GB2090993A

GB2090993A - Integrated Optical Phase Modulator

Info

Publication number: GB2090993A
Application number: GB8138953A
Authority: GB
Original assignee: Thales Optronics Ltd
Current assignee: Thales Optronics Ltd
Priority date: 1981-01-13
Filing date: 1981-12-24
Publication date: 1982-07-21
Also published as: GB2090993B

Abstract

An integrated optical phase modulator is formed by a body 10 of electro-optic material such as lithium niobate in which is formed a signal waveguide 12 and a control waveguide 11. Waveguides 11 and 12 adjoin one another over an interaction length L and the signal beam propagated along waveguide 12 from source 14 is phase shifted by the evanescent electric field established in the body 10 by an intense control beam propagated therealong from a source 13. The level of phase shift depends upon the intensity of the control beam, the interaction length L and the interwaveguide spacing. The modulator may form part of an all- optical interferometer or an all-optical analogue-to-digital converter. <IMAGE>

Description

SPECIFICATION Integrated Optical Phase Modulator This invention relates to integrated optics and in particular to an integrated optical phase modulator.

Integrated optics is a technology which utilises the passage of optical signals through waveguides and recently active integrated optical devices have been designed by forming the waveguides in an electro optic material and actively controlling the waveguide properties by the application of an electric field to the material.

A phase modulator constructed in this way has been described in a paper entitled "Efficient stripwaveguide modulator" by I. P. Kaminow et al published in Applied Physics Letter Vol. 27, No.

10 (15 November 1975). In this case the material used was lithium niobate, which is at present one of the most commonly used electro-optic materials, the waveguide being formed by diffusing in titanium along a surface strip of the material and the electric field being applied via chromium/aluminium electrodes deposited on the surface of the material on either side of the waveguide.

This known device has been used as the basis for amplitude modulators, and analogue-to-digital converters. Particular constructions are described by Dandridge et al in Electronics Letters, Vol. 16, No. 11, Goss et al in Applied Optics V. 19 No. 6, Burns et al in Applied Physics Letters Vol. 33, No.

11, Taylor et al in Applied Physics Letters, Vol 32, No. 9, and CVD Project RP7-121 (1979 Annual Report).

The known optical phase modulator and the various constructions based thereon although highly advantageous in permitting the processing of signals in optical form are all limited in that the inherent capacitance and inductance of the electrode arrangement imposes a maximum processing frequency of the order of 1 GHz which of course is the same as that available in wholly electronic devices for processing signals in electrical form and consequently the advantage of high processing frequency inherent in optical signals is lost.

According to the present invention there is provided an integrated optical phase modulator comprising a body of electro-optic material incorporating a signal waveguide for an optical signal beam to be processed, and wherein controlled processing of the signal beam is established in the signal waveguide by a relatively intense optical control beam propagated along a control waveguide formed in said body of material, said control waveguide adjoining said signal waveguide and the arrangement being such that the evanescent electric field established in the body of electro-optic material by the control beam modifies the refractive index of the signal waveguide to phase-shift the signal beam propagated therethrough.

The modulator according to the present invention may take a variety of different forms as will be explained but in each case there is no externally applied electric field, the electric field within the electro-optic material, and which effects processing of the optical- signal, being that due to the evanescent effect of the electric field established by the intense optical beam in the control waveguide.

The modulator according to the present invention does not rely upon an externally applied electric field and accordingly is not limited in processing frequency by the effects of electrode capacitance and inductance. Indeed processing frequencies greatly in excess of 1GHz are readily achieved since processing control is effected by intensity modulation of the optical beam.

It will accordingly be evident that the intensity of the optical signal to be processed must be considerably less than that of the control beam to avoid unwanted interactive effects and by way of example only, the control beam may have an intensity of 107 co/m2 or greater whilst the optical signal intensity may be of the order of 104 /m2, the former being readily established from a moderate power source propagating into a monomode waveguide typically of the order of 3 ym by 8 4m in cross-section since within a waveguide it is possible to confine optical signals beyond the diffraction limit which exists for lenses focussing radiation in free spaces.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which: Fig. 1 illustrates the basic modulator format; Fig. 2 shows the modulator of Fig. 1 incorporated into an interferometer; Fig. 3 shows several Fig. 2 interferometers incorporated into an A/D converter; and Fig. 4 illustrates a modification of a detail in Fig. 3.

As is shown in Fig. 1 the modulator comprises a body 10 of electro-optic material in which is formed a control waveguide 11 and a signal waveguide 12 which are in close proximity to one another over a length L which we refer to as the interaction length. Waveguide 11 is coupled to a power source 1 3 so that an intense optical beam is propagated along waveguide 1 waveguide 12 is coupled to a signal source 14 so that the optical signal to be processed is deiivered into waveguide 12 where the phase modulation processing occurs, the processed signal being available at output 12A.Operation of the modulator depends upon the control beam in waveguide 11 being sufficiently intense to establish an evanescent electric field (E) of sufficient intensity to overlap waveguide 1 2 and modify the refractive index of that waveguide in accordance with the wellknown electro-optic effect and it is for this reason that the two waveguides are in close proximity over the length L. The refractive index modification alters the optical path length for the optical signal delivered by source 14 so as to induce a phase shift in the signal and it will be appreciated that the level of phase shift depends upon the intensity of the control beam and the length L for a given spacing between waveguides.

In order to achieve selective levels of phase shift with the present invention either various different modulators can be constructed, preferably varying the interaction length L since there is very little constraint on this parameter or different intensities of control beam can be applied to identical modulators.

