Electrical Spectrum Analyzer Research Articles

Wideband DFS Measurement Using a Low-Frequency Reference Signal

A novel Doppler frequency shift (DFS) measurement system based on a cascaded modulator topology is presented. It is capable to measure both the object speed and moving direction on an electrical spectrum analyzer with the use of a fixed low-frequency reference signal. It has a simple single-laser and single-photodetector structure, a robust performance and a wide operating frequency range, which is only limited by the optical modulator bandwidth. Experimental results are presented that demonstrate DFS ranging from −100 kHz to +100 kHz with an error of less than ±0.1 Hz at different microwave signal frequencies. Over 35 dB suppression in unwanted frequency components present at the system output is also demonstrated.

A Comparison between off and On-Chip Injection Locking in a Photonic Integrated Circuit

The mutual and injection locking characteristics of two integrated lasers are compared, both on and off-chip. In this study, two integrated single facet slotted Fabry–Pérot lasers are utilised to develop the measurement technique used to examine the different operational regimes arising from optically locking a semiconductor diode laser. The technique employed used an optical spectrum analyser (OSA), an electrical spectrum analyser (ESA) and a high speed oscilloscope (HSO). The wavelengths of the lasers are measured on the OSA and the selected optical mode for locking is identified. The region of injection locking and various other regions of dynamical behaviour between the lasers are observed on the ESA. The time trace information of the system is obtained from the HSO and performing the FFT (Fast Fourier Transform) of the time traces returns the power spectra. Using these tools, the similarities and differences between off-chip injection locking with an isolator, and on-chip mutual locking are examined.

High-Sensitivity Sensing for Relative Humidity and Temperature Based on an Optoelectronic Oscillator Using a Polyvinyl Alcohol-Fiber Bragg Grating-Fabry Perot Filter

In this paper, a relative humidity (RH) and temperature sensing scheme with high sensitivity based on an optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. A polyvinyl alcohol-Fiber Bragg grating-Fabry Perot (PVA-FBG-FP) filter, in which the PVA film acts as an RH-strain transducer, is employed in the OEO to implement a single passband microwave photonic filter (MPF). In the OEO loop, the oscillating frequency is determined by the center frequency of the PVA-FBG-FP filter and the laser source. When the external RH or temperature changes, the central wavelength of PVA-FBG-FP filter is shifted, leading to the change in the center frequency of the MPF. By simply monitoring the variation of oscillating frequency using an electrical spectrum analyzer or a digital signal processor, the measurements for the RH and temperature can be achieved. In the experiment, the sensitivities of the RH and temperature are measured to be as high as 508.3 MHz/%RH and 1.8118 GHz/°C, respectively.

High-Resolution Temperature Sensor Based on Single-Frequency Ring Fiber Laser via Optical Heterodyne Spectroscopy Technology.

We demonstrate a high-resolution temperature sensor based on optical heterodyne spectroscopy technology by virtue of the narrow linewidth characteristic of a single-frequency fiber laser. When the single-frequency ring fiber laser has a Lorentzian-linewidth <1 kHz and the temperature sensor operates in the range of 3−85 °C, an average sensitivity of 14.74 pm/°C is obtained by an optical spectrum analyzer. Furthermore, a resolution as high as ~5 × 10−3 °C is demonstrated through optical heterodyne spectroscopy technology by an electrical spectrum analyzer in the range of 18.26–18.71 °C with the figure of merit up to 3.1 × 105 in the experiment.

Refractive Index and Temperature Sensing Based on an Optoelectronic Oscillator Incorporating a Fabry–Perot Fiber Bragg Grating

An ultrafast high-sensitivity refractive index (RI) and temperature-sensing system based on an optoelectronic oscillator (OEO) is proposed and demonstrated in this paper. A Fabry–Perot fiber Bragg grating (FP-FBG), which combines a gap with two FBGs in a silica V-shaped slot and characterizes a narrow notch in the reflection spectrum, is incorporated in the OEO to implement a microwave photonic filter and perform oscillating frequency selection. A microwave signal is generated by the OEO, whose oscillating frequency is determined by the center frequency of the FP-FBG notch, which varies with the surrounding environments. The RI or the temperature change can be accordingly measured by monitoring the frequency shift of the microwave signal using an electrical spectrum analyzer or a digital signal processor. An experiment is performed to verify the proposal. An RI sensitivity of 413.8 MHz/0.001RIU and a temperature sensitivity of 2516 MHz/°C are successively demonstrated.

