OPERATING THE SAME

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 61 /606,088 filed March 2, 201 12 and U.S. Provisional Application No. 61 /702,898 filed September 19, 2012.

BACKGROUND

[0002] The present disclosure relates to communications links, and components thereof, in which a modulated optical or RF signal is converted to a modulated millimeter wave signal, transmitted wirelessly, and converted back to a modulated optical or RF signal. Concepts of the present disclosure are broadly applicable to the fields of millimeter wave photonics, millimeter wave communications, integrated optical devices, fiber optics, optical communications, gigabit Ethernet and

synchronous optical networks (SONET), and a variety of wireless communication technologies. Particular applications include, for example, point-to-point wireless data transmission of digital information at millimeter wave frequencies for backhaul of telecommunications signals, wireless transport of uncompressed HDTV signals, mobile communications links, and temporary telecommunications networks.

BRIEF SUMMARY

[0003] According to the subject matter of the present disclosure, an input modulated signal is converted to a double sideband with suppressed carrier (DSB-SC) signal having millimeter wave separation in the optical domain using two independent laser sources. A high-speed photodiode converts the doubly modulated optical signal to a modulated millimeter wave signal. The millimeter wave signal can be transmitted wirelessly and received by a millimeter wave detector. Upon detection, the millimeter wave signal can be converted back into a modulated optical signal for retransmission via optical fiber. Particular aspects of the present disclosure, allow for the wireless transmission of digitally-modulated signals from a

telecommunications-grade transceiver at millimeter wave frequencies as part of a high speed gigabit Ethernet or synchronous optical network (SONET)

communications link.

[0004] In accordance with one embodiment of the present disclosure, a millimeter wave (MMW) transmitter is provided comprising an optical transceiver, a photonics processor, an optical transmission network, and an O/E converter. The optical transceiver comprises a Gbit modulator. The photonics processor comprises a first laser diode, a second laser diode, and a thermally floating heat transfer substrate. The first laser diode operates at a first temperature-dependent frequency and the second laser diode operates at a second temperature-dependent frequency f ₂ that collectively define a frequency spacing on the order of approximately 100 GHz. The respective positions of the first laser diode and the second laser diode on the thermally floating heat transfer substrate and the thermal conductivity of thermally floating heat transfer substrate are sufficient to ensure that temperature changes in one of the laser diodes are translated to the other laser diode at a thermal transfer quantity and thermal transfer rate that are sufficient to limit variations in the frequency spacing to less than 3% over an operating time of at least approximately 1 hour. The optical transmission network is configured to combine a first frequency output of the first laser diode and a second frequency output of the second laser diode to generate a MMW optical carrier signal characterized by the frequency spacing of the first frequency and the second frequency f ₂ . The photonics processor comprises electro-optic encoding hardware that is driven by encoded data input from the optical transceiver to generate a doubly modulated optical signal by superimposing encoded data from the Gbit modulator onto the MMW optical carrier signal. The optical transmission network is further configured to direct the doubly modulated optical signal to the O/E converter.

[0005] In accordance with another embodiment of the present disclosure, a method of operating a millimeter wave (MMW) transmitter comprising an optical transceiver, a photonics processor, an optical transmission network, and an O/E converter is provided.

[0006] In accordance with embodiments of the present disclosure illustrated herein with reference to Fig. 7, a millimeter wave (MMW) transmitter comprising a photonics processor, an optical transmission network, a plurality of O/E converters, and a polarization-sensitive orthomode transducer (OMT) is provided. The photonics processor comprises a first laser diode, a second laser diode, a third laser diode, and a fourth laser diode, each operating at distinct temperature-dependent frequencies f-i, f ₂ , f3, - The first frequency and the second frequency f ₂ define a frequency spacing GHzi on the order of approximately 100 GHz. The third frequency f ₃ and the fourth frequency f ₄ define a frequency spacing GHz ₂ on the order of approximately 100 GHz. The first frequency spacing GHz is different than the second frequency spacing GHz ₂ . The optical transmission network is configured to combine and maintain respective polarization states of a first frequency output of the first laser diode and a second frequency output of the second laser diode to generate and split a first MMW optical carrier signal characterized by the frequency spacing GHz of the first frequency and the second frequency f ₂ . The optical transmission network is configured to combine a third frequency output of the third laser diode and a fourth frequency output of the fourth laser diode to generate and split a second MMW optical carrier signal characterized by the frequency spacing GHz ₂ of the third frequency f ₃ and the fourth frequency f ₄ . The photonics processor comprises electro-optic encoding hardware that is driven by encoded data to generate four doubly modulated optical signals by superimposing encoded data onto the split MMW optical carrier signals.

