Optical signal modulation device

Abstract: An optical signal modulation device comprises first and second optical waveguides (1,2) fabricated on Z cut lithium niobate. The waveguides (1,2) are connected at one end to an optical signal splitter (7) and at the other end to a Y coupler (11). The output of the Y coupler (11) is connected to a single mode optical fibre (13). A first pair of modulator electrodes (3,4) are positioned across the waveguides (1,2) to cause relative phase shifts between signals in the two optical waveguides (1,2) in response to a first control signal V₁. These relative phase shifts are caused in both TM mode and TE mode signals. The device also includes a second pair of bias electrodes (14, 15) arranged to cause a relative phase shift between signals of the TE mode only whereby the transfer characteristics of the two modes may be shifted relatively to one another in use in accordance with the second control function.

Description: The invention relates to an optical signal modulation device. Known optical signal modulation devices comprise first and second optical waveguides connected at one end to an optical signal splitting device which couples portions of an incoming optical signal into the waveguides and at the other end to an optical signal combining device which causes optical signals received from the waveguides to interfere; and first modulating means responsive to a first control signal to cause a relative phase shift between signals in the first and second optical waveguides, the first modulating means causing relative phase shifts in both TM mode and TE mode signals. Such devices are hereinafter referred to as of the kind described. Examples of known devices of the kind described are lithium niobate integrated optical Mach-Zehnder modulators. One major problem with known modulation devices of the kind described is that they display marked polarisation sensitivity. In any practical telecommunications system this can act as a severe limitation since the state of polarisation of the signal emerging from an optical waveguide such as a length of optical fibre can vary in a random fashion. Previous proposals for developing a lithium niobate polarisation insensitive modulator have been made which involve the use of off axis propagation. These approaches are limited by significantly increased insertion loss and difficulty of fabrication respectively. The effect of polarisation sensitivity is that the transfer characteristic of the device for TM and TE modes is significantly different. This leaves a particular problem when the device is used as an optical switch in which a two state control function such as a voltage can be determined which correctly switches one mode but which has little useful affect on the other mode. This is a particular problem when it is desired to turn off the switch. The non-coincidence of minima of the two transfer characteristics means that a proportion of one zero mode polarisation will continue to pass through the device when it is controlled in the off state. Known modulation devices are described in US-A-4 291 939, I.E.E.E. Journal of Quantum Electronics, vol. QE-17 no. 6, June 1981, pages 946-959, and Optics Letters, vol 5, no. 5, May 1980, pages 176-178. In accordance with the present invention, an optical signal modulation device comprises first and second optical waveguides connected at one end thereof to an optical signal splitting device which couples portions of an incoming signal into the waveguides and at the other end thereof to an optical signal combining device which causes optical signals received from the waveguides to interfere; first modulating means responsive to a first control signal to cause a relative phase shift between signals in the first and second optical waveguides, the first modulating means causing relative phase shifts in both TM mode and TE mode signals; and second modulating means responsive to a bias signal to cause a relative phase shift between signals of one mode in the first and second waveguides and to leave signals of the other mode substantially unaffected, characterised in that the transfer characteristics of the two modes may be shifted relatively to one another in use in accordance with the bias signal such that the minima of the transfer characteristics of the TE and TM modes are substantially coincident at a first predetermined magnitude of the first control signal and such that the maxima of the transfer characteristics of the TE and TM modes are substantially coincident at a second predetermined magnitude of the first control signal. The second modulating means is provided to cause a second relative phase shift between signals of one mode in the first and second waveguides. By a suitable choice of second control signal, it can be arranged that a minimum of the transfer characteristic of the one mode is substantially coincident with a minimum of the transfer characteristic of the other mode. This property finds particular application for devices fabricated from lithium niobate, particularly z cut lithium niobate, since the electro-optic coefficients of lithium niobate which interact with the first modulating means have a nearly integral relationship (in fact related by a factor of 3). When the device is to be used as a switch, the first and second signals are generated by first and second generators respectively, the first generator producing a two-state control signal, and the second generator producing a bias voltage which is such that the maxima of the transfer characteristics of the TE and TM modes are substantially coincident when the first control signal is in a first state, and such that the minima of the transfer characteristics of the TE and TM modes are substantially coincident when the first control signal is in a second state. Thus, the two-state control signal applied to the first modulating means can be arranged to switch between potentials related by the same factor of 3, since this will have substantially the same affect on both the TM and the TE optical modes. Although in this application the maxima will not exactly coincide, this is acceptable particularly for digital signals. Although in theory, for certain devices, it may be possible for the second modulating means to act on the TM mode