SEMICONDUCTOR LASER DRIVING CIRCUIT

Abstract: A semiconductor laser driving circuit is arranged so that a predetermined number of transistors (14) can be selected from a group of transistors (12) disposed in parallel according to the characteristics of a semiconductor laser and the selected transistors (14) are connected in parallel between the semiconductor laser (1) and a current source (3). An optimum current is allowed to flow through the semiconductor laser (1) to cause it to generate a required light output. Moreover, a collector current which provides the optimum characteristics flows through each of the selected transistors (14). Therefore, an optimum high-speed response can always be realized irrespective of the magnitude of the driving pulse current of the semiconductor laser (1).

Description: 1. Field of the Invention The present invention relates to a semiconductor laser driving circuit and, more particularly, to a semiconductor laser driving circuit comprising an output circuit composed of a plurality of transistors connected in parallel and a current source. 2. Description of the Related Art Semiconductor lasers used in light output circuits for optical communication during daytime require driving circuits which accommodate variations in values of components used and variations in temperature. Fig. 1 illustrates an example of a prior art semiconductor driving circuit. This circuit comprises a parallel circuit 2 having n (n>1) transistors Tr11 to Tr1n connected in parallel and a parallel circuit 20 having n transistors Tr21 to Tr2n connected in parallel. Circuits 2 and 20 together constitute a differential pair forming a differential amplifier. A common base input V1 of parallel circuit 2 and a common base input V2 of parallel circuit 20 are supplied with pulse signals of opposite polarities. When the common base input V1 is on, transistors Tr11 to Tr1n are all turned on so that a semiconductor laser 1 is driven by a current source 3 to emit a desired light output pulse. The differential pair portion shown by broken lines in Fig. 1 is usually used in the form of an integrated circuit (refer to Japanese Unexamined Patent Publications Nos. 57-10279 and 63-119281). With the circuit of Fig. 1 , the number of transistors constituting parallel circuit 2 is preselected such that a (rated) collector current is allowed to flow through each transistor, optimizing the characteristics of each transistor, i.e., which maximizes the current amplification factor hFE, like the collector current Ic1 shown in Fig. 2 when an allowable maximum current flowing through the semiconductor laser 1 is divided equally by transistors Tr11 to Tr1n. The current source 3 is designed so that its current Io (= Ip) can be varied in order to produce a driving pulse current Ip which generates a desired light output Po to flow through the semiconductor laser 1, as shown in Fig. 3, when the current Ip flowing through the semiconductor laser 1 is varied by variations in the elements used and by variations in temperature. With such a semiconductor laser driving circuit, however, the driving pulse current Ip may be small and the driving pulse current Ip may have to be increased. This depends upon variations in the semiconductor laser 1 and variations in temperature. If the current Io of current source 3 is varied accordingly, the collector current flowing through each of the transistors will become, for example, Ic1 (where the current amplification factor has a maximum value of hFE1) or Ic2 (where the current amplification factor is hFE2 less than hFE1). These different collector currents Ic1 and Ic2 will result in different operating frequency characteristics, as shown in Fig. 4. As a result, the high-speed response characteristics of each transistor, particularly the waveform rising and falling characteristics for Ic2 shown by the broken line in Fig. 5, will be significantly degraded compared with those for Ic1 shown by the solid line in Fig. 5. The light output waveform of semiconductor laser 1 will be similarly degraded. Summary of the Invention It is accordingly an object of the present invention to provide a semiconductor laser driving circuit which avoids the degradation of high-speed response characteristics of each transistor in the driving circuit even if a driving pulse current flowing through a semiconductor laser is varied. A feature of the present invention according to claim 1 resides in a semiconductor laser driving circuit comprising a current source for supplying a current to a semiconductor laser, a transistor group in which a plurality of transistors is disposed in parallel and a predetermined number of transistors selected from said transistor group being connected in parallel between said current source and said semiconductor laser; and wherein the number of transistors constituting said transistor group is set such that an equal collector current flows through each of the transistors, thus