Timing jitter in semiconductor lasers under pseudorandom wordmodulation

Timing jitter of semiconductor lasers under pseudorandom word modulation was studied at 1 Gb/s. For lasers biased above threshold the timing jitter was Gaussian in distribution, with a turn-on timing jitter amplitude τ=2σ=±6 ps. For lasers biased below threshold, the turn-on timing jitter showed two peaks of Gaussian distribution, with an amplitude τ=60±10 ps and separated by about 80 ps. This phenomenon is due to a pattern-dependent effect of the laser turn-on delay. Theoretical calculations show that for data transmission above 2.2 Gb/s, the laser has to be biased above threshold to avoid excessive (>0.5 dB) jitter-induced power penalty     

Compressively strained 1.3 μm InAsP/InP and GaInAsP/InP multiplequantum well lasers for high-speed parallel data transmission systems

Turn-on delay times in the pulse response of compressively strained InAsP/InP double-quantum-well (DOW) lasers and GaInAsP/InP multiple-quantum-well (MQW) lasers emitting at 1.3 μm were investigated. DQW lasers with 200-μm cavity length and high-reflection coating achieved both a very low threshold current (1.8 mA) and a small turn-on delay time (200 ps), even under a biasless 30-mA pulse current. Compressively strained or lattice-matched GaInAsP MQW lasers and GaInAsP double-heterostructure (DH) lasers were also fabricated and compared. It was observed that the carrier lifetime was enhanced for InAsP DQW lasers and strained GaInAsP MQW lasers compared to the lattice-matched GaInAsP MQW lasers and conventional double-heterostructure lasers. To explain this increase in the carrier lifetime, the effect of the carrier transport on the carrier lifetime was studied. The additional power penalty due to the laser turn-on delay was simulated and is discussed     

Turn-on jitter in nearly single-mode injection lasers

Computer modeling confirms recent experimental observations and shows that photon absorption and spontaneous emission cause as much as 50-60 ps jitter in the turn-on time delay of lasers with large mode-suppression ratios. Careful selection of the bias point can reduce this jitter to about 12 ps. Statistics are presented for the expected power level at fixed sampling time during the turn-on, and for the fluctuation in time delay for the laser power to rise to one-half the equilibrium power output.     

Small turn-on delay time in 1.3 μm InAsP/InP strained doublequantum-well lasers with very-low threshold current

It is demonstrated for the first time that compressively strained InAsP/InP double quantum-well (DQW) lasers emitting at 1.3 μm performed a very small turn-on delay time by a significant reduction in threshold current. Lasers with 200 μm cavity length and high reflection coating achieved both very low threshold current of 1.8 mA and a small turn-on delay time (200 ps) even under a bias-less 30 mA pulse current. An additional power penalty was simulated, and it was shown that these small-delay and low-threshold performances are suitable for high-speed optical parallel data transmitters in computer networks     

1.3-μm InAsP modulation-doped MQW lasers

The effect of both n-type and p-type modulation doping on multiple-quantum-well (MQW) laser performances was studied using gas-source molecular beam epitaxy (MBE) with the object of the further improvement of long-wavelength strained MQW lasers. The obtained threshold current density was as low as 250 A/cm² for 1200-μm-long devices in n-type modulation-doped MQW (MD-MQW) lasers. A very low CW threshold current of 0.9 mA was obtained in 1.3-μm InAsP n-type MD-MQW lasers at room temperature, which is the lowest ever reported for long-wavelength lasers using n-type modulation doping, and the lowest value for lasers grown by all kinds of MBE in the long-wavelength region. Both a reduction of the threshold current and the carrier lifetime in n-type MD MQW lasers caused the reduction of the turn-on delay time by about 30%. The 1.3-μm InAsP strained MQW lasers using n-type modulation doping with very low power consumption and small turn-on delay time are very attractive for laser array applications in high-density parallel optical interconnection systems. On the other hand, the differential gain was confirmed to increase by a factor of 1.34 for p-type MD MQW lasers (N_A=5×10¹⁸ cm ^-3) as compared with undoped MQW lasers, and the turn-on delay time was reduced by about 20% as compared with undoped MQW lasers. These results indicate that p-type modulation doping is suitable for high-speed lasers     

Analysis of bit-error rate of vertical-cavity surface-emitting lasers modulated at high speed

A theoretical study of single and multimode vertical-cavity surface-emitting lasers (VCSELs) subject to pseudorandom modulation of the current at a rate of 10 Gb/s is performed. Eye diagrams, probability density functions of the power at the decision time, averaged turn-on delay, and timing jitter are analyzed for different values of the on- and off-state currents. Bit sequences where errors occur are identified. Extensive simulations have been performed to obtain the bit-error rate (BER) for the back-to-back configuration. We find that the BER performance of single-mode VCSELs is better than the one obtained with multimode VCSELs when the off-state current is smaller than the threshold current. The same result is obtained when the off-state current is larger than the threshold value, providing that the on-state current is large enough. However, BER in single-mode VCSELs is greater than in multimode VCSELs when the off-state current is equal to the threshold current. BER performance is also better for multimode VCSELs when the off-state current is larger than the threshold value, if the on-state current is small enough.     

