Drive circuit and method of operating a semiconductor laser

A circuit for driving a semiconductor laser, in particular a vertical cavity surface-emitting laser that comprises a differential amplifier for driving the semiconductor laser directly. The semiconductor laser is differentially driven by means of the differential amplifier, a first output of the differential amplifier being direct-current-coupled to a first terminal of the semiconductor laser and a second output of the differential amplifier being alternating-current-coupled to a second terminal of the semiconductor laser.

The invention is based on a priority application EP 03360020.6 which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a drive circuit for and a method of operating a semiconductor laser, in particular a vertical cavity surface-emitting laser (VCSEL).

The drive circuit according to the invention is, in particular, suitable for driving various types of VCSEL, i.e. VCSELs having different characteristics.

VCSELs are semiconductor laser diodes that emit their light vertically with respect to the surface of the wafer. They have many advantages compared with other semiconductor lasers. Mention may be made, for example, of the very high modulation rates, the very low power consumption, a high efficiency for optical coupling to optical fibres, and also photolithographically defined geometries. In addition, VCSELs are inexpensive. Compared with DFB lasers, the cost factor is, for example, approximately 100. However, VCSELs have a limited output power and a marked temperature dependence. Application fields for VCSELs are, for example, 10 Gbit Ethernet networks, in-office systems and also transmission systems in the short-link field, such as Metro networks. VCSELs having a laser wavelength in the region of 850 nm have been known for a fairly long time. Recently, VCSELs having a laser wavelength of 1300 nm have also been marketed. In contrast to DFB lasers, which are normally operated in the continuous-wave mode and whose laser light is modulated for information transmission via an external downstream modulator, VCSELs are modulated directly.

A circuit for driving a VCSEL having an emission wavelength of 850 nm is known that is a differential amplifier. In this instance, one terminal of the VCSEL is connected to ground or chassis respectively and the other terminal of the VCSEL to an output of the differential amplifier. Such a connection arrangement is also termed “single-ended”. The VCSEL is operated with direct modulation by means of the differential amplifier. VCSELs having an emission wavelength of 850 nm have a relatively high threshold voltage of approximately 1.8 V, a threshold current of approximately 3 mA and a limit current of approximately 10 mA. Its differential resistance is in the order of magnitude of 60 Ω. To avoid distortions, the connecting line between the drive circuit and the VCSEL chip is normally matched to the differential resistance of the VCSEL.

The recently developed VCSELs having an emission wavelength of 1300 nm have a threshold voltage of 1.3 V but a differential resistance in the order of magnitude of 120 Ω. In contrast to connecting lines having an impedance of 120 Ω, the use of connecting lines having an impedance of 55-85 Ω does not present a problem from technical and economic points of view. The above-described drive circuit of the prior art has the disadvantage that, if in general only to a limited extent, it is usable for VCSELs having a high differential resistance.

SUMMARY OF THE INVENTION

On the other hand, the object of the invention is to create a circuit and a method for driving a semiconductor with which a semiconductor laser is driven with direct modulation at a high differential resistance.

The object of the invention is furthermore to create a circuit and a method that is also usable for driving semiconductor lasers having low differential resistance.

To achieve said object, on the one hand, a circuit for driving a semiconductor laser, in particular a vertical cavity surface-emitting laser, comprising a differential amplifier for driving the semiconductor laser directly, wherein a semiconductor laser to be connected to the circuit is differentially driven by means of the differential amplifier, a first output of the differential amplifier being direct-current-coupled to a first terminal of the semiconductor laser and a second output of the differential amplifier being alternating-current-coupled to a second terminal of the semiconductor laser and, on the other hand, a method of operating a circuit, wherein the circuit is matched to the semiconductor laser used under the control of specified events,wherein characteristic-curve data of the semiconductor laser are first measured by means of the circuit,at least one start-up value of the circuit is determined and set on the basis of the measured data and
circuit operating values for operating the circuit comprising said semiconductor laser are determined and set by varying the start-up value at least as a function of a desired characteristic quantity of the semiconductor laser.