By virtue of the fact that the device of the present invention is a modulator it is desirable to avoid coupling of the control beam from waveguide 11 into waveguide 12 and likewise of the optical signal from waveguide 12 into waveguide 11 and this can be achieved by utilising waveguides of radically different propagation constants (p) and/or by arranging the wavelength of the control beam to be substantially different from the wavelength of the optical signal. However in certain constructions it may not be possible to avoid coupling of the intense control beam at least partially into waveguide 12 and whilst the desired phase shift effect upon the optical signal can still be achieved it becomes necessary to provide a de-coupling device at output 1 2A to separate the unwanted control beam from the processed optical signal.

The modulator of the present invention has an extended processing frequency range when compared with the prior art modulator but otherwise it uses the same electro-optic materials (lithium niobate is presently the preferred material but the present modulator is not limited thereto) and can be used in the same applications as the known modulator. For example it may be used to introduce a variable phase shift into one arm of an interferometer which may form part of a fibreoptic sensor, a fibre gyroscope or a Mach Zehnder.

Fig. 2 illustrates the present modulator forming part of a Mach-Zehnder for effecting amplitude modulation of an optical signal. Here the optical signal is launched into waveguide 20 which branches into waveguides 21 and 22, guide 21 having a portion formed in an electro-optic material 23 containing a further waveguide 24 into which an intensity modulated intense control beam is launched as previously described. ThuB at the output of 21 A of guide 21 there is present a phase-shifted version of the signal at the input to guide 21 and this caused to interfere with the signal at the output of guide 22 at junction 25, an amplitude modulated signal being thereafter available at output 25A as is well known provided the phase shift produced by the control beam is of the order of n. In this arrangement the arms of the interferometer may be balanced initially, if required, by a steady optical signal in waveguide 22.

Another application for the modulator of the present invention is an analogue-to-digital converter. Analogue-to-digital converters utilising a hybrid approach have been previously reported and consists of a series of Mach-Zehnder interferometers respectively equipped with electrodes of different lengths. The analogue signal, in the form of a voltage, is applied to each electrode pair and causes the largest phase shift in the longest electrode pair and the smallest phase shift in the shortest electrode pair. The arrangement allows for the analogue signal to cause phase shifts of multiples of 7r and thus threshold detectors placed at the output of the interferometers record a digital encoding of the analogue signal, the Mach-Zehnder equipped with the longest electrode producing the least significant bit.

An all-optical A/D converter could be constructed utilising this principle and one possible design is shown in Figure 3. The analogue signal is the intensity of the optical beam which is launched into waveguide 30 and divided down by 3db couplers into waveguide 30A, 30B etc. equal in number to the required number of bits. Each of these waveguides 30A, 30B etc. then forms part of a Mach-Zehnder interferometer 31 A, 31 B etc. as described in Fig.

2 but respectively constructed to produce different levels of phase shift, for example by utilising different interaction lengths, to allow encoding in a binary scale of the intensity of the optical beam according to the amplitude modulation effected upon identical optical signals introduced at the other input 33A, 33B etc. of respective Mach-Zehnders and monitored by threshold detectors 32A, 32B etc.

It will be evident that the construction of this A/D converter requires the optical signal to cross the path of the control beam and as shown in Fig.

3 this may be achieved by using optical fibres which couple into grooves 34A, 34B etc. cut in the waveguide substrate but various other approaches are possible. For example, multiple layer structures would permit the waveguides carrying the control beam to extend in a different plane from those carrying the optical signal sxcept where a phase modulator is formed. Such an arrangement is illustrated in part in Fig. 4, where waveguide 30A has a refractive index n4 and is formed between layers of refractive index n nS, n3 respectively where n4 > nS, n3 and waveguide 33A has a refractive index n2 and is formed between layers of refractive index n3, n, respectively where n2 > n3, n1 and in the region where waveguide 30A is diverted into close proximity to guide 33A the substrate material is electro-optical as previously described. Of course, depending upon the particular materials employed it may not be necessary to physically divert either waveguide.

Claims

1. An integrated optical phase modulator comprising a body of electro-optic material incorporating a signal waveguide for an optical signal beam to be processed, and wherein controlled processing of the signal beam is established in the signal waveguide by a relatively intense optical control beam propagated along a control waveguide formed in said body of material, said control waveguide adjoining said signal waveguide and the arrangement being such that the evanescent electric field established in the body of electro-optic material by the control beam modifies the refractive index of the signal waveguide to phase-shift the signal beam propagated therethrough.

2. A modulator as claimed in claim 1, wherein the propagation constants of the signal and control waveguides are different.

3. A modulator as claimed in claim 1 or claim 2, wherein the wavelength of the control beam is different from that of the signal beam.

4. A modulator as claimed in any preceding claim, inciuding a decoupling device connected to the output of the signal waveguide for separating the processed signal beam from unwanted coupling with the control beam.

5. A modulator as claimed in any preceding claim, wherein variation in phase-shift of the signal beam is provided by variation of the intensity of the control beam.

6. A modulator as claimed in any preceding claim and substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

7. An interferometer for effecting amplitude modulation of an optical signal, incorporating a phase modulator as claimed in any preceding claim.

8. An interferometer as claimed in claim 7 and substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawings.

9. Apparatus for effecting analogue-to-digital conversion of the intensity of an optical beam incorporating a plurality of interferometers each as claimed in claim 7 and substantially as hereinbefore described with reference to Fig. 3 of the accompanying drawings or as modified by Fig.

4 of the accompanying drawings.