Wavelength demultiplexer using GaInAs-InP MQW-based variable refractive-index arrayed waveguides fabricated by selective MOVPE

A GaInAs-InP multiple quantum well (MQW)-based wavelength demultiplexer composed of an arrayed waveguide in which the refractive index varies across the array was fabricated. Since optical path length differences between waveguides in the array are achieved through refractive-index differences that are controlled by SiO/sub 2/ mask design in selective metal-organic vapor phase epitaxy (MOVPE), straight waveguide gratings having reduced optical propagation losses can be achieved. Furthermore, by employing MQW waveguides, variations in the refractive index may be induced through an applied electric field, allowing the device to manipulate wavelengths dynamically. A straight arrayed waveguide device having a 1.4% difference in refractive index was fabricated using an asymmetric side mask via a single selective MOVPE growth. The achievement of a diffraction angle difference of 4.40/spl deg/ between wavelengths of 1520 and 1580 nm was confirmed experimentally. In addition, a preliminary wavelength demultiplexer with a wavelength separation of approximately 25 nm and a free spectral range (FSR) of approximately 100 nm was also fabricated.     

Intersubband quantum-box semiconductor lasers

It is shown that semiconductor lasers utilizing intersubband transitions in quantum boxes, so-called intersubband quantum-box (IQB) lasers, can have significantly lower threshold current densities and operating voltages than quantum cascade (QC) lasers. In order to achieve this result, an enhancement factor of about 20 in the LO-phonon-assisted electron relaxation time is necessary. The increased gain for the radiative stage in an IQB laser eliminates the need for a multiradiative-stage structure (typically 25 stages in QC lasers). In turn, the electron injector and Bragg mirror regions on either side of the active region can be separately optimized. Due to their inherently lower input power requirements, IQB lasers operating in the mid-IR wavelength range should be capable of higher average-output powers than QC lasers at all temperatures. Furthermore, continuous-wave (CW) operation at room temperature with high wallplug efficiency becomes possible     

Noise of mode-locked semiconductor lasers

Analytical expressions for the amplitude, frequency, timing, and carrier phase noise of mode-locked laser diodes (MLLDs) are derived. It is found both experimentally and theoretically that carrier dynamics contribute to the total noise of MLLDs. In addition, we demonstrate how to capture the high-frequency timing jitter with optical cross correlations     

Rare earth doped fluoride waveguides fabricated using molecularbeam epitaxy

We have grown planar waveguides of rare earth doped single crystal fluoride films on insulating and semiconductor substrates using molecular beam epitaxy and have formed channel waveguides by ion milling. Structural and spectral analysis demonstrates that excellent crystallinity is being achieved and that the rare earth ion is incorporated into the film at sites and in charge states similar to bulk laser hosts. Lifetime measurements confirm that the local environment of the dopant ion is essentially that found for bulk materials. Single and higher order optical mode propagation has been demonstrated for the channel waveguides. By exciting individual channels with an 800 nm pump, we have generated strong upconversion fluorescence in Er and Nd doped guides. The ability to fabricate these waveguides On semiconductor substrates substantiates the potential for on chip integration of both IR downconversion lasers and IR pumped upconversion visible and UV lasers with a diode laser pump source. The use of transition metal dopants is possible and would enable tunable operation. Waveguide propagation loss in present devices must be reduced to realize a laser oscillator and we discuss how this is being addressed     

Synchronization of high-power broad-area semiconductor lasers

Semiconductor lasers offer significant operational advantages due to their compactness and high electrical-optical conversion efficiency. The major drawback in considering semiconductor lasers for many applications is the relatively small emission power that can be obtained from a single semiconductor laser. Synchronization of laser arrays provides a unique solution to this limitation. In this paper, we describe our recent research on the synchronization of high-power broad-area semiconductor lasers and laser arrays. We demonstrate experimental results on 1) simultaneous injection locking of multiple broad-area lasers to achieve single longitudinal/transverse mode beams and 2) synchronization and coherent beam combination of an integrated 19 broad-area laser array based on a scalable external cavity. A number of issues in the synchronization of broad-area lasers have been addressed in this paper. These include the effects of laser coupling on the array synchronization performance and the gigahertz complementary intensity oscillations occurring at different transverse modes of broad-area lasers subject to the optical injection.     

