A waveguide-type optical-frequency comb generator

A waveguide type optical frequency comb (WG-OFC) generator was developed at 1.56 μm wavelength region by utilizing a waveguide type phase modulator. It was confirmed that the envelope of sideband spectra had a width of 2.7 THz and the slope of the envelope profile was 28 dB/THz. The maximum measurable frequency difference, limited by the shot noise, was estimated to be 3.2 THz     

Two-photon Ramsey fringes at 30 THz referenced to an H maser/Cs fountain via an optical-frequency comb at the 1-Hz level

Aspects of two-photon Ramsey fringes in SF/sub 6/ at 30 THz obtained with a 1-m separation between the two absorption zones are presented. The experiment is referenced to a primary standard, the Hydrogen maser/Cs fountain located at BNM-SYRTE, via a femtosecond laser frequency comb generator. This results in a stability below 1 Hz for 1000 s of averaging, and absolute frequency measurements to /spl plusmn/2 Hz which corresponds to 6/spl times/10/sup -14/ in relative frequency. Resolution of the hyperfine structure now approaches 10 Hz, while the fringe period of 200 Hz provides a frequency discriminator 100 times narrower than any other currently in use in the spectral region.     

Recent progress in integrated electro-optic frequency comb generation

Optical frequency combs have emerged as an important tool enabling diverse applications from test-and-measurement,including spectroscopy,metrology,precision distance measurement,sensing,as well as optical and microwave waveform synthesis,signal processing,and communications.Several techniques exist to generate optical frequency combs,such as mode-locked lasers,Kerr micro-resonators,and electro-optic modulation.Important characteristics of optical frequency combs include the number of comb lines,their spacing,spectral shape and/or flatness,and intensity noise.While mode-locked lasers and Kerr micro-resonators can be used to obtain a large number of comb lines compared to electro-optic modulation,the latter provides increased flexibility in tuning the comb spacing.For some applications in optical communications and microwave photonics,a high degree of integration may be more desirable over a very large number of comb lines.In this paper,we review recent progress on integrated electro-optic frequency comb generators,including those based on indium phosphide,lithium niobate,and silicon photonics.     

Limit to sensitivity of optical fibre magnetometers due to shape anisotropy

Shape anisotropy, or the demagnetising effect, limits the sensitivity achievable with optical fibre magnetometers. The optimum sensor geometry has the minimum cross-section compatible with efficient strain transfer. For a sensor head with multiple passes of magnetically sensitised fibre, there is a limit to the increase in sensitivity as the number of passes is increased.     

Highly stabilized millimeter-wave generation by using fiber-opticfrequency-tunable comb generator

A novel optical generation and transport technique for millimeter-wave (mm-wave) signals by using the fiber-optic frequency-tunable comb generator having a discrete emission spectrum as the light source is proposed. 60 GHz mm-wave signal generation and transmission over a 108 km-long optical fiber link at λ=1560 nm is demonstrated. Excellent stability of both the frequency and amplitude of the mm-wave carrier is attained, so that the 3 dB linewidth is less than 0.3 kHz and the variance of the amplitude fluctuation is 0.07 dB. The advantages include tunability of the carrier frequency, handling ease, and no need for a high-speed external light modulator     

Optical frequency comb generation using two cascaded polarization modulators

A novel approach to generate the flattened optical frequency comb (OFC) with tunable frequency spacing is proposed. The proposed generator consists of two cascaded polarization modulators driven by two radio frequency signals with different frequencies. By adjusting the parameters, 9, 12, and 16 comb lines can be generated with tunable frequency spacing. The scheme requires no DC bias voltage of the modulators and no optical filter, which makes our method simple and stable. A concept proof simulation is conducted to confirm the feasibility of the generation schemes of the 9-, 12-, and 16-line OFCs.     

