NZ264960A

NZ264960A - Optical transmitter with termination network to correct non-linearity of electro-optic modulator

Info

Publication number: NZ264960A
Application number: NZ264960A
Authority: NZ
Inventors: Heinz Krimmel; Rainer Fritschi; Jurgen Otterbach
Original assignee: Alcatel Australia
Priority date: 1993-12-10
Filing date: 1994-11-18
Publication date: 1996-11-26
Also published as: AU7904894A; ES2189796T3; EP0658016B1; DE4342184A1; AU683943B2; EP0658016A1; DE59410251D1

Description

<div class="application article clearfix" id="description"> 264960 ■ci3 <j. J;ion Fjjftd: G.Q3r.P.lte.to.C.^7./.!Si. jHP«A'D/.C^;...ri.Q.2>^//.O.fe I Publication l>at a: N.? .)L !§!?§' i ( P.O. Journal No: A^A^L. NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION "OPTICAL TRANSMITTER" WE, ALCATEL AUSTRALIA LIMITED, Ot>D »oS 3^ A Company of the State of New South Wales, of 280 Botany Road, Alexandria, New South Wales, 2015, Australia, hereby declare the invention for which we pray that a patent may be granter4 to us, and the method by which it is to be performed, to be particularly described in and by the following statement: 2649 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 lll-V Semiconductor Intensity Modulators". Figure 1 of the second literature reference shows, schematically, a Mach-Zehnder modulator with a first terminal for *he 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 2 2 © 4 $ 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 pre-distorter 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 op^'cal 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 26 4 9 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 described in Claim 1. Further developments are described in the subsidiary claims. The invention is founded on the basic idea to integrate a compensating 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 the invention Figure 2 shows a design for the termination network 15 Figure 3 shows a second design for the optical transmitter according 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 electro-optic 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 operating 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 15. 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. 26 4 8 S 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 optical output 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 35a, 35b, 36a, 36b connected in series. These diodes can, for example, be Schottky diodes. Other non-linear components, such as bipolar or field-effect transistors can also be used for the non-linear networks 33, 34. If the modulating electrical signal has such -i 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 mentioned 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 26 4 9 6 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 minimum. 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 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 of Figure 4 and Figure 5. 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 for setting the operating point of the optical 26 4 9 § 3 modulator 12, as well as being connected to the termination network 15. 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 network 15 as far as direct current is concerned. 8 6 4 9 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 22. The other side of the inductor 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 35a, 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, 35b) becomes conducting for lower voltages, and the diode 36a (or the series combination 36a, 36b) only becomes conducting 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 26 4 § 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 network 15 differs from the version of Figure 2 as follows. Each of the two 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 55, 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 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 nonlinear networks 33, 34. In this way, control becomes possible for minimising the even-order as 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. </div>

Claims

<div class="application article clearfix printTableText" id="claims"> 264 What we claim is:

1. An 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.

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 products 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 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.

5. An optical transmitter as claimed in Claim 1 or Claim 2, wherein the termination network is a non-linear network which contains a series combination of a first resistor and a second resistor connected to the 11 26 4 9 reference potential, and which contains two non-linear networks connected in parallel with each other and with the second resistor.

6. An optical transmitter as claimed in Claim 5, wherein the two parallel-connected non-linear networks consist of two diode networks connected in 5 opposite polarity, each of which contains a diode or diodes connected in series.

7. An optical transmitter as claimed in Claim 6, wherein the termination network has means for applying a separate control voltage to each of the two diode networks. 10

8. An optical transmitter as claimed in Claim 6, wherein the termination network has means for applying a control voltage to both of the diode networks. ft ^

9. An optical transmitter as claimed in any one of the preceding claims, wherein the electro-optic modulator and the termination network are 15 combined together into one unit. £

10. An optical transmitter substantially as herein described, with reference to Figures 1 - 5 of the accompanying drawings. ALCATEL AUSTRALIA LIMITED 20 B. O'Connor Authorized Agent P5/1/1703 </div>