On the injection laser contribution to mode partition noise infiber telecommunication systems

The injection laser can initially turn on in a side mode rather than in the main mode when the risetime of the driving current is short compared to the electron lifetime, causing mode partition noise in digital transmission systems which have wavelength dispersion between the laser and the receiver. It is shown that increased differential loss between the main mode and side modes reduces both the incidence and duration of side-mode turn-ons and is the most desirable approach to reducing mode partition noise. The needed differential loss is a strong inverse function of the signal pulse width. Gain compression has a lesser influence on the rate of occurrence of side-mode turn-ons. Quantitative results from a computer model indicate that significant numbers of such turn-ons occur even when the side-mode power during quiescent bias conditions never exceeds the main-mode power. Biasing slightly above threshold will always increase the pulse rate at which a given laser can be operated and provides an attractive goal for designers of laser transmitter packages     

The influence of power level on injection laser linewidth andintensity fluctuations including side-mode contributions

The contribution of side modes to the main-mode line-width is calculated as function of power output using analytic relations and verified using a series of Monte Carlo calculations. For well-behaved lasers having photon lifetimes of 0.88 and 1.57 ps and gain compression of 10% or less, it is found that the total linewidth decreases monotonically with increasing power output. These lasers have main-mode to side-mode ratios in the range 100-300 at 3 mW output; they exhibit side-mode contributions to main-mode linewidth of 17 and 57% respectively at a power level of 10 mW and modal ratios of 940 and 390. When the gain compression exceeds 10% as it may in lasers running at 10 mW or more, the linewidth can reach a minimum and then increase with increasing power level, in accordance with laboratory observation on some lasers     

On single-mode injection laser structures for reduced output fluctuations

This paper explores the possibilities of two principles for reducing the fluctuations in power output of the main mode of an injection laser: 1) including within the laser cavity a section of larger bandgap than the energy of the photons generated; and 2) use of very small wavelength-selective loss differences between the main mode and the side modes. Transient response and steady-state fluctuations are calculated from a rate-equation model. New approximate analytical expressions are given to make possible the calculation of 1) the critical gain-loss term and 2) the consequent laser characteristics. It is shown that a very small side-mode loss difference increases the coupling between the carrier density and the main-mode photons. This produces very fast transient response. It is also shown that the larger bandgap region smoothens the output power and enhances the main-mode power at constant driving current. These results give promise of a low-power-level laser capable of multigigabit direct modulation     

The semiconductor laser linewidth due to the presence of side modes

The laser linewidth is evaluated by solving the rate equations for a nearly single-mode laser with two modes. The resulting linewidth contribution due to the presence of side modes is introduced by the nonlinear gain in the laser diode. For weak side modes, the linewidth contribution is proportional to the third power of the side mode intensity. A linewidth contribution of about 20 MHz for a side-mode power of 100 μW has been found experimentally for a 1.3-μm buried-heterostructure laser     

Theory and experiments of a mode-beating noise-suppressed and mutually injection-locked Fabry-Perot laser diode and erbium-doped fiber amplifier link

By using Fabry-Perot laser diode (FPLD) as a resonant ultranarrow bandpass filter in an Erbium-doped fiber amplifier or laser (EDFA or EDFL), the theory and experiment for side-mode suppression and linewidth reduction of mutually injection-locked EDFL-FPLD and EDFA-FPLD links are demonstrated. Based on the amplified feedback injection loop, the 3-dB linewidth of 3.4 MHz for the EDFA-FPLD link is determined by using self-heterodyne interferometric spectral analysis. The EDFA-FPLD link exhibits a nearly mode-beating noise-free performance as compared to the EDFL-FPLD link. This is due to the release of the resonant cavity configuration in the EDFL-FPLD link at a cost of slightly lower side-mode suppression ratio (/spl sim/42 dB). The maximum output power of the EFDA-FPLD link is 20 mW under an FPLD input power of 0.1 mW.     

Linewidth reduction of cleaved-coupled-cavity lasers by optical feedback from a single-mode polarisation-preserving fibre external cavity

Linewidth characteristics of a cleaved-coupled-cavity laser with a single-mode polarisation-preserving fibre external cavity have been investigated as a function of feedback power level and side-mode suppression ratio. The optimum power feedback ratio is near 2×10?4 for a minimum linewidth of about 30 kHz. A weak dependence of linewidth on side-mode suppression ratio is observed as long as the suppression ratio is greater than 500:1.     