Two-section semiconductor lasers subject to optical injection

We investigate the dynamic properties of two-section semiconductor lasers subject to optical injection. A self-pulsating laser with a saturable absorber is used as the slave laser in the usual master-slave configuration of optically-injected lasers and a new model of the rate equations is applied. The dynamic properties are pictured in stability maps in the parameter space of the frequency detuning /spl omega/ and the injection strength K. New chaotic regions in the parameter space are found and investigated that are not observed in a simple injected semiconductor laser and also regions are reported where the field intensity pulsates periodically.     

Stability of injection-locked CW-emitting external-cavity semiconductor lasers

We analyze the stability of phase-locked emission modes of a semiconductor laser subject to both optical feedback and continuous-wave optical injection. We consider a large external cavity and examine the case of weak injection and feedback rates. We find that injection increases the number of excitable resonances in the external cavity and that phase-locked antimodes may bifurcate into a regime of stable bounded phase oscillations. The period of these oscillations is typically on the order of twice the round-trip time.     

Unidirectionally coupled synchronization of optically injected semiconductor lasers

Three different synchronization scenarios, namely identical chaos synchronization, chaotic driven oscillation, and chaotic optical modulation, are experimentally observed for the chaotic optical fields generated by optically injected semiconductor lasers that are unidirectionally coupled. In this fully optical system, the channel signal is different from the output field of the transmitter laser by an additional injection optical field delivered by a master laser. In the case of identical chaos synchronization, the output field of the receiver is synchronized, and frequency locked, to that of the transmitter but is not synchronized to the channel signal. In chaotic driven oscillation, the output field of the receiver is synchronized, and frequency locked, to the channel signal but is not synchronized to the output field of the transmitter. In chaotic optical modulation, the output field of the receiver is synchronized, but not frequency locked, only to the channel signal. These three different synchronization scenarios are identified in the same optical-injection system under different operating conditions.     

Dynamic modal analysis of monolithic mode-locked semiconductor lasers

We analyze the advantages and applicability limits of the mode-coupling approach to active, passive, hybrid, and harmonic mode-locking in diode lasers. A simple, computationally efficient numerical model is proposed and applied to several traditional and advanced laser constructions and regimes, including high-frequency pulse emission by symmetric and asymmetric colliding pulse mode-locking, and locking properties of hybrid mode-locked Fabry-Perot and distributed Bragg reflector lasers.     

The theory of non-Markovian gain in semiconductor lasers

In this paper, non-Markovian optical gain of a semiconductor laser is derived from recently developed time convolutionless (TCL) quantum kinetic equations for electron-hole pairs, including the many body effects. Plasma screening and excitonic effects are taken into account using an effective Hamiltonian in the time-dependent Hartree-Fock approximation. To calculate the optical gain, equation of motion for the interband pair amplitude is integrated directly. It is shown that the line shape of optical gain spectra is Gaussian for the simplest, non-Markovian quantum kinetics, and the optical gain is enhanced by the excitonic effects caused by the attractive electron-hole Coulomb interaction and the interference effects (renormalized memory effects) between the external driving field and the stochastic reservoir of the system. Enhancement of optical gain by the memory effects suggests the violation of strict energy conservation on a very short time scale, as compared with the correlation time of the system governed by non-Markovian quantum kinetics     

GaInNAs: a novel material for long-wavelength semiconductor lasers

GaInNAs was proposed and created in 1995 by the authors. It can be grown pseudomorphically on a GaAs substrate and is a light-emitting material having a bandgap energy suitable for long-wavelength laser diodes (1.3-1.55 μm and longer wavelengths). By combining GaInNAs with GaAs or other wide-gap materials that can be grown on a GaAs substrate, a type-I band lineup is achieved and, thus, very deep quantum wells can be fabricated, especially in the conduction band. Since the electron overflow from the wells to the barrier layers at high temperatures can he suppressed, the novel material of GaInNAs is very attractive to overcome the poor temperature characteristics of conventional long-wavelength laser diodes used for optical fiber communication systems. GaInNAs with excellent crystallinity was grown by gas-source molecular beam epitaxy in which a nitrogen radical was used as the nitrogen source. GaInNAs was applied in both edge-emitting and vertical-cavity surface-emitting lasers (VCSELs) in the long-wavelength range. In edge-emitting laser diodes, operation under room temperature continuous-wave (CW) conditions with record high temperature performance (T₀=126 K) was achieved. The optical and physical parameters, such as quantum efficiency and gain constant, are also systematically investigated to confirm the applicability of GaInNAs to laser diodes for optical fiber communications. In a VCSEL, successful lasing action was obtained under room-temperature (RT) CW conditions by photopumping with a low threshold pump intensity and a lasing wavelength of 1.22 μm     

