GB2131567A - Integrated optic arrangement - Google Patents

Abstract

An optical signal with optical frequency fc is applied to an electro- optic device (3) having drive electronics (2) employing sinusoidal electrical signals of frequency f0 and 2f0. The output of the device (3) is frequency upshifted to (fc + fo) or downshifted to (fc - fo) in dependence on electrical phases. The electro-optic device (3) includes three waveguide arms (6, 9, 12; 6, 10, 12; 7) of substantially identical optical length; EA and EB include sinusoidal signals of frequency f0, substantially in quadrature, whereas EC includes a sinusoidal signal of frequency 2f0. The basic device comprises an optical single sideband modulator, and it can be employed in a frequency division multiplexing optical communication system (Figs. 13 and 14). By adding a fourth (reference) waveguide arm (Fig. 6 or 7) it can be employed in an optical phase shifter (Fig. 10) or an optical phase or frequency modulator communication system (Fig. 11 or Fig. 12). <IMAGE>

Description

SPECIFICATION Integrated optic arrangement This invention relates to an integrated optic arrangement usable, inter alia, for optical frequency shifting of optical signals, and for phase control and optical transmission of electrical signals.

An optical waveguide arrangement which operates in the manner of an interferometer to provide modulation and/or switching functions is known from US Patent 4, 070, 094 (W.E.Martin). In that arrangement, two optical waveguide branches having a common connection diverge along a substantially co-extensive distance, and then reconverge thus providing first and second light paths of identical optical length. Conductive electrodes are disposed contiguous to at least one of the optical waveguide branches and are connectable via a control switch to a source of electrical energy.

In the absence of any electrical field, light entering the two branches propagates along identical optical path lengths and recombines constructively at the reconvergence of the branches. However, selectively applied electrical energy produces electrical fields in one or both branches to change the optical properties of at least one of the branches, causing phase differences in the optical energy propagated in the branches, which phase differences may be such that destructive interference occurs at the reconvergence of the two branches. Modulation is effected by varying the extent of the differences between the optical properties of the two branches.

In our co-pending Application No. 81 11096 (Serial No. ) (G.D.H. King M.C. Bone 7-1) there is disclosed an integrated optic device including a single mode optical waveguide which diverges into two waveguide branches of identical optical length that subsequentially converge into another single waveguide. The optical properties of the two branches are variable by electrical fields applied via adjacent electrodes. The electrodes are energised with a composite waveform comprising a symmetrical ramp superimposed on a square wave of the same period. The two component waveforms are in phase but of different amplitudes such that the output light is intensity modulated at a frequency which is some multiple of the input waveform (frequency upconversion). Adjustment of the square amplitude provides the means of phase shifting the output signal.The optical output of these devices consists of at least three optical frequencies.

An object of the present invention is to provide an optical waveguide arrangement of the general type referred to above which can be used for frequency shifting of optical signals. A known integrated optic frequency shifter described by B. Culshaw in "Electronics Letters" of 5th February, 1 981 includes an optical waveguide arrangement which employs square wave electrical signals on the electrodes. Thus this device can only operate at very low electrical frequencies. Another known device described in an article entitled "Integrated Optical SSB Modulator/Frequency Shifter by Izutsu in IEEE QE-17 No.

11 November 1981. This device has a complex waveguide plan with four waveguide branches and employs a single frequency signal for application to the electrodes.

According to the present invention there is provided an integrated optic arrangement including an integrated optic device and electrical drive means therefor, which device includes a substrate of electro-optically responsive material in which are defined an input optical waveguide, first second and third optical waveguide arms of substantially identical optical length, and an output optical waveguide, wherein the input waveguide is optically connected to each of the first second and third waveguide arms at their input ends, which first second and third waveguide arms converge at their output ends into the output waveguide, wherein a respective conductive electrode arrangement is disposed adjacent a portion of each waveguide arm whereby in use of the arrangement electrical fields for modifying the optical properties of the waveguide arm portions can be generated, and wherein the electrical drive means is such as to provide first and second sinusoidal electrical signals of frequency f0 for application to the electrode arrangements of the first and second waveguide arms, respectively, which first and second signals are substantially in guadrature, and such as to provide a sinusoidal electrical signal of frequency 2fo for application to the electrode arrangement of the third waveguide arm.

