Coherent light generators – Particular beam control device – Optical output stabilization
Patent
1997-03-27
1999-03-30
Davie, James W.
Coherent light generators
Particular beam control device
Optical output stabilization
372 29, 372 38, H01S 310
Patent
active
058898026
DESCRIPTION:
BRIEF SUMMARY
This invention relates to apparatus having an output which exhibits a threshold characteristic as a function of an input signal and in particular to methods of, and apparatus for, controlling the pump signal applied to such apparatus to maintain the pump signal near the threshold value.
Particular examples of such apparatus are semiconductor and optical fibre lasers in which cases the pump signal is the DC bias current and optical pump signal, respectively.
A semiconductor laser, for example, has a light output(L)/current (I) transfer characteristic having a threshold level region as shown in FIG. 1, the threshold "knee" 2 of which is shown enlarged in FIG. 2. In a digital data transmission system, for example, the laser is operated by superimposing a data pulse current on a D bias current, which combined current is used as the drive current for the laser as shown in FIG. 1.
The value of the DC bias current is ideally located near the knee of the transfer characteristic to reduce the extinction ratio penalty and switch-on delay. Whilst careful manual adjustment of the DC bias current may be acceptable in a laboratory it is necessary to provide automatic control of the bias current level for OEM and turnkey equipment.
One well known approach to providing such automatic control, which has the attraction of being simple to implement, is to control the D bias current so the mean-power output of the laser is maintained at a constant value. The mean power level is set so that the zero data level is at the knee region of the threshold region. However, this system can only maintain correct operation close to the temperature at which the control system was set up. As the temperature varies up and down so the threshold region moves to higher and lower values of drive current. The threshold also moves as the device ages.
This lateral shift of the transfer characteristic can be automatically tracked by the mean power controller. However, as the temperature variation from that at set-up gets bigger, so does the variation in laser slope efficiency of the laser. When the slope efficiency increases, as the temperature decreases, the D bias current will be reduced relative to the position of the threshold knee by the bias controller to maintain the same mean power output from the laser. This results in a turn-on delay being introduced into the system. When, on the other hand, the slope efficiency decreases as the temperature increases, the bias signal will be increased relative to the threshold knee by the bias controller to maintain the same mean power output from the laser. This results in increase in an extinction ratio penalty of the system.
In an effort to overcome the problems of the mean-power control method, control methods have been devised which are based on detection of the threshold of the transfer characteristic with the D bias current controlled to be close to this point. A second, independent, control circuit ensures the data signal amplitude is such that the mean power output remains constant despite any changes of laser slope efficiency with temperature.
Various approaches have been taken to effect detection of the threshold signal. In one method, a measurement is made of the switch-on delay of the laser the value of which is used to set the D bias current appropriately. This technique requires the use of a high-speed feedback photodiode and associated high frequency amplifier circuitry in the bias control circuit.
In a further method described in an article titled "An Automatic Bias Control (ABC) Circuit for Injection Lasers" by A Albonese, The Bell System Technical Journal Vol. 57, No. 5, May-June 1978, pages 1533 to 1544, a totally electronic method of stabilizing the laser is disclosed in which the threshold is detected by means of a feedback signal derived from the laser voltage. The laser junction voltage is monitored and an error signal proportional to the degree the laser junction is not saturated is derived and the laser D bias current is adjusted until the junction voltage is saturated. This meth
REFERENCES:
patent: 4062632 (1977-12-01), Dixon
patent: 4101847 (1978-07-01), Albanese
patent: 4344173 (1982-08-01), Straus
patent: 5020062 (1991-05-01), Cusak, Jr. et al.
Davie James W.
Hewlett--Packard Company
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