Patent Description:
Terahertz, THz, waves which cover the frequency range from <NUM> to 10THz, have been actively applied to sensing, radars, spectroscopy, measurement, and communication.

THz signal detection may be performed by a photodetector, such as a photoconductor or a photodiode, which down-converts the THz signal into an intermediate frequency, IF, signal by employing the concept of optically pumped mixing. An example of an optically pumped mixing based on a photodiode is disclosed in <NPL>. In the case of THz detection, optically pumped mixing entails down-converting a THz signal into intermediate frequency by means of a local oscillator signal that is generated through photomixing. Photomixing entails generating an optical heterodyne signal, or optical beat signal, to modulate the conductivity of a photoconductor or a photodiode. The modulated conductivity results in a modulated photocurrent when a DC bias is applied to the photoconductor or photodiode, which can be coupled out as a THz wave by means of an antenna. Conversely, when an AC bias is applied to the photoconductor or photodiode, by means of a THz wave received through a connected antenna, the optically pumped mixing process results in a photocurrent at intermediate frequency. <CIT> (<NUM>-<NUM>-<NUM>) describes a space-flight-compatible coherent receiver for submillimeter and farinfrared electromagnetic radiation.

THz detectors employing a photodiode as a photodetector generate a photocurrent resulting from the optical beat signal, typically referred to as a background photocurrent. When the photodiode is excited with a THz signal, a down-conversion photocurrent is generated in addition to the background photocurrent. The background photocurrent, however, may be larger than the down-conversion photocurrent generated by the photodiode when induced with the THz signal. This results in obscuring the photocurrent resulting from the THz signal, thereby making it difficult to separate the down-conversion photocurrent from the background photocurrent.

It is an object of embodiments of the present disclosure to provide a low-cost and compact circuit for detecting terahertz which allows eliminating the effect of the background photocurrent. It is a further object of the present disclosure to provide a circuit capable of down-converting both modulated and unmodulated THz signals. It is a yet another object of the present disclosure to provide a circuit which may be easily integrated into an integrated circuity.

This object is achieved, according to a first example aspect of the present disclosure, by a circuit for optoelectronic down-conversion of a terahertz, THz, signal, wherein THz covers a frequency range from <NUM> to 10THz, the circuit comprising:.

In other words, the photodiodes produce respective direct current, DC, components when excited or illuminated with an optical beat signal. This DC component is typically referred to as a background photocurrent. Further, the photodiodes respectively produce an additional current component with an intermediate frequency when a THz signal is received by the antenna. This component is typically referred to as the down-conversion photocurrent.

By coupling the photodiodes in series through a common antenna, i.e. by coupling the photodiodes to opposite terminals of the antenna, it is assured that the background photocurrents resulting from the optical beat signal flow in the same direction and the down-conversion photocurrents resulting from the received by the antenna THz signal flow in the opposite direction along the path connecting the photodiodes. This leads to obtaining the respective IF components, i.e. the down-conversion photocurrents, and cancelling the respective DC components, i.e. background photocurrents, at the output terminal. As a result, the circuit outputs a photocurrent comprising the IF current components resulting from the down-conversion of the THz signal.

By cancelling the respective DC components, the need for complex post-processing of the resulting photocurrent is avoided. As a result, a low-cost and a compact read-out electronic circuitry for further processing the photocurrent outputted by the optoelectronic down-conversion circuit may be used.

Furthermore, as the optoelectronic down-conversion circuit does not require complex electronic integration, it may be easily integrated into an integrated circuitry together with other electrical components.

According to an example embodiment, the photodiodes are configured to be connected at a respective terminal to a bias circuitry, the bias circuitry is configured to bias a respective photodiode such that the DC components of the respective generated currents are substantially equal.

By biasing the photodiodes, it is assured that the DC components of the respective generated currents are substantially equal or equal and having the same current direction. This ensures that the DC components of the generated currents cancel each other at the output terminal of the circuit. As a result, the DC component resulting from the optical beat signal is cancelled, thereby eliminating the need of complex read-out circuity for further processing the photocurrent outputted by the optoelectronic down-conversion circuit.

According to an example embodiment, the photodiodes are waveguide-coupled.

The photodiodes may comprise an active layer material of III-V or IV group. Examples of active layer materials include (In)GaN, (Si)Ge, (In)GaAs, and so on. By employing waveguide-couple photodiodes, the optoelectronic down-conversion circuit is realized with a minimal footprint which may be integrated into an integrated circuitry. Furthermore, this enables the incorporation of multiple of such optoelectronic down-conversion circuity into the integrated circuitry, thereby allowing the creation of an array of such optoelectronic down-conversion circuits.

According to an example embodiment, the terminals of the antenna are coupled to the output terminal through respective transmission lines.

By coupling transmission lines to the respective antenna' terminals the virtual ground at the output terminal is converted to an open, allowing the photodiodes to be biased through the antenna, while isolating the antenna from the short at the output terminal.