The arrangement of the present invention basically comprises an optical single sideband modulator, which may be employed, for example, in a frequency division multiplexing optical communication system. If a fourth waveguide arm is incorporated in the optical device it may be employed in an optical phase shifter, or an optical phase or frequency modulation communication system.

Embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic arrangement employing an electro-optic device for shifting the frequency of an optical signal either up in frequency or down in frequency; Figure 2 shows one optical waveguide arrangement which may comprise the electrooptic device of Fig. 1; Figure 3 shows another optical waveguide arrangement which may comprise the electrooptic device of Fig. 1; Figure 4 shows one embodiment of drive electronics for the arrangement of Fig. 1; Figure 5 shows another embodiment of drive electronics for the arrangement of Fig.

1; Figure 6 shows one optical waveguide arrangement, based on that of Fig. 2, which may be used in an optical phase shifter; Figure 7 shows another optical waveguide arrangement, based on that of Fig. 3, which may be used in an optical phase shifter; Figure 8 shows a block diagram of one embodiment of drive electronics for use with the waveguide arrangement of Fig. 6 for the optical phase shifting; Figure 9 shows a block diagram of another embodiment of drive electronics for use with the waveguide arrangement of Fig. 6 for the optical phase shifting; Figure 10 shows a block diagram of an optical phase shifting system, Figure 11 shows a block diagram of an optical phase modulation communication system; Figure 12 shows a block diagram of an optical frequency modulation communication system; Figure 13 shows a block diagram of an optical multiplexer transmitter; and Figure 14 shows a block diagram of an optical demultiplexer receiver.

In Fig. 1 is shown a schematic of an arrangement for shifting the optical frequency fc of an optical signal, provided by a laser 1, either up in frequency or down in frequency by an amount f0 as a result of applying sinusoidal electrical signals of frequency f, and 2fo from electronics 2 to an electro-optic device 3. The optical signal derived from the laser 1 may be fed directly to the electro-optic device 3 or via optical fibre or other coupling devices (not shown). The methods of coupling aligning an optical beam employing temperature control as described in our co-pending Applications Nos. 8227749 and 8227756 (Serial Nos and ) J. Moroz 1 and J. Moroz 2) may be employed.

In those of the drawings showing both optical and electrical signal paths, the solid interconnection lines, as between laser 1 and device 3 of Fig. 1, represent optical paths, whereas the dashed interconnection lines, as between electronics 2 and device 3, represent electrical paths.

The electro-optic device 3 comprises an optical waveguide arrangement as shown, for example, in Figs. 2 or 3.

Different waveguide plans may alternatively be employed. The essential feature of the waveguide arrangements is that three waveguides or waveguide arms derive their light from a single waveguide and then recombine their light into a single waveguide. In Fig. 2, light is input at 18 to a waveguide 4 which diverges at 5 into two waveguides 6 and 7.

Waveguide 6 diverges at 8 into two waveguides 9 and 10 which reconverge at 11 to form waveguide 1 2 which converges with waveguide 7 at 1 3 into an output waveguide 14. The waveguides are preferably all single mode, that is they only transmit single mode optical energy. Associated with each waveguide 9, 10 and 7 is a respective conductive electrode arrangement 15, 1 6, 1 7 each comprising two electrode portions arranged adjacent to the respective waveguide. One electrode portion 1 spa, 1 6a, 1 7a of each arrangement is earthed. Electric fields can be applied to the separate waveguides by applying respective electric signals EA, EB, EC to electrode portions 1 Sb, 1 6b, 1 7b.The waveguides are defined by regions of increased refractive index in a substrate of an electro-optically responsive material, for example lithium niobate (LiNbO3). The waveguide regions may be formed by indiffusion of suitable materials such as titanium or nickel oxide, or by the ion implantation of suitable materials such as helium, to provide suitable single mode waveguides. The conductive electrode arrangements 15, 16, and 1 7 comprise metallic layers deposited on the surface of the substrate substantially parallel to the respective waveguides. For optimum operation of the device the waveguides should be of similar optical length.Light is input to the waveguide 4 at point 1 8 from a laser, such as laser 1 (Fig. 1), or a fibre by some suitable coupling means and light is output at point 19 via some suitable coupling means (not shown).

