Large Differential Gain Research Articles

Spatiotemporal mode-locked fiber laser based on dual-resonance coupling long-period fiber grating.

Spatiotemporal mode-locked (STML) fiber lasers have become an excellent platform in nonlinear optics research due to the rich nonlinear evolution process. In order to overcome modal walk-off and realize phase locking of different transverse modes, it is usually crucial to reduce the modal group delay difference in the cavity. In this paper, we use long-period fiber grating (LPFG) to compensate the large modal dispersion and differential modal gain in the cavity, realizing the spatiotemporal mode-locking in step-index fibers cavity. The LPFG inscribed in few-mode fiber could induce strong mode coupling, which has wide operation bandwidth based on dual-resonance coupling mechanism. By using dispersive Fourier transform involved intermodal interference, we show that there is a stable phase difference between the transverse modes constituting the spatiotemporal soliton. These results would be beneficial for the study of spatiotemporal mode-locked fiber lasers.

Effectiveness and mechanism of broaching-to prevention using global optimal control with evolution strategy (CMA-ES)

This paper demonstrates the effectiveness of using global optimal rudder control with the covariance matrix adaptation evolution strategy, applied to a surge-sway-yaw-roll coupled numerical model, in preventing broaching-to, occurring under conventional PD rudder controls in stern quartering waves. In particular, the PD control with optimized PD parameters is more efficient than the time history-optimized approach. With the help of stability analysis of surf-riding equilibria, two different mechanisms to successfully prevent broaching-to were identified: one is to tend to a periodic orbit with additional slight rudder actions and the other is to stay in surf-riding on a position of unstable equilibrium in regular waves for a longer duration with a larger differential gain. These outcomes could facilitate real-time prevention of broaching-to in actual seas.

High performance red-emitting multiple layer InGaN/GaN quantum dot lasers

InGaN/GaN self-organized quantum dots can provide useful advantages over quantum wells for the realization of long-wavelength visible light sources because the dots are formed by strain relaxation. A III–nitride based laser emitting in the red (λ ∼ 630 nm), which has not been demonstrated with quantum wells, would be useful for a host of applications. We have investigated the epitaxy and characteristics of self-organized InGaN/GaN multiple layer quantum dots grown by plasma-assisted molecular beam epitaxy and have optimized their properties by tuning the growth parameters. Red-emitting (λ ∼ 630 nm) quantum dots have radiative lifetime ∼2.5 ns and internal quantum efficiency greater than 50%. Edge-emitting red-lasers with multi-dot layers in the active region exhibit an extremely low threshold current density of 1.6 kA/cm2, a high temperature coefficient T0 = 240 K, and a large differential gain dg/dn = 9 × 10−17 cm2.

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.

1.3-μm-Wavelength AlGaInAs Multiple-Quantum-Well Semi-Insulating Buried-Heterostructure Distributed-Reflector Laser Arrays on Semi-Insulating InP Substrate

1.3-μm-wavelength AlGaInAs distributed-reflector laser arrays on semi-insulating InP substrate for high-speed direct modulation are investigated to realize compact and low-power-consumptive optical modules. Combination of AlGaInAs quantum wells with large differential gain and semi-insulating buried-heterostructure for reduction of the volume in active region achieved high-speed direct modulation. The fabricated lasers in the array lase with different wavelengths with stable single-longitudinal mode. Clear eye-opening and large mask margin were obtained in push-pull driving at 25.8 Gb/s direct modulation with coplanar electrode and semi-insulating InP substrate. Comparison of modulation waveform under simultaneous operation of neighbor lasers shows that crosstalk between lasers in the array is small. Clear eye-openings, large mask margins, and 10-km fiber transmission of four different wavelength lasers on LAN-WDM grid are demonstrated under 28-Gb/s operation at 50 °C. For further high-speed operation, 43-Gb/s direct modulation were performed. These results indicate that the 1.3-μm AlGaInAs DR laser arrays are promising as light sources for 100-Gb/s Ethernet and higher speed transmission.

A Model for a Quantum-Dot Gain-Lever Mode-Locked Laser

An alternative explanation of achieving passive mode-locking in a quantum-dot (QD) laser without a dedicated saturable absorber section is proposed. The method employs an analytical model based on the net gain modulation within the cavity. Two differentially pumped sections, operated in forward bias, may utilize the differential gain contrast between them to enable pulse formation. The large differential gain contrast available in QD media and the relative ease with which such devices can be fabricated extend the utility of this methodology for the development of a QD gain-lever mode-locked laser as a next generation optical source. Alternative cavity configurations that exclude a dedicated saturable absorber section and the concomitant reduction of intracavity losses provides another path in the development of short cavity lasers with high pulse repetition rates.

