GHz Modulation Frequency Research Articles

The effect of modulation frequency for frequency domain diffuse optic tomography (FDDOT)

In this study, it was illustrated that extremely high frequency (EHF) Ka-band (26.5–40 GHz) modulated frequency domain diffuse optic tomography biomedical optic imaging modality is superior to the generally accepted 100 MHz modulation frequency case. The effect of modulation frequency was shown with reconstructed images for two different modulation frequency simulation cases. Ka EHF-band frequency range covers 26.5–40 GHz. In this simulation study 33 GHz modulation frequency was selected. Forward model problem photon fluencies were generated for 30 equally separated sequential phase delays. Each phase delay has different photon fluence distributions inside the imaging geometry. 3.1 cm grid sizes were set in the x, y, z cartesian grid coordinate system with 31 × 31 × 31 xyz grid elements. To test the advantage of EHF-band modulation frequecy, inverse problem solution algorithm was done and inclusion images were reconstructed for each modulation frequency simulation case. Original inclusion was embedded inside the imaging geometry at (15–16, 15–16, 15–16) x, y, z coordinate system in the three-dimensional (3D) cubic spatial form. Homogeneuous tissue background photon absorption, scattering, and anisotropy coefficients were selected as µa = 0.1 cm−1, µs = 100 cm−1, and g = 0.9. Four sources and four detectors were placed on the back-reflection geometry. Forward model problem was built between sources, voxels, and detectors. Forward model problem photon fluencies were generated based on the diffusion equation approximation of the radiative transport equation formula in the frequency domain. Original inclusion was reconstructed by simply applying mathematical pseudoinverse problem solution function. It was observed that, reconstructed inclusion image at the 33 GHz Ka-band modulation frequency simulation case is superior to the reconstructed inclusion image at the 100 MHz modulation frequency simualtion case. Since there are too many frequency selection opportunities, the best Ka-band frequency was selected to demonstrate here. Thus, 33 GHz and 100 MHz modulation frequencies were tested against each other. These two different electronic modulation frequencies were tested and compared with each other.

An X-ray frequency modulation method and its application in X-ray communication

X-ray communication (XCOM) is a novel space communication method with great potential. X-rays have much shorter wavelengths than both infrared and radio. In principle, XCOM can send more data for the same amount of transmission power, which could permit more efficient gigabits-per-second data rates for deep space missions. Besides, X-rays can pierce the hot plasma sheath that builds up as spacecraft hurdle through Earth’s atmosphere at hypersonic speeds. However, some current technical limitations have made X-ray communication unable to fully release its theoretical advantages in space communications. Among them, the modulation of X-ray emission impacts significantly. For the requirements of high-flux, collimation, quasi-monochromatic X-ray emissions, as well as the exploration of X-ray frequency modulation, we propose an X-ray frequency modulation method for generating different energy X-rays by controlling the electron beam deflection based on an external electrostatic field. Different from all traditional on-off control methods, this scheme uses two specific X-ray energies indicate the digital signal 0 and 1, which makes X-ray frequency modulation possible. Based on this, we simulated the emission flux and energy distribution of two different targets. Simulation results indicate that this X-ray modulation method produce X-rays with up to 50 GHz modulation frequency, 1014 cps X-ray emission flux, 99% monochrome, and 1.5 mrad divergence angle. Through this modulator, not only the communication distance and communication speed experience significant increases, but also the concept of X-ray communication, X-ray ranging and X-ray navigation can be realized as a three-in-one mission.

Optimization of a dispersion-tuned wavelength-swept fiber laser for optical coherence tomography.

We optimized parameters of a dispersion-tuned wavelength-swept fiber laser by numerically analyzing dynamic characteristics. The optimized laser is experimentally demonstrated and applied to the swept-source optical coherence tomography (SS-OCT) system. The dispersion-tuned wavelength-swept laser (DT-WSL) is a unique tunable fiber laser, whose lasing wavelength can be tuned rapidly without any mechanical tunable filters. Although the wavelength of a DT-WSL can be swept rapidly and widely, the broadening of the instantaneous spectral width at a high sweep rate has been a critical drawback for SS-OCT applications. Numerical simulations have shown that higher modulation frequencies for active mode-locking lead to narrower instantaneous spectral widths. However, a lower modulation frequency is needed to achieve a wider wavelength tuning range. Pulse modulation is employed to solve the trade-off between instantaneous spectral width and wavelength tuning range. In this paper, the characteristics of a sinusoidally modulated and a pulse-modulated DT-WSL are compared numerically and experimentally. The numerical simulation results show that a pulse-modulated laser can achieve spectral widths as narrow as that of the sinusoidally modulated laser with >5 GHz modulation frequency, even when the pulse modulation frequency is as low as 500 MHz. We also study the difference in the laser characteristics with different sweep directions and discover that a positive wavelength sweep leads to a narrower instantaneous spectral width. We also experimentally confirmed that pulse modulation can indeed achieve a narrower spectral width, as expected from our numerical simulation results. The pulse-modulated DT-WSL is then used in an SS-OCT system and successfully achieves a coherence length of 1.3 mm, whereas that of a sinusoidally modulated DT-WSL is limited to only 0.7 mm. Furthermore, we experimentally compare the performance difference in OCT imaging with different wavelength sweep directions, and the results proved that it is advantageous to apply a positive wavelength sweep, as predicted by our numerical simulation.

