The invention relates to a circuit arrangement for operating a laser diode. In particular, the invention relates to a circuit arrangement having a differential laser driver which makes it possible to suppress electromagnetic interference emission during operation of a laser diode.
It is known, in the context of electrical transmission or within electrical circuits, for a data signal which is to be transmitted to be transmitted differentially. In the case of a differential transmission, two mutually inverse signals are present. This has the advantage that components of a system can be driven differentially, i.e. these components respectively evaluate the difference between two mutually inverse input signals. Since the signals are the inverse of one another, twice the amplitude can be evaluated in this case. This reduces the system's susceptibility to interference and increases the stability.
In optical systems which operate with differential signals, the laser driver is also of differential design, i.e. a differential driving is effected at two input terminals by means of mutually inverse input signals. Such a differential laser driver provides a differential output signal, i.e. two mutually inverse output signals are present at two output terminals.
In the case of a customary single-ended driving of a laser diode, however, only one of said output signals is used as a driver signal for the laser diode. This is illustrated schematically in
At a laser driver 1, a differential input signal comprising two mutually inverse signals S, −S is present at two input terminals 1E1, 1E2. The laser driver 1 provides two driver signals IT, −IT at two output terminals 1A1, 1A2. One of said output signals IT provides a current through a laser diode 2, which generates a modulated light signal in accordance with the modulation of said current IT. The output signal −IT at the other driver output 1A2 is not utilized for data transmission and is fed to a complex impedance XD illustrated schematically. This is disadvantageously accompanied by the generation of interference radiation (EMI—Electro-Magnetic-Interference). The interference radiation is generated by fluctuations in the supply current of the laser driver on account of the unequal loads (XD, Laser) at the output of the laser driver.
The present invention is based on the object of providing a circuit arrangement for operating a laser diode which suppresses the production of electro-magnetic interference radiation as effectively as possible.
The invention provides a circuit arrangement for operating a laser diode, having: a laser driver, to which a differential input signal is applied and which provides a differential output signal comprising a first output signal and a second output signal that is the inverse of the first output signal, which are provided at a first and a second laser driver output; a first laser diode, which is connected to the first laser driver output and to which the first output signal is applied; and a second laser diode, which is connected to the second laser driver output and to which the second output signal is applied. In this case, the load provided at the first laser driver output and the load provided at the second laser driver output are essentially identical.
The circuit arrangement according to the invention has the effect that the supply current of the laser driver, which (in the model) results from the sum of the output signals provided at the two laser driver outputs, is essentially constant. The constancy of the supply current results from the fact that the signals provided at the two outputs of the laser driver are the inverse of one another and—since they operate on the same load—are furthermore identical in terms of magnitude. On account of the constancy of the supply current, a high-frequency interference current component that would lead to an undesirable interference emission is not superposed on said supply current. Rather, the generation of interference emission is reduced and ideally completely suppressed.
The solution according to the invention is thus based on the concept of providing identical loads at the two differential outputs of a fully differential laser driver. The instances of interference caused by the two output signals at the two differential outputs compensate for one another, so that there is no interference amplitude on the supply current of the differential laser driver. Interference amplitudes on the supply current also generate, on account of unavoidable lead inductances, instances of interference on the supply voltage that in turn amplify the interference emission.
In a preferred refinement, only the light of one laser diode is utilized for data transmission and for this purpose coupled into an optical waveguide. The light of the second laser diode is preferably covered by a light-opaque material or a screen. The second laser diode is a “dummy diode” that, for reasons of circuit symmetry, is not used for data transmission and light generation.
The differential laser driver provides two mutually inverse pulse sequences at the first laser driver output and at the second laser driver output. The sum of the two currents provided at the two laser driver outputs is essentially constant over time. This results, on the one hand, from the inverse ratio of the two pulse sequences and, on the other hand, from the fact that both pulse sequences are respectively applied to the same load.
In order to realize an identical load, the two laser diodes preferably have structurally identical design. Preferably, the two laser diodes are in this case monolithically integrated into a common laser chip. Furthermore, the two laser diodes are preferably situated at a small distance from one another in order that temperature fluctuations and other changes in the operating conditions affect the two laser diodes in the same way. For this purpose, the two laser diodes are preferably spaced apart from one another at a distance of between 20 μm and 100 μm, in particular at a distance of approximately 50 μm, and are arranged on a common substrate or monolithically integrated into such a substrate.
The invention is explained in more detail below using an exemplary embodiment with reference to the figures, in which:
A circuit arrangement for operating a laser diode in accordance with the prior art has been explained in the introduction with reference to
The inverse current signal −IT provided at the other output terminal 1A2 is fed to a complex impedance XD, which is formed by an internal termination resistance R0 and an inductance L0 in the equivalent circuit diagram. The inductance L0 is not equal to the inductance L1 plus L2 of the other branch of the circuit. On account of unavoidable fluctuations and temperature responses of the circuit and also of the laser diode, the internal resistance R0 is normally not equal to the internal resistance R2 of the laser diode 2.
The complex impedance XD may additionally also have capacitive elements that are likewise normally not equal to the capacitance of the laser.
The two diodes 2, 3 are preferably of structurally identical design and are arranged on a common laser chip at a relatively small distance, in particular at a distance of between 20 μm and 100 μm. The laser diodes are preferably vertically emitting diodes (VCSEL).
It is essential in this context that the loads provided by the two laser diodes 2, 3 and the bonding wires or other electrical contact connections in the two circuit branches are identical. It thus holds true that
L3+L4=L1+L2 (1)
C1=CO and (2)
R4=R2. (3)
The loads at the two differential outputs 1A1, 1A2 of the fully differential laser driver are thus identical.
The advantages of this solution in comparison with the solution of the prior art in accordance with
The inverse relationship, the identical profile and the identical amplitude of the two signals provided at the two output terminals 1A1, 1A2 of the laser driver result in a complete constancy of the supply current Iv illustrated in
The simulation of the currents of the fully differential laser driver reveals that the instances of interference completely compensate for one another on account of the complete identity of the loads at the two differential outputs of the laser driver. There is no longer an interference amplitude on the supply current Iv, as can be discerned in
The configuration of the invention is not restricted to the exemplary embodiments presented above. The person skilled in the art recognizes that numerous alternative embodiment variants exist which, despite their deviation from the exemplary embodiments described, make use of the teaching defined in the subsequent claims.