RAN, Moshe (58 Keshet st, R'eut, 71900, IL)
HADARIM, Motti (28 Avnei Hachoshen St, Givaat Zeev, Jerusalem, 90917, IL)
RAN, Moshe (58 Keshet st, R'eut, 71900, IL)
HADARIM, Motti (28 Avnei Hachoshen St, Givaat Zeev, Jerusalem, 90917, IL)
| We Claim:
1. A system for converting a modulated optical signal to a modulated electric signal, the system comprises: an optical interface, which is adapted to receive the modulated optical signal, wherein the modulated optical signal is modulated by a radio frequency signal; a substrate; and a thin semi-insulating microstrip, which is coupled to the substrate on one side and to the optical interface on the other side; wherein (a) a depth of the microstrip is of the scale of a penetration depth of the modulated optical signal into the microstrip; (b) an impedance of the microstrip is very responsive to a modulation of the modulated optical signal.
2. The system according to claim 1, wherein the radio frequency signal is an ultra wide band signal.
3. The system according to claim 1, adapted to facilitate an applying of a magnetic field to the microstrip.
4. The system according to claim 1, wherein the microstrip comprises a heterostructure of P-N junctions in vicinity to an applying position in which the modulated optical signal is applied to the microstrip, wherein the heterostructure is adapted to at least partially confine a generation of electron hole pairs to a vicinity of the applying position.
5. The system according to claim 1, wherein an impedance of the microstrip is selected so as to electrically match the system and an external system to which the modulated electric signal is provided.
6. The system according to claim 1, wherein the microstrip is a T-shaped microstrip.
7. The system according to claim 1, comprising multiple microstrips.
8. The system according to claim 1, further adapted to apply an unmodulated electric signal to the microstrip.
9. The system according to claim 9, further adapted to mix the unmodulated electric signal and the modulated electric signal.
10. The system according to claim 9, wherein the microstrip comprises a gap, which electrically isolates parts of the microstrip when no optical signal is applied.
11. The system according to claim 1, further comprising a resistor that is electrically coupled to the microstrip, and which is adapted to transmit the modulated electric signal.
12. The system according to claim 1, wherein the microstrip further comprises an electric signal antenna which is adapted to provide the modulated electric signal.
13. The system according to claim 12, further adapted to facilitate integration with a front end radio frequency component.
14. A method for converting a modulated optical signal to a modulated electric signal, the method comprises: receiving the modulated optical signal by an optical interface, which is modulated by a radio frequency signal; applying the modulated optical signal to a thin semi-insulating microstrip, which is coupled to a substrate; wherein (a) a depth of the microstrip is of the scale of a penetration depth of the modulated optical signal into the microstrip; (b) an impedance of the microstrip is very responsive to a modulation of the modulated optical signal; and providing the modulated electric signal, which is responsive to the modulated optical signal and that is generated as a result of the interaction between the modulated optical signal and the microstrip;
15. The method according to claim 14, wherein the receiving comprises receiving a modulated optical signal that is modulated by an ultra wide band radio frequency signal.
16. The method according to claim 14, further including applying a magnetic field to the microstrip.
17. The method according to claim 14, wherein the applying comprises applying the modulated optical signal to a microstrip that comprises a heterostructure of P-N junctions in vicinity to an applying position in which the modulated optical signal is applied to the microstrip, wherein the heterostructure is adapted to at least partially confine the generation of electron hole pairs to the vicinity of the applying position.
18. The method according to claim 14, further comprising selecting an impedance of the microstrip so as to electrically match a system that includes the microstrip and an external system to which the modulated electric signal is provided.
19. The method according to claim 14, wherein the applying comprises applying the modulated optical signal to a T-shaped microstrip.
20. The method according to claim 14, wherein the applying comprises applying the modulated optical signal to multiple microstrips.
21. The method according to claim 14, further comprising applying an unmodulated electric signal to the microstrip.
22. The method according to claim 21, further comprising mixing the unmodulated electric signal and the modulated electric signal.
23. The method according to claim 21, further comprising applying the modulated optical signal to a microstrip that comprises a gap, which electrically isolates parts of the microstrip when no optical signal is applied.
24. The method according to claim 14, wherein the providing comprises providing the modulated electric signal by a resistor that is electrically coupled to the microstrip.
25. The method according to claim 14, wherein the providing comprises providing the modulated electric signal by an electric signal antenna which is comprised in the microstrip.
26. The method according to claim 25, wherein the providing of the modulated electric signal by the electric signal antenna comprises providing the modulated electric signal by an electric signal antenna that is adapted to facilitate integration with a front end radio frequency component. |
METHOD AND SYSTEM FOR CONVERTING A MODULATED OPTICAL SIGNAL TO A MODULATED ELECTRIC SIGNAL.
Related applications
[001] This application claims priority of U.S provisional patents serial number 60/806268 , 60/806269 and 60/806270 filing date 30 June 2006, which are hereby incorporated herein, in its entirety, by this reference.
Field of the invention
[002] The present invention relates to methods and systems for converting a modulated optical signal to a modulated electric signal.