The effect of the electrical fields produced by signals EA, E, and Ec is to modify the properties of the waveguide region between the electrodes such that the velocity of light is changed. This in turn varies the transit time of the light through the respective waveguide regions. The result of this, to a good approximation up to very high frequencies of applied voltage to the electrodes, is a phase shift of the light. Thus the three waveguide arms 9, 10 and 7 with their electrode arrangements act as three indpendent optical phase shifters.

The theoretical operation of the electro-optic device of Fig. 2 will now be considered. The ideal output of the device is light whose frequency is different by a controlled amount from the frequency of the light input into the device. In practice, however, signals with frequency (fe + nf0), where n is an integer, will be inherently present together a required frequency (fc + f0). However, by suitable design of the waveguide device and its control or drive electronics it is possible to achieve an optical output which is substantially single frequency, the other optical frequencies being present only at a low level, for example more than 40dB lower in power. Assuming the light amplitude at 1 8 is of the form cos2"f0t then the output at 19 may be arranged to be substantially single frequency by arranging that the amplitudes, Sp, Sq, and Sr, Of the optical signals in the waveguide arms P, Q and R are as follows, namely: = = Pcos [2XfCt + #1 + Asin(2#f0t + +4)] S0=Osin [2#f0t+#2+Bcos(2#f0t+#5)] Sr = Rcos [2sfCt + #3 - #/4 + Ccos (2#f0t+#6)] where, P,Q,R are the peak amplitudes of the optical signals in the respective waveguide arms; fc is the optical frequency of the light input; fo,2fo are the frequencies of the electrical signals applied to specific electrodes; t is time; A,B,C are the amplitudes of the phase shifts induced by the varying electrical sig nals, and #1,#2,#3,#4,#5,#6 ar phase terms.

The above amplitude equations are obtained with the driving signals on the electrodes 15b and 16b (EA and EB)each compris ing a signal of frequency fo, which signals are substantially in quadrature, and with a signal of frequency 2fo applied to electrode 1 7b (Ec).

A variety of values of the various parameters will give the desired frequency shifter signal output, for example cos (2#(f0+f0)t), from the input amplitude cos 2#f0t. Preferably, however, the parameters tend towards the following relationships, namely: P equals Q. A equals B. #2 equals #1.

3equals W, plus sr.

+4 equals +5 equals , equals ,.

The R and C amplitudes should be such that the amplitudes of the optical signals at frequency f0 are the same in the waveguide arms R and T (12), and such that the amplitudes of the optical signals at frequency (fo+2fo) are the same in waveguide arms R and T.

For example, P:Q:R = 1:1:1.33, A; B; C = 0.5; 0.5; 0.07 respectively, #1=#2=#4=#5=#6=0 and #3=#.

The output light amplitude obtained at 19 is then predominantly of the form cos (2 #(f0=f0)t), thus the frequency has been shifted up by f0 and this comprises the upper sideband of a single sideband modulator comprised by the device.

By appropriate choice of the phase terms #1 to +6 an optical output at 19 with an amplitude of the form cos(2#(f0-f0)t) may be obtained. For example, the lower sideband cos (2#(f0-f0)t) may be obtained either by changing #4 by nradians or by changing +, and 6 by madians and #3 by #/2 radians from the situation giving the upper sideband cos[2 7r(fc + f0)t].

In practice, the electrical signal applied to each electrode consists of a d.c. bias plus a sinusoidal signal. The d.c. biases set the phase terms #1, #2 and #3. The signals EA and EB include sinusoidal signals of frequency f0.

These signals which set the values of A and B, are 90 degrees out of phase thus setting +4 and #5 to the same value. The signal Ec includes a sinusoidal signal of frequency 2f0.

The sinusoidal signals with frequency f0 and 2f0 are phase locked such that #6 = +4 = +5. A block diagram of drive electronics for obtaining the requisite sinusoidal signals is shown in Fig. 4. It comprises an oscillator 21 having an output signal of frequency 2f0 which can be applied to electrode 17b after implication at an amplifier 22 and application to a matching circuit 23. The output of oscillator 21 is also applied to a divide-by-two divider 24, subsequently amplified at amplifier 25 and then band pass filtered around frequency f0 at filter 26.The output of the filter 26 is appropriately phase-shifted at 27 and then applied to a 90 power splitter 28, whose two outputs are applied to respective matching circuits 29 and 30 before application to the electrodes 15b and 16b.