Self-Calibrated and Extinction-Ratio-Independent Microwave Characterization of Electrooptic Mach–Zehnder Modulators

A self-calibrated and extinction-ratio (ER)-independent electrical method based on frequency-shifted heterodyne is proposed for full microwave characterization of high-speed electrooptic Mach–Zehnder modulators (MZMs) with finite or infinite ER. The method eliminates the need for correcting the roll-off responsivity in the photodetection and reduces half bandwidth requirements for the photodetector and electrical spectrum analyzer. Modulation depths, half-wave voltages, chirp parameters, and asymmetric factor are experimentally measured for an MZM, which agree well with the results obtained by the optical spectrum analysis method.

Bias-Independent and Self-Calibrated Electrical Method for Microwave Characterization of Dual-Parallel Mach–Zehnder Modulators Based on Two-Tone and Bias-Swing Modulation

A bias-independent and self-calibrated measurement is proposed for the high-frequency response of dual-parallel Mach-Zehnder modulators (DPMZMs) based on two-tone and bias-swing modulation. The two-tone and bias-swing sidebands beat with each other and generate the desired low-frequency beat notes, which allows to extracting the modulation depths and half-wave voltages of DPMZMs from the sub-MHz electrical spectrum analysis. Our measurement is bias-independent due to the insensitivity to the bias voltages of the DPMZM, and it avoids any correction for the roll-off responsivity of photodetection through the specific two-tone and bias-swing modulation. Our method is experimentally demonstrated and is compared with the traditional ones for the accuracy. The proposed electrical method features bias-independence and self-calibration, providing a low-frequency photodetection and spectrum analysis for measuring high-speed DPMZMs.

Real-time broadband radio frequency spectrum analyzer based on parametric spectro-temporal analyzer (PASTA).

A real-time broadband radio frequency (RF) spectrum analyzer is proposed and experimentally demonstrated to rapidly measure the RF spectrum of broadband optical signal. Cross phase modulation in the highly-nonlinear fiber is used to convert the RF spectrum carried by the pump to the optical spectrum of the probe signal, then the optical spectrum is real-time analyzed with the parametric spectro-temporal analyzer (PASTA) technology. The system performances are investigated in detail, including bandwidth, resolution, frame rate, and dynamic range. It achieves large RF bandwidth of over 800 GHz, as well as 91-MHz frame rate without sacrificing the resolution. It is noted that 91-MHz frame rate is several orders of magnitude improvement over those previous reported all-optical RF spectrum analyzers. As a proof-of-concept demonstration, this real-time broadband RF spectrum analyzer successfully characterizes the ultra-short pulse trains with repetition rate of 160GHz, which is far beyond capability of the conventional electrical spectrum analyzer. It presents a new way to implement rapid and broadband RF spectrum measurement, and would be of great interests for some ultrafast scenarios, where the real-time RF spectrum analysis can be applied.

Simultaneous Measurement of Strain and Temperature Based on Radio-Frequency Demodulation

We propose and experimentally demonstrate a hybrid multimode fiber laser configuration for simultaneous measurement of strain and temperature based on radio-frequency demodulation. The hybrid lasers include a Brillouin erbium fiber laser (BEFL) with dual Brillouin peaks and an erbium-doped fiber laser (EDFL). The BEFL and EDFL are incorporated in a simple and compact ring laser cavity. The injection of Brillouin pump (BP) excites first-order stimulated Brillouin scattering in two sections of fibers with different Brillouin Stokes frequencies. Without BP, the laser performs as an EDFL. The simultaneous measurement of strain and temperature are achieved by monitoring the self-beating signals of the BEFL and EDFL, respectively. The longitudinal modes beat frequency originated from the EDFL and difference of Brillouin frequency shifts from the BEFL are in radio-frequency domain and detected with an electrical spectrum analyzer which makes the detection simplified. The proposed sensor is found to be characterized with simplicity and compact configuration. The maximum measurement errors of strain and temperature obtained are within $\pm 20.3~\mu \varepsilon $ and ±0.5 °C.

Ultrafast electrical spectrum analyzer based on all-optical Fourier transform and temporal magnification.

