AU670320B2 - A reduced distortion optical transmitter - Google Patents

Description

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AUSTRALIA

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ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: "A REDUCED DISTORTION OPTICAL TRANSMITTER" The following statement is a full description of this invention, including the best method of performing it known to us:- 00 00 0 0 0 0060 0 0 0@ 0 *@00 0 *900 000000 9 0 00000@ 0 2 The invention relates to an optical transmitter in which a laser generated carrier is modulated with a data signal and a dispersion signal.

It is known from G.E. Bodeep et al: "Semiconductor Lasers versus External Modulators: A Comparison of Nonlinear Distortion for Lightwave Subcarrier CATV Applications", IEEE Photonics Technology Letters, Vol 1, No 11, Nov. 1989, pages 401 to 403, that the laser of such a transmitter can be amplitude modulated by two different methods of modulation, which both have disadvantages.

With the first method, direct modulation, the laser is modulated electrically with the data signal. In this case, an undesired frequency modulation occurs in addition to the desired amplitude modulation. This effect is generally termed frequency chirping. The chirp broadens the intrinsic line width of the o° laser and causes distortion of the transmitted signal due to fibre dispersion, even for transmission paths of a few kilometres.

With the second method, indirect modulation, the carrier radiated by the go• laser is modulated with the data signal optically, in an amplitude modulator 6• following the laser. Here the laser radiation is constant, thus avoiding frequency chirping. The maximum power that can be transmitted by a transmitter operating according to this method is, however, limited by Brillouin scattering in the fibre 20 transmission path connected to the optical transmitter. The effect of Brillouin scattering already becomes noticeable with about 10 mw of transmitter power, S. This Brillouin scattering effect increases with the spectral power density and the length of the transmission path optical fibre, and likewise leads to an increase of a. B.

noise and distortion in the transmitted signal.

In Cotter et al: "Supp ession of Stimulated Brillouin Scattering during Transmission of High-Power Narrowband Laser Light in Monomode Fibre", Electronics Letters, 22 July 1982, Vol. 18, No. 15, pages 638 to 640, it was shown that the Brillouin scattering can be considerably reduced or, in an ideal situation, completely suppressed by a continuous phase or frequency variation of an optical amplitude modulated signal. To achieve this, it is mentioned that a supplementary optical phase modulator should be inserted into the optical path of the transmitter, or that an additional laser should be used. In the experiments on which the above-mentioned source is based, a solid-state laser is used which radiates in two modes which are separated by 270 MHz. However, this is only useful at the dispersion minimum of the fibre.

The object of the invention is to provide a technically simple method of avoiding the disadvantages described earlier of the two modulation methods.

This specification discloses an optical transmitter in which the laser is directly modulated with a dispersion signal.

Advantageous design variations are given in the subsidiary claims.

For better understanding of the invention and its advantages, design examples are described in the following, with the aid of the diagrams 1 to Fig. 1 shows a first design example of an optical transmitter in accordance with the invention, Fig. 2 shows an advantageous second design example with a signal S independent of the data signal, for direct modulation of the laser, Fig. 3 shows an advantageous third design example in which a signal 1, 5 derived from the data signal is used for the direct modulation of the laser, Fig. 4 shows a fourth design example with a transmitter module according to the invention, consisting of a laser which can be directly modulated and an optical amplifier, and Fig. 5 shows the static input-output load line characteristic of an optical amplifier.

A first design example shown in Fig. 1 contains an optical transmitter 1 9*e9 with a laser 2 which can be directly modulated, and an optical amplitude modulator 3. The optical input 3a of the optical amplitude modulator 3 is *4e connected to the optical output 2b of the laser 2. A dispersion signal V is superimposed on a bias current VS and applied to the signal input 2a of the laser 2 as a modulation signal. This first lead is also referred to in the following as the direct modulation path of laser 2. On a second lead a data signal N, which is amplified by a power driver amplifier 4, is applied to the electrical input 3c of the amplitude modulator 3 as a modulation signal for indirect modulation. This second lead is referred to in the following as the indirect modulation path of the laser 2. The signal which appears at the optical output 3b of the amplitude modulator 3 is the output signal A of the optical transmitter 1.

