Optical link

The present invention generally relates to optical links, and more particularly to analog optical links having a high spurious-free dynamic range.

As is known in the art, radio frequency signals are converted to the corresponding optical frequency signals for transmission to remote receivers via optical transmission media such as fiber optic cables, and then such light in the receiver. Optical links have been used to reconvert frequency signals back to radio frequency signals.

One type of optical link proposed to provide spurious-free (spurious-free) high dynamic range involves the use of two Mach-Zehnder interferometric modulators in the configuration. As a result, the three-dimensional non-linear strain component is minimized, or the two-dimensional strain component and the three-dimensional strain component are minimized at the same time. However, the results of high spurious-free dynamic range are limited to bandwidths below 1 GHz. By utilizing field absorption modulators, it is possible to bias the device to reduce 3D or 2D distortion, but compromises are needed to deal with both bandwidth and distortion. Is.

<p> While these links can be useful in some applications, the optical links used in current state-of-the-art technology are generally unsuitable for radar system applications, which have a very high spurious-free dynamic range. This is because it is necessary.</p>

<p> According to the present invention, an optical system including a main transmission link is provided, and an input radio frequency signal whose frequency is converted by a first optical frequency by modulating an input radio frequency signal with a first optical frequency signal. Generates a main signal that includes the components of the above along with the radiofrequency spurious signal components. The system includes a feedforward optical link that modulates part of the input radio frequency signal with a second optical frequency signal to capture the components of the input radio frequency signal whose frequency has been converted by the second optical frequency. Generate a feed forward signal that includes. A main signal and a feedforward signal are supplied to the receiver, and an optical frequency spurious signal component is subtracted from the main signal.</p><p> With such systems, feedforward optical links provide a way for the system to be precisely linearized over a wide bandwidth. In one embodiment, a system is provided that converts the frequency of an input radio frequency signal into an optical frequency output signal for transmission to a receiver via an optical transmission medium. Such a receiver reconverts the frequency of the optical frequency signal into the corresponding radio frequency signal. The system modulates the input radio frequency signal with the first optical frequency signal to generate a main signal that includes the component of the input signal whose frequency has been converted by the first optical frequency together with the optical frequency spurious signal component. Equipped with a transmission link. A feedforward optical link that generates a feedforward signal containing components of the input radiofrequency signal whose frequency has been converted by the second optical frequency by modulating part of the input radiofrequency signal with the second optical frequency signal. Is provided. The receiver (a) extracts the optical frequency spurious signal components generated by the main transmission link, reconverts the frequencies of these extracted optical frequency spurious signal components into radio frequency signal spurious signal components, and (b) mains. The frequency of the main light frequency component and the optical frequency spurious signal component of the signal is reconverted to the radio frequency component, and (c) the optical frequency spurious signal extracted and refrequency-converted from the reconverted frequency component of the main signal. Subtract the components.</p><p> In one embodiment, the frequency of an input radio frequency signal having a radio frequency component is converted to a corresponding radio frequency output signal for transmission via an optical transmission medium, and then the frequency of such radio frequency signal is input radio. A system is provided that reconverts to a frequency signal. The system is a first modulator to which an input radio frequency signal and a first optical frequency signal are supplied, and is driven by the input radio frequency signal into a non-linear region of the first modulator to produce an input radio frequency signal. It comprises a first modulator that generates a main signal that includes a component of the input radio frequency signal whose frequency has been converted by the first optical frequency together with an optical frequency spurious signal component by modulating with the optical frequency signal of 1. .. A second modulator that operates in its linear region and is supplied with a sample of the input radio frequency signal and a second optical frequency signal to modulate the sample of the input radio frequency signal with the second optical frequency signal. Provides a second modulator that produces a feedforward signal containing components of the input radio frequency signal whose frequency has been altered by the second optical frequency. The main signal and feedforward signal are supplied, and (a) the optical frequency spurious signal components generated by the first modulator are extracted, and the frequencies of these optical frequency spurious signal components are reconverted into radio frequency signal spurious signal components. , (B) A receiver comprising a circuit that reconverts the frequency of the main signal into the corresponding radio frequency signal and (c) subtracts the extracted optical frequency spurious signal component from the corresponding radio frequency signal is provided. ..</p><p> In one embodiment, it is the first modulator, operating in its non-linear region, with an input radio frequency signal and wavelength λ.₁A system comprising a first modulator is provided to which a first optical frequency signal having the above is supplied. The system is a second modulator that operates in its linear region and is a sample of the input radio frequency signal that has been phase-shifted by nπ radians if n is an odd integer with respect to the input radio frequency signal. And the second different radio frequency signal wavelength λ₂It is equipped with a second modulator to which and is supplied. The output of the first modulator is supplied to the first photodiode. A sample of the output of the second modulator and the output of the first modulator is supplied to the second photodiode. The subtractor is supplied with the output from the second photodiode and the output from the first photodiode.</p><p> In one embodiment, a main signal in which an input signal having a radio frequency component is modulated by a first optical frequency signal and a component of the input signal whose frequency is converted by the first optical frequency is included together with an optical frequency spurious signal component. Methods are provided that include generating. The method comprises modulating a sample of an input signal with a second optical frequency signal to generate a feedforward signal containing components of the input signal whose frequency has been converted by the second optical frequency. In this method, the optical frequency spurious signal component generated by the first modulation is extracted, and the frequency of the extracted optical frequency spurious signal component is converted into a radio frequency signal spurious signal component. The method comprises converting the frequency component of the main signal into the corresponding radio frequency component and subtracting the optical frequency spurious signal component extracted from the corresponding radio frequency component.