Showing papers on "Semiconductor optical gain published in 2012"

Monolithic Ge-on-Si lasers for large-scale electronic–photonic integration

Abstract: A silicon-based monolithic laser source has long been envisioned as a key enabling component for large-scale electronic–photonic integration in future generations of high-performance computation and communication systems. In this paper we present a comprehensive review on the development of monolithic Ge-on-Si lasers for this application. Starting with a historical review of light emission from the direct gap transition of Ge dating back to the 1960s, we focus on the rapid progress in band-engineered Ge-on-Si lasers in the past five years after a nearly 30-year gap in this research field. Ge has become an interesting candidate for active devices in Si photonics in the past decade due to its pseudo-direct gap behavior and compatibility with Si complementary metal oxide semiconductor (CMOS) processing. In 2007, we proposed combing tensile strain with n-type doping to compensate the energy difference between the direct and indirect band gap of Ge, thereby achieving net optical gain for CMOS-compatible diode lasers. Here we systematically present theoretical modeling, material growth methods, spontaneous emission, optical gain, and lasing under optical and electrical pumping from band-engineered Ge-on-Si, culminated by recently demonstrated electrically pumped Ge-on-Si lasers with >1 mW output in the communication wavelength window of 1500–1700 nm. The broad gain spectrum enables on-chip wavelength division multiplexing. A unique feature of band-engineered pseudo-direct gap Ge light emitters is that the emission intensity increases with temperature, exactly opposite to conventional direct gap semiconductor light-emitting devices. This extraordinary thermal anti-quenching behavior greatly facilitates monolithic integration on Si microchips where temperatures can reach up to 80 °C during operation. The same band-engineering approach can be extended to other pseudo-direct gap semiconductors, allowing us to achieve efficient light emission at wavelengths previously considered inaccessible.

Practicality of compensating the loss in the plasmonic waveguides using semiconductor gain medium

Abstract: We consider the issue of compensating the loss in plasmonic waveguides with semiconductor gain material and show that, independent of specific geometry, full loss compensation in plasmonic waveguides with significantly sub-wavelength light confinement (less than λ/4n) requires current density well in excess of 100 kA/cm2. This high current density is attributed to the unavoidable shortening of recombination time caused by the Purcell effect inherent to sub-wavelength confinement. Consequently, an injection-pumped plasmonic laser that is truly sub-wavelength in all three dimensions (“spaser”) would have threshold current densities that are hard to obtain in any conceivable semiconductor device.

SESAM-free mode-locked semiconductor disk laser

Abstract: Experimental demonstration of semiconductor saturable absorber-free mode-locked optically pumped semiconductor disk laser is presented. The origin of pulsed operation is attributed to the intensity dependent Kerr lens effect arising in the semiconductor gain medium. Achieved results represent a novel method to mode-lock this type of laser opening new application opportunities. The laser worked stably in both hard and soft aperture configurations. No semiconductor saturable absorber was used in the laser cavity and the operation was self-starting. The laser was mode-locked at 210 MHz repetition rate with 1.5 W average output power and 930 fs pulse width at 985 nm. A record high 6.8 kW peak power was achieved. Measured data is presented along with a discussion of the Kerr lens effect in the cavity.

Ultrafast All-Optical Half Adder Using Quantum-Dot Semiconductor Optical Amplifier-Based Mach-Zehnder Interferometer

Abstract: Interferometric devices have drawn a great interest in all-optical signal processing for their high-speed photonic activity. Quantum-dot semiconductor optical amplifier (QD-SOA)-based gate has added a new momentum in this field to perform all-optical logic and algebraic operations. In this paper, a new and alternative scheme for all-optical half adder using two QD-SOA-based Mach-Zehnder interferometers is theoretically investigated and demonstrated. The proposed scheme is driven by the pair of input data streams for one switch between which the Boolean xor function is to be executed to produce sum-bit. Then the output of the first switch and one of the input data are utilized to drive the second switch to produce carry-bit. The impact of the peak data power as well as of the QD-SOAs current density, small signal gain, and QD-SOAs length on the ER and Q-factor of the switching outcome are explored and assessed by means of numerical simulation. The operation of the system is demonstrated with 160 Gbit/s.

Loss of Time Delay Signature in Broadband Cascade-Coupled Semiconductor Lasers

Abstract: Numerical and experimental investigations of time delay concealment and chaos bandwidth enhancement in three semiconductor lasers with a cascade scheme are presented. The cascade system is composed of an external-cavity master laser, a solitary intermediate laser, and a solitary slave laser. The results show that the first two lasers are used for concealing time delay signature, and the last two lasers are used for bandwidth enhancement. For large parameter regions of injection strength and frequency detuning, time delay concealment and bandwidth enhancement of chaotic signals are simultaneously achieved by adopting the simple cascade configuration.