Real-time electrical spectrum analysis is of great significance for applications involving radio astronomy and electronic warfare, e.g. the dynamic spectrum monitoring of outer space signal, and the instantaneous capture of frequency from other electronic systems. However, conventional electrical spectrum analyzer (ESA) has limited operation speed and observation bandwidth due to the electronic bottleneck. Therefore, a variety of photonics-assisted methods have been extensively explored due to the bandwidth advantage of the optical domain. Alternatively, we proposed and experimentally demonstrated an ultrafast ESA based on all-optical Fourier transform and temporal magnification in this paper. The radio-frequency (RF) signal under test is temporally multiplexed to the spectrum of an ultrashort pulse, thus the frequency information is converted to the time axis. Moreover, since the bandwidth of this ultrashort pulse is far beyond that of the state-of-the-art photo-detector, a temporal magnification system is applied to stretch the time axis, and capture the RF spectrum with 1-GHz resolution. The observation bandwidth of this ultrafast ESA is over 20 GHz, limited by that of the electro-optic modulator. Since all the signal processing is in the optical domain, the acquisition frame rate can be as high as 50 MHz. This ultrafast ESA scheme can be further improved with better dispersive engineering, and is promising for some ultrafast spectral information acquisition applications.

Calibration-free measurement of high-speed Mach-Zehnder modulator based on low-frequency detection.

A calibration-free electrical method is demonstrated for measuring the frequency response of high-speed Mach-Zehnder modulators (MZMs) based on low-frequency detection. The method achieves the high-frequency modulation index and half-wave voltage measurement of MZMs by the low-frequency electrical spectrum analysis of the two-tone and bias-modulated optical signal. Moreover, it eliminates the need for correcting the responsivity fluctuation in the photodetector through setting a specific frequency relationship between the two-tone and bias modulation. Both absolute and relative frequency response of MZMs are experimentally measured with our method and compared with those obtained with conventional methods to check for consistency.

Calibration-free absolute frequency response measurement of directly modulated lasers based on additional modulation.

A calibration-free electrical method is proposed for measuring the absolute frequency response of directly modulated semiconductor lasers based on additional modulation. The method achieves the electrical domain measurement of the modulation index of directly modulated lasers without the need for correcting the responsivity fluctuation in the photodetection. Moreover, it doubles measuring frequency range by setting a specific frequency relationship between the direct and additional modulation. Both the absolute and relative frequency response of semiconductor lasers are experimentally measured from the electrical spectrum of the twice-modulated optical signal, and the measured results are compared to those obtained with conventional methods to check the consistency. The proposed method provides calibration-free and accurate measurement for high-speed semiconductor lasers with high-resolution electrical spectrum analysis.

Calibration-free and bias-drift-free microwave characterization of dual-drive Mach–Zehnder modulators using heterodyne mixing

An electrical method is proposed for the microwave characterization of dual-drive Mach–Zehnder modulators based on heterodyne mixing. The proposed method utilizes the heterodyne products between the two-tone modulated optical sidebands and frequency-shifted optical carrier, and achieves calibration-free and bias-drift-free microwave measurement of dual-drive Mach–Zehnder modulators with high resolution electrical-domain techniques. Our method avoids the extra calibration for the photodetector and reduces half the bandwidth requirement for the photodetector and the electrical spectrum analyzer through carefully choosing a half frequency relationship of the two-tone modulation. Moreover, our measurement avoids the bias drifting problem due to the insensitivity to the bias phase of the modulator under test. The frequency-dependent modulation depths and half-wave voltages are measured for a commercial dual-drive Mach–Zehnder modulator with our method, which agree well with the results obtained by the conventional optical spectrum analysis method.

Extinction-ratio-independent electrical method for measuring chirp parameters of Mach-Zehnder modulators using frequency-shifted heterodyne.

An extinction-ratio-independent electrical method is proposed for measuring chirp parameters of Mach-Zehnder electric-optic intensity modulators based on frequency-shifted optical heterodyne. The method utilizes the electrical spectrum analysis of the heterodyne products between the intensity modulated optical signal and the frequency-shifted optical carrier, and achieves the intrinsic chirp parameters measurement at microwave region with high-frequency resolution and wide-frequency range for the Mach-Zehnder modulator with a finite extinction ratio. Moreover, the proposed method avoids calibrating the responsivity fluctuation of the photodiode in spite of the involved photodetection. Chirp parameters as a function of modulation frequency are experimentally measured and compared to those with the conventional optical spectrum analysis method. Our method enables an extinction-ratio-independent and calibration-free electrical measurement of Mach-Zehnder intensity modulators by using the high-resolution frequency-shifted heterodyne technique.