Background of the invention
[003] Digital optical fiber communication systems are now increasingly deployed in many applications as local area networks and communications infrastructure carrying high rate, e.g. lGbps and lOGbps Ethernet, data.
[004] The main advantages of both digital and analog optical fiber systems are their wide spectral characteristics that allow transmission of broadband signals, having bandwidth ranging from few MHz to few GHz, as well as their flexibility in terms of the available choices for data rates. The combining of optical systems (such as optical systems which implement optical fibers for data transmission) with broadband wireless technologies is a promising approach to overcome the problems of the wireless channel. However, integration of optical systems with broadband wireless a viable technology is in a great need for conversion devices from the optical domain to the modulated electric signals domain (conveniently the radio frequency (RF) domain) and vice versa. Furthermore, in order to make this technology available for the residential mass market it is crucial to provide a very low-cost optical to electric signal converting device.
[005] There is therefore a great need in reliable, low cost, and simple means of converting a modulated optical signal to a modulated electric signal.
Summary of the invention
[006] A system for converting a modulated optical signal to a modulated electric signal, the system includes: (a) an optical interface, which is adapted to receive a modulated optical signal, wherein the modulated optical signal is modulated by a
radio frequency signal; (b) a substrate; and (c) a thin semi-insulating microstrip, which is coupled to the substrate on one side and to the optical interface on the other side; wherein (a) a depth of the microstrip is of the scale of a penetration depth of the modulated optical signal into the microstrip; (b) an impedance of the microstrip is very responsive to a modulation of the modulated optical signal.
[007] A method for converting a modulated optical signal to a modulated electric signal, the method includes: (a) receiving the modulated optical signal by an optical interface, wherein the modulated optical signal is modulated by a radio frequency signal; (b) applying the modulated optical signal to a thin semi-insulating microstrip, which is connected to a substrate; wherein (a) a depth of the microstrip is of the scale of a penetration depth of the modulated optical signal into the microstrip; (b) an impedance of the microstrip is very responsive to a modulation of the modulated optical signal; and (c) providing the modulated electric signal, which is responsive to the modulated optical signal and that is generated as a result of the interaction between the modulated optical signal and the microstrip;
Brief description of the drawings
[008] The foregoing and other objects, features, and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. In the drawings, similar reference characters denote similar elements throughout the different views, in which:
[009] Figure 1 illustrates a system for converting a modulated optical signal to a modulated electric signal, according to an embodiment of the invention;
[0010] Figures 2a and 2b illustrate electric circuits, which are equivalent to the system for converting a modulated optical signal to a modulated electric signal, according to an embodiment of the invention;
[0011] Figure 3 illustrates a system for converting a modulated optical signal to a modulated electric signal, according to an embodiment of the invention;
[0012] Figure 4 illustrates a system for converting a modulated optical signal to a modulated electric signal, according to an embodiment of the invention;
[0013] Figure 5 illustrates a system for converting a modulated optical signal to a modulated electric signal, according to an embodiment of the invention;
[0014] Figure 6 illustrates a system for converting a modulated optical signal to a modulated electric signal, according to an embodiment of the invention; and
[0015] Figure 7 illustrates a method for converting a modulated optical signal to a modulated electric signal, according to an embodiment of the invention.
Detailed description of the drawings
[0016] Figure 1 illustrates system 200 for converting modulated optical signal 410 to modulated electric signal 420, according to an embodiment of the invention. System 200 receives modulated optical signal 410 by optical interface 230, which conveniently receives modulated optical signal 410 from optical fiber 290, which may be a part from system 200, or a part of an external system, such as a system that generates or transmits modulated optical signal 410. It is noted that according to an embodiment of the invention, system 200 receives modulated optical signal 410 by other means than optical fiber 290. By way of example, and not intended to limit the scope of the invention in any way, according to different embodiments of the invention, an optical beam which carries modulated optical signal 410 may be taken from any optical source, optical fiber or an optical beam in a wireless optical system.
[0017] Modulated optical signal 410 is modulated by a radio frequency signal, wherein, according to an embodiment of the invention, the radio frequency signal is an ultra wide band (UWB) signal. Conveniently, modulated electric signal 420 is a radio frequency (RF) signal.
[0018] It is noted that in the herein offered details of the invention, the radio frequency signal my be any signal up to a frequency of 100 gigahertz, and conveniently between 1 GHz to 100' GHz, and that the term radio frequency refers to all the frequencies up to 100 GHz. Specifically, it is noted that the radio frequency signal may be in the domains commonly referred to as radio frequency, microwave and millimetric waves.
[0019] Optical adapter 230 applies modulated optical signal 410 to thin semi- insulating microstrip 210. Conveniently, microstrip 210 is connected to substrate 220 on one side and to optical interface 230 on the other side. Conveniently, optical adapter 230 applies modulated optical signal 410 onto an electrically open end of microstrip 210, wherein the other end of the microstrip conduct modulated electric signal 420 to a destination of modulated electric signal 420. It is noted that since
microstrip 210 is semi-insulating, it is conveniently characterized by high carrier mobility.
