Analysis of TE/TM mode selectivity in DFB lasers with a phase-shift region

In phase-shift distributed feedback (DFB) lasers, there is limited suppression of the TM mode despite extremely large submode suppressibility. The TE/TM mode selectivity of a DFB laser structure with a nonuniform waveguide region as the phase shifter is analyzed. Calculations of the threshold gain difference between the TE and TM modes are performed using the effective index method and the coupled-wave theory. It is found that the TM mode suppressibility can be doubled by optimizing the dimensions of the phase-shift region. This structure overcomes the TM mode problem.<>     

TM mode suppression property of DFB lasers with a narrow striperegion

The TM-mode suppressibility of DFB lasers with a narrow stripe region is investigated. A waveguide is designed to produce different phase shifts between the TE and TM modes. A threshold gain difference between the TE and TM modes is introduced by a phase shift difference of both modes. A TM and side mode suppression ratio of more than 40 dB is obtained under 1.8-Gb/s NRZ pseudorandom pulse modulation     

Gain margin analysis of distributed feedback lasers for bothtransverse electric and magnetic modes

A theoretical analysis of the gain margin with both transverse electric (TE) and transverse magnetic (TM) modes in distributed feedback (DFB) lasers of second-order gratings and various structural parameters is presented. Though the dominant mode is usually a TE mode, one of the TM modes often becomes the secondary mode with the second lowest threshold and seriously affects the single-mode characteristics of DFB lasers. To design DFB lasers with a large gain margin, proper amounts of facet reflectivity for both polarizations and control of the spatial phase of the grating at the facet are required     

Polarization optical bistability induced by TM mode injection to aDFB laser with complex coupled coefficient

A novel method is proposed to produce an optical bistability by using a dynamically stable complex coupled DFB (CC-DFB) laser with TM mode injection. In this paper polarization optical bistabilities are analyzed in detail using coupled mode equations and rate equations for the CC-DFB lasers considering the longitudinal hole burning and carrier dependent complex coupling coefficients. Several parameters reflecting the physical features of a complex coupled DFB laser are discussed. It is shown that for a CC-DFB laser the polarization bistability induced by the TM mode injection is much stronger for the antiphase of complex coupling than that for the in-phase. In addition, the influences of initial coupling condition for gain grating structure on the optical bistability are also investigated considering both cases of the antiphase and in-phase     

Measurement of mode partition in DFB lasers

Some DFB (distributed feedback) laser diodes have a satellite mode beside a main DFB mode even if FP modes are suppressed. In this paper, the mode partition noise is presented for several DFB lasers operating in multilongitudinal modes. The results show that under modulation at 140 Mbit/s, the mode partition coefficient k²of multimode DFB lasers is very small and at most 0.02 while that of FP lasers biased at the threshold level is 0.03 to 0.12. The numerical evaluation of the mode partition effect in two-mode DFB lasers suggests that a 20- dB suppression of the satellite mode power is enough to achieve a repeater spacing of over 100 km in the 280 Mbit/s fiber-optic transmission system with less than 0.1-dB power penalty.     

Transmission performance impairment due to degradation of optical spectrum in 1.55 ?m DFB laser

The dynamic optical spectrum and optical transmission characteristics of a 1.55 ?m conventional distributed feedback (DFB) laser with a side mode were analysed. The laser used in the experiment came to operate in two modes although initially it had operated in single-mode. When this laser was directly modulated by setting the bias current around the threshold, the mode switching between a main mode and a side mode was observed to be similar to mode partitioning in Fabry-Perot (FP) lasers. The mode switching affects considerably the bit error rate performance of 113 km single-mode fibre transmission at 140 Mbit/s.     

Distributed feedback lasers in chiral media

The combined effects of chirality and gain (or loss) on wave propagation and coupling in periodic structures is investigated here. The focus is on distributed feedback (DFB) lasers in a transversely unbounded periodic slab with spatially modulated electromagnetic parameters. The analysis uses a coupled-mode approach employing a canonical physical model of chiral materials to predict the effects of modulated chirality admittance on DFB lasers. Results for DFB laser behavior in chiral media are compared and contrasted to that in achiral media. It is found that, under certain circumstances, the electric and magnetic field coupling, which is characteristic of chiral materials, results in a lower threshold gain for DFB lasers in media with a given index of refraction and characteristic impedance. It is also found that chiral index-coupled or gain-coupled DFB lasers exhibit the same spectral mode properties as achiral DFB lasers     

