DE4404777A1 - Electro=optical actuator for high-density WDM optical waveguides - Google Patents

Abstract

The actuator includes electrodes (Ei) at equidistant lengths in a waveguide structure (W) which is formed in a substrate. The individually connected and switched electrodes are fed from a common source of voltage (UB). At least four electrodes are present in a sensing region (Sj), with at least five sections present. The switching is of electrodes are performed due to conduction of light, individual binary patterns being produced relating to measured individual wavelengths between different spectral regions. Some 100 gps. of four electrodes each form a sensing section can be constituted within the overall length (L) of 10 or 20 mm.

Description

The invention relates to an actuator for influencing light beams in optical waveguides, the optical properties of which can be changed by means of electro-optical effects, equipped with electrodes (E _i - with i = 1, 2,..., N) in an equidistant arrangement along the waveguide structure (W) which carries light beams to be influenced, at least four electrodes (E _i ) each being assigned to one scanning section (S _j ) and at least five such scanning sections (S _j ) 5 being provided.

The prior art, which is the starting point for the invention, is EP 0 260 595 A2. There are related to the continuous, reset-free polarization and phase control also essential Basics for the constructive design, material selection and the way of working of devices and arrangements for influencing light beams in optical Treated waveguides. The proposed, known measures essentially aim depends on, for predetermined operating conditions, e.g. B. a specific Operating wavelength at which the arrangement or device is to operate, the to achieve the best results.

One of the fields of application of such arrangements and facilities is the field of optical wavelength division multiplex systems. There are used for mixing and separating of channels elements and systems that have a high channel occupancy density enable sufficiently high crosstalk attenuation. In such optical Wavelength multiplexing systems (high density wavelength division multiplexing - HDWDM -) several hundred channels are supposed to be transmitted simultaneously in an optical fiber and then separate them again.  

The technical problem on which the invention is based is that of suitable ones To be able to train and build devices or devices in such a way that both a universally applicable way of working with several different components as well as such a component as a single piece of identical products boundary conditions that differ in value, e.g. B. at several wavelengths, for Use can:

For this purpose, an actuator of the type mentioned at the outset forms the solution according to the invention in that over a total length (L) of the waveguide structure (W) in the 10 mm to 100 mm range, the electrodes (E _i ) with a subsequent dimension in the direction of the waveguide structure (W) trained in the 5 µm range and provided with their own supply lines that can be switched individually to a uniform voltage potential (U _B ), and individual patterns (P _x ) for wiring states of the electrodes (E _i ) each have a specific wavelength (λ _x ) within a tunable range Spectral range (Δλ) for influenceable light beams is assigned.

The essential meaning of the invention thus lies in structurally identical Products, e.g. B. by means of fixed programming, each with different nominal values of an operating parameter or - especially by random access to Programming - each individual product within a tuning range to be able to set different nominal values of the operating parameter. The function The arrangements according to the invention are thus both generated and electro-optically controlled. It is controlled by a software-defined algorithm.

Preferred embodiments of the invention, especially those with features that in the subclaims are also given in key words below - without Completeness - compiled advantages of:

