Laser Relative Intensity Noise Research Articles

Stably suppressing laser relative intensity noise on a 3 × 3 coupler interferometric system.

A 3 × 3 coupler multiphase demodulation scheme is proposed to eliminate the impact of working point drifting and the laser relative intensity noise (RIN) on a 3 × 3 coupler interferometric system. An ellipse-fitting algorithm (EFA) is applied to fit the two interference signals of the 3 × 3 coupler in order, then the ATAN algorithm is applied to obtain three noise-containing signals with specific trigonometric relationships. By averaging the three signals, the demodulated phase noise induced from RIN can be effectively eliminated. The experimental results show that compared with the asymmetric demodulation scheme without intensity noise control, the noise floor of the proposed scheme decreases from 4.5 to 1 µrad/√Hz at 1 kHz and 2.7 to 0.8 µrad/√Hz at 3 kHz. At high frequencies, the average noise floor level is reduced from 10 to 0.9 µrad/√Hz, a reduction of about 21 dB. Furthermore, the variation range of the average noise floor is reduced from 5.4 to 0.17 µrad /√Hz within 100 s.

Diode-pumped passively mode-locked femtosecond Yb:YLF laser at 1.1 GHz

We report femtosecond pulse generation at GHz repetition rates with the Yb:YLF gain medium for the first time. A simple, low-cost, and compact architecture is implemented for the potential usage of the system as a low-noise timing jitter source. The system is pumped by 250 mW, 960 nm single-mode diodes from both sides. The semiconductor saturable absorber mirror (SESAM) mode-locked laser is self-starting and generates transform-limited 210 fs long pulses near 1050 nm. The laser's average output power is 40 mW, corresponding to a pulse energy of 36 pJ at 1.1 GHz repetition rate. The measured laser relative intensity noise (RIN) from 1 Hz to 1 MHz is 0.42%. The performance obtained in this initial work is limited by the specifications of the available optics and could be improved significantly by employing custom-designed optical elements.

Linear dual-comb interferometry at high power levels.

Detector non-linearity is an important factor limiting the maximal power and hence the signal-to-noise ratio (SNR) in dual-comb interferometry. To increase the SNR without overwhelming averaging time, photodetector non-linearity must be properly handled for high input power. Detectors exhibiting nonlinear behavior can produce linear dual-comb interferograms if the area of the detector's impulse response does not saturate and if the overlap between successive time-varying impulse responses is properly managed. Here, a high bandwidth non-amplified balanced photodetector is characterized in terms of its impulse response to high intensity short pulses to exemplify the conditions. With a 23.5 mW average power on each detector in a balanced pair, nonlinear spectral artifacts are at least 40 dB below the spectral baseline. Absorption lines of carbon dioxide are measured to reveal lines discrepancies smaller than 0.1% with HITRAN. A spectral shape independent formulation for the dual-comb figure of merit is proposed, reaching here 7.2 × 107 Hz1/2 limited by laser relative intensity noise, but corresponding to an ideal, shot-noise limited, figure of merit for an equivalent 0.85 mW average power per comb.

Improving the Performance of Optical Phased Array by Reducing Relative Intensity Noise of Optically Injection-Locked Laser Array

Relative intensity noise (RIN) is an important factor that determines the performance of optical phased arrays (OPA) that are configured using semiconductor lasers as light emission sources. This study proposes a method of improving the optical signal-to-noise ratio (OSNR) of an OPA by reducing the RIN and using high coherence of optically injection-locked (OIL) laser arrays. We numerically demonstrated a laser RIN reduction of 22.7 dB by the OIL laser compared to a free-running laser. We achieved an OPA RIN reduction of 13.2 dB by combining the coherent outputs with the uncorrelated noise of 21 OIL lasers, compared to a single OIL laser RIN. Consequently, we demonstrated an OPA OSNR increase of approximately 13.8 dB based on the OIL-based OPA compared to that of the conventional noise-correlated OPA configuration. Additionally, we confirmed the maintenance of OPA OSNR improvement during OPA operations.

