Conversion Of Laser Phase Noise Research Articles

Ring resonator-based on-chip modulation transformer for high-performance phase-modulated microwave photonic links

In this paper, we propose and experimentally demonstrate a novel wideband on-chip photonic modulation transformer for phase-modulated microwave photonic links. The proposed device is able to transform phase-modulated optical signals into intensity-modulated versions (or vice versa) with nearly zero conversion of laser phase noise to intensity noise. It is constructed using waveguide-based ring resonators, which features simple architecture, stable operation, and easy reconfigurability. Beyond the stand-alone functionality, the proposed device can also be integrated with other functional building blocks of photonic integrated circuits (PICs) to create on-chip complex microwave photonic signal processors. As an application example, a PIC consisting of two such modulation transformers and a notch filter has been designed and realized in TriPleX(TM) waveguide technology. The realized device uses a 2 × 2 splitting circuit and 3 ring resonators with a free spectral range of 25 GHz, which are all equipped with continuous tuning elements. The device can perform phase-to-intensity modulation transform and carrier suppression simultaneously, which enables high-performance phase-modulated microwave photonics links (PM-MPLs). Associated with the bias-free and low-complexity advantages of the phase modulators, a single-fiber-span PM-MPL with a RF bandwidth of 12 GHz (3 dB-suppression band 6 to 18 GHz) has been demonstrated comprising the proposed PIC, where the achieved spurious-free dynamic range performance is comparable to that of Class-AB MPLs using low-biased Mach-Zehnder modulators.

Resolution enhancement in noise spectrum by using velocity selective optical pumping in cesium vapor

We demonstrate experimentally that the resolution of amplitude noise spectrum in Cs atomic vapor can be enhanced by narrowing the absorption using velocity selective optical pumping technique. It is found that the steep atomic dispersion accompanied by high absorption leads to more conversion of laser phase noise to amplitude noise, when the field propagates throughout the atoms, and meanwhile the spectral resolution is improved. The effect of optical pumping intensity on the spectrum resolution is experimentally discussed, and a theoretical explanation for this phenomenon is given, which shows that the phase-to-amplitude noise conversion is directly proportional to the dispersion of medium.

Electromagnetically induced transparency with noisy lasers

We demonstrate and characterize two coherent phenomena that can mitigate the effects of laser phase noise for electromagnetically induced transparency (EIT): a laser-power-broadening-resistant resonance in the transmitted intensity cross correlation between EIT optical fields, and a resonant suppression of the conversion of laser phase noise to intensity noise when one-photon noise dominates over two-photon-detuning noise. Our experimental observations are in good agreement with both an intuitive physical picture and numerical calculations. The results have wide-ranging applications to spectroscopy, atomic clocks, and magnetometers.

Laser-pumped atomic clock exploiting pressure-broadened optical transitions

The alkali-vapor-cell atomic clock, of either the conventional or the coherent population trapping type, offers one of the most viable approaches to making ultraminiature and chip-scale devices. Unfortunately, this atomic clock suffers from two laser-induced noise processes: conversion of laser phase noise (PM) to amplitude noise (AM) and ac-Stark-shift fluctuations. Here we demonstrate a method for circumventing these problems in a passive and miniaturizable manner based on pressure broadening of the relevant optical transitions. For this purpose we have constructed a conventional atomic clock employing Rb^87 vapor confined with 100 Torr of N2. In this relatively high-pressure environment, both PM-to-AM conversion efficiency and the ac-Stark shift are reduced. Though we employ a phase-noisy, single-mode diode laser and lock the laser frequency to the pressure-broadened 1.6-GHz D_1 absorption line of Rb, we obtain excellent short-term frequency stability [i.e., sigma_y(tau)=1.8 × 10^−12/tau^½]. Moreover, as a single resonance cell generates locking signals for both the laser wavelength and the crystal oscillator, the atomic clock has real potential for miniaturization.

Effect of Relative Intensity Noise in Optical Transmission Systems Employing Wavelength Conversion Based on XPM and MZI

The overall intensity noise spectrum due to laser relative intensity noise (RIN) and interferometric conversion of laser phase noise after passing through wavelength conversion (WC) based on cross-phase modulation (XPM) and Mach-Zehnder interferometer (MZI) is modeled along with the resulting noise at the direct-detection receiver. It is found that the effect of the laser-induced noise at the receiver is negligible when the MZI operated at quadrature and has a free spectral range (FSR) > 0.01 GHz. Only when FSR < 0.01 GHz is a power penalty induced that increases with decreasing laser current bias, j 0 (0.8 dB, j 0 = 2; 1.7 dB, j 0 = 1.5; 5.2 dB, j 0 = 1.1).