[0007] The orthomode transducer (OMT), which is a commercially available passive MMW component capable of combining or separating two orthogonal linear polarized signals, comprises first and second optically-distinct transducer inputs for outputting first and second distinctively polarized optical signals. The optical transmission network is further configured to combine and direct two of the doubly modulated optical signals to an O/E converter and the first optically-distinct transducer input and to combine and direct a remaining two of the doubly modulated optical signals to an O/E converter and the second optically-distinct transducer input such that an output of the OMT comprises four distinct doubly modulated optical signals delineated by the first frequency spacing GHz _; the second frequency spacing GHz ₂ , and the first and second distinctive polarizations of the OMT.

[0008] In accordance with embodiments of the present disclosure illustrated herein with reference to Fig. 8, a millimeter wave (MMW) transmitter is provided wherein the photonics processor merely comprises three laser diodes, each operating at distinct temperature-dependent frequencies , , The first frequency and the second frequency f ₂ define a frequency spacing GHzi on the order of approximately 100 GHz. The third frequency f ₃ and the second frequency f ₂ define a frequency spacing GHz ₂ on the order of approximately 100 GHz. The first frequency spacing GHz is different than the second frequency spacing GHz ₂ . The first frequency and the third frequency f ₃ define a frequency spacing GHz ₃ that is beyond a maximum frequency response of the plurality of O/E converters. The photonics processor comprises electro-optic encoding hardware that is driven by encoded data to generate four modulated optical signals by superimposing encoded data onto a first pair of single wavelength signals split from the first laser diode at the first frequency fi and a second pair of single wavelength signals split from the third laser diode at the third frequency f ₃ . The optical transmission network is configured to combine a first frequency output of the first laser diode and a second frequency output of the second laser diode to generate a first MMW optical carrier signal characterized by the frequency spacing GHzi of the first frequency and the second frequency f ₂ . The optical transmission network is configured to combine a first frequency output of the first laser diode and a second frequency output of the second laser diode to generate a second MMW optical carrier signal characterized by the frequency spacing GHzi of the first frequency and the second frequency f ₂ . The optical transmission network is configured to combine a third frequency output of the third laser diode and a second frequency output of the second laser diode to generate a third MMW optical carrier signal characterized by the frequency spacing GHz ₂ of the third frequency f ₃ and the second frequency f ₂ . The optical transmission network is configured to combine a third frequency output of the third laser diode and a second frequency output of the second laser diode to generate a fourth MMW optical carrier signal characterized by the frequency spacing GHz ₂ of the third frequency f ₃ and the second frequency f ₂ . The optical transmission network is further configured to combine and direct the first and third MMW optical carrier signals to an O/E converter and a first optically-distinct transducer input of the OMT and to combine and direct the second and fourth MMW optical carrier signals to an O/E converter and a second optically-distinct transducer input of the OMT such that an output of the OMT comprises four distinct modulated optical signals delineated by the first frequency spacing GHz-,, the second frequency spacing GHz ₂ , and first and second distinctive polarizations of the OMT.

[0009] Although each of the embodiments disclosed herein are illustrated in the context of paired laser diodes that are thermally mounted on a common thermally floating heat transfer substrate, it is contemplated that the concepts of the present disclosure are not limited to such a configuration.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0010] The following detailed description of specific embodiments of the present disclosure can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which:

[0011] Fig. 1 is a schematic illustration of a MMW communications link according to one embodiment of the present disclosure;

[0012] Fig. 2 is a schematic illustration of a photonics processor suitable for use in the MMW communications link of Fig. 1 ; [0013] Fig. 3 is a schematic illustration of a MMW communications link according to another embodiment of the present disclosure;

[0014] Figs. 4 and 5 are schematic illustrations of photonics processors suitable for use in the MMW communications link of Fig. 3;

[0015] Fig. 6 is an illustration of a laser diode substrate assembly that defines thermally symmetric operating conditions for first and second laser diodes residing thereon;

[0016] Fig. 7 is a schematic illustration of a MMW transmitter and receiver according to one embodiment of the present disclosure; and

[0017] Fig. 8 is a schematic illustration of a MMW transmitter according to an alternative embodiment of the present disclosure.

