Quantum wave-converter

Abstract

A plug-and-play fiber-coupled nonlinear optical quantum wave-converter, optimized for quantum communications, comprises a commercial periodically-poled, waveguide-based, nonlinear optical chip, coupled with a pair of substrate-guided holographic (SGH) wavelength division multiplexers (WDM) and a pair of SGH filters; it offers bidirectional difference frequency conversion (DFG) and sum frequency conversion (SFG) simultaneously in a single packaged device.

Claims

1. A secure quantum communications system comprising: (a) a first light beam comprising a first wavelength from a first optical source to a first hologram adjacent to one end of a substrate-guided-holographic waveguide; (b) a second light beam comprising a second wavelength different than the first wavelength from a second optical source to a second hologram adjacent to an opposite end of the waveguide; (c) a third hologram attached to the center of the waveguide; wherein the first and second light beams bounce from their respective holograms and travel through the waveguide to combine through a third hologram are focused to a point outside the waveguide; wherein the substrate-guided-holographic waveguide comprises a periodically-poled nonlinear optical device; wherein the sum frequency generator and difference frequency generator are combined in a single unit; wherein at least one of the optical sources comprises a laser; and wherein the substrate-guided-holographic waveguide comprises a spectral filter with a bandpass comprising a target wavelength so only the target wavelength is output.

2. The system of claim 1 wherein the first wavelength comprises a short-wave infrared pump and the second wavelength comprises a quantum signal for difference frequency generation and a telecommunications-band signal for sum frequency generation.

3. The system of claim 1 including the use of optical fibers as inputs and outputs.

4. The system of claim 1 wherein a nonlinear mixing efficiency of the waveguide is nearly exponentially proportional to a quasi-matched interaction length of the waveguide.

5. The system of claim 1 wherein the first and second holograms comprise a thick holographic optical element.

6. The system of claim 1 wherein the holograms include optical power.

7. The system of claim 1 wherein the waveguide comprises a single bidirectional device performing both difference frequency generation and sum frequency generation.

8. The system of claim 1 wherein the third hologram comprises a multiplexed lens, a separated lens, a multiplexed grating, or a separated grating.

Description

CONCISE DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows coupling two wavelengths with a substrate guided holographic.

(2) FIGS. 2(a-d) shows four possible beam combining and focusing layouts.

DETAILED DESCRIPTION OF THE DRAWINGS

(3) To address this need, a new, high-efficiency, low-noise, and bidirectional plug-and-play fiber-coupled quantum wave-converter for quantum communications, has been developed. The subject invention is a commercial nonlinear optical waveguide chip, coupled with an innovative substrate guided holographic wavelength division multiplexer (WDM) at the input, and an innovative substrate guided holographic filter at the output. The substrate guided holographic WDM multiplexes the pump and the signal beams, and then couples them to the input of the WG. DFG generates the target telecommunications-band light at the output that is then coupled to the substrate guided holographic filter. The substrate guided holographic diffracts only the target light wavelength. Quantum applications are highly sensitive to very small (i.e., single photon) light leakage from Raman scattering, and other unwanted light, including out-of-band radiation, out-of-angle light, and in-band but out-of-angle radiation, which are all rejected by the substrate guided holographic filter because they do not meet its diffraction wavelength and diffraction angle at the same time. The substrate guided holographic filter offers a very high out-of-band and off-angle rejection ratio (over 60 dB). Both the WG devices and the optical fibers can be polished with angled end facets to suppress the interference effects. Thus, the SNR of the signal can be very high. The substrate guided holographic occurs in the opposite direction, where another substrate guided holographic WDM multiplexes the S-band and SWIR beams and couples them to a separate WG. At the output, a separate substrate guided holographic filter picks up the quantum band wavelengths and rejects unwanted light. Both DFG and SFG share the same pump in the SWIR. Thus, the subject invention offers bidirectional DFG and SFG simultaneously in a single plug-and-play packaged device that acts a bridge to provide secure quantum communications between remote quantum systems through telecommunications fibers.

(4) As shown in FIG. 1, the subject invention replaces the WDM couplers with couplers based on a substrate guided holographic, which can take light from the long wavelength (Red Side) and shorter wavelength (Green Side) and combine them into a single, collinear beam focused at the same point. Thus, light from the long wavelength is directed at hologram H.sub.1r where it is bounced to center hologram H.sub.2. Similarly light from the shorter wavelength is directed to hologram H.sub.1g where it is bounced to the center hologram H.sub.2 and combined with the longer wavelength light and focused to a point (FIG. 2).

(5) In FIG. 2, as applied to the subject invention, the wavelength of the Red Side will be the SWIR pump, while the Green Side will be the quantum signal for DFG and the telecommunications-band signal for SFG. The resulting coupler will be permanently fabricated on a single light guide, with permanent alignment, excellent out-of-band rejection, and high coupling efficiency.

(6) Each (HOE) hologram is a thick HOE, which has significant filtering capabilities. In this implementation, the HOEs have spectral transmission based on a squared sine function:

(7) $\begin{array}{cc}T\left(\right)={\left[\frac{\sin \left[2.783\left(-{}_0\right)\right]/{}_{\mathrm{FWHM}}}{2.783\left(-{}_0\right)/{}_{\mathrm{FWHM}}}\right]}^2.& \left(2\text{-}1\right)\end{array}$

(8) Using Covestro's Baycol 27-m photopolymer, the typical value of .sub.FWHM is 10 nm at 1.0 m, scaling with wavelength. In addition to the spectral filtering this HOE will have angular acceptance only up to 0.75 off-axis.

(9) There are four possible layouts for using an substrate guided holographic to combine two wavelengths and focus the two into a small waveguide (FIG. 2).

(10) FIG. 2(a) shows a multiplexed lens, FIG. 2(b) shows a separated lens, FIG. 2(c) shows a multiplexed grating, and FIG. 2(d) shows a separated grating.

(11) One embodiment uses individual Bragg grating HOEs with an external lens, as shown in FIG. 2(d).

(12) It will be understood that the foregoing description is of preferred exemplary embodiments of the invention and that the invention is not limited to the specific form shown or described herein. Various modifications may be made in the design, arrangement, and type of elements disclosed herein, as well as the steps of making and using the invention without departing from the scope of the invention as expressed in the appended claims.