Abstract:
The present application is directed to high speed optical system. In one embodiment, the optical system includes a photodiode which is sensitive to a wavelength of light, an image source irradiating a wavefront of a first wavelength on the photodiode to which the photodiode is sensitive, the wavefront containing an optical path difference induced phase-shift, a read source of photons irradiating photons of a second wavelength to which the photodiode is insensitive, an electric field across the photodiode in excess of the breakdown voltage thereof and configured to result in an avalanching of electrons in the photodiode when the photons from the first source strike the photodiode, the avalanching electrons resulting in a photorefractive response which changes the index of refraction in the photodiode, and a capture device in optical communication with and configured to capture light reflected from the photodiode.

Description:
BACKGROUND 
     In media having a constant index of refraction, the optical path length may be defined as the product of the geometric distance through which an optical wavefront traverses and the refractive index of the media. In contrast, as an image or optical signal propagates through a turbulent media random fluctuations in the index of refraction of the media results in variations of the optical path length. For example, an image propagating through the atmosphere encounters localized variations in the refractive index of the air. As such, the optical path length of a media having a varying index of refraction may be defined as the integral of n δ s , where δ s  is an element of length along the path, and n is the local refractive index. Variations in the optical path length leads to randomization of the phase front contour of the wavefront, thereby causing the image to be obscured. This phenomena is known as phase-shifting. 
     FIG. 1 shows a representation of an optical signal propagating through a space. As illustrated, the exemplary wavefront or optical signal  1  may be segmented into four quadrants  1 A,  1 B,  1 C, and  1 D. As the wavefront  1  propagates through the space  2 , quadrants  1 A,  1 B,  1 C, and  1 D are subjected to the optical characteristics of the space  2 . For example, the index of refraction of the space  2  may not be uniform. As shown in FIG. 1, the space  2  may include areas of uniform refractive properties  3 A,  3 B, and  3 C, as well as one or more areas of variable refractive properties  3 D. As such, the optical path length the wavefront  1  traverses is not uniform, thereby distorting, defocusing, or otherwise compromising the wavefront  1  as it propagates through the space  2 . As shown in FIG. 1, when quadrant  1 D of the wavefront  1  is subjected to a higher refractive index  3 D of the space  2 , quadrant  1 D traverses a greater optical path length than the neighboring wavefront quadrants  1 A,  1 B, and  1 C. As such, the output wavefront  4  undergoes phase-shifting. As illustrated, the output wavefront  4  includes quadrants  4 A,  4 B, and  4 C which are substantially in phase having traversed an equal optical path length. However, at a time t, quadrant  4 D has traversed an optical path length less than the adjoining quadrants  4 A,  4 B,  4 C, and is, thus, out of phase with the adjoining quadrants  4 A,  4 B,  4 C. 
     In response, a number of systems and techniques have been developed to restore the original phase state of optical signals. One system which has been developed employs adaptive optics or active optical control systems to address variations in optical path lengths. Typically, active optical systems make use of adaptive optical elements that are based on mechanical implementation. One example of an adaptive optical element is a deformable mirror. The deformable mirror includes a distortable substrate having a light-reflecting material applied thereto. The substrate includes a number of small actuators coupled thereto which push or pull segments of the substrate, thereby reconfiguring the shape of the deformable mirror. In doing so, the actuators compensate for the distortions in the beam phase by making some parts of the optical path shorter and some parts of the optical path longer. 
     While present adaptive optics or active optical control systems have proven useful in many applications, a number of shortcomings have been identified. For example, these systems transform what fundamentally is an optical problem into a mechanical problem. As such, the mechanical systems used to reconfigure the deformable mirror may introduce jitter or noise into the signal. In addition, the response times of these systems may be unacceptably slow for high date rate applications. For example, response times in the range of megahertz to several gigahertz are not uncommon. 
     Thus, in light of the foregoing, there is an ongoing need a system capable of rapidly correcting for phase-shifting errors in optical signals. 
     BRIEF SUMMARY 
     The various embodiments of the optical system disclosed herein enable a user to easily correct for phase-shifts and aberrations in optical signals. Furthermore, the various systems disclosed herein permit optical corrections of incoming signals, thereby reducing jitter and aberrations associated with presently available mechanical systems. 
