Self-heterodyne Method Research Articles

Coherence characterization of narrow-linewidth beam by C-OFDR based Rayleigh speckle analysis

A novel method for characterizing the amplitude of a coherence function with respect to a delay between two optical waves is proposed and demonstrated by using a distributional Rayleigh speckle analysis based on C-OFDR. This technique allows us to estimate both the coherence time of the laser and that of the spectral profiles from the measured amplitude of the coherence function, if the symmetry of the spectrum can be assumed. The spectral width obtained in the experiment agrees roughly with that obtained using a delayed self-heterodyne method.

Detecting single viruses and nanoparticles using whispering gallery microlasers

There is a strong demand for portable systems that can detect and characterize individual pathogens and other nanoscale objects without the use of labels, for applications in human health, homeland security, environmental monitoring and diagnostics. However, most nanoscale objects of interest have low polarizabilities due to their small size and low refractive index contrast with the surrounding medium. This leads to weak light-matter interactions, and thus makes the label-free detection of single nanoparticles very difficult. Micro- and nano-photonic devices have emerged as highly sensitive platforms for such applications, because the combination of high quality factor Q and small mode volume V leads to significantly enhanced light-matter interactions. For example, whispering gallery mode microresonators have been used to detect and characterize single influenza virions and polystyrene nanoparticles with a radius of 30 nm (ref. 12) by measuring in the transmission spectrum either the resonance shift or mode splitting induced by the nanoscale objects. Increasing Q leads to a narrower resonance linewidth, which makes it possible to resolve smaller changes in the transmission spectrum, and thus leads to improved performance. Here, we report a whispering gallery mode microlaser-based real-time and label-free detection method that can detect individual 15-nm-radius polystyrene nanoparticles, 10-nm gold nanoparticles and influenza A virions in air, and 30 nm polystyrene nanoparticles in water. Our approach relies on measuring changes in the beat note that is produced when an ultra-narrow emission line from a whispering gallery mode microlaser is split into two modes by a nanoscale object, and these two modes then interfere. The ultimate detection limit is set by the laser linewidth, which can be made much narrower than the resonance linewidth of any passive resonator. This means that microlaser sensors have the potential to detect objects that are too small to be detected by passive resonator sensors.

Characterization of White and Flicker Frequency Modulation Noise in Narrow-Linewidth Laser Diodes

Frequency modulation noise of narrow-linewidth external cavity laser diodes is characterized in terms of power spectral density (PSD) using the recirculating delayed self-heterodyne method. The measured PSD is consistent with the oscillator noise model consisting of white and flicker noise. Based on this model, beat signal spectra observed in heterodyne measurements are derived, which explain well the delay- or observation time-dependent lineshape obtained in the experiments.

Laser frequency modulation noise measurement by recirculating delayed self-heterodyne method

I propose and demonstrate the use of the recirculating delayed self-heterodyne (DSH) method for measuring FM noise power spectral densities (PSDs), which are the most fundamental measure characterizing the spectral purity of laser sources. By analyzing the DSH beat signals with 1, 10, and 160 km delays, the FM noise PSD of a narrow-linewidth fiber laser is evaluated for Fourier frequency range between 10 Hz and 100 kHz, which exhibits flicker noise as the dominant contribution.

High efficient and narrow linewidth fiber laser based on fiber grating Fabry-Perot cavity

A high Er3+-doped narrow linewidth fiber laser based on fiber Bragg grating Fabry-Perot cavity was demonstrated. The spatial hole burning effect was restrained by a fiber Faraday rotator. Two short fiber Bragg grating Fabry-Perot cavities as narrow bandwidth filters discriminated and selected laser longitudinal modes efficiently. A stable single-frequency 1534.83 nm laser was acquired. Pumped by two 976 nm laser diodes and two-ended output, the fiber laser exhibited a 12 mW threshold. Total 39.5 mW output power and one end 22 mW output power were obtained at the maximum 145 mW pump power. Optical-optical efficiency was 27% and slope efficiency was 29.7%. The output power seemed to be saturated when pump power increased. The 3 dB linewidth of the laser was less than 7.5 kHz, measured by the delayed self-heterodyne method with 15 km monomode fiber. The high power narrow linewidth fiber laser can be used in high resolution fiber sensor systems.

