There are various kinds of optical transmitters. One method of producing an optical transmitter is to directly modulate a laser diode using a laser driver. Laser drivers provide a means to switch currents in and out of laser diodes. These currents are typically driven at high data rates and are set and controlled by the laser driver for optimum performance of laser operation.
As will be appreciated by those skilled in the art, the characteristics of a laser diode change over time and with temperature. Well designed laser drivers attempt to compensate for these changes through adjustments in diode currents. To avoid laser turn-on and turn-off delays in light emission for high speed operation, a bias current, which is typically higher than the threshold current of the laser diode, is generally supplied to the laser diode. As is known in this field, when the bias current significantly exceeds the threshold value, the extinction ratio of the optical emission rapidly deteriorates, resulting in a receiver sensitivity penalty for signal transmission.
The extinction ratio of an optical signal is a ratio of the amount of optical power that is transmitted during a xe2x80x9c1/onxe2x80x9d period to the amount of optical power that is transmitted during a xe2x80x9c0/offxe2x80x9d period. In other words, the extinction ratio is the ratio of the optical-energy level of a binary xe2x80x9c1xe2x80x9d to that of a binary xe2x80x9c0xe2x80x9d level of a modulated signal. If the laser characteristics change over time and temperature with no compensation provided by the laser driver, the extinction ratio will decrease and the optical system performance will degrade, showing increased error rates. In most circumstances, a desirable extinction ratio is generally about 8.2 dB or better.
A standard laser driver has an Imod current, which is switched in and out of the laser diode providing a means to transmit a xe2x80x9c1/onxe2x80x9d or xe2x80x9c0/offxe2x80x9d signal. The optical output of the laser is therefore P1 when on, and P0 when off. The current which supplies power to the laser driver is called the modulation current Imod, and bias current Ibias.
The Ibias value sets the base line current for the laser when it is off, which is therefore the P0. The Imod current is turned on through a data input, electrically coupled to a first and second transistors and provides enough current to turn the laser on, the P1 setting. An extinction ratio is simply the ratio of P1 over P0.
PRIOR ART FIG. 1 depicts a typical laser driver 10. The laser driver 10 includes a photocurrent source, Iphoto 4, and a current source 5. A laser 1 emits light 2 to a photo current monitor 3. The photo current, Iphoto 4, is set by a user. The Imod current source 5 is controlled by the input of data through an input switch 6, which is sent through transistors 7 in a well known fashion. A block 9 is used to set an Ibias current 8. The total modulation current is simply the Imod current.
FIG. 2 graphically shows how either time or temperature decreases the extinction ratio of a laser diode of the prior art. The characteristics of laser diode optical output (vertical axis) versus laser current (horizontal axis) for an optimally functioning laser 20 has better performance and a steeper slope than a laser which is below optimal level 21 due to problems such as hysteresis or changes in the operating temperature. The optical output power at 1 versus 0 is represented by P1 and P0 for the optimal laser and P1xe2x80x2 and P0xe2x80x2 for the less functional laser. Both states of the laser have the same average optical output due to the feedback caused by the Ibias circuitry (see elements 9 and 10 of FIG. 1). The actual levels of power at 1, P1 and P1xe2x80x2 respectively, differ because of this feedback loop. So as a result the extinction ratio, which is power at 1 over power at 0 (P1/P0), gets smaller as the laser loses efficiency.
Typically, laser drivers do not compensate for the extinction ratio. Laser drivers that do are complex and require expensive peak detectors.
Some attempts to compensate for the extinction ratio can be found, for example, in U.S. Pat. No. 6,014,235. In that patent there is taught an optical-loop buffer that mitigates the reduction in the buffered signal""s extinction ratio and substantially restores (e.g., maintains or increases) the buffered signal""s extinction ratio to its original value, thereby extending the number of circulations, and hence the storage time, for which the buffered signal can remain in the buffer without the extinction ratio falling below an acceptable minimum. The optical buffer restores the extinction ratio of the buffered signal and thereby extends the length of time for which the signal can be stored. However this approach uses a complex assembly of optical components and as a result, can not be easily implemented into an existing circuit due its size and complexity.
Accordingly, what is needed is a simpler way to compensate for the deterioration of the extinction ratio and not add to circuit complexity.
It is a purpose of the present invention to provide a simplified scheme to control the extinction ratio of laser transmitters.
A standard laser driver controls laser operation through the use of an Imod current, which is switched in and out of the laser diode providing a means to transmit a xe2x80x9c1/onxe2x80x9d or xe2x80x9c0/offxe2x80x9d. The Imod current can be controlled and set externally. An Ibias current is then used to control the average current in the laser and compensates for increases in a laser""s threshold current by having its current increased, while the Imod current itself does not change. However, when this is done, the extinction ratio is reduced since there is no compensation for decreases in laser""s slope efficiency. The present invention compensates for decreases in the extinction ratio by feeding forward a portion of the Ibias current to the Imod current source. The percentage of the portion being fed forward can be controlled externally by a user to compensate for changes in the laser. Therefore the Imod total becomes a function of the set Imod current and some portion of the Ibias current. This controls the extinction ratio by making the optical output power in the xe2x80x9c1/onxe2x80x9d state, when the laser is at a higher temperature, almost the same as that as when it is functioning normally. The extinction ratio can therefore be stabilized by using embodiments of this invention.
Preferably the portion of the Ibias current being fed forward to the Imod current is between 30-200%.
One advantage of the present invention is that it prevents deterioration of the extinction ratio. Another advantage is that the maintenance of the extinction ratio is accomplished with a minimal modification of an existing circuit and therefore is economical.
These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following descriptions and a study of the various FIGS. of the drawing.
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