Ultra-sensitive electric field detection device

An electric field detection device. In one embodiment, the electric field detection device includes an interferometer having a reference arm and an active arm. The reference arm comprises a first electro-optic waveguide. The active arm comprises a first electrically conductive plate, a second electrically conductive plate spaced apart from the first electrically conductive plate defining a first gap therebetween, a third electrically conductive plate disposed in the first gap and vertically extending from the first electrically conductive plate to define a T-shape structure and a second gap between the third electrically conductive plate and the second electrically conductive plate, where the second gap is substantially smaller than the first gap; and a second electro-optic waveguide disposed in the second gap and being in electrical communication with the second and third electrically conductive plates.

FIELD OF THE INVENTION

The present invention relates generally to an electric field detection device, more particularly to an electric field detection device that utilizes a micro-antenna and an electro-optic waveguide structure to enhance the sensitivity of the electric field detection.

BACKGROUND OF THE INVENTION

Electric field sensing is often performed using an antenna and receiver combination. In these systems, the sensitivity is related to both the design of the antenna and the receiver electronics. Obtaining a system that has a high sensitivity over a broad frequency response has proven to be very challenging. In addition, since the antenna size is related to the wavelength of the signal, the electric field detectors are relatively large, especially at lower frequencies. Therefore, compact detectors and arrays are difficult to achieve. Sensor architectures that do not have an antenna, for example, the optical or high impedance based sensors, have not yet demonstrated a sensitivity comparable to that of the antenna based systems.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides an electric field detection device. In one embodiment, the electric field detection device includes an interferometer having a reference arm and an active arm. The reference arm comprises a first electro-optic (EO) waveguide.

The active arm comprises a first metallic plate, a second metallic plate spaced apart from the first metallic plate defining a first gap therebetween, a third metallic plate disposed in the first gap and vertically extending from the first metallic plate to define a T-shape structure and a second gap between the third metallic plate and the second metallic plate, where the second gap is substantially smaller than the first gap, a second electro-optic waveguide disposed in the second gap, where the second electro-optic waveguide comprises a waveguide layer, a first cladding layer disposed between the third metallic plate and the waveguide layer, and a second cladding layer disposed between the waveguide layer and the second metallic plate, and a dielectric layer disposed in the first gap and surrounding the third metallic plate and the second electro-optic waveguide. The second electro-optic waveguide is in electrical communication with the third metallic plate and the second metallic plate, respectively.

In one embodiment, the first and third metallic plates are formed integrally.

In one embodiment, the dielectric layer is formed of CYTOP™.

In one embodiment, each of the first and second cladding layers is formed of a transparent conducting oxide (TCO), where the TCO comprises one of Sn-doped In2O3, In2O3, ZnO, and NiO. In another embodiment, each of the first and second cladding layers is formed of an electrically non-conductive material.

The waveguide layer, in one embodiment, is formed of a semiconductor material selected from groups III-V of the Period Table, where the semiconductor material comprises GaAs. In one embodiment, the waveguide layer comprises quantum well structures or quantum dot structures of the semiconductor material.

In another embodiment, the waveguide layer is formed of an organic electro-optic material, where the organic electro-optic material comprises molecularly self-assembled superlattices. In one embodiment, the organic electro-optic material comprises an organic material with an electro-optic coefficient r33higher than about 100 pm/V.

In one embodiment, each of the first and second electro-optic waveguides comprises a retro-reflector at the end thereof, where the retro-reflector comprises a micro-loop mirror. The retro-reflector comprises a metallic coating at the end of a respective electro-optic waveguide.

In one embodiment, the electric field detection device further includes a first optical ring resonator coupled to the first electro-optic waveguide and a second optical ring resonator coupled to the second electro-optic waveguide.

In one embodiment, the interferometer further comprises an input/output waveguide having a first end and a second end, where the first end is optically coupled to both the first and second electro-optic waveguides, and the second end is optically coupled to an optical fiber, where the second end of the input/output waveguide is tapered to a pointed tip such that the width of the tip is less than half of the wavelength of a light beam propagating in the input/output waveguide, and the optical fiber has a lens tip.

In one embodiment, the electric field detection device further includes a gradient-index (GRIN) lens configured to facilitate the optical coupling between the second end of the input/output waveguide and the optical fiber.

In one embodiment, the reference arm has an optical path-length that is about λ/8 longer than the optical path-length of the active arm, where λ is the wavelength of a light beam propagating in the interferometer. The electric field detection device may further have a heater disposed in the vicinity of the reference arm adapted for tuning the optical path-length of the reference arm.

In another aspect, the present relates to an electric field detection device. In one embodiment, the electric field detection device includes an interferometer having a reference arm and an active arm. The reference arm comprises a first electro-optic (EO) waveguide. The active arm comprises a first electrically conductive plate, a second electrically conductive plate spaced apart from the first electrically conductive plate defining a first gap therebetween, a third electrically conductive plate disposed in the first gap and vertically extending from the first electrically conductive plate to define a T-shape structure and a second gap between the third electrically conductive plate and the second electrically conductive plate, where the second gap is substantially smaller than the first gap; and a second electro-optic waveguide disposed in the second gap, the second electro-optic waveguide having a first surface and an opposite, second surface, where the first surface is in electrical communication with the third electrically conductive plate and the second surface is in electrical communication with the second electrically conductive plate.

In one embodiment, each of the first, second and third electrically conductive plates is formed of a metal. In one embodiment, the first and third electrically conductive plates are formed integrally.

In one embodiment, the active arm further comprises a dielectric layer disposed in the first gap and surrounding the third electrically conductive plate and the second electro-optic waveguide.

In one embodiment, the second electro-optic waveguide further comprises a first cladding layer disposed on the first surface thereof and a second cladding layer disposed on the second surface thereof. In one embodiment, each of the first and second cladding layers is formed of a transparent conducting oxide (TCO). In another embodiment, each of the first and second cladding layers is formed of an electrically non-conductive material.

In one embodiment, each of the first and second electro-optic waveguides comprises a high-refractive-index-contrast waveguide, where the high-refractive-index-contrast waveguide is formed of a semiconductor material selected from groups III-V of the Period Table. In one embodiment, the semiconductor material comprises GaAs. In one embodiment, the high-refractive-index-contrast waveguide comprises quantum well structures or quantum dot structures of the semiconductor material.

In another one embodiment, each of the first and second electro-optic waveguides is formed of an organic electro-optic material. In one embodiment, the organic electro-optic material comprises molecularly self-assembled superlattices.

In one embodiment, each of the first and second electro-optic waveguides comprises a retro-reflector at the end thereof.

Additionally, the electric field detection device further includes a first optical ring resonator coupled to the first electro-optic waveguide and a second optical ring resonator coupled to the second electro-optic waveguide. In one embodiment, the interferometer further comprises an input/output waveguide having a first end and a second end, where the first end is optically coupled to both the first and second electro-optic waveguides, and the second end is optically coupled to an optical fiber.

Furthermore, the electric field detection device includes a gradient-index (GRIN) lens configured to facilitate the optical coupling between the second end of the input/output waveguide and the optical fiber. In one embodiment, the reference arm has an optical path-length that is about λ/8 longer than the optical path-length of the active arm, where λ is the wavelength of a light beam propagating in the interferometer. In one embodiment, the electric field detection device includes a heater disposed in the vicinity of the reference arm adapted for tuning the optical path-length of the reference arm.

In yet another aspect, the present invention relates to an electric field detection system. In one embodiment, the electric field detection system has an electric field detection device as disclosed above, and an optical circulator having a first port configured to receive an input light beam, a second port configured to transmit the input light beam to and to receive an output light beam from the electric field detection device, and a third port configured to transmit the output light beam received from the electric field detection device to a light detection system.

In one embodiment, the electric field detection system further has a first beam splitter, the first beam splitter having an input port configured to receive a source light beam, a first output port configured to transmit a first part of the source light beam to the first port of the optical circulator, and a second output port configured to transmit a second part of the source light beam to the light detection system.

In one embodiment, the light detection system comprises a second beam splitter, the second beam splitter having a first input port configured to receive the output light beam transmitted from the third port of the optical circulator, a second input port configured to receive the second part of the source light beam transmitted from the second output port of the first beam splitter, a first output port configured to output an enhanced light beam resulted from a constructive interference of the light beams received from the first and the second input ports of the second beam splitter, and a second output port configured to output a reduced light beam resulted from a destructive interference of the light beams received from the first and second input ports of the second beam splitter.

Additionally, the light detection system further comprises a light detector configured to receive the light beam transmitted from the second output port of the second beam splitter.

Furthermore, the light detection system comprises a phase shifter and an optical attenuator coupled in series between the second output port of the first beam splitter and the second input port of the second beam splitter, where the light detection system further comprises a feedback controller coupled between the light detector and the phase shifter and between the light detector and the optical attenuator, respectively.

In a further aspect, the present invention relates to an electric field detection module comprising a plurality of the above-disclosed electric field detection devices arranged in an array.

In one embodiment, the electric field detection module further comprises a dense wavelength division multiplexer (DWDM) coupled to the plurality of electric field detection devices via an optical fiber and a gradient-index (GRIN) lens, where the DWDM comprises a diffraction grating.

