Theory and experiment of high-speed cross-gain modulation insemiconductor lasers

We present theory and experiment for the high-speed modulation response of a quantum-well (QW) laser in the presence of an external microwave modulated optical pump in the gain region. The model includes the effects of pump-induced stimulated recombination and cross-gain saturation. Expressions for the small-signal modulation response of the test laser under gain modulation are derived. We also present experimental results using a multiple-QW InGaAlAs Fabry-Perot (FP) laser at 1.552 μm as the test laser and an external pump by a 1.542 μm DFB laser. Comparison between electrical modulation and optical cross-gain modulation (XGM) of the test laser is also presented, which shows improvement of the modulation bandwidth by optical XGM. Our data show a reduction of carrier lifetime with increasing optical pumping, a shift of the test-laser threshold current, a change in the K factor, and a variation of the relaxation frequency with different pump powers. The experimental results agree very well with the theoretical results     

SOA-Assisted Data-Polarization-Insensitive Wavelength Conversion in a PPLN Waveguide

We demonstrate a novel method of wavelength conversion in a periodically poled lithium niobate (PPLN) waveguide using polarization modulation and amplitude modulation of its pump. Polarization-insensitive, all-optical, intensity-to-polarization conversion in a semiconductor optical amplifier (SOA) is used to reduce the waveguide's polarization sensitivity from >30 dB to <1 dB. We demonstrate BER measurements of the converted signal at 2.5 Gb/s. We also demonstrate polarization modulation of the PPLN waveguide pump using cross-polarization modulation in highly nonlinear fiber (HNLF) at 10 Gb/s.     

Double Rayleigh scattering noise in Raman amplifiers using pump time-division-multiplexing schemes

The impact of signal double Rayleigh scattering (DRS) noise in time-division multiplexed counterpropagating Raman pumping schemes is theoretically and experimentally investigated. We show that, for small pump modulation duty cycles, DRS noise can be greatly enhanced with respect to continuous-wave pumping. By increasing the pump modulation frequency, however, this detrimental effect is effectively reduced, providing a powerful technique for noise figure equalization and pump four-wave mixing suppression in large bandwidth discrete and distributed amplifiers.     

Suppression of idler spectral broadening in highly efficient fiberfour-wave mixing by binary-phase-shift-keying modulation of pump wave

Proposes and demonstrates a novel method of suppressing spectral broadening of the idler wave, which is generated in fiber four-wave mixing (FWM) pumped by a continuous wave. In this scheme, binary-phase-shift-keying (BPSK) modulation of the pump wave broadens the pump spectrum so that the Brillouin threshold for the pump power is increased, whereas it never affects the spectrum of the idler wave. By the 2-Gb/s BPSK modulation of the pump wave, we achieve a FWM efficiency as high as 10 dB, maintaining a side-band suppression ratio of the idler wave larger than 25 dB     

Microwave modulation of a quantum-well laser with and withoutexternal optical injection

We compare three microwave modulation methods experimentally and theoretically using a semiconductor quantum-well (QW) Fabry-Perot test laser: (1) direct microwave current modulation of the test laser (electrical modulation); (2) optical modulation by an external single-wavelength pump laser with a modulated optical injection power; and (3) electrical modulation of the test laser that is injection locked by an external single-wavelength pump laser with a constant injection power. This is the first direct comparison of the three modulation methods on the same QW laser, to the best of our knowledge. The bandwidth of optical absorption modulation is 7.7 GHz, which is 1.45 times the direct electrical modulation bandwidth (5.3 GHz) at a bias current of 30 mA in the test laser. On the other hand, the electrical modulation of the test laser under injection-locking condition has a significantly higher modulation bandwidth (10.5 GHz) than both the electrical and optical modulation methods     

Using an injection-locked diode laser to pump a CW Raman laser

We demonstrate the use of an injection-locked diode laser (792 nm) as the pump source for a continuous-wave (CW) Raman laser with Stokes emission at 1182 nm. Phase modulation of the master signal is imparted to the slave laser output in order to generate an error signal for phase/frequency locking via the Pound-Drever-Hall method. To illustrate the utility of this technique, we achieve the highest pump rate yet observed for a CW Raman laser (>90 times threshold) and compare the data to the existing theory. We also show how current modulation of the slave laser can eliminate the need for an electrooptic modulator in the system altogether     

有损色散非线性光纤中考虑泵浦自相位调制效应下的交叉相位调制传递函数

采用小信号分析方法研究了色散和非线性共同作用下的光信号传输,导出了损耗、色散、自相位调制及交叉相位调制作用下反映交叉相位调制过程的频域传递函数,通过该传递函数给出了在强度调制直接检测系统中确定任意泵浦信号下的探测波时域波形的解析方法,发现泵浦波自相位调制一定程度上增大了频域传递函数,且增大程度随着调制频率、输入信号平均功率及传输距离的提高而逐渐变大,解析结果和数值仿真能较好吻合.     

