Coherent Optical Spectrum Analyzer Research Articles

Dynamic range of a coherent optical spectrum analyzer with a liquid-crystal matrix signal-input device

Subject of study. This study investigates the dynamic range of a coherent optical spectrum analyzer with a liquid-crystal signal-input device. Aim of study. The goal is the theoretical and experimental assessment of the false signal levels at the spectrum analyzer output and experimental determination of the dynamic range. Method. A theoretical analysis of the spectral components in the output signal is performed based on the representation of the transmission dependence of a liquid-crystal input device in terms of light amplitude on the input control signal in the form of an expansion into a series of power functions in the proximity of the operating point. The output signals of the spectrum analyzer were experimentally registered, and the values of the dynamic range were determined for the cases of one-, two- and three-frequency input signals. Main results. A calculating technique for the false signal levels at the output of an optical spectrum analyzer with a liquid-crystal matrix signal-input device is proposed, and its dynamic range is measured. Practical significance. A dynamic range higher than 40 dB is achieved for a three-frequency input signal.

Dynamic wavelength calibration based on synchrosqueezed wavelet transform.

With the developments of the tunable laser source (TLS), there are increasing demands for high-resolution dynamic wavelength calibration in recent years. Considering mutual constraints between wide measurement range and high calibration resolution, we propose a dynamic wavelength calibration method based on an auxiliary Mach-Zehnder interferometer (MZI) and the synchrosqueezed wavelet transform (SSWT). Our proposed method can achieve a calibration resolution of 5 fm and a tuning range of 10 nm. Moreover, the measurement range and spatial resolution of the optical frequency domain reflectometer (OFDR) system are improved to ∼80 m and ∼mm, respectively. Our proposed approach can substantially reduce the subtle spectrum distortion (tens of fm) in coherent optical spectrum analyzer (COSA) systems.

High resolution coherent spectral analysis method based on Brillouin scattering in an optical fiber.

High-resolution optical spectral analysis method is of significant importance for those who want to explore the physical world from the frequency domain. Aiming at the resolution degradation of classical coherent optical spectrum analysis (COSA) caused by the mirror phenomenon, this paper modifies the COSA system by introducing two homologous Brillouin scattering beams to serve as the pre-filter and local oscillator (LO), respectively. The central frequencies of the pre-filtered signal and the LO are locked by the Brillouin frequency shifts of those two Brillouin scattering beams. By means of this modification, the pre-filtered signal is located at either the upper-frequency-shifted side or the lower-frequency-shifted sides of the LO but could not exist on both sides of the LO. The proposed method could cancel the mirror phenomenon and thus improve the systematic resolution to 1.3 MHz in theory and 2 MHz in practice.

A Novel Method of Measuring Instantaneous Frequency of an Ultrafast Frequency Modulated Continuous-Wave Laser.

Ultrafast linear frequency modulated continuous-wave (FMCW) lasers are a special category of CW lasers. The linear FMCW laser is the light source for many sensing applications, especially for light detection and ranging (LiDAR). However, systems for the generation of high quality linear FMCW light are limited and diverse in terms of technical approaches and mechanisms. Due to a lack of characterization methods for linear FMCW lasers, it is difficult to compare and judge the generation systems in the same category. We propose a novel scheme for measuring the mapping relationship between instantaneous frequency and time of a FMCW laser based on a modified coherent optical spectrum analyzer (COSA) and digital signal processing (DSP) method. Our method has the potential to measure the instantaneous frequency of a FMCW laser at an unlimited sweep rate. In this paper, we demonstrate how to use this new method to precisely measure a FMCW laser at a large fast sweep rate of 5000 THz/s by both simulation and experiments. We find experimentally that the uncertainty of this method is less than 100 kHz and can be improved further if a frequency feedback servo system is introduced to stabilize the local CW laser.

Measuring complex field waveforms of quadrature amplitude modulation optical signals using a spectrally slicing-and-synthesizing coherent optical spectrum analyzer.

