Laser Intensity Noise Research Articles

Low noise photodetector in 0.1 mHz-1 Hz band

Laser intensity noise suppression in the millihertz frequency band is essential for space-based gravitational wave detection to ensure the sensitivity of the interferometer. Photonic feedback technology is one of the most effective methods for suppressing laser intensity noise. As the first-stage component in the feedback loop, the noise of the photodetector directly couples into the feedback loop, significantly impacting the laser intensity noise. Starting from the requirement to suppress laser intensity noise in the 0.1 mHz-1 Hz frequency band for space-based gravitational wave detection, this paper provides a detailed analysis of the factors influencing the electronics of photodetectors at extremely low frequencies. Leveraging the low dark current characteristic of photodiodes in photovoltaic mode, a zero-bias voltage scheme is adopted to reduce the dark noise of the photodiode. A transimpedance amplification circuit is designed using an integrated operational amplifier with zero offset voltage drift and low-temperature drift metal foil resistors, optimizing the transimpedance capacitor and follower circuit to reduce 1/f noise in the circuit. Active temperature control is employed to stabilize the photodiode's responsivity, and additional measures such as using a self-developed low-noise power supply and shielding against interference are implemented to further reduce noise. Ultimately, an ultra-low electronic noise photodetector operating in the 0.1 mHz-1 Hz frequency band is developed. A self-developed intensity noise evaluation system is used to comprehensively assess the noise in both the time and frequency domains, and experimental results demonstrate the constant noise characteristics of the developed detector. The experimental results show that the electronic noise spectral density of the developed detector reaches 2×10<sup>-6</sup>V/Hz<sup>1/2</sup> in the 0.1 mHz-1 Hz frequency band, and the detector's electronic noise does not vary with optical power. The detector achieves a gain of 35 kV/W at 1064 nm. The noise performance of the detector is two orders of magnitude lower than the laser intensity noise requirement (10<sup>-4</sup>/Hz<sup>1/2</sup>) for space-based gravitational wave detection, providing a critical component and technical support for high-gain photonic feedback control and laser intensity noise suppression in space-based gravitational wave detection.

Low-noise laser intensity noise evaluation system at Hz frequency band for ground-based gravitational wave detection

The direct detection of gravitational waves has opened a new window for understanding the universe and trailblazed multi-messenger astronomy. The frequency band of gravitational waves generated by various astronomical events can cover broadband range, and it is various for detection mechanism and scheme of gravitational waves in different frequency band. For example, The ground-based gravitational wave detection has the frequency band for 10 Hz to 10 kHz to mainly detect which based on Michelson interferometer; The space gravitational wave detection has the frequency band of 0.1 mHz to 1 Hz to mainly detect which based on space interferometer; The pulsar gravitational wave detection has the frequency band for 1×10<sup>-9</sup> Hz to 1×10<sup>-7</sup>Hz to mainly detect which based on pulsar timing array. The next generation ground-based gravitational wave project demands higher sensitivity to detect faint signals, necessitating an assessment system with minimal background noise to accurately measure the laser relative intensity noise. At present, the detection frequency band of ground-based gravitational wave detection devices in operation is mainly concentrated in the range of 10 Hz - 10 kHz. To satisfying the detection sensitivity requirements, the laser relative intensity noise should be accurately evaluated and suppressed to ≤2.0×10<sup>-9</sup>Hz<sup>-1/2</sup>@10 Hz and ≤4.0×10<sup>-7</sup>Hz<sup>-1/2</sup>@10 kHz by photoelectric feedback. This paper constructed an evaluation and characterization system for ground-based gravitational wave band laser intensity noise which based on low noise and high sensitivity photoelectric detection device and combined with LabVIEW and MATLAB algorithm programming for instrument control and data processing. This low noise evaluation system is used to test the background noise of FFT analyzer SR760, preamplifier SR560, photoelectric detector electronics noise and intensity noise of self-developed optical fiber amplifier, and then the data extraction and image processing are carried out by LabVIEW and MATLAB algorithms, and finally the evaluation of ground-based gravitational wave frequency band system is obtained. The experimental results show that the whole electronic noise for the preamplifier SR560 and FFT analyzer SR760 is lower than 3.8×10<sup>-9</sup> Hz<sup>-1/2</sup>@(10 Hz-10 kHz). The electronic noise for the photodetector is lower than 1.4×10<sup>-8</sup>V/√Hz@10 Hz&8.1×10<sup>-9</sup>V/√Hz@10 kHz and the accuracy of the system is calibrated and tested by the standard sinusoidal signal. Finally, the noise of commercial laser is evaluated and compared with the factory data to verify the accuracy of the evaluation system. Related research, device and system development provide hardware, software and theoretical basis for the preparation of high-power low-noise laser light source and gravitational wave detection and provide theoretical basis and evaluation criteria for ground-based gravitational wave detection in China.