Research and development of 40 GHz bandwidth laser relative intensity noise measurement system using PID controller and ESA spectrum analyser

Measuring and evaluating the quality of laser sources used in optical communications and in the precision measurement system is essential. Relative intensity noise (RIN) is a parameter that directly affects the system quality. Therefore, RIN needs to be measured periodically. This study presents a high-sensitivity, broadband (40 GHz) RIN measurement system using a precisely temperature-controlled photodetector and an electrical spectrum signal analyser. RIN value is determined through indirect measurement of noise components: thermal noise, total detected noise, and the shot noise of the laser source. These noises are measured by using an Electrical spectrum analyser (ESA) and a super sensitive 61/2 digital multimeter (DMM). The experimental results of the RIN measurement of the reference Distributed feedback (DFB) laser source show the similarity between the RIN value measured by the system and the RIN value provided by the manufacturer.

Impact of Laser Intensity Noise on Dual-Comb Absolute Ranging Precision.

Noise in mode-locked lasers has been a central issue for dual-comb metrological applications. In this work, we investigate the laser intensity noise on dual-comb absolute ranging precision. Two different dual-comb schemes based on linear optical sampling (LOS) and nonlinear asynchronous optical sampling (ASOPS) have been constructed. In the LOS scheme, the ranging precision deteriorates with the increase in laser relative intensity noise (RIN). This effect can be corrected by implementing a balanced photo-detection (BPD). In the ASOPS scheme, the experiment shows that the conversion from laser RIN to dual-comb ranging precision is negligible, making a balanced detection unnecessary for ranging precision improvement. The different manners of RIN’s impact on absolute ranging precision are attributed to the distinct cross-correlation signal patterns and the underlying time-of-flight (TOF) extraction algorithms.

Pulse-to-pulse relative intensity noise measurements for ultrafast lasers.

Relative intensity noise (RIN) can be used to characterize pulse-to-pulse energy variations of ultrafast lasers, and is a very important performance parameter when considering the suitability of a laser for an application. However, owing to a wide range of measurement and analysis techniques, comparison of RIN values is non-trivial. Here, we clearly layout a definition of RIN as a percentage value for ultrafast laser systems. Furthermore, we analytically describe how the RIN can be measured in the time and frequency domains, and reveal the conditions under which these two widely employed approaches are equivalent. Finally, we experimentally measure the RIN of an ultrafast supercontinuum laser to be 6.57% in the time domain and 6.98% in the frequency domain at 850 nm, and 17.06% in the time domain and 17.08% in the frequency domain at 1000 nm, thus demonstrating the expected strong agreement when the measurements and signal processing are performed appropriately.

Relative-Intensity-Noise Coupling in Heterodyne Interferometers

Laser interferometers are the core measurement tool in gravitational wave observatories. An important factor that can limit the performance is the relative power instability of the laser, a problem often called relative intensity noise (RIN). But exactly how this influences the interferometer performance is not completely understood. Therefore in this paper we analyze laser RIN coupling into the phase readout in balanced and unbalanced heterodyne interferometers. We describe the coupling theoretically, then simulate and finally measure it. Our results reveal a combination of RIN contributions from the heterodyne frequency and twice the heterodyne frequency in the interferometric phase readout. We also show that when an additional, correlated reference measurement is subtracted the combined coupling factor depends on the differential phase between the two measurements and thus can be minimized. Our results have implications for noise models in future space-based gravitational wave observatories like Laser Interferometer Space Antenna, where RIN-to-phase coupling arises directly and is modulated via spacecraft jitter, testmass position and orientation.

Inter-spacecraft offset frequency setting strategy in the Taiji program.

For controlling the beat frequency of heterodyne interferometry so that the Taiji program can detect gravitational waves in space, an offset frequency setting strategy based on a linear programming algorithm is proposed. Considering factors such as Doppler frequency shift, phase-locking scheme, laser relative intensity noise, and phase detector bandwidth, inter-spacecraft offset frequency setting results suitable for the Taiji program are obtained. During the six years of running the detection process, the use of frequency bounds in the range of [5 MHz, 25 MHz] showed that offset frequencies will remain unchanged for a maximum of 1931 days. If the upper and lower bounds are adjusted, and the relative motion between spacecraft is further constrained, the offset frequencies do not need to change during the time of the mission. These results may provide insights into selecting the phase detector and designing operation parameters such as orbit and laser modulation frequency in the Taiji program.