Linewidth measurement of Littrow structure semiconductor laser with improved methods

In this Letter, the frequency noise spectrum is analyzed in the experiment of the laser linewidth measurement. The power spectrum of three most widely used laser linewidth measurement methods, i.e., the heterodyne measurement, the self-homodyne measurement and the self-heterodyne measurement, are restudied both theoretically and experimentally. A scheme adopting an avalanche photodetector (APD) in the delayed self-heterodyne (DSHT) method is proposed, and an indirect determined result is given.

Demonstration of saturation spectra of caesium D2 line with a low linewidth distributed feedback laser diode

Low linewidths for Al-free active region distributed feedback lasers using a self-heterodyne measurement method (minimum beat width of 0.8 MHz at 20 mW and 12°C, 1.2 MHz at 852.12 nm 35 mW and 36°C) are demonstrated. Singlemode emission was achieved at 852.12 nm corresponding to the D2 caesium transition, with an optical output power of 35 mW at 140 mA under CW operation. Finally, the saturation spectra of caesium atoms with the DFB laser diode was recorded with a resolution close to the natural linewidth.

频率调制的窄线宽单偏振环形腔掺铒光纤激光器

We demonstrate a 1550-nm narrow-linewidth fiber ring laser with stable single polarization by using single-mode Er-doped fiber as active fiber and saturable absorber. A polarization-maintaining circulator is used to acquire single-polarization laser light with the degree of polarization of 99.8%-99.9%. The linewidth measured using a delayed self-heterodyne method is less than 0.5 kHz. Frequency of the fiber laser can be modulated by driving the waveguide phase modulator with proper voltage. A Mach-Zehnder interferometer with the optical path difference between two arms of about 36 km is used to study the long-distance coherent detection of the fiber laser for frequency-modulated continuous-wave application.

10 GHz regeneratively mode-locked semiconductor optical amplifier fiber ring laser and its linewidth characteristics

A 10 GHz regeneratively mode-locked semiconductor optical amplifier fiber ring laser with a picosecond pulse duration was newly constructed, and the linewidth change of a longitudinal mode was measured for various laser cavity lengths using a delayed self-heterodyne detection method. The detected line shape was approximately Gaussian for cavity lengths of 3.4-171 m. This result indicates that the laser linewidth depends strongly on thermal or acoustic cavity length fluctuations. The linewidth was proportional to the inverse square root of the cavity length, and a linewidth of 1.7 kHz was obtained for a cavity length of 171 m.

Improved Optical Heterodyne Methods for Measuring Frequency Responses of Photodetectors

An improved optical self-heterodyne method utilizing a distributed Bragg reflector (DBR) tunable laser and an optical fiber ring interferometer is presented in this paper. The interference efficiency can be increased by 7 dB compared with the scheme using the conventional Mach-Zehnder interferometer. The unsteady process that the beating frequency experiences in each tuning period is investigated. According to the measurement results, the wavelength and optical power of the tunable laser will be steady when the square-wave frequency is lower than 300 kHz. It has been shown that when a square-wave voltage is applied to the phase section of the tunable laser, the laser linewidths vary in a wide range, and are much larger than that under dc voltage tuning. The errors caused by the variations in the linewidth of the beat signal and optical power can be eliminated using the proposed calibration procedures, and the measurement accuracy can, therefore, be significantly improved. Experiments show that the frequency responses obtained using our method agree well with the data provided by the manufacturer, and the improved optical self-heterodyne method is as accurate as the intensity noise technique.

A novel self-heterodyne filtering method by wavelength shift keying Modulation for optical CDMA

We propose and demonstrate a new method of optical code-division multiple access (CDMA) based on self-heterodyne filtering. As compared to most CDMA systems, low spectrum spreading is required, thus enabling longer reach transmission capability.

