Beat Frequency Generation Research Articles

Frequency demodulation of an inhomogeneous medium multi-longitudinal mode fiber laser and its sensing application.

Dispersion characteristic could be a significant factor, which impacts the beat frequency of Multi-longitudinal mode fiber laser (MMFL). In this paper, the mechanism of beat frequency generation in inhomogeneous medium Multi-longitudinal mode fiber laser is discussed. Compared with cavity length-dependent fiber laser sensing system, the proposed model uses a several-millimeter-Fiber-Bragg-Grating (FBG) as the sensing head, which features both high sensitivity and compact size. We designed an experiment to exhibit possible sensing application based on the proposed theory as well.

광대역 레이더의 표적 반사 신호 모의 방법에 관한 연구

실환경에서 레이더 시스템의 성능 평가를 수행하는 것은 시간이나 비용의 제한조건이 많으므로, 이를 시험하기 위한 표적 신호 모의 장치가 반드시 필요하다. 본 논문은 고해상도의 거리 프로파일 또는 영상을 형성하거나 혹은 추적의 정확도를 올리기 위하여 점차적으로 광대역을 사용하는 레이더의 최신 동향에 따라 이를 시험하기 위한 표적신호 모의방법을 제시하였다. 송신신호를 직접 고속 샘플링하여 디지털 메모리를 이용한 시간 지연을 통해 구현하는 방법과 스트레치 프로세싱을 사용하는 시스템을 위한 비트 주파수 생성을 이용하는 방법에 대하여 기술하고, 각 방법이 광대역 파형의 표적을 모의할 때 가져야 할 조건을 제시하였다. 또한, 디지털 메모리 방식에 비트 주파수를 추가하는 방법을 제시하였다. 시뮬레이션을 통해 제시된 방법을 비교하고 타당함을 보였다. Testing a radar system in real environment requires a lot of time and cost. Thus various target simulators are developed to evaluate the radar performance and its functions as well. In order to enhance the range resolution and the accuracy for tracking, recent radar system tends to use the wideband signal. In this paper, we summarize two target simulation methods - the direct sampling method with the digital memories and the beat frequency generation for the stretch processing - and suggest the condition to improve their performance for a wideband radar system.

Terahertz beat frequency generation from two-mode lasing operation of coupled microdisk laser

We propose a coupled microdisk laser as a compact and tunable laser source for the generation of a coherent continuous-wave terahertz radiation by photomixing. Using the Schrödinger-Bloch model including the nonlinear effect of active medium, we find single-mode and two-mode lasings depending on the pumping strength. We explain the transitions of lasing modes in terms of resonant modes that are the solutions of the Schrödinger-Bloch model without active medium and nonlinear interaction. In particular, a two-mode lasing is shown to generate a terahertz oscillating frequency originating from the light beating of two nearly degenerated resonant modes with different symmetries.

Polarization-independent Enhanced-resolution Arrayed-waveguide Grating used in Spectral-domain Optical Low-coherence Reflectometry

The performance of an arrayed-waveguide grating (AWG) as an integrated spectrometer in spectral-domain optical low-coherence reflectometry (SD-OLCR) is significantly improved. By removing the output waveguides of the AWG, the depth range is enhanced from 1 to 3.3 mm at 800 nm and 4.6 mm at 1300 nm. Periodic signal fading, that was previously observed in the sensitivity roll-off curve in depth ranging measurements, is shown to be evoked by beat-frequency generation between the two polarizations of partially polarized signal light in a birefringent AWG. By carefully controlling the polarization state-of-light, the signal fading is eliminated. As a permanent solution to this problem, a polarization-independent AWG is demonstrated, which can reduce the size and cost of OCLR and optical coherence tomography systems further by eliminating the components for polarization control.

Channel-switchable single-/dual-wavelength single-longitudinal-mode laser and THz beat frequency generation up to 3.6 THz

We propose and demonstrate a simple approach to realize channel-switchable single-/dual-wavelength single-longitudinal-mode (SLM) lasers and switchable beat frequency generation in the THz frequency regime. It is a compact fiber-ring laser incorporating a Fabry–Perot etalon, tunable filters, and a piece of unpumped polarization maintaining erbium-doped fiber (PM-EDF) as the saturable absorber. Two parallel-arranged tunable filters are configured inside the ring cavity to support switchable dual-wavelength SLM operation and consequent THz beat frequency generation. Switchable 19-channel 200-GHz ITU-grid-compatible single-wavelength SLM operation is implemented. Moreover, the channel-switchable dual-wavelength SLM laser and switchable THz beat frequency generation ranging from 0.2 to 3.6 THz with a tuning step of 0.2 THz are demonstrated in the experiment.

