Benjamin Rudin

Optically generated microwave signals exhibit some of the lowest phase noise and timing jitter of any microwave-generating technology to date. The success of octave-spanning optical frequency combs in down-converting ultrastable optical frequency references has motivated the development of compact, robust and highly manufacturable optical systems that maintain the ultralow microwave phase noise of their tabletop counterparts. Two-point optical frequency division using chip-scale components and ~1 THz-spanning microcombs has been quite successful, but with stringent requirements on the comb source’s free-running noise and feedback control dynamics. Here we introduce a major simplification of this architecture that replaces feedback control of the frequency comb in favor of electronic feedforward noise cancelation that significantly relaxes the comb requirements. Demonstrated with both a high repetition rate solid-state mode-locked laser and a microcomb, feedforward on a 10 GHz carrier results in more robust operation with phase noise as low as -153 dBc/Hz at offsets ≥10 kHz, femtosecond timing jitter, and elimination of the large “servo bump” noise increase at high offset frequency. The system’s compatibility with a variety of highly manufacturable mode-locked laser designs and its resilience and straightforward implementation represents an important step forward towards a fully chip-scale implementation of optically generated microwaves, with applications in radar, sensing, and position, navigation and timing.

Due to their low phase noise at high carrier frequencies, photonic microwave oscillators are continuously expanding their application areas including digital signal processing, telecommunications, radio astronomy, and RADAR and LIDAR systems. Currently, the lowest noise photonic oscillators rely on traditional optical frequency combs with multiple stabilization loops that incorporate large vacuum components and complex optoelectronic configurations. Hence, the resulting systems are not only challenging to operate but also expensive to maintain. Here, we introduce a significantly simpler solution: a Photonically Referenced Extremely STable Oscillator (PRESTO). PRESTO requires only three key components: a femtosecond laser, a fiber delay element, and a pulse timing detector. The generated microwave at 10 GHz has phase noise levels of −125, −145, and <−160 dBc/Hz at 1, 10, and >100 kHz, respectively, with an integrated timing jitter of only 2 fs root mean square (RMS) over [100 Hz–1 MHz]. This approach offers a reliable solution for simplifying and downsizing photonic oscillators while delivering high performance.

Wafer-scale, nanoimprint lithography-based approaches for manufacturing of high-efficiency transmission gratings at telecom wavelengths are reported. Two microns-deep, binary gratings are thereby fabricated and combined with a subwavelength, antireflective structure to achieve a cost-efficient and reliable manufacturing process. Diffraction efficiencies of 92% are experimentally achieved in the Littrow configuration. These gratings are used to compress 8 picosecond pulses with 1W of average output power at central wavelength of 1555nm to pulse duration of 378 femtoseconds

Photoconductive emitters and receivers are widely accepted as the best combination for applications requiring broadband and high dynamic range and are nowadays deployed in most commercially available systems. Novel laser sources with higher repetition rate and power levels are a promising route towards further improvements in this area. We present our first steps in this direction by combining state-of-the-art emitters and receivers with an ultra-stable commercial fs laser (MENHIR-1550 SERIES) at 1 GHz repetition rate as the optical source. The output of the laser is amplified and compressed by a commercial fiber amplifier setup. In this experiment, we use 17 mW as the probe beam and 30 mW as the pump beam with a pulse duration of 150 fs, as these are the best operation points for the emitter and receiver available. The emitter is based on iron doped InGaAs in a strip line geometry with an active region of 50 μm x 50 μm while a fiber coupled dipole antenna with a 10 μm gap is used as the receiver. We demonstrate a 1 GHz repetition rate terahertz time-domain spectroscopy (THz-TDS) system with a dynamic range of 73 dB and a bandwidth of 3.5 THz using state-of-the-art THz photoconductive emitter and receiver with a measurement time of 60 s. This result is part of a larger effort to understand the compromises to be realized in terms of repetition rate and average power to take photoconductive emitters and receivers to the next step in dynamic range enhancement.

We demonstrate THz generation with a low noise GHz repetition rate laser with state-of-the-art photoconductive emitters. We measure a maximum dynamic range of 73 dB and a bandwidth of more than 3 THz.

Low noise and high repetition rate optical frequency combs are desirable for many applications, from timekeeping to precision spectroscopy. For example, gigahertz repetition rate sources greatly increase the acquisition speed of spectra in a dual-comb modality when compared to lower repetition rate sources, while still maintaining sufficient instantaneous resolution to resolve ro-vibrational signatures from molecules in a variety of conditions. In this paper, we present the stabilization and characterization of a turnkey commercial 1 GHz mode-locked laser that operates at telecom wavelengths (1.56 µm). Fiber amplification and spectral broadening result in high signal-to-noise ratio detection and stabilization of f _ceo with 438 mrad of residual phase noise (integrated from 10² to 10⁷ Hz). Simultaneously, we stabilize the beatnote between the nearest comb mode and a cavity stabilized continuous-wave laser at 1.55 µm with 41 mrad of residual phase noise (integrated from 10² to 10⁷ Hz). This robust, self-referenced comb system is built with off-the-shelf polarization-maintaining fiber components and will be useful for a wide range of low noise frequency comb applications that benefit from the increased repetition rate.

We present timing jitter measurements of an InGaAs quantum well vertical external cavity surface emitting laser (VECSEL) passively mode locked with a quantum dot semiconductor saturable absorber mirror (SESAM) at 2-GHz repetition rate. It generates 53-mW average output power in 4.6-ps pulses at 953 nm. The laser housing was optimized for high mechanical stability to reduce acoustic noise. We use a fiber-coupled multimode 808-nm pump diode, which is mounted inside the laser housing. No active cavity length stabilization is employed. The phase noise of the free-running laser integrated over a bandwidth from 100 Hz to 1 MHz corresponds to an RMS timing jitter of ≈212 fs, which is lower than previously obtained for mode-locked VECSELs. This clearly confirms the superior noise performance expected from a high-Q-cavity semiconductor laser. In contrast to edge-emitting semiconductor diode lasers, the cavity mode is perpendicular to the quantum well gain layers, which minimizes complex dispersion and nonlinear dynamics.

The authors present a 10-GHz mode-locked integrated external cavity surface emitting laser (MIXSEL) with 2.2 W average power in 21 ps pulses, which is to the authors knowledge, higher than any other mode locked semiconductor laser above 10 GHz. The circular output beam had excellent transverse mode quality (M ² <; 1.05). The simple straight cavity contains only two intracavity elements, the MIXSEL chip and the external output coupler.

We present timing jitter measurements of a free-running SESAM modelocked VECSEL generating 8-ps pulses with 1.88-GHz repetition rate and 80-mW average output power. We observed very good performance comparable with iondoped solid-state-lasers which typically show excellent stability. We measured the two-sided noise power spectral density at the 10th harmonic of the laser output with the von der Linde method. The rms timing jitter integrated over an offset frequency range from 100 Hz to 100 kHz gives a free-running timing jitter of ≈400 fs which is an upper limit because the measurement was already system noise limited above 10 kHz.