Abstract:
A coherent frequency modulated receiver for receiving and detecting arriving optical signals which comprises an electrically controllable optical beam scanner receiving optical input beams arriving at different angles in a field of view of the electrically controllable optical beam scanner, the electrically controllable optical beam scanner conveying a scanned optical input beam as its output optical beam; a grating coupler responsive to the output or reflected optical beam of the electrically controllable optical beams scanner, the grating coupler having a waveguided output; an optical local oscillator laser having a waveguided output; an FMCW signal generator; an optical modulator responsive to the optical waveguided outputs of the optical local oscillator laser and also to an electrical FMCW signal from the FMCW signal generator; a pair of second order non-linear optical elements for frequency upconverting respective outputs of the optical modulator and the grating coupler; and at least one photodiode optically coupled to an outputs of the pair of second order non-linear optical elements.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. patent application Ser. No. 13/754,140 filed Jan. 30, 2013 and entitled “Tunable Optical Metamaterial”, the disclosure of which is hereby incorporated herein by reference. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     None. 
     TECHNICAL FIELD 
     This invention relates a chip-scale IR (2-12 μm) frequency modulated coherent Laser Radar (LADAR) receiver with beam scanning capability. 
     BACKGROUND 
     LADAR imaging technology compared to traditional Radar technology enjoys a higher resolution due to the shorter wavelengths of light compared to RF and therefore is useful in a number of applications, which include precision target selection, automatic target recognition, agile laser designators, imaging IR seeker illuminators, 3D imaging for small unmanned airborne systems (UAS), vehicle adaptive cruise control systems, autonomous navigation systems, and speed and hazard detection for vehicle collision avoidance systems. The chip-scale scanning and coherent IR LADAR receiver concept disclosed herein has advantages of a higher Signal to Noise Ratio (SNR) due to its scanning and coherent detection capabilities. Also, because of lower atmospheric losses in the IR (and more particularly at longer IR wavelengths), the receiver SNR improves further. The ability to operate at room temperatures means that no external cooling of the components is necessary, resulting in a lower cost of manufacturing. A the chip scale size results in lower costs, a smaller size and lower power requirements compared to conventional LADAR technologies. 
     There is no prior art concerning a chip-scale IR frequency modulated LADAR receiver that the inventors are presently aware. However, there is prior art in the area of mid-IR coherent Doppler LADAR and near-IR coherent frequency modulator continuous wave (FMCW) LADAR which the presently disclosed concept significantly improves upon as explained above. None of this art 
     Frequency-modulated continuous-wave LADAR (FMCW)—also called continuous-wave frequency-modulated (CWFM) LADAR—is a short-range measuring radar set capable of determining distance. This increases reliability by providing distance measurement along with speed measurement, which is important when there is more than one source of reflection arriving at the radar&#39;s antenna. This kind of LADAR could be used as “LADAR altimeter” to measure the exact height during the landing procedure of aircraft, for example. 
     A state-of-the-art mid-IR coherent LADAR receiver, which was developed by NASA Langley Research Center, operates at a wavelength of 2 μm and is based on an optical fiber system with discrete components. See, “High Energy Double-pulsed Ho:Tm:YLF Laser Amplifier”, Jirong Yu, NASA Langley Research Center, Laser System Branch, MS 474, Hampton, Va. 23681. It uses a bulky solid-state Ho:Tm:YLF laser as the local oscillator, and dual-balanced InGaAs photodiodes for optical detection. The limitations of this LADAR receiver system are: (1) fixed wavelength operation determined by the solid-state laser material, (2) bulky, heavy, high power consuming and expensive system, (3) the use of discrete cascaded optical components results in higher overall noise and loss, (4) limitation in the wavelength of mid-IR detected signal to λ&lt;2.5 μm due to use of InGaAs photodiodes, and (4) use of high detectivity mid-IR photodetectors for wavelengths longer than 3 μm would require cooling. 
     The proposed chip-scale mid-IR coherent LADAR receiver concept disclosed herein has orders of magnitude lower size, weight and power (SWAP), has lower noise, is wavelength selectable in the long to mid-IR portions of the IR spectrum (and preferably from 2-12 μm) and can be monolithically integrated with room temperature highly sensitive Si avalanche photodiodes, as well as with CMOS electronics for post processing of the detected IR signal. 