• - Large manufacturing tolerances can be allowed by the Software control can be compensated.
• - Defective electrodes are excluded from switching through to the voltage potential (U _B ); the function of defective electrodes is taken over by other electrodes by adapting the pattern for the wiring states.
• - The area for the tuning / tuning as well as the passage width (Selectivity) are adjustable.
• - The pass curves can be influenced, e.g. B. sin², Gaussian, hat characteristics.
• - calibrations, e.g. B. with regard to temperature dependence, can be repeated one or more times be made regularly.
• - In optical wavelength division multiplex systems, channel spacing is 1 nm - in one Tuning range from z. B. 1450 nm to 1650 nm or 1950 nm Light wavelength, so easily 200, possibly 500 channels - manageable.
• - Based on InP substrate material can be buried in chips Waveguide structures made of quaternary materials with very different Refractive indices.
• - The connection of the electrodes only needs between two states - logical Values 1 and 0 - to be distinguished. Both the design effort at the Electrode equipment - only one supply line with a simple on / off switch (Transistor) per electrode - as well as the structure of the patterns for Circuit states - sequences from elements 0 and 1 - are limited to easily manageable means.
• - The potential (U _B ) to which electrodes are applied is the same for all operating electrodes once the operating state has been set, but can be different for different operating states, ie depending on the wavelength of the light beam to be influenced and the type of influencing to be carried out, at z. B. ± 50% changeable. This makes it possible to keep the amplitudes at the filter output at 100% for all wavelengths.
• - Because of the proportionality between coupling factor χ and the applied voltage, χ-tapering enables an improvement in the filter function by assigning different partial voltages, which represent a fixed division ratio of U _max , to the individual - hard-wired - electrodes. The total voltage U _max is then the same for all electrodes.

Further explanations of the essential content of the invention can be found in the post-published article: "Theoretical Investigation of an Tunable, Narrowband, Electro-Optical Filter with Low Crosstalk and a Large Number of Optical Channels" - HP Nolting, M. Gravert - on the occasion of IPR 94 , 17th-19th February 1994, San Francisco (USA). The efforts in the field of optical wavelength division multiplex systems by increasing the controllable number of channels beyond the order of 10 are closely related to the achievable bandwidth for the tuning range of electro-optical filters. Because of the proportionality between this bandwidth and the change in the refractive index, which is very small in the materials under consideration, namely about 1 ‰ (Δn _EO / n = 0.001), one has so far z. B. cascaded different filters.

Optical filters that can be tuned or tuned are based on the principle of coupled modes. The phase differences (beat length) between the two modes - TE / TM components / eigenmodes - which are dependent on the wavelength - are converted into a wavelength-dependent power at the filter output. The coupling at the nominal wavelength of the filter can be brought about with a periodic coupling structure despite the phase mismatch δ = (β₁-β₂) / 2 (β₁; β₂: propagation constants of the two eigenmodes). The length of this coupling period - in the present context in connection with the invention and its embodiments the term "scanning section-S _j " is used - is related to the phase mismatch using the expression δ (λ _o ) = π / S _j .

Such arrangements fully implement one mode in the other when the Resonance conditions are met. The boundary conditions are then fixed Periodicity of the electrode / waveguide construction together with the slight change given the refractive index difference.

In any case, four electrodes (E _i ) are present per period in the invention so that the actual beat length Λ _tune can be detected. For the dimensions of the electrodes, ie their length in the direction of the waveguide structure and their distance from the neighboring electrode, there is a follow-up measure according to the equation δ (λ _o ) = π / S _j . The pattern of the wiring states of the electrodes can be derived from the relationship

determine.

I is the atomic number of an electrode.

It is the logical value for the pattern:
1 if A <1
0 if A 1.

The principle of control behavior can be based on the theory of coupled modes (coupled mode theory - CMT) or the eigenmode development. For the In the case of uniform coupling (constant coupling factor), the filter response is one sin² function. A taped reduced or partially similar suppressed coupling along the interaction path enables special adjustments the filter response, including the reduction of side lobes.  

Example calculations result in 10 mm for the entire length L of an arrangement with even coupling (uniform χ; crosstalk <10 dB) and 20 mm at uneven coupling and tapered sin² transmission characteristic (χ-tapering; Crosstalk <25 dB). In both cases the channel spacing is 1 nm. The range of Ab or tunability is very large, it extends from 1450 in the calculated example nm to 1650 nm, so leaves 200 channels in an optical wavelength division multiplex system to.

Both the materials that can be used and the structural design allow for the invention also wavelengths of z. B. up to 1950 nm. The number of channels is then included 500.