Alternatively, the drive electronics may be as shown in the block diagram of Fig. 5, which employs an oscillator 31 having an output signal of frequency f0. The output of oscillator 31 is applied to a phase shifter 32 and a times-two multiplier 37. The output of phase shifter 32 is amplified, at an amplifier 33, before application to a 90 power splitter 34, whose two outputs are applied to respective matching circuits 35 and 36 before application to the electrodes 15b and 16b The output of multiplier 37 is amplified, at amplifier 38, before bandpass filtered around frequency 2f0 at filter 39. The output of filter 39 is applied to a matching circuit 40 before application to the electrode 17b.The design of the drive electronics is facilitated by the use of sinusoidal signals since, over the employed frequency range,typically up to around 1 5GHz, good electrical phase shifting and matching circuits can be made.

The relative values of amplitude P and Q are set by a Y-junction waveguide split at 8 (Fig. 2) The relative value of amplitude R may either be set by an optical attneuator 41 (Fig.

2) or an optical coupler 42 (Fig. 3). The optical attenuator 41 is comprised by a metal layer deposited over the waveguide 7 between junction 5 and electrode arrangement 17. The optical attenuation is determined by the length of the waveguide under the metal. Fine adjustment of the optical attenuation may be achieved by laser trimming of the metal layer.

The optical coupler 42 (Fig. 3) consists of two closely spaced waveguides 4 and 4a. Fine adjustment of the optical energy going to the waveguide arm 4a can be achieved by appling an electric field between electrodes 43 of coupler 42 arranged adjacent the waveguides 4 and 4a.

The basic three waveguide device described above can be employed to change the frequency of an optical signal either up.or down, depending on electrical phases, by a predetermined amount, and only very low levels of spurious signals are generated in the process.

Thus the device comprises an optical single sideband modulator with a high dynamic range. In comparison with the Culshaw device mentioned above the device of the present invention uses sinusoidal electrical signals on the electrodes, rather than square wave electrical signals, and can therefore operate at much higher (more than an order of magnitude) electrical frequencies. In addition the device of the present invention offers better performance in terms of the levels of unwanted frequency sidebands placed on the optical signal. In comparison with the Izutsu device mentioned above, the device of the present invention has a less complex waveguide plan, thus facilitating fabrication, and employs two sinusoidal signals of different frequencies for application to the various electrodes, rather than a single frequency signal.

The use of two such signals for generating a frequency shifter optical signal results in better performance of the device in terms cf the levels of unwanted frequency sidebands placed on the optical signal, due to the greater independence of the control factors in the device of the present invention.

The basic device of the present invention may be modified in order to provide an optical phase shifter. In that application an electrical signal frequency f0 is transmitted on an optical carrier signal, with optical frequency fc, and the detected phase of the frequency f0 is controllable. For this application an integrated optic device with waveguide arrangement as indicated in Figs. 6 or 7 is employed. The arrangement of Fig. 6 is based on that of Fig.

2, whereas the arrangement of Fig. 7 is based on that of Fig. 3. In Fig. 6 light is input at 44 to a single mode waveguide 45 which diverges at 46 into two waveguides 47 and 48. Waveguide 47 diverges at point 49 to provide two waveguides 50 and 51 which converge at 52 to form a single waveguide 53. Waveguide 48 diverges at point 54 to provide two waveguides 55 and 56 which converge at 57 to form a single waveguide 58. Waveguides 53 and 58 converge at point 59 to provide a single waveguide 60 from which light is emitted at output 61.Associated with the waveguides 50, 51, 55 and 56 are respective conductive electrode arrangements 62, 63, 64 and 65 to one portion of which electric signals EA, EB, EC and ED are applied, respectively, the other electrode portions being earthed as indicate. An optical attenuator 66 is comprised by a metal layer disposed over waveguide 47 and serves to set the relative amplitudes of the optical signals in waveguides 53 and 58. Adjustment of the attenuation may be achieved as described above with reference to Fig. 2. Alternatively, the relative amplitude of the waveguides 53 and 58 may be set by means of an optical coupler 67, as described above with reference to Fig. 3. Such an arrangement is shown in Fig. 7.An optical coupler 67, having electrodes 68 between which an electric field is generated, controls the amount of optical energy coupled to waveguide arm 45a and thus waveguide 47, using the same reference numerals for like parts in Figs. 6 and 7.

