Continuously Tunable Ultra-Broadband Low-Phase Noise Photonic Terahertz Oscillator

Oscillators with low phase noise are key to applications such as high-speed wireless communication [1], channel sounding [2] and high-resolution spectroscopy [3]. Achieving a low phase noise at terahertz frequencies (100 GHz to 10 THz) is challenging due to limitations in conventional electronic oscillators. In this paper, we present a photonic technique that employs an optical frequency comb to establish a coherent link between two cw lasers. This allows us to generate terahertz waves with significantly lower phase noise than reported to date for any tunable terahertz oscillator. The novelty of this work lies in the fact that the applied technique enables a seamless, mode-hop-free, and linear frequency sweep across multiple comb lines. Hence, the comb provides a continuous coherent link between the lasers. As a result, the low phase noise of the beat note is preserved across the entire tuning range of the system [4]. The difference frequency of the laser system can be continuously tuned from 0 to 10 THz [5]. The usable terahertz bandwidth is only limited by the response of the THz emitter and receiver photodiodes. In previous work, we have demonstrated signal generation at frequencies up to 6.5 THz [6].

Several recent publications report phase noise performance in terahertz wave generation. Shin et al. [7] demonstrated a system based on locking continuous-wave (CW) lasers to a frequency comb. However, their system lacks straightforward tunability, and the phase noise characterization is conducted relative to their own frequency comb, which serves as a common-mode reference. Consequently, their results are not directly comparable to ours, where we measure absolute phase noise. Djevahirdjian et al. [8] employed locking of CW lasers to a shared optical cavity. While their reported phase noise at 560 GHz is close to the scaled phase noise of the electronic oscillator (R&S SMB100A) used for comparison, it does not match the performance level achieved by our approach, and their system is also not readily tunable. Kuse et al. [9] presented a dual comb system, an approach involving mutual locking of two Kerr frequency combs. Their phase noise performance is inferior to ours, and the maximum generated frequency remains unclear, with reported measurements up to 560 GHz.

Figure 1 shows a schematic of the terahertz oscillator and the phase noise analysis equipment. The photonic terahertz oscillator source is based on two diode lasers (2 × CTL 1550), stabilized to a single offset-free frequency comb (DFC CORE 200 +, TOPTICA Photonics AG). We use an ultra-low-noise fiber laser as an optical reference for stabilization of the repetition rate of the difference frequency comb. The frequency of the first diode laser (Laser 1 in Fig. 1) remains fixed as it is phase-locked to one of the comb modes. For the lock of the tunable laser (Laser 2), the comb frequency is externally shifted while preserving the optical-phase-locked loop at a fixed offset [10]. The light of the two diode lasers is superimposed in a fiber splitter to create a beat signal that oscillates at the difference frequency of the two lasers, i.e., in the terahertz range. An ultrafast PIN photodiode with an integrated bow-tie antenna then converts the optical beat signal into a terahertz wave that is radiated into free space. A more detailed description of the photonic terahertz source can be found in [11, 12].

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At the detector side, we capture the terahertz signal via a horn antenna and split the signal into two paths using a 3 dB WM-570 rectangular waveguide coupler. Each signal is then fed into two separate RF-frequency converters (Rohde&Schwarz GmbH & Co. KG, R&S ZC500) driven by independent ultra-low phase noise oscillators (R&S SMA100B with B711N option) to create intermediate frequencies (IF) at a few 10 MHz. A phase noise analyzer (R&S FSWP50) receives the signal from both converters at the designated mixer inputs. Via cross-correlation, the phase noise analyzer averages the phase noise contributions of both independent combinations of R&S ZC500 converters and R&S SMA100B oscillators, which enables us to quantify the phase noise contribution of the photonically generated terahertz signal. This detection scheme is particularly useful, if the phase noise of the generator under test is expected to be lower than the phase noise of the local oscillators used in the detector unit, as is the case here [13, 14].

The difference frequency of the comb-locked laser system is seamlessly tunable from 0 to 10 THz [5]. However, the usable frequency range is limited by the bandwidth of the THz emitter and receiver PIN diodes, which support the generation and detection of terahertz signals up to 6.5 THz [6]. Phase noise measurements were conducted within the Y-band, specifically at 330 GHz, 415 GHz, and 500 GHz, corresponding to the operational range of the detection system.

Figure 2 (left) illustrates the phase noise performance of the photonic-based oscillators tuned to 330 GHz (red), 415 GHz (orange), and 500 GHz (blue), alongside the repetition rate noise of the difference frequency comb measured at 9.6 GHz with an optical frequency division microwave signal (gray) and the phase noise of the optical reference laser (green). To facilitate comparison, two y-axes are used: the left axis corresponds to a carrier frequency of 500 GHz, while the right axis shows the same data scaled to 9.6 GHz.

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All three photonic oscillators exhibit nearly identical scaled phase noise profiles across most of the offset frequency range, i.e., quadratic noise scaling up to 10³ Hz and additive noise for > 10³ Hz at the band edges and in the center, indicating consistent performance regardless of output frequency. Up to approximately 2 kHz offset, the phase noise of the photonic signals closely follows that of the frequency comb, which itself tracks the phase noise of the optical reference. This demonstrates that within the locking bandwidth, the phase stability of the generated THz signals is fundamentally limited by the quality of the optical reference. Consequently, employing a more advanced optical reference—such as a cavity-stabilized laser with sub-Hz linewidth—would directly translate into improved phase noise performance of the entire system.

Figure 2 (right) presents the phase noise performance of the photonic-based terahertz oscillator operating at 330 GHz, comparing measurements with (red curve) and without (blue curve) the cross-correlation technique. Also shown are the phase noise characteristics of a high-performance electronic oscillator (R&S®SMA100B, dark gray), measured at 20 GHz and scaled to 330 GHz, as well as reference data from prior literature [8, 9]. Across most of the frequency offset range—from a few Hz up to over 10⁴ Hz—the photonic oscillator demonstrates up to 10 dB lower phase noise compared to the electronic source. Only at higher offset frequencies around 10⁵ Hz does the phase noise of the photonic oscillator exceed that of the scaled electronic oscillator. This rise corresponds to the locking bandwidth (~ MHz) of the diode lasers involved in the setup. Additionally, the phase noise of the comb's repetition rate, measured at 9.6 GHz, closely follows the noise characteristics of the underlying fiber laser. This indicates that the frequency comb serves as an exceptionally stable phase-coherent link, effectively transferring the optical reference’s phase stability to the generated tunable THz signal with minimal degradation. Reference values from the literature generally exhibit inferior performance over most of the offset frequency range, only approaching comparable phase noise levels above 10⁴ Hz. It is important to note, however, that these reference values pertain to fixed-frequency or non-tunable oscillators, whereas the demonstrated photonic system offers both superior noise performance and tunability.

We have determined the phase noise of continuously tunable, broadband, photonics-based terahertz oscillators and compared the results to an ultra-low-noise electronic signal generator and reference values obtained from the literature. In terms of phase noise, the photonic oscillator outperforms the electronic oscillator up to 3 × 10⁴  Hz offset. Our results present the lowest phase noise reported so far for a tunable broadband terahertz source. Future work will focus on further improvements at frequency offsets above 10⁵ Hz.