     There is also prior art in a non-coherent, direct detection mid-IR LADAR operating at 3.4-3.5 μm wavelengths. This LADAR system is also optical fiber based and uses an interband cascade laser as the optical source and a TE-cooled HgCdTe photodetector in its receiver. See, “New Developments in HgCdTe APDs and LADAR Receivers”,  Proc. SPIE  8012, Infrared Technology and Applications XXXVII, 801230 (Jun. 20, 2011). 
     Finally, there is also prior art in a near-IR coherent FMCW LADAR operating at 13 μm wavelength. This LADAR system is also optical fiber based and uses a diode-pumped Nd:YAG laser as the optical source and balanced InGaAs photodetectors for the detection of the received LADAR signal. See, for example, “Chirped Lidar Using Simplified Homodyne Detection”, Journal of Lightwave Technology, Vol. 27, p. 3351, 2009. 
     In summary, the state-of-the-art IR LADAR transceivers are based on bulky modules that use discrete cascaded components which result in higher noise and optical loss, required cooling for improved photodetector performance at longer IR wavelengths and do not have electronic scanning capability. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one aspect the present invention provides a coherent frequency modulated receiver for receiving and detecting arriving optical signals which comprises an electrically controllable optical beam scanner receiving optical input beams arriving at different angles in a field of view of said electrically controllable optical beam scanner, said electrically controllable optical beam scanner conveying a scanned optical input beam as its an output or reflected optical beam; a grating coupler responsive to the output or reflected optical beam of said electrically controllable optical beams scanner, said grating coupler having a waveguided output; an optical local oscillator laser having a waveguided output; a FMCW signal generator; an optical modulator responsive to the optical waveguided outputs of the optical local oscillator laser and also to an electrical FMCW signal from the FMCW signal generator; a pair of second order non-linear optical elements for frequency upconverting respective outputs of the optical modulator and the grating coupler; and at least one photodiode optically coupled to an outputs of said pair of second order non-linear optical elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 a    is a schematic drawing of the novel chip-scale IR coherent LADAR receiver concept disclosed herein with electronic beam scanning capability and an on board FMCW generator and associated modulator. 
         FIG. 1 b    is a schematic drawing of the novel chip-scale IR coherent LADAR receiver concept disclosed herein with electronic beam scanning capability and an off board FMCW generator and associated modulator. 
         FIG. 1 c    depicts an alternative design of the electronically controlled beam scanner (compared to the electronically controlled beam scanner depicted in  FIGS. 1 a  and 1 b   ) where the electronically controlled beam scanner operates in a light reflective mode rather than a light transmissive mode. 
         FIG. 2  is a schematic drawing of an embodiment of the electronically controlled mid-IR optical beam scanner using a tunable Frequency Selective Surface (FSS) structure consisting of an array of Split Ring Resonators (SRRs) preferably of integrated varactor design. Also shown, is a simulated phase change of a reflective SRR-based FSS optical beam scanner as a function of the varactor capacitance. 
         FIG. 2 a    is graph of a simulated phase change imparted on a mid-IR optical beam reflected from the FSS as a function of the capacitance of the varactors. 
         FIG. 2 b    shows a stacked array of FSSs. 
         FIG. 3 a    depicts the structure of the tunable-FSS based mid-IR Mach-Zender optical modulator. Also shown is the dispersion characteristics and optical loss of the FSS structure as a function of the tuning integrated varactor. 
         FIG. 3 b    depicts how the on board FMCW modulator is connected to varactors in the split rings of the FSS in one leg of the Mach-Zehnder modulator and how the split rings of the FSS in the other leg of the Mach-Zehnder modulator are grounded. 
         FIG. 3 c    is a graph of the simulated dispersion characteristics and  FIG. 3 d    shows the simulated optical loss spectra of a tunable SRR-based FSS structure operated in the surface wave mode. 
         FIG. 3 e    depicts an optional design for the modulator in which utilizes two Si based IR optical waveguides, in which the phase of one of the waveguiding arms is modulated via a carrier-injection mechanism. 