Spectral changes induced by a phase modulator acting as a time lens

We show both numerically and experimentally that a phase modulator, acting as a time lens in the Fourier-lens configuration, can induce spectral broadening, narrowing, or shifts, depending on the phase of the modulator cycle. These spectral effects depend on the maximum phase shift that can be imposed by the modulator. In our numerical simulations, pulse spectrum could be compressed by a factor of 8 for a 30 rad phase shift. Experimentally, spectral shifts over a 1.35 nm range and spectral narrowing and broadening by a factor of 2 were demonstrated using a lithium niobate phase modulator with a maximum phase shift of 16 rad at a 10 GHz modulation frequency. All spectral changes were accomplished without employing optical nonlinear effects such as self- or cross-phase modulation.

Electro‐optic SSB modulator with 90° hybrid directly prepared onto LiNbO 3 substrate

Optical single-sideband (SSB) modulation is theoretically realised by applying two modulation signals with a 90° phase difference to the dual-electrode type electro-optic (EO) modulator. A branch-line coupler (BC) as the microwave 90° hybrid was directly prepared on the EO modulator substrate, LiNbO 3 , to create two modulation signals from a single input signal. Output ports of the BC were connected to the modulation electrodes on the substrate. Thereby, an EO optical SSB modulator was realised with a single-tip and a small-sized configuration. The SSB modulator was designed and fabricated for around 10 GHz modulation frequency and 1550 nm light wavelength. The optical SSB modulation operation with a sideband suppression ratio of more than 20 dB was confirmed by the experiment.

Experimental research of target uniform illumination using smoothing by spectral dispersion and continuous phase plate

To improve laser irradiation uniformity, experimental research on smoothing by spectral dispersion (SSD) and continuous phase plate (CPP) is carried out on the technical integration line (TIL). A bulk phase modulator with 9.2 GHz modulation frequency is adopted. The output spectrum of the phase modulator is stable and the residual amplitude modulation is quite small. The experimental results indicate that when the number of color cycles (Nc) is adopted to be 1, imposing of SSD in this divergence does not lead to pinhole closure in the spatial filters of the preamplifier and main amplifier. The contrast of the focal spot with 95% energy included with SSD and CPP drops to 0.47 compared with 1.71 without SSD and CPP. When the pulse width of the third harmonic wave is 1 ns and its energy is 1115 J, no damage is found in CPP and other final optics. The experiment solves some key techniques by using SSD and CPP on high-power laser facilities, and provides sound basis for the upcoming physics experiment.

Chirp Characteristics of Silicon Mach

Chirp characteristics of Silicon Mach-Zehnder Interferometer (MZI) modulators with forward-biased p-i-n and reverse-biased p-n structures are investigated by performing small-signal simulation, respectively. Simulation result shows that the chirp parameter is negative and influenced by the carrier absorption effect, the amplitude, and the frequency of applied sinusoidal modulating signals. Chirp performance at 10 GHz modulation frequency can be obtained by utilizing p-n depletion mode structure. Finally, a way to further lower the absolute value of the chirp parameter is provided by operating the MZI in a push-pull configuration.

Effect of XPM on WDM Optical Transmission System in Presence of First- and Second Order GVD

Transmission in wavelength division multiplexing (WDM) optical communication system is mainly impaired and ultimately limited by group velocity dispersion (GVD) and cross-phase modulation (XPM). In this paper, we analytically determine and compare the impact of XPM in a WDM system in presence of first- and second order GVD for standard single mode fiber (SSMF) and dispersion shifted fiber (DSF). Even at high bit rate in a first order GVD compensated system, the second order GVD plays a critical role in limiting the distance of optical signal. The results show that XPM crosstalk penalty due to second order GVD is 16 dB more in DSF than that of SSMF for 0.8 nm channel spacing at 10 GHz modulation frequency. It is also found that the spectral characteristics are strongly dependent on the channel spacing and dispersion of the fiber.