The laser 2 is lightly modulated with the dispersion signal V. This causes a small blurring of the line width of laser 2. Blurring means that continuous wavelength variations of laser 2 are occurring. These must be so small that their effect on the transmitted signal due to dispersion is kept low. The wavelength variations are, for example, in the order of 4 ppm/mA. In the simplest case, the dispersion signal V could be a clock signal generated by a crystal oscillator, or even just noise. The indirect modulation with the data signal N which carries the information takes place optically in the amplitude modulator 3. The indirect modulation of the carrier emitted by the laser 2, which is also frequency modulated by the weak direct modulation, results in the Brillouin scattering only causing noticeable deterioration, because of an increase in the distortion and noise of the transmitted signal, for larger transmitted powers or longer optical transmission paths.

S."o The laser 2 that is used must be, at least to a small degree, capable of being angle modulated, i.e. frequency or phase modulated, A semiconductor laser is especially suited for this.

Apart from its amplitude modulation, the amplitude modulator 3 should produce as li-tle additional phase or frequency modulation as possible. In el•• particular, a Mach-Zehnder (MZ) interferometer is especially suitable.

In the design example, the data signal N is an analogue wideband signal 20 such as is used, for example, in CATV arrangements. But it can also be an S. *S analogue narrowband signal or a digital signal.

eO• Fig. 2 shows an advantageous second design example of the invention. In the description, and in Fig. 2, the same reference characters are always used for 0ere ~the same or equivalent components.

The second design example achieves the same tasks, with similar a components to the first design example. However, the dispersion signal V is also applied to a first predistortion circuit 7 whose output signal AK is combined with the data signal N and the resultant signal RS is applied to a second predistortion circuit 8. This second predistortion circuit 8 is in the indirect modulation path of laser 2, so that its output 8a is connected via the power driver amplifier 4 to the electrical input 3c of the amplitude modulator 3.

After the optical output 3b of the amplitude modulator 3, a first optical amplifier 5 is connected, whose output signal is the output signal A of the optical transmitter 1.

Because of the direct modulation of laser 2 with the dispersion signal V, an undesired amplitude modulation results in addition to the desired frequency modulation, so that the information contained in the output signal A is corrupted, compared to the data signal N, The first predistortion circuit 7 is used to compensate for this additional amplitude modulation. It generates a suitable signal which compensates for the undesired direct amplitude modulation, via the indirect modulation of the carrier. To achieve a better compensation, it may be necessary to insert an additional predistortion circuit, not shown in Fig. 2, into the direct modulation path of laser 2.

The load line of a Mach-Zehnder interferometer, as used for an amplitude modulator 3, has an undesirable nonlinearity which leads to a corrupted indirect S modulation and is caused predominantly by 2nd and 3rd order distortion. If an ,o operating point is chosen for the amplitude modulator 3 at a point of inflection 15 of the load line, the 2nd order distortion is approximately zero. In order to 4.o compensate for 3rd order distortion, the second prediscortion circuit 8 is inserted into the arrangement. In addition to the method depicted in Fig. 2, it is also possible to position this second predistortion circuit 8 in the direct modulation path of laser 2. The design of the second predistortion circuit 8 must then 20 naturally be different.

0S *S S• Beside compensating for the 3rd order distortion of the amplitude 4.

S modulator 3, the second predistortion circuit 8 can also be used to compensate for nonlinearities of other optical components of the transmission system. These o04 can be within the optical transmitter 1, e.g. the laser 2 or the first optical amplifier 5, as well as outside of the optical transmitter 1, e.g. in the optical fibre of the transmission path.

Furthermore, the electrical modulation voltage required by the Mach- Zehnder interferometer for modulating the optical laser signal is quite high (approximately 6 to 12 V peak-to-peak), so that the data signal N is amplified to the required level by a power driver amplifier 4 before it is applied to the amplitude modulator 3, The first optical amplifier 5 is connected after the optical output 3b of the amplitude modulator 3 in order to be able to compensate, in the operating point 6 of the amplitude modulator 3, for the disadvantage of the lower output of the semiconductor laser compared to a solid-body laser, and of the insertion loss due to inserting the modulator. The optical amplifier produces an output signal A of the optical transmitter 1 with a high optical power and can, for example, be an Erbium-doped fibre optic amplifier (EDFA), or a differently doped fibre optic amplifier.