DETAILED DESCRIPTION

[0018] A millimeter wave (MMW) communications link comprising a MMW transmitter 100 and a MMW receiver 200 according to one embodiment of the present disclosure is illustrated in Fig. 1 . The illustrated MMW transmitter 100 comprises an optical transceiver 10, a photonics processor 20, and an O/E converter 30 linked by an optical transmission network 40. Respective transmit and receive MMW

antennae 50 are illustrated schematically in Fig. 1 . The particular configuration of the MMW receiver 200 is beyond the scope of the present disclosure. Particular details of the MMW transmitter 100 are described in further detail below.

[0019] The optical transceiver 10 comprises a Gbit modulator 12 that is configured to encode data onto an optical signal or an electrical signal at data rates on the order of 1 Gbit or higher, although it is contemplated that the concepts of the present disclosure can be practiced at lower data rates as well. In the embodiments represented by Figs. 1 and 2, for example, it is contemplated that the optical transceiver 10 may comprise a transceiver laser source 14, and that the Gbit modulator 12 may be configured to encode data onto an optical signal generated by the transceiver laser source 14 to generate an encoded data input that can be directed to the photonics processor 20.

[0020] The photonics processor 20 comprises a first laser diode λ _; a second laser diode λ ₂ , and a thermally floating heat transfer substrate 25. The first laser diode λ _; which is preferably a continuous wave laser diode, operates at a first temperature- dependent frequency and the second laser diode λ ₂ , which is also preferably a continuous wave laser diode, operates at a second temperature-dependent frequency f ₂ . The first frequency and the second frequency f ₂ define a frequency spacing on the order of approximately 100 GHz.

[0021] The respective positions of the first laser diode λι and the second laser diode λ ₂ on the thermally floating heat transfer substrate 25 and the thermal conductivity of thermally floating heat transfer substrate 25 are sufficient to ensure that temperature changes in one of the laser diodes are translated to the other laser diode at a thermal transfer quantity and thermal transfer rate that are sufficient to limit variations in the frequency spacing. For example, and not by way of limitation, it is contemplated that variations in the frequency spacing can be limited to less than 3% over operating times well in excess of 1 hour. Readily foreseeable operating times within this variation bandwidth include, for example, operating times in excess of 35 hours. In addition, it is contemplated that frequency spacing variation will often fall below approximately 2% and, in many cases, below approximately 1 %.

[0022] Accordingly, the present disclosure contemplates methods of operating MMW transmitters where data is encoded onto the MMW optical carrier signal while the temperature of the first and second laser diodes is permitted to float in an

uncontrolled manner. This methodology is particularly advantageous because it relieves system designers and operators from restrictions that typically accompany the use of active laser diode thermal control schemes. [0023] As is illustrated in Fig. 2, the optical transmission network 40, which may be formed using optical fibers or photonic waveguide components, is configured to combine a first frequency output of the first laser diode λι and a second frequency output of the second laser diode λ ₂ , via, for example, an optical Y-combiner 21 , to generate a MMW optical carrier signal characterized by the frequency spacing of the first frequency and the second frequency f ₂ . This signal can be run through an Erbium-doped amplifier 29 prior to conversion to the electrical domain. The photonics processor 20 comprises electro-optic encoding hardware in the form of, for example, a Mach Zehnder interferometer or some other type of electro-optic modulator 22. The encoding hardware is driven by encoded data input from the optical transceiver 10 to generate a doubly modulated optical signal by

superimposing encoded data from the Gbit modulator 12 of the optical transceiver 10 onto the MMW optical carrier signal. More specifically, modulated light from the optical transceiver 10 can be converted to an RF signal by a photodiode 24 and can be amplified by an RF amplifier 26 to make it suitable for driving the electro-optic modulator 22, which may, for example, comprise a conventional Lithium Niobate modulator. Typically, the electro-optic encoding hardware will comprise a bias controller 23 that can be used to maintain an optimal encoding bias. In this manner, the photonics processor 20 can be configured to translate the encoded data input from the optical domain to the electrical domain for driving the electro-optic modulator 22.

