Abstract:
An optical passive repeater is provided. The repeater is operated under a state of polariton Bose-Einstein condensation (BEC). A phase transition from a thermal polariton state to a condensed polariton state is controlled, where system temperatures and densities are lower than thermal dissociation temperatures and nonlinear saturation densities, respectively. Original input multimode laser signals are transformed into final output single-mode laser signals. Thus, the polariton BEC passive repeater becomes a power-efficient and low-cost device to increase the reach of optical links without sacrificing its signal quality and integrity.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to optical multimode communications; more particularly, relates to an optical passive repeater operated in a state of polariton Bose-Einstein condensation (BEC) for converting multimode laser signals into single-mode laser signals, where a phase transition from a thermal polariton state to a condensed polariton state is controlled with the system temperatures and densities lower than the thermal dissociation temperatures and nonlinear saturation densities. 
     DESCRIPTION OF THE RELATED ART 
     In Ethernet, due to limitation of physical wire, the distance for transmitting optical signals between two terminals are at most only about 100 meters. Therefore, a link between two terminals, such as between a local host and a remote host, or links between a series of Ethernets, often require additional repeaters to increase the reach of optical links. 
     Conventionally, digital signal processor (DSP) is used as a repeater. As shown in  FIG. 4 , the DSP processes input multimode fiber signals by digital electronics. The complex multimode fiber signals are almost exactly transformed into output single-mode fiber signals, to reduce the modal dispersion and, thus, increase the signal transmission distance. However, the power consumption and cost of DSPs are considerably high due to the active-component nature of a DSP. As a result, such an approach cannot provide a cost-effective solution to increase the reach of optical links in data centers, not to mention further applications such as optical interconnects in mass consumer markets. Hence, the prior art does not fulfill all users&#39; requests on actual use. 
     SUMMARY OF THE INVENTION 
     The main purpose of the present invention is to convert original input multimode laser signals into final output single-mode laser signals to increase the reach of multimode optical links, by controlling a phase transition from a thermal polariton state to a condensed polariton state to form a polariton BEC in a passive fashion where the system temperatures and densities are lower than the thermal dissociation temperatures and nonlinear saturation densities. 
     Another purpose of the present invention is to provide a power-efficient and low-cost (1) data-center optical links; and (2) mass-consumer-market optical interconnects with long reach, by using a polariton BEC passive repeater without sacrificing quality and integrity when optical signals are transmitted. 
     To achieve the above purpose, the present invention is a device of optical passive repeater used in optical multimode communication, comprising a vertical cavity surface emitting laser (VCSEL), a polariton BEC passive repeater and a photodetector (PD), where the VCSEL generates multimode laser signals that enter into a first multimode fiber; the repeater is connected to the VCSEL through the first multimode fiber; the repeater converts the multimode laser signals into single-mode laser signals that enter into a second multimode fiber; the PD is connected to the polariton BEC passive repeater through the second multimode fiber; and the PD senses the final laser signals. Accordingly, a novel device of optical passive repeater used in optical multimode communication is obtained. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
       The present invention will be better understood from the following detailed description of the preferred embodiment according to the present invention, taken in conjunction with the accompanying drawings, in which 
         FIG. 1  is the schematic view showing the preferred embodiment according to the present invention; 
         FIG. 2  is the cross-sectional view showing the microcavity structure of the polariton BEC passive repeater; 
         FIG. 3  is the plot showing design considerations for controlling the polariton phase transition; 
         FIG. 4  is the view of the prior art. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following description of the preferred embodiment is provided to understand the features and the structures of the present invention. 
     Please refer to  FIG. 1 , which is a schematic view showing a preferred embodiment according to the present invention. As shown in the figure, the present invention is a device of optical passive repeater used in optical multimode communication, comprising a vertical cavity surface-emitting laser (VCSEL)  1 , a polariton BEC passive repeater  2  and a photodetector (PD)  3 . 
     The VCSEL is used for generating multimode laser signals and the multimode laser signals are injected into a first multimode fiber  11 . 
     The polariton BEC passive repeater  2  is connected to the VCSEL  1  through the first multimode fiber  11  and a first lens  13  to convert input multimode laser signals, which are emitted from the VCSEL  1  and injected into the first multimode fiber  11  and focused by the first lens  13 , into output single-mode laser signals via a polariton BEC. A second lens  14  collects and collimates the single-mode laser signals. 
     Since the temporal pulse widths of the single-mode laser signals may need to be adjusted, a grating or fiber-based compressor (or stretcher) with negative dispersion (positive dispersion) is inserted after the second lens  14  given the first multimode fiber  11  has a positive dispersion. Then, the compressed (or stretched) single-mode laser signals are focused by a third lens  16  and injected into a second multimode fiber  12 . The PD  3  is connected to the compressor (or stretcher)  15  through the second multimode fiber  12  and the third lens  16  to sense the final laser signals. 
