GB2181857A

GB2181857A - Optical phase control

Info

Publication number: GB2181857A
Application number: GB08525703A
Authority: GB
Inventors: Stephen Wright
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1985-10-18
Filing date: 1985-10-18
Publication date: 1987-04-29
Also published as: DE3634563A1; GB2181857B

Abstract

An intensity-modulated optical signal is input to a first fibre Mach-Zehnder interferometer (1) which is operated such as to control (2) the relative optical path lengths of its two arms whereby a predetermined power-split ratio is obtained between the optical signals coupled, by a 3 x 3 coupler (6), to a three-armed fibre Mach-Zehnder interferometer (7) whose arms provide 120 DEG differential delays at the modulation frequency. The signals output from the second interferometer (7) are summed incoherently, the summed optical signal having a modulation phase determined by the relative optical phases set by the first interferometer (1). 360 DEG control of the modulation phase is thus achieved and the magnitude of the resultant signal following demodulation of the resultant optical signal does not vary with the optical phase difference. Control (2) may be a piezo-electric fibre stretcher. The arrangement forms an optical vector modulator used in driving antenna elements of a phased-array radar system. <IMAGE>

Description

SPECIFICATION Phase control This invention relates to controlling the phase of intensity modulation of an optical signal, in particular an optical signal modulated by an RF or microwave signal.

In phased array radar systems there are a matrix of antenna elements which are driven at microwave frequencies with different phase relationships for steering of the antenna. If there are a large number of antenna elements there are practical problems associated with the separate waveguides required for all the different antenna elements. Optical beam steering phased array systems have been suggested since they require only a single microwave feed and a plurality of optical fibres. One arrangement requires that a light source is intensity modulated at microwave frequencies and its phase be electrically controllable. Various solutions have already been proposed, many requiring sophisticated integrated optic technology.

In our co-pending Application No. 8511425 (Serial No. ) (R.E. Epworth-S. Wright-T.

Bricheno 31-13-10) there is described a method and apparatus for such phase control which employs optical fibre rather than integrated optics. Two fibre Mach Zehnder interferometers are arranged in series with a 2x2 coupler disposed therebetween. An optical signal is modulated by the microwave signal, for example, and applied to the first interferometer which is operated such that the optical signals applied to the second interferometer have a predetermined power split ratio therebetween. The second interferometer is such that the signals applied thereby to an output coupler are in-phase and quadrature. The output of this output coupler is at an electrical phase determined by the predetermined power split ratio.This arrangement only provides phase control over not much more than 90 , whereas ideally full circle (360 ) control is desirable, particularly in the phase control of an RF CW signal in a PAR radar. Such a device or arrangement is termed a vector modulator.

According to one aspect of the present invention there is provided a method of controlling the phase of intensity modulation of an optical signal comprising the steps of applying the modulated optical signal to a first fibre Mach Zehnder interferometer which has two arms and is coupled to a second Mach Zehnder interferometer, which has three arms, by a 3 x 3 coupler, controlling the relative optical path lengths of the two arms of the first interferometer whereby to obtain a predetermined optical power split ratio between the optical signals applied to the three arms of the second interferometer, the second interferometer being such that the optical signals in its arms are subject to 1200, or multiples thereof, differential delays at the modulation frequency, and incoherently summing the signals output from the three arms, the summed optical signal having a modulation phase determined by the relative optical phases set by the first interferometer.

According to another aspect of the present invention there is provided apparatus for controlling the phase of intensity modulation of an optical signal comprising a first fibre Mach Zehnder interferometer, with two arms, to which the modulated optical signal is coupled, a second Mach Zehnder interferometer, with three arms, arranged in series with the first interferometer and coupled thereto by a 3x3 coupler, means for controlling the relative optical path lengths of the two arms of the first interferometer whereby to obtain a predetermined power split ratio between the optical signals applied to the three arms of the second interferometer, which three arms include means whereby there is a differential delay therebetween of 1200, or multiples thereof, at the modulation frequency, the apparatus further comprising means for incoherently summing the signals output from the three arms whereby to produce an optical signal having a modulation phase determined by the relative optical phases set by the first interferometer.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 illustrates schematically a vector modulator comprising two fibre Mach Zehnder interferometers coupled by a 3x3 coupler, and Figures 2 and 3 are vector diagrams.

The apparatus or vector modulator illustrated in Fig. 1 comprises a balanced path length optical fibre Mach Zehnder interferometer 1 with optical phase shift control 2 in one (as illustrated), or both, of the fibre arms 3 and 4. Optical power is input to one port of a coupler/combiner 5 by, for example, intensity modulating at RF frequencies the output of a light source (not shown), for example a semiconductor laser. At the output of interferometer 1 is a 3 x 3 coupler 6 whereby the two-armed interferometer 1 is coupled to a three armed unbalanced path length optical fibre Mach Zehnder interferometer 7, having a 3 x 3 output coupler 8.The three arms 9,10 and 11 of interferometer 7, that is the output fibres from the first interferometer 1 are set to have delays of 0, 0+27r/3 and 0+47r/3, or multiples thereof, respectively at the microwave (RF) frequency.

The optical phase shift control provided by control 2 is achieved by a corresponding change in optical path length of fibre arm 4. This may be provided by a conventional piezo-electric fibre stretcher, such as a PZT modulator formed, for example, by winding fibre around a PZT cylinder for controlling phase a large amount, or by glueing a fibre onto a PZT disc for controlling phase a small amount. Any method which results in stretching of the fibre in response to an electrical signal may be employed, another example being the use of magnetostrictive materials and bonding the fibre to a strip thereof.

The 3x3 coupler 6 may be made by twisting and fusing together three single mode optical fibres, and then pulling to adjust the coupling until optical power fed into one input fibre is equally distributed at the three outputs. A property of this type of coupler is the 1200 relative phasing of its outputs.