[0024] As is illustrated in Fig. 1 , the optical transmission network 40 is further configured to direct the doubly modulated optical signal to the O/E converter 30, e.g., a photodiode, which is configured to translate the doubly modulated optical signal from the optical domain to the electrical domain for MMW transmission to the MMW transceiver 200 via a MMW amplifier 35 and the MMW antennae 50. As is noted above, the particular configuration of the MMW transceiver 200 is beyond the scope of the present disclosure and, as such, may take a variety of conventional or yet-to- be developed forms including for example, a transceiver 200 comprising MMW processing circuitry 60 and an associated microcontroller 70, an E/O converter 80, and a optical transceiver 90. More specifically, it is contemplated that the MMW processing circuitry 60 may comprise a MMW attenuator controlled by the

microcontroller 70 to maintain optimal input power to a MMW low-noise amplifier (LNA). A clock and data recovery board may also be provided to optimize data recovery. The data signal can be converted back to the optical domain using an E/O converter 80, which may comprise a laser diode and electrooptic modulator, or a transceiver. The resulting optical signal can be directed to the receiving port of the optical transceiver 90 over a single-mode optical fiber. Alternatively, a data signal in the electrical domain can be directed to a suitably equipped transceiver, as is illustrated schematically in Fig. 1 .

[0025] Suitable modulation formats include on-off keyed modulation (OOK), binary phase shift keyed modulation (BPSK), and quadrature phase shift keyed modulation (QPSK), all of which are suitable for optical signal modulation at rates of 1 -10 GB/s, or higher.

[0026] The millimeter wave (MMW) can be transmitted wirelessly using any type of antenna suitable for millimeter wave transmission and can be received using a millimeter wave detector, the respective designs of which are beyond the scope of the present disclosure and can be gleaned from conventional or yet to be developed teachings in the art. Similarly, the detected millimeter wave can be converted back into an optical signal using conventional or yet to be developed teachings in the art. By way of illustration, and not limitation, reference is made to the following US Patent for teachings related to suitable MMW antennae and detectors: US 7,486,247 B2 (Ridgway et al.).

[0027] Figs. 3-5 illustrate embodiments of the present disclosure where the photonics processor 20 is comprised within the optical transceiver 10 and is responsive to an electrical signal generated by the Gbit modulator 12. In this manner, data can be encoded onto an electrical signal within the photonics processor 20 to generate the encoded data input from the optical transceiver 10. In embodiments represented by Fig. 4, the electro-optic encoding hardware of the photonics processor 20 comprises a Mach-Zehnder interferometer or other type of electro-optic modulator 22. An RF signal is directly received as input and amplified by an RF amplifier 26. In this manner, the photonics processor 20 is configured to utilize the encoded electrical signal to drive the electro-optic modulator 22. In embodiments represented by Fig. 5, the electro-optic encoding hardware of the photonics processor 20 comprises the first and second laser diodes λ-ι , λ ₂ and the photonics processor 20 utilizes the encoded electrical signal to drive the first and second laser diodes λ-ι, λ ₂ . Although two laser diodes are illustrated in Fig. 5, it is contemplated that only one of the two lasers need be driven to modulate the millimeter wave.

[0028] Referring to Fig. 6, it is contemplated that the first laser diode λ-ι, the second laser diode λ ₂ , and the thermally floating heat transfer substrate 25 may collectively form a substantially thermally isolated substrate assembly 15 including for example, electrical interconnects and associated laser diode driving hardware. The configuration of the substrate assembly 15 and the respective positions of the first laser diode λι and the second laser diode λ ₂ relative to remaining portions of the substrate assembly define thermally symmetric operating conditions for the first and second laser diodes. In this manner, a temperature change in any selected portion of the thermally isolated substrate assembly 15, e.g., ΔΤ @ P1 , will have a corresponding temperature change in a corresponding portion of the substrate assembly, e.g., ΔΤ @ P2, and the temperature change will affect one of the laser diodes to the same extent as the corresponding temperature change will affect the other laser diode because of the thermally symmetric configuration.