Semiconductor Ring Laser With On-Chip Filtered Optical Feedback for Discrete Wavelength Tuning

Abstract: We introduce a novel concept of discretely tunable semiconductor lasers with on-chip filtered optical feedback. The integrated device is based on a semiconductor ring laser that can sustain two counter-propagating modes. By means of a directional coupler, part of the light emitted by the laser is coupled out to a feedback section integrated on the same chip. The feedback section contains two arrayed waveguide gratings and a set of semiconductor optical amplifiers to provide filtering of particular longitudinal modes sustained by the ring cavity. By controlling the current injected into the semiconductor optical amplifiers, single mode operation in both directions is achieved. In this paper, the design, characterization, and modeling of the device is presented.

Photonic Generation of Wideband Time-Delay-Signature-Eliminated Chaotic Signals Utilizing an Optically Injected Semiconductor Laser

Abstract: Photonic generation of wideband chaotic signals with time delay signature elimination is investigated experimentally and numerically based on a semiconductor laser (slave laser) with chaotic optical injection from a master laser. The master laser is subject to moderate optical feedback where the feedback strength and external-cavity length are fixed, while the slave laser stands alone. The experimental results show that wideband chaotic signals with successful time delay concealment can be generated in the slave laser by simply adjusting the coupling strength and frequency detuning between the two lasers. Furthermore, the numerical results are in accordance with the experimental observations. Finally, we propose a simple method for simultaneously generating multiple streams of high-quality chaotic signals using multichaotic lasers, where the time delay is effectively concealed in the autocorrelation function and delayed mutual information calculated from the chaotic time series.

Timing jitter from the optical spectrum in semiconductor passively mode locked lasers

Abstract: An analysis of the passively mode locked regime in semiconductor lasers is presented, leading to an explicit expression relating the timing jitter diffusion constant to the optical linewidths in these devices. Experimental results for single section quantum-dash based lasers validating the theoretical analysis are presented for the first time. Timing jitter of mode locked lasers at rates of up to 130 GHz has been experimentally estimated from the optical spectra without requiring fast photodetection.

Consistency and complexity in coupled semiconductor lasers with time-delayed optical feedback.

Abstract: Consistency of response in a system driven repeatedly by a complex signal has been observed in many nonlinear dynamical systems. We investigate the consistency of unidirectionally coupled semiconductor lasers with optical feedback and measure the complexity of the entire laser system by using the Lyapunov spectrum. The complexity strongly depends on the degree of consistency. It is found that the complexity of the coupled laser system can be classified into three regions. When the system shows consistency, the complexity of the entire laser system corresponds to that of the solitary drive laser. In the inconsistency region, the complexity of the entire laser system corresponds to the sum of the complexity of the uncoupled drive and response lasers. The complexity increases more than the sum of the two solitary lasers near the boundary of the consistency region, where new dynamical fluctuations appear due to the optical carrier interaction between the two lasers.

Polarization-Insensitive Quantum Dot Semiconductor Optical Amplifiers Using Strain-Controlled Columnar Quantum Dots

Abstract: A polarization-insensitive quantum dot semiconductor optical amplifiers (QD-SOAs) have been studied for use in future optical communication systems. A part of our work shows that the optical polarization property in QDs depends on both their aspect ratio and strain. To control these two parameters, we propose the use of strain-controlled columnar QDs (SC-CQDs), which exhibit a high aspect ratio and have strain-controlled side barriers for polarization-insensitive operation in the 1.5-μ m wavelength band. QD-SOAs with these optimized SC-CQDs demonstrated polarization-insensitive characteristics. They showed a gain of 8.0 dB with polarization dependence of the gain as low as 0.4 dB, -3-dB saturation output power of 18.5 dBm at a wavelength of 1550 nm, and error-free amplification at a bit rate of 40 Gbit/s.

Real-time frequency dynamics and high-resolution spectra of a semiconductor laser with delayed feedback

Abstract: The unstable emission of semiconductor lasers due to delayed optical feedback is characterized by combined intensity and frequency dynamics. Nevertheless, real-time experimental investigations have so far been restricted to measurements of intensity dynamics only. Detailed analysis and comparison with numerical models, therefore, have suffered from limited experimental information. Here, we report the simultaneous determination of the lasers optical emission intensity and emission frequency with high temporal resolution. The frequency dynamics is made accessible using a heterodyne detection scheme, in which a beat signal between the delayed feedback laser and a reference laser is generated. Our experiment provides insight into the overall spectral drift on nanosecond timescales, the spectral distribution of the unstable pulsations and the role of the individual external cavity modes. This opens new perspectives for the analysis, understanding and functional utilization of delayed feedback semiconductor lasers.

