AU683943B2

AU683943B2 - Optical transmitter

Info

Publication number: AU683943B2
Application number: AU79048/94A
Authority: AU
Inventors: Rainer Fritschi; Heinz Krimmel; Jurgen Otterbach
Original assignee: Alcatel NV
Current assignee: Alcatel Lucent NV
Priority date: 1993-12-10
Filing date: 1994-11-28
Publication date: 1997-11-27
Anticipated expiration: 2014-11-28
Also published as: NZ264960A; ES2189796T3; EP0658016B1; DE4342184A1; DE59410251D1; AU7904894A; EP0658016A1

Description

P/00/011 28/5/91 Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT .c Invention Title: "OPTICAL TRANSMITTER" The following statement is a full description of this invention, including the best method of performing it known to us:- S Sc S

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CO 1 9658 2 8 NIOV 94

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The invention relates to an optical transmitter for generating a modulated optical output signal, having an electro-optic modulator with a non-linear characteristic.

Such a transmitter is known from: International Cable; July 1992; pages 42-50, "External modulation system for AM fibre CATV transport, Part 2", particularly page 46, Figure 7. Such a transmitter finds a use in analogue amplitude-modulated optical communication transmission systems, particularly in cable TV systems. The optical transmitter has a laser light source, an external electro-optic modulator, a lineariser, as well as means for the detection of intermodulation products. The external modulator is, for example, an integrated optical modulator such as is known from IEEE Journal of Quantum Electronics, Volume 27, No. 3, 1991, pages 654-667, "High-Speed IIIl-V Semiconductor Intensity Modulators".

Figure 1 of the second literature reference shows, schematically, a Mach- Zehnder modulator with a first terminal for the modulating electrical signal (RF) and a second terminal to which a terminating resistor, here 50 Ohms, is connected.

The characteristic of an external electro-optic modulator is non-linear and, in particular, the characteristic of a Mach- Zehnder modulator has the form of a sine wave (see the first literature reference, page 44, right column).

The first reference shows that the electrical signal is first processed through a lineariser, before it is applied to the Mach-Zehnder modulator as a modulating electrical signal. Such a lineariser has the task of compensating the non-linear characteristic of the Mach-Zehnder modulator in such a way that the characteristic of the combination of lineariser and Mach-Zehnder modulator becomes linear. The lineariser is also frequently referred to as an electronic predistorter or compensator.

The non-linear characteristic of the modulator can be variable, e.g.

because of component ageing. If this non-linear characteristic is not compensated, or only partly compensated, disturbances will appear in the modulated optical output signal such as, for example, distortion and intermodulation products.

Like a function, a characteristic can be separated into an even and an odd component. These two components each generate characteristic intermodulation products which, are referred to as even, e.g. second order, or odd, e.g. third order, intermodulation.

The first literature reference discloses an optical transmitter which has means for measuring these intermodulation products. These measurement means additionally contain means for control of the lineariser and the operating point of the Mach-Zehnder modulator.

A problem of the known optical transmitter is that the lineariser is inserted between the output amplifier and the optical modulator, and thus leads to a power loss of the modulating electrical signal.

The task of the invention is to describe an optical transmitter of the type mentioned earlier for which the problem of the modulator non-linearity is solved without introducing any additional power loss. This task is solved as Jescribed in Claim 1. Further developments are described in the subsidiary claims.

The invention is founded on the basic idea to integrate a compensating S network into the already existing terminating resistor of the electro-optic modulator. An advantage of the invention is that it makes it possible to earth one side of the diodes used in the compensation network. Thus parasitic capacitances of these components can be reduced. Furthermore, this invention permits the control of the compensation network to be referenced to earth.

The invention will now be described by means of examples, with the aid of the diagrams, in which: Figure 1 shows a first design for the optical transmitter according to "2 the invention SFigure 2 shows a design for the termination network Figure 3 shows a second design for the optical transmitter anfvrding to the invention Figure 4 shows a first design for the termination network 15 for the optical transmitter of Figure 3 Figure 5 shows a second design for the termination network 15 for the optical transmitter of Figure 3 The optical transmitter can be described with the aid of Figure 1 as follows: The optical transmitter has a laser 9, an output amplifier 10, an electrooptic modulator 12, referred to as the modulator in the following, and a termination network 15. The modulator 12 has three electrical terminals. They are terminal 14 by which the operatin, point is set, as well as an input and output terminal.

The output amplifier 10 is connected to the electrical input of the modulator 12. The electrical output of the modulator 12 is connected via the termination network 15 to a reference potential which here, for example, is earth.