     In one embodiment, the present application is directed to a high speed optical system and discloses a photodiode which is sensitive to a wavelength of light, an image source irradiating a wavefront of a first wavelength on the photodiode to which the photodiode is sensitive, the wavefront containing an optical path difference induced phase-shift, a read source irradiating photons of a second wavelength on the photodiode to which the photodiode is insensitive, an electric field across the photodiode in excess of a breakdown voltage thereof and configured to result in an avalanching of electrons in the photodiode when the photons from the first source strike the photodiode, the avalanching electrons resulting in a photorefractive response which changes the index of refraction in the photodiode, and a capture device in optical communication with and configured to capture light reflected from the photodiode. 
     In an another embodiment, the present application is directed to a high speed optical system and discloses an InGaAsP photodiode which is sensitive to a wavelength of light, a first source of photons configured to transmit a wavefront at a first wavelength to which the photodiode is sensitive incident on the photodiode, the wavefront having an optical path difference induced phase-shift, a second source of photons at a second wavelength to which the photodiode is insensitive incident on the photodiode, an electric field across the photodiode in excess of a breakdown voltage thereof and configured to result in an avalanching of electrons in the photodiode when the photons from the first source strike the photodiode, the avalanching electrons resulting in a photorefractive response which changes the index of refraction in the photodiode, and a capture device in optical communication with and configured to capture light reflected from the photodiode. 
     In still another embodiment, the present application is directed to a high speed optical system and discloses an InGaAsP photodiode having a bandgap, the photodiode configured to operate in Geiger mode, a first photon source configured to transmit a wavefront at a first wavelength to which the photodiode is sensitive incident on the photodiode, the wavefront having an optical path difference induced phase-shift, the first wavelength less than the bandgap of the photodiode, a second photon source configured to emit light of a second wavelength, the second wavelength greater than the bandgap of the photodiode, a beam combiner positioned within an optical path and configured to combine the first and second wavelengths, an electric field applied across the photodiode greater than a breakdown voltage thereof, the electric field configured to result in avalanching of electrons in the photodiode when photons from a first photodiode are incident thereon, the avalanche of electrons resulting in a photorefractive response within the photodiode, and a capture device in optical communication with and configured to capture modulated light reflected from the photodiode. 
     The present application further discloses various methods for optically correcting for phase-shifting. One method disclosed in the present application includes baising a photodiode to operate in Geiger mode, irradiating a photodiode with a first wavelength of light to which the photodiode is sensitive, the first wavelength of light transmitting a wavefront, irradiating the photodiode with a second wavelength of light to which the photodiode is insensitive, correcting for a phase-shift of the wavefront with the photodiode by modulating light reflected from a surface of the photodiode with a photorefractive reaction within the photodiode, and capturing the modulated reflected light containing a corrected wavefront. 
     In another embodiment, the present application discloses a method for correcting for phase-shifting and includes configuring a photodiode to operate in Geiger mode, irradiating a photodiode with a first wavelength of light transmitting a wavefront, initiating a photorefractive reaction within the photodiode with the first wavelength of light, irradiating the photodiode with a second wavelength of light to which the photodiode is insensitive, modulating light reflected from a surface of the photodiode with the photorefractive reaction within the photodiode to correct for phase-shifting in the wavefront, and capturing the modulated reflected light. 
     Other features and advantages of the embodiments of the high speed optical system disclosed herein will become apparent from a consideration of the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A high speed optical system for correcting phase-shifting will be explained in more detail by way of the accompanying drawings, wherein: 
     FIG. 1 shows an illustration of a wavefront propagating through a medium having a varying index of refraction; 
     FIG. 2 shows a schematic diagram of an embodiment of an optical system having a first light source emitting a first wavelength and a second light source emitting a second wavelength to a photodiode; 
     FIG. 3 shows a cross sectional view of an embodiment of a avalanche photodiode as viewed along lines  3 — 3  of FIG. 2 having a first wavelength and a second wavelength incident thereon; 
     FIG. 4 shows a schematic diagram of the embodiment of the optical system shown in FIG. 2 wherein the first light source is transmitting a wavefront at the first wavelength to the photodiode; and 
     FIG. 5 shows a schematic diagram of the embodiment of the optical system shown in FIG. 2 wherein light at the second wavelength having a corrected wavefront thereon is reflected from the photodiode to a capture device. 
    
    
     DETAILED DESCRIPTION 
     The following discussion is directed to an optical system for phase-shifting a wavefront. More specifically, the optical system disclosed herein may be used to reduce or eliminate aberrations in a beam wavefront. In one embodiment, the optical system maybe used with coherent light systems, such as communication systems, imaging systems, sensing systems, etc. In an alternate embodiment, the optical system may used with incoherent light systems such as telescopes, imaging systems, etc. 
