Laser diode driving circuit and optical transmission device

Abstract: An optical transmission device, which is used for a short range optical data link or an optical communication system for subscribers and more particularly requires no automatic optical power control, minimizes the turn-on delay time of a laser diode and allows unformatted optical signal transmission by a fixed level decision receiving system. By applying to the laser diode a fixed-bias electric current which is no greater than the electric threshold current value of the laser diode, the turn-on delay time of the laser diode can be minimized, and the extinction level can be lowered. Thus, a fixed level decision receiving system can be used, and the optical receiver circuit can be simplified.

Description: TRANSMISSION DEVICE BACKGROUND OF THE INVENTION The present invention relates to an optical transmission device which is used for a short range optical data link or an optical communication system for subscribers and more particularly to an optical transmission device which requires no automatic optical power control. When an automatic optical power control method, that is, a control method for monitoring optical power of a laser diode, feeding back a bias current and electric modulation current, and obtaining fixed optical power from the laser diode without depending on changes in the electric threshold current value which are caused by temperature changes is used, the circuit configuration becomes complicated. Therefore, a great deal of attention has recently given to drive a laser diode by zero-bias or fixed-bias. From, "Sub-system optical interconnections using Long Wave-length Laser Diode and Single-mode Fiber Arrays" (1992 Technical Report of the Institute of Electronics, Information and Communication Engineers (Optical Communication System), OCS92-30, p 31) (hereinafter called Ref. A), zero-bias driving of a laser diode and unformatted optical signal transmission by fixed level decision receiving system are known. On the other hand, from, "APC Free Zero-Bias Modulation of an Alignment-free Optical Coupled 4-Channel Optical Module" (1992 Spring National Convention Record, The Institute of Electronics, Information and Communication Engineers, paper B-1008 (1992)) (hereinafter called Ref. B), a fixed-bias driving experiment with a laser diode is known. According to Ref. B, a bias electric current I b is set so that it becomes equal to an electric threshold current value Ith of the laser diode at a high temperature in the operating temperature area range (I b = I th (at a high temperature)). When a non-return-to-zero coding (NRZ) signal having a mark ratio which is almost 1/2 is received, "0" or "1" is identified generally on the average received optical power level, so as to prevent the optical power of the laser diode at a high temperature from lowering since the electric threshold current value increases as the temperature rises. However, in the state that the fixed-bias electric current is set so that it becomes equal to the electric threshold current value of the laser diode at a high temperature in an assumed operating temperature area range as in Ref. B, the fixed-bias electric current becomes larger than the electric threshold current value of the laser diode at a low temperature in the operating temperature area range (I b > I th (at a low temperature)). Therefore, the optical power level (extinction level) increases when an electric input signal to the laser diode is off and it disturbs unformatted data optical transmission by fixed level decision receiving system described in Ref. A. Namely, it is required to set the fixed decision level of the receiver side to a high value, so that the minimum optical power from the laser diode which is necessary to identify "l" on the receiver side increases. As shown in a measurement example of turn-on delay time of the laser diode in Fig. 4a, when zero-bias driving is carried out for the laser diode, the turn-on delay time increases compared with that when a bias electric current is applied to this laser diode. A laser diode driving circuit with the features of the first part of claim 1 is known from JP-A-59 028 396. SUMMARY OF THE INVENTION An object of the present invention is to minimize the turn-on delay time of a laser diode and to enable unformatted optical signal transmission by fixed level decision receiving system. This object is met by a laser diode driving circuit of claim 1. Preferred embodiments are disclosed in the dependent claims. By applying a bias electric current to a laser diode which is not more than an electric threshold current value of the laser diode but larger than a current value corresponding to a built-in potential of the laser diode, the turn-on delay time of the laser diode can be minimized. Since the extinction level can be lowered, the fixed level decision receiving system can be used and the optical receiver circuit can be simplified. Furthermore, the optical receiver circuit using the above constitution can be simplified compared with that when the automatic optical power control is applied. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows an embodiment of the driving circuit of laser diode of the present invention. Fig. 2 is a drawing showing a bias electric current of the driving circuit of laser diode and the dependency of electric threshold current value of the laser diode on temperature. Fig. 3 is a drawing showing an optical transmission device using the driving circuit of laser diode. Figs. 4a and 4b are drawings showing characteristics of the laser diode. Fig. 5 shows an array example of the circuit of the embodiment shown in Fig. 1. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 shows an embodiment of the driving circuit of laser diode of the present invention. An electric input signal 1 is supplied to a laser diode 3 via a current switch 2. A fixed-bias electric current I b which is not more than the electric threshold current value I th of the laser diode is applied to the laser diode 3 by a bias circuit 4. To produce the circuit, the Si bipolar IC process is used. Fig. 2 shows a bias electric current of the driving circuit of laser diode and the dependency of electric threshold current value of the laser diode on temperature which will be described later. The bias electric current of a driving circuit 5 of laser diode ranges from 1.64 mA to 2.75 mA within the operating temperature area range from 20°C to 80°C under the condition that the supply voltage variation of the driving circuit is 10% and the resistance variation due to the production process variation is 20%. On the other hand, the electric threshold current value of the laser diode 3 ranges from 2.92 mA to 10.33 mA within the operating temperature area range from 20°C to 80°C under the condition that the electric threshold current value at 25°C when the characteristic temperature is 55°C, varies within a range from 3.2 mA to 3.8 mA. When the laser diode 3 is mounted in the neighborhood of the driving circuit 5 of laser diode so as to make the temperatures of the two almost equal, the electric threshold current value of the laser diode 3 varies as shown in Fig. 2 within the entire temperature range from 20°C to 80°C and the bias electric current of the driving circuit of laser diode can be set less than the electric threshold current value of the laser diode actually on the side of the driving circuit 5 of laser diode regardless of characteristics of the laser diode 3 such as changes in the electric threshold current value due to changes in the temperature. In the embodiment shown in Figs. 1 and 2, since the bias electric current of the driving circuit 5 of laser diode is not more than the electric threshold current value of the laser diode, the extinction level is almost that of spontaneous emission light such as about -30 dBm. Furthermore, Fig. 3 shows an optical transmission device using the driving circuit of laser diode and the fixed level decision optical receiver described in Ref. A. Therefore, for example, by combining an optical transmission device having this constitution and the fixed level decision optical receiver described in Ref. A unformatted optical signal transmission can be carried out. When a bias electric current of about 2.0 mA which is smaller than the electric threshold current value 2.8 mA of the laser diode is applied as shown in the example in Fig. 4(a), for example, at 25°C, the turn-on delay time can be reduced to about 1/7 of that in the case of zero-bias driving. Figs. 4(a) and 4(b) show characteristic examples of the laser diode. Fig. 4(a) shows the relationship between turn-on delay time T d and ℓn((I d -I b )/(I d -I th ) when the driving current of laser diode I d is fixed at 20 mA and the bias electric current I b is changed at 25°C. The electric threshold current value Ith of this laser diode at 25°C is 2.8 mA. The buildup ratio for turn-on delay time decreases from around the point where Ib becomes larger than 1.0 mA. It is found that the point is in the neighborhood of the current value corresponding to a threshold voltage Vth of I b -V plot when the static characteristic graph (dependency of forward voltage and resistance on bias electric current) shown in Fig. 4(b) is compared with Fig. 4(a). The semiconductor laser is a semiconductor device having a pn junction. Therefore, when a forward bias of approximately the built-in potential of the laser diode is given beforehand, the resistance of the laser diode decreases and an electric current can be easily supplied to the laser diode. Therefore, when the bias electric current is preset above the current value corresponding to a threshold voltage V th of I b -V plot, variations of the turn-on delay time due to temperature changes of the laser diode can be minimized. The manifestation of this embodiment can be realized, for example, by the following methods. (i) An electric signal of all "0" is supplied to the driving circuit of laser diode and the emission light spectrum from the laser diode is observed. Namely, when the electric signal of all "0" is inputted, only a bias electric current smaller than the "electric threshold current value" is supplied to the laser diode, so that the laser diode generates no laser beam and the output optical spectrum is seen broad. Therefore, by observing the emission light spectrum by an optical spectrum analyzer, it is found that the bias electric current of driving circuit of laser diode is smaller than the electric threshold current of the laser diode. (ii) The coherence length of the emission light from the laser diode is changed greatly before and after the laser diode starts laser oscillation. Therefore, by observing the coherence length, it is found that the bias electric current of driving current of driving circuit of laser diode is smaller than the electric threshold current of the laser diode. In the embodiment shown in Fig. 1, the Si bipolar IC process is used to produce a driving circuit of laser diode. However, other bipolar, MOS, or FET systems may be used. The laser diode may be produced on a n-substrate, too. In the above embodiment, a bias electric current which is smaller than the electric threshold current value and larger than the current value corresponding to threshold voltage V th is applied to the laser diode. The driving circuit of the laser diode of the embodiment shown in Fig. 1 may be modified to a monolithic array circuit as shown in Fig. 5 as indicated in Ref. A. The circuit is a monolithic circuit consisting of a plurality of channels. As mentioned above, by constructing a multi-channel optical transmitter having a laser diode array and a driving circuit array of laser diode array having a means of applying a fixed-bias electric current which is not more than each electric threshold current value of the laser diode array, a multi-channel optical transmitter having a small electric crosstalk can be constructed. By combining this multi-channel optical transmitter with the fixed level decision optical receiver described in Ref. A, a high-speed, unformatted data parallel optical signal transmission device which has a small crosstalk can be constructed. Furthermore, a characteristic which is unrelated to the characteristics of the laser diode, for example, an intensive temperature characteristic may be given to the fixed bias electric current of the driving circuit of laser diode.