Selective suppression of idler spectral broadening in two-pump parametric architectures

We describe methods for selective suppression of idler spectral broadening in two-pump fiber parametric fiber amplifiers. Combinations of multiple amplification bands and dissimilar pump modulation techniques are used to generate narrow idlers at preselected wavelengths. In-phase pump modulation is shown to eliminate the need for phase-delay management in two-pump parametric amplification schemes.     

All-optical switching in 3,3'-diethyl-2,2'-thiatricarbocy-anine Iodide dye molecules

All-optical switching has been theoretically analyzed in the 3,3'-diethyl-2,2'-thiatricarbocyanine iodide(DTTCI) carbocyanine dye that exhibits large excited-state absorption to achieve high contrast and fast switching.Switching has been analyzed both ns and ps pump pulse widths.It is shown that there is an optimum value of concentration for given peak pump intensity at which maximum modulation can be achieved.We can get 93.84% modulation of transmission of a CW probe laser beam at 532 nm at peak pumping in...     

Parametric amplification and frequency conversion in optical fibers

We find that the parametric four-photon gain for light pulses decreases for fibers longer than a characteristic length. This length is related to the common experimental observation that stimulated parametric emission is usually prominent only in short fibers while in long fibers stimulated Raman scattering dominates. Despite the fact that the actual process involves an intensity dependent bandwidth and broadening of the pump linewidth from self-phase modulation, it is possible to develop a simple expression for the characteristic length which requires only the initial pump linewidth and the low-power parametric bandwidth. This bandwidth can often be estimated from the pump wavelength and the measured frequency shift between the pump and the generated waves. Expressions for gain and amplification are derived from coupled wave equations and in the Appendixes it is shown that these are of the same form as the planewave equations, but modified by coupling coefficients called overlap integrals.     

Cross-phase modulation in dispersive fibers: theoretical andexperimental investigation of the impact of modulation frequency

We investigated, theoretically and experimentally, the impact of modulation frequency on cross-phase modulation (XPM) in dispersive fibers. A simple expression for the XPM index has been derived and verified by experiment. The XPM index is found to depend on fiber length, fiber chromatic dispersion, wavelength separation between the signal and the pump, and the intensity modulation frequency. At high modulation frequencies, the XPM index is inversely proportional to the product of the modulation frequency and wavelength separation     

Impact of pump phase modulation on the gain of fiber optical parametric amplifier

In practice, fiber optical parametric amplifiers are generally driven by a phase-modulated pump wave to avoid stimulated Brillouin back-scattering. We show both analytically and numerically that the phase modulation of the pump can induce strong parametric gain modulation and that this detrimental effect depends both on the rise-fall time of the phase modulator and on the fiber dispersion slope.     

Effects of polarization-mode dispersion on cross-phase modulation in dispersion-managed wavelength-division-multiplexed systems

This paper develops a vector theory of cross-phase modulation (XPM) in optical fibers and use it to investigate the impact of polarization-mode dispersion (PMD) on the crosstalk induced by XPM in wavelength-division multiplexed lightwave systems. Under certain reasonable approximations, the theory permits us to obtain an analytic expression for the amplitude of probe fluctuations induced by a copropagating pump channel through XPM. We use this expression to calculate the average level of XPM-induced crosstalk together with its variance for several dispersion maps. We show that PMD not only reduces the crosstalk on average, but also impacts the efficiency of a commonly used polarization-interleaving technique.     

Continuous-wave and giant-pulse operations of a single-frequency erbium-doped fiber laser

We present a compact diode-pumped single-frequency single-polarization erbium-doped fiber laser with fiber Bragg gratings (FBGs) written in the active fiber itself. The single-polarization regime was obtained by applying uni-axial stress to one of the two FBGs forming the cavity. The laser is shown to operate in two regimes-quasi-continuous-wave regime with a few-percent of modulation in the output power and a giant-pulse regime accessible under sinusoidal pump modulation.     