DETAILED DESCRIPTION OF THE INVENTION

The terms “III-V”, as used herein, refers to a material selected from Groups III-V in the Periodic Table.

The description will be made as to the embodiments of the present invention in conjunction with the accompanying drawings inFIGS. 1-32. In accordance with the purposes of this invention, as embodied and broadly described herein, this invention, in one aspect, relates to an ultra-sensitive electric field detection device over a wide frequency range while being compact and power efficient.

According to embodiments of the present invention, the electric field detection device has a sensitivity about 0.1 μV/m·Hz1/2and a frequency range of about 0.5 Hz to about 1 GHz with a response bandwidth of about 1,000 Hz, however, conventional electro-optic based approaches can only achieve a typical electric field detection sensitivity of tens of mV/m·Hz1/2. In one embodiment, the electric field detection device has a size as small as about 1 mm2. In another embodiment, a plurality of such devices is fabricated into an array, for example a 5×5 array.

According to the present invention, the central component to the electric field detection device is an electro-optic (EO) waveguide. In one embodiment, the electro-optic waveguide includes a thin-film, III-V material based electro-optic waveguide structure with dimensions of around 0.3 μm. The fabrication of such an III-V electro-optic waveguide structure involves wafer bonding, thin film transfer technique and a mode enlarger for easy optical coupling, which are referred in the specification as III-V WB-TT-ME.

In another embodiment, the electro-optic waveguide includes an organic electro-optic waveguide with dimensions of around 1.5-2 μm. The organic electro-optic waveguide structure include a robust molecularly self-assembled (referred in the specification as Org-Mole-SA) organic thin film.

In yet another embodiment, the electric field detection device further includes a micro-antenna. The use of the micro-antenna enables an applied electric field to the electro-optic waveguide to be enhanced about 150 times or more. Further enhancement of the electric field is achieved with the micro antenna combined with a transparent-conducting oxide (TCO) based electro-optic waveguide structure, which is another 4-6 times enhancement over just using the micro-antenna alone.

In a further embodiment, the electric field detection device utilizes a multiple optical passing scheme having an integrated resonator structure and an integrated retro-reflector that enables effectively over 20 times longer optical interaction length and hence over 20 times higher sensitivity. The retro-reflector enables simple packaging with only one input fiber connected to the sensor instead of utilizing two fibers (one input and one output).

In yet a further embodiment, the electric field detection device includes an optical carrier power reduction scheme that enables a strong optical signal to be generated by the electric field by using a high optical power of about 100 mW in the electro-optic waveguide. The optical carrier power is reduced before it reaches the high-speed (about 1 GHz) photodetector via an interferometric technique so that the beam's power reaching the photodetector is below the detector saturation power of about 1 mW.

In one embodiment, the electric field detection device is corresponding to a sensitive wideband photodetection system in which a sensitive low-noise, wide-band photodector-amplifier module with about 10-100 fW/Hz1/2optical power sensitivity enables the optical signal to be converted to an electrical signal and measured. The use of an intensity stabilized laser and intensity noise subtraction via a balanced dual detector pair further enables the measurements to be sensitive down to about 0.5 Hz.

According to one embodiment of the present invention, with all the above-mentioned enhancements combined together, the electric field detection device provides a total of over 100,000 times enhancement in the electric field detection sensitivity. In another embodiment, another factor of 10 times improvement is achieved by using quantum well/dot structure for the III-V based electro-optic structure and organic electro-optic materials of high electro-optic tensor (coefficient) r33for the organic based structure. It turns out that a 300 times increase in sensitivity is sufficient to reach the targeted electric field detection sensitivity of 0.1 μV/m·Hz1/2, assuming an achievable 1 GHz broadband photodetection sensitivity of 10 fW/Hz1/2for the optical power.

According to one embodiment of the present invention, the ultra-sensitive electric field detection device is based on detecting the external field induced optical phase shift in an electro-optic waveguide via a number of innovative components to greatly enhance the detection sensitivity as briefly described below.

Novel Micro-antenna Design to Increase Sensitivity by about 150 Times: A micro-antenna is adapted for amplifying the electric field strength by about 150 times and applying the amplified electric field to the electro-optic material region (actual enhancement is about 300 times but reduced by 2 times due to the filling dielectric materials with a dielectric constant of about 2).

Micro-antenna Effect Enhanced by TCO Based EO Structure: The 150 times enhancement of the electric field by the use of a micro-antenna is achieved only with the use of a TCO to conduct voltages directly across the electro-optic material, otherwise the enhancement is about 3-6 times lower depending on the electro-optic material used.

Utilizing III-V and Organic EO Materials: The central component of the electric field detection device according to the present invention is an electro-optic waveguide, formed of an III-V based electro-optic material or organic electro-optic material. In one embodiment, an inorganic high refractive-index-contrast waveguide is used. In another embodiment, a low dielectric constant organic material is used. Both types of materials can give very comparable performances. The micro-antenna structure is applied to both types of material but with about 5 times smaller enhancement for the organic case.

III-V Focused on GaAs: In one embodiment, the III-V based EO waveguide initially employs bulk GaAs for ease of fabrication and demonstration. A further enhancement by about 3-10 times is achieved by using quantum dots or quantum wells. The high refractive-index-contrast of an III-V material enables the waveguide core thickness to be as thin as about 0.3 μm, which is about 5 times thinner than the organic material case (about 1.5 μm thick), enabling 5 times higher electric field enhancement utilizing the micro-antenna design. One potential drawback of GaAs is its high dielectric constant ∈=12.5 that is 5 times higher than the ∈=2.5 for the organic materials. Hence, the in-material electric field could potentially be reduced by 5 times, but it is only reduced slightly as shown later. According to the present invention, the material effect on the micro-antenna is dominated by the much larger filling material under the micro-antenna and not by the electro-optic material. The material parameter n3r33of the bulk GaAs is about 58 pm/V. Compared to the typical organic electro-optic material with the parameter n3r33of about 150 pm/V (assuming the electro-optic coefficient r33=30 pm/V), the parameter n3r33is about 3 times higher for organic materials, which indicates that the electro-optic response for GaAs is about 3 times worse. However, GaAs gained about 5 times more in the electric field strength from the micro-antenna enhancement. As a result, GaAs still comes out about 5/3 times better. If the organic case employs the recently developed high electro-optic coefficient r33organic material with the electro-optic coefficient r33of 150 pm/V, then the organic case may be better than GaAs by about 3 times. But then the use of quantum well and quantum dots in the III-V based device could also lead to another 3-10 times enhancement so they can again be very comparable. The optical wavelength to be used for the electric field detector probing is at 1.0 μm and 1.3 μm. The coupling into the GaAs waveguide is performed via a mode-enlarging tapered structure or an integrated high-NA micro GRIN lens. The process for fabricating the III-V based device include wafer bonding and thin-film transfer and the use of mode enlarger (III-V WB-TF-ME).

Organics focused on Robust Self-Assembly: The organic EO waveguide initially employs molecularly self-assembled electro-optic materials. In one embodiment, poled organic materials with a high electro-optic coefficient r33are also used. The self-assembly material currently has the electro-optic coefficient r33of about 30 pm/V but the material is covalently bonded and highly robust compared to spin-coated poled organic materials. The main advantages of a self-assembly material are that: (a) it does not need poling; (b) it can withstand high laser power of greater than 100 mW in the waveguide; (c) it has no pin holes and no poling produce them so it is very suitable for the application; (d) its optical loss is low less than 1 dB/cm; (e) it can withstand high temperatures of greater than 200° C.; (f) electro-optic modulators that demonstrate the above advantages have been developed. In one embodiment, an organic material with 5-10 times higher electro-optic coefficient r33is employed. The other organic electro-optic materials such as poled polymers can also be used for the organic based EO waveguide. The optical wavelength to be used for the electric field detector probing is at 1.0 μm and 1.3 μm.

Multi-pass and/or Resonator Structures to Increase Sensitivity about 10 Times: To achieve more phase shift within a 1 mm-size device, two methods are used to achieve multiple passing through the electro-optic region: (a) a ring resonator scheme in which a 1 mm long electro-optic section utilizing a resonator with a finesse of 10 makes the effective optical interaction length about 10 times longer. With an enhancement of about 10 times, a device having an about 1 mm-long electro-optic region acts like a device having an about 1 cm-long electro-optic region. (b) A retro-reflector based on micro-loop mirror or metallic coating enables dual pass to achieve about 2 times the phase shift. In one embodiment, a 10-20 times increase in sensitivity is achieved by combining schemes (a) and (b).

