DE19701733A1 - Optical delay line - Google Patents

Abstract

A device for temporal storage and/or respectively a time-delay of signals with carrier frequencies in the optical spectral range. The first incoming optical signals (9) are converted using a transformation unit (8) into elementary excitation of the solid (6) which are then stored and transported to least one retransformation unit (11). In the retransformation unit (11), the retransformation of the elementary excitations of the solid (6) into second optical signals (10) takes place. The delay-time results from the propagation path length of the acoustic wave(s) from the transformation unit location to the retransformation unit location and the speed of acoustic waves of the solid (6). In an embodiment comprising piezoelectric semiconductor-structures, such elementary excitations can be photogenerated electron-hole pairs, which are field-ionized and dissociated by the piezoelectric field induced by the surface acoustic wave. The acoustic wave is excited using an interdigital transducer (7). For retransformation a conducting layer (12) is used which screens the potential modulation produced by the acoustic wave (5).

Description

The invention relates to a device according to the preamble of claim 1.

Optical signals are transformed into elementary excitations of a solid and these then moved through the solid by means of acoustic waves at the speed of sound. After After a certain duration of the acoustic wave, the elementary excitations are in again optical signals transformed back. The delay time of the optical signals is included given by the duration of the acoustic wave. During this delay time is one targeted processing or manipulation of the stored signals possible.

State of the art

Diverse technological developments in the field of fiber optic technology as well rapid advances in optoelectronic components (e.g. DFB - semiconductor lasers, photodiodes, etc.) have to the rapidly growing importance of modern optical signal processing contributed.

The delay and storage of optical signals, i. H. a temporary temporary Buffering with possible manipulation and processing of a signal with a Carrier frequency in the optical frequency range is of considerable importance for the Signal processing during communication or data transfer using optical signals, as well as for data processing of electrical signals in electronics and electronic Data processing.

While numerous possibilities are technologically realized for storing signals with carrier frequencies below 10 GHz (e.g. CCD, surface acoustic and bulk wave delay lines, magnetostatic delay lines, etc.), delay lines for optical carrier frequencies are limited to a few designs:

• A) Delay lines in which the intensity- or wavelength-modulated optical Signal z. B. is detected by a photodiode and the modulation signal thus obtained  now with an electronic circuit or with a "conventional" delay line is temporarily saved and possibly manipulated, in order to then re-in modulated optical signal to be transformed back.
• B) Delay lines in which the delay time is reached by the transit time of the optical signal in a medium. Monomode optical waveguide structures 1, ^{2 have} proven to be particularly suitable for this purpose, because their small mode dispersion enables high transmission bandwidths (<20 GHz). The delay time (1 ns - 100 µs) is determined here by the optical length of the optical fiber. The carrier wavelength of the optical signal is limited to the range of 0.6 µm - 1.6 µm due to the absorption and waveguide behavior of the glass fibers. Minimal attenuation losses of 0.1 dB / µs and time-bandwidth products up to 10 ⁵ can be achieved.

By optical signal taps along the route of the delay line ("tapped delay line") ² , the z. B. can be produced by the curvature of the fiber over a critical bending radius, and optical fiber loops ^2,3 ("recirculating delay lines") generated by directional couplers also enable simple signal processing functions (turret, filter, etc.).

Disadvantages of the prior art

Optical delay lines constructed according to the principle described in section A. are electronically very complex and therefore expensive processing of the detected Modulation signal, especially at higher transmission bandwidths. The necessities Components for such delay lines (photodiode, HL laser, modulator, digital Circuit, possibly conventional delay line for the modulation signal) be produced by a wide variety of process steps, creating a monolithic Integration and a related miniaturization of the component becomes impossible.

