Patent Description:
<CIT>, in accordance with a translation of its abstract, states a laser light source control part for controlling a periodic light filter periodically having a transparent peak and a laser light source is installed subsequent to a light frequency shifter. Each transparent peak of the periodic light filter is configured to have rapid band pass characteristics. For the purpose that the light frequency of an optical angle modulation signal is about <NUM>, while the components of desired light frequency ν<NUM> are filtered, the components of light frequency ν<NUM> as a primary disturbance light and the components of a light frequency 2ν<NUM> - ν<NUM> can be suppressed. In using a light filter having rapid band pass characteristics, it is necessary to continuously have the central frequency of a transparent peak same as input light frequency, and the output light frequency of the laser light source is adjusted by a laser light source control part.

<CIT>, in accordance with its abstract, states a transmitter is proposed that provides broadband all-optical linearization of a Mach-Zehnder interferometer (MZI) modulator for use in high linearity RF photonic links and optical up-converter and down-converter schemes. It is based on an amplitude modulated (AM) MZI modulator where part of the laser Carrier is passed around the MZI modulator and added back to the AM signal, creating a Controlled Carrier-AM (CC-AM) signal. In this scheme, a dual output MZI modulator is utilized, and the alternative output (Carrier*) is used together with the Carrier from the laser to create a new signal, LO*, which when coherently combined with the AM signal can reduce or completely cancel its 3rd order intermodulation distortion.

<CIT>, in accordance with its abstract, states a phase-modulated analog optical link that uses parallel interferometric demodulation to mitigate the dominant intermodulation distortion present in the link. A receiver for demodulating phase modulated optical signals includes a splitter dividing the phase modulated signal into parallel optical paths, each optical path having an asymmetrical interferometer, the time delays of the interferometers being unequal, and each optical path includes a photodiode optically connected to an output of the interferometer. Outputs of the photodiodes enter a hybrid coupler. Alternatively, outputs of the interferometer enter a balanced photodetector. A phase shifter or time delay element can be included in one optical path to ensure inputs to the coupler or balanced photodetector have the correct phase. The input power to the parallel optical paths is split in a ratio that balances the third-order distortion in the output photocurrent.

In the paper <NPL>, there is described a total third-order intermodulation distortion elimination method for analog photonics link based on integrated dual-parallel Mach-Zehnder modulator.

There is discussed herein a method of using a photonic link, the method comprising: transmitting light using an optical emitter; splitting, using an input coupler, the light into a first path and a second path, the first path being provided to a modulator configured to modulate the light transmitted along the first path based on a radio frequency signal, and the second path being provided to a phase shifter; combining, using an output coupler, an output of the modulator and an output of the phase shifter; monitoring, using a controller, an output of a photodetector coupled to an output of the output coupler; identifying, using the controller, based on the output of the photodetector, a modulator phase angle that reduces a third order distortion at an output of the output coupler; applying, using the controller, a first bias voltage to the modulator to maintain the identified modulator phase angle; applying, using the controller, based on the output of the photodetector, a control signal to the phase shifter to maintain a phase difference between the output of the modulator and the output of the phase shifter; and generating, using the controller, a radio frequency output of the photonic link.

In some examples, the methods further include coupling, using an output coupler, an output of the modulator and an output of the phase shifter. In various examples, the methods further include extracting a low frequency component and a fundamental frequency component of an output of a photodetector coupled to the output coupler. According to some examples, the methods further include determining the control signal based, at least in part, on the low frequency component and the fundamental frequency component. In some examples, the methods further include determining the control signal based, at least in part, on the low frequency component and the fundamental frequency component as well as the output of the modulator. In various examples, the methods further include receiving, at the modulator, the radio frequency (RF) signal from an antenna. According to some examples, the antenna is mounted, at least in part, on an exterior surface of the aircraft. In some examples, the methods further include determining the first bias voltage by achieving a minimum value of a fundamental frequency of a test waveform. For example, the first bias voltage may be identified as a particular bias voltage at which an output of the photodetector at the fundamental frequency of the test waveform is reduced, or even minimized. In various examples, the methods further include determining a second bias voltage to implement a phase offset from the first bias voltage, the phase offset suppressing a third order distortion. In some examples, suppress means to substantially reduce, or to prevent or substantially prevent.