利用半导体可饱和吸收镜实现的全光纤被动锁模激光器

设计了包含半导体可饱和吸收镜(SESAM)、单包层高掺Yb增益光纤和光纤布拉格光栅(FBG)的全光纤激光器,实现了皮秒级,中心波长约为1064 nm,3 dB线宽约为0.4 nm,重复频率约为17.3 MHz的稳定的连续(CW)被动锁模脉冲输出。观察并分析了输出激光随抽运功率升高和降低的变化过程,升高过程中连续锁模启动时抽运功率阈值为50 mW,降低过程中能够实现稳定锁模的最小抽运功率为37 mW。随着抽运功率的加大,首先出现调Q现象。然后出现连续锁模,并伴有很小幅度的调Q现象。继续加大功率,脉冲会出现分裂;抽运功率越大,单脉冲分裂成的多脉冲越多,多脉冲调制越强。在较少脉冲演变为较多脉冲的过程中,会出现调制的不稳定性。当抽运功率足够大时.会出现多脉冲个数及峰值的不稳定现象。半导体可饱和吸收镜被动锁模会使输出激光谱线加宽,随着抽运功率的加大和锁模的加强,输出激光谱线逐渐加宽。随着脉冲分裂个数增多,单个脉冲脉宽变窄。在多脉冲调制阶段,外界微扰会对系统产生一定影响。     

基于硅基石墨烯的全光控太赫兹波强度调制系统研究

在太赫兹通信等系统中需要利用太赫兹波调制器对信号进行调制.基于GaAs 等传统半导体材料设计和制作的调制器在太赫兹波段的响应过低,因而很难应用于太赫兹系统.为了弥补传统调制技术在带宽和调制深度不够的缺点,设计了一种全新的基于硅基石墨烯的全光控太赫兹强度调制系统.该调制系统利用材料中光生载流子对太赫兹波的吸收特性,通过调节照射到材料上的可见光光强来改变光生载流子浓度,从而实现对太赫兹波强度调制.从理论和实验两方面对这种新型太赫兹强度调制系统的调制深度和调制带宽进行了研究.研究结果表明,在泵浦光功率密度为18 mW/mm2时,该调制系统能在实验使用的THz-TDS 测试系统(0.1~2.5 THz)的整个频谱范围内进行有效的调制,调制深度可达到12 %.且随着泵浦光能量的增大,调制深度增大.     

All-optical switching in 3,3′-diethyl-2,2′-thiatricarbocyanine Iodide dye molecules

All-optical switching has been theoretically analyzed in the 3,3＇-diethyl-2,2＇-thiatricarboeyanine iodide （DTTCI） carbocyanine dye that exhibits large excited-state absorption to achieve high contrast and fast switching. Switching has been analyzed both ns and ps pump pulse widths. It is shown that there is an optimum value of concentration for given peak pump intensity at which maximum modulation can be achieved. We can get 93.84% modulation of transmission of a CW probe laser beam at 532 nm at peak pumping intensity of 500 kW/cm^2 at 763 nm, with At =1 ns and concentration of 80 μM in alcohol, resulting in switch-off and on time of 2 ns and 8 ns, respectively. The results have been also used to design all-optical NOT and the universal NOR and NAND logic gates with multiple pump laser pulses.     

Broadband Single-Pumped Fiber-Optic Parametric Amplifiers

We analyze the impact of pump phase modulation (PM) on the bandwidth of single-pumped fiber-optic parametric amplifiers. The measured values and the calculations show that the impact on bandwidth by pump PM is small compared to the impact from the dispersion curvature. A bandwidth of 100 nm with a gain of 11.5plusmn2 dB is achieved. This is also used to build a widely tunable fiber ring laser     

Modulation of the characteristics of intense picosecond stimulated emission from GaAs

Intense picosecond stimulated emission from the face of a thin GaAs film is studied. The emission is observed when GaAs is pumped with high-power picosecond optical pulses. It is found that the dependences of the emission energy on the photon energy, the picosecond delay between two pump pulses, and the distance between the active region and the face are modulated. Modulation is taken to mean the appearance of protrusions or peaks in any of the mentioned dependences. The modulation parameters for the dependences under consideration are found to be related by expressions that make it possible to suggest the following. The modulation of characteristics is caused by a common (not yet conclusively identified) mechanism of self-modulation of the emission spectrum. This mechanism is related to an ultrafast nonlinear interaction between a highly photoexcited semiconductor and the pump radiation and stimulated emission. There is indirect evidence that this mechanism also gives rise to an amplitude modulation of the emission in a picosecond time interval.     

Vertical-cavity amplifying modulator at 1.3 μm

The modulation/switching properties of a vertical-cavity semiconductor optical amplifier operating at 1.3 μm wavelength are investigated. The device was optically pumped and operated in reflection mode. A 150-mV (100 mA) modulation of the drive to the pump source produced a 7-dB modulation of the pump power, which produced a 35-dB modulation in the output signal. The maximum extinction ratio was 35 dB, and limited by device heating. Frequency response measurements revealed a modulation bandwidth of 1.8 GHz when the amplifier was saturated. This enabled 2.5-Gb/s modulation of a -10 dBm input signal with 5.5-dB fiber-to-fiber gain