Relative Intensity Noise Reduction Research Articles

Relative intensity noise reduction for noise-like pulses based on synchronous intensity modulation

Extra-cavity synchronous intensity modulation is a straightforward and effective approach to suppress the full-bandwidth relative intensity noise (RIN) for noise-like pulse (NLP) fiber lasers. In this work, the picosecond NLPs are produced by a homemade Er/Yb co-doped mode-locked fiber laser and subsequent optimized by the synchronous intensity modulation device. The photodetector (PD) and electro-optical modulator (EOM) allow the real-time measurement and modulation for each optical pulse. A detailed description of key parameters for components is presented. The impact of DC bias voltage and RF signal amplitude on resulting RIN is experimentally investigated. We demonstrate a noteworthy reduction in RIN from 5.62% before synchronous intensity modulation to 1.91% afterward. Numerical investigations suggest that this method is potential to suppress RIN to below 1% under ideal conditions. The key factor in RIN reduction is correct operating region and high photoelectric-conversion linearity. The synchronous intensity modulation is an effective method to obtain low-RIN NLP laser sources, valuable for some practical applications.

Reduction of relative intensity noise in a diamond Raman laser

The relative intensity noise (RIN) characteristics of a continuous-wave diamond Raman laser are investigated for the first time. The results reveal the parasitic stimulated Brillouin scattering (SBS) that usually occurred with higher-order spatial modes in the diamond Raman resonator is a pivotal factor impacting the Raman longitudinal modes and deteriorating the RIN level. The diamond Raman laser automatically switches to single-longitudinal-mode operation and the RIN level is significantly decreased in the frequency range of 200 Hz to 1 MHz after the parasitic SBS is effectively suppressed through inserting a spatial aperture or a χ(2) nonlinear crystal into the cavity. Due to the introduction of additional nonlinear loss to the high intensity Raman fluctuations and the non-lasing spontaneous Raman modes, the χ(2) nonlinear crystal enables better performance in the RIN-level reduction compared to the spatial aperture which can only achieve SBS inhibition. The RIN reduction routes are well suited for various crystalline Raman media to achieve high power and low intensity noise laser at different wavelengths.

Ultra low RIN, low threshold AlGaInAs/InP BH-DFB laser

This study presents a comparative analysis of DFB lasers based on AlGaInAs/InP heterostructures, exploring the impact of different groove shapes and cavity lengths on laser output characteristics. Leveraging the vertical groove structure, we achieved a remarkable laser output of 90mW at 25°C. Compared to lasers without grooves, this represents a 3.6-fold increase in output power, coupled with an impressively low threshold current of only 4mA. In addition, we conducted a study on the temperature stability of the laser, revealing that even at elevated temperatures of 85°C, the laser maintains an edge mode suppression ratio of over 45dB. This demonstrates the robustness and reliability of the laser over a wide range of operating conditions, further enhancing its suitability for practical applications in diverse environments. Upon packaging the chip into a butterfly shape, we observed a significant reduction in relative intensity noise, with values below -162.8 dB/Hz across the frequency range of 0-40 GHz, while operating at an output power of 3mW. These findings underscore the robustness, reliability, and high-performance capabilities of the developed DFB laser, highlighting its potential for various practical applications.

Observation of RIN reduction via spectral broadening in an NPR-based stretched pulse fiber laser.

We show that an optimum mode-locking state with low relative intensity noise (RIN) can be identified by continuous broadening of an optical spectrum in a stretched-pulse fiber laser based on nonlinear polarization rotation (NPR). Under the premise of keeping the overall spectral shape unchanged, either gradually increasing the pump power or unidirectionally adjusting the polarization controller (PC) can effectively reduce RIN as the optical spectral bandwidth broadens. The optimized intensity noise performance of the laser can be attributed to the increased pulse energy and reduced intra-cavity net dispersion. Moreover, the integrated RIN will further decrease as the maximum 3-dB bandwidth extends. In our experiment, the detected minimum integrated rms RIN is below 0.003% (from 100 Hz to 100 kHz). Our experimental results find that the absolute spectral width is not a necessary key condition for obtaining low RIN mode-locked laser, whereas it may help understand and design versatile low-noise ultrafast laser sources.