Characteristics of the optical-frequency conversion in asemiconductor laser

A small signal analysis is presented for optical-frequency conversion using nearly degenerated four-wave mixing (FWM) in a semiconductor laser. The nearly degenerate FWM is mainly introduced by the carrier beating at the detuning frequency of the pump and the probe waves. Generally speaking, the influence of the nonlinear gain saturation on the frequency conversion efficiency is very small compared with that of the carrier density beating. However, the conversion efficiency is damped by the nonlinear gain saturation at higher bias current. The frequency conversion efficiency and the probe transmissivity are almost symmetrical with respect to the zero detuning frequency. The bandwidth of the converted signal is governed by the relaxation resonance frequency of the laser. A definition of the conversion. The theoretical results agree with experiments     

Frequency comb generation by four-wave mixing and the role of fiberdispersion

A fiber-optic frequency comb generator utilizes four-wave mixing (FWM) to expand a modulated continuous wave (CW) lightwave into a comb of discrete, equispaced frequencies. The sign and magnitude of the fiber dispersion are shown to control comb expansion by affecting the interference among different four-wave mixing interactions at particular comb frequencies. Positive dispersion initiates comb expansion while zero and negative dispersions suppress expansion. Dispersion mapping engineers fiber links for maximum comb bandwidths. By managing dispersion in a 35 km link of dispersion-shifted fiber, a frequency comb spanning 1.1 THz is experimentally demonstrated. Promising applications include pulse train synthesis affording low-chirp, arbitrarily profiled, short pulses     

Multiwavelength generation in an erbium-doped fiber laser using in-fiber comb filters

Simultaneous continuous wave multiwavelength operation of an erbium-doped fiber laser has been demonstrated using two types of recently developed in-fiber grating comb filters. The lasing wavelengths and linewidths were determined by the comb filter, which was inserted in a ring cavity with a single inhomogeneously broadened gain medium.     

Widely wavelength-tunable ultra-flat frequency comb generation using conventional dual-drive Mach-Zehnder modulator

Wavelength-tunable ultra-flat optical frequency comb generation is demonstrated using only a conventional Mach-Zehnder modulator that is asymmetrically dual-driven by large-amplitude sinusoidal signals with different amplitudes. A 10 GHz-spaced frequency comb with a 10 dB bandwidth of 230 GHz was experimentally generated. In addition, 50 nm widely wavelength tunable operation was achieved.     

Harmonic generation due to ballistic electron transport in GaAs

Harmonics produced by the nonlinearity in the current-voltage characteristics of a ballistic diode are studied for a GaAs sample of thickness 1 µm at 77 K under collision-free condition. Harmonic generation is found to be maximum at zero bias, the value for the second-harmonic current being 50 percent for a signal amplitude of 0.2 V.     

Small-signal analysis of optical-frequency conversion in aninjection-locked semiconductor laser

A theory of optical-frequency conversion using nearly degenerate four-wave mixing (FWM) in an injection-locked semiconductor laser is presented by small-signal analysis. The bandwidth of the converted signal is determined by the relaxation resonance frequency of the laser. The greatest conversion efficiency appears when the pump-probe detuning frequency is equal to the relaxation resonance frequency. The influence of the bias of the laser, the pump power and the other injection-locking parameters on the frequency conversion efficiency is discussed. The theoretical results agree with experiments     

Hole trap generation in the gate oxide due to plasma-inducedcharging

The paper presents results of hole trapping studies in-thin gate oxide of plasma damaged MOS transistors. Process-induced damage was investigated with antenna test structures to enhance the effect of plasma charging. In addition to neutral electron traps and passivated interface damage, which are commonly observed plasma charging latent damage, we observed and identified hole traps, generated by plasma stress. The amount of hole traps increases with increasing antenna ratio, indicating that the mechanism of hole trap generation is based on electrical stress and current flow, forced through the oxide during plasma etching. The density of hole traps in the most damaged devices was found to be larger than that in reference, undamaged devices by about 100%     

Harmonic generation due to hot electrons in surface inversion layers

Harmonic generation due to hot electron nonlinearity in Si-inversion layers at 77 K in the presence of a large high frequency signal is studied on a drifted Maxwellian model. The third harmonic content increases with increase in field amplitude but decreases to negligible values for frequencies of about 1000 GHz.     