Although monomode optical fiber structures have excellent transmission bandwidth, low losses and high time-bandwidth products, their use as an optical delay line is severely restricted by the following disadvantages:
Component size: Due to the high group speed of the optical signal in the optical waveguide structure, distances of many kilometers are necessary for delay times of a few µs. Even with a minimized fiber optic cross section and bending radius, this results in very large, heavy and expensive components. Monolithic integration with other optoelectronic components (e.g. DFB laser) is also not possible here.

Restricted accessible wavelength range: absorptions along the delay stretch limit the accessible carrier wavelengths to a range of 0.6 µm - 1.6 µm a. For long delay times, the carrier wavelength must be chosen so that corresponds to an absorption minimum of the optical waveguide used.

Signal processing options and tunability: Although some very limited signal processing functions with optical fiber structures are possible by "taps" or loops - the production requires complex and poorly reproducible, micromechanical process steps. Delay lines with electrically controllable, continuously tunable delay times over larger areas cannot be realized. A fixed small number of adjustable, discrete delay times can be produced by the combination of several optical fibers and an acousto-optical spatial modulator ⁴ , but results in large, expensive and mechanically sensitive components.

State of the art bibliography

(1) K. Wilner, "Fiber-Optic Delay Lines for Microwave Signal Processing", Proc. of the IEEE, vol. 64, p. 805, 1976
(2) KP Jackson, "Optical Fiber Delay-Line Signal Processing", IEEE Trans. Microwave Theor. Tech., Vol. 33, p. 193, 1985
(3) S. Sales, "Fiber-optic delay line filters employing fiber loops: signal and noise analysis and experimental characterization", J. Opt. Soc. At A, vol. 12, p. 2129, 1995
(4) WD Jemison, "Acoustooptically Controlled True Time Delays: ExperimentaI Results", IEEE Micro. Guided Wave Lett., Vol. 6, p. 283, 1996.

The object of the invention is to delay or store optical signals Conversion of incoming optical signals into an elementary excitation of a solid body enable and after a delay time back into optical signals for further convert optical data processing back. The conversion happens here intrinsic and the inherent solid processes used by appropriate choice of Solid body can be tailored. In addition, the optical delay Cable compatible with previously known active and passive elements of optical data processing to be.

This object is achieved by a device with the features of claim 1.  

Elementary excitations of a solid such as B. the generation of electrons, holes, Electron-hole pairs or the formation of excitonic excitations can be illuminated of a solid with light. These elementary stimuli can before Recombination in photons, i.e. the back transformation into optical signals for a certain time be stored in the solid. Essential requirement for the "long-term" storage of Optical signals transformed in elementary excitations of a solid is a clear one Enlargement of the average lifespan of the elementary excitation of the solid compared to the natural lifespan of elementary excitation in the solid.

The increase in the average lifespan of elementary excitation in the solid state is in the Invention described here according to claim 1 by spatial separation of the photo generated charge carriers in the potential of an acoustic wave. The potential modulation is either by piezoelectric fields accompanying the wave or by Deformation potentials achieved. Due to the spatial separation of the photogenerated Charge carriers, the recombination rate is lowered dramatically because of the wave function overlap between electron and hole states is greatly reduced. In the potential of Acoustic wave captured or stored photogenerated carriers can then Expiry of the delay time and possibly elsewhere on the solid by externally adjustable modification of the potential modulation brought back to the recombination and thus be transformed back into optical signals.

An advantage of the invention according to claim 1 is that compared to already known and technically implemented delay lines drastically reduced design, which ultimately on the transformation of the speed of light into five orders of magnitude smaller Sound speed of the solid body is based. So, e.g. B. for a 10 µs long Delay time requires a delay line of approx. 3 km of monomode or glass fiber, while a 3 cm long solid is sufficient in the invention.

The structuring of the solid body for the optical delay line presented can completely through conventional and rational planar lithographic process steps. Each Depending on the embodiment and choice of the solid, monolithic integration is also possible a variety of optoelectronic components (e.g. semiconductor DFB laser, Photodiodes, etc.) possible. Compared to a conventional one Optical fiber delay line is therefore the presented optical delay line be much cheaper to produce.