optimizing their operating characteristics when an allowable maximum current of said semiconductor laser is equally divided by all of the transistors in said transistor group, and means for setting the number of selected transistors such that a current is allowed to flow through said semiconductor laser, causing it to generate a required light output, and the optimum collector current flows through all of the selected transistors. The present invention is defined in independent claims 1 and 18. Embodiments of the invention are defined in the dependent claims. Brief Description of the Drawings Fig. 1 is a circuit diagram of a prior art semiconductor laser driving circuit; Fig. 2 is a graph for showing the collector current for optimum transistor characteristics; Fig. 3 is a graph illustrating a driving pulse current for obtaining a desired light output; Fig. 4 shows the collector current versus operating frequency characteristics for a transistor; Fig. 5 is a waveform diagram illustrating the high-speed response characteristics of the transistors and the semiconductor laser in the driving circuit; Fig. 6 is a schematic circuit diagram illustrating the principle of a semiconductor laser driving circuit according to the present invention; and Fig. 7 illustrates an embodiment of the semiconductor laser driving circuit according to the present invention, Fig. 8 illustrates another embodiment of the semiconductor laser driving circuit according to the present invention, and Fig. 9 illustrates a further embodiment of the semiconductor laser driving circuit according to the present invention. Detailed Description of the Preferred Embodiments As shown in Fig. 6, which illustrates the principle of the invention, the semiconductor laser driving circuit comprises a current source 3 for supplying a current to a semiconductor laser 1 and a transistor group 12 in which a plurality of transistors are connected in parallel. A predetermined number of transistors 14, connected in parallel and selected from the transistor group 12, are connected between the semiconductor laser 1 and the current source 3. The number of transistors forming the transistor group 12 is set such that, when an allowable maximum current flowing through the semiconductor laser 1 is equally divided by all the transistors of the transistor group 12, a collector current flows through each of the transistors which optimizes its characteristics (that is, the current amplification factor hFE has a maximum value). Also, the number of transistors 14 to be selected from the transistor group 12 is set such that a current Ip for generating a required light output Po flows through the semiconductor laser 1 and the optimum collector current flows through each of the transistors 14. It is also possible to supply the optimum collector current to each of the transistors 15 as well as to the selected transistors 14 in the transistor group 12, from the current source 3. By setting the number of transistors forming the transistor group 12 and the number of transistors 14 connected to the semiconductor laser 1 as described above, the present invention produces two main advantages. Firstly, the driving pulse current Ip flows through the semiconductor laser 1, which causes the laser 1 to deliver the required light output Po (refer to Fig. 3), and the same optimum collector current always flows through each of a predetermined number of transistors 14 connected to the semiconductor laser 1. Therefore, the collector current of each transistor has an optimum fixed value, such as Ic1, as shown in Fig. 2, even if the driving pulse current Ip varies from sample to sample of a given type of semiconductor laser used. This prevents degradation of the high-speed response characteristics (rising and falling characteristics) of each of the transistors used. Secondly, if the transistors 15 as well as the selected transistors 14 in the transistor group 12 are connected to the current source 3 so as to be supplied with the optimum collector current therefrom, stray capacitance can be eliminated from the transistors 15 and a driving circuit comprising the transistor group 12 can be used as one of a differential pair. This will allow the semiconductor laser to be driven with faster response characteristics. In each of the embodiments of Figs. 7 and 8, a semiconductor laser driving circuit comprises a differential pair composed of a transistor parallel circuit 20 as in the prior driving circuit shown in Fig. 1 and a transistor group 12 having the same number of transistors as the transistor parallel circuit 20. In these embodiments, the current Io of the current source 3 is selected so as to accommodate the maximum value of the driving pulse current Ip, which is subject to variation due to manufacturing processes, of the semiconductor laser 1. The number n of transistors constituting the differential pair in the transistor group 12 is selected such that n = Io/Ico, where