The pass width of electro-optical filters according to the invention is inversely proportional the number of scan sections required. So there are enough for narrow filters long arrangements to be provided with the scanning sections in terms of material achievable refractive index differences and not arbitrarily short even on lithography techniques can be interpreted. The suppression of secondary maxima at z. B. taped coupled waveguide structures require a doubling of the number required Electrodes, including the total length of such arrangements. Due to the functional generation and control of devices according to the invention and Arrangements, including filtering properties, can be achieved with little Manufacturing effort large numbers of identical components for different Operating and operating conditions are provided. It is also straightforward possible to use monolithically integrated optical or opto-electronic circuits to build up components according to the invention.

In the drawing, embodiments of the invention are shown schematically. Here demonstrate:  

Figure 1 is an actuator in plan view.

Fig. 2 is coupled an actuator with two, vertically adjacent waveguides in the cross-section and a representation of the refractive index profile;

Fig. 3 is an actuator similar to Figure 2 but with horizontally adjacent waveguides.;

4 shows a mode converter in a plan view, depicting the course of the scanning and the mode conversion M2 / M1.

FIG. 5 shows graphs of transmission curves of a filter at three different wavelengths;

Fig. 6 and Fig. 7 shows an actuator in plan view, similar to Fig. 1 and corresponding to Fig. 3 is a block diagram of a monolithic integrated circuit.

In the arrangement according to FIG. 1, a waveguide structure W is in a substrate with electrodes E _i with i = 1, 2,. . . , N equipped. Each of the electrodes E _i is provided with its own supply line, all of which lead to a common voltage source U _B. There is an on / off switch in each supply line, shown symbolically in FIG. 1. The electrodes E _{i are} wired according to individual, e.g. B. patterns supplied by a processor µP, which consist of the logical values 1 and 0. These patterns for the wiring states of the electrodes E _i generate and control the arrangement as an actuator for influencing light beams which are guided in the waveguide structure W.

Four electrodes E _i form a scanning section S _j ; five or more, in practice several 100, are located on the total length L of the actuator.

For a given wavelength of λ = 1.55 µm of the light wave to be influenced and with material that offers the desired refractive index difference Δn, the required length that the arrangement must have can be determined. If the waveguide structure W in a substrate made of InP consists of a quaternary material for a wavelength of 1.3 μm, the result for a TE / TM mode converter is Δn _{TE / TM} = 0.030, a required length L = 70 mm and a number of 1300 periods Λ _tune for a complete implementation of one fashion in the other. If a tapered coupling is also required, the number of periods must be doubled to 2600 and the length correspondingly to 140 mm.

In FIGS. 2 and 3, the arrangements of each of the coupled waveguides W1, W2 are shown in cross section in which the guided wave in the waveguide W1 light wave is influenced by an electric field between the electrodes E _i and a counter electrode. The waveguide W2 should not be influenced by the electrical field. For vertically adjacent, buried waveguides W1, W2 - FIG. 2 - the p / n transition is positioned at a corresponding distance from the waveguide W2, for horizontally adjacent waveguides W1, W2 - FIG. 3 - the waveguide W2 is located outside the electrical one Field.

Both waveguides W1, W2 should have the greatest possible difference with respect to the refractive index n _eff . The lowering of the refractive index n under the influence of an electric field strength takes place in the waveguide W1 with the already higher refractive index compared to that of the waveguide W2.

In the mode converter shown in FIG. 4, the structure and mode of operation of which is essentially described in connection with the explanations for FIG. 1, the logical values 1 and 0 of the pattern P _x for an occupancy state are given as an example for the individual electrodes E _i . For this pattern P _x , the graph shows the course of the scanning with a prevailing beat length Λ _tune and the conversion from mode 1 to mode 2 .

It is essential for the invention, as these illustrations show, that an influence by the electric field only needs to take place if between two changes in the direction of the beat by the electrical Field is supported in only one trend. A support for the other tendency does not bring an improvement in principle, but requires a second, opposite polarized voltage potential and a more complex circuit of the electrode leads. On the other hand, a shortening of the component length / operating voltage is gained the factor 2.

