Laser radio transmitter

TL;DR

This paper demonstrates a novel compact radio frequency transmitter using a semiconductor laser frequency comb, enabling wireless communication and synchronization through integrated laser and antenna design.

Contribution

It introduces a new design that couples laser frequency combs with integrated antennas for wireless RF transmission and reception, a significant advancement over traditional radio transmitters.

Findings

Laser beating generates RF current within the laser cavity.

Redesigning the laser contact enables coupling to an external antenna.

Laser modulation can encode signals onto the RF carrier.

Abstract

Since the days of Hertz, radio transmitters have evolved from rudimentary circuits emitting around 50 MHz to modern ubiquitous Wi-Fi devices operating at gigahertz radio bands. As wireless data traffic continues to increase there is a need for new communication technologies capable of high-frequency operation for high-speed data transfer. Here we give a proof of concept of a new compact radio frequency transmitter based on a semiconductor laser frequency comb. In this laser, the beating among the coherent modes oscillating inside the cavity generates a radio frequency current, which couples to the electrodes of the device. We show that redesigning the top contact of the laser allows one to exploit the internal oscillatory current to drive an integrated dipole antenna, which radiates into free space. In addition, direct modulation of the laser current permits encoding a signal in the radiated radio frequency carrier. Working in the opposite direction, the antenna can receive an external radio frequency signal, couple it to the active region and injection lock the laser. These results pave the way to new applications and functionality in optical frequency combs, such as wireless radio communication and wireless synchronization to a reference source.

Figures

Full text

Optical fields can be used to synthesize low-phase-noise microwaves by means of different techniques, such as optical frequency division Fortier et al. (2011); Li et al. (2014), optoelectronic oscillations Maleki (2011) and laser heterodyne Li et al. (2013). The latter can be realized in a medium with a nonlinear optical response, such as a photomixer Tani et al. (2005), capable of converting the frequency difference between the optical modes into a microwave tone Kim and Kärtner (2010); Rubiola and Santarelli (2013); Quinlan et al. (2013). An attractive aspect of quantum cascade lasers (QCLs) operating as optical frequency combs Udem et al. (2002); Hänsch (2006); Hugi et al. (2012); Villares et al. (2014); Burghoff et al. (2014); Lu et al. (2015); Cappelli et al. (2016) is that they can act both as sources of light whose spectrum consists of equidistant modes and as photomixers — provided that their gain dynamics are sufficiently fast — producing microwaves of high spectral purity directly inside the laser cavity. The physical process underlying the microwave generation consists in the beating among neighboring optical modes of the standing wave cavity, which induces a dynamic electronic grating with opposite ends oscillating nearly in antiphase at the beat frequency (Fig. 1a). Such mechanism was used to demonstrate a quadrature modulation scheme exploiting the alternating currents oscillating inside the laser and a system of near-field microwave probes Piccardo et al. (2018a).

In light of this phenomenon the laser can be viewed from a new perspective, namely as an ensemble of two radio frequency generators with opposite phase. Usually the top electrode of these lasers consists of an electrically continuous metal contact connecting the two generators and thus preventing the device from radiating. In this work we demonstrate that by adapting the geometry of the top contact layer of a QCL allows us to feed a dipole antenna integrated on the chip, enabling emission of radio waves into free space. The beatnote frequency itself can be tuned by modulating the laser current, thus the laser acquires a new functionality, turning into a radio transmitter capable of wireless communication at a carrier frequency of 5.5 GHz, given by the comb repetition rate. Thanks to their fast gain recovery dynamics Choi et al. (2008), QCLs have the potential of generating sub-terahertz carriers when operating in the harmonic comb regime with a wide intermodal spacing Kazakov et al. (2017). The extension of the design presented here to realize a new class of terahertz wireless communication devices Nagatsuma et al. (2016); Akyildiz et al. (2014); Kleine-Ostmann and Nagatsuma (2011) will be discussed.