</p><p> In one embodiment, the principal light frequency signal is a main transmission link through which the frequency of the input radio frequency signal is converted to the first optical carrier frequency by intensity modulation in the first light modulator. Is provided, a system with a main transmission link is provided. The principal light frequency signal includes components of the input radio frequency signal including the sideband frequency around the optical frequency signal, along with a spurious radio frequency signal that also appears as the sideband frequency around the optical carrier frequency. The system includes a feedforward optical link that uses a second light modulator to have a small portion of the input radio frequency signal (180 degrees out of phase with the principal light frequency signal). The second light modulator converts a small portion of the input radio frequency signal into the sideband frequency of the second optical carrier frequency. The feedforward optical link carries a feedforward signal with an RF amplitude that is much less intense than the main light frequency signal, thus resulting in much less distortion than that produced by the main link. The optical receiver is supplied with a sample of the main signal and a feedforward signal to generate a differential signal used to remove spurious signal components from the main signal. The two optical frequencies are selected so that the receiver does not inadvertently generate a signal due to the difference between the two optical frequencies. This is achieved by selecting a difference between two optical frequencies that is significantly larger than the bandwidth of the receiver (ie, the frequency difference between the two optical frequencies outside the bandwidth of the receiver).</p><p> In one embodiment, the input to the nonlinear element (ie, the sample from the second modulator) is subtracted from the sample of the output of the nonlinear element (ie, the sample of the output of the first modulator). If the gain of each link (ie, the main transmission link and the feedforward signal transmission link) is taken into account correctly, the resulting difference signal will be the parenchyma of the spurious component produced by the non-linear element (ie, the first modulator). It is a pure expression. By amplifying this difference signal and subtracting it from the rest of the main signal, a complete (noise-free) replica of the input radio frequency signal is reproduced at the output of the system.</p><p> In one embodiment, the receiver extracts (1) an optical frequency spurious signal component produced by the main signal transmission link. The signals are generated by superimposing radio frequency signals on the feedforward link and the sampled main link, which are reconverted from the sideband of the optical carrier frequency through the square detection operation of the photodiode. The receiver reconverts (2) the main signal component and the optical frequency spurious signal component of the main signal into radio frequency components, and (3) the reconverted component of the main signal (the "true" component and the distortion component). Subtract the extracted radio frequency spurious signal component from both).</p><p> According to another feature of the present invention, an input signal having a radio frequency component is converted into an optical frequency output signal having a radio frequency component of an input signal converted by an optical frequency in order to be transmitted via an optical transmission medium. Then, a system is provided in which the receiver reconverts such an optical frequency signal back to a radio frequency component. In this system, (1) an input signal at a level that may cause significant distortion and a first optical carrier frequency are supplied, and the input radio frequency signal is applied to this first carrier frequency through intensity modulation. , A first modulator that produces a main signal that includes a component of the input frequency signal converted by the first optical frequency along with a spurious signal component, and (2) a sample of the input signal and a second optical frequency signal. By applying a part of the input radio frequency signal to this second optical frequency signal, the components of the input frequency signal supplied and converted by the second optical frequency (amplitude modulation (AM) sideband frequency) can be obtained. It has a second modulator that produces a feedforward signal that includes. The receiver is supplied with a main signal and a feedforward signal, (1) extracts spurious signal components generated by the first modulator, and reconverts these optical frequency spurious signal components into radio frequency signal spurious signal components. Then, (2) the first optical frequency component of the main signal and the optical frequency spurious signal component are reconverted into radio frequency components, and (3) the optical frequency spurious extracted from the reconverted component of the main signal. It is equipped with a circuit that subtracts signal components.</p><p> In one embodiment, it has a first modulator, the first modulator operating in its non-linear region, with an input radio frequency signal and a wavelength λ.₁A system is provided that is supplied with a first optical frequency signal having. The system has a second modulator that operates in its linear region and shifts only nπ radians when n is an odd integer with respect to the input radio frequency signal. Input radio frequency signal sample and wavelength λ₂A second different optical frequency signal having is supplied. The output of the first modulator is supplied to the first photodiode. The second photodiode is supplied with a sample of the output of the second modulator and the output of the first modulator. A signal is supplied to the amplifier by a second photodiode. The subtractor is signaled by an amplifier and a first photodiode.</p><p> According to yet another feature of the present invention, the input signal having a radio frequency component is modulated by the first optical frequency signal, and the component of the input frequency signal converted by the first optical frequency is the optical frequency spurious signal component. Generates a main signal to include with, and modulates a sample of the input signal with a second optical frequency signal to generate a main signal containing components of the input frequency signal converted by the second optical frequency, with the first modulation. The optical frequency spurious signal component generated by is extracted, such an optical frequency spurious signal component is converted into a radio frequency signal spurious signal component, and the first optical frequency component of the main signal and the optical frequency spurious signal component are wirelessly transmitted. A method is provided that comprises converting to a frequency component and subtracting the extracted optical frequency spurious signal component from the reconverted component of the main signal.</p><p> Details of one or more embodiments of the present invention are shown in the accompanying drawings and the following description. Other features, objectives and advantages of the present invention will become apparent from their description and drawings as well as the claims.</p>