Interferometeric Optical-Carrier Reduction Enabling High Optical Power and Increase in Sensitivity by 10-100 Times: Higher optical power in the waveguide (10-100 mW) results in a 10-100 times higher sensitivity than that when only 1 mW optical power is used since the electro-optic modulated signal sideband generated has a higher power. However, most high-sensitivity photo detectors (e.g. APD) saturate at about 1 mW. According to the present invention, an optical-carrier-reduction scheme is employed to bring the 10-100 mW in-device optical carrier power down to about 1 mW so as to achieve a 10-100 times increase in sensitivity. This reduction is performed after the beam passes through the device and is converted to an amplitude modulation. In one embodiment, the optical carrier reduction is achieved using an optical interference technique with matched arms, making it insensitive to laser frequency noise. The use of intensity stabilized laser and intensity noise cancellation with dual detectors further reduces the low-frequency noise and enable electric field detection down to 0.5 Hz.

High Sensitivity Wideband Detection: To address the 0.5 Hz to 1 GHz bandwidth requirement, a sensitive photo-detection system with a wide bandwidth is used. An RF spectrum analyzer with an optical input head can reach about 10 fW/Hz1/2optical power sensitivity with about 1 GHz bandwidth. Portable APD detector module has sensitivity ranging from about 30 fW/Hz1/2to about 500 fW/Hz1/2. If 30 fW/Hz1/2detection sensitivity is assumed, to ensure 0.5 Hz is detected, equal-path-length interference is used to convert the phase-shift to an intensity modulation, which is insensitive to the laser phase noise. Thus, the resonator also utilizes a phase-shift resonance geometry instead of an intensity resonance geometry since the intensity-resonance geometry is very sensitive to laser frequency noise or phase noise, which has 1/f behavior at low spectral frequency. Lasers with intensity stabilization and the use of intensity noise subtraction with a balanced dual detector pair (if needed) further ensures that the measurements are sensitive down to about 0.5 Hz.

DWDM-On-A-Chip for Array Addressing: According to the present invention, both multi-fiber inputs and DWDM-On-A-Chip technology for 5×5 array addressing are utilized. The DWDM-On-A-Chip technology enables the use of a single fiber. It is applied to the GaAs based device and utilizes a monolithically integrated super-compact grating (Array-SCG) technology. The SCG involves only planar waveguide and is capable of substantially lower loss (<2 dB) than compact AWG based on channel waveguides (>10 dB).

In summary, the factors: micro-antenna, TCO enhancement, multi-pass, and optical carrier reduction, result in about 100,000 times increase in sensitivity compared to conventional electro-optic approaches that has a sensitivity of about tens of mV/m·Hz1/2. A 300 times increase in sensitivity is sufficient to reach an electric field detection sensitivity of 0.1 μV/m·Hz1/2, assuming an achievable 1 GHz broadband photodetection sensitivity of 10 fW/Hz1/2for the optical power. As discussed below, not all factors have to be included to meet the performance parameters listed in Table 1. For example, the use of quantum-well structure, micro-antenna, a double-pass scheme, and sensitive detector giving very low wall-plugged power is sufficient to reach the performance parameters listed under Embodiment I in Table 1. Multi-pass and optical carrier reduction can bring another over 500 times higher sensitivity than that needed for meeting the performance parameters listed under Embodiment II in Table 1, but requires more complex system control and higher wall-plugged power.

An alternative scheme to increase sensitivity by 100 times more is to use a long winding waveguide on the 1×1 mm2area, which increases the interaction length by about 100 times. However, such method cannot work unless the waveguide has very low loss. The micro-antenna approach is superior since it gets about 150 times sensitivity increase using a 1×1 mm2area for the micro-antenna. The waveguide is basically only 1 mm long so the optical throughput still is high.

According to the present invention, with all these enhancements combined together, the electric field detection device provides a total of over 100,000 times enhancement in the electric field detection sensitivity, compared to conventional electric field detection devices. Another factor of about 10 times improvement is achieved by using quantum well/dot structure for the III-V based electro-optic waveguide and organic electro-optic materials of a high EO coefficient r33for the organic based waveguide. It turns out that a 300 times increase in sensitivity is sufficient to reach the targeted electric field detection sensitivity of 0.1 μV/m·Hz1/2, assuming an achievable 1 GHz broadband photodetection sensitivity of 10 fW/Hz1/2for the optical power.

Table 2 summarizes the various enhancement factors of the electric field detection device according to embodiments of the present invention, which show that with the use of the III-V or organic electro-optic materials, the sensitivity of about 0.1 μV/m·Hz1/2is reached without employing optical carrier power reduction enhancement of 100 times or the resonator enhancement of greater than 5 times (see case A in the total enhancement row). This results in a simple low wall-plugged power system. With the optical carrier power reduction enhancement and with the use of resonator, a 500 times higher sensitivity, i.e., 0.0002 μV/m·Hz1/2is reached (see case D in the total enhancement row). However, the system control is more complex and the wall-plugged power is higher. The use of quantum well/quantum dot for the case of III-V results in another 3-10 times enhancement without increasing the wall-plugged power. The equivalent for the organic scheme is the use of high r33materials.

Referring toFIGS. 1 and 2, an electric field detection device100is shown according to one embodiment of the present invention. The electric field detection device100includes an interferometer having a reference arm181and an active arm183. The active arm183includes a micro-antenna101. In the exemplary embodiment, the micro-antenna101has a first electrically conductive plate110and a second electrically conductive plate130arranged parallel to each other and spaced apart by a distance, dF, defining a first gap142therebetween, and a third electrically conductive plate130disposed in the first gap140and vertically attached to and being in electrical contact with the first electrically conductive plate110so as to define a second gap152with a gap width, dE, between the third electrically conductive plate130and the second electrically conductive plate120. As such, the first electrically conductive plate110and the third electrically conductive plate130define a T-shape structure. In one embodiment, the first electrically conductive plate110and the third electrically conductive plate130are formed integrally.

As shown inFIG. 1, each of the first and second electrically conductive plates110and120has a width WFand a length LF. The third electrically conductive plate130has a body portion131having one end connected to the first electrically conductive plate110and a tip portion132extending from the other end of the body portion131such that the second gap152is defined between the tip portion132and the second electrically conductive plate120. The third electrically conductive plate130has a length that is the same as that of the first and second electrically conductive plate110. The body portion131has a width dAand a thickness WA. The tip portion132has a width dBand a thickness WB.

The first and second electrically conductive plates110and120constitute a receiving part of the detection device100for receiving an external electric field to be detected, and thus, are called the field receiving plates (F-RPs). The third electrically conductive plate130is adapted for applying the electric field to the electro-optic waveguide150and is called the EO-field applying plates (EOF-APs). In one embodiment, each of the first to third electrically conductive plates110,120and130is formed of an electrically conductive material such as a metal.

The electric field detection device100also has a second electro-optic waveguide structure formed in the second gap152. The electro-optic waveguide structure150has a width WEOthat is equal to the thickness WBof the tip portion132of the third electrically conductive plates130and a height dEO, as shown inFIG. 1. Specifically, the second electro-optic waveguide structure has a waveguide layer150, a first (top) cladding layer161disposed between the third metallic plate130and the waveguide layer150, and a second (bottom) cladding layer162disposed between the waveguide layer150and the second metallic plate120. Each of the top and bottom cladding layers161and162has a height dTCand a width that is the same as width WEOof the electro-optic waveguide structure150.

In one embodiment, the electro-optic waveguide layer150is formed of a material of groups III-V of the periodic table. For example, the electro-optic waveguide layer150is formed of GaAs. The GaAs electro-optic property imposes a phase shift on the TE-polarized (with polarization parallel to the substrate plane) propagating beam under the influence of an applied electric field. Additionally, the electro-optic waveguide layer150may have quantum-wells (QWs) or quantum-dots (QDs)155formed therein.

In another embodiment, the electro-optic waveguide layer150is formed of an organic electro-optic material, where the organic electro-optic material comprises molecularly self-assembled superlattices. Preferably, the organic electro-optic material comprises an organic material with an electro-optic coefficient r33higher than about 100 pm/V.

In one embodiment, the top and bottom claddings layers161and162are electrically non-conductive. In another embodiment, the top and bottom claddings layers161and162are electrically conductive, for example, formed of a transparent-conducting oxide (TCO) material, such as Sn-doped In2O3, In2O3, ZnO, NiO, or the like.

The remaining portion of the first gap142defined between the first and second electrically conductive plates110and120is filled with a dielectric material (dielectric layer)140having a dielectric constant, ∈F, so that the third metallic plate130and the second electro-optic waveguide150is surrounded by the dielectric layer140. In one embodiment, the dielectric layer140is formed of CYTOP™.

Additionally, as shown inFIGS. 1 and 2, each of the first and second electro-optic waveguides180and150comprises a retro-reflector187at the end thereof. In one embodiment, the retro-reflector187comprises a micro-loop mirror. In another embodiment, the retro-reflector comprises a metallic coating at the end of a respective electro-optic waveguide.

In one embodiment, the interferometer further comprises an input/output waveguide185having a first end and a second end, where the first end is optically coupled to both the first and second electro-optic waveguides180and150, and the second end is optically coupled to an optical fiber. In one embodiment, the second end of the input/output waveguide is tapered to a pointed tip such that the width of the tip is less than half of the wavelength of a light beam propagating in the input/output waveguide, and the optical fiber has a lens tip.