Further advantages and refinements of the invention emerge from the subclaims, from the description and the attached drawing.

It goes without saying that the above mentioned features and those still to be explained below are usable not only in the respectively specified combination, but also in other combinations or alone without departing from the scope of the present invention.

FIG. 1shows diagrammatically a drive circuit of the prior art for direct modulation of a semiconductor laser, in particular in the form of a semiconductor laser diode. Reference symbol1denotes a vertical cavity surface-emitting laser (VCSEL) whose cathode is connected to ground or chassis respectively, the VCSEL being modulated via the anode terminal. The circuit of the prior art is a known differential amplifier10that is connected upstream of the VCSEL1for the purpose of direct modulation. The differential amplifier10is a balanced direct-voltage amplifier having two inputs E1, E2and two outputs A1, A2. The differential amplifier comprises two resistors11,12, two transistors13,14and a constant-current source15that is disposed in the common emitter line of the transistors13,14. Differential amplifiers are generally known to the person skilled in the art and do not therefore require explanation in greater detail here. Owing to their low temperature drift, differential amplifiers are preferably also used if it is not a voltage difference, but only an input voltage that is to be amplified. In this case, one of the two inputs E1or E2is applied to chassis. In the circuit ofFIG. 1, the input E1is applied to chassis and the VCSEL1is driven via the input E2and the output A1, which is connected to the anode of the VCSEL1. The output A1is not connected. As shown inFIG. 1, the VCSEL1is driven unilaterally via one terminal, namely the anode, whereas the cathode is connected to chassis. Such a connection arrangement is also termed “single-ended”.

FIG. 2shows a diagram of a first embodiment of a driver or drive circuit according to the invention for a VCSEL1in which the VCSEL1is differentially driven. InFIG. 2, the some reference symbols denote the same components as inFIG. 1. The circuit according to the invention is preferably designed as an integrated circuit (IC) or a part of an integrated circuit. The circuit according to the invention is described in greater detail below.

The driver circuit according to the invention inFIG. 2likewise comprises a differential amplifier10, but both inputs E1and E2and both outputs A1and A2are used for differentially driving the VCSEL1. The transistors13,14are shown as bipolar transistors. It is obvious to the person skilled in the art that a differential amplifier comprising MOSFET transistors can also be used. The output A2is connected to the anode1aof the VCSEL1and the output A1is connected via a capacitor16to the cathode1bof the VCSEL1. Between the capacitor16and the cathode1bof the VCSEL1, a coil17and a resistor18are additionally connected to ground in series as a direct-current path. The VCSEL1is driven differentially, i.e. the resistors11,12and the current source15are specified in such a way that a voltage difference of approximately 1.5 V drops across the VCSEL1and modulation frequencies are applied via the inputs E1and E2. The cathode of the VCSEL1is alternating-current-coupled (AC-coupled) via the capacitor16to the output A2of the differential amplifier10. The circuit branch comprising output A1of the differential amplifier10, anode1aand cathode1b, respectively, of the VCSEL1is direct-current-coupled (DC-coupled) via the coil17and the resistor18. The bias for driving the VCSEL10is applied via this branch. The VCSEL1is connected to the circuit via impedance-matched terminal lines26and27.

The drive circuit according to the invention is particularly advantageous for driving a VCSEL having high differential resistance.

The VCSELs already used for a fairly long time in practice and having an emission wavelength of 850 nm have a relatively high threshold voltage of approximately 1.8 V, a threshold current of approximately 3 mA and a limit current of approximately 10 mA. Their differential resistance is in the order of magnitude of 60 Ω. Owing to the marked temperature dependence of VCSELs, an effort is made to dispose the integrated circuit comprising the drive circuit and the chip comprising the VCSEL with a spatial separation. The spacings are normally in the centimetre range. To avoid distortions, the connecting line of the drive circuit to the VCSEL chip is normally designed in such a way that the characteristic wave impedance is terminated, i.e. it is matched to the differential resistance of the VCSEL. Connecting lines having an impedance of 55 to 85 Ω are implementable and available from economic points of view at reasonable cost.

