensating aging effects is also employed for laser modules wherein laser diodes included therein and, in particular, the optical output power of laser diodes is modulated. Depending on the type of laser module, two main principles for modulating the optical laser output power are used in practice. According to one principle, a modulation voltage is applied to a modulator integrated in a laser module while a laser diode thereof itself operates in a continuous wave (CW) mode at a constant so-called laser current, i.e. a current used as power supply for the laser diode and determining the optical laser output power thereof. According to the other principle, a laser diode itself is modulated by means of a modulation current. In the latter case a resulting current actually determining the laser output power can be considered as sum of a laser current usually being a fixed value and a modulation current.
Available laser drivers for a direct modulation of a laser diode by a modulation current are designed for a so-called DC coupling between the laser driver and the laser diode.
FIG. 1
illustrates such a DC coupling of a laser driver and a laser module. The laser module includes a laser diode LD, a unit for temperature compensation, such as a thermoelectric cooler and a thermistor for stabilizing the temperature of the laser diode, and a monitoring unit including a photo diode PD which is optically coupled to the laser diode LD.
As set forth above, temperature compensation can be properly achieved by means of the temperature compensating unit, while compensation of aging effects can be obtained by a determination of the optical laser output power. For that purpose, by means of the photo diode PD in response to light from the laser diode LD being dependent from the optical laser output, power the monitoring unit provides a measure, i.e. a photo current, which characterizes the actual optical laser output power.
For a DC coupled operation of the laser driver shown in
FIG. 1
, the current actually determining the optical output power is a sum of a supply current I
source
for the laser module and a part of a modulation current I
mod
supplied from the laser driver. This relation is illustrated in FIG.
2
.
Thus, a nominal laser current providing a nominal optical output power P
nom
is a sum of the supply current I
source
and a part of the modulation current I
mod
, as shown in FIG.
2
.
In response to aging effects of the laser diode, the curve characterizing the optical output power P will drive horizontally towards higher current values, while its slope will generally remain unchanged. This phenomena is illustrated in
FIG. 2
by the solid curve representing the initial optical output power characteristics and the dashed curve representing a varied optical output power characteristics due to aging impacts.
Thus, it is complicated to obtain stable operating conditions for such a DC coupled arrangement of a laser driver and a laser module since both the supply current I
source
and the modulation current I
mod
have to be modified in order to obtain an age adjusted nominal optical output power P
nom
. Moreover, it is necessary to modify the supply current I
source
and the modulation current I
mod
in such a manner that the extinction ratio for the respective laser module is maintained, i.e. kept constant, which is an absolute requirement in most laser optical transmission systems. Due to this aspects, it is necessary to know the actual aging factor.
For DC coupled arrangement of laser drivers and laser modules, the only possible solution is a so-called pilot tone technique for keeping the modulation of the laser diode constant. In the pilot tone technique, a very low frequency signal is added to the modulation current I
mod
. As a result, the laser current for driving the laser diode comprises the supply current I
source
and a part of the modulation I
mod
, the latter of which being labeled by the low frequency signal. Thus, a photo current provided by the monitoring unit also includes a current portion comprising the low frequency signal or a signal being indicative thereof. By means of filtering measured performed with respect to the current portion including the low frequency signal, it is possible to determine the contributions of the supply current I
source
and the part of the modulation current I
mod
for the laser current driving the laser diode. On the basis of such information, it is possible to maintain the nominal extinction ration and optical output power. However, this approach requires a lot of additional circuitry.
BRIEF SUMMARY OF THE INVENTION
In general, the object of the present invention is to overcome the above problems associated with known arrangements of laser drivers and laser modules. In particular, the object of the present invention is to provide solutions for an improved direct modulation of laser diodes by laser drivers with respect to operation time effects such that nominal optical output power and extinction ratio of a laser diode can be easily maintained.
The basic idea underlying the present invention is to employ an AC coupled arrangement of a laser driver and a laser module as shown in FIG.
3
. Here, the laser module and the laser driver, both being comparable to the respective components shown in
FIG. 1
, are AC coupled by means of a capacitor C. While a supply current I
source
for the laser module is supplied comparable to
FIG. 1
, a modulation current I
mod
is provided by the laser driver to the laser module via the capacitor C.
As shown in
FIG. 4
, as a result of the AC coupled arrangement of
FIG. 3
, the laser current for driving the laser diode necessary to obtain a nominal optical output power P
nom
only is a function of the supply current I
source
, but not depending from the modulation current I
mod
. Thus, changes of the optical output power P, for example due to aging effects, can be easily compensated by a respective adjustment of the supply current I
source
in order to maintain the nominal optical output power P
nom
. For such a compensation, the modulation current I
mod
is not required to be modified. As a result, a nominal extinction ratio of the laser module can be easily maintained by utilizing the respective modulation current I
mod
, i.e. the same modulation current I
mod
independently of a value of the supply current I
source
.
In principal, such an AC coupled arrangement of a laser driver and a laser module will solve the object underlying the present invention.
In practice, any laser driver requires a minimum bias current flowing into its output stage, in particular via the output stage port providing the modulation current I
mod
. This is due to the fact, that
Leung Quyen
Telefonaktiebolaget LM Ericsson (publ)
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