The waveguide arms 50, 51 and 55 (Fig.

8) with electrode arrangements to which electric signals EA, EB and Ec are applied to operate in a similar manner to that described with reference to waveguide arms 9, 10 and 7 of Fig. 2, and serve to generato an optical signal of frequency (fc + fo) or (fc - f0), where fc is the input optical frequency and f0 is the fundamental electrical frequency. The fourth arm, waveguide arm 56, acts as a reference arm whose output is a reference optical signal with frequency f,. Application of a predetermined voltage signal ED to the one portion of electrode arrangement 65 causes a phase shift to appear on the reference optical signal having an optical frequency fc.Thus this device generates two optical signals, one at frequency (fc + fo) or (fc-fo) and a second at (fc+#). When the various optical signals coming out of the waveguide 60 at output 61 are detected, for example with a photodiode, the effect of the phase shift + on the reference signal is such as to cause the phase of the detected signal of frequency f0 to change by an amount +. Thus there is achieved a means of transmitting an electrical signal frequency f0 of controllable phase shift f on an optical carrier signal, the phase shift # being control lable by varying the voltage signal ED applied to one portion of electrode arrangement 65 of the reference arm.

Alternatively, the structure of the device may be modified such that a phase shift f appears on the frequency shifted signal (fc#Fo) rather than the reference signal (fc).

This, in a similar manner, also allows the transmission of a signal of frequency f0 of controllable phase shift # on an optical carrier signal.

Figs. 8 and 9 show embodiments of drive electronics equiv21ent to Figs. 4 and 5, respectively, and each including a data converter 69 whereby phase data input thereto is converted to a voltage signal ED for application to the reference arm electrode arrangement after application to a respective matching circuit 70. A block diagram of an optical phase shifter is shown in Fig. 10. The output of a laser 71 is rpplied t" the input of an optical device 72. comprised by he waveguide arrangement of Fig. 6, for example, hazing drive olectronics 73, comprised by the arrangement of Fig. 8, for example, to which phase information is input. The output of optical device 72 is connected to a detector 74 comprised by a photodiode, for example, which connection may be via an optio21 fibre 75. The electrical t-J'tpvt of the detcç^o 75 is bandpass filtered around fo (the fundamental electrical frequency) at a filter 76, whose output thus is of electrical frequency fro with a controllable phase shift . The four-arm waveguide arrangements of Figs. 6 and 7 used in the optical phase shifter described above may alternatively be used in an optical communication system using phase modulation and/or frequency modulation.Again the waveguide arm 56 provides a reference signal at the optical frequency, however, in the case of a phase modulation optical communication system the electrode arrangement 65 on this arm specifically serves to allow the phase of the reference signal to be changed by # radians by the application to one portion thereof of an appropriate d.c. voltage. In other words, the phase of this signal can be O or iT radians relative to the optical carrier frequency in the other waveguide arms 50, 51, 55. Thus the transmission of digital data can be achieved with, for example, a 7T radians phase shift representing a "1" and a 0 radians shift representing a "O". Such an optical phase modulation communication system is shown in block form in Fig, 11.An optical carrier input signal with frequency fc is generated by laser 77 and input to an optical device 78, which may be in the form shown in Fig. 6.

Drive electronics 79, which may be in the form shown in Fig. 4, derives sinusoidal electrical signals at frequencies f0 and 2fo for electrical signals EA, EB and Ec. Digital data to be transmitted is applied to a data converter 80 which is such as to provide a d.c. voltage for signal ED to the electrode portion of the reference arm whereby to change the phase by n, in order to indicate a "1", for example.

The output of the optical device 78 is conducted to an optical detector 81, such as a photodiode, for example, over, for example, an optical fibre link 82. The electrical output of the detector 81 is filtered around electrical frequency f0 at filter 83 and applied to a phase demodulator 84 whose output corresponds to the data input converter 80 prior to transmission over the optical link. Thus the application of a sr phase shift to the reference arm causes a sr phase shift in the signal at electrical frequency f0 as detected by detector 81 and this phase shift can then be detected by a phase sensitive detector comprising the phase demodulator 84, whose output corresponds to the data to be transmitted.