Fiber-Optic Interferometry Using Narrowband Light Source and Electrical Spectrum Analyzer: Influence on Brillouin Measurement

We observe an interference pattern using a simple fiber-optic interferometer consisting of an electrical spectrum analyzer and a narrowband light source, which is commonly employed for observing the Brillouin gain spectrum. This interference pattern expands well beyond the frequency range corresponding to the Brillouin frequency shift in silica fibers (approximately 11 GHz at 1550 nm). Using both silica single-mode and polymer optical sensing fibers, we then experimentally prove that the distinctive noise in a self-heterodyne-based Brillouin measurement with an unoptimized polarization state originates from the interference between the reference light and the Fresnel-reflected light. This noise can be almost completely suppressed by employing a delay line that is longer than the coherence length of the light source and by artificially applying a high loss near the open end of the sensing fiber.

Calibration-Free Electrical Spectrum Analysis for Microwave Characterization of Optical Phase Modulators Using Frequency-Shifted Heterodyning

A novel calibration-free electrical spectrum analysis method for microwave characterization of electrooptic phase modulators is proposed and experimentally demonstrated based on frequency-shifted optical heterodyning. The method achieves the electrical domain measurement of the modulation efficiency of phase modulators without the need for correcting the responsivity fluctuation in the photodetection. Moreover, it extends double the measuring frequency range through setting a specific frequency relationship between the driving microwave signals. Modulation depth and half-wave voltage of phase modulators are experimentally extracted from the heterodyning spectrum of two phase-modulated signals with and without frequency shifting, and the measured results are compared to those obtained with the traditional optical spectrum analysis method to check the consistency. The proposed method provides calibration-free and accurate measurement for high-speed optical phase modulators with the high-resolution electrical spectrum analysis.

Self-calibrating measurement of high-speed electro-optic phase modulators based on two-tone modulation.

We propose a self-calibrating method for high-frequency response measurement of electro-optic phase modulators based on two-tone modulation. The method utilizes the electrical domain measurement of heterodyning spectrum between the two-tone modulation optical signal and the frequency-shifted optical carrier, and eliminates the need to correct the responsivity fluctuation in the photodetection. High-frequency modulation depth and half-wave voltages are measured and compared to those with the traditional optical spectrum analysis method in the experimental demonstration. The proposed method enables calibration-free and accurate frequency response measurement of electro-optic phase modulators by using high-resolution electrical spectrum analysis.

Narrow Linewidth Tunable Terahertz Radiation By Photomixing Without Servo-Locking

A beatnote, tunable from dc to 1 THz, provided by a dual-frequency laser is used to feed an unitravelling carrier photodiode in order to produce a highly coherent THz signal radiated by a transverse-electromagnetic-horn antenna. The THz signal is detected and analyzed by a subharmonic mixer coupled to an electrical spectrum analyzer. All components involved in this experiment operate at room temperature without phase locking the beatnote. So far, the dynamic range evolves from 58 dB at 282 GHz to 15 dB at 1.026 THz, and the measured linewidth is better than 30 kHz. Linewidth narrowing using a Brillouin fiber laser pumped by the dual-frequency laser leads to a beatnote of 500-Hz linewidth at 1 THz.

Heterodyned toroidal microlaser sensor

Optical microcavity sensors have demonstrated success in detecting analytes with high sensitivity. Typically, the sensor output is monitored using oscilloscopes or optical spectrum analyzers; however, these instruments can significantly limit the sensing performance. In the present work, we address this limitation by developing a neodymium-doped toroid microlaser and heterodyning it with a reference laser. The resulting beat signal is analyzed on an electrical spectrum analyzer. By performing comparative temperature sensing experiments, we demonstrate that the heterodyning improves the detection limit over 60-fold while also improving the temporal resolution and the signal to noise ratio up to 50-fold.

High-sensitivity sensor based on mode number-encoded multi-longitudinal mode f iber laser

We propose a high sensitivity sensor based on a mode number-encoded multi-longitudinal mode fiber laser. The fiber laser incorporates a uniform fiber Bragg grating (FBG) and a fiber Fabry-Perot interferometer (FFPI) as sensitive components in the cavity. The sensor counts the number of longitudinal modes (NLM) of fiber laser, which is caused by the mismatch between the reflection band of FBG and the transmission band of FFPI resulting from the application of external perturbation to the FBG. An electrical spectrum analyzer is adopted to analyze the NLM. The strain sensor is experimentally demonstrated to have sensitivity of as high as 0.02 \mu \varepsilon/mode.