A Novel Self-Heterodyne Method for Combined Temporal and Spectral High-Resolution Measurement of Wavelength Transients in Tunable Lasers

This letter presents a new experimental optical delayed self-heterodyne method to characterize tunable laser source instantaneous wavelength transients at accuracy of electronic spectrum.

Recent Trend of Millimeter-Wave Access Systems

This paper describes the recent trend of millimeter-wave wireless access systems which utilize the advantages of millimeter-waves. The fixed wireless access systems are first discussed and newly developed testbed at Yokosuka city area for broadband transmission is introduced. Then three promising short distance wireless access systems using 60-GHz frequency are discussed. The first one is on video transmission in the house, the second on vertically -connected link for the apartment infrastructure, and the third on wireless personal area ad-hoc system. Lastly millimeter-wave self-heterodyne transmission method is briefly introduce which can decrease the complex and cost of millimeter-wave local oscillators.

Linewidth of InP-based 1.55 [micro sign]m VCSELs with buried tunnel junction

Spectral linewidth measurements of 1.55 µm InGaAlAs/InP vertical-cavity surface-emitting lasers (VCSELs) employing a buried tunnel junction are reported. A narrow linewidth around 28 MHz was obtained at a power level of 0.5 mW using the self-heterodyne method, and an estimation for the linewidth enhancement factor is given.

Delayed self-heterodyne linewidth measurement of VCSELs

We report on high-resolution linewidth measurements of proton-implanted InGaAs-GaAs VCSELs employing the delayed self-heterodyne method. Devices with 16-μm active diameter exhibit record low linewidths of 20 MHz and 4-MHz residual linewidth. The linewidth enhancement factor is accurately determined from the ratio of induced phase to amplitude modulation indexes.

Dispersion effects in optical millimeter-wave systems using self-heterodyne method for transport and generation

This paper describes the detrimental effects of chromatic and polarization mode dispersion (PMD) on systems using single-laser-based optical self-heterodyning for generation and transport of millimeter (mm)-wave signals. The decrease of the generated mm-wave power due to chromatic dispersion in conjunction with nonnegligible laser phase noise is calculated and experimentally verified. Considering statistical properties of the PMD an analytical expression for the cumulative probability distribution of the power penalty is found and used to determine the required system margin for a given system outage rate. Furthermore, two system experiments using ASK and DPSK modulation scheme, respectively, are presented showing no limitation due to the dispersion effects.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

1.5 μm tensile-strained single quantum well 20-wavelength distributed feedback laser arrays

Tensile-strained single quantum well 20-wavelength distributed feedback laser arrays have been fabricated with a wide wavelength span of 56 nm in the 1.5 μm wavelength region. The light output is TM-polarised and the average threshold current is 22.8 mA for an 875 μm long array. A minimum laser linewidth of 180 kHz is measured by a delay self-heterodyne method.

Measurement of carrier lifetime and linewidth enhancement factor for 1.5- mu m ridge-waveguide laser amplifier

Semiconductor optical amplifiers are used for investigation of the effective carrier lifetime and the linewidth enhancement factor. Contrary to semiconductor lasers, semiconductor optical amplifiers allow measurement at high levels of injected carrier density. The carrier lifetime and the linewidth enhancement factor are measured with a simple dynamic self-heterodyne method. Carrier lifetimes of 750 ps at the threshold current for the SOA without antireflection coating and 200 ps at high injection have been found. The linewidth enhancement factor is measured to be between 4 and 17 which fits with a simple empirical expression.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Simple technique for improving the resolution of the delayed self-heterodyne method

A new technique is proposed and demonstrated for improving the resolution of the delayed self-heterodyne method for laser spectral linewidth measurement. The delay time between two laser beams is increased by the use of multipass transmission in an optical fiber ring interferometer containing a frequency shifter. This technique does not require a longer fiber or complicated curve fitting of the observed spectra.

High-speed photodetector characterisation by delayed self-heterodyne method

An improved single-laser heterodyning technique for measuring the frequency response of high-speed photodetectors is described. Experimental results up to 22 GHz are presented, and the accuracy and limitations of frequency response measurements made by this method are discussed.