DE4227098A1 - Amplitude modulation circuit for laser drive signal - has differential amplifiers respectively connected to second pole of operating voltage source via voltage controlled current sources - Google Patents

Abstract

First (T1,T2), second (T3,T4) and third (T5,T6) differential amplifiers are connected together so that the respective first outputs are connected to a first pole of an operating voltage source. The second outputs are connected via the laser (LD) to the first pole of the operating voltage. The inputs of the differential amplifiers are connected to a combinational logic circuit (L) controlled by one of a data signal (D) and a pilot signal (P). Each differential amplifier is respectively connected to the second pole of the operating voltage source (VEE) via a voltage controlled current source (IP1,IP2,Imod). The voltage controlled current sources have their control inputs commonly connected to a control circuit output. ADVANTAGE - Universally applicable to customary semiconductor lasers can be operated from 5 V standard voltage source, and can be realised as monolithic integrated circuit.

Description

The invention relates to a circuit arrangement for Amplitude modulation of the control signal of a laser, which in Transmission stages of optical communication systems for digital transmission signals as electro-optical converters is used. When using the laser in high bit rates Transmission systems disrupt the history of the laser dependent on time, which leads to a bit pattern effect. The too sending light pulse is only after a certain Switch-on delay issued. To reduce this delay, the laser is operated with a bias current that is approximately equal is the threshold current of the laser. By temperature - and Aging influences change threshold current and increase in steep part of the laser characteristic. The bias current should however regardless of these influences approximately equal to the threshold current stay. For this it is necessary to change the threshold current recognize and readjust the bias current accordingly. Furthermore should the average optical power regardless of these influences be kept constant in order to keep the system data constant guarantee. To do this, the change in the characteristic slope must be recognized then a modulation current superimposed on the bias current  readjust accordingly.

Due to the separate regulation of Vorstrom and modulation current becomes the emitted optical power independent of temperature and aging. For this purpose is a Method has been specified in which the actual data signal a low-frequency pilot signal is modulated, cf. 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, April 28-30, 1980, Houston, Texas, Pages 926 to 930. By evaluating the from the laser working point dependent gain of the pilot signal and the mean optical power are the manipulated variables Vorstrom and Modulation current determined. It is regulated to the size of the Vorstroms in the kink area of the laser characteristic, that is, the Bias current is regulated approximately to the value of the threshold current.

A circuit arrangement is also known with which Vorstrom and the modulation current of the laser is a pilot current is superimposed, cf. DE 35 08 034. The circuit arrangement exists from a three-stage cascade circuit. The laser as a consumer is connected in series with the transistors of the cascade. The first Stage becomes the second according to the level of the data signal 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 mirror factors of which by negative feedback Emitter resistances can be determined.

To a strong negative feedback and a stable current injection for the differential amplifiers of the first and second cascade stages get, the emitter resistance of the current sources must be very much can be chosen larger than the reciprocal of the transistor steepness. Due to the resulting voltage drop on Emitter resistance are the collector-emitter voltages of the Transistors of the differential amplifier and the current sources  given supply voltage greatly reduced, so that Transistors come into saturation.

The output current of the current sources remains constant as long as the Transistors cannot be overdriven. The through the Differential amplifier realized power switches have dynamic only good behavior regarding settling time and jitter if the transistors do not get into the saturation state. At monolithic integrated circuits is used in transistors Saturation state the influence of the then conductive parasitic substrate transistor can no longer be neglected. The Circuit then no longer works reliably and can completely fail. These conditions result in this Circuit arrangement the problem, the working points of each Adjust transistors so that an optimal eye diagram with minimal time jitter occurs. The number of steps one Cascade connection is thus given supply voltage limited when the transistors are not in the saturation range should arrive. The following condition should be met:

U _F + _{nU BE} + U _RE V _EE

U _F forward voltage of the laser
U _BE base-emitter voltage of a transistor
U _RE Voltage drop across the emitter resistor of a current source
V _EE supply voltage
n Number of stages of the cascade connection.

When dimensioning such a circuit arrangement is too take into account that the forward voltage of a laser depends on the type  can be up to 2.5 V. The negative also has an effect Temperature coefficient of the base-emitter voltage of the transistors unfavorable to the functioning of the circuit arrangement. So takes the base-emitter voltage at -40 ° C, for example monolithically integrated RF transistors for a given Collector current up to 950 mV. Taking into account one typical voltage drop of 400 mV at the emitter resistor Power source can with the specified three-stage cascade connection a reliable function with a 5 V standard voltage source cannot be guaranteed. Rather, within one optical transmitter for such an amplitude modulator with a DC / DC converter a separate supply voltage can be generated.

The invention has for its object, one for commercial Semiconductor laser circuit arrangement for universal use Specify amplitude modulation of the control signal of a laser which can be supplied from a 5 V standard voltage source and should be realizable as a monolithically integrated circuit.

