Optical Phase Locking techniques: an overview and a novel method based on Single Side Sub-Carrier modulation

A short overview on Optical Phase locking techniques and a detailed description of the Phase Locking technique based on Sub-Carriers modulation is presented. Furthermore, a novel Single Side Sub-Carrierbased Optical Phase Locked Loop (SS-SC-OPLL), with off the shelf optical components, is also presented and experimentally demonstrated. Our new method, based on continuous wave semiconductor lasers and optical single side sub-carrier modulation using QPSK LiNbO3 modulator, allows a practical implementation with better performance with respect to the previously proposed OPLL circuits, and permits an easy use in real time WDM signal coherent demodulation

A Sub-Picosecond Hybrid DLL for Large-Scale Phased Array Synchronization

A large-scale timing synchronization scheme for scalable phased arrays is presented. This approach utilizes a DLL co-designed with a subsequent 2.5GHz PLL. The DLL employs a low noise, fine/coarse delay tuning to reduce the in-band rms jitter to 323fs, an order of magnitude improvement over previous works at similar frequencies. The DLL was fabricated in a 65nm bulk CMOS process and was characterized from 27MHz to 270MHz. It consumes up to 3.3mW from a 1V power supply and has a small footprint of 0.036mm^2

Frequency Stabilization of 4.7 THz Quantum Cascade Lasers

Heterodyne receivers for astronomy observing at 4.7 THz use Quantum Cascade Lasers (QCLs) as the local oscillator. QCLs are compact, powerful, and easy to use but prone to frequency instability from current noise, temperature fluctuations, and optical feedback, and they also lack absolute frequency reference. This work presents several solutions to this problem. A heterodyne laboratory receiver at 4.7 THz has been developed to host all the experiments. The receiver shows an uncorrected receiver noise temperature of around 5000K at an IF frequency of 5.1 GHz and a total power Allan stability time of around 500 seconds. With this receiver, measuring methanol's emission lines helped to frequency calibrate one of the QCLs. The first experiment uses a methanol absorption line for frequency discrimination and a Hot Electron Bolometer (HEB) as a total power detector. In an active control loop, the experiment locks the laser's frequency to the dip of the absorption line. This method applies a known modulation to the QCL's frequency, dominating the QCL's linewidth with an FWHM of 2.1 MHz. The second and the third experiments down-convert the QCL's frequency with a Superlattice Device (SLD) harmonic generator and mixer. A diode multiplier chain produces a 182.5 GHz signal to pump the SLD. The 26th harmonic is generated and mixed with the 4.7 THz QCL signal. The resulting IF signal at the SLD's output is 10 dB over the noise floor. The second experiment feeds the IF signal into a delay line frequency discriminator to produce the QCL frequency's error information. A power divider divides the amplified and filtered SLD's IF into two. Only one is delayed in 10 meters of coax cable, and homodyne mixing produces a DC voltage as a function of the QCL's frequency. The control electronics turn this into a correction current to the QCL. This method stabilized the QCL with more than 10 MHz of frequency deviations, to an FWHM of 780 kHz, for hours. The experiment even works at very low signal-to-noise conditions, such as 2 dB, and revealed that the optical feedback is the dominant QCL line broadening mechanism caused by the pulse tube refrigerator's forced motions. The third experiment uses a Phase Locked Loop (PLL). A phase detector compares QCL's phase with a reference, producing an error voltage. The loop filter transfers this to a correcting current, compensating the QCL's frequency disturbances. Phase locking is more challenging than frequency locking, and an exact understanding had to be developed on transfer functions and noise modeling. The experiment locked the QCL's phase, reducing the linewidth FWHM from 1.4 MHz to less than 7 kHz, stable for half an hour, and occasionally losing the lock for a few seconds. With the demonstrated works, it is possible to stabilize the frequency of 4.7 THz QCLs

Variable domain transformation for linear PAC analysis of mixed-signal systems

This paper describes a method to perform linear AC analysis on mixed-signal systems which appear strongly nonlinear in the voltage domain but are linear in other variable domains. Common circuits like phase/delay-locked loops and duty-cycle correctors fall into this category, since they are designed to be linear with respect to phases, delays, and duty-cycles of the input and output clocks, respectively. The method uses variable domain translators to change the variables to which the AC perturbation is applied and from which the AC response is measured. By utilizing the efficient periodic AC (PAC) analysis available in commercial RF simulators, the circuit’s linear transfer function in the desired variable domain can be characterized without relying on extensive transient simulations. Furthermore, the variable domain translators enable the circuits to be macromodeled as weakly-nonlinear systems in the chosen domain and then converted to voltage-domain models, instead of being modeled as strongly-nonlinear systems directly

Reducing the linewidth of a diode laser below 30 Hz by stabilization to a reference cavity with finesse above 10^5

An extended cavity diode laser operating in the Littrow configuration emitting near 657 nm is stabilized via its injection current to a reference cavity with a finesse of more than 10^5 and a corresponding resonance linewidth of 14 kHz. The laser linewidth is reduced from a few MHz to a value below 30 Hz. The compact and robust setup appears ideal for a portable optical frequency standard using the Calcium intercombination line.Comment: 8 pages, 4 figures on 3 additional pages, corrected version, submitted to Optics Letter

Fractional-N DLL for clock synchronization

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