Danny Eliyahu

RF Amplitude and Phase-Noise Reduction of an Optical Link and an Opto-Electronic Oscillator

In this paper we examine the optical sources of noise that degrade high-performance microwave photonic links. In particular, we study the residual phase noise due to laser frequency fluctuations and the detector nonlinearity on microwave signals transmitted on an optical fiber, or generated in the opto-electronic oscillator (OEO). Based on experimental findings, we identify a significant reduction of the relative intensity noise of the laser if the received optical power saturates the photodiode. Furthermore, we suggest the use of a semiconductor optical amplifier in saturation as yet another means to reduce the phase noise induced by laser intensity fluctuations. We also identify the use of multiple photodetectors to reduce the influence of associated 1/f noise, which adds to the phase noise of a transmitted microwave signal, and is the ultimate limitation to the phase noise of the high-performance OEO. Reduction of noise that is due to optical interferences is also addressed.

High spectral purity Kerr frequency comb radio frequency photonic oscillator

Femtosecond laser-based generation of radio frequency signals has produced astonishing improvements in achievable spectral purity, one of the basic features characterizing the performance of an radio frequency oscillator. Kerr frequency combs hold promise for transforming these lab-scale oscillators to chip-scale level. In this work we demonstrate a miniature 10 GHz radio frequency photonic oscillator characterized with phase noise better than −60 dBc Hz−1 at 10 Hz, −90 dBc Hz−1 at 100 Hz and −170 dBc Hz−1 at 10 MHz. The frequency stability of this device, as represented by Allan deviation measurements, is at the level of 10−10 at 1–100 s integration time—orders of magnitude better than existing radio frequency photonic devices of similar size, weight and power consumption.

Tunable, ultra-low phase noise YIG based opto-electronic oscillator

We describe a YIG tuned opto-electronic oscillator with extremely low phase noise. The oscillator can be tuned from 6 to 12 GHz in steps of 3 MHz, and exhibits a phase noise of -128 dBc/Hz at 10 KHz away from the carrier. This is nearly a 30 dB improvement over conventional YIG oscillators, and is derived from the novel approach of the opto-electronic oscillator. To our knowledge, this is the lowest noise performance of any YIG tuned oscillator previously reported.

Modulation response (S21) of the coupled opto-electronic oscillator

We report on the derivation of an analytical model for the modulation response (S21) of an open RF loop for the coupled opto-electronic oscillator (COEO). The small signal linearized model takes into account the effect of positive feedback in the optical loop of the COEO, and predicts the associated Q enhancement. Other effects, such as mode matching between the loop modes and the microresonator modes, and the effect of linear cavity configuration, are studied as well. The model is backed up by experimental results. Phase noise of a short fiber COEO is also presented

Ultralow noise miniature external cavity semiconductor laser

Advanced applications in optical metrology demand improved lasers with high spectral purity, in form factors that are small and insensitive to environmental perturbations. While laboratory-scale lasers with extraordinarily high stability and low noise have been reported, all-integrated chip-scale devices with sub-100 Hz linewidth have not been previously demonstrated. Lasers integrated with optical microresonators as external cavities have the potential for substantial reduction of noise. However, stability and spectral purity improvements of these lasers have only been validated with rack-mounted support equipment, assembled with fibre lasers to marginally improve their noise performance. In this work we report on a realization of a heterogeneously integrated, chip-scale semiconductor laser featuring 30-Hz integral linewidth as well as sub-Hz instantaneous linewidth.

Phase noise of a high performance OEO and an ultra low noise floor cross-correlation microwave photonic homodyne system

This paper describes two recent types of opto-electronic oscillators. The first is a long fiber opto-electronic oscillator, utilizing a high power laser with long delay, and consisting of low noise components. This oscillator generates a stable 10 GHz signal with phase noise of -163 dBc/Hz at 6 kHz offset from the carrier. The second is a low noise 10 GHz compact opto-electronic oscillator. This latter oscillator consists of coupled optical and microwave loops utilizing a short fiber. We also report on an automatic ultra-low noise floor measurement system, designed and built to measure the phase noise of the above (and other) oscillators. This delay line cross-correlation measurement system utilizes microwave-photonic links, eliminating the need for a second oscillator. This system provides quick and reliable measurement of the oscillator under test.

Effect of noise on the power spectrum of passively mode-locked lasers

We analyze the effects of noise on the power spectrum of pulse trains generated by a continuously operating passively mode-locked laser. The shape of the different harmonics of the power spectrum is calculated in the presence of correlated timing fluctuations between neighboring pulses and in the presence of amplitude fluctuations. The spectra at the different harmonics are influenced mainly by the nonstationary timing-jitter fluctuations; amplitude fluctuations slightly modify the spectral tails. Estimation of the coupling term between the longitudinal cavity modes or the effective saturable absorber coefficient is made from the timing-jitter correlation time. Experimental results from an external cavity two-section semiconductor laser are given. The results show timing-jitter fluctuations having a relaxation time much longer than the repetition period.

Stabilization of a Kerr frequency comb oscillator

We study stability and spectral purity of a microresonator-based Kerr frequency comb oscillator experimentally and observe a correlation between the frequency of the continuous wave laser pumping the nonlinear resonator and the repetition frequency of the comb. This correlation is used in a proof-of-principle demonstration of a Kerr frequency comb stabilized with an optical transition of 87Rb.

Optoelectronic oscillator with improved phase noise and frequency stability

In this paper we report on recent improvements in phase noise and frequency stability of a 10 GHz opto-electronic oscillator. In our OEO loop, the high Q elements (the optical fiber and the narrow bandpass microwave filter) are thermally stabilized using resistive heaters and temperature controllers, keeping their temperature above ambient. The thermally stabilized free running OEO demonstrates a short-term frequency stability of 0.02 ppm (over several hours) and frequency vs. temperature slope of −0.1 ppm/°C (compared to -8.3 ppm/°C for non thermally stabilized OEO). We obtained an exceptional spectral purity with phase noise level of -143 dBc/Hz at 10 kHz of offset frequency. We also describe the multi-loop configuration that reduces dramatically the spurious level at offset frequencies related to the loop round trip harmonic frequency. The multi-loop configuration has stronger mode selectivity due to interference between signals having different cavity lengths. A drop of the spurious level below −90 dBc was demonstrated. The effect of the oscillator aging on the frequency stability was studied as well by recording the oscillator frequency (in a chamber) over several weeks. We observed reversal in aging direction with logarithmic behavior of A ln(B t+1)-C ln(D t+1), where t is the time and A, B, C, D are constants. Initially, in the first several days, the positive aging dominates. However, later the negative aging mechanism dominates. We have concluded that the long-term aging behavioral model is consistent with the experimental results.

Noise characterization of a pulse train generated by actively mode-locked lasers

We analyze the entire power spectrum of pulse trains generated by a continuously operating actively mode-locked laser in the presence of noise. We consider the effect of amplitude, pulse-shape, and timing-jitter fluctuations that are characterized by stationary processes. Effects of correlations between different parameters of these fluctuations are studied also. The nonstationary timing-jitter fluctuations of passively mode-locked lasers and their influence on the power spectrum is discussed as well.