Wavelength switching dynamics of two-colour semiconductor lasers with optical injection and feedback

Abstract: The wavelength switching dynamics of two-colour semiconductor lasers with optical injection and feedback are presented. These devices incorporate slotted regions etched into the laser ridge waveguide for tailoring the output spectrum. Experimental measurements are presented demonstrating that optical injection in one or both modes of these devices can induce wavelength bistability. Measured switching dynamics with modulated optical injection are shown to be in excellent agreement with numerical simulations based on a simple rate equation model. We also demonstrate experimentally that time-delayed optical feedback can induce wavelength bistability for short external cavity lengths. Numerical simulations indicate that this two-colour optical feedback system can provide fast optical memory functionality based on injected optical pulses without the need for an external holding beam.

High-speed directly modulated lasers

Abstract: A collection of slides from the author's conference presentation is given. The topics discussed are: direct modulation of semiconductor lasers; frequency response of semiconductor lasers; and high-speed direct modulation.

Beam propagation method simulation and analysis of quantum dot flared semiconductor optical amplifiers in continuous wave high-saturation regime

Abstract: The authors present the investigation results of flared quantum dot (QD) semiconductor optical amplifiers (SOAs) in a continuous wave (CW) high-saturation regime using the finite difference beam propagation method (FD-BPM) model By combining the BPM model with pre-calculated results obtained from multi-population rate equations (MPREs), the authors include in a rigorous way the non-linear gain and refractive index variation of semiconductor gain medium caused by saturation effects in QD Using this model, a comprehensive analysis of the symmetric tapered QD SOAs is reported, verifying the influence of saturation effect on the characteristics of such devices and optimising the design of both gain and weakly index-guided structures for operation in the high-saturation regime

Experimental demonstration of the three phase shifted DFB semiconductor laser based on Reconstruction-Equivalent-Chirp technique.

Abstract: A three phase shifted (3PS) distributed feedback (DFB) semiconductor laser based on Reconstruction-Equivalent-Chirp (REC) technique is experimentally demonstrated for the first time. The simulation results show that the performances of the equivalent 3PS DFB semiconductor laser are nearly the same as that of the true 3PS laser. However, it only changes the μm-level sampling structures but the seed grating is uniform. So, its cost of fabrication is low. The measurement results exhibit its good single longitudinal mode (SLM) operation even at high bias current and surrounding temperature.

Progress in increasing the maximum achievable output power of broad area diode lasers

Abstract: High power broad area diode lasers provide the optical energy for all high performance laser systems, either directly or as pump sources for solid-state lasers. Continuous improvement is required in the peak achievable output power of these diode laser devices in order to enable performance improvements in full laser systems. In recent years, device technology has advanced to the point where the main limit to optical power is no longer device failure, but is instead power saturation due to various physical effects within the semiconductor device itself. For example, the combination of large optical cavity designs with advanced facet passivation means that facet failure is no longer the dominant limiting factor. Increases in the optical power therefore require firstly a clear identification of the limiting mechanisms, followed by design changes and material improvements to address these. Recent theoretical and experimental diagnostic studies at the Ferdinand-Braun-Institut have helped trace the saturation effects to three main effects: gain saturation, longitudinal-holeburning and current driven carrier leakage. Design changes based on these studies have enabled increases in the achievable emitted power density from broad area lasers. Recent experimental examples include ~100W from 100μm stripes under short-pulsed conditions, > 30W from 100μm stripes under quasi-continuous wave conditions and > 10W from 30μm stripes under continuous wave conditions. An overview of the results of the diagnostic studies performed at the FBH will be presented, and the design changes necessary to address the observed power saturation will be discussed.

Optical locking of two semiconductor lasers through high-order Brillouin Stokes components in optical fiber

Abstract: A novel technique for optical injection locking of two semiconductor lasers through high-order Brillouin Stokes components in optical fiber is proposed and experimentally demonstrated. The configuration potentially provides microwave signals generation at manifold Brillouin frequency without high-frequency external modulator and generator. Significant narrowing of the locked slave semiconductor laser linewidth was recorded.