In one embodiment, the reference arm has an optical path-length that is about λ/8 longer than the optical path-length of the active arm, where λ is the wavelength of a light beam propagating in the interferometer. The electric field detection device may further have a heater disposed in the vicinity of the reference arm adapted for tuning the optical path-length of the reference arm.

In one embodiment, the electric field detection device further includes a first optical ring resonator coupled to the first electro-optic waveguide and a second optical ring resonator coupled to the second electro-optic waveguide.

In one embodiment, the electric field detection device further includes a gradient-index (GRIN) lens configured to facilitate the optical coupling between the second end of the input/output waveguide and the optical fiber.

In one aspect of the present invention, an electric field detection system has an electric field detection device as disclosed above, and an optical circulator having a first port configured to receive an input light beam, a second port configured to transmit the input light beam to and to receive an output light beam from the electric field detection device, and a third port configured to transmit the output light beam received from the electric field detection device to a light detection system.

In another aspect of the present invention, an electric field detection module comprises a plurality of the above-disclosed electric field detection devices arranged in an array. In one embodiment, the electric field detection module further comprises a dense wavelength division multiplexer (DWDM) coupled to the plurality of electric field detection devices via an optical fiber and a gradient-index (GRIN) lens, where the DWDM comprises a diffraction grating.

Various micro fabrication techniques and materials (e.g. TCO and organic materials) required for the successful realization of the electric field detection devices have been developed. The fabrication involves wafer-level process and hence enables many devices to be fabricated on a single wafer run, resulting in economy of scale and potentially low-cost devices.

These and other aspects of the present invention are further described below. Without intent to limit the scope of the invention, exemplary devices and their related results according to the embodiments of the present invention are given below. Note again that titles or subtitles may be used in the examples for convenience of a reader, which in no way should limit the scope of the invention. Moreover, certain Theories are proposed and disclosed herein; however, in no way they, whether they are right or wrong, should limit the scope of the invention.

Electric Field Enhancement Using a Micro-Antenna

Referring now toFIG. 3, an electric field detection device300including a micro-antenna301, which provides about 100 times enhancement in sensitivity, is shown according to one embodiment of the present invention. In the exemplary embodiment, the micro-antenna301has a first metallic plate310and a second metallic plate330arranged parallel to each other and spaced apart by a distance, dF, defining a first gap342therebetween, and a third metallic plate330disposed in the first gap340and vertically attached to and being in electrical contact with the first metallic plate310so as to define a second gap352with a gap width, dEO, between the third metallic plate330and the second metallic plate330. The electric field detection device300also has an electro-optic waveguide350structure formed in the second gap352. The remaining portion of the first gap342defined between the first and second metallic plates310and320is filled with a dielectric material (dielectric layer)340having a dielectric constant, ∈F. The first and second metallic plates310and320constitute a receiving part of the detection device300for receiving an external electric field to be detected, and thus, are called the field receiving plates (F-RPs). The third metallic plate330is adapted for applying the electric field to the electro-optic waveguide350and is called the EO-field applying plates (EOF-APs).

In one embodiment, each of the first and second metallic plates310and320has a width WFand a length LF(not shown). The third metallic plate330has a thickness (width) WEOand a length LEO. The electro-optic waveguide350has a width, WEO, that is equal to the thickness of the third metallic plate330, and a height, dEO, that is the same as the gap width of the second gap352. The external electric field399to be measured is assumed to have electric field strength EAppl. The electric field between the first and second metallic plates310and320is set to be EF. If the material340in-between the first and second metallic plates310and320has a dielectric constant ∈F, then EFis scaled down from the external electric field EAppldue to the dielectric boundary conditions and is given by EF=EAppl·(∈0/∈4). Thus, it is important to choose the filling material320in-between the first pair of metallic plates310to have a low dielectric constant. In one embodiment, CYTOP having a dielectric constant of about 2.15 is used. Other dielectric materials with low dielectric constants can also be used to practice the present invention.

The electric field EFin the dielectric layer340between the first and second metallic plates310and320induces a voltage VFgiven by EFtimes dF:
VF=EF·dF(EQ1)

The voltage VFis then conducted to the third metallic plate330that applies it across the electro-optic waveguide core350. The first and second metallic plates310and320and the third metallic plate330have the same length so that LF=LEO. Since the first metallic plate310and the third metallic plate330are electrically connected, they have the same voltage difference VFbetween the first metallic plate310and the second metallic plate320, and the third metallic plate330and the second metallic plate320. Let the electric field across the second gap352be EEO, then the voltage VFis related to EEOby:
VF=EEO·dEO(EQ2)

The physics applied hereto is that by connecting one of the large first and second metal plates with a small metallic plate disposed between the first and second metal plates, the voltage across the two large metal plates and the voltage across one large metal plates and the small metallic plate are forced to be equal, and the electrical charge concentrates in the small plate region, and therefore enhances the electric field therein. It is like a lightning rod which has very strong electric field at the tip region. A more systematic explanation is described in terms of the “ideal”, “non-ideal”, and “without-top-plate” cases as follows.

Ideal Case

Equating EQ1 and EQ2 via the common voltage VF, one obtains
EEO=EF·(dF/dEO)=EAppl·(dF/dEO) ·(∈0/∈F)  (EQ3).

Thus, the electric field EEOat the electro-optic region350is increased by the plate-distance ratio (dF/dEO). As discussed below, the distance dEOis basically the height (thickness) of the electro-optic waveguide core350that is much smaller than the distance dF. In one embodiment of the GaAs waveguide discussed below, dEOis about 0.3 μm. The distance dFis about 10 times or 100 times larger than dEO. Foe example, dF=30 μm that is 100 times larger than dEO. As the induced voltage VFacross dFis now applied across a 100 times smaller gap dEOat the third metallic plate330, the field strength is increased by 100 times as given by the ratio (dF/dEO)=30 μm/0.3 μm=100 for this example. As discussed below, in reality, it works only at the limit when the electric field receiving plates are dominantly large. Let's call this limit the “ideal scaling” and define EIDEAL=EF·(dF/dEO) so that when EEO=EIDEAL, the ideal case is approached. In reality, EEOis always less than EIDEAL. Under what condition can the ideal case be obtained is the subject of discussion below.

Non Ideal Case and Saturation Region

From the above discussions, a question is whether this means that one can make dFvery large, say dF=1 mm and obtain an incredible field enhancement of 3,300 times (assuming dEO=0.3 μm)? A related question is whether the widths WFand WEOof the first to third plates play any role. What happens if the plate width WFof the first and second plates is small compare to the plate separation dF? A quick answer is that in order to approach the ideal case, the “capacitance CF” of the field receiving plates (F-RPs)310and320shall be larger than the “capacitance CEO” of the EO-field applying (third) plate (EOF-AP)330and the second plate320in order for the voltage transfer to be determined by the field receiving plates310. That is, one needs CF>>CEO.

Let the dielectric constant of the dielectric material340in-between the field receiving plates310and320be ∈Fand the dielectric constant of the dielectric material350in-between the EO-field applying plate330and the second plate320be EEO. Since C=∈ A/d, one has CF=(WF·LF·∈F)/dFthen the condition requiring CF>>CEOleads to (WF·∈F/dF)>>(WEO·∈EO/dEO) or WF>>WEO·(∈EO/∈F)·(dF/dEO).

Detailed field simulations show that if CFand CEOare exactly equal, the EEOachieved is at 50% of its ideal value EIdeal. Similarly, if CFis 9 times larger than CEO, then the EEOachieved is at about 90% of its ideal value EIdeal, and one can call such a region the field-scaling saturation region. More precisely, if the saturation factor S is defined the ratio of EEOto EIdeal, i.e., S=(EEO/Eidea), the field saturation region is corresponding to S approaching its maximum value of 1. Numerical simulation shows that in terms of the capacitances, the saturation factor S is approximately given by S≈CF/(CF+CEO). Hence, when the capacitance of the field receiving plates dominates so that CF>>CEO, S approaches 1.

Case without Top Plate

What is interesting is that from the simulation, it turns out that when WFgoes to zero, the field scaling does not disappear but approaches about 20% of the ideal case. This is because the “vertically-connecting metal” plate330is used to connect the lower plate320to the upper plate330still interacts with the external electric field EAppland has fringing field pattern that still gives a significant effective capacitance value for CF. As the top plate is not really needed but it does not mean that one can put another device next to it as the fringing field still occupies some effective space around the vertically-connecting metal. A good design is still to have the top plate to confine the fringing field.

Numerical Simulation Results

In one embodiment, as shown inFIG. 4, a 0.01 V voltage is applied across 10 mm to obtain the external field EAppl=1 V/m.

FIG. 5, shows: (1): in order to achieve EIdeal, WFneeds to be large enough so CFdominates the micro-antenna structure. For example, in order for saturation factor S=90%, WFis larger than 128 μm, and (2) the dielectric constants of the materials in the CEOregion and the CFregion scale down EF, and thus EEOdue to the relative dielectric constant of the material in the CFregion, or ∈F/∈0.

