Leaky-wave spatial modulator with integrated photonic crystal for all-optical analog-to-digital converters

Abstract

Performance improvement of an all-optical analog-to-digital converter (AOADC) addresses both RF and optical modeling of a leaky waveguide based optical spatial light modulator (SLM) using electro-optic (E-O) material. The E-O polymer provides improved sensitivity for SLM and achieves a broader bandwidth due to better velocity matching between RF and optical waves.

Claims

1. A spatial light modulator (SLM) device within an all-optical analog to digital converter (AOADC) comprising: a first radio frequency (RF) electrode; a second RF electrode positioned in-plane and adjacent to the first RF electrode; an integrated electro-optic (E-O) polymer based optical waveguide positioned in-plane and sandwiched between the first RF electrode and the second RF electrode, wherein the integrated E-O polymer based optical waveguide is integrated with a photonic crystal (PhC) layer separated from the integrated E-O polymer based optical waveguide by a buffer layer defined by the first RF electrode and the second RF electrode such that the PhC layer is offset transversely with respect to the first RF electrode, the second RF electrode, and the integrated E-O polymer based optical waveguide to form a leaky waveguide structure that re-directs light to form the SLM, wherein a leaky wave exit angle from the integrated E-O polymer based optical waveguide is configured to be controlled by an RF signal applied to the leaky waveguide structure, and wherein the applied RF signal is quantized at discrete time intervals based on a sampling rate of the applied RF signal.

2. The SLM of claim 1, wherein the integrated E-O polymer based optical waveguide is compatible with Si-Photonics.

3. The SLM of claim 1, wherein DC contacts rendering realization of a lowpass filter are used for poling of the integrated E-O polymer based optical waveguide.

4. The SLM of claim 1, wherein a superstrate material prism is comprised in the leaky waveguide structure.

5. The SLM of claim 1, wherein the applied RF signal causes a change in a leakage angle of an optical wave in a superstrate.

6. The SLM of claim 1, wherein a travelling wave push-pull topology of the RF electrodes provides a broad bandwidth of the RF modulated SLM.

7. The SLM of claim 1, wherein the RF signal is a voltage applied to push-pull electrodes.

8. The SLM of claim 1, wherein a gap between push-pull electrodes is configured to control an RF electric field.

9. The SLM of claim 1, wherein an applied RF electric field changes an index of refraction due to the Pockels effect.

10. The SLM of claim 1, wherein a change in an index of refraction in the optical waveguide adjusts an exit angle from the optical waveguide based on a dispersion diagram and the Snell's law.

11. The SLM of claim 1, wherein the PhC layer is configured to decrease a wave velocity through the integrated E-O polymer based optical waveguide.

12. The SLM of claim 11, wherein the PhC layer is configured to influence an effective group index at an operating optical wavelength.

13. The SLM of claim 11, wherein SLM sensitivity is configured to be controlled by the PhC layer integrated with the buffer layer.

14. The SLM of claim 1, wherein optical leakage of the leaky waveguide structure is configured to be controlled by adjusting an applied electric field in a gap of a push-pull travelling wave structure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows an all-optical ADC based on SLM sampler realized using 1D PhC and optical aperture based optical quantizer.

(2) FIG. 2A depicts the configuration of ADCs that includes two major processes: sampling and quantization.

(3) FIG. 2B is an example of quantization noise for a sinusoidal signal.

(4) FIG. 3A depicts the conceptual representation of a spatial light modulator based AOADC.

(5) FIG. 3B depicts top-view of the RF and DC contacts of the designed spatial light modulator and an optical binary coding mask as quantizer.

(6) FIG. 4 depicts the simulated RF performances in terms of insertion loss (S21) and return loss (S11).

(7) FIGS. 5A and 5B are graphs of effective index of refractive of the first six guided TE/TM modes inside the E-O polymer.

(8) FIG. 6A depicts propagation dispersion in a phase matching diagram.

(9) FIG. 6B is a graph of calculated leaky angle and output angle swing against the applied RF electrical field.

(10) FIGS. 7A-7C show conceptual demonstrations of the leaky-wave sweeping as a function of magnitude of the applied RF electrical field.

(11) FIG. 8A is a graph of photonic band structure.

(12) FIG. 8B is a graph of calculated effective group index.