Effect of axial hole-burning on TM mode suppression in λ/4phase-shift DFB lasers

The TM mode suppressibility in λ/4 phase-shift distributed feedback (DFB) lasers is analysed by considering the effect of the axial hole-burning. The calculated result shows that the hole-burning always decreases the threshold gain difference between the TE and TM modes     

Bit-error-rate saturation due to mode-partition noise induced by optical feedback in 1.5-µm single longitudinal-mode C3and DFB semiconductor lasers

We report first measurements of the effect of optical feedback on the bit error rate (BER) as a function of received power of single longitudinal mode cleaved-coupled cavity (C³) and distributed-feedback (DFB) semiconductor lasers. Mode-partition noise, induced by the optical feedback, can cause a bit error-rate floor for large optical feedbacks, but it is shown that high-performance C³lasers and especially DFB lasers can withstand very large optical feedbacks without system degradation.     

Spatial hole burning effects in distributed feedback lasers

The effects of spatial hole burning in a steady-state distributed feedback (DFB) laser are examined by numerically solving the coupled mode equations that describe the system. An approximate solution for the gain above threshold is derived and compared to the exact solution. It is shown that the self-induced grating that arises due to spatial hole burning significantly reduces the mode discrimination of index-coupled DFB lasers. This makes it difficult for these lasers to maintain single-longitudinal-mode behavior above threshold. However, it is found in addition that bulk-modulated (gain-coupled) DFB lasers do not lose their mode selectivity above threshold, indicating that these lasers may be better choices for narrow-linewidth operation     

Modeling of distributed feedback semiconductor lasers withaxially-varying parameters

A numerical model that is capable of predicting important laser characteristics such as the threshold gain and the gain margin between the main and side modes for a distributed-feedback (DFB) semiconductor laser of arbitrary complexity is described. The method consists of solving the coupled-mode equations with axially varying parameters iteratively until the boundary conditions at the two facets are satisfied. The numerical model is applied to two DFB laser structures. In the case of a multiple-phase-shift DFB laser the results show that such devices can have a more uniform axial distribution than that of a conventional quarter-wave-shifted DFB laser while maintaining sufficient gain margin between the main and side modes. In the case of a dual-pitch DFB laser it is shown that the incorporation of a slightly different grating period (~0.1%) over a small section can provide a gain margin that is comparable to that achieved in quarter-wave-shifted DFB lasers     

Design and performance of externally modulated 1.5-μm lasertransmitter in the presence of chromatic dispersion

Key laser and modulator characteristics that impact the use of externally modulated lasers in the presence of chromatic dispersion, excluding effects due to fiber nonlinearities, are reviewed. After a brief consideration of transmission performance with directly modulated 1.5-μm DFB lasers which have limited application of up to 80 km at 2.5 Gb/s, the key design characteristics of externally modulated transmitters are discussed. Experimental results showing the effects of modulator chirp and laser linewidth at a 2.5-Gb/s transmission rate are presented. It is found that lasers with CW linewidth under 100 MHz have less than 2-dB dispersion penalty for 600 km of non-dispersion-shifted fiber. Lower dispersion penalties can be realized if the modulator chirp is tuned so as to narrow the transmitted pulses. Excellent modulator stability is demonstrated for 60 days of error- and degradation-free 2.5-Gb/s operation     

Effects of radiation loss on the performance of second-order DFBsemiconductor lasers

The effects of radiation loss on the performance of second-order distributed-feedback (DFB) semiconductor lasers with a symmetrical grating are analyzed systematically. The threshold gains of the two lowest modes, the differential quantum efficiency, and the spectral linewidth are calculated as a function of the complex coupling coefficient. The comparison is also made between a conventional DFB and a quarter-wave phase-shifted DFB laser. It is shown that the effects of radiation loss are different in these two laser types. In addition, a simple approximate expression for threshold gain is derived. By way of example, a DFB laser with a rectangular grating is analyzed. It is shown that the performance of a second-order DFB laser is not sensitive to the grating pitch over a fairly wide range, provided that it is close to the pitch which gives the maximum coupling coefficient     

Full-duplex radio-on-fiber transport systems based on main and multiple side modes injection-locked DFB laser diode

A full duplex radio-on-fiber (ROF) transport system based on main and multiple side modes injection-locked distributed feedback laser diode (DFB LD) is proposed and experimentally investigated. DFB LD is successfully injection-locked not only in the main mode but also in other side modes. A data signal of 70 Mbps/10 GHz (WiMAX) signal is transmitted over long-haul single-mode fiber (SMF) transmission. Good bit error rate (BER) performances were achieved for both down-link and up-link transmissions in our proposed full-duplex ROF transport systems.     