[0020] In order to clarify the invention, an numerical example is offered, which is not intended to limit the scope of the invention in any way, wherein, according to an embodiment of the invention, substrate 220 is relatively thick in comparison with microstrip 210 (e.g. 400 micrometer), while in comparison, according to an embodiment of the invention, the thickness of microstrip 210 is selected in response to an absorption depth of modulated optical signal 410 in microstrip 210, e.g. 10 micrometers.
[0021] According to an embodiment of the invention, substrate 220 is made of a high resistivity material (wherein, according to an embodiment of the invention, substrate 220 is nearly intrinsic), which facilitates the limiting of losses of electric signals (such as modulated electric signal 420 and unmodulated electric signal 430).
[0022] Microstrip 210 is adapted to generate and to conduct modulated electric signal 420, wherein the generating is responsive to modulated optical signal 410. Conveniently, modulated electric signal 420 is a linear transformation of the modulated optical signal.
[0023] In order for microstrip 210 to generate modulated electric signal 420, the following conditions apply, according to the teaching of the invention: (a) a depth of microstrip 210 is of the scale of a penetration depth of modulated optical signal 410 into microstrip 210; and (b) an impedance of microstrip 210 is very responsive to a modulation of modulated optical signal 410.
[0024] It is noted that according to an embodiment of the invention, the depth of microstrip 210 is further responsive to a diffusion depth of modulated electric signal 420, or of unmodulated electric signal 430, which is described below.
[0025] Furthermore, according to an embodiment of the invention, microstrip 210 is made out of a semi-insolating material whose energy band gap is responsive to one or more wavelengths of modulated optical signal 410. Respectively, a photon energy of modulated optical signal 410 is conveniently larger than the energy gap of microstrip 210. Having the photon energy of modulated optical signal 410 larger than the energy gap facilitates the physical phenomena that result in the conversion of
modulated optical signal 410 to modulated electric signal 420, and which is described below.
[0026] By way of examples only, and not intended to limit the scope of the invention in any way, typical wavelengths of modulated optical signal 410 that may be used in different embodiments of the invention are in the near infrared spectrum (e.g. 880 nm for AlGaAs, 950 nm for GaAs, and so forth), so that the photon energy is larger that the energy gap of the semi conducting material of microstrip 210.
[0027] According to an embodiment of the invention, a depth of microstrip 210 is selected in response to a light intensity of the modulated optical signal, wherein conveniently the selecting is carried out so as to facilitate a sufficient change in the concentration of electron hole pairs when modulated optical signal 410 light microstrip 210.
[0028] As modulated optical signal 410 is applied to microstrip 210 having a photon energy larger than the energy gap, a process of generation of electron holes pairs is resulted by the incident radiation, which leads to a local state of electron-hole plasma inside microstrip 210, in which the concentration of electron hole pairs is significantly higher than before the applying of modulated optical signal 410 (by way of an example only, and not intending to limit the scope of the invention in any way, in response to the applying of modulated optical signal of a first intensity, the concentration of electron hole pairs in the vicinity of the applying positions may raise from the 10 14 cm "3 domain to the 10 17 cm "3 domain).
[0029] It is clear to any person who is skilled in the art that the electron-hole plasma causes local changes in the relative permittivity and conductivity of at least a portion of microstrip 210. It is noted that the modulation of modulated optical signal 410 results in respective modulations in the concentration of electron holes pairs in the vicinity of the applying position, and therefore the relative permittivity and conductivity of the vicinity of the applying position will change in response to the modulation of modulated optical signal 410. The herein related process enables system 200, and especially microstrip 210, to have an optically controlled impedance.
[0030] It is noted that by choosing the material out of which microstrip 210 is made, and different attributes of microstrip 210 and/or of other components of system 200, it is possible to match a characteristic impedance, and or other electrical
characteristics, of microstrip 200 to any desired value, and therefore it is further possible to choose characteristics of microstrip 210 so that there will be no need for additional impedance matching between system 200 and an external system the is adapted to receive modulated electric signal 420.
[0031] Explicitly, it is noted that according to an embodiment of the invention, an impedance of microstrip 210 is selected so as to electrically match system 200 and an external system to which modulated electric signal 420 is provided.
[0032] It is known to any person skilled in the art that microstrip 210 could be made from a variety of different semi conducting materials or structures which are known in the art. In order to clarify the invention, though not intending to limit the scope of the invention in any way, few alternatives are herein offered. The material of microstrip 210 can be adopted from a variety of semiconductor materials or structures such as heterostructures or epitaxial layers in order to optimize the device performance according to the specific requirements of the application.
[0033] It is further clear to any person who is skilled in the art, that the choice of the semi conducting material of microstrip 210 is subject to restrictions posed by the optical wavelengths used in the system. Specific examples for material which can be used to construct system 200 are Silicon (Si) and Gallium arsenide (GaAs).