     FIG. 2 shows a schematic diagram of a high speed optical system for phase-shifting the wavefront of an incident beam. As shown in FIG. 2, the optical system  10  includes an image source or first light source  12  configured to emit light at a first wavelength  14 . In one embodiment, the image source  12  is configured to emit a first wavelength of light  14  having a wavelength shorter than the bandgap of an avalanche photodiode (APD)  16 . For example, in one embodiment, the first wavelength of light  14  is less than 1.59 microns. As a result, the first wavelength of light  14  will be absorbed by the APD  16 , and may thus be considered an input to the APD  16 . The first wavelength of light  14  is incident upon a beam director  18 , which directs the light through a beam combiner  20  to the APD  16 . As shown in FIG. 2, at least one optical filter  22  may be positioned within the optical path. In the illustrated embodiment, a λ/4 plate  22  is positioned within the optical path between the beam combiner  20  and the APD  16 . Optionally, any number or variety of optical filters  22  may be used with the optical system  10 . 
     Referring again to FIG. 2, the optical system  10  further includes a read source or second light source  24  configured to emit light  26  at a second wavelength to the APD  16 . In one embodiment, the second wavelength of light  26  has a wavelength longer than the bandgap of the APD  16 . As such, the second wavelength of light  26  will not be absorbed by the APD  16 . The second wavelength of light  26  is incident upon and traverses through a beam splitter  28 . Optionally, the beam splitter  28  may comprise a polarizing beam splitter. Thereafter, the second wavelength of light  26  is incident upon the beam combiner  20  and is combined with the first wavelength of light  14  emitted by the image source  12 . The second wavelength of light  26 , which is combined with the first wavelength of light  14 , is directed through the λ/4 plate  22  and is incident upon the APD  16 . 
     As shown in FIG. 2, reflected light  30  is reflected off a surface of the APD  16  and is incident upon the λ/4 plate  22  positioned within the optical path, which modulates the reflected light  30 . As such, the reflected light  30  may be considered an output of the APD  16 . The modulated reflected light  32  is incident upon the beam combiner  20  which directs the modulated reflected light  32  into a capture device  34  in optical communication with the beam splitter  28 . Exemplary capture devices  34  include, without limitation, cameras, CCD devices, imaging arrays, photometers, and like devices. The reflected light  30  and modulated light  32  comprise the second wavelength of light  26  which is greater than the bandgap of the APD  16 . As such, the reflected light  30  and modulated light  32  will not be absorbed by the APD  16 . Optionally, additional optical components  36  may be positioned anywhere within the optical system  10 . For example, additional optical component  36  may be positioned proximate to the image source  12 . In an alternate embodiment, additional optical components  36  are positioned approximate to the read source  24 . Exemplary additional optical components  36  include, without limitation, wavelength filters, spatial filters, shutters, light modulators, light valves, lens, objectives, and/or the like. 
     Referring again to FIG. 2, the high speed optical system of the present application utilizes an ADP  16  to phase-shift the incident wavefront. Optionally, any number of APDs  16  may be used within the optical system  10 . For example, the APD  16  may comprise an array of multiple photodiodes. The APD  16  may be manufactured from any variety of material, including, without limitation, Indium Gallium Arsenide (InGaAs), Indium Gallium Arsenide Phosphide (InGaAsP), Silicon (Si), Germanium (Ge), Gallium Nitride (GaN), Silicon Carbide (SiC), or any other suitable materials. In addition, the APD  16  may be manufactured in any number of sizes or shapes as desired. For example, in one embodiment, the APD  16  may be configured to form an asymmetric Fabry-Perot etalon. 
     In one embodiment, the APD  16  is configured to operate in Geiger mode. When operating in Geiger mode, a voltage greater than a breakdown voltage is applied to the APD  16 . FIG. 3 shows an exemplary APD  16  configured to operate in Geiger mode. As shown, the APD  16  includes a first layer  40 , a second layer  42 , and a third layer  44 . In one embodiment, the first layer  40  comprises a positively doped semiconductive material configured to permit an avalanche of electrons to be freed when struck with a photon. For example, in one embodiment the positively doped semiconductive material comprises silicon. In an alternate embodiment, the first layer  40  is comprised of indium phosphide and is heavily doped with a P-type material such as zinc. As a result, the first layer  40  loses its semiconductive properties and functions similar to a conductor. The second layer  42  is either a negative layer or an insulator. For example, the second layer  42  maybe manufactured without doping or with low doping. The third layer  44  is a negative layer. In one embodiment, the third layer  44  is moderately doped with an N-type material. In another embodiment, the third layer  44  is heavily doped with an N-type material such as sulfur, for example, such that the third layer no longer behaves as a semiconductor but instead has a reasonable good conductivity. Optionally, the first, second, and/or third layers  40 ,  42 ,  44 , respectively, may include at least one surface which may be partially reflective to light of a selective wavelength. 