Ico is the optimum collector current for a transistor and Io is the current supplied by current source 3. Thus, the collector current Ico supplied to the semiconductor laser 1 by one of the transistors is given by Io/n. In the embodiment of Fig. 7, the optimum driving pulse current Ip (refer to Fig. 3) for the semiconductor laser 1 is 2Io/n. For this reason, two transistors Tr11 and Tr12 are connected in parallel between the semiconductor laser 1 and the current source 3. The other transistors Tr13 through Tr1n have their emitters connected to the current source 3 and their collectors connected to ground. In the embodiment of Fig. 8, on the other hand, the semiconductor laser 1 requires a driving pulse current of n x Io/n. Thus, in this case, all the transistors Tr11 through Tr1n are connected in parallel between the semiconductor laser 1 and the current source 3. In general, where the semiconductor laser 1 requires m x Io/n (m ≦ n) as its optimum driving current Ip, m transistors are arbitrarily selected from the transistor group 12 and connected between the semiconductor laser 1 and the current source 3. The embodiments of Figs. 7 and 8 are distinct from each other in the optimum driving pulse current Ip for the semiconductor laser 1. However, in both embodiments, the current flowing through each of the transistors in the transistor group 12 has a constant value of Io/n, independent of the magnitude of the driving pulse current for the semiconductor laser. For this reason, even if the characteristics of the semiconductor laser 1 vary from sample to sample, the switching current flowing through the transistor group 12 always amount to Io, which is identical to that of the transistor parallel circuit 20, thus forming a differential pair with the transistor group. Thus, the degradation in the high-speed response characteristics of the driving circuit can be minimized. In the above embodiment, the driving circuit is formed of a differential pair. However, a differential pair configuration may not be needed so long as bipolar transistors are used. In addition, the transistors 15 as well as the transistors 14 connected to the semiconductor laser 1 are connected to ground. This is to allow the same current to flow through each of the transistors in transistor group 12 and transistor parallel circuit 20, which form the differential pair, and to eliminate stray capacitances and thereby achieve a high-speed response. Even where the transistors 15 are not supplied with current, degradation of waveforms is prevented. Furthermore, if the transistors constituting transistor group 12 have their emitters connected together and their bases connected together, their collector currents remain unchanged. Thus, it is not necessarily required to directly connect all of the transistors 15 to ground as shown in Fig. 7. For example, suitable resistors may be connected between the transistors 15 and ground. Alternatively, another semiconductor laser 11 may be connected between the transistors 15 and ground as shown in Fig. 9. In this case, if the semiconductor lasers 1 and 11 have different characteristics, the number of transistors to be connected to them will vary according to these characteristics. The transistors 14 may be connected to semiconductor laser 1 by bonding with a conductive material. Alternatively, the selection of transistors may be made by means of a switch between the transistor group 12 and the semiconductor laser 1. In the above embodiments, npn bipolar transistors are used. Of course, pnp bipolar transistors may be used instead. In this case, each pnp transistor has its emitter electrode (one of its main electrodes) connected to the semiconductor laser 1 and its collector (the other of its main electrodes) connected to the current source 3. If the driving circuit is arranged such that currents flow in the opposite direction, the transistors are disposed accordingly. As described above, according to the present invention, by connecting a predetermined number of transistors selected from a group of transistors connected in parallel between a semiconductor laser and a current source, a semiconductor laser driving circuit can be arranged such that a current flows through the semiconductor laser to deliver a required light output and an optimum collector current flows through all of the predetermined transistors. Accordingly, the collector current of all of the transistors always has a constant optimum value irrespective of the magnitude of the current necessary for the semiconductor laser. Thus, optimum high-speed response is always achieved. If, in that case, transistors other than those connected to the semiconductor laser are equally supplied with an equal optimum collector current, the driving circuit can be used in the form of a differential pair, thus eliminating stray capacitances. Accordingly, a semiconductor laser driving circuit with faster response characteristics can be implemented.