[0034] Continuing the same example, it is well known to any person who is skilled in the art that Silicon is characterized by a relatively high dark conductivity, and relatively low mobility, and an absorption coefficient which strongly depends on the wavelength, wherein all of these attributes facilitates the conversion of modulated optical signal 410 to modulated electric signal 420, according to the teachings of the invention.
[0035] It is noted that conveniently, the material out of which microstrip 210 is made is characterized by a low density of electrons and hole, and therefore even a relatively small intensity of modulated optical signal 410 is sufficient for changing considerably the density of electron hole pairs, and thus also of modulated electric signal 420, as was explained before.
[0036] According to an embodiment of the invention, microstrip 210 is made out of a high carrier mobility semiconductor, such as III-V compound semiconducting materials. According to different embodiments of the invention, the material of
microstrip 210 may be used in different structures, such as bulk, multiple-quantum well (MQW), quantum dots (QD), heterostructures, and the like.
[0037] According to an embodiment of the invention, system 200 includes a direct current (DC) source 240, which applies a DC voltage to microstrip 210 and to substrate 220, conveniently by electrodes 250. According to an embodiment of the invention, one or more resistors 260 are electrically connected between DC source 240 and microstrip 210. Conveniently, changes of the current that passes through resistor 260 facilitate a transmission of modulated electric signal 420 via resistor 260.
[0038] Conveniently, the DC voltage results a current that passes via microstrip 210, and especially via the vicinity of the applying position, and which is subject to modulation in the relative permittivity and conductivity of said area. It is clear to any person who is skilled in the art that the motion of charge carriers is dependant of the DC voltage as well as of the optically controlled impedance of microstrip 210.
[0039] According to an embodiment of the invention, in accordance with the properties of microstrip 210, DC voltage source 240 is isolated from modulated electric signal 420 by way of a DC bridge (not shown) which includes a choke that is adapted to a frequency of modulated electric signal 420 (e.g. an RF choke for radio frequency modulated electric signal 420, and so forth) and capacitors so that the outgoing signal from system 200 (which is modulated electric signal 420 after being manipulated by there herein related electrical components) contains no DC bias.
[0040] The variations of the line voltage at the open edge of microstrip 210 produce a varying voltage that consists of modulated electric signal 420 which contains of an envelope of modulated optical signal 410 which is a high fidelity replica of modulated optical signal 410, and propagates along microstrip 410, such as towards an RF circuit or system.
[0041] According to an embodiment of the invention, the DC voltage which is applied by DC source 240 is around 0.1 Volts, which results in the saturated carrier velocity (e.g. 10 7 cm/sec in the case of a silicon microstrip 210), hence reducing a response time of system 200 down to the 10 "10 regime. It is noted that said demonstrative response time corresponds to bandwidths which are in the regime of 10GHz, which are used in broadband wireless signals (e.g. UWB radio signals).
[0042] Referring now to figure 2a, which illustrates electric circuit 201, which is equivalent to an embodiment of system 200. A resistance of substrate 220 is represented in the equivalent electric circuit by resistor 221 (also denoted as R 2 ), and a capacitance of substrate 220 is represented by capacitor 222 (also denoted as C). The resistance of microstiϊp 210, which is as previously mentioned varying in response to modulated optical signal 410, is represented by variable resistor 211(also denoted as R 1 ).
[0043] Since R 2 and C are constant, the load on Ri only depends on the resistance of microstrip 210, which in turn depends on modulated optical signal 410, as related before.
[0044] It is understood by any person who is skilled in the art, that conveniently only a limited area of microstrip 210 is susceptible to changes in modulated optical signal 410 (the area in the vicinity of the interaction area between modulated optical signal 410 and microstrip 210). Therefore, the changes in conductivity and relative permittivity in microstrip 210 are thus conveniently limited to said limited area.
[0045] It is clear however to any person who is skilled in the art, that the capacity of substrate 220 (which is denoted as C) is parasitical, and reduces the performances of system 200. Different embodiments of the invention implement different techniques for reducing said capacity (such as by using a thicker substrate 220), thus facilitating the conversion of higher frequencies optical signals 410.
[0046] As aforementioned, according to an embodiment of the invention, system 200 is biased by the DC voltage, wherein the variations of the photoconductivity level lead to instantaneous variations of the DC voltage near the end of microstrip 210, which in turn produces a line voltage wave whose amplitude is responsive to the power of modulated optical signal 410, and which propagates along microstrip 210 towards the closed end of microstrip 210.
[0047] It will be clear to any person who is skilled in the art that the continuity equations for the photo excited charge carriers, in accordance with the drift diffusion model, are:
Where n ph ,p ph ,μ np ,D np ,τ nφ dxt the photo excited electron and hole concentrations, motilities, diffusion coefficients and lifetimes, respectively, and where E is the electric field, and G n is the generation rate of electron hole pairs.
[0048] In the photo excited plasma n ^ Pph "i*»( z ) + V( z O j an d the
generation rate has the form
Where r is the reflection coefficient, η is the quantum efficiency, hv≥E g is the photon energy, α is the absorption coefficient of the semiconductor material, I 0 =P 0 /A eff and Po are the optical beam intensity and power at the surface z=0,
2 .