We propose a spectrally slicing-and-synthesizing coherent optical spectrum analyzer to measure complex field waveforms of quadrature amplitude modulation (QAM) optical signals with ultralong periods. The optical spectrum of a measured optical signal is divided into multiple narrowband spectral components, called slices. The slices are sequentially measured using low-speed coherent detection. After phase noise suppression and frequency fluctuation compensation on each slice, the measured slices are synthesized to recover the original signal spectrum. Our numerical and experimental results confirm that the proposed method can overcome the limitation of the measurement bandwidth because the signal spectrum can synthesize more than 100 slices. We experimentally demonstrate complex field measurements of 16QAM optical signals. Our method can measure high-speed optical complex field waveforms with no bandwidth limitation.

OPTICAL METHOD FOR QUALITY CONTROL OF TRANSPARENT FABRICS

Background. The main element of optical information processing systems is a coherent optical spectrum analyzer (COSA). In the textile industry in the manufacture and use of thin transparent fabrics (gauze, bandages, etc.) there is a problem of controlling the shape and geometric dimensions of transparent cells. For this purpose, microscopes are used with the help of which the geometrical parameters of many transparent cells are measured. Such process is time consuming, requires a considerable amount of time for measurement. Using COSA allows you to directly measure the average cell size.Objective. The purpose of the paper is development of an optical model of a transparent fabric, on the basis of which a laser method and an experimental stand for measuring the average geometric dimensions of transparent fabrics cells are proposed.Methods. The proposed method of quality control of transparent fabrics is based on the use of COSA. If a transparent fabric is positioned in the front focal plane of a Fourier lens, then diffraction maximums are formed in the back focal plane, the position of which depends on the size of the transparent fabric cells.Results. The geometric model of the structure of a transparent fabric, which describes the amplitude transmittance coefficient of the fabric depending on the period and the size of the transparent fabrics cells, is proposed. An analytical formula is obtained for calculating the spatial spectrum of the transmittance of such a model. The COSA laboratory stand was developed, which allows measuring the geometric dimensions of transparent fabric cells.Conclusions. The research of the transmission model of transparent fabric showed that the fabric structure can be viewed as a two-dimensional diffraction grating, the period of which is determined by the thickness of the filament and the transparent part of the fabric cell. To measure the size of the cells of the transparent fabric, it is proposed to use the COSA, which allows you to directly measure the spatial frequencies of the diffraction peaks and calculate the average values of the fabric cell sizes. To validate the proposed method, an experimental stand of digital COSA has been developed.

Ultrahigh‐resolution coherent optical spectrum analysis based on electrical frequency sweeping with a doubled measurement range

Based on electrical frequency sweeping, an ultrahigh-resolution coherent optical spectrum analyser with a doubled measurement range is proposed and experimentally demonstrated. In the scheme, a frequency-swept electrical signal is converted to the optical domain. By selecting one of the second-order sidebands of the generated optical signal, a wavelength-swept signal with a frequency range that is twice the bandwidths of the electro-optic and RF components is generated. With the wavelength-swept signal, high-resolution optical spectrum analysis with a doubled measurement range is implemented. In a proof-of-concept experiment, the spectrum of a multi-tone optical signal with a bandwidth of 20 GHz is measured with a resolution of 1.5 MHz by employing 10 GHz components.

Influence of the Matrix Structure of the Modulator and Detector on the Optical Spectrum Analyzer Output Signal