RIN Reduction in Gain-Switched InAs-InP(113)B Quantum Dot Laser Based on Multi-Population Rate Equations

For the first time, relative intensity noise (RIN) of an InAs-InP(113)B quantum dot (Q-Dot) laser is examined theoretically under the gain-switching condition with the application of a Gaussian pulse beam (GPB) to excited state (Exs) of the laser. The multi-population rate equations considering nonlinear gain are solved by the Runge–Kutta method. Noises are added to rate equations as Langevin noise sources and a different method is applied here for the addition of these noise sources to equations. In this method, new rate equations are defined to eliminate the cross-correlations between the noise sources to make them independent and simulate with independent white Gaussian random variables. Obtained results showed that RIN decreases with the increasing ac peak current and the increasing inhomogeneous broadening. It was also observed that further RIN reduction of about 30 dB/Hz and gain-switched short pulses with a high power due to Exs emission being obtained with the application of GPB to Exs of the laser. Moreover, it was demonstrated that while carrier noise generated from Exs and ground state (Grs) affect the RIN spectrum, there is no effect of wetting layer (Wly) carrier noise.

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.

Reduction of relative intensity noise in a broadband source-driven RFOG using a high-frequency modulation technique.

A broadband source-driven resonant fiber-optic gyroscope (RFOG) can reduce coherence-related noise, thus achieving a better sensitivity with a much simpler configuration than the traditional system with a coherent source. Its detection sensitivity, however, is still limited by the excess relative intensity noise (RIN) of the broadband source. In this paper, the RIN error mechanism in this broadband source-driven RFOG is revealed and countermeasures are presented. We demonstrate that the use of a high-finesse fiber-optic ring resonator and a high-frequency modulation-demodulation technique can reduce the RIN-induced error. It is indicated that the optimal modulation parameters can provide a RIN-induced error reduction of 6.1 dB, allowing the broadband source-driven RFOG to operate near the shot-noise-limited theoretical sensitivity. With the optimal high-frequency modulation-demodulation technique, an angle random walk of 0.0013°/√h is achieved with a 200-m-long fiber-optic ring resonator of 7.6 cm diameter. This is the best result reported to date, to the best of our knowledge, for fiber-optic gyroscopes of this size.

Intensity instability and correlation in amplified multimode wave mixing

The dynamics of optical nonlinearity in the presence of gain and feedback can be complex leading to chaos in certain regimes. Temporal, spectral, spatial, or polarization instability of optical fields can emerge from chaotic response of an optical chi ^{(2)} or chi ^{(3)} nonlinear medium placed between two cavity mirrors or before a single feedback mirror. The complex mode dynamics, high-order correlations, and transition to instability in these systems are not well known. We consider a chi ^{(3)} medium with amplified four-wave mixing process and study noise and correlation between multiple optical modes. Although individual modes show intensity instability, we observe relative intensity noise reduction close to the standard quantum noise, limited by the camera speed. We observe a relative noise reduction of more than 20 dB and fourth-order intensity correlation between four spatial modes. More than 100 distinct correlated quadruple modes can be generated using this process.

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.

Quantum-dot semiconductor lasers with prominent relative intensity noise and spectral characteristics.

Relative intensity noise (RIN) behavior of a quantum dot laser (QDL) at free running and under optical injection locking (OIL) has been reported in detail. Considering the mutual effects of the inhomogeneous and homogeneous broadenings, different and also non-routine RIN characteristics of a QDL at low, intermediate, and high homogeneous broadening regimes have been discussed. Results demonstrate that an injection-locked quantum dot laser exhibit enhanced characteristic over free-running operation along with low-frequency noise behavior in a wide range of homogeneous broadening values. RIN reduction of about 25, 55, and 7 dB/Hz have been achieved for low, intermediate, and high values of homogeneous broadening which implies the spontaneous emission noise suppression even at low injection power. The promising impact of the OIL on side-mode suppression for the intermediate homogeneous broadening regime has also been discussed. Our results guarantee the single-mode and low-noise operation of the OIL-QDL at any homogeneous broadening value.

Excess Relative-Intensity-Noise Reduction in a Fiber Optic Gyroscope Using a Faraday Rotator Mirror

