Circuit in CMOS technology for high speed driving of optical sources

Abstract: The circuit in CMOS technology allows high speed driving of optical sources, in particular laser diodes, operating efficiently at speeds that meet the requirements imposed by optical fibre communication systems and avoiding interference between driving currents and biasing current in the optical source. The circuit comprises a bias current generator, a modulation current generator and a cascade of CMOS inverter stages that supplies a driving voltage to the modulation current generator and receives digital signals at its input. It is possible to independently adjust the bias current, so as to allow driving sources with different threshold currents, and to adjust the modulation current.

Description: The invention described herein relates to transmission devices for communication systems using optical fibres and in particular it concerns a circuit in CMOS technology for high speed driving of optical sources. The use of optical fibres in communication systems nowadays is quite widespread and it allows increasing the speed of transmissions and reducing the size of the means used to carry the signals. However, at the current state of the art, a large portion of the processing performed on the information transmitted is carried out on electrical signals. There is, therefore, the need to create interfacing devices which allow coupling on one side the information sources and on the other the associated receivers with the optical fibre segments utilised for the connection. In particular, the sources are provided with transducers that are capable of converting the electrical signals into optical radiations to send into the fibre, while the receivers are provided with transducers that are capable of converting the optical radiation received into electrical signals. In the former case, opto-electronic sources are used, such as laser diodes or LEDs, whereas in the latter case photo-detectors are typically used. The use of laser diodes is particularly widespread since the radiation emitted by these devices has advantageous characteristics for optical fibre transmission: in particular, the optical radiation emitted is coherent and typically monochromatic, with the additional advantage that the small wavelength of such a radiation allows the use of optical fibre cables with a smaller cross section compared to that of the cables carrying the radiation emitted by other sources, such as LEDs. Once the optical source to be used has been chosen, the problem is how to drive it in such a way that, taking into account the source characteristics, appropriate optical signals to send through the fibre will correspond to the electrical signals emitted by the generator. In laser diodes, the emission of optical radiation occurs only when the current flowing through the directly biased device exceeds a minimum value, called threshold current and hereinafter indicated as I s . Such threshold current in general depends on various factors: first of all, on the level of technology and accuracy with which the laser diodes are manufactured and, among the devices manufactured with the same process, on the inevitable spread of the actual characteristics with respect to the nominal ones. In any case, it is possible to set the variation of the threshold current of the most commonly used laser diodes roughly between 5 mA and 30 mA. Another characteristic of laser diodes is the fact that the power of the optical radiation emitted is directly proportional to the intensity of current flowing in the device in excess of the threshold current; such excess current is called modulation current and hereinafter it is indicated with I m . Typical values for I m vary roughly between 0 mA and 30 mA. When information is to be sent in digital form, it is common practice to make a laser driving current only slightly higher than I s , correspond to one logic level. This way, the device is always on and therefore, when switching from a logic level to the other one, there is no delay due to the need to turn the device on. A current given by the sum of I s and a certain value of I m is made to correspond to the other logic level; this value of I m is proportional to the desired difference between the optical power associated to logic "1" and that associated to logic "0". Such difference is essentially chosen through a trade-off between the need to increase the margin of noise immunity (which corresponds to high levels of I m ) and the need for fast switching (which is accomplished by reducing I m ). Incidentally, by keeping the laser diode always above the threshold, the absence of input signal corresponds to one of the two logic levels and no indefinite conditions can occur, which could cause a strong dissipation. The laser diode is therefore a current-controlled device; typically, however, generators supply the signals as voltages at logic levels; thus, circuits are necessary which allow imposing current I s to the laser diode and converting the voltages received from the information generators into suitable modulation currents I m to be superimposed on said I s . Moreover, the driving circuits must allow the conversion of signals having the highest possible frequency, typically of the order of hundreds of MHz, reducing dissipated powers to the minimum possible level. Several driving circuits are known in the art: they are made using both GaAs technology (where the basic component is the MESFET) and silicon technology (where the basic component is the BJT = Bipolar Junction Transistor). In the former case, fast circuits that can be integrated on the same chip with the laser diode are obtained, but the technology is very expensive and does not allow the implementation of high component densities. In the latter case, the bit rate and integration density can be high, but there is a high power dissipation. As an alternative to the two approaches mentioned above, another technology can be used; it uses silicon and its basic component is the CMOS. Circuits in CMOS technology assure a high integration density, low power dissipation and low costs, but on the other hand their operating frequency is not very high and the power they dissipate is proportional to the square of the frequency. As an example of application of CMOS technology to driving circuits for optical sources, one can mention what is described by