Enhancement Limitations

From the above discussions, it is clear that given the area constrain of 1 mm2and assuming a square area so that the width of the electric receiving plate310is constrained to 1 mm, the smaller the EO-plate distance dEO, the larger is the enhancement of the electric field. In one embodiment, as shown inFIG. 6, the electric field detection device600has an electro-optic waveguide structure350formed of GaAs. In order to avoid metal-induced optical loss, a low-refractive-index top cladding layer361is formed between the third plate330and the GaAs waveguide350and a low-refractive-index bottom cladding layer362is formed between the GaAs waveguide350and the second plate320in the second gap352. In one embodiment, each of the low-refractive-index top and bottom cladding layers361and362has a thickness about 0.5 μm, as shown inFIG. 6. In the example, the EO-plate (second gap) distance dEO=0.5 μm×2+0.3 μm=1.3 μm, if these claddings layers361and362are electrically non-conductive.

Similarly for an organic waveguide based device700, each of the top and bottom cladding layers361and362typically has a thickness about 3 μm, as shown inFIG. 7, resulting in dEO=3 μm×2+1.5 μm=7.5 μm, if the cladding layers361and362are electrically non-conductive, which results in 4 times to 5 times loss in potential field strength gain had there been no claddings. One way to gain the enhancement factor back is to make the cladding conductive.

Enhancement Using TCO based EO Waveguide Structure with Micro-Antenna

Referring to nowFIG. 8, an electric field detection device800is shown according to one embodiment of the present invention. The detection device800is structurally similar to the detection device600shown inFIG. 6, except the top and bottom cladding layers361and362are electrically conductive, and formed of a transparent-conducting oxide (TCO) material. As discussed above, the use of TCO enables another 4 times to 5 times gain in the electric field detection sensitivity and still maintain the 1 GHz frequency response with the use of appropriate TCO materials and device structure. When using TCO layers as cladding, it is necessary to maintain the cladding to be low in optical loss. In one embodiment, the TCO is engineered to have the appropriate material figure of merits FTCOand electrical conductivity σ, so as to have a low optical loss.

The material figure of merit F is defined by the ratio of the electrical conductivity (σ) to optical loss (α) (FTCO=σ/α). It has been shown that high-frequency (greater than 2.5 GHz) and low voltage can be achieved with FTCO>0.1 and σ≈10 S. Appropriate TCO materials meeting such requirements have been successfully engineered. TCO based modulators that show the significant TCO advantages have been successfully realized. Similar conditions also apply to the application to the electric field detector. Hence, similar materials and structures already developed for the TCO-based modulators is used to achieve the 4-5 times higher field strength and still maintain the required high frequency response of greater than 1 GHz.

High Frequency Response with TCO

In one embodiment, the TCO bridge electrodes together with the electro-optic waveguide core is modeled as a RC circuit, in which the TCO layers361and362are modeled as two resistors R and the electro-optic region350is modeled as a capacitor.

Since the device length is short, compared with the wavelength of the electric field (λ=c/f=30 cm when f=1 GHz), the frequency response is mainly RC limited.

FIG. 8shows the equivalent RC circuit and the extracted frequency response near 1 GHz range. In the RC circuit model R=dTC/(σWEOΔL) and C=∈TCWEOΔL/dEO. From the curves shown inFIG. 9, it can be seen that the frequency response near 1 GHz region is quite flat, which means the sensitivity of the TCO enhanced micro-antenna does not change when the frequency is below 1 GHz.

GaAs Based EO Waveguide Device

Referring toFIG. 10, an electric field detection device1000is shown according to one embodiment of the present invention. The electric field detection device1000is similar to the electric field detection device100shown inFIG. 1. The electric field detection device1000includes a micro-antenna1001. In the exemplary embodiment, the micro-antenna1001has a first electrically conductive plate1010and a second electrically conductive plate1030arranged parallel to each other and spaced apart by a distance, dF, defining a first gap1042therebetween, and a third electrically conductive plate1030disposed in the first gap1040and vertically attached to and being in electrical contact with the first electrically conductive plate1010so as to define a second gap1052with a gap width, dE, between the third electrically conductive plate1030and the second electrically conductive plate1020. As such, the first electrically conductive plate1010and the third electrically conductive plate1030define a T-shape structure. In one embodiment, the first electrically conductive plate1010and the third electrically conductive plate1030are formed integrally.

The first and second electrically conductive plates1010and1020constitute a receiving part of the detection device1000for receiving an external electric field to be detected, and thus, are called the field receiving plates (F-RPs). The third electrically conductive plate1030is adapted for applying the electric field to the electro-optic waveguide1050and is called the EO-field applying plates (EOF-APs).

In one embodiment, each of the first and second electrically conductive plates1010and1020has a width WFand a length LF. The third electrically conductive plate1030has a body portion1031having one end connected to the first electrically conductive plate1010and a tip portion1032extending from the other end of the body portion1031such that the second gap1052is defined between the tip portion1032and the second electrically conductive plate1020. The third electrically conductive plate1030has a length that is the same as that of the first and second electrically conductive plate1010. The body portion1031has a width dAand a thickness WA. The tip portion1032has a width dBand a thickness WB.

In one embodiment, each of the first to third electrically conductive plates1010,1020and1030is formed of an electrically conductive material such as a metal.

The electric field detection device1000also has an electro-optic waveguide structure1050formed in the second gap1052. The electro-optic waveguide structure1050has a width WEOthat is equal to the thickness WBof the tip portion1032of the third electrically conductive plates1030and a height dEO, as shown inFIG. 10. The electro-optic waveguide structure1050is formed of a material of groups III-V of the periodic table. In one example, the electro-optic waveguide structure1050is formed of GaAs. Additionally, the electro-optic waveguide structure1050may have quantum-wells (QWs) or quantum-dots (QDs)1055formed therein.

In the exemplary embodiment, a low-refractive-index top cladding layer1061is formed between the tip portion1032of the third electrically conductive plate1030and the electro-optic waveguide structure1050and a low-refractive-index bottom cladding layer1062is formed between the electro-optic waveguide structure1050and the second electrically conductive plate1020in the second gap1052. Each of the top and bottom cladding layers1061and1062has a height dTCand a width that is the same as width WEOof the electro-optic waveguide structure1050. In one embodiment, these claddings layers1061and1062are electrically non-conductive. In another embodiment, the claddings layers1061and1062are electrically conductive, for example, formed of a transparent-conducting oxide (TCO) material. The GaAs electro-optic property imposes a phase shift on the TE-polarized (with polarization parallel to the substrate plane) propagating beam under the influence of an applied electric field.

The remaining portion of the first gap1042defined between the first and second electrically conductive plates1010and1020is filled with a dielectric material (dielectric layer)1040having a dielectric constant, ∈F.

In one embodiment, the waveguide structure1050formed of GaAs with a refractive index of nGaAs=3.5, a height (thickness) dEOof 0.3 μm, and a width WEOof 0.5 μm, and surrounded on both sides by a low refractive index polymer 1040 with a refractive index n<1.5 is used as a single-mode strongly-guiding waveguide to guide the optical beam. In one embodiment, the top and bottom cladding layers1061and1062are formed of a TCO material, with the height dTC=0.5 μm and the width equal to WEO=0.5 μm. The TCO layers1061and1062are fabricated using a self-aligned process as discussed in detail below. The TCO material has a low refractive index of n=1.7 but can act as a conducting material with low optical loss.

In one embodiment, the micro-antenna1001is fabricated on the top of the top-TCO layer of the GaAs waveguide. The EO-field applying plates (EOF-APs) of the micro-antenna have a top-plate width WEOof about 0.6 μm. As described below, the placement of such a structure on the top of the 0.5 μm-wide TCO can be done within 50 nm alignment accuracy using E-Beam lithography or stepper photolithography. Such an E-beam pattern realignment is routinely performed for nanophotonic device fabrications so it can be comfortably achieved.

Referring now toFIGS. 11-13, the micro-antenna1001has a vertically connected metallic plate1030of about 1 mm high (dA). Its top is electrically connected to the first (top) plate1010with the width WFthat forms the top plate1010of the pair of field receiving plates (F-RPs). In order to achieve the high electric field only the width of the tip1032(i.e., WEO) has to be narrow as it determines the CEO, which is made low comparing to CFof the large parallel plates1010and1020. Thus, it is only required that the vertically connected metallic plate1030to have a narrow (tip) width WEOof 0.6 μm for the length dBof 3-5 μm. This has a structural width-to-height aspect ratio of less than 1:10, which can be easily fabricated using the standard nano-fabrication technique. The width WAof the vertically connected metallic plate is enlarged to 50 μm for ease of fabricating a dA=1 mm high structure. This structure has a width-to-height aspect ratio of less than 1:20 and is quite manageable to fabricate.