The fiber optic gyroscope (FOG) suffers from the excess relative intensity noise (RIN) caused by the broadband light source, which significantly impacts the angular random walk (ARW) of the FOG. The existing suppression methods for the excess RIN usually require complex configuration or devices with big volume, and thus are not suitable for practical applications especially in small-sized FOGs. What is more, as the key parameters or components, that determine the practical suppression effects of the excess RIN, and the corresponding impacting mechanisms are not clarified, there is no efficient way in evaluating or optimizing the excess-RIN reduction configuration in practice. In this work, we propose a simple but effective method to achieve the reduction of the excess RIN at the demodulation frequency, in which a Faraday rotator mirror is inserted to the dead arm of the source coupler to realize the delay line filter for the excess RIN. Experimental results show that the ARW is improved from 1.70 mdeg/h <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> to 0.58 mdeg/h <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> for a navigation-grade FOG. More importantly, this work provides a comprehensive understanding on how the main defects in the excess-RIN reduction scheme impact the practical excess-RIN suppression effects both theoretically and experimentally, which demonstrates the inherent robustness of the proposed scheme against internal and external influencing factors. This method is especially suitable for realizing the excess-RIN reduction in small-sized FOGs and upgrading the existing FOG products with minimal modifications, and is also potentially referable in evaluating and optimizing other similar excess-RIN reduction schemes.

On the Phase Noise Enhancement of a Continuous Wave in Saturated SOA Used for RIN Reduction

We report on the spectral characteristics of the phase noise enhancement of continuous-wave (CW) optical signals passing through a semiconductor optical amplifier (SOA) operating in the saturation regime and used for relative intensity noise (RIN) reduction. We show that the known effect of phase noise enhancement, attributed to the linewidth enhancement factor of the device, happens only at a limited band of the phase noise spectrum, and the actual measurable linewidth of the output CW signal may not be affected. While this phase noise enhancement is not shown as an increase of spectral linewidth, it can still affect system performance when coherent detection is used, especially in applications with relatively low symbol rates. Numerical simulations and experimental results are used to support the observation. A single spectral line from a quantum-dot mode-locked laser is used as the light source, which is known to have relatively high RIN (> −120 dB/Hz in the low frequency region). Experimental transmission of 16-QAM modulation with coherent detection has been performed at 5 GBd to assess the implication on system performance.

Dither-free stabilization of a femtosecond doubly resonant OPO using parasitic sum-frequency mixing.

Stable operation of a doubly resonant femtosecond optical parametric oscillator (OPO) requires submicron matching of the OPO and pump laser cavity lengths, which is normally implemented using a dither-locking feedback scheme. Here we show that parasitic sum-frequency mixing between the pump and resonant pulses of a degenerate femtosecond OPO provides an error signal suitable for actuating the cavity length with the precision needed to maintain oscillation on a single fringe and at maximum output power. Unlike commonly used dither-locking approaches, the method introduces no modulation noise and requires no additional optical components, except for one narrowband filter. The scheme is demonstrated on a Ti:sapphire-pumped sub-40-fs PPKTP OPO, from which data are presented showing a tenfold reduction in relative intensity noise compared with dither locking.

Application of the Wiener filter for intensity noise reduction in fiber optic gyroscopes.

An essential issue for the low-noise system application of the fiber optic gyroscope (FOG) is to reduce its noise level. The relative intensity noise (RIN) of the light source is the dominant noise of the FOG when the light power on the detector reaches a certain level. The noise subtraction method is effective for RIN reduction and easy to implement in a FOG. This paper theoretically analyzes the factors that influence the result of the method and deduces the function to calculate the noise suppression ratio that can be achieved. A method that uses an optimum filter design based on the Wiener filter in the reference detector signal is proposed to improve the subtraction result. A FOG system is set up to test the feasibility of the method. The experiment results meet with the theoretical analysis, and by using the Wiener filter, the achieved noise subtraction factor reaches the limitation that restrains the optical system and detection circuit.

Dynamic behavior of injection-locked two-state quantum dot lasers

Quantum dot (QD) lasers with excited state (ES) emission exhibit superior characteristics compared with their ground state (GS) emission. Here, we have developed a model that predicts the small-signal modulation response and relative intensity noise (RIN) of a QD laser capable of emitting from both GS and ES resonance energies in free-running and also under optical injection-locking conditions. The GS resonance wavelength in the injection-locked laser benefits a considerable RIN reduction of ∼20 dB/Hz before a lasing threshold of ES emission, while the ES resonance wavelength is promoted more regarding the modulation bandwidth around the lasing threshold, which is almost doubled, especially at very low injection ratios. Contrary to the GS, the injected locked RIN is the poorest property of ES emission, which shows ∼10 dB/Hz enhancement at the lasing threshold and larger injection ratios. It was affirmed that the RIN response of ES emission can be significantly reduced at the cost of setup complexity through simultaneous injection locking of both wavelengths. A 3 dB modulation bandwidth of ∼18 GHz is achieved for ES emission, which is more than twice that of the free-running operation of ES emission.