This object is achieved by the main claim described circuit arrangement solved. Details and variants of the embodiments are described in the subclaims.

The essence of the invention is that as a power switch working differential amplifier of the laser driver depending from the logical combination of the data signal with the pilot signal activated at the power switch level and of those in one second level arranged voltage-controlled current sources be fed. Only this one power switch level is the only one Signal level with direct logical combination of data signal and Pilot signal at this level. The logical link of the Data signal with the pilot signal causes in the semiconductor laser Vorstrom, modulation current and the two pilot currents to Laser current are superimposed so that the optical output signal  of the semiconductor laser symmetrical and out of phase is amplitude modulated and the average optical output power of the semiconductor laser is constant.

In the event of a deviation from the operating point of the semiconductor laser set in this way, a control signal is derived from the mean optical output power, which then fluctuates with the frequency of the pilot signal, with which the currents superimposed on the laser current are readjusted so that the original mean optical output power is reached again. With the circuit arrangement according to the invention, synchronism of the modulation current and two pilot currents is made possible with high accuracy, so that an adjustment-free and temperature-independent amplitude modulation of the optical output signal and a constant mean optical output power of the semiconductor laser are always guaranteed over the entire operating temperature range. With a cascade circuit which is only two-stage and very simple in its construction, it is possible that even under worst-case conditions when using a 5V standard voltage source and in a temperature range from T = -40 ° C to 100 ° C, the transistors of the differential amplifiers and the voltage-controlled current sources work outside their saturation range and thus their switching speed is not restricted. Like the data signal, the pilot signal can be transmitted at high frequencies. With the circuit arrangement according to the invention, a better pulse shape of the transmitted signals is achieved compared to circuit arrangements which have become known. The circuit arrangement can be implemented using npn transistors with uniform technology and can be integrated completely monolithically. The comparatively low supply voltage is advantageous here, since it contributes to an increase in the reliability of the integrated circuit due to the reduced power consumption. It is easily possible to systematically expand the circuit arrangement from two input variables to n input variables for a maximum of 2 ⁿ states of the optical output signal.

The invention is illustrated below using an exemplary embodiment explained. The accompanying drawing shows:

Fig. 1 is a block diagram of the inventive circuit arrangement,

Fig. 2 is a timing diagram of a pilot signal, of an exemplary data signal, an amplitude modulated laser current and a representation of the four states of the optical output power,

Fig. 3 is a laser characteristic curve P _opt = f (I _F) with the representation of the envelope of the laser current and the envelope of the optical power,

Fig. 4 shows a circuit arrangement of the invention, wherein the logic circuit is implemented with AND gates,

Fig. 5 shows a circuit arrangement according to the invention, in which the logic circuit is implemented with OR gates and

Fig. 6 is a detailed circuit diagram of the circuit arrangement according to the invention.

Referring to FIG. 1, the circuit arrangement for amplitude modulation of the drive signal of a semiconductor laser LD composed of a first differential amplifier T1, T2, a second differential amplifier T3, T4 and a third differential amplifier T5, T6 with a respectively associated first voltage controlled current source I _P2, a second voltage-controlled current source I _P1 and a third voltage-controlled current source I _mod and a logic circuit L.

The three differential amplifiers are connected in parallel on the output side. The first common connection point of the collectors is then via the semiconductor laser LD at the first pole of the operating voltage source. The second common connection point of the collectors lies directly at the first pole of the operating voltage source; however, it is also possible, if necessary, to connect the collector connections to the first pole of the operating voltage source via a respective resistor. The logic amplifier L controls the differential amplifiers on the input side. The logic circuit L combines the data signal D with the pilot signal P in such a way that the transistors of the differential amplifier operating in large-signal operation as current switches switch the two pilot currents I _P1 , I _P2 and the modulation current I _mod to the semiconductor laser LD in such a way that the optical output power P _{opt is in} phase opposition is amplitude modulated. The voltage-controlled current sources I _P1 , I _P2 , I _mod are controlled together by a control voltage U _s .

Fig. 2 shows the time course of the laser current I _F as a function of the data signal D and the pilot signal P for an exemplary impulse pattern. The four current components I _v , I _mod , I _P1 , I _P2 are superimposed on the laser current I _F in the semiconductor laser LD. The optical output power P _opt assumes four states P0, P1, P00, P11.

This situation is shown in FIG. 3 with the aid of the laser characteristic P _opt = f (I _F ). The optical output power P _opt is symmetrical and amplitude-modulated in opposite phase. Since the two modulation strokes P _P1 , P _{P2 are the} same, the average optical output power has no fluctuations with the frequency of the pilot signal

so that the spectral component of the pilot signal is missing in the frequency spectrum. This state shown in Fig. 3 assumes that all current components are set to a defined value. The ratio of the two pilot currents I _P1 : I _P2 corresponds to a ratio η ₂ : η ₃ , in the example shown η ₂ : η ₃ = 2: 1; where η _{2 is} the slope of the laser characteristic in the modulation range of the upper optical power level P1 and η _{3 is} the average gradient of the laser characteristic in the modulation range of the lower optical power level P0.