An optical frequency modulation communication system is illustrated in block form in Fig. 12. An optical carrier input signal with frequency fc is generated by a laser 85 and input to an optical device 86, which may be basically in the form shown in Fig. 6 and employs sinusoidal electrical signals at frequencies fo and 2fo to provide electrical signals EA, EB and E0 by means of drive electronics as in Fig. 4, for example. Digital data to be transmitted is applied to a data converter 88.

Instead of being employed to change the phase of the reference signal in arm 56 (Fig.

6), the output of converter 88 is employed to change the basic frequency f0 and hence 2fo (or vice versa) between two values, in dependence on whether a "O" or a "1" is to be transmitted, and thus correspondingly charge EA, EB and Ec. In this arrangement ED is a constant to produce a constant reference signal at optical frequence fc The output of optical device 86 is transmitted to an optical detector, for example a photodiode, 89 via, for example, an optical fibre link 90. The electrical output of detector 89 is filtered around electrical frequency f0 by filter 91 and applied to a frequency demodulator 92 whose output corresponds to the data input to converter 80 prior to transmission over the optical link.

The basic optical device described above with reference to Figs. 1 to 5 can also be employed for frequency division multiplexing in optical communications. This application makes use of the fact that the basic three waveguide-arm optical device can be used either for frequency upshifting or for frequency downshifting. The direction of the frequency shift is determined by the phases #1, #2, #3, #4, #5, and #6 of the signals. At a transmitter, phase or frequency modulation, in response to data to be transmitted, may be imposed on a number of different optical carrier frequencies, which are derived from a single carrier frequency by sequentially adding fO (n) to the optical frequency before modulation.These various optical carrier frequencies may then be transmitted down a single optical fibre, with a second fibre carrying a reference signal. The various signals may be demultiplexed in a receiver by sequentially reducing the optical frequency by fO(n). This allows the different channels to be separated because the respective detectors and band pass filters will only respond to signals around their respective frequency. The channels are spaced in frequency such that they cannot give rise to signals which would come within the bandpass region of the wrong filter.

A multiplexer transmitter and a demultiplexer receiver for a system with two multiplexed channels are shown in block diagram form in Fig. 13 and 14, respectively. More channels may be used in practice. An optical carrier signal with optical frequency fc is input at 93 to the transmitter (Fig. 13).By means of a first optical device 94, as for example shown in Fig. 2, and drive electronics 95, as shown for example in Fig. 4, employing sinusoidal electrical frequencies f0 and 2fo, the optical signal output from device 94 is frequency upshifted to frequency (fc + fO) from the input value f0. This signal output is applied via line 96,for example an optical fibre or free-space link, to a frequency or phase modulator 97 whereby it is modulated in accordance with a first digital data stream to be transmitted "Data 1".The signal output from device 94 is also applied via a line 98, for example an optical fibre or a free-space link, to a second optical device 99, also as shown, for example, in Fig. 2, and having drive electronics 100, as shown for example in Fig. 4 and employing sinusoidal electrical frequencies f,' and 2fowl. Thus the output of device 99 is further upshifted in frequency to (fc + fro + fO) from the transmitter input value fc. The output device 99 is frequency or phase modulated in a modulator 101 in accordance with a second digital data stream to be transmitted "Data 2". The outputs of modulators 97 and 101 are combined and transmitted to a receiver over an optical fibre link 102.A second fibre link 103 carries a reference signal, namely the optical carrier signal with optical frequency fc.

At the demultiplexer receiver (Fig. 14) the optical signal transmitted over link 102 is applied to an optical device 104, as shown for example in Fig. 2, and having drive electronics 105, as shown for example in Fig. 4 and employing sinusoidal electrical frequencies f0 and 2f,, whereby the optical signal input thereto is downshifted in frequency by fO. By means of couplers 106 and 107 a part of the signal output from device 104 and a part of the reference signal transmitted over link 103 are applied to a detector 108, the electrical output of which is bandpass filtered around fd/2 at 109, where fd is the bandwidth required for the data, and then demodulated at 110 in order to provide an output corresponding to "Data 1".The output of device 104 is also applied to an optical device 111, which can be as described with reference to Fig. 2, having drive electronics 11 2, which can be as described with reference to Fig. 4, whereby the optical signal is downshifted by a further amount f01. The remaining signal transmitted along link 103 is applied to a detector 11 3 together with the output of device 111. The electrical output of detector 11 3 is bandpass filtered around fa/2 where fd is the bandwidth required for the data, in filter 114 and demodulated at demodulator 11 5, the output of which corresponds to "Data 2".

