Intensity Noise Power Research Articles

Intensity noise in difference frequency generation-based tunable femtosecond MIR sources.

We characterize the intensity noise of two mid-infrared (MIR) ultrafast tunable (3.5-11 μm) sources based on difference frequency generation (DFG). While both sources are pumped by a high repetition rate Yb-doped amplifier delivering 200 μJ 300 fs at a central wavelength of 1030 nm, the first is based on intrapulse DFG (intraDFG), and the second on DFG at the output of an optical parametric amplifier (OPA). The noise properties are assessed through measurement of the relative intensity noise (RIN) power spectral density and pulse-to-pulse stability. The noise transfer mechanisms from the pump to the MIR beam is empirically demonstrated. As an example, improving the pump laser noise performance allows reduction of the integrated RIN (IRIN) of one of the MIR source from 2.7% RMS down to 0.4% RMS. The intensity noise is also measured at various stages and in several wavelength ranges in both laser system architectures, allowing us to identify the physical origin of their variation. This study presents numerical values for the pulse to pulse stability, and analyze the frequency content of the RINs of particular importance for the design of low-noise high repetition rate tunable MIR sources and future high performance time-resolved molecular spectroscopy experiments.

Mid-infrared homodyne balanced detector for quantum light characterization.

We present the characterization of a novel balanced homodyne detector operating in the mid-infrared. The challenging task of revealing non-classicality in mid-infrared light, e. g. in quantum cascade lasers emission, requires a high-performance detection system. Through the intensity noise power spectral density analysis of the differential signal coming from the incident radiation, we show that our setup is shot-noise limited. We discuss the experimental results with a view to possible applications to quantum technologies, such as free-space quantum communication.

Performance Optimization of Fiber Optic Interferometric Accelerometer Based on Phase Noise Analysis

The phase noise of a Michelson interferometer-based fiber optic accelerometer is analyzed to determine the optimal modulation depth of the laser light frequency and the length difference between the interference arms. The output intensity fluctuation of the interferometer, originating from the phase noise of the modulated laser source, is examined theoretically, revealing that both the interference intensity noise power and the phase modulation depth of the interferometer are proportional to the frequency modulation depth of the laser and the interference arm length difference. In addition, it is determined experimentally that the laser linewidth, or equivalently, the laser phase noise, varies approximate linearly with the light frequency. As the optimum phase modulation depth is a constant, the interference intensity noise power has a minimum value, when the light frequency modulation depth is at its maximum. The proposed theory is demonstrated experimentally, with results showing that a high-performance broadband fiber optic interferometric accelerometer achieves the lowest noise level when the light frequency modulation depth of the source laser is at its maximum value.

Long-to-short wavelength swept source.

Most swept external cavity diode lasers tune in the short-to-long wavelength direction (red tuning). Lower relative intensity noise (RIN) and higher output power are typically possible in this direction. We show here that long-to-short tuning (blue tuning) is possible for a short, linear cavity laser that has both low noise and high power. This mode of operation is made possible by nonlinear frequency broadening in the semiconductor optical amplifier (SOA) followed by clipping of the red portion of the spectrum by the micro-electro-mechanical systems (MEMS) tunable Fabry-Perot filter. Blue shifting during gain recovery is an important broadening mechanism. There is an approximate 50% advantage in coherence length for the same filter bandwidth for blue over red tuning, which allows deeper imaging in optical coherence tomography (OCT) applications. Calculations contrasting the blue tuning mechanism with red tuning are presented. The accuracy of the blue-tuning model is confirmed by coherence and coherence revival measurements and simulations.

Sensitivity of a phase-sensitive optical time-domain reflectometer with a semiconductor laser source

In the present paper we perform, for the first time, an analysis of the average sensitivity of a coherent phase-sensitive optical time-domain reflectometer (phase-OTDR) with a semiconductor laser source to external actions. The sensitivity of this OTDR can be defined in a conventional manner via average SNR at its output, which in turn is defined by the average useful signal power and the average intensity noise power in the OTDR spatial channels in the bandwidth defined by the OTDR sampling frequency. The average intensity noise power is considered in detail in a previous paper. In the current paper we examine the average useful signal power at the output of a phase-OTDR.The analysis of the average useful signal power of a phase-OTDR is based on the study of a fiber scattered-light interferometer (FSLI) which is treated as a constituent part of a phase- OTDR. In the analysis, one of the conventional phase-OTDR schemes with a rectangular dual-pulse probe signal is considered. The FSLI which corresponds to this OTDR scheme has two scattering fiber segments with additional time delay, introduced between backscattered fields. The average useful signal power and the resulting average SNR at the output of this FSLI are determined by the degree of coherence of the semiconductor laser source, the length of the scattering fiber segments, and by the additional time delay between the scattering fiber segments. The average useful signal power characteristic of the corresponding phase-OTDR is determined by analogous parameters: the source coherence, the time durations of the parts constituting the dual-pulse, and the time interval which separates these parts.In the paper an expression for the average useful signal power of a phase-OTDR is theoretically derived and experimentally verified. Based on the found average useful signal power of a phase-OTDR and the average intensity noise power, derived in the previous paper, the average SNR of a phase-OTDR is defined. Setting the average signal SNR to 1, at a defined spectral band the minimum detectable external action amplitude for our particular phase-OTDR setup is determined. We also derive a simple relation for the average useful signal power and the average SNR which results when making the assumption that the laser source coherence is high. The results of the paper can serve as the basis for further development of the concept of phase-OTDR sensitivity.