In addition, the running track can also be used as an active component for analysis or external Manipulation of the optical signals transformed into elementary excitations. There  in an embodiment with a surface acoustic wave the elementary excitations are carried out directly on the surface of the solid, manipulation z. B. by "Tapes" that can be produced by planar technology are possible in a simple manner. A variety of Results and techniques of advanced signal processing with acoustic waves can be used in this context. So optical signals after the Transformation in elementary excitations of a solid by both external electrical Signals and acoustic waves are shifted, switched or switched on the solid be modulated before they are transformed back into optical signals. So it is possible, in addition to a simple time delay of an optical signal, also complex Signal processing functions (filters, convolvers, multiplexers) for optical signals on them Way to realize.

In contrast to known embodiments with optical delay lines monomode optical waveguide structures can be according to the invention Claim 1, set the delay time externally and continuously within wide limits.

Embodiments of the invention are shown in the drawings and are in following described in more detail.

By illuminating a solid according to FIG. 1 - in this case a semiconductor structure - with light ( 1 ) of an energy which is equal to or greater than the effective energy gap Eg of the semiconductor, electron-hole pairs ( 3 ) are photogenerated. Here, an electron is lifted by the absorbed photon from the valence into the conduction band ( 2 ) of the semiconductor. These electron-hole pairs ( 3 ) can form one of the elementary excitations of the solid body, in this case an exciton. Normally they recombine either radiating with the emission of photons ( 4 ) or non-radiating with loss of energy due to processes other than radiative processes after the end of the mean life. In the present example, this phenomenon is the known photoluminescence of the solid and is shown in FIG. 1 (b).

The electron-hole pairs ( 3 ) or excitons can first be polarized and finally dissociated both by static lateral or vertical electric fields and by dynamic lateral or vertical electric fields of sufficient field strength. The photogenerated charge carriers ( 3 ) are spatially separated, as shown in Fig. 2. In the case of an acoustic wave ( 5 ) and the electric fields accompanying it at the speed of sound, the spatial separation of the charge carriers leads to their storage in the potential minima of the line ( 2 ) or the potential maxima of the valence band ( 2 ). These potential extremes then move through the solid at the speed of sound and are laterally approx. Λ / 2 apart.

A possible simple embodiment of an optical delay line according to claim 1 is shown in FIG. 3.

By external shielding of the piezoelectric fields, e.g. B. by means of a conductive layer ( 12 ) in the sound path, the spatial separation of the charge carriers - in this case, the electron-hole pairs ( 3 ) photogenerated by an incoming optical signal ( 9 ) - can no longer be maintained and the charge carriers are emitted recombine by light ( 10 ). In this case, the electron-hole pairs ( 3 ) are recombined at the front edge of the conductive layer ( 12 ) and forms the decoupling device ( 11 ). The energy of this light is determined by the energy of the radiating transition. During the transit time of the acoustic wave between the generation and recombination area on the solid ( 6 ), the optical signals transformed in elementary excitations of the solid in the optical coupling element ( 8 ) have been time-delayed or stored. The acoustic wave ( 5 ) can be operated both in continuous operation and, as shown here, in a pulsed manner and is generated in a known manner with sound transducers ( 7 ). The conductive layer ( 12 ) can be implemented either by metallization or else by free charge carriers in the semiconductor structure ( 6 ). In this case, the conductivity of the layer can be controlled via the field effect, for example, and the targeted recombination at definable points can even be set externally. It is also possible, instead of a simple, passive conductive layer, an active component, for. B. use a semiconductor laser for coupling ( 11 ) of the signal. For this purpose, the electrons and holes transported by the acoustic wave can also be separated transversely to this by a static lateral electric field directed perpendicular to the sound direction of two planar electrodes, in addition to the spatial separation in the direction of sound propagation. If the electrons are now fed to the n-doped region and the holes to the p-doped region of a semiconductor laser biased just below the threshold voltage, so that this is raised above the laser threshold, an amplification of the time-delayed outcoupled optical signal ( 10 ) can be achieved.