(13) FIG. 9 is a graph of simulated angular distribution of the leaky wave intensity.

DETAILED DESCRIPTION OF THE DRAWINGS

(14) The summary, as well as the following detailed description, is further understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, the drawings show exemplary embodiments of the invention. The invention, however, is not limited to the specific methods, compositions, and devices disclosed. In addition, the drawings are not necessarily drawn to scale.

(15) FIGS. 1, 3A, and 3B depicts an-all optical ADC 200 based on SLM sampler realized using 1D PhC and optical aperture based optical quantizer. There are two portions for the designed AOADC: 1) a sampler including of a short optical periodic light source 204 and a spatial light modulator, and 2) a quantizer, like an optical coding mask (either binary code or Gray code) 208. The device may be made on the first part and primarily on SLM.

(16) The performance of an AOADC device is generally categorized and expressed by two parameters, namely, the sampling rate and the resolution. The sampling rate defines how fast a digitalized code is generated by the ADC 200. The resolution refers to the dynamic range or the noise performance of the ADC 200 providing high quantization level accuracy. These parameters are largely influenced by the sampler portion of an ADC.

(17) An RF electrode configuration of lateral coupled microstrip (CMS) is employed for the SLM design for broad modulation bandwidth. As an example of E-O polymer, CPO-1/PMMI guest-host system is used as the optical core 212 material, and it has a refractive index of n.sub.0=1.63 and a conservative E-O coefficient of r.sub.33=70 pm/V at 1550 nm. Norland Optical Adhesive 65 (NOA65) may be used as cladding material 216 with a refractive index of 1.51, a loss tangent of tan =2.210.sup.2. The in-plane optical waveguide 212 is placed in the gap 214 between the electrodes 213 where the RF electrical field maximized to ensure the highest optical-electrical waves overlap efficiency. There are DC access 210 and RF access 211 zones locates on the ADC 200.

(18) An RF transition from coplanar waveguide (CPW) terminals to microstrip (MS) line is CPW-MS providing a single ended input and then from MS to CMS transitions generates a differential structure required for push-pull topology. Virtually ground technique using capacitive effect is adopted for the CPW terminals at high frequencies. The transitions are combined with capacitively grounded CPW (CG-CPW) for ease of fabrication and avoid expensive via hole realization. The CG-CPW is employed for high-speed RF connections of digital receiver and is usually driven by drive/power amplifier. RF design optimization of overall electrode structure is achieved to match for a 50- reference system, but could be applied for any other impedance references.

(19) To optimize the excitation of E-O effect, CPO-1 chromophore may be poled by a high DC electric field as that of propagation direction of odd-mode excitation. Even though the E-O polymers used to suffer from long terms stability, however, new synthesized chromophores such as CPO-1 exhibited a longer-term stability; nonetheless, for long-term operation one could consider re-poling process. Therefore, DC poling contacts 210 are connected to the CMS electrodes; in one embodiment the DC contacts 210 could be realized using Hi-Lo impedance lowpass filter designs to avoid possible leakage of RF signal from the DC contacts 210 during the E-O modulation period. This leakage could influence the passband characteristics of CG-CPW to MS and then from MS to CMS behavior.

(20) The electrode structure is shallowly buried inside the cladding 216 to form a buffer layer, which controls how much dispersive effect is incorporated from the 1D or 2D or 3D PhC structure(s) 220 to the optical waveguide.

(21) As an example of an invention embodiment, 1D PhC model is a combination of alternating layers of PMMI and air and could interact with optical waveguide in superstrate. Other embodiment could use SiO.sub.2 and Si.sub.3N.sub.4 as substrate material. In the 1D PhC as superstrate, the lattice constant of a=500 nm is selected for PMMI/Air to enable 1550 nm light wave to propagate in the 1.sup.st dielectric band. The filling fraction, f, is the ratio between the thickness of the low-s material (air) and the periodicity a; such value is optimized in our design to be 0.5 to achieve a slow-wave structure. The thickness of PhC layer 220 may also be optimized to avoid excessive optical energy dissipating inside the layer, but also ensure effective dispersion-effect be generated.