         FIG. 3 f    depicts an optional scanner design based on an array of scanning MEMS micromirrors; 
         FIG. 4  is a schematic drawing of the sum-frequency-generation (SFG) components of the chip-scale coherent LADAR receiver. 
     
    
    
     DETAILED DESCRIPTION 
     The chip-scale IR coherent LADAR receiver concept disclosed herein has potential advantages of: (1) electronically controlled beam scanning capability which results in enhanced receiver efficiency as well as provides the angle of arrival information, (2) capability of using frequency modulated LADAR signals for optimum receiver operation, (3) operation at room temperature due to optical frequency upconversion of an incoming IR signal, and/or (4) high signal-to-noise ratio (SNR) as a result of coherent detection and use of near-IR photodetectors. The possibility of room temperature operation of the disclosed IR LADAR receiver eliminates the complexity and cost associated with cooling devices to near cryogenic temperatures (&lt;80° K) which would otherwise be required for optimum operation. Furthermore, the coherent detection capability of the disclosed device allows the detection of weak IR LADAR return signals as a result of the gain associated with the disclosed detection scheme. Finally, the compact nature of this potentially chip-scale device can result in a smaller size, weight and lower power compared with using traditional technologies. 
     Schematic diagrams of two slightly different embodiments of the chip-scale mid-IR coherent LADAR receiver  100  of the present invention with electronic beam scanning and frequency modulation signal processing capabilities are shown in  FIGS. 1 a  and 1 b   . The embodiment of  FIG. 1 a    has six main components (many of which are preferably integrated together on a common substrate (preferably as an optical integrated circuit chip)): (i) an electronically controlled IR optical beam scanner  110 , (ii) a IR optical modulator  120 , (iii) a grating coupler  130 , (iv) a local oscillator laser  140  (which is preferably locked to a local oscillator in the transmitter via an optical path  134 ), (v) an optical, frequency upconverter  150 , (vi) balanced photodiodes  160 , (vii) a network of low loss integrated optical waveguides  131 , splitters  132  and combiners  133 , and (viii) a frequency modulated signal processing unit  180 . 
     The optical chip for both embodiments can be implemented using Si-based semiconductor technologies, although III-V semiconductors technologies such GaAs or InP can also be used as the chip substrate for the receiver  100 . In the embodiment of  FIG. 1 b    the optical modulator  120  is not disposed on the integrated circuit chip of receiver  100  but rather a small amount (a sample, preferably less than 5%) of the output of an optical modulator  120 ′ associated with the LADAR transmitter  200  is utilized instead in the receiver, which is communicated from a (or the) chip of the transmitter  200  via an optical path  134 ′. In the embodiment of  FIG. 1 b    the optical modulator  120 ′ of the LADAR transmitter  200  is effectively shared with the receiver  100  although only a small amount of the output of the optical modulator  120 ′ associated with the LADAR transmitter  200  is needed by the receiver  100  in order to achieve coherent detection. So in the embodiment of  FIG. 1 b    the LO (local oscillator) signal is extracted from the modulated QCL laser 140 ′ in the transmitter  200 , using a sample (typically &lt;5%) of its output power. The modulated transmit signal is also typically amplified for even a higher output power, using, for example, a quantum cascade amplifier (QCA). The transmitter&#39;s output port is labeled  135 ′ in  FIG. 1   b.    
     As mentioned above,  FIGS. 1 a  and 1 b    show two embodiments of the chip-scale LADAR receiver  100 . Each embodiment relies on coherent detection of a received optical beam  12  which is reflected off some object (see the objects represented by boxes  1 ,  2 ,  3  on  FIG. 1 a   ) to be detected and which reflected beam was originally transmitted by the LADAR transmitter  100 —not shown in detail—but a portion of the transmitter is shown by the chip labeled  200  in  FIG. 1 b   . In coherent detection the received optical beam  13  is compared with the transmitted beam (or a replica of the transmitted beam). In the embodiment of  FIG. 1 a    the aforementioned comparison is made with a replica of the transmitted beam while in the embodiment of  FIG. 1 b    the aforementioned comparison is made with the sample of the transmitted beam (as outputted by modulator  120 ′) and therefore the embodiment of  FIG. 1 b    does not need an on-chip a IR optical modulator  120  since an off-chip modulator  120 ′, associated with the LADAR transmitter  135 , is utilized instead. 