The recombination of the photogenerated charge carriers that are spatially separated in the field of the wave ( 5 ) can be made possible again not only by external shielding of the piezoelectric fields by a conductive layer ( 12 ). The presence of another acoustic wave ( 13 ) can also result in the spatial separation of the photogenerated charge carriers stored in the field of the first wave being locally eliminated. "Collision" of two wave packets, which need not necessarily be collinear, results in a wave field in which the spatial separation of the charge carriers is canceled again. This is shown in FIG. 4. The delay time between the generation of the two acoustic wave packets ( 5 ) and ( 13 ) determines the location and thus the time of the "collision" at which the recombination of the charge carriers is initiated. This time and thus the delay or storage time of the optical signals can thus be set and controlled externally.

The optically generated elementary excitations ( 3 ), which are stored in an acoustic wave ( 5 ) in the solid ( 6 ), can be spatially displaced in the solid at the speed of sound. This makes it possible to transport the elementary excitations to suitable locations on the solid and to divide the optically generated and stored elementary excitations ( 3 ) into smaller subunits (multiplexing, demultiplexing). A possible embodiment of such a multiplex device is outlined in FIG. 5. Here, by triggering the recombination in various decoupling devices ( 11 ), z. B. by externally controllable conductive layers or colliding acoustic waves, the elementary excitations ( 3 ) stored in the acoustic wave ( 5 ) at different locations are transformed back into one or different time-delayed optical signals ( 10 ).

By using not only an acoustic wave, it is also possible to transport the optically generated charge carriers stored in the solid in the form of an elementary excitation in directions on the solid that are not in the direction of propagation of the primary storing acoustic wave (beam steering). A possible embodiment for this is outlined in FIG. 6. Here an incoming optical signal ( 9 ) is transformed into elementary excitations of a solid ( 6 ) and stored in a sound wave by the sound transducer ( 7 ) and initially transported in the direction of its propagation. A second wave from the sound transducer ( 14 ) "takes over" the stored optical signal and transports it - in this example orthogonal to the original direction of propagation - to the decoupling device ( 11 ).

By using not only a primary storage acoustic wave is a modification the spatial separation of photogenerated elementary excitations by changing the Length scale possible. This can be done by using the secondary storage shaft has a different wavelength than the primary storing wave.

Experimental evidence of the time delay of an optical signal is shown in Fig. 7 and Fig. 8. An embodiment of an optical delay line according to claim 1 in the form of a structured semiconductor quantum well structure ( 18 ) is used here.

The layer structure of the type I quantum well structure is shown schematically in FIG. 7 (a) and is not true to scale. For this purpose, a 10 nm thick In _0.15 G _0.85 As layer ( 16 ) and a 10 nm thick GaAs cover layer ( 15 ) are grown epitaxially (for example by molecular beam epitaxy) on a 0.5 mm thick undoped GaAs substrate ( 17 ).

In Fig. 7 (b) shows the view is displayed on the laterally structured surface. The surface acoustic wave (f = 840 MHz) is generated by means of a high-frequency signal generator ( 22 ) connected to a conventional sound transducer ( 7 ). The optical input signal is coupled in by a semiconductor laser ( 23 ) focused on the location ( 19 ). In this case, the optical signal is a simple signal of constant intensity and wavelength. After a running distance ( 20 ) of 1.1 mm here, the semi-transparent metal layer ( 21 ) transforms the electron-hole pairs ( 3 ) photogenerated in ( 19 ) back into an optical signal ( 10 ) and decouples them again at the edge of the metal layer ( 21 ) . The exact temporal process of the acoustic transport of the elementary excitations is shown in Fig. 8. At the time t = t ₁ = 290 ns ( FIG. 8 (a)), the 200 ns long acoustic surface ( 5 ) is exactly at the location ( 19 ) in the coupling-in area of the optical signal. The photogenerated excitons in the In _0.15 Ga _0.85 As quantum well are _dissociated by the y component of the piezoelectric field of the surface wave ( 5 ) into spatially separated electron-hole pairs and cannot recombine due to the very small wave function overlap. The optical scatter signal measured by the photomultiplier ( 25 ) at this point in time by photoluminescence of excitons at location ( 19 ) therefore shows a minimum. The elementary excitations stored in this way are now transported at the speed of sound (here v _s = 2865 m / s) over the running distance ( 20 ) to the semitransparent metal layer ( 21 ). Once the acoustic wave pulse (5) has passed through the coupling-in point (19) of the semiconductor laser (23) is turned to experimental artifacts by scattered light from the place (19) be completely ruled out. At the time t = t ₂ = 670 ns (ie 380ns later) ( FIG. 8 (b)), half the acoustic wave pulse has arrived under the semitransparent metal layer ( 21 ). The lateral potential modulation by the piezoelectric fields of the surface wave ( 5 ) is electrostatically shielded by this conductive layer. The spatial separation of electrons and holes is eliminated and there is a radiant recombination. The optical signal ( 10 ) transformed back in this way is fed to a photomultiplier ( 25 ) with a converging lens ( 26 ). At time t = t ₂ , the photoluminescence intensity detected in FIG. 8 (c) therefore shows a clear maximum. The experimentally proven transport of the optical signal stored in spatially separated electron-hole pairs is already very efficient and robust in this simple structure and robust against external influences (e.g. temperature, magnetic field, electromagnetic interference, mechanical shocks).

Reference list

1

Irradiated photon

2nd

Valence and conduction band edges

3rd

Elementary excitation, here an electron hole pair

4th

Radiated photon

5

Acoustic wave / acoustic wave pulse with stored and transported elementary excitations

6

Solid

7

Sound transducer

8th

Coupling device for transforming optical signals in elementary excitations of a solid

9

Incoming optical signal

10th

Delayed outgoing optical signal

11

Decoupling device for transforming elementary excitations of a solid into an optical signal

12th

Conductive layer

13

Colliding acoustic wave

14

Sound transducer

15

GaAs top layer, 10 nm thick

16

InGaAs layer, quantum well, 10 nm thickness

17th

GaAs substrate layer, 0.5 mm thick

18th

Semiconductor heterostructure

19th

Place of focus of the with a converging lens (

24th

) bundled semiconductor laser (

23

)

20th

Delay line

21

Semitransparent metal layer, (5 nm NiCr)

22

Radio frequency source

23

Semiconductor laser

24th

Coupling device in the form of a converging lens

25th

Photomultiplier for the detection of the outgoing optical signal (

10th

)

26

Decoupling device in the form of a converging lens

Claims (26)

