Low-frequency Relative Intensity Noise Research Articles

Coherent beam combining of two all-PM thulium-doped fiber chirped pulse amplifiers

In this paper, we report a coherent beam combining (CBC) system that involves two thulium-doped all-polarization maintaining (PM) fiber chirped pulse amplifiers. Through phase-locking the two channels via a fiber stretcher by using the stochastic parallel gradient descent (SPGD) algorithm, a maximum average power of 265 W is obtained, with a CBC efficiency of 81% and a residual phase error of λ/17. After de-chirping by a pair of diffraction gratings, the duration of the combined laser pulse is compressed to 690 fs. Taking into account the compression efficiency of 90% and the main peak energy proportion of 91%, the corresponding peak power is calculated to be 4 MW. The laser noise characteristics before and after CBC are examined, and the results indicate that the CBC would degrade the low frequency relative intensity noise (RIN), of which the integration is 1.74% in [100 Hz, 2 MHz] at the maximum combined output power. In addition, the effects of the nonlinear spectrum broadening during chirped pulse amplification on the CBC efficiency are also investigated, showing that a higher extent of pulse stretching is effective in alleviating the spectrum broadening and realizing a higher output power with decent combining efficiency.Graphical

Relative Intensity Noise in a Two‐Section Distributed Bragg Reflector Tapered Laser

The relative intensity noise (RIN) in a two‐section distributed Bragg reflector (DBR) tapered laser emitting around 1060 nm is being studied experimentally. By analyzing the RIN spectra, a resonance frequency and damping is obtained. Herein, the dependence of the resonance frequency and the maximum RIN at the resonance frequency is studied as functions of the ridge waveguide (RW) and tapered section currents independently. The low frequency RIN is approximately −162 dB Hz−1, and the RIN peak at currents providing high output power is as low as −154 dB Hz−1. The observed nonlinearity of the squared resonance frequency with the RW section current is attributed to carrier saturation. The measured high values of the resonance frequency, exceeding 2.5 GHz for RW currents higher than 100 mA at a tapered current of 3 A, indicate gain lever enhancement.

Noise analysis of optoelectronic oscillators in the presence of the relative intensity noise of the laser

Optoelectronic oscillators (OEOs) have recently been considered very good candidates for producing ultra-low phase noise radio frequency (RF) oscillations. One of the most important noise sources of any OEO is the relative intensity noise (RIN) of the optical field in the optical part of this system. It is the dominant noise source in some OEO cases. The initial source of the RIN is the laser; however, it may be largely enhanced in the fiber by some phenomena such as the guided entropy mode Rayleigh scattering. Here a frequency domain analysis approach is introduced to separately analyze the effect of both the low-frequency RIN (LFRIN) and the high-frequency RIN (HFRIN), i.e., the RIN around the RF harmonics, on both the phase and amplitude noises of the delay-based single-loop/dual-loop OEOs. The presented approach can take into account the RIN at both the input and the output of the optical fibers. The measured or analytical nonlinear gain function of the photodetector and the RF amplifier can be taken into account to accurately characterize the important effect of the amplitude noise to phase noise (AN-PN) conversion. It is shown that the AN-PN conversion can largely enhance the LFRIN-induced phase noise. Also, to a much lesser degree, it can enhance the HFRIN-induced phase noise. Furthermore, it is shown that the AN-PN conversion reduces the HFRIN- and LFRIN-induced amplitude noise. It is also shown that the fibers’ dispersion has a small effect on the phase/amplitude noise power induced by either LFRIN or the HFRIN, especially for smaller fiber lengths. The validity of the new analysis approach is verified by comparing its results with those of the previously published works in the literature.

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.

Simultaneously improving the linewidth and the low-frequency relative intensity noise of a single-frequency fiber laser

By optimizing the pump intensity noise, simultaneous improvements in the linewidth and the low-frequency (<20 kHz) relative intensity noise (RIN) of a 1550 nm single-frequency fiber laser (SFFL) are demonstrated. A lower-RIN distributed feedback laser diode with optoelectronic feedback replaces the fiber Bragg grating laser diode to pump the SFFL. As a result, the linewidth of the SFFL is narrowed from 1.3 to 0.65 kHz and the RIN is decreased by about 7–11.5 dB from 0.2 to 5 kHz, and by about 3–7 dB from 5 to 10 kHz. The improved SFFL is an ideal laser source for fiber-optic sensing applications.

Near quantum-noise limited and absolute frequency stabilized 1083 nm single-frequency fiber laser.

The Earth's magnetic field has significant effects that protect us from cosmic radiation and provide navigation for biological migration. However, slow temporal variations originating in the liquid outer core invariably exist. To understand the working mechanism of the geomagnetic field and improve accuracy of navigation systems, a high-precision magnetometer is essential to measure the absolute magnetic field. A helium optically pumping magnetometer is an advanced approach, but its sensitivity and accuracy are directly limited by the low-frequency relative intensity noise and frequency stability characteristics of a light source. Here, we demonstrate a near quantum-noise limited and absolute frequency stabilized 1083nm single-frequency fiber laser. The relative intensity noise is only 5dB higher than the quantum-noise limit, and the root mean square of frequency fluctuation is ∼17 kHz after locked. This fiber laser could suppress the fluctuation of magnetic resonant frequency and improve the signal-to-noise ratio of the magnetic resonance signal detection.