A widely tunable fiber ring laser with closed loop control based on high-precision stepper motor

A tunable single-longitudinal mode erbium-doped fiber ring laser based on stepper motor and closed loop control is proposed and demonstrated. The system consists of an erbium-doped fiber (EDF), a tunable fiber Bragg grating (FBG) filter and a wavelength detector. The characteristics of output laser, such as output power, power stability and 3-dB linewidth, are investigated in the operation range of 1 531—1 569 nm. The repeated experimental results of the fiber laser show that the 3-dB linewidth is less than 17 ps, the side-mode suppression ratio (SMSR) is up to 60 dB, the output power is up to 1.37 dBm, and the power variation is less than 0.61 dB.     

On the probabilistic characterization of side mode fluctuations inpulse-modulated nearly-single-mode semiconductor lasers

Occasionally, a weak side mode in a pulse-modulated nearly-single-mode semiconductor laser can process a significant portion of the total laser output power during the leading edge of a pulse. When combined with the group velocity dispersion of single-mode optical fibers, this mode partitioning can influence the bit error ratio performance of high-speed lightwave systems. Simulation-based solutions of stochastic rate equations, which describe the modulation dynamics of the laser, are used to characterize the side-mode fluctuations probabilistically     

Side-mode gain in grating-tuned extended-cavity semiconductorlasers: investigation of stable single-mode operation conditions

In semiconductor lasers, nonlinear phenomena inside the active medium change the side-mode gain with respect to the static threshold-gain. For a given side mode, the sign and the magnitude of the change depend on the lasing-mode optical power and on the wavelength detuning from the lasing mode. If the side-mode gain is enhanced, mode-hopping can occur toward a side mode whose loss is higher than the lasing-mode one. Conversely, in the case of side-mode-gain suppression, the side-mode loss can be smaller with no mode-hop. In this paper, effects of carrier-density pulsation, carrier heating, and spectral-hole burning on the conditions of stable single-mode operation in grating-tuned single-mode extended-cavity semiconductor lasers are investigated. Taking into account the external cavity spectral selectivity and mode coupling, we present a theoretical analysis of experimental results. We perform calculations that compare well with the experimental data obtained with a 1.55-μm extended-cavity laser     

Amplification properties of a tunable single transverse mode laser diode

The amplification properties of a tunable single transverse mode laser diode can be exploited to be used as ann times 1wavelength division switch. This kind of switch is expected to play an important role in telecommunications systems, especially when signals are not synchronous or when signal bit rates are various. The light amplification ratio between a desired main-mode wavelength and undesired submode wavelengths are improved by decreasing injection light power, and a ratio of 8.5 dB is achieved. This experimental result is in good agreement with theory. It is predicted theoretically using rate equations that the ratio is also improved by increasing the laser cavity mirror loss difference between the main mode and the submodes. A light amplification ratio of more than 11 dB is obtained when the cavity mirror loss difference is more than 50 cm^-1.     

Power dropout statistics of nearly-single-longitudinal-mode semiconductor lasers

An experimental study of power dropouts in the main mode of a nearly-single-longitudinal-mode GaAlAs semiconductor laser is presented. Knowledge of the statistics of main-mode power dropout interarrival times, depths, and widths is necessary to estimate the performance of a communication system using such laser diodes. We found that at a typical laser operating bias (I/I_{th} = 2.1), the mean dropout width was 1.8 ns, dropout depths were exponentially distributed, and dropouts falling below 90 percent of the average main-mode power occurred at a rate of1.5 times 10^{3}s^-1. Also, the rate of dropouts falling below 90 percent of the average main-mode power was found to increase substantially near mode hops, sometimes by several orders of magnitude.     

Effect of cross saturation on frequency fluctuations in a nearly single-mode semiconductor laser

The effect of side-mode cross-saturation on the frequency noise of the main mode of a nearly-single-mode laser is investigated by considering the Langevin rate equations which contain the nonlinear gain with both the self-saturation and cross-saturation terms. When cross-saturation is stronger than self-saturation, the frequency noise of the main mode is found to be significantly enhanced in the low-frequency regime (<1 GHz). An increase of more than 20 dB is predicted due to a side mode suppressed by 15-20 dB. The enhanced frequency noise results in broadening of the laser linewidth and may affect the performance of coherent optical communication systems.<>     

1.5 Gbit/s FSK transmission system using two-electrode Fabry-Perot buried heterostructure laser

The performance of a tunable two-electrode FP laser has been studied experimentally. An FSK transmission system up to 1.5 Gbit/s has been demonstrated using this laser. This new laser showed single longitudinal mode (20 dB side-mode suppression) lasing at different wavelengths through a range of 6.5 nm and minimum spectral linewidth of 30 MHz.<>     

