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Register a new userUltra-Low Noise Microwave Extraction from Fiber-Based Optical Frequency Comb
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In this letter, we report on all-optical fiber approach to the generation of ultra-low noise microwave signals. We make use of two erbium fiber mode-locked lasers phase locked to a common ultra-stable laser source to generate an 11.55 GHz signal with an unprecedented relative phase noise of -111 dBc/Hz at 1 Hz from the carrier.The residual frequency instability of the microwave signals derived from the two optical frequency combs is below 2.3 10^(-16) at 1s and about 4 10^(-19) at 6.5 10^(4)s (in 5 Hz bandwidth, three days continuous operation).
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We demonstrate a remote microwave/radio-frequency (RF) transfer technique based on the stabilization of a fiber link using a fiber-loop optical-microwave phase detector (FLOM-PD). This method compensates for the excess phase fluctuations introduced in fiber transfer by direct phase comparison between the optical pulse train reflected from the remote site and the local microwave/RF signal using the FLOM-PD. This enables sub-fs resolution and long-term stable link stabilization while having wide timing detection range and less demand in fiber dispersion compensation. The demonstrated relative frequency instability between 2.856-GHz RF oscillators separated by a 2.3-km fiber link is $7.6 times 10^{-18}$ and $6.5 times 10^{-19}$ at 1000 s and 82500 s averaging time, respectively.
In this paper, we present a very simple design based on commercial devices for the all-optical generation of ultra-low phase noise microwave signals. A commercial, fibered femtosecond laser is locked to a laser that is stabilized to a commercial ULE Fabry-Perot cavity. The 10 GHz microwave signal extracted from the femtosecond laser output exhibits a single sideband phase noise $mathcal{L}(f)=-104 mathrm{dBc}/mathrm{Hz}$ at 1 Hz Fourier frequency, at the level of the best value obtained with such microwave photonics laboratory experiments cite{Fortier2011}. Close-to-the-carrier ultra-low phase noise microwave signals will now be available in laboratories outside the frequency metrology field, opening up new possibilities in various domains.
We describe a Yb-fiber based laser comb, with a focus on the relationship between net-cavity dispersion and the frequency noise on the comb. While tuning the net cavity dispersion from anomalous to normal, we measure the amplitude noise (RIN), offset frequency (f_CEO) linewidth, and the resulting frequency noise spectrum on f_CEO. We find that the laser operating at zero net-cavity dispersion has many advantages, including an approximately 100x reduction in free-running f_CEO linewidth and frequency noise power spectral density between laser operation at normal and zero dispersion. In this latter regime, we demonstrate a phase-locked f_CEO beat with low residual noise.
A compact and robust coherent laser light source that provides spectral coverage from the ultraviolet to infrared is desirable for numerous applications, including heterodyne super resolution imaging[1], broadband infrared microscopy[2], protein structure determination[3], and standoff atmospheric trace-gas detection[4]. Addressing these demanding measurement problems, laser frequency combs[5] combine user-defined spectral resolution with sub-femtosecond timing and waveform control to enable new modalities of high-resolution, high-speed, and broadband spectroscopy[6-9]. In this Letter we introduce a scalable source of near-single-cycle, 0.56 MW pulses generated from robust and low-noise erbium fiber (Er:fiber) technology, and we use it to generate a frequency comb that spans six octaves from the ultraviolet (350 nm) to mid-infrared (22500 nm). The high peak power allows us to exploit the second-order nonlinearities in infrared-transparent, nonlinear crystals (LiNbO$_3$, GaSe, and CSP) to provide a robust source of phase-stable infrared ultra-short pulses with simultaneous spectral brightness exceeding that of an infrared synchrotron[10]. Additional cascaded second-order nonlinearities in LiNbO$_3$ lead to comb generation with four octaves of simultaneous coverage (0.350 to 5.6 $mu$m). With a comb-tooth linewidth of 10 kHz at 193 THz, we realize a notable spectral resolving power exceeding 10$^{10}$ across 0.86 PHz of bandwidth. We anticipate that this compact and accessible technology will open new opportunities for multi-band precision spectroscopy, coherent microscopy, ultra-high sensitivity nanoscopy, astronomical spectroscopy, and precision carrier-envelope phase (CEP) stable strong field phenomena.
We propose a method to generate stabilized radio-frequency polarization modulation based on optical frequency combs. Two pulse trains with the same repetition rate and different offset frequencies generate arbitrary polarization states that are modulated at the offset frequency difference. Long-term stability of the polarization modulation is demonstrated with the modulation frequency at frep/2. Modulation at frep/4 is also demonstrated to show the flexibility of the technique. We employ an electrical delay line to fine-tune the polarization states that constitute the time-dependent modulation.
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