Analysis of the spectral linewidth of distributed feedback laser diodes

The spectral linewidth of distributed feedback (DFB) laser diodes is theoretically studied. Numerical calculation shows that DFB lasers with long cavity lengths and large coupling coefficients have very narrow spectral linewidth less than 1 MHz, The effects of the phase shift and mirror facets on the spectral characteristics of DFB lasers are also analyzed, It is shown that the phase-shifter further narrows the spectral linewidth of DFB lasers. Its numerical result and physical meaning are also shown.     

分布反馈光纤激光器模式特性分析

根据耦合波理论,在分析分布反馈(DFB)光纤激光器纵模特性的基础上,着重阐述其偏振特性。另外给出了偏振态同耦合系数和双偏振态相移量差的关系。理论分析结果表明,当分布反馈光纤激光器输出为0阶模时,输出激光的偏振状态由耦合系数和双偏振态的相移量差共同决定,即在耦合系数一定的情况下,通过增加双偏振态相移量的差,或在双偏振态相移量差一定的情况下,通过减小耦合系数,可以实现单偏振输出。实验中在经载氢处理的掺铒光纤上制作分布反馈光纤激光器,由于耦合系数较大和双折射效应过小,输出为双偏振态。     

Measurements and simulation of multipath interference for 1.7-Gb/slightwave transmission systems using single- and multifrequency lasers

Power penalties due to multipath interference (MPI) have been measured for 1.7-Gb/s lightwave systems that use single-frequency (SF) or multifrequency (MF) lasers. Systems that use SF lasers potentially exhibit worse degradation than those using MF lasers. Bit-error-rate (BER) floors occur only under the worse-case conditions of poor receiver margin and large multiple reflections. The use of optical isolation to reduce laser feedback is ineffective in reducing multipath interference, and in many cases may worsen the penalty. It is shown that for a typical transmission system, these degradations are reduced if optical interconnection reflections are maintained below -20.5 dB. The experimental study is in good agreement with theoretical predictions using an analytic expression of the MPI noise power spectral density and with computer simulations using multimode laser rate equations     

Analysis of TE and TM modes in DFB lasers by two-dimensional theory

The two-dimensional theory of a distributed feedback (DFB) laser (which was previously presented and applied to the analysis of the laser threshold conditions for the transverse-electric (TE) mode in a simple three-layer waveguide structure) is developed to treat both TE and transverse-magnetic (TM) modes in a four-layer waveguide structure with a thin grating layer, which more closely reflects actual DFB laser structure. The differences between TE and TM modes for the dispersion relations and the laser threshold conditions are clarified. The effects of the waveguide structure (including grating layer thickness, refractive indexes of layers, coupling constant, and corrugation period) on the threshold gains and the gain differences between the two longitudinal modes on both sides of the Bragg frequencies are studied in detail for both TE and TM modes     

Modeling the static and dynamic behavior of quarter-wave-shiftedDFB lasers

High-coupling (grating coupling constant=3.0) phase-shifted distributed-feedback (DFB) lasers are studied using a transmission-line laser model (TTLM) which includes spatial hole burning (SHB), the material gain spectrum, refractive index dependence on carrier concentration, and random spontaneous emission. Good agreement for CW spectra is shown with other models and experimental results. Dynamic simulation of laser transients shows SHB-induced deterministic mode hopping and chirping at moderate output powers. The effects of mode hopping and chirping on system performance are studied using a laser model combined with a fiber model     

Longitudinal spatial instability in symmetric semiconductor lasersdue to spatial hole burning

A novel type of longitudinal instability due to spatial hole burning in symmetric semiconductor laser structures (DFB lasers in particular) is examined analytically and numerically. It is shown that, at a certain output power, the gain and refractive index spatial distributions of the lasing mode become unstable. Above this output power, the modal gains and oscillation frequencies change drastically, which often causes multimode operation. A measure of the cavity stability is introduced and derived analytically for a Fabry-Perot and a single phase-shifted DFB laser. Results from numerical simulations of a multiple phase-shifted DFB laser are presented