[0029] More specifically referring to Fig. 6, the thermally isolated substrate assembly 15 comprises a first sub-assembly 15A dedicated to the first laser diode λι and a second sub-assembly 15B dedicated to the second laser diode λ ₂ . According to the illustrated embodiment, thermal symmetry is achieved by arranging the first and second sub-assemblies 15A, 15B illustrated in Fig. 6 on the thermally floating heat transfer substrate 25 such that one of the sub-assemblies is rotated 180 degrees relative to the other. Further, for particular laser diode configurations, it may be preferable to arrange the first and second sub-assemblies 15A, 15B so that the subassemblies 15A, 15B are inter-leaved relative to one another over the heat transfer substrate 25, as is illustrated in Fig. 6. In a further contemplated embodiment, the laser diodes λ _; λ ₂ are fabricated on the same chip and point in the same direction to provide a more convenient means to interface with fiber arrays or planar lightwave circuits. To help further ensure the aforementioned thermal symmetry, it is contemplated that the thermally floating heat transfer substrate 25 can be

constructed of materials exhibiting high thermal conductivity as well as thermal interface materials, or combinations thereof. For example, and not by way of limitation, suitable substrate compositions include metals, ceramics, polymer-based and silicate-based pastes, and solder. Additional suitable materials can be gleaned from teachings in the art, including for example the teachings of Chung, "Materials for thermal conduction," Applied Thermal Engineering, 21 , (2001 ) 1593-1605.

[0030] It is noted that the concepts of the present disclosure can be extended to millimeter wave (MMW) transmitters that allow multiple data signals to be

multiplexed onto a MMW signal using both frequency and polarization diversity. For example, referring to Fig. 7, contemplated MMW transmitters 300 are similar to those described with reference to Figs. 1 -5 in that they also comprise a photonics processor 20, a plurality of O/E converters 30, and an optical transmission network 40. The contemplated MMW transmitters 300 will also comprise a polarization- sensitive orthomode transducer (OMT), which is a commercially available passive MMW component capable of combining or separating two orthogonal linear polarized signals.

[0031] In the embodiment illustrated in Fig. 7, the photonics processor 20 comprises a first laser diode λ _; a second laser diode λ ₂ , a third laser diode λ ₃ , and a fourth laser diode λ ₄ . The first laser diode λ operates at a first temperature-dependent frequency - The second laser diode λ ₂ operates at a second temperature- dependent frequency f ₂ . The third laser diode λ ₃ operates at a third temperature- dependent frequency f ₃ . The fourth laser diode λ ₄ operates at a fourth temperature- -in

dependent frequency f ₄ . The first frequency and the second frequency f ₂ define a frequency spacing GHz-i, e.g., 92 GHz, that is on the order of approximately 100 GHz. The third frequency f ₃ and the fourth frequency f ₄ define a different frequency spacing GHz ₂ , e.g., 102 GHz, that is also on the order of approximately 100 GHz.

[0032] In the embodiment of Fig. 7, as is the case with the other embodiments disclosed herein, the first laser diode λι and the second laser diode λ ₂ are thermally mounted on a common thermally floating heat transfer substrate 25A. Similarly, the third laser diode λ ₃ and the fourth laser diode λ ₄ are thermally mounted on a common thermally floating heat transfer substrate 25B. It is contemplated that the two substrates 25A, 25B may comprise a single substrate or two independent substrates. As is described with reference to the embodiments of Figs. 1 -5, the respective positions of the laser diodes on the common thermally floating heat transfer substrates and the thermal conductivity of the substrates are preferably sufficient to ensure that temperature changes in one of the laser diodes are translated to the paired laser diode at a thermal transfer quantity and thermal transfer rate that are sufficient to limit variations in the frequency spacing GHz-,.

[0033] In Fig. 7, the optical transmission network 40 is configured to combine a first frequency output of the first laser diode λι and a second frequency output of the second laser diode λ ₂ to generate and split a first MMW optical carrier signal characterized by the frequency spacing GHz-, . The optical transmission network 40 is also configured to combine a third frequency output of the third laser diode λ ₃ and a fourth frequency output of the fourth laser diode λ ₄ to generate and split a second MMW optical carrier signal characterized by the frequency spacing GHz ₂ .