The VCSELs recently developed and having an emission wavelength of 1300 nm have a threshold voltage of 1.3 V, but a differential resistance in the order of magnitude of 120 Ω. Connecting lines having an impedance of 120 Ω are technically expensive to produce and handle and are, consequently, not inexpensive.

As a result of the drive circuit according to the invention having differential drive, the characteristic wave impedance of the drive/connecting lines can easily be terminated even for VCSELs having high differential resistance. As a result of the two connecting lines, a termination using simple technical means can also be achieved in this case as a consequence of the differential drive, i.e. the available lines having a line impedance in the range from 50 to 85 Ω can be used.

The circuit according to the invention inFIG. 2has, in addition, the advantage that, as a result of the chosen circuit arrangement—namely the use of a direct-current-coupled branch and an alternating-current-coupled branch for connecting to the VCSEL —the capacitor16, the coil17and the resistor18, which are also described as a bias-T, are dimensioned in such a way that they can be integrated into the IC that comprises the drive circuit. In terms of order of magnitude, the capacitor16has a value of 2 pF, the coil17has a value of 4.7 nH and the resistor has a value of 50 Ω. This is an appreciable advantage for the manufacture and the operation of the circuit. The circuit according to the invention consequently has the advantage that the VCSEL is operated differentially only in the “high-frequency” range, whereas the VCSEL is driven “quasi single-ended” in the direct-current range to apply the bias.

The circuit according to the invention furthermore has the advantage that it is also usable for the known VCSELs having fairly low differential resistance.

In other words: the circuit or is suitable not only for driving a VCSEL having a fairly high differential resistance, that is to say, for example, a VCSEL having an emission wavelength of 1300 nm and a differential resistance of approximately 120 Ω as described previously, but can also easily be used for the VCSELs already known for a fairly long time having an emission wavelength of 850 nm and having a fairly low resistance of approximately 60 Ω. To drive an 850 nm VCSEL, one input, that is to say E1or E2, is simply applied to a constant potential and the cathode of the VCSEL is not connected to the output A2via the capacitor16, but simply applied to chassis, thereby achieving a “single-ended” drive of the VCSEL.

FIG. 3shows an advantageous refinement of the circuit according to the invention inFIG. 2. For balance reasons, between the output A2and the capacitor16, a coil19and two diodes20,21are connected in series to ground via a resistor22. This achieves the result that the working points are the same on both sides, thereby avoiding distortions.

FIG. 4shows a further advantageous refinement of the driver circuit according to the invention inFIG. 3. A transistor23is connected upstream of the parallel connected resistors11and12in the collector branch of the transistors13and14. In addition, a further transistor24and25is connected in parallel with each resistor11and12. The transistors23,24and25shown inFIG. 4are MOSFET transistors. The connection arrangement shown for the resistors advantageously makes possible a variable setting of the working point of the differential amplifier10if suitable voltages are applied to the gates of the transistors23,24and25.

A driver circuit according to the invention designed in this way is suitable, in particular, for an automatic setting or adjustment of the drive circuit to the respectively used VCSEL1. Said adjustment may take place in regard to very varied factors and may be desirable, for example, for the use of the circuit comprising VCSELs that emit laser light at different wavelengths. The characteristics or characteristic curves normally differ in this connection markedly from one another so that considerable matching is required in this case. However, the adjustment may be performed, in particular, also in regard to the use of the circuit in the “single-ended” operating mode or the differential operating mode or “dual-ended” operating mode. The automatic matching/adjustment of the circuit may furthermore also be provided for absorbing less significant deviations, for example if VCSELs of identical or similar construction from different manufacturers are used, or for compensating for production tolerances of a certain VCSEL. Finally, the automatic adjustment may also serve for matching to different environmental effects during the operation of the VCSEL, for example temperature effects. Depending on the desired purpose of use, an appropriately large adjustment range and an appropriate adjustment precision should consequently be provided.