     One of the important components of the chip-scale LADAR receiver  100  as disclosed herein is the electronically controlled beam scanner  110 . The electronically controlled beam scanner  110  redirects a received optical beam or signals  12  an as outputted beam  13  at some fixed angle relative to the grating coupler  130 . One of the received (or incoming) beams  12  (three are shown in the embodiments of  FIGS. 1 a  and 1 b    for ease of illustration—there many be more of or fewer such beams  12 ) is selected for transmission (in the case of the embodiments of  FIGS. 1 a  and 1 b   ) or reflection (in the case of the embodiment of the electronically controlled beam scanner  110  of  FIG. 1 c   ). The beams  12  are typically a reflected LADAR signal (originally transmitted by transmitter  135  to illuminate one or more objects  1 ,  2 ,  3  to be detected), which is a reflection and/or scattering off the one or more objects  1 ,  2 ,  3  be detected by a LADAR transmitted FMCW signal). The beams  12  are incident on the electronically controlled optical beam scanner  110  of the LADAR receiver  100  within the field-of-view FOV of the beam scanner  110  and then redirected to a grating coupler  130  (preferably disposed below it on chip  10 ) and preferably at a fixed angle relative thereto, as shown in  FIGS. 1 a  and 1 b   . The redirection of the received optical beams or signals  12  at a fixed angle to the grating coupler  130  optimizes its coupling efficiency to optical waveguides on the chip for further processing. Another feature of the beam scanner  110  is that it can determine the angle of arrival of the LADAR received beams or signals  12 . 
     The disclosed IR LADAR receiver  100  is described herein as being chip-scale. The reason for doing so is that it may be largely embodied using semiconductor substrates which are modified to include the electrical and optical components shown in  FIGS. 1 a    and/or  1   b  using standard manufacturing techniques to form same. The electronically controlled beam scanner  110  is depicted as being spaced from substrate  10 , but its bottom surface in the embodiments of  FIGS. 1 a  and 1 b    might be separated from the coupler  130  on substrate  10  by only a small distance, typically less than one cm and perhaps only a mm or so. So the electronically controlled beam scanner  110  is preferably supported by (and bonded to) substrate  10  with that spacing kept in mind, unless of course, the light reflective embodiment for the electronically controlled beam scanner  110  of  FIG. 1 c    is utilized instead, in which case the spacing between the substrate of he electronically controlled beam scanner  110  and substrate  10  would necessarily be greater. 
     One embodiment of the electronically controlled beam scanner  110  is a tunable frequency selective surface (FSS)  111 , as shown in  FIGS. 1 a  and 1 b   , which operates an an IR light transmissive mode. That is the IR light from the FSS is directed to a grating coupler  130  formed on substrate  10  after passing through the substrate  119  on which the FSS  111  is formed. Alternatively, the light can be reflected off the surface of the FSS  111  before being directed to the grating coupler  130 . This alternative embodiment is shown in  FIG. 1 c   . The reflective embodiment of the FSS  155  of  FIG. 1 c    differs from the light transmissive FSS embodiments of  FIGS. 1 a  and 1 b    in that (i) the FSS includes a reflective layer  118  (preferably formed by a layer of Au) and (ii) by the fact that an external mirror  14  will typically needed to be utilized in order for the incoming beams  12  to reach the IR light sensitive surface of the FSS  111 . Since there needs to be room for the light from mirror  14  to reach the light sensitive surface of the FSS  155 , in the embodiment of  FIG. 1 c    the FSS  155  will typically be more widely spaced from substrate  10  than it is in the embodiments of  FIGS. 1 a    and  1   b.    
     The FSS  111  is preferably formed by a two dimensional array of split rings  112  one of which is shown in greater detail in  FIG. 2  and in still greater detail in the above-identified U.S. patent application Ser. No. 13/754,140. The FSS  111  preferably comprises a periodic array of metallic resonant elements or cells  112  integrated with a plurality of voltage tunable impedance structures  113  for each element or cell  112  and disposed on an electrically insulating and preferably IR transmissive (at the IR frequencies utilized) layer or substrate  117  such as SiO 2 . The SiO 2  may in turn be disposed on a layer or substrate of semiconductor material  119  preferably transmissive to IR (for the light transmissive embodiment) such as Si especially when the reflective layer  118  is omitted from the electronically controlled beam scanner  110  such as in the case of the light transmissive embodiments of the electronically controlled beam scanner  110  utilized in receiver  100  embodiments of  FIGS. 1 a    and  1   b.    