[0034] The photonics processor 20 comprises electro-optic encoding hardware including electro-optic modulators 22 that are driven by encoded data inputs S-i, S ₂ , S ₃ , S ₄ to generate four doubly modulated optical signals GHZ-I (S-I), GHz ₂ (S ₂ ),

GHzi (S ₃ ), GHz ₂ (S ₄ ) by superimposing the encoded data onto the split MMW optical carrier signals. The OMT comprises first and second optically-distinct transducer inputs for outputting first and second distinctively polarized optical signals, e.g., a vertically polarized output and a horizontally polarized output. The optical transmission network 40 is further configured to combine and direct two of the doubly modulated optical signals GHZ-I(S-I), GHz ₂ (S ₂ ) to an O/E converter and the first optically-distinct transducer input of the OMT and to combine and direct a remaining two of the doubly modulated optical signals GHzi (S ₃ ), GHz ₂ (S ₄ ) to an O/E converter and the second optically-distinct transducer input of the OMT such that an output of the OMT comprises four distinct doubly modulated optical signals delineated by the first frequency spacing GHz-,, the second frequency spacing GHz ₂ , and the first and second distinctive polarizations of the OMT.

[0035] More specifically, referring to Fig. 7, four continuous wave (CW) laser diodes can be used to provide the optical signals used to create the millimeter wave carriers GHz-, , GHz ₂ . As an example, if the wavelength of λ is equal to 1549 nm and the wavelength of λ ₂ is equal to 1549.736 nm, the frequency difference between the two laser lines would be 92 GHz. These two laser signals can be joined together using a single-mode polarization maintaining 3dB coupler 27 and sent to two different electro-optic modulators 22 in the form of, e.g., Mach-Zehnder Interferometer modulators. Further, if the wavelength of λ ₃ is 1552.000 nm and the wavelength of λ ₄ is 1552.819 nm, then the frequency difference between the two lines would be 102 GHz. Further, it is noteworthy that the wavelengths of λ and λ ₂ are more than 250 GHz away from the corresponding wavelengths of λ ₃ and λ ₄ . These two laser signals can also be joined together using a single-mode 3dB coupler 27 and sent to two different electro-optic modulators 22.

[0036] Four high speed data signals (S-i, S ₂ , S ₃ , and S ₄ ), each at for example, 2.5 Gb/s, are provided to the transmitter. The first digital signal Si can be used to drive an electro-optic modulator 22 to modulate the millimeter wave carrier GHz in an on- off keying modulation format. Similarly the second digital signal S ₂ can be used to drive an electro-optic modulator 22 to modulate the millimeter wave carrier GHz ₂ in an on-off keying modulation format. The resulting modulated optical signals

GHZ-I (S-I), GHz ₂ (S ₂ ) can be combined using an optical Y-combiner and transmitted over the waveguide network to, for example, an Erbium-doped fiber amplifier 29 for amplification to, for example, approximately 20 mW of optical power. A high speed photodetector O/E, such as a unitraveling-carrier photodiode, converts the combined optical signals GHZ-I(S-I), GHz ₂ (S ₂ ) to two millimeter wave signals: a 92 GHz signal that is on-off-key modulated with Si and a 1 02 GHz signal that is on-off-key modulated with S ₂ .

[0037] Similarly, the third digital signal S ₃ can be used to drive an electro-optic modulator 22 to modulate the millimeter wave carrier GHz in an on-off keying modulation format. The fourth digital signal S ₄ can be used to drive an electro-optic modulator 22 to modulate the millimeter wave carrier GHz ₂ in an on-off keying modulation format. The resulting modulated optical signals GHzi (S ₃ ), GHz ₂ (S ₄ ) can be combined using an optical Y-combiner and transmitted over the waveguide network to, for example, an Erbium-doped fiber amplifier 29 for amplification to, for example, approximately 20 mW of optical power. A high speed photodetector O/E, such as a unitraveling-carrier photodiode, converts the combined optical signals GHz-i (S ₃ ), GHz ₂ (S ₄ ) to two millimeter wave signals: a 92 GHz signal that is on-off-key modulated with S ₃ and a 1 02 GHz signal that is on-off-key modulated with S ₄ .

[0038] As is noted above, an orthomode transducer (OMT) is a passive millimeter wave component capable of combining or separating two orthogonal linear polarized signals. The output waveguide of the OMT used in the transmitter is a circular waveguide capable of supporting both linear polarizations. Amplified MMW signals GHZ-I (S-I), GHz ₂ (S ₂ ) can be connected to the vertically polarized port of the OMT and amplified MMW signals GHzi (S ₃ ), GHz ₂ (S ₄ ) can be connected to the horizontally polarized port of the OMT. The circular waveguide port of the OMT is connected to the feed of a MMW antenna 50. The four millimeter wave signals emerge from the antenna with GHZ-I(S-I) and GHz ₂ (S ₂ ) vertically polarized and GHz-i (S ₃ ), GHz ₂ (S ₄ ) horizontally polarized.