The functional relationship of the binary input variables data signal D and pilot signal P and the four states of the laser current I _F and the optical output power P _opt for an exemplary pulse pattern of the data signal D and the pilot signal P is shown in FIG. 2. Depending on the combination of the input variables D and P chosen arbitrarily from four possibilities, the four current components I _v , I _mod , I _P1 and I _P2 overlap to the laser current I _{F in} such a way that the optical output power P _opt with the four states P0, P1 , P00 and P11 results.

Here means:

The three differential amplifiers T1, T2; T3, T4; T5, T6 are now controlled so that the current superimposition for the laser current I _F is produced. From this follows the following truth table with the auxiliary variables Q ₁ , Q ₂ , Q ₃ assigned to the respective differential amplifier:

The following disjunctive normal forms are from the truth table refer to:

Q₁ = · P

Q₂ = D

Q₃ = · + · P

The following applies to Q₃:

Q₃ = (+ P)
=

The corresponding circuit implementation with AND gates is shown in FIG. 4.

Further implementation options result from reshaping the above normal forms with the help of Boolean algebra, for example

Fig. 5 shows the resulting form of implementation with OR gates.

An advantageous circuit implementation of the drive circuit of the semiconductor laser LD is shown in FIG. 6. The collector-emitter path of the first transistor T1 of the first differential amplifier T1, T2 and the collector-emitter path of the first transistor T3 of the second differential amplifier T3, T4 each have a further transistor T11, T33 connected in parallel with its collector-emitter path , so that the OR functions can be realized with this arrangement. The differential amplifiers are each supplied by the operating voltage source VEE via a voltage-controlled current source, which consists of a series connection of a transistor T7, T8, T9 and an emitter resistor R _E7 , R _E8 , R _E9 . The ratios of the three currents I _mod : I _P1 : I _{P2 of} the voltage-controlled current sources are essentially determined by appropriate dimensioning of the respective emitter resistors R _E7 , R _E8 , R _E9 . In order to achieve the required amplitude modulation of the laser current I _F , the differential amplifiers are controlled as follows: The first differential amplifier T11, T1, T2, which is fed by the second pilot current I _P2 , is supplied with the data signal D and via the OR function of the first input driven the negated pilot signal, there is a constant reference signal PR at the second input.

The second differential amplifier T33, T3, T4, which is fed by the first pilot current I _P1 , is driven by the negated data signal and the pilot signal P via the OR function of the first input, and the constant reference signal PR is also present at the second input. The third differential amplifier T5, T6 is driven by the data signal D at its first input, the negated data signal or a constant reference signal DR is connected to its second input.

This circuit arrangement has the advantage that the two pilot currents I _P1 , I _{P2 are provided in a} particularly simple manner for a pilot control without interfering with the third differential amplifier T5, T6, which switches at high speed, for the modulation current I _mod .

The ratio of the pilot currents I _P1 : I _P2 corresponds to the ratio η ₂ : η ₃ , cf. Fig. 3. The degree of modulation of the amplitude modulation m = I _P2 : I _mod is set by the amplitude ratio of the superimposed second pilot current I _P2 and modulation current I _mod . The synchronism of the pilot currents I _P1 , I _P2 is achieved with high accuracy if the ratio of the emitter areas A _E7 : A _{E8 of} the transistors T7, T8 of the first and the second voltage-controlled current sources is selected to be the ratio of the pilot currents I _P2 : I _P1 assigned to them becomes. The temperature responses of the first and second voltage-controlled current sources are then compensated for and the optical output power P _{opt of} the semiconductor laser LD is amplitude-modulated symmetrically and in opposite phase over the entire operating temperature range, without adjustment and temperature-independently.

If the slope η₂ of the laser characteristic changes in the modulation range of the upper optical power level P1, the amplitude modulation of the optical output power P _opt becomes asymmetrical, the average optical output power fluctuates with the frequency f _{P of} the pilot signal, and thus the spectral component of the pilot signal appears in the frequency spectrum. From this, a control voltage U _{s is} derived via a control circuit, not shown here, with which the three voltage-controlled current sources are controlled in such a way that the mean optical output power remains constant regardless of the change in the characteristic slope η ₂ . For this purpose, the second pilot current I _P2 must be changed to the same extent as the modulation current I _mod . The synchronization of these currents is achieved with high accuracy if the ratio of the emitter areas A _E7 : A _{E9 of} the transistors T7, T9 of the first and third voltage-controlled current sources is chosen to be the corresponding current ratio I _P2 : I _mod .