       FIG. 1 c    shows an embodiment of the tunable FSS  111  beam scanner  110  operated in a light reflective mode and therefor it includes a reflective layer  118  preferably formed of Au as mentioned above. For the transmissive embodiment of the FSS  111  beam scanner  110 , the reflective layer  118  is omitted. The reflective and transmissive embodiments of the FSS  111  each include a 2D periodic array of metallic split ring resonators (SRR) unit cells  112  with a tunable capacitor  114  placed in gaps in the sides of the preferably square shaped SRRs  112 . The tunable capacitors  114  are preferably implemented by tunnel diode varactors or using an alternative tunable device as taught by U.S. patent application Ser. No. 13/754,140 referenced above. The tunable capacitors  114  are controlled by electronics preferably disposed on the Si layer of substrate associated with the FSS beam scanner  110  to scan the Field of View FOW of the LADAR system for incoming LADAR signals reflected by (or scattered by) objects  1 ,  2 ,  3  which the LADAR system (with which the disclosed LADAR receiver it utilized) is intended to detect. 
     The metallic split ring resonators (SRR) unit cells  112  preferably are formed of (i) a layer  116  of Au disposed on the aforementioned layer  117  of SiO 2 , (ii) a layer  115  of Ti formed on layer  116  and a layer  114  of hydrogen silsesquioxane (HSQ) formed on layer  115  as taught in greater detail by U.S. patent application Ser. No. 13/754,140 referenced above. 
       FIG. 2 a    shows a graph of a simulated phase change imparted on a mid-IR optical beam reflected from FSS  111  as a function of the capacitance of the varactors  112 . Multiple FSS structures  111  can be stacked in order to obtain a larger phase change (see the embodiment of  FIG. 2 b   ) where each FSS  111  in that embodiment preferably has a Si substrate (not shown) covered by an insulator  117  such as SiO 2  with for supporting a 2D array of metallic split ring resonators (SRR) unit cells  112  with a tunable capacitor  114  placed in gaps between the SRRs  112 , but in this embodiment a single metallic substrate  118  (preferably formed of Au) is provided for all of the FSS structures  111  of this stacked embodiment when used in a reflective mode (if a light transmissive mode FSS  15  is utilized then the light reflective layer is of course omitted). By employing multiple FSS structures  111  in a stacked arrangement, a larger beam  12  scanning angle can be obtained. The layers in this stacked arrangement may be bonded together at the peripheries by bonds  121 . Substrate  119  is not shown in  FIGS. 2 and 2   b  for ease of illustration 
     Only a relatively small number of metallic split ring resonators (SRR) unit cells  112  are shown for each FSS  111  in the embodiments of  FIGS. 1 a -1 c   , it being understood that numbers of metallic split ring resonators (SRR) unit cells  112  forming the 2D arrays of same of a FSS  111  may be far greater than that depicted. 
     Another approach for tuning the FSS beam scanner  110  is to integrate nano-electro-mechanical (NEMs) cantilevers in the FSS unit cell structure  112  as taught by U.S. patent application Ser. No. 13/754,140 referenced above. Moving the cantilevers in a vertical or horizontal direction will result in a change in the effective capacitance and/or inductance of the FSS unit cell structure  112  and hence phase tuning. 
     Yet another possible embodiment for the mid-IR optical beam scanner  110  is a 2-D scanning micro-mirror structure having a plurality of small mirrors whose angles are preferably individually controlled using MEMS devices such as that shown in  FIG. 3 f   . A disadvantage of the micro-mirror scanning approach is that the scanning speed is slower due to the needed mechanical motion of the micro-mirrors, and potentially another disadvantage is smaller deflection angles. 