Demonstration of balanced coherent detection using polymer optical waveguide integrated distributed traveling-wave photodetectors

We have demonstrated balanced coherent detection using distributed balanced traveling-wave photodetectors integrated with single mode polymer optical waveguides. Balanced distributed traveling-wave photodetectors having 3 dB bandwidth of 20 GHz exhibited 20 dB signal to noise ratio improvement measured at 15 GHz modulation frequency in a balanced coherent detection demonstration.

Phase shift keyed systems based on a gain switched laser transmitter

Return-to-Zero (RZ) and Non-Return-to-Zero (NRZ) Differential Phase Shift Keyed (DPSK) systems require cheap and optimal transmitters for widespread implementation. The authors report on a gain switched Discrete Mode (DM) laser that can be employed as a cost efficient transmitter in a 10.7 Gb/s RZ DPSK system and compare its performance to that of a gain switched Distributed Feed-Back (DFB) laser. Experimental results show that the gain switched DM laser readily provides error free performance and a receiver sensitivity of -33.1 dBm in the 10.7 Gbit/s RZ DPSK system. The standard DFB laser on the other hand displays an error floor at 10(-1) in the same RZ DPSK system. The difference in performance, between the two types of gain switched transmitters, is analysed by investigating their linewidths. We also demonstrate, for the first time, the generation of a highly coherent gain switched pulse train which displays a spectral comb of approximately 13 sidebands spaced by the 10.7 GHz modulation frequency. The filtered side-bands are then employed as narrow linewidth Continuous Wave (CW) sources in a 10.7 Gb/s NRZ DPSK system.

High-Power High-Linearity InGaAs/InP Photodiodes

The third-order intermodulation distortions of an InGaAs/InP modified uni-traveling carrier photodiode (MUTC-PD) are characterized using a two-tone setup. At 0.3 GHz modulation frequency and photocurrents above 40 mA the third order local intercept point (IP3) reaches 52 dBm and remains above 35 dBm up to the photodiode's 3 dB bandwidth. A simple equivalent circuit model is used to explain the frequency characteristics of the intermodulation distortions. Based on experimental data, we find that the frequency characteristics of the IP3 can be attributed to the change of both the responsivity and device capacitance induced by the photocurrent and the transient voltage drop at the load and series resistors. Further investigations of the non-linear phenomena suggest that the Franz-Keldysh effect and impact ionization are the primary reasons for voltage-dependent responsivity, while low doping concentration in the p-type InGaAs absorber and space charge screening are responsible for the voltage dependent capacitance and photocurrent dependent capacitance, respectively.

Enhancing light slow-down in semiconductor optical amplifiers by optical filtering

We show that the degree of light-speed control in a semiconductor optical amplifier can be significantly extended by the introduction of optical filtering. We achieve a phase shift of approximately 150 degrees at 19 GHz modulation frequency, corresponding to a several-fold increase of the absolute phase shift as well as the achievable bandwidth. We show good quantitative agreement with numerical simulations, including the effects of population oscillations and four-wave mixing, and provide a simple physical explanation based on an analytical perturbation approach.

A simple model for cavity enhanced slow lights in vertical cavity surface emission lasers

We develop a simple model for the slow lights in vertical cavity surface emission lasers (VCSELs), with the combination of cavity and population pulsation effects. The dependences of probe signal power, injection bias current and wavelength detuning for the group delays are demonstrated numerically and experimentally. Up to 65 ps group delays and up to 10 GHz modulation frequency can be achieved at room temperature at a wavelength of 1.3 μm. The most significant feature of our VCSEL device is that the thickness of the active region is only several micrometers long. Based on the experimental parameters of quantum dot VCSEL structures, we show that the resonance effect of the laser cavity plays a significant role in enhancing the group delays.

Harmonic distortion in 1.55-μm vertical-cavity lasers

We investigate second and third order harmonics in the analog modulation response of InP-GaAs fused 1.55-μm vertical-cavity lasers (VCLs). These devices exhibit the lowest threshold current as well as the highest temperature of continuous-wave operation of any electrically pumped long wavelength VCL. Near the resonance frequency, harmonic distortion is dominated by electron-photon interaction, and it is close to the distortion measured with in-plane lasers. At low frequencies, distortions seem to be dominated by carrier transport effects. At 1 GHz modulation frequency, we measure a maximum spur-free dynamic range of 64 dB Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> 2/ for second-order harmonic distortion and 81-dB Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> 3/ for third-order harmonic distortion.