According to a further example embodiment, one terminal of a respective photodiode is coupled to a respective terminal of the antenna and another terminal of the respective photodiode is coupled to a respective open transmission line.

By coupling open transmission lines to the respective photodiodes, the circuit is isolated at THz frequency from the influence of parasitics of the biasing circuitries and shorts at THz frequency are provided between the terminals of the photodiodes coupled to the respective open transmission lines.

According to an example embodiment, the transmission lines have a length of a quarter of the wavelength of the THz signal.

By dimensioning the transmission lines as a quarter of the wavelength of the THz signal, opens at the terminals of the antenna and shorts at the terminals of the photodiodes opposite the terminals connected to the antenna are introduced at THz frequencies. This allows the photodiodes to be connected to the antenna while providing a short between them at DC frequency.

According to an example embodiment, the respective photodiodes are coupled to opposite terminals of the antenna via a matching network.

By coupling the respective photodiodes through a matching network to the opposite terminals of the antenna, it is assured that maximum voltage is transferred from the antenna to the photodiodes.

According to example embodiments, the THz signal is a modulated or an unmodulated THz signal.

In this case that the THz signal is unmodulated, and both the frequency and phase of the optical beat frequency matches that of the THz signal, the current component resulting from the down-conversion of the THz signal comprises a direct current, while in the case of a modulated THz signal, the resulting current component comprises an alternate current, AC, component centred around the intermediate frequency.

According to an example embodiment, the antenna is a differential antenna.

According to an example embodiment, the antenna is a narrowband differential antenna such as a dipole antenna.

According to a further embodiment, the antenna is a planar antenna.

According to a second example aspect, an integrated circuit is disclosed comprising the circuit for optoelectronic down-conversion of a modulated terahertz, THz, signal according to the first example aspect.

According to an example embodiment, the integrated circuit further comprising an amplifying circuitry configured to amplify the current at the output of the circuit.

<FIG> shows a block scheme of the optoelectronic down-conversion circuit <NUM> according to an embodiment of the present disclosure. The optoelectronic down-conversion circuit <NUM> down-converts THz signals by employing the principle of optically pumped mixing. This principle involves down-converting a THz signal <NUM> by means of an optical beat signal <NUM> acting as a local oscillator signal to output a downconverted THz signal. The optoelectronic down-conversion circuit <NUM> and its operation will be described in detail below with reference to <FIG>.

The circuit <NUM> comprises a first waveguide-coupled photodiode <NUM> and a second photodiode <NUM> coupled in series through a common differential antenna <NUM>. That is, a second terminal, i.e. the anode, of the first photodiode <NUM> is coupled to a first terminal, i.e. the cathode, of the second photodiode <NUM> via the antenna <NUM>. The respective photodiodes are configured to be excited or illuminated with the same optical beat signal <NUM>. The photodiodes may be for example waveguide-coupled photodiodes. The antenna is configured to receive a THz signal <NUM> which may be modulated or unmodulated.

The terminals <NUM> and <NUM> of the antenna <NUM> are coupled together via respective transmission lines <NUM> and <NUM> to form the output terminal <NUM>. Therefore, it suffices to use a narrowband antenna, such as a dipole or similar antenna. The terminals <NUM> and <NUM> are further coupled to the respective terminals of the photodiodes via matching networks <NUM> and <NUM>. In other words, the anode of the first photodiode <NUM> is coupled to the cathode of the second photodiode <NUM> via a pair of transmission lines <NUM> and <NUM> with the antenna <NUM> connected in between the transmission lines. Further, the opposite terminals of the photodiodes <NUM> and <NUM>, i.e. terminals <NUM> and <NUM>, are coupled to respective open transmission lines <NUM> and <NUM>. The transmission lines <NUM> to <NUM> are sized to have a length a quarter of the wavelength of the THz signal. In other words, the photodiodes <NUM> and <NUM> are effectively placed in between a pair of quarter-wavelength transmission lines <NUM>-<NUM> and <NUM>-<NUM>. The terminals <NUM> and <NUM> of the respective photodiodes are further configured to be connected to respective bias circuitries, i.e. bias circuitry <NUM> and <NUM>. The bias circuitries may respectively apply a DC bias to reverse-bias the photodiodes.

Dimensioning the transmission lines <NUM>-<NUM> to have a length of a quarter of the wavelength of the THz signal, allows to introduce a π/<NUM> phase shift between the circuit paths defined by transmission lines <NUM> and <NUM> and by transmission lines <NUM> and <NUM>, respectively. For the transmission lines <NUM> and <NUM> with respective open terminals <NUM> and <NUM> this introduces a short at terminals <NUM> and <NUM> at THz frequencies, while for the transmission lines <NUM> and <NUM> the π/<NUM> phase shift converts the virtual ground at the output terminal <NUM> to an open at terminals <NUM> and <NUM> at THz frequencies, thereby allowing to connect the photodiodes to the same antenna while providing a short between them at DC frequency. Furthermore, the transmission lines <NUM> and <NUM> allow reducing the influence of parasitics effects caused the biasing circuitries.