In this article, we investigate the physical and mathematical model of a coherent optical spectrum analyzer (COSA), which uses a matrix light modulator and a matrix detector as input and output devices. This model allows to define distortions in the output signal of the spectrum analyzer and the error in determining the signal spatial frequency. The study of this model showed that form of the signal at the COSA’s output depends on the pixels sizes of modulator and detector matrices, as well as on the aberrations of the Fourier lens entrance pupil diameter. The output signal is a convolution of an ideal input signal spectrum with a discrete spatial transmission spectrum of the modulator, which is followed by convolution with a discrete sensitivity of the matrix detector. This means that the spectrum of the signal under investigation is distorted by the spatial spectrum of the modulator and the matrix structure of the matrix detector. An important feature of the signal is its independence from the phase shift, which is caused by the displacement of the modulator center relative to the optical axis of the spectrum analyzer. The output signal of COSA consists of an infinite number of diffraction maximum, each of which has three maximum, the distance between which is proportional to the spatial frequency of the test signal. The position (frequency) of the maximum is determined by the pixel size, and their width by the size of the modulator. Obtain the formulas for determining the spatial frequency of the test signal, which differ substantially from the traditional formula and depend on the position of the central and lateral maximum in the diffraction maximum. The error in measuring the frequency depends on the size of the detector pixel, focal length of the Fourier lens, and the modulator matrix size. Developed the method for determining the error in measuring the spatial frequency of a harmonic signal. The error is defined as the difference between the true frequency corresponding to the position of the center of the diffraction maximum and the measured frequency corresponding to the position of the pixel center which has the maximum signal.

Investigation and development of a high spectral resolution coherent optical spectrum analysis system.

Focusing on high resolution optical spectroscopy, a coherent optical spectrum analysis (COSA) system is investigated in this paper. Principle is built to demonstrate the operation of COSA and its signal processing in both time and frequency domain. According to COSA principle, resolution bandwidth (RBW) filters are found to have significant influence on power accuracy and spectral resolution of the optical spectrum analysis (OSA). Much effort is paid to design RBW filters, including center frequency, bandwidth and type of filters. Two RBW filters are optimized to reduce the power uncertainty of different spectral resolution and satisfy different signal under test. Then, simulations and experiments are conducted to verify COSA principle and results show that the power uncertainty is less than 0.5% and 1.2% for high and medium spectral resolution application, respectively. Finally, experiments on the OSA of actual spectra indicate that COSA system can achieve a 6 MHz spectral resolution and has an excellent capacity in analysis of fine spectrum structures.

Geometrical noise bandwidth of the optical spectrum analyzer

Background. Coherent optical spectrum analyzers (COSA) are widely used in data processing systems. The efficiency of the spectrum analyzers is determined by their generalized characteristics such as the operating range of spatial frequencies, spatial bandwidth, the spatial spectral resolution and the energy resolution. One of such characteristics is geometric noise bandwidth GNBW that determines the spatial spectral resolution of spectrum analyzer.Objective. The purpose of this article is to provide a physical and mathematical model of COSA for calculating its geometric noise bandwidth, which determines the spatial spectral resolution of the spectrum analyzer.Methods. Based on the analysis of physical-mathematical model of COSA invited to determine the field amplitude at the output of the spectrum analyzer using the geometric noise bandwidth.Results. It was found that the spectrum analyzer can be considered as a coherent optical invariant linear system characterized by the impulse response and a transfer function. The geometric noise bandwidth of COSA is equal the square of the input transparency, amplitude transmittance of which is determined by the amplitude of the investigated signal.Conclusions. Analysis of the developed physical and mathematical model of the COSA showed that the geometric noise bandwidth is one of the important characteristics of the spectrum analyzer as it allows us to calculate the spatial spectral resolution. It was found that the coherent transfer function of the optical spectrum analyzer is equal to the square of the input transparency, amplitude transmittance of which is determined by the amplitude of the investigated signal.

Coherent optical three-dimensional spectrum-correlation processing of wave signals based on space–time integration

The architectures of classical analog coherent optical (ACO) spectrum analyzers and correlators are not designed to process the wave signal as a whole, i.e., simultaneously in three dimensions. In this paper, the theory of ACO three-dimensional direct spectrum-correlation processing of spatial-temporal optical replicas (copies) of wave signals is discussed. In the single-stage and two-stage ACO systems, the spatial power spectrum and spatial correlation function of the wave signal (envelope) are obtained on the basis of space-time integration. The geometry of the final compressed signal in the output plane of either optical system allows one to evaluate the angle of wave arrival. The wave signal to be processed can theoretically have any form (due to autocorrelation properties of the systems) and an unlimited duration (due to time integration of wave energy and possibility of electronic subtraction of the intermediate bias terms of the time integration).