At the output end, the optical mode is enlarged using a down-taper structure that tapers to a small pointed tip, as shown inFIG. 14. When the width WEOof the dielectric waveguide1050is narrower than half a wavelength in the material, the optical mode actually becomes larger enabling coupling of 30-50% into an optical fiber1090. This is shown inFIG. 14. Alternatively, a unique beam enlarger based on a miniature integrated super-high NA GRIN lens (referred as “super-GRIN lens”) has been developed. The super-GRIN lens1095has a micro-lens size of only 15 μm in height by 20 μm in length and width as shown inFIG. 15. The GRIN structure1095has a graded refractive index that can be realized using two alternating material layers (referred as the dual material approach). A preliminary version of the lens has been successfully realized on silicon substrate. In one embodiment, the lens is fabricated on the GaAs waveguide using standard multi-layer deposition technique for optical coating such as that used for DWDM filter. The lens efficiently enlarges the optical mode size to match the mode size of optical fiber. The super-GRIN lens1095is better than down taper as it does not need lens tip fiber and can use the typical cleaved fiber. It has much less stringent alignment tolerance. It enables higher coupling efficiency of >70% and much easier fiber alignment and device packaging.

A process for fabricating an epi-layer structure including a GaAs/AlGaAs layer structure is shown inFIGS. 16A-16O, according to one embodiment of the present invention. A 1 μm-thick Al0.6Ga0.4As structure1051is grown on a GaAs substrate1001followed by a thin 0.3 μm-thick top GaAs layer1050, as shown inFIG. 16A. Through a wafer-bonding and etched-back process, the 0.3 μm-thick top GaAs layer1050is fabricated to form the GaAs electro-optic waveguide1050of the GaAs based electric field detector. First, a 0.3 μm-thick TCO layer1062is deposited on the GaAs layer1050as the bottom transparent conductive cladding layer1062, as shown inFIG. 16B. The bottom metal layer1020(such as gold, aluminum, etc.) is then deposited on the top of the TCO layer1062as the bottom metallic plate1020of the T shape structure, as shown inFIG. 16C.

A layer1071of BCB is then spin-coated on the bottom metallic plate1020as the wafer bonding material, as shown inFIG. 16D. The substrate1001is then bonded with another substrate1002(called transfer substrate, typically also formed of GaAs) with a BCB layer1072on it, as shown inFIG. 16E. When the two wafers are bonded together, the original substrate1001is thinned down by polishing it till the 1 μm-thick Al0.6Ga0.4As layer1051is exposed. Selective wet etching using HF then completely removes the Al0.6Ga0.4As layer1051, resulting in the thin 0.3 μm-thick GaAs layer1050on the TCO layer1062, as shown inFIG. 16E. Below the TCO layer1062is the metal layer1020, followed by the BCB layer1071. Another 0.3 μm-thick TCO layer1061is deposited on the GaAs layer1050as the top transparent conductive cladding layer, as shown inFIG. 16G. A metal etch-mask with 0.3 μm width is then formed by e-beam lithography, as shown inFIG. 16H. The metal mask is used as a dry-etching mask to etch through the two TCO layers and the GaAs layer, as shown inFIG. 16I. A 5 μm-thick CYTOP1040is then spin coated to cover the etched structure and then a 0.3 μm wide window is opened above the GaAs/TCO waveguide, as shown inFIGS. 16J and 16KElectro-plating fills the 0.3 μm wide, 5 μm high trench to form a narrow metal tip1032, as shown inFIG. 16L. A 1 mm-thick CYTOP layer1041is casted on the top of the wafer, and a 50 μm window1045is opened above the narrow tip1032and a second electro-plating form the wider metal portion1031, which is connected with the narrower tip1032, as shown inFIGS. 16M and 16N. Finally a 1 mm wide metal layer1010is deposited on the top of the structure as the top plate1010for the T-shape structure, as shown inFIG. 16O.

Organic Material Based EO Waveguide Device

Referring toFIG. 17, an electric field detection device1700is shown according to one embodiment of the present invention. The electric field detection device1700is similar to the electric field detection device1000shown inFIG. 10, except that the device1700utilizes an organic electro-optic material to achieve the required electric field induced optical phase shift. The organic based structure has the potential advantage of low fabrication cost and larger beam size enabling reasonable coupling to be achieved without using mode-size enlarger. The recently engineered organic electro-optic materials with r33of 150 pm/V opens up the possibility of further increase its sensitivity once the organic-EO based structure is demonstrated. In one embodiment, molecularly self-assembled organic electro-optic material is used. The molecularly self-assembled organic electro-optic material has been proven to be highly robust. There are, among other things, advantages including (a) it does not need poling; (b) it can withstand high laser power of greater than 100 mW in the waveguide; (c) it has no pin holes and no poling that could produce them so it is very suitable for the application; (d) its optical loss is lower than 1 dB/cm; (e) it can withstand high temperature of greater than 200° C.; (f) electro-optic modulators that demonstrated the above advantages have been made; (g) its r33can be engineered to a higher value in later generation.

As shown inFIG. 17, the electric field detection device1700includes a micro-antenna1701. In the embodiment, the micro-antenna1701has a first electrically conductive plate1710and a second electrically conductive plate1730arranged parallel to each other and spaced apart by a distance, dF, defining a first gap1742therebetween, and a third electrically conductive plate1730disposed in the first gap1740and vertically attached to and being in electrical contact with the first electrically conductive plate1710so as to define a second gap1752with a gap width, dE, between the third electrically conductive plate1730and the second electrically conductive plate1720. As such, the first electrically conductive plate1710and the third electrically conductive plate1730define a T-shape structure. In one embodiment, the first electrically conductive plate1710and the third electrically conductive plate1730are formed integrally.

Each of the first and second electrically conductive plates1710and1720has a width WFand a length LF. The third electrically conductive plate1730has a body portion1731having one end connected to the first electrically conductive plate1710and a tip portion1732extending from the other end of the body portion1731such that the second gap1752is defined between the tip portion1732and the second electrically conductive plate1720. In one embodiment, each of the first to third electrically conductive plates1710,1720and1730is formed of an electrically conductive material such as a metal. The third electrically conductive plate1730has a length that is the same as that of the first and second electrically conductive plate1710. The body portion1731has a width dAand a thickness WA. The tip portion1732has a width dBand a thickness WB.

In one embodiment, each of the first to third electrically conductive plates1010,1020and1030is formed of an electrically conductive material such as a metal. The first and second electrically conductive plates1710and1720constitute a receiving part of the detection device1700for receiving an external electric field to be detected, and thus, are called the field receiving plates (F-RPs). The third electrically conductive plate1730is adapted for applying the electric field to the electro-optic waveguide1750and is called the EO-field applying plates (EOF-APs).

The electric field detection device1700also has an electro-optic waveguide structure1750formed in the second gap1752. The electro-optic waveguide structure1750has a width WEOthat is equal to the thickness WBof the tip portion1732of the third electrically conductive plates1730and a height dEO, as shown inFIG. 17.

In the exemplary embodiment, a low-refractive-index top cladding layer1761is formed between the tip portion1732of the third electrically conductive plate1730and the electro-optic waveguide structure1750and a low-refractive-index bottom cladding layer1762is formed between the electro-optic waveguide structure1750and the second electrically conductive plate1720in the second gap1752. Each of the top and bottom cladding layers1761and1762has a height dTCand a width that is the same as width WEOof the electro-optic waveguide structure1750. In one embodiment, these claddings layers1761and1762are electrically non-conductive. In another embodiment, the claddings layers1761and1762are electrically conductive, for example, formed of a transparent-conducting oxide (TCO) material. As shown inFIG. 17, the organic waveguide structure1750with a refractive index of norg=1.65, a thickness of dEO=1.5 μm, and a width of WEO=2 μm, surrounded by a low refractive index polymer 1730 with a refractive index of n<1.5 (such as CYTOP™) is used as a single-mode waveguide1750to guide the optical beam by fabricating a ridge1771of about 2 μm wide formed by BCB material with a refractive index of nBCB=1.6. The organic electro-optic property imposes a phase shift on the TM-polarized (with polarization perpendicular to the substrate plane) propagating beam under the influence of an applied electric field.

In one embodiment, the micro-antenna is fabricated on the top of the organic waveguide. The EO-field applying plates (EOF-APs) of the micro-antenna has two top-plates, one on the left and one on the right of the waveguiding region, each with a width of 1 μm (i.e. WEO). The gap-space between the two top-plates is about 5 μm to avoid metal absorption for the optical beam. The voltage is conducted to the beam propagating region from these two top-plates via a short length TCO with a side-conduction geometry very similar to the TCO-based modulator structure. As described below, the placement of such metallic top-plate structure of 1 μm in width on top of the waveguide can be done within 50 nm alignment accuracy using e-beam lithography or stepper photolithography. Such e-beam pattern realignment is routinely performed so it can be comfortably achieved. The top plate is then connected to two vertically connected metallic plates with a narrow width of 1 μm for a length of 5 μm. These two 5 μm tall structures are then connected to another vertically connected metallic plate of about 1 mm high and 50 μm wide. Its top is then connected to a large parallel plate with width WFthat forms the top plate of the pair of field receiving plates (F-RPs). In order to achieve the high electric field only the width of the tips (i.e. WEO) have to be narrow as it determines the CEO, which is low comparing to CFof the large parallel plates. It has a width-to-height aspect ratio of less than 1:10, which can be easily fabricated using the standard nano-fabrication technique. Above that, the width of the vertically connected metallic plate is enlarged to 50 μm for ease of fabricating a 1 mm high structure. Such structure has a width-to-height aspect ratio of less than 1:20 and shall be quite manageable to fabricate.