In a further preferred refinement of the invention, digital-to-analogue converters are provided at least for the inputs E1, E2of the differential amplifier10and of the control input of the constant-current source15(not shown). In this way, adjustments/matchings of the circuit or the drive of the semiconductor laser can be performed by means of a program executed on a microprocessor. The microprocessor together with the input/output chips and memory (RAM, ROM, etc.) may be provided externally or be on the IC of the drive circuit. In a preferred development of the circuit, it comprises at least the microprocessor (not shown) for performing a program for driving the semiconductor laser and/or adjustment of the circuit.

The method according to the invention for operating the drive circuit in which the circuit is matched to the respectively used semiconductor laser is described below.

The matching of the circuit is initiated by specified events. Specified events are, for example, an initial start-up or a repeat start-up, a replacement of the semiconductor loser, a specified number of operating hours or an occurrence of a fault.

In accordance with the methodcharazcteristic-curve data of the semiconductor lasers are first measured by means of the circuit,at least one start-up value of the circuit is determined and set on the basis of the measured data andcircuit operating values for operating the circuit comprising said semiconductor laser are determined and set by varying the start-up value at least as a function of the desired characteristic quantity, for example the emission rate, of the semiconductor laser.

The method of automatically matching the drive circuit to the respectively connected VCSEL is described more precisely below. By way of example, the control current is provided as the start-up value or parameter for setting the circuit. However, other characteristic quantities, for example voltage, temperature, etc. may be used instead or additionally for the setting. Preferably, the circuit is calibrated during start-up or the characteristic curve of the VCSEL is measured, and the circuit is set on the basis of the measured data obtained. For this purpose, the circuit is connected to the previously described microprocessor (not shown), which executes a program for determining the characteristics of the VCSEL. The microprocessor is connected via appropriate input/output chips and digital-to-analogue converters to the inputs E1, E2of the transistors13and14, to a control input of the constant-current source15and the gate inputs of the transistors23,24and25. The output of a monitor diode, which is normally available on a semiconductor laser, for example VCSEL, is made available to the microprocessor for evaluation via an analogue-to-digital converter. Appropriate sensors and interfaces may be provided to determine further characteristic data, for example the temperature.

To set/match the circuit, the characteristic curve of the VCSEL is preferably measured first. This is done using the drive circuit and, to be precise, by setting suitable current/voltage values in steps, in which case equidistant steps or even variable step widths may be provided depending on characteristic-curve range. The characteristic curves determined may be stored, for example for later evaluation, for documenting, etc. The maximum control current is then set and the emission rate is measured by means of the monitor diode. The maximum control current is then lowered in steps and the current for the desired emission rate is determined during this operation using the monitor diode. The working point determined in this way is stored and the circuit is aligned to this setting. Depending on the respective requirements (tolerance values, etc.), the characteristic curves for certain semiconductor lasers may already be stored in a memory, with the result that determination of the characteristic curve may be dispensed with.

The adjustment of the driver circuit may, in addition, be controlled by different events, for example, the initial start-up, the replacement of a semiconductor laser, for example a VCSEL, the number of operating hours, the occurrence of a fault, etc. In addition, certain configurations of the driver/drive circuit may be stored in the memory for certain types of semiconductor lasers and/or operating modes (dual-ended, single-ended). Depending on the requirement, no fine adjustment or a fine adjustment only to be performed rapidly is necessary. In addition, provision may be made for the operation of the semiconductor laser to be monitored continuously and/or measured data of the semiconductor laser to be evaluated at short time intervals in order to ensure in this way a largely optimized operation of the semiconductor laser.