Also discussed herein is a system configured to implement a photonic link, the system comprising: an optical emitter; an input coupler configured to split an output of the optical emitter into a first path and a second path; a modulator configured to modulate light transmitted along the first path based on a radio frequency signal; a phase shifter configured to modify a phase of light transmitted along the second path based on a bias voltage; an output coupler coupled to the output of the modulator and the output of the phase shifter; a photodetector coupled to an output of the output coupler; and a controller configured to: identify, based on an output of the photodetector, a modulator phase angle that reduces a third order distortion of the photonic link; apply a first bias voltage to the modulator to maintain the identified modulator phase angle; apply, based on an output of the photodetector, a control signal to the phase shifter to maintain a phase difference between an output of the modulator and an output of the phase shifter; and generate a radio frequency output of the photonic link.

In various examples, the controller is configured to extract a low frequency component and a fundamental frequency component of an output of the photodetector. According to some examples, the controller is configured to determine the first bias voltage and the control signal based, at least in part, on the low frequency component and the fundamental frequency component. In some examples, the first bias voltage is applied to the modulator, and wherein the control signal is applied to the phase shifter. In various examples, the modulator is coupled to an antenna, and wherein the modulator and the antenna are implemented in a vehicle. According to some examples, the vehicle is an aircraft, and wherein the antenna is mounted, at least in part, on an exterior surface of the aircraft.

Further disclosed herein are devices that include a modulator configured to receive a first optical signal from an optical emitter via a first path, the modulator being configured to modulate light transmitted along the first path based on an RF signal, and a phase shifter configured to receive a second optical signal from the optical emitter via a second path, the phase shifter being configured to modify a phase of light transmitted along the second path based on a bias voltage. The devices further include a controller configured to identify a modulator phase angle that reduces a third order distortion of the photonic link, apply a first bias voltage to the modulator to maintain the identified phase angle, and apply a control signal to the phase shifter to maintain a phase difference between an output of the modulator and an output of the phase shifter.

In some examples, the modulator is coupled to an antenna, and the modulator and the antenna are implemented in an aircraft. In various examples, the controller is configured to extract a low frequency component and a fundamental frequency component of an output of the photodetector. According to some examples, the controller is configured to determine the first bias voltage and the control signal based, at least in part, on the low frequency component and the fundamental frequency component.

In the following description, numerous specific details are set forth in order to provide a thorough understanding of the presented concepts. The presented concepts may be practiced without some or all of these specific details. In other instances, well known process operations have not been described in detail so as to not unnecessarily obscure the described concepts. While some concepts will be described in conjunction with the specific examples, it will be understood that these examples are not intended to be limiting.

Communications links and components within such links may have performance characteristics that affect operation of the communications links. For example, such communications links may include photonic links that experience distortion, such as intermodulation distortion, which may be caused by characteristics of the photonic links, such as harmonic resonances. Such photonic links may also experience signal noise. As will be discussed in greater detail below, such performance characteristics may, at least in part, determine an acceptable operational amplitude or intensity range of the photonic link, also referred to herein as its dynamic range. Conventional photonic links are limited in their ability to enhance their dynamic range because they are not able to suppress intermodulation distortion and reduce a noise figure concurrently. In various examples, a noise figure is the degradation in radio frequency (RF)-input to RF-output signal-to-noise ratio due to the photonic link. The noise figure may be represented in a dB scale, and a noise factor may be represented as a numerical value. In one example, the noise figure may be a ratio between a direct current (DC) photocurrent and the square of the photocurrent at a signal frequency or a test waveform frequency, which, as will be discussed in greater detail below, are two components of an output of a photodetector.

Various examples disclosed herein provide photonic links that are configured to suppress intermodulation distortion and also reduce a noise figure to achieve an enhanced dynamic range. In various examples, a photonic link includes a modulator and a phase shifter as well as a controller configured to determine and generate bias and control voltages for both the modulator and phase shifter. Accordingly, an optical emitter, such as a laser, may provide an optical signal to both the modulator and the phase shifter. The controller may monitor signals produced by the modulator and phase shifter and may determine bias and control voltages to be applied to modulator and phase shifter. As will be discussed in greater detail below, the controller may utilize the applied voltages to maintain a relative phase angle between the two paths associated with the modulator and the phase shifter, while at the same time maintaining a particular phase angle of the modulator. In this way, the photonic link may simultaneously suppress distortion characteristics such as third order intermodulation distortion, and may also reduce the noise figure of the photonic link.