A~ = πr b is the optical beam cross section, and f(t) is the signal envelope.
[0049] In the steady state part D a "ζ 10 - ^SL + GJz) = O, where dz τ
G 0 (z) = ( l~ y η<X I 0& x V (-az) nv
(l-r)ηaτ ssinh(z/L) + (DJL)co$h(z/L) '
I o exp(—ad) hv[l-(aL) 21 ssmh(d/L) + (DJL)cosh(d/L)
(s+D a
a)smh((d-z)/L) ssinh(d/L) + (DJL)cosh(d/L)
and where s is the surface recombination velocity at Z=O and ^ " is the ambipolar diffusion length.
[0050] For the modulated part,
[0051] Assuming that G λ (z,t) = G 0 (z) f (t) , the Fourier transform of equation (1) with respect to time is
N phl
{z,ω)
F (ω) = \ f (t)exp(-jωήdt
[0052] It is noted that equation (2) can be solved analytically, wherein the inverse
Fourier transform n plll [z,t) = — J N phι (z,ω)exp(jωt)dω is determined by the
spectrum of modulated optical signal F(ω), taking into account the distortions caused by the time delay.
[0053] It is clear to any person who is skilled in the art that this is solvable in the df_ 1 quasistatic case, wherein dt * , which leads to (3) Hphl ^ = U?m ( z ' ? W . Thus, it is clear that the solution (3) is proportional to the modulated optical signal.
[0054] Referring now to figure 2b, which illustrates an electric circuit which is also equivalent to system 200, wherein impedance 209 (which is also denoted as Z) is equivalent to the combination of Rl, R2 and C which are illustrated in figure 2a. Transmission lines 207 and 208 are equivalent to the portion of system 200 that separates impedance 209 from ports of system 200, by which electric signals are applied to and/or received from system 200.
[0055] Referring now back to figure 1. According to an embodiment of the invention, system 200 further includes an electric signal antenna (not shown) that is adapted to transmit modulated electric signal 420. It is noted that conveniently, the antenna is an RF antenna.
[0056] It is further noted that according to an embodiment of the invention, system 200, and especially the electric signal antenna according to some embodiments of the invention, are adapted to facilitate an integration of system 200 with one or more front end RF components (not shown), such as amplifiers, filters, automatic gain control units, and so forth, wherein the front end RF component may and may not be part of system 200, and may and may not be implemented on microstrip 210.
[0057] Any person who is skilled in the art is familiar with the implementation of an antenna by a microstrip, and it is noted that according to an embodiment of the invention, microstrip 210 includes an electric signal antenna which is adapted to transmit modulated electric signal 420. It is further noted that according to some such embodiment of the invention, the implementation of the electric signal antenna (on microstrip 210 or otherwise) requires the implementation of one or more additional electric circuit (e.g. matching circuits that are used in order to avoid reflections).
[0058] Further more, it is noted that according to different embodiments of the invention, system 200, and especially microstrip 210, implement different techniques that are used to avoid reflections, wherein said technique may include additional dedicated electric circuits, structural designing techniques, and so forth. According to an embodiment of the invention, a geometry of system 200, and especially of microstrip 210, can be optimized so as to greatly reduce the response time of system 200.
[0059] According to an embodiment of the invention, microstrip 210 is made of a special structure that enhances the conversion efficiency, e.g. a multi-quantum well (MQW) structure, heterostructures, epitaxial layers, and so forth.
[0060] It is clear to any person who is skilled in the art, that as the conversion of modulated optical signal 410 to modulated electric signal 420 is depending on a concentration of electron hole pairs in the limited area of microstrip 210 which is susceptible to changes in modulated optical signal 410 (the area in the vicinity of the
interaction area between modulated optical signal 410 and microstrip 210). Therefore, some embodiments of the invention implement one or more techniques which are used to facilitate a concentration of electron hole pairs in said area.
[0061] According to an embodiment of the invention, microstrip 210 further includes a heterostmcture of P-N junctions in vicinity to an applying position in which the modulated optical signal is applied to microstrip 210, wherein the heterostmcture is adapted to at least partially confine the generated electron hole pairs to the vicinity of the applying position.
[0062] According to an embodiment of the invention, system 200 is adapted to facilitate the applying of a magnetic field to microstrip 210 (and conveniently to substrate 220), wherein the magnetic field may be generated by a magnetic field source of system 200, or externally to system 200. The magnetic field improves the confinement of the photo-carriers at the modulated optical signal applying point at the open end of microstrip 210.
[0063] According to an embodiment of the invention, an electric and/or magnetic field is applied on system 200 so as to enhance a conversion response time of system 200. The applied field results in a drift assisted diffusion of photo-carriers, and thus in a reduced response time in comparison with a pure (conventional) photoconductivity.
[0064] The use of an external field may be used, for example, to facilitate a conversion of higher frequencies optical signals 410, or, for conversion of lower intensity optical signals 410.
[0065] Figure 3 illustrates system 200, according to an embodiment of the invention. According to the illustrated embodiment of the invention, microstrip 210 is T-shaped. The T-shape formation of microstrip 210 has several benefits; among them are ease of production, and reduction of edge effects.