Mitigation of relative intensity noise of quantum dash mode-locked lasers for PAM4 based optical interconnects using encoding techniques.

Quantum dash (Q-Dash) passively mode-locked lasers (PMLLs) exhibit significant low frequency relative intensity noise (RIN), due to the high mode partition noise (MPN), which prevents the implementation of multilevel amplitude modulation formats such as PAM4. The authors demonstrate low frequency RIN mitigation by employing 8B/10B and Manchester encoding with PAM4 modulation format. These encoding techniques reduce the overlap between the modulation spectral content and the low-frequency RIN of the Q-dash devices, at the expense of increased overhead. The RIN of the 33.6 GHz free spectral range Q-dash PMLL was characterized, and the results obtained show very high levels of RIN from DC to 4 GHz, but low levels for higher frequencies. The performance improvement for 28 GBaud 8B/10B and Manchester encoded PAM4 signal has been demonstrated compared to the case when no encoding is used. Finally, the effect of RIN on the system performance was demonstrated by comparing the bit error rate (BER) performance of the PAM4 signaling obtained with an external cavity laser (ECL) to those obtained with Q-dash PMLL.

Cross-Correlation Measurements of Phase Noise Induced by Relative Intensity Noise in Photodetectors

Up-converted phase noise, which is induced by the low-frequency relative intensity noise (RIN) of a laser through AM-PM conversion within a photodetector (PD), is first measured here by means of a cross-correlation method. Our proposed measurement system can isolate the RIN-induced phase noise from noise contributions of other components, such as amplifiers, modulators, and mixers. In particular, shot noise and thermal noise generated from the PD are also suppressed by this method, so that standalone characteristics of the RIN-induced phase noise can be obtained. Experimental results clearly show the quantitative relationship between the RIN-induced phase noise and the incident optical power of the PD. Our findings indicate that the least RIN-induced phase noise appeared at the saturation point of the PD, which is about -162 dBc/Hz at 10 kHz offset.

Compact Bragg Grating Stabilized Ridge Waveguide Laser Module With a Power of 380 mW at 780 nm

A single ridge waveguide laser wavelength stabilized by a compact external Bragg grating cavity with an output power as much as 380 mW and a beam quality factor M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of 1.3 is presented. A characterization concerning tunability by current, short-term optical linewidth, and intensity fluctuations is performed. The device exhibits a wavelength tuning factor at 780 nm of 0.5 pm/mA, a short-term optical linewidth of 18 kHz, and a low-frequency relative intensity noise of -136 dB/Hz. These characteristics allow a high resolution absorption spectroscopy of the D2 line of rubidium.

Low-frequency noise suppression of a fiber laser based on a round-trip EDFA power stabilizer

We have designed a power stabilizer based on a round-trip erbium-doped fiber amplifier (EDFA) structure to suppress the low-frequency relative intensity noise (RIN) for a narrow linewidth fiber laser. The noise suppressor is analyzed theoretically and its feasibility is verified experimentally. For a short-cavity single-frequency fiber laser with this device, about 20 dB low-frequency RIN improvement is achieved (down to −120 dB Hz−1 at 10 Hz). The corresponding frequency noise is also reduced by a factor of 1.6. The proposed method is an effective solution to achieve a low-frequency low RIN laser source for highly coherent detection applications.

Pump-to-Stokes relative intensity noise transfer and analytical modeling of mid-infrared silicon Raman lasers

An analytical model for mid-infrared (mid-IR) silicon Raman lasers (SRLs) is developed. The relative intensity noise (RIN) transfer from the pump to the Stokes in the lasers is also investigated. The analytical model can be used as a versatile and efficient tool for analysis, design and optimization of mid-IR SRLs. It is shown that conversion efficiency of 70% is attainable and the low-frequency RIN transfer may be suppressed to below 1 dB by pumping low-loss waveguides at high intensities.

Gain-clamped discrete Raman amplifier with suppressed low-frequency relative intensity noise pump-to-signal transfer

We demonstrate an optically gain-clamped discrete Raman amplifier with a suppressed low-frequency relative intensity noise transfer from pump sources to an input amplified signal, which is accomplished by employing a cascaded second-order Raman fiber resonator as an optical amplification. The input signal dynamic range for a 3-dB gain compression is controllable without affecting the clamping level. In addition, we find that the Raman net gain is unsusceptible to fiber parameters such as the gain and attenuation coefficients at the pump, first-, and second-order Stokes lines. These characteristics fit our proposed gain-clamped discrete Raman amplifier for practical deployment although it works at the expense of the efficiency.