The modulator 12 also has an optical input to which the output light 13 from the laser 9 is applied, and an optical output at which an optical output signal 17 appears.

The output amplifier 10 amplifies an electrical signal, for example a TV signal. The amplified electrical signal 11 is applied to the modulator 12 which modulates the output light 13 of the laser 9 to produce an amplitude-modulated output signal 17. The electrical signal 11 is connected to the reference potential 16 via the termination network The termination network 15 has two functions. First, it provides an electrical termination for the modulator 12, normally 50 Ohms. Second, it compensates the non-linear characteristic of the modulator 12.

Figure 2 shows a termination network 15 which can be used in the optical transmitter of Figure 1. This termination network 15 has a non-linear characteristic which compensates for odd-order intermodulation products of the S optical outout signal 17.

The termination network 15 of Figure 2 consists of a series combination of a first adjustable resistor 31 and a second adjustable resistor 32 whose second terminal is connected to the reference potential 16. Two non-linear networks 33, 34 are connected in parallel with the second adjustable resistor 32, and with each other. These are preferably two diode networks, connected in opposite polarity, each of which consists of one diode, or several diodes

I-

36a, 36b connected in series. These diodes can, for example, be Schottky diodes. Other non-linear components, such as bipolar or field-effect transisto, can also be used for the non-linear networks 33, 34.

If the modulating electrical signal has such a small amplitude that the intermodulation products of the optical output signal of the modulator are negligible, then the termination network 15 has a constant total resistance which, for example, equals the 50 Ohm value mentionea earlier.

With increasing amplitude of the modulating electrical signal, due to the non-linear characteristic of the modulator the disturbances of the optical signal such as, for example, distortion and intermodulation, will increase. At a sufficiently large amplitude of the electrical signal, the voltage drop across the second adjustable resistor 32 grows so large that the non-linear networks 33, 34 become activated. The second adjustable resistor 32 should be set so that the disturbance of the optical signal is a Ininimum. In the preferred design of the non-linear networks 33, 34, the voltage drop across the second adjustable resistor 32 is equal to the threshold voltage of the diodes 35a, 35b, 36a, 36b.

If this threshold voltage, approximately 0.3 V for Schottky diodes, is exceeded, the total resistance of the termination network 15 is reduced in a non-linear manner and the non-linear characteristic of the modulator is compensated in this way.

Figure 3 shows a second design of the optical transmitter according to S the invention. This optical transmitter has the output amplifier 10 and the modulator 12, as for the first version of Figure 1. Furthermore, the optical transmitter according to Figure 3 has a termination network 15 which differs from that of the first version of Figure 2, as will be discussed later with the aid S of Figure 4 and Figure The first version of Figure 1 is augmented by a measuring unit 21 for the measurement of the intermodulation products of the modulated optical signal 17, and by a control unit 22. The output amplifier 10 is connected to the electrical input of the modulator 12. The electrical output of the modulator 12 is connected via the termination network 15 to a reference potential 16 which here, for example, is earth. The control unit 22 is connected to the terminal 14 6 for setting the operating point of the optical modulator 12, as well as being connected to the termination network The optical output signal 17 passes through the means 21 for measuring the intermodulation products. The measuring unit 21 determines the size and type of these intermodulation products and provides corresponding signals to the control unit 22. Techniques and devices for determining the distortion and intermodulation products of the modulated optical signal 17 are known, for example from the first literature reference, and are not the subject of this invention. The control unit 22 controls the operating point of the modulator 12 when even- order intermodulation products are present, and the termination network 15 of Figure 4 (described in more detail below) when odd-order intermodulation products are present, in such a way that the occurring intermodulation products are minimised. The advantage of this design version is that the modulator can always be adjusted optimally.

Figure 4 shows a first design version of the controllable termination network 15, which could be used in the optical transmitter of Figure 3. Its construction and function differ from those of the first design version of the termination network 15 of Figure 2 as follows: It has additionally a coupling capacitor 43, and inductor 42 and a control terminal 41. The coupling capacitor is connected in series with the first adjustable resistor 31 and separates the modulator 12 from the termination S network 15 as far as direct current is concerned.

The inductor 42 connects the termination network 15 with the control terminal 41, to which is applied a control voltage produced by the control unit "2 22. The other side of the indllctor connects to the non-linear networks 33, 34 and transfers the control voltage to them.