Referring also toFIGS. 18A-18K, the fabrication process of organic electro-optic material based E-Field detector is shown according to one embodiment of the present invention. At first, it starts from the substrate1701with a 0.3 μm thick bottom cladding layer (e.g. SiO2or BCB layer)1772on it, as shown inFIG. 18A. A metal layer1720is deposited on the bottom cladding layer1772and a 5 μm window1721is opened by photo lithography and wet etching, as shown inFIG. 18B. The bottom TC layer1762is then deposited to fill in the 5 μm window1742as the bottom TC layer1762, as shown inFIG. 18C. A layer1750of an organic electro-optic material about 1 μm thick is spin coated or self-assembled on the top of the bottom electrode1720and1762, as shown inFIG. 18D. Another TC layer1761which is also 5 μm wide is then patterned on the top of the electro-optic layer1750, aligned with the bottom TCO electrode1762, as shown inFIG. 18E. A BCB top cladding layer1771about 1 μm high and 1 μm wide is patterned on the top TC electrode1761, in order to confine the optical mode laterally, as shown inFIG. 18F. A 5 μm thick CYTOP layer1740is spin coated to cover the whole structure, as shown inFIG. 18Gand one 0.5 μm wide window is opened in order to fill in metal so that the parallel TC plates1761and1762connect with the wide metal tip1732, as shown inFIG. 18H. After filling in the two 0.5 μm wide windows with metals like gold or aluminum by electroplating, another 1 mm CYTOP layer1741is coated on the structure, as shown inFIG. 18I. A 50 μm wide window is then opened and filled in with a metal material by electro-plating to form a wide metal tip1731, as shown inFIG. 18J. Finally, a metal layer1710is deposited on the top as the top plate1710of the T shape structure, as shown inFIG. 18K.

Mach-Zehnder Structure and Multiple Passing Scheme via Resonator and Retro-Reflector

Referring toFIG. 19, this example shows a complete Mach Zehnder structure1900common to both the GaAs and organic based EO-waveguide electric field detection device. To form the Mach Zehnder interferometer structure1900, from the top view, the micro-antenna1901is actually placed on only one (active) arm1983of the Mach Zehender interferometer structure1900. An input beam splitter (a Y-junction) splits the beam by 50:50 to two GaAs waveguides1950and1980. One waveguide1950goes under the micro-antenna1901and is called the electro-optic (active) arm1983, while the other waveguide1980does not and is called the referenced arm1981. To ease the manufacturing process, a reflector1987is placed at the end of the waveguides1950and1980so the split beams are retro-reflected back and recombined at the input Y-junction that now acts as the output combiner of the Mach Zehnder interferometer1900. The return beam is then coupled back into the same input optical fiber and can be split out using a beam circulator.

The optical path-length of the reference arm1981is fabricated to be λ/8 longer than the electro-optic arm1983so that over the round trip it gains a λ/4 optical path-length difference before combining with the beam from the electro-optic arm1983. This enables the default operating point to be at the linear slope of the Mach-Zehnder interferometer response curve. A resistive heater1989is fabricated very near the reference arm, which enables an electrical tuning of the relative phase shift between the reference arm1981and the electro-optic arm1983if needed. If the fabrication process can be fined tuned, or the phase difference can be tuned in advance using laser to physically chisel the waveguide width, such heating element may not be needed.

As shown inFIG. 19, the retro reflection is achieved by fabricating a micro-loop mirror1987or with the use of a metallic coating. Such a micro-loop mirror based high-reflector1987has been successfully fabricated for an InP based micro-loop mirror laser. The loop needs only a length of less than 20 μm and hence can be very compact and low loss.

FIG. 20shows another embodiment of a Mach Zehnder structure2000according to the present invention. In addition to the setup of the Mach Zehnder structure1900, shown inFIG. 19, the Mach Zehnder structure2000further has a coupling waveguide1991at close to the retro reflector of the reference arm1981, which is used to couple about 1% of the light energy emitted out from the back of the reference arm1981. A small 100 μm-large photodetector chip1995is mounted close to the output spot of the waveguide1991so as to measure the optical power in the reference arm1981. Likewise, the Mach Zehnder structure2000also has a coupling waveguide1993at close to the retro reflector of the electro-optic arm1983and coupled to a 100 μm-large external photodetector chip1997to measure the optical power in the electro-optic arm1981. Such a power detection aids in coupling the beam output from a lens-tip optical fiber into the input waveguide. They can also be used to help to diagnose the device2000operations such as checking on the 50:50 power splitting.

In one embodiment, the length of the waveguide1991/1993is about 5 mm, which gives a long interaction length to achieve the phase-shift needed to reach the performance parameters listed under Embodiment I in Table 1. To achieve the performance parameters listed under Embodiment II in Table 1, the waveguide length is reduced to 1 mm. In order to gain the interaction length back, the waveguide1991/1993is integrated with a ring resonator (resonant cavity)1992/1994that has a diameter of around 1 mm, as shown inFIG. 21. The resonator1992/1994is designed to achieve the appropriate Finesse to optimize its performance depending on the optical loss of the waveguide fabricated.

In one embodiment, the cavity coupling strength is designed so that the effective number of round trips in the cavity1992/1994gives a total optical path length is a few times shorter than the optical loss length given by the inverse of the optical loss coefficient (α). This operates the resonator at the under damped region, making it acting like a good phase shifter without too much amplitude change at the optical resonance. Such a resonator based phase-shifter2100is shown inFIG. 21. It gives a 5 times more increase in sensitivity (depending on the waveguide loss coefficient) when the straight waveguide is only 1 mm long.

Usually, an ideal lossless ring resonator with a single coupling waveguide always transmits 100% of the optical power but at on-resonance, the energy in the ring resonators interfere with the laser beam that goes straight through to result in a π phase shift compared to the beam that is tuned off resonance. So the optical phase at the output of this waveguide goes from zero to π from below the resonance frequency to on resonance then go to 2π when tuned to above the resonance frequency. Thus, a frequency shift of the resonator translates to a phase shifting of the optical beam when it is at the resonance frequency. With two coupling waveguides, the ring resonator becomes an intensity modulator if the beam is sit at the slop of the resonance. However, such an intensity resonance configuration is sensitive to a laser frequency or phase noise that can have large 1/f behavior at low frequency. Thus, the intensity resonance configuration does not work if one wants the electric field detector to be sensitive at down to 0.5 Hz. The phase shifting configuration with two balanced arms (i.e. with the same resonator at the reference arm) enables one to have equal-path-length interference that makes the beam output insensitive to the laser frequency or phase noise. Hence, to meet the low frequency 0.5 Hz specification, it is essential to use the phase resonance configuration for the resonator together with the equal-arm Mach Zehnder inteferometer to convert phase shift to intensity change.

Molecularly Self-Assembled EO Materials Engineering for Electric Field Sensors

A new class of high-performance molecular electro-optic materials has been developed at Northwester University which is intrinsically acentric (no electric field poling is required) and can be applied to virtually any surface as a thermally/chemically robust, conformal coating by solution phase self-assembly techniques. The fabrication process uses successive layer-by-layer organosilane self-assembly reactions to build up structurally well-defined low-loss superlattices with the electro-optic coefficient r33about 65 pm/V, as shown inFIG. 22. These materials are stable to 350° C. in air, are radiation-hard, and are impervious to common organic solvents. In one embodiment, such materials are used to coat the surfaces of the electric field sensor(s) disclosed herein.

Transparent Conducting Oxide Materials Engineering for Electric Field Sensors

Transparent conducting oxides (TCOs) are degenerately doped metal oxide n-type semiconductors with large band gaps for excellent optical transparency in the visible range. TCO conductivity can be tuned to as high as 17,000 S/cm by tuning the carrier density through adding other ions as dopants or by creating oxygen vacancies. TCOs have found extensive commercial use in photovoltaic cells, flat-panel displays, and smart windows, and are a well-established technology. In one embodiment of the present invention, ion-assisted deposition (IAD) is used for TCO film growth. IAD utilizes two beams, a deposition beam which impinges on a rotating target of exactly known composition (FIG. 23) and an assist beam which continuously bathes the growing film in a second ion flux. For TCO films, the net result is excellent control of film dopant level, conductivity, optical absorption characteristics, densification, adhesion, and surface morphology at temperatures as low as room temperature. At Northwester University, the available IAD tool is located in a clean room and has precision control of O2partial pressure, growth and assist beam ion energies, and sample temperature. It has been used to grow excellent quality films of Sn-doped In2O3, In2O3, ZnO, NiO, and other TCOs on a variety of substrates, including Si/SiO2and plastic. Depending on the desired electrical and optical properties, these TCO films have been used as electrodes in ultra-low Vπ electro-optic modulators, as high-mobility semiconductors in thin-film transistors, as hole extracting/-electron blocking layers in high-efficiency photovoltaic cells, and as electrodes for flexible organic light-emitting diodes. In the present invention, TCO films, beginning with In2O3are grown as described in previous sections with conductivity and optical characteristics tuned to meet the specifications.

