Response of a BaTiO3 phase conjugate mirror to broadband and narrowband radiation

The bandwidth of a BaTiO₃ self-pumped phase conjugate mirror is measured in three different ways. 1) The crystal is exposed to infrared light with a phase change that is sinusoidal in time and the degree of phase modulation on the reflected wave is measured. As the modulation frequency is increased from 0.1 to 3 GHz, the modulation transfer function of the phase conjugate mirror decreases by a factor of 2. 2) The crystal is exposed to visible light from a laser operated both in a single longitudinal mode and in multiple longitudinal modes. When the laser bandwidth is increased from 20 MHz to 2 GHz the reflectivity of the phase conjugate mirror increases by up to a factor of 7. 3) A laser cavity is formed with the crystal as one end mirror and the lasing bandwidth is measured. Depending on the exact conditions, lasing bandwidths range from 2 to 240 GHz.     

Self-induced high-speed modulation in microchip solid-state lasers with asymmetric end pumping

We applied laser-diode sheetlike end pumping to a multimode Nd:YVO(4) laser and observed high-speed (>400-MHz) modulation of the intensity of chaotic pulsation near 1 MHz. The frequencies of modulation were the beat frequencies for pairs of closely spaced lasing modes. Asymmetric optical confinement and the resultant modal interference are shown to lead to oval-hollow-mode operation in which modal beat notes induce high-speed modulation, the frequency range of which is 2 orders of magnitude higher than the intrinsic relaxation oscillation frequency. Good numerical reproduction of the observed chaotic pulsations and their high-speed modulation was obtained with model equations in which such effects as nonlinear gain coupling among modes and field interference between pairs of modes were included. High-speed pulsations in nonchaotic lasers were also demonstrated.     

Generation of Transform-Limited Pulse Train in New Scheme of Harmonically Mode-Locked Er-Doped Fiber Ring Lasers

A new scheme of an amplitude modulated harmonically mode-locked Er-doped fiber ring laser is proposed, and the lasing characteristics are experimentally investigated. The transform-limited sech² pulses at the repetition frequency of 1.64 GHz are stably generated. The pulse width and spectral bandwidth are independent of both the amplitude modulated (AM) modulation signal frequency and the AM modulation depth.     

Dynamic analysis of directly modulated Fabry–Perot laser diodes using time-domain modeling with random noise fluctuations

Based on time-domain laser modeling, we have systematically evaluated multi-mode excitation and mode competition in directly modulated Fabry–Perot laser diodes biased with different current levels. In our simulation, random noise fluctuations depending on the bias current are included since they play an important role in determining the number of lasing modes. Multi-mode excitation and mode competition occurred at the biased current below the threshold current can be explained with the aid of frequency chirp at turn-on and gain difference in between '0' and '1' bits. The positive frequency chirp (25 GHz for lasers biased below the threshold current and 15 GHz for lasers biased above the threshold current) affects to enhance the probability of multi-mode excitation in low-frequency region during turn-on delay.     

Optimization and Characterization of InGaAsN/GaAs Quantum-well Ridge Laser Diodes for High Frequency Operation

Optimization and characterization of multiple InGaAsN/GaAs quantum-well laser diodes for high frequency operation are reported. From the modelling of the dilute nitride quantum well, we investigate how to design the structure to achieve a high frequency operation. The gain characteristics are optimized by incorporating the minimum amount of nitrogen in the well to obtain the emission at 1.3 μm with a low transparency density and a high differential gain. We show that the number of wells must be adjusted to three to benefit of the best compromise between the threshold current and the differential gain. The effects of the cavity losses on the dynamic characteristics are evaluated and demonstrate the interest for high cavity losses to reach high relaxation frequency despite a lower characteristic temperature. An optimized structure has been realized and exhibits an emission at 1.34 μm with a transparency current density of 642 A/cm² and a characteristic temperature T₀ ~ 80 K. Dynamic properties for ridge devices are evaluated from relative intensity noise measurements and small-signal modulation. A relaxation frequency as high as 7.4 GHz and a 9.7 GHz small-signal bandwidth are reported. We demonstrate transmission up to 10 Gb/s at 25°C without penalty and bit error floor.     