For clarity, additional driver amplifier components, and components for propagation time compensation, are not shown.

Fig. 3 shows a further, especially advantageous, third design example of the invention. Again the same reference characters are used for identical or equivalent components.

The third design example achieves the same tasks with similar S components to the previous design examples. The data signal N is applied to a splitter 10 which applies it via the power driver amplifier 4 to the electrical input :4 15 3c of the amplitude modulator 3, and, with reduced amplitude, also to a third 4** predistortion circuit 6 whose function corresponds to that of the second predistortion circuit 8 of design example 2. The output signal from the predistortion circuit 6 is here the dispersion signal V, which has been derived from the data signal N. As in the second design example, the first optical 20 amplifier 5 is connected after the output terminal 3b of the amplitude modulator .9 3. As already mentioned earlier, this amplifier has the function of amplifying the O laser output power to a higher level. Additional propagation-time or amplifying *components are not shown in Fig. 3.

The third predistortion circuit 6 produces a signal for compensating for the nonlinearity of the Icad characteristic of the amplitude modulator 3, so that the amplitude modulation produced by the direct modulation is effectively utilised, in contrast to the second design example. A further advantage of locating the p,'edistortion circuit 6 in the direct modulation path of the laser 2 lies in relieving the load on the power driver amplifier 4, on which heavy demands are placed in this application. It must cope with severe requirements for 2nd and 3rd order linearity, and must simultaneously provide the relatively high modulation voltage for the amplitude modulator 3. Since it does not here have to deliver the correction power needed for compensating for the nonlinearity of the load 7 characteristic of the amplitude modulator 3, it can be designed for less severe requirements, or it can be more lightly driven and provide better signal linearity.

The direct modulation with dispersion signal V therefore simultaneously achieves two separate goals in this case, which can be simply and effectively implemented with the arrangement according to the invention of this design example.

In order to achieve better compensation in this case also, it can be advisable to insert predistortion circuits both in the direct modulation path and in the indirect modulation path of laser 2 which, however, are not shown in Fig. 3.

Fig. 4 shows a further advantageous design example according to the invention. Again, the same reference characters are used to denote components which are the same, or equivalent.

.oo• The fourth design example achieves the same tasks with similar S components to the previous design examples. The dispersion signal V for direct modulation is combined with the bias current VS and is here applied to a transmitter module 11, consisting of the laser 2 and a second optical amplifier 9.

Such a transmitter module 11 can be a generally available module with these *ego components, and can also be used for other optical transmitters than the ones described in the previously mentioned design examples. Here the second optical 20 amplifier 9 is a fibre optic amplifier which, among other items, contains a light-

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S amplifying length of fibre doped with Er 3 ions and a pump light source for exciting the Er 3 ions. It can also be an optical semiconductor amplifier, or an optical amplifier of some other design which can be operated in the saturation mode. The second optical amplifier 9 is connected after the optical output of the laser 2, and in the design example is inserted into the optical path between the laser 2 and the amplitude modulator 3, so that its input 9a is connected to the optical output 2b of laser 2, and its output 9b is connected to the optical input 3a of the amplitude modulator 3.