independently of the TE mode, preferably the second modulating means causes a relative phase shift between TE mode signals in the two waveguides. Typically, the first and second modulating means each comprise electric field generating means for generating respective electric fields which are substantially orthogonal to each other and to the waveguides. In this case, the electric field generated by the second modulating means extends across only one of the waveguides. Typically, the second modulating means will apply a substantially constant bias field. In this specification, the term "optical" is intended to refer to that part of the electromagnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultra-violet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres. Preferably, when the device is incorporated into an optical switch, the output of the combining device is coupled with a single mode waveguide. This provides a simple way of causing the device to act as a switch since when the first control function takes up one state, only first or higher order mode signals will be generated at the combining device and will therefore not be guided by the single mode waveguide. When the first control function takes up its other state, zero order modes will result which are guided by the single mode waveguide. An example of an optical switch incorporating an optical signal modulation device according to the invention will now be described with reference to the accompanying drawings, in which:- Figure 1 is a schematic view of the device; Figure 2 illustrates graphically the TM and TE mode transfer characteristics of the Figure 1 device for two different TE bias potentials; and, Figure 3 shows the output for the TE and TM modes when an electrical pulse of amplitude equal to three times the switching voltage for the TM mode is applied to the modulation electrodes. Figure 1 illustrates a conventional Mach-Zehnder modulator which has been modified in a manner to be explained below. The modulator comprises first and second optical waveguides 1, 2 fabricated on Z cut lithium niobate by the standard Ti diffusion technique. An oxide buffer layer is employed in order to reduce the propagation loss for the TM optical mode. As in a conventional modulator, a pair of modulation electrodes 3, 4 are aligned directly above the optical waveguides 1, 2 thus ensuring that electro-optically induced phase shifts occur as a result of the interaction of the vertical component or the applied field and the r₃₃(32.2 x 10⁻¹²mV⁻¹) or r₁₃(10 x 10⁻¹²mV⁻¹) electro-optic coefficient of the substrate material for the TM and TE optical modes respectively. The modulator electronics are driven from a modulation voltage generator 21 to which a modulation potential is applied and where appropriate combined with a bias potential to generate the required control potential. The optical waveguides 1, 2 are coupled with respective arms 5, 6 of an optical beam splitter 7 having an input arm 8 along which incoming optical signals are fed. The output ends of the waveguides 1, 2 are coupled to respective input arms 9, 10 of a Y coupler 11 having an output arm 12 coupled with a single mode optical fibre 13. The device shown in Figure 1 differs from conventional devices by the inclusion of a second electrode pair 14, 15 aligned such that the gap between the electrodes 14, 15 overlies the waveguide 1 only. A horizontal electric field is generated between the electrodes 14, 15 the size of which is controlled by a bias potential generator 16 which generates a bias potential V₂. This second electrode pair 14, 15 induces electro-optical phase shifts only in the TE optical mode passing along the waveguide 1 via the interaction of the horizontal component of the applied electric field and the r₂₂(6.8 x 10⁻¹⁰mV⁻¹) electro-optic coefficient. The transfer characteristics of a conventional modulator device (equivalent to the device shown in Figure 1 in which the bias potential V₂ is zero) are indicated by the lines 17, 18 in Figure 2. The line 17 indicates the characteristic for the TM mode while the line 18 indicates the characteristic for the TE mode. It will be seen in Figure 2 that in general the maxima and minima of the characteristics do not occur at the sane modulation potential V₁. This is particularly undesirable where the device is used as a switch since it is desired to select two values for the modulation potential V₁ which cause optical signals to be coupled into the single mode optical fibre 13 or not coupled respectively. In general, for a single mode injected into the device, an optical signal will be guided by the single mode fibre 13 if the portions of the signal fed along the waveguides 1, 2 are in phase on reaching the Y coupler 11 since this will produce a zero order mode at the Y coupler. There will be no guidance due to the formation of first and higher order modes at the Y coupler if there is a π phase shift between the signals at the Y coupler. These different phase shifts can be obtained by applying suitable potentials V₁ to the modulator electodes 3, 4. It will be apparent from Figure 2 that the potentials suitable for switching the TE mode in the conventional device are V E1 and V E2 . However, at these potentials, the TM mode takes some intermediate values. To deal with this, a bias potential V₂ is generated by the generator 16 and applied to the electrodes 14, 15. The bias potential generates an electric field between the electrodes 14, 15 which causes an additional phase shift between the TE mode signals in the optical waveguide 1 and the same signals in the waveguide 2. This causes the TE mode signals to interfere in a different manner at the Y coupler 11 and effectively shifts the transfer characteristic for the TE mode. By suitably tuning the bias voltage V₂ the transfer characteristic can be shifted to take up a form similar to that indicated by the line 19 in Figure 2 in which a minimum of the TE mode characteristic 19 is coincident with a minimum of the TM mode characteristic 17. This occurs at a particular value of the modulation potential V₁ indicated at V off in Figure 2. Due to the near integral relationship between the electro-optic coefficients of the TE and TM modes which interact with the vertical field generated between the