When an optical beat signal <NUM> excites or illuminates the photodiodes, the photodiodes <NUM> and <NUM> generate charge carriers with a fixed frequency corresponding to the frequency of the optical beat signal. This causes the photodiodes to respectively produce a DC current, commonly referred to as a background photocurrent, which flows from the cathode to the anode terminals of the respective photodiodes. As the photodiodes are coupled in series the background photocurrents will effectively flow from the cathode of the second photodiode <NUM> to the anode of the first photodiode <NUM>, i.e. from terminal <NUM> towards terminal <NUM>.

When a THz signal <NUM> is received by the antenna <NUM>, the antenna will induce a voltage over the photodiodes causing a change in the velocity of the generated charge carriers. The matching networks <NUM> and <NUM> ensure that the maximum voltage is transferred or induced by the antenna over the photodiodes. As the antenna is provided in between the photodiodes, i.e. the photodiodes are respectively connected to a different terminal of the antenna, the voltage induced over the photodiodes will be with opposite polarity. If the phase and frequency of the optical beat signal and the THz signal match, optically pumped mixing is observed at the photodiodes resulting in the generation of a maximum down-conversion photocurrent in addition to the background photocurrent. This is referred to as a coherent homodyne down-conversion process. As the voltage induced over the photodiodes is with opposite polarity, the resulting down-conversion photocurrents will flow in the opposite direction along the path connecting both photodiodes.

Depending on whether the THz signal is modulated or not, the induced voltage and hence the charge carriers' velocity, will either be modulated or unmodulated. In case the THz signal is modulated, e.g. amplitude or phase modulated, the down-conversion photocurrent generated by the respective photodiodes is also modulated. In other words, intermediate frequency, IF, down-conversion photocurrents are generated which change magnitude and/or phase continuously with time in accordance with the modulation in the THz signal. In case the THz signal is unmodulated, and both the frequency and phase of the optical beat frequency matches that of the THz signal, the IF down-conversion photocurrent generated by the respective photodiodes will comprise DC components.

The current components generated by the respective photodiodes, i.e. the DC component resulting from the optical beat signal and the down-conversion IF current component resulting from the received THz signal, are summed at the output terminal <NUM> of the circuit <NUM> to output a resulting photocurrent. As the output terminal <NUM> is formed in between the photodiodes <NUM> and <NUM>, the DC components from the respective photodiodes flowing in the same direction along the path connecting the photodiodes effectively subtract one from another at the output terminal, while the IF components flowing in the opposite direction along the path connecting the photodiodes, effectively sum together.

Tuning the DC biasing of the respective photodiodes allows evening out the magnitudes of the DC components generated by them. Doing so assures that the DC components are cancelled out at the output terminal. This further allows adjusting the time-average photoconductance of the photodiodes to achieve optimal impedance matching between the respective photodiodes and the antenna. If the time-average conductivity is too low then the resulting down-conversion current is decreased, while if too high then voltage is lost due to the radiation resistance of the antenna itself. As a result, this further assures that the photodiodes generate IF components with an optimum magnitude.

The photocurrent outputted by the optoelectronic down-conversion circuit <NUM> may be read out by an external read-out circuitry. For this purpose, an external read-out circuitry may be coupled to the output terminal <NUM> of the circuit <NUM>. The read-out circuitry may comprise an amplifying circuitry, a filtering circuitry, and a processor. In the figure, only the amplifying circuitry <NUM>, i.e. a transimpedance amplifier, TIA, is shown. In this case, the transmission lines <NUM> and <NUM> coupled to respective terminals of the antenna further prevent shorting the antenna terminals at THz frequencies and isolate the optoelectronic down-conversion circuit from parasitics caused by the amplifier.

Furthermore, the optoelectronic down-conversion circuit enables determining the phase of the THz signal by for example employing two optoelectronic down-conversion circuits <NUM> respectively illuminated with <NUM>° phase-shifted optical beat signals. The phase of the THz signal may then be determined from the in-phase and quadrature down-conversion IF photocurrents outputted by the respective optoelectronic down-conversion circuits.

Although the present invention has been illustrated by reference to specific embodiments, it will be apparent to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied with various changes and modifications without departing from the scope of the claims.

Claim 1:
A circuit (<NUM>) for optoelectronic down-conversion of a terahertz, THz, signal, wherein THz covers a frequency range from <NUM> to 10THz, the circuit comprising:
a first photodiode (<NUM>) and a second photodiode (<NUM>) configured to be excited by an optical beat signal, the photodiodes being coupled in series through a common antenna (<NUM>), wherein terminals of the antenna are coupled to form an output terminal (<NUM>), and wherein the antenna is configured to receive the terahertz, THz, signal; the photodiodes thereby, by means of the optical beat signal, respectively down-converting the THz signal and generating a current comprising an intermediate frequency, IF, component and a direct current, DC, component, wherein the respective generated currents are summed at the output terminal (<NUM>), thereby obtaining the IF components and cancelling the DC components.