Reduction in Optical Feedback Noise of Atomic Force Microscopy by High-Frequency Current Modulation

Introduction: With the advancement of technology, nanotechnology has emerged as a prominent research field. The Atomic Force Microscope (AFM) serves as a vital tool in nanoscience and technology, playing an indispensable role across various domains. Method: However, in the AFM photoelectric sensing system, the optical feedback noise from the beam deflection method can lead to inaccuracies in signal identification and analysis, impacting the accuracy and reliability of AFM measurements. To mitigate this interference, this study proposes a highfrequency current superposition system aimed at reducing optical feedback noise. By superimposing high-frequency currents, the semiconductor laser transitions from a single-mode to a multi-mode operational state, thereby altering its mode of operation and consequently reducing optical feedback noise during sensing. Initially, mathematical modeling and simulation analysis were conducted on the high-frequency current superposition noise reduction system to examine the impact of high-frequency current on the intensity noise of the semiconductor laser. Subsequently, the design of the high-frequency current superposition noise reduction system was outlined, encompassing the development of a constant current drive circuit, a voltage-controlled oscillator circuit, and a biasing circuit. Finally, the high-frequency current superposition noise reduction system underwent testing. Results: During the high-frequency current superposition noise reduction test, the system's Signal-to-Noise Ratio (SNR) increased from 15.57 dB to 17.81 dB, and the system's noise peak-to-peak value decreased from 8 mV to 6 mV. Analysis of the superposition frequency and noise reduction effect determined the optimal superposition frequency of the AFM photoelectric sensor system to be 400 MHz. Characterization experiments of the high-frequency superposition noise reduction system compared the clarity of Escherichia coli images before and after noise reduction. Conclusion: The aforementioned experimental results demonstrated that high-frequency current superposition is an effective noise reduction method capable of mitigating optical noise in the AFM photoelectric sensing system.

Improvement of optical noise in optical-injection-locked quantum dot lasers epitaxially grown on silicon by reducing external carrier noise

Abstract This study theoretically investigates the impact of external carrier noise from pumping sources on the optical noise of epitaxial quantum dot (QD) lasers on silicon. The findings indicate that the spectral linewidth and relative intensity noise (RIN) of silicon-based QD lasers using a quiet pump are significantly reduced. At 5.5 times the threshold current, the spectral linewidth decreases from 337.2 kHz to 213.2 kHz, and the RIN decreases from −141.3 dB Hz−1 to −168.8 dB Hz−1. This reduction is attributed to the lower external carrier noise level of the quiet pump, which also suppresses the spectral linewidth rebroadening effect at a high bias current. Moreover, when external optical injection locking is applied, the spectral linewidth further decreases to 24 kHz at an injection ratio of −70 dB and to 1.8 mHz at 0 dB. The RIN also slightly decreases to −172.4 dB Hz−1 with an injection ratio of 10. These results demonstrate that using a quiet pump is an effective and manageable strategy for significantly reducing both the spectral linewidth and RIN of QD lasers, thereby facilitating their application in next-generation photonic integrated circuits, continuous-variable quantum key distribution, quantum computing and ultra-precise quantum sensing.