Amplitude-modulation-free optoelectronic frequency control of laser diodes

A novel method is described for fast frequency modulation or frequency control of diode lasers that avoids problems associated with bias current modulation, namely, amplitude modulation and thermal phase delays. The method is based on amplitude-modulated, noninterfering control light with a wavelength near the transparency region of the laser diode, which specifically modifies the spectral gain profile to yield a constant gain but a controllable refractive index at the lasing wavelength. This permits amplitude-modulation-free frequency modulation at modulation frequencies up to the relaxation oscillation frequency. A phase lock between the emissions of two extended-cavity diode lasers that could not be achieved with bias current modulation was achieved by this method.     

Direct intensity modulation of a rectangular ring laser with bidirectional lasing characteristics

The direct intensity modulation of a three-guide coupled rectangular ring laser having bidirectional lasing characteristics is reported for the first time. The rectangular laser cavity consists of four low loss total internal reflection mirrors and an output coupler made out of three passive coupled waveguides. The laser is fabricated using a total cavity length of 580 μm with active section lengths of 250 μm. For both the clockwise and counterclockwise circulating directions, a lasing threshold of around 38 mA is obtained at room temperature under continuous wave operation. A maximum 3-dB modulation bandwidth of approximately 3.2 GHz is observed in both circulating directions.     

Multiline coherent oscillation in photorefractive crystals with two species of movable carriers

The coherent oscillation, because of nearly degenerate four-wave mixing in photorefractive crystals with two types of movable charge carriers, occurs at two spectral lines symmetrically shifted with respect to the pump frequency. Consequently the output oscillation exhibits the high contrast intensity modulation. The frequency separation of two oscillation modes (and modulation frequency of the output intensity) depend on the incident light intensity and spatial frequency of the developing grating. A model is presented explaining this type of oscillation by the two-maxima shape of the gain spectrum in crystals with sufficiently different relaxation times of two space-charge gratings, one formed by movable electrons and the other one by movable holes. The experimental data for coherent oscillator with tin hypothiodiphosphate (Sn₂P₂S₆) are in reasonable quantitative agreement with the calculations. Received: 12 November 1998 / Revised version: 11 January 1999 / Published online: 7 April 1999     

Modulation frequency dependence of actively mode-locked oscillation of A He-Ne 3.39 μm laser

The dependence of mode-locked oscillation of homogeneous lasers on the modulation frequency of locking signal is studied. An experiment using a He-Ne 3.39 μm laser and applying intracavity phase (FM) modulation was performed on the frequency range of 40 MHz to 1.20 GHz. The oscillation bandwidth was observed to be nearly proportion to $\text{√}\text{?}{}_{\mathrm{m}}$ and ?_m (?_m: modulation frequency) for the modulation frequency range of ?_m<Δ? and ?_m>Δ? (Δ?: linewidth), respectively. The possibility to generate ultra-short pulses by using larger modulation frequency than the linewidth is also discussed. For 1.20 GHz modulation frequency, the pulsewidth was estimated to be about 70 ps from the observed power spectrum, while the reciprocal of the linewidth was about 3 ns.     

Enhancement of chaotic carrier bandwidth in laser diode transmitter utilizing external light injection

Semiconductor laser with optical feedback emitting chaotic optical signal can be treated as chaotic carrier transmitter. Based on laser rate equations, we numerically study the effect of external injection light on the bandwidth of chaotic carrier transmitter. Our numerical simulation shows that the bandwidth of the chaotic carrier transmitter can be enhanced significantly by external photons injection. Compared with the 2 GHz relaxation oscillation frequency of a solitary laser diode without optical injection, the bandwidth of a chaotic carrier transmitter is expanded to 14.5 GHz with injection parameter at k_inj = 0.39. Simulation results also demonstrate that the enhanced bandwidth depends obviously on the frequency detuning between the external injection laser diode and the chaotic carrier transmitter. The maximum bandwidth of the chaotic transmitter can be obtained when the frequency of the injected light is higher than the central frequency of the carrier transmitter between 2 GHz and 4 GHz.     