The interaction between fibre dispersion and laser phase noise in an ASK self-heterodyne millimetre-wave fibre-radio system

An analysis is presented of the BER achievable in an ASK modulated millimetre-wave self-heterodyne fibre radio system. The analysis shows that such systems require a depressed decision threshold if they are to exhibit optimum performance. This is due to the conversion of laser phase noise to intensity noise after the two optical carriers have experienced different delays due to propagation through the dispersive fibre. Results are presented that show the practicality of such systems over a very wide range of operating conditions.

Influence of fiber nonlinearity on the phase noise to intensity noise conversion in fiber transmission: theoretical and experimental analysis

Intensity noise resulting from the phase modulation to intensity modulation conversion of laser phase noise can be a major impairment in direct detection systems. In this paper, we investigate theoretically and experimentally the influence of fiber nonlinearity on the conversion of laser and optical amplifier phase noise to intensity noise by fiber transmission. Very good agreement of relative intensity noise (RIN) spectra at the output of a standard singlemode fiber between experimental data and theoretical predictions has been achieved. Results reveal that the fiber nonlinearity can enhance significantly the RIN magnitude and lead to a shift of the RIN dips toward higher frequencies, and consequently to a broader RIN spectrum at fiber output.

Conversion of laser phase noise to amplitude noise in an optically thick vapor

As laser light propagates through a resonant vapor, laser phase noise (PM) is converted to laser intensity noise (AM) because of the sensitivity of atomic coherence to laser phase fluctuations. In experiments reported here it is shown that this PM-to-AM conversion process is highly efficient and can cause the relative intensity noise of transmitted diode laser light to be 1 to 2 orders of magnitude larger than the laser’s intrinsic relative intensity noise. By use of a semiclassical description of the phenomenon, including the effect of optical pumping, reasonably good agreement between theory and experiment is obtained. The PM-to-AM conversion process discussed here has important consequences for atomic clock development, in which diode-laser optical pumping in thick alkali vapors holds the promise for orders-of-magnitude improvement in atomic clock performance.

Performance analysis of DPSK direct detection optical systems in the presence of interferometric intensity noise

The performance of DPSK direct detection optical systems using a dual detector receiver is analyzed by means of a Gauss quadrature rules method. The filtering effects on the intensity noise, resulting from the interferometric conversion of laser phase noise, and the influence of laser relaxation oscillations are also investigated. Linewidth requirements are estimated and compared with that of DPSK heterodyne systems. It is shown that linewidth requirements can be relaxed by a factor of about 0.56, when baseband filtering effects are taken into account.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Theoretical and experimental investigation of conversion of phase noise to intensity noise by Rayleigh scattering in optical fibers

The conversion of laser phase noise to intensity fluctuation noise in optical fibers due to mixing with Rayleigh scattered light is considered. A theory combining the laser quantum phase dynamics and the statistical scattering in the fiber leads to simple expressions for the spectral densities of the intensity fluctuations in a number of generic cases. These are compared with experiments involving distributed feedback semiconductor lasers and low-loss fibers with lengths up to 20 km.

Effects of resonance asymmetries on the conversion of laser phase noise into amplitude noise in fiber optic ring-resonators

Fiber optic ring resonators constructed from a continuous strand of single-mode fiber and an evanescent wave directional coupler can show an asymmetric resonance characteristic due to coupler losses. The effects of this asymmetry on the conversion of laser phase noise into amplitude noise is discussed. A mathematical model, valid for highly coherent signals, is used to analyze even high-finesse ring resonators with a minimum of CPU time. >

Effects of phase-to-intensity noise conversion by multiple reflections on gigabit-per-second DFB laser transmission systems

Large power penalties and bit-error-rate floors have been observed in some Gb/s systems using distributed feedback (DFB) lasers, which could be attributed to interferometric conversion of laser phase noise to intensity noise by multiple reflections at connectors and splices. The authors calculated the power spectral density of the interferometric noise and its impact on system performance as a function of both the magnitude and number of reflections, and they compare the theoretical predictions with experimental results. Their studies indicate that connectors and splices with return losses of more than about 25 dB are required for the reliable operation of Gb/s fiber transmission systems, even if optical isolators are used.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Degradations in Gbit/s DFB laser transmission systems due to phase-to-intensity noise conversion by multiple reflection points

Severe system degradations and bit error rate floors are observed in Gbit/s fibre transmission systems due to the interferometric conversion of laser phase noise to intensity noise by multiple reflections from connectors and splices. Studies of penalties as a function of both the magnitude and number of reflections indicate that fibre joints with guaranteed minimum return losses of better than 20dB may be required for high-speed direct detection systems as well as future coherent systems, even though optical isolators may be employed in such systems.

Interferometric conversion of laser phase noise to intensity noise by single-mode fibre-optic components

The intrinsic phase noise of semiconductor lasers is interferometrically converted into high levels of intensity noise by reflections at single-mode connectors and other fibre-optic components. This noise can have significant adverse impact on both direct detection and coherent transmission systems, causing bit-error-rate floors in extreme cases.