Swept Coherent Optical Spectrum Analysis

Swept coherent optical spectrum analysis is becoming a viable method for subpicometer spectral characterization of optical communications signals. The performance of this technique depends on a number of factors that we have investigated both theoretically and experimentally. This includes the influence of signal linewidth, polarization, optical phase, intensity noise, and electrical filtering methods. These effects are discussed in detail and illustrated through spectral measurements on communications transmitters. We extend the heterodyne method to permit measurement of both spectral amplitude and phase of the spectral sidebands. The transformation of the complex optical field spectrum to the time domain waveform is demonstrated.

Frequency resolution and amplitude accuracy of the coherent optical spectrum analyzer with a swept local oscillator

We present a model for coherent optical spectrum analysis that relates the key parameters of frequency resolution, power uncertainty, and sweep speed. The theoretical model is compared with experimental results obtained from spectral measurements of an external-cavity laser.

Coherent optical spectrum analyzer

We demonstrate coherent optical spectrum analysis with /spl sim/70-dB dynamic range based on a swept-tuned optical local oscillator and a coherent receiver operating in the 1.5-μm wavelength hand. Measurement principles are presented along with high-resolution measurements of distributed feedback laser spectra.

Generalization of the falling raster-folded spectrum relationship

A family of raster mappings for the optical spectrum analysis of broadband signals is presented. This family contains as one member the conventionally used falling raster of C. E. Thomas. An analytical model, appropriate for space-integrating coherent optical spectrum analyzers, is given for the raster input and the Fourier plane distribution. The family of raster mappings and its Fourier transform is shown to be a continuous domain analog of the I. J. Good fast Fourier transform algorithm.

Contrast effect on imagery power spectra

Recent work has demonstrated the effectiveness of coherent optical power spectrum (COPS) analysis for solving diverse problems in image evaluation. This paper extends previous work to include the prominent aspects of imagery contrast, as they pertain to COPS analysis. It is shown that the contrast effect on power spectra can and should be maximized or minimized, via different normalization techniques, depending on the particular class of image evaluation problems being addressed. In addition, the utility of using a COPS analyzer for contrast measurement of complex (or random) imagery is given.

Frequency variant optical signal analysis

A class of coherent optical spectrum analyzers for one-dimensional signals is described that is characterized by a frequency variant response to the spectral components of the input signal. Operations performed exploit the second degree of freedom inherent in the optical systems. One example considered is a constant proportional bandwidth, log-frequency spectrum analyzer. Experimental and analytical results are presented.

Holographic Storage of Electric Signals

In the holographic storage of electric signals, a pulsed electric input signal is converted to a one dimensional spatial transmittance function by an ultrasonic light modulator. This transmittance function, which travels in the light modulator at a speed characteristic of the acoustic medium, may be viewed as a one-dimensional scene undergoing constant-speed linear motion. It is possible to record the Fourier transform hologram of the pulsed signal by compensating the reference beam in a straightforward manner. Reference beam compensation permits the use of a longer exposure time than would otherwise be allowed in the formation of the hologram of a moving scene. Reconstruction of the original electric signal is effected by forming the Fourier transform of the holographic recording in a coherent optical spectrum analyzer. Electric signals with 19-microsec duration, 1-MHz bandwidth, and 20-MHz carrier frequency were successfully recorded and reconstructed by this technique.

Optical Spectrum Analysis of Large Space Bandwidth Signals

Coherent optical spectrum analyzers are available for determining the frequency content of many analog signals written simultaneously on film. The space bandwidth product of each signal must not exceed one thousand for a typical 35 mm system. By sacrificing the multichannel capability of such a computer, single signals with space bandwidth products greater than one million (10(6)) can be analyzed. Two optical systems are described. The first analyzes long signals but maintains the frequency resolution of the multichannel system (one part in one thousand). The second system yields frequency resolution of at least one part in one million. Near real-time, continuous spectrum analysis of analog signals is within the current state of the art. A breadboard system is being built.