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Coherent & Tunable THz Emission

We demonstrate coherent and tunable THz emission by excitation of a unitraveling-carrier photodiode by a dualfrequency III–V semiconductor laser emitting up to 80 mW of optical power around 1 μm. The laser is an optically-pumped vertical-external-cavity surface-emitting laser that operates simultaneously on two transverse Laguerre–Gauss modes.

Modes frequency difference is driven by thermal effects, band-filling effects and/or phase masks, allowing THz emission from 50 GHz to few THz. To reach THz emission from a pigtailed photodiode, we detail quantitatively how orthogonal transverse modes can be coupled within a single-mode fiber, leading to more than 20% beat efficiency.

The figure shown above depicts the dual-frequency optically-pumped QW VeCSEL design. The laser cavity is formed by a wavelength-thick GaAs-based gain mirror and a concave dielectric output coupler. The cavity length is set by a 1 to 10 mm-long air gap leading to a Free Spectral Range in the 15–150 GHz range. It defines a stable short linear cold cavity of high axial symmetry, supporting a degenerate petals-like LG_pm eigenmode orthogonal basis.

We developed a dual-frequency CW laser that exploits this GaAs-based Quantum-Well (QW)VeCSEL technology integrating an intracavity sub-λ thick spatial filter on the end gain mirror, that consists of a metallic mask and eventually a metamaterial-based phase mask.
The dual-frequency VeCSEL is based on the stabilization of two Laguerre-Gauss (LG) transverse eigenmodes, in this high-Q single-axis short laser cavity.

THz emission was successfully achieved by exciting a commercial UTC-PD (from NTT Electronics corp.) that integrates a high-resistivity Si lens for broadband emission.
As shown in the set-up illustrated above, this emitter is pigtailed with a single-mode (SM) polarization-maintaining (PM) fiber (panda-type). We use a fiber coupler to control the fiber injection by monitoring the optical spectrum of the injected signal, while injecting this signal in the UTC-PD.
The THz beam that comes from the UTC-PD is collimated using a 10-cm focal-length Teflon lens, then focused using an identical lens into a calibrated heterodyne head receiver that allows to observe the THz spectrum on a RF analyzer.

As shown above, we observe a very coherent signal around 256 GHz with a full-width at half-maximum of 150 kHz for a 3-ms measuring time, that tends to be limited by the 100-kHz resolution bandwidth of the RF analyzer. The output power of −35 dBm is limited by the maximum output power of the UTC-PD.
We also observe a very large signal-to-noise ratio, that exceeds 60 dB, thanks to the active heterodyne receiver head.

The frequency difference between the two modes increases with pump, as illustrated above.
Furthermore, using another cavity mode couple, the beat frequency can be tuned from 50 GHz up to 700 GHz using a same structure. Additionally, the beat frequency depends on the meta-material spatial distribution, and thus allows to reach higher beat frequencies.

Among all characteristics, the main interest of a two frequency laser is the correlation in time that may exist between the frequency fluctuations of the two emitted modes: this correlation gives rise to a low frequency noise THz beat-note.
In a single-frequency VeCSEL, the strongest part of the frequency fluctuations is due to both the mechanical and to the thermally induced index fluctuations that occur in the gain chip due to the intensity noise of the pump power.
At high frequency, the linewidth is limited by the spontaneous emission quantum noise. This contribution of course depends on the specific VeCSEL configuration under study, but it is as low as 100 Hz2/Hz.
The frequency noise of each optical mode was measured thanks to a traditional Fabry-Perot discriminator set-up.

Because the laser emits two optical modes, we have to ensure that only one mode fluctuations are observed at a time. For that reason, the frequency comb of the frequency discriminator was carefully designed to permit the frequency-to-intensity conversion of one VeCSEL mode only, while the power of the second one is rejected by the cavity. This operation was performed for each of the two optical modes.

The results of these experiments, shown above, demonstrate the efficiency of this two-frequency laser concept. We indeed obtain a THz beating exhibiting low frequency noise compared to the optical frequency noise of the two laser modes (up to four decades). This is due to the fact that the same fluctuations feed simultaneously the two optical modes. Because the two modes are mostly uncorrelated, the frequency noise fundamental limit is simply due to the spontaneous emission quantum noise of the two optical fields. This white noise limit is almost reached at a frequency as low as 10 kHz.

 

For more details, see our last article describing the coherent and tunable room-temperature THz emission we obtained :
S. Blin et al., Coherent and Tunable THz Emission Driven by an Integrated III–V Semiconductor Laser, IEEE Journal of Selected Topics in Quantum Electronics, 23, 4, July/August 2017

 

THz Communication & Detection

We report the first successful terahertz heterodyne communication using a field-effect transistor for detection. The communication is a real-time transmission of an uncompressed high-definition TV signal at a data rate of 1.5 Gbps with a 307-GHz carrier frequency. The emitter is a frequency-multiplied amplifier chain whose last stage is a second harmonic mixer that multiplies the carrier signal by the data. The receiver only consists of a GaAs high-electron-mobility transistor that acts as a quadratic receiver, and two 20-dB-gain amplifiers, no limiting amplifier or forward error correction were used.
A direct communication would be impossible with such a combination of modulation scheme at emission and quadratic detection at reception, while it is possible in a heterodyne configuration. In addition, for the same source power, the heterodyne scheme allows to increase the communication bandwidth from 80 MHz to more than 2 GHz for a local oscillator power of –8 dBm.

The experimental setup is presented above and consists of two sources, one being the radio frequency source (RF) used to carry the signal, the other being a continuous-wave source used as a local oscillator (LO).
The signal at the output of the RF source presents a double-side-band amplitude-modulation with a suppressed carrier. Thus, such a modulation scheme at emission would not permit any digital communication using a quadratic receiver at detection, therefore the heterodyne scheme (addition of the LO signal) is at the same time necessary to the communication, and interesting as it improves the possible communication bandwidth by improving the effective incoming power.

The IF signal is retrieved at the output of the transistor by measuring the drain–source voltage. The sources and the transistor have horn antennas. The transistor output signal is amplified with two identical amplifiers whose gain is 20 dB, bandwidth is 100 kHz – 1 GHz, and noise figure is 2.9 dB. The serial digital interface (SDI) signal is then converted to a HDMI signal that is observed successfully on a HD TV.


As illustrated above, and as detailed in a previous work, cf for instance S. Blin et al., Wireless communication at 310 GHz using GaAs high-electron-mobility transistors for detection, J. Commun. Netw., vol. 15, no. 6, pp. 559–568, Dec. 2013, the transistor is packaged within a homemade horn with a rectangular waveguide to increase the gathered THz power, thus improving the pHEMT effective sensitivity up to a few V/W around 300 GHz.
The rectangular waveguide was designed to assure a small incoming beam waist at the transistor, and to reduce detection of undesired noise. This screening is important because the undesired coupled signals might be amplified by the transistor, thus reducing the signal-to-noise at the transistor output. 

To check the ability of this heterodyne scheme to transmit real data, we demonstrated the transmission of a realtime uncompressed high-definition video signal at a rate of 1.5 Gbps. The transmission was successful and very robust, as the video could be observed on the HD TV without any interruption. The previous figure shows the spectrum of the video signal at the transistor output (after amplification).
The inset of this figure shows the eye diagram of the signals, exhibiting a very open eye and a peak-to-peak amplitude of about 350 mV.

For further information, see our last article describing this THz heterodyne communication :
S. Blin et al., Terahertz Heterodyne Communication Usingb GaAs Field-Effect Transistor Receiver, IEEE Electron Device Letters, 38, 1, January 2017