As will also be discussed in greater detail below, when the optical power incident on a photodetector is relatively high, as would be the case with positive photonic link gain, a noise figure of the photonic link may be dependent on the average photocurrent, since some contributions to the noise in the photonic link output are an optical emitter's relative intensity noise (RIN) and the shot noise of the light coupled to the photodetector. More specifically, a noise figure of a photonic link may be proportional to the average photocurrent, and a net RIN may be proportional to a square of the average photocurrent. Accordingly, photonic links disclosed herein are configured to enable the suppression of such noise (which may be from a large photocurrent) and also, at the same time, suppress the intermodulation distortion at various different operating points and operational conditions, including those where a signal gain is relatively large.

<FIG> illustrates an example of a photonic link, configured in accordance with some examples. In various examples, a system for implementing a photonic link, such as system <NUM>, includes optical emitter <NUM> which is configured to emit light. For example, optical emitter <NUM> is a laser that is configured to emit a continuous laser beam with a controlled output, such as optical waveform or pulse duration and intensity. Accordingly, optical emitter <NUM> is configured to generate and provide an optical signal used by other downstream components of system <NUM>. In various examples, optical emitter <NUM> is coupled with other downstream components via optical transmission mediums such as optical fibers.

As shown in <FIG>, optical emitter <NUM> is coupled to input coupler <NUM>. In various examples, input coupler <NUM> is configured to split the output provided by optical emitter <NUM> into a first path and a second path. As will be discussed in greater detail below, the first path is provided to a first component, such as modulator <NUM>, and the second path is provided to a second component, such as phase shifter <NUM>. In various examples, input coupler <NUM> is a variable coupler that is configured to have a variable coupling ratio. Accordingly, input coupler <NUM> is configured to adjust an optical power, or a strength or amplitude, of signals sent along the first path and the second path. In various examples, the operation of input coupler <NUM> and its variable coupling ratio are controlled by controller <NUM>, also discussed in greater detail below.

In some examples, modulator <NUM> is configured to modulate a signal received from input coupler <NUM>. For example, modulator <NUM> receives an optical signal from input coupler <NUM> via the first path, and modulates the optical signal based on a received input. In one example, the modulation may be phase modulation. In some examples, modulator <NUM> is configured to receive an input from a source such as antenna <NUM>. Accordingly, modulator <NUM> receives a radio frequency (RF) signal from antenna <NUM>, and modulates the optical signal based on the received RF signal. In this way, the optical signal is modulated to encode information included in the RF signal, and the modulated optical signal is provided as an output of modulator <NUM>. Moreover, in various examples, modulator <NUM> is configured to receive a signal from a controller, such as controller <NUM> that configures modulator <NUM> to maintain a designated bias angle. As will be discussed in greater detail below, the bias angle of modulator <NUM> may be identified and specified to reduce distortion that may otherwise occur in system <NUM>. For example, a bias angle may be maintained such that a third order distortion is minimized.

According to various examples, phase shifter <NUM> is configured to receive an optical signal from input coupler <NUM> via the second path. Moreover, phase shifter <NUM> is configured to implement a shift in phase of the received signal based on a received input. As will be discussed in greater detail below, the input may be signal generated by controller <NUM>. Accordingly, phase shifter <NUM> introduces a phase shift to the received signal and generates an output that includes the phase shift. As will also be discussed in greater detail below, phase shifter <NUM> is configured to maintain a relative phase angle between the first path and the second path. In various examples, a relative optical phase may produce destructive interference between the optical fields for the optical-carrier frequencies from the two paths, and the average optical power at an input of photodetector <NUM>, discussed in greater detail below, can be reduced, and thus the noise figure of the photonic link can be reduced. Accordingly, the input to phase shifter <NUM> received from controller <NUM> may be used to adjust phase shifter <NUM> to maintain a designated phase angle between the first path and the second path, and reduce a noise figure of system <NUM>.