Intensity noise and modulation dynamics of an epitaxial mid-infrared interband cascade laser on silicon

Interband cascade lasers typically have significantly lower threshold current and power consumption than quantum cascade lasers. They can also have advantages regarding costs and compactness with the photonic integration onto silicon substrates by epitaxial growth. This research introduces a novel examination of the relative intensity noise and the modulation dynamics of a silicon-based Fabry–Perot interband cascade laser emitting at 3.5 μm. The investigation delves into crucial parameters, such as relaxation oscillation frequency, differential gain, gain compression, and K-factor. The resonance patterns identified in relative intensity noise curves can provide essential insights for the thorough characterization of high-defect mid-infrared semiconductor structures intended for high-speed applications. Moreover, this study demonstrates the feasibility of reaching 10 Gbit/s free-space transmission using a silicon-based interband cascade laser in conjunction with an interband cascade infrared photodetector.

SESAM-assisted Kerr-lens mode-locked Cr:ZnS laser.

Mode-locking in Cr:ZnS/Se lasers typically rely on Kerr-lensing (KLM) or a semiconductor saturable absorber mirror (SESAM). The former allows generation of shorter pulses, but, unlike the latter, does not support self-starting mode-locking. Here, we combine the advantages of these two techniques and demonstrate the SESAM-assisted KLM Cr:ZnS laser. Our self-starting oscillator generates up to 1 W of average power with 54 fs pulses at a central wavelength of 2360 nm. We identify a general limitation for further pulse shortening in SESAM mode-locked Cr:ZnS/Se lasers, which is related to the finite operation bandwidth of the semiconductor absorbers. In our experiment, we fully exploit the potential of commercially available GaSb SESAMs and fill their entire reflection bands. Furthermore, we compare the performance of a SESAM-assisted KLM laser with a pure KLM oscillator producing broadband, yet not self-starting, 33 fs pulses with 780 mW power. We also show that the choice of saturable absorbers has a negligible impact on the laser intensity noise, which is exceptionally low with sub-0.005% integrated noise.

Self-Supervised Image Denoising of Third Harmonic Generation Microscopic Images of Human Glioma Tissue by Transformer-Based Blind Spot (TBS) Network.

Third harmonic generation (THG) microscopy shows great potential for instant pathology of brain tumor tissue during surgery. However, due to the maximal permitted exposure of laser intensity and inherent noise of the imaging system, the noise level of THG images is relatively high, which affects subsequent feature extraction analysis. Denoising THG images is challenging for modern deep-learning based methods because of the rich morphologies contained and the difficulty in obtaining the noise-free counterparts. To address this, in this work, we propose an unsupervised deep-learning network for denoising of THG images which combines a self-supervised blind spot method and a U-shape Transformer using a dynamic sparse attention mechanism. The experimental results on THG images of human glioma tissue show that our approach exhibits superior denoising performance qualitatively and quantitatively compared with previous methods. Our model achieves an improvement of 2.47-9.50 dB in SNR and 0.37-7.40 dB in CNR, compared to six recent state-of-the-art unsupervised learning models including Neighbor2Neighbor, Blind2Unblind, Self2Self+, ZS-N2N, Noise2Info and SDAP. To achieve an objective evaluation of our model, we also validate our model on public datasets including natural and microscopic images, and our model shows a better denoising performance than several recent unsupervised models such as Neighbor2Neighbor, Blind2Unblind and ZS-N2N. In addition, our model is nearly instant in denoising a THG image, which has the potential for real-time applications of THG microscopy.