[0066] According to an embodiment of the invention, microstrip 210 includes one or more P-N junctions 215 in vicinity to an applying position in which the modulated optical signal is applied to microstrip 210, which are used to confine the generated electron hole pairs to the vicinity of the applying position.
[0067] Figure 4 illustrates system 200, according to an embodiment of the invention, wherein system 200 includes multiple microstrips 210, such as the microstrips denoted as 210(1) through 210(4). According to an embodiment of the
invention, an impedance of a first microstrip 210 differs from an impedance of a second microstrip 210, so as to facilitate the transmission of modulated electric signal 420 to multiple external systems or circuits, without necessitating additional impedance matching systems or additional power dividing.
[0068] It is noted that according to this embodiment of the invention, system 200 converts a single modulated optical signal 410 to multiple electric signals 420 (such as electric signals 420(1) through 420(4)), which are all responsive to modulated optical signal 410. Data that is included in modulated optical signal 410 is converted to the electric domain (conveniently to the RF domain) and simultaneously fed into the multiple microstrips 210 which are connected to different systems.
[0069] It is noted that according to another embodiment of the invention, a single microstrip 210 includes multiple transmission lines, wherein an impedance of a first transmission line of microstrip 210 may differ from an impedance of a second transmission line of microstrip 210.
[0070] Figure 5 illustrates system 200, according to an embodiment of the invention. According to the illustrated embodiment, system 200 further includes unmodulated electric signal source 270, which is adapted to apply unmodulated electric signal 430 to microstrip 210. It is noted that unmodulated electric signal 430 could be also referred to as a reference electric signal. According to an embodiment of the invention, unmodulated electric signal source 270 is an RF oscillator. According to an embodiment of the invention, unmodulated electric signal 430 is fed to microstrip 210 via transmission line 272 (e.g. a coaxial cable). It is noted that, according to an embodiment of the invention, a characteristic impedance of transmission line 272 is the same as the nominal impedance of microstrip 210.
[0071] According to an embodiment of the invention, system 200 is adapted to mix unmodulated electric signal 430 and modulated electric signal 420.
[0072] According to this embodiment of the invention, a frequency band of modulated electric signal 420 is shifted in response to a desired center frequency that is set by unmodulated electric signal 430. It is noted that conveniently, unmodulated electrical signal 430 is selected so as to conform modulated electric signal 420 to the desired center frequency.
[0073] It will be appreciated by any person who is skilled in the art that said embodiment of the invention facilitates mainly the following two interactions, which occur simultaneously in a confined volume in microstrip 210 which is conveniently located at the open end of microstrip 210, underneath the applying position: (a) interaction between modulated optical signal 410 and the semi-insulating material of microstrip 210, which results in a modulation of the voltage reflection or transmission coefficient of microstrip 210 in response to modulated optical signal 410, as described above; and (b) interaction between unmodulated electric signal 430 which propagates along microstrip 210 and between the photo-induced plasma, in regard to the reflection or transmission coefficient of at least a portion of microstrip 210.
[0074] A non-linear interaction between modulated optical signal 410 and unmodulated electric signal 430 is facilitated through the photo-induced plasma which results from the applying of modulated optical signal 410 to microstrip 210. Said interaction results in modulated electric signal 420 that includes two main frequency ranges, which are expressed as f c -f R ±B/2 and f c +f R ±B/2, wherein f c represents a central frequency of modulated optical signal 410, fR represents the frequency of unmodulated electric signal 430 and B represents the bandwidth of modulated optical signal 410. It is noted that modulated electric signal 420 also include higher frequencies components with rather low power.
[0075] According to an embodiment of the invention, the high ordered harmonics are filtered out by at least one filter (not shown) such as a band-pass filter, which may and may not be implemented within system 200.
[0076] It is clear to any person that is skilled in the art that, according to an embodiment of the invention, band pass filters may be used to separate modulated electric signal 420 into two electric signals, f c -f R ±B/2 and f c +fR±B/2. It is further noted that, according to an embodiment of the invention, the two electric signals are propagating in two different microstrips 210, wherein the different microstrips 210 or may and may not have different impedances, as related above.
[0077] Figure 6 illustrates system 200, according to an embodiment of the invention. According to the illustrated embodiment of the invention, microstrip 210 is cut into two collinear transmission lines which are separated by a small gap 218, in which the substrate is exposed. Gap 218 is sufficiently wide, so that under dark
conditions, i.e. when no modulated optical signal 410 is applied, the two transmission lines of microstrip 210 are totally disconnected, and each transmission line is practically open-ended, wherein unmodulated electric signal which is fed from first port 291 is totally reflected back.