M. Steyaert et al. in the document entitled "150 Mbit/s CMOS LED-driver and PIN-receiver IC for Optical Communication", presented at the IEEE 1992 Custom Integrated Circuits Conference. This publication presents a circuit, integrated in a single chip, for driving an LED at the frequency of 150 Mbit/s. The circuit has at its input side a cascade of CMOS inverters whose function is to couple the CMOS or TTL circuits upstream with the LED driving stage. This driving stage consists essentially of a current mirror circuit, which makes a bias current flow through the LED, the value of the current being imposed once and for all by dimensioning an external resistor, and of a transistor that controls the modulation current, arranged in parallel to the current mirror. This circuit has some drawbacks: in particular, it works poorly at high frequencies (>200 MHz), since in these conditions there is an accentuation in the phenomenon of the production of disturbances that originate on the switching fronts and propagate from the gate input of the transistor controlling the modulation current, toward the branch of the current mirror circuit that sets the threshold current. This brings about a reduction in the signal-to-noise ratio of the output of the laser diode, since the current peaks caused by the aforesaid disturbances reduce the dynamic range of the optical signal. Moreover, it is impossible to adjust the modulation current, whilst the nominal bias current remains rigidly fixed by means of the external resistor. The aforesaid drawbacks are obviated by the circuit provided by the present invention, which allows driving optical sources, in particular laser diodes. This circuit operates efficiently at speeds that are well suited for the requirements imposed by optical fibre communication systems, and prevents interference between the voltage and current driving signals in the optical source. It is possible to adjust the bias current, so as to allow driving sources with different threshold currents, as well as to adjust the modulation current. Thanks to the use of CMOS technology, the circuit has low power dissipation and is cost-efficient. The circuit in CMOS technology for high speed driving of optical sources according to the present invention comprises: a bias current generator; a modulation current generator; a cascade of CMOS inverter stages which supplies a driving voltage to the modulation current generator and receives digital signals at its input; said bias current generator being a MOS transistor, whose source is connected to one of the power supply terminals, whose gate is connected to a terminal for controlling the bias current and whose drain is connected to one of the terminals of the optical source to be driven, characterised in that the modulation current generator is a pair of MOS transistors, arranged in series, one of which has its drain connected to said terminal of the optical source, its gate connected to the output of said cascade of inverter stages and its source connected to the drain of the second transistor of the pair, whose own source is connected to said power supply terminal and whose gate is connected to a terminal for modulation current control, the value of said bias current and the maximum value of said modulation current depending only on the voltages imposed externally on the respective control terminals. The characteristics of the present invention will be rendered more evident by the description that follows and by the enclosed drawings, related to a preferred embodiment thereof, where: Fig. 1 and 2 represent two dual examples of integrated embodiment of the driving circuit according to the present invention; Fig. 3 represents the eye diagram related to the schematic diagram represented in Fig. 1. In Fig. 1, a data source, not shown, supplies a driving voltage to input I of the first stage of a cascade of d.c. coupled CMOS inverters. Each stage of this cascade is composed of two CMOS transistors, one p-type and one n-type, with gates and drains connected together and with sources connected respectively to the positive power supply terminal Vdd and to ground. In static conditions, one transistor is saturated while the other one is cut off, so no current flows between Vdd and ground and no power dissipation occurs. When there is a transition from one logic level to the other, both transistors can conduct but the phenomenon is very rapid and therefore dissipated power is modest. The output of the last stage of this cascade is connected to the gate of a MOS transistor MS, of the p-type. The drain of this transistor is connected to a terminal L, which can be connected to the anode of an optical source to be driven, for example a laser diode with grounded cathode; the source of MS, instead, is connected to the drain of a transistor MM, whose gate is in turn connected to an external terminal M which receives a voltage controlling the modulation current, and whose source is connected to power supply terminal Vdd. Between Vdd and L there is also a p-type transistor MP, whose gate is connected to the outside terminal P which receives the voltage controlling the bias current. The dimensions of the transistors used in the various stages of the cascade of inverters increase as the final stage is approached. The parasitic capacitances at the input of these inverter stages and the output currents that they can supply or absorb are directly proportional to the surface areas of the transistors utilised. Proceeding from the first to the last stage, therefore, the input capacity of the inverter stage increases, as well as the available output current. In general, the charging and discharging time of the input capacitances of a driven stage, inversely proportional to the switching speed of the input voltage, decreases as the output current respectively supplied or absorbed by the driving stage increases. Therefore the cascade of inverters having the transistor-dimensioning characteristics described above allows driving with sufficient speed the input capacitance of transistor MS, whose area is relatively large precisely to allow it to provide fast enough current variations on the load. The problem of driving in an analogous way the gates of transistors MM and MP, both of which have an area that is comparable to that of MS, does not arise, since they are subjected to voltages that remain constant once they have been set at the