Dependence of timing jitter on bias level for single-modesemiconductor lasers under high speed operations

The dependence on the bias level of some quantities characterizing optical pulse statistics, such as the turn-on time, pulsewidth, maximum output photon number, and average output power, of single-mode semiconductor lasers is numerically analyzed at frequencies in the gigahertz range. Periodic modulation and pseudorandom word modulation are considered. In the former regime, timing jitter is shown to be rather independent of the bias current. In the latter regime, timing jitter becomes larger when biasing above threshold than when biasing below threshold. This larger jitter is found to be associated with a bimodal probability distribution of the turn-on time, which yields undesirable pattern effects. A privileged bias, slightly below threshold, suppresses these pattern effects making the laser response almost independent of previous input bits. For such bias value the probability distribution functions of the turn-on time in the case of the periodic and pseudorandom word modulation coincide     

Doping-type dependence of turn-on delay time in 1.3-/spl mu/m InGaAsP-InP modulation-doped strained quantum-well lasers

The dependence of turn-on delay time on doping type in 1.3-/spl mu/m InGaAsP-InP modulation-doped (MD) strained quantum-well (QW) lasers is theoretically investigated, based on the detailed band structure model including the band mixing effects. It is found that the turn-on delay time in n-type MD lasers is reduced to 1/4 that of undoped lasers due to both a lower threshold current and a reduced carrier lifetime. The reduction of the delay time is smaller in p-type MD lasers, however, because of the increased threshold current. These results show that the n-type MD-QW lasers are superior for high-speed modulation under zero-bias conditions.     

Turn-on jitter of external-cavity semiconductor lasers

Analytical expressions, validated by numerical simulations, are obtained for the turn-on delay jitter of semiconductor lasers subjected to weak optical feedback in short external cavities. The results show explicitly that displacement of the external reflector on optical wavelength scales causes significant changes in the switch-on dynamics of the laser. It is found that more than a 400% increase of jitter can occur under certain circumstances. The demonstrated sensitivity of laser switch-on dynamics to reflector location is considered to be particularly relevant to the performance of packaged laser diodes     

Statistics of transverse mode turn-on dynamics in VCSELs

The turn-on process of a multimode VCSEL is investigated from a statistical point of view. Special attention Is paid to quantities such as time jitter and bit error rate. The single-mode performance of VCSELs during current modulation is compared to that of edge-emitting lasers     

The effect of external optical feedback on the turn-on delaystatistics of laser diodes under pseudorandom modulation

The turn-on delay statistics of laser diodes with optical feedback are investigated by numerical modeling. Under pseudorandom modulation the optical feedback causes an increase in the average turn-on delay and multiplies and jitter considerably. The jitter increase due to optical feedback is largest at highest bias currents. Optical feedback from an external reflectivity of greater than 0.1% can destroy the pattern effects which occur for some operating conditions     

Reduced timing jitter of two-section 1.55-μm laser diodes undergain-/loss-switching regime at multigigahertz rates

Picosecond pulses at multigigahertz frequencies with low timing-jitter have been generated from two-section 1.55-μm semiconductor lasers. A pulse jitter of ~1 ps was measured at a 5-GHz repetition rate with the laser diodes operated in gain-switching regime. More than 2x lower values were obtained in the loss-switching regime. A timing jitter model has been adapted to treat two-section laser diodes in the two regimes. The superiority of the loss-switching regime is explained by faster laser dynamics around threshold. A good agreement is obtained between calculations and timing jitter measurements     

Operation range of VCSEL-interconnect links with“below-threshold”-biasing

Biasing lasers below threshold in interconnect links yields a lower effort for the monitoring circuitry, but it leads to a significant turn-on jitter due to the bit-pattern effects and spontaneous emission. An analytical expression describing the probability density function (pdf) of the total turn-on delay for a single-mode vertical-cavity surface-emitting laser (VCSEL) biased below threshold is derived, which accounts for both bit-pattern effects and spontaneous emission. In a high speed digital transmission system both timing jitter as well as the signal-to-noise ratio (SNR) limit the system-performance, which can be measured by the resulting bit-error rate (BER). The measured BER is compared with the calculated BER yielding good agreement. Therefore, following the quite general guideline as presented here, the operation range for “below-threshold”-biased VCSEL-interconnect links can be determined     