Small signal modulation characteristics of red-emitting (λ = 610 nm) III-nitride nanowire array lasers on (001) silicon

The small signal modulation characteristics of an InGaN/GaN nanowire array edge- emitting laser on (001) silicon are reported. The emission wavelength is 610 nm. Lattice matched InAlN cladding layers were incorporated in the laser heterostructure for better mode confinement. The suitability of the nanowire lasers for use in plastic fiber communication systems with direct modulation is demonstrated through their modulation bandwidth of f-3dB,max = 3.1 GHz, very low values of chirp (0.8 Å) and α-parameter, and large differential gain (3.1 × 10−17 cm2).

Healing Near-PT-Symmetric Structures to Restore Their Characteristic Singularities: Analysis and Examples

PT-symmetric structures, such as a pair of coupled waveguides with balanced loss/gain, exhibit a singularity of their eigenvalues around an exceptional point, hence a large apparent differential gain. In the case of fixed losses and variable gain, typical of plasmonic systems, a similar behavior emerges but the singularity is smoothened, especially in more confined structures. This reduces the differential gain around the singular point. Our analysis ascribes the origin of this behavior to a complex coupling between the waveguides once gain is present in an unsymmetrical fashion, even if guides feature the same modal gains in isolation. We demonstrate that adjunction of a real index variation to the variable waveguide heals the singularity nearly perfectly, as it restores real coupling. We illustrate the success of the approach with two geometries, planar or channel, and with different underlying physics, namely dielectric or plasmonic.

High-speed VCSELs for short reach communication

We present the design of a high-speed 850 nm multimode vertical cavity surface-emitting laser (VCSEL) and demonstrate record performance in terms of small signal modulation bandwidth (23 GHz) and error-free operation at high bit rates (40 Gb s−1). The large bandwidth was enabled by an active region design for large differential gain and small gain compression, a low reflectivity top mirror for photon lifetime reduction and multiple oxide layers for a reduction of the capacitance. Error-free operation at 40 Gb s−1 was achieved in a back-to-back configuration with less than 0 dBm of received optical power.

The antiguiding parameter in mid-infrared optically pumped semiconductor lasers

We describe measurements of the antiguiding parameter, α, for several optically pumped semiconductor lasers. Three laser structures were investigated; two of the lasers utilize W-quantum wells (QWs) in which 14 InAs/In0.4 GaSb/InAs QWs are imbedded in lattice-matched In0.25 GaAsSb layers. The emission wavelengths of the W lasers were ∼3.5 and 4.5 μm, respectively. The other laser, a double heterostructure (DH) design, contained a ∼1.5 μm InAsSb active region embedded in ∼2.5 μm thick AlAsSb clad regions. The emission wavelength of the DH was λ∼3.8 μm. We employed the Hakki–Paoli method [B. W. Hakki and T. L. Paoli, J. Appl. Phys., 44, 4113, (1973)] in conjunction with a Fourier transform infrared spectrometer to measure subthreshold gain and index variations as a function of pump intensity. To reduce errors associated with incoherent background emission a full spectral curve fit was used to determine the differential gain and index. The results reveal the antiguiding factor in the W lasers to be low with α∼1.0. The antiguiding factor for the DH was markedly larger with α=9.4±1.3. We attribute the low α for the W lasers to the higher QW gain as well as to inhomogeneous broadening induced by the 14 QWs. The differing well widths and the independent optical pumping of the wells, leads to a net gain spectrum that is symmetrical about the gain peak. This symmetry, in turn, leads to small differential index shifts at the gain peak; the result of the small differential index and large differential gain is low antiguiding

1.3-$\mu$m AlGaInAs Multiple-Quantum-Well Semi-insulating Buried-Heterostructure Distributed-Feedback Lasers for High-Speed Direct Modulation

This paper describes 1.3-mum AlGaInAs multiple-quantum-well semi-insulating buried-heterostructure distributed-feedback lasers for high-speed direct modulation. Combination of large differential gain AlGaInAs quantum wells and semi-insulating buried-heterostructure for reduction of active region achieved 25 and 40 Gb/s direct modulation with the device having the cavity length of 150 mum. The device whose Bragg wavelength is longer than gain peak wavelength showed 25 Gb/s direct modulation characteristics with superior temperature characteristics. The relaxation oscillation frequency of this device was 15 GHz and higher up to 70degC. Clear eye-opening was observed and single-mode fiber transmission experiments showed low power penalty up to 70degC. With the device of negative detuning with larger differential gain, we achieved 40 Gb/s direct modulation. This device showed high relaxation oscillation frequency and slope of 20.5 GHz and 3.2 GHz/mA1/2, respectively. The 40 Gb/s eye opened clearly at 25degC and eye-opening was observed up to 50degC.

