MXPA98000036A - La actuator circuit - Google Patents

Abstract

A laser diode (1) is driven by a first integrated amplifier (5) which receives a reference voltage of direct current V ref, input data Vin, and negative feedback from a back-face photodiode (2) and by a second amplifier broadband that receives data and negative feedback

Description

CI RCUITO LASER ACTUATOR DESCRIPTION OF THE INVENTION The present invention relates to laser driver circuits, and more particularly to drive circuits to which a modulation signal can be applied. Before considering such circuits in detail, it would be useful to examine the laser behavior. Figure 1 shows a graph of light output energy versus input current for a typical semiconductor laser diode. The curve marked A illustrates the typical characteristics at room temperature. It is characterized by an inclination S and an ignition current It. This characteristic is however dependent on the temperature, the typical graph for higher and lower temperatures shows the curves B and C respectively. It is evident from an examination of these graphs that in order to drive the laser with a modulation signal it is necessary to provide some constant current to bring the laser into a region of operation and to vary this current in accordance with the modulation signal. Reference D indicates a typical scale of driving current for the operation of curve A in which it can be seen that the light output varies between substantially zero and some desired maximum. It can also be observed that applying on the same current scale at a higher temperature (Graph B) results in a lower maximum energy output and also results in the laser being driven considerably below the cut. This is particularly unsatisfactory since, once it is driven below the cut, a rising current is returned to the device within the operating region, and introduces a delay that can degrade the performance when trying to modulate with high-scale digital signals of bit. On the other hand, applying the same scale of currents to the case of temperature (Graph C), a much higher light output is obtained, although with a considerably minimal light output (this is referred to as a low extinction ratio); this however causes problems in the demodulation. One solution to this problem is to use a Peltier cooler that controls the circuit appropriately to keep the temperature of the device reasonably constant. This however, results in an increase in cost. The variations in the ignition threshold can be accommodated by using the known average power controller, a schematic diagram of which is shown in Figure 2. Here a laser diode 1 is driven with a current I and produces a light output L = S.1 Watts. The light output is detected by a subsequent monitor photodiode 2 which drives a current KL amps inside a resistor load 3 of resistance R. The average light output is determined by a voltage reference source 4 which produces a voltage Vref and the voltage developed through the load resistor 3 is compared to an integrated transconductance amplifier 5 with the voltage developed through the load resistor 3 to control the current and load to the diode. A modulation current is fed to the laser diode 1 from an external power source connected to an input 6. If the amplifier 5 has a product G of transductance width, then the laser output ignoring any modulation input is: It is seen that the light output for (0 = 0 is independent of S, and therefore the set average energy is kept constant.In the case of a modulation current I ata is applied to the input 6, the light output is given by: It is observed that here at high frequencies the gain is dependent on S, and therefore the situation shown in Figure 3 is obtained, where the operating regions for the same of current drive are shown. At high temperatures, a low extinction ratio is obtained, considering that at low temperatures, the laser can be deflected below the cut, or even inverted, with the penalty of connection delay, it can also be observed that the gain of the data low frequency is low, falling to zero in direct current, the feedback control of amplifier 5, effectively removing from the direct current component from the data. Therefore, this type of drive is suitable only for data that have a symmetric waveform; Specifically, it is extremely unsuitable for drive data drive as it can be used in TDMA systems such as passive optical networks. In accordance with the present invention, there is provided a laser actuator comprising data input for receiving data signals; means for providing a feedback signal representative of the laser light output; a first amplifier having a gain in direct current and lower frequencies connected to receive the data signals, a direct current reference signal, and the feedback signal to supply current to the laser; and a second amplifier having gain at higher frequencies connected to receive the data signals from the feedback signal to provide current to the laser. Preferably the first amplifier is an integrated amplifier having, below a threshold frequency, a gain greater than the second amplifier and above the threshold frequency, a gain lower than the second amplifier. If it is desired that the actuator may have means of gain adjustments by means of which the gain provided to the actuator for data signals may become equal to the direct current and at a higher frequency the pass band of the first amplifier.

Some embodiments of the invention will now be described by way of example with reference to Figures 4 and 5 of the accompanying drawings. Figure 4 is a schematic circuit diagram of an embodiment of the laser driver circuit according to the invention. A laser diode 1 is shown again, with a back face monitor photodiode 2 and its charge resistor 3, a voltage reference source 4 and the transconductance amplifier 5. In this case, however, the amplifier 5 receives (in its inverting input) not only the voltage Vret from the reference source 4 but also the data Vn from a data input 10. Its input without inversion is connected to receive the feedback voltage in the resistor 3. In addition, a second broadband amplifier 11, is also provided, with an operational amplifier 12 which is connected to receive the same feedback voltage at its input without inversion and the data Vn at its reversing input by means of an input resistor 1 3. The voltage gain of this amplifier is determined by the input resistor and a feedback resistor 14. The amplifier drives a current inside the laser diode 1 by means of the resistor. load stor 15.

In operation, the amplifier 5 operates the diode 1 with a current (in the direction shown in the arrow in Figure 4) of l- | . With the annotation previously used this current is given by: On the other hand, to obtain the voltage output * 2 from the broadband amplifier, the currents in the inversion input are summed, where R and RfD are the values of the input and feedback resistors 13, 14. vm - (KSIR - vs) V: -. { KSIR ~ VS) = 0 R. R * for so much and, if R | _ is the load resistor value 1 5.