Stabilization of the output power from a Raman fiber laser by generated-amplified spontaneous emission

In this work, an optical approach is theoretically presented for suppressing the low-frequency relative intensity noise transfer from the pump source to the Raman fiber laser output. This is accomplished by generating a sufficiently high-amplified spontaneous emission intensity at one Stokes shift in the wavelength from the lasing line to stabilize the latter at the cost of lasing efficiency. In addition, the clamped laser output can be easily controlled in a large dynamic range. These special properties suit the proposed Raman fiber laser to usage in Raman fiber amplifiers as the pump source.

Low-Frequency Relative Intensity Noise due to Nonlinear Mode Competition in InGaAsP–InP Microring Lasers

An enhanced low frequency relative intensity noise due to mode hopping induced from the asymmetric gain suppression in InGaAsP-InP microring lasers, is observed experimentally for the first time. The experimental results are in good agreement to those predicted by a multimode model based on the rate equation approximation accounting for both symmetric and asymmetric nonlinear gain suppression

Output-power-clamped Raman fibre laser with suppression of low-frequency RIN transfer from pump sources

An optical approach is demonstrated for suppressing low-frequency relative intensity noise (RIN) transfer from pump sources to the author's proposed Raman fibre laser (RFL). Low-frequency RIN transfer suppression is accomplished by outputting not the second- but first-order Stokes, which output power is clamped by the lasing second-order Stokes in the cavity and can be easily tuned in a large dynamic range. Thus, this technique fits the RFL for use as pump sources in Raman fibre amplifiers.

Polarization-resolved intensity noise in erbium-ytterbium codoped fiber lasers

The intensity-noise properties and polarization dynamics of a two-polarization Er:Yb codoped fiber laser are investigated and discussed. For a laser without polarization control, the low-frequency relative intensity noise reaches a minimum of approximately −90 dB/Hz. Polarization-sensitive detection reveals that the low-frequency intensity noise is influenced by a strong resonance associated with anticorrelated intensity fluctuations of the orthogonal linearly polarized modes. The frequency of the resonance varies linearly with laser power and leads to an increase in low-frequency noise of more than 45 dB for the polarized output of the laser.

Double-cavity Raman fibre lasers with suppressed pump-to-Stokes transfer of low-frequency RIN

Double-cavity Raman fibre lasers with two longitudinally concatenated Stokes cavities are shown to permit the suppression of low-frequency relative-intensity-noise (RIN) transfer from the pump laser to the Stokes output. This concept can provide low-noise Raman fibre lasers for use as pump sources in co-pumped Raman amplifiers.

Configurable three-wavelength Raman fiber laser for Raman amplification and dynamic gain flattening

A three-wavelength Raman fiber laser is presented that is suitable for the generation of a flattened Raman gain curve over a broad range of extended telecommunications bandwidths. This laser utilizes an 1100-nm Yb-doped cladding pumped fiber laser and a cascaded Raman resonator to generate output radiation at 1427, 1454, and 1480 nm. The slope efficiency for conversion from the 1100- to 14xx-nm radiation is 0.38, the total output power is 1.1 W, and the fiber laser "wall-plug" efficiency is 0.10. The output power at 1454 and 1480 nm is adjusted by varying the reflectivity of the fiber grating output couplers and the suppression of lower order spectral components is between 15 and 20 dB. Low frequency relative intensity noise (RIN) measurements indicate RIN values less than -90 dB/Hz below 100 kHz.

Dynamics and chaos stabilization of semiconductor lasers with optical feedback from an interferometer

This paper studies dynamical behaviors and noise properties of semiconductor lasers with optical feedback from an interferometer, i.e., two external cavities. Bifurcation diagrams versus external-cavity length and reflectivity reveal the existence of stable regions for fixed states and limit cycles. The steady-state analysis for a laser diode with two external feedbacks is performed, which shows that oscillation modes are locked at minimum excess gain modes. Oscillation frequencies of limit cycles show discrete transitions with the variation of ratios between two external-cavity lengths and reflectivities. It is proposed that such a model might be applied to stabilization of the feedback-induced chaos in external-cavity semiconductor lasers since the stable region is quite robust for wide parameter ranges. Intensity and phase noises are examined and the results indicate that the low-frequency relative intensity noise level and the linewidth can be greatly improved when the laser output is a fixed state or a limit cycle.

Noise equivalent circuit of a two-mode semiconductor laser with the contribution of both the linear and the nonlinear gain

An equivalent circuit model of a semiconductor laser diode and its MDS (microwave and RF design systems) implementation are described that allow a straightforward calculation of the noise characteristics of a laser diode combined with electronic components. This noise equivalent circuit model that is derived from the two-mode rate equations including both the self-saturation and cross-saturation contributions to the nonlinear gain and Langevin noise sources demonstrates the influence of the sl demode on the low-frequency relative intensity noise (RIN) enhancement.