(22) The PhC layer 220 may be covered by a superstrate material prism 224 with a greater than the effective index of the optical waveguide to create a leaky wave structure. Therefore, the guided mode of light is coupled to radiation mode, and then leak out from the waveguide. The leaky waves in time T, 2T, 3T . . . for instance, where T is an arbitrary time period and reciprocal of clock frequency for Nyquist sampling rate, propagate inside the superstrate. The prism like shape of superstrate 224 translates the leaky wave to be refracted to formulate an image line at output. The applied RF signal causes change in refractive index of the optical waveguide and hence alters such deflection angle. The image line therefore shifts up and down of an optical aperture of 2.sup.m by m with desired coding mask 208. Behind the optical apertures are high-speed photo-detectors (not shown) with m separate outputs as a digital word. With an optical coding mask 208 placed in an appropriate location behind a lens 228, the moving image is read as successive digitized signals.

(23) A silicon wafer 232 with a ground plane 236 resting thereon forms the base for the ADC 200.

(24) FIG. 2A depicts the configuration of ADCs which includes two major processes: sampling 25 and quantization 35. The inputted RF signal 20 is sampled by accurate optical clock 10 and quantized at discrete time intervals related to sampling rate. For example, in an RF signal 20 with bandwidth of 20 GHz, a Nyquist clock 10 at rate of 40 GSPS with optical pulses with duration of T=25 ps is required.

(25) FIG. 2B is an example of quantization noise for a sinusoidal signal. V.sub.FS is the full-scale voltage and Q is the size of quantization level. Based on the quantization level, the signal is represented by a m-bit digital code. The quantization error is defined as the difference between the amplitude of analog and digitized waveforms, and such error associated with the quantization process is within Q/2, where Q is the smallest quantizing step. For the shown example a 10 dBm RF power applied to AOADC in a 50 load system requires a 1V peak voltage that provides a 2V of V.sub.FS. For a 3 bits ADC, V.sub.FS of 2 V is subdivided to 8 levels, each with quantization of Q=250 mV.

(26) FIG. 3A depicts the conceptual representation of a spatial light modulator SLM 300 based AOADC with an optical source clock 10, an E-O polymer based leaky optical waveguide 50 separated from integrated 1D PhC layer 40 with buffer layer 140 that is driven by RF signal 20 using in-plane CMS electrodes 160. The superstrate material 30 causes leaked optical waves 90 to exit the SLM. A cylindrical lens of 100 focuses exiting leaked wave of 90 on an optical binary coding mask of 110 as quantizer. The superstrate 30 acts as a prism layer (As also discussed with respect to FIG. 1) and glued on top of the 1D PhC layer 40 to deflect optical leaky wave at an angle of 120 degrees. The optical leakage losses are optimized using buffer layer 140 to achieve large enough active aperture on prism 30 to achieve angular resolution of 130.

(27) FIG. 3B depicts a top-view of the RF and DC contacts of the designed spatial light modulator 300l and an optical binary coding mask as quantizer. Optical waveguide 50 may be sandwiched between CMS electrodes 160. The DC contact 150 renders realization of Hi-Lo lowpass filter used for poling of E-O polymer, such as COP-1/PMMI. A PMMI layer 60 acts as cladding layer. The CG-CPW contact 170 provides access to RF drive/power amplifier and 50 W load, which are external to this SLM 300. As an example of 40 GSPS AOADC that is realized based on the available materials, the optimized physical dimensions of the designed SLM are: W.sub.gnd=2 mm, W=94 m, s=60 m, =90, r=0.994 mm, L.sub.CMS=1 cm, g.sub.CMS=10 m, h.sub.high=4 min, W.sub.high=10 m, h.sub.low=2 mm and W.sub.low=3 mm.

(28) FIG. 4 depicts the simulated RF performances in terms of insertion loss (S.sub.21) and return loss (S.sub.11) of the designed electrode structure using transitions from CG-CPW 170 to MS and then from MS to CMS 160 and then in return back from CMS to MS and from MS to CG-CPW transition with the DC poling contacts 150 in the form of a second order Hi-Lo lowpass filter. The insertion loss represents a bandpass behavior with first pass-band corner frequency of about 4 GHz and second pass-band corner frequency of about 20 GHz. The high-pass behavior is caused by CG-CPW and when via holes are used, the lowpass behavior is achieved. 20 pF eternal capacitors loading the CG-CPW of 170 could lower this first pass-band to as low as 300 MHz. The observed ripples may be due to the second order Hi-Lo lowpass filter used to block off RF signal leakage through the DC contacts 150, while providing high DC voltage required for poling of E-O polymer in the leaky wave optical waveguide 50.