With reference to the only drawing here, the optical link 10 is shown, which is the input radio frequency (RF) signal IN and the wavelength λ if c is the speed of light in the optical fiber.₁1st optical frequency f₁= 2πc / λ₁Includes a first light modulator 12, for example an electric field absorption type modulator, driven into a non-linear operating region to which a first optical frequency signal having is supplied. Similarly, here, for example, the second light modulator 14, which is an electro-absorption modulator, operates in its linear region and is (a) a sample, a small portion (part) of the input radio frequency signal IN, which is transferred. For the input radio frequency signal IN in the phase device 13, a sample in which nπ radians are shifted when n is an odd integer and (b) c is the wavelength of light in the optical fiber, the wavelength λ₂1st optical frequency f₂= 2πc / λ₂A second different optical frequency signal having is supplied.

More specifically, the optical link 10 includes a directional coupler 16 to which the input radio frequency signal IN is supplied. The sample (ie, part) of the input radio frequency signal IN is 180 degrees phase-shifted and then the second optical frequency signal wavelength λ.₂It is supplied to the second modulator 14 together with.

Modulators 12 and 14 are matched modulators and thus guarantee equivalent transfer characteristics. The outputs of the first modulator 12 and the second modulator 14 are optical frequency signals indicated by dotted lines, and solid lines represent RF signals. More specifically, the input radio frequency signal IN is the first optical carrier frequency signal λ through intensity modulation in its first light modulator 12 by the first modulator 12.₁The frequency is converted to. Therefore, the output from the first modulator 12 is a spurious radio frequency signal in which the component of the input radio frequency signal IN that appears as a sideband around the optical signal is similarly used as a sideband around the optical carrier frequency. It is the "main" optical signal contained together with the components of. The output of the first light modulator 12 may be referred to as the main transmission link or the main optical link 18.

The feed-forward optical link 20 is a second light modulator that transmits a sample (ie, a portion) of the input radio frequency signal IN after 180 degrees phase shift to the input signal to the second light modulator 14. 14 outputs are provided. The second modulator 14 uses the frequency of the input radio frequency signal IN as the second optical carrier signal λ.₂Convert to the sideband frequency of. Due to the fractional (partial) nature of the RF signal provided to the second modulator 14, very low distortion is lower than the distortion of the main link.

These optical frequency signals at the main link 18 and the feedforward link 20 are supplied to the optical receiver 22 via an unsigned optical fiber cable. The optical frequency signal carried by the main transmission link 18 may be referred to as a main signal in the present specification. The optical frequency signal carried by the feedforward link is sometimes referred to herein as a feedforward signal, which provides the receiver 22 with a feedforward signal. The effects of any variability resulting from different delays in the two fibers can be overcome by adjusting the two lengths of the fibers.