     Turning again to the embodiments of  FIGS. 1 a  and 1 b   , a grating coupler  130 , such as one constructed from Si (for example), couples the incoming mid-IR optical signal from the FSS  111  that is to be detected to a low loss (preferably &lt;1 dB/cm) Si-based mid-IR photonic waveguide  131  with high efficiency (preferably &gt;90%). The mid-IR photonic waveguide  131  is preferably realized on a silicon-on-insulator (SOI) substrate  10  using either a Si photonic waveguide with an underlying air bridge to eliminate optical losses in the underlying SiO 2  layer for wavelengths above 3 μm, or a Ge photonic waveguide on Si whose losses are below 1 dB/cm. The use of an SOI substrate  10  is preferred as it allows the realization of these low loss mid-IR waveguiding structures. 
     An IR optical modulator  120  is another important element of the chip-scale IR frequency modulated LADAR receiver embodiment of  FIG. 1 a   . A preferred embodiment of this IR optical modulator  120  is shown in  FIG. 3 a    while the electrical connections to the FMCW Generator are shown in greater detail in  FIG. 3   b.    
     It should perhaps be recalled that two embodiments of the receiver are shown by  FIGS. 1 a  and 1 b   . In the embodiment of  FIG. 1 a    the IR an optical modulator  120  is on board the receiver chip  10  (it being understood of course that addition that an optical modulator would also be associated with the transmitter of the LADAR system) while in the embodiment of  FIG. 1 b    the IR optical modulator  120 ′ is associated with the transmitter and therefor a separate modulator in the receiver is not needed. When the optical modulator  120  is on board the receiver it needs to mimic the optical modulator  120 ′ in the transmitter for coherent detection to work properly. So in addition to having optical modulator  120  on board the chip in the embodiment of  FIG. 1 a   , a laser source  140 , and a FMCW Generator  124  are also shown preferably on board the receiver chip  10  and they would mimic similar components of the transmitter (see the laser source  140 ′ and FMCW Generator  124 ′ in block  135  on  FIG. 1 b   ). 
     It should also be understood that when consideration is given to mimicking the optical modulator  120 ′, the laser source  140 ′ and the FMCW Generator  124 ′ of the transmitter on the receiver chip  10 , one does not necessarily need to mimic all of them. For example, instead of mimicking the unmodulated laser of the transmitter by laser source  140 ′, the source could instead be a sampled portion of the laser of the transmitter (before the laser is modulated at the transmitter). Similarly, the FMCW Generator  124  on the receiver chip  10  (which mimics the FMCW Generator  124 ′ associated with the transmitter of the LADAR system) may be replaced with a signal from the FMCW Generator  124 ′ associated with the transmitter. 
     The IR optical modulator  120  shown in  FIGS. 3 a  and 3 b    includes a pair of FSS structures  122  which support surface waves which are arranged in a Mach-Zehnder configuration. The IR light is coupled from the IR waveguides  131  to the tunable FSS structures  122  of the optical modulator  120  which is preferably arranged in a Mach-Zehnder configuration. The tuning of the FSS structures  122  in the modulator  120  can be obtained preferably using the varactor approach as discussed above and thus FSS structures  122  of the modulator are preferably of the same basic design of the FSS  111  described above with reference to  FIG. 2 , although only two opposing varactors are needed when a pair of FSS  122  is used in the modulator embodiment of  FIGS. 3 a    and  3   b.    
       FIG. 3 c    is a graph of the simulated dispersion characteristics and  FIG. 3 c    shows the simulated optical loss spectra of a tunable SRR-based FSS  111  structure operated in a surface wave mode. Simulation results indicate that the tunable FSS based mid-IR optical modulator can be as small as 4 μm in the propagation direction (with 2-4 μm in the lateral direction) due to the large effective index change achievable with this structure. The small size and low optical propagation loss (&lt;2 dB) of this mid-IR optical modulator implementation is advantageous for its integration into the chip-scale mid-IR LADAR receiver disclosed herein. 