(29) FIGS. 5A and 5B show graphs of effective index of refractive of the first six guided TE/TM modes inside the E-O polymer, such as COP-1/PMMI, optical waveguide wherein the dimensions of the waveguide are fixed by 1.6 m1.6 m in FIG. 5A. Optical field profile of these six TE/TM modes are also depicted in inset. Single mode operation for fixed wavelength of .sub.0=1550 nm is depicted in FIG. 5B. In this, the dimensions of the optical waveguide re fixed by 1.6 m1.6 m to assure single TE/TM modes propagation over 1.2 to 1.75 m region.

(30) FIG. 6A shows propagation dispersion in a phase matching diagram. A short optical clock 10 is inputted to optical guide 50. With the phase forced to be matched along the tangential direction of the interfaces by the boundary condition, and all interfaces are parallel to each other, the k.sub.z components in each layer are therefore identical based on Snell's law. The effective index and thickness of the 1D PhC layer 40 combined with the buffer substrate affect the leaky coefficient and hence the leaky angle .sub.leak 190, so for simplicity and avoiding a complicated dispersion diagram, those layers are not shown in the dispersion diagram. At interface of the superstrate 30 with the air interface 180, the leaky waves hit the edge at an incident angle .sub.i 90 and are transmitted into the air at an output angle .sub.o 200. Si-photonics based substrate is the basis of realizing this SLM.

(31) FIG. 6B is a graph of calculated leaky angle and output angle swing against the applied RF electrical field. The leaky angle is defined by Snell's law:

(32) $\begin{array}{cc}{}_{\mathrm{leak}}={\sin }^{-1}\left(\frac{n_{\mathrm{eff}}}{n_s}\right),& \mathrm{EQ}.1\end{array}$
where n.sub.eff is the effective index of refractive of the waveguide by solving the optical mode inside the waveguide surrounding by cladding material. The output angle swing is then derived by

(33) $\begin{array}{cc}{}_{\mathrm{out}}=\frac{1}{2}\frac{\cos {}_i}{n_o\cos {}_{\mathrm{leak}}\sqrt{1-{\left(\frac{n_s}{n_o}\sin {}_i\right)}^2}}{n}_1^3{r}_{33}\stackrel{.fwdarw.}{E}.& \mathrm{EQ}.2\end{array}$
where r.sub.33 is the linear E-O coefficient and E is the magnitude of applied RF signal.

(34) FIGS. 7A, 7B and 7C are conceptual demonstrations of the leaky-wave sweeping as a function of magnitude of the applied RF electrical field. FIG. 7A illustrates E=500 V, FIG. 7B illustrates E=0 V, and FIG. 7C illustrates E=500 V. Consecutive images show changes in the deflected beam angle out of the leaky waveguide and optical power level in substrate. The higher the sensitivity of SLM (i.e., a higher effective E-O coefficient of r.sub.33) results in a larger angle sweep of deflection angle of 120. The longer the light is maintained in the optical waveguide by keeping a low optical loss (i.e., low optical leakage coefficient and for example 5 dB/cm) by control of thickness of buffer layer of 140, a higher resolution AOADC is realized with the optical deflected beam angle resolution of of 130. The ENOB is calculated as log.sub.2 (/). The color bars shows optical power levels of 0 dBm (red) to 27 dBm (blue), which indicates small leakage to achieve ENOB of 7.5 for about 8 bits of resolution.

(35) FIG. 8A is a graph of photonic band structure of the 1D PhC slow-wave structure with lattice constant a=500 nm and filling factor f=0.5.

(36) FIG. 8B is a graph of calculated effective group index of 1D PhC structure of FIG. 8A versus normalized wave vector for lattice constant of a=500 nm and filling factor of f=0.5. If group index n.sub.g is far larger than refractive index n of the material at ultra-high frequencies, dispersive effect is then expressed by

(37) $\begin{array}{cc}{n}_g\frac{n}{}=\frac{n}{}=\frac{n}{\text{/}}& \mathrm{EQ}.3\end{array}$
where / is normalized bandwidth and n is index variation with respect to frequency response.