A >20-W, linearly polarized single-frequency continuous-wave all-fiber laser at ∼1540 nm

We have developed both theoretically and experimentally an all polarization-maintaining fiber-based master oscillator power amplifier (MOPA) system, which allows for amplifying the single-frequency continuous-wave (CW) fiber laser to >20 W without compromising the laser intensity noise (−110 dBc/Hz@ relaxation oscillation peak) at the specific wavelength of ∼1540 nm. The experimental results agree well with the simulated results. To the best of our knowledge, this is the first demonstration of a ∼1540-nm single-frequency all fiber laser and reaching such a high-power level. The amplified laser features a narrow linewidth of ∼1.4 kHz and a spectral side-mode-suppression ratio of >38 dB, even as the laser wavelength is tuned from 1538.216 nm to 1540.616 nm. It is believed to be the narrowest laser linewidth realized in the <1550-nm wavelength band of an Erbium-Ytterbium co-doped fiber MOPA system. The all polarization-maintaining configuration of the MOPA ensures the amplified laser linearly polarized with the polarization extinction ratio of >22 dB and power root mean square fluctuations of <0.2 %@1h. This high-performance single-frequency fiber laser is expected to be an ideal pump source in the spin-exchange relaxation-free (SERF)-based atomic sensors after second harmonic generation.

Broadband suppression of laser intensity noise based on second-harmonic generation

Broadband suppression of laser intensity noise based on second-harmonic generation

Linewidth compression of a single longitudinal mode ytterbium-doped fiber laser based on femtosecond laser fabricated fiber Bragg gratings.

We demonstrate a single longitudinal mode distributed Bragg reflection (DBR) fiber laser by directly fabricating fiber Bragg gratings (FBGs) on an ytterbium-doped fiber (YDF) using a femtosecond laser. A simple optical self-injection feedback method was used to effectively compress the linewidth and reduce relative intensity noise (RIN) of a single longitudinal mode DBR fiber laser. Further, we investigated the effect of self-injection feedback cavity length and reflectivity on linewidth compression and determined that the linewidth tends to decrease with the increase of the external cavity photon lifetime. By a self-injection feedback, the laser linewidth was compressed from 31.8kHz to 1.4kHz. Meanwhile, the relaxation oscillation peak from -103.2d B/H z at 1.51MHz was suppressed to -122.3d B/H z at 0.16MHz. This low-noise narrow linewidth single longitudinal mode fiber laser is expected to be a promising candidate for applications such as active detection of neutral atmosphere and distributed fiber sensing.

Time delay Measurement Method Based on the Frequency-Spectrum Distribution of Laser Intensity or Phase Noise Signal

Time delay Measurement Method Based on the Frequency-Spectrum Distribution of Laser Intensity or Phase Noise Signal

Laser Frequency Modulation and PM-to-AM Noise Conversion in Atomic Clocks.

Laser wavelength stability is a necessity in present-day chip-scale atomic clocks (CSACs), in next-generation atomic clocks planned for Global Navigation Satellite Systems (GNSSs), and in many other atomic devices that generate their signals with lasers. Routinely, this is accomplished by modulating the laser's frequency about an atomic or molecular resonance, which in turn induces modulated laser-light absorption. The modulated absorption then generates a correction signal that stabilizes the laser wavelength. However, in addition to creating absorption modulation for laser wavelength stabilization, the modulated laser frequency can produce a time-dependent variance in transmitted laser intensity noise because of laser phase-noise (PM) to transmitted laser intensity-noise (AM) conversion. Here, we show that the time-varying PM-to-AM conversion can have a significant influence on the short-term frequency stability of vapor-cell atomic clocks. If diode-laser enabled vapor-cell atomic clocks are to break into the [Formula: see text] frequency-stability range, the amplitude of laser frequency modulation for wavelength stabilization will need to be chosen judiciously.