Transverse-junction superluminescent diodes at the 1.1μm wavelength regime

We demonstrate, for the first time to our knowledge, GaAs-based transverse-junction (TJ) superluminescent diodes (SLDs) that operate at a wavelength of 1.1 microm. Due to lateral current injection by use of TJ, specified as transverse carrier flow spread in each quantum well horizontally instead of vertical well-by-well injection, nonuniform carrier distribution can be minimized among different multiple quantum wells (MQWs), which is a problem in vertical-junction (VJ) SLDs whose electroluminescent (EL) spectrum is governed by the center wavelength of QWs near the p side. In contrast with a VJ SLD, the EL spectrum of our device is determined by QWs that have a larger differential gain than the positions of QWs neighbored with a p side layer.

Reduction of Internal Loss and Threshold Current in a Laser Diode with a Ridge by Selective Re-Growth (RiS-LD)

The origin of the internal loss in ridge-type laser diodes (LDs) fabricated using selective re-growth is investigated through a systematic device characterization and additional optical measurements. We found that the internal loss of this LD is mainly caused by the absorptive layers at the re-growth boundary and Mg-doped GaN layer. The internal loss can be significantly reduced through a re-design of the LD structure to avoid these absorptive regions by shifting the perpendicular optical field to the n-cladding side. The re-designed LDs had a very low threshold current of 10 mA and superior gain characteristics. These results indicate, that the InGaN-quantum-well (QW) active layer has a large differential gain and fewer non-radiative defects. The fabrication method of this LD, i.e. epitaxial growth on low-dislocation-density GaN substrates combined with a process without dry-etching, is responsible for the high quality of the QWs.

Photonic bandgap materials: towards an all-optical micro-transistor

We describe all-optical transistor action in photonic bandgap (PBG) materials doped with active atoms and analyse the advantages of this system over other all-optical transistor proposals. In the presence of a PBG material, a coherent laser beam with the frequency slightly detuned from the resonant atomic transition frequency can drive a collection of two-level atoms to an almost totally inverted state, a phenomenon strictly forbidden in ordinary vacuum. By varying the laser field intensity in the neighbourhood of a threshold value, it is possible to drive the atomic system through a transition from states in which the atoms populate preferentially the ground level to almost totally inverted states. In this process, the atomic system switches from a passive medium (highly absorptive) to a active medium (highly amplifying). The large differential gain exhibited by the atomic medium is very robust with respect to nonradiative relaxation and dephasing mechanisms. The switching action in a PBG material is not associated with operation near a narrow cavity resonance with conventional trade-off between switching time and switching threshold intensity. Rather it is associated with an abrupt discontinuity in the engineered broad-band electromagnetic density of states of the PBG material. We demonstrate all-optical transistor action in PBG materials by analysing the absorption spectrum of a second probe laser beam and we show that the probe beam experiences a substantial differential gain by slight intensity modulations in the control laser field. Under certain conditions, the fluctuations in the number of totally inverted atoms that contribute to the amplification process are strongly diminished (the statistics of the excited atoms becomes sub-Poissonian), which, in turn, determines a very low-noise regime of amplification.

Extremely large differential gain of 1.26 µmGaInNAsSb-SQW ridge lasers

The differential gain of long wavelength GaInNAs-based quantum film (QF) lasers and highly strained GaInAs-based QF lasers have been investigated for the first time. These lasers were grown by gas-source molecular beam epitaxy, and include a small amount of Sb to improve the crystalline quality. GaInNAsSb single quantum well (SQW) ridge lasers that oscillate at 1.258 µm have an extremely large differential gain of 1.06 × 10-15 cm2 in spite of the SQW lasers; therefore GaInNAsSb lasers are suitable for high-speed lasers in the long wavelength region.

High-performance CW 1.26-μm GaInNAsSb-SQW ridge lasers

Long wavelength GaInNAsSb-SQW lasers and GaInAsSb-SQW lasers that include a small amount of Sb were successfully grown by gas-source molecular beam epitaxy (GSMBE). We confirmed that Sb reacts in a highly strained GaInAs-GaAs system and GaInNAs-GaAs system like a surfactant, which increases the critical thickness at which the growth mode changes from two-dimensional (2-D) growth to three-dimensional (3-D) growth. The lasers were processed into ridge lasers. The GaInNAsSb lasers oscillated under continuous-wave (CW) operation at 1.258 /spl mu/m at room temperature. The low CW threshold current of 12.4 mA and high characteristic temperature (T/sub c/) of 157 K were obtained for GaInNAsSb lasers, which is the best result for GaInNAs-based narrow stripe lasers. Further, the GaInNAsSb laser oscillated under CW conditions over 100/spl deg/C. On the other hand, GaInAsSb lasers oscillated under CW operation at 1.20 /spl mu/m at room temperature. The low CW threshold current of 6.3 mA and high characteristic temperature (T/sub c/) of 256 K were obtained for GaInAsSb lasers, which is also the best result for 1.2-/spl mu/m-range highly strained GaInAs-based narrow stripe lasers. We can say that GaInNAsSb lasers are very promising material for realizing an access network. Further, the differential gain of GaInNAs-based SQW lasers was estimated for the first time. GaInNAsSb-SQW lasers have the extremely large differential gain of 1.06-10/sup -15/ cm/sup 2/ in spite of the single-QW lasers; therefore, GaInNAsSb lasers are also suitable for high-speed lasers in the long wavelength region.