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 Hugi et al. (2012) in the mid-infrared spectral range with a narrow (kilohertz) linewidth beat note at $f_{B}=5.5$ GHz (Fig. 1b). A 400 $\mu$m wide gap is etched in the top contact layers of the device creating two contact sections with an open-circuit resistance of 250 $\Omega$. 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 (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 set-up is shown in Fig. 1c. The LRT is mounted on a rotation stage and the microwave radiation emitted at angles between $-90^{\circ}$ and $90^{\circ}$ on the horizontal plane is detected by a directive horn antenna (gain 18.5 dBi) at a distance of 0.9 m (16 $\lambda_{B}$) from the source, amplified and then measured with a spectrum analyzer (Supplementary Sec. 1). Fig. 1d shows the measured radiation pattern of the device (continuous line). The central peak emitted around the normal to the surface of the LRT originates from the dipole antenna and QCL, while the side lobes are due to emission from the wirebonds (Supplementary Sec. 2). 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. Even though 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 Supplementary Sec. 3. The antenna is connected to the QCL via wirebonds, whose inductive behavior 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 (Supplementary Sec. 3). 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 (Supplementary Sec. 4). On the other hand, the source impedance mismatch due to the pad admittance can be corrected using a buried heterostructure geometry with a Fe-doped InP insulating layer Nida et al. (2017), promising to improve the extraction performances both at microwave and sub-terahertz frequencies, as the capacitive pad admittance would be substantially reduced.

Next, we provide a proof-of-concept of wireless communication using the laser radio transmitter. The experimental set-up 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 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 (1.9-5.5 GHz bandwidth), amplified and then downconverted to 1.5 GHz by mixing with a local oscillator (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 $\Delta I$ 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 $\Delta I/I=0.2\%$. 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 $\Delta I$. 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; see Supplementary Material for the audio file). 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 (Supplementary Sec. 5), which lies 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, though this was limited in the present work by the bandwidth of the software demodulator. While the present 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 Maineult et al. (2010); Gellie et al. (2010); Calvar et al. (2013); Hinkov et al. (2016). 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. In particular, at gigahertz modulation frequencies, we expect that the laser beat note will be predominantly amplitude modulated by plasma effects, as the cut-off frequency of thermal effects responsible for frequency modulation lies in the megahertz range Hangauer et al. (2014).

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 schematic of the set-up is shown in Fig. 3a. A local oscillator (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, whose power is swept between -30 and 24 dBm for a set of different frequencies. Examples of the behavior exhibited by the QCL beat note upon 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 Gellie et al. (2010). The increase of the power threshold with the detuning from $f_{0}$ follows a quadratic dependence, as expected from injection-locking theory Siegman (1986); Adler (1973). 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 new 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 utilized as a wireless radio transmitter. Thanks to the recently discovered harmonic comb operation of QCLs Kazakov et al. (2017); Piccardo et al. (2018a) — where the intermodal comb spacing lies 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 sub-terahertz 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 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 Piccardo et al. (2018b). 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 Piccardo et al. (2018a). Adapting the antenna 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, travelling- and standing-wave antenna designs, which have already been proven successful in cryogenically-cooled terahertz QCL systems Tavallaee et al. (2011); Masini et al. (2017), or surface grating outcouplers, used in difference-frequency-generation terahertz QCLs Pflugl et al. (2008); Kim et al. (2018), could be used to efficiently radiate the generated sub-terahertz signal. Besides increasing the carrier wave frequency, the harmonic state offers a spectrum constituted by 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 as compared to QCLs operating in the fundamental comb regime. 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 (Supplementary Sec. 4).

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 new radio frequency source can generate tones of high spectral purity, leading to a very narrow (kHz or sub-kHz) 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, as compared to existing composite terahertz wireless communication systems Nagatsuma et al. (2016), unifying in a single device the capability of room-temperature generation, modulation and emission of sub-terahertz waves, and may find applications in fields ranging from telecommunications and spectroscopy, to radioastronomy and quantum optics.

This work was supported by the National Science Foundation (Grants No. ECCS-1614631, 1541959, DGE1144152), Assistant Secretary of Defense for Research and Engineering (Air Force Contract FA8721-05-C-0002 and/or FA8702-15D-0001), Swiss National Science Foundation (177836), Austrian Science Fund (P28914−N27). We thank D. Kazakov for discussions that motivated this demonstration and for a careful reading of the manuscript, and A. Amirzhan for useful discussions.