In employing the laser in high bit rate transmission systems, the turn-on time, which is dependent on the prior firing history of the laser, interferes, leading to a bit pattern effect. The light pulse to be transmitted is released only after a certain switching delay. In order to lessen this delay, the laser is operated with a bias current which is approximately equal to the threshold current of the laser. The threshold current and the rise of the portion of the characteristic curve of the laser vary due to temperature and the influence of aging. But the bias current is to remain approximately equal to the threshold current, independent of these influences. In order for this to take place, it is necessary to recognize the alteration of the threshold current and correspondingly readjust the bias current. In addition, the average optical power must be held constant, independent of these influences, in order to insure constant system data. In order for this to take place, the alteration of the slope of the characteristic curve must be recognized in order to then correspondingly readjust a modulating current combined with the bias current. The emitted optical power becomes independent of temperature and aging through a separate adjustment of the bias current and modulating current. A procedure has been indicated for this purpose for which a low frequency pilot signal is modulated on the actual data signal, compare Smith, D. W. and Hodgkinson, T. G.: "Laser level control for high bit rate optical fiber systems", Proc. of IEEE, 13th International Symposium on Circuits and Systems, 28th to 30th April 1980, Houston, Tex., pages 926 through 930. The manipulated variables of bias current and modulating current are determined by evaluation of the amplification of the pilot signal, dependent on operating point of the laser, as well as evaluation of the average optical power. The amplitude of the bias current is adjusted at the inflection region of the characteristic curve of the laser, e.g., the bias current is adjusted to approximately the value of the threshold current.
A circuit arrangement is also known, in which a pilot current is combined with the bias current and the modulating current of the laser, compare DE 3,508,034. The circuit arrangement consists of a three stage cascade. The laser as load is connected in series to the transistors of the cascade. The first stage is switched corresponding to the level of the data signal; the second stage is switched according to the level of the pilot signal. The third stage consists of current-controlled current sources, so-called current mirrors, the image factors of which are determined by negative feedback with emitter resistors.
In order to maintain a powerful reverse feedback and a stable current injection for the differential amplifiers of the first and second cascade stages, the emitter resistance of the current sources must be selected to be much greater than the reciprocal value of the transistor steepness. The resulting voltage drop at the emitter resistor sharply reduces the collector-emitter voltages of the transistors in the differential amplifiers and the current sources for a given supply voltage, so that the transistors reach the state of saturation.
The output current of the current sources remains constant as long as the transistors are not overloaded. The current switches realized by means of the differential amplifiers have a good dynamic behavior with reference to response time and jitter only if the transistors do not reach the state of saturation. For solid-state integrated circuits with transistors in the state of saturation, the influence of the parasitic substrate transistor then becoming conductive can no longer be neglected. The circuit no longer operates dependably and can break down entirely. Following from these conditions for this circuit arrangement is the problem of regulating the operating points of the individual transistors such that an optimum optical diagram with minimal time jitter results. The number of stages in a cascade is consequently limited for a given supply voltage if the transistors are not to reach the saturation region. The following conditions should be fulfilled: EQU U.sub.F +n.multidot.U.sub.BE +U.sub.RE .times.V.sub.EE,
where
U.sub.F =forward voltage of the laser, PA1 U.sub.BE =base-emitter-voltage of a transistor, PA1 U.sub.RE =voltage drop at the emitter resistor of a current source, PA1 V.sub.EE =supply voltage, and PA1 n=number of stages of the cascade.
In dimensioning this type of circuit arrangement it is to be considered that the forward voltage of a laser can amount to, according to type, up to 2.5 V. Likewise the negative temperature coefficient of the base-emitter-voltage of the transistors has an unfavorable effect on the function of the circuit arrangement. Thus, for example, the base-emitter voltage of monolithic integrated HF transistors increases at -40.degree. C. up to 950 mV for a given collector current. Taking into consideration a typical voltage drop of 400 mV at the emitter resistor of a current source, a dependable function with a standard 5 V source cannot be insured for the three stage cascade indicated. Rather, a separate supply voltage must be generated within an optical transmitter with a DC/DC converter for this type of amplitude modulator.
USPTO Backgrounds
USPTO Backgrounds