System <NUM> further includes output coupler <NUM> which is configured to couple an output of modulator <NUM> and phase shifter <NUM> to a downstream component, such as photodetector <NUM> discussed in greater detail below. Thus, according to some examples, output coupler <NUM> is an optical coupler that is configured to provide the signals generated by modulator <NUM> and phase shifter <NUM> to photodetector <NUM> via a first output port. In some examples, output coupler <NUM> has multiple output ports that may include a second output port, and the second output port may be left uncoupled or hanging.

System <NUM> also includes photodetector <NUM> that is coupled to an output port of output coupler <NUM>, and is configured to detect optical intensity of the signal received from output coupler <NUM>. Accordingly, photodetector <NUM> is configured to detect the square of the optical field, and is configured to mix the optical signal received via the first path with the optical signal received via the second path. In this way, photodetector <NUM> is configured to mix modulated light, as well as its various modulation sidebands, received via the first path with the phase shifted light received via the second path.

System <NUM> additionally includes controller <NUM> which is configured to generate one or more signals that are provided to modulator <NUM> and phase shifter <NUM>, respectively. As will be discussed in greater detail below, controller <NUM> is configured to generate a bias voltage that is used to set a bias angle of modulator <NUM>. Controller <NUM> is further configured to generate a first control signal used to control and maintain a phase angle of modulator <NUM>, and a second control signal used to control the operation of phase shifter <NUM> to maintain a phase difference between the first path and the second path. Additional details of the determination and generation of the bias voltage and control signals is discussed in greater detail below with reference to <FIG>. In various examples, controller <NUM> is further configured to generate an output which may be an RF output. Accordingly, the output of the controller may be an output of system <NUM>, and an output of the photonic link.

<FIG> illustrates another example of a photonic link, configured in accordance with some examples. As similarly discussed above, system <NUM> includes optical emitter <NUM>, output coupler <NUM>, photodetector <NUM>, and controller <NUM>. In various examples, system <NUM> further includes optical splitter <NUM>, modulator <NUM>, and phase shifter <NUM>. In various examples, system <NUM> is configured such that optical splitter <NUM> is configured to split an optical signal generated by optical emitter <NUM>, and provide a first optical signal to modulator <NUM> via a first path, and provide a second optical signal to phase shifter <NUM> via a second path. Moreover, in system <NUM> modulator <NUM> is configured to include an attenuator configured to implement a designated attenuation to the optical signal received via the first path. In this way, system <NUM> may be implemented using a simple optical splitter, such as optical splitter <NUM> instead of the variable coupler discussed above.

<FIG> illustrates an additional example of a photonic link, configured in accordance with some examples. As similarly discussed above, system <NUM> includes optical emitter <NUM>, input coupler <NUM>, modulator <NUM>, phase shifter <NUM>, output coupler <NUM>, photodetector <NUM>, and controller <NUM>. As shown in <FIG>, controller <NUM> may include various components configured to implement the determination and generation of the bias voltages and control signals discussed above. In various examples, controller <NUM> includes first bias tee <NUM>, second bias tee <NUM>, RF coupler <NUM>, and processor <NUM>.

In some examples, first bias tee <NUM> is configured to receive an output of photodetector <NUM>, and is further configured as a diplexer that generates a low frequency output and a high frequency output. As shown in <FIG>, the low frequency output is provided to processor <NUM>, and the high frequency output is provided to second bias tee <NUM>. In various examples, second bias tee <NUM> further generates two outputs based on the high frequency output received from first bias tee <NUM>. More specifically, second bias tee <NUM> provides an output representing a fundamental frequency to processor <NUM>, and provides an additional output including a higher frequency component to RF coupler <NUM>. In various examples, RF coupler <NUM> is configured to provide an RF output at an output port, and also provide a third harmonic component to processor <NUM>. In this way, controller <NUM> is configured to extract specific frequency components of the output of photodetector <NUM>, and determine and generate bias voltages and control signals based on such extracted frequency components.