Hydrogen generation due to decomposition of carboxym ethyl-cellulose in muddy water

The letter describes a novel hydrogen generation phenomenon observed for carboxymethyl-cellulose (CMC), used as a typical material for water-swellable tape in waterproof optical cable. A graded-index optical fibre set in CMC solution containing muddy water exhibits optical loss increase due to hydrogen gas generation, and the viscosity of the CMC solution decreases because of micro-organism decomposition.     

30-THz span optical frequency comb generation by self-phasemodulation in an optical fiber

The width of an optical frequency comb (OFC) was increased to 30 THz by using self-phase modulation (SPM) in an optical fiber. This value is 2.7 times larger than the maximum OFC span obtained by the OFC generator alone. We compare the resulting spectrum to numerical simulations to confirm that the SPM and the higher order dispersion of the fiber contribute to broaden the spectral profile     

基于级联相位调制器和光衰减器的光频梳产生研究

提出了一种基于级联相位调制器和光衰减器的光 频梳产生方案,建立了方案的理论模型并进行了 系统验证,研究了射频(radio frequency,RF) 信号幅度和光衰减器衰减系数对光频梳平坦度的影响。结果表明数值 计算与实验 结果一致性较好,通过调整RF驱动信号幅度和光衰减器衰减系数可产生梳线数量为15条、平坦度为 0.8 dB,边模抑制比(side-mode suppression ratio,SMSR) 为5.05 dB的光频梳。方 案中引入滤波器后,在梳线数量保持不变的前 提下,其平坦度和SMSR可分别提高62.5%和61.4%。     

Small signal analysis of THz optical-frequency conversion in aninjection-locked semiconductor laser

A theory of terahertz optical-frequency conversion using highly nondegenerate four-wave mixing (FWM) in an injection-locked semiconductor laser is presented, using small-signal analysis. The optical frequency conversion can be realized through the use of cavity-enhanced highly nondegenerate FWM in an injection-locked semiconductor laser in a range of 1-THz detuning frequency between the pump and the probe waves, when the probe frequency is tuned close to one of the resonance modes. The frequency conversion is mainly attributed to the nonlinear gain effect. The maximum bandwidth of the converted signal is increased by shortening the laser cavity length. The frequency conversion efficiency is asymmetrical with respect to both the zero detuning frequency and the resonance mode where the converted signal appears. The theoretical results agree with experiments     

Noise due to generation and recombination of carriers in p-n junction transition regions

The theory for the current noise associated with carrier generation and recombination in ap-njunction transition region is presented. The noise model is derived directly from the SRH defect center model. In a reverse biased junction the noise from this mechanism varies from two-thirds to full shot noise, depending upon frequency, and in a forward biased junction the noise ranges from three-fourths to full shot noise, depending upon injection level. These results approximately agree with published experimental results, but the agreement is not conclusive, especially in forward biased junctions. The theory shows that the noise associated with transition region generation-recombination current is not particularly significant in bipolar transistors.     

基于级联外部调制器产生宽光梳系统的研究

本文研究了基于双边带调制方式采用级联两个强度 调制器(MZM)或级联MZM和相位调 制器(PM)分别生成平坦宽光梳的两种方案。通过理论分析得到合适的调制器关联变量-调 制电压和偏置电压的关系。在此基础上,我们分别讨论了高阶边带以及相位偏转对两种方案 产生宽光梳平坦度的影响。两种方案如果要达到相同的平坦度,级联相同MZM所输出的光梳 频带宽度可高达500 GHz,比级联MZM和PM的输出频带更宽;如果两种 方案在相同的输出光梳 频带宽度(300 GHz)下,级联MZM的平坦度可以达到0. 2 dB,而级联MZM和PM系统的平坦度只 有2.1 dB。因此级联两个强度调制器产生的宽光梳在光梳平坦度方面 性能更为良好。这为产生平坦光梳及调制器的灵活运用提供了一种解决方案。