Claims: An optical transmission device comprising a laser diode (3) and a laser diode driving circuit (5) having a current switch (2) for supplying a driving current to the laser diode, and a bias circuit (4) for fixing a bias current (I b ) supplied by the driving circuit (5) to the laser diode (3), characterised in that said bias circuit (4) is adapted to fix said bias current (I b ) supplied by the driving circuit (5) to the laser diode (3) independently from a laser oscillation output such that said bias current is smaller than the threshold current value (I th ) of the laser diode within a predetermined operating temperature area but is larger than a current value corresponding to a built-in potential of said laser diode so that the turn-on delay time of the laser diode (3) is shorter than a turn-on delay time at said current value corresponding to said threshold voltage. A system comprising an optical transmission device according to claim 1 and a fixed decision-level optical receiver coupled to said laser diode (3) via an optical transmission path. An optical transmission device according to claim 1 comprising an array of laser diodes and an array of driving circuits each as specified in claim 1. A system comprising an optical transmission device according to claim 3 and a fixed decision-level parallel optical receiver. An optical transmission device according to claim 1, wherein the transistors in said current switch (2) and the transistors in said bias circuit (4) are bipolar transistors. An optical transmission device according to claim 1, wherein the transistors in said current switch (2) and the transistors in said bias circuit (4) are MOS transistors. An optical transmission device according to claim 1, wherein the transistors in said current switch (2) and the transistors in said bias circuit (4) are field effect transistors.

Inventors: ABE HAJIME, TAKAI ATSUSHI, ABE, HAJIME, TAKAI, ATSUSHI
Applicants: OPNEXT JAPAN INC, OPNEXT JAPAN, INC.
Application year: 1993
Main classification: H04B10
IPC codes: H01S5, H04B10