A main signal and a feedforward signal are supplied to the optical receiver 22. As will be described in more detail later, the receiver 22 produces a differential signal, which is used to remove spurious signal components from the main signal. Two optical frequencies λ₁And λ₂Is selected so that it does not occur at the receiver 22 due to the difference between the two optical frequency links. This is a frequency λ that is significantly larger than the bandwidth of receiver 22₁And λ₂Achieved by choosing the difference between.

More specifically, the receiver 22 includes a first photodiode 24, a second photodiode 26, an amplifier 28 and a subtractor 30. More specifically, the second photodiode 26 is supplied with a sample (ie, a fractional portion) of the output of the second modulator 14 and the output of the first modulator 12. More specifically, a portion of the output of the first modulator 12, i.e. a sample, is fed to the second photodiode 26 via the directional coupler 30. Therefore, the signal generated by superimposing the radio frequency signals of the feed forward link 20 and the sample main link 18 is reconverted from the sideband of the optical carrier frequency through the square detection operation of the second photodiode 26. The first photodiode 24 is signaled by the first modulator 12, more specifically by the main part of the output of the directional coupler 30. A signal is supplied to the amplifier 28 by a second photodiode 26. A signal is supplied to the subtractor 30 by an amplifier 28 and a second photodiode 26. Fractional portion of the sample signal in the RF directional coupler 16, fractional portion of the sample signal in the optical directional coupler 18, gain of amplifier 28 and laser output to ensure that the resulting OUT signal is distortion-free. Is adjusted.

The frequency spectrum of the input radio frequency signal IN is represented by reference numeral 40 for purposes of illustration and description. The frequency spectrum of the signal produced by the directional coupler 16 is unchanged, but the phase of the spectrum is phase-shifted 180 degrees before being fed to the second modulator 14, as described above. Please note.

The effect of the modulator 12 is to λ the frequency spectrum of the output of the directional coupler 16 by the optical frequency of the first optical frequency signal having a wavelength.₁Is to convert to. However, since the first modulator 12 is driven by the input radio frequency signal to operate in the non-linear region to provide maximum output power, such non-linear operation is an unwanted spurious (SPUR) indicated by reference numeral 42. Note that it also produces frequency components.

The main part of the output of the first modulator 12 carried by the main link 18 is supplied to the first photodiode 24 as described above. Therefore, the main transmission link 18 has (1) a spurious (SPUR) optical frequency component generated by the modulator 12 operating in the non-linear region and (2) a frequency of f.₁It carries the frequency component of the component of the input signal IN converted by.

At its output, the first photodiode 24 has a spurious optical frequency component generated by the modulator 12 operating in the non-linear region and f.₁Provided together with the frequency component of the component of the input signal IN whose frequency has been converted by. The frequency spectrum of the signal generated by the first photodiode 24 is shown by reference numeral 42.

A small sample of the signal produced by the first modulator 12 provided by the directional coupler 30 is fed to the second photodiode 26 along with the output of the second modulator 14 as described above. Therefore, the feedforward transmission link 20 is (1) f.₁The frequency component of the input signal IN whose frequency has been converted by (2) the spurious (SPUR) optical frequency component generated by the modulator 12 operating in the non-linear region, and (3) f.₂It carries the frequency component of the component of the input signal IN whose frequency is converted by.

The second photodiode 26, at its output, provides only spurious signals converted to radio frequencies. Therefore, the signal generated by the second photodiode 26 is essentially an error signal representing the spurious frequency component produced by the non-linear operation of the first modulator 12. The frequency spectrum of the signal generated by the second photodiode 26 is indicated by reference numeral 44.

In this way, the second photodiode 26 extracts the optical frequency spurious signal component generated by the main transmission link 18, and reconverts the frequency of the thus extracted optical frequency spurious signal component into the radio frequency signal spurious signal component. To do. The first photodiode 24 reconverts the frequencies of the main light frequency component of the main signal and the optical frequency spurious signal component into radio frequency components. The subtractor 30 subtracts the spurious signal component obtained as a result of extraction produced by the second photodiode 26 from the reconverted frequency component of the main signal produced by the first photodiode 24.

