Coherent Optical Detection Research Articles

Performance of free-space optical coherent detection systems under imprecise channels with nonzero boresight pointing errors

Performance of free-space optical coherent detection systems under imprecise channels with nonzero boresight pointing errors

Suppression of intensity noise and phase noise for thulium-holmium co-doped fiber laser by self-injection locking

The present study proposes and demonstrates a narrow-linewidth thulium-holmium co-doped fiber laser based on self-injection locking. The laser with single-longitudinal-mode operation is realized using a fiber Bragg grating as a wavelength-selection component and a dual-ring compound cavity as a mode-selection filter. The linewidth was compressed by increasing the photon lifetime by utilizing delay fibers of different lengths in the feedback cavity. The results show that the optical signal-to-noise ratio of the output laser is 57.2 dB, and the relative intensity noise is -130.43 dB/Hz with self-injection locking. The intrinsic linewidth is compressed from 39.45 kHz to 3.21 kHz, a reduction of nearly an order of magnitude. The linewidth compression mechanism solves the problems associated with deep linewidth compression in fiber lasers with long ring cavities and fosters the development of practical and reliable all-fiber structures. The laser source is characterized by low cost, high coherence, and low noise, which are highly desirable features in coherent optical detection, high-resolution spectrometers, microwave photonics, and optical sensing.

Non-contact sub-nanometer displacement sensing based on optical digital coherent detection using frequency-domain filters

Non-contact sub-nanometer displacement sensing based on optical digital coherent detection using frequency-domain filters

Length of Polarization-Correlation Based on Speckle Pattern for Optical Coherent Detection

Length of Polarization-Correlation Based on Speckle Pattern for Optical Coherent Detection

Phase and frequency-resolved microscopy of operating spin Hall nano-oscillator arrays.

Coherent optical detection is a powerful technique for characterizing a wide range of physical excitations. Here, we use two optical approaches (fundamental and parametric pumping) to microscopically characterize the high-frequency auto-oscillations of single and multiple nano-constriction spin Hall nano-oscillators (SHNOs). To validate the technique and demonstrate its robustness, we study SHNOs made from two different material stacks, NiFe/Pt and W/CoFeB/MgO, and investigate the influence of both the RF injection power and the laser power on the measurements, comparing the optical results to conventional electrical measurements. To demonstrate the key features of direct, non-invasive, submicron, spatial, and phase-resolved characterization of the SHNO magnetodynamics, we map out the auto-oscillation magnitude and phase of two phase-binarized SHNOs used in Ising machines. This proof-of-concept platform establishes a strong foundation for further extensions, contributing to the ongoing development of crucial characterization techniques for emerging computing technologies based on spintronics devices.

Nano-displacement sensing by phase-diversity optical digital coherent detection utilizing alternating quadrature phase-modulated reference light

We have introduced a nanometer-scale non-contact displacement sensing method that relies on phase-diversity optical digital coherent detection. In our prior work, we used a conventional setup involving a 90°optical hybrid, two balanced amplified photodetectors (BAPs), and a narrow-linewidth (NLW) laser, which is complex and costly. However, in this paper, we have streamlined the system configuration by employing alternating quadrature phase modulation (AQPM) reference light, implemented using a phase modulator and a BAP. Moreover, we’ve employed an economical distributed feedback (DFB) laser, enabling us to achieve displacement sensing at 1.6 nm with a resolution of 0.6 nm. It is notable that there is some degradation in the performance due to the phase noise compared to the NLW laser, which achieves a displacement sensing down to 0.6 nm with a 0.2 nm resolution. Nevertheless, the DFB-AQPM system holds a significant potential for cost-effective, high-resolution nanometer-scale sensing applications.

An All-Digital Optical Phase-Locked Loop Suitable for Satellite Downlinks

The optical signal propagation used in satellite uplinks and downlinks is influenced by absorption, scattering, and changes in the atmospheric refractive index or turbulence, causing optical signal attenuation. A free space optics (FSO) communications system using coherent communication can improve the link sensitivity and reach higher distances. This article proposes a new architecture for the phase detector in an all-digital optical phase-locked loop (OPLL) for coherent optical detection. Firstly, the performance of the proposed phase detector is evaluated under Gaussian noise, where the best operation point is found for the OPLL working with two sample rates: 625 MSa/s and 10 GSa/s. The system analyses also take a non-negligible delay into account. Then, it will be evaluated and compared with an OPLL using an analog phase detector in the presence of atmospheric turbulence. Finally, in three different atmospheric turbulence conditions, the effect of wind speed on communication quality is investigated through the obtained bit error rate (BER) from the recovered data for a bit rate of 20 Gbps. The results show that the proposed digital phase detector can track a signal under longer feedback loop delays and fading signals.