Optical chaos and hysteresis in a laser-diode pumped Nd doped fibre laser

Modulated pumping of a Nd³⁺ doped silica monomode fibre laser has been investigated experimentally. For small modulation depths the laser exhibits a resonance at the relaxation oscillation frequency, providing pulses of a few microseconds duration at a repetition rate in the region of 10 kHz, dependent on the average pump level above threshold. As the pump modulation depth is increased, hysteresis is observed as a function of modulation frequency, along with optical bistability. For further increase in modulation depth, optical chaos and period doubling are observed. Gain-switching has also been demonstrated.     

Mode and modulation characteristics for microsquare lasers with a vertex output waveguide

The mode and high-speed modulation characteristics are investigated for a microsquare laser with a side length of 16 ?m and a 2-?m-wide output waveguide connected to one vertex. The longitudinal and transverse mode characteristics are analyzed by numerical simulation and light ray model, and compared with the lasing spectra for the microsquare laser. Up to the fifth transverse mode is observed clearly from the lasing spectra. Single mode operation with the side mode suppression ratio of 41 d B is realized at the injection current of 24 m A, and the maximum output power of 0.53(0.18) m W coupled into the multiple(single) mode fiber is obtained at the current of 35 m A, for the microsquare laser at the temperature of 288 K. Furthermore, a flat small-signal modulation response is reached with the 3-d B bandwidth of 16.2 GHz and the resonant peak of 3.6 d B at the bias current of 34 m A. The K-factor of 0.22 ns is obtained by fitting the damping factor versus the resonant frequency, which implies a maximum intrinsic 3-d B bandwidth of 40 GHz.     

大幅度增加弛豫振荡频率来实现毫米级外腔半导体激光器的外腔机制转换

混沌外腔半导体激光器输出明显存在弛豫振荡特征,弛豫振荡频率小于外腔振荡频率时,外腔半导体激光器输出态是短腔机制;反之,外腔半导体激光器输出态是长腔机制.首先对比分析了弛豫振荡频率为5.6 GHz,腔长对频谱有效带宽的影响.然后同时调节注入电流和载流子寿命来大幅度地增加弛豫振荡频率.最后在弛豫振荡频率为40 GHz、腔长为毫米级(4—20 mm)时,实现由短腔机制到长腔机制的转换,进而分析了外腔反馈率和外腔长对外腔半导体激光器频谱带宽的影响.分析结果表明:短腔机制下,输出混沌态不稳定,0.1 mm的偏差就会导致混沌态与非混沌态之间的转化;长腔机制下,输出混沌态稳定,输出混沌区域较大,证明长腔机制下更有益于获得宽带连续的混沌区域.在弛豫振荡频率为40 GHz、外腔长度为毫米级时,实现了外腔半导体激光器的长腔机制,从而增大了高带宽混沌的参数空间.     

Global rate equation description of a laser

The global integro-differential rate equations describing a multimode laser are analyzed. Expressions for the relaxation oscillation frequencies and their damping rates in the single-mode and two-mode regimes are obtained without specifying either the cavity geometry or the longitudinal pump profile. On the same level of generality, we prove the existence of universal relations relating the peaks of the power spectra in the two-mode regime. For a Fabry-Perot with arbitrary longitudinal pump profile, series expansions of all the physical functions are derived in powers of the pump moments. These moments are averages of the pump profile over cavity modes at linear combinations of the lasing frequencies and their harmonics. These results apply to end-pumped and/or partially filled lasers. For a single mode Fabry-Perot laser, we prove that the contribution to the steady state intensity from the lasing mode varies from 75% close to the lasing threshold to zero at high intensity. The remainder comes from the harmonics of the lasing mode. Analyzing the steady state single mode intensity equation in terms of the pump gratings, we prove that close to the lasing threshold only the space average of the pump and its grating oscillating at twice the lasing wave number do not vanish. This provides a hint towards the justification of the usual modal rate equations which retain only these two functions in the dynamical evolution of a laser. For a Fabry-Perot with constant pump profile, an exact expression for the upper boundary of the stable single mode regime is derived. In that two-mode regime, we prove that there is a critical value of the pump at which the ratio of the two relaxation oscillation frequencies is 2, leading to an internal resonance.     