Detuning and Thermal Effects on the Dynamics of Passively Mode-Locked Quantum-Well Lasers

Abstract: The impact of the wavelength-dependent properties of quantum-well (QW) saturable-absorbers (SAs) on the mode-locked operation of monolithic two-section QW lasers is examined with a travelling-wave description that includes a model for the optical response of the active material at finite temperatures. Good-quality mode-locking regimes occur only in small regions of parameter space due to the wavelength-dependent absorption and saturation energy of the SA section. Joule heating associated with photocurrent generation in the SA section dynamically modifies the relative detuning, and may eventually lead to the disappearance of mode-locking. Either proper thermal management of the absorber section or absorbers with wavelength-independent properties are needed in order to improve the breadth of the regime of good-quality mode-locked operation.

Semiconductor ring lasers coupled by a single waveguide

Abstract: We experimentally and theoretically study the characteristics of semiconductor ring lasers bidirectionally coupled by a single bus waveguide. This configuration has, e.g., been suggested for use as an optical memory and as an optical neural network motif. The main results are that the coupling can destabilize the state in which both rings lase in the same direction, and it brings to life a state with equal powers at both outputs. These are both undesirable for optical memory operation. Although the coupling between the rings is bidirectional, the destabilization occurs due to behavior similar to an optically injected laser system.

Ultrafast Vertical-External-Cavity Surface-Emitting Semiconductor Lasers

Abstract: We describe recent advances in the development of optically pumped passively mode-locked semiconductor lasers; ultrashort pulse sources that begin to offer levels of pulse duration, beam quality, and average power that formerly belonged only to diode-pumped solid-state lasers (DPSSLs) based on impurity-doped dielectric gain media. Unlike dielectric gain media, however, III–V semiconductors exhibit immense spectral versatility, with alloy compositions allowing emission wavelengths spanning the spectrum from visible through to the mid-infrared. Within the past few years, it has been shown that strained InGaAs/GaAs quantum well lasers operating around 1 μm are capable of generating transform-limited pulse durations of 100 fs or less; and moreover, that sub-400-fs pulses with > 300 W peak power, and 1.5-ps pulses with ∼ 500 W peak power can be generated. Very recently, material systems other than InGaAs quantum wells have been used to demonstrate femtosecond mode locking, with results reported for a self-assembled quantum dot laser, a 2-μm antimonide laser and a 1.5-μm indium phosphide device. The vertical-external-cavity surface-emitting semiconductor laser, or VECSEL, mode-locked under the influence of a semiconductor saturable absorber mirror in the external cavity, is thus capable of bridging the gap in performance between mode-locked edge-emitting diodes and DPSSLs. A particular advantage of VECSELs is that they operate easily at repetition frequencies in the 1–20 GHz range, where dielectric lasers tend toward Q-switching instability, whereas monolithic diodes become inconveniently large – the range addressed both by electronics, and by the optical resolution of simple grating devices.

Experiment on synchronization of semiconductor lasers by common injection of constant-amplitude random-phase light.

Abstract: We experimentally and numerically observe the synchronization between two semiconductor lasers induced by common optical injection with constant-amplitude and random-phase modulation in configurations with and without optical feedback. Large cross correlation (~0.9) between the intensity oscillations of the two response lasers can be achieved although the correlation between the drive laser and either one of the two response lasers is very small (~0.2). High quality synchronization is achieved in the presence of optical feedback in response lasers with matched feedback phase offset. We investigate the dependence of synchronization on parameter values over wide parameter ranges.

Model for direct-transition gain in a Ge-on-Si laser

Abstract: This paper describes a laser gain model for the direct, Γ-point transition in bulk Ge. The model allows calculation of gain and spontaneous-emission spectra for arbitrary strain, as well as doping and injected carrier densities. Many-body effects are included at the level of the screened Hartree-Fock approximation to account for energy renormalization modifications, especially to the impact of n-doping on magnitude and spectral extension of laser gain. Application of the model is illustrated by predicting the net material peak gain versus injected current density under different combinations of tensile strain and n-doping density.

Microwave frequency comb attributed to the formation of dipoles at the surface of a semiconductor by a mode-locked ultrafast laser

Abstract: The generation of terahertz radiation by focusing a mode-locked ultrafast laser on the surface of a semiconductor was demonstrated by Zhang in 1990, and others have made numerous measurements and analyses of this effect. We have measured the surge current which causes this radiation, showing that this current, and presumably the radiation, are frequency combs with harmonics at integer multiples of the pulse repetition rate of the laser. The harmonics in the current are enhanced by placing the semiconductor in a tunneling junction, where the fundamental is increased by 8 dB with a DC tunneling current of 100 pA.