     Another embodiment (see  FIG. 3 e   ) for the mid-IR optical modulator  120  is a Mach-Zehnder configuration consisting of two Si based mid-IR optical waveguides, in which the phase of one of the waveguiding arms (the lower arm as depicted in  FIG. 3 e   ) is modulated via a carrier-injection mechanism. The p-Si and n-Si regions of this embodiment of the optical modulator  120  (which is known per se in the prior art), form a pn junction-based optical modulator which can operate in two modes: (i) a charge injection mode wherein the pn junction is forward biased in order to inject electrons and holes from the n and p sides of the junction respectively, thus changing the steady-state charge density, and hence the optical index of refraction for the propagating optical mode, and (ii) a charge depletion mode wherein the pn junction is reverse biased in order to deplete both the electrons and holes in the vicinity of the junction, again resulting in a net change in the charge density, and hence the optical index of refraction for the propagating optical mode. This embodiment of the IR optical modulator  120  will be longer (a few millimeters in length) than the FSS-based approach described above, but should be simpler to fabricate. 
     In either implementation of the mid-IR optical modulator  120 , a frequency modulated electrical signal (V m ), preferably identical to the one in a transmitter analog of this receiver disclosed herein, is applied to the modulator  120  preferably by the FMCW generator  124  in order to obtain a corresponding frequency modulated optical signal at the output of the modulator  120 . A linear FMCW signal generated by the on chip FMCW generator  124  (see  FIG. 1 a   ) is one example of the frequency modulated signal used to modulate the IR optical modulator  120 . 
     The optical local oscillator (LO)  140  for coherent detection is preferably embodied as a quantum-cascade mid-IR semiconductor laser (QCL) based on III-V semiconductors such as GaAs or InP substrates in the embodiment of  FIGS. 1 a  and 1 b   . The layer of the QCL device  140  are preferably monolithically integrated with the main SOI chip  10  that the coherent LADAR receiver is based on using conventional semiconductor wafer bonding techniques. The QCL  140  can provide local oscillator optical beams in the 3-12 μm wavelength range, compatible with the IR optical detection signal. The output of the QCL  140  is coupled to low-loss Si photonic waveguides  131 , as shown in  FIG. 1  and hence to the modulator  120  via a splitter  132 . In the embodiment of  FIG. 1 b    the master laser of the transmitter  140 ′ is sampled and a very small portion of that signal is used instead of mimicking that signal by the QCL laser  140  mentioned above. 
     The outputs of the photonic waveguides  131  carrying the IR signal beam (from grating coupler  130 ) and LO optical beams (after modulation by, for example, modulator  120 ) are coupled via a combiner  133  to two semiconducting regions  152  (which form the frequency upconverters  150  with high second order optical nonlinearity). 
     Second order nonlinearity is one of the properties of nonlinear optical material that is used for optical frequency conversion such as in second harmonic or sum-frequency generation. III-V semiconductors, such as GaAs and related compounds (e.g. AlGaAs), have a high second order nonlinearity (˜100 pm/V), which is preferably utilized by upconvertors  150 . The fabrication of such nonlinear elements for frequency conversion is explained in “Continuous-wave sum-frequency generation in AlGaAs Bragg reflection waveguides”, Optics Letters, Vol. 34, p. 3656, 2009. 
     These optical nonlinear regions  152  preferably consist of GaAs or InP semiconductor compound layers, which are bonded to the Si substrate  10  preferably as part of the overall semiconductor layers also containing the QCL device  140  (if an onboard embodiment thereof is used), described above. Related compounds of GaAs, such as AlGaAs can be used as the nonlinear element depending on the wavelength of the frequency converted signal. In the disclosed embodiment, the converted signal preferably has a wavelength of 805 nm which is absorptive in GaAs, hence AlGaAs is used instead for the upconvertors  150  due to its transparency at this wavelength. Both the signal and the LO optical beams are preferably mixed with an optical beam generated by a high power pump laser  142  (see  FIGS. 1 a  and 1 b   ) operating at higher frequencies (preferably corresponding to near-IR wavelengths and preferably at 980 nm) in the two nonlinear optical regions  152 , as shown in  FIGS. 1 a  and 1 b   . The mixing of the signal/LO beams and the higher frequency pump beam from laser  142  in the nonlinear material of regions  152  of upconvertor  150  results in sum-frequency generation (SFG) process which upconverts both the signal and LO beams to higher frequencies given by the formula below: 