So the total laser diode current is - * _ R, n + ¿. { r »+ v« + v,) Simplifying The variable time component of this is in terms of the alternating current component Vn ac of the input signal which is And the corresponding light output is It is observed that if Rj_ / KSR is much smaller than the unit, then that expression is independent of the inclination of the laser S; in addition if R¡nO + R | _'KSR) / RfD is much smaller than the unit the light output is only dependent on fixed parameters of the photodiode load R, and the optical feedback transfer function K. In fact, A compromise is necessary here, as excessively large R values can cause a tendency towards instability. However, a considerable reduction in the inclination sensitivity can be obtained. The direct current component of current I in terms of the direct current component Vjna < c is given by dc Vní + Vs' * = KSR (10) and the light output «Dt • __ KR (11) which again is independent of the inclination and also maintains the direct current component of the input.

In operation, the laser deviation point is set by adjusting Vref. Since the circuit is coupled to direct current, the usual deflection setting would be just in the threshold. This fixation would correspond to V¡n = or volts. Any change in the laser threshold is followed by the transconductance amplifier 5. The low frequency modulation, virtually all the laser drive is provided by this amplifier; as the frequency increases the contribution from the broadband amplifier 1 1 becomes more important and the proportion of the drive current carried by it increases uniformly according to the ratio of its gain (which is substantially constant) to that of the amplifier of transconductance, (which is inversely proportional to the frequency) increases. Therefore, the low speed device provides the constant laser threshold deviation, while the broadband amplifier contributes just to the high frequency modulation component. In the embodiment of figure 4 it is not a convenient form for implementation, since the reference voltage is in series with the data signal. In addition, the photodiode is derived by the inputs of both amplifiers, which can cause I I degradation. performance if the transconductance amplifier 5 has, as is typical for lower broadband direct current amplifiers, a low impedance at radio frequencies. A more practical version is shown in figure 5. In figure 5, the identical components according to those of figure 4 are assigned with the same reference numbers. The reference voltage is generated by a potentiometer 16 and is added to the input voltage Vn by an amplifier 1 7 with input resistor 18, 19 for reference voltages and V, respectively, and the feedback resistor. variable 20 to adjust to gain and therefore the direct current balance. Due to the inversion of this amplifier, the reference and input voltages are now applied to the same polarity input of the amplifier 5, since it is in the amplification signal from the photodiode 2, although by means of separate input resistors 21, 22. The amplifier 5 has an operational amplifier 23, with a feedback resistor 24, which is powered by means of a resistor 25, the value R tnt > u a second of such amplifier 26 having a feedback capacitor 27 of value C tn of value to form an integrator. It operates by means of a resistor 28 a transmitter follower transistor pnp 29 which supplies the current by means of a resistor R0ut a 'laser diode 1. The construction of a broadband amplifier does not change; however, note that the photodiode 2 now operated in photovoltaic mode, and operates the broadband amplifier 1 1 directly and the first stage 23 of the integration amplifier 5 by means of the resistor 22, avoiding excessive load of the diode by the input of this stage. A capacitor is also connected from the inverting input of the amplifier 23, to avoid excessive currents R. F. at the amplifier input. The broadband amplifier 12 may be a high performance operational amplifier such as HFA1 100 manufactured by Harris Corporation or CLC401 of Comlinear Corporation. With the HFA1 100 amplifier good results were obtained in data scale of up to 300 Mbit / s. The others (1 7.23 and 26) require good direct current stability although their frequency response is not critical. The CA3140 BiMOS op-amp is suitable. The cutoff frequency of the amplifier 5 is f = GKRS / 2Ó, where G = 1 / (C¡ntR¡ntRL) - Noting that the amplifier 1 7 and the associated components are outside the feedback loop, it is necessary to adjust the gain by direct current balance control 20 so that the transfer function for the data is equivalent in both high and low frequencies. A possible adjustment procedure is as follows: (a) with a zero volt data input, set the direct current balance to a nominal start value with the resistor 20 and set the threshold with the potentiometer 1 6 to zero volts; (b) observing the laser light output using a direct current coupled monitor, adjusting the threshold control 16 so that the laser operating point is within the laser illumination region; (c) apply asymmetric data from 0 to 0.5 volts in 300 Mbit / s to the data input (for example, an individual mark followed by 100 spaces), followed by the inverse sequence and the direct current balance control setting 20 to that there is no displacement of the baseline on the monitor to the repeated switching between the two signals; (d) readjusting the threshold control 16 until the 0 volt data input signal corresponds to the laser deviation just at the threshold.

Typical component values for circuit a 5 are as follows: Reference Value Symbol Value 13 Rin 100 O 14 Rfb 510 O 15 RL 75 O 16 50 kO 18 100 kO 19 10 kO 20 50 kO (nominal) 21 2 kO 22 51 O 24 2 kO 25 Rínt 10 kO 27 Cínt 1 μF 28 1 kO 30 Rnut 75 O

Claims (3)

REVIVAL NAME IS

1 . A laser driver characterized in that it comprises a data input for receiving data signals; means for providing a feedback signal representative of the laser light output; a first amplifier having a direct current gain and low frequencies connected to receive the data signals, a direct current reference signal and the feedback signal to supply current to the laser; and a second amplifier having a high frequency gain connected to receive the data signals and the feedback signal to provide current to the laser.

2. The laser actuator according to claim 1, characterized in that the first amplifier is an integrated amplifier having, below a threshold frequency, a gain greater than the second amplifier and, above the threshold frequency, a lower gain than the second amplifier.

3. The laser actuator according to claim 1 or 2, characterized in that it has gain adjustment means, whereby the gain provided by the actuator for data signals can become equal to the direct current and to a frequency above the band of step of the first amplifier.