In this frequency division multiplexing application, frequency shifts up to n times the maximum frequency can be generated on the optical signal by shifting the optical frequency n times. The limitation on n will be the bandwidth of the optical fibre or waveguide and signal to noise limitations in the system.

In certain applications the bandwidth will be limited by the detector to around 1 5GHz.

Claims (26)

1. An integrated optic arrangement including an integrated optic device and electrical drive means therefor, which device includes a substrate of electro-optically responsive ma terial in which are defined an input optical waveguide, first second and third optical waveguide arms of substantially identical opti cal length, and an output optical waveguide, wherein the input waveguide is optically con nected to each of the first and second and third waveguide arms at their input ends, which first second and third waveguide arms converge at their output ends into the output waveguide, wherein a respective conductive electrode arrangement is disposed adjacent a portion of each waveguide arm whereby in use of the arrangement electrical fields for modifying the optical properties of the wave guide arm portions can be generated, and wherein the electrical drive means is such as to provide first and second sinusoidal electri cal signals of frequency f0 for application to the electrode arrangements of the first and second waveguide arms, respectively, which first and second waveguide arms, respec tively, which first and second signals are substantially in quadrature, and such as to provide a sinusoidal electrical signal of fre quency 2fo for application to the electrode arrangement of the third waveguide arm.

2. An arrangement as claimed in claim 1, wherein the waveguides and waveguide arms are comprised by single mode waveguides.

3. An arrangement as claimed in claim 1 or claim 2, wherein the input waveguide diverges into two waveguide portions,one of which diverges to form the portions of the first and second waveguide arms and the other of which comprises the third waveguide arm.

4. An arrangement as claimed in claim 3, including a metal layer comprising an optical attenuator arranged on the surface of the substrate over a length of the third waveguide arm in front of the respective electrode ar rangement.

5. An arrangement as claimed in claim 1 or claim 2, wherein the input waveguide diverges to form the portions of the first and second waveguide arms and is optically con nected to the third waveguide arm by means of an optical coupler.

6. An arrangement as claimed in any one of the preceding claims, wherein each elec trode arrangement comprises two portions, which two electrode portions are arranged on the substrate surface on opposite sides of the waveguide, wherein one electrode portion is earthed and the respective electrical signal is applied to the other electrode portion.

7. An arrangement as claimed in any one of the preceding claims, wherein the electrical signals include a d.c. bias.

8. An arrangement as claimed in any one of the preceding clairns and in the form of an optical single sideband modulator, wherein in use and in combination with a source of optical signal, having an optical frequency fc, connected to the input waveguide, the optical frequency at the output waveguide is shifted up to (fc + fO) or down to (fc - f0) in dependence on electrical phases.

9. An arrangement as claimed in claim 1, wherein the device includes a fourth optical waveguide arm of substantially identical optical length to that of the first second and third waveguide arms, which fourth waveguide arm is optically connected between the input and output waveguides and a portion of the fourth waveguide arm has a respective conductive electrode arrangement deposed adjacent thereto, and wherein the electrical drive means is such as to provide an electrical signal to the fourth waveguide arm electrode arrangement such as to shift the phase of the optical signal transmitted along the fourth waveguide arm by an amount .

10. An arrangement as claimed in claim 9, wherein the electrical signal to the fourth waveguide arm electrode arrangement is a constant voltage whereby to obtain a constant phase shift +.

11. An arrangement as claimed in claim 9 and in the form of an optical phase shifter, wherein in use and in combination with a source of optical signal, having an optical frequency fc, connected to the input waveguide and an optical detector arranged to detect the output of the output waveguide, the electrical signal frequency f0 carried by the optical signal as a result of application of the sinusoidal electrical signals is, at the optical detector, phase shifted by the amount +.