[0078] According to the illustrated embodiment of the invention, unmodulated electric signal source 270 feeds unmodulated electric signal 430 to microstrip 210 via transmission lines 272. According to said embodiment of the invention, modulated optical signal 410 is applied to microstrip 210 at gap 218, i.e. at the ends of both transmission lines of microstrip 210. The interaction between modulated optical signal 410 and microstrip 210 results in a local discontinuity in terms of both conductivity and permittivity of microstrip 210, in the aforementioned confined volume. This local discontinuity introduces a photo-induced load for both transmission lines of microstrip 210, which otherwise would be open ended. The variations in the intensity of modulated optical signal 410 result in a modulation of the transmission coefficient of unmodulated electric signal 430 which propagates along microstrip 210. Thus, upon passing through the photo-induce load region, unmodulated electric signal 430 undergoes a modulation and hence modulated electric signal 420 that is transmitted via second port 292 is modulated similarly to modulated optical signal 410, whereas the spectrum of modulated electric signal 420 is transposed by a frequency which is equal to the frequency of unmodulated electric signal 430. Modulated electrical signal 420 is provided via port 292, and could be sampled, for example, in location 299.
[0079] Figure 7 illustrates method 500 for converting a modulated optical signal to a modulated electric signal, according to an embodiment of the invention. It is noted that according to an embodiment of the invention, method 500 is intended to be carried out by a system for converting a modulated optical signal to a modulated electric signal according to one of the embodiments of the invention. It is further noted that different embodiments of method 500 are intended to be carried out by different embodiments of system 200 which is described above. A person skilled in the art may benefit from reading the description of method 500 in relation to the descriptions of system 200. It is noted, however, that method 500 may be carried out by other systems as well.
[0080] It is noted that according to an embodiment of the invention, the radio frequency signal is an ultra wide band signal, and the modulated electric signal is conveniently an RF signal.
[0081] It is noted that in the herein offered details of the invention, the radio frequency signal my be any signal up to a frequency of 100 gigahertz, and conveniently between 1 GHz to 100 GHz, and that the term radio frequency refers to all the frequencies up to 100 GHz. Specifically, it is noted that the radio frequency signal may be in the domains commonly referred to as radio frequency, microwave and millimetric waves.
[0082] According to an embodiment of the invention, method 500 starts with stage 510 of selecting an impedance of a microstrip so as to electrically match a system that includes the microstrip and an external system to which the modulated electric signal is provided.
[0083] Referring to the examples set forward in the previous drawings, stage 510 includes selecting the impedance of microstrip 210 so as to electrically match system 200 to the external system.
[0084] According to an embodiment of the invention, method 500 includes stage 520 of selecting a depth of the microstrip in response to a light intensity of the modulated optical signal. It is noted that, according to an embodiment of the invention, the selecting of stage 520 is further responsive to a diffusion depth of the modulated electric signal, or of an unmodulated electric signal, that is described below.
[0085] Referring to the examples set forward in the previous drawings, stage 520 includes selecting the depth of microstrip 210 in response to the light intensity of modulated optical signal 410.
[0086] Method 500 continues with stage 530 of receiving the modulated optical signal by an optical interface. It is noted that conveniently, the optical interface is connected to the microstrip, on the side opposing the side to which a substrate is connected.
[0087] Referring to the examples set forward in the previous drawings, stage 530 includes receiving modulated optical signal 410 by optical interface 230.
[0088] Stage 530 is followed by stage 540 of applying the modulated optical signal to a thin semi-insulating microstrip, which is connected to the substrate; wherein (a) a depth of the microstrip is of the scale of a penetration depth of the modulated optical signal into the microstrip; (b) an impedance of the microstrip is very responsive to a modulation of the modulated optical signal.
[0089] It is noted that the applying of stage 540 results an interacting between the modulated optical signal and the microstrip, which are described in the descriptions of the previous figures.
[0090] According to an embodiment of the invention, stage 540 includes stage
541 of applying DC voltage to the microstrip and to the substrate. Conveniently, the applying of the DC voltage results a current that passes via microstrip 210, and especially via the vicinity of the applying position, and which is subject to modulation in the relative permittivity and conductivity of said area. It is clear to any person who is skilled in the art that the motion of charge carriers is dependant of the DC voltage as well as of the optically controlled impedance of microstrip 210.
[0091] According to an embodiment of the invention, the DC voltage which is applied during stage 541 is around 0.1 Volts, which results in the saturated carrier velocity (e.g. 10 7 cm/sec in the case of a silicon microstrip 210), hence reducing a response time of the converting down to the 10 "10 regime. It is noted that said demonstrative response time corresponds to bandwidths which are in the regime of 10GHz, which are used in broadband wireless signals (e.g. UWB radio signals).
[0092] According to an embodiment of the invention, stage 540 includes stage
542 of applying the modulated optical signal to multiple microstrips. According to an embodiment of the invention, different microstrips may have different impedances, so as an impedance of a first microstrip differs from an impedance of a second microstrip. This is useful in order to provide the modulated electric signal to multiple external systems which are characterized by different impedances, without need for additional matching.
[0093] According to an embodiment of the invention, the applying of stage 540 includes applying the modulated optical signal to a microstrip that includes a heterostructure of P-N junctions in vicinity to an applying position in which the modulated optical signal is applied to the microstrip, wherein the heterostructure is
adapted to at least partially confine the generation of electron hole pairs to the vicinity of the applying position.
