AU609541B2 - A laser pulse shaper - Google Patents

Description

609541 UMQRiGa ocl )0 00 COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952-1969 COMPLETE SPECIFICATION FOR THE INVENTION ENTITLED "A LASER PULSE SHAPER" The following statement is a full description of this invention, including the best method of performing it known to us:j a

F

j ,r The present invention relates to a circuit arrangement for modulating an injection laser, comprising a predistorter which predistorts an electric signal containing rectangular pulses for generating an optical signal containing rectangular pulses.

A circuit arrangement of this kind is disclosed in German Patent 25 43 570. A "pulse-wave-modulating facility" is mentioned there which "varies the width and/or amplitude of the modulating pulse" (column 2, line 66, to column 3, °rLQ line 9).

During digital modulation, the following effects, which are undesirable for optical communication, may occur at high o 00 00°° 0 bit rates: 1. The optical output pulses of the laser have different amplitudes depending on the pulse sequence of the input signal (pattern effect). A 1-state output following a 0 0",000 state (no voltage pulse at the laser input) is smaller 0 othan if a 1 was modulated immediately beforehand.

.o o 2. The optical signal approaches the rectangular waveform 08 0 0 much less than the electric signal does. The trailing edge, in particular, is much less steep than that of the 3lectric signal.

3. Due to the phase relationship between photon density and electron density, the optical signal exhibits damped oscillations after the trailing edge.

4. This phenomenon entails an increase in the linewidth of the emitted radiation.

The circuit arrangement disclosed in the above German patent specification predistorts the modulating current.

However, the patent specification does not describe how each individual pulse must be predistorted in order for the laser to produce an optical pulse equivalent to the respective electric pulse.

It is desirable to provide a circuit arrangement which predistorts each electric pulse so as to compensate for distortions caused by the laser.

This specification discloses a method of predistorting a pulse signal comprising the steps of causing the pulse to "o o rise to its full amplitude via an intermediate amplitude, o 00 and causing a backswing in the amplitude of the trailing edge of the pulse.

There is also disclosed a circuit for performing the predistortion.

One of the advantages of the invention is that no discriminator for recognizing the bit sequence is needed, o 0 since each pulse is predistorted independently of the preceding pulse and since even very high pulse frequencies can 000 be processed.

o o A further advantage is that the other effects referred to above are also largely suppressed. The backswing added to the trailing edge of each pulse eliminates the pattern effect and provides sufficient edge steepness. By the preliminary step of the pulse, the damped oscillations and, thus, the increases in linewidth are suppressed.

An embodiment of the invention will now be explained with reference to the accompanying drawings, in which: Fig 1 shows the shape of' a predis tortoed po Iso l~n oomt parison with an input pulse, and Fig. 2 shows a predistorter circuiit emibodyinig Uthi-,r~ vention.

Fig. 1 shows the rectangular input pulse VE (above) and the predistorted pulse (below), consisting of a pre"Liminary step, a constant portion, and a backswing at the trailing edge.

Fig. 2 shows a field-effect transistor, In a ooinmon- ~lQ~source configuration, as is described, Cor examplo, 'in U.

Tietze, Ch. Schenk,, "H albleiter-Schltng3t;echnrlk" llerln, 8th edition, 1986, p. 89. The drain of the Vlold-e~f'ent transistor Fl. :Is connected via a resistor TID to tho poiltive Germninal of a voltage source, and the source of the Moeldeffect transistor is grounded. At t, e inpt end, a ignal Osource providing a voltage VE is connected In tir'ies wit~h the internal resistance ci. to the gate of' the Cede~c 0:00:transistor Fl.; at the output, a voltage VA Is takeon between 0:00 the drain terminal and ground.

A series combination of,' a waveguide TI., haiving a predetermined characteristic Impedance ZI and a prodetomined delay TI, and a reoistor RI in connected between the gate of the field-effect tranW3iitor F)1 and ground, wIth the waveguide having one end connected to the gate,, and the, other grounded through the res~istor Thia aeries combination is shunted by a resistor RG.

Connected to the oarain of the Pield-cff'eot tranoistor is a second waveguide Z2, T2 having a predetermlned characteristic impedance Z2 and a predetermIned delay T2. Thi1s waveguide has an open end at which the waveo are reClected.

The waveguides are preferably implemented with microstrip lines; coaxial lines are also suitable. The network formed by the waveguide Z1, Ti and the resistors Rl and RG deforms the input voltage VE. The gate of the field-effect transistor Fl is thus subjected to a voltage consisting of a component proportional to VE and a superposed component approximately proportional to the time derivative of VE. At that point, the pulse consists of an overshoot, a constant portion, and a backswing. The overshoot of the output volt- Sl"'d age VA is suppressed by suitable choice of the operating point of the field-effect transistor Fl and the stepping of the leading edge. The second waveguide Z2, T2 produces the preliminary step of the pulse. The output of the S predistorter thus provides a voltage VA as shown in the lower part of Fig. 1.

o o a 4 9

Claims (4)

1. A method of predistorting an electric pulse signal for modulating an injection laser, the method comprising the steps of applying an input electric pulse to pulse shaping circuit to cause the pulse to rise to its full amplitude via an intermediate amplitude step, the pulse shaping circuit also causing a backswing in the amplitude of the trailing edge of the pulse, whereby, when the electric pulse signal 0 is used to modulate an injection laser, a substantially rec- o" tangular laser pulse is produced. a 0

2. A method of predistorting an electric pulse signal for a modulating an injection laser, the method as herein de- S0 0 scribed with reference to the accompanying drawings. 0 0

3. A circuit arrangement for modulating an injection la- ser, comprising a predistorter which predistorts an electric 0 0 signal containing rectangular pulses for generating subtantially rectangular laser pulses, wherein the predistorter causes each pulse to rise to its full amplitude in two steps and produces a backswing at the trailing pulse S edge, wherein the predistorted electric signals are applied 0~ to the injection laser to produce substantially rectangular 0 00 laser pulses.

4. A circuit arrangement as claimed in claim 3, wherein Sthe predistorter includes a field-effect transistor in a common-source configuration, the gate of the field-effect transistor is grounded through a first waveguide having a first predetermined characteristic impedance and a first 6 *with a first resistor, wherein the gate is grounded through RA41 a second resistor in parallel with the first waveguide and the first resistor and wherein the drain of the field-effect transistor is connected to a second waveguide having a sec- ond predetermined characteristic impedance and a second pre- determined delay and whose distal end is open. A pulse predistorter for generating an electric pulse signal to modulate an injection laser, the distorter com- prising a pulse shaping circuit as herein described with reference to the accompanying drawings. 0 0 0 0 0 o oQ 0 0 00 0 0 0 00 0 0 0 I SDATED THIS TWENTY-FOURTH DAY OF JANUARY, 1991 I o"o I ALCATEL N.V. 0 0 0 00 o 00 0 o 0 o 'O /^di