EIT as a tool to observe dual wavelength operation of a semiconductor laser

We propose a simplified technique for dual wavelength operation of an extended cavity semiconductor laser, and its characterization using electromagnetically induced transparency (EIT). In this laser cavity scheme light beam is made converging before it incidences on the cavity grating. The converging angle of the beam creates two longitudinal oscillating modes of resonating cavity. Frequency separation between the longitudinal modes are measured with the help of beat frequency generation in a photodiode and creating pair of EIT spectra in Rb vapor. The pair of EIT dips that are generated due to dual wavelength of this laser (that is used as control laser) can be used to estimate frequency difference between the generated wavelengths. Width of EIT spectra can be used to estimate linewidth of individual wavelength components.

Tuneable and switchable dual wavelength lasers using optical fibre Bragg grating external cavities

Two novel external cavity lasers are demonstrated experimentally. The first utilises a Bragg grating fabricated in highly birefringent optical fibre and offers the ability to switch between modes that are separated in polarisation and wavelength. The second utilises two spatially separate Bragg gratings fabricated in mono-mode fibre. Both can be used for tunable beat frequency generation.

Beat frequency generation up to 3.4 THz from simultaneoustwo-mode lasing operation of sampled-grating DBR laser

Simultaneous two-mode lasing operation for 0.56 – 3.4 THz beat signal generation is achieved from a single sampled-grating DBR laser by simply controlling the bias current. A high autocorrelation modulation depth and > 30 dB suppression of the other cavity modes is demonstrated.

Dual tunable wavelength Er,Yb:glass laser for terahertz beat frequency generation

Dual tunable wavelength operation of Er,Yb:phosphate glass laser is reported. The spatial separation of the laser eigenstates and the use of two properly designed intracavity etalons permit one to tune the two wavelengths independently from 1540.5 to 1562.7 nm. The generated beat note, monitored using a Michelson interferometer, is experimentally shown to be adjustable from dc to 2.7 THz. The linewidth of this beat note is found to be less than 10 kHz. Several applications are discussed.

THz optical beat frequency generation from a single mode locked semiconductor laser

We demonstrate ultrahigh repetition rate beat signal generation by filtering two modes from the optical spectrum of a 110 GHz pulse train produced by a 1.56 μm wavelength, subharmonic hybrid mode locked monolithic semiconductor distributed Bragg reflector laser. The beat signal repetition rate can be tuned to any higher harmonic of the 110 GHz frequency up to 1.34 THz. Furthermore, the filtering technique facilitates low phase noise and synchronization with these signals.

Tuneable THz beat frequency generation in optical fibers

Polarization modulation instability (PMI) has been utilised to generate amplitude modulated light pulses where the modulation frequency is continuously tuneable from 3.5 THz to 7.0 THz. The tuneability is achieved by varying the tension in a low-birefringent single-mode fiber which is wrapped around a spool.

Optical beat frequency generation up to 40.6 THzby mode-locked semiconductor lasers

Optical beat frequency generation up to 40.6 THz is achieved by beating two mode-locked semiconductor lasers optical pulses using a wavelength division multiplexing fibre coupler. To the author's knowledge, this is the fastest amplitude modulated signal observed so far.

THz optical beat frequency generation by modelockedsemiconductor lasers

THz optical beat frequency generation has been demonstrated by beating the outputs of two external cavity modelocked semiconductor lasers. Beat frequencies up to 3.6 THz were generated with >95% modulation depth.

Beat frequency generation between visible lasers with frequency differences in the 80-GHz band

Millimeter-wave GaAs Schottky barrier diodes were used as fast light detectors. It was possible to observe beat signals between a HeNe laser oscillating at 633 nm and a cw dye laser with approximately 80-GHz frequency offset. Thus, great frequency differences between lasers emitting in the visible can be determined with the high accuracy inherent in heterodyning techniques.

Nonlinear plasma effects in the Alouette recordings

Recordings from the ionosonde receivers in the Alouette II satellite commonly show narrow-band, resonant-like signals at frequencies below the fundamental ionospheric resonances, f <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</inf> , f <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</inf> , and f <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</inf> . These are attributed to nonlinear processes in the plasma near the spacecraft, and the on-board electronic equipment: studies of these signals are reviewed in this paper. Particular attention is given to the subsidiary resonances, which appear at simple fractions of the ionospheric resonances. It is shown that an explanation involving harmonic stimulation of the plasma resonances and beat-frequency generation is consistent with most of the observations. Several other types of satellite data are also examined and further substantiation is found for nonlinear plasma behavior. The importance of ionospheric nonlinearities is discussed briefly for both transmission and reception of radio signals in space.

Satellite Ionosonde Records: Resonances Below the Cyclotron Frequency

Resonant responses observed by the topside ionosonde in the Canadian satellite Alouette II are examined. In addition to the well-known plasma resonances, several subsidiary resonances are identified below the electron cyclotron frequency. Their patterns of occurrence are not consistent with a suggested explanation of induced magnetic dipole radiation; rather they appear to result from harmonic stimulation of the plasma resonances and beat-frequency generation.