[0094] According to an embodiment of the invention, the applying of stage 540 includes applying the modulated optical signal to a T-shaped microstrip.
[0095] According to an embodiment of the invention, method 500 includes stage 551 of applying a magnetic field onto the microstrip. According to an embodiment of the invention, method 500 includes stage 552 of applying an electric field onto the microstrip.
[0096] The magnetic field improves the confinement of the photo-carriers at the modulated optical signal applying point at the open end of the microstrip. The electric and/or magnetic field is applied to the microstrip (and conveniently onto the substrate) so as to enhance a conversion response time. The applied field results in a drift assisted diffusion of photo-carriers, and thus in a reduced response time in comparison with a pure (conventional) photoconductivity.
[0097] The use of an external field may be used, for example, to facilitate a conversion of higher frequencies optical signals, or, for conversion of lower intensity optical signals.
[0098] According to an embodiment of the invention, method 500 includes stage
560 of applying an unmodulated electric signal to the microstrip. It is noted that the unmodulated electric signal could be also referred to as a reference electric signal.
[0099] According to this embodiment of the invention, a frequency band of the modulated electric signal is shifted in response to a desired center frequency that is set by the unmodulated electric signal. It is noted that conveniently, stage 560 includes selecting the unmodulated electrical signal so as to conform the modulated electric signal to the desired center frequency.
[00100] According to an embodiment of the invention, stage 560 includes stage
561 of mixing the unmodulated electric signal and the modulated electric signal.
[00101] It will be appreciated by any person who is skilled in the art that the applying of the unmodulated electric signal to the microstrip facilitates mainly the following two interactions, which occur simultaneously in a confined volume in the microstrip which is conveniently located at the open end of the microstrip, underneath
the applying location of the modulated optical signal: (a) Interaction between the modulated optical signal and the semi-insulating material of the microstrip, which results in a modulation of the voltage reflection or transmission coefficient of the microstrip in response to the modulated optical signal, as described above; and (b) Interaction between the unmodulated electric signal which propagates along the microstrip and between the photo-induced plasma through its reflection or transmission coefficient.
[00102] A non-linear interaction between the modulated optical signal and the unmodulated electric signal is facilitated through the photo-induced plasma which results from the applying of the modulated optical signal to the microstrip. Said interaction results in the modulated electric signal that includes two main frequency ranges, which are expressed as f c -f R ±B/2 and f c +f R ±B/2, wherein f c represents a central frequency of the modulated optical signal, fβ represents the frequency of the unmodulated electric signal and B represents the bandwidth of the modulated optical signal. It is noted that the modulated electric signal also include higher frequencies components with rather low power.
[00103] According to an embodiment of the invention, method 500 includes filtering out high ordered harmonics, e.g. by a filter, such as a band-pass filter, which may and may not be implemented within the same system. According to an embodiment of the invention, method 500 further includes separating the modulated electric signal into two electric signals, f c -f R ±B/2 and f c +f R ±B/2 (e.g. by band pass filters) .It is further noted that, according to an embodiment of the invention, the two electric signals are propagating in two different microstrips, or in two different transmission lines of the microstrip, wherein the different microstrips or transmission lines may and may not have different impedances, as related above.
[00104] According to an embodiment of the invention, stage 560 is carried out for a microstrip that consists of a gap, which electrically isolates parts of the microstrip when no optical signal is applied. It is noted that, like other features of method 500, the benefits of such an embodiment are described in the descriptions of the previous drawings.
[00105] Method 500 continues with stage 570 of providing the modulated electric signal, which is responsive to the modulated optical signal and that is generated as a result of the interaction between the modulated optical signal and the microstrip.
[00106] According to an embodiment of the invention, stage 570 includes stage
571 of providing the modulated electric signal by a resistor that is electrically connected to the microstrip.
[00107] According to an embodiment of the invention, stage 570 includes stage
572 of providing the modulated electric signal by a modulated electric signal antenna, which may be implemented in the microstrip, or external to it.
[00108] According to an embodiment of the invention, stage 572 includes stage
573 of providing the modulated electric signal by an electric signal antenna that is adapted to facilitate integration with a one or more front end radio frequency components, such as amplifiers, filters, automatic gain control units, and so forth, wherein the front end RF component may and may not be part of the conversion system, and may and may not be implemented on the microstrip.
[00109] It is stressed again that many features of method 500, and the benefits which results from different embodiments of method 500, are described in the descriptions of the previous drawings, which may offer a more comprehensive and clear explanation of the embodiment.
[00110] The present invention can be practiced by employing conventional tools, methodology and components. Accordingly, the details of such tools, component and methodology are not set forth herein in detail. In the previous descriptions, numerous specific details are set forth, in order to provide a thorough understanding of the present invention. However, it should be recognized that the present invention might be practiced without resorting to the details specifically set forth.
[00111] Only exemplary embodiments of the present invention and but a few examples of its versatility are shown and described in the present disclosure. It is to be understood that the present invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein.