Radio frequency transmitter based on a laser frequency comb

The laser radio transmitter (LRT) is illustrated in Fig. 1B. It consists of a continuous wave, ridge waveguide, uncoated Fabry–Perot QCL with an 8-mm-long cavity operating as a fundamental frequency comb (12) in the mid-IR spectral range with a narrow (kilohertz) linewidth beat note at $f_B=5.5$ GHz (Fig. 1B). A 400 -$\mathrm{\mu }$m-wide gap is etched in the top contact layers of the device, creating two contact sections with an open circuit resistance of 250 $\mathrm{\Omega }$. The two top laser contacts are connected through wirebonds to a low-impedance half-wave dipole antenna designed to radiate at $f_B$ consisting of two gold stripes fixed on a polyactide dielectric substrate. The laser current is injected from the dc power source through the antenna into the QCL (Materials and Methods).

To characterize the emission pattern of the system, we carry out far-field measurements of the QCL microwave radiation. A schematic of the experimental setup is shown in Fig. 1C. The LRT is mounted on a rotation stage, and the microwave radiation emitted at angles between $-90^{○}$ and $90^{○}$ on the horizontal plane is detected by a directive horn antenna (gain of 18.5 dBi) at a distance of 0.9 m (16 ${\lambda }_B$) from the source, amplified, and then, measured with a spectrum analyzer (SI Appendix). Fig. 1D shows the measured radiation pattern of the device (Fig. 1D, continuous line). The central peak observed around $0^{○}$ originates from the dipole antenna and QCL, while the side lobes are due to emission from the wirebonds (SI Appendix). The maximum radiated power is approximately −80 dBm. When the gap of the QCL and antenna is closed using wirebonds (Fig. 1D, dashed line), the maximum power drops to −88 dBm. Although this does not correspond to the case of an unstructured device with a continuous metal electrode— because of the inductive nature of the wirebonds, which prevents them from being a perfect short—this result shows the fundamental role that the gap geometry plays for the wireless emission.

From the measured microwave radiation, taking into account the directivity of the emitter and receiver antenna and using an equivalent circuit model of the LRT, we can estimate the microwave power available at the source. The QCL active region can be modeled as a radio frequency generator with low-output impedance. The bonding pads on the sides of the waveguide behave as capacitors in parallel to the generator. A full model of the impedance of the pads and of the other elements in the equivalent circuit is described in SI Appendix. The antenna is connected to the QCL via wirebonds, the inductive behavior of which at microwave frequencies is also taken into account in the model. Finally, the antenna constitutes the load connected at the other end of the circuit. The impedance mismatch loss (mostly caused by the presence of the capacitive pads) is estimated to be −22 dB, which means that the power actually radiated by the antenna is 22 dB lower than the available power at the QCL. Using this fact and knowing that the total received power is −80 dBm, it is possible to compute the wireless link power budget with the Friis formula (SI Appendix). The total radiated power is found to be −58 dBm, meaning that the available power at the QCL is estimated to be −36 dBm. Based on numerical simulations of the QCL radio frequency generation, the available power is expected to increase by several orders of magnitude when operating the laser in the harmonic comb regime due to the higher optical power per mode and lower number of beat harmonics that are generated in this state (SI Appendix). The source impedance mismatch due to the pad admittance can be corrected using a buried heterostructure geometry with an iron-doped indium phosphide insulating layer (23), promising to improve the extraction performances both at microwave and subterahertz frequencies, as the capacitive pad admittance would be substantially reduced.

Next, we provide a proof of concept of wireless communication using the LRT. Note that here information is encoded in the laser beat note rather than in an optical carrier as it was done in previous works in optical wireless communication (24). The experimental setup is schematized in Fig. 2A. The laser current is modulated by an audio analog signal, which in turn, modulates the frequency of the laser beat note, allowing it to encode the baseband information onto the 5.5-GHz carrier wave. The radio signal is received by a horn antenna at a distance of 0.9 m, filtered by a receiving filter (1.9- to 5.5-GHz bandwidth), amplified, and then, down-converted to 1.5 GHz by mixing with a local oscillator (LO; 7.0 GHz) to fit into the bandwidth of a software-defined radio (SDR) used for demodulation. The physical process underlying the current-driven beat note modulation is the following: the current modulation $\mathrm{\Delta }I$ induces a thermal variation of the QCL active region, thus changing the group refractive index $n_g$ of the cavity. This, in turn, changes the intermodal spacing of the comb and the beat note frequency. In Fig. 2B, we present a waterfall plot of the signal demodulated by the SDR when the laser current is modulated at $f_{mod}=0.1$ Hz with a relative current modulation amplitude of $\mathrm{\Delta }I/I=0.2\%$. The beat note exhibits a nearly constant power, and its instantaneous frequency is modulated with a period of 10 s defined by $f_{mod}$, spanning a range of frequency deviation of 120 kHz determined by $\mathrm{\Delta }I$. In essence, the QCL behaves as a current-controlled oscillator, which can generate a frequency-modulated (FM) signal. This scheme is used for radio transmission of an audio track, which can be correctly retrieved after demodulation (Fig. 2C and Audio File S1). Unwanted slow thermal fluctuations of the laser cavity, which persist despite the use of a temperature controller, induce a jittering of the beat note appearing as a slow modulation of the baseline of the received audio signal. The effect of these fluctuations is to add a noise contribution below 10 Hz (SI Appendix), which is outside of the audio frequency range (20–20,000 Hz). The high-frequency background noise that is audible in the track is due to the level of the noise floor. The signal-to-noise ratio could be improved by increasing the range of frequency deviation of the modulated beat note, although this was limited in this work by the bandwidth of the software demodulator. While this demonstration deals with a low-frequency audio modulation signal, QCL modulation of several tens of gigahertz has been demonstrated using microstrip and coplanar waveguide geometries (25–28). These studies investigated the high-frequency modulation of the optical field in single-mode QCLs and can aid in understanding the effect of multimode QCL modulation on the generated beat note, although additional research will be needed to assess the impact of high-power modulation on frequency combs. In particular, at gigahertz modulation frequencies, we expect that the laser beat note will be predominantly amplitude modulated by plasma effects, as the cutoff frequency of thermal effects responsible for frequency modulation is below 1 MHz (29, 30).

Due to the presence of the antenna, the laser is also sensitive to wireless radio frequency signals. Here, we show that the laser beat note can be wirelessly injection locked to an external microwave reference. A similar injection locking scheme was used in semiconductor mode-locked lasers equipped with patch antennas and operating in the telecommunication region for radio-over-fiber applications (31). A schematic of the setup is shown in Fig. 3A. An LO, tunable in power and frequency, is connected to a horn antenna directed at the QCL chip. A microwave probe is placed in proximity of the QCL antenna to monitor the changes in the laser beat note induced by the LO, the power of which is swept between −30 and 24 dBm for a set of different frequencies. Examples of the behavior exhibited by the QCL beat note on wireless microwave injection are shown in Fig. 3B. In the lower range of powers of the LO, the QCL behaves as free running; its beat note is unlocked and shows small-frequency oscillations around $f_0=5.501$ GHz due to thermal fluctuations. When the LO power overcomes a threshold dependent on the detuning from $f_0$, the QCL beat note locks to the external oscillator and stops jittering. This phenomenon is preceded by the appearance of a weaker sideband next to the QCL beat note as typically observed in wired injection locking experiments (26). The increase of the power threshold with the detuning from $f_0$ follows a quadratic dependence as expected from injection locking theory (32, 33). For the maximum explored power of the LO (24 dBm), the wireless locking range is about 40 kHz. We note that an enhancement of the microwave emission from the laser based on a buried heterostructure design as discussed above will also improve the coupling of external radio signals into the system, thus lowering the wireless locking threshold curve. The demonstration of wireless injection locking of the laser beat note shows the possibility of remote control of laser frequency combs and may open applications in the field, such as wireless synchronization of multiple comb generators to a single reference oscillator, without the need of integrating complex interconnected microwave architectures.

This work is a proof of concept demonstration of a QCL frequency comb used as a wireless radio transmitter. Thanks to the recently discovered harmonic comb operation of QCLs (17, 19)—where the intermodal comb spacing is in the hundreds of gigahertz range due to longitudinal mode skipping—the frequency range of radio transmission of the system can be potentially extended to subterahertz carriers. Moreover, the frequency of such carriers holds promise of broad tunability, since the intermodal spacing of a harmonic frequency comb may be varied from a few 100 GHz up to over 1 THz in a single device, as it was recently experimentally demonstrated by optical injection of an external seed (34). In the harmonic regime, the internal dynamic grating will exhibit a number of spatial cycles corresponding to the number of skipped longitudinal modes in the frequency comb (17). Adapting the waveguide design demonstrated here to match the spatial periodicity of this higher-order grating will provide for extraction of a substantial fraction of the available radio frequency power generated in the harmonic state. For instance, traveling and standing wave antenna designs, which have already been proven successful in cryogenically cooled terahertz QCL systems (35, 36), or surface grating outcouplers used in difference frequency generation terahertz QCLs (37, 38) could be used to efficiently radiate the generated subterahertz signal. Other than increasing the carrier wave frequency, the harmonic state offers a spectrum of a few powerful optical modes. In principle, this spectral distribution of the optical power presents the net advantage of a much more efficient radio frequency generation for a given beat tone compared with QCLs operating in the fundamental comb regime, as it allows one to generate a radio frequency spectrum consisting of fewer but much stronger beat notes. Numerical simulations suggest that a QCL in the harmonic state can generate a beat note in the 100-GHz range with a 37-dB power enhancement with respect to that produced at 5.5 GHz by a QCL operating as a fundamental frequency comb in similar current and optical power conditions (SI Appendix). Considering that the available power at 5.5 GHz for the QCL studied in this work is −36 dBm, the estimated available power that would be obtained in the harmonic comb regime from this device is 1 dBm, which exceeds the −20-dBm level considered to be the lower limit for practical communication applications (22).

QCL radio frequency sources benefit from good impedance matching with extraction elements, such as antennas and waveguides, thanks to the low impedance of their active region. This is a clear advantage with respect to existing terahertz photomixers, which suffer from high impedance in the order of tens of kiloohm, thus losing several orders of magnitude in power efficiency. Thanks to the frequency comb nature of the light beating inside the cavity, this radio frequency source can generate tones of high spectral purity, leading to a very narrow (kilohertz or sub kilohertz) linewidth. Another attractive feature of the LRT is that the carrier frequency may be controlled by the laser current, allowing in principle to phase lock it to a reference microwave source using frequency division and thus, stabilize it with high accuracy. Ultimately, the system introduced here will benefit from unprecedented compactness compared with existing composite terahertz wireless communication systems (20), unifying in a single device the capability of room temperature generation, modulation, and emission of subterahertz waves, and it may find applications in fields ranging from telecommunications and spectroscopy to radioastronomy and quantum optics.

We thank D. Kazakov for discussions that motivated this demonstration and a careful reading of the manuscript, and A. Amirzhan for useful discussions. We acknowledge support from NSF Award ECCS-1614631. The work conducted by Lincoln Laboratory was sponsored by Assistant Secretary of Defense for Research and Engineering Air Force Contracts FA8721-05-C-0002 and/or FA8702-15D-0001. This work was performed in part at the Center for Nanoscale Systems, a member of the National Nanotechnology Coordinated Infrastructure Network, which is supported by NSF Award 1541959. M.T. acknowledges the support of Swiss National Science Foundation Grant 177836. B.S. was supported by Austrian Science Fund Project NanoPlas P28914-N27. N.A.R. is supported by NSF Graduate Research Fellowship Program Grant DGE1144152. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the Assistant Secretary of Defense for Research and Engineering or the NSF.