Mode-locking in broad-area semiconductor lasers enhanced by picosecond-pulse injection

We present combined experimental and theoretical investigations of the picosecond emission dynamics of broad-area semiconductor lasers (BALs). We enhance the weak longitudinal self-mode-locking that is inherent to BALs by injecting a single optical 50-ps pulse, which triggers the output of a distinct regular train of 13-ps pulses. Modeling based on multimode Maxwell-Bloch equations illustrates how the dynamic interaction of the injected pulse with the internal laser field efficiently couples the longitudinal modes and synchronizes the output across the laser stripe. Thus, our results reveal insight into the complex interplay between lateral and longitudinal dynamics in BALs, at the same time indicating their potential for short optical pulse generation.     

Tapered-cavity surface-emitting distributed-Bragg-reflector semiconductor lasers: modeling and experiment

We report on the design and fabrication of tapered cavity grating coupled surface-emitting distributed Bragg reflector (DBR) lasers in the 980-nm regime. A curved second-order grating is used at the end of a tapered gain section to provide feedback as well as collimated surface out-coupling. A detailed numerical analysis shows that operation up to approximately twice the threshold is possible without significant degradation of the far field. Near diffraction-limited collimated surface-emitting output with moderate power of about 150 mW is obtained under continuous operation.     

Nonlinear dynamics of semiconductor lasers with mutual optoelectronic coupling

Mutually coupled oscillators are currently of great interest because of the important insight they provide into coupled physical, chemical, and biological systems. Two semiconductor lasers with optoelectronic feedback are used as two nonlinear oscillators, and the effect of mutual coupling on these lasers is found to be significant. Depending on the operating conditions, mutual coupling can act as a negative feedback to stabilize the coupled oscillators, or it can increase the complexity of the system inducing a highly complex chaos. A quasi-periodicity and period-doubling bifurcation, or a mix of them, is found in such a system. Although the chaotic waveforms are very complex with broad spectra, a high quality of synchronization between the chaotic waveforms is observed. Such synchronization is achieved because of the effect of mutual coupling and the symmetric design between the two lasers. It is found that the time delay of coupling plays an important role on the dynamics and synchronization in the mutually coupled semiconductor lasers.     

Large- and small-signal dynamic behavior of high-speeddual-polarization quantum-well semiconductor lasers

The transmission-line laser model is modified to model both transverse-electric (TE) and transverse-magnetic (TM) modes so that it is applicable to quantum-well (QW) dual-polarization lasers and polarization-insensitive semiconductor optical amplifiers (SOAs). The effects of carrier transport are also included in the model. The resulting dual-polarization transmission-line laser model is used to study large- and small-signal dynamic behavior of dual-polarization lasers. We find from large-signal simulations that the polarization asymmetry (ratio of the transverse-modal powers) varies on a nanosecond time scale in dual-polarization single-quantum-well (SQW) devices. We show that unequal transverse-modal differential gains and gain nonlinearities are responsible for this temporal polarization asymmetry. In addition, our numerical simulations show that the steady-state polarization asymmetry is a strong function of the gain nonlinearity. Small-signal dynamic simulations show that the modulation response of the polarization-unresolved output of dual-polarization SQW lasers follows that of the transverse mode with the lowest gain nonlinearity coefficient, regardless of the transverse-modal differential gains     

Transmission-line modelling of semiconductor lasers: The transmission-line laser model

The application of the transmission-line modelling technique to the design of semiconductor lasers and optical systems is reviewed. Generalized scattering matrices are developed to allow the technique to be applied to novel devices and systems.