Pump RIN reduction of the TMI threshold powers in Yb-doped kW class fiber amplifiers.

A 3D spatially resolved coupled mode and perturbation analysis for the transverse mode instability threshold powers in Yb-doped fiber amplifiers is extended to include threshold power reduction induced by the pump laser relative intensity noise (RIN). A static intensity grating combines with the fluctuating pump intensity to generate an inversion standing wave. The forward component of the standing wave contributes to the nonlinear coupling between the fundamental mode and the higher-order mode (HOM), thereby reducing the threshold power. It is estimated that a pump RIN level <-150dB/Hz is required to achieve quantum limited performance. The model also quantifies the impact of unwanted laser power in the HOM on the threshold power reduction. The threshold power calculations as a function of RIN and laser power in the HOM are in qualitative agreement with recently published experimental measurements.

High-power 1560 nm single-frequency erbium fiber amplifier core-pumped at 1480 nm

Abstract High-power continuous-wave single-frequency Er-doped fiber amplifiers at 1560 nm by in-band and core pumping of a 1480 nm Raman fiber laser are investigated in detail. Both co- and counter-pumping configurations are studied experimentally. Up to 59.1 W output and 90% efficiency were obtained in the fundamental mode and linear polarization in the co-pumped case, while less power and efficiency were achieved in the counter-pumped setup for additional loss. The amplifier performs indistinguishably in terms of laser linewidth and relative intensity noise in the frequency range up to 10 MHz for both configurations. However, the spectral pedestal is raised in co-pumping, caused by cross-phase modulation between the pump and signal laser, which is observed and analyzed for the first time. Nevertheless, the spectral pedestal is 34.9 dB below the peak, which has a negligible effect for most applications.

The relative intensity noise of a vertical-cavity surface-emitting laser with a fiber Bragg grating external cavity

The research deals with the topical problem of stabilizing the optical parameters of a single-mode vertical-cavity surface-emitting laser. A solution to the problem is proposed by forming an external cavity based on a Bragg grating induced into an optical polarization-maintaining fiber. In this experimental research, the authors estimated the contribution of an external cavity to the relative intensity noise of a vertical-cavity surface-emitting laser. The stability of the laser generation with a varying phase of back reflections at various pump currents is investigated. The relative intensity noise of laser radiation in the pump current range of 1.1–6.3 mA is estimated using a photodetector with a bandwidth of 8.5 MHz. The phase change of the back reflections was carried out by shifting the optical fiber end with a 100 nm step. As a result of the research, it was obtained that the vertical-cavity surface-emitting laser with such an external cavity shows a stable emission in the current range of 1.8–3.2 mA. However, one can observe instability with constant switching between adjacent longitudinal modes outside this range. The most stable laser operation mode with an external cavity against a change in the phase of back reflections is registered at a pump current of 2.78 mA. Utilizing an external cavity with the fiber Bragg grating made it possible to reduce the relative intensity noise from 7.2∙10–10 1/Hz to 1.5∙10–11 1/Hz in the currents range 1.86–3.2 mA. The study can be useful in applications that require an optical radiation coherent source with a low relative intensity noise level (such as coherent optical communication or fiber-optic sensors).

Intensity noise characteristics of intracavity contacted VCSELs with rhomboidal oxide current aperture for the magnetometric sensor with Cs133 vapor cell used in magnetoencephalography