Optical Carrier Power Reduction Scheme

In one embodiment of the present invention, an optical carrier power reduction scheme is employed to bring the high 10 mW-100 mW in-device optical carrier power down to 1 mW to achieve a 10-100 times increase in sensitivity of the electric field detection. This reduction is done after the beam passes through the electric field detection device and converted to the amplitude modulation. As shown inFIG. 24, the return beam is taken out via an optical circulator and goes to a fiber-based 50:50 beam splitter or coupler. The return beam has a high carrier power carries the amplitude modulation from the electric field detector.

FIG. 24shows an electric field detection system2400according to one embodiment of the present invention. The electric field detection system2400has an electric field detection device2410that is the same as the detection device2100ofFIG. 21in the exemplary embodiment. Other embodiments of the electric field detection devices disclosed above according to the present invention can also be used. The electric field detection system2400also has an optical circulator2420having a first port configured to receive an input light beam, a second port configured to transmit the input light beam to and to receive an output light beam from the electric field detection device2410, and a third port configured to transmit the output light beam received from the electric field detection device2410to a light detection system2450.

The electric field detection system2400further has a first beam splitter2430having an input port configured to receive a source light beam from a light source2440, a first output port configured to transmit a first part of the source light beam to the first port of the optical circulator2420, and a second output port configured to transmit a second part of the source light beam to the light detection system2450.

The light detection system2450has a second beam splitter2460having a first input port2461configured to receive the output light beam transmitted from the third port of the optical circulator2420, a second input port2462configured to receive the second part of the source light beam transmitted from the second output port of the first beam splitter2430, a first output port2463configured to output an enhanced light beam resulted from a constructive interference of the light beams received from the first and the second input ports2461and2462of the second beam splitter2460, and a second output port2464configured to output a reduced light beam resulted from a destructive interference of the light beams received from the first and second input ports2461and2462of the second beam splitter2460.

Additionally, the light detection system2450further includes a light detector2470configured to receive the light beam transmitted from the second output port2464of the second beam splitter2460, a phase shifter2482and an optical attenuator2484coupled in series between the second output port of the first beam splitter2430and the second input port2464of the second beam splitter2460, and a feedback controller2486coupled between the light detector2470and the phase shifter2482and between the light detector2470and the optical attenuator2484, respectively.

As shown inFIG. 24, two (first and second) input ports2461and2462of the second beam splitter2460are labeled as input port A and input port B, respectively. Suppose the return beam goes to input port A. A portion of the original source laser beam that powers the electric field detector2410is split out and is introduced into input port B. As the carriers in port A and port B come from the same laser source, they can be made to interfere destructively at the A−B output port2464of the beam splitter and constructively at the A+B output port2463. To ensure such interference, the beam that goes into port B is passed through an optical delay line so that it travels the same optical path length from the laser2440to Port B and the beam that goes from the laser2440to the electric field detector2410and back to Port A. Each of the A−B and A+B output ports has half the amplitude modulated side band power obtained from the beam at input port A. However, A−B port has much reduced carrier power. A phase shifter2482and attenuator2484allow the amplitude and phase control of the beam that goes to port B. By shifting the phase of port B beam, the carrier power level at the A−B output port can be controlled. These enable one to achieve an effective reduction of the optical carrier power and retain the strong amplitude modulated side band.

The beam at output port A−B is then sent to a sensitive 1 GHz photodetector2470to measure the amplitude modulated signal. The optical carrier power at A−B is reduced to about 1 mW so that it does not saturate the photodetector2470. In fact the DC output from the photodetector2470can be used to feedback to control the phase shifter to maintain the carrier power level. The feedback-loop shall be designed to be slower than 0.5 Hz so as not to affect the signal being detected.

As the optical carrier reduction is be achieved by using optical interference technique with matched optical path lengths, it makes the resultant beam insensitive to laser frequency noise. Intensity stabilized laser and intensity noise cancellation scheme involving dual detectors could be used to further reduce the low-frequency noise and enable electric field detection down to 0.5 Hz.

Packaging of a Single Electric-Field Detection Device

In one embodiment, as shown inFIG. 25, to enable ease in fiber mounting, a slightly larger substrate2501is provided so that the extended edge of the substrate2501is etched down about 175 μm. The edge has an extended length of about 400 μm, which gives enough room to epoxy down the lens-tip fiber2550after alignment. For a waveguide down taper, a lens-tip fiber2550is used for coupling light from the fiber to the down taper. The intensity monitoring of light from the retro-reflector end of the detection derive (chip)2510utilizes a waveguide coupler to split out 1% of the light and emit it towards the back-end of the chip. This enables one to measure the light power inside the waveguide, which is useful for aligning the fiber to the chip.

In another embodiment, the reflected power returning from the retro-reflector2610is used to monitor the return power after the circulation. The entire fiber coupled chip2610is mounted into a ceramic substrate2601. The fiber mounted chip2610is turned upside-down into a ceramic miniature can2620, as shown inFIG. 26.

5×5 Array Realization with Hybrid and Monolithic (DWDM) Approach

In one embodiment as shown inFIG. 27, an electric field detection module2700has 5×5 above-disclosed electric field detection devices2710arranged in an array. Other numbers of the electric field detection devices2710can also be used to practice the present invention. The electric field detection module also has a dense wavelength division multiplexer (DWDM) coupled to the plurality of electric field detection devices via an optical fiber and a gradient-index (GRIN) lens. The DWDM may include a diffraction grating.

For the array fabrication, two processes are provided: (a) a hybrid process involving the hybrid mounting of 25 chips (electric field detection devices) onto a single substrate, and (b) an monolithically integrated process involving on-chip wavelength division multiplexing of 25 wavelengths in which each wavelength is used for measuring a single detector.

In the process (a), the 25 detectors are fabricated individually and then mounted back onto a single substrate with appropriate fiber mounting management so the chip at the outer rim of the array has fiber connected to the center of the chip, as shown inFIG. 27and the chip inside the array has fiber connected to the right or left edge of the chip.

In the process (b), the array is monolithically fabricated on a single substrate and an on-chip DWDM wavelength de-multiplexer (DeMux) with 25 DWDM wavelength channels is used to channel the 25 wavelengths to 25 detectors. In this example, there is only one single input fiber needed, as shown inFIG. 28. The on-chip DWDM DeMux is fabricated on the GaAs based case to demonstrate the approach.

The DWDM DeMux is based on a WDM-On-A-Chip technology that is developed recently enabling the realization of “external grating laser” on a single monolithically integrated chip. The key element is an integrated curved diffraction grating on III-V chip with very small physical size call the super-compact grating (SCG). For example, wavelength multiplexers (Mux's) with 100 GHz DWDM channel spacing is realizable with a grating size less than 0.5 mm2. The novel curved grating is computationally generated with full spatial aberration correction, fabricated by vertical etching into the side wall of a planar waveguide on an III-V chip. This results in an integrated curved diffraction grating with the highest spectral resolution and the smallest physical size. These gratings actually operate with a very small slit size defined by the integrated waveguide to achieve high resolution. The small slit size results in large beam diffraction angle, and the aberration-free grating enables focusing back to a small slit giving ultra-high spectral resolution. The grating region is based on a planar waveguide (instead of channel waveguides such as in arrayed waveguide grating or AWG) and hence the propagation loss in SCG is substantially lower (<1 dB) than III-V based AWG (typically >10 dB). An example of the highly integrated multi-wavelength InGaAsP laser chip based on the SCG that is realized recently is shown inFIG. 29.FIG. 30shows a spectrum of the simultaneous outputs at channel 1 (1547 nm) and channel 2 (1531 nm) separated by 16 nm according to one embodiment of the present invention.

Experimental results verifying the ultra-large-angle aberration-corrected grating (i.e. SCG) design has been achieved.FIG. 31shows an integrated WDM grating optical spectrometer on InP chip only 2 mm2in size capable of 100 GHz channel spacing (50 GHz resolution) based on the SCG. It involved integration of SCG with a detector array to pick up powers in multiple channels.FIG. 32shows the attractive device performances of the spectrometer with channel cross talks <−25 dB, experimentally confirming the SCG design.

Overall System Engineering and Power/Cost Estimates

There are a number of choices in terms of the actual combination of technologies. They are not all equal in cost and power requirement. Here are some best case analyses to give an idea of the wall-plug power requirement. As discussed, the GaAs and organic waveguide based electric field detectors are comparable in some way. The analyses are focused on the GaAs waveguide based electric field detector. Quantum well structures is used to gain about 3 times higher electro-optic response than that provided by bulk GaAs. With a 150 times enhancement by the micro-antenna, a 2 times enhancement by a dual pass scheme, and a detector sensitivity of 30 fW/Hz1/2, the performance parameters listed under Embodiment II in Table 1 can be reached. A module-type photodetector is also used.

For this configuration, the wall plugged power is basically dominated by the 20 mW fixed wavelength DFB laser diode with controller giving 2-3 W wall-plugged power requirement and the RF photodetctor module with a few Watts wall-plugged power requirement. Thus, the total wall-plugged power is about 10 W or lower.