Electrical modulation of the complex refractive index in mid-infrared quantum cascade lasers

Abstract: We have demonstrated an integrated three terminal device for the modulation of the complex refractive index of a distributed feedback quantum cascade laser (QCL). The device comprises an active region to produce optical gain vertically stacked with a control region made of asymmetric coupled quantum wells (ACQW). The optical mode, centered on the gain region, has a small overlap also with the control region. Owing to the three terminals an electrical bias can be applied independently on both regions: on the laser for producing optical gain and on the ACQW for tuning the energy of the intersubband transition. This allows the control of the optical losses at the laser frequency as the absorption peak associated to the intersubband transition can be electrically brought in and out the laser transition. By using this function a laser modulation depth of about 400 mW can be achieved by injecting less than 1 mW in the control region. This is four orders of magnitude less than the electrical power needed using direct current modulation and set the basis for the realisation of electrical to optical transducers.

Pulse shortening of gain switched single mode semiconductor lasers using a variable delay interferometer.

Abstract: We propose a pulse shaping and shortening technique for pulses generated from gain switched single mode semiconductor lasers, based on a Mach Zehnder interferometer with variable delay. The spectral and temporal characteristics of the pulses obtained with the proposed technique are investigated with numerical simulations. Experiments are performed with a Distributed Feedback laser and a Vertical Cavity Surface Emitting Laser, emitting at 1.5 µm, obtaining pulse duration reduction of 25-30%. The main asset of the proposed technique is that it can be applied to different devices and pulses, taking advantage of the flexibility of the gain switching technique.

Generation and Modulation of a Millimeter-Wave Subcarrier on an Optical Frequency Generated via Optical Injection

Abstract: A highly tunable millimeter-wave subcarrier signal is generated by optically injecting a Fabry-Perot semiconductor laser. The optically injected light, which enables microwave subcarrier frequencies well beyond the injected laser's free-running relaxation-oscillation frequency, is then on-off keyed by direct-current (dc) modulation of the injected slave laser. Adjustment of the subcarrier frequency is easily accomplished by changing either the dc bias current and/or junction temperature of the injected slave or the injecting master laser. In this paper, we theoretically and experimentally investigate the purity of the modulated microwave subcarrier. The generated microwave signal was then transmitted over 50 km of single-mode fiber, demonstrating the applicability of a directly modulated slave laser optically injected into the period-one state for radio-over-fiber applications.

All-optical latches based on two-photon absorption in semiconductor optical amplifiers

Abstract: The design and performance of two optical latches, the set–reset (S-R) latch and D flip-flop have been studied. These latches are the building blocks of large optical processors. The latches are built using two optical logic operations NAND and NOT. Both NAND and NOT operations are realized by using the ultrafast phase response during two-photon absorption process in semiconductor optical amplifiers. Rate equations for semiconductor optical amplifiers, for input data signals with high intensity, configured in the form of a Mach–Zehnder interferometer, have been solved. The input intensities are high enough that the two-photon induced phase change is larger than the regular gain induced phase change. Results show that this scheme can realize the functions of the S-R latch and D flip-flop at high speeds (∼250 Gb/s) with good signal-to-noise ratio.

Investigation of active filter using injection-locked slotted Fabry-Perot semiconductor laser

Abstract: We investigate the selective amplification and filtering of injection-locked slotted Fabry-Perot semiconductor lasers. Current and temperature tuning are used to selectively filter and amplify subcarriers of coherent optical combs with a selectivity of at least 10 GHz with an optical gain of up to 18 dB for filtered lines. A side mode suppression ratio in excess of 20 dB is also achieved.

Multiwavelength fiber ring laser using a gain clamped semiconductor optical amplifier

Abstract: We demonstrate a novel multi-wavelength fiber ring laser based on a gain clamped semiconductor optical amplifier The number of lasing lines can be tuned by adjusting the loss inside the cavity The wavelength interval between the wavelengths is 100 GHz The proposed laser shows a stable operation with total intensity fluctuation for a single laser line within ±002 dB at room temperature for a period of 30-minutes

Analysis of Gain-Switching Characteristics Including Strong Gain Saturation Effects in Low-Dimensional Semiconductor Lasers

Abstract: The effects of gain nonlinearities on gain-switched short-pulse-generation characteristics are analyzed via rate equations assuming a nonlinear-gain model including a gain saturation parameter gs to quantitatively describe the strong gain-saturation nonlinearity in low-dimensional semiconductor lasers at high carrier densities. It was found that the minimum pulse width and the delay time are mainly determined by gs rather than a differential gain coefficient g0 and a gain compression factor e. By tracing the temporal evolution of carrier density, photon density, and material gain during gain switching, distinctly different effects of gs, e, and cavity lifetime τp on pulse generation were clarified.