A Compensation Method for Nonlinearity Errors in Optical Interferometry.

Optical coherent detection is widely used for highly sensitive sensing applications, but nonlinearity issues pose challenges in accurately interpreting the system outputs. Most existing compensation methods require access to raw measurement data, making them not useful when only demodulated data are available. In this study, we propose a compensation method designed for direct application to demodulated data, effectively addressing the 1st and 2nd-order nonlinearities in both homodyne and heterodyne systems. The approach involves segmenting the distorted signal, fitting and removing baselines in each section, and averaging the resulting distortions to obtain precise distortion shapes. These shapes are then used to retrieve compensation parameters. Simulation shows that the proposed method can effectively reduce the deviation caused by the nonlinearities without using the raw data. Experimental results from a silicon-photonics-based homodyne laser Doppler vibrometry prove that this method has a similar performance as the conventional Heydemann correction method.

Optical coherent detection through multi-scattering media by wave-mixing cleaning effect in liquid-crystal OASLM.

Liquid-crystal (LC) optically addressable spatial light modulators (OASLMs) allow control of the phase and amplitude of optical beams. By performing wave mixing in an OASLM, we show that coherent phase detection can be achieved for light beams passing through highly scattering media, such as foam layers with several cm thicknesses. Thanks to the adaptive response of our OASLM, the phase information on the speckle signal is transferred at the output of the OASLM to the plane wave reference beam, allowing the cleaning of optical distortions and the direct measurement of amplitude phase modulations with a small diameter single photodiode. A good signal-to-noise ratio (SNR) is demonstrated for foam thickness up to 3 cm. These properties, together with the recently demonstrated sub-ms response time of our OASLM, make the method compatible with foreseen applications for imaging in biomedical tissues and turbid media.

Use of Optical Coherent Detection for Environmental Sensing

We discuss the use of the fiber channel transfer matrix extracted from a digital coherent receiver for environmental sensing. We show that the polarization rotation vector describing the unitary part of the matrix is highly sensitive to perturbations affecting the optical fiber link. This vector can be readily extracted from the time-dependent transfer matrix reconstructed by the coherent receiver during system operation and may serve as an effective observable to monitor environmental changes.

Experimental demonstration of non-contact and quasi OPD-independent nanoscale-displacement measurement by phase-diversity optical digital coherent detection and comb filtering.

We experimentally demonstrated and quantitatively evaluated a non-contact nanometer-displacement measurement using phase-diversity optical digital coherent detection implemented by a 90 ° optical hybrid and a narrow-linewidth probe laser without fine-tuning of optical path length difference (OPD). Combined with a comb filter, the system exhibits 99.99% linearity detection with a scale and resolution of approximately 7 nm and 2 nm respectively, as well as a wideband vibration of 5.85 MHz. We also experimentally analyzed the effect of noise arising from the OPD and demonstrated the detection of displacement down to 85 nm with a resolution of 24 nm at an OPD of 342 cm.

Research Progress of Wide Tunable Bragg Grating External Cavity Semiconductor Lasers.

In this paper, we review the progress of wide tunable Bragg grating external cavity semiconductor lasers (BG-ECSLs). We concentrate on BG-ECSLs based on the wide tunable range for multicomponent detection. Wide tunable BG-ECSLs have many important applications, such as wavelength-division multiplexing (WDM) systems, coherent optical communications, gas detection and atom cooling. Wide tunability, narrow linewidth and a high side-mode suppression ratio BG-ECSLs have attracted much attention for their merits. In this paper, three main structures for achieving widely tunable, narrow linewidth, high side-mode suppression ratio BG-ECSLs are reviewed and compared in detail, such as the volume Bragg grating (VBG) structure, fiber Bragg grating (FBG) structure and waveguide Bragg grating (WBG) structure of ECSLs. The advantages and disadvantages of different structures of BG-ECSLs are analyzed. The results show that WBG-ECSLs are a potential way to realize the integration, small size, wide tuning range, stable spectral output and high side-mode suppression ratio laser output. Therefore, the use of WBG as optical feedback elements is still the mainstream direction of BG-ECSLs, and BG-ECSLs offer a further new option for multicomponent detection and multi-atoms cooling.