1. Device for time delay and storage of optical signals ( 9 ) with at least one device ( 8 ) for coupling optical signals by transforming the optical signals into elementary excitations ( 3 ) of a solid ( 6 ) and at least one device ( 11 ) by the expiry a backward transformation of the elementary excitations into optical signals ( 10 ) is effected after a storage or runtime, characterized in that the elementary excitations ( 3 ) are stored or transported in a solid by acoustic waves ( 5 ). 2. Device according to claim 1, characterized in that the transformation of optical signals ( 9 ) in elementary excitations ( 3 ) of a solid ( 6 ) by photogeneration of electrons, holes, electron-hole pairs, or excitatory excitations. 3. Apparatus according to claim 1, characterized in that the transformation of optical signals ( 9 ) in elementary excitations ( 3 ) of a solid ( 6 ) is caused by thermal generation of electrons, holes, electron-hole pairs or excitonic excitations. 4. Device according to one of the preceding claims, characterized in that the solid body ( 6 ) is a semiconductor, insulator or metal. 5. Device according to one of the preceding claims, characterized in that the solid body ( 6 ) is a connection of different semiconductors or insulators and / or metals (epitaxial layers, heterostructures, MOS, MIS structures or hybrid structures made of said materials). 6. Device according to one of the preceding claims, characterized in that regions of the solid body ( 6 ) are designed in the form of a so-called quantum well structure or combinations of quantum well structures or equivalent potential wells. 7. The device according to claim 1, characterized in that the acoustic waves ( 5 ) are surface waves or bulk waves. 8. The device according to claim 1, characterized in that the device ( 8 ) for transforming optical signals ( 9 ) in elementary excitations ( 3 ) of the solid body ( 6 ) by external illumination of the solid body is represented either directly or by at least one optical waveguide. 9. The device according to claim 1, characterized in that the device ( 8 ) for transforming optical signals in elementary excitations ( 3 ) of the solid ( 6 ) represents a light source which is already integrated in the device (light emitting diode, laser structure, waveguide structure). 10. The device according to claim 1, characterized in that the storage of the optical signal ( 9 ) by a spatial separation or accumulation of the elementary excitations ( 3 ) is effected by a potential modulation due to a sound wave ( 5 ). 11. Device according to one of the preceding claims, characterized in that the potential modulation is mediated by piezoelectric fields of the sound wave ( 5 ) or by coupling the sound wave ( 5 ) to the deformation potential of the solid body ( 6 ). 12. The apparatus according to claim 1, characterized in that the device ( 11 ) for transforming back the elementary excitations ( 3 ) into optical signals ( 10 ) is designed as a conductive layer ( 12 ) for shielding the piezo potentials. 13. The apparatus according to claim 1, characterized in that the device ( 11 ) for transforming the elementary excitations ( 3 ) into optical signals ( 10 ) as a wave sump for extinguishing the sound wave ( 5 ) and thus to cancel the potential modulation generated by the wave . 14. The apparatus according to claim 12, characterized in that the conductive layer ( 12 ) is a metal layer. 15. The apparatus according to claim 12, characterized in that the conductive layer ( 12 ) is designed in the form of a doping in the solid ( 6 ). 16. The apparatus according to claim 12, characterized in that the conductive layer ( 12 ) is formed by a region of the solid body ( 6 ) which can be externally tuned in its conductivity and can thereby also be “non-conductive” (inversion layer, accumulation layer, depletion layer). 17. Device according to one of the preceding claims, characterized in that the tunability of the conductivity of the conductive layer ( 12 ) is achieved by electrical means. 18. Device according to one of the preceding claims, characterized in that the tunability of the conductivity of the conductive layer ( 12 ) is achieved by illuminating the conductive layer ( 12 ). 19. Device according to one of the preceding claims, characterized in that the tunability of the conductivity of the conductive layer ( 12 ) is achieved by changing the temperature. 20. Device according to one of the preceding claims, characterized in that the tunability of the conductivity of the conductive layer ( 12 ) is achieved by pressure changes on the conductive layer ( 12 ). 21. The apparatus according to claim 1, characterized in that the device ( 11 ) for transforming back the elementary excitations ( 3 ) into optical signals ( 10 ) is in the form of a local potential modulation. 22. The apparatus according to claim 21, characterized in that the local potential modulation is carried out in the form of at least one further sound wave ( 13 ) next to the primary wave ( 5 ). 23. The device according to claim 21, characterized in that the local potential modulation in the form of at least one electrostatically defined potential modulation is carried out. 24. The device according to claim 21, characterized in that the local potential modulation in the form of at least one local change of material properties defined potential modulation is carried out. 25. The device according to claim 1, characterized in that the device ( 11 ) for transforming the elementary excitations ( 3 ) back into optical signals ( 10 ) is formed by at least one optically active element with amplification function. 26. The device according to claim 25,  characterized in that the optically active element with a gain function in the form a laser structure.