modulator electrodes 3, 4, the two transfer characteristics 17, 19 will have substantially coincident maxima at another potential V on . Thus, the device can be operated as a switch by causing the voltage V₁ to take up one of the two values V off and V on . In practice, a switch control signal is supplied along a line 20 to the generator 21 which adds to the signal a suitable bias voltage to produce the desired value for V₁. When used as an intensity modulator with the modulating voltage V₁ applied to the modulator electrodes 3, 4 the output intensity I is related to the input intensity I₀ by: I = I₀Cos² (/2) (1) where  is the phase difference between the two optical signals propagating in the two parallel waveguide arms 1, 2. In practice the phase shift  is proportional to the electro-optic coefficient r nm being employed and thus the optical response of the device is dependent on the state of polarisation of the optical mode. The device described will still display a marked polarisation sensitivity for analogue modulation signals supplied along the line 20 but will produce quasi polarisation independent operation for digital modulation signals. Figure 3 shows the output for the two orthogonal modes when an electrical pulse of amplitude equal to three times the switching voltage for the TM mode is applied to the modulation electrodes. The optical output for the TM mode is driven to its maximum value but ringing appears on the leading and trailing edges due to the strong overdriving of the optical response. Since the ringing is totally confined to the leading and trailing edges of the optical pulse it is unlikely to introduce signficant system penalties at data rates where the rise and fall times of a binary 1 are far less than the bit time ie. ∼lGbit/s. For the TE mode on the other hand the transition to the peak of the optical pulse occurs without ringing although additional insertion loss will arise since the peaks of the optical responses for the TE and TM modes do not exactly coincide since r₃₃ and r₁₃ are not precisely integrally related. In practice the excess loss for the TE mode is estimated to be less than 0.1dB assuming the values of the electro-optic coefficients r₁₃ r₃₃ quoted earlier.

Claims: An optical signal modulation device comprising first and second optical waveguides (1, 2) connected at one end thereof to an optical signal splitting device (7) which couples portions of an incoming optical signal into the waveguides and at the other end thereof to an optical signal combining device (11) which causes optical signals received from the waveguides to interfere; first modulating means (3, 4) responsive to a first control signal (V₁) to cause a relative phase shift between signals in the first and second optical waveguides, the first modulating means causing relative phase shifts in both TM and TE mode signals; and second modulating means (14, 15) responsive to a bias signal (V₂) to cause a relative phase shift between signals of one mode in the first and second waveguides and to leave signals of the other mode substantially unaffected, characterised in that the transfer characteristics of the two modes may be shifted relatively to one another in use in accordance with the bias signal (V₂) such that the minima of the transfer characteristics of the TE and TM modes are substantially coincident at a first predetermined magnitude (V off ) of the first control signal (V₁) and such that the maxima of the transfer characteristics of the TE and TM modes are substantially coincident at a second predetermined magnitude (V on ) of the first control signal (V₁). A device according to claim 1, wherein the second modulating means (14, 15) causes a relative phase shift between TE mode signals in the two waveguides (1, 2). A device according to claim 1 or claim 2, wherein the first and second modulating means (3, 4 and 14, 15) each comprises electric field generating means for generating respective electric fields which are substantially orthogonal to each other and to the waveguides. A device according to claim 3, wherein the electric field generated by the second modulating means (14, 15) extends across only one (1) of the waveguides. A device according to any one of the preceding claims fabricated on z cut lithium niobate. A device according to any one of the preceding claims, wherein the second modulating means (14, 15) is adapted to cause a substantially constant relative phase shift between the signals of said one mode. An optical switch comprising an optical signal modulation device according to any one of claims 1 to 6, wherein, first and second generators (21, 16) generate first and second control signals (V₁, V₂) respectively, the first control signal (V₁) having two voltage states (V on , V off ) and the second control signal (V₂) being a bias voltage which is such that the maxima of the transfer characteristics of the TE and TM modes are substantially coincident when the first control signal is in a first state (V on ), and such that the minima of the transfer characteristics of the TE and TM modes are substantially coincident when the first control signal is in a second state (V off ). An optical switch as claimed in claim 7, wherein the first generator (21) is such that, when the first control signal (V₁) is in the first state (V on ), both zero order TE and TM modes are substantially cut off over the combining device (11), and, when the first control signal is in the second state (V off ), the zero order TE and TM modes are substantially at respective maximum intensities. An optical switch according to claim 7 or claim 8, wherein the output of the combining device (11) is coupled with a single mode waveguide (13). An optical switch according to any one of claims 7 to 9, wherein the ratio of the magnitudes of the first and second states (V on and V off ) of the first control signal (V₁) is about 3.

Inventors: BEAUMONT ALAN ROY, BOOTH RICHARD CHARLES, CUNNINGHAM DAVID GEORGE, DAYMOND-JOHN BARBARA ELIZABETH, STALLARD WILIAM ANDREW, BEAUMONT, ALAN ROY, BOOTH, RICHARD CHARLES, CUNNINGHAM, DAVID GEORGE, DAYMOND-JOHN, BARBARA ELIZABETH FELINDRE, STALLARD, WILIAM ANDREW
Applicants: BRITISH TELECOMM, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1987
Main classification: G02F1
IPC codes: G02B6, G02F1