Intensity noise limit in a phase-sensitive optical time-domain reflectometer with a semiconductor laser source

In the present paper we perform, for the first time, the analysis of the average intensity noise power level at the output of a coherent phase-sensitive optical time-domain reflectometer (phase-OTDR) with a semiconductor laser source. The origin of the considered intensity noise lies in random phase fluctuations of a semiconductor laser source field. These phase fluctuations are converted to intensity noise in the process of interference of backscattered light. This intensity noise inevitably emerges in every phase-OTDR spatial channel and limits its sensitivity to external phase actions. The analysis of intensity noise in a phase-OTDR was based on the study of a fiber scattered-light interferometer (FSLI) which is treated as the constituent part of OTDR. When considered independently, FSLI has a broad intensity noise spectrum at its output; when FSLI is treated as a part of a phase-OTDR, due to aliasing effect, the wide FSLI noise spectrum is folded within the spectral band, determined by the probe pulse repetition frequency. In the analysis one of the conventional phase-OTDR schemes with rectangular dual-pulse probe signal was considered, the FSLI, which corresponds to this OTDR scheme, has two scattering fiber segments with additional time delay introduced between backscattered fields. The average intensity noise power and resulting noise spectrum at the output of this FSLI are determined by the degree of coherence of the semiconductor laser source, the length of the scattering fiber segments and by the additional time delay between the scattering segments. The average intensity noise characteristics at the output of the corresponding phase-OTDR are determined by the analogous parameters: the source coherence, the lengths of the parts constituting the dual-pulse and the time interval which separates the parts of the dual-pulse. In the paper the expression for the average noise power spectral density (NPSD) at the output of FSLI was theoretically derived and experimentally verified. Based on the found average NPSD of FSLI, a simple relation connecting the phase-OTDR parameters and the limiting level of full average intensity noise power at its output was derived. This relation was verified by experimental measurement of the average noise power at the output of phase-OTDR. The limiting noise level, considered in the paper, determines the fundamental noise floor for the phase-OTDR with given parameters of the source coherence, probe pulse length and time delay between two pulses constituting the dual-pulse.

Passive coherent discriminator using phase diversity for the simultaneous measurement of frequency noise and intensity noise of a continuous-wave laser

The frequency noise and intensity noise of a laser set the performance limits in many modern photonics applications and, consequently, must often be characterized. As lasers continue to improve, the measurement of these noises however becomes increasingly challenging. Current approaches for the characterization of very high-performance lasers often call for a second laser with equal or higher performance to the one that is to be measured, an incoherent interferometer having an extremely long delay-arm, or an interferometer that relies on an active device. These instrumental features can be impractical or problematic under certain experimental conditions. As an alternative, this paper presents an entirely passive coherent interferometer that employs an optical 90° hybrid coupler to perform in-phase and quadrature detection. We demonstrate the technique by measuring the frequency noise power spectral density of a highly-stable 192 THz (1560 nm) fiber laser over five frequency decades. Simultaneously, we are able to measure its relative intensity noise power spectral density and characterize the correlation between its amplitude noise and phase noise. We correct some common misconceptions through a detailed theoretical analysis and demonstrate the necessity to account for normal imperfections of the optical 90° hybrid coupler. We finally conclude that this passive coherent discriminator is suitable for reliable and simple noise characterization of highly-stable lasers, with bandwidth and dynamic range benefits but susceptibility to additive noise contamination.

Superlinear growth of Rayleigh scattering-induced intensity noise in single-mode fibers.

Rayleigh scattering generates intensity noise close to an optical carrier that propagates in a single-mode optical fiber. This noise degrades the performance of optoelectronic oscillators and RF-photonic links. When using a broad linewidth laser, we previously found that the intensity noise power scales linearly with optical power and fiber length, which is consistent with guided entropy mode Rayleigh scattering (GEMRS), a third order nonlinear scattering process, in the spontaneous limit. In this work, we show that this behavior changes significantly with the use of a narrow linewidth laser. Using a narrow linewidth laser, we measured the bandwidth of the intensity noise plateau to be 10 kHz. We found that the scattered noise power scales superlinearly with fiber length up to lengths of 10 km in the frequency range of 500 Hz to 10 kHz, while it scales linearly in the frequency range of 10 Hz to 100 Hz. These results suggest that the Rayleigh-scattering-induced intensity noise cannot be explained by third-order nonlinear scattering in the spontaneous limit, as previously hypothesized.