80~200 GHz erbium doped fibre laser using a rationalharmonic mode-locking technique

Pulse trains with repetition rates as high as 80~200 GHz were obtained at 1.55 µm, using rational harmonic mode-locking with an erbium doped fibre laser. A fundamental repetition frequency (fosc) of 40 GHz was first generated by applying a 40 GHz modulation frequency to an LN intensity modulator in the fibre cavity, in which the longitudinal mode spacing (Δf) was 1.5 MHz. By rationally detuning the modulation frequency by Δf/n, the authors succeeded in obtaining a repetitive oscillation of (nfosc+Δf). The maximum frequency was 200 GHz, at which n corresponds to 5.

High-speed polarization scrambler with adjustable phase chirp

We report on the operation and performance of a novel electrooptic polarization scrambler that is capable of generating any desired combination of optical phase and polarization modulation. The scrambler comprises a dual-drive Mach-Zehnder-type intensity modulator on lithium niobate and an external microoptic polarization splitter which combines the two complementary outputs of the integrated-optic intensity modulator in orthogonal polarization states. The modulation indexes of the sinusoidal phase and polarization modulation in the output light can be varied independently of each other via the amplitudes and phases of the two drive voltages to the Mach-Zehnder modulator. The scrambler has a 3-dB modulation bandwidth of 14 GHz and operates in the 1.5-1.6-/spl mu/m wavelength range with an overall fiber-to-fiber insertion loss of less than 5 dB. At 5 GHz modulation frequency and electric drive powers between 390-2030 mW, we demonstrate phase-chirped and chirp-free depolarization with a residual degree of polarization of less than 0.02 and continuously variable phase modulation to 2.3-rad peak phase deviation.

1.5-μm InGaAsP-InP multigain-levered-MQW-DFB-LD with high-efficiency and large-bandwidth FM response

A 1.5-/spl mu/m multigain-levered (MGL)-MQW-DFB-LD which utilizes a new concept of multigain-levering is proposed and realized by two-step selective MOVPE growth. The device consists of three sections with different bandgap energies. High- and flat-FM-efficiency value of 10 GHz/mA was theoretically and experimentally demonstrated up to the 2 GHz modulation frequency. >

Millimeter-wave asymmetric Fabry-Perot modulators

We have designed, fabricated, and characterized GaAs-AlGaAs (/spl lambda/=864 nm) asymmetric Fabry-Perot modulators with /spl ap/37 GHz modulation frequency response, comparable to the fastest waveguide modulators. The modulation response saturates at high optical powers due to saturation of the excitonic absorption and heating effects, but the frequency response is independent of the incident optical intensity, since it depends only on the RC time constant, and not on the carrier transit time. The device design takes advantage of the fact that the quantum-confined Stark effect is more pronounced at some distance from the absorption edge to achieve a modulator with /spl ges/20 dB contrast and /spl ap/3 dB insertion loss for /spl plusmn/2 V operating voltage, but only 21 fF capacitance. The DC bias used to move the operating point off the absorption edge has the additional benefits of improving the linearity and chirp of the device, as well as the saturation intensity. Here we present measurements of the modulation and photocurrent responses of the modulators, calculate the RC and transit times for the device, analyze the saturation mechanisms, and discuss the linearity and chirp of the device from the perspective of a high-speed digital optical communications system.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Transient resistive photoresponse of YBa2Cu3O7−δ films using low power 0.8 and 10.6 μm laser radiation

Thin YBa2Cu3O7−δ laser deposited films were patterned into devices consisting of ten parallel 1 μm wide strips. Nonequilibrium picosecond and bolometric photoresponses were studied by the use of 17 ps full width at half-maximum laser pulses and amplitude modulated radiation from an AlGaAs laser up to 10 GHz and from a CO2 laser up to 1 GHz. The time and frequency domain measurements were in agreement. The fast response can be explained by electron heating. The use of low optical power and a sensitive measurement system excluded any nonlinear transient processes and kinetic inductance changes in the superconducting state. At 1 GHz modulation frequency, the responsivity was ∼1.2 V/W both for 0.8 and 10.6 μm wavelengths. The sensitivity of a fast and spectrally broadband infrared detector is discussed.

Generation of transform-limited optical pulses up to 20 GHz from a monolithic electroabsorption modulator/DFB laser

Stable generation of transform-limited optical pulses using a monolithic electroabsorption (EA) modulator/DFB laser was demonstrated. The integrated device had a small α-parameter less than 0.3 and a wide electrical bandwidth over 20GHz. The time-bandwidth products of the generated optical pulses were less than 0.33 in the modulation frequency between 5 and 20GHz. Temporal forms of th e pulses were in good agreement with the sech2 pulse form. The over 4dBm output peak power was achieved from the EA modulator facet for 20GHz modulation frequency.