Claims: A semiconductor laser driving circuit comprising: a current source (3) for supplying a current to a semiconductor laser (1); a transistor group (12) in which a plurality of transistors is disposed in parallel and a predetermined number of transistors (14) selected from said transistor group being connected in parallel between said current source and said semiconductor laser; the number of transistors constituting said transistor group being set such that an equal collector current flows through each of the transistors, thus optimizing their operating characteristics when an allowable maximum current of said semiconductor laser is equally divided by all of the transistors in said transistor group; and means for setting the number of selected transistors such that a current is allowed to flow through said semiconductor laser, causing it to generate a required light output, and the optimum collector current flows through all of the selected transistors. The semiconductor laser driving circuit according to claim 1, in which each transistor (15) other than said selected transistors in said transistor group is connected to said current source to be supplied with the optimum collector current therefrom. A semiconductor laser driving circuit according to claim 1, in which all of said transistors constituting said transistor group have the same structure and the same characteristics. A semiconductor laser driving circuit according to claim 1, in which the optimum collector current is a collector current at which the current amplification factor of a transistor has a maximum value. A semiconductor laser driving circuit according to claim 1, in which said transistors constituting said transistor group are bipolar transistors having their corresponding main electrodes connected together to said current source. A semiconductor laser driving circuit according to claim 1, in which all of said transistors constituting said transistor group have their base electrodes connected in common and are driven simultaneously by a signal applied to said base electrodes. A semiconductor laser driving circuit according to claim 1, further comprising a transistor parallel circuit (20) having the same number of parallel connected transistors as said transistor group, said transistor parallel circuit being connected to said transistor group to form a differential pair. A semiconductor laser driving circuit according to claim 1, in which said selected transistors are connected to said semiconductor laser by bonding with a conductive material. A semiconductor laser driving circuit according to claim 1, in which said selected transistors are connected to said semiconductor laser by a change-over switch disposed between said selected transistors and said semiconductor laser. A semiconductor laser driving circuit according to claim 2, in which all of said transistors constituting said transistor group have the same structure and the same characteristics. A semiconductor laser driving circuit according to claim 10, in which the optimum collector current is a collector current at which the current amplification factor of a transistor has a maximum value. A semiconductor laser driving circuit according to claim 11, in which said transistors constituting said transistor group are bipolar transistors having their corresponding main electrodes connected together to said current source. A semiconductor laser driving circuit according to claim 12, in which the other main electrodes of said transistors other than said selected transistors in said transistor group are connected to ground either via elements or directly. A semiconductor laser driving circuit according to claim 13, in which all of said transistors constituting said transistor group have their base electrodes connected in common and are driven simultaneously by a signal applied to said base electrodes. A semiconductor laser driving circuit according to claim 14, further comprising a transistor parallel circuit (20) having the same number of parallel connected transistors as said transistor group, said transistor parallel circuit being connected to said transistor group to form a differential pair. A semiconductor laser driving circuit according to claim 15, in which said selected transistors are connected to said semiconductor laser by bonding with a conductive material. A semiconductor laser driving circuit according to claim 16, in which said selected transistors are connected to said semiconductor laser by a change-over switch disposed between said selected transistors and said semiconductor laser. A semiconductor laser driving circuit comprising: a current source (3) for supplying a current to a semiconductor laser (1); a transistor group (12) having a plurality of transistors disposed in parallel and means for selecting a desired number of transistors (14) from said transistors of said transistor group according to a current necessary for a required light output of said semiconductor laser, said selected transistors being connected in parallel between said semiconductor laser and said current source.

Inventors: FUKUDOME, FUJITO
Applicants: FUJITSU LIMITED
Application year: 1990
Main classification: H01S3
IPC codes: H01S5, H03K17