     Thus, a novel device of optical passive repeater used in optical multimode communication is obtained. 
     Please refer to  FIG. 2 , which is a cross-sectional view showing a microcavity structure of a polariton BEC passive repeater. As shown in the figure, a microcavity structure comprises a substrate  21  and a polariton cavity  22  on the substrate  21 . The polariton cavity  22  has a p-n junction and comprises a first Bragg reflector  221  on the substrate  21 ; a half-wavelength (λ/2 plus a integer multiple of λ) cavity  222  on the first Bragg reflector  221 ; a second Bragg reflector  223  on the half-wavelength cavity  222 ; and a quantum-well part  224  distributed in the first Bragg reflector  221 , the half-wavelength cavity  222  and the second Bragg reflector  223 . The first Bragg reflector  221  comprises a plurality of alternating coatings of aluminum arsenide (AlAs) and gallium aluminum arsenide (GaAlAs) to form a plurality of pairs of quarter-wavelength (λ/4) GaAlAs/AlAs alternating coatings  2211 . The half-wavelength cavity  222  comprises a plurality of AlAs layers. The second Bragg reflector  223  comprises a plurality of alternating coatings of GaAlAs and AlAs to form a plurality of pairs of λ/4 AlAs/GaAlAs alternating coatings  2231 . The quantum-well part  224  has multiple quantum wells sitting at microcavity electric field antinodes, and can be GaAlAs quantum wells, gallium arsenide (GaAs) quantum wells, indium gallium arsenide (InGaAs) quantum wells, or indium arsenide (InAs) quantum wells. 
     The polariton cavity  22  has a n +  doped layer and a p +  doped layer. The n +  doped layer is a most-bottom coating layer adjacent to the substrate  21  and the p +  doped layer is a most-top coating layer corresponding to the most-bottom coating layer. Or, the p +  doped layer is the most-bottom coating layer adjacent to the substrate  21  and the n +  doped layer is the most-top coating layer corresponding to the most-bottom coating layer. The n +  and p +  regions may also exist in the first Bragg reflector  221  and the second Bragg reflector  223  without touching the quantum well part  224  to avoid performance degradation of the polariton BEC passive repeater  2 . A forward (or reverse) bias applied on the p-n junction can further provide an optical gain (attenuation) to the multimode-to-single-mode laser signal conversion process. 
     On using the present invention, multimode laser signals are inputted into the microcavity structure at an oblique angle such as a 45 degree (°) angle (indicated by an arrow in  FIG. 2 ), and then being absorbed to excite thermal polaritons. The thermal polaritons subsequently become condensed polariton if the polariton phase transition occurs (to be disclosed in the next paragraph), and eventually decay to re-emit single-mode laser signals due to the spontaneous phase coherence inherited from polariton BEC. Through the polariton cavity  22 , multimode laser signals are converted into single-mode laser signals outputted at a nearly vertical angle such as a 0° angle (indicated by an arrow in  FIG. 2 ) via polariton BEC. 
     Please refer to  FIG. 3 , which is a plot showing design considerations for controlling a polariton phase transition. As shown in the figure, solid curves show a phase transition crossover from a thermal polariton state (high temperature, low density) to a condensed polariton state (low temperature, high density). Horizontal dashed lines  4   b  show polariton nonlinear saturation densities. Vertical dashed lines  4   c  show polariton thermal dissociation temperatures. Three sets of curves are calculated according to three different microcavity designs. By operating the system temperatures and densities in a region enclosed by an upper part (or left-hand side) of a solid curve  4   a  (polariton phase transition crossover), a lower part of a horizontal dashed line  4   b  (polariton nonlinear saturation density), and a left-hand side part of a vertical dashed line  4   c  (polariton thermal dissociation density), a polariton BEC passive repeater is obtained and original multimode laser signals are converted into final single-mode laser signals. Note that, with an appropriate microcavity design, e.g., the lowest (or the rightmost) solid curve plus the highest horizontal dashed line plus the rightmost vertical dashed line, a room temperature operation can be obtained. 
     To sum up, the present invention is a device of optical passive repeater used in optical multimode communication, where the device is operated at a polariton BEC state to control a phase transition from a thermal polariton state to a condensed polariton state with system temperatures and densities below thermal dissociation temperatures and nonlinear saturation densities, respectively, for converting original multimode laser signals into final single-mode laser signals; and, thus, a power-efficient and low-cost optical link with long reach is provided by using a disclosed polariton BEC passive repeater. 
     The preferred embodiment herein disclosed is not intended to unnecessarily limit the scope of the invention. Therefore, simple modifications or variations belonging to the equivalent of the scope of the claims and the instructions disclosed herein for a patent are all within the scope of the present invention.