GS-DFB半导体激光器的光自注入技术

报道了一种光脉冲自注入的新方法 ,它能使增益开关 DFB激光器输出光脉冲的时间抖动从 5.7ps减小到 1.2 ps,分析了注入光延迟时间及功率对时间抖动的影响 ,指出为取得抑制时间抖动的最佳效果 ,必须选择合适的反馈光脉冲延迟时间和适当的反馈光功率。实验中观察到在增益开关 DFB激光器光脉冲建立期间注入反馈光时 ,输出光脉冲会发生严重畸变。     

Reduced turn-on delay time in 1.3 μm InGaAsP/lnP n-typemodulation-doped strained multiquantum well lasers

Reductions in carrier lifetime, threshold current, and thus turn-on delay time, due to n-type modulation doping, have been experimentally demonstrated in 1.3 μm InGaAsP strained multiquantum well lasers for the first time     

Low-threshold distributed feedback lasers fabricated on material grown completely by LP-MOCVD

GaInAsP-InP distributed feedback (DFB) lasers emitting at 1.57 μm have been fabricated on material grown completely by low-pressure metalorganic chemical vapor deposition (LP-MOCVD). The CW threshold current of 60 mA and an output power of 6 mW per facet at room temperature have been obtained. The lasing wavelength λLunder CW operation showed a temperature coefficient (d_{lambdaL}/dT) of 0.9 Å/°C for this DFB laser over the range of10-90degC. A stable single longitudinal mode was maintained under high speed pulse modulation up to 500 ps, and sinusoidal modulation at 1 Ghz.     

Dual Semiconductor Laser System With Rapid Time-Delay for Ultrafast Measurements

We present a dual semiconductor laser system at 1.55 mum with femtosecond pulse widths and very low timing jitter for rapid pump-probe measurements. Synchronizing the two lasers to the same low-noise radio frequency-oscillator allows the use of an electrical phase shifter for the relative time delay between the lasers. This leads to a large scanning window that nearly matches the pulse period of 2.5 ns, as well as achieving a discrete time step of below 100 fs. The timing jitter of the complete dual laser system including all electronics is only 540 fs across the whole time delay. The nonlinear pulse compression using especially designed comb-like dispersion profiled fiber leads to autocorrelation widths of 310 fs. The system performance, i.e., the high time resolution is demonstrated by optical cross correlation of the pump and probe pulse, showing a very low full-width at half maximum of 1.3 ps     

Low threshold operation of 1.5-µm DFB laser diodes

Highly reliable distributed feedback (DFB) laser diodes operating at 1.5-μm wavelength range are fabricated through optimizing the device parameters. Thickness control of the active layer is found to be an essential factor in achieving low threshold operation of DFB lasers. The threshold current as low as 11 mA and stable single longitudinal mode CW operation up to 106°C is achieved with these DFB lasers.     

Extremely low threshold current operation in 1.5-μm MQW-DFBlaser diodes with semi-insulating InP current blocking region

Threshold current operation of 1.5 mA was achieved for 1.5-μm multiple-quantum-well distributed feedback (MQW-DFB) laser diodes (LDs) with semi-insulating current blocking layers entirely grown by metalorganic vapor phase epitaxy (MOVPE). Such low-threshold current is attained by reducing leakage current and mirror loss in the laser structure. The required bias current for achieving several gigahertz bandwidth is markedly reduced due to the enhanced differential gain and low threshold current. Due to the reduced lasing delay time in such low threshold LDs, up to 5-GHz zero-bias current modulation, with a clear eye opening, is successfully demonstrated     

Performance characteristics of buried facet quarter-wave shifted distributed feedback lasers

The performance characteristics of quarter-wave shifted GaInAsP distributed feedback lasers emitting near 1.3 mu m are described. The quarter-wave shifted grating is fabricated on a substrate using the double-exposure holographic technique. The low reflectivity required for this quarter-wave shifted DFB laser is obtained using buried facets at both ends of the laser. The lasers have threshold current of 30 mA, quantum efficiency of 0.18 mW/mA/facet, bandwidth of 11.5 GHz at 10 mW and 10 dB chirp width of 2.5 AA under 40 mA modulation current at 5 Gbit/s.<>