Optical detectors work by generating electron-Hole pairs from absorbed photons in the presence of an electric field. Each absorbed photon produces an electron-hole pair (ie, more light power = more photons = more current). An electric current is generated by generating an electron-hole pair in the presence of an electric field. This is a square detector because the current (rather than power) of the optical detector is proportional to the input light power. With a single input signal (one input light wavelength), the resulting current will be proportional to the AM modulation of the input light signal. In the case of the main signal, this includes a spurious optical signal generated by an electronic / optical modulation process. The optical sideband is converted to RF at the optical receiver (main and spurious). Similarly, the optical sideband in a feedforward signal with little distortion is converted to RF mode. In other words, the detector produces an RF current from each of the supplied optical signals. At this time, since these currents are superposed in the diode and have the same frequency, they cause constructive interference (intensifying interference) or canceling interference (weakening interference) with each other. If phase alignment is set correctly, it will result in weakening interference. The difference frequency of the optical wavelengths is chosen to be well above the passband of the detector so that no heterodyne can occur. By guaranteeing a large difference frequency, any current generated thereby will be short-circuited due to the parasitic capacitance and travel time of the detector. Therefore, the difference frequency is not maintained. When the two wavelengths of light are selected to be close to each other (within the passband of the receiver), the difference between the two signals produces a mixed component. This is an undesirable choice as it produces another strain component.

The weakening interference is brought about by ensuring that the two RF signals are 180 degrees out of phase with each other. Therefore, the two RF signals are 180 degrees out of phase due to the phase shift at the input. This can be achieved at the input of the receiver, but a third optical detector is required at the output to perform the phase shift. Note that this phase shift can be incorporated into the amplifier. Note that the use of a third photodiode adds design flexibility. More specifically, the third wavelength can add to the flexibility of the device, although it can make it more difficult to manage.

Thus, with such a configuration, the input radio frequency signal IN, here the wavelength λ₁By modulating with the first optical frequency signal of, the first optical frequency f when c is the speed of light in the optical fiber.₁= 2πc / λ₁An optical system having a main transmission link is provided that produces a main signal that includes a component of the input frequency signal IN converted by, along with an optical frequency spurious signal component. Part of the input radio frequency signal IN is the second optical frequency signal f₂= 2πc / λ₂By modulating with, a feedforward optical link is provided that produces a main signal containing components of the input frequency signal converted by a second optical frequency. The receiver is supplied with a main signal and a feedforward signal, has a first photodiode 24, a second photodiode 26, an amplifier 28, and a subtractor 30, and has an optical frequency extracted from the main signal. Subtract the spurious signal component.

In this way, the receiver 22 supplies the main signal and the main signal, extracts the optical frequency spurious signal component generated by the first modulator, and converts such an optical frequency spurious signal component into a radio frequency signal spurious signal component. Reconvert, reconvert the first optical frequency component and optical frequency spurious signal component of the main signal into radio frequency components, and subtract the extracted optical frequency spurious signal component from the reconverted component of the main signal. , Including the circuit.

Wavelength λ₁And λ₂Must be chosen not to be heterodyne in the optical receiver. That is, the difference between the two optical frequencies must be significantly greater than the bandwidth of the optical receiver. The RF power of the signal routed to the feedforward arm is significantly less than the RF power of the signal fed to the main arm. Therefore, it produces much lower distortion in electronic and optical modulators than in the main arm. A fractional portion of the feedforward signal (inverted in the electrical domain) and the signal in the main arm is fed to a single optical receiver to generate an error signal. The combined fraction of the main arm signal is the amount of signal first passed to the feedforward arm and λ₁It depends on the light power in. The resulting error signal is fed to the error amplifier, which increases the distortion signal to the distortion signal carried by the main arm. The resulting error signal is subtracted from the main arm signal to produce a relatively distortion-free output signal.

The feedforward optical link configuration may be implemented with additional differential light signals to eliminate common mode noise (common mode noise, i.e., relative intensity noise). Compensation for the delay between the main arm and the feedforward arm may be accommodated by lengthening the fiber. The optical receiver 22 is mounted in a monolithically integrated optical receiver to ensure matching of components and proper integration with different amplifiers. Similarly, the modulator should be a matched component. On the other hand, an electric field absorption type modulator may be required.

A plurality of embodiments of the present invention have been described. However, it will be appreciated that various changes may be made without departing from the spirit and scope of the invention. Therefore, other embodiments are also included in the claims.

<figref num="1">It is a schematic block diagram which shows the exemplary embodiment of the optical link by this invention.</figref>