CLADISS-a longitudinal multimode model for the analysis of thestatic, dynamic, and stochastic behavior of diode lasers withdistributed feedback

A computer model called CLADISS is presented for the analysis of multisection diode lasers. The model allows for the analysis of a wide variety of multisection devices with discrete or distributed internal reflections. The simulator can carry out a threshold, DC, AC, and a noise analysis. The threshold analysis determines the threshold of the various longitudinal modes of the laser. The power versus current and the wavelength versus current characteristics are found with the self-consistent DC analysis. CLADISS includes all of the longitudinal variations by dividing each laser section in many short segments. Both the optical field and carrier density are discretized according to this segmentation. To demonstrate the capabilities of CLADISS some nonlinear effects in DFB lasers are treated. Instabilities induced in the side-mode suppression ratio by spatial hole burning are considered. The effects of spatial hole burning and side modes on the FM response on the linewidth are discussed     

Relative intensity noise measurements of a widely tunablesampled-grating DBR laser

The intensity noise of a sampled-grating distributed Bragg reflector laser with 50-nm tuning range and 45-dB side-mode suppression ratio has been measured. The resonance frequency, damping factor, and modified Schawlow-Townes linewidth are extracted from the noise spectra. At high output power, the relative intensity noise (RIN) of the laser is below the photodiode shot noise limit, which is -160 dB/Hz. The laser has uniform shot noise limited RIN properties along the whole tuning range. The maximum resonance frequency is 5.4 GHz at a bias current of 120 mA and the K factor is 0.58 ns     

High performance single frequency fiber grating-basederbium/ytterbium-codoped fiber lasers

The device characteristics of Er³⁺,Yb³⁺ single frequency fiber lasers are reported. A 5-cm long 1550-nm distributed feedback fiber laser with 4 mW output power is shown to have excellent specifications in terms of optical linewidth, signal-to-noise ratio (SNR), relative intensity noise, side-mode suppression and polarization purity. For higher power applications, a 1.5 cm single frequency Er³⁺,Yb³⁺ grating-based fiber laser with 60 mW output power and a net efficiency of 12% is demonstrated     

Linewidth of 1.3?m cleaved-coupled-cavity lasers

The linewidth and side-mode suppression ratio of three variable-gap C3 lasers were measured as a function of output power and drive currents. With feedback carefully eliminated, the narrowest linewidth obtained was 8 MHz at an output power of 13.6 mW. There was little correlation between line-width and suppression ratio.     

InP-based passive ring-resonator-coupled lasers

The design of passive ring-coupled lasers based on InGaAsP waveguides is investigated using a beam propagation method. Mode coupling, propagation loss due to bending, and scattering loss from waveguide sidewall roughness are taken into account. By compromising threshold gain, linewidth and side-mode suppression ratio (SMSR), suitable waveguide width and coupling strength are determined for different ring sizes. Using a ring with radius ranging from 20 to 200 /spl mu/m, it is possible to design passive ring-coupled lasers with threshold gain less than 60/cm and 80/cm for waveguide sidewall roughness 5 and 10 nm, respectively, SMSR larger than 50 dB, and linewidth in the range of /spl sim/3-500 kHz.     

Integrated low-noise lasers

Using new small-signal analytic relations, the linewidth, amplitude fluctuations, main-mode transient response and photon-electron ringing frequency are presented for a laser with fixed active length and variable-length passive region joined with negligible reflection. With 150 ?m active length and 1000 ?m total length, a linewidth below 1 MHz is achievable at 5 mW output.     

平坦功率输出的宽带波长可调光纤环型激光器

为了获得一种平坦功率输出的宽带波长可调的掺铒光纤环型激光器,提出采用一高双折射光纤环镜获得其平坦功率输出,在高双折射光纤环镜中,采用了大量的高双折射光纤段和偏振控制器,它们的反射光谱可补偿掺铒光纤环型激光器输出功率光谱的不平坦,通过压缩或者延伸激光腔里的分布式光纤布喇格光栅即可实现波长调谐。实验验证可以获得一种宽达38nm的宽带波长调谐(1527nm～1565nm),输出功率的不平坦被控制在±0．8dB范围之内,总输出功率大约4dBm、3dB线宽为0．01nm、旁瓣抑制比为48dB的光纤环型激光器。