Gain-dynamics-dependent noise evolution in a narrow-linewidth single-frequency Yb-fiber amplifier chain

The relative intensity noise (RIN) and frequency noise (FN) of a single-frequency fiber laser (SFFL) are essential parameters that directly determine the sensitivity of high-precision optical metrology. Here, we report the gain-dynamics-dependent evolution of the RIN and FN in an SFFL Yb-fiber amplifier chain. Three types of single-mode Yb-doped fibers constructed with diverse mode-field diameters (MFD), core diameter, overlapping factor and doping concentrations were separately employed to achieve three SFFL amplifiers. The noise characteristics of the SFFL during the fiber amplification processes were investigated using contrast measurement curves of the RIN and FN. These results indicate that the amplifier-induced RIN and FN exhibit an iconic corner frequency of approximately 10 kHz. The relationships between the noise and signal input power exhibit opposite trends in the low- and high-frequency regions. In particular, the RIN curves of the noise power spectral density (PSD) reveal that a gain fiber with a larger overlapping factor can optimize the RIN. Moreover, higher doping concentration gain fiber can suppress the FN of the low-frequency region, realizing a narrower-linewidth SFFL. We believe that this study provides significant guidance for optimizing high-power, low-noise SFFL generation.

Enhancing the sensitivity of atom-interferometric inertial sensors using robust control

Atom-interferometric quantum sensors could revolutionize navigation, civil engineering, and Earth observation. However, operation in real-world environments is challenging due to external interference, platform noise, and constraints on size, weight, and power. Here we experimentally demonstrate that tailored light pulses designed using robust control techniques mitigate significant error sources in an atom-interferometric accelerometer. To mimic the effect of unpredictable lateral platform motion, we apply laser-intensity noise that varies up to 20% from pulse-to-pulse. Our robust control solution maintains performant sensing, while the utility of conventional pulses collapses. By measuring local gravity, we show that our robust pulses preserve interferometer scale factor and improve measurement precision by 10× in the presence of this noise. We further validate these enhancements by measuring applied accelerations over a 200 μg range up to 21× more precisely at the highest applied noise level. Our demonstration provides a pathway to improved atom-interferometric inertial sensing in real-world settings.

Theoretical analysis of relative intensity noise in semiconductor QD lasers

Theoretical analysis of relative intensity noise in semiconductor QD lasers

140-Hz Narrow-Linewidth Brillouin/Erbium- Ytterbium Fiber Laser

We demonstrate an ultra-narrow-linewidth Brillouin/Erbium-Ytterbium co-doped fiber ring laser, which uses a length of 80-cm Erbium-Ytterbium co-doped fiber as both the Brillouin gain and linear gain media. The 976-nm pump threshold is only 80 mW when the Brillouin pump power is 20 mW. The output power of this laser reaches about 200 mW. The laser linewidth is as narrow as 140 Hz. The relative intensity noise of the fiber laser is -145 dB/Hz. The phase noise at 1 kHz is as low as to -107 dB re rad/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> (1-m optical path difference). Moreover, the 140-Hz-linewidth Brillouin laser light is transferred from a 2-MHz-linewidth Brillouin pump light. This fiber laser presents outstanding linewidth-narrowing characteristic. It presents many potential applications in coherent communications, optical fiber sensors, and etc.

Sensitivity of quantum gate fidelity to laser phase and intensity noise

The fidelity of gate operations on neutral atom qubits is often limited by fluctuations of the laser drive. Here, we quantify the sensitivity of quantum gate fidelities to laser phase and intensity noise. We first develop models to identify features observed in laser self-heterodyne noise spectra, focusing on the effects of white noise and servo bumps. In the weak-noise regime, characteristic of well-stabilized lasers, we show that an analytical theory based on a perturbative solution of a master equation agrees very well with numerical simulations that incorporate phase noise. We compute quantum gate fidelities for one- and two-photon Rabi oscillations and show that they can be enhanced by an appropriate choice of Rabi frequency relative to spectral noise peaks. We also analyze the influence of intensity noise with spectral support smaller than the Rabi frequency. Our results establish requirements on laser noise levels needed to achieve desired gate fidelities.