High-speed spin-polarized intersubband lasers

Spin-polarized intersubband lasers based on optically pumped type-II antimonide quantum wells in a magnetic field are proposed. Complete discretization of the electron energy spectrum is predicted to extend the electron lifetime considerably. Continuous-wave room-temperature operation is projected for both interband and intersubband pumping configurations lasing at λ=16–24 μm. Furthermore, the parasitic capacitances associated with electrical injection are eliminated, and the large differential gain and fast intrinsic time scale should give modulation bandwidths in excess of 100 GHz.

Effects of increased growth rate of well layer on 2.1% compressive strained InAsP-MQWs grown by metalorganic molecular beam epitaxy (MOMBE)

Effects of growth rate on metalorganic molecular beam epitaxy (MOMBE) growth of 2.1% compressive strained InAsP-multi-quantum wells (MQWs) was investigated using photoluminescence (PL), double-crystal X-ray diffraction (DCXRD) analysis and cross-sectional transmission electron microscopy (XTEM). As the growth rate of a well layer increases from 0.17 to 0.33 nm/s, the PL intensity increases remarkably and the width of satellite peaks in the DCXRD spectra becomes narrower. The amplitude of a well layer surface undulation, ascertained by XTEM, decreases as the growth rate increases, simultaneously. This indicates that increasing of the PL intensity and narrowing of the satellite peak width are attributed to decreasing of amplitude of the well layer surface undulation caused by increasing the growth rate. Utilizing this finding, we can increase the number of strained InAsP wells up to 20 without any evidence of material degradation due to well layer surface undulation. In addition, we fabricated buried heterostructure MQW lasers emitting a 1.56 μm wavelength light with a low threshold current of 7.0 mA and a large differential gain of 9.0×10 −16 cm 2.

CBE growth of 2%-strained InAsP MQWs for 1.5 μm wavelength laser diodes

Structural and optical properties of the compressively strained multiple quantum wells (MQWs), where InAsP well layer strain ranges from 1.9–2.15%, have been studied. Transmission electron microscopy reveals that 1.9%-strained MQWs have flat interfaces whereas 2.15%-strained MQWs have wavy interfaces as growth proceeds. The 1.9%-strained MQW photoluminescence (PL) intensities stay quite strong in the barrier tensile strain range of 0–0.4% but are reduced by 90% in the 0.6–0.9% range, i.e. strain compensation is not useful in the highly strained MQWs. A new type of MQW structure, where a thin compositional-graded InAs x P 1− x layer is inserted between the well and the barrier layers, provides six-well MQWs having high PL intensities. These results show that the shearing stress at the well–barrier interface rather than net strain plays an important role in growing the 1.9%-strained MQWs. We have fabricated the buried heterostructure MQW lasers emitting at a 1.5 μm wavelength light with a low threshold current of 6.5 mA and a large differential gain of 9.3×10 −16 cm 2.

Enhanced modulation bandwidth for strain-compensated InGaAlAs-InGaAsP MQW lasers

Strain-compensated (SC) multiple-quantum-well (MQW) lasers were designed using tensile-strained InGaAlAs barrier layers in order to enhance the modulation bandwidth of MQW lasers at 1.55 /spl mu/m. The design scheme simultaneously ensures the pseudomorphic growth of a large stack of highly strained wells, a uniform hole injection into large number of wells, a large conduction band discontinuity to suppress the carrier overflow effect, and a large differential gain by suppressing the band mixing effect. The SC-MQW structures were processed into a mushroom stripe laser structure to obtain a low parasitic capacitance. The number of wells and the cavity length were optimized to maximize the modulation bandwidth. Both the relaxation oscillation and RC cutoff frequencies increased with reducing the cavity length, and a maximum 3-dB modulation bandwidth of 30 GHz was obtained at a short cavity length of 120 /spl mu/m for 20-well SC-MQW lasers. Moreover, a high internal quantum efficiency and large differential gain were obtained for the SC-MQW lasers with well numbers of up to 20 as a result of the reduced carrier transport and overflow effects. The differential gain, gain compression factor, and K factor were evaluated experimentally from the modulation characteristics and compared to the theoretical calculation based on the spectral hole burning theory. The observed experimental results were well explained by the model using the identical intraband relaxation times typically used for 1.55-/spl mu/m bulk lasers.