The transmission curves - Fig. 5 - of a filter as an embodiment of the invention show for three nominal wavelengths, 1450 nm, 1550 nm and 1650 nm, the same shape for tapered coupling (solid lines), at 1550 nm for comparison (shown in dashed lines) also for uniform Coupling. The filter can be tuned, ie the desired nominal wavelength and also the transmission characteristic are set by means of the associated patterns for occupancy states of the electrodes. The pass width enables channel spacing of 1 nm.

The arrangement shown in FIG. 6 largely corresponds to that of FIG. 1 with the difference that an asymmetrical coupler has two coupled waveguides W1, W2 (cf. also FIG. 3) made of different quaternary materials (λ _Q1 = 1.3 μm, t₁ = 0.5 µm and λ _Q2 = 1.05 µm, t₂ = 1.0 µm). The difference in refractive index is Δn _1/2 = 0.055. Electrodes E _i (Δn _EO = 0.004) above the waveguide with the higher refractive index result in a uniform coupling in an arrangement with a total length L = 10 mm in only 440 beat periods, with tapered coupling with a total length L = 20 mm in 880 beat periods.

The change in the mode distribution and the dispersion of course influence the Coupling ratios. The coupling factor is therefore not constant over the entire  Kept tuning range, resulting in a slight reduction in the Filter passage properties leads (reduced efficiency, little variation of the Half width - FWHM). This can be done completely by adjusting the total voltage be compensated.

In Fig. 7 is a monolithic integrated circuit structure is indicated as a block diagram. Two actuators CD (control device) according to the invention are provided. An incoming light wave is separated into two arms for one polarization - TE and TM - in each arm filters z. B. the actuator CD located there has the same wavelength - the switching pattern can be different for both polarizations - which are then given for further transmission to a common light guide. In both arms, the actuators CD are identical products. The processors µP supply the patterns for the wiring states of the electrodes in the actuators CD and, if necessary, also carry out further control and regulating functions.

Claims (8)

1.Actuator for influencing light beams in optical waveguides, the optical properties of which can be changed by means of electro-optical effects, equipped with electrodes (E _i - with i = 1, 2,...; N) in an equidistant arrangement along the waveguide structure (W ), which leads to the light rays to be influenced, at least four electrodes (E _i ) per scanning section (S _j ) and at least five such scanning sections (S _j ) 5 being provided, characterized in that over a total length (L) of the waveguide structure (W) in the 10 mm to 100 mm range, the electrodes (E _i ) are designed with a follow-up dimension in the direction of the waveguide structure (W) in the 5 mm range and provided with their own feed lines that can be switched individually to a uniform voltage potential (U _B ) and individual patterns (P _x ) for the wiring states of the electrodes (E _i ) each have a specific wavelength (λ _x ) within a tunable spectral spectrum realm (Δλ) is assigned for influenceable light beams. 2. Actuator according to claim 1, characterized in that the equipment contains a processor (µP) which controls the wiring states of the electrodes (E _i ). 3. Actuator according to claim 1 or 2, characterized in that the waveguide structure (W) from two coupled, asymmetrical waveguides (W1, W2) exists. 4. Actuator according to one of claims 1 to 3, characterized in that the arrangement on InP substrate material with quaternary material for the Waveguide structure (W) is formed. 5. Actuator according to claim 3 and 4, characterized in that  the waveguides (W1, W2) made of quaternary materials with very different Refractive indices (n1, n2) exist. 6. Actuator according to one of claims 3 to 5, characterized in that individual patterns (P _x ) for wiring states of the electrodes (E _i ) is assigned a tapered coupling characteristic. 7. Actuator according to one of claims 1 to 6, characterized in that several, also different types of elements and / or links of the integrated optics as monolithically integrated optical circuit (OIC) are constructed. 8. Actuator according to one of claims 1 to 7, characterized in that several, also different types of elements and / or links of the integrated optics as well electronic components and / or circuits as monolithically integrated opto electronic circuit (OEIC) are built.