Comments and Corrections Comments on “Frequency Response of the Noise Conversion From Relative Intensity Noise to Phase Noise in the Photodetection of an Optical Pulse Train”

Abstract — It is shown that the results presented in the article by Wu et al. (IEEE Photon. Technol. Lett. 23 (n° 8) 2011, pp. 468-470), and attributed by the authors to amplitude-to-phase conversion in the photodetector, do not originate in the photodetector under test, due to the power dependence observed. More recent results show that conversion effects with the same size and frequency dependence are observed in passively mode-locked NPR (non-linear polarization rotation) mode-locked lasers. Index Terms — Phase measurement, Phase noise, Photodetectors, mode-locked lasers. n their article, K. Wu et al. [1] address the problem of the amplitude-to-phase conversion in photodetectors, already observed in earlier publications. They present a particularly simple set-up, that may provide a convenient means to characterize the AM-to-PM conversion in high bandwidth photodetectors. They use a 2.4 mW free-running passively mode-locked laser at 147 MHz repetition rate, and modulate the power of the laser at audio frequencies by modulating the power of the pump diode. They subsequently detect the pulse train emitted by the mode-locked laser using a 2GHz photodiode. The signal of the photodiode is analyzed by a signal analyzer FSUP26, which can deliver - the relative intensity noise power spectrum and the phase noise of the 147 MHz signal, separately, (Fig. 2 a- and b-, respectively) as a function of the offset frequency - or the spectrum, in dBm of the signal around 147 MHz, Fig. 4. Both figures show that the 147 MHz signal is phase-modulated at the frequency of the power modulation and at its harmonics. In Fig. 4, the phase modulation appears as a an asymmetry in the side bands: the asymmetry

Implementation and performance investigation of radio‐over‐fiber systems in wireless sensor networks

AbstractWe report the performance investigation and implementation of four different radio‐over‐fiber (ROF) systems as backhaul for wireless sensor networks based on frequency shift keying modulation at 915 MHz. Its applicability was experimentally verified as a function of packet error rate, received signal strength indicator, and system reach. Furthermore, a performance noise analysis was carried out by taking into account the optical transmitter and receiver parameters, such as: equivalent input noise, noise figure, relative intensity noise, laser wavelength, and output power. The implementation of the proposed technical solution implies in a outstanding enhancement in the system reach, management, and flexibility. It has been achieved an improvement of 17.45 dB in the system performance by using high‐quality pieces of equipment of ROF technology. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:2669–2675, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27168

Temperature-Dependent Saturation Characteristics of Injection Seeded Fabry–PÉrot Laser Diodes/Reflective Optical Amplifiers

In this paper, the temperature dependence of the gain and saturation power of injection seeded Fabry-Perot laser diodes/reflective semiconductor optical amplifiers are analyzed theoretically and experimentally. For a constant gain, the saturation power increases with the ambient temperature. This dependency explains the observed variation in relative intensity noise versus injection power, as a function of temperature.

Theory of noise in p-n junction light emitters

The intensity noise of light generated by semiconductor lasers and light-emitting diodes is treated by semiclassical Langevin equations. An independent equation for the junction voltage dynamics is considered, and the non-Markoffian nature of the pump current is decomposed into Markoffian carrier injection and a regulation mechanism due to charging effect at the junction. The intensity noise power spectrum and squeezing bandwidth predicted by these equations agree well with recent experimental results. External current noise generated as a result of the internal noise process and subsequent relaxation process is calculated. Also, correlations between the carrier-number fluctuation and the junction-voltage fluctuation, and between the emitted photon flux fluctuation and the junction-voltage fluctuation are studied in detail.

Saturated absorption spectroscopy using diode-laser phase noise.

We have investigated theoretically the applicability of phase-fluctuating laser fields in saturated absorption spectroscopy. The fluctuations of pump and probe fields are fully correlated if they are derived from the same laser source. Inside the saturable medium, phase fluctuations are converted into intensity noise. This nonlinear mixing modifies the statistics of the transmitted fields. By measuring higher-order correlations one can deduce additional spectroscopic information. Apart from the mean intensity, we have examined the intensity noise and intensity power spectrum of the weak probe field. The resonances of these correlation functions are also unaffected by large inhomogeneous broadening since they are inherently related to the usual saturated absorption dip. We find qualitative agreement with results of a recent experiment employing this technique [D. H. McIntyre et al., Opt. Lett. 18, 1816 (1993)], which demonstrates the advantages of noise spectroscopy using spectrum analyzers.

Phase noise in semiconductor lasers: A theoretical approach

Noise phenomena in semiconductor lasers have been extensively studied from an experimental point of view because of their importance in designing optical communication systems. Recently, a semiclassical theory of laser noise has been presented [1]. In this paper, following a different and simpler approach, we obtain analytical expressions for the intensity and phase noise power spectra which agree with experimental results.