More Than 20 W, High Signal-to-Noise Ratio Single-Frequency All-Polarization-Maintaining Hybrid Brillouin/Ytterbium Fiber Laser

A high-power all-polarization-maintaining (PM) hybrid Brillouin/ytterbium fiber laser (BYFL) at 1064 nm is demonstrated by adopting large-mode-area fiber with core/cladding diameter of 20/400 µm as the hybrid gain medium. Single-frequency laser output with a record output power of 23 W and an excellent optical signal-to-noise ratio of 72 dB is achieved. The BYFL also presents linear-polarized operation with a polarization extinction ratio of over 16.5 dB, excellent output beam profile and long-term stability. Meanwhile, the relative intensity noise of the Brillouin pump laser is significantly reduced with a maximum factor of 36 dB by the BYFL at the high frequency range, while the corresponding frequency noise is reduced by 15–30 dB across the whole examined frequency range. In addition, the estimated laser linewidth was compressed by 7.8 times from 260 kHz to 33 kHz. To the best of our knowledge, this is the highest output power that emitted from any kind of single-frequency fiber lasers and Brillouin lasers. It is believed that the combination of the hybrid gains and larger mode area fiber provides a promising scheme for realizing further power scaling of a single-frequency laser with all-fiber configuration.

Programmable precision voltage reference source for space-based gravitational wave detection

Gravitational wave detection plays an important role in exploring the universe and opening up a new chapter for multi-messenger astronomy. As the most common device used for space and ground-based gravitational wave detection, large-scale laser interferometer requires a low-noise laser as a beam source. The noise of the laser can be suppressed by utilizing the optoelectronic negative-feedback noise reduction technology to improve the sensitivity of large-scale laser interferometer. The optoelectronic negative-feedback control system can suppress laser noise by subtracting the photodetector signal from the voltage reference signal, and then calculating the modulated signal by a proportional integral differentiator to control the output power of the pump current driver. Since the photodetector signal is affected by the laser intensity, its output voltage varies within a certain range, which requires that the output voltage of the voltage reference source signal is variable. In addition, the performance of the voltage reference directly affects the overall performance of the feedback control loop, therefore it is the lower limit of laser noise suppression. We develop a high-precision low-noise program-controlled voltage reference by selecting low-noise reference chip and digital-to-analog conversion chip, designing external control circuit, using low-temperature drift coefficient components and using temperature control and electromagnetic shielding. The digital-to-analog conversion is controlled through the FPGA module programming to accurately realize the reference voltage change. The output voltage range of the developed voltage reference source is from negative 10 V to positive 10 V and the minimal precision of the voltage variation is 20 bit. The voltage noise spectral density of the developed voltage reference is below &lt;inline-formula&gt;&lt;tex-math id="M1"&gt;\begin{document}$9.6 \times 1{0^{ - 6}}/\sqrt {\rm Hz}$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20222119_M1.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20222119_M1.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; and the noise performance of the reference source is less than the laser intensity noise in the space-based gravitational wave frequency band. The developed voltage reference source provides an important technical support for laser intensity noise suppression in gravitational wave detection.

Low-noise and high-power second harmonic generation of 532nm laser for trapping ultracold atoms.

Optical lattices for coherently manipulating ultracold atoms demand high-power, low-noise, narrow-line-width, and continuous-wave lasers. Here, we report the implementation of a 30W 532nm low-noise laser by second harmonic generation from a 1064nm fiber laser, which is capable to generate optical lattices for a quantum gas microscope of Rb87 atoms. The overall conversion efficiency is 59% at an input power of 51W with a lithium triborate crystal coupled to a ring cavity. The relative intensity noise of the output laser is suppressed to -120dBc/Hz in the range of 10Hz-100kHz with a high dynamic range of over 50dB, which is suitable for long-term trapping and coherent manipulation of the quantum gases.