                 f     S   ⁢           ⁢   F       =         f   S     +       f   P     ⁢           ⁢   or   ⁢           ⁢     λ     S   ⁢           ⁢   F           =         λ   P     ⁢     λ   S           λ   P     +     λ   S             ,         
where f SF (λ SF ) are the sum-frequency (wavelength), and f S (λ S ) and f P (λ P ) are the signal/LO and pump frequencies (wavelengths), respectively. For example, a pump laser with a wavelength of 980 nm will upconvert a 4500 nm mid-IR signal/LO beam to a near-IR wavelength of about 805 nm. The upconversion process is given by the following relationship:
 
     
       
         
           
             
               
                 
                   
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     where, P i (i=SF,P,S), λ i (i=SF,P,S), α i (i=SF,P,S), and n i (i=SF,P,S) are the optical power levels, wavelengths, optical losses, and optical indices of the sum-frequency (SF), pump (P) and signal (S) optical power levels, respectively, Δk=k SF -k P -k S  is the phase mismatch, η NL  is the second order nonlinearity, L is the length of the nonlinear waveguide and υ is the optical frequency. As an example, a received optical signal at a wavelength of 4500 nm and a power level of 350 μW at the input of a 3 mm long GaAs nonlinear region with a second-order nonlinearity of 100 pm/V will upconvert to an optical beam at a wavelength of 805 nm and a power level of about 35 μW using a pump signal with an optical power level of 500 mW at a wavelength of 980 nm. This translates into a frequency conversion efficiency of about 10%. The optical pump power is preferably provided by a high power near infrared distributed Bragg reflector (DBR) pump laser  142 . 
       FIG. 4  shows a schematic of the SFG components of one of the frequency upconvertors  150 . There are two such upconvertors  150  in each of the embodiments of  FIGS. 1 a  and 1 b    and each upconvertor  150  has an associated input waveguide  131  in which either the output of the Si grating coupler  130 , or that of the Mid-IR modulator  120  is combined (at a combiner  133 ) with the GaAs pump laser  142  to perform the optical frequency upconversion in the nonlinear waveguide element  152 . The output waveguide  135  carrying the upconverted optical signals from the two upconvertors  150  are applied to a combiner  132  then thence to the frequency modulated signal processing unit  180 . 
     The output waveguides  135  carrying the upconverted optical signals is preferably based on a III-V material, such as GaAs or InP, which is transparent to the wavelength of this signal. Again, the III-V semiconductor-based layers which form the output waveguides  135  are preferably part of an overall semiconducting layer structure bonded to the Si substrate  10 , which also supports or contains the QCL  140  (if implemented on board) and SFG elements  152  of the device. 
     The final section of the receiver  100  preferably contains a pair of balanced photodiodes  160  which mix the upconverted received mid-IR optical signal and LO beams to thereby provide a coherently converted electrical signal. A single photodiode could be used instead at the expense of signal SNR. This coherent detection of the mid-IR optical signal results is an additional gain which is proportional to the optical power of the LO signal. The photodiode pairs  160  are preferably based on Si avalanche photodiodes (APDs), which are advantageous because of their high signal-to-noise ratio (SNR), as well as being compatible with the SOI substrate  10  utilized in a the preferred embodiment of the chip-scale mid-IR coherent LADAR receiver  100 . The upconversion of both the mid-IR optical signal and the optical LO enables their mixing in Si-based APDs which can operate efficiently at room temperature. The outputs of the balanced photodiodes  160  are preferably fed into CMOS-based electronics  180  for post receiver processing of the coherently detected mid-IR signal. 
     For frequency modulated LADAR signals, post processing electronics  180  are coupled to the output of the LADAR receiver  100  and preferably includes a bandpass filter (BPF)  181 , a frequency (FMCW) demodulator  182 , amplifiers, an analog-to-digital (ADC) converter  183 , fast Fourier transformer (FFT)  184 , and data analysis processors  185 . The FMCW demodulator  182  is also connected to the FMCW generator whether or not the FMCW generator of the transmitter is mimicked on board chip  10  as FMCW generator  124  (as in the embodiment of  FIG. 1 a   ) or the FMCW generator  124 ′ of the transmitter is utilized directly (as in the embodiment of  FIG. 1 b   ). The post detection LADAR electronic processing provided by post processing electronics  180  may be conventional and therefore does not need to be further described. 
     This concludes the description of embodiments of the present invention. The foregoing description of these embodiments and the methods described have been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms or methods disclosed. Many modifications and variations are possible in light of the above teachings. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.