The present invention relates to a directly modulated optical module. FIG. 10 shows an exemplary configuration of a conventional directly modulated optical module. In FIG. 10, 1′ is a directly modulated optical module, 2′ is a semiconductor laser, 3′ is a driver circuit which drives the semiconductor laser 2′ with a current changed based on a transmission signal, 4′ is a lens which converges the laser light emitted from the semiconductor laser 2′, 5′ is an optical fiber which transmits the laser light and 7′ is a bias circuit allowing a bias current to flow into the semiconductor laser 2′.
In this configuration with the bias current provided by the bias circuit 7′, the driver circuit 3′ drives the semiconductor laser 2′ which in turn generates laser light depending on the current (hereinafter denoted as the input current as the case may be) entered from the driver circuit 3′. This laser light is converged by the lens 4′ and then goes into the optical fiber 5′ as an optical signal for transmitting information. FIG. 11(a) shows an example of the waveform of the aforementioned input current 20. FIG. 11(b) shows an example of the waveform of the optical output (laser light output) 21 emitted from the semiconductor laser 2′. FIG. 11(c) shows an example of the eye pattern waveform formed by superimposing the optical output at intervals of one bit data period through a filter determined according to an optical waveform evaluation standard for optical transmission equipment. In the prior art, as shown, the driver circuit 3′ outputs such a trapezoidal waveform that the rise time of the input current 20 is almost same as the fall time, that is, the rate of change in the rise time is almost the same as the rate of change in the fall time (FIG. 11(a)). Since the semiconductor laser 2′ has relaxation oscillation when turned on, the waveform of the optical output 21 shows ringing at the rising edge and rises sharper than falls (FIG. 11(b)). If the ringing is large, it may be impossible to remove the ringing even through the filter determined by the optical waveform evaluation standard, leading to a deterioration of the eye opening. Thus, a technique has been devised which adds feed forward circuitry to the driver circuit in order to enlarge the eye opening by suppressing the ringing of the optical output waveform, as disclosed in Japanese Patent Laid-open No. 11-214781.
Further, another method intends to allow the drive current waveform to sharply fall while suppressing the ringing of the optical output waveform at the rising edge as disclosed in Japanese Patent Laid-open No. 2003-17800. Japanese Patent Laid-open No. 2003-17800 is not necessarily prior art to the invention described and claimed herein. That document is discussed herein only as providing information related to this application. In the laid-open document, the driver circuit comprises transistor and resistor combinations which are selectively used in order to supply a pulsed current waveform whose rising edge consists of a predetermined number of appropriate steps
In the case of the conventional drive circuit which outputs a drive current whose waveform is designed to rise and fall in the same length of time, making sharper the falling edge of the optical output of the semiconductor laser 2′ in order to improve the eye opening results inevitably in making the rising edge sharper too. Making the rising edge sharper enlarges the relaxation oscillation 21b (ringing) at the rising edge, which makes the eye opening of the eye pattern waveform smaller and the transmission distance shorter.
Conversely, making the rise time of the input 20 longer, i.e., making the rising edge more gradual (lowering the rate of change in the rise time), makes the fall time 21d of the optical output waveform longer and the falling edge more gradual although the ringing at the rising edge is reduced. This may cause interference with the adjacent data, which makes it impossible to realize a required transmission rate, for example, 10 G bps.
In addition, the ringing at the rising edge is dependent on the amount of rising (amplitude) as well as the rising sharpness (the rate of change in the rise time). This means that the ringing can also be decreased by dividing the rising edge into a predetermined number of steps of small amplitudes and imposing a settlement period after each rising step as described in the above-cited Japanese Patent Laid-open No. 2003-17800. The ringing due to the last rising step can be suppressed by this method.
In this method described in the above-cited Japanese Patent Laid-open No. 2003-17800, however, since each rising step of the input current is a rectangular waveform having a sharp rising edge, the respective steps cause ringing which is superimposed at the rising edge of the waveform of the optical output, resulting in deteriorating the eye opening. If the rising edge is divided more finely so as not to deteriorate the eye opening, it is inevitable to not only enlarge the scale of the driver circuit but also control the driver circuit at an unrealistic speed. For example, for 10 G bps transmission, since the rise time must be set to 20 to 30 ps, dividing the rise time into several steps makes it necessary to control the circuit within 10 ps.
In addition, the methods disclosed in Japanese Paten Laid-open No. 11-214781 and Japanese Patent Laid-open No. 2003-17800 have a common problem which also deteriorates the eye opening as described below.
Following a transient period (a portion 21b of the waveform) in which the optical intensity falls due to a ringing caused by the relaxation oscillation, the optical output waveform of the semiconductor laser 2′ rises gradually to the peak. In the prior art, this factor also causes the eye opening to deteriorate.
This invention extends the transmission distance of a directly modulated optical module by enlarging the eye opening. In particular, the present invention is intended to extend the transmission distance of an optical transmission module for 10 Gbps or higher rate transmission.