As shown in this design example, using the transmitter module 11 in the optical transmitter 1, in accordance with the invention, provides a simple method of suppressing the undesired amplitude modulation caused by the direct modulation of the laser 2. The second optical amplifier 9 is operated in the saturation mode, i.e. an operating point AP in the saturation region is chosen on '-e its static output-input load line of Fig. 5, so that small input variations cause no changes in the output power of the second optical amplifier 9. The pump power of its pump light source is then limited. The operating point AP is therefore set according to this requirement by the setting of the pump light source and, via the bias current VS, by the selection of the power of the input signal to the optical amplifier 9. Static behaviour of the second optical amplifier 9 occurs when the frequency of the dispersion signal V, and therefore the frequency of the input power variations of the second optical amplifier 9, are chosen to be much smaller than the reciprocal of the mean lifetime of the energy states of the light-amplifying substance of the second optical amplifier 9, which is excited by the light of the pump light source; in the case of an Erbium-doped fibre optic amplifier (EDFA), the frequency must therefore lie below 1 MHz. In order to Sachieve a good saturation condition of the second optical amplifier 9, the frequency of the dispersion signal V should be as small as possible. On the other 15 hand, it is not possible to make it arbitrarily small, since the frequency and amplitude of the dispersion signal V determine the frequency chirping behaviour.

•o Thus the smaller the chosen frequency is, the bigger the amplitude has to be, in order to guarantee the necessary frequency chirping behaviour for reduction of the Brillouin scattering. Admittedly, the bigger the amplitude is, the worse the 20 saturation behaviour of the second optical amplifier 9 becomes. Frequencies in SG @0 the range of a few kilohertz have been found to be particularly advantageous o within the limitations mentioned above.

The frequency modulation produced because of the frequency chirps *000 remains unaffected by the introduction of the second optical amplifier 9. In an ideal situation, the output signal S of the transmitter module 11 thus represents a purely frequency modulated signal, which is best suited to the reduction of the Brillouin scattering occurring in the fibre of the transmission path.

Claims (12)

1. An optical transmitter for a communication system in which the carrier generated by a laser is modulated with a data signal and a dispersion signal,wherein the carrier is modulated in the laser directly with the dispersion signal, wherein the carrier is modulated with the data signal in an optical amplitude modulator, and wherein the dispersion signal is applied to compensation means for producing a compensating signal for compensating the nonliunearity of at least one optical component.

2. An optical transmitter in accordance with Claim 1, wherein the dispersion signal is a signal derived from the data signal.

3. An optical transmitter in accordance with Claim 2, wherein the dispersion signal is a signal derived from the data signal via a predistortion circuit in order to compensate the nonlinearity of the amplitude modulator.

4. An optical transmitter in accordance with Claim 1 or Claim 2, wherein the laser is a semiconductor laser and that an optical amplifier is connected after the amplitude modulator.

An optical transmitter in accordance with any one of claims 1 to 4, i.:o wherein the amplitude modulator is a Mach-Zehnder interferometer.

6. An optical transmitter in accordance with any one of Claims 1 to 20 wherein the dispersion signal is additionally applied to a predistortion circuit o which serves to compensate the amplitude modulation occurring in direct modulation and whose output signal, combined with the data signal, is applied to the amplitude modulator.

7. An optical transmitter in accordance with any one of claims 1 to 6, 25 wherein the data signal is an analogue broadband signal.

8. A transmitter module for an optical transmitter for a communication system with a laser, wherein the carrier generated by the laser is modulated directly in the laser with a dispersion signal and is amplified by an optical amplifier located in a transmitter module in such a way that the amplitude variations of the output signal from the transmitter modulo are suppressed.

9. A transmitter module in accordance with Claim 8, wherein the frequency of the dispersion signal is smaller than the reciprocal of the average lifetime of the energy states of the light-amplifying substance in the optical amplifier which are excited by the pump light produced by a pump source intograted into optical amplifier.

A transmitter module in accordance with Claim 8 or Claim 9, wherein the optical amplifier is operated at an operating point which lies in the saturation region of its static output-input load line.

11. A transmitter module in accordance with any one of claims 8 to wherein the optical amplifier is a fibre optic amplifier.

12. An optical transmitter substantially as herein described with reference to the accompanying drawings. DATED THIS ELEVENTH DAY OF APRIL 1996 ALCATEL N.V 9* 000 0** 0* 0 0 -x ii ABSTRACT Distortion in optical transmissions caused by Brillouin scattering can be reduced by applying a frequency jitter signal to the laser signal. This invention provides an arrangement for directly applying the jitter signal to the laser together with the laser bias signal Additionally, the jitter signal can be used to compensate for non-lineanties in the transmission system. Figure 1. a a a ao *o o a C a