As the optical phase in arm 4 of the Mach Zehnder interferometer 1 is varied by control 2, the magnitude of the optical fields transmitted down each of the fibres 9,10 and 11, that is the power split ratio, will vary. See the vector diagram in Fig. 2. In other words, as the relative phases of the two inputs of 3 x 3 coupler 6 are varied by the control 2, the amplitudes of the three output waves are described by the projections of the three vectors phased optically at 1200. These waves are then transmitted through the fibres 9,10 and 11 of interferometer 7 with the aforementioned delays which are multiples of 1200 at the RF frequency. It is necessary that the polarisation states at coupler 6 be identified, otherwise this will limit the possible extinction.

To achieve this a respective polarisation controller 12 and 13 is. included in the fibre arms 3 and 4. If it is not possible to maintain extinction for any input state of polarisation, then an additional polarisation controller 14 may be employed at the input to pre-adjust the input state.

At the output coupler 8 the powers in the three signals are summed, summing the RF modulation terms at their relative phases. This process can easily be achieved if power addition (incoherent addition) takes place. The power transmitted down each of the three fibres 9,10 and 11 can be represented by the following expressions, namely: P (o) =cos26 P(2 n/3)=sin2 (30"+8) P(4 7r/3 =sin2 (300o) If the three signals are added incoherently at coupler 8 then the RF modulation envelopes will be summed vectorially with different relative phases because of the 2 z/3 relative delays in the three fibre arms 9,10 and 11. This simmation can be represented by the vector diagram of Fig.

3 showing the sum of three vectors mutually phased at 1200 with amplitudes as given above.

Summation can be carried out readily by resolving into in-phase and quadrature components a and b.

a =cos26- sin2(300 + O).sin300 - sin2(300 - 6).sin300 =cos26--21 .sin300( 1 -cos(600 + 20) + 1- cos(600 - 20)) = 21.(1 +cos20)- -41.(2-2.cos600cos20) =-.cos20 b = sin2(300 + 0).cos300-sin2(300 - 0) .cos300 = 21.cos300(1 - cos(60 +20)-1+cos(60-20)) =cos30 .sin60 .sin20 a4 sin20 Therefore, the magnitude A of the resultant RF signal is given by A= (a2+b2)t a That is, the magnitude of the output RF signal does not vary with the size of the optical phase difference or in other words there is no variations in the output signal amplitude with optical phase. The phase a of the RF signal is given by tan a=b/a=tan 20 thus a=20 In the case of the 2 x 2 Mach Zehnder system described in the aforementioned co-pending application the output RF signal varies with optical phase and RF phase and a third interferometer is required if the output amplitude is to be held constant.

The extra problem with the 3 x 3 coupler system that can be overcome by the 2 x 2 coupler approach is the incoherent addition. With the 2x2 approach the two outputs of the second interferometer can be maintained in orthogonal polarisations to provide power addition. This is clearly not possible in a case with three outputs. One way to maintain the condition is by limiting the source coherence so that the three outputs are mutually incoherent. Essentially this requires that the source line width is greater than the RF modulation frequency. However, a minimum coherence length of at least a few centimetres is required so that the first interferometer 1 can be balanced to within the source coherence length without great difficulty. A semiconductor laser with an number of longitudinal modes could meet both of these conditions.

Claims

1. A method of controlling the phase of intensity modulation of an optical signal comprising the steps of applying the modulated optical signal to a first fibre Mach Zehnder interferometer which has two arms and is coupled to a second Mach Zehnder interferometer, which has three arms, by a 3x3 coupler, controlling the relative optical path lengths of the two arms of the first interferometer whereby to obtain a predetermined power split ratio- between the optical signals applied to the three arms of the second interferometer, the second interferometer being such that the optical signals in its arms are subject to 1 20C, or multiples thereof, differential delays at the modulating frequency, and incoherently summing the signals output from the three arms, the summed optical signal having a modulation phase determined by the relative optical phases set by the first interferometer.

2. A method as claimed in claim 1, wherein controlling the relative optical path lengths of the two arms of the first interferometer comprises stretching of one of its fibre arms.

3. A method as claimed in claim 2, wherein the stretching is achieved by a PZT modulator.

4. A method as claimed in any one of the preceding claims wherein the optical signal is obtained from a source whose coherence is limited whereby the output signals from the three arms of the second interferometer are mutually incoherent.

5. Apparatus for controlling the phase of intensity modulation of an optical signal comprising a first fibre Mach Zehnder interferometer, with two arms, to which the modulated optical signal is coupled, a second Mach Zehnder interferometer, with three arms, arranged in series with the first interferometer and coupled thereto by a 3x3 coupler, means for controlling the relative optical path lengths of the two arms of the first interferometer whereby to obtain a predetermined power split ratio between the optical signals applied to the three arms of the second interferometer, which three arms include means whereby there is a differential delay therebetween of 120 , or multiples thereof, at the modulation frequency, the apparatus further comprising means for incoherently summing the signals output from the three arms whereby to produce an optical signal having a modulation phase determined by the relative optical phases set by the first interferometer.

6. Apparatus as claimed in claim 5 wherein the means for controlling the relative optical path lengths of the two arms of the first interferometer comprises means to stretch one of its fibre arms.

7. Apparatus as claimed in claim 6 wherein said stretching means comprises a PZT modulator.

8. Apparatus as claimed in any one of claims 5 to 7 wherein the 3x3 coupler comprises a 3x3 fibre coupler.

9. A method of controlling the phase of intensity modulation of an optical signal substantially as herein described with reference to the accompanying drawings.

10. Apparatus for controlling the phase of intensity modulation of an optical signal substantially as herein described with reference to the accompanying drawings.