<FIG> illustrates a flow chart of a method of using a photonic link, implemented in accordance with some examples. As discussed above, the operation of modulators and phase shifters included in photonic links may be controlled by a controller to simultaneously suppress distortion characteristics of the photonic link as well as noise characteristics of the photonic link. Accordingly, a method, such as method <NUM>, may be used to configure a modulator and phase shifter to suppress such distortion and noise characteristics, as well as generate control signals that maintain suppression of such distortion and noise characteristics during operation.

Method <NUM> may commence with operation <NUM> during which a phase angle may be identified for a modulator in a photonic link. Accordingly, a modulator phase angle may be identified that minimizes or reduces a third order distortion of the photonic link. As will be discussed in greater detail below with reference to <FIG> and <FIG>, the phase angle may be identified based on the utilization of a test waveform and a varying DC component to identify a phase angle at which a distortion characteristic, such as a third order distortion, is reduced, and in some examples, minimized.

Method <NUM> may proceed to operation <NUM> during which the modulator may be configured based on the identified phase angle. A system component, such as a controller, may generate a bias voltage and apply the bias voltage to the modulator to set the modulator phase angle to the identified phase angle. As will be discussed in greater detail below with reference to <FIG> and <FIG>, the setting of such a phase angle may reduce or minimize a third order distortion of the photonic link.

Method <NUM> may proceed to operation <NUM> during which a control signal may be generated. In various examples, the control signal is generated and provided to the phase shifter to maintain a phase difference between the modulator and the phase shifter. Accordingly, the control signal is configured to maintain the phase difference during operation of the photonic link, and thus account for any changes or drift in the phase of the modulator. In this way, noise characteristics of the photonic link may also be suppressed, and both distortion and noise characteristics may be suppressed simultaneously.

In various examples, an accumulated optical phase in each path can fluctuate with changes in the thermal and mechanical environment. For example, a particular path may have <NUM> meter of a silica optical fiber. The temperature dependence of such fibers results in a change in the refractive index of the fiber and the accumulated phase shift is proportional to the length of the path and also dependent on the wavelength of the light. For example, for a path length of <NUM> meter, the accumulated shift would change by <NUM>. 13π (or a change in the relative phase angle φp of <NUM>°) for each <NUM> change in temperature. Examples disclosed herein adjust the phase shifter to compensate for these drifts and thereby stabilize the value of φp.

<FIG> illustrates a flow chart of another method of using a photonic link, implemented in accordance with some examples. As will be discussed in greater detail below, a method, such as method <NUM>, may be implemented to simultaneously suppress distortion characteristics of a photonic link as well as reduce noise characteristics of the photonic link. More specifically, a bias voltage may be used to suppress distortion characteristics, such as second or third order distortion, and a control signal may be used to reduce, and in some cases even minimize, noise characteristics such as a noise figure.

Accordingly, method <NUM> may commence with operation <NUM> during which a first test waveform may be applied to an input port of a modulator. In various examples, the first test waveform has a fundamental frequency and is applied to a bias port of the modulator. The first test waveform may have a DC component and a time-varying component. For example, the first test waveform may be a single tone sinusoidal waveform.

Method <NUM> may proceed to operation <NUM> during which the DC component of the first test waveform may be varied, and an output of a photodetector coupled to the modulator may be monitored. Accordingly, effects of modulation of the DC component may be monitored via the output of the photodetector, and more specifically, an output at the fundamental frequency may be monitored.

Method <NUM> may proceed to operation <NUM> during which a first bias voltage may be identified. In various examples, the first bias voltage is a bias voltage at which a minimum value occurs at the fundamental frequency. Accordingly, the variations in the output of the photodetector monitored at operation <NUM> may identify a particular bias voltage at which the output of the photodetector at the fundamental frequency is reduced, and even minimized. In some examples, this first bias voltage may be a zero-bias set point. In one example, the first bias voltage may set a modulator bias angle to <NUM> degrees.

Method <NUM> may proceed to operation <NUM> during which a control signal may be provided to a phase shifter to achieve a designated time-average photocurrent value which may be determined based on one or more properties of the photonic link. In various examples, the designated time-average photocurrent value may be determined based on a low-frequency output of a bias tee, such as first bias tee <NUM> discussed above. In various examples, the photocurrent value may be measured at an output of the photodetector.