In the present version of the non-linear networks 33, 34 containing diodes 35a, 36a, or a series combination of diodes 35a, 35b, 36a, 36b, the inductor is connected to the junction of the first adjustable resistor 31 and the second adjustable resistor 32. The control voltage acts as a bias voltage for the diodes 35a, 36a, or for the series combination of diodes 35a, 35b, 36a, 36b.

If the non-linear networks 33, 34 are implemented by means of a diode rsa~sll~B 36a, or by a series combination of diodes 35a, 35b, 36a, 36b, then for a positive control voltage the diode 35a (or the series combination 35a, becomes conducting for lower voltages, and the diode 36a (or the series combination 36a, 36b) only becomes conducting for numerically greater negative voltages, than is the case with the termination network of Figure 2.

Other versions of the means of coupling to 41, 42, 43 are also possible.

For example, the control voltage can also be coupled in between the optical modulator and the termination network 15. Also, the coupling capacitor 43 can be omitted, since the modulator has a network for coupling in the operating point voltage which, with a coupling capacitor fitted between the optical modulator and the termination network 15, separates these two units as far as direct current is concerned.

Figure 5 shows a second version of the termination network 15 for the optical transmitter according to Figure 3. This second version of the termination S network 15 differs from the version of Figure 2 as follows. Each of the two o non-linear networks 33, 34 has a separate network for coupling in a separate control signal. The coupling networks have at least one coupling capacitor 56 and one inductor 53, 54. Each of the coupling capacitors 55, 56 is connected in series with one non-linear network 33, 34 and thus, as far as •2tP direct current is concerned, separates the non-linear networks from each other and from the modulator. One of each of the inductors 53, 54 is connected to the junction of the coupling capacitor 55, 56 and the non-linear network 33, 34 with the other end of the inductor forming the corresponding control terminal 51, 52. This version permits separate control of the two non-linear networks "2 33, 34. In this way, control becomes possible for minimising the even-order as S well as the odd-order inter- modulation products. Control of the operating point via the terminal 14 (for setting the operating point of the modulator), as mentioned in the description for Figure 3, can therefore be omitted.

In the design examples the invention is implemented with discrete components, but it can also be wholly or partly integrated. This has the advantage that interconnections with their parasitic effects are minimised.

Claims

1. Azi optical transmitter for generating a modulated optical output signal, having an electro-optic modulator with a non-linear characteristic, said transmitter including a termination network which connects the electro-optic modulator to a reference potential, and that the termination network compensates the non-linear characteristic of the electro-optic modulator, wherein the termination network is a non-linear network which comprises a series combination of a first resistor and a second resistor connected to the reference potential, and which comprises two non-linear networks connected in paralle' with each other and with the second resistor.

2. An optical transmitter as claimed in Claim 1, including an intermodulation-measuring arrangement for measuring the intermodulation products of the modulated optical output signal, and a control unit for controlling the termination network.

3. An optical transmitter as claimed in Claim 2, wherein the termination network is controlled by the control unit in such a way that the intermodulation proa'icts of the modulated optical signal are minimised.

4. An optical transmitter as claimed in Claim 3, wherein the control unit is connected to a terminal for setting the operating point of the electro-optic 20 modulator, where its operating point is so controlled by the control unit that the even-order intermodulation is minimised, and where the control unit controls the termination network in such a way that the odd-order intermodulation is minimised. i

5. An optical transmitter as claimed in any one of Claims 1-4, wherein the 25 two parallel-connected non-linear networks consist of two diode networks connected in opposite polarity, each of which contains a diode or diodes connected in series.

6. An optical transmitter as claimed in Claim 5, wherein the termination network has means for applying a separate control voltage to each of the two diode networks.

A7. An optical transmitter as claimed in Claim 5, wherein the termination ''v network has means for applying a control voltage to both of the diode networks.

8. An optical transmitter as claimed in any one of the preceding claims, wherein the electro-optic modulator and the termination network are combined together into one unit.

9. An optical transmitter substantially as herein described, with reference to Figures 1 5 of the accompanying drawings. DATED THIS TWENTY-EIGHTH DAY OF AUGUST 1997 ALCATEL N.V ,*re *o I m ABSTRACT The optical transmitter has a laser light source and an electro-optic modulator (12) which has a non-linear characteristic. The modulator e.g. a Mach-Zehnder modulator, receives its electrical input signal (11) direct from the output amplifier The modulator (12) is connected to a non-linear termination network (15) which simultaneously forms the l r.',al terminating resistor and compensates the non-linear characteristic o a' o-optic modulator (12). (Figure 1) goes C. *O IC* i