Heat treatment to regulate bismuth valence toward enhanced radiation resistance in barium gallo‐germanate glass

AbstractDue to the excellent infrared transparency, low phonon energy, and high rare‐earth solubility, germanate laser glass has attracted much attention in mid‐infrared fiber lasers for potential coherent laser radar systems, optical detection, remote sensing, and laser surgery. However, radiation‐induce darkening often occurs in fiber lasers that operate in radiation environment. Here, we report a useful strategy to improve the radiation resistance by adding multivalence Bi ions and discuss its radiation resistance mechanism. In order to study the effect of valence states on the radiation resistance of barium gallo‐germanate glass, we adjust the valence states of Bi ions by heat treatment, the potential mechanism of which is discussed in detail based on the absorption, photoluminescence (PL), and Raman spectra. The absorption, electron paramagnetic resonance, and photoluminescence spectra have proved the interconversion of Bi ions between low and high valence, which inhibits the formation of Ge‐related electron center (GEC) and non‐bridging oxygen hole center defects in the irradiation process. In addition, the Bi3+ content increased by heat treatment is beneficial to serve as electron‐trapping centers in γ‐ray irradiation, thus further reducing the formation of GEC. This study provides a simple method to achieve Bi valence regulation, so as to improve the radiation resistance.

Tens of hertz ultra-narrow linewidth fiber ring laser based on external weak distributed feedback.

We suggest and demonstrate a single-frequency fiber ring laser with an ultra-narrow linewidth based on an external weak distributed feedback. A π phase-shifted fiber Bragg grating (PSFBG) is used to improve mode selection and enable single-longitudinal mode (SLM) laser operation. The linewidth is then further strongly compressed using a signal generated by a weak distributed feedback structure (WDFS) and injected into the main laser cavity to suppress spontaneous emission. The resulting ultra-narrow linewidth fiber ring laser achieves a side-mode suppression ratio (SMSR) of ∼72 dB, and low white frequency noise of ∼10.3 Hz2/Hz, which correspond to an instantaneous linewidth of ∼32.3 Hz in the normal operating condition of the laser. Our linewidth compression mechanism not only solves the problems associated with deep linewidth compression in long-cavity fiber laser, but also fosters the development of practical and reliable all-fiber structures. Our laser source is characterized by low cost, high coherence, and low noise, which are highly desirable features in coherent optical detection, high-resolution spectrometers, microwave photonics, and optical sensing.

On‐Chip High‐Speed Coherent Optical Signal Detection Based on Photonic Spin‐Hall Effect

AbstractThe use of coherent optical signal processing in long‐distance optical communication systems has dramatically increased data capacity enabling encoding of multiple‐bit information in the phase of a light beam. Direct detection of phase information of a high‐speed modulated light remains challenging and requires an external, local oscillator for referencing, which is expensive for short‐reach optical communications, for example, in data centers. The availability of less‐complex integrated photonics devices for coherent signal detection will alleviate this bottleneck. On the other hand, phase information of coherent, orthogonally polarized light beams can be extracted from their polarization states, and it is, therefore, possible to achieve phase measurements via fast polarization detection. Here, an on‐chip nanodisk device is demonstrated for high‐speed coherent optical signal detection enabled by spin–orbit coupling in Si‐photonics circuitry. In a coherent communication experiment with up to 32 GBaud rate, the high‐speed phase‐shift keying and quadrature amplitude modulation signals detected by a Si nanodisk based polarization measurements at multiple wavelengths in the C‐band are recovered with a bit error rate below the forward error correction threshold. The proposed on‐chip nanodisk device shows promise in high‐speed coherent optical communication applications where phase detection is required at low cost and small footprint.

Material and thickness selection of dielectrics for high transmittance terahertz window and metasurface

Dielectric materials, commonly used in conventional terahertz devices, are applied in all-dielectric metasurfaces for many functional applications in recent years. Various measurement systems, including time-domain spectrometer (TDS), vector network analyzer (VNA) and Fourier transform infrared (FTIR) spectrometer, have been used to analyze the dielectric properties of common materials in terahertz region. However, only the influence of thickness or Fabry-Perot (FP) term on transmittance under a specified incident angle was considered. In this paper, the influences of refractive index, absorption coefficient and thickness on the extrema and fluctuation period of transmittance spectra are systematically analyzed for the first time. Moreover, the transmittance spectra of four plates, which are two different thicknesses of each TPX and HDPE material, were measured by 3 THz systems. These three systems are frequency multiplication chain (FMC), VNA and TDS which are corresponding to direct and coherent detection methods by electronic, and optical coherent detection method. For each plate, the transmittance spectra measured by three systems have been compared, which can obtain the average error of each system. These measurement results are in good agreement with the theoretical calculation values. The electronic coherent detection technique shows half of the test error compared with the other two techniques. The criteria for dielectrics selection proposed in this paper can provide some guidance for the selection of suitable material and thickness of high transmittance windows and the substrate of dielectric metasurfaces.