Moreover, the designated time-average photocurrent value may be a target value that corresponds to a relative phase angle. For example, an initial calibration procedure may have been previously implemented in which average photocurrent values corresponding to relative phase angles may have been determined and stored. Accordingly, a designated time-average photocurrent value may have been previously identified for a relative phase angle of <NUM> degrees, and during operation <NUM>, the bias voltage applied to the phase shifter may be adjusted until the output of the photodetector achieves the designated time-average photocurrent value. In various examples, an average photocurrent value depends strongly on the value of the relative phase angle between paths. Moreover, that average photocurrent value has only a weak dependence on the value of the modulator bias angle if the bias angle is close to zero. In some examples, a desired value for the average photocurrent depends on the relative levels of laser power coupled, for example, from emitter <NUM> via coupler <NUM> into the first path containing modulator <NUM> versus into the second path containing phase shifter <NUM>.

Method <NUM> may proceed to operation <NUM> during which a second bias voltage may be identified. In various examples, the second bias voltage is a voltage that implements a phase offset relative to the first bias voltage. Furthermore, the phase offset may be selected such that the implementation of the phase offset suppresses a distortion characteristic of the photonic link. For example, the implementation of the phase offset may suppress a third order distortion of the photonic link.

Method <NUM> may proceed to operation <NUM> during which a control signal is provided to the phase shifter. Accordingly, the control signal may be adjusted to maintain a phase difference between the modulator output and the phase shifter output even after the application of the second bias voltage. In this way, the control signal provided to the phase shifter reduces a noise characteristic of the photonic link. For example, the adjusted control signal may minimize a noise figure of the photonic link. An estimate of the noise figure may be determined from the measured values of the DC component of the photodetector output and the fundamental-frequency component of the photodetector output. In some examples, the fundamental-frequency component is obtained at an output of a bias tee, such as second bias tee <NUM>. In some examples, the phase difference is set at <NUM> degrees.

As noted above, a similar methodology may be implemented for suppressing 2nd order distortion and 2nd harmonic spurs. More specifically, a system component, such as a controller, may extract a 2nd harmonic frequency of a test tone, and may then adjust the modulator bias voltage to reduce or minimize that 2nd harmonic component. The control signals for the phase shifter as well as the other control signals may be then be generated, as discussed above, but with regards to the bias voltage determined for the <NUM>nd harmonic component.

<FIG> illustrates a flow chart of yet another method of using a photonic link, implemented in accordance with some examples. For example, another method, such as method <NUM>, may be implemented to simultaneously suppress distortion characteristics of a photonic link as well as reduce noise characteristics of the photonic link. More specifically, control signals may be generated based on extracted frequency components, and may be used to suppress distortion characteristics, such as second or third order distortion, as well as reduce, and even minimize noise characteristics such as a noise figure.

Accordingly, method <NUM> may commence with operation <NUM> during which a test waveform may be generated and applied to an input port of a modulator. In various examples, the test waveform has a fundamental frequency and is applied as a bias voltage to a bias port of the modulator.

Method <NUM> may proceed to operation <NUM> during which an output of a photodetector coupled to the modulator may be monitored. Accordingly, a system component, such as a controller, may monitor an output of the photodetector in order to obtain information used for the generation of control signals described in greater detail below with reference to operations <NUM>-<NUM>.

Method <NUM> may proceed to operation <NUM> during which a low-frequency component and a fundamental frequency component may be extracted from the output of the photodetector. As discussed above, one or more components of a system component, such as a controller, may be used to extract the low-frequency and fundamental frequency components. In various examples, bias tees included in the controller are configured to extract the specific frequency components.

Method <NUM> may proceed to operation <NUM> during which a higher-frequency component may be extracted from the output of the photodetector. In various examples, the high-frequency component may be a component such as a third-order harmonic of the fundamental frequency. This higher-frequency component may be obtained from an output of a component, such a coupler <NUM>. Accordingly, during operation <NUM>, a third-order harmonic of the fundamental frequency may be extracted.