[0039] The four linearly polarized millimeter wave signals can be captured at a receive antenna 50 of a MMW receiver 400 and fed into a circular waveguide of a receiver-based OMT that separates the two orthogonal polarizations. Signals

GHZ-I (S-I) and GHz ₂ (S ₂ ) emerge from the vertical polarization port of the OMT and signals GHzi(S ₃ ), GHz ₂ (S ₄ ) emerge from the horizontal port of the OMT. These signals can be amplified using low-noise amplifiers 410 and can be fed into directional couplers 420 that divide the millimeter wave power into two equal signals, one of which is fed through a band pass filter Δίι centered, for example, at 92 GHz to remove Δί ₂ and the second of which is fed through a band pass filter Δί ₂ centered, for example, at 102 GHz to remove Δί-ι. Schottky diode millimeter wave detectors D1 , D2, D3, D4 are fast enough to demodulate the on-off-key modulation of signals to produce output data signals. Clock and data recovery circuitry can be used to recover the clock from the detected signals and recondition the data.

[0040] Fig. 8 illustrates an alternate embodiment of a MMW transmitter 500 according to the present disclosure where an unmodulated optical reference is mixed with a modulated optical signal. In the embodiment, four electro-optic modulators 22 and three laser diodes λ-ι , λ ₂ , λ ₃ are used to create four millimeter wave signals GHz^S^, GHz ₂ (S ₂ ), GHz^Ss), GHz ₂ (S ₄ ). The four electro-optic modulators 22 modulate only single wavelengths - one from each of the first and second laser diodes λ-ι , λ ₂ . In this example, λι and λ ₃ are 194 GHz apart and beyond the frequency response of the high speed photodiodes O/E. A third unmodulated wavelength λ ₂ is added to each of the combined signals. In this example, the wavelength λ ₂ is chosen to be 92 GHz from λ and 102 GHz from λ ₃ . Both of these difference frequencies are chosen to be within the frequency response of the high speed photodiodes O/E. Since λι and λ ₃ are on-off-key modulated, the high speed photodiode O/E will create the two on-off-key modulated millimeter wave signals at 92 GHz and 102 GHz. As with the original configuration, signals can be oriented to induce horizontally and vertically polarized MMW signals.

[0041] In addition to the embodiments of Figs. 7 and 8, it is contemplated that the sideband generation approach detailed by Ridgway et al. in US Pub. Nos. 201 1 /0122477, 2010/0263001 , 2009/0016729, and 2008/0199124 can also be used to generate suitable MMW carrier frequencies in the optical domain.

[0042] It is noted that recitations herein of a component of the present disclosure being "configured" in a particular way, to embody a particular property, or function in a particular manner, are structural recitations, as opposed to recitations of intended use. More specifically, the references herein to the manner in which a component is "configured" denotes an existing physical condition of the component and, as such, is to be taken as a definite recitation of the structural characteristics of the

component.

[0043] It is noted that terms like "preferably," "commonly," and "typically," when utilized herein, are not utilized to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to identify particular aspects of an embodiment of the present disclosure or to emphasize alternative or additional features that may or may not be utilized in a particular embodiment of the present disclosure.

[0044] For the purposes of describing and defining the present invention it is noted that the terms "substantially" and "approximately" are utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The terms "substantially" and

"approximately" are also utilized herein to represent the degree by which a

quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.

[0045] Having described the subject matter of the present disclosure in detail and by reference to specific embodiments thereof, it is noted that the various details disclosed herein should not be taken to imply that these details relate to elements that are essential components of the various embodiments described herein, even in cases where a particular element is illustrated in each of the drawings that accompany the present description. Rather, the claims appended hereto should be taken as the sole representation of the breadth of the present disclosure and the corresponding scope of the various inventions described herein. Further, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. More specifically, although some aspects of the present disclosure are identified herein as preferred or particularly advantageous, it is contemplated that the present disclosure is not necessarily limited to these aspects.

[0046] It is noted that one or more of the following claims utilize the term "wherein" as a transitional phrase. For the purposes of defining the present invention, it is noted that this term is introduced in the claims as an open-ended transitional phrase that is used to introduce a recitation of a series of characteristics of the structure and should be interpreted in like manner as the more commonly used open-ended preamble term "comprising."