(38) FIG. 9 is a graph of simulated angular distribution of the leaky wave intensity with the optical deflected beam swing angle of (120) and diffracted beam resolution angle of (130).

(39) A combination of analytical calculations, Matlab numerical calculations, and commercial software modeling and simulations were used for physical understanding and optimized design of SLM to operate effectively as an optical sampler in 40 GS/S AOADC with ENOB of about 8. Optimized performance was attained with addition of PhC structure for slow wave effect. Even though modeling was provided for 1D PhC of PMMI and air as superstrate, other designs of 2D and 3D superstrates as well as 1D, 2D, and 3D substrates of Si.sub.3N.sub.4 with SiO2 could also be used. The following predictions are made of 1D PhC of structure depicted in FIG. 8A.

(40) Half-wave voltage (V.sub.) of the designed modulator without PhC layer is calculated by 7.2 Vcm using the following formula:

(41) $\begin{array}{cc}{V}_{}=\frac{{}_0.Math.g_{\mathrm{CMS}}}{r_{33}.Math.n_{\mathrm{eo}}^3.Math..Math.L}& \mathrm{EQ}.4\end{array}$
where L of 1 cm is the interaction length and F is the field overlap efficiency between RF field and optical field with estimated 80%. g.sub.CMS is gap of 10 m for CMS lines, n.sub.eo is index of refraction of E-O material and is 1.55 at wavelength of .sub.o=1550 nm, and r.sub.33 is E-O coefficient of CPO-1/PMMI of 70 pm/V.

(42) The simulated figure of merit of V.sub.L using commercial optical simulator, OptiBPM, is approximately 3.7 Vcm; therefore, the effective E-O coefficient is theoretically calculated to be increased from 70 pm/V to 136 pm/V. The propagating light is then effectively retarded.

(43) The spatial light modulator structure is simulated using commercial optical simulator, OptiFDTD. The total optical attenuation observed at the point of 1 cm away from the input is 5 dB/cm; 77% of which is enticed from the optical waveguide by the 1D PhC layer and then leaked into the prism layer.

(44) The total leaky optical field is derived by
=.sub.leakAe.sup.jk.sup.s.sup.h.sup.buffer.sub.0.sup.Le.sup.[(.sup.leak.sup.+.sup.opt.sup.)j(k.sup.s.sup.sin .sup.leak.sup.)]zdzEQ. 5
Where =k (n.sub.eff+0.5n.sup.3.sub.effr.sub.eoE.sub.0e.sup.RFz)z is the phase propagation constant. .sub.RF is total RF loss, .sub.leak is optical attenuation due to leaky effect, .sub.opt is optical attenuation inside waveguide due to material loss, k.sub.x is the optical wavenumber in prism and A is the magnitude of input light field.

(45) By solving the leaky optical field in a custom Matlab program with the simulated values, the total angular sweep and leaky wave divergence resolution are defined in FIG. 9. The resolution of the designed AOADC expressed by effective number of bits (ENOB) was then calculated as log.sub.2(/). As shown, the ENOB is 7.5, which is close to targeted 8 bits of resolution.

(46) The present invention may be understood more readily by reference to the following detailed description taken in connection with the accompanying figures and examples, which form a part of this disclosure. It is to be understood that this invention is not limited to the specific devices, methods, applications, conditions or parameters described and/or shown herein, and that the terminology used herein is for the purpose of describing particular embodiments by way of example only and is not intended to be limiting of the claimed invention. Also, as used in the specification including the appended claims, the singular forms a, an, and the include the plural, and reference to a particular numerical value includes at least that particular value, unless the context clearly dictates otherwise. The term plurality, as used herein, means more than one. When a range of values is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent about, it will be understood that the particular value forms another embodiment. All ranges are inclusive and combinable.

(47) It is to be appreciated that certain features of the invention which are, for clarity, described herein in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention that are, for brevity, described in the context of a single embodiment, may also be provided separately or in any sub-combination. Further, reference to values stated in ranges include each value within that range.

(48) A person of ordinary skill in the art would understand that various changes or modifications may be made thereto without departing from the scope of the claims.