Method <NUM> may proceed to operation <NUM> during which a first control signal may be generated. In various examples, the first control signal is generated based on a monitored third order harmonic of the photodetector output. Accordingly, the first control signal is configured to maintain a distortion characteristic of the photonic link below a designated value. For example, the first control signal may maintain a third order harmonic of the fundamental frequency below a designated threshold value. In various examples, the designated threshold value is a predetermined value stored during a configuration process.

Method <NUM> may proceed to operation <NUM> during which a second control signal may be generated and applied to the phase shifter. In various examples, the second control signal is generated based on a monitored low-frequency component and a fundamental frequency component of the photodetector output. In this way, the second control signal is generated based on both of these monitored components. The second control signal is configured to maintain a phase difference between the modulator and the phase shifter. For example, the second control signal may maintain a phase difference between the modulator and the phase shifter at a phase angle of <NUM> degrees or <NUM> degrees. In this way, the second control signal reduces a noise characteristic of the photonic link. For example, the second control signal may minimize a noise figure of the photonic link by maintaining this relative phase between the two paths.

As discussed above, various examples of photonic links disclosed herein, such as those discussed above with reference to <FIG>, may be integrated with aircraft and spacecraft. Accordingly, the manufacture of such photonic links may be described in the context of an aircraft manufacturing and service method <NUM> as shown in <FIG> and an aircraft <NUM> as shown in <FIG>. During pre-production, illustrative method <NUM> may include specification and design <NUM> of aircraft <NUM> and material procurement <NUM>. During production, component and subassembly manufacturing stages <NUM> and system integration stage <NUM> of aircraft <NUM> takes place. Thereafter, aircraft <NUM> may go through certification and delivery <NUM> in order to be placed in service <NUM>. While in service by a customer, aircraft <NUM> is scheduled for routine maintenance and service <NUM> (which may also include modification, reconfiguration, refurbishment, and so on).

As shown in <FIG>, aircraft <NUM> produced by illustrative method <NUM> may include an airframe <NUM> with plurality of systems <NUM> and an interior <NUM>. Examples of high-level systems <NUM> include one or more of a propulsion system <NUM>, an electrical system <NUM>, a hydraulic system <NUM>, and an environmental system <NUM>. Any number of other systems may be included. In various examples, photonic links, such as systems and devices discussed above with reference to <FIG>, may be implemented with systems included in systems <NUM> or components implemented in interior <NUM>. Although an aerospace example is shown, the principles of the examples described herein may be applied to other industries.

Devices and methods embodied herein may be employed during any one or more of the stages of method <NUM>. For example, components or subassemblies corresponding to stages <NUM> and <NUM> may be fabricated or manufactured in a manner similar to components or subassemblies produced while aircraft <NUM> is in service. Also, one or more device examples, method examples, or a combination thereof may be utilized during stages <NUM> and <NUM>, for example, by expediting or substantially expediting assembly of an aircraft <NUM>, or reducing the cost of an aircraft <NUM>. Similarly, one or more of device examples, method examples, or a combination thereof may be utilized while aircraft <NUM> is in service, for example and without limitation, to maintenance and service <NUM>.

It is to be understood that the scope of protection is provided by the claims.

Claim 1:
A method of using a photonic link (<NUM>), the method comprising:
transmitting light using an optical emitter (<NUM>);
splitting, using an input coupler (<NUM>), the light into a first path and a second path, the first path being provided to a modulator (<NUM>) that modulates the light transmitted along the first path based on a radio frequency signal, and the second path being provided to a phase shifter (<NUM>);
combining, using an output coupler (<NUM>), an output of the modulator (<NUM>) and an output of the phase shifter (<NUM>);
monitoring, using a controller (<NUM>), an output of a photodetector (<NUM>) coupled to an output of the output coupler (<NUM>);
identifying, using the controller (<NUM>), based on the output of the photodetector (<NUM>), a modulator phase angle that reduces a third order distortion at an output of the output coupler (<NUM>);
applying, using the controller (<NUM>), a first bias voltage to the modulator (<NUM>) to maintain the identified modulator phase angle;
applying, using the controller (<NUM>), based on the output of the photodetector (<NUM>), a control signal to the phase shifter (<NUM>) to maintain a phase difference between the output of the modulator (<NUM>) and the output of the phase shifter (<NUM>); and
generating, using the controller (<NUM>), a radio frequency output of the photonic link.