1 2. An arrangement as claimed in claim 11, wherein the amount 9 is controllable by varying the electrical signal applied to the fourth waveguide arm electrode arrangement.

1 3. An arrangement as claimed in any one of the claims 9 to 12, wherein the input waveguide divides into two portions, one of which diverges to form the portions of the first and second waveguide arms and the other of which diverges to form the portions of the third and fourth waveguide arms, which first and second and third and fourth waveguide portions converge to form respective further waveguide portions, which further waveguide portions converge to form the output waveguide.

14. An arrangement as claimed in claim 1 3 including a metal layer comprising an optical attenuator arranged on the surface of the substrate over the one waveguide portion.

1 5. An arrangement as claimed in any one of claims 9 to 12, wherein the input waveguide diverges to form the portions of the third and fourth wave guide arms and is optically connected to another waveguide portion; which diverges to form the portions of the first and second waveguide arms, by means of an optical coupler.

16. An arrangement as claimed in claim 15, wherein the first and second, and third and fourth, waveguide arm portions converge to form respective waveguide portions which converge to form the output waveguide.

1 7. An arrangement as claimed in claim 9 and in the form of an optical phase shifter for an optical digital data communication system, wherein in use and in combination with a source of optical signal, having an optical frequency fcf connected to the input waveguide and an optical detector arranged to detect the output of the output waveguide, the electrical signal frequency f0 carried by the optical signal as a result of application of the sinusoidal electrical signals is, at the optical detector, phase shifted by the amount +, and wherein the electrical signal applied to the fourth waveguide arm is such that < p is O or sr radians, or vice versa, in dependence on whether the digital data to be transmitted comprises a "O" or a "1".

1 8. An arrangement as claimed in claim 17, wherein the detector produces an electrical output, and in combination with a fibre optic link between the output waveguide and the optical detector, a filter for filtering the detector output around the frequency f0 and a phase sensitive detector to which the filter output is applied, whereby to form an optical phase modulation digital data communication system, the output of the phase sensitive detector comprising the digital data.

1 9. An arrangement as claimed in claim 9 wherein in use and in combination with a source of optical signal, having an optical frequency fc connected to the input waveguide, a digital data signal applied to the electrical drive means serve to change the electrical frequency fO, carried by the optical signal, between two values in dependence on whether the data signal respresents a "O" or a "1".

20. An arrangement as claimed in claim 1 9 in combination with an optical detector producing an electrical output, a fibre optic link between the output waveguide and the optical detector, a filter for filtering the detector output around f0 and a frequency sensitive detector to which the filter output is applied, whereby to form an optical frequency modulation digital data communication system, the output of the frequency sensitive detector comprising the digital data.

21. An optical communications system employing frequency division multiplexing including a transmitter and a receiver, wherein the transmitter includes a respective integrated optic arrangement as claimed in claim 8 and a respective modulator for each channel to be transmitted, whereby the optical frequency of the optical carrier signal is correspondingly sequentially shifted in a first direction before modulation for each channel, wherein the modulated optical carrier frequencies are transmitted to a receiver along a first optical fibre link, wherein a reference optical signal comprising the optical signal before frequency shifting is transmitted to the receiver along a second optical fibre link, wherein the receiver includes a respective integrated optic arrangement as claimed in claim 8 and a respective detector and demodulator for each channel to be received, whereby the transmitted signals can be demultiplexed by sequentially shifting the optical frequency in the opposite direction, and combining with the reference signal before application to the detector and applying to the respective demodulator.

22. An integrated optic arrangement substantially as herein described with reference to Figs. 1 and 2 or Figs. 1 and 3 with or without reference to Fig, 4 or Fig 5; or Fig. 6 or Fig.

7 with or without reference to Fig. 8 or Fig. 9 of the accompanying drawings.

23. An optical phase shifter substantially as herein described with reference to Fig. 10 of the accompanying drawings.

24. An optical phase modulation communication system substantially as herein described with reference to Fig. 11 of the accompanying drawings.

25. An optical frequency modulation communication system substantially as herein described with reference to Fig. 12 of the accompanying drawings.

26. An optical communications system employing frequency division multiplexing substantially as herein described with reference to Figs. 13 and 14 of the accompanying drawings.