Numerical investigations on phase-diversity optical digital coherent receiver-based noncontact photoacoustic signal detection

Numerical investigations on phase-diversity optical digital coherent receiver-based noncontact photoacoustic signal detection

Optical Versus RF Free-Space Signal Transmission: A Comparison of Optical and RF Receivers Based on Noise Equivalent Power and Signal-to-Noise Ratio

To compare the power efficiency of free-space signal transmission over an optical carrier with that of signal transmission over an RF carrier, we compare the signal to noise ratio and the noise equivalent power of various optical detection schemes with that of simple antenna reception of an RF signal. We point out that for direct optical detection schemes, the noise equivalent power for optical detection is orders of magnitude larger than for RF signal detection with an antenna, with only coherent optical detection requiring similar power levels at the receiver as RF detection with an antenna and mixer, for the same available bandwidth. This is often ignored in articles in which the relative advantages of optical and RF transmission are discussed. Optical transmission mostly has advantages when very high capacity is required and where the available RF bandwidth is small.

Low-noise Brillouin random fiber laser with Auto-tracking dynamic fiber grating based on a saturable absorption ring

A low-noise Brillouin random fiber laser (BRFL) with stabilized lasing output and narrow linewidth is proposed and demonstrated experimentally. Stimulated Brillouin scattering and Rayleigh scattering are exploited to work as the gain and random distributed feedback mechanisms, respectively. A simple unpumped Erbium-doped fiber (EDF) loop structure which consists of a section of EDF and an optical coupler is introduced into the BRFL cavity, forming a saturable absorption ring (SAR). When light with proper intensity is incident into the SAR, a dynamic fiber grating (DFG) can be built up to work as a narrow-bandwidth filter with the ability of auto-tracking the center frequency. The SAR can effectively filter out dense random longitudinal modes and suppress multi-mode resonances and frequency jitters by restricting the mode-competition-induced mode hopping within a limited frequency range of ∼ 1 MHz and auto-tracking the center frequency of light in the cavity, simultaneously. Thus, the output of BRFL with SAR is much more stabilized than existing free-running BRFLs. In the experiment, a stable lasing output with an ultra-narrow linewidth (less than2kHz) is obtained from the proposed BRFL. The measured relative intensity noise and frequency noise demonstrate that the noise level of the proposed laser is significantly lower than that of the free-running BRFLs. This proposed technique not only provides a solution to the dilemma of previously reported BRFLs with severe lasing instabilities, but also makes a step towards the development of practical and reliable BRFLs. This high-quality laser source with advantages of high stability, low cost, low threshold power, high coherence, narrow linewidth, and low noise can find useful applications in coherent optical detection, high-resolution spectrometers, microwave photonics, and optical sensing fields.

Capacity expansion of chaotic secure transmission system based on coherent optical detection and space division multiplexing over multi-core fiber.

We propose and experimentally study a coherent optical chaotic secure transmission system through a multi-core fiber (MCF). The messages are encrypted by the chaotic carrier and transmitted through the outer cores of the MCF, whereas the chaotic carrier signal is concealed by transmitting through the center core. The MCF provides large transmission capacity expansion and security enhancement against eavesdroppers due to its physical structure. In addition, the designed optical chaos self-homodyne coherent detection strategy has high detection sensitivity and simple physical structure. Due to the prevalence of devices and digital signal processing (DSP) algorithms used in this system, it can be well compatible with a commercial coherent optical communication system. Error free 40 Gb/s/core encrypted 16 quadrature amplitude modulation (QAM) signal transmission over 10 km 7-core fiber is achieved, and 20 Gb/s quadrature phase shift keying (QPSK) signal transmission over a 100 km standard single-mode fiber (SSMF) is demonstrated to verify the long-distance transmission capability. The sensitivity to the secret key is also studied.