We demonstrate noise characterization of novel 894.4 nm vertical-cavity surface emitting lasers with intracavity contacts and a rhomboidal oxide current aperture (IC-VCSELs), dedicated to 133Cs D1 line compact optically pumped atomic magnetometers (OPM). The laser relative intensity noise, measured to be -139 dB/Hz at 10 kHz frequency in 1 Hz bandwidth for a laser optical power of 0.8 mW, is decreased with optical power growth. The IC-VCSELs polarization-resolved relative intensity noise is 143 dB/Hz at 10 kHz frequency in 1 Hz bandwidth for 0.8 mW. The emission linewidth of the VCSEL is about 55 MHz. The IC-VCSEL parameters are determined, such as emission linewidth ~ 50-60 MHz, optical power ~ 0.5-1.0 mW, at which the polarization-resolved RIN becomes close to the RIN, which makes it possible to use these lasers in various OPM Mz and Mx schemes. The ultimate sensitivity of OPM was estimated by the ratio of the magnetic resonance to the signal to noise level. It is shown that a OPM based on the IC-VCSELs, assuming magnetic resonance FWHM ~ 1 kHz, can achieve a shot noise-limited sensitivity around 20 fT in 1 Hz bandwidth without any polarization improvements by polarizer or polarization beam splitter cube (PBC). Developed IC-VCSELs is acceptable for use in compact OPM for magnetoencephalography.

Evolution of Relative Intensity Noise in High-Power Narrow-Linewidth Fiber Laser Systems

In this work, we carry out a systematic study on the evolution of relative intensity noise (RIN) in a high-power narrow-linewidth fiber laser system based on phase-modulated single-frequency seed laser. The experimental results clarify that enhancement of the RIN mainly occurs when the seed laser gets amplified from low power to medium power. In addition, further power scaling could lead to reduction of the RIN until it reaches saturation on the condition that the transverse mode instability (TMI) effect is effectively suppressed. Apart from the power scaling process, the phase-modulation would also lead to enhancement of the RIN at specific frequencies corresponded to the modulation frequency. More importantly, the contrast experiments reveal that the TMI effect could lead to obvious enhancement of the RIN of the signal laser during amplification well below the TMI threshold defined by the traditional method. Therefore, apart from degeneration of the RIN during amplification, potential enhancement of the RIN induced by the phase-modulation or the TMI effect below its traditional threshold should be carefully considered and controlled to achieve high-power low-noise fiber laser. Overall, our experimental results could provide a well reference for design of high-power low-noise fiber lasers and high-power narrow-linewidth fiber lasers.

Noise conversion in delay-line optoelectronic microwave oscillators

We report the results of theoretical and experimental studies of noise in a delay-line optoelectronic microwave oscillator with optical gain. The effect of various noise sources on the parameters of an optoelectronic microwave oscillator is considered, and, in addition to the noise of its individual components, several linear and nonlinear mechanisms for converting optical carrier noise into microwave signal noise are taken into account. It is shown that, due to the nonlinearity of the energy characteristics of high-power microwave photodiodes in an optoelectronic generator, the low-frequency components of the laser relative intensity noise are converted into microwave signal noise, as a result of which sources of correlated amplitude and phase noise appear in the oscillator loop.

DR-RINS: Digital real-time relative intensity noise suppressor for pump-probe spectroscopy and microscopy.

Relative intensity noise (RIN) inherent in fiber lasers poses a serious obstacle to their use in pump-probe spectroscopy and imaging. RIN can be removed through an analog balanced detector, or, as we have previously shown, software adaptive noise cancellation (ANC) on digitized signals. One major drawback to software ANC is the added time required for digitizing and post-processing. In this article, we describe a design for ANC on a field-programmable gate array (FPGA), making use of high-level synthesis tools and fixed-point arithmetic to achieve real-time laser RIN suppression at 25MHz sample rates. Unlike the software-ANC approach, the FPGA-ANC device can serve as a dedicated drop-in denoiser, placed between the detectors and a commercial lock-in amplifier. We demonstrate its application to transient absorption spectroscopy and microscopy, lowering the noise floor to ∼17 dB above the shot noise limit. Furthermore, we demonstrate a dramatic improvement in data acquisition time from ∼6 h to ∼5min in a real-time imaging scenario.