desired value. To ensure that the laser diode is under minimum optical emission conditions when at input I there is a signal of low logic level, the number of inverter stages is odd. Thus, when the voltage on I is at low logic level, the gate of MS is at high logic level and, since the transistor is of the p-type, it is cut off, so that only the bias current supplied by MP can flow toward terminal L; conversely, when the voltage on I is at high logic level, the gate of MS is at low logic level and the transistor is saturated, i.e. it behaves like a closed switch, and therefore the current that can flow toward terminal L is given by the sum of the bias current and of the modulation current supplied by MM. These two currents depend on the voltages to which terminals M and P are subjected. These voltages are obtained with external circuits, not shown in the Figure: in particular, both the voltage on M and that on P can be adjusted independently from each other, for instance by connecting to each terminal the sliding contact of a multi-turn potentiometer connected between the power supply and ground and by varying the positions of these sliding contacts according to the voltages one desires to impose. Fig. 2 represents a circuit similar to the one described above, suitable for driving an optical source whose terminals are insulated from the case. The signal supplied by an information source, not shown, is supplied to input I of a cascade of inverters, which drives the gate of a n-type MOS transistor, indicated as MS'. This transistor, which acts as a switch for the modulation current, is connected through terminal L to the cathode of an optical source. A n-type MOS transistor. indicated as MM', is arranged in series with transistor MS' and its source is grounded while its gate is connected to a terminal M. The voltage on M can be adjusted externally, for instance by means of a variable resistor, and on this voltage depends the maximum value of the modulation current. A n-type MOS transistor MP' has its gate connected to a terminal P, its source connected to ground and its drain connected to terminal L. By setting the voltage on terminal P externally, it is possible to control the bias current flowing through the optical source. Note that the voltage drop on the optical source has no influence on the values of the voltages between gate and source of transistors MM', MP' and MS'. In this case, an even number of stages is used in the cascade of inverters: to a high logic level at the input of the cascade of inverters, a high logic level corresponds on the gate of MS', which is thus saturated and lets the modulation current supplied by MM' flow to ground. In this operating condition, a current given by the sum of the modulation and bias currents flows in the optical source. Conversely, when at the input of the cascade of inverters there is a low logic level, the gate of MS' is itself at low logic level and that transistor is cut off, so that only the bias current is drained from the optical source. Fig. 3 represents the eye diagram related to the schematic diagram in Fig. 1 for a data flow at 622 Mbit/s, where time (625 ps/division) is reported on the x-coordinates and the signal amplitude (100 mV/division) is reported on the y-coordinates. From this diagram it is evident that noise is limited and inter-symbol error is practically absent. This means that the voltage that determines the value of the bias current and the voltage that determines the value of the modulation current are not influenced by the voltage variations on the gate of the modulation switch transistor even for high bit rates. It is evident that modifications, adaptations, integrations, variations and replacements of elements with others functionally equivalent may be made in the example of embodiment described above without departing from the scope of the claims that follow; in particular, nothing prevents realising the circuit described above with discrete components, rather than integrating it on a single chip.

Claims: Circuit in CMOS technology for high speed driving of optical sources, comprising: a bias current generator, a modulation current generator, a cascade of CMOS inverter stages which supplies a driving voltage to the modulation current generator and receives digital signals at its input said bias current generator being a MOS transistor (MP or MP') whose source is connected to one of the power supply terminals, whose gate is connected to a terminal (P) for controlling the bias current and whose drain is connected to one of the terminals of the optical source to be driven characterised in that the modulation current generator is a pair of MOS transistors of the same conductivity type [(MM and MS) or (MS' and MM')] arranged in series, one of which (MS or MS') has its drain connected to said terminal of the optical source, its gate connected to the output of said cascade of inverter stages and its source connected to the drain of the second transistor of the pair (MM or MM') whose own source is connected to said power supply terminal and whose gate is connected to a terminal (M) for controlling the modulation current, the value of said bias current and the maximum value of said modulation current depending only on the voltages imposed externally on the respective control terminals. Circuit in CMOS technology as in claim 1, characterised in that said cascade is composed of an odd number of CMOS inverter stages, said MOS transistor (MP) and said pair of MOS transistors (MM and MS) are of p-type, said power supply terminal is the positive one (Vdd) and said optical source terminal is the anode. Circuit in CMOS technology as in claim 1, characterised in that said cascade is composed of an even number of CMOS inverter stages, said MOS transistor (MP') and said pair of MOS transistors (MS' and MM') are of n-type, said power supply terminal is the ground terminal and said optical source terminal is the cathode. Circuit in CMOS technology as in any of claims 1 to 3, characterised in that the voltages imposed externally on the terminals controlling said currents can be adjusted independently by means of variable resistors.

Inventors: BELLA VALTER, PELLEGRINO PAOLO, BELLA, VALTER, PELLEGRINO, PAOLO
Applicants: CSELT CENTRO STUDI LAB TELECOM, CSELT CENTRO STUDI E LABORATORI TELECOMUNICAZIONI S.P.A.
Application year: 1995
Main classification: H01S3
IPC codes: H01S5