     To operate the APD  16  in Geiger mode at least one electric field is applied across the APD  16 . As shown in FIG. 3, in one embodiment, a first set of electrodes  46 ,  48  is connected to a voltage source  50  and configured to apply a charge across the APD  16 . Optionally, a circuit resistor  52  may be positioned between the voltage source  50  and at least one of the electrodes  46 ,  48 . As a result, a first electric field  54  may be created across the APD  16  and configured to permit the APD  16  to be operated in Geiger mode. Optionally, the APD  16  may also include a second set of electrodes  56 ,  58  coupled to a second voltage source  60 . As such, a second electric field  62  may be created within or surrounding the APD  16 . In the illustrated embodiment, the second electric field  62  is perpendicular to the first electric field  54 . Optionally, any number of electric fields or field directions may be used. Furthermore, the APD  16  may be manufactured in any number of sizes or shapes as desired. When operated in Geiger mode, the incidence of a photon on the APD  16  causes a chain reaction of freeing electrons in a photodiode material which continues until the current within an electrical field applied to the APD  16  drops to zero or until the voltage falls below the breakdown voltage. 
     FIGS. 4-5 show an embodiment of the optical system  10  during use. As shown in FIG. 4, the first wavelength of light  14  emitted by the image source  12  is directed by the beam director  18  to the APD  16 . During propagation, the wavefront  70  may encounter a media having a varying index of refraction. As such, an optical path difference may exist between various portions of the wavefront  70 , thereby resulting in a phase-shift. As a result, the wavefront  70  may become defocused. Referring again to FIG. 4, the APD  16  is simultaneously irradiated with the second wavelength of light  26  emitted by the read source  24 . The first wavelength of light  14  containing the wavefront  70  and a second wavelength of light  26  are combined by the beam combinder  20  and are directed through the λ/4 plate  22  to the APD  16 . The incidence of the first wavelength of light  14  on the APD  16  causes localized pixel heating due to absorption within the photodiode material, thereby inducing a modulation of the refractive index of the photodiode material. Further, the localized pixel heating due to absorption within the photodiode material quickly normalizes thereby compensating for the defocusing of the wavefront  70  caused by the optical path difference. For example, normalization of the pixel heating may occur within about 1 nanosecond. 
     As described above, the APD  16  may be configured to approximate an asymmetric Fabry-Perot etalon. Like the modulation of refractive index described above, the reflectivity of the APD  16  is modulated at a point where the photon of the first wavelength of light  14  is incident upon the APD  16 . The index of refraction and reflectivity of the photodiode material is modulated in the same pattern as the wavefront  70  from the first wavelength of light  14 . However, the phase-shift contained within the wavefront  70  has been reduced or eliminated within the modulated image pattern formed within the photodiode material. As such, the light  30  reflected from the APD  16  at the second wavelength, which is greater than the bandgap of the APD,  16  is modulated to reproduced the corrected wavefront  70 ′ while reducing or eliminating the phase-shift phenomena present in the incident wavefront  70 . 
     As shown in FIG. 5, the reflected light  30  carrying the corrected wavefront  70 ′ is incident upon the λ/4 plate  22  which permits light of a selected polarization to traverse therethrough and which is captured by the capture device  34  coupled to the beam splitter  28 . The capture device  34  captures the corrected wavefront  70 ′ at the second wavelength  26 . As such, the reflected light  32  at the second wavelength received at the capture device  34  is modulated to include the corrected wavefront  70 ′ and has the same intensity pattern as the first wavelength  14 . However, unlike prior art systems, the high speed optical system  10  disclosed herein optically corrects for aberrations caused by phase-shifting without mechanical actuators to compensate for optical path differences. Further, the high speed optical system  10  is capable of correcting phase-shift aberrations in time ranges on the order of 1 nanosecond. 
     Embodiments disclosed herein are illustrative of the principles of the invention. Other modifications may be employed which are within the scope of the invention, thus, by way of example but not of limitation, alternative photodiode configurations, alternative beam director devices, alternative optical filters, and alternative electronic components. Accordingly, the devices disclosed in the present application are not limited to that precisely as shown and described herein.