Numerical investigation of relative intensity noise in frequency-doubled multimode fiber lasers

In this paper, we study the intensity noise performance of frequency-doubled lasers. In particular, we investigate how the relative intensity noise (RIN) of multimode fiber lasers is affected by the second harmonic generation process. We first develop an analytical approach and show that, in contrast with a single-mode laser, the low frequency RIN (or excess noise) of a two-mode laser can increase of more than 6 dB after the frequency-doubling operation. This occurs when the intensities of both modes are different and this is explained by a nonlinear coupling between noise and intensity of modes. To deal with more commonly-used multimode fiber lasers, we have extended our study to any number of modes. For this purpose, we have developed a model to numerically simulate the dynamics of a multimode fiber laser. This model includes noise sources and mode competition dynamics due to spatial hole burning. It gives access to the complex amplitude of the electric field of the laser. Using this model, we have confirmed that the excess noise of frequency-doubled multimode fiber lasers can be more than 6 dB higher than the excess noise of the laser before frequency-doubling.

Adaptive noise canceling for transient absorption microscopy.

.Significance: Ultrafast fiber lasers are an attractive alternative to bulk lasers for nonlinear optical microscopy for their compactness and low cost. The high relative intensity noise (RIN) of these lasers poses a challenge for pump-probe measurements such as transient absorption and stimulated Raman scattering, along with modalities that provide label-free contrast from the vibrational and electronic structure of molecules.Aim: Digital adaptive filtering was applied to determine the applicability for canceling laser RIN in a transient absorption microscope with an ultrafast fiber laser source.Approach: Digitized signals from the transmitted probe and reference photodetectors were fed to an adaptive filter in MATLAB, running in a noise canceling configuration. This result was then fed to a software lock-in algorithm to demodulate the pump-probe signal. Images were built up one line scan at a time with a 3.5-kHz resonant scanner, with averaging. The imaging target was , which exhibits nondegenerate two-photon absorption at the pump/probe wavelengths used (530-nm pump and 490-nm probe).Results: Without adaptive noise cancellation, the lock-in output primarily passes the laser RIN within its detection bandwidth, resulting in images that closely follow the linear transmissivity and lack sensitivity to pump-probe time delay. With adaptive noise cancellation in front of the lock-in, the RIN rejection is enough to restore the z-sectioning and sensitivity to pump-probe delay, as expected for transient absorption. Results were limited primarily by noise from the photodetector and analog-to-digital converter.Conclusions: Digital adaptive noise cancellation, even when limited by electronics noise, can recover pump-probe signals by removal of laser RIN, under conditions where averaging alone fails.

Multi-Objective Laser Rate Equation Based Parameter Extraction Using VCSEL Small Signal Response and RIN Spectra

Rate equation parameters used to simulate semiconductor lasers must be carefully chosen in order to accurately model high-speed laser characteristics in optical link simulations. Previous parameter extraction methods have largely characterized these lasers through their small signal frequency response. We show that capturing both the small signal frequency response and relative intensity noise (RIN) spectrum of the laser is crucial for accurate parameter extraction. We present a multi-objective parameter extraction method alongside a stochastic rate equation model to account for a vertical cavity surface emitting laser's (VCSEL's) bandwidth and RIN characteristics. A genetic algorithm extracts VCSEL parameters from a set of experimentally measured small signal responses and RIN spectra at multiple drive currents. Through Pareto sorting we can extract a single set of parameters which accurately captures the laser's BW and RIN characteristics.

Brownian motion-induced amplitude noise in vapor-cell frequency standards

We demonstrate that the Brownian motion of alkali metal atoms in buffer gas gives rise to a significant source of frequency instability in vapor cell clocks. We consider, in particular, laser pumped cell devices working in pulsed operation and using a resonant Gaussian light beam to detect the clock transition. It is well known that the diffusion motion through the buffer gas results from many random walks performed by the atoms, as a consequence of the collisions with other atoms/molecules. Owing to this random-walk behavior, the atoms explore different intensity regions of the Gaussian laser beam, reducing the forward light transmission and causing amplitude fluctuations at the photodetector. The contribution of this so called transit noise to the clock frequency stability turns out in the low 10−14 region for a centimeter-scale cell, at the same level of other amplitude noises, like laser relative intensity noise and shot noise. As a consequence, even if it is not the main source of instability in currently used vapor cell clocks, Brownian motion-induced noise represents a novel source of frequency fluctuations and it should be accounted for in the clock stability budget. A preliminary evaluation of the transit noise is also reported for microcell devices.