Angular random walk improvement of a fiber-optic gyroscope using an active fiber ring resonator.

High precision fiber-optic gyroscopes (FOGs) interrogated with broadband light have angular random walk (ARW) limited by the source relative intensity noise (RIN). A passive fiber ring resonator (passive FRR) reduces the RIN and improves the ARW, but the low power transmission results in a low signal-to-noise ratio at the detection. For a great ARW improvement, the fiber ring resonator should have not only a great RIN reduction but also a large power transmission. An erbium-doped fiber amplifier (EDFA) is inserted in the passive resonator to construct an active fiber ring resonator (active FRR). The EDFA compensates for the loss of the resonator and leads to a high finesse and a greater reduction in the RIN. The coupling ratios of the couplers in the active FRR are both 50%-50% tap. By adjusting the EDFA gain properly, a 17-dB decrease in the RIN is demonstrated, which reduces the standard deviation by 8.5dB. Adding the active FRR between the Sagnac interferometer and the broadband light, the detected power reaches 150μW with the FOG at rest and without modulation, which is about one hundred times of the detected power when using a passive FRR. The phase noise is reduced by 6dB at the proper frequency. The ARW is improved by 4.9dB from 1.40 to 0.45 mdeg/hour1/2.

Increased Performance of Single-Channel Analog Photonic Links Enabled by Optical Wavelength-Division Multiplexing

Laser-intensity noise often sets the limitation to the performance of an analog photonic link. In particular, optimization of the all-important noise figure requires reduction of the laser’s relative-intensity noise (RIN) in the spectral band of interest. In this paper, we build on earlier work and analyze an approach for RIN reduction using multiple wavelengths. We extend the analysis and calculate all the important link parameters with no assumptions on modulator bias, RIN spectra, or signal power strength. We then impose constraints that conform with various practical situations and we calculate the performance tradeoffs among the three link parameters: 1) direct current photocurrent; 2) number of wavelengths; and 3) ratio of RIN-to-non-RIN noise. An experiment was performed with five lasers, and the results are in excellent agreement with theory.

Reduced relative intensity noise of a coherently combined single-mode semiconductor laser array under injection locking

A scheme for reducing the relative intensity noise (RIN) of laser source by an injection locked single-mode semiconductor laser array is proposed and demonstrated. Intensity and phase noise of the slave laser under injection locking are first analyzed by rate equations, and the RIN of the injection locked laser array after coherent combining is then estimated. The RIN of the injection-locked $N$-element laser array can be reduced by a factor about $1/N$ over the entire frequency range in the ideal case. Measurements with a two-element distributed feedback laser array confirm the main theoretical predictions and a nearly 3 dB reduction in RIN is obtained. The effect of the key parameters on the RIN of the injection locked array is investigated as well.

Fiber optic gyroscope noise reduction with fiber ring resonator.

A method of improving the angular random walk (ARW) of a fiber optic gyroscope (FOG) using a fiber ring resonator (FRR) to reduce the relative intensity noise (RIN) is demonstrated. An FRR reduces the RIN near the proper frequency and improves the ARW. The analytical forms for the RIN transfer function and the power transmission of the FRR are derived when the free spectral range of the FRR is much narrower than the source bandwidth. Building the FRR using couplers and fiber with low excess loss enhances the RIN reduction. For the coupling ratios, there is a trade-off between the RIN reduction and the power transmission. The coupling ratios are 90%-10% tap in the experiment. When the FRR resonator length is shorter than the FOG coil length, the ARW improvement is further enhanced by adjusting the average window length in the signal processing. An experiment demonstrated a 7dB decrease in the RIN at the loop proper frequency, which reduces the standard deviation by 3.5dB. The ARW was reduced from 970 to 570 μdeg/h1/2, which amounts to a 2.3dB reduction.

Relative intensity noise reduction with fiber ring resonator

Abstract A method of reducing the relative intensity noise (RIN) using a fiber ring resonator to improve the angular random walk (ARW) of a fiber optic gyroscope (FOG) is demonstrated. The RIN at the proper frequency f p degrades the ARW of the FOG when the detected power reaches tens of microwatts. Adding a fiber ring resonator between the source and the interferometer periodically modulates the optical spectrum and consequently reduces the RIN at f p . The reduction in the RIN is analyzed theoretically and experimentally. An experiment demonstrated a 5.2-dB decrease in the RIN at f p , which reduces the standard deviation by 2.6-dB. The ARW was reduced from 870 to 580 μ deg/h 1∕2 , which amounts to a 1.8-dB reduction.