The temperature responses of the current sources compensate and the degree of modulation of the amplitude modulation and optical output power P _{opt of} the semiconductor laser LD are adjustment-free and constant regardless of temperature in the entire operating temperature range.

It is easily possible to use the circuit arrangement even if the amplitude modulation is not to be symmetrical and not in phase opposition. Depending on the control method and setting of the bias current I _v in relation to the threshold current I _{s of} the laser LD, the ratio of the pilot currents I _P1 : I _P2 can be selected differently compared to the example described above. In the limit case, a pilot current can become zero. The amplitude modulation can then be symmetrical and in phase, asymmetrical and out of phase or asymmetrical and in phase. Since the optical modulation bursts P _P1 , P _{P2 of} the optical output power P _{opt are} thus unequal, the average optical output power fluctuates with the frequency of the pilot signal f _P. A component in the frequency spectrum then occurs in the desired state, the amplitude of which the control method can be designed for. In the circuit arrangement described, the number of input variables can be increased from two to any number n, depending on the application. Correspondingly, the optical output power P _opt then assumes not only four states but a maximum of 2 ⁿ states in the case of an expanded number of differential amplifiers with assigned current sources. Depending on the number n of input variables, a maximum of 2 ⁿ combinations can be specified, each of which can be linked by a differently constructed logic circuit. According to the circuit arrangement shown in FIG. 6, further transistors are connected in parallel to the collector-emitter paths of the differential amplifiers, if necessary, to implement the OR functions collector-emitter paths.

Claims (5)

1. Circuit arrangement for amplitude modulation of the control signal of a laser (LD), which consists of a cascade connection of differential amplifiers and voltage-controlled current sources, with which the modulation current (I _mod ) of the laser (LD) is amplitude-modulated with two pilot currents (I _P1 , I _P2 ), characterized in that a first (T1, T2), a second (T3, T4) and a third differential amplifier (T5, T6) are interconnected so that the respective first outputs with the first pole of an operating voltage source and the respective second outputs via the Lasers (LD) are connected to the first pole of the operating voltage source and the inputs of the differential amplifiers (T1, T2; T3, T4; T5, T6) to a combinatorial logic circuit (L) controlled by a data signal (D) and a pilot signal (P). are connected and that each differential amplifier (T1, T2; T3, T4; T5, T6) with the second via a voltage-controlled current source (I _P1 , I _P2 , I _mod ) n pole of the operating voltage source (V _EE ) is connected and the voltage-controlled current sources (I _P1 , I _P2 , I _mod ) are connected together with their control input to the output of a control circuit. 2. Circuit arrangement according to Claim 1, characterized in that the first differential amplifier (T1, T2) with the first voltage-controlled current source (I _P2 ) supplying the second pilot current, the second differential amplifier (T3, T4) with the second voltage-controlled current source supplying the first pilot current (I _P1 ) and the third differential amplifier (T5, T6) is connected to the third voltage-controlled current source (I _mod ) supplying the modulation current. 3. Circuit arrangement according to claims 1 and 2, characterized in that the differential amplifier (T1, T2; T3, T4; T5, T6) each of a first (T1, T3, T5) and a second transistor (T2, T4, T6 ) exist that to the collector-emitter path of the first transistor (T1) of the first differential amplifier (T1, T2) and to the collector-emitter path of the first transistor (T3) of the second differential amplifier (T3, T4) the collector-emitter Route of a respective third transistor (T11, T33) is connected in parallel and that the base connections of the transistors of the differential amplifier are connected as follows: The base of the third transistor (T11) of the first differential amplifier and the base of the first transistor (T5) of the third differential amplifier the data signal (D), the base of the first transistor (T1) of the first differential amplifier with the negated pilot signal (), the base of the second transistor (T2) of the first differential amplifier and the base de s second transistor (T4) of the second differential amplifier with a constant reference signal (PR), the base of the third transistor (T33) of the second differential amplifier with the negated data signal (), the base of the first transistor (T3) of the second differential amplifier with the pilot signal ( P) and the base of the second transistor (T6) of the third differential amplifier with the negated data signal () or a constant reference signal (DR) and that the voltage-controlled current sources (I _P1 , I _P2 , I _mod ) each consist of a series connection of a transistor ( T7, T8, T9) and an emitter resistor (R _E7 , R _E8 , R _E9 ) exist and the base connections of these transistors are common to a control voltage (U _s ). 4. Circuit arrangement according to claim 1, characterized in that the number of input variables of the combinatorial Logic circuit (L) and the number of differential amplifiers and these associated current sources can be enlarged. 5. Circuit arrangement according to claims 1, 2 and 4, characterized characterized in that depending on the number of Input variables and depending on their logical Link to the collector-emitter paths of the transistors Differential amplifier each collector-emitter paths further Transistors are connected in parallel to OR the Realize input variables.