Lasing from single, stationary, dye-doped glycerol/water microdroplets located on a superhydrophobic surface

We report laser emission from single, stationary, Rhodamine B-doped glycerol/water microdroplets located on a superhydrophobic surface. In the experiments, a pulsed, frequency-doubled Nd:YAG laser operating at 532 nm was used as the excitation source. The microdroplets ranged in diameter from a few to 20 μm. Lasing was achieved in the red-shifted portion of the dye emission spectrum with threshold fluences as low as 750 J/cm². Photobleaching was observed when the microdroplets were pumped above threshold. In certain cases, multimode lasing was also observed and attributed to the simultaneous lasing of two modes belonging to different sets of whispering gallery modes.     

Relaxation oscillations and damping factors of 1.3 μm In(Ga)As/GaAs quantum-dot lasers

In(Ga)As/GaAs quantum-dot (QD) lasers with emission wavelength at 1295 nm at room temperature are fabricated. The laser active region contains a threefold stack of QD layers with surface dot density of 4.56 × 10¹⁰ cm^–2. The laser structure is aluminum-free with InGaP as cladding layers. Threshold current density of a narrow stripe laser of 8 m wide and 3.5 mm long is 152.5 A/cm². The highest relaxation oscillation frequency measured at room temperature is 1.8 GHz, corresponding to a modulation bandwidth of 2.8 GHz due to the small damping factor. From the above measurement, the differential gain and gain compression factor were extracted to be 4.3 × 10^–16 cm² and 3.4 × 10 ^–17 cm ³, respectively. Using these parameters, the maximum modulation bandwidth f _{3 dB max} is estimated as 7.9 GHz.     

Enhancing the relaxation oscillation frequency of a chaotic semiconductor laser transmitter using optical dual-feedback light

A method to enhance the relaxation oscillation frequency and carrier bandwidth of a delayed feedback semiconductor laser transmitter is presented using additive optical feedback light. A formula for frequency detuning of the optical dual-feedback is developed to demonstrate that frequency detuning is added with the additive optical feedback light level. The function of the relaxation oscillation frequency of the chaotic laser is theoretically determined to show that it can be broadened by including the delay time and feedback level. Numerical results demonstrate that the bandwidth with the additive optical feedback light can be enhanced 1.7 times more than the bandwidth without it and the relaxation oscillation frequency of the chaotic laser is increased to 1.8 times more than that of the laser without it. The current can enhance the relaxation oscillation frequency and bandwidth of the transmitter more efficiently.     

Harmonic and intermodulation distortions and noise associated with two-tone modulation of high-speed semiconductor lasers

We present results of modeling and simulation of the harmonic and intermodulation distortions as well as the intensity noise of high-speed semiconductor lasers under two-tone modulation. Multiple quantum-well lasers are considered, which are characterized by large differential gain and a modulation bandwidth of about 25GHz. The study is based on the rate equation model of semiconductor lasers excited by injection current with two sinusoidal tones separated by a radio frequency. The modulated laser signal is modeled in both the time and frequency domains. The time domain characteristics include the fluctuating waveform, while the frequency domain characteristics include the frequency spectrum of the relative intensity noise (RIN), carrier-to-noise ratio, modulation response, harmonic distortion, and the second- and third-order intermodulation distortions (IMD2 and IMD3). The analysis is performed for three frequencies of 5, 15, and 24 GHz, which are, respectively, lower, comparable, and higher than the laser relaxation frequency. The range of the modulation depth covers the regimes of small and large-signal modulation. We show that both RIN and IMD3 of two-modulated laser are minimum when the modulation frequency is 5GHz, and maximum when the modulation frequency is 24 GHz. The second-order harmonic distortion, IMD2, and IMD3 values are larger in the vicinity of relaxation oscillations and increase with the modulation index, especially under large-signal modulation.     

半导体激光器的超高频调制

本文从理论上分析了半导体激光器的超高频调制特性和脉动电流引起的张弛振荡现象。分析了国产DHGaAlAs半导体激光器调制过程中出现的一些问题。实验证明,采用微带匹配技术,可使这类激光器的调制频率达1.3GHz。